JPH0339967B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a heat-resistant composite oxide powder that is optimal for producing mullite and mullite-based sintered bodies that are dense and have excellent heat resistance and mechanical strength. [Prior art] Mullite is a composite oxide (3Al ₂ O ₃ .2SiO ₂ ) consisting of alumina and silica, and its stoichiometric composition ratio is Al ₂ O ₃ :SiO ₂ = 71.8:28.2 by weight. It is. By the way, conventionally, mullite with this structure is extremely stable thermally, and it is extremely difficult to obtain a high-density sintered body without pores by sintering powder consisting only of mullite. Ta. For this reason, what has traditionally been called mullite porcelain has a two-phase structure in which it contains more silica than the stoichiometric ratio of mullite, and the mullite particles are bound together by a matrix of this excess silica. The silica content here is 40 to 60% by weight, and the silica glass bonds the mullite particles. In such conventional mullite porcelain with an excess of silica, its thermal properties are largely controlled by the silica matrix, leading to deterioration in hot strength, creep properties, and heat resistance. Therefore, in order to eliminate the above-mentioned drawbacks of the conventional method and obtain a thermally stable high-purity mullite sintered body through the bonding of true mullite crystals, there is a method for producing high-purity mullite powder using chemical means. was considered. A representative method is the alkoxide method. This method involves hydrolyzing a mixed solution of aluminum and silicon alkoxides, separating the resulting hydroxide gel, drying it, and calcining it (Mazdiyasni and Brown, “Synthesis and Mechanism of Stoichiometric Aluminum Silicates (Mullite)”). ”Jornal of the American
Ceramics Society, Vol55, No.11, P548−552
(1972) etc.). In addition to this method, the easiest way to synthesize stoichiometric high-purity mullite is to mechanically mix high-purity crystalline alumina and silica and then react at high temperature to synthesize mullite. . However, this method requires extremely high temperatures to cause the alumina particles and silica particles to react. Furthermore, sintering requires a high temperature of 1800° C. or higher, and it is difficult to obtain a mullite sintered body with a dense stoichiometric ratio and no glass phase. Therefore, methods such as the alkoxide method were considered, but in this method, the conditions for hydrolysis of aluminum alkoxide and silicon alkoxide are different, so aluminum and silicon are not necessarily homogeneous. do not have. Furthermore, the alkoxide used as a raw material is very expensive and the reaction operation is complicated, so it is difficult to use it in an industrial production method. for example,
Even when a hydrolyzate containing a uniform mixture of aluminum and silicon on the atomic order is obtained using the alkoxide method, it is necessary to prevent the fine particles of aluminum silicate that precipitate during the drying and firing process of the hydrolyzate from agglomerating. If strong agglomeration occurs here, even if the powder is uniform,
In order to obtain a dense sintered body, it is necessary to use the pressure sintering method as in the Mazdiyasni literature. Next, as reported in recent years by Nils Claussen et al.
It is known that a sintered body in which zirconia particles, which are mainly present as tetragonal particles, are dispersed in a matrix of alumina or the like has significantly improved fracture strength, fracture toughness, and the like. However, this method does not have a sufficient effect on ceramics whose matrix itself cannot be sintered densely, and mullite is a typical example of ceramics that are difficult to sinter densely. A conventional method for obtaining this zirconia-dispersed mullite sintered body is to mechanically mix zirconia powder with mullite powder. However, in this method, the sinterability of mullite itself is important, and it is difficult to control the particle size, dispersibility, and crystal phase of the tetragonal zirconia particles, making it difficult to obtain a sintered body with sufficient performance. It is difficult. Therefore, the aforementioned Nils _Claussen et _al .
We are proposing a special method in which alumina and zircon (ZrSiO ₄ ) are used as starting materials, and a sintered body with zirconia particles dispersed in mullite is obtained through the reaction of alumina and zircon. However, this method has the disadvantage that it is difficult to change the content of zirconia particles. [Object of the Invention] An object of the present invention is to provide a method for producing a composite oxide powder, that is, a mullite powder, which overcomes the drawbacks of the above-mentioned mullite or mullite-based powder and has excellent properties. [Summary of the Invention] The gist of the present invention to achieve the above object is to include a water-soluble zirconium salt or a water-soluble zirconium salt and yttrium or cerium in an aqueous solution (A) consisting of a water-soluble aluminum salt and a silica sol. With or without mixing the aqueous solution (B),
Add an alkali to form a precipitate, dehydrate the water-containing precipitate by heating distillation in the presence of an organic solvent, separate the organic solvent, dry the precipitate, and further
A method for producing a heat-resistant mixed oxide powder, characterized by heating and firing at ~1600°C. The present invention uses water-soluble aluminum salt and silica sol as raw materials to mix Al ions and SiO ₂ particles as uniformly as possible in an aqueous solution state,
In addition, in order to prevent particle agglomeration during drying of the generated precipitate, azeotropic dehydration is performed in an organic solvent.
