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1.
A. S. Fedorov M. V. Serzhantova A. A. Kuzubov 《Journal of Experimental and Theoretical Physics》2008,107(1):126-132
The stability of magnesium hydride (MgH x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ~ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH2. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH2 are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH2. To attain high diffusion rates, the structures of Mg53Sc and Mg53Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg53ScH108 near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH2, while the frequency of jumps in Mg53TiH108 near Ti atoms is higher by approximately a factor of 2.5 × 106. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles. 相似文献
2.
Nuclear magnetic resonance spectroscopy has had extensive applications for the characterization of numerous metal-hydrogen
systems. Although the greatest emphasis of proton NMR has been to evaluate diffusion behavior, increasing attention has been
addressed upon the correlation of proton Knight shifts and the conduction electron contributions to proton spin-lattice relaxation
times to the electronic structure properties of the hydride. The general principles of NMR, that pertain to the usual situations
for most transition metal hydrides, will be briefly reviewed. Several specific examples from some recent research will be
discussed in greater detail. In particular, the roles of host crystal structure and hydrogen site occupancy to hydrogen diffusion
behavior are examined for the Ti1-y
Cu
y
H
x
and Zr1-y
Pd
y
H
x
systems. The proton hyperfine parameters in TiH
x
and ZrH
x
, as well as several related ternary hydrides, are used to qualitatively assess the character of the Fermi level electronic
states. The relationship between the tetragonal distortions of the Ti and Zr dihydrides and a solid-state Jahn-Teller mechanism
will also be examined.
Mound is operated by the Monsanto Research Corporation for the U.S. Department of Energy under Contract No. DE-AC04-76-DP00053. 相似文献
3.
Measurements of the work functions of films of TiHx and ZrHx during the FCC/FCT transition have given the change of Fermi level between these two phases of the hydrides. The results agree with modern calculations of the band structures of these hydrides. 相似文献
4.
We present a calculation of the electronic density of states in TiHx alloys (x=2.0, 1.7, and 1.5) with various degrees of long-range atomic order, η, in the subsystem of hydrogen and vacancies
in the nonstoichiometric hydrides. In comparing the calculated results with the features of the photoelectron spectrum of
TiH1.5 we find agreement for η=0.5. We determine the temperature range for the establishment of atomic vacancy order.
Siberian V. D. Kuznetsov Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii,
Fizika, No. 10, pp. 7–14, October, 1998. 相似文献
5.
Activation energies EA for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by 1H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T1) and rotating (T1p) frames. For the HfHx system both activation energies obtained from T1 data, and the value of T1 at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH2 (where M = metal), there is excellent agreement between activation energies obtained from T1 and T1p data. Mean activation energies obtained were 0.51 eV for TiH1.98 and 0.83 eV for ZrH1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T1 and T1p respectively. 相似文献
6.
Ti40Zr40Ni20 in the form of elemental powders was mechanically alloyed in a planetary ball-mill under argon and hydrogen atmospheres at an acceleration of 55 m s?2 for different time intervals. The samples were analyzed by X-ray diffraction (XRD), vibrating-sample magnetometry (VSM) and scanning electron microscopy (SEM), which revealed the large impact of the ambient gas, i.e. argon and hydrogen, on the magnetic properties, morphology and structure. Namely, during mechanical alloying in argon the saturation magnetization decreased due to the modification of the Ni d-states upon alloying with paramagnetic Ti and Zr, whereas in hydrogen the Ni d-states remained largely unmodified, since the formation of TiH2 and ZrH2 was faster than the alloying with the ferromagnetic nickel. However, after 40 h we obtained a mixture of nanocrystalline Ni and ZrH2/TiH2 hydrides, which in equilibrium contained 1.55 mass% of hydrogen. In the case of argon we determined welding of the Ti40Zr40Ni20 amorphous particles, whereas in hydrogen such a process was suppressed by the brittle ZrH2/TiH2 hydrides. In addition, we revealed that the mechanical alloying of pure Ni powder for 100 h in argon does not affect its magnetic properties. 相似文献
7.
