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Tuesday, 7 February 2012

Thermal management: Beating the Heat

Feb 7, 2012

Credit: ASME

Although technically a sub-category of thermal engineering, heat transfer affects all engineering disciplines, including mechanical engineering. Heat transfer refers to the exchange of thermal energy from one system to another.

Thermal management, a practical embodiment of heat transfer, involves the removal of heat from the point of generation to the surroundings. Thermal management involves the use of fans, cooling fluids, or specialized materials to draw heat from temperature-sensitive components like computer chips, solar cells, and lighting. Components that overheat have a poor performance and shortened lifespan.

Electronic devices are notorious for generating heat. All central processing units in modern computers incorporate some sort of heat-dissipating mechanism such as a fan, heat-drawing materials, or both. As computers shrink in size, thermal management becomes even more critical. Components are close together, so heat from one part can easily damage a nearby component. Because space is limited, a suitable fan becomes impractical.

Passive Removal

Several options exist for thermal management in miniaturized computers. Passive thermal management uses materials with specialized thermal properties that transmit heat only in one direction–away from a hot component. You may have noticed how your notebook computer warms your thighs as you type away. Were it not for these "thermal spreaders" the computer would be literally too hot to handle.

Another passive strategy involves heat-spreading, which prevents hot spots by dissipating heat evenly across and away from microprocessor packages. Joel Plawsky, professor of chemical engineering at Rensselaer Polytechnic Institute, Troy, NY, conceived of a composite microcoating that conducts heat away from delicate components while simultaneously absorbing stresses associated with thermal expansion.

"Because computers cycle on and off through sleep mode, they never reach a steady state temperature," Dr. Plawsky says. "Thermal problems are much worse now than when computers were kept on all the time."

The nanocomposite consists of two phases. Nonconductive silicone, epoxy, or silicone-epoxy copolymer provide a matrix for the conductive elements and a means of absorbing stresses as chips heat and cool. Heat is drawn away by silver or copper nanoparticles. During polymer curing, the metal particles "find" one another and form heat-conducting networks. According to Dr. Plawsky, the bifunctional, 25-mm-thick composite is barely visible to the unaided eye.
This infrared image shows an ionic-cooling system in action. After the device is turned on, air carries heat away from the system. Image: Tessera

Thermal Spreading

Alexis R. Abramson, associate professor of mechanical and aerospace engineering at Case Western Reserve University, Cleveland, OH, is investigating new materials that draw heat away from sensitive components in laptop and desktop components. At stake may be the future of computing itself. "Thermal management is the reason we're not able to increase processor speeds as significantly as we have in the past," she says.

One of Dr. Abramson's research interests is thermal spreading, which draws heat away from critical components and operators through simple conduction. Next-generation thermal spreaders are expected to carry even more heat, perhaps in a single direction.

Ionic Wind Pumps

Another project, the ionic wind pump, achieves the same heat-dissipative effect as a fan but with no moving parts.

Ionic pumps are microfabricated electronic devices consisting of two opposing structures, each measuring 1 mm x 1 mm x 150 mm. One structure serves as an emitter electrode, the other as the collector. When a voltage is applied across the electrodes, air close to the emitter becomes positively ionized. That air is attracted to the negatively charged collector electrode. The distance the air travels is minuscule, but hundreds or thousands of the devices can move the air far enough to move it out of the laptop's case.

Unlike fans, micropumps can be placed anywhere inside the device, in any configuration, and almost any density. Dr. Abramson has used them with thermal spreaders to take advantage of the best properties of each.

"We've shown that an array of ionic wind pumps works better than fans and consumes much less electricity," she says.

Heat transfer and dissipation become even more critical with electronic devices. As circuits become smaller, the ability of their materials to dissipate heat generally decreases. Bulk silicon, for example, has a thermal conductivity of 149 W/m-K. In silicon nanowires, however, conductivity falls to about 50. Dr. Abramson attributes the decline to heat "bouncing off the walls" instead of moving directly through the material.

Ironically, the thermal conductivity of single-walled carbon nanotubes, 3,500 W/m-K, is the highest of any known material (copper's is 385 W/m-K).

Source: American Society of Mechanical Engineers (ASME)


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