Site Competition Effects Research Articles

Ab initio informed solute drag assessment for ferritic steels

Linking atomistic information on solute interactions with microstructure evolution is a key challenge for predictive modelling of chemistry effects on material properties. The objective of the present work is to provide a link between grain boundary segregation energies with GB migration kinetics on the example of recrystallization in multi-component ferritic steels. For that purpose, the segregation of 64 elements from the periodic table to a representative grain boundary is computed with ab initio density functional theory. To connect this data to grain boundary migration kinetics, the solute trend parameter from a simplified solute drag treatment is employed. The solute trend parameter for individual solutes is presented, which highlights solutes with large impact on grain boundary migration. Furthermore, an extension of the solute trend parameter is introduced that allows to evaluate the solute drag potential of realistic steel compositions. The necessity to include solute co-segregation, site competition, and precipitation effects is shown in a comparison to experimental data on recrystallization kinetics. The comparison to experimental data demonstrates the qualitative predictability of recrystallization kinetics by the extended solute trend parameter.

Insight into the effect of phosphate on ferrihydrite colloid-mediated transport of tetracycline in saturated porous media.

Colloid-mediated contaminant mobility is absolutely critical for the environmental behavior of contaminants such as antibiotics in water resources. In this study, the influences of phosphate (a commonly inorganic ligand in the environment) on the ferrihydrite colloid-mediated transport of tetracycline (TC, a typical antibiotic) in porous media were investigated. In the absence of colloids, phosphate promoted TC mobility due to the competitive deposition of phosphate and TC on the sand surface as well as the electrostatic repulsion. Interestingly, ferrihydrite colloids could inhibit TC transport; however, the inhibitory effect of the colloids was weakened by the addition of phosphate. This phenomenon stemmed from colloid-associated TC mobility, the increased electrostatic repulsion induced by adsorbed phosphate, and deposition site competition effect. Another interesting finding was that the impacts of phosphate on the colloid-mediated mobility of TC were pH-dependent. That is, phosphate exhibited a weaker effect on the inhibitory role of ferrihydrite colloids in TC mobility at pH 5.0 than that at pH 7.0; specially, ferrihydrite colloids acted as possible carriers of TC and facilitated antibiotic transport at pH 9.0. The observations were ascribed to different influences of phosphate on the binding affinity of ferrihydrite toward TC and the mobility of free TC under different pH conditions. Therefore, the findings of this study provide useful information about the fate and co-transport of antibiotics and natural mineral colloids in the presence of inorganic ligands in the aquatic environment.

The mechanisms involved into the inhibitory effects of ionic liquids chemistry on adsorption performance of ciprofloxacin onto inorganic minerals

The ionic liquids (ILs) can influence the adsorption of antibiotics on minerals but there was ambiguity about mechanisms. Herein, we demonstrated the adsorption characteristics of ciprofloxacin (CIP) onto ferrihydrite and montmorillonite by various soluble ILs. Five imidazolium-based ILs with different alkyl chain lengths ([C2mim]Cl, [C4mim]Cl, [C6mim]Cl) and counteranions ([C4mim][TOS] and [C4mim][PF6]) were employed. The results validated that ILs could suppress CIP adsorption on both minerals due to the adsorption site competition between cations and CIP molecules, steric hindrance induced by long alkyl chains of ILs, and electrostatic interaction. Interestingly, the adsorption-inhibition effects of ILs were almost unchanged with the lengthening alkyl chain for ferrihydrite, which were ascribed to the comparable adsorption capacities of the three ILs on ferrihydrite. Whereas, the effects of ILs on CIP adsorption onto montmorillonite followed the order of [C6mim]Cl > [C4mim]Cl > [C2mim]Cl owing to the adsorption site competition and steric hindrance effects on the ILs increased with the lengthening alkyl chain. Furthermore, the inhibitory effects of ILs on CIP adsorption onto both studied minerals were also dependent on the counteranions (kept the same cation ([C4mim]+)) and in the following order of [C4mim][PF6] > [C4mim][TOS] > [C4mim]Cl, which was mainly ascribed to the differences of anions in the ion sizes and the binding affinities to minerals. Additionally, the extent of the inhibitory impact of [C4mim][TOS] on CIP adsorption onto ferrihydrite declined with increasing solution pH from 5.0 to 9.0, which was attributed to the amounts of adsorbed [TOS]– declined with increasing pH. Contrarily, for montmorillonite, the extent increased with increasing pH, owing to increase in adsorption site competition and steric effect at higher pH conditions. Our findings emphasize the significance of emerging contaminants in understanding the antibiotic environmental behavior in eco-environmental systems.

