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Vacuum Insulation of High Voltages Utilizing Dielectric Coated ElectrodesL. Jedynak Citation: Journal of Applied Physics 35, 1727 (1964); doi: 10.1063/1.1713728 View online: http://dx.doi.org/10.1063/1.1713728 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/35/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in New perspectives in vacuum high voltage insulation. II. Gas desorption J. Vac. Sci. Technol. A 16, 720 (1998); 10.1116/1.581052 New perspectives in vacuum high voltage insulation. I. The transition to field emission J. Vac. Sci. Technol. A 16, 707 (1998); 10.1116/1.581051 High-voltage vacuum insulation with epoxy-coated cathodes J. Vac. Sci. Technol. 11, 472 (1974); 10.1116/1.1318657 Insulation of High Voltage Across Solid Insulators in Vacuum J. Vac. Sci. Technol. 2, 234 (1965); 10.1116/1.1492433 The Insulation of High Voltages in Vacuum J. Appl. Phys. 18, 327 (1947); 10.1063/1.1697654
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JOURNAL OF APPLIED PHYSICS VOLUME 35, NUMBER 6 JUNE 1964
Vacuum Insulation of High Voltages Utilizing Dielectric Coated Electrodes* L. JEDYNAK
Electrical Engineering Department, University of Wisconsin, Madison, Wisconsin (Received 24 June 1963; in final form 5 December 1963)
An experimental research program has demonstrated that the high-voltage performance of a vaCuum gap can be improved by the simple expedient of coating the cathode surface with a suitable thin insulating film. Steady voltages ranging to 340 kV were obtained with 5-mm gaps composed of 15-cm-diam Rogowski electrodes. Simultaneously the average gap currents were suppressed to the 10-8 to l(}1O A range. Further, it was shown that an insulating film on the anode can be severely detrimental to gap performance. Experiments were performed, at voltages up to 380 kV, involving 12 different film materials. Film thicknesses ranged from 0.2 to 135 J1- and film resistivities ranged from Ion to 1019 n-cm. The dielectric constants were between 1.7 and 3.5, except for one at 90. Based upon the experimental results and their interpretation, a tentative set of specifications can be given for a good cathode film: (1) resistivity of at least lO" Q-cm; (2) dielectric constant in the range 1.5 to 4; (3) dielectric strength of at least 106 V/cm; (4) film thickness between 10 and 25 J1-; (5) mechanically hard and smooth with high abrasion resistance and high adhesion strength; (6) no gas bubbles within the film. If bubbles are unavoidable they must be substantially smaller than the film thickness; (7) low vapor pressure; (8) the cathode substrate should be of a highly polished metal suitable for quality deposition of the desired film material.
INTRODUCTION
M ANY vacuum-insulated devices are used under constant-potential conditions and utilize one or
more vacuum gaps, a vacuum gap being a pair of electrodes separated by a region of electrically insulating vacuum. The condition of vacuum insulation exists when the mean free path of electrons in the residual atmosphere is large compared to the interelectrode gap, resulting in negligible gaseous ionization. For the nonnal electrode spacings of physics and engineering, pressures below 10-4 Torr are generally adequate.
Frequently, the perfonnance of high-voltage, vacuuminsulated devices is limited by the failure of the electrical insulating strength of the vacuum gap. The failure of the vacuum gap may be evidenced in several ways. A stable but excessive current may flow, A transient discharge, dissipating part of the stored electrostatic energy, may take place. A complete breakdown may occur, its final phase being a spark or arc depending upon the resistance in the external circuit and the character-istics of the voltage source. .
