Glass fusion point

The clear point, also called the clear melting point, complete melting point, complete fusion point or capillary melting point, is the temperature at which a sample of fat becomes visibly completely clear, indicating the disappearance of all traces of solid fat (Rossell, 1986 Stauffer, 1996). Its measurement is specified by the American Oil Chemists Society (AOCS) Official Method Cc 1-25 (Firestone, 1998). Samples of the tempered fat contained in at least three vertical glass capillary tubes, sealed at their lower ends, are attached to a vertical mercury-in-glass thermometer such that their lower ends are level with the lower end of the thermometer s bulb. This assembly is immersed in a water bath and heated, and the... 

Fig. 4 Free energy diagram for a material in the amorphous and crystalline states (Fg denotes the glass transition temperature Ff indicates the melting/fusion point).

Since the process of glass formation is a central interest of this chapter, we consider first some results of dynamic simulation experiments similar to those referred to schematically in Fig. 5. As the density of a system of spherical particles is increased from the fusion points, three significant, continuous, effects occur in g(r) as shown in Figs. 14 and 16 (Section IV.D.2) for LJ particles and soft spheres. First, the peaks become narrower and more sharply defined as a result of restricted particle displacements. Second, discernible structure begins to extend to larger distances. Third, the second-nearest-neighbor peak begins to split into two components. 

Fig. 21. Entropy versus log-temperature diagram for the hard-sphere model. The solid curves give the computer simulation values for the supercooled fluid, glass, and crystal. The dashed curves have the following bases (a) a calculation from the virial equation using the known first seven coefficients and higher coefficients obtained from the conjectured closure (the plot corresponds quite closely with that calculated from the Camahan-Starling equation ) and (i>) an extrapolation of higher temperature behavior such as that used by Gordon et al., which implies a maximum in the series of virial coefficients. The entropy is defined in excess of that for the ideal gas at the same temperature and pressure. Some characteristic temperatures are identified 7, fusion point 7 , upper glass transition temperature T/, Kauzmann isoentropic point according to closure virial equation.

When quartz, cristobalite or tridymite is heated above 1710 C (the fusion point) and then cooled rapidly, again the silica tetrahedra do not have time to arrange themselves in a definite order, and so they link in a random fashion to form a glass (more accurately a super-cooled liquid) which, like silica gel, is also amorphous. It is therefore not strictly correct to call it fused quartz , as is commonly done. 

Flux. A substance that, even in small quantities, lowers the fusion point of material in which it is naturally present (e.g. alkalis in clays) or of material to which it has been added (e.g. borax added to glazes). The term is also used for the prepared low-melting glasses that are added to ceramic colours to fuse them to the ware on which they are being used as decoration. 

Figure 4.3b is a schematic representation of the behavior of S and V in the vicinity of T . Although both the crystal and liquid phases have the same value of G at T , this is not the case for S and V (or for the enthalpy H). Since these latter variables can be written as first derivatives of G and show discontinuities at the transition point, the fusion process is called a first-order transition. Vaporization and other familiar phase transitions are also first-order transitions. The behavior of V at Tg in Fig. 4.1 shows that the glass transition is not a first-order transition. One of the objectives of this chapter is to gain a better understanding of what else it might be. We shall return to this in Sec. 4.8. 

Another approach to nuclear fusion is shown in Figure 19.6. Tiny glass pellets (about 0.1 nun in diameter) filled with frozen deuterium and tritium serve as a target. The pellets are illuminated by a powerful laser beam, which delivers 1012 kilowatts of power in one nanosecond (10 9 s). The reaction is the same as with magnetic confinement unfortunately, at this point energy breakeven seems many years away. 

To calculate for this transition, it is necessary to have heat capacity data for both glassy and crystaUine glycerol from near 0 K to the melting point and the heat of fusion of both glass and crystal. Such data [7] lead to a ASm for Equation (11.7) of 19.2 J K mol. Thus, glassy glycerol cannot be assigned zero entropy at OK rather, it possesses a residual entropy of 19.2 J moP. ... 

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