Scope of the NATO ARW workshop

Abstract

The workshop will review new materials and examine mechanisms that could lead to new thermoelectric devices with an enhanced figure-of-merit. The main topic is the heat, charge and spin transport in strongly correlated systems, and the final objective is to acquire the basic knowledge about the relevant quantum degrees of freedom, which is required to achieve the control and engineer new thermoelectric and magneto-caloric materials with specific quantum mechanical properties.

Scientific summary

Thermoelectric devices are heat engines that either convert heat into electricity or use the electricity to pump the heat from a cold to hot reservoir. The possibilities arising from the fact that electricity can be generated directly from heat, the Seebeck effect known since 1821, are beginning to be more widely appreciated. This is due to current environmental concerns to reduce waste heat loss and to find new, sustainable energy sources. The thermoelectric devices can reduce the petrol consumption in motor vehicles by 5 to 10%, reducing significantly the oil needs. They are also used for power generation in remote regions, where the thermoelectricity ensures a continuous power supply of electronic equipment. This is an important, but only one type of application of a thermoelectric effect. The other thermoelectric effects, the Peltier effect and Thompson effect, can be used for cooling without moving parts, providing microcooling for the electronics industry and refrigeration without the use of environmentally damaging CFCs and FCs. All of these can play an important role in development and efficient use of sustainable energy resources. The scientific and technological advances in this field could have important implication for modern society.

The main problem in nearly all of these applications is the rather low efficiency of the processes of energy conversion. The important factor which determines the efficiency is the dimensionless ratio, ZT, known as the figure of merit. This needs to be optimized to give a value of ZT of the order of 1 or higher for the more widespread use of thermoelectric devices. A value of ZT of the order 1 requires use of a material with a large thermopower and electrical conductivity and a low thermal conductivity. These tend to be incompatible requirements; for example, a good metal has a high electrical conductivity but also a high thermal conductivity. Materials which have a high thermopower tend also to have a low electrical conductivity. The aim of this research field, therefore, is to find or fabricate materials with the properties that enhance ZT. It is a multidisciplinary field, requiring the expertise of material physicists, chemists, metallurgists and the support of theory. There have been important recent developments in innovative synthesis techniques, the discovery of new materials, and a deeper understanding of the parameters that affect the performance of materials in thermoelectric devices. These have brought the goal of producing materials with the required characteristics for commercial application a significant step closer. The aim of this workshop is to build on the success achieved so far.

The workshop will bring together the experts in the different fields, to exchange the latest results and ideas, and to discuss the directions for future work. The programme will focus on a particular issue each day. Several strategies for increasing the figure of merit will be considered. One of them is the use of materials with strong electron correlation, which are promising due to their enhanced thermopower. Another is the use of nanostructured, layered materials and composites. There have been important developments here, in the fabrication and design of these materials, and they have considerable potential, due to the possibility of combining materials with quite different attributes to influence the various factors which contribute to the overall figure of merit. Another important class of materials are those with cages or rattlers. These tend to be rather complicated compounds as they have atoms within local rigid cages, which are relatively free to rattle, and so reduce the thermal transport by the phonons. The aim is to combine these with the favorable aspects from strong electron correlation. Yet another way of enhancing the figure of merit is by using disordered, correlated semiconductors. Semiconductors are already used in some thermoelectric devices. Disorder and increasing the electronic correlation in these materials could be a way of improving their efficiency. There will also be theoretical contributions using various of approaches, some based on simplified models, others aiming at first principles calculations for particular materials, to get a deeper understanding of the interplay of the factors that influence thermoelectric properties.

Integration of younger scientists

Every effort will be made to involve younger scientists in the program. The organization and form of the workshop will be such as to encourage the informal exchange of ideas, and to promote discussion. The one-hour poster session will be held each afternoon, before the afternoon lecture session. The posters will be continuously on the display and the organizers will print-out all the presentations and put them on poster-boards as well. The lecturers will be available at their posters for answering additional questions. The island of Hvar is an ideal location as it provides an informal and relaxing atmosphere, free from the usual time pressures and distractions of the big city. We have had a very positive feedback from earlier occasions.