bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal site disorder in zinc tin phosphide
Authors: Ryan M. A. , Peterson Mark W. , Williamson D. L. , Frey James S. , Maciel Gary E. , Parkinson B. A.
Published: September 2007

The optoelectronic properties of the II-IV-V2 semiconductor ZnSnP2 are studied as a function of the cooling rate of the crystal growth melt. The structure of the material, as studied by x-ray diffraction, is seen to change from chalcopyrite to sphalerite as the cooling rate is increased. Photoelectrochemical measurements show that the bandgap of the material decreases from 1.64 eV for the chalcopyrite to 1.25 eV as the structure approaches sphalerite. The 119Sn Mössbauer spectroscopy shows both an isomer shift and a broadening of the 119Sn resonance as a result of new tin environments produced by disordering of zinc and tin sites at the faster cooling rates. The 31P solid-state nuclear magnetic resonance spectroscopy clearly shows new resonances associated with the additional phosphorus environments produced by metal site disordering. A model based on zinc and tin site exchange with the introduction of compensating donor and acceptor states is proposed and discussed.
Comment on “The diffusion of antimony in heavily doped and (sic) n- and p-type silicon” [J. Mater. Res. 1, 705 (1986)]
Authors: Morehead F. F.
Published: September 2007

R. B. Fair et al. have analyzed original data on the effect of heavy, uniform background doping with As and B on the diffusivity of Sb in Si at 1000°–1200°C. Their analysis attributed a role to “electric fields” and led to the conclusion that, for n-type background doping, Sb diffusion is dominated by double-negatively charged vacancies with no contribution from V−. For p-type backgrounds they concluded that donor-acceptor pairing alone retards the diffusion. Here it is shown (1) that electric fields do not play a role in their experiments, (2) that the contribution of V− to Sb diffusion in an n-type background is not zero but is roughly equal to that of V= for n/ni = 10, and (3) that the retardation of Sb diffusion in p-type Si is due both to donor-acceptor pairing and, principally, to the elimination of the contributions of V− and V=.
Neutron irradiation scoping study of twenty-five copper-base materials
Authors: Harling O. K. , Grant N. J. , Kohse G. , Ames M. , Lee T-S , Hobbs L. W.
Published: September 2007

A scoping irradiation test was carried out to help define the performance of copper-base materials in potential fusion applications. Twenty-five different copper materials including oxide dispersion-stabilized and precipitation-hardened powder metallurgy alloys, as well as pure copper and solid solution-strengthened ingot alloys, were neutron irradiated to 13.5 dpa at 400°C in the Experimental Breeder Reactor II (EBR-II) fast reactor. Volumetric swelling and electrical conductivity data were measured for all irradiated materials, and four selected materials were characterized for mechanical property changes using a miniaturized disk bend test. A number of these copper alloys, especially those prepared by powder metallurgy techniques, showed low swelling (less than 0.3%), small changes in conductivity (less than ± 5%), and relatively small changes in yield strength with good post-irradiation ductility. Preliminary electron microscopy results show microstructural changes that are consistent with the results of the macroscopic studies. The pure copper materials showed significant changes in conductivity and high levels of swelling as a result of the neutron exposure. It is apparent that a variety of copper alloys can survive fast neutron exposures to at least 13.5 dpa at 400°C without unacceptable changes in density, conductivity, or yield strength. Further irradiation testing to much higher doses and experimental investigation of the effect of transmutation product gases are needed to fully define the response of available copper alloys to fusion reactor environments.
Thermodynamic study at high temperature of the superconducting system YBa2Cu3Oz with 6 <z<7
Authors: Marucco J. F. , Noguera C. , Garoche P. , Collin G.
Published: September 2007

The YBa2Cu3Oz system has been investigated by thermogravimetry in a controlled partial oxygen pressure PO2 ranging from 105-10 Pa. A compound with z = 6.290 was prepared at T = 950°C and PO2 = 105 Pa and identified as a metastable β phase. Subsequent isothermal or isobar phase stability studies were performed. This β phase has a narrow metastability domain 6.25 <z < 6.50 at high temperature. By decreasing the temperature or increasing the PO2, the β phase transforms irreversibly into an α phase characterized by large deviations 7—x from stoichiometry. Near the stoichiometry z = 7, the structure of the a phase is orthorhombic, and fully ionized vacancies are likely to be predominant. By decreasing the PO2, the β phase transforms into a new phase with a lower oxygen content (z < 6.1).
Electron paramagnetic resonance assessment of the electronic charge of Cu in YBa2Cu3O7−δ
Authors: Benakki S. , Christoffel E. , Goltzene A. , Meyer B. , Schwab C. , Besnus M. J. , Meyer A. , Vilminot S. , Drillon M.
Published: September 2007

Electron paramagnetic resonance (EPR) has been measured on powdered YBa2Cu3O7−δ at 9 GHz and at variable temperature 4.2 K ≤ T ≤ 293 K. The decrease of the mean Landé factor value ḡ from 2.22 to 2.07, between T = 4 K and T = Tc, is ascribed to a hole being transferred from copper to oxygen.
Pressure dependence of the superconducting transition temperatures in La1.85Sr0.15CuO4 to 8 GPa
Authors: Erskine D. , Hess E. , Yu P. Y. , Stacy A. M.
Published: September 2007

