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Electrical behaviour of ceramic composite materials for aero-engine igniters
Aerospace Science and Technology 10 (2006) 207–216 www.elsevier.com/locate/aescte
Electrical behaviour of ceramic composite materials for aero-engine igniters A. Jankowiak, P. Blanchart ∗ GEMH, ENSCI, Limoges, France Received 25 January 2005; received in revised form 26 May 2005; accepted 9 January 2006 Available online 31 January 2006
Abstract High energy igniters are extensively used in aero-engines and this study describes the specific ceramic composite materials used at the igniter tips. These ceramics favour the formation of a plasma-like process on their surfaces under an electrical field not exceeding 1 kV mm−1 . The elevated temperature which is reached, and the high energy released during sparks are very favourable for engine ignition, even when internal engine temperature, pressure and fuel flow are unfavourable. Currently used ceramics and possible igniter designs are described. For composite ceramics, the physical mechanisms involved during sparking are presented, together with the high temperature and pressure effect in engines. Degradation mechanisms of materials are also examined to understand the operational life of igniters when working conditions vary. Problems associated with continuous ignition and the type of surface discharge materials are also discussed. 2006 Elsevier SAS. All rights reserved. Keywords: Ceramic; Igniter; Electrical property
1. Introduction This study describes the design of ceramic composite materials used for spark igniters or more generally for aeronautical ignition systems. Descriptions will be focussing on specific low-tension igniters as they have been studied for years [1], and are currently used for the aero-engine ignition. The rather special characteristic of these igniters is the plasma-like process occurring on the ceramic surface during the electrical discharge under an electric field in the 0.5–1 kV mm−1 range. The localized high energy level (1–5 J) of “hot electrons” is very useful for gas or liquid ignition, even when the stoechiometry ratio in engines is unfavourable. To understand the role of a ceramic composite material, it is necessary to present the whole ignition systems, the various types of spark igniters and the whole assembly of spark igniters in which ceramics are used.
form the DC low-tension source (28 V) into high voltage pulses. Whereas for simple air gaps up to 30 kV are required, composite ceramic igniters require only 1–2 kV. The high voltage source must be precisely calibrated (duration of pulses, energy level, periodicity) to obtain a high energy level during sparks on the ceramic composite surface. There are three main groups of igniters currently used in civilian or military aero-engines. Different technologies and designs are used, with various high voltage sources and a large range of delivered energy. This review describes three typical types of igniter designs. 1.2. Air-gap igniters Fig. 1 presents the air-gap igniter end. It is similar to standard spark plugs for automotive internal-combustion engines.
1.1. The ignition system The whole ignition system includes four parts: a direct current source, a high voltage unit, an electric cable and the igniter itself. The HV ignition unit is an electronic assembly to trans* Corresponding author.
E-mail address: [email protected] (P. Blanchart). 1270-9638/$ – see front matter 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.ast.2006.01.001
Fig. 1. Air-gap igniter.
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These igniters have two coaxial electrodes with an 0.2–1 mm air gap. The electrical insulation uses a ceramic insulator (alumina ceramic) and a glass sealing between metal and ceramic. The spark across the air gap requires at least 15 kV, but the breakdown voltage increases drastically with pressure, up to 30 kV for 15 bars. Therefore, these igniters cannot always meet ignition needs in combustion chambers when the atmosphere pressure exceeds 15 bars. Whereas these igniters present the largest lifespan under operation, their use is not generalized in all aeronautical engines, because of the electromagnetic disturbances from high voltage pulses. 1.3. Surface discharge igniters High energy surface discharge igniters (H.E. Igniter) are devices used to initiate combustion with a low voltage source. Early surface discharge igniters were developed in the forties for aircraft engines in Germany. The story of the utilisation of H.E. Igniters is linked to the history of gas turbine ignition. In general, problems encountered in the early days of gas turbine ignition included: – fouling and shorting of high voltage igniters by combustion products; – inability of low energy spark plugs to ignite droplets of the less volatile kerosene fuels; – disturbance and insulation problems when carrying high voltage power sources on board aircrafts. These requirements mean that at present, low voltage H.E. Igniters are always used. There are two main categories of H.E. Igniters, the first one having a thin layer of semiconductor or resistor coated on an insulating material acting as the surface discharge material. The alternative is to use a specific ceramic material at the igniter tip, which is a source of a plasma-like process.
recently, improvements were also obtained with doping an insulating ceramic surface, such as alumina, by various metals (e.g. copper). As for igniters with a simple air-gap between metal and electrodes, the whole assembly has two coaxial electrodes close to the semi-conducting surface of the ceramic (Fig. 2). Metal and ceramics parts are sealed by a glass layer. The main difference between an air gap igniter and a semi-diffused igniter is the existence of a significant surface conductivity. This material favours the decrease of the breakdown voltage down to 1.5 kV. Voltage is almost independent of the atmosphere pressure, within the common range of the atmosphere pressure encountered in engines (up to 15 bars). Nevertheless, the major disadvantage of these igniters is their lifespan, which is considerably reduced because of the wear and of the fast chemical erosion of the conducting layer. The most recent type of igniter uses composite ceramic materials. They are often named semiconductors, even if this terminology is incorrect because the physical mechanisms involved are somewhat different. Such igniters are often used for the ignition of a large range of aero-engines. These ceramic materials, insulating materials, metal parts and the whole igniter design are at present under constant development. 1.5. Igniters ceramic composites The igniter design described in Fig. 3 is commonly used. Two coaxial electrodes are connected to the composite ceramic material at the igniter tip. Underneath, an insulating ceramic holds the assembly and a glass seal is used for metal to ceramic bonding. In general, the breakdown voltage is about 1 kV for a gap between electrodes of about 1.5 mm. The significant physico-chemical strength of some ceramics at high temperature reduces the phenomenon of wear and increases the lifespan of igniters. Besides, the breakdown voltage and the electrical discharge characteristics at high pressure are important characteristics, which can be maintained within
1.4. Surface diffused or coated igniter This range of novel devices was investigated years ago and the first successful solution was found for Rolls Royce Dart Engines, which used a primitive form of the high energy igniter. This device named Aquadag, was a colloidal dispersion of graphite powder in an aqueous media, deposited onto the ceramic between electrodes, and dried to form an adherent film. The problem of erosion of such a surface discharge material was overcome by the deposition of soot from the combustion products and the soot layer acted effectively to allow sparks. Nevertheless, problems with igniter life resulting from spark plug erosion were encountered as soon as more efficient combustion chambers were developed, where the necessary carbon layer did not withstand working conditions. Subsequently some new ceramics have been investigated as surface discharge materials to prevent erosion at high temperature and improve ignition efficiency at any pressure or fuel to air ratio. These ceramics include various combinations of materials such as oxides of aluminum, copper, iron or titanium. More
Fig. 2. Semi-diffused igniter.
Fig. 3. Ceramic gap igniter.
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Fig. 4. Typical microstructure of the composite ceramic. SiC grains appear in bright grey and the matrix material in dark grey.
average values. The lifespan of these igniters with composite ceramics is higher than that obtained with diffused igniters, but it is lower than that obtained with simple air-gap igniters. Physical phenomena involved in the breakdown process are detailed hereafter of the document as well as common type of ceramic material used for such igniters. As ceramics are assembled together, mechanical engineering problems arise from the need to accommodate thermal stresses and from bonding requirements of components made of differing materials. This paper provides some insight into the operation of igniters. A number of features of engine applications and operations are discussed in relation to igniter performance. The surface discharge materials used are composite ceramics, investigated for several years; a wide range of possible compositions must be considered. In this work, we studied particle-reinforced ceramic matrix composites with micro-sized SiC grains as the more conductive phase and SiAlON as the matrix phase. The volume fraction of SiC varies from 50 to 70%. Despite the high quantities of SiC conductive grains the global electrical conductivity is relatively low (1.5 × 10−6 S/cm). This is due to partially oxidized SiC grain interfaces, but also to the presence of porosity within the material (about 20 vol%) [2]. A typical microstructure is presented in Fig. 4, where SiC grains appear in light grey and the matrix material in dark grey. It is seen that the distribution of the SiC conductive phase within the matrix phase should be optimised, but further development is still necessary. 2. Properties of composite ceramics
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Fig. 5. Variation of the electrical conductivity of the composite ceramic with the nickel content.
