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Effects of surface “micro-holes” on the flashover properties of a disk-type ceramic-vacuum insulator
Vacuum 141 (2017) 124e129
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of surface “micro-holes” on the flashover properties of a disktype ceramic-vacuum insulator Xiao-Liang Sun*, Hui-Huang Zhong, Tao Xun, Jun Zhang College of Opto-electric Science and Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2016 Received in revised form 28 March 2017 Accepted 29 March 2017 Available online 31 March 2017
A surface treatment method is proposed which creates surface “micro-holes” to improve the flashover properties of large disk-type alumina ceramic-vacuum insulators operating under high pulsed voltages. This method eliminates residual surface flashover traces and recovers the insulation strength. A structure optimized ceramic vacuum insulator was treated with this method and tested using a high-voltage pulse modulator (600 kV, 100 ns). Experimental results show that this treatment can improve the insulation reliability greatly. Scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) were applied to investigate the influence of surface micro-hole on flashover properties. It was found that the width (W) and depth (D) of micro-holes greatly influenced flashover properties. A D/W ratio was introduced to explain the mechanism of this improvement. The surface micro-holes had the greatest beneficial effect on insulation strength when the ratio was between 1.2 and 2.0. A mechanism explaining the effect of surface “micro-hole” on the flashover properties was proposed. Simulation results of VSimPIC code corroborate our observations and explanation. © 2017 Published by Elsevier Ltd.
Keywords: Surface micro-hole Ceramic insulator Surface flashover Vacuum discharge
1. Introduction Vacuum insulators are widely used in a variety of applications, including high-voltage pulsed power systems, high-current electron accelerators, high-power microwave devices, and vacuum circuit breakers [1] [2], [3] [4], [5] [6], [7] [8], [9] [10], and [11]. These insulators work as interfaces which separate the vacuum on one side and the liquid dielectric such as oil, deionized water on the other side. With the application of a high electric field, surface flashover of insulators usually occurs from the triple junctions of conductor-insulator-vacuum [3]. This flashover can lead to insulator breakdown which is a serious limiting factor in the field of high voltage pulsed power systems [1] [12] [13], and [32]. With the development and practical application of pulsed power and high power microwave (HPM) technology, there is a trend for high-current diodes to be more compact and better able to withstand high vacuum conditions for long periods of time. Organic materials are the conventional insulators used in diodes due to their low relative permittivity and better machining characteristics [12] [13], and [14]; however, gases released from the bulk material
* Corresponding author. E-mail address: [email protected] (X.-L. Sun). http://dx.doi.org/10.1016/j.vacuum.2017.03.036 0042-207X/© 2017 Published by Elsevier Ltd.
to surface restricts their application in high vacuum conditions. Because of better high temperature resistance, lower out-gassing rate, and better metal-welding ability compared with organic material, the ceramics are superior as high vacuum insulators [14]. Whereas higher dielectric constant can create stronger polarization electric field between electrode and insulator, this stronger electric filed can make surface flashover occur easier [33] and [34]. Dielectric constant of ceramic is about 9.8 higher than organic materials which is unfavorable its application as vacuum insulators. So it is significant to improve surface flashover characteristics of high vacuum ceramic insulators. Due to the brittleness and fragility of ceramics, the conventional machining methods used to improve the flashover properties of organic materials, such as surface grooving and arbitrary shaping, are not suitable for ceramic insulators. In this condition, surface treatment method is a convenient and feasible way to ameliorate the flashover performance of ceramic insulators. Surface treatment methods including varying the surface roughness [15] [16], [17] [18], surface coating or quasimetallizing [19] [20], [21] [22], hydrogen treatment [23] [24], and metal ion implantation [25] [26] [27], have been studied and these researches have greatly improved the insulation characteristics for ceramic vacuum insulators used in pulsed power systems. However, ceramic insulators reported in these researches are relatively small (<200 mm) and the operating high pulse voltages are usually
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no more than 500 kV with a pulse duration less than 100 ns. In this paper, a surface treatment method to improve flashover properties for ceramic vacuum insulator is reported. This disk-type insulator is 300 mm in diameter with a center conductor which is biased to negative potential and the outer vacuum chamber and anode are grounded. By sputtering SiC-sand onto the vacuum side surface of the insulator, “micro-holes” are created and surface roughness increases. Insulation tests for structurally optimized ceramic insulator treated with this method were conducted using a high-voltage pulse modulator (600 kV, 100 ns). Scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) were then employed to investigate the parameters of surface “micro-holes”. A theory of D/W ratio was introduced to explain the influence of surface micro-holes on flashover properties, where D and W are the depth and width, respectively. Lastly, a VSim-PIC simulation was conducted to verify the effects of the surface micro-holes on the flashover characteristics. 