Regulation Of Oxygen Vacancy Defects Of Lithium Niobate Powder And The Preparation Of High Density Ceramics

As a new generation of mechanical and electrical energy conversion materials,lead-free piezoelectric ceramics are a research hotspot in the field of functional ceramics.Alkaline metal niobate piezoelectric ceramics have attracted much attention due to their low dielectric constant,high frequency constant,and excellent piezoelectric properties.However,most research has focused on ceramic systems such as Na Nb O3,KNb O3,（K0.5Na0.5）Nb O3,（Li0.06Na0.94）Nb O3,and LiNbO₃ ceramics with high Curie temperature and spontaneous polarization strength are difficult to sinter At high temperatures,the loss of Li element during sintering makes it difficult to prepare highdensity ceramics,and there is limited research.Therefore,it is necessary to explore new methods that can solve the poor sintering activity of LiNbO₃ powder and achieve high densification at lower temperatures.Spark plasma sintering has the advantages of fast sintering speed and low sintering temperature,which can reduce the loss of alkali metal Li element in LiNbO₃ during the sintering process and is suitable for rapid densification of ceramics.Therefore,this study prepared stoichiometric LiNbO₃ powder through solid-state synthesis,and reduced LiNbO₃ powder with metal Al powder to introduce oxygen vacancy defects.Then,high-density LiNbO₃ ceramics were prepared using SPS sintering technology.The relationship between reduction temperature and oxygen vacancy concentration,as well as the effect of oxygen vacancies on crystal structure,was studied.The densification process of oxygen rich vacancy LiNbO₃ powder during SPS sintering was analyzed,the optimal sintering process parameters were determined,and the densification mechanism was explored.Finally,the LiNbO₃ ceramics sintered by SPS were annealed and oxygenated to obtain high-density LiNbO₃ ceramics with low oxygen vacancy defects.The conclusions drawn from the study are as follows:（1）As the reduction temperature increases,the oxygen vacancy concentration in LiNbO₃ powder first increases and then decreases.The oxygen vacancy concentration reaches its maximum at 700 ℃,and the second phase Li Nb O2 appears at 800 ℃,resulting in a decrease in the oxygen vacancy concentration in LiNbO₃ powder.The introduction of oxygen vacancies will weaken the vibration of the niobium oxygen octahedron,which is introduced near the Nb atom.（2）Using oxygen rich vacancy LiNbO₃ powder for SPS sintering,the relative density of LiNbO₃ ceramics shows a trend of first increasing and then decreasing with the increase of sintering temperature.At a sintering temperature of 950 ℃,the relative density reaches the highest of 98.62%.Continuing to increase the sintering temperature will cause the grain boundary migration rate to be greater than the pore migration rate during the sintering process,resulting in the formation of pores within the grains and a decrease in relative density.The optimal SPS sintering process was determined to be at a sintering temperature of 950 ℃,a heating rate of 25 ℃/min,an applied pressure of50 MPa,and a soaking time of 20 minutes.（3）The introduction of oxygen vacancies promotes SPS sintering.As the concentration of oxygen vacancies increases,the relative density of ceramics increases.During the sintering process,the diffusion of oxygen vacancies can drive the diffusion of cations,thereby promoting the migration of sintered materials.Moreover,oxygen vacancies also increase the conductivity of the powder,causing tip discharge in the micro areas between LiNbO₃ particles,promoting the sintering densification process.（4）White LiNbO₃ ceramics with a relative density of 99.44% were obtained under the reduction process of annealing at 800 ℃ and holding for 24 hours.After annealing and oxygenation treatment,the concentration of oxygen vacancies in LiNbO₃ sharply decreases,and O atoms fill the normal lattice positions,making the lattice structure of LiNbO₃ more complete.