by Richard Mandel

Nanotubes as semiconductors.
 A team of researchers led by Michael Fuhrer, assistant professor of physics at University of Maryland’s Center for Superconductivity Research, has fabricated a semiconducting nanotube transistor with a mobility almost 25% higher than any previous semiconducting material at room temperature, and more than 70 times higher than the mobility of the silicon used in today’s computer chips. Mobility, a measure of how well a semiconductor conducts electricity, is the conductivity of a material divided by the number of current-carrying charges, and is the number typically used to compare the conduction properties of one semiconductor to another. The results, published online in the journal Nano Letters, provide new evidence that semiconducting carbon nanotubes hold great promise for replacing conventional semiconductor materials in applications ranging from computer chips to biochemical sensors. "This is the first measurement of the intrinsic conduction properties of semiconducting nanotubes," said Fuhrer, who heads the university’s Nanoelectronics Research Group. The team synthesized nanotubes on flat silicon chips with lengths up to 0.3 mm, about 100,000 times the nanotubes’ diameter, and some 100 times longer than nanotubes previously studied in electronic measurements. A special technique using a scanning electron microscope had to be developed in order to locate the nanotubes on the chip so that wires could be connected to them. The International Technology Roadmap for Semiconductors, an assessment of the semiconductor industry’s technology requirements, says that a replacement material for silicon with higher mobility will be necessary by the year 2010. According to Fuhrer, the new findings by the team indicate nanotubes could fill that role. "Many challenges remain before nanotubes can be used instead of silicon in computer chips," notes Fuhrer. "The contact resistance between nanotube and metal electrodes must be controlled. Nanotube batches must be prepared that contain only semiconducting nanotubes. And nanotubes must be placed with precision on substrates." However, he adds, "significant progress is taking place in all these areas, and the challenges do not seem insurmountable."

University of Maryland Center for Superconductivity Research,
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Compact cooling.
Two technologies for removing heat from electronic devices — synthetic jets that rely on trains of turbulent air puffs, and a system that uses vibration to atomize cooling liquids such as water — were developed by Professor Ari Glezer and co-workers at the Georgia Institute of Technology’s School of Mechanical Engineering, and licensed to Atlanta-based Innovative Fluidics. "There is a lot of concern in the electronics industry about thermal management," said Raghav Mahalingam, a research engineer in Georgia Tech’s School of Mechanical Engineering. "New processors are consuming more power, circuit densities are getting higher, and there is pressure to reduce the size of devices." Traditional metallic heat sinks and circulating fans have limitations like circulating air bypassing the heat sinks, and "dead areas" where motor assemblies block air flow. Boosting air flow to increase cooling involves fans that use more energy. Synthetic jet ejector arrays (SynJets) produce two to three times more cooling than fans, with two-thirds less energy input. Possessing no friction parts to wear out, a synthetic jet module has a diaphragm mounted within a cavity that has one or more orifices. Electromagnetic or piezoelectric drivers vibrate the diaphragm 100 to 200 times per second, sucking surrounding air into the cavity and then expelling it to where cooling is needed. Although the jets move 70% less air than fans of comparable size, the air flow they produce contains tiny vortices which make the flow turbulent, encouraging efficient mixing with ambient air and breaking up thermal boundary layers. The modules take up less space in cramped equipment housings, and can be conformed flexibly to components that need cooling — even mounted directly within the heat sink fins. For larger-scale applications like high-powered military electronics, automotive components, blade servers, radars and lasers, cooling may be handled by vibration-induced droplet atomization (VIDA). VIDA uses high-frequency vibrations produced by piezoelectric actuators to create sprays of tiny droplets from liquid coolants, such as water, within a closed cell attached to the component in need of cooling. The droplets form a thin film on the heated surface, allowing thermal energy to be removed by evaporation. The heated vapor condenses, either on the walls of the cooling cell or on tubes carrying liquid coolant through the cell. The condensate then returns to the vibrating diaphragm for re-use. "We have so far been able to cool about 420W/cm2, and ultimately expect to increase that to 1000W/cm2," said Samuel Heffington, a research engineer in the School of Mechanical Engineering and director of technology development for Innovative Fluidics. The company has produced several prototype SynJets and expects to have commercial products available within 15 months. The VIDA technology will take longer.

Innovative Fluidics, 
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