BACKGROUND OF THE INVENTION 0001 1. Field of the Invention 0002 This invention relates to a method of manufacturing a molding material, a method of manufacturing a molded body, a method of manufacturing a sintered body, a molding material, a molded body and a sintered body that can be utilized for ceramics or powder metallurgy. 0003 2. Related Art Statement 0004 Aqueous injection molding technique using agar has been known in the fields of ceramic powder molding and powder metallurgy. Particularly, in the injection molding method of ceramic powder, it has been known to mold a ceramic powder by using a binder constituted with water and agar. For example, in Japanese Patent No. 2604592 (U.S. Pat. No. 4, 734, 237), a basic patent in the relevant field, a metal powder or ceramic powder, an agaroid gel-forming material and water are mixed, and the mixture is heated to a temperature exceeding the gel point of the agaroid gel-forming material. The mixture is then molded at a temperature lower than the gel point thereby obtaining a molded body comprising the ceramic powder or metal powder. For example, an aqueous agar solution is admixed to a mixture of an alumina powder and deionized water, the mixture is mixed under heating at 77 C. and injection molded, and the resultant molded body is sintered at 1600 C. Further, Japanese Patent No. 3105225 (U.S. patent application Ser. No. 748,151: PCT/US92/06799) also discloses a similar manufacturing method.

SUMMARY OF THE INVENTION 0005 The present inventors have made a study, based on the patents described above, for obtaining a molded body of a complicate shape and its sintered body by the injection molding method. In the course of the study, it was observed that scattering was caused in the strength of the sintered body or the strength of the sintered body was lowered depending on the conditions. When the sintered body showing a low strength was analyzed and studied, it was found that lowering in the relative density was extremely small. Further, in the relevant field of the art using agar and water as the binder, the amount of agar is extremely small compared with those of conventional resin binders or wax binders. In addition, in the course of elevating the temperature of the molded body, since agar is easily released together with water, it is considered that no special degreasing step is required. That is, in the technique of using agar and water as the binder, it would be unlikely that remarkable lowering of the strength in the sintered body is caused depending on mixing, molding and degreasing conditions, and the cause therefor was unknown. Since ceramics are brittle, it is desirable to increase the strength in view of avoiding the risk leading to fracture. 0006 An object of this invention is to improve the strength of a sintered body upon manufacturing of the sintered body from a molding powder by using a gel-forming material as a binder. 0007 Further, another object of this invention is to improve the strength of a sintered body and suppress scattering in the strength upon manufacturing the sintered body from a molding powder while using a gel-forming material as a binder. 0008 This invention provides a method of manufacturing a molding material. The method comprises the steps of; 0009 mixing at least a molding powder, a powder of a gel-forming material and a solvent at a temperature not higher than a dissolving point of the gel-forming material to provide a mixture, while applying a treatment of pulverizing agglomerates containing the molding powder, the powder of said gel-forming material and the solvent absorbed in the material; 0010 heating the mixture to a temperature not lower than the dissolving point while kneading said mixture to form a sol; and 0011 cooling the sol to a temperature not higher than a gel point of the gel-forming material. 0012 Further, in the molding method according to this invention, a molded body comprising the molding material is obtained. Specifically, the molding product is obtained by filling or supplying the molding material into a predetermined mold, and molding the same in the mold. Alternatively, the molded body is obtained by cooling the sol in the mold. Further, this invention also provides a method of obtaining a sintered body by sintering the molded body. 0013 As has been described above, the present inventors have made a detailed study over the entire process for manufacturing the sintered body for the reason that the strength of the sintered bodies is lowered while the density of the sintered body is relatively high and have attained the following knowledge. That is, a molding powder, a gel-forming material, for example, an agar powder and a solvent were mixed at a room temperature and the resultant mixture was heated to a temperature not lower than the dissolving point of the gel-forming material, which was fluidized and kneaded. When the mixed slurry is observed closely, inhomogeneous suspensions were observed on the surface of the slurry. Further, after discharging the slurry out of the mixing vessel, adhesion of agglomerates was observed on the inner wall surface of the mixing vessel. The adhesion of the agglomerates on the inner wall surface of the mixing vessel were scraped off into the slurry and mixing was tried again, but the agglomerates were suspend again and adhered on the inner wall surface of the mixing vessel. 0014 When suspensions or adhesion on the inner wall surfaces were closely studied microscopically, it was found that a number of molding powders were firmly bonded by agar and agglomerated. Although the reason is not apparent at present, it may be estimated as below. That is, agar powder has an extremely small water content and has a property of rapidly absorbing water to the inside of the powder and swells while including water. When the agar powder is charged into a slurry, it abruptly absorbs water in the vicinity of the agar powder. In the field of the relevant art, the water content is generally small while the amount of the molding powder is large in a slurry. Therefore, when water in the vicinity of the agar powder is absorbed rapidly, the water content is locally lost at the periphery thereof to agglomerate the molding powder. It is considered that agar just after absorbing water performs as a binder in the inside of the agglomerates, resulting in strong agglomeration. 0015 Based on the findings described above, the present inventors have adopted a process comprising the steps of mixing a molding powder, a gel-forming material powder and a solvent at a temperature not higher than the dissolving point of the gel-forming material and applying treatment of pulverizing agglomerates containing the molding powder, the gel-forming material powder and the solvent absorbed in the gel-forming powder and then heating and kneading the mixture at a temperature not lower than the dissolving point of the gel-forming material. As a result, the strength of the sintered body was improved. Further, it has been found that scattering of the strength of the sintered body is reduced. 