Effect of Stoichiometry on Properties of Rare-Earth-Based Hydrogen Storage Alloy for Nickel-Metal Hydride Secondary Battery

Effect of stoichiometry on microstructures, electrochemical properties and PCT characteristics of the alloys MI(Ni0.71Co0.15-Al0.06Mn0.08)x (MI=Lanthanum-rich Michmetal, x=4.6~5.2) have been investigated. The lattice constants a, c, and cellvolumes of non-stoichiometric alloys are bigger than those of the stoichiometric alloy. With the increasing stoichiometry x,the value of a decreases, and the value of c and cell volume increases except for those of the stoichiometric alloy; the plateaupressure of PCT curve, discharge capacity and cycling stability all increase. The alloy with x=5.2 shows the highest dischargecapacity and the best cycling stability among the studied alloys.     

Surface Structures and Their Relative Stabilities of Orthorhombic YAlO3: A First-Principles Study

First-principles energetics calculations were performed to investigate the structures and relative stabilities of six low millerindex surfaces of orthorhombic YAlO₃(YAP).The stoichiometric YAP (100) and (001) were predicted to have the lowest surface energies of 1.91 and 1.96 J/m ²,respectively.Using a thermodynamic defect model,non-stoichiometric YAP surface energies were further predicted as a function of ■ and temperature (T).All the results were combined to construct th...     

Stoichiometry determination of VOx thin films by O-RBS spectrometry

This paper reports on the stoichiometry determination of epitaxially grown vanadium monoxide (VO_x) thin films on MgO(100) substrates. The epitaxial growth was confirmed by RHEED, LEED and XRD techniques. The oxygen content of VO_x thin films, as a function of oxygen flux, was determined using Rutherford backscattering spectrometry. The ¹⁸O isotope was used for film growth, in order to distinguish between the oxygen of film and substrate. The upper and lower stoichiometry limit found are consistent with the ones known for bulk material.     

Hartree–Fock study of near-edge gap states in CaF2 with Na, Cl or Sr impurities

The energy of formation and electronic structure of Na⁺, Cl⁻ and Sr²⁺ impurity centers in CaF₂ have been computed using ab initio Hartree–Fock theory and the supercell approach. The work extends and complements recent results [Solid State Commun. 118 (2001) 569] for Mg²⁺ as a substitutional impurity in CaF₂. For Na⁺ substituting for Ca²⁺ [S(Na)], charge compensation by an F⁻ vacancy [V(F)] or by a second, interstitial Na⁺ [I(Na)] are both considered. In all cases, geometry optimization is done by relaxing the positions of nearest- and next-nearest-neighbors to minimize the total energy. After correction for electron correlation, the energies of formation increase in the order Mg²⁺2+−+ results are in agreement with previous Mott–Littleton formation energies. Ion charges, charge density maps and Mulliken bond populations are obtained to show the nature of bonding in the vicinity of the defect. Na⁺ leads to states just above the CaF₂ valence band maximum (VBM), and Na⁺ (and also Mg²⁺) produce states just below the conduction band minimum. The results are in qualitative agreement with available optical data for Na⁺ and Mg²⁺ impurity effects on CaF₂ near-edge absorption but show that gap states are important in addition to perturbed excitons. A Cl⁻ impurity yields a narrow band of states above the VBM which may significantly affect the deep-ultraviolet transmission of CaF₂. Sr²⁺ does not appear to produce states in the CaF₂ gap.     

Using bioprocess stoichiometry to build a plant-wide mass balance based steady-state WWTP model

Steady-state models are useful for design of wastewater treatment plants (WWTPs) because they allow reactor sizes and interconnecting flows to be simply determined from explicit equations in terms of unit operation performance criteria. Once the overall WWTP scheme is established and the main system defining parameters of the individual unit operations estimated, dynamic models can be applied to the connected unit operations to refine their design and evaluate their performance under dynamic flow and load conditions. To model anaerobic digestion (AD) within plant-wide WWTP models, not only COD and nitrogen (N) but also carbon (C) fluxes entering the AD need to be defined. Current plant-wide models, like benchmark simulation model No 2 (BSM2), impose a C flux at the AD influent. In this paper, the COD and N mass balance steady-state models of activated sludge (AS) organics degradation, nitrification and denitrification (ND) and anaerobic (AD) and aerobic (AerD) digestion of wastewater sludge are extended and linked with bioprocess transformation stoichiometry to form C, H, O, N, chemical oxygen demand (COD) and charge mass balance based models so that also C (and H and O) can be tracked through the whole WWTP. By assigning a stoichiometric composition (x, y, z and a in C_xH_yO_zN_a) to each of the five main influent wastewater organic fractions and ammonia, these, and the products generated from them via the biological processes, are tracked through the WWTP. The model is applied to two theoretical case study WWTPs treating the same raw wastewater (WW) to the same final sludge residual biodegradable COD. It is demonstrated that much useful information can be generated with the relatively simple steady-state models to aid WWTP layout design and track the different products exiting the WWTP via the solid, liquid and gas streams, such as aerobic versus anaerobic digestion of waste activated sludge, N loads in recycle streams, methane production for energy recovery and green house gas (CO₂, CH₄) generation. To reduce trial and error usage of WWTP simulation software, it is recommended that they are extended to include pre-processors based on mass balance steady-state models to assist with WWTP layout design, unit operation selection, reactor sizing, option evaluation and comparison and wastewater characterization before dynamic simulation.     

