Effects of heat treatment on the temperature coefficient of resistivity of laser fabricated NiCr flexible sensors

Advancements in data collection and automation require reliable flexible sensors due to their versatility and customization. One challenge with the practical use of flexible sensors is the change in resistance due to the temperature coefficient of resistivity (TCR) at different use temperatures. This research investigates the heat treatment process for a common metal sensing element, Evanohm (80/20 nickel chromium alloy), to reduce its TCR. NiCr thin films were heat treated in an inert argon environment at various temperatures and hold times, then fabricated into flexible sensors on a polymer substrate using UV nanosecond laser ablation. Sensors were mounted onto an aluminum substrate to provide mechanical backing during testing. Following heat treatment, TCR values as low as 1.44 ppm/°C were achieved at a heat treatment temperature of 475°C for 180 min of hold time. Electrical and microstructural changes were also analyzed to determine the mechanism for TCR change. The results demonstrate the effectiveness of heat treatment in reducing the TCR of NiCr flexible sensors, minimizing the impact of use temperature on resistance change. This research contributes to the development of NiCr flexible sensors with stable and linear resistance behaviour over the temperature testing range from -10°C to 150°C.