US3397083A

US3397083A - Insulator body having an electrically conductive surface and method

Info

Publication number: US3397083A
Application number: US448188A
Authority: US
Inventors: Milton E Poland
Original assignee: Champion Spark Plug Co
Current assignee: Federal Mogul Ignition LLC
Priority date: 1965-04-14
Filing date: 1965-04-14
Publication date: 1968-08-13
Anticipated expiration: 1985-08-13
Also published as: DE1590215A1; GB1155363A; DE1590215C3; DE1590215B2

Description

Aug. 13, 1968 M.'E. POLA INSULATOR BODY HAVING AN ELECTRICALLY CONDUCTIVE SURFACE AND METHOD Filed April 14, 1965 INVENTOR. MIL TEN E. PULAND.

BY QWJQMH ATT D's.

United States Patent "ice 3,397,083 INSULATOR BODY HAVING AN ELECTRICALLY CONDUCTIVE SURFACE AND METHOD Milton E. Poland, Royal Oak, Micl1., assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware Filed Apr. 14, 1965, Ser. No. 448,188 11 Claims. (Cl. 117-201) ABSTRACT OF THE DISCLOSURE A method of producing an electrically conductive surface on an insulator body consisting principally of alumina which includes the steps of applying to a surface of the insulator a coating of a copper oxide containing composition capable of forming an electrically semiconducting on the insulator, the composition including at least about 3 percent of an oxide of chromium, the amount of the oxide being sufficient to stabilize the surface resistance of the coating upon firing to temperatures above approximately 2400 F., and firing the insulator and coating to a temperature above approximately 2400 F., but not sufficiently high that the surface resistance of the coatin is appreciably higher than the surface resistance of the coating fired to maturation, and for a time sufficiently short that, adjacent the original insulator surface and immediately therebelow there is a copper rich electrically semiconductive region which is substantially devoid of free alumina. Also disclosed is an insulator body produced in accordance with the above method.

The present invention relates to insulator bodies having a surface layer of a semiconductive material thereon; and more particularly to spark producing devices having spaced-apart electrodes in contact with a semiconductive surface layer of an insulator body as occurs, for example, in jet igniters.

It has been common practice heretofore to use a semiconductive material between and in abutment with the electrodes of jet engine igniters to reduce the onset voltage that is necessary to induce a spark between the electrodes. In some instances, the prior art has used a disc of semiconductive material placed between and in abutment with the electrodes, while in others the prior art has used a thin coating, commonly called an engobe coating, applied to an electrical insulator body in such manner that a thin semiconductive layer of the engobe abuts and bridges the gap between the electrodes of the igniter. The insulator bodies used in substantially all instances have contained approximately 85 percent 1 to 95 percent of alumina, and have been fired into hard, erosion resistant bodies having but a few percent of voids therein. The engobe coatings which have been used heretofore have an oxide of either iron or copper as the electrically conductive ingredient thereof. These ingredients have usually been mixed with one or more other oxides. Perhaps the most commonly used material has been one comprising oxides of copper mixed with a minor percentage of either chromic oxide or ferric oxide or a mixture of both, and in some instances copper powder mixed with chromic oxide or ferric oxide and made into a slurry has been applied to the insulator body and fired in an oxidizing atmosphere to produce an oxide of the copper in situ.

semiconductive material when used to bridge the spark gap of an igniter will, of course, erode away. Discs of appreciable thickness can provide considerable material be- The terms percent and parts are used herein, and in the appended claims, to refer to percent and part by weight, unless otherwise indicated,

3,397,083 Patented Aug. 13, 1968 fore being eroded away by the spark but it has been found that the thickness of discs required to give long service life do not confine the fiow of electricity to the surface of the semiconductor and, therefore, require too high a current flow before sparking. It has also been found that prior art engobe coatings tend to be soft so that they erode away quickly without providing acceptable service life.