The area surrounding the precipitated particles is protected with an organic solvent to prevent particle agglomeration. Therefore, the mullite powder obtained by firing has less agglomeration and Al
and Si are uniform powders. The method for producing the powder according to the present invention will be described in more detail below. Water-soluble aluminum salt and silica sol are added to distilled water and sufficiently dissolved and mixed so as to have a predetermined mullite composition. The concentration of this mixed solution (hereinafter referred to as aqueous solution (A)) is preferably about 0.005 to 0.3 mol/approximately 0.005 to 0.3 mol/molar amount of the mullite powder to be finally synthesized. is 0.03 to 1.8 mol/, and the concentration of silica sol is 0.01 to 0.6 mol/ as SiO ₂ component. Mullite and mullite composition as used in the present invention mean a certain composition range in which the aluminum oxide content is centered around 71.8% by weight, which is the stoichiometric ratio of mullite. This range defined by the inventors is that the aluminum oxide content is 65 to 80% by weight. This mullite composition has a great influence on the properties of mullite, especially its high-temperature properties. That is,
From the phase diagram of the Al ₂ O ₃ −SiO ₂ system, it is thought that the single phase region of mullite is in the range of about 65 to 80% by weight of Al ₂ O ₃ , and the content of Al ₂ O When ₃ decreases, the glass phase mainly composed of silica increases, and the density during sintering increases, but high-temperature properties (strength, etc.)
decreases. Furthermore, when Al ₂ O ₃ increases to 80% by weight or more, an Al ₂ O ₃ phase appears in addition to the mullite phase, making it difficult to increase the density during sintering. By the way, no matter how much the raw material powder is prepared so that the chemical composition is in the single phase region of mullite, if Al ₂ O ₃ and SiO ₂ are non-uniform, it is impossible to obtain a sintered body consisting of the single phase of mullite. Can not. Therefore, it is necessary to synthesize a powder having a composition in the one-phase region, which is a uniform mixture of Al and Si as described in the present invention. Furthermore, even within the composition in the single-phase region, the high-temperature properties of the sintered body are thought to be better on the side with more Al ₂ O ₃ . As water-soluble aluminum salts used in the present invention, nitrates, sulfates, hydrochlorides, acetates, etc. are used. In addition, silica sol can be both acidic and alkaline, and there are different colloid particle sizes and Na content, but it is preferable to use one that is acidic, has as small a particle size as possible, and has a low Na content. . Other SiO ₂ sources include ethyl silicate,
SiCl ₄ Na ₂ SiO ₃ etc. are considered, but Na ₂ SiO ₃ is Na
It is not suitable because ethyl silicate becomes an impurity, and it takes time for ethyl silicate to be hydrolyzed by acid, and furthermore, the precipitated silica does not all precipitate as a colloid, but rather a considerable amount is dissolved in water and gels. easy. Therefore, it is unsuitable for coprecipitation by adding alkali. Moreover, the biggest drawback is that it is expensive. SiCl ₄ is a corrosive gas that is inconvenient to handle and difficult to add quantitatively. Compared to the SiO ₂ sources mentioned above, silica sol is a fine silica colloidal particle (approximately 100 Å), can be handled without any concerns, can be added quantitatively, and is inexpensive. Next, add an alkaline aqueous solution to this mixed aqueous solution of aluminum salt and silica sol while stirring.
Add until the pH is 6-8, preferably 6.5-7.5. This aqueous alkali solution is preferably an aqueous ammonia solution or an aqueous ammonium carbonate solution. Thus, the aluminum ions form a precipitate as a hydroxide gel with colloidal particles of silica. Next, after separating the precipitate from the mother liquor by an appropriate method such as filtration or centrifugation, the solution remaining in the precipitate and the products attached to the precipitate (for example, when aluminum nitrate is used, NH ₄ NO ₃ )
It is recommended to wash the precipitate with water to remove it. The precipitate obtained in this way contains a large amount of water, and if it is dried as it is, the fine particles in the precipitate will aggregate and bond firmly in the subsequent firing process, resulting in a sintering product. It is not preferred as a fine powder. Therefore, an organic solvent is added to these precipitates, and distillation is performed with the precipitates dispersed in the organic solvent.
