JP2010228932A - Method of manufacturing porous sintered compact - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous sintered compact having arbitrary porosity and pore size. <P>SOLUTION: The method of manufacturing porous sintered compact includes: a slurry kneading step of preparing slurry; a foamable slurry preparing step of comprising a bubble nucleus forming step of dispersing and forming the bubble nucleus comprising an addition gas into de-bubbled slurry; a pressure reducing step of expanding the bubble nucleus by evacuating the slurry containing the bubble nucleus to a predetermined pressure; a green body forming step of: including a vacuum-freeze step for vacuum-freeze drying the slurry having a volume increased by the expansion of the bubble nucleus and forming a green body formed by keeping the binder in raw material powder; and a sintering step of sintering the green body to form the porous sintered compact. Wherein, at least the pressure reducing expanding step is carried out while keeping a preliminary cooling temperature above the freezing point and less than boiling point under a predetermined pressure. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a porous sintered body.

  Porous sintered bodies are lightweight structural materials, electrodes for fuel cells and alkaline secondary batteries, various filters, catalyst carriers, biomaterials such as artificial bones, silencers, heat insulating materials, vaporization promoting parts, electromagnetic wave shielding materials, etc. As a material, it is used for various purposes.

  Conventionally, as a method for producing such a porous sintered body, a method (slurry foaming method) in which a slurry containing raw material powder is produced and fired is known (Patent Document 1). In addition, it has also been proposed to manufacture a metal porous sintered body by heating the foamed resin whose surface is plated to eliminate the resin portion (Patent Document 2).

JP-A-9-118901 JP-A-4-2759

  The porous sintered body is required to have an arbitrary porosity and pore size according to various applications. Conventionally, the porosity and pore size of the porous sintered body can be adjusted to any desired shape by adjusting the slurry composition or processing temperature in the case of the slurry foaming method, and when the foamed resin is used as the base material. A certain degree of control is possible by using a foamed resin. However, in the conventional manufacturing method, it is difficult to precisely control the porosity and the pore size, and further, it is a technical problem to suppress the variation in pore diameter. In addition, these problems are more difficult with a thick plate having a thickness of 5 mm or more and a 3D (three-dimensional arbitrary shape) porous sintered body.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a porous sintered body having an arbitrary porosity and pore size.

  The present invention is a method for producing a porous sintered body in which a green body is formed from a slurry comprising a raw material powder, a binder, and a material containing water, and the green body is sintered to produce a porous sintered body. A slurry kneading step of kneading the material to produce a slurry, a defoaming step of removing bubbles and dissolved gas from the slurry, and stirring while introducing an additive gas into the defoamed slurry, A foaming slurry manufacturing step including a bubble nucleus forming step of dispersing and forming bubble nuclei composed of the additive gas in the slurry, and a decompression for expanding the bubble nuclei by reducing the slurry containing the bubble nuclei to a predetermined pressure. An expansion step, and a vacuum freeze-drying step in which the slurry whose volume increased due to expansion of the bubble nuclei is vacuum-lyophilized, wherein the raw material powder is the binder A green body forming step for forming a green body that is held, and a sintering step for forming a porous sintered body by sintering the green body, wherein at least the reduced pressure expansion step is performed as the predetermined step. The pressure is maintained while maintaining a precooling temperature exceeding the freezing point of the slurry and lower than the boiling point.

According to this manufacturing method, since the slurry is cooled to a temperature at which the slurry is not frozen and boiled in the decompression / expansion step, the bubble core can be expanded without boiling the components (particularly water) in the slurry by the decompression. Therefore, the porous sintered body can be manufactured while the porosity is precisely controlled without causing the additive gas introduced into the slurry to escape.
Moreover, in the conventional manufacturing method, a certain amount of slurry viscosity is required for maintaining bubbles and maintaining the shape, and a binder and other additives for that purpose are mixed with the slurry. However, these binders and other additives may remain as impurities after sintering. On the other hand, in the production method of the present invention, the viscosity of the slurry can be increased by cooling, so the amount of binder and additives for maintaining bubbles and maintaining the shape can be reduced, and the porous sintered body Impurities can be reduced.

