Thermodynamic properties and thermal stability of ionic liquid-based nanofluids containing graphene as advanced heat transfer fluids for medium-to-high-temperature applications

Here the experimental technique for measuring the thermal conductivity of nanofluids at the temperatures above 100 °C has been developed, and a systematic research on the thermodynamic properties including thermal conductivity, viscosity, specific heat and density, of the graphene-dispersed nanofluids based on the ionic liquid 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4), has been conducted at the temperatures ranging from room temperature to around 200 °C. The thermal conductivity of the nanofluid containing graphene of as low as 0.06 wt% increases by 15.2%–22.9% as the tested temperature varies from 25 to 200 °C, as compared with that of the base fluid. The viscosity of [HMIM]BF4 and its graphene-dispersed nanofluids dramatically decreases to 6.3 cp with the temperature increasing to 210 °C, which just favors their medium-to-high-temperature applications. The specific heat and density of the graphene-dispersed nanofluids exhibit a slight decrease as compared with those of [HMIM]BF4. It is found that the thermodynamic properties of [HMIM]BF4 and its GE-dispersed nanofluids are superior to those of the commercial heat transfer fluid Therminol VP-1. The thermogravimetric analysis shows that the initial decomposition temperature of the GE-dispersed nanofluids is very close to 440.6 °C of [HMIM]BF4, indicating that all of them possess good thermal stability. This novel class of fluids based on the ionic liquid shows great potential for use as advanced heat transfer fluids in medium- and high-temperature systems such as solar collectors.