Sensors for the ultra-fast monitoring of explosive gas concentrations

The target of this paper is answering the question about the minimum response time of metal oxide semiconductor gas sensors. This question is very important at the monitoring of sudden explosive emission of high concentrations of combustible gases in mines, chemical and oil refinery factories, nuclear power stations, etc. To reach this target, we eliminated the influence of internal and external diffusion to the sensor response time by the optimization of the thickness of sensing layer, deposition conditions, material, and working temperature of the sensor. As a result, it was possible to fabricate the sensor with very short response time ∼ 160 ms (hexane) in a concentration range up to 12 vol%. The response time of the sensor was compared with the response time of IR optic sensor based on LED (λ = 3.4 μm) using testing gas mixing installation equipped with ultra-fast valves. The effect of sensing layer thickness and the conditions necessary to exclude the influence of reagent and product diffusion through the sensing layer to the response time were studied in detail. It was shown that, if the thickness of thin film sensing layer exceeds 200 nm, the sensor response rate is diffusion limited, the diffusion coefficient corresponding to inter-grain diffusion at 550 °C was estimated as (1.5 ± 0.5)·10−10 cm2/s. The results were proven in field experiment with spike emission of petrol aerosol.