Thermodynamic trapping and diffusion model for multiple species in systems with multiple sorts of traps

This work presents a trapping and diffusion model for multiple species of solute atoms in a system with multiple sorts of traps based on irreversible thermodynamics. We use two established approaches describing the interaction between lattice and traps, i.e. transient trapping and theory based local thermodynamic equilibrium, and extend them to systems with multiple species. Consequently, the presented theory and its numerical implementations cover effectively any kinetics of exchange between the lattice and traps as well as site competition effects within traps for any system size. The presented theory can be applied for several phenomena in material science. For example, (co-) segregation of solutes at grain boundaries, trapping of interstitials during precipitation processes and hydrogen interaction with material defects and other interstitials. Simulations of charging and discharging are presented for several plate-like samples containing multiple sorts of traps occupied by multiple species. The role of trapping parameters is demonstrated and discussed for charging and discharging behavior, site competition effects and the interaction of trapping kinetics with diffusion kinetics for multiple species.

Grain boundary segregation and toughness of friction-stir-welded high-phosphorus weathering steel

Friction stir welding (FSW) of a high-P (0.3 wt%) weathering steel was performed at temperatures above A3 and below A1. Three-dimensional atom probe tomography analysis revealed that the grain boundaries in the stir zone formed below A1 (which had a considerably finer grain size) had higher P and lower C concentrations than those of the grain boundaries in the stir zone formed above A3. The effect of grain size on non-equilibrium grain boundary segregation during cooling after FSW was analyzed using the diffusion equation, assuming grain boundaries acted as sink sites for solutes. The fast diffusion of C is postulated to result in an almost equilibrium segregated state, such that its segregation decreases with decreasing grain size. In contrast, P diffuses only an extremely small distance, presumably leading to the grain-size-independent segregation of P. Therefore, the experimental result that the grain boundary concentration of P in the stir zone formed below A1 was larger than that in the stir zone formed above A3 is assumed to stem from the site competition effect between P and C and/or the enhanced P diffusion to grain boundaries due to dynamic recrystallization. Moreover, the segregation heat treatment was performed after the FSW to investigate the effect of the grain boundary segregation of P on the toughness. An investigation into the correlation between grain boundary segregation and toughness revealed that when the grain boundary concentration is less than approximately 7 at%, the effect of P segregation on intergranular fracture is relatively small and the toughness can be effectively improved by grain refinement.

Mutually facilitated co-transport of two different viruses through reactive porous media

In this study, we quantified transport and retention behaviors of a human pathogenic virus (Adenovirus 41, abbreviated as Ad-41) and a model bacteriophage (ϕX174) through metal oxide-removed and goethite-coated sand under saturated flow conditions. Two sets of bench scale column experiments were conducted: transport of each type of viruses alone and co-transport of both viruses. All experiments were conducted at pH 7.5 using 2 mM artificial ground water (AGW) buffer as background solution. Experimental results revealed that goethite-coated sand (i.e., reactive sand) retained much more viruses relative to metal oxide-removed sand (i.e., non-reactive sand), with the difference more pronounced for ϕX174 (effluent concentration decreased by 92.1%) than for Ad-41 (effluent concentration decreased by 59.7%). Interestingly, lower retention of both viruses on goethite-coated sand was observed when they co-existed in the influent. The mutual promotion to the transport of the two viruses may be attributed to attachment site competition and steric hindrance effect (illustrated in the graphical abstract). Mass recovery results revealed that ϕX174 was largely reversibly attached, whereas Ad-41 was mostly irreversibly bound or inactivated. Force measurements using atomic force microscopy (AFM) demonstrated that ϕX174 had doubled affinity to goethite-coated sand compared with Ad-41, consistent with the transport behavior of each type of viruses when they existed alone. As expected, the extended DLVO theory cannot accurately estimate interfacial energy profiles for viruses and reactive sand that have heterogeneous surface properties (i.e., roughness and charge heterogeneity). The results of this study clearly demonstrate that caution must be taken when applying laboratory results, which are generally obtained from experiments employing a single virus species, to predict the mobility and environmental risks in natural systems where multiple agents are present.

