Van der Waals solid

Contrast the bonds between atoms in metals, in van der Waals solids, and in network solids in regard to ... 

Van der Waals solid 123 Vapour transport 201 Vickers hardness 122 Viscoelastic 203... 

Simpson WT, Peterson DL (1957) Coupling strength for resonance force transfer of electronic energy in Van der Waals solids. J Chem Phys 26 588-593... 

In van der Waals solids (e.g., solid Ne figure 1.2C), the reticular positions are occupied by inert atoms held together by van der Waals forces. The van der Waals bond is rather weak, and the bond does not survive a high thermal energy (Fm3m solid Ne, for instance, is stable below 20 K). The van der Waals bond also exists, however, in molecular compounds (e.g., solid CO2). The reticular positions in this case are occupied by neutral molecules (CO2). Within each molecule, the bond is mainly covalent (and interatomic distances are considerably shorter), but... 

As has been discussed in this article, C60 fullerene has shown its rich cohesive properties in various environments. It can form a van der Waals solid in the pristine phase and in other compound materials with various molecules. In fullerides, i.e., the compounds with metallic elements, valence electrons of metal atoms transfer to C60 partly or almost completely, depending on the lattice geometries and electronic properties of the metallic elements. So fullerides are ionic solids. Interestingly, these ionic fullerides often possess metallic electronic structure and show superconductivity. The importance of the superconductivity of C60 fullerides is not only in its relatively high Tc values but also in its wide... 

Figure 14.8 shows stress-strain curves for polycarbonate at 77 K obtained in tension and in uniaxial compression (12), where it can be seen that the yield stress differs in these two tests. In general, for polymers the compressive yield stress is higher than the tensile yield stress, as Figure 14.8 shows for polycarbonate. Also, yield stress increases significantly with hydrostatic pressure on polymers, though the Tresca and von Mises criteria predict that the yield stress measured in uniaxial tension is the same as that measured in compression. The differences observed between the behavior of polymers in uniaxial compression and in uniaxial tension are due to the fact that these materials are mostly van der Waals solids. Therefore it is not surprising that their mechanical properties are subject to hydrostatic pressure effects. It is possible to modify the yield criteria described in the previous section to take into account the pressure dependence. Thus, Xy in Eq. (14.10) can be expressed as a function of hydrostatic pressure P as... 

Equation (5.7.5], and variants thereof, have been widely invoked to assess the surface tension of solid surfaces by working with organic liquids (such as hydrocarbons) in which dispersion forces prevail. For those one may set = y " so that the equation can be solved for y , which, in turn, may be equated to y for a Van der Waals solid. However, we see from [5.7.7] that the situation is more complicated. First, neglection of the TAS term is not allowed, it may account for 20-30% of the interaction Helmholtz energy. Second, even if an assumption is made on, or if it is directly measured from the heat of adhesion, only the... 

One may attempt to derive the ideal shear strength So of the van der Waals solid normal to the chain axis from the value of the lateral surface free energy, a. This value is well known for common polymers such as PE or polystyrene (PS) (Hoffman et al, 1976) or else can be calculated from the Thomas-Stavely (1952) relationship a = /a Ahf)y, where a is the chain cross-section in the crystalline phase, Ahf is the heat of fusion, and y is a constant equal to 0.12. If one now assumes that a displacement between adjacent molecules by Si within the crystal is sufficient for lattice destruction then the ultimate transverse stress per chain will be given by So = cr/31. The values so obtained are shown in Table 2.1 for various polymers. In some cases (nylon, polyoxymethylene, polyoxyethylene (POE)) the agreement with experiment is fair. In the others, deviations are more evident. In order to understand better the discrepancy between the experimentally observed and the theoretically derived compressive strength one has to consider more thoroughly the micromorphology of polymer solids and the phenomena caused by the applied stress before lattice destruction occurs. 

Recently, Eklund s group has reported that solid Ceo transforms from a van der Waals solid into a covalently bonded network upon irradiation with visible or ultraviolet light.[Ra93] The transformed phase is resistant to toluene, a good solvent for Ceo-Significant changes in the infrared, Raman, UV-vis, and x-ray diffraction spectra were noted. However, the spectra are sufficiently similar to untransformed Ceo that it appears that the fullerene framework is largely intact in the phototransformed phase. 

The preceding was developed mostly for atomic or molecular targets, and, indeed, much experimental work has been carried on gaseous samples. If one thinks along the lines of the Bragg rule [21], then one would not expect such considerations to differ much from results obtained on solid samples, and for most molecular or van der Waals solids - indeed they do not [22]. 

Our purpose in these last two subsections has been to show how the simplest fundamental description of SEE for van der Waals solids can emerge from the hard-sphere model and mean field theory. Much of the remainder of the chapter deals with how we extend this kind of approach using simple molecular models to describe more complex solid-fiuid and solid-solid phase diagrams. In the next two sections, we discuss the numerical techniques that allow us to calculate SEE phase diagrams for molecular models via computer simulation and theoretical methods. In Section IV we then survey the results of these calculations for a range of molecular models. We offer some concluding remarks in Section V. 

Figure 3.1 8 Bulk modulus of PMMA at different pressures vs. a function of molar volume. The straight line is the prediction of Eq. (3.26) for a van der Waals solid with = 137kJ mol. ...

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