CA1080590A - Electrically semi-conducting ceramic body - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A fired, electrically semi-conducting ceramic body is disclosed.
The body consists essentially of silicon carbide particles having a median particle size from about 3 to about 25 microns dispersed in a bonding matrix.
The silicon carbode particles constitute from about 50 to about 70 percent of the body. The body has an "apparent porosity" as subsequently defined herein from 20 to 40. The production of such a body from a batch composed of silicon carbide, alumina, silica and at least one oxide, carbonate or hydroxide of calcium, magnesium, barium or strontium is also disclosed.
The batch is milled and pressed into a shape, and the resulting shape is then fired in an inert gas atmosphere to a temperature from about 2300 to 2800°F for a time sufficiently long to produce a semi-conductor body having the requisite apparent porosity of from 20 to 40 percent. These ceramic bodies find use in high energy igniters, suitable for use in jet engines.

Description

The terms "percent~ and "parts" are used herein to refer to per-cent and parts by weight, unless otherwise indicated.
The term "apparent porosity" is used herein to refer to volume percent open porosity of a body.
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, 108()590 BACKGROUND OF THE INVENTION

2 This invention relates to a method for producing a

3 semi-conductor body useful in a jet engine igniter of the

4 high energy t~pe. In service such an igniter is fired by a capacitor discharge ignition system. The semi-conductor ~ body is incorporated in the high energy igniter so that a 7 portion of a surface thereof is adjacent a spark gap between 8 a center electrode and a ground electrode. It has been g found that a semi-conductor, so positioned, reduces the voltage required to cause a spark discharge, by comparison 11 with an igniter where there is an alumina insulator in 12 this position. Although the mechanism by which a semi-13 conducting body operates to reduce the voltage requirement 14 is not fully understood, two theories have been proposed.
These theories are stated below, but the statement lff should in no way be construed as a limitation on the 17 scope of this invention.

19 One theory suggests that when a voltage is applied to the center electrode, there is a limited flow of cur-21 rent along the semi-conductor surface. This current 22 flow causes ionization of gas in the spark gap. The 23 ionization enables a spark discharge to occur at a lower 24 voltage than would be required without the ionization.
26 Another theory suggests that because a small space 27 of about 0.0005 inch exists between the center electrode 28 and the semi-conducting body, electrical charges of 29 opposite polarity build up on the surfaces of the center .

108~5910 1 electrode and of the semi-conductor, as in the polariza-2 tion of opposing faces of a capacitor. Ionization of 3 gas in this small space or microgap within the igniter 4 gap enables an initial spark discharge at a low applied voltage. This partial discharge is believed to cause a 6 cascade ionization and discharge across the main gap.

8 In either case, discharge of the previousIy g charged capacitor occurs when there is a spark between the ground and center electrodes. The large size of the 11 capacitor is responsible for the high energy nature of 12 the spark.

14 An extension of the second thqory proposes that the porosity of the semi-conductor surface assists the 1~ cascade process by providing a series of microgaps be-q tween conducting silicon carbide grains which may become 18 charged, ionized and discharged in rapid s7~ccession. The 19 presence of a non-conducting phase such as alumina serves not only to bond the conducting grains of silicon carbide, 21 but also to prevent a direct short circuit. Controlled 22 spacing and contact of the silicon carbide grains is ob-23 tained by means of the porosity and the alumina as well 24 as the grain size and amount of silicon carbide.

