Study Of Synthesis And Microwave Electromagnetic Property Of Ni/Co/Fe-doped MnO₂

To study the doping effects on the microwave dielectric property, Ni/Co/Fe-doped MnO2was prepared by water bath synthesis and hydrothermal synthesis methods. Powder X-ray diffraction diffractometer, scanning electron microscope, transmission electron microscope, X-ray fluorescence spectrometry, and vector network analyzer were used to characterize the phase structure, morphology, crystal structure, chemical composition and electromagnetic performances. Moreover, first-principles calculation was employed to study the magnetic coupling state, electronic density of state, band structure, crystal parameters before and after ion doping, and hence to explain the doping mechanism at microscopic scale.Results showed that the microwave dielectric property of a-MnO2was significantly affected by Ni/Co doping. After Ni/Co doping, the phase structure and morphology of a-MnO2were kept the same as those before doping, while the imaginary part of complex permittivity and the dielectric loss tangent were increased with evident frequency response character. Dipoles relaxation polarization was the main polarization mechanism. Ionic doping extended the relaxation time, resulting in more obvious frequency dispersion. The partial density states of O2p, Mn3d, and Ni3d hybridized with each other, enhancing the strength of dipole polarization and electronic polarization, which certified the enhanced dielectric loss capacity.The influences of reaction time and temperature on the microwave dielectric property were also studied based on Ni-doped MnO2. Results showed that when the temperature was increased to100℃, time extended to3h, the product would transform from a-MnO2and y-MnO2to Mn3O4, with the morphology being kept the same. The dielectric performance was also changed:the real part and imaginary part of the complex permittivity and the dielectric loss tangent were all decreased with the increased temperature and extended time. This was related with the reduced crystal defects companied with the increased crystallinity.Ni-doped, Co-doped, and Ni/Co-codoped β-MnO2were synthesized by hydrothermal method, and the effects of crystal defects, including extrinsic ions and Mn/O vacancies, on the microwave dielectric performance were studied systematically. Doping of Ni/Co did not change the phase structure, but increased the dielectric loss capacity. The dielectric polarization process was ascribed to ionic polarization. Calculation results showed that defect states appeared near Fermi energy after incorporating crystal defects, and weak bonded electrons and weak related ions were formed at these defect states, enhancing the strength o electronic and ionic relaxation polarization.The effects of Fe doping on dielectric property of MnO2were different from those o Ni/Co doping. Both the phase structure and morphology were changed after Fe doping. The real and imaginary parts of complex permittivity and dielectric loss tangent were decreasec with the increased Fe doping concentration. The electromagnetic impedance matching condition was improved after Fe doping, but the resulted microwave absorbing property was bad due to the poor dielectric and magnetic loss strength. Both the impedance matching condition and loss capacity should be considered at the same time during the study of wave absorbing materials.