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High-temperature UV-VIS-NIR absorption and emission spectroscopy of soda-lime-silica glasses doped with Nd2O3
Absorption spectra were recorded from a glass with the basic composition 16Na2O ∙ 10CaO ∙ 74SiO2 doped with 4 wt% Nd2O3 at temperatures in the range from 25 to 1400°C. The effective width of the observed absorption peaks increased with increasing temperature, while the peak positions remained nearly constant. Some absorption coefficients decreased with temperature while that at a wavelength of 657 nm increased notably. Emission spectra were recorded from glass melts exhibiting a temperature gradient at the surface (cold surface). The spectra showed distinguished minima at those wavelengths where the absorption maxima occurred. Numerical simulation of the emission spectra assuming a constant temperature gradient at the surface is in agreement with the experimental spectra.
High-temperature UV-VIS-NIR spectroscopy of chromium-doped glasses
Chromium-doped glasses with the basic composition (in mol%) 16 Na2O · 10 CaO · 74 SiO2 were melted under different redox conditions. From these glasses, UV-VIS-NIR absorption spectra were recorded at temperatures up to 1200 °C. While the intensity of the peak attributed to Cr6+ decreases, some of the peaks caused by Cr3+ increase in intensity at higher temperature. All peaks are slightly shifted to larger wavelengths and get broader with increasing temperature. Glasses melted under oxidizing conditions were slowly cooled as well as quenched. Using EPR spectroscopy, in the quenched sample, Cr5+ was detected in a larger concentration than in the slowly cooled sample. Otherwise, the Cr6+ concentration was larger in the slowly cooled sample. This is explained by a redox reaction, i.e. a disproportionation of Cr5+ to Cr6+ and Cr3+ during cooling. As shown by high-temperature spectroscopy of the quenched sample, this redox reaction is frozen in below 550 °C.
Thermal radiation of chromium-doped glass melts
Glass melts are semitransparent radiators whose emission depends on the type and concentration of colouring ions in the melt. Furthermore the temperature profile in the melt especially in regions near the surface is of major importance. In this study, numerical calculations based on experimentally determined absorption coefficients of chromium-doped melts are presented. The model used is that of an isothermal glass melt layer covered by a further layer with a constant temperature gradient. The effect of temperatures, layer thickness and concentrations on the emission spectra is described. Emission spectroscopy should enable the quantitative determination of colouring ions in the melt as well as of the temperature profile near the melt surface.