bluuebunny/crossref_metadata_2025 huggingface dataset:
Materials characterization and effect of purity and ion implantation on the friction and wear of sublimed fullerene films
Authors: Gupta B.K. , Bhushan Bharat , Capp C. , Coe J.V.
Published: September 2007

In previous studies, sublimed C60-rich fullerene films on silicon, when slid against a 52100 steel ball under dry conditions, have exhibited low coefficient of friction (∼0.12). Films with different purities can be produced by sublimation at different substrate temperatures. In this paper, effects of purity of fullerene films and ion implantation of the films with Ar ions on the friction and wear properties of sublimed fullerene films are reported. C60-rich films (called here films with high purity) exhibit low macroscale friction. An increased amount of C70 and impurities in the fullerene film determined using Raman and Fourier transform infrared (FTIR), increases its coefficient of friction. Microscale friction measurements using friction force microscopy also exhibited similar trends. Low coefficient of friction of sublimed C60-rich films on silicon is probably due to the formation of a tenacious transfer film of C60 molecules on the mating 52100 steel ball surface. Based on scanning tunneling microscopy (STM), transmission electron microscopy (TEM), and high resolution TEM (HRTEM), we found that fullerene films primarily consisted of C60 molecules in a fcc lattice structure. Nanoindenter was used to measure hardness and elastic modulus of the as-deposited films. Ion-implantation with 1 × 1016 Ar+ cm−2 reduced macroscale friction down to about 0.10 from 0.12 with an increase in wear life by a factor of 4; however, doses of 5 × 1016 ions cm−2 gave three times higher friction and poorer wear life; higher doses disintegrated the C60 molecules. Based on STM, TEM, Raman, FTIR, and laser desorption Fourier-transform ion cyclotron resonance mass spectrometer (LD/FT/ICR) studies, we found that the ion implantation with a dose of 1 × 1016 Ar+ cm−2 resulted in smoothening of the fullerene film surface probably by compacting clusters, but without disintegrating the C60 molecules. However, a high dose of 5 × 1016 Ar+ cm−2 damaged the C60 molecules, converting it to an amorphous carbon. Nanoindentation studies show that ion implantation with a dose of 1 × 1016 Ar+ cm−2 resulted in an increase in the hardness from about 1.2 to 4.0 GPa and in elastic modulus from about 70 to 75 GPa and modified the elastic-plastic deformation behavior.
Raman scattering characterization of (100) and (111) oriented diamond films grown in the same run by hot filament chemical vapor deposition
Authors: Popovici G. , Khasawinah S. , Sung T. , Prelas M.A. , Spitsyn B.V. , Loyalka S. , Tompson R. , Chamberlaine J. , White H.
Published: September 2007

The crystalline quality of a diamond film with two different preferential orientations (100) and (111), obtained in the same run by hot filament chemical deposition, has been studied. The quality of the film determined by Raman spectroscopy measurements was found to be nearly the same for both orientations. The second order Raman spectrum for diamond film was observed by using an infrared excitation.
Particle size distributions of aerosols formed by laser ablation of solids at 760 Torr
Authors: Whitlock Robert R. , Frick Glendon M.
Published: September 2007

The size distributions of aerosol particles generated in N2 and O2 at ambient atmospheric pressure by repetitive excimer laser ablation of solids have been measured in quasi-steady-state from 10 nm to 1 μm diameter with a differential mobility analyzer (DMA). A light scattering technique extended the range of measurable sizes up to 20 μm, beyond the upper limit of sizes observed in the aerosol. Irradiation near focus was required to observe metal particles above 0.1 μm diameter. YBCO ceramic (YBa2Cu3O7) produced copious smaller particles with mild focusing. For an ablated polymer, PMMA, particulate production increased and the size distribution altered shape with continued energy deposition.
Mechanism of solid-gas reaction for formation of metastable niobium-nitride alloy powders by reactive ball milling
Authors: Sherif El-Eskandarany M. , Sumiyama K. , Aoki K. , Masumoto T. , Suzuki K.
Published: September 2007

We report on the formation of a new metastable phase of niobium-nitride (Nb-N) alloy powders. This nonequilibrium phase has been synthesized by milling elemental niobium (Nb) powders under purified nitrogen gas flow using a high energy ball mill at room temperature. The progress of the solid-gas reaction for the Nb-N system has been followed by means of x-ray diffraction, differential thermal analysis, transmission electron microscopy, and chemical analysis. After 720 ks of milling, the initial bcc-Nb is completely transformed to nonequilibrium-fcc-NbN containing about 50 at. % N. The lattice parameter of the end product fcc-NbN was calculated to be 0.433 nm. In addition, the particle and grain sizes of the completely reacted alloy powders are 3 μm and 5 nm, respectively. This transforms to a mixture of hcp-Nb4N3 and hcp-NbN at the high temperatures, as high as 1150 K.
Structural variations and transformation behavior of the Al68Cu11Co21 decagonal phase
Authors: Grushko B. , Wittmann R. , Urban K.
Published: September 2007

The Al68Cu11Co21 decagonal phase was studied after annealing at 1000 °C for 24-760 h by transmission and scanning electron microscopy. The strong superstructure odd-n reflections in the [1,-2, 1, 0, 0] electron diffractograms were stable under annealing up to about 40 h. As a possible origin of the increased intensities of the odd-n reflections the formation of vacancy-ordered structures is discussed. The structure was modified by prolonged annealing. In several annealed samples a dense net of extra reflections overlapping the quasiperiodic reflections was observed. This observation was explained as an indication of the formation of metastable states during cooling. Differently ordered decagonal structures exhibited different transformation kinetics during cooling from high temperatures.
X-ray photoelectron spectroscopic study of hydrated aluminas and aluminas
Authors: Tsuchida Takeshi , Takahashi Hideaki
Published: September 2007

