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DC discharge with high secondary electron emission oxide cathode: Effects of gas pressure and oxide cathode structure
Vacuum 166 (2019) 114–122
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DC discharge with high secondary electron emission oxide cathode: Effects of gas pressure and oxide cathode structure
T
Xiaomei Yaoa,b, Nan Jianga,c,∗, Bangfa Pengc, Yun Xiaa,b, Na Lua,c, Kefeng Shanga,c, Jie Lia,c, Yan Wua,c a
Key Laboratory of Industrial Ecology and Environmental Engineering, MOE, Dalian University of Technology, Dalian, 116024, China School of Environmental Science & Technology, Dalian University of Technology, Dalian, 116024, China c School of Electrical Engineering, Dalian University of Technology, Dalian, 116024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: DC discharge Oxide cathode Secondary electron emission Gas pressure Oxide layer structure
In present study, DC discharge with high secondary electron emission (SEE) oxide cathodes (NiO/Ni and MgO/ NiO/Ni cathode) is characterized with a significant increase in discharge current compared to the one with normal Ni cathode at different gas pressures from 400 to 700 torr. The increase multiple value of the discharge current decreases as the gas pressure increases, which can be related to a relative weakened ion-induced SEE process at high gas pressure. The NiO intermediate layer plays an important role in a sandwich MgO/NiO/Ni cathode for DC discharge, which may be due to the fact that its high resistance favors field-enhanced SEE process. Regardless of the gas pressure, the structural characteristics (thickness and roughness) of oxide layer have an effect on the enhancement of the discharge current, which can be attributed to field-enhanced SEE process. Additionally, the increase in the number of needle electrodes is conducive to the increase of discharge current with oxide cathode, which is related to an increase in the discharge area on the cathode surface according to the discharge images.
1. Introduction Nonthermal plasma (NTP), characterized as electrons (the main energy carriers) and heavier particles (ions, gas atoms and molecules), has recently become a subject of great interest for a wide variety of technologies including chemical treatment of gases such as VOCs, combustion gases; water treatment; surface modification of material, etc. [1–7]. Among all of the applications, energetic electrons play a major role, which is due to the fact that electrons with sufficient energy collide with the neutral atoms and molecules in the feed gas, generating a series of reactive species (radicals, ions, excited species, etc.), which is the basis for NTP applications. Therefore, improving the density and energy of electrons in the NTP is a top priority in the study of plasma sources techniques and the relative applications [8,9]. Various plasma generation methods and plasma sources are determined largely by the particular applications for which the plasma is intended [10]. The most widely used method for plasma generation is electrical breakdown of neutral gas in the present of an applied electric field, where electrons are accelerated by the electric field to collide with neutral gas molecules, producing an electron avalanche process in the discharge space. Therefore, most researches on electric field plasma ∗
source technology mainly focus on density and distribution of electron, as well as the related electron impact ionization source, which are determined by excitation power, discharge mode (corona, glow, streamer, spark, etc.) and geometric structure of discharge reactor [8,11–14]. In several studies, it is an effective way to produce more electrons for plasma source by using cathode material with high γ, which is often called the secondary electron emission (SEE) coefficient [15–17]. According to Townsend's theory, which is the theoretical basis of gas discharge plasma by electric field as mention in almost all textbooks, two important Townsend ionization coefficients α and γ are proposed to describe the production of electrons in the gas discharge process. Coefficient α indicates the probability that electrons emitted from the cathode will collide with the neutral gas molecule and form an ion and additional electron per unit length, depending mainly on the gas pressure and inversely proportional to the mean free path of the electrons. γ is equal to the average number of emitted electrons per an ion upon impact on the cathode from the discharge plasma. γ is very important for ionization and formation of a steady-state current according to Eq. (1). In addition, the role of secondary electron avalanches in the electrical breakdown of gases discharges has been discussed in detail in numerous studies [18–20].
Corresponding author. Key laboratory of Industrial Ecology and Environmental Engineering, MOE, Dalian University of Technology, Dalian, 116024, China. E-mail address: [email protected] (N. Jiang).
https://doi.org/10.1016/j.vacuum.2019.04.035 Received 13 March 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 16 April 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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γ [exp (αd ) − 1] = 1
(1)
plate is 20 mm. The galvanometer is connected between the cathode (plate) and the ground to test the entire circuit discharge current as shown in the previous study [30]. The gas pressure in the chamber can be set in the range of 400–700torr and pure nitrogen is set as the background gas. A cannon 700D digital camera is used to capture discharge images. The preparation and characterization methods of oxide cathodes have been described in our previous study [30]. MgO particles are deposited on the Ni base by electrophoretic deposition, the thickness of MgO layer is determined by the deposition time when the voltage is fixed at 50 V, and the specific thickness values are shown in Fig. S2. Then, it is dried in an oven at 100 °C for 30 min, and calcined at 1125 °C for 2 h in a muffle furnace. During the calcination process, a green intermediate layer is formed between the white MgO layer and the Ni base, and it has been confirmed to be NiO by results of both Energy Dispersive X-Ray Spectroscopy (EDX, Bruker Quantax 400) and X-ray diffraction (XRD, a Rigaku D/Max-2400 diffractometer equipped with a Cu Kα radiation), which are shown in Fig. 2. When the calcination process is carried out in inert gas argon, there is no formation of intermediate NiO layer. Thus, the prepared cathode with a single MgO layer is MgO/Ni cathode. The resistance of the oxide layer is measured by a surface volume resistivity meter (ZST-121). The thickness and surface roughness are determined by 120 mm phase grating interference roughness profile (PGI 840, Taylor Hobson Ltd, England) and ZYGO Surface Profiler (NV5000 5022S), respectively.
Generally speaking, γ of ceramic materials is larger than that of metal, because the secondary electrons lose less energy when they pass through the solid and finally emit [21–24]. Oxide cathodes, i.e. a metal base coated with a single oxide layer or doped oxide material, have been considered effective SEE sources and widely used in vacuum devices. Among them, MgO is a recognized material with a high γ, which has been used in plasma displays to reduce the initial voltage of discharge and energy consumption, as well as serving as a strong protective material against positive ions bombardment on metal electrodes [25]. It has also been reported that the addition of some other oxides, such as NiO, ZnO, TiO2, can improve the γ property of MgO, resulting in a good plasma discharge efficiency [26–29]. In our previous study, we proposed a MgO/NiO/Ni sandwich cathode to ignite atmospheric pressure DC discharge, in which U-I characteristics, spectroscopic investigations, visual observation of the discharge, analysis of ozone active substances and the possible application for the toluene treatment were explored [30]. However, the role of the intermediate layer NiO in the MgO/NiO/Ni sandwich cathode for DC discharge generation needs to be further discussed. Correspondingly, the DC discharge ignited by the NiO/Ni cathode (with a single NiO layer) also need to be characterized for additional explanation. In order to further enhance the discharge, the effect of its structural properties (such as oxide layer thickness and roughness) related to the SEE process, is also required to be discussed. Besides, as gas pressure is an important factor affecting the gas discharge, the effect of gas pressure on the SEE process of the oxide cathode also required further analysis. The innovation of the present work is to analyze the difference of DC discharge caused by different gas conditions and structural properties of oxide cathode, basing on the SEE theory. Characteristics of the DC discharge with NiO/Ni cathode (with a single NiO layer) are detected in a vacuum chamber with multi-needle plate structure at different gas pressures (400–700 torr). Besides, the role of the intermediate layer NiO in the MgO/NiO/Ni sandwich cathode for DC discharge is analyzed. Additionally, the effect of oxide cathode structural characteristics including the thickness and roughness of oxide layer, as well as the number of needle electrodes (discharge anode) on DC discharge are discussed.