Pressure dependence of the superconducting transition temperature in La1.85Sr0.15CuO4 has been measured to 8 GPa using a diamond anvil cell. The experimental results are discussed within the conventional electron-phonon model of superconductivity.
Transformation toughening of glass ceramics
Authors: Clarke D. R. , Schwartz B.
Published: September 2007

The utilization of transformation toughening has hitherto been restricted to increasing the fracture resistance of polycrystalline ceramic materials. Although a number of investigators have attempted to extend the concept to toughening glasses and glass ceramics with tetragonal zirconia, no successful reports have been published. It is argued that the approaches employed are inevitably limited primarily because they do not take into account the necessity of nucleating the tetragonal-to-monoclinic transformation away from the crack tip itself. By concentrating on the nucleation event and using standard ceramic processing techniques, it has been demonstated that transformation toughening can be used to increase the toughness of glass-ceramic materials, and this approach is illustrated by increasing the fracture toughness of a cordierite glass ceramic.
Luminescent properties of x-irradiated rare-earth-doped barium copper oxides
Authors: Cooke D. W. , Rempp H. , Fisk Z. , Smith J. L. , Jahan M. S.
Published: September 2007

Thermally stimulated luminescence (TSL) of x-ray-induced defects in R-Ba–Cu–O (R = Gd, Ho, and Eu) has been measured. In the interval 25–400°C, Ho- and Eu-doped samples exhibit similar TSL with peaks at 65, 135, and 185°C. The GdBa2Cu3Ox has, in addition to the three aforementioned peaks, a maximum at 310°C. All TSL peaks are attributed to recombination of F+s – and F−s center electrons with Vs-type holes, requiring typical thermal activation energies 0.5–1.0 eV. Spectral emission data support this conclusion. Irreversible quenching of TSL is found to occur in each sample. It is postulated that adsorbed oxygen molecules interact with radiation-induced F+s and Fs centers to produce O−2 and O2−2 molecular ions, respectively, thereby reducing the surface oxygen vacancy concentration and quenching the luminescence. In addition to activation energies, other TSL parameters are extracted from the data. It is suggested that surface TSL in these oxide superconductors is independent of rare-earth dopant.
Raman microprobe measurements of residual strains at the interfaces of Si on quartz
Authors: Cheong Y. M. , Marcus H. L. , Aclar F.
Published: September 2007

In order to quantitatively determine the residual stresses at the interfaces of laminate composite materials, a model involving exponential stress gradient in the substrate and no stress gradient in the film was derived. The measurements of residual strains at the Si/quartz interfaces using the Raman microprobe were compared to expected strains by the model. The model shows that a small volume of substrate near the interface about 2 times the film thickness was affected by the thermal mismatch of the two regions. Approximately 5–10 times higher residual strains were expected at the substrate-side interfaces compared to the measured results. This is explained by the experiments averaging along the probe thickness of about 10μm resolution. The recrystallization process of Si film by thermal annealing was also investigated using Raman spectroscopy.
Rapidly solidified Al3Ti-base alloys containing Ni
Authors: Huang S. C. , Hall E. L. , Gigliotti M. F. X.
Published: September 2007

Two Ni-modified Al3Ti alloys (Al65Ni10Ti25 and Al62Ni8Ti30) were rapidly solidified by melt spinning. The resulting microstructure was studied using light microscopy and analytical electron microscopy. Significant variations in the microstructure and phases were observed between the two ribbons and through the thickness of each ribbon.A single-phase γ-TiAl structure was seen near the wheel side of the Al62Ni8Ti30 ribbon, having microcrystalline grains ∼ 100 nm in diameter. Second-phase particles of Λ-AlNiTi were found in the remaining regions of that ribbon as the structure became columnar due to reduced rates of cooling. The Al65Ni10Ti25 alloy exhibited a primary phase of π-Al6.5 NiTi2.5. A second phase of μ-Al2NiTi formed with morphology and distribution varying through thickness. Microchemistry measurements on the phases indicated substantial deviations (up to 14 at. %) from the stoichiometric compositions. Further, the π, γ, and μ are low-temperature phases that do not form by solidification under equilibrium conditions. The observation of these phases thus suggests significant undercoolings achieved during the melt-spinning processing of the present alloys. Both ribbons are brittle as spun.
A new type of brittle fracture in a fcc metal bicrystal with intergranular segregation
Authors: Wang Jian-Sheng
Published: September 2007

A new type of brittle fracture was discovered in a Cu–Bi alloy bicrystal with a random misorientation interface. Macroscopically, fracture occurred along the interface with a low-energy release rate of order 50 Jm−2. Microscopically, the crack apparently propagated in a zigzag manner such that a cleavagelike fracture was produced, which consists of two sets of smooth {110} facets and one set of {100} facets with serrations corresponding to their intersections with {111} slip planes. Facet sizes are typically of order 1 by 5 μm or smaller. The very smooth facets and low-energy absorption suggest that a decohesion separation process has occurred along {110} planes, and the serrations suggest that substantial dislocation slip accompanied fracture of the {100} facets. The fracture surface morphology may be explained as the result of a similar morphology of Bi-induced microscopic faceting of the bicrystal interface or, instead, may result from fracture propagating in the vicinity of, but not quite along, the interface. Evidence is presented that favors the latter so that this is a rapid, cleavagelike, semiintergranular fracture that has not been observed in fee metals or alloys when an environment effect is not involved. Its faceted features resemble those of transgranular stress corrosion cracks in certain fee metals.
On the nature of the quasicrystalline phases obtained in alloys of Al86Mn14 under slow solidification rates
Authors: Reyes-Gasga J. , Pérez-Ramírez J. G. , Pérez R.
Published: September 2007

