Zero heating rate

Figure 5.9. Least-squares extrapolation or peak temperatures to zero heating rate (64j.

Tm Melting point at the extrapolated zero heating rate... 

Charrier et al. accurately measured the equilibrium temperature of the solid-solid phase transition of high purity caesium nitrate by stepwise heating and by the method of extrapolation to zero heating rate. The authors reported a mean value of 154.3 0.1 C, that was obtained by using two different heat-flux DSC instruments. Because the transition temperature of caesium nitrate is close to that of the fusion of indium, indium can be used as a single-point calibrant. This approach decreases the error that may arise when the calibration value for the material of interest is determined by interpolation between two-point or multiple-point calibrations [66],... 

The process of temperature calibration and measurement has been considered in considerable detail in connection with a program of work by GEFTA (German Thermal Analysis Society). The authors raise the fundamental issue that whereas the temperatures of the fixed points are defined for the substances in phase equilibrium the experimental results are measured under dynamic conditions. The authors recommend a procedure based on extrapolation of results to zero heating rate. The details are contained in a series of publications. The authorshave also considered the problem of temperature calibration under conditions of decreasing temperatures. 

Quantitatively, these dependences can be seen in Fig. 7, in which enthalpy and true Tt values have been plotted vs. MW. True Tt is the term used to describe the Tt value obtained by extrapolation to zero heating rate (v = 0) of the DSC peak temperature. 

We can now compare the endothermic peak temperatures extrapolated to small mass and zero heating rate with the results of the dilatometric experiments. The dilatometrically determined melting temperatures represent the true melting temperatures (9). This comparison is made in Fig. 2. The temperature for the onset of melting, as determined by the DSC experiments is also indicated in this figure. ... 

The effect on the sample of this substantial temperature drop across the cup can be determined with greater generality with the sample placed directly on the bottom of the cup rather than in a sample pan. With this arrangement it is clear that the temperature drop across the cup induces a flow of heat from the bottom of the cup through the enclosed sample and the gas that surrounds it to the sides and lid of the cup. As in the case of the cup itself, the external temperature drop produces within the sample a pseudosteady state temperature distribution that depends on the temperature of the bottom and lid of the cup in exactly the same way in the true steady state as at non-zero heating rates (after the passage of a transient period). This distribution in the sample, as with that in the calorimeter cups, is modified at non-zero... 

The true melting points of crystalline polymers can be determined by plotting the differential scanning calorimetry (DSC) melting peak temperatures as a function of the square root of heating rate and linear extrapolations to zero heating rate. 

The heat capacity, as the first derivative of the enthalpy, goes to infinity at the phase transition temperature. In reality, this pulse-like event is smeared into a more or less sharp peak (Figure 6.2). Traditionally, the phase transition temperature is assigned to the extrapolated onset temperature at zero heating rate. 

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