In this manufacturing method, it is preferable that the bubble nucleus forming step is performed in a state where the slurry is held at the preliminary cooling temperature.
In this case, since the viscosity of the slurry is increased by cooling, the introduced additive gas can be prevented from escaping from the slurry by buoyancy. Therefore, the amount of added gas in the slurry can be controlled more precisely.

After the defoaming step, it is preferable to vacuum knead the slurry while maintaining the slurry at the preliminary cooling temperature.
In this case, since the slurry before introducing the additive gas is cooled and kneaded in vacuum, degassing is performed at the preliminary cooling temperature, so that the amount of gas in the slurry can be controlled more precisely.
Further, by maintaining the defoaming step and the decompression / expansion step in a cooled state, the additive gas does not escape from the slurry in the middle, and the amount of additive gas in the slurry can be maintained until sintering. Therefore, a porous sintered body having an arbitrary porosity can be obtained.

  In the foamable slurry manufacturing step of the method for manufacturing the porous sintered body, a filling step of filling the molding die while holding the slurry containing the bubble nuclei at the preliminary cooling temperature after the bubble nucleation step. It is preferable to have. In this case, since the slurry is put in the mold in the cooled state and then the decompression expansion step is performed to expand the bubble nuclei, an arbitrary outer shape corresponding to the shape of the mold can be formed and deformation due to its own weight can be suppressed. Therefore, it is possible to obtain a porous sintered body having an arbitrary outer shape or a thick porous sintered body.

  In this production method, it is preferable that a surfactant is added to the slurry in the slurry kneading step. By adding an appropriate amount of the surfactant, pores having a large window size can be formed by connecting the pores.

  In this production method, the preliminary cooling temperature is preferably less than 5 ° C. By cooling to less than 5 ° C., the slurry can reliably hold the added gas, and more precise control of the porosity and formation of a green body with a homogeneous porosity and pore diameter can be realized.

  According to the method for producing a porous sintered body of the present invention, a porous sintered body having an arbitrary porosity can be easily produced by setting the amount of added gas, the pressure in the reduced pressure expansion step, and the precooling temperature. be able to. Further, since the porosity can be controlled, the size of the porous sintered body can be easily controlled. Furthermore, in contrast to these parameters for controlling the porosity, the parameters for controlling the pore size are the particle size of the raw material powder, the composition of the slurry, the formation conditions, etc., and these do not affect the porosity so much. The pore size can be adjusted independently of the control, and the pore size of the porous sintered body can be easily controlled.

It is a schematic diagram which shows the window size of the bubble in the manufacturing process of a porous sintered compact. In the manufacturing method of the porous sintered compact of this invention, it is a schematic diagram of the stirring apparatus which shows a bubble nucleus formation process. In the manufacturing method of the porous sintered compact of this invention, the schematic diagram which shows the filling process (a) of a slurry, a decompression expansion process (b), a freeze-solidification process, a vacuum freeze-drying process (c), and a sintering process (d). It is. In the manufacturing method of the porous sintered compact of this invention, it is a figure which shows the change of the porosity of a porous sintered compact when the pressure of a pressure reduction expansion process is changed.

Hereinafter, the manufacturing method of the porous sintered compact of this invention is demonstrated. This manufacturing method includes a foamable slurry manufacturing process for creating a slurry having bubbles, a green body forming process for forming the slurry into a green body, and a sintering process for sintering the green body.
The foaming slurry production process includes a slurry kneading process for producing a slurry by kneading a material containing raw material powder, a binder and water, a defoaming process for removing bubbles and dissolved gas from the slurry, and bubbles in the slurry. Bubble nucleation step of dispersing and forming nuclei.
The step of forming a green body includes a decompression expansion step for expanding bubble nuclei in the slurry, a freeze solidification step for freezing the slurry in which the bubble nuclei have expanded, and a vacuum freeze-drying step for forming a green body from the frozen slurry. Including.

