Altaf H. Carim

Symmetry and crystal structure of Ti3SiC2

Convergent beam electron diffraction (CBED) and X-ray diffractometry (XRD) have been used to examine the symmetry, structure and lattice parameters of Ti3SiC2. The CBED analysis confirms that the material is hexagonal with a point group of 6mmm. The XRD diffraction peak positions and intensities are in good agreement with calculations based on previously proposed atomic positions.

Interfacial reaction products and film orientation in YBa 2 Cu 3 O 7− x on zirconia substrates with and without CeO 2 buffer layers

Thick film (1.2 [mu]m) YBCO superconductors grown by pulsed laser deposition on unbuffered and CeO[sub 2]-buffered single crystal (001)-oriented yttria-stabilized zirconia (YSZ) substrates have been investigated. YBCO and YSZ react to form BaZrO[sub 3] (BZO), whereas YBCO and CeO[sub 2] react to form BaCeO[sub 3]. Reaction phases were examined by [theta]-2[theta] and four-circle [ital x]-ray diffraction and high resolution electron microscopy. Three orientation relationships identified for the unbuffered films were: (i) (001)[sub YBCO][parallel](011)[sub BZO][parallel](001)[sub YSZ] with [110][sub YBCO][parallel][100][sub BZO][parallel][100][sub YSZ], (ii) (001)[sub YBCO][parallel](001)[sub BZO][parallel](001)[sub YSZ] with [110][sub YBCO][parallel][100][sub BZO][parallel][100][sub YSZ], and (iii) (001)[sub YBCO][parallel](001)[sub BZO][parallel](001)[sub YSZ] with [100][sub YBCO][parallel][100][sub YSZ]. The results suggest that for films grown at typical deposition temperatures, YBCO epitaxy is established before the interfacial reaction occurs. The presence of BaCeO[sub 3] in buffered films grown at high temperatures (790 [degree]C) was confirmed by [theta]-2[theta] scans and selected area diffraction patterns.

Phase Equilibria in the Ti‐AI‐O System at 945°C and Analysis of Ti/Al2O3 Reactions

Phase relations in the Ti-Al-O system were evaluated experimentally at 945 C. The tie lines were established using equilibrated samples with phase compositions determined by electron probe microanalysis. The phase relations were in agreement with previous estimates but the phase fields of {alpha}-Ti[O,Al] and Ti{sub 3}Al[O] were significantly different. The Ti{sub 3}Al[O] phase has a maximum solubility of 22 at.% O, corresponding to a nominal stoichiometry of Ti{sub 3}AlO, whereas the {alpha}-Ti[O,Al] phase, at a maximum O solubility of 33.33 at.%, has a negligible amount of Al in solution. The disagreement between these results and previous studies was attributed to the differences in experimental techniques for sample preparation and analysis. The reported layer sequences at the Ti/Al{sub 2}O{sub 3} interface were evaluated based on the ternary section and the corresponding O activity diagram. The layered interfaces were found to be stable, with the evolution of the reaction products governed by the thickness of the initial Ti layer and the partial pressure in the ambient.

A TEM study of microstructures of YBa2Cu3O7−x thin films deposited on LaAlO3 by laser ablation

The microstructures of YBa{sub 2}Cu{sub 3}O{sub 7{minus}}{sub {ital x}} thin films deposited by laser ablation on single crystal (001) LaAlO{sub 3} substrates have been investigated. The orientation of the YBa{sub 2}Cu{sub 3}O{sub 7{minus}}{sub {ital x}} layer next to the interface is found to be completely {ital c}-perpendicular, with a high degree of epitaxy between the film and the substrate. Misfit dislocations, with a periodic spacing of around 13 nm, are present at the interface. Two distinct interfacial structures are seen in these films. At a film thickness of around 400 nm, nucleation of {ital c}-parallel grains occurs, leading to a switchover from a {ital c}-perpendicular to a {ital c}-parallel microstructure. Amorphous particulates, ejected from the target during processing, lead to the formation of misoriented grains, giving rise to high-angle grain boundaries in the film.

