phase changes of water
Smart Materials can be divided into two typologies: Property Changing (Type 1) and Energy Exchanging (Type 2). PCMs are Type 1, which are characterized by an intrinsic response (at the molecular level) to an external stimuli. Specifically, PCMs respond to temperature and pressure changes - but most often the thermal response is the most worked with.
The phase changes which are possible are Solid-Solid, Solid-Liquid, Liquid-Gas, Gas-Solid. The only one that PCMs are truly applicable with are Solid-Liquid changes; the others either have too much volume, high pressure requirements, or a slow and low heat of transformation.
A solid PCMs temperature will rise as it absorbs heat, but differs from conventional materials in that when it reaches its melting point, it will begin to absorb heat at an almost constant temperature, without a significant raise in its own temperature as it changes into liquid phase. Once the ambient temperature falls, the PCM re-solidifies, releasing the latent heat being stored.
There are three classifications of PCMs: Organic (fatty acids and paraffin), Inorganic (salt hydrates) and Eutectics (organic-organic, organic-inorganic, inorganic-inorganic compounds). Each of these has its advantages and disadvantages over the others; a full list of these can be seen here.
Due to their capability for thermal storage, PCMs have been uesd as a medium in thermal transfer since the late 1800s. As ther thermal cycle involves a solid-liquid phase change, encapsulation was the obious way to harness their abilities. Microscopic PCMs, now with their protective coating, could be suspended in materials and control (even prevent) heat exchange through the substance.
microencapsulated particle of Thermusol
There are many examples of uses of PCMs incorporating microencapsulation, all of which related to the control of heat transfer in regards to human comfort. The combination of a PCM with other materials (usually solids) is referred to as thermal-composites. These materials are usually created with a specific property in mind, and thermal conductivity is the most commonly targeted. For example, a highly conductive solid material could be integrated with a PCM so that once the metal is allowed to gain a certain temperature, the two substances corroborate to sustain higher thermal conductivity.
Smart Wall Assignment Integration
Many Type 1 smart materials respond to thermal stimuli, and the combining of PCMs with other thermally responsive materials seems like an interesting possibility. For example, a thermally massive concrete wall which incorporates microencapsulated PCMs (there are many versions of this available today) could have a thermochromic film which informs you (by color change) as to what is occuring within the wall, and whether the wall is heating or cooling. There are many prospects of combinations with other smart materials, as well as the development of PCMs themselves. I have not located any examples yet, but it seems likely that there could also be an application of PCM films - controlling thermal variances at a thinner and more portable scale.
