Über den Dissoziationszustand von geschmolzenem Natriumdiwolframat (Na2W2O7)

According to E. Kordes and G. Nolte (1969), the dissocn. of molten Na2W2O7 was assumed to give 2Na+, WO42-, and WO3 whereas R. J. H. Gelsing et al. (1966) assumed a dissocn. into Na+ and a mixt. contg. chains of WO4 tetrahedra (WO42-, W2O72-, W3O102- etc.). It was demonstrated that the dissocn. scheme proposed by Gelsing et al. explained well the cryometric results of Kordes and Nolte. Regarding the data of surface tension measurements from Gossink and Stevels, the dissocn. scheme according to Gelsing et al. is more probable than that proposed by Kordes and Nolte. [on SciFinder (R)

Viscosity of molten alkali molybdates, tungstates, and sulfates

The viscosities of molten M2MoO4 and M2WO4 (M = Li, Na, K) have been detd. as a function of temp. up to 1000 Deg by the method of the oscillating hollow cylinder. Viscosity values, ranging from 2 to 10 cP, are higher for the tungstates than for the corresponding molybdates and decrease in the order Li -> Na -> K. In addn., viscosity values of Li2SO4 and Na2SO4 are reported, in order to allow a comparison with the behavior of other high-melting salts with complex anions. The relations between viscosity and temp. can be represented satisfactorily by an Arrhenius equation. Values of the activation energy for viscous flow Eh are as follows (in kcal mole-1): Li2MoO4, 7.0; Na2MoO4, 6.8; K2MoO4, 5.1; Li2WO4, 12.0; Na2WO4, 9.2; K2WO4, 8.6; Li2SO4, 12.8; Na2SO4, 10.0. The Eh values are predicted reasonable well by the semiempirical relation Eh = 3.7 RTm. The relatively low values of the ratio Eh/EL, i.e., activation energy of viscosity vs. activation energy of equiv. conductivity, in the case of molten alkali molybdates, tungstates, and sulfates would suggest that the melts have a more or less assocd. character. [on SciFinder (R)

Density and surface tension of molten alkali molybdates and tungstates in connection with structure and glass formation

The densities and surface tensions of the binary molten systems M2MoO4¿MoO3 and M2WO4¿WO3 (M = Li, Na, K) have been measured by the method of the buoyancy exerted on a Pt bob and by the ring method respectively. From density and surface tension values molar volumes and molar free surface energies were calculated. The isotherms of all these properties show characteristic differences between molybdate and tungstate systems. The trends of these isotherms give an indication of the occurrence of large complexes in the molten molybdates, including Mo6+ ions with co-ordination numbers higher than 4. In the molten tungstates probably only complexes exist of small average dimension, consisting of chains of WO4-tetrahedra. The proposed structures agree with those assumed by Gelsing et al. and Van der Wielen et al. for the vitreous alkali tungstates and molybdates respectively

Nucleation kinetics and critical cooling rate of glass-forming liquids

The critical cooling rate of a substance is the minimum linear rate at which its melt must be cooled to prevent crystallization entirely. In a number of systems this critical cooling rate has proved to be a useful quantitative experimental measure of the tendency to glass formation. The present paper describes a method of deriving theoretical values of the critical cooling rate from classical nucleation kinetics. In general, the calculated values show the correct tendency to glass formation

Nucleation kinetics and critical cooling rate of glass-forming liquids

The critical cooling rate of a substance is the minimum linear rate at which its melt must be cooled to prevent crystallization entirely. In a number of systems this critical cooling rate has proved to be a useful quantitative experimental measure of the tendency to glass formation. The present paper describes a method of deriving theoretical values of the critical cooling rate from classical nucleation kinetics. In general, the calculated values show the correct tendency to glass formation

