Concentrated, yet stable silver- and copper-in-water nanocolloids are prepared using a novel method combining formation of a metal ammine complex and use of a strong NaBH4 reductant. Maximum solid contents of the stable silver and copper nanofluids are 2000 and 5000 ppm (reported as mass fractions), respectively. The metallic nanoparticles are reduced in micellar microreactors, favoring formation of small nanoparticles. Use of stable metal ammine complexes ([Ag(NH3)2]+ and [Cu(NH3)4(H2O)2]2+) as metal ion sources prevent the formation of sparingly-soluble metal salts and thus, aid the nanocolloid synthesis. Several different stabilizers and combinations of them are tested for nanofluid synthesis: anionic sodium dodecyl sulfate, polymeric polyvinylpyrrolidone, sodium citrate, nonionic sorbitan trioleate and polysorbate 20. The particle sizes and size distributions are studied using dynamic laser scattering and transmission electron microscopy. Stability of the nanofluids is assessed by zeta potential measurements, repetitive particle size measurements and visual observations. The average particle sizes of the silver and copper nanofluids with optimized surfactants are < 20 nm and ~40 nm, respectively, and the fluids with optimized stabilizer compositions are stable over the storing period of a month. Specific heat and thermal conductivities of the fluids are measured using differential scanning calorimetry and modified transient plane source technique (TCi Thermal conductivity analyzer), respectively. In addition, the nanofluid viscosities are measured in order to assess the usability of the nanofluids in convective heat transfer. The chemistry of stabilizers is found to have a significant impact on the viscosity of nanofluids. Commonly used polymeric polyvinylpyrrolidone stabilizer produces viscous fluids, whereas the viscosities of the fluids stabilized with small size surfactants are close to that of water.
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.