bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation, microstructure, chemical long-range order, and stability of quasicrystals in Al–Pd–Mn alloys
Authors: Tsai An-Pang , Yokoyama Yoshihiko , Inoue Akihisa , Masumoto Tsuyoshi
Published: September 2007

The icosahedral (i-) phases are found over a wide composition range in melt-quenched Al–Pd–Mn alloys. The i-phase has a simple icosahedral lattice in the low Pd content region (Pd <10 at. %) and transforms to a face-centered icosahedral with increasing Pd content (Pd >10 at. %). A decagonal phase with a periodicity of 1.6 nm exists at a low Mn content region. On the basis of the change in diffraction peak intensity, it was presumed that the chemical ordering between Al and Pd exists in a face-centered icosahedral lattice in which Pd and Mn occupy one superlattice site and Al occupies the other one. A dendritic growth morphology in a mixed structure containing i- and fcc Al phases changes to a planar growth morphology in a fully single i-phase. An interpretation of dependence of morphology on composition is given in terms of the interface stability criterion using a metastable i–Al phase diagram.
Evolution of crystalline zirconia structure in heat-treated ceria stabilized zirconia gels
Authors: Nagarajan V.S. , Rao K.J.
Published: September 2007

Transformation characteristics of CeO2 stabilized ZrO2 gels have been studied during heating in the temperature region 1273–1473 K. The nature of the major phase present changes drastically in this temperature region. The crystallite sizes of tetragonal and monoclinic ZrO2 crystallites have been calculated by x-ray line broadening. The change in the nature of the major phase observed during heat treatment seems to be related to the presence of a barrier for transformation from a metastable to a stable regime, which in turn appears to be related to the diffusion barriers of Ce4+ in ZrO2.
A kinetic study of the phase transformations in Al2O3 and Na2O · 6Al2O3 by a combined use of 27Al MAS NMR spectroscopy and x-ray diffraction: Importance of nucleation
Authors: Prabakar S. , Rao K.J. , Rao C.N.R.
Published: September 2007

Phase transformations of Al2O3 and Na2O · 6Al2O3 prepared by the gel route have been investigated for the first time by 27Al MAS NMR spectroscopy in combination with x-ray diffraction. Of particular interest in the study is the kinetics of the γ → α and γ → β transformations, respectively, in these two systems. Analysis of the kinetic data shows the important role of nucleation in both these transformations.
Silicon oxycarbide glasses: Part I. Preparation and chemistry
Authors: Renlund Gary M. , Prochazka Svante , Doremus Robert H.
Published: September 2007

Silicone polymers were pyrolyzed to form silicon oxycarbides that contained only silicon, oxygen, and carbon. The starting polymers were mainly methyl trichlorosilane with a small amount of dimethyl dichlorosilane. NMR showed that the polymers had a silicon-oxygen backbone with branching and ring units. When the polymer was heated in hydrogen, toluene and isopropyl alcohol, used in production of the polymer, were given off in the temperature range 150 °C to 500 °C. Substantial decomposition of the polymer itself began only above about 700°by evolution of methane. The network of silicon-oxygen bonds and silicon-carbon bonds did not react and was preserved; the silicon-carbon bonds were linked into the silicon-oxygen network. The silicon oxycarbide was stable above 1000 °C, showing no dimensional changes above this temperature. The interior of the silicon oxycarbide was at very low effective oxygen pressure because oxygen diffused slowly in it. There was also a protective layer of silicon dioxide on the surface of the silicon oxycarbide.
Effects of process history and aging on the properties of polyimide films
Authors: Denton Denice D. , Buncick Milan C. , Pranjoto Hartono
Published: September 2007

Polyimide is used extensively in a variety of integrated circuit packaging applications. It is a good dielectric material with excellent planarizing capabilities, but like most polymers, it absorbs moisture. This hygroscopic behavior can lead to reliability problems in integrated circuit packages. The effects of variations in process history on moisture uptake are examined using gravimetric measurement techniques. In particular, the effects of cure schedule and exposure to high temperature/high humidity environments (85 °C/85% RH) on steady state moisture uptake are reported. Steady state moisture uptake is shown to be a decreasing function of cure temperature. Samples cured at 250 °C absorb 25% more moisture by weight than do samples cured at 400 °C. Moreover, the steady state moisture uptake in polyimide is greater after the samples have been “aged” in a high temperature and humidity ambient. The bulk and surface chemical composition are also monitored as a function of aging using Fourier transform infrared spectroscopy (FTIR) and electron spectroscopy for chemical analysis (ESCA), respectively. The PI surface chemistry degrades after 700 h in an 85 °C/85% RH environment. The bulk chemical composition appears to be unaffected.
Effect of CO2 on the processing of Y–Ba–Cu–O superconductors
Authors: Selvaduray G. , Zhang C. , Balachandran U. , Gao Y. , Merkle K.L. , Shi H. , Poeppel R.B.
Published: September 2007

