Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires

Extensive molecular dynamics (MD) simulations have been performed in a B2-NiAl nanowire using an embedded atom method (EAM) potential. We show a stress induced B2 -> body-centered-tetragonal (BCT) phase transformation and a novel temperature and cross-section dependent pseudo-elastic/pseudo-plastic recovery from such an unstable BCT phase with a recoverable strain of similar to 30% as compared to 5-8% in polycrystalline materials. Such a temperature and cross-section dependent pseudo-elastic/pseudo-plastic strain recovery can be useful in various interesting applications of shape memory and strain sensing in nanoscale devices. Effects of size, temperature, and strain rate on the structural and mechanical properties have also been analyzed in detail. For a given size of the nanowire the yield stress of both the B2 and the BCT phases is found to decrease with increasing temperature, whereas for a given temperature and strain rate the yield stress of both the B2 and the BCT phase is found to increase with increase in the cross-sectional dimensions of the nanowire. A constant elastic modulus of similar to 80 GPa of the B2 phase is observed in the temperature range of 200-500 K for nanowires of cross-sectional dimensions in the range of 17.22-28.712 angstrom, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

Phase separation versus spin glass behavior in La0.85Sr0.15CoO3

We present comprehensive studies of dc magnetization, ac susceptibility, and magnetotransport of two sets of La0.85Sr0.15CoO3 samples, one exhibits phase separation and the other exhibits spin glass behavior. Our study reveals that the phase separation in La0.85Sr0.15CoO3 is neither inherent nor ubiquitous; rather, it is a consequence of preparation condition. It is realized that the low temperature annealed sample exhibits phase separation while the high temperature annealed one shows the characteristic of spin glass behavior. This study shows that the most probable magnetic state of La0.85Sr0.15CoO3 is spin glass.

Continuation of limit cycles near saddle homoclinic points using splines in phase space

We present a new algorithm for continuation of limit cycles of autonomous systems as a system parameter is varied. The algorithm works in phase space with an ordered set of points on the limit cycle, along with spline interpolation. Currently popular algorithms in bifurcation analysis packages compute time-domain approximations of limit cycles using either shooting or collocation. The present approach seems useful for continuation near saddle homoclinic points, where it encounters a corner while time-domain methods essentially encounter a discontinuity (a relatively short period of rapid variation). Other phase space-based algorithms use rescaled arclength in place of time, but subsequently resemble the time-domain methods. Compared to these, we introduce additional freedom through a variable stretching of arclength based on local curvature, through the use of an auxiliary index-based variable. Several numerical examples are presented. Comparisons with results from the popular package, MATCONT, are favorable close to saddle homoclinic points.

Thermodynamic properties of Pb2PtO4 and PbPt2O4 and phase equilibria in the system Pb–Pt–O

The compounds Pb2PtO4 and PbPt2O4 were synthesized from an intimate mixture of yellow PbO and Pt metal powders by heating under pure oxygen gas at 973 K for periods up to 600 ks with intermediate grinding and recompacting. Both compounds were found to decompose on heating in pure oxygen to PbO and Pt, apparently in conflict with the requirements for equilibrium phase relations in the ternary system Pb–Pt–O. The oxygen chemical potential corresponding to the three-phase mixtures, Pb2PtO4 + PbO + Pt and PbPt2O4 + PbO + Pt were measured as a function of temperature using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the solid electrolyte and pure oxygen gas at 0.1 MPa pressure as the reference electrode. The standard Gibbs free energies of formation of the ternary oxides were derived from the measurements. Analysis of the results indicated that the equilibrium involving three condensed phases Pb2PtO4 + PbO + Pt is metastable. Under equilibrium conditions, Pb2PtO4 should have decomposed to a mixture of PbO and PbPt2O4. Measurement of the oxygen potential corresponding to this equilibrium decomposition as a function of temperature indicated that decomposition temperature in pure oxygen is 1014(±2) K. This was further confirmed by direct determination of phase relations in the ternary Pb–Pt–O by equilibrating several compositions at 1023 K for periods up to 850 ks and phase identification of quenched samples using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Only one ternary oxide PbPt2O4 was stable at 1023 K under equilibrium conditions. Alloys and intermetallic compounds along the Pb–Pt binary were in equilibrium with PbO.

NMR spectrum of pyridine-N-oxide in a thermotropic nematic phase

Absract is not available.

NMR studies of phase transition in ammonium hydrogen bischloroacetate, NH4H(CH2ClCOOO)2

Proton second moment (M2) and spin-lattice relaxation time (T1) of Ammonium Hydrogen Bischloroacetate (ABCA) have been measured in the range 77-350 K. A value of 6.5 G2 has been observed for the second moment at room temperature, which is typical of NH4+ reorientation and also a second moment transition in the range 170-145 K indicates the freezing of NH4+ motion. The NMR signal disappears dicontinuously at 128 K. Proton spin-lattice relaxation time (T1) Vs temperature, yielded only one sharp miniumum of 1.9 msec which is again typical of NH4+ reorientation. A slope change at 250 K is also observed, prbably due to CH2 motion. Further, the FID signal disappears at 128 K. Thus the Tc appears to be 128 K (of two reported values 120 K and 128 K). Activation energies have been calculated and the mechanism of the phase transition is discussed.

Photoacoustic investigation of phase transitions in solids

The use of the photoacoustic effect in the investigation of first- and second-order phase transitions has been examined. Changes in the amplitude of the photoacoustic signal across the phase transition are compared with changes in thermal properties such as specific heat and thermal diffusivity. The systemsstudied include NaN02, TlN03, CsN03, NH4N03, BaTiO,, COO, Cu,HgI,, V02 andV305. The current photoacoustic studies are discussed in the light of the theoretical models available.

