The effect of G/R (temperature gradient, G; growth rate, R) on the crystal alignment and the crystallization mechanism of YBa2Cu3Oy (123) from the partially melting state [in which Y2BaCuO5 (211) and liquid coexist in equilibrium] grown by the horizontal Bridgman method was investigated. Highly aligned 123 crystals containing fine 211 particles were obtained at high G/R ratios. The limit of G/R for continuous growth, necessary for obtaining textured structure, was confirmed to exist between 301 and 482°Ch/cm2. A high critical current density (Jc) of 1.9 × 104 A/cm2 (77 K, 1 T) was measured for a sample grown at the high G/R condition of 482°Ch/cm2, and the 123 grains grown under a condition of G/R lower than the lower limit became coarser with decreasing GR (cooling rate). Entrapment of the 211 particles in the growing 123 crystals occurs after significant coarsening of the particles in the liquid since observed radii of the 211 particles were independent of the distance from the solid/liquid interface in the 123 crystals.

High temperature reaction calorimetry using molten lead borate as a solvent is used to study the thermochemistry of phases in the Y2O3–BaO–Cu–O system. Enthalpies of formation of YBa2Cu3Ox (6 < x < 7), YBa2Cu4O8, Y2BaCuO5, Y2Cu2O5, and BaCuO2.01 phases have been determined. The thermodynamic relationships among these phases at room temperature have been assessed. The results show that the 123 phase is not thermodynamically stable with respect to assemblages containing combinations of Y2BaCuO5, Y2Cu2O5, BaCuO2, Y2O3, and CuO. The fully oxidized 123 phase is not stable with respect to the 124 and the above assemblages. Reactions at room temperature are shown as follows: YBa2Cu3O7 (s) + CuO (s) → YBa2Cu4O8 (s); δH = −31.9 ± 22.3 kJ mol−1 and YBa2Cu3O6.76 (s) → ½YBa2Cu4O8 (s) + ¼Y2BaCuO5 (s) + ¾BaCuO2.01 (s); δH = −78.9 ± 21.0 kJ mol−1.

Grain growth in the GdBa2Cu3O7−δ high Tc superconductor was investigated. The composition Gd1.09Ba1.91Cu3O7−δ, within the solid solubility region, was selected for the present grain growth study. Differential thermal analysis did not reveal any thermal event except the incongruent melting point, which is indicative of the absence of a liquid second phase during grain growth. The final densities of isothermally annealed samples ranged from 91.3% to 93.7% of theoretical density. The microstructure observation showed a greater grain aspect ratio in this material than in YBa2Cu3O7−δ. The average grain aspect ratio was about 4.7. A very low grain growth exponent of 0.07 was found in the isothermal annealing temperature range from 965 °C to 1020 °C. By comparison with the results on the YBa2Cu3O7−δ system previously reported, it was concluded that the grain growth kinetics in these materials are strongly controlled by the anisotropy of the grain boundary energy. The activation energy of grain growth was calculated to be about 77 kJ/mole.

Pb(ZrxTix)O3 (PZT) powders of 0.2–0.4 μm particle sizes could be obtained from the complex alkoxides synthesized from lead acetate, Zr(OBun)4, and Ti(OEt)4. The powders crystallized to well-crystalline perovskite phases at a temperature as low as 250 °C, independent of the compositional ratio of the B site (Zr/Ti). For the preparation of powders, the complexing of alkoxides, addition of the second solvent such as acetone, acetonitrile, or methyl acetate, and base-catalyzed hydrolysis with ammonia water were essential.

We have observed a rapid amorphization reaction at ambient temperature in the Gd/Co system by employing grazing incidence x-ray scattering. We find that a 135 Å crystalline Gd film is amorphized in less than 30 min by deposition of Co. We postulate that the rapidity of the reaction is due to surface diffusion of Co atoms after deposition to fast diffusion sites such as grain boundaries in the Gd film. Once the interfacial region has been amorphized these fast diffusion paths are sealed off from the surface, rapid diffusion of Co into the Gd crystalline layer is prevented, and the amorphization reaction stops.

