▪ Abstract Recently there has been a resurgence of research efforts related to the investigation of new and novel materials for small-scale thermoelectric refrigeration and power generation applications. These materials need to couple and optimize a variety of properties in order to exhibit the necessary figure of merit, i.e. the numerical expression that is commonly used to compare one potential thermoelectric material with another. The figure of merit is related to the coefficient of performance or efficiency of a particular device made from a material. The best thermoelectric material should possess thermal properties similar to that of a glass and electrical properties similar to that of a perfect single-crystal material, i.e. a poor thermal conductor and a good electrical conductor. Skutterudites are materials that appear to have the potential to fulfill such criteria. These materials exhibit many types of interesting properties. For example, skutterudites are members of a family of compounds we call...

SKUTTERUDITES : A phonon-glass-electron crystal approach to advanced thermoelectric energy conversion applications

In recent years, many soft-chemical reactions of layered perovskites have been reported, and they can be classified into sets of similar reactions. Simple ion-exchange and intercalation reactions replace or modify the interlayer cations of layered perovskites, and more complex metathesis reactions replace interlayer cations with cationic structural units. Topochemical condensation reactions that involve dehydration and reduction provide access to a variety of metastable structural features in three-dimensional perovskites, and similar reactions can be used to convert among higher order layered perovskite homologues. Other techniques, such as high pressure and anion intercalation/deintercalation, also yield interesting metastable phases. When combined, the individual reactions complement each other, and a powerful toolbox of solid-state reactions emerges. By using layered perovskites as templates, it is possible to retrosynthetically design new product perovskites that retain the structural features of the...

Perovskites by Design: A Toolbox of Solid-State Reactions

We present evidence of a relatively high dimensionless figure of merit (ZT) in a polycrystalline skutterudite partially filled with ytterbium ions. The small-diameter yet heavy-mass Yb atoms partially filling the voids of the host CoSb3 system exhibit low values of thermal conductivity while the quite favorable electronic properties are not substantially perturbed by the addition of Yb. This combination is ideal for thermoelectric applications exemplifying the “phonon-glass electron-crystal” concept of a thermoelectric material, resulting in ZT=0.3 at room temperature and ZT∼1 at 600 K for Yb0.19Co4Sb12.

High figure of merit in partially filled ytterbium skutterudite materials

This review highlights the use and great potential of liquid metals as exotic and powerful solvents (i.e. fluxes) for the synthesis of intermetallic phases. The results presented demonstrate that considerable advances in the discovery of novel and complex phases are achievable utilizing molten metals as solvents. A wide cross-section of examples of flux-grown intermetallic phases and related solids are discussed and a brief history of the origins of flux chemistry is given. The most commonly used metal fluxes are surveyed and where possible, the underlying principal reasons that make the flux reaction work are discussed.

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The metal flux: a preparative tool for the exploration of intermetallic compounds.

Publisher Summary This chapter describes the relevant physical, chemical, and materials issues pertaining to skutterudites and includes the latest exciting developments. As the search for novel, promising thermoelectrics intensifies; skutterudites have emerged as prospective candidates for achieving figures of merit well in excess of unity. The name “skutterudite” is derived from a small mining town in Norway called “Skutterud,” where a CoAs3-based mineral had been mined extensively. The compounds identical in structure to CoAs3—the structures first identified by Oftedal in 1928—have since been known as skutterudites. Skutterudite structure offers interesting possibilities to alter both the electronic and lattice properties. The chapter discusses the structural aspects and bonding of binary skutterudites and filled skutterudites. The inability to prepare pure binary skutterudites with transition metals other than those of the Co group does not mean that cobalt cannot be partially replaced by its immediate neighbors in the periodic table—iron and nickel. The lattice thermal conductivity is one of the key parameters determining the thermoelectric figure of merit. It is, thus, essential to understand the lattice dynamics in skutterudites and to have a good grasp of the phonon density of states. Skutterudites are compounds with the highest pnicogen concentration forming stable compounds with Co, Rh, and Ir at ambient conditions. The fundamental point about the binary skutterudites is that they are diamagnetic solids.

