Improved Methanol Detection Using Carbon Nanotube-Coated Carbon Fibers Integrated with a Split-Ring Resonator-Based Microwave Sensor

A novel microwave sensor based on the electrically small split ring resonator (SRR) integrated with the hierarchal carbon nanotube coated carbon fiber (CNTCF) for gas-sensing applications is proposed. The CNTCF is synthesized via catalytic chemical vapor deposition process and investigated by scanning electron microscopy and transmission electron microscopy for their structural information. The CNTCF integrated SRR is excited by the electromagnetic field through high impedance microstrip line. The use of highly conductive CNTCF as sensing material facilitated extremely large surface area causing a substantial change in the inherent resonant frequency as a sensor response at room temperature (25 °C) for methanol, nitrogen dioxide (NO₂), ethanol, and chloroform gases. This change in resonant frequency facilitates the detection and sensing of the analyte gases under controlled environment and constant relative humidity of 32%. The CNTCF-SRR sensor exhibited high selectivity and sensitivity toward the methanol gas with large resonance frequency shift of 400 MHz and gas sensitivity of 4.2 ± 0.03% at 300 ppm. Both response and recovery times are on the order of seconds for the gas concentration ranging from 100 to 300 ppm. In addition, sensor exhibited good reproducibility and stability with variations of 0.03% and 2%, respectively. The plausible gas-sensing mechanism of CNTCF-SRR sensor has been discussed. Therefore, in view of the above-mentioned sensor responses it is speculated that the proposed CNTCF integrated planar microwave sensor can effectively be used for selective sensing of methanol gas in harsh condition.