We report here on the appearance of a hysteresis in the current-voltage characteristic, when a voltage is applied to a GdBa2Cu3O7−δ rod at room temperature. This hysteresis was caused by the appearance and disappearance of a hot spot. When a gradually increasing dc voltage (0–8 V) was applied to a GdBa2Cu3O7−δ rod, a hot spot appeared at a given voltage, Va. Further increasing of the voltage increased the spot size, while the current stayed constant. When a voltage of 8 V was reached, the voltage was gradually decreased. The spot did not disappear at Va, but at a voltage 0.2 to 1.3 V lower than Va. The current remained constant until the spot disappeared. By the authors' best knowledge, this is the first report of a hysteresis in the current-voltage characteristic at room temperature for a high Tc superconductor.

Dislocations in the misfit epitaxial film system In0.25Ga0.75As on GaAs(001) were imaged using a modified electron channeling contrast technique in a LaB6 SEM. We obtained images at an incident beam energy of 30 keV, a beam divergence of less than 1 mrad and a specimen tilt of 70° in conjunction with a movable scintillator detector mounted at a takeoff angle of approximately 3° to 5°. We achieved a spatial resolution of approximately 80 to 100 nm with this technique. Such resolution allowed rapid imaging of clusters consisting of only a few closely spaced dislocations in a 55 nm thick film. At such small film thicknesses, we did not require accurate knowledge of the incident beam direction in order to obtain sufficiently strong channeling contrast for qualitative characterization. The observed defect arrangements included features that we believe represent clustered threading segments.

The decomposition of almost fully reacted (Bi, Pb)2Sr2Ca2Cu3Ox (BSCCO-2223) tapes caused by heating in 1 atm of pure O2 at 825 °C has been studied. It was found that partially decomposing 2223 tapes to a mixture of Bi2Sr2Ca1Cu2Oy, (Ca, Sr)2PbO4, and other secondary phases reduced the critical current density (77 K, 0 T) from ∼20 kA/cm2 to nearly zero. Reheating the tapes in 7.5% O2 restored the 2223 phase and, while there was some degradation of the 2223 grain alignment due to residual secondary phase growth, the critical current density was also restored to nearly its original value. We hypothesize that such a decomposition/reformation process can be useful in increasing the connectivity and relative density of polycrystalline 2223, by encouraging the formation of a liquid phase which heals residual cracks in the BSCCO core.

In melt-processed YBa2Cu3O7−x (123) microstructures, often unreacted Y2BaCuO5 (211) particles are observed to be present in an inhomogeneous manner delineating distinguishable patterns. The presence of these patterns in 123 is more clearly observed in the case of low 211 volume concentration and also when the 211 particle size is small. The observed patterns are believed to be due to 211 particle segregation in 123 domains in specific planes during melt texture growth. In the present paper a software program is used to draw a three-dimensional visualization model to demonstrate a possible structure of 211 particle segregation in 123 domains and to explain the presence of observed patterns in the microstructures. The formation of such a 211 particle segregation is explained in the light of previously proposed 123 growth mechanisms.

Rapidly quenched melt-spun ribbons of palladium-based borocarbides with large fractions of the superconducting phase (Tc > 20 K) have been obtained. Superconductivity in the Pd-based alloys seems to be metastable and exists only in the rapidly quenched state. Thus, while in the as-cast state the YPd2B2C ingot is found to be paramagnetic, the rapidly quenched ribbon superconducts and has a simple XRD pattern characteristic for a fcc lattice with a lattice parameter of 4.15 Å. A systematic study in an annealing sequence shows decay of the superconducting phase above 750 °C. The reduction in the integral intensities of XRD fcc peaks correlates well with the loss of the superconducting fraction on successive annealing. The recently reported tetragonal phase, attributed to the superconducting state in this system, is not found in any of our rapidly quenched samples. We present the phase diagrams indicating the optimum composition regimes for superconductivity in the Y–Ni–B–C and Y–Pd–B–C systems. From our magnetic data the values for the upper critical field Hc2(0), and the coherence length ξ(0) are estimated to be 10 Tesla and 57 Å, respectively, for the rapidly quenched YPd2B2C superconducting ribbons.

Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the c-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with a Burgers vector. In addition, dislocation hal-loops, elongated along the c-axis of GaN, were also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of were found to be accommodated by localized elastic bending of GaN (0001)GaN planes in the vicinity of the film/substrate interface. Observations show that the region of the film, with a thickness of ∼100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN “buffer” layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression.

Extruded eutectoid Zn–Al alloy was welded by a melt of the same eutectoid alloy. Two different microstructures were observed in the joint part and in the bulk of the welded alloy. Typical dendritic structure of as-cast Zn–Al alloy was observed in the joint part of the welded alloy. The bulk of the welded Zn–Al alloy appeared as fine grain structure. Two different metastable phases, η′T decomposed from η′s of the chilled as-cast state and η′E of the extruded state, were found to be unstable during the early stages of aging. A four-phase transformation occurred after the decompositions of these two metastable phases of η′T. Microstructures of both the joint part and bulk of the welded alloy were investigated simultaneously during the aging processes.

Elemental powders of zirconium and aluminum in the atomic ratio of 70:30 were mechanically alloyed in an attritor under argon atmosphere using zirconia balls as milling media. Samples have been taken out for characterization after different durations of milling. The process of alloying and resultant amorphization had been studied using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) was carried out to study the morphological changes occurring during repeated cold welding and breaking of the particles. Samples for TEM study were prepared by dispersing the mechanically attrited particles in the nickel foil by electrochemical codeposition. TEM study of the initial stages of milling revealed that localized structural changes precede the bulk amorphization process during mechanical alloying (MA). The sequence of phase evolution has been identified as (i) the formation of nanocrystalline supersaturated solid solution of aluminum in α-zirconium, (ii) amorphization of localized regions at powder interfaces, (iii) ordering of aluminum-rich regions in the metastable Zr3Al (DO19) phase, and, finally, (iv) bulk amorphization of the powders.

Stage 2 CsC24 graphite-cesium derivatives were synthesized and characterized using x-ray diffraction. Pure CsC24 specimens are single phase stage 2 in the range 77–300 K. From the 00l scans, we observed on samples slightly polluted during the transfer in the glove box, the tridimensional segregation of stage 2 in a main CsC26 structure and an additional dense CsC20 structure. The 2D Laue diffraction photographs are the same as already reported, but are interpreted as a mixture of 2 × 2 R 0° lattice commensurate and 2.54 × 2.54 R 14.5° lattice incommensurate structures. In oxidized compounds, we identified another 2.23 × 2.23 R 25.5° 2D additional incommensurate structure corresponding to the composition CsC20 already evidenced by analysis of the 00l diffractograms. We suggest that oxygen impurities are at the origin of this particular stiff and dense structure at low temperature.

The interruption of long-range polar order in rhombohedral ferroelectricPb(Zr1−xTix)O3 (PZT) ceramics has been systematically studied by incorporating La onto the A-site of the perovskite (ABO3) structure for Zr/Ti ratios of 65/35 and 80/20 and various La contents. Studies have been performed by hot-stage transmission electron microscopy, dielectric spectroscopy, and Sawyer–Tower polarization (P-E) techniques. The evolution of a polar nanodomain state from a normal micron-sized domain state with increasing La content was observed. The emergence of this polar cluster state was characterized by the onset of strong frequency dispersion in the dielectric response, indicative of relaxor behavior. The La content that drives the structure into the relaxor state was found to be related to the lattice distortion of the undoped base composition.