An investigation of the quasicrystalline phases obtained in alloys of Al86Mn14 under slow solidification rates is carried out. A quasicrystalline phase similar to the so-called T phase [L. Bendersky, R. J. Schaefer, F. S. Biancaniello, M. J. Boettinger, M. J. Kaufman, and D. Schechtman, Scr. Metall. 19, 909 (1985)] is found. However, this phase also shows a pseudofivefold axis and a threefold diffraction pattern similar to the same orientation in the reported phase of Al4Mn. The analysis of pseudofivefold diffraction patterns from different quasicrystalline grains shows elliptical arrangements of spots with different eccentricities. The presence of planar faults indicates the existence of slip pseudoplanes and slip directions in the quasicrystal. Different eccentricities might suggest that some of the reported quasicrystalline phases (T, T′, T″, …, etc.) are structural states obtained from the icosahedral quasicrystalline phase when particular stress environments are acting.
Decoupled bulk and surface crystallization in Pd85Si15 glassy metallic alloys: Description of isothermal crystallization by a local value of the Avrami exponent
Authors: Calka A. , Radliński A. P.
Published: September 2007

Isothermal devitrification of Pd85Si15 amorphous alloys has been analyzed using differential scanning calorimetry (DSC) and x-ray diffractometry. Both as-quenched and aged amorphous ribbons were investigated. Crystallization of aged samples starts from the surface and proceeds several micrometers into the bulk. The product of this process is a layer of strongly textured palladium (111) followed by a mixture of Pd2Si, Masumoto MSI phase, and untextured palladium. Next, the crystallization occurs via a different (bulk) mechanism, resulting in a mixture of Masumoto MSII phase and untextured palladium. The bulk mechanism is the only one observed in as-quenched samples. The surface and bulk crystallization mechanisms are spatially decoupled and, therefore, the corresponding DSC data can be analyzed separately. This has been done according to the Kolmogorov–Johnson–Mehl–Avrami model and also using the recently developed concept of local value of Avrami exponent n. For both the surface and bulk crystallization the phase transition process cannot be characterized by a single value of n. Observed variation of n with the crystallized fraction x is explained by a considerable variation of the nucleation rate that takes place during devitrification.
Comparison of low-voltage field emission from TaC and tungsten fiber arrays
Authors: Cochran J. K. , Lee K. J. , Hill D. N.
Published: September 2007

Field emitter array (FEA) devices were constructed using thin-film fabrication techniques based on in situ eutectic composites containing either TaC or NbC fibers in a NiCr alloy matrix. The emission characteristics of the TaC devices were measured and compared to those of W–ZrO2 FEA's. Both devices produced the linear Fowler-Nordheim plots common to all field emission devices; however, the array current density of the TaC FEA's was more than two orders of magnitude lower than that of the W–ZrO2 FEA's. This was primarily due to the lower fiber density and lower field enhancement of the carbide fiber devices. If it were possible to produce cathode geometries comparable to those of the W–ZrO2 FEA's, a theoretical analysis indicates that emission performance of the TaC devices would be superior because of the lower work function of TaC. However, post-emission observations indicated that the carbide fiber emitters failed more readily than the tungsten emitters under comparable operating conditions.
c-Axis conductivity and conductivity anisotropy in graphite intercalation compounds
Authors: McRae E. , Marêché J. F.
Published: September 2007

The stage (s) and temperature (4.2≤T≤295 K) dependence of the c-axis resistivity ρc and the conductivity anisotropy (ρc/ρa), taking into account all available literature data as well as those from the laboratory, are examined. For all intercalants, the ρc (s, T) variations are highly similar. The temperature coefficient of resistivity is generally positive over the entire T range for the richest compounds; it slowly decreases in magnitude and changes sign as s increases. The sign change occurs for a compound between the second and fifth stage depending on the intercalant. At 4.2 Kρc is T independent. Except for a few stage 1 compounds, the anisotropy always rises upon decreasing T. An analysis of the existing c-axis conductivity models is given: these invoke phonon- and impurity-assisted hopping mechanisms, interlayer conducting paths, and stacking faults. Some unanswered questions are raised that may indicate directions of future research.
Crystallographic and transport studies on AsF5 intercalated graphite from 4.2 to 295 K. I. Structural ordering and phase separation
Authors: Lelaurain M. , Marêché J. F. , McRae E. , Furdin G. , Hérold A.
Published: September 2007

