JP2532753B2 - Slurry for producing porous ceramics and porous ceramics obtained using this slurry - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slurry for producing porous ceramics and a porous ceramics obtained by using this slurry. More specifically, it is excellent in stability, water resistance and strength. Slurry for producing porous ceramics, which provides excellent mold, and porous ceramics obtained by using this slurry,
For example, it relates to a mold for precision casting of highly active metal such as titanium or titanium alloy.

[0002]

2. Description of the Related Art Generally, a porous mold used for precision casting is removable by heat such as wax, naphthalene, low melting point alloy, urea, Styrofoam or chemicals such as solvent, that is, thermal or chemical. Slurry in which refractory fine particles and a binder are mixed in a cast model made of a removable material in a desired shape, and the refractory particles which are the same as or different from the refractory fine particles are alternately applied and dried to form a mold. Is manufactured, the internal casting model is removed by thermal or chemical treatment, and the obtained raw mold is fired.

For the slurry as the casting material, various ceramic casting materials such as silica type, magnesia type, zirconia type and calcia type have been developed. However, when a highly active metal such as titanium or a titanium alloy is cast, using a mold made of a silica-based mold material that is generally widely used for casting high melting point metals, the metal-template reaction is remarkably excellent. No cast can be obtained. For this reason, the application of magnesia-based, zirconia-based, and calcia-based casting materials that have low reactivity with these highly active metals has been investigated, but these are not only the strength of the casting mold, thermal shock resistance, but also the strength of the raw casting mold. However, it was not satisfactory in terms of heat shrinkage.

In view of the above circumstances, the applicant of the present patent has investigated a mold for precision casting of a highly active metal such as titanium or a titanium alloy, and a method for manufacturing a mold using a slurry for producing a zirconia-based ceramics. Patent application (Japanese Patent Application No. 1-124128). The slurry used here is one prepared by adding electrofused calcia-stabilized zirconia fine particles as refractory fine particles to a transparent zirconia sol, and titanium or titanium alloy precision casting products are industrially produced using the resulting mold. Has been done.

[0005]

However, in the industrial world, there is a need for larger and more complex titanium-based precision cast products such as aircraft materials and automobile parts. Higher performance is required even more. Specifically, it is required to improve the stability of the slurry for producing ceramics, improve the strength and water resistance of the raw mold before firing, and further suppress the reactivity between the mold and molten titanium.

The present invention is a further development of the method of the above-mentioned Japanese Patent Application No. 1-124128. One object of the present invention is to provide a slurry for producing porous ceramics which is excellent in stability. That is.

Another object of the present invention is to provide a slurry for producing porous ceramics, which can be applied to a casting model to obtain a green mold having high strength and improved water resistance.

Another object of the present invention is to obtain a porous ceramics mold whose reactivity with molten titanium is further suppressed and which enables high quality and high dimensional accuracy to be produced even for titanium-based castings of large size and complicated shape. It is to provide a slurry for producing a porous ceramics. Another object of the present invention is to provide a porous ceramic obtained by using the above slurry, particularly a porous ceramic suitably used as a casting mold for titanium or titanium alloy.

[0009]

As a result of intensive studies, the present inventors have obtained the following findings.

(1) In order to stabilize a slurry for producing porous ceramics (hereinafter sometimes referred to simply as "slurry"), an acidic type to basic type transparent zirconia sol depending on the properties of refractory fine particles. (Hereinafter, it may be simply referred to as “zirconia sol”). For example, when using refractory fine particles containing a basic or basic component, use a neutral to basic zirconia sol. By this, a stable slurry is obtained.

(2) In addition to the zirconia sol and the refractory fine particles, the combined use of an organic binder improves the strength of the green mold and also improves the water resistance thereof. Further, when a basic zirconia sol is used. It is possible to prevent the generation of cracks in the green mold.

The present invention has been completed based on the above findings.

The present invention relates to a slurry for producing porous ceramics, which comprises a transparent zirconia sol, refractory fine particles and an organic binder.

The present invention further relates to a porous ceramic obtained by using the above slurry. Hereinafter, the present invention will be described in detail.

As the zirconia sol used in the present invention, an acidic type zirconia sol, a neutral type zirconia sol or a basic type zirconia sol is used depending on the properties of the refractory fine particles. Can be obtained by

(1) Acidic Zirconia Sol This acidic zirconia sol is a transparent zirconia sol produced from an aqueous zirconium salt solution and urea.

First, urea is added to a zirconium salt aqueous solution selected from water-soluble zirconium salts such as zirconium oxychloride, zirconium nitrate, zirconium sulfate or zirconium acetate, and heated to produce a zirconia sol.

In this reaction, the concentration of ammonia produced by hydrolysis of urea according to the following reaction formula is extremely uniform in the aqueous solution, and therefore the zirconia sol produced from this ammonia and zirconium salt has an extremely small particle size and is uniform. It has the characteristic of being

(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂ The reaction is stopped while the sol thus obtained maintains transparency. The sol after the reaction has a pH in the range of 1.2 to 2.9.

After cooling the sol, it is washed and concentrated using an ultrafiltration membrane, and the ions in the sol are discharged out of the system together with water. When the concentration of ions in the sol after concentration is high, the steps of adding pure water to the sol, washing and concentrating it are repeated until the desired concentration of ions is reached. As a method for removing ions, it is also effective to wash by ultrafiltration until the ion concentration reaches a certain level, and thereafter to remove ions using an ion exchange resin.

