RONG Jinghao, WANG Yige, CAI Tingqing, GU Zongli, SUN Wenqiang, WANG Leiming

(College of Chemical and Textile Engineering, Xinjiang University of Science and Technology,

Korla 841000, Xinjiang Uygur Autonomous Region, China)

Extended abstract:

[Background and purposes] Co-Mn-Ni-O oxides have garnered significant attention, due to their high sensitivity, rapid response, good structural stability and low cost. However, their internal conduction mechanism remains elusive, while their long-term aging stability requires further improvement. In this study, Fe3⁺ doping was adopted to systematically modulate the electrical properties of Co-Mn-Ni-O based NTC thermistor ceramics, with a focus on their microstructure and electrical behavior. Fe³⁺ doping influenced the hopping behavior of variable-valence Mn³⁺/Mn⁴⁺ cations. Combined with an 800-hour aging test, intrinsic factors affecting the aging stability of the material were systematically examined, providing both theoretical foundation and experimental support for the development of high-performance NTC thermistor ceramics.

[Methods] Co_4−xMn_1.8Ni_0.2Fe_xO₈ (x=0–1.0) ceramics were prepared by using the solid-state reaction method. The raw powders were mixed through ball milling in alcohol, pre-calcined at 850 ℃ for 2 h, pressed into pellets and sintered at 1225 ℃. Silver electrodes were coated on surfaces of the sintered samples for electrical performance testing, followed by an aging experiment conducted at 125 ℃ for 800 h. Phase composition, microstructure and elemental chemical states of the materials were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Sintering behavior was analyzed by using a dilatometer, while the resistivity-temperature characteristics were measured from room temperature to 850 ℃ with a digital multimeter.

[Results] The temperature for maximum densification of this series of ceramics was determined to be 1225 ℃ using a DIL 402 dilatometer. XRD results confirmed the formation of cubic spinel phase in all ceramic samples with x=0 to 0.8 (JCPDS card #84-0482), with no traces of impurity phases. For x=1.0, the Fe₂O₃ phase was observed, due to the segregation of excess Fe³⁺ ions at the grain boundaries, which were resulted from their inability to fill the octahedral vacancies in the spinel structure. Scanning electron microscopy (SEM) observations revealed that, in the samples sintered at 1225 ℃, the undoped (x=0) samples exhibited some surface porosity and relatively large grain size. In contrast, the Fe³⁺-doped samples demonstrated a uniform microstructure, dense packing and clearly discernible grain boundaries. Within the temperature testing range of −50–100 ℃, resistivity (ρ) of all samples decreased with increasing temperature, where lnρ exhibited a linear relationship with 1000/T, displaying typical NTC thermistor characteristics. Among them, the resistivity at 25 ℃ (ρ25) of the sample with x=0 was 1972 Ω·cm, while the material constant at 25/50 ℃ (B25/50) was 3237 K. With increasing content of Fe³⁺ (x), both ρ₂₅ and B25/50 showed a gradual upward trend. Since the migration energy is positively correlated with the interionic distance, the contraction of the unit cell volume directly leads to a decrease in the B-value. X-ray photoelectron spectroscopy (XPS) results revealed that Mn existed in multiple valence states of Mn²⁺, Mn³⁺ and Mn⁴⁺, while the Fe coexisted as Fe²⁺ and Fe³⁺. Additionally, peak deconvolution was performed on Mn 2p and Fe 2p. After aging at 125 ℃ for 800 h, the resistance drift rate stabilized after 400 h, within a range of 5.16% to 6.94%.

[Conclusions] Electrical conduction behavior of the Co_4−xMn_1.8Ni_0.2Fe_xO₈ (x=0–1.0) ceramic system primarily originates from the hopping conduction of the Mn³⁺/Mn⁴⁺ ion pairs, while the increase in resistivity is attributed to the enhanced hopping resistance of Mn³⁺/Mn⁴⁺ and Fe³⁺/Fe²⁺ ion pairs. This synergistic effect, constructed by multivalent metal ions and oxygen vacancies, not only improves the structural stability of the lattice but also significantly facilitates the rapid migration of charge carriers and interfacial charge exchange. By constructing a schematic model of the concave grain aging of Co_4−xMn_1.8Ni_0.2Fe_xO₈ ceramics, the intrinsic mechanism for the stabilization of the resistance drift rate was clarified. The underlying mechanism involves the concave morphology, which allows for more direct exposure of grain boundaries to the environment, consequently promoting oxidation reactions. As oxygen ions embed in the grain boundary regions, the valence state of iron ions increases, accompanied by enhanced electron localization, leading to a decline in carrier concentration and a rise in resistivity. This oxidation process proceeds at a higher rate initially but slows downwards, due to the formation of a surface passivation layer, ultimately reaching a dynamic equilibrium. The establishment of this dynamic equilibrium serves to explain the stabilization of the resistivity drift rate over time, while offering further verification that grain boundary oxidation dominates the aging behavior. Therefore, by regulating the Fe doping concentration, long-term stability of the Co_4−xMn_1.8Ni_0.2Fe_xO₈ ceramics can be optimized, providing a feasibility basis for their application in high-temperature electronic devices. Fe doping not only modulates the chemical stability of grain boundaries but also further enhances the anti-aging performance of the material by suppressing the migration kinetics of oxygen vacancies. By constructing Fe doping concentration gradients, thermal stress mismatch can be effectively mitigated and interfacial compatibility enhanced. The stability mechanism of this material system provides universal design principles for multi-element oxides. Based on this foundation, multifunctional ceramic composites integrating high electrical conductivity with thermal stability can be developed to meet the requirements of integrated devices under extreme operating conditions in the future. Through precise regulation of Fe gradients and grain boundary chemistry, gradual matching of thermal expansion coefficients can be achieved, significantly reducing interfacial stress concentration.

Key words: NTC ceramic; doping; microstructure; electrical properties