Here, the volume ratio of the pores in the entire porous sintered body is referred to as the porosity. The porosity can be calculated from the actually measured weight with respect to the calculated weight of the solid body of the same shape.
Moreover, the arithmetic mean of the equivalent circle diameter calculated by measuring the area of the pores in the field of view of the 5-100 times photomicrograph on the surface of the porous sintered body is called the average pore diameter.

  Further, as shown in FIG. 1, a portion w in which bubbles C are connected in the slurry is called a window. In the porous sintered body in which the moisture in the slurry is removed and the raw material powder is sintered, the window w is in an open state. Therefore, the larger the size of this window w (window size), the greater the degree of communication of the continuous pores, that is, a porous sintered body having high air permeability and water permeability.

[Foaming slurry production process]
(Slurry kneading process and defoaming process)
First, a kneading step of preparing a slurry S by kneading a material containing raw material powder, a binder, and water is performed. By performing this kneading in a vacuum atmosphere, the slurry S can be created while defoaming. That is, the slurry kneading step and the defoaming step are performed simultaneously.

  The raw material powder is a powder of a material that can be sintered, such as metals, ceramics, such as Ni, Cu, Ti, Al, Ag, and stainless steel, and atomization methods such as water atomization method and plasma atomization method, oxide reduction Manufactured by chemical process methods such as chemical methods, wet reduction methods, and carbonyl reaction methods. The content rate of the raw material powder in a slurry shall be 30-80 mass%. The average pore size of the porous sintered body and the size of the window in the slurry S increase as the average particle size (particle size) of the raw material powder increases. Therefore, the average pore diameter and the window size of the porous sintered body can be adjusted by adjusting the average particle diameter of the raw material powder.

  The binder binds the particles of the raw material powder, and a water-soluble resin binder such as methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, carboxymethyl cellulose ammonium, ethyl cellulose, polyvinyl alcohol or the like can be used. The content rate of the binder in a slurry shall be 0.5-20 mass%.

  Water is a solvent of the binder which is a water-soluble resin binder, and fixes the shape of the slurry S by freezing. Since the window size increases as the viscosity of the slurry S decreases, the degree of pore communication in the porous sintered body can be controlled by controlling the water content ratio and controlling the viscosity of the slurry S.

In addition, you may add additives, such as surfactant and a plasticizer, to the slurry S further as needed.
By adding the surfactant to the slurry S, the average pore diameter and window size of the porous sintered body can be increased. Surfactants include alkylbenzene sulfonates, α-olefin sulfonates, alkyl sulfate esters, alkyl ether sulfate esters, alkane sulfonates, and other anionic surfactants, polyethylene glycol derivatives, polyhydric alcohol derivatives, alkyls. Nonionic surfactants such as betaine and amphoteric surfactants can be used. When the surfactant is mixed, the content ratio in the slurry is preferably 0.05 to 10% by mass.

  A plasticizer is added when it is necessary to impart plasticity to the green body to prevent collapse. For example, polyhydric alcohols such as ethylene glycol, polyethylene glycol, and glycerin, fats and oils such as coconut oil, rapeseed oil, and olive oil, petroleum ether Ethers such as diethyl phthalate, di-N-butyl phthalate, diethyl hexyl phthalate, dioctyl phthalate, sorbitan monooleate, sorbitan trioleate, sorbitan palmitate, sorbitan stearate, and the like can be used. When mixing a plasticizer, it is preferable that the content rate in a slurry shall be 0.05-3 mass%.

  In addition, in order to improve the characteristics and moldability of the slurry S, an optional additive component may be further added. For example, a preservative can be added to improve the storage stability of the slurry S, or a polymer compound can be added as a binding aid to improve the strength of the molded body. However, in order to produce a porous sintered body with few impurities, it is preferable that there are few materials other than the raw material powder and water.

