[China Aluminum Network] Alumina ceramics are currently divided into high-purity and ordinary types. High-purity alumina ceramics ceramic materials with a content of 99.9% or more of Al2O3 have a sintering temperature as high as 1650-1990°C and a transmission wavelength of 1 to 6 μm. They are generally made of molten glass to replace platinum crucibles: their light transmission properties are used. Alkali metal corrosion resistance can be used as a sodium lamp; in the electronics industry, it can be used as an integrated circuit substrate and high-frequency insulating material. Ordinary type alumina ceramics are divided into 99 porcelains, 95 porcelains, 90 porcelains, 85 porcelains and so on according to the content of Al2O3. Sometimes the content of Al2O3 is 80% or 75%, which is also classified as ordinary alumina ceramics. Among them, 99 alumina ceramic materials are used to make high-temperature crucibles, refractory furnace tubes and special wear-resistant materials, such as ceramic bearings, ceramic seals, and water valve plates; 95 alumina ceramics are mainly used as corrosion-resistant and wear-resistant components; 85 porcelain As part of the talc is often incorporated, the electrical properties and mechanical strength are improved, and it can be sealed with molybdenum, tantalum, niobium, and other metals, and some are used as electrical vacuum devices. The production process is as follows:
First, powder preparation:
The incoming alumina powder is prepared into powder materials according to different product requirements and different molding processes. The particle size of the powder is below 1μm micron. If the high-purity alumina ceramic product is manufactured with the exception that the purity of the alumina is 99.99%, ultrafine pulverization is required and the particle size distribution is uniform. When using extrusion molding or injection molding, binders and plasticizers must be introduced into the powders, usually thermoplastic plastics or resins with a weight ratio of 10-30%. The organic binder should be uniformly mixed with the alumina powder at a temperature of 150-200°C to facilitate the molding operation. Powdered raw materials formed by hot pressing process do not need to be added with a binder. If semi-automatic or fully automatic dry pressing is used, there is a special process requirement for the powder. The powder must be treated with a spray granulation method to make it be spherical, so as to facilitate the improvement of the powder flowability and facilitate the automatic filling of the mold during molding. wall. In addition, in order to reduce the friction between the powder and the mold wall, it is necessary to add 1 to 2% of a lubricant such as stearic acid and a binder PVA.
For dry compression molding, the powder must be spray granulated, and polyvinyl alcohol is introduced as a binder. In recent years, a research institute in Shanghai developed a water-soluble paraffin to be used as a binder for Al2O3 spray granulation, which has good fluidity under heating conditions. After the powder spray granulation must have good flowability, bulk density, flow angle of friction temperature is less than 30 ℃. Ideal conditions such as particle size ratios are preferred to achieve greater green density.
Second, the molding method:
Alumina ceramic molding methods are dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing and other methods. In recent years, domestic and foreign companies have also developed pressure molding, direct solidification injection molding, gel injection molding, centrifugal injection molding and solid free forming molding technology methods. Different product shapes, sizes, complex shapes, and precision products require different molding methods. Pick out the commonly used molding introduction:
1. Dry compression molding: Alumina ceramic dry press molding technology is limited to articles whose shape is simple and whose inner wall thickness exceeds 1mm and the length to diameter ratio is not more than 4:1. The molding method is uniaxial or bidirectional. There are two kinds of hydraulic presses and mechanical presses, which can be semi-automatic or fully automatic. The pressure of the press is 200Mpa. The output can reach 15 to 50 pieces per minute. Since the stroke pressure of the hydraulic press is even, the height of the pressed parts is different when there is a difference in powder filling. The pressure applied by the mechanical press varies depending on how much the powder is filled, which can easily lead to differences in dimensional shrinkage after sintering and affect the product quality. Therefore, uniform distribution of powder particles during dry pressing is very important for mold filling. The accuracy of the filling amount has a great influence on the dimensional accuracy control of the manufactured alumina ceramic parts. Powder particles larger than 60μm, between 60-200 meshes can get a larger free flow effect, get better pressure molding effect.
2. Grouting method: Grouting is an earlier method of forming alumina ceramics. Due to the use of a plaster mold, the cost is low and it is easy to form large-size, complex-shaped components. The key to grouting is the preparation of alumina slurry. Water is usually used as a flux medium, and then a debonding agent and a binder are added. After grinding, the mixture is evacuated and poured into a plaster mold. Due to the adsorption of water by the plaster mold capillary, the slurry crucible is solidified in the mold. In the hollow grouting, when the mold wall absorbs the slurry to the required thickness, excess slurry must be poured out. In order to reduce the shrinkage of the green body, high concentration slurry should be used as much as possible.
