969 research outputs found
Nanocrystalline and Thin Film Germanium Electrodes with High Lithium Capacity and High Rate Capabilities
Germanium nanocrystals (12 nm mean diam) and amorphous thin films (60-250 nm thick) were prepared as anodes for lithium secondary cells. Amorphous thin film electrodes prepared on planar nickel substrates showed stable capacities of 1700 mAh/g over 60 cycles. Germanium nanocrystals showed reversible gravimetric capacities of up to 1400 mAh/g with 60% capacity retention after 50 cycles. Both electrodes were found to be crystalline in the fully lithiated state. The enhanced capacity, rate capability (1000C), and cycle life of nanophase germanium over bulk crystalline germanium is attributed to the high surface area and short diffusion lengths of the active material and the absence of defects in nanophase materials
Magnetomechanical effects in textured polycrystalline Tb76Dy24
Uniaxial stress-strain measurements were performed on polycrystalline Tb76Dy24 alloys which exhibit "giant magnetostriction" at cryogenic temperatures. The Young's moduli were reduced by up to a factor of five at 77 K, in comparison to their values at 300 K. We attribute this reduction to a mechanical compliance from domain rotation. Large mechanical hysteresis is also found in nominally elastic stress-strain curves measured below the Curie temperature. Hysteretic curves from 0 to 25 MPa demonstrate up to 19% dissipation of the applied mechanical energy. The anisotropy of thermal expansion was also measured and used as a parameter for the degree of crystallographic texture. This anisotropy was correlated to bulk magnetostriction and to mechanical hysteresis
Highly Reversible Lithium Storage in Nanostructured Silicon
Anode materials of nanostructured silicon have been prepared by physical vapor deposition and characterized using electrochemical methods. The electrodes were prepared in thin-film form as nanocrystalline particles (12 nm mean diameter) and as continuous amorphous thin films (100 nm thick). The nanocrystalline silicon exhibited specific capacities of around 1100 mAh/g with a 50% capacity retention after 50 cycles. The amorphous thin-film electrodes exhibited initial capacities of 3500 mAh/g with a stable capacity of 2000 mAh/g over 50 cycles. We suggest that the nanoscale dimensions of the silicon circumvents conventional mechanisms of mechanical deterioration, permitting good cycle life
Magnetostriction of single crystal and polycrystalline Tb0.60Dy0.40 at cryogenic temperatures
At cryogenic temperatures, single crystals of TbDy alloys exhibit giant magnetostrictions of nearly 9000 ppm, making these materials promising for engineering service in cryogenic actuators, valves, and positioners. The preparation of single crystals is difficult and costly. Preliminary results on the magnetostriction of textured polycrystalline materials are presented here. For instance, polycrystalline Tb0.60Dy0.40, plane-rolled (one direction of applied stress) to induce crystallographic texture, has shown magnetostrictions at 77 K of 3000 ppm for an applied field of 4.5 kOe and an applied load of 23 MPa, or 48% that of a single crystal under similar conditions. Comparisons are presented between the magnetostrictive response of plane- and form-rolled (two orthogonal directions of applied stress) polycrystalline Tb0.60Dy0.40 at 10 and 77 K. It is reported that at 10 K plane-rolled Tb0.60Dy0.40 exhibits 1600 ppm magnetostriction at an applied field of 4.4 kOe with a minimal applied load of 0.28 MPa. An observed restoration of the initial unstrained state may be a useful feature of polycrystalline materials for engineering service. Finally it is reported that thermal expansion measurements provide a measure of crystallographic texture for comparison with the magnetostriction
Structure and Thermodynamics of the Mixed Alkali Alanates
The thermodynamics and structural properties of the hexahydride alanates
(M2M'AlH6) with the elpasolite structure have been investigated. A series of
mixed alkali alanates (Na2LiAlH6, K2LiAlH6 and K2NaAlH6) were synthesized and
found to reversibly absorb and desorb hydrogen without the need for a catalyst.
