Sintering characteristics of activated alumina powder and its application in refractories
1. Sintering characteristics of activated alumina powder
The sintering process refers to the transfer of substances between the particles when the surface tension is not neglected. The purpose is to cause adhesion between the powders, thereby increasing the strength of the sintered body.
P. 1 is a schematic diagram of the powder sintering.
All along, alumina powder has been widely used in the field of high temperature due to its high strength, strong corrosion-resistance, and high hardness. However, because alumina belongs to the ionic bond crystal form, the crystal form is large, and the structure is stable, so a certain amount of thermal energy must be provided to achieve sintering. At present, refining raw material particles is one of the main methods to reduce the sintering temperature. This is because when the powder size is small, the surface energy and surface activity of the powder will be significant. When the surface energy of the powder is greater than the grain boundary energy of the polycrystalline sintered body, the surface energy of the dust and the grain boundary energy will have a gradient and become the driving force for sintering. After the sintering is completed, only the grain boundary energy exists inside the material, and this substitution effect can keep the state of the equipment constant.
Activated alumina powder is made by calcining industrial alumina. It is an ultra-finely ground product. The particle size D50 is generally between 0-2um and has the following properties:
(1) The smaller particles can quickly react with other chemical components inside the material, and the new mineral phase generated by the reaction is conducive to increasing the service life of the content;
(2) The small particle size provides high specific surface area and high activity for the particles;
(3) Excellent filling performance;
(4) The smaller particle size can effectively fill the pores between large particles in different particle gradations, thereby reducing the water demand of the castable and improving the fluidity of the sample;
(5) Strengthen the ceramic combination of the matrix to enhance the corrosion resistance, wear-resistance, and mechanical strength of the sample.
Optimized the structure and performance of the material by adding the nanometer-sized fine powder to the casting material. The results show that the addition of nano-Al2O3 to the corundum casting material system can reduce the sintering temperature by 100-200 ° C. Also, after the heat treatment at the same temperature, the nano-Al2O3 sample has higher bending strength at room temperature.
2. Application of activated alumina powder in refractories
Activated alumina powder as a high-quality ultrafine powder is often used to improve the flowability and mechanical properties of different types of castables. Bai Chen et al. Used corundum castable as the object to study the effect of alumina powder on the physical properties, high-temperature elastic modulus, thermal shock stability, and slag erosion resistance. The results show that the increase in the content of fine dust can significantly improve physical material properties. Alumina powder further enhances the slag erosion resistance of the castable by controlling the pore size and CA6 morphology in the matrix.
The Researchers Studied the reaction mechanism of different activated alumina powders and orthophosphoric acid at lower temperatures. The study found that the mechanical properties of the material are inversely proportional to the particle size of the micro powder. The smaller particle size of the reactant, the easier it is to react with phosphoric acid to form aluminum phosphate. Jagannath et al. Discussed the application of nanotechnology in amorphous refractory materials, and he concluded that nano-sized particles could significantly improve the mechanical strength of amorphous refractory materials at high temperatures. Nanometer-sized fine powder can occupy the pores between the particles, promote the sintering of the content, and reduce the sintering temperature. Nanoparticles can enhance the oxidation resistance, erosion resistance, and thermal shock resistance of the castable by improving the accumulation system between the particles. Zhao Huizhong et al. Prepared the RH furnace dip tube castable by controlling the content of the fine powder. He found that the micropores in the Al2O3-MgO castable will be filled with activated alumina powder. Besides, the reaction between Al2O3 and cement will change The looser microstructure of the sample improves the low, medium, and high-temperature strength and slag resistance of the material.
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