| Hexagonal boron nitride (h-BN) is a new inorganic material with various excellent properties. h-BN is in well prospects of application in industry, science and technology, military engineering and so on. This thesis mainly consists of three aspects: synthesis of BN ceramic fibers from precursor, preparation of BN powders and hot-pressed BN-AlN ceramic composites.The BN powders preparation was briefly presented. The BN powders were prepared by using boric acid (H3BO3) and melamine (C3N6H6) as raw materials. The raw materials were mixed by dry ball-milling. The content of nitrogen of BN powders was 49.45%. The particle size was 0.2～5μm. The BN powders were characterized by XRD. It was found that crystallization level of the powder was low.The BN fibers synthesized from precursor were studied in this work. The preparation process included the synthesis of precursor fibers using H3BO3 and C3N6H6 by wet chemical method and the transformation from precursor fibers to BN fibers. BN fibers were usually synthesized using the polyborazine polymer as the precursor. The process include preparing the precursor by the reaction of BCl3 and HNSi(Me3)2(HMDS) , melting spinning to produce precursor fibers, curving of the precursor fibers and heat-treatment at high temperature. In this work, it was the first time that the BN fibers were synthesized using H3BO3 and C3N6H6. The preparation process was simple and easy to be controlled. The chemical qualities of the precursor fibers were stable. The N content of BN fibers was high. The optimum process conditions for preparing precursor fibers were as follows: H3BO3 : C3N6H6=1:1(mol ratio), solution concentration: 0.05g/ml. The chemical bonds in precursor fibers were characterized by IR spectroscopy. The results showed that the precursor fibers may contain bonds of C-N,-NH2,B-N,-OH and so on. The appearance of precursor fibers was observed by SEM. The cross section dimension was 2～10μm, and the length/diameter ratio is 20～100. The precursor were heat-treated at 350～1700℃. The N content of products increased while theheat-treatment temperature elevated. The spectrum of IR showed that the characteristic absorbing band of BN appeared at 900℃. The process for precursor to transform into BN fibers was investigated. The optimum processing conditions were as follows: heat-treatment temperature: 1700℃, soaking time: 2～4h, nitrogen flow:2.0L/min, atmospheric pressure: 0.1MPa. The N content of the BN fibers was 53.41%. The XRD analysis of BN fibers showed that the crystalline level was low, but it was enhanced at higher temperature. The appearance of BN fibers was observed by SEM. The diameter of fibers was 1～10μm and length/diameter was 10～60. Comparing BN fibers with precursor fibers, the volume of BN fibers was contracted. The BN fibers were relatively regular.BN-AlN ceramic composites were prepared by means of hot-pressed technology from powder of h-BN and AlN. The sintering dopant consisted of Y2O3 and CaF2. The density and bending strength of BN-AlN ceramic composites added with Y2O310wt% were better than which added with Y2O3+CaF2. When adding single sintering dopant, the influence about the density with the amount of sintering dopant was different. The influence of dopant on the bending strength was small. When the content of Y2O3 was 8wt%, the density of BN-AlN ceramic was the highest. The density of specimens was up to 2.637g/cm3 and the porosity was 0.265%. Phase analysis by XRD showed that major phases of BN-AlN ceramic were BN and AlN. When Y2O3 was single added, there were minor phases Y4Al2O9, YAlO3, Y3Al5O12, YBO3, Y3BO6. Meanwhile, the minor phases of Y4Al2O9, YAlO3, Y3Al5O12, YBO3, Y3BO6, Ca3Al2O6 were founded in ceramics with combined dopants of Y2O3 and CaF2. SEM fracture morphology analysis of the BN-AlN ceramic was carried out. The BN-AlN ceramic was dense and having flake-house-shaped structure. BN particles were flaky, and AlN particles were multilateral. The BN-AlN ceramic was fractured along grain boundary.
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