The conductivity of such a composite material is described along with some percolation theories, which account for the current percolation through a random network of conductive grains. Whereas the bulk grain properties are considered in these models, grain boundaries effects are also significant and must be added. Under a high electric field, tunnelling at grain boundaries must be considered as a significant phenomenon. The theory, which explains the current density j through a single grain boundary of two adjacent grains, is represented as: 3 (1) j = Em exp(− φd) + BEm where Em is the applied electrical field across the thickness d of the interface and φ is the nickel work function. When the electrical field increases, an ohmic regime is followed by a power law regime with a large increase of current. For a composite material, the “spreading” of current through the volume is obtained by the increase of voltage favouring the opening of many conducting channels, even at significant inter particle distances. It leads to the activation of many emission sites at micro-protrusions on the surface, which becomes active for electronic emission [4]. This extended surface ionisation is the onset of the surface breakdown. To be suitable for ignition applications, composite materials must present an electrical conductivity value within a narrow range just above the threshold point of the conductivity against the conductive phase content (region II of Fig. 5). For materials in region I, inter particles distances are large enough to break many percolation paths. In region III, at the opposite of the nickel content range, highly conductive materials behave as ohmic conductors. Most of the energy from the external supply is dissipated by Joule effect.
2.1. Volume properties 2.2. Surface properties The electrical properties of the surface in relation to volume properties were studied with a representative composite material [3], i.e., a mix of a conductive nickel powder and an insulating silica glass powder as the matrix phase. Thus, it was possible to change the nickel quantity over a large range, to obtain a large range of conductivity values.
The surface micro-morphologies of composite materials were obtained by a non-contact optical profiler [5] (NOP) (Fogale Nanotech). In general, surface characteristics include sharp grains, protruding out of the core with a grain size over the 3–10 µm range. Fig. 6 shows a typical material microstruc-
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ture where a thick layer of the secondary phase is distributed between SiC grains of different sizes. The small intergranular pores and the matrix are clearly visualized with different grey scales. The non-contact optical profiler measures the surface height variations by optical phase shifting and white light vertical scanning interferometry [6], with a vertical measurement performance of 2 nm. Along the material surface, the light intensity of optical fringes is related to the surface profile and discredited into the measurement array of a camera. Within individual pixel and for each sampling interval, the light intensity represents the averaged value of surface heights within the elementary surface. The lateral spatial sampling is fixed by the magnification of the optical system. We used an objective lens magnification of 10 and the sampling interval was 1 µm in both directions. Roughness (short waves) and waviness (long waves) of surface profiles are preliminary separated by a filtering operation, to obtain the reference mean surface. This procedure uses a Gaussian filter whose parameters are adjusted to remove most of the waviness. From profiles (Fig. 7), we computed the bearing ratio curves (Abott curves), which are the depth distribution from the highest peak depth versus the bearing ratio. Smr is expressed in percent of the sum of material-filled surfaces Si to the evalu-
Fig. 6. Typical surface morphology where a thick layer of secondary phase is distributed between SiC grains.
ation surface Sm at a given profile section. It is the distance between the measured intersection level and the reference level: 1 Smr = Si · 100 (2) Sm i
The quantitative description of surface uses the specific group of Sk parameters [7]: Spk , Sk , and Svk which are derived from the bearing ratio graphic (Fig. 7). In particular, Spk is the measure of the peak height above the core roughness (peak to valley), Svk is the measure of the valley depths below the core roughness and Sk is a measure of the core roughness minus the peaks and valleys characterized by Spk and Svk respectively. A high Spk means that the surface is composed of sharp peaks with a large aspect ratio value. The peak material ratio Sr1 , is the percentage of material volume that includes the peak structures associated with Spk . Sr2 is a measure of the valley material ratio, with (100%-Sr2 ) representing the percentage of material that includes the valley structures associated with Svk . We will discuss later the strong relation between the Spk parameter and the breakdown voltage. 2.3. Electrical breakdown mechanisms of high energy igniters A distinguishing feature of the H.E. Igniter is the breakdown voltage required for sparking when the atmosphere pressure increases up to 40 bars, as in combustion chambers of some aeroengines. In Fig. 8, breakdown voltage is plotted against pressure in comparison with the simple air-gap characteristic [8]. Here, the very weak variation of breakdown voltage with pressure of H.E. Igniters is pointed out. To perform electrical breakdowns, composite materials are laboratory-tested between two metal electrodes connected to the electrical discharge circuit. A gap of 1.2 mm is adjusted between the two electrodes on the surface of the material. The energy is stored by a high voltage capacitance connected to our device through a trigger circuit and a series resistor. In general, the stored energy ranges from 1 to 5 J. The discharge voltage and current is measured with a large bandwidth oscilloscope (500 MHz, LeCroy Waverunner LT322). All electronic components are selected to obtain a smooth repeatable voltage and
Fig. 7. Spk , Sk , Svk morphology parameters which are derived from the bearing ratio curve.
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current signal for the discharge. For our experiments, the capacitive discharge circuit is electronically switched to the ceramic device thought a damping resistor placed in series. The resistor value was adjusted to reduce high frequency oscillations of the discharge current, but also to modify the whole duration of the electrical discharge. Typical current and voltage characteristics during the surface discharge process are illustrated in Fig. 9 for a composite material with 65 Vol% of SiC. On the curves, two main stages are distinguished: – a first and rapid increase of voltage is followed by a characteristic peak, where higher voltage is attained (Vm ); – a subsequent and very rapid decrease of voltage is accompanied by a large increase of current and the formation of a hot plasma-like process. The first stage is attributed to the material ionisation stage when the applied voltage between the electrodes reaches its maximum value. The second stage is the breakdown stage when the voltage decreases due to the formation of preferential breakdown paths and sparking occurs for engine ignition. For our composite material, the current-voltage characteristic extends to a broad range of voltage and current (up to 1000 V and
Fig. 8. Breakdown voltage versus pressure for the composite ceramic, in comparison with the simple air-gap characteristic.
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300 A) within the 1.2 mm gap between electrodes. The whole duration of discharges is about 100 µs and the process ends when the capacitor discharge is exhausted. It is proposed that the sequence of phenomena can be explained using the knowledge from electric discharges in lowpressure gas, under the Paschen’s law [9,10]. This model reflects the electrical breakdown mechanism in gases at the Vm voltage. Vm is related to the p.d product where p is the gas pressure and d the gap between electrodes: Vm =
Bpd ln(Apd) − ln(αd)
(3)
where A and B are gas dependent parameters and obtained from the literature [11]. α is the Townsend’s first ionisation coefficient, which relates the mean free path for electron secondary ionisation to the total scattering path. It is pressure and electric field dependent. In the continuation of the discharge process, the acceleration of ions in the electric field leads to secondary electron emissions at the electrodes. This secondary emission replenishes the population of ions and the discharge becomes self-sustaining when: 1 (4) αd = ln 1 + γ where γ is the secondary ionisation coefficient for the electrode surface. When p increases, α decreases from the limiting lowpressure. α is the number of electron-ion pairs generated per unit distance, which is related to the secondary electron emission coefficient γ at the cathode through Eq. (4). A large decrease of α means an increase of γ , which exceeds realistic values. Therefore, it is also assumed the occurrence of the Fowler– Nordheim’s mechanism [12] which gives the expression of the current flowing between electrodes. The current density j is: Φ 3/2 aE 2 exp −b (5) j= Φ E where Φ represents the work function, E the electric field, a and b depend on Φ. For SiC the work function is in the 2.7 to 3.5 eV range [13]; a and b can be estimated as [14]: a = 1.54 × 10−6 A m−2 , b = 6.83 × 109 V−1/2 m.
(6)
For composite materials having true surfaces, the abovedescribed Fowler–Nordheim’s law is modified [15,16] to take into account the enhanced field emission, which is observed with significant current values even at low electric fields. The significant enhancement of the microscopic field on surface is the so-called enhanced field emission (EFE). It seems that the current emission is enhanced at local microscopic emitting sites, which can be micro-protrusions on surfaces [17] or chemically modified surfaces by doping or local deposits. In recent years, progress has been made in the understanding of the various contributions to EFE, which can be classified into different categories: Fig. 9. Typical current and voltage characteristics during the surface discharge process for a 65 Vol% SiC composite.
