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

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

Thermal fluids used to alleviate wear on components and tools and for machining operations like stamping and drilling, medical therapy and diagnosis, biopharmaceuticals, fuel cells, power transmission systems, solar cells, micro- and nanoelectronic mechanical systems and cooling systems could benefit from the nanoparticle treatment, researchers found. In each application, the fluids must balance an ability to flow with thermal transport properties. Thin fluids like water and ethylene glycol flow easily but do not conduct enough heat away from the tools while traditional heat transfer fluids can be affected by stability, viscosity, surface charge, layering, agglomeration and other factors that limit essential flow.

Researchers have been looking for more than a decade for efficient, customizable nanofluids that offer a middle ground. They use sub-100 nanometer particles in low-enough concentrations that they do not limit flow but still make efficient use of their heat transfer and storage properties.

According to Rice University, nanodiamonds are proving to be the best additive yet. In tests, the researchers dispersed nanodiamonds in mineral oil and found that a very small concentration — one-tenth of a percent by weight — raised the thermal conductivity of the oil by 70 percent at 373 K (~211°F). The same concentration of nanodiamond at a lower temperature still raised the conductivity, but to lesser effect (about 40 percent at 323 K).

They suggested a mechanism somewhat like percolation – but perhaps unlike anything else yet seen — takes hold as oil and diamond molecules collide when heated.

“Brownian motion and nanoparticle/fluid interactions play an important role,” said 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. “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.”

The work that 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 Taha-Tijerina. The results appeared in the American Chemical Society journal Applied Materials and Interfaces.

Co-authors are former Rice postdoctoral researcher Tharangattu Narayanan, now at the CSIR-Central Electrochemical Research Institute, Karaikundi, India; Chandra Sekhar Tiwary, who has a research appointment at Rice and is a scientist at the Indian Institute of Science, Bangalore, India; and Rice alumna Karen Lozano, a professor of mechanical engineering, and Mircea Chipara, an assistant professor of physics and geology, both of the University of Texas Pan American, Edinburg, Texas. Ajayan is Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry.

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