Silicate-Based Melts

Silicate-Based Melts. Silicate or Si02 melts have been studied by several investigators in an effort to develop a commercial process for electrowinning silicon. The molten solutions most often studied contained Si02 in cryolite. 

Monnier and his co-workers were able to obtain 99.9 to 99.99% pure silicon from SiOj - cryolite solutions in a two step process. The first step was the deposition of silicon to form a molten copper-silicon alloy at the 

The main problem in using the Hall process for silicon electrodeposition is that silicon melts at a much higher temperature than aluminum (1412°C compared to 660°C). Cryolite can not be used at this temperature due to volatization problems, so a binary or ternary melt containing SiOg had to be developed that would be stable above this temperature. Johnson (29) indicated that calcium and magnesium based silicate melts looked favorable, while other alkaline earth and alkali metal silicates were less desirable. 

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Mold fluxes are routinely used in both continuous casting and bottom pouring of steel. These fluxes are generally calcium silicate based compositions with alkali oxides [(Li, Na, K)20] and fluorides [CaF2, NaF] added as fluidizers. The compositions frequently use fly ash as the base material because it provides a significant concentration of silica in a prefused form easily dissolved as the powder melts on the liquid steel. 

Among test specimens that had been heated to 900°C some exhibited no visible changes, some were moderately bent, and one even melted, depending on the aggregate employed. The strength was generally increased by exposure to elevated temperature. There are several fields of applicatiorr, where the use of an alkali silicate based binder may be advantageous ... 

Similarly to PCL layered silicate nanocomposites, melt intercalation of plasticized PLA with a constant amount of nanoclays (3 wt%) leads to an intercalated nanostructure , even for the unmodified natural montmorillonite (Cloisite Na. This particularity can be explained by the sole intercalation of the plasticizer (PEG chains) into the interlayer spacing of the filler, leading to an increase of the interlayer distance from 1.21 to 1.77 nm, as already observed for simple blends of natural montmorillonite with PEG alone. Selective PEG intercalation was further confirmed by the impossibility to form a nanocomposite by melt blending (non plasticized) PLA with Cloisite Na, only microcomposites could be recovered. XRD analysis performed on organoclay based blends (Figure 8) does not allow... 

For optical applications, inorganic silicate-based oxide [2], non-oxide sulfide [3], and hybrid [4] and nanocomposite thin films [5] have been extensively studied, for passive or active optical components in integrated optical applications, including power splitters, Er " -dof>ed optical amplifiers for dense wavelength division multiplexer (DWDM) systems, and Au nanoparticle (NP)-doped optical sensors. However, sol-gel-derived materials may often differ from their melt-quenched analogues, even for the simple compoimd Si02- In these... 

Intercalated nanocomposites are usually formed by mixing in the melt or in situ polymerisation whereas exfoliation may require more complex processing depending on the properties of the clay (Usuki et al, 1993). However, such layered silicate-based polymer nanocomposites have attracted considerable recent interest after the commercialisation of polypropylene-and nylon-6-based materials (Krishnamoorti and Yurekli, 2001, Kiersnowski and Piglowski, 2004). The major barrier to commercialisation has been developing techniques to ensure a reliable and reproducible product which has now been addressed for clay-based composites some thirty or so years after they were first developed. 

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

The rheological properties of insitu polymerized nanocomposites with end-tethered polymer chains were first described by Krisnamoorti and Giannelis [33]. The flow behavior of PCL- and Nylon 6-based nanocomposites differed extremely from that of the corresponding neat matrices, whereas the thermorheological properties of the nanocomposites were entirely determined by the behavior of the matrices [33]. The slope of G (co) and G"(co) versus flxco is much smaller than 2 and 1, respectively. Values of 2 and 1 are expected for linear mono-dispersed polymer melts, and the large deviation, especially in the presence of a very small amount of layered silicate loading, may be due to the formation of a network structure in the molten... 

The geochemical interpretation of silicate melts is essentially based on three main concepts ... 

In silicate melts and other nonprotonated solvents, the Bronsted-Lowry equation is not applicable and is conveniently replaced by the Lux-Flood acid-base definition (Lux, 1939 Flood and Forland, 1947), according to which free oxygen 0 replaces A basic oxide is one capable of furnishing oxygen ions, and an acidic oxide is one that associates oxygen ions ... 

It is obvious that the acid-base property of a dissolved oxide may markedly affect the structure of a silicate melt. A dissolved acidic oxide associates the free oxygen, thus displacing reaction 6.4 toward the right, resulting in a marked correlation between the field strength of the dissolved cation and the polymerization constant of the melt. This correlation is shown in the values listed in table 6.2. 

Figure 6.12 Miscibility gap in natural and synthetic silicate melts. Dots unmixing for a tholeiitic magma, based on model of Ghiorso et al. (1983). E FeO total iron as FeO.

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 lb of lime must be used to manufacture a ton of steel. 

In one case, therefore, triple silicates are the proximate constituents of the glass in the others, double silicates. In foot, the proportions of the three essential constituents, namely, silica, potassa, and oxide of lead, vary according as the melting furnace is heated with wood or coal. In the latter case, the proportion of oxide of lead is increased. The following analysis by Dumas has been already given at page 192, but is here repeated, to show the proportion of oxygen in the bases, as compared with that of the silica or silicic acid.—... 

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