Silica growth rates

In upwelling areas, the resupply of DSi via Ekman transport leads to a silica trapping effect as illustrated in Figure 16.4. Under these conditions, growth rates in the surface waters are controlled by the supply rate of DSi via upwelling. An example of nutrient... 

In principle, silica growth kinetics may be controlled by (1) slow release of monomer via alkoxide hydrolysis in the particle-free reverse micelles, (2) slow surface reaction of monomer addition to the growing particle, and (3) slow transport processes as determined by the dynamics of intermicellar mass transfer. There is strong experimental evidence to support the view that the rate of silica growth in the microemulsion environment is controlled by the rate of hydrolysis of TEOS (23,24,29). Silica growth kinetics can be analyzed in terms of the overall hydrolysis and condensation reactions ... 

Fig. 2. Devitrification rate of vitreous silica for surface-nucleated cristobalite as a function of temperature (99). Growth rate is proportional to the square...

Increased pressures can lower the temperature at which crystallization occurs. Experiments performed using Spectrosil (Thermal Syndicate Ltd.) and G.E. Type 204 (General Electric Company) fused silicas (see Fig. 2) show that at pressures above 2.5 GPa (<25,000 atm), devitrification occurs at temperatures as low as 500°C and that at 4 GPa (<40,000 atm), it occurs at as low as 450°C (107). Although the temperatures and pressures were in the coesite-phase field, both coesite and quartz were observed. Both the devitrification rate and the formation of the stable phase (coesite) were enhanced by the presence of water. In the 1000—1700°C region at 500—4000 MPa (<5,000 40,000 atm), oc- and J3-quartz were the primary phases. Crystal growth rates... 

That nucleation and growth rate are the limiting steps in quartz cementation has no particular imphcations with respect to the ultimate source of the sihca or the mechanism of transport. Potential sources of silica for quartz cementation are numerous (McBride, 1989) and include all documented silicate dissolution reactions in sandstones and shales. 

A variation to the supply of reactant SiO has been demonstrated by Milewski et al. [79] who used an SiO generator that is simply Si02 brick impregnated with silica and carbon to produce SiO at temperature. The SiO then dissolves into the liquid catalyst. Carbon was introduced as CH4 carried by a mixture of H2, CO, and N2. The kinetics of the growth rate are then controlled by the composition of the surrounding atmosphere, which controls the partial pressure of SiO to the liquid. The stoichiometry... 

Low-temperature (e.g., ambient temperature) aging can increase the rate of nucleation from the parent mixture, which is equivalent to a low-temperature reaction. However, the growth rate of a crystal at ambient temperature is very slow and can be neglected. Previous studies indicate that the aging process is necessary for the synthesis of both high-silica zeolites (e.g., TS-1) and low-silica zeolites (e.g., zeolite A and X). 

Figure 2.30 Crystal growth rate of cristobalite in Si02 and three soda silica melts containing lowNa20. (After Dietzel and Wickert, 1956).

Table III. Average Product Dimension and Growth Rate for All Synthetic Runs (The suffix A in the sample number indicates runs made using Ludox AS-40 as the silica source ) ...

Commercial silica grades, 146 Compaction, 202, 213, 568 Center-point load and third-point load, 252 Comparison, 252, 270 Compatibilizer, 84, 86, 161 Cesa-mix, 174 Complex viscosity, 631 Composite handrails and railing systems, 238, 253, 258, 264, 276, 303-310, 427 Composite railing post, 429 Compound annual growth rate (CAGR), WPC, 45... 

Important features of the selective oxidation process are shown schematically in Figure 1. The slow growth rates of alumina and silica, illustrated in the plot of parabolic rate constants versus temperature at lower right, makes the formation of one of these oxides as a continuous surface layer necessary for long term oxidation protection. This requires that the protective oxide be more stable thermodynamically than the more rapidly growing oxides. The plot of standard free energy of formation as a function of temperature at lower left shows that the Ni-Al system satisfies this condition. Alumina is stable, relative to NiO, even when the activity of aluminum in the alloy is very low. However, when the Al concentration is low the alumina forms as internal oxide precipitates and is non-protective allowing an external layer of NiO to form (illustrated in the cartoon at top). Therefore, a critical concentration of Al exists above which out-... 

Catalyzing influence on reactions in silicate systems may be excited by an increase of crystal-forming solution alkalinity. This is evidenced by zeolite synthesis experience (17) and by direct experiments of Campbell and Fyfe (3) and Kerr (8). However, an increase of pH may affect not only growth rate, but displacement of desilication reactions towards products poorer in silica (15, 16). These 2 effects must be distinguished when analyzing the influence of alkalinity growth. 

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