High-temperature piezoelectric crystals such as langasite (LGS, La3Ga5SiO14) can be piezoelectrically excited even at temperatures above 1400 °C. However LGS, like many other extensively studied members of the langasite family exhibits a disordered structure which may lead to increased electromechanical losses, in particular, at high temperatures. In contrast, CTGS (Ca3TaGa3Si2O14) is a fully-ordered crystal with lower conductivity and lower damping in comparison to LGS. However, partly due to their later historical development, the high-temperature properties of CTGS crystals are studied not yet in detail. In particular, the defect chemistry of CTGS is not fully understood and the stability of the crystal properties over long time periods at elevated temperatures needs to be examined. This work focuses on fundamental properties, defect chemistry and transport mechanisms in CTGS from different manufacturers at high temperatures. In addition, changes in properties over long time periods at elevated temperatures are analyzed.
The CTGS bulk acoustic wave resonators used in this study are fabricated from Czochralski-grown single crystals. Acoustic, electrical, and dielectric properties are determined by electrical impedance spectroscopy on the samples, produced by three different manufacturers (IKZ, Berlin, Germany; FOMOS, Moscow, Russia; SICCAS, Shanghai, China). The samples are coated with the keyhole-shaped screen-printed platinum electrodes (thickness about 3 mkm). The operation temperature of the resonators is demonstrated up to 1270 °C (fig. 1) which is already close to the melting point (1350 °C). The elastic and piezoelectric constants are determined from measurements of the velocities of bulk acoustic waves. Oxygen diffusion coefficients are determined from 18O-tracer diffusion runs and subsequent secondary ion mass spectrometry.
The long-term properties of CTGS samples are examined by determining their conductivity (fig. 2) and resonance frequency at 1000 °C in air for about 5000 hours. Finally, the sample structure after long-term thermal treatment is studied by secondary neutral mass spectroscopy and compared with an untreated sample.
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|Manuskript||Fig. 1||Fig. 1. Temperature-dependent relative change of the resonance frequency of CTGS samples.||28 KB||Download|
|Manuskript||Fig. 2||Fig. 2: Electrical conductivity of CTGS samples at 1000 °C as a function of time.||95 KB||Download|