Alumina ceramics are inorganic non-metallic materials with alumina as the main body. They have good conductivity, high mechanical strength and high temperature resistance. It is a kind of widely used ceramics. Thanks to its superior performance, it has been more and more widely used in modern industry.
According to the content of alumina, alumina ceramics can be divided into precision type and common type. Precision alumina ceramics with 99.9% alumina content can be sintered at 1650-1990 C with transmission wavelength of 1-6 micron. Fused glass is usually made to replace platinum crucible. It can be used as sodium lamp tube by its light transmittance and alkali metal corrosion resistance. It can be used as IC substrate and high frequency insulating material in electronic industry. Because of its insulation, acid and alkali resistance, high temperature resistance and other characteristics, alumina ceramics began to show prominence in mechanical structural parts. Ordinary alumina ceramics can be divided into 99, 95, 90 and 85 kinds according to the content of alumina.
Alumina powder is made into powder materials according to different product requirements and different forming processes. The particle size of the powder is less than 1 micron. In addition to the purity of 99.99% of alumina, the high purity alumina ceramics need to be superfine crushed and the particle size distribution is uniform. In extrusion or injection moulding, binders and plasticizers should be added to the powder, usually thermoplastic plastics or resins with a weight ratio of 10-30%. Organic binders and alumina powders are evenly mixed at 150-200 temperature to facilitate the moulding process. The powder material formed by hot pressing process does not need to add binder. If semi-automatic or fully automatic dry pressing is used, there will be special technological requirements for the powder. Spray granulation is needed to treat the powder and make it appear round, so as to improve the fluidity of the powder and facilitate the automatic filling of the mould wall.
In addition, in order to reduce the friction between powder and wall, it is necessary to add 1-2% lubricant, such as stearic acid, and binder PVA. In dry pressing, powder granulation is needed, and polyvinyl alcohol is used as binder. In recent years, a research institute in Shanghai has developed a water-soluble paraffin as a binder for Al203 spray granulation and has good fluidity under heating conditions. After spray granulation, the powder must have good fluidity and density, and the friction temperature of flow angle is less than 30 C. In order to obtain a larger green density, the particle size ratio is ideal and other conditions. Molding methods: Alumina ceramic products are usually formed by dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing. In recent years, pressure molding, direct solidification casting, gelcasting, centrifugal casting and solid free molding have been developed. Different product shapes, sizes, complex shapes and precision require different forming methods.
Introduction to Common Forming of Alumina Ceramics:
1. Dry pressing: Alumina ceramics (AL2O3) dry pressing technology is limited to single shape and inner wall thickness of more than 1 mm, the ratio of length to diameter is not more than 4:1. The forming methods are uniaxial or bidirectional. Press has two types: hydraulic and mechanical. It can be semi-automatic or fully automatic. The maximum pressure of the press is 200 Mpa. The output can reach 15-50 pieces per minute. Because the stroke pressure of hydraulic press is uniform, the height of pressing parts is different when powder filling is different. The magnitude of pressure exerted by mechanical press varies with the filling of powder, which easily leads to the difference of size shrinkage after sintering and affects the quality of products. Therefore, the uniform distribution of powder particles during dry pressing is very important for die filling. The accuracy of filling quantity has a great influence on the dimensional accuracy control of alumina ceramic (AL2O3) parts. The maximum free flow effect can be obtained when the powder particles are larger than 60 micron and between 60 and 200 meshes, and the best pressure forming effect can be obtained.
2. Grouting: Grouting is the earliest method of forming alumina ceramics. Because of gypsum mould, low cost and easy to form large size and complex shape parts. The key of grouting is the preparation of alumina slurry. Usually water is used as flux medium, followed by gelling agent and binder, exhaust after full grinding, and then pour into the gypsum mould. The slurry solidified in the mould because of the adsorption of water by the gypsum mould capillary. When hollow grouting, the excess slurry should be poured out when the wall adsorbent slurry reaches the required thickness. In order to reduce the shrinkage of green body, high concentration slurry should be used as far as possible.
Addition of organic additives to alumina ceramic slurry is also needed to form a double layer on the surface of slurry particles so that slurry can be suspended stably without precipitation. In addition, vinyl alcohol, methyl cellulose, alginate amine and other binders as well as polyacrylamide, gum arabic and other dispersants should be added in order to make the slurry suitable for grouting operation.