A study of the (00l), (hk0), and (hkl) reflections of stage 1 AsF5 intercalated graphite between 4.2 and 295 K has been done using synchrotron radiation for single-crystal samples and a linear detector setup for those based on highly oriented pyrographite (HOPG). The nature and temperature dependence of the structural ordering allow the materials to be classified into (at least) two types related to the degree to which the AsF5 has been converted into AsF−6 and AsF3. At 295 K a small amount of in-plane order is detected within the essentially two-dimensional (2-D) liquid intercalate attributed to small quantities of AsF5 and AsF−6 − AsF5-ordered phases. Lowering the temperature leads to increased phase separation through crystallization and to changes in the in-plane unit cells associated with each. The most marked structural change is an incommensurate-to-commensurate (I⇉C) transformation within the AsF5 phase, which starts at 215 ± 5 K. No new structural order is detected below 180 K. The (00l) studies give clear confirmation of the existence of separate phases with different values of interplanar distance. A smaller number of stage 2 compounds were examined. The most clearly different feature is that the I⇉C transformation is downshifted by ∼ 70 K. At room temperature, the stacking sequences are A/A/A … for stage 1 and A/AB/BC/CA … for stage 2.
Crystallographic and transport studies on AsF5 intercalated graphite from 4.2 to 295 K. II. Effects of structural transformations and demixing on basal plane and c-axis electrical resistivity
Authors: McRae E. , Lelaurain M. , Marêché J. F. , Furdin G. , Hérold A. , Saint Jean M.
Published: September 2007

Part I of this study has shown that first stage AsF5 intercalated graphite samples can be classified into two types of compounds, depending upon the nature of the structural modifications they undergo upon lowering the temperature from 295 to 4.2 K. These structural changes are related to demixing of the species contained within the intercalate resulting in the formation of phases rich in AsF5, AsF−6 − AsF5, or AsF3 depending on the degree to which the AsF5 has been converted into AsF−6 and AsF3. Resistivity studies have been carried out in the basal plane [ρa (T)] and along the c axis [ρc (T)]. The type 1 compounds, in which the AsF5 has undergone little conversion, manifest a ρa (T) transition related to the incommensurate-to-commensurate (I⇉C) transformation of the AsF5 in-plane unit cell and a rapid, structureless 300 to 200 K decrease in ρc (T). The type 2 compounds involving a greater degree of conversion of AsF5 into AsF3 and AsF6 yield more complex ρc (T) behavior from 300 to 200 K attributed to the more involved ordering phenomena; no anomalies are seen in ρa (T). In the case of the stage 2 compounds, changes in ρc (T) are seen down to lower temperatures in accord with structural data indicating a downshift of the I⇉C transformation by ∼ 70 K. The transport results are discussed in the light of the crystallographic data and the low-temperature results are analyzed within the framework of proposed conductivity models.
Rapid crystallization of thin solid films
Authors: van der Poel C. J.
Published: September 2007

Laser-beam-controlled heating appears to be an excellent technique for driving isothermal transformations in a thin solid film on a thick substrate. The transformation is detected by the change in optical properties of the film as it evolves from the initial to the final state. Results are presented for the amorphous-to-crystalline transition in 100 nm thick films of InSb and of some Te alloys on thick glass substrates. Discrimination between interface growth and homogeneous crystallization can be made from the data. The crystallization of InSb films can be desribed by an Avrami equation with a single activation energy of 154 ± 6 kJ/mol over the full range of measured crystallization times between 100μs and 1000s. For low temperatures the results are consistent with differential scanning calorimetry (DSC) measurements. For Te-alloy films, the large temperature interval covered by the experimental method enables clear observation of the curvature of the temperature-time-transmission (T–t–t) plot.
Crystallization temperatures of 5d–5d alloys
Authors: van der Kolk G. J.
Published: September 2007

Binary alloys of Re–Ta, Os–Ta, Ir–Ta, Os–W, and Ir–W were vapor deposited. A wide amorphous formation range was found for all systems, except Ir–W. The crystallization temperatures were generally very high—between 800 and 1000 K.
Surface mechanical properties of C implanted Ni
Authors: Nastasi M. , Hirvonen J. -P. , Jervis T. R. , Pharr G. M. , Oliver W. C.
Published: September 2007

Nickel foils, 165 μm thick, have been carbon implanted at 300 K with 2, 3, and 4.2 × 1017 C ions/cm2 and implanted with a two-step implantation of 2.1 × 1017 C/cm2 at 300 K followed by 2.1 × 1017 C/cm2 at 77 K. All implantations performed at 300 K result in the formation of the metastablc phase Ni3C while the two-step implantation produces an amorphous Ni/C alloy. Surface mechanical property studies showed that both the surface hardness and wear properties are correlated with chemistry (carbon dose), and that the friction coefficient is additionally dependent on the surface microstructure. It was found that both the wear rate and coefficient of friction were reduced as the volume fracion of Ni3C increased. At the highest dose implanted 4.2 × 1017 C/cm7, the coefficient of friction was found to be lower for the sample implanted half at 300 K and half at 77 K and possessing an amorphous structure compared to the sample implanted entirely at 300 K and possessing a crystalline Ni3C structure. Increases in the surface hardness were also observed with increasing carbon content, with the greatest hardness observed in samples implanted to a total dose of 4.2 × 1017 C/cm2. The hardness at this dose was not dependent on the implant conditions or the metastable phase formed.
Toward a simple density functional theory of nonuniform solids
Authors: Stoessel James P.
Published: September 2007

