Dual Gradient Thick-Film Metal Oxide Gas Sensors

Abstract

We describe the technology and characteristics of a family of thick-film gas sensors as simple to manufacture as a one element sensor, but whose signal can be as rich as that of an array of twenty-five or more discrete sensors. This sensor design is particularly well adapted to using pattern recognition and related methods of analyte identification and quantitation based on the co-interpreted response of multiple sensors. Our prototype sensors are constructed on alumina Snapstrates intended for 20-pin DIP hybrid circuit substrates. A printed precious metal alloy electrode pattern provides pin connection pads, electrodes for mapping the gas-sensitive resistance of the overprinted metal oxide thick-film, and connections to an overprinted ohmic heater. Gradients in one or two different catalytic properties can be imposed across the short dimension and along the long dimension of the film. Each gradient can be continuous, for example a catalyst concentration gradient across or along the film, or discrete, for example three stripes across or nine stripes along the film, each of different but uniform catalyst concentration and composition. Depending on the electrode pattern used, the resistances of up to twenty-five usefully distinct surface regions can be measured. While these regions are effectively separate sensors, they share an identical history, and in operation share an identical environment. Thus differential effects from region to region can be attributed with certainty to differential responses to the same sample. Several electrode patterns and thick-film formulations have been investigated. Measurements to date confirm the validity of the design, and suggest directions in which evolution of the fabrication technology might be directed. Future possibilities include making essentially continuous surface-location-resolved measurements, adding gradients in the third spatial dimension, and using temporal as well as spatial gradients.