US3876455A

US3876455A - Electric insulating porcelain article

Info

Publication number: US3876455A
Application number: US360423A
Authority: US
Inventors: Noboru Higuchi; Yutaka Ogawa; Kyosuke Tunekawa
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1972-05-18
Filing date: 1973-05-15
Publication date: 1975-04-08
Anticipated expiration: 1992-04-08
Also published as: JPS4913699A; CH582406A5; DE2325100A1; DE2325100C3; JPS5250399B2; DE2325100B2; FR2184991A1; FR2184991B3; GB1427989A

Abstract

An electric insulating porcelain article whose excellent electrical resistance properties and mechanical strength are not deteriorated even when used in an atmosphere containing the decomposition products obtained with SF.sub.6 gas is subjected to spark discharge. The electric insulating porcelain article comprises an electric porcelain body and a porcelain coating applied to the surface of the porcelain body, the coating containing not less than 50 % by weight of zircon and having a thermal expansion coefficient lower than that of the porcelain body.

Description

[ 1 Apr. 8, 1975 1 ELECTRIC INSULATING PORCELAIN ARTICLE [75] Inventors: Noboru Higuchi; Yutaka Ogawa,

both of Nagoya; Kyosuke Tunekawa, Aichi, all of Japan [73] Assignee: NGK Insulators, Ltd., Mizuho Nagoya, Japan [22] Filed: May 15, 1973 [2]] Appl. No.: 360,423

[30] Foreign Application Priority Data May 18, 1972 Japan 47-49367 [52] US. Cl. 117/125; 106/396; 106/48; 117/169 A; 174/137 A; 174/209; 174/D1G. l [51] Int. Cl C03c 5/02; B32b 9/00 [58] Field of Search 174/DIG. 1, 137 A, 209; 117/125, 169 A; 200/148 G, 144 C; 106/48, 39.6

[56] References Cited UNITED STATES PATENTS 2,157,100 5/1939 Rowland 117/125 2,170,387 8/1939 Morgan 106/48 2,325,553 7/1943 Schleicher.... 117/125 2,534,327 12/1950 Whitman 117/125 3,024,303 3/1962 Smothers 117/125 3,078,186 2/1963 Trevney.... 106/48 3,166,430 1/1965 Seabright 106/48 FOREIGN PATENTS OR APPLICATIONS 735,243 8/1955 United Kingdom 106/48 1,168,817 4/1964 Germany 106/48 1,091,399 10/1960 Germany 106/48 Primary ExaminerRalph I-Iusack Assistant Examiner-William H. Schmidt Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT An electric insulating porcelain article whose excellent electrical resistance properties and mechanical strength are not deteriorated even when used in an atmosphere containing the decomposition products obtained with SF gas is subjected to spark discharge. The electric insulating porcelain article comprises an electric porcelain body and a porcelain coating applied to the surface of the porcelain body, the coating containing not less than 50 by weight of zircon and having a thermal expansion coefficient lower than that of the porcelain body.

14 Claims, 2 Drawing Figures PATENTEUAPR 8197s 3 876,455

susnzpgz v F G. 2

x i v lNlTlAL VALUES OF EACH ZIRCON PORCELAIN ',-O----0-"" l2 lxl IO VALUES AFTER |oo HOURS OF EACH H ZIRCON PORCELAIN 9 lo I" U) E'z Z o I, 9 I0 I '3; 1 X 3 2 I09 x 0 IO 20 3O 4O 5O 6O 7O 8O AMOUNT OF ZIRCON BY WEIGHT) ELECTRIC INSULATING PORCELAIN ARTICLE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an electric insulating porcelain article. More particularly, the invention pertains to an electric insulating porcelain article, which is composed of a generally used electric porcelain body and, applied to the surface thereof, a porcelain coating containing not less than 50 by weight of zircon, and hence is not deteriorated in its excellent insulation resistance and mechanical strength even when used in an atmosphere containing decomposed products of SF gas which have been produced due to spark discharge.