3. Results and discussion 3.1. Characterization of oxide layer Fig. 2 shows EDX of two different oxide layers and the Ni base, as well as XRD of the green intermediate layer. The Ni metal base contains trace impurities such as Ti, Fe, Ca, Al, Mg, etc. in addition to the main Ni element (Fig. 2(a)). Since the green intermediate layer is formed by self-growth during calcination, it also contains trace amounts of impurity components such as Ca and Mg, except for the main Ni and O components (Fig. 2(b)). The surface oxide layer is prepared by commercially available MgO particles, only containing Mg and O elements, as shown in Fig. 2(c). Besides, the green intermediate layer are recognized as NiO according to the corresponding peaks at 2θ = 37.09°, 43.23°, 62.88°, 75.29° and 79.28° from the XRD results. In addition, peak positions of 44.52° and 51.84° corresponding to metallic Ni are also detected on NiO intermediate layer, which may be attributable to a partially exposure of Ni metal base. Anyway, the elemental analysis by EDX in Fig. 2 (b) is consistent with the XRD results.
2. Experimental methods The discharge system includes a DC power supply (TRC2020P50, TSLAMAN), a galvanometer, a vacuum system and a six-needle-plate device in the vacuum chamber. The schematic diagram of the sixneedle-plate structure in the vacuum chamber is shown in Fig. 1, and the six stainless steel needle electrodes are evenly distributed on a circle with a radius of 15 mm. The discharge gap between the needle and
3.2. Current-voltage characteristics of DC discharges with oxide cathodes Fig. 3 shows current-voltage characteristics of DC discharge with different cathodes (Ni cathode, MgO/Ni cathode, MgO/NiO/Ni cathode and NiO cathode) in different gas pressure from 400 to 700 torr. It is obvious that the U-I curves of MgO/Ni cathode and Ni cathode are separated at different initial voltages (corresponding to different gas pressures), and then as the voltage increases, the discharge current of MgO/Ni cathode is larger than that of Ni cathode. However, the increase of discharge current under near atmospheric pressure (700 torr) is not so obvious, compared to that at a relative low gas pressure (400 torr). In the discharge with MgO/Ni cathode, the maximum increase multiple of discharge current at the same voltage compared to the one with Ni cathode is 2.98, 2.57, 1.79 and 1.24 under different gas pressures of 400, 500, 600 and 700 torr, respectively, as shown in Fig. S1 (b). As MgO is a recognized material with a high γ, the increase of discharge current in the discharge with MgO/Ni cathode is mainly related to an enhancement of SEE process according to Townsend's theory of gas discharge, when the applied voltage, discharge gap and gas
Fig. 1. A schematic diagram of multi-needle structure in the vacuum chamber (a); the distribution of 6-needle electrodes (b). 115
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Fig. 2. EDX of Ni base (a), NiO intermediate layer (b), and MgO layer (c); XRD of the green intermediate layer (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Current-voltage characteristics of DC discharges with different cathodes (Ni cathode, MgO/Ni cathode, MgO NiO/Ni cathode and NiO/Ni cathode) in different gas pressures from 400 to 700 torr. 116
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on its surface, leading to a stronger electric field within NiO layer that induces more secondary electrons as mentioned above. On the other hand, when excluding the influence of field-enhanced SEE, SEE yield value of different oxides depends on its physical characteristics, such as work function, surface morphologies and crystal orientations and so on. These properties depend on the method and condition of the oxide layer preparation process in different studies [41,42], and it needs a further study in future. For the latter, it may attribute to the fact that electrons which are accelerated to release from the single NiO layer lack a further acceleration process in the surface MgO layer, so that γ of single NiO layer is smaller than the double layer of MgO/NiO, according to the above explanation. In addition, since both NiO an MgO layer are insulating material, there is a resistive characteristic to the whole discharge current at the initial stage of discharge, that is, the discharge current is slightly smaller than the one with Ni metal cathode under a low discharge voltage, as shown in the small block diagram of Fig. 3. The current reduction is more pronounced especially at near atmospheric conditions (600–700 torr). This is because the presence of the insulating oxide layer, on the one hand, has a positive effect on the SEE process, and on the other hand suppresses the discharge current due to the increase of the total resistance between the discharge gaps (gas resistance plus dielectric layer resistance). Obviously, under high gas pressure and low discharge voltage, (i.e. the reduced electric field is relative small), the negative effect for the discharge current is more significant. Additionally, it is apparent that the resistance characteristic of the discharge current of the individual NiO layer is higher than that of the double layer MgO/NiO, although the latter has a higher total resistance value. The possible reason may be that the surface MgO layer can further accelerate the electrons generated by the NiO layer, resulting in a relatively good SEE process that counteracts the negative effect of partial resistance characteristic.
