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Atmospheric pressure plasmas: A review
Spectrochimica Acta Part B 61 (2006) 2 – 30 www.elsevier.com/locate/sab
Review
Atmospheric pressure plasmas: A review Claire Tendero a,*, Christelle Tixier a, Pascal Tristant a, Jean Desmaison a, Philippe Leprince b a
Laboratoire Sciences des Proce´de´s Ce´ramiques et Traitements de Surfaces, UMR CNRS 6638, Universite´ de Limoges, France b Laboratoire de Physique des Gaz et des Plasmas, UMR CNRS 8578, Universite´ Paris XI, Orsay, France 2
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Received 17 July 2005; accepted 5 October 2005 Available online 18 November 2005
Abstract This article attempts to give an overview of atmospheric plasma sources and their applications. The aim is to introduce, in a first part, the main scientific background concerning plasmas as well as the different atmospheric plasma sources (description, working principle). The second part focuses on the various applications of the atmospheric plasma technologies, mainly in the field of surface treatments. Thus this paper is meant for a broad audience: non-plasma-specialized readers will find basic information for an introduction to plasmas whereas plasma spectroscopists who are familiar with analytical plasmas may be interested in the synthesis of the different applications of the atmospheric pressure plasma sources. D 2005 Elsevier B.V. All rights reserved. Keywords: Plasma; Atmospheric; Review; Surface treatment; DBD; Corona; Torch
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Part A. Basic and fundamentals . . . . . . . . . . . . . . . . . 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Plasma generation . . . . . . . . . . . . . . . 2.1.2. Plasmas classification. . . . . . . . . . . . . . 2.2. Atmospheric pressure plasmas: LTE or non-LTE? . . . . 2.2.1. LTE plasmas . . . . . . . . . . . . . . . . . . 2.2.2. Non-LTE plasmas. . . . . . . . . . . . . . . . 2.2.3. Atmospheric pressure plasmas . . . . . . . . . 2.3. Overview of various atmospheric plasma sources . . . . 2.3.1. DC and low frequency discharges . . . . . . . 2.3.2. RF discharges. . . . . . . . . . . . . . . . . . 2.3.3. Microwave induced plasmas (MIPs) . . . . . . 2.3.4. Summary . . . . . . . . . . . . . . . . . . . . Part B. Applications of the various atmospheric plasma sources 3.1. Spectroscopic analysis . . . . . . . . . . . . . . . . . . 3.2. Gas treatments . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Gas cleaning . . . . . . . . . . . . . . . . . . 3.2.2. Gas synthesis . . . . . . . . . . . . . . . . . .
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* Corresponding author. Tel.: +33 6 16 77 73 47; fax: +33 5 55 42 36 80. E-mail address: [email protected] (C. Tendero). 1 ENSIL, 16 rue d’Atlantis, Parc d’ESTER Technopole, BP 6804, 87068 Limoges, France. 2 LPGP, Universite´ Paris Sud, Bat 210, ave G. Clemenceau, 91405 Orsay Cedex, France. 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.10.003
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3.3.
Material processing . . . . . . . . 3.3.1. Surfaces treatments . . . 3.3.2. Surface coating . . . . . 3.3.3. Bulk material treatments . 3.4. Lamps . . . . . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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1. Introduction Plasmas are chemically active media. Depending on the way they are activated and their working power, they can generate low or very high ‘‘temperatures’’ and are referred correspondingly as cold or thermal plasmas. This wide temperature range enables various applications for plasma technologies: surface coatings, waste destruction, gas treatments, chemical synthesis, machining. . . Nevertheless, many of these techniques have not been industrialized, albeit environmental norms are strictly followed. Thermal plasmas (especially arc plasma) were extensively industrialized, principally by aeronautic sector. Cold plasma technologies have been developed in the microelectronics but their vacuum equipment limits their implantation. To avoid drawback associated with vacuum, several laboratories have tried to transpose to atmospheric pressure processes that work under vacuum for the moment. Their researches have led to various original sources that are described here. This paper is a review about atmospheric plasmas. It is mainly about the technologies that are still under development in laboratories whereas arc and inductive plasmas are briefly discussed. After a summary about the different kinds of plasmas, the various sources are described in term of design, working conditions and plasma properties. Then the study focuses on their applications (spectroscopic analysis, gas treatments and materials proceedings) and concludes with a comparative synthesis of each system (applications, advantages, limits).
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The atmospheric plasmas described in this paper are generated from electrical energy. The electric field transmits energy to the gas electrons (which are the most mobile charged species). This electronic energy is then transmitted to the neutral species by collisions. These collisions [2] follow probabilistic laws and can be divided in: & Elastic collisions: they do not change the internal energy of the neutral species but slightly rise their kinetic energy & Inelastic collisions: when electronic energy is high enough, the collisions modify the electronic structure of the neutral species. It results in the creation of excited species or ions if the collisions are energetic enough. Most of the excited species have a very short lifetime and they get to ground state by emitting a photon. The ‘‘metastable species’’ are also excited states but with a long lifetime because their decay by emission of radiation is hampered as there are no allowed transitions departing from the respective state: decay can only take place by energy transfers through collisions. 2.1.2. Plasmas classification Depending on the type of energy supply and the amounts of energy transferred to the plasma, the properties of the plasma change, in terms of electronic density or temperature. These two parameters distinguish plasmas into different categories, presented in see Fig. 1. The atmospheric plasma sources described in this paper are supposed to be positioned near the glow discharges and the arcs.
2. Part A. Basic and fundamentals 2.1. Definitions Plasma is a more or less ionized gas. It is the fourth state of matter and constitutes more than 99% of the universe. It consists of electrons, ions and neutrals which are in fundamental and excited states. From a macroscopic point of view, plasma is electrically neutral. However, it contains free charge carriers and is electrically conductive. 2.1.1. Plasma generation A plasma is created by applying energy to a gas [1] in order to reorganize the electronic structure of the species (atoms, molecules) and to produce excited species and ions. This energy can be thermal, or carried by either an electric current or electromagnetic radiations.
Fig. 1. 2D classification of plasmas (electrons temperature versus electrons density) [5].
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Table 1 Main characteristics of LTE and non-LTE plasma LTE plasmas
Non-LTE plasmas
Current name
Thermal plasmas
Cold plasmas
Properties
Te = T h High electron density: 1021 – 1026 m 3 Inelastic collisions between electrons and heavy particles create the plasma reactive species whereas elastic collisions heat the heavy particles (the electrons energy is thus consumed)
Te H T h Lower electron density: <1019 m 3 Inelastic collisions between electrons and heavy particles induce the plasma chemistry. Heavy particles are slightly heated by a few elastic collisions (that is why the electrons energy remains very high)
Arc plasma (core) Te = T h å 10,000 K
Glow discharges Te å 10,000 – 100,000 K T h å 300 – 1000 K
Examples [117]
In this classification, a distinction can be made between: & Local thermodynamic (or thermal) equilibrium plasmas (LTE) & Non-local thermodynamic equilibrium plasmas (non-LTE). 2.2. Atmospheric pressure plasmas: LTE or non-LTE? The Local Thermodynamic Equilibrium notion [3] is really important, especially for a spectroscopic study of the plasma, since the determination of the plasma parameters (particles distribution functions; electron, excitation, vibration temperatures. . .) is based on relationships which differ for plasmas in LTE or not. 2.2.1. LTE plasmas LTE plasma requires that transitions and chemical reactions are governed by collisions and not by radiative processes. Moreover, collision phenomena have to be micro-reversible. It means that each kind of collision must be balanced by its inverse (excitation/deexcitation; ionization/recombination; kinetic balance) [4]. Moreover LTE requires that local gradients of plasma properties (temperature, density, thermal conductivity) are low enough to let a particle in the plasma reach the equilibrium: diffusion time must be similar or higher than the time the particle need to reach the equilibrium [5]. For LTE plasma, the heavy particles temperature is closed to the electrons temperature (ex: fusion plasmas). According to the Griem criterion [6], an optically thin homogeneous plasma is LTE if the electron density fulfills: 3 kT 23 E21 ne ¼ 9:10 m3 EHþ EHþ where ˝ E 21 represents the energy gap between the ground state and the first excited level,
˝ E H+ = 13.58 eV is the ionization energy of the hydrogen atom ˝ T is the plasma temperature. This criterion shows the strong link that exists between the required electron density for LTE and the energy of the first excited state. Those rules for LTE are very strict. Thus most of the plasmas deviate from LTE, especially all types of low density plasma in laboratories. 2.2.2. Non-LTE plasmas Departure from Boltzmann distribution for the density of excited atoms can explain the deviation from LTE. Indeed, for low-lying levels, the electron-induced deexcitation rate of the atom is generally lower than the corresponding electroninduced excitation rate because of a significant radiative deexcitation rate [4]. Another deviation from LTE is induced by the mass difference between electrons and heavy particles. Electrons move very fast whereas heavy particles can be considered static: electrons are thus likely to govern collisions and transitions phenomena. Deviations from LTE are also due to strong gradients in the plasma and the associated diffusion effects. It has been shown that the LTE distribution can be partial. For example, LTE can be verified for the levels close to ionization threshold [7] (e.g., 5p levels and higher, in argon plasma): such plasmas are pLTE (partial LTE). The non-LTE plasmas can be described by a twotemperature model: an electron temperature (Te) and a heavy particle temperature (T h). Regarding the huge mass difference between electrons and heavy particles, the plasma temperature (or gas temperature) is fixed by T h. The higher the departure from LTE, the higher the difference between Te and T h is. Table 1 sums up the main characteristics of LTE and nonLTE plasmas. More details on LTE and deviations from LTE are developed in the books by Huddlestone and Leonard [8], Griem [9], Lochte-Holtgreven [10] and Mitchner and Kruger [11].
Fig. 2. Evolution of the plasma temperature (electrons and heavy particles) with the pressure in a mercury plasma arc [5].
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2.2.3. Atmospheric pressure plasmas Fig. 2 shows the influence of the pressure on the transition from a glow discharge (Te > T h) to an arc discharge. Low pressure plasmas (10 4 to 10 2 kPa) are non-LTE. Heavy particles temperature is lower than the electronic one. The inelastic collisions between electrons and heavy particles are excitative or ionizing. These collisions do not rise the heavy particles temperature. When the pressure becomes higher, collisions intensify. They induce both plasma chemistry (by inelastic collisions) and heavy particles heating (by elastic collisions). The difference between Te and T h is reduced: plasma state becomes closer to LTE but does not reach it. The significant gradient of properties in plasma restricts a particle, moving in the discharge, achieving equilibrium. The density of the feeding power influences a lot the plasma state (LTE or not). On the whole, a high power density induces LTE plasmas (e.g. arc plasmas) whereas non-LTE plasmas are favored by either a low density of feeding power or a pulsed power supply. In this latter case, the short pulse duration prevents the equilibrium state from establishing. Finally, it is important to note that an atmospheric plasma jet can be divided in two zones: & a central zone or plasma core which is LTE & a peripheral zone which is non-LTE. In this plume, heavy particles temperature is much lower than electrons one. Indeed, for a free-burning argon arc [6], operating conditions (a pressure of 300 kPa, currents of 300 to 400 A) are necessary to reach a LTE state in the central portion. These conditions lead to an electron density of 1024 m 3 in the center. Departures from LTE occur in the outer regions of such arcs where the electron density decreases below 1024 m 3. Thus, the local thermodynamic equilibrium is a primordial notion since it induces the temperature of the plasma. It strongly depends on the kind of plasma source and is
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determining for its applications. In the next part, the various atmospheric plasma sources are described in terms of design, operating conditions (power supply, plasma excitation. . .). 2.3. Overview of various atmospheric plasma sources The excitation frequency is important since it influences the behavior of the electrons and the ions. Fig. 3 shows an example of the variation range for f pe (frequency of the electrons in the plasma) and f pi (ions frequency) in cold plasmas (e.g. glow discharges). The atmospheric plasma sources can be classified regarding their excitation mode. Three groups are then highlighted: & the DC (direct current) and low frequency discharges; & the plasmas which are ignited by radio frequency waves; and & the microwave discharges. Table 4 reports the main characteristics of these various plasma sources. Among them, the emerging of original microplasmas [12] is quite interesting. The trend of miniaturization of plasma systems is important in order to create low-powered directive portable systems and to reduce instrument and operation costs. 2.3.1. DC and low frequency discharges Depending on their design, the DC and low frequency discharges can work either with a continuous or a pulsed mode. A pulsed working mode enables the injection of large energy amounts in the discharge while the system warming up is limited. On the other hand, a pulsed power supply is technically more complex than a DC source and compromises the reproducibility of the process. 2.3.1.1. Continuous working mode: the arc plasma torches. The arc plasma torches [13] are fed by a DC power supply. They can be
Fig. 3. Electrons and ions frequencies in cold plasmas [99].
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Fig. 4. Principle of arc plasma torches (left: current-carrying arc, right: transferred arc).
divided into two categories: current-carrying arc and transferred arc (see Fig. 4). They both consist of: & a cathode where electrons are emitted; & a plasma gas injection system; and & a nozzle which confines the plasma. In a current-carrying arc torch, the nozzle which is positively polarized is the anode. In the case of a transferred arc torch, the treated material is the anode whereas the nozzle is at a floating potential. The arc is ignited between the cathode and the anode and ionizes the plasma gas. The plasma temperature varies from 8 000 K (plasma envelop) to 15 000 K (plasma core) which enables high temperature applications (use of the thermal effect of the plasma). An arc plasma is a very conductive media (I = 50– 600 A). The gas is highly ionized and the electronic density is about 3.1023 m 3. Through the years, the arc plasma torches have been improved and are strongly implanted in industries: & a high-energy, high velocity torch: Plazjet [14] (see Fig. 5); & a structure with three cathodes and a segmented anode: TRIPLEX (Fig. 6);
Fig. 5. High velocity Plazjet [100] (Tafa, Praxair).
& a torch working with a long plasma column stabilized by vortices [15]; and & A miniature seal-less plasma torch designed by CEA [15] (licensed by Europlasma). 2.3.1.1.1. Pencil-like torches. Those last 5 years, lowpowered flexible and innovative arc plasma torches have been commercialized: & Plasmapen and Plasmapen Xtension to increase the size of the treated surface (PVA-TEPLA [16]): see Fig. 7; & Plasma-JetR from Corotec Corporation [17]; & Openair technology (patterned by PlasmatreatR [18] ) is well implanted in production lines (automobile, textile, packaging. . .). They all use a homogeneous, low-powered, currentcarrying arc plasma jet to prepare a surface prior to joining it with adhesives, coating it, or printing upon it. On the opposite of the classical arc plasma torches, the discharge generates very little heat, which allows surface treatment of various materials including low temperature degradable materials (polymers). The classical arc torches can be classified as LTE discharges. They are characterized by rather high temperatures and are used
Fig. 6. Sultzer Metco Triplex II Plasma Spray Gun [101].
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Fig. 9. Hollow needle to plate discharge [102] (left: positive needle, right: negative needle). Fig. 7. Plasmapen (left) and Plasmapen Xtension (right), PVA-TEPLA [16].
for applications where heat is required (welding, cutting, spraying. . .), whereas the low-powered pencil-like torches involve a non-LTE discharge: they create a ‘‘chemically rich environment’’ which is used for low temperature applications. 2.3.1.2. Pulsed working mode 2.3.1.2.1. Corona discharge. Corona [19] discharge is a non-LTE discharge with low current density. The device (Fig. 8) consists of a cathode-wire and an anode (the treated material), the DC power supply is pulsed. The plasma creates a lighting crown around the wire: that is why this discharge is called ‘‘Corona’’. When a negative high voltage is applied to the wire, the discharge is a negative corona. The positive ions are accelerated towards the wire where secondary electrons are emitted and accelerated into the plasma: this moving front of high-energy (about 10 eV) electrons followed by a tail of lower energy electrons (about 1 eV) is called a streamer [96]. Inelastic collisions occur between these high-energy electrons and heavy particles and induce the formation of chemically reactive species. The pulses duration is shorter than the time necessary for the arc creation: when each pulse ends, the discharge extinguishes before it becomes too conductive. The transition into spark [20] is then avoided. The current discharge is very low: 10 10 to 10 5 A. A positive corona also exists: the positive polarized wire acts as the anode (see Fig. 9).
As the plasma volume is very small, the main corona drawback regarding surfaces treatments is the thin size of the treated surface. To increase the size of the surface treatment, the cathode wire can be replaced by a planer electrode which is parallel to the treated surface: this system generates micro-arcs (streamers) that are perpendicular to the gap between the electrodes. The streamers always initiate at the same place (default on the surface) causing a non-homogeneous treatment on the material surface. To avoid this problem, a dielectric barrier discharge was developed. 2.3.1.2.2. Dielectric barrier discharge. The DBD device (see Fig. 10) consists of two plane-parallel metal electrodes: at least one of these electrodes is covered by a dielectric layer. To ensure stable plasma operation, the gap which separates the electrodes is limited to a few millimeters wide. Plasma gas flows in the gap. The discharge is ignited by means of a sinusoidal [21] or pulsed [22] power source. Depending on the working gas composition, the voltage and frequency excitation, the discharge can be either filamentary or glow [23,24]. A filamentary discharge is formed by micro-discharges or streamers [25] that develop statistically on the dielectric layer surface. The use of helium as plasma gas seems to favor a glow discharge (high energetic He metastable species [24], Penning effect [23,26]). The dielectric layer plays an important part by: & limiting the discharge current and avoiding the arc transition that enables to work with a continuous or pulsed mode. & distributing randomly streamers on the electrode surface and ensuring a homogeneous treatment. The streamer creation is due to the electrons accumulation on the dielectric layer. The described DBD device is the most common one and other systems have been developed:
Fig. 8. Principle of a corona discharge.
& a DBD with a brush-style cathode [27] that consists of 25 fine stainless wires. This process is convenient for nonconductive sample treatment, & a system with a special designed cathode [28] which can be a applied to metallic and dielectric surface treatment,
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Fig. 10. Principle of dielectric barrier discharge (picture: a non-equilibrium diffuse plasma at atmospheric pressure [103]).
& a DBD with spiral electrodes [29] that are used for coating in the tube interior, & a device where the dielectric layer consists of capillary dielectrics [30] or a disc of glass beads [31]. 2.3.1.2.3. The derived-from-Corona processes. A lot of industries have developed processes which are similar to corona treating systems, even if they use an alternative power supply rather than a pulsed one: & ALDYNEi (created by Air Liquide and Softal [32]) is mainly adapted to on-line surfaces treatments of polymers (Fig. 11). & Corona treatment systems are manufactured by 3DT [33]: BottleDynei, FlexyDynei, PlasmaDynei which make it possible to Corona treat any three-dimensional polymeric surface delicately, quickly and efficiently. & The ‘‘AcXys Technologies’’ system [34] was developed by T. Sindzingre et al. (Socie´te´ AcXys Technologies [35], France). It consists of two cylindrical concentric electrodes. The plasma gas is introduced in the gap between the electrodes through an inlet in the outer electrode (see Fig. 12). The inner electrode is connected with a high voltage and low frequency source. The discharge is initiated in the gap. The afterglow, where long lifetime species are accessible, exits by an outlet in
Fig. 11. Principle of Aldynei process.
the outer electrode situated on the opposite side of the gas inlet. Physical and chemical treatments occur in this afterglow. 2.3.1.2.4. Microplasma. DC-operating microplasma on glass chips (see Fig. 13) were developed by J.K. Evju et al. [36] for the chemical modification of microchannel walls. The microchannels are formed by hot imprinting PS between glass microscope slides. Microelectrodes for plasma generation are fashioned from platinum wires that are sharpened by etching with alternating current in 6 M NaOH. A similar micro-source was designed by Bessoth and coworkers [37] for gas detection: the plasma is generated in a 70-Am deep and 500-Am wide channel between two tungsten electrodes. 2.3.2. RF discharges Regarding their structure, the RF sources can work with a high or low power supply. It influences the properties of the plasma and thus its potential applications. The impedance matching can be either inductive (high powered discharges) or capacitive. 2.3.2.1. High powered discharges 2.3.2.1.1. The ICP torches. The inductive discharges have been known for a long while. The RF torch is simply designed (see Fig. 14). The plasma is initiated and maintained by an RF fed helical coil.
