Metallic Semiconductor
An n type transition metal oxide acts as the primary active material in many solid state gas sensors. Tin dioxide is valued for its chemical stability and its strong response to a wide range of combustible and toxic gases. It naturally forms an oxygen deficient structure that provides a baseline level of electronic conductivity.
Sensing Performance
Interactions between the solid surface and gas molecules alter the thickness of the electronic depletion region. This change is particularly pronounced when the material is heated to temperatures between 200 and 400 degrees Celsius.
Grain Boundary Control
Conduction in the thick film or thin film layer is limited by the bottlenecks formed at the contact points between individual crystallites. These boundaries create high resistance paths that dominate the total impedance of the sensor. Small adjustments to the sintering temperature can alter the neck width and modify the sensitivity of the tin dioxide film.
Material Certification
Industrial suppliers provide high purity powders with specific surface areas measured by the Brunauer Emmett Teller method. The presence of trace contaminants like iron or copper can drastically alter the sensitivity and selectivity of the final sensor. Qualification involves scanning electron microscopy to verify grain size and X-ray photoelectron spectroscopy to confirm the oxidation state of the tin.
Finished devices are tested for long term stability to ensure that the grain structure does not coarsen over time, which would lead to a loss of sensitivity.
