Experimental Concentration Profiles Research Articles

Oxidation and interdiffusion behavior of a germanium-modified silicide coating on an Nb–Si-based alloy

To investigate the interdiffusion behavior of Ge-modified silicide coatings on an Nb–Si-based alloy substrate, the coating was oxidized at 1250°C for 5, 10, 20, 50, or 100 h. The interfacial diffusion between the (Nb,X)(Si,Ge)2 (X = Ti, Cr, Hf) coating and the Nb–Si based alloy was also examined. The transitional layer is composed of (Ti,Nb)5(Si,Ge)4 and a small amount of (Nb,X)5(Si,Ge)3. With increasing oxidation time, the thickness of the transitional layer increases because of the diffusion of Si from the outer layer to the substrate, which obeys a parabolic rate law. The parabolic growth rate constant of the transitional layer under oxidation conditions is 2.018 μm·h−1/2. Moreover, the interdiffusion coefficients of Si in the transitional layer were determined from the interdiffusion fluxes calculated directly from experimental concentration profiles.

Study of chloride penetration in concretes exposed to high-mountain weather conditions with presence of deicing salts

The durability of reinforced concrete (RC) is a challenge. One of the most aggressive and frequent ions is chloride, which frequently affects RC structures by steel corrosion. Many studies have concluded that the use of mineral admixtures improves concrete durability. This paper examines the transport process of chloride ions in non-saturated concrete. Three types of concrete have been designed and tested: the first one with only CEM I 52,2 R (CP), the second with 10% of silica fume (CPHS) and the third with 20% of blast furnace slag (CPEAH). An experimental programme was designed with the aim of studying the influence of external condition variation in a high-mountain environment with presence of deicing salts. The total time duration of the programme was 210days. The specimens were conditioned at different temperatures, relative humidity and chloride surface concentrations in five phases to reproduce the seasons annually. The experimental chloride concentration profiles were obtained at the end of each phase for each concrete. These profiles were mainly influenced by two transport mechanisms: diffusion and capillary suction. The chemical chloride binding and physical adsorption of chlorides in the concretes with silica fume (CPHS) and blast steel furnace (CPEAH) were two decisive factors in the chloride penetration processes. A numerical model for simulating chloride ion transport in non-saturated concrete is presented. It considers the diffusive and advective flux. In addition, the concrete pore network and the associate properties, such as chloride binding capacity and chloride precipitation, are also taken into account. The model has been implemented in a finite element programme. The chloride concentration profiles obtained by the model at the end of each phase for each concrete properly fit the experimentally obtained.

Effect of particle concentration on sediment and turbulent diffusion coefficients in open-channel turbulent flow

To achieve a complete knowledge about the effect of particle concentration on sediment and turbulent diffusion coefficients in open-channel turbulent flow is a long-standing problem for the community of researchers. The effect of particle concentration is investigated on the sediment and turbulent diffusion coefficients through the inverse of turbulent Schmidt number or β which is defined by the ratio of sediment diffusion coefficient to turbulent diffusion coefficient. It is observed that with increasing particle concentration, the sediment diffusion coefficient decreases more in comparison with the turbulent diffusion coefficient for both dilute and non-dilute sediment-laden flows. The physical characteristics of β observed are expressed mathematically in terms of normalized settling velocity, reference level and reference concentration. The applicability of the mathematical formulae is confirmed by the agreement analysis between experimental data and particle concentration profile computed from the Rouse equations modified through the newly proposed expression of β. Apart from the better agreement between dilute particle concentration data and the developed Rouse equation, the striking observation is that the modified Rouse equation shows reasonable computational accuracy for non-dilute particle concentration data also. Minimum error is obtained from the present model when it is compared with the models proposed by the previous researchers.