Fig. 12. Principle of AcXys process.
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Table 2 RF torches characteristics with a 6000 K argon plasma [118] Working power (kW)
Torch diameter(mm)
Working frequency (kHz)
50 80 300 700
42 54 104 159
1020 630 166 72
Fig. 13. DC microdischarge design.
The current that flows in the RF coil induces a timevarying magnetic field nearby the plasma zone. The resulting electric ring field (see Faraday law) accelerates the electrons and thus maintains the discharge. The frequency of the generated plasma is higher than 1 MHz. This frequency level implies that electrons follow the electric field oscillations and neither ions nor electrons can reach the torch wall. This lack of contact between the plasma and the wall reduces the pollution of plasma and torch walls which enables to work with different gases: inert, reductive, oxidant, nitriding gas [38]. The plasma is enclosed in a ceramic tube (quartz, silicon nitride) that is cooled by air or water, depending on the working power. The inductive torches work in a wide power range: 20 kW– 1000 kW, with a gas flow rate of 10 –200 slm. A higher working power is accompanied with lower torch diameter and lower plasma frequency (see Table 2). These sort of inductive torches are presently very well developed (in particular by EFD induction SA [39], Tekna Plasma System Inc [40]. . .). They can be applied to spectroscopic analysis, toxic waste treatment. . . 2.3.2.1.2. The IST system. An intelligent electrodes design makes it possible to treat very complex surfaces in a fast and efficient way. The IST society has developed an RF pulsed discharge to decontaminate the interior of plastic bottles [41]. This process is the DBD adaptation to the complex surfaces treatment. Without introducing an electrode inside the bottle, the plasma is however generated inside the bottle with the help of surrounding and top central electrodes (see Fig. 15) and is stabilized by the dielectric walls. To make the discharge initiation easier, argon can flow nearby the central electrode.
This system needs high power source (20 kW) but the use of impulsions makes it possible to treat low temperature degradable materials. 2.3.2.2. Low-powered discharges. To initiate the discharge in the gas, a voltage must be applied between the electrodes. This breakdown voltage depends on the value of P d where P is the pressure and d the gap between the electrodes. Paschen laws [19] show that working at atmospheric pressure implies a thin gap (several millimeters) in order to have a realistic voltage. In these discharges, the impedance matching is capacitive. 2.3.2.2.1. The atmospheric pressure plasma jet (APPJ). The APPJ [19] is a small (L < 20 cm) RF plasma torch that works at low power. It was developed by J.Y. Jeong et al. (University of California, Los Angeles) in collaboration with J. Park et al. (Los Alamos National Laboratory [42]). This system (see Fig. 16) consists of two concentric electrodes through which the working gas flows. By applying RF power to the inner electrode at a voltage between 100 and 150 V, the gas discharge is ignited. The ionized gas exits through a nozzle since the gas velocity is about 12 m s 1. The low injected power enables the torch to produce a stable discharge and avoids the arc transition. The same research team (Park and coworkers [43], UCLA) designed a rectangular version of the APPJ. This source produces a volumetric and homogeneous discharge in a 1.6 mm wide gap between two planar aluminum electrodes. Both electrodes are perforated to let the plasma gas flow through them. The upper electrode is connected to the RF power supply while the lower electrode is grounded. It has been recently applied to the deposition of hydrogenated amorphous silicon [44] with silane added downstream of the hydrogen– helium plasma.
Fig. 14. RF plasma torch (Teckna Plasma System Inc [40]).
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Fig. 15. Principle of IST process.
Both sources are commercialized by Surfx Technologies [45] (Atomfloi: see Fig. 17). 2.3.2.2.2. The cold plasma torch. The cold plasma torch [46] developed by H. Koinuma et al. (Institute of Technology, Tokyo) lies between the DBD and APPJ structures. The device is shown in Fig. 18 and its properties are reported in Table 4. The RF electrode is a stainless-steel needle. A quartz tubing is inserted between the cathode and anode to ensure both plasma stability and homogeneity. The plasma gas flows into the gap between the cathode and the dielectric tube. 2.3.2.2.3. The hollow cathode systems. Designed by Jane`a et al. (Masaryk University, Brno, Republic Czech), the RF pencil (see Fig. 19) is very close [47] to the cold plasma torch. This coaxial device is very small (L < 10cm). The RF electrode is a hollow needle; the plasma gas flows inside it. This cathode is inserted into a quartz tube. The plasma is generated inside the hollow electrode. The gas speed is high enough to make the plasma jet flow outside the sharp top of the needle. A barrier torch [48] which is similar to the RF pencil was also developed in Czech Republic by Hubie`ka et al. (Academy
of Sciences, Prague): see Fig. 20. A quartz tube is inserted into the RF hollow electrode. The working gas is injected inside the dielectric tube. The dielectric layer stabilizes the discharge and limits the electrode heating. The discharge remains stable in the case of a multi-nozzle torch-barrier device. A larger surface zone can be treated. In the same category, the HEIOS and HELIOS systems [49] were introduced by Bardos and Barankova. They create stable and uniform RF plasma over large area. HEIOS is a cylindrical hollow cathode whose open structure is made with 900 channels where the working gas flows: it is a 900 hollow simultaneously working cathodes assembling that generates a stable discharge over 7 cm2. HELIOS [50] design is based on the same idea: it is a rectangular hollow cathode that generates stable and uniform RF plasma over 20 cm2. The design of these sources allows their direct scaling up for large area plasma applications. 2.3.2.2.4. Microplasma. Recently, small-scale and low power plasmas have been investigated by several authors. Those microplasmas can be applied in various fields, as summarized in Table 3 (Fig. 21).
Fig. 16. APPJ design.
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Fig. 17. Atmofloi plasma can clean and activate surface prior to bonding [45].
2.3.3. Microwave induced plasmas (MIPs) The electrode-less microwave systems all work according the same principle. Microwaves are guided along the system and transmitted energy to the plasma gas electrons. Elastic collisions between electrons and heavy particles occur. Due to the large mass of heavy particles, the collided electrons rebound whereas the heavy particles remain static. The electrons are thus accelerated (they get kinetic energy) and the heavy particles are slightly heated. After several elastic collisions (which follow probabilistic laws), the electrons get enough energy to produce inelastic exciting or even ionizing collisions. The gas is partially ionized and becomes plasma which supports microwave propagation [51]. The next described microwave plasma sources are all designed according to the same idea. They consist of: ˝ a microwave power source (power supply, magnetron and circulator to protect the magnetron from the reflected power); ˝ microwave equipment (wave guides, tuning system); ˝ an ignition system; and ˝ gas injections.
Fig. 18. Cold plasma torch design.
Fig. 19. RF pencil design.
The discharge ignition is the key to microwave sources. Indeed, a self-ignition of the discharge ensures flexible operating conditions and enables the industrialization of the process. The energy, transmitted to the gas electrons, has to be high enough to initiate the plasma. In the literature, several methods are suggested to concentrate the microwave energy: ˝ The indirect ignition with a conductive rod [52] which plays the part of an antenna. The microwave are then caught and concentrated at the tip of the rod. ˝ A resonator cavity [53] that makes the electric field maximum where the plasma gas flows. ˝ A helical coil [54] that induces a circularly polarized wave. The energy which is transferred to electrons is then enhanced. ˝ A material with electrically conductive and heat-resistive properties [55]. This material can generate easily stable plasma when irradiated by microwaves in an argon flow.
Fig. 20. Barrier torch design.
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Table 3 Examples of RF microplasma sources Source
Source characteristics
Applications
Micro-structured electrode (MSE) [119,120]
Capacitively coupled discharge Micro-reactor volume: 60.5 mm3 Multi-reactor (MSE array) volume: 16 60.5 mm3
Fluorinated waste gas treatment Diamond-like carbon deposition Sterilization
Capacitively coupled microplasma CCAP [121]
Microchannel: 10 mm long, 200 – 500 Am deep See Fig. 21
Detector for gas chromatography
Miniaturized plasma jet source [122]
Inductively coupled discharge Discharge tube: 1 1 30 mm3 Plasma source chip: 15 30 mm2
Optical emission spectroscopic analysis of liquid sample
These atmospheric MIPs can be classified in three categories: ˝ resonant cavity plasmas; ˝ free expanding torches; and ˝ microplasmas. 2.3.3.1. Resonant structures. The Beenakker [56] cavity is the most-well known resonant cavity to generate microwave plasmas. A resonator cavity uses resonance phenomena to amplify a wave. Its interior surfaces reflect one type of wave. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low-energy loss: a standing wave is thus generated. As more waves enter the cavity, they combine with the standing wave and reinforce it, increasing its energy. This energy is used to initiate the discharge. The resonance frequency of the cavity strongly depends on its geometry parameters, especially on its radius (see Fig. 22). Thus, it is necessary to fit the geometry of the cavity with the frequency delivered by the microwave generator (2.45 GHz), in order to ensure the ignition of the plasma. 2.3.3.2. Free expanding plasma torches. These sources create a plasma that flows in open air. Regarding the structure of the torches, a distinction can be made between the metallic and the semi-metallic torches. 2.3.3.2.1. Metallic torches. The TIA (torche a` injection axiale) microwave plasma torch was developed by Moisan et al. [57]. It works with a conventional wave guide-to-coaxial line transition. The plasma gas flows in the inner coaxial line conductor and exits through a nozzle (see Fig. 23). The microwaves are generated by the magnetron and reach the working gas using rectangular and coaxial guides. The discharge
Fig. 21. Design of a capacitively coupled microplasma (CCMP).
is ignited on top of the nozzle. The plasma consists of a needlelike converging filamentary cone with a tail flame on top of it (see Fig. 24). The guide selects the propagating modes: TE10 in the rectangular guide (E z = 0 with z the wave propagation direction) and TEM in the coaxial line (E z = Hz = 0), which limits the energy loss that could come from parasite modes excitation. Impedance matching functions are achieved using two intrinsic tuning elements (movable plungers: short circuit and initiator): the transition between TE10 and TEM modes is then optimized and the reflected power is minimized. This metallic microwave torch was patented by P. Leprince et al. [58] (Laboratoire de Physique des Gaz et des Plasmas, Orsay, France). Other laboratories have studied other microwave torches using the same wave guide-to-coaxial line transition. The design changes slightly depending on the torch application field. In order to limit the interactions with surrounding atmosphere, Jasinski et al. [59] confined the plasma by putting a quartz tube around the nozzle. Okamoto et al. [54] have put a quartz tube (longer than the coaxial line) inside the inner conductor to inject aerosol samples and confine the plasma (torch for spectroscopic
Fig. 22. Resonance modes and frequencies of a cylindrical cavity [104].
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Fig. 23. TIA design.
analysis). The discharge ignition is induced by a helical coil. This torch produces annular-shaped plasma. Suzuki et al. [60] have designed a torch that uses the guidecoaxial line transition without movable plunger initiator. Water flows inside the inner conductor whose top is made of molybdenum to prevent thermal damage. A copper nozzle is fitted at the end of the torch. The plasma gas is injected in the coaxial line gap. The discharge is generated in the space between the molybdenum top and the nozzle where the electrical field concentrates. As the TIA design is quite sophisticated, its construction requires advanced and expensive workmanship. Thus, this coaxial torch has been simplified. Moisan et al. have designed the TIAGO (TIA sur guide d’onde). It is a simple and compact one-nozzle unit [61] but electromagnetic radiation can be emitted from the gap between the nozzle and the wave guide.
Fig. 24. The three different zones that can be distinguished in typical plasmas produced by the TIA [105].
To protect the personnel and instrumentation from those radiations, Jasinski et al. [62] have placed a cylindrical metal grid inside the reactor, coaxially around the nozzle. This grid influences the plasma flame, improving its stability. This system is called the Microwave Torch Discharge (MTD). In United Kingdoms, a very simple structure was developed by S.R. Wylie et al. [63]: the Microwave Plasma Jet (MPJ). The plasma cavity consists of a rectangular wave guide section that ends at a copper plate acting as a short circuit. The gas nozzle is situated inside the cavity directly above an aperture in the wave guide, one-quarter wavelength from the short circuit, where the maximum electric field is produced. This system can work either at 2.45 GHz or at 896 MHz [64] (this latter wavelength limits the electromagnetic radiation, in spite of the wave guide aperture). 2.3.3.2.2. Semi-metallic torches. The design of this torches is very similar the one of the metallic torches. The main difference is the propagation mode of the electromagnetic waves, as there is no guide-to-coaxial line transition. In the semi-metallic torches, the plasma gas flows through a quartz tube (which is transparent to microwave radiations). Instead of burning out on top of the nozzle, the discharge is initiated in the zone where the quartz tube intersects the rectangular wave guide [65] (see Fig. 25). This system (also called ‘‘guide-surfatron [66]’’) involves surface waves: the propagation mode is converted from the TE01 mode (in the rectangular wave guide) into the TM01 mode (in the quartz capillary). The microwaves propagate along the interface between the quartz tube and the plasma: then, the plasma can be extended over long distances. In the literature, this simplest semi-metallic torch is called Microwave Plasma Torch (MPT). It was first developed by Jin and coworkers [67] for atomic emission spectroscopy, as a possible alternative to the ICPs in analytical chemistry. The MPT creates a flame-like discharge: in general the flame is luminous and white-colored with a fainter central channel [68].
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Fig. 25. MPT design.
Fig. 26. Pulsed microwave torch design (Baeva et al.).
The key remains the way to initiate the discharge: it has to be self-ignited in order to ensure flexible operating conditions and to make the industrialization of the process possible. Thus Y.C. Hong et al. [69] have designed an original system made of a three microwave plasma torches line. Each torch is turned on by tungsten igniters (similar to spark plugs) positioned slightly away from the center of the wave guide. This kind of ignition enables the use of a very low cost microwave power supply (magnetrons used in typical home microwave ovens). Still in order to achieve flexible operating conditions, Pott et al. [70] have developed a torch with a resonant cavity (see Fig. 26). Moreover the working gas flows with a superimposed vortex. This kind of injection avoids wall burning and stabilizes the discharge. This torch works with a pulsed power supply, which limits the system heating while working with high-energy amounts (and thus very energetic plasma species). Another way of igniting the discharge is the one developed by Sugiyama et al. [71]. They found that an atmospheric argon cold plasma discharge could be generated by microwave
irradiation on the perovskite type powders. These powders are electrically conductive and thermally resistant. All those microwave plasma torches are still in development in laboratories. CyrannusRI (cylindrical resonator with annular slots) is an example of industrialized microwave atmospheric plasma source. It generates a homogeneous plasma in a quartz tube (Fig. 27) and was developed in Germany by Iplas [72].
Fig. 27. The CyrannusRI source (Iplas, Germany) [72].
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Table 4 Characteristics of atmospheric plasma sources Excitation
Source
Plasma properties
Operating conditions
Industrialized sources DC/Low frequency
Arc torch [5,123]
Te = T h å 8000 – 14,000 Ks n e = 1021 – 1026 m 3
PlasmatreatR [124]
T h < 700 K
Gas: Ar /He Gas flow: 10 – 150 slm Power: 10 – 100 kW Gas: air Gas flow: 117 slm
Corona [19]
Te = 40,000 – 60,000 K T h < 400 K n e = 1015 – 1019 m 3 Te = 10,000 – 100,000 Km T h < 700 K n e å 1018 – 1021 m 3
Gas: air
Pulsed DC/Low frequency
DBD [43]
Some 100 W Plasma gas: 5 – 40 slm
Radio frequency
ICP [5,123]
Te = T h = 6000 – 11,000 Ks n e = 1021 – 1026 m 3
Gas: Ar/He Gas flow: 10 – 200 slm Power: 50 – 700 kW
Pulsed radio frequency
IST [41]
T h < 400 K
Gas: surrounding air No gas flow Power: 20 kW
Microwave
Cyrannus [72]
T h < 700K
Gas: Ar/O2 Power: 6 kW
Still in laboratory development sources DC
Microplasma
–
Power: 500 V; 250 AA
Radio frequency
APPJ [19]
Te = 10,000 – 20,000 Ks, T h < 600 K n e = 1017 – 1018 m 3 Te = 10,000 – 20,000 Ks, T h < 700 K n e = 1017 – 1018 m 3 Te = 3000 – 11,000 Ks T h < 800 K n e å 1017 – 1018 m 3 Te = 1850 – 2300 K
Cold plasma torch [19]
Hollow cathode [48,49]
Microplasma CCAP [12]
Microwave
TIA [125]
MTD [62]
MPJ [63]
MPT [126,127]
Baeva et al. [70]
Sugiyama et al. [128]
s
: temperatures calculated from spectrometric measurements. : temperatures calculated from models. l : temperatures calculated from Langmuir probe measurements. t : temperatures measured by thermocouple. m
Te = 13,000 – 14,000 K s T h = 2400 – 2900 Ks n e å 1021 m 3 Te = 17,000 – 20,000 Ks T h = 1500 – 4000 Ks n e = 1020 – 1021 m 3 Te = 12,000 – 17,000 Ks T h = 5000 – 10,000 Ks n e å 1022 m 3 Te = 16,000 – 18,000 Ks T h = 3000 – 3500 Ks n e å 1020 – 1021 m 3 Te å 7000 Ks T h å 7000 Ks n e å 1019 m 3 Te å 90,000 Ks T h å 1000 Kt n e å 1017 m 3
m, l
l
Gas: O2 /He Gas flow: 50 – 90 slm Power: some 100 W Gas: Ar Gas flow <1 slm Power: 100 W Gas: Ar, He Gas flow <2 slm Power: some 100 W Gas: Ar Gas flow: <0.2 slm Power: 5 – 25 W Gas: He Gas flow: 2 – 6 slm Power: 100 W – 2 kW Gas: N2 Gas flow: 1 – 3 slm Power: 100 W – 400 W Gas: Ar Gas flow: 2 – 7 slm Power: 2 – 5 kW Gas: Ar Gas flow <1 slm Power: some 100 W Resonant cavity Pulsed MW power supply Gas: N2 Gas flow: 30 slm Power: 800 W Ignition by perovskite powder Gas: Ar/H2 Gas flow: 0.3 – 1.2 slm Power: some 100 W
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C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30 Table 6 Ozone density in atmospheric plasma discharges [19]
Fig. 28. Classification of atmospheric pressure plasma sources.
