Multi-wall carbon nanotubes decorated with ZnO nanocrystals: Mild solution-process synthesis and highly efficient microwave absorption properties at elevated temperature

Light weight and high efficiency are two key factors for microwave absorption materials. In particular, it is extremely important that absorption materials meet the harsh requirements of thermal environments. In this work, multi-wall carbon nanotubes decorated with ZnO nanocrystals (ZnO@MWCNTs) were synthesized by a mild solution-process synthesis. The high-temperature dielectric and microwave absorption properties of SiO₂-based composites loaded with ZnO@MWCNTs (ZnO@MWCNTs/SiO₂) are investigated in 8.2-12.4 GHz and in the 373-673 K temperature range. The imaginary permittivity ε′′ of the composite with 5 wt% loading presents a weak downward trend, while those of the composites with 10 and 15 wt% loading show an upward trend with increasing temperature, which reveals different temperature dependences of ε′′. The ε′′ for 15 wt% loading is about 10 times that for 5 wt% loading. The maximum loss tangent tan δ values of the composites with 10 and 15 wt% loading exceed 0.8, while that of the composites with 5 wt% loading is less than 0.3. High tan δ is mainly attributed to the conductivity of ZnO@MWCNTs, which is dominated by the hopping of electrons in the ZnO@MWCNT network, which increases with elevated temperature. The addition of ZnO properly adjusts the complex permittivity to endow the ZnO@MWCNT/SiO₂ composites with highly efficient and thermally stable microwave absorption coupled with a broad attenuation bandwidth, which almost covers the full X-band for RL ≤ -10 dB. A series of outstanding properties of ZnO@MWCNTs imply that it is a promising functional material in the world of microwave absorption.