164
Another consideration is the clusters formed from aggregations of nanoparticulates. Aggregated Al 2 O 3 nanoparticles were observed by Lu and Huang [38] with the
sizes of clusters on the order of 1 μm in nitrate salts in both solid and liquid states.
Lasfargues et al. [41] found clusters formed from aggregations of CuO nanoparticles (Fig. 3) in the solidified nitrate salt, whilst larger interconnected layers of TiO 2
nanoparticles (Fig. 4) rather than clusters were observed in the same work. Ho and
Pan [13] suggested that the optimal pattern of particles or clusters may maximise
the specific heat capacity. However, Lu and Huang [38] conjectured the nanolayer
effect, instead of the formation of aggregates, might be the important factors affecting the specific heat capacity of Al 2 O 3 nanoparticles doped binary nitrate salts, and
proposed a theoretical model taking into account the nanolayer effect on the specific
heat capacity of the nanofluid. The model could predict the specific heat capacity of
the studied nitrate based nanofluids doped with Al 2 O 3 nanoparticles of a given size
at any mass fraction. The prediction was made based on the measured specific heat
capacity of the nanofluid at a certain mass fraction, whilst the specific heat capacity
of the nanoparticle and nanolayer is not required.
It is still uncertain which factor is decisive for the variation of the specific heat
capacity, and also unclear how the presence of these structures would impact the
enhancement of specific heat capacity in a positive way. This argument is related to
several main concerns. (1) No unambiguous empirical evidence on the direct and
positive correlation between the quantities of nano/microstructures and the specific
heat capacity has been reported before, less than the association with the specific
surface area and surface energy. (2) Few studies have been conducted to illustrate
Fig. 3 SEM image of 1 wt.% CuO in a solidified molten nitrate salt (75,000 magnification). The
blue arrow indicates the surface of the solidified salt free from nanoparticles, whilst the red circle
shows an agglomerate of CuO. Bottom right side shows a magnified image of CuO nanoparticles [41]
X. Guo et al.
Another consideration is the clusters formed from aggregations of nanoparticulates. Aggregated Al 2 O 3 nanoparticles were observed by Lu and Huang [38] with the
sizes of clusters on the order of 1 μm in nitrate salts in both solid and liquid states.
Lasfargues et al. [41] found clusters formed from aggregations of CuO nanoparticles (Fig. 3) in the solidified nitrate salt, whilst larger interconnected layers of TiO 2
nanoparticles (Fig. 4) rather than clusters were observed in the same work. Ho and
Pan [13] suggested that the optimal pattern of particles or clusters may maximise
the specific heat capacity. However, Lu and Huang [38] conjectured the nanolayer
effect, instead of the formation of aggregates, might be the important factors affecting the specific heat capacity of Al 2 O 3 nanoparticles doped binary nitrate salts, and
proposed a theoretical model taking into account the nanolayer effect on the specific
heat capacity of the nanofluid. The model could predict the specific heat capacity of
the studied nitrate based nanofluids doped with Al 2 O 3 nanoparticles of a given size
at any mass fraction. The prediction was made based on the measured specific heat
capacity of the nanofluid at a certain mass fraction, whilst the specific heat capacity
of the nanoparticle and nanolayer is not required.
It is still uncertain which factor is decisive for the variation of the specific heat
capacity, and also unclear how the presence of these structures would impact the
enhancement of specific heat capacity in a positive way. This argument is related to
several main concerns. (1) No unambiguous empirical evidence on the direct and
positive correlation between the quantities of nano/microstructures and the specific
heat capacity has been reported before, less than the association with the specific
surface area and surface energy. (2) Few studies have been conducted to illustrate
Fig. 3 SEM image of 1 wt.% CuO in a solidified molten nitrate salt (75,000 magnification). The
blue arrow indicates the surface of the solidified salt free from nanoparticles, whilst the red circle
shows an agglomerate of CuO. Bottom right side shows a magnified image of CuO nanoparticles [41]
X. Guo et al.