bluuebunny/crossref_metadata_2025 huggingface dataset:
Electroreduction of Nitrate at Copper Electrodes and Copper-PANI Composite Layers
Authors: Christophe J. , Tsakova V. , Buess-Herman C.
Published: September 2007

The electroreduction of nitrate ions is investigated in acid and neutral aqueous solutions (HClO4 and NaClO4 as electrolytes) at polycrystalline copper electrodes, copper single crystals and at copper particles deposited in polyaniline (PANI) layers. In the presence of low nitrate concentrations (5 mM), the reduction of nitrate is not significantly different on various copper atomic surface structures but is greatly dependent on the local pH at the electrode. In contrast to strong acidic solutions, two separate waves are detected when nitrate ions are present in neutral solutions irrespective of the use of copper polycrystalline or single crystal electrodes. The first of the two waves leads to the formation of nitrite ions. When copper particles are dispersed in polyaniline layers it is demonstrated that the electrocatalytic activity is strongly dependent on the way of depositing copper in the polymer layer. A clear difference is observed in the current response in absence and presence of nitrate ions for copper deposited in the reduced state of the PANI layer, whereas copper deposited in the oxidized state of the PANI layer remains still electrocatalytically rather inactive. Copper crystalline species act effectively for the investigated reaction only if copper conducting paths are available through the polymer matrix up to the underlying metal surface.
A Way for Determining the Effective Three Phase Boundary Width of Solid State Electrochemical Reactions from the Primary and Secondary Current Distribution at Microelectrodes
Authors: Fleig Jürgen
Published: September 2007

A way is suggested for determining the effective three phase boundary width of electrodes on solid electrolytes from the difference between the electrolyte resistance of the primary and the secondary current distribution at a circular microelectrode. It is shown by means of numerical finite element calculations how the potential distribution in the solid electrolyte depends on the frequency and how the corresponding variation of the electrolyte resistance can lead to an additional semicircle in the complex impedance plane. An equivalent circuit is introduced for analyzing such a situation and thus for determining the additional ohmic resistance due to current constriction close to the three phase boundary region of the electrode. This additional resistance is quantitatively related to the effective three phase boundary width and thus can serve as a tool to estimate the size of the electrochemically active region of electrode reactions such as oxygen reduction or hydrogen oxidation on oxide ion conducting solid electrolytes.
Electrochemical Nucleation and Growth Kinetics of Nanostructured Gold on Rotating Disc and Stationary Electrodes
Authors: Yevtushenko Oleksandra , Piskarov Vladyslav , Natter Harald , Hempelmann Rolf
Published: September 2007

The kinetics of the electrocrystallization of nanostructured gold is investigated on both rotating disc and stationary glassy carbon electrodes. A possible mechanism of gold deposition is presented. A general expression of the current-time transient for the superposition of adsorption, three-dimensional nucleation and mixed charge transfer/diffusion controlled growth based on existing theories is reported for the kinetics on stationary electrodes. The experimentally recorded current-time transients are analyzed in terms of this model. Kinetic parameters are determined and their consequence is discussed.
Theoretical Trends in Particle Size Effects for the Oxygen Reduction Reaction
Authors: Greeley J. , Rossmeisl J. , Hellmann A. , Norskov J. K.
Published: September 2007

A simple, first principles-based model of the oxygen reduction reaction (ORR) is used to determine ORR kinetics on the (111), (100), and (211) facets of eleven transition metals (Au, Ag, Pt, Pd, Ir, Cu, Rh, Ni, Ru, Co, Fe). For most metals, the unreconstructed (100) facets are found to have an activity comparable to, or slightly higher than, the (111) facets. In contrast, (211) steps are found to be significantly less active than the terraces, with the exception of the most noble metals. These results are combined with simple models of the geometries of catalytic nanoparticles to estimate the average ORR activity of Pt and Au nanoparticles of various sizes. On Pt, a modest decrease in the activity with decreasing particle size is predicted, while for Au, the opposite trend is found.
Metal Deposition by Inducing a Microgalvanic Cell with the Scanning Electrochemical Microscope (SECM)
Authors: Radtke Valentin , Heß Christian , Heinze Jürgen
Published: September 2007

A new mechanism of metal deposition on conducting surfaces is presented. But in contrast to former procedures where metal ions were deposited by electrolysis on a conducting cathodically polarised surface, now the deposition occurs without any additional electrochemical energy even if the substrate metal is more noble than the deposited metal. This phenomenon can be explained by the formation of a microgalvanic cell as a consequence of the tip reaction. The process is similar to the effects of a local element well known in corrosion science. The latter one is an undesirable effect due to surface impurities whereas the former one can be targeted on generating microstructures. In this paper, we will show that the mechanism of deposition is congenerous to that one of the positive feedback, since both may occur on a nonpolarised conducting surface. Both are “tip-induced” electrochemical cells with a positive electromotive force.
Confined Spatio-Temporal Chaos During Metal Electrodissolution: Simulations
Authors: Birzu Adrian , Krischer Katharina
Published: September 2007

