Steady-state Composition Research Articles

Spontaneous near-substrate composition modulation in electrodeposited Fe–Co–Ni alloys

Conventional and reverse depth profile analysis of electrodeposited Fe–Co–Ni alloys was performed by secondary neutral mass spectrometry (SNMS). It was found that the reverse sputtering method gave a much better depth resolution at the vicinity of the substrate. The reverse SNMS spectra showed that the deposition of Fe–Co–Ni alloys starts with the formation of an Fe-rich zone followed by an increase in Co concentration, then the nickel content increases and a steady-state alloy composition is achieved. At high current density, the initial depth pattern reproduces itself twice before the composition becomes stable. It was concluded that the varying depth profile is a consequence of the anomalous nature of the codeposition of the alloy components, the depletion of the electrolyte with respect to the metal salts, and the dependence of the intensity of the hydrogen evolution on the deposit surface composition.

Plasma parameters and etching mechanisms of metals and semiconductors in hydrogen chloride

The parameters and steady state composition of HCl plasma are investigated under dc glow discharge conditions (p = 30–250 Pa and ip = 10–30 mA)). The simulation of plasma has shown that the balance of neutral particles is largely determined by radical-chain processes. It is shown that dissociative attachment of electrons to HClV > 0 does not have a crucial effect on the kinetics of generation and loss of charged particles. The concentrations of neutral and ionic components of the plasma are calculated; the validity of these calculations is confirmed by the satisfactory agreement between experimental and computed values of the reduced field strength and the flux density of positive ions incident onto the surface of a discharge tube. A model analysis is carried out of the effect of external discharge parameters on the kinetics of interaction between the active particles of plasma and the surface.

Specificity of Cytoplasmic Dynein Subunits in Discrete Membrane-trafficking Steps

The cytoplasmic dynein motor complex is known to exist in multiple forms, but few specific functions have been assigned to individual subunits. A key limitation in the analysis of dynein in intact mammalian cells has been the reliance on gross perturbation of dynein function, e.g., inhibitory antibodies, depolymerization of the entire microtubule network, or the use of expression of dominant negative proteins that inhibit dynein indirectly. Here, we have used RNAi and automated image analysis to define roles for dynein subunits in distinct membrane-trafficking processes. Depletion of a specific subset of dynein subunits, notably LIC1 (DYNC1LI1) but not LIC2 (DYNC1LI2), recapitulates a direct block of ER export, revealing that dynein is required to maintain the steady-state composition of the Golgi, through ongoing ER-to-Golgi transport. Suppression of LIC2 but not of LIC1 results in a defect in recycling endosome distribution and cytokinesis. Biochemical analyses also define the role of each subunit in stabilization of the dynein complex; notably, suppression of DHC1 or IC2 results in concomitant loss of Tctex1. Our data demonstrate that LIC1 and LIC2 define distinct dynein complexes that function at the Golgi versus recycling endosomes, respectively, suggesting that functional populations of dynein mediate discrete intracellular trafficking pathways.

Resemblance in gas composition of Ar–N2–O2 plasmas and Ar–NO plasmas

We measured the steady-state gas composition of plasmas produced from Ar–N2–O2 mixtures and Ar–NO mixtures with quantitative mass spectrometry. In the former, mainly N2 and O2, but also a significant amount of nitric oxide (NO) was formed, i.e. up to 5% of the background gas was NO. In the inverse experiment, in which NO was admixed to an argon plasma, up to 92% of the NO was converted into N2 and O2. The observed molecules are mostly generated in wall association processes but also by gas phase reactions between N atoms and O2 molecules leading to NO. The two types of plasmas show a strong mutual resemblance in the steady-state gas composition if substantial dissociation can be reached in the residence time of the gases in the plasma, i.e. ≈5% NO and ≈95% N2 and O2, although the starting conditions are completely different. It seems that in first order the system prefers to produce the most thermodynamically stable molecules.

