Study On The Polycrystallization Of Y-Cut Surface Of Lithium Niobate Crystal Under External Field

Lithium niobate?LiNbO₃,LN?is a synthetic crystal which is called"optical silicon".In the three crystal planes of X-Cut,Y-Cut and Z-Cut,Y-Cut of LN,whose Miller indices is?100?,has the lowest hardness,the smallest modulus of elasticity,and the widest range of applications.However,in the process of grinding,because of the soft and brittle nature of lithium niobate itself,it is very difficult to obtain ultra-thin wafers with sub-nanometer surface roughness,which leads to the extremely easy breakage of lithium niobate during the grinding processing and the efficiency is very low.The lithium niobate crystal is affected by its own thermoelectric effect and piezoelectric effect during the processing.Therefore,the external field effect is added during the grinding processing to explore the influence factors of the structural change,which is very important to enrich the corresponding grinding processing theory.In this paper,the Y-Cut of lithium niobate crystal was used as the research object,and the mechanical grinding was carried out under the condition of external temperature field and electric field.After the thickness of LN wafer was reduced from 200?m to 80?m,the microstructure changes were characterized and related simulation software was used to predict the mechanism of the change,which provided experimental and theoretical basis for obtaining ultra-thin lithium niobate wafers.The main research contents are as follows:?1?The crystal structure change of the LN Y-Cut whose thickness reduced from 200?m to 80?m with mechanical grinding in the different temperature field and different electric field was analyzed.It was found from the results of characterization that the ratio of Li/Nb of the sample decreased gradually and the fragmentation area of the sample became larger as the grinding temperature increased,and the fragmentation was concentrated at the edge of the wear scar.The samples all showed single crystal state at 45?and 60?,but the sample exhibited polycrystalline state at75?.Under the action of electric field,it was found that the atomic ratio of Li/Nb of the sample decreased first and then increased as the electric strength increased,and the fragmentation area changed as same as it,the fragmentation part was concentrated at the the edge of the wear scar.under the electric field of 2×10⁵ V/m,the sample showed single crystal state,and the sample exhibited polycrystalline state under the electric fields of 1×10⁵V/m and 3×10⁵V/m.?2?The Forcite module of Materials Studio was used to simulate the molecular dynamics of lithium niobate model with corresponding Li vacancies at different temperatures field.From the change of the mean square displacement curve and the Mulliken's population,it was found the higher the temperature,the more unstable the crystal structure.However,it was also found that the effect of the vacancy rate on the crystal structure was stronger than that of the temperature change.The CASTEP module in Materials Studio was used to simulate the molecular dynamics of the lithium niobate model with corresponding Li vacancies at different electric field.The results of mean square displacement and radial distribution function showed that the stability of the crystal increased first and then decreased with the increasing of electric field strength.In other words,certain electric field strength can enhance the stability of the crystal.?3?COMOSOL Multiphysics software was used to simulate the stress and strain generated by lithium niobate wafers under different temperatures and different electric fields.The results showed that the higher the temperature,the greater the strain displacement and stress generated by the wafer.As the electric field strength increased,the strain displacement and stress generated by the wafer first decreased and then increased,which means it had a minimum value at the electric field of 2×10⁵ V/m.The above results also illustrated that the electric field with 2×10⁵ V/m can suppress the fragmentation of the wafer to some extent.This study explored the reason why the lithium niobate wafer was fragile during the grinding process.Through the results of microstructural characterization,it was found that polycrystallization was the main cause of crystal fragility.Through the molecular dynamics simulation of the crystal,it was verified that the stability of the crystal was reduced after being ground and the disorder was enhanced by the external field.Through the multiphysics coupling analysis of the wafer model,it was verified that the internal force of the wafer was uneven during the grinding process,which was mainly concentrated in the edge part.The results of computation and simulation were consistent with the characterization results.This work may contribute a lot in the grinding field of lithium niobate.