Superconducting properties of Ce-substituted TaSr2(Gd,Ce)2 {\rm Cu}_2{\rm O}_y

A layered cuprate TaSr₂ Cu₂ for x = 0.4–0.8 has been successfully synthesized by solid state reaction. X-ray powder diffraction analysis indicates that nearly all the peaks from the samples can be indexed to a single phase of Ta-1222. The sample with nominal composition TaSr₂Gd_1.6Ce_0.4Cu₂ showed a tetragonal structure with lattice parameters a = 3.858 Å, c = 28.81 Å  with space group most likely I4/mmm. The TaSr₂ Cu₂ compound exhibits a narrow superconducting region near x = 0.6 with T_c-onset = 30 and T_c-zero=10 K as determined by d.c. electrical resistance versus temperature measurements.     

Photoconductivity in Thin Film of a-(Ge20Se80)0.90Sn0.10

Steady state and transient photoconductivity measurements have been done on thin film of a-(Ge₂₀Se₈₀)_0.90Sn_0.10 as a function of temperature and intensity. Dark conductivity (σ_d) and photoconductivity ( ) measurements show that the conduction in this glass is through an activated process having single activation energy in the temperature range 283–350 K. The intensity dependence of steady state photoconductivity ( ) follows a power law with intensity (F), , where the power γ has been found between 0.5 and 1.0, suggesting bimolecular recombination. Rise and decay of photocurrent at different temperatures and intensities show that photocurrent (I_ph) rises monotonically to the steady state value and the decay of photocurrent is quite slow. A detailed analysis of photoconductive decay shows that the recombination within localized states may be predominant recombination mechanism in this glassy alloy.     

TEM study of Mg-Zn precipitates in Mg-Zn-Y alloys

The precipitates in Mg-Zn-Y alloys in the as-cast state with nominal atomic composition of Mg₉₆Zn₁Y₃, Mg₉₆Zn₂Y₂ and Mg₉₇Zn₁Y₂ were studied by means of TEM as well as XRD techniques. The results show that there is a phase separation of Zn on the nanometer scale in these alloys. Two kinds of nano-sized precipitates were found, namely MgZn and MgZn₂. TEM observation shows that MgZn precipitates are distributed in both the Mg matrix and in Mg₂₄Y₅ grains, which is the typical precipitate in Mg-Zn-Y alloys. There is an inherent crystallographic relationship between MgZn and Mg₂₄Y₅ precipitates: // , // . The size of MgZn₂ precipitates is much smaller than that of MgZn precipitates. They are distributed only in Mg₂₄Y₅ grains, not in the Mg matrix. It is suggested that the nano-sized precipitates, MgZn and MgZn₂, can improve the mechanical properties of the Mg-Zn-Y alloys studied.     

Microstructure and microwave properties of {CaT{i}O}<Subscript>3</Subscript>–LaGaO<Subscript>3</Subscript> {solid solutions}

The phase assemblage, microstructure and microwave (MW) properties of xCaTiO₃-(1-x)LaGaO₃ (CT-LG) ceramics prepared by solid state synthesis from raw oxides and carbonates have been investigated. LG was predominantly single phase perovskite with space group, Pnma, but a small quantity of pyrochlore structured La₂Zr₂O₇ second phase was present due to the contamination from the ZrO₂ milling media. At MW frequencies, the temperature coefficient of resonant frequency (τ_{f}) of LG was --80~ppm/\({\circ}\)C, its permittivity, \(\varepsilon_{r} {=} \)27 and MW quality factor, \(Q^{*}f_{o} {=} \)97,000 (@5~GHz). As CT concentration increased, \(\tau_{f}\) and \(\varepsilon_{r}\) increased to The phase assemblage, microstructure and microwave (MW) properties of xCaTiO₃-(1-x)LaGaO₃ (CT-LG) ceramics prepared by solid state synthesis from raw oxides and carbonates have been investigated. LG was predominantly single phase perovskite with space group, Pnma, but a small quantity of pyrochlore structured La₂Zr₂O₇ second phase was present due to the contamination from the ZrO₂ milling media. At MW frequencies, the temperature coefficient of resonant frequency (τ_{f}) of LG was --80~ppm/ C, its permittivity, 27 and MW quality factor, 97,000 (@5~GHz). As CT concentration increased, and increased to $+$850~ppm/ C and 160, respectively but decreased to 20,000. Zero was achieved at 0.65 with 47, and 40,000, properties comparable with commercial compositions. However, for 0.5, a different second phase was observed which was rich in Ca and Ga. Electron diffraction patterns could be indexed according to a body centred cubic lattice, 12.5 ?. It is suggested that the presence of second phases in the CT-LG compounds may, in part, be responsible for the deterioration in .     