Concentration with an ultrafiltration membrane is possible up to 20% by weight as ZrO ₂ although there are restrictions from the ultrafiltration membrane. When the concentration is made higher than this, the sol is heated to be concentrated. This concentration needs to be performed at a sol temperature of 80 ° C. or lower, preferably 60 ° C. or lower, and more preferably 40 ° C. or lower, and can be efficiently performed by stirring under reduced pressure. The transparent zirconia sol thus obtained can be concentrated to a concentration of 50% by weight as ZrO ₂ and is stable for a long period of time.

The refractory fine particles and an organic binder are added to the acidic zirconia sol obtained as described above to prepare the slurry for producing porous ceramics of the present invention. In order to obtain a high-strength green mold, zirconia is used. It is desirable that the concentration of the sol is high, and it is generally preferable to use a sol containing 20% by weight or more of ZrO ₂ . However, 20% by weight
Since the zirconia sol having the above concentration has a high viscosity, nitric acid, hydrochloric acid, a mineral acid such as sulfuric acid, acetic acid, an organic acid such as formic acid, and the above-mentioned zirconium salt aqueous solution used as a starting material of the zirconia sol. It is preferable to add the zirconia sol after ultrafiltration or after heating and concentrating the zirconia sol to adjust the viscosity of the zirconia sol in the range of 3 to 80 cp. The pH of the zirconia sol at this time is about 1 to 3.

The amount of urea used in the above reaction is usually
Urea (mol number) / zirconia (mol number) is 0.5 to 2, and the heating temperature is 80 to 120 ° C.

(2) Neutral Zirconia Sol This neutral zirconia sol is obtained by hydrolyzing a reaction product obtained by mixing an aqueous zirconyl ammonium carbonate solution and a chelating agent.

The above zirconyl ammonium carbonate is usually obtained from zirconyl carbonate and an alkali carbonate such as ammonium carbonate or ammonium hydrogen carbonate according to the following reaction formula. 2ZrOCO ₃ In _{· 8H 2 O + 4NH 4 HCO} 3 + (NH 4) 2 CO 3 → 2 (NH 4) 3 ZrOH (CO 3) 3 · 2H 2 O + 5H 2 O The above reactions, the alkali carbonate is used in excess However, it is difficult to uniquely define an aqueous zirconyl ammonium carbonate solution because the structure of zirconyl ammonium carbonate is complicated and there are many unclear points about the behavior of zirconyl ammonium carbonate in an aqueous solution. The zirconyl ammonium aqueous solution includes all that are generally called zirconyl ammonium carbonate aqueous solutions. Since the raw materials used for the production of zirconyl ammonium carbonate may contain a relatively large amount of impurities, in order to obtain a high-purity zirconia sol, the zirconyl ammonium carbonate prepared from the examined raw materials is used. It is also necessary to use an aqueous solution. In the present invention, the pH
Is 8 to 10 and the concentration is 10 to 20% by weight as ZrO _2.
An aqueous solution of zirconyl ammonium carbonate to a degree is preferably used.

The neutral zirconia sol can be obtained by mixing the above aqueous zirconyl ammonium carbonate solution with a chelating agent, and then hydrolyzing the obtained reaction product. Specifically, for example, when an aqueous zirconyl ammonium carbonate solution is placed in a stirred tank reactor and then the chelating agent is added under stirring, the zirconyl ammonium carbonate and the chelating agent react rapidly at room temperature. After the reaction, when the reaction liquid is heated to 60 ° C. or higher, the hydrolysis reaction of the reaction product of zirconyl ammonium carbonate and the chelating agent proceeds while a gas mainly containing carbon dioxide and ammonia is generated. During this reaction, the reaction liquid does not thicken and the reaction can be completed while maintaining the transparency of the reaction liquid. By adding an alkaline substance to this reaction solution, a neutral zirconia sol stable in the neutral or alkaline range can be obtained without causing thickening or gelation of the reaction solution.

One of the features of the method for preparing the neutral zirconia sol is that during the hydrolysis reaction, ammonium ions and carbonate ions which are impurity ions for the sol are discharged out of the system as ammonia and carbon dioxide. Therefore, the reaction solution obtained by hydrolysis of the reaction product of zirconyl ammonium carbonate and the chelating agent can be used as it is as a zirconia sol without any particular washing. Further, this reaction liquid can be concentrated to a high concentration by ordinary heating evaporation. However, when a minute amount of unreacted substances, carbonate ions, ammonium ions, etc. remaining in the reaction solution are removed to obtain a zirconia sol of higher purity, an ultrafiltration membrane is used and the above acidic zirconia is used. It may be washed by the same method as described in the method of preparing the sol. Examples of the chelating agent include oxyphenols such as catechol and pyrogallol; amino alcohols such as diethanolamine and triethanolamine;
Glycolic acid, citric acid, tartaric acid, lactic acid, mandelic acid,
Oxyacids such as malic acid and hydroxyacrylic acid and their esters such as methyl, ethyl and hydroxyethyl; oxyaldehydes such as glycolaldehyde; polycarboxylic acids such as oxalic acid and malonic acid; amino acids such as glycine and alanine Acetylacetone,
Β- of benzoylacetone, stearoylacetone, stearoylbenzoylmethane, dibenzoylmethane, etc.
Examples thereof include diketones; β-ketone acids such as acetoacetic acid, propionylacetic acid and benzoylacetic acid, and their esters such as methyl, ethyl, n-propyl, isopropyl, n-butyl and t-butyl. These chelating agents may be used alone or in combination of two or more.

Of the above chelating agents, glycolic acid,
Oxyacids such as citric acid, tartaric acid, lactic acid, mandelic acid, malic acid and hydroxyacrylic acid, and β-diketones such as acetylacetone are preferably used. Particularly, carbonyl compounds having oxygen atoms on the carbon atoms of α, β and γ, such as α, β and γ-oxy acids, are preferably used.