The most commonly observed characteristics of vacuum breakdown are:
1. Total voltage effect. For electrode separations of less than 1 mm and breakdown voltages of less than 50 k V, the breakdown is essentially dependent upon the electric field intensity. For larger separations and higher voltages, breakdown becomes progressively more dependent upon the total voltage applied across the vacuum gap and less dependent upon field intensity.1-6
2. Prebreakdown currents. At voltages below complete breakdown there exist both a current and a transfer of electrode material through the vacuum separating the electrodes. The current may exhibit one or more of three forms: (1) a constant value dependent upon applied voltages, (2) a random fluctuation, and (3) pulses of microsecond duration and of varying amplitude. Measurement of prebreakdown currents provides a useful index of vacuum gap perfonnance, though the currents do not always act as a signal of approaching complete collapse of the vacuum insulation.1,2,7.8,9-19
3. Electrode dependence. The prebreakdown current and breakdown voltage are dependent upon the electrode material, surface roughness, surface impurities, and electrode geometry. 7,8,20--28
A number of theories have been developed during the last thirty-five or so years to explain the mechanism of
1 P. F. Browne, Proe. Phys. Soc. (I.ondon) B68, 564 (1955), 8 R. Arnal, Ann. Phys. Paris 10, 830 (1955). 9 H. G. Heard and E. J. Lauer, USAEC UCRL-1622 (1952). 10 D. Leader, Proc. lnst. Elec. Engrs. I.ondon 100 IIA, 3, 138
(1953). 11 A. Maitland, J. Appl. Phys. 32, 2399 (1961). 12 A. 1. Bennett, J. App!. Phys. 28, 1251 (1957). 13 W. H. Bennett, Phys. Rev. 37, 582 (1931). 14 S. Schwabe, Z. Angew. Phys. 12, 244 (1960). 16 R, Hawley and C. A. Walley, Nature 190,252 (1961). 16 L. V. Tarasova and A. A. Razin, Zh. Techn. Fiz. 29, 967
(1959) [English trans!.: Soviet Phys.-Tech. Phys. 4, 879 (196O)J. 11 L. I. Pivovar and V. I. Gordienko, Zh. Techn. Fiz. 28, 2289
(1958) [English trans!': Soviet Phys.--Tech. Phys. 3, 2101 (1958)J.
18 W. J. R. Calvert. Proc. Phys. Soc. (London) B69,651 (1956). 19 H. C. Bourne, Jr. J. App!. Phys. 26, 625 (1955). 2U A. S. Denholm, Can. J. Phys. 36, 476 (1958). 211. L. Sokol'skaya, Zh. Techn. Fiz. 26, 1177 (1956) [English
trans!.; Soviet Phys.-Tech. Phys. 26, 1177 (1956)]. * This paper is based on a thesis submitted in partial fulfillment 22 C. C. Chambers, J. Franklin lnst. 218, 463 (1934).
of the requirements of the degree of Doctor of Science in the 23 H. G. Heard, USAEC UCRL-1697 (1952). Department of Electrical Engineering, MIT (30 August 1962). 24 W. D. Kilpatrick, Rev. Sci. lnstr. 28, 284 (1957).
1 H. W. Anderson, Am. Inst. Elec. Engrs. 54, 1315 (1935). 25 W. D. Kilpatrick, USAEC UCRL-2321 (1953). 2 J. G. Trump and R. J. Van De Graaff, J. App!. Phys. 18, 327 261. N. Slivkov, Zh. Tekhn. Fiz. 27,2081 (1957) [English trans).:
(1947). Soviet Phys.-Tech. Phys. 2,1928 (1957)]. 3 P. H. Gleichauf, J. App!. Phys. 22, 535 (1951). 21 A. Maitland, J. App!. Phys. 33, 248 (1962). 4 L. Cranber~, J. Appl. Phys. 23, 518 (1952). 28 E. S. Borovik and B. P. Batrakov, Zh. Techn. Phys. 3, 1811 5 J. L. McKibben and R. K. Beuchamp, AECD-2039 (1948). (1958) [English trans!.: Soviet Phys.-Tech. Phys. 28, 1971 6 H. G. Heard, USAEC UCRL-2252 (1953). (1959)].
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1728 L. JEDYNAK
the initiation and growth of the vacuum spark. These theories can be divided into the following groups:
1. Field emission. The early work of Millikan et al.29
in 1928, Fowler and Nordheim30 in 1928, and others, resulted in a gradient-dependent theory for vacuum breakdown adequate for small gaps at low dc voltages and high gradients. This theory was based on the field emission phenomenon quantitatively described by the Fowler-Nordheim equation.
2. Field emission plus positive ions. This theory was advanced by Chambers22 in 1934 and later in a modified form by Boyle et al.31 in 1955. Basically it states that field emission electrons from the cathode bombard the anode producing positive ions. Breakdown occurs when the number and energy of the ions become great enough.
3. Field emission plus anode vapors. In this theory it was proposed that field emission electrons from the cathode bombard the anode producing vapors in which a gaseous discharge occurs. This mechanism was considered by Hull and Burger32 in 1928 and by othersll ,21,31-37 since. Maitlandll in 1961 mathematically elaborates on this theory and supports his work with experimental measuremen ts.
