Amorphous structures, modelling

To develop amorphous structure model for the direct interpretation of adsorption isotherms measured on CPGs and porous glasses, one could in... 

We now ask how well does the bond fluctuation model with these bond lengths and bond angle potentials reproduce the properties of real polymer melts quantitatively. First of all, it must be admitted that the model yields a qualitatively reasonable picture of the amorphous structure, as exemplified by... 

The structure of a vapor-quenched alloy may be either crystalline, in which the periodicity of the unit cell is repeated within the crystallites, or amorphous, in which there is no translational periodicity even over a distance of several lattice spacings. Mader (64) has given the following criteria for the formation of an amorphous structure the equilibrium diagram must show limited terminal solubilities of the two components, and a size difference of greater than 10% should exist between the component atoms. A ball model simulation experiment has been used to illustrate the effects of size difference and rate of deposition on the structure of quench-cooled alloy films (68). Concentrated alloys of Cu-Ag (35-65%... 

FIGURE 2.14 Schematic two-dimensional representation of a modified micelle model of the crystalline-amorphous structure of polymers. 

The existence of the surface contribution to the effective magnetostriction of nanocrystalline alloys has been confirmed theoretically in terms of the dipolar model (Szumiata et al. 1999). These authors showed that, due to the limited radius of the nanoparticles, additional magnetostrictive stresses are localised at the interfaces. The evaluation of the influence of the dipolar interaction on the magnetostriction in crystalline grains of perfect spherical shape surrounded by a magnetic environment of about 0.S nm with either crystalline or amorphous structure has been calculated. A similar method was previously used to obtain the surface and volume anisotropy (Draaisma and de Jonge 1988) and to... 

Y. Cao and A.N. Cormack. A structural model for interpretation of an anomaly in alkali aluminosilicate glasses at Al/alkali = 0.2-0.4. In H. Jain and D. Gupta, editors, Diffusion in Amorphous Materials, pages 137-151, Warrendale, PA, 1994. The Minerals, Metals and Materials Society. 

The assignments of the v (CH2) bands in terms of the assumed model are not entirely unambiguous. Two strong a bands and two weak 7t bands [re (CH2)0 is probably close to zero intensity] are predicted. In pure PVdC four bands are found, all apparently of a polarization. Two of these, however, at 2850 and 2930 cm-1, are of highly variable intensity in the copolymer spectra they become quite weak in comparison to the other two at 2948 and 2990 cm-1 [Narita, Ichinohe, and Enomoto (747) quote frequency values of 2966 and 3010 cm-1 for these two bands]. A possible explanation is that the 2850 and 2930 cm-1 bands are associated with amorphous structures, while the 2948 and 2990 cm-1 bands are associated with the Fuller structure. We would then have to assume that the vtt (CH2)0 mode is too weak to be observed. It is clear that these proposals require additional confirmation. 

Model systems can be, on the one hand, subjected to a static structure optimization. There, the fact is considered that the potential energy of a relaxed atomistic system (cf. Equation 1.1) should show a minimum value. Static optimization then means that by suited numeric procedures the geometry of the simulated system is changed as long as the potential energy reaches the next minimum value [16]. In the context of amorphous packing models, the main application for this kind of procedure is the reduction of unrealistic local tensions in a model as a prerequisite for later molecular dynamic (MD) simulations. 

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

In addition, one hypothesis for the secondary structure in spidroin suggests that there are amorphous phases, highly oriented crystals, and oriented noncrystalline phases coexisting (Grubb et al., 1997). This structure model has been used to explain the super-contraction of dragline (Liu et al., 2005b). 

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