Glasses and Amorphous Solids

From vector algebra, we know that any ordered set of three numbers that can be obtained from (u, Uy, u ) by multiplying all of them by the same positive constant k is also a set of direction numbers for the vector, in that they define the direction of the vector. Hence choosing k to be (1/-0.176776) gives  

Therefore, an equivalent axis-angle pair is rotation by 109.47° about 041). 

In the supercooled liquid state, BMGs have very high yield strength and a high elastic-strain limit (often exceeding 2 percent, compared with crystalline materials that are almost always less than 1 percent), which makes them very springy. However, 

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At lower temperatures, weaker intercluster interactions may lead to further aggregation into superclusters (clusters of clusters). In this manner the unit at one temperature becomes the subunit at a lower temperature, leading to a hierarchy of organizational levels. Clusters and superclusters may underlie the properties of many amorphous solids and glasses, but such materials are beyond the scope of the present work. 

However, the identicalness of protein molecules possessing the same macroconformation is not absolute. Within each structurally determined conformational macrostructure, there exists a microdisordering which is similar to that observed in amorphous solids and glasses.(U,14) It is associated with the presence of multiple relative minima of the free energy depending on small shifts and variations in orientation of certain groups within the limits of available space. 

The precision with which this relationship between the imposed impulse and the physical property needs to be expressed depends on a number of factors. Of these, the nature of the sample is of considerable importance. Gases, liquids, amorphous solids and glasses, and polycrystalline arrays are isotropic. That is, the physical property is the same in all directions. In these cases there is little to be gained by specifying the imposed impulse and response as vectors, and scalars give aU the information about the physical property that is needed. In many other cases the directional nature of the processes becomes aU important, and the material is regarded as anisotropic. This happens when measuring molecular properties, properties of objects such as nanotubes, and most crystals. In this case it is necessary to specify the direction of... 

Several new thermal analytical techniques are potentially valuable for the study of second-order transitions in the characterization of amorphous solids and for the accurate determination of glass transition temperatures. These modem techniques can detect and characterize glass transitions and other second-order transitions that are not detectable by conventional thermal analytical techniques such as DSC, TGA, or TMA. 

To illustrate this more quantitatively, consider the hypothetical sucrose example discussed by Ahlneck and Zografi [80]. Assuming that all the sorbed water is taken up by the amorphous portion of material, 0.1% total moisture would correspond to approximately 20%, 10%, 4%, and 2% moisture content in the amorphous material, respectively, for 0.5%, 1%, 2.5%, and 5% of amorphous solid. The glass transition temperatures for the amorphous portions... 

FIG. 31 Schematic diagram illustrating the transition between a supercooled liquid state (rubber) and an amorphous solid state (glass). The glass transition event is typically caused by a decrease in water content and/or temperature. The reversibility of the transition, as indicated by the dotted arrow, is material dependent (see text for further discussion of the reversibility of the transition). 

Solubility and speciation. Minimum requirements for reliable thermodynamic solubility studies include (i) solution equilibrium conditions (ii) effective and complete phase separation (iii) well-defined solid phases and (iv) knowledge of the speciation/oxidation state of the soluble species at equilibrium. Ideally, radionuclide solubilities should be measured in both oversaturation experiments, in which radionuclides are added to a solution untU a solid precipitates, and undersaturation experiments, in which a radionuchde solid is dissolved in aqueous media. Due to the difference in solubilities of crystalline versus amorphous solids and different kinetics of dissolution, precipitation, and recrystalhzation, the results of these two types of experiments rarely agree. In some experiments, the maximum concentrahon of the radionuchde source term in specific water is of interest, so the sohd that is used may be SF or nuclear waste glass rather than a pure radionuclide solid phase. 

This Chapter deals with some basic notation and nomenclature of solid-state chemistry. In some areas, such as amorphous systems and glasses, the nomenclature needs further development. The reader is also referred to the work of the International Union of Crystallography. 

A crystal may be defined as a solid composed of atoms arranged in a pattern periodic in three dimensions. As such, crystals differ in a fundamental way from gases and liquids because the atomic arrangements in the latter do not possess the essential requirement of periodicity. Not all solids are crystalline, however some are amorphous, like glass, and do not have any regular interior arrangement of atoms. There is, in fact, no essential difference between an amorphous solid and a liquid, and the former is often referred to as an undercooled liquid. ... 

I Tsukushi, O Yamamuro, H Suga. Heat capacities and glass transitions of ground amorphous solid and liquid-quenched glass of tri-O-methyl-P-cyclodextrin. J Non-Crystall Solids 175 187, 1994. 

Liquids, when strongly supercooled, are as a rule frozen, and so the examination of them down to very low temperatures is rendered impossible. We know, however, a large number of exceptions, in particular the glasses quartz glass may be mentioned as a striking example. Molten quartz, if cooled sufficiently rapidly, does not crystallize, but passes continuously into the condition of amorphous solid, quartz glass the specific heat of this can be measured without any special difficulty down to temperatures as low as may be desired. 

Glass is a special kind of amorphous solid, and it serves as an analog for comparison for other types of materials. For example, when proteins fold into macroscale structures, they can sometimes be referred to as being in a glassy state. 

Examples of amorphous solids are glass, tar and the naturally occurring gums and resins. Such resins formed the basis of many early paints, because the distinctive property of an amorphous solid is that, when a melt is poured into a tray, or when a solution of the solid is poured upon a surface and the solvent is allowed to evaporate, the resultant solid forms a continuous film. Because it has no natural shape of its own, it takes the shape into which it has been cast. A crystalline material would give a discontinuous film, consisting of many hundreds of tiny individual crystals. It is the amorphous continuity that makes the resin film extremely suitable for protecting surfaces. 

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