Deformation-Thermal Co-Induced Ferromagnetism of Austenite Nanocrystalline FeCoCr Powders for Strong Microwave Absorption

Abstract

Nanocrystalline soft magnetic alloy powders are promising microwave absorbents since they can work at diverse frequencies and are stable in harsh environments. However, when the alloy powders are in austenite phase, they are out of the screen for microwave absorbents due to their paramagnetic nature. In this work, we reported a strategy to enable strong microwave absorption in nanocrystalline austenite FeCoCr powders by deformation-thermal co-induced ferromagnetism via attritor ball milling and subsequent heat treatment. Results showed that significant austenite-to-martensite transformation in the FeCoCr powders was achieved during ball milling, along with the increase in shape anisotropy from spherical to flaky. The saturation magnetization followed parabolic kinetics during ball milling and rose from 1.43 to 109.92 emu/g after milling for 4 h, while it exhibited a rapid increase to 181.58 emu/g after subsequent heat treatment at 500 °C. A considerable increase in complex permeability and hence magnetic loss capability was obtained. With appropriate modulation of complex permittivity, the resultant absorbents showed a reflection loss of below −6 dB over 8~18 GHz at thickness of 1 mm and superior stability at 300 °C. Our strategy can broaden the material selection for microwave absorbents by involving Fe-based austenite alloys and simply recover the ferromagnetism of industrial products made without proper control of the crystalline phase.

High temperature microwave absorbing materials

“…Fu et al investigated a way to improve the absorbing performance of nanocrystalline austenite FeCoCr powders via attritor ball milling and subsequent heat treatment. 108 The results showed that the saturation magnetization of the FeCoCr powders increase from 1.43 to 109.92 emu g −1 after ball milling for 4 h. When the heat treatment reached 500 °C, its saturation magnetization exhibited a great increase to 181.58 emu g −1 . Therefore, the strategy improved the material selection of high temperature materials.…”

Enhanced Thermal Stability of Carbonyl Iron Nanocrystalline Microwave Absorbents by Pinning Grain Boundaries with SiBaFe Alloy Nanoparticles

“…From a kinetic perspective, curvature-driven migration of the grain boundaries accounts for the grain growth [24]. Chemically ordered crystal structures, such as the Fe 3 Si superlattice [25], can inhibit atomic diffusion and, thus, hinder the migration of grain boundaries [26], while their formation in Fe-based alloys is limited to certain elements. The pinning effect has been widely used to stabilize the grains of nanocrystalline particles, such as solute/second-phase dragging [27][28][29].…”

“…With the progress in high-power and miniaturized electronic equipment, electromagnetic interference among heated components raises the urgent need for heat-resistant microwave-absorbing materials [1]. Additionally, heat-resistant microwave-absorbing materials are also required to control the radar cross-section (RCS) of hot parts of military devices [1][2][3]. At present, the widely used heat-resistant absorbents, such as silicon carbide [4], carbon-based composites [4,5], and barium titanate ceramics [6], exhibit excellent oxidation resistance and stable phases, while their microwave absorption mainly relies on dielectric loss, and there are difficulties in tuning permittivity dispersion over frequency [7,8], giving rise to narrow-band absorption and large thickness.…”

“…At present, the widely used heat-resistant absorbents, such as silicon carbide [4], carbon-based composites [4,5], and barium titanate ceramics [6], exhibit excellent oxidation resistance and stable phases, while their microwave absorption mainly relies on dielectric loss, and there are difficulties in tuning permittivity dispersion over frequency [7,8], giving rise to narrow-band absorption and large thickness. In contrast, magnetic metallic absorbents, such as carbonyl iron (CI) [9], FeSiAl [10], FeCo [3], etc., exhibit both magnetic and dielectric loss to microwaves, in which the resonance and working band can be regulated by the composition and morphology [7]. When the grain size of the magnetic absorbents is on the nanoscale, strong inter-grain exchange coupling and consequent high permeability can be achieved [11,12].…”

Microstructures, absorption and adhesion evolution of FeCoCr/silicone resin coatings at elevated temperature

“…However, carbon materials generally can work at temperature lower than 400 °C, and the regulation of their permittivity is complex, leading to large thickness and narrow absorption bandwidth. Magnetic metallic absorbents have great potential in preparation of thin-layer, lightweight, wide-band and strong absorbing microwave materials since they possess both dielectric and magnetic loss mechanisms (Magisetty et al, 2018;Fu et al, 2022). However, when they are used at elevated temperature, many challenges appear, such as growth of grain, oxidation and mechanical deterioration when combining with polymeric matrix.…”