bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural characterization and thermal stability of W/Si multilayers
Authors: Brunel M. , Enzo S. , Jergel M. , Luby S. , Majkova E. , Vavra I.
Published: September 2007

Tungsten/silicon multilayers with tungsten layers of a thickness of 1–2 nm were prepared by means of electron beam deposition. Their structure and thermal stability under rapid thermal annealing were investigated by a combination of x-ray diffraction techniques and cross-sectional transmission electron microscopy. The crystallization behavior was found to depend on the interdiffusion and mixing at the tungsten/silicon interfaces during deposition as well as during annealing. The as-deposited tungsten/silicon multilayers were amorphous and remained stable after annealing at 250 °C/40 s. Interdiffusion and crystallization occurred after annealing all samples from 500 °C/40 s up to 1000 °C/20 s. By performing the same heat treatment in the tungsten/silicon multilayers, the formation of body-centered cubic W was observed with a layer thickness ratio δW/δsi = 1, whereas tetragonal WSi2 was detected in tungsten/silicon multilayers with a layer thickness ratio of δw/δsi ∼0.25. This dependence of the crystallization products on the layer thickness ratio δw/δsi originates from the different phenomena of interdiffusion and mixing at the tungsten/silicon interfaces. The possible formation of bcc tungsten as a first stage of crystallization of tungsten-silicon amorphous phase, rich in tungsten, is discussed.
Single crystal wurtzite GaN on (111) GaAs with AlN buffer layers grown by reactive magnetron sputter deposition
Authors: Ross Jennifer , Rubin Mike , Gustafson T.K.
Published: September 2007

We report the growth conditions necessary for highly oriented wurtzite GaN films on (111) GaAs, and single crystal GaN films on (111) GaAs using AlN buffer layers. The GaN films and AlN buffers are grown using rf reactive magnetron sputter deposition. Oriented basal plane wurtzite GaN is obtained on (111) GaAs at temperatures between 550 and 620 °C. However, using a high temperature 200 Å AlN buffer layer epitaxial GaN is produced. Crystal structure and quality are measured using x-ray diffraction (XRD), reflection electron diffraction (RED), and a scanning electron microscope (SEM). This is the first report of single crystal wurtzite GaN on (111) GaAs using AlN buffer layers by any growth technique. Simple AlN/GaN heterostructures grown by rf reactive sputter deposition on (111) GaAs are also demonstrated.
Epitaxy, microstructure, and processing-structure relationships of TiO2 thin films grown on sapphire (0001) by MOCVD
Authors: Chang H.L.M. , Zhang T.J. , Zhang H. , Guo J. , Kim H.K. , Lam D.J.
Published: September 2007

TiO2 thin films have been deposited on sapphire (0001) substrates under various conditions by metal-organic chemical vapor deposition. The structural properties of the deposited films were characterized by x-ray diffraction and transmission electron microscopy. The important growth parameters were found to be the deposition temperature and the deposition rate. The ranges studied for the two parameters were 400 to 850 °C and 10 to 120 Å/min, respectively. Depending on the growth conditions, most of the deposited films were either single-phase anatase or rutile, or a mixture of the two. These films were all epitaxial, but none of them were single-crystal films. Three distinct epitaxial relationships were observed between the films and the substrates, and, depending on the growth conditions, a deposited film can contain one, two, or all three of them. The fact that the films we obtained, although epitaxial, were never single crystal is explained based on the consideration of the difference in the rotational symmetries of the substrate surface and the film growth plane. We believe that it should be generally true that, in heteroepitaxial growth, a true single-crystal film can never be obtained as long as the point symmetry group of the substrate surface is not a subgroup of that of the film growth plane.
RuO2 films by metal-organic chemical vapor deposition
Authors: Si Jie , Desu Seshu B.
Published: September 2007

Pure and conducting RuO2 thin films were successfully deposited on Si, SiO2/Si, and quartz substrates at temperatures as low as 550 °C by a hot wall metal-organic chemical vapor deposition (MOCVD). Bis(cyclopentadienyl)ruthenium, Ru(C5H5)2, was used as the precursor. An optimized MOCVD process for conducting RuO2 thin films was established. Film structure was dependent on MOCVD process parameters such as bubbler temperature, dilute gas flow rates, deposition temperature, and total pressure. Either pure RuO2, pure Ru, or a RuO2 + Ru mixture was obtained under different deposition conditions. As-deposited pure RuO2 films were specular, crack-free, and well adhered on the substrates. The Auger electron spectroscopy depth profile showed good composition uniformity across the bulk of the films. The MOCVD RuO2 thin films exhibited a resistivity as low as 60 μω-cm. In addition, the reflectance of RuO2 in the NIR region had a metallic character.
Characterization of europium implanted LiNbO3
Authors: Moretti P. , Canut B. , Ramos S.M.M. , Brenier R. , Thévenard P. , Poker D. , Da Cunha J.B.M. , Amaral L. , Vasquez A.
Published: September 2007

LiNbO3 single crystals were implanted at room temperature with Eu+ ions at 70 keV with fluence ranging from 0.5 to 5 × 1016 ions · cm−2. The damage in the implanted layer has been investigated by Channeling Rutherford Backscattering (RBS-C), and the oxidation states of the cations have been determined by x-ray photoelectron spectroscopy (XPS). Following implantation, a fully amorphized layer of 60 nm is generated, even for the lowest fluence employed. Subsequent annealing in air, in the range 800–1250 K, was applied to restore tentatively the crystallinity and promote the substitutional incorporation of Eu in the crystal. Only a partial recrystallization of the damaged layer was observed. For as-implanted samples, XPS spectra clearly reveal europium in Eu2+ and Eu3+ states, and the Nb5+ ions are driven to lower charge states.
Preparation of ultrafine tungsten carbide powder by CVD method from WCl6–C2H2–H2 mixtures
Authors: Won Chang-Whan , Chun Byong-Sun , Sohn H.Y.
Published: September 2007