The electrodissolution of metal electrodes exhibits often sustained oscillations of the current density. We study the spatio-temporal dynamics of a metal disk electrode embedded in an insulator in a cylindrical electrochemical cell with a ring-shaped CE in the oscillatory region. The chosen cell geometry introduces a strong radial parameter dependence into the system. For low conductivity the dynamics is spatio-temporally chaotic. A small aspect ratio of the cell supports 2-dimensional space-time chaos on the entire disk electrode. For larger aspect ratios and thus larger parameter gradients, however, we observe a ‘self-confinement’ of the turbulent dynamics to the central part of the disk electrode, the outer ring of the electrode exhibiting angularly uniform oscillations. It can be expected that the state of confined chaos is not restricted to electrochemical systems but can be established also in other dynamical systems by introducing an appropriate parameter gradient.
The Adsorption and Growth of Copper on As-Terminated GaAs(001): Physical Vapour versus Electrochemical Deposition
Authors: Zegenhagen J. , Lee T.-L. , Gründer Y. , Renner F. U. , Fimland B.-O.
Published: September 2007

The adsorption of copper deposited from an effusion cell in ultra high vacuum and electrodeposited from aqueous solution on As-terminated GaAs(001) is investigated by scanning tunneling microscopy, X-ray diffraction, as well as X-ray standing waves in combination with X-ray fluorescence spectroscopy. The analysis of the adsorbate is performed in situ with the samples either in ultra high vacuum or in aqueous electrolyte under potential control. Deposited as Cu ions from sulphuric acid solution, submonolayer coverages of Cu diffuse into the GaAs interface region assumimg predominantly Ga substitutional positions. Different positions with lower symmetry are occupied by Cu if submonolayer are deposited at room temperature in ultra high vacuum. At higher coverage, epitaxial Cu clusters grow by electrodeposition on the As-terminated GaAs(001), but with the Cu lattice of the islands rotated and inclined with respect to the GaAs substrate lattice, leading in total to eight equivalent domains. At comparable coverage, no epitaxial growth was observed for Cu deposited in ultra high vacuum.
Normalized Differential Reflectance Spectroscopy at Polycrystalline Platinum Electrodes in Aqueous Acidic Electrolytes: Quantitative Aspects
Authors: Shi Ping , Fromondi Iosif , Scherson Daniel A.
Published: September 2007

The intensity of monochromatic light (λ = 633 nm) reflected off the surface of a polycrystalline Pt electrode, R, has been measured in situ as a function of potential, E, during continuous voltammetric cycles in the range -0.25 < E < 1.05 V vs. SCE, both in 0.5 M HClO4 and 0.5 M H2SO4 aqueous solutions. Plots of the normalized differential reflectance, Δ R/R = [R(E
sam) - R(E
ref)]/R(E
ref) vs. Qn
, where E
sam and E
ref refer to the sampling and reference potentials, respectively, and Qn
is the coulometrically determined relative oxide charge (in terms of e/Pt surface atom), were found to deviate from linearity in the PtOH formation region. Improved proportionality between Δ R/R and Qn
was achieved up to Qn
= 1, upon correcting Δ R/R for double layer effects using the method originally reported by Conway et al. [3]. In the case of 0.5 M H2SO4, however, remaining discrepancies could only be resolved by accounting for the charge due to bisulfate desorption induced by PtOH formation on the surface. On the basis of these results, it has been concluded that for both types of electrolytes, Δ R/R is proportional to the PtOH coverage, θ, and independent of the applied potential for Qn
≤ 1, opening new prospects for monitoring θ quantitatively during the course of faradaic reactions.
Voltammetry and Electrocatalysis of Achromobacter Xylosoxidans Copper Nitrite Reductase on Functionalized Au(111)-Electrode Surfaces
Authors: Welinder Anna C. , Zhang Jingdong , Hansen Allan G. , Moth-Poulsen Kasper , Christensen Hans E. M. , Kuznetsov Alexander M. , Bjørnholm Thomas , Ulstrup Jens
Published: September 2007

A long-standing issue in protein film voltammetry (PFV), particularly electrocatalytic voltammetry of redox enzyme monolayers, is the variability of protein adsorption modes, reflected in distributions of catalytic activity of the adsorbed protein/enzyme molecules. Use of well-defined, atomically planar electrode surfaces is a step towards the resolution of this central issue.

We report here the voltammetry of copper nitrite reductase (CNiR, Achromobacter xylosoxidans) on Au(111)-electrode surfaces modified by monolayers of a broad variety of thiol-based linker molecules. These represent positively charged and electrostatically neutral, hydrophobic and hydrophilic, aliphatic and aromatic, and variable-length micro-environments, as well as their combinations. Optimal conditions for enzyme function seems to be a combination of hydrophobic and hydrophilic surface linker properties, which can lead to close to complete non-catalytic monolayer interfacial electron transfer function and electrocatalysis with activity approaching enzyme activity in homogeneous solution. Thiophenol (combined hydrophobic stacking and interdispersed water molecules), 4-methyl-thiophenol (hydrophobic and water molecules), and 3- and 4-aminothiophenol (hydrophilic, hydrophobic) offer the overall most efficient micro-environments. Subtle differences with even small structural linker differences, however, lead to widely different electrocatalytic properties, strikingly illuminated by the ω-mercaptoamines. CuNiR thus shows highly efficient, close to ideal reversible electrocatalytic voltammetry on cysteamine-covered Au(111)-electrode surfaces, most likely due to two cysteamine orientations previously disclosed by in situ scanning tunnelling microscopy. Such a dual orientation exposes both a hydrophobic and a positively charged, hydrophilic surface feature. In contrast, the higher cysteamine homologues expose only the hydrophilic component with no electrocatalytic activity on these surfaces. These results offer a basis for rational surface design in forthcoming biological electrocatalysis useful both fundamentally and in novel biosensor technology.
Icosahedral ordering in quasicrystals and metallic glasses
Authors: Phillips J. C.
Published: September 2007