Per?Erik?Vullum Mark?Pitt John?Walmsley Bj?rn?Hauback Randi?Holmestad 《Applied Physics A: Materials Science & Processing》2009,94(4):787-793
Transmission electron microscopy has been used to study ball milled and H cycled NaAlH4 with 10 mol% TiCl3. Isolated from the main phases in this hydrogen storage system, nanocrystalline aggregates of fcc TiH x (0≤x<0.67) were found. The value of x was determined based on the assumption of a linear increase of the TiH x lattice parameter by increasing H content. The size of the TiH x crystallites was in the range 10 to 20 nm, and the lattice parameter decreased from 4.22 Å in TiH0.67 to 4.10 Å in pure fcc Ti. Non-equilibrium ball milling and subsequent H cycling in combination with a small crystallite size are believed to make the TiH x phase stable. The present results are the first observations of fcc TiH x with low hydrogen content, and the measured fcc lattice parameter of Ti matches first-principles calculations. 相似文献
8.
NMR pulse techniques have been used to measure proton relaxation times in high purity β-TiFeH1.03 between 94K and 413K. Recent crystal structure parameters from neutron diffraction studies of orthorhombic β-TiFeDx have been explicitly included in the analysis of the T2 relaxation times to obtain more accurate hydrogen diffusion parameters than were possible from previous NMR studies of β-TiFeHx. The room temperature (300K) hydrogen diffusion constant in β-TiFeHx is verified to be about 1.2 ± 0.8 × 10?12cm2/s, which is 10?4–10?5times smaller than in many other metal hydrides such as PdHx or LaNi5Hx. 相似文献
9.
The electronic structure of TiH2 has been studied using the augmented-plane-wave method and the LCAO interpolation. The density of states and its orbital components show that the conduction band is Ti d-like and that the valence band is largely derived from the hydrogen orbitals with small Ti 3d hybridization. The electronic charges on the hydrogen atom are ~ 1.5 as compared to 1.6–1.7 of the rare-earth metal hydrides. 相似文献
10.
FeTi is an interesting hydrogen storage material which has to be activated at ≈670 K for the absorption of hydrogen. We review critically the great number of previously published results and models on this activation process and emphasize the controversial points. To eliminate the controversy we analysed the variation of the surface composition of FeTi upon activation in the high-pressure cell of a photoelectron spectrometer. The initially passivating surface oxide is shown to be converted into a mixture of TiO2 and Fe by surface segregation and chemical reduction. No evidence for the formation of Fe2Ti4O x , FeTiO x , and TiH x is found. H2/D2 exchange reactions show that H2 dissociates rapidly on Fe and FeTi, but not on TiO2. The surface of FeTi is activated easily at 670 K. Difficulties encountered with the initial hydrogen absorption by virgin high purity FeTi are probably related to bulk (H diffusion, fracture toughness) rather than surface properties. 相似文献
11.
S. N. Nemnonov V. M. Cherkashenko V. N. Kozhanov A. V. Skripov É. Z. Kurmaev E. P. Romanov 《Physics of the Solid State》2006,48(1):30-36
The TiKβ5 spectra and the electronic structure of HfTi2Hx hydrides (x = 0, 0.5, 3.0, 4.0, 4.5, 5.95) are investigated. The location of hydrogen atoms in the structure of the HfTi2H
x
hydrides is determined, and the hydrogen occupancies of the positions e and g are calculated. The inference is made that the hydrogen atoms occupy only the e positions at a low hydrogen content up to x = 3.0 (this is confirmed by the experimental crystallographic data) and the positions e and g at a higher hydrogen content. 相似文献
12.
Calculations are made of the electronic structures of TiH0.73 and TiH hydrides having a NaCl crystal structure and TiH1.5, TiH0.73, and TiH having a CaF2 structure. Estimates of their superconductivity temperature indicate that this should be significantly higher for the stoichiometric
NaCl phase compared to nonstoichiometric phases. The results are compared with the characteristics of the χ phase of titanium
hydride TiH0.73 which undergoes a superconducting transition at 4.3 K.
Fiz. Tverd. Tela (St. Petersburg) 40, 195–197 (February 1998) 相似文献
13.
