ELECTROCONDUCTIVE CERAMIC MATERIAL

Abstract: The invention relates to the field of producing electroconductive ceramics on the basis of lanthanum chromite and intended for high-temperature applications (about 2000K). The object of the invention is to increase the heating rate of the ceramics, which will make it possible to reduce the time of the working operation, and ensure its improved thermal stability, which will increase the reliability of operation at high temperature. The goal is achieved by supplementing ceramics based on lanthanum chromite and containing zirconium dioxide, with alloying and stabilizing additives of lanthanum aluminate, yttrium chromite, magnesium chromite and cerium dioxide.

Description: ELECTROCONDUCTIVE CERAMIC MATERIAL The present invention relates to the field of producing high-temperature ceramics and can be used in manufacturing high-temperature heaters, semiconductors, thermocouples, temperature sensors, as additives in manufacturing conventional ceramics and in other fields requiring high thermal and chemical stability and high electric conductivity when operating in air, and in medical practice. An elcctroconductive material is known on the basis of rare-earth chromites, such as lanthanum and/or yttrium chromite with additions of zirconium dioxide and rare-earth oxides, which is intended for stable operation at high temperatures, about 2 000°K. In this case the amount of zirconium dioxide must exceed 5 raol%, preferably 30-50 mol% (see US Patent # 3 475 352, "Electrically conductive ceramic material", national classification 252- 520, published 28.10.1969). Better results are obtained with compositions containing 33 mol% LaCr0 ., 16 mol% Gd 2 0 3 , the rest being Zr0 2 (Composition 1) and 33 mol% LaCr0 3 and the rest, Zrϋ (Composition 2). Hereinafter, the properties of the novel material are compared with those of the ceramics of the two compositions. A serious shortcoming of the said ceramic material is its low permissible rate of heating, relatively low reliability of operation at 1 600°C, and low thermal stability when operating at about 1 500°C. The object of the present invention is to create a ceramic material that will have a high heating rate, a higher thermal stability and a higher reliability when operating at high temperatures. The goal is achieved by modifying the target material, based on lanthanum chromite and containing zirconium dioxide, witli alloying and stabilizing additives of lanthanum aluminate, yttrium chromite, magnesium chromite and cerium dioxide, in the following ratio of ingredients, wt%: lanthanum aluminate 0.50~10.0 yttrium chromite 0.50~3.0 magnesium chromite 1.0~15.0 cerium dioxide 0. X1.0 zirconium dioxide 0.50X.0 lanthanum chromite the rest The material was obtained in the following manner. After preparing a charge of the necessary composition it was mixed in a planetary mill using plexiglass drums and teflon balls as milling bodies. The resulting powder was dried and melted in a solar furnace. The melt was cooled, ground and pressed into specimens of 50x6x6 mm in the middle and 50x6x12 at the ends for measuring maximum heating rate, of 40x4x4 mm for measuring electric conductivity, and of 15 mm in diameter and height for the rest of the tests. The specimens were sintered in a lanthanum chromite furnace at 1 600°C for 12 hours. The specimens thus prepared were held at 1 500°C for 60 hours for determining thermal stability at that temperature, and at 1 600°C for 20 hours for determining weight loss. To measure conductivity, the specimens of 40x4x4 mm were metallized at the ends. To measure maximum heating rate the appropriate specimens were put in a corundum chamber and, after having a thermocouple attached to them, were energized and heated at various rates. Then the specimens were examined on a cross-section. When a certain heating rate was exceeded the specimens disintegrated owing to cracking at the surface and the inner layer melting through because of the low thermal conductivity and inverse temperature dependence of electric conductivity. The results obtained for various ceramic compositions are listed in Tables 1 to 1 1. Given below are examples of preparing the ceramic material according to the present invention. Example 1. A charge was prepared of the following composition, wt%: lanthanum aluminate 0.30 yttrium chromite 0,30 magnesium chromite 0.50 cerium dioxide 0.05 zirconium dioxide 0.30 lanthanum chromite the rest The charge was mixed in plexiglass drums in a planetary mill using teflon balls, dried, melted in a solar furnace, cooled, ground and pressed into specimens of various dimensions and shapes. For measuring the maximum heating rate, plates were used of 50x6x6 mm in the middle and 50x6x12 mm at the ends. For measuring electric conductivity, plates of 40x4x4 mm were used. For measuring weight loss and thermal stability, cylinders of 15 mm in diameter and height were used. The specimens were sintered in a lanthanum chromite furnace at 1 600°C for 12 hours. Then they were held at 1 500°C for 60 hours to determine thermal stability at that temperature, and at 1 600°C for 20 hours to measure weight loss. The specimens of 40x4x4 mm, for measuring electric conductivity, were metallized at the ends. In measuring maximum heating rate the specimens — plates with a different thickness in the middle and at the ends — were put in a corundum chamber, had a thermocouple attached to them, were energized and heated at various rates. Then the specimens were examined on a cross-section. When heated at a rate of 5K/min the specimens were in a good condition, but at a rate of 1 OK/min they exhibited cracking. The weight loss after 20 hours hold at 1 600°C was 1.2%, which is twice as much than the prototype material was. The electric resistivity was 312.6 Ohmxcm. The compressive strength after 60 hours hold at 1 500°C decreased from 72.6 MPa to 18.4 MPa. Thus, almost all the properties of this composition turned out to be inferior to those of the prototype. Example 2. The ceramic material was prepared in the same way as in Example 1 , except that the ceramic composition was that given in column II of Table 1. In that case all the properties of the novel ceramic material were better than those of the prototype (see Table 1). Example 3. All the steps followed were the same as in Example 1 , except that the ceramic composition was that given in column III of Table 1 . In that case all the properties of the target material were greatly improved: maximum heating rate increased 5 times, weight loss decreased to 0.2%, resistivity decreased to 8.2 Ohmxcm, compressive strength was 144.1 MPa, and its degradation after 60 hours hold at 1 500°C was only 3.8. Example 4. All the steps followed were the same as in Example 1 , except that the composition was that given in column IV of Table 1. In that case the main properties of the novel material were also higher than those of the prototype except resistivity. Example 5. All the steps followed were the same as in Example 1 , except that the 5 composition was that given in column V of Table 1. Resistivity increased so much that a specimen could not be sufficiently heated to measure maximum heating rate. Example 6. All the steps followed were those of Examples 1 to 5, except that all the i() compositions used the minimal (Table 2) and maximal (Table 3) concentrations of lanthanum aluminate. As can be seen from the data given, the main properties for columns II to IV of the tables exceed those of the prototype material. Example 7. 15 All the steps followed were those of Examples I to 6, except that all the compositions of the novel ceramic used the minimal (Table 4) and maximal (Table 5) concentrations of yttrium chromite. As can be seen from the data given, the main properties for columns II to IV of the tables exceed those of the prototype material. 20 Example 8. All the steps followed were those of Examples 1 to 7, except that all the compositions of the novel ceramic used the minimal (Table 6) and maximal (Table 7) concentrations of magnesium chromite. As can be seen from the data given, the main properties for columns IT to TV of the tables exceed 25 those of the prototype material. Example 9. All the steps followed were those of Examples 1 to 8, except that all the compositions of the novel ceramic used the minimal (Table 8) and maximal (Table 9) concentrations of cerium dioxide. As can be seen from the data given, the main properties for columns II to IV of the tables exceed those of the prototype material. Example 10. All the steps followed were those of Examples 1 to 9, except that all the compositions of the novel ceramic used the minimal (Table 10) and maximal (Table 1 1) concentrations of zirconium dioxide. As can be seen from the data given, the main properties for columns II to IV of the tables exceed those of the prototype material. Given below are tables comparing properties for various compositions of the novel ceramic material with the corresponding properties for the prototype material. Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Ingredients and Ingredient content, wt% main properties II HI IV VI lanthanum aluminate 0.30 0.50 3.00 10.00 12.00 prototype yttrium chromite 0.30 0.50 2.50 3.00 4.00 magnesium chromite 0.50 1.00 10.00 15.00 20.00 cerium dioxide 0.05 0.10 1.00 1.00 2.00 zirconium dioxide 0.50 0.50 0.50 0.50 0.50 lanthanum chromite the rest the rest the rest the rest the rest maximum heating rate K/min 5 15 50 20 - 10 weight loss, % 0.9 0.5 0.3 0.4 1.0 0.55-0.6 resistivity, Ohmxcm 372.8 124.0 7.1 768.3 1987.7 50-4800 compressive strength, MPa 92.6 96.3 123.1 103.3 89.8 96-135 compressive strength, MPa (1 500°C/60h) 21.3 71.4 101.6 36.9 32.2 10.2-16.1 Table 11 As can be seen from the tables, for the prototype material obtained by the same technology, maximum heating rate is 10 degrees per minute, weight loss is 0.55 to 0.6%, resistivity is 50 to 4800 Ohmxcm, compressive strength is 96 to 135 MPa, and compressive strength after 60 hours hold at 1 500°C is 10.2 to 16.1 MPa. Thus, the use of this technical decision to permits increase maximum heating rate 5 times, reduce weight loss 3 times (these two parameters are the principal shortcomings of the prototype), and decrease compressive strength degradation after 60 hours hold at 1 500°C 9 times while increasing this property about 1 .5 times.

