Alumina microcrystalline ceramics refer to alumina ceramic materials that use high-purity α-Al2O3 powder as the main raw material, the grain size of the crystal phase produced by ceramic technology is less than 6 μm, and the main crystal phase is corundum. It has a high melting point, High hardness, good mechanical properties, corrosion resistance, insulation and other excellent characteristics.

Wear-resistant alumina microcrystalline ceramic ball

Alumina microcrystalline ceramics are usually divided into two types: high-purity type and ordinary type. High-purity microcrystalline alumina ceramics refer to alumina ceramic materials with an Al2O3 content of more than 99.9%. Its sintering temperature is as high as 1650-1990 ° C, and its transmission wavelength is in the range of 1-6 μm. Its light transmission and alkali metal corrosion resistance are used. Performance, often used as high pressure sodium lamp tube. Ordinary microcrystalline alumina ceramics can be divided into 99, 95, 92, 90, 85 porcelain and other varieties according to Al2O3 content (sometimes Al2O3 content of 80% or 75% is also classified as ordinary alumina ceramic series). Among them, 99 alumina ceramic materials are often used to make high-temperature crucibles, refractory furnace tubes and other special wear-resistant materials (such as ceramic bearings, ceramic seals and water valves), and can be used as integrated circuit substrates and high-temperature components in the electronics industry. High-frequency insulating material, commonly used as catalyst carrier in the chemical industry; 95, 92, 90 alumina ceramics are mainly used as corrosion-resistant, wear-resistant materials and wear-resistant parts; 85 ceramics are often mixed with some talc, which improves the electrical properties And mechanical strength, can be sealed with metals such as niobium and tantalum, and used as electric vacuum device devices, etc.

1. Preparation process of alumina microcrystalline ceramic material

Preparation Technology of Alumina Microcrystalline Ceramic Material

Fig. 2 Flow chart of preparation process of alumina microcrystalline ceramic material

1 Raw material preparation

The alumina raw materials generally used in the ceramic industry mainly include industrial alumina and fused corundum.

(1) Industrial alumina

Industrial alumina is generally based on natural mineral bauxite with high aluminum content (composed of aluminum hydroxide, such as diaspore, boehmite, gibbsite and other alumina hydrates)) Raw materials are processed by chemical methods (mainly alkali method, Bayer soda lime method is mostly used) to remove impurities such as silicon, iron, titanium, etc., and prepare aluminum hydroxide by calcination. Most of its mineral components are γ- Al2O3. Industrial alumina is a white loose crystalline powder. The particles are porous spherical aggregates composed of many γ-Al2O3 crystals with a particle size of less than 0.1 μm. The porosity is about 30%, and the average particle size is 40-70 μm. Among the three main impurity components of industrial alumina, Na2O and Fe2O3 will reduce the electrical properties of alumina ceramics. The content of Na2O should be 0.5%~0.6%, and the content of Fe2O3 should be less than 0.04%. In addition, in the electric vacuum porcelain, the industrial alumina must not contain chlorides, fluorides, etc., because they can corrode the electric vacuum device.

(2) Fused corundum

Fused corundum is melted in an electric arc furnace with industrial alumina or aluminum-rich raw materials, and slowly cooled to crystallize out. The Al2O3 content can reach more than 99%, and the Na2O content can be reduced to 0.1% to 0.3%. The mineral composition of fused corundum is mainly α-Al2O3, and pure fused corundum is white, called white corundum; adding chromium oxide during melting can make red chrome corundum; adding zirconia can make zirconium corundum; Fused corundum containing TiO2 is called titanium corundum. Due to its high melting point and high hardness, this series of fused corundum is a high-quality raw material for manufacturing high-grade refractory materials and high-hard abrasives.

