High Purity Alumina ceramic is a high purity ultra-fine alumina powder (crystal phase mainly α-Al2O3) as the main raw material composed of important ceramic materials. High purity alumina ceramic because of high mechanical strength, hardness, high temperature, corrosion resistance and other excellent performance by the people's attention.
The preparation of high purity alumina ceramics is higher than that of the original powder. It is generally the main raw material of alumina powder with purity> 99.99% crystal phase as α phase. The characteristics of high purity ultrafine alumina powder determine the properties of the final preparation of high purity alumina ceramics. In the preparation of high purity alumina powder, the purity of the powder is high, the particle size is small and the distribution is uniform, the activity of the powder is high, and the degree of agglomeration is low. This allows high purity alumina ceramics to be produced at relatively low temperatures. Therefore, in order to prepare high purity alumina ceramics, first to prepare high purity alumina powder.
At present, high purity ultrafine alumina powder mainly improved Bayer process, aluminum hydroxide thermal decomposition method, precipitation method, active high purity aluminum hydrolysis method and other preparation methods.
A. Improved Bayer Law
Bayer method is commonly used in the preparation of alumina powder method. In the process of preparing alumina by this method, the purity of the prepared alumina powder is reduced due to the large amount of impurities such as Si, Fe, K and Ti in the raw material sodium aluminate. On the basis of the traditional preparation process, the aluminate powder with high purity was prepared by dehybridization of sodium aluminate and crystallized alumina. This method was modified Bayer method.
The method used in the main raw materials for the sodium aluminate, a wide range of sources, the whole process will not produce pollution. However, due to its relatively complex preparation process, resulting in low alumina production efficiency, thus limiting its application in more areas.
B. Aluminum hydroxide thermal decomposition method
Aluminum hydroxide thermal decomposition method is to high purity aluminum hydroxide heated to a certain temperature after calcination decomposition of high purity alumina powder preparation method. However, the complete conversion of aluminum hydroxide to α-Al2O3 usually requires a high temperature of about 1200 ° C. Such a high calcination temperature will cause the alumina grains to grow rapidly, forming a hard agglomerated powder that affects the performance of the product. Therefore, the reduced α-Al2O3 phase transition temperature became a hot topic.
Precipitation method refers to the introduction of an appropriate amount of precipitant into the raw materials, so that the various forms of Al3 + ions in the raw material precipitation of aluminum hydroxide precipitation, aluminum hydroxide precipitate by filtration, dehumidification, drying, high temperature calcination process Preparation of high purity ultrafine alumina powder.
D. Active high purity aluminum hydrolysis
Active high-purity aluminum hydrolysis method is the use of highly reactive metal aluminum and water reaction to produce aluminum hydroxide, the product after drying, high temperature calcination after the preparation of high purity alumina. The method is simple to operate and has low production cost, but the purity of the prepared alumina may be reduced.
After the high purity alumina powder was obtained, it was prepared into a good green body, and the high purity alumina ceramic was prepared according to the requirements and cost of the product. At present, the commonly used sintering methods include: atmospheric sintering, hot pressing sintering, two-step sintering, discharge plasma sintering, microwave sintering and so on.
A. Atmospheric pressure sintering (PS)
The use of atmospheric pressure sintering method, high purity alumina ceramics usually need to be higher than 1600 ℃ in the sintering dense, high sintering temperature can lead to abnormal growth of alumina grains, sintered compact density decreased, thus affecting high purity oxidation The performance of aluminum ceramics. Reduce the average size of the powder particles, add the appropriate additives, the use of special molding methods can usually reduce the high purity alumina ceramic sintering temperature.
B. Hot pressing sintering (HPS)
Hot pressing sintering is the application of a certain pressure during the sintering process. The presence of pressure increases the rate of diffusion of atoms and increases the sintering driving force, thus accelerating the sintering process. However, under high pressure conditions, grains are oriented perpendicularly to the direction of pressure in the sintered body. To avoid this phenomenon, hot isostatic pressing (HIP) can be used.
C. Two-step sintering (TSS)
The two-step sintering method heats the green body to a specific temperature T1 to exclude the subcritical pores in the green body and then drops to a lower temperature T2 to make the body dense. In the low-temperature sintering stage of the two-step sintering method, the grain boundary migration is dominated by grain boundary diffusion because the grain boundary migration is higher than the activation energy required for grain boundary diffusion. Thus, in the second stage of the two-step sintering process, the green body is densified, but the grains do not grow too fast.
D. Discharge plasma sintering (SPS)
Discharge plasma sintering (SPS) is the raw material powder into the mold made of graphite and other materials, the use of the upper and lower punch and the electrode with a specific sintering power and pressing pressure applied to the raw material powder, the discharge activation, thermoplastic deformation and Cooling to complete the sintering process. Due to the rapid sintering of SPS and the densification of fine ceramic structure at lower temperature, it can effectively inhibit the abnormal growth of grain, and it is suitable for the preparation of high purity alumina ceramics with high degree of densification and small grain size.
E. Microwave sintering (MWS)
Microwave sintering is a new type of sintering technology which utilizes the fine structure of the material and the special band in the microwave to generate heat. The new sintering technology is realized by the material to reach the sintering temperature as a whole. The heating rate is fast and the grain growth is fast specialty. Microwave sintering has similar sintering mechanism as conventional sintering, but microwave sintering can produce sintered body with good grain size in a short time. Microwave sintering method also has some defects, that is, too fast heating rate may lead to a certain position of the grain growth, so that the sintered body pores increased.
High purity alumina ceramic has excellent mechanical properties. High purity alumina ceramics with flexural strength of about 250MPa can be prepared by atmospheric pressure sintering method. The high purity alumina ceramics prepared by hot press sintering method can have a flexural strength of 500MPa and a hardness of 9GPa (Mohs hardness ). The use of these properties of high purity alumina ceramics can be used as grinding wheels, ceramic nails, etc., of which the most widely used is high purity alumina ceramic cutting tools and high purity alumina ceramic balls. Due to the high fracture toughness and thermal shock resistance of high purity oxidation ceramics, it is usually necessary to introduce a second phase, such as ZrO2, into alumina to improve the toughness and thermal shock resistance of high purity alumina ceramic materials.
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