The superconducting properties of YBa2Cu3O6+x reacted with various known ratios of O2/CO2 gas mixtures during sintering at different temperatures were studied. Jc was found to decrease drastically upon reaction with CO2, becoming zero at certain CO2 activities. The stability region for the 123 superconductor, as a function of CO2 activity and temperature, was empirically formulated as follows: log pCO2 < (−45,000)/T + 33.4. The grain boundaries in sintered samples with Jc = 0 were investigated with HRTEM in conjunction with EDS. Two distinct types of grain boundaries were observed. Approximately 10% of the grain boundaries were wet by a thin layer of a second phase, deduced to be BaCuO2. The remaining boundaries were sharp grain boundaries. The grain structure near the sharp grain boundaries was tetragonal. These two types of grain boundaries are thought to be responsible for Jc being zero.
The influence of test frequency, temperature, and environment on the fatigue resistance of an Ni3Al–B/Cr/Zr intermetallic alloy
Authors: Camus G.M. , Duquette D.J. , Stoloff N.S.
Published: September 2007

Stress-controlled fatigue tests have been carried out on an Ni3Al–B/Cr/Zr alloy, at 600 °C and 800 °C in air and in vacuum at various test frequencies. Decreasing the test frequency and/or increasing the temperature leads to a decrease in the number of cycles to failure and a gradual disappearance of a fatigue fracture zone. This trend is shown to be related to a true creep component. Environment has a weak interacting effect on fatigue life but strongly influences the fracture path in the fatigue zones, with fracture becoming partly or entirely intergranular when the environment is changed from vacuum to air. It is suggested that most of the fatigue life is spent in initiating a crack. Comparisons are made with some creep data in terms of fracture paths and time to rupture. Fatigue life at 800 °C is shown to be entirely controlled by creep damage at the lowest test frequency of 0.2 Hz.
Preparation of molybdenum nitride thin films by N+ ion implantation
Authors: Choi J-G. , Choi D. , Thompson L.T.
Published: September 2007

A series of molybdenum nitride films were synthesized by implanting energetic nitrogen ions into molybdenum thin films. The resulting films were characterized using x-ray diffraction to determine the effects of nitrogen ion dose (4 × 1016−4 × 1017 N+/cm2), accelerating voltage (50–200 kV), and target temperature (∼298–773 K) on their structural properties. The order of structural transformation with increased incorporation of nitrogen ions into the Mo film can be summarized as follows: Mo → γ−Mo2N → δ−MoN. Nitrogen incorporation was increased by either increasing the dose or decreasing the ion energy. At elevated target temperatures the metastable B1–MoN phase was also produced. In most cases the Mo nitride crystallites formed with the planes of highest atomic density parallel to the substrate surface. At high ion energies preferential orientation developed so that the more open crystallographic directions aligned with the ion beam direction. We tentatively attributed this behavior to ion channeling effects.
Electrochemical, hydrothermal, and electrochemical-hydrothermal synthesis of barium titanate thin films on titanium substrates
Authors: Bacsa R. , Ravindranathan P. , Dougherty J.P.
Published: September 2007

Well-crystallized polycrystalline films of cubic BaTiO3 have been synthesized on Ti metal substrates by electrochemical, hydrothermal, and electrochemical-hydrothermal reaction of a Ti metal with a saturated solution of Ba(OH)2 in the temperature range 80 °C−200 °C. The films can be produced in the electrochemical process by the anodization of a Ti substrate in Ba(OH)2 solution at pH = 13 and current densities of the order of 10–50 mA/cm2. The films can also be synthesized by the hydrothermal reaction of Ba(OH)2 and a Ti substrate without electrical bias. A combination of these two techniques reduces the reaction time to 30–45 min. The films were found to have good adherence on the substrate and films of thickness up to 2 μm could be formed within a reaction time of 45 min. Capacitance measurements have yielded a dielectric constant of 300. The films prepared by the three different techniques have been compared for crystallinity and microstructure. The thickness of the films increased with temperature and time of reaction. The crystallinity decreased with extended time of reaction for a given temperature. It is likely that a reactive intermediate such as Ti(OH)4 (aq.) is formed in all the three cases which leads to the formation of crystalline BaTiO3.
Sol-gel processing of varistor powders
Authors: Hohenberger Gert , Tomandl Gerhard
Published: September 2007

A new sol-gel method for the preparation of ZnO varistor powders, using inexpensive source materials such as acetates and nitrates, is described. It yields powders with a more homogeneous distribution of dopants compared to the commercially used mixed oxide technique. Varistor ceramics made from sol-gel powders can be sintered at lower temperatures and show improved electrical behavior. This is a consequence of formation of more varistor-active grain boundaries within the ceramics.
Compositional homogeneity in processing precursor powders to the Ba2YCu3O7−x high Tc superconductor
Authors: Kelly James F. , Ritter Joseph J.
Published: September 2007

We have used scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) to study compositional homogeneity at several stages in the processing of powders to the Ba2YCu3Ox high Tc superconductor. We investigated the effects of solution pH and ionic concentration in the precipitation preparation of mixed hydroxycarbonates of Ba, Y, and Cu as precursor to the Ba2YCu3O7−x high Tc superconductor. Our results show that both powder homogeneity and particle morphology are strongly dependent on these parameters. Additionally, studies of the compacted powders by EDS mapping at successive stages of processing show the development of phase inhomogeneities even in homogeneous starting powders.
Properties of Bi2Sr2CaCu2O8 thick films melt-processed at temperatures up to 950 °C
Authors: McGinnis W.C. , Briggs J.S.
Published: September 2007

