Nucleation-and-growth process

In an amorphous material, the aUoy, when heated to a constant isothermal temperature and maintained there, shows a dsc trace as in Figure 10 (74). This trace is not a characteristic of microcrystalline growth, but rather can be well described by an isothermal nucleation and growth process based on the Johnson-Mehl-Avrami (JMA) transformation theory (75). The transformed volume fraction at time /can be written as... 

The high sensitivity of the MS [160] makes it a particularly appropriate tool for the investigation of nucleation and growth processes, since it is possible to measure rates during the early part of the reaction using small samples or individual crystals. The influence of residual gases [160] on the initiation of reaction can also be determined. Short scan times enable very rapid reactions e.g. detonations, to be studied, and it is also possible to measure simultaneously the rate of evolution of several different product molecules. 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

These various distributions reflect the variations in reactivity of the reacting sites. Johnson and Kotz [444] discuss in detail the Weibull and other distributions which find application when conditions of strict randomness of the exponential distribution are not satisfied. From an empirical point of view, the power transformation is a practical and convenient method of introducing a degree of flexibility into a model. Gittus [445] has discussed some situations in which the Weibull distribution may be expected to find application, including nucleation and growth processes in alloy transformations. 

Before discussing such theories, it is appropriate to refer to features of the reaction rate coefficient, k. As pointed out in Sect. 3, this may be a compound term containing contributions from both nucleation and growth processes. Furthermore, alternative definitions may be possible, illustrated, for example, by reference to the power law a1/n = kt or a = k tn so that k = A exp(-E/RT) or k = n nAn exp(—nE/RT). Measured magnitudes of A and E will depend, therefore, on the form of rate expression used to find k. However, provided k values are expressed in the same units, the magnitude of the measured value of E is relatively insensitive to the particular rate expression used to determine those rate coefficients. In the integral forms of equations listed in Table 5, units are all (time) 1. Alternative definitions of the type... 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

Metal salts of carboxylic acids obviously possess some organic character, but decompositions of these substances can be considered in the present context. Many metal carboxylates decompose at a reactant—product interface and their nucleation and growth processes are similar to the behav-... 

An unusual variation in kinetics and mechanisms of decomposition with temperature of the compound dioxygencarbonyl chloro-bis(triphenyl-phosphine) iridium(I) has been reported by Ball [1287]. In the lowest temperature range, 379—397 K, a nucleation and growth process was described by the Avrami—Erofe ev equation [eqn. (6), n = 2]. Between 405 and 425 K, data fitted the contracting area expression [eqn. (7), n = 2], indicative of phase boundary control. At higher temperatures, 426— 443 K, diffusion control was indicated by obedience to eqn. (13). The... 

Another possibility is that one of the reactants is particularly mobile, this is apparent in certain solid—gas reactions, such as the reduction of NiO with hydrogen, which is a well-characterized nucleation and growth process [30,1166]. Attempts have been made to use the kinetic equations developed for interface reactions to elucidate the mechanisms of reactions between the crystalline components of rocks under conditions of natural metamorphism [1167,1168]. 

Turkevich, J., Stevenson, P.C. and Hillier, J. (1951) A study of the nucleation and growth processes in the synthesis of colloidal gold. Discussions of the Faraday Society, 11, 55-75. 

TBP and injected into a hot ( 350 °C) solution of TOPO (12 g). The injection of CdSe precursors into the hot solution ofTOPO resulted in spontaneous nucleation of CdSe nanocrystals and a decrease in temperature. Once the temperature was stabilized, an additional amount (0.4 mL) of the precursor solution was added for the growth of the nucleated nanocrystals. Here, Ostwald ripening was avoided by separating the nucleation and growth processes. All the reagents and the reaction were kept under an Ar atmosphere to avoid fire hazard and surface oxidation of the nanocrystals. 

The kinetics and mechanisms of the C —> G transition in a concentrated solution of PS-fr-PI in the PS-selective solvent di-n-butyl phthalate was studied [137,149]. An epitaxially transformation of the shear-oriented C phase to G, as previously established in melts [13,50,150], was observed. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexag-onally perforated layer (PL) state. The C -> G transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. 

The electrodeposited TiAl3 described above displays crystallographic features of distinctly different length scales. The deposit grain size is on the order of 0.1 to 0.5 pm while the Ll2 domain size is on the order of 5-10 nm. The domains appear to have grown through a first order nucleation and growth process that is independent... 

The fracture mode of both forgings was by the microvoid nucleation and growth process (Figure 4). Microvoids nucleate at nonmetallic inclusions in the steel (sulfides, oxides, etc) and grow under strain until they coalesce at fracture (10). While the CF heats had a fairly uniform microvoid size distribution on the fracture surface, the HERF steel fracture surface had a bimodal distribution of microvoids with large microvoids surrounded by clusters of fine microvoids. 

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