Nucleation range

Nucleation and growth of gas hydrate crystals have been investigated with optical methods under different pressures and temperatures. The particle sizes measured during gas hydrate nucleation ranged from 2 to 80 imi [1334,1335]. The nucleation process is nondeterministic, because of a probabilistic element within the nucleation mechanism [1393]. 

The stationary layer thickness (i ) has to be much larger than the particle size. Usually, d is believed to he on the order of 10 fan, decreasing with increasing stirring intensity. For particles in the nucleation range (2-20 nm) this condition is very well fulfilled for seeded experiments with larger particles the assumption may be doubtful. 

Type V isotherms of water on carbon display a considerable variety of detail, as may be gathered from the representative examples collected in Fig. 5.14. Hysteresis is invariably present, but in some cases there are well defined loops (Fig. 5.14(b). (t ), (capillary-condensed water. Extreme low-pressure hysteresis, as in Fig. 5.14(c) is very probably due to penetration effects of the kind discussed in Chapter 4. 

Cellulose Acetate. The extmsion process has also been used to produce ceUular ceUulose acetate (96) ia the deasity range of 96—112 kg/m (6—7 lbs /fT). A hot mixture of polymer, blowiag ageat, and nucleating agent is forced through an orifice iato the atmosphere. It expands, cools, and is carried away on a moving belt. 

Economic Aspects. The first company to introduce nucleated PET mol ding resins was Ak2o Plastics BV with their Amite range. This business has been taken over by DSM NV as part of a portfoHo exchange between the two companies. Du Pont introduced their Rynite fast crystallising materials in 1978, and other manufacturers followed. 

Modifications to Precipitates. Silicon is sometimes added to Al—Cu—Mg alloys to help nucleate S precipitates without the need for cold work prior to the elevated temperature aging treatments. Additions of elements such as tin [7440-31-5] Sn, cadmium [7440-43-9] Cd, and indium [7440-74-6] In, to Al—Cu alloys serve a similar purpose for 9 precipitates. Copper is often added to Al—Mg—Si alloys in the range of about 0.25% to 1.0% Cu to modify the metastable precursor to Mg2Si. The copper additions provide a substantial strength increase. When the copper addition is high, the quaternary Al CuMg Si Q-phase must be considered and dissolved during solution heat treatment. 

Nucleation of particles in a very short time foUowed by growth without supersaturation yields monodispersed coUoidal oxide particles that resist agglomeration (9,10). A large range of coUoidal powders having controUed size and morphologies have been produced using these concepts (3,14). 

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