HEAT RESISTING LOW EXPANSION ZIRCONYL PHOSPHATE-ZIRCON COMPOSITE BODIES AND PROCESS FOR PRODUCING THE SAME

Abstract: Heat resisting low expansion zirconyl phosphate-­ zircon composite bodies are disclosed, which contain zirconyl phosphate and zircon as a main crystalline phase and a secondary crystalline phase, respectively. The heat resisting low expansion zirconyl phosphate-­ zircon composite bodies have a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C being not more than 30×10⁻⁷/°C and a melting point being not less than 1,600°C. The composite bodies have a chemical composition essentially consisting of 58.2 to 65.4% by weight of ZrO₂, 17.4 to 37.1% by weight of P₂O₅, and 1.5 to 19.0% by weight of SiO₂. The heat resisting low expansion zirconyl phosphate-zircon composite bodies optionally contain MgO and Aℓ₂O₃ in a total amount of not more than 2.5% by weight or 0.1 to 4% by weight of Nb₂O₅. A process for producing such zirconyl phosphate-zircon composite bodies is also disclosed.

Description: The present invention relates to low expansion ceramic materials and a process for producing these ceramic materials. More particularly, the invention relates to zirconyl phosphate-zircon base low expansion ceramic materials having excellent thermal shock resistance and heat resistance, and to a process for producing such ceramic materials. With advances in recent industrial technology, demands for materials having excellent heat resistance and thermal shock resistance have been increasing. The thermal shock resistance of the ceramic materials is influenced by physical properties of the materials such as a coefficient of thermal expansion, heat conductivity, strength, modulus of elasticity, Poisson ratio, etc, and is also influenced by sizes and configurations of resulting articles and a heating and cooling process, that is, a heat flow speed. Among these factors influencing the thermal shock resistance, the coefficient of thermal expansion particularly has a greater contributory percentage. It is known that particularly when the heat flow speed is large, the thermal shock resistance is largely influenced by the coefficient of thermal expansion only. Under these circumstances, development of low expansion materials having excellent thermal shock resistance have earnestly been demanded. As relatively low expansion ceramic materials having a coefficient of thermal expansion of around 5 to 20×10⁻⁷ (1/C°) in a temperature range from 40°C to 800°C, cordierite (MAS), lithium-aluminum-silicate (LAS), etc. are known. Their melting points are low, that is the melting point of cordierite is 1,450°C and LAS has a melting point of 1,423°C. For instance, in the case of ceramic honeycomb structures used as catalyst substrate for automobile catalyst purifiers, a fitting position of a catalyst converter is changed from a conventional underbed to near an engine to improve catalytic purifying efficiency, or a temperature of exhaust gases increases due to design changes, for instance, attachment of a turbocharger to improve fuel mileage and engine output. Consequently, since a temperature of catalyst beds increases by 100 to 200°C, it has been turned out that even a cordierite base honeycomb structural substrate having a high melting point may clog due to melting. Accordingly, development of low expansion materials having thermal shock resistance equivalent to or more excellent than that of cordierite as well as excellent heat resistance have been strongly desired. As ceramic materials having relatively low thermal expansion and high heat resistance, mullite (3Aℓ₂O₃·2SiO₂, coefficient of thermal expansion: 53×10⁻⁷/°C, melting point: 1,750°C) and zircon (ZrO₂·SiO₂, coefficient of thermal expansion: 42×10⁻⁷/°C, melting point: 1,720°C) only are available. However, they have drawbacks that their coefficients of thermal expansion are both high and their thermal shock resistance is low. As known examples of low expansion ceramic materials mainly consisting of zirconyl phosphate, there are high strength zirconyl phosphate sintered bodies disclosed in Japanese patent publication No. 61-12,867 which each contain 2 to 10 mol% of a mixture of SiO₂/Nb₂O₅ at a mixing molar ratio of 1 to 8 and 1 to 6 mol% of Aℓ₂O₃, low expansion zirconium phosphate porcelains disclosed in Japanese patent application laid-open No. 61-21,853 which each contain 0.5 to 0.6% by weight of magnesium phosphate as sintering aid, low thermal expansion zirconyl phosphate ceramic materials disclosed in Japanese patent application Laid-open No. 61-219,753 which contain at least one kind of a material selected from each of a group consisting of ZnO, MgO, Bi₂O₃, MnO₂, Co₂O₃, NiO, TiO₂, CeO₂, Nb₂O₅ and Ta₂O₅ as a sintering aid and a group consisting of SiO₂ and a silicate as a grain growth retardant in a total amount of 0.3 to 10% by weight, an amount of said at least one kind of the material from each of the groups being not less than 0.1% by weight, zirconium phosphate ceramic materials disclosed in Ceramic Technology Study Annual Report No. 9, pp 23 to 30 (1982) published by Nagoya Institute of Technology, which each contain 2% by weight of an additive such as MgO, MnO₂, Fe₂O₃, ZnO, etc. However, none of the above ceramic materials have good heat resistance because their sintering mechanism is a liquid phase sintering utilizing production of a liquid phase a having a low melting point. Thus, such techniques could not meet the above-mentioned demands. It is an object of the present invention to eliminate the above-mentioned problems. The invention provides zirconyl phosphate-zircon composite bodies having