1~80590 1 Various electrically semi-con~ucting ceramic 2 bodies have heretofore been suggested and used in igniters 3 for low voltage ignition systems. The prlor art has 4 emphasized, insofar as semi-conductors containing silicon carbide are concerned, such semi-conductors having a 6 crystalline bonding phase. For example, U.S. Patent No.
7 3,558,959 discloses alumina and silicon carbide semi-8 conductors having a crystalline bond produced by hot 9 pressing the alumina and silicon carbide. U.S. Patent Nos.
.10 3,376,367 and 3,573,231 disclose the production of 11 crystalline bonded semi-con2uctors from silicon carbide 12 - and aluminum silicate or the li~e by.forming an article 13 of the desired shape, iring in air to achieve a controlled 14 oxidation of silicon carbide to silica, embedding the 1~ article in a mass o~ silicon carbide particles, and firing 16 the article while so embedded. Alternatively, the aluminum .
17 silicate can be a part of the batch from which t~e original 18 shape is formed. In either case the bonding phase is a 19 crystalline aluminum silicate or the like.
The patents described above disclose high 21 energy igniters containing silicon carbide and a bonding .22 matrix which is essentially a crystalline phase~ While 23 these crystalline bonded igniters perform well under service 24 cor.ditions, several advantages are.obtained with a bonding matrix that is a glass. With such a matrix a lower iring , 26 temperature can be used and the porosity of the fired 27 semi-conductor can be controlled more effecti~ely to obtain~ .
28 an increased open porosity, and decreased spark erosio~ rate.
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108~)590 A silicon carbide semi-conductor having a glassy bon~ing phase can be produced from a shape of a particular composition by a two-step f~ring procedure. The shape is first fired in air to reduce the size of the silicon carbide and to introduce SiO2, and is then fired in an inert atmosphere. An igniter including a silicon carbide semi-conductor having a glassy bonding phase has been produced commercially since 1973. Ihe semi-conductor composition, after firing, was 30.0 percent SiO2, 9.0 per-cent A1203, 7.2 percent CaO, 1.8 percent MgO and 52 percent SiC. m eglassy bonding phase of this semi-conductor contained 62.5 percent SiO2, 18.8 percent A1203 and 18.8 percent CaO plus MgO. The production of semi-conductors by the two-step firing procedure is described in detail in our copending application Serial No. 185,374, filed November 8, 1973; it can be used to produce semi-conductors where the conposition of the glassy bonding phase, after firing, is from 48.8 to 71.5 percent SiO2; from 13.6 to 32.7 percent A1203; and from 9.1 to 30.1 percent CaO and MgO.
An improved semi-conductor having a substantially reduced erosion rate when sparked at a pressure of 400 psi has now been discovered.
The improved semi-conductor has an apparent porosity of 20 to 40 and con-sists essentially of silicon carbide particles dispersed in a bonding 20 matrix; it can be produced by a one-step firing in an inert atmosphere, by the two-step procedure discussed in the preceding paragraph, and, perhaps, by hot pressing. The method, in any case, involves careful control of the apparent porosity , 10805~0 of the semi-conductor body; the composition of the bonding matrix appears to be of only minor importance.
The instant invention is based upon the discovery of an improved silicon carbide semi-conductor having an apparent porosity ~rom 20 to 40, and consisting essentially of silicon carbide particles dispersed in a bonding matrix. The matrix, in a preferred embodiment, constitutes from 55 to 70 parts of the body, and consists essentially Or from 10 to 70 percent of at least one oxide of calcium, magnesium, barium or strontium, from 20 to 75 percent SiO2 and from 10 to 40 percent A12O3.
The silicon carbide particles have a median particle size from about 3 to about 25 microns. The preferred semi-conductor can be produced by milling a mixture of appropriate composition, forming the milled mixture into a shape, and bringing the shape in an inert gas atmosphere to a temperature from 2300 to 2800F for a period of time sufficient to produce a body having the requisite apparent porosity of from 20 to 40 ~ `
percent. As indicated above, an electrically semi-conducting body according to the invention has improved resistance to erosion when in-corporated in an igniter which is sparked at a pressure of 400 psi.
The low thermal conductivity imparted by the high porosity of a semi-conductor according to the invention is believed to minimize conduction of the heat at a high temperature generated by the spark discharge, so that, as a consequence, essentially only the surface of the body is ~ subjected to excessively high .~ :
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: , temperatures. This tends to limit the destructive melting and expansion stresses to the surface. In general, it is preferred that the surface elec-trical resistance, measured at 500 volts D.C., of a body according to the in-vention be between 1 and 200 megohms.
Accordingly, the present invention seeks to provide an improved silicon carbide semi-conductor.
The present invention also seeks to provide a method for producing a silicon carbide semi-conductor having an electrically non-conducting glass bonding phase.
In a first embodiment this invention provides a fired, electrically semi-conducting ceramic body having a volume percent open porosity from 20 to 40, and consisting essentially of silicon carbide particles having a median particle size from abou~ 3 to about 25 microns dispersed in a bonding matrix, the silicon carbide particles constituting from about 50 to about 70 percent of the body.
In a second embodiment this invention provides a process for the preparation of a fired, electrically semi-conducting body having an apparent porosity of from 20% to 40%, and consisting essentially of silicon carbide particles having a median particle size from about 3 to about 25 microns dis-persed in a bonding matrix, said silicon carbide particles constituting from about 50 to about 70 percent of the body, which process comprises:
(a) milling a mixture of silicon carbide, and bonding matrix components to a suitable particle size;
(b) combining the thus obtained mixture with paraffin wax and a solvent;
(c) pressing the combination of mixture and wax to provide a desired shape;
; (d) heating the pressings to about 1000F to remove the solvent; and (e) firing the pressings by either heating the pressing at about 2600 F in an inert atmosphere for a suitable time in a furnace, or heating the pressing in air to about 2000F for a short period of time; cooling the pressing; and re-heating the pressing at about 2600 F in an inert atmosphere for a suitable ~ - 8 -J

