Ultraviolet Laser Sintering of Printed Nickel Oxide Nanoparticles for Thin-Film Thermistor via Aerosol Jet Printing Technology

In this study, nickel oxide (NiO) thin films were printed by an aerosol jet printer, which is capable of fabricating thin films on the curve substrate via air stream. To approach high efficiency fabricating thin film thermistors in small batch sizes, the printed NiO nanoparticle thin films were sintered by using a 355 nm wavelength ultraviolet (UV) laser; this novel fabrication method reduced several steps of the conventional manufacturing process of the thermistor. Compared with furnace heat treatments of the NiO thermistor in previous studies, the UV laser sintering not only significantly improved the electrical properties but decreased the treatment time from an hour to a second. Since the resistance declined, the thermistor has been operated at an ambient temperature, which provides ready measurement. The resistance and morphology of the thin films were analyzed for evaluating the effect of the laser treatment. To identify the proper UV laser parameters, three laser parameters, including laser output energy, frequency, and scanning speed, were studied. Due to the laser-sintering parameters, namely, 2 W, 150 mm/s, 90 kHz, and a B value of 4683 K, the resistance has been reduced from 106.8 MΩ to 6.15 MΩ at 100 °C. The experiments exhibited a series of analyses for sintering states and defects of printed NiO nanoparticle thin film, which were sintered by UV laser. For NiO nanoparticles, UV laser has higher absorption energy than that of other wavelength lasers, when excess laser output was applied to the NiO thin film, cracks were observed on the surface. It was found that the crystal plane distances were not affected by recrystallization, but the cracks were based on the XRD analysis. Based on the analysis, there were obvious regional compressive stains before the appearance of cracks, and the uneven shrinking strains caused the cracks on the surface as energy irradiation increased.