DSC Measurements at High Heating Rates - Advantages and Disadvantages

Introduction 

DSC measurements are usually performed at heating rates of 10 to 20 K/min. This is a good compromise between accuracy, resolution, sensitivity and actual measurement time.

The use of high heating rates represents an attempt to measure the sample in its original state, i.e. so rapidly that the sample has no time to change during the measurement (e.g. cold crystallization).

The recent increased discussion on this topic can be summarized as follows: Measurements at high heating rates lead to

• much shorter experiment times,
  
• increased sample throughput,
  
• better sensitivity with certain effects (e.g. the determination of weak glass transitions) because of the greater heat flow signals at higher heating rates and
  
• less reorganization in the sample during the DSC measurement.
  

In principle, the software allows high heating rates (up to 400 K/min and more). The accuracy of the results so obtained must be checked with respect to the physical and chemical facts. The laws governing physical and chemical events such as reactions and transitions must be taken into account. 

Such events include: 

• Heating rate-dependent effects:
  - kinetic events
  - crystallization
  - chemical reactions
  - glass transitions

• Heating rate-independent effects:
  - melting
  - physical properties such as specific heat capacity

Besides this, sample measurement is influenced by the following sample properties and instrument parameters:
Sample

•  Situation within the sample:
  - sample size
  - sample homogeneity
  - thermal gradients
  - thermal conductivity
  - decomposition
  - thermal contact

Instrument

• Instrumental effects:
  - crucible
  - signal-time constant
  - baseline stability
  - stabilization time
  - calibration

To gain a better understanding of the effect of these different factors, a number of measurements will now be presented and discussed. All the measurements were performed with a DSC822e equipped with an automatic sample robot. The instrument was calibrated with indium and zinc.

Application Examples 

Melting of Metals 

In general, the melting point of pure substances is determined as the onset temperature of the peak. For physical reasons, the side of the melting peak before melting is a straight line. The enthalpy of fusion is determined from the peak area. As a result of the given thermal conductivity, melting takes place over a certain period of time. The peaks are therefore much broader at higher heating rates.

With the DSC822e, the melting point of indium can be determined over a wide range of heating rates (Fig. 1). Due to the integrated FlexCal^® calibration system, onset temperatures and the enthalpies of fusion are independent of the heating rate. This ensures that the DSC measures reliably at high heating rates and that accurate temperature and heat flow values are obtained. To improve heat transfer, it is advisable to perform fast measurements in light crucibles. The measurements used for the evaluation in Figure 1 were done in 20- µl aluminum crucibles.

Conclusions 

The DSC822^e can measure samples at very high heating rates. FlexCal^® technology ensures that the measured temperatures remain independent of the heating rate. The short response time of the DSC sensor results in rapid stabilization of the signal so that starting temperatures do not have to be very low.

The influence of the heating rate on the effect to be investigated should always be carefully considered beforehand. In general, higher heating rates lead to larger thermal gradients in the sample. This can influence the kinetics of processes, e.g. nucleation in connection with crystallization is significantly affected. 

Furthermore, peaks are broadened due to smearing. To reduce these effects, it is best to use light crucibles and optimize the thermal contact between the sample and the crucible. Small, thin samples reduce the formation of temperature gradients within the sample. The use of commercially available crucibles allows an automatic sample changer to be used.

Higher heating rates permit kinetic processes to be studied over a large dynamic range. Undesired processes that occur during the measurement (e.g. cold crystallization) can be suppressed so that the measurement curve characterizes the original material.

_{DSC Measurements at High Heating Rates -Advantages and Limitations | Thermal Analysis Application No. UC 191 | Application published in METTLER TOLEDO Thermal Analysis UserCom 19}