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June 2010
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13 Reads
ChemInform
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
October 2007
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27 Reads
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6 Citations
Festkoerperprobleme
Light particles in metals are studied for low temperatures where the nonadiabatic coupling to conduction electrons strongly affects the tunneling between adjacent interstitial sites. Special attention is given to two-level systems formed by trapped hydrogen in niobium. The basic principles governing the tunneling dynamics of such systems are explained and they are applied to determine the dynamic structure factor. The theoretical findings are compared with neutron spectroscopic measurements. These experiments demonstrate a transition from low-temperature coherent tunneling with a well-defined tunneling frequency to hopping with an incoherent tunneling rate at elevated temperatures. The agreement between theory and experiment over a large range of temperature shows the dominant effect of conduction electrons on the motion of light interstitials in metals at low temperatures.
July 2007
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10 Reads
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1 Citation
Quasi-static anelastic experiments on YBa2Cu3Ox (x 6.45, T = 100 and 120 °C) demonstrate a relaxation process with a wide spectrum of activation enthalpies H for the relaxation times (H 1 eV, σH = (0.07 ± 0.01) eV) and an extremely large relaxation strength Δ = 0.75 ± 0.15. The large Δ identifies the process as an oxygen redistribution between differently oriented sites in the CuO plane. The relaxation process characterizes the rate-determining step in long-range oxygen diffusion.
July 2007
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13 Reads
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12 Citations
The local jump diffusion of trapped H in two Nb(OH)x samples was investigated between 10 and 160 K by neutron spectroscopy. The jump rates vary by less than a factor of 7. Below ~ 70 K, they exhibit Kondo's T2K-1 temperature power law for a diffusion process that is controlled by nonadiabatic coupling to conduction electrons. Above ~ 70 K, the diffusion is dominated by the interaction with phonons.
July 2007
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34 Reads
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10 Citations
The self-diffusion coefficient of hydrogen in the dihydrides TiHx and YHx was investigated by neutron spectroscopy (1.59 < x < 2.09, 350 °C < T < 950 °C). The two hydrides differ in that hydrogen in TiHx occupies solely tetrahedral interstitial sites, whereas a small percentage (< 10%) of the hydrogen in YHx is also located on octahedral sites. The measurements demonstrate that these (small) differences in the interstitial occupancies cause a distinctly different concentration dependence of the self-diffusion coefficient. For both investigated systems, the concentration dependence of the self-diffusion coefficient can quantitatively be described with the model assumptions of a concentrated lattice gas.
July 2007
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163 Reads
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10 Citations
The generalized phonon density of states (PDOS) of the shape memory alloy NiTi was measured in three structural phases by means of inelastic neutron scattering for the first time. The PDOS are dominated by two pronounced peaks near 17 meV and at 33 meV. Within experimental accuracy the PDOS of the austenitic and the intermediate phase are identical, but a strong hardening of predominantly acoustic phonons in the martensitic phase is observed. In contrast to model calculations, based on fits to the measured dispersion of acoustic phonons, we find no pronounced gap between the acoustic and the optic modes in the PDOS. Data for the lattice vibrational part of the specific heat as derived from the PDOS are presented.
July 2007
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31 Reads
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1 Citation
The pressure and, thus, lattice-parameter dependence of H diffusion in NbH0.033 was investigated at 297.6 K by neutron spectroscopy. For 5000 bar (0.1% reduced lattice parameter), the diffusion coefficient was found to increase by 1.5% with an experimental accuracy that does not exclude a zero pressure dependence. We performed quantitative theoretical model calculations on the lattice-parameter dependence of the diffusion coefficient. The comparison with the experimental result demonstrates the importance of lattice anharmonicity. We predict a significant pressure-induced decrease of the diffusion coefficient below ~ 200 K.
July 2007
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10 Reads
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18 Citations
We studied by neutron spectroscopy H tunnelling states in Nb below the superconducting transition temperature in both the superconducting and normal-conducting electronic state. Our results prove the direct (nonadiabatic) interaction between tunnelling states and conduction electrons. The electronic coupling parameter is derived independently of the damping (relaxation) of the tunnelling states and of the renormalization of the tunnelling matrix elements.
July 2007
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14 Reads
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3 Citations
The jump rate of a particle in quantum diffusion is theoretically expected to be proportional to the square of the tunnel splitting (or tunnel matrix element). This basic relationship is exemplified in neutron spectroscopic experiments on the tunnelling of trapped-H interstitials in Nb.
January 2007
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3 Reads
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2 Citations
We have performed a mechanical-spectroscopy study on hcp YHx single crystals (x ≤ 0.1, vibrating-reed technique, 280 to 1750 Hz). In agreement with previous data from polycrystalline YHx, we find an anelastic relaxation peak at ~ 280 K. Our single crystals show a large anisotropy in the relaxation strength which demonstrates that this peak results from a Zener relaxation of hydrogen (H) pairs. Two types of H jumps determine the relaxation kinetics. Comparison with previous QNS data identifies the jumps that cause the relaxation, showing further that their jump rates follow, over ten orders of magnitude, an Arrhenius law with an activation energy of (0.60 ± 0.03) eV.
... As a consequence, the resulting hydrogen charging of these steels followed hydrogen ingress that lead to alloy degradation such as in the cases of austenitic stainless steels [14][15][16][17] and high carbon Fe-C austenitic alloys [18] are well documented. In order to understand the mechanisms by which the decomposition of austenitic steels is accomplished, varied interpretations are documented [14,16,[19][20][21][22][23]. Similarly, while there are different ways of introducing hydrogen in a steel material. ...