2. Experimental setup The disk-type alumina ceramic-vacuum insulator discussed in this paper has already been structure optimized by Xun et al. [12], [13]. However, surface flashovers still occurred from time to time during the experiments under the pulse voltage conditions of ~600 kV and ~100 ns [12]. Due to the cumulative effect, surface flashover traces can be visually examined after the whole test. This trace damage may be severe enough to prevent the insulator from performing its function. In order to eliminate these traces, a surface treatment method was employed. The method comprises the following three steps: firstly, the vacuum sides of ceramic insulators were sputtered by SiC-sand and blew to clean up the surface, secondly, the ceramics were washed by anhydrous ethanol and acetone for more than 30 min, respectively and after that the ceramics were placed in vacuum oven for about 30 min. A complete description of this surface treatment method can be found on China Design Patent, Number 201510729054. After the surface treatment, the flashover traces were removed from the ceramic insulator surface. The ceramic insulator recovers its insulating function and the surface flashover increase. To investigate the effect of this treatment, an insulation test was conducted using a high-voltage pulse system on the structure optimized ceramic vacuum insulator. The experimental setup is schematically illustrated in Fig. 1, where the ceramic insulator separates transformer oil from vacuum, a conductor on-axis protrudes the insulator and is at negative high voltage (~600 kV, ~100 ns). At the end of this conductor is affixed a cathode electrode
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of a diode that generates electrons through explosive electron emission which are accelerator across the anode-cathode gap into a high power microwave source. A turbo-molecular vacuum pump was applied to maintain the vacuum conditions in the experimental chamber at 103 Pa during the tests. The voltage waveform at the ceramic-vacuum insulator was measured by a capacitive voltage divider in the shape of an umbrella [28]. The division factor of the capacitive divider was 950, and it operated in a self-integral mode [12]. Six new ceramic insulators were put forward to carry out the insulation test and they were equally divided into two groups: the untreated group (group A) and the treated group (group B). Ceramic insulators in treated group were surface treated in the same method and ceramic insulators in untreated group were carried out the comparison test. The high pulse voltage levels of 400 kV, 500 kV and 600 kV were applied gradually with the pulse width near 100 ns. The total pulse shot numbers for the two groups at each level are the same and the test was done in single shot mode. To further study the changes of ceramic insulator surface with this treatment, scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) were used. Surface morphology of the ceramic samples was observed using a Quanta 200 FEI SEM at an operating voltage of 20.0 keV. Surface roughness and parameters of the surface microstructures were measured using the LEXT OLS4100 LSCM. 3. Results and discussion Fig. 2 shows the voltage waveforms of the ceramic insulator before and after the surface treatment. As shown in the figure, once the surface flashover occurred, the voltage pulse-width of the insulator was severely shortened from 100 ns to no more than 60 ns. It can also be found that the surface flashover voltage waveform of ceramic insulator discussed in this paper dropped relatively slowly, not steeply as in the most surface flashovers. This was because the impedance mismatch between high current diode and pulsed power source which were dozens to hundreds of ohm and 10e20 U, respectively. When these insulators were surface treated, they recovered their insulating function. By comparing the voltage waveforms before and after the surface treatment, it can be concluded that this treatment improved the waveforms while also increasing the flashover voltage. The insulation test data is summarized in Table 1. From the table, the follows can be concluded surface flashover doesn't occur at the voltage level of 400 kV and it occurs from the voltage level of 500 kV, flashover numbers of ceramic with surface treatment
Fig. 1. Schematic illustration of the insulation test arrangement showing the experimental chamber and ceramic-vacuum insulator.
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Mechanism of surface micro-holes on improving flashover properties of ceramic insulators was analyzed. These evenly distributed surface holes influence the flashover characteristics by changing electrons movement along the ceramic surface and the surface charge statement as shown in Fig. 4. Size of these surface holes is in the order of microns which is the same level of electron movement between the electrons impact on the ceramic surface. With deeper and wider micro-holes, more electrons will be restricted in the holes and these accumulated electrons can further ! change the electric filed Ee along the surface. This electric field reduces the electric field between the cathode and micro-holes which decreases the attraction of anode for electrons. Consequently, fewer electrons will move to anode. This can be called the “effect of resistance” which causes electrons need more time and energy move to the anode. The effect contributes to a longer establishment time for flashover and a higher flashover voltage. Besides, these accumulated electrons can also accelerate the formation of flashover by attracting positive charges and this effect can be called the “effect of promotion”. These two effects both can influence the development of surface flashover and which effect is dominant depending on the position of surface micro-holes. As for the disk-type ceramic insulator discussed in this paper, the radial distance is about 150 mm. It means that the resistance effect is dominant and the promote effect can be not taken into account especially for cathode initiated surface flashover. The D/W ratio was approximately 0.82 without surface treatment, this effect is weak at this time and electrons emitted from cathode triple junction (CTJ) move to anode easily. When the ceramic insulator was surface treated, this effect became stronger for the D/W ratio increased to 1.42. As a result, establishment time of surface flashover becomes longer for the same voltage level applied over the ceramic insulator and reliability of the insulator increases. These micro-holes improve flashover properties by delaying the development of a flashover rather than preventing its establishment. However, it doesn't mean better flashover properties for higher D/W ratio. This is because too much big micro-holes can change the structure of the insulator which may decrease the flashover voltage. In addition, the SiC-sand may cause wear to the ceramic surface and the