Atom Probe Tomography of a Cu-Doped TiNiSn Thermoelectric Material: Nanoscale Structure and Optimization of Analysis Conditions.

Cu-doping and crystallographic site occupations within the half-Heusler (HH) TiNiSn, a promising thermoelectric material, have been examined by atom probe tomography. In particular, this investigation aims to better understand the influence of atom probe analysis conditions on the measured chemical composition. Under a voltage-pulsing mode, atomic planes are clearly resolved and suggest an arrangement of elements in-line with the expected HH (F-43m space group) crystal structure. The Cu dopant is also distributed uniformly throughout the bulk material. For operation under laser-pulsed modes, the returned composition is highly dependent on the selected laser energy, with high energies resulting in the measurement of excessively high absolute Ti counts at the expense of Sn and in particular Ni. High laser energies also appear to be correlated with the detection of a high fraction of partial hits, indicating nonideal evaporation behavior. The possible mechanisms for these trends are discussed, along with suggestions for optimal analysis conditions for these and similar thermoelectric materials.

Facile Surfactant-Free Synthesis of p-Type SnSe Nanoplates with Exceptional Thermoelectric Power Factors.

A surfactant-free solution methodology, simply using water as a solvent, has been developed for the straightforward synthesis of single-phase orthorhombic SnSe nanoplates in gram quantities. Individual nanoplates are composed of {100} surfaces with {011} edge facets. Hot-pressed nanostructured compacts (Eg ≈0.85 eV) exhibit excellent electrical conductivity and thermoelectric power factors (S(2) σ) at 550 K. S(2) σ values are 8-fold higher than equivalent materials prepared using citric acid as a structure-directing agent, and electrical properties are comparable to the best-performing, extrinsically doped p-type polycrystalline tin selenides. The method offers an energy-efficient, rapid route to p-type SnSe nanostructures.

Toward New Thermoelectrics: Tin Selenide/Modified Graphene Oxide Nanocomposites.

New nanocomposites have been prepared by combining tin selenide (SnSe) with graphene oxide (GO) in a simple aqueous solution process followed by ice templating (freeze casting). The resulting integration of SnSe within the GO matrix leads to modifications of electrical transport properties and the possibility of influencing the power factor (S 2σ). Moreover, these transport properties can then be further improved (S, σ increased) by functionalization of the GO surface to form modified nanocomposites (SnSe/GOmod) with enhanced power factors in comparison to unmodified nanocomposites (SnSe/GO) and "bare" SnSe itself. Functionalizing the GO by reaction with octadecyltrimethoxysilane (C21H46O3Si) and triethylamine ((CH3CH2)3N) switches SnSe from p-type to n-type conductivity with an appreciable Seebeck coefficient and high electrical conductivity (1257 S·m-1 at 539 K), yielding a 20-fold increase in the power factor compared to SnSe itself, prepared by the same route. These findings present new possibilities to design inexpensive and porous nanocomposites based on metal chalcogenides and functionalized carbon-derived matrices.

Topotactic anion-exchange in thermoelectric nanostructured layered tin chalcogenides with reduced selenium content.

Anion exchange has been performed with nanoplates of tin sulfide (SnS) via "soft chemical" organic-free solution syntheses to yield layered pseudo-ternary tin chalcogenides on a 10 g-scale. SnS undergoes a topotactic transformation to form a series of S-substituted tin selenide (SnSe) nano/micro-plates with tuneable chalcogenide composition. SnS0.1Se0.9 nanoplates were spark plasma sintered into phase-pure, textured, dense pellets, the ZT of which has been significantly enhanced to ≈1.16 from ≈0.74 at 923 K via microstructure texturing control. These approaches provide versatile, scalable and low-cost routes to p-type layered tin chalcogenides with controllable composition and competitive thermoelectric performance.

Large-Scale Surfactant-Free Synthesis of p-Type SnTe Nanoparticles for Thermoelectric Applications.

A facile one-pot aqueous solution method has been developed for the fast and straightforward synthesis of SnTe nanoparticles in more than ten gram quantities per batch. The synthesis involves boiling an alkaline Na2SnO2 solution and a NaHTe solution for short time scales, in which the NaOH concentration and reaction duration play vital roles in controlling the phase purity and particle size, respectively. Spark plasma sintering of the SnTe nanoparticles produces nanostructured compacts that have a comparable thermoelectric performance to bulk counterparts synthesised by more time- and energy-intensive methods. This approach, combining an energy-efficient, surfactant-free solution synthesis with spark plasma sintering, provides a simple, rapid, and inexpensive route to p-type SnTe nanostructured materials.

Effect of Spark Plasma Sintering on the Structure and Properties of Ti1-xZrxNiSn Half-Heusler Alloys.

XNiSn (X = Ti, Zr and Hf) half-Heusler alloys have promising thermoelectric properties and are attracting enormous interest for use in waste heat recovery. In particular, multiphase behaviour has been linked to reduced lattice thermal conductivities, which enables improved energy conversion efficiencies. This manuscript describes the impact of spark plasma sintering (SPS) on the phase distributions and thermoelectric properties of Ti0.5Zr0.5NiSn based half-Heuslers. Rietveld analysis reveals small changes in composition, while measurement of the Seebeck coefficient and electrical resistivities reveals that all SPS treated samples are electron doped compared to the as-prepared samples. The lattice thermal conductivities fall between 4 W·m-1·K-1 at 350 K and 3 W·m-1·K-1 at 740 K. A maximum ZT = 0.7 at 740 K is observed in a sample with nominal Ti0.5Zr0.5NiSn composition.