Wall Reaction Research Articles

Sticking Coefficient-Orchestrated Selection in a Kinetic Model for the First Origin

The mathematical formalism of the steady-state Poisson equation is applied to a variant of Freeman Dyson's “Toy Model” for a first origin. Our kinetic approach allows for an examination of the requisite conditions under which metabolism is quantized into discrete eigenstates (e.g. Dyson's disordered, saddle point, and metabolically active toy cell states). The surface reaction machinery additionally allows for more realistic modeling, whence the crucial role of sticking coefficients (catalyst precursors) as prebiotic selectors emerges. In our interior source model, a steady influx of vent nutrients fuels the intracellular synthesis of (impermeable) monomers within a rock-encradled cavity. Random adsorptions and desorptions occur at inactive “cell” wall sites (where the inert monomers remain impermeable until their eventual return to the intracellular metabolite pool). Occasionally, metabolizing reactions also occur due to endogeneous source monomers adsorbing at their “active” sites. Dyson's mean field approach is used to simplify the species-specific sticking coefficients at empty active (substrate) sites to functions of the fraction x of sites occupied by (catalytically) active monomers. In short, our work suggests that disorder–order transition models based on random drift between discrete metabolic eigenstates (Dyson's Toy Model) do not, in general, extend to more realistic metabolisms. From a perspective based on quasi-random feedback kinetics, the contraindication for discretization (spontaneous generation) in non-autocatalytic metabolisms is consistent with the emergence of ordered metabolism under hydrothermal driving forces, a provisio the occurrence of each period of vent dormancy coincides with a discrete zero-source (dormant) metabolic state. Cell drift to higher order is induced by the random reactions which happen to enhance the substrate specificity (chemical selectivity) of the sticking coefficients for active monomers. The result is stronger sink effects for metabolizing species, whence active adsorptions are promoted in favor of inactive adsorptions at substrate sites. Positive feedback plays a crucial role in preserving (“propagating”) order in the cell wall reaction kinetics and is held in check by negative feedback inhibition of excessive cell growth. Finally, the eventual desorption of randomly growing dysfunctional proteins is postulated as a deterrent to deterioration catastrophes.

Biocompatibility and Performance of the Wallstent and the Wallgraft, Jostent, and Hemobahn Stent-Grafts in a Sheep Model

Three recently developed stent-grafts and the Wallstent were compared directly in an ovine animal model with regard to performance and biocompatibility. Three stent-grafts, the Hemobahn (polytetrafluoroethylene [ePTFE]/nitinol), Wallgraft (polyester/Ni-Co-Ti-steel alloy), and Jostent peripheral stent-graft (balloon-expandable ePTFE/stainless steel), and the Wallstent (Ni-Co-Ti-steel alloy), were implanted in sheep iliac arteries (one type of each stent or stent-graft per animal, n = 8). Pre- and postimplantation luminal diameters were measured for each prosthesis and implantation site. Angiography, intravascular ultrasonography (IVUS), and histomorphometric, histologic, and scanning electron microscopic analyses were performed at 3 months. Early lumen gain, late lumen loss, and patent vessel diameter at angiography were not significantly different. Two stent-grafts had significantly more neointima formation (Hemobahn, 9.88 mm(2) +/- 0.94; Wallgraft, 14.98 mm(2) +/- 0.90) than the other stent-graft (Jostent, 6.52 mm(2) +/- 0.46) and the Wallstent (5.24 mm(2) +/- 0.62; P <.01). Patent lumen area was not significantly different (Hemobahn, 42.57 mm(2) +/- 1.41; Jostent, 39.76 mm(2) +/- 2.04; Wallgraft, 40.22 mm(2) +/- 1.04; Wallstent, 41.64 mm(2) +/- 1.59; P =.57). The Hemobahn had significantly more inflammatory reaction (inflammation score of 0.83 +/- 0.03) than the Jostent (0.58 +/- 0.03), Wallgraft (0.55 +/- 0.04), or Wallstent (0.16 +/- 0.01). Angiography and IVUS demonstrated absence of anastomotic neointima formation. Endothelialization was incomplete and immature for all prostheses. The stent-grafts caused a greater degree of neointima formation and inflammatory vessel wall reaction than the bare stent. However, these changes did not interfere with patent lumen areas and occurred in the absence of excessive anastomotic neointima formation.

