Conductivity relaxation studies on sol-gel-derived Li 2 O-SiO 2 glasses by a heterogeneous conductor model
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Conductivity relaxation spectra of sol-gel-derived glasses in the system Li2O (1 - x) SiO 2 , with x varying from 0.15 to 0.25, have been analyzed using a heterogeneous conductor model. The latter comprises diagonal layers of phases, one of them being rich in alkali ions and the other deficient in the same. Satisfactory agreement between the experimental data and the theoretical curves has been obtained over the frequency range 100 Hz to 1 MHz. The Kohlrausch-Williams-Watts (KWW) exponent, ft, has been calculated from the conductivity spectra for all the glasses. The sol-gel-derived glasses have ft values smaller than those obtained in the case of the corresponding melt-quenched ones. The ft value is found to be correlated to the conductivity fluctuation in the alkali-rich phase.
I. INTRODUCTION Investigation of conductivity relaxation in a variety of materials has been of considerable interest because it throws much light on the ion movement mechanism in these systems.1"3 The dielectric data for these materials have been analyzed by using the Kohlrausch-Williams-Watts (KWW) stretched exponential function4
F{t) =
(1)
where TR denotes the conductivity relaxation time, and the exponent ft has a value less than one. The distribution of relaxation times is related to the magnitude of ft. A small value of the latter indicates a wide distribution and vice versa. To explain the occurrence of such nonDebye relaxation in amorphous materials, a many-body interaction has been invoked.1 It has been proposed that the jumping ion contributing to conductivity relaxation interacts with the other ionic species within the glass structure. Detailed models have been proposed, delineating the ion jump mechanism in disordered materials.4-5 Recently, we have looked at this problem considering a possible heterogeneous distribution of the mobile ion within the glass structure. We have used a diagonal layer model,6 modified to take into account the spatial variation of mobile ions in the glass system for calculating the electrical modulus spectra of different oxide glasses containing alkali ions.7 By fitting the experimental modulus spectra to the KWW model specified by Eq. (1), it has been possible to correlate the value of the exponent, ft, to the parameters describing the spatial
a) Present
address: Durgapur Government College, Durgapur-713214, West Bengal, India.
1442 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 6, Jun 1993 Downloaded: 06 Apr 2015
conductivity variation in the inhomogeneous conductor model mentioned as above. So far our analyses have been restricted to melt-quenched oxide glasses. We have now extended these investigations to glasses prepared by the sol-gel method. In this paper we report on the results obtained on sol-gel-derived lithium silicate glasses. II. EXPERIMENTAL In Table I the compositions of the glasses prepared in the present work are summarized. A weighed amount of lithium acetate (CH3COOLi, 2H2O) is first of all dissolved in a mixture comprising 2 ml water and 4 ml acetic acid (CH 3 C
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