G. Majer

Hydrogen in the mechanically prepared nanostructured graphite

Nanostructured graphite was prepared by mechanical milling under hydrogen atmosphere. Several samples obtained after different milling times were systematically examined to get fundamental information about the structures and hydrogen concentrations. After the expansion of the graphite interlayer, the long-range ordering of the interlayer disappears continuously with increasing milling time. The hydrogen concentration reaches up to 7.4 mass % (CH0.95) after milling for 80 h. Judging from the radial distribution function determined by the neutron diffraction measurement, there are two types of deuterium coordinations: deuterium atoms in the graphite interlayers and that with the CDx covalent bonds, respectively.Nanostructured graphite was prepared by mechanical milling under hydrogen atmosphere. Several samples obtained after different milling times were systematically examined to get fundamental information about the structures and hydrogen concentrations. After the expansion of the graphite interlayer, the long-range ordering of the interlayer disappears continuously with increasing milling time. The hydrogen concentration reaches up to 7.4 mass % (CH0.95) after milling for 80 h. Judging from the radial distribution function determined by the neutron diffraction measurement, there are two types of deuterium coordinations: deuterium atoms in the graphite interlayers and that with the CDx covalent bonds, respectively.

Hydrogen desorption property of mechanically prepared nanostructured graphite

Two desorption peaks of hydrogen molecule (mass number=2), starting at about 600 and 950 K, respectively, are observed in thermal desorption mass spectroscopy of nanostructured graphite mechanically milled for 80 h under hydrogen atmosphere. It follows from a combined analysis of thermal desorption mass spectroscopy and thermogravimetry, that ∼6 mass % of hydrogen (corresponding to 80% of the total amount of hydrogen) is desorbed at the first desorption peak as a mixture of pure hydrogen and hydrocarbons. Below the temperature of the second desorption peak, at which recrystallization related desorption occurs, nanostructured graphite is expected to retain its specific defective structures mainly with carbon dangling bonds as suitable trapping sites for hydrogen storage. The formation process of the nanostructures during milling under hydrogen atmosphere is also discussed on the basis of the profile of Raman spectroscopy.

Hydrogen in mechanically prepared nanostructured h-BN: a critical comparison with that in nanostructured graphite

Nanostructured h-BN was prepared by mechanical milling under hydrogen atmosphere. The hydrogen concentration reaches up to 2.6 mass% after milling for 80 h, and this value corresponds to ca. 35% of that of nanostructured graphite as was previously reported. In addition to the hydrogen desorption starting at about 570 K, nitrogen desorption was also detected at about 700 K. There was no recrystallization phenomenon at least below 1173 K. The dissimilarities on the (de-)hydriding properties between nanostructured h-BN and graphite might be due to the different local electronic structure near the specific defects.

Hydrogen interaction with carbon nanostructures - current situation and future prospects

Recent research on hydrogen in various carbon nanostructures is reviewed. Based on these research activities, we focus on a defect mediated hydrogen sorption in carbon nanostructures. Mechanically prepared nanostructured graphite has been reported to exhibit a specific interaction with hydrogen, probably due to the partial formation of the defect mediated hydrogen sorption. Current situations and future prospects of carbon nanostructures providing hydrogen storage functions are critically, but still positively, described in this paper.

Hydrogen and deuterium diffusion in titanium dihydrides/dideuterides

Abstract The macroscopic diffusivities D of hydrogen in titanium dihydrides. Ti 1 H 1 (1.65≤ x ≤2.02), and of deuterium in titanium dideuterides. Ti 2 H 1 ( x =1.74, 1.98), have been measured by means of pulsed-field-gradient (PFG) nuclear magnetic resonance over wide temperature ranges. The effective activation enthalpy for hydrogen diffusion H a 1 H , obtained by fitting an Arrhenius expression to the diffusivities, increases with increasing hydrogen concentration from H a 1 H = 0.55 eV ( x = 1.65) to H 4 1 H = 0.92 eV ( x = 2.02). The proton spin-lattice relaxation rate Γ 1 , measured on the same samples, reveals also a rapid increase in H a 1 H for x →2. The effective activation enthalpy in the deuterides, H a 2 H = 0.60 eV ( x = 1.74) and H a 2 H = 0.67 eV ( x = 1.98), depends only weakly on x . A comparison of PFG and Γ 1 data indicates that hydrogen atoms jump predominantly between adjacent tetrahedral (T) sites. The observed increase in the effective activation enthalpy with increasing x stands in contradiction to published H a 1 H values based on Γ 1 or NMR line width data. The literature values H a 1 H ≈0.50 eV, which reflect primarily measurements at lower temperature and lower x -values, are nearly independent of the hydrogen concentration x . The present studies show, however, that the concentration dependence of the diffusion parameters of hydrogen in TiH 1 is similar to that in ZrH 1 , where H 4 increases sharply as x approaches the stoichiometric limit. If the sublattice formed by the T-sites is almost filled, additional diffusion processes besides the jumps from occupied to empty T-sites must contribute significantly to the hydrogen diffusivity. Several models for such an additional diffusion mechanism are discussed.

