High temperature ultrasonic gas flow sensor based on lead free piezoelectric material

The review of current technologies for measurement of gas velocity in stack flow applications is undertaken and it is shown that the ultrasonic time-of-flight method is the most suitable and offers a number of advantages over alternatives. Weakness of current piezoelectric based transducers are identified as the inability to operate at temperatures above 400 °C due to limitation of piezoelectric materials used, and a case for development of an alternative high temperature material is put forward. A novel and highly enhanced, lead free piezoelectric material, suitable for continuous operation at temperatures above 400 °C has been engineered for ultrasonic gas velocity sensor applications. Structural modification of pure bismuth titanate (Bi4Ti3O12) or BIT compound, through multi-doping at the Ti-site, has been found to enhance piezoelectric properties accompanied with a mild reduction in Curie temperature, Tc. Initially, compounds doped with tungsten and chromium were found to increase the piezoelectric coefficient (d33) from around 5 pC N¯¹ in pure bismuth titanate, to above 20 pC N¯¹ in doped compounds. This increase is attributed to lower conductivity and improved poling conditions. Further increases in d33 (up to 35 pC N¯¹) were then realised through controlled grain growth and reduction in conductivity for niobium, tantalum and antimony doped compounds. The Curie temperature of the material with best properties is found to be 667 °C, which is a slight reduction from 675 °C for pure bismuth titanate ceramic. The enhancements in modified bismuth titanate achieved in present work allow the material to be considered as suitable for high temperature ultrasonic transducer applications. Integration of bismuth titanate material into a working high temperature transducer is then considered and the investigation of suitable, high temperature bonding method is undertaken. It is shown that reactivity of bismuth titanate with the titanium based fillers makes brazing unsuitable as a bonding method between piezo-ceramics and stainless steel. A novel assembly method, using liquid gallium as an electrically conductive bond, and a mechanical restraint for the piezo actuator is then presented as an alternative with the potential to reduce the negative effects of differences in thermal expansion coefficients between constituents of the transducer assembly.This work was supported by funding from the EPSR