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DSC and T-history: Complementary methods for measuring the melting and freezing ranges of PCMs

04-Jan-2016

Differential scanning calorimetry and the T-history method are used to measure the latent heat and melting/freezing ranges of phase change materials. Heating/cooling rates, sample size and thermal conductivity of the sample can have an adverse impact on the accuracy of the DSC results. T-history analysis minimizes such effects and can be used to verify the results obtained via DSC.

By Mohammed Farid, Ph.D.

Q: What is differential scanning calorimetry (DSC)?
A: DSC is a measure of the difference in the heat flow between a sample and standard reference. Heating and cooling are applied at a specified constant rate. DSC provides information about latent heat of melting, specific heat capacity and melting and freezing range of materials.

Q: What are the problems associated with a DSC measurement?
A: DSC was originally developed for measuring materials with a high thermal conductivity such as metals. Since then, the use of DSC has been extended to cover polymers and PCMs, which have very low thermal conductivity in the range of 0.2-0.4 J/s moC. 

Q: Why does the low thermal conductivity of PCMs cause a problem in the DSC measurements?
A: All DSC measurements are based on the assumption of uniform temperature distribution within the sample and the reference. This is true for the high thermally conductive reference but may not be true for the low thermal conductivity PCM sample. In this case, the measured temperature does not represent the true sample temperature and there will be a time lag in the measured temperature. 

Q: What needs to be done to get accurate DSC measurements?
A: The sample size should be kept small and the heating/cooling rate low. This will give the sample sufficient time to equilibrate as heating/ cooling progresses. However, using very low heating/ cooling rates could reduce the measurements sensitivity and extend measurement time.

Q: What PCM properties are most affected by DSC heating/cooling rates, sample size and thermal conductivity?
A: The effect of these parameters on the measured latent heat of PCM is small; however, their effect on the measured melting range can be very dramatic. The effects of heating rate and sample size are very significant on the measured melting range as shown in Figure 1 and 2. However, all measurements gave very similar latent heat values.

Farid DSC paper, Figure 1
 
Figure 1. The effect of heating rates on DSC measurements.

Farid DSC paper, Figure 2
 
Figure 2. The effect of sample mass on DSC measurements.

Q: How can the effect of heating/cooling rate, sample size and thermal conductivity be reduced when analysing PCM thermal properties?
A: The effect of heating/cooling rate, sample size and thermal conductivity can be minimized by using the T-history method of analysis to determine the melting point and latent heat. 

Q: How is T-history analysis conducted?
A: Thermal characterization via the T-history method is usually done by placing a reasonable amount of PCM in a test tube, which is placed in a programmable cycling water bath or air cabinet provided with heating and cooling. A thermocouple placed preferably at the center of the sample should be used to measure the change in sample temperature with time. This method can be monitored for a single cycle or multiple cycles depending on what type of data is desired.

Q: What are the benefits of the T-history method?
A: The benefit of the T-history method is that it provides information about the freezing and melting points of the PCM more accurately than to DSC. This is because slower heating/cooling rates are usually used when compared to those employed in DSC. This gives a better simulation of the true use of PCM in most practical applications. The melting/freezing range of PCM is the most critical point in the selection of PCM for a specific application. In addition, a larger volume of PCM is used in this testing method, which reduces the likelihood of supercooling being observed.

Q: What is the best way of doing the T-history analysis?
A: The PCM samples should be placed in a programmable water bath or temperature chamber. The heating and cooling temperatures should be set 30 °C above and below the melting point of the PCM, preferably Thot-Tf= Tf-Tcold. The DSC in Figure 3, which has been done at a rate of 3 °C/min, shows that PCM freezing point is 20 °C, while cooling in the T-history method shows that it is 23 °C with no supercooling when cooling is done at a rate of less than 0.2 °C/min. 


Figure 3: T-history vs DSC analysis   

Figure 3. Comparison between DSC and thermal cycling tests for T21 sample

About the author

Dr. Mohammed Farid, a professor of chemical and materials engineering at the University of Auckland, New Zealand, is a leading authority on thermal energy storage and phase change materials. He has been working since 1980 on the development of PCMs, their encapsulation and applications, from buildings and cold storage to the cooling of batteries and electronic devices. He joined the Entropy Solutions advisor board in December 2014.