Polarization Mechanisms And Dielectric Relaxations Of Double-Doped Ba（Sr）TiO₃ Ceramics

BaTiO3 and SrTiO3 play an important role in modern electronic devices,and doping is an effective method for promoting their electrical properties and expanding their application areas.With the research development of Ba（Sr）TiO3 in recent years,the colossal dielectric constant（CDC）phenomenon was also observed in titanates.The CDC behavior was first observed in CaCu3Ti4O12 and then was found to widely exist in many transition-metal oxides.CDC materials exhibit temperature and frequency stability of dielectric constant and non-Ohmic effect,which are different from the traditional ferroelectrics.However,the origins of CDC for Ba（Sr）TiO3 are still unclear,and both nonintrinsic space charge polarization and intrinsic charge hopping are popular.In this thesis,we prepared double-doped Ba（Sr）TiO3 ceramics with different charge compensation mechanisms by means of solid state reaction method,and studied the structure,valence states,polarization type,dielectric relaxations and conduction mechanisms.We explored the relationship between nonintrinsic interfacial polarization and intrinsic charge transport.The polarization types in different frequency ranges were distinguished,and the ionic displacement polarization and charge hopping contributions to dielectric constant of BaTiO3 and SrTiO3 were identified.The main contents are as follows:1.BaTi0.7（Cu0.1Nb0.2）O3 ceramic was studied by means of XRD and Raman Spectroscopy.The crystal structure was highly distorted due to the mismatch of ionic radii and Jahn–Teller effect of Cu²⁺,resulting in the localization of charges.The palarons,which are originated from the interaction of localized charge and the distorted structure,hop among the different valence states of Cu ions and form the variable-range-hopping（VRH）conduction.The VRH conduction was also confirmed by the results of electron paramagnetic resonance.The dielectric relaxation and conduction have the same behavior,and both of which originate from charge hopping.The impedance spectroscopy under different electrode and bias voltage was used to distinguish the electric responses of grain,grain boundary and electrode,and the response of electrode was significant at high temperatures.Compared with Mg/Nb co-doped BaTiO3 ceramic,the electrically conductive grain and interfacial polarization are both crucial for the CDC of BaTi0.7（Cu0.1Nb0.2）O3.2.Sr doping decreases the dielectric constant and loss of BaTi0.7（Cu0.1Nb0.2）O3 in both high-and low-frequency ranges.With the Sr content increasing,the resistance of grain increases,and the quantity of mobile carrier decreases,resulting in the weakening of interfacial polarization,which may be related to the higher distortion of lattice caused by Sr doping.3.Cu/Nb co-doped SrTiO3 ceramics with different doping content have a distorted cubic structure and VRH is still the best way for charge transport,and the dielectric relaxation also shows the same behavior with conduction.The high-frequency dielectric constant decreases with increasing temperature and is related to the ionic displacement polarization,while space charge polarization is responsible for the low-frequency dielectric constant,which is enhanced by high temperature.With doping content increasing,the ionic displacement polarization is destroyed and high-frequency dielectric constant decreases,and resistances of grain and grain boundary as well as capacitance of grain decreases.However,the capacitances of grain boundary increases with doping content.These results confirm that the space charges polarization at grain boundary mainly come from the mobile electrons of grain.The activation energy for charge hopping doesn’t decrease in highly doped ceramics,which may be caused by the higher distortion of lattice.4.BaTi（1-3x）（Mgx Nb2x）O3 ceramics have larger resistivity than pure BaTiO3,suggesting that the invariable valence of Mg²⁺ions are important for charge compensation and localization.Although the samples also show dielectric constant plateaus in low-frequency range,the dielectric constants are much lower than Cu/Nb doped samples due to the smaller quantity of mobile charges.We compared the dielectric constant of BaTi（1-3x）（MgxNb2x）O3 with pure BaTiO3 and Cu/Nb co-doped BaTiO3,and conclude that the high-frequency polarization is a combined results of ion displacement and charge hopping.The low-frequency dielectric constant contains the contributions of ions polarization and space charge polarization,and the latter will be dominant when doping content and conductivity are high.We studied the low-temperature dielectric relaxation mechanism and defect chemistry,and supposed that both dielectric relaxation and conduction originate from the electron hopping between different valence states of Ti ions.5.La/Cu co-doping has less influence on crystal structure of BaTiO3 than Cu/Nb co-doped samples,indicating that doping at Ti sites is beneficial to the phase transition from tetragonal to cubic.The dielectric relaxation at low temperatures and high frequencies has a T^-1/4 relation with temperature,which coincides with the VRH conduction of grain.Ba0.8La0.2Ti0.9Cu0.1O3 has larger resistivity and lower dielectric constant in low-frequency range than BaTi0.7（Cu0.1Nb0.2）O3,which is supposed to be caused by2La_Ba-Cu?_Ti defect complexes.The charges are strongly bounded in defect complexes and affect the space charge polarization.