Polyvinylpyrrolidone induced uniform coating of nickel nanoparticles on carbon nanotubes for efficient microwave absorption.

The impedance matching performance of carbon nanotubes (CNTs) can be effectively enhanced by developing a uniform magnetic impedance matching layer, which can take on critical significance in achieving the desirable microwave absorption (MA) performance. To obtain a uniform coating of Nickel (Ni) nanoparticles on CNTs, several methods have been developed (e.g., the γ-irradiation technique, electroless deposition, as well as microwave welding method). However, the intricate and complicated conditions of the above-mentioned methods limit their wide application. Therefore, controlling the distribution of Ni nanoparticles with the aid of a concise and effective method remains a great challenge. Herein, in view of the uniform dispersion effect of polyvinylpyrrolidone (PVP) on CNTs and its complexation with Ni ions, uniform coating of Ni nanoparticles on CNTs is well developed after it is introduced in the hydrothermal process. The prepared Ni/CNTs composites exhibited excellent MA performance in comparison with those of reported Ni/CNTs composites for the ideal impedance matching performance and microwave attenuation ability. When the filler content was only 15 wt%, the minimum reflection loss (RLmin) reached -39.5 dB, and the effective bandwidth (EB) with RL < -10 dB reached 5.2 GHz at the thickness of 1.15 mm. A scalable strategy of regulating the distribution of Ni nanoparticles and preparing a lightweight microwave absorber based on CNTs was developed in this study, which can serve as a vital guideline for preparing novel MA composite materials.

Mechanically Reinforced Rigid Polyimide Foam via Chemically Grafting Isocyanate Acid for Ultrabroad Band Microwave Absorption.

Polyimide (PI) foam with excellent microwave absorption (MA) performance and desirable compressive strength is highly critical and in demand in the structural MA components. Although the satisfactory MA performance of the present PI-based MA foams has been achieved by employing diverse methods, the relatively low compressive strength (∼KPa) restricted them from use as structural MA foams in practical application. Herein, isocyanate acid was introduced to the backbone of PI resin, which not only increased the PI backbone polarity and strength as rigid chain segment, but also served as a self-foaming component. The porous structure of PI foams was readily regulated by adjusting the water and carbon nanotube (CNT) filler contents of precursor dispersion. As a result of the improved polarity of the PI backbone resulted from the isocyanate group and high dielectric loss of CNT, the high compressive strength of 7.04 MPa and impressive MA property of the resultant PI foam with a low CNT loading ratio of 1.5 wt % were achieved, which were much higher than those reported previously. Especially, the effective absorption bandwidth (EAB) (RL < -10 dB) was up to 10.7 GHz (at the thickness of 3 mm), covering the C, X, and Ku bands simultaneously. Meanwhile, the EAB of the as-prepared PI foam retained 9.3 and 9.7 GHz even after being subjected to liquid nitrogen (-196 °C) and high temperature (300 °C) treatments due to the desirable stability of PI. In addition, the excellent thermal insulation resulted from the pores structure and low filler content was achieved, where the top surface only presented 60 °C after placing on 300 °C platform for 30 min. The high compressive strength, impressive MA property, and thermal insulation endowed the resultant CNT/PI foam with great potential application as structural MA foam in a harsh service environment.