cumulative effect can damage the insulator too. Consequently, the D/W ratio should be chosen appropriately. Based on our experiment results, the optimum range of D/W ratio for the ceramic insulator discussed in this paper is between 1.2 and 2. In order to validate our interpretation of the surface micro-holes for surface flashover, a simulation was conducted via the multiphysics particle in cell (PIC) code by VSim [30] and [31]. In the two-dimension simulation model, an insulator with a relative permittivity of 8.9 is placed between two parallel plate electrodes. The electric field is polarized in the direction perpendicular to the surface of insulator, and electrons are emitted by the FowlerNordheim model. The electrons will move under the control of the EM-field until they strike the anode or the insulator, and some of the electrons absorbed by the insulator can emit secondary electrons. The model of secondary electron emissions was proposed by Perkins et al. [30]. The simulation results for three different D/W rates of surface
Fig. 2. Waveforms of the voltage across the ceramic-vacuum insulators. The black line represents the waveform when surface flashover occurred, and the blue and red lines represent the voltage before and after surface treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decreases. Further, reliability of the ceramic insulator was analyzed. It was defined as the ratio of flashover number and the total shots number. The results show that this surface treatment can further improve the reliability of the structure optimized ceramic insulator discussed in this paper for high pulse voltage (>500 kV). Surface microstructures have a great influence on the performance of vacuum insulators, so surface metrology [35] was employed to study the improving mechanism of the surface treatment. SEM images of the ceramic insulators are shown in Fig. 3(a) and (b). With this treatment, the ceramic surface becomes rougher more and larger surface micro-holes are created. The LSCM was applied to measure the ceramic surface morphology and results were presented in Fig. 3(c)e(e). This treatment can greatly increase the average depth of micro-holes from 17.9 mm to 34.9 mm while the width changes relatively, only from 21.7 mm to 24.6 mm. The average surface roughness (Ra) of the ceramic insulator increased from 1.4 mm to 2.9 mm after the treatment. These surface micro-holes are approximately evenly distributed in the study and the density changes a little after the treatment. Kawata et al. [29] once used a model of a bowl structure to investigate the influence of the surface roughness on the secondary electron emissions from a beryllium surface. The H/W rate of the bowl was used to characterize the secondary electron yields s, where H and W are the height and width, respectively. In that study, electrons bombarding the surface were low-energy (<1 keV) and the bowl dimensions was on the nanometer level. In this paper, a similar method was introduced to study the influence surface micro-holes on the flashover properties for high-energy electrons. A D/W ratio was used to characterize the surface micro-holes, where D and W are the depth and width of the holes, respectively. In this model, the average D/W ratio of surface micro-holes increased from 0.82 to 1.42 after the surface treatment.
Table 1 Insulation test data. Pulse voltage (kV)
Total shots
Flashover in group A
Flashover in group B
Reliability in group A
Reliability in group B
433 ± 17 541 ± 14 618 ± 19
70 335 384
0 17 26
0 4 9
100% 94.9% 93.2%
100% 98.8% 97.8%
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Fig. 3. Surface morphology of the ceramic insulators before and after surface treatment, (a) and (b) are the SEM images, (c) and (d) are the corresponding LSCM images, (e) represents the ups and downs on the ceramic surface in (c) and (d).
Fig. 4. Influence of the surface micro-holes on the electric field distribution along the ceramic surface. These holes can restrict the movement of electrons and as a result, the accumulated electrons change the electric field around the holes.
“micro-holes”: 0, 1.2e2,>2 are shown in Fig. 5 ~ Fig. 7, respectively. Comparing these figures, it is clear that surface flashover occurs regardless of the surface conditions, but the establishment time varies. Among these three surface conditions, electrons move quickest along the smooth surface, slower for the large D/W ratio,
and the slowest for the small D/W ratio. The numbers of electrons that arrive at the anode also change similarly. These results agree with our previous explanation. The surface micro-holes improved the flashover properties by delaying the establishment of a flashover. This again indicates the differences after the surface grooving treatment which cuts off the development of a flashover, although the degree of improvement depends on the D/W ratio. According to the results of our experiments and simulation, the effects of the treatment are the greatest when the D/W ratio is in the range of 1.2e2.0, and the improvement is weaker when the rate exceeds this range. The simulation results agree with our experimental data and explanation (see Fig. 6).
4. Summary Surface flashover is one of the most critical factors limiting the application of ceramic insulators for high electric fields. In this paper, we proposed a surface treatment method by SiC-sand sputtering to improve flashover properties for disk-type large-
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Fig. 5. Movement of electrons along the smooth ceramic surface: (a) Electrons are emitted from the cathode, (b) and (c) moving quickly toward the anode, and (d) a channel was established between cathode and anode after 7 ns.
Fig. 6. Movement of electrons along the ceramic surface when the D/W ratio was between 1.2 and 2.0: (a) the electrons are emitted from cathode, (b) and (c) can be restricted by micro-holes, (d) so fewer electrons arrive at the anode.
Fig. 7. Movement of electrons along the ceramic surface when the D/W ratio was greater than 2.0: micro-holes with large D/W ratio can also restrict the movement of electrons (b), (c) emitted from the cathode (a). But the resistance effect became weak, so there were more electrons arriving at the anode as compared to the number for the proper D/W ratio, and a longer time was needed to establish the flashover, compared to the smooth surface (d).
