Negative temperature coefficient (NTC) thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature. They are widely used in various industrial and domestic applications, such as temperature measurements and controls, temperature compensation and the suppression of in-rush current.    

YCrO₃ perovskites have received broad attention due to their magnetic and electronic properties, especially the promising applications as high temperature electrodes, thermoelectric and magnetoelectric materials. In particular, YCr_1-xMn_xO₃ (0≤x≤0.5) having an orthorhombic perovskite structure, has been considered as a candidate for NTC thermistor applications. However, little physical and chemical information is available on YCr_1-xMn_xO₃, particularly on its electrical conduction mechanism.  

Professor  CHANG Aimin's group from Xinjiang Technical Institute of Physics & Chemistry (XTIPC) of the Chinese Academy of Sciences, discovered electrical conductivity anomaly of YCr_1-xMn_xO₃ NTC ceramics, and   revealed the mechanism of electrical conductivity anomaly for these ceramics by using defect chemistry theory combination with X-ray photoelectron spectroscopy analysis. The major carriers in YCrO₃ are holes. The Mn ions act as an n-type dope, and partly compensate for the effect of metal vacancies, thus leading to an increase in the resistivity. The Mn⁴⁺ ions increase as the Mn content increases from 0.2 to 0.5, which promote the rise in charge carriers and the electron hopping, thereby resulting in a decrease in the resistivity (Fig.1). The proposed conduction mechanism  will provide theoretical foundation for designing new NTC thermistor materials by controlling atmosphere and doping ions.   

                               Fig. 1 (a) XPS spectra of Cr 2p regions of YCrO₃ ceramics. (b) Relationship between lnρ and 1000/T for the YCr_1-xMn_xO₃ NTC thermistors(image by XTIPC). 

The research results have been published in Applied Physics Letters, 104, 102109 (2014).