Pulmonary pharmacokinetics of levofloxacin in rats after aerosolization of immediate-release chitosan or sustained-release PLGA microspheres

A comparative pharmacokinetic study was conducted in rats after intratracheal aerosolization of levofloxacin, as a solution, as immediate-release chitosan microspheres or as sustained-release PLGA microspheres. A pharmacokinetic model was constructed to model levofloxacin concentrations both in plasma and in the lung epithelial lining fluid (ELF). The plasma and ELF experimental concentration profiles versus time were similar for the intravenous and intratracheal levofloxacin solutions and for the intratracheal levofloxacin-loaded chitosan microsphere dry powder, indicating that levofloxacin diffused almost instantaneously through the broncho-alveolar barrier and that the chitosan microspheres released levofloxacin very rapidly, as anticipated from in vitro release studies. The bioavailability for the intratracheal levofloxacin solution and intratracheal chitosan microspheres was estimated to be 98% and 71%, respectively, both with a direct release into the ELF compartment. The ELF-to-unbound plasma AUC ratios were slightly above 2 and may result from an efflux transport. For the intratracheal PLGA microspheres, a high ELF-to-unbound plasma AUC concentration ratio (311) was observed and high levofloxacin concentrations were maintained in ELF for at least 72h in consistency with the in vitro release studies. The bioavailability was 92%, with 19% of the dose released immediately (burst release) into the ELF and 73% released slowly into the ELF from a depot compartment, i.e. the PLGA microspheres, according to a Weibull model. These results highlight the benefit of using sustained-release microspheres administered as aerosols to provide and to maintain high pulmonary concentrations of an antibiotic characterized with a high permeability profile through the broncho-alveolar barrier. The sustained-release microsphere dry powder aerosol may therefore provide advantages over solutions or pure drug dry powders for inhalation in terms of treatment efficiency, ease of use and frequency of administration.

Development of multi-component diesel surrogate fuel models – Part I: Validation of reduced mechanisms of diesel fuel constituents in 0-D kinetic simulations

In the present work, development and validation of reduced chemical kinetic mechanisms for several different hydrocarbons are performed. These hydrocarbons are potential representative for practical diesel fuel constituents. n-Hexadecane (HXN), 2,2,4,4,6,8,8-heptamethylnonane (HMN), cyclohexane (CHX) and toluene are selected to represent straight-alkane, branched-alkane, cyclo-alkane and aromatic compounds in the diesel fuel. A five-stage chemical kinetic mechanism reduction scheme formulated in the previous work is applied to develop the reduced HMN and CHX models based on their respective detailed mechanisms. Alongside with the development of the reduced CHX model, a skeletal toluene sub-mechanism is constructed since the elementary reactions for toluene are subset of the detailed CHX mechanism. The final reduced HMN mechanism comprises 89 species with 319 elementary reactions, while the final reduced CHX mechanism which includes the toluene sub-mechanism consists of 80 species with 287 elementary reactions. Both reduced models are approximately 92% smaller than their respective detailed models in terms of total number of species and elementary reactions. Following that, both the newly developed fuel constituent reduced mechanisms, together with the formerly derived reduced HXN mechanism are comprehensively validated in zero-dimensional chemical kinetic simulations under a wide range of shock tube and jet-stirred reactor (JSR) conditions. Well agreement between the reduced and detailed mechanisms is achieved for ignition delay (ID) and species concentration predictions under both auto-ignition and JSR conditions, with a maximum relative error of 40%. In addition, the reduced models are further validated against the JSR experimental results for each diesel fuel constituents. The surrogate models are able to reasonably reproduce the experimental species concentration profiles in view of their simplified fuel chemistries. Deviations within one order of magnitude in the absolute values are recorded between the computations and measurements. Validation of these reduced models for each diesel fuel constituents in this work serves as a prerequisite for constructing a multi-component diesel surrogate fuel model. The compact yet accurate chemical models proposed here aid to reduce the chemistry size of the final multi-component diesel surrogate model such that it remains computationally efficient when it is incorporated with multi-dimensional CFD simulations. The reduced mechanism of each fuel constituent can also be used individually for other CFD applications.