high heat resistance and a low coefficient of thermal expansion and a process for producing the same. According to a first aspect of the present invention, there are provided heat resisting low expansion zirconyl phosphate-zircon composite bodies which contain zirconyl phosphate and zircon as a main crystalline phase and a secondary crystalline phase, respectively, wherein a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C and a melting point is not less than 1,600°C. According to a second aspect of the present invention, there are provided the heat resisting low expansion zirconyl phosphate-zircon composite bodies which contain 58.2 to 65.4% by weight of ZrO₂, 17.4 to 37.1% by weight of P₂O₅ and 1.5 to 19.0% by weight of SiO₂ in the chemical composition. According to a third aspect of the present invention, there are provided heat resisting low expansion zirconyl phosphate-zircon composite bodies which have a chemical composition essentially consisting of 61.3 to 65.4% by weight of ZrO₂, 18.3 to 37.1% by weight of P₂O₅ and 1.6 to 16.4% by weight of SiO₂, wherein zirconyl phosphate and zircon are contained as a main crystalline phase and a secondary crystalline phase, respectively, a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C, and a melting point is not less than 1,600°C. According to a fourth aspect of the present invention, there are provided heat resisting low expansion zirconyl phosphate-zircon composite bodies which have a chemical composition essentially consisting of 58.2 to 65.3% by weight of ZrO₂, 17.4 to 37.1% by weight of P₂O₅, 1.6 to 19.0% by weight of SiO₂, and the balance being MgO and Aℓ₂O₃ in a total amount of not more than 2.5% by weight, wherein zirconyl phosphate and zircon are contained as main crystalline phase and a secondary crystalline phase, respectively, a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C, and a melting point is not less than 1,600°C. According to a fifth aspect of the present invention, there are provided heat resisting low expansion zirconyl phosphate-zircon composite bodies which have a chemical composition essentially consisting of 58.8 to 65.3% by weight of ZrO₂, 17.6 to 37.1% by weight of P₂O₅, 1.5 to 16.4% by weight of SiO₂, and 0.1 to 4% by weight of Nb₂O₅, wherein zirconyl phosphate and zircon are contained as a main crystalline phase and a secondary crystalline phase, respectively, a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C and a melting point is not less than 1,600°C. According to a sixth aspect of the present invention, there is provided a process for producing zirconyl phosphate-zircon composite bodies which contains zirconyl phosphate and zircon as a main crystalline phase and a secondary crystalline phase, respectively, and have a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C being not more than 30×10⁻⁷/°C and a melting point being not less than 1,600°C, said process comprising adding 5 to 50% by weight of zircon (ZrSiO₄) to zirconyl phosphate {(ZrO)₂P₂O₇} and sintering a thus obtained mixture. In the above compositions, the composite bodies are obtained by coexisting zircon (ZrSiO₄) having high heat resistance and relatively low expansion with zirconyl phosphate {(ZrO)₂P₂O₇} as a low expansion ceramic material. Thus, ceramic materials having excellent heat resistance and thermal shock resistance in which a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C and a melting point is not less than 1,600°C can be obtained. None of the ceramic materials of the prior art based on zirconyl phosphate described above contain zircon as a secondary phase. These and other optional features and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a diagram showing dependency of the coefficient of thermal expansion of zirconyl phosphate-zircon composite bodies according to the third aspect of the present invention upon an addition amount of zircon; Fig. 2 is a diaphragm showing dependency of a four point flexural strength of the zirconyl phosphate-zircon composite bodies upon the addition amount of zircon; Fig. 3 is a diagram showing relation between the self-weight softening percentage at 1,300°C of the zirconyl phosphate-zircon composite bodies and the total amount of oxides of an alkali metal and a alkaline earth metal; Fig. 4 is a diagram showing relation between the molar ratio of ZrO₂/P₂O₅ in a zirconyl phosphate starting material used for the production of the zirconyl phosphate-zircon composite bodies and the coefficient of thermal expansion of the zirconyl phosphate-zircon composite bodies; Fig. 5 is a diagram showing an X ray diffraction pattern of a zirconyl phosphate-zircon composite body in Example 3 in which ZR denotes peaks of zircon; Fig. 6 is a diagram showing a thermal expansion curve of the zirconyl phosphate-zircon composite body in Example 3; Fig. 7 is a view showing a method of measuring the self-weight softening percentage; Fig. 8 is a diagram showing dependency of the coefficient of thermal expansion of zirconyl phosphate-zircon composite bodies according to the fourth aspect of the present invention upon the addition amount of zircon; Fig. 9 is a diagram showing dependency of the four point flexural strength of the zirconyl phosphate-zircon composite bodies upon the addition amount of zircon; Fig. 10 is a diagram showing relation between the self-weight softening percentage at 1,300°C of the zirconyl phosphate-zircon composite bodies and the total amount of oxides of an alkali metal and an alkaline earth metal excluding MgO; Fig. 11 is a diagram showing relation between the self-weight softening percentage at 1,300°C of the zirconyl