Silica growth process

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

Figure 1.27 Double-walled silica nanotubes with monodisperse diameters self-orga-nize into highly ordered centimetre-sized fibres, using a synthetic octa-peptide as a template. The growth mechanism is proposed to be the fundamental mechanism for growth processes in biological systems. (Reproduced from ref. 53, with permission.)...

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 ... 

Thermal uniformity in the cold zone was found to be from 0.01 to 0.02 °C, and that in the hot zone was found to be better than + 0.5 °C vertically and + 0.1 °C horizontally. Thermal gradients near the solid-liquid interface were achieved in excess of 30 °C cm " in the crystal region and up to 20 °C cm" in the melt. The growth of crystals was performed in a sealed transparent silica ampoule, which has two rooms for As source and GaAs polycrystalline, respectively, separated by a quartz diffusion barrier. For details of the growth process the reader is referred to Ref. 43. In this experiment the As source temperature T. was systematically reduced by 2 °C at 3 h intervals from 620 °C to 614 °C. 

Zeolite ZSM-23 can be readily synthesized with pyrrolidine as organic template and fumed silica as silica source. Pyrrolidine seems to intervene in the nucleation and crystal growth process, in the latter case most probably as pore-filling agent. By proper optimization of the gel composition and the synthesis conditions, ZSM-23 can be synthesized in less than one day. The synthesis of ZSM-23 has some... 

In a recent study, Baker and Chludzinski (10) used the basis of this proposed mechanism to develop methods of inhibiting the growth of filamentous carbon. The approach was aimed at the introduction of additives into the metal catalyst particles, which had the potential of reducing the rate of the critical steps involved in the growth process i.e., carbon solubility or carbon diffusion through the catalyst particle. Among several additives investigated, silica was the most effective as it reduced the rates of both of these processes. 

This invention teaches a procedure to maintain constant the original number of nuclei particle in the sol while the particles are grown so that no nuclei are lost in the growth process and therefore more concentrated products of small and quite uniformly sized silica particles are obtained. 

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