Design
Materials
Process
Nanoscale Suspensions for Heat Transfer Applications
S.J. Chung, J.P. Leonard, I. Nettleship April 2010
Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh PA 15261
Thermal conductivity and viscosity are the most important physical parameters of fluids selected for use in heat transfer applications. Of the commonly available fluids, water has the highest conductivity (0.57-0.68 Wm-1K-1) and a low viscosity (0.855 cP at 20C). The thermal conductivity of water is, however, well below that of most solids such as metals (10-400 Wm-1K-1) or metal oxides (1-100 Wm-1K-1). It is therefore compelling to explore the possibility of increasing the thermal conductivity of a liquid by adding solid particles of high thermal conductivity. Such liquid-solid suspensions have have been widely studied in many fields and the increase in thermal conductivity with volume fraction of solid added is known to obey the effective medium model and its extensions.
When solid particles are added to a liquid the viscosity increases linearly with volume fraction (j) at low dilution. At higher volume fractions, typically above 3 vol% (φ=0.03), hydrodynamic interactions between particles cause sharper increases in viscosity, including transition to solid-like behavior and other complex rheological effects. Additional effects such as particle-particle interaction and surfactants can increase the viscosity even further and introduce other effects such as gellation. In heat transfer applications, a low fluid viscosity is critical requirement for maximizing advective heat transport efficiency, as well as minimizing pressure gradients and drag effects. It can be demonstrated using basic convection theory that the viscosity of aqueous suspensions should remain well below 10 cP to obtain any meaningful improvement in heat transfer efficiency relative to pure water.
Figure 1. Stability map calculated for a ZnO suspension in a static volume 100mm in depth. The solid line is the characteristic height above the floor that particles can reach due to Brownian agitation. Dashed lines are the upper extent for larger particles as they settle out of the suspension.
An equally important consideration for a fluid suspension is chemical stability. Many metal particles such as Al or Cu oxidize readily when dispersed into water. Additionally, suspensions of simple solid particles in a liquid lack the thermodynamically stability of a true solution, as a driving force for particle aggregation is nearly always present. Aggregation rates are determined the mean time between collisions and the strength of the barrier associated with particle-particle bonding. In suspensions with volume fractions sufficiently high to effect an increase in therrmal conductivity, interparticle spacing approaches nanometer scales and high collision rates cannot be avoided. Here, stability to aggregation is commonly achieved using a kinetic barrier at the surface of the particles that depends on the density of charge, steric hindrance of adsorbed surfactant molecules or both. Gellation is a term corresponding to a different set of related processes involving the formation of weaker networks in the fluid that can involve interactions between particles as well as the surfactant molecules. Gels can exhibit a wide range of rheological behaviors as well as temperature and time dependence. Although the immobilizing effects of gels on partciles can be used to stabilize suspensions, the corresponding increase in viscosity is a concern when considering heat transfer fluid applications.
Even if the solid particles are chemically stabilized and well dispersed in the fluid, sedimentation due to gravity will change the configuration of the particles in the fluid when it is at rest. Sedimentation velocities are governed by Stokes law, with large micron-scale particles settling much faster than smaller nano-scale particles. Thus a narrow size distribution of small particles will be most suitable for maintaining configurational stability. Nanoparticles offer the extra advantage that they are small enough to be subject to Brownian motion. In the case where Brownian motion is sufficient to counter the sedimentation effects, a quiescent suspension can be considered to ‘kinetically stable’ and its dispersed configuration should remain indefinitely. Susepensions posessing both chemical stability to avoid aggregation, vided that chemical stability is also present. and the there is no reason to expect fractionation of the nanofluid by sedimentation. Depending on the system this is usually transition occurs when the particles are a few tens of nanometers in size.
Figure 2. The effect of Darvan-C dispersant concentration on viscosity of 20 percent volume fraction ZnO suspension in water measured at 2000 s-1
The present work concerns the preparation and characterization of zinc oxide particles in an a high volume fraction aqueous suspension. Using this high thermal conductivity oxide (116 Wm-1K-1) in solid loadings up to 0.02, we produce a fluid with a thermal conductivity significantly above that of water. By optimizing the preparation and chemical composition we obtain a fluid with a low viscosity, as well as posessing long term chemical and kinetic stability.
Publications relating to this project
- J.P. Leonard, S.J. Chung, I. Nettleship, Y. Soong, D.V. Martello and M.K. Chyu, "Stability of Zinc Oxide Nanofluids Prepared with Aggregated Nanocrystalline Powders", Journal of Nanoscience and Nanotechnology 8, 1 (2008).
- S.J. Chung, J.P. Leonard, I. Nettleship, J.K. Lee, Y. Soong, D.V. Martello and M.K. Chyu, "Characterization of ZnO nanoparticle suspension in water-- Effectiveness of ultrasonic dispersion", Powder Techonolgy 194, 75 (2009).