phosphate-zircon composite bodies and the addition amount of cordierite; Fig. 12 is a diagram showing relation between the molar ratio of ZrO₂/P₂O₅ in zirconyl phosphate starting materials used for producing the zirconyl phosphate-zircon composite bodies and the coefficient of thermal expansion of the zirconyl phosphate-zircon composite body; Fig. 13 is a diagram showing an X ray diffrac tion pattern of a zirconyl phosphate-zircon composite body in Example 33 in which ZR denotes peaks of zircon; Fig. 14 is a diagram showing a thermal expansion curve of the zirconyl phosphate-zircon composite body in Example 33; Fig. 15 is a diagram showing dependency of the coefficient of thermal expansion of the zirconyl phosphate-zircon composite bodies according to the fifth aspect of the present invention upon the addition amount of zircon; Fig. 16 is a diagram showing dependency of the four point flexural strength of the zirconyl phosphate-zircon composite bodies; Fig. 17 is a diagram showing relation between the self-weight softening percentage at 1,300°C of the zirconyl phosphate-zircon composite bodies and a total amount of oxides of an alkali metal and an alkaline earth metal; Fig. 18 is a diagram showing the relation between the self-weight softening percentage at 1,300°C of the zirconyl phosphate-zircon composite bodies and the addition amount of Nb₂O₅; Fig. 19 is a diagram showing relation between the molar ratio of ZrO₂/P₂O₅ in a zirconyl phosphate starting material used for the production of the zirconyl phosphate-zircon composite bodies and the coefficient of thermal expansion of the zirconyl phosphate-zircon complex sintered bodies. Fig. 20 is a diagram showing an X ray diffraction pattern of a zirconyl phosphate-zircon composite body in Example 63 in which ZR denotes peaks of zircon; and Fig. 21 is a diagram showing a thermal expansion curve of the zirconyl phosphate-zircon composite body in Example 63. Zircon coexisisting with zirconyl phosphate is to compensate difficult sinterability of zirconyl phosphate, thereby promoting sintering. Further, since zirconyl phosphate readily produces a liquid phase of a low melting point together with an oxide of an alkali metal or an alkaline earth metal, it gives a low strength sintered body through abnormal grain growth or causes a softening deformation at high temperatures if such an impurity exists. However, such abnormal grain growth and softening deformation at high temperatures can be restrained by coexisting zircon. In the sintered body in which zircon is coexisting, an open porosity is reduced with strength being increased by further addition of MgO, Aℓ₂O₃ and SiO₂ without lowering heat resistance. The reason why the addition amount of zircon to zirconyl phosphate is limited to 5 to 50% by weight in the producing process according to the present invention is that if zircon is less than 5% by weight, a desired strength cannot be obtained, while if it exceeds 50% by weight, the coefficient of thermal expansion becomes larger. In this case, it is necessary that with respect to the chemical composition ZrO₂, P₂O₅, and SiO₂ are 58.2 to 65.4% by weight, 17.4 to 37.1% by weight, and 1.5 to 19.0% by weight, respectively. If ZrO₂ exceeds 65.4% by weight, m-ZrO₂ precipitates to result in high expansion, while if it is less than 58.2% by weight, ZrP₂O₇ precipitates to also result in high expansion. If P₂O₅ is over 37.1% by weight, ZrP₂O₇ precipitates to result in high expansion, while if it is less than 17.4% by weight, m-ZrO₂ precipitates to result in high expansion. If SiO₂ exceeds 19.0% by weight, over 50% by weight of zircon precipitates to result in high expansion, while if it is less than 1.5% by weight, less than 5% by weight of zircon precipitates to exhibit no sufficient strength effects. A range from 5 to 35% by weight of zircon to zirconyl phosphate is preferable. In this case, it is necessary that with respect to the chemical composition, ZrO₂, P₂O₅ and SiO₂ are 58.2% to 64.8% by weight, 22.5 to 37.1% by weight, and 1.5 to 14.1% by weight, respectively. The reason by MgO, Aℓ₂O₃ and SiO₂ are added in a form of a cordierite compound is that since a cordierite phase has a relatively high melting point and stable, abnormal grain growth occurring in the sintered body when these additives are independently added can be controlled. It is preferable from the standpoint of improving the heat resistance that the total content of oxides of an alkali metal, and an alkaline earth metal contained in the heat resisting low expansion ceramic materials according to the present invention excluding MgO is not more than 0.5% by weight. For this purpose, it is preferable to use, as starting materials, a zirconyl phosphate starting material and a zircon starting material each containing not more than 0.5% by weight of the oxides of the alkali metal and the alkaline earth metal and a cordierite starting material containing not more than 1% by weight of the oxide of the alkali metal and an alkaline earth metal excluding MgO so as to control the content of the oxide of the alkali metal and alkaline earth metal in the sintered body. It is preferable that the molar ratio of ZrO₂/P₂O₅ in the zirconyl phosphate starting material is from 1.80 to 2.00. By using the zirconyl phosphate starting material having the molar ratio restricted in such a range, m-ZrO₂ can be prevented from precipitating in the sintered body, the coefficient of thermal expan sion of the sintered body can be lessened, and abnormal expansion-shrinkage due to a phase transformation of m-ZrO₂ precipitated can be restrained. Since the abnormal expansion-shrinkage