Development of high catalytic activity disordered hydrogen-storage alloys for electrochemical application in nickel–metal hydride batterie

Multi-element, multiphase disordered metal hydride alloys have enabled the widespread commercialization of nickel–metal hydride (NiMH) batteries by allowing high capacity and good kinetics while overcoming the crucial barrier of unstable oxidation/corrosion behavior to obtain long cycle life. Alloy-formula optimization and advanced materials processing have been used to promote a high concentration of active hydrogen-storage sites vital for raising NiMH specific energy. New commercial applications demand fundamentally higher specific power and discharge-rate kinetics. Disorder at the metal/electrolyte interface has enabled a surface oxide with less than 70 ? metallic nickel alloy inclusions suspended within the oxide, which provide exceptional catalytic activity to the metal hydride electrode surface. Received: 14 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Cubic BN Sintered with A1 under High Temperature and High Pressure

Sintering of cubic boron nitride （cBN） with addition of A1 is carried out in the temperature range 1300-1500℃ and under the pressure 5.5 GPa. When sintered at 1300℃, a weak diffractive peak of hexagonal BN （hBN） is observed in the Al-cBN sample, indicating the transformation from cBN to hBN. No nitrides or borides of A1 are observed, which indicated that A1 does not react with cBN obviously. When the sintering temperature is increased to 1400℃, the diffractive peak of hBN disappears and new phases of A1N and A1B2 are observed, due to reactions between A1 and cBN. When the sintering temperature is further increased to 1500℃, the contents of A1N and A1B2 phases increase and the A1 phase disappears completely.     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Dynamics of gold cluster systems

x surface of Si(111) wafers was studied as a function of annealing time at relatively low annealing temperatures (50 and 100 °C) by application of scanning force microscopy. A fast increase in cluster diameter and height as a function of annealing time was detected. After more than 3 h of annealing depletion zones and nucleation exclusion zones were observed. Received: 13 August 1998 / Published online: 10 February 1999     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Theory of spectral hole burning for the study of ultrafast electron dynamics in metal nanoparticles

In order to study the ultrafast relaxation dynamics of surface plasmon excitation in metal nanoparticles in the presence of inhomogeneous line broadening and investigate the influence of the reduced dimensions on the dephasing time T₂ in the size regime below about 10 nm, we have recently demonstrated a novel technique based on persistent spectral hole burning [1]. Here, we describe a theoretical model that has been developed for evaluation of the experimental data and precise determination of T₂ for particles of different size and shape. Comparison of the model to experimental data for Ag nanoparticles on sapphire shows that the theoretical treatment does not only reproduce the shape of the generated holes but also the dependence of their widths on the applied laser fluence. As a result, we have a reliable and versatile tool at hand making possible systematic studies of the ultrafast electron dynamics in small metal particles, and the dependence of the femtosecond dephasing time on their size, shape and surrounding dielectric. Received: 12 September 2001 / Published online: 15 October 2001     

Experimental and theoretical studies of instabilities of metal nanoparticles: a new kind of quasimelting

In this work we report experimental and theoretical studies of structural instabilities of gold nanoparticles supported on a carbon substrate using high-resolution transmission microscopy and molecular dynamics simulations. It is shown that particles undergo structural fluctuations in which a particle exhibits a change in orientation whilst maintaining the overall structure. These fluctuations are of a different kind than the ones reported in previous work, in which transitions between different structures or the appearance, movement or vanishing of twin boundaries are observed. Computer calculations were performed using a model that included both the particle and the substrate. It is found that during the fluctuations there is a collective displacement of the atoms at the interface between the gold and the carbon substrate that results in a larger contact area. Calculations of particle stability that include the interaction with the carbon substrate show that different orientations of the particle have similar energies. As a result, during the fluctuations the particle becomes trapped between different orientations of the same structure. This kind of phenomenon can be considered as a new kind of quasimelting. Received: 26 September 2000 / Accepted: 27 January 2001 / Published online: 23 May 2001     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

Carbon nanotubes formed in graphite after mechanical grinding and thermal annealing

Multi-walled carbon nanotubes with cylindrical and bamboo-type structures are produced in a graphite sample after mechanical milling at ambient temperature and subsequent thermal annealing up to 1400 °C. The ball milling produces a precursor structure and the thermal annealing activates the nanotube growth. Different nanotubular structures indicate different formation mechanisms: multi-wall cylindrical carbon nanotubes are probably formed upon micropores and the bamboo tubes are produced because of the metal catalysts. A two-dimensional growth governed by surface diffusion is believed to be one important factor for the nanotube growth. A potential industrial production method is demonstrated with advantages of large production quantity and low cost. Received: 17 May 2002 / Accepted: 12 September 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8338, E-mail: ying.chen@anu.edu.au     

