Gas sensors can provide information about the presence of dangerous gases in industrial and residential sites, allowing for improved environmental protection and human health and safety. In order to enable ubiquitous wireless monitoring of combustible and toxic gases, sensor elements with low power consumption are required. Two common types of gas sensors, namely calorimetric and conductometric sensors, rely on heated sensor elements to activate the appropriate reactions with the gas of interest, resulting in high power requirements to maintain the necessary operating temperatures. Miniaturized gas sensors made with microfabricated heater platforms and nanoscale sensing materials can lead to low power sensor elements with high performance. The minimization of the power consumption cannot be at the expense of the sensor’s ability to detect dangerous combustible and toxic gases. The sensing performance is impacted by the properties of both the microheater and the sensing material, as well as the integration of the two elements, creating significant opportunity for research and development to improve the performance parameters, such as sensitivity, detectable concentration range, selectivity, response and recovery time, stability, size, manufacturability, and cost, as well as power consumption.
The development approach described in the following thesis relies on an optimized microheater platform as well as novel sensing materials. The design, fabrication, and characterization of a low-power microheater platform is described, with polycrystalline silicon as the heater material for moderate temperature and polycrystalline silicon carbide for improved reliability at high temperature operation. A small, isolated heated area decreases the required power consumption and a closed membrane facilitates easier deposition of sensing materials. Sensing materials for combustible and toxic gas detection have been developed based on ultra high surface area aerogels. For combustible gas sensing, graphene or boron nitride aerogel acts as a scaffold for catalytic platinum or palladium nanoparticles. Bare graphene aerogel and graphene aerogel coated with single to few-layer molybdenum disulfide sheets are used for toxic gas detection.
The microheater platform with fast response and recovery times provides opportunities for localized, in-situ synthesis of high surface area sensing materials. The use of the microheater platform for localized growth of a nanocrystalline tin oxide sensing film is described. The fast thermal response of the microheater leads to rapid heating of the precursor, creating a highly porous film that is proven beneficial for toxic gas sensing.