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Nano-layered thin films of quaternary bismuth telluride lead selenide for low-dimensional thermopile devices

Final Activity Report Summary - BITEFILM (Nano-layered thin films of quaternary bismuth telluride lead selenide for low-dimensional thermopile devices)

Thermoelectric devices have been known as a viable and flexible principle of direct energy conversion for cooling and power generation in manifold scenarios for practical application. CoSb3-based materials showing Skutterudite crystal structure are known to exhibit promising thermoelectric properties for power generation at temperatures up to 600 degrees Celsius. The advantage of thermoelectric devices over conventional conversion techniques is their extremely high reliability (over 250,000 hours of operation free of failure under space conditions have been reported), silent and vibration-less operation, low volume, no need of maintenance, light weight, free scalability from the sub-microwatt up to the MW region, no involvement of compressed gases or cycling fluids or even green-house gases, and environmentally friendly operation. The development and investigation of new thermoelectric materials has become recently an attractive area for both scientific research and device application. The main disadvantage of thermoelectric devices is their to-date still moderate efficiency.

The main objective of the proposal was the development of nano-structured CoSb3 skutterudite based thermoelectric materials with enhanced performance. Nano-structured and filled Skutterudite thermoelectrics offer the possibility of considerably enhancing the thermoelectric figure of merit of those materials by "phonon blocking and electron transmitting". Nanostructuring in bulk samples may produce strong phonon scattering and thereby induce a major reduction of thermal conductivity, leading to an improved figure of merit. The figure of merit is the most important parameter of thermoelectric materials' performance. Its enhancement would lead to an increase of the conversion efficiency of thermogenerators, of the coefficient of performance of Peltier cooling and to improved responsivity of thermal sensors.

The project focuses on filled skutterudite nano-structured thermoelectric materials prepared by ball milling and sintering techno¬logy. This method combination is suitable for effectively consolidating nano-materials from nano-powders. The work experimentally focuses on fabricating and characterising Indium-filled cobalt antimonide.

The goals of the two-year R&D project were to develop doped and filled nano-Skutterudite and to optimise the doping and filler concentration to achieve an improved figure of merit. To understand the Skutterudite filling mechanism in nano-powders under high pressure and elevated temperature for each of the processed materials has been pursued as the specific scientific-technical objective.

Main achievements are
- Improvement of the figure of merit of In-filled CoSb3 Skutterudite. The lattice structure of cobalt antimonide contains large voids which can be filled by different sorts of atoms to obtain the filled skutterudite structure. The filler can act as doping to control the electrical properties of the material but also may dramatically decrease the thermal conductivity by a rattling motion. In certain cases, the thermal conductivity may be reduced by a factor of up to ten or even more as compared to binary, un-filled skutterudites.
- Verification of phonon and charge carrier scattering phenomena due to the nano-structure. The thermal conductivity and electrical conductivity were compared between samples of nano-structured In-filled CoSb3 and crystalline bulk material. In the filled sample strong phonon scattering decreases the lattice thermal conductivity but also charge carrier scattering phenomena are substantially affected. An increase of the figure of merit has been observed, and all thermoelectric properties are strongly influenced.