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Solidification in Heat Packs

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Sequence of photographs illustrating the onset and development of crystallisation in a heat pack: (a) the subcooled solution and metal trigger; (b) the metallic trigger is flexed; the solidification front at times (c) 3 s, (d) 5 s and (e) 6 s after triggering; and (f) the solution is completely solidified 7 s after triggering.
The ability of an aqueous sodium acetate solution to remain liquid at temperatures well below its liquidus temperature has been known for at least one hundred years and was originally proposed as a means of storing solar energy in the form of latent heat. Nowadays, the most common use for this method of heat storage is in portable heat packs, used for warming drinks, soothing muscle aches and as a treatment for hypothermia in remote locations. A heat pack consists of a sealed, flexible PVC envelope (typically 20 x 10 cm2) containing a subcooled aqueous solution of sodium acetate. A metallic trigger located within the container initiates the solidification of the liquid to release latent heat.

In this PhD project, the solidification of sodium acetate trihydrate in a commercial heat pack was investigated. First, we used classical nucleation theory to predict the range of temperatures and pressures for which a subcooled solution of sodium acetate solidifies spontaneously. We then determined experimentally the nucleation temperature. Finally, we used the knowledge gained to design a new, more reliable trigger.

Prize: Danckwerts-Pergamon Prize for best thesis of 2002, Dept of Chemical Engineering, University of Cambridge.

  

(click images to enlarge)

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The metallic disc
 

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