Showing papers by "Silke Paschen published in 2014"
TL;DR: The combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, Ba(8)Si(46), identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure.
Abstract: Perfectly crystalline solids are excellent heat conductors. Prominent counterexamples are intermetallic clathrates, guest-host systems with a high potential for thermoelectric applications due to their ultralow thermal conductivities. Our combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$, identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure. Above a critical wave vector, the purely harmonic guest-host interaction leads to a drastic transfer of spectral weight to the guest atoms, corresponding to a localization of the propagative phonons.
99 citations
TL;DR: In this article, single crystals of the intermetallic phase InPd were grown by the Czochralski method and determined their electronic, thermal, magnetic and hydrogen-absorption properties.
32 citations
TL;DR: In this article, the first mixed clathrate-skutterudite unicouple thermoelectric generator was presented, which achieved an average contact resistivity of 16'µΩ'cm2 with Pb-Ag (hot side) and In-Sn (cold side) brazings combined with and intermediate Ni layer.
Abstract: The type I clathrates A8Ga16X30 (A = Sr, Ba, Eu; X = Si, Ge, Sn) and filled skutterudites Ay(Fe1−xMx)4Sb12 (A = rare-earth, M = Co, Ni,) are intermetallic compounds with good thermoelectric performance in the temperature range 400–800 K. Here we report on the first mixed clathrate–skutterudite unicouple thermoelectric generator and address important technical issues such as upscaling, contacting, and electrode material selection. At 600 K, the materials Eu8Ga16Ge30, Ba8Ga16Ge30, and Mm0.7Fe3.5Ni0.5Sb12 synthesized for this purpose have figures of merit ZT = 0.32, 0.15, and 0.46, respectively. The clathrate and skutterudite legs were brazed to a hot side nickel electrode. An average contact resistivity of 16 µΩ cm2 was obtained with Pb–Ag (hot side) and In–Sn (cold side) brazings combined with and intermediate Ni layer. For a temperature difference ΔT = 250 K (hot side temperature = 573 K), this generator produced an open circuit voltage Uopen = 54 mV and an electrical specific power P = 178 mW cm−2 as our best result with the couple Mm0.7Fe3.5Ni0.5Sb12 and Eu8Ga16Ge30.
17 citations
TL;DR: In this article, the authors review the field, with focus on Ce-based systems with temperature-magnetic field or temperature-pressure phase diagrams that may host a quantum critical point.
Abstract: Quantum criticality in cubic heavy fermion compounds remains much less explored than in quasi-two-dimensional systems. However, such materials are needed to broadly test the recently suggested global phase diagram for heavy fermion quantum criticality. Thus, to boost these activities, we review the field, with focus on Ce-based systems with temperature-magnetic field or temperature-pressure phase diagrams that may host a quantum critical point. To date, CeIn3 and Ce3Pd20Si6 are the only two among these compounds where quantum critical behaviour has been systematically investigated. Interestingly, both show Fermi surface reconstructions as function of the magnetic field that may be understood in terms of Kondo destruction quantum criticality.
13 citations
TL;DR: In this paper, the authors review the field, with focus on Ce-based systems with temperature-magnetic field or temperature-pressure phase diagrams that may host a quantum critical point.
Abstract: Quantum criticality in cubic heavy fermion compounds remains much less explored than in quasi-two-dimensional systems. However, such materials are needed to broadly test the recently suggested global phase diagram for heavy fermion quantum criticality. Thus, to boost these activities, we review the field, with focus on Ce-based systems with temperature–magnetic field or temperature–pressure phase diagrams that may host a quantum critical point. To date, CeIn3 and Ce3Pd20Si6 are the only two among these compounds where quantum critical behavior has been systematically investigated. Interestingly, both show Fermi surface reconstructions as function of the magnetic field that may be understood in terms of Kondo destruction quantum criticality.
11 citations
TL;DR: In this article, the authors reported thermopower measurements on single-crystalline intermetallic compounds with temperatures between 2.4 and 650 K along the main crystallographic directions.