This system is well adapted to surface preparation (polymer activation) but its closed design avoids direct integration to production lines. 2.3.3.3. The microstrip plasma. Bilgic et al. [73] have developed a low-powered, compact microwave induced plasma source. It basically consists of a planar microstrip line on fused silica as dielectric substrate and a massive copper ground plate. It involves surface waves that propagate in a gas channel along the interface between the dielectric substrate and the plasma. The tiny size of the system limits the energy loss in the dielectric. This microstrip plasma technology has been mainly applied to atomic emission spectroscopy [74]. 2.3.4. Summary Table 4 shows the characteristics of the various atmospheric plasma sources in terms of plasma properties (electron temperature and density, gas temperature) and working conditions (power supply, gas flow). The nature of the plasma gas is important since it influences the plasma temperature. The temperature [75] does not change if the working gas is argon, argon/hydrogen, nitrogen, air, oxygen, argon/helium because the ionization energies are very closed (between 13.5 and 16 eV). On the other hand, in a helium plasma, the ionization
Excitation
Source
Ozone density (cm 3)
DC Pulsed DC Low frequency RF
Arc plasma torch Corona DBD APPJ
<1010 1018 1018 1016
energy is much higher (24 eV), resulting in a 4000 or 5000 K gap temperature. The plasma temperatures evaluation must be carefully considered. Plume temperatures evaluated from thermocouple measurements are likely to correspond to heavy particles temperature (that fixes the gas temperature). The plasma core temperature is recorded by spectroscopic measurements. Spatially resolved measurements have to be carefully done with the appropriate techniques depending on whether LTE applies or not. It is now interesting to place the various discussed sources on Fig. 1, in order to know more about the properties of the different plasmas which condition their applications. Indeed, glow discharges involve high chemical reactivity and low plasma temperatures whereas arc and ICP discharges provide much higher gas temperatures. Thus glow-discharge-like plasmas suit for applications that need both chemical reactivity and low temperature of the plasma (surface activation, coatings. . .). Arc-discharge-like plasmas are required for high temperature applications (welding, melting. . .). Fig. 28 classifies the various plasma sources regarding their electronic density and temperature. It puts the light on general trends: ˝ Corona, APPJ, Sugiyama et al. microwave torch and DBD involve the same temperature and density as a glow discharge. They can be applied to low temperature processes [23,43]. The DBD characteristics are between those of glow discharge and those of arc discharge. This can be correlated to two working modes that have been observed experimentally: a glow and a filamentary discharge.
Table 5 Gas cleaning by atmospheric plasma Excitation
Source
Working conditions
Plasma
Pollutant
Destruction efficiency
Observations
Semi-metallic microwave torch
Kiyokawa and al. torch [129] Baeva and al. torch [130,131]
0.6 slm 90 W
Ar Ar/12% O2/2% H2O N2/NO N2/NO/10%O2 N2/NO/2%O2
NO 2000 ppm
Reaction products: N2 and O2
NO 500 ppm
98% 18% 50% – 50%
TIA [132]
1 slm 220 W
Air/CHCl3
CHCl3 (3%)
100%
MTD [62]
2 slm 400 W
air
CFC 50%
100%
MPJ [133]
5 slm 400 W
N2/NO
NO 100 ppm
90%
Reaction products: CO2, CO, NOx , HCl, COCl2, H2O Destruction is more efficient in air than in N2 No production of NOx torch is more efficient than low pressure plasma No production of NOx
Metallic microwave torch
20 slm A few pulsed kW 10 slm 1 MW pulsed
– Reaction products: NO and NO2 Complex design
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Fig. 30. Professor Hicks’ group (UCLA: Chemical Engineering Department, Semiconductors Manufacturing) has developed the APPJ for cleaning, etching and deposition of materials. The ‘‘cold flame’’ (T ¨75 -C) is impinging on the hand of a student. [107].
intermediary between a homogeneous and an arc discharge. This behavior enables a wide range of applications for those plasma sources. Depending on the plasma zone that is involved for the treatment, it is possible to use either rather high temperatures or chemical reactivity of the plasma [70].
Fig. 29. A cold atmospheric plasma is sterilizing a dental needle at Loughborough University (plasma and pulsed power research group) [106].
˝ The properties (e.g. n e, Te) of the microwave metallic torches (TIA, MPJ) are similar to those of the arc plasmas. These sources suit for high temperature processes [168]. ˝ Microwave torches (Baeva et al., MTD, MPT) produce plasmas with characteristics (n e, Te) that are just between those of arc plasmas and those of glow discharge. The plasma generated by those microwave torches is thus likely to be an
3. Part B. Applications of the various atmospheric plasma sources The described atmospheric plasma sources are very different (structure, power supply, plasma temperature, working conditions) and therefore they have various applications.
Table 7 Surface cleaning by atmospheric plasma Excitation
Source
Contaminant (substrate)
Plasma
Treatment duration
Observations
Low frequency
DBD
Ag2S (Ag) [26] Oil (Al, Si) [134,135]
Ar
180 s
Ag2S layer is removed
Air, O2
A few sec
Fe2O3 (Fe) [136]
Ar/N2
60 s
Lubricant is totally removed if the plasma gas flow rate is low (1 – 5 slm). With a high flow rate, polymerization of the oil occurs Plasma cleaning is more efficient in air than in O2: importance of the metastable N2 species Surface is cleaned Complete cleaning mechanism is not yet established: it is different from a simple etching mechanism by nitrogen active species)
Pulsed low frequency
Glow DBD [137]
Oil (Fe)
O2
10 min
Oil is removed DBD is as efficient for oil removal as ultrasonic cleaning in acetone
Radio frequency
Plasma pencil [47]
Corrosion (archeological metal artifacts)
Ar
30 s
APPJ [138]
Biological, chemical agents (glass)
He/O2
30 s
Pulsed radio frequency
IST system [41]
Micro-organism (PET bottle)
Air
15 ms
Reduction of the corrosion products on antique metallic artifacts The object is immersed in a chemically reactive liquid in order to combine the efficiency of the plasma treatment and the selectivity of the chemical processes Neutralization of chemical and biological agents (e.g. mustards, anthrax) APPJ operates at low temperature and does not generate harmful or toxic products: thus it is suitable for rapid decontamination of material and safe for personnel Sterilization and deodorization inside PET bottles No damage (mechanical, thermal) on the surface Industrial process: 36,000 bottles per hour
Semi-metallic microwave torch
MPT [136,139]
Fe2O3 (Fe)
He, Ne, Ar
120 s
Sugiyama and al torch [128]
Iron oxide (Fe)
Ar/H2
15 s
Surface is cleaned: FeOOH groups are completely removed. Importance effect of metastable species for breaking bounds Negligible influence of the temperature and the UV photons on cleaning Oxide layer is removed but the surface is slightly damaged
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Table 8 Surface etching by atmospheric plasma Excitation
Source
Plasma
Substrate
Etching rate 1
Observations
Low frequency
DBD [31]
He/O2
Organic materials
0.2 Am min
Not uniform etching
Radio frequency
APPJ [140]
He/O2 He/O2/CF4
He/CF4
8 Am min 1 1.2 Am min 1 1 Am min 1 2 Am min 1 0.3 Am min 1
Chemical process: influence of the oxygen metastable species and atoms
Cold plasma torch [47]
Kapton SiO2 W Ta Si
The following part mainly focuses on the applications of the plasma sources that are still under development in laboratories. The well-developed and industrialized processes are only mentioned.
Emission intensity (OES) of F* is related to the etching rate of Si
3.2. Gas treatments 3.2.1. Gas cleaning Gas cleaning is a huge economical and ecological issue. Pollutants are various:
3.1. Spectroscopic analysis As the RF inductively coupled plasmas (ICPs), atmospheric plasma sources that have been described can be used as excitation sources for spectroscopic analysis [76]. The microplasmas are particularly interesting as this technology has the potential of further integration with complementary devices onto a single ‘‘chip’’, resulting in miniaturized total analytical systems [73]. Since this article is aimed at making analytical spectrochemists more familiar with atmospheric plasmas in a wider range of applications, the analytical applications will not be detailed. Standard works on this field have been reported in various reference books [77 – 79].
& volatile organic compounds (VOCs): carbon monoxide, chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs). . . & inorganic compounds: nitrogen oxides (NOx ), sulfur dioxide (SO2). . . Those toxic pollutants come from engines exhaust gases, electric plants, chemical industry (solvents, paints, varnishes), petrochemical industry. . . Those compounds are emitted to the atmosphere and cause environmental problems [80] such as the depletion of the ozone layer, the greenhouse effect, the smog (which is a mixture of solid particles and ozone that comes from the reaction between
Table 9 Surface activation by atmospheric plasma Excitation
Source
Plasma
Substrate
Observations
Pulsed DC
Corona [141]
Air
PP (E: 26 mJ m 2)
Increase in PP surface energy: 43 mJ m 2 Surface energy value remains stable during 100 days
Low frequency
DBD [23]
He
PP (E: 26 mJ m 2)
Aldynei [142]
Gas mixture based on N2 Air
PP (E: 26 mJ m 2) PP (h: 95-)
Plasmatreat [124]
Air
PP (E: 26 mJ m 2)
Radio frequency
HELIOS [50,144]
Air/Ne
PE (E: 33 mJ m 2)
Activation efficiency depends on the discharge mode Filamentary discharge increases the PP surface energy value to 45 mJ m 2 Values as high as 62 mJ m 2 are obtained with a glow discharge Improvement of wettability is due to O implantation and N atoms density at the PP surface O comes from impurities in the plasma gas (N2, H2O) that are excited, ionized by highly energetic He metastable species Increase in PP surface energy: 60 mJ m 2 Surface energy value remains stable during 100 days Decrease in water contact angle: 25Contact angle value remains stable over 3 weeks Increase in PP surface energy: 56 mJ m 2 Improvement of wettability due to the increase of oxygen concentration and the changes of the topology substrate surface induced by the thermal component of the plasma Increase in PE surface energy: 57 mJ m 2 Dehydrogenation + C=C bonds formation (FTIR) Uniformity of the activation over 20 cm2
Semi-metallic microwave torch
Sugiyama and al. torch [145]
Ar/O2
PP (h: 100-)
AcXys [143]
Ar/CF4
Decrease in water contact angle: 80CO, O – CO – O bounds formation (XPS) Increase in water contact angle: 125CF, CF2, C-CFn, CF-CFn bounds formation (XPS)
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NOx and VOCs under UV irradiation). . . Therefore, effective processes are developed to limit and remove these pollutants. Among them, atmospheric pressure plasma techniques are becoming important because of their high reactivity. The plasma treatment principle can be described as following: toxic molecules are decomposed by collisions with energetic plasma species. This decomposition leads to free radicals that combine to form inoffensive molecules. The performances of a few atmospheric plasma sources, used for NOx or VOCs destruction in various atmospheres, are illustrated in Table 5. The removal efficiency is estimated by the analysis of outgoing treated gas by FTIR spectroscopy [130,62], optical emission spectroscopy and TCD gas chromatography [129]. Those results reveal that atmospheric plasma sources are suitable for gas cleaning even if the process still needs to be improved: the efficiency has to be increased and no toxic products must be generated by the reactions. The gas injection seems to be a key point. It must be designed in order to increase the pollutants stay-time in the plasma and then the removal [131]. Moreover, an optimal flow rate must be adapted: lower flow rate increase pollutants dwell time in the discharge, enhancing the process efficiency but may result in poor stability of the plasma. The admixture of oxygen lowers the reductive potential of plasma reactors for NOx destruction. The O2 molecule ionization consumes plasma energy: the energy amount which is available for NOx decomposition is then reduced. Moreover, collisions between N2 and O2 can form NOx and then compete with destruction reactions [130]. As this NO creation is enhanced by high temperature, the system must not warm up. That is why Baeva and al. [131] have chosen a pulsed power source. 3.2.2. Gas synthesis As the plasma is a high chemically reactive media, various products can be synthesized: hydrocarbons, ozone, etc. . . Downstream of the reactor, the products are identified by gas chromatograph (GC) coupled with mass spectroscopy (MS). Their amounts can be determined by GC detectors [81]: FID (flame ionization detector) for hydrocarbons and TCD (thermal conductivity detector) for H2, O2.
19
Fig. 32. Ink spreading on a sample before (left side of the sample) and after (right side of the sample) treatment by the Plasma pen (Tepla-PVA) [16].
3.2.2.1. Hydrocarbons. Acetylene production by arc plasma (C1 – C4 hydrocarbons cracking in H2 presence) has been industrialized for years (Hu¨ls [38]: Marl, Germany). This process is very flexible: production can be adjusted to acetylene needs (no storage). In Japan, methane conversion [81] using a pulsed discharge (corona) has been investigated in laboratory in order to produce higher hydrocarbon fuels such as alcohol and formaldehyde. As those chemical compounds are liquid under normal conditions, it is more economical and safer to transport them over large distances than gaseous methane or hydrogen. In China, a diesel engine fuel additive has been synthesized from DME conversion [82] by DBD in order to suppress the emissions of soot and smoke. 3.2.2.2. Ozone. Ozone has various applications in chemical and pharmaceutical industries [83] and treatment of water, of paper dough, of food. Ozone can be generated in oxygen, air or N2/O2 plasma. First a diatomic oxygen molecule has to be split. The resulting free radical oxygen is thereby free to react with diatomic oxygen to form the triatomic ozone molecule [84]. Plasma ozone laboratory generators work following the corona discharge principle (wire-to-cylinder reactor [85]) whereas industrialized generators are bases on the DBD principle (cylinder – cylinder reactor). Table 6 gives an idea of the ozone density in various plasma discharges and confirms that DBD and corona discharges suit for ozone generation.
Fig. 31. Plasma-JetR corona [17] (left) and BottleDynei [33] treating systems for 3D products (right). The plasma increases the surface tension of the material to strengthen its wettability. This creates a powerful bond between the printing ink, adhesive or coating and the material surface.
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Fig. 33. In-line PlasmatreatR treatment of EPDM/TPE extrusion before flocking, ice-coating, or taping [18].
Basically there are two ozone generation technologies [86]: & Ultraviolet & Plasma (corona or DBD). The plasma generators are preferable for large quantities of ozone. UV ozone generators produce concentrations between 0.1% and 0.001% by weight, while corona discharge systems generate between 1% and 6% by weight. 3.3. Material processing In this part a distinction is made between: & surfaces treatments: cleaning, activation, etching, coatings. . . & bulk material treatment: powder treatment, machining (cutting, welding. . .), toxic waste treatment. . . 3.3.1. Surfaces treatments There are many ways to treat surfaces: cleaning (decontamination, grease removal), activation (adherence or antiadherence properties), etching, functionalization (electrical conductivity, protection against corrosion, chemical barrier. . .). Note that the cleaning and activation steps often precede the deposition phase, and the surface quality is determinant for the coating quality.
3.3.1.1. Surface cleaning. Surface cleaning consists of removing contaminants (oil, dust, oxides, biological and chemical agents: see Fig. 29. . .) from the substrate surface. Surfaces have been degreased by halogenated solvents for a long time. However, because of the strict environmental norms (Montreal protocol [87], CE 2037/2000 regulation [88]) and the hazardous effect of the solvents on the environment [89] (ozone layer depletion, greenhouse effect, smog), solvent alternative have been developed. Among those alternatives, plasma seems to be suitable for surfaces cleaning, as shown in Table 7. The cleaned samples are analyzed by XPS in order to estimate the contaminant removal. Those plasma treatments operate at low temperature (see Fig. 30) which enables the cleaning of low temperature degradable materials (e.g. PET). Even if the cleaning mechanisms are not yet identified, they seem to depend on the kind of plasma source [136]. Metastable energetic species [134,136] (e.g. N2, He) seem to play a determining part in contaminant destruction process. The temperature influence is clearly less important, even negligible. 3.3.1.2. Surface etching. Surface etching consists in removing material from the treated sample surface in order to create a relief (e.g. a hole in a dielectric material which will be metallized after the etching step). Table 8 presents some results for surface etching by atmospheric plasma.
Fig. 34. Plasma spray principle [108].
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
21
Table 10 Examples of APS coatings [146 – 149]
Fig. 35. SEM observation of an alumina APS coating section [109].
The etching rate depends on several parameters: plasma composition (influence of F* [46], oxygen species [140]), substrate nature, working conditions (power, gas flow, substrate position). Helium is used as plasma gas in order to stabilize the discharge. The metastable energetic species enhance the etching efficiency and they play an essential part in excitation, ionization and dissociation phenomena. 3.3.1.3. Surface activation. Surface activation consists in grafting chemical functions (plasma active species) on the material surface in order to give it specific properties by varying its surface energy. The plasma composition influences the treated material superficial properties (see Table 9). For example, an argon – oxygen plasma leads to the grafting of polar and hydrophilic functions (oxygen groups), which increase the material surface energy. This kind of activation is useful to prepare the surface before other treatments: metallization, painting, printing, coating, sticking, bonding (see Fig. 31). On the other hand, Ar-CF4 plasma leads to the surface fluorination and induces anti-adherence properties. The treatment efficiency can be characterized by two methods: & the measurement of the contact angle h between the treated surface and a drop of water: h < 90- means a hydrophilic
APS coating
Functions
Examples of applications
Zn, Al
Resistance to wet corrosion
Al2O3 CoCrAlY
Electromagnetic protection for electronic equipment Electrical insulation Resistance to dry corrosion
Water or gas pipes (petrochemistry), bridge metallic structures. . . Computers
Zn, Sn, Cu Al2O3 hydroxyapatite
Electrical conductivity Biocompatibility
ZrO2 – Y2O3
Thermal barrier
Cr2O3 ZrO2 – NiCrAlY
Wear resistance
Ozonors, oven inductors Aeronautic: gas turbine Nuclear Connecting welding Biomedical: artificial limb (hip, knee. . .), implants (tooth. . .) Turbine combustor, rocket nozzle Mechanic, army, aeronautic, metallurgy, paper industry, petrochemistry
surface whereas h < 90- is measured with a hydrophobic surface. & the surface energy evaluation by calibrated inks (as shown by Fig. 32). Spectroscopic analysis of the treated surfaces (FTIR, XPS) can link the surface energy evolution to the surface composition and the chemical bounds. Atmospheric plasma can treat various materials even those which are low temperature degradable (as polymers are). The surface activation remains stable over a quite long period: treated substrates can thus be stored. Nevertheless, handling a sample between two treatment steps can damage the surface. Then, it is interesting to integrate the plasma system into the production line (see Fig. 33). 3.3.2. Surface coating Deposits functionalize the material surface (chemical barrier, resistance to corrosion, electrical conductivity. . .) while the intrinsic bulk properties (mechanical especially) remain unchanged.
Fig. 36. Coating of a wide range of materials, including ceramics, is performed with (fully automated) atmospheric plasma spray (left: Plasma and Thermal Coatings Ltd. [110], right: Plasma Giken co [111]).