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scale ceramic insulators. Surface flashover traces can be eliminated and the insulation strength recovers with this treatment. Insulation test results show that the ceramic insulator reliability increases for high pulse voltage after the treatment. Surface morphology of the insulator was observed via SEM and LSCM and a D/W model to explain the effect of surface micro-holes on flashover properties was introduced. Surface roughness of the insulator after the treatment was 2.9 mm. The influence of D/W rate of the holes and the radial distance of disk-type structure insulator on surface flashover was analyzed. Experimental results show that the best D/ W rate for the large disk-type ceramic discussed in this paper is between 1.2 and 2. A VSim-PIC simulation was conducted and the results agree with our explanation. In the future, the effects of the surface coating for ceramic insulators and the influence of distribution of surface micro-holes will be studied. Acknowledgments The authors gratefully thank A. K. Li for his help in operating the scanning electron microscope. We also acknowledge the work of R. P. Wang for the surface treatment, and the assistance of L. Luo and W. X. Zhu in the insulation tests. References [1] J.Z. Gleizer, Y.E. Krasik, J. Leopold, Time-and space-resolved light emission and spectroscopic research of the flashover plasma, J. Appl. Phys. 117 (2015) 073301. [2] Yamano Y. Suharyanto, S. Kobayashi, S. Michizono, Y. Saito, Secondary electron emission and surface charging evaluation of alumina ceramics and sapphire, IEEE Trans. Diel. Electr. Insul. 13 (2006) 72e78. [3] H.C. Miller, Flashover of insulators in vacuum: the last twenty years, IEEE Trans. Diel. Electr. Insul. 22 (2015) 3641e3657. [4] M.A. Furman, M.T.F. Pivi, Probabilistic model for the simulation of secondary electron emission, Phys. Rev. St. Accel. Beams 5 (2002) 124404. [5] R.V. Latham, High Voltage Vacuum Insulation: Basic Concepts and Technological Practice, Elsevier, 1995. [6] J. Benford, J.A. Swegle, E. Schamiloglu, High Power Microwaves, CRC Press, 2015. [7] G. Shafir, M. Kreif, J.Z. Gleizer, S. Gleizer, Y.E. Krasik, A.V. Gunin, O.P. Kutenkov, V. PegeI, V.V. Rostov, Experimental research of different plasma cathodes for generation of high-current electron beams, J. Appl. Phys. 118 (2015) 193302. [8] Y.W. Fan, X.Y. Wang, Z.C. Zhang, et al., A high-efficiency repetitively pulsed magnetically insulated transmission line oscillator, Vacuum 128 (2016) 39e44. [9] T. Xun, H. Yang, J. Zhang, et al., Effects of vacuum pressures on the performance of a velvet cathode under repetitive high-current pulse discharges, Vacuum 85 (2) (2010) 322e326. [10] X. Zhao, T. Xun, L. Liu, et al., Maintenance of high vacuum level in a compact and lightweight sealed hard-tube magnetically insulated line oscillator system, Vacuum 111 (2015) 55e59. [11] T. Xun, X. Zhao, G. Li, et al., High-current, pulsed electron beam sources with SiC nanowire cathodes, Vacuum 125 (2015) 81e84. [12] T. Xun, H.-W. Yang, J.-D. Zhang, A high-vacuum high-electric-field pulsed power interface based on a ceramic insulator, IEEE Trans. Plasma. Sci. 43 (2015) 4130e4135. [13] T. Xun, H.-W. Yang, J.-D. Zhang, Z.-X. Liu, Y. Wang, Y.-S. Zhao, A ceramic radial insulation structure for a relativistic electron beam vacuum diode, Rev. Sci. Instrum. 79 (2008) 063303.
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[14] T. Xun, J.-D. Zhang, H.-W. Yang, J.-M. Gao, Hydrodynamic loading of ceramic components due to pulsed discharge in water, IEEE Trans. Plasma. Sci. 37 (2009) 1975e1979. [15] T. Hosono, K. Kato, A. Morita, H. Okubo, Surface charges on alumina in vacuum with varying surface roughness and electric field distribution, IEEE Trans. Diel.Electr.Insul. 14 (2007) 627e633. [16] T. Shao, W.-J. Yang, C. Zhang, Z. Niu, P. Yan, E. Schamiloglu, Enhanced surface flashover strength in vacuum of polymethylmethacrylate by surface modification using atmospheric-pressure dielectric barrier discharge, Appl. Phys. Lett. 105 (2014) 071607. [17] H. Naruse, O. Yamamoto, Estimation of flashover voltage along cylindrical insulator in vacuum, IEEE Trans. Diel.Electr.Insul 23 (2016) 22e28. [18] Suharyanto, Y. Yamano, S. Kobayashin, S. Michizono, Y. Saito, Tumiran, Effect of mechanical finishes on secondary electron emission of alumina ceramics, IEEE Trans. Diel.Electr.Insul 14 (2007) 620e626. [19] A. Nikolaeva, E.M. Oksa, K. Savkina, G.Y. Yushkova, D.J. Brennerb, G. Johnsonb, G.R. Pehrsonb, I.G. Brownc, R.A. MacGillc, Surface resistivity tailoring of ceramic insulators for an ion microprobe application, Surf. Coat. Technol. 201 (2007) 8120e8122. [20] K.-K. Yu, G.-J. Zhang, G.-Q. Liu, X.-P. Ma, G.-X. Li, Effect of surface shallow traps on flashover characteristics across machinable ceramic in vacuum, IEEE Trans. Diel.Electr.Insul 15 (2008) 1464e1470. [21] N. Zhang, G.-J. Zhang, X.-Z. Huang, P. Zhai, X.-P. Ma, G.-X. Li, Influence of surface ion exchange on pulsed flashover characteristics of machinable ceramics in vacuum, IEEE Trans. Diel.Electr.Insul 18 (2011) 1011e1016. [22] H.C. Miller, Flashover of insulators in vacuum: review of the phenomena and techniques to improved holdoff voltage, IEEE Trans. Electr. Insul. 