Model catalyst studies with single crystals and epitaxial thin oxide films

The paper describes recent results from our relatively new program to perform detailed studies of the catalytic properties of metal-oxide materials; in particular, to effect a determination of the active catalytic site(s) and the mechanism for reactions over this especially important class of heterogeneous catalysts. Issues of structure-sensitivity, poisoning and promotion, and competing reaction mechanisms are critical questions that need to be addressed in a detailed manner for catalysis by oxides. As just one important example, both surface (Langmui–Hinshelwood) and direct (Eley–Rideal) reaction mechanisms have been proposed for the selective catalytic reduction (SCR) reaction of nitrogen oxides (NO_x) over vanadia/titania catalysts. For this program, we are using a number of unique, state-of-the-art capabilities available in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory; for example, the first molecular beam epitaxy (MBE) system dedicated to the growth of model metal-oxide films, and a unique moderate-pressure catalytic reactor/surface science apparatus. We describe the growth, characterization, and water adsorption properties of a thin Fe₃O₄(0 0 1) film grown on a lattice-matched MgO(0 0 1) substrate. Because our moderate pressure catalysis studies are preliminary at this point, we instead describe our previous results on the CO oxidation reaction over a Ru(0 0 0 1) model catalyst to demonstrate the utility of the experimental approach. We specifically discuss the possibility that this reaction occurs by an Eley–Rideal mechanism.     

Influences of interface morphology and thermally grown oxide thickness on residual stress distribution in thermal barrier coating system

Calculation of residual stress with finite element method is a basic work in failure mechanism investigation in thermal barrier coating (TBC) system because the residual stress is main driving force for crack nucleation and propagation. In this work, a complicated cosine curve with gradually increasing amplitude was used to simulate interface morphologies between layers so as to study the residual stress behavior during the cooling process in air plasma spraying TBC system by finite element method. The substrate, thermally grown oxide (TGO) and top coat (TC) are considered to be elastic and bond coat (BC) elastic-perfectly plastic. The material properties are all temperature dependent. The stress result comparison between models with and without substrate shows the effect of substrate on the residual stress distribution around layers interfaces should not be ignored as the substrate influences the value of normal residual stress as well as the stress distribution along undulating interfaces. Then the model with substrate was used to study the residual stress evolution along interfaces during cooling down from the temperature of 1000 °C to room temperature. The influences of the thickness of TGO and the amplitude and wavelength of interface on the residual stress distributions near interfaces were considered. The results show that these influences are very complicated. Meanwhile, it's found that the hybrid roughness parameter containing information for height and spacing is more suitable to describe the interface complicacy. The results facilitate understanding the failure mechanism relevant to interface morphology and TGO thickness.     

Influence of surface roughness on the semiconducting properties of oxide films formed on 304 stainless steel

Oxide films were formed at 350 °C in borate buffer solution on AISI 304 stainless steel priory abraded with wet SiC paper of different grit size. The films were characterised by Atomic Force Microscopy (AFM) and studied by capacitance, impedance, and photocurrent measurements. The images obtained by AFM microscopy show the evolution of the surface roughness of the films with increasing grit size. Capacitance measurements show that, in all cases, the electronic structure of the films is comparable to that of a p–n heterojunction. This structure is due to the development of space charge layers in the outer iron oxide region at the film/electrolyte interface and in the inner chromium oxide region at the film/metal interface. However, donor and acceptor densities are closely related to the surface roughness via the grit size of the wet SiC paper. An increase in capacitance leading to higher doping densities is manifested by an increase in grit size. These parameters are also frequency dependent. An investigation of the frequency dispersion in relation to the evolution of the surface roughness was performed by analysing the Bode plots through the impedance measurements. The photocurrent results, obtained for the oxide films formed on the different abraded AISI 304, show an increasing photoresponse with decreasing grit size. In spite of this, a constant value of the band gap energy was obtained whatever the oxide film considered. The photocurrent response near the absorption edge is also discussed. The photocurrent response at fixed wavelength and as a function of the applied potential is also influenced by the grit size.     

Microstructural characterization of the oxide scale on nitride bonded SiC-ceramics

The microstructure of the oxide scale formed at 1510 °C by oxidation of silicon nitride-bonded SiC-ceramics was studied by scanning electron microscopy (SEM). Etching by diluted HF etchants was used to help microstructural observation. This method revealed individual cristobalite crystallites as well as interfaces or interlayers between silica and non-silica phases. The ceramics–scale interface was covered by cristobalite crystallites showing that the devitrification of oxide scale begins here. Crystallites grown on SiC were smaller than those grown on binding phase. A thin HF-soluble interlayer was observed between SiC grains and cristobalite. No interlayer was found between cristobalite and oxynitride-type binding phases. The applied etching procedure gave supplementary information on the cracking, too. Partial etching by diluted HF delineated cracks in the scale. These cracks originate in the cristobalite and extend nearly perpendicularly to the substrate through the whole glassy part of the scale.     