22
Two kinds developed:
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
of
atmospheric
plasma
coatings
are
& air plasma spray (APS), & plasma enhanced chemical vapor deposition (PECVD). 3.3.2.1. APS coatings. The coating material, fine powder suspended in the carrier gas, is injected into the plasma jet,
where the powder particles are accelerated and heated [5]. Then the high velocity molten (or semi molten) particles strike the substrate surface where they are flattened (splat) and dramatically frozen: during this rapid solidification, metastable (or even glassy) phases can be created (see Fig. 34). The coating is formed when millions of particles are deposited on top of each other: it consists of layered splats [90]. Its structure is lamellar with an interconnected porosity
Table 11 Examples for oxide coatings by atmospheric plasma Excitation
Source
Plasma
Coating (substrate) [deposition rate]
Observations
Applications
Low frequency
DBD (glow)
N2/SiH4/N2O
SiOx [150] (Si) [2.2 Am h 1]
–
He/HMDSO
SiO2 [151] (Al) [7.2 Am h 1)
Nano and micro-particles formation Morphology (dense or powder) and thickness of the film depend on the substrate position in the discharge Substrate is not heated Si – O bonds and low C impurities (XPS, FTIR analysis) The mass transport by precursor ions plays an important part in the film formation Protection of Al surface against NaOH (0.1 N)
He/aerosol of water solution of Ce salts
CeOx (Al) [0.5 Am h 1]
He/vapors of solid or liquid precursor containing In, Sn
InOx , SnOx (polymer) [2.1 Am h 1]
APPJ [154]
He/O2/TEOS
SiO2 (Si) [18 Am h 1]
Cold plasma torch [155,156]
Ar/TMOS/H2
SiO2 (Si) [36 Am h 1]
He/O2/TEOT
TiO2 (Si) [54 Am h 1]
ICP torch [157]
Liquid precursor carried by O2
YBa2Cu3O7 (CaO – ZrO2) [6 Am h 1]
Cavity [158]
N2/HMDS
SiO2 (C fibers)
CyrannusR-I [159]
Ar/O2/HMDSO
SiO2 (Si) [9 Am h 1]
Radio frequency
Microwave
Barrier torch [152,153]
Stoichiometry: excess of oxygen (in comparison with CeO2), carbon contamination (XPS, electron microprobe analysis) Pollution by the surrounding atmosphere and incomplete precursor decomposition High adhesion and transparency of the film, electrical conductivity r InO: 104 S cm 1 Stoichiometry ¨In2O3 and carbon contamination <10% (electron microprobe analysis) Treatment temperature ¨300 K Si – O bonds, no carbon impurities (FTIR), good electric properties Good surface morphology (peak-to-valley surface ˚ V by AFM) roughness: 20 A Sample heating: 350 -C FTIR spectrum similar to the one of thermal CVD SiO2 film; homogeneous coating Stoichiometry SiO2, carbon impurities <1% (XPS analysis) Sample heating: 500 -C Influence of H2 as plasma gas Amorphous TiO2 film (XPS, X-ray analysis), good electric properties Influence of H2 as plasma gas on the film structure: it changes from anatase to rutile by admixing H2 Sample heating: 500 -C Films are black, dense smooth, highly textured (X-ray diffraction analysis) Sample heating: 450 -C Coating is dense, homogeneous, amorphous (TEM analysis), good adhesion SiO2 stoichiometry (EDX analysis), SiC particles formation (precursor decomposition) Plasma process (remote mode) doesn_t degrade the mechanical properties of the fibers but has no significant effect on improving the C fiber/Al matrix interface E – H tuning at low pressure (5 mbar) before reaching the atmospheric pressure to ensure a stable discharge Si – O bonds (FTIR analysis) Problem: NOx generation with an air plasma
Food wrapping (chemical barrier)
Optic
TCO films (transparent conductive oxides)
Microelectronic (dielectric layer)
Food wrapping Microelectronic
Microelectronic
Superconductor
Metal-matrix composites
Chemical barrier, protection against corrosion
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
23
Table 12 Examples for polymer coatings by atmospheric plasma Excitation
Source
Plasma
Coating (Substrate) [deposition rate]
Observations
Applications
Low frequency
DBD (glow)
He/C2H4 [28]
PE (Si) [1 Am h 1]
No difference in the chemical structures between a low and an atmospheric pressure plasma-polymerized film (FTIR analysis) Distribution of the film thickness depends on the high-voltage electrode design High power induces powder formation Hydrophobic layer (h: 98-) Chemical structure: CF2 monomers (FTIR, ATR, XPS analysis) Fluoro-polymer coating onto the inner surface of PVC tube
Protective layer
Radio frequency
Plasma pencil [47]
He/C2F4 [29]
(CF2)n (PVC tube) [3 Am h 1]
Ar/He/HMDS
Si based polymer [6 Am h 1]
Plasma polymers are stable, cross-linked but the film is not uniform Deposition on a substrate immersed in liquid
Adhesion layer
Non adhesive layer Biomedical (blood circulating tube)
Protective layer
Table 13 Examples for carbon coatings by atmospheric plasma Excitation
Source
Plasma
Coating (Substrate) [deposition rate]
Observations
Applications
Low frequency
DBD (glow)
He/H2/CH4
Carbon nanotubes [160] (Ni)
Multi-wall carbon nanotubes deposition with outer diameter and number density of 40 – 50 nm and 109 – 1010 cm 2 respectively (SEM, TEM analysis) Wall defects associated with Ni particles aggregation Substrate heating: 600 -C Uniform, black coating Substrate heating: 400 -C
Nanotechnology
Carbon [27] (quartz) [1.4 Am h 1] Microwave semi-metallic torch
DC
MPT [161]
Arc plasma torch [162]
Ar/C2H4/iron vapor
CH4/H2 *precursor can also be liquid [163]
Carbon nanotubes (metal)
Diamond (Mo) [150 Am h 1]
Single wall carbon nanotubes both in bundles and insolated with diameters ranging from 0.9 to 1.5 nm Results comparable to laser ablation and arc discharge techniques Problem of carbon deposition in the plasma tube: discharge destabilization Surface morphology and crystal structure of the synthetic diamond is strongly dependant on process conditions (CH4 amount, substrate temperature Best quality film with 4% CH4, at 950 -C
rate that can reach 30% (see Fig. 35). Its thickness goes from 50 Am to a few millimeters. As the plasma jet temperature reaches 15 000 K, lots of materials (metals, ceramics, cermets) can be sprayed (Fig. 36) provided they melt: the difference between their melting and decomposition or evaporation temperature must be greater than 300 K.
H2 storage Material reinforcement Mechanic
Nanotechnology H2 storage Material reinforcement Mechanic
Electronic
DC torches [90] are widely used for this spray process but ICP torches [91] can also suit. APS coatings have been implanted in industry for years. The development of this process is strongly due to the aeronautic sector [92].
Table 15 Waste treatment by atmospheric plasma Table 14 Powder treatments by atmospheric plasma Excitation
Source
Applications
DC
Arc plasma torch
RF Microwave
ICP torch Semi-metallic torch
Ultra fin powder synthesis (metallic: silver, copper; ceramic: nitrides, carbides, composites; carbon) [164,165] Densification: sintering, spheroidization [5] Oxide ceramic melting, spheroidization [127]
Excitation
Source
Applications
DC
Arc plasma torch
Microwave
Metallic torch
REFIOMS vitrification: EDF process [38], EUROPLASMA process [166] Medical waste pyrolysis [5] Hazardous waste decomposition [167]: PLASMION process [38] Vitrification of chloride waste [60], radioactive products [168] Decontamination of nuclear waste [60]
24
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Table 16 Material machining by atmospheric plasma Excitation
Source
Applications
DC
Arc plasma torch
Microwave
Metallic torch
Welding (see Fig. 37): MIG, TIG PTA recharging Cutting of ceramics [169], metals (see Fig. 37) Ceramic processing [168]: production of alumina beads, quartz glass fibers Cutting by a hybrid plasma-laser system [166]
Table 10 gives examples of APS coatings and the properties they confer to the treated materials. Although plasma spraying has been widely developed, the fundamentals of this process are not completely identified yet (e.g. plasma jet behavior, coating formation, interaction between the plasma and the injected particles) [5]. 3.3.2.2. PECVD coatings. PECVD deposits precursors are gas (or liquid carried by gas). The plasma (thermal or cold) is used as a chemically reactive media to activate the coating reactions. The reactive species are carried to the surface substrate where they adsorb, they react and the reaction byproducts desorb. The coating grows by a germination-growth phenomenon. Two PECVD configurations can exist: direct or remote. In the direct mode, the gas plasma and the precursor are injected in the discharge. This mode ensures the complete precursor decomposition. In the remote mode, only the gas plasma is excited in the discharge. The precursor is introduced in the afterglow where the long lifetime species are the only ones to exist. This configuration enables a better reaction control as there are fewer reactive species. The precursor is partially broken: it enables the adsorption of larger molecule fragments at the substrate surface. The working mode (glow or afterglow) strongly influences the nature and thus the properties of the coating. Moreover, in a remote mode, the substrate is positioned far from the glow, in a low temperature zone: it is then possible to make a deposit on polymers. Various substrates can be coated, even fine particles provided the reactor design includes a fluidized bed [93] or a system to vehicle particles to the discharge (ultrasonic horn [94], cyclone [95]) and to store them.
All materials can be deposited assuming that a gas (or liquid) precursor exists: then oxide, polymer, carbon coatings can be realized: see Tables 11 –13. Plasma is a very complex and reactive media. Thus, deposition mechanisms are far from being elucidated even if studies and models are developed [21,154,163,151]. Working parameters influence the film quality: substrate heating and H2 used as plasma gas seem to favor the impurities desorption and to improve the surface morphology [155]. 3.3.3. Bulk material treatments The following applications use the high temperature that can be generated by atmospheric thermal plasmas (the flame core in particular). The plasma sources are thus arc or microwave plasma essentially. Arc plasmas have been implanted in industries for years: their applications as heat source are then numerous and various. Microwave plasma sources are still developed in laboratories. Four kinds of applications are presented: & & & &
Fine particles treatment, Toxic waste treatment, Material machining, and Metallurgy.
3.3.3.1. Powder treatment. Particles synthesis and treatment require both the high temperature (> 1 500 K) and the chemical reactivity of the plasma. Table 14 gives examples of powder treatments by atmospheric plasma. The main problem is the collection and handling of those fin explosive particles (use of PTFE filtering films, on line treatment in glove-boxes to avoid any reaction with the surrounding atmosphere) [90]. 3.3.3.2. Toxic wastes treatment. Plasma treatment of toxic wastes (asbestos products, hazardous industrial waste, radioactive residues) is an essential issue: it takes part in the waste recycling. The plasma high temperatures induce fast and complete pyrolysis of organic hazardous wastes. Inorganic wastes are melted and vitrified, resulting in the waste passivation and in its volume reduction, as shown in Table 15.
Fig. 37. Material machining by atmospheric plasma (welding [112,113] on the left, cutting [114] on the right).
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Fig. 38. High intensity plasma arc lamp (Courtesy of OSRAM Sylvania [115]).
3.3.3.3. Material machining. As this kind of application requires high temperatures, plasma sources such as arc plasma torch or microwave torch are used for material machining, as shown in Table 16. For welding or cutting by arc plasma, an inert gas (Ar, He) sheath is used to avoid contamination from the surrounding media (Fig. 37). 3.3.3.4. Metallurgy. The use of plasmas in metallurgy is based on the very high temperature that can be generated. Thus only the high power arc plasmas suit for this kind of application. The working power can reach several megawatts. Such high-energy levels are necessary for: & extractive metallurgy: metallurgic ores are reduced into various metallic alloys at high temperature [5], & melting –purification: the metal melts under inert atmosphere and slowly solidifies in a cooled copper mould. The solidification can be well controlled with an inductive heating system (e.g. Leybold Heraeus process for large scale titanium remelting, Germany [39]), & heating: cements industry (EDF, Lafarge Coppe´e [13]), & ... 3.4. Lamps High pressure plasmas are very radiative media. If they emit in the visible domain, they can be used as light sources. Two kinds of lamps have to be considered: &Electroded lamps [96]
25
They work following an arc mode (see Fig. 38). Among the most developed lamps, the high pressure mercury (HMP) emits mainly green lines. Other colors can be obtained by adding other materials into the discharge. &Electrode-less lamps There is an increasing interest in electrode-less lamps since they avoid reactions between the electrode and the plasma. These discharges are fed with microwave [97]. In 1994 Fusion Lighting Systems produced the Solar 1000, a microwavedriven high pressure lamp with sulfur as the radiating medium [98]. This sulfur lamp (see Fig. 39) displays lots of advantages: excellent coloring, very high radiative efficiency, no electrode (then a long bulb lifetime is likely to be expected) and less environmentally damaging than mercury. The main problem is the magnetron power supply (low lifetime and efficiency). 4. Conclusion The potential applications of the atmospheric plasma sources are conditioned by the plasma properties (especially the gas temperature), and therefore by the plasma excitation. For example, polymer surface treatment requires low temperatures (below 500 K) whereas plasma cutting and welding of metals necessitate high temperatures (higher than 1500 K). Table 4 shows that low temperatures can be obtained either with low working power (some hundreds of watts) or in the plasma plume with higher power (especially for the microwave plasmas). Nevertheless, the distance from the plasma core should be limited to have sufficient density of active species. Thus it is not possible for arc or inductive plasmas to combine low temperature and active zone. High gas temperatures can only be obtained with high working power and inductive or microwave plasmas are the most suitable. This study shows that the microwave plasmas have the widest range of applications (both low and high temperatures), even if their role in the surface coating has not been explored yet (see Table 17). Arc and inductive plasma are required where high working power is necessary, whereas low and radio frequency plasmas suit for applications that need active species and low temperature. Regarding the development of these sources, the arc and inductive plasmas are the most implanted in the industry, in various domains (see Table 18). DBD and
Fig. 39. Principle of the sulfur lamp [116].
26
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Table 17 Synthesis of laboratory atmospheric plasma sources Excitation
Source
Applications
Advantages
Limits
Radio frequency
Hollow cathode sources, APPJ, cold plasma torch
Surface treatments: cleaning, decontamination, activation, etching, coating
Complex surface treatments Easy handling Simple design
Only low temperatures applications Small treated surface
Microwave
Metallic torches
Spectroscopic analysis
Complex surface treatments
Gas cleaning Toxic waste treatment Machinings Spectroscopic analysis
Suitable for both low and high temperature applications
Microwave device (expensive power supply, very accurate machining, safety rules) Discharge ignition Small treated surface
Semi-metallic torches
Complex surface treatments
Gas cleaning Surface treatments: cleaning, activation, coating Powder treatment Lamps
Large range of applications (rather low temperatures)
Difficult to industrialize (stability problems, lifetime) Microwave device Discharge ignition
Small treated surface
corona sources have also been known for a long time and now they can be adapted to the treatment of threedimensional pieces (line of 3DT products: BottleDynei, FlexyDynei. . .).
It is also interesting to note the emerging of various simple and easy handling torches (plasmatreatR, plasmajetR). They are very well adapted to surface activation (even polymer) and can treat very complex pieces (inside of holes, etc. . .).
Table 18 Synthesis of industrial atmospheric plasma sources Excitation
Source
Applications
Advantages
Limits
DC
Arc plasma torch
Coatings (APS) Machining Toxic waste treatment
Can be adapted to a robot Complex surfaces treatment High deposition rate, thick coatings, wide range of deposited materials
Noise, powder emission, radiations Cathode erosion Various parameters make the process control difficult
Spectroscopic analysis Coatings (TPCVD)
Can work with very high power No electrode
Noise, powder emission, radiations Not easy maneuverable: the substrate has to move
Toxic waste treatment Powder treatment
Complex surfaces treatment
Powder treatment Lamps RF
ICP torch
Pulsed DC
Corona
Ozone production Surface activation
Complex surfaces treatment Easy handling
Inhomogeneous treatment Surface can be damaged
Low frequency
DBD
Ozone production
Treatment of large plane surfaces
Surface activation, cleaning
Easy to handle
Plasma treatR
Surface cleaning, activation
AcXys
Surface activation, cleaning
Multi-nozzle system Complex surfaces treatment Can be adapted to a robot Treatment of large surfaces, complex pieces Can be adapted to a robot
Problems of stability (deposition on the electrode) Gap size limits the thickness of the treated piece High flow rate Not enough energy to remove oil
Pulsed radio frequency
IST
Sterilization, deodorization
Fast treatment of complex surfaces 36,000 bottles per hour Reduced cost comparing to other sterilization methods
–
Microwave
CyrannusR
Cleaning and activation of polymer surfaces
Stable and homogeneous discharge
The quartz tube diameter limits the size of the treated surface The closed design avoids direct integration to production lines
Complex surfaces treatment
High flow rate Not enough energy to remove oil
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Moreover, it is possible to increase the surface treatment area with a multi-nozzle array. To further end, it is important to underline the development of microplasma sources. They represent a great interest because of their miniaturization (easy to handle and to transport) enabling a wide range of applications (spectrometric analysis, gas treatment, sterilization, coatings). List of acronyms Acronym
Signification
Page
APPJ APS DBD DC HEIOS HELIOS ICP LTE MPJ MPT MTD PECVD pLTE PTA RF TIA
Atmospheric pressure plasma jet Air plasma spray Dielectric barrier discharge Direct current Hollow electrode with integrated open structure Hollow electrode with linear integrated open structure Inductively coupled plasma Local thermodynamic equilibrium Microwave Plasma Jet Microwave plasma torch Microwave torch discharge Plasma enhanced chemical vapor deposition Partial local thermodynamic equilibrium Plasma transferred arc Radio frequency Torche a` injection axiale
8 20 6 5 9 9 7 3 12 12 12 20 3 23 7 11
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[149] G. Montavon, B. Hansz, C. Coddet, F. Tourenne, F. Kassabji. Les applications industrielles de la projection thermique: marche´s et domaines sectoriels. Cahiers de l’Inge´nierie, 1998, no.67 [on line]. Available on: www.trs-online.com/trs-online/techniqu/cadtech.htm (consulted on 02.28.2003). [150] S. Martin, I. Roca, P. Cabarrocas, C. Jimenez, N. Gherardi, J. Durand, D. Cot, F. Massines, De´poˆt de couche mince par de´charge luminescente a` la pression atmospherique, Mate´riaux 2002: de la conception a` la mise en oeuvre, 21 – 25Octobre 2002, Tours, France, Tours VINCI, Tours, 2002, CD-ROM. [151] R. Foest, F. Adler, F. Sigeneger, M. Schmidt, Study of an atmospheric pressure glow discharge (APG) for thin film deposition, Surf. Coat. Technol. 163 – 164 (2003) 323 – 330. [152] M. Cada, O. Churpita, Z. Hubicka, H. Sichova, L. Jastrabik, Investigation of the low temperature atmospheric deposition of TCO thin films on polymer substrates, Surf. Coat. Technol. 177 – 178 (2004) 699 – 704. [153] L. Soukup, Z. Hubicka, A. Churpita, M. Cada, P. Pokorny, J. Zemek, P. Jurek, L. Jastrabik, Investigation of the atmospheric RF torch-barrier plasma jet for deposition of CeOx thin films, Surf. Coat. Technol. 169 – 170 (2003) 571 – 574. [154] S.E. Babayan, J. Jeong, A. Schu¨tze, V.J. Tu, M. Moravej, G.S. Selwyn, R.F. Hicks, Deposition of silicon dioxide films with a non-equilibrium atmospheric-pressure plasma jet, Plasma Sources Sci. Technol. 10 (2001) 573 – 578. [155] K. Inomata, H. Ha, K.A. Chaudhary, H. Koinuma, Open air deposition of SiO2 film from a cold plasma torch of tetramethoxysilane-H2 – Ar system, Appl. Phys. Lett. 64 (1) (1994) 46 – 48. [156] H. Ha, B. Moon, T. Horiuchi, T. Inushima, H. Ishiwara, H. Koinuma, Structure and electric properties of TiO2 films prepared by cold plasma torch under atmospheric pressure, Mater. Sci. Eng., B 41 (1996) 143 – 147. [157] H. Zhu, Y.C. Lau, E. Pfender, Radio frequency thermal plasma chemical vapor deposition of supraconducting Y1Ba2Cu3O7 – x films, J. Appl. Phys. 69 (5) (1991) 3404 – 3406. [158] M. Brown, P. Hayes, P. Prangnell, Charaterisation of thin silica films deposited on carbon fibre by an atmospheric pressure non-equilibrium plasma (APNEP), Compos., Part A 33 (2002) 1403 – 1408. [159] A. Pfuch, R. Cihar, Deposition of SiOx thin films by microwave induced plasma CVD at atmospheric pressure, Surf. Coat. Technol. 2 – 3 (2004) 134 – 140. [160] T. Nozaki, Y. Kimura, K. Okazaki, Carbon nanotubes deposition in glow barrier discharge enhanced catalytic CVD, J. Phys. D: Appl. Phys. 35 (2002) 2779 – 2784. [161] O. Smiljanic, B.L. Stansfield, J.-P. Dodelet, A. Serventi, S. Desilets, Gasphase synthesis of SWNT by an atmospheric pressure plasma jet, Chem. Phys. Lett. 356 (2002) 189 – 193. [162] D.-W. Park, J.-S. Yun, Structural characterizationof diamond thin films prepared by plasma jet, Thin Solid Films 345 (1999) 60 – 66. [163] M. Asmann, D. Kolman, J. Heberlein, E. Pfender, Experimental confirmation of thermal plasma CVD of diamond with liquid feedstock injection model, Diamond Relat. Mater. 9 (2000) 13 – 21. [164] B. Hansz, A. Roux, Q. Saif, Elaboration de poudres par plasma HF, Premier Forum CERAMI Plasma – applications des traitements par plasma d’arc de petite et moyenne puissance, 1er Octobre 1992, Limoges, France, 1992. [165] J. Karthikeyan, C.C. Berndt, J. Tikkanen, S. Reddy, H. Herman, Plasma spray synthesis of nanomaterials powders and deposits, Mater. Sci. Eng., A 328 (1997) 275 – 286. [166] EUROPLASMA. Et si stocker c’e´tait de´passe´? [on line]. Available on: www.europlasma.com/pdf/dossier.pdf (consulted on 09.09.2003). [167] C. Girold. Une torche plasma sans raccords ni joints! CEA Technologies [on line]. Available on: www-drt.cea.fr/cea-tech/environ/48-801.html (consulted on 03.26.03). [168] A.I. Al-Shamma’a, S.R. Wylie, J. Lucas, R.A. Stuart, Microwave Plasma Jet for material processing at 2.45GHz, J. Mater. Process. Technol. 121 (2002) 143 – 147. [169] W.J. Xu, J.C. Fang, Y.S. Lu, Study on ceramic cutting by plasma arc, J. Mater. Process. Technol. 129 (2002) 152 – 156.
