%0 Journal Article
%T Determination of Mobility and Charge Carriers Concentration from Ionic Conductivity in Sodium Germanate Glasses above and below
%A Marcio Luis Ferreira Nascimento
%J ISRN Electrochemistry
%D 2013
%R 10.1155/2013/240571
%X The ionic conductivity and viscous flow data of , , have been collected in a large temperature range, below and above their glass transition temperatures ( ). A microscopic model is proposed, assuming that the ionic displacement would result from the migration of interstitial positively charged cationic pairs whose concentration is an activated function of temperature. Below , their migration is also an activated mechanism, but a ˇ°free volumeˇ± would prevail above this temperature. This discontinuity in the migration mechanism justifies a Dienes-Macedo-Litovitz (DML) relationship to be representative of conductivity data above and an Arrhenius law below. According to this model, the enthalpy deduced by the fit of high temperature data using a DML equation would correspond to the charge carrier formation, whose migration enthalpy, below , could be deduced by the difference between the activation energy measured in the Arrhenius domain and the charge carrier formation enthalpy. To reduce the number of adjustable parameters numerical values were physically justified. We also applied a complete test for conductivity below , using the so-called weak electrolyte model, splitting activation enthalpy into formation and migration enthalpies and also explaining the variation of pre-exponential term of conductivity with composition. 1. Introduction For glass-forming mixtures in the solid or supercooled liquid state, ionic transport due to alkali cations strongly depends on temperature . The variations of the conductivity-temperature product in an Arrhenius representation show two distinct behaviors. At the lowest temperatures, that product follows an activated relationship: where and are constants; is the gas constant. At higher temperatures, experimental data obey another temperature behavior: where , and are constants. This expression will be deduced in more details below. Such equation suggests an asymptotic decrease of conductivity towards and is similar to the empirical Vogel-Fulcher-Tammann-Hesse (VFTH) relationship originally established to describe the viscosity-temperature dependence of molten silicates [1¨C3]: where (the glass transition temperature) is an empirical constant at which the viscosity diverges, and has the same meaning of the preexponential constant of the Arrhenius expression: where and are constants. The regime of viscosity below follows (4) and is called isostructural (i.e., the viscosity of the glassy state where the structure is frozen). Unfortunately (4) was not used because there is no data in such temperature range. Some authors have
%U http://www.hindawi.com/journals/isrn.electrochemistry/2013/240571/