  These materials are vacuum-kneaded for 1 to 1.5 hours, and slurry S is created while removing contained gases (bubbles and dissolved gas). Furthermore, vacuum kneading is performed for about 1 hour in the state cooled to the precooling temperature. The precooling temperature is set to a temperature exceeding the freezing point of the slurry at a predetermined pressure and less than the boiling point (for example, 5 ° C. or less). By maintaining the preliminary cooling temperature, the viscosity of the slurry S is higher than that at room temperature.

(Bubble nucleation process)
Next, a bubble nucleus forming step of dispersing and forming bubble nuclei in the defoamed slurry S is performed. In this step, as shown in FIG. 2, the additive gas C <b> 1 is introduced into the slurry S while the slurry S is continuously charged into the stirring device 10. The slurry S and the additive gas C1 are each introduced at a constant volume flow rate. The additive gas C1 is preferably a gas that does not denature the slurry S and does not remain as an impurity after sintering, such as air, oxygen, nitrogen, argon, helium, carbon dioxide, and the like. The additive gas C1 is introduced in such an amount that the volume of the degassed slurry S: volume of the additive gas = 10: 0.1-8.

  Also in the stirring device 10, the slurry S is cooled to the preliminary cooling temperature. By stirring the slurry S at a high speed, the additive gas C1 is dispersed in the slurry S, and bubble nuclei C2 are formed. At this time, since the slurry S is in a cooled state and a viscosity is increased, the introduced additive gas C1 is prevented from escaping from the surface of the slurry S. Further, the slurry S is defoamed in advance. Therefore, almost all of the introduced additive gas C1 is dispersed as bubble nuclei C2, and all the bubble nuclei C2 in the slurry S are formed by the additive gas C1. In other words, in this step, the amount of gas contained in the slurry S can be precisely controlled by controlling the amount of the additive gas C1 introduced into the slurry S.

  The average diameter of the bubble nucleus C2 is controlled by controlling the stirring speed and the stirring time in the bubble nucleus forming step. Further, the lower the precooling temperature, the higher the viscosity of the slurry S, and the bubble nuclei C2 are likely to be formed smaller. That is, the average diameter of the bubble nucleus C2 can be controlled by controlling the stirring speed, stirring time, and precooling temperature.

  Further, as shown in FIG. 2, the stirring device is controlled by controlling the cross-sectional area of the outlet 11 of the stirring device 10 from which the slurry S is taken out and the flow rates of the slurry S and the additive gas C1 introduced into the stirring device 10. The internal pressure of 10 is controlled. By appropriately setting the internal pressure, the bubble nuclei C2 can be uniformly dispersed in the slurry S. For example, in this bubble nucleation step, the average diameter and dispersion are obtained by stirring the slurry S for about 0.5 to 5 minutes while maintaining the internal pressure of the stirring device 10 at 0 to 1 atm as a gauge pressure. Is controlled to form the bubble nucleus C2.

(Filling process)
Next, the slurry S containing an arbitrary amount of the additive gas C1 as the bubble nucleus C2 is filled into the molding die 12 made of polystyrene or the like while maintaining the preliminary cooling temperature (FIG. 3A). In this step, the slurry S filled in the mold 12 is impacted by striking or ultrasonic waves, or left for about 1 hour while being maintained at the precooling temperature, and the filled slurry S and Bubbles embraced between the mold 12 and large voids formed in the slurry S during filling can be removed.

[Green body formation process]
(Decompression expansion process)
Next, while maintaining the preliminary cooling temperature, the slurry S is depressurized to a predetermined pressure, and the bubble nuclei C2 in the slurry S are expanded to form bubbles C3. Specifically, the mold core 12 filled with the slurry S is placed in a decompression vessel and decompressed to 0.01 to 0.4 atm to expand the bubble nucleus C2 (FIG. 3B). In addition, since the slurry S is cooled to the preliminary cooling temperature, it does not boil even if the pressure is reduced in the decompression vessel, the bubble nucleus C2 expands to become the bubble C3, and the volume of the entire slurry S increases.

