Study On Microstructure, Properties Of Spark Plasma Sintered AIN/BN Composites And Its Modification

AlN/BN ceramics composite with high thermal conductivity and machinability is a new promising composite in extensive application fields. They are perfect materials that could be used in such fields as microelectronics, microwave transmission devices and those in high temperature corrosion. However, AlN and BN are both covalent compounds with high melting point and low self-diffusion coefficient. In system of Al-B-N, AlN and BN could not form a solid solution because they are in different crystal structure and valence electron structure. Furthermore, lamellar h-BN crystals grow in card room structure which impedes the sintering constriction and densification. These factors make complete densification of AlN/BN composites a technical problemand exert unfavourable effects on material performance.Spark Plasma Sintering (SPS) is an advanced method for materials preparation far from balanced state. Its productive efficiency is very high. The materials prepared by SPS method are in highly-densified and fine structure. And the phase composition and structure of the materials are adjustable and controllable.In this thesis, the densification process and mechanism of spark plasma sintered AlN ceramics and AlN/BN ceramic composites are elaborately studied. The effects of additives on microstructure and thermal conductivity of AlN ceramics are investigated. The thesis is also focused on the microstructure, thermal conductivity and machinability of AlN/BN composites prepared by SPS. It describes the controlling and modification of microstructure and properties of AlN/BN composites through the processing of powders preparation and grain boundary purification, which is useful for the improvement of thermal conductivity of AlN/BN composites containing good machinability.The AlN ceramic with density of 97.5% could be achieved when pure AlN powders are sintered at 1900℃ or at 1850℃ for 15 minutes. Such additives as Y₂O₃, Sm₂O₃ or Li₂O could significantly accelerate the sintering of AlN ceramics. They can depress the requisite sintering temperature and enhance the density of the ceramics while improving the thermal conductivity remarkably. The thermal conductivity could...