GREMAN Laboratory UMR-CNRS 7347, University of Tours, 16 rue Pierre et Marie Curie, BP 7155, 37071 TOURS Cedex 2, France.
IMS Laboratory UMR-CNRS 5218, University of Bordeaux, 351 cours de la libération, 33405 TALENCE Cedex, France.
Ultrasonics. 2021 Jul;114:106410. doi: 10.1016/j.ultras.2021.106410. Epub 2021 Mar 11.
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 µm 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 mm. To validate the general principle of the proposed technique, two binary gas mixtures of CO/N and H/N, with varying concentrations, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3% for CO in N and 0.15% for H in N.
本工作旨在开发用于二元混合气体分析的超宽带空气耦合电容式微机械超声换能器（CMUT）。检测原理基于飞行时间（ToF）测量，以监测气体超声速度变化。为了进行这种测量，CMUT 被特别设计为工作在离谐模式下，就像一个麦克风。选择的膜片尺寸为 32×32µm，间隙高度为 250nm。共振频率和崩溃电压分别为 8MHz 和 58V。如前所述，CMUT 被用于准静态工作模式，在非常低的频带内，从 1MHz 到 1.5MHz 的频率。对换能器脉冲响应进行了表征，测量得到的-6dB 相对分数带宽（FBW）高于 100%，这使得 CMUT 能够用于目标应用。此外，还设计了一个测量单元，用于将制造的 CMUT 发射器和接收器原型面对面放置。单元内的体积保持在 3mL 以下，发射器/接收器的表面为 1.6×8mm。为了验证所提出技术的一般原理，对两种具有不同浓度的 CO/N 和 H/N 二元混合气体进行了测试。结果非常有前景，在 N 中 CO 的检测限（LOD）为 0.3%，在 N 中 H 的检测限为 0.15%。
