Preparation And Dielectric Properties Of Lithium And Niobium Co-Doped Strontium Titanate Ceramics

Due to their great application potential in many fields such as modern electronics,sensors,energy storage and multifunctional devices,materials with high dielectric constant have received extensive attention.At present,there are both opportunities and challenges in the field of high dielectric constant materials.How to prepare high-performance dielectric materials with high dielectric constant,low dielectric loss and high frequency and temperature stability is an urgent problem to be solved.In this paper,Sr Ti O₃-based ceramics were studied,and high-performance dielectric ceramics were prepared by（Li,Nb）Co-doped,and their microstructure and dielectric properties were studied.In the follow-up experiments,different annealing processes were used to improve the dielectric properties of Sr Ti O₃-based ceramic system.Finally,high-performance dielectric ceramic materials with high dielectric constant,low dielectric loss and high frequency and temperature stability were obtained.Sr(Li_1/4 Nb_3/4)x Ti_1-xO₃（0.000≤x≤0.04）（STLN）ceramics were synthesized by the traditional solid phase reaction.The co-doped of Li⁺and Nb⁵⁺significantly improves the dielectric properties of Sr Ti O₃ ceramics.When the co-doped concentration x reaches 0.012,the dielectric properties of the ceramic sample are enhanced with the increase of the concentration;when the co-doped concentration x reaches 0.012,the dielectric properties of the ceramic sample begin to decline.When the co-doped concentration x=0.012,the dielectric constant of STLN ceramics is1.73×10⁴（1 k Hz）,the dielectric loss is 0.017（1 k Hz）with excellent frequency（20-10⁶Hz）and temperature（room temperature-450°C）stability.Their dielectric relaxation mechanism and point defect structure were studied.When the co-doped concentration x value is too high,the formation of point defects is inhibited,leading to the deterioration of dielectric properties of STLN ceramics.Further studies showed that acceptor doping Li⁺and N₂ atmosphere sintering provided a certain concentration of oxygen vacancy.The oxygen vacancy helps to form some defects related to the oxygen vacancy dipole-dipole and defect clusters(Ti_Ti^′-V_O^?··-Ti_Ti^′,Nb_TI^·-Ti_Ti^′).These defect-dipoles and defect dipole clusters can limit the long-range movement of non-localized electrons and ultimately localized electrons to enhance local polarization.Therefore,the sample exhibits great dielectric constant and low dielectric loss.In addition,the results show that higher grain boundary resistance also contributes to the reduction of dielectric loss.Annealing process is also beneficial to improve the dielectric properties of ceramic materials.Therefore,based on the defect structure regulation of Sr(Li_1/4Nb_3/4)_0.012Ti_0.988O₃ ceramics,a novel annealing strategy is proposed to improve the dielectric properties of ceramic samples.T The strategy is composed of two annealing steps under different atmospheres,each of the two steps under either nitrogen or air for 1 h.The results show that the dielectric properties of annealed samples are optimized in both nitrogen and air atmospheres.At room temperature,the dielectric constant value of Sr(Li_1/4Nb_3/4)_0.012Ti_0.988O₃ ceramic sample annealed in nitrogen atmosphere and then in air atmosphere is 1.96×10⁴（1 k Hz）,and the dielectric loss is0.020（1 k Hz）.In addition,the sample show significant temperature and frequency stability in the temperature range from room temperature to 500℃.Further studies show that annealing in different atmospheres has a significant effect on the structure of the defects,especially on the oxygen vacancy.Oxygen vacancies with appropriate concentration can ionize electrons,which are fixed by defect clusters(Ti_Ti^′-V_O^?··-Ti_Ti^′)to form a large number of defect dipoles,so as to obtain a large dielectric constant with very low dielectric loss.But too much oxygen vacancy will lead to high dielectric loss,which is not conducive to the application of dielectric materials.The adjustment of defect structure is the reason for the improvement of comprehensive dielectric properties.This work not only provides an annealing strategy to optimize dielectric properties of materials,but also helps subsequent researchers to better understand the principle of dielectric properties of materials and the relationship between defect structure and dielectric properties of materials.