Phase Change Material Particles and Their Application in Heat Transfer Fluids - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 461

enhancement observed in the Indium-PAO PCM fluids using the 3x-wire tech-
nique can be explained using the Maxwell model, and no anomalous enhancement
of thermal conductivity is observed in this study. However, many other studies in
nanofluids have shown a thermal conductivity increase beyond the Maxwell pre-
dictions [ 38 ].

4.1.3 Phase Change Behavior of Metallic PCM Fluids

Knowledge of the phase change behavior of these low-melting metallic nano-
particles is critical for their use as thermal fluids [ 39 - 42 ]. The melting-freezing
phase transition of the as-prepared Field's metal and Indium nanoparticles is
measured using DSC measurements which were taken at an ordinary cyclic ramp
mode, with a scan rate of 10 C/min. Figure 10 shows the cyclic DSC heating and
cooling curves for the nanoemulsions containing Field's metal and Indium

nanoparticles and similar data for each material in the bulk state.
A relatively large melting-freezing hysteresis, about 45 C for Field's metal
nanoparticles and about 50 C for Indium nanoparticles, can be seen in Fig. 10 .
Based on the classical nucleation theory, the melting and freezing of these
nanoparticles are dependent on the interface energies between the solid metal
nanoparticles and the oil matrix, the liquid metal and oil matrix, and the solid and
liquid metals [ 43 ]. The observed T m slightly below the bulk value implies that
c SM [ c SL þ c LM (or c LM \c SM þ c SL ), where c is the interfacial energy and the
subscripts S, L, and M represent the solid phase, the liquid phase, and the oil
matrix. In this situation, the molten phase would not ''presolidify'' at the inter-
faces, instead, it would require critical nuclei inside these nanoparticles, i.e.,
homogeneous nucleation. Therefore, these liquid nanoparticles supercool to tens of
degrees below the bulk melting temperature, until critical nuclei associated with
solidification are formed. This characteristic may provide a mechanism to tailor
the phase transition behavior of nanoparticles by varying their interfacial energy or
size for different applications.
The phase change within the metallic PCM fluid has its greatest impact on the
effective specific heats of these fluids. The effective specific heat can be defined as
C eff ¼ C 0 þ / H particle DT, where / is the volume fraction of the phase-
changeable nanoparticles, H particle is the latent heat of the phase-changeable
nanoparticles per unit volume, and DT is the temperature difference between the
heat transfer surface and the bulk fluid or the difference between the nanoparticle
melting and freezing temperature. If assuming DT = 47 C, the effective volu-
metric-specific heat can be increased by about 20 % for the PCM fluids containing
8 vol% Indium nanoparticles. If DT could reduce to 10 C, which is feasible by
introducing external nucleating agents to suppress the melting-freezing hysteresis,
the effective volumetric-specific heat of the metallic PCM fluids would be

increased by up to 100 %.