Synthesis and characterization of thermoelectric materials : bulk and nanostructure approach

Abstract

The potential of a material for thermoelectric applications is determined by it's dimensionless thermoelectric figure of merit. A promising route to enhance ZT is the use of guest atoms in oversized cages as in intermetallic clathrates or filled skutterudites. The new cage compound Ce4[Ce tief 4]Pt12[Pt tief 12]Sn25[Sn tief 25] is investigated in this aspect. Nevertheless, our experimental result shows that the thermoelectric figure of merit of Ce4[Ce tief 4]Pt12[Pt tief 12]Sn25[Sn tief 25] is small compared to the traditional thermoelectric materials. This is primarily due to its metallicity. However, Ce4[Ce tief 4]Pt12[Pt tief 12]Sn25[Sn tief 25] turned out to be interesting in a different context. It is a strongly correlated material with a low- lying antiferromagnetic transition that can be continuously suppressed by a magnetic field of 1 T and thus exhibits a quantum critical point.<br />The zero-field resistivity reveals Landau Fermi liquid behavior and a gapped spin wave contribution with [Delta] = 0.54 K. Above TN [T tief N] a sublinear temperature dependence is observed up to 1.9 K that might be explained by short-range order fluctuations and/or the opening of the spin gap [Delta]. The low-temperature electrical resistivity measurements of Ce4[Ce tief 4]Pt12[Pt tief 12]Sn25[Sn tief 25] in various magnetic fields show the evolution of its antiferromagnetic state with magnetic field. Another cage compound, Ce3[Ce tief 3]Ru4[Ru tief 4]Sn13[Sn tief 13], is an interesting compound in view of both its peculiar crystal structure and its rich low-temperature properties. At room temperature, Ce3[Ce tief 3]Ru4[Ru tief 4]Sn13[Sn tief 13], behaves like an ordinary paramagnetic metal. The susceptibility shows deviations from Curie-Weiss behavior only below 50 K. These deviations are due to the formation of a gap, due to crystalline electric field (CEF) splitting of the ground state multiplet of Ce3+[Ce hoch 3+] or due to antiferromagnetic correlations. The existence of crystal fields, as well as the high-temperature effective moment for Ce3[Ce tief 3]Ru4[Ru tief 4]Sn13[Sn tief 13], suggests a localized Ce3+[Ce hoch 3+] state. There is a well-defined specific heat anomaly confirming the transition observed in the electrical resistivity measurement below 2 K. The resistivity and specific heat measurements confirm that this anomaly is due to antiferromagnetic (AFM) ordering. By optimizing nanowire diameters, the power factor can be increased by quantum confinement and the lattice component of the thermal conductivity can be reduced by scattering. FeSb2[FeSb tief 2] has been considered as a novel thermoelectric material due to a giant Seebeck coefficient at low temperatures. However, due to the large lattice thermal conductivity, the dimensionless thermoelectric figure of merit is small. Various techniques were attempted to synthesize FeSb2[FeSb tief 2] nanowires; the solvothermal technique in an autoclave, a membrane filling technique using alumina nanochannel matrices, and a vapor-liquid-solid mechanism.<br />The former route yielded so far the best results. FeSb2[FeSb tief 2] nanowires were also fabricated by focussed ion beam (FIB) cutting.<br />PtSn4[PtSn tief 4] single crystals were obtained as a by-product in the process of synthesizing Ce4[Ce tief 4]Pt12[Pt tief 12]Sn25[Sn tief 25].<br />The high quality of the samples made it possible to observe quantum oscillations in both the magnetization M(H) and the resistivity with fields along the three principal crystallographic directions. Due to their proximity with the theoretical value we could assign most of the experimentally observed dHvA frequencies with different sheets of the Fermi surface of PtSn4[PtSn tief 4]. Another approach to reduce phonon thermal conductivity is site substitution with iso-electronic elements to preserve a crystalline electronic structure while creating large mass contrast to disrupt the phonon path. The thermoelectric figure of merit of FeSi is low, mostly due to the high lattice thermal conductivity.<br />This can be reduced for instance by a partial substitution of Fe by an iso-electronic element.