Structure of the melts

The local structure of the melt was determined with the calculation of the radial distribution function g(r), and the dynamic information was directly obtained from an estimation of the self-diffusion coefficient. 

In a melt, the NMR spectrum consists of a single sharp peak, the position of which (called the chemical shift, S, in ppm) is the average value of the chemical shifts of the different ionic species, weighted by their molar 

Moreover, it is known that molten alkali fiuoride mixtures (LiF, NaF, KF) behave as a bath of polarizable spheres containing free cations (Li, Na , K ) and free F anions [15]. In that case, the fluorine chemical shift for a given composition is given by  

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To the extent that the segmental friction factor f is independent of M, then Eq. (2.56) predicts a first-power dependence of viscosity on the molecular weight of the polymer in agreement with experiment. A more detailed analysis of f shows that segmental motion is easier in the neighborhood of a chain end because the wagging chain end tends to open up the structure of the melt and... 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

Table 55 presents the results discussed above. Fluoride melts containing tantalum contain two types of complex ions, namely TaF6 and TaF72 . The equilibrium between the complexes depends on the concentration of fluoride ions in the system, but mostly upon the nature of the outer-sphere cations. The complex ionic structure of the melts can be adjusted by adding cations with a certain polarization potential. For instance, the presence of low polarization potential cations, such as cesium, leads primarily to the formation of TaF72 complexes, while the addition of cations with relatively high polarization potentials, such as lithium or sodium, shifts the equilibrium towards the formation of TaF6 ions. 

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

Cast film extrusion of polyolefins has been developed to obtain flexible films with a high level of transparency by freezing the amorphous polymer structure of the melt on a chill roll. Cast films are mono-oriented in extrusion direction. 

While in aqueous solutions an electrical double layer is formed, in molten salts an electrical multilayer is formed because of their structure [116]. The Russian school, particularly Dogonadze and Chizmadzhev [117], presented the first theoretical description of this layer. These authors used a binary distribution function to interpret X-ray diffraction measurements in molten salts. By means of these results they tried to describe the structure of the melt at the interface and concluded that the charge distribution was characterized by an attenuated oscillation of the charge distribution in the melt. 

As shown in Fig. 11, the melting peak obtained by DSC is often structured and the clearing transition can be rather broad. The structuring of the melting peak may be an indication of polymorphism in the solid state but more often it is caused by successive meltings and recrystallizations of regions of imperfect crystallinity. The existence of more than one mesophase in the melt has also been observed, and this behavior leads to the presence of two or more liquid crystal transitions in the thermogram... 

From the literature it follows that the electro-deposition of molybdenum from the binary MeF-Me2Mo04 mixtures is impossible. However, a small addition (1 mole %) of boron oxide or Si02 to the electrolyte facilitates the electro-deposition of molybdenum. The presence of boron or silicon oxide most probably modifies the structure of the melt, which results in changes in the cathode process. The survey of electrochemistry of molybdenum deposition was given by Danbk et al. (1997). 

The above-described trend is maintained up to approximately 30 mole % of alkali metal oxide. Above this concentration, non-bridging oxygen atoms arise as the result of the reverse transition of some boron atoms from the tetrahedral into the triangular coordination. The number of boron atoms in the tetrahedral coordination decreases, approaching zero at 70 mole % of the alkali metal oxide. It should be, however, noted that the picture of the structure of the melt is only approximate and differences between the liquid and crystalline phase may occur. 

The two kinds of oxygen atoms have evidently different energetic states. Their mutual molar ratio defines the structure of the melt, i.e. its polymerization degree as well as the chemical potentials of the components. With regard to this structural aspect of silicate systems, the chemical potential of an arbitrary component may be defined as the sum of chemical potentials of all atoms forming the component considered, when their particular energetic states are taken into account. The chemical potential of the ith component in an arbitrary solution is defined by the relation... 

The Ca0-Ah0j,-Si02 system. The coordination of Al(III) atoms in aluminosilicate melts is one of the most important research directions in the structure of the melts containing alumina. This interest is associated with an extensive utilization of these melts in different sections of the silicate industry, such as glass, cement, porcelain production, etc. 

The theoretical calculation of phase diagrams is an important tool both in forecasting the properties of materials, as well as in the study of the structure of the melts, and became a self-sustaining field of science. Several characteristics of the phase diagrams can be related to the calculations and to particular terms in the theory. Extension of the deviation from ideal behavior is an important feature, which can explain the probability of the formation of compounds. 

Theoretical interpretation of the concentration dependence of equivalent conductivity for simple binary mixtures was first presented by Markov and Shumina (1956). It should be emphasized that this theory, even when considering simple structural aspects, represents rather a method of interpretation of the experimental data than a genuine picture of the structure of the melt. In molten salts generally only ions and not molecules are present, hence the conception of Markov and Shumina (1956) is to be considered also from this aspect. Their theory is based on the assumption that the electrical conductivity of a mixture of molten salts varies with temperature like pure components. In this respect, general character of the electrical conductivity dependence on composition, indicating the interaction of components in an ideal solution, could be expected. 

In-Bi [259], Au-Al [256], and Al-Te [258] systems. Martin-Garin et al. [255] correlated the thermodynamic properties of the Ag-Ge melt obtained by them with the results on the structure of the melt evaluated from neutron diffraction measurements. 

Glass is a product of the super-cooUng of a melted hquid mixture consisting primarily of sand (silicon dioxide) and soda ash (sodium carbonate) to a rigid condition, in which the super cooled material does not crystaUize and retains the organization and internal structure of the melted hquid. When waste glass is crushed to sand size particles, similar to those of natural sand, it exhibits properties of an aggregate material [207-214]. 

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