Production and Dispersion Stability of Ultrafine Al, Cu and Al-Cu Particles in Base Fluid for Heat Transfer Applications

Abstract

Nanofluid is a stable colloidal suspension of low volume fraction of ultrafine solid particles in nanometric dimension dispersed in conventional heat transfer fluid to offer a dramatic enhancement in conductivity. A two step approach of synthesis of Al, Cu & Al-Cu nanoparticles by mechanical alloying and then dispersing them in base fluid to prepare nanofluids has been adopted here. Elemental Al & Cu powders were milled separately in a high energy planetary mill at a speed of 300 rpm and at 10:1 ball to powder weight ratio. Milling was carried out for 50 hours in wet condition (about 50 ml of toluene) to prevent undue oxidation. In another set of experiment, mixture of elemental Al and Cu (50 atomic wt. %) were milled in planetary mill at the same milling conditions. Powders were picked up from the mill after selected milling time to see the change in size reduction and morphology of powders. Powder particles were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), particle size analyzer, and transmission electron microscopy (TEM). It is found from XRD that the crystallite size is around 31 nm for both Al and Cu whereas 6 nm for Al-Cu alloy. The lattice strain value is around 0.37, 0.29 & 1.42 % for Al, Cu & Al-Cu respectively. After 50 hours of milling, particles size has been reduced to 500 nm for Al, 400 nm for Cu and 300 nm for Al-Cu. It is also found from transmission electron microscopy (TEM) that each particles consists of large number of crystallites of size around 10-15 nm. The milled powders were then dispersed by ultrasonication and agnetic stirring in deionized water to prepare nanofluids. The stability of nanofluids was also studied by nanozeta meter at different pH of nanofluids for constant ultrasonication time and magnetic stirring. It has been found from Nanozeta meter that the suspension is best stable at pH value of 5.5, 9.8 and 9.5 corresponding to zeta potential value of -55.50, -74.55 and -90.60 mV for Al, Cu and Al-Cu
respectively with the presence of surfactant.