Comparison of Small Intestinal Submucosa–covered and Noncovered Nitinol Stents in Sheep Iliac Arteries: A Pilot Study

To compare the biocompatibility and performance of nitinol endografts covered externally or internally with small intestinal submucosa (SIS) with bare nitinol stents in medium-sized arteries. Eighteen nitinol Zilver stents were used: six externally SIS-covered endografts (ECEs), six internally SIS-covered endografts (ICEs), and six bare stents (BSs). Devices were implanted in the balloon-injured external iliac arteries (EIAs) of nine female sheep via carotid approach. Arteriograms were obtained before and after implantation and before animal sacrifice at 1, 3, and 6 months. Histologic studies of explanted specimens were performed. Implantation of all BSs, ECEs, and ICEs was successful, but slight luminal narrowing of 19% +/- 5.3% (range, 12%-28%) was seen in ICEs on postimplantation angiograms. At sacrifice, all six BSs and ECEs were patent, with BSs showing a mean angiographic luminal narrowing of 8.4% +/- 7.2% (range, 0%-18%) and ECEs showing a mean angiographic luminal narrowing of 16% +/- 7.5% (range, 6.5%-26%) as a result of neointimal hyperplasia. Four ICEs showed luminal narrowing of 21% +/- 17% (range, 0%-35%) as a result of marked neointimal hyperplasia and two were occluded, one at 3 months and the other at 6 months. ECEs and BSs showed approximately 80% lumen endothelialization at 1 month, which increased to 100% at 3 and 6 months. ICEs did not show complete endothelialization. The BS had the least vessel wall reaction. ECEs exhibited early endothelialization with early mild to moderate wall reaction decreasing at the late study stages. ICEs showed extensive wall reaction, possibly as a result of technical problems with SIS attachment.

Shape normalization and analysis of the mass transfer controlled regime in catalytic monoliths

We present a shape normalization for solving the convection–diffusion equation for the case of laminar flow in a duct of uniform cross-section but of arbitrary shape and with a wall catalyzed reaction. We show that when the flow is hydrodynamically developed and the wall reaction is infinitely fast, the reactant mixing-cup exit conversion ( χ m ) depends mainly on the transverse Peclet number, P (=R Ω 2〈u〉/LD m , R Ω=A Ω/P Ω ; where 〈 u〉 and D m are the average velocity, and molecular diffusivity of the reactant species; and A Ω, P Ω , and L are the channel cross-sectional area, perimeter, and length, respectively) and is a very weak function of the axial Peclet number, Pe (=〈u〉L/D m ) for P≫1. We also show that the curve χ m versus P is universal (for all common channel geometric shapes such as circular, square, triangular, etc.) and is described by the two asymptotes χ m =1 for P≪1 and χ m ≈ P −2/3 for P≫1 with a transition around a P value of unity. For the case of developing flow with a finite Schmidt number ( Sc= ν/ D m ), we show that χ m =1 for P≪1 and χ m ≈ Sc −1/6 P −1/2 for P≫1 with a transition around a P value of unity. We give formulas for estimating the conversion in any arbitrary channel geometry for finite values of P and show that the commonly used two-phase models with a constant Sherwood number can be in considerable error (≈20–30%) even for the case of long channels ( P≪1). We also extend the shape normalization to the local Sherwood number ( Sh) for fully developed as well as simultaneously developing flow and compare the analytical results with numerical computations and literature correlations. The asymptotic results and formulas presented here are useful for determining an upper bound on conversion and a lower bound on the Sherwood number for a given set of flow conditions and physical dimensions of the monolith. Finally, we present simple criteria for optimal design of catalytic monoliths and packed-beds operating in the mass transfer controlled regime.

