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Fig. 6 DSC curves of
dispersions of paraffin
microcapsules in PAO
It is interesting to note that on the cooling side of the DSC curve, the liquid-rotator
and rotator-triclinic phase transition processes are well separated from each other,
peaking at 19.7 and 21.0 C, representatively. The latent heat of the liquid-rotator
phase transition dominates in the freezing process, and the subcooling for the
liquid-rotator phase transition is about 3.2 K, as shown in the DSC curve in Fig. 6 .
4 PCM Nanoparticles Without Shell
PCM microcapsules are often synthesized using the chemical process, as discussed
in the previous section. Physical methods, including direct emulsification and
spray drying, are capable of producing PCM particles smaller than 1 lm without
shell. The following sections will discuss two types of PCM nanoparticles: low-
melting-point metallic nanoparticles and NPG nanoparticles.
4.1 Low-Melting-Point Metallic Nanoparticles
(Solid-Liquid PCM)
Low melting metals, such as Indium and BiSn alloy, have thermal conductivity
much higher than conventional dielectric thermal fluids; for example, thermal
conductivities of Indium and PAO are 82 and 0.14 W/(mk), respectively. The use
of low-melting metallic PCM nanoparticles provides a way to simultaneously
improve the effective thermal conductivity and heat capacity of the base fluids [ 5 ,
7 ]. When the metallic PCM particles are small enough (e.g., below 50 nm in size)
and stabilized with surfactant molecules, the nanoparticle suspensions could
remain stable during freezing and melting of the PCM particles.
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