28 (1993) 512e527. [23] J.M. Elizondo, K. Meredith, N. Lapetina, Ceramic secondary electron emission and surface charge measurements, IEEE Trans. Plasma. Sci. 30 (2002), 1995e1960. [24] H. Fujii, Control of charge on insulating glass in vacuum by plasma processing, IEEE Trans. Diel. Electr. Insul. 09 (2002) 230e235. [25] V.I. Gushenets, A.G. Nikolaev, E.M. Oks, K.P. Savkin, G. Yushkov Yu, I.G. Brown, High-energy metal ion implantation for reduction of surface resistivity of alumina ceramic, Rev. Sci. Instrum. 83 (2012) 02B908. [26] K.P. Savkin, A.S. Bugaev, A.G. Nikolaev, E.M. Oks, I.A. Kurzina, M.V. Shandrikov, G. Yushkov Yu, I.G. Brown, Decrease of Ceramic Surface Resistance by Implantation Using a Vacuum Arc Metal Ion Source 25th Discharges and Electrical Insulation in Vacuum (Tomsk, Russia, 2-7 September 2012), 2012, pp. 89e92. [27] N. Asari, T. Shioiri, J. Sato, K. Matuso, H. Kojima, N. Hayakawa, Influence of Heat Treatment in Atmosphere on Surface Flashover Characteristics of Alumina in Vacuum Discharges and Electrical Insulation in Vacuum (Mumbai, India, 28 September-3 October 2014), 2014, pp. 554e557. [28] Z.-C. Zhang, J.-D. Zhang, B.-L. Qian, C.-B. Liu, T. Xun, H. Zhang, B. Liang, Compact rep-rate GW pulsed generator based on forming line with built-in high-coupling transformer, IEEE Trans. Plasma. Sci. 42 (2014) 241e248. [29] J. Kawata, K. Ohya, K. Nishimura, Simulation of secondary electron emission from rough surfaces, J. Nucl. Mater. 220 (1995) 997e1000. [30] M.P. Perkins, T.L. Houck, J.B. Javedani, G.E. Vogtlin, D.A. Goerz, Progress on Simulating the Initiation of Vacuum Insulator Flashover Pulsed Power Conference (Washington, DC, 28 June-2 July 2009), 2009, pp. 441e446. [31] M.P. Perkins, T.L. Houck, A.R. Marquez, G.E. Vogtlin, Simulations for Initiation of Vacuum Insulator Flashover Power Modulator and High Voltage Conference (Atlanta, GA, 23-27 May 2010), 2010, pp. 727e730. [32] Y.E. Krasik, J. Leopold, Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination, Phys. Plasmas. 22 (2015) 083109. [33] M. Akahane, K. Kabda, K. Yahagi, Effects of dielectric constant on surface discharge of polymer insulators in vacuum, J. Appl. Phys. 44 (1973), 2927e2927. [34] T. Suzuki, Flashover voltage over ceramic in high vacuum, J. J. Appl. Phys. 13 (1974) 1541e1542. [35] D.J. Whitehouse, Surfaces and Their Measurement, Butterworth-Heinemann, 2004.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of surface “micro-holes” on the flashover properties of a disktype ceramic-vacuum insulator Xiao-Liang Sun*, Hui-Huang Zhong, Tao Xun, Jun Zhang College of Opto-electric Science and Engineering, National University of Defense Technology, Changsha 410073, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2016 Received in revised form 28 March 2017 Accepted 29 March 2017 Available online 31 March 2017
A surface treatment method is proposed which creates surface “micro-holes” to improve the flashover properties of large disk-type alumina ceramic-vacuum insulators operating under high pulsed voltages. This method eliminates residual surface flashover traces and recovers the insulation strength. A structure optimized ceramic vacuum insulator was treated with this method and tested using a high-voltage pulse modulator (600 kV, 100 ns). Experimental results show that this treatment can improve the insulation reliability greatly. Scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) were applied to investigate the influence of surface micro-hole on flashover properties. It was found that the width (W) and depth (D) of micro-holes greatly influenced flashover properties. A D/W ratio was introduced to explain the mechanism of this improvement. The surface micro-holes had the greatest beneficial effect on insulation strength when the ratio was between 1.2 and 2.0. A mechanism explaining the effect of surface “micro-hole” on the flashover properties was proposed. Simulation results of VSimPIC code corroborate our observations and explanation. © 2017 Published by Elsevier Ltd.
Keywords: Surface micro-hole Ceramic insulator Surface flashover Vacuum discharge
1. Introduction Vacuum insulators are widely used in a variety of applications, including high-voltage pulsed power systems, high-current electron accelerators, high-power microwave devices, and vacuum circuit breakers [1] [2], [3] [4], [5] [6], [7] [8], [9] [10], and [11]. These insulators work as interfaces which separate the vacuum on one side and the liquid dielectric such as oil, deionized water on the other side. With the application of a high electric field, surface flashover of insulators usually occurs from the triple junctions of conductor-insulator-vacuum [3]. This flashover can lead to insulator breakdown which is a serious limiting factor in the field of high voltage pulsed power systems [1] [12] [13], and [32]. With the development and practical application of pulsed power and high power microwave (HPM) technology, there is a trend for high-current diodes to be more compact and better able to withstand high vacuum conditions for long periods of time. Organic materials are the conventional insulators used in diodes due to their low relative permittivity and better machining characteristics [12] [13], and [14]; however, gases released from the bulk material
* Corresponding author. E-mail address: [email protected] (X.-L. Sun). http://dx.doi.org/10.1016/j.vacuum.2017.03.036 0042-207X/© 2017 Published by Elsevier Ltd.
