## 4 Applications  
### High Temperature Piezoelectricity  
As ferroelectric materials, Aurivillius compounds are piezoelectric after electrical poling. Two characteristics are exploited: their high ferro-paraelectric phase transitions, which make them good candidates for uses as high temperature piezoelectrics; and the high coercive fields, which facilitate the uses as actuators with difficult depolarization.  
Figure 9: Cross-sectional TEM image along the [110] SrTiO\({}_{3}\) zone axis of the same (103)SrBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\)/ (111) SrRuO\({}_{3}\)/(111) SrTiO\({}_{3}\) film from which the Polarization–electric-field hysteresis curve shown at the right Figure was measured (taken from [69]).  
Nowadays, the Ferroperm company is commercializing bismuth titanate ceramics (code Pz46, with values of \(d_{33}\) = 18 pC/N; \(d_{31}\) = 2 pC/N; \(K_{t}\) = 0.20; \(T_{C}\) = 650 \({}^{\circ}\)C) [74]. It is recommended mainly for high temperature applications, as flow-meters or pressure sensors.  
There are two main applications for piezoelectrics at high temperature: as sensors or as transducers [75]. Thus, as sensors, _accelerometers_ are used to detect abnormal behaviors in combustion engines working at harsh conditions, at high pressure (80 bars) and high temperature (1.227 \({}^{\circ}\)C); _high temperature SAW_ (Surface Acoustic Wave) sensors can be suitable for monitoring temperatures, pressures or gas concentrations for high-temperature applications such as HT furnaces or process chambers for semiconductors or ceramics manufacturing. The possibility of using wireless systems without traditional thermocouples or thermistors makes HT SAW a promising technique for these high-temperature applications, as shown in Figure 9 in reference [75]; _Acoustic Emission Sensor_. The generation of a crack or its propagation produces the release of elastic energy that can be detected by means of piezoelectric sensors. High-temperature non-destructive inspection techniques are frequently required for particular applications such as the nuclear power industry, for safety reasons; _Gas Sensor_. By monitoring the shifts in resonance frequency produced by changes in mass, the gas molecules absorbed at the surface can be detected, for example in high-temperature combustion processes; _Pressure Sensor_. They are commonly used in internal combustion engines and turbines, with high pressures and temperatures.  
In the case of actuators or transducers, applications can be found as ultrasound transducers for non-destructive testing (NDT) and non-destructive evaluation (NDE) in high-temperature environments. In this sense, potential applications have generated an important interest in the field of Space exploration. The working conditions in the Solar System can be very extreme. Temperatures in Venus can be as high as 460 \({}^{\circ}\)C. Most of the currently available electronic electromagnetic actuators are limited in their ability to produce sufficiently high stroke, torque, o3r force at the extreme conditions, which can be achieved by electromechanical devices. The actuators that are needed for robotic exploration include lander pedal motors, drive/steering motors, manipulator joint motors, latching and deployment motors and sampling tool motors [76]. Some of these applications are motors as important elements of automatic and robotic mechanisms.  
As examples in this field of Space exploration, [PERSON] et al. from the Jet Propulsion Laboratory at NASA published the study of a high temperature piezoelectric drill, as the one shown in [77]. In that work, the authors designed a testbed to simulate the conditions in Venus. Basalt or brick samples were drilled at 500 \({}^{\circ}\)C by a piezoelectric drill with different piezoelectric materials for the transducer that drives the USDC. They found advantages in the use of bismuth titanate in terms of drilling depth and time duration over other materials as LiNbO\({}_{3}\). This opened new perspectives for high temperatures applications for Aurivillius in Space exploration.