| contribute to the piezoelectric coefficients d14 and d36, respectively.(2) The orientation dependences of piezoelectric coefficients were studied in double-rotated coordinates. Take CaNdGa3O7 for example, d12*, d13* and d33* obtained highest values of 4.5,4.7,3.9 pC/N, with the corresponding cuts of XYtw46°/1°,XZlt68°/46°, ZXtl46/56°,respectively.(3) The high temperature resistivities of Melilite crystals were measured. The factors that affect the resistivities were discussed from the aspect of crystal structure. The layered crystal structures against the charge transportation and as a result, increasing the resistivity. For another, the disordered structure induced neutron scattering, and thus, reducing the resistivity of the crystal.(4) The temperature dependences of dielectric, elastic and piezoelectric properties of Melilite-family crystals were investigated. The results show that SrNdGa3O7 has high resistivity (9.2×106Ohm·cm at 600 ℃), excellent piezoelectric coefficient (d14=9.4 pC/N) and high thermal stability of elastic and piezoelectric constants (the variation is less than 3%), which makes it a good candidate for piezoelectric sensing applications at high temperatures.5. The Electrical and Elastic Properties of Sillenite-type Crystals(1) Design proper piezoelectric cuts, the electromechanical properties of Sillenite-type Bi12TiO20 (BTO) and Nd0.06Bi11.94SiO20 (BSO) crystals were investigated, with relative dielectric permittivities ε11T/ε0, piezoelectric coefficient d14 and mechanical coupling factor k14 were measured to be 48, 42.8-47.7 pC/N, and 32.8%-36.3%, respectively.(2) The orientation dependence of piezoelectric d33 was studied in double-rotated coordinates, with the highest d33 obtained from ZXtl45°/54°cuts. The experiments show that the measured d33 values matches well with the theoretical values for BSO and BTO.(3) The temperature dependence of electromechanical properties of BTO and BSO were studied in the temperature range of 25-350 ℃ and 25-500 ℃, respectively, with the variations of piezoelectric coefficients less than 6%, showing BTO and BSO crystals are potential candidates for electromechanical applications in medium and high temperature range.Piezoelectric materials have important applications in aerospace, space exploration, oil exploration and nuclear safety inspections. At present, a-SiO2、GaPO4、LiTaO3、 La3Ga5SiO14 (LGS)-family and ReCa4O(BO3)4-family crystals are the mainly piezoelectric crystals studied in domestic and international which were used for sensors. However, because of the low transformation temperature, low resistivity, low piezoelectric coefficient, high dielectric loss, or low crystal symmetry etc., it is difficult to meet the needs of high-temperature piezoelectric applications. Therefore, continuous efforts are need to make to explore piezoelectric crystals which have large piezoelectric sensitivity, high thermal stability, small noise signal and high symmetry.In order to explore high temperature piezoelectric crystals used for high temperature sensors, we grown Fresnoite (Ba2TiSi207) and Melilite-family (Ca2MgSi2O7、Ca2Ga2SiO7、CaNdGa3O7、SrNdGa3O7) crystals. The electrical properties of Fresnoite, Melilite-family and Sillenite-type (Bi12TiO20, BTO; Nd0.06Bi11.94SiO20, BSO) crystals were systematically investigated. The main contents and conclusions are as following:1. Crystal Growth(1) Based on the introduction of equipment and Czochralski method, combining the characteristics of fresnotie and melilite crystals, the synthesis of polycrystalline materials, the process of crystal growth and the annealing process after the completion of crystal growth were determined. Using JTL-400B single-crystal growth furnace, large size of Fresnoite Ba2TiSi2O7 (with size of φ25×50 mm3) crystals and Melilite-family Ca2MgSi2O7、Ca2Ga2SiO7. CaNdGa3O7、SrNdGa3O7 (with size of φ25×40 mm3) crystals were grown by the Czochralski method.(2) The crystal phase and structure were investigated by High-resolution X-ray diffraction, Jade 10 and VESTA software. We found that Ba2TiSi2O7 with Fresnoite structure belongs to P4bm space group. The crystal structure of Ba2TiSi2O8 was analyzed, consisting of SiO4 tetrahedra and TiO5 square pyramids which are linked to give flat sheets parallel to the{001} plane. The sheets are linked together by Ba2+ ions. The bonds within the sheet (Ti-O and Si-O) are predominantly covalent, and the inter-sheet bonds (Ba-O) are mainly ionic.