Many workers have recently conjectured that the underlying forces responsible for icosahedral clustering in crystals may also explain the origin of metallic glass formation. Critical examination of thermochemical structural diagrams and radial distribution functions of good metallic glasses shows that this assertion is unfounded.
Creep behavior of a polycrystalline nickel aluminide: Ni-23.5 at.% A1-0.5 at.% Hf-0.2 at.% B
Authors: Schneibel J. H. , Petersen G. F. , Liu C. T.
Published: September 2007

The creep behavior of a poly crystalline nickel aluminide with the composition Ni-23.5 at.% Al-0.5 at. % Hf-0.2 at. % B has been measured as a function of stress, temperature, and grain size. At high stresses, of the order of 100 MPa, the strain rate is nonlinear in the stress, with a stress exponent greater than two. Below approximately 10 MPa, at 1033 K, the steady-state strain rate is almost proportional to the stress, indicating that diffusional creep is rate controlling. Calculations of expected Nabarro—Herring and Coble creep rates did not answer whether diffusive mass transport through the grains, or along the grain boundaries, is rate controlling. The grain-size dependence of the strain rate, however, indicates predominance of volume diffusion control, i.e., Nabarro—Herring creep, for our experimental conditions.
Formation of LanxideTM ceramic composite materials
Authors: Newkirk M. S. , Urquhart A. W. , Zwicker H. R. , Breval E.
Published: September 2007

An overview is given of a new process that has been used successfully to make numerous ceramic/metal composite materials by directed oxidation of molten metallic precursors. As an example, the formation of A12O3/A1 composites from Al is discussed in detail.
Electron microscopy and krypton adsorption characterization of high-purity MgO powder
Authors: Bretz M. , Shastri A. G. , Schwank J.
Published: September 2007

A promising method for the preparation of sizeable quantities of structurally well-defined, highpurity MgO powder is reported. The morphology and surface uniformity of the powder is comparable to that of MgO smokes but with narrow size distribution of particles. Sample characterization of these oxide powders is accomplished by combining structural TEM/STEM examination with krypton gas adsorption isotherms. The latter technique is sensitive to the presence of surface hydroxyl groups and of surface roughness on an atomic scale. Highresolution TEM indicates a perfect cubic morphology, intra-crystallite orientation, and dendritic sintering of cubes. In the STEM mode sharp convergent beam electron diffraction patterns are obtained, and in thick specimen regions Kikuchi lines appear, indicating the absence of crystal defects. After prolonged outgassing to remove surface hydroxyl groups, a krypton adsorption isotherm contains a near vertical submonolayer riser and second layer step along with partial wetting features near saturation. These near-ideal dendritic ceramic powders, therefore, provide a research bridge between single crystal surface studies and large-scale powder technology.
Transition metal ions in silicate melts. IV. Cobalt in sodium silicate and related glasses
Authors: Nelson Carolynn , White William B.
Published: September 2007

Optical measurements have been made on glasses quenched from a series of sodium silicate melts as well as germanate, borate, and phosphate liquids containing CoO. The spectral data show that Co+2 is the species stabilized in oxide melts heated in air. The observed bands occur at 2700–3850, 5750–7900, and 15 500–18 600 cm-1. The independence of the optical parameters from melt composition and known melt structures, the high relative intensity of the highfrequency band, and the low calculated value of the Racah B parameter relative to the free ion value indicate that Co+2 forms a distinct tetrahedral complex in the quenched melt.
High-resolution transmission electron microscopy on KHx–GIC's
Authors: Salamanca-Riba L. , Yeh N.-C. , Dresselhaus M. S. , Endo M. , Enoki T.
Published: September 2007

The in-plane and c-axis structure of KHx—GIC's and KDy—GIC's is studied using transmission electron microscopy (TEM) and x-ray diffraction as a function of intercalation temperature and time. With the TEM, two commensurate in-plane phases are found to coexist in these compounds with relative concentrations depending on intercalation conditions. When the direct intercalation method is used, the first step of intercalation is the formation of a stage n potassium-GIC and the final compound is a stage n KHx—GIC (or KDy—GIC). Highresolution (00l) lattice images show direct evidence for intermediate phases in the intercalation process. These intermediate phases are hydrogen (deuterium) deficient and are found at the boundary between pure potassium regions and regions with high hydrogen (deuterium) content. A comparison of the structure for the two methods of intercalation of KH is also presented.
Kinetics of formation of silicides: A review
Authors: d'Heurle F. M. , Gas P.
Published: September 2007

The kinetics of silicide growth are classified into three different categories: (a) diffusion controlled, (b) nucleation controlled, (c) others (reaction rate controlled). These are analyzed with the aim of understanding both the phenomenology of growth and the specific atomic mechanisms of phase formation. Diffusion-controlled growth is discussed with respect to the Nernst-Einstein equation. Stress relaxation is considered as a possible cause of reaction-rate control. The relative merits of two different types of marker experiments are compared. A few silicides are discussed in terms of what can be inferred about diffusion mechanisms. The competition between reaction-rate and diffusion control phenomena is shown to have specific effects on the sequence of phase formation; it is also related to the formation of some amorphous compounds. Reactions between silicon and alloyed metal films are used to illustrate the respective influences of mobility and driving force factors on the kinetics of silicide growth; they can also be used to underline the dominance of nucleation over diffusion in some silicide formation processes.
Amorphization of rapidly quenched quasicrystalline Al-transition metal alloys by the addition of Si
Authors: Dunlap R.A. , Dini K.
Published: September 2007

