Phase change materials
What are Phase change Materials
• A phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy
• Heat is absorbed or released when the material changes from solid to liquid and vice versa
Physical, technical & Economic Requirements of PCM
· Suitable Phase change Temperature
· Large Phase change Enthalpy
· Reproducible phase change or Cycling Ability
· Good Thermal Conductivity
· Technical Requirements:
· Low Vapour Pressure
· Chemical Stability of the PCM
· Compatibility of PCM with other materials
· Economical Requirements
· Low Price
· Good recyclability
· Cost effective
Classification of Phase Change Materials
Organic PCMs
Paraffin (CnH2n+2) and Fatty acids (CH3 (CH2)2nCOOH)
Advantages
- Availability in a large temperature range
- Freeze without much super cooling
- Ability to melt congruently
- Self nucleating properties
- Compatibility with conventional material of construction
- No segregation
- Chemically stable
- High heat of fusion
- Safe and non-reactive
- Recyclable
Disadvantages
· Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle
· Volumetric latent heat storage capacity is low
· Flammable. This can be easily alleviated by a proper container
· Due to cost consideration only technical grade paraffin may be used which are essentially paraffin mixture and are completely refined of oil
Inorganic PCMs
Inorganic Phase Change Materials (PCMs) are generally Hydrated Salt based materials.
Advantages
· High volumetric latent heat storage capacity
· Low cost and easy availability
· Sharp melting point
· High thermal conductivity
· High heat of fusion
· Low volume change
· Non-flammable
Disadvantages
- Change of volume is very high
- Super cooling is major problem in solid-liquid transition
- Nucleating agents are needed and they often become inoperative after repeated cycles
Applications of PCM
Microencapsulation:
Micro-encapsulation is a process in which tiny particles or droplets are surrounded by polymeric material to form capsules. In a relatively simplistic form, a microcapsule is a small sphere with a uniform wall around it. The material inside the microcapsule is referred to as the core, whereas the wall is called a shell. Most microcapsules have diameters between a few micrometers and a few millimeters.
Reasons for encapsulation
The reasons for microencapsulation are
1. To isolate Core material from its surroundings (External agency)
2. Control the rate of release of core material
3. To keep the core material intact within the desired boundary, so that oozing out from the fabric during its transition (from solid to liquid) is avoid
Materials:
· Melamine (97.5% pure)
· Formaldehyde (37% pure)
· Sodium lauryl sulphate
· n –Octadecane (90% pure)
· Polyvinyl alcohol
· Anhydrous sodium carbonate
· Sulfuric acid (99% pure)
Products:
Microcapsules (Encapsulation Efficiency of 65-70%)
Challenges pursued:
· Excess foam formation
· Reproducibility of capsule
· Yield of capsules
Achievements:
1. Problem of excess foam formation has been encountered by reducing the Emulsifier content in the emulsion.
2. High yield of capsules has been attained by allowing sufficient time to react the molecules between the addition of different reactants in the reaction mixture
3. Capsules of same quality have been reproduced successfully.
Application areas:
1. Protective clothing (Cooling Jackets)
2. Helmet
SEM images of Microcapsules:
DSC Graph of microcapsules:
Thermal stability of microcapsules