Lead-strontium titanate glass ceramics: I – crystallization and microstructure

The crystallization behaviour of glasses in the system [(Pb Sr ) O·TiO₂]-[2SiO₂· B₂O₃]-[K₂O]-[BaO] (0.2 ≤ x ≤ 0.9) have been studied. Perovskite titanate was found to be the major phase in all the glass ceramic samples investigated. The actual composition of crystalline phases could not be confirmed on the basis of shift in the X-ray diffraction (XRD) peak positions because of similar effects due to solid solution formation and strain due to crystal clamping. Comparison of the observed intensities of various XRD peaks of the perovskite titanate phase with the calculated intensities for (Pb Sr )TiO₃ with same lead/strontium ratio confirmed the formation of lead strontium titanate solid solution. Microstructural characteristics of various glass ceramics are also discussed. The advantages of using K₂O and BaO as additives instead of only K₂O are also discussed.     

TG/DTA-based techniques for the determination of equilibrium vapour pressures of N,N′-propylenebis(2,4-pentanedion-iminoato)nickel(II) for CVD applications

The Schiff’s base complexes of nickel(II) prepared by condensing 1,2-diaminopropane (pn), 1,3-diaminopropane (trien), 1,4-diaminobutane (tren) or 1,2-diaminobenzene (opdn) with 2,4-pentanedione (acac) in a 1:2 mole ratio followed by chelation with nickel(II) were examined for their volatility/decomposition behaviour for CVD applications. Among the complexes screened, only one complex namely N,N′-propylenebis (2,4-pentanedion-iminoato) nickel(II) (designated as [Ni(acac)₂pn], Ni′) exhibited a single stepped volatilisation commencing from above its melting point (T_o) of 431.9 K and ending up with nil residue at about 570 K. Fast Atom Bombardment Mass Spectrometry was employed to determine the molecular mass of the vapour species to be 295 in accordance with the molecular mass for the monomeric Ni(C₁₃H₂₀O₂N₂). The equilibrium vapour pressure (p_e) of Ni′ over the range of 434–498 K was determined to be log p_e/Pa = 13.771 (±0.574)–4925.4 (±258.2) K/T by employing a TG-based transpiration technique, which yielded a value of 94.3±5.0 kJ mol⁻¹ for its standard enthalpy of vapourisation . The DTA-based melting point depression (T_o–T) studies were carried out on four mixtures of Ni′ (as a volatile solvent) with bis(2,4-pentanedionato)nickel(II) (designated as Ni(acac)₂ or Ni″) as the non-volatile solute. The dependence of log X_Ni′ against 1/T(K) for the four mixtures with the solvent mole fraction X_Ni′ = 0.910, 0.897, 0.881 or 0.849 exhibited near constant slope leading to an average value of 19.4±1.6 kJ mol⁻¹ for the standard enthalpy of fusion . Combining and , a value of 113.7 ± 6.6 kJ mol⁻¹ for standard enthalpy of sublimation was derived to facilitate the estimation of vapour pressures for solid/vapour equilibrium below the melting point.     