Effect of C/Ti ratio on densification,microstructure and mechanical properties of TiCx prepared by reactive spark plasma sintering

Titanium carbide (TiC) has been widely used as reinforcement in metal matrix composites and is known to exist over a wide range of stoichiometry. In this study, the effect of C/Ti ratio on the densification kinetics, grain size, lattice parameter, hardness and elastic modulus of TiC_x prepared by reactive spark plasma sintering (RSPS) is presented. Commercial purity titanium was ball milled with 5, 7.5, 10, 12.5, 15 and 17.5 wt% carbon black powder for 5 h and subjected to RSPS to prepare TiC_x samples with different C/Ti ratio. Dense TiC_x samples with ‘x′ ranging from 0.34 to 0.78 could be prepared by RSPS at 1400 °C. Increasing C/Ti ratio was found to increase the activation energy thereby reducing the rate of sintering and also resulted in finer grain size. The lattice parameter and the ratio of intensities of (200) to (111) peaks were correlated with the C/Ti ratio. The hardness and elastic modulus were shown to increase significantly with increase in C/Ti ratio.     

Fabrication and characterization of hot-pressed B-rich boron carbide

Boron carbide has a wide solubility range owing to the substitution of B and C atoms in the crystal. In this study, boron carbides with different stoichiometric ratios were prepared using a hot-pressing sintering method, and the influences of the B/C atomic ratio on the microstructures and properties were explored in detail. X-ray diffraction analysis showed that excessive B atoms caused lattice expansion. Raman spectroscopy analysis showed disordered substitution of B atoms in the chains and icosahedra. Analysis of the densification process and microstructure evolution revealed that the addition of B promoted densification, and more stacking faults and twins occurred in B-rich boron carbide, and result in the densification mechanism gradually changes from atomic diffusion mechanism driven by thermal energy to plastic deformation mechanism dominated by the proliferation of dislocation and substructures. The introduction of chemical composition changes by dissolving excessive B into boron carbide further affected the microstructure and consequently the mechanical properties. The Vickers hardness, modulus, and sound velocity all decreased with the increase in B content. Moreover, the fracture toughness improved with increased B content. The flexural strength of the samples was optimised at the B/C stoichiometric ratio of 6.1.     

Multidimensional Modelling of Polymer Electrolyte Fuel Cells under a Current Density Boundary Condition

A three‐dimensional numerical model of the polymer electrolyte fuel cell (PEFC) is applied to study current distribution and cell performance under a current density boundary condition. Since the electronic resistance in the along‐channel direction in the current collector plate is much larger than in the other two directions, i.e., 50 mΩ cm² vs. 0.5 mΩ cm², it significantly affects current flow, and current and cell voltage distributions in a PEFC. An identical polarization curve results with two different boundary conditions, constant cell voltage and constant current density, however, the current density profiles in the along‐channel direction differ significantly; it is much flatter for the constant current boundary condition. Increasing the electronic conductivity of the bipolar plate diminishes the difference in the current density distribution under the two boundary conditions. The results also point out that an experimental validation of a PEFC model based on the polarization curve alone is insufficient, and that detailed current density distribution data in the along‐channel direction is essential.     

Stoichiometry distribution of thin films deposited by laser ablation: Monte Carlo simulation

An expansion of a bi-component laser plume into a dilute background gas is simulated using a combined Monte Carlo simulation method. The effects of different types of collisions taking place during the transport of the ablated species on their distributions when arriving at a flat substrate are shown. Furthermore, we demonstrate that the film thickness distributions of both components and, hence, the final stoichiometry distribution, depend strongly on the incorporation probability of the species. This probability is expressed as a function of both deposited particle energy and the substrate absorbing properties. These studies can facilitate the comparison of the simulation results with experiments and are of particular interest for pulsed laser deposition (PLD) of multicomponent materials.     

STOICHIOMETRIC RELATIONS OF CHEMICAL OSCILLATION SYSTEMS AND STOICHIOMETRIC NUMBER THEORY

The stoichiometry of chemical oscillation systems, especially that of the Oregonator proposed by Field and Noyes (1974), is discussed with use of the generalized stoichiometric number theory by Horiuti and Nakamura (1957). The independent stoichiometric equations derived by Noyes (1976) are shown to correspond to Horiuti and Nakamura's reaction routes, each of which was expressed in terms of a set of stoichiometric numbers. It is found that the relations derived by Noyes are partly incorrect.