The synthesis of ultrafine tungsten carbide powder from WCl6–C2H2-H2 mixtures was investigated. Under most experimental conditions, a mixture of α–WC and β–W2C was produced. The specific surface area and morphology of the product prepared under various reaction conditions were determined. The product particle size increased with temperature up to 1373 K and then decreased, went through a minimum with increasing WCl6 partial pressure, and decreased with increasing H2 flow rate. The specific surface area of the product showed the opposite trend in all cases.
Chemistry and localization effects in the C-KVV Auger line shapes of transition metal carbides and related compounds
Authors: Pehrsson Pehr E. , Ramaker David
Published: September 2007

The C-KLL Auger line shapes of the transition metal carbides and some related compounds are examined as a function of the metal and nonmetalloid components (C, N, and O). They are discussed in the context of hole-hole repulsion and screening effects. The effect of π versus σ-bonding and the proportion of ionic versus covalent bonding are correlated with various aspects of the X-CVV Auger line shape, such as relative peak shifts, intensity of a high energy shoulder, and the multiplet splitting of the sp component.
UV induced dielectric loss in AlN ceramics
Authors: Harris J.H. , Enck R.C.
Published: September 2007

The dissipation factor of aluminum nitride ceramics (AlN), doped with various concentrations of oxygen, is probed at 1 kHz before and after exposure to UV radiation. The results of this study show that UV photogenerated carriers, which are trapped at charged oxygen impurity related defect sites, are responsible for the observed dielectric loss. A simple energy level diagram is presented that outlines trapped carrier distributions within the AlN bandgap and is consistent with the experimentally observed changes in dissipation factor as a function of time, UV exposure, and exposure to visible light, which liberates carriers from charged trap sites.
A novel brazing technique for aluminum and other metals
Authors: Timsit R.S. , Janeway B.J.
Published: September 2007

In this brazing technique, at least one of the metal surfaces to be joined is coated with a powder-mixture of silicon and a potassium fluoroaluminate flux. Brazing of aluminum is carried out by heating at ∼600 °C in nitrogen gas. During heating, the flux melts and dissolves the oxide layers on the contacting surfaces, thus allowing the silicon to come into intimate contact with bare aluminum. At a temperature exceeding the Al–Si eutectic temperature of 577 °C, the silicon diffuses rapidly into the aluminum and generates in situ a layer of Al–Si liquid alloy of near-eutectic composition. The filler material then flows into the joint and forms a fillet. This brazing technique may be exploited with aluminum using intermediary elements other than Si, and may be used for joining a variety of metals.
TEM studies of dislocations in deformed melt-textured YBa2Cu3Ox superconductors
Authors: Mironova M. , Selvamanickam V. , Lee D.F. , Salama K.
Published: September 2007

TEM studies have been conducted on melt-textured YBa2Cu3Ox samples that were uniaxially and isostatically deformed at high temperatures and compared with those of undeformed samples. Dislocation pile-ups along [100] and [010] are found to be the common feature between undeformed samples with the best Jc and the uniaxially deformed samples, and are suggested to be responsible for enhanced pinning when the magnetic field (H) is applied parallel to the a-b plane. Dislocation loops, tangles, and arrays are also observed, and are considered to contribute to pinning in field orientations other than H ‖ a-b. In addition to these dislocations, 〈301〉 type partial dislocations are found to be present in isostatically deformed samples. The strain field around these dislocations is considered to be an additional source of pinning in the intermediate field orientations.
Quantitative image analysis of superconductor thin film microstructure: The use of conditional, multiparametric, shape-analysis algorithms
Authors: Amato Philip A. , Lelental Mark , Bowen Ray C.
Published: September 2007

We have developed a novel approach for quantifying the microstructure of granular thin films using digital image processing and analysis. In the past, conventional scanning electron microscopy of thin films has generated qualitative information on the surface topography and film microstructure. However, when coupled to digital image analysis, the amount or degree of surface contours (i.e., granularity) in SEM micrographs can be quantified in a rapid and reproducible manner. Briefly, SEM micrographs are digitized and the edge boundaries on the film surface are enhanced by a gradient filter; granularity is then quantified by calculating the %AREA covered by the edges with respect to the entire field. Objects of a particular shape, such as phase impurity particles, can be selectively deleted from the image using a specific sequence of shape analysis algorithms and parameter values. In this manner, the contributions of edges from the phase impurity particles is minimized in the final measurement of real surface contours. Statistical analysis of the data yields quantitative information concerning variations in microdomains within single thin films and can detect statistically significant differences among samples. This method is being used in the characterization of the microstructure of superconducting thin films for optimization of their electrical and magnetic properties.
The formation of metastable Ti–Al solid solutions by mechanical alloying and ball milling
Authors: Oehring M. , Klassen T. , Bormann R.
Published: September 2007

