| Manganese nitrides with different reaction temperatures, different reaction time and different ammonia flowing rates were prapared by solid-gas reaction method with ammonia as nitrogen source on raw manganese powders. The phase structure, micromorphology, static magnetic and microwave electromagnetic properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), vibrating sample magnetometer (VSM) and vector network analyzer (VNA). First principles calculations have been performed for manganese nitrides by Castep program of Material Studio5.0. Structure optimization, spin-polarized total density of states, spin-up and spin down partial density of states, Electron density difference map were adopted, aiming to examine the magnetic and dielectric properties on different phases from the aspect of atoms.Results showed that reaction temperature, reaction time and ammonia flowing rates have a great effect on phase structure of manganese nitrides. When temperature reached400℃, manganese nitride began to produce. When temperature reached600℃, manganese turned to Mn4N5completely. With the raising of reaction temperature, reaction time and ammonia flowing rates, manganese nitrides tended to high nitrogen phase under600℃. Mn6Ns began to denitride upon600℃. There is no influence of prolong reaction time on products, while slow the decomposition of nitrogen by enhancing ammonia flowing rate. Upon results attributed to the decomposition rates of ammonia, amount of active nitrogen atoms and the thermodynamic stability of phase. High ammonia decomposition as well as high ammonia flowing rate make high nitrogen density gradient, which promote nitrogen enter into manganese lattice.Magnetic properties results showed that manganese nitride samples, major phase of Mn4N prepared at900℃, got the highest saturation magnetization of18.23emu/g and coercivity of54Oe. From the complex permeability graph, we can see an obvious resonance peak at2-6GHz, the largest imaginary part reach0.34at2.75GHz. To explore resonance mechanism of the sample, the complex permeability figure was fitted by the Gilbert modification of the Landau-Lifshitz equation (LLG equation). The matched results demonstrated the resonance peak come from natural resonance loss. Besides, calculated results showed that eddy-current loss is one of the loss mechanisms. Compared to permeability properties, the imaginary parts of complex permittivity properties are almost zero at low frequency. While the imaginary parts enhanced with the frequency increasing due to the frequency dispersion effect. Reflection loss results showed that Mn4N possess excellent microwave absorption properties at2-6GHz. Above results indicated Mn4N is an excellent magnetic loss absorbent. Effects of particle size on static magnetic, microwave electromagnetic and absorption properties were investigated. The results show that the saturation magnetization of Mn4N and complex permeability of the compositions increase, and complex permittivity decreases with the decrease of particle size. The reflection loss of70-100mesh Mn4N/wax composites show the best microwave absorption properties with2mm.Based on the standard PDF data, first principle method was taken to research the electromagnetic properties of structure cells of Mn, Mn4N, Mn3N2, MnN. Density of states of Mn4N clearly leads to spin splitting compared with Mn. The calculated spin-up and spin-down density of states are evidently dissymmetry with the magnetic moment of0.98μb, exhibit good magnetic properties. Diversely other unit cell show symmetry density of states with the magnetic moment of0μb. This is in line with the experiment results. The bond analysis results showed the bonding nature in Mn and MnN are metallic bonds and ionic bonds, respectively. However, Mn3N2and Mn4N are a complex mixture of covalent, ionic and metallic characters. The covalent and ionic bonds can easily effected by microwave, and increase the complex permittivity. |
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