Abstract
Using the slow-cooling method in selected MoO3-based fluxes, single-crystals of GeO2 and GaPO4 materials with an α-quartz-like structure were grown at high temperatures (T ≥ 950 °C). These piezoelectric materials were obtained in millimeter-size as well-faceted, visually colorless and transparent crystals. Compared to crystals grown by hydrothermal methods, infrared and Raman measurements revealed flux-grown samples without significant hydroxyl group contamination and thermal analyses demonstrated a total reversibility of the α-quartz ↔ β-cristobalite phase transition for GaPO4 and an absence of phase transition before melting for α-GeO2. The elastic constants CIJ (with I, J indices from 1 to 6) of these flux-grown piezoelectric crystals were experimentally determined at room and high temperatures. The ambient results for as-grown α-GaPO4 were in good agreement with those obtained from hydrothermally-grown samples and the two longitudinal elastic constants measured versus temperature up to 850 °C showed a monotonous evolution. The extraction of the ambient piezoelectric stress contribution e11 from the CD11 to CE11 difference gave for the piezoelectric strain coefficient d11 of flux-grown α-GeO2 crystal a value of 5.7(2) pC/N, which is more than twice that of α-quartz. As the α-quartz structure of GeO2 remained stable up to melting, a piezoelectric activity was observed up to 1000 °C.
Highlights
Piezoelectric materials that can operate under very high temperature without degradation are sought for the control of structure materials and control system in turbines, engines, nuclear reactors, etc. [1,2].the use of a piezoelectric material at elevated temperature presents many challenges such as possible phase transition, chemical degradation or structural defect propagation which can cancel or lead to instability of the piezoelectric properties.Non-pyroelectric single crystals with the α-quartz-like structure exhibiting both higher piezoelectric constants and a higher thermal stability as compared to α-quartz would be promising materials to build miniaturized high temperature piezoelectric-operated devices without cooling
This paper presents an overview of the main results obtained from several structural and physical characterizations undertaken on flux-grown α-GaPO4 and α-GeO2 piezoelectric crystals
An excellent agreement was obtained between experimental and theoretical Raman lines for both wavenumbers and relative intensities which permitted to unambiguously assign the symmetry and the nature of α-quartz GeO2 modes. This Raman study pointed out that high temperature flux-grown GeO2 single crystals of α-quartz-like structure were of high structural quality, and that vibrations in the α-quartz GeO2 structure were relatively quasi-harmonic as the calculated frequencies at −273 °C were almost the same as the experimental values at 25 °C
Summary
Piezoelectric materials that can operate under very high temperature without degradation are sought for the control of structure materials and control system in turbines, engines, nuclear reactors, etc. [1,2]. When epitaxially-grown in the temperature region where it is metastable, the hydrothermally-grown alpha-quartz form of GeO2 contains high OH impurities which rather catalyze its return to the thermodynamically stable rutile-like structure when heated as low as 180 °C [77,78,79,80,81] In this context, it appeared that another suitable growth technique for oxides could be applied for the crystallization of α-GaPO4 and α-GeO2 materials to get single crystals with a high degree of structural and chemical perfection; the high temperature solution growth technique known as the fluxed melt growth [82,83,84,85]. The results are compared and discussed in the view of hydrothermally-grown α-quartz-like SiO2, GeO2 and GaPO4
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