25.
Core–shell structures have been proposed to improve the electrical properties of negative-temperature coefficient (NTC) thermistor ceramics. In this work, Al
2O
3-modified Co
1.5Mn
1.2Ni
0.3O
4 NTC thermistor ceramics with adjustable electrical properties were prepared through citrate-chelation followed by conventional sintering. Co
1.5Mn
1.2Ni
0.3O
4 powder was coated with a thin Al
2O
3 shell layer to form a core–shell structure. Resistivity (
ρ) increased rapidly with increasing thickness of the Al
2O
3 layer, and the thermal constant (B) varied moderately between 3706 and 3846 K. In particular, Co
1.5Mn
1.2Ni
0.3O
4@Al
2O
3 ceramic with 0.08 wt% Al
2O
3 showed the increase of
ρ double, and the change in its B was less than 140 K. The Co
1.5Mn
1.2Ni
0.3O
4@Al
2O
3 NTC ceramics showed high stability, and their grain size was relatively uniform due to the protection offered by the shell. The aging coefficient of the ceramic was less than 0.2% after aging for 500 hours at 125°C. Taken together, the results indicate that as-prepared Co
1.5Mn
1.2Ni
0.3O
4@Al
2O
3 NTC ceramics with a core–shell structure may be promising candidates for application as wide-temperature NTC thermistor ceramics.
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