Abstract: The intermetallic compound $$\hbox {CeRu}_4\hbox {Sn}_6$$
has been tentatively classified as Kondo insulator. This class of material, especially non-cubic representatives, is not yet fully understood. Here we report thermopower measurements on single-crystalline $$\hbox {CeRu}_4\hbox {Sn}_6$$
between 2 K and 650 K, along the main crystallographic directions. Large positive thermopower is observed in the directions along which the hybridization is strong and a Kondo insulating gap forms. A negative contribution to the thermopower dominates for the crystallographic $$c$$
axis where hybridization is weak and metallicity prevails.
8 citations
TL;DR: In this paper, Ba-Cu-Si clathrate powders were synthesized by mechanical milling using different amounts of process control agent (PCA), and the effects of PCA on the phase constitution and crystallite size of nanopowders using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
Abstract: Nanostructured Ba-Cu-Si clathrate powders were synthesized by mechanical milling using different amounts of process control agent (PCA). We investigated systematically the effects of PCA on the phase constitution and crystallite size of nanopowders using X-ray diffraction (XRD) as well as the particle size and morphology by scanning electron microscopy (SEM). The PCA increases the powder yield by reducing the powder agglomeration. No detectable reaction occurred between the PCA and the clathrate phase, and thus the composition of the clathrate phase is unchanged after milling. Compared to the powders milled without PCA, the crystalline size of powders with PCA is reduced from about 70 to about 50 nm.
7 citations
TL;DR: The cage compound Ba3Ge16Ir4 as discussed by the authors is a Pauli-paramagnetic metal which becomes superconducting below Tc = 5.1 K. The phase was obtained after 10 d at 910 °C with small impurities of clathrate-I, BaGe7Ir2 and α-Ge remaining at the grain boundaries.
Abstract: The cage compound Ba3Ge16Ir4 crystallizes with the Ba3Ge16Rh4 type of crystal structure, which represents a hierarchical derivative of the BaAl4 type. The crystal structure [Pearson symbol tI46, space group I4/mmm; a = 6.5312(2) A, c = 22.2845(6) A] was refined from single-crystal X-ray diffraction data. The phase was obtained after 10 d at 910 °C with small impurities of clathrate-I, BaGe7Ir2 and α-Ge remaining at the grain boundaries. Ba3Ge16Ir4 is a Pauli-paramagnetic metal, which becomes superconducting below Tc = 5.1 K. Electronic structure and analysis of the chemical bonding were performed based on density functional theory calculations. The physical properties are discussed in comparison to the isotypic phase Ba3Ge16Rh4.
6 citations
TL;DR: In this paper, a global phase diagram of antiferromagnetic (AF) heavy fermion metals was proposed and applied to the case of Kondo insulators, and the future prospect to study the interplay among Kondo coherence, magnetism, and topological states.
Abstract: Strongly correlated electron systems at the border of magnetism are of active current interest, particularly because the accompanying quantum criticality provides a route towards both strange-metal non-Fermi liquid behavior and unconventional superconductivity. Among the many important questions is whether the magnetism acts simply as a source of fluctuations in the textbook Landau framework, or instead serves as a proxy for some unexpected new physics. We put into this general context the recent developments on quantum phase transitions in antiferromagnetic (AF) heavy fermion metals. Among these are the extensive recent theoretical and experimental studies on the physics of Kondo destruction in a class of beyond-Landau quantum critical points (QCPs). Also discussed are the theoretical basis for a global phase diagram of AF heavy fermion metals, and the recent surge of materials suitable for studying this phase diagram. Furthermore, we address the generalization of this global phase diagram to the case of Kondo insulators, and consider the future prospect to study the interplay among Kondo coherence, magnetism, and topological states. Finally, we touch upon related issues beyond the AF settings, arising in mixed valent, ferromagnetic, quadrupolar, or spin glass f-electron systems, as well as some general issues on emergent phases near QCPs.
5 citations
TL;DR: In this paper, the crystal growth process of type-I transition metal clathrates was studied in two different regimes: a regime of moderate cooling rate, realized with the floating zone technique, and an regime of ultra rapid cooling, realized by the melt spinning technique.
5 citations
TL;DR: In this paper, the influence of the hot pressing temperature on the thermoelectric properties of a type-I clathrate of nominal composition Ba8Cu4.5Ge35.5Si6.
