J. Bai

Measurements and ab initio molecular dynamics simulations of the high temperature ferroelectric transition in hexagonal RMnO3

Measurements of the structure of hexagonal RMnO3 (R=rare earths (Ho) and Y) for temperatures significantly above the ferroelectric transition temperature (TFE) were conducted to determine the nature of the transition. The local and long range structural measurements were complemented by ab initio molecular dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find no large atomic (bond distances or thermal factors), electronic structure changes or rehybridization on crossing TFE from local structural methods. The local symmetry about the Mn sites is preserved. With respect to the local structure about the Ho sites, a reduction of the average Ho-O bond with increased temperature is found. Ab initio molecular dynamics calculations on HoMnO3 reveal the detailed motions of all ions. Above ~900 K there are large displacements of the Ho, O3 and O4 ions along the z-axis which reduce the buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to towards central points between pairs of Ho ions on the z-axis. These structural changes make the coordination of Ho sites more symmetric thus extinguishing the electric polarization. At significantly higher temperatures, rotation of the MnO5 polyhedra occurs without a significant change in electric polarization. The born effective charge tensor is found to be highly anisotropic at the O sites but does not change appreciably at high temperatures.

Structural, magnetic, and transport studies of La0.8MnO3 films

A study of the properties of La0.8MnO3 films of varying thickness was performed. Transport and magnetization measurements show that the ferromagnetic (metallic) volume fraction of the film varies from ∼ 1/4 for ultrathin 60u200au200aA films to ∼ 1/2 for 1600u200au200aA films. Multilength scale structural measurements reveal that near 300u200au200aA, a transition from highly strained ultrathin films to relaxed bulk-like films occurs. The transition region is characterized by low surface roughness, high crystallite orientation, and broad d-spacing distributions.

Structural studies of annealed ultrathin La0.8MnO3 films

A detailed study of the long-range, nanoscale, and local structure of La0.8MnO3 films of varying thickness was performed. These measurements give insight on the relative volumes of the insulating and metallic regions. A thin metallic surface region is found in all films. The nature of the film growth is also discussed.

Observation of Anomalous Phonons in Orthorhombic Rare-earth Manganites

We observe the appearance of a phonon near the lock-in temperature in orthorhombic REMnO3 (RE denotes rare earth) (RE: Lu and Ho) and anomalous phonon hardening in orthorhombic LuMnO3. The anomalous phonon occurs at the onset of spontaneous polarization. No such changes were found in incommensurate orthorhombic DyMnO3. These observations directly reveal different electric polarization mechanisms in the E-type and incommensurate-type orthorhombic REMnO3.

Residual Stress Analysis of Boronized AISI 1018 Steel by Synchrotron Radiation

AISI 1018 steel substrates were powder-pack, diffusion boronized at 850xa0°C for 4xa0h, followed by air quenching. Optical microscopy in conjunction with color etching was used to obtain the average penetration depth of the iron monoboride layer (9xa0μm) and the iron diboride layer (57xa0μm). X-ray diffraction by synchrotron radiation, conducted at the National Synchrotron Light Source in Brookhaven National Laboratory, confirmed the presence of iron monoboride and iron diboride in the boronized plain steel substrates. The sin2xa0ψ technique was employed to calculate the residual stress found in the iron monoboride layer (−237xa0MPa) and in the substrate layer (−150xa0MPa) that is intertwined with the needle-like, iron diboride penetration.

Determination of the order parameter of CuPt-B ordered GaInP2 films by x-ray diffraction

We present quantitative characterization of atomic ordering in semiconductor alloy films by x-ray diffractometry. In particular, we show that the order parameter of CuPt-B ordered GaInP2 films can be determined without measuring the fundamental reflections or examining structural details of the ordered domains. Our method is based on the fact that the ordering peak is modulated by statistical displacements of atom planes, which is a function of the degree of ordering. Therefore, by comparing two or more ordering peaks in an x-ray spectrum, the order parameter of an ordered film can be extracted solely for those regions that are, in fact, ordered. The method can straightforwardly be extended to other ordered alloys.

Promotion of Water-mediated Carbon Removal by Nanostructured Barium Oxide/nickel Interfaces

The existing Ni-yttria-stabilized zirconia anodes in solid oxide fuel cells (SOFCs) perform poorly in carbon-containing fuels because of coking and deactivation at desired operating temperatures. Here we report a new anode with nanostructured barium oxide/nickel (BaO/Ni) interfaces for low-cost SOFCs, demonstrating high power density and stability in C3H8, CO and gasified carbon fuels at 750°C. Synchrotron-based X-ray analyses and microscopy reveal that nanosized BaO islands grow on the Ni surface, creating numerous nanostructured BaO/Ni interfaces that readily adsorb water and facilitate water-mediated carbon removal reactions. Density functional theory calculations predict that the dissociated OH from H2O on BaO reacts with C on Ni near the BaO/Ni interface to produce CO and H species, which are then electrochemically oxidized at the triple-phase boundaries of the anode. This anode offers potential for ushering in a new generation of SOFCs for efficient, low-emission conversion of readily available fuels to electricity.

Scaled Up Pulsed Deposition Technology: Carburization Resistant Ablation Coatings for Ethylene Pyrolysis Coils

Products derived from ethylene have and will continue to replace metallic materials traditionally used for transportation, building materials, and products we use in our everyday lives. As the demand continues to increase, a more suitable material for the outlet coils of ethylene pyrolysis heaters will have to be identified. In this study, we discuss utilization of scaled up pulsed deposition technology to deposit adherent carburization resistant coatings on the inner diameter of ethylene pyrolysis tubing with the intent of extending tube life. Ablation target material selection was based primarily on elevated temperature properties and the ability of the coating to prevent transformation of the inherent protective chromium oxide surface film to metal carbides while in service. The near optimal settings of the processing parameters for pulsed laser deposition of ceramic SiC on heat resistant tubing traditionally used for ethylene service were investigated using a semi quantitative controlled random search methodology. Minimization of the objective function which was based on width, thickness and coverage of the thin film resulted in an optimal deposition time of 4.3 minutes and surface finish of 272 nm.

X-ray characterization of atomic-layer superlattices

We have characterized the structure of AlAs/GaAs atomic-layer superlattices by x-ray diffraction. We show that when the superlattice layers are only a few monolayers (MLs) thick, lateral domains of vertical extent of 1-2 MLs exist. The small layer thickness also magnifies the growth error, leading to periodic compositional stacking faults in the growth direction. As the layer thickness increases, the lateral domain structures tend to behave like interfacial roughness. Growth interruption between successive layers enlarges the lateral domains, but does not remove the vertical stacking faults.

X-ray study of antiphase boundaries in the quadruple-period ordered GaAs0.87Sb0.13 alloy

The x-ray diffraction intensity from antiphase boundaries (APBs) in a quadruple-period ordered GaAsSb alloy is derived introducing statistical probabilities for these APBs and their associated phase shifts. Through experimental fits, we obtain an average distance between the neighboring APBs of approximately 130, 400, and 5 nm along [110], [−110], and [001] directions, respectively. The short distance along the [001] growth direction leads to a narrow streak along [001] in an intensity contour map. In addition, we find that the APBs broaden the ordering peaks, while their effect on the intensity ratio between the ordering peaks is negligible. Static atomic displacements, associated with bond length disparities are also included in the calculations.