Validity of the second Fick's law for modeling ion-exchange diffusion in non-crystalline viscoelastic media (glasses)

It is shown that, in general case, the diffusion equation (or the second Fick's law) does not provide an adequate description of ion-exchange transport phenomena in viscoelastic media, including glassy or any other non-crystalline media. In this connection the general phenomenological model of ion-exchange diffusion in viscoelastic media has been developed. A theoretical analysis of the model shows that, in the case of a linear dependence of medium density on the concentration of diffusing ions, the necessary and sufficient condition of the absolute validity of the diffusion equation in viscoelastic media is Φ ≫ 1, where Φ = τD/τR is the dimensionless value (or criterion of similarity), with τD = L2/D being the characteristic time of diffusion and τR = η/G being the characteristic time of stress relaxation, where L, D, η, and G are the characteristic length of diffusion, the diffusivity, the viscosity, and the shear modulus, respectively. The value of 1/Φ characterizes the accuracy which is provided if the second Fick's law is used in the simulation of ion-exchange diffusion in viscoelastic media. We have demonstrated the applicability of this criterion experimentally. Our experimental studies on ion-exchange diffusion in an oxide glass (typical viscoelastic media) have shown that under the condition the Φ > 105 the experimental concentration profiles are close to those predicted by the second Fick's law to within an accuracy of 1%.

Modeling solute segregation during the solidification of γ-phase U-Mo alloys

Using first principles calculations, it is demonstrated that solute segregation during U-Mo solidification can be modeled using the classic Brody-Fleming limited diffusion framework. The necessary supporting equations specific to the U-Mo alloy, along with careful verification of the assumptions underpinning the Brody-Fleming model are developed, allowing for concentration profile predictions as a function of alloy composition and cooling rate. The resulting model is compared to experimental solute concentration profiles, showing excellent agreement. Combined with complementary modeling of dendritic feature sizes, the solute segregation model can be used to predict the complete microstructural state of individual U-Mo volume elements based upon cooling rates, informing ideal processing routes.

Investigation of Ion-Implanted Photosensitive Silicon Structures by Electrochemical Capacitance–Voltage Profiling

The method of electrochemical capacitance–voltage profiling is used to study boron-implanted silicon structures for CCD matrices with backside illumination. A series of specially prepared structures with different energies and doses of ion implantation and also with various materials used for the coating layers (aluminum, silicon oxide, and their combinations) is studied. The profiles of the depth distribution of majority charge carriers of the studied structures are obtained experimentally. Also, using the Poisson equation and the Fredholm equation of the first kind, the distributions of the charge-carrier concentration and of the electric field in the structures are calculated. On the basis of the analysis and comparison of theoretical and experimental concentration profiles, recommendations concerning optimization of the structures’ parameters in order to increase the value of the pulling field and decrease the effect of the surface potential on the transport of charge carriers are suggested.

Evaluating the Performance of the Mixture Model Coupled with High and Low Reynolds Turbulence Closures in the Numerical Description of Concentrated Solid-Liquid Flows of Settling Particles

In this manuscript, Computational Fluid Dynamics (CFD) studies applying the Mixture Model coupled with both a High Reynolds and a Low Reynolds k-ε turbulence closures, were used to describe experimental pressure gradients and particle concentration profiles from the literature, for concentrated solid-liquid flows of settling particles in a horizontal pipe above the critical deposition velocity. With this work a new Mixture Model formulation is presented, incorporating the Jones-Launder Low Reynolds k-ε turbulence closure, to overcome the excessive turbulence production observed in the numerical studies using the High Reynolds k-ε turbulence model. Additionally, this formulation provides a more accurate representation of the wall turbulence damping phenomena, observed in the experimental testing for the flow of higher particle concentrations suspensions and higher flow velocities. Knowledge on the conditions that cause turbulence attenuation on solid-liquid flows is still scarce in the literature and of considerable value for industrial applications.