With analogy to the “highly accurate” summation of cluster diagrams for hard sphere fluids à la Carnahan-Starling, a simple real space free-energy density functional for arbitrary potential systems is proposed, based on a generalization of the second virial coefficient to inhomogeneous systems, which when applied to ordered and amorphous solid hard-sphere systems yields pressures in remarkable agreement with experiment. Possibilities for corrections and extensions toward a simple density functional theory of nonuniform solids are noted.
Response of a stringlike dislocation loop to an external stress
Authors: Lund Fernando
Published: September 2007

The dynamics of a very thin dislocation loop under the influence of an externally applied, time dependent, stress field is studied in the context of continuum elasticity, where very thin means that the dislocation core is small compared to the loop's typical radius of curvature as well as to any relevant acoustic wavelengths. This is done using energy and momentum conservation as derived from a variational principle for conservative motion of the loop. Energy conservation alone does not suffice, since it is insensitive to forces that do no work. The idea is to have a theory of sources (dislocation loops) interacting with a field (particle displacement) in the same sense that classical electrodynamics is a theory of point-charged particles interacting with the electromagnetic field. The sum of elastic strain and particle velocity generated by a dislocation loop and those generated by external agents are replaced in the action functional whose extrema give the equations of classical dynamic elasticity, thus obtaining a functional of the loop's trajectory. Extrema of the action with respect to variations of the dislocation history select the trajectory that will be followed by the loop under prescribed external stresses. In general, the evolution of the loop will be governed by an integrodifferential equation. Differential equations are obtained when the work done by external forces is much greater than the elastic energy radiated, and the motion of any one point of the loop is affected only by those other loop points in its immediate neighborhood (local approximation). These equations are explicitly written down. They describe the dynamics of a string with mass and line tension of purely elastic origin. The cutoff procedure needed to give meaning to logarithmically divergent expressions is carefully described. The main ideas can be understood in the case of a screw dislocation, which is worked out in detail. The general case with two characteristic velocities, although algebraically more cumbersome, is not essentially different physically. Additional examples include the gliding edge, pinned dislocation segments, and kinks. Results presented are valid in a homogeneous, isotropic, infinite elastic solid, and ways in which these various restrictions might be lifted is discussed.
On the growth and composition of conductive filaments on the surface of As–Se–Te bulk glassy semiconductors
Authors: Márquez E. , Villares P. , Jiménez-Garay R.
Published: September 2007

A memory (lock-on) phenomenon was observed in the semiconducting glasses belonging to the As–Se–Te system, and this phenomenon was studied through electrical, calorimetric, and x-ray diffraction methods. The structural characteristics were also correlated to the time it takes to reach the memory state. The growth process in composition As0.40Se0.30Te0.30 exhibited a dependence on electrode polarity in such a way that it began at the positive electrode and finished at the negative one. The growth speed of the conductive filament was found to be approximately 22μm s−1. The crystalline nature of the lock-on filament was confirmed, and the peaks corresponding to the x-ray diffraction pattern of the surface of a sample of composition As0.45 Se0.10Te0.45, partially covered by filaments, correspond to monoclinic As2Te3. The memory effects found agree with those predicted through Ovshinsky's criterion, as the average coordination numbers for the noncrystalline structures of the alloys analyzed are 2.4 and 2.3, respectively.
Hydrogen incorporation mechanisms in the preparation of a-Si:H by ion bombardment-activated reactive evaporation
Authors: Strauven H. , Stesmans A. , Winters J. , Spinnewijn J. , Verbeke O. B.
Published: September 2007

Here a-Si:H is prepared by ion bombardment-activated reactive evaporation of Si in a H2O residual gas pressure ranging from 10−9 to 10−7 Torr. The Si+ ions (2.7keV) are bombarding the substrate and the walls during evaporation. Two hydrogen incorporation mechanisms are revealed by H evolution experiments, depending on the H2O residual gas pressure during evaporation. In the first mechanism H is sputtered from the walls of the system by the ion bombardment; this mechanism contributes 10 at. % to the hydrogen content. In a second mechanism Si+ bombardment on the growing layer injects H from H2O molecules adsorbed on the film surface; at least 5 at. % H is incorporated by this process. The second mechanism has a remarkable influence on the microstructure as revealed from the electrical conductivity, electron spin resonance, and infrared transmission. Indeed, Si+ bombardment-induced injection of H changes the conductivity type from variable range hopping to an activated behavior, while the dangling bond density remains low (< 1018 cm −3). The growth of [SiH2]n bundles, observed by the resonance frequency and absorption strength of the stretch mode of the Si–H dipole, is also a consequence of the H injection mechanism. It is concluded that the properties of the a-Si:H, prepared by ion bombardment-activated reactive evaporation, are explained by a microstructure, dependent on the specific hydrogen incorporation mechanism.
X-ray depth profiling of iron oxide thin films
Authors: Toney Michael F. , Huang Ting C. , Brennan Sean , Rek Zophia
Published: September 2007