pressure are constant. The SEE mechanism of oxides is really complex, it is generally believed that SEE is caused by the bombardment of positive ions with kinetic energy no matter whether the cathode is a metal or an oxide [31–33]. When the applied field E is constant, the larger the gas pressure P is, the smaller reduced electric field value is, and the energy obtained for the positive ions from the electric field in an average free path is reduced. Therefore, the SEE process due to positive ions bombardment is weakened, resulting in a relative low γ [34]. This is why the increased discharge current multiple of the MgO/NiO cathode is reduced at relatively high gas pressures (such as 700 torr). Besides, when there is an insulating layer on the mental cathode, field-enhanced SEE process may also occur within the insulating layer, as a result of the loss of electrons by ion bombardment during DC discharge, as described in our previous study and other studies [30,35–37]. When an oxide cathode is subjected to ion bombardment in positive DC discharge, ion-induced SEE occurs from the oxide layer surface as mentioned above. This leaves a net positive charge on the oxide layer surface causing it to become polarized. Since MgO is an insulator (with a measured resistivity of 1012–1013 Ω•cm), the positive charge does not neutralize as fast as it is built up. Thus, a strong gradient of electric field occurs across the oxide layer according to a very thin oxide layer (about micrometers as shown in Fig. S2 in the present study). When the electric field reaches 104–105 V/cm or more, fieldenhanced secondary electrons can generate, which is consistent with most of the studies on the SEE characteristics of MgO cathode [38]. Field-enhanced SEE can explain the phenomenon that the SEE yield of the oxide cathode is still improved under relatively high gas pressure conditions (in the case where the effect of ion bombardment is not very significant). As shown in Fig. 3, a sandwich MgO/NiO/Ni cathode (i.e. an intermediate NiO layer exists between MgO layer and Ni base) can cause a better discharge current enhancement than the one with MgO/Ni cathode at different gas pressures, as shown in Fig. 3. The maximum multiple of the discharge current is 10.1, 5.3, 3.9 and 3.1 times at different gas pressure of 400, 500, 600 and 700 torr, respectively, as shown in Fig. S1 (a). One of the possible reasons may be that the NiO layer can enhance the insulation performance of the total oxide layer for a better field-enhanced SEE process. Because the measured resistivity of the intermediate NiO layer is about 1014–1015 Ω•cm, which is much larger than MgO layer about 1012–1013 Ω•cm. Thus, NiO layer can help to effectively accumulate positive charges on its surface, finally promoting the field-enhanced SEE process. Similar study also supports that in the sandwich MgO/NiO/Ni cathode structure, holes form due to the continuous loss of electrons in the MgO layer. Thus, a strong electric field is also formed in the NiO layer, and electrons in the NiO layer are promoted as a source of electrons that migrate into the MgO layer and then are accelerated to be released from MgO layer [39]. Besides, it is reported that small amount of NiO additives in MgO thin films can enhance the SEE compared with the pure MgO thin films due to the change of surface properties of MgO film, such as the surface morphology, crystal orientations and density, and it needs a further research [40]. In order to investigate the possibility of SEE caused by the presence of a separate NiO layer, DC discharge characteristics of the NiO/Ni cathode is also explored, as shown in Fig. 3. The NiO/Ni cathode is prepared by ultrasonic removal of the surface MgO layer from the sandwich MgO/NiO/Ni cathode. The results show that discharge with NiO/Ni cathode also shows an obvious increase in the discharge current at different gas pressures compared to the one with Ni cathode. We believe that it is related to the enhancement by field-enhanced SEE process for its high resistivity as mentioned above. Besides, the discharge current increase multiple with NiO/Ni cathode is higher than the one with MgO/Ni cathode, while lower than the one with MgO/ NiO/Ni cathode, as shown in Fig. S1. One of the possible reasons for the former is that the resistivity of the intermediate NiO layer is larger than single MgO layer, which can accumulate much more positive charges
3.3. Effect of oxide layer thickness on DC discharge The thickness of the oxide layer optimized to obtain a large γ is expected that a large amount of continuous electrons can flow through the oxide layer and then escape to the vacuum, finally resulting in an increase in discharge current in DC discharge. Fig. S2 shows thickness of different oxide layer with different electrophoretic deposition time. As the time for electrophoretic deposition increases, the thickness of both the single oxide (MgO and NiO) layer and the double MgO/NiO layer increases, all on the order of micrometers in 2.5–8 min. The multiple of the discharge current in discharge with MgO/NiO/Ni cathode, compared to the one with Ni cathode at different gas pressure is shown in Fig. 4, and Fig. 5 is related to the NiO/Ni cathode. The related current-voltage characteristics of DC discharges with MgO/ NiO/Ni cathode and NiO/Ni cathode in different oxide layer thickness are shown in Fig. S3 and Fig. S4, respectively. The results show that the thickness of the oxide layer (both MgO/ NiO layer and single NiO layer) has an effect on DC discharge under different gas pressure conditions. It indicates that, except for the influence of gas pressure conditions mainly on ion-induced SEE process, the effect of the structural characteristics of oxide layer thickness on DC discharge may be related to the field-enhanced SEE process under the same gas pressure. The corresponding MgO/NiO oxide layer thickness of 89.2 μm at the deposition time of 5 min are optimized under different gas pressure from 400 to 700 torr, as shown in Fig. 4, which is consistent with our previous study under atmospheric pressure in air [30]. If the oxide layer is too thick, the generated secondary electrons have difficulty to escape to the vacuum through the total oxide layer. However, when the oxide layer is too thin, a small potential drop will be formed through the oxide layer, resulting in a few secondary electrons [43,44]. However, the optimum thickness of single NiO layer has not been obtained within the current thickness range of 4.6–15 μm in this study. As shown in Fig. 5, the multiple of the discharge current is 117
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Fig. 4. The multiple of the discharge current in discharge with MgO/NiO/Ni cathode compared to the one with Ni cathode, at different gas pressures: the effect of MgO/NiO layer thickness.
preferential NiO thickness, it is necessary to use other experimental methods to prepare NiO/Ni cathode for the optimize research.
positively correlated with this thickness range of the NiO layer. Since NiO is self-generated during calcination, the thickness of NiO layer is limited to a dozen microns in this study. It is because if the electrophoretic deposition time is too long, it will cause cracking and shedding of MgO layer during calcination. Therefore, in order to obtain a
Fig. 5. The multiple of the discharge current in discharge with NiO/Ni cathode compared to the one with Ni cathode, at different gas pressures: the effect of NiO layer thickness. 118
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Fig. 6. Surface morphology and relative average roughness (Ra) of different cathodes: (a) a smooth Ni base (Ra1 = 0.06 μm); (b) a rough Ni base (Ra2 = 1.60 μm); (c) NiO/Ni cathode prepared by (a) (Ra3 = 0.40 μm); (d) NiO/Ni cathode prepared by (b) (Ra4 = 2.90 μm).
Fig. 7. DC discharge characteristics ignited by NiO/Ni cathode and Ni cathode with different roughness, in different gas pressures from 400 to 700 torr: Ni (S), (Ra1 = 0.06 μm); Ni (R), (Ra2 = 1.60 μm); NiO/Ni (S), (Ra3 = 0.40 μm); NiO/Ni (R), (Ra4 = 2.90 μm).
