JP3017066B2 - Method for freeze-drying ceramic frozen molded body and drying jig used for the method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for freeze-drying a ceramic frozen compact, and more particularly, to a freeze-drying method which can dry a ceramic frozen compact in a short time and has a good surface condition after drying. The present invention relates to a drying method and a drying jig (pedestal) used therefor.

[0002]

2. Description of the Related Art Conventionally, various methods such as press molding, injection molding, and cast molding are known as a method of molding ceramics. One of such molding methods has recently been called a freeze molding method. The law is drawing attention. Freezing molding of ceramics is performed by mixing ceramic powder with a dispersion medium such as water, filling the resulting mixture (raw material slurry) into a mold, and freezing and hardening the mixture to obtain a desired shape. This is a molding method for obtaining a body, and has the advantages that a complicated shape can be given and that the degree of freedom of a molding die is large.

[0003] The freeze-formed product obtained by the freeze-forming method must be dried before baking to remove the dispersion medium. Usually, this drying is performed by a freeze-drying method in which the frozen molded body is placed in a vacuum in a frozen state and the dispersion medium is sublimated. In general, a shelf-type freeze vacuum dryer is used for the freeze-drying, and the freeze-formed product is dried in a chamber (vacuum tank) of the freeze vacuum dryer while being directly mounted on a shelf plate.

[0004] The shelf on which the frozen compact to be dried is placed is usually capable of being cooled or heated by itself, and if the frozen compact may melt during drying. In order to promote sublimation and drying, if the melting of the frozen compact is not considered due to the relationship between the pressure and the temperature, the shelf Temperature control such as heating to increase the temperature of the frozen molded body is performed.

[0005]

In the above-mentioned conventional freeze-drying method, as shown in FIG.
The freeze-drying is performed in a state where 0 is directly placed on the shelf 14, and the installation surface (the surface on the side in contact with the shelf) 16 of the frozen molded body 10 that is in direct contact with the shelf 14 with respect to the shelf 14 is ,
Since it is in close contact with the surface of the shelf plate 14 and has no air permeability with the atmosphere around the frozen molded body 10, sublimation drying hardly proceeds from the installation surface 16, so that it takes time to dry the frozen molded body. There was such a problem. In addition, the installation surface 16 of the frozen molded body 10 that is in direct contact with the shelf board 14 has problems that the surface state is deteriorated (discoloration, color unevenness, etc.) and easily deformed.

The present invention has been made in view of such circumstances, and an object of the present invention is to reduce the drying time of a frozen molded body and to prevent deterioration and deformation of the surface state of the frozen molded body. It is an object of the present invention to provide a freeze-drying method for a ceramic frozen molded article that can be used.

[0007]

According to the present invention, a mixture of a ceramic powder and a dispersion medium is filled in a mold, frozen and hardened, and then a frozen molded body obtained by releasing the mold is obtained. Lyophilization method, the thermal conductivity is 15 W / m
K is a drying jig made of a material of K or more, and when a frozen molded body is placed on the drying jig, a space between an installation surface of the frozen molded body with respect to the drying jig and an atmosphere around the frozen molded body. A freeze-drying method for a ceramic freeze-formed body, characterized in that a freeze-drying method is performed by using a drying jig having a structure capable of securing air permeability to the freeze-formed body while the freeze-formed body is placed on the drying jig. Is done.

Further, according to the present invention, a mixture of a ceramic powder and a dispersion medium is filled in a mold, frozen and hardened, and then a freeze-molded product obtained by releasing the mold is freeze-dried. Drying jig used when the thermal conductivity is 15 W /
When the frozen molded body is made of a material of mK or more and the frozen molded body is placed on the drying jig, air permeability is secured between the installation surface of the frozen molded body with respect to the drying jig and the atmosphere around the frozen molded body. A drying jig characterized by having a structure capable of being provided.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The freeze-drying method of the present invention is different from the conventional method in that a freeze-formed product is not placed directly on a shelf plate of a freeze-vacuum dryer and vacuum freeze-drying is performed. A drying jig (pedestal) of a specific material and structure is laid on a shelf plate, and freeze-drying is performed with the frozen molded body placed on the drying jig.

[0010] The drying jig used in the present invention, when the frozen molded body is placed, the installation surface of the frozen molded body with respect to the drying jig (the surface in contact with the drying jig) and the periphery of the frozen molded body. It has a structure that can ensure air permeability with the atmosphere (hereinafter, simply referred to as “air permeability”). When freeze-drying of the frozen molded body is performed on a drying jig having such a structure, sublimation drying hardly progresses in the conventional method.
Sublimation also proceeds from the installation surface of the frozen molded body on the shelf (in the present invention, the installation surface on the drying jig), and the drying time is greatly reduced.

