JPH0631174B2 - Method for producing reticulated silica whiskers-ceramics porous body composite - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a porous material used for a liquid, a gas filtering material, a bioceramic as a biochemical reaction catalyst, an adsorption sheet for a liquid chromatograph, and the like. Therefore, the reticulated silica whiskers have a reticulated structure and have a particularly high porosity.
The present invention relates to a method for manufacturing a ceramics porous body composite.

(Prior Art) Conventionally, there are roughly the following three methods for manufacturing a ceramic porous body.

(A) A ceramic powder is mixed with an organic substance or the like that is volatilized and burned by heating, shaped, and sintered.

(B) Ceramic powder and a foamable substance are mixed, molded, and then sintered.

(C) Water and a binder are added to the ceramic powder to make a slurry slurry, and the slurry slurry is molded into a foamed form by blowing a gas and then sintered.

However, among the above-mentioned manufacturing methods, (a) and (b), when the ceramics must be sintered in a non-oxidizing atmosphere gas depending on the type of ceramics, mixed organic substances, foamable substances, etc. Since it cannot be decomposed and removed easily, it tends to remain as a foreign substance in the finally obtained ceramic porous body, and (c) has the disadvantage that the pore size is not uniform and not uniform. In addition, when sintering a ceramic porous body, it is necessary to keep it in a semi-sintered state without completely sintering it. However, depending on conditions such as firing temperature, it should disappear and volatilize when mixed with ceramic powder. There are problems that organic substances, foaming substances, etc. may remain after sintering, and that a high porosity cannot be obtained or a porous body having a uniform structure cannot be obtained.

On the other hand, from the structural viewpoint, the pores of the porous ceramic body are classified into voids formed between the semi-sintered particles or voids formed by three-dimensional entanglement of whiskers and sponge-like voids. Since each void of these ceramic porous bodies has small pores and it is difficult to obtain a large porosity, when the ceramic porous body is used as a fluid permeation material such as a filtering material, the apparent unit area is The filtration resistance is large and the filtration flow rate is small, or the pressure loss is large. However, the three-dimensional entanglement itself of fibers or whiskers has a poor bonding force between themselves, and such a porous body has insufficient mechanical strength and pressure resistance. However, there are drawbacks such as that it is difficult to form relatively small pores of about several hundred μm or less.

(Problems to be Solved by the Invention) As described above, when the ceramic porous body produced by the conventional manufacturing method is applied to a filtering material or the like, even if the pore diameter is about several tens of μm or less, the filtration resistance (pressure) is reduced. The loss is small and the pressure resistance is excellent, but the one with a small pore diameter has a low porosity and is therefore inferior in terms of filtration resistance (pressure loss).
When the porosity is increased, the strength and pressure resistance are deteriorated, and therefore, those satisfying these required characteristics at the same time have not been obtained.

The present invention provides a reticulated silica whisker-ceramic porous body composite which has a significantly higher porosity and a higher pressure resistance as compared with a conventional ceramic porous body produced by a conventional manufacturing method, and is excellent in fluid permeability. It is the one we are trying to provide.

(Means for Solving Problems) In the present invention, as a result of earnest research, a silica whisker formed by three-dimensional entanglement of fibers or whiskers in the pores of a ceramic porous body having a large pore diameter
By producing a ceramic porous body composite, the defects of the ceramic porous body produced by the conventional manufacturing method have been eliminated. The means is to impregnate the pores of a ceramic porous body having a pore size of several tens of μm to several hundreds of μm with metallic silicon, and then calcine it in an atmosphere containing a reducing gas as a main component, The water vapor dew point of the atmosphere is 25 ° C. or lower.

(Operation) In the present invention, the mechanism of the formation of silica whiskers in the pores of the porous ceramic body is not clear, but it is presumed that the mechanism is as described below.

First, in the present invention, metallic silicon (Si) is used as a SiO ₂ source which is a component of whiskers, and this is fired in an atmosphere of a reducing gas containing an oxygen source. At this time, the starting material metallic silicon (Si) is oxidized little by little by the trace amount of oxygen contained in the firing atmosphere.
To generate. This SiO has high volatility and gradually evaporates.Furthermore, due to the equilibrium reaction of SiO ← → SiO ₂ , the highly reactive SiO is transferred to SiO ₂ , and the SiO ₂ crystal particles once generated at that time serve as nuclei. The SiO ₂ crystals grow like filaments to form whiskers, which intersect three-dimensionally and finally form a network structure.