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pressure from 400 to 700 torr. The results show that when the number of needles changes from 1 to 3 and then to 6, the discharge current increases both in the discharge with Ni cathode and NiO/Ni cathode. This is because as the number of needles increases, the total area of the ionization region around the needle electrode tips increases, resulting in a large discharge current. In addition, it is obvious that the discharge current enhancement in the discharge with NiO/Ni cathode is more remarkable caused by the increase of the number of needle electrodes than the one with Ni cathode, especially when it is increased from 3 to 6. This may be related to the discharge phenomenon on the oxide cathode surface found in our previous study [30]. In our previous study, we have found that the discharge mode caused by the MgO/NiO/Ni cathode is different from the normal discharge with a Ni metal cathode. That is, in addition to the discharge around the needle tips, some blueviolet glow points also occur on the MgO/NiO/Ni cathode surface. Similar discharge phenomenon is observed in the discharge with NiO/Ni cathode, as shown in Fig. 9. This luminescence is a striking feature of SEE process due to an electron-hole recombination process within the oxide layer in vacuum [47,48]. Besides, it may also attribute to the fact that the emitted secondary electrons induce the gas excitation and ionization around the oxide layer surface due to a very short mean free path of the electrons at a relative high gas pressure (700 torr, near atmospheric pressure). In addition, we also find a very interesting phenomenon about the oxide layer surface luminescence in the discharge with NiO/Ni cathode, caused by the change of the number of needle electrode, as Fig. 9 shows. The discharge area on the NiO/Ni cathode surface is concentrated vertically below the needle tips and gradually weakens toward the periphery. For example, the area on the NiO layer surface corresponding to the vertical of the needle-free electrode, i.e. in the center of the 6-needle circle, is dark, as shown in Fig. 9 (c). Besides, when three needles are removed, only the semicircular luminous region is left, as shown in Fig. 9 (d). The reason may be that the electric field of the vertical space corresponding to the needle tip is relative strong, resulting in large energy for ion bombardment, as well as large amount of positive charges accumulated on the oxide layer surface, finally
3.4. Effect of oxide layer roughness on DC discharge Fig. 6 shows the surface morphology and relative average roughness (Ra) of different cathodes (Ni cathode and NiO/Ni cathode). A relative smooth Ni base (Fig. 6 (a), Ra1 = 0.06 μm) and a rough one with vertical gullies (Fig. 6 (b), Ra2 = 1.60 μm), are used to prepare NiO/Ni cathodes (Fig. 6 (c), Ra3 = 0.40 μm) and (Fig. 6 (d), Ra4 = 2.90 μm), respectively. Since the intermediate NiO layer is formed by self-growth during calcination, the surface morphology may greatly affected by the Ni base from Fig. 6 (a) and 6 (b), exhibiting an irregular pattern (Fig. 6 (c)) and undulating gullies (Fig. 7 (d)), respectively. Correspondingly, the surface roughness becomes larger after calcination process. The related DC discharge current-voltage characteristics with NiO/ Ni cathode and Ni cathode is shown in Fig. 7. The results show that for discharge with Ni metal cathode, a smooth surface is helpful to improve the discharge current. This is because when the surface is rough, some of the secondary electrons are absorbed by the original substance [45]. In general, roughening the surface or spraying a rough layer of metal powder is one of the effective ways to reduce the SEE coefficient [45]. However, for discharge with NiO/Ni cathode, the rough surface facilitates an increase in discharge current regardless of the gas pressure conditions. The same rule is also shown in the discharge with the sandwich MgO/NiO/Ni cathode, as shown in Fig. S5. This indicates that the effect of the structural surface roughness of the oxide layer on DC discharge is mainly related to the occurrence of field-enhanced SEE process, regardless of the gas pressure condition [46]. It is believed that undulating gullies of NiO surface with a large drop, as seen from the height profiles across the white lines drawn in Fig. 6 (d), contribute to the uneven distribution of the surface potential caused by the accumulated charges on the oxide layer, which is an important factor contributing to the emission process of secondary electrons. 3.5. Effect of the number of needle electrode on DC discharge Fig. 8 shows the effect of the number of needle electrode on DC discharge with NiO/Ni cathode and Ni cathode, in different gas
Fig. 8. The effect of the number of needle electrode on DC discharge with NiO/Ni cathode, in different gas pressures from 400 to 700 torr: single-needle electrode (1N), three-needle electrode (3N) and six-needle electrode (6N). 120
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Fig. 9. Images of DC discharge ignited by NiO/Ni cathode with different needle electrodes: (a) side view of discharge with 6-needle (U = 20 kV, I = 170 μA); (b) side view of discharge with 3-needle (U = 20 kV, I = 67 μA); (c) NiO/Ni cathode surface discharge image associated with (a); (d) NiO/Ni cathode surface discharge image associated with (b). Exposure time of 10 s, gas pressure of 700 torr.
whole discharge space, as well as further increase of discharge current, can be optimized by changing the geometry of the discharge electrode and the corresponding oxide cathode in future study, such as using multi-needle/wire plate type, wire-cylinder type, etc., which can further expand its chemical applications, such as exhaust gas removal and so on.
leading to a better SEE process and a more obvious luminescence. Correspondingly, luminescence phenomenon on the oxide layer surface, where is far away from the needle tips, is weakened due to the weakening of the spatial electric field. It indicates that the number of the needle electrode, i.e. discharge region of the discharge electrodes, has a significant influence overall spatial discharge. This should be an important reason for the significant increase in current with NiO/Ni cathode as the number of needle electrode increases. In addition, the size of the oxide cathode is also designed to coincide with the area of the discharge electrode. The design of both discharge electrode and oxide cathode has an important influence on the distribution of electrons and reactive species in the discharge chamber. Therefore, plasma volume and discharge current in the cell space can be optimized by changing the structure of the discharge electrode and oxide cathode (such as using multi-wire/needle plate type, wire-cylinder type, etc.) to further expand its chemical applications, like exhaust gas removal and so on.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51477025, 51877028). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.04.035. References
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In summary, the effect of the gas pressure, as well as the structural properties of oxide cathode on DC discharge with oxide cathodes have been deeply analyzed, basing on the SEE theory. Under different gas pressure conditions, DC discharge with oxide cathodes (NiO/Ni and MgO/NiO/Ni cathode) exhibits a significant increase in discharge current compared to the one with normal Ni cathode. As the gas pressure increases from 400 to 700 torr, the enhancement of discharge current with oxide cathode is weakened, which is due to the fact that the reduced electric field is relative small under a high gas pressure condition, resulting in a weakened ion-induced SEE process. Besides, we believe that the effect of the structural characteristics of oxide layer on DC discharge, such as the thickness and roughness, may be related to the field-enhanced SEE process regardless of gas pressure condition. A rough NiO layer surface with undulating gullies contributes to the uneven distribution of the surface potential caused by the accumulated charges, resulting in a good field-enhanced SEE process. The NiO intermediate layer plays an important role in the sandwich MgO/NiO/Ni cathode for DC discharge, which is due to the fact that its high resistance favors the field-enhanced SEE processes. In addition, the increase in the number of needle electrodes is conducive to enhancement of discharge current with oxide cathode, which is related to the increase discharge area on the oxide layer surface. Thus, plasma volume in the 121
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DC discharge with high secondary electron emission oxide cathode: Effects of gas pressure and oxide cathode structure
T
Xiaomei Yaoa,b, Nan Jianga,c,∗, Bangfa Pengc, Yun Xiaa,b, Na Lua,c, Kefeng Shanga,c, Jie Lia,c, Yan Wua,c a
Key Laboratory of Industrial Ecology and Environmental Engineering, MOE, Dalian University of Technology, Dalian, 116024, China School of Environmental Science & Technology, Dalian University of Technology, Dalian, 116024, China c School of Electrical Engineering, Dalian University of Technology, Dalian, 116024, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: DC discharge Oxide cathode Secondary electron emission Gas pressure Oxide layer structure
In present study, DC discharge with high secondary electron emission (SEE) oxide cathodes (NiO/Ni and MgO/ NiO/Ni cathode) is characterized with a significant increase in discharge current compared to the one with normal Ni cathode at different gas pressures from 400 to 700 torr. The increase multiple value of the discharge current decreases as the gas pressure increases, which can be related to a relative weakened ion-induced SEE process at high gas pressure. The NiO intermediate layer plays an important role in a sandwich MgO/NiO/Ni cathode for DC discharge, which may be due to the fact that its high resistance favors field-enhanced SEE process. Regardless of the gas pressure, the structural characteristics (thickness and roughness) of oxide layer have an effect on the enhancement of the discharge current, which can be attributed to field-enhanced SEE process. Additionally, the increase in the number of needle electrodes is conducive to the increase of discharge current with oxide cathode, which is related to an increase in the discharge area on the cathode surface according to the discharge images.