A nominally γ-Fe2O3 thin film (oxidized from an Fe3O4 film) has been structurally depth profiled using x-ray scattering in a grazing incidence, asymmetric Bragg geometry. By varying the grazing incidence angle, the x-ray penetration depth is varied from tens of Angstroms to several thousand Angstroms, slightly larger than the film thickness. At small incidence angles a diffraction pattern characteristic of α-Fe2O3 is observed, while at larger angles the pattern is predominantly from γ-Fe2O3, showing that there is a surface layer of α-Fe2O3. These results are quantified and the thickness of the α phase found to be 90 Å. The presence of the α phase explains a nonferromagnetic layer observed previously. These data together with magnetic and chemical data suggest that the nonferromagnetic layer forms during oxidation of the Fe3O4 film due to outward diffusion of Fe ions and their subsequent oxidation to form α-Fe2O3
Interaction of NF3 with melt confinement materials in fluoride glass processing
Authors: Atkins R. M. , Broer M. M.
Published: September 2007

Nitrogen trifluoride (NF3) was recently used by others as a reactive atmosphere during the melting of heavy metal fluoride glasses. This compound fluorinates oxide impurities, dehydrates the melt, and controls the oxidation state of both glassformer and impurity cations. The reaction of NF3 with nickel, silica, platinum, and vitreous carbon between 200 and 1100°C using IR absorption spectroscopy was investigated. This is an important issue, since reaction products (impurities) from the crucible and containment vessel walls may be incorporated in the melt. All materials, except for nickel, reacted below 600°C yielding corresponding fluorides (and oxides in the case of silica). The results indicate that, directly heated, vitreous carbon crucibles are preferred for melting of fluoride glasses in a NF3 atmosphere.
Preparation and characterization of high and low CaO/SiO2 ratio “pure” C–S–H for chemically bonded ceramics
Authors: Li Shiqun , Roy Delia M.
Published: September 2007

A type of solidified “pure” C–S–H (calcium silicate hydrate) has been prepared by hydrating mixtures of synthesized Ca3SiO5(C3S) and silica fume at very low water/solid (w/s) ratios (< 0.15). The mechanical properties (splitting tensile strength), stoichiometry, and microstructure of hydration products have been investigated. Results are reported here in which 12.03%−41.56% (by weight) silica fume was added to C3S. It has been found that the strength of the materials with CaO/SiO2(C/S) molar ratio of 1.5 is comparatively higher than that of the others, after curing from 1 day to 28 days. The homogeneity and rheology are of direct importance to the formation of the calcium silicate bonding, which influences greatly the mechanical properties. The hydrates are primarily C–S–H with only small amounts of Ca(OH)2. The compositions of the solidified C–S–H are (1.33–1.37) CaO·SiO2. (1.02–1.21) H2O and (1.40–1.19)CaO·SiO2.(0.68–0.91)H2O for mixtures of C/S = 1.5 and 0.83, respectively. The characterization of the chemically bonded materials by Wn (nonevaporable water), density, x-ray diffraction, and scanning electron microscopy (SEM) studies has been carried out. The relationships between these characteristics and the strength are discussed.
Correlations between sintering conditions and microstructure in ceramics of composition Li0.80Mg0.20 (Ta0.80Ti0.20)O3
Authors: Bonnet J. P. , Ravez J. , GiTae Joo , Hagenmuller P.
Published: September 2007

Two quite different types of microstructures have been detected for MgTiO3-enriched LiTaO3 ceramics corresponding either to small size grains and open porosity or to large size grains with closed porosity and appearance of microcracks. The first structure appears in short sintering processes at relatively low temperature. The textural change is explained by transitory formation of a grain boundary phase that leads to quick coalescence of the small grains.
Direct measurement of surface forces between sapphire crystals in aqueous solutions
Authors: Horn R. G. , Clarke D. R. , Clarkson M. T.
Published: September 2007

Measurements are presented of the electrical double layer and van der Waals forces between the (0001) surfaces of two single-crystal sapphire platelets immersed in an aqueous solution of NaCl atpH values from 6.7 to 11. The results fit the standard Deryaguin-Landau-Verwey-Overbeek (DLVO) theory, with a Hamaker constant of 6.7 × 10−20J. These are the first measurements made using the Israelachvili surface forces apparatus without mica as a substrate material, and they demonstrate the possibility of using this technique to explore the surface chemistry of a wider range of materials.
Icosahedral phase formation in rapidly quenched aluminum-ruthenium alloys
Authors: Anlage Steven M. , Fultz Brent , Krishnan Kannan M.
Published: September 2007

Systematic rapid quenching experiments on the icosahedral phase-forming system Al1 − xRux were performed for × < 20 at. %. The solidified alloys have been studied by electron microscopy and x-ray diffraction to determine their composition, constituent phases, and phase morphology. It has been determined that the icosahedral phase must form directly from the liquid and, at these quench rates, is always found in the presence of second or third phases. The results have been summarized in a metastable phase diagram appropriate for rapid solidification of Al1 − xRux at piston and anvil quench rates. This metastable phase diagram describes the results for Ru concentrations less than 14 at. %.
Simple embedded atom method model for fcc and hcp metals
Authors: Oh D. J. , Johnson R. A.
Published: September 2007

A procedure based on the embedded atom method (EAM) is presented for developing atomistic models for use in computer simulation calculations, with an emphasis on simple but general schemes for matching experimental data with fitting parameters. Both the electron density function and the two-body potential are taken as exponentially decreasing functions and the model is derived for any choice of cutoff distance. The model has been applied successfully to seven fcc and three hcp metals, but the extension to bcc metals was unsuccessful because of difficulty in matching the shear anisotropy ratio.
The nature of residual stress, defects, and device characteristics for thick single-crystalline Si films on oxidized Si wafers
Authors: Trimble L. E. , Celler G. K. , Schimmel D. G. , Lu C. Y. , Nakahara S. , Fauchet P. M.
Published: September 2007