Mechanistic modeling of the wall reactions in the pyrolysis of pentachloroethane

AbstractThe thermal dehydrochlorination C2HCl5 → C2Cl4 + HCl has been studied in a static system between 565 and 645 K at pressures ranging from 5 to 21 torr. The course of the reaction was followed by measuring the pressure rise in the conditioned quartz reaction vessel and by analyzing the products by gas chromatography. The observed experimental results and data from the literature for flow systems can be explained quantitatively in terms of a radical reaction model involving heterogeneous chain initiation and termination steps. The rate constants have been deduced for reactions of Cl, Cl2, and C2HCl5 over reactor walls covered with a pyrolytic carbon film and for reactions of adsorbed Cl atoms. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 322–330, 2002

Multistage reaction textures in xenolithic high‐MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration‐driven metasomatism

AbstractHigh‐MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh‐temperature (UHT) metamorphism and near‐isothermal decompression, the rocks were intruded in a lit‐par‐lit manner by felsic melts (charnockite), which caused local‐scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions.The sequence of multiphase reaction textures in the high‐MgAl xenoliths reflects the changes of physico‐chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, &gt; 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)1 + garnet (grt)1 + sapphirine (spr)1 + spinel (spl)1 + rutile (rt). A subsequent phase of near‐isothermal decompression in the order of 1–2 kbar (stage 2) resulted in extensive replacement of grt1 and opx1 megacrysts by lamellar (opx2 + spr2) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg3 + opx2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx2 + spr2) symplectites by biotite and sillimanite and/or plg3, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain‐size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4–7 kbar) as indicated by local growth of late garnet (grt5) at the expense of (opx + spr + plg) domains at static conditions.Recently published isotope data suggest that the multistage evolution of the high‐MgAl granulites at Anakapalle followed a discontinuous P–T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep‐crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid‐crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high–T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.

Platelet deposition in stagnation point flow: an analytical and computational simulation

A mathematical and numerical model is developed for the adhesion of platelets in stagnation point flow. The model provides for a correct representation of the axi-symmetric flow and explicitly uses shear rate to characterise not only the convective transport but also the simple surface reaction mechanism used to model platelet adhesion at the wall surface. Excellent agreement exists between the analytical solution and that obtained by the numerical integration of the full Navier–Stokes equations and decoupled conservation of species equations. It has been shown that for a constant wall reaction rate modelling platelet adhesion the maximum platelet flux occurs at the stagnation point streamline. This is in direct contrast to that found in experiment where the maximum platelet deposition occurs at some distance downstream of the stagnation point. However, if the wall reaction rate is chosen to be dependent on the wall shear stress then the analysis shows that the maximum platelet flux occurs downstream of the stagnation point, providing a more realistic model of experimental evidence. The analytical formulation is applicable to a large number of two-dimensional and axi-symmetrical surface reaction flows where the wall shear stress is known a priori.

3D Transport in Acid-Fracturing Treatments: Theoretical Development and Consequences for Hydrocarbon Production

SummaryWhile the effect of 2D proppant transport during hydraulic fracturing has been studied extensively and debated frequently, relatively little attention has been given to acid-fracturing treatments. In addition to fluid-density differences and gravity-driven segregation, spatial variation of the acid temperature and of the rock chemical properties results in more complicated physical phenomena for acid fracturing. Acid diffusion in the fracture has been estimated empirically with no considerations made to model correctly the acid flow across the fracture width. The etched-width profile resulting from 3D flow could differ significantly from that predicted by the 1D, piston-like displacement normally included in acid-fracture simulators. An important consequence of this variation in the etched pattern is a substantially different prediction of the fracture conductivity, and hence the post-stimulation hydrocarbon production. Acid flow in the fracture can affect the outcome of reservoir stimulation and must be considered when designing acid-fracturing treatments.The equations governing fluid flow are developed initially for a 2D pressure profile. These equations are coupled with the formulations for acid wall reaction, heat transfer, and diffusion within the framework of a P3D hydraulic fracturing simulator. The theoretical formulation for acid transport across the fracture width is presented then and coupled with the equations for 2D acid flow. The equation of acid diffusion is solved across the fracture width, hence solving the equations of fluid flow for three dimensions in the fracture. The resulting fracture geometry and conductivity are used subsequently with a reservoir simulator to illustrate the consequences of the 3D acid formulation on hydro-carbon production. Simulated and field examples are presented to illustrate the effects of 3D acid flow on the etched-width distribution and post-stimulation production.