Deposition and characterization of epitaxial Ta-doped TiO2 films for ultraviolet photoelectric detectors

Undoped and tantalum-doped titania (TiO₂:Ta) films were synthesized via metalorganic chemical vapor deposition (MOCVD). The crystallization qualities, surface morphologies and optical properties of the deposited films were systematically characterized. The results indicated that the films having low doping levels were epitaxial anatase titania along [001] orientation with high transparency in visible region. The optical band gap could be modulated from 3.38 to 3.52?eV by controlling Ta doping levels. Ultraviolet (UV) photoelectric detectors with metal-semiconductor-metal (MSM) structure were designed and fabricated based on the undoped and Ta-doped films. The maximum spectral response of 32.3?A/W was detected at about 315?nm for the 1% Ta-doped TiO₂ film-based detector. The detectors based on the undoped and 1% Ta-doped TiO₂ films also presented good temporal responses and visible-blind characteristics, showing excellent UV light detection performances.     

The effect of electrolyte anions incorporated into anodic oxide films on the experimental transport numbers of ions

The growth of barrier anodic film is considered theoretically with regard to the migration of three ionic carriers: oxygen and metal ions and electrolyte anions. It is shown that the consideration of anion transport leads to the conclusion that the film grows at three interfaces: the metal/oxide and oxide/electrolyte interfaces and the interface between an oxide layer containing electrolyte anions (contaminated layer) and the oxide layer free of them (“pure” layer). The error in the measured transport numbers of metal and oxygen, which is caused by ignoring a contribution of electrolyte anions to the total charge transport, is maximum in the absence of anion motion.     

Effect of the local structure of Ti-oxide species on the photocatalytic reactivity and photo-induced super-hydrophilic properties of Ti/Si and Ti/B binary oxide thin films

From the results of various spectroscopic investigations of Ti-oxide-based binary oxides, it was found that tetrahedrally coordinated Ti-oxide species are formed in the thin films of Ti/Si binary oxides with low TiO₂ content, while octahedrally coordinated TiO₂ nano-particles are formed in the Ti/B binary oxide thin films, reflecting the effect of the crystalline structures of the host SiO₂ or B₂O₃ on the local structure of the guest Ti-oxide species, respectively. The photocatalytic reactivity of the TiO₂ thin films was found to be remarkably enhanced by the dispersion of the Ti-oxide moiety into both the SiO₂ and B₂O₃ matrices, whereas the photo-induced super-hydrophilic properties of the TiO₂ thin films were enhanced only by a combination or mixing of the Ti-oxide moiety with B₂O₃.     

Correlation between matrix structural order and compressive stress exerted on silicon nanocrystals embedded in silicon-rich silicon oxide

Abstract

Silicon nanocrystals embedded in a silicon oxide matrix were deposited by radio frequency reactive magnetron sputtering. By means of Raman spectroscopy, we have found that a compressive stress is exerted on the silicon nanocrystal cores. The stress varies as a function of silicon concentration in the silicon-rich silicon oxide layers varies, which can be attributed to changes of nanocrystal environment. By conducting the Fourier transform infrared absorption experiments, we have correlated the stresses exerted on the nanocrystal core to the degree of matrix structural order.

PACS

78.67.Bf, 78.67.Pt, 73.63.Bd, 78.47.D, 74.25.Nd     

Orientationally ordered and patterned discotic films and carbon films from liquid crystal precursors

This article demonstrates techniques for fabricating novel organic and carbon films, in which liquid crystal surface anchoring and flow are exploited to precisely control molecular structure (in organic films) or crystal structure (in carbon films). Surface anchoring states were first measured for AR mesophase on spin-coatable organic resins, including commercial polyimide and photoresist. These results were used to develop a lithographic technique for ordering AR surfaces in preprogrammed orientational micropatterns. AR was also processed into radial or star symmetry films by forced spreading combined with edge-on anchoring templates. Additional thin films were prepared from alternative liquid crystalline precursors composed of sulfonated polyaromatic dyes. These disk-like planar molecules undergo massive π-stacking in aqueous solution to form rod-like aggregates. At high concentrations or on surfaces, these rods or molecular columns align by repulsive interactions (lyotropic behavior), giving raise to a transverse alignment of the stacked polyaromatic disks. Here the lyotropic dye indanthrone disulfonate is used to make fully dense ordered carbon films by spin coating or Meyer-bar coating thin films on quartz followed by direct carbonization (without oxidative stabilization). These films exhibit surfaces rich in graphene edge-sites and are either anisotropic unidirectional (by bar coating) or multi-domain with long-range isotropy (by spin coating).     