2. Description of the Prior Art An electric insulating porcelain article, such as electric insulator, bushing or the like, is ordinarily prepared by applying a glaze to a porcelain body containing a crystal phase consisting of 3 to 35 (by weight based on the weight of said porcelain body; the same shall apply hereinafter) of quartz and 6 to 30 of rnullite; to 45 of cristobalite, 2 to 15 of quartz and 15 to 30 of mullite; 15 to 45 ofcristobalite,'2 to 15 of quartz, 15 to 35 of mullite and 3 to 45 of corundum; up to 35 of quartz, 6 to 30 of mullite and l to 45 of corundum; 3 to 35 of quartz, 6 to 30 of mullite and l to 30 of zircon; or up to 35 of quartz, 6 to 30 of mullite, l to 45 of corundum and l to 30 of zircon. Further, such porcelain body contains a glass phase in addition to such crystal phase as mentioned above.

It is well known that if, in the above case, the thermal expansion coefficient of the glaze applied to the porcelain body is made smaller than that of the porcelain body, the porcelain article is increased in mechanical strength due to the so-called effect of compression glaze, and hence can be more effectively designed, e.g. made compact in size, etc. (see US. Pat. Nos. 2,157,100 and 3,024,303). However, the use of such a porcelain article in a system employing SF gas as the insulating medium has heretofore yielded the following drawback: Namely, silica components constituting the glaze on the surface of the porcelain article are corroded by the decomposition products obtained when SF gas is subjected to spark discharges, and the fiuorides produced through this corrosion, having a low electric resistance, result in reducing the electric resistance of the porcelain article. In order to overcome the above-mentioned drawback, a required portion of the surface of such porcelain article has sometimes been coated with an epoxy resin containing as'a filler an alumina powder or the like which is not attacked by the decomposed products of SE, gas. However,-the surface of the glaze is too smooth to apply a resincoating which firmly adhere to the porcelain'article. Further, since the surface of an unglazed porcelain bodyis somewhat coarse, an attempt has been made to apply a resin coating to the surface of such unglazed porcelain body so as to firmly adhere the resin-coating tothe porcelain body. However, the unglazed porcelain body has no such effect of compression glaze as mentioned previously, so thatthere has been such drawback that no desired mechanical strength can be attained unless the porcelain bodyis made large in size. Moreover, the thermal expansion coefficient of the resin coating is larger than that of the porcelain body, so that there has been suc-hdrawback that the resin coating is peeled off when the porcelain article is used over a long period of time.

There has been also well known a zircon porcelain article, the whole body of which is composed of a zircon article. However, the zircon porcelain is narrow in firing temperature range and great in firing deformation, so that it is extremely difficult from the technical standpoint to use the zircon porcelain for commercial scale production of large-sized insulators. Even if the zircon porcelain is forcibly used for production of largesized insulators, while overcoming the said difficulty, delicate producton techniques of high degree are required to make the resulting insulators high in cost. Thus, the zircon porcelain has had such drawback that it is not suitable for commercial scale production of insulating porcelain articles.

SUMMARY OF THE INVENTION The present invention is based on the finding that when a porcelain containing not less than 50 by weight of zircon is coated on a generally used electric porcelain body, the resulting porcelain article is not deteriorated in insulation resistance even when used in an atmosphere containing decomposed products of SF, gas which have been produced due to spark discharge, and gives the same effect as the so-called effect of compression glaze, since the porcelain coating formed on the generally used electric porcelain body is smaller in thermal expansion coefficient than the electric porcelain body. In the present invention, the zircon porcelain as a coating is composed of a glass phase and a crystal phase comprising a major proportion of zircon and a minor proportion of mullite, zirconia, quartz and the like crystals.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, FIG. 1 is a graph showing the variations in insulation resistance of various electric porcelain bodies in SF, gas contaminated due to spark discharge; and FIG. 2 is a graph showing the variations in insulation resistance of zircon porcelain bodies containing varying amounts of zircon in SF gas contaminated due to spark discharge.

DETAILED DESCRIPTION OF THE INVENTION The reasons why the zircon porcelain according to the present invention is not deteriorated in insulation resistance even when used in an atmosphere containing decomposed products of SF gas which have been produced due to spark discharge, and why this effect is particularly marked in the case of a zircon porcelain containing not less than 50 by weight of zircon, are as follows:

Using raw materials having the chemical compositions shown in Table 1, test pieces of 10 mm. in diame- 'ter and mm. in length were prepared from such electric porcelain batches (l) to (3) and zircon porcelain batch as shown in Table 2, and the thus prepared test pieces were individually fired in an ordinary kiln for firing of porcelain insulator to obtain samples.