– metallic surface roughness; – metallic dust and micro-particles;
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– – – – –
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grain boundaries; molten craters after breakdown; dielectric impurities and layers; absorbed gas; metal-insulator-vacuum (MIV) or metal-insulator-metal (MIM) layers.
The microscopic electric field is enhanced by a βi factor by which the ideal surface field E is increased to a local value Em : βi = Em /E.
(7)
For material surfaces having a surface micro-roughness, the appropriate Fowler–Nordheim’s law is modified to take into account the enhanced field emission. When βi is added, Eq. (5) becomes: aβi2 E 2 Φ 3/2 j= exp −b (8) Φ βi E The βi value is calculated considering the linear relationship between log(I /E 2 ) and 1/E during the pre-breakdown voltage period of the electrical discharge sequence. From experiments, only macroscopic electrical data are available, and a global field enhancement factor βg is calculated. Microscopic βi contribute to the global βg which proves that experimental data are significant. A typical example of such plot is in Fig. 10, where experiments are satisfactorily fitted by a straight line. From line slopes, βg values are within the βg = 1 × 104 –3 × 104 range, which means a very large accentuation of the field emission [18]. Similar data were obtained elsewhere with electrodes having micro-protrusions that support the validity of the theoretical treatment [19,20]. Such very high values of βg are explained by the peculiar surface microstructure, which allows multiple tip on tip configurations at conductive grains. The enhancement factors are therefore multiplied and contribute to a very large increase of the global βg . Furthermore, thermionic electron emission due to current flowing on the surface and Joule heating should have an important role. As the surface temperature increases, the electron energy also increases until a point is reached when a few electrons have sufficient energy to escape the binding forces of
Fig. 10. F.N. plot of log(I /E 2 ) against 1/E during the pre-breakdown voltage period.
the atoms and thus escape the surface of the material. In addition, we must consider that thermo-field (T-F) emission is favoured by both the high temperature and the occurrence of the high electric field. The T-F emission current densities can be significantly larger than the simple addition of thermionic and field emission mechanisms [21]. On the material surface, the plasma process induces the formation of a thin volume, characterized by a high temperature and ion density. It causes a large voltage gradient on the surface [22] and induces an intense local heating [23,24]. The contribution from the T-F emission to the current density can be calculated according to the Richardson– Dushman equation for thermionic emission corrected for the Schottky effect, i.e., φ = e3 E/(4πεo ), then: 4πkB eme 2 ϕ − φ (9) T exp − jT (E, T , ϕ) = kB T h3 where E is the electric field, T the temperature, φ the material work function, e the electron charge, kB the Boltzmann’s constant, h the Planck constant, εo the vacuum permittivity. It is seen that j increases with E 1/2 and with T 2 . Besides these phenomena, repeated sparking induces flaws and craters on the material surface (Fig. 11) and a local melt can also be evidenced. Explosive electron emission inside material flaws and pores is a particular phenomenon, which occurs when a field emission phenomenon is extended to intense conditions. When an extreme current density occurs, electrons from emitters form a very localised and dense plasma on the surface, but also inside pores. This phenomenon is similar to microscopic explosions [25], as a high energy level is concentrated inside micro-volumes, resulting in local material degradation. 2.4. Influence of the surface morphology Sparking on a material surface is affected by surface characteristics. As described above, the surface micro-morphologies of the materials were obtained by a non-contact optical profiler.
Fig. 11. SEM evidence of a local degradation due to explosive electrons emission.
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Fig. 12. Correlation between the breakdown electric field Em and the surface morphology parameter Spk .
Fig. 13. Breakdown voltage variation versus the oxidation time at 850 ◦ C under air at atmospheric pressure.
Surfaces include sharp SiC grains (5–20 µm) protruding out of the secondary phase. SiC grains are distributed in the core in which small inter-granular pores are also observed. From surface roughness, we calculated the Spk parameter, which is a measure of the peak height above the core (peak to valley). This parameter is supposed to be strongly correlated to the discharge process. It can be experimentally changed by using various SiC powders with different grain sizes. Results (Fig. 12) show that the macroscopic electric field for breakdown Em is correlated with Spk . An empirical expression of the macroscopic breakdown electric field Em is given by:
3.1. Thermal stresses and surface erosion
Em = aSpk + Eo
(10)
where a and Eo are parameters related to material. Em appears to be correlated with the number of surface protrusions acting as emitting sites. When Spk increases, the macroscopic breakdown electric field drastically decreases. This trend can be attributed to the increase of the aspect ratio of emitters together with the βi field enhancement factor. Here the specific role of emitter morphology and density for the surface ionisation is pointed out and similar observations were made in the literature with metal tips [26]. 3. Determination of the operational life of H.E. Igniters The operational life of H.E. Igniters for different applications must range from 40 to 4000 hours. The potential failure mechanisms, which determine life time, can be identified as follows: – – – – –
thermal stresses and surface erosion oxidation gas pressure spark energy and high temperature fuel use and engine type.
The process and extent of spark erosion are also modified by a number of variables as the erosion rate of the ceramic material when the energy level and the atmosphere type vary. Influent variables are discussed below.
Thermal stresses arise from both temperature gradients and differences in the thermal expansion coefficients of various materials used. In addition to thermal stresses induced by engine operation, extreme transient temperatures can be produced during start up and shut down of the engine. It is accompanied by a very fast heating or cooling from the outer surfaces of igniters, leading to large temperature differences from the centre to the outside. The igniter design must be developed to accommodate these thermal stresses. Sparks induce erosion of both the ceramic material and electrodes. A high erosion rate of electrodes leads to increased erosion at the interface with the composite material, due to local breakdowns through thin interfaces. In general, electrode erosion influences the igniter life and sometime leads to unexpected failure. For the ceramic material, erosion is generated by sparks on the ceramic surface. The spark is a plasma-like process, which acts as a source of thermal shock, and significant thermal transformations as vaporisation of the ceramic can occur. This is not surprising when it is remembered that the arc core temperature should attain 10000 ◦ C. It occurs very closely to the ceramic surface and an increase in pressure leads to a closer plasma. 3.2. Oxidation A very high oxidation resistance is necessary but the high temperature in engines restricts the choice of suitable materials. SiC-SiAlON composites are suitable for extreme environment applications. Fig. 13 represents the breakdown voltage variation versus the oxidation time at 850 ◦ C under air at atmospheric pressure. Whereas the large stress induced by the air atmosphere at 850 ◦ C, the breakdown voltage increase by only 30% after 8 h. In general, the principal chemical reactions that occur on SiC grain surface are: SiC+3/2O2 → SiO2 +CO SiC+O2 → SiO2 +CO2 .
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Fig. 14. SEM photo of the surface after a 850 ◦ C, 7 h heating under air.
Fig. 15. SEM photo of the surface after 500 sparks (1 J) at room temperature under air.
In addition, secondary reactions determine the equilibrium between species at the reaction interface with the atmosphere: SiC+2CO → SiO2 +3C 2C+O2 → 2CO. Global oxidation reactions under air follow a parabolic curve, which is indicative of an oxidation process controlled by diffusion mechanisms [27]. Oxidation of composites results in an increase of the breakdown voltage from a local insulating layer on SiC grains. Correspondingly, Fig. 14 shows the ceramic microstructure after heating under air at 850 ◦ C, 7 hours. Local oxidation zones are not clearly evidenced, but a charging effect under the SEM spot (white zones) suggests the occurrence of an insulating phase such as silica. Fig. 16. Relative erosion rate of the composite ceramic under different working conditions.
3.3. Gas pressure Pressure in combustion chambers reaches 10 to 40 bars. A pressure increase reduces both the electron mean free path in the plasma and the extend of the secondary ionisation phenomenon. The plasma is restricted to a closer volume onto the material surface, whose temperature increases. Stronger thermal shocks and acoustic waves during sparks induce more pronounced erosion. It is illustrated in Fig. 15, where the relative erosion is significantly increased when pressure increases from 1 bar to 12 bars, at the same energy level (1 J). 3.4. Spark energy and high temperature The energy for the discharge Ei is preliminary stored in a high voltage capacitor. The expression of Ei against the capacitance C and the voltage V is: Ei = 0.5CV 2
(13)
where Ei commonly varies from 1 to 5 J, depending on requirements from the engine manufacturer. Furthermore, the energy delivered during sparks also varies with the peak voltage, during the ionisation stage. V is from the high voltage source, but it has to increase when the material characteristics change after a long working period at high temperature.
