  The thermal emissivity spectrum in the mid infrared range (3 to 21 $\mu$m) as
well as its dependence on temperature between 225 and 320 $^{\circ}$C has been
obtained for the Mg-51%Zn (weight %) eutectic alloy, a candidate for thermal
storage. The spectral curves show the typical behaviour of metals and alloys,
with emissivity values between 0.05 and 0.2. It was also found that the
emissivity spectrum shows variations in each heating cycle during the first few
cycles. These changes are associated with the presence of metastable phases in
the solid solid phase transition, present in the alloy below the melting point.
The absence of signs of oxidation in air is very favourable for the use of this
alloy in thermal energy storage systems. Moreover, the total normal emissivity
curves obtained from dynamic spectral measurements have allowed analysing the
behaviour phase transition sequence present in this alloy. These experimental
results indicate that accurate emissivity measurements can be sensitive enough
to account for the structural phase transitions in metals and alloys.