Further, if the above-mentioned drying jig is interposed between the frozen molded body and the shelf plate without directly placing the frozen molded body on the shelf plate, the surface condition of the dried molded body is deteriorated or deformed. Is also suppressed. Although the reason for this is not clear, as described above, since the sublimation drying proceeds in the same manner as the surface of the other frozen molded body from the installation surface for the drying jig of the frozen molded body, the frozen molded body is uniformly dried. At the same time, one possible factor is that the entire surface of the freeze-formed body becomes a dried layer at the initial stage of drying.

FIGS. 1 and 2 show specific examples of the structure of a drying jig. FIG. 1 (a) is a plan view, FIG. 1 (b) is a side view, and FIG. It is. The drying jig 2 in FIG. 1 has a mesh-like structure provided with a large number of through holes 4 and grooves 6. In this example, when the frozen molded body is placed on the drying jig, the installation surface of the frozen molded body with respect to the drying jig is:
Through the through hole 4 and the groove 6, the atmosphere around the frozen molded body is communicated. That is, as shown in FIG. 3, in the chamber of the freeze vacuum dryer, the drying jig 2
When the frozen molded body 10 is placed on the drying jig 2, the surface of the shelf 14, the installation surface 12 of the frozen molded body 10 for the drying jig 2, and the through-hole 4 of the drying jig 2 A space is formed, and this space communicates with the atmosphere around the frozen molded body 10 by the groove 6. Therefore, when freeze-drying is performed in this state, sublimation drying proceeds from the installation surface 12 of the freeze-formed body 10 exposed in the through holes 4 and the grooves 6 of the drying jig 2.

The drying jig 2 shown in FIG. 2 is provided with a large number of grooves 8 so as to make the surface uneven, and by adopting such a structure, when the frozen molded body is placed on the drying jig. In addition, air permeability is ensured. That is, as shown in FIG. 3, when the frozen molded body 10 is placed on the drying jig 2, the installation surface 12 of the frozen molded body 10 with respect to the drying jig 2 communicates with the atmosphere around the frozen molded body 10 through the groove 8. Therefore, if freeze-drying is performed in this state, sublimation drying proceeds from the installation surface 12 of the frozen molded body 10 exposed in the groove 8 of the drying jig 2.

The drying jig used in the present invention is made of a material having a thermal conductivity of 15 W / mK or more, preferably 50 W / mK or more. As described above, in freeze-drying, by cooling or heating the shelf plate, the temperature of the freeze-formed body during freeze-drying is adjusted and controlled to prevent the freeze-formed body from melting and promote sublimation drying. As in the case where a drying jig is interposed between the frozen molded body and the shelf board, compared to the case where the frozen molded body is directly placed on the shelf board,
The ability of the frozen molded body to follow the shelf temperature is poor,
It becomes difficult to control the temperature of the frozen molded body.

In order to solve this problem, it is necessary to make the drying jig from a material having a high thermal conductivity. Therefore, the thermal conductivity is specified as described above. When a drying jig made of a material having a thermal conductivity of less than 15 W / mK is used, for example, even if the shelf is cooled to prevent the frozen molded body from melting, the temperature of the frozen molded body is not sufficiently lowered. During the freeze-drying, the inside of the molded body is melted, and sublimation drying with completely frozen solid is not performed, and defects occur inside the molded body after drying.

Similarly, the thickness of the drying jig is preferably as small as possible in order to alleviate the deterioration of the followability of the temperature of the frozen compact to the temperature of the shelf due to the interposition of the drying jig. Is preferably 10 mm or less.

Further, the drying jig used in the present invention as shown in FIGS. 1 and 2 is preferably a structure having a volume ratio of 5% or more (a space portion ratio of 95% or less). Here, the volume ratio means the ratio of the actual volume of the drying jig to the volume of the drying jig including the volume of a space portion such as a hole or a groove formed in the drying jig. This volume ratio is 5
% (The ratio of the space portion exceeds 95%), even if the drying jig is made of a material having a high thermal conductivity, the temperature of the shelf plate is hardly transmitted to the frozen molded body, and the above-described deterioration of the temperature followability is deteriorated. Problems can arise.

As the drying jig used in the present invention, in addition to the drying jig provided with holes and grooves as described above to ensure air permeability, the drying jig is formed of a porous material, A material having air permeability in the material itself can also be used. In this case, the porosity of the porous body is preferably 10% or more. If the porosity is less than 10%, sublimation drying from the installation surface of the frozen molded body to the drying jig is not sufficiently performed, and the effect of the present invention is weakened.

