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

Abstract

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.

Peelable Microwave Absorption Coating with Reusable and Anticorrosion Merits

“…However, there is no magnetic ingredient involved in our specimens; hence, the effect of the relative complex permeability (μ r ) on MA results is negligible. Typically, the real parts of ε′ and the imaginary parts of ε″ indicate the storage capacity and the dissipation capacity of microwave energy, respectively . The corresponding dielectric loss tangent (tan δ ε = ε″/ε′) is highly suited for the MA performance.…”

“…Typically, the real parts of ε′ and the imaginary parts of ε″ indicate the storage capacity and the dissipation capacity of microwave energy, respectively. 50 The corresponding dielectric loss tangent (tan δ ε = ε″/ε′) is highly suited for the MA performance. Obviously, the growing values of ε′ and ε″ of all specimens in Figure 4a with the increasing loadings of CNT@PANI indicate the improved polarization storage and dissipation capacity of microwaves, which is attributed to the elevated electrical conductivity (Figure S2a) and polarization behavior.…”

Lightweight Porous Carbon Foams via Pyrolysis of Robust Polyimide Foams for Efficient Broadband Microwave Absorption

“…The highest point before falling is considered the compressive strength due to the large fluctuation of curves, and the related results are exhibited in Figure b. The compressive stress was transferred in the 3D skeleton of foams during compression, and the skeleton was broken and further compacted as the stress increased (Figure c) . PIF-300 displayed the optimal compressive strength with a value of 0.85 MPa, which was related to the cross-linked structure and the closed cell structure.…”

Research Progress of Flexible Piezoresistive Sensors Based on Polymer Porous Materials

“…PI is a high-performance material used as a substrate for flexible electronic devices . It exhibits excellent stability and mechanical properties, and temperature resistance, adapting to a wide range of temperature changes. , It also has good corrosion resistance . These properties have generated significant interest among researchers in recent years.…”