High purity alumina ceramics is an important ceramic material composed of high purity ultrafine alumina powders (crystalline facies mainly Α-al2o3) as main raw materials. High-purity alumina ceramics are widely concerned because of their high mechanical strength, large hardness, high temperature resistance, corrosion resistance and other excellent properties.
I.Preparation of high-purity alumina ceramics
The preparation of high purity alumina ceramics is of high requirement to the original powders, and generally the alumina powders with the purity 99.99% crystalline phase as α phase are the main raw materials. The characteristics of high purity ultrafine alumina powders Determine the properties of the final preparation of high-purity alumina ceramics. In the preparation of high purity alumina powders, it is required that the purity of powders is high, the particle size is small and evenly distributed, the powder activity is high and the degree of agglomeration is low. This allows high purity alumina ceramics to be made at relatively low temperatures. Therefore, in order to prepare high purity alumina ceramics, high purity alumina powders should be prepared first.
1.Preparation of high purity alumina powders
At present, the high purity ultrafine alumina powders mainly include modified Bayer method, Aluminum hydroxide pyrolysis method, precipitation method and active high purity aluminum hydrolysis method.
A.Improved Bayer method
Bayer method is commonly used in industry to prepare alumina powders. In the process of preparing alumina by this method, the purity of alumina powders was reduced due to the large amount of impurities such as Si, Fe, K and ti in sodium aluminate. On the basis of traditional preparation process, the alumina powder with high purity was prepared by removing impurities from sodium aluminate and crystalline alumina, which is the modified Bayer method.
The main raw materials used in this method are sodium aluminate, which has a wide range of sources and will not cause pollution in the whole process. However, due to its relatively complex preparation process, alumina production efficiency is low, which limits its application in more fields.
b.Thermal decomposition method of aluminum hydroxide
The thermal decomposition method of aluminum hydroxide is the method of calcination decomposition and preparation of high-purity alumina powders after heating the high purity aluminum hydroxide to a certain temperature. However, the total conversion of aluminum hydroxide to Α-al2o3 is usually required at around 1200 ℃ high temperature, so high calcination temperature will induce the rapid growth of alumina grains, forming a hard agglomeration powder, affecting the performance of the product. Therefore, the reduced Α-AL2O3 phase transition temperature has become a popular research topic.
Precipitation method refers to the process of preparing high-purity ultrafine alumina powders through the process of filtration, desorption, drying and calcination of aluminum hydroxide precipitates through the introduction of appropriate amount of precipitation agent into raw materials and the precipitation of various forms of al3+ ions in the raw material solution.
D.Solution of active high purity aluminum water
The reactive high purity aluminum solution is a method of producing aluminum hydroxide by reaction of high activity aluminium and water, and producing high purity alumina after drying and high-temperature calcination. The method is simple in operation and low in production cost, but the purity of alumina prepared may be reduced.
2.Preparation method of high purity alumina ceramics
After obtaining the high purity alumina powder, it was prepared into a good forming plain, and the suitable sintering method was selected to prepare high purity alumina ceramics according to the product requirement and cost. At present, the common sintering methods include: Atmospheric sintering, hot pressing sintering, two-step sintering, plasma sintering, microwave sintering and so on.
A.Sintering at atmospheric pressure (PS)
Using the normal pressure sintering method, high purity alumina ceramics usually need to be higher than 1600 ℃ to sinter dense, higher sintering temperature can lead to the abnormal growth of alumina grains, reduce the degree of structure densification, and thus affect the performance of high purity alumina ceramics. The sintering temperature of high purity alumina ceramics can be reduced by reducing the average size of powder particles, adding appropriate additives and adopting special moulding methods.
b.Hot-Pressing sintering (HPS)
Hot-pressing sintering, which exerts a certain pressure on the sintering process, increases the atom diffusion rate and the sintering driving force, thus speeding up the sintering process. However, in order to avoid this phenomenon, it is possible to choose the method of hot isostatic pressing sintering (HIP) in the sintering body under high pressure.
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