THE LAMINAR FLOW TUBULAR REACTOR WITH HOMOGENEOUS AND HETEROGENEOUS REACTIONS II. Application to Reactor Modeling Involving Free Radical Reactions

The simulation of the complex polychlorination of methane in a non-isothermal photoreactor with laminar flow regime shows the usefulness of the integral equations deduced in Part I of this contribution. Improved iterative schemes for numerical solution have been implemented by using the particular equations proposed for species having slow, very fast and instantaneous reaction regimes. From such integral equations, in a mathematically consistent form, deviations in the fields of predicted concentrations with and without the application of the local steady-state approximation for homogeneous reactions involving atomic and free radical species have been obtained. Similarly the fields of predicted concentrations applying the usual assumption of negligible wall reactions for the same highly reactive intermediate species has been compared with the reaction mechanisms that includes the wall reactions. A comprehensive analysis of the validity of these simplifying assumptions is presented.

Biocompatibility and Performance of the Wallstent and Several Covered Stents in a Sheep Iliac Artery Model

To compare the biocompatibility and performance of various stent-grafts to those of a bare stent in an ovine model with a subchronic (3 months) endpoint. Three different types of stent-grafts (ePTFE/nitinol, n = 8; polyester/nitinol, n = 8; and polycarbonate urethane/cobalt-alloy, n = 8) and a bare stent as a control (Ni-Co-Ti-steel-alloy, n = 8) were implanted in the iliac arteries in eight female sheep. One type of each stent-graft was implanted per animal, two implants at each side. The implantation sites for each type varied from animal to animal. Angiographic control and intravascular ultrasound (IVUS) imaging were performed before and after implantation, after 2 months, and before explantation at 3 months and were used to characterize patency and to assess intimal hyperplasia. After 3 months, the implants were retrieved and subjected to histologic evaluation (after methacrylate embedding, cutting, and histologic staining) to characterize the biologic response. Implantation was technically successful in all procedures. At 2 and 3 months after implantation, all segments in which stents had been implanted were patent. Marked neointima formation was found in the polyester-covered stent-graft that showed significant luminal narrowing of 50%, compared to the ePTFE-covered (24%) and polycarbonate urethane-covered endoprostheses (22%), as well as the bare stent (Wallstent; 9%; P < .001). A minimal inflammatory vessel wall reaction was demonstrated for the polyester-covered and ePTFE-covered endoprostheses; the polycarbonate urethane-covered stent-graft's response was demonstrable but not significantly different from that of the Wallstent. At 3 months, the ePTFE-covered stent-graft showed incomplete (>90%) endothelial coverage; in the other endoprostheses, complete but partially immature endothelialization was found. All stent-grafts induced an inflammatory vessel wall reaction with neointimal hyperplasia. The polyester-covered endoprosthesis caused a marked reaction with 50% luminal stenosis. Endothelialization was retarded with the ePTFE-covered stent-graft. The bare stent performed best in regard to neointimal formation and caused the least inflammatory response.

Intermuscular coordination during pendulum rebound exercises

In this study, we assessed coordination during pendulum rebound jumps. To gain insight into the movement coordination strategy, nine experienced male volleyball players performed maximal rebound jumps in a pendulum swing device using three different seat arrangements (90±x00B0;, 135±x00B0; and 180±x00B0;). Two-dimensional filming was performed in the right sagittal plane (200Hz) synchronized with a force platform fitted to the wall (1000Hz). The surface electromyograms of five muscles were recorded (200Hz), in conjunction with kinematic and kinetic assessment. During the countermovement phase, the impact forces were attenuated by eccentric contractions of most muscles, which helped to reduce the energy input into the system. The wall reaction forces, net moments and joint power profiles were comparable between conditions. The small differences found between the extreme seat arrangements were attributed to differences in muscle length and the position of the feet. The strategy used during landing was similar to that observed in unconstrained vertical and drop jumps, where the neuromuscular system attenuates the impact forces. During the push-offphase, most muscles were found to contribute to positive work generation, except the semitendinosus, which was stretching throughout the propulsive phase. Despite not being able to exert a large influence over the trunk segment, this muscle was deemed to play an important role in regulating and synchronizing the onset of knee extension, enabling hip extension to occur before extension in the other more distal joints. Our findings show that the neuromuscular system is able to produce consistent movement coordination across experimental conditions and in accordance with the specific task demands and constraints imposed in the movement structure.

