NTC (Negative Temperature Coefficient) thermistors are widely used in industrial applications for temperature measurement, control and compensation. One important problem in the use of NTC thermistors is to overcome aging phenomenon. Their electrical properties change with time particularly when the temperature increases and it is the main reason why they are unable to be used at temperatures above 150℃.
Aging of electrical properties is attributable to non-equilibrium states that exist inherently in semiconducting NTC ceramics.
Sol-gel auto combustion is an advanced process that is widely used in nano-powder synthesis such as magnetic material.
Yu Nam Chol, a researcher at the Science Engineering Institute, successfully prepared Fe0.8Mn1.54Ni0.66O4 nano-ceramics by sol-gel auto combustion and observed the microstructure and phase of these samples by using Scanning Electron Microscope and X-Ray Diffraction. The results showed that powdered Fe0.8Mn1.54Ni0.66O4 ceramics pre-fired at 800℃ and sintered at about 1 050℃ had more compact and uniform spinel structure. In addition, the investigation of the electrical characteristics of these thermistors manufactured at different sintering temperatures proved that the sample sintered at 1 200℃ was suitable for the improvement of the properties of thermistors. It was also shown that the thermistor annealed for 72 hours at 450~550℃ in N2 atmosphere had aging ratio of less than 0.7%.
The details about this are carried in his paper “Preparation of Fe-Mn-Ni-O Nano-Ceramics by sol-gel auto combustion for NTC thermistor” presented to the SCI Journal “International Journal of Materials”.