An object of the present invention is the provision of a new and improved insulator body having a semiconductive surface layer which when placed into abutment with spaced-apart electrodes is highly resistant to spark erosion.

Another object of the present invention is the provision of a new and improved insulator body of the abovedescribed type which includes a semiconductive layer at and just beneath the surface of a fired insulator body, and formed by diffusion of semiconductive oxides into the structure of the insulator body.

A further object of the present invention is the provision of a new and improved insulator body having a surface semiconductive layer of the above-described type wherein a minimum of the material of the original insulator body is diffused out of the insulator body during the time that the conductive materials are diffused into the surface of the insulator body.

A more specific object of the present invention is the provision of a new and improved insulator body having a semiconductive surface layer of the above-described type wherein the semiconductive material is a conductive aluminate formed in situ by reaction between the materials of which the insulator body is made and another oxide diffused into the surface of the insulator body.

A still more specific object of the present invention is the provision of a new and improved alumina insulator body in which cuprous oxide is diffused into the crystal lattice of the alumina adjacent the surface of the insulator body to form a dense aluminate without appreciable migration of the alumina out of the surface of. the alumina body.

Another object of the present invention is the provision of a new and improved method of making an insulator body having a semiconductive surface layer thereon and wherein the surface of an alumina insulator body is coated with a slip comprising a minor amount of alumina and a major amount of an oxide which will form an electrically conductive auminate with the alumina of the insulator body and the coated body is then heated to produce a dense electrically conductive aluminate in the insulator adjacent its surface.

Other objects and advantages of the invention will be apparent from the description which follows, reference being had to the accompanying drawing, in which the sole figure is a sectional view through a jet engine igniter embodying principles of the present invention.

Although the invention has utility in various types of applications wherein it is desired to make the surface of an insulator electrically conductive, it has particular advantages when embodied in jet engine igniters wherein the electrically conductive material is subject to severe mechanical and thermal shock conditions as well as corrosive atmospheres and alternately reducing and oxidizing conditions. In order that a better understanding of the invention can be had as quickly as possible, the invention will first be described in detail as embodied in a jet engine igniter, and thereafter some of the various modifications of which the invention is capable will be explained.

As above mentioned a preferred application of the present invention occurs in a jet engine igniter. The jet engine igniter shown in the drawing is designated by the numeral 11 and has a thin electrically semiconductive layer which bridges two electrodes to provide a path for the discharge of a high energy spark. The igniter 11 comprises a metal shell 12 threaded as at 13 for insertion into the combustion chamber, for example, of a jet engine, and threaded at 14 for reception of an ignition harness. A ceramic insulator 15 attached to a second ceramic insulalator 16 is sealed inside the metal shell 12, and a center electrode 17 is sealed inside the insulator 15. An electrically semiconductive layer produced according to the invention is provided on the nose end of the insulator 15 and is designated by the numeral 18. The layer 18 provides an electrical path interconnecting the nose end 19 of the center electrode 17 and a ground electrode 20 attached to the shell 12. When a comparatively low voltage charge is applied to the center electrode of the igniter 11, for example, from a condenser, a high energy electric discharge occurs along the surface of the semiconductive layer 18. The electrical resistance of the layer 18 will in general depend upon its thickness as well as the material from which it is made, and the resistance will decrease as the thickness of the layer increases. The thickness of the layer 18 can be controlled by the manner of production.

The present invention is concerned primarily with the composition of the semiconducting layer 18 as well as the method by which such layer is produced. Accordingly, one of the best known modes of producing this layer will be explained by means of the following example.

