BIBO And BZBO Crystals:Kyropoulos Method Growth And Electro-elastic Properties

Borate crystals have always been the research hotspot of inorganic functional crystal materials,and more than 1000 kinds of borate crystals have been developed.To develop new applications of old borate crystals,and new properties of new borate crystals,bismuth borate crystals have been selected as the research objects.In this thesis,?-BiB3O6(BIBO),Bi2ZnB2O7(BZBO)and Nd:BZBO crystals were grown by Kyropoulos method.The thermal properties and optical properties were measured and characterized.In order to explore the potential piezoelectric application,the electro-elastic properties and temperature dependence behaviors of BIBO and BZBO crystals were studied systematically.Therefore,the vibration modes were discussed,large piezoelectric constants and pure vibration mode were designed for piezoelectric transducer used for photoacoustic gas detection.Main contents and conclusions are as following:(?)Crystal growth(1)The ?-BiB3O6(BIBO),Bi2ZnB2O7(BZBO)and Nd:BZBO crystals were grown by Kyropoulos method.The crystal growth technique had been optimized by studying the synthesis of polycrystalline materials,exploration of the flux system,adjustment of thermal field etc.The high quality and large size of as-grown crystals were achieved in a short time.(2)Crystal quality was characterized by using High Resolution X-Ray Diffraction(HRXRD)measurement.The rocking curve had a sharp and symmetrical shape with the full width at half maximum,which indicated the high quality of the as-grown crystals.The high quality crystals established the foundation for the research on nonlinear optical and laser properties.(3)The defect parts with brown and inclusion areas on the sides of the BIBO crystals were studied by using the X-ray Photoelectron spectroscopy.The causes of the defects were analyzed by analyzing the peak positions and contents,which provided the reference direction for crystal growth.(?)Physical properties(1)The X-ray diffractometer was used to determine the crystal phase and the cell parameters.By the X-ray Fluorescence method,Nd3+ concentrations in all doped crystals were determined.(2)Thermal properties of BIBO and BZBO crystals were measured.BIBO and BZBO crystals are congtuent melting compounds,and the melting points are 718and 698,respectively.The specific heat of BIBO and BZBO crystals was 0.48 and 0.62 J·g-1·K-1 at room temperature.The specific heat was increased with increasing temperature,the larger specific heat means that the crystal can possess higher laser damaged threshold.The thermal expansion properties of BIBO and BZBO crystals showed great difference.BIBO crystal had strong thermal expansion anisotropy,the coefficients ?11 and ?22(a11=7.3×10-6 K-1,a22=44.1×10-6 K-1)exhibited a positive vibration,while the a33 showed a negative change(?33=-21.7×10-6 K-1).The zero expansion property was studied based on the thermal expansion property.While for BZBO crystal,the thermal expansion coefficients ?11,?22 and ?33 were 6.9×10-6,11.2×10-6 and 1.9×10-6 K-1,respectively.And all the thermal expansion exhibited positive vibration with temperature increased.In addition,BIBO and BZBO had high thermal conductivity.They were 11.583.4.775 and 6.954 W·m-1·K-1 for BIBO and 4.3,3.2 and 4.0 W·m-1·K-1 for BZBO crystal,which were suitable for high power laser.(3)The optical properties of BIBO,BZBO and Nd:BZBO crystals were measured.The UV-vis diffuser reflectance indicated that the absorption edges of BIBO,BZBO and Nd:BZBO crystals were 315 nm,358 nm and 365 nm,respectively.The transmission spectra from 300 nm to 2700 nm showed that the as-grown crystals possessed high transmittance(70%),which indicated the high quality of the crystals.The absorption and fluorescence spectra of Nd:BZBO crystals were investigated at room temperature,and the characteristic absorption and emission peaks were recorded.With the measured spectra,the continuous wave laser were realized at a wavelength of 1064nm for the Nd:BZBO crystal(0.5 mol%Nd3+),which indicated Nd:BZBO might be another kind of important laser and nonlinear optical crystal in the oxyborate family.(?)Electro-elastic properties of BIBO and BZBO crystals(1)The full set of dielectric,elastic,electromechanical and piezoelectric properties for BIBO and BZBO crystals were determined by using E49904A Impedance analyzer,capacitance determined and ultrasonic pulse echo method.The piezoelectric coefficient d22 for BIBO crystal was determined to be 40 pC/N,d32 for BZBO crystal was determined to be 40 pC/N at room temperature.