Dielectric Responses in Multilayer Cf/Si3N4 as High-Temperature Microwave-Absorbing Materials

Abstract

High-temperature microwave-absorbing materials are in great demand in mili-tary and aerospace vehicles. The high-temperature dielectric behavior of multilayer C f /Si 3 N 4 composites fabricated by gelcasting has been intensively investigated at temperature coverage up to 800°C in the X-band (8.2–12.4 GHz). Experimental results show that the permittivity of Si 3 N 4 matrix exhibits excellent thermo-stability with temperature coefficient lower than 10 − 3 °C − 1 . Taking temperature-dependent polarized bound charge and damping coefficient into consideration, a revised dielectric relaxation model with Lorentz correction for Si 3 N 4 ceramics has been established and validated by experimental results. Besides, a general model with respect to permittivity as a function of temperature and frequency has been established with the help of nonlinear numerical analysis to reveal mechanisms of temperature-dependent dielectric responses in C f /Si 3 N 4 composites. Temperature-dependent permittivity has been demonstrated to be well distributed on circular arcs with centers actually kept around the real ( ε ′ ) axis in the Cole-Cole plane. Furthermore, space charge polarization and relaxation are discussed. These findings point to important guidelines to reveal the mechanism of dielectric behavior for carbon fiber functionalized composites including but not limited to C f /Si 3 N 4 composites at high temperatures, and pave the way for the development of high-temperature radar absorbing materials. dielectric Si 3 permittivity and loss thermo-stability temperature coefficient − 3 dielectric as-prepared 3 of for permittivity of electric polarization and relaxation of migration for graphitic basal planes of short carbon fibers. addition room-temperature model concerning frequency-dependent permittivity available when extended into the full range of temperature coverage up to 800°C, an empirical equation with respect to temperature-dependent permittivity of multilayer C f /Si 3 N 4 composites has been established. It is concluded that the measured complex permittivity of multilayer C f /Si 3 N 4 composites is well distributed on circular arcs with centers actually kept around the real ( ε ′ ) axis in ( ε ′ , ε ″ ) complex plane. Furthermore, the relaxation time as a function of temperature also has been derived. Results suggest that the relaxation time for multilayer C f /Si 3 N 4 composites increases from 216.1 to 250.2 ps when heated from