Roles of BLOC-1 and adaptor protein-3 complexes in cargo sorting to synaptic vesicles.

Neuronal lysosomes and their biogenesis mechanisms are primarily thought to clear metabolites and proteins whose abnormal accumulation leads to neurodegenerative disease pathology. However, it remains unknown whether lysosomal sorting mechanisms regulate the levels of membrane proteins within synaptic vesicles. Using high-resolution deconvolution microscopy, we identified early endosomal compartments where both selected synaptic vesicle and lysosomal membrane proteins coexist with the adaptor protein complex 3 (AP-3) in neuronal cells. From these early endosomes, both synaptic vesicle membrane proteins and characteristic AP-3 lysosomal cargoes can be similarly sorted to brain synaptic vesicles and PC12 synaptic-like microvesicles. Mouse knockouts for two Hermansky-Pudlak complexes involved in lysosomal biogenesis from early endosomes, the ubiquitous isoform of AP-3 (Ap3b1(-/-)) and muted, defective in the biogenesis of lysosome-related organelles complex 1 (BLOC-1), increased the content of characteristic synaptic vesicle proteins and known AP-3 lysosomal proteins in isolated synaptic vesicle fractions. These phenotypes contrast with those of the mouse knockout for the neuronal AP-3 isoform involved in synaptic vesicle biogenesis (Ap3b2(-/-)), in which the content of select proteins was reduced in synaptic vesicles. Our results demonstrate that lysosomal and lysosome-related organelle biogenesis mechanisms regulate steady-state synaptic vesicle protein composition from shared early endosomes.

Modelling phagosomal lipid networks that regulate actin assembly

BackgroundWhen purified phagosomes are incubated in the presence of actin under appropriate conditions, microfilaments start growing from the membrane in a process that is affected by ATP and the lipid composition of the membrane. Isolated phagosomes are metabolically active organelles that contain enzymes and metabolites necessary for lipid interconversion. Hence, addition of ATP, lipids, and actin to the system alter the steady-state composition of the phagosomal membrane at the same time that the actin nucleation is initiated. Our aim was to model all these processes in parallel.ResultsWe compiled detailed experimental data on the effects of different lipids and ATP on actin nucleation and we investigated experimentally lipid interconversion and ATP metabolism in phagosomes by using suitable radioactive compounds.In a first step, a complex lipid network interconnected by chemical reactions catalyzed by known enzymes was modelled in COPASI (Complex Pathway Simulator). However, several lines of experimental evidence indicated that only the phosphatidylinositol branch of the network was active, an observation that dramatically reduced the number of parameters in the model. The results also indicated that a lipid network-independent ATP-consuming activity should be included in the model. When this activity was introduced, the set of differential equations satisfactorily reproduced the experimental data. On the other hand, a molecular mechanism connecting membrane lipids, ATP, and the actin nucleation process is still missing. We therefore adopted a phenomenological (black-box) approach to represent the empirical observations. We proposed that lipids and ATP influence the dynamic interconversion between active and inactive actin nucleation sites. With this simple model, all the experimental data were satisfactorily fitted with a single positive parameter per lipid and ATP.ConclusionBy establishing an active 'dialogue' between an initial complex model and experimental observations, we could narrow the set of differential equations and parameters required to characterize the time-dependent changes of metabolites influencing actin nucleation on phagosomes. For this, the global model was dissected into three sub-models: ATP consumption, lipid interconversion, and nucleation of actin on phagosomal membranes. This scheme allowed us to describe this complex system with a relatively small set of differential equations and kinetic parameters that satisfactorily reproduced the experimental data.

A geophysiologist's thoughts on geoengineering

The Earth is now recognized as a self-regulating system that includes a reactive biosphere; the system maintains a long-term steady-state climate and surface chemical composition favourable for life. We are perturbing the steady state by changing the land surface from mainly forests to farm land and by adding greenhouse gases and aerosol pollutants to the air. We appear to have exceeded the natural capacity to counter our perturbation and consequently the system is changing to a new and as yet unknown but probably adverse state. I suggest here that we regard the Earth as a physiological system and consider amelioration techniques, geoengineering, as comparable to nineteenth century medicine.

