A PDMS-Assisted Back-to-Back SIW Re-Entrant Cavity Microwave Resonator Film for VOC Gas Detection

In this article, a back-to-back (BTB) substrate integrated waveguide (SIW) re-entrant cavity resonator (RECR) is proposed to perform volatile organic compound (VOC) gas detection at room temperature. First, the structural parameters are optimized by finite difference time domain (FDTD) simulation to obtain high
${Q}$
-factor and sensitivity. The strongly induced
${E}$
-field can be further highly concentrated between two BTB capacitive posts, which can effectively improve the filling factor and enhance the interaction of the induced
${E}$
-field with the material under test (MUT). Second, polydimethylsiloxane (PDMS) as a VOC gas-sensitive material is deposited in the gap sensing area, where strongly induced
${E}$
-field is highly concentrated, to achieve high sensitivity to VOC gas when VOC gas penetrates into the pores distributed in the PDMS, resulting in the change of the permittivity of the PDMS due to the swelling of the PDMS. Finally, by injecting VOC gases, such as isopropyl alcohol (IPA), ethanol, and acetone gas, into the sensing bottle, it is experimentally verified that the proposed sensor demonstrates good stability, reliability, and repeatability during the VOC gas absorption and recovery process. The sensitivities demonstrated by the sensor for IPA, ethanol, and acetone gases are 7.82, 4.58, and 9.78 kHz/ppm, respectively, and the limit of detection (LOD) for IPA, ethanol, and acetone gases are 292, 498, and 233 ppm, respectively. The measured results indicate that the sensor demonstrates a great potential for highly sensitive detection of VOC gas concentration in air at room temperature and variable relative humidity (RH).