nano 2008-04-13 19:57
Nanofluid could cool tiny electronic devices
[box=Lime]【纳米科技世界快讯】Researchers in India have shown that the thermal properties of a magnetic nanofluid can be tuned by applying a magnetic field. The effect comes thanks to the magnetic particles lining up in chains when the field is applied. The nanofluid, which is made from a colloidal suspension of magnetite nanoparticles, could find use in a variety of technology applications, including "smart" cooling devices.[/box]
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[color=darkslategray]Schematic of the mechanism of heat transport from a cylindrical device immersed in the nanofluid without and with magnetic field. Credit: J Philip.[/color]
Magnetic nanofluids are unique materials that can be used in applications such as optical modulators, optical fibre filters, optical switches and gratings. Many of the physical properties of these materials can be tuned by simply varying the applied magnetic field. Now, John Philip, PD Shima and Baldev Raj of the Indira Gandhi Centre for Atomic Research in Tamilnadu have shown that the nanofluid's thermal properties can be tuned in this way as well. This new result could be useful because nanofluids are often touted as being ideal coolants for future electronic devices and engines.
Philip and co-workers developed a stable colloidal suspension of Fe3O4 nanoparticles, with an average diameter of 6.7 nm. They found that the thermal conductivity of the suspension increased by 300 times when a magnetic field of just 10 mT was applied. The phenomenon occurs because the nanoparticles aggregate into linear chains (the length of which can also be precisely controlled from the nano to micron scales by changing the magnetic field strength) in the fluid. The effect is reversible because the nanofluid returns to its previous state once the magnetic field is switched off.
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John Philip
The researchers measured the thermal conductivity of the material using a thermal property analyser, which uses a single-needle sensor for heating and monitoring the temperature of the sample.
The result also puts to rest a long-standing debate over the exact mechanism responsible for the anomalous thermal enhancement in such nanofluids. Previously, researchers were unsure as to whether the effect was due to Brownian-motion-induced convection of nanoparticles or nanoparticle aggregation. Philip's team has proved once and for all that thermal conductivity is enhanced when nanoparticles in the fluid form linear chain-like aggregates.
"The observed reversibly tunable thermal property of our nanofluid may find many technological applications in nanoelectromechanical and microelectromechanical-based devices," Raj told nanotechweb.org. "For example, depending on the cooling requirement, the magnetic field can be precisely programmed to obtain the desired level of thermal conductivity enhancement or cooling," added Philip.
The team would now like to adopt the same concept for electrically tunable fluids, where electric fields are used instead of magnetic fields to control the linear aggregation of nanoparticles.
The work was published in [url=http://link.aip.org/link/?APPLAB/92/043108/1][color=#0000ff][i]Appl. Phys. Lett.[/i] [b]92[/b] 043108[/color][/url].
来源： nanotechweb.org