Thick Bi2Sr2CaCu2O8 films have been produced by melting Bi2Sr2CaCu2O8 powder on MgO substrates at temperatures just above the melting temperature of the powder. X-ray diffraction measurements indicate enhanced c-axis alignment throughout the thickness of the films. Films melted at 900 °C show greater alignment and contain less Bi2Sr2CuO6 compared to those processed at 950 °C. Both the degree of alignment and the Bi2Sr2CuO6 content increase as the cooling rate is decreased. Transport measurements show that films quickly cooled from low melt-processing temperatures have the highest critical current densities (Jc = 13 kA/cm2 at 30 K). The temperature dependence of Jc is best described by a flux creep model. Finally, Jc at 4.2 K (in both parallel and perpendicular magnetic fields) undergoes a sudden drop at low fields, but levels off above 0.5 T to about 30% of the zero field value.
Phase separation during co-deposition of Al–Ge thin films
Authors: Adams C.D. , Atzmon M. , Cheng Y-T. , Srolovitz D.J.
Published: September 2007

We present the results of a combined experimental and theoretical investigation of phase separation and microstructure development in co-deposited Al–Ge thin films. For small film thicknesses and deposition temperatures above 150 °C the phase-separated films consist of an array of domains of the Al- and Ge-rich terminal phases (lateral phase separation). Films deposited at 100 °C or less contained one or both of the terminal phases plus a metastable phase. We show that the domain structure evolves during deposition in a manner consistent with a surface interdiffusion controlled process. As film thickness increases we observe a transition from the laterally phase-separated microstructure to a layered microstructure exhibiting phase separation perpendicular to the film/substrate interface (transverse phase separation), with Al segregating to the film surface. We present a thermodynamic argument based on the competition between surface and interfacial free energies to explain this transition. Finally, we discuss the stability of the transverse phase-separated microstructure in the thick-film limit in terms of the transport of Ge through the Al-rich surface layer.
Aerosol synthesis of gadolinium iron garnet particles
Authors: Xu H.K. , Sorensen C.M. , Klabunde K.J. , Hadjipanayis G.C.
Published: September 2007

Particles of gadolinium iron garnet, Gd3Fe5O12, were produced primarily by an aerosol spray pyrolysis technique starting with solutions of gadolinium and iron nitrates. The as-prepared particles were polydisperse solid spheres. Average diameters in the range 0.05 to 0.8 μ could be obtained by variation of the initial solution concentration. Larger particles to 2 μ were created by direct, non-aerosol, pyrolysis of the solutions. Heat treatment caused sintering and particle coalescence and yielded ∼95% garnet phase. The reaction time to create the garnet phase scaled with the square of the particle diameter, the smallest particles transforming the quickest. Magnetic measurements showed bulk behavior for the saturation magnetization, but the coercivity could be varied with particle size with a maximum near the single domain size.
The effect of annealing environments on the epitaxial recrystallization of ion-beam-amorphized SrTiO3
Authors: Rankin J. , McCallum J.C. , Boatner L.A.
Published: September 2007

Time-resolved reflectivity and Rutherford backscattering spectroscopy were used to investigate the effects of regrowth environments on the thermally induced solid phase epitaxial (SPE) regrowth of amorphous near-surface layers produced by ion implantation of single-crystal SrTiO3. Water vapor in the regrowth atmosphere was found to alter both the apparent rate and activation energy of the SPE regrowth. For relatively dry atmospheres, a single constant regrowth rate is observed at any given temperature, and the activation energy is 1.2 ± 0.1 eV. When the concentration of H2O vapor in the atmosphere is increased, however, the regrowth activation energy effectively decreases to ∼0.95 eV. When regrown in atmospheres containing H2O vapor, the SrTiO3 amorphous layer exhibits two distinct stages of SPE regrowth as compared to the single rate found for dry anneals. This two-stage process apparently results from the diffusion of H/OH from the regrowth atmosphere at the surface of the crystal through the amorphous layer to the regrowing crystalline/amorphous interface.
Growth mechanisms and near-interface structure in relation to orientation of MoS2 sputtered thin films
Authors: Moser J. , Lévy F.
Published: September 2007

The growth of sputter-deposited MoS2 thin films is investigated by high-resolution transmission electron microscopy. Pure high-temperature grown films are compared with H2O-contaminated films and amorphous annealed films. In the first case, the films are oriented. They have a first interface layer with crystallites having their (002) planes parallel with the substrate. The subsequent growth leads to the already described lamellar structure, with flakes perpendicular to the substrate. This structure can be explained in terms of a local branching process during crystal growth. The orientation relations between the crystallites in the parallel layer and the lamellae are determined. The local structure at the root of the lamellae, as well as at the interface, is investigated by image calculation. Water contamination in the plasma is shown to result in an amorphization of the interfacial region, followed by lamellar growth. Amorphous films annealed under vacuum do not show a lamellar structure, but have isotropic crystallization. In each of these cases, the mechanism determining the film structure is different.
Slow crack growth of CVD diamond
Authors: Drory M.D. , Gardinier C.F.
Published: September 2007

Slow crack growth in CVD diamond has been observed from indentation experiments. Diamond films of 400 μm thickness were prepared by plasma-enhanced CVD on a silicon substrate, subsequently removed from the substrate, and polished for indentation testing. A microhardness tester was used to produce Vickers indentations under low load. Crack length measurements over time revealed susceptibility to slow crack growth in ambient testing conditions. The stress-corrosion susceptibility coefficient, N, was measured as 9.3.
Buried oxide layers formed by low-dose oxygen implantation
Authors: Nakashima S. , Izumi K.
Published: September 2007