' ' 108VSgo time in a furnace, and thereafter cooling slowly from about 2600F to at least 700F.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The production of silicon carbide semi-conductor bodies having a glass bonding phase is described in the following Examples, which illustrate the best presently known modes. Examples 1-7 disclose a preferred bonding ccmposition, and the production therewith of semi-conductors at two different silicon carbide contents and at different apparent porosities, while Examples 8-10 illustrate a second and Examples 11-14 a third preferred bonding compo-sition and the production therewith of semi-conductors of different silicon carbide contents and of different porosities. -~ .

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1 EXAMPI.E 1 2 A batch of 65 parts substantially 600 mesh silicon carbide, 10.7 parts A1203, 12.2 parts SiO2, 4 11.5 parts whiting,* 11.3 parts dolomite** and 1/2 part ; 5 oleic acid was milled for two hours in a ball mill.
6 The milled batch was mixed with 17 parts of a ~5 percent 7 solution of paraffin wax in Stoddard solvent and dried.
8 Bored c~linders having an outside diameter of about 0.5 9 inch, an inside diameter of about Q.l inch and a height of about 0.3 inch were then pressed from the milled charge 11 under a pressure of about 20,000 pounds per square inch.

13 The cylinders were heated to about 1000F. in 14 air to volatilize the paraffin, cooled and placed on a silicon carbide bed in a molybdenum boat. The boat was 1~ then placed inside a tube, and the tube was purged with 17 helium. The tube and the cylinders contained therein 18 were then heated to 2600F., held at that temperature 19 for one hour and cooled to 700F. The tube was purged continually with helium during the entire heating cycle 21 to maintain a helium atmosphere therein. Eeating to 22 2600F. required about three-fourths hour, while cooling 23 from 2600F. to 700F. required about one and three-24 quarters hours. The boat and the fired semi-conductors were then removed from the tube. The fired semi~conductors 2~ were ground so that the bore had a diameter of 0.1 inch, 28 * CaC03; the sample used as described in this and other examples contained 55.2 percent CaO.

