METHOD FOR PRODUCING CERAMICS WITH THERMAL SHOCK RESISTANCE

Abstract: A ceramics having improved thermal shock resistance is produced by a method comprising firing a mixture compris­ ing at least one compound selected from the group consist­ ing of sulfate, nitrate, carbonate, oxide, hydroxide, acetate and oxalate of an alkali metal and an alkaline earth metal, titanium oxide, rehydratable alumna, optionally silica and optionally a rare earth element compound at a temperature in the range of 1,000°C to 1,300°C.

Description: The present invention relates to a method for producing ceramics having a small coefficient of thermal expansion and improved thermal shock resistance. More particularly, it relates to a method for producing ceramics having good thermal shock resistance suitable for use in making a catalyst carrier, a combustion apparatus and the like. Description fo the Prior Arts Conventional ceramics having a small coefficient of thermal expansion and good thermal shock resistance include cordierite ceramics, lithia type ceramics, aluminum titanate ceramics and the like. The cordierite ceramic is a MgO-Al₂O₃-SiO₂ type one. The lithia type ceramic is a Li₂O-Al₂O₃-SiO₂ type one. The cordierite is prepared by mixing talc [Mg₃(Si₄O₁₀)(OH)₂], kaolin [Al₂Si₂O₅(OH)₄] and alumina (Al₂O₃) in an adequate ratio, followed by dehydration, molding, drying and calcinating. For example, calcination of the mixture is carried out at about 1,400°C for 4 to 5 days (cf. Japanese Patent Publication Nos. 1564/1979 and 20358/1976). The aluminum titanate ceramics are prepared by calcinating a mixture of titanium oxide (anatase type) and highly pure α-alumina in an equimolar amount at 1,600°C to 1,700°C (cf. "Yogyokogaku Handbook" (Ceramic Industry Handbook), p 1274, Gihodo, Japan). However, the conventional thermal shock resistant ceramics have sufficient mechanical strength only when they are calcinated at a high temperature for a long period of time. In addition, the synthesized ceramics may not be dense or may tend to be cracked at particle boundaries. Therefore, they lack dimensional accuracy and their thermal shock resistance is not satisfactory. SUMMARY OF THE INVENTION US-A-3,316,108 teaches a method for preparing a furnace lining ceramic, mainly consisting of magnesia, alumina and titania, wherein the alumina can be rehydratable. The mixture should be fired at temperatures between above about 1200° C to as high as 2400° C and temperatures over 1700° C are most preferred. The mixture may contain small amounts of silica, although the presence of silica in the mixture is discouraged. EP-A-O 037 868 teaches to make low-expansion ceramics for car exhaust catalysts. The use of rehydratable alumina is not disclosed in this prior art, but the mixture contains up to 20% by weight iron oxide to prevent variation of the thermal expansion coefficient. Firing temperatures range between 1300° C and 1700° C. A silica content of the mixture is not discloses in this prior art. One object of the present invention is to provide a method for producing a ceramic having improved thermal shock resistance even at a comparatively high temperature. Another object of the present invention is to provide a method for producing a ceramic having dimensional accuracy. Further object of the present invention is to provide a method for producing a ceramic having good mechanical strength, for example, compressive strength. These and other objects of the present invention are accomplished by a method for producing a ceramic comprising firing a mixture comprising at least one compound selected form the group consisting of sulfate, nitrate, carbonate, oxide, hydroxide, acetate and oxalate of an alkali metal and an alkaline earth metal (hereinafter referred to as "metal compound"), titanium oxide, rehydratable alumina and 60% to 90% of silica at a temperature in the range of 1,000°C to 1,300°C. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing compressing strength and the volume increase factor of the ceramic against the content of the fused silica in the ceramics, and Fig. 2 is a graph showing CO, HC and NO x removing rates of the catalyst comprising a carrier made of the ceramic produced according to the present invention against the air/fuel ratio. DETAILED DESCRIPTION OF THE INVENTION Examples of the alkali metal and alkaline metal are lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium, and their sulfate, nitrate, carbonate, oxide, hydroxide, acetate and oxalate are preferably used. Titanium oxide includes the anatase type one, rutile titanium dioxide and brookite. Among them, rutile titanium dioxide is preferred due to stability of the produced ceramics when used as the catalyst carrier. In the present specification, the rehydratable alumina is used to mean transition alumina except α-alumina (e.g., ρ-alumina and amorphous alumina), which is prepared by pyrolisis of hydrated alumina. Industrially, the transition alumina is obtained by heating hydrated alumina such as alumina trihydrate obtained from, for example, Bayer's process at a temperature of about 400°C to 1,200°C for some tenths to ten seconds or by firing the hydrated alumina at a temperature of about 250°C to 900°C for one minute to four hours and usually has about 0.5 % to 15 % by weight of weight loss on heating. The rehydratable alumina is used in the form of powder as well as sol, namely a dispersion in water. A combination of powder and sol of the rehydratable alumina improves the