0016 Further, the present inventors have also made a study for sufficiently dissolving the gel-forming material, for example, agar by increasing the amount of a solvent, for example, water and using a great amount of water upon mixing the gel-forming material and the molding powder. That is, it has been attempted to suppress formation of agglomerates by supplying a sufficiently great amount of water even when agar rapidly absorbs water. However, since agar rapidly absorbs water to firmly adhere a fine molding powder at the periphery of agar particles, it is necessary to use a great amount of water. This extremely increases the water content in the mixture. However, since water in the mixture has to be removed in the subsequent drying step, this is extremely time and energy consuming in the drying step. According to this invention, since a sintered body of a high strength can be manufactured without increasing the cost in the drying step, it is useful for the reduction of the cost. 0017 Further, this invention provides a method of manufacturing a molding material. The method comprises the steps of: 0018 mixing at least a molding powder, a powder of a gel-forming material and a solvent at a temperature not higher than a dissolving point of the gel-forming material to obtain a mixture, the powder of the gel-forming material requiring 50 minutes or more of time to increase the weight by 5% when the powder is maintained in an atmosphere at a temperature of 30 C. and at a relative humidity of 100%; 0019 heating and kneading the mixture at a temperature not lower than the dissolving point to form a sol; and 0020 obtaining a molding material form the sol. 0021 Further, this invention provides a method of manufacturing a molded body comprising the steps of mixing at least a molding powder, a gel-forming material powder and a solvent at a temperature not higher than the dissolving point of the gel-forming material powder thereby obtaining a mixture, the gel-forming powder used requiring 50 minutes or more of time to increase the weight by 5% when the gel-forming material powder is maintained in an atmosphere at a temperature of 30 C. and at a relative humidity of 100%, then heating and kneading the mixture at a temperature not lower than the dissolving point thereby forming a sol, then forming a molding material from the sol, and then molding the molding material thereby obtaining a molded body. This invention also provides a method of manufacturing a sintered body by sintering the molded body described above. 0022 Further, this invention also provides a molding material, a molded body and a sintered body obtained, respectively, by the methods described above. 0023 As has been described above, the present inventors have made a detailed studied over the entire process of manufacturing the sintered body for the reason that the strength of the sintered body is lowered while the density of the sintered body is relatively high and have obtained the following knowledge. That is, a molding powder, a gel-forming material, for example, an agar powder and a solvent were mixed at a room temperature, the resultant mixture was heated to a temperature not lower than the dissolving point of the gel-forming material to be fluidized and then kneaded. In this process, when mixed slurry was observed closely, inhomogeneous suspensions were suspended on the surface of the slurry. Further, after discharging the slurry from a mixing vessel, adhesion of the agglomerates was observed on the inner wall surface of the mixing vessel. Agglomerates adhered on the inner wall surface of the mixing vessel were scraped off into the slurry and mixing was tried again but it was found that the agglomerates were suspended again and adhered on the inner wall surface of the mixing vessel. 0024 When the suspensions or the adhesion on the inner wall surface were microscopically observed specifically, it was found that a number of molding powders were firmly bonded by agar and agglomerated. Although the reason is not apparent but it may be estimated as below. That is, the agar powder is highly water absorbing, and has a property of rapidly absorbing the water content to the inside of the agar powder and swelling with inclusion of water. When the agar powder is charged into a slurry, it rapidly absorbs water in the vicinity of the agar powder. In the field of the relevant art, the water content is relatively small, while the amount of the molding powder is large in the slurry. Therefore, when water in the vicinity of the agar powder is absorbed rapidly, water is locally deprived at the periphery thereof to agglomerate the molding powder. It is considered that agar just after absorption performs as the binder in the inside of the agglomerates, resulting in strong agglomeration. 0025 Further, the present inventors have also made a study for sufficiently dissolving the gel-forming material, for example, agar in mixing the gel-forming material and the molding powder by increasing the amount of the solvent, for example, water and using a great amount of water. That is, it has been attempted to suppress the formation of agglomerates by using a great amount of water even when agar rapidly absorbs water. However, since agar rapidly absorbs water and a fine molding powder is firmly adhered around the agar particles, it is necessary to use a great amount of water. Therefore, the water content in the mixture is extremely large. However, since water in the mixture has to be removed in the subsequent drying step, this results in an enormous loss of time and energy in the drying step. 0026 The present inventors have found that the inhomogenity of the molded body and the low strength of the sintered body, due to the agglomeration of the molding powder upon mixing, can be suppressed by using such a gel-forming material powder having a lower initial solvent absorption speed upon mixing into the solvent as requiring 50 minutes or more of time to increase the weight by 5% when the material is maintained in an atmosphere at a temperature of 30 C. and at a relative humidity of 100% and have accomplished the present invention. 0027 In this invention, a gel-forming material powder requiring 50 minutes or more of time to increase the weight by about 5% based on the initial weight when the material is maintained in an atmosphere at a temperature of 30 C. and at a relative humidity of 100% is used and mixed with the molding powder and the solvent. The necessary time is preferably 50 minutes or more with a view point of this invention and most preferably 250 min or more. The upper limit of the necessary time has no particular restriction in view of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS 0028 FIG. 1 is a graph showing a relation between the humidifying time and the change of weight when agar powder was put to various pre-treatments. 0029 FIG. 2 is a graph showing a relation between the water absorption time for absorbing 5% by weight of water based on the initial weight and the 4-point bending strength. 0030 FIG. 3 is a graph showing a relation between the water absorption time for absorbing 5% by weight of water based on the initial weight and the residue on a screen (parts by weight).