Nanotube growth during annealing of mechanically milled Boron

Boron powder, finely ground in a tungsten carbide ball mill in an ammonia atmosphere, has been annealed at 1200 °C in flowing nitrogen to produce small quantities of cylindrical BN nanotubes, both as isolated individuals and grouped into ropes. Thick-walled conical BN tubes are abundant in specimens annealed for longer times, and their growth was catalysed once WC debris was converted into W metal particles. Some catalytic effect of small W nanoparticles could be necessary for nanotube formation, though no tip particles have been imaged here. Given the low temperature of mechanical milling and annealing, BN growth must involve surface diffusion and solid-state reconfiguration. It could be possible to engineer desirable physical and chemical properties by exploiting the variation in cylindrical versus conical BN structures as a function of annealing time. Received: 19 December 2001 / Accepted: 3 April 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8253, E-mail: john.fitzgerald@anu.edu.au     

EXAFS study on ultrafine Ni-Co-B amorphous catalysts

\chem{\chemvar{A}C_{60}} with , , . From the temperature dependent line shift of we have extracted information about the electron density distribution on the molecules. The data also support the existence of an like bonding type between the molecules. The magnetic phase transition observed at low temperature in and is monitored by an increase of the second moment as well as an increase of the spin lattice relaxation rates of the and alkali nuclei. The results are compared to NMR data of where no signs of a magnetic ordering are found. The magnetically ordered phases are discussed in terms of antiferromagnetic spin density wave. We also report on NMR measurements of orthorhombic one dimensional and rhombohedral two dimensional polymers of obtained under high pressure. By spinning the samples up to , we are able to identify two resonances at and for the orthorhombic polymer and six resonances at , , , , and for the rhombohedral one. The static distortion of the molecules induced by the transformation under pressure must be at the origin of the observed inequivalent carbons in both systems. The NMR lineshape simulations of the obtained spectra are compatible with the suggested polymeric structures where the molecules are connected by cycloadditions. Received: 10 October 1996/Accepted: 13 November 1996     

Deposition and diffusion of size-selected (Ag400+) clusters on a stepped graphite surface

400 clusters on a stepped graphite surface by a combination of scanning electron microscopy experiments and computer simulations (molecular dynamics and Monte Carlo methods). We find that the shape of the clusters is only partially deformed by the impact with the surface, moreover the clusters do not create surface defects upon landing, and so are able to diffuse freely over the surface. Many clusters are found to become trapped at surface steps, where their mobility is reduced by the higher binding energy. Exploring the 1-D diffusion of clusters along the steps reveals the low mobility for larger islands, as well as the importance of defects on the step (for example kinks), which trap the mobile clusters. Received: 9 April 1998/Accepted: 25 August 1998     

Kinetics of nanocrystallization in FeCoNbB(Cu) alloys

HITPERM alloys (FeCoMBCu; M=Nb, Zr, Hf...) have been recently developed and proposed as competitive soft magnetic materials for high-temperature applications. To our knowledge, this work contains the first results on nanocrystallization isothermal kinetics for these alloys. Analysis of nanocrystallization of the studied FeCoNbB(Cu) alloys in the frame of the Johnson–Mehl–Avrami theory shows a slowing-down of the kinetics and anomalously low values of the Avrami exponent, in a similar way to that reported for FeSiBNbCu (FINEMET)-type alloys. Compositional effects of Co substitution and Cu addition are considered. A more realistic kinetic model developed by Hermann et al. accounts for the experimental data. Received: 14 November 2001 / Accepted: 24 June 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-95/461-2097, E-mail: conde@us.es     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn     

A Raman study of polymerised C60

60 are reported. Although prepared according to different routes the Raman spectra of the two polymeric phases of C₆₀ show a quantitative agreement with respect to mode positions and intensity. We conclude from this that both materials have the same structure at least in the short range order, i.e. the same type of bonding and co-ordination between neighbouring C₆₀ molecules. An investigation of the time dependence of the thermal decomposition of high pressure polymerised C₆₀ is also presented. The rate of decomposition of the polymeric phase is found to be multi-exponential at all temperatures investigated. From an Arrhenius-type analysis of the short time data and the long time data, respectively, the activation energy for thermal dissociation of polymeric bonds was found to increase with time. Received: 20 September 1996/Accepted: 11 November 1996     

