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Vacancy diffusion along two different high-angle twist grain boundaries (Σ5 and Σ13) was studied using the Embedded Atom Method (EAM). Vacancy formation energies in all the possible sites were calculated and found to be directly related to the degree of coincidence with the neighboring crystal planes. Optimal migration paths and migration energies were determined and found to be very low. The activation energies for self-diffusion at the boundaries were found to be less than half of the bulk value.
The atmospheric pressure chemical vapor deposition of aluminum nitride coatings using hexakis(dimethylamido)dialuminum, Al2(NMe2)6, and ammonia precursors is reported. The films were characterized by ellipsometry, transmission electron microscopy, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry. The films were deposited at 200–250 °C with growth rates up to 1000 Å/min. They displayed good adhesion to silicon, vitreous carbon, and glass substrates and were chemically inert, except to concentrated hydrofluoric acid. Rutherford backscattering analysis showed that the N/Al ratio ranged from 1.1 to 1.2. Refractive indexes were 1.8–1.9. The films were smooth and amorphous by transmission electron microscopy.
The evolution of the size and shape of molybdenum nanocrystals fabricated by sputtering in a thermal gradient has been studied as a function of the argon gas pressure, p. For 4 < p < 100 mTorr, continuous Mo films are deposited. At p = 150 mTorr, isolated and well-faceted Mo nanocrystals of two sizes (20 and 5 nm average size) are formed. For 200 ≤ p ≤ 400 mTorr, the particle size decreases with increasing pressure and is about 7 nm at 400 mTorr. On increasing p further, larger particles start to form and at p = 700 mTorr, particle agglomerates are observed. Possible mechanisms leading to these results are suggested.
An alternative synthetic route for obtaining bulk forms of the high temperature superconductor YBa2Cu3O7−x has been investigated. The approach is based on first preparing the phases Y2Cu2O5 and BaCuO2, followed by a single sintering of an appropriate mixture of these intermediate compounds to produce the superconducting phase. The resulting materials are largely single-phase as shown by x-ray diffraction, and have densities as high as 86% of the theoretical value, and superconducting onset temperatures of 93 K with magnetic shielding factors ranging from 0.85 to 1.02 (±0.05). Metallography and scanning electron microscopy data were also obtained on the best (high Tc, high shielding factors) of the samples. This synthesis approach is believed to be simpler, more reproducible, and has the potential of producing better materials than previously used bulk synthesis methods.
Both YBa2Cu4O8 and (Y0.95Ca0.05)Ba2Cu4O8 were successfully prepared by firing for 160 h at 850 °C and at oxygen partial pressure of 3 atm without using any catalysts. These samples were characterized in terms of the crystallographic structure and thermal and superconducting properties. The x-ray powder diffraction patterns and superconducting properties for these samples were little changed after post-annealing for 6 h at 1070 °C and at oxygen partial pressure of 400 atm. However, when heat-treated at 700 °C in air and then quenched into liquid nitrogen, samples without post-annealing showed broader superconducting transitions than those post-annealed. Actually, the sharpness of the superconducting transition for a post-annealed sample was little affected by quenching. These observations were in agreement with the results of both transmission electron microscopy and thermal analyses.
Fission fragment damage was introduced into aligned sintered samples of various superconductors containing small additions of UO2 by irradiation with thermal neutrons. Samples of aligned, sintered YBa2Cu3Ox, powdered Bi2Sr2Ca1Cu2Oy, powdered Bi1.7Pb0.3Sr2Ca2Cu3Oz, and epitaxial films of YBa2Cu3Ox on (100)SrTiO3 were used. Magnetic hysteresis, with the critical state model, was used to evaluate changes in the intragranular critical current. In the case of the Bi1.7Pb0.3Sr2Ca2Cu3Oz powders an increase in intragranular Jc at 0.8 T of greater than 70 times was produced by the radiation. The epitaxial film showed no change in Jc on irradiation. This behavior upon irradiation is attributed to the pinning caused by damage produced by the fission products of uranium.