PREFERRED EMBODIMENTS OF THE INVENTION 0031 This invention is to be explained more in details. There is no particular restriction on the molding powder as an object of this invention so long as the powder may be sintered by heating to form a sintered body. Further, it is preferred that the molding powder is not dissolved into a solvent or does not react with a solvent. The molding powder is preferably an inorganic powder and it is particularly preferably an inorganic powder intended to be used for sintering. The inorganic powder can include, typically, ceramic powder, metal powder, a powder of ceramic-metal composite material and a powder mixture thereof. The ceramics can include, for example, oxide series ceramics such as, for example, alumina, zirconia, titania, silica, magnesia, ferrite, cordielite and oxides of rare elements such as yttria; composite oxides such as barium titanate, strontium titanate, lead zirconate titanate, manganites of rare earth elements and chromites of rare earth elements; nitride series ceramics such as aluminum nitride, silicon nitride and sialon; and carbide series ceramics such as silicon carbide, boron carbide, and tungsten carbide. Further, the metal can include, for example, iron series metals such as iron, stainless steel and carbonyl iron, non-iron metals such as titanium, copper and aluminum or alloys of non-iron metals. The inorganic powder can further include graphite, glass and carbon. Further, the molding powder other than the inorganic powder can include, for example, resin powders. 0032 When the molding powder is mixed with the solvent and the gel-forming material, the molding powder tends to agglomerate more intensely at the periphery of gel-forming material particles when the solvent is deprived from the periphery of the gel-forming material as the grain size of the molding powder is smaller (the grains are smaller). Accordingly, this invention is particularly useful in a case where the average particle size of the molding powder is small. In view of the above, the average particle size of the molding powder is 10 m or less in a preferred embodiment and 1 m or less in a further preferred embodiment. 0033 There is no particular restriction on the lower limit for the average particle size of the molding powder in view of the invention. It can be 0.1 m or more with an economical view point, with no particular restriction thereto. 0034 A gel-forming material is such a material as forming a flowable sol upon heating to a temperature not lower than the dissolving point and then forming a gel upon cooling the sol to a temperature not higher than the gel point. As described in Japanese Patent No. 2604592, the gel-forming material having a gel strength of at least 500 g/cm2 at a temperature between 0 - 22 C. is preferred. In a preferred embodiment, the gel-forming material forms a sol at a temperature not lower than the dissolving point and forms a gel at a temperature not higher than the gel point reversibly. 0035 Particularly preferably, the gel-forming material is a natural or synthetic hydro-colloid. The hydro-colloid can include, for example, polysaccharides such as agar, carrageenan, furcellaran, alginic acid and derivatives thereof, Azotobactor-vinelandii-gum, gellan gum (deacyl type or native type), pectin, curdlan, starch and derivatives thereof, psyllium seed gum; and proteins such as gelatin, chitin, chitosan and modified milk protein. Further, it can include gel-forming materials with addition of at least one of locust bean gum, tara bean gum, casia gum, guar gum, fenugreek gum, konyac mannan (also including purified glucomannan), xanthan gum, tamarind gum, locust, gum arabic, arabino galactan, crystalline cellulose, sodium cellulose glucolate, methyl cellulose, gum ghatti, almond gum, welan gum, tragancanth gum, pllulan, and soy bean polysaccharides. 0036 In a particularly preferred embodiment, polysaccharides having galactose as the basic skeleton are used. The polysaccharide may preferably be an agaroid gel-forming material. The agaroid gel-forming material include all the agaroid gel-forming materials and includes not only agarose at high purity but also agar at low purity containing a great amount of an agaroid pectin ingredient and also includes modified agar. Pure agar is referred to as agarose, which is a neutral polysaccharide comprising -D-galactopyranose bonded at the 1,3-positions and 3,6-anhydro-L-garactopyranose bonded at the 1,4-positions as repeating units. Further, the agaroid gel-forming material may also include derivatives of agarose. Agarose partially containing sulfate esters, methoxyl and pyruvic acid are referred to as agaropectin. Most of commercial agars are mixtures of agarose and agaropectin. 0037 In a case where agarose and agarose derivatives (typically agaropectin) are contained in the agaroid gel-forming material, the ratio of agarose and agarose derivative may be varied depending on the aimed application use. The agarose derivative contains ashes such as sodium, potassium, calcium and sulfuric acids. Therefore, it is preferred that the amount of the agarose derivative is smaller in such application use as fine ceramics, in which impurities should be avoided. Specifically, agarose at high purity with an ash content of 2% by weight or less is preferred and with an ash content of 1% by weight or less is particularly preferred. 