Guided-wave-based structural health monitoring (SHM) technology is of great importance for real-time inspection of high-temperature structures. The fundamental shear horizontal (SH₀) wave is believed to be an ideal wave mode for developing SHM systems due to its nondispersive characteristics. However, currently very limited SH₀ wave transducers can be used for SHM of high-temperature structures due to the limitation of materials. Herein, a novel YSr₃(PO₄)₃ (YSP) piezoelectric crystal in the space group I43d was grown. Experiments show that the face-shear piezoelectric coefficient d₁₄ (d₁₄ = d₂₅ = d₃₆) is 9.7 pC/N and varies little from 25 to 800 °C. Then a beam-focused SH₀ wave piezoelectric transducer is developed based on face-shear-mode YSP wafers. Both finite element simulations and experimental results indicate that the YSP-based transducer can excite pure SH₀ wave and focus the wave energy along two opposite main directions. Especially, the obtained SH₀ wave beam is highly concentrated with a small divergence angle of less than 30°, originating from the high working frequency range from 300 to 400 kHz. The excellent temperature stability of the as-grown YSP crystal makes the proposed SH₀ wave piezoelectric transducer very suitable for SHM of high-temperature structures.

The yttrium calcium oxyborate crystal (YCa₄O(BO₃)₃, YCOB) has been actively studied for high-temperature piezoelectric sensing applications. In this work, the stability of electric properties of YCOB crystal annealed in critical conditions (high-temperatures of 900–1100 ℃ with a low oxygen partial pressure of 4 × 10⁻⁶ atm for 24 h) was investigated and the recovery mechanism for the electrical resisitivity, dielectric permittivity and dielectric loss were studied, taking advantage of the X-ray photoelectron spectra and the first principle calculations. The electrical resistivity of the annealed YCOB crystal was slightly decreased when compared to the pristine counterpart, being (2–5) × 10⁷ Ω·cm at 850 ℃. The dielectric permittivity and dielectric loss were found to increase after annealing, showing recoverable behaviours after thermal treatment above 650 ℃ in air. The calculated vacancy formation energy indicates that the oxygen vacancy is the dominant defects in YCOB. The formation of oxygen vacancy weakens the chemical bonding strength between B (Ca or Y) and O atoms, introduces extra donor levels in the band gap, which excites the electrons to conduction band more easily thus enhances the electrical conductivity and dielectric loss. The recovered electrical properties are believed to be associated with the reduced vacancy defects at elevated temperatures in air.

Lithium niobate (LiNbO₃, LN) crystal is a multi-functional material with favorable piezoelectric, nonlinear optical and electro-optic properties. In this study, the electromechanical properties of the radial extensional (RE) and the thickness extensional (TE) modes of the congruent LN are studied and the temperature dependent behaviors are revealed. The RE mode electromechanical coupling factors (k_p) for the Y- and Z-oriented discs are calculated and found to be 3.8% and 24.7%, respectively, which are nearly the same as the experimental results of 3.8% and 25.2%, respectively. The maximum RE and thickness shear (TS) modes electromechanical coupling factors are obtained to be 47.6% and 68.5% for the Y_x/25° and Y_x/167° crystal cuts, respectively. The LN crystal possesses good temperature stability of the electromechanical coupling factors (RE and TE modes) from 20 ℃ to 500 ℃, where the variations of k_p and k_t for the Y-oriented discs are < 8.0% and <1.8%, respectively.