Stability and thermophysical characterization of ZrO₂ nanoparticles by surface-modified

Nanofluids have attracted considerable interest in advanced thermal properties; however, poor stability limits practical application. Then, vacuum drying methods were used to graft β-cyclodextrin (β-CD) onto ZrO₂ nanoparticles to improve stability of nanofluids. The morphological features, surface functional groups, and molecular structure of the nanoparticles before and after modification were compared. A two-step method was used to prepare ZrO₂/EG:DI and β-ZrO₂/EG:DI nanofluids at volume fraction of 0.06 vol.%. The stability of the nanofluids were characterized by TEM and sedimentation observation method, and the viscosity and thermal conductivity were evaluated in the temperature range of 20–60 ℃. The results indicated that when β-ZrO₂ nanoparticles are modified with β-CD, the polymer attachment introduces spatial site resistance between the nanoparticles. This resistance mitigates particle agglomeration and promotes the long-term stability of the nanofluid. Therefore, the modified β-ZrO₂ nanoparticles were more uniformly dispersed, which had a 57.90% lower settling velocity in base fluid than ZrO₂ nanofluid at room temperature. Besides, the thermal conductivity of β-ZrO₂ nanofluid was10.25% higher than that of ZrO₂ nanofluid at 60 ℃ with the constant viscosity. The enhancement in nanofluid thermal conductivity results from the polymer wrapped around the β-ZrO₂ nanoparticles, forming an elastic layer and leading to elastic collisions, as well as micro-convection. Therefore, surface modification of nanoparticles is one of the effective methods to improve the stability of nanofluids without affecting the thermal conductivity.