Porous ceramic development began in the 1870s, is a kind of internal contains a large number of different sizes and morphological holes in the inorganic material. It is made from a variety of pellets and binder composed of blank, through molding, firing and many other complex process. Conductive Ceramics not only have the advantages of high temperature, corrosion resistance and high chemical stability of traditional ceramics because of its large number of pores, large surface area, low density and adjustable pore size distribution has been widely used in aerospace, energy, Metallurgy, chemical industry, environmental protection, military, electronics, biology and medicine and other scientific fields. Because of its wide range of applications caused by the scientific community great concern. Conductive Ceramics can be divided into microporous materials (<2nm), mesoporous materials (2 ~ 50nm) and macroporous materials (> 50nm) according to the pore size. The porosity is up to 90%.
2 porous ceramic preparation process
2.1 organic foam impregnation process
Organic foam impregnation process is impregnated with organic foam ceramic slurry, dry and burn organic foam, so that ceramic holes. The organic foam has a special structure of an open three - dimensional mesh skeleton. So the pores obtained after burning the organic foam are mesh type. The method is suitable for the preparation of high porosity, open porosity Conductive Ceramics. The pore size of the organic foam and the thickness of the slurry thereon play a decisive role in the pore size of the final product.
2.2 foaming process
The foaming process was invented by Sundermann in 1973. The principle is to add organic or inorganic matter in the ceramic component, in the firing stage of the material through the chemical reaction to produce gas, thus forming holes, made of Conductive Ceramics. The method is particularly suitable for the preparation of closed porcelain materials. The foam method can better control the size and shape of the pores, so that the product has better controllability.
2.3 Add the pore-forming process
In the ceramic ingredients by adding pore-forming agent, pore-forming agent in the body to occupy a certain space. During the firing stage, the pore-forming agent is subjected to high temperature action to form a gas volatilization and leave pores in the article. Compared with the traditional ceramic technology, this process is only in the ceramic ingredients to join the pore-forming agent, the manufacturing process is not much change, suitable for low-cost enterprise selection. The types and addition of pore-forming agents play a decisive role in the pore size distribution and porosity of Conductive Ceramics.
2.4 sol-gel process
The sol-gel process mainly utilizes the accumulation of colloidal particles during gelation and the small pores in the process of gel treatment and heat treatment to form a controlled porous structure. Most of this method produces nanoscale stomata, which is used to produce microConductive Ceramics. However, this method uses a large number of organic matter, high cost, low yield is not conducive to the industrialization of large-scale production.
2.5 particle accumulation process
With the aggregate particles can be formed by a certain accumulation of particles in the gap. In the sintering process, the adhesive produces a liquid phase at a high temperature, and the portions where the ceramic particles are in contact with each other are sintered together, and the voids between the particles form micropores that penetrate each other. By controlling the particle size and particle size distribution of the aggregate, microConductive Ceramics having a pore size of 0.1 to 600 m can be obtained. The shape, particle size, particle size distribution of the aggregate particles, the content of various additives and the firing system have a direct effect on the pore size distribution and pore size of the microporous bodies.
2.6 new preparation process
Conductive Ceramics to meet people's higher requirements, the preparation process must continue to innovate and develop. Therefore, a new type of preparation process such as freeze-drying process, gradient construction technology and biological morphology Conductive Ceramics appeared. Among them, the biological principle of porous ceramic preparation process is the use of biological materials as a template to prepare a unique pore structure of the grained material, and then use the material made of porous materials. Biological materials with diversity, multi-dimensional multi-level pore structure. The porous materials prepared by this process also have structural diversity and great potential for development.
Properties of Conductive Ceramics
An important feature of Conductive Ceramics is that it has more uniform and controllable pores. The stomata have the opening of the stomata and the closed stomata, which is the percentage of the volume of the open stomata (the stomata associated with the atmosphere) and the total volume of the sample. Open pores with the role of filtration, absorption, adsorption, elimination of echo and so on, and closed pores are conducive to blocking heat, sound and liquid and solid particles.
3.2 pore size and pore size distribution
The pore size and pore size distribution is an extremely important property of Conductive Ceramics, and the size and distribution of pore size directly affect other series of properties. Therefore, in the choice of Conductive Ceramics, the size and distribution of pore size is an important factor to be considered. Determination of porous ceramic pore size and pore size distribution methods are microscopic, steam permeability and gas bubble method. With the decrease of the pore size, the convective heat transfer in the pores will be reduced, and the better insulation effect can be achieved.
3.3 Mechanical properties
Most of the porous materials used to meet certain mechanical properties, these mechanical properties include the compressive strength, bending strength. Porous ceramic materials are generally made of metal oxides, silicon dioxide, silicon carbide and other high-temperature calcination, the material itself has a high strength, the process of calcining the raw material particles in the boundary part of the melting and bonding, forming a high strength Of the ceramic. But because of the existence of pores will significantly reduce the mechanical properties of ceramics. Stomatal distribution, stomatal size, porosity will have an impact on mechanical properties, with the increase in porosity, porous ceramic mechanical properties will decline sharply.
3.4 Thermal insulation performance
The effective thermal conductivity of porous thermal insulation is: Ke = (1-P) Ks + PKg + 4dσT3. According to the formula can be seen, with the increase in porosity, the size of the pores to reduce the thermal conductivity of the material worse, the better the thermal insulation performance. But with the increase in porosity, ceramic mechanical properties will decline sharply. Therefore, in the choice of Conductive Ceramics, we must consider the balance between the two to achieve the best results.
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