A high-temperature calorimetric flow sensor employing ion conduction in zirconia

This paper presents the use of the temperature-dependent ion conductivity of 8mol% yttria-stabilized zirconia (YSZ8) in a miniature high-temperature calorimetric flow sensor. The sensor consists of 4 layers of high-temperature co-fired ceramic (HTCC) YSZ8 tape with a 400 mu m wide, 100 mu m deep, and 12 500 mu m long internal flow channel. Across the center of the channel, four platinum conductors, each 80 mu m wide with a spacing of 160 mu m, were printed. The two center conductors were used as heaters, and the outer, up- and downstream conductors were used to probe the resistance through the zirconia substrate around the heaters. The thermal profile surrounding the two heaters could be made symmetrical by powering them independently, and hence, the temperature sensing elements could be balanced at zero flow. With nitrogen flowing through the channel, forced convection shifted the thermal profile downstream, and the resistance of the temperature sensing elements diverged. The sensor was characterized at nitrogen flows from 0 to 40 sccm, and resistances at zero-flow from 10 to 50 M Omega. A peak sensitivity of 3.1M Omega/sccm was obtained. Moreover, the sensor response was found to be linear over the whole flow range, with R-2 zeta of around 0.999, and easy to tune with the individual temperature control of the heaters. The ability of the sensor to operate in high temperatures makes it promising for use in different harsh environments, e.g., for close integration with microthrusters. (C) 2015 AIP Publishing LLC.