X-ray photoelectron spectra of hydrated aluminas (boehmite, diaspore, bayerite, and gibbsite), transition aluminas (y, δ, η, θ, X, and k –Al2O3) and corundum (α-Al2O3) have been studied for spectral characterization of each compound. The O1s spectra are shifted 0.2–1.2 eV to higher binding energy (Eb) in the order of α-Al2O3 < boehmite, diaspore < bayerite, gibbsite, and this agrees with the order of bulk OH/Al molar ratio in samples. The Eb and FWHM values of O1s spectra of transition aluminas depend on the ratio OH/O, i.e., the amount of OH− ions chemisorbed on them, and tend to decrease toward those of α-Al2O3 with increasing calcination temperature. Therefore, it is considered that an attracting effect of the proton on valence electrons in the hydroxyl oxygen causes the increased binding energy between core electrons and oxygen atomic nuclei. The broad O1s spectra of boehmite and diaspore can be deconvoluted into equal contribution from the two oxygen species in O2− and OH− ions in their structures.
Molecular beam homoepitaxial growth of MgO(001)
Authors: Chambers S.A. , Tran T.T. , Hileman T.A.
Published: September 2007

We describe homoepitaxial growth and detailed in situ characterization of MgO(001). We have used, for the first time, high-speed Auger electron spectroscopy as a real-time probe of film composition during growth. Excellent short-range and long-range crystallographic order are achieved in films grown to a thickness of several hundred angstroms in the substrate temperature range of 450 °C to 750 °C. Moreover, the films become more laminar as the growth temperature increases, suggesting that MgO grows homoepitaxially by the step-flow growth mechanism at elevated temperature. The surfaces of films grown at 650°and 750 °C are smoother than those obtained by cleaving MgO(001).
Reactive ion etching of PbZr1−x TixO3 and RuO2 films by environmentally safe gases
Authors: Pan Wei , Desu S.B. , Yoo In K. , Vijay Dilip P.
Published: September 2007

A new process for the reactive ion etching (RIE) of both PbZr1−x TixO3 (PZT) thin films and RuO2 electrodes is presented, employing etching gases with low ozone depletion potential (ODP) and global warming potential (GWP). The etching process has been investigated as a function of etching time, discharge power density, chamber pressure, and additive gas. Etch rates were in the range of 250–650 Å/min and 100–400 Å/min for PZT and RuO2, respectively. A large etch selectivity between PZT and RuO2 was optimized. Etched surfaces exhibited smooth morphologies. Furthermore, the ferroelectric properties of PZT were not altered significantly by the etching process. A surface residue containing Cl and F was found after etching, but this organic substance was totally removed by an after-etch bake. In addition, the etched profile of the PZT films was studied through SEM.
Rare gas bubbles in muscovite mica implanted with xenon and krypton
Authors: Hishmeh G.A. , Cartz L. , Desage F. , Templier C. , Desoyer J.C. , Birtcher R.C.
Published: September 2007

Xenon and krypton have been implanted into muscovite mica at room temperature and at liquid nitrogen temperature. The behavior of the implanted Xe and Kr was followed by low-temperature transmission electron microscopy and energy dispersive x-ray analysis. An electron diffraction pattern of diffuse bands is observed at room temperature due to the presence of fluid rare gas and to noncrystalline mica. Visible cavities with diameters 10–300 nm formed in the Xe-implanted mica. Visible cavities in room-temperature Kr-implanted mica ranged from 5–50 nm in diameter. The gas pressures at room temperature in the cavities are estimated, assuming all of the implanted gas precipitated in cavities to be ∼10 MPa for Xe and ∼20 MPa for Kr. These pressures are considerably lower than found for rare gases implanted in metals and ceramics, but sufficient to liquefy the rare gases at room temperature. The Xe and Kr were observed by dark-field microscopy to form fcc crystalline solids within the cavities at temperatures below their triple points, with lattice parameters of a(xe) = 0.630 ± 0.0015 nm and a(Kr) = 0.565 ± 0.005 nm. The solid Xe within bubbles was unstable under the electron beam of the transmission electron microscope at temperatures above 80 K, while the solid Kr within bubbles was unstable at temperatures as low as 35 K. The crystalline mica matrix undergoes a transformation from a crystalline structure to an amorphous structure as a result of implantation.
Energetics in the brownmillerite-perovskite pseudobinary Ca2Fe2O5-CaTiO3
Authors: Prasanna T.R.S. , Navrotsky A.
Published: September 2007

The energetics of intermediate phases in the Ca2Fe2O5-CaTiO3 pseudobinary system was determined by high temperature solution calorimetry. The results suggest that both the intermediate compounds Ca3Fe2TiO8 and Ca4Fe2Ti2O11 are entropically stabilized because their enthalpies of formation are positive with respect to a mixture of Ca2Fe2O5 and CaTiO3. At high temperatures, the configurational entropy of the random distribution of Fe and Ti ions on the octahedral sites appears sufficient to stabilize the intermediate phases. The enthalpies of formation from the oxides at 1073 K of Ca2Fe2O5, Ca3Fe2TiO8, and Ca4Fe2Ti2O11 are −45.0 ± 3.8, −123.7 ± 8.0, and −192.7 ± 11.2 kJ/mol, respectively. Their enthalpies of formation from the elements at 298 K are −2139.8 ± 4.4, −3798.4 ± 8.6, and −5447.3 ± 12.2 kJ/mol, respectively.
Intrinsic stresses in compositionally modulated Au-Ni thin films and the supermodulus effect
Authors: Baker Shefford P. , Nix William D.
Published: September 2007