The structure and thermal stability of rapidly solidified Al-Cr-Si, Al-Mn-Si, Al-Fe-Si, Al-Co-Si, and Al-Ni-Si alloys have been investigated using x-ray diffraction and thermal analysis measurements. Each series of alloys shows a region of stoichiometry that yields icosahedral symmetry and a region that yields an amorphous phase. Thermal and structural properties of these alloys are reported as a function of stoichiometry and quench rate.
Probability of detection of internal voids in structural ceramics using microfocus radiography
Authors: Baaklini George Y. , Roth Don J.
Published: September 2007

The reliability of microfocus x radiography for detecting internal voids in structural ceramic test specimens was statistically evaluated. The microfocus system was operated in the projection mode using low x-ray photon energies (<20 keV) and a 10 μm focal spot. The statistics were developed for implanted internal voids in green and sintered silicon carbide and silicon nitride test specimens. These statistics were compared with previously obtained statistics for implanted surface voids in similar specimens. Problems associated with void implantation and characterization are discussed. Statistical results are given as probability-of-detection curves at a 95% confidence level for voids ranging in size from 20–528 μm in diameter.
Light-ion-beam-induced annealing of implantation damage in diamond
Authors: Adel M. , Kalish R. , Richter V.
Published: September 2007

The removal of defects in diamond by light-ion bombardment has been studied by means of Rutherford backscattering spectroscopy (RBS) channeling techniques. The damage produced by 1 × 1014 Sb ions cm−2 at 300 keV (below the critical dose for graphitization) was observed to diminish by as much as 50% under bombardment with H and He ions. The ion-beam-induced annealing has been studied as a function of ion dose and incident angle (channeling and random). Although the data sets differ markedly, they nearly coincide when the dose is normalized to the energy deposited by elastic collisions in the damaged region. This may indicate that nuclear and not electronic collisions contribute primarily to the in situ annealing in a reasonably good insulator such as diamond.
On tunneling effects in metal-deposited polyethylene-carbon black and polycarbonate-carbon black systems
Authors: Ezquerra T.A. , Baltá Calleja F.J. , Plans J.
Published: September 2007

Electrical conductivity for polyethylene-carbon black, polycarbonate-carbon black composites was measured for conducting compositions well above the percolation limit. Three conductivity methods were employed in our studies: sputtering coated metal electrodes, painted metal electrodes, and the four-point method. Results suggest that while the two latter methods do not modify substantially the material surface, the former method introduces a low conductivity region at the surface. The effect of contact resistance developed during metal coating is discussed in the light of both fluctuation-induced tunneling predictions and “hopping” transport mechanisms. Experimental results favor the tunneling alternative across the evaporated metalcomposite interphase.
Ordering and decomposition in semiconductor alloys
Authors: Martins José Luís , Zunger Alex
Published: September 2007

The stability of ordered semiconductor alloys has been studied, using total energy pseudopotential calculations. The ordered alloys are found to be stabilized with respect to disordered alloys via reduction of the internal strain and by chemical interactions. The Si–C and Si–Ge systems are used as illustrations, finding that ordered Six Ge1−x should be a metastable alloy, in agreement with experimental observations.
A method for interpreting the data from depth-sensing indentation instruments
Authors: Doerner M.F. , Nix W.D.
Published: September 2007

Depth-sensing indentation instruments provide a means for studying the elastic and plastic properties of thin films. A method for obtaining hardness and Young's modulus from the data obtained from these types of instruments is described. Elastic displacements are determined from the data obtained during unloading of the indentation. Young's modulus can be calculated from these measurements. In addition, the elastic contribution to the total displacement can be removed in order to calculate hardness. Determination of the exact shape of the indenter at the tip is critical to the measurement of both hardness and elastic modulus for indentation depths less than a micron. Hardness is shown to depend on strain rate, especially when the hardness values are calculated from the data along the loading curves.
The influence of applied stress on precipitate shape and stability
Authors: Berkenpas M.B. , Johnson William C. , Laughlin D.E.
Published: September 2007

Shape transitions and orientation alignment of elastically misfitting precipitates in the presence of an external stress field have been predicted using bifurcation and group theory and by performing actual energy calculations for elliptical cylinders under plane strain conditions. Under the assumption of system isotropy, the applied field acts to break or perturb the bifurcation. Both first- and second-order size-induced shape transitions are observed for elastically soft precipitates. Stress-induced shape transitions are shown to be either first-order or continuous for elastically soft precipitates. Only continuous stressinduced shape transitions are observed for hard precipitates.
Laser alloying of Cu and Cr
Authors: Westendorp J.F.M. , Koelewijn W. , van Sark W.G.J.H.M. , Saris F.W. , van der Pers N.M. , de Keijser Th.H.
Published: September 2007