Tailoring the composition of self-assembledSi1−xCx quantum dots: simulation of plasma/ion-related controls

Precise control of composition and internal structure is essential for a variety of noveltechnological applications which require highly tailored binary quantum dots (QDs) withpredictable optoelectronic and mechanical properties. The delicate balancing act betweenincoming flux and substrate temperature required for the growth of compositionally graded(Si1−xCx;x varies throughout the internal structure), core–multishell (discreteshells of Si and C or combinations thereof) and selected composition(x set) QDs on low-temperature plasma/ion-flux-exposed Si(100) surfaces is investigated via ahybrid numerical simulation. Incident Si and C ions lead to localized substrateheating and a reduction in surface diffusion activation energy. It is shown thatby incorporating ions in the influx, a steady-state composition is reached morequickly (for selected composition QDs) and the composition gradient of aSi1−xCx QD may be fine tuned; additionally (with other deposition conditions remaining the same),larger QDs are obtained on average. It is suggested that ionizing a portion of the influx isanother way to control the average size of the QDs, and ultimately, their internal structure.Advantages that can be gained by utilizing plasma/ion-related controls to facilitate thegrowth of highly tailored, compositionally controlled quantum dots are discussed as well.

A versatile membrane system for bulk storage and shipping of produce in a modified atmosphere

Modified atmosphere packaging, MAP, employs gas permeable membranes to achieve the specific atmosphere needed to prolong the shelf-life of produce; commercial implementation of this concept is growing for small, disposable retail packages. Membrane technology can also be used to create appropriate atmospheres in reusable large-scale bulk containers for storage and shipping of produce. However, this approach would be more viable if a versatile system were available that could accommodate the requirements of different types of produce without altering the hardware, i.e., one membrane system could be used to create different compositions of oxygen and carbon dioxide, depending on what produce is being shipped or stored at a given time. A scheme is proposed here that uses a selective membrane and a non-selective membrane acting in parallel. The relative amount of gas exchange through the non-selective membrane can be adjusted by varying the volumetric air feed rate to its upstream surface; this will, in turn, adjust the steady-state composition of the gas around the produce. A quantitative model for this scheme and sample calculations are presented to illustrate the concept and how to design such a system where the atmosphere created can be set to the desired range by adjusting the air feed rate.

Yeast Pgc1p (YPL206c) Controls the Amount of Phosphatidylglycerol via a Phospholipase C-type Degradation Mechanism

The product of the open reading frame YPL206c, Pgc1p, of the yeast Saccharomyces cerevisiae displays homology to bacterial and mammalian glycerophosphodiester phosphodiesterases. Deletion of PGC1 causes an accumulation of the anionic phospholipid, phosphatidylglycerol (PG), especially under conditions of inositol limitation. This PG accumulation was not caused by increased production of phosphatidyl-glycerol phosphate or by decreased consumption of PG in the formation of cardiolipin, the end product of the pathway. PG accumulation in the pgc1Delta strain was caused rather by inactivation of the PG degradation pathway. Our data demonstrate an existence of a novel regulatory mechanism in the cardiolipin biosynthetic pathway in which Pgc1p is required for the removal of excess PG via a phospholipase C-type degradation mechanism.

