Epitaxial Stabilization and Persistent Nucleation of the 3C Polymorph of BaSrMnO.

Ba-rich compositions in the BaSrMnO (BSMO) cubic perovskite (3C) system are magnetic ferroelectrics and are of interest for their strong magnetoelectric coupling. Beyond = 0.5, they only form in hexagonal polymorphs.

Grain boundary velocity and curvature are not correlated in Ni polycrystals.

Grain boundary velocity has been believed to be correlated to curvature, and this is an important relationship for modeling how polycrystalline materials coarsen during annealing. We determined the velocities and curvatures of approximately 52,000 grain boundaries in a nickel polycrystal using three-dimensional orientation maps measured by high-energy diffraction microscopy before and after annealing at 800°C. Unexpectedly, the grain boundary velocities and curvatures were uncorrelated.

Influence of pH and Surface Orientation on the Photochemical Reactivity of SrTiO.

The photochemical reactivity of the SrTiO surface is affected by the pH of the surrounding aqueous solution. Scanning electron microscopy and atomic force microscopy have been used to quantify the amount of silver that is photochemically reduced on the surfaces of (100), (110), and (111) oriented crystals as a function of pH. For all orientations, the reactivity increases from pH 3, reaches a maximum, and then decreases at higher pH.

Influence of the Magnitude of Ferroelectric Domain Polarization on the Photochemical Reactivity of BaTiO.

The spontaneous polarization of domains in ferroelectric materials has been used to spatially separate photogenerated electrons and holes, reducing recombination and thereby improving the efficiency of photochemical reactions. Here, the influence of the magnitude of the polarization on photochemical reactivity is investigated. The magnitude of the out-of-plane component of the polarization was characterized by scanning Kelvin probe force microscopy (KFM).

Three-dimensional geometrical and topological characteristics of grains in conventional and grain boundary engineered 316L stainless steel.

The three-dimensional microstructures of a conventional 316L stainless steel and the same material after grain boundary (GB) engineering have been measured by serial sectioning coupled with electron backscatter diffraction mapping. While it is well known that GB engineered materials are differentiated from conventional materials because of the proportion of coincidence site lattice boundaries, the size of their twin-related domains, and their reduced random boundary connectivity, this work provides a quantitative comparison of the geometrical and topological characteristics of grains in 316L stainless steel before and after GB engineering. Specifically, the numbers of grain faces, triple lines, and quadruple unions per grain have been measured and compared.

Enhanced ionic conductivity in electroceramics by nanoscale enrichment of grain boundaries with high solute concentration.

The enhancement of oxygen ionic conductivity by over two orders of magnitude in an electroceramic oxide is explicitly shown to result from nanoscale enrichment of a grain boundary layer or complexion with high solute concentration. A series of CaCeO polycrystalline oxides with fluorite structure and varying nominal Ca solute concentration elucidates how local grain boundary composition, rather than structural grain boundary character, primarily regulates ionic conductivity. A correlation between high grain boundary solute concentration above ∼40 mol%, and four orders of magnitude increase in grain boundary conductivity is explicitly shown.

Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy.

The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains.

Controlling the termination and photochemical reactivity of the SrTiO(110) surface.

SrTiO(110) orientated crystals have been heated to temperatures between 1000 °C and 1200 °C in air, alone or in the presence of powder reservoirs of TiO or SrTiO. In these conditions, the surface is terminated by two types of atomically flat terraces. One has a relatively higher surface potential and promotes the photochemical reduction of silver (it is photocathodic) and the other has a relatively lower surface potential and promotes the photochemical oxidation of lead (it is photoanodic).

Buried Charge at the TiO/SrTiO (111) Interface and Its Effect on Photochemical Reactivity.

High-temperature annealing in air is used to produce SrTiO (111) surfaces with two types of atomically flat terraces: those that promote photoanodic reactions and those that promote photocathodic reactions. Surface potential measurements show that the photocathodic terraces have a relatively more positive surface potential than the photoanodic terraces. After depositing thin TiO films on the surface, from 1 to 13 nm thick, the surface of the film above the photocathodic terraces also has photocathodic properties, similar to those of the bare surface.

Nano-Photoelectrochemical Cell Arrays with Spatially Isolated Oxidation and Reduction Channels.

Photoelectrochemical conversion of solar energy is explored for many diverse applications but suffers from poor efficiencies due to limited solar absorption, inadequate charge carrier separation, redox half-reactions occurring in close proximity, and/or long ion diffusion lengths. We have taken a drastically different approach to the design of photoelectrochemical cells (PECs) to spatially isolate reaction sites at the nanoscale to different materials and flow channels, suppressing carrier recombination and back-reaction of intermediates while shortening ion diffusion paths and, importantly, avoiding mixed product generation. We developed massively parallel nano-PECs composed of an array of open-ended carbon nanotubes (CNTs) with photoanodic reactions occurring on the outer walls, uniformly coated with titanium dioxide (TiO), and photocathodic reactions occurring on the inner walls, decorated with platinum (Pt).

Photocatalysts with internal electric fields.

The photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron-hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p-n junctions, polar surface terminations, and polymorph junctions.

Visible-light photochemical activity of heterostructured core-shell materials composed of selected ternary titanates and ferrites coated by tiO2.

Heterostructured photocatalysts comprised of microcrystalline (mc-) cores and nanostructured (ns-) shells were prepared by the sol-gel method. The ability of titania-coated ATiO3 (A = Fe, Pb) and AFeO3 (A = Bi, La, Y) catalysts to degrade methylene blue in visible light (λ > 420 nm) was compared. The catalysts with the titanate cores had enhanced photocatalytic activities for methylene blue degradation compared to their components alone, whereas the catalysts with ferrite cores did not.

Measuring relative grain-boundary energies in block-copolymer microstructures.

The (relative) energies of symmetric tilt grain boundaries in a strongly segregated lamellar block copolymer are determined by analysis of the dihedral angles at grain-boundary triple junctions. The analysis reveals two regimes: at low and intermediate misorientations (corresponding to a tilt-angle range 0≤θ≤85°) the grain-boundary energy is found to depend on the tilt angle as E(θ)∼θ(x), with 2.5>x≥0.

Enhanced photochemical activity of α-Fe2O3 films supported on SrTiO3 substrates under visible light illumination.

The visible light photochemical reactivity of a 50 nm thick α-Fe(2)O(3)(0001) (hematite) film on a SrTiO(3)(111) substrate is compared to the reactivities of bulk hematite and the same film supported by α-Al(2)O(3)(0001). The hematite film supported by SrTiO(3)(111) is far more reactive then the other two cases.

Effect of crystal and domain orientation on the visible-light photochemical reduction of Ag on BiFeO₃.

The reduction of silver from an aqueous solution on BiFeO₃ surfaces, activated by visible light, was investigated as a function of crystal and ferroelectric domain orientation. When excited by light with energy between 2.53 and 2.

Crystallographic texture in pulsed laser deposited hydroxyapatite bioceramic coatings.

The orientation texture of pulsed laser deposited hydroxyapatite coatings was studied by X-ray diffraction techniques. Increasing the laser energy density of the KrF excimer laser used in the deposition process from 5 to 7 J/cm(2) increases the tendency for the c-axes of the hydroxyapatite grains to be aligned perpendicular to the substrate. This preferred orientation is most pronounced when the incidence direction of the plume is normal to the substrate.