The Design and Use of a Photochemical Flow Reactor: A Laboratory Study of the Atmospheric Chemistry of Cyanoacetylene on Titan

The laboratory investigation of the atmospheric photochemistry of planets and satellites is mainly carried out in static systems. These studies are often poor models of chemical processes in atmospheres because: (1) much higher mixing ratios of minor constituents must be used to accurately determine the amount of reactant consumed and to obtain sufficient products for analysis, (2) secondary photolysis of the initial photoproducts often occurs, (3) wall reactions occur, and (4) most of the starting material is converted to products to obtain enough for spectroscopic analysis. The use of a photochemical flow reactor either circumvents or minimizes these problems by using gas mixtures and photolysis conditions more representative of a planetary atmosphere. A gas mixture, composed of a small amount of a reactant gas diluted in a much larger amount of carrier gas, is flowed past a UV lamp for an extended period of time. Unconsumed reactants and products are collected in traps downstream until amounts sufficient for spectral analysis are collected. FTIR and NMR analysis provides structural information and quantitative data on their rates of formation. The feasibility of this approach for the investigation of planetary atmospheres has been demonstrated by the photolysis of mixing ratios of 10 −3–10 −6 of cyanoacetylene, (2-propynenitrile, HC 3N) in nitrogen gas. Hydrogen cyanide (HCN), acetonitrile (CH 3CN), acrylonitrile (CH 2CHCN), and a polymer have been identified as reaction products. The quantum yields for reactant loss and product formation have been determined. Aspects of polymer structure have been determined by FTIR. Its empirical formula has been determined on the basis of the reaction products produced, and its morphology has been examined by scanning electron microscopy. It is concluded from the high quantum yields for HCN and CH 3CN formation that the C/N ratio of the polymer is high. This was confirmed by infrared analysis of the polymer where it was observed that the intensity of the C≡N stretching frequency decreases as the HC 3N mixing ratio is lowered to a mixing ratio closer to that of Titan.

Peculiarities of somatovisceral reactions in early postnata ontogenesis

To reveal possible ways of the somatovisceral interaction, in the 0–5-day, 11–13-day and one-month-old rats, reactions of the spontaneous contractile activity of the gastric antrum were studied to a rhythmic stimulation of the tibial nerve for 30 s with frequency 30 imp/s and strength equal to 3–4 thresholds of the gastrocnemial muscle motor response. Only the stimulations of the central, but not of peripheral, nerve segments were effective, which indicates the reflex nature of the reactions. It has been established that in the process of development, a shift occurs of the predominantly inhibitory reactions to predominantly activating the gastric wall reactions. Similar data were obtained by skin pain stimulation in the region of the tail base. Thus, the age-dependent change of direction of the observed somatoautonomous reactions can be produced by stimulation of different high-threshold somatic afferents. The efferent pathway both of inhibitory and of stimulatory reactions is mediated mainly, but not exclusively, by the gastric branches of the vagus nerve, which is confirmed by consequences of vagotomy and of section of the splanchnic nerve. As a whole, the results of the study can suggest that one of possible pathways of effects of the skeletal muscle contraction on rhythmic activity of the gastrointenstinal tract can be activation of muscle receptors.

