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

Abstract

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.