The amount of the chelating agent used varies depending on the kind of the chelating agent used, but the chelating agent (mole number)
/ Zirconia (the number of moles) is 0.02 / 1 to 4/1, preferably 0.1 / 1 to 3/1, more preferably 0.5 / 1
It is preferable to select it in the range of ˜2 / 1.

When the amount of the chelating agent used is too small, a certain organic zirconium salt produced by the reaction between the chelating agent and zirconyl ammonium carbonate is similar to the case of using zirconyl ammonium carbonate alone in the above hydrolysis reaction. It behaves, the hydrolysis reaction cannot be continued, and the effect of the chelating agent cannot be obtained. On the other hand, even if the chelating agent is used in a large amount exceeding the chelating agent / zirconia = 4/1, no further special effect can be obtained and it is not economical.

The above hydrolysis reaction may be carried out at 60 ° C. or higher, and it is desirable to carry out the reaction in a pressurized atmosphere in order to accelerate the reaction. The practical reaction temperature is 60 to 300 ° C.

The reaction liquid after the hydrolysis reaction has a transparent appearance and a pH of about 7, and is used as a neutral zirconia sol. This neutral zirconia sol uses a chelating agent in its preparation process,
It has the characteristic that the stability of the sol is extremely high. Further, even when the concentration is high, the viscosity of the sol is low, it is stable over time, and it is very easy to prepare a zirconia-based sol containing a compound of a metal other than zirconium.

Examples of the zirconia-based sol include compounds such as yttrium oxide (Y ₂ O ₃ ), calcium oxide (CaO), magnesium oxide (MgO), which are known as stabilizers of zirconia, as starting materials, or a sol thereof. It is also possible to use a stabilized or partially stabilized zirconia sol obtained by mixing the above with a neutral zirconia sol. That is, the neutral zirconia sol in the present invention includes such a stabilized or partially stabilized zirconia sol.

(3) Basic type zirconia sol When a mold is produced using a slurry prepared by using the neutral type zirconia sol, the wax model is repeatedly coated with the slurry to form a coating layer on the surface thereof. When the wax model provided with the coating layer is dipped in the slurry in the process of forming, the coating layer may be softened and deviated from the wax model, or a uniform coating layer may not be formed.

It has been found that the water resistance of such a neutral type zirconia sol can be improved by decreasing the chelating agent / zirconia molar ratio used for its production. However, when an aqueous zirconyl ammonium carbonate solution and a chelating agent are mixed so that the above molar ratio becomes small, and heating is performed in the same manner as in the case of the neutral zirconia sol to cause hydrolysis, the reaction solution becomes thickened during the reaction. Moreover, gelation is likely to occur, which makes it difficult to control the hydrolysis reaction. Therefore, once a neutral zirconia ammonium sol is prepared from a chelating agent and a zirconyl ammonium carbonate aqueous solution, and an aqueous zirconyl ammonium carbonate aqueous solution is further added thereto to cause hydrolysis, which causes problems such as thickening and gelation. It was found that a basic type zirconia sol having a small chelating agent / zirconia molar ratio was obtained.

That is, the basic zirconia sol used in the present invention is obtained by adding zirconyl ammonium carbonate to the neutral zirconia sol (2) and then hydrolyzing it. This neutral zirconia sol is prepared by the same method as described in (2) above, and the description of the preparation method is omitted. As the zirconyl ammonium carbonate used here, the one used in the preparation of the neutral zirconia sol (2) above can be used.

An aqueous solution of zirconyl ammonium carbonate is added to the neutral zirconia sol, and the resulting solution is heated as it is or at 60 ° C. or lower and concentrated, and then at 60 ° C. or higher as in the case of the preparation of the neutral zirconia sol. It is heated to hydrolyze, and if necessary, further concentrated to obtain a basic zirconia sol having a pH of 7 to 11 and a concentration of ZrO ₂ of up to 45% by weight. Further, as in the case of the neutral type zirconia sol, the reaction solution after hydrolysis is filtered and washed with an ultrafiltration membrane, and the content of ammonium ions, carbonate ions and the like is smaller, and high purity basic type zirconia is used. It is also possible to obtain a sol.

The amount of zirconyl ammonium carbonate added to the neutral zirconia sol is 10 to 90% by weight, preferably 30 to 50% by weight, based on the total amount of ZrO ₂ in the basic zirconia sol, as ZrO _2. It is better to be When the amount exceeds 90% by weight, the above hydrolysis reaction cannot be continued, and when the amount less than 10% by weight is added, the basic type zirconia sol can be obtained, but the above-mentioned improvement in water resistance is substantially not recognized. .

Generally, the chelating agent (mole number) / zirconia (mole number) in the basic zirconia sol is 0.
05-2.

An aqueous solution obtained by adding a chelating agent and an aqueous zirconyl ammonium carbonate solution so that the ratio of the chelating agent and zirconia is the same as that of the basic zirconia sol, or heating the aqueous solution, The partially hydrolyzed reaction liquid obtained by hydrolyzing and stopping the hydrolysis before the reaction liquid gels is also basic and has the performance as an inorganic binder. When used as, it behaves similarly to an aqueous solution of zirconyl ammonium carbonate, and the object of the present invention cannot be achieved.

The above acidic type, neutral type and basic type zirconia sols have a transparent appearance, which means that the sol particles in the zirconia sol have a very small particle size. ing. Moreover, since all of these zirconia sols can be obtained in high concentration, they can be suitably used as an inorganic binder in the slurry of the present invention.

The acidic, neutral and basic zirconia sols are all stabilized with yttrium oxide, calcium oxide, magnesium oxide, etc. as described in (2) Neutral zirconia sol. Alternatively, it also includes a partially stabilized zirconia sol.

