Materials RD European labs shed light on high temperature piezoelectrics
2013/07/15Middlesex-based National Physical Laboratory is developing techniques to characterise high-temperature piezoelectrics.
The materials are increasingly popular in engineering. For example: “Most modern diesels use piezoelectrics in the injectors,” NPL’s Dr Paul Weaver told Electronics Weekly.
As firms are developing higher temperature materials, measuring their properties is becoming increasingly difficult because temperature, applied voltage and physical size have a complex relationship in any piezoelectric structure.
“Voltage expansion you want, temperature expansion you need to account for,” said Weaver.
Within a European project called METCO (metrology of electro-thermal coupling), NPL and fellow standards laboratories are developing measurement techniques, and will offer services to industry – for example to companies developing actuators.
NPL is working on two techniques: optical interferometry and electrical resonant frequency.
Optical interferometry is capable of sub-picometre accuracy, according to Weaver.
However, the piezoelectric materials have to be in an oven with the interferometer outside. This means any measuring lasers have to pass through a window and the gas inside the oven, both of which will have temperature gradients, and one of which will be convecting.
“The most powerful piezoelectrics are oxide materials, which can have problems with chemical reactions at high temperature. You may need a compromise atmosphere between the best atmosphere for measurement and the best atmosphere for the material,” said Weaver.
As an example of what can be done to remove errors, Weaver said an ac voltage can be superimposed on the test voltage “to see what is noise and what is signal”. The situation is complicated. “This is why you have to understand piezoelectrics. It is one thing to make a measurement and another thing to interpret the measurement to provide information to someone building a piezoelectric valve.”
Displacement resolution is expected to be “in the low nanometres,” said Weaver.
The resonant frequency method is purely an electrical measurement on a sample, examining its complex impedance as it vibrates under ac excitation.
“It’s a well-known technique. We are taking it to temperatures which people have not really studied how to get to,” said Weaver. “You can find all of the small-field piezoelectric properties without optical access, but you do need to understand the resistance of the wires and the connections to the piezoelectric.”
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