The present work aims to develop ultra-wide bandwidth air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for binary gas mixture analysis. The detection principle is based on time-of-flight (ToF) measurements, in order to monitor gas ultrasound velocity variations. To perform such measurements, CMUTs were especially designed to work out of resonance mode, like a microphone. The chosen membrane size is 32 × 32 µm² and gap height is 250 nm. The resonance frequency and collapse voltage were found at 8 MHz and 58 V respectively. As mentioned, the CMUTs were exploited in quasi-static operating mode, in a very low frequency band, from 1 MHz to 1.5 MHz frequencies. The transducer impulse response was characterised, and a −6 dB relative fractional frequency bandwidth (FBW) higher than 100% was measured, enabling to use CMUT for the targeted application. Additionally, a measuring cell has been designed to hold the fabricated CMUT emitter and receiver prototypes facing each other. The volume inside the cell was kept lower than 3 mL and the surface of emitter/receiver was 1.6 × 8 mm². To validate the general principle of the proposed technique, two binary gas mixtures of CO₂/N₂ and H₂/N₂, with varying concentrations, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3% for CO₂ in N₂ and 0.15% for H₂ in N₂.

本工作旨在开发用于二元气体混合物分析的超宽带空气耦合电容微机械超声换能器（CMUT）。该检测原理基于飞行时间（ToF）测量，以便监视气体超声速度变化。为了执行此类测量，CMUT特别设计为在共振模式下工作，就像麦克风一样。所选的膜尺寸为32×32 µm ²间隙高度为250 nm。发现共振频率和崩溃电压分别为8 MHz和58V。如前所述，CMUT是在准静态操作模式下使用的，该模式处于1 MHz至1.5 MHz频率的非常低的频带中。表征了传感器的脉冲响应，并测量了高于100％的-6 dB相对分数频率带宽（FBW），从而能够将CMUT用于目标应用。此外，还设计了一个测量单元，以使制造的CMUT发射器和接收器原型彼此面对。电池内部的体积保持在3 mL以下，发射器/接收器的表面为1.6×8 mm ²。为了验证所提出技术的一般原理，使用了CO ₂ / N _2的两种二元混合气已经测试了具有不同浓度的H ₂和N ₂和H ₂ / N ₂。结果非常有检测（LOD）中的0.3％的测量极限为CO有为₂的N ₂和0.15％的用于h ₂的N ₂。