0038 The solvent used herein is those for the gel-forming material that dissolve the gel-forming material powder, form a sol upon heating and form a gel upon lowering of the temperature. As the solvent, water is particularly preferred but it may be a hydrophilic organic solvent such as alcohols or a mixture of water and a hydrophilic organic solvent. The hydrophilic organic solvent preferably includes alcohols such as methanol, ethanol and propanol, and alcohols substituted with chlorine or bromine atoms. This may be presumed that an alcohol penetrates into molecules of agar and, further, causes swelling by substitution with water, so that it no more absorbs water rapidly and less forms agglomerates. 0039 In the mixing step of the mixture, existence of the agglomerates can be confirmed by passing the mixture after mixing through a screen with a wire diameter of 390 m and a mesh of 600 m (JIS Z8801) and observing the presence of residues on the screen. Further, whether the agglomerates have been pulverized or not in the mixture can be confirmed by passing the mixture after pulverization through a screen with a wire diameter 390 m and a mesh of 600 m (JIS Z8801) and measuring the amount of residues on the screen. 0040 The amount of the residues when the mixture after pulverization is passed through the screen in this invention is preferably 0.4 parts by weight or less, further preferably, 0.2 parts by weight or less and, particularly preferably, 0.1 parts by weight or less based on 100 parts by weight of the molding powder. For the measurement for the amount of the residues it is tested as below. That is, 100 parts by weight of a molding powder, 27.4 parts by weight of a solvent, 3 parts by weight of a gel-forming material and one part by weight of a dispersant (40% solution of ammonium polyacrylate polymer) are mixed. After the mixing, the mixture is passed through the screen described above and the weight ratio of the residues remaining on the screen is measured. 0041 Whether a certain treatment constitutes a pulverization treatment or not can be determined depending on the ratio between the residual amount of the agglomerates on the screen when the treatment was applied upon mixing and the residual amount of the agglomerates on the screen when the treatment was not applied upon mixing. It is necessary that the residual amount is reduced at least to 50% or less by applying the treatment and, preferably, it is reduced to 20% or less, further preferably, 15% or less and, particularly preferably to 5% or less. So long as the condition is satisfied, there is no particular restriction for the concrete method of the pulverizing treatment. For example, it may be considered a method of cutting or disconnecting the agglomerates in the mixture by a cutting mechanism such as a chopper blade, as well as a method of crushing the agglomerates or providing the mixture with flow, thereby colliding the agglomerates to each other to decompose them. 0042 Since the agglomerates are relatively firm, they are less pulverized by flowing and stirring a mixture slurry of mixture by a usual mixing process. Therefore, it is preferred to provide a cutting mechanism for cutting the agglomerates in the slurry. Particularly preferably, a mixing vessel is provided with a flowing mechanism for circulating and flowing the slurry and a cutting mechanism for cutting agglomerates contained in the slurry during flowing. There are no particular restrictions on the flowing and cutting mechanisms. Preferably, the flowing mechanism is a stirring mechanism for stirring the slurry in the mixing vessel, including a blade, stirrer or the like. A planetary blade is particularly preferred. Further, the flowing mechanism may be a pipeline for flowing the slurry in the mixing vessel. 0043 The slurry of mixture during movement under flowing is passed in front of the cutting mechanism disposed in the flow channel to disconnect the agglomerates. The cutting mechanism is preferably a cutting blade or cutting tooth, and a chopper blade is, particularly preferred. A mixing vessel provided with such stirring mechanism and cutting mechanism can include, for example, high speed dispersing machines T. K. Homodisper, T. K. Homojetter, T. K. Pipeline Homomixer and T. K. Highvismix (manufactured by Tokushu Kika Kogyo K. K.). 0044 In view of the efficient pulverization, the rotation speed of the cutting blade or the cutting tooth is preferably 500 rpm or more and, further preferably, 1000 rpm or more. 0045 However, when the rotation speed is excessively high, cost of the apparatus is increased and metal impurities may possibly intrude into the products by the abrasion of the blade. In view of the above, the rotation speed of the cutting blade or the cutting tooth is preferably 5000 rpm or less and, further preferably, 3000 rpm or less. 0046 The temperature upon mixing of the molding powder, the gel-forming material and the solvent is not higher than the dissolving point of the gel-forming material. The dissolving point of the gel-forming material is such a temperature that the gel-forming material dissolves into the solvent to form a sol when the gel-forming material and the solvent are mixed and then the temperature is elevated. Accordingly, the dissolving point may vary even for an identical gel-forming material when the solvent is different. In this invention, it is particularly preferred that mixing is conducted at a temperature somewhat lower than the dissolving point and, it is preferred to conduct mixing, for example, at a temperature lower by at least 30 C. than the dissolving point and, further preferably, lower by at least 50 C. than the dissolving point. When the gel-forming material is of agaroid, the mixing temperature is preferably 50 C. or lower and, further preferably 40 C. or lower. 