Elemental Ti–Al powder blends were mechanically alloyed in order to study phase formation during the alloying process. In addition, the stability of intermetallic phases upon milling was investigated separately in order to determine the origins of phase selection during the milling process. It was found that by mechanical alloying of powder blends, as well as by ball milling of Ti-aluminides for long milling times, the same metastable phases were formed for corresponding compositions, i.e., the hep solid solution for Al concentrations up to 60 at. % and the fcc solid solution for 75 at. % Al. X-ray diffraction (XRD) analyses indicated that the process of mechanical alloying occurred via the diffusion of Al into Ti. By lowering the milling intensity, a two-phase mixture of the hcp solid solution and the amorphous phase was observed for Ti50Al50 and confirmed by transmission electron microscopy (TEM). The results show that phase selection in the final state during mechanical alloying of Ti–Al powder blends and milling of intermetallic compounds is mainly determined by the energetic destabilization of the competing phases caused by the milling process. The destabilization is most pronounced in the case of intermetallic compounds due to the decrease in long-range order upon milling. For the final milling stage, phase formation can be predicted by considering the relative stabilities of the respective phases calculated by the CALPHAD method using the available thermodynamic data for the Ti–Al system.
Deposition of diamond phase carbon films on surface pretreated stainless steel substrate
Authors: Heidarpour Ebrahim , Namba Yoshikatsu
Published: September 2007

The deposition of diamond phase carbon films on stainless steel substrates by an ionized deposition technique has been studied. A molybdenum grid used during argon ion sputtering had a decisive role in improving the morphology and adhesion ability of the substrate surface. The chemical composition of the surface was obtained by x-ray photoelectron spectroscopy, indicating the reduction of oxygen, carbon, and other contamination, while the surface morphology of the substrate obtained by scanning electron microscopy showed less roughness with a partially smooth surface. Attempts to extract the deposited films from the pretreated substrate surface by a superadhesive agent with an adhesion of 250 kg/cm2 failed, yielding a much stronger adhesion for the pretreated surface. This fact was also supported by examining the surface morphology, hardness, and the resistivity of the films deposited on the same substrates. As for the crystal structure of diamond phase carbon films on stainless steel, selected area diffraction patterns obtained from transmission electron microscopy suggested a mixture of amorphous carbon and polycrystalline diamond components.
Quantitative nucleation and growth studies of PACVD diamond film formation on (100) silicon
Authors: Bauer R.A. , Sbrockey N.M. , Brower W.E.
Published: September 2007

Diamond crystallites and continuous films were deposited on (100) silicon with various surface treatments by microwave plasma assisted CVD at times varying from 2 min to 1600 min. In each experiment, the average diameter of the crystallites increased linearly with time, while the density of crystallites was essentially constant. Thus, nucleation of the diamond occurred within a short time interval early in the deposition process. After the nucleation event, only growth occurred. Various surface treatments were used: untreated, polished with 1 μm diamond, scratched with 350 mesh SiC, scratched with 1 μm alumina, wiped with 350 mesh graphite powder, and spin coated with polymethyl methacrylate. Only the diamond polishing affected the crystallite density, and none of the surface treatments had any effect on crystallite morphology or growth rate. Growth rates were determined by least squares fits to average diameter versus time for crystals and average thickness versus time for films. The growth rate data extrapolate to zero size at zero deposition time. Applying the Volmer–Weber model, an activation energy for nucleation of diamond on silicon was calculated to be 52 kcal/mole.
In situelectron microscopy studies of the inhibition of graphite oxidation by phosphorus
Authors: Oh S.G. , Rodriguez N.M.
Published: September 2007

A combination ofin situtransmission electron microscopy and thermogravimetric techniques has been used to follow the manner by which phosphorus addition to graphite influences its interaction with oxygen. Direct observation of the process shows that the additive completely inhibits the reaction at temperatures below 830 °C. At higher temperatures phosphorus species are found to bond preferentially to the graphite “armchair”{1120} faces leaving the “zigzag” {100} faces vulnerable to attack by oxygen.In situelectron diffraction analysis indicates the formation of a chemical bond between the phosphorus and graphite edge atoms at high temperatures, which involves the formation of a complex believed to become an integral part of the structure. This unique type of chemical bonding is believed to be responsible for the observed thermal stability of P–O species on the graphite atoms at temperatures up to 1050 °C. In a further series of experiments, phosphorus was found to poison the catalytic activity of cobalt, which in its unadulterated state is a very effective promoter of the graphite-oxygen reaction.
Synthesis of ceramic oxide powders in a microwave plasma device
Authors: Vollath Dieter , Sickafus Kurt E.
Published: September 2007

Synthesizing oxide ceramic powders by application of a microwave plasma is a great advantage. There are two ways the microwave plasma can be used: The first is as a source of heat for the pyrolysis of solutions and the second is to excite gas reactions to obtain nanosized powders. Both applications are superior to standard methods. A microwave cavity well suited for these experiments and its operating characteristics are described. Using a microwave plasma as a source of heat for pyrolytic decomposition of nitrates in aqueous solutions leads to a fine-grained product with particle sizes from 100 to 1000 nm. Crystallite sizes in those particles are in most cases less than 10 nm. This is demonstrated with zirconia-based ceramics, such as ZrO2−3 mol % Y2O3−20 mol % Al2O3. Depending on the conditions during pyrolysis, it is possible to obtain a product in which alumina is either dissolved in zirconia or the onset of the phase separation is observed. The energy efficiency of this process is better than 80%. If the reactants are gaseous, e.g., ZrCl4, it is possible to produce powders with mean particle sizes of about 4 nm. In the case of zirconia, these particles are monocrystalline with a cubic structure. This structure is not in equilibrium under the experimental conditions.
Preparation of heat-resistant alumina aerogels
Authors: Mizushima Yasuyuki , Hori Makoto
Published: September 2007

