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(Al1-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(Al1-xCrx)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, B500/800 and Ea 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(Al1-xCrx)2O4 ceramic；(b) Relationship between lnρ and 1000/T for the Mg(Al1-xCrx)2O4 thermistors.(Image by XTIPC)
Xinjiang Technical Institute of Physics & Chemistry,CAS