2 Preparation of alumina powder

According to the requirements of product performance and the characteristics of the molding process, the qualified high-temperature alumina powder is processed to the required fineness by grinding equipment (ball mill, stirring mill, etc.) according to the formula and ingredients, and the qualified powder slurry for billet is prepared. . The use of semi-automatic or fully automatic dry pressing molding requires certain process parameters for the powder. It is necessary to use the spray granulation method to process the powder (the powder particles are spherical) in order to improve the fluidity of the powder and facilitate the automatic molding process. Fill the mold wall. In addition, in order to reduce the friction between the powder and the mold wall, it is necessary to add 1% to 2% lubricant (such as aluminum stearate, etc.) and binder PVA. The powder after spray granulation must have good fluidity, loose density, ideal particle gradation ratio and other conditions, in order to obtain a larger green body density. When extrusion molding or injection molding is used, binders and plasticizers need to be introduced into the powder; organic binders should be evenly mixed with alumina powder to facilitate molding operations. Binder may not be added when forming by hot die casting process.

3 forming

The molding methods of alumina microcrystalline ceramic products are: dry pressing, grouting, extrusion, isostatic pressing (dry method, wet method), casting, casting, hot die casting, centrifugal grouting, etc. Different products require reasonable molding methods due to their different shapes, sizes, complex shapes and precision.

4 fired

At present, except for some high value-added alumina microcrystalline ceramic products or special parts required for national defense and military industry, such as ceramic bearings, mirrors, nuclear fuel and gun barrels, etc., which are fired by hot isostatic pressing, most of them are fired by ordinary methods. pressure sintering technology. Some alumina microcrystalline ceramic materials need to be cold worked after sintering. Due to the high hardness of alumina microcrystalline ceramic materials, it needs to be finished with harder grinding and polishing brick materials, such as SiC, C or diamond. Usually, it is grinded step by step from coarse to fine abrasives, and the final surface is polished. Generally, Al2O3 micropowder or diamond paste less than 1 μm can be used for grinding and polishing. In addition, laser processing and ultrasonic processing grinding and polishing methods can also be used, and some alumina microcrystalline ceramic parts need to be packaged with other materials.

Second, the application of alumina microcrystalline ceramics

1 Non-metallic ore deep processing industry

At present, billions of tons of non-metallic minerals are crushed and ground every year in the world, which requires a large amount of microcrystalline alumina ceramic grinding media and other grinding media. Due to the excellent wear resistance of microcrystalline alumina ceramic grinding media and the requirements of high-quality ceramic products for grinding media, it will become an inevitable trend for microcrystalline alumina ceramic grinding media to gradually replace other grinding media in the future.

2 petrochemical industry

In the production of modern fuel industry and chemical industry, more than 80% use catalytic process. Catalyst is an important medium that affects chemical reactions, and is the key to the development and production of many chemical products. As an adsorbent, catalyst, and catalyst carrier, alumina has the characteristics of large specific surface area, high activity, strong adsorption capacity, good pressure resistance, and corrosion resistance. It is widely used in petroleum refining, petrochemical, fertilizer, natural gas, and environmental protection industries.

3 Electronics industry

Alumina electronic ceramic substrate products are widely used in the microelectronics industry as the basic components of thick film integrated circuits, focusing potentiometers, refrigerators, copper clad laminates, network resistors, ozone generators, etc. With the rapid development of my country's electronics industry, especially the microelectronics industry, the demand for alumina ceramic substrates continues to increase.

4 other industries

The wear resistance of alumina microcrystalline ceramics is 260 times that of manganese steel and 171 times that of high chromium cast iron. Reduce the material wear of equipment, prolong the service life of equipment, reduce the comprehensive cost of production and operation, and will not cause any harm to the environment. Under harsh working environments such as high speed, high temperature, heavy wear, and strong corrosion, the corrosion damage of equipment is serious. Because microcrystalline alumina ceramic products have good corrosion resistance and wear resistance, they are ideal materials to replace metals and plastics in strong corrosive environments, and their applications have shown a growing momentum, becoming the most promising wear-resistant materials. , One of the corrosion-resistant materials.

Alumina microcrystalline ceramics also have many applications in the military field, such as bulletproof armor for aircraft, vehicles and personnel. At present, high-purity alumina microcrystalline ceramics have been used as the selected ceramic material for high-temperature structures at home and abroad, and key research and development have been carried out in military and civilian fields.