Colossal Permittivity Properties In Co-doped TiO₂

The giant dielectric performance is obtained in In+Nb co-doped TiO₂ ceramic system with excellent dielectric constant?>10⁴?and a low dielectric loss?<0.05?.The electron created by the incorporation of Nb into Ti sites is explained for the colossal permittivity?CP?and pinned by the oxygen vacancy because of the In doping in Ti.Therefore,the CP is achieved with a low loss in In+Nb co-doped TiO₂.Moreover,the CP property keeps stable with the change of frequency and temperature,which is better than the other CP systems.Although the new system of In+Nb co-doped TiO₂ has improved the dielectric properties but the dielectric loss is still too high to satisfy the demand in manufacture.It is necessary for us to research the co-doped Ti O₂ system.The work of this paper is mainly studied from three aspects of the co-doped Ti O₂.In the first place,the CP is achieved in Mg+Nb co-doped TiO₂ ceramics.It is found that the dielectric property is significantly improved with the introduction of Mg and Nb.The dielectric loss decreases to 0.008 at 1kHz with a colossal permittivity?>10⁴?and the CP is stable at the frequency range of 100 Hz to 1 MHz.Based on the X-ray photoelectron spectroscopy,it can be confirmed that the production of electron is ascribed to Nb⁵⁺and the oxygen vacancy is associated with Mg⁵⁺.We speculate that the electron-pinned defect-dipoles can be formed in the internal of the material,which is beneficial to decrease the dielectric loss.Secondly,we investigated the effect on the dielectric property of Nb-doping in TiO₂ by the introduction of isovalent Zr elements in Ti sites.The dielectric property is different with the change of doping level.Comparing to Nb-only doped TiO₂,the dielectric loss decrease and the frequency-and temperature-stability is better,which is comparable with the donor and acceptor co-doped TiO₂.Therefore,the excellent CP obtained in Zr+Nb co-doped TiO₂ break the pattern of donor and acceptor co-doping.Impedance analysis and valence state analysis of samples were carried out.It is found that the increase of Zr ion doping concentration can decrease the concentration of oxygen vacancy and the corresponding activation energy is higher.Therefore,we speculate that theTi⁴⁺?e-V_O^??-Ti⁴⁺?e defect clusters play the decisive role in the improvement of dielectric property.Finally,considering that all the acceptor elements in the current literature are fixed valence and the research of the variable valence acceptor element is none,we select the valence Mn element as the acceptor element to co-doping with donor Nb.The ion radius of manganese is very close to the titanium ion radius.By testing the dielectric properties,it is found that the CP disappear with a small amount of Mn ions doping into Ti sites,but the CP is recovered for the Mn+Nb co-doped TiO₂ when post-sintering in N₂atmosphere.The XPS and impedance analysis were carried out,and we speculate that the disappearance of giant dielectric constant may be due to the suppression of conversion of Ti⁴⁺to Ti³⁺by the Mn element of the variable valence.The Mn⁴⁺is easier to be reduced than the Ti⁴⁺element and Mn³⁺and Mn²⁺ions are obtained.The electron produced from the donor element is prior to reduce the high valence of Mn element,which inhibiting the change price of Ti ion.Therefore,the polarization of the material weakens,resulting in the disappearance of the CP.However,the CP of the sample regains after the post-sintering in N₂,which may be attribute to the semiconducting grain and an insulating grain boundary.