Dilatometric softening temperature

Another viscosity point can be obtained from thermal expansion curves. The dilatometric softening temperature, T, is usually defined as the temperature where the sample reaches a maximum length in... 

I d = Dilatometric softening temperature where the sample reaches maximum expansion. 

If the melt were contained in a crucible, expansion would continue to occur to the highest temperature of the experiment. However, the sample used in thermal expansion measurements is actually under a small load introduced by the push rod. Eventually, deformation will occur as the push rod sinks into the sample or the sample bends under the load. The temperature of maximum expansion is called the dilato-metric softening temperature, or T (Figure 7.8). This temperature is an artifact of the dilatometric method used to measure the thermal expansion coefficient. As a result, it is slightly dependent upon the load applied by the push rod and the cross-sectional area of the sample, which, in combination, determine the stress on the sample. Values obtained from most laboratories, surprisingly, agree within a few degrees. 

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.

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