The effect of firing conditions on the characteristics of thick-film resistors for temperature sensors

Abstract

An integrated miniature electrochemical sensor (ES) that offers rapid, sensitive, and selective detection of chemical and biological contaminants in a variety of samples requires temperature control to work accurately. To address this, one approach is to locate temperature sensor (TS) next to the ES components. However, this integration poses a challenge as different firing processes are required for the sensor components and the TS. Commercially available thick film materials for the realisation of TS are designed for screen printing on alumina and firing in air at 850 °C for 10 minutes. However, a key component of an ES, a carbon-based working electrode, must be fired in an oxygen-lean atmosphere. In this study, we investigated the influence of the firing atmosphere, i.e., air and argon, on the characteristics of thick-film resistors, including thickness, roughness, phase composition, resistivity, and temperature dependence. For the study, we used two commercially available thick-film materials, NTC2114 and NTC2113, as TS with nominal sheet resistivities of 10 kΩ/sq and 1 kΩ/sq at 25 °C, respectively. Using X-ray powder diffraction analyses, we detected RuO2 and spinel phases in the samples heated at 850 °C in air. However, when the samples were fired in argon, we detect metallic ruthenium and alloys. As a result of these changes, the resistivity of the NTC2114 and NTC2113 increased significantly. However, despite these changes, the relative resistance and the coefficient of temperature sensitivity did not vary significantly, indicating the suitability of these materials as TS. These findings have important implications for the future integration of TS into various screen-printed ES systems, fostering the design and development of systems with enhanced accuracy and reliability in temperature measurements.