Pulham, Colin

Kirk, Caroline

Stamatiou, Anastasia

Ravotti, Rebecca

2024-07-18T10:26:19Z

2024-07-18T10:26:19Z

2024-07-18

In view of the challenges associated with flexible energy generation and storage required to reach a net-zero emissions future, thermal energy storage is of great importance. In particular, latent heat storage (LHS) using phase-change materials (PCMs) is increasingly attracting the attention of both researchers and policymakers. While several PCMs have already been developed and commercialised, there remain many challenges that have yet to be overcome. One of the key challenges associated with many PCMs is that of polymorphism, i.e. the ability of a compound to crystallise as different crystal structures known as polymorphs. Different polymorphs can show different properties (e.g. solubility, melting point, enthalpy of fusion) thereby affecting their performance in LHS applications. Although polymorphism is widespread in other sectors (e.g. pharmaceuticals, food processing), it remains relatively poorly understood within the context of thermal energy storage. In this thesis, the polymorphic behaviours of three PCM candidates of current interest (tristearin, methyl behenate, disodium hydrogen phosphate dodecahydrate) have been studied in depth. The thermodynamic and structural properties of polymorphs of tristearin were studied using a variety of analytical methods including differential scanning calorimetry (DSC), hot-stage microscopy (HSM) and powder X-ray diffractometry (PXRD). The studies reveal that the polymorph of tristearin present in commercial samples at ambient temperature (denoted as the δ-form) does not recrystallise after melting, leading to different thermal behaviour between the first and the second heating cycles. This polymorph then transforms into the β-form upon heating. These observations challenge the current knowledge in the literature on the relative thermodynamic stabilities of the main polymorphs of tristearin. Changes in the polymorphic behaviour were also observed from DSC, PXRD and molecular dynamic (MD) simulations when treating tristearin above a certain temperature threshold. These findings were correlated with thermal memory effects involving the potential existence of liquid-crystals clusters. Calorimetric and structural studies were performed on methyl behenate, and led to the discovery of three new polymorphs. Single crystals of two of these were successfully grown and their crystal structures solved. Their relative stabilities under dynamic heating and cooling conditions were elucidated using DSC, PXRD, single-crystal XRD and HSM. Analogies with the polymorphic behaviour of other fatty acid esters were observed, allowing broader conclusions to be drawn about the polymorphism of this class of materials. The crystallisation behaviour of disodium hydrogen phosphate dodecahydrate was studied for the first time using X-ray computed tomography (XCT). Combined with the results of DSC and PXRD studies, this research casts doubts on the published phase diagram. It appears more likely that under certain conditions the heptahydrate and dodecahydrated forms can coexist and this is responsible for the observed crystallisation properties. A new polymorph of the dodecahydrate was also structurally characterised using single crystal X-ray diffraction.

https://hdl.handle.net/1842/42002

http://dx.doi.org/10.7488/era/4724

en

The University of Edinburgh

R. Ravotti, X. Liu, C. R. Pulham, P. Camp, I. Prentice, J. Worlitschek, and A. Stamatiou. “An exploration of the polymorphic behaviour of Tristearin: effects of thermal conditioning and thermal history”. In: J. Mat. Chem. A tbd (under submission), tbd

R. Ravotti, X. Liu, C. R. Pulham, J. Worlitschek, and A. Stamatiou. “The polymorphism of Methyl Behenate”. In: Cryst. Eng. Comm. tbd (under submission), tbd

R. Ravotti, X. Liu, C. R. Pulham, J. Worlitschek, and A. Stamatiou. “Disodium hydrogen phosphate: the role of dehydration on the relative stability of its hydrated forms and on the polymorphism”. In: Crystals tbd (under preparation), tbd

R. Ravotti, J. Wortlischek, C.R. Pulham, and A. Stamatiou. “Triglycerides as Novel Phase-Change Materials: A Review and Assessment of Their Thermal Properties”. In: Mol 25,23 (2020), p. 5572

R. Ravotti et al. “Synthesis and Investigation of Thermal Properties of Highly Pure Carboxylic Fatty Esters to Be Used as PCM”. In: Appl. Sci. 8 (2018), p. 1069

R. Ravotti et al. “Investigation of the Thermal Properties of Diesters from Methanol, 1-Pentanol, and 1-Decanol as Sustainable Phase Change Materials.” In: Mat. 13,4 (2020), p. 810

R. Ravotti et al. “Analysis of Bio-Based Fatty Esters PCM’s Thermal Properties and Investigation of Trends in Relation to Chemical Structures.” In: Appl. Sci. 9 (2019), p. 225

R. Ravotti et al. “Assessment of the Thermal Properties of Aromatic Esters as Novel Phase Change Materials”. In: Cryst 10,10 (2020), p. 919

2026-07-18

latent heat storage

phase change materials

triglycerides

salt hydrates

Polymorphism

Investigation of polymorphism in phase-change materials for latent heat storage applications

An investigation of polymorphism in phase-change materials for latent heat storage applications

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy

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