Table 1 Raw material Alumina Silica Feldspar Clay Zircon Dolomite Porcelain sand shred Component lg. loss 0.18 0.20 0.58 13.50 1.05 45.90 510 Trace 99.58 67.95 48.89 32.31 0.38 73.19 66.28 A120,, 99.47 0.02 17.22 33.73 0.12 21.04 Fe o 0.02 0.04 0.14 1.54 0.13 0.23 0.91 Ti Trace Trace Truce 0.98 0.33 0.10 0.05 Trace 0.15 0.33 35.78 0.15 MgO Trace Trace 0.02 0.21 17.35 0.17 K20 0.02 0.02 10.59 0.75 1.66 Na o 0.24 0.02 3.35 0.12 1.95

Table 2 bonate, zinc white, etc. on an unfired body of a generally used electric porcelain, which, when fired, comes (wt%) to have such crystal phase as mentioned previously, d f r f ch Tt 20 and then firing at a temperature of 1,250 to 1,350C.

1n 0 IXllIg ra IO 0 emica composl I OI'I I porcelain raw materials an firing The fired zircon porcelam coating cons1sts of.

a. 37 to 62 by weight of ZrO Generally used Alumina SiO 67.52 K: 2.88 b 18 5 t 465 b i ht f sio electric porcelain Silica :30 A1 0 27.85 Nap. 0.89 5 to 30 y 8 z ar Feldspar: 23 n.0,: 0.65 d. 0.1 to 5.0 by we1ght of at least one oxide se- Clay 37 226 lected from the group consisting of K 0 and Na O,

M 01 107 e. 1.0 to 10.0 b wei ht of at least one oxide ses y Same as above Alumina I 35 S10 41.16 K102 3.07 lected from the group cons st ng of Ca o and S111ca 2 5 A1203: 53.58 Na oz 1.01 f h 07 b ht f t] t peldspar; 25 P6205062 .1101. moret an 0 ywe 1g O 3. eas one OX1 6 Clay :35 'Ti0,: 0.41 selected from the group consisting of ZnO and BaO,

CaO: 0.19 and M 0.08 Same as above Alumina 50 SiO 3l.6l K 0: 2.52 g. not more than 5.0 by weight of at least one oxide 3 1 0? 9 28-113 2 L04 selected from the group consisting of TiO and Fe O ii E the sum of said oxides (d) to (g) being 1.1 to 12 by shred 10 we1ght and the ratio ZrO,/S1O being 0.8 to 2.0.

38 f 86 Synthetic zircon may be used in place of the raw ma- Zircon Zircon sand265 510.: 36.87 MgO: 0.61 terial zircon sand. Porcelain This zircon porcelain coating has a thermal expan- 2. $2,32 ,22 Dolomite 3 45 98 Bao 49 s1on coefficient of 3.9 to 4.9 X 10 /C. (25 to 650C.), coating of Feldspar :2 A1203: 9.34 K20: 0.43 which is identical with that of a generally used com- Barium pression glaze, and hence has the same effect as the socarbonate 3 Fe o z 0.51 Na oz 0.11 called effect of compression glaze on the aforesaid genl :25 erally used electric porcelain body having a thermal ex- Z1nc whlte 2 C210 1.23 o o panslon coefficient of 5.0 to 8.8 X 10 C. (25 to The thus obtained samples were individually placed in SF 8 gas, and were first measured in initial insulation resistance with application of electrodes to both ends of each sample, and then the variations with time in insulation resistance of the samples were measured, while continuously causing spark discharge by use of another sparking means provided in SF gas, to obtain such results as shown in FIG. 1. As is clear from FIG. 1, the zircon porcelain (containing 69 by weight of zircon) is far more excellent in insulation resistance than other 3 kinds of porcelains.

Funt her, 10 kinds of zircon porcelain samples varying in amiaunt of zircon were measured in insulation resistance iii the same manner as above to obtain the results as shown in FIG. 2. As is clear from FIG. 2, the effects of the zircon porcelains containing not less than by weight of zircon are particularly marked.

The porcelain article according to the present invention can be easily obtained by coating to a thickness of about 0.2 to 0.6 mm. a green layer for zircon porcelain coating comprising a mixture of zircon sand, feldspar,

clay, alumina, talc, dolomite, limestone, barium car- Further, the coating is completely vitrified by firing at the aforesaid temperature to show a water absorption ratio of 0 so that the porcelain article is not de-,

teriorated in insulating property.

