Thermoelectricity of inverse clathrates

In dieser Arbeit wurden Synthesemethoden für inverse Clathrate verbessert und deren Kristallstruktur, sowie thermoelektrische Eigenschaften untersucht. Zunächst wurde eine alternative Methode, die aus 2 Schritten, nämlich mechanischem Legieren und Heißpressen, besteht, zur Synthese von Sn19,3Cu4,7P22I8 entwickelt. Dann wurden elektrischer Widerstand und Seebeck-Koeffizient dieser Probe gemessen und versucht durch gezielte Substitutionen im Gitter hinsichtlich einer thermoelektrischen Anwendung zu verbessern. Es wurden die Elemente Cu, Zn, Ag und Au in die Struktur eingebaut. Der Einbau wurde mit Hilfe von Röntgenpulveruntersuchungen und Bestimmung der Gitterkonstanten überprüft. Die neu hergestellten Verbindungen wurden ebenfalls hinsichtlich ihrer thermoelektirschen Eigenschaften (elektrischer Widerstand, Seebeck-koeffizient, thermische Leitfähigkeit) untersucht. Die Substitutionen zeigten eine Verschiebung der Eigenschaften in die gewünschte Richtung. Weiters wurde der Schermodul der Verbindung Sn19,3Cu4,7P22I8 gemessen.In this work synthesismethods for inverse clathrates were improved and their crystal structure as well as their thermoelectric properties were investigated. First an alternative method, including two steps, namely mechanical alloying and hot pressing, for the synthesis of Sn19,3Cu4,7P22I8 was developed. Then electrical resistivity and seebeck-coeffizient of this sample were measured and we tried by applying specific substitutions in the lattice to improve the properties for thermoelectric applications. The elements Cu, Zn, Ag and Au were incorporated into the structure. This was confirmed by X-ray powder diffraction and determination of lattice parameters. The thermoelectric properties (electrical resistivity, Seebeck-coefficient and thermal conductivity) of the new compounds were measured. The substitutions showed a shift of the properties into the assigned direction. Furthermore the sheer modulus of Sn19,3Cu4,7P22I8 was measured

Misfit Layer Compounds and Ferecrystals: Model Systems for Thermoelectric Nanocomposites

A basic summary of thermoelectric principles is presented in a historical context, following the evolution of the field from initial discovery to modern day high-zT materials. A specific focus is placed on nanocomposite materials as a means to solve the challenges presented by the contradictory material requirements necessary for efficient thermal energy harvest. Misfit layer compounds are highlighted as an example of a highly ordered anisotropic nanocomposite system. Their layered structure provides the opportunity to use multiple constituents for improved thermoelectric performance, through both enhanced phonon scattering at interfaces and through electronic interactions between the constituents. Recently, a class of metastable, turbostratically-disordered misfit layer compounds has been synthesized using a kinetically controlled approach with low reaction temperatures. The kinetically stabilized structures can be prepared with a variety of constituent ratios and layering schemes, providing an avenue to systematically understand structure-function relationships not possible in the thermodynamic compounds. We summarize the work that has been done to date on these materials. The observed turbostratic disorder has been shown to result in extremely low cross plane thermal conductivity and in plane thermal conductivities that are also very small, suggesting the structural motif could be attractive as thermoelectric materials if the power factor could be improved. The first 10 compounds in the [(PbSe)1+δ]m(TiSe2)n family (m, n ≤ 3) are reported as a case study. As n increases, the magnitude of the Seebeck coefficient is significantly increased without a simultaneous decrease in the in-plane electrical conductivity, resulting in an improved thermoelectric power factor

Effect of Local Structure of NbSe₂ on the Transport Properties of ([SnSe]_1.16)₁(NbSe₂)_<i>n</i> Ferecrystals

([SnSe]_1.16)₁(NbSe₂)_<i>n</i> ferecrystals were synthesized through the modulated elemental reactants technique by increasing the number of Nb|Se layers in the precursor from 1 to 4. The <i>c</i>-lattice parameter of the intergrowth was observed to change as a function of <i>n</i> by 0.635(2) nm. The <i>c</i>-lattice parameter of SnSe was observed to be 0.588(8) nm and independent of <i>n</i>. The electrical resistivity does not decrease as <i>n</i> increases as expected from simple models, but instead the trend in the resistivity is (1,3) > (1,4) ≥ (1,1) > (1,2). The carrier concentration increases with <i>n</i> as expected, so the unusual trend in resistivity is a result of the carrier mobility decreasing with increasing <i>n.</i> In-plane X-ray diffraction line widths and STEM images of the (1,4) compound show that it has small in-plane grain sizes and a large diversity of stacking sequences respectively, providing a potential explanation for the reduced carrier mobility

Structure, Stability, and Properties of the Intergrowth Compounds ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>, where <i>m</i> = <i>n</i> = 1–20