CuCr multilayers, 0.5−1 /um total thickness, on Cu substrates have been laser irradiated. Threshold energy densities for complete alloying with different laser wavelengths and different multilayer structures were determined using Rutherford backscattering. Results are discussed in terms of absorbance of Cu and Cr as a function of laser wavelength, overall chemical composition, and thicknesses of the individual Cu and Cr layers. Also, x-ray diffraction was used to study the microstructure of the CuCr before and after laser irradiation. A method is outlined for unraveling the contributions to peak shift of stacking faults, stresses, and change in'chemical composition. The CuCr alloy produced by the laser irradiation consisted of small, very defective Cu-rich and Cr-rich crystallites. The CuCr layer was subjected to a high tensile stress. The distinct change in preferred orientation of crystallites on laser irradiation indicated a complete melting of the CuCr multilayer. A high tensile strength (> 935 MPa) of the CuCr before and after laser alloying is suggested by the microstructure as observed by x-ray diffraction and sustained by hardness measurements. In the Cu-rich crystals 4.0 at. % Cr was in solid solution, i.e., five times the maximum equilibrium solid solubility.
Wear properties of shock-consolidated amorphous and microcrystalline Ni-38 wt % Mo−8 wt % Cr-1. 5 wt % B alloy powders
Authors: Vreeland Thad , Thadhani Naresh N. , Mutz Andrew H. , Thomas Susan P. , Nibert Roger K.
Published: September 2007

Powder flakes prepared from 50/um thick melt-spun ribbons of Markomet 1064 (Ni52.5 Mo38 Cr8 B 1.5 wt %) were shock consolidated in the unannealed and annealed condition. The unannealed powder flakes (microhardness 933 kg/mm2) are amorphous while flakes annealed at 900°C for 2 h have a fcc crystal structure, with a microcrystalline grain size of 0.3 μm and microhardness of 800 kg/mm2. The shock-consolidated amorphous powder compact (∼250 kJ/kg shock energy) shows no crystal peaks in an x-ray diffractometer scan. Compacts of annealed powder (400–600 kJ/kg shock energies) contain amorphous material (18%–21%), which was rapidly quenched from the melt formed at interparticle regions during the consolidation process. The microhardness of the amorphous interparticle material is 1100 kg/mm2. Wear properties of the compacts measured in the low velocity friction apparatus (LVFA) pin-on-disk tests show low average friction values (−0.03). The 60 h cumulative wear appears to correlate with the energy of shock compaction rather than the metallic glass content. The Falex ring-on-block test also showed that both the Markomet amorphous compact and the compact containing a mixture of amorphous and microcrystalline phases performed almost equally and wore half as much as the base-line SAE-01 steel-polished sample.
The diffusion of antimony in heavily doped and n- and p-type silicon
Authors: Fair R.B. , Manda M.L. , Wortman J.J.
Published: September 2007

The diffusion of Sb in heavily doped n- and p-type Si has been studied to determine the activation energies and charge states of the point defects responsible for Sb diffusion. It is shown that neutral point defects, probably Vx, dominate under intrinsic doping conditions. For samples doped with high-concentration As or P backgrounds, Sb diffusion is dominated by a double-negatively charge point defect that causes an n2 concentration-dependent Sb diffusivity. Electric-field effects also are important. The measured diffusion coefficients are Dix = 17.5 exp(−4.05 eV/kT), and Di= = 0.01 exp(−3.75 eV/kT). The activation energies are consistent with diffusion via Vx and V= vacancies. Retarded diffusion of Sb in p+-doped samples with uniform B profiles fits an ion pairing model where Sb+B− pairs form to reduce the flux of Sb atoms.
A (001) x-ray diffraction study of stage 1 α and β phases of graphite-CsBix
Authors: Yang Ming Hong , Eklund Peter C.
Published: September 2007

Results are reported of (00l) x-ray studies of the α(Ic = 10.65 Å) and β(Ic = 11.49 Å) phases of stage 1 graphite-CsBix ternary intercalation compounds that are air stable. The integrated intensities of the (OOl) x-ray lines are analyzed in terms of a centrosymmetric sandwich model that consists of two laycrs each of Cs, Bi, and C. The results of these analyses indicate that the Bi layers are nearly centered in the cell as Suggested by Lagrange et al. [Synth. Met. 12, 201 (1985)]. Furthermore, stoichiometries of the α and β phases are found to be Cs0.99 Bi0.43C4 and Cs0.83Bi0.81C4, respectively. The β phase is found to contain nearly twice as many Bi atoms'. The sample mass change upon intercalation is in good agreement with the x-ray analyses. The substantial difference in the Bi concentration in the α and β phases would be expected to have strong influence on the superconducting properties of these graphite intercalation compounds (GIC's).
Pore growth kinetics in heat-treated glass-like carbons
Authors: Lachter Jalil , Henry Leo G. , Bragg Robert H. , Spooner Stephen
Published: September 2007

The kinetics of changes in void size during annealing of glass-like carbons in the temperature range 1000°−2800 °C for times up to 150 h were studied using small-angle x-ray scattering determinations of the radius of gyration Rg. The results show that Rg ranged from 9 Å at 1000°C to about 24 Å at 2800 °C. A pore coarsening analysis and a superimposition kinetic analysis applied to Rg gave activation energies of 76 ± 4 kcal/mole and 74 ± 9 kcal/mole, respectively, which are associated with migration of vacancies within graphitic layers in the matrix material.
Microstructural defects in γ-Fe2O3 particles
Authors: Hall Ernest L. , Berkowitz Ami E.
Published: September 2007

