Nickel Oxide-based Flexible Thin-Film NTC Thermistors by using reverse offset printing

In recent years, the use of printing methods to fabricate electronic devices (printed electronics) has attracted attention because of their low cost and low environmental impact. Printing technology enables the high-throughput fabrication of electrical circuits on film substrates, providing inexpensive personal healthcare devices to monitor health status in real-time. Temperature detection is one of the central concerns as a fundamental physical quantity in various fields. In 2013, a highly sensitive flexible thermistor was reported by formulating aqueous inks of nickel oxide nanoparticles for inkjet printing. However, the calcinating of the nickel oxide (NiO) layer required a high-temperature process of more than 200°C, which required expensive polyimide films with high heat resistance. It is necessary to promote further the development of low-temperature processes for printed thermistors to realize flexible NTC thermistors at low cost using printed electronics technology. In screen printing and inkjet printing, the definition of the ink pattern applied on the substrate changes due to spreading and coffee distortion phenomena, and the thickness between sensors becomes non-uniform, which is a structural consistency problem that can lead to variations in sensing performance. This study developed a printing and low-temperature calcinating method of NTC thermistors with a temperature-sensitive layer of nickel oxide by using reverse offset printing. The NTC thermistors were fabricated by printing a comb-like pattern of silver nanoparticles and a thin nickel oxide film on a glass substrate. In addition, the low-temperature formation of a nickel oxide layer by oxygen plasma treatment was investigated, and XPS was used to carry out compositional analysis of the surface. Together with the plasma-assisted calcinating, a flexible NTC thermistor formed on polyethylene terephthalate (PEN) film is demonstrated.

Cu-Mn Co-doped NiFe2O4 based thick ceramic film as negative temperature coefficient thermistors

NTC (Negative Temperature Coefficient) thermistors are widely used as temperature sensors in industrial and medical applications due to their high-temperature sensitivity, durability, and low cost. Generally, NTC thermistors are made from spinel structured ceramics formed by transition metal oxides with the general formula AB2O4. One of the spinel structured ceramics that can be made for NTC thermistors is NiFe2O4 nanoparticles. This work aimed to prepare Cu-Mn co-doped NiFe2O4 based thick ceramic film using Jarosite mineral as a precursor. The synthesis method used was a simple coprecipitation method, while the technique used in making a thick ceramic film was a simple screen printing technique. The sintering temperatures used were 1000 °C, 1100 °C, and 1200 °C. Based on x-ray diffraction analysis, the thick films consist of spinel phase, hematite phase, and some unidentified phase. The constant thermistor values (B) for thick films obtained with 1000, 1100, and 1200 °C sintering temperatures were 4740 K, 5669 K, and 5731 K, respectively. These results showed that all obtained thick films had passed the minimum value in market needs (B ≥2000 K).