Enhanced conductivity and stability of Co <sub>0.98</sub>Cu <sub> <i>x</i> </sub>Mn <sub>2.02− <i>x</i> </sub>O <sub>4</sub> ceramics with dual phases and twin structures
Chengjian Ma, Longhua He, Lei Bi, Hong Gao, Jianxiang Ding
Abstract
Semiconductor materials with heterogeneous interfaces and twin structures generally demonstrate a higher concentration of carriers and better electrical stability. A variety of Cu-doped Co<sub>0.98</sub>Cu<em><sub>x</sub></em>Mn<sub>2.02</sub><sub>−</sub><em><sub>x</sub></em>O<sub>4</sub> (0 ≤ <em>x</em> ≤0.5) negative temperature coefficient (NTC) ceramics with dual phases and twin structures were successfully prepared in this study. Rietveld refinement indicates that the content of cubic spinel phase increases with increasing Cu content. The addition of Cu can promote grain growth and densification. Atomic-level structural characterization reveals the evolution of the twin morphology from large lamellae with internal fine lamellae (L<sub>IT</sub> lamellae) to large lamellae without internal fine lamellae (L lamellae) and the distribution of twin boundary defects. First-principles calculations reveal that the dual phases and twin structures have lower oxygen-vacancy formation energies than those in case of pure tetragonal and cubic spinel, hence enhancing the transmission of carriers. Additionally, the three-dimensional charge-density difference shows that metal ions at the interface lose electrons and dwell in high valence states, thereby enhancing the electrical stability of the NTC ceramics. Furthermore, the additional Cu ions engage in electron-exchange interactions with Mn and Co ions, therefore reducing the resistivity. In comparison to previous Cu-containing systems, the Co<sub>0.98</sub>Cu<em><sub>x</sub></em>Mn<sub>2.02−<em>x</em></sub>O<sub>4</sub> series exhibit superior stability (aging value ≤ 2.84%) and tunable room-temperature resistivity and <em>B</em> value (17.5 Ω·cm ≤ <em>ρ</em> ≤ 7,325 Ω·cm, 2,836 K ≤ <em>B</em> ≤ 4,315 K). These discoveries lay a foundation for designing and developing new NTC ceramics with ultra-high performance.