to surface restricts their application in high vacuum conditions. Because of better high temperature resistance, lower out-gassing rate, and better metal-welding ability compared with organic material, the ceramics are superior as high vacuum insulators [14]. Whereas higher dielectric constant can create stronger polarization electric field between electrode and insulator, this stronger electric filed can make surface flashover occur easier [33] and [34]. Dielectric constant of ceramic is about 9.8 higher than organic materials which is unfavorable its application as vacuum insulators. So it is significant to improve surface flashover characteristics of high vacuum ceramic insulators. Due to the brittleness and fragility of ceramics, the conventional machining methods used to improve the flashover properties of organic materials, such as surface grooving and arbitrary shaping, are not suitable for ceramic insulators. In this condition, surface treatment method is a convenient and feasible way to ameliorate the flashover performance of ceramic insulators. Surface treatment methods including varying the surface roughness [15] [16], [17] [18], surface coating or quasimetallizing [19] [20], [21] [22], hydrogen treatment [23] [24], and metal ion implantation [25] [26] [27], have been studied and these researches have greatly improved the insulation characteristics for ceramic vacuum insulators used in pulsed power systems. However, ceramic insulators reported in these researches are relatively small (<200 mm) and the operating high pulse voltages are usually
X.-L. Sun et al. / Vacuum 141 (2017) 124e129
no more than 500 kV with a pulse duration less than 100 ns. In this paper, a surface treatment method to improve flashover properties for ceramic vacuum insulator is reported. This disk-type insulator is 300 mm in diameter with a center conductor which is biased to negative potential and the outer vacuum chamber and anode are grounded. By sputtering SiC-sand onto the vacuum side surface of the insulator, “micro-holes” are created and surface roughness increases. Insulation tests for structurally optimized ceramic insulator treated with this method were conducted using a high-voltage pulse modulator (600 kV, 100 ns). Scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) were then employed to investigate the parameters of surface “micro-holes”. A theory of D/W ratio was introduced to explain the influence of surface micro-holes on flashover properties, where D and W are the depth and width, respectively. Lastly, a VSim-PIC simulation was conducted to verify the effects of the surface micro-holes on the flashover characteristics. 2. Experimental setup The disk-type alumina ceramic-vacuum insulator discussed in this paper has already been structure optimized by Xun et al. [12], [13]. However, surface flashovers still occurred from time to time during the experiments under the pulse voltage conditions of ~600 kV and ~100 ns [12]. Due to the cumulative effect, surface flashover traces can be visually examined after the whole test. This trace damage may be severe enough to prevent the insulator from performing its function. In order to eliminate these traces, a surface treatment method was employed. The method comprises the following three steps: firstly, the vacuum sides of ceramic insulators were sputtered by SiC-sand and blew to clean up the surface, secondly, the ceramics were washed by anhydrous ethanol and acetone for more than 30 min, respectively and after that the ceramics were placed in vacuum oven for about 30 min. A complete description of this surface treatment method can be found on China Design Patent, Number 201510729054. After the surface treatment, the flashover traces were removed from the ceramic insulator surface. The ceramic insulator recovers its insulating function and the surface flashover increase. To investigate the effect of this treatment, an insulation test was conducted using a high-voltage pulse system on the structure optimized ceramic vacuum insulator. The experimental setup is schematically illustrated in Fig. 1, where the ceramic insulator separates transformer oil from vacuum, a conductor on-axis protrudes the insulator and is at negative high voltage (~600 kV, ~100 ns). At the end of this conductor is affixed a cathode electrode
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of a diode that generates electrons through explosive electron emission which are accelerator across the anode-cathode gap into a high power microwave source. A turbo-molecular vacuum pump was applied to maintain the vacuum conditions in the experimental chamber at 103 Pa during the tests. The voltage waveform at the ceramic-vacuum insulator was measured by a capacitive voltage divider in the shape of an umbrella [28]. The division factor of the capacitive divider was 950, and it operated in a self-integral mode [12]. Six new ceramic insulators were put forward to carry out the insulation test and they were equally divided into two groups: the untreated group (group A) and the treated group (group B). Ceramic insulators in treated group were surface treated in the same method and ceramic insulators in untreated group were carried out the comparison test. The high pulse voltage levels of 400 kV, 500 kV and 600 kV were applied gradually with the pulse width near 100 ns. The total pulse shot numbers for the two groups at each level are the same and the test was done in single shot mode. To further study the changes of ceramic insulator surface with this treatment, scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) were used. Surface morphology of the ceramic samples was observed using a Quanta 200 FEI SEM at an operating voltage of 20.0 keV. Surface roughness and parameters of the surface microstructures were measured using the LEXT OLS4100 LSCM. 3. Results and discussion Fig. 2 shows the voltage waveforms of the ceramic insulator before and after the surface treatment. As shown in the figure, once the surface flashover occurred, the voltage pulse-width of the insulator was severely shortened from 100 ns to no more than 60 ns. It can also be found that the surface flashover voltage waveform of ceramic insulator discussed in this paper dropped relatively slowly, not steeply as in the most surface flashovers. This was because the impedance mismatch between high current diode and pulsed power source which were dozens to hundreds of ohm and 10e20 U, respectively. When these insulators were surface treated, they recovered their insulating function. By comparing the voltage waveforms before and after the surface treatment, it can be concluded that this treatment improved the waveforms while also increasing the flashover voltage. The insulation test data is summarized in Table 1. From the table, the follows can be concluded surface flashover doesn't occur at the voltage level of 400 kV and it occurs from the voltage level of 500 kV, flashover numbers of ceramic with surface treatment
Fig. 1. Schematic illustration of the insulation test arrangement showing the experimental chamber and ceramic-vacuum insulator.
