Linear Growth Region Research Articles

Investigations of the adsorbed layer of polysulfone: Influence of the thickness of the adsorbed layer on the glass transition of thin films.

This work studies the influence of the adsorbed layer on the glass transition of thin films of polysulfone. Therefore, the growth kinetics of the irreversibly adsorbed layer of polysulfone on silicon substrates was first investigated using the solvent leaching approach, and the thickness of the remaining layer was measured with atomic force microscopy. Annealing conditions before leaching were varied in temperature and time (0-336h). The growth kinetics showed three distinct regions: a pre-growth step where it was assumed that phenyl rings align parallel to the substrate at the shortest annealing times, a linear growth region, and a crossover from linear to logarithmic growth observed at higher temperatures for the longest annealing times. No signs of desorption were observed, pointing to the formation of a strongly adsorbed layer. Second, the glass transition of thin polysulfone films was studied in dependence on the film thickness using spectroscopic ellipsometry. Three annealing conditions were compared: two with only a tightly bound layer formed in the linear growth regime and one with both tightly bound and loosely adsorbed layers formed in the logarithmic growth regime. The onset thickness and increase in the glass transition temperature increases with annealing time and temperature. These differences were attributed to the distinct conformations of the formed adsorbed layers.

Effect of urban street canyon aspect ratio on fire-induced air pollutants dispersion under cross wind

The incidence of vehicle fires in urban street canyons has continuously increased with the process of urban modernization. The dispersion and accumulation of fire-induced air pollutants seriously affect residents’ safety. The effects of the street canyon aspect ratio and fire source location on fire-induced air pollutants dispersion under cross wind were numerically investigated in the paper. The results show that the street canyon aspect ratio can affect flow patterns and vortex structure and then lead to different accumulation states of air pollutants. Smoke development can be divided into four different regimes according to cross wind velocity. The re-entrainment phenomenon occurs when the fire source is close to the windward building or in the center of the street, but it does not occur when the fire source is close to the leeward building. Smoke tends to accumulate in street canyons when the aspect ratio is smaller and the fire source is closer to the leeward building. The variation of critical velocity with the aspect ratio can be divided into three regions, i.e., initial linear growth region (0.5 ≤ W/H < 1), constant region (1 ≤ W/H ≤ 1.5), and second linear growth region (1.5 < W/H ≤ 2). The dimensionless critical velocity and critical Froude number are used to quantify the re-entrainment phenomenon in different fire scenarios, and prediction models are finally proposed. The main findings can provide a more detailed reference for urban layout and air pollution control to reduce fire-induced air pollutants.

Tensile damage self-monitoring of carbon fiber/epoxy 3D braided composites with electrical resistance method

Electrical resistance method (ERM) has great potential in self-monitoring structural health for carbon fiber composites due to carbon fiber conductivity. Here we report the correlation between tensile damages and electrical resistance changes of 3D braided composites subjected to monotonic and cyclic tensile loading. Experimental and finite element methods were utilized to reveal the electrical resistance variation and damage evolution under monotonic tensile loading. We found that the electrical resistance changes under monotonic tensile loading could be segmented into three regions, namely, linear growth region, transition region and exponential growth region, corresponding to different damage modes. Synchronous and reversible electrical resistance changes at a low strain level (12% of fracture strain) cyclic loading proved the stability and repeatability of self-monitoring capability. While, the abrupt electrical resistance changes at a high strain level (75% of fracture strain), indicated catastrophic failure of the composites. Such kind of correspondence between tensile damages and electrical resistance change under tensile loading enables ERM to achieve the in-situ structural health monitoring of 3D braided composites without relying on external sensors.

Numerical simulation of an acoustically excited plane jet in an incompressible framework and comparison with experimental data.

This paper is concerned with the interaction of a two-dimensional plane jet with transverse plane acoustic waves, which occur, for example, in flue instruments in the vicinity of the nozzle exit. The acoustic excitation is modeled with fluctuating boundary conditions within the framework of an incompressible simulation. This method can be easily implemented in commercial CFD software. The simulations are compared with well-documented measurement data from other authors. It is shown that the model can predict not only the growth rate but also the amplitude of the perturbation, thus it captures the key features of the jet behavior in the receptive and in the linear growth region.

Liutex (vorex) cores in transitional boundary layer with spanwise-wall oscillation

The vortex core detection method based on the Liutex vector is utilized to investigate the alternation of vortical structures on a boundary layer transition subjected to spanwise-wall oscillation. Compared with iso-surface based methods, the Liutex core line method is shown to be precise, free of threshold and capable to capture both strong and weak vortices simultaneously. Tollmien-Schlichting (T-S) waves in the linear growth region, Λ — and hairpin vortices in the transition region and twisted vortices in the turbulent region are all well captured by Liutex core lines. The cyclic wall movement accelerates the transition process while reducing the turbulent drag by 21.8% with selected parameters. For the wall oscillation case, the development from T-S wave to Λ — vortex is advanced about one T-S wave length in the streamwise direction. In the transition region, the Λ — vortex and legs of hairpin vortex are shortened in the wall oscillation case, and the symmetry of the vortical structures is lost in the late transition region since the introduction of asymmetry disturbances by the cyclic wall movement. Extrusions of weak vortices at the edge of boundary layer are found in the turbulent section which is often omitted by iso-surface based vortex identification method. Thus, it is demonstrated that for the transitional boundary layer the Liutex core line method provides a systematic and threshold-free vortex definition, which could serve as a powerful tool to understand and guide flow control.