The stresses that arise in compositionally modulated Au-Ni thin films as a result of the constraint of the substrate were determined from the curvatures of micrometer-scale bilayer cantilever beams consisting of a silicon dioxide layer upon which the metal film had been deposited. These substrate interaction stresses were found to vary strongly with the wavelength of the composition modulation. Simulations of the bulge test show that such stresses can account for the major features of the “supermodulus effect” as artifacts of the analysis method.
Change in mechanical properties and microstructure of 201 stainless steel with increased nitrogen alloying
Authors: Rawers J.C. , Asai G. , Dunning J.S.
Published: September 2007

It is well known that mechanical properties of commercial stainless steel are improved by alloying with nitrogen. In this study a series of nitrogenated commercial 201 stainless steel alloys with nitrogen levels as high as 2.6 wt. % were obtained by melting in a hot-isostatic-pressure furnace using nitrogen as the pressurizing gas. Nitrogen concentrations in excess of 1.25 wt. % formed a series of different chromium nitride precipitates and morphologies depending upon the nitrogen concentration. Five different nitrogen levels were fabricated using the same processing conditions recommended for 201 stainless steel including hot-and cold-working, and heat-treating at two different temperatures. Tensile strength of the nitrogenated materials at each processing step was related to the interstitial nitrogen concentration and the presence or absence of precipitates. The presence of chromium precipitates did reduce the fracture ductility and changed the fracture features. This U.S. Bureau of Mines study shows that increasing the nitrogen concentration in commercial steels above their current level has positive effects on mechanical properties as long as the nitrogen solubility level is not exceeded and chromium nitride precipitates begin to form.
The microstructure and morphology of carbon black: A study using small angle neutron scattering and contrast variation
Authors: Hjelm Rex P. , Wampler Wesley A. , Seeger Philip A. , Gerspacher Michel
Published: September 2007

This is a study of the form and structure of particles and particle aggregates of an experimental high surface area carbon black (HSA) using small-angle neutron scattering and the method of contrast variation. Contrast variation was effected by studying suspensions of the carbon black in cyclohexane containing different fractions of deuterocyclohexane. We find that the approximately 29 nm diameter USA particles are arranged as small, linear aggregates with an average aggregation number between 4 and 6. The structure averaged over the particle population is best represented by a prolate ellipsoid of revolution with semiaxes 14.5 and 76.4 nm. The surface of the aggregates appears smooth over length scales larger than 1 nm, which places an upper limit on the surface roughness observed by other methods. The internal structure of the aggregates is described by a shell-core model, with the shell density being consistent with a graphitic structure and the core being of lower density, more like amorphous carbon. Some fraction of the core volume (0.1 to 0.2) is taken up by voids that are not accessible to the solvent. An estimate of the shell thickness gives 1 to 2 nm along the ellipsoid minor axis and 6 to 10 nm along the major axis. The particles of the aggregate appear to be fused so that the less dense amorphous core is continuous through the inner parts of the aggregate. The information that can be obtained on the internal structure using contrast variation is limited by heterogeneity in the chemical composition of carbon black aggregates.
Morphological and structural properties of high quality YBCO thin films
Authors: Cassinese A. , Di Chiara A. , Miletto Granozio F. , Saiello S. , di Uccio U. Scotti , Valentino M.
Published: September 2007

High-quality YBCO thin films have been grown by Inverted Cylindrical Magnetron Sputtering (ICMS) on LaAlO3(100), SrTiO3(100), SrTiO3(110), and MgO(100) substrates. Transition temperatures of c-axis films exceed 90 K, and transition widths are within 1 K. Critical currents range up to 5 × 106 A/cm2 at 77 K. Structural and morphological features analyzed by x-ray diffraction and scanning electron microscopy, respectively, are found to be strongly dependent on film orientation and deposition temperature. In order to understand such dependence, a simple interpretation is proposed in terms of Gibbs energies and growth dynamics of the nucleation process.
Strontium aluminum tantalum oxide and strontium aluminum niobium oxide as potential substrates for HTSC thin films
Authors: Guo Ruyan , Bhalla A.S. , Sheen Jyh , Ainger F.W. , Erdei S. , Subbarao E.C. , Cross L.E.
Published: September 2007

Single crystal fibers of A(B11/2B21/2)O3 perovskites type with compositions Sr(Al1/2Ta1/2)O3 (SAT) and Sr(Al1/2Nb1/2)O3 (SAN) were grown successfully for the first time, using a laser-heated pedestal growth (LHPG) technique. Their crystallographic structures were found to be simple cubic perovskite with lattice parameters a = 3.8952 Å (SAT) and a = 3.8995 Å (SAN) that are close lattice matches to the YBCO superconductors. No structural phase transitions or twins have been found, and the average coefficients of the thermal expansion match well with the YBCO superconductor materials. We report that SAT is one of the most promising substrates to date for the epitaxial growth of HTSC thin films suitable for microwave device applications as it has low dielectric constants (K ∼ 11–12, at 100 Hz–10 GHz and 300 K) and low dielectric loss (∼4 × 10−5 at 10 kHz and 80 K), together with lattice parameter matching, thermal expansion matching, and chemical compatibility with the high Tc superconductors (YBCO).
Crystal orientation and near-interface structure of chemically vapor deposited MoS2 films
Authors: Lee Woo Y. , More Karren L.
Published: September 2007

