Christina S. Birkel

Silver-Based Intermetallic Heterostructures in Sb2Te3 Thick Films with Enhanced Thermoelectric Power Factors

In this work, Ag(x)Te(y)-Sb(2)Te(3) heterostructured films are prepared by ligand exchange using hydrazine soluble metal chalcogenide. Because of the created interfacial barrier, cold carriers are more strongly scattered than hot ones and thereby an over 50% enhanced thermoelectric power factor (~2 μW/(cm·K(2))) is obtained at 150 °C. This shows the possibility of engineering multiphases to further improve thermoelectric performance beyond phonon scattering through a low-temperature solution processed route.

A Mesoporous Anisotropic n‐Type Bi2Te3 Monolith with Low Thermal Conductivity as an Efficient Thermoelectric Material

A mesoporous Bi_2Te_3 monolith with ≈20% porosity is fabricated by hot-pressing mesoporous powders. The mesostructures are able to transport carriers and scatter phonons efficiently. The reduction (≈60%) of the thermal conductivity is sufficient to compensate for the loss of electrical conductivity perpendicular to the direction of applied pressure, leading to an enhanced zT of 0.7, highest among all reported self-doped, n-type Bi_2Te_3 at similar temperatures.

Phase stability and property evolution of biphasic Ti–Ni–Sn alloys for use in thermoelectric applications

Thermoelectric properties and phase evolution have been studied in biphasic Ti–Ni–Sn materials containing full-Heusler TiNi2Sn embedded within half-Heusler thermoelectric TiNiSn. Materials, prepared by levitation induction melting followed by annealing, were of the nominal starting composition of TiNi1+xSn, with x between 0.00 and 0.25. Phases and microstructure were determined using synchrotron X-ray diffraction and optical and electron microscopy. The full-Heusler phase is observed to be semi-coherent with the half-Heusler majority phase. Differential thermal analysis was performed to determine melting temperatures of the end-member compounds. The thermal conductivity is reduced with the introduction of a dispersed, full-Heusler phase within the half-Heusler material. This leads to an increased thermoelectric figure of merit, ZT, from 0.35 for the stoichiometric compound to 0.44 for TiNi1.15Sn. Beyond x = 0.15 ZT decreases due to a rise in thermal conductivity. Density functional theory calculations usi...

Enhanced thermoelectric properties of bulk TiNiSn via formation of a TiNi2Sn second phase

The effect of phase-segregated Heusler TiNi2Sn on high temperature thermoelectric properties of bulk half-Heusler TiNiSn has been studied. In samples expressed by the composition TiNi1+xSn, a significant decrease in thermal conductivity (between 10% and 30%) is observed for two-phase TiNi1.15Sn, despite the second-phase particles existing at the micrometer scale; a 50% increase in the electrical conductivity is also measured. These result in a maximum figure of merit, ZT, of 0.44 at 800 K, which is 25% greater than is observed for the x = 0 sample. Density functional calculations of TiNiSn and TiNi2Sn suggest that the latter should deplete carriers at 0 K.

Hot carrier filtering in solution processed heterostructures: a paradigm for improving thermoelectric efficiency.

An approach based on a solution-based synthesis that produces a thermally stable Ag/oxide/S₂ Te₃ -Te metal-semiconductor heterostructure is described. With this approach, a figure of merit of zT = 1.0 at 460 K is achieved, a record for a heterostructured material made using wet chemistry. Combining experiments and theory shows that the large increase in the materials Seebeck coefficient results from hot carrier filtering.

Spectroscopic Signature of the Superparamagnetic Transition and Surface Spin Disorder in CoFe2O4 Nanoparticles

Phonons are exquisitely sensitive to finite length scale effects in a wide variety of materials. To investigate confinement in combination with strong magnetoelastic interactions, we measured the infrared vibrational properties of CoFe(2)O(4) nanoparticles and compared our results to trends in the coercivity over the same size range and to the response of the bulk material. Remarkably, the spectroscopic response is sensitive to the size-induced crossover to the superparamagnetic state, which occurs between 7 and 10 nm. A spin-phonon coupling analysis supports the core-shell model. Moreover, it provides an estimate of the magnetically disordered shell thickness, which increases from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that the associated local lattice distortions take place on the length scale of the unit cell. These findings are important for understanding finite length scale effects in this and other magnetic oxides where magnetoelastic interactions are important.