The structure of buried oxide layers formed by low-dose 16O+ implantation of 0.4 and 0.7 × 1018 cm−2 at 180 keV and by subsequent annealing in the temperature range of 1150 to 1350 °C has been investigated using cross-sectional transmission electron microscopy (XTEM). At a dose of 0.4 × 1018 cm−2, an 80-nm continuous uniform buried oxide layer having a breakdown voltage of approximately 40 V is formed after annealing at 1350 °C. At a dose of 0.7 × 1018 cm−2, multiple buried oxide layers having Si islands between them are formed at an anneal temperature of 1150 °C. The number of multiple layers is reduced as the annealing temperature increases, but the Si islands do not dissolve even after annealing at 1350 °C. The existence of the Si islands causes the breakdown voltage to fall to 0 V despite the higher dose.
Reaction mechanism of Y-system superconductors in the MPMG method
Authors: Izumi Teruo , Nakamura Yuichi , Sung Tae-Hyun , Shiohara Yuh
Published: September 2007

The effect of the melt-quenching step on the refinement of Y2BaCuO5 (211) particles, and phase transformations during crystallization in the MPMG (Melt-Powder-Melt-Growth) process were investigated. The quenched powders, which mainly consisted of Y2O3 and BaCu2O2, were transformed into the 211 and liquid phases by way of the YBa2Cu3Oy (123) phase. The transformation was examined on samples quenched from various temperatures. Through crystallographic examination using several kinds of precursors, it has been demonstrated that the addition of platinum, which is a contaminant from the crucible during the first partial melting step in the MPMG method, can refine the 211 particles.
Bulk YBa2Cu3Ox superconductors through pressurized partial melt growth processing
Authors: Hu S. , Hojaji H. , Barkatt A. , Boroomand M. , Hung M. , Buechele A.C. , Thorpe A.N. , Davis D.D. , Alterescu S.
Published: September 2007

We have developed a novel pressurized partial melt growth process for producing large pieces of bulk Y–Ba–Cu–O superconductors. During long-time partial melt growth stage, an additional driving force for solidification is obtained by using pressurized oxygen gas. The microstructure and superconducting properties of the resulting samples were investigated. It was found that this new technique can eliminate porosity and inhomogeneity, promote large-scale grain-texturing, and improve interdomain coupling as well.
Modulated microwave absorption in single crystal Bi–Sr–Ca–Cu–O
Authors: Rubins R.S. , Trybula Z. , Waplak S. , Drumheller John E. , Shi Donglu , Xu Ming
Published: September 2007

Modulated microwave absorption measurements at 9.3 GHz were made on the low field absorption observed in the superconducting phase of Bi–Sr–Ca–Cu–O. The line intensity observed with a low modulation amplitude of 50 mOe showed a maximum between 70 K and 80 K and dropped to zero near 95 K. With a modulation amplitude of 10 Oe, an additional broad line absorption was observed above 88 K, peaking in intensity near 95 K, and disappearing above 100 K. Hysteresis, associated with flux trapping, occurred when the maximum field Hmax in a sweep cycle exceeded a critical value H*(T), which varied from about 7 Oe at 59 K to 1 Oe at 82 K. For Hmax > H*, the hysteresis splitting at first varied linearly with (Hmax – H*), later saturating at a value δHmax(T). Above 70 K, the relationship δHmax = b(T0 – T) was obeyed with T0 = 92.3 K and b· = −0.3 Oe/K for H perpendicular and b· = −0.23 Oe/K for H parallel to the c-axis.
Chemistry and structural modulations in Bi2Sr2CuO6
Authors: Shen Y. , Richards D.R. , Hinks D.G. , Mitchell A.W.
Published: September 2007

A series of samples along the composition lines Bi2+xSr2−xCuOy, and Bi2Sr2−xCuOy have been used to study the structural modulation, chemistry, and superconducting properties of pseudo-tetragonal Bi2Sr2CuO6 (2201). The 2201 phase can be formed from crystallization of thin glassy platelets. The sample displayed a strong crystallographic (00l) orientation which made it possible to determine incommensurate modulations near (00l) reflections using a conventional x-ray θ-2θ scan. From the crystallization of the 2201 phase, it was found that structural modulation was intrinsic to the phase, and ordering of the structure required a long time at high temperature. High temperature in situ x-ray diffraction of a 2201 Bi2Sr1.85CuOy platelet showed that the modulation existed at 875 °C in O2 (Tmelt ≍ 892 °C in O2). These suggest that the structural modulation cannot be caused solely by oxygen ordering and that metal-ion displacement must be involved. By removing 0.04 to 0.05 oxygen atom per formula unit from Bi2Sr2CuOy and Bi2Sr1.85CuOy, the c* components of the modulation changed from 0.31 to 0.26 and from 0.38 to 0.31, respectively, while the b* component of the modulation remained approximately 0.2. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of bmod with respect to the change in oxygen content does not support the model that explained the modulation by inserting extra oxygen in the BiO plane. By varying metal-ion concentrations of Bi and Sr we found that both the lattice parameters and the modulation vectors depended more on the Bi/Sr ratio than on the Sr concentration alone. As the Bi/Sr ratio increased from 1.0 to 1.35, the modulation lines moved toward the (00l) reflections. The corresponding superstructural periodicities were calculated to vary from ∼1/5b* + 0.32 c* to ∼1/5 b* + 0.63 c*. Effects of oxygen content and metal-ion concentration on the 2201 phase formation and the superconducting properties will also be discussed.
Ion irradiation induced phase formation in Al–Ni
Authors: Kyllesbech Larsen K. , Karpe N. , B⊘ttiger J. , Bormann R.
Published: September 2007

