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- Hydrogen detection (1)
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A hydrogen sensor based on a silicon Metal/Solid Electrolyte/Insulator/Semiconductor (MEIS) structure with thin layers of the super-ionic conductor LaF3 and Pd gate metal was investigated in a hybrid structure on a resistance heater mounted on a ceramic substrate. The sensor was operated at room temperature, however, a short heating impulse once a day ensures a fast and reproducible dynamic behavior. The sensor was characterized in the concentration range typical for alarm levels up to 40% of the lower flammability limit and shows a logarithmic dependency of the sensor signal from the hydrogen concentration and a mean sensitivity of about 140 mV/decade. The response time (t90) is about 8 s. In a long term test for a period of three month, the sensor was exposed to hydrogen every two weeks. The sensor signal was shown to be stable in dynamic behavior, sensitivity and signal difference.
The efficiency of gas sensor application for facilitating the safe use of hydrogen depends to a considerable extent on the response time of the sensor to change in hydrogen concentration. The response and recovery times have been measured for five different hydrogen sensors, three commercially available and two promising prototypes which operate at room temperature.
Experiments according to ISO 26142 show that most of the sensors surpass much for a concentration change from clean to hydrogen containing air the demands of the Standard for the response times t(90) and values of 2 to lös were estimated. For an opposite shift to clean air, the recovery times t(10) are from 7 to 70 s. Results of transient behaviour can be fitted with an exponential approach. It can be demonstrated that results on transient behaviour depend not only from investigation method, and the experimental conditions, like gas changing rate and concentration jump, as well as from operating
Parameters of sensors. In comparison to commercial MOS and MIS-FET hydrogen sensors new sensor prototypes operating at room temperature possesses in particular longer recovery times.