Dielectric property characterization at high temperature (RT to 1000 °C) of BN-based electromagnetic transparent materials for hypersonic applications

In this work, the dielectric properties of two dense, hot-pressed, commercial hexagonal boron nitride (h-BN, referred to as BN) samples and fabricated reinforced silicon carbonitride (SiCN) samples were evaluated from room temperature to 1000 °C across the Ka frequency band from 26.5 to 40 GHz in an air atmosphere. The two reinforcements in the fabricated samples are boron nitride nanotube (BNNT) and boron nitride nanobarb (BN-NanoBarb™). The ceramic matrix was prepared via the polymer-derived route. Each of the four types of samples was evaluated via free-space and waveguide methods. When the BNNT or BNNB loading was 10 wt%, the average permittivity of BNNT-SiCN and BNNB-SiCN were 2.52 and 3.42, with the loss tangent of 0.001 and 0.004, respectively. The porosity was detected to decrease from BN (31.14–41.61%) to BNNT - SiCN (28.11%) and BNNB - SiCN (23.28%) after the filler loading. The reduction of Van der Waals attraction by non-covalent functionalization ensures excellent dispersion between BNNT or BNNB and SiCN, as well as mechanical performance. This work aims to enhance the limited amount of dielectric data at high temperatures and across a high-frequency range. The results presented in the work establish the knowledge base for electromagnetic transparent materials in high-temperature and extreme environment applications.