A TiV system, which does not amorphize in an inert gas atmosphere, has been mechanically alloyed (MA) and mechanically ground (MG) using a planetary ball mill in a hydrogen and nitrogen atmosphere. The products were characterized by x-ray diffractometry, transmission electron microscopy, differential scanning calorimetry, and chemical analysis as a function of milling time and the gas pressure. Amorphization occurs by MA and MG in a hydrogen atmosphere and by MG in a nitrogen atmosphere. Amorphization by MA proceeds by the reaction between TiH2 and pure V in a hydrogen atmosphere, while it proceeds by MG by hydrogen and nitrogen absorption into the metastable β-TiV alloy and subsequent milling. The difference in the phase formation by MA and MG of the TiV system is discussed based on the gas absorption rate and the solubility of gases.

The electromigration boundary value problem is investigated for the three physically reasonable boundary conditions, assuming a perfectly blocking boundary on one side. The solution to this problem is believed to be that for nucleation dominated electromigration lifetime. The three boundary conditions investigated are the semi-infinite constant vacancy source of Shatzkes and Lloyd,9 the closed system of De Groot13 and Kirchheim and Käber,19 and the heretofore unsolved constant vacancy source at a finite distance from the blocking boundary. It is argued that the first is unrealistic in that there is no length effect possible, which has been repeatedly observed experimentally. The second is argued to be too restrictive to account for failure, leaving the last as the most physically reasonable under most circumstances. The deceptively simple appearance of the boundary conditions belies a complex, double infinite series solution arrived at by a unique approach to inverting the Laplace transform of the solution. The solution correctly predicts the experimental observations of a length effect and, combined with the understanding provided by the solutions under the other two boundary conditions, the effect of a thick passivation layer on electromigration lifetime.

Bi—(Pb)—Sr—Ca—Cu—O superconductors were synthesized by sol-gel processing from nitrates by complexation with citric acid. Their grain growth and sintering above 800 °C were studied by Scanning Electron Microscopy (SEM) and Brunauer, Emmett, Teller (BET) porosimetry. The sintering was limited by anisotropic grain growth, and microcracks in 2212 phase grains were created due to the formation of 2223 phase.

Atomistic computer simulation with embedded atom method potentials has been performed to study the energetics and structures of point defects in L12Ni3A1 and B2 NiAl. The large size of Al atoms plays a dominant role in the relaxed atomic configurations of vacancies and antisite defects in the system. In both Ni3Al and NiAl, it was found that excess Ni can be accommodated by Ni in Al sites. Accommodation of excess Al by locating Al in Ni sites is not energetically favorable in NiAl. The most stable di-vacancy configuration is two vacancies as first nearest neighbors in Ni sublattices for Ni3Al and as second nearest neighbors in Al sublattices for NiAl. General attraction of two vacancies in Ni3Al was found. On the contrary, vacancies in NiAl show repulsive interaction in several cases. This accounts for the existence of structural vacancies in B2 NiAl. The effects of different degrees of random disorder on the lattice parameters were evaluated, and it was found that these effects can be correlated with the local distortion around antisite defects using a simple model.

The early stages of phase formation during mechanical alloying of Ti/Al powder blends were investigated using x-ray diffraction and transmission electron microscopy. After the initial formation of an hep solid solution by the diffusion of Al into Ti, the first phase nucleated at the Ti/Al interface is a (partially) Ll2-ordered fcc phase with a crystallite size of about 10–30 nm. This observation is ascribed to the kinetics of phase formation and the energetic destabilization of the equilibrium intermetallic compounds during milling due to chemical and structural disordering. A mechanism of phase formation during mechanical alloying is proposed, and comparisons are made with the thermal annealing behavior of Ti/Al multilayered thin films.

Co/Cu superlattices with total thicknesses ranging from 10 nm to 60 nm and with periodicities of 1.6–8.5 nm were sputtered on single-crystalline sapphire (1120)-substrates. Sputtering with low rates at room temperature yields samples of high epitaxial and crystalline quality. By careful choice of the sputtering parameters, either the fee [100] or the fcc [111] orientation can be selected as growth direction on one and the same substrate orientation. The preference for a particular film orientation appears to be kinetically driven. In all cases, the average lattice spacings d and the appearance of satellite reflections in x-ray Bragg-scans point to coherent growth up to thicknesses of 30 nm. X-ray small angle reflectivity measurements reveal clear oscillations and satellites indicative for smooth interfaces. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations supplement the characterization of the films.