Review
Atmospheric pressure plasmas: A review Claire Tendero a,*, Christelle Tixier a, Pascal Tristant a, Jean Desmaison a, Philippe Leprince b a
Laboratoire Sciences des Proce´de´s Ce´ramiques et Traitements de Surfaces, UMR CNRS 6638, Universite´ de Limoges, France b Laboratoire de Physique des Gaz et des Plasmas, UMR CNRS 8578, Universite´ Paris XI, Orsay, France 2
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Received 17 July 2005; accepted 5 October 2005 Available online 18 November 2005
Abstract This article attempts to give an overview of atmospheric plasma sources and their applications. The aim is to introduce, in a first part, the main scientific background concerning plasmas as well as the different atmospheric plasma sources (description, working principle). The second part focuses on the various applications of the atmospheric plasma technologies, mainly in the field of surface treatments. Thus this paper is meant for a broad audience: non-plasma-specialized readers will find basic information for an introduction to plasmas whereas plasma spectroscopists who are familiar with analytical plasmas may be interested in the synthesis of the different applications of the atmospheric pressure plasma sources. D 2005 Elsevier B.V. All rights reserved. Keywords: Plasma; Atmospheric; Review; Surface treatment; DBD; Corona; Torch
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Part A. Basic and fundamentals . . . . . . . . . . . . . . . . . 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Plasma generation . . . . . . . . . . . . . . . 2.1.2. Plasmas classification. . . . . . . . . . . . . . 2.2. Atmospheric pressure plasmas: LTE or non-LTE? . . . . 2.2.1. LTE plasmas . . . . . . . . . . . . . . . . . . 2.2.2. Non-LTE plasmas. . . . . . . . . . . . . . . . 2.2.3. Atmospheric pressure plasmas . . . . . . . . . 2.3. Overview of various atmospheric plasma sources . . . . 2.3.1. DC and low frequency discharges . . . . . . . 2.3.2. RF discharges. . . . . . . . . . . . . . . . . . 2.3.3. Microwave induced plasmas (MIPs) . . . . . . 2.3.4. Summary . . . . . . . . . . . . . . . . . . . . Part B. Applications of the various atmospheric plasma sources 3.1. Spectroscopic analysis . . . . . . . . . . . . . . . . . . 3.2. Gas treatments . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Gas cleaning . . . . . . . . . . . . . . . . . . 3.2.2. Gas synthesis . . . . . . . . . . . . . . . . . .
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* Corresponding author. Tel.: +33 6 16 77 73 47; fax: +33 5 55 42 36 80. E-mail address: [email protected] (C. Tendero). 1 ENSIL, 16 rue d’Atlantis, Parc d’ESTER Technopole, BP 6804, 87068 Limoges, France. 2 LPGP, Universite´ Paris Sud, Bat 210, ave G. Clemenceau, 91405 Orsay Cedex, France. 0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2005.10.003
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3.3.
Material processing . . . . . . . . 3.3.1. Surfaces treatments . . . 3.3.2. Surface coating . . . . . 3.3.3. Bulk material treatments . 3.4. Lamps . . . . . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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1. Introduction Plasmas are chemically active media. Depending on the way they are activated and their working power, they can generate low or very high ‘‘temperatures’’ and are referred correspondingly as cold or thermal plasmas. This wide temperature range enables various applications for plasma technologies: surface coatings, waste destruction, gas treatments, chemical synthesis, machining. . . Nevertheless, many of these techniques have not been industrialized, albeit environmental norms are strictly followed. Thermal plasmas (especially arc plasma) were extensively industrialized, principally by aeronautic sector. Cold plasma technologies have been developed in the microelectronics but their vacuum equipment limits their implantation. To avoid drawback associated with vacuum, several laboratories have tried to transpose to atmospheric pressure processes that work under vacuum for the moment. Their researches have led to various original sources that are described here. This paper is a review about atmospheric plasmas. It is mainly about the technologies that are still under development in laboratories whereas arc and inductive plasmas are briefly discussed. After a summary about the different kinds of plasmas, the various sources are described in term of design, working conditions and plasma properties. Then the study focuses on their applications (spectroscopic analysis, gas treatments and materials proceedings) and concludes with a comparative synthesis of each system (applications, advantages, limits).
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The atmospheric plasmas described in this paper are generated from electrical energy. The electric field transmits energy to the gas electrons (which are the most mobile charged species). This electronic energy is then transmitted to the neutral species by collisions. These collisions [2] follow probabilistic laws and can be divided in: & Elastic collisions: they do not change the internal energy of the neutral species but slightly rise their kinetic energy & Inelastic collisions: when electronic energy is high enough, the collisions modify the electronic structure of the neutral species. It results in the creation of excited species or ions if the collisions are energetic enough. Most of the excited species have a very short lifetime and they get to ground state by emitting a photon. The ‘‘metastable species’’ are also excited states but with a long lifetime because their decay by emission of radiation is hampered as there are no allowed transitions departing from the respective state: decay can only take place by energy transfers through collisions. 2.1.2. Plasmas classification Depending on the type of energy supply and the amounts of energy transferred to the plasma, the properties of the plasma change, in terms of electronic density or temperature. These two parameters distinguish plasmas into different categories, presented in see Fig. 1. The atmospheric plasma sources described in this paper are supposed to be positioned near the glow discharges and the arcs.
2. Part A. Basic and fundamentals 2.1. Definitions Plasma is a more or less ionized gas. It is the fourth state of matter and constitutes more than 99% of the universe. It consists of electrons, ions and neutrals which are in fundamental and excited states. From a macroscopic point of view, plasma is electrically neutral. However, it contains free charge carriers and is electrically conductive. 2.1.1. Plasma generation A plasma is created by applying energy to a gas [1] in order to reorganize the electronic structure of the species (atoms, molecules) and to produce excited species and ions. This energy can be thermal, or carried by either an electric current or electromagnetic radiations.
Fig. 1. 2D classification of plasmas (electrons temperature versus electrons density) [5].
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Table 1 Main characteristics of LTE and non-LTE plasma LTE plasmas
Non-LTE plasmas
Current name
Thermal plasmas
Cold plasmas
Properties
Te = T h High electron density: 1021 – 1026 m 3 Inelastic collisions between electrons and heavy particles create the plasma reactive species whereas elastic collisions heat the heavy particles (the electrons energy is thus consumed)
Te H T h Lower electron density: <1019 m 3 Inelastic collisions between electrons and heavy particles induce the plasma chemistry. Heavy particles are slightly heated by a few elastic collisions (that is why the electrons energy remains very high)
Arc plasma (core) Te = T h å 10,000 K
Glow discharges Te å 10,000 – 100,000 K T h å 300 – 1000 K
Examples [117]
In this classification, a distinction can be made between: & Local thermodynamic (or thermal) equilibrium plasmas (LTE) & Non-local thermodynamic equilibrium plasmas (non-LTE). 2.2. Atmospheric pressure plasmas: LTE or non-LTE? The Local Thermodynamic Equilibrium notion [3] is really important, especially for a spectroscopic study of the plasma, since the determination of the plasma parameters (particles distribution functions; electron, excitation, vibration temperatures. . .) is based on relationships which differ for plasmas in LTE or not. 2.2.1. LTE plasmas LTE plasma requires that transitions and chemical reactions are governed by collisions and not by radiative processes. Moreover, collision phenomena have to be micro-reversible. It means that each kind of collision must be balanced by its inverse (excitation/deexcitation; ionization/recombination; kinetic balance) [4]. Moreover LTE requires that local gradients of plasma properties (temperature, density, thermal conductivity) are low enough to let a particle in the plasma reach the equilibrium: diffusion time must be similar or higher than the time the particle need to reach the equilibrium [5]. For LTE plasma, the heavy particles temperature is closed to the electrons temperature (ex: fusion plasmas). According to the Griem criterion [6], an optically thin homogeneous plasma is LTE if the electron density fulfills: 3 kT 23 E21 ne ¼ 9:10 m3 EHþ EHþ where ˝ E 21 represents the energy gap between the ground state and the first excited level,
˝ E H+ = 13.58 eV is the ionization energy of the hydrogen atom ˝ T is the plasma temperature. This criterion shows the strong link that exists between the required electron density for LTE and the energy of the first excited state. Those rules for LTE are very strict. Thus most of the plasmas deviate from LTE, especially all types of low density plasma in laboratories. 2.2.2. Non-LTE plasmas Departure from Boltzmann distribution for the density of excited atoms can explain the deviation from LTE. Indeed, for low-lying levels, the electron-induced deexcitation rate of the atom is generally lower than the corresponding electroninduced excitation rate because of a significant radiative deexcitation rate [4]. Another deviation from LTE is induced by the mass difference between electrons and heavy particles. Electrons move very fast whereas heavy particles can be considered static: electrons are thus likely to govern collisions and transitions phenomena. Deviations from LTE are also due to strong gradients in the plasma and the associated diffusion effects. It has been shown that the LTE distribution can be partial. For example, LTE can be verified for the levels close to ionization threshold [7] (e.g., 5p levels and higher, in argon plasma): such plasmas are pLTE (partial LTE). The non-LTE plasmas can be described by a twotemperature model: an electron temperature (Te) and a heavy particle temperature (T h). Regarding the huge mass difference between electrons and heavy particles, the plasma temperature (or gas temperature) is fixed by T h. The higher the departure from LTE, the higher the difference between Te and T h is. Table 1 sums up the main characteristics of LTE and nonLTE plasmas. More details on LTE and deviations from LTE are developed in the books by Huddlestone and Leonard [8], Griem [9], Lochte-Holtgreven [10] and Mitchner and Kruger [11].
Fig. 2. Evolution of the plasma temperature (electrons and heavy particles) with the pressure in a mercury plasma arc [5].
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
2.2.3. Atmospheric pressure plasmas Fig. 2 shows the influence of the pressure on the transition from a glow discharge (Te > T h) to an arc discharge. Low pressure plasmas (10 4 to 10 2 kPa) are non-LTE. Heavy particles temperature is lower than the electronic one. The inelastic collisions between electrons and heavy particles are excitative or ionizing. These collisions do not rise the heavy particles temperature. When the pressure becomes higher, collisions intensify. They induce both plasma chemistry (by inelastic collisions) and heavy particles heating (by elastic collisions). The difference between Te and T h is reduced: plasma state becomes closer to LTE but does not reach it. The significant gradient of properties in plasma restricts a particle, moving in the discharge, achieving equilibrium. The density of the feeding power influences a lot the plasma state (LTE or not). On the whole, a high power density induces LTE plasmas (e.g. arc plasmas) whereas non-LTE plasmas are favored by either a low density of feeding power or a pulsed power supply. In this latter case, the short pulse duration prevents the equilibrium state from establishing. Finally, it is important to note that an atmospheric plasma jet can be divided in two zones: & a central zone or plasma core which is LTE & a peripheral zone which is non-LTE. In this plume, heavy particles temperature is much lower than electrons one. Indeed, for a free-burning argon arc [6], operating conditions (a pressure of 300 kPa, currents of 300 to 400 A) are necessary to reach a LTE state in the central portion. These conditions lead to an electron density of 1024 m 3 in the center. Departures from LTE occur in the outer regions of such arcs where the electron density decreases below 1024 m 3. Thus, the local thermodynamic equilibrium is a primordial notion since it induces the temperature of the plasma. It strongly depends on the kind of plasma source and is
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determining for its applications. In the next part, the various atmospheric plasma sources are described in terms of design, operating conditions (power supply, plasma excitation. . .). 2.3. Overview of various atmospheric plasma sources The excitation frequency is important since it influences the behavior of the electrons and the ions. Fig. 3 shows an example of the variation range for f pe (frequency of the electrons in the plasma) and f pi (ions frequency) in cold plasmas (e.g. glow discharges). The atmospheric plasma sources can be classified regarding their excitation mode. Three groups are then highlighted: & the DC (direct current) and low frequency discharges; & the plasmas which are ignited by radio frequency waves; and & the microwave discharges. Table 4 reports the main characteristics of these various plasma sources. Among them, the emerging of original microplasmas [12] is quite interesting. The trend of miniaturization of plasma systems is important in order to create low-powered directive portable systems and to reduce instrument and operation costs. 2.3.1. DC and low frequency discharges Depending on their design, the DC and low frequency discharges can work either with a continuous or a pulsed mode. A pulsed working mode enables the injection of large energy amounts in the discharge while the system warming up is limited. On the other hand, a pulsed power supply is technically more complex than a DC source and compromises the reproducibility of the process. 2.3.1.1. Continuous working mode: the arc plasma torches. The arc plasma torches [13] are fed by a DC power supply. They can be
Fig. 3. Electrons and ions frequencies in cold plasmas [99].
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Fig. 4. Principle of arc plasma torches (left: current-carrying arc, right: transferred arc).
divided into two categories: current-carrying arc and transferred arc (see Fig. 4). They both consist of: & a cathode where electrons are emitted; & a plasma gas injection system; and & a nozzle which confines the plasma. In a current-carrying arc torch, the nozzle which is positively polarized is the anode. In the case of a transferred arc torch, the treated material is the anode whereas the nozzle is at a floating potential. The arc is ignited between the cathode and the anode and ionizes the plasma gas. The plasma temperature varies from 8 000 K (plasma envelop) to 15 000 K (plasma core) which enables high temperature applications (use of the thermal effect of the plasma). An arc plasma is a very conductive media (I = 50– 600 A). The gas is highly ionized and the electronic density is about 3.1023 m 3. Through the years, the arc plasma torches have been improved and are strongly implanted in industries: & a high-energy, high velocity torch: Plazjet [14] (see Fig. 5); & a structure with three cathodes and a segmented anode: TRIPLEX (Fig. 6);
Fig. 5. High velocity Plazjet [100] (Tafa, Praxair).
& a torch working with a long plasma column stabilized by vortices [15]; and & A miniature seal-less plasma torch designed by CEA [15] (licensed by Europlasma). 2.3.1.1.1. Pencil-like torches. Those last 5 years, lowpowered flexible and innovative arc plasma torches have been commercialized: & Plasmapen and Plasmapen Xtension to increase the size of the treated surface (PVA-TEPLA [16]): see Fig. 7; & Plasma-JetR from Corotec Corporation [17]; & Openair technology (patterned by PlasmatreatR [18] ) is well implanted in production lines (automobile, textile, packaging. . .). They all use a homogeneous, low-powered, currentcarrying arc plasma jet to prepare a surface prior to joining it with adhesives, coating it, or printing upon it. On the opposite of the classical arc plasma torches, the discharge generates very little heat, which allows surface treatment of various materials including low temperature degradable materials (polymers). The classical arc torches can be classified as LTE discharges. They are characterized by rather high temperatures and are used
Fig. 6. Sultzer Metco Triplex II Plasma Spray Gun [101].
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7
Fig. 9. Hollow needle to plate discharge [102] (left: positive needle, right: negative needle). Fig. 7. Plasmapen (left) and Plasmapen Xtension (right), PVA-TEPLA [16].
for applications where heat is required (welding, cutting, spraying. . .), whereas the low-powered pencil-like torches involve a non-LTE discharge: they create a ‘‘chemically rich environment’’ which is used for low temperature applications. 2.3.1.2. Pulsed working mode 2.3.1.2.1. Corona discharge. Corona [19] discharge is a non-LTE discharge with low current density. The device (Fig. 8) consists of a cathode-wire and an anode (the treated material), the DC power supply is pulsed. The plasma creates a lighting crown around the wire: that is why this discharge is called ‘‘Corona’’. When a negative high voltage is applied to the wire, the discharge is a negative corona. The positive ions are accelerated towards the wire where secondary electrons are emitted and accelerated into the plasma: this moving front of high-energy (about 10 eV) electrons followed by a tail of lower energy electrons (about 1 eV) is called a streamer [96]. Inelastic collisions occur between these high-energy electrons and heavy particles and induce the formation of chemically reactive species. The pulses duration is shorter than the time necessary for the arc creation: when each pulse ends, the discharge extinguishes before it becomes too conductive. The transition into spark [20] is then avoided. The current discharge is very low: 10 10 to 10 5 A. A positive corona also exists: the positive polarized wire acts as the anode (see Fig. 9).
