Silicate formation reactions

If several compounds occur in the phase diagram between the quasi-binary reactants (for example, between the oxides AO and B2O3), then a multiphase product layer will be formed during the solid state reaction. This situation is shown in Fig. 6-7 where the activity of one component is plotted. 

Schematic plot of the activity of a reactant in a multiphase reaction product layer under the assumption that (1) local thermodynamic equilibrium is maintained, (2) local thermodynamic equilibrium is not maintained. 

In the ternary system, the values of all thermodynamic variables will be fixed on all phase boundaries if local equilibrium is maintained and if the partial pressure of oxygen, as well as P and r, is fixed. Therefore, the defect gradient, and thus the particle flux, in every phase of the reaction product will be inversely proportional to the corresponding thickness of the phase. This, then, results in a parabolic growth law for each phase of the reaction product, as follows from the discussion of section 6.2.1. Let (p) be an index which denotes the phase, and let Ic be the practical rate constant. Then  

Therefore, a parabolic growth law also applies to the total reaction layer thickness. Furthermore, since 

In order to make theoretical calculations, we assume first that the thermodynamics of the total quasi-binary system and the mobilities of the ions in the individual product phases are known. This means according to eq. (6-23) that the rational rate constants of the individual phases are known. If the rational rate constant of phase (p) is designated by then the differential equation for the increase in thickness of this phase is as follows  

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Dolomite, MgC03CaC03, occurs widely on calcining, it yields the mixed oxide. On using this in place of MgO, the activity of the silica product is much reduced by formation of calcium silicate, CaSi03. This helps formation of magnesium, which is gaseous at the temperature of commercial operation, 1450 K. The silicate formation reaction ... 

Surface diffusion can also provide a fast transport path, and can thus also give rise to rapid growth along the surface. Although this situation is quite different from that of gas transport which was just described, the resultant morphologies can often be indistinguishable. Particularly striking examples of such phenomena are found for silicate formation reactions as illustrated in Fig. 8-12. 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

The adsorption of ligands (anions and weak acids) on metal oxide (and silicate) surfaces can also be compared with complex formation reactions in solution, e.g.,... 

Primarily, Fe is released from the lithosphere into surface environments including soils by veathering of primary silicate and sulphide minerals (Tab. 16.1). In the presence of O2 and H2O and in the common pH range (>2) of surface environments, the released Fe" is oxidized to Fe " which in turn, is immediately hydrolysed to form Fe " oxides and oxide hydroxides. For Fe" silicates these reactions involve breakage of an Fe"-0-Si bond and the formation of Fe "OH and SiOH groups. For example, goethite may be formed from the oxidation and hydrolysis of olivine (fayalite) through the reaction ... 

The acidic character of silica is shown by its reaction with a large number of basic oxides to form silicates. The phase relations of numerous oxide systems involving silica have been summarized (23). Reactions of silica at elevated temperatures with alkali and alkaline-earth carbonates result in the displacement of the more volatile acid, C02, and the formation of the corresponding silicates. Similar reactions occur with a number of nitrates and sulfates. Silica at high temperature in the presence of sulfides gives thiosilicates or silicon disulfide, SiS2. 

Silicon and lead oxide Mixtures of powdered silicon and lead oxide/lead dioxide/red lead burn fiercely and rapidly with the formation of metallic lead and fusible lead silicate. The reactions may be shown in Equation 5.9 ... 

The comparison between Table 3.1 and Table 3.2 shows that the Gibbs energy and the difference of electronegativities correlate with each other. Fig. 3.1 shows this dependence for the reactions of sulphates and silicates formation. 

Kalinin, Dimitrii, Mechanism and Kinetic of Hydrothermal Reactions of Silicates Formation. Novosibirsk Nauka, 1973. 

The reaction in this mixture was also studied by IR spectroscopy. The formation of the product, calcium silicate, is characterized by the appearance of a new intense band at 1005 cm". A comparison of data on basicity and the IR spectra shows that the silicate formation is accompanied... 

