Water-like thermal conductivity of ionanofluids containing high aspect ratio multi-walled carbon nanotubes and 1-ethyl-3-methylimidazolium-based ionic liquids with cyano-functionalized anions

Abstract

In the era of growing climatic requirements, the search for efficient heat transfer and storage fluids for both industrial and domestic applications is a very important point on the road map of energy transformation in the economies of countries around the world. To achieve the assumed process parameters, working fluids must be characterized by many, often seemingly contradictory, parameters, such as high thermal conductivity and low viscosity, for effective heat transfer. Therefore, the media usually have a very complex structure, which is often designed on the atomic level. An example of such are ionanofluids (INFs) – colloidal dispersions of solid nanoparticles in ionic liquids (ILs). This work aimed to determine the impact and comparison of various cyano-functionalized anions ([SCN]−, [N(CN)2]−, [C(CN)3]−) in 1-ethyl-3-methylimidazolium ([Emim]+) ILs on key properties, such as thermal conductivity, density, viscosity, and structure of INFs, in a wide range of temperature and concentration (up to 10 wt%) of in-house synthesized multi-walled carbon nanotubes (MWCNTs) and carboxyl-functionalized (oxidized) MWCNTs, as well as commercially-available helical carbon nanotubes. Very promising results have been obtained, especially in the case of 1-ethyl-3-methylimidazolium tricyanomethanide [Emim][C(CN)3] with 5.0 wt% loading of ultra-high aspect ratio (up to 11,000) in-house 16 h MWCNTs – thermal conductivity reached 0.532 W·m−1·K−1 at 25 °C which is a threefold improvement (200 %) over the base IL. The thermal conductivity of this value is close to that of water, i.e., an absolute record holder among the conventional heat transfer fluids (0.606 W·m−1·K−1 at 25 °C). The nanofluids were characterized by long-term stability and a wide spectrum of rheological properties, from liquid-like INFs to solid-like ‘bucky gels’, which can be controlled mainly by the morphology and concentration of MWCNTs. For instance, liquid-like [Emim][N(CN)2] + 1.0 wt% in-house 16 h MWCNTs had a viscosity of 34.0 mPa·s (at 186 s−1, 25 °C), which was lower than those of many conventional heat transfer fluids including propylene glycol (42.3 mPa·s, 25 °C), Duratherm® S (49.2 mPa·s, 27 °C), Therminol® 66 (70.8 mPa·s, 27 °C), or Xiameter® PMX-210 (100 mPa·s, 25 °C), while solid-like [Emim][SCN] + 3.0 wt% in-house 16 h MWCNTs had a viscosity up to 8.9 Pa·s (at 18.6 s−1, 25 °C) and may be used in next-generation phase-change materials for efficient thermal energy storage.