Transient thermal response of micro-thermal conductivity detector (µTCD) for the identification of gas mixtures: An ultra-fast and low power method

Micro-thermal conductivity detector (µTCD) gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems. Conventional TCDs use steady-state resistance (i.e., temperature) measurements of a micro-heater. In this work, we developed a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step. The method was implemented for a 100-µm-long and 1-µm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer. Transient resistance variations of the µTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen. Simulations and experimental results are presented and compared for the time resolved and steady-state regime of the sensor response. Thermal analysis and simulation show that the sensor response is exponential in the transient state, that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient, and that the sensor was able to quantify the mixture composition. The level of detection in nitrogen was estimated to be from 25 ppm for helium to 178 ppm for carbon dioxide. With this novel approach, the sensor requires approximately 3.6 nJ for a single measurement and needs only 300 µs of sampling time. This is less than the energy and time required for steady-state DC measurements. Microsensors: Efficient thermal gas detection Researchers in the USA have developed a fast, energy-efficient measurement technique for use in micrometer-sized thermal gas sensors. Small and fast sensors are crucial for gas chromatography and other applications. Thermal gas sensors operate by measuring the characteristic thermal conductivity of gasses and gas mixtures. Peter Hesketh and Alireza Mahdavifar from the Georgia Institute of Technology and co-workers heated a 100-micrometers-long silicon bridge on a chip using pulsed electrical currents then measured the electrical resistance of the element as it changed with temperature. The time-dependent changes of the electrical resistance were characteristic of the thermal conductivity of the surrounding gas, providing a gas-specific detection mechanism that takes less than 300 μs and uses only 3.6 nJ per measurement. These robust sensors are particularly attractiv