Analysis of rotator phase transitions in the linear alkanes hexacosane to triacontane by adiabatic scanning calorimetry and by photopyroelectric calorimetry

Organic Phase Change Materials for Thermal Energy Storage: Influence of Molecular Structure on Properties

Materials that change phase (e.g., via melting) can store thermal energy with energy densities comparable to batteries. Phase change materials will play an increasing role in reduction of greenhouse gas emissions, by scavenging thermal energy for later use. Therefore, it is useful to have summaries of phase change properties over a wide range of materials. In the present work, we review the relationship between molecular structure and trends in relevant phase change properties (melting temperature, and gravimetric enthalpy of fusion) for about 200 organic compounds from several chemical families, namely alkanes (paraffins), fatty acids, fatty alcohols, esters, diamines, dinitriles, diols, dioic acids, and diamides. We also review availability and cost, chemical compatibility, and thermal and chemical stabilities, to provide practical information for PCM selection. Compounds with even chain alkyl lengths generally give higher melting temperatures, store more thermal energy per unit mass due to more efficient packing, and are of lower cost than the comparable compounds with odd alkyl chains.

Molecular dynamics simulation of octacosane for phase diagrams and properties via the united-atom scheme

We used the united-atom scheme to build three types of crystalline structures for octacosane (C₂₈H₅₈) and carried out molecular dynamics simulations to investigate their phase properties. By gradually heating the three polymorphs, we managed to reproduce the sequence of experimentally reported crystalline phases and rotator phases. By studying the system density, molecule morphology, chain tilt angle and cell anisotropy, we hypothesized three mechanisms behind the observed system deformations and phase transformations during the annealing process. Furthermore, our model successfully predicted the melting temperature and heat of fusion. We also reproduced the characteristics of the rotator phases and the liquid phase, validating the transferability of the united-atom scheme among the different condensed phases of octacosane. Our methodology represents an effective and efficient means of numerical study for octacosane and may be used for other members of the n-alkane family.

Potential new reference materials for caloric measurements on PCM

Current applications of PCM focus on the temperature range from about − 40 °C to about + 80 °C, with the interest expanding up to about 400 °C for future applications in power plants and industry. Calorimetric measurements to determine the ability of PCM to store heat as a function of temperature are done with a variety of methods and instruments. Some of them have been developed only in the last decade, especially to allow characterization of larger samples, even encapsulated PCM with volume in the order of 100 ml and larger. Currently there are not enough reference materials for calibration and testing available to sufficiently cover the required temperature range, being suitable for the used variety of samples regarding size, as well as other requirements of the new methods and instruments. In this paper, we give an introduction, review, and recommendations for new reference materials for calibration and testing.