0047 Then, the mixture of the molding powder, the gel-forming material and the solvent is heated and kneaded at a temperature not lower than the dissolving point of the gel-forming material to form a sol and then the sol is cooled to a temperature not higher than the gel point of the gel-forming material, thereby forming a molding material. 0048 During heat kneading, the mixture is heated to a temperature not lower than the dissolving point simultaneously with kneading to obtain a flowable sol. The heating temperature is different depending on the kind of the gel-forming material and it is, preferably, 85 to 120 C. in a case of the agaroid gel-forming material. At a temperature of 100 C. or higher, it is preferably pressurized in order to avoid boiling of water. Then, a molding product of a desired shape can be obtained by cooling the heat-kneaded product in a mold. Alternatively, a molding material is obtained by cooling the heat-kneading product and then the molding material is supplied into a mold, to obtain a molded body. 0049 In a preferred embodiment, the molding material is pelleted and, simultaneously, the water content of the pellet is controlled within a certain range. The pellet after controlling the water content is injected into an injection molding die to obtain a molded body. However, other molding methods, for example, a casting method or an extrusion molding method may also be used. 0050 In the mixture of this invention, the amount of the gel-forming material powder based on 100 parts by weight of the molding powder is preferably 1 part by weight or more and, further preferably, 3 parts by weight or more. This further increases the strength of the molded body. Further, the strength of the molded body can further be increased by using the gel-forming material having a high gel strength as described above. 0051 Further, the amount of the gel-forming material powder based on 100 parts by weight of the molding powder is preferably 10 parts by weight or less and, further preferably, 7 parts by weight or less. Since this decrease the amount of water deprived from the slurry of the mixture upon mixing the gel-forming material powder, micro-cracks formed in the sintered body can be suppressed effectively. 0052 In the mixture of this invention, the amount of the solvent based on 100 parts by weight of the molding powder is preferably 40 parts by weight or less and, further preferably, 30 parts by weight or less. This reduces the cost in the drying step. By decreasing the amount of the solvent, formation of the agglomerates described above becomes remarkable to enhance the effect of this invention. On the other hand, with a view point of enabling the mixing effectively in a short period of time and increasing the strength of the sintered body, the amount of the solvent based on 100 parts by weight of the molding powder is preferably 40 parts by weight or more and, further preferably, 50 parts by weight or more. 0053 The gel-forming material powder having the hygroscopic property described above can be prepared by applying a treatment to an appropriate commercially available starting material powder for suppressing the rapid absorption of the solvent. There are no particular restrictions on the kind and the shape of the starting material powder and the shape can include powdery, flaky, granular, network-like or other special shapes. Further, when the gel-forming material is agar, it may be agar prepared from Gelidum amansii Lamuroux, Pterocladia tenuis Okamura, Acanthopeltis japonicum Okamura, G. conferoides Papen., thymi Campylaephora hypnaeoides Okamura, Ceramium boidenii Gepp. and Eucheuma muricaum Weber van Bosse. 0054 As the treatment for suppressing rapid absorption of the solvent, the following treating methods are preferred. 0055 (1) Water, a hydrophilic solvent or a mixed solvent of water and the hydrophilic solvent is added to the gel-forming material powder. The hydrophilic solvent can include, preferably, alcohols such as methanol, ethanol and propanol, and alcohols substituted with chlorine or bromine atoms. 0056 In this case, water, the hydrophilic solvent or the mixed solvent can be sprayed to the gel-forming material powder. Preferably, water, hydrophilic solvent or mixed solvent is sprayed to the starting powder for the gel-forming material after spreading the same on a plane. Further, the starting powder for the gel-forming material can be kept in a highly humid atmosphere. In this case, the relative humidity is preferably 80% or higher and the temperature is preferably 10 to 30 C. in the atmosphere. Particularly preferably, the powder is humidified by being kept in an atmosphere at a temperature of 30 C. and at a relative humidity of 100%. 0057 (2) The gel-forming material powder is coated with a water soluble polymer. For example, an agar powder is immersed in an aqueous solution of a water soluble polymer such as polyvinyl alcohol or polyethylene glycol. After solid-liquid separation by filtration, the agar powder is dried.

