Narrow Stability Range Research Articles

Investigation of the Pd(1−x)Zn x alloy phase diagram using ab initio modelling approaches

The identification of the stable phases in alloy materials is challenging because composition affects the structural stability of different intermediate phases. Computational simulation, via multiscale modelling approaches, can significantly accelerate the exploration of phase space and help to identify stable phases. Here, we apply such new approaches to understand the complex phase diagram of binary alloys of PdZn, with the relative stability of structural polymorphs considered through application of density functional theory coupled with cluster expansion (CE). The experimental phase diagram has several competing crystal structures, and we focus on three different closed-packed phases that are commonly observed for PdZn, namely the face-centred cubic (FCC), body-centred tetragonal (BCT) and hexagonal close packed (HCP), to identify their respective stability ranges. Our multiscale approach confirms a narrow range of stability for the BCT mixed alloy, within the Zn concentration range from 43.75% to 50%, which aligns with experimental observations. We subsequently use CE to show that the phases are competitive across all concentrations, but with the FCC alloy phase favoured for Zn concentrations below 43.75%, and that the HCP structure favoured for Zn-rich concentrations. Our methodology and results provide a platform for future investigations of PdZn and other close-packed alloy systems with multiscale modelling techniques.

Pilot scale production, extraction and purification of a thermostable phycocyanin from Synechocystis sp. PCC 6803

Phycocyanin (PC) is a soluble blue pigment-protein primarily harvested from the cyanobacterium Arthrospira platensis. PC is in high demand from several industries, but its narrow stability range limits potential applications. Here, a pilot scale (120 L total) batch production, extraction and purification process for thermostable PC (Te-PC) from a Synechocystis sp. PCC 6803 'Olive' strain expressing the PC operon cpcBACD from Thermosynechococcus elongatus BP-1 on a self-replicating vector is presented. Batch cultivation without antibiotics had no impact on growth or Te-PC production and optimisation of growth conditions resulted in Te-PC contents of 75.3 ± 1.7 mg g DW−1. Wet biomass was harvested following chitosan-based flocculation with a 97 ± 2% efficiency, and Te-PC was extracted by high pressure homogenisation. Subsequent purification by heat-treatment and two-step ammonium sulfate precipitation removed chlorophyll and allophycocyanin contamination, resulting in Te-PC purities of 2.9 ± 0.7 and a mean Te-PC recovery of 84 ± 12%.

Experimental study on isolated operation of hydro-turbine governing system of Lunzua hydropower station in Zambia

This paper focused on the stable operation and control strategy of the hydro-turbine governing system (HTGS) in an isolated-grid operation mode. The stable operation of a hydropower station in isolated-grid operation mode is important for the safe operation of a power system. A regional power grid can be isolated from a main grid due to factors such as natural disasters, grid failures, and human errors. The Lunzua hydropower station in Zambia is often forced to operate as an isolated grid. To ensure its safe and stable operation, we conducted isolated-grid regulation tests in conjunction with field testing of HTGS and determined the parameters of the HTGS for stable operation of the isolated grid. In the field test, we first set the no-load and initial parameters for the isolated-grid mode through the no-load disturbance tests. Through simulated isolated-grid operation tests, we verified and further tuned the initial parameters. Meanwhile, we propose a control strategy for the transition between large-grid and isolated-grid modes. Through operational tests, we determined the parameters for stable operation in isolated-grid mode. During testing, we found that the water flow inertia time constant Tw was too large and the inertial time constant Ta of the impulsive generator was too small, which resulted in a narrow stability range of proportional–integral–derivative (PID) controller parameters for isolated-grid operation. The governing stability for isolated-grid operation was thus sensitive to the PID parameters. Since the isolated-grid field operation test of the HTGS of the Lunzua hydropower station was completed in August 2016, the generator units have maintained safe and stable operation in large-grid and isolated-grid modes, which validated the isolated-grid operation parameters and control strategy for the transition between large-grid and isolated-grid modes as determined by our test method. The methods of field test and control strategies used for isolated-grid operation have important practical value for the isolated grid operation of hydropower stations.