At this time, water in the precipitate flows out together with the organic solvent, so the organic solvent replaces the water in the precipitate that causes aggregation of primary particles, making it possible to dehydrate while preventing aggregation. At this time, the amount of organic solvent added must be sufficient to sufficiently dehydrate the water in the precipitate (this amount is 4 or more in the case of butanol per 100 g of mullite to be synthesized). If an organic solvent is added to the aqueous solution containing the initial precipitate for dehydration without precipitation, a large amount of organic solvent is required and by-products remain, which is not a practical method. The organic solvent used in this distillation includes alcohol containing at least one type of alcohol having 3 to 10 carbon atoms, benzene, and the like, with butanol and isoamyl alcohol being particularly preferred. It is also possible to add a small amount of nonionic surfactant to the organic solvent in order to promote dispersion of the precipitated particles in the organic solvent. Distillation is usually carried out in a temperature range from the boiling point of the mixed solution with the precipitate to the boiling point of the organic solvent alone, taking into account the azeotropic point of the solvent and water, the degree of distillation, etc. After distillation, it is cooled, and suspended matter and precipitates in the liquid phase are separated and recovered from the liquid phase by means such as filtration or centrifugation, and the precipitates are dried under normal pressure or reduced pressure. For this drying, drying at a temperature up to the boiling point of the organic solvent is sufficient. The composite oxide powder of the present invention, that is, the mullite powder, is obtained by calcining this dried product preferably at 900 to 1600°C. When firing at temperatures below 900°C, a small amount of undecomposed substances still remain, and these undecomposed substances are likely to volatilize during sintering, creating voids in the sintered body. Furthermore, when firing at a temperature of 1600° C. or higher, crystal growth of mullite or mullite and zirconia particles occurs, and the agglomeration of these particles becomes strong, resulting in deterioration of sinterability. In this case, crystallization of mullite is not observed by X-ray diffraction when fired at temperatures below 1200°C, and the fired product is amorphous or has a structure similar to that of r-Al ₂ O ₃ . Although it has no effect on the sintering of mullite. Next, a case will be described in which a water-soluble zirconium salt is added as a third component to a mixed aqueous solution of a water-soluble aluminum salt and silica sol. Water-soluble zirconium salts include nitrates, hydrochlorides, acetates, etc., but zirconium oxychloride and zirconium nitrate are preferred. Further, the amount of the zirconium salt added may be such that the amount of zirconia is 3 to 40% by volume based on the total volume of mullite and zirconia finally obtained. Here, when the volume ratio of zirconia is 3% or less, the effects of the tetragonal zirconia particles cannot be expected to be great, but they are effective in obtaining a dense sintered body of mullite. Further, it becomes difficult to have 40% or more of zirconia particles present in the mullite matrix, and the heat resistance at high temperatures is easily influenced by the properties of zirconia, which is not preferable. Since the addition of this water-soluble zirconium salt is meaningful in that it is present as zirconium oxide in the synthesized mullite powder, methods of addition other than those described above are possible. For example, a method of adding a predetermined amount of zirconium hydroxide sol produced by adding aqueous ammonia to an aqueous solution of a water-soluble zirconium salt to an aqueous solution of aluminum salt and silica sol and mixing it; In addition, it is not impossible to mix the zirconium hydroxide sol with the sol produced, or to add it to the precipitate, dried material, etc. after filtration.
When adding this zirconium salt, a stabilizer can be further added, and as this stabilizer, itria or ceria is preferable since it is easy to obtain tetragonal zirconia particles. In this case, a water-soluble yttrium salt may be added at the same time as the zirconium salt so that yttrium oxide is contained in an amount of 1 to 8 mol % based on zirconium oxide. Further, a water-soluble salt of cerium may be used in place of the yttrium salt, and the amount of cerium oxide to be added may be 1 to 20 mol% relative to zirconium oxide. When a sintered body is produced using the mullite powder obtained according to the present invention, the mullite sintered body exhibits excellent physical properties. That is, the powder obtained by the present invention has Al and
Si is uniformly dispersed and has fine particles with weak aggregation. Therefore, when producing a sintered body, it is possible to obtain a dense sintered body at a lower temperature (1600 to 1700℃) than conventional mullite, which contains almost no glass phase mainly composed of silica. . In addition, in mullite powder containing zirconium oxide particles, Al and Si ions are uniform, and at the same time,
By containing zirconium oxide particles, the formation of glass phase is further reduced, and the sintering temperature can be lowered to 50 to 100°C compared to those that do not contain zirconium oxide particles.