Crystalline MoS2 films were deposited on Si and graphite substrates using MoF6 and H2S as precursors. The crystal orientation and near-interface structure of the MoS2 films were studied using transmission electron microscopy. In general, the preferred orientation of the (002) basal planes of the MoS2 films with respect to the substrate surface changed from parallel to perpendicular with increased deposition temperature from 320 to 430 °C. At 430 °C, the basal planes were primarily oriented perpendicular to the Si substrate, except for the presence of a ∼5 nm interface region in which the basal planes were oriented in the parallel direction. The formation of this transitional region was also observed on the graphite substrate.
Laser-induced microstructural changes and decomposition of aluminum nitride
Authors: Cao Siqi , Pedraza A.J. , Allard L.F.
Published: September 2007

The microstructural changes induced by pulsed laser irradiation in the surface layer of AlN and the initial stage of electroless copper deposition in laser processe specimens have been investigated using transmission electron microscopy (TEM). It was found that a dislocation microstructure is generated by laser processing at laser energy densities of 1.5 J/cm2 or higher. A very sharp change in the dislocation microstructure was seen at a depth of 0.2 to 0.3 μm from the free surface. The dislocation Burgers vector is 〈100〉 and the slip plane is {001}, in agreement with previous reports. AlN was melted and resolidified homo-epitactically from the solid substrate forming a mosaic microstructure with very fine cells having a misorientation of up to 15°. Patches of metallic aluminum were found at the surface of all the specimens irradiated at a laser energy density of 1.5 J/cm2 or higher. Very fine particles of AlN, 20 to 50 nm in diameter, were randomly distributed inside the patches. Immersion of these specimens in an electroless copper bath showed that the electroless solution preferentially etched away aluminum at the Al-AlN interface. At the same time copper islands were deposited in cavities left by AlN particles as well as at the interface with the underlying substrate. These regions are the seeds for further electroless deposition. The TEM observations of laser-induced microstructural changes reported in this paper help to unravel further the mechanisms of adhesion enhancement and surface activation by pulsed laser irradiation.
Synthesis and characterization of sol-gel derived lanthanum hexaluminate powders and films
Authors: Cinibulk Michael K.
Published: September 2007

Powders and thin films of high-yield lanthanum hexaluminate (LaAl11O18) were prepared by a sol-gel route and compared with yields obtained by conventional hot-pressing of oxide powders. X-ray diffraction and transmission electron microscopy (TEM) were used to characterize powders and thin films deposited on TEM grids. While the solid-state kinetics of formation of LaA11O18 are known to be extremely sluggish, the yield of LaAl11O18 formed by the sol-gel route was much higher than that obtained by processing under similar conditions by solid-state reaction of elemental oxides. The development of a very fine grained microstructure at 1200 °C and a coarser, much more mature microstructure at 1450 °C, with strong texturing of the magnetoplumbite phase, was observed by TEM. Isolated grains of LaAlO3 were present in all powders and films. Trace impurities, introduced most likely as impurities in the initial alumina sol, appear to have segregated to both the grain boundaries and to the external surfaces of grains in as-prepared films.
Factors influencing the formation of hollow ceramic microspheres by water extraction of colloidal droplets
Authors: Liu Jay G. , Wilcox David L.
Published: September 2007

Hollow ceramic microspheres of Al2O3, SiO2, and mullite have been prepared by the combination of an emulsion technique with a water extraction sol-gel method. Concentration of sol, initial droplet size, and water extraction rate of the system are found to be the important process parameters controlling the size and wall thickness of the hollow microspheres, and their influences are shown. A model that correlates the morphology of microspheres to concentration and water extraction rate is proposed and is in good agreement with the experimental observations. The capability and limitation of this process for forming hollow microspheres are demonstrated. It was shown that hollow microspheres with sizes greater than 5 μm could be readily prepared, while a limitation was met for sizes less than 1 μm, in which case solid microspheres were normally formed.
An experimental study of the temperature and stoichiometry dependence of diamond growth in low pressure flat flames
Authors: Kim J.S. , Cappelli M.A.
Published: September 2007

A study of the temperature and stoichiometry dependence of diamond synthesis in low pressure premixed acetylene-oxygen flames is presented. A specially designed low pressure flat flame operating at 40 Torr is employed to deposit diamond films uniformly over areas of at least 2 cm2. Under optimized conditions of substrate temperatures and flame equivalence ratios, high quality translucent diamond that is well faceted is synthesized exhibiting first-order Raman fullwidths (half maximum) of about 2.5 cm−1. Diamond growth rates under these optimum conditions are approximately 4 μm/h. The film growth rate is found to drop off substantially at high substrate temperatures, with little or no carbon deposited beyond a temperature of 1070 °C. The growth behavior in response to changes in flame equivalence ratio and substrate temperature is discussed in terms of the possible role that oxygen-containing species may have on surface chemistry. The results described here are also used to project a base cost for manufacturing diamond under these process conditions.
Formation of highly oriented diamond film on carburized (100) Si substrate
Authors: Maeda Hideaki , Irie Miki , Hino Takafumi , Kusakabe Katsuki , Morooka Shigeharu
Published: September 2007

