Results and discussion
3.1. SEM observations
The observations of PCMs by the Scanning Electron Microscopy
(SEM) technique showed the presence of macrocapsules and
microcapsules of PCMs. The diameters of the macrocapsules are
ranging between 50 μm and 200 μm and they present quite
spherical agglomerates composed of many thousands of microcapsules
ranging between 5 μm and 10 μm (Fig. 1(a), and (b)). The
spherical shape appears sometimes somewhat deformed and not
very smooth. In addition, it was noticed that some macrocapsules
were initially damaged and even broken before their use. The
damage of PCMs can be initiated by the intensified shear forces
during the mixing process. It can be observed from the SEM results
(Fig. 1(c), and (d)) the emergence of many fragments derived from
broken PCMs as well as smaller microcapsules of diameter ranging
between 5 and 10 μm due to the deterioration of PCMs agglomerations
after their damage.
3.2. Heat flux, temperature peak
The endothermic segment of the DSC signal of damaged and
non-damaged PCMs, recorded at different scanning rates using the
same test sample mass of 12 mg, is presented in Fig. 2. Note that in
the case of damaged PCMs, the results were averaged over three
new samples recovered from the same lot of PCMs subjected to the
damage phenomenon. The results tests have shown a good
repeatability for both damaged and non-damaged PCMs. The
experimental dispersion of DSC measurements was less than 1%.
The average result over the different repeated tests is presented
here. Note that the minus sign in the DSC values is related to the
nomenclature of the International Confederation for Thermal
Analysis and Calorimetry (ICTAC) often used in the thermal
analysis.
One can notice that for the heating rates lower than 5 K/min,
the DSC curves for both damaged and non-damaged PCMs are
confused in the considered temperature range. However, when the
heating rate is higher than 5 K/min, a slight difference is noticed in
the heat fluxes between damaged and non-damaged PCMs cases.
For illustration, a maximal deviation of 8.8% is recorded in the
peak temperature at the heating rate of 20 K/min between
damaged and non-damaged PCMs.