162
Furthermore, it deserves to notice that clusters or aggregates of nanoparticulates
may be formed before use or in the process of preparation, in which case the actual
size may differ very much from the nominal size of the nanomaterial on the package. Thus, the simple comparison of the nominal nanomaterial size before experiments may not be feasible to give the reliable trend or explain the variation of
specific heat capacity.
• Nanomaterial concentration
Noticeably different results of the effect of nanomaterial concentration on the variation of specific heat capacity of molten salts were reported by many researchers. It
has been found that the largest enhancement of specific heat capacity was usually
achieved at a concentration of 1 wt.% [9, 10, 12], but not constricted to the same
kind of nanomaterials in different studies. Besides, other optimal concentrations for
the largest enhancement were also reported. For example, Ho and Pan [13] found
that the optimal concentration of 0.063 wt.% Al 2 O 3 provided the largest enhancement of specific heat capacity. Tao et al. [21] achieved the largest specific heat
capacity enhancement at the concentration of 1.5 wt.% graphene after examining
four kinds of carbon nanomaterials with different mass fractions. Generally, concentrations lower or higher than the optimal level did not show the same enhancement or even had a detrimental influence on the specific heat capacity.
It may be deduced that there should be a saturation concentration that would
promote the best dispersion of the nanoparticulates in the molten salt, leading to
optimal interactions between the nanomaterial and the molten salt. Otherwise, deficient interactions may result from either insufficient doping, or formation of clusters and aggregates of the nanoparticulates at concentrations higher than saturation.
• Nanostructures
Most explanations proposed for the enhancement have focused on the contribution
of unique structures or nanolayers observed in solidified salts after mixing with
nanomaterials and the resulting larger specific surface area. Shin and Banerjee [39]
observed a substructure–percolation network with higher density in the eutectic
mixture of Li 2 CO 3 –K 2 CO 3 with added SiO 2 nanoparticles. They suggested that the
high specific surface energy associated with the high specific surface area may have
resulted in the enhancement of specific heat capacity. Likewise, special needle-like
structures were observed all over the SiO 2 -doped eutectic mixture of Li 2 CO 3 –K 2 CO 3
at micro/nanoscales, which had very large specific surface area and were thought to
be the reason for the enhancement of specific heat capacity [22]. Chain-like nanostructures were observed by Shin et al. [40] for the nanofluid composed of 1 wt.%
alumina nanoparticles in the eutectic Li 2 CO 3 –K 2 CO 3 mixture, and it was thought
that the chain-like nanostructures induced by nanoparticles were mainly responsible
for the enhanced specific heat capacity instead of the direct contribution from the
nanoparticles. Additionally, Luo et al. [12] studied the binary mixture of nitrate salts
(NaNO 3 –KNO 3 , 60:40, mass ratio) doped with CuO nanoparticles at various concentrations (Fig. 2). They observed enhanced TES capacity when the doping was at
0.1 and 0.5 wt.%, whilst the respective solidified salts presented some needle-like
structures as shown in Fig. 2b, c.
X. Guo et al.
Furthermore, it deserves to notice that clusters or aggregates of nanoparticulates
may be formed before use or in the process of preparation, in which case the actual
size may differ very much from the nominal size of the nanomaterial on the package. Thus, the simple comparison of the nominal nanomaterial size before experiments may not be feasible to give the reliable trend or explain the variation of
specific heat capacity.
• Nanomaterial concentration
Noticeably different results of the effect of nanomaterial concentration on the variation of specific heat capacity of molten salts were reported by many researchers. It
has been found that the largest enhancement of specific heat capacity was usually
achieved at a concentration of 1 wt.% [9, 10, 12], but not constricted to the same
kind of nanomaterials in different studies. Besides, other optimal concentrations for
the largest enhancement were also reported. For example, Ho and Pan [13] found
that the optimal concentration of 0.063 wt.% Al 2 O 3 provided the largest enhancement of specific heat capacity. Tao et al. [21] achieved the largest specific heat
capacity enhancement at the concentration of 1.5 wt.% graphene after examining
four kinds of carbon nanomaterials with different mass fractions. Generally, concentrations lower or higher than the optimal level did not show the same enhancement or even had a detrimental influence on the specific heat capacity.
It may be deduced that there should be a saturation concentration that would
promote the best dispersion of the nanoparticulates in the molten salt, leading to
optimal interactions between the nanomaterial and the molten salt. Otherwise, deficient interactions may result from either insufficient doping, or formation of clusters and aggregates of the nanoparticulates at concentrations higher than saturation.
• Nanostructures
Most explanations proposed for the enhancement have focused on the contribution
of unique structures or nanolayers observed in solidified salts after mixing with
nanomaterials and the resulting larger specific surface area. Shin and Banerjee [39]
observed a substructure–percolation network with higher density in the eutectic
mixture of Li 2 CO 3 –K 2 CO 3 with added SiO 2 nanoparticles. They suggested that the
high specific surface energy associated with the high specific surface area may have
resulted in the enhancement of specific heat capacity. Likewise, special needle-like
structures were observed all over the SiO 2 -doped eutectic mixture of Li 2 CO 3 –K 2 CO 3
at micro/nanoscales, which had very large specific surface area and were thought to
be the reason for the enhancement of specific heat capacity [22]. Chain-like nanostructures were observed by Shin et al. [40] for the nanofluid composed of 1 wt.%
alumina nanoparticles in the eutectic Li 2 CO 3 –K 2 CO 3 mixture, and it was thought
that the chain-like nanostructures induced by nanoparticles were mainly responsible
for the enhanced specific heat capacity instead of the direct contribution from the
nanoparticles. Additionally, Luo et al. [12] studied the binary mixture of nitrate salts
(NaNO 3 –KNO 3 , 60:40, mass ratio) doped with CuO nanoparticles at various concentrations (Fig. 2). They observed enhanced TES capacity when the doping was at
0.1 and 0.5 wt.%, whilst the respective solidified salts presented some needle-like
structures as shown in Fig. 2b, c.
X. Guo et al.