Boron nitride(BN) powders were synthesized by pyrolysis at various temperatures to investigate the hydrolysis mechanism of borazine-derived BN pyrolysized below 1200 C.The BN was hydrolysized near room temperature at 65% or 90% relative humidity(RH) over 45 days.The long-term hydrolysis mechanism and structure evolution were investigated by a measurement of mass growth and by X-ray diffraction(XRD),Fourier-transform infrared(FT-IR) spectroscopy,and scanning electron microscopy(SEM) analyses.The samples pyrolysized below 1200 C were very sensitive to moisture,and their mass growth was closely related to the pyrolysis temperature.At 25 C and 65% RH,the sample pyrolysized at 400 C exhibited almost 100 wt% mass growth within 8 days,while the sample pyrolysized at 1200 C exhibited about 20 wt% mass growth.The XRD analysis suggested a hydrolysis mechanism that corresponded to the interlayer spacing in the BN,which was consistent with the results reported.On the other hand,the instability of borazine-derived BN should be directly due to the residual N-H bonds in the compound,as suggested by FT-IR analysis.
The silica fiber reinforced silica and boron nitride-based composites(SiO2f/SiO2-BN) were prepared firstly via the sol-gel method and then the urea route,and the effects of oxidation treatment on the component,structure,mechanical and dielectric properties of the composites were investigated.The results show that the oxidation treatment at 450 °C will not impair the structure of boron nitride,and carbon is the main impurity with the excessive urea.The density of SiO2f/SiO2-BN composites is 1.81 g/cm3,and the flexural strength and elastic modulus are 113.9 MPa and 36.5 GPa,respectively.After oxidation treatment,the density varies to 1.80 g/cm3,and the flexural strength and elastic modulus are decreased to 58.9 MPa and 9.4 GPa,respectively.The mechanical properties of the composites are severely damaged,but they still exhibit a good toughness.The composites show excellent dielectric properties with the dielectric constant and loss tangent being 3.22 and 0.003 9,respectively,which indicates that the oxidation treatment is ineffective to improve the dielectric properties of SiO2f/SiO2-BN composites.
The boron nitride fibers were heated at the range of 600-1400oC in flowing nitrogen and air, respectively, and the effects of heat treatment on the structure, composition and morphology of BN fibers were studied. The results showed that BN fibers exhibited smooth surfaces, and that t-BN was the main phase with a little B2O3 included. After heat treatment at 1400oC in nitrogen atmosphere, the fibers displayed rough surfaces with little change in mass. Better crystallinity was obtained with the increasing temperature. During heat treatment in air, the fibers were oxidized severely as the temperature went up, especially at 1400oC. The volatilization of B2O3, HBO2 and H3BO3 led to the pores on the surfaces of the fibers, while the boron oxide glaze and nitrogen gas produced during the oxidation process protected the fibers from further oxidation.
