Thermoelectric modules for high and low temperature applications: progress and challenges

Kurzfassung

Alternative thermoelectric materials which can substitute the commercially dominant bismuth telluride technology are highly desirable for near-room-temperature heat conversion and thermal management applications. The combination of MgAgSb and n-type Mg2(Si,Sn) is highly promising due to the excellent thermoelectric properties of both materials and the higher availability of the constituting elements compared to BiTe. We have fabricated prototypes with conversion efficiencies > 6% (T_c=25 °C,T_h=300 °C) and power densities of ~1 W/cm2, comparable in performance to commercial bismuth telluride modules. Analysis of the module measurement and comparative calculations using the constant property model indicates that a further performance gain of ~10% can be achieved by improved interface design. Progress beyond that can only be achieved by improvement of the materials. Mg2Si1-xSnx, originally optimized for hot side temperatures of > 400 °C, can be improved by an (re)-optimizing of the Si:Sn ratio and the carrier concentration according to the targeted lower hot side temperature. Optimal compositions can be estimated using transport models considering the relevant conduction and valance band(s). Advanced strategies for material improvement include selective scattering of charge carriers on grain boundaries or nanostructures (“energy filtering”). This can be achieved by a locally varying electronic band structure, feasible in Mg2Si1-xSnx by changing the band gap via the Sn content x and adjusting the energy levels of the electronic bands in grains of different compositions by selective doping. To test selective doping strategies a local charge carrier concentration measurement is highly useful. We’ll show that transient microprobe measurements of the Seebeck coefficient can be used to that purpose with a local resolution down to ~ 5 µm. For carrier concentration determination with sub-µm resolution we employ a combin¬ation of the AFM-based Kelvin Probe Force Microscopy for work function measurements, SEM for determination of the composition, first-principles calculations for band offset predictions and modelling based on the Boltzmann transport equation. First results indicate an inhomogeneous distribution of the dopant Sb among several observed Mg2Si1-xSnx phases, and confirm the feasibility to tune the local carrier concentration and influence the electronic band structure alignment.