Dynamic behaviour and conditioning time of a zirconia flow sensor for high-temperature applications

The temperature dependent ion conductivity of yttria stabilized zirconia (YSZ) can be used to create a miniaturized flow sensor using a calorimetric measurement scheme. Such a sensor is compatible with harsh environments, and can sustain temperatures of up to 1000 °C, although thermal crosstalk will limit its performance as the temperature rises. This paper investigates if the integration of thermal isolation in the form of sealed cavities can mitigate the detrimental effect of the thermal crosstalk, particularly by studying the conditioning time of the sensor to temperature changes. To this end, high temperature co-fired ceramic (HTCC) sensors were fabricated from tapes of 8 mol-% YSZ that were screen printed with platinum paste. Definition of channels and structures were made by milling the green tapes, and sacrificial inserts were placed in all cavities to give mechanical support during lamination and sintering. Cavities with widths of 240 μm, 400 μm and 560 μm were investigated, and sensors without cavities were also made to serve as references. Additionally, two different positions of the sensor element with respect to the edge of the cavity (560 or 800 μm) were investigated. The results showed that it was possible to improve the conditioning time of the sensor by up to five times by the use of isolating cavities, and that this improvement is translated into a reduction in rate-dependent hysteresis for measurements with long elapse times. The latter effect is most pronounced for the sensors with the largest cavities.