Recently it was reported that high quality, 10–100 μm thick, single-crystalline Si films were formed on oxidized single-crystalline Si wafers by the lateral epitaxial growth over oxide (LEGO) process. Although this recrystallization process is reliable and reproducible, periodic regions of dislocations in the otherwise relatively dislocation-free Si film were not well understood. In this paper, therefore, the film stress and defect properties are investigated in detail, and devices made in recrystallized wafers are compared with devices in conventional wafer structures. Stress levels were found to be too low to cause defects, with TEM data suggesting an impurity mechanism (SiO2 precipitation) for small dislocation loops and slight crystalline misorientation for long dislocation lines in the periodic, defective areas. Device results confirmed that LEGO is a viable alternative to the dielectric isolation (DI) technology.
The effect of off-axis Si (100) substrates on the defect structure and electrical properties of β-SiC thin films
Authors: Kong H. S. , Wang Y. C. , Glass J. T. , Davis R. F.
Published: September 2007

Antiphase domain-free, monocrystalline β-SiC thin films have been epitaxially grown on off-axis Si (100) substrates. The effects of degree of misorientation and substrate preannealing on the antiphase domain boundaries (APB's) have been investigated. Wet oxidation, optical microscopy, and transmission electron microscopy were used to characterize the defect structures in both β-SiC thin films grown on exact Si (100) and on off-axis Si (100) substrates. The results revealed that many dislocations were included in APB's and that APB's were eliminated in the β-SiC thin films grown on Si (100) substrates that were oriented 2°–4° from [100] toward the [011] direction. Some APB's were observed near the edge of the β-SiC film on the 2° off-axis Si (100) substrates; however, they were eliminated by substrate preannealing. The carrier concentration and electron mobility of these β-SiC films were similar to those of β-SiC films grown on exact Si (100) substrates as determined by differential capacitance-voltage and Hall effect measurements. Au-β-SiC Schottky diodes were also fabricated on the β-SiC thin films on 4° off-axis substrates and had an ideality factor of 1.4. Differential capacitance-voltage measurements and current-voltage characteristics of Au-β-SiC Schottky diodes indicated that APB's caused significant leakage current in the β-SiC thin films.
Elastic and plastic anisotropy during growth from the melt of single semiconductor crystals
Authors: Lambropoulos John C.
Published: September 2007

The effect on dislocation density of elastic anisotropy (relation of growth direction to elastic stiffness tensor of cubic symmetry) and of plastic anisotropy (relation of growth direction to crystallographic slip systems of {111} ≪110≫ type) is investigated for the growth from the melt of shaped, single-crystal semiconductors of III–V compounds (typically GaAs, InP) by the Czochralski method. The thermal stresses are determined numerically by using three-dimensional finite element techniques for growth along the ≪100≫, ≪111≫ or ≪110≫ directions. The equivalent shear stress is calculated and it is assumed that the dislocation density is proportional to the sum over all slip systems of the excess of the resolved thermal shear stress over the crystal's yield stress in shear. It is shown that proper account of both elastic and plastic anisotropy is necessary in order to correlate numerical estimates of dislocation density to experimentally determined patterns. In particular, it is shown that the effect of elastic anisotropy is to produce significantly higher dislocation densities, especially for the case of ≪111≫ and ≪110≫ growth. For moderate levels of elastic anisotropy, the dislocation density levels during ≪111≫ and ≪110≫ growth become significantly larger than density, during ≪100≫ growth, at specific locations within the crystal. The correlation between dislocation density and equivalent shear stress is presented, and the effect of plastic anisotropy is thus discussed. It is shown that the assumption of elastic isotropy leads to dislocation density levels that considerably underestimate the levels but, for a given growth direction, are qualitatively similar to the dislocation density patterns corresponding to the three-dimensional numerical solutions in which elastic anisotropy is accounted in full.
Elaboration of LiNbO3 ceramics from sol–gel process powders
Authors: Puyôo-Castaings Nadine , Duboudin Françoise , Ravez Jean
Published: September 2007

LiNbO3 powders have been prepared by appropriate hydrolysis of lithium and niobium alkoxides using a sol-gel process. Ceramics with pretty high shrinkage coefficients and compactness have been obtained by sintering of the particles at 1075 °C.
Crystal structure of hafnia–zirconia mixtures obtained by calcination of hydrous oxide prepared by precipitation
Authors: Tau Li-Min , Srinivasan Ram , De Angelis Robert J. , Pinder Tim , Davis Burtron H.
Published: September 2007

The pH of the solution in contact with a hydrous zirconium oxide plays a dominant role in determining the crystal phase, tetragonal or monoclinic, in the calcined material. The substitution of low concentrations of hafnium for zirconium destabilizes the tetragonal phase so that only the monoclinic phase is formed; the amount of Hf required for destabilization depends upon the pH used for the preparation of the hydrous oxide. While this study has defined a phenomenon, the results do not permit a definition of the mechanism for it.
Improvement of polycrystalline Y–Ba–Cu–O by the addition of silver
Authors: Peterson G. G. , Weinberger B. R. , Lynds L. , Krasinski H. A.
Published: September 2007