Highly oriented diamond film was grown on a (100) Si substrate by a bias-enhanced microwave-plasma chemical vapor deposition. The Si surface was carburized at a faster rate by bias treatment than by carburization alone, but the initial carburization stage was indispensable. During the bias treatment, the flat surface was changed to a textured structure on the nanometer scale. The formation of this structure was required for the synthesis of a highly oriented diamond film. Diamond microcrystals formed subsequently were irregular and of a few to a few tens nanometers in size. They then grew to oriented film in the following growth process.
Diamond nucleation on unscratched silicon substrates coated with various non-diamond carbon films by microwave plasma-enhanced chemical vapor deposition
Authors: Feng Z. , Brewer M.A. , Komvopoulos K. , Brown I.G. , Bogy D.B.
Published: September 2007

The efficacy of various non-diamond carbon films as precursors for diamond nucleation on unscratched silicon substrates was investigated with a conventional microwave plasma-enhanced chemical vapor deposition system. Silicon substrates were partially coated with various carbonaceous substances such as clusters consisting of a mixture of C60 and C70, evaporated films of carbon and pure C70, and hard carbon produced by a vacuum are deposition technique. For comparison, diamond nucleation on silicon substrates coated with submicrometer-sized diamond particles and uncoated smooth silicon surfaces was also examined under similar conditions. Except for evaporated carbon films, significantly higher diamond nucleation densities were obtained by subjecting the carbon-coated substrates to a low-temperature high-methane concentration hydrogen plasma treatment prior to diamond nucleation. The highest nucleation density (∼3 × 108 cm−2) was obtained with hard carbon films. Scanning electron microscopy and Raman spectroscopy demonstrated that the diamond nucleation density increased with the film thickness and etching resistance. The higher diamond nucleation density obtained with the vacuum are-deposited carbon films may be attributed to the inherent high etching resistance, presumably resulting from the high content of sp3 atomic bonds. Microscopy observations suggested that diamond nucleation in the presence of non-diamond carbon deposits resulted from carbon layers generated under the pretreatment conditions.
The behavior of screw dislocations dynamically emitted from the tip of a surface crack during loading and unloading
Authors: Huang C.C. , Yu C.C. , Lee Sanboh
Published: September 2007

The behavior of screw dislocations dynamically emitted from the tip of a surface crack during loading and unloading has been investigated using a discrete dislocation model. The critical stress intensity factor at the crack tip for dislocation emission is a function of friction stress, core radius of dislocation, and dislocations near the crack tip. During motion, the velocity of dislocation is assumed to be proportional to the effective shear stress to the third power. The effect of crack length and friction stress on dislocation distributions, plastic zone, and dislocation-free zone during loading and unloading was examined.
Luminescence of polymer electrolytes containing europium (III)
Authors: Carlos L.D. , Assunção M. , Alcácer L.
Published: September 2007

The excitation and emission spectra of polymeric matrices—poly(ethylene oxide), PEO, and poly(propylene oxide), PPO—containing different concentrations of EuBr3 were recorded and most of the observed transitions identified. The Stark components of the 7F0−4, 5D0 levels and the corresponding barycenters were calculated based on the emission lines assignment to the 5D0,1 → 7F0−4 transitions. The spectra were discussed in terms of a C2v local Eu3+ coordination. The relative intensity of the 5D0 → 7F2 hypersensitive transition was related to the electrolyte morphological changes due to increasing europium concentration.
Nanophase Ni particles produced by a blown arc method
Authors: Teng M.H. , Host J.J. , Hwang J.-H. , Elliott B.R. , Weertman J.R. , Mason T.O. , Dravid V.P. , Johnson D.L.
Published: September 2007

Nanophase Ni particles (<10 nm in diameter) were produced by a blown arc method. A helium gas stream directed at the arc reduces the Ni vapor concentration and increases the quench rate. The helium gas velocity is the predominant factor influencing the size of the Ni particles. Gas velocities of 20 m/s and 56 m/s (at 26.6 kPa total helium pressure) resulted in Ni particle sizes of 13 nm and 7 nm, respectively.
Carbon nanostructures in silica aerogel composites
Authors: Song Xiang-Yun , Cao Wanqing , Ayers Michael R. , Hunt Arlon J.
Published: September 2007

A new method of preparing carbon nanotubes and their derivatives using silica aerogels as a matrix for the deposition of carbon is repeated. We present results of observations of graphite tubes and rings including nested structures in nanometer dimensions using high resolution transmission electron microscopy. Furthermore, we propose a model for the growth of carbon nanotubes in three steps including nucleation, growth, and closure of tubes.
Simulated growth and microstructure of DyBa2Cu3O7−x with and without Dy2BaCuO5 addition
Authors: Vandewalle N. , Cloots R. , Ausloos M.
Published: September 2007

We present optical observations of magnetically melt-textured DyBa2Cu3O7−x with and without 20 wt. % excess of Dy2BaCuO5. From these observations, we propose some kinetic mechanism of the growth of 123 compounds. Kinetic processes can be simulated on computers. Two (very) simple models derived from the well-known Eden model are presented. They simulate the growth of the grain front. The simulated patterns agree with the observations. The microstructure of such materials cannot be explained by thermodynamic and chemical considerations alone, but explanations must include the kinetics of the growth front as well. From our observations, we conclude that the growth probability ratios g110/g100 and g100/g001 are of the order of 10 and 50, respectively.
Magnetic hardening of melt-spun 2:14:1-based materials by high heating rate and short time crystallization treatments
Authors: Murillo N. , González J. , González J.M. , de Julián C. , Cebollada F.
Published: September 2007

