Influence of nano-sized cobalt oxide additions on the structural and electrical properties of nickel-manganite-based NTC thermistors

Abstract

The structural and electrical properties of NiMn2O4 and Ni0.5CoxMn2.5-xO4 (where x = 0.5, 0.8 and 1.1) NTC thermistors have been investigated. The samples, prepared by conventional ceramic processing techniques, were calcinated at 900°C for 2 h and then sintered at 1100°C and 1200°C for 5 h. The cubic spinel phase was observed by XRD analysis in the NiMn2O4 and Ni0.5Co0.8Mn1.7O4 samples sintered at 1100°C for 5 h. The sintering at 1200°C resulted in much denser microstructures with a larger grain size. The room-temperature electrical resistivity (r25) and material constant (B) value of the NiMn2O4 sample sintered at 1100°C were 7710 W cm and 3930 K, respectively. The electrical resistivity of the samples decreased significantly with the addition of Co3O4. The B25/85 values of the Ni0.5CoxMn2.5-xO4 (where x = 0.5, 0.8 and 1.1) samples sintered at 1100°C were found to be 3820 K, 3525 K and 3270 K, respectively. Keywords: cobalt oxide, electrical properties, microstructure, NTC thermistor Preiskovana je bila zgradba in elektri~ne lastnosti NiMn2O4 in Ni0.5CoxMn2.5-xO4 (kjer je x = 0,5, 0,8 in 1,1) NTC termistorjev. Vzorci, pripravljeni po obi~ajni tehniki priprave keramike, so bil kalcinirani 2 h na 900°C in potem 5 h sintrani na 1100°C in 1200°C. Kubi~na {pinelna faza je bila opa`ena pri XRD-analizi, v vzorcih NiMn2O4 in Ni0.5Co0.8Mn1.7O4, sintranih 5 h na 1100°C. Sintranje na 1200°C je povzro~ilo mnogo bolj gosto mikrostrukturo z ve~jimi zrni. Vrednosti za elektri~no upornost pri sobni temperaturi (r25) in materialne konstante (B) vzorca NiMn2O4, sintranega na 1100°C, sta bili 7710 W cm in 3930 K. Elektri~na upornost vzorcev se je ob~utno zmanj{ala po dodatku Co3O4. Vrednosti B25/85 pri vzorcih Ni0.5CoxMn2.5-xO4 (kjer je bil x = 0,5, 0,8 in 1,1) sintranih na 1100°C so bile: 3820 K, 3525 K in 3270 K.

Nano–Crystalline Mn–Ni–Co–O Thermistor Powder Prepared by Co–Precipitation Method

“…These changes in microstructure explain the promotion of densification, as well as the RD increase with respect to sintering temperature shown in Figure 6. It should be noted that the sintering temperature of 1100 • C, which enabled an RD value larger than 97%, is lower than those reported for most conventional ceramics that possess the same RD level [32][33][34]. On the other hand, at the sintering temperature of 1100 • C, the pure cubic spinel structure is retained without any other secondary phase being detected, even though the dwell time was extended to 6 h (Figure 8).…”