A serious problem for Ag-sheathed Bi-2212 wires produced by a partial melt process is oxygen release which causes effluence of the oxide materials from the Ag sheath. Higher oxygen partial pressure during the melt process cuts the oxygen release. We investigated the effect of the oxygen partial pressure, melting temperature, and cooling rate on superconducting properties of Ag-sheathed wires. It was found that (i) the formation of the Bi-2212 phase under a higher oxygen partial pressure of 1 atm is slower than that with a lower oxygen partial pressure of 0.2 or 0.5 atm; therefore, the cooling rate should be as slow as 0.25 °C/min to get homogeneity of the intragrain superconductivity as well as formation of the Bi-2212 phase, and (ii) melting temperature should be just above the incipient melting point; otherwise, Jc significantly decreases due to the formation and growth of voids together with degradation of superconductivity.

The YBa2Cu3O7−x formation kinetics from a spray-roasted precursor powder containing Y2O3, BaCO3, and CuO was followed via in situ, time-resolved x-ray diffraction as a function of gas atmosphere and temperature. In inert atmospheres, BaCO3 and CuO form BaCu2O2 which subsequently reacts with Y2O3 to form YBa2Cu3O6. However, YBa2Cu3O6 decomposes at temperatures exceeding 725 °C with Y2BaCuO5 being one of the decomposition products. In oxidizing atmospheres, YBa2Cu3O7−x formation involves the BaCuO2. At high temperatures (800–840 °C), oxygen increases the yield of YBa2Cu3O6. A nuclei growth model assuming two-dimensional, diffusion-controlled growth with second-order nucleation rate fits the experimental data.

A review and some case studies on the interatomic distances of candidate substrates in comparison to the high Tc superconductor (HTSC) phase YBa2Cu3O7−δ (YBCO) is presented, in an attempt to enhance the basis for substrate selection for YBCO film epitaxy. This preliminary study was carried out by examining a variety of interatomic distances in the structure rather than merely matching the lattice parameters. Interatomic structure matching planes of selected YBCO orientations in contact with substrates were identified. The surface termination of the substrate was found to be a crucial parameter in determining the oriented or epitaxial growth. Possible composition dependence of the orientation of the films at the nucleation stage was also anticipated depending on the comparison. Several currently most commonly used substrates are discussed in some detail.

Syntheses of Al-based metastable alloys from powder mixtures of elemental Al and amorphous Fe78Si12B10 [x at. % Al + (100 — x) at. % (Fe78Si12B10)] alloy by mechanical milling (MM) using a planetary ball mill are investigated. X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are used to characterize their structure during the MM process. For the powder mixture with low content of Al (x = 75, 82), fully amorphous material can be obtained by MM, while for the milled product with a high content of Al (x = 90), nanocrystalline Al and amorphous phases are obtained. During the initial milling stage, the Al atoms are dissolved into the amorphous Fe78Si12B10 matrix by heavy deformation. Consequently, the Al-enriched homogeneous amorphous alloys are produced with the disappearance or shrinkage of diffraction peaks of Al in the XRD pattern. Further milling of the powder mixture with 75 at. % Al results in the crystallization of amorphous phase and the formation of nanocrystalline Al3Fe type phase. The crystallization products of all as-milled samples are very similar, composed of Al13Fe4 and AlFe3 phases. It is suggested that the kinetics of nucleation and growth favor the formation of amorphous phase due to the existence of amorphous phase initially. The amorphization reaction by mechanical milling is diffusion process, but defects and strain also play an important role.

Bi–Sr–Ca–Cu–O (BSCCO) superconducting whiskers (2212 phase) were prepared by heating in air the compacted specimens of the mixtures of melt-quenched Bi2SryCa2Cu4Al1Ox powders and alumina powders. Formation of the whiskers depends on the composition and the applied pressure of the compacts. The optimum composition of the melt-quenched products for preparing the long whiskers is Bi2Sr2Ca2Cu4Al1Ox. Superconducting whiskers <1 mm in length (2212 phase) containing an excess amount of copper were grown numerously from the specimen compacted at 20 MPa; long whiskers (2212 phase) of 1–5 mm in length were obtained from that compacted at 180 MPa. These whiskers showed diamagnetic signals below Tc ≃ 80 K.