Systematic studies have been carried out about the effects of the thermal treatment parameters on melt-spun materials quenched at different cooling rates and based on the 2:14:1 hard magnetic phase. Samples of nominal compositions Dy3Nd10.2Fe79.6B6Si1.2 and Pr3Nd10.2Fe79.6B6Si1.2 were annealed at temperatures above that of crystallization of the amorphous phases present upon quenching, for times ranging from 1 to 30 min and by using different heating rates up to the annealing temperature. It is concluded that best hysteretic properties can be achieved in samples quenched at intermediate cooling rates by means of short-time thermal treatments performed by using high heating rates up to the treatment temperature. Low heating rates and long time anneals led to the deterioration of the hard magnetic behavior, due to the segregation of soft crystalline phases.
Microwave characteristics of BaPr2Ti4O12 and BaPr2Ti5O14 ceramics
Authors: Fukuda Koichi , Kitoh Ryozo , Awai Ikuo
Published: September 2007

Microwave dielectric properties and microstructures of BaPr2Ti4O12 and BaPr2Ti5O14 ceramics which have extremely similar crystal structures, were investigated, including those of the solid solution represented by the formula of Ba6−xPr8+2x/3Ti18O54 ceramics. It is suggested that BaPr2Ti5O14 ceramics are composed of mixed phases, approximately 85.0 vol. % Ba4.05Pr9.3Ti18O54 (x = 1.95), 8.1 vol. % TiO2, and 6.9 vol. % BaTi4O9. The microwave dielectric properties of the Ba6−xPr8+2x/3Ti18O54 system are strongly dependent on the x value, which is interesting for studying the relationship between microstructures and dielectric properties. Far infrared reflection spectra were also measured to study the mixed phases formed in these ceramics in more detail.
Characterization and stabilization of Si3N4 suspensions
Authors: Galassi C. , Rastelli E. , Roncari E. , Ardizzone S. , Cattania M.G.
Published: September 2007

Two commercial Si3N4 powders were mixed with Al2O3 and Y2O3 through coprecipitation of the oxides from solutions of nitrates. The samples were characterized by XPS spectroscopy and electrophoretic mobility determinations on the suspensions. Sedimentation tests indicated that an oxylignin-based polyelectrolyte proved effective in the dispersion of the powders. The adsorption of the deflocculant followed Langmuir type isotherms characterized by values of the standard free energy of adsorption in the range typical of physisorption processes. The results of the different characterizations are discussed point by point, and different localizations of the dispersant in the interfacial region of the different samples are proposed.
Rapid synthesis of transition-metal borides by solid-state metathesis
Authors: Rao Lin , Gillan Edward G. , Kaner Richard B.
Published: September 2007

A rapid self-sustaining solid-state precursor route to transition-metal borides, boride solid solutions, and boride composites has been developed. Solid-state metathesis (SSM) reactions between transition-metal chlorides and magnesium boride (MgB2) produce crystalline borides and magnesium chloride. Boride solid solutions are formed using mixed chloride precursors. By using a third precursor, such as NaN3, boride-nitride composites are synthesized. The reaction products are characterized by powder x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and inductively coupled plasma atomic absorption spectroscopy. These boride reactions become self-propagating when the adiabatic temperature is greater than the melting point of the by-product salt, MgCl2(mp 987 K).
Development of fullerene-reinforced aluminum
Authors: Barrera E.V. , Sims J. , Callahan D.L.
Published: September 2007

Powder metallurgy and casting have been used to produce aluminum with 1.3, 4, and 8 vol. % fullerene additions. Fullerene extract was mixed with Al and heat-treated to obtain various levels of dispersion of the fullerenes. Intergranular dispersion of stable fullerenes was accomplished by both powder metallurgy and casting; however, x-ray diffraction indicated the formation of some Al4C3. Homogeneous dispersion did not occur because of limited diffusion in the solid state or limited solubility of fullerene in Al in the liquid state. Enhancements in hardness over that for Al were observed yet were not comparable to precipitation hardened Al alloys since a less homogeneous dispersion was achieved. Interest in Al having fullerene additions is for development of fullerene strengthened materials where fullerenes act as nanosize dispersoids for dispersion strengthening of metals or as a lightweight reinforcement in metal-matrix composites.
An experimental fracture mechanics study of a strong interface: The silicon/glass anodic bond
Authors: Hurd D.S. , Caretta R. , Gerberich W.W.
Published: September 2007

The fracture behavior of the silicon/Pyrex glass anodic bond was investigated using the methods of linear elastic bimaterial fracture mechanics. Due to the high bond strength, interfacial cracks were invariably observed to kink away from the interface into the more compliant glass under approximately mode I remote tensile loading. Kink angles measured by profilometry increased from 14 to 28°as bonding temperature increased from 300 to 450 °C. A regime of stable cracking accompanied penetration of cracks into the glass, with maximum load and corresponding fracture toughness measurement occurring at a location significantly removed from the interface. Approximately mode I, near-interface, plane-strain fracture toughness values (KIC) measured by rising load testing of chevron-notched and straight-thru-cracked compact-tension specimens increased from 0.63 to 0.68 MPa-m1/2 and 0.66 to 0.75 MPa-m1/2, respectively, as bonding temperature increased from 300 to 450 °C. In addition, XPS measurements revealed a sodium depletion zone of decreasing size and depletion magnitude with increasing bonding temperature over the same range. The near-interface region of the glass also experiences compressive residual stresses which decrease linearly with distance from the interface according to linear elastic computations. These stresses increase in magnitude with increasing bonding temperature due to enhanced differential thermal contraction upon cooling to room temperature. It is proposed that the trends in toughness and in kink angle with bonding temperature can be at least partially accounted for by variation of crack-tip shielding with compressive residual stress magnitude, the effects of interfacial. crack-tip shear stresses induced by the thermal mismatch, and by an increase in Young's modulus of the near-interface glass accompanying sodium depletion.
Effect of hydrogenated amorphous carbon films on nucleation of diamond particles by hot-filament chemical vapor deposition
Authors: Tamaki Kazunori , Nakamura Yoshikazu , Watanabe Yoshihisa , Hirayama Shigekazu
Published: September 2007