of the precipitated m-ZrO₂ reversibly occurs at a temperature of around 1,000°C, the sintered body is damaged during use in heat cyclining, which is very harmful from the standpoint of the practical application because of lowered strength and dimensional change due to microcrack growth. In the following, an aspect of the present invention will be explained with reference to specific examples. They are merely given in illustration of the invention, but should never be interpreted to limit the scope of the invention. After adjustment of the grain size, zirconyl phosphate, zircon, magnesia, mullite, aluminum phosphate, alumina, spinel, and kaolin were mixed together according to a compounding recipe given in Table 1. The grain size of the zirconyl phosphate was adjusted by using a vibration mill, a pot mill or an attritor mill in which ZrO₂ sintered grinding media having about 5 mm in diameter were filled. As the ZrO₂ sintered grinding media, ZrO₂ stabilized with MgO or Y₂O₃ was used. The chemical compositions of the grinding media used are shown in Table 2. Chemical analysis values of the starting materials used are shown in Table 3. 5 parts by weight of a 10% PVA aqueous solution was added to 100 parts by weight of a formulated mixture shown in Table 1, which were fully mixed, press molded by a mold of 25×80×6 mm at a pressure of 100 kg/cm², and pressed with rubber at a pressure of 2 ton/cm², followed by drying. The dried molding was fired in air in an electric furnace under conditions shown in Table 1. The temperature rising speed was from 5°C/h to 1,700°C/h. After the firing, the fired sintered body was machined in a form of 3×4×40 mm as a flexural strength test piece shown in JIS R 1601(1981), and a coefficient of thermal expansion at a temperature range from 40 to 1,400°C, a four point flexural strength, a self-weight softening percentage, an open porosity and a melting point were measured. In the measurement of the coefficient of thermal expansion, a push rod indication type thermal expansion meter using a high purity alumina sintered body was employed. The measurement temperature range was from 40 to 1,400°C. The four point flexural strength was measured according to a method described in JIS R 1601. The self-weight softening percentage was determined according to the following equation by placing the above flexural strength test piece of 3×4×40 mm on supports separated as in Fig. 7 at a span of 30 mm, heating it in air at 1,300°C for 5 hours, and measuring a self-weight displacement Δx. Self-weight softening percentage=Δx/ℓ × 100(%) In the above equation, "ℓ" is a distance between the supports(=30 mm). The open porosity was measured according to an Archimedean method. With respect to the melting point, whether a sintered body cut in a form of 3×4×5 mm was melt or not was visually judged when it was heated in an electric furnace at 1,650°C for 10 minutes. Amounts of crystalline phases in the sintered body were quantitatively measured by the intensity of a (101) reflected peak of zircon (ZrSiO₄) and the intensity of a (002) reflected peak of zirconyl phosphate {β(ZrO)₂P₂O₇}. With respect to other different crystalline phases, their presence or absence only were detected from an X-ray diffraction pattern. MgO-stabilized ZrO₂ grinding media Y₂O₂-stabilized ZrO₂ grinding media SiO₂ 0.28 <0.05 Aℓ₂O₃ 0.037 <0.02 Fe₂O₃ 0.14 0.10 TiO₂ 0.12 <0.01 CaO 0.29 <0.005 MgO 3.37 0.007 K₂O 0.010 <0.005 Na₂O 0.047 <0.01 Y₂O₃ - 5.04 ZrO₂ 94.19 92.90 HfO₂ 1.65 2.11 Total 100.13 100.16 From results in Examples 1 to 7 and Comparative Examples 11 to 23 shown in Table 1, it is seen that the sintered bodies aimed at by the present invention in which the coefficient of thermal expansion in a temperature range from room temperature to 1,400°C was not more than 30×10⁻⁷/°C and a melting point was not less than 1,600°C could be obtained when ZrO₂, P₂O₅, and SiO₂ were contained 61.3 to 65.4% by weight, 18.3 to 37.1% by weight, and 1.6 to 16.4% by weight, respectively, and zirconyl phosphate and zircon were contained as a main crystalline phase and a secondary crystalline phase, respectively. Such sintered bodies could be obtained when a mixture in which 5 to 50% by weight of zircon was added to zirconyl phosphate was sintered under firing conditions shown in Table. 1. In this case, it is necessary that with respect to the chemical composition, ZrO₂, P₂O₅, and SiO₂ are 61.3 to 65.4% by weight, 18.3 to 37.1% by weight, and 1.6 to 16.4% by weight, respectively. If ZrO₂ exceeds 65.4% by weight, m-ZrO₂ precipitates to result in high expansion, while if it is less than 61.3% by weight, ZrP₂O₇; precipitates to result in high expansion. Also, if P₂O₅ exceeds 37.1% by weight, ZrP₂O₇ precipitates to result in high expansion, while if it is less than 18.3% by weight, ZrO₂ precipitates to result in high expansion. If SiO₂ exceeds 16.4% by weight, an amount of zircon precipitated exceeds 50% by weight to result in high expansion, while if it is less than 1.6% by weight, the amount of zircon precipitated is less than 5% by weight, so that an effect of enhancing strength cannot fully be obtained. The amount of precipitated zircon is preferably 5 to 35% by weight. In this case, the chemical composition is that ZrO₂ is 61.3 to 64.8% by weight, P₂O₅ is 23.7 to 37.1% by weight, and SiO₂ is from 1.6 to 11.5% by weight, and the coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is as low as not more than 20×10⁻⁷/°C. Fig. 1 shows relation between the addition amount of zircon and the coefficient of thermal expansion. Fig. 2 shows relation between the addition amount of zircon and