Wireless sensing in hostile environments

Microwave acoustic devices have long been shown to provide sensitive platforms for physical and gas sensors. Piezoelectric substrates introduced over the past two decades, such as the langasite (LGS) family of crystals, gallium orthophosphate, and aluminum nitride have enabled the exploration of microwave acoustics sensing at temperatures above 500°C. However the ability of the substrate to withstand high temperatures is only one of the requirements for sensor operation in harsh environments. Other prerequisites are: the development of stable high-temperature device electrodes; appropriate packaging; and resilience of the entire system to thermal shock and thermal cycling. In this paper, the wireless operation of microwave acoustic wave sensors in harsh environments is reviewed, and recent achievements are highlighted. Particular focus is given to surface acoustic wave (SAW) sensor technology because it has the advantage of being wireless, battery-free, and allows interrogation of arrays of multiple devices. During the last decade, wireless hostile environment microwave acoustic sensors that can detect gases and monitor temperature and pressure have transitioned from laboratory proof-of-concept to commercial devices and systems. Recent accomplishments in thin film technology and packaging are extending the technology to even higher temperatures, beyond 1000°C. These technological advances are enabling wireless microwave acoustic sensor applications in hostile environments including power plants, turbine engines, oil/gas refineries, and high temperature industrial processing.