Boyle et al. ( ) measured the enthalpy of combustion of metallic cobalt at 303.16 K. Their analyses of the combustion products indicated that some overoxidation of the metal had occurred. Correction of their results to correspond to CoO was made on the assumption that the excess oxygen was combined as Co O. Also the analyses indicated that the combustion products had attacked the silica-glass capsules, used to hold the metal samples, resulting in appreciable amounts of silicate formation. Corrections for these two side reactions amounted to three percent of the total measured energy of combustion. From these data, we derive A.H (CoO, cr, 298.15 K) = -57.0 0.3 kcal mol". Roth and Havekoss ( ) also investigated calorimetrically the... 

An even more impressive result was reported by Takeda and coworkers [33] (O Scheme 11). In their synthetic studies on the spermatozoa-derived glycolipid, the disaccharide donor 20 was coupled with 21 by the action of silver silicate. The reaction proceeded in a remarkably selective manner a P = l) to afford the desired disaccharide in 70% yield. This is one of a very few examples of the direct formation of a p-manno-g yco idQ using an oligosaccharide... 

The secondary amine of the AEP group is responsible for the supported enamine formation with acetone (aldol reaction), the deprotonation of nitromethane (Henry reaction) and the generation of a potential nucleophile from trimethylsilyl cyanide through hypervalent silicate formation (cyanosilylation reaction). Therefore, the presence of both AEP and UDP groups in close proximity can cooperatively activate the electrophile (through hydrogen bond) and the nucleophile by enamine formation, thus enhancing the reaction rate. 

The removal of silica from solution by magnesium compounds is a nonstoi-chiometric reaction and proceeds by a surface adsorption mechanism with the formation of magnesium silicate see reaction (10.15) ... 

An obvious idea is to use a layer of inert insulating material between the Si and the superconductor, such as Si02- However, here we nm into the problem of silicate formation via the following reaction ... 

The effect of sodium and potassium carbonates can be derived from the effect of their hydroxides which appear as a result of reaction between the carbonates and CaCOH). The calcium carbonate is then precipitated. The 0.01 mol/1 NaOH solution gives the liberation of 43 and 24.4% of CH in case of C3S and P-C2S respectively, after 24 h of hydration [45]. Yoshi and Sudoh [142] attributed the catalytic effect of NaOH to the sodium silicate formation. This sodium silicate is then reacting with Ca(OH)2 giving the C-S-H. The NaOH liberated during this process can react again with the anhydrous phases. 

The reaction between Si02 and PbO is relevant for PZT films on Si containing substrates like 96% Alumina, Si wafer and LTCC. The existence of silicon oxide in tire substrate and in commercial electrode materials causes the diffusion into the PZT thick film and the reaction to Pb based silicates, which deteriorate ftie ferroelectric behavior of the PZT thick film. Therefore a special Au electrode was developed by Fraunhofer KTS with modified composition preventing silicate formation during firing of the PZT thick film. For ZtOr and AI2O3 substrates a commercial Au electrode (Heraeus 5789) fired at 850°C/ 30 min can bee used. 

An FIA method for the simultaneous spectrophotometric determination of phosphate and silicate was developed. This method is based on the different reaction rates of heteropolymolybdate formation reactions. Concentrations within 0.026-0.485 mM P/L and 0.125-2.848 mM Si/L ranges were determined at a frequency of 30 samples/h. A relafive standard deviation of 2.1% was obfained for 0.162 mM P/L and of 1.1% for 1.424 mM Si/L. The method was suitable to determine phosphate and silicate in wastewater. 

Here AG° is the free energy for reaction (7-45) above. At 900 °C, the minimum a(Na20) is 10 ", meaning any activity greater than this will lead to silicate formation. Since the a(Na20) is set by the partial pressure of S03(g), this means that a P(S03)... 

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