Alumina ceramic is a kind of ceramics with a wide range of uses. Because of its superior properties, it has become more and more widely used in modern society to meet the needs of daily use and special properties.
1.Overview of alumina ceramics
1.1 Properties and classification of alumina ceramics
Alumina ceramic is a ceramic material mainly composed of alumina (Al2O3), which is used for thick film integrated circuits. Alumina ceramics have good conductivity, mechanical strength and high temperature resistance.
Alumina ceramics are currently divided into two types: high-purity and ordinary. High-purity alumina ceramics are ceramic materials with an Al2O3 content of 99.9% or more. Because their sintering temperature is as high as 1650-1990 ° C and transmission wavelength is 1 to 6 μm, they are generally made of molten glass to replace platinum crucibles. It is resistant to alkali metal corrosion and used as sodium lamp; it can be used as integrated circuit substrate and high-frequency insulation material in the electronics industry.
Ordinary alumina ceramics are divided into 99 porcelain, 95 porcelain, 90 porcelain, 85 porcelain and other varieties according to different Al2O3 content. Sometimes those with Al2O3 content of 80% or 75% are also classified as ordinary alumina ceramic series. 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 discs; 95 alumina porcelain is mainly used as corrosion-resistant and wear-resistant parts; 85 porcelain Because it is often mixed with talc, it improves the electrical performance and mechanical strength. It can be sealed with metals such as molybdenum, niobium, and tantalum. Some are used as electrical vacuum devices.
2. Preparation of alumina ceramics
2.1 Preparation of powder
Alumina powder is prepared into powder materials according to different product requirements and different molding processes. The particle size of the powder is less than 1 μm. If the high-purity alumina ceramic products are manufactured in addition to the alumina purity of 99.99%, ultrafine crushing is required to make the particle size distribution uniform. When extrusion molding or injection molding is used, a binder and a plasticizer need to be introduced into the powder, generally a thermoplastic plastic or resin with a weight ratio of 10-30%, and the organic binder should be 150-200 with alumina powder. Mix uniformly at ℃ to facilitate the molding operation.
The powder raw material formed by the hot pressing process does not need to add a binder. If semi-automatic or full-automatic dry pressing is used, the powder has special process requirements, and the powder needs to be processed by spray granulation to make it spherical, so as to improve the fluidity of the powder and facilitate automatic filling of the mold during molding. wall. [1]
For dry pressing, powder granulation is required, and polyvinyl alcohol is introduced as a binder. In recent years, a research institute in Shanghai has developed a water-soluble paraffin used as a binder for Al2O3 spray granulation, which has good flowability under heating. The powder after spray granulation must have good fluidity and loose density, and the flow angle friction temperature is less than 30 ° C. The particle grading ratio is ideal and other conditions in order to obtain a larger density of the green body.
2.2 forming method
Alumina ceramic products can be formed by dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing. In recent years, domestic and foreign countries have developed molding technology methods such as filter press molding, direct solidification injection molding, gel injection molding, centrifugal grouting, and solid free molding. Products with different product shapes, sizes, complex shapes and precision require different molding methods.
2.2.1 Dry pressing
Alumina ceramic dry pressing technology is limited to objects with simple shapes and inner wall thickness exceeding 1mm, and the ratio of length to diameter is not greater than 4: 1. The molding method is uniaxial or bidirectional. There are two types of presses: hydraulic and mechanical. They can be semi-automatic or fully automatic. The maximum pressure of the press is 200Mpa, and the output can reach 15-50 pieces per minute. [1]
Due to the uniform stroke pressure of the hydraulic press, the height of the pressed part is different when there is a difference in the powder filling. However, the amount of pressure applied by a mechanical press varies depending on how much the powder is filled, which can easily lead to differences in dimensional shrinkage after sintering and affect product quality. Therefore, the 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. The powder particles with a size greater than 60 μm and between 60 and 200 meshes can obtain the maximum free-flowing effect and obtain the best pressure forming effect.
2.2.2 Grouting
Grouting is the earliest forming method for alumina ceramics. Due to the use of gypsum molds, low cost, and easy to form large-sized, complex-shaped parts, the key to grouting is the preparation of alumina slurry. Generally, water is used as the flux medium, and then a degumming agent and a binder are added. After being sufficiently ground, the air is exhausted, and then poured into a gypsum mold. Due to the adsorption of moisture by the capillary of the plaster mold, the slurry was solidified in the mold. In the case of hollow grouting, the excess slurry must be poured out when the mold wall absorbs the slurry to the required thickness. In order to reduce the shrinkage of the green body, high concentration slurry should be used as much as possible.