EXAMPLES

Example 10058 As an alumina powder, 6000 g of an easily sinterable low soda alumina content powder (AES-11, manufactured by Sumitomo Chemical Industry Co., purity: 99.5%, BET specific surface area: 7 m2/g, average particle size: 0.5 m) was used. As a dispersant, one part by weight of ammonium polyacrylate copolymer (A-6114, manufactured by Toa Gosei Co., molecular weight: 10000 MW, pH: 7 - 9, 39 - 41% solution) was added based on 100 parts by weight of the alumina powder, ion exchanged water was added so as to adjust the water content to 21.5 parts by weight and mixed thoroughly. Then, the mixture was pulverized for 5 hours in a pot mill using nylon balls as pulverization media. The resultant slurry was passed through a screen having a wire diameter of 390 m and a mesh of 600 m (JIS Z8801) and then placed in a mixing vessel. As a mixing vessel, T. K. Highvismix (manufactured by Tokushu Kika Kogyo K. K.) was used. The mixing vessel was provided with a planetary blade (twisted blade) and a chopper blade. 0059 Powdered agar (XG89, manufactured by Ina Shokuhin Kogyo K. K. having gel strength of 840 g/cm2) was weighed at a ratio of 3 parts by weight based on 100 parts by weight of the alumina powder. While stirring the mixture, the agar powder was charged stepwise into the mixing vessel. 5 min after charging the entire amount of the weighed agar powder, the lid of the mixing vessel was opened, solid adhesion on the planetary blade, on the chopper blade and on the inner wall surface were scraped into the slurry and then mixed again for 5 min. The resultant slurry was passed through a screen having a wire diameter of 390 m and a mesh of 600 m (JIS Z8801), and the weight of the solids remaining on the screen was measured. The solids were agglomerates. The rotation speed for the planetary blade and the rotation speed for the chopper were changed as shown in Table 1. 0060 Then, the mixing vessel was heated to dissolve and, simultaneously, knead the agar powder. The temperature was elevated from a room temperature to 95 C. for 30 min. While keeping the temperature of the material at 95 C., the planetary blade was rotated at a rotation speed of 30 rpm to knead the slurry (without rotating the chopper). Upon temperature elevation, rotation of the planetary blade was stopped. Kneading was conducted at 95 C. for 30 min, the resultant kneading product was cooled down to a room temperature to obtain a molding material. The molding material was taken out of the mixing vessel. 0061 The molding material was pelleted while controlling the humidity so that the water content of the pellet was adjusted to 17.5%. The pellets after the humidity control was molded by an electromotive injection molding machine at 30 ton die clamping to mold flat plates each of 60 mm for each side and 6 mm in thickness. After drying the molding product at a room temperature over one day and one night, the residual water content was removed in a drying apparatus at 130 C., and it was sintered under the conditions at a temperature elevation rate of 300 C./hr, at a maximum temperature of 1620 C. for a retention time at 1620 C. of 2 hours. Cooling was conducted by spontaneous cooling. The resultant sintered body was fabricated and a test specimen sized 4 mm3 mm40 mm according to JIS (R1601) was cut out and density and 4-point bending strength were measured for the test specimen. 1TABLE 1rotationdensityfourspeed ofrotationresidueofpointplanetaryspeed ofon screensinteredbendingbladechopper(parts bybodystrengthrpmrpmweight)g/ccMPaexample 11010000.43.93403example 21030000.33.94413example 32510000.23.93411example 42530000.13.94425example 55010000.13.94423example 65030000.03.94432comparative100103.91339example 1comparative25073.92345example 2comparative5001.73.92378example 3

0062 As can be seen from the results, the amount of residues on the screen when the slurry after mixing was passed through the screen in Examples 1 to 6 was remarkably reduced compared with Comparative Examples 1 to 3. In Comparative Examples 1 to 3, agglomerates suspended in the vicinity of the surface of the slurry and suspensions were adhered on the surface of the blade or on the inner wall surface of the mixing vessel. The adhesion were sprayed off into the mixing vessel and the mixing was tried again but the agglomerates were hard and could not be pulverized by the planetary blade. In Examples 1 to 6, such agglomerates were pulverized. 0063 The density of each sintered body of Comparative Examples 1 to 3 was as large as about 3.92 g/cc, which was not substantially different from the density of each sintered body of Examples 1 to 6. This is because the amount of agar (organic material) in the mixture is small and most of the slurry comprises alumina powder. Indeed, it is considered that when agglomerates are formed in the slurry, scattering may occur in the amount of the alumina powder or agar also in the molded body. However, since agar is dissipated in the course of the sintering, it gives no substantial effect in view of the density of the entire sintered body. Accordingly, even when agglomerates are formed in the mixture, the density of the sintered body is scarcely lowered. 0064 On the other hand, the strength of the sintered body has a close concern with the size, the shape and the frequency of occurrence of defects in the sintered body. It is considered that when the agglomerates are present in the slurry, the portions where agar and water are dissipated in the course of sintering remain as coarse portions or pores to act as defects. It is considered that such defects act as stress concentration sources, leading to deterioration of the strength.

