Advances in Thermoelectric Materials Could Further Accelerate Chips

The dramatic improvement in chip performance embodied in Moore's Law went in lock-step with an exponential increase in power consumption. Indeed, the problem of the "power wall" causing chips to melt at further increased speeds is now acknowledged to be a likely first hard limit to Moore's Law.

A possible solution for this critical challenge are relatively modest enhancements of thermoelectric materials, acting as integrated cooling elements in chips. Better performance for oxides would create huge potential for environmentally friendly applications for cooling in electronic circuits. Oxides are particularly attractive, since they are already extensively employed in integrated circuits and, ultimately, it should be possible to include them in the chip production process.

In recent years, thermo-electric materials have enabled the re-use of otherwise wasted thermal energy as electrical power. Driven by the quest to efficiently cool densely packed micro-electronics chips, they are also used as solid-state refrigerators. One of the difficulties involved in developing thermo-electric systems that convert heat into electric current is the need for materials exhibiting high electrical conductivity but low thermal conductivity, which is only possible with complicated crystal structures.

Scientists have now discovered a way of suppressing thermal conductivity in sodium cobaltate, opening new paths for energy scavenging. Led by Jon Goff from Royal Holloway, University of London, the international team of scientists conducted a series of experiments on crystals of sodium cobaltate grown in the University's Department of Physics. X-ray and neutron scattering experiments were carried out at the European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL) in Grenoble, with calculations key to their interpretation performed using the UK's national supercomputer facility HECToR.

The scientists believe their approach can easily be applied also to other substances since it only requires tiny crystals, and this will facilitate the design of a next generation of thermoelectric materials.

The application of a temperature difference across a conductor causes charged carriers to diffuse from hot to cold regions, in a similar manner to the expansion of a gas upon heating. Mobile carriers leave behind their oppositely-charged immobile nuclei in the hot regions, giving rise to a thermoelectric voltage. This phenomenon is known as the Seebeck effect, and it enables the conversion of waste heat to useful electricity.

"The global target to reduce carbon emissions has brought research into thermoelectric materials centre stage," said Professor Jon Goff from the Department of Physics at Royal Holloway.  "If we can design better thermoelectric materials, we will be able to reduce the energy consumption of cars by converting waste heat in exhausts into electrical power, as well as cooling hot spots on computer chips using solid state refrigerators."

Thermoelectric coolers are also used in air conditioners and in scientific equipment where a rapid response to changes in temperature is required. Energy harvesting is important in miniaturized electronic devices, including "systems on a chip", and power recovery using this method is competitive for any off-grid electricity applications, including in space. -- ESRF

Reference:

D. J. Voneshen et al., Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate, Nature Materials, 2013, DOI: 10.1038/nmat3739