Sunday, 31 August 2014
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Diamond nanoparticles hailed for thermal conductivity

A mixture of diamond nanoparticles and mineral oil easily outperforms other types of fluid created for heat-transfer applications, according to new research by Rice University.

Rice scientists are said to have mixed low concentrations of diamond particles measuring approximately 6nm in diameter with mineral oil to test the nanofluid’s thermal conductivity and how temperature would affect its viscosity. They found it to be much better than nanofluids that contain higher amounts of oxide, nitride or carbide ceramics, metals, semiconductors, carbon nanotubes and other composite materials.

The Rice results have appeared in the American Chemical Society journal Applied Materials and Interfaces.

The work, which could improve applications where control of heat is paramount, was led by Pulickel Ajayan, chair of Rice’s Materials Science and NanoEngineering Department, and Rice alumnus Jaime Taha-Tijerina, now a research scientist at Viakable Technology and Research Center in Monterrey, Mexico, and a research collaborator at Carbon Sponge Solutions in Houston.

Thermal fluids are used to alleviate wear on components and tools and for machining operations like stamping and drilling, medical therapy and diagnosis, biopharmaceuticals, air conditioning, fuel cells, power transmission systems, solar cells, micro- and nanoelectronic mechanical systems and cooling systems for everything from engines to nuclear reactors.

Fluids for each application have to balance an ability to flow with thermal transport properties. Thin fluids like water and ethylene glycol flow easily but don’t conduct heat well, while traditional heat-transfer fluids can be affected by stability, viscosity, surface charge, layering, agglomeration and other factors that limit flow.

According to Rice, researchers have been looking since the late 1990s for efficient, customisable nanofluids that offer a middle ground. They use sub-100nm particles in low-enough concentrations that they don’t limit flow but still make efficient use of their heat-transfer and storage properties.

Nanodiamonds are proving to be the best additive yet as they carry most of the properties that make bulk diamond so outstanding for heat-transfer applications at the macro scale. Single diamond crystals can be 100 times better at thermal conductivity than copper while still acting as an efficient lubricant.

‘The great properties of nanodiamond - lubricity, high thermal conductivity and electrical resistivity and stability, among others - are quite impressive,’ Taha-Tijerina said in a statement. ‘We found we could combine very small amounts with conventional fluids and get extraordinary thermal transport without significant problems in viscosity.’

In tests, the researchers dispersed nanodiamonds in mineral oil and found that one-tenth of a per cent by weight raised the thermal conductivity of the oil by 70 per cent at 373 kelvins (about 211 degrees Fahrenheit). The same concentration of nanodiamond at a lower temperature still raised the conductivity, but to lesser effect at around 40 per cent at 323K.

They suggested a mechanism somewhat like percolation takes hold as oil and diamond molecules collide when heated.

‘Brownian motion and nanoparticle/fluid interactions play an important role,’ Taha-Tijerina said. ‘We observed enhancement in thermal conductivity with incremental changes in temperature and the amount of nanodiamonds used. The temperature-dependent variations told us the changes were due not just to the percolation mechanism but also to Brownian motion.’

Mexico’s National Council for Science and Technology and the Army Research Office through the Multidisciplinary University Research Initiative supported the research.


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