As the plasma volume is very small, the main corona drawback regarding surfaces treatments is the thin size of the treated surface. To increase the size of the surface treatment, the cathode wire can be replaced by a planer electrode which is parallel to the treated surface: this system generates micro-arcs (streamers) that are perpendicular to the gap between the electrodes. The streamers always initiate at the same place (default on the surface) causing a non-homogeneous treatment on the material surface. To avoid this problem, a dielectric barrier discharge was developed. 2.3.1.2.2. Dielectric barrier discharge. The DBD device (see Fig. 10) consists of two plane-parallel metal electrodes: at least one of these electrodes is covered by a dielectric layer. To ensure stable plasma operation, the gap which separates the electrodes is limited to a few millimeters wide. Plasma gas flows in the gap. The discharge is ignited by means of a sinusoidal [21] or pulsed [22] power source. Depending on the working gas composition, the voltage and frequency excitation, the discharge can be either filamentary or glow [23,24]. A filamentary discharge is formed by micro-discharges or streamers [25] that develop statistically on the dielectric layer surface. The use of helium as plasma gas seems to favor a glow discharge (high energetic He metastable species [24], Penning effect [23,26]). The dielectric layer plays an important part by: & limiting the discharge current and avoiding the arc transition that enables to work with a continuous or pulsed mode. & distributing randomly streamers on the electrode surface and ensuring a homogeneous treatment. The streamer creation is due to the electrons accumulation on the dielectric layer. The described DBD device is the most common one and other systems have been developed:
Fig. 8. Principle of a corona discharge.
& a DBD with a brush-style cathode [27] that consists of 25 fine stainless wires. This process is convenient for nonconductive sample treatment, & a system with a special designed cathode [28] which can be a applied to metallic and dielectric surface treatment,
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Fig. 10. Principle of dielectric barrier discharge (picture: a non-equilibrium diffuse plasma at atmospheric pressure [103]).
& a DBD with spiral electrodes [29] that are used for coating in the tube interior, & a device where the dielectric layer consists of capillary dielectrics [30] or a disc of glass beads [31]. 2.3.1.2.3. The derived-from-Corona processes. A lot of industries have developed processes which are similar to corona treating systems, even if they use an alternative power supply rather than a pulsed one: & ALDYNEi (created by Air Liquide and Softal [32]) is mainly adapted to on-line surfaces treatments of polymers (Fig. 11). & Corona treatment systems are manufactured by 3DT [33]: BottleDynei, FlexyDynei, PlasmaDynei which make it possible to Corona treat any three-dimensional polymeric surface delicately, quickly and efficiently. & The ‘‘AcXys Technologies’’ system [34] was developed by T. Sindzingre et al. (Socie´te´ AcXys Technologies [35], France). It consists of two cylindrical concentric electrodes. The plasma gas is introduced in the gap between the electrodes through an inlet in the outer electrode (see Fig. 12). The inner electrode is connected with a high voltage and low frequency source. The discharge is initiated in the gap. The afterglow, where long lifetime species are accessible, exits by an outlet in
Fig. 11. Principle of Aldynei process.
the outer electrode situated on the opposite side of the gas inlet. Physical and chemical treatments occur in this afterglow. 2.3.1.2.4. Microplasma. DC-operating microplasma on glass chips (see Fig. 13) were developed by J.K. Evju et al. [36] for the chemical modification of microchannel walls. The microchannels are formed by hot imprinting PS between glass microscope slides. Microelectrodes for plasma generation are fashioned from platinum wires that are sharpened by etching with alternating current in 6 M NaOH. A similar micro-source was designed by Bessoth and coworkers [37] for gas detection: the plasma is generated in a 70-Am deep and 500-Am wide channel between two tungsten electrodes. 2.3.2. RF discharges Regarding their structure, the RF sources can work with a high or low power supply. It influences the properties of the plasma and thus its potential applications. The impedance matching can be either inductive (high powered discharges) or capacitive. 2.3.2.1. High powered discharges 2.3.2.1.1. The ICP torches. The inductive discharges have been known for a long while. The RF torch is simply designed (see Fig. 14). The plasma is initiated and maintained by an RF fed helical coil.
Fig. 12. Principle of AcXys process.
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Table 2 RF torches characteristics with a 6000 K argon plasma [118] Working power (kW)
Torch diameter(mm)
Working frequency (kHz)
50 80 300 700
42 54 104 159
1020 630 166 72
Fig. 13. DC microdischarge design.
The current that flows in the RF coil induces a timevarying magnetic field nearby the plasma zone. The resulting electric ring field (see Faraday law) accelerates the electrons and thus maintains the discharge. The frequency of the generated plasma is higher than 1 MHz. This frequency level implies that electrons follow the electric field oscillations and neither ions nor electrons can reach the torch wall. This lack of contact between the plasma and the wall reduces the pollution of plasma and torch walls which enables to work with different gases: inert, reductive, oxidant, nitriding gas [38]. The plasma is enclosed in a ceramic tube (quartz, silicon nitride) that is cooled by air or water, depending on the working power. The inductive torches work in a wide power range: 20 kW– 1000 kW, with a gas flow rate of 10 –200 slm. A higher working power is accompanied with lower torch diameter and lower plasma frequency (see Table 2). These sort of inductive torches are presently very well developed (in particular by EFD induction SA [39], Tekna Plasma System Inc [40]. . .). They can be applied to spectroscopic analysis, toxic waste treatment. . . 2.3.2.1.2. The IST system. An intelligent electrodes design makes it possible to treat very complex surfaces in a fast and efficient way. The IST society has developed an RF pulsed discharge to decontaminate the interior of plastic bottles [41]. This process is the DBD adaptation to the complex surfaces treatment. Without introducing an electrode inside the bottle, the plasma is however generated inside the bottle with the help of surrounding and top central electrodes (see Fig. 15) and is stabilized by the dielectric walls. To make the discharge initiation easier, argon can flow nearby the central electrode.
This system needs high power source (20 kW) but the use of impulsions makes it possible to treat low temperature degradable materials. 2.3.2.2. Low-powered discharges. To initiate the discharge in the gas, a voltage must be applied between the electrodes. This breakdown voltage depends on the value of P d where P is the pressure and d the gap between the electrodes. Paschen laws [19] show that working at atmospheric pressure implies a thin gap (several millimeters) in order to have a realistic voltage. In these discharges, the impedance matching is capacitive. 2.3.2.2.1. The atmospheric pressure plasma jet (APPJ). The APPJ [19] is a small (L < 20 cm) RF plasma torch that works at low power. It was developed by J.Y. Jeong et al. (University of California, Los Angeles) in collaboration with J. Park et al. (Los Alamos National Laboratory [42]). This system (see Fig. 16) consists of two concentric electrodes through which the working gas flows. By applying RF power to the inner electrode at a voltage between 100 and 150 V, the gas discharge is ignited. The ionized gas exits through a nozzle since the gas velocity is about 12 m s 1. The low injected power enables the torch to produce a stable discharge and avoids the arc transition. The same research team (Park and coworkers [43], UCLA) designed a rectangular version of the APPJ. This source produces a volumetric and homogeneous discharge in a 1.6 mm wide gap between two planar aluminum electrodes. Both electrodes are perforated to let the plasma gas flow through them. The upper electrode is connected to the RF power supply while the lower electrode is grounded. It has been recently applied to the deposition of hydrogenated amorphous silicon [44] with silane added downstream of the hydrogen– helium plasma.
Fig. 14. RF plasma torch (Teckna Plasma System Inc [40]).
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Fig. 15. Principle of IST process.
Both sources are commercialized by Surfx Technologies [45] (Atomfloi: see Fig. 17). 2.3.2.2.2. The cold plasma torch. The cold plasma torch [46] developed by H. Koinuma et al. (Institute of Technology, Tokyo) lies between the DBD and APPJ structures. The device is shown in Fig. 18 and its properties are reported in Table 4. The RF electrode is a stainless-steel needle. A quartz tubing is inserted between the cathode and anode to ensure both plasma stability and homogeneity. The plasma gas flows into the gap between the cathode and the dielectric tube. 2.3.2.2.3. The hollow cathode systems. Designed by Jane`a et al. (Masaryk University, Brno, Republic Czech), the RF pencil (see Fig. 19) is very close [47] to the cold plasma torch. This coaxial device is very small (L < 10cm). The RF electrode is a hollow needle; the plasma gas flows inside it. This cathode is inserted into a quartz tube. The plasma is generated inside the hollow electrode. The gas speed is high enough to make the plasma jet flow outside the sharp top of the needle. A barrier torch [48] which is similar to the RF pencil was also developed in Czech Republic by Hubie`ka et al. (Academy
of Sciences, Prague): see Fig. 20. A quartz tube is inserted into the RF hollow electrode. The working gas is injected inside the dielectric tube. The dielectric layer stabilizes the discharge and limits the electrode heating. The discharge remains stable in the case of a multi-nozzle torch-barrier device. A larger surface zone can be treated. In the same category, the HEIOS and HELIOS systems [49] were introduced by Bardos and Barankova. They create stable and uniform RF plasma over large area. HEIOS is a cylindrical hollow cathode whose open structure is made with 900 channels where the working gas flows: it is a 900 hollow simultaneously working cathodes assembling that generates a stable discharge over 7 cm2. HELIOS [50] design is based on the same idea: it is a rectangular hollow cathode that generates stable and uniform RF plasma over 20 cm2. The design of these sources allows their direct scaling up for large area plasma applications. 2.3.2.2.4. Microplasma. Recently, small-scale and low power plasmas have been investigated by several authors. Those microplasmas can be applied in various fields, as summarized in Table 3 (Fig. 21).
Fig. 16. APPJ design.
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Fig. 17. Atmofloi plasma can clean and activate surface prior to bonding [45].
2.3.3. Microwave induced plasmas (MIPs) The electrode-less microwave systems all work according the same principle. Microwaves are guided along the system and transmitted energy to the plasma gas electrons. Elastic collisions between electrons and heavy particles occur. Due to the large mass of heavy particles, the collided electrons rebound whereas the heavy particles remain static. The electrons are thus accelerated (they get kinetic energy) and the heavy particles are slightly heated. After several elastic collisions (which follow probabilistic laws), the electrons get enough energy to produce inelastic exciting or even ionizing collisions. The gas is partially ionized and becomes plasma which supports microwave propagation [51]. The next described microwave plasma sources are all designed according to the same idea. They consist of: ˝ a microwave power source (power supply, magnetron and circulator to protect the magnetron from the reflected power); ˝ microwave equipment (wave guides, tuning system); ˝ an ignition system; and ˝ gas injections.
Fig. 18. Cold plasma torch design.
Fig. 19. RF pencil design.
The discharge ignition is the key to microwave sources. Indeed, a self-ignition of the discharge ensures flexible operating conditions and enables the industrialization of the process. The energy, transmitted to the gas electrons, has to be high enough to initiate the plasma. In the literature, several methods are suggested to concentrate the microwave energy: ˝ The indirect ignition with a conductive rod [52] which plays the part of an antenna. The microwave are then caught and concentrated at the tip of the rod. ˝ A resonator cavity [53] that makes the electric field maximum where the plasma gas flows. ˝ A helical coil [54] that induces a circularly polarized wave. The energy which is transferred to electrons is then enhanced. ˝ A material with electrically conductive and heat-resistive properties [55]. This material can generate easily stable plasma when irradiated by microwaves in an argon flow.
Fig. 20. Barrier torch design.
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Table 3 Examples of RF microplasma sources Source
Source characteristics
Applications
Micro-structured electrode (MSE) [119,120]
Capacitively coupled discharge Micro-reactor volume: 60.5 mm3 Multi-reactor (MSE array) volume: 16 60.5 mm3
Fluorinated waste gas treatment Diamond-like carbon deposition Sterilization
Capacitively coupled microplasma CCAP [121]
Microchannel: 10 mm long, 200 – 500 Am deep See Fig. 21
Detector for gas chromatography
Miniaturized plasma jet source [122]
Inductively coupled discharge Discharge tube: 1 1 30 mm3 Plasma source chip: 15 30 mm2
Optical emission spectroscopic analysis of liquid sample
These atmospheric MIPs can be classified in three categories: ˝ resonant cavity plasmas; ˝ free expanding torches; and ˝ microplasmas. 2.3.3.1. Resonant structures. The Beenakker [56] cavity is the most-well known resonant cavity to generate microwave plasmas. A resonator cavity uses resonance phenomena to amplify a wave. Its interior surfaces reflect one type of wave. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low-energy loss: a standing wave is thus generated. As more waves enter the cavity, they combine with the standing wave and reinforce it, increasing its energy. This energy is used to initiate the discharge. The resonance frequency of the cavity strongly depends on its geometry parameters, especially on its radius (see Fig. 22). Thus, it is necessary to fit the geometry of the cavity with the frequency delivered by the microwave generator (2.45 GHz), in order to ensure the ignition of the plasma. 2.3.3.2. Free expanding plasma torches. These sources create a plasma that flows in open air. Regarding the structure of the torches, a distinction can be made between the metallic and the semi-metallic torches. 2.3.3.2.1. Metallic torches. The TIA (torche a` injection axiale) microwave plasma torch was developed by Moisan et al. [57]. It works with a conventional wave guide-to-coaxial line transition. The plasma gas flows in the inner coaxial line conductor and exits through a nozzle (see Fig. 23). The microwaves are generated by the magnetron and reach the working gas using rectangular and coaxial guides. The discharge
Fig. 21. Design of a capacitively coupled microplasma (CCMP).
is ignited on top of the nozzle. The plasma consists of a needlelike converging filamentary cone with a tail flame on top of it (see Fig. 24). The guide selects the propagating modes: TE10 in the rectangular guide (E z = 0 with z the wave propagation direction) and TEM in the coaxial line (E z = Hz = 0), which limits the energy loss that could come from parasite modes excitation. Impedance matching functions are achieved using two intrinsic tuning elements (movable plungers: short circuit and initiator): the transition between TE10 and TEM modes is then optimized and the reflected power is minimized. This metallic microwave torch was patented by P. Leprince et al. [58] (Laboratoire de Physique des Gaz et des Plasmas, Orsay, France). Other laboratories have studied other microwave torches using the same wave guide-to-coaxial line transition. The design changes slightly depending on the torch application field. In order to limit the interactions with surrounding atmosphere, Jasinski et al. [59] confined the plasma by putting a quartz tube around the nozzle. Okamoto et al. [54] have put a quartz tube (longer than the coaxial line) inside the inner conductor to inject aerosol samples and confine the plasma (torch for spectroscopic
Fig. 22. Resonance modes and frequencies of a cylindrical cavity [104].
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Fig. 23. TIA design.
analysis). The discharge ignition is induced by a helical coil. This torch produces annular-shaped plasma. Suzuki et al. [60] have designed a torch that uses the guidecoaxial line transition without movable plunger initiator. Water flows inside the inner conductor whose top is made of molybdenum to prevent thermal damage. A copper nozzle is fitted at the end of the torch. The plasma gas is injected in the coaxial line gap. The discharge is generated in the space between the molybdenum top and the nozzle where the electrical field concentrates. As the TIA design is quite sophisticated, its construction requires advanced and expensive workmanship. Thus, this coaxial torch has been simplified. Moisan et al. have designed the TIAGO (TIA sur guide d’onde). It is a simple and compact one-nozzle unit [61] but electromagnetic radiation can be emitted from the gap between the nozzle and the wave guide.
Fig. 24. The three different zones that can be distinguished in typical plasmas produced by the TIA [105].
To protect the personnel and instrumentation from those radiations, Jasinski et al. [62] have placed a cylindrical metal grid inside the reactor, coaxially around the nozzle. This grid influences the plasma flame, improving its stability. This system is called the Microwave Torch Discharge (MTD). In United Kingdoms, a very simple structure was developed by S.R. Wylie et al. [63]: the Microwave Plasma Jet (MPJ). The plasma cavity consists of a rectangular wave guide section that ends at a copper plate acting as a short circuit. The gas nozzle is situated inside the cavity directly above an aperture in the wave guide, one-quarter wavelength from the short circuit, where the maximum electric field is produced. This system can work either at 2.45 GHz or at 896 MHz [64] (this latter wavelength limits the electromagnetic radiation, in spite of the wave guide aperture). 2.3.3.2.2. Semi-metallic torches. The design of this torches is very similar the one of the metallic torches. The main difference is the propagation mode of the electromagnetic waves, as there is no guide-to-coaxial line transition. In the semi-metallic torches, the plasma gas flows through a quartz tube (which is transparent to microwave radiations). Instead of burning out on top of the nozzle, the discharge is initiated in the zone where the quartz tube intersects the rectangular wave guide [65] (see Fig. 25). This system (also called ‘‘guide-surfatron [66]’’) involves surface waves: the propagation mode is converted from the TE01 mode (in the rectangular wave guide) into the TM01 mode (in the quartz capillary). The microwaves propagate along the interface between the quartz tube and the plasma: then, the plasma can be extended over long distances. In the literature, this simplest semi-metallic torch is called Microwave Plasma Torch (MPT). It was first developed by Jin and coworkers [67] for atomic emission spectroscopy, as a possible alternative to the ICPs in analytical chemistry. The MPT creates a flame-like discharge: in general the flame is luminous and white-colored with a fainter central channel [68].
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Fig. 25. MPT design.
Fig. 26. Pulsed microwave torch design (Baeva et al.).
The key remains the way to initiate the discharge: it has to be self-ignited in order to ensure flexible operating conditions and to make the industrialization of the process possible. Thus Y.C. Hong et al. [69] have designed an original system made of a three microwave plasma torches line. Each torch is turned on by tungsten igniters (similar to spark plugs) positioned slightly away from the center of the wave guide. This kind of ignition enables the use of a very low cost microwave power supply (magnetrons used in typical home microwave ovens). Still in order to achieve flexible operating conditions, Pott et al. [70] have developed a torch with a resonant cavity (see Fig. 26). Moreover the working gas flows with a superimposed vortex. This kind of injection avoids wall burning and stabilizes the discharge. This torch works with a pulsed power supply, which limits the system heating while working with high-energy amounts (and thus very energetic plasma species). Another way of igniting the discharge is the one developed by Sugiyama et al. [71]. They found that an atmospheric argon cold plasma discharge could be generated by microwave
irradiation on the perovskite type powders. These powders are electrically conductive and thermally resistant. All those microwave plasma torches are still in development in laboratories. CyrannusRI (cylindrical resonator with annular slots) is an example of industrialized microwave atmospheric plasma source. It generates a homogeneous plasma in a quartz tube (Fig. 27) and was developed in Germany by Iplas [72].
Fig. 27. The CyrannusRI source (Iplas, Germany) [72].
