Dispersion Stability Of Hybrid Nanosuspension Containing Differently Shaped TiO₂ Nanoparticles Under Depletion Interaction

Nanosuspension is prepared by adding small amounts of nanoscaled metallic or nonmetallic nanoparticles in the base fluid. The pre-exiting researches have indicated nanosuspension can be applied as heat transfer work fluid and phase change materials for thermal energy storage. However, nanoparticles would aggregate to clusters of larger size due to the high surface energy, the cluster will sediment under gravity once formed, and thus heat transfer conductivity of nanosuspension will decrease seriously. Therefore, improvement of nanosuspension stability is the primary issue of theoretical research and practical application.By analyzing depletion interaction and other interactions in colloidal suspension, mixing differently shaped TiO2 nanoparticles for improving stability of TiO2-H2O nanosuspension was proposed. The main works are as following:(1) Synthesis of TiO2 nanotubes (TiNTs) and nanoplatelets (TiNPs). TiNTs were synthesized using NaOH and anatase TiO2 nanoparticles by hydrothermal synthesis method. TiNPs were synthesized using titanate isopropoxide and hydrofluoric acid by hydrothermal synthesis method as well. TiO2 nanospheres (TiNSs) were bought directly. TiNTs, TiNPs, TiNSs were analyzed by TEM, SEM, and nano analyzer after synthesis. TiNTs, TiNPs, TiNSs showed regular shape, well-distributed size and high purity.(2) Stability of hybrid TiO2-H2O nanosuspension. Stability of hybrid nanosuspension containing TiNPs and TiNTs, TiNPs and TiNSs, TiNSs and TiNTs were investigated respectively. Stability of hybrid nanosuspension was quantitatively appraised by ultraviolet light absorbance method. Stabilization condition and rule were shown by stability phase diagram. The results indicated that stability of hybrid nanosuspension of same total concentration containing TiNPs and TiNTs were much more superior to individual TiNPs and TiNTs nanosuspension at higher TiNTs concentration. The sedimentation velocity of nanoparticles decreased as much as 68.7%. Stability of hybrid nanosuspension wasn’t improved at at higher TiNPs concentration. Stability of hybrid nanosuspension containing TiNPs and TiNSs was greatly improved with increase of TiNPs concentration, but declined with further increase of TiNPs concentration. Stability of hybrid nanosuspension containing TiNSs and TiNTs is independent on concentration ratio of TiNSs and TiNTs. Stability of hybrid nanosuspension wasn’t improved compared with individual TiNSs and TiNTs nanosuspension.(3) Theoretical analysis of hybrid nanosuspension. Stability of hybrid nanosuspension was analyzed and explained by depletion interaction and other interactions. Depletion interaction under suitable concentration ratios can decrease nanoparticles aggregation and make nanoparticles more dispersed, thus stability of nanosuspension can be improved. Furthermore, steric hindrance of two-dimensional nanoplatelet can also improve stability of hybrid nanosuspension because it can decrease frequency of nanoparticle collision, thus decrease nanoparticle aggregation and improve nanosuspension stability.(4) Dispersion stability of hybrid nanosuspension with SDS. Stability of hybrid nanosuspension containing TiNSs and TiNPs with dispersant SDS was studied. Combining dispersant addition and nanoaprticles mixing is found to be a more excellent method to improve nanofluid stability. The method could greatly improve stability of nanofluid, especially at high concentration. The main reason is synergistic effect of depletion interaction, steric repulsion and electrostatic interaction of dispersant.