Investigation on syntheses of nanocolloids and their thermophysical properties

This thesis explores the thermophysical properties of nanocolloids. We focus here on preparation and thermal conductivity measurements of various colloidal systems consisting of different gold and ceramic particles, which are studied both in their natural state as well as chemically (surface) modified. The colloidal suspensions of nanoparticles (so-called nanofluids) have recently attracted particular attention in applied research as fluids with advanced thermal conductivity combined with good transport properties. To enhance the properties of nanofluids, it is crucial to obtain high concentrations of the solid particles. The direct synthesis of stable, highly concentrated colloids is also very important for more complex studies. We demonstrate a new way 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 by triethyleneglycolmono-11-mercaptoundecylether (EGMUDE). This leads to the immediate precipitation and formation of composite assemblies. The prepared gold colloid can be easily concentrated up to 20 times by separation of the flocculated part. Moreover, we show that the gold nanoparticles were successfully self-redispersed after few days and stay stable at high concentrations over months. UV–visible spectra, transmission electron microscopy (TEM), and dynamic light scattering (DLS) are used to characterize the products thus formed and a new model of surfactant composite assembly formation is proposed. As a model system to evaluate the effect of the particle size, concentration, stabilization method and particle clustering on the thermal conductivity, we use gold nanofluids. We synthesized spherical gold nanoparticles of different size (from 2 nm to 45 nm) and prepared stable gold colloids in the range of volume fraction of 0.00025–1 %. The particles are either protected solely by citrate ions or covered by chemically bound ionic or nonionic stabilizers. We investigate the influence of these parameters as well as the temperature on the thermal conductivity of gold nanofluids by both steady state parallel plate (GAP) and transient hot-wire (THW) methods. Obtained thermal conductivity data are consistent with effective medium theory. In order to test the effect of dispersed materials, different kinds of commercial ceramic nanoparticles (AlN, Al₂O₃, MgO^*Al₂O₃, ZnO, CuO, TiO₂, SiO₂, SiO₂ ^*Al₂O₃) suspended in water were investigated. Thermal conductivity of nanofluids was measured by the transient hot wire technique. The effects of particle volume fraction, particle shape and dispersal agent were studied. Different models of heat transfer in nanofluids were applied in order to estimate the thermal conductivity of ceramics suspensions and to compare with experimental data. Finally, we investigate the thermal conductivity of concentrated colloids in fluid, glass and gel states at equal volume fractions. We use two kinds of nanoparticles (SiO₂ and Al₂O₃) with significantly different thermal conductivity in the solid state. Thermal conductivity of the three states was measured as a function of volume fraction. Different local dynamical properties of the particles allow us to gain insight on various mechanisms of heat transfer in nanofluids. While in fluid (nanofluid) and gel (interconnected particles) states we observed expected enhancement of thermal conductivity, glassy samples (isolated frozen particles) exhibit a significant decrease of the thermal conductivity compared to the base fluid. Obtained data are analyzed in terms of existing models and possible explanations of the thermophysical properties of different colloidal states are proposed

Thèse de doctorat : Université de Neuchâtel, 2009 ; Th. 2115