B11-P-04Limit of Detection for Dopant in Si Using Large Solid Angle Silicon Drift Detector

Sensitivity of compositional analysis with energy dispersive X-ray spectrometer (EDS) has been improved, since a large solid angle Si drift detector (SDD) and a system with multiple detectors were realized [ 1 , 2 ]. Up to now, transmission electron microscopy (TEM)-EDS has been mostly untouched in the analysis of semiconductor dopants, because of their low concentration. There has been a strong desire to see “limit of detection (LOD)”, using a multiple SDD analysis system. In this study, we explore a concentration at LOD for a dopant in Si device using an FETEM (JEM-2800, JEOL) with two SDDs, whose sensing area is 100 mm 2 each. We examined an As-doped Si wafer as a sample. The concentration profile along z direction has been verified by secondary ion mass spectrometry (SIMS). We made a cross sectional sample for TEM observation. We define an dopant concentration ( C dopant ) at LOD as !!...!! where the I net is a net count of Si-K, the P/B is a ratio of a net count of Si (P) to the background count at an energy of the dopant element line (B), and k is the K factor for the line of a dopant[ 3 ]. The C dopant are calculated to be 30 ppm for As-K and 70 ppm for As-L using the eq. ( 1 ). The experimental concentration profiles were extracted using a quantitative analysis system (Thermo Fisher). The expected LODs by eq. ( 1 ) agree well with the experimental results.

Effect of TaC and NbC addition on the microstructure and hardness in graded cemented carbides: Simulations and experiments

By using the gradient sintering process, a tough surface zone in the cemented carbides is usually created to prevent crack propagation from the hard-coating. The microstructure and hardness distribution in the gradient zone are crucial to crack propagation resistance. In the present work, the influence of TaC and NbC contents on the microstructure and hardness distribution in WC–Ti(C,N)–Co-based graded cemented carbides was chosen as the target and systematically studied via a hybrid approach of the simulations and key experiments. Under the guide of the thermodynamic calculation, the cemented carbides with different TaC and NbC addition have been designed and sintered at 1450°C for 2h in an N free atmosphere. The microstructure and concentration distributions were investigated using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Based on the thermodynamic and diffusion databases, the gradient zone formation of the cemented carbides was simulated by DICTRA software. The simulation can reasonably reproduce most of the experimental concentration profiles. Microindentation was used to measure hardness as a function of depth below the surface. On the basis of the simulations and experiments, key microstructural parameters in the hardness model were obtained to predict the hardness distribution in the gradient layer. The relationship among the composition, concentration and hardness distribution can be developed via the present approach.

Axial Mixing and Mass Transfer Performance of an Annular Pulsed Disc-and-Doughnut Column

The annular pulsed disc-and-doughnut column (APDDC) is an important type of extraction equipment in the PUREX process, which has been used in several commercial reprocessing plants. As there are few mass transfer experimental results reported in the literature, the axial mixing and mass transfer performance of an APDDC was studied for both extraction and stripping processes in the present work. Two parameters in the axial dispersion model (ADM), namely, the axial dispersion coefficient of the continuous phase and the number of mass transfer units, were regressed by correlating ADM with experimental concentration profiles. The influence of flow rate and pulsation intensity on these parameters was also investigated. Models developed for the PDDC were tested for correlation with APDDC experimental data and suitable models and conditions were determined. The height of a mass transfer unit was also calculated, which highlights the impact of axial mixing on mass transfer performance. Moreover, the influence of internal wettability on mass transfer performance was discussed.

Pattern-fluid interpretation of chemical turbulence.

The spontaneous formation of heterogeneous patterns is a hallmark of many nonlinear systems, from biological tissue to evolutionary population dynamics. The standard model for pattern formation in general, and for Turing patterns in chemical reaction-diffusion systems in particular, are deterministic nonlinear partial differential equations where an unstable homogeneous solution gives way to a stable heterogeneous pattern. However, these models fail to fully explain the experimental observation of turbulent patterns with spatio-temporal disorder in chemical systems. Here we introduce a pattern-fluid model as a general concept where turbulence is interpreted as a weakly interacting ensemble obtained by random superposition of stationary solutions to the underlying reaction-diffusion system. The transition from turbulent to stationary patterns is then interpreted as a condensation phenomenon, where the nonlinearity forces one single mode to dominate the ensemble. This model leads to better reproduction of the experimental concentration profiles for the "stationary phases" and reproduces the turbulent chemical patterns observed by Q. Ouyang and H. L. Swinney [Chaos 1, 411 (1991)].