In the quest for new targets for pathogen eradication: the adenylosuccinate synthetase from the bacterium Helicobacter pylori

Adenylosuccinate synthetase (AdSS) is an enzyme at regulatory point of purine metabolism. In pathogenic organisms which utilise only the purine salvage pathway, AdSS asserts itself as a promising drug target. One of these organisms is Helicobacter pylori, a wide-spread human pathogen involved in the development of many diseases. The rate of H. pylori antibiotic resistance is on the increase, making the quest for new drugs against this pathogen more important than ever. In this context, we describe here the properties of H. pylori AdSS. This enzyme exists in a dimeric active form independently of the presence of its ligands. Its narrow stability range and pH-neutral optimal working conditions reflect the bacterium’s high level of adaptation to its living environment. Efficient inhibition of H. pylori AdSS with hadacidin and adenylosuccinate gives hope of finding novel drugs that aim at eradicating this dangerous pathogen.

Carlsonite, , and huizingite-(Al), (NH4)9Al3(SO4)8(OH)2·4H2O, two new minerals from a natural fire in an oil-bearing shale near Milan, Ohio

Carlsonite, , and huizingite-(Al), (NH₄)₉Al₃(SO₄)₈(OH)₂·4H₂O, two new minerals from a natural fire in an oil-bearing shale near Milan, Ohio

Aqueous zinc-organic polymer battery with a high rate performance and long lifetime

A novel redox-active polymer based on a 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF) system in combination with a conjugated backbone was synthesized via rhodium (Rh)-catalyzed polymerization of 2-ethynyl(exTTF), leading to polymers with low polydispersities. Composite electrodes containing this polymer exhibited chemically reversible two-electron oxidation in aqueous media. The application of these electrodes as active cathode materials in hybrid zinc-organic batteries using an aqueous electrolyte enabled the production of air-stable charge storage systems with a theoretical capacity of 133 mAh g−1. These batteries featured high performance, charge/discharge rates of up to 120 C (30 s) and an ultra-long lifetime, of over 10 000 charge/discharge cycles (accompanied by a minor capacity loss of 14%). Finally, the polymer was compared with its nonconjugated derivative, revealing the positive influence of the conjugated backbone on the material activity owing to improved electron transfer within the polymer chain. Researchers in Germany have unlocked the secret to using water as an electrolyte in rechargeable batteries with a new conjugated polymer. Polymer batteries are flexible and lightweight, but a reliance on flammable organic solvents for electrolytes has raised concerns. Ulrich S. Schubert and colleagues synthesized a poly(acetylene)-based compound containing charge-active organosulphurs to improve battery safety while maintaining high performance. Water has a narrow range of stability under applied voltages, making it tricky to use as an electrolyte. The team's polymer, however, is capable of a reversible, two-electron oxidation reaction in this range. A prototype with the polymer cathode, a zinc anode and an aqueous electrolyte revealed the possibilities of this approach — the battery could be charged to its 1-volt potential within 30 seconds and had a lifetime exceeding 10,000 recharge cycles. The application of a conjugated polymer with 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF) units as cathode and zinc as anode enables an aqueous-based hybrid organic battery. This energy storage device features ultra-high rate performance of up to a full charge in 30 s and an extended lifetime up to 10 000 cycles.

Complexes with pyrazine-tetrathiafulvalene-dithiolate (pztdt) ligand [M(pztdt)2], M = Ni, Pd, Pt; Synthesis and characterisation

The synthesis and characterisation of three metal bisdithiolene complexes based on the recently reported highly extended ligand incorporating fused tetrathiafulvalene and pyrazine moieties, [M(pztdt)2], M=Ni, Pd, Pt and pztdt=pyrazinetetrathiafulvalenedithiolate, are reported. These complexes were obtained under anaerobic conditions as crystals of dianionic tetrabutylammonium salts. The crystal structures of Ni and Pd complexes as determined by single crystal X-ray diffraction confirm the ability of these molecules to established interactions with their neighbours through the pyrazine moieties. These dianionic complexes easily undergo oxidations to their monoanionic and neutral states, with a narrow range of stability for the monoanionic and an extended one for the neutral forms that is rather insoluble in common solvents.