Effect of gas mixing ratio on etch behaviors of Ba 2Ti 9O 20 (BTO) and Pt thin films in Cl 2/Ar inductively coupled plasma

An investigation of the Ba 2Ti 9O 20 (BTO) and Pt thin films etch mechanism in the Cl 2/Ar inductively coupled plasma was carried out. It was found that an increase in Ar mixing ratio at fixed gas pressure and input power causes a fast decrease in the BTO etch rate (26.9–1.2 nm/min for 0–100% Ar) while the Pt etch rate increases slightly from 17.4–23.0 nm/min. Langmuir probe diagnostics and zero-dimensional plasma modeling provided the data on plasma parameters, steady-state composition and fluxes of active species on the etched surface. From the model-based analysis of etch kinetics, it was shown that the behavior of the BTO etch rate corresponds to the reaction-rate-limited etch regime, where the etch rate is limited neither by physical sputtering of the main material nor by the ion-stimulated desorption of low-volatile reaction products. The etch process of Pt appears in the transitional regime and is controlled by the neutral and ion fluxes together.

Influence of the interaction potential and of the temperature on the thermodiffusion (Soret) coefficient in a model system

In this paper the thermodiffusive behavior of an equimolar binary mixture subject to repulsive potentials of the form (sigma/r)(n) is investigated by using nonequilibrium molecular dynamics (NEMD) and the thermodiffusion (Soret) coefficient, S(T), is computed in a wide range of temperatures. With the aim to contribute to the study of the dependence of the Soret coefficient on the interaction potential, the exponent n of the potential is varied from 1 to 12, that is from a pseudocoulombian to a pseudohard-sphere interaction. The steady state equation is integrated for the composition function under reasonable assumptions and it is shown that in some cases the request for it to be linear cannot be satisfied. For this reason nonlinear functions are used to fit the NEMD composition data. The simulations indicate a negligible dependence of S(T) on the composition (in the composition range here considered) while the dependence on the temperature is more marked. The computed values of S(T) as a function of the temperature are fitted with analytical functions. It is found that with n> or =3 (medium and short range interaction) the model system behaves like a dilute gas mixture with the Soret coefficient varying with the temperature almost like 1/T. In the case of n=1 (long range interaction), S(T) has a more complex dependence on T: in particular it shows a change of sign. The analytical fitting functions, S(T)(T), are used in the integrated steady state equation thus obtaining the steady state composition profile and its comparison with the NEMD results indicates the grounding of the approach here proposed.

Lattice Parameter and Sintering Temperature Dependence of Bulk and Grain-Boundary Conduction of Garnet-like Solid Li-Electrolytes

The dependence of the bulk and grain-boundary lithium-ion conduction on the lattice parameter by substitution of trivalent La by divalent Mg, Ca, Sr, , or Ba and monovalent Li in garnet-like , and the effect of sintering temperature were investigated. The ionic bulk conductivity increases with increasing ionic radius of the divalent alkaline earth ion and corresponding increased lattice parameter. An exception is Mg, which is too small for replacing La and forms a second phase. The lattice parameter is also found to increase with increased sintering temperature, except for the mixed substituted sample. The Ca, Sr, , and Ba compounds show mainly bulk resistances with minor boundary contribution at room temperature, which decreases with increasing size of the alkaline earth ion. In contrast, the multiphase Mg-substituted sample exhibits an appreciable grain-boundary contribution to the total resistance. Microstructural investigations indicate the dependence of the grain-boundary resistance on the grain size, sinterability, and formation of transient or steady-state phase boundary compositions, which are caused by different chemical diffusion coefficients of the components. This is related to the higher conductivities of and annealed at compared to samples annealed at .

A comparative study of plasma parameters and gas phase compositions in Cl 2 and HCl direct current glow discharges

The investigations of plasma parameters and active particles kinetics in Cl 2 and HCl direct current glow discharges ( p = 40–300 Pa, j d = 8.5 × 10 − 3 A/cm 2) were carried out. The investigation combined plasma diagnostics by Langmuir probes, absorption spectroscopy and self-consistent 0-dimensional plasma modeling based on the solution of Boltzmann kinetic equation. It was found that the model provides an outstanding agreement with experiment on the reduced electric field strength ( E/N), Cl atom density and the total flux of positive ions and thus, gives an adequate description of basic kinetic effects determining the steady-state plasma parameters and composition. Under the same operating conditions, Cl 2 plasma provides higher E/N, mean electron energy, dissociation degree and electronegativity that can be explained by the peculiarities of formation-decay kinetics analyzed in both systems.

