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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.
The Vickers hardness number Hv of a typical glassy inorganic polymer, a-Se, is studied as a function of temperature with the heating rate varied as a parameter from 0.032 to 3 °C/min, over two decades. It is shown that Hv(T), as a function of temperature, goes through a sharp drop in the glass transformation region following the similar drop for the shear modulus G(T) reported previously. By defining an empirical glass transition temperature TG at the inflection point of Hv vs T behavior, the heating rate dependence of TG is examined and interpreted via the kinetic structural relaxation model of glass transformation. It is shown that over the temperature range 36–50 °C the rate of structural relaxation processes controlling the mechanical properties obeys an Arrhenius type of temperature dependence with an activation energy ∼2.75 cV/atom. Furthermore, over the temperature range accessed, the structural relaxation rate seems to follow the viscosity-temperature behavior.
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.
The kinetics of the oxidation of dense and porous samples of Ba2 YCu3Ox ceramic have been determined by gravimetric analysis at 400–700 °C. At 600 °C and above, the rate decreases as the thickness of the oxidized layer increases. At 500 °C and below, the kinetics show a linear relation that indicates that the oxidized layer does not protect the ceramic. Dilatometric, microscopic, and high-temperature x-ray data suggest that fractures in the oxide layer at the lower temperatures are caused by the large volume decrease that accompanies the change in oxygen stoichiometry.
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 both in situ irradiations in 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–ms from 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.
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
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.
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.
Titanium aluminum intermetallic compound is a possible candidate for a high-temperature structural material, except for a problem of lack of room-temperature ductility. Recently, this problem was found to be overcome possibly by the addition of Mn, but this mechanism has not been fully understood yet. In order to understand the fundamental mechanism of the ductility improvement by Mn addition, microanalyses have been carried out. The results are as follows. Twin structures in a TiAl intermetallic compound in the as-cast state can be climinated by high-temperature annealing, while those in Mn-added TiAl are thermally more stable and exist even after annealing for 86.4 ks at 1273 K. The reason for this thermal stabilization of twin structures is considered to be due to the pinning effect of twin dislocations by Mn addition. The enhancement of twin deformation in TiAl by Mn addition is regarded to be caused by two factors. One is the stabilization of twin partial dislocations, becoming the nucleation sites for twin formation. The other is the decrease in stacking fault energy, which makes twin deformation energetically easier.
This paper analyzes the effect of grain size on yield stress of ordered Ni3Al and Zr3Al, and mild steels that show Lüders band propagation after yielding, using the Hall-Petch relation, σy = σ0 + kyd−½, and the new relation proposed by Schulson et al., σy = σ0 + kd −(p − 1)/2 [Schulson et al., Acta Metall. 33, 1587 (1985)]. The major emphasis is placed on the analysis of Ni3Al data obtained from published and new results, with a careful consideration of the alloy stoichiometry effect. All data, except for binary stoichiometric Ni3Al prepared by powder extrusion, fit the Hall-Petch relation, whereas the data from boron-doped Ni3Al and mild steels do not follow the Schulson relation. However, no conclusion can be made simply from the curve fitting using either relation. The results are also discussed in terms of Lüders strain and alloy preparation methods. On the basis of the Hall-Petch analysis, the small slope ky is obtained only for hypostoichiometric Ni3Al with boron, which would be related to a stronger segregation of boron in nickel-rich Ni3Al. In addition, the potency for the solid solution strengthening effect of boron is found to be much higher for stoichiometric Ni3Al than for hypostoichiometric alloys.
A simple atomistic model of a crack tip is used to demonstrate the existence of chaotic motion of crack-tip atoms. The model, which has been developed in detail elsewhere in the literature, consists of a linear chain of four atoms. Nearest neighbors interact via Morse-function potentials, with environment-induced lattice decohesion simulated by reducing the strength of the inner bond. Dynamic calculations are carried out by allowing the two inner atoms to move freely, starting from rest in a given initial configuration, with the two end atoms being held rigidly in place, Under certain conditions, associated with large departures from minimum potential energy, the motion of the inner atoms is shown to be chaotic in a manner that is consistent with the Kosloff-Rice description of chaotic dynamics in a classical Hamiltonian system. Possible implications of the results relative to the fracture of actual materials are discussed.