Effect of boundary plane on the atomic structure of [0001] Σ 7 tilt grain boundaries in ZnO

The atomic structure of [0001] Σ 7 tilt grain boundaries with { }‖ { }, { }‖ { }, and { }‖ { } boundary planes in ZnO was investigated through high-resolution transmission electron microscopy observation of fiber-textured thin films and atomistic calculations. These boundaries were found to comprise three kinds of common structural units that are characterized by fourfold- to eightfold-coordinated channels along the [0001] direction in contrast to sixfold-coordinated channels in wurtzite structure. The boundary structural units are very similar to the multiple core structures of edge dislocations with Burgers vectors of 1/3 < > . Transformation between two of the three configurations can easily occur through an atom flipping corresponding to dislocation glide. Depending on the orientation of boundary planes with respect to the Burgers vectors, the dislocation-like units exhibit straight or zigzag arrangements with periodicities corresponding to the Σ 7 misorientation.     

Microstructure and ionic conductivity of Sr- and Mg-doped LaGaO3

Samples of Sr- and Mg-doped LaGaO₃ (LSGM) with various concentrations of Sr and Mg were prepared by using solid-state reaction method. Results show that ionic conductivities increase with the increase of relative densities. It can also be known that the optimized concentration in with high conductivity is LSGM1520 or LSGM2015. The results also show that, in various concentrations of LSGM, equiaxed, rode-like, polygonal secondary phases such as LaSrGaO₄ or LaSrGa₃O₇ were detected besides (La,Sr)(Ga,Mg)O₃ by means of SEM and EDX. With the increase of doped elements, i.e. x + y, the grain size increases.     

Microstructure, topology and X-ray diffraction in Ag-metal reinforced polymer of polyvinyl alcohol of thin laminates

A polymer composite of Ag-metal reinforced polyvinyl alcohol (PVA) is synthesized in shape of thin laminates of 200–300 μm thickness. The process involves a chemical Ag⁺ dispersion in PVA and in-situ reduction-reaction with active PVA molecules under hot conditions (with stirring) in water at 60–70°C temperature. The product results in a metal Ag-polymer complex dispersed in the solution. After evaporating part of water, a derived viscous solution is casted (in hot conditions) in shape of a thin laminate in a glass mould. In addition to chemical reducer, active OH-groups (free from H-bonding) in PVA molecules of refreshed surfaces act as head groups to adsorb Ag⁺ and drive a directional growth. Short fibrils of Ag-metal thus occur in reaction over the PVA molecules. Casting thin laminates from a liquid sample Ag-PVA allows the fibrils (also the polymer molecules) to align along the surface. Selected Ag-contents up to 5.0 wt.% in Ag-PVA laminates are studied in terms of scanning electron micrograph, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. Average size, morphology and aspect ratio (ϕ) vary in Ag-metal depending on the Ag-content. As long Ag-metal fibrils as 2–5 μm, ϕ=35, occur in a sample of 2.0 wt.% Ag. The Ag-metal reflects in two characteristic 3d_5/2 and 3d/_3/2 XPS bands of 368.3 and 374.1 eV respectively.     

Nonlinear DC response in high-density polyethylene/graphite nanosheets composites

In this paper, the nonlinear conduction behavior of high-density polyethylene (HDPE)/graphite nanosheets composites above the percolation threshold was investigated. For some samples, the I–V curves are smooth and the nonlinear term is cubic, rather than quadratic. However, for other samples, the onset of nonlinearity is accompanied by the appearance of steps in I–V curves. Furthermore, the crossover current I_c for two types of curves at which nonlinearity occurs scales with the linear conductance Σ₁ as , with x≈1.28. And the second-order conductance, Σ₂, also scales with Σ₁ as , with y≈1.00. Two classical models, NLRRN (nonlinear random resistor network) and DRRN (dynamic random resistor network), cannot fully explain our experimental results. It is likely that a combination of these two models may account for the nonlinear feature better.     

