الفهرس 
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Abstract
The present work aims to study the mixed alkali effect in borate glasses. Density and electric conduction measurements have been carried out to throw more light on that phenomenon. Various glasses were prepared for this purpose. The glasses prepared include two main classes, namely binary alkali borate glasses and mixed alkali borate glasses. The prior contains Li2OB2O3, Na2OB2O3 and K2OB2O3 glasses with 20<U+F0A3> R2O <U+F0A3>50 mol%. Mixed alkali glasses include two groups. The first one is free of nonbridging oxygen ions, with 24 mol% R2O. The second one contains 40 mol% R2O, i.e. there is a certain concentration of NBOs. In both groups the glasses contain (Li2O+Na2O), (Li2O+K2O) or (Na2O+K2O). In each case one type of alkali oxides is increased at the expense of the other. Li2OB2O3, Na2OB2O3 and K2OB2O3 glasses were investigated as an introductory investigation. The density of these glasses increases, with various rates, when increasing the R2O content. The density tends to be constant for Li2O<U+F0B3>35 mol%. In the case of Na2OB2O3 and K2OB2O3 glasses there is a marked decrease in the rate of change of density in the same region (R2O<U+F0B3>35 mol%). Up to about 38 mol% R2O there is a linear increase in D with increasing the fraction of BO4 units (N4) in Li2OB2O3, Na2OB2O3 and K2OB2O3 glasses. The increase in density, up to 33 mol% R2O, is correlated with the increase in N4 (the denser borate unit). The molar volume of Li2OB2O3, Na2OB2O3 and K2OB2O3 glasses changes linearly over two regions, with a kink around 33 mol% R2O. The slope of the linear region is greater for R2O>33 mol%. The changes in D and Vm in that region are attributed to formation of NBOs. The electric conductivity of Li2OB2O3, Na2OB2O3 and K2OB2O3 glasses can be described by Arrhenius relation. At a certain temperature the logarithm of conductivity (log<U+F073>) increases with a decreasing rate when increasing the R2O content in glass. An opposite behavior is observed for the activation energy of conduction. For a certain R2O content the conductivity decreases from Li2OB2O3 glasses to K2OB2O3 glasses. The results reveal that E predominates the change in conductivity of these glasses whereas log<U+F073>0 has a minor effect. A unique dependence of log<U+F073> on the R+R+ distance (d) is found for Li2OB2O3, Na2OB2O3 and K2OB2O3 glasses, in spite of the different volumes of the alkali ions. Further, at a certain temperature, there is also a unique dependence of log<U+F073> on the concentration (N) of alkali ions. A linear dependence is then found between log<U+F073> and N3/2. Both the slope and intercept of straight lines change linearly with the reciprocal of temperature. Similar trends are observed between log<U+F06D> and N 3/2. These effects could be correlated to get empirical relations that give log<U+F073> and log<U+F06D> of any of the glasses investigated as a function of N and T. It is then concluded that, at a certain temperature, only N determines the conductivity. In the case of mixed alkali glasses there is a linear change in both D and Vm with increasing X = (R2O)a/R2O. Relations are proposed to calculate D in terms of the number, mass and volume of the structural units present in glass. The results of the present study indicate that the values of calculated D and Vm agree well with the experimental data when using the volumes of structural units present in the corresponding R2OB2O3 glasses. The same ideas are applied for glasses containing three types of alkali oxides. The electric conductivity of mixed alkali glasses shows a minimum at about equal concentrations of alkali oxides. This behavior is attributed to change in the mobilities of charge carriers. It is deduced that the mobility of any type of alkali ions increases when increasing its concentration. Relations are given to fellow the change of mobility with composition for various species. It has been indicated that the minimum conductivity might be observed at the composition of equal contribution to conductivity of each type of charge carriers.By separating the individual contributions to conductivity, a linear dependence of log<U+F073> on the temperature reciprocal could be obtained for various charge carriers. The activation energy for conduction for each type of alkali ions could be obtained from the slope of the straight line. The activation energy for the conduction process in the glass as a whole is assumed as the resultant of the individual contributions of alkali ions in glass. A reasonable agreement is attained between the calculated and experimental activation energies of a selected glass series. This indicates that the presented model is able to describe the mixed alkali effect as revealed by electric conduction in glass.The mixed alkali effect is not limited for glasses containing three types of alkali ions. It is also observed in solid mixtures, obtained from melt, of alkali carbonates. In literature, the phenomenon is reported for nonoxide glasses as well as metallic alloys.