Solid State Ionics 136–137 (2000) 375–380 www.elsevier.com / locate / ssi The effect of humidity on the conductivity processes in 2 1 1 polycrystalline Cu -stabilised K -b0-ferrite a b, a c * M. Karus , O. Kalogirou , W. Gunsser , A. Reller a Institute of Physical Chemistry, University of Hamburg, Bundesstrasse 45, 20146 Hamburg, Germany b Third Laboratory of Physics, Department of Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece c ¨ Solid State Chemistry, University of Augsburg, Universitatsstr. 2, 86135 Augsburg, Germany Abstract 21 1 The effect of humidity on the conductivity of Cu -stabilised K -b0-ferrite was investigated by impedance spectroscopy. Several conductivity processes could be determined in the temperature range between 20 and 5008C in humidified atmospheres with water partial pressures between 26 and 2510 Pa. Bulk conductivity, conductivity through grain boundaries and electrode processes were found for as-prepared samples. In aged samples an additional conductivity at the grains surfaces was revealed. Thermogravimetric analysis was used to explain this behaviour. A non-uniform and time dependent water distribution profile along the diameter of pressed polycrystalline samples is suggested.  2000 Elsevier Science B.V. All rights reserved. Keywords: b0-ferrite; Ionic conductivity; Humidity 1. Introduction occupation of free alkali sites in the conduction plane. On the contrary, a humidity dependent in- 21 Potassium b0-ferrite stabilised by Cu is a single crease in conductivity has been observed at tempera- phase b0-ferrite formed with less than 5% of the tures below 1008C. Recently, we have reported the related b-phase [1,2]. The material is isostructural to determination of several conductivity processes in a b0-alumina. The compound is a good potassium ion wet atmosphere [2]. A reversible uptake and loss of conductor in dry atmospheres; the electronic con- water has been found to correspond with changes in ductivity is about two orders of magnitude smaller conductivity. This work provides a more detailed [1]. The influence of water taken up from a sur- view on these processes, using different water partial rounding wet atmosphere on the electric properties of pressures and an ageing process of the material. 1 K-b0-ferrites has been investigated previously [3– 5]. The water molecules tend to decrease the con- ductivity at temperatures above 1008C due to an 2. Experimental 21 1 Powder of Cu -stabilised K -b0-ferrite (rhom- *Corresponding author. Tel.: 130-31-998-148; fax: 130-31- ¯ bohedral symmetry, space group R3m) was prepared 998-003. E-mail address: orestis@lab3.physics.auth.gr (O. Kalogirou). by the standard ceramic technique as described in 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(00)00457-4