Current voltage measurements (a typical example is in Fig. 9) with varying the energy level show that both the duration of discharges and the current peak intensity increase when the stored energy is increased from 1 to 5 J. It is clear that high current values enhance both the ceramic and electrode erosion during the discharge phase. This point is evidenced in curves of Fig. 16, showing relative ceramic erosion under 12 bars of pressure. When the energy level increases from 1 to 5 J, the relative erosion increased by 30%. Correspondingly, Fig. 16 shows the ceramic surface after 500 sparks (1 J) at room temperature under air. Whereas the released energy attains only 1 J per spark, a strong modification of the surface morphology is evidenced. Rounded shapes of grains reveal the high temperature during sparking. The high temperature in engines has a limited influence on the igniter life. Whereas a temperature of up to 800 ◦ C is attained, the local temperature within the plasma on the ceramic surface is significantly higher. The whole igniter assembly is composed not only of a composite ceramic, but also of metal parts, ceramic insulator and materials for joining. Differential thermal stresses of materials and higher temperatures for met-
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als and other parts joining are key parameters to obtain a high working temperature. 3.5. Fuel use The presence of liquid fuel at the igniter tip tends to reduce the spark extends and modify the erosion conditions. Fuel influence in pellet erosion depends on pellet type. This point will be investigated further. Under totally submerged conditions, the current peak takes place partially through the material volume. High internal current values may lead to cracks and chipping of the surface discharge ceramic. Breakdowns inside material porosity can be highly destructive. 3.6. Engine type and igniter life In normal working conditions, the operational life of igniters is mainly determined by the type of the surface discharge material. This reflects the highly developed state of the metallurgy of electrode materials, leaving only little progress for further developments. Shortcomings in the oxidation or erosion of the electrode materials lead to greater demands being placed on the quality of the surface discharge material, which may provide more opportunities for improvements. When considering spark erosion of the ceramic as a key factor for igniter life, it is clear that it is appropriate to measure life time in terms of the number of sparks achieved. If the igniter performance is mainly assessed on this basis, experiments show a significant variation of material working life, but the resistance to spark erosion was highly improved in recent years. Engine working conditions vary greatly in different engine types. Particularly, igniter tip temperatures range from 300 to 1000 ◦ C and engine pressures from 10 to 40 bars. Engine design should favour the flooding of the igniter’s tip with liquid fuel, but should leave it comparatively dry. These variables are important in determining the spark extend. Furthermore, it becomes frequent to use igniters during the whole duration of engine runs. Continuous ignition greatly increases the number of sparks during an operational period. Whereas sparking only at start up might deliver 100000 sparks during a 1000 hours operational life, an igniter used for continuous ignition during 100 hours may deliver over 250000 sparks. The higher number of sparks clearly increases the erosion of both the metal electrodes and the surface discharge material. Besides, sparks occurring at start up occur at low pressure and surrounding temperature, whereas sparks during continuous ignition occur at full engine pressure and high temperature, which favours erosion. 4. Conclusion High-energy discharge materials are very specific ceramic composite materials for aero-engines ignition. Fundamental mechanisms involved in spark formation are already known: they are related to ceramic compositions, microstructure characteristics and surface morphology. The operational life of ceramic materials depends mostly on the high temperature
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reached, the atmosphere type and the fuel spraying, but also on the whole assembly of igniter components. Degradation mechanisms are recognized, but more investigations are required to increase the working life of future engines, operating at even higher temperatures. Acknowledgement This research project is supported by the Association Nationale pour la Recherche Technologique (ANRT), which is acknowledged. References [1] G.N. Burland, K.A. Goreham, R.J. Taunt, The development of the high energy surface discharge spark igniter, Amer. Soc. Mechanical Engineers 84 GT (51) (1984) 1–7. [2] A. Jankowiak, F. Collardey, P. Blanchart, Electrical behaviour at high voltage on surface of SiC-β -SiAlON ceramic composites, J. Eur. Ceram. Soc. 25 (1) (2005) 13–18. [3] A. Jankowiak, P. Blanchart, Electrical discharges under high voltage on surface of silica–nickel ceramic composites, Adv. Engrg. Mater. 12 (6) (2004). [4] E.O. Forster, Progress in the understanding of electrical breakdown in condensed matter, J. Phys. D. 23 (1990) 1506–1514. [5] G.F. Batalha, M. Stipkovic Filho, Quantitative characterization of the surface topography of cold rolled sheets-new approaches and possibilities, J. Mater. Process. Technol. 113 (2001) 732–738. [6] M. Takeda, H. Ina, K. Kobayashi, Fourier-transform method of fringe pattern analysis for computer-based topography and interferometry, J. Opt. Soc. Am. 72 (1) (1982) 156–160. [7] DIN 4776 1990 measurement of surface roughness; Parameters Rk, Rpk, Rvk, Mr1, Mr2 for the Description of the Material Portion in Roughness German standard. [8] A.R. Von Hippel, Molecular Science and Molecular Engineering, The Technology Press of MIT/J. Wiley, New York, 1959, pp. 39–47. [9] N.St.J. Braithwaite, Introduction to gas discharge, Plasma Sources Sci. Tech. 9 (2000) 517–527. [10] S. Vacquié, L’arc électrique, CNRS Ed., Eyrolles, 2000. [11] J.D. Von Engel, Cobine, Gaseous Conductors, New York, 1958. [12] M. Lenzlinger, E.H. Snow, Fowler–Nordheim tunnelling into thermally grown SiO2 , J. Appl. Phys. 40 (1969) 278. [13] L.M. Porter, R.F. Davis, A critical review of ohmic and rectifying contacts for silicon carbide, Mater. Sci. Engrg. B 34 (2–3) (1995) 83–105. [14] K. Yuasa, A. Shimoi, I. Ohba, C. Oshima, Modified Fowler– Nordheim field emission formulae from a non-planar emitter model, Surf. Sci. 520 (1–2) (2002) 18–28. [15] A.A. Evtukh, N.I. Klyui, V.G. Litovchenko, Yu.M. Litvin, O.B. Korneta, V.M. Puzikov, A.V. Semenov, Peculiarities of field emission from silicon carbide films, Appl. Surf. Sci. 215 (1–4) (2003) 237–241. [16] D.C. Lim, H.S. Ahn, D.J. Choi, C.H. Wang, H. Tomokage, The field emission properties of silicon carbide whiskers grown by CVD, Surf. Coat. Technol. 168 (1) (2003) 37–42. [17] M. Jimenez, R.J. Noer, G. Jouve, J. Jodet, B. Bonin, Electron field emission from large-area cathodes: evidence for the projection model, J. Phys. D: Appl. Phys. 27 (1994) 1038–1045. [18] B.M. Cox, The nature of emission sites, J. Phys. D: Appl. Phys. 8 (1975) 2065–2073. [19] A.N. Obraztsov, Al.A. Zakhidov, A.P. Volkov, D.A. Lyashenko, Nonclassical electron field emission from carbon materials, Diamond Related Mater. 12 (2003) 446–449. [20] A.A. Evtukh, N.I. Klyui, V.G. Litovchenko, Y.M. Litvin, O.B. Korneta, V.M. Puzikov, A.V. Semenov, Peculiarities of field emission from silicon carbide films, Appl. Surf. Sci. 215 (1–4) (2003) 237–241. [21] J. Paulini, T. Klein, G. Simon, Thermo-field emission and the Nottingham effect, J. Phys. D: Appl. Phys. 26 (1993) 1310.
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[22] S. Mackeown, The cathode drop in an electric arc, Phys. Rev. 34 (1929) 611. [23] A.E. Guile, Arc-electrode phenomena, Proc. IEE, IEE Rev. 118 (1971) 1131. [24] G.A. Farrall, Electrical breakdown in vacuum, in: J.M. Lafferty (Ed.), Vacuum Arcs: Theory and Application, Wiley, New York, 1980. [25] G.A. Mesyats, et al., Explosive electrons emission, IEEE Trans. Elec. Insul. EI 18 (1983) 218.