Interface structure and strain development during compression tests of Al2O3/Nb/Al2O3 sandwiches

The interface structure of an Al₂O₃/Nb/Al₂O₃ sandwich produced by solid-state diffusion bonding was investigated in detail by various transmission electron microscopy (TEM) methods. The joint possessed at one interface a , , and on the other interface a and orientation relationship. At both interfaces, misfit dislocations formed to compensate the lattice mismatch as found by high-resolution transmission electron microscopy (HRTEM). Electron energy-loss near edge structure (ELNES) studies revealed that the interface is terminating with an Al layer resulting in Al–Nb bonds. Identical sandwiches were investigated on the meso- and macroscopic scale by performing compression tests and simultaneously monitoring the strain development at (001)_Nb and crystal faces. The full-field optical strain measurements (FFOM) revealed that the strain is localized at the interfaces when observed at the (001)_Nb face while it is along the maximum shear directions of 36–54° inclined to the interface when observed at the face. The strain localization along a specific maximum shear direction results in the cleavage of Al₂O₃, always initiating from the interface possessing the and orientation relationship.     

Effects of material properties on the fragmentation of brittle materials

We present a fundamental investigation of the influence of material and structural parameters on the mechanics of fragmentation of brittle materials. First, we conduct a theoretical analysis (similar to Drugan’s single wave problem, Drugan, W.J. (2001), Journal of Mechanical and Physics Solids 49, 1181–1208.) and obtain closed form solutions for a problem coupling stress wave propagation and single cohesive crack growth. Expressions for a characteristic fragment size s ₀ and a characteristic strain-rate are given. Next, we use a numerical approach to analyze a realistic fragmentation process that involves multiple crack interactions. The average fragment size s is calculated for a wide variety of strain-rates and a broad range of material parameters. Finally, we derive an empirical function that relates the normalized fragment size s/s ₀ to the normalized strain-rate and that fits all of the numerical results with a single master curve.     

Manufacturing of cellular β-SiAlON/β-SiC composite ceramics from cardboard

Light-weight, cellular β-SiAlON/SiC ceramics were produced via dip-coating of an Al/Si-powder containing preceramic polymer slurry into corrugated cardboard. The coated cardboard preforms were pyrolyzed in Ar-atmosphere at 1200°C, where the cellulose fibres decomposed into carbon. Simultaneously the Al/Si melt infiltrated into the porous carbon and formed β-SiC. Subsequent nitridation at temperatures between 1200–1530°C resulted in the formation of a β-SiC-containing composite. Different pre-oxidation treatment resulted in a variation of the oxygen content in the solid solution phase (z = 0.6–1.2).     

The origin of asterism in almandine-pyrope garnets from Idaho

We have shown that the asterism in garnets emanating from Emerald Creek, Latah County, Idaho, is due to the precipitation of acicular crystals in the garnet crystal host. These inclusions have been identified, using electron microscopy, as tetragonal rutile, the stable phase of TiO₂. One set of rutile (r) needles was found to have the following crystallographic relationship with its garnet (g) host: . From meticulous examinations of the asteriated garnets and the angular relationships between the rutile inclusions and the garnet matrix it was possible to determine the orientations of the other sets of needles for both four and six rayed star garnets. A six rayed star garnet exhibits needles aligned along the and directions of the garnet while a four rayed star garnet exhibits needles aligned along the directions only. These results are believed to be applicable to asteriated garnets emanating from all sources.     

The properties of AlGaN films and AlGaN/GaN heterostructures grown on (11\bar{2}0) sapphire substrates

Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN heterostructures were prepared on the ( ) sapphire substrates at elevated temperatures by alternate supply of trimethylgallium (TMG)/trimethylaluminum (TMA) and ammonia (NH₃) in an inductively heated quartz reactor. X-ray studies reveal the monocrystalline nature of these Al-containing structures. The results of absorption measurements of the Al _x Ga_1-x N films exhibit clear cut-off energies of the films. Based on the investigations of transmission electron microscopy (TEM), Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN structures were found to deposit on the ( ) sapphire substrates with < 0001 > _AlGaN and being parallel to and , respectively.     

