Preparation And Functional Properties Of Cubic Boron Nitride Based Polycrystalline Materials

Cubic boron nitride (cBN) has fascinating functional properties, such as wide band gap, high thermal conductivity, and relatively small dielectric constant, etc. The potential applications of cBN based polycrystalline materials as ceramic substrate, heat sink and microwave window materials need to be developed urgently. In this thesis, cBN polycrystalline materials were sintered under high temperature and high pressure conditions. The influences of grain, grain boundary phase, and defects on the sintering and functional properties of cBN based polycrystalline materials were studied through microstructure observation and investigations on the materials'electrical, thermal, and optical properties. Meanwhile, the preparation of silver electrode with ohmic contact and some properties of low melting glass frits for surface metallization of cBN based polycrystalline materials were generally researched. The main results of this thesis are as follows:1. cBN based polycrystalline materials could be rapidly sintered from cBN micro-powders under high temperature and high pressure conditions, and they showed near-ultraviolet(UV) light-emitting properties. The photoluminescence intensities of Si-cBN polycrystalline materials increased with the increase of Si additions. Heat treatment showed little effect on the luminescence properties of pure polycrystalline cBN sintered materials, but significantly improved the near-UV photoluminescence intensity of Si-cBN polycrystalline materials. The short-wavelength light emission and spectral vibration structure of cBN polycrystalline materials were directly related to the phonon interaction of cBN crystals. Moreover, n-type shallow doping of cBN polycrystalline materials was realized by Si during the high temperature and high pressure sintering process.2. Mg was an effective additive for improving the relative densities of cBN polycrystalline materials. As-prepared Mg-cBN polycrystalline materials had low electrical resistivity (within the semiconducting range) and near-UV light-emitting property. Heat treatment displayed a greater impact on the surface morphology of Mg-cBN polycrystalline materials than the adjustment of cBN grain morphology within the specimens. The interaction between phonons of annealing-related MgO crystals and phonons of hBN led to phonon replica structure. 3. Al reacted with cBN to form grain boundary phase, AlN and AlB12, which significantly promoted the sintering of Al-cBN polycrystalline materials and showed notable influence on their thermal conductivities and optical absorption properties. When the amount of Al was increased, Al-cBN polycrystalline materials maintained increasing density and thermal conductivity, and their optical absorption characteristics were dominated by those of AlN. Existence of grain boundary phase in general resulted in the lower thermal conductivities and the narrower bandgap for Al-cBN polycrystalline materials.4. Al-cBN polycrystalline materials had high volume resistivities and good resistance stability at high voltage, high stability of dielectric properties at high frequency and the similar dielectric properties as growth-sintered type cBN polycrystals. The investigations on the electrical and dielectric properties of Al-cBN polycrystalline materials revealed the I-V relation exhibited a quadratic nonlinear growth at low voltage and an e exponential growth at high voltage, and indicated an e exponential decline of dielectric constant, the product of dielectric constant and dissipation vs porosity were existed.5. Ohmic contact with cBN polycrystalline materisls were formed using silver paste. With the increase of Si addition in cBN based polycrystalline materisls, the Ohmic contact character of the electrodes gradually weakened.