To enhance a nucleation rate of diamond particles, hydrogenated amorphous carbon (a-C: H) intermediate layers have been formed by radio frequency plasma chemical vapor deposition (CVD) on silicon substrates prior to diamond deposition by hot filament CVD, and the effect of a-C: H intermediate layers on the nucleation and growth rate of diamond particles is studied by varying the thickness of a-C: H films. It is found that diamond particles are well synthesized on thin a-C: H intermediate layers and the nucleation density and growth rate are decreased with increasing the thickness of a-C: H films. Atomic force microscope observations show that a-C: H intermediate layers with rough surface are more effective than the smooth surface for diamond synthesis. Raman spectroscopy shows that the bonding state of carbon atoms in a-C: H films does not change by varying the thickness of a-C: H films. It is proposed that diamond nucleation is affected by the surface morphology rather than the bonding state of carbon atoms in a-C: H films.
Study of polycarbonate degradation induced by irradiation with He+ ions
Authors: Švorčik Václav , Rybka Vladimír , Miček Ivo , Hnatowicz Vladimír , Kvítek Jiří
Published: September 2007

A stack of five polycarbonate foils, each 1.4 μm thick, was irradiated with 1.3 MeV 4He+ ions to the dose of 1.1 × 1014 cm−2. Ion beam induced polymer degradation, as a function of the particle energy, was studied by UV-VIS and IR spectroscopy of individual foils. In the irradiated foils, a significant reduction of characteristic absorption bands is observed, indicating polymer degradation. Significant increase of the surface polarity, characterized by a polar component of the surface free energy, is also found. Both the degree of the polymer degradation and the surface polarity correlate with the total energy deposited by 4He+ ions in the foils.
Morphology of instability of the wetting tips of eutectic SnBi, eutectic SnPb, and pure Sn on Cu
Authors: Kim H.K. , Liou H.K. , Tu K.N.
Published: September 2007

The Pb-based solder used in microelectronics industry is becoming an environmental issue. To understand the wetting behavior of solders with and without Pb, we have studied the surface morphology and wetting reaction of eutectic SnBi, eutectic SnPb, and pure Sn on Cu through the measurements of wetting angle change and wetting tip stability by SEM and EDX. The wetting angle remains constant after the initial spread, but the eutectic SnPb/Cu continues to react and forms a reaction band in front of the solder edge as well as intermetallic compounds at the interface. For eutectic SnBi/Cu, there is no reaction at the wetting tip, and the wetting angle does not change much; however, the interfacial reaction between eutectic SnBi and Cu forms intermetallic compounds at the solder joint; the wetting tip is not in a static equilibrium. A rough surface and edge was observed on the eutectic SnBi/Cu joint, but the eutectic SnPb/Cu has a smoother surface and edge.
Monte Carlo simulations of long-range order kinetics in B2 phases
Authors: Yaldram K. , Pierron-Bohnes V. , Cadeville M.C. , Khan M.A.
Published: September 2007

The thermodynamic parameters that drive the atomic migration in B2 alloys are studied using Monte-Carlo simulations. The model is based on a vacancy jump mechanism between nearest neighbor sites, with a constant vacancy concentration. The ordering energy is described through an Ising Hamiltonian with interaction potentials between first and second nearest neighbors. Different migration barriers are introduced fur A and B atoms. The results of the simulations compare very well with those of experiments. The ordering kinetics are well described by exponential-like behaviors with two relaxation times whose temperature dependences are Arrhenius laws yielding effective migration energies. The ordering energy contributes significantly to the total migration energy.
Fabrication, microstructure, and mechanical properties of TaC particulate and SiC fiber-reinforced lithia-alumina-silica composites
Authors: Shin Hyun-Ho , Kirchain Randolph , Speyer Robert F.
Published: September 2007

Additions of O to 9 mol % Ta2O5 to a lithia-alumina-silica glass-ceramic matrix Nicalon SiC-reinforced composite increased the elastic modulus and ultimate strength of the composite. The additive fostered sphereulitic growth of β-eucriptite solid solution crystals which concentrated Ta2O5 at sphereulite boundaries and adjacent to the fiber-matrix carbon-rich interphases. These regions reacted with the interphases as well as soluble carbon monoxide gas to convert them to TaC. The former reaction was shown to be thermodynamically favorable above 983 °C, while the latter was favorable above 1249 °C. The improvement in mechanical properties was attributed to TaC particulate reinforcement, and suggests a simple glass-ceramic route to the fabrication of particulate-reinforced ceramic matrix composites.
Investigation of SiC–AlN system: Part I. Microstructure and solid solution
Authors: Huang Jow-Lay , Jih Jyh-Ming
Published: September 2007

