Influence of B³⁺- and Na⁺-ions on electrical property and temperature sensitivity of NiO-based ceramics

For various applications of negative temperature coefficient (NTC) thermistors, it is useful to develop a material system to achieve different room-temperature resistivities (ρ₂₅) and temperature sensitivity (B value) by adjusting slightly the chemical composition. Here, B³⁺/Na⁺-modified NiO-based ceramics (denoted as xB/yNa-NiO, 0 ≤ x ≤ 0.04 and 0 ≤ y ≤ 0.07) were prepared for NTC thermistors. The phase component and microstructure of the ceramics were detected respectively by using X-ray diffraction and scanning electron microscope. The related electrical properties and temperature sensitivity were investigated by analyzing the resistivity-temperature characteristic and related complex impedance spectra. The results show that all the prepared ceramics have a cubic crystalline structure and present typical NTC characteristics. By changing the concentrations of B³⁺- and Na⁺-ions in the compounds, ρ₂₅ from 47.94 Ω cm to 1.024 MΩ cm and B values from 2582 to 8019 K were achieved. The analysis of complex impedance spectra reveals that both grain effect and grain boundary effect contribute to the electrical conduction and NTC feature. The conduction mechanisms combining with band conduction and polaron hopping model are proposed for the NTC effect in xB/yNa-NiO thermistors.