Preparation,Properties And Polarization Mechanism Of Co-doped TiO₂-based And LaGaO₃-based Colossal Permittivity Ceramics

Colossal permittivity（CP）dielectric materials have been attracting increased attention due to their promising applications in high energy density capacitors and device miniaturization.In recent years,a kind of（In,Nb）co-doped rutile Ti O₂ colossal permittivity ceramics exhibit excellent dielectric performance,providing a new method for the preparation of CP materials.Based on this co-doping strategy,we prepared rutile TiO₂ and perovskite LaGaO₃ based colossal permittivity ceramics,their dielectric properties and polarization mechanism were studied in detail.（1）（Cu,Nb）co-doped rutile TiO₂ ceramics with a nominal composition of Ti_1-x(Cu_1/3Nb_2/3)_xO₂ have been prepared by solid-state reaction method,and x varies from0.05%to 30%.For ceramics with x=0.5%and 1%,colossal permittivity and relatively low dielectric loss are obtained simultaneously over a wide temperature and frequency range.For 0.5%（Cu,Nb）co-doped rutile TiO₂,the samples show low dielectric loss（below 0.07）from 100 Hz to 1 MHz,the dielectric loss can be as low as 0.05 at 1 kHz.（2）For Ti_1-x(Cu_1/3Nb_2/3)_xO₂ ceramics with x=0.5%and 1%,the electrical modulus as a function of frequency shows three peaks,which indicates there are three kinds of polarization mechanisms exist in the material.The colossal permittivity of Ti_0.99(Cu_1/3Nb_2/3)_0.01O₂ ceramics originates from non-intrinsic polarization mechanisms like electrode effect and internal barrier layer capacitor effect（IBLC）,as well as intrinsic polarization mechanism like variable-range-hopping polarization.The colossal permittivity of Ti_0.995(Cu_1/3Nb_2/3)_0.005O₂ ceramics originates from surface barrier layer capacitor effect（SBLC）,IBLC and variable-range-hopping（VRH）polarization mechanism.The impedance spectroscopy results indicate high grain boundary impedance exists in Ti_0.995(Cu_1/3Nb_2/3)_0.005O₂ ceramics,the XPS results as well as the color difference between the surface and the inside of Ti_0.995(Cu_1/3Nb_2/3)_0.005O₂ suggests the existence of surface barrier layer.Grain boundary and resistive surface layer can block the long-range hopping of charge carriers,making the dielectric loss of Ti_0.995(Cu_1/3Nb_2/3)_0.005O₂ lower than that of Ti_0.99(Cu_1/3Nb_2/3)_0.01O₂.By adjusting the co-doping concentration and using appropriate post annealing process,the grain boundary and surface resistivity of the ceramic samples can be increased.And this is a good method to obtain high performance colossal permittivity dielectric ceramics.（3）（Mg,Mn）co-doped LaGa_0.7-xMg_xMn_0.3O₃（x=0,0.05,0.10,0.15）ceramics were prepared by solid-state reaction method.The dielectric and electric properties of the LaGa_0.7-xMg_xMn_0.3O₃ ceramics were studied in detail by dielectric spectra,impedance spectra,and I-V characteristic analysis.Colossal permittivity up to 10⁴ could be obtained across the frequency range up to 10⁴ Hz.The impedance analysis of the co-doped LaGaO₃ ceramics indicated that the Mott’s variable range hopping（VRH）polarization should be the main origin of colossal permittivity.（Mg,Mn）co-doping suppressed the formation of Mn³⁺and enhanced the VRH polarization,resulting in increased permittivity.Partially localization of electrons by Mg reduced the long-range electron hoping and led to the decrease of dielectric loss.