Low-temperature Preparation And Sintering Behavior Study Of α-Al₂O₃ Nanoparticles

Alumina ceramics show wide applications as high-temperature materials, composite materials, electronics, refractories, and air source device capacitors due to their unique physicochemical properties such as high strength, high hardness, high temperature-resistance, resistance to wear, high corrosion-resistance, thermal insulation, and low dielectric loss. However, the brittleness of alumina ceramics is the major defect which not only increases the difficulty of manufacturing but also limits their wider application in engineering field. The reported nanocrystalline ceramics with fine grains（such as CaF2 and TiO₂ nanocrystalline ceramics with a grain size of 8 nm） have exhibited considerable ductility at low temperatures, which provides a possible strategy to overcome the brittleness of alumina ceramic. The idea of this work is that firstly prepare α-Al₂O₃ nanoparticles with good sintering activities（disperse, fine in particle, equiaxed, pure） and then obtain dense α-Al₂O₃ nanocrystalline ceramic by an appropriate sintering approach. We expect the α-Al₂O₃ nanocrystalline ceramic has excellent mechanical performance.In this article, we use a large amount of α-Fe₂O₃ as both seed and isolation phase to prepare disperse fine α-Al₂O₃ nanoparticles, and the agglomeration of particles is effectively inhibited. At first, heterogeneous precipitation method was adopted to prepare disperse α-Al₂O₃ nanoparticles with an average particle size of 10 nm. However, the size distribution is wide and the Fe impurity content is high. Then we chose homogeneous precipitation method to improve the experimental. With appropriate experimental parameters, disperse fine pure α-Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution from 2 to 27 nm were synthesized on the basis of inhibiting the agglomeration of particles, and what’s more, the experiment have a high yield. The result accomplished the target of obtaining α-Al₂O₃ nanoparticles with good performance and get ready for preparing fine fully dense α-Al₂O₃ nanocrystalline ceramic.After studying the sintering behavior of α-Al₂O₃ nanoparticles with average particle size of 9 nm and size distribution from 2 to 27 nm prepared by homogeneous precipitation, the result shows that the rapid densification and rapid grain growth of the α-Al₂O₃ sample occur in different temperature ranges, which provides an opportunity to densify α-Al₂O₃ nanocrystalline ceramic without promoting rapid grain growth. We use two-step sintering method to obtain α-Al₂O₃ nanocrystalline ceramic by first sintering the green body to a high temperature for a certain intermediate density and then holding the sample at a low temperature to finish the densification. The α-Al₂O₃ nanocrystalline ceramic with an average grain sizes of 55 nm and a relative density of 99.6% was obtained by two step sintering: 10°C/min to 1175°C without hold and then 10°C/min decreasing to 1025°C with a 20 h hold in air. Through two-step sintering, fully dense nanocrystalline α-Al₂O₃ ceramic was produced and the densification was realized without grain growth in the final stage of sintering.