Modified Random Network model

The reader should recall that the fitting of a structure to diffraction data is not unique. We have shown that both the constructed modified random network model of Polk, as well as the network simulated by allowing Gaussian distributions of atom-atom distances can fit the observed structure functions for low density H20(as), and the latter, with modification to include small OOO... 

Alben and Boutron suggest that the peak in the X-ray and neutron scattering functions at 1.7 A-1 is indicative of an anisotropic layer structure extending over at least 15 A in Polk type continuous random network models. To show this better Fig. 52 displays the radial distribution function of the Alben-Boutron modified... 

A good example of the role of such calculations is provided by the recent work of Vessal et al. (amplified in Chapter 12) who have developed by MD quenching procedures, models for both silica and silicate glasses. Plate III shows their model of vitreous Na2Si2C>5, an intriguing feature of which is the clear indication of a microsegregation of the framework modifying Na+ cations into loosely defined channels in the silicate matrix. Such behaviour is in accordance with the Modified Random Network (MRN) model of silicate systems... 

More recently, Greaves (1985) introduced a modified random network where modifiers form zones connecting silica-rich volumes as shown for a modified silica glass in Figure 5.5. This model offers interesting predictions and... 

These network models are comprised of different-sized, interconnected elements of uniform shape (e.g., Fig 3-17B). The configuration of elements within the network can be either systematic or random. Marie and Defrenne (1960) were the first to use this type of model to predict solute dispersion. Their network was a modified capillary bundle model with regularly spaced interconnections between parallel tubes of radii r and r2. This model does not consider diffusion. Spreading of a solute in the model is given by ... 

Model calculations (Mackinnon, 1980 Mackinnon and Kramer, 1981) that employ a random network (Weaire et al., 1979) have been utilized to calculate the expected H NMR linewidths. In these calculations the atomic positions of the original hand-built model were relaxed to the energy minimum given by a Keating Hamiltonian that was modified to include a repulsive term between nonbonded atoms and a coulombic term to simulate the polarization of the Si-H bond. With these two extra terms, linewidths appropriate to the broad H NMR line can be obtained, but unfortunately these calculations provide no indication of the origin of the narrow line. As mentioned above, it is interesting to note that the broad line is produced from an H-H distribution that is broad and essentially structureless. 

The structure of vitreous silica consists of a continuous, random network of corner-sharing Si04 tetrahedra. Extending this model to alkali silicate glasses requires that the alkali cations be regarded as network modifiers, as shown in Figure 11 The addition of each alkali oxide unit results in the replacement of... 

The remarkable properties of electrospun CNTs nanocomposites continue to draw attention in the development of multifunctional properties of nanostructures for many applications.. Multiscale model for calculation macroscopic mechanical properties for fibrous sheet is developed. Effective properties of the fiber at microscale determined by homogenization using modified shear-lag model, while on the second stage the point-bonded stochastic fibrous network at macroscale replaced by multilevel finite beam element net. Elastic modulus and Poisson s ratio dependence on CNT volume concentration are calculated. Effective properties fibrous sheet as random stochastic network determined numerically. We conclude that an addition of CNTs into the polymer solution results in significant improvement of rheological and structural properties. 

Although the environment of the network former cation is relatively well known, that of the modifier cation is much less so, due to the lack of appropriate spectroscopic techniques. The absence of direct experimental data has given rise to the coexistence in the literature of very different hypotheses ranging from models based on a totally random distribution of ionic bonds to those based on zones rich in modifier cations which alternate with less rich zones (Greaves, 1985). 

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