Pressure-composition isotherms were measured to investigate the thermodynamics
of the absorption and desorption reactions with hydrogen. Isotherms for
catalyzed (4 mol% TiCl3) and uncatalyzed Na2LiAlH6 exhibited an increase in
kinetics, but no change in the bulk thermodynamics with the addition of a
dopant. A structural analysis using synchrotron x-ray diffraction showed that
these compounds favor the Fm-3m space group with the smaller ion (M') occupying
an octahedral site. These results demonstrate that appropriate cation
substitutions can be used to stabilize or destabilize the material and may
provide an avenue to improving the unfavorable thermodynamics of a number of
materials with promising gravimetric hydrogen densities.Comment: 6 pages, 7 figures,3 tables, submitted to PR
X-ray absorption study of Ti-activated sodium aluminum hydride
Ti K-edge x-ray absorption near edge spectroscopy (XANES) was used to explore
the Ti valence and coordination in Ti-activated sodium alanate. An empirical
relationship was established between the Ti valence and the Ti K-edge onset
based on a set of standards. This relationship was used to estimate oxidation
states of the titanium catalyst in 2 mol% and 4 mol% Ti-doped NaAlH4. These
results demonstrate that the formal titanium valence is zero in doped sodium
alanate and nearly invariant during hydrogen cycling. A qualitative comparison
of the edge fine structure suggests that the Ti is present on the surface in
the form of amorphous TiAl3.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
A micrometeorological study of the contributions of the individual elements of a community of plants to the momentum, heat and vapour fluxes from that community as a whole
Thermochemistry of Alane Complexes for Hydrogen Storage: A Theoretical and Experimental Comparison
Knowledge of the relative stabilities of alane (AlH3) complexes with electron
donors is essential for identifying hydrogen storage materials for vehicular
applications that can be regenerated by off-board methods; however, almost no
thermodynamic data are available to make this assessment. To fill this gap, we
employed the G4(MP2) method to determine heats of formation, entropies, and
Gibbs free energies of formation for thirty-eight alane complexes with NH3-nRn
(R = Me, Et; n = 0-3), pyridine, pyrazine, triethylenediamine (TEDA),
quinuclidine, OH2-nRn (R = Me, Et; n = 0-2), dioxane, and tetrahydrofuran
(THF). Monomer, bis, and selected dimer complex geometries were considered.
Using these data, we computed the thermodynamics of the key formation and
dehydrogenation reactions that would occur during hydrogen delivery and alane
regeneration, from which trends in complex stability were identified. These
predictions were tested by synthesizing six amine-alane complexes involving
trimethylamine, triethylamine, dimethylethylamine, TEDA, quinuclidine, and
hexamine, and obtaining upper limits of delta G for their formation from
metallic aluminum. Combining these computational and experimental results, we
establish a criterion for complex stability relevant to hydrogen storage that
can be used to assess potential ligands prior to attempting synthesis of the
alane complex. Based on this, we conclude that only a subset of the tertiary
amine complexes considered and none of the ether complexes can be successfully
formed by direct reaction with aluminum and regenerated in an alane-based
hydrogen storage system.Comment: Accepted by the Journal of Physical Chemistry
White Lines and 3d-Occupancy for the 3d Transition-Metal Oxides
Electron energy-loss spectrometry was employed to measure the white lines at
the L23 absorption edges of the 3d transition-metal oxides and lithium
transition-metal oxides. The white-line ratio (L3/L2) was found to increase
between d^0 and d^5 and decrease between d^5 and d^10, consistent with previous
results for the transition metals and their oxides. The intensities of the
white lines, normalized to the post-edge background, are linear for the 3d
transition-metal oxides and lithium transition-metal oxides. An empirical
correlation between normalized white-line intensity and 3d occupancy is
established. It provides a method for measuring changes in the 3d-state
occupancy. As an example, this empirical relationship is used to measure
changes in the transition-metal valences of Li_{1-x}Ni_{0.8}Co_{0.2}O_2 in the
range of 0 < x < 0.64. In these experiments the 3d occupancy of the nickel ion
decreased upon lithium deintercalation, while the cobalt valence remained
constant.Comment: 6 pages, 7 figure
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