No abstract

We present a relationship between piezoelectric property and crystal structure in melilite-type crystals. Two independent piezoelectric constants, d 14 and d 36 , increase and decrease with the polyhedral distortion, respectively. Based on this knowledge, single crystals made of strontium-substituted calcium magnesium silicate Ca 2 MgSi 2 O 7 (CMS) have been submitted to the Czochralski method, which made them grown. The CMS crystal has a high d 36 ; however, its transition temperature is around 85°C. Therefore, the strontium substitution is conducted to decrease this phase transition temperature. A single crystal of strontium substituted CMS, with nominal chemical composition Ca 1.7 Sr 0.3 MgSi 2 O 7 (CSMS30), shows no phase transition when temperature rises from room to melting temperature. Its piezoelectric d¤ 31 constant is 2.1 pC/N and its compressive strength is 830 MPa on crystal substrate with (ZXt)45°cut. The CSMS30 single crystal is a superior candidate material for pressure sensors at high operating temperatures.

Ba 3 TaGa 3 Si 2 O 14 (BTGS) bulk single crystals were grown by the Czochralski method. The crystal structure of BTGS has been refined using single-crystal X-ray diffraction data with a precision corresponding to an R index of 0.018. The crystal structure is isostructural to La 3 Ga 5 SiO 14 which has the trigonal space group P321 and Z = 1, and the distribution of each cation is ordered in each site. Material constants and resistivity of the crystal were measured up to 550 °C. The results indicate that the BTGS crystal is a good candidate for piezoelectric applications in elevated temperatures.

We present a possible method to reduce the anisotropy of the thermal stress generated on langasite-type La3Ta0.5Ga5.5O14 (LTG) piezoelectric crystals arising from the mismatch of the thermal expansion coefficients and Young’s moduli of the crystals and metals at high temperatures. To formulate this method, the thermal stresses of order-type langasite crystals, in which each cation site is occupied by one element only, were calculated and compared to each other. Our results suggest that the largest cation site affects the thermal stress. We attempted to replace La3+ in LTG by a larger ion and considered Sr2+. Single crystals of strontium-substituted LTG (Sr-LTG) were grown using the Czochralski method. The thermal stress along the crystallographic c-axis decreased but that perpendicular to the c-axis increased by strontium substitution into the LTG crystal. The anisotropic thermal stress was reduced effectively. The Sr-LTG single crystal is a superior candidate material for pressure sensors usable at high temperatures.

The effects of strontium substitution on the electrical properties of La 3 Ta 0.5 Ga 5.5 O 14 (LTG) are reported. Strontium substituted LTG (Sr-LTG) with a nominal chemical composition La 2.9 Sr 0.1 Ta 0.55 Ga 5.45 O 14 crystals were grown using the conventional Czochralski method under high and low oxygen partial pressures. The electrical properties and thermal behaviors of the Sr-LTG crystals were also compared with those of pure crystals. Sr substitution was found to affect the electroacoustic constants and thermal expansion coefficient clearly. These characteristics are discussed from crystallographic perspectives. The Sr-LTG crystals grown at low oxygen partial pressures exhibited higher electrical resistivity µ for pressure sensors used at high temperatures.