L. Rabenberg

High Capacity Surface-Modified LiCoO2 Cathodes for Lithium-Ion Batteries

The surface of LiCoO 2 cathodes was modified with Al 2 O 3 , TiO 2 , and ZrO 2 by a chemical processing procedure followed by heat treatment at 300°C in air for 4 h. The surface-modified LiCoO 2 samples show much better capacity retention at both 25 and 60°C than the unmodified LiCoO 2 cathode to higher cutoff charge voltages of as high as 4.7 V vs. lithium. For example, Al 2 O 3 -modified LiCoO 2 shows approximately 180 mAh/g at 4.5 to 3.2 V with a capacity fade of only 8% in 100 cycles, compared to 32% for the unmodified LiCoO 2 . Transmission electron microscopic studies reveal that the guest materials are present as loose oxides (Al 2 O 3 and ZrO 2 ) or as monolayers (TiO 2 ) on the surface of LiCoO 2 particles. The improved capacity retention and the higher reversible capacity (180 mAh/g) of the surface-modified LiCoO 2 compared to the unmodified LiCoO 2 (140 mAh/g) could be due to a suppression of the chemical and structural instabilities of the charged Li 1 - x CoO 2 cathodes and/or reduction of interparticle stresses and strains.

Electron diffraction and microscopy study of oxygen ordering in YBa sub 2 Cu sub 3 O sub 7 minus. delta

Electron-diffraction techniques have been used to study the ordering of oxygen atoms within the Cu(1)O{sub 1{minus}{delta}} plane of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. The results show that ordering along a single axis and ordering along two orthogonal axes both occur and alternate systematically across the composition range 0.06{lt}{delta}{lt}0.75. Where the ordering is restricted to a single axis, it is found to be the crystallographic {bold a} axis. Various models of oxygen ordering are proposed to explain the observed diffraction phenomena. In addition to the ordering of oxygen atoms that give the diffuse diffraction spots at ({ital h}*/2,0,0) and (0,{ital k}*/2,0), a second type of ordering reflection that occurs at ({ital h}*/3,0,0) and (0,{ital k}*/3,0) was also observed. These sharper and more strongly streaked diffraction maxima are explained in terms of a defect in the cation sublattice.

IN SITU TRANSMISSION ELECTRON MICROSCOPY STUDY OF PLASTIC DEFORMATION IN PASSIVATED AL-CU THIN FILMS

Plastic deformation in passivated Al-1 wt %Cu thin films was studied in situ using a straining device in the transmission electron microscope. Both edge and screw dislocations were found to have caused slip on inclined {111} planes. Multiple slip was frequently observed as two or more sets of intersecting slip traces. Microstructural investigations of both unpassivated and passivated Al-1 wt %Cu films indicate that grain size and encapsulation by passivating layers are major contributors to strength of a thin film with a particular thickness. Additional strengthening is also provided by interactions between dislocations on multiple slip systems. The roles of grain orientation and precipitates in plastic deformation are also discussed.

Enhancement of room temperature magnetoresistance in double perovskite ferrimagnets

The double perovskite Sr2FeMoO6 is a half-metallic ferrimagnet exhibiting significant magnetoresistance (MR) at 300 K in a magnetic field H<2 kOe. A preliminary study of the influence of isovalent and aliovalent substitutions for Sr2+ on the magnitude of the room-temperature MR is reported. Of the polycrystalline samples, Sr1.9A0.1FeMoO6 (A=Ca, Ba, La or Pr), the best result was obtained with the Ba(0.1) sample, which showed a factor of 2 enhancement of the MR over that previously reported for Sr2FeMoO6.

