Design and Fabrication of Flexible Thin-Wire Multispot Thermocouples Using Hot-Wire Coating

Temperature measurement is still one of the most important applications for sensor devices. Besides resistive thermal sensors, semiconductor devices, and infrared thermal sensors, thermoelectric devices (or thermocouples) are widely used. This work presents a sensor concept and an associated fabrication method for flexible thermoelectric sensors based on a single metal wire substrate with a diameter of only 80 μm covered by one or multiple coating layers of isolating and conductive materials. These devices are fabricated using hot-wire coating technology, where an electric current heats the substrate via Joule heating, causing applied inks to cure nearly instantly. Due to the small diameter of the used wires, the sensors are not only highly flexible but also feature ultra-low response times. The samples are fabricated by coating a commercially available 80 μm thick copper wire with multiple layers of poly(3,4-ethylene-dioxythiophene:styrene sulfonate) (PEDOT:PSS) and silver ink separated by layers of polyamide-imide (PAI) for insulation. Intentional gaps (“holes”) in the insulation allow for the electrical connection of different layers, creating the hot junction of a thermocouple. This design allows for any number of thermocouples to be integrated into a single wire, provided that enough layers of conductive material are provided. The fabricated samples feature thermocouples utilizing copper-PEDOT:PSS as well as silver-PEDOT:PSS junctions, showing different sensitivities to temperature. The thermocouples were characterized to investigate possible differences between material combinations and solvent mixtures and showed a large but expected deviation when comparing two different PEDOT:PSS solutions. The sensitivity was measured to be around 6.3 μV/K and 13 μV/K respectively, for both copper-PEDOT:PSS-thermocouple variants and 9.4 μV/K for the silver-PEDOT:PSS-thermocouples.