4. Multiple exchange of elementary particles. This theory, originally suggested by Van Atta, Van De Graaff, and Barton38 in 1933 and again by others,2,5,8,18,39 relates to an interchange of elementary particles between the cathode and anode due to secondary emission at each electrode. If the interchange process becomes cumulative breakdown ensues.
5. M icroparticle initiated breakdown. Suggested by Cranberg4 in 1952 and supported by others,2o,21,40-43 this theory states that breakdown occurs in vapors produced when a sufficiently energetic, charged microparticle collides with an electrode. It was suggested that the microparticles are composed of loosely attached particles on either electrode. Denholm20 in 1958 elaborates on this theory and suggests that the microparticles can also originate from the growth and fracture of a surface projection due to the large electric forces in the gap.
Variations and refinement of the above theories have been developed, but none has been proven to be the primary cause of breakdown. The work described in the
29 R. A. Millikan, C. F. Eyring, and S. S. Mckeown Phys. Rev. 31, 900 (1928).
30 R. H. Fowler and L. W. Nordheim. Proc. Roy. Soc. (London) A119, 173 (1928).
31 W. S. Boyle, P. Kisliuk, and L. H. Germer J. Appl. Phys. 26, 720 (1955).
32 A. W. Hull and E. E. Burger, Phys. Rev. 31, 1121 (1928). 33 L. B. Snoddy,Phys. Rev. 37, 1678 (1931). 34 J. W. Beams, Phys. Rev. 44, 803 (1933). 35 G. Schmidt, Acta. Physic. Acad. Sci. Hung. 9, 1 (1958). 36 W. J. Wijker, Appl. Sci. Res. Sec. B 9, 1 (1960). 37 J. A. Chiles, J. Appl. Phys. 8, 632 (1937). 38 L. C. Van Atta R. J. Van De Graaff, and H. A. Barton, Phys.
Rev. 43, 158 (1933). 39 R. Arnal, Compt. Rend. 240, 610 (1955). 40 See Ref. 26. 41 W. E. Meyer, Z. Angew. Phys. 13, 51 (1961). 42 G. A. Farrall, J. App!. Phys. 33, 96 (1962). 43 H. G. Heard and E. J. Lauer, USAEC, UCRL-2051 (1953).
L.l.....j inch ..
FIG. 1. Vacuum insulation test system.
following sections is related primarily to theories 1,2, and 3, in which field emission is the essential phenomenon.
Since a number of breakdown theories specify electronic field emission from the cathode as a significant factor in vacuum breakdown, it was proposed that coating the cathode ",ith a thin insulating film would reduce the field emission and thus lead to improved vacuum-gap performance. The work of L. Hershoff and R. W. Cloud in the High Voltage Research Laboratory at MIT from 1947 to 1950 demonstrated that some improvement could be obtained by coating the cathode with a thin insulating film. The work reported in this paper, conducted from 1960 to 1962 in the same laboratory, supports this early evidence and expands upon it.
EQUIPMENT
The equipment used in the research is shown in Fig. 1 and described below.
1. Electrodes. The 1S-cm-diam electrodes (1 in Fig. 1) were of the Rogowski shape, for which the electric field intensity is uniform and maximum in the center and becomes gradually less toward the periphery. They were formed from 1.6-mm-thick 2S-0 aluminum by the metal spinning process. Several experiments were performed with spun electrodes of 0.63-mm-thick No. 347 stainless steel. In general, the electrodes were buffed to a mirror finish before receiving the special treatment prescribed by each experiment.
2. High-voltage generator. The high-voltage generator (2 in Fig. 1) was a 500-kV Van de Graaff generator of special design using stabilized corona belt-charging of
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VACUUM INSULATION OF HIGH VOLTAGES 1729
35 p,A capacity. Its column and HV tenninal were inserted directly into the vacuum chamber. This eliminated the troublesome high-voltage bushing, as well as external high-voltage equipment.
Measurement of high voltage was accomplished with a calibrated high-voltage resistor, situated within the generator column, and a precision microammeter. Accuracy of the measurement was within ±5%. By subsequent calibration of the displacement charge of the vacuum gap, for various gap settings, it was possible to recheck the high voltage resistor's calibration at frequent intervals, thus insuring that any change in the resistor's characteristics would be quickly detected.