Influence of Growth Temperature on Site Competition Effects during Chemical Vapor Deposition of 4H-SiC Layers

After presenting an exhaustive experimental study of aluminum incorporation in epitaxial 4H-SiC and 3CSiC films grown by chemical vapor deposition (CVD), we focalize once more on what is called site competition effects. We observed that the influence of C/Si ratio on dopant (Al, N) incorporation in SiC was qualitatively different depending on whether the growth experiments were performed in “low temperature” (LT) or “high temperature” (HT) regime. Partial explanation of observed phenomena basing on thermal evolution of carbon coverage of SiC surface is proposed.

Influence of Site Competition Effects on Dopant Incorporation during Chemical Vapor Deposition of 4H-SiC Epitaxial Layers

An exhaustive experimental study of the influence of C/Si ratio on voluntary incorporation of nitrogen (N) and aluminum (Al) in 4H-SiC thin films is presented. The films were grown by chemical vapor deposition (CVD) in a horizontal, hot wall CVD reactor on Si- and C-face substrates, under Si-rich and C-rich conditions. Under some conditions the observed variation of dopant incorporation with C/Si ratio could be clearly attributed to the site competition effects, while in several cases other mechanisms have to be taken into account.

Effect of growth gas flow rate on the SiC crystal resistivity

Abstract

A simple and colorimetric fluoride receptor and its fluoride-responsive organogel

In this paper, a new p-nitrophenylhydrozine-based anion receptor 1 containing cholesterol group had been designed and synthesized. It could selectively recognize fluoride among different anions tested with color changes from pale yellow to red for visual detection. Simultaneously, it could gel in cyclohexane, and the gel was also fluoride-responsive. When treated with TBAF (tetra-n-butylammonium fluoride), the gel could undergo gel–sol transition accompanied by color, morphology and surface changes. The binding mechanism had been investigated by UV–vis and 1HNMR (proton nuclear magnetic resonance spectra) titrations. From SEM (scanning electron microscope), SAXS (small-angle X-ray scattering), IR (Infrared Spectroscopy) and CA (contact angle) experiments, it was indicated that the addition of F− could destroy the molecule assembly of host 1 in the gel state, thus resulting in the gel-to-sol transition due to the binding site competition effect. To the best of our knowledge, this was the simplest fluoride-responsive organogel with high selectivity.

Etch rates near hot-wall CVD growth temperature for Si-face 4H-SiC using H 2 and C 3H 8

During the growth of silicon carbide (SiC) by hot-wall chemical vapor deposition, there is a competition between etching and growth processes due to hydrogen etching of the SiC surface, which is a function of temperature and the etching atmosphere. The interplay of the etch rates and growth rates and the influence of the doping level of the growing layer on these rates must be taken into account when growing device structures with stringent thickness constraints. In this work, a comparison of secondary ion mass spectrometry data with etch rates for a variety of individual epilayers is used to differentiate the contribution of etching processes from that of site competition effects between precursor and incorporated dopant species on the net growth rate. It is found that the presence of nitrogen at the growing surface decreases the net growth rate by competing with carbon species for incorporation into surface lattice sites at a level far beyond what would be expected for most dopant atoms. This is a result of the high nitrogen flux required at the surface due to the inefficiency of nitrogen incorporation at growth temperatures. Furthermore, etch rates of SiC epilayers were found to increase with increasing carrier concentrations for both p- and n-type 4H-SiC. Finally, small additions of propane to the hydrogen atmosphere suppresses the etch rate of SiC; however, too much propane leads to degradation of the surface morphology. The activation energy for the hydrogen–propane etching mechanism is determined to be 120.5 kcal mol −1, in good agreement with values reported in the literature.

Characterising forest spatial structure through inhomogeneous second order characteristics

Point process theory plays a fundamental role in the analysis and modelling of spatial forest patterns. For instance, the Ripley’s K function and its density with respect to the area, i.e. the pair correlation function, have been extensively used to analyse and characterise stationary forest configurations. However, the stationarity condition is not often met in practice when analysing real data. Thus, the development and application of new statistics to measure the degree of inhomogeneity suggests the use of inhomogeneous statistics to describe forest stands. In this paper, we restrict our attention to the inhomogeneous pair correlation function in the context of replicated spatial data. We then analyse the spatial configuration of pure and mixed conifer stands in a case study in Central Catalonia, North-East of Spain. Our results suggest that whilst P. sylvestris tend to be aggregated for short inter-tree distances, P. nigra and P. halepensis keep a minimum inter-event distance between trees. Regarding the mixed stands, trees of distinct species tend to be segregated from each other. Tentative explanations for these results are related with site properties, competition effects, shade tolerance and silviculture practices applied in this forest region.