The acidic type, neutral type and basic type zirconia sol are properly selected in consideration of the properties of the refractory fine particles used in the preparation of the slurry of the present invention, the material of the cast model, the economical efficiency and the like. . For example, when the refractory fine particles contain an acidic or acidic component, an acidic zirconia sol, and when the refractory fine particles contain a basic or basic component, a neutral zirconia sol and / or a base. A basic zirconia sol, preferably a basic zirconia sol, is used. As a result, the slurry of the present invention has excellent stability over time and does not cause problems such as gelation even when stored for a long period of time. In the slurry of the present invention, a combination of acidic refractory fine particles and acidic zirconia sol, and a combination of basic refractory fine particles and basic zirconia sol are particularly preferably used.

Furthermore, by adding an acidic or basic substance to the above slurry and adjusting and controlling the pH of the slurry, it is possible to make the slurry more stable, for example, for a month or more.

As the refractory fine particles, any of those generally used in the production of ceramics as a refractory substance can be used, but it is particularly considered to be used in the production of a casting mold for titanium or titanium alloy casting. if you did this,
Due to its low reactivity with molten titanium, in particular zirconia; stabilized zirconia fully or partially stabilized with CaO, Y ₂ O ₃ or MgO; WO ₃ ; Y
₂ O ₃ ; ThO ₂ ; Mg that becomes MgO by firing
Magnesium compounds such as CO ₃ and Mg (OH) ₂ ; and CaCO ₃ and Ca (O that become CaO and the like when fired.
Calcium compounds such as H) ₂ are preferably used. Of these, high-purity stabilized zirconia such as electro-fused calcia-stabilized zirconia, which has been widely used as a refractory material, high-purity calcium carbonate, etc., are particularly preferable in terms of stability at high temperature and economical efficiency. Used for.

The above-mentioned high-purity stabilized zirconia is, for example,
It can be manufactured according to the method for manufacturing so-called high-purity zirconia powder for fine ceramics. By the way, as the high-purity stabilized zirconia fine particles, it is necessary to use heat-treated particles at a temperature higher than the firing temperature of the raw mold in order to prevent the mold from shrinking when firing the raw mold. This heat treatment is usually 1000 ° C. or higher, and when heat-treated at such a high temperature, the high-purity stabilized zirconia obtained by the above-mentioned method undergoes sintering to form a hard lump, and to make it into fine particles, A crushing process is required. However, this pulverization is inefficient, and a washing step is required because impurities may be mixed in, so the above method is not always economically advantageous.

In addition, high-purity stabilized zirconia produced by the following method can also be used. First, for example, nitrates, chlorides, sulfates, carbonates of zirconium, for example, nitrates, chlorides, sulfates, carbonates or acetates and metals serving as stabilizers of zirconia such as yttrium, calcium, magnesium. Alternatively, a method of mixing with an acetate salt and adding a basic substance to the mixed aqueous solution to obtain coprecipitation or mixed hydroxide of zirconium and a metal constituting the stabilizer, and then filtering and washing the same. Alternatively, the precursor of the stabilized zirconia is prepared by any wet or dry method such as a method of calcining a mixture of a zirconium salt and a metal salt as a stabilizer. Next, the precursor is subjected to heat treatment as it is or at a low temperature and then crushed by a ceramic ball mill, hammer mill, jet mill or the like to be sized. Finally, the sized precursor is fired at a temperature of about 1200 to 1700 ° C. to obtain high-purity stabilized zirconia. This method is industrially advantageous as compared with the above-mentioned method because the crushing step is easy and efficient.

The high-purity stabilized zirconia used in the present invention is not limited by the production method thereof, and any of the high-purity stabilized zirconia obtained by the above method can be used.

As described above, this high-purity stabilized zirconia also includes high-purity partially stabilized zirconia, and the amount of the stabilizer added in this high-purity partially stabilized zirconia is not particularly limited. Absent. Usually, the amount of the stabilizer added is about 15 mol%, but in the present invention, highly pure stabilized zirconia having the amount of the stabilizer added of 15 mol% or more can also be used.

High-purity calcium carbonate is a purified natural product.
Alternatively, it can be produced relatively easily and economically by synthesizing calcium salt or lime milk as a starting material. There is no particular limitation on the average particle size and particle size distribution of the refractory fine particles, and it is appropriately determined in consideration of the properties such as the porosity, pore size and strength of the target raw template or mold, and the stability of the slurry. can do. Usually, those having an average particle size of 1 to 30 μm are used.

When the addition amount of the refractory fine particles is large, the strength of the porous ceramics such as the mold obtained is increased, but the viscosity of the slurry is increased, which causes a problem that the handling becomes difficult. Therefore, the amount of refractory particles added is
The amount is appropriately determined in consideration of the strength of the target porous ceramics and the viscosity of the slurry, but is usually 1 to 5 times by weight, preferably 2 to 4 times by weight that of the zirconia sol.

Using a slurry prepared by blending the above-mentioned acidic type, neutral type or basic type zirconia sol with refractory fine particles and, if necessary, a small amount of additives such as a surfactant,
When manufacturing a large-sized and complex-shaped raw mold for precision casting of titanium or titanium alloy, the water resistance problem as described below occurs.

One of them is that when the coating operation of the wax model is repeated to form a coating layer in a laminated form, when the coated wax model is immersed in the slurry to form the next layer, the water in the slurry causes That is, the coating layer provided in 1 is softened and eluted, the coating layer is displaced from the wax model, or a uniform coating layer cannot be obtained. Another problem arises when the wax model is eluted from the wax model with the coating layer by steam in an autoclave, and the condensed hot water reduces the strength of the coating layer and destroys the coating layer and the raw mold. Or the inner surface of the green mold may be peeled off.