[26] D. Chen, S.P. Wong, W.Y. Cheung, J.B. Xu, Influence of surface morphology on the field emission properties of planar SiC/Si heterostructures formed by ion beam synthesis, Solid State Comm. 128 (2003) 435–439. [27] A. Zymla et al., Oxidation in air of nitride bonded silicon carbide ceramic, in: La Revue de la Métallurgie, Paris CIT, Sciences et Génie des matériaux, mai 2004, pp. 427–439.
Electrical behaviour of ceramic composite materials for aero-engine igniters A. Jankowiak, P. Blanchart ∗ GEMH, ENSCI, Limoges, France Received 25 January 2005; received in revised form 26 May 2005; accepted 9 January 2006 Available online 31 January 2006
Abstract High energy igniters are extensively used in aero-engines and this study describes the specific ceramic composite materials used at the igniter tips. These ceramics favour the formation of a plasma-like process on their surfaces under an electrical field not exceeding 1 kV mm−1 . The elevated temperature which is reached, and the high energy released during sparks are very favourable for engine ignition, even when internal engine temperature, pressure and fuel flow are unfavourable. Currently used ceramics and possible igniter designs are described. For composite ceramics, the physical mechanisms involved during sparking are presented, together with the high temperature and pressure effect in engines. Degradation mechanisms of materials are also examined to understand the operational life of igniters when working conditions vary. Problems associated with continuous ignition and the type of surface discharge materials are also discussed. 2006 Elsevier SAS. All rights reserved. Keywords: Ceramic; Igniter; Electrical property
1. Introduction This study describes the design of ceramic composite materials used for spark igniters or more generally for aeronautical ignition systems. Descriptions will be focussing on specific low-tension igniters as they have been studied for years [1], and are currently used for the aero-engine ignition. The rather special characteristic of these igniters is the plasma-like process occurring on the ceramic surface during the electrical discharge under an electric field in the 0.5–1 kV mm−1 range. The localized high energy level (1–5 J) of “hot electrons” is very useful for gas or liquid ignition, even when the stoechiometry ratio in engines is unfavourable. To understand the role of a ceramic composite material, it is necessary to present the whole ignition systems, the various types of spark igniters and the whole assembly of spark igniters in which ceramics are used.
form the DC low-tension source (28 V) into high voltage pulses. Whereas for simple air gaps up to 30 kV are required, composite ceramic igniters require only 1–2 kV. The high voltage source must be precisely calibrated (duration of pulses, energy level, periodicity) to obtain a high energy level during sparks on the ceramic composite surface. There are three main groups of igniters currently used in civilian or military aero-engines. Different technologies and designs are used, with various high voltage sources and a large range of delivered energy. This review describes three typical types of igniter designs. 1.2. Air-gap igniters Fig. 1 presents the air-gap igniter end. It is similar to standard spark plugs for automotive internal-combustion engines.
1.1. The ignition system The whole ignition system includes four parts: a direct current source, a high voltage unit, an electric cable and the igniter itself. The HV ignition unit is an electronic assembly to trans* Corresponding author.
E-mail address: [email protected] (P. Blanchart). 1270-9638/$ – see front matter 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.ast.2006.01.001
Fig. 1. Air-gap igniter.
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These igniters have two coaxial electrodes with an 0.2–1 mm air gap. The electrical insulation uses a ceramic insulator (alumina ceramic) and a glass sealing between metal and ceramic. The spark across the air gap requires at least 15 kV, but the breakdown voltage increases drastically with pressure, up to 30 kV for 15 bars. Therefore, these igniters cannot always meet ignition needs in combustion chambers when the atmosphere pressure exceeds 15 bars. Whereas these igniters present the largest lifespan under operation, their use is not generalized in all aeronautical engines, because of the electromagnetic disturbances from high voltage pulses. 1.3. Surface discharge igniters High energy surface discharge igniters (H.E. Igniter) are devices used to initiate combustion with a low voltage source. Early surface discharge igniters were developed in the forties for aircraft engines in Germany. The story of the utilisation of H.E. Igniters is linked to the history of gas turbine ignition. In general, problems encountered in the early days of gas turbine ignition included: – fouling and shorting of high voltage igniters by combustion products; – inability of low energy spark plugs to ignite droplets of the less volatile kerosene fuels; – disturbance and insulation problems when carrying high voltage power sources on board aircrafts. These requirements mean that at present, low voltage H.E. Igniters are always used. There are two main categories of H.E. Igniters, the first one having a thin layer of semiconductor or resistor coated on an insulating material acting as the surface discharge material. The alternative is to use a specific ceramic material at the igniter tip, which is a source of a plasma-like process.
recently, improvements were also obtained with doping an insulating ceramic surface, such as alumina, by various metals (e.g. copper). As for igniters with a simple air-gap between metal and electrodes, the whole assembly has two coaxial electrodes close to the semi-conducting surface of the ceramic (Fig. 2). Metal and ceramics parts are sealed by a glass layer. The main difference between an air gap igniter and a semi-diffused igniter is the existence of a significant surface conductivity. This material favours the decrease of the breakdown voltage down to 1.5 kV. Voltage is almost independent of the atmosphere pressure, within the common range of the atmosphere pressure encountered in engines (up to 15 bars). Nevertheless, the major disadvantage of these igniters is their lifespan, which is considerably reduced because of the wear and of the fast chemical erosion of the conducting layer. The most recent type of igniter uses composite ceramic materials. They are often named semiconductors, even if this terminology is incorrect because the physical mechanisms involved are somewhat different. Such igniters are often used for the ignition of a large range of aero-engines. These ceramic materials, insulating materials, metal parts and the whole igniter design are at present under constant development. 1.5. Igniters ceramic composites The igniter design described in Fig. 3 is commonly used. Two coaxial electrodes are connected to the composite ceramic material at the igniter tip. Underneath, an insulating ceramic holds the assembly and a glass seal is used for metal to ceramic bonding. In general, the breakdown voltage is about 1 kV for a gap between electrodes of about 1.5 mm. The significant physico-chemical strength of some ceramics at high temperature reduces the phenomenon of wear and increases the lifespan of igniters. Besides, the breakdown voltage and the electrical discharge characteristics at high pressure are important characteristics, which can be maintained within
1.4. Surface diffused or coated igniter This range of novel devices was investigated years ago and the first successful solution was found for Rolls Royce Dart Engines, which used a primitive form of the high energy igniter. This device named Aquadag, was a colloidal dispersion of graphite powder in an aqueous media, deposited onto the ceramic between electrodes, and dried to form an adherent film. The problem of erosion of such a surface discharge material was overcome by the deposition of soot from the combustion products and the soot layer acted effectively to allow sparks. Nevertheless, problems with igniter life resulting from spark plug erosion were encountered as soon as more efficient combustion chambers were developed, where the necessary carbon layer did not withstand working conditions. Subsequently some new ceramics have been investigated as surface discharge materials to prevent erosion at high temperature and improve ignition efficiency at any pressure or fuel to air ratio. These ceramics include various combinations of materials such as oxides of aluminum, copper, iron or titanium. More
Fig. 2. Semi-diffused igniter.
Fig. 3. Ceramic gap igniter.
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Fig. 4. Typical microstructure of the composite ceramic. SiC grains appear in bright grey and the matrix material in dark grey.
average values. The lifespan of these igniters with composite ceramics is higher than that obtained with diffused igniters, but it is lower than that obtained with simple air-gap igniters. Physical phenomena involved in the breakdown process are detailed hereafter of the document as well as common type of ceramic material used for such igniters. As ceramics are assembled together, mechanical engineering problems arise from the need to accommodate thermal stresses and from bonding requirements of components made of differing materials. This paper provides some insight into the operation of igniters. A number of features of engine applications and operations are discussed in relation to igniter performance. The surface discharge materials used are composite ceramics, investigated for several years; a wide range of possible compositions must be considered. In this work, we studied particle-reinforced ceramic matrix composites with micro-sized SiC grains as the more conductive phase and SiAlON as the matrix phase. The volume fraction of SiC varies from 50 to 70%. Despite the high quantities of SiC conductive grains the global electrical conductivity is relatively low (1.5 × 10−6 S/cm). This is due to partially oxidized SiC grain interfaces, but also to the presence of porosity within the material (about 20 vol%) [2]. A typical microstructure is presented in Fig. 4, where SiC grains appear in light grey and the matrix material in dark grey. It is seen that the distribution of the SiC conductive phase within the matrix phase should be optimised, but further development is still necessary. 2. Properties of composite ceramics
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Fig. 5. Variation of the electrical conductivity of the composite ceramic with the nickel content.
