Study On Defect Structure And Dielectric Properties Of Nd-doped SrTiO₃-based Ceramics With High Energy Storage Density

Strontium titanate,a quantum paraelectrics,is one of the main electronic materials with ABO₃-type cubic perovskite structure at room temperature.Strontium titanate not only has relative high dielectric constant(ε_r～300),but also relative high breakdown strength(E_b～10kV/mm).For its unique properties,strontium titanate has broad vistas of application as a high-voltage energy storage dielectric material.Doping modification of strontium titanate is an important research project for the purpose of expanding its applications.In recent years,more and more research works have been done on heterovalent cation substitution in strontium titanate ceramics. This kind of SrTiO₃-based ceramics shows many interesting properties,such as quantum ferroelectricity,superconductivity,very high permittivity and so on,but many mechanisms in these researches are not clear.In this thesis,the dielectric properties of the strontium titanate ceramics by substituting divalent Sr²⁺with trivalent Nd³⁺were investigated.The research work focuses on finding out the relationship between structures and properties.The defect chemistry mechanism basing on the experimental data and theoretical analysis is also discussed.Aiming at understand the mechanism of heterovalent cation substitution in strontium titanate, this research project is of great significance to both the theoretical meaning and the practical utility.Considering three possible charge compensation mechanisms of Nd³⁺ion doped SrTiO₃ ceramics,the compositions Nd_xSr_1-3x/2TiO₃(NST),Nd_xSr_1-xTi_1-x/4O₃(NTO), Nd_xSr_1-xTiO₃(NSTO)were designed and the corresponding ceramics were prepared by solid state reaction and conventional ceramic processing method.The crystal structure and microstructure of the ceramics were characterized by X-ray diffraction (XRD)and scanning electronic microscopy(SEM).The chemical compositions of the final ceramic samples were analyzed by x-ray photoelectron spectroscopy(XPS)and energy dispersive x-ray spectroscopy(EDX).Combined with the dielectric properties of the three series of ceramics,the defect structure mechanism of ion Nd³⁺doped SrTiO₃ ceramics was discussed.Crystal structure analysis shows that each of the three series of ceramics form single phase all over the corresponding doping range.The tendency of the crystal structure transformation is similar,i.e.,with the increasing of Nd³⁺doping concentration,the crystal structure incline cubic to tetragonal and change to tetragonal at last.But the tendency of lattice parameters transformation is different with different Nd³⁺doping manner.The microstructure analysis indicates that doped Nd operated as grain inhibitor.The appropriate Nd doping concentration makes the grain size of NTO and NSTO ceramics smaller than 1μm.The dielectric relaxation properties of the three series of ceramics are studied systematically.The results suggest that all the dielectric relaxations are related to the thermally activated process by following Arrhenius law.At room temperature and the frequency of 1kHz for NSTO ceramics with x=0.1,the dielectric properties for the real part of complex-permittivity reach to the maximum 10100 in the order of magnitude of 10⁴, so-called "giant permittivity effect".Impedance spectroscopy results indicate that the three series of ceramics are electrically heterogeneous and the origin of the high permittivity in NTO and NSTO ceramics is Maxwell-Wagner interface relaxation at room temperature.Under the same measured conditions,the breakdown strength of the three series of ceramics has the direct relations with the ceramics microstructure. Higher breakdown strength can be obtained when the grain size is smaller and homogeneous.The highest breakdown strength is E_b=13.52kV/mm in NSTO ceramics with Nd doping concentration x=0.01.Basing on the chemical compositions analysis of the ceramic samples, combining with the dielectric properties of Nd doped SrTiO₃ ceramics,the conception "compulsive charge compensation mechanism" is proposed in this thesis,which means a factitious charge compensation method is introduced in the chemical formula for ceramics preparation.Analysis shows that Ti ion has a valence of four in Nd doped SrTiO₃ ceramics and the chemical state of Ti ion do not change.For the NST and NTO ceramics prepared on the basis of "compulsive charge compensation mechanism",the charge compensation mechanism is conducted according to the compulsive mode,i.e.,the charge compensation mechanism in NST ceramics is Sr vacancy,while in NTO ceramics Ti vacancy.However,for the NSTO ceramics prepared in the light of Sr ion equimolar substituted by Nd ion,the "self charge compensation" mode is both Sr vacancy and Ti vacancy.In Nd doped SrTiO₃ ceramics,the dielectric properties show close relationship with defect structure and oxygen vacancy in each ceramics system.The coupling of defects and electrons which produced by first-ionization and second-ionization of oxygen vacancy has close relationship with the dielectric relaxation properties.The dielectric relaxation is also attributed to the movement of the defect association composed of different defects.