NMR study of the magnetic properties of the polymerized phase of Cs1C60

\chem{Cs_{1}C_{60}} . The NMR spectrum allows, when the Magic Angle Spinning (MAS) technique is used, to get information about the electronic structure of this phase, which points out that the spin density is not uniformly distributed on the ball. An estimation of the strength of the quadrupolar effects on is then given, which shows that it is possible to analyse the spin-lattice relaxation time in terms of magnetic effects. A comparison with the of is then justified and allows us to point out the importance of low dimensional antiferromagnetic fluctuations in this compound. We also investigate the nature of the magnetic ground state in : the broadening of the spectra states unambiguously its magnetic nature and a detailed analysis of their shape gives further indications on the antiferromagnetic nature of the magnetic order, which seems characterized by a large amount of disorder. Received: 19 November 1996/Accepted: 19 December 1996     

The phase diagram of charge- and spin-density waves in polymeric C60

\chem{(C_{60})_{\chemindex{x}}} at interfullerene distances larger than . At smaller ball separation, a special charge-density-wave ground state occurs, which exhibits polarized spheres. At around experimental bond distances, both CDW and SDW are present with rather small amplitudes. Received: 13 September 1996/Accepted: 11 November 1996     

Do Mie plasmons have a longer lifetime on resonance than off resonance?

The dynamics of laser-excited electrons in elliptically shaped silver nano-particles is studied using time-resolved two-photon photo-emission. By changing the polarization of the laser pulse at a fixed frequency, a different dynamics is observed near and far from resonance. To analyze these data, the effect of radiation damping on Mie plasmons of Ag nano-particles is studied as a function of laser frequency and polarization. It is shown that the off-resonance lifetime can be shorter or longer than the lifetime at resonance, depending on the incidence conditions of the laser light and on the depolarization factors characterizing the optical response of the nano-particles. Received: 28 August 2001 / Published online: 15 October 2001     

High-quality GaN nanowires synthesized using a CVD approach

High-quality GaN nanowires were synthesized on a large-area Si substrate by direct reaction of gallium with ammonia using InCl₃ as a catalyst. The morphology and microstructure of the resulting products were characterized using a field-emission scanning electron microscope (SEM), a high-resolution transmission electron microscope, and X-ray diffraction (XRD). XRD and electron diffraction revealed that the nanowires are of a hexagonal GaN phase with the wurtzite structure. The SEM study showed that the nanowires are straight and have a smooth morphology with lengths up to 500 μm. The present results reveal that InCl₃ is an optimal catalyst in GaN nanowire production. Received: 2 April 2002 / Accepted: 12 April 2002 / Published online: 19 July 2002     

The formation and thermostability of MgO and MgAl2O4 nanoparticles in oxidized SiC particle-reinforced Al-Mg composites

Interfacial reactions and their products in oxidized SiC particle-reinforced Al-Mg matrix composites were investigated using X-ray diffraction and Field EmissionScanning Electron Microscopy (FE-SEM). Observation of the interfacial reaction between oxidized SiC particles and aluminum alloys containing Mg showed that nanoparticles of MgO form initially and do not change form when more than 4 wt. % Mg is in the matrix. However, MgO transforms into octahedral MgAl₂O₄ crystals when less than 2 wt. % Mg is in the matrix .Comparison of the amounts and the sizes of the reaction products MgAl₂O₄ and MgO between the Al-Mg alloyswith different matrix compositions shows that fewer MgAl₂O₄ crystals form at the surface of the particles in the 2014Al matrix composite than in the Al-2 wt. % Mg (Al-2Mg) matrix composite. Also, the size of MgAl₂O₄ in the former composite is greater than that of the latter composite under the same conditions. However, the amount and the size of MgO crystals that form in the Al-4 wt. % Mg (Al-4Mg) matrix composite is almost the same as that of the Al-8 wt. % Mg (Al-8Mg) composite, and the size of MgO changes a little during heat-treatment at elevated temperatures. The amount of the reaction product (either MgO or MgAl₂O₄) depends on nucleation rates and density of nucleation sites on the oxidized SiC particles at the initial reaction. The more completely the nuclei cover the surface of the oxidized SiC particles, the smaller the resulting size. According to the results, an addition of Mg into the matrix can be used to control the interfacial characteristics in the oxidized SiC/Al composites. Received: 25 January 2001 / Accepted: 26 January 2001 / Published online: 23 May 2001