1.1. Piezoelectric Sensors
Electro-mechanical sensors are essential components for various industry and laboratory applications in automotive, aerospace, energy, chemical, biomedical and electronic industries. Currently, electromechanical sensors can be categorized into the following main types based on their sensing principles: piezoresistive, capacitive, piezoelectric, and optic sensing [
1,
2]. Among these sensors, piezoelectric sensors are known to have features such as fast response, high linearity, high resolution, simple integration, and low cost [
3,
4,
5]. Piezoelectric sensing can be divided into two main types: resonant and non-resonant (
Figure 1). In non-resonant piezoelectric sensing, the direct piezoelectric effect, which is defined as the generation of charge (voltage) by applying a stress, is utilized as a sensing mechanism [
6]. For non-resonant piezoelectric sensors, the typical operation frequency range is usually much lower than the fundamental resonant frequency of the sensor (e.g., <30% of the resonant frequency). This type of sensors usually operates in different sensing modes including compression, shear, and flexural mode. Each mode provides specific benefits in sensitivity, stability, and reliability for specific applications.
Figure 1.
Schematic illustration of piezoelectric sensing technology.
Figure 1.
Schematic illustration of piezoelectric sensing technology.
On the other hand, converse piezoelectric effect referred as the generation of strain (stress) upon an applied electric field is also adopted for sensor designs, namely resonant-type sensors or acoustic wave sensors [
7]. The resonant piezoelectric sensors are usually designed based on the fact that the pressure amplitude, wave velocity, and center frequency of the generated acoustic waves are dependent on the external stimulus including pressure, temperature, mass loading,
etc. In acoustic wave sensors, acoustic waves can be generated at the resonance frequency of the sensor [
8], and this is why the acoustic wave sensors are usually called the resonant-type sensors.
Although piezoelectric sensors are known with many favorable features, one limitation of piezoelectric sensors is often mentioned that: non-resonant piezoelectric sensors can only be used for dynamic measurements but not for static measurements. The reason is that non-resonant piezoelectric sensors respond only to a variation of the input load. However, proper circuit design can mitigate this limitation. For example, with a large capacitance, non-resonant piezoelectric sensors can be used for quasi-static measurements [
5]. In addition, the resonant piezoelectric sensors exhibit stable performance in static measurements. Therefore, various resonant and non-resonant piezoelectric sensing of vibration, force, flow, pressure, bio, and chemical stimuli have been actively utilized for both dynamic and static measurements in many industrial applications [
9,
10,
11,
12,
13,
14,
15].
A few review papers on piezoelectric sensors have been published and well received. These papers reviewed piezoelectric sensors based on quartz crystals and lead-based ceramics [
6,
16,
17], and the key features of well-known piezoelectric sensor designs were introduced as well as the associated applications. Typical sensor configurations such as piezoelectric bars, disks, cylinders, spheres, and benders were studied for hydrophones, accelerometers, and bio-sensors [
17]. Concerning miniaturization of piezoelectric sensors, Tadigadapa
et al. [
6] presented a comprehensive review of micro-machined piezoelectric sensor technology with a particular focus on the available micro-fabrication methods. Recently, Jiang
et al. [
18] reviewed the high temperature piezoelectric sensing technology. In these review papers, many piezoelectric materials were investigated, but relaxor-ferroelectric materials were scarcely mentioned in spite of their ultrahigh piezoelectricity. As a future trend of piezoelectric sensors, Tressler
et al. [
17] briefly mentioned the promising aspects of relaxor ferroelectric single crystals.
1.2. Relaxor-PT Ferroelectric Crystals
Among all components of a piezoelectric sensor, properties of the piezoelectric material have the most dominant effect on the sensor performance. Due to the high piezoelectricity, possibility of application-adapted shaping and ease of fabrication, ferroelectric ceramics-based on Pb-containing perovskites have been chosen as active materials for most of the piezoelectric sensors [
19,
20,
21]. Over the last two decades, relaxor-based ferroelectric single crystals such as Pb(Mg
1/3Nb
2/3)O
3-PbTiO
3 (PMN-PT) and Pb(Zn
1/3Nb
2/3)O
3-PbTiO
3 (PZN-PT) have been spotlighted due to their ultrahigh piezoelectric properties [
22,
23]. For example, the ultra-high longitudinal piezoelectric strain coefficient
d33 (>2000 pC/N) and electromechanical coupling factor
k33 (>0.9) of the relaxor-PT single crystals far outweigh the mostly used polycrystalline ceramics Pb(Zr,Ti)O
3 (PZT), and these merits are promising for a broad range of electromechanical applications [
24,
25].