Intergrowth compounds of ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>, where 1 ≤ <i>m = n ≤ </i>20, with the same atomic composition but different <i>c</i>-axis lattice parameters and number of interfaces per volume were synthesized using the modulated elemental reactant technique. A <i>c</i>-axis lattice parameter change of 1.217(6) nm as a function of one unit of <i>m</i> = <i>n</i> was observed. In-plane X-ray diffraction shows an increase in distortion of the rock salt layer as a function of <i>m</i> and a broadening of the NbSe₂ reflections as <i>n</i> increases, indicating the presence of different coordination environments for Nb (trigonal prismatic and octahedral) and smaller crystallite size, which were confirmed via scanning transmission electron microscopy investigations. The electrical resistivities of all 12 compounds exhibit metallic temperature dependence and are similar in magnitude as would be expected for isocompositional compounds. Carrier concentration and mobility of the compounds vary within a narrow range of 2–6 × 10²¹ cm^–3 and 2–6 cm² V^–1 s^–1, respectively. Even at a thickness of 12 nm for the SnSe and NbSe₂ blocks, the properties of the intergrowth compounds cannot be explained as composite behavior, due to significant charge transfer between them. Upon being annealed at 500 °C, the higher order <i>m</i> = <i>n</i> compounds were found to convert to the thermodynamically stable phase, the (1,1) compound. This suggests that the capacitive energy of the interfaces stabilizes these intergrowth compounds

The Influence of Interfaces on Properties of Thin-Film Inorganic Structural Isomers Containing SnSe–NbSe₂ Subunits

Inorganic isomers ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>([SnSe]_1+δ)_<i>p</i>(NbSe₂)_<i>q</i>([SnSe]_1+δ)_<i>r</i>(NbSe₂)_<i>s</i> where <i>m</i>, <i>n</i>, <i>p</i>, <i>q</i>, <i>r</i>, and <i>s</i> are integers and <i>m</i> + <i>p</i> + <i>r</i> = <i>n</i> + <i>q</i> + <i>s</i> = 4 were prepared using the modulated elemental reactant technique. This series of all six possible isomers provides an opportunity to study the influence of interface density on properties while maintaining the same unit cell size and composition. As expected, all six compounds were observed to have the same atomic compositions and an almost constant <i>c</i>-axis lattice parameter of ≈4.90(5) nm, with a slight trend in the <i>c</i>-axis lattice parameter correlated with the different number of interfaces in the isomers: two, four and six. The structures of the constituents in the <i>ab</i>-plane were independent of one another, confirming the nonepitaxial relationship between them. The temperature dependent electrical resistivities revealed metallic behavior for all the six compounds. Surprisingly, the electrical resistivity at room temperature decreases with increasing number of interfaces. Hall measurements suggest this results from changes in carrier concentration, which increases with increasing thickness of the thickest SnSe block in the isomer. Carrier mobility scales with the thickness of the thickest NbSe₂ block due to increased interfacial scattering as the NbSe₂ blocks become thinner. The observed behavior suggests that the two constituents serve different purposes with respect to electrical transport. SnSe acts as a charge donor and NbSe₂ acts as the charge transport layer. This separation of function suggests that such heterostructures can be designed to optimize performance through choice of constituent, layer thickness, and layer sequence. A simplistic model, which predicts the properties of the complex isomers from a weighted sum of the properties of building blocks, was developed. A theoretical model is needed to predict the optimal compound for specific properties among the many potential compounds that can be prepared

Superconducting ferecrystals: Turbostratically disordered atomic-scale layered (PbSe)1.14(NbSe2)n thin films

Hybrid electronic heterostructure films of semi- and superconducting layers possess very different properties from their bulk counterparts. Here, we demonstrate superconductivity in ferecrystals: turbostratically disordered atomic-scale layered structures of single-, bi- and trilayers of NbSe2 separated by PbSe layers. The turbostratic (orientation) disorder between individual layers does not destroy superconductivity. Our method of fabricating artificial sequences of atomic-scale 2D layers, structurally independent of their neighbours in the growth direction, opens up new possibilities of stacking arbitrary numbers of hybrid layers which are not available otherwise, because epitaxial strain is avoided. The observation of superconductivity and systematic Tc changes with nanostructure make this synthesis approach of particular interest for realizing hybrid systems in the search of 2D superconductivity and the design of novel electronic heterostructures

Ferro- and antiferro-magnetism in (Np, Pu)BC

Two new transuranium metal boron carbides, NpBC and PuBC, have been synthesized. Rietveld refinements of powder XRD patterns of {Np,Pu}BC confirmed in both cases isotypism with the structure type of UBC. Temperature dependent magnetic susceptibility data reveal antiferromagnetic ordering for PuBC below TN = 44 K, whereas ferromagnetic ordering was found for NpBC below TC = 61 K. Heat capacity measurements prove the bulk character of the observed magnetic transition for both compounds. The total energy electronic band structure calculations support formation of the ferromagnetic ground state for NpBC and the antiferromagnetic ground state for PuBC.JRC.E.6-Actinide researc