To meet the sensing demands of extreme environments, such as those encountered in aircraft engines and nuclear industries, there is an urgent need to develop new piezoelectric crystals with excellent stability in piezoelectric performance. In this study, rare-earth mixed Er_xGd_{1− x}Ca₄O(BO₃)₃ (Er_xGd_{1− x}COB, x = 0.30, 0.40, and 0.45) crystals were grown via the Czochralski method, and the electro-elastic properties were characterized over the temperature range of − 100 ℃ to 900 ℃. The results demonstrated that the Er_0.45Gd_0.55COB crystal exhibited exceptional temperature stability across the entire temperature range, with a negligible variation of only − 3% in the piezoelectric coefficient d₂₆. The fatigue behaviors of the electrical resistivity and piezoelectric coefficient were further investigated at 850 ℃ for 100 h. The results indicated that the electrical resistivity of Er_0.45Gd_0.55COB crystal remained within the same order of magnitude, approximately 8× 10⁷ Ω·cm, whereas the piezoelectric coefficient d₂₆ decreased only slightly by − 4.2%. Furthermore, a prototype piezoelectric vibration sensor with an average sensitivity of 0.75 pC/g was fabricated using the Er_0.45Gd_0.55COB crystal, and the sensor exhibited a negligible sensitivity variation of ~4% over the temperature range of 25–800 ℃. These results demonstrate the strong potential of Er_xGd_{1− x}COB crystals for high-temperature piezoelectric sensing applications.

Enhancing the stability of piezoelectric properties is essential for ensuring the reliability of high-temperature piezoelectric sensors. In this study, we have synthesized AlN piezoelectric crystals as representative materials and employed first-principles methods to investigate their temperature-dependent piezoelectric properties. By integrating the effects of lattice expansion and electron–phonon interactions, we accurately constructed the crystal structure of AlN across a wide temperature range and successfully predicted its piezoelectric behavior. Theoretical analysis reveals that ion polarization driven by lattice distortion and elastic softening of chemical bonds maintains the overall structural integrity of defect-free AlN single crystals, resulting in a stable piezoelectric coefficient d₃₃ with a deviation of only 8.55% at temperatures up to 1300 K. However, experimental results indicate that the stability of the piezoelectric performance of the grown AlN crystals is disrupted at temperatures above 870 K. This temperature limitation is attributed to point defects within AlN crystals, particularly those caused by oxygen-substituted nitrogen (O_N). These findings provide valuable guidance for enhancing the piezoelectric temperature stability of AlN crystals through optimized experimental conditions, such as oxygen atmosphere treatment and defect modification during crystal growth.

A new piezoelectric crystal, KCsMoP₂O₉ (KCMP), was successfully grown by the Kyropoulos method. The electro-elastic properties of KCMP crystal were characterized by the impedance method. The full set of elastic constants was determined by solving the Christoffel equation. Remarkably, the crystal exhibits a large face-shear piezoelectric coefficient of d₁₄ = 16.2 pC/N, surpassing that of the extensively studied ordered langasite crystal Ca₃TaGa₃Si₂O₁₄ (d₁₄ = 10.4 pC/N). A fundamental shear horizontal (SH₀) wave piezoelectric transducer was developed, leveraging the face-shear-mode of KCMP wafers. Finite element simulations have conclusively demonstrated the KCMP-based transducer's exceptional ability to efficiently excite and capture the pure SH₀ wave, independently along two orthogonal main directions (0°/180° and 90°/270°). These results were subsequently corroborated through experimental validation at temperatures up to 300 ℃, highlighting the considerable promise of KCMP crystals for utilization in non-destructive testing and structural health monitoring applications.

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.

Rare-earth calcium oxyborate crystals (i.e., RECa₄O(BO₃)₃, RECOB, RE: Y and rare-earth elements) are promising piezoelectric materials for high-temperature sensing application due to their high melting point, highs electrical resistivity, low dielectric loss and moderate piezoelectric activity. Single crystal growth of RECOB crystals was introduced, the characterization of electro-elastic properties and the research progress of RECOB piezoelectric crystals in the high-temperature sensing field were represented. High-temperature surface acoustic wave sensor for temperature sensing, high-temperature piezoelectric transducers and vibration sensors were described. The potential applications and technical advantages of RECOB crystals and related sensors were introduced.

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.