The conductivity of such a composite material is described along with some percolation theories, which account for the current percolation through a random network of conductive grains. Whereas the bulk grain properties are considered in these models, grain boundaries effects are also significant and must be added. Under a high electric field, tunnelling at grain boundaries must be considered as a significant phenomenon. The theory, which explains the current density j through a single grain boundary of two adjacent grains, is represented as: 3 (1) j = Em exp(− φd) + BEm where Em is the applied electrical field across the thickness d of the interface and φ is the nickel work function. When the electrical field increases, an ohmic regime is followed by a power law regime with a large increase of current. For a composite material, the “spreading” of current through the volume is obtained by the increase of voltage favouring the opening of many conducting channels, even at significant inter particle distances. It leads to the activation of many emission sites at micro-protrusions on the surface, which becomes active for electronic emission [4]. This extended surface ionisation is the onset of the surface breakdown. To be suitable for ignition applications, composite materials must present an electrical conductivity value within a narrow range just above the threshold point of the conductivity against the conductive phase content (region II of Fig. 5). For materials in region I, inter particles distances are large enough to break many percolation paths. In region III, at the opposite of the nickel content range, highly conductive materials behave as ohmic conductors. Most of the energy from the external supply is dissipated by Joule effect.
2.1. Volume properties 2.2. Surface properties The electrical properties of the surface in relation to volume properties were studied with a representative composite material [3], i.e., a mix of a conductive nickel powder and an insulating silica glass powder as the matrix phase. Thus, it was possible to change the nickel quantity over a large range, to obtain a large range of conductivity values.
The surface micro-morphologies of composite materials were obtained by a non-contact optical profiler [5] (NOP) (Fogale Nanotech). In general, surface characteristics include sharp grains, protruding out of the core with a grain size over the 3–10 µm range. Fig. 6 shows a typical material microstruc-
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ture where a thick layer of the secondary phase is distributed between SiC grains of different sizes. The small intergranular pores and the matrix are clearly visualized with different grey scales. The non-contact optical profiler measures the surface height variations by optical phase shifting and white light vertical scanning interferometry [6], with a vertical measurement performance of 2 nm. Along the material surface, the light intensity of optical fringes is related to the surface profile and discredited into the measurement array of a camera. Within individual pixel and for each sampling interval, the light intensity represents the averaged value of surface heights within the elementary surface. The lateral spatial sampling is fixed by the magnification of the optical system. We used an objective lens magnification of 10 and the sampling interval was 1 µm in both directions. Roughness (short waves) and waviness (long waves) of surface profiles are preliminary separated by a filtering operation, to obtain the reference mean surface. This procedure uses a Gaussian filter whose parameters are adjusted to remove most of the waviness. From profiles (Fig. 7), we computed the bearing ratio curves (Abott curves), which are the depth distribution from the highest peak depth versus the bearing ratio. Smr is expressed in percent of the sum of material-filled surfaces Si to the evalu-
Fig. 6. Typical surface morphology where a thick layer of secondary phase is distributed between SiC grains.
ation surface Sm at a given profile section. It is the distance between the measured intersection level and the reference level: 1 Smr = Si · 100 (2) Sm i
The quantitative description of surface uses the specific group of Sk parameters [7]: Spk , Sk , and Svk which are derived from the bearing ratio graphic (Fig. 7). In particular, Spk is the measure of the peak height above the core roughness (peak to valley), Svk is the measure of the valley depths below the core roughness and Sk is a measure of the core roughness minus the peaks and valleys characterized by Spk and Svk respectively. A high Spk means that the surface is composed of sharp peaks with a large aspect ratio value. The peak material ratio Sr1 , is the percentage of material volume that includes the peak structures associated with Spk . Sr2 is a measure of the valley material ratio, with (100%-Sr2 ) representing the percentage of material that includes the valley structures associated with Svk . We will discuss later the strong relation between the Spk parameter and the breakdown voltage. 2.3. Electrical breakdown mechanisms of high energy igniters A distinguishing feature of the H.E. Igniter is the breakdown voltage required for sparking when the atmosphere pressure increases up to 40 bars, as in combustion chambers of some aeroengines. In Fig. 8, breakdown voltage is plotted against pressure in comparison with the simple air-gap characteristic [8]. Here, the very weak variation of breakdown voltage with pressure of H.E. Igniters is pointed out. To perform electrical breakdowns, composite materials are laboratory-tested between two metal electrodes connected to the electrical discharge circuit. A gap of 1.2 mm is adjusted between the two electrodes on the surface of the material. The energy is stored by a high voltage capacitance connected to our device through a trigger circuit and a series resistor. In general, the stored energy ranges from 1 to 5 J. The discharge voltage and current is measured with a large bandwidth oscilloscope (500 MHz, LeCroy Waverunner LT322). All electronic components are selected to obtain a smooth repeatable voltage and
Fig. 7. Spk , Sk , Svk morphology parameters which are derived from the bearing ratio curve.
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current signal for the discharge. For our experiments, the capacitive discharge circuit is electronically switched to the ceramic device thought a damping resistor placed in series. The resistor value was adjusted to reduce high frequency oscillations of the discharge current, but also to modify the whole duration of the electrical discharge. Typical current and voltage characteristics during the surface discharge process are illustrated in Fig. 9 for a composite material with 65 Vol% of SiC. On the curves, two main stages are distinguished: – a first and rapid increase of voltage is followed by a characteristic peak, where higher voltage is attained (Vm ); – a subsequent and very rapid decrease of voltage is accompanied by a large increase of current and the formation of a hot plasma-like process. The first stage is attributed to the material ionisation stage when the applied voltage between the electrodes reaches its maximum value. The second stage is the breakdown stage when the voltage decreases due to the formation of preferential breakdown paths and sparking occurs for engine ignition. For our composite material, the current-voltage characteristic extends to a broad range of voltage and current (up to 1000 V and
Fig. 8. Breakdown voltage versus pressure for the composite ceramic, in comparison with the simple air-gap characteristic.
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300 A) within the 1.2 mm gap between electrodes. The whole duration of discharges is about 100 µs and the process ends when the capacitor discharge is exhausted. It is proposed that the sequence of phenomena can be explained using the knowledge from electric discharges in lowpressure gas, under the Paschen’s law [9,10]. This model reflects the electrical breakdown mechanism in gases at the Vm voltage. Vm is related to the p.d product where p is the gas pressure and d the gap between electrodes: Vm =
Bpd ln(Apd) − ln(αd)
(3)
where A and B are gas dependent parameters and obtained from the literature [11]. α is the Townsend’s first ionisation coefficient, which relates the mean free path for electron secondary ionisation to the total scattering path. It is pressure and electric field dependent. In the continuation of the discharge process, the acceleration of ions in the electric field leads to secondary electron emissions at the electrodes. This secondary emission replenishes the population of ions and the discharge becomes self-sustaining when: 1 (4) αd = ln 1 + γ where γ is the secondary ionisation coefficient for the electrode surface. When p increases, α decreases from the limiting lowpressure. α is the number of electron-ion pairs generated per unit distance, which is related to the secondary electron emission coefficient γ at the cathode through Eq. (4). A large decrease of α means an increase of γ , which exceeds realistic values. Therefore, it is also assumed the occurrence of the Fowler– Nordheim’s mechanism [12] which gives the expression of the current flowing between electrodes. The current density j is: Φ 3/2 aE 2 exp −b (5) j= Φ E where Φ represents the work function, E the electric field, a and b depend on Φ. For SiC the work function is in the 2.7 to 3.5 eV range [13]; a and b can be estimated as [14]: a = 1.54 × 10−6 A m−2 , b = 6.83 × 109 V−1/2 m.
