Nucleation site sources

An important consideration of reactions in which a new solid phase appears is the number, source and location of nucleation sites and the form of the new phase. A needle-like new phase can result in a marked apparent expansion of the material leading to particle disintegration in packed beds. 

The oscillating gas bubbles are a continuous source of free-radicals as long as they remain in the antinode area of the sonic field, but one has also to envisage a catastrophic collapse that generates in addition to the free radicals a number of smaller bubbles. These serve as further nucleation sites for subsequent cavitation processes. 

Crevice nucleation can be observed in a glass of carbonated beverage. A series of bubbles can be seen rising from the same place. The source of these bubbles is a favored nucleation site, probably a minute scratch in the bottom of the glass. 

The mechanism of bubble formation by nucleation requires supersaturation of the dissolved gas [11-13] and a nucleus radius greater than the critical [7], The main sources of heterogeneous nucleation are usually surface irregularities capable of containing entrapped gas, e.g. pits and scratches. The bubbles typically develop over the electrode surface, grow in size until they reach a break-off diameter and subsequently detach into the electrolyte. After detachment, some residual gas remains at the nucleation site and another bubble will form at the same place [2,13,14], In most two-phase flow simulations [15-19], it is assumed that bubbles detach with a constant diameter, although from experiments [20,21] it is know that electrochemically formed bubbles show a size distribution. 

The structured carbon source must have steps or ledges to serve as diamond nucleation sites. 

Although the nucleation of cavities does not seem to be well understood at present it is clear that cavitation depends on microstructure. Pbrosity and second-phase particles, which are sources of stress concentration (see Chapter 18), can act as nucleation sites for cavitation and subsequent crack growth. Remember pores can be found in most ceramics even pore-free materials such as hot-pressed alumina may contain small pores. Cavitation also occurs in ceramics with IGFs. Nucleation of the cavities will usually occur at regions where the IGF is not homogeneous, e.g., nonwetted regions, gas bubbles, and impurity particles. 

The relationships between solubility, supersaturation, pressure, and temperature depend upon the details of the microscopic mechanisms operative in the context of the detailed active impurity chemistry and the specific geometric relationship set up between the source carbon, molten metal solution, and nucleation site, and or growing diamond. For an introduction into these extensive subjects the reader is referred to Burns and Davies [16] where the issues such as nitrogen and boron active impurity chemistries, diamond surface reconstruction, morphology and solvent/catalyst are discussed. 

Another source of nucleating sites are crystalline regions of the parent material entrapped in micro cavities in the container walls or in motes (Turnbull (1950d)). For a cylindrical microcavity of radius the crystalline phase will be stable for all cavities for which... 

Sources of Nucleation Sites on Surfaces, Steps and Dislocations... 

Zeolite synthesis occurs at temperatures between 100 and 200°C in a basic medimn under hydrothermal conditions. Usually high concentrations of sUica and almnina sources are used and synthesis occurs from a state where initially a gel is formed. Upon heating, silicate and aluminate species dissolve from the gel and chemistry relevant to crystallization occurs in the solvent phase. Nucleation itself is assisted by nucleation sites offered by the gel material. 

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