Browsing by Author "Künstler, Klaus"

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    Electrochemical determination of the oxygen activity in tin melts by means of the solid electrolyte method
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Künstler, Klaus; Lang, Hans-Jürgen; Herrmann, Jens; Tomandl, Gerhard
    The measurements of oxygen activity in the tin melt were carried out under inert (light oxidizing) as well as under reducing (N2 with 2 vol.% H2 and 10 vol% H2, respectively) conditions. As solid electrolyte (SE) material Y2O3-, CaO- and MgO-stabilized zirconia was used in the form of a long tube closed at one end. A short SE tube closed at one end within an alumina tube was applied as industrial probe. The galvanic cells worked with a Pt/air and Me/MeO reference electrode and with electrical leads of steel, tantalum as well as rhenium wires. Under inert conditions (argon with 30 vol.ppm O2) a saturation of the tin with oxygen is always obtained and SnO2 is formed. Under reducing conditions the saturation of the melt with oxygen does not take place and no SnO2 is formed. An equilibrium is adjusted between oxygen in the atmosphere and solute oxygen in the melt. At extremely low oxygen contents no equilibrium will be achieved. The lower operating temperatures of the probes were between 500 and 700 °C. The data taken from the literature were confirmed by EMF measurements (solubility of oxygen in liquid tin, molar free solution enthalpy of oxygen in tin, molar free standard enthalpy of formation of SnO2). This kind of investigations is new as to the behaviour of the tin melt under forming gas and the defined addition of elements and compounds to the tin melt (Na2SnO3, Na2O, Na, Fe2O3, Fe, MgO, Mg). Under reducing conditions and the addition of sodium or magnesium to the tin melt a quick decline of the oxygen activity can be recognized at the moment of addition. The melt becomes strongly deoxidized by the addition of these metals. The subsequent processes of deoxidation, which are different from metal to metal, have still to be investigated. No change of the oxygen activity in the melt was observed by addition of iron in the ppm range to the tin melt. For the measurements alumina and a fireclay brick were used as crucible materials. The tin surface was either in immediate contact with the gas phase or was covered with a float glass melt. The oxygen activity of the tin melt was influenced by the crucible material and the float glass melt, too.
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    Electrochemical determination of the sodium activity in tin melts by means of the solid electrolyte method
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Künstler, Klaus; Lang, Hans-Jürgen; Rozumek, Michael; Just, Tino; Tomandl, Gerhard
    A sensor with β"-Al2O3 as solid electrolyte was used in the tin melt for the determination of sodium activity. The chemical, microstructural and functional properties of the home-made β"-alumina were determined. The properties of the home-made ceramics do not much differ from those of commercially obtained materials, which have been used for the purpose of comparison. The behaviour of probes with the reference electrodes sodium, tin-sodium alloys and sodium borosilicate glass was investigated within the scope of emf measurements. No longtime-stable measurements could be carried out with the use of the sensors with sodium as well as tin-sodium reference electrodes. The observed drift of the emf signal is explained by a partial electronic conductivity of the solid electrolyte. Nernst behaviour was found while using a probe with sodium reference electrolyte, i.e. the emf shows a linear dependence on the logarithm of the sodium content in the melt. A proportionally stable behaviour of the emf signal was indicated by measurements with a sodium glass reference system. With the alteration of the sodium concentration the probe reacted spontaneously and reversibly. Besides, an exact Nernst behaviour is indicated. Applying several probes of the same type a displacement of Nernst straight lines is observed, which diverge up to about 400 mV. The family of the Nernst straight lines at multiple use of a single probe moves within a relatively narrow range of only about 25 mV. This permits comparatively precise measurements (error is dependent on the position in the concentration decade). Each probe of the type developed here has to be tested and calibrated by repeated application. A probe delivery with individual characteristic is proposed. The determination of the sodium contents in melts of the basic material and of tin from the float glass process yielded in both cases about 2 wt ppm. These values show that the sodium contents of the tin melt do not differ before and after use.
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    Oxygen behaviour in the process of float glass manufacturing
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Künstler, Klaus; Just, Tino; Lang, Hans-Jürgen; Tomandl, Gerhard
    By means of solid electrolyte probes the oxygen content was determined in inert gas atmosphere and in the tin melt. The probes can be used within a temperature range of 500 °C for tin melt and of 600 °C for inert gas atmosphere up to 1100 °C. A higher emf value causes an oxygen decrease, a lower emf value leads to an oxygen increase in the system. From the measured emf values the oxygen partial pressures in the inert gas atmosphere and the oxygen contents in the tin melt can be calculated by means of the respective mathematical relations. The investigations confirmed that the leveling of an oxygen-unsaturated tin melt is very time-consuming if forming gas of 10 vol.% H2 is used. However, at temperatures above 700 °C (900 °C may be best) the oxygen unsaturated tin melt is reached relatively quickly. At 500 and 600 °C the melt will stay in the oxygen-saturated state for days. Setting of an oxygen-unsaturated Sn melt is most efficient under pure hydrogen. If the Sn melt is covered by a glass melt, the oxygen content above the glass melt and in the tin melt is always higher than without the glass. Small amounts of oxygen are steadily transferred from the glass melt to the atmosphere and to the tin. Furthermore, the effects of air, H2O vapour, CO and CO2 on the system were investigated in detail. A rather complicated behaviour in inert gas atmosphere and in the melt was observed. It depends on doping, temperature and type of inert gas atmosphere. Carrying SO2 in the carrier gas stream (forming gas and H2, respectively) leads to changes of oxygen content both in the tin melt and in the gas phase. The equilibrium state in the Sn melt is reached more slowly than in the gas phase. The emf curve for the gas phase falls below that of the tin melt, i.e. the main part of SO2 is decomposed in the gas atmosphere and only then is available as oxygen for the Sn melt. The restoration of the starting state of oxygen in both phases takes much time (about 40 h). However, at the Sn surface and at the wall of the vessel there are depositions consisting of SnS2, SnS and SnO2. As a further impurity in the floating chamber H2S is considered. The measurements show that there are no essential changes in oxygen behaviour in the system. The Sn melt, however, is strongly interspersed with SnS, such that it is no longer suitable for the float glass process. For the float glass process a tin melt unsaturated with oxygen is required. If saturation is achieved, SnO2 is formed, which is deposited as slag onto the tin surface, and thus is detrimental to the float glass process. Oxygen dissolved in tin can be removed by the addition of an dement, the oxide of which is more stable than that of the oxide of tin. Na, Mg, C and Zr are used as deoxidation agents. Using Zr, which is dissolved in Sn, a nearly oxygen-free melt is obtained, too. The advantage of the use of this getter consists in the fact that no slag is formed on the Sn surface. The introduction of a getter into the production process of float glass manufacture could lead to an essential improvement of the process technology.