Laboratory-numerical models of rapidly rotating convection in planetary cores

Author(s): Cheng, JS; Stellmach, S; Ribeiro, A; Grannan, A; King, EM; Aurnou, JM | Abstract: We present laboratory and numerical models investigating the behavioural regimes of rapidly rotating convection in high-latitude planetary core-style settings. Our combined laboratorynumerical approach, utilizing simplified geometries, can access more extreme parameters (e.g. Rayleigh numbers Ra ≲ 1013; Nusselt numbers Nu ≲ 103; Ekman numbers E ≳ 3 × 10-8) than current global-scale dynamo simulations. Using flow visualizations and heat transfer measurements, we study the axialized flows that exist near the onset of rotating convection, as well as the 3-D flows that develop with stronger forcing. With water as the working fluid (Prandtl number Pr ≲ 7), we find a steep scaling trend for rapidly rotating convective heat transfer, Nu~(Ra/RaC)3.6, that is associated with the existence of coherent, axialized columns. This rapidly rotating trend is steeper than the trends found at moderate values of the Ekman number, and continues a trend of ever-steepening scalings as the rotation rate of the system is increased. In contrast, in more strongly forced or lower rotation rate cases, the heat transfer scaling consistently follows a shallower slope equivalent to that of non-rotating convection systems. The steep heat transfer scaling in the columnar convection regime, corroborated by our laboratory flow visualizations, imply that coherent, axial columns have a relatively narrow range of stability. Thus, we hypothesize that coherent convection columns are not stable in planetary core settings,where the Ekman number is estimated to be~10-15. As a consequence, convective motions in the core may not be related to the columnar motions found in presentday global-scale models. Instead, we hypothesize that turbulent rotating convection cascades energy upwards from 3-D motions to large-scale quasi-2-D flow structures that are capable of efficiently generating planetary-scale magnetic fields. We argue that the turbulent regimes of rapidly rotating convection are essential aspects of core dynamics and will be necessary components of robust, next-generation and multiscale dynamo models.

Novel Si Networks in the Ca/Si Phase Diagram under Pressure

The rich Ca/Si binary phase diagram is explored here theoretically and over a range of pressures, through four stoichiometries: Ca2Si, CaSi, CaSi2, and CaSi3. The calculations are calibrated by recovering the known P = 1 atm structures (even as they suggest metastable alternatives for some) and by predicting correctly the range of pressures at which CaSi3, which has a positive heat of formation at P = 1 atm, can be synthesized. Ca2Si, in accord with its Zintl compound nature, features isolated Si4– ions then chains until decomposition. CaSi, which begins as a Zintl phase (albeit metallic) chain structure evolves to rings and zeolitic-channel structures, while CaSi2 explores a variety of triply connected Si polytypes to Laves phases with increasing pressure. CaSi3 has a calculated narrow range of stability of 8–18 GPa. Interestingly, all the phases studied are calculated to decompose to the elements at P ∼ 250 GPa. This unusual behavior is traced to the special compressibility of elemental Ca and the backward charge transfer from Si to Ca. All the phases studied are metallic, except that Ca2Si is a semiconductor at 1 atm.

NMR and computational study of Ba8CuxGe46-x clathrate semiconductors

Ba8CuxGe46-x is a type-I clathrate material that forms as a semiconductor in a narrow composition range corresponding to the electron-balanced Zintl composition, with x = 5.3. We use NMR spectroscopy combined with ab initio electronic structure calculations to probe the electronic and structural behavior of these materials. Computational results based on a superstructure model for the atomic configuration of the alloy provide good agreement with the electric quadrupole-broadened NMR lineshapes. Modeling using the modified Becke–Johnson (TB-mBJ) exchange potential is also shown to agree well with experimental NMR Knight shifts. The results indicate that the Cu–Ge balance is the main factor determining the carrier density, within a narrow stability range near the ideal Zintl composition.

Kinetics of crystallization of the metastable polytype of cerium upon melt quenching

Using a mathematical model of nonequilibrium crystallization of polymorphous metals with a narrow range of stability of the high-temperature modification, we have conducted numerical analysis of the kinetics of the formation of the metastable polytype of cerium (μ-Ce) that is fixed by melt quenching at cooling rates more than ∼105 K/s. Agreement between the calculation data and the results of X-ray diffraction phase analysis of rapidly quenched foils has been obtained at a physically reasonable value of the model parameter having the meaning of the degree of metastability of μ-Ce.

