Inline Microfluidic Thermal Conductivity Sensor Using a Suspended Highly-Doped Silicon Heater

This paper reports on a microfluidic sensor for inline measurement of the thermal conductivity of liquids at flow rates up to 1 g/h. The sensor contains a highly-doped silicon heater suspended in a 1 mm long microchannel that is 60 μm wide and 60 μm deep. The heater element is positioned 30 μm deep inside the microchannel to increase the contact area with the fluid and to reduce heat losses to the environment. Moreover, cavities at either side are designed to thermally isolate the microchannel, reducing the power consumption of the heater to 10 mW. The sensor is characterised through a range of different binary liquid mixtures, yielding a thermal conductivity range from 0.169 W/(m·K) (ethanol) up to 0.617 W/(m·K) (water). The analytical model coincides closely with the measurements. For practical applications, calibration curves are provided, yielding accuracies within 4% and 5% at 0 g/h and 1 g/h, respectively. Furthermore, the fabrication technology allows for further on-chip integration with other inline microfluidic sensors, which is relevant for compensating the response of microfabricated thermal flow sensors and fluid composition analysis.