Enhanced aging and thermal shock performance of Mn_1.95-xCo_0.21Ni_0.84Sr_xO₄ NTC ceramics

The Mn_1.95-xCo_0.21Ni_0.84Sr_xO₄ (MCNS) (0 ≤ x ≤ 0.15) based negative temperature coefficient (NTC) materials are prepared by co-precipitation method. The replacement of Mn by Sr plays a critical role in controlling the lattice parameter, relative density, microstructure, and electrical properties. The lattice parameter and relative density increase with the increase of Sr content. A small amount of Sr restrains the grain growth and increases the bulk density. Moreover, the room resistivity ρ₂₅, material constant B_25/50, activation energy E_a, and temperature coefficient α values of MCNS ceramics are influenced by the Sr content and ranged in 1535.0-2053.6 Ω·cm, 3654-3709 K, 0.3149-0.3197 eV, and (-4.173%)-(-4.111%), respectively. The X-ray photoelectron spectroscopy (XPS) results explain the transformation of MCNS ceramics from n- to p-type semiconductors. The conduction could arise from the hopping polaron between Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺ in the octahedral sites. The impedance data analysis also discusses the conduction mechanism of the MCNS ceramic, whereas grain resistance dominates the whole resistance of the samples. Furthermore, the aging coefficient (ΔR/R) of MCNS ceramics is found to be < 0.2%, which indicates the stable distribution of cations in the spinel. Finally, the MCNS ceramics demonstrate excellent thermal durability with < 1.3% of resistance shift after100 thermal shock cycles.