3. Cathode electrode assemhly. The cathode electrode assembly (3 in Fig. 1) was insulated from the main chamber by a ring of insulating glass. The use of flexible bellows and spacers provided a finely adjustable highvoltage working space ranging in length up to a maximum of 18 em.
4. Vacuum chamber. The 50-em-diam 68-cm-long cylindrical vacuum chamber (4 in Fig. 1) was constructed almost entirely of buffed aluminum. All vacuum seals were of compressed 1.6-mm-diam annealed indium wire. During experimentation the vacuum was monitored by a Bayard-Alpert ionization gauge and maintained between 5X 10-7 and 10-6 Torr by parallel operation of a mercury diffusion pump and a barium getter pump.
5. Gap measurements. Gap current and charge transfer measurements were made with an electronic ammeter and current integrator circuit. The current scales were 0.2-2-20-200 p,A. Full scale reading of the current integrator corresponded to 2 P,c. Average currents as low as 2X 10-10 A could be estimated in 1 min using the integrator circuit. Current pulses involving as little as 10-2 /-tC were detectable.
EXPERIMENTAL RESULTS
The vacuum breakdown experiments were done in groups designed to test the effect of specific parameters, such as film material, film thickness, and substrate preparation. The experimental results of each group are displayed graphically in the following sections.
0 ~ -- 0
i1 ~ ~ ~
1: /
---~
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.~--- -=::--:-
,..;,.l ],{ JJ" --~:: ~lJ-" !--" ~-
o 40 80 120 160 200 240 280 NUMBER OF SPARKS
I I I 1
-11
j
FIG. 2. Group 1. MgF2 film on cathode-effect of film thickness. 0, a-voltage and current, respectively, for 0.2 J< film. 0, .-3.5 micron film. A, .... -lO-p film. X, x-bare aluminum cathode. All anodes of bare aluminum. All gaps at 6.3 mm.
Wherever appropriate, the curves are accompanied by a performance curve of two highly polished aluminum electrodes as a standard of comparison. Most of the data are displayed in the form of spark-conditioning curves in which the maximum steady voltage and corresponding gap current are plotted as a function of the number of sparks preceding the measurement. The remaining data are presented in the form of maximum steady voltage as a function of electrode separation.
The results of a typical experimental group are shown in Fig. 2. The solid curves indicate the maximum steady voltage which could be held for at least 5 min. The dashed curves indicate the average gap current at the corresponding maximum steady voltage. The absicissa gives the total number of sparks which occurred prior to the measurement. The data points represent actual 5-min or longer measurements. The portions of the curves between data points, especially for the current curves, represent shorter duration measurements.
A sparking, or breakdown, voltage curve lies above the steady voltage curve and is not shown. The general procedure was to raise the voltage in 10-k V steps at 5-min intervals until the first spark occurred. The electrodes were then allowed to spark-condition until the voltage could be held for 5 min without a spark. Frequently a gap would continue to spark once or twice during a 5-min interval. A dozen or so rapid sparks at a slightly higher voltage would often condition the electrodes so that the gap would then hold the original voltage for 5 min. Subsequently the voltage would be raised another 10 kV and the process repeated until no further improvement could be attained.
The performance curves of two bare aluminum electrodes, used as the standard of comparison, were obtained using two highly polished solvent-cleaned spun aluminum electrodes with a 6.3-mm separation. The maximum steady voltage obtained with these electrodes was 4% below that shown for steel electrodes by Trump and Van de Graal12
Group 1: MgF 2 Film on Cathode. Effect of Film Thickness
This group was the first to be processed and was designed to test the concept that an insulating film on the cathode would suppress electron field emission. Film thickness was chosen as an independent parameter. Magnesium flouride was vapor deposited on highly polished spun aluminum electrodes in film thickness of 0.2, 3.5, and 10 /-t. de measurements on the films indicated a film resistivity of about 1013 g-cm. The vacuum gap data using these electrodes as cathodes, with spun aluminum anodes, are given in Fig. 2. Although the voltage increase was not outstanding, the average gap current was decreased by 2 to 4 orders of magnitude below that of a pure aluminum cathode. No apparent dependence on film thickness, in this range of thicknesses, is evident from these data. The significant
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1730 L. JEDYNAK
,------- ----- --280 f--- ---
240 -- # ~
20C -- ~ ~ /' 160 ----
/" ~W 120
j:2 ~ 80 ."