Alumina aerogels were prepared using supercritical drying methods, and their thermal properties were examined. The effects of several additives to the alumina aerogel and supercritical drying methods were examined in order to improve heat resistance. Silica, phosphoric oxide, barium oxide, lanthanum oxide, and SiC whisker were effective for maintaining a high specific surface area of the alumina aerogel at a temperature over 1200 °C. Silica was found to be the most effective among the additives. The addition of 10 mol % silica resulted in an alumina aerogel with the highest specific surface area, 114.3 m2/g at 1200 °C, and increased the transformation temperature to α alumina. Barium oxide and lanthanum oxide formed smaller crystals within the alumina, compared with those of alumina alone. SiC whisker caused many cracks in the alumina aerogel. When supercritically dried, the alumina aerogel was strengthened by treatment at higher temperature and pressure.
Amorphous and nanocrystalline Fe–Ti prepared by ball milling
Authors: Zaluski L. , Tessier P. , Ryan D.H. , Doner C.B. , Zaluska A. , Ström-Olsen J.O. , Trudeau M.L. , Schulz R.
Published: September 2007

Nanocrystalline FeTi has been prepared in two ways: by ball milling the intermetallic compound and mechanically alloying a mixture of the elemental powders. The materials obtained in each case are identical. The reaction proceeds via the formation of interfacial β–Ti(Fe) which then grows to include all of the material present. Oxygen levels above 3 at. % suppress this reaction and lead to the formation of amorphous Fe–Ti.
High temperature mechanical properties of Cr2Nb-based intermetallics
Authors: Takasugi T. , Hanada S. , Miyamoto K.
Published: September 2007

The results of high-temperature compressive deformation tests on a two-phase alloy consisting of Cr2Nb, a C15 Laves phase, and a soft Nb-based solid solution, are presented along with measurements of oxidation kinetics at 1273 K in air. These alloys are deformable only at temperatures above 1273 K. The measured 0.2% yield stress decreases steadily with increasing temperature and is only slightly sensitive to alloy composition. The steady state flow stress decreases steadily with increasing temperature and depends on alloy composition. A constitutive equation was fitted to the experimental data with a composition-independent stress exponent of about 2.7 and an apparent activation energy which ranges between 477 and 391 kJ/mol, also depending on alloy composition. Microstructural examination shows that cracking (cavitation) and interfacial sliding between the two constituent phases, in addition to bulk deformation of the constituent phases, are responsible for the deformation. The oxidation resistance of these alloys is very good.
Calorimetric analysis of the system Ag2S–Ag2Se between 25 and 250 °C
Authors: Pingitore N.E. , Ponce B.F. , Estrada L. , Eastman M.P. , Yuan H.L. , Porter L.C. , Estrada G.
Published: September 2007

Differential scanning calorimetry (DSC) demonstrates that compounds in the pseudobinary system Ag2S–Ag2Se undergo rapid, reversible solid-state phase changes at temperatures between approximately 70 and 178 °C. These temperatures vary systematically with composition, with highs at the pure end members, Ag2S (178 °C) and Ag2Se (134 °C), and a low in the compositional range of approximately Ag2S0.4Se0.6to Ag2S0.3Se0.7(70 °C). These data are consistent with the presence of two solid solutions in this system at ambient conditions: the Ag2S–III-type, monoclinic, ranging to approximately Ag2S0.4Se0.6and the Ag2Se-II-type, orthorhombic, extending from Ag2Se to Ag2S0.3Se0.7. Entropies of transition of 6 to 9 e.u. characterize compositions within the Ag2S-III-type solid solution, whereas values of 13 to 16 e.u. mark members of the Ag2Se-II-type solid solution. The high-temperature allotrope is presumably a continuous solid solution between Ag2S-II and Ag2Se-I, which have similar body-centered cubic structures.
Indium-tin oxide thin films by metal-organic decomposition
Authors: Gallagher Dennis , Scanlan Francis , Houriet Raymond , Mathieu Hans Jörg , Ring Terry A.
Published: September 2007

In2O3–SnO2 films were produced by thermal decomposition of a deposit which was dip coated on borosilicate glass substrates from an acetylacetone solution of indium and tin acetoacetonate. Thermal analysis showed complete pyrolysis of the organics by 400 °C. The thermal decomposition reaction generated acetylacetone gas and was found to be first order with an activation energy of 13.6 Kcal/mole. Differences in thermal decomposition between the film and bulk materials were noted. As measured by differential scanning calorimetry using a 40 °C/min temperature ramp, the glass transition temperature of the deposited oxide film was found to be ∼462 °C, and the film crystallization temperature was found to be ∼518 °C. For film fabrication, thermal decomposition of the films was performed at 500 °C in air for 1 h followed by reduction for various times at 500 °C in a reducing atmosphere. Crystalline films resulted for these conditions. A resistivity of ∼1.01 × 10−3 Ω · cm, at 8 wt. % tin oxide with a transparency of ∼95% at 400 nm, has been achieved for a 273 nm thick film.
Microwave-hydrothermal processing for synthesis of electroceramic powders
Authors: Komarneni Sridhar , Li Q. , Stefansson Karin M. , Roy Rustum
Published: September 2007