Effect of solidification cooling rate and phosphorus inoculation on number per unit volume of primary silicon particles in hypereutectic aluminium—silicon alloys

Collected data on derived number per unit volume of primary silicon particles in hypereutectic Al-Si alloys show a power relationship with solidification cooling rate of the form where typically and mm^-3 (K/s)^-1 in the absence of phosphorus and mm^-3 (K/s)^-1 in its prescence. Significantly lower apparent values of from one set of results appear to stem from measurement of a mean long dimension rather than diameter of particle sections as well as lower measured undercoolings than in Bridgman experiments at similar .     

Orientation-dependent surface tension functions for surface energy minimizing calculations

Previous numerical methods that calculate equilibrium particle shape to study thermodynamic and kinetic processes depend on interfacial (surface) free energy functions γ( ) that have cubic symmetry and thus produce Wulff shapes W of cubic symmetry. This work introduces a construction yielding the minimal surface energy density γ^convex(W) that can be determined for any W. Each γ( ) that belongs to the equivalence class γ(W) bounded by γ^convex(W) can be used in an energy-minimizing calculation that depends only on W. For practical numerical calculations, this work gives two methods taking directional distance from specified orientation minima as a parameter to produce analytic forms of γ( ) giving W as the equilibrium shape for (an otherwise unconstrained) fixed volume. Included are several two- and three-dimensional examples that demonstrate the application and utility of the model γ( ) functions.     

The melting behavior of faceted particles embedded in the solid state: A family of Wulff shapes

We consider the melting behavior of nanoscale particles embedded in a crystalline solid. Calculations of three-dimensional shapes following from a minimization of interfacial free energy show that melt configurations depend on the melt volume fraction, particle shape, and the relative energy densities of particle interfaces. Five members of the family of Wulff shapes with cubic symmetry containing only and facets are studied in detail. Although general trends are noted, the melting behavior is found to be complex and strongly dependent on particle shape. We assume that the melt volume is located in one contiguous region. In such a case, the driving force to replace high-energy facets decreases with the relative energy of those facets.     

Model for kink-like deformation in CoTi single crystals

Single crystals of the B2 CoTi intermetallic compound were deformed in compression at temperatures between room temperature and 1000 K. The orientation was about 10° off the [001] cube axis. For B2 compounds with `strong’ AB bonding, the slip dislocations are often type gliding on planes although and slip cannot be ruled out especially when, as in this case, the Schmid factors for glide are very small. In the lower half of the temperature range, deformation progressed by a series of load drops which manifests itself as coarse bands formed at intervals along the gauge length. The bands are roughly parallel to the (001) plane but it is concluded that the slip planes are and that the kink-like behaviour is due to the formation of twist boundaries. A model for this behaviour is presented which is based on a pole mechanism and the glide of dislocations.     

Structural features of Σ = 19, [110] GaAs tilt grain boundaries

= 19, [110] tilt grain boundaries have been observed to facet parallel to particular planes; the facets lie along _A/ _B, ( )_A/ )_B and ( )_A/( )_B. The structural unit of the = 19 ( )_A/( )_B [110] boundaries consists of 5- and 7-member rings, which are similar to the core structure of a/2[110] edge dislocations. The polarities in each grain on either side of the boundaries has been confirmed by CBED methods; a lower number of anti-site type cross-boundary bonds occur along the boundaries compared to when the polarity of one grain is reversed. The presence of 7-member rings and anti-site cross-boundary bonds results in a more open atomic structure at the boundary, shortening the distance between the first and the second {331} atomic planes from the boundary plane by 40%.