Lanthanum-modified lead zirconate titanate ceramics Pb1−3/2xLax(Zr1−yTiy)O3 [PLZT 100x/100(1 − y)/100y] with Zr/Ti ratios close to the antiferroelectric-ferroelectric (AFE-FE) phase boundary were investigated by dielectric spectroscopy, Sawyer–Tower polarization techniques, and electron microscopy. An incommensurate antiferroelectric (AFEIn) phase was found to be stabilized from the rhombohedral FE state in the compositional series 100x/90/10 for x ≥ 0.02. The La content required to induce the AFEIn state increased as the Ti content was increased. For 100x/85/15, a state with relaxor-like dielectric behavior and nanodomains was observed to develop with increasing La content; however, the double-loop-like P-E curves were suggestive of antiferroelectric behavior. Investigations for the composition 6/85/15 revealed the formation of nanodomains from the AFEIn modulation, where the size of the nanodomains equaled the value of the AFEIn modulation wavelength. For this composition, P-E studies revealed double hysteresis characteristics, whereas dielectric investigations revealed relaxor-like behavior. It is suggested that the order within the nanodomain state may be antipolar over a range of compositions in high La content rhombohedral PLZT ceramics.

Studies of the reactions between rare earth salts and phosphoric acid in aqueous or ethanolic media have shown that in both cases stable gels can be formed. Upon drying, gels prepared in aqueous environments yield macrocrystalline REPO4 products similar to those produced by conventional precipitation and drying. Gels prepared in ethanol, on the other hand, undergo dehydration to form dense microcrystalline products. This observation is based on optical and scanning electron microscopy, as well as on x-ray diffraction studies and infrared spectroscopy. These techniques, as well as differential thermal analysis, indicate that crystal growth of these products takes place around 600−700 °C. The composition of the dehydrated gels produced in both the aqueous and ethanolic systems corresponds to an orthophosphate structure. Other characteristics of the microcrystalline REPO4 products include high resistance to attack by water, absence of coloration upon exposure to gamma rays, and a high index of refraction.

The order-disorder states of the A-site vacancy of PBN solid solution affected by different thermal treatments were studied with the aid of High Resolution Electron Microscopy (HREM). PBN ceramics around the morphotropic phase boundary were prepared through two routes to control ordering degree of the A-site vacancy: (1) samples through quenching processes resulted in chaotic states of the A-site vacancies and misfit anti-phase boundary; (2) samples through slow cooling led to an ordered structure of the vacancies in the A1-site. The ordered A1-site vacancies were modulated by interchanges of the sublattices of the ordered vacancies and the Pb2+ cations in the A1-sites along both and orientations, forming a narrow discommensurate wall between two anti-phase domains. The anti-phase domains were observed as a regular belt structure with dimensions of about 45 nm × (≥120) nm. The belt nano-domain structures were a result of quasi-equilibrium thermodynamic processes.

Compressive creep behavior of fine-grained (5 μm) La0.9Sr0.1MnO3 with a relative theoretical density between 85 and 90% was investigated over the temperature range 1150–1300 °C in air. The fine grain size, brief creep transients, stress exponent close to unity, and absence of deformation-induced dislocations, suggested that the deformation was controlled by a diffusional creep mechanism. The activation energy for creep of La0.9Sr0.1MnO3 was 490 kJ/mole. A comparison of the activation energy for creep of La0.9Sr0.1MnO3 with existing diffusion and creep data for perovskite oxides suggested that the diffusional creep of La0.9Sr0.1MnO3 was controlled by lattice diffusion of the cations, either lanthanum or manganese.

To avoid the formation of hollow spheres during spray pyrolysis, NH3 was employed to change the mechanism of forming NiFe2O4 from aerosols, containing Ni(II) and Fe(III) nitrates in the required stoichiometric ratio. Nearly spherical, solid submicron NiFe2O4 particles with narrow size distribution were produced in one step using a dilute aqueous solution at pyrolysis temperatures as low as 823 K. However, higher pyrolysis temperatures (≥1023 K) reduced the oxides to metallic alloy of Ni and Fe due to dissociation of NH3. The forming steps and possible reaction mechanisms for aerosol droplets involved in the process were discussed.