4C --
o 2 3 4 5 6 7 GAP (mm)
FIG. 3. Group 2. MgF2 film on cathode-maximum steady voltage vs gap. D-3.5-~ film. b.-l0-~ film .• -18-~ film. 129-permanent damage to cathode.
reduction of average gap current indicates that the film is acting to suppress the charge transfer mechanisms of the vacuum gap.
Group 2: MgF2 Film on Cathode. Maximum Steady Voltage vs Gap
The purpose of this group was to evaluate the dependence of maximum steady voltage on electrode separation, with film thickness as an independent parameter. As is evident in Fig. 3, there was no apparent dependence on film thickness. Further, no simple dependence on gap length was observed. The permanent damage to the cathodes sustained at small gaps was in the form of transfer of anode material to the cathode during a spark.
Group 3: MgF 2 Film on Stainless Steel Substrate. Aluminum and Stainless Steel Anodes
Vapor deposition of MgF2 on aluminum resulted in a film with a crazed, or crystalline, appearance while a glass-like film was obtainable on stainless steel. A 2.5-J.I. MgF2 film was deposited on a highly polished 15-cmdiam spun electrode of 0.63-mm-thick No. 347 stainless steel. The results obtained with this electrode as the cathode, with aluminum and stainless steel anodes, are
o 100 200 300 400 NUMBER OF SPARKS
rIG. 4. Group 3. Thin (2.5-~) MgF2 film on stainless steel cathode-aluminum and stainless steel anodes. 0, .-voltage and current, respectively, for MgF2 coated stainless steel cathode with stainless steel anode. 5-mm gap. 0, .-MgF2 coated stainless steel cathode with bare aluminum anode. 5-mm gap. 6, Ao.-MgF2
coated aluminum cathode with bare aluminum anode. 5-mm gap. x, x-Bare aluminum cathode and anode. 6.3-mm gap.
shown in Fig. 4. Note the change in gap length from 6.3 to 5 mm in this experimental group. Although no significant change in maximum steady voltage was obtained over that for MgF 2 on aluminum, the average gap currents were much more dependable suppressed, as comparsion between Figs. 2 and 4 shows.
The curves representing MgF 2 on aluminum in Fig. 4 correspond to the retesting of a cathode which had been spark conditioned in a prior experiment and then removed from the vacuum. As is evident from these MgF 2-on-aluminum curves, the removal of the electrode from the vacuum and then the later re-insertion did not alter the prior spark conditioning of the electrode insofar as gap currents were concerned. One explanation for this would be that the initial erratic behavior of the gap current is the result of film or substrate imperfections which are modified by sparks, and not due to loosely adhering particles. This also explains the more uniform current supression obtained with the glass-like MgF 2 film on stainless steel.
Figure 5 shows the voltage vs gap curve for the MgF2
2 80 ,...---<
---" ~
/
~ ~
2 / ~ I---4 ~
'"
a 2 3 4 5 6 7 8 GAP (mm)
FIG. 5. Voltage vs gap for coated cathodes.·o-MgF2 on stainless steel with bare stainless steel anode .• -MgF2 on aluminum with bare aluminum anode. D-silicon monoxide on aluminum with bare aluminum anode. iZI-permanent damage to cathode.
coated stainless steel cathode and stainless steel anode in relation to curves for MgF 2 on aluminum and SiO on aluminum. The SiO coated cathode is discussed in detail later. Note that the maximum voltages obtained with the MgF2 stainless steel cathode and the SiO coated cathode show less dependence on the gap length than does the voltage for the MgF2 aluminum cathode. Both of these cathodes were characterized by glass-like insulator films. Permanent damage was again in the form of transfer of anode material to the cathode.
Group 4: MgF2 Film on Cathode, Anode, or Both
These experiments were performed to evaluate the dependence of gap performance on the polarity of the insulator-coated electrode. Figure 6 shows that a MgF2 insulating film on the anode is detrimental to good gap performance. Further, the eventual deterioration of performance with coated anodes is an anode effect. This was shown by retesting the cathode of the coated
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VACUUM INSULATION OF HIGH VOLTAGES 1731
electrode pair with a plain aluminum anode. The results of this test are shown by the curve corresponding to a bare aluminum anode. At the end of this test the coated cathode had been sparked approximately 850 times but was still able to sustain 230 kV with less than)0-9 A in a 5-mm gap.
