Explosion-proof monitoring of hydrocarbons by mechanically stabilised, integrable calorimetric microsensors

Abstract Safe detection of combustible gases is demonstrated by a new type of integrated micropellistor and heat conductivity sensor. The basic element in both types of calorimetric devices is a thermally isolated microhotplate with a Pt heater, embedded in non-stoichiometric silicon-nitride. The pellistors are formed from a pair of an active and a passive resistor element in a Wheatstone-bridge arrangement, and heated up to 200–600 °C in the environment to be monitored with a power dissipation of 20–60 mW. A novel, proprietary one-side porous silicon micromachining process was developed for the formation of the air gap around the devices, offering a thin single crystalline Si support for the membrane. 1 Owing to the better mechanical stability obtained, the relatively large mass of porous substance, containing the finely dispersed catalysts of Pt, Pd or Rh on one hand, and for forming the chemically inert reference device on the other hand, can be deposited on top of the microheaters without jeopardising the heater integrity. The sensors, utilising the heat conductivity principle, are composed of a pair of chemically passive elements, having different heat exchange properties due their dissimilar surface. Explosion-proof detection of hydrocarbons was demonstrated by measuring concentration levels between the lower and upper explosion limits (LEL, UEL) with no protective encapsulation, whatsoever.

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