August 2004
... The opposite behavior of E 0 (R) for the monohydrides of different d-metals (Fig. 7) thus indicates a significant increase in the hydrogen-metal interaction in the series of the 3d-metals Ni→Co→Fe→Mn→Cr and also of the 4d-metals Pd→Rh→Mo. This effect agrees with predictions provided by the first-principles calculations [98] that the reduction in the number of valence electrons from right to left along the transition metal rows in the periodic table should lead to a , γ-CrH [21], γ-MnH 0.41 [25], ε-MnH 0.86 [23], ε′-FeH [94], [29], γ-CoH [30], γ-NiH [95], ε-MoH 1.1 [96], γ-RhH [47], and γ-PdH [50]. For the convenience of comparison, the curves are shifted along the y-axis. ...
December 1997
Polish Journal of Chemistry
... A detailed discussion on the selection of this moderator material is provided by Mehta et al. [1]. Temperature gradients in the moderator result in the self-diffusion of hydrogen in metal hydrides due to Soret diffusion [2]. Accordingly, YH 2−x can transform into YH 2−x±∆ due to the hydrogen migration. ...
January 1989
Zeitschrift für Physikalische Chemie
... These data in general were in good agreement with the results of Einstein-model calculations employing the Morse interaction potential [261,262]. A higher population at low frequencies vibrational modes (<15 meV) was also observed in Fe [263,264] and Ni 3 Fe [263][264][265] nanocrystals, nanocrystalline Ni [266][267][268], Fe [267,269], Pd [270], isolated Fe nanoparticles [271], nanoparticles of hematite (α-Fe 2 O 3 ) [272] and Pb [273], nanocrystals of CuFe [274] using inelastic neutron scattering (INS) from which the time-of-flight spectra can directly give the vibrational DOS. The enhancement in the PhDOS at low energies (below 15 meV) was also observed in nanocrystalline Fe [275] and Sn nanoparticles [276] by synchrotron radiation 12 . ...
August 1998
Physical Review Letters
... Intermetallics have been attracted great attention for researchers due to their potential for combining hardness with low density, high strength, ductility, fracture and required resistance are used in various engineering fields such as batteries and green energy technologies to o find new materials improve the efficiency of energy storage [3]. Intermetallics which have heavy elements give generally modest mass storage capacities (i.e. ...
August 2003
Journal of Alloys and Compounds
... More numerous are the studies of hydrogen composition to determine the maximum hydrogen content, which varies from Nb 3 SnH 1.0 to Nb 3 PtH 5.1 [16]. For the related pair Ti 3 SbH n and Ti 3 IrH n , the maximum values of n are, respectively, ∼3 [17] and 3.8 [18] with Ti 3 IrH n reported to have a two-phase region for n between 0.9 and 2.3. Furthermore, superconductivity is eliminated generally before the maximum hydrogen composition is reached, e.g., in Ti 3 SbH n no superconductivity is observed for n ≥ 1 [19]. ...
August 1990
... 14 With the decrease of the grain size, hydrogen solution in the a phase of PdH becomes bigger, whereas smaller in the b phase of PdH. 14,15 Interestingly, Stuhr et al. made a quite different conclusion that compared with coarse-grained Pd, no change of hydrogen solubility was discovered for nanocrystalline Pd. 16 In addition, hydrogen diffusion in grain boundaries of Pd seems much faster than that within the grains, 8 and is also found to be concentration dependent, i.e., hydrogen diffusion coefficients are lower than single crystalline value at low hydrogen contents, and vice versa for higher contents. 14 It is well known that the PdCu membrane is a kind of excellent candidates for hydrogen separation and purication, and compared with pure Pd and other Pd alloys, PdCu possesses excellent hydrogen selectivity, high thermal stability, superior resistance against poisoning, moderate mechanical properties, relatively low price, and wide operating temperature range. ...
December 1995
Nanostructured Materials
... It then reaches a second pressure P 2 where the sorption curve shows the linear characteristic. Finally we consider a monolayer model since it is stated in the literature [34,35] among many statements that the absorption of hydrogen into the metal compounds forms a chemisorbed hydrided monolayer. The adjustment of the experimental isothermal hydrogen over the alloy LaNi 4.75 Fe 0.25 293 K (Fig. 2), 303 K (Fig. 3) and 313 K (Fig. 4) shows that the model correlates well with the experimental curves. ...
December 2005
Journal of Alloys and Compounds
... The problems of diffusion of hydrogen, of the interaction of hydrogen with other interstitials, the phase diagram of metal-hydrogen systems have been studied by mechanical spectroscopy or internal friction for a long time [10]. During the last decades many papers have been published about hydrogen related anelastic relaxation: Gorsky relaxation in Pd 3 Mn alloys [11], Snoektype relaxation in Nb 0.8 Mo 0.2 [12]. ...
August 2003
Journal of Alloys and Compounds
... The X-ray diffraction patterns of both untreated and pretreated biomass were measured. The powdered samples were dispersed onto a stub and placed within the chamber of an analytical X-ray powder diffractometer (Danilkin et al. 1998). The relative crystallinity indices were calculated according to Eq. 2, [(I(002)-Iam)/I(002)]×100 ...
February 1998
Journal of Alloys and Compounds