Example I A preferred embodiment of igniter was made using a previously fired ceramic insulator 15 containing approximately 92% of A1 0 fired to approximately 95% of theoretical density. A slip comprising the following materials was prepared by mixing with an equal weight of water and was brushed on the lower end surface of the insulator 15:

Percent Copper metal powder 70.2 Chromic oxide 7.8 Alumina 20.0 Pure kaolin 2.0

The insulator with the coating of the above-described slip was advanced through a furnace, the internal temperature of which was maintained at 2500 F. in a manner which allowed the insulator to remain in the hot zone of the furnace for ten minutes. The coated insulator was thereafter allowed to cool to room temperature, another coating of the above-described slip was brushed thereon and the re-coated insulator was refired for the same time and temperature described above. The coated insulator was again allowed to cool to room temperature, and a third coating of the slip was applied and fired in the same manner as were the two previously described coatings.

In a development program of the type involved in the invention of the above-described igniter, it is necessary that a great number of specimens be spark tested, and that a standard specimen and test arrangement be used for spark testing each insulator body having a semiconductive layer thereon. The test specimens which were used were tubular bodies 1% inch long and having an CD. of 0.355 inch and an ID. of approximately 0.100 inch. These insulator bodies were made of the same material as the insulator 15, above-described, and comprised approximately 92% of alumina. Slips of various materials were brushed onto the lower annular surface of the test insulators and were then fired at a predetermined temperature. Each insulator was cooled to room temperature, and second and third applications of the slip were usually applied as described above in Example I. Test specimens on which slips were applied and fired were placed in a sparking fixture having a headed central electrode, the head of which was spring biased against the end having the fired slip, and this assembly is then placed within a test casing having a shoulder which abutted the outside edge of the fired slip of the specimen. A gap of approximately 0.050 inch was provided between the head of the center electrode of the sparking fixture, and the edge of the shoulder of the casing body, and the test specimens installed with the fired slip end facing upwardly in a sparking machine. Ten drops per minute of a JP4 jet fuel was dripped onto the fired slip end of the specimen, and two sparks per second were produced across the fired slip end between the electrodes. These sparks were produced by discharging a IO-microfarad condenser charged with 2000 volts so that 20 joules of energy were dissipated during each spark.

Five test specimens prepared using the same slip described above in Example I and fired at 2500 F. gave an average service life of 57.7 hours. The firing of the abovementioned slips was done in an oxidizing atmosphere so that the copper metal Was converted to a copper oxide, which at approximately 2000 F. is primarily cuprous oxide. At the above firing temperature, cuprous oxide is believed to react with chromic oxide and alumina to form aluminates and chromates. An examination of the specimen after firing showed that very little of the slip remained, and that a black penetration band of cuprous oxide had penetrated the alumina insulator body to varying depths which in some instances was approximately inch. A small degree of swelling of the insulator body occurred and spectographic analysis showed that a large amount of cuprous oxide was present in the surface layer of the insulator in a form combined with alumina and chromium sesquioxide. The electrical resistance acnoss the electrodes of the test fixture varied between 30 and 40 thousand ohms as initially produced. The specimens were sparked in the above-described testing machine for seven hours at room temperature; the specimens were thereafter baked at 1000 F. to remove carbon deposits; new electrodes were fitted onto these test specimens; and the onset voltage was thereafter determined. A failure of the specimen was deemed to have occurred and the tests were discontinued when the onset voltage required to initiate sparking exceeded 1800 volts at a capacitance of 0.1 mfd.