the four point flexural strength. Further, it is evident from Fig. 3 showing the relation between the self-weight softening percentage at 1,300°C of the zirconyl phosphate-zircon composite sintered bodies and the total amount of oxides of an alkal metal and an alkaline earth metal that if the total amount of the oxides of the alkali metal and the alkaline earth metal exceed 0.5%, the self-weight softening percentage at 1,300°C increases and the heat resistance lowers. In order to obtain the sintered bodies free from the above disadvantages, it is necessary that the total amount of the alkali metal and the alkaline earth metal contained in the zirconyl phosphate starting material and the zircon starting material is not more than 0.5% by weight. It is also important to control the molar ratio between ZrO₂ and P₂O₅ in the zirconyl phosphate starting material in a range from 1.80 to 2.00. If it exceeds 2.00, the sintered bodies cannot be practically used, because monoclinic ZrO₂ precipitates to increase the coefficient of thermal expansion of the sintered body, and the sintered body undergoes serious damages due to rapid shrinkage caused by phase transformation of ZrO₂ from the monoclinic system to the tetragonal system and a rapid expansion caused by the phase transformation from the tetragonal phase to the monoclinic phase. On the other hand, if it is less than 1.80, an amount of a precipitated (ZrO)₂P₂O₇ phase is not sufficient, so that the coefficient of thermal expansion of the sintered body increases. Thus, the sintered body cannot be used as a low expansion material. Fig. 4 shows a relation between the molar ratio of ZrO₂/P₂O₅ and the coefficient of thermal expansion. Fig. 5 shows an X-ray diffraction pattern of a zirconyl phosphate-zircon composite body in Example 3. It is seen that a main component of the crystalline phase is zirconyl phosphate and the secondary crystalline phase is zircon ("ZR" denotes peak of zircon). Fig. 6 is a thermal expansion curve of the zirconyl phosphate-zircon composite body in Example 3. It is seen that the sintered body did not soften in a temperature range from room temperature to 1,400°C. Next, another aspect of the present invention will be explained with reference to the following examples. Zirconyl phosphate, zircon, magnesia, mullite, aluminum phosphate, alumina, spinel, kaolin and cordierite which had preliminarily been adjusted in grain size were mixed according to a compounding recipe shown in Table 4. The zirconyl phosphate was adjusted in grain size by using a vibration mill, a pot mill or an attritor mill which was filled with ZrO₂ sintered grinding media having about 5 mm in diameter. ZrO₂ sintered grinding media stabilized with MgO or Y₂O₃ was used. As in the case of the aforementioned Examples, the chemical compositions of the grinding media used are shown in Table 2. Chemical analysis values of the starting materials used are shown in Table 3. 5 parts by weight of a 10% PVA aqueous solution was added to 100 parts by weight of a formulated mixture shown in Table 4, which were sufficiently mixed, press molded at a pressure of 100 kg/cm² by using a mold of 25×80×6 mm, and rubber pressed at a pressure of 2 ton/cm², followed by drying. The thus dried molding was fired in air in an electric furnace under conditions shown in Table 4. A temperature elevating speed was 5°C/h to 1,700°C. After the firing, the thus sintered body was machined in a form of 3×4×40 mm as a flexural strength test piece shown in JIS R 1601 (1981). In the same manners as in the aforementioned Examples, coefficient of thermal expansion in a temperature range from room temperature to 1,400°C, four point flexural strength, self-weight softening percentage, open porosity, and melting point were measured. From results of Examples 31 to 39 and Comparative Examples 40 to 53 shown in Table 4, the sintered bodies aimed at by the present invention in which the coefficient of thermal expansion in a temperature range from room temperature to 1,400°C was not more than 30×10⁻⁷/°C and a melting point was not less than 1,600°C could be obtained when the sintered body consisted of 58.2 to 65.3% by weight of ZrO₂, 17.4 to 37.1% by weight of P₂O₅, 1.6 to 19.0% by weight of SiO₂, and the balance being MgO and Aℓ₂O₃ in a total amount of not more than 2.5% by weight, and contained zirconyl phosphate and zircon as a main crystalline phase and a secondary crystalline phase, respectively. Such sintered bodies could be obtained by sintering a formulated mixture in which 0.1 to 5 parts by weight of cordierite was added to 100 parts by weight of a batch mixture in which 5 to 50% by weight of zircon was added to zirconyl phosphate under firing conditions shown in Table 4. It is necessary that with respect to the chemical composition, ZrO₂, P₂O₅, SiO₂, and MgO+Aℓ₂O₃ are 58.2 to 65.3% by weight, 17.4 to 37.1% by weight, 1.6 to 19.0% by weight, and not more than 2.5% by weight, respectively. If ZrO₂ exceeds 65.3% by weight, m-ZrO₂ precipitates to result in high expansion, while if it is less than 58.2% by weight, ZrP₂O₇ precipitates to result in high expansion. Also, if P₂O₅ is more than 37.1% by weight, ZrP₂O₇ precipitates to result in high expansion, while if it is less than 17.4% by weight, m-ZrO₂ precipitates to result in high expansion. If SiO₂ exceeds 19.0% by weight, an amount of zircon precipitated exceeds 50% by weight to result in high expansion, while if it is less than 1.6% by weight, the amount of precipitated zircon is less than 5% by weight, so that an effect of improving strength cannot fully be obtained. If a total amount of MgO+Aℓ₂O₃ is