The zircon porcelain coating according to the present invention may be applied to the whole surface of the porcelain body, but it is preferable that the coating is applied to at least a surface to be brought into contact with SP gas, and a generally used compression glaze is applied to the remaining surface.

It is needless to say that the same effect as above can be attained even when the coating according to the present invention is applied to '"a fired body of an unglazed generally used electric porcelain. In this case, however, it is necessary to carry out the firing two times, i.e., firing of the porcelain body and firing of the coating. It is therefore desirable to obtain the electric insulating porcelain article of the present invention by such one time firing operation that a green layer for zircon porcelain coating according to the present invention is applied to a required surface of an unfired body of generally used electric porcelain, and the resulting coated porcelain is subjected to firing, as mentioned previously.

The present invention is illustrated below with reference to test examples, in which the invention was applied to test pieces.

Test Example 1 A rod-1ike test piece of mm. in diameter and 120 mm. in length was prepared from a green body for porcelain insulator comprising 37 (by weight; the same shall apply hereinafter) of Amakusa pottery stone, 26 of Tsushima feldspar and 37 of Gaerome clay. Subsequently, the test piece was coated with a green layer of about 0.35 mm. in thickness for zircon porcelain coating comprising 70 of zircon sand, 5 of dolomite, 2 of feldspar; 3 of barium carbonate and 20 of clay, and then fired in a firing kiln at a maximum temperature of 1,300C. to'prepare a sample.

This sample was placed in SE gas in a l8-1iter vessel kept under a gas pressure of 1.2 atm., measured in initial insulationresistance with application of electrodes to both ends of the sample, and then measuredin variation of insulation resistance thereof, while contaminatto the Segers formula: and a sample (3) comprising the aforesaid test piece having no coating. From Table 3, it is understood that the sample of the present invention is excellent in properties.

Test Example 2 A cylindrical test piece of 100 mm. in outer diameter, 60 mm. in inner diameter and 200 mm. in height was prepared from a green body for porcelain insulator comprising 35 of Amakusa pottery stone, 30 of Tsushima feldspar and 35 of Gaerome clay. Subsequently, the test piece was coated on the inner and outer surfaces with a green layer of about 0.3 mm. in thickness for zircon porcelain coating comprising 63 of zircon sand, 4 of dolomite, 2 of feldspar, 6 of alumina, 3 of barium carbonate, 2 of zinc white and 20 of clay, and then fired in a firing kiln at a maximum temperature of l,300C. to prepare a sample. This sample was measured in variation of insulation resistance at the inner surface in the same manner as in Test Example 1 to obtain the results as shown in ing the atmosphereby continuously effecting spark discharge using-spark discharge electrodes (needle and plane plate) under such conditions as a gap between two electrodes of 2 mm., an applied voltage of 15 kv. and a currentof 5 mA. to obtain the results as shown in (1) of Table 3, in which were also shown the results ner as above, except that the coating according to the present invention was replaced by a generally used insulator glaze of the composition 0.20 K 0. 0.10 Na O. 0.30 CaO. 0.40 MgO. 0.70A1 O 7.00 SiO according (l) of Table 4, in which were also shown the results of measurement in inner pressure strength thereof. In Table 4, there were also shown, for comparison, the results of measurement in said properties of a sample (2) prepared in the same manner as above, except that the coating according to the present invention was replaced by an insulator glaze of the composition 0.20 K 0. 0.10 Na O. 0.28 CaO. 0.42 MgO. 0.75 A1 0 6.80 Si0 according to the Segers formula; a sample (3) comprising the aforesaid test piece having no coating; and a sample (4) prepared by coating the aforesaid test piece with an epoxy resin containing alumina powder as a filler, and curing theresin at C. for 2 hours and then at 160C. for 2 hours to form a coating of about 0.4 mm. in thickness. From Table 4, it is understood that the sample of the present invention is excellent in properties.