Modeling pH-zone refining countercurrent chromatography: A dynamic approach

A model based on mass transfer resistances and acid–base equilibriums at the liquid–liquid interface was developed for the pH-zone refining mode when it is used in countercurrent chromatography (CCC). The binary separation of catharanthine and vindoline, two alkaloids used as starting material for the semi-synthesis of chemotherapy drugs, was chosen for the model validation. Toluene/CH3CN/water (4/1/5, v/v/v) was selected as biphasic solvent system. First, hydrodynamics and mass transfer were studied by using chemical tracers. Trypan blue only present in the aqueous phase allowed the determination of the parameters τextra and Pe for hydrodynamic characterization whereas acetone, which partitioned between the two phases, allowed the determination of the transfer parameter k0a. It was shown that mass transfer was improved by increasing both flow rate and rotational speed, which is consistent with the observed mobile phase dispersion. Then, the different transfer parameters of the model (i.e. the local transfer coefficient for the different species involved in the process) were determined by fitting experimental concentration profiles. The model accurately predicted both equilibrium and dynamics factors (i.e. local mass transfer coefficients and acid–base equilibrium constant) variation with the CCC operating conditions (cell number, flow rate, rotational speed and thus stationary phase retention). The initial hypotheses (the acid–base reactions occurs instantaneously at the interface and the process is mainly governed by mass transfer) are thus validated. Finally, the model was used as a tool for catharanthine and vindoline separation prediction in the whole experimental domain that corresponded to a flow rate between 20 and 60mL/min and rotational speeds from 900 and 2100 rotation per minutes.

A contribution to the Ni-based mobility database: Fcc Ni–Fe–Ti ternary alloy

In order to establish a general Ni-based mobility database, diffusion couples for Ni–Fe–Ti ternary systems are prepared, all of which are annealed at 1573K for 50h. Interdiffusion coefficients are retrieved from the common compositions of two independent diffusion paths, which are then combined with thermodynamic description to inversely explore atomic mobilities of Ni, Fe and Ti around the Ni-rich region. Good agreements are obtained from comprehensive comparisons between calculated and experimental diffusion coefficients and concentration profiles, which can be used to verify the quality of atomic mobilities obtained in this work.

Observation and modeling of α-NiPtAl and Kirkendall void formations during interdiffusion of a Pt coating with a γ-(Ni-13Al) alloy at high temperature

During the last 15years, Pt-rich γ–γ′ bond-coatings have been studied extensively for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings in thermal barrier coating systems. To optimize their fabrication and durability, it is essential to investigate their interdiffusion with Ni-based superalloys. This study reports on experimental results and modeling of the interdiffusion of the model Pt/γ-(Ni-13Al) alloy system. Pt coatings were deposited either by electroplating or by spark plasma sintering using a Pt foil. Heat treatments at 1100°C for 15min to 10h were performed either in a high-temperature X-ray diffraction device under primary vacuum or in a furnace under argon secondary vacuum. The α-NiPtAl phase with L10 crystal structure formed very rapidly, implying fast uphill Al diffusion toward the surface. For Pt electroplating, α-phase transformed to γ′-(Ni,Pt)3Al after only 45min–1h at 1100°C. The resulting two-phased γ–γ′ microstructure remained up to 10h. When using a Pt foil coating, the continuous layer of α-NiPtAl phase disappeared after 10h and the γ′-(Ni,Pt)3Al or γ-(Ni,Pt,Al) phase appeared, resulting in two different diffusion paths in the Ni–Pt–Al phase diagram. Voids also formed at the interdiffusion zone/substrate interface for both systems after 1h or more. Composition analyses confirmed that voids were located at the Pt diffusion front corresponding to the Al-depleted zone. Experiments performed with the samples coated with a Pt foil confirmed that voids are due to a Kirkendall effect and not to the Pt deposition process. Numerical simulations including the cross-term diffusion coefficients in the diffusion flux equations reproduced the experimental concentration profiles for the γ-phased systems.

