Preparation of bixbyite phase (Mn xFe 1− x) 2O 3 for NTC thermistor applications

Abstract

Spinel compounds like doped Mn 3O 4 and ferrites are conventionally used as negative temperature coefficient (NTC) thermistors. Incidentally, Mn 2O 3 has got two orders of magnitude lower room temperature resistivity than that of Mn 3O 4 and it is expected that doped Mn 2O 3 should perform better than doped Mn 3O 4 as NTC thermistor. However, Mn 2O 3 can not be fabricated in the ceramic form as it decomposes to Mn 3O 4 above 900 °C. We have found that iron, as solid solution, can stabilize Mn 2O 3 as bixbyite (Mn x Fe 1− x ) 2O 3 phase even at high temperatures We have successfully prepared phase-pure nano-size powders of (Mn x Fe 1− x ) 2O 3 (where x=0.98, 0.66 and 0.37) by citrate–nitrate gel method. After sintering the precursor powders at high temperature (1500 °C), their electrical behaviour was studied. It has been found that the NTC sensitivity index ( β) of the bixbyite phase (Mn x Fe 1− x ) 2O 3 (where x=0.37 and 0.66) can be as high as 6000 K (in the temperature range of 50–150 °C) which is appreciably higher than that of conventional NTC materials.