Microwave-hydrothermal processing has so far been used only todissolveinorganic solids for chemical analysis. We report herein the use of microwave-hydrothermal processing to synthesize various ceramic powders in binary and polynary systems. We describe the synthesis of some electroceramic powders such as BaTiO3, SrTiO3, Sr0.5Ba0.5TiO3, PbTiO3, BaZrO3, SrZrO3, Pb(Zr0.52Ti0.48)O3, and pyrochlore phases with the Pb(Mg1/3Nb2/3)O3and Pb(Zn1/3Nb2/3)O3compositions by this novel microwave-hydrothermal processing technique.
The effect of siloxane spin-on-glass and reaction bonded silicon oxycarbide coatings with a self-propagating interfacial reaction treatment (ASPIRE) in the synthesis of carbon/graphite fiber-reinforced aluminum metal matrix composites
Authors: Balaba Willy M. , Weirauch Douglas A. , Perrotta Anthony J. , Armstrong George H. , Anyalebechi Princewill N. , Kauffman Suzanne , MacInnes Andrew N. , Winner Angela M. , Barron Andrew R.
Published: September 2007

Carbon fibers were treated with siloxane spin-on-glass and reaction bonded silicon oxycarbide coatings. The spin-on-glass (SOG) coatings were prepared by pyrolyzing solutions of polymethylsilsesquioxane (PMSO), polydimethoxysilane (PDSO), and poly(ethoxysilane)ethyltitanate copolymer (ESET). Since the flexibility of the coatings was found to be dependent on the concentration of the siloxane solution, only those of PMSO and PDSO below 1.25% were determined to be suitable for fiber coatings, and an alternative approach to the formation of a pliable silicon-based ceramic coating on the fibers was developed. Carbon fiber tows were impregnated by ethanolic solutions of organosilicon chlorides and fired at temperatures up to 900 °C to form a flexible reaction bonded silicon oxycarbide (RB–SiOC) coatings. Uncoated, SOG coated, and RB–SiOC coated carbon fibers were embedded in aluminum metal at 1000 °C. While both silica-based coatings protected the carbon surface, no wetting was observed, leading to fiber pull-out. When the coated fibers were treated with a mixture of Ti and B prior to immersion into the molten aluminum, complete wetting of the fibers occurred. In the presence of molten aluminum, the Ti/B coating enabled the exothermic formation of TiB2 and titanium aluminides, which facilitate wetting. This reaction is termed ASPIRE (Aluminum Self-Propagating Interfacial Reaction) and in combination with silicon-based ceramic coatings provides a scientific approach to the formation of stable carbon fiber/aluminum metal-matrix composites. The coated fibers and composites were characterized by scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) analysis, and x-ray photoelectron spectroscopy (XPS).
Alloy thermodynamics in nanostructures
Authors: Weissmüller Jörg
Published: September 2007

The importance of the interactions between alloy atoms and topological defects for the thermodynamic properties of nanostructured alloys is pointed out. The McLean model for grain boundary segregation is extended to yield an expression for the total Gibbs free energy of an alloy polycrystal. This provides a simple conceptual basis for a qualitative discussion of the thermodynamic properties of nanocrystalline alloys. It is demonstrated that certain alloy poly- or nanocrystals may reach a metastable state, where the alloy is stable with respect to variation of its total grain boundary area.
La2CuO4+δ: Synthesis under high oxygen pressure and study of phase relations and energetics
Authors: Rapp R.P. , Mehta A. , DiCarlo J. , Navrotsky A.
Published: September 2007

High oxygen pressures have been achieved in a piston-cylinder apparatus using a double capsule assembly consisting of a sealed outer Au capsule, containing an oxygen source (KMnO4), and an inner, open Pt capsule containing the sample. Using this technique, La2CuO4 was annealed at 800 °C, 5–25 kbar for 2–4 h. Transposed temperature drop calorimetry at 704 °C was used to determine the enthalpy of oxidation, and weight loss measurements characterized the oxygen nonstoichiometry, δ, in La2CuO4+δ, in the high-pressure, oxygen-annealed samples. For samples analyzed at room temperature, x-ray diffraction measurements show that beyond δ ≍ 0.10–0.13, additional oxygen is accommodated in a perovskite-like LaCuO3−α phase. An analysis of the thermochemical measurements indicates that the nature of holes in La2CuO4+δ could change in the range of δ ≍ 0.03–0.06.16,17 It is further suggested that the observed change in the thermochemical behavior in the range of δ ≍ 0.03–0.06 could be the driving influence behind the spinodal decomposition of La2CuO4+δ at low temperatures (Dabrowski et al.10).
Friction and wear of argon-implanted silicon crystals
Authors: Lekki J. , Stachura Z. , Preikschas N. , Cleff B. , Cholewa M. , Legge G.J.F.
Published: September 2007

Silicon 〈111〉 single crystals were implanted with 70 keV Ar ions to the dose of 1017 ions/cm2. Next, the friction coefficient between a Si crystal and a hard steel ball was measured using a pin-on-disk setup in air and in vacuum. The wear tracks were measured using a surface profilometer. For measurements performed in vacuum, a strong influence of implantation on friction force and wear tracks was found. The microstructure of the samples was subsequently investigated using RBS, ERD, and x-ray diffraction (XRD) techniques. Micro-RBS measurements showed that Ar had been removed from the wear tracks, despite their continued exhibition of low friction.
Interface states in ZnO varistor with Mn, Co, and Cu impurities
Authors: Yano Yoshihiko , Takai Yoshizo , Morooka Hisao
Published: September 2007