  At this time, if the decompression speed is too high, the bubbles C3 may burst and escape from the slurry S, and the stability may not be achieved, and the filling property of the slurry S into the mold 12 may be deteriorated. On the other hand, if the decompression speed is too slow, the bubbles C3 may be integrated and the bubbles C3 may become too large. Therefore, the pressure in the decompression vessel is adjusted so that the predetermined pressure is obtained over 1 to 15 minutes.

  Further, the lower the precooling temperature, the higher the viscosity of the slurry S, so that the adjacent bubbles C3 are less likely to be integrated with each other, and the window size is not easily increased. That is, the average pore diameter and the degree of communication of the porous sintered body can be controlled by controlling the preliminary cooling temperature in this decompression and expansion step.

(Antistatic process)
The slurry S thus expanded is placed at a predetermined holding time (1 to 60 minutes) while maintaining the pressure and temperature. In this process, the movement of the raw material powder and the gas is settled because the window of the bubble C3 is expanded by the movement of the raw material powder and the bubble C3 itself is expanded and contracted by the movement of the gas in the slurry S. The slurry S can be stabilized.

(Freeze solidification process)
Next, this slurry S is cooled to a freezing temperature not lower than −80 ° C. and not higher than the freezing point, and is maintained for 0.1 to 2 hours. In this step, when the water in the slurry S is solidified, the shape of the slurry S including the bubbles C3 is fixed.

(Vacuum freeze-drying process)
Next, the water | moisture content of the frozen slurry S is sublimated in a vacuum. In this step, the slurry S is sublimated in a vacuum to form a green body G in which the raw material powder is held by the binder while maintaining its shape (FIG. 3C). In the green body G, pores H are formed in a state where the shape of the bubble C3 obtained by growing the bubble nucleus C2 is substantially maintained. In addition, since the slurry S is solidified by the freeze solidification step, this step may be performed in a state of being taken out from the mold 12. The green body G is brittle when the amount of the binder mixed is small and may collapse due to its own weight, but by mixing a plasticizer with the slurry, the strength can be improved and the collapse can be prevented. In addition, since binder and a plasticizer remain | survive as an unnecessary impurity in a porous sintered compact, in order to reduce the impurity of a porous sintered compact, it is preferable to reduce the content rate of a binder or a plasticizer.

[Sintering process]
The green body G is sintered to form a porous sintered body P (FIG. 3D). At this time, when the green body G can be sintered while being put in the mold 12, the green body G is less likely to collapse. Therefore, the binder content is reduced, and the porous sintered body P is formed. It is also possible to further reduce the remaining impurities.

As described above, the porous sintered body P having an arbitrary porosity can be manufactured by reducing the pressure of the slurry S having the arbitrary amount of the additive gas C1 as the cell nucleus C2 in a cooled state.
Here, the porous sintered body obtained when the amount of added gas and the precooling temperature in the reduced pressure expansion step are different will be described with examples and comparative examples.

In the following examples and comparative examples, the slurry A is
As raw material powder, stainless steel (SUS316) powder having an average particle diameter of 8 μm: 73 mass%,
As binder, water-soluble resin binder: 2% by mass,
Surfactant: 0.05% by mass
Water: The one containing the remainder was used. The freezing point of this slurry A at 0.015 atm is around 0 ° C., and the boiling point is around 13 ° C.
Slurry B is
As raw material powder, Ti powder having an average particle diameter of 16 μm: 49% by mass,
As binder, water-soluble resin binder: 2% by mass,
Surfactant: 3% by mass
Water: The one containing the remainder was used.

  In each of these Examples and Comparative Examples, a porous sintered body was manufactured by changing the precooling temperature and the amount of added gas to 0.015 atm in the reduced pressure expansion step. In forming the green body, the precooling temperature is maintained from the slurry forming step to before the freeze solidification step.