The larger and more complicated the wax model is, the longer the raw mold is dipped in the slurry and the longer it takes to remove it. The water resistance is a serious problem in the production of green molds.

The above-mentioned problem of water resistance is caused by the particle size of sol particles of zirconia sol used as an inorganic binder,
Crystallinity, the content of ions and organic substances in the sol, that is, the purity of the sol depends on the properties of the zirconia sol itself. In the present invention, by using in combination with an organic binder, It complements the drawbacks of zirconia sol. Furthermore, when producing a green mold using a basic type zirconia sol, cracks may occur in the green mold due to dry shrinkage of the green mold, but even in this case, by using the basic type zirconia sol in combination with an organic binder. The generation of cracks can be effectively prevented. That is, according to the present invention, by using a combination of zirconia sol and an organic binder, it is possible to efficiently produce a green mold having a large and complicated shape and high water resistance. It has become.

As the above-mentioned organic binder, those which are dissolved or suspended in the above-mentioned zirconia sol or a slurry prepared from zirconia sol and refractory fine particles are preferable. In the slurry composed of zirconia sol, refractory fine particles and organic binder, Any organic compound can be used as long as it is dispersed in the slurry under stirring, and a compound that is separated on the upper portion of the slurry while stirring is stopped can also be used. Further, an organic polymer compound is preferably used, which does not cause an increase in the viscosity of the slurry, exhibits an effect even when used in a small amount, and does not leave ash and metals after thermal decomposition and disappearance.

Examples of the organic binder that can be used in the present invention include celluloses such as methyl cellulose, carboxymethyl cellulose and hydroxy cellulose;
Vinyl-based polymers such as polyvinyl alcohol and polyvinyl acetate; polyolefin glycols such as polyethylene glycol; acrylic-based polymers containing (meth) acrylic acid and / or its ester; isobutylene-maleic anhydride copolymer; polyacrylic Ammonium acid etc. can be mentioned. In addition, polyvinyl acetate emulsion, vinyl acetate-acrylic acid ester copolymer emulsion, vinyl oxyacid-ethylene copolymer emulsion, vinyl acetate-maleic acid ester copolymer emulsion, vinyl acetate-vinyl chloride copolymer emulsion, acetic acid Vinyl acetate-based vinyl-vinyl propionate copolymer emulsion, vinyl acetate-ethylene-vinyl chloride copolymer emulsion, vinyl acetate-ethylene-vinyl chloride copolymer emulsion, vinyl acetate-ethylene-acrylic acid copolymer emulsion, etc. Polymer emulsions; polyacrylic ester emulsions,
Acrylic acid-based polymer emulsions such as acrylic acid ester-styrene copolymer emulsions; Vinyl chloride-based polymer emulsions such as vinyl chloride copolymer emulsions; Vinylidene-based polymer emulsions such as vinylidene chloride copolymer emulsions Can also be used. These can be used alone or in combination of two or more.

Of these, celluloses, polyvinyl alcohol and vinyl acetate polymer emulsions are preferably used. The above organic binder is as it is,
Alternatively, it is dissolved in an organic solvent such as alcohols and added to a zirconia sol or a slurry containing the same to use.

The amount of the organic binder added is 0.5 to 20 parts by weight, preferably 1 to 10 parts by weight, based on 100 parts by weight of the zirconia sol, based on the solid content. If the addition amount is 0.5 parts by weight or less, the effect of adding the organic binder cannot be obtained, while if it exceeds 20 parts by weight, the mold strength after firing the raw mold is lowered, which is not preferable.

The method for preparing the slurry of the present invention is not particularly limited, and for example, a basic type zirconia sol having a concentration of ₂₀ to 35% by weight as ZrO _{2 and a} high-purity stabilized zirconia as refractory fine particles can be used as a zirconia sol. ~ 5 times the weight,
A vinyl acetate-ethylene copolymer emulsion as an organic binder is added in an amount of 0.5 to 100 parts by weight per 100 parts by weight of the zirconia sol.
It can be prepared by sequentially adding at a ratio of 20 parts by weight. In addition to the above components, a small amount of a surfactant, a defoaming agent, etc. can be added to the slurry of the present invention as in the case of the slurry used in ordinary precision casting.

The above-mentioned surfactant is for improving the wettability between the wax model and the slurry, and a nonionic surfactant containing no metal ion is preferably used. The amount of the surfactant used is 0.1 to 100 parts by weight of the slurry.
It is about 10 parts by weight. Further, the defoaming agent is for eliminating bubbles in the slurry, and preferably does not contain silicon. The amount of defoaming agent used is about 0.1 to 10 parts by weight per 100 parts by weight of the slurry.

When a mold is manufactured as porous ceramics, the viscosity of the slurry is adjusted to Zahn cup No. 5
It is better to adjust within the range of 10 to 90 seconds.

Since the slurry of the present invention can be used to produce various kinds of porous ceramics moldings such as crucibles and setters in addition to the above-mentioned molds, the porous ceramics of the present invention are these porous ceramics. It includes all molded articles.

The method for producing the porous ceramics of the present invention will be described below by taking a mold as a typical example.
The method for producing the mold is not particularly limited, and it can be produced according to a method generally called the lost wax method.

First, a casting model is made of wax. Next, this casting model is immersed in the slurry of the present invention, or the slurry is poured into the casting model to adhere the slurry to the surface of the casting model. Before the slurry layer dries, the refractory particles are sprinkled on the slurry layer to cover the casting model with the refractory particles. By repeating the operation of forming this slurry layer and sprinkling the refractory particles to form a coating layer on the casting model, the casting model is removed by a usual method such as heating or chemical treatment to produce a green mold. .