Superconducting films in the Bi–Sr–Ca–Cu–O systems were made using metal alkoxides. To prepare a dip-coating solution using a mixed alkoxide solution, insoluble Cu and Bi alkoxides were dissolved by modification with 2-dimethylaminoethanol and formation of a double alkoxide, respectively. Formation of the double alkoxides of Bi with Ca or Sr was confirmed using FT-IR and 1H-NMR. Bi–Sr–Ca–Cu–O films on yttria-stabilized ZrO2 and single crystal MgO(100) substrates were made using this solution. The films were closely oriented along the c-axis perpendicular to the substrate. The film on MgO(100) fired at 850 °C for 48 h showed two resistance drops around 115 and 85 K, corresponding to the high-Tc and low-Tc phases, respectively, and zero resistance at 72 K.
The temperature dependence of electrical resistivity of Nd2−xCexCuO4−y (x = 0.0, 0.1, 0.15, 0.18) was measured at oxygen partial pressures of 2.08 × 10−1 1.8 × 10−2, 8.1 × 10−4, and 3.3 × 10−6 atm. The oxygen-partial-pressure dependence of resistivity indicated that the charge carriers in Nd2−xCexCuO4−y were electrons. According to the resistivity data of these compounds at temperatures above 770 K, Ce doping affected the relation between the oxygen deficiency and oxygen partial pressure in Nd2CuO4−y: the doping of Ce worked to hinder the formation of oxygen vacancies in the lattice. Moreover, the carrier density after Ce doping was found to be much less than the value anticipated from the amount of the dopant. This suggested that not all the doped Ce ions worked as donors. That is, the relative amount of Ce3+ ions compared to that of Ce4+ ions increased as the total amount of the doped Ce ions increased.
The precipitation kinetics of the δ′ (Al3Li) phase in two rapidly solidified samples and one conventionally cast sample of an Al–2.3Li–6.5Mn–0.65Zr (in wt. %) alloy are compared. Following high cooling rates, manganese is retained in solid solution in the aluminum matrix (αAl) up to 6.0 wt.%, far beyond the thermodynamic equilibrium value (0.36 wt.% at 500 °C). Extended solid solution of manganese in aluminum induces strain gradients, similar to those produced by dislocations. The effect of such gradients, the size of which is proportional to the solute atomic fraction, is to enhance lithium precipitation by lowering the activation energy, as observed, and also by affecting the rate parameter. Kinetic thermal analysis has been performed in a series of nonisothermal experiments in the heat flux differential scanning calorimetry (DSC) mode. The precipitation of the δ′ (Al3Li) phase is evidenced by an exothermic peak whose characteristics were analyzed. The rate of transformation (precipitation) is assumed to obey the Johnson–Mehl–Avrami equation. The activation energy for the precipitation process as well as the kinetic rate parameter have been evaluated for the rapidly solidified and the conventionally cast specimens. The activation energy for precipitation is lowered, from 107.0 kJ mol−1 for the conventionally cast material, down to 81.8 kJ mol−1 and 77.0 kJ mol−1 for samples that exhibit manganese solid solubility extensions of 2.10 and 6.00 wt.%, respectively. The rate parameter for the precipitation reaction, which has the generally admitted value of 1.50, for a transformation involving diffusion controlled growth, is affected by the strain gradients, too. Its value is reduced from 1.40 for the slowly cast sample to 1.32 and 1.20, respectively, for the two rapidly solidified samples, as a result of competing mechanisms, namely: growth controlled by diffusion and strain-assisted precipitation.
