High temperature sensors are desirable in aerospace and automotive industries for monitoring component conditions to optimize the propulsion system, with operations at temperatures up to 1000 °C, to enable safer, more fuel-efficient and more reliable vehicles; in addition, they are important for nondestructive
in situ inspection of the structure health of the furnace component or systems in electric generation plants, to improve the safety and reduce life-cycle costs. Compared with the commercial strain gauge and optical fiber sensors,
etc., piezoelectric sensors have greater potentials in realizing high temperature (>600 °C) sensing with the merits of high accuracy, fast response time and ease of integration [
1,
2,
3,
4].
Of all the investigated high temperature piezoelectric materials up to date, crystals in trigonal, tetragonal and monoclinic systems have been extensively investigated. Among these materials, the trigonal lithium niobate (LiNbO
3, LN) crystals with
3m symmetry were reported to possess high piezoelectric coefficient, being on the order of 6–70 pC/N at room temperature, approximately 3–30 times that of commercial α-quartz (SiO
2) (2–3 pC/N). However, the maximum operating temperature of LN-based piezoelectric devices, restricted by their low electrical resistivity (a requirement of >10
6 Ohm·cm was proposed for comparison, where the materials with low resistivity yet can be used for high frequency applications [
1]) at elevated temperature is limited to <600 °C, though the Curie temperature is above 1150 °C [
5]. Other important trigonal piezoelectric crystals include the langasite family with the general formula of A
3BC
3D
2O
14 [
6,
7,
8,
9,
10,
11,
12] and gallium orthophosphate GaPO
4, in the point group of
32 [
13,
14,
15,
16,
17,
18,
19], these crystals were reported to show modest piezoelectric coefficients (5–7 pC/N) and high melting points (1300–1500 °C for langasite family crystals and ~1670 °C for GaPO
4), prior to which, there are no phase transitions observed for langasite family crystals (the phase transition for GaPO
4 is about 970 °C). However, the costly component Ga
2O
3 restricted their further implements. The newly developed Ca
3TaAl
3Si
2O
14 (CTAS) crystals, substituting the Ga with Al elements, were found to possess improved higher temperature properties than La
3Ga
5SiO
14 (LGS) and to significantly decrease the cost of raw materials; nevertheless, the crystal quality needs to be improved, due to the core defect observed inside the crystals [
12]. The tetragonal melilite crystals (point group
42m, such as SrLaGa
3O
7 (SLG), Ca
2Al
2SiO
7 (CAS),
etc.) and fresnoite crystals (point group
4mm, such as Ba
2TiSi
2O
8) were investigated for piezoelectric applications. These crystals show the merits of high melting points (1400–1700 °C) and high effective piezoelectric coefficients
deff (5–18 pC/N) [
20,
21,
22,
23,
24,
25,
26]; the evaluations of the temperature dependence of dielectric, piezoelectric and electromechanical properties, however, are limited. Of particular significance is that the monoclinic rare-earth calcium oxyborate crystals (ReCa
4O(BO
3)
3, ReCOB, Re: rare earth), which have been extensively investigated for nonlinear optical applications in the last two decades [
27,
28,
29,
30,
31,
32,
33], were reported to exhibit good piezoelectric properties and high electrical resistivity at an elevated temperature of 1000 °C, with no phase transition prior to their melting points (~1400–1520 °C) [
1,
2,
3,
34,
35,
36,
37,
38].
In this review article, crystal growth, dielectric, elastic and piezoelectric property characterizations of the monoclinic ReCOB crystals are surveyed. Different crystal growth techniques, including the Bridgman and Czochralski (Cz) pulling methods, are discussed in
Section 2. The crystal orientation related to the physical axes and crystallographic axes for electro-elastic property investigations is studied in
Section 3. In
Section 4, characterizations of the dielectric, elastic and piezoelectric properties of ReCOB crystals are reviewed. In
Section 5, the maximum piezoelectric coefficients for different crystal cuts and the optimized crystal cuts free of cross-talk are discussed. Finally, the significance and challenges of ReCOB crystals are summarized; future research is proposed in
Section 6.