Regardless of whether a hole or a groove is provided or a porous body is used, the area between the frozen molded body and the drying jig relative to the area of the installation surface of the frozen molded body on the drying jig is determined. It is preferable that the ratio of the contact area is 5% or more.
From the viewpoint of increasing the surface area (sublimation area) of the surface of the frozen molded body in which sublimation drying can proceed, it is preferable that the ratio of the contact area is small. Occurs.

In the present invention, the freeze-molded object to be freeze-dried is a freeze-molding method of ceramics, that is, a mixture of a ceramic powder and a dispersion medium (raw material slurry) is filled in a mold, which is frozen and hardened. After that, it is obtained by a method of releasing the mold. If it is in a frozen state, the shape of the frozen molded body is not limited.

The type of the ceramic powder used for producing the frozen molded body is not particularly limited, and powders such as alumina, silicon nitride, silicon carbide, zirconia, and sialon can be used. Further, in order to improve the properties of these sintered bodies, those to which various auxiliaries are added can be used.

As the dispersion medium, any of an aqueous dispersion medium and a non-aqueous dispersion medium can be used. The kind of the non-aqueous dispersion medium is not particularly limited, but benzene, xylene, toluene, methanol, ethanol and the like can be preferably used. In addition, a mixture of an aqueous dispersion medium and a non-aqueous dispersion medium can be used, and a mixture to which additives such as a dispersant, a binder, an antifoaming agent, and a surfactant are added as necessary can be used. As for the ratio between the ceramic powder and the dispersion medium, the volume concentration of the ceramic powder in the mixture is preferably 60% or less.

The molded body dried by the freeze-drying method of the present invention can be made into a sintered body by a general ceramic firing step. Since the molded body dried by the freeze-drying method of the present invention has the surface state deteriorated and deformed by freeze-drying suppressed, the finally obtained ceramic sintered body also has a good surface state and excellent shape accuracy. It will be.

[0024]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 38 vol% of silicon nitride powder, 2 vol% of a dispersant (ammonium polycarboxylate), 2 vol% of a binder (polyvinyl alcohol) and 58 vol% of water were mixed, and the mixture was vacuumed at -70 cmHg for 3 minutes. The raw material slurry was prepared by defoaming. Using this raw material slurry, a square plate having a size of 70 × 50 × 20 mm was formed. As a forming method, an aluminum alloy forming die having a forming part of the above dimensions,
The raw material slurry is filled at a pressure of 0.5 kg / cm ² ,
The raw material slurry was frozen by immersion in a refrigerant at 7 ° C for 7 minutes.

The freeze-formed body thus obtained was freeze-dried. Freeze-drying is performed by using a mesh-shaped drying jig 2 (aluminum alloy, thickness 3) having air permeability as shown in FIG.
mm), the drying jig 2 was laid on the shelf 14 as shown in FIG. 3 in the chamber of the freeze vacuum dryer, and the frozen molded body 10 was placed on the drying jig. As the lyophilization conditions, the eutectic point of the freeze-formed body was −5 ° C., and the shelf temperature was set to −1 to prevent melting during lyophilization.
The temperature was set at 0 ° C., and the degree of vacuum was about 0.1 Torr.

The time required for freeze-drying under the above conditions, the flatness of the dried compact, and the internal state of the dried compact were examined. The results are shown in Table 1. As shown in the table, freeze-drying was completed in a short time of 52 hours, and the flatness of the dried compact was also good. In addition, since the thermal conductivity of the drying jig material was sufficiently high, the thermal conductivity from the shelf plate was also good, and no defects such as cracks were found inside the molded body after drying.

Example 2 Example 1 was used except that a drying jig 2 (stainless steel, 3 mm in thickness) having a rugged surface as shown in FIG. 2 was used as the drying jig. The freeze-drying was performed in the same manner as described above, and the required time for freeze-drying, the flatness of the dried compact, and the internal state of the dried compact were examined. The results are as shown in Table 1, and almost the same results as in Example 1 were obtained.

Comparative Example 1 Freeze-drying was carried out in the same manner as in Example 1 except that the freeze-formed product 10 was directly placed on the shelf plate 14 as shown in FIG. 5 without using a drying jig. Then, the required time for freeze-drying, the flatness of the dried compact, and the internal state of the dried compact were examined. The results are as shown in Table 1. Since sublimation drying was not possible from the installation surface of the frozen molded body that was in direct contact with the shelf, a long time of 72 hours was spent for drying. Further, the state of the surface of the frozen molded body that was in direct contact with the shelf board deteriorated, and the flatness of the molded body after drying was considerably larger than those in Examples 1 and 2.