Phase equilibria in the ternary system Pr–Fe–Al have been established in an isothermal section at 800 °C from room temperature x-ray powder diffraction analysis of about 50 alloys, which were melted, annealed at 800 °C, and quenched. Phase equilibria are characterized by the formation of rather extended homogeneous regions, i.e., by a random substitution of Fe/Al in Pr(Al1−xFex)2, 0 ≤ x ≤ 0.15, in Pr2(Fe1−xAlx)17, 0 ≤ x ≤ 0.65, as well as by the formation of at least four ternary compounds. Whereas the existence of PrFe4Al8 with the CeMn4Al8-type structure has been confirmed, there were no indications for a compound “PrFe6Al6” earlier claimed to crystallize with the ThMn12-type structure. Pr6(Fe1−xAlx)14, 0.16 ≤ x ≤ 0.36 with a homogeneous region parallel to the Fe–Al binary, was found to be isotypic with the La6Co11Ga3-type of structure. Pr-rich alloys are liquid at 800 °C, and all the alloys Pr2(Fe1−xAlx)17 with aluminum concentrations less than 5 at.% Al (x ∼ 0.07) enter a two-phase equilibrium with the Pr-rich liquid. At temperatures below 800 °C, alloys with compositions close to 30 at.% Pr and 5 at.% Al show a further ternary phase on solidification, whose crystal structure is related to the La6Co11Ga3-type. PrFe2Al8 is a new representative of the CeFe2Al8-type structure. The crystal structure of the ternary compound richest in Al, PrFe2Al10, has not been solved yet.
An alloy of composition (Co78Fe22)3V, which orders to an L12 superlattice below a critical temperature (Tc) of 910 °C, was rolled to 25–50% reduction in the initially ordered condition and annealed at various temperatures above and below Tc and examined by hardness, tensile testing, optical and electron microscopy and dilatometry, in order to study the progress of recovery and recrystallization. Recrystallization was severely retarded on annealing below Tc; close to Tc, recrystallization was ≈ 300 × slower in the ordered than the disordered range. Although recrystallization started promptly, predominantly at grain boundaries, very rapid recovery-softening of the unrecrystallized regions progressively reduced the driving force for recrystallization and slowed it down drastically. However, at 770°and 500 °C, recovery-softening was replaced by some recovery-hardening (i.e., strain-age hardening). Above Tc, recrystallization was complete in a few seconds and a special annealing method was needed to measure such times accurately. Dilatometric measurements showed that most of the order destroyed by rolling was restored long before recrystallization began, but the restoration was never complete unless the alloy was heated up through Tc and then slow cooled. Electron microscopy showed no sign of any antiphase domains in recrystallized grains except for a few isolated domain boundaries on annealing at 770 °C. A model is proposed to rationalize the incidence of recovery-softening or strain-age hardening at different annealing temperatures.
The reaction of SiH4/H2 mixtures with iron, cobalt, nickel, copper, chromium, molybdenum, and tungsten, at temperatures between 350 and 800 °C and 1 atm of total pressure, was studied. When the amount of water vapor in the gas mixture is carefully controlled, a metal silicide diffusion coating forms at the appropriate treatment temperature. Cu silicide coatings form at temperatures as low as 350 °C. Metal silicide coatings for Fe, Ni, Co, and Cr form at intermediate temperatures (500–700 °C), and higher temperatures (above 700 °C) are required for W and Mo. Composition and structure of the metal silicide coatings were determined by Auger depth profiling and x-ray diffraction. Kinetics of the surface reaction between SiH4 and the metal substrate as well as the behavior of these coatings in oxidizing environments at high temperatures were studied by a microgravimetric technique. The metal silicide coatings provide oxidation protection for Fe, Ni, and Cr in pure O2 up to 1000 °C, for W and Mo in air up to 900 °C, and for Cu exposed to air up to 700 °C.
The modification of film properties in evaporated tungsten was studied as a function of deposition environment. Using concurrent argon ion bombardment of the growing film, the stress varied in the same manner at all ion energies and substrate temperatures. Initial increases in tensile stress are followed by a monotonic trend toward compressive stress, for all sets of films. On the other hand, the qualitative changes in film resistivity with concurrent bombardment were dependent on the ion energy and substrate temperature, showing increases at high temperature and energy and decreases at low temperature and energy. Changes in the microstructure and impurity content in deposited films were found to be strongly linked to stress and resistivity changes. The trend toward compressive stress induced by high levels of ion bombardment is primarily reflected in an increase in (110) orientation. Increased resistivity is related to decreased grain size, increased (110) texture, and increased levels of film argon and oxygen content. By choice of deposition conditions, both the resistivity and stress can be minimized.