A. Budziak M. ?urekJ. ?ukrowski M. Ba?andaA. Pacyna M. Czapla 《Journal of magnetism and magnetic materials》2012,324(5):735-741
The HoMn2 compound crystallizes in the cubic C15 or hexagonal C14 Laves phases depending on preparation. The effect of hydrogen absorption on structural and magnetic properties of HoMn2Hx hydrides for the C14 phase has been investigated by XRD and AC/DC magnetometry in the temperature ranges of 75-380 K and 4-390 K, respectively. In addition to general features revealed by RMn2Hx compounds (R=rare earth or Yttrium), unusual behavior of these hydrides was found. In particular, a transformation from the hexagonal to the monoclinic structure was detected, the same as that observed for cubic HoMn2Hx compounds. The structural transformations are correlated to the magnetic behavior. The presented results are compared mainly with the properties of the cubic HoMn2Hx hydrides as well as with those of other RMn2Hx hydrides. Tentative magnetic and structural phase diagrams are proposed. 相似文献
14.
I. P. Chernov Yu. M. Koroteev O. V. Gimranova Yu. I. Tyurin 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2007,1(2):186-191
Ab initio calculations of the electronic structure of pure Pd, pure Ti, and PdHx and TiHx (x = 1, 2, 3) systems are performed within the local density approximation. It is found that the electronic subsystem of metals containing dissolved hydrogen increases their capacity to absorb the energy of electromagnetic radiation and accumulate it for a longer time than pure metals. These two factors promote the nonequilibrium migration of hydrogen atoms and their release from metals upon exposure to ionizing radiation. 相似文献
15.
Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti1?yVyHx in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration. 相似文献
16.
A proton NMR spin echo study of the system HfV2Hx and ZrV2Hx (0 ? x ? 4.5) hydrides are reported. The T1 spin lattice relaxation rate enables us to extract the activation energy for diffusion, Ea, as well as the attempt frequency v0; both depend strongly on the hydrogen concentration. The existence of a correlation between Ea and v0 in these and other hydride systems yield the temperature dependence of the activation energy. 相似文献
17.
NMR measurements of proton rotating-frame relaxation times were made to deduce hydrogen diffusion parameters for crystalline and amorphous Zr2PdHx. Enhanced proton mobility was observed in glassy a-Zr2PdH2.9 relative to diffusion behavior in any crystalline Zr2PdHx sample. Both host crystal structure and proton site occupancy make significant contributions to the diffusion process where smaller activation energies correspond to jumps involving octahedral sites. 相似文献
18.
Andrea Di Vita 《Central European Journal of Physics》2012,10(1):86-95
Metal hydrides are solutions of hydrogen in a metal, where phase transitions may occur depending on temperature, pressure
etc. We apply Le Chatelier’s principle of thermodynamics to a particular phase transition in TiH
x
, which can approximately be described as a second-order phase transition. We show that the fluctuations of the order parameter
correspond to fluctuations both of the density of H+ ions and of the distance between adjacent H+ ions. Moreover, as the system approaches the transition and the correlation radius increases, we show -with the help of statistical
mechanics-that the statistical weight of modes involving a large number of H+ ions (‘collective modes’) increases sharply, in spite of the fact that the Boltzmann factor of each collective mode is exponentially
small. As a result, the interaction of the H+ ions with collective modes makes a tiny suprathermal fraction of the H+ population appear. Our results hold for similar transitions in metal deuterides, too. A violation of an -insofar undisputed-upper
bound on hydrogen loading follows. 相似文献
19.
Shigenobu Hayashi Kikuko Hayamizu Osamu Yamamoto 《Journal of Physics and Chemistry of Solids》1983,44(6):601-606
The behavior of hydrogen in the hydrides of vanadium metal containing small amounts of nickel was studied by 1H NMR spectroscopy. FT spectra and spin-spin and spin-lattice relaxation times for VH0.77, V0.95Ni0.05H0.73, and V0.90Ni0.10H0.65 were measured at temperatures between 77 and 400 K. On the addition of nickel the number of hydrogen atoms on O22 sites decreases and the superstructure of hydrogen is altered. Different effects of nickel on hydrogen diffusion are observed above and below about 200 K, and, therefore, the mechanism of hydrogen diffusion is assumed to change at this temperature. 相似文献
20.
Jean Senegas Michel Pezat Jean Pierre Darnaudery Bernard Darriet 《Journal of Physics and Chemistry of Solids》1981,42(1):29-35
Wide line and pulsed NMR studies are reported for MgH2; and Mg2NiH4 hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH2, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgHx, (x<0.04).Mg2NiH4 shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T1 and T2 relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10?8 < D < 4.6 × 10?8 cm2/s. 相似文献