The amount of oxygen contained in the atmosphere at this time can usually be adjusted by the dew point temperature of water vapor. When the dew point of the firing atmosphere is too high, that is, when the amount of oxygen contained in the reducing gas is too large, the reaction from Si to SiO ₂ proceeds at a stretch, and it cannot effectively exist as SiO. It is not preferable because the effect of forming whiskers cannot be obtained sufficiently. Therefore, the firing atmosphere of the present invention must be mainly composed of a reducing gas in order to prevent rapid oxidation of metallic silicon (Si), and an ammonia decomposition gas (hydrogen It is preferable to use a mixed gas of nitrogen). In order to contain a trace amount of oxygen in this reducing gas, the oxygen gas may be mixed as it is, but it is somewhat difficult in terms of concentration control and safety. Therefore, humidify the oxygen source in the steam state. Is used industrially, and a method of utilizing oxygen generated by decomposition thereof is preferable because it is relatively easy to control. Further, at this time, hydrogen peroxide may be included in the steam.

On the other hand, by lowering the dew point of the firing atmosphere, it is possible to effectively generate SiO while suppressing the rapid oxidation reaction to SiO ₂ and promote the growth of whiskers, and it is considered that the desired network structure is formed. To be It was experimentally confirmed that this whisker formation reaction proceeds by setting the water vapor dew point to 25 ° C or lower.

There are several methods for producing a ceramics porous body that serves as a matrix, for example, (a) after forming a ceramic powder of a starting material with a kneaded material containing a binder of an organic polymer and isostatically pressing it, A method of firing, (b) a ceramic powder of the starting material into a slurry into a green sheet by a doctor blade method, a method of firing this, (c) a method of pouring the slurry slurry into a gypsum mold and firing, (d) a kneader Rolling-molding the soiled raw material or firing the slurry by granulating the slurry by spray drying and press-molding, (e) Mixing water and a quick-drying binder with the starting ceramic powder to form a slip for molding. , Adhesion to a substrate made of organic porous polymer having a three-dimensional network structure, for example, flexible polyurethane foam, multiple times I was able to select a production method according to the purpose and shape from the method, such as baking a material obtained by drying and solidifying it.

Further, in the slurry slurry pouring method of the above (c), in addition to using a gypsum mold, ceramic material slurry is injected under pressure into the mold cavity of a mold lined with a porous material such as water-absorbent paper. There is also a method in which the molded product is cured by removing water contained in the molded product by water-absorbent paper, removed from the molded product, further dehydrated, cured, dried, and then baked.

In order to impregnate this ceramic porous body with metallic silicon, a slurry in which metallic silicon is dispersed is prepared in a depressurizable container such as a vacuum chamber, the ceramic porous body is immersed in the slurry, and the inside of the chamber is vacuumed. After degassing and filling the pores of the ceramic porous body with the slurry of metallic silicon, the porous ceramic body is taken out and dried. More desirably, metallic silicon can be sufficiently impregnated by repeating this operation a plurality of times.

When firing this impregnated ceramic porous body,
The firing temperature is preferably in the range of 1250 ° C to 1420 ° C. At below 1250 ° C, the growth of reticulated silica whiskers is extremely slow, and at above 1420 ° C, the melting of metallic silicon causes the pores of the ceramic porous body to be blocked, and the diffusion of atmospheric gas into the porous body is obstructed. It is not preferable because the growth of whiskers is hindered.

The water vapor dew point of the firing atmosphere is preferably 25 ° C or lower. If the temperature exceeds 25 ° C, whisker-like silica is unlikely to be formed as described above, and the reaction with the porous ceramic body becomes remarkable, and the silica is fused to the pore wall surface, which is not preferable. On the other hand, if the dew point is too low, the amount of oxygen contributing to the oxidation reaction will be small. Therefore, if the dew point is lower than −25 ° C., the production of SiO will be small and it will take a long time to grow the network whiskers. Therefore, the most preferable dew point range is 5 ° C to -20 ° C.