** The sample of dolomite used as described in this and other examples contained 30.5 percent CaO and 21.6 percent MgO.

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1 the outside diameter was 0.35 inch and the height was 2 0.27 inch. The resistance of the buttons, measured with a 500 volt Megger, was found to be 75 megohms. The 4 ground buttons were then placed in a sparking fixture, spring loaded in position so that a portion of one flat 6 surface o~ each was in mechanical and electrical contact 7 with the ground electrode of the fixture, and spaced by 8 0.001 to 0.002 inch from a center electrode thereof, 9 spark gap 0.050 inch. The minimum voltage required to 1~ sustain sparking in an ignition circuit which included 11 a 1.5 microfarad condenser was found to be 1650 volts 12 under atmospheric pressure. Subsequently, the minimum 13 voltage required to sustain sparking on the same piece 14 was found to be 1020 volts under a pressure of 100 pounds per square inch gauge. Thereafter, the minimum voltage re-16 quired to sustain sparking on the same piece was found to 17 be 750 volts under a pressure of 400 pounds per square 18 inch gauge. The ground semi-conductors, mounted in the 19 assembly, were sparked for one hour at 12 joules under a pressure of 100 pounds per square inch gauge, 70 sparks 21 per minute, and the erosion measured as weight loss in 22 grams, was found to be 0.0004. Under a pressure of 400 23 pounds per square inch gauge,the erosion was found to be 24 0.0025. After sparking for thirty minutes~under a pressure of 400 pounds per square inch gauge, it was 28 estimated that sparking had occurred along 17 percent of 27 the available surface of the buttons. The ignition 28 s~stem, during the erosion test described above, supplied 29 2000 ~olts, and had a total capacitance of 6 microfarads.

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1 During firing, in the procedure described above 2 in Example 1, the whiting and the dolomite were converted 3 to oxides, with evolution of carbon dioxide. On an 4 oxide/carbide basis the semi-conductors produced as described contained 65 percent silicon carbide, 10.7 6 percent A1203, 12.2 percent SiO2, 9.7 percent CaO and 7 2.4 percent MgO. The semi-conduc~ors consisted essentially 8 of silicon carbide particles dispersed in a glassy bond-9 ing matrix; the matrix constituted 35 percent of the total r so its overall composition can be calculated by dividing 11 each of the foregoing percentages by 0.35. This composition 12 is 34.8 percent SiO2, 30.4 percent A1203, 27.8 percent CaO
13 and 7.0 percent MgO. The material had an apparent porosity of 14 25.6 The procedure described above in Example 1 has 16 been repeated, either precisely or with the variation 17 noted below, to produce semi-conductors from different 18 batches. In some cases, denatured alcohol, 125 parts, was 19 substituted for the oleic acid; where this occurred, the milling ti~e of the batch was increased to four hours.
21 The identities of several of the batches are set forth 22 in the following Table:

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108~590 l The properties and performance of the ground 2 semi-conducting bodies of Examples 2 through 14, 3 determined as described above, are set forth in the 4 following Table:

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28 ~ ~ ~ I o u~ o o o o o o o o o o o 29 0 ~ ~ ~ u~ o , - ~ . _, _, _l ,, ,_1 .

' , 1 The compositions of the bonding matrices of the 2 bodies of Examples 2 through 14 are set forth in the 3 following Table:

Bonding Matrix, Percent 7 Examples SiO~ A123 CaO + MgO

9 2-7 34.8 30.4 34.8

8-10 60 . 20 20 11 11-14 50.0 30.4 19.6 14 The body of Example 5 was also produced at an apparent porosity of 19.3, and the erosion rate, during 1~ sparking at 400 psi. was found to be 0.0129 gram per hour.
17 By a comparison of this data with that given in Example 5 18 it will be noted that the apparent porosity of a body 19 according to the invention strongly influences erosion rate during sparking at 400 psi. Specifically, at an 21 apparent porosity of 19.3, the erosion rate during such 22 sparking was also double that when the apparent porosity was 29Ø. It has been found that further decreases in 24 apparent porosity below the 19.3 value for the bodies of Example 5 cause further increases in erosion rate 2~ during such sparking.

, The procedures described in all of the foregoing Examples describe a single firing in an inert atmosphere, specifically helium, to produce semi-conducting bodies. It has also been found that the two-step firing procedure to which reference is made above can be modified to produce improved semi-conductor bodies of higher porosity and improved resistance to erosion when sparked at a comparatively high pressure. The procedure described in the following Example illustrates such modification o the t~o-step firing procedure.