Mechanism of formation and growth of sunflower-shaped imperfections in anodic oxide films on niobium

Anodizing of niobium has been investigated to develop niobium solid electrolytic capacitors. Chemically polished niobium specimens were anodized in a diluted phosphoric acid solution, initially galvanostatically at i_a = 4 A m⁻² up to E_a = 100 V, and then potentiostatically at E_a = 100 V for t_pa = 43.2 ks. During the galvanostatic anodizing, the anode potential increased almost linearly with time, while, during potentiostatic anodizing, the anodic current decreased up to t_pa = 3.6 ks, and then increased slowly before decreasing again after t_pa = 30.0 ks. Images of FE-SEM and in situ AFM showed that nuclei of imperfections were formed at the ridge of cell structures before t_pa = 3.6 ks. After formation, the imperfection nuclei grew, showing cracking and rolling-up of the anodic oxide film, and crystalline oxide was formed at the center of imperfections after t_pa = 3.6 ks. The growth of imperfections caused increases in the anodic current between t_pa = 3.6 and 30.0 ks. Long-term anodizing caused a coalescence of the imperfections, leading to decreases in the anodic current after t_pa = 30.0 ks. As the imperfections grew, the dielectric dispersion of the anodic oxide films became serious, showing a bias voltage dependence of the parallel equivalent capacitance, C_p, and a dielectric dissipation factor, tan δ. The mechanism of formation and growth of the imperfections, and the correlation between the structure and dielectric properties of anodic oxide films is discussed.     

The crystallographic relationship of heteroepitaxial diamond films on scratched silicon substrates

By using the straight hot filament chemical vapor deposition method with one falt horizonatal filament, diamond films were rapidly grown on a scratched silicon substrate. Observing two kinds of interface structures of the samples by cross-section high-resolution transmission electron microscopy, we found that diamond {111}-oriented films are epitaxially grown on β-SiC{111} planes with a tilt angle of about 7° around the common [110] axis. We also found that diamond771 planes are parallel to silicon111 planes on which diamonds are directly epitaxially grown on silicon substrate. The interface dislocations are of either 60°-type or Schockley partial dislocation in relation to our observations.     

The transfer of stress through a steel to concrete adhesive bond

This paper describes part of a programme of research aimed at investigating the potential for strengthening reinforced concrete beams in shear by means of externally bonded steel plates. This may be a useful strengthening technique following the assessment of older bridge and building structures designed to outdated codes of practice. In order to produce a design guide for such shear plate bonding, a method for determining the anchorage length needs to be devised. By measuring the strain distribution in a steel plate adhesively bonded to a concrete block, the shear stress distribution within the adhesive and the effective anchorage length can be determined. A series of 15 experimental tests have been conducted to investigate the transfer of stress through a steel-concrete adhesive bond. The experimental programme was supported by theoretical and finite element analysis. The shear stress in a steel-concrete adhesive bond was found to be distributed exponentially, peaking at the loaded end of the specimen. For the specimens used, the stress distribution was distributed over a length of up to 155 mm for serviceability loads.     

Growth and structural characteristics of perovskite-wurtzite oxide ceramics thin films

Wurtzite ZnO thin films were grown on single-crystal perovskite SrTiO₃(STO) (1 0 0) substrates at various temperatures. The ZnO/STO thin films thus formed exhibit a preferred (1 1 0)-orientation at a growth temperature of 600-700 °C. A high growth temperature enhances not only the (1 1 0)-texture of ZnO/STO thin films but also the crystalline quality of the film. (La_0.7Sr_0.3)MnO₃ (LSMO) thin films were subsequently grown on ZnO(1 1 0)/STO(1 0 0) substrates with various thicknesses, and were polycrystalline. A thicker LSMO film has a stronger (0 0 l)-preferred orientation than the thinner one. The lattice distortion of LSMO decreases as the LSMO thickness increases. Magnetization vs. temperature curves show that both crystalline quality and lattice distortion influence the magnetic properties of LSMO thin films. The physical properties of the manganite oxide can be modulated by forming a heterostructure with wurtzite ZnO.     

Effect of iodine incorporation on the electrical properties of amorphous conducting carbon films

In this work, the influence of iodine incorporation on the electrical properties of amorphous conducting carbon films, prepared by the vapor phase pyrolysis of maleic anhydride, is reported and discussed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies reveal structural changes in the system. The anomalous behavior in the electrical properties of the intercalated system at low temperatures is investigated. The system shows a positive temperature coefficient of resistance (TCR) at low temperatures, which suggests reasons for the induced ordering of the system at low temperatures with the iodine incorporation. Also, a systematic increase in the conductivity of the sample is observed. The crossover temperature depends on the disorder in the system. The results indicate the possibility of metal-insulator (M-I) transition as a function of preparation temperature, iodine concentration and magnetic field.     

