Abstract
Iron impurity in raw material remains a major challenge in producing colourless glass. In this investigation, we report microwave (MW) heating capable of altering Fe-redox ratio (Fe2+/∑Fe) enabling preparation of colourless phosphate glass. The effect of Sn concentration in retention of Fe[II] in glass melted in MW was compared with conventional glasses. Colourimetric study developing Fe2+-ferrozine colour complex reveals Fe-redox ratio ≥0.49 required to obtain colourless phosphate glass. In microwave heating, addition of 1 wt.% Sn metal powder can impart the desired effect whereas addition of 1.9 wt.% Sn metal powder is required in conventional heating. The correlation equation of Fe-redox ratio with concentration of Sn metal is found to be different in microwave and conventional heating. Thus, exploiting this different redox changes in MW heating optical properties can be tailored. Preservation of higher Fe[II] in MW melted glass is also confirmed by XPS and TGA. 31P MAS NMR spectra suggest that transition from cross linked ultra phosphate to linear polymer metaphosphate network in incorporation of Sn is found different in glass prepared adopting microwave irradiation. 27A1 MAS NMR spectra suggest higher relative content of Al6+ in glass obtained from MW heating. Energy consumption analysis revels 3.4 kWh in MW heating while 14 kWh in conventional glass melting using resistance heating. Further, glass melting in MW can be completed within 2 h unlike ~5 h needed in conventional. MW heating plays a significant role in improving properties to make colourless phosphate glass in addition to significant energy and time saving.
Highlights
Iron plays an important role in glass making as a colourant either by deliberate addition or as impurity
The objective of this study is to investigate preparation of colourless phosphate glass containing Fe in MW heating
Vis-NIR absorbance spectra recorded for the glasses (GM 0Sn, Glass melted in heating (GM) 0.5Sn, GM 1.0Sn and GM 1.5Sn) obtained in MW heating have been illustrated in Fig. 3a showing broad absorbance band centred at ~1050 nm, which is ascribed to the ferrous ions in glass
Summary
Iron plays an important role in glass making as a colourant either by deliberate addition or as impurity. Despite many precautions iron enter the glass melt either from furnace atmosphere or from raw material. Property of glass depends on the concentration of iron, but its oxidation state . Iron present in combination with Fe2+ and Fe3+ in glass melt, trivalent state is predominate under oxidizing atmosphere melting, resulting yellow or brown colour in glass. Colour of glass depends on many factors including iron-redox ratio (Fe2+/∑Fe). In conventional glass melting under air atmosphere, Fe- redox ratio remains low signifying more retention of trivalent oxidation state and makes the glass yellow or brown in colour. Improved properties and low contamination from the crucible wall have been observed in glasses prepared adopting microwave radiation[18,19,20]. Determination of threshold Fe-redox ratio required to make the glass colourless is another aim of this investigation
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