Attempts are made to measure activities of both components of a binary alloy (A–B) at 650 K using a solid-state galvanic cell incorporating a new composite solid electrolyte. Since the ionic conductivity of the composite solid electrolyte is three orders of magnitude higher than that of pure CaF2, the cell can be operated at lower temperatures. The alloy phase is equilibrated in separate experiments with flourides of each component and fluorine potential is measured. The mixture of the alloy (A–B) and the fluoride of the more reactive component (BF2) is stable, while (A–B) + AF2 mixture is metastable, Factors governing the possible use of metastable equilibria have been elucidated in this study. In the Co–Ni system, where the difference in Gibbs energies of formation of the fluorides is 21.4 kJ/mol, emf of the cell with metastable phases at the electrode is constant for periods ranging from 90 to 160 ks depending on alloy composition. Subsequently, the emf decreases because of the onset of the displacement reaction. In the Ni–Mn system, measurement of the activity of Ni using metastable equilibria is not fully successful at 650 K because of the large driving force for the displacement reaction (208.8 kJ/mol). Critical factors in the application of metastable equilibria are the driving force for displacement reaction and diffusion coefficients in both the alloy and fluoride solid solution.
Electron beam melting has been used to obtain ultrapure refractory metals that are gaining importance in metal oxide semiconductor-very large scale integration (MOS-VLSI) processing technology, fusion reactor technology, or as superconducting materials. Although the technology of electron beam melting is well established in the field of production of very clean refractory metals, little is known about the limitations of the method because the impurity level of the final products is frequently below the detection power of common methods for trace analysis. Characterization of these materials can be accomplished primarily by in situ methods like neutron activation analysis and mass spectrometric methods [glow discharge mass spectrometry (GDMS), secondary ion mass spectrometry (SIMS)]. A suitable method for quantitative multielement ultratrace bulk analysis of molybdenum with SIMS has been developed. Detection limits of the analyzed elements from 10−7g/gdown to 10−12g/g have been found. Additional information about the distribution of the trace elements has been accumulated.
The intercalation of SbCl4F and SbCl3F2 into graphite is described. The c-axis identity period, measured by (00/) x-ray diffraction, is 9.30, 12.69, and 16.04 Å for stages 1, 2, and 3, respectively, of SbCl4 F-intercalated graphite. The c-axis identity period for stage 1 SbCl4F-intercalated graphite is 8.85 Å. Mass spectroscopy shows that the molecule in SbCl4F-intercalated graphite is a (SbCl4F)4 tetramer. The molecule in SbCl3F2-intercalated graphite is a (SbCl3F2)4 tetramer. Mossbauer measurements show that Sb is in a 5 + state in SbCl4F-intercalated graphite.
A silicon single-crystal slab 0.15 mm in thickness was bent to produce small, nonuniform surface strains of the order of 0.2%. The electron channeling patterns were observed in a JSM 840 SEM (scanning electron microscope) at an accelerating voltage close to 25 kV. Proper choice of the triangles formed by intersecting channeling lines of zero-order and of higher-order Laue zones allows one to measure the changes in their dimensions caused by imposed strain. It was estimated that the lower limit of detectable elastic strain is close to 0.1%. The possibilities of using this method for estimation of the average elastic strains in thin epitaxial layers are discussed.
