Conductivity behavior of n -type semiconducting ferrites from hydrothermal powders
- PDF / 166,942 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 21 Downloads / 223 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Conductivity behavior of n-type semiconducting ferrites from hydrothermal powders Anderson Dias Departamento de Engenharia Metal´urgica e de Materiais, UFMG, 30160-030, Belo Horizonte, MG, Brazil
Roberto Luiz Moreira Departamento de F´ısica, ICEx-UFMG, CP 702, 30123-970, Belo Horizonte, MG, Brazil (Received 27 January 1997; accepted 17 March 1998)
The frequency and temperature dependencies of the dielectric permittivity and of the electrical conductivity of excess ferrous ions hydrothermal NiZn ferrites were analyzed before and after sintering. A decreasing tendency with frequency of the dielectric responses was observed, but the high permittivities attained (e 0 ø 105 ) masked any relaxation in these materials. This behavior is characteristic of n-type semiconducting ferrites, where electron hopping between Fe12 and Fe13 ions leads to very high conductivity values. The temperature dependence of the dielectric permittivities revealed the existence of broader peaks. The electron hopping mechanism leads to a frequency dispersion of the temperature where the permittivities attain their maxima. The electrical conductivity variations with temperature exhibited Arrhenius-type behaviors, with activation energies ranging from 0.34 eV (hydrothermal powder) to 0.16 eV (for the highest sintering temperature). These results were correlated to the variations in Fe12 concentration and microstructure.
I. INTRODUCTION
The interest in the electrical characteristics of interfaces has been stimulated by the development of grain boundary controlled electronic ceramics, such as MnZn and NiZn ferrites.1 These materials are in the category of semiconductive ceramics, which are characterized by microstructures consisting of relatively more conductive grains and less conductive grain boundaries. Most of the external electrical field applied to the specimens is concentrated in the higher resistive region. Thus, the characteristics of the grain boundary phase control the electrical properties of these materials. Also, the resistivity of the ferrites depends strongly on their stoichiometry; for instance, for the NiZn ferrites, it can vary by up to 8 orders of magnitude depending on the (Ni, Zn) to Fe ratio.2 The electrical response of polycrystalline materials such as NiZn ferrites must be deconvoluted to enable a meaningful analysis of the bulk and grain boundary contributions. Measurements of the frequency-dependent complex impedance can, in principle, accomplish this goal.3 Indeed, this technique has been successfully applied to characterize the electrical properties of ferrites.4,5 The interpretation of the data is often based on the ability to represent the sample by an equivalent electrical circuit. A number of reports on dielectric dispersion in ferrites over a wide frequency range show that a simple two-component model does not give an appropriate description.5–7 In such cases, a random network model 2190
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 8, Aug 1998
Download
Data Loading...
