Fictive temperature, glass

Figure 10.13 Schematic of a thermomechanical analysis (TMA) thermal expansion curve of an inorganic compound glass (solid curve). The geometrical construction to obtain the dilatometric softening point, A4g, at 10 i =Pas is given. The analogous TMA thermal expansion curve is shown (dotted curve) when a high-fictive-temperature glass sample is heated under load relaxation to a lower-fictive-temperature configurational arrangement can produce an observable contraction as illustrated.

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). 

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... 

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 above equations can also predict the celebrated memory effect of Kovacs (1964). The memory effect can be observed by quenching a glass former, such as B2O3, from equilibrium 583.2 K to a lower temperature, 498.7 K, and then aging the sample at this lower temperature long enough to bring its fictive temperature Tf down to some lower temperature. 

Fast firing, 295 Fayalite, 307, 387 Feldspars, 42—45 Ferric oxide, 32 Ferrite ceramics, 328—333 Ferrites, 328—331,379—380 synthesis of, 331 — 332 Ferroelectric ceramics, 322—327 Ferroelectric glass-ceramics, 236 Ferromagneticceramics, 328—333 Fibre optics, 216 Fibreous insulations, 394 Fibres, 215-217 Fictive temperature, 58 Fireclay, 34, 370-371 Fireclay brick, deformation of, 356—358 Fireclay refractories, 370—373 Firing, 273—297 fast, 295... 

Since glasses are inherently in a non-equilibrium state around Tg, thermodynamic formulation relevant for this region has to include one important additional feature and that is a time scale. Even in laboratory time scales profound changes in properties can occur. It is the existence of this time scale, which led to the definition of a "fictive temperature" by Tool (Tool and Eichlin, 1931 Tool, 1946). If we call this reference or fictive temperature as 7/ (which is more accurately written as T/J) since it is time-dependent), then the Gibb s free energy of a glass can be described in terms of three parameters, T, P and Tf. Also the heat balance equation in... 

Enthalpy relaxation is one of the most widely studied in the context of both non-linearity and non-exponentiality of the measured glass properties. A convenient technique for these studies is scanning calorimetry. In simple cooling and heating experiments, heat capacity curves exhibit normal increase with characteristic hump of Cp above the glass transition as represented in Figure 9.08(A). The fictive temperature,... 

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