Defoliation and competition effects in a productivity gradient for a semiarid Mediterranean annual grassland community

Abstract Grazing and competition are two main factors shaping range plant communities; however, few studies have investigated their interaction. The current study aimed to investigate the effects of defoliation, competition and their interaction on production of annual grasses in semiarid Mediterranean areas. Competition treatments (absence/presence of neighbors) were combined with three defoliation intensities (0%, 30% and 60%) in a complete factorial design. Competition significantly reduced grass biomass. However, the role of competition was eliminated under heavy defoliation or under dry growth conditions. Defoliation showed variable results on final biomass (FB) and cumulative biomass (CB). While heavy defoliation (60% clipping intensity) reduced grass FB down to 80% during the two growing seasons, light defoliation (30%) significantly increased CB. Results showed that competition may limit the direct effect of defoliation on dominant grass species. Further, the relationship between site productivity and competition effect was best explained by a negative linear model. This hypothesized model may suggest that facilitation and competition alternatively affect grassland communities along a productivity gradient. The results suggest that light grazing may sustain or even enhance grassland productivity. The results also indicated the suitability of annual grass species to re-vegetate degraded rangeland in semi-arid climate. Further, optimum grazing practices to conserve biodiversity of Avena grassland may involve moderate stocking rate.

Growth and characterization of nitrogen-doped C-face 4H-SiC epilayers

In this paper, nitrogen-doped 4H-Silicon carbide (SiC) epilayers are grown on 4° off-axis 〈 0 0 0 1 ¯ 〉 C-face 4H-SiC substrates by horizontal hot-wall chemical vapor deposition. Surface morphology of the epilayers shows a strong dependence on N 2 flow, C/Si ratio and growth temperature. Surface defect densities are higher for samples grown under high C/Si ratio and high N 2 flow condition. Lightly doped epilayers with low defect densities have been achieved by growing under optimized conditions. Macrostep bunching is observed on epilayers grown under high N 2 flow condition. The degree of step bunching manifests a dependence on N 2 flow. Possible impurity-induced step-bunching mechanism is discussed to explain the results. Lightly doped C-face 4H-SiC epilayers exhibit a site-competition effect, while heavily doped epilayers do not. Schottky barrier diodes fabricated on C-face 4H-SiC epilayers are shown to be comparable to or outperform those on Si-face epilayers.

Growth of SiC Boules with Low Boron Concentration

The effects of growth conditions, diffusion barrier coatings, and hot zone materials on B incorporation in 6H-SiC crystals grown by physical vapor transport (PVT) were evaluated. Development of high purity source material with a B concentration less than 1.8x1015 atoms/cm3, was critical to the growth of boules with a B concentration less than 3.0x1016 atoms/cm3. Application of refractory metal carbide coatings to commercial graphite to serve as boron diffusion barriers and the use of very high purity pyrolytic graphite components ultimately led to the growth of SiC boules with boron concentrations as low as 2.4x1015 atoms/cm3. The effect of growth temperature and pressure were closely examined over a range from 2100°C to 2300°C and 5 to 13.5 Torr. This range of growth conditions and growth rates had no effect on B incorporation. Attempts to alter the gas phase stoichiometry through addition of hydrogen gas to the growth environment also had no impact on B incorporation. These results are explained by considering site competition effects and the ability of B to diffuse through the graphite growth cell components.

Electrical Properties of Undoped 6H- and 4H-SiC Bulk Crystals Grown by Halide Chemical Vapor Deposition

Undoped 6H- and 4H-SiC crystals were grown by Halide Chemical Vapor Deposition (HCVD). Concentrations of impurities were measured by various methods including secondary-ion-mass spectrometry (SIMS). With increasing C/Si ratio, nitrogen concentration decreased and boron concentration increased as expected for the site-competition effect. Hall-effect measurements on 6H-SiC crystals showed that with the increase of C/Si ratio from 0.06 to 0.7, the Fermi level was shifted from Ec-0.14 eV (nitrogen donors) to Ev+0.6 eV (B-related deep centers). Crystals grown with C/Si > 0.36 showed high resistivities between 1053 and 1010 4cm at room temperature. The high resistivities are attributed to close values of the nitrogen and boron concentrations and compensation by deep defects present in low densities.