Claims: CLAIM An electroconductive ceramic material containing lanthanum chromite and zirconium dioxide, whose distinguishing feature is that it also contains lanthanum aluminate, yttrium chromite, magnesium chromite and cerium dioxide in the following ratio of ingredients, wt%: lanthanum aluminate 0.50-10.0 yttrium chromite 0.50-3.0 magnesium chromite 1.0-15.0 cerium dioxide 0.1-1 .0 zirconium dioxide 0.50-5.0 lanthanum chromite the rest AMENDED CLAIMS [received by the International Bureau on 31 October 1998 (31.10.98); original claim replaced by amended claim (1 page)] An electroconductive ceramic material containing magnesium chromite, yttrium chromite, zirconium dioxide, cerium dioxide, lanthanum chromite, whose distinguishing feature is that it also contains lanthanum aluminate in the following ratio of ingredients, wt%: lanthanum aluminate La Al 2 O 0.50-10.0 magnesium chromite Mg CrO 4 1.0-15.0 yttrium chromite Y CrO 3 0.50-3.0 zirconium dioxide Zr O 2 0.50-5.0 cerium dioxide Ce O 2 0.1-1.0 lanthanum chromite LaCrO 3 remainder STATEMENT UNDER ARTICLE 19 The Applicant wishes to amend the claim in connection with substitution of the prototype. As a result of the International search the new information has been found out: Patent WO 93 26011 A (MITECH SCIENT CORP) 23 December 1993. In this Patent is given a description of ceramic material, which is the most close to the applicated ceramic material according to this PCT Application PCT /IB 98/ 00824 and evidently must be taken as a prototype to this invention.

Inventors: RAKHIMOV RUSTAM, RAKHIMOV, RUSTAM
Applicants: RAKHIMOV RUSTAM, RAKHIMOV, RUSTAM
Application year: 1998
Main classification: C04B35
IPC codes: C04B35, H01B1, H05B3