Swelling Kinetics of HPMC Tablets

Hydroxypropyl methylcellulose (HPMC) is a cellulosic polymer widely employed in tablets formulation. In contact with biological fluids, it undergoes a glassy–rubbery transition and drug release is strongly influenced by swelling. We study the kinetic parameters of a classical phenomenological model (Astarita and Sarti), which describes the velocity of a glassy–rubbery interface as a function of the local solvent concentration. One-dimensional mass transport equations with moving boundaries are numerically solved by finite elements method in order to fit Astarita–Sarti parameters k and n on experimental swelling fronts and concentration profiles taken from literature for HPMC K4M, HPMC K15M, and HPMC K100M.

Mathematical modeling and simulation of hexane degradation in fungal and bacterial biofilters: effective diffusivity and partition aspects

Mathematical modeling in the biofiltration of volatile organic compounds is a valuable tool for performance prediction and in scaling up. Majority of the published models include parameters obtained from fitting experimental data, thus masking their real influence as they are lumped generally. The present work aims to evaluate experimentally some of the most relevant parameters including kinetic constant, partition coefficient in the biofilm, biofilm thickness, superficial area, and effective diffusivity. For the fungal biofilm, all the parameters mentioned above were obtained experimentally; and for the bacterial biofilm, the biofilm thickness and some intrinsic parameters used to obtain the first-order kinetic constant were taken from the literature. These parameters were then incorporated in a mathematical model to describe the steady-state degradation of hexane in bacterial and fungal biofilters operating under continuous mode. Experimentally, the dimensionless partition coefficients (mG) indicated that hexane was 4 and 35 times more soluble in the bacterial (mG = 9·14) and fungal (mG = 0·88) biofilters, respectively, than in water (mG = 30·4). Comparison of model estimates with experimental concentration profiles of the pollutant along the height of the biofilters proves that the first-order limited by reaction model was appropriate to interpret the experimental results with a small error of ∼1%.

A Kinetic Investigation of the Initialization of Catalyzed Chain Growth of Styrene: The Reaction of Cp*2ZrCl2 with Dibenzylmagnesium

The system Cp*2ZrCl2 and dibenzyl magnesium in toluene, which is a model system for initialization in the catalyzed chain growth (CCG) of styrene using a bifunctional organometallic chain transfer agent, is studied via time‐resolved NMR spectroscopy. Based on these data, a mechanism of initialization is proposed. The experimental concentration profiles of the individual species can successfully be modeled to the proposed kinetic scheme. Refined kinetic coefficients of high significance can be obtained by applying analytical methods using pseudo‐first‐order rate approximations. It is found that the adduct of the Zr catalyst and the Mg compound only slowly transforms via a ligand exchange reaction, yielding the alkylated Zr species. The temperature dependence of these reactions indicates a high reaction barrier of this type of rearrangement, which constitutes the core reaction of CCG in terms of molecular‐weight control. image

Dissolution of dehydroxylated lizardite at flue gas conditions: II. Kinetic modeling

A non-steady state kinetic model was developed that describes the dissolution kinetics of partially dehydroxylated lizardite at far-from-equilibrium operating conditions, at temperatures of 30–120°C and CO2 partial pressures of 0.1–2.0bar. Based on two simplified dehydroxylation pathways that represent the extreme cases, namely of a fully homogeneous dehydroxylation and a heterogeneous dehydroxylation, two different particle structures for the dehydroxylated material were considered. The model uses surface complexation mechanisms to describe the specific dissolution rates. Both particle structures were able to reproduce equally well the experimental concentration profiles which were the result of multiple dissolving species in the particles. The model estimates the kinetic parameters for these species from the large set of dissolution data reported in the first part of this publication series. The model was able to describe the evolution of the non-stoichiometric aqueous concentration profiles of magnesium and silica accurately, taking into account the dynamic evolution of the reactor pH at non-steady state dissolution conditions. The kinetic parameters could be used to compare the dissolution performances of differently activated minerals in their application as potential feed materials for an ex situ mineral carbonation process.