The time and temperature at which an insulator body coated with a slip is fired determines the depth to which the cuprous oxide of the slip diffuses into the insulator and determines the proportion of the aluminate that is formed relative to the nonconductive material from which the insulator body is made. By controlling the time and temperature of firing, therefore, layers of various resistances can be produced, and if the insulator body coated With the slip is fired at a temperature below approximately 2400 F. an engobe coating is produced on the insulator body without appreciable diffusion of the euprous oxide into the insulator body. For most igniter applications it is desired to have a relatively high concentration of cuprous oxide in a thin dense layer adjacent the outer surface of the insulator, so that the surface will have a fairly high conductivity and yet be dense and erosion resistant. It appears that appreciable diffusion of the cuprous oxide into the insulator body to form aluminates and chromates does not occur below approximately 2400 F. Accordingly, upon firing below this temperature, an engobe coating on the surface of the insulator body is produced. Upon firing above this critical temperature, diffusion into the insulator body takes place at a rate which increases with temperature. A satisfactory rate of diifusion takes place at about 2500 F. and too high a rate of diffusion takes place above approximately 2700 F. Where an engobe coating is formed rather than an aluminate or chromate layer within the surface of the insulator body, a greatly reduced service life of the igniter is obtained. The engobe coatings above referred to are relatively porous and soft and have a service life only a fraction of that of an insulator body having a good dense layer of aluminates and chromates adjacent the surface of the insulator body. This was demonstrated by the following procedure, which was practiced for purposes of comparison, and not in accordance with the invention:

A slip, hereafter called Slip No. 2, comprising 93% of copper, 5% of chromic oxide, and 2% of kaolin was painted on an alumina test insulator of the above-described type comprising 92% of alumina, and the insulator body coated with the slip was thereafter fired at 2100 F. The insulator body was cooled, and the process was repeated to apply two further additions of the slip coating to form an engobe approximately 0.006 inch thick. Visual examination showed that the insulator had an engobe coating produced thereon with substantially no diffusion of the material from which the slip was made into the 1 insulator. This insulator body with the engobe so formed was tested in the manner above-described, and the inservice life for samples produced at any given time. In order that valid comparisons can be had, therefore, it is necessary to run control specimens fired at previously evaluated times and temperatures each time a series of test specimens is made. Table II gives the results of another series of tests of insulator bodies produced in the same manner described above for the specimens tested in Table I, but fired for the times and temperatures given in Table II. Table II indicates that improved results are obtained when the insulator bodies are fired for only approximately seven minutes, but at a temperature of 2700 Fahrenheit.

sulator body with the thus formed engobes had a service life of only approximately 6 hours.

Optimum sparking life is obtained when a highly dense concentration of electrically conductive cuprous chromates dispersed throughout aluminates are produced adjacent the surface of the insulator with .a minimum of diffusion of the insulator material out of the surface of the insulator. The temperature as shown above must be high enough to produce the aluminates and chromates, and the firing time must not be so long as to allow the oxide applied to the outer surface of the insulator body to be depleted by diffusion too far into the insulator to leave a porous region deficient in the aluminates and chromates. This is shown by the following tests. Numerous A1 0 insulators of the type above-described were coated with a slip comprising 88.3% of copper, 9.8% of chromic oxide, and 1.9% kaolin, hereafter designated Slip No. 3, and were heated for the time and temperature indicated in the following Table I. Each insulator was cooled, a second coating of slip was applied, and the insulator again fired at the same temperature and for the same time as was used to fire the first coating of slip. The process was repeated for a third time and the finished insulator body was then tested in the manner above-described. Although a considerable diiference in service life is obtained between specimens fired at some of the higher temperatures, a comparison of the average service life of insulators fired at the various times and temperatures is indicative of the service life to be expected for insulator bodies fired at any particular time and temperature.

TABLE I.-THREE APPLICATIONS AND FIRINGS Hours Sparking to Failure at Different Firing Temperature Firing Time Min 10.7 12. 2 20. 6 27. 5 35.0 67. 4 34. 2 11.3 12. 3 24. 0 41.9 39. 2 71. 0 52.1 15. 2 12.8 38.1 44. 7 42. 2 81. 6 65.0 17. 5 17. 6 23. 0 21. 1

Avg 15.5 15. 2 27. 6 38. 0 38. 8 73.3 50. 4

Min 14 0 7.0 13.8 23. 5 35. 5 15.2 9. 8 14 0 12. 0 20. 7 33.1 51. 4 42.3 15.0 25. 5 11.1 55. 7 36. 2 61. 2 76. 0 71.5