Morphology, structure, and nucleation of out-of-phase boundaries (OPBs) in epitaxial films of layered oxides

Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-bmisfit, and inclined-cmisfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.

Bismuth volatility effects on the perfection of SrBi2Nb2O9 and SrBi2Ta2O9 films

The volatility of bismuth and bismuth oxide species complicates the growth of phase-pure films of SrBi2Nb2O9 and SrBi2Ta2O9. Films that appear phase-pure by x-ray diffraction can have microstructural defects caused by transient bismuth nonstoichiometry which have a significant impact on properties. Such defects are resolved by transmission electron microscopy. Post-growth loss of bismuth from a slowly cooled SrBi2Ta2O9 film resulted in the generation of a high density of out-of-phase boundaries (OPBs), which are demonstrated to be ferroelectrically inactive. In another film, the difference in the rate of desorption of bismuth oxides from SrTiO3 versus that from SrBi2Nb2O9 led to bismuth enrichment at the film–substrate interface, and the formation of an epitaxial reaction layer in an otherwise stoichiometric SrBi2Nb2O9 film. This different-composition layer would be expected to alter the electrical properties of the film as a whole. These results help explain the scatter in electrical data reported for si...

Suppression of superconductivity by crystallographic defects in epitaxial Sr2RuO4 films

Epitaxial Sr2RuO4 thin films grown by pulsed-laser deposition from high-purity (99.98%) Sr2RuO4 targets on (001) LaAlO3 were found to be not superconducting down to 0.4 K. Structural disorder is believed to be responsible. A correlation was observed between higher resistivity ratios in electrical transport measurements and narrower x-ray diffraction rocking curve widths of the Sr2RuO4 films. High-resolution transmission electron microscopy revealed that the dominant structural defects, i.e., the defects leading to the observed variation in rocking curve widths in the films, are {011} planar defects, with a spacing comparable to the in-plane superconducting coherence length of Sr2RuO4. These results imply that minimizing structural disorder is the key remaining challenge to achieving superconducting Sr2RuO4 films.

Al solubility in M6X compounds in the Ti-Cu-O system

Abstract M6X-type compounds in the Ti-Cu-O system, Ti3Cu3O and Ti4Cu2O, have been identified as reaction products at Al2O3/ Ag-Cu-Ti braze alloy interfaces. These compounds which form as a result of the brazing process usually have other elements in solution, the most predominant being Al released from reduction of Al2O3. Microprobe analysis and X-ray diffraction show that the M6X structure accommodates a maximum of 15 at% Al, corresponding to a stoichiometry of Ti3Cu2AlO. The compositions Ti3Cu2AlO and Ti3Cu3O belong to the same phase field, whereas Ti4Cu2O has an independent phase field with a marginal (1.5 at%) solubility of Al. The Al atoms occupy Wyckoff position c in the Fd3m M6X structure.

Brazing of alumina with Ti4CU2O and Ti3CU3O interlayers

Abstract The joining of alumina with the compounds Ti 4 Cu 2 O and Ti 3 Cu 3 O, which are normally found as reaction products in conventional active metal brazing, has been demonstrated. The final phase assemblages correspond to those expected based on the TiCuO ternary phase diagram. No cavitation is observed at the interfaces in the final microstructure. While the samples are robust under normal handling, many joints failed during preparation of mechanical test specimens and those few that remained exhibited very low four-point bend strengths.

SiCAl4C3 interfaces in aluminum-silicon carbide composites

Abstract In transmission electron microscopy examinations of cast Al matrix-SiC particulate composites, it was noted that a coating of Al4C3 had been formed in contact with some SiC particles. The observed epitaxial orientation relationships at the SiC Al 4 C 3 interface and between the carbides and interfacial Al formed by Al4C3 decomposition are described. The lattice matching between SiC, Al4C3, and reprecipitated Al particles may result in strong bonding and thus permit crack propagation across such interfaces, contributing to the detrimental effects on fracture and fatigue associated with the presence of Al4C3 in aluminum-silicon carbide composites.