The primary aim of this present work was to investigate the solubility, microstructure, thermal stability, AlN polytypes, and interfacial reactions in the SiC-AlN system. Alpha SiC particles of different sizes were incorporated into AlN and hot pressed at 2050 °C for 1 to 3 h. XRD, OM, STEM, WDS, and SAD patterns were utilized in analyzing phases, morphology, and distribution of elements. The surface area of SiC particles was found to have substantial effects on the formation of solid solution. Multilayered polytypes, which are usually associated with stacking faults, grew and impeded one another. Additionally, the number of planes between subsequent stacking increased with the increase of either hot-pressing temperature or time. Affinity of oxygen to AlN probably played an essential role in the formation of polytypes and stacking faults.
Synthesis and characterization of TiO2 thin films on organic self-assembled monolayers: Part I. Film formation from aqueous solutions
Authors: Shin H. , Collins R.J. , De Guire M.R. , Heuer A.H. , Sukenik C.N.
Published: September 2007

Self-assembled monolayers (SAMs) bearing sulfonate (-SO3H) surface functional groups, on single-crystal Si wafers, were used as substrates for the deposition of TiO2 thin films from aqueous solutions. Polycrystalline TiO2 thin films over 50 nm thick formed in 2 h by hydrolysis of TiCl4 in aqueous HCI solutions at 80 °C. The films were pore-free, showed excellent adherence and uniformity, and consisted of anatase crystallites 2–4 nm in diameter. Annealing at temperatures up to 600 °C caused coarsening of the anatase grains, but no loss of adherence or structural integrity.
Structures and electrical properties of barium strontium titanate thin films grown by multi-ion-beam reactive sputtering technique
Authors: Peng C-J. , Krupanidhi S.B.
Published: September 2007

The structure and electrical properties of multi-ion beam reactive sputter (MIBERS) deposited barium strontium titanate (BST) films were characterized in terms of Ba/Sr ratio, substrate temperature, annealing temperature and time, film thickness, doping concentration, and secondary low-energy oxygen ion bombardment. Films deposited onto unheated substrates, followed by annealing at 700 °C showed lower dielectric constant (<200), compared to a dielectric constant of about 560 for those deposited at elevated temperatures, probably due to reduced voids. Two types of microstructures (type I and type II) were observed depending on the incipient phase of the as-grown films, which also led to two types of time domain dielectric response, Curie-von Schweidler and Debye type, respectively. The current-voltage (I-V) characteristics of type II films doped with high donor concentration showed a bulk space-charge-limited conduction (SCLC) with discrete shallow traps embedded in a trap-distributed background at high electric fields. The I-V characteristics of bombarded films deposited at higher substrate temperatures showed promising results of lower leakage currents and trap densities.
A combustion synthesis process for synthesizing nanocrystalline zirconia powders
Authors: Venkatachari K.R. , Huang Dai , Ostrander Steven P. , Schulze Walter A. , Stangle Gregory C.
Published: September 2007

Materials with nanocrystalline features are expected to have improved or unique properties when compared to those of conventional materials. Methods for the practical and economical production of nanoparticles in large quantities are not presently available. A method based on combustion synthesis for preparing nanocrystalline powders was investigated in this work. Yttria-doped zirconia powders with an average crystallite size of 10 nm were synthesized. The characteristics of the powder (e.g., surface area and phase content) were found to depend strongly on the fuel content in the starting mixture and on the ignition temperature used in the process. The method is expected to be suitable for commercial fabrication of nanocrystalline multicomponent oxide ceramic powders.
Preparation of nanocrystalline yttria-stabilized zirconia
Authors: Venkatachari K.R. , Huang Dai , Ostrander Steven P. , Schulze Walter A. , Stangle Gregory C.
Published: September 2007

Nanocrystalline powder with an average crystallite size of 8–12 nm, which was produced by a combustion synthesis process, was used to prepare dense, nanocrystalline articles. Green compacts of high green density were prepared by dry pressing and densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final grain size and final density. On the other hand, the ranges of heating rates and dwell times that were used had a much less significant effect on the final density and final grain size. It was determined, however, that a high final density (>99% ρth) and a very fine final average grain size (<200 nm) can be simultaneously achieved under three different firing conditions. The high densification rates are, in part, a result of the minimal coarsening that the particles undergo when the sample is taken rapidly through the temperature regime in which surface diffusion predominates to the temperature regime in which the densification mechanisms of grain boundary and lattice diffusion predominate.
A novel process for combustion synthesis of AlN powder
Authors: Lee Wei-Chang , Tu Chang-Lin , Weng Chang-Yueh , Chung Shyan-Lung
Published: September 2007

A SHS process has been established for synthesis of AlN powder under low nitrogen pressures. Al and NaN3 powders were used as the metal and nitrogen sources, respectively. The compact of the mixture of the reactants was wrapped up with aluminum foil and then wrapped up with an igniting agent (i.e., Ti + C). The synthesis reaction was triggered by the combustion of the igniting agent. The wrappings were found necessary in achieving high product conversions under low nitrogen pressures (<1 MPa). The product conversion was also affected by the reactant composition and the nitrogen pressure. High conversions were obtained when the mass ratio of Al to NaN3 was 1/2 or lower and the nitrogen pressure was 0.5 MPa or higher. The AlN powders as synthesized were observed to have two major types of morphology, i.e., granular particles with 0.5–3 μm in diameter and fibers with aspect ratios of 10–800.