The growth morphology and interface structure of Nd1.85Ce0.15CuO4−y (NCCO) films grown by pulsed laser deposition on two different types of substrates, “perovskite” LaAlO3 (LAO) and SrTiO3 (STO) and “fluorite” Y2O3-stabilized ZrO2 (YSZ), were studied using cross-sectional electron microscopy. Structurally, the NCCO films are different when grown on the two types of substrates in three aspects: (i) epitaxy, (ii) substrate-film intermixing, and (iii) substrate-film interface roughness. In general, films deposited on “fluorite” substrates showed better superconducting properties than the films grown on “perovskite” substrates, especially for thinner films. Lattice mismatch considerations are not sufficient to explain the observed differences since films grown on the YSZ substrate showed sharp substrate-film interface in spite of their large lattice misfit. The atomic arrangements at the interface were analyzed in terms of electrostatic energy (charge balance) and matching of the oxygen sublattices in order to account for the experimental results.

X-ray photoelectron spectroscopy (XPS) was performed in as-received, thermally annealed, and laser-irradiated sapphire and alumina specimens in order to study the effects of the different treatments on surface chemistry and properties. Laser irradiations with a 308 nm wavelength laser were performed in air and in a reducing atmosphere consisting of a mixture of Ar and 4% of hydrogen. The atomic percentages of carbon, aluminum, and oxygen were measured in all the specimens. Particular attention was paid to the percentages of oxygen in the oxide and in a hydroxyl state. The XPS analyses clearly established that a very thin film of metallic aluminum is formed on the surface of both alumina and sapphire substrates when they are irradiated under a reducing atmosphere. However, the film is discontinuous because it is electrically insulating. Substrates irradiated in air have metallic aluminum only for fluences below 0.4 J/cm2. The valence band photoemission spectra of as-received, annealed, and laser-irradiated specimens were measured. In irradiated specimens, the width of the valence band spectra was found to decrease by ∼10%. One possible cause of this decrease is the generation of point defects during laser irradiation. Electroless copper deposition occurs on sapphire and alumina substrates if their surface has been activated by laser irradiation. The time required for copper deposition was monitored by measuring the electrical resistivity in the irradiated area while the substrates were immersed in an electroless bath. The kinetics of deposition on laser-activated substrates and the XPS results show that the presence of metallic aluminum accelerates the deposition process. However, the presence of aluminum is not the sole reason for laser activation in alumina. Very strong metal-ceramic bonding is produced after thermal annealing of samples having preirradiated substrates. This result is explained in terms of the excess oxygen that is present at the ceramic surface after irradiation.

Bi-Sr-Ca-Cu-O phases provide continuous manufacturing of tapes in long lengths using conventional techniques. Thick films of the BPSCCO 2223 phase [(Bi, Pb)2Sr2Ca2Cu3O10+x] were successfully prepared on Ag tapes by combining the processes of screen-printing with organic binders, cold deformation, and sintering.1 Technical and economical studies revealed that these processes have optimal properties for conductor fabrication. The effect of atmosphere and powder composition on the formation of the 2223 superconductor phase was studied on two different composites: BPSCCO/Ag (open system) and Ag/BPSCCO/Ag (closed system). For the open system, both the evaporation of lead had to be avoided and the presence of additional “catalytic” Ag grains within the layer is necessary to promote the reaction of 2223 phase. Critical current densities jc of 6–12 ± 103 A/cm2 (77 K, O T) were achieved in samples of the closed system.

The temperatures and pathways of crystallization for amorphous Bi–Sr–Ca–Cu–O are strongly dependent on oxygen partial pressure (Po2), the oxidation state of the glass, and the presence of Ag. Reducing the Po2 from 1 to 0 bar lowers the onset of melting 100 °C, but does not change the onset of crystallization. Decreasing Po2 does inhibit the formation of (CaSr)CuO3 (011). Although Ag appears to be immiscible in Bi–Sr–Ca–Cu–O, finely distributed Ag lowers the onset of melting by 20 to 35 °C, depending on Po2. In oxygen-deficient glass, two exotherms are observed upon heating. The first exotherm at 460 °C is independent of Po2, and Ag and corresponds with the formation of Bi2Sr2CuO6 (Bi-2201). The second exotherm occurs at 486 °C with Ag or 500 °C without Ag. The onset temperature of the second exotherm does not change with Po2, but the exotherm decreases in intensity with decreasing Po2. EXAFS indicates that in the as-quenched amorphous material, Cu is in a reduced state. Annealing the amorphous material in oxygen below the first exotherm (i.e., glass relaxation) increases the oxidation state of the Cu to that necessary for the formation of the Bi2Sr2CaCu2O8 (Bi-2212) without additional oxygen diffusion. This relaxation of the glass increases the crystallization temperature of the Bi-2201 to 500 °C. The crystallization sequence of Bi-2201 to Bi-2212 occurs at all Po2 ≥ 0.1 bar, but the composition of the secondary phases changes in the unrelaxed glass. Relaxing the glass eliminates liquid formation at low temperatures, allowing for rapid formation of the Bi-2212 phase with minimal formation of secondary phases for 0.1 ⋚ Po2 ⋚ 0.2 bar.

