Negative temperature coefficient (NTC) thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature. They have various industrial applications. However, the popular NTC materials based on spinel forming oxide systems are commonly limited to temperatures below 300 °C as a result of the instability and changing electrical characteristics. Therefore, developing new materials with good electrical characteristics at high temperatures are needed.  

A research group led by Prof. CHANG Aimin at Xinjiang Technical Institute of Physics & Chemistry (XTIPC) of the Chinese Academy of Sciences, designed and prepared a new high temperature NTC thermistor material Mg(Al1-xCrx)₂O₄ through the substitution of Cr³⁺ for Al³⁺ in MgAl₂O₄.    

Researchers selected MgO, Al₂O₃ and Cr₂O₃ as raw material and employed the conventional solid-state reaction to synthesize Mg(Al_1-xCrx)₂O₄ (x = 0,0.1,0.3,0.4,0.5) ceramic samples.

The crystalline phases and the microstructure of the sintered ceramics were observed by X-ray diffraction (XRD) and the Scanning Electron Microscopy (SEM). 

Besides, researchers investigated the structure, conductive mechanism and electrical properties of Mg(Al_1-xCr_x)₂O₄ ceramics.The samples consist of a single phase isomorphic to the cubic spinel MgAl₂O₄. The X-ray photoelectron spectroscopy (XPS) results show that there are two different Cr ions in these samples, basically Cr³⁺ and Cr⁴⁺.

The electron hopping takes place between Cr³⁺ and Cr⁴⁺ ions. The obtained value of ρ₅₀₀, B_500/800 and E_a are in the range of 2.67×10³–3.72×10⁷ Ωcm, 6731–11886 K, 0.581–1.025 eV, respectively.   

The study will provide theoretical foundation for designing new high temperature NTC thermistor materials and it has been published in Journal of the Alloys and Compounds.  

This work was supported by the National Natural Science Foundation of China, the West Light Foundation of the Chinese Academy of Sciences, etc.  

Figure: (a) XPS spectra result showing the Cr 2p region of Mg(Al_1-xCr_x)₂O₄ ceramic；(b) Relationship between lnρ and 1000/T for the Mg(Al_1-xCr_x)₂O₄ thermistors.(Image by XTIPC)