The phase formation of elemental multilayered thin Al100−xNix films during 500 keV Xe+ irradiation has been investigated using transmission electron microscopy. The whole composition range has been covered for various substrate temperatures during the irradiation. At low temperatures, only structurally simple crystalline and amorphous phases are observed. Thermodynamic data for these simple phases were calculated using the CALPHAD method, and nonequilibrium phase diagrams were derived and discussed in the context of the experimental findings.
Phase equilibria and ternary phase formation in the In–Ni–P system
Authors: Mohney S.E. , Chang Y.A.
Published: September 2007

Isothermal sections of the In–Ni–P phase diagram have been determined at 600 °C and 470 °C. Two new ternary phases not previously identified in bulk samples were found. A phase with the composition Ni57In22P21 is present at both 600 °C and 470 °C and was found by differential thermal analysis to melt at 736 °C. The phase Ni2InP was found to be in equilibrium with InP at 470 °C, but it does not appear in the 600 °C isotherm since it melts at 526 °C. The phases (In), Ni2P, Ni5P4, and NiP2 are in equilibrium with InP at both temperatures studied.
Nanoindentation of nanocrystalline ZnO
Authors: Mayo M.J. , Siegel R.W. , Liao Y.X. , Nix W.D.
Published: September 2007

A number of nanocrystalline ceramics have been fabricated by the gas phase condensation technique. The mechanical properties of one of the first ceramics produced by this method, nanophase TiO2, have been discussed in an earlier study.1 This paper reports a similar study undertaken to examine the properties of nanocrystalline ZnO. Nanoindenter techniques are used to determine hardness, Young's modulus, and strain rate sensitivity in ultra-fine grained ZnO. Significant properties variations are experienced within a given sample, indicating a large degree of microstructural inhomogeneity. Nevertheless, a distinct evolution in properties can be observed as a function of sintering temperature. Young's modulus and hardness values increase almost linearly with increasing sintering temperature, and, in addition, there also appears to be a linear correlation between the development of the two materials properties. In contrast, strain rate sensitivity is shown to have an inverse dependence on sintering temperature. This dependence appears to be linked to the strong influence of grain size on strain rate sensitivity, so that the lower sintering temperatures, which provide the finer grain sizes, tend to promote strain rate sensitivity. The results of this study are strikingly similar to those obtained earlier for nanophase TiO2, and they indicate that the earlier results could probably be generalized to a much broader range of nanocrystalline ceramics.
Crystallization of various compositions in the Y2O3–SiO2 system
Authors: Kumar Suresh , Drummond Charles H.
Published: September 2007

In as-sintered silicon nitride with yttria as a sintering aid, the intergranular glassy phase contains Y, Si, O, and N. Results of crystallization of partly crystalline, as-quenched melt samples containing 28–40 mol% Y2O3 in the Y2O3–SiO2 system are presented. Three different compositions were melted in tungsten crucibles at 2100 °C in 1 and 50 atm nitrogen and heat treated in air in the temperature range of 1200 to 1600 °C. Phase identification by x-ray diffraction (XRD) and transmission electron microscopy (TEM) showed crystallization to β, γ, and δ polymorphs of Y2Si2O7.
Investigation of the heat-affected zone fracture in Ni3Al welds
Authors: Chen G.H. , Chen C.
Published: September 2007

Ternary Ni77Al23−yXy (X = Zr or Hf, y = 0.5 or 1) + 500 ppm boron compounds with various grain sizes were welded by a CO2 laser. Fractographic examinations of the heat-affected zone (HAZ) in the welds with or without postweld heat treatment (PWHT) were performed on the impact-fractured specimens. In laser welds, the fracture appearance of the HAZ was mixed transgranular/intergranular modes for fine-grained alloys and intergranular mode for coarse-grained materials. However, an entirely transgranular mode was observed in the base metal regardless of the grain size of the compounds. Boron desegregation at high temperatures during the thermal cycle of welding could be used to explain the fractographic change from originally ductile mode into less-ductile or even brittle fracture in the HAZ. Short-term PWHTs along with slow cooling provided sufficient time for boron segregation back to the grain boundary, resulting in a completely transgranular fracture mode in the fine-grained HAZ. Nevertheless, such a phenomenon was not observed in the HAZ of coarse-grained welds. Cracks in the HAZ of coarse-grained welds after long-term PWHT, if not so severe, could be healed by a sintering process.
Laser induced deposition of copper and lead powders
Authors: Narasimharao Dontula , Babu Suryadevara V. , Rasmussen Don H.
Published: September 2007

Copper and lead powders were precipitated by laser-induced reduction of copper(II) formate, copper(II) acetate, and lead(II) acetate dispersed/dissolved in glycerine by exposure to 248 nm KrF pulsed laser radiation. The deposited powders consisted of pure copper and a mixture of pure lead and partially reduced metallo-organic complexes, respectively. The reduction of the metallo-organic complex of lead was completed thermally. Temperature, concentration, laser pulse repetition rate, and stirring influenced the deposition rate in both cases. The copper particles were predominantly hexagonal in shape and ranged in size from 0.01–0.04 μm. The lead particles were football shaped and had a very narrow size distribution, with an axial dimension of about 0.5 μm. It was shown from a spectroscopic analysis of the reaction products that the initial nucleation was caused by photochemical processes. However, exposure of the nonaqueous solutions to CO2laser pulses at room temperature did not yield any particles.
An investigation of XeCl laser ablation of polyetheretherketone (PEEK)-carbon fiber composite
Authors: Dyer P.E. , Lau S.T. , Oldershaw G.A. , Schudel D.
Published: September 2007