1. Introduction Nonthermal plasma (NTP), characterized as electrons (the main energy carriers) and heavier particles (ions, gas atoms and molecules), has recently become a subject of great interest for a wide variety of technologies including chemical treatment of gases such as VOCs, combustion gases; water treatment; surface modification of material, etc. [1–7]. Among all of the applications, energetic electrons play a major role, which is due to the fact that electrons with sufficient energy collide with the neutral atoms and molecules in the feed gas, generating a series of reactive species (radicals, ions, excited species, etc.), which is the basis for NTP applications. Therefore, improving the density and energy of electrons in the NTP is a top priority in the study of plasma sources techniques and the relative applications [8,9]. Various plasma generation methods and plasma sources are determined largely by the particular applications for which the plasma is intended [10]. The most widely used method for plasma generation is electrical breakdown of neutral gas in the present of an applied electric field, where electrons are accelerated by the electric field to collide with neutral gas molecules, producing an electron avalanche process in the discharge space. Therefore, most researches on electric field plasma ∗
source technology mainly focus on density and distribution of electron, as well as the related electron impact ionization source, which are determined by excitation power, discharge mode (corona, glow, streamer, spark, etc.) and geometric structure of discharge reactor [8,11–14]. In several studies, it is an effective way to produce more electrons for plasma source by using cathode material with high γ, which is often called the secondary electron emission (SEE) coefficient [15–17]. According to Townsend's theory, which is the theoretical basis of gas discharge plasma by electric field as mention in almost all textbooks, two important Townsend ionization coefficients α and γ are proposed to describe the production of electrons in the gas discharge process. Coefficient α indicates the probability that electrons emitted from the cathode will collide with the neutral gas molecule and form an ion and additional electron per unit length, depending mainly on the gas pressure and inversely proportional to the mean free path of the electrons. γ is equal to the average number of emitted electrons per an ion upon impact on the cathode from the discharge plasma. γ is very important for ionization and formation of a steady-state current according to Eq. (1). In addition, the role of secondary electron avalanches in the electrical breakdown of gases discharges has been discussed in detail in numerous studies [18–20].
Corresponding author. Key laboratory of Industrial Ecology and Environmental Engineering, MOE, Dalian University of Technology, Dalian, 116024, China. E-mail address: [email protected] (N. Jiang).
https://doi.org/10.1016/j.vacuum.2019.04.035 Received 13 March 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 16 April 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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γ [exp (αd ) − 1] = 1
(1)
plate is 20 mm. The galvanometer is connected between the cathode (plate) and the ground to test the entire circuit discharge current as shown in the previous study [30]. The gas pressure in the chamber can be set in the range of 400–700torr and pure nitrogen is set as the background gas. A cannon 700D digital camera is used to capture discharge images. The preparation and characterization methods of oxide cathodes have been described in our previous study [30]. MgO particles are deposited on the Ni base by electrophoretic deposition, the thickness of MgO layer is determined by the deposition time when the voltage is fixed at 50 V, and the specific thickness values are shown in Fig. S2. Then, it is dried in an oven at 100 °C for 30 min, and calcined at 1125 °C for 2 h in a muffle furnace. During the calcination process, a green intermediate layer is formed between the white MgO layer and the Ni base, and it has been confirmed to be NiO by results of both Energy Dispersive X-Ray Spectroscopy (EDX, Bruker Quantax 400) and X-ray diffraction (XRD, a Rigaku D/Max-2400 diffractometer equipped with a Cu Kα radiation), which are shown in Fig. 2. When the calcination process is carried out in inert gas argon, there is no formation of intermediate NiO layer. Thus, the prepared cathode with a single MgO layer is MgO/Ni cathode. The resistance of the oxide layer is measured by a surface volume resistivity meter (ZST-121). The thickness and surface roughness are determined by 120 mm phase grating interference roughness profile (PGI 840, Taylor Hobson Ltd, England) and ZYGO Surface Profiler (NV5000 5022S), respectively.
Generally speaking, γ of ceramic materials is larger than that of metal, because the secondary electrons lose less energy when they pass through the solid and finally emit [21–24]. Oxide cathodes, i.e. a metal base coated with a single oxide layer or doped oxide material, have been considered effective SEE sources and widely used in vacuum devices. Among them, MgO is a recognized material with a high γ, which has been used in plasma displays to reduce the initial voltage of discharge and energy consumption, as well as serving as a strong protective material against positive ions bombardment on metal electrodes [25]. It has also been reported that the addition of some other oxides, such as NiO, ZnO, TiO2, can improve the γ property of MgO, resulting in a good plasma discharge efficiency [26–29]. In our previous study, we proposed a MgO/NiO/Ni sandwich cathode to ignite atmospheric pressure DC discharge, in which U-I characteristics, spectroscopic investigations, visual observation of the discharge, analysis of ozone active substances and the possible application for the toluene treatment were explored [30]. However, the role of the intermediate layer NiO in the MgO/NiO/Ni sandwich cathode for DC discharge generation needs to be further discussed. Correspondingly, the DC discharge ignited by the NiO/Ni cathode (with a single NiO layer) also need to be characterized for additional explanation. In order to further enhance the discharge, the effect of its structural properties (such as oxide layer thickness and roughness) related to the SEE process, is also required to be discussed. Besides, as gas pressure is an important factor affecting the gas discharge, the effect of gas pressure on the SEE process of the oxide cathode also required further analysis. The innovation of the present work is to analyze the difference of DC discharge caused by different gas conditions and structural properties of oxide cathode, basing on the SEE theory. Characteristics of the DC discharge with NiO/Ni cathode (with a single NiO layer) are detected in a vacuum chamber with multi-needle plate structure at different gas pressures (400–700 torr). Besides, the role of the intermediate layer NiO in the MgO/NiO/Ni sandwich cathode for DC discharge is analyzed. Additionally, the effect of oxide cathode structural characteristics including the thickness and roughness of oxide layer, as well as the number of needle electrodes (discharge anode) on DC discharge are discussed.