In situ transmission electron microscopy study of plastic deformation and stress-induced voiding in Al–Cu interconnects

Plastic deformation in submicron wide Al-1 wt %Cu interconnects was studied in situ using a straining device in the transmission electron microscope. Dislocation motion occurred readily in unpassivated lines but was nonexistent in passivated lines due to the presence of stiff oxide sidewalls. Instead heterogeneous void nucleation occurred on straining to a critical limit. The void morphology was always near hemispherical and the nucleation always took place at the line edges. Further stretching of the lines led to a rupture of the sidewalls away from the lines, resulting in immediate dislocation motion. Void nucleation, cross slip, and operation of dislocation sources at line edges were all recorded on video. It was noted that dislocations almost parallel to the plane of the lines are rarely observed and furthermore, their movement is sluggish. Based on the dislocation configuration observed in these lines, a generalized geometrical model was arrived at in order to determine the significance of grain orientation on yield stress of passivated lines with columnar, bamboo grains. Frequent occurrence of twinning within the grains indicated that plastic deformation was indeed restricted in confined metal lines.

Evolution of the microstructure of rapidly solidified Nd‐Fe‐B permanent magnets

Nd‐Fe‐B permanent magnets prepared by rapid solidification have been extensively characterized by transmission electron microscopy at all stages of their development. Grains in the as‐spun ribbons range in size up to several micrometers in their largest dimension. Excess Nd is trapped within some of the larger grains during solidification; this excess Nd separates from the lattice during subsequent processing, forming Nd‐rich, coherent, precipitates. During consolidation of the ribbons, the surface oxide films are broken, creating intimate metallurgical bonds and allowing grain growth to occur across the original ribbon interfaces. The bulk of the grains in the final magnet are plate shaped, having a c axis in common and parallel to the axis of die upsetting; these grains are surrounded by the Nd‐rich grain‐boundary phase.

Extended x‐ray‐absorption fine‐structure study of the position of Zr within the unit cell of Sm2Co17

Extended x‐ray‐absorption fine‐structure spectroscopy (EXAFS) has been used to determine the position of Zr within the unit cell of Sm2Co17. Induction‐melted Sm2Co17:Zr ternary alloys, aged at 1180 °C, then quenched, consisted of intimately mixed H2:17 and R2:17 having Zr in solid solution as well some regions of R2:17 that were poor in Zr. EXAFS spectroscopy of these specimens indicates that the most probable position for Zr is a site having 2 Sm near‐neighbor atoms and 11 Co atoms distributed over three different interatomic distances. This is consistent with a direct substitution of Zr for Co in the Co site in the mixed planes (12j in P63/mmc or 18f in R3m).

Superconducting Ba0.6K0.4BiO3 : thin film preparation by RF magnetron sputtering

Superconducting Ba 0.6 K 0.4 BiO 3 thin films with transition temperatures up to 25 K have been successfully grown on SrTiO 3 substrates using RF magnetron sputtering and postgrowth oxygen annealing. Systematic variation of the sputtering process parameters showed that optimum films can be grown on substrates heated to 400 °C in a 10 mTorr atmosphere containing 96% Ar and 4% O 2 and using a target containing Ba, K, and Bi in ratios of 0.6:1.2:1.4. High transition temperature superconductivity was observed only in highly oriented, crystalline films having the ideal stoichiometry, Ba 0.6 K 0.4 BiO 3 .

Crystallographic characterization of GexSi1−x/Si superlattices grown by remote plasma‐enhanced chemical vapor deposition

Single‐crystal GexSi1−x/Si superlattices have been successfully fabricated using remote plasma‐enhanced chemical vapor deposition, a novel low‐temperature thin‐film growth technique. Reflection high‐energy electron diffraction, cross‐sectional transmission electron microscopy (XTEM), plan view TEM, x‐ray‐diffraction, and secondary‐ion mass spectroscopy techniques have been applied to study the crystallographic properties of the superlattice structures. Arrays of dislocation lines, which are either parallel or perpendicular to each other, have been observed in the superlattices for those cases in which the total layer thickness exceeds the critical layer thickness. The location, orientation, and Burgers vectors of the misfit dislocation lines have been analyzed. Possible mechanisms of the generation of the misfit dislocations are also discussed.