|Enhancement in thermal conductivity. Credit: ACS, Taha-Tijerina et al. Click to enlarge.|
Rice University scientists have created a nano-infused mineral oil that could greatly enhance the ability of devices as large as electrical transformers and as small as microelectronic components to shed excess heat. The approach, described in a paper in the journal ACS Nano, could increase the thermal conductivity of such thermal oils by as much as 80% without compromising the electrically insulating properties.
The researchers discovered that a very tiny amount of hexagonal boron nitride (h-BN) particles, two-dimensional cousins to carbon-based graphene, suspended in standard mineral oils are highly efficient at removing heat from a system. The team found that 0.1 wt. % of h-BN in mineral oil enhanced thermal conductivity by nearly 80%; at 0.01 wt. %, the enhancement was around 9%.
Nanofluid-based heat transfer plays an important role in diverse fields such as microelectronics, high voltage power transmission systems, automobiles, solar cells, biopharmaceuticals, medical therapy/diagnosis, and nuclear cooling. The miniaturization and high efficiency of electrical/electronic devices in these fields demand successful heat management and energy-efficient fluid-based heat-transfer systems. High thermal conductivity is essential for such heat-transfer fluids. Conventional heat-transfer fluids such as water, ethylene glycol, and engine/transformer oils are typically low-efficiency heat-transfer fluids.
...Heat transfer using fluids is a complex phenomenon, and various factors such as fluid stability, composition, viscosity, surface charge, interface, and morphology of the dispersed particles influence the observed results. The reported high thermal conductivity values of nanofluids are far from satisfactory for practical implementations. The improvement in thermal conductivity cannot be achieved by increasing the solid filler amount beyond a limit because increase in filler concentration will increase the viscosity which will adversely affect the fluid properties. Hence, the search for new nanofillers which can get high thermal conductivities at lower filler fractions is important. Moreover, fillers that would provide increased thermal conductivities but do not increase electrical conductivity are mostly ceramic particles, and conventional ceramic particles often have dispersion or settling problems and are not well-dispersible.—Taha-Tijerina et al.
The h-BN particles, about 600 nanometers wide and up to five atomic layers thick, disperse well in mineral oil and, unlike highly conductive graphene, are highly resistant to electricity. The team determined that the oil’s viscosity is minimally affected by the presence of the nanoparticle fillers.
We have demonstrated here a stable Newtonian nanofluid with 2D fillers of h-BN in a mineral oil, normally used in transformers, having high thermal conductivity. The h-BN/MO nanofluid is also an electrically insulating fluid and has a lower freezing point than the pure MO. The h-BN/MO results are compared with graphene/MO, which appears to be more electrically conductive though it is also highly thermally conductive. Our electrical and thermal analysis of these unique h-BN/MO fluids shows that these may possibly be the next generation thermal nano-oils for lubrication, capable of efficient thermal management in heavy duty machinery such as transformers.—Taha-Tijerina et al.
Support for the research came from Prolec GE Internacional, Monterrey, Mexico; the National Council of Science and Technology, Mexico; Nanoholdings LLC; and the MURI program on novel, free-standing 2-D crystalline materials focusing on atomic layers of nitrides, oxides and sulfides, by the Army Research Office.
Jaime Taha-Tijerina, Tharangattu N. Narayanan, Guanhui Gao, Matthew Rohde, Dmitri A. Tsentalovich, Matteo Pasquali, and Pulickel M. Ajayan (2012) Electrically Insulating Thermal Nano-Oils Using 2D Fillers. ACS Nano. doi: 10.1021/nn203862p