A calcination-free sintering approach for Ni-Mn-Co-Fe NTC thermistors via $ \mathrm{B_2O_3} $ addition

The traditional solid-state processing of negative temperature coefficient (NTC) thermistors typically requires energy-intensive calcination and high-temperature sintering steps. In this study, a calcination-free route was successfully developed for the processing of \mathrm{Ni_{0.5}Co_{0.5}Fe_{0.3}Mn_{1.7}O_4} thermistors by employing B2O3 as a sintering additive. The un-doped Ni-Co-Fe-Mn thermistor samples were synthesized through conventional calcination at 900°C followed by sintering at 1100°C, whereas the \mathrm{B_2O_3} -added Ni-Co-Fe-Mn thermistor were directly sintered at 1100°C without any prior calcination. The structural and electrical NTC characteristics of \mathrm{B_2O_3} -added \mathrm{Ni_{0.5}Co_{0.5}Fe_{0.3}Mn_{1.7}O_4} thermistors were investigated. The XRD patterns of the samples exhibit the formation of the cubic spinel \mathrm{NiMn_2O_4} phase structure as a major phase with the coexistence of a minor tetragonally distorted spinel phase. The values of electrical resistivity at 25°C, material constant ( \mathrm{B_{25/85}} ), and activation energy ( \mathrm{E_a} ) of the un-doped Ni-Co-Fe-Mn sample were in the range of 4138 Ω.cm, 3787 K, and 0.326 eV, respectively. The electrical resistivity increased to 2709 Ω.cm, the activation energy decreased to 0.316 eV with the \mathrm{B_2O_3} addition. The increase in the concentration of divalent ions Co^{2+} , Fe^{2+} , Ni^{2+} observed by XPS analysis indicates that boron doping acts as a charge compensation mechanism within the structure and causes the formation of mixed-valence pairs in the structure. This increases the active sites required for the hopping mechanism, positively affecting the electrical conductivity of the material. The addition of \mathrm{B_2O_3} provides a highly effective strategy to eliminate the calcination step, significantly saving energy while enhancing microstructural and electrical characteristics for NTC thermistor applications. This outcome of research is important for saving energy during the traditional production route.