The XeCl laser ablation of a polyetheretherketone (PEEK)-carbon fiber composite (APC-2) is reported. Etch rates and measurements of the ablation products have been carried out, together with scanning electron microscope evaluation of the etch craters. Selective removal of the PEEK matrix occurs for fluences ∼70–400 mJ cm−2. Net composite etching commences at ∼420 mJ cm−2, with an etch rate above this value that is determined by the carbon fibers and is consistent with thermal vaporization at high temperature (≥4000 K).
Preferred orientation of pitch precursor fibers and carbon fibers prepared from isotropic pitch
Authors: Hamada T. , Furuyama M. , Tomioka T. , Endo M.
Published: September 2007

The preferred orientation of pitch precursor fibers and carbon fibers from an isotropic pitch was studied using x-ray diffraction techniques. The φ scan profile of a pitch precursor fiber was divided into two parts: φ scan-dependent (anisotropic) and φ scan-independent (isotropic) profiles. The half width at half maximum (HWHM) intensity of the anisotropic part became smaller as the pitch viscosity increased during spinning and as the filament diameter decreased. The area ratio of the anisotropic part to the total area increased when spinning at higher pitch viscosities and when making the diameter thinner; no preferred orientation existed for pitch precursor fibers with very large diameters. A thermodynamic model is proposed for the preferred orientation formation when spinning an isotropic pitch. The carbon fiber exhibited no skin-core difference in preferred orientation. A crystallite with a larger crystallite size Lc(002) in the carbon fiber was shown to be better aligned along the fiber axis than that with a smaller Lc(002). Furthermore, a well-aligned crystallite possessed a larger Lc(002) than one that was misaligned. The pitch precursor fiber also exhibited such a correlation between Lc(002) and the preferred orientation, but the correlation was weak.
Selective growth of diamond crystals on the apex of silicon pyramids
Authors: Ramesham R. , Ellis C.
Published: September 2007

Diamond crystals have been selectively grown on the apex of anisotropically chemically etched silicon pyramids. A novel process sequence is developed which exposes a patterned sharp apex of silicon pyramids surrounded by thermally grown silicon dioxide to a high pressure microwave plasma-assisted chemical vapor deposition (HPMACVD) process where the reactant feed gases are methane and hydrogen. The nucleation rate of diamond is very high on the sharp edge of a silicon mesa structure or an apex of a silicon pyramid, as anticipated. Selective growth of diamond particles on the apex of silicon pyramids fabricated using various approaches was analyzed by scanning electron microscopy.
Diamond growth using carbon monoxide as a carbon source
Authors: Cerio F.M. , Weimer W.A. , Johnson C.E.
Published: September 2007

Polycrystalline diamond films were produced in a microwave plasma assisted CVD reactor using CO as the carbon source gas. Reactor exhaust gas compositions were determined by mass spectrometry using 2–10% CO and 0–1.5% O2 in H2 feed gas mixtures. The chemistry involved in the gas phase is similar to that which occurs when diamond is grown using hydrocarbons as carbon source gases. A chemical mechanism for the oxidation of CH4 in flames appears to be applicable to this system. Addition of O2 to the reactor feed gas results in increased growth rates for low addition levels possibly due to activation of the diamond surface, while lower growth rates result at high addition levels due to oxidation of carbon from the surface and depletion of diamond growth precursors in the gas phase. The chemical reactions that take place in the plasma are similar to those that occur in flames and hot filament reactors, indicating that the plasma acts to induce reactions that are normally associated with high temperature combustion processes.
MBE growth of compound semiconductors: Part I. Stochastic modeling
Authors: Venkatasubramanian R.
Published: September 2007

A stochastic model for the MBE growth kinetic study of compound semiconductors is developed based on the master equation approach, the solid-on-solid restriction, and the quasi-chemical approximation. The developed model is suitable for the zinc blende crystals with 001 as the growth direction. In the modeling, the diamond cubic structure and the two sublattice nature of the zinc blende crystal are taken into account. The stochastic model is extended to compound semiconductor alloys such as GaAlAsSb to make it suitable for the MBE kinetic studies of alloys. Up to four elements with two in each sublattice can be accommodated. The presence of two elements in the same sublattice was taken into account. A procedure for the evaluation of the model parameters based on the available thermodynamic and experimental data is discussed. Advantages and limitations of the stochastic model over the available theoretical models are discussed.
Polymer-derived Si3N4−ZrO2 nanocomposite powders
Authors: Sorarù Gian Domenico , Ravagni Alberto , Maschio Roberto Dal , Carturan Giovanni , Babonneau Florence
Published: September 2007

A Zr-modified polycarbosilane has been obtained reacting a zirconium alkoxide with a polycarbosilane. This new preceramic polymer has been nitridated in flowing ammonia up to 1000 °C. The conversion process of the polymer precursor to the Si–N–Zr–O ceramic has been followed mainly by FT-IR, XRD, and TEM investigations. The formation of the Si–N–Si network starts at 600 °C. At 1000 °C the system can be described as an amorphous silicon nitride ceramic in which very fine zirconia-based particles are dispersed. Increasing the temperature to 1300 °C results in the crystallization of t-ZrO2 microcrystals (35 Å in size). At higher temperatures the crystallization of the silicon nitride matrix into β–Si3N4 has been observed.
Direct observation of nucleation embryos during NiFe2O4 precipitation in NiO
Authors: Summerfelt Scott R. , Carter C. Barry
Published: September 2007