The deposition process at 500 °C with SiH4–GeH4–B2H6–He mixtures, which yields the amorphous Si–Ge–B alloy, was studied. Although in crystalline Si and Ge the maximum B content is limited to the solid solubility, any amount of B can uniformly be contained in amorphous Si–Ge–B. Thus, films with a B content up to 64 at.% have been prepared. The deposition rate of atoms, defined as the number of atoms deposited in a unit time interval, is obtained for each element by analyzing the growth rate together with the composition and the mass density of the film. When the SiH4 and the B2H6 partial pressures are constant, the Si and the B deposition rates are almost independent of the GeH4 partial pressure. In contrast, the Si deposition rate increases remarkably as the B2H6 partial pressure increases, even when the SiH4 partial pressure is maintained constant. A simple model is proposed for explaining the relationship between the Si and the B deposition rates.
High temperature x-ray diffractometry (HT-XRD) and differential scanning calorimetry (DSC) measurements confirm that two kinds of structural phase transitions exist in LaGaO3. One is the first order transition from the orthorhombic to the rhombohedral structure at ∼150 °C. The other is the second order transition from the rhombohedral to the monoclinic structure from ∼750 °C to ∼1000 °C.
Carbon films deposited by a modified dc cathodic arc technique were characterized by several analytical methods. The coatings consist of two constituents originating from process characteristics of the technique applied. Small fragments of graphite are embedded in a matrix material of an amorphous structure with diamond-like short-range order. The coatings exhibit friction and wear reducing properties.
During and following fracture of a number of materials, the emission of photons, electrons, ± ions, and neutral species are observed; these emissions are collectively known as fracto-emission. In this work, we present measurements of the neutral particle emission following fracture of two single crystal fcc alkali halides: NaCl and LiF. We observe no measurable emission attributable to release during the fracture event itself. However, after relatively long time intervals of ∼0.5–250 ms, we observe rapid bursts of alkali atoms, as well as molecular species which include NaCl and (LiF)n where n = 1,2,3. Bursts of alkali containing species also occur during loading prior to fracture and for unloaded specimens during heat treatment. We argue that these bursts are due to energetic emergence (“popout”) of dislocations at free surfaces.
Raman microprobe studies of pulsed laser damaged TiO2 films deposited using three different methods are reported. Phase transformation and redeposition of coating materials were observed in selected regions of amorphous films deposited by ion beam sputtering and electron beam evaporation. Preferential removal of a specific phase or transformation to a second phase were observed in reactively sputtered films. Some damage sites exhibited regions of stress heterogeneity which can be explained in terms of the return electron stream model of plasma/target interaction and rapid quenching.
We have implanted Nd and Er ions into x- and z-cut LiNbO3 single crystals. Rutherford backscattering spectrometry and channeling shows the recrystallization of the host during annealing and the rare earth diffusion. Nd and Er have different solubilities and different diffusion constants in LiNbO3. The solubility is strongly temperature dependent. The diffusion is substitutional, fastest parallel to c-axis of the LiNbO3 crystal and characterized by an activation energy of approximately 3.6 eV.
As part of a research program devoted to the microstructural characterization of Al2TiO5-based compounds, high resolution electron microscopy (HREM) has been undertaken in order to study the crystallographic arrangement, especially ordering possibilities, of Al and Ti cations in the metallic sublattice of aluminum titanate. It is seen that adequate experimental conditions, mainly defocus setting, for a resolution of at least 2.5 Å point-to-point, enable the disordered model to be directly and unambiguously proved on 100-oriented micrographs.