Investigation Of Dielectric Behaviour And Energy Storage Properties For High Dielectric Constant Ceramics

Energy storage dielectrics are used to storage charge by generating polarisation effect under electric field,which can be applied to the storage and release of capacitor electrical energy,and are widely used in electronics and energy industries.With the lightweight and miniaturisation of electronic and power devices,the demand for high dielectric constant and high energy density dielectrics has become increasingly urgent,stimulating the research and development of dielectric ceramics.Therefore,this thesis systematically investigates the giant dielectric TiO₂-based and relaxation ferroelectric Bi_0.5Na_0.5TiO₂-based dielectric ceramic materials for different scales of applications,and analyses in detail their relaxation behaviours and dielectric mechanisms,as well as explores the dielectric enhancement and optimization mechanisms to improve the dielectric and energy storage properties of the ceramics with consideration of the applications,and to provide the basis for the development of new principles and new methods.The main research contents and conclusions are as follows:1.Co-doped TiO₂ ceramics with the composition of(Tm_0.5A_0.5)_xTi_1-xO₂（A=Ta and Nb）were successfully prepared using the conventional solid-phase method.The crystal structure,microscopic morphology,dielectric properties and dielectric mechanism of(Tm_0.5A_0.5)_xTi_1-xO₂ ceramics were systematically investigated.(Tm_0.5A_0.5)_xTi_1-xO₂（A=Ta and Nb）ceramics exhibit rutile structure and dense microstructure.The formation of defective dipoles is proved by XPS analysis.The ceramics have semi-conducting grains and insulating grain boundaries as shown by impedance spectroscopy analysis,proving the formation of internal barrier layer effect.The dielectric behaviour at different electrodes proves the existence of electrode effect in ceramics.Both ceramics have high dielectric properties（?>10⁴）due to defective dipoles,internal blocking layer and electrode response induced.And both have excellent temperature stability over a wide frequency/temperature range.2.Ta+Gd co-doped TiO₂ ceramics and single crystals were prepared by conventional solid-phase method and optical floating zone method,respectively.The ceramics and single crystals have the rutile crystal structure.The doping elements are uniformly distributed in the ceramics and single crystals.And the dielectric relaxation behaviour and impedance analysis show that the dielectric mechanism of the ceramics is not only related to the internal defect clusters,but also heavily influenced by the external interface.Huge dielectric constants（2.65?10⁴@1k Hz,2.37?10⁴@1MHz）and low dielectric losses（0.007@1k Hz,0.03@1MHz）were observed for ceramics with composition x=0.01 at 1k Hz.In addition,by eliminating the grain boundaries of the co-doped single crystals,we exclude the effect of grain boundary effect.The co-doped single crystals have very high dielectric constants（>10⁴）.Combined with the ceramic analysis,it can be seen that the co-dopant-induced defect clusters can significantly increase the dielectric constant;however,the defects do not completely localise the free electrons,and some of the free electrons can move inside the grains under the electric field.3.Ho³⁺can significantly enhance the grain boundary resistance of TiO₂,therefore,Ho³⁺was chosen to further prepare（Ta+Ho）co-doped TiO₂ ceramics,and its dielectric properties and polarization relaxation behaviour were investigated.From the analysis,it can be seen that（Ta+Ho）co-doped TiO₂ ceramic（x=0.01）shows high dielectric properties（ε?～3.35×10⁴,tanδ～0.021 at 1 k Hz）.The giant dielectric constant of the ceramics originates from the synergistic effect of defect clusters,grain boundaries and electrode interfaces.In order to further obtain high energy storage performance,(Ho_0.5Ta_0.5)_0.01Ti_0.99O₂-x Si O₂（x=0,1,3,5,and 7 wt%）ceramics were prepared by introducing Si O₂ glass phases at the grain boundaries of(Ho_0.5Ta_0.5)_0.01Ti_0.99O₂ ceramics through grain boundary engineering.The moderate amount of Si O₂ phase forms an insulating layer at the grain boundaries,which is conducive to achieving short-range electron migration,leading to a significant increase in breakdown field strength.The(Ho_0.5Ta_0.5)_0.01Ti_0.99O₂-5wt%Si O₂composite ceramics exhibit low dielectric loss（tanδ～0.012）,high dielectric constant（ε?～1.29×10⁴）,and enhanced energy storage properties（E_b～1.86k V/cm,W～1.97m J/cm³）at 1 k Hz.4.Bi_0.5Na_0.5TiO₃ ceramics with high energy storage density at low electric field were prepared by introducing composite ions（Nb+Al）⁴⁺ions and Ca²⁺ions.The microstructure,dielectric properties,energy storage and pulse charge/discharge performance of（1-x）Bi_0.5Na_0.5TiO₃-x Ca(Nb_0.5Al_0.5)O₃ ceramics were investigated.Notably,the 0.85Bi_0.5Na_0.5TiO₃-0.15Ca(Nb_0.5Al_0.5)O₃ ceramic exhibits high energy storage density(W_rec=4.41 J/cm³)and efficiency（η=88%）at low electric field（210k V/cm）.In addition,a high power density（P_d）of 49.8 WM/cm³ and a fast charge/discharge rate(t_0.9=61.2ns)can be achieved simultaneously.The excellent performance of the Pb-free 0.85Bi_0.5Na_0.5TiO₃-0.15Ca(Nb_0.5Al_0.5)O₃ ceramics from the P4bm polar nanodomains（PNRs）,enhanced bandgap,and refined grains in the modified non-homogeneous structure.5.（1-x）Bi_0.47Na_0.47Ba_0.06TiO₃-x CaTi_0.8Sn_0.2O₃ ceramics were prepared by introducing CaTi_0.8Sn_0.2O₃ into a Bi_0.47Na_0.47Ba_0.06TiO₃ matrix with a polycrystalline phase boundary（MPB）.By transforming the rhombic phase（R3c）into tetragonal phase（P4bm）,the ceramics exhibit domain structure of multiphase nanometres in the superparabolicstateatroomtemperature.Thelead-free0.75Bi_0.47Na_0.47Ba_0.06TiO₃-0.25CaT_i0.8Sn_0.2O₃ ceramics not only have excellent energy storage properties(W_rec=5.81 J/cm³,η=90.5%at 315 k V/cm)but also have good frequency/temperature stability（1-200Hz,20-200°C）and a fast discharge rate of72 ns(t_0.9).In addition,the 0.75Bi_0.47Na_0.47Ba_0.06TiO₃-0.25CaTi_0.8Sn_0.2O₃ sample has excellent dielectric constant thermal stability（Δε’/ε’25°C≤±15%at 1 k Hz）in the temperature range of-120°C～204°C,which meets the requirements of X9R-type capacitors.