Composition control, nucleation

In addition to growth of SiNWs, one-dimensional composite structures prepared by vacuum deposition of silicon clusters on CNTs also have been investigated. Hybrid silicon/carbon nanotube one-dimensional nanostructures were synthesized using a two-step CVD process (see Fig. 15.8) [18]. The spaces between the CNTs, formed through controlled nucleation in the first deposition step, allow for subsequent penetration of silane gas and a homogenous deposition of silicon clusters on the surfaces of the CNTs during the second CVD deposition step. The hybrid silicon/CNTs exhibit a high reversible capacity of 2,000 mAh/g with a 0.15% capacity loss per cycle over 25 cycles. When compared with commercially available silicon particles, in situ prepared silicon by CVD always has a smaller irreversible capacity loss in the first cycle. This may be related to the reduced amount of SiOx on the surface of the silicon particles which usually traps lithium in... 

All these different ways of forming microstructures involve controlled nucleation and crystallization, as well as the choice of parent glass composition. 

Since the most important glass-forming systems are based on silicate compositions, the key crystalline components of glass-ceramics are therefore silicates. Certain oxide minerals, however, are important, both in controlling nucleation as well as forming accessory phases in the final product. 

In all of the above discussion we have been concerned with systems which crystallize without change of composition, i.e., pure materials or congruently melting compounds. In the more general case, with composition as an additional variable, the techniques of controlled nucleation and crystal growth have been fruitfully employed in recent years. Applied in various contexts they form the basis of several important commercial processes... 

A wide variety of minerals are deposited by organisms including metal oxides, sulfides, carbonates, and phosphates (6). Thus, biomineralization appears to be a general phenomenon the concepts of which should be applicable to position of minerals not found in nature. Furthermore, the mineral deposition process is not under direct cellular control and only the structure and composition of the nucleation protein appear to be controlling the mineralization. With this rationale, our researches have focused on understandmg how organic substrates control nucleation and crystal growth of both biominei s and technically important minerals (7-ii). 

Precipitatioa (2,13—17) techniques employ a combination of nucleation and growth iaduced by adding a chemical precipitant, or by changing the temperature and/or pressure of the solution. Chemical homogeneity is controlled by controlling the rate of precipitation. FFeterogeneous precipitation iavolves the precipitation of a soHd of different composition from the solution, and the composition of the precipitate may change as precipitation continues. Coprecipitation iavolves the simultaneous precipitation of similar size cations ia a salt as a soHd solutioa. 

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

Pronounced discrepancies between observed composition and the calculated equilibrium composition illustrate that the formation of the solid phase, for example, the nucleation of dolomite and calcite in seawater, is often kinetically inhibited, and the formation of phosphates, hydrated clay and pyrite is kinetically controlled. 

The polymerization temperature, through its effects on the kinetics of polymerization, is a particularly effective means of control, allowing the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The effect of the temperature can be readily explained in terms of the nucleation rates, and the shift in pore size distribution induced by changes in the polymerization temperature can be accounted for by the difference in the number of nuclei that result from these changes [61,62]. For example, while the sharp maximum of the pore size distribution profile for monoliths prepared at a temperature of 70 °C is close to 1000 nm, a very broad pore size distribution curve spanning from 10 to 1000 nm with no distinct maximum is typical for monolith prepared from the same mixture at 130°C [63]. 

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