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Nb/Al-modified CuO ceramics (yNb/0.02Al-CuO, 0 ≤ y ≤ 0.07, denoted as NACO)  were synthesized using sol-gel method for applications in negative  temperature coefficient (NTC) thermistors. The phase structure,  microstructure and electrical properties of the ceramics were investigated.  XRD investigation reveals that the NACO ceramics has the main phase with  monoclinic crystalline structure. The analysis of X-ray photoelectron  spectroscopy proved the existence of Cu2+/Cu+ and Nb5+/Nb4+ ions.  Temperature dependence of the resistivity indicated that the NACO ceramics  present typical NTC characteristic. The NTC materials’ constant, B value,  can be adjusted from 2430K to 3805K by changing the Nb-concentration in the  Al-doped CuO ceramics. Among four applied calibration equations the Hoge-3  equation is the most effective one for the resistancetemperature calibration  of the prepared NTC thermistors. The complex impedance analysis was  performed and revealed that both grain effect and grain boundary effect  similarly contribute to the electrical conductive behaviour and NTC feature  of the NACO ceramics. The band conduction and polaron hopping conduction are  proposed as the conduction mechanisms in the NACO thermistors.

Electrical properties of Nb/Al-doped CuO-based ceramics for NTC thermistors