101 research outputs found
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Voltammetry in a sulfur and iron-containing soda-lime-silica glass melt
With the aid of square-wave voltammetry soda-lime-silica melts with sulfate fining were investigated in order to enable a quantitative in-situ determination of sulfur and iron. In this study, glass melts with low iron and high sulfate contents, typical for technical white glasses, were examined. The current-potential curves are predominantly influenced by sulfur and not solely controlled by diffusion. This behavior is supposedly caused by deposition of a sulfur layer on the surface of the working electrode. However, a simultaneous quantitative in-situ determination of iron and sulfur in melts of white glasses is possible
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Electric melting of glass: Influence of cathodic currents on the formation of protective layers on molybdenum electrodes
The influence of cathodic currents on the corrosion of molybdenum electrodes during electrical melting of glass was studied with the aid of laboratory experiments. It is shown that cathodic currents lead to the formation of molybdenum silicide layers on the electrode. Best results were obtained using a DC current density of 3.75 mA/cm² at a heating current density of 1 A/cm². Higher DC current densities resulted in molybdenum silicide layers, which dissolve in the melt as silicide particles. Simultaneous to the molybdenum silicide layers, at the counter electrodes MoOâ layers are formed, which also dissolve as particles in the melt
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Influence of process parameters on the formation of protective MoSiâ layers on molybdenum electrodes during electric melting of glass
With the aid of laboratory experiments, the cathodic passivation of molybdenum electrodes during electrical melting of glass was studied. The formation of molybdenum silicide layers does not only depend on the cathodic passivation currents, but also on time and the AC heating current density If the melt has freshly been prepared from raw materials, the molybdenum silicide layer is thinner than under otherwise same conditions. While at an AC heating current density of 1 A/cm², a cathodic current density of 3.75 mA/cm² led to Optimum layer formation, at a heating current density of 2 A/cm², higher cathodic current densities are required
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Influence of nucleating agents on the crystallization of Mg-Ca-Si-Al-O-N oxynitride glasses
Oxynitride glasses in the system M g - C a - S i - A l - O - N were prepared with the aid of a polymeric preceramic aluminum nitride precursor as nitrogen source and transformed into glass-ceramics at temperatures in the ränge of 950 to 1350°C To support the devitrification process, TiOâ , CrâOâ, MgFâ and ZrOâ were added to the batches as potentially nucleating agents. Under the reducing meldng conditions applied. TiOâ led to undesired reactions in the glass melt and was rejected as nucleadng agent. CrâOâ could only be dissolved in the glasses to a maximum content of 1 wt% and did not lead to obvious effects concerning nucleation and crystal growth. The addition of fluorine results in the formation of a fine-grained microstructure but by analogy to CrâOâ, did not lead to phase Separation. Adding ZrOâ, provoked phase Separation in the oxynitride glasses, which was even promoted by the presence of nitrogen. These glass-ceramics possessed an extremely fine-grained microstructure containing nonstabilized tetragonal ZrOâ, which results in an additional nitrogen content independent improvement of the mechanical properties. By comparison to Ti02, Cr203 and MgF2, zirconia has a twofold effect: it is not only an efficient nucleating agent in the case of oxynitride glasses but also an efficient toughening agent for the resulting glass-ceramics
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Kinetics of phase separation in a 6.5 Na2O â 33.5 B2O3 â 60 SiO2 glass
Industrially melted glasses with the composition (in mol%) 6.5 Na2O â 33.5 B2O3 â 60 SiO2 were thermally treated at temperatures in the range of 660 to 750 °C. This resulted in phase separation, i.e. in the formation of a silica- and a sodium borate-rich phase with an interconnected microstructure. Both, the volume content of the borate-rich phase and the mean structure thickness (the correlation length) increased with time as well as with temperature. The volume content approached to a limiting value at constant temperature. The correlation length increased with time according to a power law (~ t^1/n). By contrast to previous studies, n was in the range of 1 to 1.2 within the temperature range and time scale studied. The correlation lengths were much larger (up to 12 Âľm) and the viscosities much lower than in most previous studies. The kinetic law was explained as controlled by visous flow
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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 â 10CaO â 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
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High-temperature spectroscopic study of redox reactions in iron-and arsenic-doped melts
Glasses with the basic composition 16Na2O - 10CaO - 74SiO2 doped with iron or with both iron and arsenic were studied by means of high-temperature UV-VIS-NIR spectroscopy. Increasing temperatures led to a shift of the UV absorption edge caused by Fe3+ -charge transfer bands to larger wavelengths. All other bands, especially the Fe2+ absorption band at around 1100 nm, decreased in intensity at higher temperatures. For glasses, solely doped with iron, the temperature dependency of the extinction coefficient was quantitatively determined. Glasses doped with both arsenic and iron showed a different behaviour: the intensity of the bands decreased up to a temperature of 600 to 650 °C and then increased again. This can be explained by the temperature-dependent redox reaction 2Fe3+ + As3+ â 2Fe2+ + As5+. Increasing temperatures lead to a shift of the reaction to the right. This reaction is in equilibrium at temperatures > 650 °C and gets frozen in at smaller temperatures, depending on the respective iron and arsenic concentrations. The latter is explained by a numerical simulation assuming the redox reactions to be controlled by diffusion
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Redox behaviour of polyvalent ions in phosphate glass melts and phosphate glasses
Glass melts and glasses with the basie composition NaPOâ ¡ 2Sr(POâ)â doped with VâOâ
and FeâOâ were studied with the aid of square-wave voltammetry and spectroscopic methods. From the square-wave voltammetry in the glass melt, a linear dependence of the peak potentials on the temperature was obtained. The Standard enthalpy ÎHâ°, the Standard entropy ÎSâ° and lg(C_ox/C_red) of the transitions FeÂłâş/Fe²⺠and Vâ´âş/VÂłâş were calculated. The diffusion coefficients calculated from the peak currents measured follow Arrhenius' law. By means of electron spin resonance, the concentration of Vâ´âş was determined. The corresponding extinction coefficients for the Vâ´âş bands were calculated from the absorption spectra. Concerning the vanadium-containing glass, the results of square-wave voltammetry and spectroscopy were compared quantitatively. For the iron-doped glass, the comparison was only qualitative
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Preparation of glass-ceramics with needle-like apatite crystals of different aspect ratios and their behavior during extrusion
Glass-ceramics containing needle-like apatite crystals were produced from glasses in the system SiO2-Al2O3-CaO-P2O5-K2O-F- . Crystallization was achieved by rapid heating to 1200°C and keeping the sample for 0.5 to 15 h. With increasing time of heat treatment, length, L, width, W, as well as the aspect ratio, L/W, of the apatite crystals increased. This also led to an increase in the Newtonian viscosity. At higher shear rates, non-Newtonian flow behavior was observed. This effect was more pronounced with increasing aspect ratio. In all cases, the result of the extrusion process was highly oriented glass-ceramics, if it was carried out in the non-Newtonian range
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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
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