Amorphization dose required

The dose required for amorphization is a function of the kinetics of simultaneous dynamic recovery processes. The recovery process is accelerated at elevated temperatures and, in many cases, is greatly increased by radiation-enhanced defect migration. These simultaneous recovery processes may be associated with defect recombination or annihilation, epitaxial recrystallization at crystalline-amorphous interfaces (Carter and Nobes 1991), or nucleation and growth recrystallization in the bulk of the amorphous state. For any crystalline solid, there is a critical temperature, above which the rate of amorphization is less than the rate of recovery, thus amorphization cannot occur. However, Tc also depends on the energy and mass of the incident beam, as well as the dose rate. 

Nc is the radionuclide content required to cause the metamict state, in ppm uranium, Do is the extrapolated ion-beam amorphization dose at 0 K, C is Ec/kTc from the model of Weber et al. (1994), n is the average number of a-decay events in the decay chain, and x is the number of atomic displacements per a-decay event. For a given temperature, a specimen containing a uranium concentration above Nc should be metamict. 

Chemical transformations, such as crosslinks or double bonds, which are radiation induced defects [92, 97, 101, 102], result in a degree of crystallite destruction. Therefore, as the absorbed dose increases, the crystallinity should decrease. In Table 4 [103], there are presented the doses required for crystallographic effects of electron irradiation in various polymers. The dose necessary for the destruction of crystallinity in linear PE is much larger than that corresponding for the other polymers. In fact, it was shown [97] that radiation damage occurs preferentially within the amorphous regions of the material. 

Table 3.13 gives the doses of electron irradiation required for crystallographic effects in various polymers. The dose required for the destruction of crystallinity in linear PE is much larger than that for the other polymers, due to the fact that that radiation damage occurs preferentially within the amorphous regions of the material. 

Quartz variety Major impurity Dose Z>o for which damage is first observed (nvt) D required to produce the amorphous state (nvt) Annealing conditions necessary to recover specimens damaged by a dose... 

It is very difficult to bombard most metals to an extent high enough to achieve the crystalline to amorphous transition. This requires in any case doses 10 ions/cm. Very often recrystallization processes take place even at RT. Thus one may get a saturation dose but not a completely amorphous structure. The situation is quite different for the semiconductors Si and Ge which easily suffer a transformation from the crystalline to amorphous states at doses of 10 —10 ions/cm. These... 

If the drug is insufficiently soluble to allow delivery of the required dose as a solution (the maximum delivered dose for each nostril is 200 p,L), then a suspension formulation will be required. There are additional issues for suspension products, for example crystal growth, physical stability, resuspension, homogeneity and dose uniformity. Suspension products will also require information on density, particle size distribution, particle morphology, solvates and hydrates, polymorphs, amorphous forms, moisture and/or residual solvent content and microbial quality (sterile filtration of the bulk liquid during manufacture is not feasible). 

During ion implantation, each ion produces a region of disorder around the ion track. As the implantation dose increases, the disorder increases until all the atoms have been displaced and an amorphous layer is produced over a depth Rp. The buildup and saturation of disorder are shown in Fig. 10.1 for 40 keV phosphorus ions incident on Si. In this example, about 4 x 1014 phosphorus ions cnT2 are required to form an amorphous layer. Except for low doses or implantation with light ions, we can anticipate that an amorphous layer is formed during the implantation process. This assumes that no recovery of lattice order occurs around the ion track. 

The obtained results imply that the prediction of amorphization resistance of composites in the irradiation environment requires taking into consideration the particle size and the relationship between the formation heat of phases and the heat of their mixing. On the other hand, low radiation doses fail to allow us to draw a unique conclusion on radiation tolerance of these nanocomposites, whose thickness of layers does not exceed tens of nanometers. By varying the thickness of layers... 

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