Thermal and piezoelectric properties of La3Ta0.5Ga5.1Al0.4O14 (LTGA) for high temperature sensors

Abstract

La3Ta0.5Ga5.5-xAlxO14 (LTGA) is a piezoelectric crystal from the langasite family which presents advantageous properties for high temperature sensor applications. It possesses a high piezoelectric constant and its structure remains stable up to 1500 °C. In this work, a LTGA single crystal has been grown by using the Czochralski technique under N2 atmosphere with 1% O2. The high crystalline quality of grown crystal is confirmed by the 26 arcsec FWHM of X-ray rocking curves. High temperature X-ray diffraction measurements indicate the absence of phase transitions up to 1300 °C. The thermal properties of the crystal, including thermal expansion, specific heat, thermal diffusivity and thermal conductivity, are investigated as a function of temperature. The average thermal expansion coefficients are αa = 8.48 × 10−6 K−1 and αc = 6.32 × 10−6 K−1. The thermal conductivities along the a and c axes at 50 °C are 1.2 and 2.0 W m−1 K−1, respectively. The resistivity of grown crystal decreases from 4 × 1010 to 2 × 105 Ω cm between 200 and 900 °C. The measured piezoelectric coefficient d11, electromechanical coupling factor k12 and corresponding mechanical quality factor Q at room temperature are 6.94 pC/N, 17% and 12,000, respectively.