In-situ Modulating Electromagnetic Behaviors Of Ferromagnetic Microwires And Their Composites By Magnetic Field And Electric Current

Ferromagnetic amorphous microwires(abbreviated as microwires)have excellent soft magnetic properties and unique high-frequency electromagnetic characteristics.They can be employed as resonance units to achieve a so-called “left-handed” transmission performance in certain frequency bands with an apparent response to the variation of external excitation.Moreover,owning to their micron-sized dimensions and good mechanical properties,microwires are attractive functional fillers for structural composites with minimal dimension mismatch with the reinforcing fibres.Therefore,a type of multi-functional composite can be constructed by using microwire array and a suitable resin system that synthesizes the excellent electromagnetic and mechanical properties.Such kind of multifunctional materials will find a wide range of applications in high-precision imaging and electromagnetic stealth.The key research challenge of this type of composite is how to improve its response sensitivity to external stimili and widen the characteristic band.This research started with the optimization of the intrinsic structure and the arrangement design of microwires,focusing on the relationship between magnetic structure and electromagnetic behavior of microwires,the dipole interaction between microwires and the exploration of the tunable band with applied magnetic field and electric current.These are followed by the design and preparation of microwire composites,in order to(i)clarify the physical essence of the relationship among the electromagnetic behavior of monolithic composites,interface control,and individual characteristics of microwires,(ii)explore the regulation of external field to electromagnetic properties in different mesoscopic structures,and(iii)provide both theoretical and technical supports for the preparation of highly sensitive,broadspectrum,strong-response microwire composites.Studies have shown that the high-frequency electromagnetic behavior(the transmission spectra)of microwires is dominated by ferromagnetic resonance(FMR).FMR is dependent on the internal magnetic domain distribution and magnetic anisotropy of microwire that can be conveniently tailored by controlling the alloy composition and geometric dimension.The results indicate that Co(22,24)and Co(18.6,21.6)microwires exhibit a transmission “window”(a typical phenomenon of left-handed behavior)in the frequency band of 8.8-10.4 GHz and 9.2-10.7 GHz,respectively.The application of external excitations(magnetic field and electric current)has promoted the movement and reorganization of domain and adjusted the FMR frequency,which can be reasonably explained by the Landau-Lifsthiz-Gilbert theory.In the interest of practical applications where many microwires are usually embedded in a matrix,the parallel array with equal wire spacing has been investigated.It has been shown that any microwire in the array is subject to the additional influence due to the long-range dipole interaction between microwires,the wire array as a whole exhibits different electromagnetic behavior from that of the a single wire,but it still maintains a prominent response to external magnetic fields and electric currents.Since the number and spacing of microwires take control of the internal distribution in the waveguide and the influence from the surroundings,different wire arrays possess unique electromagnetic behavior and external field response sensitivity.Based on the understanding of the electromagnetic behavior of individual wire and wire arrays,microwires are embedded in the silicone matrix and glass fiber reinforced plastic composite prepregs,respectively,to obtain two types of microwire composites.Excited by a static magnetic field of 8 Oe,the silicone-based composites containing 6 and 8 parallel microwires exhibits the left-handed transmission characteristics in the 9-11.5 GHz frequency band.An additional DC current(0-50 mA)had profound regulation effect on the electromagnetic behavior.Unlike the monolithic microwire,the interface introduced in the composite is believed to alter the stress state of the microwire.Therefore,in the microwire-glass fiber reinforced composite,changing the orientation between microwires and glass fibers or adopting different curing processes have had the important influence on the interface condition and affected the electromagnetic behavior of composite and its external field tunability.All these provide useful insights in to the design and development of ferromagnetic microwire composite materials for a wide range of ingenious engineering application.