Metastable phase relations and europium(II) luminescence of barium hexa-aluminates prepared by gel to crystallite conversion

Wet chemical reaction of hydrated alumina gels, Al2O3.yH(2)O(80Metastability of this nonstoichiometric phase is evident from the splitting of (00l) reflections, on heat treatment above 1600 degrees C, indicating the subsolidus decomposition to 0.82BaO 6Al(2)O3 and 1.32BaO 6Al(2)O(3). High resolution electron microscopy reveals extensive distribution of the blocking defects and structural intergrowths. The metastable Ba-hexa-aluminates doped with Eu(II), exhibit predominantly the 436nm emission band for all the 'x' values. No deterioration in emission intensity is noticed on long term preservation of the phosphors. Differences prevailing in literature on the phase relations in Ba-hexa-aluminates can be attributed to preparative route dependent metastability.

Universal Stability and Temperature Dependent Phase Transformation in Group VIIIB-IB Transition Metal FCC Nanowires

Atomistic simulation of Ag, Al, Au, Cu, Ni, Pd, and Pt FCC metallic nanowires show a universal FCC -> HCP phase transformation below a critical cross-sectional size, which is reported for the first time in this paper. The newly observed HCP structure is also confirmed from previous experimental results. Above the critical cross-sectional size, initial < 100 >/{100} FCC metallic nanowires are found to be metastable. External thermal heating shows the transformation of metastable < 100 >/{100} FCC nanowires into < 110 >/{111} stable configuration. Size dependent metastability/instability is also correlated with initial residual stresses of the nanowire by use of molecular static simulation using the conjugant gradient method at a temperature of 0 K. It is found that a smaller cross-sectional dimension of an initial FCC nanowire shows instability due to higher initial residual stresses, and the nanowire is transformed into the novel HCP structure. The initial residual stress shows reduction with an increase in the cross-sectional size of the nanowires. A size dependent critical temperature is also reported for metastable FCC nanowires using molecular dynamic, to capture the < 110 >/{111} to < 100 >/{100} shape memory and pseudoelasticity.

Nitride synthesis using ammonia and hydrazine - a thermodynamic panorama

Because of the wide variety of projected applications of ultrapure nitrides in advanced technologies, there is interest in developing new cost-effective methods of synthesis. Explored in this study is the use of ammonia and hydrazine for the synthesis of nitrides from oxides, sulfides and chlorides. Even when the standard Gibbs energy change for the nitridation reactions involved are moderately positive, the reaction can be made to proceed by lowering the partial pressure of the product gas below its equilibrium value. Use of a metastable form of precursor in the nanometric size range is an alternative method to facilitate nitridation. Ellingham-Richardson-Jeffes diagrams are used for a panoramic presentation of the driving force for each set of reactions as a function of temperature. Oxides are the least promising precursors for nitride synthesis; sulfides offer a larger synthetic window for many useful nitrides such as BN, AlN, InN, VN, TiN, ThN and Si3N4. The standard Gibbs free energy changes for reactions involving chlorides with either ammonia or hydrazine are much more negative. Hydrazine is a more powerful nitriding agent than ammonia. The metastability of hydrazine requires that it be introduced into a reactor through a water-cooled lance. The use of volatile halides with ammonia or hydrazine offers the potential for synthesis of pure and doped nanocrystalline nitrides. Nitride thin films can also be prepared by suitable adaptations of the chloride route. (C) 2002 Kluwer Academic Publishers.

Size and temperature dependent stability and phase transformation in single-crystal zirconium nanowire

A novel size dependent FCC (face-centered-cubic) -> HCP (hexagonally-closed-pack) phase transformation and stability of an initial FCC zirconium nanowire are studied. FCC zirconium nanowires with cross-sectional dimensions < 20 are found unstable in nature, and they undergo a FCC -> HCP phase transformation, which is driven by tensile surface stress induced high internal compressive stresses. FCC nanowire with cross-sectional dimensions > 20 , in which surface stresses are not enough to drive the phase transformation, show meta-stability. In such a case, an external kinetic energy in the form of thermal heating is required to overcome the energy barrier and achieve FCC -> HCP phase transformation. The FCC-HCP transition pathway is also studied using Nudged Elastic Band (NEB) method, to further confirm the size dependent stability/metastability of Zr nanowires. We also show size dependent critical temperature, which is required for complete phase transformation of a metastable-FCC nanowire.