It has a depressing effect. When the zirconium oxide particles contained in this mullite powder are sintered, they exist as zirconium oxide particles consisting mainly of tetragonal crystal particles uniformly dispersed in the mullite matrix, and depending on the sintering temperature, The particle size is then controlled. The presence of these tetragonal zirconium oxide particles makes the mullite sintered body excellent in strength and other physical properties. The strength of sintered bodies made of mullite alone that has been reported so far has a maximum bending strength of about 30 kg/mm ² . However, in a mullite sintered body containing 10% by volume of zirconia using the powder obtained according to the present invention, the weight loss was 50Kg/mm ² at room temperature.
It has a bending strength of over 40°C even at high temperatures of 1300°C.
A strength of approximately Kg/mm ² was obtained, which indicates that it has excellent heat resistance as a high-temperature structural material. [Examples of the Invention] The present invention will be specifically described below with reference to Examples, but the present invention is not limited thereto. Example 1 Aluminum nitrate aqueous solution (Al ₂ O ₃ equivalent content: 9
(% by weight) and 140 g of an aqueous silica sol solution (silica content: 20.0% by weight) were mixed and diluted to 5.
The composition of this formulation was adjusted to be mullite with an Al ₂ O ₃ /SiO ₂ weight ratio of 72/28. Aqueous ammonia was added to this aqueous solution with stirring, and the resulting colloidal precipitate was filtered out. This precipitate was then dispersed in n-butanol from step 4, and the water in the precipitate was removed by heating and distillation. Heating was stopped when the temperature reached ℃. After cooling, the precipitate in the liquid phase was filtered to collect the precipitate. This precipitate was dried at 80°C and calcined at 1300°C for 1 hour to obtain about 95g of mullite powder. The X-ray diffraction pattern of this powder confirmed that it was complete mullite, and the crystallite particle size determined from the diffraction peak from the <100> plane was 530 Å. This powder was pulverized using ethyl alcohol in a vibrating ball mill for 8 hours, dried and made into a powder for molding, molded into a disc with a diameter of 20 mm and a thickness of 6 mm using a rubber press at a pressure of 2 t/cm ² and sintered at 1650°C for 3 hours. I went there. The density of the obtained mullite was 3.12 g/cc. The atmosphere during sintering was air. Table 1 shows the above results, as well as the results of experiments using the same precipitate as above but with different firing temperatures, different sintering temperatures for compacts, and different sintering atmospheres. Indicated. Example 2 Experiments were conducted in the same manner as in Example 1, but with different mullite compositions, and the results are shown in Table 1. Example 3 Aluminum nitrate aqueous solution (Al ₂ O ₃ equivalent content 9.0
weight%) 800g, 140g of silica sol aqueous solution (silica content 20% by weight), and 113g of zirconium oxychloride aqueous solution (ZrO ₂ equivalent content 19% by weight)
were mixed and diluted to 5. Thereafter, a sintered body was manufactured in the same manner as in Example 1. Sintering was performed at 1620°C for 3 hours. The density of the obtained sintered body is 3.43g/cc
It was hot. Furthermore, when the sintered body was analyzed by X-ray diffraction, it was found to consist of two phases: mullite and zirconia, with 80% by weight of zirconia being tetragonal and 20% monoclinic.
% by weight, and the particle size of the zirconia particles was approximately 0.7 μm based on observation using a scanning electron microscope. The zirconia particles were uniformly dispersed in the mullite matrix. Table 2 shows the above results and the results of an experiment conducted in the same manner as above except that the sintering temperature of the compact was changed. Example 4 In the same manner as in Example 3, experiments were conducted with different mullite compositions, different zirconium oxychloride formulations, and different conditions for calcination of the precipitate and sintering of the compact, and the results were reported. It is listed in Table 2. Furthermore, an experimental example in which Y ₂ O ₃ was added in the form of YCl ₃ to an aqueous zirconium oxychloride solution at a concentration of 3 mol % relative to zirconia in terms of ZrO ₂ in the aqueous solution was also described. Example 5 Aluminum nitrate aqueous solution (Al ₂ O ₃ equivalent content 9.0
(% by weight) and 140 g of an aqueous silica sol solution (silica content: 20% by weight) were mixed and diluted to 5. Next, 0.81 g of Y ₂ O ₃ dissolved in 20 c.c. of HCl was added to 113 g of zirconium oxychloride aqueous solution (ZrO ₂ equivalent content 19% by weight), and this solution was diluted to 1.