The local order in a single crystal of YBa2Cu2.53Co0.47O7.13 has been studied with anomalous diffuse x-ray scattering. (For such a Co concentration the compound is nonsuperconducting.) Intensity measurements were carried out at two energies below the Co edge. The difference data could then be expressed in terms of the local structure around a Co atom. The short-range order parameters (α's) indicate that the Co and Cu atoms are nearly randomly distributed on the Cu1 sites. The estimated size of the Co-free “domains” is 5-7 Å. The first neighbor in-plane Co-Co distances are significantly shortened, indicating that the Co atoms are displaced from their average positions. The data also show a significant decrease of the Co-O1 distance, leading to an increase of the Cu2-O1 distance. The lengthening of the Cu2-O1 distance implies a lowering of the Cu2 formal valence. The Co substitution also affects the in-plane Cu2 positions. The present study shows that the Cu-O2 structural coherence is altered on a scale smaller than the superconducting coherence length. As far as the superconductivity is concerned, the Cu2 valence remains one of the most important parameters in determining the superconducting properties of the Co-doped 123 compounds. On the other hand, there is some evidence that in order for superconductivity to occur in this and other doped cuprate compounds, the size of dopant-free regions in the basal plane may have to exceed the superconducting coherence length.

The artificial layering of metals can change both physical and structural characteristics from the bulk. The stabilization of polymorphic metallic phases can occur on a dimensional scale that ranges from single overgrowth layers to repetitive layering at the nanoscale. The sputter deposition of crystalline titanium on nickel, as both a single layer and in multilayer form, has produced a face-centered cubic phase of titanium. The atomic structure of the face-centered cubic titanium phase is examined using high resolution electron microscopy in combination with electron and x-ray diffraction.

The formation of BaCeO3 and its effects on microstructure were studied in sintered/melt-textured Y-Ba-Cu-O oxides containing 5 wt. % CeO2 and various amounts of Y2Ba1Cu1O5. The added CeO2 was converted to fine particles of BaCeO3 near 930 °C, which is the conventional sintering temperature for Y-Ba–Cu-O. Y2Ba1Cu1O5 and CuO are formed as by-products of the reaction between CeO2 and Y1Ba2Cu3O7−y phase. The CeO2 addition reduced the particle size of Y2Ba1Cu1O5 which was trapped in the Y1Ba2Cu3O7−y matrix after the melt-texture growth. During the peritectic decomposition stage of Y1Ba2Cu3O7−y phase into Y2Ba1Cu1O5 and liquid phase, the morphology of the decomposed Y2Ba1Cu1O5 was changed from a blocky shape in the undoped sample to an acicular shape of high anisotropy in the CeO2-added sample. The formation of the highly anisotropic Y2Ba1Cu1O5 particles appears to be responsible for the refinement of Y2Ba1Cu1O5 particle after the melt-texture processing.

Dislocation dissociations in Al3Ti alloys modified with Mn to stabilize the L12 cubic structure have been studied with transmission electron microscopy and computer simulation of images. Dissociations of a〈110〉 superdislocations into pairs of a/2〈110〉 partials bounding APB's were observed at all temperatures from room temperature to 700 °C. Asymmetrical image contrast, in which one of the a/2〈110〉 partials gives a much more intense image than the other, was observed at small separations of the partials. Although some investigators have taken such asymmetry to suggest SISF-type dissociations in similar alloys, the current work demonstrates that the asymmetry is fully consistent with APB-type dissociation. Further, the degree of image asymmetry decreases as the spacing of the partials increases. It is concluded that identification of the partial dislocations with “near-invisibility criteria” for fractional values of g · b is unreliable, and that computer simulation of images is useful for identification of the partials. However, as expected, the ability to distinguish simulated bright-field images of APB- and SISF-type dissociations also becomes difficult as the separation of the partials becomes very small. Under these conditions, both weak-beam imaging and simulations are necessary to identify the dissociations. Weak-beam simulations have shown that fringe contrast must be present under certain imaging conditions for SISF dissociations, and this contrast has never been observed in this study or in previously published studies of dissociated single superdislocations in cubic Al3Ti alloys. Finally, APB contrast formed with superlattice reflection imaging has been observed between partials on both {111} and {100} after deformation at 700 °C.

Internal cavities often develop during the deformation of superplastic alloys. Experiments were conducted to determine whether a nondestructive photoacoustic method may be used to detect the presence of internal cavities in two different commercial Al-based alloys. An analytical procedure was developed to provide quantitative information on the volume fraction of cavitation. The results confirm that the photoacoustic signal can be used to detect the presence of cavities, and it is demonstrated that the quantitative measurements derived from the photoacoustic data are consistent with observations obtained by sectioning and standard metallographic techniques.

It was shown through two experiments that submicron-size pits formed on the surfaces of a dense fused quartz substrate and a Y2O3-ZrO2 ceramic thin film that were coated with gold and then heat-treated in air at 1200 °C. The formation of the pits is believed to be caused by the positive capillary pressure (approximately 22 MPa) generated in the molten gold droplets (<1 μm in diameter) that were formed as the gold coating melted.