Ignition and Extinction in Catalytic Partial Oxidation of Methane-Oxygen Mixtures with Large H2O and CO2 Dilution

The ignition and extinction in catalytic partial oxidation (CPO) of CH4/O2 mixtures with large exhaust gas dilution (46.3% H2O and 23.1% CO2 vol.) has been investigated experimentally and numerically at 5 bar. Experiments were carried out in a short contact time Rh-coated honeycomb reactor and involved temperature measurements along the reactor and exhaust gas analysis. Numerical predictions were performed with a 2-D transient elliptic code that included detailed chemical reaction schemes and relevant heat transfer mechanisms in the solid. The employed heterogeneous reaction scheme reproduced the measured minimum inlet temperatures required for catalytic ignition (light-off), the elapsed times for the propagation of the reaction front, and the steady-state exhaust gas compositions at a fuel-to-air equivalence ratio of ϕ = 4.0. The chemical impact of the added H2O, although important already at the early light-off stages, was minimal on the ignition delay times because the latter were dominated by total oxidation and not by partial oxidation or reforming reactions. The key reaction controlling catalytic ignition was the surface oxidation of CO to CO2, which was the main exothermic heat release step in the induction zone. Measurements and predictions indicated that vigorous combustion could be sustained at inlet temperatures at least as low as 473 K and 298 K in CPO with and without exhaust gas dilution, respectively. The extended stability limits of CPO combustion were due to a shift from partial to total oxidation products, and hence to higher exothermicity, with decreasing inlet temperature. The key parameter controlling extinction was the CO(s) coverage, which led to catalyst poisoning. Finally, operation at non-optimal stoichiometries (ϕ = 2.5) was shown to be beneficial in CPO of power generation systems with large exhaust dilution, due to the moderating effect of dilution on the maximum reactor temperature.

The fractional ionization in dark molecular clouds

Aims. We have studied the mechanisms which govern the degree of ionization of the gas in molecular clouds and prestellar cores, with a view to interpreting the relative abundances of the carbon-chain species C n H and their negative ions, C n H - . Methods. We followed the chemical evolution of a medium comprising gas and dust as it evolves towards its steady-state composition. Various assumptions were made concerning the grain size-distribution and the fraction of very small grains (in practice, PAH), as well as the cosmic ray ionization rate. Particular attention was paid to reactions which determine the fractional ionization of the gas and the charge of the grains. Results. We found that the abundance ratio n(C 6 H - )/n(C 6 H) is determined essentially by the ratio of the free electron density to the density of atomic hydrogen. A model with a high fractional abundance of PAH and a low fractional abundance of electrons yields agreement to a factor of 2 with the value of the ratio C 6 H - :C 6 H observed recently in TMC-1. However, the fractional abundances of the molecular ions HCO + and DCO + are then higher than observed. The best overall fit with the observations of TMC-1 is obtained when the cosmic ray ionization rate is reduced, together with the rate of removal of atomic hydrogen from the gas phase (owing to adsorption on to grains).

Theoretical investigation of the adsorption of a binary mixture in a chromatographic column using the nonlinear frequency response technique

The nonlinear frequency response of a chromatographic column for the adsorption of two dissolved components is analyzed using the concept of higher order frequency response functions (FRFs) which is based on the Volterra series and generalized Fourier transform. By applying this concept a nonlinear model of a system is replaced by an infinite series of the FRFs of the first, second, etc. order. The FRFs up to the third order are derived theoretically starting from the equilibrium-dispersive model, which is used for description of a chromatographic column, and applying the harmonic probing method. The functions that relate outlet concentration changes of each component to the corresponding inlet concentration changes are derived. At the inlet of a chromatographic column, it is considered: (a) the concentration change of one of the components keeping the concentration of the other component constant and (b) the concentration change of both components keeping their ratio constant. The FRFs are calculated numerically for different steady-state concentrations and relative mixture compositions. It has been found that, despite certain differences in initial conditions, the FRFs exhibit similar behavior. For higher frequencies, the amplitudes of the FRFs tend to zero and phases to −∞. In the low frequency range, which is of interest for investigation of equilibrium parameters, these functions have similar behavior, but tend to different asymptotic values. Correlations between coefficients of competitive adsorption isotherms, i.e. partial isotherm derivatives, and the derived FRFs are established. This theoretical result offers the potential to use the analysis of the nonlinear frequency response of a chromatographic column for estimation of competitive adsorption isotherms.

