Effects of structural relaxation on cationic tracer diffusion in silicate melts

The glass transition in silicate melts is a curve in time-temperature space marking the transition of the melt structure from an unrelaxed, disequilibrium glass to a relaxed, equilibrium liquid. Tracer diffusivity data obtained in glasses vs. liquids cannot be compared without consideration of the effects of this transition. For tracer diffusivity experiments, two time scales are important, the time duration of the experiment (τd) and the inverse of the jump frequency (τp) of the tracer. When the time duration of the experiments reaches the relaxation time-scale (τd = τs) of the melt a transition occurs from diffusion in an unrelaxed matrix (undergoing vibrational thermal expansion) to diffusion in a relaxed matrix (undergoing equilibrium, configurational and elastic, thermal expansion). At this transition, an inflection is observed in the temperature dependence of cationic tracer diffusivity. At temperatures below the inflection, the diffusivity is Arrhenian whereas at temperatures above the diffusivity is non-Arrhenian. At high temperatures the tracer diffusivities of the cations approach the value of diffusivity obtained from the Eyring relation (τp = τs). The contrasting, high-temperature, composition dependence of Na and Li vs. Co, Cs, Sr, Ba, Eu, Fe and C diffusivities can be explained in terms of the Eyring (network O and Si) diffusivity influencing the latter group. The contrasting high- vs. low-temperature, composition dependence of Ba and Sr diffusivities can be similarly explained. These latter observations indicate that all cationic diffusivities will be within a log10 unit of the Eyring oxygen diffusivity in melts with viscosities below 10 P

Shear viscosity of alkali and alkaline earth titanium silicate liquids

The shear viscosities of l3 silicate liquids along the NarSiOr-TiO, and CaSiOr-TiO, joins as well as six liquids based on the sphene stoichiometry X#TiSiO. (where X represents Li, Na, K, Rb, Cs, Ca, and Sr) have been measured in equilibrium with air using the concentric cylinder method. The NarSiOr-TiO, join was investigated from l0 to 50 mol0/oT iO, in the temperaturer ange 1000-1150 "C, whereast he CaSiO3-TiO,jo in was investigated from l0 to 80 molo/oT iO, in the temperature range of 1400-1625' C. The shear viscosities of liquids in the CaSiOr-TiO, and NarSiOr-TiO2 systems decrease with the addition of TiO,. The decreaseis linear in the CaSiOr-TiO, system at 1400- I 600 'C but nonlinear in the NarSiO3-TiO, system at 1000-1 150'C. Viscosities of melts of sphene stoichiometry, X1;TiSiO, (where X: Cs, Rb, K, Na, Li, Ba, Sr, and Ca), decrease with increasing field strength. Similar to the behavior of ferro-, alumino-, and galliosilicate melts, this decreaseis strong for the alkalies but very weak for the alkaline earths

Redox viscometry of some Fe-bearing silicate melts

The dependence of shear viscosity on the oxidation state of six ferrosilicate melts has been measured using the concentric cylinder method and a gas mixing furnace. The measurements were performed under air, COr, and COr-CO mixtures at I atm and in a temperature range of 1345 to 1470"C. The experimental procedure involved a continuous measurement of viscosity during stepwise reduction of the melts. Melt chemistry was controlled by dip sampling the tiquids at each oxidation state. The resulting glassesw ere analyzed by electron microprobe, a volumetric FeO itration, and 57Fe Mdssbauer spectroscopy. The electron microprobe data indicate Fe loss for some of the most reduced samples.T he wet chemical (+ microprobe) and spectroscopicd eterminations of theseF erich samples yield oxidation states that are in excellent agreement. The viscosity of all melts investigated herein decreasesw ith melt reduction. The viscosity decrease is, in general, a nonlinear function of oxidation state expressed as Fe2*/F€,o,

Density of Ga2O3 Liquid

The density of Ga2O3 liquid in equilibrium with air has been measured at 18000 to 19000C using an Ir double-bob Archimedean method. The data yield the following description of the density of Ga2O3 liquid: ρ= 4.8374(84)–0.00065(12)(T −18500C). This density-temperature relationship is compared with the partial molar volume of Ga2O3 in glasses in the systems CaO–Ga2O3–SiO2 and Na2O–Ga2O3–SiO2, corrected to the glass transition temperature using thermal expansivities. The comparison illustrates that a positive excess volume term is required in these systems at low temperature. This observation is similar to those deduced from studies of the partial molar volumes of Fe2O3 and Al2O3 in silicate melts

Viscosity-temperature relationships in the system Na2Si2O5-Na4Al2O5

The viscosity-temperature relationships of five melts on the join Na2Si2O2-Na4Al2O5 (5, 10, 20, 30 and 40 mole percent Na4Al2O5) have been measured in air, at 1 atm and 1000–1350°C with a concentric cylinder viscometer. All the melts on this join of constant bulk polymerization behave as Newtonian fluids, in the range of shear rates investigated, and the melts exhibit Arrhenian viscosity-temperature relationships. Isothermal viscosities on this join initially decrease and then increase with increasing mole percent Na4Al2O5. The minimum viscosity occurs near 20 mole percent Na4Al2O5 at 1000°C and moves to higher Na4Al2O5 content with increasing temperature. The observation of a viscosity minimum along the join Na2Si2-O5-Na4Al2O5 is not predicted based on earlier viscosity data for the system Na2O-Al2O3-SiO2 (RlEBLlNG, 1966) or based on calculation methods derived from this and other data (Bottinga and Weill, 1972). This unexpected behavior in melt viscosity-temperature relations emphasizes the need for a more complete data set in simple silicate systems. Previous spectroscopic investigation of melts on the join Na22Si2O5-Na4Al2O5 offer a structural explanation for the observed viscosity data in terms of a disproportionation reaction involving polyanionic units. Macroscopically, the viscosity data may be qualitatively reconciled with the configurational entropy model for viscous flow (Richet, 1984)