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Table 4 Characteristics of atmospheric plasma sources Excitation
Source
Plasma properties
Operating conditions
Industrialized sources DC/Low frequency
Arc torch [5,123]
Te = T h å 8000 – 14,000 Ks n e = 1021 – 1026 m 3
PlasmatreatR [124]
T h < 700 K
Gas: Ar /He Gas flow: 10 – 150 slm Power: 10 – 100 kW Gas: air Gas flow: 117 slm
Corona [19]
Te = 40,000 – 60,000 K T h < 400 K n e = 1015 – 1019 m 3 Te = 10,000 – 100,000 Km T h < 700 K n e å 1018 – 1021 m 3
Gas: air
Pulsed DC/Low frequency
DBD [43]
Some 100 W Plasma gas: 5 – 40 slm
Radio frequency
ICP [5,123]
Te = T h = 6000 – 11,000 Ks n e = 1021 – 1026 m 3
Gas: Ar/He Gas flow: 10 – 200 slm Power: 50 – 700 kW
Pulsed radio frequency
IST [41]
T h < 400 K
Gas: surrounding air No gas flow Power: 20 kW
Microwave
Cyrannus [72]
T h < 700K
Gas: Ar/O2 Power: 6 kW
Still in laboratory development sources DC
Microplasma
–
Power: 500 V; 250 AA
Radio frequency
APPJ [19]
Te = 10,000 – 20,000 Ks, T h < 600 K n e = 1017 – 1018 m 3 Te = 10,000 – 20,000 Ks, T h < 700 K n e = 1017 – 1018 m 3 Te = 3000 – 11,000 Ks T h < 800 K n e å 1017 – 1018 m 3 Te = 1850 – 2300 K
Cold plasma torch [19]
Hollow cathode [48,49]
Microplasma CCAP [12]
Microwave
TIA [125]
MTD [62]
MPJ [63]
MPT [126,127]
Baeva et al. [70]
Sugiyama et al. [128]
s
: temperatures calculated from spectrometric measurements. : temperatures calculated from models. l : temperatures calculated from Langmuir probe measurements. t : temperatures measured by thermocouple. m
Te = 13,000 – 14,000 K s T h = 2400 – 2900 Ks n e å 1021 m 3 Te = 17,000 – 20,000 Ks T h = 1500 – 4000 Ks n e = 1020 – 1021 m 3 Te = 12,000 – 17,000 Ks T h = 5000 – 10,000 Ks n e å 1022 m 3 Te = 16,000 – 18,000 Ks T h = 3000 – 3500 Ks n e å 1020 – 1021 m 3 Te å 7000 Ks T h å 7000 Ks n e å 1019 m 3 Te å 90,000 Ks T h å 1000 Kt n e å 1017 m 3
m, l
l
Gas: O2 /He Gas flow: 50 – 90 slm Power: some 100 W Gas: Ar Gas flow <1 slm Power: 100 W Gas: Ar, He Gas flow <2 slm Power: some 100 W Gas: Ar Gas flow: <0.2 slm Power: 5 – 25 W Gas: He Gas flow: 2 – 6 slm Power: 100 W – 2 kW Gas: N2 Gas flow: 1 – 3 slm Power: 100 W – 400 W Gas: Ar Gas flow: 2 – 7 slm Power: 2 – 5 kW Gas: Ar Gas flow <1 slm Power: some 100 W Resonant cavity Pulsed MW power supply Gas: N2 Gas flow: 30 slm Power: 800 W Ignition by perovskite powder Gas: Ar/H2 Gas flow: 0.3 – 1.2 slm Power: some 100 W
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Fig. 28. Classification of atmospheric pressure plasma sources.
This system is well adapted to surface preparation (polymer activation) but its closed design avoids direct integration to production lines. 2.3.3.3. The microstrip plasma. Bilgic et al. [73] have developed a low-powered, compact microwave induced plasma source. It basically consists of a planar microstrip line on fused silica as dielectric substrate and a massive copper ground plate. It involves surface waves that propagate in a gas channel along the interface between the dielectric substrate and the plasma. The tiny size of the system limits the energy loss in the dielectric. This microstrip plasma technology has been mainly applied to atomic emission spectroscopy [74]. 2.3.4. Summary Table 4 shows the characteristics of the various atmospheric plasma sources in terms of plasma properties (electron temperature and density, gas temperature) and working conditions (power supply, gas flow). The nature of the plasma gas is important since it influences the plasma temperature. The temperature [75] does not change if the working gas is argon, argon/hydrogen, nitrogen, air, oxygen, argon/helium because the ionization energies are very closed (between 13.5 and 16 eV). On the other hand, in a helium plasma, the ionization
Excitation
Source
Ozone density (cm 3)
DC Pulsed DC Low frequency RF
Arc plasma torch Corona DBD APPJ
<1010 1018 1018 1016
energy is much higher (24 eV), resulting in a 4000 or 5000 K gap temperature. The plasma temperatures evaluation must be carefully considered. Plume temperatures evaluated from thermocouple measurements are likely to correspond to heavy particles temperature (that fixes the gas temperature). The plasma core temperature is recorded by spectroscopic measurements. Spatially resolved measurements have to be carefully done with the appropriate techniques depending on whether LTE applies or not. It is now interesting to place the various discussed sources on Fig. 1, in order to know more about the properties of the different plasmas which condition their applications. Indeed, glow discharges involve high chemical reactivity and low plasma temperatures whereas arc and ICP discharges provide much higher gas temperatures. Thus glow-discharge-like plasmas suit for applications that need both chemical reactivity and low temperature of the plasma (surface activation, coatings. . .). Arc-discharge-like plasmas are required for high temperature applications (welding, melting. . .). Fig. 28 classifies the various plasma sources regarding their electronic density and temperature. It puts the light on general trends: ˝ Corona, APPJ, Sugiyama et al. microwave torch and DBD involve the same temperature and density as a glow discharge. They can be applied to low temperature processes [23,43]. The DBD characteristics are between those of glow discharge and those of arc discharge. This can be correlated to two working modes that have been observed experimentally: a glow and a filamentary discharge.
Table 5 Gas cleaning by atmospheric plasma Excitation
Source
Working conditions
Plasma
Pollutant
Destruction efficiency
Observations
Semi-metallic microwave torch
Kiyokawa and al. torch [129] Baeva and al. torch [130,131]
0.6 slm 90 W
Ar Ar/12% O2/2% H2O N2/NO N2/NO/10%O2 N2/NO/2%O2
NO 2000 ppm
Reaction products: N2 and O2
NO 500 ppm
98% 18% 50% – 50%
TIA [132]
1 slm 220 W
Air/CHCl3
CHCl3 (3%)
100%
MTD [62]
2 slm 400 W
air
CFC 50%
100%
MPJ [133]
5 slm 400 W
N2/NO
NO 100 ppm
90%
Reaction products: CO2, CO, NOx , HCl, COCl2, H2O Destruction is more efficient in air than in N2 No production of NOx torch is more efficient than low pressure plasma No production of NOx
Metallic microwave torch
20 slm A few pulsed kW 10 slm 1 MW pulsed
– Reaction products: NO and NO2 Complex design
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Fig. 30. Professor Hicks’ group (UCLA: Chemical Engineering Department, Semiconductors Manufacturing) has developed the APPJ for cleaning, etching and deposition of materials. The ‘‘cold flame’’ (T ¨75 -C) is impinging on the hand of a student. [107].
intermediary between a homogeneous and an arc discharge. This behavior enables a wide range of applications for those plasma sources. Depending on the plasma zone that is involved for the treatment, it is possible to use either rather high temperatures or chemical reactivity of the plasma [70].
Fig. 29. A cold atmospheric plasma is sterilizing a dental needle at Loughborough University (plasma and pulsed power research group) [106].
˝ The properties (e.g. n e, Te) of the microwave metallic torches (TIA, MPJ) are similar to those of the arc plasmas. These sources suit for high temperature processes [168]. ˝ Microwave torches (Baeva et al., MTD, MPT) produce plasmas with characteristics (n e, Te) that are just between those of arc plasmas and those of glow discharge. The plasma generated by those microwave torches is thus likely to be an
3. Part B. Applications of the various atmospheric plasma sources The described atmospheric plasma sources are very different (structure, power supply, plasma temperature, working conditions) and therefore they have various applications.
Table 7 Surface cleaning by atmospheric plasma Excitation
Source
Contaminant (substrate)
Plasma
Treatment duration
Observations
Low frequency
DBD
Ag2S (Ag) [26] Oil (Al, Si) [134,135]
Ar
180 s
Ag2S layer is removed
Air, O2
A few sec
Fe2O3 (Fe) [136]
Ar/N2
60 s
Lubricant is totally removed if the plasma gas flow rate is low (1 – 5 slm). With a high flow rate, polymerization of the oil occurs Plasma cleaning is more efficient in air than in O2: importance of the metastable N2 species Surface is cleaned Complete cleaning mechanism is not yet established: it is different from a simple etching mechanism by nitrogen active species)
Pulsed low frequency
Glow DBD [137]
Oil (Fe)
O2
10 min
Oil is removed DBD is as efficient for oil removal as ultrasonic cleaning in acetone
Radio frequency
Plasma pencil [47]
Corrosion (archeological metal artifacts)
Ar
30 s
APPJ [138]
Biological, chemical agents (glass)
He/O2
30 s
Pulsed radio frequency
IST system [41]
Micro-organism (PET bottle)
Air
15 ms
Reduction of the corrosion products on antique metallic artifacts The object is immersed in a chemically reactive liquid in order to combine the efficiency of the plasma treatment and the selectivity of the chemical processes Neutralization of chemical and biological agents (e.g. mustards, anthrax) APPJ operates at low temperature and does not generate harmful or toxic products: thus it is suitable for rapid decontamination of material and safe for personnel Sterilization and deodorization inside PET bottles No damage (mechanical, thermal) on the surface Industrial process: 36,000 bottles per hour
Semi-metallic microwave torch
MPT [136,139]
Fe2O3 (Fe)
He, Ne, Ar
120 s
Sugiyama and al torch [128]
Iron oxide (Fe)
Ar/H2
15 s
Surface is cleaned: FeOOH groups are completely removed. Importance effect of metastable species for breaking bounds Negligible influence of the temperature and the UV photons on cleaning Oxide layer is removed but the surface is slightly damaged
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Table 8 Surface etching by atmospheric plasma Excitation
Source
Plasma
Substrate
Etching rate 1
Observations
Low frequency
DBD [31]
He/O2
Organic materials
0.2 Am min
Not uniform etching
Radio frequency
APPJ [140]
He/O2 He/O2/CF4
He/CF4
8 Am min 1 1.2 Am min 1 1 Am min 1 2 Am min 1 0.3 Am min 1
Chemical process: influence of the oxygen metastable species and atoms
Cold plasma torch [47]
Kapton SiO2 W Ta Si
The following part mainly focuses on the applications of the plasma sources that are still under development in laboratories. The well-developed and industrialized processes are only mentioned.
Emission intensity (OES) of F* is related to the etching rate of Si
3.2. Gas treatments 3.2.1. Gas cleaning Gas cleaning is a huge economical and ecological issue. Pollutants are various:
3.1. Spectroscopic analysis As the RF inductively coupled plasmas (ICPs), atmospheric plasma sources that have been described can be used as excitation sources for spectroscopic analysis [76]. The microplasmas are particularly interesting as this technology has the potential of further integration with complementary devices onto a single ‘‘chip’’, resulting in miniaturized total analytical systems [73]. Since this article is aimed at making analytical spectrochemists more familiar with atmospheric plasmas in a wider range of applications, the analytical applications will not be detailed. Standard works on this field have been reported in various reference books [77 – 79].
& volatile organic compounds (VOCs): carbon monoxide, chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs). . . & inorganic compounds: nitrogen oxides (NOx ), sulfur dioxide (SO2). . . Those toxic pollutants come from engines exhaust gases, electric plants, chemical industry (solvents, paints, varnishes), petrochemical industry. . . Those compounds are emitted to the atmosphere and cause environmental problems [80] such as the depletion of the ozone layer, the greenhouse effect, the smog (which is a mixture of solid particles and ozone that comes from the reaction between
Table 9 Surface activation by atmospheric plasma Excitation
Source
Plasma
Substrate
Observations
Pulsed DC
Corona [141]
Air
PP (E: 26 mJ m 2)
Increase in PP surface energy: 43 mJ m 2 Surface energy value remains stable during 100 days
Low frequency
DBD [23]
He
PP (E: 26 mJ m 2)
Aldynei [142]
Gas mixture based on N2 Air
PP (E: 26 mJ m 2) PP (h: 95-)
Plasmatreat [124]
Air
PP (E: 26 mJ m 2)
Radio frequency
HELIOS [50,144]
Air/Ne
PE (E: 33 mJ m 2)
Activation efficiency depends on the discharge mode Filamentary discharge increases the PP surface energy value to 45 mJ m 2 Values as high as 62 mJ m 2 are obtained with a glow discharge Improvement of wettability is due to O implantation and N atoms density at the PP surface O comes from impurities in the plasma gas (N2, H2O) that are excited, ionized by highly energetic He metastable species Increase in PP surface energy: 60 mJ m 2 Surface energy value remains stable during 100 days Decrease in water contact angle: 25Contact angle value remains stable over 3 weeks Increase in PP surface energy: 56 mJ m 2 Improvement of wettability due to the increase of oxygen concentration and the changes of the topology substrate surface induced by the thermal component of the plasma Increase in PE surface energy: 57 mJ m 2 Dehydrogenation + C=C bonds formation (FTIR) Uniformity of the activation over 20 cm2
Semi-metallic microwave torch
Sugiyama and al. torch [145]
Ar/O2
PP (h: 100-)
AcXys [143]
Ar/CF4
Decrease in water contact angle: 80CO, O – CO – O bounds formation (XPS) Increase in water contact angle: 125CF, CF2, C-CFn, CF-CFn bounds formation (XPS)
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
NOx and VOCs under UV irradiation). . . Therefore, effective processes are developed to limit and remove these pollutants. Among them, atmospheric pressure plasma techniques are becoming important because of their high reactivity. The plasma treatment principle can be described as following: toxic molecules are decomposed by collisions with energetic plasma species. This decomposition leads to free radicals that combine to form inoffensive molecules. The performances of a few atmospheric plasma sources, used for NOx or VOCs destruction in various atmospheres, are illustrated in Table 5. The removal efficiency is estimated by the analysis of outgoing treated gas by FTIR spectroscopy [130,62], optical emission spectroscopy and TCD gas chromatography [129]. Those results reveal that atmospheric plasma sources are suitable for gas cleaning even if the process still needs to be improved: the efficiency has to be increased and no toxic products must be generated by the reactions. The gas injection seems to be a key point. It must be designed in order to increase the pollutants stay-time in the plasma and then the removal [131]. Moreover, an optimal flow rate must be adapted: lower flow rate increase pollutants dwell time in the discharge, enhancing the process efficiency but may result in poor stability of the plasma. The admixture of oxygen lowers the reductive potential of plasma reactors for NOx destruction. The O2 molecule ionization consumes plasma energy: the energy amount which is available for NOx decomposition is then reduced. Moreover, collisions between N2 and O2 can form NOx and then compete with destruction reactions [130]. As this NO creation is enhanced by high temperature, the system must not warm up. That is why Baeva and al. [131] have chosen a pulsed power source. 3.2.2. Gas synthesis As the plasma is a high chemically reactive media, various products can be synthesized: hydrocarbons, ozone, etc. . . Downstream of the reactor, the products are identified by gas chromatograph (GC) coupled with mass spectroscopy (MS). Their amounts can be determined by GC detectors [81]: FID (flame ionization detector) for hydrocarbons and TCD (thermal conductivity detector) for H2, O2.
19
Fig. 32. Ink spreading on a sample before (left side of the sample) and after (right side of the sample) treatment by the Plasma pen (Tepla-PVA) [16].
3.2.2.1. Hydrocarbons. Acetylene production by arc plasma (C1 – C4 hydrocarbons cracking in H2 presence) has been industrialized for years (Hu¨ls [38]: Marl, Germany). This process is very flexible: production can be adjusted to acetylene needs (no storage). In Japan, methane conversion [81] using a pulsed discharge (corona) has been investigated in laboratory in order to produce higher hydrocarbon fuels such as alcohol and formaldehyde. As those chemical compounds are liquid under normal conditions, it is more economical and safer to transport them over large distances than gaseous methane or hydrogen. In China, a diesel engine fuel additive has been synthesized from DME conversion [82] by DBD in order to suppress the emissions of soot and smoke. 3.2.2.2. Ozone. Ozone has various applications in chemical and pharmaceutical industries [83] and treatment of water, of paper dough, of food. Ozone can be generated in oxygen, air or N2/O2 plasma. First a diatomic oxygen molecule has to be split. The resulting free radical oxygen is thereby free to react with diatomic oxygen to form the triatomic ozone molecule [84]. Plasma ozone laboratory generators work following the corona discharge principle (wire-to-cylinder reactor [85]) whereas industrialized generators are bases on the DBD principle (cylinder – cylinder reactor). Table 6 gives an idea of the ozone density in various plasma discharges and confirms that DBD and corona discharges suit for ozone generation.
Fig. 31. Plasma-JetR corona [17] (left) and BottleDynei [33] treating systems for 3D products (right). The plasma increases the surface tension of the material to strengthen its wettability. This creates a powerful bond between the printing ink, adhesive or coating and the material surface.
20
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Fig. 33. In-line PlasmatreatR treatment of EPDM/TPE extrusion before flocking, ice-coating, or taping [18].
Basically there are two ozone generation technologies [86]: & Ultraviolet & Plasma (corona or DBD). The plasma generators are preferable for large quantities of ozone. UV ozone generators produce concentrations between 0.1% and 0.001% by weight, while corona discharge systems generate between 1% and 6% by weight. 3.3. Material processing In this part a distinction is made between: & surfaces treatments: cleaning, activation, etching, coatings. . . & bulk material treatment: powder treatment, machining (cutting, welding. . .), toxic waste treatment. . . 3.3.1. Surfaces treatments There are many ways to treat surfaces: cleaning (decontamination, grease removal), activation (adherence or antiadherence properties), etching, functionalization (electrical conductivity, protection against corrosion, chemical barrier. . .). Note that the cleaning and activation steps often precede the deposition phase, and the surface quality is determinant for the coating quality.
3.3.1.1. Surface cleaning. Surface cleaning consists of removing contaminants (oil, dust, oxides, biological and chemical agents: see Fig. 29. . .) from the substrate surface. Surfaces have been degreased by halogenated solvents for a long time. However, because of the strict environmental norms (Montreal protocol [87], CE 2037/2000 regulation [88]) and the hazardous effect of the solvents on the environment [89] (ozone layer depletion, greenhouse effect, smog), solvent alternative have been developed. Among those alternatives, plasma seems to be suitable for surfaces cleaning, as shown in Table 7. The cleaned samples are analyzed by XPS in order to estimate the contaminant removal. Those plasma treatments operate at low temperature (see Fig. 30) which enables the cleaning of low temperature degradable materials (e.g. PET). Even if the cleaning mechanisms are not yet identified, they seem to depend on the kind of plasma source [136]. Metastable energetic species [134,136] (e.g. N2, He) seem to play a determining part in contaminant destruction process. The temperature influence is clearly less important, even negligible. 3.3.1.2. Surface etching. Surface etching consists in removing material from the treated sample surface in order to create a relief (e.g. a hole in a dielectric material which will be metallized after the etching step). Table 8 presents some results for surface etching by atmospheric plasma.
Fig. 34. Plasma spray principle [108].
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
21
Table 10 Examples of APS coatings [146 – 149]
Fig. 35. SEM observation of an alumina APS coating section [109].