The microstructure of three different types of γ-Fe2O3 particles were examined using transmission electron microscopy. These included pure γ-Fe2O3, γ-Fe2O3 that had been surface modified using Co, and γ-Fe2O3 that had been doped with Co. The major internal microstructural defects found in the particles in all of the samples were pores and antiphase boundaries. Some particles also had a very high density of dislocations and low-angle boundaries. In general, the particles could be described as single crystals with symmetric cross section. The structure is based on a tetragonal unit cell, and each particle is divided into antiphase domains in which the c axis is oriented at 90°with respect to adjoining domains. The particles often exhibited very irregular shapes. No effect of Co modification was seen on the internal or surface structure of the particles. The Co-doped particles were found to be smaller in size and contained a lower density of internal defects. The effect of the microstructural defects and morphological irregularities in these particles on magnetic behavior is discussed.
Plastic properties of thin films on substrates as measured by submicron indentation hardness and substrate curvature techniques
Authors: Doerner M.F. , Gardner D.S. , Nix W.D.
Published: September 2007

Substrate curvature and submicron indentation measurements have been used recently to study plastic deformation in thin films on substrates. In the present work both of these techniques have been employed to study the strength of aluminum and tungsten thin films on silicon substrates. In the case of aluminum films on silicon substrates, the film strength is found to increase with decreasing thickness. Grain size variations with film thickness do not account for the variations in strength. Wafer curvature measurements give strengths higher than those predicted from hardness measurements suggesting the substrate plays a role in strengthening the film. The observed strengthening effect with decreased thickness may be due to image forces on dislocations in the film due to the elastically stiffer silicon substrate. For sputtered tungsten films, where the substrate is less stiff than the film, the film strength decreases with decreasing film thickness.
Structural ceramics based on Si3N4–ZrO2(+ Y2O3) compositions
Authors: Lange F. F. , Falk L. K. L. , Davis B. I.
Published: September 2007

An investigation was made of the effect of Y2O3 in solid solution in ZrO2 on the phases that are formed when ZrO2 is reacted with either N2 or Si3N4. The results suggest that the Zr–oxynitride phase can be predcluded as a reaction product when the Y2O3 content of the ZrO2 is ≥4 mol %. Dense Si3N4/ZrO2(+ Y2O3) composites were fabricated, which did not degrade during oxidation at temperatures %le; 1000°C. Severe degradation was observed for composites containing the Zr-oxynitride phase. The fracture toughness of the Si3N4/ZrO2(+ Y2O3) composites increased with ZrO2 content.
Multitechnique surface spectroscopic studies of plasma-modified polymers I: H2O/Ar plasma-modified polymethylmethacrylates
Authors: Hook Thomas J. , Gardella Joseph A. , Salvati Lawrence
Published: September 2007

Results from x-ray photoelectron spectroscopy (XPS or ESCA), low-energy ion scattering spectrometry (LEIS or ISS); and Fourier transform infrared spectroscopy (FTIR) analyses are presented for unmodified and modified poly (methylmethacrylate) (PMMA) polymer films. Analysis of the unmodified PMMA polymers (isotactic, syndiotactic, and atactic) via ESCA, ISS, and FTIR, established the surface composition, bonding, and functionality before the modification was employed. An rf-plasma glow discharge created from an Ar/H2O gas mixture at different exposure times and power levels was used to treat the polymer surface. Subsequent ESCA, ISS, and FTIR analyses of these modified PMMA's show the effects of surface modification in terms of a model of structural differences, over a limited depth (50–100 Å). The composition and functionality changes of the resulting surfaces are discussed with respect to proposed mechanisms of the plasma reaction and differences in tacticity of the reactant. A two-step reaction mechanism involving reactive decarboxylation/reduction followed by H2O adsorption is proposed to understand the spectroscopic results.
An investigation of the Al86Mn14 alloy and its quasicrystalline phases
Authors: Pérez-Ramirez J. G. , Pérez R. , Gómez A. , Cota-Araiza L. , Martinez L. , Herrera R. , José-Yacamán M.
Published: September 2007

A study of the phases obtained in an aluminum manganese (Al86Mn14) alloy prepared under relatively slow solidification rates is presented. The identification of the phases presented in the alloy has been carried put using different techniques. Two of the phases were found to be crystalline in nature (Al, Al6Mn), two were amorphous (Al2O3, AlMn–O), and one of them was quasicrystalline (T phase). In the case of the quasicrystalline T phase an investigation on its structural characteristics was carried out, and a possible model for its structure is suggested, based on high-resolution images under fivefold diffraction conditions.
High-energy (MeV) ion-beam mixing of Ti with SiC and Si3N4
Authors: Bhattacharya R. S. , Rai A. K. , Pronko P. P.
Published: September 2007

Ion-beam mixing of Ti layers with sintered α-SiC and hot-pressed Si3N4 was measured for 1 McV Au+ at doses of 1X1016 cm−2 and 5X1016 cm−2. Rutherford backscattering (RBS) and cross-section transmission electron microscopy (XTEM) were used to evaluate the mixing. Mixing was observed in Ti/SiC system; however, there was no mixing in Ti/Si3N4 system. Results are discussed in light of the enthalpy of mixing criterion for metal-insulator systems.
Silicon oxidation and Si–SiO2 interface of thin oxides
Authors: Ravindra N. M. , Narayan J. , Fathy Dariush , Srivastava J. K. , Irene E. A.
Published: September 2007

