NOx gas ppm level monitoring using gravure printed gas sensor

Different kind of gas sensors have been widely used in several applications in industry, vehicle technologies, health care, household appliances and alarm systems, etc. The problems of the currently available gas sensors are their often rather expensive fabrication costs, high energy consumption, and low selectivity for detected gases. Solution based processing eg. printing and coating, offer a cost efficient way to produce novel functionalities and provide a way to develop completely new applications fields. In this paper gravure printed resistive gas sensor is used for ppm level monitoring of NOx gas. NOX gas sensor was manufactured by gravure printing WO3 nanoparticles onto lithographically made silver finger electrodes. These electrodes were manufactured onto a flexible plastic PEN substrate using three different channel lengths of 10, 20, and 50 µm. The nanoparticles were dispersed into solvent together with binders and surfactants to make the ink printable and adjust its viscosity and dispersion stability. The printing was done with a table-top gravure printing using different ink transfer volumes. After printing, the layer was dried in an oven at 200 °C for 2 h. The print quality depended on the ink transfer volume. As the transfer volume increased, thicker layers were obtained but at the same time the amount of particle agglomerates increased. With thin layers, the amount of agglomerates decreased but the ink layer coverage became poorer. The sensitivity to NOX gases was determined by measuring the resistance of the sensor as a function of gas concentration and chamber temperature. The concentrations were 5, 25, 50, and 90 ppm and temperatures 125, 150, 175, and 200 °C. The printed resistive gas sensor was sensitive to NOX gases since the conductivity of the nanoparticle layer increased as gas was introduced into the chamber. A clear response was obtained as the gas concentration was 25 ppm or higher. The increase in the chamber temperature, ink layer thickness, and gas concentration as well as the decrease in the channel length of the finger electrodes improved the sensor response. The effect of the layer thickness was not that significant since thicker layers contained more agglomerates that interfere with the gas sensing than thinner ones. The substrate limited the operating temperature to 200 °C at which the gas response curve became ragged. The sensitivity was 1.15 at 25 ppm and 2.40 at 90 ppm. The response time of the sensor was 20-30 s and the recovery time depended on the gas concentration and chamber temperature. The higher the concentration and temperature, the longer the recovery time was due to the better sensor response. The recovery time was 20 s - 4.5 min.