The stability of YBa2Cu3O7−x (the 123 compound) in contact with silver (Ag) at temperatures below 900 °C was investigated by conducting SEM and EDX analyses on 123 agglomerates that were enclosed in a dense Ag matrix and subjected to various thermal treatments. The stability of the 123 agglomerates was found to depend heavily on the oxygen content in the Ag matrix. In the case of insufficient oxygen content in Ag, the 123 agglomerates, which were as large as 150 μm thick, decomposed readily at temperatures above 500 °C. The complete decomposition process can be summarized as continuous extraction of Ba from the 123 oxide into the surrounding Ag matrix and is proposed to be driven by high mutual solubilities between Ag and Ba under the oxygen-lean condition. Prolonged preoxygenation of the (123 + Ag) mixture powders at temperatures above 400 °C prevents the occurrence of 123 decomposition in compacted samples during subsequent heat treatment, suggesting that the critical oxygen content in Ag for stabilizing the 123 compound to be no higher than 10−3 at. % (the oxygen saturation solubility at 400 °C). The findings may have implications for the processing of other Ba-containing high-Tc superconducting oxides as well.

Transmission electron microscopy and powder x-ray diffraction methods have been used to investigate the evolution of two-phase (L12 + DO22) microstructures from the quenched fcc phase of the Ni-5Al-20V (at. %) alloy. The microstructure after annealing in a temperature range from 650 to 900 °C differs from the eutectoid structure which might be expected for the alloy according to the eutectoid-type phase diagram of the Ni3Al-Ni3V section. This structure results from fast kinetics of ordering in the fcc → L12 and fcc → DO22 phase transitions. Four main stages in the microstructural evolution were observed. Stage I is the formation of spheroidal coherent L12 clusters in a disordered fcc matrix. During stage II the L12 clusters transform into cuboidal precipitates, and the fcc matrix orders into three DO22 variants (which may have interfaces that are wetted by thin fcc layers). In stage III accommodation of misfit (elastic energy) between different phases and variants occurs by formation of (110) twins or a single variant of the DO22 phase and tetragonally strained lamellae of the L12 phase. Stage IV is a discontinuous coarsening process in which a coarse incoherent two-phase structure replaces the fine coherent one. Grains of the coarse structure are nucleated on high-angle boundaries of primary fcc or other surfaces. Many of the grains are found twinned.

The introduction of more reactive precursors for Pb and Sr (oxalates), as well as Ca (citrate) and the use of a Bi nitrate decomposition route, has increased the percentage of the high-Tc (2223) phase in the Bi—Sr—Ca—Cu—O (BSCCO) system. Partial substitution of Bi(Pb) with Sb gives an almost single (2223) phase sample. In addition, a single (2212) phase sample is obtained when high purity Bi2O3 is used as a precursor, whereas Bi acetate leads to semiconducting behavior. The morphology of the samples is studied with a scanning electron microscope (SEM), the stoichiometry with energy-dispersive x-ray analysis (EDAX), and the structure with x-ray diffraction (XRD), while the superconducting properties are investigated by dc-resistivity, ac-susceptibility, and SQUID magnetometry techniques.

We have studied the oxidation of thin epitaxial Fe(100) films on MgO(100) with and without an Au(100) protecting cap by x-ray reflectivity measurements. The oxidation was carried out under atmospheric conditions between 20 δC and 200 δC. The results are compared to the oxidation of Fe(110) oriented films on Al2O3(1120) substrates with an Au(111) cap. Auger electron spectroscopy before and after oxidation was carried out for sublimentary chemical information of the surface. For the uncovered Fe films we observe smoothly growing oxide films at the surface during oxidation at elevated temperatures. As expected, the Au(100) cap serves as an effective shield against oxidation, while the Au(111) cap, surprisingly, does not. In the case of Au/Fe/Al203, we find Fe2O3 formation at the surface of the Au layer at 200 δC. The different behavior of Au(100) and Au(111) is discussed in terms of stacking faults and/or domain structure occurring in the latter case during epitaxial growth.

Acetate, nitrate, and oxalate precursors for YBCO have been spray pyrolyzed under different conditions. Shelled and nonhollow microparticles were obtained from acetate and nitrate precursors, while nonhollow agglomerates were obtained from the oxalate suspension. At low furnace temperatures, the temperature and residence time of the particles were insufficient for complete decomposition of the precursors leading to Cu2O and Cu metal in the product. At 900 °C and above, reduced forms of CuO were not detected by x-ray measurements, and up to ∼60 wt.% YBCO was obtained. An approximate model predicting the particle and gas temperatures along the reactor under different operating conditions was developed. The model demonstrates that under the experimental conditions used here, the absorbed radiation heat by the particles from the furnace walls is significant in heating the gas. The gas and the particle temperatures are fairly close due to the effective heat transfer to the particles. At furnace temperatures of 700 °C, the maximum predicted particle temperature is about 500 °C (for ∼1 s). This explains the incomplete reactions obtained under these conditions. Above 900 °C the reactions are predicted to be complete within the first half of the furnace, leaving sufficient residence time for partial conversion into YBCO. Finally, an approximate expression predicting the relative contribution to the gas heating by the walls and the aerosol has been developed.

