Influence Of Defects To The Conductivity And Dielectric Loss Of Microwave Ceramics BaNd₂Ti₄O₁₂

In this thesis, BaNd₂Ti₄O₁₂ is selected as a model material to investigate the influence of quenching-annealing and atmospheres on the conductivity and the dielectric loss of titanate-based microwave dielectric ceramics. Conductivity spectra in low frequency range (5-10⁷Hz) and dielectric loss in microwave range(～10⁹Hz) of BaNd₂Ti₄O₁₂ ceramics were characterized by using AC impedance spectroscopy and microwave dielectric resonator method. The intrinsic defect, such as v_o^x, Ti_Ti'and weakly bonded electrons, were controlled and modified by the various thermal treatments and atmospheres. Variations of the conductivity and the dielectric loss related to the contents of these defects were characterized by using AC impedance spectroscopy and microwave dielectric resonator method. Variations of defects, conductivity and microwave dielectric loss and the relations among these factors were investigated comprehensively to reach conclusions about the influence mechanism of defects on microwave dielectric loss, which could be helpful on the improvement of quality factors for the titanate-based microwave dielectric ceramics. The main results in this thesis are summarized as below:1. AC impedance spectroscopy has proved to be a powerful technique to investigate the electrical properties of microwave dielectric ceramics. Not only can we design the corresponding equivalent circuit, but also analyze and differentiate the influence factors to the electric characters of the materials. Generally, there are three components in the complex impedance plot(Z"/Z'plot) of microwave dielectric ceramics: the bulk impedance, the grain boundary impedance and the impedance between electrodes and ceramic materials. However, there is only bulk impedance in the BaNd₂Ti₄O₁₂ ceramics studied in this thesis. The typical value of capacitance for bulk in this ceramics is～10^-12 F·cm^-1. For this value, the Z"/Z'plot in the complex plane is composed by one semicircles.2. The intrinsic defects including oxygen vacancies, weakly bonded electrons and Ti_Ti' (Ti³⁺) in the BaNd₂Ti₄O₁₂ ceramics form during the sintering procedure at high temperature. Hopping conduction is engendered by the electrons and vacancies in alternant current electric field. The conductivity is predominated by the concentration of intrinsic defects, and influence of temperature is crucial to the concentrations of intrinsic defects. The defects concentration is changed and controlled through thermally treating the BaNd₂Ti₄O₁₂ ceramics in various conditions. Air-quenched BaNd₂Ti₄O₁₂ ceramics possess high conductivity duo to high concentration of these intrinsic defect, anneal in air at low temperature can eliminate these intrinsic defects and reduce the conductivity. On the contrary, quenching in air and annealing in N₂ at low temperature can increase the concentrations of these intrinsic defects and conductivity. Similarly, annealing in O₂ can also reduce the conductivity. Under certain conditions, the concentration of the electrons is related to that of oxygen vacancies, which make it reasonable to detect the variation of oxygen vacancies by the conductivity measurements.3. Though air-quenched and annealed BaNd₂Ti₄O₁₂ ceramics can influence the value of microwave dielectric loss duo to alternant current concentration of defect, no obvious influence on the microwave dielectric loss when the ceramics were treated in different atmospheres. We consider that most possibly the microwave dielectric loss is determined by the thermal strain introduced by the quenching, but not the defects in the BaNd₂Ti₄O₁₂ ceramics. In order to improve the Q×f factor of the materials, annealing the materials in relatively low temperature is necessary.4. Doping of Ta⁵⁺/Nb⁵⁺ with proper content can eliminate oxygen vacancies and weakly bound electrons in BaNd₂Ti₄O₁₂ ceramics, thus the conductivity decrease with the pentavalent-ions dopants. Whereas the conductivity rise appreciably, because doping of Ta⁵⁺/Nb⁵⁺ can import more oxygen vacancies and weakly bound electrons in BaNd₂Ti₄O₁₂ ceramics. In addition, integrality of BaNd₂Ti₄O₁₂ ceramics is breached by doping of Ta⁵⁺/Nb⁵⁺, so as to decrease of the microwave dielectric loss.