Example 20065 (Starting material powder) 0066 As the starting material powder for agar, agar powders of each of kinds and jelly strength shown in Table 2 and Table 3 were used. Among them, UM-11 is instantly soluble agar. 0067 (Pre-treatment) 0068 As the pre-treatment for the agar powder, one of the following methods was adopted. 0069 (1) Water by 1 to 3 times the weight of agar was sprayed by using a sprayer and they were left as they were for 1 hour. The pre-treatment method is indicated in Table 2 and Table 3 as water added (equivalent weight) or water added(three times by weight). 0070 (2) An agar powder was thinly scattered on a vat and maintained in a thermostable and humidity stable vessel at a temperature of 30 C. and at a relative humidity of 100% for 2 hour or 8 hours. The pre-treatment method is indicated in Table 2 and Table 3 as vapor added. 0071 (3) Ethanol was added by five times the weight of agar, and stirred lightly. The pre-treatment method is indicated in Table 2 and Table 3 as ethanol added. 0072 (4) An agar powder was charged in an aqueous 10 wt % solution of polyvinyl alcohol by ten times the weight of agar and then stirred lightly. It was filtered by using qualitative filter paper and the filtration product was dried at 80 C for 1 hour. The pre-treatment method is indicated in Table 2 and Table 3 as coating. 0073 (Measurement for absorption time) 0074 The water absorption time of each agar powder was measured as described below. That is, the agar powder was thinly scattered on a vat, kept in a thermostable and humidity stable vessel at a temperature of 30 C. and at a relative humidity of 100%, and the time at the instance absorbing 5 wt % of water based on the initial weight was measured. As the absorption time is shorter, agglomerates tend to be formed more when the agar powder is charged into the slurry. Relation between the humidifying time and change of weight when XG-89 (jelly strength 840 g/cm2) was applied with various types of pre-treatments is shown in FIG. 1. 0075 (Preparation of slurry of mixture) 0076 As an alumina powder, 6000 g of an easily sinterable low soda alumina content powder (AES11 C, manufactured by Sumitomo Chemical Industry Co., purity: 99.5%, BET specific surface area: 8 m2/g, average particle size: 0.5 m) was used. As a dispersant, 0.9% by weight of ammonium polyacrylate copolymer (A-6114, manufactured by Toa Gosei Co., molecular weight: 10000 MW, pH: 7-9, 39-41% solution) was added based on the alumina powder, ion exchanged water was added so as to adjust the water content to 25% by weight and mixed thoroughly. Then, it was mixed for 12 hours in a pot mill using monomalon media as a pulverization media. The resultant slurry was passed through a screen having a wire diameter of 390 m and a mesh of 600 m (JIS Z8801) and then charged in a mixing vessel. 0077 Each of the previously prepared agar powders was weighed at a ratio of 3 parts by weight (based on the weight of the agar powder before pre-treatment) based on 100 parts by weight of the alumina powder. While stirring the mixture, the agar powder was charged stepwise into the mixing vessel. 5 min after charging all the weighed agar powder, the lid of the mixing vessel was opened, solids adhered on the inner wall surface were sprayed off into the slurry and then mixed again for 5 min. The resultant slurry was passed on a screen having a wire diameter of 390 m and a mesh of 600 m (JIS Z8801) and the weight of solids remaining on the screen was measured. The solids were agglomerates. 0078 (Heating and kneading) 0079 Then, the mixing vessel was heated to dissolve and, simultaneously, knead the agar powder. The temperature was elevated from a room temperature to 95 C. for 30 min. While keeping the temperature of the material at 95 C., the slurry was kneaded. Kneading was conducted at 95 C. for 30 min, the resultant kneading product was cooled down to a room temperature to obtain a molding material. The molding material was taken out of the mixing vessel. 0080 (Molding and sintering) 0081 The molding material was pelleted while controlling the humidity and the water content of the pellet was adjusted to 17% by weight. The pellets after humidity control was molded by an electromotive injection molding machine at 30 ton die clamping to mold flat plates each of 60 mm for each side and 6 mm in thickness. After drying the molding product at a room temperature over one day and one night, the residual water content was removed in a drier at 130 C., and it was sintered under the conditions at a temperature elevation rate of 300 C./hr, at a maximum temperature of 1620 C. for a retention time at 1620 C. of 2 hours. Cooling was conducted by spontaneous cooling. The resultant sintered body was fabricated and a test specimen sized 4 mm3 mm40 mm according to JIS (R1601) was cut out and density and 4-point bending strength were measured for the test specimen. The result of the measurement described above is shown in Table 2 and Table 3. Further, a relation between the water absorption time for absorbing 5% by weight of water based on the initial weight, and the 4-point bending strength is shown in FIG. 2, while a relation between the water absorption time for absorbing 5% by weight of water based on the initial weight, and the residues on the screen (parts by weight) is shown in FIG. 3. 