Nucleation of crystallites

Particularly interesting is the examination of the effects of temperature and relative humidity on the nucleation of crystallites interlinked... 

Rapid and dense nucleation, or initial rapid surface growth, followed by advance of interface from all, or certain specific, surfaces into the bulk of crystallites... 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

All of these parameters are not under control. To deliver a more uniform distribution of crystallites, specific nucleating agents are added whilst processing. It has also been shown that many of the desirable properties are the result of small regular spherulites (or nascent crystals). 

Hence, the decision to use a heated substrate with simultaneous evaporation of the component metals as an aid to homogenization requires consideration of whether or not it might have an adverse effect, i.e., causing preferential nucleation of one component, which interdiffusion may not be able to remedy. It was believed (60) that in preparing Pd-Rh alloys by simultaneous deposition on a substrate at 400°C, rhodium nucleated preferentially and that crystallites grew by the addition of palladium (and rhodium) atoms. The diffusion of palladium atoms into this kernel formed a phase with 88 =t 5% Rh (phase II). The outer shell of the crystallite, phase I, was in effect a solid solution deficient in rhodium compared with the overall film composition, and the Rh content of phase I therefore increased as the Rh flux was increased. 

Composition range 30-80% Rh. In this composition range phase separation occurs, and the structure of such Pd-Rh alloy films has been reviewed (Section II). Phase I varied in composition and phase II contained 88 5% Rh. It was proposed that these results could be explained by the preferential nucleation of rhodium so that the crystallites consisted of a phase II kernel surrounded by an outer shell (phase I), the Rh content of which increased with an overall increase in the Rh content of the alloy film. Note the essential difference to the Cu-Ni films (38, 33) discussed in Section IV.A where complete separation into two phases of fixed equilibrium composition is envisaged, and over a wide composition range the crystallite surfaces have the same composition. 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

Another demonstration of the impact of upd on bulk deposition is provided by Pb and T1 deposition on Ag(lll) and Ag(lOO), where the orientation of the three-dimensional crystallites reflects the epitaxially relationship established by the upd layer [341]. For example, in the case of Pb deposition on Ag(lll) [395], a two-dimensional layer, Ag(lll)[110] compressed 2D hep Pb [110] R 4.5°, is initially formed followed by nucleation of a three-dimensional cluster having the same orientational relationship, Ag(lll)[110] 3DPb(lll)[110] R4.5°. 

The ratio of the instantaneous solute concentration c to the solute s solubility s, where the latter is the solute concentration in equihbrium with its crystalline or precipitated phase. Hence, RS = c/s, and a supersaturated solution experiences a thermodynamic driving force (AG = RT ln[RS]). A supersaturated solution will remain as a metastable state, because crystallization or precipitation requires a mechanism for relieving the supersaturated condition (eg., nucleation or addition of crystallite/precipitate). See Biomineralization... 

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