Metal crystallites formed upon UV irradiation of benzophenone (BP) solutions in octane containing oleoyl sarcosine as a particle stabilizer and Ag neodecanoate (AgOOR), as well as the complexes Pd(acac)2 and Pt(acac)2, with acac = acetylacetonate anion. Initial quantum yields of metal formation for Pd(acac)2 and Pt(acac)2 were 7 × 10–3 and 4.5 × 10–3, respectively, which are about 7 times higher than those for analogous reactions conducted in the absence of the BP photosensitizer. Direct irradiation of AgOOR, on the other hand, resulted in no reaction, but silver particles were formed with a quantum yield of 1.2 in the presence of BP. Whereas the Pd colloids decay within 1 week of preparation, the Ag and Pt particles have remained stable, thus far, in octane for more than 4 months. The resulting octane colloids were evaluated for enhancements in thermal conductivity using the thermal hot disk method. Enhancements of up to 10% were observed for the Ag and Pt systems at metal concentrations of 5 mM, which are far larger than what Maxwell’s theory predicts for a colloid of low volume fraction (∼5 × 10–5 vol %).

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As a fundamental process during production of composite thermal energy storage systems, infiltration of phase change materials (PCM) leads to formation of voids (air pockets) inside the pores of graphite foams. The presence of voids inside graphite cells (i.e. the presence of air pockets next to the conductive walls of the porous structure) markedly affects the thermal and phase change behavior of the composite. Therefore, it is vitally important to investigate the effect of voids on phase change behavior of latent heat energy storage composites. In complementing recent work devoted to modeling of the infiltration of PCM into graphite foams, a numerical approach was employed to study the solidification of PCM infiltrated into a graphite pore in the presence of a void. For this purpose, a two-dimensional model of the porous structure was developed based on the typical geometrical features of the pores. Grid independence study was performed on different unstructured grid systems. Since more than one fluid phase is present in this problem (PCM being the liquid phase and air pocket or void as the gas phase), the volume-of-fluid (VOF) method was utilized for investigation of solidification problem and tracking the interface. Considering various forces operating at the scale of the pore (i.e. 500 microns in diameter), this problem is under the influence of surface tension, gravity, and pressure gradient. The simulation was transient and continued until the entire liquid PCM inside the pore freezes. The volume of final void space will represent a combination of infiltration and shrinkage voids. Results of the simulation indicate the presence of 9.8% void (from the infiltration process) that can greatly alter the solidification rate of the PCM inside the pore. It is concluded that formation of shrinkage void during solidification can be predicted using this multi-phase model. For verification purposes, the volume of the predicted infiltration void was compared to reported experimental measurements and the volume of shrinkage void was compared to theoretical volume change. Good agreements were found in both cases.

Infiltration of phase change materials (PCM) into highly conductive porous structures effectively enhances the thermal conductivity and phase change (solidification and melt ing) characteristics of the resulting thermal energy storage (TES) composites. However, the infiltration process contributes to formation of voids as micron-size air bubbles within the pores of the porous structure. The presence of voids negatively affects the thermal and phase change performance of TES composites due to the thermophysical properties of air in comparison with PCM and porous structure. This paper investigates the effect of voids on solidification of PCM, infiltrated into the pores of graphite foam as a highly con ductive porous medium with interconnected pores. A combination of the volume-of-fluid (VOF) and enthalpy-porosity methods was employed for numerical investigation o f solid ification. The proposed method takes into account the variation of density with tempera ture during phase change and is able to predict the volume shrinkage (volume contraction) during the solidification of liquids. Furthermore, the presence of void and the temperature gradient along the liquid-gas interface (the interface between void and PCM) can trigger thermocapillary effects. Thus, Marangoni convection was included during the solidification process and its importance was elucidated by comparing the results among cases with and without thermocapillary effects. The results indicated that the presence of voids within the pores causes a noticeable increase in solidification time, with a sharper increase for cases without thermocapillary convection. For verification purposes, the amount of volume shrinkage during the solidification obtained from numer ical simulations was compared against the theoretical volume change due to the variation o f density for several liquids with contraction and expansion during the freezing process. The two sets of results exhibited good agreement.