Growth and Doping Modeling of SiC‐CVD in a Horizontal Hot‐Wall Reactor

AbstractModeling and simulation of the SiC epitaxial growth, and doping in a horizontal hot‐wall reactor from common precursors (SiH4; C3H8 diluted in H2 for growth; N2 and Al(CH3)3 for n‐type and p‐type doping) are presented. The growth and doping features of SiC thin layers on both Si‐terminated and C‐terminated surfaces are analyzed as a function of various inlet source gas conditions, i.e., various C/Si ratios. The role of the actual surface mass fluxes of both Si‐containing and C‐containing species and their ratio is analyzed and compared to the inlet experimental parameters. It is demonstrated that the doping level resulting from lattice site competition effects can be quantified by the actual C/Si ratio calculated above the growing surface. Moreover, the surface morphology of the epitaxial layer is explained on the basis of the mass fluxes at the growing surface.

Residual impurities and native defects in 6H-SiC bulk crystals grown by halide chemical-vapor deposition

A variety of defect-sensitive techniques have been employed to detect, identify, and quantify the residual impurities and native defects in high-purity (undoped) 6H-SiC crystals grown by halide chemical-vapor deposition technique. The incorporation efficiencies of N and B are determined by the site-competition effect. Most notably, material with low residual N levels (∼1014cm−3) can be produced. In addition, the nitrogen concentrations obtained from Hall-effect measurements and low-temperature photoluminescence are systematically lower than those determined from secondary-ion-mass spectrometry. The difference is ascribed to nitrogen forming complexes with native defects. The energy level of this complex is approximately 0.27eV below the conduction band. Four major electron traps with activation energies of 0.4, 0.5, 0.65, and 1eV and five hole traps with activation energies of 0.3, 0.4, 0.55, 0.65, and 0.85eV were observed by deep-level transient spectroscopy. The concentration of all traps decreased strongly with increasing C∕Si ratio during growth. Increasing the C∕Si flow ratio also led to increased resistivity of the crystals and change of conductivity from conductive n type to high-resistivity p type. The Fermi level in p-type material is pinned either to highly compensated shallow B acceptors or to deep B-related center at 0.6eV above the valence band. Electron paramagnetic resonance shows the presence of positively charged carbon vacancies in such high-resistivity material.

Competitive Effects of Natural Organic Matter: Parametrization and Verification of the Three-Component Adsorption Model COMPSORB

Natural organic matter (NOM) hinders adsorption of trace organic compounds on powdered activated carbon (PAC) via two dominant mechanisms: direct site competition and pore blockage. COMPSORB, a three-component model that incorporates these two competitive mechanisms, was developed in a previous study to describe the removal of trace contaminants in continuous-flow hybrid PAC adsorption/membrane filtration systems. Synthetic solutions containing two model compounds as surrogates for NOM were used in the original study to elucidate competitive effects and to verify the model. In the present study, a quantitative method to characterize the components of NOM that are responsible for competitive adsorption effects in natural water was developed to extend the application of COMPSORB to natural water systems. Using batch adsorption data, NOM was differentiated into two fictive fractions, representing the strongly competing and pore blocking components, and each was treated as a single compound. The equilibrium and kinetic parameters for these fictive compounds were calculated using simplified adsorption models. This parametrization procedure was carried out on two different natural waters, and the model was verified with experimental data obtained for atrazine removal from natural water in a PAC/membrane system. The model predicted the system performance reasonably well and highlighted the importance of considering both direct site competition and pore blockage effects of NOM in modeling these systems.

Comparison of the electrical activation of P+ and N+ ions co-implanted along with Si+ or C+ ions into 4H-SiC

The annealing behavior of P+ and N+ ions implanted into p-type 4H-SiC epilayers is studied by a temperature-dependent Hall-effect. Detailed investigations reveal that the electrical activation of implanted P+ ions is governed by the site competition effect, while implanted N+ ions react with intrinsic defects during the annealing step and can form thermally stable and electrically inactive complexes. In this way, the electrical activation of implanted N+ ions is strongly reduced.