Highly Sensitive, Fully Screen-Printed Sensor Matrix Based on a PTC Material for Sensing Thermal Energy Flow

Positive temperature coefficient (PTC) thermistors offer unique sensing capabilities based on their thermoelectric response.[1,2] In this study we take advantage of the extraordinary temperature-resistance-relation of a commercial printable PTC-ink originally designed for self-limiting heaters to develop novel and highly sensitive sensors. The sensors can detect minimal changes in the thermal conductivity of the surrounding enabling its usage as material-, liquid level- and caloric sensors.<br/>The sensor concept relies on the step-like temperature-resistivity relation of the PTC material. The material exhibits a relatively constant electrical resistance up to 60 °C. At this threshold a large increase in resistivity occurs with over one order of magnitude. At temperatures higher than 65 °C the resistance is relatively constant again yet at elevated level. This results in a temperature window of &lt; 5 K with an extremely high temperature sensitivity. The PTC thermistor is brought to and kept in this sensitive temperature window by powering it with constant voltage and therefore internally heating it. Around the operation temperature a slight temperature variation is easily detectable.<br/>The sensor is fabricated only by industrial scalable printing techniques like screen printing allowing different designs and functional concepts. The characteristics of the sensor will be presented and the obstacles for the integration of the high-sensitive sensors in a matrix for spatially resolved sensing will be discussed. In particular, we fabricated a fully screen-printed 4x4 matrix with a spatial resolution of 15 mm. As the PTC material requires to be powered continuously to maintain its working temperature at the steep slope of resistivity both measurement and heating must run simultaneously. For this, each pixel consists of the sensing PTC element and a small measurement resistor in series forming a voltage divider. The voltage over the measurement resistor depends on the resistance and therefore temperature of the PTC and is read out with an Arduino pixel by pixel.<br/>With our device we can visualise differences in thermal energy flow and effectively create real time heat maps or detect different thermal surroundings. We will highlight different applications, namely directly measuring temperature distribution, sensing fluid level and phase separation.<br/>[1] Horn, M., Umar, L. & Ruser, H. Self-controlled PTC sensor for Reliable Overfill Protection of Liquids. <i>IEEE Instrum. Meas.</i> (2002).<br/>[2] Horn, M., Ruser, H. & Umar, L. Self-calibrated PTC air flow sensor. in <i>Proceedings of IEEE Sensors</i> vol. 2 1771–1774 (IEEE, 2002).