A. R. Moodenbaugh

Formation of SEI on Cycled Lithium-Ion Battery Cathodes: Soft X-ray Absorption Study

The formation of a solid electrolyte interface (SEI) on LiNi 0 . 8 5 Co 0 . 1 5 O 2 cathodes from lithium-ion cells cycled at 40 and 70°C was observed and characterized using soft X-ray absorption spectroscopy (XAS). XAS measurements were made in the energy region between 500 and 950 eV, encompassing the Ni and Co L 3 - and L 2 -edges and at the K-edges of O and F. Measurements, obtained in the total electron yield mode, are surface sensitive, probing to a depth of ∼5 nm. XAS at the F K-edge demonstrates the presence of poly(vinylidene fluoride) (PVdF) in addition to LiF on the surface of cycled electrodes. The results indicate that the PVdF in the cycled electrodes is largely intact and that the LiF comes from decomposition of LiPF 6 from the electrolyte. XAS also suggests Fe contamination of cycled cathodes.

Large magnetic entropy change in the metallic antiperovskite Mn3GaC

A large positive magnetic entropy change ΔSM is observed in the metallic antiperovskite Mn3GaC near its first-order metamagnetic transition temperature 159 K where the stoichiometric compound transforms from an antiferromagnetic to a canted ferromagnetic state accompanied by a discontinuous volume change of −0.46% without change of symmetry. The unusual field dependence of the ΔSM of Mn3GaC shows a very rapid linear increase from zero to a saturation value within a field interval smaller than 1 T. The broadening of the peak of ΔSM to low temperature with increasing field change creates an ΔSM plateau in the temperature dependence of the MCE, which is of significance for practical application of these materials in the Ericsson-cycle magnetic refrigerator.

Microstructure and structural defects in MgB2 superconductor

Abstract We report a detailed study of the microstructure and defects in sintered polycrystalline magnesium diboride (MgB2). Both transmission electron microscopy and X-ray data reveal that MgO is the major second-phase in our bulk samples. Although MgB2 and MgO have different crystal symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg and B (or O) and lattice spacings in certain crystallographic orientations are very similar. The size of MgO varies from 10–500 nm, and its mismatch with the MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a Burgers vector of 〈1 0 0〉 . 1/3 〈1 −1 0〉 and 1/3 〈2 1 0〉 partial dislocations and their associated stacking faults were also observed. Since both dislocations and stacking faults are located in the ( 0 0 1 ) basal plane, flux pinning anisotropy is expected. Diffuse scattering analysis suggests that the correlation length along the c-axis for defect-free basal planes is about 50 nm. ( 0 0 1 ) twist grain boundaries (GBs), formed by rotations along the c-axis, are major grain boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and B atoms. An excess of Mg was observed in some grain boundaries. High-resolution nano-probe electron-energy loss spectroscopy reveals that there is a difference in near edge structure of the boron K-edge acquired from GBs and grain interiors. The change at the edge threshold may be suggestive of variation of the hole concentration that would significantly alter boundary superconductivity.

Synthesis and characterization of submicron single-crystalline Bi2Fe4O9 cubes

Single-crystalline, submicron-sized Bi2Fe4O9 cubes of reproducible shape have been successfully prepared using a facile, large-scale solid-state reaction employing a molten salt technique in the presence of a nonionic surfactant. The role of surfactant as well as alterations in the molar ratio of Bi3+ to Fe3+ precursors have been examined under otherwise identical reaction conditions and correlated with the predictive formation of different shapes of Bi2Fe4O9 products. Extensive structural characterization of as-prepared samples has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), Mossbauer spectroscopy, and X-ray diffraction (XRD). Magnetic measurements were obtained using a superconducting quantum interference device (SQUID).

A neutron powder diffraction study of Ba2YCu3O7

Abstract The structure of an orthorhombic sample of Ba2YCu3O7 with a superconducting transition at 89 K has been determined by Rietveld profile analysis of neutron powder diffraction data. The space group chosen was Pmmm (a = 3.817 A , b = 3.882 A and c = 11.671 A ) . The structure may be viewed as a heavily-distorted oxygen-defective ordered derivative of a perovskite-type structure. Two-thirds of the Cu atoms have four near-neighbor oxygens and one more distant neighbor in a tetragonal pyramidal arrangement, and form a two-dimensional network in the ab planes linked through corner shared oxygens about 0.3 A out of the planes. The remaining one-third of the Cu atoms have fourfold planar coordination in the bc planes and form chains linked through corner-shared oxygens along the b axis.

