Silicate viscous

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

As in die case of die diffusion properties, die viscous properties of die molten salts and slags, which play an important role in die movement of bulk phases, are also very stiiicture-seiisitive, and will be refeiTed to in specific examples. For example, die viscosity of liquid silicates are in die range 1-100 poise. The viscosities of molten metals are very similar from one metal to anodier, but die numerical value is usually in die range 1-10 centipoise. This range should be compared widi die familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for die temperature dependence of die viscosity of liquids as an AiTlienius expression is... 

Elastic modulus Up to the fracture stress, glass behaves, for most practical purposes, as an elastic solid at ordinary temperatures. Most silicate-based commercial glasses display an elastic modulus of about 70GNm", i.e. about 1/3 the value for steel. If stress is applied at temperatures near the annealing range, then delayed elastic effects will be observed and viscous flow may lead to permanent deformation. 

The dispersions are very viscous they set to jelly-like masses known as gels. Examples sols of silicic acid, tin(IV) oxide, gelatin. 

With chemical treatment, the natural surfactants in crude oil can be activated [1384]. This method has been shown to be effective for highly viscous crude oil from the Orinoco Belt that has been traditionally transported either by heating or diluting. The precursors to the surfactants are preferably the carboxylic acids that occur in the crude oil. The activation occurs by adding an aqueous buffer solution [1382,1383]. The buffer additive is either sodium hydroxide in combination with sodium bicarbonate or sodium silicate. Water-soluble amines also have been found to be suitable [1506]. 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

The product of the fusion of silica with sodium carbonate, sodium silicate (strictly called sodium poly trioxosilicate but usually metasilicate), dissolves in water to give a clear, viscous solution known as waterglass . It hydrolyses slowly and silica is precipitated. Besides the metasilicate, other silicates of sodium are known, e.g. the poly-tetroxosilicate (orthosilicate), Na4Si04. Only the silicates of the alkali metals are soluble in water. Other silicates, many of which occur naturally, are insoluble, and in these substances the polysilicate anions can have highly complicated structures, all of which are constructed from a unit of one silicon and four oxygen atoms arranged tetrahedrally (cf. the structure of silica). Some of these contain aluminium (the aluminatesilicates) and some have import ant properties and uses. 

Figure 15. Amplitude -time plots obtained from AFM measurements for the decay of sinusoidal 1-D gratings with the periods indieated on the surface of a silicate glass(Corning 1737) annealed in air at827C.[42], The exponential decay constant scales approximately as q as expected for a viscous flow controlled process[l].

Whereas electric conduction in the molten silicates is predominantly a cationic process, viscous flow involves the motion of both ionic species and is expected to furnish more information on the larger anions. Some recent systematic studies have indeed confirmed this (5, 6). The addition of even small amounts of metal oxides to silica can reduce the viscosity very appre-... 

Fig. 4. Energy of activation for viscous flow in binary liquid silicates (28).

Although electrical measurements have confirmed the ionic nature of borate melts, viscous flow and volumetric studies clearly indicated their difference from liquid silicates. Shartsia and co-workers (4 ) have suggested that an equilibrium exists in the melt between BOj triangles and B04 Letrahedra, this being both temperature and composition dependent. The only ionic model for the borates is that of Bockris and Mellors (8) which... 

The carbide impurities tend to distill and are reoxidized in the upper, cooler region of the charge. This process forms crusts near the top of the charge or around the cooler part of the reaction cmcible and causes trouble in furnace operation. Large amounts of dissolved calcium silicate and aluminate may form a viscous melt and impede the tapping process. Ferrosilicon is commonly removed from the crushed carbide by electromagnets. 

Si NMR comes close to detecting the fundamental step in viscous flow in silicates with good agreement between the time constant of the exchange process determined by NMR and the shear relaxation time of the K SiztOg liquid. 

Thus, the ideal flow thickness yields one bar at the top and 2 bars at the bottom of a flow (about 3 m) but we have found that flows anywhere from 1 to 5 m are quite well suited for analysis. A 10 m flow would be too thick as the lava overburden would overwhelm the atmospheric pressure in Equation (1), while a 50 cm flow would be so thin that atmospheric pressure would dominate, thus reducing sensitivity below the threshold necessary for analysis based on current ability to measure size distributions. This is one of the reasons why the method works better on basaltic rather than on more silicic and, thus, viscous and thick flows. 

At the concentration levels characteristic of zeolite synthesis, highly viscous gels are formed upon mixing silicate and aluminate solutions. The use of liquid-line NMR techniques to observe such gels results in very hroad spectral features due to the fact... 

The product is called water glass, because when solid, it actually is a glass but unlike lime-soda glass (ordinary window glass), it is soluble in water. The process is carried out in large tank furnaces similar to window glass furnaces. The materials are introduced in at intervals, but the products may be drawn off continuously if desired. A mixture of salt cake and coal may replace a portion of the soda ash. As the melt leaves the furnace, a stream of cold water shatters it into fragments. These are dissolved with superheated steam in tall, narrow steel cylinders with false bottoms,14 and the product liquor is clarified.15 Sodium silicates are sold as solutions that vary from the most viscous, 69°Be, to the thinner solu-... 

The tendency to hydrolyze by a relatively slow and controllable stepwise reaction is characteristic of all the silicon orthoesters. When ethyl silicate, for example, is poured into water it remains immiscible and hydrolyzes at the interface over a period of many days or even months. If both the ester and the water are dissolved in alcohol, however, a much faster reaction in the homogeneous medium becomes possible. If a drop of strong acid then is added to the solution, the hydrolysis is further accelerated to such an extent that the solution becomes warm and silica is precipitated as solid or gel. If less than the equivalent amount of water is used, there is formed a viscous nonvolatile liquid which may be stored for long periods. The addition of further water (as in mixing an ethyl silicate paint) then... 

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