Small additions of MnO2 up to 15 at. % in YBa2Cu3O7−δ in place of Cu have been carried out to study their influence on the superconducting properties. It has been found that transition temperature and microstructure of Mn-doped samples are highly sensitive to the sintering temperature and Mn content. The normal state resistance increases with Mn content. Microstructure shows plate-like grain growth for low Mn contents (up to x = 0.05) at higher sintering temperature. XRD and microstructure reveals the presence of additional Y2BaCuO5 and Ba3Mn2O8 phases with an increase in Mn content. Also, reduction in the diamagnetic signal and broadening of transition is observed beyond certain Mn content. A detailed discussion of microstructure, XRD, normalized resistance, and magnetic properties has been carried out in correlation with sintering temperature and Mn content.

The effect of the doping on the growth and superconducting properties of whiskers in the Bi–Sr–Ca–Cu–X–O system (where X = Na, B, Ga, and Pb, respectively), prepared by heating a glassy melt-quenched plate in flowing oxygen, is reported. The thermal behavior of the glassy samples (studied by DTA, XRD, and SEM techniques) was found to be dependent on the dopant as well as on its concentration. The Bi2Sr2CaCu2Ox (2212 phase) whiskers were grown only from the Ga-doped system annealed at 855 °C. The dependence of the mechanism of the whiskers' growth on the dopant is also discussed.

Grain sizes were measured after various heat treatments in three Fe3Al alloys having similar composition that were fabricated using the techniques of ingot metallurgy (cast and wrought), hot extrusion of prealloyed powder, and hot isostatic pressing (HIP) of elemental powders. The ingot metallurgy (I/M) material exhibited normal grain growth behavior at temperatures above 750 °C, in agreement with previous observations. Both powder metallurgy (P/M) materials displayed unusual resistance to grain growth compared to the I/M alloy. In the case of the prealloyed P/M material, the initial (recrystallized) grain size was larger than the initial grain size of the I/M material, although little grain growth was observed for heat-treatment temperatures up to 1100 °C. At higher temperatures grain growth occurred at a rate comparable to that observed to the I/M alloy. The elemental powder P/M material exhibited similar grain growth behavior to the prealloyed P/M material, although the initial (as-HIPed) grain size was considerably smaller. Transmission electron microscopy (TEM) indicated that the grain growth resistance of the P/M materials could be attributed to grain boundary pinning by oxide particles presumed to originate from the powder particle surfaces. The difference in the stable grain size between the prealloyed and elemental powder P/M materials was attributed to the nature of the particle dispersions resulting from processing.

A gradient in the density of polytypoidal stacking faults was observed through the thickness of chemically derived epitaxial Ba2YCu3O7−x (BYC) films on (001) LaAlO3. Cross-sectional TEM studies indicated that films of less than 100 nm thickness were faulted, with a high density of polytypoidal stacking faults. A decrease in stacking fault density in thicker films (300-500 nm thick) was found with increasing distance from the most defective layer near the film/substrate interface. An abrupt transition from highly faulted material near the substrate to essentially stacking fault-free BYC in the upper part of the films was observed in several cases. The present observations are compared with the previously reported1 decrease in critical current density with increasing thickness of these films. Possible implications for flux pinning in BYC thin films are discussed.

Diamond films exhibiting contiguous epitaxial crystallites (∼3-4 μm) have been grown on single crystal (100) silicon by 2.45 GHz microwave plasma deposition. The diamond films were deposited by a three-step process. In the initial stage the silicon wafer was pretreated for 3 h in a 1.8% methane in hydrogen plasma at low pressure. The subsequent nucleation stage was performed, under identical processing conditions, except that a dc bias of −340 V was applied for 25 min. During this period the current increased from 38 mA at the start to 102 mA at the end of the bias. The final growth stage was performed, with an earthed substrate, utilizing a carbon monoxide/methane/hydrogen gas mixture whose composition is conducive to uniformly faceted (100) diamond growth. Scanning electron micrographs showed that a large fraction of the (100) faces of the diamond crystallites are aligned with the (100) plane of the underlying silicon lattice with the crystallite edges parallel to the (110) direction. Raman scattering was used to confirm this finding by measuring the angular dependence of the Raman backscattering intensity at 1332 cm−1 using plane polarized excitation of individual crystallites within the diamond films.