Molecular probe dynamics reveals suppression of ice-like regions in strongly confined supercooled water

The structure of the hydrogen bond network is a key element for understanding water's thermodynamic and kinetic anomalies. While ambient water is strongly believed to be a uniform, continuous hydrogen-bonded liquid, there is growing consensus that supercooled water is better described in terms of distinct domains with either a low-density ice-like structure or a high-density disordered one. We evidenced two distinct rotational mobilities of probe molecules in interstitial supercooled water of polycrystalline ice Banerjee D, et al. (2009) ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water. Proc Natl Acad Sci USA 106: 11448-11453]. Here we show that, by increasing the confinement of interstitial water, the mobility of probe molecules, surprisingly, increases. We argue that loose confinement allows the presence of ice-like regions in supercooled water, whereas a tighter confinement yields the suppression of this ordered fraction and leads to higher fluidity. Compelling evidence of the presence of ice-like regions is provided by the probe orientational entropy barrier which is set, through hydrogen bonding, by the configuration of the surrounding water molecules and yields a direct measure of the configurational entropy of the same. We find that, under loose confinement of supercooled water, the entropy barrier surmounted by the slower probe fraction exceeds that of equilibrium water by the melting entropy of ice, whereas no increase of the barrier is observed under stronger confinement. The lower limit of metastability of supercooled water is discussed.

Stabilizing the Native Trimer of HIV-1 Env by Destabilizing the Heterodimeric Interface of the gp41 Postfusion Six-Helix Bundle

The HIV-1 envelope glycoprotein (Env) is a trimer of gp120-gp41 heterodimers and is essential for viral entry. The gp41 subunit in native, prefusion trimeric Env exists in a metastable conformation and attains a stable six-helix bundle (6-HB) conformation comprised of a trimer of N-heptad repeat (NHR) and C-heptad repeat (CHR) heterodimers, that drives the fusion of viral and cellular membranes. We attempted to stabilize native Env trimers by incorporation of mutations at the NHR-CHR interface that disrupt the postfusion 6-HB of gp41. The mutations V570D and I573D stabilize native Env of the HIV-1 JRFL strain and occlude nonneutralizing epitopes to a greater extent than the previously identified I559P mutation that is at the interface of the NHR trimers in the 6-HB. The mutations prevent soluble-CD4 (sCD4)-induced gp120 shedding and 6-HB formation. In the context of cell surface-expressed JRFL Env, introduction of a previously reported additional disulfide between residues A501 and T605 perturbs the native conformation, though this effect is partially alleviated by furin coexpression. The data suggest that positions 570 and 573 are surface proximal in native Env and that the NHR homotrimeric coiled coil in native Env terminates before or close to residue 573. Aspartic acid substitutions at these positions stabilize native trimers through destabilization of the postfusion 6-HB conformation. These mutations can be used to stabilize Env in a DNA vaccine format. IMPORTANCE The major protein on the surface of HIV-1 is the envelope (Env) glycoprotein. Env is a trimer of gp120-gp41 heterodimers. gp120 is involved in receptor/coreceptor binding and gp41 in the fusion of viral and cellular membranes. Like many other viral fusion proteins, the gp41 subunit in native trimeric Env exists in a metastable conformation. gp41 readily forms a stable six-helix bundle (6-HB) conformation comprised of a trimer of N-heptad repeat (NHR) and C-heptad repeat (CHR) heterodimers that drives fusion of viral and cellular membranes. While it is expected that native Env is a good immunogen, its metastability results in exposure of immunodominant nonneutralizing epitopes. In the present study, we stabilize native Env trimers by incorporation of a number of different mutations at the NHR-CHR interface that disrupt the postfusion 6-HB of gp41. The stabilized constructs described here can be incorporated into DNA vaccine candidates.

Large-eddy simulations and measurements of a small-scale high-speed coflowing jet

Measurements and predictions are made of a short-cowl coflowing jet with a bypass ratio of 8:1. The Reynolds number is 300,000, and the inlet Mach numbers are representative of aeroengine conditions. The low Reynolds number of the measurements makes the case well suited to the assessment of large-eddy-simulation-related strategies. The nozzle concentricity is carefully controlled to deal with the emerging metastability issues of jets with coflow. Measurements of mean quantities and turbulence statistics are made using both laser Doppler anemometry and particle image velocimetry. The simulations are completed on 6× 106, 12× 106, and 50 × 106 cell meshes. To overcome near-wall modeling problems, a hybrid large-eddy-simulation-Reynolds-averaged-Navier-Stokesrelated method is used. The near-wall Reynolds-averaged-Navier-Stokes layer is helpful in preventing nonphysical separation from the nozzle wall.Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Comparison of les to LDA and PIV measurements of a small scale high speed coflowing jet

Measurements and predictions are made of a short cowl co-flowing jet with a bypass ratio of 8:1. The Reynolds number for computations and measurements are matched at 300,000 and the Mach numbers representative of realistic jet conditions with core and co flow velocities of 240m/s and 216m/s respectively. The low Reynolds number of the measurements makes the case well suited to the assessment of large eddy resolving computational strategies. Also, the nozzle concentricity was carefully controlled to deal with the emerging metastability issues of jets with coflow. Measurements of mean quantities and turbulence statistics are made using both two dimensional coincident LDA and PIV systems. The computational simulations are completed on a modest 12×106 mesh. The simulation is now being run on a 50×106 mesh using hybrid RANSNLES (Numerical Large Eddy Simulation). Close to the nozzle wall a k-l RANS model is used. For an axisymmetric jet, comparison is made between simulations which use NLES, RANSNLES and also a simple imposed velocity profile where the nozzle is not modeled. The use of a near wall RANS model is shown to be beneficial. When compared with the measurements the NLES results are encouraging. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Light and annealing induced changes in Si-H bonds in undoped a-Si : H

