Synthesis, Magnetic And Microwave Properties Of Co-B Spherical Particles And Synthesis Of Co Particles With Different Morphologies

With the increasing use of the mobile telephones, intelligent transport systems, electronic toll collection systems, and local area network systems, and other electromagnetic tools, the electromagnetic interference problems become severer. These problems have prompted the research of electromagnetic wave absorbing materials.Transation metal-boron nanocomposites exhibit special magnetic properties and microwave properties due to their special microstructures. Therefore, these nanocomposites have become the subject of intense studies because of their potential as the microwave absorption materials. However, the magnetic properties and microwave absorption properties of the nanocomposites have not been studied systematically to date. The Fe-B nanocomposites have been studied widely. However, for the study of the magnetism and microwave properties of Co-B nanocomposites, very little work has been done. For these reasons, it is critical to produce transition metal-boron nanocomposites and investigate their microsturctures, magnetic properties and microwave properties.In the present work, nanocomposite powders of the Co-B amorphous alloy particles have been prepared by liquid reduction method. The obtained Co-B amorphous alloy particles then were annealed in the vacuum at the different temperatures for two hours. Their microstructures, magnetic properties and microwave characteristics were investigated. The effects of microstructure on properties were discussed. Preliminary research of transation metal Co was also done. ( 1) Synthesis of the Co-B amorphous alloy particles and the Co-B alloy particles obtained by the vacuum annealed method were studied. With the vacuum annealed temperature increasing, the average sizes of the spherical particles decrease from 500 nm to 75 nm; the precipitated phases of crystalline Co and Co₃B increase. When annealed at 500℃, Co-B alloy particles only contain the crystalline Co and Co₃B phases.(2) The saturation magnetization and coercivity of the Co-B amorphous alloy particles and Co-B alloy particles were investigated. The saturation magnetization increased with an increase of the annealing temperature. The maximum value of saturation magnetization (119.7 emu/g) is obtained after 500℃annealing which is due to more crystallization of Co and Co₃B from the amorphous matrix that improves the magnetic characteristics of the materials; And the maximum value of coercivity (403 Oe) is obtained after 300℃annealing. This is because that the exchange coupling between crystals averages out magnetocrystalline anisotropy.(3) The complex permittivity was investigated for the Co-B amorphous alloy particles-paraffin wax composites and Co-B alloy particles-paraffin wax composites. For all the composites the complex permittivity shows almost constant and low value indicating high resistivity of the composites which is needed for impedance matching condition.(4) The complex permeability was investigated for the Co-B amorphous alloy particles-paraffin wax composites and Co-B alloy particles-paraffin wax composites. Three resonance peaks appear for all the samples in the frequency range of 0.1-18 GHz. The first resonance peak (at about 4.1-5.5 GHz) may result from the natural resonance; the second and third resonance peaks (at about 10.0 GHz and 12.9 GHz) may be related to the exchange resonance modes. The nature resonance peak becomes broadening and lower with the vacuum annealing temperature increasing. This may be due to the fact as follows: With the annealing temperature increasing, the initial phase was gradually decomposed into crystal Co and Co₃B. This means that the value of the nature resonance peak of the Co-B amorphous phase becomes lower. Synchronously, the nature resonance peaks of the Co₃B and fcc-Co appeared. However, due to the low contents of fcc-Co and the Co₃B crystals, the corresponding peaks of them may be lower. Consequently, the overlapped peak becomes broadening and lower as the annealing temperature increasing.( 5 ) The microwave absorbing properties of the Co-B amorphous alloy particles-paraffin wax composites and Co-B alloy particles-paraffin wax composites were investigated in the 0.1-18 GHz frequency range. The reflection loss (RL) of the composites is less than -10 dB in a broad frequency range indicating excellent microwave absorbing properties.( 6) Synthesis of flower-like Co particles and rod-like Co particles by hydrothermal methods and Synthesis of dendritic Co particles by liquid reduction method was studied. From the TEM, the particle size distributions of the prepared Co nano-particles were large and the morphologies were inhomogenous. The products synthesized by liquid reduction method were hexagonal close packed (HCP) structure.