Pulsed DC reactive magnetron sputtering of vanadium dioxide thermochromic thin films

<title/>Vanadium oxides are an important class of materials with a large diversity of physical and chemical properties which derive from a range of single or mixed valences and a large variety of structures. They are already being used in many technological applications such as electrical and optical switching devices, light detectors, temperature sensors, etc. There has been a great interest in a particular phase, VO2(M), due to its thermochromic behaviour near room temperature which allows the development of smart windows with active control of the solar spectrum, for energy efficiency purposes. However, stoichiometric VO2 is difficult to deposit because of a narrow stability range due to the complex vanadium-oxygen reactive system. In this work, vanadium oxide thin films were synthesised on glass substrates by reactive pulsed direct current magnetron sputtering from a vanadium metal target in an O2/Ar atmosphere. Different processing conditions have been chosen in order to evaluate their influence on the crystal phases formed, surface morphologies and thicknesses and optical performance. The films were characterised by X-ray diffractometry in order to examine the crystal structure and identify the phases present in different films. The obtained VO2(M) films were thereafter analysed in terms of surface morphology by scanning electron microscopy and the characteristic reversible semiconductor-metal transition of the VO2 films was evaluate by optical spectrophotometry in the ultraviolet-visible-near infrared.

Formation and characterization of Y : 247 film through spray pyrolysis technique

The synthesis of high temperature superconducting films of Y : 247 (T c ∼ 73 K) have been successfully achieved. The difficulty in synthesis owing to narrow range of stability of Y : 247 has been taken care of through several quenching modes, e.g. quenching of the films synthesized at ∼850°C, in air or in liquid nitrogen. The energy dispersive analysis of X-rays (EDAX) and transmission electron microscopy (TEM) studies of the as processed film, Y2Ba4Cu7O14+x, exhibit nearly correct cationic stoichiometry of 2 :4 :7; also narrow regions (< 50 Å) of minority Y : 124 phase and stacking faults capable of working as flux pinning sites have been invariably found to be present. In addition to Y : 247, Ag admixed films have also been investigated. TheT c here is ∼ 70–75 K which is similar to that of the film without silver. Representative estimates of transport critical current density (J c) for Y : 247 films is ∼103 A/cm2, and with silver corresponding to Y2Ba4Cu7O14+x (Ag0.1) is found to be ∼ 104 A/cm2.

Molecular Lego®: non-centrosymmetric alignment within interdigitating layers

(E)-N-Hexadecyl-4-[2-(4-octadecyloxynaphthyl)ethenyl]quinolinium bromide, which has a wide-bodied chromophore and terminal n-alkyl groups, adopts a U-shape when spread at the air–water interface but a stretched conformation when compressed to ca. 35 mN m−1. The high-pressure phase has a narrow stability range prior to collapse but may be extended from 40 to 60 mN m−1 by co-spreading the dye in a 1 ∶ 1 ratio with docosanoic acid. The mixed Langmuir–Blodgett (LB) film has a monolayer thickness of 4.6 ± 0.2 nm which decreases to 2.5 ± 0.1 nm layer−1 in the bulk, the reduction arising from an interdigitating layer arrangement, both top and bottom. It is the first example of LB-Lego® and, in addition, represents the only fully interdigitating structure with non-centrosymmetrically aligned chromophores. They are tilted 38° from the substrate normal. The second-harmonic intensity increases quadratically with the number of layers, i.e. as I2ω(N) = I2ω(1)N2, with a second-order susceptibility of χ(2)zzz = 30 pm V−1 at 1064 nm for refractive indices of nω = 1.55 and n2ω = 1.73, d = 2.5 nm layer−1 and ϕ = 38°. Angle resolved X-ray photoelectron spectra (XPS) of these films provide no evidence of the bromide counterion, which suggests that it is replaced by OH− or HCO3−, which occur naturally in the aqueous subphase, or C21H43COO− from the co-deposited fatty acid. This probably applies to all cationic dyes deposited by the LB technique.