High-resolution transmission electron microscopy (HRTEM) and ellipsometry techniques have been employed to measure thicknesses of silicon oxide, grown at 800°C in dry oxygen, in the thickness range of 2–20 nm. While the oxide growth data measured from TEM obey a nearly linear behavior, those obtained from ellipsometry are seen to vary nonlinearly. The interface structure as function of the increasing oxide thickness was studied using HRTEM. At these oxidation temperatures, the earlier reported variations of roughness at the interface on the oxide thickness for oxides grown at 900°C are not seen. Attempts aimed at correlating the high-resolution transmission electron micrographs with some physical parameters like the refractive index and the dielectric breakdown lead to considerations of the importance of the effect of protrusions of silicon atoms of 1 mm size into SiO2 layers on the interface properties. These findings lead to explanations of some key features concerning the refractive index and density of thin SiO2.
Ni, Pd, and Pt on GaAs: A comparative study of interfacial structures, compositions, and reacted film morphologies
Authors: Sands T. , Keramidas V. G. , Yu A. J. , Yu K-M. , Gronsky R. , Washburn J.
Published: September 2007

The reactions between (100) GaAs and the near-noble metals Ni, Pd, and Pt have been investigated by application of high-resolution transmission electron microscopy (TEM), energy-dispersive analysis of x-rays in the scanning TEM and Rutherford backscattering spectrometry. Emphasis is placed on the evolution of the phase distributions, film compositions, and interface morphologies during annealing at temperatures up to 480°C. The first phase in the Ni/GaAs reaction is shown to have the nominal composition Ni3GaAs. Ternary phases of the type PdxGaAs are also found to be the dominant products of the Pd/GaAs reaction. Conversely, only binary phases result from the Pt/GaAs reaction. These observations are used to construct isothermal sections of the M-Ga-As thin-film phase diagrams. The behavior of a thin (1–2 nm) native oxide-hydrocarbon layer during the Ni/GaAs, Pd/GaAs, and Pt/GaAs reactions is also investigated. Only the Ni/GaAs reaction is noticeably impeded in some regions by this intervening layer. In contrast, the Pd/GaAs and Pt/GaAs reactions tend to mechanically disperse the native oxide layers.
Spark erosion: A method for producing rapidly quenched fine powders
Authors: Berkowitz A. E. , Walter J. L.
Published: September 2007

Spark erosion is a method for producing fine powders of metals, alloys, semiconductors, and compounds. The technique involves maintaining repetitive spark discharges among chunks of material immersed in a dielectric liquid. As a result of the spark discharge there is highly localized melting or vaporization of the material. The powders are produced by the freezing of the molten droplets or the condensation and freezing of the vapor in the dielectric liquid. Since the powders are quenched in situ, they may be extremely rapidly cooled. Particles can be produced in sizes ranging from 5 nm to 75 μm. The average powder size and production rate depend on the power parameters, material used, and the dielectric liquid.
A comparative electron microscopic study of Al-based and Mg-based quasicrystals
Authors: Mukhopadhyay N. K. , Thangaraj N. , Chattopadhyay K. , Ranganathan S.
Published: September 2007

It is shown that both icosahedral Al–Mn–Si and Mg–Al–Zn alloys give rise to the same variety of electron diffraction patterns as documented for icosahedral Al–Mn alloys. Subtle variations in intensity are ascribed to a different decorational motif in terms of the Mackay icosahedron and the Pauling triacontahedron. Icosahedral Al–Mn–Si alloys do not appear to be ordered on a superlattice basis.
Low-voltage field emission from tungsten fiber arrays in a stabilized zirconia matrix
Authors: Cochran J. K. , Chapman A. T. , Hill D. N. , Lee K. J.
Published: September 2007

Field emitter array cathodes were fabricated from unidirectionally solidified composites of tungsten fibers in an insulating yttria-stabilized-zirconia (YSZ) matrix. A close-spaced molybdenum gate film (extractor) was formed utilizing c-beam evaporation of alumina as an insulator, which was overlayed by the molybdenum extractor. The high resistivity of the composite matrix coupled with the alumina insulator resulted in low leakage current and permitted dc operation of the device. Emission testing demonstrated current densities of 1–5 A/cm2 with leakage in the μA range for applied potentials of 125–200 V. Variation of emitter tip geometries from hemispheres to right circular cylinders to pointed cones produced increases in emission consistent with reduced tip radii.
High-temperature crystal chemistry of sodium zirconium phosphate (NZP)
Authors: Hazen R. M. , Finger L. W. , Agrawal D. K. , McKinstry H. A. , Perrotta Anthony J.
Published: September 2007

High-temperature crystal structures of NZP (Na1+xZr2P3−xSixO12) have been determined by x-ray measurements made on single crystals. Thermal expansion of NZP (x = 0.11) parallel to the hexagonal c axis is positive (about 22.4 ⊠ 10−6°C−1 between 25° and 700°C), whereas expansion perpendicular to c is slightly negative (about 5.4 ⊠ 10−6°C−1), resulting in an average volume thermal expansion of 11.8 ⊠ 10−6°C−1. A proposed structural model to interpret this anisotropic thermal expansion of NZP is tested to prove the model's validity. In this model the rotation of the phosphate tetrahedron is coupled to the rotation of the zirconium octahedron. The observed thermal expansions of sodium, zirconium, and phosphorus cation coordination polyhedra are 10.8, 0.00, and −0.23 (all × 10−6°C−1), respectively. The large thermal expansion of the sodium site is offset by rotations in the Zr–P polyhedral framework, thus yielding the low net expansion of NZP.
Crack resistance by interfacial bridging: Its role in determining strength characteristics
Authors: Cook Robert F. , Fairbanks Carolyn J. , Lawn Brian R. , Mai Yiu-Wing
Published: September 2007