Very small NiFe2O4 particles between 1 and 4 nm in size have been observed in an Fe-doped NiO sample quenched during early stages of spinel precipitation. The differences between the crystal structure of NiFe2O4 and NiO made it possible to use three different TEM imaging techniques to observe these very small particles. These very small particles are distributed throughout the volume of the sample and hence were not produced by specimen preparation. The large concentration of the very small precipitates compared with the much larger but less numerous growing precipitates indicates that the very small precipitates are actually embryos which form due to compositional fluctuations. The critical size for spinel nucleation was estimated to be 4.3 nm based on the distribution in the size of very small particles. The embryos are bounded by 〈111〉 planes with an octahedral morphology which is the minimum energy morphology.
Two different low friction mechanisms of diamond-like carbon with silicon coatings formed by plasma-assisted chemical vapor deposition
Authors: Oguri K. , Arai T.
Published: September 2007

Diamond-like carbon with silicon (DLC-Si) coatings formed by plasma-assisted chemical vapor deposition showed low friction coefficients of the order of 0.01 against steel without a lubricant, not only in dry atmosphere but also in humid atmosphere, where conventional DLC coatings showed higher friction coefficients of 0.1–0.2. DLC-Si coatings with 1 μm thickness deposited on steel were slid against steel using a conventional ball-on-disk type of apparatus to compare with a low friction mechanism of DLC-Si in dry and humid atmospheres. Analyses of wear scars indicated that formation and/or transfer of graphite-like carbon including hydrogen that originated in a DLC-Si coating occurred in dry atmosphere, while oxidation of contained silicon with water vapor formed silica-sol by sliding in humid atmosphere. The latter, peculiar to DLC-Si, was considered to cause the low friction coefficient in humid atmosphere through adsorbed water on silica.
Critical current and microstructure of uniaxially aligned, polycrystalline YBa2Cu3O7−δ
Authors: Tkaczyk J.E. , Briant C.L. , DeLuca J.A. , Hall E.L. , Karas P.L. , Lay K.W. , Narumi E. , Shaw D.T.
Published: September 2007

Three processing routes that generate uniaxial alignment but otherwise yield very different microstructure and critical current are compared. Fine grain size and c-axis alignment are obtained in magnetically aligned ceramics, pyrolyzed thick films, and in situ deposited thin films. The dense, well-aligned microstructure of the in situ process produces the highest zero field critical current Jc > 104 A/cm2 at 77 K. However, the critical current is suppressed in low magnetic field, suggesting that uniaxial alignment is not sufficient to avoid Josephson-type intergranular coupling. Above 1 T, the critical current of the aligned ceramic dominates in spite of its less ideal microstructure. The critical current in this high field region is one to two orders of magnitude greater than that of nonaligned material. This result implies the existence of a 3-d percolative network of strong links.
Chemical stability of CVD source materials for high-Tc superconducting films
Authors: Hashimoto Takuya , Koinuma Hideomi , Nakabayashi Masaaki , Shiraishi Tadashi , Suemune Youichi , Yamamoto Takakazu
Published: September 2007

The stability of chemical vapor deposition (CVD) source materials for high-Tc superconducting films was examined by 1H-NMR (nuclear magnetic resonance), 13C-NMR, IR (infrared) absorption, and TG-DTA (thermogravimetry and differential thermal analysis) measurements. Highly purified Ca(DPM)2 (DPM: dipyvaloylmethane: (CH3)3CCOCHCOC(CH3)3) and Sr(DPM)2, utilizing recrystallization and sublimation, easily decomposed at room temperature and changed their thermal properties when they were handled and stored in air or even in an Ar atmosphere without removing adsorbed species such as water. The stability increased by removing a trace of adsorbed species and storing in an Ar atmosphere at −30 °C. Bi(C6H5)3 and Y(DPM)3 showed higher chemical stability than Ca(DPM)2 and Sr(DPM)2.
The relationship between mass transport and oxide chemistry in oxidation of Ni3Al alloys
Authors: Haasch R.T. , Venezia A.M. , Loxton C.M.
Published: September 2007

Isotope diffusion and surface analysis has been used to investigate mass transport processes and oxide chemistry during oxidation of Ni3Al alloys in oxygen between 450 and 800 °C. In all cases, the oxide has an outer layer rich in NiO that grows by outward metal diffusion. Below 650 °C, an inner mixed oxide layer allows fast inward oxygen diffusion and growth. A porous inner layer of alumina forms at 700 °C that also allows oxide growth by oxygen in-diffusion. At 800 °C, the alumina becomes protective and halts the inner oxide growth mechanism. When macroalloyed with chromium, oxide chemistry is similar, but with an additional chromium oxide in the outer layer. There is an increased oxidation protection at temperatures ⋚600 °C that results from the inhibition of oxygen in-diffusion as an oxidation process. Above 700 °C, however, protective alumina formation is retarded and faster outward metal diffusion enhances the oxide growth rate when compared to Ni3Al.
Nucleation of an intermetallic at thin-film interfaces: VSi2 contrasted with Al3Ni
Authors: Ma E. , Clevenger L.A. , Thompson C.V.
Published: September 2007

