Multi-scale structural regulation of negative temperature coefficient effect and electrical conduction mechanism of xSm2O3-(1-x) CaCu3Ti4O12 ceramics

In this work, a series of x Sm 2 O 3 -(1- x ) CaCu 3 Ti 4 O 12 ( x Sm 2 O 3 -(1- x )CCTO, x  = 0.00–0.05) negative temperature coefficient (NTC) ceramics were prepared by a solid-state method. The influence of Sm 2 O 3 content on electrical properties was examined using microstructural, band gap, morphological, Direct Current (DC) resistivity, and impedance spectroscopy analysis. The increase in Sm 2 O 3 content resulted in a gradual decrease of the resistivities ( ρ 25 ), thermal constants ( B 25/75 ), activation energies for electrical conduction ( E a ) and activation energies of the relaxation process ( E relax ) from 1.16 × 10 7  Ω cm, 6387 K, 0.5456 eV and 0.5592 eV to 3.43 × 10 5  Ω cm, 4149 K, 0.4088 eV and 0.4115 eV respectively. First-principles calculations revealed a reduction in the band gap by adding Sm 2 O 3 , partially explaining the decrease in electrical resistivity. In-situ XPS analysis elucidated a relationship between conduction mechanism of Sm 2 O 3 -doped CCTO ceramics and electron hopping. Impedance analyses showed that both grains and grain boundaries contributed to the NTC effect of ceramics. The electrical inhomogeneity arose from grains and grain boundaries, while local relaxation was due to the short-range motion of charge carriers.