Diffusion-enhanced amorphous

If the weak bonding model is valid for high angle grain boundaries (>20°) it follows that diffusion in amorphous preparations of a given material should also show this measure of enhanced diffusion, when compared widr die crystalline material. 

Radiochemical studies indicate that the pore base is the actual site of formation of aluminium oxide, presumably by transport of aluminium ions across the barrier-layer, although transport of oxygen ions in the opposite direction has been postulated by some authorities. The downward extension of the pore takes place by chemical solution, which may be enhanced by the heating effect of the current and the greater solution rate of the freshly formed oxide, but will also be limited by diffusion. It has been shown that the freshly formed oxide, y -AljOj, is amorphous and becomes slowly converted into a more nearly crystalline modifipation of y-AljO . 

Preparation method. Mild acid-dealumination will generally result in a more active material than the parent zeolite due to (a) removal of amorphous materials from the zeolite channels, thus lowering the diffusion resistance for the reacting molecules and (b) generation of stronger acid sites during the dealumination process, which enhances the catalytic activity of the zeolite for acid-catalyzed reactions. However, thermal dealumination will generally result in less... 

The chemical stability of an amorphous formulation is usually also a function of its storage temperatme relative to Tg. The enhanced molecular mobility achieved near the glass transition translates into an increase in translational diffusion-dependent degradation pathways, such as aggregation in proteins. It should be noted that the reaction kinetics near the Tg do not obey Arrhenius kinetics, and that extrapolation of the accelerated stability data generated near the Tg to stability at the storage temperature should be viewed with extreme caution. Amorphous materials must be stored well below the glass transition (at least 10°C, and typically 40 to 50°C below Tg) to maintain their physical and chemical stability. 

In summary, there are at least four ways in which residual moisture in the amorphous state can impact on chemical reactivity. First, as a direct interaction with the drug, for example, in various hydrolytic reactions. Second, water can influence reactivity as a by-product of the reaction, by inhibiting the rate of the forward reaction, for example, in various condensation reactions, such as the Maillard reaction. Third, water acting locally as a solvent or medium facilitating a reaction, without direct participation. Finally, by virtue of its high free volume and low Tg, water can act as a plasticiser, reducing viscosity and enhancing diffusivity [28]. 

A sample calculation using these conditions is shown in Figure 26. B + was implanted (1 X 1015/cm2 and 50 keV) and annealed by RTA at 1150 °C for 10 s. Then Si+ was implanted to create an amorphous layer, and the RTA cycle was repeated. These data were compared with a B profile obtained with no Si+ implant but with the same heat treatment. The enhanced diffusion of B was calculated by using equation 52, and good agreement was achieved. This model has been verified for RTA and furnace anneals at temperatures as low as 750 °C. 

Historically most of the microscopic diffusion models were formulated for amorphous polymer structures and are based on concepts derived from diffusion in simple liquids. The amorphous polymers can often be regarded with good approximation as homogeneous and isotropic structures. The crystalline regions of the polymers are considered as impenetrable obstacles in the path of the diffusion process and sources of heterogeneous properties for the penetrant polymer system. The effect of crystallites on the mechanism of substance transport and diffusion in a semicrystalline polymer has often been analysed from the point of view of barrier property enhancement in polymer films (35,36). 

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