A batch of 40 paTtS substantially 400 mesh silicon carbide, 40 parts substantially 800 mesh silicon carbide, 10 parts A1203, 14.3 parts CaC03, 2 ~ parts Mg(OH~2 and 125 parts denatured alcohol was milled for four hours in a ball mill. The milled batch was dried and mixed with 17 parts of a 25 percent solution of paraffin wax in Stoddard solvent. Bored cylinders having an outside diameter of about 0.5 inch, an inside diameter o about 0.1 inch and a height of about 0.3 inch were then pressed from the milled charge under a pressure of about 20,000 pounds per square inch.

l The cylinders were heated to about 1000F.
2 in air to volatilize the paraffin, cooled and weighed.
3 After weighing, the cylinders were fired in an electric 4 furnace and in an air atmosphere to 2000F. for five minutes. After cooling, the cylinders were reweighed 6 and were found to have lost weight during this firing.
7 In twelve samples the weight loss ranged from 0.9 to 8 2.5 percent. This weight loss was the consequence

9 of the loss of carbon dioxide from calcium carbonate and of water from magnesium hydroxide, partially offset by 11 the oxidation of silicon carbide to silicon dioxide.

, 12 The cylinders were then placed on a silicon carbide 13 bed in a molybdenum boat, and the boat was placed in a i 14 tube. The tube and cylinders were then heated to 2600F., held at that temperature for one hour, and cooled to 700F.;

16 during this firing the tube was purged to maintain a 17 helium atmosphere therein. Heating to 2600F. requi~ed l~ about three-fourths hour; while cooling from 2600 to ; l9 700F. required about one and three-quarters hours.

The fired bodies had an apparent porosity of 31.9.

21 Their composition has been calculated, based upon the 28 weight loss after the five minute firing in air.-Such 23 calculations can be made by letting X equal the number 24 of parts by weight of silicon carbide oxidized to silicon dioxide. The final body, then, contains 80 2~ minus X parts of silicon carbide, 1.5 X parts of 27 silicon dioxide, lO parts of alumina, 8 parts of cal-28 cium oxide and 2 parts of magnesium oxide. The sum of ~9 these divided by the initial weight, 107.2 parts, is 3~

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1 the fraction representing the weight loss during firing:
2 0.991 in the case of a 0.9 percent loss and 0.975 3 in the case of a 2.5 percent loss. The calculated 4 compositions were as follows:

0.9% Loss . 2.5 ~Loss 6 . .
7 5i2 17.6 12.9 8 SlC 53.6 68.2 9 A123 9.4 9.5 CaO 7.5 7.6 11 MgO 1.9 .1.9 13 The fired bodies had an apparent porosity of 14 31.9; they were tested as described above, with the re-15 sults set forth below: .

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1 When, for purposes of comparison, but not 2 according to the instant invention, the procedure of Example 15 was repeated except that the initial firing 4 in air was to 2070F., sixty minutes at temperature, and the second firing, in helium, was to 2625F., the B fired bodies contained 54.2 percent silicon carbide, 7 27.6 percent silica, 9.2 percent A1203, 7.1 per cent CaO and 1.9 percent MgO and had an apparent porosity 9 of 17.9 percent. Buttons produced as described in this paragraph but at an apparent porosity of 19.9, 11 and sparked as described above at 400 psi., eroded at 12 a rate of 0.0183 gram per hour.