Organic additives need to be added to the alumina ceramic slurry to form an electric double layer on the surface of the slurry particles so that the slurry is stably suspended without precipitation. In addition, it is necessary to add binders such as vinyl alcohol, methyl cellulose, alginate, and dispersants such as polyacrylamine and acacia gum, all of which are intended to make the slurry suitable for slurry molding operations.
2.3 firing technology
The technical method of densifying the granular ceramic body and forming a solid material is called sintering. Sintering is a method of removing voids between particles in the body, excluding a small amount of gas and impurity organic matter, and making the particles grow and combine with each other to form a new substance.
The most widely used heating device for firing is an electric furnace. In addition to normal pressure sintering, that is, pressureless sintering, there are also hot pressing sintering and hot isostatic sintering. Although continuous hot-pressing sintering increases the output, the cost of equipment and molds is too high. In addition, due to axial heating, the length of the product is limited. Hot isostatic firing uses high temperature and high pressure gas as the pressure transmission medium. It has the advantage of being heated uniformly in all directions, and is very suitable for sintering of products with complex shapes. Due to the uniform structure, the material performance is improved by 30-50% compared with cold-press sintering. 10 ～ 15% higher than general hot press sintering. In addition, microwave sintering, arc plasma sintering, and self-propagating sintering are also being developed. [2]
2.4 Finishing and packaging process
Some alumina ceramic materials require finishing after sintering. If used as artificial bone, the surface is required to have a high finish, like a mirror surface, to increase lubricity. Because of the higher hardness of the alumina ceramic material, it is necessary to finish it with a harder abrasive polishing tile material. Such as SiC, B4C or diamond. Usually use coarse to fine grinding step by step, the final surface polishing. Generally, <1 μm Al2O3 fine powder or diamond drilling paste can be used for grinding and polishing. In addition, laser processing and ultrasonic processing can be used for grinding and polishing. Some alumina ceramic parts require packaging with other materials.
2.5 Alumina ceramic strengthening process
In order to strengthen alumina ceramics and significantly increase their mechanical strength, a new type of alumina ceramic strengthening process has been introduced abroad. The process is novel and simple. The technical method adopted is to apply a layer of silicon compound film on the surface of alumina ceramics by electron beam vacuum coating, sputtering vacuum coating, or chemical vapor deposition, and heat at 1200 ° C to 1580 ° C. Treatment to toughen alumina ceramics. [3]
3.Application of alumina ceramics
3.1 Mechanical aspects
The bending strength of Al2O3 ceramic sintered products can reach 250MPa, and the hot-pressed products can reach 500MPa. Al2O3 ceramics can reach Mohs hardness of 9, plus excellent anti-wear properties, so they are widely used in the manufacture of tools, ball valves, grinding wheels, ceramic nails, bearings, etc. Among them, Al2O3 ceramic tools and industrial valves are the most widely used. .
3.1.1 Pure Al2O3 ceramic tools
Pure Al2O3 ceramic tools refer to high-purity Al2O3 ceramics containing only a small amount of oxides, of which the purity of Al2O3 is greater than 99%. In pure Al2O3 ceramics, ZrO2 can be added as a sintering aid to improve its fracture toughness. At present, the grain size of Al2O3 ceramics prepared by ordinary sintering is all on the micrometer level, and fine-grained Al2O3 ceramics can obtain higher strength and fracture toughness and better high-temperature performance, which is an ideal material for preparing pure Al2O3 ceramic tools.
3.1.2 Composite Al2O3 ceramic tool
In composite ceramics, there are several composite directions: Al2O3-carbide ceramic tools, Al2O3-carbide-cermet tools, Al2O3-nitride or boride ceramic tools, and so on. Al2O3-carbide ceramic tools are added with certain carbides in Al2O3 to improve its strength, wear resistance, impact resistance and high temperature performance. [4]
Among the additives, TiC is the most widely used. Compared with pure Al2O3 ceramics, the bending strength of Al2O3-carbide composite ceramics is better than that of pure Al2O3 ceramics at normal temperature or high temperature. This composite tool is suitable for high-speed rough and precision machining of difficult-to-machine materials such as wear-resistant cast iron, hardened steel and high-strength steel. Al2O3-carbide-cermet tool. In addition to carbides, a small amount of bonding metal is added to Al2O3. Due to the addition of metal, the strength of the connection between Al2O3 and carbides is improved, and the performance is improved. Ceramic tools are suitable for machining hardened steels, alloy steels, manganese steels, chilled cast irons, nickel-based and cobalt-based alloys, and non-metallic materials.
