Phase change materials (PCMs) are substances that can absorb or release large amounts of latent heat. This happens when their physical state changes, generally, either from solid to liquid or vice versa.
Microcalorimetry is a technique for measuring the energy absorbed or released during a phase change process thus enabling the amount of energy to be quantified in real-time.
This technique also enables the successive melting and crystallisation of PCMs used in various applications to test them for long term usefulness and stability.
A phase change material (PCM) is a substance that stores and releases thermal energy. When a PCM crystallises, it releases large amounts of energy (exothermic) and, when it melts, changing from solid to liquid form, it absorbs an equal amount of energy (endothermic).
PCMs are useful for thermal energy storage, where you can store either hot or cold energy up for later release.
They also work as insulation or thermal barriers, in practical applications such as temperature-controlled transport.
The use of PCMs have been used for many years, simply because the most fundamental and widely used phase change material is water-ice
There are various broad types of PCMs:
Of these broad categories, inorganic PCMs are the cheapest to produce and obtain, with a high latent heat capacity and high thermal conductivity.
Organic PCMs, consisting of paraffin and non-paraffin groups, will freeze without supercooling (unlike inorganic PCMs) and have a high latent heat capacity, but producing them is costly, and can
be highly flammable.
Bio-based PCMs are non-toxic, economical and have a high latent heat capacity. However, their thermal conductivity is poor.
PCMs have applications in a range of sectors and products, including the construction industry, transport and electrical appliances.
Successive melting and crystallisation can make PCMS more effective and efficient for use in building insulation systems where they are required to be used in many heat-cool cycles without losing thermal efficiency.
What these processes do is to enable the material to dampen the effects of day and night-time outdoor temperature variations while maintaining acceptable indoor temperatures.
Building materials such as bricks, flooring, ceiling and roofing materials can incorporate these PCMs.
To maximise the design and effectiveness of PCMs in this way, the melting and crystallisation processes must be fully understood and accurately characterised.
The technique for doing this is microcalorimetry.
Calorimetry is the science of measuring heat changes, and microcalorimetry is an ultra-sensitive development of this.
Microcalorimetry measures very small heat changes in samples, and it is a proven technique for accurately predicting the storage life of materials.
In terms of thermal analysis, it can measure changes in heat flow versus temperature in real-time, with modern devices for microcalorimetry incorporating highly sensitive heating and cooling systems.
The Microcalvet range of microcalorimeters from Setaram combine high sensitivity with very accurate temperature measurements.
These devices consist of a sensor that is made up of many thermocouples (thermopile) surrounding the sample and reference vessel thus measuring heat absorbed/evolved in all directions unlike a conventional plate differential scanning calorimeter.
Different types of measurement cell enable different samples sizes to be studied under a range of conditions including under controlled pressure environments (up to 1000bar) so not only can we measure heat flow versus temperature and time but also versus pressure.
Example of heat flow versus temperature profile for a PCM studied at different heating and cooling rates is shown below.
The slower heating rates that can be employed in the Calvet calorimeters are representative of real-world conditions for PCMs, and thus for testing and analysing the capabilities of PCMs under simulated real-life conditions, microcalorimetry is an ideal technique.