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    Investigation of the mixed alkali effect in a range of phosphate glasses

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    This study investigated the mixed alkali effect in a series of phosphate based glasses. These glasses were of the composition 0.5P(2)O(5)-0.2CaO-0.3-xNa(2)O-xK(2)O where x=0 to 0.3 in steps of 0.05. This study considered density measurements using Archimedes's principle, thermal characterisation using differential scanning calorinictry, phase analysis following crystallisation using X-ray powder diffraction (XRD), and degradation studies combined with ion release. The results showed that these mixed alkali glasses showed a linear decrease in density, with the ternary single alkali glass with 0.3mol K2O showing a 3% reduction in density as compared to that with 0.3mol Na2O which correlated well with the difference in ionic diameter and atomic weight of both cations. These glasses also showed intermediate glass transition temperature (T,) values, compared to those of the ternary single alkali glasses having the same alkali oxide content, and the minimum T, value was recorded for equimolar amounts of both alkali oxides. However, they did not show any significant change in the degradation rate compared to the glass with 0.3mol Na2O with the exception of the 0.25mol K2O glass. The single alkali glass with 0.3mol KO showed a significant increase in the degradation rate by an approximate one order of magnitude. For the mixed alkali glasses with low molar concentration of K2O, only sodium phosphate-rich phases NaCa(PO3)(3) and Na4Ca(PO3) were detected from XRD; at high molar concentrations however, potassium phosphate-rich phases KCa(PO3)(3) and KPO were detected. At equimolar concentration of both alkali cations, KCa(PO3)(3) and Na4Ca(PO3)(6) were identified. K+, Ca2+, and P3O9 release followed the degradation behaviour where the highly degrading glasses with 0.25 and 0.3mol K2O released the highest amount of these ions; however, there was no definite trend in the remaining glass compositions.

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    Description

    Title : Investigation of the mixed alkali effect in a range of phosphate glasses
    Author(s) : EA Neel, I Ahmed, JC Knowles
    Abstract : This study investigated the mixed alkali effect in a series of phosphate based glasses. These glasses were of the composition 0.5P(2)O(5)-0.2CaO-0.3-xNa(2)O-xK(2)O where x=0 to 0.3 in steps of 0.05. This study considered density measurements using Archimedes's principle, thermal characterisation using differential scanning calorinictry, phase analysis following crystallisation using X-ray powder diffraction (XRD), and degradation studies combined with ion release. The results showed that these mixed alkali glasses showed a linear decrease in density, with the ternary single alkali glass with 0.3mol K2O showing a 3% reduction in density as compared to that with 0.3mol Na2O which correlated well with the difference in ionic diameter and atomic weight of both cations. These glasses also showed intermediate glass transition temperature (T,) values, compared to those of the ternary single alkali glasses having the same alkali oxide content, and the minimum T, value was recorded for equimolar amounts of both alkali oxides. However, they did not show any significant change in the degradation rate compared to the glass with 0.3mol Na2O with the exception of the 0.25mol K2O glass. The single alkali glass with 0.3mol KO showed a significant increase in the degradation rate by an approximate one order of magnitude. For the mixed alkali glasses with low molar concentration of K2O, only sodium phosphate-rich phases NaCa(PO3)(3) and Na4Ca(PO3) were detected from XRD; at high molar concentrations however, potassium phosphate-rich phases KCa(PO3)(3) and KPO were detected. At equimolar concentration of both alkali cations, KCa(PO3)(3) and Na4Ca(PO3)(6) were identified. K+, Ca2+, and P3O9 release followed the degradation behaviour where the highly degrading glasses with 0.25 and 0.3mol K2O released the highest amount of these ions; however, there was no definite trend in the remaining glass compositions.
    Keywords : phosphate glasses, mixed alkali effect, glass transition, x ray powder diffraction

    Subject : unspecified
    Area : Other
    Language : English
    Year : 2007

    Affiliations UCL
    Journal : Key Engineering Materials
    Volume : 330-332
    Publisher : TRANS TECH PUBLICATIONS LTD
    Pages : 161-164
    Url : http://discovery.ucl.ac.uk/182476/

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