Study On The Preparation And Properties Of Langatate And The Isostructural Crystals

Piezoelectric materials and devices are very important in modern social life and development of science and technology.They play an important role in the field of high tech and in economy.Piezoelectric crystals are the earliest used piezoelectric materials and are still dominant today.At present the crystals used widely in piezoelectric engineering areα-quartz and LiNbO₃.Quartz possesses zero frequency-temperature coefficient and thus with high frequency-temperature stability, but the small electromechanical coupling coefficients and large insert loss restrict the application of quartz in wideband filter.LiNbO₃ has high electromechanical coupling coefficients,however it does not have zero frequency-temperature coefficient orientation and it has low thermostability,which restrict its application.With the rapid development of electronic technology and communication,the researchers focus on the search for new piezoelectric materials.The aimed materials must be superior to quartz and LiNbO₃ and can be used in wideband filters and oscillators with high frequency-temperature stabilization.Recently researchers show great interest in the following three kinds of piezoelectric crystals:AlPO₄ and GaPO₄, A₃BC₃D₂O₁₄-type crystals,Li₂B₄O₇(LBO).A₃BC₃D₂O₁₄-type crystals belong to 32 crystal class,P321 space group.They have better piezoelectric properties than LGS crystal,zero temperature coefficient orientations,high Q value and low acoustic loss,no phase transformation between room temperature and melting temperature,low thermal expansion,and other excellent properties.Furthermore,they melt consistently and can be grown with Cz pulling method.In this thesis,many experiments were carried out to grow the La₃Ga_5.5Ta_0.5O₁₄ (LGT)crystals and their isostructural compounds using the Czochralski technology. The improved crystal growth technique was investigated and crystals with high quality were obtained.Their thermal,optical,mechanical,and electrical properties were measured and analyzed.The piezoelectric parameters,dielectric coefficients and elastic coefficients were mainly studied.Furthermore,the study on the growth and optical properties of the rare earth doped LGT crystals were carried out.New isostructural La₃Al_5.5Nb_0.5O₁₄(LAN),La₃Al_5.5Ta_0.5O₁₄(LAT)and RE³⁺:LAN(RE= Dy,Eu,Nd)nanocrystals were synthesized and their properties were studied.The main works were listed as follows:1.According to the stoichiometric ratio,LGT crystals with good quality were grown along c axis using suitable temperature field and grow parameters by the Czochralski technique.The crystals were transparent with the size ofφ30×35mm and no crack was observed.Based on traditional Cz pulling method,we adopted melting-point-holding technique and multi-step-pulling-out technique to grow LGT crystals and the temperature field was optimized by some measures.Melting-point-holding technique can reduce the volatilization of Gallium oxide obviously.At the end of the crystal growth procedure,using multi-step-pulling-out technique can reduce the thermal impact and thermal stress in crystals.The optimized temperature field improved the effect of heat preservation under iridium crucible and the status of temperature grads, helpful for improving the crystal quality.As a result,the crystals with high quality were obtained,which have better piezoelectric properties.2.The X-ray powder diffraction method was used to identify the structure of LGT crystal.The results showed that the crystal is pure LGT phase.The calculating results by using DICVOL91 program showed that a=0.82200±0.0021 nm, c=0.51217±0.0015 nm,V=0.29967nm³.Infrared spectra of LGT crystal were examined using a NEXUS 670 FTIR spectrophotometer.There were two absorption bands centered at 478 cm^-1and 666 cm^-1respectively,corresponding to the vibration of GaO₄ and GaO₆ radical groups.The transmittance spectrum of LGT crystal was recorded on a Hitachi U-3500 spectrophotometer in the range of 200-3200nm.We can find that the optical transmittance is about 80%from 450 to 3200 nm.There were two strong absorption peaks centered at 290 nm and 350 nm and two weak absorption peaks at 1868 nm and 2910 nm,respectively.The thermal expansion coefficients along x and z direction were measured from 20 to 500℃.It showed that LGT had small expansion coefficients and expansion anisotropy,which were beneficial to the crystal's application under high temperature. The expansion results also showed that the binding energy was high in LGT,which may have some influence on other properties of LGT crystals.The relationship between specific heat and temperature was determined from 20 to 500℃.It showed that the specific heat increased with temperature.The relationship between specific heat and temperature