Curve Interpretation, Part 2: Variation of Heating and Cooling Rates - METTLER TOLEDO

Curve Interpretation, Part 2: Variation of Heating and Cooling Rates

In practice, heating measurements are frequently performed at just one heating rate. The properties of many substances and materials however depend on their production and storage conditions. In such cases, important additional information can be obtained by simply varying the heating and cooling rates. These possibilities are illustrated with the aid of typical examples.


Introduction

Thermal analysis methods are now well established as standard analytical methods in industry and research because they provide comprehensive and reliable information about a material or a substance in a relatively short time. The full potential of these methods is often not fully realized because the analysis is based on measurements at one single heating rate. For example, in differential scanning calorimetry (DSC) this is 10 K/min.

This series of articles describes analytical approaches that greatly expand the information obtainable from thermal analysis measurement curves and that improve the quality of the evaluation and interpretation.

In Part 1 [1] we showed how further information about the sample can be obtained by varying experimental conditions such as the temperature range or the atmosphere. In this article, Part 2, we discuss the additional information that can be gained by varying heating and cooling rates.

Here, we will concentrate mainly on conventional DSC, temperature-modulated DSC (TMDSC) and fast DSC (Flash DSC). The analysis of reaction kinetics is demonstrated using thermogravimetric (TGA) measurements.


Variation of the cooling rate between two heating measurements

If a substance forms metastable structures, for example polymorphic phases, information about the structures involved can be obtained by varying the cooling rate between heating measurements. On cooling from the melt, crystallization or partial crystallization often occurs.

At high cooling rates, the crystallization process shifts to lower temperatures. Different structures can then be formed that differ in their stability. At a sufficiently high cooling rate, vitrification (glass formation) may also occur.

The sample can be cooled externally or in the instrument. The cooling rate can be controlled or the sample can be allowed to cool freely in contact with a cold medium.



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Conclusions

In practice, the thermal analysis methods are frequently used for just one single heating measurement.

The information content of such measurements, the reliability of the interpretation and not least the accuracy of the measurement result can, however, be greatly increased by using a series of different heating and cooling cycles and an intelligent variation of the experimental conditions. This prevents errors and saves time for the analysis.



Curve Interpretation, Part 2: Variation of Heating and Cooling Rates | Thermal Analysis Application No. UC391 | Application published in METTLER TOLEDO Thermal Analysis UserCom 39