Polycrystalline oxide materials exhibit semiconductor properties due to grain boundary and grain characteristics, which enrich the variety of applications. However, how to regulate the energy band structure of grains and the potential barriers at grain boundaries through defect engineering is crucial for electronic devices to achieve high performance .

Recently, Prof. Bo Zhang (Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Science) et al. have deeply analyzed the nonlinear physical mechanism of electrical properties in the high temperature region by studying the grain and grain boundary effects of CaCu₃Ti₄O₁₂ (CCTO) materials; and achieved the linearization by changing the energy band structure induced by Fe³⁺ doping.

They found that the grain resistivity of CCTO ceramic materials exhibits anomalous PTC (PTC, Positive Temperature Coefficient) characteristics after 575 K, which is the main reason for the nonlinearity of lnρ-1000/T curves in the high temperature region. First principles calculation indicates that Fe³⁺ doping narrows the forbidden band and induces new impurity energy levels in the forbidden band, which matches the conclusion that the resistivity-temperature dependence of grains shifts toward the low-temperature region as derived from impedance spectroscopy. This shift results in no monotonic variation in grain resistivity within the application temperature region, thus enhancing the linearity of the lnρ-1000/T curve of CCTO materials in the high temperature region. In addition, Fe³⁺ ions can modulate the activation energy of CCTO materials in a wide range by changing the activation energy of grain boundaries, which broadens the temperature range of CCTO. This approach is based on the modulation of grain and grain boundary by Fe³⁺ doping, which provides a new avenue for the study of polycrystalline semiconductor materials.

Relevant research results were published in the Appl. Phys. Lett.. with the title of “Enhanced linearity of CaCu₃Ti₄O₁₂ by changing energy band structure induced by Fe³⁺ doping for high temperature thermistor application”. The research work is supported by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of CAS, and the West Light Foundation of the Chinese Academy of Sciences.

Article:https://doi.org/10.1063/5.0096124

Linearization mechanism of CaCu₃Ti₄O₁₂-based thermistor ceramics in the high-temperature region.