The refractory particles used for sprinkling are:
It is usually coarser than the refractory fine particles used for preparing the slurry, and for example, those having 20 to 200 mesh are preferably used. The refractory material may be the same as or different from the refractory material used for preparing the slurry,
For example, when the refractory fine particles used for preparing the slurry are calcium carbonate, the stabilized zirconia particles are preferably used as the sprinkling refractory particles used for forming the coating layer.

The mold using the slurry of the present invention is
Although it is more expensive than conventional silica-based molds and may become heavy, in this case, when casting, the slurry of the present invention is used from the first layer to the second to third layers that come into contact with the molten metal. A conventional method, for example, using a silica-based slurry, is practically recommended for forming the coating layer and thereafter forming the coating layer. In the mold thus obtained, the coating layer obtained by using the silica-based slurry does not substantially participate in the reaction with the molten metal, and the obtained mold also has sufficient strength.

A raw mold is obtained by separating the wax casting model from the coating layer. The casting model can be removed by a thermal shock method, an autoclave method, a solvent method, or microwave heating. It can be carried out by a commonly used method such as a method. Industrially, it is preferable to use steam at about 150 ° C. in an autoclave and heat for a short time.
At this time, the green mold obtained using the slurry of the present invention,
As described above, since the water resistance is excellent, the casting model can be removed without causing problems such as cracks in the coating layer or peeling of the inner surface layer.

A mold can be obtained by removing the remaining wax and organic binders from the thus obtained green mold by heat treatment and then firing it in a temperature range of 800 to 1600 ° C. This mold is excellent in strength and further suppressed in reactivity with a highly active metal, so that it is suitably used for precision casting of a highly active metal, particularly titanium or titanium alloy.

The slurry of the present invention can be used not only for the production of the above-mentioned porous ceramic molded body but also as a coating agent for imparting heat resistance to the substrate,
Further, by impregnating the porous ceramics, it is possible to make the porous ceramics highly dense and improve the strength.

[0071]

The slurry for producing porous ceramics of the present invention is an acidic zirconia sol, a neutral zirconia sol or a basic zirconia sol, particularly an acidic zirconia sol or a basic zirconia sol, depending on the nature of the refractory fine particles. Since the type zirconia sol is used, it has excellent stability over time and can be stably stored for a long period of time. Also, since the slurry viscosity is low even if the content of refractory fine particles is increased,
Easy to handle. Since the slurry for producing porous ceramics of the present invention uses a combination of a transparent zirconia sol and an organic binder as a binder, a raw mold having excellent water resistance and strength can be obtained using this slurry.

The mold obtained by using the slurry for producing porous ceramics of the present invention has excellent strength, and the reactivity with highly active metal such as titanium or titanium alloy is further suppressed. It is extremely useful as a mold for precision casting of such a highly active metal.

[0073]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Reference Example 1 (Preparation of acidic zirconia sol) ZrO ₂ 18 wt% aqueous solution of zirconyl nitrate 30
kg and 3 kg of urea were added to 200 liters of pure water (hereinafter referred to as L) and then heated to obtain a transparent zirconia sol. After cooling this sol, it was put into a circulation type ultrafiltration device, and the sol was continuously washed while adding pure water to the sol. Impurities such as ions in the sol were discharged out of the system together with the filtrate, and a high-purity sol was obtained. After the above washing, the sol concentration was adjusted to 10% by weight as ZrO ₂ , and then acetic acid was added, followed by heating and concentration under vacuum while maintaining the temperature at 50 ° C. to obtain transparent acidic zirconia having a concentration of 25% by weight as ZrO _2. I got a sol.

The zirconia sol had a pH of 1.3 and a viscosity of 30 cp and was stable for a long period of time.

Reference Example 2 (Preparation of Basic Type Zirconia Sol) 13% by weight of a commercially available zirconyl ammonium carbonate aqueous solution (pH 8.6) 13 as ZrO ₂ was added to a 15 L flask.
000g was put. Glycolic acid 1040 with stirring
g was added slowly. At this time, odorless gas was generated.
Then, the flask was heated by a mantle heater to carry out the hydrolysis reaction. Ammonia odor gas begins to be generated from the reaction solution temperature of 50 to 60 ° C., and foams violently as the temperature rises, and gases such as ammonia and carbon dioxide gas generated from insoluble ions in the sol are discharged to the outside of the system. While the reaction proceeded. By reacting the reaction solution at a temperature of about 100 ° C for about 3 hours, the foaming subsides, and heating is continued for 12 hours while appropriately adding pure water to the flask. The transparency of ZrO ₂ is 25% by weight and the pH is 7. A neutral zirconia sol was obtained.

To 5000 g of this neutral type zirconia sol, 9600 g of the aforementioned aqueous zirconyl ammonium carbonate solution was added, and mixed by stirring. Then, the mixed solution is depressurized at 60 ° C.
After concentrating to a concentration of about 30% by weight as ZrO ₂ below, heat treatment is performed at 95 ° C. under normal pressure. During this heat treatment, there is foaming due to hydrolysis of zirconyl ammonium carbonate. After heat treatment for about 1 hour, the mixture is cooled, and pure water is added to the mixture to obtain a transparent base having a pH of 9 and a concentration of ZrO ₂ of 30% by weight. A soluble zirconia sol was obtained. Reference Example 3 (Production of high-purity yttria-stabilized zirconia fine powder) 112 kg of zirconium carbonate and 41 kg of nitric acid were mixed to prepare an aqueous zirconyl nitrate solution. An yttrium chloride aqueous solution was added to this aqueous solution as Y ₂ O _{3 so} as to be 8 mol% added zirconia, and then pure water was added to obtain a mixed aqueous solution of zirconium salt and yttrium salt having a total volume of 800 L. Separately, the above mixed aqueous solution was added to 800 L of a basic aqueous solution prepared by adding aqueous ammonia under stirring to obtain a coprecipitated hydroxide consisting of zirconium hydroxide and yttrium hydroxide.

