Diffusion of Silicophosphate Melts
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KNOWLEDGE of the mechanism of transport of anions in complex ionic melts is important for a better understanding of both the nature of ionic liquids and the various refining applications for which these liquids are used, but, at present, there is no well defined theory of transport. The majority of the earlier work on silicates was concerned with high silica contents and from viscosity, density and conductance rneasuremenca.v" it was deduced that the addition of metal oxide to pure silica breaks up the network and at about 15 mole pet, the structure consists of discrete ions and further addition of metal oxide gives rise to increased depolymerization until, at 50 mole pet, the melt is thought to consist of rings of the general formula (Si n03n )2n - and chains of general formula Si n03n + 1 (2n + 2)-. At higher concentrations of metal oxide the chain lengths are reduced. In recent years, there has been considerable work directed towards linking the thermodynamic properties of melts with theories of ionic constitution." For melts containing greater than 50 mole pet metal oxide the results indicate that, for silicate melts, the ions may be considered as existing of either linear chains" or, with the more sophisticated models ,'?" branched chains, all in a state of dynamic equilibrium. These views were reinforced by studies correlating density changes with predicted degrees of polymer-ization," However, constderation of the available viscosity data for these melts indicates that each chain must be short lived since the dependence of the viscosity upon the molecular weight of the melt is not typical of a long chain polymer." Moreover, the application of various models which relate surface tension and viscosity to the structure shows that the melt is more likely to consist of single ions rather than long chains although there is no direct experimental evidence for the existence of short chains in the melt ,'" In the case of phosphates, it is possible to quench the melt to a glass and thence determine the distribution of chains by means of paper chromatography':' but, consideration of density, surface tension and viscosity data for sodium phosphate melts indicates that a short active structural unit is probable .12 ,13 Due to hydrolysis in aqueous solutions paper chromatography may not be performed on silicates but gas J. H. CLELAND and D. J . FRAY are Postdoctoral Research Assistant and University Lecturer, respectively, Department of Metallurgy and Materials Science, University of Cambridge, Cambridge CB2 3QZ, England. Manuscript submitted November 22, 1977. METALLURGICAL TRANSACTIONS B
chromatography has been used to help identify the chain structure in the gtasses ." When mixed silicate-phosphate melts are quenched to a glass and the glass analyzed using paper chromatography it is found that, compared to the pure phosphate glass, the chains are much shorter due to the incorporation of silicate ions into the phosphate chains and subsequent hydrolysis of the bonds. From the change in chain distribution with silicate c
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