Liquid, definition glass

Amorphous Selenium.—(1) Vitreous Selenium.—When molten selenium is cooled in not too protracted a manner, no definite solidification or crystallisation ensues, but the mass gradually hardens and the product really represents a strongly undercooled liquid like glass. Vitreous selenium is a brittle reddish-brown substance, exhibiting a conchoidal fracture. When finely powdered and viewed in thin layers it has a deep red colour. This form has an average density of 4-28 5 the value varies slightly, possibly owing to the presence of other allotropic modifications of the element. 

Temperahire is commonly measured with liquid-in-glass thermometers, wherein the hquid expands when heated. Thus a uniform hibe, partially filled with mercury, alcohol, or some otherfluid, can indicate degree of "hotness" simply by the length of tire fluid colunm. However, numerical values are assigned to the various degrees of hotness by arbitrary definition. 

Such a definition is equivalent to what is customary in glass physics, where the transition from an equilibrium liquid to a non-equilibrium supercooled liquid (a glass) is characterized by a glass transition temperature, Tg, which is typically defined as the temperature at which the a-relaxation time scale approaches a certain laboratory time scale, typically 100 s. 

Clearly, the LL- and SL-environment components of a-Si presented above are a direct parallel of the 5-7-sided and 6 6-sided polyhedra in the 2D simulation of a sub-cooled metallic liquid and glass discussed in Section 1.7 and the examples shown in Figs. 1.1 l(a)-(c). Both types of these representations of LL and SL atom environments are in full conformity with the definition introduced by Cohen and Grest (1979). The direct role of these special environments, their formation, and alterations during plastic flow are the subject of Chapter 7. 

There are two hypothetical limiting cases of interest. In one, an infinitely slow cooling rate maintains thermodynamic equilibrium to the ideal glass, and the equilibrium formalism is applicable. In the other a fluid in equilibrium (at its fictive temperature) is quenched infinitely fast to a temperature low enough so that no molecular transport occurs. In this case, what were dynamic fluctuations in time becomes static fluctuations in space. The most elementary treatment of this glass is then as a thermodynamic system with one additional parameter, the fictive temperature. In an actual experiment, of course, relaxations take place and the state of the system is dependent upon its entire thermal history and requires many parameters for its definition. Detailed discussion of the use of irreversible thermodynamics for the study of relaxation processes in liquids and glasses is contained in reviews by Davies (1956, 1960). 

Quantitative, empirical temperature scales were developed in the eighteenth century. In principle, one can choose any function of length for a definition of temperature based on the liquid-in-glass thermometer, but certainly one wants to make it as simple as possible and transferable from one thermometer to another. The liquid-in-glass temperature scales were thus all linear, as shown in Fig. 13. The two constants a and b are fi ee to be chosen. The easiest way of fixing these two constants is to specify two known and reproducible temperatures. 

Endotherm (3) indicates the melting at 526 K, and finally, there is a broad, exotherm with two peaks (4) due to decomposition. Again, this DTA curve is quite characteristic of poly(ethylene terephthalate) and can be used for its identification. Optical observation to recognize glass and melt by their clear appearances is helpful. Microscopy between crossed polars is even more definitive for the identification of an isotropic liquid or glass (see Sect. 3.4.4). 

The strict definition of a phase is any homogeneous and physically distinct region that is separated from another such region by a distinct boundary . For example a glass of water with some ice in it contains one component (the water) exhibiting three phases liquid, solid, and gaseous (the water vapour). The most relevant phases in the oil industry are liquids (water and oil), gases (or vapours), and to a lesser extent, solids. 

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

A distinction between a solid and liquid is often made in terms of the presence of a crystalline or noncrystalline state. Crystals have definite lines of cleavage and an orderly geometric structure. Thus, diamond is crystalline and solid, while glass is not. The hardness of the substance does not determine the physical state. Soft crystals such as sodium metal, naphthalene, and ice are solid while supercooled glycerine or supercooled quartz are not crystalline and are better considered to be supercooled liquids. Intermediate between the solid and liquid are liquid crystals, which have orderly structures in one or two dimensions,4 but not all three. These demonstrate that science is never as simple as we try to make it through our classification schemes. We will see that thermodynamics handles such exceptions with ease. 

A crystalline solid is a solid in which the atoms, ions, or molecules lie in an orderly array (Fig. 5.16). A crystalline solid has long-range order. An amorphous solid is one in which the atoms, ions, or molecules lie in a random jumble, as in butter, rubber, and glass (Fig. 5.17). An amorphous solid has a structure like that of a frozen instant in the life of a liquid, with only short-range order. Crystalline solids typically have flat, well-defined planar surfaces called crystal faces, which lie at definite angles to one another. These faces are formed by orderly layers of atoms (Box 5.1). Amorphous solids do not have well-defined faces unless they have been molded or cut. 

In order to discuss the various techniques we must distinguish between diffusive and non-diffusive systems (J8). Diffusive systems, such as liquids, are characterized by the eventual diffusion of particles over all of the available space non-diffusive systems such as solids, glasses and macromolecules with a definite average structure are characterized by time independent average positions around which the atoms fluctuate. 

Solutions are mixtures, and therefore do not have definite compositions. For example, in a glass of water it is possible to dissolve 1 teaspoonful of sugar or 2 or 3 or more. However, for most solutions there is a limit to how much solute will dissolve in a given quantity of solvent at a given temperature. The maximum concentration of solute that will dissolve in contact with excess solute is called the solubility of the solute. Solubility depends on temperature. Most solids dissolve in liquids more at higher temperatures than at lower temperatures, while gases dissolve in cold liquids better than in hot liquids. 

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