We have compared the quality of Ba2YCu3O7−δ (BYCO) films grown by the BaF2 process on (100) substrates of the perovskites LaAlO3, LaGaO3, NdAlO3, NdGaO3, LiBaF3, and SrTiO3. The films were grown by coevaporation of Y, Cu, and BaF2 followed by a two-stage anneal. The high temperature stage of the anneal, the part of the process during which the BYCO crystal structure and morphology develop, has been varied. LaAlO3 and SrTiO3 support much better films, both electrically and structurally, than the other substrates. Of the oxides, NdGaO3 supports the worst films, while films on LiBaF3 are nonconducting. These results emphasize the overriding importance of chemical compatibility in determining the suitability of a potential substrate material. Unless this criterion is satisfied, the issue of lattice matching is unimportant.

An evaluation of the effects of oxygen partial pressure (pO2) on sintering behavior and the resulting microstructure of YBa2Cu3Ox (YBCO) indicates that sintering kinetics are enhanced at reduced pO2. The density of specimens sintered at 910 °C increased from 79 to 94% theoretical when pO2 was decreased from 0.1 to 0.0001 MPa. It is believed that increase in density with decrease in pO2 is the result of enhanced sintering kinetics, due probably to increased defect concentration, decreased activation energy of the rate-controlling species, and possibly the presence of a small amount of liquid phase. Sintering at 910 °C resulted in a fine-grain microstructure, with an average grain size of ≍4 μm. Such a microstructure results in reduced microcracking. Consequently, strength as high as 191 MPa is achieved. Reduced microcracking may have important implications for developing microstructures with improved critical current density.

The agglomeration of Co silicide films formed on Si substrates processed with evaporated Co film thicknesses from 9 to 28 nm was investigated by TEM and four-point-probe resistivity measurements. It was found that the upper portion of a reacting Co or Co silicide film can agglomerate independently from the main body of the silicide layer. This phenomenon is designated partial agglomeration in contrast to whole film agglomeration which generally occurs at higher temperatures. Partial agglomeration appears to develop more extensively for thinner films and poses a serious limitation for the application of thin silicide contact layers for advanced VLSI devices. The formation mechanism of partial agglomeration and the reason for its variation with film thickness are explained on the basis of a previously presented [MRS Proc. Vol. 202, p. 101 (1991)] theoretical model of grain boundary grooving and the onset of islanding in silicide films. Kirkendall voids and phase transformation induced volume changes play an important role in the process.

Epitaxy of cupric oxides, such as the high temperature superconductor YBa2Cu3O7, using “vacuum” techniques requires either activated forms of oxygen, such as atomic oxygen, oxygen plasma, or ozone, or a relatively high pressure of molecular oxygen. In contrast, cuprous oxides (those with formal valence of copper less than +2) can be grown epitaxially in molecular oxygen at pressures below 10−4 Torr. We have explored this regime of epitaxial growth because of the possibility of forming DyBa2Cu3O7 through low temperature ex situ oxidation of DyBa2Cu3O6. We find that the dominant phases growing epitaxially on MgO are CuDyO2, Cu2O, CuBa2O2, DyBa2Cu3O6, and the barium-rich perovskite solid solutions. Sticking coefficients of barium and dysprosium depend on substrate temperature and flux composition for substrate temperatures between 550° and 700 °C. We have obtained superconducting films by annealing Dy-rich, Cu-deficient films in oxygen at 400 °C. The nonstoichiometry (with respect to DyBa2Cu3O6) appears to stabilize “DyBa2Cu3O6,” at low oxygen pressures. We also discuss the use of copper in effusion cells.

Nanocrystalline FexCu100−x solid solutions (x < 60) with single-phase fcc structure have been prepared by mechanical alloying. The average grain size of the powders (8–20 nm) depends on the composition of the material. Varying the composition changes the grain size reversibly. This can be explained by the underlying mechanism of plastic deformation and solution hardening during mechanical alloying coupled with the recovery behavior of the material.

