9 - Shape-stabilized and form-stable PCMs

Abstract

Phase change materials (PCMs) are materials whose functionality depends on a phase transition affecting their properties. Although in principle any phase transition can be exploited in a PCM, solid-liquid transitions are particularly convenient, having a lower volume change with respect to gas-based transitions and a higher latent heat with respect to solid-solid transitions; moreover, their transition kinetics is faster with respect to solid-solid transitions. Nevertheless, solid-liquid PCMs have some significant drawbacks, as the need of macro- or micro-encapsulation to prevent leakage of liquid phase from the container, significant undercooling during solidification and low thermal conductivity. Shape-stabilized, or form-stable, PCMs are an effective solution to these issues. They are composites materials consisting of an active phase undergoing the transition (i.e., the actual PCM) and a higher-melting passive phase, which can prevent leakage of molten active phase, provide structural properties and enhance other features like thermal conductivity. In this way, coupling thermal storage with one or more functions, the form-stable PCM becomes a multifunctional material. The systems considered for this application can be fully organic, fully metallic or hybrid (e.g., metal-polymer, ceramic-polymer, ceramic-metal). In this chapter, shape-stabilized PCMs will be described focusing on the mechanism that allows to obtain form stability and on the characterization techniques necessary to verify the desired conditions. For every composite material class, some examples are presented. The aim of the chapter is providing an overview of the possible form-stable PCM systems, explaining the concepts behind their design to support further development of these systems.