Comparative Example 2 The same procedure as in Example 1 was carried out except that a flat drying jig 2 (aluminum alloy, 3 mm thick) having no air permeability as shown in FIG. 4 was used as the drying jig. Then, freeze-drying was carried out, and the required time for freeze-drying, the flatness of the dried compact, and the internal state of the dried compact were examined. The results are as shown in Table 1, and were almost the same as Comparative Example 1.

[Comparative Example 3] Freeze-drying was carried out in the same manner as in Example 1 except that the material of the drying jig was made of plastic. The time required for freeze-drying, the flatness of the dried compact,
The internal state of the molded body after drying was examined. The results are as shown in Table 1. The drying was completed in a short time of 44 hours.
The flatness of the dried compact was also a relatively good result.
However, since the thermal conductivity of the drying jig material was low, the heat conduction from the shelf was poor, and the frozen compact was not sufficiently cooled. Was not carried out, and a defect occurred inside the molded body after drying. The reason why the drying was completed in a shorter time than in Examples 1 and 2 is that, as described above, since the cooling of the frozen molded body was insufficient, the temperature of the molded body increased during drying, and the drying was accelerated. It is considered that

[0032]

[Table 1]

In the above embodiment, the eutectic point of the frozen molded product was -5.
This is an example of a case where cooling of a shelf plate is required at the time of freeze-drying due to a low temperature of ℃. When melting of the frozen molded article is not considered from the relationship between pressure and temperature, the shelf board may be heated and freeze-dried in reverse to promote sublimation drying. By applying the freeze-drying method, the freeze-drying time can be shortened, and the deterioration and deformation of the surface state of the molded body can be suppressed.

[0034]

As described above, according to the present invention,
Sublimation drying hardly progressed in the conventional method. Sublimation drying proceeds from the installation surface of the frozen molded body on the shelf plate (in the present invention, the installation surface of the drying jig). Is shortened to In addition, the deterioration and deformation of the surface state of the molded body are suppressed, and a dry molded body with good surface state and shape accuracy can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an example of a drying jig, wherein (a) is a plan view, (b) is a side view, and (c) is a partially enlarged view of a side face.

FIGS. 2A and 2B are explanatory views showing another example of the drying jig, wherein FIG. 2A is a plan view, FIG. 2B is a side view, and FIG.

FIG. 3 is an explanatory diagram showing a state during freeze-drying by the freeze-drying method of the present invention.

FIG. 4 is an explanatory view showing a drying jig used in a comparative example, and FIG.
Is a plan view, and (b) is a side view.

FIG. 5 is an explanatory view showing a state during freeze-drying by a conventional freeze-drying method.

[Explanation of symbols]

2 ... drying jig (pedestal), 4 ... through hole, 6 ... groove, 8 ... groove,
10: frozen molded body, 12: installation surface for drying jig, 1
4: shelf, 16: installation surface for shelf

Claims (6)

(57) [Claims] 1. A method of filling a mixture of a ceramic powder and a dispersion medium into a mold, freezing and hardening the mixture, and then releasing a freeze-formed body obtained by releasing the mold, wherein a freeze-drying method is used. Is a drying jig made of a material of 15 W / mK or more, and when a frozen molded body is placed on the drying jig,
A state in which a frozen jig is placed on the drying jig using a drying jig having a structure such that air permeability can be secured between an installation surface of the frozen molded body with respect to the drying jig and an atmosphere around the frozen molded body. A freeze-drying method for a ceramic frozen molded article, wherein the freeze-drying is performed by using 2. The freeze-drying method according to claim 1, wherein the drying jig is a structure having a volume ratio of 5% or more. 3. The freeze-drying method according to claim 1, wherein the drying jig is a porous body having a porosity of 10% or more. 4. The freeze-drying method according to claim 1, wherein the thickness of the drying jig is 10 mm or less. 5. The freeze-drying method according to claim 1, wherein a ratio of a contact area between the freeze-formed body and the drying jig to an area of an installation surface of the freeze-formed body on the drying jig is 5% or more. 6. A drying jig used for filling a mixture of a ceramic powder and a dispersion medium into a mold, freezing and hardening the mixture, and then releasing the frozen molded body obtained by freeze-drying. And when the frozen molded body is made of a material having a thermal conductivity of 15 W / mK or more and the frozen molded body is placed on the drying jig, the installation surface of the frozen molded body with respect to the drying jig and the periphery of the frozen molded body A drying jig having a structure capable of securing air permeability between the drying jig and the atmosphere.