14 Semi-conducting bodies were produced as des-cribed above in Example 1, except that the batch was milled four hours with denatured alcohol, and no oleic 17 acid was used, from 27 parts 400 mesh silicon carbide, . ,, `; 18 27 parts 800 mesh silicon carbide and 46 parts, on an 19 oxide basis, bonding matrix. The compositions of the bonding matrices, and the properties and performance of . .
21 the conducting bodies so produced, determined as des-22 cribed above, are set forth in the following Table:

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, 1 Semi-conductors of a preferred family accord-2 ing to the invention have a glassy bonding matrix. The S overall composition of the preferred bonding matrix 4 can vary widely, preferably containing from 15 percent to 75 percent of SiO2, from 10 percent to 80 percent of A1203 6 and from 10 percent to 70 percent of at least one alkaline 7 earth oxide. It has been found that the glassy bonding `8 matrix can contain, in some instances, a crystalline ma-9 terial, e.g., alumina, mullite, cristobalite, quartz, or any of various alkaline earth metal silicates. The 11 presence of the crystalline material has not been found 12- to be detrimental, provided that the body has an apparent 13 porosity within the indicated limits, i.e., from 20 to 40 14 percent.
~! 15 16 It will be apparent that various changes and 17 modifications can be made from the specific disclosure 18 hereof without departing from the spirit and scope of 19 the invention as defined in the appended claims.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1, A fired, electrically semi-conducting ceramic body having a volume percent open porosity from 20 to 40, and consisting essentially of silicon carbide particles having a median particle size from about 3 to about 25 microns dispersed in a bonding matrix, the silicon carbide particles constituting from about 50 to about 70 percent by weight of the body.

2. A fired, electrically semi-conducting ceramic body as claimed in claim 1 wherein the bonding matrix is glassy, and the matrix and any secondary crystalline phase present consist essentially, in percentages by weight, of from 15 percent to 75 percent of SiO2, from 10 percent to 80 percent of Al2O3 and from 10 percent to 70 percent of at least one alkaline earth oxide.

3. A fired, electrically semi-conducting ceramic body as claimed in claim 1 wherein the bonding matrix is glassy, and the matrix and any secondary crystalline phase present consists essentially of from 15% to 75% SiO2, from 10% to 80% Al2O3 and from 10% to 70% of a mixture of CaO and MgO, by weight.

4, A fired, electrically semi-conducting ceramic body as claimed in claim 3 wherein in the bonding matrix and any secondary crystalline phase present, the ratio of CaO to MgO is about 4:1, in percentage by weight.

5. Process for the preparation of a fired, electrically semi-conducting body having an apparent porosity of from 20% to 40%, and consist-ing essentially of silicon carbide particles having a median particle size from about 3 to about 25 microns dispersed in a bonding matrix, said silicon carbide particles constituting from about 50 to about 70 percent by weight of the body, which process comprises:

(a) milling a mixture of silicon carbide, and bonding matrix components to a suitable particle size;
(b) combining the thus obtained mixture with paraffin wax and a solvent;

(c) pressing the combination of mixture and wax to provide a desired shape;
(d) heating the pressings to about 1000°F to remove the solvent;
and (e) firing the pressings by either heating the pressing at about 2600°F in an inert atmosphere for a suitable time in a furnace, and thereafter cooling slowly from about 2600°F to at least 700°F.

6 Process according to claim 5 wherein in step (e) the pressing is heated in air to about 2000°F for a short period of time and cooled before being heated at 2600°F in an inert atmosphere in a furnace.

7. Process according to claim 5 wherein in step (e) the pressings are heated at 2600°F for about one hour.

8. Process according to claim 6 wherein in step (e) the pressings are heated to about 2000°F for about 5 minutes.

9. Process according to claims 5 or 6 wherein the cooling period from about 2600°F to about 700°F is about one and three quarter hours.

Process according to claim 5 wherein the milled mixture used in step (a) comprise silicon carbide, alumina, silica whiting, and dolomite.

11. Process according to claim 5 wherein the milled mixture used in step (a) comprises silicon carbide, alumina, calcium carbonate, and magnesium hydroxide.

12. Process according to claim 11 wherein the milled mixture comprises from about 45% to about 65% SiC; from about 7% to about 27% SiO2; from about 5% to about 35% Al203; from about 5% to about 20% CaCO3; and from about 1% to about 7% Mg(OH)2, in percentages by weight.