The crystal structures of YBa2Cu3O7−x thin films deposited on MgO(100) substrates have been investigated by cross-sectional transmission electron microscopy (TEM). At a substrate temperature of 670 °C, Cu–O chains become wavy. Stacking faults are formed between the Cu–O chains. Therefore, the domains with c-axis lattice constants of 15 Å, 27 Å, and 39 Å are observed. By applying a biasing voltage of + 300 V, deviation in the c-axis orientation is reduced compared to that at an unbiased condition; even some grain boundaries remain. Near substrate surfaces, deviation of the c-axis direction from the substrate normal increases with a decreasing substrate temperature from 700 °C to 670 °C. Application of a substrate bias voltage of +300 V improved the heteroepitaxial growth of the c-axis oriented YBa2Cu3O7−x.

A new sol-gel method for the preparation of ZnO varistor powders, using inexpensive source materials such as acetates and nitrates, is described. It yields powders with a more homogeneous distribution of dopants compared to the commercially used mixed oxide technique. Varistor ceramics made from sol-gel powders can be sintered at lower temperatures and show improved electrical behavior. This is a consequence of formation of more varistor-active grain boundaries within the ceramics.

The stability of chemical vapor deposition (CVD) source materials for high-Tc superconducting films was examined by 1H-NMR (nuclear magnetic resonance), 13C-NMR, IR (infrared) absorption, and TG-DTA (thermogravimetry and differential thermal analysis) measurements. Highly purified Ca(DPM)2 (DPM: dipyvaloylmethane: (CH3)3CCOCHCOC(CH3)3) and Sr(DPM)2, utilizing recrystallization and sublimation, easily decomposed at room temperature and changed their thermal properties when they were handled and stored in air or even in an Ar atmosphere without removing adsorbed species such as water. The stability increased by removing a trace of adsorbed species and storing in an Ar atmosphere at −30 °C. Bi(C6H5)3 and Y(DPM)3 showed higher chemical stability than Ca(DPM)2 and Sr(DPM)2.

The crystalline structure of the Ba2Y1−xCuxWO6−y system was obtained from x-ray diffraction measurements as a function of copper concentration. Five different x regions are observed, four of which correspond to solid solutions and lie at x ≤ 0.75. The crystalline structure of the phases at these copper concentrations has cubic symmetry with lattice parameters between 0.8382 and 0.8275 nm. At 0.75 < x ≤ 1.0 the samples are a mixture of two phases. In this study we have identified the existence of only two stoichiometric compounds: Ba8Y3CuW4O24 with a cubic crystalline structure with space group Pm3m and lattice parameter of 0.8350(1) nm, and Ba2CuWO6 with a crystalline tetragonal structure and space group I4/mmm and lattice parameters a = 0.5566(1) nm and c = 0.8634(1) nm. The crystalline structure at all copper concentrations is based in the ordered perovskite structure. The samples show electrical insulator behavior.

Classical sintering of YBa2Cu3O7−δ always leads to poor critical transport currents whatever the powder process and the thermal cycle are. Best results are obtained by synthesis of textured ceramics. Fabrication of YBa2Cu3O7−δ with well-oriented grain structure by the melt textured growth process was performed in order to understand the mechanism that controls the texture formation. The evolution of the microstructure was studied by quenching samples at different stages of the MTG cycle. The importance in the texture formation process of the presintering temperature, of the high temperature plateau, of the thermal gradient and its fluctuations, of the presence of second phase inclusions (essentially 211 phase), and of the substrate and its reactivity with Y–Ba–Cu–O is shown.

