B. L. Pedersen

Thermally stable thermoelectric Zn4Sb3 by zone-melting synthesis

The thermal stability of thermoelectric Zn4Sb3 has been investigated on samples produced by a new zone-melting technique, as well as by the conventional quench method. The multitemperature synchrotron powder diffraction data reveal that while conventionally synthesized, Zn4Sb3 samples have almost 40% degradation in the first heating cycle at 625K, samples prepared by zone melting only have 3% degradation. Repeated thermal cycling induces additional degradation of the quenched sample of up to 58%, compared to ∼9% degradation in the zone-melted sample. Thus, zone-melting produces Zn4Sb3 samples that are significantly more thermally stable, which make them promising for commercial implementation.

Influence of sample compaction on the thermoelectric performance of Zn4Sb3

Five compacted samples of thermoelectric Zn4Sb3 have been prepared from the same synthesis batch by spark plasma sintering. Four samples were made from powder with a grain size 45μm. Thermoelectric properties were evaluated, and an apparent strong correlation with sample density is found. ZnSb impurity contents obtained from powder x-ray diffraction cannot explain the variation in properties, which the authors’ suggest may be caused by slight changes in Zn content. The results show that minute changes in sample compaction conditions can have a larger effect on ZT than doping.

The effect of Mg doping on the thermoelectric performance of Zn4Sb3

The effect of Mg doping of Zn4Sb3 on thermal and thermoelectric properties has been studied on 4 samples with a doping degree ranging from 0.1 to 2 at.% Mg, and compared to a pure, undoped sample. Laboratory X-ray powder diffraction shows that all samples are pure single phase materials as further evidenced with higher confidence by Rietveld refinement of high resolution synchrotron powder diffraction of the as synthesized 1 at.% and 2 at.% Mg doped samples. Zn4Sb3 undergoes a transition from the room temperature disordered beta phase to an ordered alphapsila phase, going over an intermediate alpha phase; differential scanning calorimetry show only small effects of doping on the phase transition temperature. Physical properties have been evaluated from 2-400 K for all samples. Some physical property parameters are affected by doping, but no immediate improvement of ZT was achieved by the doping procedure. The reason for this is discussed in relation to differences in sample compaction.

Low temperature transport and structural properties of misch-metal-filled skutterudites

Skutterudites, such as CoSb3, are a promising class of thermoelectric materials, particularly when the voids in the crystal structure are filled with guest atoms. We report a comprehensive study of the effects of filling skutterudites with misch-metal (Mm), a rare-earth alloy having the naturally occurring La, Ce, Pr, and Nd composition. Our power diffraction experiments show that Mm filling causes a larger expansion and an unusual distortion of the CoSb3 lattice compared with single-element-filled skutterudites. We probed the response of crystal lattice, electronic structure, and carrier and phonon scattering mechanisms to Mm filling using neutron powder diffraction, Hall effect, electrical resistivity, thermopower, and thermal conductivity measurements between 2 and 300 K on a series of MmyFe4−xCoxSb12 samples. The thermoelectric properties of these Mm-filled skutterudites in this low temperature range are comparable to those of pure Ce-filled skutterudites despite the anomalous lattice expansion and di...

High Temperature stability of thermoelectric Zn4Sb3

The thermal stability of high performance thermoelectric Zn4 Sb3 has been investigated using multi-temperature, high resolution synchrotron powder diffraction. Rietveld refinement of the data show ~30 % impurity phases at 500 K, which is inconsistent with previous reports of stability up to 523 K. At 700 K almost 70% of the sample has degraded. The influence of anion doping of Zn4Sb 3 on the thermal stability is examined for a sample doped with 2.5% tellurium. The doping changes the decomposition pattern, but no immediate improvement in the thermal stability is observed

The effect of Mg doping on the thermoelectric performance of Zn 4 Sb 3

The effect of Mg doping of Zn4Sb3 on thermal and thermoelectric properties has been studied on 4 samples with a doping degree ranging from 0.1 to 2 at.% Mg, and compared to a pure, undoped sample. Laboratory X-ray powder diffraction shows that all samples are pure single phase materials as further evidenced with higher confidence by Rietveld refinement of high resolution synchrotron powder diffraction of the as synthesized 1 at.% and 2 at.% Mg doped samples. Zn4Sb3 undergoes a transition from the room temperature disordered beta phase to an ordered alphapsila phase, going over an intermediate alpha phase; differential scanning calorimetry show only small effects of doping on the phase transition temperature. Physical properties have been evaluated from 2-400 K for all samples. Some physical property parameters are affected by doping, but no immediate improvement of ZT was achieved by the doping procedure. The reason for this is discussed in relation to differences in sample compaction.

Graphene inclusion controlling conductivity and gas sorption of metal–organic framework

A general approach to prepare composite films of metal–organic frameworks and graphene has been developed. Films of copper(II)-based HKUST-1 and HKUST-1/graphene composites were grown solvothermally on glassy carbon electrodes. The films were chemically tethered to the substrate by diazonium electrografting resulting in a large electrode coverage and good stability in solution for electrochemical studies. HKUST-1 has poor electrical conductivity, but we demonstrate that the addition of graphene to HKUST-1 partially restores the electrochemical activity of the electrodes. The enhanced activity, however, does not result in copper(II) to copper(I) reduction in HKUST-1 at negative potentials. The materials were characterised in-depth: microscopy and grazing incidence X-ray diffraction demonstrate uniform films of crystalline HKUST-1, and Raman spectroscopy reveals that graphene is homogeneously distributed in the films. Gas sorption studies show that both HKUST-1 and HKUST-1/graphene have a large CO2/N2 selectivity, but the composite has a lower surface area and CO2 adsorption capacity in comparison with HKUST-1, while CO2 binds stronger to the composite at low pressures. Electron paramagnetic resonance spectroscopy reveals that both monomeric and dimeric copper units are present in the materials, and that the two materials behave differently upon hydration, i.e. HKUST-1/graphene reacts slower by interaction with water. The changed gas/vapour sorption properties and the improved electrochemical activity are two independent consequences of combining graphene with HKUST-1.

Insights into thermoelectric candidate materials from powder diffraction

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s43 Acta Cryst. (2007). A63, s43 MS17 O1 Structural reaction of PZT under in situ conditions using Synchrotron powder diffraction influence and stability of nanostructures Hartmut Fuess, Kristin A. Schoenau, Ljubomira A Schmitt , Michael Knapp, Matteo Leoni , Mario Maglione Materials Science, Darmstadt University of Technology, Germany. CELLS, Barcelona, Spain. Dept. Materials Engineering and Industrial Technologies, University of Trento, Italy. d ICMCB-CNRS, Universite Bordeaux 1, France. E-mail: hfuess@tu-darmstadt.de