mechanical strength of the produced ceramics. In view of the compressive strength and thermal shock resistance of the produced ceramic, preferably, a mixing ratio of the metal compound and titanium oxide is such that 0.1 to 4 gram atom of titan in titanium oxide is present per gram atom of the alkakine or alkaline earth metal in the metal comppound. A weight ratio of titanium oxide to the rehydratable alumina is from 1:1 to 1:10. When an additive as described below is used, the ratios among the metal compound, titanium oxide and the rehydratable alumina are within the above ranges. A total amount of the metal oxide, titanium oxide and the rehydratable alumina is preferably at least 10 % by weight based on the whole weight of the ceramics in view of the compressive strength and thermal shock resistance of the ceramic. When the metal compound is used in the form of a coating on titanium oxide particles, the properties of the ceramic are further improved. According to the present invention, a mixture comprising the metal compound, titanium oxide, rehydratable alumina and silica is fired to produce ceramics having improved thermal shock resistance. In addition to the metal compound, titanium oxide rehydratable alumina and silica, the mixture may contain water-soluble cellulose derivatives such as carboxymethylcellulose and methylcellulose as a molding aid and glycerin and vaseline as a plasticizer. Further, 60%-90% of silica powder is used but as an aggregate, other low thermal expansion heat resistant materials such as cordierite powder, mullite powder and chamotte powder may be used. Silica may be naturally occurring quartz, agate, quartz rock, sands as well as synthetic fused silica. Among them, fused silica is preferred in view of improvement of thermal shock resistance of the produced ceramics. Silica has a small coefficient of thermal expansion. Particularly, the fused silica has a coefficient of thermal expansion of 0.5 x 10⁻⁶/deg in the range of room temperature to 1,000°C and is one of the materials having the smallest thermal expansion. Since silica is a glass-forming oxide, when it is heated at a temperature of 1,000°C or higher in the presence of an alkaline compound, it forms a crystalline compound such as cristobalite and tridymite whereby its coefficient of thermal expansion considerably increases, for example, to 4 x 10⁻⁶ to 5 x 10⁻⁶/deg. in the range of room temperature to 1,000°C. From this view point, it has been believed that silica cannot be used in the production of a thermal shock resistant ceramics. When silica is combined with the metal compound, titanium oxide and the rehydratable alumina and fired at a temperature of 1,000°C to 1,300°C according to the present invention, a ceramic having good thermal shock resistance and a comparatively large specific surface area can be produced. When the ceramic is used as a catalyst carrier, it may further contain a compound of a rare earth element (hereinafter referred to as "rare earth element compound"). Examples of the rare earth element compound are oxide of the rare earth element such as La, Ce, Pr, Nd and Sm and chloride, nitrate, acetate and hydroxide which will be converted to the oxide by the thermal treatment at a temperature of 1,000°C to 1,300°C according to the present invention. Among the rare earth element compounds, those of La, Ce and Nd are preferred in view of the catalytic properties of the carrier. Practically and presently preferred embodiments of the present invention will be illustrated by the following examples, in which part(s) and % are by weight unless otherwise indicated. Procedure of manufacture A mixture of rehydratable of alumina, titanium oxid, potassium carbonate as the metal compound and silica methylcellulose (4.0 parts) as the molding aid, glycerin (2.0 parts) as the plasticizer and water (32 parts) were mixed and kneaded by a screw kneader for 10 minutes and then extruded by a screw extruder to form a cylindrical molded ceramic article of 100 mm in outer diameter and 100 mm in length having a honeycomb structure consisting of square cells each of 0.3 mm in wall thickness and 1.5 mm in side length. The article was heated to 1,200°C at a rate of 10°C/hr. and fired at that temperature for one hour. Physical properties of the fired ceramic article are shown according to different examples below. The compressive strength was measured in a direction of the C axis of the honeycomb structure. The thermal shock resistance temperature was measured as follows: The fired ceramic article having a volume of one liter is heated in an electric furnace to a high temperature and suddenly cooled to an ambient temperature in the air and the maximum temperature of the electric furnace at which the article is firstly cracked after cooling is used as the thermal shock resistance temperature. Example 1 Withe the procedure described above but using the rehydratable alumina, titanium oxide and potassium carbonate in a weight ratio of 2:1:1 and varying the content of the fused silica in the range of 55 % to 95 %. a ceramic article having the honeycomb structure was produced. Compressive strength and volume increase factor after thermal cycling were measured. The volume increase factor was measured as follows: One thermal cycling consisted of heating the ceramic article from room temperature to 1,100°C over one hour, keeping it at the same temperature for 15 minutes and cooling it to room temperature over 30 minutes. After repeating 500 cycles, the volume