over 2.5% by weight, heat resistance of the sintered body lowers and abnormal grain growth occurs to lower strength. The amount of precipitated zircon is particularly desirably 5 to 35% by weight. In this case, the chemical composition is that ZrO₂, P₂O₅, SiO₂, and MgO+Aℓ₂O₃ are 58.2 to 64.7% by weight, 22.5 to 37.1% by weight, 1.6 to 14.1% by weight, and not more than 2.5% by weight, respectively, and the coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is as low as not more than 20×10⁻⁷/°C. Fig. 8 shows relation between the addition amount of zircon and the coefficient of thermal expansion, and Fig. 9 does the relation between the addition amount of zircon and the four point flexural strength. Moreover, as obvious from Fig. 10 showing the relation between the self-weight softening percentage at 1,300°C and the total amount of oxides of an alkali metal and an alkaline earth metal excluding MgO in the zirconyl phosphate-zircon composite sintered bodies and Fig. 11 showing relation between the self-weight softening percentage and the total amount of MgO and Aℓ₂O₃, the total amount of the oxides of the alkali metal and the alkaline earth metal in the sintered body excluding MgO exceeds 0.5% by weight or the total amount of MgO and Aℓ₂O₃ exceeds 2.5% by weight, the self-weight softening percentage at 1,300°C increases and the heat resistance lowers. In order to obtain the sintered bodies according to the present invention, it is necessary that the total amount of the oxides of the alkali metal and the alkaline earth metal contained in each of the zirconyl phosphate starting material and the zircon starting material is not more than 0.5% by weight and the total amount of the oxides of the alkali metal and the alkaline earth metal in the cordierite starting material excluding MgO is not more than 1% by weight. It is also important to control the molar ratio between ZrO₂ and P₂O₅ in the zirconyl phosphate starting material in a range from 1.80 to 2.00. If this molar ratio exceeds 2.00, the sintered bodies undergo serious damages because of increased coefficient of thermal expansion of the sintered bodies due to the precipitation of monoclinic ZrO₂, rapid shrinkage due to the phase transformation of ZrO₂ from the monoclinic system to the tetragonal system and rapid expansion due to the phase transformation from the tetragonal system to the monoclinic system. Thus, the sintered bodies cannot be practically used. If it is less than 1.80, the coefficient of thermal expansion of the sintered body increases because a sufficient amount of (ZrO)₂P₂O₇ phase is not precipitated. Thus, the obtained sintered bodies cannot be practically used as low expansion materials. Fig. 12 shows relation between the molar ratio of ZrO₂/P₂O₅ and the coefficient of thermal expansion. Fig. 13 shows an X-ray diffraction pattern of a zirconyl phosphate-zircon complex sintered body in Example 33. It is seen that a main component of the crystalline phase is zirconyl phosphate and a secondary crystalline phase is zircon ("ZR" denotes peaks of zircon). Fig. 14 shows a thermal expansion curve of the zirconyl phosphate-zircon complex sintered body in Example 33. It is seen that the sintered body did not soften at a temperature range from room temperature to 1,400°C. Next, the third aspect of the present invention will be explained with reference to the following examples. They are merely given in illustration of the invention, but should never be interpreted to limit the scope thereof. Zirconyl phosphate, zircon, magnesia, mullite, aluminum phosphate, alumina, spinel, kaolin, and Nb₂O₅ which had been adjusted in grain size were mixed according to a compounding recipe shown in Table 6. The grain size of the zirconyl phosphate was adjusted by using a vibration mill, a pot mill, or an attritor mill which was filled with ZrO₂ sintered grinding media of about 5 mm in diameter. As the ZrO₂ sintered grinding media, ZrO₂ stabilized with MgO or Y₂O₃ was used. The chemical composition of the grinding media used was shown in Table 2 as in the same as in the above Examples. Table 3 shows chemical analysis values of the starting materials used. 5 parts by weight of a 10% PVA aqueous solution was added to 100 parts by weight of a formulated mixture shown in Table 6, which were fully mixed, press molded at a pressure of 100 kg/cm² by using a mold of 25×80×6 mm, and rubber pressed at a pressure of 2 ton/cm², followed by drying. The thus dried molding was fired in air in an electric furnace under conditions shown in Table 6. A temperature elevating speed was in a range from 5°C/h to 1,700°C/h. After the firing, the sintered body was machined in a form of 3×4×40 mm as a flexural strength test piece shown in JIS R 1601 (1981), and in the same manners as in the aforementioned examples, coefficient of thermal expansion, four point flexural strength, self-weight softening percentage, open porosity, and melting point were measured. The coefficient of thermal expansion was measured by using a push rod indicating type thermal expansion meter employing a high purity alumina sintered body. From results of Examples 61 to 68 and Comparative Examples 70 to 83 shown in Table 6, the sintered bodies aimed at by the present invention in which the coefficient of thermal expansion in the temperature range from room temperature to 1,400°C was not more than 30×10⁻⁷ and the melting point was not less than 1,600°C could be obtained when ZrO₂, P₂O₅, SiO₂, and Nb₂O₅ were contained in amounts of 58.8 to 65.3% by weight, 17.6 to 37.1% by weight, 1.5 to 16.4% by weight, and 0.1 to 4% by weight, respectively, and zirconyl