Iron oxide-doped alkali-lime-silica glasses Part 2. Voltammetric studies

Alkali-lime-silica glass melts doped with different quantities of Fe2O3 in the range of 0.1 to 2 mol% were studied by means of square-wave voltammetry. At low iron concentrations a one-step reduction of Fe^III to Fe^II was observed. The shift of the standard potential with the alkali concentration is explained using a structural model where FeIII occurs as a tetrahedrally coordinated complex which forms an ion pair with an alkali cation. At higher Fe2O3 quantities a two-step reduction was observed. This means that FeIII-containing compounds occur as two thermodynamically different species. In agreement with the EPR-spectroscopic behaviour, this was explained by the formation of iron-containing clusters

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

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 MoO₂ layers are formed, which also dissolve as particles in the melt

Iron in glass melts - Α voltammetric investigation

By the aid of square-wave voltammetry, a fast potentiostatic pulse method, iron oxide-doped soda-lime-silica glass melts were investigated. The iron concentration (0.1 to 2 mol% Fe2O3) and the temperature (400 to 1300 °C) were varied in a wide range. At low iron concentrations as well as at high temperatures the reduction of the Fe3+ to the Fe2+ ion is a one-step process, while at high iron concentrations and at relatively low temperatures a two-step process is observed. By comparison with thermodynamic data of solid state iron oxides, especially of Fe3O4, it is assumed that this is due to the formation of aggregated species containing Fe3+ as well as Fe2+ ions

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, TiO₂ , Cr₂O₃, MgF₂ and ZrO₂ were added to the batches as potentially nucleating agents. Under the reducing meldng conditions applied. TiO₂ led to undesired reactions in the glass melt and was rejected as nucleadng agent. Cr₂O₃ 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 Cr₂O₃, did not lead to phase Separation. Adding ZrO₂, 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 ZrO₂, 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

Kinetics of phase separation in a 6.5 Na2O ∙ 33.5 B2O3 ∙ 60 SiO2 glass

Industrially melted glasses with the composition (in mol%) 6.5 Na2O ∙ 33.5 B2O3 ∙ 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 µm) and the viscosities much lower than in most previous studies. The kinetic law was explained as controlled by visous flow

Influence of process parameters on the formation of protective MoSiₓ 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

Voltammetric methods for determining polyvalent ions in glass melts

Voltammetry is an in situ technique to investigate redox equilibria in glass melts. Its sensitivity and resolution have been improved by introducing the square-wave voltammetry. The results reported in this paper clearly demonstrate the superiority of this method over the cyclic voltammetry, which had been used so far in glass melts

Voltammetric studies in a soda-lime-silica glass melt containing two different polyvalent ions

By the aid of electrochemical methods, a quantitative determination of polyvalent ions in glass melts is possible. This paper investigates the square-wave voltammetry in soda-lime-silica glass melts doped with iron and additionally with arsenic or antimony. In principle, a simultaneous, quantitative determination of these elements is possible, but the experimental parameters should be chosen with care and the recorded current-potential curves should be carefully analyzed

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