| High temperature piezoelectric single crystals play an important role in the aerospace, nuclear energy, oil exploration and chemical industries. The currently commercialized crystals, for example crystal quartz and LiNbO3 (LN), hardly meet the requirements for high temperature piezoelectric application since the fact that the crystals don’t possess satisfying high temperature dielectric and piezoelectric properties. Of the studied piezoelectric crystals to date, the langasite (La3Ga5SiO14, LGS) family crystals which possess high melt points (1450℃-1510℃), no phase transformation, proper hardness and ease of growth, become an important candidate for high temperature piezoelectric applications. Among the langasite type crystals, the structural ordered Ca3TaGa3Si2O14 (CTGS) crystals were found to possess even higher electrical resistivity and mechanical quality factor, implying their prospects for sensing at elevated temperatures. In this dissertation, the vertical Bridgman growth of CTGS crystals was studied, the prototypes of the compressing and shear modes vibration sensors were fabricated and their properties were investigated. The main works and results are as follows:(1) Two-temperature-zone vertical Bridgman furnaces were designed and utilized to grow CTGS single crystals. By adjusting the position of MoSi2 heaters, suitable temperature gradients (10～20℃/cm) were obtained for single crystal growth.(2) By improving the preparation technology, pure CTGS polycrystalline raw materials were obtained. The critical crystal growth parameters were achieved to be: seeding temperature is 1420±2℃, growth velocity at seeding period is 0.25mm/h, and growth velocity after seeding is 0.35mm/h. High quality CTGS crystals with dimension of Φ25mm×80mm were successfully grown along the physical X direction, while the crystal quality of the CTGS crystals grown along other special directions, such as the Y-axis and the 30° rotated from Y to -Z axis, which is noted as Y(-30°) direction, need further improvement.(3) The quality of crystal was investigated by the high resolution X-ray diffraction. The rocking curves of different sections in the crystal shew sharp peak with FWHM<50", which implied that there were not twin boundaries and obvious lattice distortion in the crystal. The Energy Dispersive Spectrometer was applied to analyze ratio of elements in defects. The results shew that there are deficiency of Ca and enrichment of O in the defects. The X-ray photoelectron spectroscopy of crystal and impurity zones at the bottom shew that there was deficiency of Ga and enrichment of Ta in crystal at bottom. The impurity at bottom existed the deficiency of Ga, Si and the enrichment of O.(4) The resonance method and balanced bridge were used to characterize the dielectric constant, elastic compliance coefficient, electromechanical coupling coefficient and piezoelectric constant as function of temperature. The variation of temperature coefficient of resonance frequency for crystal grown along special direction were measured. The results indicated that the piezoelectric parameters of different sections in crystal were consistent and existed uniform tendency as temperature rising. The dielectric constant ε11T, electromechanical coupling coefficient k12, elastic compliance.s22E, and piezoelectric coefficient d12 increased as the temperature rising. The variations of k12 and d13 were not exceed 30% and 21% respectively. The dielectric loss displayed obvious rising after 600℃ (tanδ>0.05). There is a gap at dielectric properties at high temperature between the crystals grown by vertical Bridgman method with crystals grown by Czochralski method. The variation of dielectric constant of crystal grown by vertical Bridgman method reached to 12% at 900℃, while the crystal grown by Czochralski method exhibited more stable trend(the variation of dielectric constant<4%). Compared with the crystal grown by Czochralski method, the crystal grown by vertical Bridgman method displayed more quickly increase at dielectric loss after 600℃. The dielectric loss reached to 2.3 at 900℃, however, the dielectric loss of crystal grown by Czochralski method increased up to 0.8. The results implied that the crystal grown by Czochralski method possess more high quality. The resonance frequency of disk cut directly from crystal grown along special direction Y(-30°) was tested as the temperature rising. After fitting, the turnover temperature was ～250℃, and the TCF(1) and TCF(2) were 17ppm/℃ and -38ppb/℃2 respectively. The results was consistent with crystal grown by Czochralski method, which implied that the crystal grown by vertical Bridgman method was not deviate direction and the property of crystals grown by two methods was without distinction.(5) According to the electro-elastic property of CTGS, the vibration sensors with longitudinal and shear mode were designed and fabricated. The varying temperature vibration test system was set up to measure the property of vibration sensors. The results indicated that the two mode vibration sensors can function with the bandwidth 10-1000Hz as temperature rising from room temperature to 200℃. With the condition of 100Hz, The sensitivity of longitudinal mode vibration sensor was 0.36±0.03pC/g and displayed slightly rising tendency as the temperature rising. While the sensitivity of shear mode vibration sensor was 0.44±0.02pC/g, which didn’t show obvious fluctuation. At the condition of 500Hz and 1000Hz, the sensitivity of longitudinal mode vibration sensor were 0.36±0.03pC/g and 0.32±0.02pC/g respectively. The sensitivity of shear mode vibration sensor were 0.45±0.02pC/g and 0.46±0.02pC/g respectively. According to the relation of sensitivity, the variation of d11 as function of temperature aroused change of sensitivity of longitudinal vibration sensor. At the condition of 100Hz, the thermal sensitivity drift of longitudinal mode vibration sensor was 0.1%, while the thermal sensitivity drift of shear mode vibration sensor was 0.02%, which show stable sensitivity. In the temperature range, the maximum non-linear error of longitudinal and shear mode sensors reached to 3.52% and 3.16% respectively. The two mode sensors maintained stably intact signal output at the high temperature situation. So the range of acceleration can be confirm to 0～8g. |
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