EFFECT OF INTERPHASE MASS TRANSFER ON UNSTEADY CONVECTIVE DIFFUSION:PART I, PLANE-POISEUELLE FLOW OF A POWER-LAW FLUID IN A CHANNEL

The paper presents an all-time analysis of unsteady convective diffusion of solute undergoing an irreversible first-order chemical reaction at the bounding walls of a parallel-plate channel. The heterogeneous chemical reaction significantly influences the convective and diffusive coefficients. A new coefficient arises exclusively due to the incorporation of wall reaction. The effects of wall-catalysed reaction and power-law index ‘N’ on dispersion are investigated against the background of the no-reaction problem. The analytical result on dispersion of solute with wall catalysed reaction at long times is compared with the analytical solution when.reaction is absent. The Taylor [1 ] and Ans [2] regimes of dispersion for the present problem are obtained as limiting cases from the study. The graphical results of the study serve as a jury on any numerical study that might be undertaken considering non-asymptotic all-time analysis.

Increased hyaluronan and hyaluronidase production and hyaluronan degradation in injured aorta of insulin-resistant rats.

Diabetic patients have a greater incidence of restenosis, which has been shown to be related to exaggerated intimal hyperplasia. Hyaluronan (HA) has been shown to be closely involved in arterial smooth muscle cell proliferation and migration, which provoke intimal hyperplasia after balloon catheter injury. Our aim was to determine the effect of fructose feeding, which produces certain characteristics of non-insulin-dependent diabetes (ie, insulin resistance, hyperinsulinemia, and hypertriglyceridemia), on production of HA and hyaluronidase and degradation of HA in rat aorta. Treated rats received fructose (25% in tap water) 12 weeks before balloon catheter injury and 14 days afterward. Fructose-fed rats had hyperinsulinemia and hypertriglyceridemia. Injury increased intima-media wet weight (7.5%) and DNA content (20%) in control rats. This increase was significantly greater in fructose-fed rats (22% for wet weight and 34% for DNA content) and was associated with greater HA and hyaluronidase production (123% and 41%, respectively) than in control rats (49% and 7%, respectively). Determination of HA molecular mass showed that balloon catheter injury increased the number of HA fragments in the aorta of control rats. Normal aorta of fructose-fed rats contained more HA fragments than that of control rats. Injury to the aorta of fructose-fed rats increased HA fragments and induced the appearance of a very-high-molecular-mass (>2000 kDa) HA. In conclusion, fructose treatment, which induced hyperinsulinemia and hypertriglyceridemia, increased HA and hyaluronidase production and HA degradation in injured aorta. This finding suggests that HA, which has been shown to play a crucial role in proliferation and migration of arterial smooth muscle cells, may be involved in the promotional effect of long-term fructose feeding on arterial wall reaction to injury.

Dependence of Fluorocarbon Plasma Chemistry on the Electron Energy Distribution Function

Recent diagnostics of high-density plasmas suggest a difference in the electron energy distribution function (EEDF) between a surface wave plasma and an inductively coupled plasma, which may be a main reason for the difference between the radical compositions of the two types of fluorocarbon plasmas at the same electron density. In order to account for the experimental result, a simple model is described based on a bi-Maxwellian EEDF with a bulk-electron temperature Tb and a tail-electron temperature Tt (>Tb). For simplicity, the conventional C4F8 etching gas is replaced by feeding CF3, CF2 and CF radicals into a vessel. A set of particle-balance equations is formulated with wall reactions (radical sticking, charge neutralization) and inelastic electron collisions are taken into account. Numerical solutions for the discharge in 2 mTorr C4F8/18 mTorr Ar show that the plasma becomes less dissociative and CFx radical rich (F radical poor) with increasing Tt and decreasing Tb. For example, the radical density ratio of CFx/F at the electron density of 3×1011 cm-3 is two times larger in the bi-Maxwellian EEDF of Tb=1.5 eV and Tt=5.0 eV than in the `single' Maxwellian EEDF of Tb=2.75 eV.

