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

Abstract

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.

A Phase-Domain Readout Circuit for a CMOS-Compatible Hot-Wire CO2Sensor

“…Rather than measuring the steady-state temperature rise of a heated wire, an alternative method is to characterize its thermal time constant τ th , which is the product of its thermal resistance to ambient (R th ) and its thermal capacitance (C th ) [15]- [17]. When the wire is driven with a current I d pulsed at a frequency f drive , and is thus periodically heated, its temperature transients are delayed relative to the driving pulses.…”

“…To measure TC accurately, a steady-state approach is usually employed, in which the power dissipation of a transducer is maintained at a well-defined level, and the resulting temperature rise is measured [15]- [17]. To relax the requirements on the stability of the transducer's power dissipation, Cai et al [14] propose a ratiometric approach, in which the temperature and power dissipation of CO 2 -sensing and reference transducers are measured, and then used to calculate a CO 2 -dependent TC ratio.…”

“…As an alternative to a steady-state measurement, TC can also be derived from a transient measurement of the transducer's thermal time constant τ th , i.e., the product of its thermal capacitance (C th ) and its thermal resistance to ambient (R th ), since this also depends on the TC of the surrounding air [15]- [17]. This approach has the important advantage that the measurement of τ th relies on a time reference, and hence the absolute temperature and power levels do not need to be accurately stabilized or measured.…”

Thermal Noise-Boosting Effects in Hot-Wire-Based Micro Sensors

“…Among the sensors that exploit this principle are vacuum sensors [1][2][3], gas concentration sensors [4], thermal conductivity probes [5], and anemometers [6][7][8]. An evolution of the single hot wire is represented by the pair of thermally coupled wires.…”

Novel method for the detection of short trace gas pulses with metal oxide semiconductor gas sensors

“…Detectors for µGC are therefore an important research topic. In addition to the miniaturization of common detector principles like, e.g., TCD (thermal conductivity detector) (Mahdavifar et al, 2015) and PID (photoionization detector) (Narayanan et al, 2014), the combinations with other detector principles, e.g., nanocantilevers (Chapman et al, 2007) or ion mobility spectroscopes (Eiceman et al, 2002), have been demonstrated. The combination with metal oxide semiconductor (MOS) gas sensors also goes back many decades (Bârsan and Ionescu, 1994).…”