The interface states in ZnO with impurities of transition-metals, Mn, Co, and Cu, were investigated by the DLTS (deep-level transient spectroscopy) measurements in ZnO/PrCoOx/ZnO junctions as model systems of ZnO ceramic varistors and by the SCF-Xα-SW molecular orbital calculations using simplified cluster models. The DLTS signals, correlated to the doping of Mn and Co, are obtained with ZnO/PrCox/ZnO junctions. The signals correspond to the interface states due to the transition-metal doping. Xα calculations indicate that the interface states attributed to the doping of transition-metals, Mn, Co, and Cu, in ZnO are created between the valence band and the conduction band, which consist of transition-metals 3d character. The impurities of transition-metals affect interface states as well as the adsorbed excess oxygen.
The effect of the distribution of vacancies on the magnetic properties of γ – Fe2O3 particles
Authors: Morales M.P. , de Julián C. , González J.M. , Serna C.J.
Published: September 2007

Samples of γ-Fe2O3 have been prepared from uniform α-Fe2O3 particles obtained by homogeneous hydrolysis in solution and directly by spray pyrolysis methods. The crystalline structure of these samples was analyzed by using x-ray and electron diffraction. It was concluded that, depending on the preparation method and parameters, different degrees of order in the distribution of vacancies were present. The study of the magnetic properties of these samples included the measurement of both intrinsic (saturation magnetization) and extrinsic properties (coercive force and magnetization behavior in the approach to saturation region). It was found that all the above-mentioned quantities depended on the degree of order in the distribution of vacancies.
The relationship between the MOCVD parameters and the crystallinity, epitaxy, and domain structure of PbTiO3 films
Authors: Bai G.R. , Chang H.L.M. , Foster C.M. , Shen Z. , Lam D.J.
Published: September 2007

Lead- and titanium-based oxide thin films were prepared by the metal-organic chemical vapor deposition technique (MOCVD) and the relationship between the film structures and the processing parameters, such as the ratio of Pb/Ti precursors in the gas phase, substrate materials, substrate surface orientation, and growth temperature, was systematically studied. It was found that whether a single-phase stoichiometric PbTiO3 film could be obtained depended on both the Pb/Ti precursor ratio in the gas phase and the deposition temperature. Under appropriate conditions, stoichiometric PbTiO3, films could be obtained on all the substrates including silicon, MgO, α-Al2O3, SrTiO3, and LaAlO3. The PbTiO3 films grown on silicon substrates were always polycrystalline, whereas epitaxial PbTiO3 films were obtainable on all the other substrates. For epitaxial PbTiO3 films, the epitaxial relationship, crystallinity, and domain structures were found to be a function of both the substrate materials and surface orientation as well as the deposition temperature. X-ray rocking curves (ω scan) of the (100) and (001) planes of PbTiO3 epitaxial film and PbTiO3 single crystal revealed the inherent nature of the domain structures in PbTiO3.
Preparation of zinc oxide seed grains
Authors: Liangbin Wang , Xiao-Guang Li , Xinyu Xiang , Chun Chen , Yuheng Zhang
Published: September 2007

ZnO seed grains are derived from washing ZnO-BaO sintered disks in boiling water. The grain growth of ZnO results from the energy difference across a curved grain boundary. The growth dynamics of ZnO grains and the dissolution of BaO in the boiling water are studied. The classical model shows that the growth of ZnO grains depends on grain boundary diffusion. The dissolution of BaO in boiling water can be considered as the diffusion of BaO in boiling water. The weight percent of BaO dissolved in boiling water varies linearly with the thickness of ZnO-BaO sintered disks and the reciprocal of the square root of the boiling time.
EPR and magnetization of La2NiO4
Authors: González-Calbet J.M. , Vallet-Regí M. , Sayagués M.J. , Sánchez R.D. , Causa M.T.
Published: September 2007

EPR and magnetization measurements of La2NiO4 samples prepared by two different methods are discussed. From the comparison with magnetization and EPR properties of metallic nickel it can be concluded that the sample prepared by the ceramic method is contaminated with ∼ 1% of metallic nickel whereas no evidence of contamination is found in a sample prepared by the so-called liquid-mix technique.
Raman and infrared investigations of glass and glass-ceramics with composition 2Na2O · 1CaO · 3SiO2
Authors: Ziemath Ervino C. , Aegerter Michel A.
Published: September 2007

Precursor glass and glass-ceramics with molar composition 2Na2O · ICaO · 3SiO2 are studied by infrared, conventional, and microprobe Raman techniques. The Gaussian deconvoluted Raman spectrum of the glass presents bands at 625 and 660 cm−1, attributed to bending vibrations of Si-O-Si bonds, and at 860, 920, 975, and 1030 cm−1, attributed to symmetric stretching vibrations of SiO4 tetrahedra with 4, 3, 2, and 1 nonbridging oxygens, respectively. The Raman microprobe spectrum of a highly crystallized sample presents two narrow and intense bands at about 590 and 980 cm−1, associated with vibrations of SiO4 tetrahedra with two nonbridging oxygens, in agreement with the predicted chain-like structure of crystalline metasilicates. Scanning electron microscopy shows that the crystals distributed in partially crystallized samples have a spherical shape, built up by radially oriented needle-like single crystals. The Raman microprobe spectra of these spherulites show that they still contain residual amorphous material. A comparison of Raman and infrared spectra of amorphous and highly crystallized samples is presented.
The permittivity at X-band frequencies of nickel-coated graphite fibers in an epoxy matrix
Authors: Ho Yung-Shou , Schoen P.
Published: September 2007

