Manufacture of alumina ceramic products
1 Preparation of raw materials
Alumina is very rich in the earth's crust, with an average content of 15.34% in rocks, which is second only to SiO2 in nature. There are two main types of alumina used in the ceramic industry, namely industrial alumina and fused corundum.
Industrial alumina generally uses natural minerals bauxite, bauxite and kaolin with high aluminum content as raw materials, and is treated by chemical methods such as alkali method and Bayer method to remove impurities such as silicon, iron and titanium, and prepare hydrogen oxide Aluminum is obtained by calcining. Industrial alumina is a white loose crystalline powder with an average particle size between 40 and 70 μm and a porosity of about 30%.
Fused corundum is a crystal obtained by melting industrial alumina or aluminum-rich raw materials in an electric arc furnace, slowly cooling, and crystallizing. The content of Al2O3 can reach more than 99%, and the content of Na2O can be reduced to 0.1% to 0.3%. . The mineral composition of fused corundum is mainly α-Al2 O3, which is a trigonal columnar crystal. In the crystal structure, the oxygen ions form the hexagonal closest packing, and the aluminum ions are in the center of the octahedron surrounded by six oxygen ions. The most stable crystal form in the state. Because α-Al2O3 has excellent characteristics such as high melting point, high hardness, chemical corrosion resistance, and good dielectric properties, the alumina ceramic material prepared with α-Al2O3 as a raw material has excellent mechanical properties, high temperature properties, dielectric properties and resistance to corrosion. The chemical corrosion performance is very good.
Different alumina ceramic products and different molding processes have different requirements for alumina powder. If high-purity alumina ceramic products are manufactured, the purity of alumina powder must be above 99.99%, and it needs to be ultra-fine Crush to distribute the alumina particles evenly. If the injection molding process and the extrusion molding process are adopted, the alumina powder also needs to be mixed with a binder and a plasticizer to facilitate the molding operation. If semi-automatic or fully automatic dry pressing is used, the powder needs to be treated with spray granulation method to make the powder into a spherical shape, so as to improve the fluidity of the powder and facilitate automatic filling and molding.
The molding of alumina ceramics is the process of making alumina raw materials into ceramic bodies. There are many molding methods, such as grouting, injection, dry pressing, extrusion, cold isostatic pressing, hot pressing and hot pressing. Various methods such as isostatic pressing. Products of different shapes, sizes and precision require different forming methods.
Among them, grouting molding is the earliest method adopted. Because this method mainly uses plaster molds as molds for slurry casting molding, the cost of using this method to manufacture alumina ceramics is low, and it can make complex shapes and large sizes. product. The key to the grouting molding technology is to prepare the slurry. Generally, water is used to dissolve the alumina powder, and an appropriate amount of debonding agent and binder are added. The mixed slurry is fully ground and exhausted, and finally injected into the plaster mold. . Since the capillary of the plaster mold will absorb the moisture in the slurry, the slurry will slowly solidify and form in the mold. In order to reduce the shrinkage of the green body, high-concentration slurry should be prepared and used.
Dry pressing technology can realize the standardized and mass production of alumina ceramics, which provides a production basis for the market promotion of alumina ceramics. However, dry pressing technology can only produce standard parts with simple shapes, and has restrictions on the specifications of the product, that is, the thickness of the inner wall should exceed 1 mm, and the ratio of length to diameter should be less than 4:1. In the process of dry pressing, the uniform distribution of powder particles and the accuracy of the filling amount during mold filling have a great influence on the quality, size, precision and other properties of alumina ceramic products. Through practice, the best molding effect can be obtained when the particle size of alumina powder is between 60 and 200 μm.
Sintering is a technique of densifying ceramic blanks at high temperatures to finally form solid materials. The most widely used device on the market today is the electric stove. There are three common sintering methods: atmospheric pressure sintering, hot pressing sintering and hot isostatic pressing sintering. Among them: hot pressing sintering can be produced continuously, which improves the efficiency and output of alumina ceramic production, but the purchase cost and mold cost of hot pressing equipment are higher than other methods, and the economic benefits are not good. Secondly, hot pressing sintering needs to limit the size of ceramic products. Hot isostatic pressing sintering is a method of compressing and densifying ceramic green bodies with high-temperature and high-pressure gas. This method has the advantages of uniform force and heat, and is very suitable for the molding of products with complex shapes. The performance of ceramic materials fired by this method is more than 30% higher than that of cold-pressed sintered products, and more than 10% higher than that of general hot-pressed sintered products. Therefore, some high value-added products and special parts All adopt the hot isostatic pressing firing method. After the products such as ceramic bearings, reflectors, nuclear fuel and gun barrels are processed, they are generally packaged with containers such as plastics.