Computational Modeling of Microstructure Evolution in Solidification of Aluminum Alloys

In this article, a front tracking (FT) model and a modified cellular automaton (MCA) model are presented and their capabilities in modeling the microstructure evolution during solidification of aluminum alloys are demonstrated. The FT model is first validated by comparison with the predictions of the Lipton–Glicksman–Kurz (LGK) model. Calculations of the steady-state dendritic tip growth velocity and equilibrium liquid composition as a function of melt undercooling for an Al-4 wt pct Cu alloy exhibit good agreement between the FT simulations and the LGK predictions. The FT model is also used to simulate the secondary dendrite arm spacing as a function of local solidification time. The simulated results agree well with the experimental data. The MCA model is applied to simulate dendritic and nondendritic microstructure evolution in semisolid processing of an Al-Si alloy. The effect of fluid flow on dendritic growth is also examined. The solute profiles in equiaxed dendritic solidification of a ternary aluminum alloy are simulated as a function of cooling rate and compared with the prediction of the Scheil model. The MCA model is extended to the multiphase system for the simulation of eutectic solidification. A particular emphasis is made on the quantitative aspects of simulations.

Numerical and experimental observations of spherical diffusion flames

Spherical diffusion flames supported on a porous sphere were studied numerically and experimentally. Experiments were performed in 2.2 s and 5.2 s microgravity facilities. Numerical results were obtained from a Chemkin-based programme. The programme simulates flow from a porous sphere into a quiescent environment, yields both steady state and transient results and accounts for optically thick gas-phase radiation. The low flow velocities and long residence times in these diffusion flames lead to enhanced radiative and diffusive effects. Despite similar adiabatic flame temperatures, the measured and predicted temperatures varied by as much as 700 K. The temperature reduction correlates with flame size but characteristic flow times and Lewis number also influence temperature. The numerical results show that the ambient gas Lewis number would have a strong effect on flame temperature if the flames were steady and nonradiating. For example, a 10% decrease in Lewis number would increase the steady state flame temperature by 200 K. However, for these transient, radiating flames the effect of Lewis number is small. It was also observed that when hydrocarbon fuel is supplied from the ambient the large diffusion distances associated with these flames can lead to unusual steady state compositions near the outer boundary because decomposition products can diffuse to the outer boundary. This results in a loss of chemical enthalpy from the system but the effect on flame temperature is small. Transient predictions of flame sizes are larger than those observed in microgravity experiments. Close agreement could not be obtained without either increasing the model's thermal and mass diffusion properties by 30% or reducing mass flowrate by 25%.

Stability of ZnO{0 0 0 1} against low energy ion bombardment

The steady-state surface compositions of the polar (O and Zn terminated) faces of ZnO{0 0 0 1} produced by low energy (0.3–2 keV) Ar + ion bombardment were studied by Auger electron spectroscopy and electron energy loss spectroscopy. The alterations produced by the ion bombardment using different ion energies were monitored by calculating the intensity ratios of the low and high energy Zn Auger peaks (59 eV and 994 eV, respectively); Zn and O Auger peaks (59 eV and 510 eV, respectively). Based on the dependence of these ratios on the ion energy and termination of the surface, we could conclude that the stability of the Zn face is higher against the low energy argon ion bombardment-induced compositional changes than that of the O face.