3. Results and discussion 3.1. Characterization of oxide layer Fig. 2 shows EDX of two different oxide layers and the Ni base, as well as XRD of the green intermediate layer. The Ni metal base contains trace impurities such as Ti, Fe, Ca, Al, Mg, etc. in addition to the main Ni element (Fig. 2(a)). Since the green intermediate layer is formed by self-growth during calcination, it also contains trace amounts of impurity components such as Ca and Mg, except for the main Ni and O components (Fig. 2(b)). The surface oxide layer is prepared by commercially available MgO particles, only containing Mg and O elements, as shown in Fig. 2(c). Besides, the green intermediate layer are recognized as NiO according to the corresponding peaks at 2θ = 37.09°, 43.23°, 62.88°, 75.29° and 79.28° from the XRD results. In addition, peak positions of 44.52° and 51.84° corresponding to metallic Ni are also detected on NiO intermediate layer, which may be attributable to a partially exposure of Ni metal base. Anyway, the elemental analysis by EDX in Fig. 2 (b) is consistent with the XRD results.
2. Experimental methods The discharge system includes a DC power supply (TRC2020P50, TSLAMAN), a galvanometer, a vacuum system and a six-needle-plate device in the vacuum chamber. The schematic diagram of the sixneedle-plate structure in the vacuum chamber is shown in Fig. 1, and the six stainless steel needle electrodes are evenly distributed on a circle with a radius of 15 mm. The discharge gap between the needle and
3.2. Current-voltage characteristics of DC discharges with oxide cathodes Fig. 3 shows current-voltage characteristics of DC discharge with different cathodes (Ni cathode, MgO/Ni cathode, MgO/NiO/Ni cathode and NiO cathode) in different gas pressure from 400 to 700 torr. It is obvious that the U-I curves of MgO/Ni cathode and Ni cathode are separated at different initial voltages (corresponding to different gas pressures), and then as the voltage increases, the discharge current of MgO/Ni cathode is larger than that of Ni cathode. However, the increase of discharge current under near atmospheric pressure (700 torr) is not so obvious, compared to that at a relative low gas pressure (400 torr). In the discharge with MgO/Ni cathode, the maximum increase multiple of discharge current at the same voltage compared to the one with Ni cathode is 2.98, 2.57, 1.79 and 1.24 under different gas pressures of 400, 500, 600 and 700 torr, respectively, as shown in Fig. S1 (b). As MgO is a recognized material with a high γ, the increase of discharge current in the discharge with MgO/Ni cathode is mainly related to an enhancement of SEE process according to Townsend's theory of gas discharge, when the applied voltage, discharge gap and gas
Fig. 1. A schematic diagram of multi-needle structure in the vacuum chamber (a); the distribution of 6-needle electrodes (b). 115
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Fig. 2. EDX of Ni base (a), NiO intermediate layer (b), and MgO layer (c); XRD of the green intermediate layer (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Current-voltage characteristics of DC discharges with different cathodes (Ni cathode, MgO/Ni cathode, MgO NiO/Ni cathode and NiO/Ni cathode) in different gas pressures from 400 to 700 torr. 116
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on its surface, leading to a stronger electric field within NiO layer that induces more secondary electrons as mentioned above. On the other hand, when excluding the influence of field-enhanced SEE, SEE yield value of different oxides depends on its physical characteristics, such as work function, surface morphologies and crystal orientations and so on. These properties depend on the method and condition of the oxide layer preparation process in different studies [41,42], and it needs a further study in future. For the latter, it may attribute to the fact that electrons which are accelerated to release from the single NiO layer lack a further acceleration process in the surface MgO layer, so that γ of single NiO layer is smaller than the double layer of MgO/NiO, according to the above explanation. In addition, since both NiO an MgO layer are insulating material, there is a resistive characteristic to the whole discharge current at the initial stage of discharge, that is, the discharge current is slightly smaller than the one with Ni metal cathode under a low discharge voltage, as shown in the small block diagram of Fig. 3. The current reduction is more pronounced especially at near atmospheric conditions (600–700 torr). This is because the presence of the insulating oxide layer, on the one hand, has a positive effect on the SEE process, and on the other hand suppresses the discharge current due to the increase of the total resistance between the discharge gaps (gas resistance plus dielectric layer resistance). Obviously, under high gas pressure and low discharge voltage, (i.e. the reduced electric field is relative small), the negative effect for the discharge current is more significant. Additionally, it is apparent that the resistance characteristic of the discharge current of the individual NiO layer is higher than that of the double layer MgO/NiO, although the latter has a higher total resistance value. The possible reason may be that the surface MgO layer can further accelerate the electrons generated by the NiO layer, resulting in a relatively good SEE process that counteracts the negative effect of partial resistance characteristic.
