The lower explosive limit, or LEL, concentration of methane gas (CH 4) is approximately 5% by volume in the air. Methane gas is a flammable, odorless, and colorless gas.
(a) The developed Love wave-based sensor system, (b) the contribution from the waveguide effect on sensor sensitivity, (c) stability test, and (d) sensitivity evaluation. (Image credit: IOA)
Methane gas explosions or poisoning in underground mines not only lead to property losses but also result in huge casualties. Hence, a sensitive and rapid methane gas monitoring system could be effective in responding to such major concerns.
Prof. WANG Wen and his coworkers from the Institute of Acoustics (IOA) of the
Chinese Academy of Sciences have recently suggested a unique Love wave-based device for sensing methane gas. The device integrates Cryptophane-A (CrypA) thin-film composed of a differential oscillation structure as well as a sensitive CrypA thin-film coated along the wave propagation path of the surface of the sensing device. The study has been reported in Sensors.
The wave propagation velocity is modulated by the adsorption in the CrypA thin-film toward methane gas, and the corresponding frequency shift can be possibly collected to assess the detected methane gas.
Owing to the waveguide effect, the acoustic wave is restricted within the thin-guiding layer, and it turns out to be more susceptible to surface mass perturbations. The Love wave mode for gas sensing provides another benefit, that is, temperature compensation of the device itself, and this is accomplished by selecting correct guiding materials that possess reverse polarization of the temperature coefficient to the piezoelectric substrate.
With the help of the FEM analyses, the scientists were able to extract the feature parameters for coupling of modes, or COM, simulation, and optimized the Love wave device accordingly.
A standard two-step method was used to synthesize the CrypA, which was then coated onto the wave propagation path through a dropping technique. The Love wave sensing device, thus developed, was linked to a differential oscillation loop, and defined in gas exposure experiments.
Next, excellent temperature stability, fast response, and high sensitivity were realized at room temperature, that is, 25 °C. Apparently, the recommended Love wave-based device specifically has three times higher sensitivity when compared to the earlier Rayleigh-surface acoustic wave-based sensor.