Thermal expansion of mixed-alkali silicate glasses

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Abstract
Thermal-expansion measurements were made for five varieties of mixed-alkali silicate glasses (Na-K, Na-Rb, Na-Cs, K-Rb, and K-Cs), with special emphasis on the Na-K silicates. Negative deviations from additivity were observed in the glass transformation temperature in every case, with the deviations increasing with total alkali-oxide concentration. Positive deviations from additivity were noted in the thermal-expansion coefficients in all but one case (Na-Cs), where a small negative deviation was observed. The deviations in both properties were maximized when the radius ratio of the two alkali ions was approximately 1.4, i.e., for the Na-K glasses. These results are quite similar to those reported for mixed-alkali germanate glasses which suggests that they are due to ion-ion interactions rather than to gross alterations of the glass network.

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