It is currently the best tool for precision machining of chilled cast iron rolls, and can be applied to intermittent cutting and cutting with cutting fluid.
3.1.3 Toughened Al2O3 Ceramic Tool
Toughened Al2O3 ceramic tools refer to the addition of toughened or reinforced materials to the Al2O3 matrix. At present, the commonly used toughening methods are: ZrO2 phase transformation and toughening, whisker toughening, and second phase particle dispersion toughening. ZrO2 phase transformation and toughening is an effective method of toughening. When ZrO2 is transformed at about 1150 ° C, a volume change occurs, and many cracks are induced in the matrix, so that it absorbs most of the energy at the main crack tip and increases its strength. Tough purpose. Al2O3 ceramic tools made of micron or submicron ZrO2 phase transformation toughened Al2O3 can effectively improve the fracture toughness of the tool.
3.2 Electronics and electricity
In terms of electronics and power, there are various Al2O3 ceramic substrates, substrates, ceramic films, transparent ceramics and various Al2O3 ceramic electrical insulating ceramics, electronic materials, magnetic materials, etc. Among them, Al2O3 transparent ceramics and substrates are the most widely used.
3.2.1 Al2O3 transparent ceramic
Transparent ceramics can be divided into two categories according to their applications: light transmission, wave transmission applications and special light functional applications.
In light transmission and wave transmission applications, new energy-saving lamps made of transparent Al2O3 ceramics, metal halide lamps, high-strength transparent armor materials, infrared wave transmission materials, etc., are important materials in civilian and defense equipment. MgAl2O4 transparent ceramic belongs to this kind of material. It has both the characteristics of ceramics and the optical properties of sapphire crystal and quartz glass. It can be used for transparent armor and lighting fixtures. [5]
In the application of special light functional characteristics, there are thin-film light-emitting materials, high-power all-solid-state lasers, and transparent scintillation ceramics. Al2O3 materials in thin-film light-emitting materials have proven to be the most promising thin-film light-emitting materials because of its high transparency, good thermal stability, and relatively high light-emitting brightness.
3.2.2 Al2O3 ceramic substrate
Al2O3 ceramic substrates have excellent properties such as high mechanical strength, good insulation, and high light shielding. They are widely used in multilayer wiring ceramic substrates, electronic packaging and high-density packaging substrates. The commonly used forming methods in the preparation of Al2O3 ceramic substrates include dry pressing, casting, etc., and casting forming is currently the most widely used forming method. Casting is divided into non-aqueous and water-based: non-aqueous casting is a simple process, but it will cause environmental pollution and higher cost; water-based casting is more environmentally friendly, and the cost is lower, but the process is more difficult.
At present, most of the industrial applications use non-aqueous casting to form Al2O3 ceramic substrates. Non-aqueous casting can be used to prepare Al2O3 ceramic substrates with smooth, flat, and dense surfaces. However, in the preparation process, the substrates The sintering temperature is high and the energy consumption is large. [6] Therefore, some additives can be added to the Al2O3 ceramic substrate to reduce the firing temperature. For example, a black Al2O3 ceramic substrate can be prepared by adding a Fe-Cr-Mn black colorant, which can effectively reduce the performance under other conditions. The sintering temperature of Al2O3 ceramic substrate reduces energy consumption.
3.3 Chemical industry
In chemical applications, Al2O3 ceramics also have a wide range of uses, such as Al2O3 ceramic chemical filler balls, inorganic microfiltration membranes, corrosion-resistant coatings, etc. Among them, Al2O3 ceramic membranes and coatings have been the most studied and applied.
Al2O3 ceramic membranes have a large number of applications in the purification of industrial water processing, seawater desalination, gas separation, and catalytic reactions. Therefore, ceramic inorganic membranes have increasingly received widespread attention from the scientific and industrial circles. There are many methods for preparing Al2O3 ceramic film, including sol-gel method, solid particle sintering method, chemical vapor deposition method, and anodizing method. [7]
3.4 Medical aspects
In terms of medicine, Al2O3 is more used in the manufacture of artificial bones, artificial joints, artificial teeth, etc. Al2O3 ceramics have excellent biocompatibility, biological inertness, physical and chemical stability, high hardness, and high wear resistance. They are ideal materials for preparing artificial bones and artificial joints. However, it has the same disadvantages as other ceramic materials, such as large brittleness, low fracture toughness, high difficulty in machining technology, and complicated technology, so it needs further research and application. [7]
3.5 Other aspects
Al2O3 ceramic is currently one of the most studied and widely used materials in new materials. In addition to the above applications, it is also widely used in other high-tech fields, such as aerospace, high-temperature industrial furnaces, and composite reinforcement.