Avg 17. 8 10.0 30.1 30. 9 49. 4 44. 5 32. 1

50 Min 7.0 19. 6 18. 6 15. 5 37. 6 21.0 14. 0 21. 3 12. 1 7.0 14. 0 36. 0 77. 1 28. 0 10.7 12.3 21.0 (42. 0) 48. 2 57. 8 14. 0

Avg 13.0 14. 7 15.5 23. 8 40. 6 52.0 18. 7

It has been found that the consistency of the slip, the

atmosphere of the furnace and other variables affect the Other tests have shown that a single application and firing of a slip produces a service life approximately twothirds that achieved when three applications and firings of the slip are used to produce the insulator bodies.

In another series of tests it has been found to be beneficial to incorporate the same material from which the insulator is made in the copper containing material applied to the insulator body. Cuprous oxide is very fluid at 2500 F., and the viscosity of the material applied to the insulator is increased considerably by the addition of alumina. The addition of alumina and chromium sesquioxide, therefore, helps to hold the cuprous oxide in place to promote uniform penetration of the insulator body. In addition, alumina additions to the slip increases the density of materials remaining on the surface of the insulator body to increase its resistance to erosion.

Test specimens were produced by applying coatings of suitable slips to insulator bodies in the manner abovedescribed, firing at the temperatures indicated in Table III for ten minutes, and repeating the coating and firing steps for a total of three applications. The coatings were composed of Slip No. 3 and of Slip No. 3 plus varying amounts of alumina. The compositions used and the average service life in hours are presented in Table IH, below:

Slip No. 2, parts, plus alumina, 20 parts- 2, 500

Cupric oxide is stable in air at temperatures up to approximately 1026 C. at which temperature it dissociates to the cuprous state. Cuprous oxide is stable above 1026 C. and melts at 1235 C. Cupric oxide combines with A1 0 to form the spinel which is stable below approximately 900 C. Above approximately 900 C., a copper aluminate is formed having the formula CuAlO It appears, therefore, that the presence of A1 0 lowers the temperature at which transformation from cupric oxide to cuprous oxide takes place.

At temperatures above approximately 1165 C. a liquid forms which consists principally of Cu O. This liquid can be in equilibrium with C-uAlO depending upon the amount of A1 0 in the mixture. Above approximately 1260 C. the CuAlO breaks down to A1 0 and a copper oxide rich melt. It appears, therefore, that Cu O fluxes with the alumina of the insulator body and 7 quickly migrates beneath the surface of the insulator body.

Applicant has found that the presence of chromium sesquioxide when added to the material applied to the insulator bodies stabilizes the conductive phase which is formed so that the resistance of the body so formed does not change appreciably with use at a temperature of 1600 F. It appears that chromium sesquioxide has a strong affinity for cuprous oxide, probably forming CuCrO and that the large size of the chromium sesquioxide molecule, prevents the chromium sesquioxide from appreciable diffusion or migration into the alumina insulator body, and therefore holds the cuprous oxide in place. It may also be that chromium sesquioxide lowers the transition temperature of cupric oxide to cuprous oxide.

In any event, applicant has found that something more than an engobe is formed when copper or any form of copper which establishes equilibrium with oxygen at elevated temperatures are applied to an alumina insulator body and fired at temperatures between approximately 2500 F. and 2700 F. in an oxidizing atmosphere for a controlled short period of time which holds diffusion of cuprous oxide to within approximately 0.010 inch of the insulator surface and preferably to less than 0.005 inch of its surface. In addition, the presence of chromium sesquioxide has been found to prevent appreciable change in the resistance of the electrically conductive layer so formed at use temperatures above 1600 F. and even at temperatures approaching 2000 F.