This hydroxide was continuously filtered and washed with a rotary filter press equipped with two tanks. The filtration, the washed hydroxide was dispersed again in pure water, filtered until further chloride ion concentration in the wastewater is 1ppm or less, by repeating the operation of washing as Y ₂ O ₃ 8
As a precursor of yttria-stabilized zirconia fine powder containing mol%.

The precursor was dried at 140 ° C., crushed with a hammer mill and classified with a 100 mesh filter. By calcining the obtained powder at 1500 ° C. for 3 hours, a weakly agglomerated lump-like powder or granule is obtained, and the powder or granule is crushed by a ceramic jet mill crusher to obtain an average particle diameter. A high-purity yttria-stabilized zirconia fine powder having a particle size of 12 μm or less was obtained.

A high-purity yttria-stabilized zirconia fine powder of 60 to 120 mesh and a filter of 4 to 10 mesh were obtained in the same manner as above except that after crushing with the hammer mill, classification was performed using a filter of 8 to 14 mesh. High-purity yttria-stabilized zirconia fine powder of 24-70 mesh was obtained in the same manner as above except that the fine powder was classified.

Example 1 Using the acidic zirconia sol obtained in Reference Example 1, the yttria-stabilized zirconia fine powder having an average particle size of 12 μm obtained in Reference Example 3 and an acetic acid-ethylene copolymer emulsion as an organic polymer compound, A slurry having the composition of was prepared.

Parts by Weight Acidic Zirconia Sol 100 Yttria-Stabilized Zirconia Fine Powder 300 Acetic Acid-Ethylene Copolymer Emulsion 5 (Brand Name: Sumikaflex, Sumitomo Chemical Co., Ltd.) Nonionic Surfactant 0.5 ( Product name: Discol, manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Defoamer 0.3 (Product name: SN-deformer, manufactured by San Nopco Co., Ltd.) First, nitric acid is added to acidic zirconia sol to obtain viscosity of sol. Was adjusted to 30 cp, and high-purity yttria-stabilized zirconia fine powder having an average particle diameter of 12 μm was added to this zirconia sol to prepare a slurry. Next, an acetic acid-ethylene copolymer emulsion, a nonionic surfactant and an antifoaming agent were sequentially added to this slurry with stirring, whereby the viscosity of Zahn cup No. 5 seconds, 29 seconds, pH 1.
A slurry for producing a porous ceramics No. 6 was prepared. This slurry was stable for over a month.

A brass plate having a width of 125 mm, a length of 250 mm, and a thickness of 5 mm, which was thinly coated with wax, was dipped in the above slurry and then pulled up to obtain a 60 to 120 mesh high-purity yttria stabilized product obtained in Reference Example 3. After sprinkling the powdered zirconia powder, the operation of drying at room temperature was repeated to form three coating layers. Continuing, 24 ~
Two more coating layers were formed in the same manner as above except that 70-mesh high-purity yttria-stabilized zirconia powder was used to form a total of five coating layers.

No dissolution of the coating layer was observed in this coating operation, and a coating layer having a uniform thickness could be formed. Next, while heating one end of the brass plate to melt the wax, the brass plate was taken out from the coating layer to obtain a raw mold. The wax remaining in the raw mold was washed and removed with kerosene. In this way, a plurality of green molds were produced and the following tests were conducted.

Water resistance of raw mold A test piece was cut from the raw mold and immersed in boiling pure water. Even when this boiling was continued for 60 minutes, no turbidity of the aqueous solution was observed, and neither cracking of the test piece nor collapse of the test piece was observed.

From these results, it was found that the green mold has sufficient resistance to hot water and can be dewaxed in the autoclave without any problem.

Molds and Strength of Molds Molds were prepared by firing the green molds at 1300 ° C. for 2 hours. A raw mold and a mold were cut out with a diamond cutter to prepare a test piece, and its bending strength was measured according to the three-point bending strength test method.

Test piece dimensions: width 12 mm, length 40
mm, thickness 6 mm Measurement conditions: span 39 mm, measurement temperature room temperature, bending speed 0.5 mm / min The average value of the bending strength measured for each of 10 test pieces was as follows.
Raw mold: 31 kg / cm ² , mold: 96 kg / cm ^{2 From} these results, it was found that both the raw mold and the mold have sufficient bending strength.

Reactivity of Mold The molten pure titanium was cast into the mold and the thickness of the reaction layer on the surface of the casting was measured and found to be 100 μm. From this result, it was found that the mold had a low reactivity with titanium and that this mold could be sufficiently used as a titanium precision casting mold.

Example 2 A slurry having the following composition was prepared using the basic zirconia sol obtained in Reference Example 2, high-purity calcium carbonate having a purity of 99.9%, and polyvinyl alcohol as an organic polymer compound.

Weight part Basic type zirconia sol 100 High-purity calcium carbonate 200 (trade name: CS-3N, manufactured by Nippon Lime Industry Co., Ltd.) Polyvinyl alcohol 2 (trade name: Poval, manufactured by Kuraray Co., Ltd.) Amphoteric surfactant Agent 0.5 (trade name: Amphitol, manufactured by Kao Co., Ltd.) Defoaming agent 0.3 (trade name: Formaster P, manufactured by San Nopco Co., Ltd.) First, basic zirconia sol with three types of particle sizes A high-purity calcium carbonate having a mean particle size of 9 μm was added to prepare a slurry. Next, an aqueous solution prepared by separately dissolving polyvinyl alcohol, a surfactant and an antifoaming agent in 15 parts by weight of water was mixed with the above slurry to prepare a slurry for producing a porous ceramic. The viscosity of this slurry is Zahn cup No. 5 was 70 seconds and its pH was 9.2.