In this study, we have investigated the microwave dielectric behavior of a composite formed by embedding nickel-coated graphite fibers in an epoxy matrix. Permittivities of composites in the X-band frequency range as a function of fiber concentration, fiber length, and the degree of fiber aggregation were studied. Fiber aggregation was reduced significantly by the addition of silica particles to the composite mixture before epoxy curing. Predictions from the mean field theory fit the experimental data well at dilute fiber concentrations.
Stability of cubic phase in nanocrystalline ZrO2
Authors: Chatterjee A. , Pradhan S.K. , Datta A. , De M. , Chakravorty D.
Published: September 2007

Stable cubic ZrO2 up to a temperature of 1173 K has been synthesized by a chemical precipitation technique. Such a stability appears to be driven by particle size. The critical value below which cubic ZrO2 has been found to be stable is 17 nm. The x-ray diffraction pattern of such ultrafine cubic particles is similar to that obtained by stabilization of ZrO2 by the addition of 20 mole % CaO.
Elastic strain gradients and x-ray line broadening effects as a function of temperature in aluminum thin films on silicon
Authors: Venkatraman Ramnath , Besser Paul R. , Bravman John C. , Brennan Sean
Published: September 2007

Grazing incidence x-ray scattering (GIXS) with a synchrotron source was used to measure elastic strain gradients as a function of temperature in aluminum and aluminum alloy thin films of different thicknesses on silicon. The stresses in the films are induced as a result of the difference in thermal expansion coefficient between film and substrate. Disregarding minor deviations at the surface, it is shown that there are no gross strain gradients in these films in the range of temperatures (between room temperature and 400 °C) considered. Significant x-ray line broadening effects were observed, suggesting an accumulation of dislocations on cooling the films and their annealing out as the films were being reheated. The variation of the dislocation density during thermal cycling compares well in nature with that of the concurrent variation in film stress, indicating that large strain hardening effects contribute toward the film flow stress.
Elevated temperature mechanical behavior of CoSi and particulate reinforced CoSi produced by spray atomization and co-deposition
Authors: Baskin Don , Wolfenstine Jeff , Lavernia Enrique J.
Published: September 2007

Monolithic CoSi and TiB2reinforced CoSi materials were produced by spray atomization and co-deposition. The creep behavior of both materials at elevated temperature was investigated. The unreinforced material of grain size ≍25 μm exhibited a stress exponent of three, activation energy for creep of 320 kJ/mole, dislocation substructure of homogeneously distributed dislocations, and inverse creep transients upon stress increases. These results suggest that the creep behavior of CoSi is controlled by a dislocation glide mechanism. In contrast, the reinforced material of a finer grain size (≍10 μm) exhibited a stress exponent of unity, activation energy for creep of 240 kJ/mole, and negligible creep transients upon stress increases, suggesting that the creep behavior of the reinforced material is controlled by a diffusional creep mechanism. The creep resistance of the reinforced material was lower than that for the unreinforced material. This is a result of the finer grain size and higher porosity in the reinforced material.
Crystallization of amorphous melt-spun Nd15Fe77Bx (x = 6–14) alloys
Authors: Harada T. , Kuji T.
Published: September 2007

In the process of crystallization of amorphous melt-spun Nd15Fe77Bx (x = 6–14) alloys, it was found that the crystallization behavior strongly depended upon the boron content even in the same equilibrium three-phase coexistence range. The Nd2Fe14B phase was crystallized directly from the amorphous state in the amorphous Nd-Fe-B materials with relatively lower boron content (Nd15Fe77B6 and Nd15Fe77B8 alloys). In contrast, the metastable body-centered-cubic (bcc) α-iron phase crystallized from the amorphous state prior to the crystallization of the Nd2Fe14B phase in the amorphous Nd-Fe-B materials with relatively higher boron content (Nd15Fe77B10, Nd15Fe77B12, and Nd15Fe77B14 alloys). The existence of the metastable bcc α-iron phase in the amorphous matrix was confirmed by TEM studies, magnetic measurements, and in situ observation by x-ray diffraction. The formation of the metastable phase stabilized the amorphous state, increasing the crystallization temperature of the Nd2Fe14B phase.
The migration of interstitial H in diamond and its pairing with substitutional B and N: Molecular orbital theory
Authors: Mehandru S.P. , Anderson Alfred B.
Published: September 2007

We present results of atom superposition and electron delocalization molecular orbital (ASED-MO) calculations of interactions of interstitial H with substitutional B and N in diamond. Nearest-neighbor and next-nearest-neighbor C atoms were relaxed in geometry depending on the cluster size, XC34H36 or XC70H60, respectively, where X = B or N and the H atoms saturate the surface dangling radical orbitals of the models. A small Jahn-Teller distortion occurs for interstitial B, a shallow acceptor which, in the B− state, sits in a tetrahedral lattice site. For interstitial N distortions are large, with a long C-N distance which stabilizes a ŝ∗ orbital that would otherwise be in the conduction band. This orbital has one electron in it and has its greatest amplitude on C; the bonding counterpart has its greatest amplitude on N and is similar to the N lone-pair orbital in amines. The calculations indicate that N is a deep donor and N+ relaxes to the tetrahedral lattice site. Interstitial H is a mid-band-gap donor and is possibly also an acceptor with a high 1.9 eV calculated activation energy barrier to migration. Interstitial H+ is expected to be very mobile, with a migration barrier of 0.1 eV. H− is predicted to be relatively immobile with an activation barrier for migration of 2.5 eV. The mobility of bond-inserted H around B in BH pairs should be high, with a calculated activation energy of 0.13 eV, but for N the comparable process has an activation energy of 2.50 eV. In NH pairs the interstitial H has formed a bond with the radical orbital on the C, so donation would be from the lone-pair orbital on N, which lies deep in the band gap; hence, the donor property is passivated.
Ion beam mixing study on Cu/Al2O3 bilayer samples using Rutherford backscattering spectrometry
Authors: Balogh A.G. , Macht M-P. , Naundorf V.
Published: September 2007