Solution synthesis of nanoparticular binary transition metal antimonides.

The preparation of nanoengineered materials with controlled nanostructures, for example, with an anisotropic phase segregated structure or a regular periodicity rather than with a broad range of interparticle distances, has remained a synthetic challenge for intermetallics. Artificially structured materials, including multilayers, amorphous alloys, quasicrystals, metastable crystalline alloys, or granular metals, are mostly prepared using physical gas phase procedures. We report a novel, powerful solution-mediated approach for the formation of nanoparticular binary antimonides based on presynthesized antimony nanoparticles. The transition metal antimonides M-Sb (M = Co, Ni, Cu(2), Zn) were obtained with sizes ranging from 20 and 60 nm. Through careful control of the reaction conditions, single-phase nanoparticular antimonides were synthesized. The nanophases were investigated by powder X-ray diffraction and (high resolution) electron microscopy. The approach is based on activated metal nanoparticles as precursors for the synthesis of the intermetallic compounds. X-ray powder diffraction studies of reaction intermediates allowed monitoring of the reaction kinetics. The small particle size of the reactants ensures short diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis.

Wet chemical synthesis and a combined x-ray and Mössbauer study of the formation of FeSb2 nanoparticles

Understanding how solids form is a challenging task, and few strategies allow for elucidation of reaction pathways that are useful for designing the synthesis of solids. Here, we report a powerful solution-mediated approach for formation of nanocrystals of the thermoelectrically promising FeSb(2) that uses activated metal nanoparticles as precursors. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. A time- and temperature-dependent study of formation of nanoparticular FeSb(2) by X-ray powder diffraction and iron-57 Mössbauer spectroscopy showed the incipient formation of the binary phase in the temperature range of 200-250 °C.

Non-conventional synthesis and magnetic properties of MAX phases (Cr/Mn)2AlC and (Cr/Fe)2AlC

A few years after the theoretical prediction of magnetic MAX phases, a number of such materials have been experimentally reported, especially in the form of thin films. Yet, due to a relatively small number of studies, we have only just begun to discover the intriguing magnetic properties that are associated with this class of materials. The preparation of bulk MAX phases with later transition metals has been proven to be particularly challenging. Consequentially, there is a great need to develop synthetic strategies to obtain the respective materials in suitable quantities for magnetic investigations. Here, bulk Mn- and Fe-substituted Cr2AlC are prepared using non-conventional synthesis methods such as microwave heating and spark plasma sintering. Synchrotron X-ray diffraction coupled with detailed elemental analyses is used to confirm the successful doping of the MAX phase with the later transition metals as well as to elucidate the microstructure of the obtained dense materials. 57Fe Mossbauer spectroscopy data are presented showing signals of the doped MAX phase and Fe-containing secondary phases. Based on PPMS and SQUID measurements the non-trivial magnetic behavior of the obtained samples is discussed in the context of the existing studies.

Thermoelectric properties of spark-plasma sintered nanoparticular FeSb2 prepared via a solution chemistry approach

Nanoparticular FeSb2 was prepared in solution from cyclopentadienyl iron(ii) dicarbonyl dimer [Fe(Cp(CO)2)]2 and antimony nanoparticles. Spark plasma sintering was used as consolidation method to maintain the particle size. The thermoelectric performance of FeSb2 is limited by its high thermal conductivity. In this work, the thermal conductivity was suppressed by nearly 80% compared to the bulk value by introducing grain boundary scattering of phonons on the nanoscale. The thermoelectric properties of the consolidated FeSb2 emphasize the possibility of altering thermal transport of promising thermoelectric compounds by phonon scattering by engineering the interfaces at the nanoscale.