Rate-controlled sintering (RCS) of isostatically pressed particulate compacts of ZnO showed lower average grain sizes and intragranular pore densities than constant heating rate temperature controlled sintering. Valid comparisons of this form could only be made after corrections to hardware and software which reduced specimen creep under dilatometer pushrod load, nonuniform pushrod expansion, reproducible specimen temperature determination, thermal expansion during sintering, and instantaneous termination of sintering at the specified end of RCS. The improved microstructures from RCS were attributed to maximized efficiency of densification, optimizing the time and temperatures permitted for grain growth.

In this work the normal reflectance, R, at a planar silica aerogel interface and the normal transmittance, T, of a silica aerogel slab were measured using a Fourier Transform Infrared Spectrometer. Two procedures were used to obtain the effective optical constants, i.e., the refractive index n and the absorption index κ, of silica aerogel. One procedure determined κ from the measured transmittance T and then determined n from the results for κ and from the measured reflectance R using the Kramers–Kronig relation; the other procedure determined n and κ of silica aerogel from n and κ of fully dense silica glass by using the Clausius–Mossotti equation, Maxwell Garnett formula, and Bruggeman formula. The first procedure has a relatively large error due to the inaccuracy of the transmission and reflection measurements. The second procedure, especially the Clausius–Mossotti equation, yields values of n that are consistent with experiments and may be used for the calculation of the effective optical constants and the extinction coefficient of silica aerogel.

Based on results from chemical kinetic model calculations, a method to improve diamond film growth in a dc arcjet chemical vapor deposition reactor has been developed. Introducing the carbon source gas (CH4) into an Ar/H2 plasma in close proximity to the substrate produced diamond films exhibiting simultaneous improvements in quality and mass deposition rates. These improvements result from a reduced residence time of the methane in the plasma which inhibits the hydrocarbon chemistry in the gas from proceeding significantly beyond methyl radical production prior to encountering the substrate. Improvements in growth rate were modest, increasing by only a factor of two. Optical emission actinometry measurements indicate that the flux of atomic hydrogen across the stagnation layer to the substrate is mass diffusion limited. Since diamond growth depends upon the flux of atomic H to the substrate, these results suggest that under the conditions examined here, a low atomic H flux to the substrate poses an upper limit on the attainable diamond growth rate.

The PbO/MoO3 system with 47%: 53%, 53%: 47%, and 50%: 50% molar ratios at various processing temperatures has been studied with x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and differential thermal analysis (DTA) methods. It is found that in addition to the crystallization of primary PbMoO4 phase, subphases such as Pb2MoO5 and PbMo2O7 are also formed. The remaining PbO and MoO3 are detected at certain stages of the thermal process due to localized powder inhomogeneity. Physical processes, such as sublimation, eutectic melting, solid to liquid, and liquid to vapor transformations are also investigated. In particular, evaporations of excessive PbO or MoO3 in the nonstoichiometric PbO/MoO3 can be correlated to thermal processing parameters. The current study has led to the following three processing guidelines to obtain stoichiometric PbMoO4: (i) for high temperature application, such as the Czochralski melt growth, it is suggested an excessive MoO3 (a few mol %) must be included and a slow heating rate should be employed; (ii) for low temperature synthesis, the stoichiometric PbO–MoO3 can be used, but with a fast heating rate; and (iii) PbO-rich PbO/MoO3 system is not recommended in PbMoO4 synthesis.

Phase transformations, including chemical vapor deposition (CVD) of diamond, taking place by nucleation and growth are commonly described by Avrami or Johnson-Mehl type models. In order to avoid the restrictions of such models with respect to assumptions concerning nucleation rates and growth velocities, the variation with time of nucleation and growth of diamond particles during the deposition of microwave plasma-assisted CVD was studied. The size distributions obtained from image analysis enabled us to trace back details of the nucleation and growth history. Three sources of particle formation were operating during deposition. A general growth law suitable for all particles did not exist. These observations limited the applicability of Avrami-type models to describe space filling. Computer simulation of surface coverage and particle growth was successful because one particular mode of particle formation and growth dominated surface coverage. Based on image analysis and the determination of the film growth rate, the evolution of the diamond volume fraction with time, starting from three-dimensional particle growth followed by a continuous transition to one-dimensional film growth, was described.