A procedure for modifying the surface composition of catalytically active metals with silicon-containing gaseous reactants has been developed. This new gas-solid reaction method is unique in that it can be used for the in situ synthesis of catalytically interesting materials, which cannot be done by conventional solid-solid reaction techniques. Using an oxygen-free silicon compound (e.g., hexamethyldisilazane, HMDS), the metals studied fall into two categories: those that involve reaction followed by diffusion, and those that exhibit surface reaction only. The first group, consisting of the metals Ni, V, Rh, Pt and Pd, formed thick (up to 0.6μ) Si diffusion layers, after reaction at 430 °C for a few hours, with either H2/HMDS or Ar/HMDS mixtures. Under the same conditions, the second group of metals, Fe and Co, showed thin (∼ 500 Å) overlayers containing silicon, but with no diffusion. The only nonmetal (graphite) studied so far showed no reaction, within the detectability limits of Auger spectroscopy. This observation shows that these reactions are catalytically induced by certain metal surfaces; in other words, they selectively take place on metals. These findings clearly have important implications for catalysis. For example, the metal surfaces of oxide-supported metal catalysts can, in principle, be selectively modified by gas-phase reactants. This treatment can readily be accomplished in situ in the catalytic reactor. The above reactions may well result in a new class of metallic catalysts, with one of the components being silicon. Furthermore, gas-phase compounds containing the elements aluminum, boron, and germanium are known to react with metals in an analogous manner, which further extends the range of possibilities for the synthesis of new catalytic materials.
The effect of hydrogen plasma exposure on the properties of transparent conducting indium-tin oxide films has been studied. The exposure reduces the film surface to elemental indium. The thickness of the reduced layer increases with increasing exposure and finally saturates to a thickness of about 100 nm. The reduced surface is rough and decreases the visible transmittance of these films drastically due to increased absorptance and reflectance. The reduced metal layer decreases the sheet resistance of the films. Annealing of the plasma-exposed film in oxygen recovers the visible transmittance except in the case of the severely damaged films.
The wetting of ceramic surfaces by aluminum alloys has been reexamined using a chemical system where interfacial reactions and oxide film effects could be isolated. The system Al–Mg–O was chosen since it is technologically important and high-purity, well-characterized materials are readily available. Magnesium alloyed with the aluminum sessile drop and silicon picked up from the experimental apparatus cause an initial reduction in contact angle by altering the protective nature of the oxide film formed on the sessile drop. Evidence of spreading is observed as an intermediate process in the reactive sessile drop pairs. Reaction products formed between the Al–Mg alloys and sapphire (Al2O3), spinel (MgAl2O4), or periclase (MgO) can be interpreted with predicted phase equilibria and the measured loss of magnesium from the sessile drop. Only the rate of the periclase alloy interaction was rapid enough to result in a continuous product layer after 24 h at 800 °C. The volatilization of all of the magnesium from the sessile drop resulted in the formation of a true Al–Al2O3 interface. The contact angle for a true Al–Al2O3 interface is 88 ± 5 deg at 800 °C. The liquid-solid interfacial energy is 1688 ergs/cm2.
Zinc oxide films were deposited using the reactants dimethyl zinc and tetrahydrofuran. Growth kinetics were determined from the dependence of the growth rate on substrate temperature and reactant partial pressure. A complex temperature dependence was observed over the temperature range of 300–500 °C. For a deposition temperature of 400 °C, the growth kinetics were modeled using a bimolecular surface reaction rate-limited mechanism. The layers were characterized using x-ray diffractometer and Auger spectroscopy measurements.
The ZnO varistor degradation has been attributed to the field-assisted, temperature-activated diffusion of interstitial zinc in the depletion layer. To improve stability, one approach is to reduce the formation of interstitials, and then further, to prevent their migration through empty interstitial sites. Based on this concept, an amphoteric dopant, such as Na or K, has been incorporated in the ZnO varistor grain boundary wherein a dopant is substituted both in the lattice and in the interstitial sites. A grain boundary defect model has been developed for this dual mode of substitution, with the dopant acting as an acceptor at the lattice site and as a donor at the interstitial site. Under these conditions, and given a desired neutrality range, the concentration of zinc interstitial is indeed shown to be reduced and stability greatly improved. The experimental data presented here validate the grain boundary defect model presented in this and in an earlier paper [J. Mater. Sci. 20, 3487 (1985)].