pressure are constant. The SEE mechanism of oxides is really complex, it is generally believed that SEE is caused by the bombardment of positive ions with kinetic energy no matter whether the cathode is a metal or an oxide [31–33]. When the applied field E is constant, the larger the gas pressure P is, the smaller reduced electric field value is, and the energy obtained for the positive ions from the electric field in an average free path is reduced. Therefore, the SEE process due to positive ions bombardment is weakened, resulting in a relative low γ [34]. This is why the increased discharge current multiple of the MgO/NiO cathode is reduced at relatively high gas pressures (such as 700 torr). Besides, when there is an insulating layer on the mental cathode, field-enhanced SEE process may also occur within the insulating layer, as a result of the loss of electrons by ion bombardment during DC discharge, as described in our previous study and other studies [30,35–37]. When an oxide cathode is subjected to ion bombardment in positive DC discharge, ion-induced SEE occurs from the oxide layer surface as mentioned above. This leaves a net positive charge on the oxide layer surface causing it to become polarized. Since MgO is an insulator (with a measured resistivity of 1012–1013 Ω•cm), the positive charge does not neutralize as fast as it is built up. Thus, a strong gradient of electric field occurs across the oxide layer according to a very thin oxide layer (about micrometers as shown in Fig. S2 in the present study). When the electric field reaches 104–105 V/cm or more, fieldenhanced secondary electrons can generate, which is consistent with most of the studies on the SEE characteristics of MgO cathode [38]. Field-enhanced SEE can explain the phenomenon that the SEE yield of the oxide cathode is still improved under relatively high gas pressure conditions (in the case where the effect of ion bombardment is not very significant). As shown in Fig. 3, a sandwich MgO/NiO/Ni cathode (i.e. an intermediate NiO layer exists between MgO layer and Ni base) can cause a better discharge current enhancement than the one with MgO/Ni cathode at different gas pressures, as shown in Fig. 3. The maximum multiple of the discharge current is 10.1, 5.3, 3.9 and 3.1 times at different gas pressure of 400, 500, 600 and 700 torr, respectively, as shown in Fig. S1 (a). One of the possible reasons may be that the NiO layer can enhance the insulation performance of the total oxide layer for a better field-enhanced SEE process. Because the measured resistivity of the intermediate NiO layer is about 1014–1015 Ω•cm, which is much larger than MgO layer about 1012–1013 Ω•cm. Thus, NiO layer can help to effectively accumulate positive charges on its surface, finally promoting the field-enhanced SEE process. Similar study also supports that in the sandwich MgO/NiO/Ni cathode structure, holes form due to the continuous loss of electrons in the MgO layer. Thus, a strong electric field is also formed in the NiO layer, and electrons in the NiO layer are promoted as a source of electrons that migrate into the MgO layer and then are accelerated to be released from MgO layer [39]. Besides, it is reported that small amount of NiO additives in MgO thin films can enhance the SEE compared with the pure MgO thin films due to the change of surface properties of MgO film, such as the surface morphology, crystal orientations and density, and it needs a further research [40]. In order to investigate the possibility of SEE caused by the presence of a separate NiO layer, DC discharge characteristics of the NiO/Ni cathode is also explored, as shown in Fig. 3. The NiO/Ni cathode is prepared by ultrasonic removal of the surface MgO layer from the sandwich MgO/NiO/Ni cathode. The results show that discharge with NiO/Ni cathode also shows an obvious increase in the discharge current at different gas pressures compared to the one with Ni cathode. We believe that it is related to the enhancement by field-enhanced SEE process for its high resistivity as mentioned above. Besides, the discharge current increase multiple with NiO/Ni cathode is higher than the one with MgO/Ni cathode, while lower than the one with MgO/ NiO/Ni cathode, as shown in Fig. S1. One of the possible reasons for the former is that the resistivity of the intermediate NiO layer is larger than single MgO layer, which can accumulate much more positive charges
3.3. Effect of oxide layer thickness on DC discharge The thickness of the oxide layer optimized to obtain a large γ is expected that a large amount of continuous electrons can flow through the oxide layer and then escape to the vacuum, finally resulting in an increase in discharge current in DC discharge. Fig. S2 shows thickness of different oxide layer with different electrophoretic deposition time. As the time for electrophoretic deposition increases, the thickness of both the single oxide (MgO and NiO) layer and the double MgO/NiO layer increases, all on the order of micrometers in 2.5–8 min. The multiple of the discharge current in discharge with MgO/NiO/Ni cathode, compared to the one with Ni cathode at different gas pressure is shown in Fig. 4, and Fig. 5 is related to the NiO/Ni cathode. The related current-voltage characteristics of DC discharges with MgO/ NiO/Ni cathode and NiO/Ni cathode in different oxide layer thickness are shown in Fig. S3 and Fig. S4, respectively. The results show that the thickness of the oxide layer (both MgO/ NiO layer and single NiO layer) has an effect on DC discharge under different gas pressure conditions. It indicates that, except for the influence of gas pressure conditions mainly on ion-induced SEE process, the effect of the structural characteristics of oxide layer thickness on DC discharge may be related to the field-enhanced SEE process under the same gas pressure. The corresponding MgO/NiO oxide layer thickness of 89.2 μm at the deposition time of 5 min are optimized under different gas pressure from 400 to 700 torr, as shown in Fig. 4, which is consistent with our previous study under atmospheric pressure in air [30]. If the oxide layer is too thick, the generated secondary electrons have difficulty to escape to the vacuum through the total oxide layer. However, when the oxide layer is too thin, a small potential drop will be formed through the oxide layer, resulting in a few secondary electrons [43,44]. However, the optimum thickness of single NiO layer has not been obtained within the current thickness range of 4.6–15 μm in this study. As shown in Fig. 5, the multiple of the discharge current is 117
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Fig. 4. The multiple of the discharge current in discharge with MgO/NiO/Ni cathode compared to the one with Ni cathode, at different gas pressures: the effect of MgO/NiO layer thickness.
preferential NiO thickness, it is necessary to use other experimental methods to prepare NiO/Ni cathode for the optimize research.
positively correlated with this thickness range of the NiO layer. Since NiO is self-generated during calcination, the thickness of NiO layer is limited to a dozen microns in this study. It is because if the electrophoretic deposition time is too long, it will cause cracking and shedding of MgO layer during calcination. Therefore, in order to obtain a
Fig. 5. The multiple of the discharge current in discharge with NiO/Ni cathode compared to the one with Ni cathode, at different gas pressures: the effect of NiO layer thickness. 118
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Fig. 6. Surface morphology and relative average roughness (Ra) of different cathodes: (a) a smooth Ni base (Ra1 = 0.06 μm); (b) a rough Ni base (Ra2 = 1.60 μm); (c) NiO/Ni cathode prepared by (a) (Ra3 = 0.40 μm); (d) NiO/Ni cathode prepared by (b) (Ra4 = 2.90 μm).
Fig. 7. DC discharge characteristics ignited by NiO/Ni cathode and Ni cathode with different roughness, in different gas pressures from 400 to 700 torr: Ni (S), (Ra1 = 0.06 μm); Ni (R), (Ra2 = 1.60 μm); NiO/Ni (S), (Ra3 = 0.40 μm); NiO/Ni (R), (Ra4 = 2.90 μm).
