Microwave-absorbing materials in a hightemperature harsh environment are highly desired for electronics and aerospace applications. This study reports a novel high-temperature microwave-absorbing ceramic composites made of polymer-derived SiOC ceramic and in situ partially surface-oxidized ultra-high-temperature ceramic (UHTC) ZrB 2 nanoparticles. The fabricated composites with a normalized weight fraction of ZrB 2 nanoparticles at 40% has a significantly wide microwave absorption bandwidth of 13.5 GHz (26.5−40 GHz) covering the entire K a -band. This is attributed to the extensive nanointerfaces introduced in the composites, attenuation induced by the interference of electromagnetic wave, attenuation from the formed current loops, and the electronic conduction loss provided by the partially surface-oxidized ZrB 2 nanoparticles. The minimum reflection coefficient (RC) was −29.30 dB at 29.47 GHz for a thickness of 1.26 mm for the composites with a normalized weight fraction of ZrB 2 nanoparticles at 32.5%. The direct current (dc) conductivity of the nanocomposites showed a clear percolation phenomenon as the normalized weight fraction of ZrB 2 nanoparticles increases to 30.49%. The results provide new insights in designing microwave-absorbing materials with a wide absorption frequency range and strong absorption loss for hightemperature harsh environment applications.

We report the thermal properties of boron nitride nanotube (BNNT) reinforced ceramic composites using the polymer derived ceramic (PDC) processing route. The nano‐composites had a BNNT loading of up to 35.4 vol.%. TGA results showed that nano‐composites have good thermal stability up to 900°C in air. BNNTs in nano‐composites survived in an oxidizing environment up to 900°C, revealing that nano‐composites can be used for high temperature applications. Thermal conductivity of PDC reinforced with 35.4 vol.% BNNT was measured as 4.123 W/(m·K) at room temperature, which is a 2100 % increase compared to that of pristine PDC. The thermal conductivity value increases with the increase of BNNT content. A thermal conductivity percolation phenomenon appeared when the BNNT content increased to 36 ± 5 vol.%. The results of this study showed that BNNTs could effectively improve the thermal conductivity of PDC materials. BNNT reinforced PDC could be used as thermal structural materials in a harsh environment at temperatures up to 900°C.

This paper studies the effect of pyrolysis temperature on the semiconductor-conductor transition of pristine polymer-derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C-1800°C is presented. At relatively lower pyrolysis temperatures (1200°C-1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C-1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal-like conductive response and their resistivity increases monotonically with the increasing measuring temperature. K E Y W O R D Smetal-like conduction mechanism, pristine polymer derived ceramics, semiconductor-conductor transition, turbostratic carbon

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