We analyze the formation of VSi2 at the amorphous-vanadium-silicide/amorphous-Si interface by linear-heating and isothermal calorimetry, and cross-sectional transmission electron microscopy. We show evidence that indicates sporadic VSi2 nucleation with a steady-state nucleation rate after a transient period. The results are contrasted with those obtained for Al2Ni nucleating at the polycrystalline-Al/polycrystalline-Ni interface, where the kinetics appears to be controlled by growth of a fixed number of nuclei at quickly consumed preferred nucleation sites.
Elastic and nanostructural properties of Cu/Pd superlattices
Authors: Davis B.M. , Li D.X. , Seidman D.N. , Ketterson J.B. , Bhadra R. , Grimsditch M.
Published: September 2007

A series of Cu/Pd superlattices with composition modulation wavelengths (Λ's) ranging from 1.6 to 3.5 nm and a strong [111] growth texture were prepared by electron beam evaporation. The elastic properties of the films were examined using the methods of uniaxial tension tests [a Young's modulus (1/s11), where sij is an elastic compliance] with the applied load parallel to the plane of the Cu/Pd interface and Brillouin scattering [a shear modulus (1/s44) with the shear waves parallel to the plane of the Cu/Pd interface]. Also, the films were characterized using both x-ray diffraction and high-resolution electron microscopy; this was done to assess the effect of the nanostructure on a possible “supermodulus effect.” The films are nanostructurally very similar to the superlattice films employed in previous studies at Northwestern in which a supermodulus effect was reported. But, contrary to previous studies, no anomalous behavior was observed for the measured elastic properties of the thin films. Therefore the present results negate the earlier results and cast a serious doubt on the existence of a supermodulus effect.
Electron channeling effect on highly oriented graphites–Size evaluation and oriented mapping of crystals
Authors: Yoshida Akira , Hishiyama Yoshihiro
Published: September 2007

Structural studies on kish graphite (KG) and highly oriented pyrolytic graphite (HOPG) were performed using the techniques of electron channeling pattern (ECP) and electron channeling micrograph (ECM) in the scanning electron microscope. In the KG specimens the ECP for the single crystal was observed for each specimen, while in the HOPG specimens those of polycrystalline graphites with parallel c-axes were observed. The crystal sizes along the a-axis in the HOPG specimens were evaluated by ECM's. The average crystal size was related to a parameter sensitive to their crystallinity, i.e., the ratio of the resistivity at room temperature to that at 4.2 K (residual resistivity ratio). A mapping of the a-axis orientation of HOPG specimens was carried out.
Phase relationships in the Ti–Si–C system at high pressures
Authors: Sambasivan S. , Petuskey W.T.
Published: September 2007

The compatibility of phases in the Ti–Si–C system was studied for pressures from 10 to 20 kbars (1 to 2 GPa) and temperatures from 1200 °C to 1500 °C via an interfacial reaction between titanium and silicon carbide. The interface was characterized by backscattered electron imaging and electron probe microanalysis. Ti3Si was stabilized at high pressures exhibiting an appreciable solubility for carbon (up to 9 at. %). At higher temperatures and lower pressures, it decomposed to Ti5Si3, Ti(Si, C), and TiCx according to the relation Ti3Si (Cz) = (0.70588)TiCx + (0.32437)Ti5Si3 + (0.67227)Ti (Si, C), where the mathematical coefficients were calculated using z = 0.4 and x = 0.5. No detectable quantities of carbon dissolved in Ti5Si3 under these conditions. A clapeyron slope of 0.08 (±0.01) kbar/K was measured for this reaction. The microstructure of the reaction zone and other general thermodynamic and kinetic characteristics are discussed for different experimental conditions.
An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments
Authors: Oliver W.C. , Pharr G.M.
Published: September 2007

The indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%.
Growth of (100) oriented diamond thin films on ball structure diamond-like particles
Authors: Chow Lee , Horner Alan , Sakouri Hooman , Roughani Bahram , Sundaram Swaminatha
Published: September 2007

The morphology of typical CVD diamond thin films has been shown to be controlled by the concentration of methane during deposition. For example, for CH4 concentrations c < 0.4% the (111) faces dominate, while at 0.4% < c < 1.2% (100) faces dominate. Here we showed that the (100) oriented diamond films can be grown on top of the microcrystalline ball-like particles under suitable conditions. These (100) oriented diamond films are grown under the condition of 1.5% methane in hydrogen, substrate temperature of 680 °C–750 °C, and pressure of 30–80 Torr. The bombardment of the diamond thin films by ions in the plasma is believed to be an important factor for the formation of (100) oriented films on top of the ball-like particles. SEM, Raman, and x-ray techniques were used to characterize the deposited (100) oriented diamond thin films.
Ion-assisted pulsed laser deposition of cubic BN films on Si (001) substrates
Authors: Ballal A.K. , Salamanca-Riba L. , Doll G.L. , Taylor C.A. , Clarke R.
Published: September 2007

We report on oriented and adherent cubic BN films grown on (001) faces of silicon using the method of ion-assisted pulsed laser deposition. The structure of the films is cubic zinc-blende with a lattice constant of 3.62 Å, as determined by infrared absorption and transmission electron microscopy. The films were found to be polycrystalline, and oriented with the cubic BN [110] axis parallel to the Si [001] direction.