Magnetism and the defect state in the magnetocaloric antiperovskite Mn3GaC1−δ

Magnetic and spectroscopic techniques were used to study the intermetallic antiperovskite Mn3GaC. An antiferromagnetic–ferromagnetic magnetostructural transition at 160 K underlies a remarkable magnetocaloric effect; these phenomena are suppressed in the substoichiometric composition Mn3GaC1−δ. X-ray absorption spectroscopy (XAS) data reported for three compositions Mn3GaC1−δ, δ = 0, 0.10, 0.22, are the basis for drawing inferences concerning the mechanism controlling magnetic order as a function of carbon stoichiometry. While the temperature dependence of the Mn3GaC carbon K edge reveals no observable change across the first-order magnetic transition, a clear splitting of the carbon absorption bands is observed that increases with increasing carbon deficiency. The room temperature Mn and Ga K edges indicate no significant variation with C content. FEFF 8.2 code calculations are in good qualitative agreement with data for the stoichiometric sample, but do not predict the changes in XAS observed in C-deficient samples. These results and the Goodenough–Anderson–Kanamori rules are the basis for a phenomenological model that attributes the carbon content dependence of the low temperature transition to the promotion of weak near-neighbour 90° Mn–Mn pairs in the carbon-deficient compound over the stronger 180° Mn–C–Mn interaction, locking in dominant ferromagnetism at low temperatures.

Inversion of two-band superconductivity at the critical electron doping of (Mg,Al)B-2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.

Alignment and analyses of MnBi∕Bi nanostructures

A Mn5Bi95 alloy was rapidly solidified into a mixture of nanocrystalline Bi and metastable Bi(Mn). Heating the ribbons to temperature T=525K in a dc magnetic field causes formation and c-axis alignment of low-temperature phase (LTP) MnBi nanorods along the applied field direction. Nanorod alignment increases with increased magnetic field, with a calculated alignment half-angle of 47° for a sample heated to 520K at 50kOe. In situ magnetization changes suggest that nanorod alignment is achieved by rotation of MnBi particles. Particle alignment enables the measurement of the MnBi nanorod spin reorientation temperature of 100K, the same as its bulk counterpart.

Neutron-diffraction study of reduced and reoxidized YBa2(Cu0.90Co0.10)3O7 − δ

Abstract Three samples of YBa 2 (Cu 0.90 Co 0.10 ) 3 O 7 − δ were prepared by different heat treatments and studied by both neutron- and high- resolution X-ray diffraction. Standard samples, prepared using sintering temperatures up to 950°C in flowing oxygen, were superconducting with a midpoint near 30 K. About 0.75 formular unit of oxygen was removed by heating in flowing argon near 830°C, producing a non-superconducting reduced material. Reoxidized samples were prepared by heating the latter in flowing oxygen near 400°C. These regained the lost oxygen, but are not superconducting above 5 K. High-resolution synchrotron X-ray diffraction demonstrated the tetragonal structures of the standard and reduced specimens, as well as the orthorhombic structure of the reoxidized material. Portions of each sample were used for neutron powder diffraction studies, and the data were analyzed with the Rietveld technique. Structural parameters, including the Co occupancies on the two copper sites, were refined. The results are evaluated with respect to other neutron-diffraction, X-ray absorption fine structure, and Mossbauer data. The implications of these results in terms of a physical basis for the shift from tetragonal to orthorhombic symmetry are explored. We feel that oxygen reordering in the chain layers is the underlying reason for the lowering of the crystal symmetry.

MnBi nanostructures: Size dependence of magnetostructural transition and matrix templating

Rapidly solidified MnxBi(1−x) (x=0.05 Mn5, x=0.10 Mn10) alloys form as NiAs-type low-temperature phase MnBi nanoparticles in a Bi matrix. Microstructural differences underlie diverse magnetic behavior: Mn5 has well-separated nanorods along the basal plane axial directions of Bi, while Mn10 consists of MnBi nanorods and submicron-sized MnBi grains. The Curie transition of Mn10 is TC=630K with a second-order thermodynamic character, while Mn5 nanorods exhibit a reduced and hysteretic TC=520K. Magnetic field annealing fosters (00n) alignment of the MnBi phase. Mn10 also shows alignment of the Bi matrix by a proposed templating mechanism.