Table 3 Chemical compositionof coating after Amount Insulation resistance (.0) Bending strength Sample firing (wt /z) of (kg/cm) [Chemical composition of porcelain zircon After After After Before After body in the case of the sample (3)] (WV/t lni- 10 30 trcat- 100 tial hrs. hrs. hrs. ment' hrs. 1 1) t it SiO 36.16 A1 0 7.68 TiO 0.21 2.9 3.2 3.2 1.9 Sample of MgO 0.97 Na. 0.10 ZrO :49.68 73 X X X X 1.380 1.360 the present Fe. .O,, 0.43 010: 1.99 K 0: 0.39 10"" 10 10" 10"" invention 8:10 2.49

2) Sample SiO. 76.02 11: 0,; 0.35 CaO: 3.06 4.2 4 2 1.7 9.8 coated with K 0 3.43 A1 O,,:12.9l TiO- 0.17 X X X X 1.350 970 conventional MgO 2.93 Na O: 1.13 10 10' 10 10 glaze 3) Sample SiO. [73.23 'Fe O z 0.83 CaO: 0.17 3 9 9.1 3 3 7 6 having no K 0 2.00 A1 O;,:2l.5l TiO 0.37 X X X X 1,010 980 coating MgO 0.18 Na 1.65 10'' l0" 10" 10 Table 4 Chemical compisition of coating after Amount Insulation resistance (.0) Inner pressure Sample firing (wt /r) of Strength k 1 [Chemical composition of porcelain zircon lnitial After After After Before After body in the case of the sample (3)] (M71) 10 30 100 treat- 100 V hrs. hrs. hrs. ment hrs.

(1) Si0 :33.30 A1 O:,:13.89 TiO-,: 0.21 Sample of MgO: 0.97 ZrO :44. 15 Fe- ,O,-,:0.42 66 3.2 1.9 3.1 2.0 241 235 the present CaO:- 1.98 K 0: 0.38 Na O: 0.11 X 10 X 10 X 10' X 10' invention 3110: 2.46 ZnO: 2.1 1

Table 4 Continued resin Chemical compisition of coating after 7 Amount lnsulation resistance (9) Inner pressure Sample firing(wt/() of strength (kg) [Chemical composition of porcelain zircon lnitial After After After Before After body in the case of the sample (3)] (WW!) 10 30 100 treat- 100 hrs. hrs. hrs. ment hrs.

Sample SiO- :75.08 PC2031 3 CaO: 2.86 6.5 3.3 6.1 7.6 236 169 coated with K 0: 3.45 Al O;,:l3 89 TiO. 0.19 X X 10" X 10 X 10" conventional MgO: 3.08 Na .O: 1.12 glaze Sample 510 :7357 Fe 0 0.87 CaO: 0.19 3.6 1.0 4.9 3.1 173 170 having no K 0: 1.60 A1- O ,:21.06 TiO 0.31 X l0 X 10* X 10" X 10' coating MgO: 0.16 Na ,O: 2.24

Sample 1.1 5.3 6.3 4.2 181 176 coated with X 10" X 10 X 10' X 10' As is clear from the above explanation, the electric insulating porcelain article of the present invention is free from the drawbacks of the conventional electric insulating article, such as insulator or insulating bushing, and can be effectively used over a long period of time as an electric insulating article for electric appliances using SF gas as an insulating medium, without any deterioration in insulation resistance and mechanical strength. Thus, the industrial value of the present invention is extremely great.

What we claim is:

lLln a porcelain electric insulating article comprising a porcelain body and a porcelain coating thereon coming into contact with the decomposition products formed when SP gas is subjected to an electrical spark, the improvement wherein the porcelain coating coming into contact with said decomposition products contains not less than 50% by weight of zircon and has a thermal coefficient of expansion lower than the thermal coefficient of expansion of said body.

2. An electric insulating porcelain article as recited in claim 1, wherein said coating comprises of:

a. 37 to 62 by weight of ZrO b. 18.5 to 46.5 by weight of SiO c. 5 to by weight of A1 0 d. 0.1 to 5.0 by weight of at least one oxide selected from the group consisting of K 0 and Na O,

e. 1.0 to 10.0 by weight of at least one oxide selected from the group consisting of CaO and MgO,

f. not more than 7.0 by weight of at least one oxide selectedfrom the group consisting of ZnO and BaO, and g. not more than 5.0 by weight of at least one oxide selected from the group consisting of TiO and 2 3, the sum of said oxides (d) to (g) being 1.1 to 12 by weight and the weight ratio ZrO /SiO being 0.8 to 2.0. 3. An electric insulating porcelain article as recited in claim 1, wherein said body contains a crystal phase comprising quartz and mullite.

