James Eilertsen

Influence of tungsten substitution and oxygen deficiency on the thermoelectric properties of CaMnO3−δ

Polycrystalline tungsten-substituted CaMn1−xWxO3−δ (0.00 ≤ x ≤ 0.05) powders were synthesized from a polymeric precursor, pressed and sintered to high density. The impact of tungsten substitution on the crystal structure, thermal stability, phase transition, electronic and thermal transport properties is assessed. Tungsten acts as an electron donator and strongly affects high-temperature oxygen stoichiometry. Oxygen vacancies form in the high figure-of-merit (ZT)-region starting from about T = 1000 K and dominate the carrier concentration and electronic transport far more than the tungsten substitution. The analysis of the transport properties yields that in the investigated regime the band filling is sufficiently high to overcome barriers of polaron transport. Therefore, the Cutler-Mott approach describes the electrical transport more accurately than the Mott approach for small polaron transport. The lattice thermal conductivity near room temperature is strongly suppressed with increasing tungsten concen...

Multiband Transport in CoSb3 Prepared by Rapid Solidification

Abstract Nano‐grained CoSb3 was prepared by melt‐spinning and subsequent spark plasma sintering. The phonon thermal conductivity of skutterudites is known to be sensitive to the kind and the amount of guest atoms. Thus, unfilled CoSb3 can serve as model compound to study the impact of a nanostructure on the thermoelectric properties, especially the phonon thermal conductivity. Therefore, a series of materials was prepared differing only by the cooling speed during the quenching procedure. In contrast to clathrates, the microstructure of meltspun CoSb3 was found to be sensitive to the cooling speed. Although the phonon thermal conductivity, studied by means of Flash and 3ω measurements, was found to be correlated with the grain size, the bulk density of the sintered materials had an even stronger impact. Interestingly, the reduced bulk density did not result in an increased electrical resistivity. The influence of Sb and CoSb2 as foreign phase on the electronic properties of CoSb3 was revealed by a multi‐band Hall effect analysis. While CoSb2 increases the charge carrier density, the influence of the highly mobile charge carriers introduced by elemental Sb on the thermoelectric properties of the composite offer an interesting perspective for the preparation of efficient thermoelectric composite materials.

Thermoelectric and Structural Characterization of In x Rh 4 Sb 12 (0 <x< 0.2) Skutterudites

Thermoelectric materials play a unique and import role in the global effort toward energy diversification. Skutterudite-based materials have been extensively studied due to their highly tunable transport properties and show promise as a viable substitute for current thermoelectric (TE) materials. The skutterudite crystal (a naturally occurring mineral) exhibits a unique open crystal structure with two icosahedral void-sites per unit cell. Interstitial filling of the icosahedral void-sites has been a heavily exploited approach that reliably reduces the lattice thermal conductivity and produces a concomitant enhancement of the thermoelectric figure of merit (ZT). Partial indium-void-site filling of cobalt antimonide skutterudites has proven an effective strategy in achieving a ZT namely through a reduction in lattice thermal conductivity. As little work has been published on void-filled rhodium antimonide (RhSb3) skutterudite compounds, a series of In-filled rhodium antimonides skutterudites (InxRh4Sb12) were synthesized and their thermoelectric properties and microstructure characterized. Polycrystalline samples of InxRh4Sb12 (0 <x< 0.3) were prepared by solid-state reaction [1]. The crystal structure was characterized by powder X-Ray diffraction. The principle thermoelectric properties; Seebeck coefficient, electrical resistivity and thermal conductivity, were measured from 300–600K. Temperature dependence of the Seebeck coefficients and electrical resistivity of InxRh4Sb12 are shown in Figs. 1 and 2, respectively. The unfilled RhSb3 structure exhibits typical semiconducting behavior over the temperature range; however the In-filled compounds become increasingly degenerate with greater indium filling at high temperatures. Unexpectedly no reduction of lattice thermal conductivity was observed with indium filling (Fig. 3). Microstructural studies were performed using SEM (Fig. 4) and TEM/HREM (Fig. 5) analysis in order to further understand the unexpected trend in thermal transport with indium filling as well as to verify the presence and distribution of indium within the sample. All samples exhibit a high degree of porosity with an increasing crystallite size with indium content. The bright contrast in STEM image may indicate a variation in composition, e.g on possible segregation of In. EDS spectra of local areas of In0.2Rh4Sb12 sample (not shown) indicate that there are In-rich areas in the sample. The microstructural analysis reveals that the unusual increase in lattice thermal conductivity with indium filling can be correlated with an increasing grain size which results in fewer acoustic-phonon grain boundary scattering events.