Chapter 5 Skutterudites: Prospective novel thermoelectrics

A new metastable binary compound and a series of new, metastable ternary crystalline compounds with the skutterudite crystal structure have been prepared through controlled crystallization of amorphous reaction intermediates formed by low-temperature interdiffusion of modulated elemental reactants. Discussed in this paper are La{sub 1-x}Fe{sub 4}Sb{sub 12}, FeSb{sub 3}, Hf{sub 1-x}Fe{sub 4}Sb{sub 12}, and Y{sub 1-x}Fe{sub 4}Sb{sub 12}. The amorphous reaction intermediate for each system crystallizes exothermically near 200 {degree}C forming the desired skutterudite structure. At temperatures above 500 {degree}C, they decompose exothermically forming a thermodynamically more stable mixture of binary compounds and elemental components. We propose that the desired compounds nucleate from the amorphous precursor because slow solid state diffusion rates hinder disproportionation into the more stable mix of binary compounds and elements. This general synthetic approach to prepare new metastable compounds give control of both the composition and structure of the product compounds. 15 refs., 6 figs., 2 tabs.

Rational synthesis of metastable skutterudite compounds using multilayer precursors

Modulated elemental reactants were used to form amorphous ternary reaction intermediates of the desired compositions. The amorphous reaction intermediate for each system crystallizes exothermically below 200 °C forming kinetically stable “filled” skutterudites of general formula M1-xFe4Sb12 where M = Sn, Al, Ga, In, and Zn. These metastable compounds can only be prepared by controlling the reaction intermediates, avoiding the formation of more thermodynamically stable binary compounds. We propose that the desired compounds nucleate from the amorphous precursor because slow solid-state diffusion rates hinder disproportionation into the more stable mix of binary compounds and elements. If modulated elemental reactants are prepared with layer thicknesses above a critical value (∼40 A), the thermodynamically stable binary compound FeSb2 interfacially nucleates, preventing formation of the desired metastable ternary compounds. The use of low-angle X-ray diffraction and calorimetry provide the data necessary to...

Synthesis of Metastable Post-Transition-Metal Iron Antimony Skutterudites Using the Multilayer Precursor Method

This review focuses on the use of designed reactants, consisting of ultrathin elemental layers sequentially deposited in ultrahigh vacuum, to control reaction pathways and the structure of compounds formed Two reaction pathways are discussed The first uses controlled nucleation of an amorphous intermediate to kinetically access compounds under conditions where they may be thermodynamically unstable The second approach takes advantage of the controlled diffusion lengths in elementally modulated reactants and slow diffusion rates at low temperatures to prepare superlattice and heterostructure compounds with controlled composition and structure

Control of Reaction Pathway and the Nanostructure of Final Products through the Design of Modulated Elemental Reactants

We have synthesized a series of stable and metastable rare earth element−iron antimonides, controlling both composition and structure using multilayer reactants. The stable rare earth iron element antimonides have the filled skutterudite structure (LaFe4P12) with a stoichiometry of 1 for the rare earth element. The metastable rare earth element−iron antimonides could be prepared with the entire sequence of rare earth elements. The lattice parameters of these metastable compounds were always larger than those of the corresponding more stable analog, and the occupancy of the rare earth element cation was found to be less than 1. Reitveld analysis of Lu1-xFe4Sb12 suggests that the late rare earth element compounds have a new distorted filled skutterudite structure. Structural trends are discussed in terms of their potential use as thermoelectric materials.

Systematic Study of New Rare Earth Element−Iron−Antimony Skutterudites Synthesized Using Multilayer Precursors

This paper presents a study of the reaction of thin film niobium/selenium multilayers as a function of layer thickness. Diffraction and calorimetry data show a distinct difference in reactions between films with layer thicknesses above and below 60 A. The thicker films are shown to nucleate NbsSe4 heterogeneously at the niobium-selenium interfaces. Thinner films form kinetically stable amorphous reaction intermediates before crystallizing NbSSe4 homogeneously. The nucleation barrier was measured using a Kissinger analysis demonstrating the kinetic stability of the amorphous intermediate. An energy versus reaction progress diagram is presented that contrasts the two reaction mechanisms. ~~~~

http://liquids.seas.harvard.edu/masa/NB5SE4.pdf

Marc D. Hornbostel

Papers

Rational synthesis of metastable skutterudite compounds using multilayer precursors

Synthesis of Metastable Post-Transition-Metal Iron Antimony Skutterudites Using the Multilayer Precursor Method

Control of Reaction Pathway and the Nanostructure of Final Products through the Design of Modulated Elemental Reactants

Systematic Study of New Rare Earth Element−Iron−Antimony Skutterudites Synthesized Using Multilayer Precursors

Use of Superlattice Structure To Control Reaction Mechanism: Kinetics and Energetics of Nb5Se4 Formation