  First, as shown in Table 1, using the slurry A, each porous material produced by changing the precooling temperature with the volume ratio of the degassed slurry A and the additive gas in the bubble nucleation step being 10: 1. The sintered bodies (Examples 1 and 2 and Comparative Examples 1 to 5) were compared in terms of porosity and average pore diameter.

In Example 1, as a result of performing the decompression expansion process at a precooling temperature of 5 ° C., the porosity of the porous sintered body was 95%. Further, the formed pores were almost the same as the shape in which the bubble nuclei uniformly formed in the bubble nucleation step were grown, and the dispersion of the average pore diameter was small.
In Example 2 where the precooling temperature was lower than that in Example 1, the porosity was 95%. However, the average pore diameter of Example 2 was smaller than that of Example 1.

In Comparative Example 1, since the slurry A boiled, large pores were scattered, and uniform pores were not formed. For this reason, the average pore diameter could not be measured. Moreover, the porosity became low compared with Example 1.
In Comparative Examples 2 and 3, the slurry A did not boil, but because of its low viscosity, bubbles increased in the slurry A, and large bubbles concentrated at the top. For this reason, the pore diameter distribution is inclined in the thickness direction, making it impossible to evaluate the pore diameter as a whole.

In Comparative Example 4, the porosity and the average pore diameter were small as compared with Examples 1 and 2.
In Comparative Example 5, the bubbles were not frozen at the preliminary cooling stage, so that the bubbles did not expand even under reduced pressure.

  Next, as shown in Table 2, by using the slurry A, the volume ratio of the degassed slurry C and the additive gas in the bubble nucleation step was set to 10: 3, and the precooling temperature in the vacuum expansion step was changed. For each of the produced porous sintered bodies (Examples 21 and 22 and Comparative Examples 21 to 25), the porosity and the average pore diameter were compared.

  When Example 21 was compared with Example 22, the porosity was the same, but Example 22 had a smaller average pore diameter. In addition, when Example 21 and Example 1 having the same precooling temperature were compared, Example 21 with a larger amount of added gas had a higher porosity, but the average pore diameter did not change. Similarly, when Example 22 and Example 2 having the same precooling temperature were compared, the porosity was higher in Example 22 with a larger amount of additive gas, but the average pore diameter was not changed.

In Comparative Example 21, since the slurry A boiled, uniform pores were not formed. For this reason, the average pore diameter could not be measured. Moreover, the porosity became small compared with Example 21. Further, even when compared with Comparative Example 1, the porosity was low.
In Comparative Examples 22 and 23, the slurry A did not boil, but because the viscosity was low, bubbles increased in the slurry A and large bubbles concentrated on the top. For this reason, the pore diameter distribution is inclined in the thickness direction, making it impossible to evaluate the pore diameter as a whole.

In Comparative Example 24, the porosity and average pore diameter were small as compared with Examples 21 and 22.
In Comparative Example 25, the bubbles were not frozen even in the pre-cooling stage, so the bubbles did not expand even under reduced pressure.

  From the Examples and Comparative Examples shown in Table 1 and Table 2, it was confirmed that pores can be appropriately formed by setting the precooling temperature so that the slurry A does not boil and solidify. That is, when the slurry A boils, it is considered that the additive gas escapes from the slurry A and bubbles are formed by boiling. Further, when the slurry A is solidified, it is considered that the introduced gas cannot be reliably held in the slurry A even if the additive gas is introduced while stirring the solidified slurry A in the bubble nucleus forming step. Further, the solidified slurry A is considered to be less likely to increase the porosity because the bubble nuclei are less likely to expand even if the pressure is reduced.

  Further, it was confirmed that the porosity was changed by changing the amount of the added gas in the temperature range where the pores were appropriately formed. That is, it is considered that the precooling temperature does not affect the porosity. Furthermore, in such a temperature range, it has been confirmed that the average pore diameter of the porous sintered body becomes smaller as the preliminary cooling temperature is lower. That is, in the slurry A having a low temperature and a high viscosity, the shape of each bubble is easily maintained, and even if adjacent bubbles are connected to each other, it is difficult to integrate them.