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Mechanism of surface micro-holes on improving flashover properties of ceramic insulators was analyzed. These evenly distributed surface holes influence the flashover characteristics by changing electrons movement along the ceramic surface and the surface charge statement as shown in Fig. 4. Size of these surface holes is in the order of microns which is the same level of electron movement between the electrons impact on the ceramic surface. With deeper and wider micro-holes, more electrons will be restricted in the holes and these accumulated electrons can further ! change the electric filed Ee along the surface. This electric field reduces the electric field between the cathode and micro-holes which decreases the attraction of anode for electrons. Consequently, fewer electrons will move to anode. This can be called the “effect of resistance” which causes electrons need more time and energy move to the anode. The effect contributes to a longer establishment time for flashover and a higher flashover voltage. Besides, these accumulated electrons can also accelerate the formation of flashover by attracting positive charges and this effect can be called the “effect of promotion”. These two effects both can influence the development of surface flashover and which effect is dominant depending on the position of surface micro-holes. As for the disk-type ceramic insulator discussed in this paper, the radial distance is about 150 mm. It means that the resistance effect is dominant and the promote effect can be not taken into account especially for cathode initiated surface flashover. The D/W ratio was approximately 0.82 without surface treatment, this effect is weak at this time and electrons emitted from cathode triple junction (CTJ) move to anode easily. When the ceramic insulator was surface treated, this effect became stronger for the D/W ratio increased to 1.42. As a result, establishment time of surface flashover becomes longer for the same voltage level applied over the ceramic insulator and reliability of the insulator increases. These micro-holes improve flashover properties by delaying the development of a flashover rather than preventing its establishment. However, it doesn't mean better flashover properties for higher D/W ratio. This is because too much big micro-holes can change the structure of the insulator which may decrease the flashover voltage. In addition, the SiC-sand may cause wear to the ceramic surface and the cumulative effect can damage the insulator too. Consequently, the D/W ratio should be chosen appropriately. Based on our experiment results, the optimum range of D/W ratio for the ceramic insulator discussed in this paper is between 1.2 and 2. In order to validate our interpretation of the surface micro-holes for surface flashover, a simulation was conducted via the multiphysics particle in cell (PIC) code by VSim [30] and [31]. In the two-dimension simulation model, an insulator with a relative permittivity of 8.9 is placed between two parallel plate electrodes. The electric field is polarized in the direction perpendicular to the surface of insulator, and electrons are emitted by the FowlerNordheim model. The electrons will move under the control of the EM-field until they strike the anode or the insulator, and some of the electrons absorbed by the insulator can emit secondary electrons. The model of secondary electron emissions was proposed by Perkins et al. [30]. The simulation results for three different D/W rates of surface
Fig. 2. Waveforms of the voltage across the ceramic-vacuum insulators. The black line represents the waveform when surface flashover occurred, and the blue and red lines represent the voltage before and after surface treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decreases. Further, reliability of the ceramic insulator was analyzed. It was defined as the ratio of flashover number and the total shots number. The results show that this surface treatment can further improve the reliability of the structure optimized ceramic insulator discussed in this paper for high pulse voltage (>500 kV). Surface microstructures have a great influence on the performance of vacuum insulators, so surface metrology [35] was employed to study the improving mechanism of the surface treatment. SEM images of the ceramic insulators are shown in Fig. 3(a) and (b). With this treatment, the ceramic surface becomes rougher more and larger surface micro-holes are created. The LSCM was applied to measure the ceramic surface morphology and results were presented in Fig. 3(c)e(e). This treatment can greatly increase the average depth of micro-holes from 17.9 mm to 34.9 mm while the width changes relatively, only from 21.7 mm to 24.6 mm. The average surface roughness (Ra) of the ceramic insulator increased from 1.4 mm to 2.9 mm after the treatment. These surface micro-holes are approximately evenly distributed in the study and the density changes a little after the treatment. Kawata et al. [29] once used a model of a bowl structure to investigate the influence of the surface roughness on the secondary electron emissions from a beryllium surface. The H/W rate of the bowl was used to characterize the secondary electron yields s, where H and W are the height and width, respectively. In that study, electrons bombarding the surface were low-energy (<1 keV) and the bowl dimensions was on the nanometer level. In this paper, a similar method was introduced to study the influence surface micro-holes on the flashover properties for high-energy electrons. A D/W ratio was used to characterize the surface micro-holes, where D and W are the depth and width of the holes, respectively. In this model, the average D/W ratio of surface micro-holes increased from 0.82 to 1.42 after the surface treatment.
Table 1 Insulation test data. Pulse voltage (kV)
Total shots
Flashover in group A
Flashover in group B
Reliability in group A
Reliability in group B
433 ± 17 541 ± 14 618 ± 19
70 335 384
0 17 26
0 4 9
100% 94.9% 93.2%
100% 98.8% 97.8%
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Fig. 3. Surface morphology of the ceramic insulators before and after surface treatment, (a) and (b) are the SEM images, (c) and (d) are the corresponding LSCM images, (e) represents the ups and downs on the ceramic surface in (c) and (d).
Fig. 4. Influence of the surface micro-holes on the electric field distribution along the ceramic surface. These holes can restrict the movement of electrons and as a result, the accumulated electrons change the electric field around the holes.
