Effects of grain size distribution on Cole-Cole plots of polycrystalline spinels
- PDF / 118,866 Bytes
- 5 Pages / 595 x 842 pts (A4) Page_size
- 17 Downloads / 194 Views
Effects of grain size distribution on Cole-Cole plots of polycrystalline spinels
M. P. Gutiérrez-Amador and R. Valenzuela Institute for Materials Research, National University of México, México, D.F., 04510 México.
ABSTRACT Spinel ferrites of composition Ni0.5Zn0.5Fe2O4 were prepared by coprecipitation. Different grain size and grain size distributions were obtained by various heat treatments. Their electrical properties were investigated by impedance spectroscopy in the temperature range 25-200oC and in the frequency range of 5Hz-13MHz. A decrease in resistivity was observed as grain size increased, which can be explained by an increase in the mean free path of electrons. Cole-Cole plots showed two well-resolved semicircles for samples with a narrow grain size distribution. As the width of the distribution increased, the semicircles exhibited a deformation and eventually became unresolved. These results are interpreted on the basis of a distribution of time-constant of the impedance response, associated with the grain size distribution.
INTRODUCTION Ferrites remain one of the best magnetic ceramics in many technological applications because of their excellent combination of good magnetic and electrical properties. Their electrical conductivity has been the subject of continuous investigations. In particular, electrical properties of Ni-Zn ferrites have been studied as a function of composition [1,2] and sintering atmosphere [3,4]. Impedance spectroscopy has been among the most useful investigation techniques since the impedance of grains can be separated from the other sources of impedance, namely, grain boundaries and electrodes. Microstructural effects have been considered as one of the most important factors influencing the impedance response in ceramic materials. When the grain boundary resistance is larger than the bulk’s, two semicircles are often obtained in Cole-Cole plots. Deviations from ideal semicircles have been attributed to the distribution of the relaxation times arising from the microstructural inhomogeneity or disorder in the samples [3-5]. In spite of the importance of this aspect, to our knowledge, systematic studies about this subject have not been carried out. The aim of this work is to investigate the influence of grain size distribution on the impedance response of ferrites. Ni-Zn ferrites were chosen because they usually show a high grain boundary resistivity, which allows a clear resolution of impedance contributions from grains (bulk), grain boundaries and electrodes.
R3.13.1
EXPERIMENTAL PROCEDURE
Polycrystalline Ni0.5Zn0.5Fe2O4 ferrite was prepared by coprecipitation from aqueous solutions of Fe(NO3)3•9H2O, NiCl2•6H2O and ZnCl2, using Na2CO3 in order to coprecipitate the reagents. During precipitation the temperature was maintained at 80-90oC in a pH of 911. The precipitate was calcined at 600oC/2h. A single ferrite phase was observed by X-ray diffraction powders with very fine particles (
Data Loading...