Light and annealing induced changes in Si-H bonds in undoped a-Si:H have been investigated by a differential infrared spectroscopy method. The light-induced changes in Si-H bonds are not monotonic, quite different from the usual Staebler-Wronski effect in electronic properties, and involve more complicated physics. The magnitude of the light-induced changes in Si-H bonds is proportional to the hydrogen content in the film. There may exist more than one microscopic process which determine the light-induced changes in Si-H bonds. Almost the whole a-Si:H network is affected when a-Si:H is subjected to Light-soaking or to annealing. The light-induced changes in Si-H bonds may be an independent light-induced phenomenon or an auxiliary process of the metastable SWE defect creation. (C) 2000 Elsevier Science Ltd. All rights reserved.

Influence of crystal perfection on the reverse leakage current of the SiGe Si p-n heterojunction diodes

Double-crystal X-ray diffraction and I-V characterization have been carried out on the GSMBE grown SiGe/Si p-n heterojunction materials. Results show that the SiGe alloys crystalline quality and the misfit dislocations are critical influences on the reverse leakage current. The crystal perfection and/or the degree of metastability of the Sice alloys have been estimated in terms of the model proposed by Tsao with the experimental results. High-quality p-n heterojunction diodes can be obtained by optimizing the SiGe alloy structures, which limit the alloys in the metastable states. (C) 1999 Elsevier Science B.V. All rights reserved.

THEORETICAL INVESTIGATION OF THE DYNAMIC PROCESS OF THE ILLUMINATION OF GAAS

The dynamic process of light illumination of GaAs is studied numerically in this paper to understand the photoquenching characteristics of the material. This peculiar behavior of GaAs is usally ascribed to the existence of EL2 states and their photodriven metastable states. To understand the conductivity quenching, we have introduced nonlinear terms describing the recombination of the nonequilibrium free electrons and holes into the calculation. Though some photoquenching such as photocapacitance, infrared absorption, and electron-paramagnetic-resonance quenching can be explained qualitatively by only considering the internal transfer between the EL2 state and its metastability, it is essential to take the recombination into consideration for a clear understanding of the photoquenching process. The numerical results and approximate analytical approach are presented in this paper for the first time to our knowledge. The calculation gives quite a reasonable explanation for n-type semiconducting GaAs to have infrared absorption quenching while lacking photoconductance quenching. Also, the calculation results have allowed us to interpret the enhanced photoconductance phenomenon following the conductance quenching in typical semi-insulating GaAs and have shown the expected thermal recovery temperature of about 120 K. The numerical results are in agreement with the reported experiments and have diminished some ambiguities in previous works.

A NEW METHOD FOR DETECTING SUBTLE CHANGES IN SI-H BONDS OF A-SI-H

A new method developed for detecting possible subtle changes in Si-H bonds of a-Si:H after light soaking is described in detail. The method promises a sensitivity orders of magnitude higher than that reached by the usual IR spectrometer. Some preliminary results on phosphorus doped a-Si:H are given.

Yellow luminescence mechanism and persistent photoconductivity in n-GaN single crystal films grown on alpha-Al2O3(0001) substrates by LP-MOCVD

Unintentionally doped and Si-doped single crystal n-GaN films have been grown on alpha-Al2O3 (0001) substrates by LP-MOCVD. Room temperature photoluminescence measurement showed that besides the bandedges, the spectrum of an undoped sample was a broad deep-level emission band peaking from 2.19 to 2.30eV, whereas the spectrum for a Si-doped sample was composed of a dominant peak of 2.19eV and a shoulder of 2.32eV. At different temperatures, photoconductance buildup and its decay were also observed for both samples.. The likely origins of persistent photoconductivity and yellow luminescence, which might be associated with deep defects inclusive of either Ga vacancy(V-Ga)/Ga vacancy complex induced by impurities or N antisite (N-Ga), will be proposed.

Influence of crystal perfection on the reverse leakage current of the SiGe Si p-n heterojunction diodes

Double-crystal X-ray diffraction and I-V characterization have been carried out on the GSMBE grown SiGe/Si p-n heterojunction materials. Results show that the SiGe alloys crystalline quality and the misfit dislocations are critical influences on the reverse leakage current. The crystal perfection and/or the degree of metastability of the Sice alloys have been estimated in terms of the model proposed by Tsao with the experimental results. High-quality p-n heterojunction diodes can be obtained by optimizing the SiGe alloy structures, which limit the alloys in the metastable states. (C) 1999 Elsevier Science B.V. All rights reserved.