  Next, as shown in Table 3, by using the slurry B, the volume ratio between the defoamed slurry B and the additive gas in the bubble nucleation step was 10: 1, and the precooling temperature in the vacuum expansion step was changed. About each manufactured porous sintered compact (Examples 31 and 32 and Comparative Examples 31-35), the porosity and the average pore diameter were compared.

  In Examples 31 and 32, the porosity and the average pore diameter were slightly increased as compared with Examples 1 and 2 of the slurry A set to the same precooling temperature.

In Comparative Example 31, because the slurry B boiled, uniform pores were not formed. Moreover, the porosity of the comparative example 31 became lower than the comparative example 1 of the slurry A set to the same precooling temperature.
In Comparative Examples 32 and 33, the slurry A did not boil, but since the viscosity was low, bubbles increased in the slurry A, and large bubbles concentrated on the top. For this reason, the pore diameter distribution is inclined in the thickness direction, making it impossible to evaluate the pore diameter as a whole.

In Comparative Example 34, the porosity and the average pore diameter were increased as compared with Comparative Example 4 of Slurry A set at the same precooling temperature.
Further, in Comparative Example 35, the air bubbles were not expanded even in the reduced pressure because they were already frozen in the preliminary cooling stage.

  Next, as shown in Table 4, using slurry B, the volume ratio of the defoamed slurry and the additive gas in the bubble nucleation step is 10: 3, and the precooling temperature in the vacuum expansion step is changed. For each of the porous sintered bodies (Examples 41 and 42 and Comparative Examples 41 to 45), the porosity and the average pore diameter were compared.

  In Example 41, the porosity was higher than that in Example 31, but the average pore diameter was the same. In addition, Example 42 also had a larger porosity than the Example 32, but the average pore diameter was the same. When Example 41 and Example 42 were compared, Example 42 having a lower precooling temperature had a lower average pore diameter.

In Comparative Example 41, since the slurry B boiled, uniform pores were not formed. The porosity of Comparative Example 41 was higher than that of Comparative Example 21 of Slurry A set to the same precooling temperature, and was the same as that of Comparative Example 31 of Slurry B with a small amount of added gas.
In Comparative Examples 42 and 43, the slurry A did not boil, but since the viscosity was low, bubbles increased in the slurry A and large bubbles concentrated on the upper part. For this reason, the pore diameter distribution is inclined in the thickness direction, making it impossible to evaluate the pore diameter as a whole.

In Comparative Example 44, the porosity and average pore diameter were increased as compared with Comparative Example 24 of slurry A set to the same precooling temperature. Moreover, compared with the comparative example 34 of the slurry B with few additive gas amounts, the porosity was high in the comparative example 44, and the average pore diameter was the same.
In Comparative Example 45, the bubbles were not frozen at the preliminary cooling stage, so that the bubbles did not expand even under reduced pressure.

  From the examples and comparative examples shown in Table 3 and Table 4, it was confirmed that pores can be appropriately formed by setting the precooling temperature so that the slurry B does not boil and solidify. Further, it was confirmed that, in the range where the pores are appropriately formed, it is not the preliminary cooling temperature but the amount of added gas that changes the porosity. Furthermore, in such a temperature range, it was confirmed that the average pore diameter of the porous sintered body was smaller as the preliminary cooling temperature was lower. These results were the same even when the porous sintered body was manufactured using either the stainless steel slurry A or the Ti slurry B.

  However, when the precooling temperature and the amount of added gas are the same, the porous sintered body using the titanium slurry B is more porous and the average than the porous sintered body using the stainless steel slurry A. The pore diameter increased. This is probably because Ti is lighter than stainless steel, so that the viscosity of slurry B is lower than that of slurry A, and bubble nuclei are likely to grow.