Furthermore, a gel of zirconium and yttrium hydroxides was prepared using aqueous ammonia. This gel was added to the previously prepared mixed aqueous solution of aluminum nitrate and silica sol and thoroughly stirred, and then aqueous ammonia was further added to this solution to adjust the pH to 7.0 to obtain a precipitate. Thereafter, the same steps as in the other examples described above were carried out, and the dried precipitate was calcined at 1300° C. for 1 hour.
The powder obtained consisted of mullite and tetragonal _ZrO2 . This powder was pulverized using ethyl alcohol in a vibrating ball mill for 5 hours, molded in the same manner as in the previous example, and sintered at 1600°C. The obtained sintered body has a density of 3.40 g/cc and a zirconia particle size.
0.6 μm, and the tetragonal content of the zirconia particles was 96%. Table 2 shows examples with different contents of zirconium salt and itria. The content of ittria is expressed in mol% relative to zirconia obtained from the zirconium salt added.

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[Effects of the present invention]

As explained above, the present invention is a method for producing a raw material powder that provides a mullite sintered body with excellent properties. The effects of this invention will be specifically shown below. It is possible to produce powder that has uniform Al and Si content and can yield a dense sintered body. Therefore, the sintered body contains almost no glass phase and has extremely good heat resistance and creep properties. It is possible to produce a powder capable of obtaining a sintered body in which zirconium oxide or the like is finely and uniformly dispersed as a second phase in a mullite sintered body. Therefore, the zirconium oxide particles existing in the tetragonal form increase the strength of the sintered body through a stress-induced transformation strengthening mechanism. Compared to raw materials such as alkoxides, raw materials that are cheaper and easier to handle can be used. It has excellent strength at room and high temperatures, creep properties, thermal shock resistance, chemical resistance, abrasion resistance, etc., so it can be used for the following purposes. Gas turbine engine components such as combustion chambers, casings, and turbine blades; reciprocating engine components such as tappets, cams, auxiliary combustion chambers, and piston heads; acid- and alkali-resistant industrial equipment; chemical-resistant pumps, mechanical seals, and food products. Machining machine parts, precision measuring instruments, cutting and cutting knives, dies, wear-resistant parts, skid buttons for steel manufacturing, bearings and sliding parts, medical equipment parts, and insulating insulators. However, it goes without saying that these examples do not limit the scope of use of the present invention.

Claims (1)

[Claims] 1. Adding an alkali to an aqueous solution (A) consisting of a water-soluble aluminum salt and silica sol to form a precipitate,
A heat-resistant composite oxide powder characterized in that the water-containing precipitate is dehydrated by heating distillation in the presence of an organic solvent, the organic solvent is separated, the precipitate is dried, and then further heated and calcined at 900 to 1600°C. manufacturing method. 2. Mix an aqueous solution (A) consisting of a water-soluble aluminum salt and silica sol, and an aqueous solution (B) containing a water-soluble zirconium salt or a water-soluble zirconium salt and yttrium or cerium, add an alkali to form a precipitate, and add the water-soluble Production of heat-resistant composite oxide powder, characterized in that the precipitate is dehydrated by heating distillation in the presence of an organic solvent, the organic solvent is separated, the precipitate is dried, and then further heated and calcined at 900 to 1600°C. Method. 3 Mix an aqueous solution (A) consisting of a water-soluble zirconium salt and silica sol and a hydroxide gel prepared by adding an alkali to the water-soluble zirconium salt or the aqueous solution (B) containing the water-soluble zirconium salt and yttrium or cerium. , further add alkali to this to form a precipitate, dehydrate the hydroxide by heating distillation in the presence of an organic solvent, separate the organic solvent, dry the precipitate, and further heat at 900 to 1600 ° C. A method for producing a heat-resistant composite oxide powder, which comprises firing.