Research On The Low Temperature Sintering And Anisotropic Grain Growth Of Alumina

This work is concentrated on the study of low temperature sintering and anisotropic grain growth of alumina. Effect of two additive systems, CuO-TiO₂/SiO₂ and Nb₂O₅/SiO₂, on the sintering and microstructure of alumina were investigated. Mechanical properties of the alumina with addition were measured. When CuO-TiO₂ was added to alumina, the sintering of alumina was enhanced by the formation of liquid phase between CuO and TiO₂ at sintering temperature. When the mass ratio of CuO:TiO₂ was 1:2, addition content was 2.0wt%, alumina could be densified at a temperature as low as 1300℃. Liquid content had great effect on the sintering, the more liquid contained, the faster the sintering rate obtained. The grain growth of alumina was greatly promoted by the addition of CuO-TiO₂, and the grain size was equiaxed. The sintering kinetic has been studied through isothermal sintering and the results indicated that the densification was controlled by diffusion of Al3+ and O₂-in the liquid phase. The relative density decreased after added SiO₂ to the system. This indicated that SiO₂ inhabited the sintering of alumina, but it could avoid the phenomena by rising up the temperature. SEM showed that there were some anisotropy grains in the matrix after adding SiO₂. Mechanical properties test indicated that, the fracture toughness was improved after adding SiO₂ to alumina sintered at 1400℃. But this was limited by the number of anisotropy grains. The densification of the alumina was greatly improved by doping small amount of Nb₂O₅. Maximum density was obtained at 1500℃ with a doping level of 1wt% Nb₂O₅. The microstructure was characterized by a great promotion of Nb₂O₅ to the grain growth of alumina. The more the addition contained, the bigger the grains grown. The mechanism associated with this process is considered attributable to the dissolution of Nb2O₅ in alumina grains. When a little more Nb₂O₅ was added, AlNbO4 segregated in the grain boundaries(GB). When the alumina doped with both Nb₂O₅ and fractionary SiO₂, density rised up a little. Synergetic effect of both Nb₂O₅ and SiO₂ promoted the anisotropic grain growth(AGG) for nearly all of the grains in the matrix. AGG might be attributed to the precipitation of GB liquid phase at high temperature. HREM revealed that the basal surfaces of the platelets were coated with thin amorphous layers of about several nanometers. The liquid film was thermodynamical and it changed the surface energy, so that promoted AGG. The resultant mearsurement of mechanical properties showed that, with the addition of SiO2, high bending strength and fracture toughness rised up exaggerately, especially that the fracture toughness rised 42.2%. This was attributed to the AGG in the matrix of alumina. In conclusion, the sintering temperature of alumina was lowered down and good mechanical properties were obtained with addition of CuO-TiO2/SiO2 and Nb2O5/SiO2 respectively. At the same time, a thorough explanation and discussion on the anisotropic grain growth of alumina was also given based on the materials analysis technology. Therefore, this research is valuable to the low cost preparation of alumina ceramics.