Dielectric And Microwave Attenuation Properties Of Several Dielectric Materials

PMN-PST and BiFeO₃, the classic dielectric materials with perovskite structure, possess excellent physical and chemical properties, which exhibit potential applications in civil and military fields. In this dissertation, we have investigated the dielectric, ferroelectric and other electrical properties of Mn-doped PMN-PST ceramics and the microwave electromagnetism and attenuation of Nd-doped, La-doped BiFeO₃ and BiFeO₃/ ferrite composites. The effect of doping and recombination on electrical, magnetic properties and microwave attenuation of the materials has been explored, and the relationship between structure and performances has been discussed.（1） The dielectric properties of Mn-doped PMN-PST ternary ceramics.Mn-doped PMN-PST ceramics were prepared by two step precursor method. The sturecture, dielectric, ferroelectric and other electrical properties were investigated. Moderate Mn dopant increases density and grain size of ceramics. With the increase of Mn dopant, the diffuse phase transition（DPT） behavior of Mn-doped PMN-PST is weakened, indicating the change from relaxor to normal ferroelectric. Mn doping improves the ferroelectric properties and mechanical quality and factor, and decreases dielectric loss. The optimum electircal properties were obtained when the addition of Mn is 0.75 mol%.（2） The microwave electromagnetism and attenuation properties of Nd-doped BiFeO₃.Nd-doped BiFeO₃ nanoparticles were prepared by Sol–gel method. The microwave electromagnetism properties were investigated in the frequency range from 8.2-12.4 GHz at room temperature. With the increase of Nd concentration, the imaginary permittivity decreases which is dominated by polarization rather than conduction, and the imaginary permeability increases. The experimental and calculated results demonstrate that Nd doping generates the ordering domains structure and changes the electrons coupling states, which induce difficult polarization rotation and strong natural ferromagnetic resonance, leading to the decrease of dielectric loss and the increase of magnetic loss with increasing Nd concentration. The decrease of the tanδe and the increase of the tanδm achieve the development of electromagnetism parameters from mismatching to matching, leading to the increase of microwave attenuation and the broadening of effective absorption bandwidth.（3） The high-temperature microwave electromagnetism and attenuation properties of La-doped BiFeO₃.BLFO20 nanoparticles were prepared by Sol–gel method. The microwave electromagnetism and attenuation properties of BFO and BLFO20 were investigated in the temperature range from 323-673 K. The imaginary permittivity of BLFO2 O increases greatly above 473 K, which is attributed to the increase of conductivity due to the change of conduction mode from ionic to hopping-electron conduction. La doping increases natural ferromagnetic resonance, leading to the increase of magnetic loss at high temperature. BLFO20 exhibits high-efficiency and stable attenuation at dynamic temperature and frequency condition. With the increase of temperature, the attenuation capacity enhances, where the minimum reflection loss is less than-10 dB from 323 K to 673 K, and the best reflection loss reaches-54 dB. The effective reflection loss（≤-10 dB） almost covers 3 GHz in the full temperature range.（4） The microwave electromagnetism and attenuation properties of BiFeO₃/ BaFe12-x（Mn0.5Ti0.5）xO19 composites.BFO and BaFe12-x（Mn0.5Ti0.5）xO19 were prepared by molten-salt and conventional solid state method, respectively. The microwave electromagnetism and attenuation properties of BFO and BaFe12-x（Mn0.5Ti0.5）xO19 were investigated. The results show that BFO possesses low microwave attenuation due to the low magnetic loss and electromagnetism mismatching. Mn and Ti tune the frequency of magnetic resonance. With the increase of Mn and Ti concentration, the resonance peak shifts towards lower frequency. As to x = 5, the resonance frequency is tuned to the range 8.2- 12.4 GHz. BaFe12-x（Mn0.5Ti0.5）x O19 exhibits weak microwave attenuation owing to low dielectric loss. The imaginary permittivity of BiFeO₃/ BaFe12-x（Mn0.5Ti0.5）xO19 composites demonstrates multi-relaxation behavior, showing higher dielectric loss than BaFe12-x（Mn0.5Ti0.5）x O19. Compared with BiFeO₃, the composites possess higher magnetic loss. The improvement of dielectric and magnetic loss achieves electromagnetism parameters matching, resulting in enhancing the microwave attenuation of the composites. When volume ratio is 3:1, the minimum reflection loss reaches up-45 dB.