“micro-holes”: 0, 1.2e2,>2 are shown in Fig. 5 ~ Fig. 7, respectively. Comparing these figures, it is clear that surface flashover occurs regardless of the surface conditions, but the establishment time varies. Among these three surface conditions, electrons move quickest along the smooth surface, slower for the large D/W ratio,
and the slowest for the small D/W ratio. The numbers of electrons that arrive at the anode also change similarly. These results agree with our previous explanation. The surface micro-holes improved the flashover properties by delaying the establishment of a flashover. This again indicates the differences after the surface grooving treatment which cuts off the development of a flashover, although the degree of improvement depends on the D/W ratio. According to the results of our experiments and simulation, the effects of the treatment are the greatest when the D/W ratio is in the range of 1.2e2.0, and the improvement is weaker when the rate exceeds this range. The simulation results agree with our experimental data and explanation (see Fig. 6).
4. Summary Surface flashover is one of the most critical factors limiting the application of ceramic insulators for high electric fields. In this paper, we proposed a surface treatment method by SiC-sand sputtering to improve flashover properties for disk-type large-
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Fig. 5. Movement of electrons along the smooth ceramic surface: (a) Electrons are emitted from the cathode, (b) and (c) moving quickly toward the anode, and (d) a channel was established between cathode and anode after 7 ns.
Fig. 6. Movement of electrons along the ceramic surface when the D/W ratio was between 1.2 and 2.0: (a) the electrons are emitted from cathode, (b) and (c) can be restricted by micro-holes, (d) so fewer electrons arrive at the anode.
Fig. 7. Movement of electrons along the ceramic surface when the D/W ratio was greater than 2.0: micro-holes with large D/W ratio can also restrict the movement of electrons (b), (c) emitted from the cathode (a). But the resistance effect became weak, so there were more electrons arriving at the anode as compared to the number for the proper D/W ratio, and a longer time was needed to establish the flashover, compared to the smooth surface (d).
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scale ceramic insulators. Surface flashover traces can be eliminated and the insulation strength recovers with this treatment. Insulation test results show that the ceramic insulator reliability increases for high pulse voltage after the treatment. Surface morphology of the insulator was observed via SEM and LSCM and a D/W model to explain the effect of surface micro-holes on flashover properties was introduced. Surface roughness of the insulator after the treatment was 2.9 mm. The influence of D/W rate of the holes and the radial distance of disk-type structure insulator on surface flashover was analyzed. Experimental results show that the best D/ W rate for the large disk-type ceramic discussed in this paper is between 1.2 and 2. A VSim-PIC simulation was conducted and the results agree with our explanation. In the future, the effects of the surface coating for ceramic insulators and the influence of distribution of surface micro-holes will be studied. Acknowledgments The authors gratefully thank A. K. Li for his help in operating the scanning electron microscope. We also acknowledge the work of R. P. Wang for the surface treatment, and the assistance of L. Luo and W. X. Zhu in the insulation tests. References [1] J.Z. Gleizer, Y.E. Krasik, J. Leopold, Time-and space-resolved light emission and spectroscopic research of the flashover plasma, J. Appl. Phys. 117 (2015) 073301. [2] Yamano Y. Suharyanto, S. Kobayashi, S. Michizono, Y. Saito, Secondary electron emission and surface charging evaluation of alumina ceramics and sapphire, IEEE Trans. Diel. Electr. Insul. 13 (2006) 72e78. [3] H.C. Miller, Flashover of insulators in vacuum: the last twenty years, IEEE Trans. Diel. Electr. Insul. 22 (2015) 3641e3657. [4] M.A. Furman, M.T.F. Pivi, Probabilistic model for the simulation of secondary electron emission, Phys. Rev. St. Accel. Beams 5 (2002) 124404. [5] R.V. Latham, High Voltage Vacuum Insulation: Basic Concepts and Technological Practice, Elsevier, 1995. [6] J. Benford, J.A. Swegle, E. Schamiloglu, High Power Microwaves, CRC Press, 2015. [7] G. Shafir, M. Kreif, J.Z. Gleizer, S. Gleizer, Y.E. Krasik, A.V. Gunin, O.P. Kutenkov, V. PegeI, V.V. Rostov, Experimental research of different plasma cathodes for generation of high-current electron beams, J. Appl. Phys. 118 (2015) 193302. [8] Y.W. Fan, X.Y. Wang, Z.C. Zhang, et al., A high-efficiency repetitively pulsed magnetically insulated transmission line oscillator, Vacuum 128 (2016) 39e44. [9] T. Xun, H. Yang, J. Zhang, et al., Effects of vacuum pressures on the performance of a velvet cathode under repetitive high-current pulse discharges, Vacuum 85 (2) (2010) 322e326. [10] X. Zhao, T. Xun, L. Liu, et al., Maintenance of high vacuum level in a compact and lightweight sealed hard-tube magnetically insulated line oscillator system, Vacuum 111 (2015) 55e59. [11] T. Xun, X. Zhao, G. Li, et al., High-current, pulsed electron beam sources with SiC nanowire cathodes, Vacuum 125 (2015) 81e84. [12] T. Xun, H.-W. Yang, J.-D. Zhang, A high-vacuum high-electric-field pulsed power interface based on a ceramic insulator, IEEE Trans. Plasma. Sci. 43 (2015) 4130e4135. [13] T. Xun, H.-W. Yang, J.-D. Zhang, Z.-X. Liu, Y. Wang, Y.-S. Zhao, A ceramic radial insulation structure for a relativistic electron beam vacuum diode, Rev. Sci. Instrum. 79 (2008) 063303.
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