(3) The crystal phase and structureof melilite crystals were investigated. We found that Ca2MgSi2O7、Ca2Ga2SiO7、CaNdGa3O7 and SrNdGa3O7 crystals with Melilite structure belong to p421m space group. All the crystals have the same structure. Take Ca2Ga2Si07 for example, it consisted of regular GaO4 tetrahedrons and corner-sharing irregular tetrahedrons (GaO4/SiO4) forming the tetrahedral layers in theXY plane, while the layers are linked together along the Z-axis by Ca cations on eight-fold coordinate sites.2. The Thermal Properties(1) The thermal properties, including specific heat, thermal expansion, thermal conductivity and thermal diffusivity were investigated. The specific heat was measured using a simultaneous thermal analyzer (Perkin Elmer Diamond: DSC) and we found that the specific heat of all the grown crystals increased with increasing temperature. The thermal expansion coefficients were measured using a thermal mechanical analyzer (PerkinElmer model:Diamond TMA) in the temperature range of 30-300 ℃, the results show that the crystal enlongated with the increasing temperature. The thermal diffusivity was measured by the laser flash method using Laser flash 44 apparatus (NETZSCH LFA 447 and 457 Nanoflash). The temperature dependence of thermal conductivity were calculated based on thermal diffusivity and specific heat, and the thermal conductivity along X-direction were found to be larger than that of along Z-direction, which were caused by the layered structure that act against the heat transportation.(2) Using the Archimedes method, the density of Ba2TiSi2O7、Ca2MgSi2O7、 Ca2Ga2SiO7、CaNdGa3O7 and SrNdGa3O7 were measured to be 4.47、2.96、3.80、 5.14 ' 5.47 g/cm3, respectively. Based on the temperature dependence of thermal expansion, the temperature dependence of density was calculated and we found that the density decreased with increasing temperature.3. The Electrical and Elastic Properties of Fresnoite Crystals(1) The stress-strain, piezoelectric equations, and the piezoelectric properties, including the elastic constants, piezoelectric constants, electromechanical coupling coefficients were introduced. The testing methods, piezoelectric vibration modes and piezoelectric cuts were also introduced.(2) The electromechanical properties of Ba2TiSi2O7 were measured, with relative dielectric permittivities ε11T/ε0 and ε33T/ε0 were found to be 16.5 and 10.8, the piezoelectric coefficients d15 、d31 and d33 were 17.5、2.7 and 4.0 pC/N, respectively. The orientation dependence of longitudinal piezoelectric coefficient d33* were investigated, when the rotation angle equals to 50°, d33** obtained the highest value of 8.7 pC/N。(3) The resistivities of Ba2TiSi2O7 at high temperature were measured, with values at 600 ℃ along X-, Z- and Z*-directions were found to be 1.9×107 Ohm·cm, 3.6 ×109 Ohm·cm and 2.1 ×107 Ohm·cm, respectively. The layered structure act against the charge transportation, so the resistivity along Z-axis is larger than that along X-axis.(4) The temperature dependence of electromechanical properties of Ba2TiSi2O7 were investigated, d15 and d33* show high thermal stability (with variations of 6% and 4.5% in the temperature range of 25-700 ℃). With the high resistivity, high piezoelectric and coupling factor value, all the excellent properties make fresnoite BTS crystals promising candidates for high temperature sensing applications.4. The Electrical and Elastic Properties of Melilite-type Crystals(1) The electromechanical properties of Ca2MgSi2O7、Ca2Ga2SiO7、CaNdGa3O7 and SrNdGa3O7 were investigated, with relative dielectric permittivities ε11T/ε0, piezoelectric coefficient d14 and mechanical coupling factor k14 were measured to be 11.7-16.8,5.3-9.4 pC/N, and 8.0-16.3%, respectively. The relationship between microstructure and electromechanical properties were established, and the distortion of eight-fold antiprisms and the distortion/rotation of tetrahedrons... |
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