Epitaxial films of semiconducting iron disilicide (β-FeSi2) have been grown by pulsed laser deposition. We find that pulsed laser deposition creates conditions favorable to the formation of films with the smallest geometric misfit possessed by this material system. In situ reflection high energy electron diffraction results indicate a layer by layer growth of the silicide. Analysis of transmission electron diffraction data has determined that the films are single phase and that this growth method reproduces the epitaxial relationship: β-FeSi2 (001) ‖ Si(111).

Bulk nearly single phase Bi-2223 materials doped with Pb and Sb were prepared using the metallurgical route. Li-doped samples were also synthesized, but Li leads to the 2212 phase and prevents the formation of the 2223 phase. The dependence of the sintering conditions on the phases obtained was studied and the influence of different factors on the melting temperature and the temperature required for the formation of the 2223 phase is discussed. For the materials with nominal composition Bi1.5Pb0.3Sb0.1Sr2Ca2Cu3Sy, Tc(zero) = 111 K was achieved. It was found that the Sb-rich phase segregates on the surface of the superconducting grains.

Highly oriented diamond thin films grown on silicon via microwave plasma chemical vapor deposition were examined by transmission electron microscopy. In the plan view, defects appearing at the grain boundary were easily observed. (100) faceted grains that appeared to have coalesced were connected at their interfaces by dislocations characteristic of a low angle grain boundary. From Burgers vector calculations and electron diffraction patterns, the azimuthal rotation between grains was measured to be between 0 and 6°. The defect densities of these films are compared to reports from (100) textured randomly oriented films, and the relative improvement due to the reduction of misorientation and grain boundary angles is discussed.

The joints formed by brazing Al2O3 to itself or to a Ti-alloy (Ti-6Al-4V) with a Ag–Cu–Ti braze filler were investigated. In the brazing process, Ti in the braze alloy reduces Al2C3 to form a series of reaction products which have a layered morphology. The formation of M6X-type compounds, Ti4Cu2O or Ti3Cu3O, at the interface is characteristic of these joints. The other reaction products also belong to the Ti–Cu–O system (with the reduced Al in solution), and hence this subsystem was chosen to assess the thermodynamic stability of the joints. The Ti–Cu–O section was established experimentally at 945 °C, and activities of elements in three of the three-phase regions were estimated based on the phase boundaries of the ternary section and available binary thermodynamic data. The estimated free energies of formation of the two M6X-type compounds, Ti4Cu2O and Ti3Cu3O, are −120 kcal/mol and −122 kcal/mol, respectively. The highly negative values for the free energies of formation suggest that these compounds are thermodynamically stable. The activity data were also used to generate activity diagrams for Ti–Cu–O system. The layer sequences at the joints satisfied the stability requirements based on the ternary section and the activity diagrams, indicating that even though the interfaces formed in a matter of minutes, they were at local thermodynamic equilibrium.

YBa2Cu3Ox ceramic superconductors were levitated, laser-beam melted, and undercooled in pure oxygen, dry air, and pure argon atmospheres. Solidified material from this containerless melting was examined by electron microscopy and x-ray diffraction analysis. The solubility of yttrium oxide in the melt was found to depend on ambient oxygen pressure such that complete dissolution was readily achieved in argon, but not in oxygen. The material formed in argon could be deeply undercooled and resolidified to produce a fine-grained and chemically homogeneous material. Use of ambient oxygen pressure to control the product morphology is discussed.

We prepared the superconducting Bi-Sr-Ca-Cu-O thin films on Y3Al5O12 (YAG) single crystal substrates by metal-organic chemical vapor deposition (MOCVD). This film of 50 nm thickness on a YAG substrate showed c-axis orientation clearly, and the zero-resistivity (Tc-zero) was achieved at 66 K. The film has the advantageous property of surface smoothness compared with the film fabricated on MgO and SrTiO3 single-crystal substrates. The surface property of the YAG substrate seems to influence the film quality.

The effects of milling and Ag addition on the decomposition of single-phase LaBa2Cu3Oy have been evaluated. It was found that the decomposition of milled single-phase LaBa2Cu3Oy powders when sintered in pure N2 is attributed to the introduction of strain in the lattice which causes the instability of the structure. The possible reasons why single-phase LaBa2Cu3Oy could be synthesized by sintering in pure N2 at high temperatures and its transition width is always broadened are proposed. The decomposition of compacts of unmilled powders of single-phase LaBa2Cu3Oy when sintered in pure N2 for a long time is due to the fact that oxygen diffuses along the grain boundaries and evolves through the surface of the specimens. Silver might segregate to the grain boundaries and prevent decomposition. It is suggested that to obtain a high-quality LaBa2Cu3Oy, sintering in the reduced atmosphere to achieve a proper oxygen content is required. Reduced atmosphere and Ag addition could enhance the densification rate. For Ag-doped specimens, Tc is highest for x = 0.0001, but decreases for x > 0.0001.

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.

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.