To determine the effect of chromium and aluminum, six test specimens of the type above-described were made of the compositions given in Table IV. In each instance 2% of Kaolin was incorporated with the percentage of Cr O or A1 indicated in the table, and the balance was copper powder. Slips of these compositions were brushed in a band around the outer cylindrical surface of alumina insulator bodies of the type above described and the coated bodies were fired for ten minutes at the temperature indicated. A second coating of the same slip was applied over the first band and the coated body refired at the temperature indicated, after which a third coating was applied and fired in the same manner. The resistance was measured by clamping the treated surface f the specimens between flat plates and measuring the resistance therebetween with a 500 volt megger. The average resistance measured for the six test specimens coated with each mixture are given in Table IV.

TABLE IV. RESISTANCE AFTER THIRD FIRED COAT (K-OI-IMS) Firing Temperature Engobe Addition Pure Copper. 508 236 293 1, 188 7, 944 25, 721 176 158 168 444 10, 333 24, 610 149 170 111 101 3,000 8, 611 81 112 104 119 5, 611 28, 2'22 53 61 85 116 221 10, 222 17 18 9 15 21 33 8 9 9 10 20 18 4 4 4 l1 6 9 539 204 188 1,959 8, 389 30, 277 280 224 231 2, 131 11, 889 26, 833 346 208 172 454 6, 111 12, 167 213 158 142 1, 467 7, 500 l 28, 705 125 117 91 3, 551 9,333 30, 555 81 74 74 3, 889 12, S66 22, 333 30% Al:O 88 77 70 133 15,000 29, 722

1 Less than 6 specimens included in this average.

From the above and other tests it appears that chromium sesquioxide should comprise between 3 and 30% by weight of the solids applied to the insulators, preferably between and 20%, and for economic reasons most preferably about 10% by weight. These tests also indicate that alumina additions improve life of the resulting article. Improved results are had when the alumina comprises between approximately 10% and 50% by weight of the solids. It appears that no advantage is had in using more than approximately 30% by weight of alumina, and

optimum results appear to be had when the alumina comprises approximately 20% of the solid mixture. Minor percentages of other materials can be used without shifting the Cu O-Al O and Cu OCr O phase relationships to harmful degrees, and a considerable amount of materials which do not shift phase relationships can be incorporated without blocking the synergistic effect. Evidence of this is bad by the large amount of alumina which can be used in the solid materials applied to the alumina insulator bodies without greatly reducing the utility of the fired insulator. Although alumina is not inert, it is the same material as the material from which the insulator is made, and therefore, among other things acts as a diluent.

It will be apparent that various changes and modifications can be made from the specific details disclosed and described without departing from the spirit of the attached claims.

What I claim is:

1. A method of producing an electrically conductive surface on an insulator body consisting principally of alumina, said method comprising: bringing the surface of said insulator into contact with cuprous oxide at a temperature of at least about 2400 F., and controlling the time and temperature of contact between said cuprous oxide and said surface of said insulator body to provide an electrically conductive phase beneath its surface.

2. A method of producing an electrically conductive surface on an insulator body consisting principally of alumina, said method comprising: bringing the surface of said insulator into contact with a mixture of materials comprising a major percentage of cuprous oxide and a minor percentage of alumina at a temperature of at least about 2400 F., and controlling the time and temperature of contact between said mixture of cuprous oxide and alumina and said surface of said insulator body to change the surface layer of said insulator body into an electrically conductive layer comprising CuAlO and alumina.

3. A method of producing an electrically conductive surface on an insulator body consisting principally of alumina, said method comprising: bringing the surface of said insulator into contact with a mixture of materials comprising a major percentage of cuprous oxide and at least about 3 percent of chromium sesquioxide at a temperature of at least about 2400 F., and controlling the time and temperature of contact between said mixture of cuprous oxide and chromium sesquioxide and said surface of said insulator body to change the surface layer of said insulator body into an electrically conductive layer of reacted cuprous oxide, alumina and chromium sesquioxide.