During the experiment with the coated anode and bare aluminum cathode it was observed that the MgF2-
coated anode emitted a bluish light under electron bombardment from the cathode. One or more bright points of light were observed, in addition to large areas of luminescence. These point glows were observed to disappear and appear elsewhere without being accompanied by sparks. These observations indicate that the electron current from a bare aluminum cathode consists of one or more dense electron beams plus a diffuse flow of electrons. In contrast to the bare aluminum cathode a coated cathode resulted in no discemable light emission from the coated anode except at the instant of a spark.
a.. .. 40 '"
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16
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, , ~ 8 , , ~
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40 80 120 160 200 o NUMBER OF SPARKS
Fro. 6. Group 4. MgF2 film on cathode and/or anode. 0, .voltage and current, respectively, for MgF2 film on aluminum anode with bare aluminum cathode. 0, .-MgF. film on aluminum cathode and on aluminum anode. x, x-MgF. film on aluminum cathode with bare aluminum anode. b., A-Both cathode and anode of bare aluminum. All gaps at 5 mm except bare aluminum gap at 6.3 mm.
Group 5: Thick (O.13-mm) Epoxy Film on Cathode. Effect of Substrate Roughness
It was shown earlier that an insulating film on a highly polished cathode will improve vacuum-gap performance. This e~erimental group was devised to evaluate the ability of a relatively thick film to shield a rough cathode substrate. The material used was Epon 826 epoxy with a resistivity of about 1014 Q-cm. The film thickness was 0.13 mm. Figure 7 shows the results of this experimental group in which one of the aluminum substrates was highly polished, another was etched with NaOH to a velvet finish, and the last was abraided with 200-grit emory. The small gap-currents shown in the figure demonstrate that the thick insulating film was able to compensate for the substrate roughness. Further, it was possible to obtain a remarkable 340 kV across a 5-111111 gap with an average current of only 10-9 A. This is far superior to the 220 kV and 10- 5 A obtained with a bare aluminum cathode in a 6.3-m111 gap.
The permanent damage sustained by each cathode
Fro. 7. Group 5. Thick (0.13 mm) epoxy film on cathode-effect of metal surface roughness. 0, e-voltage and current, respectively, for epoxy film on buff-finished aluminum cathode. b., A-epoxy film on etched aluminum cathode. 0, .-epoxy film on 200 grit emery-finished aluminum cathode. x, X-bare aluminum electrodes. @-permanent damage to cathode. Epoxy used was Epon 826 with Versimid 140. All gaps 5 mm except 6.3-rnm bare aluminum gap.
was in the form of detachment of a disk of epoxy at a spark site. These disks were generally about 0.8 mm or more in diameter and exposed an area of bare aluminum which was splattered with molten metal. Subsequent sparks always occurred at these exposed metallic sites and required substantially reduced voltages to initiate them. It was further observed that, in contrast with bare aluminum and MgF 2 coated cathodes, virtually no spark damage occurred to anodes which were used with cathodes having thick epoxy films.
Group 6: Comparison of Three Epoxy Films
Three different epoxies were tested to evaluate their performances in a vacuum gap. The films were approximately 25 p. thick and applied to highly polished spun aluminum electrodes. Figure 8 shows the results of these tests. The 1016 Q-cm C-26 epoxy twice demonstrated that its 5-mm vacuum gap could reach 300 kV without a single spark. As remarkable as this may be it was further able to condition to its maximum voltage of 340 kVin about a dozen sparks. With subsequent spark-
I" ~ ~~ u.-... r-2"-
2"" ~ r-_ f ~~
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~"""-""'- ~ 120
, [S":~--- -~""-""",-
: ~--- ------- .. I 40 ' _ -.-o 100 200 300 400
NUMBER OF SPARKS
FIG. 8. Group 6. Thin (0.025 mm) epoxy fIlm on polished aluminum cathode-comparison of three epoxies. 0, .-voltage and current, respectively, for Eccocoat C-26. b., A-Epon 826 with Versimid 140 (toluene thinned). 0, .-Eccocoat VE. x, X-bare aluminum cathode. @-Permanent damage to cathode. All gaps at 5 mm except 6.3-mm bare aluminum gap.
