**1. Introduction**

With their wide industrial and civil applications, heat transfer fluids (HTFs) have been potentially used in lubrication, energy storage, heat exchange, electronic cooling, and so on [1–5]. However, for conventional HTFs (such as water, polyalphaolefin (PAO), fluorocarbons, and glycols), the main drawback of deficient heat transfer performance owing to their low thermal conductivities has limited their practical applications. For the purpose of improving their heat transfer properties, earlier research efforts have been carried out by dispersing those materials, which have high thermal conductivities, such as silver, copper, alumina, copper oxide, silicon carbide, and carbon nanotubes, into HTFs [6–12].

Until recently, adding nano-sized phase change materials (nano-PCMs) into base HTFs attracts considerable attentions. The most frequently used PCMs include inorganic PCMs, such as metal, alloy and salt hydrates, and organic PCMs, such as paraffins, polyethylene glycols, fatty acids, and esters [13–19]. Different types of nano-PCMs can be synthesized by using various synthetic methods [20–22]. By encapsulating or coating the nano-PCM with a suitable layer, the nano-PCMs can be dispersed in a base fluidic phase. However, nano-sized particles have a strong tendency to agglomerate and easily lead to precipitate in HTFs, which restricts their application as thermal energy storage media. Therefore, encapsulation or surface modification of nanoparticles to increase their dispersion ability in the carrier fluid is of primary importance. Various techniques such as interface polymerization [23] and coacervation [24] and emulsion polymerization [25, 26] were explored to make encapsulated nano-PCM. Meanwhile, some modified nanoparticles using certain way exhibit excellent dispersion stability in some HTFs [27–29]. As inexpensive and stable dielectric HTFs, PAO and HFE7100 are usually applied in cooling of avionic systems [30, 31].

Among those PCMs, the thermal conductivity of lots of low melting point metals, such as indium (In), bismuth (Bi), tin (Sn), and lead (Pb) and their eutectics, is at least two orders higher than that of inorganic PCMs. Meanwhile, their latent heat density and other thermal properties are comparable to inorganic PCMs, which makes low melting point metals or eutectic alloys highly attractive PCMs in practical applications. By taking advantages of nano-PCMs (such as their small size, large surface-to-volume ratio, good dispersion ability in base fluid, and large latent heat of fusion), these HTFs have some distinct merits such as high energy density thermal storage, large specific heat capacity, low flow drag, and enhanced thermal conductivity. Meanwhile, the fluids still keep the fluidic properties. All these aforementioned merits make these thermal fluids containing nano-PCM a promising HTF for electronic cooling equipment, thermal control, and those systems requiring high heat transfer rates [32–34]. The tight contact of nanoparticle and base fluid decreases the heat transfer resistance between nanoparticles and fluid, thus enabling fast exchange of heat transfer between phases [35]. Therefore, the slurry with nano-PCM could decrease the total pumping power in a heat transfer loop due to the increased heat capacity of the carrier fluid.

For a liquid, when the flow rate and thermal conductivity keep constant, the heat transfer capability is predominantly depending on its heat absorbing capacity [36]. Frequently, in high-flux heat removal case, dielectric fluids (such as HFE7100) are usually used to take the heat away through utilizing their latent heat of vaporization. As a low melting point alloy, Field's alloy is a eutectic alloy melt at approximately 62°C (144 F), in which composes with the weight percentage of 32.5% bismuth (Bi), 51% indium (In), and 16.5% tin (Sn). As a low melting point alloy, Field's alloy is selected due to its melting temperature a litter higher than the boiling point of HFE7100. Therefore, during liquid-vapor phase transition of HFE7100, the thermal fluid heat capacity can be increased significantly when the nano-PCM changes from the solid to liquid phase.

This chapter mainly summarized our works in recently few years [37, 38], which include: (1) synthesis and modification of Field's alloy nanoparticles; (2) characterization of as-prepared Field's alloy nanoparticles; (3) jet impingement heat transfer of Field's alloy nanoparticles-HFE7100 slurry.
