Harvesting unused energy has been the object of research since the days of the windmill and the waterwheel. 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. The results are published in Nature Materials dated 25 August 2013 (“Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate”).
The supercell of Na0.8CoO2 showing the rattling mode at E ~ 13meV comprising mainly displacements of the (red) Na 2b ions inside tri-vacancy clusters. The introduction of these approximately flat rattling modes at low energies reduces the group velocity along the thermal gradient, suppressing the thermal conductivity by a factor six compared to vacancy-free NaCoO2. The supercell of Na0.8CoO2 showing the rattling mode at E ~ 13meV comprising mainly displacements of the (red) Na 2b ions inside tri-vacancy clusters. The introduction of these approximately flat rattling modes at low energies reduces the group velocity along the thermal gradient, suppressing the thermal conductivity by a factor six compared to vacancy-free NaCoO2.
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 Universitys 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 UKs 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.
Given the small size of the crystals studied, the ESRF was chosen to work in parallel with the ILL, combining inelastic X-ray and neutron scattering experiments in order to study and understand the mechanisms involved in obtaining low thermal conductivity in a thermoelectric material. “In general, this type of research is carried out with neutrons. However, the size of the samples was so small that the team called on the powerful X-rays available at the ESRF to extract useful signals “, says Michael Krisch, scientist on the ESRF’s Inelastic Scattering beamline, and member of the research team.
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.
The corollary to the dramatic improvement in chip performance embodied in Moores Law is the exponential increase in power consumption. Indeed, the problem of the power wall is now acknowledged to be a likely first hard limit to Moores Law. A relatively modest enhancement of thermoelectric 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.
The development of thermoelectric oxides offers an environmentally clean alternative to current materials that contain elements that are harmful, such as lead, bismuth or antimony, or are in limited supply, such as tellurium, adds Jon Goff.
Source: European Synchrotron Radiation Facility
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