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1732 L. JEDYNAK
ing at slightly above 340 kV its current at 340 kV gradually decreased from a maximum of only 10-7 A to approximately 5X 10-10 A. Continued sparking eventually led to a deterioration of performance and finally to complete failure. In this case, the permanent damage to the cathode was not a simple detachment of an epoxy disk. It was not possible to determine which of many spark punctures of the film was responsible for the ultimate failure of the cathode. The 1014 Q-cm Epon 826 epoxy was also superior to bare aluminum but failed with high voltage sparking by detachment of an epoxy disk, as did the thicker epoxy films of Group 5. The 1012 Q-cm VE epoxy was a soft and gassy material not suitable for high-vacuum work, but it did perform better than bare aluminum. However, it required considerable spark conditioning before it surpassed bare aluminum in performance.
Group 7; Comparison of Cathode Films
A number of film materials were tested to evaluate their performances in vacuum gaps. These materials were: silicon monoxide, Mylar tape, Formvar, titanium dioxide, cerium oxide, iron oxide, and tin oxide. Of these materials only the first four provided an improvement
80
~
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~ ~
2C r ... --- I , , --- , I
8 '''' , I
4C~~~~ '" I ,
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I
o 100 200 300 400 NUMBER OF SPARKS
FIG. 9. Group 7. Comparison of cathode films. £:,., ... -voltage and current, respectively, for 3-J.I film of SiO on polished aluminum cathode. 0, .-2.S-J.l-thick Mylar tape. 0, .-2-J.I film of Formvar. x, x-bare aluminum electrodes. All anodes of polished aluminum. All gaps at 5 mm except 6.3-mm bare aluminum gap.
over bare aluminum. The others resulted in performances inferior to that of a bare aluminum cathode.
Silicon monoxide, with a measured resistivity of about 5X 1013 Q-cm, resulted in the most unusual vacuum-gap performance. Figure 9 shows the voltage and current curves for a 5-mm gap. The SiO cathode conditioned quickly to its peak voltage of 260 kV with a uniform suppression of current to less than 10-9 A. The peak voltage was dearly defined in that a few kV increase would cause both a very rapid rise in current and violent, but nondestructive, sparking. In Fig. 5 is shown the voltage vs gap curve obtained with this cathode. A variation in gap length from a minimum of 3 mm to a maximum of 8 mm resulted in a voltage variation of only 25%. Further, the data taken over this gap range was repeatable at any gap length until permanent
damage occurred at 3 mm. This damage was in the form of transfer of anode material to the cathode.
Figure 9 also shows the gap performances obtained with a Formvar coated cathode and a cathode covered with overlapping 1.3-cm-wide strips of 0.025-mm Mylar tape having a resistivity of about 1019 Q-cm. Not shown is the gap performance of a O.13-mm-thick titanium dioxide film on a cathode. The Ti02 film had a resistivity greater than 1010 Q-cm and a dielectric constant of about 90. All of the other materials had dielectric constants of 4 or less. The Ti02 5-mm gap conditioned from 210 kV to 280 kV with only 4 sparks. Permanent damage in the form of film erosion occurred at 280 kV. The current rose to 10-10 A at 240 kV and reached 10-7 A at 280 kV prior to permanent damage.
DISCUSSION
The experimental results given in the preceding section have demonstrated that a thin insulating film on a metallic cathode can provide vacuum gap performance superior to that of a bare metallic cathode. The improved performance was evidenced by increases of up to 70% in breakdown voltage for 5-mm gaps, accompanied by decreases in pre breakdown currents of 2 to 4 orders of magnitude. Although the 70% increase in breakdown voltage is unusual, its importance is overshadowed by the great reduction in prebreakdown current. In applications where gap currents must be kept to a minimum, the insulating film may be of significant value.
It is perhaps worthwhile at this point to suggest possible explanations for the observed behavior of gaps having cathode films. The substantial decrease in prebreakdown current may be due to a reduction of cathode field emission currents. Although the potential barrier at an insulator's surface is similar to that for a metal,44 the small number of conduction electrons in an insulator as compared to a metal corresponds to a greatly reduced supply of electrons available for field emission. Thus field emission currents from an insulator or semiconductor can be well below that obtained from a metal. For example, the current density from a 4-eV work function metal would be about 10-0 .9 Ajcm2 lfor an applied field of 2X 107 V jcm, while the current density from a hypothetical 4-e V work function semiconductor, similar to silicon carbide, was shown by Stratton44 to be about 10-10 Ajcm2 for the same applied field. However, Stratton points out that the presence of surface energy states is highly significant in causing this low emission and that at 3X 107 V jcm the current, neglecting bulk resistivity effects, could approach that obtainable from a metal. The field intensity for which this transition from low to high current densities occurs is a strong function of the semiconductor's surface properties. Field emission from an insulating film is somewhat more complicated than for a semiconductor or metal due to the
44 R. Stratton, Proc. Phys. Soc. (London) B68, 746 (1955).
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VACUUM iNSULATION OF HIGH VOLTAGES 1733
high volume resistivity which limits the rate of charge flow to the field emitting surface.
