One-step synthesis of Ni-Mn-Al-O thin film on Al2O3 substrate via hydrothermal method

The hydrothermal reaction method was firstly introduced to deposit high quality negative temperature coefficient (NTC) thermistor thin films directly on Al2O3 substrates. The crystal phase, microstructure and electrical properties of as-synthesized film were investigated in terms of reaction time. Basically, the crystallinity of Ni-Mn-Al-O film increased by varying the reaction time from 2 to 15 h, however, the cubic spinel phase was the only phase in relation to the film prepared from 15 h. On the other hand, it was found that the grain size increased from 2.91 μm to 5.46 μm with the increase of reaction time. Further, all prepared films exhibited typical NTC characteristic when the temperature ranges from room temperature to ~ 250 °C. Remarkably, the B25/50 constant and activation energy increased with the increase of reaction time were distributed within the range of 4160-4645 K, 0.359-0.4008 eV, respectively, both of which were increased with the reaction time as well.