Ni-Carbon Microtube/Polytetrafluoroethylene as Flexible Electrothermal Microwave Absorbers

Abstract

Flexible microwave absorbers with Joule heating performance are urgently desired to meet the demands of extreme service environments. Herein, a type of flexible composite film is constructed by homogeneously dispersing a hierarchical Ni-carbon microtube (Ni/CMT) into a processable polytetrafluoroethylene (PTFE) matrix. The Ni/CMT are interconnected into a 3D conductive network, in which the huge interior cavity of the carbon microtube (CMT) improves impedance matching and provides additional hyper channels for electromagnetic (EM) waves dissipation, and the hierarchical magnetic Ni nanoparticles enhance the synergistic interactions between confined heterogeneous interfaces. Such an ingenious structure endows the composites with excellent electrothermal performance and improves their serviceability for application under extreme environments. Moreover, under a low fill loading of 3 wt.%, the Ni/CMT/PTFE (NCP) can achieve excellent low-frequency microwave absorption (MA) property with a minimum reflection loss of -59.12 dB at 5.92 GHz, which covers almost the entire C-band. Relying on their brilliant MA property, an EM sensor is designed and achieved by the resonance coupling of the patterned NCP. This work opens up a new way for the design of next-generation microwave absorbers that meet the requirements of EM packaging, proofing water and removing ice, fire safety, and health monitoring.

Keywords: carbon materials; electrothermal effect; extreme environments; flexible film; microwave absorption.

Figure 1

Fabrication, structural characterization, and EM properties of Ni‐MOF/CMT. a) Schematic illustration of fabricating hierarchical Ni‐MOF/CMT. b) XRD patterns and c) XPS spectra of Ni‐MOF, Ni‐MOF/CMT‐50, Ni‐MOF/CMT‐100, and Ni‐MOF/CMT‐150. d) XPS spectrum of Ni 2p. SEM images of e) CMT and f) Ni‐MOF/CMT‐150. g) TEM image and h) EDS elemental mappings of Ni‐MOF/CMT‐150. i) ε′ and ε″ values of Ni‐MOF/CMT samples. Reflection loss (RL) curves of j) CMT, k) Ni‐MOF/CMT‐50, l) Ni‐MOF/CMT‐100, and m) Ni‐MOF/CMT‐150, respectively.

Figure 2

Identification of Ni/CMT and structural characteristics of NCP films. a) XRD pattern, b) SEM image, c) TEM image, and d) SAED image (inset is HRTEM image) of Ni/CMT. e) HAADF‐STEM image and corresponding EDS elemental mapping of Ni/CMT. f) Schematic illustration for preparing NCP films. g) Digital photos of NCP‐7. h) Tensile stress−strain curves. SEM cross‐sectional images of i) NCP‐1, j) NCP‐3, k) NCP‐5, and l) NCP‐7.

Figure 3

Joule heating, hydrophobic, and fire resistance properties of NCP. a) TC of NCP samples. b) The surface temperature of NCP‐7 with various driven voltages. c) The photograph and thermal infrared images for NCP‐7 at different driven voltages. d) Experimental data and linear fitting of saturation temperature for NCP‐7 versus U ². e) The tailored surface temperature of NCP‐7 under voltage gradient variation. f) The surface temperature−time curve of NCP‐7 during the cyclic on−off tests at 30 V. g) Water contact angles of NCP films. h) Digital photos and thermal infrared image for NCP‐7 at a working voltage of 30 V during the deicing process. i) Fire retardance test of NCP films.

Figure 4

MA performance of NCP samples. RL curves of a,e) NCP‐1, b,f) NCP‐3, c,g) NCP‐5, and d,h) NCP‐7. i) RL values and j) EAB values of NCP samples under different thicknesses. k) Optimal RL plots in the C‐band (4−8 GHz), X‐band (8−12 GHz), and Ku‐band (12−18 GHz) of the series NCP. l) Comparison of the reported Ni‐based absorbers.

Figure 5

MA mechanism of NCP samples. a) ε′ and ε″, b) tan δ _ε , c) µ′ and µ″, d) α, and e) Cole‐Cole plots of NCP samples. Z curves of f) NCP‐1, g) NCP‐3, h) NCP‐5, and i) NCP‐7. j) The electric field polarization direction and the distribution of the electric field norm. k) Schematic illustration of MA mechanisms for NCP.

Figure 6

RSC simulation results and pressure‐driven EM sensor. 3D RCS diagram for the PEC substrate covered with a) PCN‐1, b) PCN‐3, c) PCN‐5, and d) PCN‐7. e) RCS simulation model. f) RCS curves and g) RCS reduction values of NCP composites. h) Schematic illustration of the sensor (0.1 mm thickness of copper). i) Magnetic energy density distributions, j) magnetic field, k) electric energy density distributions, l) electric field of NCP‐7 pattern at l of 0.3 mm. m) Equivalent circuit model. n) Frequency‐dependence S ₁₁ at l = 0.3−1.5 mm. o) Resonance frequency response fitted curve with r ² = 0.9926 at various l. p) Fitted pressure‐response curve with r ² = 0.9925 (pressures are represented by the l of rubber).