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pressure from 400 to 700 torr. The results show that when the number of needles changes from 1 to 3 and then to 6, the discharge current increases both in the discharge with Ni cathode and NiO/Ni cathode. This is because as the number of needles increases, the total area of the ionization region around the needle electrode tips increases, resulting in a large discharge current. In addition, it is obvious that the discharge current enhancement in the discharge with NiO/Ni cathode is more remarkable caused by the increase of the number of needle electrodes than the one with Ni cathode, especially when it is increased from 3 to 6. This may be related to the discharge phenomenon on the oxide cathode surface found in our previous study [30]. In our previous study, we have found that the discharge mode caused by the MgO/NiO/Ni cathode is different from the normal discharge with a Ni metal cathode. That is, in addition to the discharge around the needle tips, some blueviolet glow points also occur on the MgO/NiO/Ni cathode surface. Similar discharge phenomenon is observed in the discharge with NiO/Ni cathode, as shown in Fig. 9. This luminescence is a striking feature of SEE process due to an electron-hole recombination process within the oxide layer in vacuum [47,48]. Besides, it may also attribute to the fact that the emitted secondary electrons induce the gas excitation and ionization around the oxide layer surface due to a very short mean free path of the electrons at a relative high gas pressure (700 torr, near atmospheric pressure). In addition, we also find a very interesting phenomenon about the oxide layer surface luminescence in the discharge with NiO/Ni cathode, caused by the change of the number of needle electrode, as Fig. 9 shows. The discharge area on the NiO/Ni cathode surface is concentrated vertically below the needle tips and gradually weakens toward the periphery. For example, the area on the NiO layer surface corresponding to the vertical of the needle-free electrode, i.e. in the center of the 6-needle circle, is dark, as shown in Fig. 9 (c). Besides, when three needles are removed, only the semicircular luminous region is left, as shown in Fig. 9 (d). The reason may be that the electric field of the vertical space corresponding to the needle tip is relative strong, resulting in large energy for ion bombardment, as well as large amount of positive charges accumulated on the oxide layer surface, finally
3.4. Effect of oxide layer roughness on DC discharge Fig. 6 shows the surface morphology and relative average roughness (Ra) of different cathodes (Ni cathode and NiO/Ni cathode). A relative smooth Ni base (Fig. 6 (a), Ra1 = 0.06 μm) and a rough one with vertical gullies (Fig. 6 (b), Ra2 = 1.60 μm), are used to prepare NiO/Ni cathodes (Fig. 6 (c), Ra3 = 0.40 μm) and (Fig. 6 (d), Ra4 = 2.90 μm), respectively. Since the intermediate NiO layer is formed by self-growth during calcination, the surface morphology may greatly affected by the Ni base from Fig. 6 (a) and 6 (b), exhibiting an irregular pattern (Fig. 6 (c)) and undulating gullies (Fig. 7 (d)), respectively. Correspondingly, the surface roughness becomes larger after calcination process. The related DC discharge current-voltage characteristics with NiO/ Ni cathode and Ni cathode is shown in Fig. 7. The results show that for discharge with Ni metal cathode, a smooth surface is helpful to improve the discharge current. This is because when the surface is rough, some of the secondary electrons are absorbed by the original substance [45]. In general, roughening the surface or spraying a rough layer of metal powder is one of the effective ways to reduce the SEE coefficient [45]. However, for discharge with NiO/Ni cathode, the rough surface facilitates an increase in discharge current regardless of the gas pressure conditions. The same rule is also shown in the discharge with the sandwich MgO/NiO/Ni cathode, as shown in Fig. S5. This indicates that the effect of the structural surface roughness of the oxide layer on DC discharge is mainly related to the occurrence of field-enhanced SEE process, regardless of the gas pressure condition [46]. It is believed that undulating gullies of NiO surface with a large drop, as seen from the height profiles across the white lines drawn in Fig. 6 (d), contribute to the uneven distribution of the surface potential caused by the accumulated charges on the oxide layer, which is an important factor contributing to the emission process of secondary electrons. 3.5. Effect of the number of needle electrode on DC discharge Fig. 8 shows the effect of the number of needle electrode on DC discharge with NiO/Ni cathode and Ni cathode, in different gas
Fig. 8. The effect of the number of needle electrode on DC discharge with NiO/Ni cathode, in different gas pressures from 400 to 700 torr: single-needle electrode (1N), three-needle electrode (3N) and six-needle electrode (6N). 120
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Fig. 9. Images of DC discharge ignited by NiO/Ni cathode with different needle electrodes: (a) side view of discharge with 6-needle (U = 20 kV, I = 170 μA); (b) side view of discharge with 3-needle (U = 20 kV, I = 67 μA); (c) NiO/Ni cathode surface discharge image associated with (a); (d) NiO/Ni cathode surface discharge image associated with (b). Exposure time of 10 s, gas pressure of 700 torr.
whole discharge space, as well as further increase of discharge current, can be optimized by changing the geometry of the discharge electrode and the corresponding oxide cathode in future study, such as using multi-needle/wire plate type, wire-cylinder type, etc., which can further expand its chemical applications, such as exhaust gas removal and so on.
leading to a better SEE process and a more obvious luminescence. Correspondingly, luminescence phenomenon on the oxide layer surface, where is far away from the needle tips, is weakened due to the weakening of the spatial electric field. It indicates that the number of the needle electrode, i.e. discharge region of the discharge electrodes, has a significant influence overall spatial discharge. This should be an important reason for the significant increase in current with NiO/Ni cathode as the number of needle electrode increases. In addition, the size of the oxide cathode is also designed to coincide with the area of the discharge electrode. The design of both discharge electrode and oxide cathode has an important influence on the distribution of electrons and reactive species in the discharge chamber. Therefore, plasma volume and discharge current in the cell space can be optimized by changing the structure of the discharge electrode and oxide cathode (such as using multi-wire/needle plate type, wire-cylinder type, etc.) to further expand its chemical applications, like exhaust gas removal and so on.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51477025, 51877028). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.04.035. References
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In summary, the effect of the gas pressure, as well as the structural properties of oxide cathode on DC discharge with oxide cathodes have been deeply analyzed, basing on the SEE theory. Under different gas pressure conditions, DC discharge with oxide cathodes (NiO/Ni and MgO/NiO/Ni cathode) exhibits a significant increase in discharge current compared to the one with normal Ni cathode. As the gas pressure increases from 400 to 700 torr, the enhancement of discharge current with oxide cathode is weakened, which is due to the fact that the reduced electric field is relative small under a high gas pressure condition, resulting in a weakened ion-induced SEE process. Besides, we believe that the effect of the structural characteristics of oxide layer on DC discharge, such as the thickness and roughness, may be related to the field-enhanced SEE process regardless of gas pressure condition. A rough NiO layer surface with undulating gullies contributes to the uneven distribution of the surface potential caused by the accumulated charges, resulting in a good field-enhanced SEE process. The NiO intermediate layer plays an important role in the sandwich MgO/NiO/Ni cathode for DC discharge, which is due to the fact that its high resistance favors the field-enhanced SEE processes. In addition, the increase in the number of needle electrodes is conducive to enhancement of discharge current with oxide cathode, which is related to the increase discharge area on the oxide layer surface. Thus, plasma volume in the 121
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