X-ray photoelectron spectroscopy, scanning Auger, and optical microscopy studies of polycrystalline superconducting pellets of Y–Ba–Cu–O/Ag are presented. Silver-laced samples have a lower porosity and a drastically reduced hydrocarbon contamination. Results indicate no detectable substitution of A g into the Y–Ba–Cu–O but a collection of metallic silver in voids and possibly along grain boundaries Intergranular silver could mitigate adverse grain boundary effects in polycrystalline Y–Ba–Cu–O.
Superconducting thin films of the Bi–Sr–Ca–Cu–O system prepared by multilayer metal deposition
Authors: Mikalsen D. J. , Roy R. A. , Yee D. S. , Shivashankar S. A. , Cuomo J. J.
Published: September 2007

Superconducting thin films of the Bi–Sr–Ca–Cu–O system (2 and 10 kÅ) have been successfully prepared by multilayer deposition from elemental metal sources and annealed in N2/O2 mixtures. The onset of the superconducting transition is greater than 80 K with zero resistance at 74 K. The films contain primarily a Bi2(SrCaBi)3Cu2O8 + δ phase with a high degrees of (001) preferred orientation.
A thermodynamic and kinetic basis for understanding metastable phase formation during ion-beam mixing of nickel-aluminum alloys
Authors: Eridon James , Was Gary S. , Rehn Lynn
Published: September 2007

A quantitative thermodynamic explanation for the formation of metastable phases in the nickel-aluminum alloy system through heavy-ion irradiation is presented. The role of kinetics in the transformation to a metastable state is also investigated. Experiments involved the irradiation of both layered nickel-aluminum samples and ordered intermetallic compounds with 500 keV krypton ions over a range of temperatures and compositions. Samples were formed by alternate evaporation of layers of nickel and aluminum. A portion of these samples was subsequently annealed to form intermetallic compounds. Irradiations were performed at both room temperature and 80 K using the 2 MV ion accelerator at Argonne National Laboratory. Phase transformations were observed during bothin situ irradiationsin the high-voltage electron microscope at Argonne and also in subsequent electron diffraction analyses of an array of irradiated samples. Metastable phases formed included disordered crystalline structures, an amorphous structure, and a hexagonal-close-packed structure. These phase structures were modeled using the embedded atom method to compute heats of transformation ΔHs–msfrom stable to metastablestates. It was found that metastable states that have moderate heats of transformation, ΔHs–ms≍ 15%–20% of the heat of formation of the stable phase, form under irradiation. Metastable states with high heats of transformation, ΔHs–ms≍ 50% of the heat of formation of the stable phase, do not form under irradiation. Kinetics also play an important role in determining the effect of temperature and initial structure on the formation of metastable phases.
Elements of the radiation-induced structural self-organization in materials
Authors: Abromeit C. , Wollenberger H.
Published: September 2007

A crystal irradiated by energetic particles is an open system capable of self-organization. The minimum requirements for self-organization of the microstructure (defect sinks) are derived by a linear stability analysis based upon a rate equation treatment of the defect reactions. The selected description eases the application also for proper chemical reactions. Coupling of defect fluxes to solute fluxes and resulting compositional self-organization in alloys is also considered
Dislocation dynamics in Al–Mg–Zn alloys: A nuclear magnetic resonance and transmission electron microscopic study
Authors: De Hosson J. Th. M. , Kanert O. , Schlagowski U. , Boom G.
Published: September 2007

Pulsed nuclear magnetic resonance (NMR) proved to be a complementary new technique for the study of moving dislocations in Al–Mg–Zn alloys. The NMR technique, in combination with transmission electron microscopy (TEM), has been applied to study dislocation motion in Al–0.6 at. % Mg–1 at. % Zn and Al–2 at. % Mg–2.5 at. % Zn. Spin-lattice relaxation measurements clearly indicate that fluctuations in the nuclear quadrupolar interactions caused by moving dislocations in Al–Mg–Zn are different compared to those in ultra pure Al. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been determined as a function of strain. From the NMR data it is concluded that moving dislocations advance over a number of solute atoms in these alloys as described by Mott-Nabarro's model. At large strains there exists a striking difference between the mean jump distances in Al–0.6 at. % Mg–1 at. % Zn and in Al–1.2 at. % Mg–2.5 at. % Zn. The latter is about five times smaller than the former one. This is consistent with TEM observations that show dislocation cell formation only in Al–0.6 at. % Mg-1 at. % Zn and the macroscopic stress-strain dependences of these alloys.
Evolution of dislocation structure during inverse creep of a nickel aluminide: Ni–23.5 Al–0.5 Hf–0.2B (at. %)
Authors: Schneibel J. H. , Horton J. A.
Published: September 2007

A well-annealed polycrystalline nickel aluminide of composition Ni–23.5Al–0.5 Hf–0.2B (at. %) shows inverse creep behavior at 1033 K and 250 MPa. The minimum creep rate does not correspond to a steady-state creep condition. The increase in the creep rate with strain and time is accompanied by an increase in the volume fraction of dislocation-containing regions. The inverse transient can be eliminated by prestraining at room temperature. It is absent in the diffusional creep regime.