(2)The macroscopic piezoelectric properties were qualitatively analyzed from the perspective of crystal structure.The contributions of the crystal symmetry space group to the piezoelectric property were analyzed from the viewpoint of polyhedral distortion and net dipole moment.The calculated results indicated that the BiO6 octahedron showed the larger polarization.However,the directions of the dipole moment for the two types of BiO6 octahedron in the BZBO crystal were nearly opposite.Therefore,the net dipole moment weakened.This occurrence might be the reason that the piezoelectric properties of the BZBO crystal were weaker than those of the BIBO crystal.(3)The temperature dependence of the electro-elastic properties for BIBO crystal was investigated over the temperature range of-150 ??150 ?.The BIBO crystal was found to possess low dielectric loss(<0.5%)and minimal variation of relative dielectric permittivities,with variations of ?11T/?0,?22T/?0 and ?33T/?0 were 9.4%,-9.2%and 4.5%.The elastic constants s?E on the primary axis showed a positive linear variation with the temperature increased.The S22E and S66E were little influenced by the temperature with the variations of 4.0%and 2.6%,the corresponding first order temperature coefficients were 0.91×10-4/? and 0.74×10-4/?,respectively.The piezoelectric constants with different vibration modes showed different trends with temperature increased.The longitudinal extensional vibration mode d22 had the maximum value,and the variation was 10.7%from-150 ? to 150 ?.The variation for length extensional vibration mode d21 was only 1.3%,which demonstrated good temperature stability.(4)The temperature dependent behaviors of the electro-elastic properties of BZBO crystals were characterized in the temperature range from room temperature to 500 ?.The variations of relative dielectric constants as a function of temperature were 20.9%,26.7%and 50.3%.The elastic compliances increased linearly with increasing temperature,showing positive temperature coefficients,with the variations were 8.05,7.8%,8.0%,8.1%and 7.5%,respectively.From room temperature to 500 ?,the largest piezoelectric coefficient d32 varied from 6.4 pC/N to 8.0 pC/N,with the first order temperature coefficients were 3.0×10-4/? The piezoelectric coefficieIts d31 and d24 increased with the temperature increased,which were 2.5 and 6.1 pC/N at room temperature,slightly increased to 1.5 and 5.9 pC/N at 500 ?,respectively.Notably,the piezoelectric coefficient d33 was small but remained constant at 1.1 pC/N at temperatures ranging from room temperature to 500 ?.In addition,the high temperature resistivity and dielectric permittivities for Nd:BZBO crystals were measured,and the values were reduced with the temperature increased.(?)Vibration modes analysis and the design for piezoelectric transducers(1)The radial extensional mode electromechanical coupling factors(kp)for BIBO and BZBO crystals with different orientations were deduced based on the internal energy and piezoelectric equations.Therefore,the finite element method and experiments were used to verify.The results showed that the kp for the Y-oriented BIBO disc was found to be 30.82%,while 5.8%for the Z-oriented BZBO disc.The relative errors were in 6%with the experimental results,which indicated the reliablity of the deduced formula.(2)The face shear vibration modes(the piezoelectric coefficients were d14,d25 and d36)of BIBO and BZBO crystals were discussed by finite element analysis.The pure face shear vibration modes were obtained by designing the appropriate side ratio Taking advantage of the anisotropy of crystal materials,the maximum piezoelectric constant were obtained.The optimal face shear mode piezoelectric crystal cuts were found to be Y/37° and Y/-530 for BIBO crystals,and Z/±45° for BZBO crystals,respectively.The optimal crystal cuts with high piezoelectric constans and pure vibration modes layed a foundation for the design of piezoelectric transducer.(3)The longitudinal extension vibration mode was studied and the corresponding electromechanical coupling factor(kt)was about 44%at room temperature.The ratio relationship was studied based on the finit element analysis.The pure vibration mode can be got when the ratio of diameter to thickness were 5:7.The BIBO based transducer was installed to pick up the acoustic signal generated by photoacoustic effect,the detection limit for SF6 reached 750 parts per quadrillion.A new application of BIBO crystals in piezoelectric field is developed.