Collectively Orientated Magnetic Needles for S-Band Microwave Absorption and Thermal Conduction: The Factor of Composition

Abstract

Efficient thermal management and electromagnetic interference shielding are crucial for high-power integrated circuits to prevent overheating and ensure reliable operation. Asymmetric CoFe magnetic needles (MNs), which are synthesized via the wet chemical method, serve as fillers in composites for microwave absorption and thermal conduction. The atomic ratio of Co2+ to Fe3+ (γCo/Fe) in the precursor is used as the key parameter to control the composition and microstructure of CoFe MNs. A high γCo/Fe value increases the content of metallic Co, leading to the high electrical conductivity, and the residual permeability in the microwave frequency range. The collective orientation of CoFe MNs along the magnetic field does not significantly change the real part of the permittivity, but it facilitates the formation of long-range conductive networks, resulting in enhanced dielectric losses. The composite composed of oriented Co3Fe1 with a thickness of 4.0 mm is able to effectively absorb S-band microwaves from 2.4 to 4.0 GHz. This effective absorption bandwidth is the broadest one, to our knowledge, for a single composite, when the lower limit of the EAB is at S-band frequencies. CoFe MNs with a high content of metallic Co improves significantly the thermal conductivity. The long-range conductive networks in the oriented CoFe-MN composites are available for thermal energy transfer with minimized phonon scattering. The composition design, as well as the collective orientation, of magnetic fillers is a highly feasible strategy for adapting the properties of the composites into many application scenarios of conformal shielding.

Keywords: CoFe alloy, Asymmetric magnetic particle, Collective orientation, Microwave absorption, Thermal conductivity