Study On Microstructure And Property Of AlN Ceramics And BN/AlN Composites Prepared By Spark Plasma Sintering

AlN ceramics and BN/AlN composites were prepared by Spark Plasma Sintering (SPS) method. The effects of sintering additives and sintering technology on microstructure and thermal and dielectric properties of AlN ceramics and BN/AlN composites were systematically investigated. The densification mechanism during SPS process and the mechanism of multiple component sintering additives were revealed. Mcirostructures were controlled and thermal and dielectric properties of AlN ceramics and BN/AlN composites were improved by heat treatment, and the feasibility of heat treatment technology was investigated.The experimental results shows that only 2³wt.% sintering additive is enough to realize the liquid phase sintering mechanism in the short SPS term. When introducing single sintering additive, the change of the c-axis size is different from that of thermal conductivity of AlN ceramics, and the purifying effect of the sintering additive isn't obvious. The distribution and content of the secondary phases play important roles on the thermal and dielectric properties of AlN ceramics. The AlN ceramics with uniform microstructures with Sm₂O₃ as sintering additive have higher thermal concuctivity than that of other samples. There is no obvious difference about the thermal conductivity and dielectric property between AlN ceramics with different sintering additives.The multiple component sintering additive not only removes oxygen impurity by reacting with Al₂O₃ but also reduces the content of grain boundaries by forming the evaporable secondary phases, thermal conductivity and dielectric property of AlN ceramics are greatly improved. The change of the c-axis size is basically same as that of thermal conductivity of AlN ceramics with multiple component sintering additives, and the purifying effect of the sintering additive is obvious. The sintering additive composition of 2:1(Y₂O₃(Sm₂O₃):CaF₂) is beneficial to improve thermal and dielectric properties of AlN ceramics, which has uniform microstructure, suitable distribution of secondary phase and perfect AlN grain. That is to say, the sintering additive composition of 2:1 is favourable to realize the double effect of multiple component sintering additive. The HREM results show that a little amphous phase existes between AlN grains formes during the liquid cooling process, which has impact on the property on the AlN ceramics.Dense BN/AlN composites can be successfully fabricated by SPS method with suitable sintering additives. The multiple component sintering additive can also play double effects during preparing BN/AlN composites, and plays important role on promoting densification process and improving property. H-BN don't have the tendency of directional arrangement, and the flaky BN grain locates at the around of AlN grains, which inhabites the growth of AlN grain and has impact on the property of materials. The AlN grain size increases and thermal conductivity firstly increases and then reduces with increasing multiple component sintering additive content. The change tendency of dielectric constant is same as that of relative density, but the dielectric loss firstly deduces and then increases. The BN cross stacking tendency is obvious, the uniformity of microstructures decreases, the relative density and dielectric constant reduces and dielectric loss increases with increasing BN content.The heat treatment is introduced to lower the content and to modify the distribution of grain boundary, which is beneficial to purify the lattice and accelerate the AlN growth. The AlN ceramics with multiple component sintering additives, especially the speciman introducing more CaF₂, the effect of purify AlN lattice is obvious and thermal conductivity is greatly improved. After the heat treatment, both the dielectric constant and dielectric loss deduces. The HREM result showes that the amphous phase cann't be removed through the heat treatment.Both the size of AlN and BN in the BN/AlN composites increases after heat treatment. In addition, the content of secondary phase deduces and thermal conductivity improves. Because of the evaporating secondary phase, both the relative density and dielectric constant deduces. The dielectric loss is improved because of the decreasing defect and impurity element.