Influence of the silicon carbide particle size on dielectric characteristics, thermal conductivity,  and microwave radiation absorption of pressureless-sintered  AlN–(20–50)% SiC composites

Abstract

The application of ceramic dielectrics in various electronic microwave instruments and devices is determined by properties such as dielectric constant, dielectric loss coefficient, thermal conductivity, and absorptance. Alumina nitride composites with dielectric losses (high dielectric loss tangent tgδ varying from 0.1 to 0.6) are promising functional materials, but there are few publications with detailed dielectric characteristics, along with thermal conductivity, which is especially important for devices with high output power. To determine the dielectric characteristics (ε’ and tgδ), the resonance measurement method with a cylindrical resonator was employed. The absorptance of electromagnetic energy L (attenuation in a bulk absorber with respect to absorber length) was used to compare absorbers of different sizes. Microwave attenuation in the absorber ring located in a resonator of the delay system of the traveling wave lamp model was measured employing a P2–61 a panoramic meter for voltage standing wave ratio and attenuation. Experimental values of the real ε’ and imaginary ε” parts of the complex dielectric constant in pressureless-sintered AlN–SiC composites with different contents and sizes of semiconducting silicon carbide particles are presented. When SiC increases from 20 to 50% in the AlN-based composite, ε’ becomes 1.7–2.1 times higher and ε’ by 5.4–6.8 times. The smaller the SiC particle size, the greater the increase in ε’ and ε”, same silicon carbide content being the same. The relationship between the thermal conductivity and electromagnetic energy absorptance in the AlN–SiC composites was studied. A range of compromise values was found: they combine a relatively high thermal conductivity of 45–55 W/(m · K) and significant absorptance L = 2.8–3.5 dB/mm, corresponding to the highest silicon carbide content (40–50%) and the microsized range of SiC particles (2.3–4.4 µm).A relationship was established between the imaginary part of the complex dielectric constant and electromagnetic energy,L (dB/mm)=  allowing the absorptance to be determined from known ε” at a frequency of 3.3 GHz.