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

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 2430 K to 3805 K 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 resistance-temperature 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.