Gap breakdown at the moderately increased breakdown voltage can be attributed to an initial intrinsic dielectric breakdown of the film leading eventually to a full gap breakdown. This initial dielectric breakdown could occur at field intensification points situated at either the metal-film interface or at the film's surface. If the dielectric constant of the film is small, then the field penetrating the film prior to establishment of a surface charge and intensified by a projection of the metal substrate may exceed the dielectric strength of the material. In this event a dielectric breakdown would occur, inserting large quantities of vapors and charged particles into the gap and causing a full gap breakdown. Projections on the surface of the dielectric film could also intensify the field to beyond the dielectric strength of the material resulting in breakdown. Even after the film surface reaches its equilibrium charge density; this could be minutes or hours depending on the relaxation time of the material; field emission currents from a surface projection could produce sufficient resistive heating to initiate a breakdown sequence. It is of interest to note that a number of epoxy films revealed Lichtenberg discharge figures on their surfaces. It is apparent that during the gap discharge sequence the charge trapped on the film's surface unloads into the primary site of instability by means of a transverse, or surface, discharge. In one instance the center of a Lichtenberg figure was occupied by a piece of lint partially imbedded in the epoxy film. No other spark damage was evident in this area of the electrode.
Epoxy films also provided information on the effect of gas bubbles within a cathode film. It was invariably observed that wherever the size of the gas bubble was comparable to the thickness of the film, the bubble was punctured by a discharge. Small isolated bubbles, of which there were many in several instances, seldom showed evidence of spark damage. However, clusters of small bubbles were clearly involved in interbubble discharges. These discharges occasionally involved the substrate metal as well. It is likely that bubble discharges are due to internal gas discharges which subsequently trigger volume discharges in the dielectric.
An examination of the electrodes after each experiment revealed that electrode spark damage was a function of cathode film thickness. For films of thickness less than several microns the spark damage to both electrodes was similar to that encountered with bare electrodes. For thicker films the anode damage became progressively less, until, at a film thickness of 0.005 in., there was no anode damage observable at magnifications of up to 90. At the same time the cathode damage became progressively more severe as the film thickness was increased.
Based on the collection and interpretation of the experimental data, and giving consideration to normal vacuum criteria, the following initial set of specifications can be written for a good cathode film.
(1) Resistivity of at least 1011 f.l-cm, (2) dielectric constants in the range of 1.5 to 4, (3) dielectric strength of at least 106 V / cm, (4) film thickness between 10 and 25/-1, (5) mechanically hard and smooth with high abrasion resistance and high adhesion strength, (6) no gas bubbles within the film. If bubbles are unavoidable then they must be substantially smaller than the film thickness, (7) low vapor pressure and low moisture absorption, (8) chemically resistant to water and solvents, and (9) the cathode substrate should be brought to a mirror polish before coating. The choice of metal should be dictated by film deposition requirements and normal vacuum criteria.
It is emphasized that the above specifications are tentative in that they are based on a limited set of experiments. For example, the minimum resistivity of lOll f.l-cm represents the lowest resistivity material which has been adequately tested to date, thus it well may be that dielectric materials with substantially lower resistivities will also improve gap performance. Also, the range of dielectric constants specified above represents the approximate range of values encountered in the experiments, and therefore are known to be acceptable.
ACKNOWLEDGMENTS
The author wishes to thank Professor John G. Trump of the High Voltage Research Laboratory at the Massachusetts Institute of Technology for suggesting and supervising this research project. The National Science Foundation is gratefully acknowledged for its support of the research.
ADDENDUM
A recent paper by Little and Whitney45 describes an excellent experimental sequence which has revealed that even on optically polished electrodes there occur slender "hairs" which are capable of intensifying the applied field by a factor of 100. They have shown that prebreakdown currents can, and do, originate from these points at gross field strengths of only 105 V / cm. These authors also used thin films of Teflon and aluminum oxide and report that the voltage required to obtain prebreakdown currents was substantially raised. The experimental results reported by this author are consistent with the results obtained by the above authors.
45 R. P. Little and W. T. Whitney, NRL-5944 (1963).
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