Ferrite-based room temperature negative temperature coefficient printed thermistors

Two screen printing inks were developed for the low-temperature fabrication of printed and flexible thick film negative temperature coefficient (NTC) thermistors able to operate at room temperature. The first of the two screen printing inks developed utilised cobalt ferrite (CoFe₂O₄) as the temperature sensing material with the second ink incorporating manganese ferrite (MnFe₂O₄). These were then screen printed onto lithographically printed silver interdigitated electrodes with a 200-micron track and gap using a synthetic paper (Teslin) as the substrate. The inks required a 10-minute curing step at 80^oC. Preannealing of the ferrite powders before ink formulation enabled the avoidance of high-temperature processing post-fabrication typically required in industrial thermistor production. The printed thermistors were tested at a controlled constant humidity between 15 and 50^oC. Both materials demonstrated typical natural logarithmic responses with high linearity and sensitivity.