Carbon nanotubes as fast-thickening agents in polyalphaolefin greases: Partial crystallinity and intertube joints toward multifunctionality

Abstract

Scalable and economic manufacturing of carbon nanotube (CNT) greases addresses the current challenges of nanofluids. The key problems remain the complex and non-obvious nature of nanotube-base fluid interactions with the diversity of nanotube morphological variants as the main aspect. Here, we prove that successful formation of stable and multifunctional CNT-polyalphaolefin (PAO) greases is possible only by employing two types of multi-wall CNTs (MWCNTs) as the thickening as well as thermo- and electro-active agents, i.e., (1) thin, entangled, less crystalline nanotubes and (2) thick nanotubes connected via few-mismatched-layer graphenoid joints. Using mechanical spectroscopy, SEM, TEM, Raman spectroscopy, inverse gas chromatography (IGC), and Hansen solubility parameters, we analyze the mechanisms governing fast-thickening behavior and CNT-PAO affinities, revealing a reversible formation of a 3D CNT-network which mimics a mesh of the conventional lithium soaps. Finally, we reveal the multifunctionality of the CNT-PAO greases, expressed in as high as 58%- and 700%-enhancements of thermal and alternating current conductivity, respectively, compared to PAO base fluid.