Cu/Al2O3 bilayer samples were produced by vapor deposition and irradiated successively with 150 keV Ar ions up to a dose of 500 dpa. Ion beam mixing effects were studied by 2 Me V He+ Rutherford backscattering spectroscopy (RBS). Concentration depth profiles show the mixing of Cu, Al, and O atoms in the sample. Because of the low mixing efficiency found in this study, Al2O3-dispersion strengthened Cu alloys seem to be good candidates as structural materials in fusion reactors.
Sedimentation in flocculating colloidal suspensions
Authors: Abel Jeffrey S. , Stangle Gregory C. , Schilling Christopher H. , Aksay Ilhan A.
Published: September 2007

A combined experimental and theoretical investigation of the sedimentation of unstable colloidal ceramic suspensions has been performed. Suspensions containing submicron-sized α-Al2O3 particles were prepared at various pH values in order to modify suspension stability. Particle volume fraction during sedimentation was determined as a function of position and time by gamma-ray densitometry. A population balance model was developed to account for various coagulation and decoagulation mechanisms that affect sedimentation behavior in flocculating suspensions. Model predictions were then compared with experimental measurements, in order to establish the validity of the theoretical model.
Densification kinetics of binary borosilicate glass composite
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

Densification kinetics and mechanism of a binary borosilicate glass composite, containing low-softening borosilicate (BSG) and high-softening high silica (HSG) glasses, have been studied. Apparent activation energy of densification varies from 200 to 400 kJ/mol, and decreases with increasing BSG content at a given densification factor. At a given BSG content, the activation energy of densification initially remains relatively unchanged with increasing densification factor (DF), but increases with densification when DF reaches a critical value (DF∗). Moreover, the value of DF∗ increases with increasing BSG content. From the activation energy estimates of densification, it is concluded that the predominant densification mechanism for BSG ≥ 30 vol. % with DF < DF∗ is viscous flow of low-softening BSG. For BSG ≥ 30 vol. % with DF < DF∗ and BSG ⋚ 20 vol. % with all DF investigated, the activation energies are within the range governed by viscous flow of both BSG and HSG, indicating that the densification is controlled by viscous flow of a new glass with a composition between BSG and HSG. The latter evidence stems from the microstructural observation that as sintering proceeds, the HSG particle undergoes an extensive dissolution process.
A novel surface modification technique for aluminum
Authors: Timsit R.S. , Lauzon D.C.
Published: September 2007

Surface modification is achieved by coating the aluminum surface with a powder mix of {Si + flux + a metal (Me)} and heating at ∼600 °C for a time interval of the order of 1 min. During heating, the flux melts and dissolves the native surface oxide film on aluminum, thus allowing Si to react with the substrate to form a film of liquid Al-Si alloy of near-eutectic composition. The component Me of the powder coating then reacts with the molten metal layer to modify the metallurgical properties of the surface after solidification.
Molecular dynamics simulations of surface reconstruction at the edges of a crack in ruthenium aluminum
Authors: Becquart C.S. , Clapp P.C. , Rifkin J.A.
Published: September 2007

Using molecular dynamics computer simulations and interatomic potentials derived partly by Voter and Chen1 and Rifkin et al.,2 we studied the surface reconstruction taking place on free surfaces of arrays of RuAl. Surface reconstruction appears to be very important on {111} and {110} types of planes and almost nonexistent on {100} type of planes. Cracks oriented so that their crack planes were either {111} types or {110} types exhibit on their internal free surface important surface reconstruction. It is believed that this effect may have some contribution in the brittle versus ductile behavior of the crack.
Identification of partial dislocations and faults in cubic Al3Ti
Authors: Zhang S. , Milligan W.W. , Mikkola D.E.
Published: September 2007

Dislocation dissociations in Al3Ti alloys modified with Mn to stabilize the L12 cubic structure have been studied with transmission electron microscopy and computer simulation of images. Dissociations of a〈110〉 superdislocations into pairs of a/2〈110〉 partials bounding APB's were observed at all temperatures from room temperature to 700 °C. Asymmetrical image contrast, in which one of the a/2〈110〉 partials gives a much more intense image than the other, was observed at small separations of the partials. Although some investigators have taken such asymmetry to suggest SISF-type dissociations in similar alloys, the current work demonstrates that the asymmetry is fully consistent with APB-type dissociation. Further, the degree of image asymmetry decreases as the spacing of the partials increases. It is concluded that identification of the partial dislocations with “near-invisibility criteria” for fractional values of g · b is unreliable, and that computer simulation of images is useful for identification of the partials. However, as expected, the ability to distinguish simulated bright-field images of APB- and SISF-type dissociations also becomes difficult as the separation of the partials becomes very small. Under these conditions, both weak-beam imaging and simulations are necessary to identify the dissociations. Weak-beam simulations have shown that fringe contrast must be present under certain imaging conditions for SISF dissociations, and this contrast has never been observed in this study or in previously published studies of dissociated single superdislocations in cubic Al3Ti alloys. Finally, APB contrast formed with superlattice reflection imaging has been observed between partials on both {111} and {100} after deformation at 700 °C.