Binodal and spinodal curves of an L3 (sponge) phase

In a previous paper (T. D. Le, U. Olsson, H. Wennerström and P. Schurtenberger, Phys. Re. E, 1999, 60, 4300) we presented a quantitative thermodynamic analysis of an L3 (sponge) phase formed in the C12E5–n-decane–water system where the focus was on the narrow stability range of the phase. In this study, we continue to develop and test the free energy model used in that analysis by extending the light scattering measurements towards the two-phase region. This is done by using the Joule heating temperature jump (JHTJ) technique to instantaneously jump (microsecond pulse) from an equilibrium state to a metastable state at a higher temperature. With multiple photosensors placed around the sample, we simultaneously measured the relaxation of the scattered light intensity at different scattering vectors, q. Our best-stretch of this set-up yielded a data set containing the q-dependent scattered intensity and turbidity measurements for a broad range of temperatures and concentrations. The combined phase equilibrium and T-jump data set shows agreement between experimental and theoretical binodal and spinodal curves of this L3 phase.

Redox competition and stability: Rare earth misfit layer compounds as an example

Abstract Recent studies of the electronic structure of the rare earth misfit layer compounds (LnS)1 + xTS2 (Ln = rare earth, T = Ti, V, Cr, Nb, Ta) allow one to present a model which explains their stability. Two redox competitions, one between the [LnS] and the [TS2] slabs, and the second between the T nd and S 3p bands, are found to govern the stabilization of these compounds. This study shows how the modification of the electronic structure of each slab, by moving along the rare earth or the transition elements, defines a narrow stability range for the misfit layer structure.

Determination of the Width of the Single Phase Stability Field of Nickel Titanate Spinel

ABSTRACTThe width of the single-phase stability field of nickel titanate spinel (Ni2(1+x)Ti1−xO4) has been determined in order to resolve an ambiguity in the analysis of the microstructure of equilibrated and quenched spinel-containing specimens. Bulk polycrystalline diffusion couples of the constituent oxides, NiO and NiTiO3, have been prepared at several reaction temperatures between 1400°C and 1550°C. The microstructure of the reaction layers of slow-cooled specimens has been examined by SEM and the composition has been mapped using x-ray microanalysis. It is concluded that the nickel titanate spinel phase has a narrow range of stability, and that the stoichiometry of the spinel is temperature dependent. The microstructure of equilibrated and quenched specimens is discussed in light of these findings.

Crystallization of spodumene-lithium magnesium orthosilicate glasses

The crystallization of some glasses in the LiAlSi 2O 6Li 2MgSiO 4 system with and without nucleation catalysts has been investigated using thermal, X-ray diffraction and microscopic techniques. Fine-grained glass-ceramic materials with good thermal properties were obtained. Different phase assemblages, including γ 0-lithium magnesium orthosilicate, β-eucryptite solid solution, β-spodumene, lithium metasilicate and forsterite, were formed under different conditions of heat treatment. The γ 0-phase showed a limited and narrow stability range and dissociated rapidly around 850°C. The β-eucryptite→ β-spodumene transformation was delayed with increase of the lithium magnesium component in the glass. The presence of TiO 2 enhanced the decomposition of the γ 0-phase, whereas ZrO 2 favoured its formation and stability. The type, stability and compatibility of the crystallized phases are discussed in relation to glass composition and thermal treatment.

Pyroelectric detection of high-order commensurate phases with a narrow range of stability in betaine calcium chloride dihydrate.

Betaine calcium chloride dihydrate exhibits a sequence of structural transitions from a $\mathrm{Pnma}$ phase to different commensurate or incommensurate phases modulated along the c axis. By measurements of the pyroelectric effect we have obtained the first experimental evidence of the existence of three new intermediate high-order commensurate phases, in agreement with recent theoretical predictions. The effect of a dc-bias electric field in the temperature range of stability of these phases is also reported.

Effect of large scale structures on turbulent flame propagation

The effect of laminar burning velocity, Reynolds number, and turbulent fluctuations on the propagation of a turbulent premixed flame stabilized behind a sudden expansion is investigated numerically using a reaction sheet model and line interface calculations of the front motion. Within this framework, flame propagation as an interface between reactants and products is due to the combined action of the advection of products and burning of reactants. Results indicate that turbulent flames have a narrow range of stability when propagating inside channels. At lean conditions, blowoff is caused by the low burning velocity and large turbulent fluctuations. At near-stoichiometric conditions, flashback may occur when the flame blocks the flow and propagates along the wall. Turbulent fluctuations establish an extra mass flux across the flame front that enhances burning at high Reynolds numbers and moderate burning velocities.