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Sintering technology for nano-structured thermoelectrics

Final Activity Report Summary - NANOSINTHER (Sintering Technology for Nano-structured Thermoelectrics)

Thermoelectric materials are attracting increasing attention due to their particular thermal-electrical properties. Nowadays, thermoelectric material technology and performance is under dynamic development and research aimed at device application covering the fields of energy converters, thermoelectric coolers and heat pumps (Peltier devices), and thermal sensors. Devices built from thermoelectric materials exhibit high reliability but no noise or vibration as there is no mechanically moving part involved. They have portable size and are light in weight.

The development and investigation of new thermoelectric materials has become recently an attractive area for both scientific research and application of thermoelectric devices with improved performance to meet higher requirements and to overcome the economic threshold towards mass application of energy conversion modules. Nano-structured thermoelectrics offer the possibility of considerably enhancing the figure of merit Z of those materials. Z is the most important parameter of thermoelectric materials performance. Enhancement of Z would lead to an increase of the conversion efficiency, of the coefficient of performance of cooling and to improved thermal sensors.

Nano-effects are allowing for exceeding the limits of thermoelectric performance met for bulk materials. The low-dimensional quantum confinement deforms the density of states of the charge carriers in a beneficial manner leading to an increasing thermoelectric power factor. On the other hand, nano-structures in scales between about 5 and 100 nanometers may drastically reduce the thermal conductivity. When leaving the electrical conductivity less effected, this considerably improves the thermoelectric figure of merit.

The scientific topic of this proposal focuses on fabrication of nano-structured thermoelectric materials by sintering technology. Sintering is a suitable and effective process for the consolidation of nano-materials from nano-powders. The work experimentally focuses on Skutterudites based on the compound cobalt antimonide; furthermore on bismuth telluride and the highly effective thermoelectric material zinc antimonide. Basic studies were made using iron disilicide. Drastic increase of these materials' performance could be achieved but further reserves of improvement have been identified particularly regarding optimisation of compaction technology.

The goals of the two-year R&D project 'NanoSinTher' on nano-structured thermoelectric materials in the EU Marie Curie programme were to develop sintering technologies for materials compaction and to optimise sintering conditions to fabricate nano-structured thermoelectric materials with improved figure of merit. These goals were achieved by pursuing specific scientific-technical objectives:
- understand the sintering dynamics of nano-powders under pressure and elevated temperature for each of the processed materials, describe beneficial and harmful effects of microstructural evolution during sintering and annealing in terms of their functional (electrical, thermal) effects of the material properties, and conclude how they act on the materials performance (figure of merit);
- elaborate technological recipes on optimal compaction procedures giving detailed instruction on the regime of temperature, pressure, heating rate, surrounding atmosphere during the process, and on the influence of original material quality (composition, size, shape, shroud atmosphere); and
- develop and apply a theoretical model to describe the observed microstructural evolution, conclude theoretically and experimentally on upper temperature limits where beneficial nanostructure (and phase integrity of the materials) and high density are still conserved.

Main achievements are:
- improvement of the figure of merit Z via the optimisation of processing conditions;
- verification of phonon and charge carrier scattering phenomena due to the dispersion on nano-inclusions;
- improvement of the figure of merit by dispersion of appropriate nano-particles.