Microwave Magnetic Properties And Microwave Absorption Properties Of Plate-like Metal And Alloy Nanoparticles

Ferromagnetic metal particles have gained a considerable attention because of their higher saturation magnetization, higher working frequency, and higher complex permeability than ferrites. The plate-like ferromagnetic metal and alloy nanoparticles of large shape anisotropy can exceed the Snoek's limit and have higher permeability in the GHz frequency range. Hence the ferromagnetic metal and alloy nanoparticles of plate-like shape can be expected to be good microwave absorption filler in GHz frequency range. However, no studies on the microwave magnetic properties and microwave absorption properties of the plate-like ferromagnetic metal and alloy nanoparticles (nanoplatelets) have been reported so far.In this work, ferromagnetic metal and alloy nanoplatelets [Co, Ni, Ni_1-xFe_x(x=0.1-0.5), and Fe_0.2(Co_1-x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8)] have been synthesized by the selective absorption of the stabilizing agents (OH^-). The nanoplatelets were coated with a thin MnO₂ layer by a simple solution phase chemical method. The composites with higher resistivity were prepared by homogenously mixing the MnO₂-coated nanoplatelets and paraffin wax. The microwave charaeteristics and microwave absorption properties of the composites have been studied. And the intrinsic permeability of the nanoplatelets has been extracted. The main results are shown as follows:(1) The prepared metal and alloy particles [Co, Ni, Ni_1-xFe_x(x=0.1-0.5), and Fe_0.2(Co_1-xNi_x)_0.8 (x=0.2, 0.4, 0.6, 0.8)] are plate-like in shape. The diameters of the nanoplatelets are in the range from 50 to 150nn. The thicknesses of the nanoplatelets are in the range from 10 to 20 nm.(2) The nanoplatelets [Co, Ni, Ni_1-xFe_x(x=0.1-0.5), and Fe_0.2(Co_1-x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8)] were coated with a thin MnO₂ layer by a simple solution phase chemical method.(3) The complex permittivity of the Co, Ni, Ni_1-xFe_x(x=0.1-0.5), and Fe_0.2(Co_1-x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8) nanoplatelet composites has been studied in the microwave band. All of the composites have a relative low permittivity. The complex permittivity of the composite with 13% Co nanoplatelet volume fraction shows one resonance peak (at about 8.5 GHz) related to the interfacial polarization. The complex permittivity of the composites with different volume fractions of the Ni nanoplatelets is almost constant in the 0.1-10 GHz frequency range and increases with increasing Ni nanoplatelet volume fraction. The complex permittivity of the composites with 75 wt% Ni_1-xFe_x(x=0.1-0.5) nanoplatelets is almost constant and the dielectric loss tangent have a small value (tanδ<0.05) in the 0.1-18 GHz frequency range. The complex permittivity of the composites with 80 wt% Fe_0.2(Co_1-x, Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8) nanoplatelets is almost constant in the 0.1-18 GHz frequency range. The real partε'is at about 9.5-13.5 and the imaginary partε"is around 0.5.(4) The complex permeability of the Co, Ni, Ni_1-xFe_x(x=0.1-0.5), and Fe_0.2Co_1-x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8) nanoplatelet composites has been studied in the microwave band. The complex permeability of all the composites shows several resonance peaks in the microwave band. The first resonance peak may result from the natural resonance; and the others may be related to the exchange resonance modes. The complex permeability of the Co nanoplatelet composites shows two broad resonance peaks in the 0.1-18 GHz frequency range. More resonance peaks are observed in the composites with low concentration of the Ni nanoplatelets; and the resonance frequencies decrease with increasing Ni nanoplatelet volume fraction. The permeability of the Ni_1-xFe_x(x=0.1-0.5) nanoplatelet composites has a slight increase with increasing Fe content. The resonance frequency of the natural resonance peak increase with increasing Fe content. The natural resonance peak of the Fe_0.2(Co_1-x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8) nanoplatelet composites becomes narrower with increasing Ni content, the peak value increases with increasing Ni content and the resonance frequency decreases with increasing of Ni content.(5) The microwave absorption properties of the Co, Ni, Ni_1-xFe_x(x=0.1-0.5), and Fe_0.2(Co_1x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8) nanoplatelet composites have been studied. With thickness from 4 to 1 mm, the reflection loss values of the composite with 13% Co nanoplatelet volume fraction are less than -10 dB in the 2-12.5 GHz frequency range. The microwave absorption properties of the Ni nanoplatelet composites can be controlled by the thickness of the composites and the Ni nanoplatelet volume fraction. The microwave absorption properties of the Ni_1-xFe_x(x=0.1-0.5) nanoplatelet composites can be controlled by the content of the Ni_1-xFe_x(x=0.1-0.5), the thickness of the composites and the Ni_1-xFe_x(x=0.1-0.5) nanoplatelet volume fraction. By increasing of the Ni_1-xFe_x(x=0.1-0.5) nanoplatelet volume fraction in the composites, a good microwave absorption properties have been attained in the quasi-microwave band (1-3 GHz). The composites of 80 wt% Fe_0.2(Co_1-x Ni_x)_0.8 (x=0.2, 0.4, 0.6, 0.8) nanoplatelet have a good microwave absorption properties in the 2.6-18 GHz frequency range and the composite with 1 mm thickness still have a good microwave absorption properties.(6) The intrinsic microwave permeability of the Ni nanoplatelets has been extracted by the isotropic model. Based on the intrinsic properties of the Ni nanoplatelets, the microwave permeability of the composites with different volume fractions of the Ni nanoplatelets has been predicted. This indicates that the model can be used to extract the intrinsic microwave permeability of the plate-like magnetic particles.