Temperature fictive

Density. The density of transparent vitreous sihca is approximately 2.20 g/cm. Translucent and opaque glasses have lower densities owing to the entrapped bubbles. The density of translucent Vitreosil, for example, is 2.07—2.15 g/cm (87,119). The density of transparent vitreous sihca decreases with increasing hydroxyl content and with lower fictive (glass stmcture equihbrium) temperatures. The fictive temperature depends on the thermal history and on glass viscosity (120). 

Shear velocity and acoustic absorption have been studied as a function of OH content and fictive temperature for four different vitreous siUca samples (170). AH showed a shear—velocity minimum at 80—100 K. The magnitudes of the minima are influenced by OH content and fictive temperature. The samples having the highest OH content and lowest fictive temperature display the lowest losses. 

Temperature(s). See also Blackbody temperature sensor Cure temperature Curie temperature Eutectic temperature Fictive temperature Furnace temperature Glass- transition temperatures Heat entries Heating Hot entries Refrigeration Target temperature emperature measurement Thermal entries Thermo-entries Transition temperatures in analysis of water, 26 35 biofiltration system, 10 76 in biological wastewater treatment,... 

Glass transition temperature or the fictive temperature may be investigated or diagrammed using different methods, resulting in different definitions. These... 

Fictive temperature as a function of temperature and heating/cooling rate... 

Table 10.1. Comparison of full-width at half-maximum (nm) of two Nd emission lines with Th concentration of different natural monazites and determination of equivalent annealing fictive temperature (Madagascar sample considered as reference material), (nd not determined) (age Scharer and Deutsch 1990 Paquette et al. 1994 Seydoux-GuiUaume et al. 2002b Scharer et al. 1994)...

Figure 6 Schematic of the temperature dependence of configurational entropy definition of fictive temperature. Data calculated using input data for sucrose (see text).

As the glass or liquid ages and its volume decreases, the fictive temperature also decreases, finally reaching the actual temperature T if and when the glass or liquid reaches thermodynamic equilibrium. From Figs. 4-10 and 4-17, it can readily be deduced that Tf can be related to the fractional free volume by ... 

Figure 4.17 Graphical determina-tion of the volume-based fictive tem-perature for a glass cooled to temperature T with specific volume ui and then aged so that its specific volume drops to V2, V3, and finally V4. The fictive temperature for the glass at any point in its history is obtained by ex-tending a line with slope ag through the specific volume of the glass until it intersects the extrapolated liquidus line with slope Off. If aging continues until the specific volume lies on the liquidus line, then Tf — T, and aging stops.

The configurational entropy Sc is a structural property and is therefore assumed to be a function of the fictive temperature Tf. The term entropy is used here loosely, since rigorously speaking, entropy is only defined at equilibrium. Thus, modifying Eq. (4-13) by making Sc AS dependent on Tf rather than T, and inserting this into Eq. (4-23), we obtain... 

Since glassy relaxation is definitely not monoexponential. Tool s equation is not adequate for quantitative predictions. However, Moynihan et al. (1976) and Kovacs et al. (1979) have developed a simple and successful way of incorporating a spectrum of relaxation times into nonlinear relaxation. First, consider a single step change in temperature AT from T to T2 = T + AT at time t = t. Moynihan et al. (1976) proposed that the fictive temperature varies with time after this step according to... 

For a small step in temperature, the fictive temperature Tf is never far from the actual temperature T hence r, as given by the Narayanaswamy or the Adam-Gibbs equations, doesn t vary much with time. Equation (4-27) then simplifies to the ordinary linear KWW equation, Eq. (4-1). For large AT, varies during the relaxation, and the asymmetry discussed earlier is predicted. Note, however, that in Eq. (4-27) is assumed to be a constant this is not strictly valid for large changes in temperature, but is usually acceptable even when AT is a few tens of degrees. 

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