Gas discrimination by simultaneous sound velocity and attenuation measurements using uncoated capacitive micromachined ultrasonic transducers

Luis Iglesias Hernandez,Isabelle Dufour,Jean-François Michaud,Daniel Alquier,Dominique Certon,Priyadarshini Shanmugam

doi:10.1038/s41598-021-04689-4

Abstract

Chemically functionalized or coated sensors are by far the most employed solution in gas sensing. However, their poor long term stability represents a concern in applications dealing with hazardous gases. Uncoated sensors are durable but their selectivity is poor or non-existent. In this study, multi-parametric discrimination is used as an alternative to selectivity for uncoated capacitive micromachined ultrasonic transducers (CMUTs). This paper shows how measuring simultaneously the attenuation coefficient and the time of flight under different nitrogen mixtures allows to identify hydrogen, carbon dioxide and methane from each other and determine their concentration along with identification of temperature and humidity drifts. Theoretical comparison and specific signal processing to deal with the issue of multiple reflections are also presented. Some potential applications are monitoring of refueling stations, vehicles and nuclear waste storage facilities.

Highlights

Functionalized or coated sensors are by far the most employed solution in gas sensing
Some parameters such as the resonant frequency of a c antilever[6], the gas sound v elocity[7] and the acoustic attenuation c oefficient[8,9] depend on the gas physical properties. This allowed the development of gas density sensors[10] and in some cases these principles can be exploited to measure a binary gas mixture concentration[11,12]. Some of these sensors consist of capacitive micromachined ultrasonic transducers (CMUTs)[13]
We propose to measure simultaneously two other gas properties; the acoustic attenuation coefficient, α, for frequencies ranging from 1 to 4.5 MHz, and the time of flight, τ, which is the time taken by an acoustic ultrasonic wave to travel from an emitter to a receiver passing through the gas to be characterized

Summary

Introduction

Functionalized or coated sensors are by far the most employed solution in gas sensing. For this reason and despite their typically lower selectivity and worse limits of detection (LOD)[2], the development of uncoated gas sensors has increased in popularity over the past decade Some parameters such as the resonant frequency of a c antilever[6], the gas sound v elocity[7] and the acoustic attenuation c oefficient[8,9] depend on the gas physical properties. Possibility to use CMUTs to determine the concentration of a binary gas mixture by measuring either τ17,18 or α15 separately and in which the best selectivity obtained was slightly over 10 This value is high enough for some applications, it can be considered very low for others. In “Methods” section, details are given about the setup such as the microfabrication of the CMUTs and the signal processing employed for a robust measurement, even in case of spurious reflections and electrical coupling between the emitter and the receiver

Methods

Results

Conclusion