Thereafter, in the same manner as in Example 1, three zirconia-based coating layers were first formed by using the slurry and the high-purity yttria-stabilized zirconia powder of 60 to 120 mesh obtained in Reference Example 3. A mold was created. In this coating operation, dissolution of the coating layer was not observed, and a strong coating layer with a uniform thickness could be formed.

Next, a silica-based slurry prepared separately by mixing a colloidal silica sol having a concentration of 30% by weight as SiO ₂ and zircon powder # 200, and 24-70 mesh zircon powder as sprinkling particles are used. Three silica-based coating layers were further formed on the zirconia-based coating layer of the green mold.

In the same manner as in Example 1, the wax model was removed to form a green mold having a silica-based outer layer and a zirconia-based coating layer as the inner layer. 110 this raw mold
By firing at 0 ° C for 2 hours, the outer layer is silica-based,
A zirconia-based mold was manufactured for the inner layer. Since the inner layer of this mold collapses due to moisture absorption, it must be stored in vacuum.

Molten pure titanium was cast into the mold thus obtained, and the thickness of the reaction layer on the surface of the obtained casting was measured and found to be 100 μm or less. From this result, it was found that this casting can be sufficiently used as a titanium precision casting mold.

Example 3 Using the basic type zirconia sol obtained in Reference Example 2, electromelted calcia-stabilized zirconia fine powder, and a vinyl acetate-based emulsion containing vinyl acetate as a main component as an organic polymer compound, the following composition was prepared. A slurry was prepared.

Weight part Basic type zirconia sol 100 Electrofused calcia-stabilized zirconia fine powder 300 (trade name: Zirbon, Fukushima Steel Co., Ltd.) Vinyl acetate emulsion 10 (trade name: Sumika Flex, Sumitomo Chemical Co., Ltd. )) Amphoteric surfactant 0.5 (Brand name: Amphitol, manufactured by Kao Co., Ltd.) Defoaming agent 0.3 (Brand name: SN-deformer, manufactured by San Nopco Co., Ltd.) First, basic zirconia sol A fine powder of fused zirconia stabilized with fused-calcium having an average particle diameter of 15 μm was added with stirring, and then the vinyl acetate emulsion, the nonionic surfactant and the defoaming agent were sequentially added to obtain a Zancup No. A slurry for producing porous ceramics having a pH of 9.0 and a pH of 9.0 was prepared.

The above-mentioned electrofused calcia-stabilized zirconia fine powder also contains a basic calcia component that is not solid-dissolved in zirconia, but by using a basic zirconia sol as the zirconia sol, it is stable for one month or more. A slurry could be prepared.

Hereinafter, a raw mold and a mold were produced in the same manner as in Example 1 except that the above slurry was used as the slurry and electrofused calcia-stabilized zirconia was used as the powder for sprinkling.

When the water resistance of this green mold and the strength of the green mold and the mold were measured in the same manner as in Example 1, almost the same results as in Example 1 were obtained.

A titanium alloy having a composition of Ti-6Al-4V was cast into this mold, and the thickness of the reaction layer on the surface of the obtained casting was measured and found to be 200 μm or less. From this result, it was found that the above casting can be sufficiently used as a mold for precision casting of titanium alloy.

Comparative Example 1 A slurry having the same composition as in Example 1 was prepared in the same manner as in Example 1 except that the acetic acid-ethylene copolymer emulsion was not used.

Thereafter, five coating layers were formed in the same manner as in Example 1. However, since the coating layer was partially dissolved when immersed in the slurry, the coating layer became uneven.

Comparative Example 2 A slurry having the same composition as in Example 2 was prepared in the same manner as in Example 2 except that polyvinyl alcohol was not used.

Thereafter, three coating layers were formed in the same manner as in Example 2. However, cracks occurred in the dried green mold.

[0106]

Front Page Continuation (72) Inventor Teruyuki Hongo, Himeji-shi, Hyogo Prefecture, Kamitaka Itabashi, Inspector, Catalysis Research Laboratory, Catalysis Research Laboratory, No. 1 Nishi-oki, Kohama, Aki-ku, Japan

Claims (8)

(57) [Claims] 1. A slurry for producing porous ceramics, comprising a transparent zirconia sol, refractory fine particles and an organic binder. 2. The slurry for producing porous ceramics according to claim 1, wherein the transparent zirconia sol is an acidic zirconia sol obtained by heating an aqueous zirconium salt solution in the presence of urea. 3. The slurry for producing porous ceramics according to claim 1, wherein the transparent zirconia sol is a neutral zirconia sol obtained by hydrolyzing a reaction product of zirconyl ammonium carbonate and a chelating agent. 4. A basic zirconia sol obtained by further hydrolyzing a transparent zirconia sol obtained by hydrolyzing a reaction product of zirconyl ammonium carbonate and a chelating agent with zirconia ammonium ammonium. The slurry for producing a porous ceramic according to claim 1, wherein 5. The slurry for producing porous ceramics according to claim 1, wherein the organic binder is at least one selected from celluloses, polyvinyl alcohol and vinyl acetate emulsions. 6. The slurry for producing porous ceramics according to claim 1, wherein the refractory fine particles are at least one selected from stabilized zirconia and calcium carbonate. 7. A porous ceramic obtained by firing the slurry for producing a porous ceramic according to claim 1. 8. The porous ceramic according to claim 7, which is a casting mold for titanium or titanium alloy casting.