Study On The Dielectric Properties Of H-BN Ceramic For Wave Transmission At High Temperature

The research on the dielectric properties of materials is a hot subject as well as a foundational one in the field of electrical engineering. With the great development of the aerospace industry, the hexagonal boron nitride (h-BN) ceramic has been used as an important wave-transparent material more and more widely in the spaceflights. The wave-transparent property of materials depends on their dielectric properties. However, it is very difficult to measure the dielectric properties in the extra high temperature, especially the measurement at temperature over 1200°C and in the frequency range from 1 GHz to 20GHz hasn't been carried out successfully till now. So the dielectric properties of h-BN ceramic under extra high temperature and extra high frequency condition have attracted much interest of those scientists in different countries all over the world. On the basis of dielectric theory, this dissertation is aimed to investigate the dielectric properties of h-BN ceramic. Firstly, the polarization mechanisms and the conduction properties are studied. Secondly, considering the effects of effective electric field (EEF) and conductivity in high temperature, the classical model for calculating the complex dielectric constant is modified, thus the calculation model for h-BN ceramic is obtained. Then, this calculation model is validated by experiments that are carried out under the present technical condition. Finally, the dielectric properties of actual h-BN ceramic and their dependence on temperature and frequency are studied for engineering application. The studies in this dissertation should be important for obtaining more dielectric information of the h-BN ceramic, and useful to predict the dielectric properties of high temperature and high frequency, which are important for application of h-BN ceramic as a wave-transparent material.The traditional calculation model of the electron polarizabilities is modified due to the h-BN crystal structure, and an asymmetric diatomic molecule model is presented, in which the interaction among the atoms in a molecule are taken into consideration. This modified model for calculating the electron polarizabilities of h-BN ceramic is much fitter than the model based on the isolated atoms and the symmetrical diatomic model. The total electronic charge density and the differential electronic charge density of h-BN ceramic are calculated by the molecular calculation technique, indicating the existence of ionic bonds in this material. And then the calculation formula of the ionic polarizabilities in the covalent-bond crystals containing a certain ionic bonds is deduced. The Madelung constant, repelling energy index and the ionic polarizability of h-BN ceramic are calculated. The thermionic polarization and rotation polarization are studied too. The research on the polarization characteristics of h-BN has laid the foundations for understanding its dielectric properties changing with temperature and frequency in wide range.After analysis of the classical model for calculating complex dielectric constant, it is found that the model is not suitable for h-BN ceramic due to the neglect of EEF and conduction loss at high temperature. By analyzing the suitability of Lorentz and Onsager EEF for h-BN ceramic, it is found that the Lorentz EEF can not be used in h-BN ceramic, which shows that the interaction between the polar particles can not be ignored and the Lorentz EEF should be modified for h-BN ceramic. Considering the overlap and permeation of electron cloud, the EEF is calculated by introducing a modifying parameter. But this parameter can not be calculated quantitatively. Therefore, based on the microstructure of h-BN ceramic, the local electric field of the adjacent polar particles and EEF has been calculated directly. Then, the model for calculating complex dielectric constant of h-BN ceramic is established by introducing the effects of EEF and conduction loss, which is useful for application of h-BN ceramic at different temperature and frequency.The values of key unknown parameters in this calculation model are obtained by experiments and calculation. The molecular simulation technique is used to study dielectric properties of materials at the first time in this field. By this technique, the formation energies of typical defects in h-BN ceramic are calculated, which are important parameters for calculating the conduction loss at high temperature. Under the present technical condition of experiments for dielectric properties, the model for calculating complex dielectric constant is validated. Moreover, the dielectric spectroscopy of h-BN ceramic at high temperature is analyzed and the relaxation characteristics of h-BN ceramic are obtained.A new calculation model for effective dielectric constant of two-phase materials is put forward due to the actual h-BN ceramic. This model assumes that the spatial inclusions are embedded in another material with random radius under a certain volume ratio, and it is close to the real two-phase composite. The random changing radius is fulfilled by Monte Carlo simulation, and then a mapping transformation with a power exponent is used in order to build the relationship between the radius and the produced random number. Thus, the effective permittivities with different total volume ratio of inclusions are calculated. Due to the simulation of non-uniform sizess of inclusions, this calculation model is much closer to the real two-phase materials than the traditional ones. By using this model, the effects of air void and boron oxide on the dielectric properties of actual h-BN ceramic are studied. The results are very useful for application of actual h-BN ceramic.