Far infrared reflectance of sintered nickel manganite samples for negative temperature coefficient thermistors

Abstract

Single phase complex spinel (Mn, Ni, Co, Fe)(3)O-4 samples were sintered at 1050, 1200 and 1300 degrees C for 30 min and at 1200 degrees C for 120 min. Morphological changes of the obtained samples with the siDtering temperature and time were analyzed by X-ray diffraction and scanning electron microscope (SEM). Room temperature far infrared reflectivity spectra for all samples were measured in the frequency range between 50 and 1200 cm(-1). The obtained spectra for all samples showed the presence of the same oscillators, but their intensities increased with the sintering temperature and time in correlation with the increase in sample density and microstructure changes during sintering. The measured spectra were numerically analyzed using the Kramers-Kronig method and the four-parameter model of coupled oscillators. Optical modes were calculated for six observed ionic oscillators belonging to the spinel structure of (Mn, Ni, Co, Fe)(3)O-4 of which four were strong and two were weak.