Proprietes des molybdates et tungstates vitreux et fondus

There is a tendency to form infinite chains in which common tetrahedral and octahedral coordination (the latter for the alkali ions in groups [RO6]) is interlinked with [MO5] and even (with M = Mo) [MoO5]4- groups. The [MoO5]4- anion is in this case in the shape of a tetragonal-pyramidal configuration. With M = W there is a lower tendency to appear in coordinations above [WO4], preferring the coordinations [WoO4] and perhaps [WoO6]. Glass formation in Mo and W oxy salts require an extremely rapid quenching techniques, e.g., by application of the principle of a glass drop in a Pt-PtRh thermocouple loop and a rate of cooling of 10-1 to 102 degree/sec on a microscope stage. \"Splash\"-cooling devices can be used to produce layers of less than 1m thickness on a \"collector\" plane, when the sample drop is molten in a thermocouple loop and then \"explosively\" splashed onto the substrate. The rate of cooling may then reach 106 degrees/sec. The ir absorption spectra of such molybdate and tungstate glasses show important bands at 250 to 650 cm-1 for the bonds Mo-O and W-O. Evaluation of the vibrational bands confirms the possibility for formation of variable bands [MOn], with n > 4, and the existence even of chainlikeconfigurations. The viscosity of such molybdate and tungstate glasses are extremely low at 950 Deg, with a monotonic isotherm as a function of concn. in MO3. The same is valid for calcd. mol. vols. which show linear curves for W, more curved ones for Mo salts, about in the same dimensions. With a stepwise removal of R2O from the longer chains, typical changes in the mol. vol. data indicate structural effects of widening in the structure. Specifically, for Li+ in its tetrahedral coordination, the glass formation is distinctly reduced, in comparison with the conditions in the vitrification of Na and K molybdates and tungstates. Measurements of the surface-tension parameters, by using the ring balance method at 950 Deg, show decreasing surface tension for glasses with increasin alkali ionic radii, smaller for the molybdates than for the tungstates. Evidently, the surface layer of the molten glass is enriched in MO3, the alkali ions tending to move away from the surface. Calcn. of the Eotvos free mol. surface enthalpy isotherms shows a surprising coincidence of those for the molybdates and tungstates with Na+ and Li+ ions, whereas that for K+ is lower by seyeral percent. [on SciFinder (R)

Nucleation kinetics and critical cooling rate of glass-forming liquids

The critical cooling rate of a substance is the minimum linear rate at which its melt must be cooled to prevent crystallization entirely. In a number of systems this critical cooling rate has proved to be a useful quantitative experimental measure of the tendency to glass formation. The present paper describes a method of deriving theoretical values of the critical cooling rate from classical nucleation kinetics. In general, the calculated values show the correct tendency to glass formation

Proprietes des molybdates et tungstates vitreux et fondus

There is a tendency to form infinite chains in which common tetrahedral and octahedral coordination (the latter for the alkali ions in groups [RO6]) is interlinked with [MO5] and even (with M = Mo) [MoO5]4- groups. The [MoO5]4- anion is in this case in the shape of a tetragonal-pyramidal configuration. With M = W there is a lower tendency to appear in coordinations above [WO4], preferring the coordinations [WoO4] and perhaps [WoO6]. Glass formation in Mo and W oxy salts require an extremely rapid quenching techniques, e.g., by application of the principle of a glass drop in a Pt-PtRh thermocouple loop and a rate of cooling of 10-1 to 102 degree/sec on a microscope stage. \"Splash\"-cooling devices can be used to produce layers of less than 1m thickness on a \"collector\" plane, when the sample drop is molten in a thermocouple loop and then \"explosively\" splashed onto the substrate. The rate of cooling may then reach 106 degrees/sec. The ir absorption spectra of such molybdate and tungstate glasses show important bands at 250 to 650 cm-1 for the bonds Mo-O and W-O. Evaluation of the vibrational bands confirms the possibility for formation of variable bands [MOn], with n > 4, and the existence even of chainlikeconfigurations. The viscosity of such molybdate and tungstate glasses are extremely low at 950 Deg, with a monotonic isotherm as a function of concn. in MO3. The same is valid for calcd. mol. vols. which show linear curves for W, more curved ones for Mo salts, about in the same dimensions. With a stepwise removal of R2O from the longer chains, typical changes in the mol. vol. data indicate structural effects of widening in the structure. Specifically, for Li+ in its tetrahedral coordination, the glass formation is distinctly reduced, in comparison with the conditions in the vitrification of Na and K molybdates and tungstates. Measurements of the surface-tension parameters, by using the ring balance method at 950 Deg, show decreasing surface tension for glasses with increasin alkali ionic radii, smaller for the molybdates than for the tungstates. Evidently, the surface layer of the molten glass is enriched in MO3, the alkali ions tending to move away from the surface. Calcn. of the Eotvos free mol. surface enthalpy isotherms shows a surprising coincidence of those for the molybdates and tungstates with Na+ and Li+ ions, whereas that for K+ is lower by seyeral percent. [on SciFinder (R)