expansion factor was measured. In view of suppression of crack formation, the smaller the volume increase factor, the better. The results are plotted in fig. 5 Althrough the compressive strength of the ceramic article did not change substantially with increase of the content of the fused silica till 90 %, it was suddenly fell over 90 %. The volume increase factor increased below 60 % of the fused silica content. From these results, a preferred content of the fused silica is between 60 % and 90%. Examples 2(1) - 2(5) In the same manner as in Example 1 but using the rehydratable alumina (10 parts), titanium oxide (5 parts), potassium carbonate (5 parts), the fused silica (75 parts), nitrate of the rare earth element (5 parts), methylcellulose (4.0 parts) as the molding aid, glycerin (2.0 parts) as the plasticizer and water, a ceramic article having the honeycomb structure was produced. The rare earth element used in each example was as follows: Example 2(1): La Example 2(2): Ce Example 2(3): Pr Example 2(4): Nd Example 2(5): Sm A ceramic article having the same composition but containing no rare earth element compound was also produced (Example 2(6). By using the ceramic article having the honeycomb structure as a carrier, a catalyst was produced as follows: In a mixed aqueous solution of chloroplatinic acid and rhodium nitrate in an adequate concentration, the ceramic article was soaked for 1 minute at room temperature and dried followed by heating at 500°C in an atmosphere of hydrogen containing nitrogen to produced a catalyst carrying 1.0 g of platinum and 200 mg of rhodium per liter of the ceramic carrier. The six catalysts were used for cleaning exhaust gas from an automobile and their catalytic property was avaluated by following two methods: (1) Before and after heating the catalyst in an electric furnace kept at 1,000°C for 100 hours, a CO removing rate of the catalyst was measured at 200°C, the space velocity of 40,000 r.h⁻¹ and a CO content of 600 ppm (in air) at the entrance. (2) In an exhaust line of an automobile carrying a 2,800 cc engine, the catalyst was placed and its removing rates of CO, hydrocarbons (HC) and nitrogen oxide (NO x ) were measured by varying the air/fuel ratio from 14.0 to 15.5 at intervals of 0.1. The initial rates and the rates after 100 hours bench time are plotted in Fig. 6. The catalytic efficiency was evaluated from the removing rates at the point A where the NO x curve and the CO curves intersect and at the point B where the NO x curve and the HC curve intersect. The results obtained by the method (1) are shown in Table 1 and those obtained by the method (2) are show in Table 2. As is apparent from the results in Tables 1 and 2, the properties of the ceramic article as the catalyst carrier was improved by the addition of the rare earth element compound, particularly the La, Ce or Nd compound. In Examples, the rare earth element compound was used alone although two or more rare earth element compounds may be simultaneously used. Further, the chloride, acetate, hydroxide and the like of the rare earth elements have the same effects. Examples 3(1)-3(7) In the same manner as in Example 1 but using the rehydratable alumina (10 parts), titanium oxide (5 parts), potassium carbonate (5 parts), fused silica (80 parts), metylcellulose (4.0 parts) and water (31 parts), a fired ceramic article having the honeycomb structure was produced (Example 3(1)). In the same manner as in the above but using, in place of potassium carbonate, magnesium carbonate (Example 3(2)), magnesium sulfate (Example 3(3)), or other potassium compounds (Examples 3(4)-3(7)), a fired ceramic article having the honeycomb structure was produced. The volume increase factors of the articles are shown in Table 3. As is apparent from the results in Table 3, the potassium compounds have better volume increase factors than the magnesium compound.

Claims: 1. A method for producing a ceramic characterized in that a mixture comprising at least one compound selected from the group consisting of sulfate, nitrate, carbonate, oxide, hydroxide, acetate and oxalate of an alkali metal and an alkaline earth metal, titanium oxide, rehydratable alumina and silica is fired at a temperature in the range of 1000° C to 1300° C, the content of silica in the mixture being 60% to 90%. 2. The method according to claim 1, wherein the mixing ratio of the alkali metal or alkaline earth metal compound and titanium oxide is such that 0.1 to 4 gram atom of titanium in titanium oxide is present per gram atom of the alkaline or alkaline earth metal in the metal compound and the weight ratio of titanium oxide to the rehydratable alumina is from 1:1 to 1:10. 3. The method according to Claim 1 or 2, wherein the alkaline metal or alkaline earth metal compound is a potassium compound or a magnesium compound. 4. The method according to any one of claims 1 to 3, wherein the silica is fused silica. 5. The method according to any one of claims 1 to 4, wherein the mixture further comprises a compound of a rare earth element. 6. The method according to claim 5, wherein the compound of the rare earth element is oxide or a compound which is converted to oxide by firing. 7. The method according to any one of claims 1 to 6, wherein the ceramic is molded in the form of an article having a honeycomb structure.

Inventors: NISHINO, ATSUSHI, NUMOTO, HIRONAO, ONO, YUKIYOSHI, TAKEUCHI, YASUHIRO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: C04B35
IPC codes: C04B35, B01J23