  From the above Examples and Comparative Examples, it was confirmed that appropriate pores were formed by setting the precooling temperature to an appropriate temperature at which the slurry does not solidify and boil regardless of the material of the raw material powder. In addition, it was confirmed that if the precooling temperature was appropriately set, the porosity of the porous sintered body could be controlled by controlling the amount of additive gas. Furthermore, it was confirmed that the average pore diameter of the porous sintered body can be controlled by changing the precooling temperature within an appropriate range.

  Next, the relationship between the amount of added gas in the bubble nucleus forming step, the pressure in the reduced pressure expansion step, and the porosity of the porous sintered body will be described. FIG. 4 shows the pressure in the decompression expansion process and the pores of the porous sintered body when the volume ratio of the defoaming slurry and the additive gas is changed to 10: 1, 10: 3, 10: 5, 10: 8, respectively. An example of the relationship with the rate is shown. As shown in this figure, the porosity increases as the amount of added gas increases. The porosity increases as the pressure in the decompression / expansion step decreases. Therefore, the porosity of the porous sintered body can be controlled over a wide range by controlling the pressure in the decompression expansion step and the amount of added gas in combination. Even if the porosity is the same, increasing the pressure in the reduced pressure expansion process by increasing the amount of added gas reduces the growth of bubble nuclei, so the average pore size of the porous sintered body is reduced.

As described above, according to the present invention, by reducing the pressure of a slurry containing an arbitrary amount of additive gas while maintaining an appropriate precooling temperature, and freezing and solidifying the slurry while maintaining its shape, the porosity and average gas content are reduced. It is possible to manufacture a porous sintered body having a controlled pore size and an arbitrary degree of communication.
In addition, this invention is not limited to the thing of the structure of the said embodiment, In a detailed structure, it is possible to add a various change in the range which does not deviate from the meaning of this invention.

10 Stirrer 11 Outlet C1 Addition Gas C2 Bubble Core C3 Bubble G Green Body H Pore P Porous Sintered S Slurry

Claims (6)

A method for producing a porous sintered body, in which a green body is formed from a slurry comprising a raw material powder, a binder, and a material containing water, and the green body is sintered to produce a porous sintered body,
A slurry kneading step of kneading the material to produce a slurry, a defoaming step of removing bubbles and dissolved gas from the slurry, and stirring while introducing an additive gas into the defoamed slurry. A foaming slurry manufacturing step including a bubble nucleus forming step of dispersing and forming bubble nuclei comprising the additive gas;
A decompression / expansion step of expanding the bubble nucleus by depressurizing the slurry containing the bubble nucleus to a predetermined pressure, and a vacuum freeze-drying step of vacuum freeze-drying the slurry whose volume has expanded due to the expansion of the bubble nucleus. Including a green body forming step of forming a green body in which the raw material powder is held by the binder;
A sintering step of sintering the green body to form a porous sintered body;
Have
A method for producing a porous sintered body, comprising performing at least the reduced pressure expansion step while maintaining a precooling temperature that exceeds the freezing point of the slurry at the predetermined pressure and is lower than the boiling point.   The method for producing a porous sintered body according to claim 1, wherein the bubble nucleus forming step is performed in a state where the slurry is held at the preliminary cooling temperature.   3. The method for producing a porous sintered body according to claim 1, wherein after the defoaming step, the slurry is kneaded under vacuum while maintaining the preliminary cooling temperature. 4.   2. The foaming slurry manufacturing step, further comprising a filling step of filling the molding die while holding the slurry containing the bubble nuclei at the preliminary cooling temperature after the bubble nucleation step. 4. A method for producing a porous sintered body according to any one of 3 above.   The method for producing a porous sintered body according to any one of claims 1 to 4, wherein a surfactant is added to the slurry in the slurry kneading step.   The method for producing a porous sintered body according to any one of claims 1 to 5, wherein the preliminary cooling temperature is less than 5 ° C.

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