The origin of relaxor single crystals’ prominence and various compositions, orientations, and crystal growth methodologies were reported by numerous published articles [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50]. A strong crystal anisotropy in these single crystals were considered as the key factor of their high piezoelectric properties [
26,
27]. As an example, piezoelectric properties of relaxor-PT single crystals poled along different crystallographic directions can be significantly different, and many of these crystal cut and poling configurations can be unique in many applications [
25,
28,
29,
30,
31,
32,
33,
34,
35,
36]. In the matter of crystal growth methods, various growth techniques including high temperature solution, Bridgman, and solid state conversion methods have been developed and reported [
37,
38,
39,
40]. Relaxor-PT single crystals with different compositions grown by different growth methods were characterized considering their poling directions and crystal groups. Among many material properties measurement methods, impedance measurement—also called resonant measurement—and the ultrasonic method were commonly used to determine the full matrix of elastic, piezoelectric, and dielectric constants of relaxor-PT crystals [
34,
41,
42,
43,
44,
45,
46,
47,
48]. Material properties of [001] poled relaxor-PT single crystals are compared with those of conventional piezoelectric materials in
Table 1, where
TC,
d,
k, ε
T, and
QM denote a Curie temperature, a piezoelectric constant, an electro-mechanical coupling coefficient, a dielectric permittivity at constant stress, and a mechanical quality factor, respectively. ε
0 is the permittivity of free space, 8.854 × 10
−12 F/m. Based on the published material properties, many papers presented promising applications of relaxor-PT single crystals. For instance, Park
et al. [
28,
49,
50] reported the feasibility of the PMN-PT and PZN-PT single crystals for the actuator and transducer applications taking advantage of the ultrahigh piezoelectric coefficients with low hysteresis and large strain behavior of relaxor single crystals.
Table 1.
The material properties of conventional piezoelectric materials and relaxor-PT single crystals [
25,
51,
52,
53,
54,
55,
56,
57,
58,
59].
Table 1.
The material properties of conventional piezoelectric materials and relaxor-PT single crystals [25,51,52,53,54,55,56,57,58,59].
Material | TC (°C) | d31 (pC/N) | d33 (pC/N) | k31 | k33 |  | QM | Reference |
---|
BaTiO3 | 115 | −78 | 190 | 0.21 | 0.50 | 1700 | 300 | [51] |
PZT-4 | 330 | −123 | 289 | 0.33 | 0.70 | 1300 | 600 | [52] |
PZT-5A | 370 | −171 | 374 | 0.34 | 0.71 | 1700 | 75 | [52] |
PZT-5H | 195 | −274 | 593 | 0.39 | 0.75 | 3400 | 65 | [52] |
PVDF | – | 21 | −32.5 | – | – | 7.6 | 8.5 | [53,54,55] |
PMN-33%PT | 155 | −1335 | 2820 | 0.59 | 0.96 | 8200 | 100 | [25,52] |
PZN-8%PT | – | −1075 | 2200 | 0.59 | 0.94 | 5100 | – | [56] |
PIN-PMN-PT | 197 | −1337 | 2742 | 0.65 | 0.95 | 7244 | 120 | [57] |
PMN-PZT | 216 | −718 | 1530 | 0.44 | 0.93 | 4850 | 100 | [58] |
Mn:PIN-PMN-PT | 193 | – | 1120 | – | 0.90 | 3700 | 810 | [25] |
Mn:PMN-PZT | 203 | −513 | 1140 | 0.45 | 0.92 | 3410 | 1050 | [59] |
Recently, Zhang
et al. [
25] reported in-depth review papers summarizing key features of published works and suggesting a perspective on future development of relaxor-PT crystals. In those articles, crystal growth technique, characterization process, measured results with high piezoelectric properties and low loss characteristics, and performance at a specific application were discussed [
25,
60,
61]. Referring to these papers, the concept of classification of crystals was proposed by Smith [
25]. According to this concept, the first generation crystals, e.g., PMN-PT and PZN-PT, show high electromechanical coupling and piezoelectricity which suggests wider bandwidth, higher sensitivity, and higher source level in ultrasonic transducer applications than their conventional PZT ceramic counterparts. The second generation crystals, e.g., PIN-PMN-PT and PMN-PZT, are advanced with a broader operation range of temperature, higher coercive field, and higher mechanical stress limit compared to the first generation crystals. The key feature of the third generation crystals is the greatly increased mechanical quality factor. For example, Mn-doped relaxor-PT crystals, such as Mn:PIN-PMN-PT and Mn:PMN-PZT, exhibit high mechanical quality factor of ~900, which is about 9 times higher than other generation crystals. However, the third generation crystals are known to have ~20% decreased piezoelectric and dielectric constants, in comparison with the first and second generation relaxor-PT crystals [
25].
As mentioned earlier, well-organized review articles with topics covering material properties of relaxor-PT single crystals and their applications in piezoelectric actuators, transducers, and energy harvesters have been published. However, relaxor-PT crystals as a sensor material have not been extensively reported on so far, largely because these crystals are known to have relatively low transition temperature, coercive field, and mechanical quality factor. As a result, there are a relatively small number of published articles and patents on sensor application of relaxor-PT crystals compared to other applications, such as actuators, transducers, and energy harvesters. Nevertheless, the advancement of relaxor single crystals poses for promising piezoelectric sensing applications.
In this paper, relaxor-PT crystals-based piezoelectric sensors were reviewed. Specifically, relaxor-PT accelerometers, hydrophones, surface load sensors, and bio-chemical sensors were presented with details. These sensors were selected to review because that they showed the most significant advancement of sensor performance in comparison with their ceramic counterparts. Analysis of emerging applicable fields and future trends of relaxor-PT sensors were also presented in this review paper.