The etching rate depends on several parameters: plasma composition (influence of F* [46], oxygen species [140]), substrate nature, working conditions (power, gas flow, substrate position). Helium is used as plasma gas in order to stabilize the discharge. The metastable energetic species enhance the etching efficiency and they play an essential part in excitation, ionization and dissociation phenomena. 3.3.1.3. Surface activation. Surface activation consists in grafting chemical functions (plasma active species) on the material surface in order to give it specific properties by varying its surface energy. The plasma composition influences the treated material superficial properties (see Table 9). For example, an argon – oxygen plasma leads to the grafting of polar and hydrophilic functions (oxygen groups), which increase the material surface energy. This kind of activation is useful to prepare the surface before other treatments: metallization, painting, printing, coating, sticking, bonding (see Fig. 31). On the other hand, Ar-CF4 plasma leads to the surface fluorination and induces anti-adherence properties. The treatment efficiency can be characterized by two methods: & the measurement of the contact angle h between the treated surface and a drop of water: h < 90- means a hydrophilic
APS coating
Functions
Examples of applications
Zn, Al
Resistance to wet corrosion
Al2O3 CoCrAlY
Electromagnetic protection for electronic equipment Electrical insulation Resistance to dry corrosion
Water or gas pipes (petrochemistry), bridge metallic structures. . . Computers
Zn, Sn, Cu Al2O3 hydroxyapatite
Electrical conductivity Biocompatibility
ZrO2 – Y2O3
Thermal barrier
Cr2O3 ZrO2 – NiCrAlY
Wear resistance
Ozonors, oven inductors Aeronautic: gas turbine Nuclear Connecting welding Biomedical: artificial limb (hip, knee. . .), implants (tooth. . .) Turbine combustor, rocket nozzle Mechanic, army, aeronautic, metallurgy, paper industry, petrochemistry
surface whereas h < 90- is measured with a hydrophobic surface. & the surface energy evaluation by calibrated inks (as shown by Fig. 32). Spectroscopic analysis of the treated surfaces (FTIR, XPS) can link the surface energy evolution to the surface composition and the chemical bounds. Atmospheric plasma can treat various materials even those which are low temperature degradable (as polymers are). The surface activation remains stable over a quite long period: treated substrates can thus be stored. Nevertheless, handling a sample between two treatment steps can damage the surface. Then, it is interesting to integrate the plasma system into the production line (see Fig. 33). 3.3.2. Surface coating Deposits functionalize the material surface (chemical barrier, resistance to corrosion, electrical conductivity. . .) while the intrinsic bulk properties (mechanical especially) remain unchanged.
Fig. 36. Coating of a wide range of materials, including ceramics, is performed with (fully automated) atmospheric plasma spray (left: Plasma and Thermal Coatings Ltd. [110], right: Plasma Giken co [111]).
22
Two kinds developed:
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
of
atmospheric
plasma
coatings
are
& air plasma spray (APS), & plasma enhanced chemical vapor deposition (PECVD). 3.3.2.1. APS coatings. The coating material, fine powder suspended in the carrier gas, is injected into the plasma jet,
where the powder particles are accelerated and heated [5]. Then the high velocity molten (or semi molten) particles strike the substrate surface where they are flattened (splat) and dramatically frozen: during this rapid solidification, metastable (or even glassy) phases can be created (see Fig. 34). The coating is formed when millions of particles are deposited on top of each other: it consists of layered splats [90]. Its structure is lamellar with an interconnected porosity
Table 11 Examples for oxide coatings by atmospheric plasma Excitation
Source
Plasma
Coating (substrate) [deposition rate]
Observations
Applications
Low frequency
DBD (glow)
N2/SiH4/N2O
SiOx [150] (Si) [2.2 Am h 1]
–
He/HMDSO
SiO2 [151] (Al) [7.2 Am h 1)
Nano and micro-particles formation Morphology (dense or powder) and thickness of the film depend on the substrate position in the discharge Substrate is not heated Si – O bonds and low C impurities (XPS, FTIR analysis) The mass transport by precursor ions plays an important part in the film formation Protection of Al surface against NaOH (0.1 N)
He/aerosol of water solution of Ce salts
CeOx (Al) [0.5 Am h 1]
He/vapors of solid or liquid precursor containing In, Sn
InOx , SnOx (polymer) [2.1 Am h 1]
APPJ [154]
He/O2/TEOS
SiO2 (Si) [18 Am h 1]
Cold plasma torch [155,156]
Ar/TMOS/H2
SiO2 (Si) [36 Am h 1]
He/O2/TEOT
TiO2 (Si) [54 Am h 1]
ICP torch [157]
Liquid precursor carried by O2
YBa2Cu3O7 (CaO – ZrO2) [6 Am h 1]
Cavity [158]
N2/HMDS
SiO2 (C fibers)
CyrannusR-I [159]
Ar/O2/HMDSO
SiO2 (Si) [9 Am h 1]
Radio frequency
Microwave
Barrier torch [152,153]
Stoichiometry: excess of oxygen (in comparison with CeO2), carbon contamination (XPS, electron microprobe analysis) Pollution by the surrounding atmosphere and incomplete precursor decomposition High adhesion and transparency of the film, electrical conductivity r InO: 104 S cm 1 Stoichiometry ¨In2O3 and carbon contamination <10% (electron microprobe analysis) Treatment temperature ¨300 K Si – O bonds, no carbon impurities (FTIR), good electric properties Good surface morphology (peak-to-valley surface ˚ V by AFM) roughness: 20 A Sample heating: 350 -C FTIR spectrum similar to the one of thermal CVD SiO2 film; homogeneous coating Stoichiometry SiO2, carbon impurities <1% (XPS analysis) Sample heating: 500 -C Influence of H2 as plasma gas Amorphous TiO2 film (XPS, X-ray analysis), good electric properties Influence of H2 as plasma gas on the film structure: it changes from anatase to rutile by admixing H2 Sample heating: 500 -C Films are black, dense smooth, highly textured (X-ray diffraction analysis) Sample heating: 450 -C Coating is dense, homogeneous, amorphous (TEM analysis), good adhesion SiO2 stoichiometry (EDX analysis), SiC particles formation (precursor decomposition) Plasma process (remote mode) doesn_t degrade the mechanical properties of the fibers but has no significant effect on improving the C fiber/Al matrix interface E – H tuning at low pressure (5 mbar) before reaching the atmospheric pressure to ensure a stable discharge Si – O bonds (FTIR analysis) Problem: NOx generation with an air plasma
Food wrapping (chemical barrier)
Optic
TCO films (transparent conductive oxides)
Microelectronic (dielectric layer)
Food wrapping Microelectronic
Microelectronic
Superconductor
Metal-matrix composites
Chemical barrier, protection against corrosion
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23
Table 12 Examples for polymer coatings by atmospheric plasma Excitation
Source
Plasma
Coating (Substrate) [deposition rate]
Observations
Applications
Low frequency
DBD (glow)
He/C2H4 [28]
PE (Si) [1 Am h 1]
No difference in the chemical structures between a low and an atmospheric pressure plasma-polymerized film (FTIR analysis) Distribution of the film thickness depends on the high-voltage electrode design High power induces powder formation Hydrophobic layer (h: 98-) Chemical structure: CF2 monomers (FTIR, ATR, XPS analysis) Fluoro-polymer coating onto the inner surface of PVC tube
Protective layer
Radio frequency
Plasma pencil [47]
He/C2F4 [29]
(CF2)n (PVC tube) [3 Am h 1]
Ar/He/HMDS
Si based polymer [6 Am h 1]
Plasma polymers are stable, cross-linked but the film is not uniform Deposition on a substrate immersed in liquid
Adhesion layer
Non adhesive layer Biomedical (blood circulating tube)
Protective layer
Table 13 Examples for carbon coatings by atmospheric plasma Excitation
Source
Plasma
Coating (Substrate) [deposition rate]
Observations
Applications
Low frequency
DBD (glow)
He/H2/CH4
Carbon nanotubes [160] (Ni)
Multi-wall carbon nanotubes deposition with outer diameter and number density of 40 – 50 nm and 109 – 1010 cm 2 respectively (SEM, TEM analysis) Wall defects associated with Ni particles aggregation Substrate heating: 600 -C Uniform, black coating Substrate heating: 400 -C
Nanotechnology
Carbon [27] (quartz) [1.4 Am h 1] Microwave semi-metallic torch
DC
MPT [161]
Arc plasma torch [162]
Ar/C2H4/iron vapor
CH4/H2 *precursor can also be liquid [163]
Carbon nanotubes (metal)
Diamond (Mo) [150 Am h 1]
Single wall carbon nanotubes both in bundles and insolated with diameters ranging from 0.9 to 1.5 nm Results comparable to laser ablation and arc discharge techniques Problem of carbon deposition in the plasma tube: discharge destabilization Surface morphology and crystal structure of the synthetic diamond is strongly dependant on process conditions (CH4 amount, substrate temperature Best quality film with 4% CH4, at 950 -C
rate that can reach 30% (see Fig. 35). Its thickness goes from 50 Am to a few millimeters. As the plasma jet temperature reaches 15 000 K, lots of materials (metals, ceramics, cermets) can be sprayed (Fig. 36) provided they melt: the difference between their melting and decomposition or evaporation temperature must be greater than 300 K.
H2 storage Material reinforcement Mechanic
Nanotechnology H2 storage Material reinforcement Mechanic
Electronic
DC torches [90] are widely used for this spray process but ICP torches [91] can also suit. APS coatings have been implanted in industry for years. The development of this process is strongly due to the aeronautic sector [92].
Table 15 Waste treatment by atmospheric plasma Table 14 Powder treatments by atmospheric plasma Excitation
Source
Applications
DC
Arc plasma torch
RF Microwave
ICP torch Semi-metallic torch
Ultra fin powder synthesis (metallic: silver, copper; ceramic: nitrides, carbides, composites; carbon) [164,165] Densification: sintering, spheroidization [5] Oxide ceramic melting, spheroidization [127]
Excitation
Source
Applications
DC
Arc plasma torch
Microwave
Metallic torch
REFIOMS vitrification: EDF process [38], EUROPLASMA process [166] Medical waste pyrolysis [5] Hazardous waste decomposition [167]: PLASMION process [38] Vitrification of chloride waste [60], radioactive products [168] Decontamination of nuclear waste [60]
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Table 16 Material machining by atmospheric plasma Excitation
Source
Applications
DC
Arc plasma torch
Microwave
Metallic torch
Welding (see Fig. 37): MIG, TIG PTA recharging Cutting of ceramics [169], metals (see Fig. 37) Ceramic processing [168]: production of alumina beads, quartz glass fibers Cutting by a hybrid plasma-laser system [166]
Table 10 gives examples of APS coatings and the properties they confer to the treated materials. Although plasma spraying has been widely developed, the fundamentals of this process are not completely identified yet (e.g. plasma jet behavior, coating formation, interaction between the plasma and the injected particles) [5]. 3.3.2.2. PECVD coatings. PECVD deposits precursors are gas (or liquid carried by gas). The plasma (thermal or cold) is used as a chemically reactive media to activate the coating reactions. The reactive species are carried to the surface substrate where they adsorb, they react and the reaction byproducts desorb. The coating grows by a germination-growth phenomenon. Two PECVD configurations can exist: direct or remote. In the direct mode, the gas plasma and the precursor are injected in the discharge. This mode ensures the complete precursor decomposition. In the remote mode, only the gas plasma is excited in the discharge. The precursor is introduced in the afterglow where the long lifetime species are the only ones to exist. This configuration enables a better reaction control as there are fewer reactive species. The precursor is partially broken: it enables the adsorption of larger molecule fragments at the substrate surface. The working mode (glow or afterglow) strongly influences the nature and thus the properties of the coating. Moreover, in a remote mode, the substrate is positioned far from the glow, in a low temperature zone: it is then possible to make a deposit on polymers. Various substrates can be coated, even fine particles provided the reactor design includes a fluidized bed [93] or a system to vehicle particles to the discharge (ultrasonic horn [94], cyclone [95]) and to store them.
All materials can be deposited assuming that a gas (or liquid) precursor exists: then oxide, polymer, carbon coatings can be realized: see Tables 11 –13. Plasma is a very complex and reactive media. Thus, deposition mechanisms are far from being elucidated even if studies and models are developed [21,154,163,151]. Working parameters influence the film quality: substrate heating and H2 used as plasma gas seem to favor the impurities desorption and to improve the surface morphology [155]. 3.3.3. Bulk material treatments The following applications use the high temperature that can be generated by atmospheric thermal plasmas (the flame core in particular). The plasma sources are thus arc or microwave plasma essentially. Arc plasmas have been implanted in industries for years: their applications as heat source are then numerous and various. Microwave plasma sources are still developed in laboratories. Four kinds of applications are presented: & & & &
Fine particles treatment, Toxic waste treatment, Material machining, and Metallurgy.
3.3.3.1. Powder treatment. Particles synthesis and treatment require both the high temperature (> 1 500 K) and the chemical reactivity of the plasma. Table 14 gives examples of powder treatments by atmospheric plasma. The main problem is the collection and handling of those fin explosive particles (use of PTFE filtering films, on line treatment in glove-boxes to avoid any reaction with the surrounding atmosphere) [90]. 3.3.3.2. Toxic wastes treatment. Plasma treatment of toxic wastes (asbestos products, hazardous industrial waste, radioactive residues) is an essential issue: it takes part in the waste recycling. The plasma high temperatures induce fast and complete pyrolysis of organic hazardous wastes. Inorganic wastes are melted and vitrified, resulting in the waste passivation and in its volume reduction, as shown in Table 15.
Fig. 37. Material machining by atmospheric plasma (welding [112,113] on the left, cutting [114] on the right).
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Fig. 38. High intensity plasma arc lamp (Courtesy of OSRAM Sylvania [115]).
3.3.3.3. Material machining. As this kind of application requires high temperatures, plasma sources such as arc plasma torch or microwave torch are used for material machining, as shown in Table 16. For welding or cutting by arc plasma, an inert gas (Ar, He) sheath is used to avoid contamination from the surrounding media (Fig. 37). 3.3.3.4. Metallurgy. The use of plasmas in metallurgy is based on the very high temperature that can be generated. Thus only the high power arc plasmas suit for this kind of application. The working power can reach several megawatts. Such high-energy levels are necessary for: & extractive metallurgy: metallurgic ores are reduced into various metallic alloys at high temperature [5], & melting –purification: the metal melts under inert atmosphere and slowly solidifies in a cooled copper mould. The solidification can be well controlled with an inductive heating system (e.g. Leybold Heraeus process for large scale titanium remelting, Germany [39]), & heating: cements industry (EDF, Lafarge Coppe´e [13]), & ... 3.4. Lamps High pressure plasmas are very radiative media. If they emit in the visible domain, they can be used as light sources. Two kinds of lamps have to be considered: &Electroded lamps [96]
25
They work following an arc mode (see Fig. 38). Among the most developed lamps, the high pressure mercury (HMP) emits mainly green lines. Other colors can be obtained by adding other materials into the discharge. &Electrode-less lamps There is an increasing interest in electrode-less lamps since they avoid reactions between the electrode and the plasma. These discharges are fed with microwave [97]. In 1994 Fusion Lighting Systems produced the Solar 1000, a microwavedriven high pressure lamp with sulfur as the radiating medium [98]. This sulfur lamp (see Fig. 39) displays lots of advantages: excellent coloring, very high radiative efficiency, no electrode (then a long bulb lifetime is likely to be expected) and less environmentally damaging than mercury. The main problem is the magnetron power supply (low lifetime and efficiency). 4. Conclusion The potential applications of the atmospheric plasma sources are conditioned by the plasma properties (especially the gas temperature), and therefore by the plasma excitation. For example, polymer surface treatment requires low temperatures (below 500 K) whereas plasma cutting and welding of metals necessitate high temperatures (higher than 1500 K). Table 4 shows that low temperatures can be obtained either with low working power (some hundreds of watts) or in the plasma plume with higher power (especially for the microwave plasmas). Nevertheless, the distance from the plasma core should be limited to have sufficient density of active species. Thus it is not possible for arc or inductive plasmas to combine low temperature and active zone. High gas temperatures can only be obtained with high working power and inductive or microwave plasmas are the most suitable. This study shows that the microwave plasmas have the widest range of applications (both low and high temperatures), even if their role in the surface coating has not been explored yet (see Table 17). Arc and inductive plasma are required where high working power is necessary, whereas low and radio frequency plasmas suit for applications that need active species and low temperature. Regarding the development of these sources, the arc and inductive plasmas are the most implanted in the industry, in various domains (see Table 18). DBD and
Fig. 39. Principle of the sulfur lamp [116].
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Table 17 Synthesis of laboratory atmospheric plasma sources Excitation
Source
Applications
Advantages
Limits
Radio frequency
Hollow cathode sources, APPJ, cold plasma torch
Surface treatments: cleaning, decontamination, activation, etching, coating
Complex surface treatments Easy handling Simple design
Only low temperatures applications Small treated surface
Microwave
Metallic torches
Spectroscopic analysis
Complex surface treatments
Gas cleaning Toxic waste treatment Machinings Spectroscopic analysis
Suitable for both low and high temperature applications
Microwave device (expensive power supply, very accurate machining, safety rules) Discharge ignition Small treated surface
Semi-metallic torches
Complex surface treatments
Gas cleaning Surface treatments: cleaning, activation, coating Powder treatment Lamps
Large range of applications (rather low temperatures)
Difficult to industrialize (stability problems, lifetime) Microwave device Discharge ignition
Small treated surface
corona sources have also been known for a long time and now they can be adapted to the treatment of threedimensional pieces (line of 3DT products: BottleDynei, FlexyDynei. . .).
It is also interesting to note the emerging of various simple and easy handling torches (plasmatreatR, plasmajetR). They are very well adapted to surface activation (even polymer) and can treat very complex pieces (inside of holes, etc. . .).
Table 18 Synthesis of industrial atmospheric plasma sources Excitation
Source
Applications
Advantages
Limits
DC
Arc plasma torch
Coatings (APS) Machining Toxic waste treatment
Can be adapted to a robot Complex surfaces treatment High deposition rate, thick coatings, wide range of deposited materials
Noise, powder emission, radiations Cathode erosion Various parameters make the process control difficult
Spectroscopic analysis Coatings (TPCVD)
Can work with very high power No electrode
Noise, powder emission, radiations Not easy maneuverable: the substrate has to move
Toxic waste treatment Powder treatment
Complex surfaces treatment
Powder treatment Lamps RF
ICP torch
Pulsed DC
Corona
Ozone production Surface activation
Complex surfaces treatment Easy handling
Inhomogeneous treatment Surface can be damaged
Low frequency
DBD
Ozone production
Treatment of large plane surfaces
Surface activation, cleaning
Easy to handle
Plasma treatR
Surface cleaning, activation
AcXys
Surface activation, cleaning
Multi-nozzle system Complex surfaces treatment Can be adapted to a robot Treatment of large surfaces, complex pieces Can be adapted to a robot
Problems of stability (deposition on the electrode) Gap size limits the thickness of the treated piece High flow rate Not enough energy to remove oil
Pulsed radio frequency
IST
Sterilization, deodorization
Fast treatment of complex surfaces 36,000 bottles per hour Reduced cost comparing to other sterilization methods
–
Microwave
CyrannusR
Cleaning and activation of polymer surfaces
Stable and homogeneous discharge
The quartz tube diameter limits the size of the treated surface The closed design avoids direct integration to production lines
Complex surfaces treatment
High flow rate Not enough energy to remove oil
C. Tendero et al. / Spectrochimica Acta Part B 61 (2006) 2 – 30
Moreover, it is possible to increase the surface treatment area with a multi-nozzle array. To further end, it is important to underline the development of microplasma sources. They represent a great interest because of their miniaturization (easy to handle and to transport) enabling a wide range of applications (spectrometric analysis, gas treatment, sterilization, coatings). List of acronyms Acronym
Signification
Page
APPJ APS DBD DC HEIOS HELIOS ICP LTE MPJ MPT MTD PECVD pLTE PTA RF TIA
Atmospheric pressure plasma jet Air plasma spray Dielectric barrier discharge Direct current Hollow electrode with integrated open structure Hollow electrode with linear integrated open structure Inductively coupled plasma Local thermodynamic equilibrium Microwave Plasma Jet Microwave plasma torch Microwave torch discharge Plasma enhanced chemical vapor deposition Partial local thermodynamic equilibrium Plasma transferred arc Radio frequency Torche a` injection axiale
8 20 6 5 9 9 7 3 12 12 12 20 3 23 7 11
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