The addition of metal nanoparticles to coolants used in thermal control systems can dramatically increase the thermal conductivity of the base fluid. Such metal nanoparticle-fluid composite materials are referred to as nanofluids and their use as coolants has the potential to reduce the weight and power requirements of spacecraft thermal control systems. The thermal conductivity of nanofluids is dependent on the concentration, size, shape, surface chemistry, and aggregation state of the constituent nanoparticles. The effects of nanoparticle loading concentration and the aspect ratio of the nanoparticles on the thermal conductivity and viscosity of water and ethylene glycol based coolants were investigated. Silver nanorods with a diameter of 55 ± 12 nm and an average length of 12.8 ± 8.5 m at a concentration of 0.5% by volume increased the thermal conductivity of water by 68%. The thermal conductivity of an ethylene glycol based coolant was increased by 98% with a silver nanorod loading concentration of 0.5% by volume. Longer nanorods had a greater effect on the thermal conductivity than shorter nanorods at the same loading density. However, longer nanorods also increased the viscosity of the base fluid to a greater extent than shorter nanorods. TCi導(dǎo)熱系數(shù)儀