2TABLE 2time required forincreasing theresidue ondensity offour pointjellyweight by 5 percentscreensinteredbendingtradestrengthpre-treatmentof agar with(parts bybodystrengthnameg/c m2of agarabsorbing waterweight)g/cc(Mpa)Comparative example A1XG-89840None251.93.93300Comparative example A2XG-89840Water added (equivalent weight)301.83.93305example A3XG-89840water added three times by weight1801.33.93374example A4XG-89840vapor added4200.53.94434example A5XG-89840ethanol added7200.03.94440example A6XG-89840Coating6200.13.93428comparative example B1XG-171870None202.03.92308comparative example B2XG-171870water added (equivalent weight)351.93.92310example B3XG-171870water added three times by weight1451.13.92360example B4XG-171870vapor added2400.83.92388example B5XG-171870ethanol added5400.13.93422example B6XG-171870Coating5400.13.93435comparative example C1XG-2581350None102.53.91289comparative example C2XG-2581350water added (equivalent weight)252.33.92299example C3XG-2581350water added three times by weight952.33.91368example C4XG-2581350vapor added2100.73.91390example C5XG-2581350ethanol added5200.23.93419example C6XG-2581350Coating4000.23.93428

0082

3TABLE 2time required forincreasing theresidue ondensity offour pointjellyweight by 5 percentscreensinteredbendingtradestrengthpre-treatmentof agar with(parts bybodystrengthnameg/c m2of agarabsorbing waterweight)g/cc(Mpa)comparative example D1XF-3302100None82.63.91285comparative example D2XF-3302100water added (equivalent weight)222.43.92301example D3XF-3302100water added three times by weight602.03.92362example D4XF-3302100vapor added2000.23.92380example D5XF-3302100ethanol added7800.13.92432example D6XF-3302100Coating3500.23.94440comparative example E1UM-111040None82.83.91260comparative example E2UM-111040water added (equivalent weight)152.73.91270example E3UM-111040water added three times by weight602.23.91346example E4UM-111040vapor added1500.83.91377example E5UM-111040ethanol added4000.23.92411example E6UM-111040Coating5000.13.94425comparative example F1Daiwa430None402.33.92288comparative example F2Daiwa430water added (equivalent weight)452.03.92300example F3Daiwa430water added three times by weight1101.03.93360example F4Daiwa430vapor added3200.73.93412example F5Daiwa430ethanol added7200.03.93432example F6Daiwa430coating6100.13.93436

0083 As shown in Table 2, when the slurry after the mixing was passed through the screen, the amount of residues on the screen was reduced in Examples A3, A4, A5 and A6 compared with those in Comparative Examples A1 and A2. Particularly, in Example A5, since ethanol intrudes into molecules of agar and further causes swelling by substitution with water, it no more absorbs water rapidly and forms less agglomerates. Then, the strength of the sintered body was remarkably improved in Examples A3, A4, A5 and A6 compared with Comparative Examples 1 and 2. On the other hand, the density of the sintered body was not substantially changed among the Examples A1 to A6. 0084 The reason is considered as below. Since agar is lost in the course of sintering, agar has no concerns with the lowering of the density of the sintered body also in a case where agar has caused agglomeration. On the other hand, the strength of the sintered body has a close concern with the size, the shape and the frequency of occurrence of defects in the sintered body. It is considered that when the agglomerates are present in the slurry, the portions where agar and water are dissipated in the course of sintering remain as coarse portions or pores to act as defects. Such defects acts as stress concentration sources, leading to deterioration of the strength. 0085 Further, similar effects were also obtained in a case of comparison between Comparative Examples B1 and B2 with Examples B3, B4, B5 and B6, between Comparative Examples C1 and C2 with Examples C3, C4, C5 and C6, between Comparative Examples D1 and D2 with Examples D3, D4, D5 and D6, between Comparative Examples E1 and E2 with Examples E3, E4, E5 and E6, and between Comparative Examples F1 and F2 with Examples F3, F4, F5 and E6. 0086 As can be seen from the result described above, when the water absorption amount till the instance absorbing 5% by weight of the water based on the initial weight is less than 50 min, the strength is as low as less than 310 MPa, while in a case where the water absorption time is 50 to 250 min, the strength increases as 360 to 390 MPa. Further, when the water absorption time is 250 min or more, the strength further increases as 410 to 436 MPa.