(Example) Next, the manufacturing method of this invention is concretely demonstrated by an Example.

(Production of ceramics porous body) -Alumina (commercial product, purity 99.9%, average particle size 0.5 μm) 100 g-Polystyrene beads (average particle size 120 μm) 30 g-Water 60 ml Mix the above mixture into a polyethylene container with an internal volume of 300 ml 120 with 300 g of rubber-lined spheres with a diameter of 15 mm
The mixture was mixed at rpm for 18 hours to obtain a slurry slurry.

Next, pour this slurry into a plaster mold and use size 20
A plate-shaped molded body of mm × 40 mm × 50 mm was produced.

Next, this plate-shaped compact was fired in an electric furnace at 1600 ° C. for 1 hour to produce a porous ceramic body made of alumina. The average pore size of this ceramic porous body is 100μ.
m, and the porosity was 60%.

(Preparation of metallic silicon slurry) -Metallic silicon powder (commercial product 200 mesh pass) 100g-Tertiary butyl alcohol 60ml Disperse the above mixture in a polyethylene container with an internal volume of 300ml and make alumina porcelain ball with a diameter of 15mm. It was put together with 500 g and mixed at 120 rpm for 18 hours to prepare a metallic silicon slurry.

(Impregnation of Porous Ceramics Body with Metallic Silicon) The porous ceramic body prepared above is immersed in a metallic silicon slurry, placed in a vacuum chamber, and vacuum degassed.
The pressure was returned to atmospheric pressure, the product was taken out and dried, and the porous silicon body was impregnated with metallic silicon.

(Baking) A porous ceramic body impregnated with metallic silicon was placed in a furnace and fired under the conditions of hydrogen gas moistened with steam (dew point temperature 3 ° C.) as an atmosphere gas and maintained at a temperature of 1360 ° C. for 1 hour.

The microstructure of the composite thus obtained was observed with a scanning electron microscope. As a result, as shown in the photographs in FIG. 1 (magnification: 500 times) and FIG. 2 (magnification: 2000 times), silica whiskers were observed. It was confirmed that they were formed in the pores of the ceramic porous body. Each whisker-like crystal has a diameter of about 1 to 2 μm and a length of about 10 to 200 μm, and these three-dimensionally intersect and branch to combine to form a network structure, with sufficient voids between the whisker crystals. Was secured.

(Effects of the Invention) The reticulated silica whiskers / ceramics porous body composite obtained by the present invention has three-dimensional reticulated whiskers of silica formed in the pores of the ceramics porous body, and the whiskers are entangled with each other. Since sufficient voids are secured inside, the permeation resistance is small and excellent fluid permeability is expected.

Further, since the composite of the present invention uses a ceramics porous body having a relatively large pore size as a matrix, the matrix itself has a small pressure loss when a fluid passes therethrough, so that a reticulated silica whisker is formed in the pores. Even if the pressure loss as a whole can be suppressed to a low level, it is possible to construct a filter having excellent pressure resistance, low pressure loss, and high efficiency when such a composite is used as a filtering material. Become.

Since the composite of the present invention is also sintered with the ceramic porous body in which a part of the mesh-like whiskers is a matrix, the whiskers can be reliably retained, and the conventional porous whiskers are only entangled. In addition to exhibiting a higher pressure resistance than the body, it also exhibits a high transverse rupture strength by being retained by the ceramic porous body having a relatively high strength. Therefore, it is also useful as a filtering material for liquids and gases, bioceramics as a biochemical reaction catalyst, or various materials used for liquid chromatograph adsorption sheets, heat exchangers, and the like.

[Brief description of drawings]

FIG. 1 is a scanning electron micrograph (500 times) showing the network structure of silica whiskers (crystals) formed in the pores of the porous body according to the present invention, and FIG. 2 is the same scanning electron micrograph (2000 times). ).

Claims (1)

[Claims] 1. A molded body in which metallic silicon is impregnated in a ceramics porous body is mainly composed of a reducing gas and has a water vapor dew point of 25 ° C.
A method for producing a reticulated silica whisker-ceramics porous body composite, which comprises firing in the following atmosphere.