Publications (13)18.36 Total impact
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Article: An Alternative Approach to Improve the Thermoelectric Properties of Half-Heusler Compounds
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ABSTRACT: In this report an alternative approach for optimization of the thermoelectric properties of half-Heusler compounds is presented. The common approaches are partial substitution of elements by elements of nearby groups and substitution with homologs. In this approach we substitute one element by one neighboring element with fewer valence electrons and by one with more electrons. The amounts of the substitutions are chosen such that the amount of deficiency and excess electrons are compensated. In the solid solution TiCox(Ni0.5Fe0.5)1-xSb\hbox{TiCo}_{x}(\hbox{Ni}_{0.5}\hbox{Fe}_{0.5})_{1-x}\hbox{Sb}, Co was substituted equally by Fe and Ni. The aim of the substitution was to improve the figure of merit by a reduction of the thermal conductivity accompanied by an unchanged high Seebeck coefficient. The solid solution TiCox(Ni0.5Fe0.5)1-xSb\hbox{TiCo}_{x}(\hbox{Ni}_{0.5}\hbox{Fe}_{0.5})_{1-x}\hbox{Sb} was synthesized by arc-melting. The structure of the as-cast samples was analyzed by x-ray diffraction. Rietveld refinements yielded the C1bC1_b structure type with a small amount of antisite disorder between Co and Sb. The thermoelectric properties of the solid solution were investigated in the temperature range from 2K to 400K. A Seebeck coefficient of -260mVK-1-260\,\mu\hbox{V\,K}^{-1} at 400K and a reduction of the thermal conductivity to 3Wm-1K-13\,\hbox{Wm}^{-1}\,\hbox{K}^{-1} were measured. The figure of merit was enhanced by a factor of about seven to a value of 0.04at 400K for TiCo0.8(Ni0.1Fe0.1)Sb\hbox{TiCo}_{0.8}(\hbox{Ni}_{0.1}\hbox{Fe}_{0.1})\hbox{Sb}. KeywordsHeusler compounds–thermal conductivity–thermoelectricsJournal of Electronic Materials 04/2012; 40(5):702-706. · 1.47 Impact Factor -
Article: Anomalous transport properties of the half-metallic ferromagnets Co₂TiSi, Co₂TiGe and Co₂TiSn.
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ABSTRACT: In this work, the theoretical and experimental investigations of Co₂TiZ (Z=Si, Ge or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfil the Slater-Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic-like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting-like behaviour. A large negative magnetoresistance (MR) of 55 per cent is observed for Co₂TiSn at room temperature in an applied magnetic field of μ(0)H=4T, which is comparable to the large negative MRs of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is -52 μVK(-1) for Co₂TiSn, which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations.Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 09/2011; 369(1951):3588-601. · 2.77 Impact Factor -
Dataset: Epitaxial growth and thermoelectric properties of TiNiSn and Zr 0.5 Hf 0.5 NiSn thin films
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ABSTRACT: Thermoelectric Thin films Multilayers TiNiSn Zr 0.5 Hf 0.5 NiSn Due to their exceptional thermoelectric properties Half-Heusler alloys like MNiSn (M = Ti,Zr,Hf) have moved into focus. The growth of single crystalline thin film TiNiSn and Zr 0.5 Hf 0.5 NiSn by dc magnetron sputtering is reported. Seebeck and resistivity measurements were performed and their dependence on epitaxial quality is shown. Seebeck coefficient, specific resistivity and power factor for Zr 0.5 Hf 0.5 NiSn at room temperature were measured to be 63 μV K − 1 , 14.1 μΩ m and 0.28 mW K − 2 m − 1 , respectively. Multilayers of TiNiSn and Zr 0.5 Hf 0.5 NiSn are promising candidates to increase the thermoelectric figure-of-merit by decreasing thermal conductivity perpendicular to the interfaces. The epitaxial growth of multilayers containing TiNiSn and Zr 0.5 Hf 0.5 NiSn is demonstrated by measuring satellite peaks in the X-ray diffraction pattern originating from the additional symmetry perpendicular to the film surface. -
Article: Itinerant half-metallic ferromagnets Co(2)TiZ (Z=Si, Ge, Sn): Ab initio calculations and measurement of the electronic structure and transport properties
Physical Review B 02/2010; 81(6). · 3.69 Impact Factor -
Article: Phase-separation-induced changes in the magnetic and transport properties of the quaternary Heusler alloy Co2Mn1-xTixSn
Physical Review B 01/2010; 82(10). · 3.69 Impact Factor -
Article: Tuning the carrier concentration for thermoelectrical application in the quaternary Heusler compound Co2TiAl(1 − x)Six
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ABSTRACT: The family of half-metallic ferromagnets Co2TiZ exhibits exceptional transport properties. The investigated compounds Co2TiAl(1−x)Six (x = 0.25, 0.5, 0.75) show Curie temperatures (TCs) that vary between 250 and 350 K, depending on the composition. Above TC the Seebeck coefficient remains constant. This makes them promising candidates for thermoelectric devices such as thermocouples with a tunable working range. The electrical resistivity data show an anomaly at TC which is attributed to changes in the electronic structure and therefore in the carrier concentration.Scripta Materialia. 01/2010; 63(9):925-928. -
Article: Investigation of the Thermoelectric Properties of the Series TiCo1–xNixSnxSb1–x
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ABSTRACT: The effect of the simultaneous substitution of cobalt by nickel and antimony by tin in the solid solution TiCo1–xNixSnxSb1–x was systematically investigated. The number of valence electrons does not change by this substitution and therefore the resistivity stays semimetallic or semiconducting. The series were synthesized by arcmelting and the thermoelectric properties were determined. It was found out that the substitution of cobalt and antimony by nickel and tin reduces the thermal conductivity to 2 W·m–1·K–1 at 400 K. The reduction is caused by titanium rich prolate micro structures that were found by energy dispersive X-ray spectroscopy investigations. The Seebeck coefficient and the resistivity increase with increasing amounts of cobalt and antimony. The highest Power factor was found for the composition TiCo0.8Ni0.2Sn0.8Sb0.2 at 900 K with the value of 9.2 μW·cm–1·K–1.Zeitschrift für anorganische Chemie 12/2009; 636(1):132 - 136. -
