Doping Effect on the Thermal Conductivity of Metal-Oxide Nanofluids: Insight and Mechanistic Investigation

Nanofluids which are dispersions of nanoparticles in liquids, are known to exhibit anomalous heat transfer properties compared to conventional base liquids. Numerous mechanisms were proposed to explain the unusual enhancement in thermal conductivity of nanofluids, including Brownian motion, interfacial thermal resistance, and conduction due to particle aggregation. In the present study, the individual contributions of the various mechanisms are detailed. Nanofluids of pristine metal oxides (ZnO and CuO) and of Zn²⁺ doped CuO in water as base fluid were sonochemically prepared, without a surfactant, using a probe sonicator. Varying the specific heat capacity (C_p) of the synthesized nanomaterials was exploited to understand the interfacial resistance (Kapitza resistance) in the base liquid, which influences the thermal flow between the particle and the liquid molecules wrapping over the particle surface (the nanolayer). The thermal conductivity was evaluated at two different concentration ranges. The enhancement at low concentrations is attributed to Brownian motion and thermophoresis, whereas the rise in the heat transfer at the higher concentration range was ascribed to the conduction mechanism that results from particle aggregation.