(6)
For composite materials having true surfaces, the abovedescribed Fowler–Nordheim’s law is modified [15,16] to take into account the enhanced field emission, which is observed with significant current values even at low electric fields. The significant enhancement of the microscopic field on surface is the so-called enhanced field emission (EFE). It seems that the current emission is enhanced at local microscopic emitting sites, which can be micro-protrusions on surfaces [17] or chemically modified surfaces by doping or local deposits. In recent years, progress has been made in the understanding of the various contributions to EFE, which can be classified into different categories: Fig. 9. Typical current and voltage characteristics during the surface discharge process for a 65 Vol% SiC composite.
– metallic surface roughness; – metallic dust and micro-particles;
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– – – – –
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grain boundaries; molten craters after breakdown; dielectric impurities and layers; absorbed gas; metal-insulator-vacuum (MIV) or metal-insulator-metal (MIM) layers.
The microscopic electric field is enhanced by a βi factor by which the ideal surface field E is increased to a local value Em : βi = Em /E.
(7)
For material surfaces having a surface micro-roughness, the appropriate Fowler–Nordheim’s law is modified to take into account the enhanced field emission. When βi is added, Eq. (5) becomes: aβi2 E 2 Φ 3/2 j= exp −b (8) Φ βi E The βi value is calculated considering the linear relationship between log(I /E 2 ) and 1/E during the pre-breakdown voltage period of the electrical discharge sequence. From experiments, only macroscopic electrical data are available, and a global field enhancement factor βg is calculated. Microscopic βi contribute to the global βg which proves that experimental data are significant. A typical example of such plot is in Fig. 10, where experiments are satisfactorily fitted by a straight line. From line slopes, βg values are within the βg = 1 × 104 –3 × 104 range, which means a very large accentuation of the field emission [18]. Similar data were obtained elsewhere with electrodes having micro-protrusions that support the validity of the theoretical treatment [19,20]. Such very high values of βg are explained by the peculiar surface microstructure, which allows multiple tip on tip configurations at conductive grains. The enhancement factors are therefore multiplied and contribute to a very large increase of the global βg . Furthermore, thermionic electron emission due to current flowing on the surface and Joule heating should have an important role. As the surface temperature increases, the electron energy also increases until a point is reached when a few electrons have sufficient energy to escape the binding forces of
Fig. 10. F.N. plot of log(I /E 2 ) against 1/E during the pre-breakdown voltage period.
the atoms and thus escape the surface of the material. In addition, we must consider that thermo-field (T-F) emission is favoured by both the high temperature and the occurrence of the high electric field. The T-F emission current densities can be significantly larger than the simple addition of thermionic and field emission mechanisms [21]. On the material surface, the plasma process induces the formation of a thin volume, characterized by a high temperature and ion density. It causes a large voltage gradient on the surface [22] and induces an intense local heating [23,24]. The contribution from the T-F emission to the current density can be calculated according to the Richardson– Dushman equation for thermionic emission corrected for the Schottky effect, i.e., φ = e3 E/(4πεo ), then: 4πkB eme 2 ϕ − φ (9) T exp − jT (E, T , ϕ) = kB T h3 where E is the electric field, T the temperature, φ the material work function, e the electron charge, kB the Boltzmann’s constant, h the Planck constant, εo the vacuum permittivity. It is seen that j increases with E 1/2 and with T 2 . Besides these phenomena, repeated sparking induces flaws and craters on the material surface (Fig. 11) and a local melt can also be evidenced. Explosive electron emission inside material flaws and pores is a particular phenomenon, which occurs when a field emission phenomenon is extended to intense conditions. When an extreme current density occurs, electrons from emitters form a very localised and dense plasma on the surface, but also inside pores. This phenomenon is similar to microscopic explosions [25], as a high energy level is concentrated inside micro-volumes, resulting in local material degradation. 2.4. Influence of the surface morphology Sparking on a material surface is affected by surface characteristics. As described above, the surface micro-morphologies of the materials were obtained by a non-contact optical profiler.
Fig. 11. SEM evidence of a local degradation due to explosive electrons emission.
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Fig. 12. Correlation between the breakdown electric field Em and the surface morphology parameter Spk .
Fig. 13. Breakdown voltage variation versus the oxidation time at 850 ◦ C under air at atmospheric pressure.
Surfaces include sharp SiC grains (5–20 µm) protruding out of the secondary phase. SiC grains are distributed in the core in which small inter-granular pores are also observed. From surface roughness, we calculated the Spk parameter, which is a measure of the peak height above the core (peak to valley). This parameter is supposed to be strongly correlated to the discharge process. It can be experimentally changed by using various SiC powders with different grain sizes. Results (Fig. 12) show that the macroscopic electric field for breakdown Em is correlated with Spk . An empirical expression of the macroscopic breakdown electric field Em is given by:
3.1. Thermal stresses and surface erosion
Em = aSpk + Eo
(10)
where a and Eo are parameters related to material. Em appears to be correlated with the number of surface protrusions acting as emitting sites. When Spk increases, the macroscopic breakdown electric field drastically decreases. This trend can be attributed to the increase of the aspect ratio of emitters together with the βi field enhancement factor. Here the specific role of emitter morphology and density for the surface ionisation is pointed out and similar observations were made in the literature with metal tips [26]. 3. Determination of the operational life of H.E. Igniters The operational life of H.E. Igniters for different applications must range from 40 to 4000 hours. The potential failure mechanisms, which determine life time, can be identified as follows: – – – – –
thermal stresses and surface erosion oxidation gas pressure spark energy and high temperature fuel use and engine type.
The process and extent of spark erosion are also modified by a number of variables as the erosion rate of the ceramic material when the energy level and the atmosphere type vary. Influent variables are discussed below.
Thermal stresses arise from both temperature gradients and differences in the thermal expansion coefficients of various materials used. In addition to thermal stresses induced by engine operation, extreme transient temperatures can be produced during start up and shut down of the engine. It is accompanied by a very fast heating or cooling from the outer surfaces of igniters, leading to large temperature differences from the centre to the outside. The igniter design must be developed to accommodate these thermal stresses. Sparks induce erosion of both the ceramic material and electrodes. A high erosion rate of electrodes leads to increased erosion at the interface with the composite material, due to local breakdowns through thin interfaces. In general, electrode erosion influences the igniter life and sometime leads to unexpected failure. For the ceramic material, erosion is generated by sparks on the ceramic surface. The spark is a plasma-like process, which acts as a source of thermal shock, and significant thermal transformations as vaporisation of the ceramic can occur. This is not surprising when it is remembered that the arc core temperature should attain 10000 ◦ C. It occurs very closely to the ceramic surface and an increase in pressure leads to a closer plasma. 3.2. Oxidation A very high oxidation resistance is necessary but the high temperature in engines restricts the choice of suitable materials. SiC-SiAlON composites are suitable for extreme environment applications. Fig. 13 represents the breakdown voltage variation versus the oxidation time at 850 ◦ C under air at atmospheric pressure. Whereas the large stress induced by the air atmosphere at 850 ◦ C, the breakdown voltage increase by only 30% after 8 h. In general, the principal chemical reactions that occur on SiC grain surface are: SiC+3/2O2 → SiO2 +CO SiC+O2 → SiO2 +CO2 .
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Fig. 14. SEM photo of the surface after a 850 ◦ C, 7 h heating under air.
Fig. 15. SEM photo of the surface after 500 sparks (1 J) at room temperature under air.
In addition, secondary reactions determine the equilibrium between species at the reaction interface with the atmosphere: SiC+2CO → SiO2 +3C 2C+O2 → 2CO. Global oxidation reactions under air follow a parabolic curve, which is indicative of an oxidation process controlled by diffusion mechanisms [27]. Oxidation of composites results in an increase of the breakdown voltage from a local insulating layer on SiC grains. Correspondingly, Fig. 14 shows the ceramic microstructure after heating under air at 850 ◦ C, 7 hours. Local oxidation zones are not clearly evidenced, but a charging effect under the SEM spot (white zones) suggests the occurrence of an insulating phase such as silica. Fig. 16. Relative erosion rate of the composite ceramic under different working conditions.
