Preparation of copper-bearing nanofluids for thermal applications

Stable suspension of nanometer solid particles in suitable solvents, so-called nanofluids, has shown enhanced thermal conductivity when compared with the fluid base; therefore, the preparation and characterization of these type of suspension will enable the development of more efficient and effective thermal management systems. In general, nanofluids hosting metal nanoparticles, (e. g., Ag or Cu) would exhibit better thermal conductivities than those bearing oxide nanoparticles. Accordingly, the present research addressed the optimization of the size-controlled synthesis conditions of copper nanoparticles, determination of the most suitable conditions to stabilize those nanoparticles in ethylene glycol and the reproducible measurement of the thermal conductivity as a function of nanoparticle volumetric load. The research work was also focused on the improvement of the stability and reliability of the system used to measure the thermal conductivity of copper-bearing nanofluids.;The synthesis of nanoparticles was achieved through the reduction of copper ions using hydrazine as well as by taking advantage of the reducing power of polyol solutions. The preparation of the copper-bearing stable nanofluid was attempted by treating nanoparticles with surfactant agents to disperse them in the base fluid (ethylene glycol). In the aqueous route, the rate of the Cu reduction reaction and the corresponding average crystallite size of the nanoparticles were strongly dependent on the copper ion and hydrazine concentration. From starting 0.016M Cu solution, the time at which the reduction of Cu was realized was shortened from 12 hours down to only 30 minutes when the concentration of hydrazine was increased from 0.059M to 0.7M. The corresponding average crystallite size decreased from 25nm to 17.8nm. In the polyol approach, the reaction time was as short as 30 seconds when a NaOH/Cu mole ratio = 50 was used. The corresponding average crystallite size, estimated at 21.2nm, went down to 12.7nm when a 5E-7M of Polyvinylpyrrolidone was used. The formation of Copper nanoparticles from starting Cu(II) species, in water and polyol solutions, involved the formation of precursor cuprous oxide (Cu2O), which underwent dissolution and subsequent reduction into elemental Cu. This dissolution-reduction step controls the average size of nanoparticles of elemental Copper.;The thermal conductivity of nanofluids produced was measured by using the transient hot-wire technique. The relationship between nanoparticle size, volumetric concentration in ethylene glycol and the relative thermal conductivity values are presented and discussed. The thermal conductivity of the base ethylene glycol was increased 32% when copper nanoparticles (7.5 %v/v), averaging 21nm, were suspended in the base fluid.