| High temperature piezoelectric crystal as the core components of piezoelectric sensors have been widely used in military,petrochemical,production control,aerospace,oil,energy and other special areas.Traditional ceramic materials,such as lead zirconate titanate,have low temperature Curie point;the actual use of the maximum temperature is lower than 400 ?,which is less than half of Curie temperature point,due to heat activation aging and large losses.Commercial piezoelectric materials,mainly including quartz and lithium niobate,quartz have phase transition around 350?,relative low piezoelectric coefficients;lithium niobate crystal have relative high temperature ion conductivity,low resistivity,electromechanical performance and other reasons,the temperature of actual use is below 500?,it is difficult to meet the high temperature applications.Rare earth oxyborate crystal ReCa4O(BO3)3(Re:Y,Gd,Sm and other rare earth elements)have high melting point(13000C?1500?)and ordered langasite crystal Ca3TaGa3Si2O14(CTGS)are free of phase transition before melt point,can be grown by czochralski method,together with good piezoelectric,mechanical and electrical properties,showing its potential for high temperature application.Nowadays,there are still some shortcomings for high temperature piezoelectric sensing,and it is urgent to further optimize the performance:(1)The oxyborate ReCOB crystals have pyroelectric effect,which tends to interfere with the charge measurement,we need to measure its pyroelectric properties,to provide a reference for high temperature piezoelectric sensor design.Second,monoclinic crystal have more independent parameters,we need to reduce the error,due to variety of vibration modes,we need to optimize crystal cuts,to enhance the sensitivity of the sensor.The previous results show that there are some regularities in the oxyborate ReCOB crystals,and it is necessary to design the new crystal or new component to further optimize its high temperature piezoelectric performance.(2)There different reported datas about the electro-elastic properties of the langasite crystal CTGS,which affecte the application of this crystal.At the same time,there are few paper bout the high temperature elastic properties.(3)Ca2Nb2O7(CN)crystal has a high Curie point,about 1575?,can be growtn by czochralski method,it have been reported that the ceramic materials have relative high resistivity and electromechanical coupling factors,but its basic thermal properties,dielectric,elastic,piezoelectric and ferroelectric properties have not been reported yet.In the view of the current high-temperature piezoelectric crystal development problems,the main research work and the results are as following:I.Structure analysis,crystal growth,piezoelectric properties characterization and optimization of ReCOB oxyborate single crystal(1)The pure phase TmCOB polycrystals were synthesized by solid state reaction method.The suitable reaction temperature was 1100?1150?.ReCOB(Re:Tm?Y?Gd ? Sm)?YxGd1-xCa4O(BO3)3?LaxGd1-xCa4O(BO3)3?SmxY1-xCa4O(BO3)3 and TmCa4O(BO3)3 were grown by Czochralski method using the TDJ-J40 single crystal growth furnace,the defects in the crystal growth were analyzed,the crystal quality was characterized by high resolution X-ray instrument.(2)The basic thermal properties of TmCOB crystals were studied,including specific heat,thermal diffusion coefficient,thermal expansion coefficient,thermal conductivity,and compared with the same type of materials.(3)The full set of the elastic constant of the m point group YCOB and TmCOB have been investigated,including the dielectric constants,the elastic constants,the piezoelectric coefficients and the electromechanical coupling coefficients,and the piezoelectric vibration modes were analyzed.Some vibration modes are optimized for sensor design.The temperature dependence of dielectric constant,dielectric loss,elastic constants and piezoelectric coefficients of YCOB and TmCOB crystals was measured by high temperature test platform.At 900?,the TmCOB piezoelectric crystal has a high resistivity ?11 about 6 × 107? · cm and a lower dielectric loss tan?22 of less than 15%.While the TmCOB piezoelectric coefficiients d26 is 7.8pC/N,about three times of quartz crystal,its variation is less than 12.9%,showing the advantages for high temperature piezoelectric sensor.