What is an MGA?


MGA stands for Miscibility Gap Alloy and is used to describe an exciting new type of second generation thermal energy storage material. When two elements do not dissolve or react to form a compound, but instead stay as a simple mixture, we say that they exhibit a miscibility gap or they are immiscible. A miscibility gap alloy is formed between metallic (or semi-metallic) elements. Many metals that dissolve in each other as liquids, show this behavior as solids i.e. they separate into two solid phases. Some like iron and lead don't even mix when molten. An instrumented MGA test block is shown below.
To make a miscibility gap alloy that is useful for thermal storage, the constituent metals must meet several conditions. The most basic of these are:

  • They must have fairly different melting temperatures (generally by more than 100° Celsius).
  • They should not be composed of metals that are very costly (noble metals, rare earth metals etc.) or those that are dangerous (alkali metals, arsenic, beryllium etc.)
  • It must be possible to manufacture them in such a way that the lower melting temperature metal (the active phase) is present as discreet particles within the solid higher melting temperature metal (the matrix phase). Our team has developed successful techniques for doing this.
  • The active phase particles should ideally be small.
  • There should be as much as possible of the active phase present (typically >50% by volume).
  • The active phase particles shouldn't touch each other.
An example of typical internal structure of an MGA (Iron-Copper) is shown in the image below.

When an MGA is heated it begins to absorb thermal energy. When the melting temperature of the active phase is reached, that phase begins to melt, the temperature rise ceases and an increased amount of energy called latent heat is stored. When all the active phase has melted, the temperature begins to rise again.  The matrix phase remains solid throughout and so the block of MGA storage material retains its shape and general physical properties.

In our work, we typically quote the operating temperature of the MGA as the melting temperature of the active phase and the energy stored is quoted as the latent heat plus the sensible heat (temperature rise) for +/- 50° Celsius on either side. This is illustrated in the next figure.
Many combinations of metals can form an MGA. Some examples and their properties are listed in the table below. A more complete analysis is given in MGA Advantages. 

No comments:

Post a Comment