Material screening for fully printed polymer-based thermocouples designed for use in harsh environments

Thermocouples are widely used as temperature sensors and most commonly made of two different metallic electrodes which are in contact at the measuring junction. In this work we present an approach to embed thermocouples in the painted surface of machine components, facilitating measurement directly at the point of interest for certain applications. The utilized spray process allows a cost-effective and fast fabrication method. In order to be competitive with available sensors, the spray-processed sensor ideally has to provide an output voltage in the same range as commercial ones and should withstand temperatures up to 200 °C while providing reliable adhesion to the surface at the same time. To meet these requirements, a material screening was performed including commercial as well as custom-fabricated paints. In particular, different commercial paints based on silver were combined with a carbon black polyamide-imide paint to form thermocouples which were then characterized regarding adhesion, noise and sensitivity (Seebeck coefficient). Furthermore, custom-fabricated paints based on iron and silver in a polymer binder were evaluated and compared to a commercial type J thermocouple (iron-constantan thermocouple). The paper reports on identified suitable material combinations and the associated sensor performances.