was nearly linear before 350℃and then get steady gradually.The specific heat was 0.3635J/g.K at 25℃and 0.4019J/g·K at 100℃respectively,greater than that of quartz,so LGT has high temperature stability than quartz.The density of LGT crystal was larger than LGS, and its hardness was also larger than LGS.3.The dielectric,piezoelectric and elastic properties of LGT crystals were studied in detail.We processed and measured many kinds of wafers with different orientations.The dielectric coefficients,the piezoelectric coefficients and the elastic coefficients of LGT crystals were determined by the LCR bridge and resonant-antiresonant method.It showed that d₁₁=+7.1×10^-12C/N,d₁₄=-5.1×10^-12C/N. The piezoelectric coefficients of LGT crystals are 3-7 times that of quartz,and they are also larger than LGS.At the same time,LGT crystal has low acoustic loss.So LGT crystals have large application potential in the field of piezoelectricity.It was discussed for the relationship between effective piezoelectric coefficient d′₁₂(θ)and rotation angleθfor(xyt)θcuts.The values of the effective piezoelectric coefficient change with the cutting orientation and the maximum value can be obtained withθin the first quadrant.For the LGT sample,it was 17.85°.A more strongly piezoelectric effect can be obtained by using the cuts with the maximum effective coefficients.The maximum piezoelectric effects can be applied on piezoelectric devices and films under the circumstance that the temperature effects need not be considered.4.Single crystals of Dy³⁺:LGT and Nd³⁺:LGT were grown along c-axis using the Czochralski technique.The structure,microhardness,absorption spectrum,and emission spectrum were measured.Dy³⁺:LGT crystals with the size ofΦ25×35 mm were grown.The segregation coefficient of Dy³⁺was measured to be 0.252.The structure of the crystal was studied by x-ray powder diffraction method and similar to undoped crystals.The cell parameters of Dy³⁺:LGT wore a=0.822030nm,c=0.512202 nm,V=0.29974 nm³. The mohs hardness of(001)plane was 6.5.It showed by fluorescence spectrum that the emission peak at 574 nm was the most intense under the excitation of 350nm light.Transparent Nd³⁺:LGT crystals with the size ofΦ22.5×29 mm wore also obtained.It can be seen from the absorption spectrum that there were strong absorption peaks at 587nm,746nm and 807nm.The fluorescence spectrum showed the emission peak at 1066 nm was the most intense under excitation of 595 nm light.5.Two new langatate-type compounds La₃Al_5.5M(M=Nb,Ta)_0.5O₁₄were synthesized for the first time by citrate sol-gel method.TG-DTA,XRD and FTIR wore employed to investigate the transformation process of gel to LAM powder.The results showed that,after the calcinations at 900℃,LAM nanocrystalline powder with a narrow particle size distribution was obtained,which has the same structure as LGN and LGT.The relationship between cell parameters and the content of A1 in La₃Ga_5.5-xAl_xNb_0.5O₁₄(LGAN)was determined.It showed that the cell parameters of LGAN reduce gradually with the increase of A1 content.The cell parameters of LAN wore a=0.8058nm,c=0.4977nm,V=0.2799nm³.The cell parameters of LAT were a=0.8127nm,c=0.4985nm,V=0.2851nm³.The differential scanning calorimetric (DSC)experiment showed that LAN had no melting point,so it might not melt consistently and can not be grown by Cz method.We think LAN crystal might be grown by other growth method,or be grown as LPE films.Because smaller aluminum ions substitute for gallium ions in the structure,LAM has a larger crystal lattice distortion.It results in larger vibrating frequency of radicals and larger polarization in the lattice under applied stress.So we may expect that LAM has the nearly same piezoelectric properties with LGN and LGT.In addition,LAM has the price advantage with no Ga₂O₃ content,so these kinds of compounds have tremendous potential valuables in the piezoelectric application.The same synthesis technique as LAM was used to synthesize rare-earth doped LAN nanocrystals.The results of XRPD showed they had the same structure with LAN.In the fluorescence spectrum of Dy³⁺:LAN,there were three emission peaks centered at 482nm,576nm and 666nm,which were due to the energy level transitions of ⁴F_9/2→⁶H_15/2,⁴F_9/2→⁶H_13/2and ⁴F_9/2→⁶H_11/2,respectively.The intensity of 576 nm peak was the most intense.The intensity of emission peaks increased with the content of Dy³⁺ions and it reached the maximum at 2mol%of Dy³⁺ions.In the excitation spectrum of Eu³⁺:LAN,there were two stronger emission peaks at 395nm and 465nm.The 395 nm light can excite several emission peaks at 580,588,594,614, 619,656,700 nm,and the 614 nm peak was the most intense.It was showed in the emission spectrum of Nd³⁺:LAN that there were two emission peaks.The peak at 1066 nm was the most intense under excitation of 808nm light.