phosphate and zircon were contained as a main crystalline phase and secondary crystalline phase, respectively. Further, such sintered bodies could be obtained by firing a formulated mixture in which 0.1 to 4 parts by weight of Nb₂O₅ was added to 100 parts by weight of a batch mixture in which 5 to 50% by weight of zircon was added to zirconyl phosphate under firing conditions shown in Table 5. In this case, it is necessary that with respect to the chemical composition, ZrO₂, P₂O₅, SiO₂, and Nb₂O₅ are 58.8 to 65.3% by weight, 17.6 to 37.1% by weight, 1.5 to 16.4% by weight, and 0.1 to 4% by weight, respectively. If ZrO₂ exceeds 65.3% by weight, m-ZrO₂ precipitates to result in high expansion, while if it is less than 58.8% by weight, ZrP₂O₇ precipitates to result in high expansion. Also, if P₂O₅ exceeds 37.1% by weight, ZrP₂O₇ precipitates to result in high expansion, while if it is less than 17.6% by weight, m-ZrO₂ precipitates to result in high expansion. If SiO₂ exceeds 16.4% by weight, an amount of zircon precipitated exceeds 50% by weight to result in high expansion, while if it is less than 1.5% by weight, the amount of zircon precipitated is less than 5% by weight, so that an effect of improving strength cannot fully be obtained. If Nb₂O₅ exceeds 4% by weight, heat resistance of the sintered body unfavorably lowers. The amount of zircon precipitated is particularly desirably 5 to 35% by weight. In this case, the chemical composition is that ZrO₂, P₂O₅, SiO₂, and Nb₂O₅ are 58.5 to 64.7% by weight, 22.7 to 37.1% by weight, 1.5 to 11.5% by weight, and 0.1 to 4% by weight, respectively, and the coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is as low as not more than 20×10⁻⁷/°C. Fig. 15 shows relation between the addition amount of zircon and the coefficient of thermal expansion, and Fig. 16 does relation between the addition amount of zircon and the four point flexural strength. Further, as obvious from Fig. 17 showing relation between the self-weight softening percentage at 1,300°C and the total amount of oxides of an alkali metal and an alkaline earth metal in the zirconyl phosphate-zircon complex sintered bodies and Fig. 18 showing relation between the self-weight softening percentage at 1,300°C and the amount of Nb₂O₅, if the total amount of the oxides of the alkali metal and the alkaline earth metal in the sintered body exceeds 0.5% by weight, the self weight softening percentage at 1,300°C increases and the heat resistance lowers. In order to obtain sintered bodies free from the above disadvantages, it is necessary that the total amount of the oxides of the alkali metal and the alkaline earth metal contained in the zirconyl phosphate starting material, the zircon starting material, and the Nb₂O₅ starting material is not more than 0.5% by weight. It is also important to control the molar ratio between ZrO₂ and P₂O₅ in the zirconyl starting material in a range from 1.80 to 2.00. If the molar ratio exceeds 2.00, the sintered body undergoes serious damages, because of increased coefficient of thermal expansion of the sintered body due to the precipitation of monoclinic ZrO₂, rapid shrinkage due to the phase transformation of ZrO₂ from the monoclinic system to the tetragonal system, and rapid expansion due to the phase transformation of ZrO₂ from the tetragonal phase to the monoclinic system. Thus, the sintered body cannot be practically used. If this value is less than 1.80, since a sufficient amount of the (ZrO)₂P₂O₇ phase does not precipitate, the coefficient of thermal expansion of the sintered body increases. Thus, the sintered body cannot be used as a low expansion material. Fig. 19 shows relation between the molar ratio of ZrO₂/P₂O₅ and the coefficient of thermal expansion. Fig. 20 shows an X-ray diffraction pattern of a zirconyl phosphate-zircon complex sintered body in Example 63. It is seen that a main component of the crystalline phase is zirconyl phosphate and a secondary crystalline phase is zircon (ZR denotes peak of zircon). Fig. 21 shows a thermal expansion curve of the zirconyl phosphate-zircon complex sintered body in Example 63. It is seen that the sintered body did not soften in a temperature range from room temperature to 1,400°C. As obvious from the above explanation, according to the heat resisting low expansion zirconyl phosphate-zircon composite sintered bodies and their producing process in the present invention, the heat resisting low expansion ceramic materials in which the coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C and the melting point is not less than 1,600°C can be obtained when a chemical composition in which ZrO₂, P₂O₅ and SiO₂, and optionally further MgO and Aℓ₂O₃ or Nb₂O₅ are contained in a specific amounts, and zirconyl phosphate and zircon are contained as the main crystalline phase and the secondary crystalline phase, respectively. Owing to this, applicable fields in which the zirconyl phosphate-zircon complex sintered bodies may be used as low expansion materials required to be thermal shock resisting are wide. The complex sintered bodies according to the present invention have sufficiently wide practical applicability, for instance, when the moldings are shaped in a honeycomb structure by extrusion molding or the like, they are used as rotary heat regenerating type ceramic heat exchangers, heat conductive type heat exchangers, etc. Further, when the moldings are shaped by a slip casting or, a press molding, an injection molding, or the like, the complex sintered bodies are used as heat insulating materials for use in housings of ceramic turbocharger rotors, or engine manifolds.

