By Luke Haun, M.E.
Phase change materials (PCMs), due to their inherent large storage capacity, have been used across a multitude of industries for energy conservation and storage. Applications range from cooling vests and warming blankets to temperature-control packaging and thermal energy storage tanks. Choosing the correct material for each application is necessary to ensure optimum design and performance. The defining characteristics of an ideal material are its thermal properties: latent heat, specific heat, thermal conductivity, and melt and solidification temperatures.
Differential scanning calorimetry (DSC) is the leading method for determining a phase change material's melt and solidification points. This technique uses a small sample of the PCM, roughly 10µL to 40µL. The sample is subjected to a temperature profile alongside a well-documented reference material, usually water. As a constant heating rate is applied to both samples in a differential scanning calorimeter, the voltage signals are measured to obtain a curve like the one below. The specific heat is a simple algebraic calculation using voltage and mass values, and the latent heat is calculated as an integration of signal over a temperature interval.
Several factors can influence the results of the measurement, including heating rate and sample size. While a skilled user of the equipment is able to decipher the effects of heating rates, the effect of sample size produces some inaccuracies that prove to be problematic when designing a system on a larger scale. The onset of solidification (nucleation) of a material starts at a single point and then propagates until the entire sample is solid, assuming the ambient temperature is lower than the solidification temperature. Due to the extremely small sample size, many materials experience a degree of supercooling; that is, the material is capable of temporarily being in a liquid state at a temperature lower than its normal solidification temperature. Solidification is a statistical process governed by the volume of the material; the smaller the volume, the lesser the chance of solidification. Therefore the solidification temperature measured and indicated on a DSC is often significantly lower than that experienced in applications when larger volumes are used.
Another calorimetric method is useful in such situations. The “t-history method,” developed by Zhang et al.1, uses significantly greater sample volumes, between 15 ml to 30 ml. That’s nearly 1,000 times the volume used in a DSC measurement. The possibility of supercooling is eliminated, and all characteristics measured by t-history will be seen in an application or process using the PCM.
While no commercial instrument is available for t-history, the setup is extremely simple and the materials are readily available in most laboratories, at far less than the cost of a $250,000 differential scanning calorimeter. The PCM and the reference material are placed in long glass tubes (to prevent axial heat flux) and put in an environmental chamber. The temperature is monitored in the axial and radial center. A typical t-history curve and experimental setup are shown below.
After the data has been obtained, the enthalpy curve is calculated using the equations developed by Zhang and modified for improvements by H. Hong et al.2 The latent heat [kJ/kg] is calculated as follows:
With sub/super scripts:
A2 and A'2 are the integrated area between the curves as indicated below. Note that the modified t-history method takes into account the supercooling found in many phase change materials and clearly defines the end of solidification with an inflection point (b, in the first graphic).
Several laboratories have compared enthalpy values between DSC and t-history and have found that the results coincide well. H. Hong et al.2 found that the heat of fusion measured by t-history was within 4 percent of that measured by DSC.
DSC will continue to be the main method for characterizing and benchmarking PCMs, thanks to its precision, reproducibility and widespread acceptance by industry and academia. For many engineering applications, however, the degree of supercooling must be well documented with a realistic volume. Collecting more calorimetric data via the t-history test can be highly beneficial for designing and augmenting systems that use phase change materials.
About the author
Luke Haun graduated from University of Minnesota’s Institute of Technology with a B.S. in biomedical engineering, specializing in biotransport phenomena and heat transfer. After joining Entropy Solutions, he began working with phase change material and helped design the award-winning Greenbox temperature-control shipping system. He is now focused on application engineering and research for the thermal energy storage market.
1 Yinping, Zhang, and Jiang Yi. "A simple method, the-history method, of determining the heat of fusion, specific heat and thermal conductivity of phase-change materials." Measurement Science and Technology 10.3 (1999): 201.
2 Hong, Hiki, Sun Kuk Kim, and Yong-Shik Kim. "Accuracy improvement of T-history method for measuring heat of fusion of various materials." International Journal of Refrigeration 27.4 (2004): 360-366.