Effect of deposition voltage on the microstructure of electrochemically deposited hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited on Si substrates by electrolysis in a methanol solution at ambient pressure and a low temperature (50 °C), using various deposition voltages. The influence of deposition voltage on the microstructure of the resulting films was analyzed by visible Raman spectroscopy at 514.5 nm and X-ray photoelectron spectroscopy (XPS). The contents of sp³ bonded carbon in the various films were obtained by the curve fitting technique to the C1s peak in the XPS spectra. The hardness and Young’s modulus of the a-C:H films were determined using a nanoindenter. The Raman characteristics suggest an increase of the ratio of sp³/sp² bonded carbon with increasing deposition voltage. The percentage of sp³-bonded carbon is determined as 33–55% obtained from XPS. Corresponding to the increase of sp³/sp², the hardness and Young’s modulus of the films both increase as the deposition voltage increases from 800 V to 1600 V.     

Effect of gas-timing technique on structure and optical properties of sputtered zinc oxide films

Zinc oxide thin films were prepared by the RF magnetron sputtering using a gas-timing technique whereby the flow of argon into the sputtering chamber was controlled by an on–off sequence. With this technique, polycrystalline ZnO thin films on glass substrates have been achieved without any thermal treatment of the substrate. In addition, the RF power and the gas-timing sequence can be fine-tuned to produce the hexagonal structure of ZnO thin films. X-ray diffraction (XRD) measurements confirm a (0 0 2) plane oriented wurtzite structure ZnO thin films. The optimized conditions for this hexagonal structure are an RF power of 30 W and an on–off gas-timing sequence of 50:2 s. The root mean square surface roughness of ZnO thin films measured by atomic force microscopy are in the range of 6.4–11.5 nm. The optical transmittance of ZnO thin films is over 85% in the visible range.     

Effects of the sulfurization temperature on sol gel-processed Cu2ZnSnS4 thin films

As a promising and alternative solar absorber material, the copper–zinc–tin–sulfide compound (Cu₂ZnSnS₄) has been drawing attention in recent years for the production of cheap thin-film solar cells owing to the high natural abundance and non-toxicity of all the constituents, a tunable direct-band-gap energy and a large optical absorption coefficient. In addition, to overcome the problem of expensive vacuum-based methods, solution-based approaches are being developed for Cu₂ZnSnS₄ deposition. In this study, we have produced Cu₂ZnSnS₄ thin films via the sol–gel technique and subsequent sulfurization. The effects of the sulfurization temperature on the structural, morphological, compositional and optical properties of the films were investigated. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. The crystallinity of the films increased with an increasing sulfurization temperature. From the surface images and the results of the composition analysis, it was found that the films are uniform, composed of homogenously distributed grains and have compositions with Cu deficit. The values of the optical absorption coefficients for the films were found to be 10⁴ cm^?1 based on absorbance spectroscopy. The optical band-gap values were estimated to be between 1.32 and 2.27 eV depending on the sulfurization temperature.     

Hydrothermal epitaxy and resultant properties of EuTiO3 films on SrTiO3(001) substrate

We report a novel epitaxial growth of EuTiO₃ films on SrTiO₃(001) substrate by hydrothermal method. The morphological, structural, chemical, and magnetic properties of these epitaxial EuTiO₃ films were examined by scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffractometry, X-ray photoelectron spectroscopy, and superconducting quantum interference device magnetometry, respectively. As-grown EuTiO₃ films with a perovskite structure were found to show an out-of-plane lattice shrinkage and room-temperature ferromagnetism, possibly resulting from an existence of Eu³⁺. Postannealing at 1,000°C could reduce the amount of Eu³⁺, relax the out-of-plane lattice shrinkage, and impact the magnetic properties of the films.

PACS

81.10.Aj; 81.15.-z; 61.05.-a     

A three-dimensional finite-element stress analysis and strength evaluation of stepped-lap adhesive joints subjected to static tensile loadings

Stress distributions in stepped-lap adhesive joints subjected to static tensile loadings are analyzed using three-dimensional finite-element calculations. For establishing an optimum design method of the joints, the effects of the adhesive Young's modulus, adhesive thickness and number of steps on the interface stress distributions are examined. The results show that the maximum value of the maximum principal stress σ₁ occurs at the edge of the adhesive interfaces. The maximum value of the stress σ₁ decreases as the adhesive Young's modulus and number of steps increase and as the adhesive thickness decreases under static loadings. A method for estimating the joint strength under static loadings is proposed using interface stress distributions. For verification of the finite-element method calculations, experiments were carried out to measure the strains and the joint strengths under static loadings. Fairly good agreements were found between the numerical and the experimental results.     

Effects of a magnetic field on growth of porous alumina films on aluminum

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.