An indentation-strength formulation is presented for nontransforming ceramic materials that show an increasing toughness with crack length (T curve, or R curve) due to the restraining action of interfacial bridges behind the crack tip. By assuming a stress-separation function for the bridges a microstructure-associated stress intensity factor is determined for the penny-like indentation cracks. This stress intensity factor opposes that associated with the applied loading, thereby contributing to an apparent toughening of the material, i.e., the measured toughness in excess of that associated with the intrinsic cohesion of the grain boundaries (intergranular fracture). The incorporation of this additional factor into conventional indentation fracture mechanics allows the strengths of specimens with Vickers flaws to be calculated as a function of indentation load. The resulting formulation is used to analyze earlier indentation-strength data on a range of alumina, glass-ceramic, and barium titanate materials. Numerical deconvolution of these data determines the appropriate T curves. A feature of the analysis is that materials with pronounced T curves have the qualities of flaw tolerance and enhanced crack stability. It is suggested that the indentation-strength methodology, in combination with the bridging model, can be a powerful tool for the development and characterization of structural ceramics, particularly with regard to grain boundary structure.
High-pressure studies on lithium fast-ion conductors
Authors: Mazumdar D. , Bose D. N. , Parthasarathy G. , Gopal E. S. R.
Published: September 2007

The variation of resistivity of the lithium fast-ion conductor Li3+y Ge1−yO4 (y = 0.25, 0.6, 0.72) has been studied with hydrostatic pressure up to 70 kbar and compared with that of Li16−2x Znx (GeO4)4(x = 1, 2). Both types showed pronounced resistivity maxima between 20–30 kbar and marked decrease thereafter. Measurements as a function of temperature between 120–300 K permitted the determination of activation energies and prefactors that also showed corresponding maxima. The activation volumes (ΔV) of the first type of compound varied between 4.34 to −4.90 cm3/mol at 300 K and decreased monotonically with increasing temperature. For the second type ΔV was much smaller, varied with pressure between 0.58 and −0.24 cm3/mol, and went through a maximum with increasing temperature. High-pressure studies were also conducted on aged samples, and the results are discussed in conjunction with results of impedance measurements and nuclear magnetic resonance (NMR) studies. The principal effect of pressure appears to be variations of the sum of interatomic potentials and hence barrier height, which also causes significant changes in entropy.
A nuclear magnetic resonance study of Al–Mn quasicrystals and related materials
Authors: Carduner Keith R. , Suits B. H. , DiVerdi J. A. , Murphy Michael D. , White David
Published: September 2007

Nuclear magnetic resonance (NMR) results are presented for several aluminum alloy samples prepared using the melt-spinning technique including orthorhombic Al6Mn, Al–Mn quasicrystals both with and without doping with Si and Ru, and a T-phase alloy of Al and Pt. With the exception of the orthorhombic material, all of the NMR spectra show a broad distribution of sites. No features unique to the quasicrystal phase are observed. For the orthorhombic material the quadrupole field parameters are found to be ∥VQ∥ − 1.0±0.1 MHz and η = 0.4±0.1.
Grazing incidence synchrotron x-ray diffraction method for analyzing thin films
Authors: Lim G. , Parrish W. , Ortiz C. , Bellotto M. , Hart M.
Published: September 2007

A method using synchrotron radiation parallel beam x-ray optics with a small incidence angle α on the specimen and 2Θ-detector scanning is described for depth profiling analysis of thin films. The instrumentation is the same as used for Θ:2Θ synchrotron parallel beam powder diffractometry, except that the specimen is uncoupled from the detector. There is no profile distortion. Below the critical angle for total reflection αc, the top tens of Angstroms are sampled. Depth profiling is controlled to a few Angstroms using a small α and 0.005° steps. The penetration depth increases to several hundred Angstroms as α approaches αc. Above αc there is a rapid increase in penetration depth to a thousand Angstroms or more and the profiling cannot be sensitively controlled. At grazing incidence the peaks are shifted several tenths of a degree by the x-ray refraction and an experimental procedure for calculating the shifts is described. The method is illustrated with an analysis of iron oxide films.
Alloying and entropy effects in predicting metal/compound–semiconductor interface reactivity
Authors: McGilp John F.
Published: September 2007

A previous bulk thermodynamic model, which used enthalpies of compound and alloy formation to predict metal/compound–semiconductor interface reactivity, is extended to include entropy. It is shown that, for most metals on CdTe, GaAs, GaSe, InP, and MoS2, solid-state reactions are energetically favored up to semiconductor dissociation temperatures and, consequently, entropy effects are minimal. Gold and silver, with their small enthalpies of metal–semiconductor anion compound formation, can be exceptions. Even here, the results for gold/III–V systems favor solid-state reaction at room temperature, but at higher temperatures entropy drives the reaction via vapor-phase production of the group V element. The binary phase bulk thermodynamic model is not sufficient to predict absolute reactivity, but can rank the reactivity of the various metal–semiconductor combinations successfully, as long as possible alloy formation is included. It is suggested that the limitations of the model are due to the specific effects of the interface.