Quantitative stability analyses of multiwall carbon nanotube nanofluids following water/ice phase change cycling

Multiwall carbon nanotube nanofluids are regularly investigated for phase change enhancement between liquid and solid states owing to their improved heat transfer properties. The potential applications are numerous, the most notable being latent heat thermal energy storage, but the success of all nanofluid-assisted technologies hinges greatly on the ability of nanoparticles to remain stably dispersed after repeated phase change cycles. In this report, the stability of aqueous nanofluids made from oxygen-functionalized multiwall carbon nanotubes (f-MWCNTs) was profiled over the course of 20 freeze/thaw cycles. Sonication was used after each cycle to redisperse clusters formed from the crystallization process. This study offers a quantitative evaluation of f-MWCNT-nanofluid stability as a result of phase change through optical characterization of concentration and particle size. It also provides insight into the integrity of the surface functionalities through zeta potential and XPS analyses. Concentration and particle size measurements showed moderate and consistent recoverability of f-MWCNT dispersion following ultrasonication. XPS measurements of solid-state MWCNTs exposed to freeze/thaw cycling in water, and zeta potential analyses of the nanofluids indicate that the surface oxygen content is preserved throughout phase change and over repeated cycles. These results suggest a resilience of oxygen-functionalized MWCNTs to the freezing and thawing of water, which is ideal for their utilization as phase change enhancers.

由于多壁碳纳米管纳米流体改善了传热性能，人们经常研究其在液固相变强化方面的应用。其潜在应用众多，其中最显著的是潜热热能储存，但所有纳米流体辅助技术的成功在很大程度上取决于纳米粒子在反复相变循环后保持稳定分散的能力。在本报告中，对由氧官能化多壁碳纳米管（f - MWCNTs）制成的水性纳米流体在20次冻融循环过程中的稳定性进行了分析。在每个循环后使用超声处理来重新分散由结晶过程形成的聚集体。本研究通过对浓度和粒径的光学表征，对因相变导致的f - MWCNT - 纳米流体稳定性进行了定量评估。还通过ζ电位和XPS分析深入了解了表面官能团的完整性。浓度和粒径测量表明，超声处理后f - MWCNT分散体具有适度且一致的可恢复性。对在水中经历冻融循环的固态MWCNTs进行的XPS测量以及对纳米流体的ζ电位分析表明，在整个相变过程和多次循环中，表面氧含量得以保留。这些结果表明氧官能化MWCNTs对水的冻融具有耐受性，这对于将其用作相变增强剂是理想的。