3.3. Gas pressure Pressure in combustion chambers reaches 10 to 40 bars. A pressure increase reduces both the electron mean free path in the plasma and the extend of the secondary ionisation phenomenon. The plasma is restricted to a closer volume onto the material surface, whose temperature increases. Stronger thermal shocks and acoustic waves during sparks induce more pronounced erosion. It is illustrated in Fig. 15, where the relative erosion is significantly increased when pressure increases from 1 bar to 12 bars, at the same energy level (1 J). 3.4. Spark energy and high temperature The energy for the discharge Ei is preliminary stored in a high voltage capacitor. The expression of Ei against the capacitance C and the voltage V is: Ei = 0.5CV 2
(13)
where Ei commonly varies from 1 to 5 J, depending on requirements from the engine manufacturer. Furthermore, the energy delivered during sparks also varies with the peak voltage, during the ionisation stage. V is from the high voltage source, but it has to increase when the material characteristics change after a long working period at high temperature.
Current voltage measurements (a typical example is in Fig. 9) with varying the energy level show that both the duration of discharges and the current peak intensity increase when the stored energy is increased from 1 to 5 J. It is clear that high current values enhance both the ceramic and electrode erosion during the discharge phase. This point is evidenced in curves of Fig. 16, showing relative ceramic erosion under 12 bars of pressure. When the energy level increases from 1 to 5 J, the relative erosion increased by 30%. Correspondingly, Fig. 16 shows the ceramic surface after 500 sparks (1 J) at room temperature under air. Whereas the released energy attains only 1 J per spark, a strong modification of the surface morphology is evidenced. Rounded shapes of grains reveal the high temperature during sparking. The high temperature in engines has a limited influence on the igniter life. Whereas a temperature of up to 800 ◦ C is attained, the local temperature within the plasma on the ceramic surface is significantly higher. The whole igniter assembly is composed not only of a composite ceramic, but also of metal parts, ceramic insulator and materials for joining. Differential thermal stresses of materials and higher temperatures for met-
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als and other parts joining are key parameters to obtain a high working temperature. 3.5. Fuel use The presence of liquid fuel at the igniter tip tends to reduce the spark extends and modify the erosion conditions. Fuel influence in pellet erosion depends on pellet type. This point will be investigated further. Under totally submerged conditions, the current peak takes place partially through the material volume. High internal current values may lead to cracks and chipping of the surface discharge ceramic. Breakdowns inside material porosity can be highly destructive. 3.6. Engine type and igniter life In normal working conditions, the operational life of igniters is mainly determined by the type of the surface discharge material. This reflects the highly developed state of the metallurgy of electrode materials, leaving only little progress for further developments. Shortcomings in the oxidation or erosion of the electrode materials lead to greater demands being placed on the quality of the surface discharge material, which may provide more opportunities for improvements. When considering spark erosion of the ceramic as a key factor for igniter life, it is clear that it is appropriate to measure life time in terms of the number of sparks achieved. If the igniter performance is mainly assessed on this basis, experiments show a significant variation of material working life, but the resistance to spark erosion was highly improved in recent years. Engine working conditions vary greatly in different engine types. Particularly, igniter tip temperatures range from 300 to 1000 ◦ C and engine pressures from 10 to 40 bars. Engine design should favour the flooding of the igniter’s tip with liquid fuel, but should leave it comparatively dry. These variables are important in determining the spark extend. Furthermore, it becomes frequent to use igniters during the whole duration of engine runs. Continuous ignition greatly increases the number of sparks during an operational period. Whereas sparking only at start up might deliver 100000 sparks during a 1000 hours operational life, an igniter used for continuous ignition during 100 hours may deliver over 250000 sparks. The higher number of sparks clearly increases the erosion of both the metal electrodes and the surface discharge material. Besides, sparks occurring at start up occur at low pressure and surrounding temperature, whereas sparks during continuous ignition occur at full engine pressure and high temperature, which favours erosion. 4. Conclusion High-energy discharge materials are very specific ceramic composite materials for aero-engines ignition. Fundamental mechanisms involved in spark formation are already known: they are related to ceramic compositions, microstructure characteristics and surface morphology. The operational life of ceramic materials depends mostly on the high temperature
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reached, the atmosphere type and the fuel spraying, but also on the whole assembly of igniter components. Degradation mechanisms are recognized, but more investigations are required to increase the working life of future engines, operating at even higher temperatures. Acknowledgement This research project is supported by the Association Nationale pour la Recherche Technologique (ANRT), which is acknowledged. References [1] G.N. Burland, K.A. Goreham, R.J. Taunt, The development of the high energy surface discharge spark igniter, Amer. Soc. Mechanical Engineers 84 GT (51) (1984) 1–7. [2] A. Jankowiak, F. Collardey, P. Blanchart, Electrical behaviour at high voltage on surface of SiC-β -SiAlON ceramic composites, J. Eur. Ceram. Soc. 25 (1) (2005) 13–18. [3] A. Jankowiak, P. Blanchart, Electrical discharges under high voltage on surface of silica–nickel ceramic composites, Adv. Engrg. Mater. 12 (6) (2004). [4] E.O. Forster, Progress in the understanding of electrical breakdown in condensed matter, J. Phys. D. 23 (1990) 1506–1514. [5] G.F. Batalha, M. Stipkovic Filho, Quantitative characterization of the surface topography of cold rolled sheets-new approaches and possibilities, J. Mater. Process. Technol. 113 (2001) 732–738. [6] M. Takeda, H. Ina, K. Kobayashi, Fourier-transform method of fringe pattern analysis for computer-based topography and interferometry, J. Opt. Soc. Am. 72 (1) (1982) 156–160. [7] DIN 4776 1990 measurement of surface roughness; Parameters Rk, Rpk, Rvk, Mr1, Mr2 for the Description of the Material Portion in Roughness German standard. [8] A.R. Von Hippel, Molecular Science and Molecular Engineering, The Technology Press of MIT/J. Wiley, New York, 1959, pp. 39–47. [9] N.St.J. Braithwaite, Introduction to gas discharge, Plasma Sources Sci. Tech. 9 (2000) 517–527. [10] S. Vacquié, L’arc électrique, CNRS Ed., Eyrolles, 2000. [11] J.D. Von Engel, Cobine, Gaseous Conductors, New York, 1958. [12] M. Lenzlinger, E.H. Snow, Fowler–Nordheim tunnelling into thermally grown SiO2 , J. Appl. Phys. 40 (1969) 278. [13] L.M. Porter, R.F. Davis, A critical review of ohmic and rectifying contacts for silicon carbide, Mater. Sci. Engrg. B 34 (2–3) (1995) 83–105. [14] K. Yuasa, A. Shimoi, I. Ohba, C. Oshima, Modified Fowler– Nordheim field emission formulae from a non-planar emitter model, Surf. Sci. 520 (1–2) (2002) 18–28. [15] A.A. Evtukh, N.I. Klyui, V.G. Litovchenko, Yu.M. Litvin, O.B. Korneta, V.M. Puzikov, A.V. Semenov, Peculiarities of field emission from silicon carbide films, Appl. Surf. Sci. 215 (1–4) (2003) 237–241. [16] D.C. Lim, H.S. Ahn, D.J. Choi, C.H. Wang, H. Tomokage, The field emission properties of silicon carbide whiskers grown by CVD, Surf. Coat. Technol. 168 (1) (2003) 37–42. [17] M. Jimenez, R.J. Noer, G. Jouve, J. Jodet, B. Bonin, Electron field emission from large-area cathodes: evidence for the projection model, J. Phys. D: Appl. Phys. 27 (1994) 1038–1045. [18] B.M. Cox, The nature of emission sites, J. Phys. D: Appl. Phys. 8 (1975) 2065–2073. [19] A.N. Obraztsov, Al.A. Zakhidov, A.P. Volkov, D.A. Lyashenko, Nonclassical electron field emission from carbon materials, Diamond Related Mater. 12 (2003) 446–449. [20] A.A. Evtukh, N.I. Klyui, V.G. Litovchenko, Y.M. Litvin, O.B. Korneta, V.M. Puzikov, A.V. Semenov, Peculiarities of field emission from silicon carbide films, Appl. Surf. Sci. 215 (1–4) (2003) 237–241. [21] J. Paulini, T. Klein, G. Simon, Thermo-field emission and the Nottingham effect, J. Phys. D: Appl. Phys. 26 (1993) 1310.
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