(4)The pyroelectric properties of ReCOB crystals were investigated from 20 to 180? by charge integral method.And the distributions of the pyroelectric coefficients in the space were calculated.The pyroelectric coefficients of the ReCOB series crsytal at room temperature were between-59.3 to-65.5?C/(m2·?),and the absolute value of the pyroelectric coefficient decreases with the increase of temperature.ReCOB crystals have high detection value of Fd,about 7.6?11.4 ×10-5Pa-1/2,about twice of the traditional pyroelectric crystal TGS and LiTaO3,indicating that the ReCOB crystal were expected to be used for infrared detection applications.?.Crystal growth and optimiezed piezoelectric properties of CTGSThe growth process of CTGS crystal was studied,and the stable growth parameters were obtained,and the thermal field was improved.For the first time,the full set of electro-elastic properties of the 32-point group crystal CTGS was measured by the resonance method combined with ultrasonic method.The dielectric constant,the elastic constant,the piezoelectric coefficient and the electromechanical coupling factors were tested,and the self-consistency have been analyzed.The relationship between acoustic velocity and elastic stiffness constants were obtained by deriving the Christopher equation.The relative permittivity ?11/?0 is relative stable from RT to 900? with the variation less than 5%,fot the ?33/?0 the variation is 8.0%.At 600?,the dielectric loss of the CTGS crystal is 1.6%,which is two orders of magnitude(150%)smaller than the same series of LGS crystals.The relative dielectric permittivities were determined to be?11/?0=17.5,?33/?0=21.8,and the piezoelectric coefficients were verified to be on the order of d11=4.17pC/N and d14=-13.7pC/N.The elastic stiffness constants were obtained to be c11=15.63,C12=7.52,C13=8.55,C14=0.051,C33= 22.51,c44=4.15 and C66=4.06×1010 N/m2,respectively.The couplings of piezoelectric vibration mode were analyzed by Matlab program,and the optimized vibration modes of CTGS crystal were obtained.Based on the achieved material constants,the lateral field excitation piezoelectric coupling factors and the acoustic wave phase velocity are calculated,and then the cut of pure-LFE for CTGS crystal is attained.By testing the impedance curve of the devices,the validity of the cut is verified.The experimental results agree with the theoretical prediction well.?.Growth and piezoelectric properties of CN crystalCN crystals were grown in different directions and high quality single crystal materials were obtained,the crystal quality was characterized by high resolution diffractometer.The thermal properties of CN were studied.The thermal expansion coefficients were ?11=5.83??22=7.94??33=13.32×10-6/?,?33/?11=2.28.The specific heat?thermal diffusion coefficient?thermal conductivity coefficient were 3.52 W/(m·?)?1.332 mm2/s and 2.64 J/(g·?),respectively,at room temperature.The thermal diffusion coefficient decreases with increasing temperature,shows the variation about-33.2%at 600?.The thermal conductivity coefficient is 2.32 W/(m·?)at 600?.The specific heat show relative stable properties with variation less than 20%.We have found that the CN crystals belong to the orthorhombic system,space group Pn21a,a=26.471A,b = 5.49A,c = 7.688A,and the unit cell volume is 1117.2 A3.The crystal structure of the CN crystal is analyzed.The crystal density is 4.494 g/cm3.The distortion of the anionic groups in the CN crystal was calculated.The relationship between the piezoelectric properties and the structure was analyzed.The CN crystal has a layered structure,and the distorted perovskite octahedral structure is connected by a common angle form along the[001]direction,while the perovskite-like flat plate is stacked along the(001)plane.The main dissociation surfaces were(100)and(401)planes,because of higher bulk density,(100)and(401)planes have lower surface energy of the crystal;the low surface makes the crystal face easier to form and crack.Through reducing the thermal stress,we successful grown the high-quality single crystal without dissociation.The dipole momen of the NbO6 octahedron are calculated.The value of the dipole moment of the niobium-oxygen polyhedral can be obtained from the bond length and bond angle of the octahedron.The single niobium oxide polyhedral dipole moment Nb1O6,Nb2O6,Nb3O6,Nb4O6 were 3.9809,3.5934,1.2072,2.7085 debye,respectively.At the same time for the niobium oxide octahedral corresponding to the total cell dipole moment were 0.028435,0,025668,0.008623,and 0.019347 debye,respectively.We selected the Z-cut cut to check the chemical stability,corroded with hydrogen fluoride(30%),+Z direction seems have more corrosion pits than-Z direction,using the optical microscope,we confirmed the existence of twins,belonging to the natural perfect match.The elastic constants of CN crystals are very cdose to Sr2Nb2O7,where s11=6.68 pm2/N,which is slightly larger than Sr2Nb2O7(5.3 pm2/N),s22=9.2 pm2/N,slightly larger than 9.4 pm2/N.The dielectric constants,elastic constant and piezoelectric coefficient of CN crystal were characterized.CN crystal has a large relative permittivity constant,its?22 is 45.4,and its electromechanical coupling coefficient k33 is 19.2%,which is comparable to YCOB,which indicating that CN crystal is a potential piezoelectric crystal.CN crystal length vibration mode d31 piezoelectric coefficient is 5.8 pC/N,about twice of d32,the longitudinal vibration mode d33 =8.5 pC/N,about 3.7 times the ?-SiO2,indicating the advantages for piezoelectric sensor.The spontaneous polarization and the coercive field were 6.5?C cm-2 and 110kv/cm,respectively,and the test voltage was 170kv/cm.The filling factor in the perovskite structure is calculated to be about 0.832,indicating that the Ca element has a larger moving space... |
PDF Full Text Request