Article: Seebeck coefficients of half-metallic ferromagnets
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ABSTRACT: In this article the Co2 based Heusler compounds are discussed as potential materials for spin voltage generation. The compounds were synthesized by arc melting and consequent annealing. Band structure calculations were performed which revealed the compounds to be half-metallic ferromagnets. Magnetometry was performed on the samples and the Curie temperatures and the magnetic moments were determined. The Seebeck coefficients were measured from low to ambient temperatures for all compounds. For selected compounds high temperature measurements up to 900 K were performed.Solid State Communications 11/2009; 150(11-12):529-532. · 1.65 Impact Factor -
Article: Thermoelectric properties of CoTiSb based compounds
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ABSTRACT: Several CoTiSb based compounds were synthesized and investigated on their thermoelectric properties. The aim was to improve the thermoelectric properties of CoTiSb by the systematic substitution of atoms or the introduction of additional Co into the vacant sublattice. The solid solutions Co1+xTiSb, Co1−yCuyTiSb and CoTiSb1−zBiz were synthesized. X-ray diffraction was used to investigate the crystal structure. The resistivity, the Seebeck coefficient and the thermal conductivity were determined for all compounds in the temperature range from 2 to 400 K. The highest figure of merit for each solid solution is presented. We were able to improve the figure of merit by a factor of approximately seven.Journal of Physics D Applied Physics 08/2009; 42(18):185401. · 2.54 Impact Factor -
Article: Anomalous transport properties of the halfmetallic ferromagnets Co2TiSi, Co2TiGe, and Co2TiSn
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ABSTRACT: In this work the theoretical and experimental investigations of Co2TiZ (Z = Si, Ge, or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfill the Slater-Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting like behavior. A large negative magnetoresistance of 55% is observed for Co2TiSn at room temperature in an applied magnetic field of 4T which is comparable to the large negative magnetoresistances of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is -52muV/K m for Co2TiSn which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations. Comment: 4 pages 4 figures07/2009; -
Article: A spatially resolved investigation of the local, micro-magnetic domain structure of single and polycrystalline Co2FeSi
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ABSTRACT: The Heusler compound Co2FeSi is a promising material for magneto-electronic devices. With a Curie temperature of 1100 K and a saturation magnetization of 6 Bohr magnetons and a high spin polarization at the Fermi edge it fulfils the essential requirements for magnetic sensors or spin valve structures. An essential feature for such devices is the micro-magnetic domain structure. X-ray magnetic circular dichroism–photo emission electron microscopy has been used for a direct observation of the domain structure of single- and polycrystalline samples. The polycrystalline material exhibits a micro-magnetic ripple structure, as it is well known for pure Co and other polycrystalline Heusler compounds. The (1 1 0)-oriented surface of the single crystal exhibits a multi-domain pattern characteristic for systems with an easy axis that might point out of the surface. Asymmetric 180° Bloch walls with changing sense of rotation are observed between oppositely magnetized domains. Spin polarized photo emission from a single domain of the single crystal shows a spin polarization of 16% at the Fermi energy and up to 35% in the d-bands, at room temperature.Journal of Physics D Applied Physics 03/2007; 40(6):1570. · 2.54 Impact Factor -
Article: Bulk sensitive photo emission spectroscopy of C1b compounds
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ABSTRACT: This work reports about bulk-sensitive, high energy photoelectron spectroscopy from the valence band of CoTiSb excited by photons from 1.2 to 5 keV energy. The high energy photoelectron spectra were taken at the KMC-1 high energy beamline of BESSY II employing the recently developed Phoibos 225 HV analyser. The measurements show a good agreement to calculations of the electronic structure using the LDA scheme. It is shown that the high energy spectra reveal the bulk electronic structure better compared to low energy XPS spectra.10/2006; -
Article: Phase separation in the quaternary Heusler compound CoTi(1−x)MnxSb – A reduction in the thermal conductivity for thermoelectric applications
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ABSTRACT: We investigate the phase separation of the solid solution CoTi(1−x)MnxSb into the two Heusler compounds CoTiSb and CoMnSb. Energy-dispersive X-ray spectroscopy measurements on the two-phase material reveal the presence of size- and shape-tunable CoTiSb regions in a CoMnSb matrix. We demonstrate that the formed phase and grain boundaries have a considerable influence on the phonon scattering processes, which leads to a reduction in the thermal conductivity by a factor of three compared to single-phase CoTiSb.Scripta Materialia.
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Institutions
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2006–2012
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Johannes Gutenberg-Universität Mainz
- Institute of Inorganic and Analytical Chemistry
Mainz, Rhineland-Palatinate, Germany
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