4. An electric insulating procelain article as recited in claim 1, wherein said body contains a crystal phase comprising cristobalite, quartz and mullite.

5. An electric insulating porcelain article as recited in claim 1, wherein said body contains a crystal phase comprising cristobalite, quartz, mullite and corundum.

6. An electric insulating porcelain article as recited in claim 1, wherein said body contains a crystal phase comprising quartz, mullite and corundum.

7. An electric insulating porcelain article as recited in claim 1, wherein said body contains a crystal phase comprising quartz, mullite and zircon.

8. An electric insulating porcelain article as recited in claim 7, wherein said crystal phase further contains corundum.

' 9. The article of claim 1, wherein said coating contains at least about 60% zircon.

10. The article of claim 9, wherein said coating contains about 69% zircon.

13. The article of claim 12, wherein said porcelain body has a thermal expansion coefficient of 5.0 to 8.8

x 10- /C. at 25 to 650C.

14. The article of claim 1, wherein at least a portion of the surfaces of the porcelain body not coming into contact with the decomposition products of SF 6 gas are coated with a compression glaze.

Claims

1. IN A PORCELAIN ELECTRIC INSULATING ARTICLE COMPRISING A PORCELAIN BODY AND A PORCELAIN COATING THEREON COMING INTO CONTACT WITH THE DECOMPOSITION PRODUCTS FORMED WHEN SF6 GAS IS SUBJECTED TO AN ELECTRICAL SPARK, THE IMPROVEMENT WHEREIN THE PROCELAIN COATING COMING INTO CONTACT WITH SAID DECOMPOSITION PRODUCTS CONTAINS NOT LESS THAN 50% BY WEIGHT OF ZIRCON AND HAS A THERMAL COEFFICIENT OF EXPANSION LOWER THAN THE THERMAL COEFFICIENT OF EXPANSION OF SAID BODY.

2. AN ELECTRIC INSULATING PORCELAIN ARTICLE AS RECITED IN CLAIM 1, WHEREIN SAID COATING COMPRISES OF: A. 37 TO 62% BY WEIGHT OF ZRO2, B. 18.5 TO 46.5% BY WEIGHT OF SIO2, C. 5 TO 30% BY WEIGHT OF AL2O3, D. 0.1 TO 5.0% BY WEIGHT OF AT LEAST ONE OXIDE SELECTED FROM THE GROUP CONSISTING OF K2O AND NA2O, E. 1.0 TO 10.0% BY WEIGHT OF AT LEAST ONE OXIDE FROM THE GROUP CONSISTING OF CAO AND MGO, F. NOT MORE THAN 7.0% BY WEIGHT OF AT LEAST ONE OXIDE SELECTED FROM THE GROUP CONSISTING OF ZNO AND BAO, AND G. NOT MORE THAN 5.0% BY WEIGHT OF AT LEAST ONE OXIDE SELECTED FROM THE GROUP CONSISTING TIO2 AND FE2O3, THE SUM OF SAID OXIDES (D) TO (G) BEING 1,1 TO 12% BY WEIGHT AND THE WEIGHT RATIO ZRO2.SIO2 BEING 0.8 TO 2.0.

3. An electric insulating porcelain article as recited in claim 1, wherein said body contains a crystal phase comprising quartz and mullite.

9. The article of claim 1, wherein said coating contains at least about 60% zircon.

10. The article of claim 9, wherein said coating contains about 69% zircon.

11. The article of claim 1, wherein said coating is composed of a glass phase and a crystal phase comprising a major portion of zircon and a minor portion of mullite, zirconia and quartz.

12. The article of claim 1, wherein said coating has a thermal expansion coefficient of 3.9 to 4.9 .times. 10.sup.-.sup.6 /.degree.C. at 25.degree. to 650.degree.C.

13. The article of claim 12, wherein said porcelain body has a thermal expansion coefficient of 5.0 to 8.8 .times. 10.sup.-.sup.6 /.degree.C. at 25.degree. to 650.degree.C.

14. The article of claim 1, wherein at least a portion of the surfaces of the porcelain body not coming into contact with the decomposition products of SF.sub.6 gas are coated with a compression glaze.