Process diagnostics and thickness metrology using in situ mass spectrometry for the chemical vapor deposition of W from H2/WF6

Quadrupole mass spectrometry has been used to monitor reactant and product partial pressures in a selective W chemical vapor deposition process. A 4/1H2/WF6 molar reactant ratio was used to produce W films on Si wafers, at 67 Pa (0.5 Torr) total pressure, and for wafer temperatures around 400 °C. A relatively fast response time (∼4 s) sensor system sampled gas directly from a commercial Ulvac ERA-1000 reactor in order to minimize the effect of wall reactions. The signal from the volatile HF product, integrated over the deposition cycle, and corrected for contributions from reactions in the ion-source region of the quadrupole and for sensor drifts, was found to vary linearly with the weight of the W film deposited, to within an uncertainty of ∼7%. This provides the basis for real-time, noninvasive thickness metrology to drive process control. Depletion of both H2 and WF6 reactants was observed. The time integral of the H2 reactant depletion was also linearly related to the film weight, though the data exhibited a somewhat larger scatter due to the low conversion efficiency of the process. In addition, volatile SiF4 and SiHF3 products of the initial rapid W nucleation reaction on the Si surface were clearly observed, indicating that initial surface conditions may be monitored in real time under selective growth conditions.

Unsteady convective diffusion with heterogeneous chemical reaction in a plane-Poiseuille flow of a micropolar fluid

The paper examines the influence of heterogeneous chemical reaction on the exchange coefficient, convective coefficient and diffusive coefficient arising in the study of dispersion in a micropolar fluid flow. The first of the three coefficients emanates exclusively from the incorporation of the catalytic wall reaction. The wall reaction also influences the other two coefficients. The effect of wall-catalysed reaction on dispersion is investigated against the background of the no-reaction problem. The analytical result on dispersion of solute with wall catalysed reaction at long times is compared with the analytical solution when reaction is absent. The Taylor [1] and Aris [2] regimes of dispersion for the present problem are obtained as limiting cases from the study. The graphical results of the study serve as a jury on any numerical study that might be undertaken considering non-asymptotic all-time analysis. The problem may find application in industries letting away waste into the environment, in crude oil conveyance, in chromatography and in extracorporeal biomechanical problems.

New biocompatible polymer surface coating for stents results in a low neointimal response.

Clinical studies indicate a more pronounced endothelial response after stent implantation than after balloon inflation. This might be related to the metal surface of the stent, and therefore it is speculated that coating of the stent might partially prevent hyperplasia. One coated and one noncoated Palmaz-Schatz stent were implanted in two separate coronary arteries of seven pigs. The coating was composed of methylmethacrylate (MMA) (hydrophobic, 70 mol %) and 2-hydroxyethyl methacrylate (HEMA) (hydrophilic 30 mol %). After sacrifice (3 weeks), cross sections were made of the stented areas. Vessel wall reaction was calculated both independently and dependently of local vessel wall injury due to the stent struts. Overall, vessel wall reaction of the coated stents was lower than that of the noncoated stents. The degree of hyperplasia was linearly related to the degree of stent-induced vessel wall injury. Analyses of all the struts showed that significantly less hyperplasia occurred in the coated versus noncoated stents. In this porcine coronary artery model, the MMA/HEMA stent coating resulted in significantly reduced vessel wall response. However, it remains to be determined whether this favorable outcome will also be present in humans.

Detailed chemical kinetic mechanisms for combustion of oxygenated fuels

Thermodynamic properties and detailed chemical kinetic models have been developed for the combustion of two oxygenates: methyl butanoate, a model compound for biodiesel fuels, and methyl formate, a related simpler molecule. Bond additivity methods and rules for estimating kinetic parameters were adopted from hydrocarbon combustion and extended. The resulting mechanisms have been tested against the limited combustion data available in the literature, which was obtained at low temperature, subatmospheric conditions in closed vessels, using pressure measurements as the main diagnostic. Some qualitative agreement was obtained, but the experimental data consistently indicated lower overall reactivities than the model, differing by factors of 10 to 50. This discrepancy, which occurs for species with wellestablished kinetic mechanisms as well as for methyl esters, is tentatively ascribed to the presence of wall reactions in the experiments. The model predicts a region of weak or negative dependence of overall reaction rate on temperature for each methyl ester. Examination of the reaction fluxes provides an explanation of this behavior, involving a temperature-dependent competition between chain-propagating unimolecular decomposition processes and chain-branching processes, similar to that accepted for hydrocarbons. There is an urgent need to obtain more complete experimental data under well-characterized conditions for thorough testing of the model.