The mechanism of hydrogen diffusion in zirconium dihydrides

Hydrogen diffusion in ZrHx (1.58<or=x<or=1.98) in the temperature range 600 K to 970 K has been measured by means of pulsed-field-gradient nuclear magnetic resonance. The activation enthalpy for hydrogen diffusion, Ha, obtained by fitting an Arrhenius expression D=D0exp(-Ha/kBT) to the diffusivities, increases sharply as x approaches the limiting value of two in good agreement with results deduced from the proton spin-lattice relaxation rate, Gamma 1, measured on the same samples. Hydrogen atoms jump predominantly between nearest-neighbour tetrahedral sites in ZrHx. The observed concentration dependence of both the effective value of H,and the pre-exponential factor D0 suggests, however, that at high hydrogen concentrations and high temperatures another interstitial site is occupied in addition. At x to 2 a small fraction of hydrogen atoms located on an interstitial site other than the tetrahedral site appears to contribute significantly to the diffusivity. The temperature and concentration dependence of the diffusion data can quantitatively be described by such a model. The activation enthalpies for all possible jumps in this system with two different kinds of site are shown to be independent of the hydrogen concentration x. The corresponding attempt frequencies nu a approximately=1013 s-1 are compatible with the picture of a classical diffusion mechanism.

Low-temperature growth of silicon nanotubes and nanowires on amorphous substrates

Silicon one-dimensional (Si 1D) materials are of particular relevance due to their prospect as versatile building materials for nanoelectronic devices. We report the growth of Si 1D structures from quasi-hexagonally ordered gold (Au) nanoparticle (NP) arrays on borosilicate glass (BSG) and SiOx/Si substrates. Using hydrogen instead of oxygen plasma during NP preparation enhances the catalytic activity of AuNPs (diameters of 10-20 nm), enabling Si 1D growth at temperatures as low as 320 degrees C. On BSG, Si nanowires (SiNWs) are identified and reasonable vertical alignment is achieved at 420 degrees C. On SiOx/Si, only Si nanotubes (SiNTs) are obtained right up to 420 degrees C. A mixture of SiNTs and SiNWs is observed at 450 degrees C and only SiNWs grow at 480 degrees C.

NMR studies of hydrogen motion in nanostructured hydrogen–graphite systems

Abstract Nanostructured hydrogen–graphite systems, C nano H x ( x =0.24, 0.31, 0.96), have been characterized by first nuclear magnetic resonance (NMR) measurements. The NMR spectrum of C nano H 0.96 is well represented by the sum of a Lorentzian and a Gaussian line, indicating two types of hydrogen coordinations. These two components may be ascribed to hydrogen in graphite interlayers and hydrogen chemisorbed at dangling bonds. Information on the hydrogen hopping frequencies is provided by the spin–lattice relaxation rate Γ 1 . The temperature dependence of Γ 1 yields high hydrogen diffusivities and low activation energies of E a ≈0.1 eV. A change in the Γ 1 data of C nano H x with x =0.24 and 0.31 occurred after the samples had been heated to about 400–430 K. This suggests that in this temperature range hydrogen atoms start to occupy sites with different site energies, resulting in a distribution of the activation energies for hydrogen motion.

Hydrogen diffusion in metallic and nanostructured materials

The diffusion mechanisms of hydrogen in metallic and nanostructured materials have been studied systematically by different nuclear magnetic resonance techniques. The present paper reviews three examples of our recent work: (i) The hydrogen-stabilized Laves-phase compound C15-HfTi2H4, with rather complex mechanisms of hydrogen diffusion. Long-range diffusion and localized motion coexist on different time scales in this compound. (ii) Nanostructured vanadium-hydrides n-VHx, in which the dynamical properties of hydrogen are fundamentally changed compared to that in a crystalline compound. The diffusion parameters of hydrogen in the grain boundary regions could be determined independently of the hydrogen motion inside the crystalline grains. (iii) Hydrogen in nanostructured hydrogen-graphite-systems n-CHx, where the NMR spectra reveal two types of hydrogen coordinations. The relaxation data indicate high hydrogen mobilities at ambient temperatures.

A pulsed-field-gradient NMR study of hydrogen diffusion in the Laves-phase compounds ZrCr2Hx

Hydrogen diffusivities D in C14-ZrCr2H0.4 and C15-ZrCr2Hx (0.2<or=*<or=0.5) have been measured by means of pulsed-field-gradient nuclear magnetic resonance over the temperature range 130-430 K. In all the samples studied the temperature dependence of D follows an Arrhenius law above 200 K but shows marked deviations from Arrhenius behaviour below about 180 K. This suggests that different diffusion mechanisms dominate at high and low temperatures. The jump length of H atoms estimated by comparing D(T) and the proton spin-lattice relaxation rates appears to be greater than the closest distance between tetrahedral (Zr2Cr2) sites occupied by hydrogen in these compounds. A diffusion model that considers jumps between next-nearest-neighbour (Zr2Cr2) sites via metastable (ZrCr3) sites is discussed.