Structure packing density

Starting material Deposition method8 evaporation temperature Film composi tion Film structure, - packing density (p), and substrate temperature (Ts) Transmittance range (pm) a<103 cm 1 Refractive index ( , 2) , wavelength (pm), and substrate temperature (Ts) Mechanical and chemical film properties... 

This section provides a systematic account of proton transport mechanisms in water-based PEMs, presenting studies of proton transport phenomena in systems of increasing complexity. The section on proton transport in water will explore the impact of molecular structure and dynamics of aqueous networks on the basic mechanism of proton transport. The section on proton transport at highly acid-functionalized interfaces elucidates the role of chemical structure, packing density, and fluctuational degrees of freedom of hydrated anionic surface groups on concerted mechanisms and dynamics of protons. The section on proton transport in random networks of water-filled nanopores focuses on the impact of pore geometry, the distinct roles of surface and bulk water, as well as percolation effects. 

S has the same sign as e, i/ = 0.5 corresponds to volume conservative deformation. As detailed in the main part of this book, Poisson s ratio depends on glass type and increases as glass structure packing density increases (Rouxel et al, 2008 Greaves et al., 2011). It is —0.2 for oxide glasses (Cg — 0.5) and 0.4 for metallic glasses (Cg > 0.7). 

Metal atoms tend to behave like miniature ball-bearings and tend to pack together as tightly as possible. F.c.c. and c.p.h. give the highest possible packing density, with 74% of the volume of the metal taken up by the atomic spheres. However, in some metals, like iron or chromium, the metallic bond has some directionality and this makes the atoms pack into the more open b.c.c. structure with a packing density of 68%. 

Fig. 2.4. The structure of a typical grain boundary. In order to "bridge the gap" between two crystals of different orientation the atoms in the grain boundary have to be packed in a less ordered way. The packing density in the boundary is then as low as 50%.

The secondary and tertiary structures of myoglobin and ribonuclease A illustrate the importance of packing in tertiary structures. Secondary structures pack closely to one another and also intercalate with (insert between) extended polypeptide chains. If the sum of the van der Waals volumes of a protein s constituent amino acids is divided by the volume occupied by the protein, packing densities of 0.72 to 0.77 are typically obtained. This means that, even with close packing, approximately 25% of the total volume of a protein is not occupied by protein atoms. Nearly all of this space is in the form of very small cavities. Cavities the size of water molecules or larger do occasionally occur, but they make up only a small fraction of the total protein volume. It is likely that such cavities provide flexibility for proteins and facilitate conformation changes and a wide range of protein dynamics (discussed later). 

AFM is used in the surface analysis. Figure 16 is the AFM topography of the monolayer and the multilayer L-B films. It shows that the monolayer L-B film is well packed and highly ordered on the mica surface. The surface of the monolayer film (shown in Fig. 16(a)) has a higher packing density than that of the four-layer L-B film (shown in Fig. 16(b)). This is because the molecules form the different structures in the monolayer film from those in four-layer... 

What this means is that elements (e.g.- metals) can have more than one structure. For example, Fe exists in 4 structures, i.e.- Fe is tetramorphous. a-Fe is the one stable at room temperature. But, it transforms to p-Fe, then y-Fe and finally 8 -Fe as the temperature is raised. These changes are all reorganizations in packing density before the melting temperature is reached. In constrast, heterogeneous solids usually exist in one strucural... 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

By using this definition, increasing values of the (weighted) CNW coordination number are obtained for the structures diamond (4.54), simple cubic (6), body-centred cubic (10.16), face-centred cubic (12) (in agreement with the increasing packing density). 

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