In a previous paper we studied vacancy diffusion in two high-angle twist grain boundaries in Cu, using the EAM. In this paper, we discuss vacancy diffusion along four additional twist grain boundaries, from 8.8–43.6°. Vacancy formation energies in all the possible sites were calculated (0.14–1.42 eV) and found to be directly related to the degree of coincidence with the neighboring crystal planes. The optimal migration paths were found to coincide with the screw dislocations which comprise the boundary. Vacancy migration energies were found to be low (0.02–0.52 eV). The activation energies for self-diffusion at the boundaries were found to be less than half of the bulk value, in general agreement with experiment. Calculated diffusion rates, δD, for medium-high angle twist grain boundaries were in reasonable agreement with experimental data for polycrystalline material. Diffusion rates were found to decrease with increasing twist angle, in contrast with two sets of conflicting experimental data.

Shock-induced chemical reactions between nickel and aluminum powders (mixed in Ni3Al stoichiometry) are used for the synthesis of nickel aluminides. It is shown that the extent of shock-induced chemical reactions and the nature of the shock-synthesized products are influenced by the morphology of the starting powders. Irregular (flaky type) and fine morphologies of the powders undergo complete reactions in contrast to partial reactions occurring in coarse and uniform morphology powders under identical shock loading conditions. Furthermore, irregular morphology powders result in the formation of the equiatomic (B2 phase) NiAl compound while the Ni3Al (L12 phase) compound is the reaction product with coarse and regular morphology powders. Shock-induced reaction synthesis can be characterized as a bulk reaction process involving an intense “mechanochemical” mechanism. It is a process in which shock compression induces fluid-like plastic flow and mixing, and enhances the reactivity due to the introduction of defects and cleansing of particle surfaces, which strongly influence the synthesis process.

The interfacial reactions between the Y1Ba2Cu3O7−x (123) compound and a Ag70Pd30 alloy have been studied for several heat treatments below and above the melting point of the superconductor. Scanning and transmission electron microscopy, energy dispersive x-ray analysis, Auger spectroscopy, and x-ray diffraction have been used to characterize the reaction products. For the heat treatments below the melting point of the superconductor (950 °C), the Ba and Cu of the 123 compound migrate to the interface and react with the Pd to form a body-centered cubic phase Ba(Pd, Cu)O2. This structure is formed by substitution of Cu by Pd on the Cu+2 sites of the BaCuO2 structure (Im3m space group). CuO is also found in some areas as a product of the reaction. For the thermal treatments above the melting point of the superconductor, the reaction products are the same but, in this case, a large number of Y2BaCuO5 precipitates are found in the bulk of the superconductor near the interface. Pure Ag particles have also been observed in the 123 compound at distances relatively far from the interface.

Epitaxial films of Ba2YCu3O7−δ (BYCO), as thin as 250 Å and with Jc's approaching those for the best in situ grown films, can be formed by coevaporating BaF2, Y, and Cu followed by a two-stage anneal. These results extend the work of R. Feenstra et al. [J. Appl. Phys. 69, 6569 (1991)] for film thicknesses >2000 Å. This involves using low oxygen partial pressure [p(O2) = 4.3 Torr] during the high temperature portion of the anneal, which we vary from Ta = 600 to 950 °C. The BYCO melt line is seen to be the upper limit for Ta. The use of lower p(O2) shifts the window for stable BYCO film growth to lower temperature. The lower growth temperature required for the low p(O2) process allows the formation of smooth films with greater microstructural disorder than for films grown in p(O2) = 740 Torr at higher Ta, resulting in higher Jc values by a factor of four. The relationship between the Ta required to grow films with the strongest pinning force and p(O2) is log [p(O2)] ∝ Ta−1, independent of growth method (in situ or ex situ) over a range of five orders of magnitude in p(O2).

The extent of phase transformation occurring in silicon during room-temperature indentation experiments has been examined by transmission electron microscopy of low-load microindents. The results show that the entire hardness impression arises from structural transformation and extrusion of a ductile high pressure phase. In particular, there is no dislocation activity or other mechanism of plastic deformation operating outside the clearly demarcated transformation zone. The observable impression consists of an amorphous transformation zone with an adjacent region of plastically extruded material and a layer of polycrystalline silicon at the near-surface transformation interface.