Claims: A heat resisting low expansion zirconyl phosphate-zircon composite body which contains zirconyl phosphate and zircon as a main crystalline phase and a secondary crystalline phase, respectively, wherein a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C and a melting point is not less than 1,600°C. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, which contains 58.2 to 65.4% by weight of ZrO₂, 17.4 to 37.1% by weight of P₂O₅ and 1.5 to 19.0% by weight of SiO₂ in the chemical composition. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, which contains 58.2 to 64.8% by weight of ZrO₂, 22.5 to 37.1% by weight of P₂O₅ and 1.5 to 14.1% by weight of SiO₂ in the chemical composition and contains zirconyl phosphate and zircon as the main crystalline phase and the secondary crystalline phase, respectively, wherein the coefficient of thermal expansion in the temperature range from room temperature to 1,400°C is not more than 20×10⁻⁷°C. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, which has a chemical composition essentially consisting of 61.3 to 65.4% by weight of ZrO₂, 18.3 to 37.1% by weight of P₂O₅, and 1.6 to 16.4% by weight of SiO₂. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, which contains 61.3 to 64.8% by weight of ZrO₂, 23.7 to 37.1% by weight of P₂O₅ and 1.6 to 11.5% by weight of SiO₂ in the chemical composition and contains zirconyl phosphate and zircon as the main crystalline phase and the secondary crystalline phase, respectively, wherein the coefficient of thermal expansion in the temperature range from room temperature to 1,400°C is not more than 20×10⁻⁷/°C. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, wherein a total amount of oxides of an alkali metal and an alkaline earth metal is not more than 0.5% by weight. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, which comprises 58.2 to 65.3% by weight of ZrO₂, 17.4 to 37.1% by weight of P₂O₅, 1.6 to 19.0% by weight of SiO₂, and the balance being MgO and Aℓ₂O₃ in a total amount of not more than 2.5% by weight. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 7, which contains 58.2 to 64.7% by weight of ZrO₂, 22.5 to 37.1% by weight of P₂O₅, 1.6 to 14.1% by weight of SiO₂, and the balance being MgO and Aℓ₂O₃ in a total amount of not more than 2.5% by weight, wherein the coefficient of thermal expansion in the temperature range from room temperature to 1,400°C is not more than 20×10⁻⁷/°C. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 7, wherein a total amount of oxides of an alkali metal and an alkaline earth metal excluding MgO is not more than 0.5% by weight. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 1, which has a chemical composition essentially consisting of 58.8 to 65.3% by weight of ZrO₂, 17.6 to 37.1% by weight of P₂O₅, 1.5 to 16.4% by weight of SiO₂, and 0.1 to 7% by weight of Nb₂O₅. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 10, which has the chemical composition essentially consisting of 58.8 to 64.7% by weight of ZrO₂, 22.7 to 37.1% by weight of P₂O₅, 1.5 to 11.5% by weight of SiO₂, and 0.1 to 4% by weight of Nb₂O₅, wherein the coefficient of thermal expansion at the temperature range from room temperature to 1,400°C is not more than 20×10⁻⁷/°C. A heat resisting low expansion zirconyl phosphate-zircon composite body according to claim 10, wherein the total amount of oxides of an alkali metal and an alkaline earth metal is not more than 0.5% by weight. A process for producing zirconyl phosphate-zircon composite body containing zirconyl phosphate and zircon as a main crystalline phase and a secondary crystalline phase, respectively, wherein a coefficient of thermal expansion in a temperature range from room temperature to 1,400°C is not more than 30×10⁻⁷/°C and a melting point is not less than 1,600°C, said process comprising adding 5 to 50% by weight of zircon (ZrSiO₄) to zirconyl phosphate [(ZrO)₂P₂O₇] to obtain a batch mixture and sintering the batch mixture. A process for producing a zirconyl phosphate-zircon composite body according to claim 13, wherein an addition amount of zircon is from 5 to 35% by weight and the coefficient of thermal expansion in the temperature range from room temperature to 1,400°C is not more than 20×10⁻⁷/°C. A process for producing a zirconyl phosphate-zircon composite body according to claim 13, wherein zirconyl phosphate and zircon which totally contain not more than 0.5% by weight of oxides of an alkali metal and an alkaline earth metal are used. A process for producing a zirconyl phosphate-zircon composite body according to claim 13, wherein 0.1 to 5 parts by weight of cordierite (2MgO·2Aℓ₂O₃.5SiO₂) is added to 100 parts by weight of the batch mixture, and a thus obtained mixture is then sintered. A process for producing a zirconyl phosphate-zircon composite body according to claim 16, wherein an Nb₂O₅ starting material containing not more than 0.5% by weight of the oxides of the alkali metal and the alkaline earth metal oxides in the total amount is used. A process for producing a zirconyl phosphate-zircon composite body according to claim 16, wherein 0.1 to 4 parts by weight of Nb₂O₅ is added to 100 parts by weight of the batch mixture, and then a thus obtained mixture is sintered. A process for producing a zirconyl phosphate-zircon composite body according to claim 18, wherein a cordierite starting material containing not more than 1% by weight of the oxides of the alkali metal and the alkaline earth metal excluding MgO is used. A process for producing a zirconyl phosphate-zircon composite body according to claim 13, wherein a molar ratio of ZrO₂/P₂O₅ in a zirconyl phosphate starting material is 1.8 to 2.00.

Inventors: OHASHI, TSUNEAKI NGK TAKEMI-RYO, WATANABE, KEIICHIRO 4E CO.NAKANOMACHI HOUSE
Applicants: NGK INSULATORS, LTD.
Application year: 1987
Main classification: C04B35
IPC codes: C04B35