Viewpoint: Bringing the heat - piezoelectric thermal capacity for new applications
Raising the thermal capacity of piezoelectric materials will open up a host of new applications. Dr Tim Stevenson, postdoctoral researcher at the University of Leeds, UK, discusses a newly launched three-year project.
Piezoelectric materials are all around us in sensors and actuators, and are essential to modern devices such as parking sensors, medical ultrasound, fuel injection valves and the ubiquitous piezoelectric buzzer. The high performance materials we use today were originally developed in the 1960s. But these materials only work well up to around 200°C and for applications such as ultrasound, which requires stable material performance, at even lower temperatures. This severely restricts the applications that can beneﬁt from this technology. For instance, valves and transducers that operate reliably at temperatures in excess of 500°C would enable the development of cleaner, more efficient transport systems.
If our piezoelectric materials could withstand higher temperatures without degradation, it would be possible to improve sterilisation of medical ultrasound probes, preventing contamination associated with diseases such as Creutzfeldt-Jakob (CJD), which can survive normal sterilisation procedures. There is also a demand for high operating temperatures for ultrasonic transducers used for down-hole oil and gas surveying, which need to operate above 210°C.
A difficulty in developing new materials that can operate under these extreme conditions is that measuring the piezoelectric properties at high temperatures is not easy. Reliable measurement is essential to provide the data required for development and design of new materials technology and eﬀective design of new devices, for reproducibility in characterisation and test, and to ensure quality.
Ultrasonic non-destructive evaluation (NDE) operating at temperatures up to 750°C would allow power and process industries to improve safety and reliability while reducing cost and downtime associated with inspection and maintenance of critical plant components.
An effect closely related to piezoelectricity is the electrocaloric eﬀect, which is generating a lot of interest as a potential new solid-state cooling technology. These materials promise to provide solid state cooling for electronic chips and could potentially replace refrigerant gas in cooling applications.
To accelerate development in piezoelectric technologies, close collaboration between industry, materials scientists, metrology and instrumentation is required. To this end, a £2.24m project has recently been set up under the European Metrology Research Programme, jointly funded by the EU and national metrology research programmes. This project brings together leading European metrology research labs (NPL, UK, PTB, Germany, LNE, France, MIKES, Finland, CMI, Czech Republic) with materials scientists from the University of Leeds, and European piezoelectric test equipment manufacturer aixACCT.
The aim of the project is to develop the metrological infrastructure and facilities within Europe for the traceable metrology of piezoelectric and electrocaloric properties at high temperatures and high electric ﬁelds. Novel high-precision interferometric and electromechanical resonance techniques will be developed to measure piezoelectric coefficients up to 1,000°C. An integral part of this is to apply the new measurement technologies to the development of new materials. For example, the University of Leeds is developing high Curie temperature piezoceramic materials (more than 450°C) for use in new applications and to develop reference standards for industrial measurement assurance.
The ﬁrst phase of the three-year project, which started in June 2012, will work with industrial and academic stakeholders to deﬁne the measurement and materials requirements, and to design the measurement systems. The main challenges relate to increased conductivity of the ceramics at high temperature, mounting and connecting the samples without disrupting the measurement, and performing precision optical measurements in these thermally extreme environments. The development of new high-temperature functional materials technologies will begin in 2013.