Effective Thermal Conductivity of Various Surface-Modified Cerium Dioxide Nanoparticles Dispersed in Cyclohexane

Nanofluids with ultrasmall nanoparticles below 10 nm stabilized in organic liquids lack a deeper characterization with respect to the effective thermal conductivity λ eff . This study investigates nanofluids consisting of cerium dioxide (CeO 2 ) nanoparticles and cyclohexane. To elaborate the influence of particle size, surface modification, and particle volume fraction φ p , nearly spherical nanoparticles with very small core sizes d core from (3 to 6) nm covered by modifiers of varying chain length were examined at φ p of up to 0.045. λ eff was measured using a guarded parallel-plate instrument at temperatures from (283.15 to 313.15) K and at atmospheric pressure with an expanded uncertainty of 2.5%. All samples were characterized using transmission electron microscopy, thermogravimetric differential thermal analysis, small-angle X-ray scattering, and dynamic light scattering to determine d core , the mass and size of the modifier shell, the effective particle size and d core , and the hydrodynamic diameter, respectively. The thorough analysis allowed for a better understanding of the influence of the particle characteristics on λ eff . The enhancement of λ eff shows no temperature dependence and an increase as a function of φ p . With increasing d core and chain length of the modifier, λ eff exhibits the tendency to decrease and increase, respectively. The comparison of the experimental λ eff data with empirical prediction models shows agreement, especially with the geometric mean model that accounts for an interfacial resistance between the core particle and cyclohexane. This work demonstrates a novel multi-method approach for investigating λ eff together with particle morphology in nanofluids with ultrasmall surface-modified nanoparticles relevant for colloid science and technology.