Abstract: It is well known that the process variables for sample synthesis affect the electrical, magnetic, mechanical, etc. properties of the final product. In this study, we present the influence of the hot pressing temperature on the thermoelectric properties of a type-I clathrate of nominal composition Ba8Cu4.5Ge35.5Si6. The samples are prepared in a high-frequency furnace, annealed, and subject to a long-time ball milling. Finally, the nanopowders from ball milling are hot pressed at different temperatures. All hot pressed samples are characterized by X-ray diffraction, energy dispersive X-ray spectroscope, scanning electron microscopy, and transport property measurements. The results show that lower hot pressing temperatures are desired to prevent the nanopowders from coarsening, but are unfavorable for the densification: Samples hot pressed at lower temperatures have smaller crystallite sizes, larger porosities, and lower relative densities and, consequently, have higher electrical resistivity, and lower thermal conductivity. No significant differences are seen in ZT as the positive effect of reduced thermal conductivity is compensated by the enhanced electrical resistivity. The highest ZT of 0.42 is reached at 873 K for a sample hot pressed at 780 °C.
01 Jan 2014
TL;DR: In this paper, a large body of work focussed on thermoelectric properties of Eu-containing type-I clathrates are generally attributed to the pronounced rattling of EU in oversized host cages and the occurrence of split sites in the larger of the two cages of the structure.
Abstract: Clathrates that contain rare-earth elements as guest atoms have been of active interest since the discovery of intermetallic clathrates A large body of work focussed on thermoelectric properties of Eu-containing clathrates The very low lattice thermal conductivities that are reached in Eu-containing type-I clathrates are generally attributed to the pronounced rattling of Eu in oversized host cages and to the occurrence of split sites in the larger of the two cages of the structure The potential of Eu-containing clathrates for magnetic refrigeration has been recognized more recently Here, key features are the large magnetic moment of Eu, together with the second order character of the paramagnetic to ferromagnetic phase transition The incorporation of other magnetic rare-earth elements into the clathrate cages has long remained elusive Only very recently the successful synthesis of a cerium containing type-I clathrate was reported Interestingly, a sizable enhancement of the thermopower is observed and attributed to a rattling enhanced Kondo interaction This discovery may trigger a wealth of future investigations
TL;DR: Sol–gel processing of the AEAPTS derivative resulted in incorporation of the cyanometallate in silica, and the obtained material is magnetically ordered below 22 K with an effective magnetic moment μeff of 4.46 μB.
Abstract: Compound [Ni(AEAPTS)2]3[Fe(CN)6]2 (AEAPTS=N-(2-aminoethyl)-3-aminopropyltrimethoxysilane), in which Ni2+ and Fe3+ ions are ferromagnetically coupled through cyano bridges, was prepared. Sol–gel processing of the AEAPTS derivative resulted in incorporation of the cyanometallate in silica. The obtained material is magnetically ordered below 22 K with an effective magnetic moment μeff of 4.46 μB at room temperature, a maximum of 8.60 μB at approximately 15 K and a narrow hysteresis at 2 K, with a saturation remanence of about 300 emu mol−1 and a coercitivity of 0.03 T.
16 Jun 2014
TL;DR: In this article, the title compound was prepared from a stoichiometric mixture of BaGe, Ge, and Ir (Ar atmosphere, glassy carbon crucible, 910 °C, 10 d).
Abstract: The title compound is prepared from a stoichiometric mixture of BaGe, Ge, and Ir (Ar atmosphere, glassy carbon crucible, 910 °C, 10 d).
TL;DR: In this paper, the authors reported the successful incorporation of cerium as guest atom into the clathrate crystal structure, which led to a 50% increase in thermopower compared to a rare earth-free reference material.
Abstract: The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities [1]. Here we report the successful incorporation of cerium as guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, via the Kondo interaction, to strong correlation phenomena including the ocurrence of giant thermopowers at low temperatures [2]. Indeed, we observe a 50% enhancement of the thermopower compared to a rare earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a `rattling' enhanced Kondo interaction [3] underlies this effect.
16 Jun 2014