Nanofluids have been proposed to improve the performance of microchannel heat sinks. In this paper, we present a systematic characterization of aqueous silica nanoparticle suspensions with concentrations up to 31  vol %. We determined the particle morphology by transmission electron microscope imaging and its dispersion status by dynamic light scattering measurements. The thermophysical properties of the fluids, namely, their specific heat, density, thermal conductivity, and dynamic viscosity were experimentally measured. We fabricated microchannel heat sinks with three different channel widths and characterized their thermal performance as a function of volumetric flow rate for silica nanofluids at concentrations by volume of 0%, 5%, 16%, and 31%. The Nusselt number was extracted from the experimental results and compared with the theoretical predictions considering the change of fluids bulk properties. We demonstrated a deviation of less than 10% between the experiments and the predictions. Hence, standard correlations can be used to estimate the convective heat transfer of nanofluids. In addition, we applied a one-dimensional model of the heat sink, validated by the experiments. We predicted the potential of nanofluids to increase the performance of microchannel heat sinks. To this end, we varied the individual thermophysical properties of the coolant and studied their impact on the heat sink performance. We demonstrated that the relative thermal conductivity enhancement must be larger than the relative viscosity increase in order to gain a sizeable performance benefit. Furthermore, we showed that it would be preferable to increase the volumetric heat capacity of the fluid instead of increasing its thermal conductivity.

The thermal conductivity of concentrated colloids in fluid, glass, and gel states was analyzed. SiO₂ colloids at 10−31 vol % and Al₂O₃ colloids at 4.8 vol % in the fluid, the gel, and the glassy states were studied by dynamic light scattering, rheology, and transmission electron microscopy. Thermal conductivity of the three states was measured as a function of volume fraction. For the fluid and gel states the thermal conductivity increases almost linearly with concentration, reaching roughly 18% enhancement for silica at a volume fraction of 31 vol %. In contrast, in the glass state thermal conductivity strongly decreases with increasing volume fraction.

Nanofluids (colloidal suspensions of nanoparticles) have been reported to display significantly enhanced thermal conductivities relative to those of conventional heat transfer fluids, also at low concentrations well below 1% per volume (Putnam, S. A., et at. J. Appl. Phys. 2006, 99, 084308; Liu, M.-S. L., et al. Int. J. Heat Mass Transfer. 2006, 49; Patel, H. E., et al. Appl. Phys. Lett. 2003, 83, 2931−2933). The purpose of this paper is to evaluate the effect of the particle size, concentration, stabilization method and particle clustering on the thermal conductivity of gold nanofluids. We synthesized spherical gold nanoparticles of different size (from 2 to 45 nm) and prepared stable gold colloids in the range of volume fraction of 0.00025−1%. The colloids were inspected by UV−visible spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The thermal conductivity has been measured by the transient hot-wire method (THW) and the steady state parallel plate method (GAP method). Despite a significant search in parameter space no significant anomalous enhancement of thermal conductivity was observed. The highest enhancement in thermal conductivity is 1.4% for 40 nm sized gold particles stabilized by EGMUDE (triethyleneglycolmono-11-mercaptoundecylether) and suspended in water with a particle-concentration of 0.11 vol%.

We have developed a simple method for the preparation of nearly mono-dispersed stable gold colloids with a fairly high concentration using a two step procedure. First we synthesize citrate capped gold nanoparticles and then exchange the citrate ions with triethyleneglycolmono-11-mercaptoundecylether (EGMUDE). This leads to the immediate precipitation and formation of composite assemblies. The gold nanoparticles were successfully self-redispersed after a few days. The prepared gold colloid can be easily concentrated up to 20 times by separation of the flocculated part. UV-visible spectra, transmission electron microscopy (TEM), and dynamic light scattering (DLS) were used to characterize the products thus formed.

A combination of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, UV−vis spectroscopy and transmission electron microscopy was used to study the adsorption of thiol-protected gold nanoparticles on TiO₂ films and the behavior of the resulting composite films upon UV irradiation. The gold nanoparticles were covered by charged thiols N-acetyl-l-cysteine and l-glutathione and had a mean core diameter of about 1 nm. The TiO₂ film was prepared by deposition of a slurry of TiO₂ nanoparticles with a particles size of 21 nm. The combination of the two spectroscopic techniques showed that the adsorption of the gold nanoparticles onto the TiO₂ films is significantly limited by intrafilm diffusion. Upon illumination the IR spectra revealed the removal of the adsorbed thiolates and the appearance of sulfates. These species were also observed when N-acetyl-l-cysteine adsorbed on TiO₂ was illuminated, i.e., in the absence of gold. In the latter case oxalate was observed in large quantity on the TiO₂ surface, in contrast to the illumination of the N-acetyl-l-cysteine-protected gold particles. This indicates a different pathway for the decomposition of the adsorbed thiol when adsorbed on the gold or directly on the TiO₂ surface. In situ UV−vis spectroscopy also shows the formation of larger particles upon illumination, which is confirmed by transmission electron microscopy.