Ca₃Co₄O₉-Based Planar Thermoelectric Gas Sensor with High Sensitivity Fabricated by Powder Aerosol Deposition for...

Increasingly stringent emission regulations in combustion systems drive the demand for robust, cost-effective gas sensors capable of operating under harsh, high-temperature conditions. Thermoelectric ceramic gas sensors represent a promising approach. Their application, however, may be limited owing to a low response. In this work, calcium cobaltite (CCO), a p-type thermoelectric oxide with a Seebeck coefficient higher than most metals used for thermocouples, is investigated as an alternative material to enhance sensor performance. CCO powder was synthesized via the mixed oxide route and deposited as dense ceramic films onto alumina substrates at room temperature using the powder aerosol deposition method (PAD). The thermoelectric properties of the deposited films were characterized up to 850 °C, with the Seebeck coefficient showing only minor dependencies on variations in oxygen and water vapor concentrations. Following these results, planar exothermic gas sensors based on Au/CCO thermocouples utilizing laser-cut polyimide masks for patterning the PAD films were fabricated and compared to reference sensors with screen-printed metallic Au/Pt thermocouples. Laboratory gas measurements with CO and hydrocarbons demonstrated that the Au/CCO-based sensors exhibited an average sensitivity increase by a factor of 8-9, while maintaining high linearity and low cross-sensitivity to variations in oxygen and humidity. Furthermore, the additive response to gas mixtures was confirmed. Initial tests in real flue gas from a wood-burning stove showed an excellent correlation between the sensor signal and relevant flue gas components (measured using precise gas analyzers). The presented results highlight the potential of calcium cobaltite as a thermoelectric material for high-temperature gas sensors based on the exothermic principle and demonstrate the suitability of the PAD method for fabricating fine-structured functional films.