4. A method of producing an electrically conductive surface on an insulator body consisting principally of alumina, said method comprising: bringing the surface of said insulator into contact with a mixture of materials comprising a major percentage of cuprous oxide, at least about 3 percent of chromium sesquioxide and a minor percentage of alumina, at a temperature of at least 2400 F., and controlling the time and temperature of contact between said mixture and said surface of said insulator body to change the surface layer of said insulator body into aluminates and chromates of cuprous oxide.

5. A method of producing an electrically conductive surface on an insulator body consisting principally of alumina, said method comprising: bringing the surface of said insulator into contact with a mixture consisting essentially of 3 to 30 percent by weight of chromium sesquioxide, 10 to 30 percent by weight of alumina and the balance copper in a form which establishes equilibrium with oxygen at elevated temperatures at a temperature of at least about 2400 F., and controlling the time and temperature of contact between said mixture and said insulator body to change the surface layer of said insulator body into chromates and aluminates of cuprous oxide without appreciable migration of the alumina out of said insulator body.

6. A method of producing an electrically conductive surface on an insulator body consisting principally of alumina, said method comprising: bringing the surface of said insulator into contact with a mixture consisting essentially of 3 to 30 percent by weight of chromium sesquioxide, to 30 percent by weight of alumina and the balance copper in a form which establishes equilibrium with oxygen at elevated temperatures, and heating said mixture and insulator body to a temperature between approximately 2400 F. and approximately 2700 F. for a period of time between approximately 10 minutes and approximately 6 minutes to change the surface layers of said insulator body into chromates and aluminates of cuprous oxide without appreciable migration of the alumina out of said insulator body.

7. A method of producing an electrically conductive surface On an insulator body consisting essentially of alumina, said method comprising: bringing the surface of said insulator into contact with material consisting essentially of approximately 90% by weight of copper in a form which establishes equilibrium with oxygen at elevated temperatures and approximately 10% by Weight of Cr O at a temperature of between approximately 2400 F. and approximately 2700 F. for between approximately ten minutes to approximately six minutes in an oxidizing atmosphere to produce an electrically semiconductive layer adjacent the surface of said insulator.

8. A method of producing an electrically conductive surface on an insulator body consisting essentially of alumina, said method comprising: bringing the surface of said insulator into contact with material consisting essentially of approximately 75% by weight of copper in a form which establishes equilibrium with oxygen at elevated temperatures, approximately 16.5% by weight of alumina, and approximately 8.5% by weight of chromium sesquioxide at a temperature above approximately 2400 F. for approximately ten minutes in an oxidizing atmosphere.

9. A method for producing an electrically conductive copper oxide containing surface region on an insulator consisting principally of alumina, said method comprising: applying to a surface of the insulator a coating of a copper oxide containing composition capable of forming an electrically semiconducting coating on the insulator, the composition including at least about 3 percent of an oxide of chromium, the amount of the oxide being sufficient to stabilize the surface resistance of the coating upon firing to temperatures above approximately 2400 F., and firing the insulator and coating to a temperature above approximately 2400 F., but not sufiiciently high that the surface resistance of the coating is appreciably higher than the surface resistance of the coating fired to maturation, and for a time sufiiciently short that, adjacent the origin-a1 insulator surface and immediately therebelow there is a copper rich electrically semiconductive region which is substantially devoid of free alumina.

10. A new and improved insulator body having an electrically conductive surface comprising a body of alumina fired to approximately of theoretical density, said body having a surface layer between 0.001 inch and 0.010 inch thick containing Cu O united with the alumina of said insulator body to form CuAlO therewith.

11. A new and improved insulator body having an electrically conductive surface comprising a body of alumina fired to approximately 95 of theoretical density, said body having a surface layer between 0.001 inch and 0.010 inch thick containing cuprous oxide and chromium sesquioxide united with the alumina of said insulator body to form a stabilized electrically conductive layer in said insulator body.

References Cited WILLIAM L. JARVIS, Primary Examiner.