Electrochemical phase formation (ECPF) and macrogrowth Part I. Hemispherical models

The phenomenological theory of hemispherical growth is generalised to time-dependent nucleation and growth-rates. Special cases, which include models with diffusion-controlled rates, are analysed. Expressions are obtained for small and large time behaviour and peak characteristics of potentiostatic transients, and their use in model parameter estimation is discussed. Two earlier equations are corrected. Numerically calculated transients which are presented exhibit some interesting features such as a maximum preceding the steady state, oscillations and shoulder.

Electrochemical phase formation (ECPF) and macrogrowth Part II. Two-rate models

A general theory is evolved for a class of macrogrowth models which possess two independent growth-rates. Relations connecting growth-rates to growth geometry are established and some new growth forms are shown to result for models with passivation or diffusion-controlled rates. The corresponding potentiostatic responses, their small and large time behaviours and peak characteristics are obtained. Numerical transients are also presented. An empirical equation is derived as a special case and an earlier equation is corrected. An interesting stochastic result pertaining to nucleation events in the successive layers is proved.

Enhancement of organic phase controlled extraction of copper by activated carbon

The addition of activated carbon particles (Darco-G, average size 4.3,μm) is shown to enhance the initial rate of extraction of copper in a Lewis cell by a mixture of α- and β-hydroxyoximes, when the rate of extraction is controlled by resistances in the organic phase. It is likely that the copper complex is adsorbed by carbon near the interace and partially released in the bulk. The enhancing effect of carbon vanishes when toluene is used as a diluent instead of heptane, presumably because toluene preferentially adsorbs on its surface.

A novel investigation of vapor-phase charge-transfer complexes of halogens with n-donors by electron energy loss spectroscopy

Complexes of I2 with diethyl ether and triethylamine and of Br, with diethyl ether have been investigated in the vapor phase for the first time by employing electron energy loss spectroscopy. Besides the CT bands, blue-shifted vacuum-UV bands of the halogens have been assigned; the amine-I, system appears to exhibit two CT bands,associated with two different excited states of the complex.

Electron-Energy Loss Spectroscopy (Eels) Of Organic-Molecules In Vapor-Phase - Design And Fabrication Of An Indigenous Eel Spectrometer

An indigenous electron energy loss spectrometer has been designed and fabricated for the study of free molecules. The spectrometer enables the recording of low-resolution electronic spectra of molecules inthe vapour phase with ready access to the vacuum ultraviolet region. Electron energy loss spectra of aliphatic alcohols and carbonyl compounds as wellas of benzene derivatives have been recorded with the indigenous spectrometer and the electronic transitions in these molecules discussed.

Analysis of EXCOBULLE two-phase expansion tests

analysis of a complex physical problem and the close agreement they achieved with observations. However, the following points need to be clarified. First of all the authors assume that during the initial phases of expansion, the Tayior's instability sets in due to the acceleraacceleration of lighter fluid against the more dense cold water.

Re-entrant Phase Behavior of a Concentrated Anionic Surfactant System with Strongly Binding Counterions

The phase behavior of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the strongly binding counterion p-toluidine hydrochloride (PTHC) has been examined using small-angle X-ray diffraction and polarizing microscopy. A hexagonal-to-lamellar transition on varying the PTHC to SDS molar ratio (alpha) occurs through a nematic phase of rodlike micelles (N-C) -> isotropic (I) -> nematic of disklike micelles (N-D) at a fixed surfactant concentration (phi). The lamellar phase is found to coexist with an isotropic phase (l') over a large region of the phase diagram. Deuterium nuclear magnetic resonance investigations of the phase behavior at phi = 0.4 confirm the transition from N-C to N-D on varying alpha. The viscoelastic and flow behaviors of the different phases were examined. A decrease in the steady shear viscosity across the different phases with increasing alpha suggests a decrease in the aspect ratio of the micellar aggregates. From the transient shear stress response of the N-C and N-D nematic phases in step shear experiments, they were characterized to be tumbling and now aligning, respectively. Our studies reveal that by tuning the morphology of the surfactant micelles strongly binding counterions modify the phase behavior and rheological properties of concentrated surfactant solutions.

In Situ Phase Separation Following Dehydration in Bimetallic Sulfates: A Variable-Temperature X-Ray Diffraction Study

Phase separation resulting in a single-crystal-single-crystal transition accompanied by a polycrystalline phase following the dehydration of hydrated bimetallic sulfates [Na2Mn1.167(SO4)(2)S0.33O1.167 center dot 2H(2)O and K4Cd3-(SO4)(5)center dot 3H(2)O] has been investigated by in situ variable-temperature single-crystal X-ray diffraction. With two examples, we illustrate the possibility of generating structural frameworks following dehydration in bimetallic sulfates, which refer to the possible precursor phases at that temperature leading to the mineral formation. The room-temperature structure of Na2Mn1.167(SO4)(2)S0.33O1.167 center dot 2H(2)O is trigonal, space group R (3) over bar. On heating the crystal in situ on the diffractometer, the diffraction images display spherical spots and concentric rings suggesting phase separation, with the spherical spots getting indexed in a monoclinic space group, C2/c. The structure determination based on this data suggests the formation of Na2Mn(SO4)(2). However, the diffraction images from concentric rings could not be indexed. In the second example, the room-temperature structure is determined to be K4Cd3(SO4)(5)center dot 3H(2)O, crystallizing in a monoclinic space group, P2(1)/n. On heating the crystal in situ, the diffraction images collected also have both spherical spots and diffuse rings. The spherical spots could be indexed to a cubic crystal system, space group P2(1)3, and the structure is K4Cd3(SO4)(3). The possible mechanism for the phase transition in the dehydration regime resulting in this remarkable single-crystal to single-crystal transition with the appearance of a surrogate polycrystalline phase is proposed.