Tip‐vortex breakdown of wind turbines subject to shear

AbstractSheared velocity profiles pervade all wind‐turbine applications, thus making it important to understand their effect on the wake. In this study, a single wind turbine is modeled using the actuator‐line method in the incompressible Navier–Stokes equations. The tip vortices are perturbed harmonically, and the growth rate of the response is evaluated under uniform inflow and a linear velocity profile. Whereas previous investigations of this kind were conducted in the rotating frame of reference, this study evaluates the excitation response in the fixed frame of reference, thus necessitating a frequency transformation. It is shown that increasing the shear decreases the spatial growth rate in the upper half of the wake while increasing it in the lower half. When scaled with the local tip vortex parameters, the growth rate along the entire azimuth collapses to a single value for the investigated wavenumbers. We conclude that even though the tip‐vortex breakdown is asymmetric in sheared flow, the scaled growth rates follow the behavior of axisymmetric helical vortices. An excitation amplitude reduction by an order of magnitude extends the linear growth region of the wake by one radius for uniform inflow. In the sheared setup, the linear growth region is extended further in the top half than in the bottom half because of the progressive distortion of the helical tip vortices. An existing model to determine the stable wake length was shown to be in close agreement with the observed numerical results when adjusted for shear.

Effect of the thiourea incorporation velocity and RTA post-deposit treatments, on the properties of CdS films deposited by CBD

In this work the influence of the thiourea incorporation velocity, or drip time (td), on the properties of the CdS films deposited by chemical bath deposition technique (CBD) was studied. The thiourea was employed as the sulfur source in the bath solution. The films were deposited on glass substrates keeping the bath solution temperature (Tb) at 90 °C and varying the td from 10 to 60 min. The other reagents used were cadmium acetate, ammonium acetate, ammonium hydroxide and deionized water as an aqueous medium, whose concentrations were constant. The dependence of the thickness with td showed three regions associated with three growth stages: i) induction period, ii) linear growth and iii) saturation stage. The best structural, optical and electrical properties were obtained in the linear growth region (20 min ≤ td ≤ 40 min). These films exhibited a crystallite size around 35 nm and a resistivity value of ∼100 Ω-cm. With respect to their optical properties, an average transmittance value of 70% for λ ≥ 500 nm and Eg of 2.35 eV were obtained. The presence of organic compounds was determined by FTIR spectra. The main organic compound identified was cyanamide, whose concentration was greater and constant at linear growth region. Afterwards, the effect of rapid thermal annealing (RTA) treatments on the properties of the CdS films, was evaluated for three films of the linear growth region. Such treatments did not modify the thickness, crystalline structure and crystallite size of the films. However, there was a change of color from light orange to dark orange, and a shift of Eg to 2.27 eV without the detriment of the transmittance. The CdS optical constants (n, k, ε) obtained from an optical model, confirmed the shift. In addition, photoluminescence spectra after RTA treatment showed a decrease of the defects associated to the radiative bands: green (2.21 eV), red (1.88 eV) and nanoparticles (2.94 eV), this without a change in the orange band (2.04 eV). The cyanamide, main organic compound present in the films, decreased significantly with thermal treatment, producing a resistivity drop from ∼100 to 4 Ω-cm. The drop of resistivity was attributed to the decrease of both, cyanamide and radiative defects concentration. The RTA treatments lead to CdS films with low resistivity and good transmittance, making them appropriate to be used as window material in solar cells.

Experimental investigation on oxidation kinetics of germanium by ozone

Oxidation kinetics of germanium surface by ozone at low temperature (≤400°C) is experimentally investigated. The growth process contains two regions: initial linear growth region and following parabolic growth region. The GeOx thickness vs. oxidation time plot obeys the well-known Deal-Grove or linear parabolic model. The linear growth region contains reaction of oxygen atoms with surface bond and back bonds of outmost Ge layer. And the activation energy is experimentally estimated to be 0.06eV. Such small activation energy indicates that the linear growth region is nearly barrier-less. The parabolic growth region starts when the oxygen atoms diffuse into back bonds of second outmost Ge layers. And the activation energy for this process is found to be 0.54eV. Furthermore, in the ozone oxidation it is not O3 molecules but O radicals that go through the GeOx film.

Dynamics of a membrane interacting with an active wall.

Active motions of a biological membrane can be induced by nonthermal fluctuations that occur in the outer environment of the membrane. We discuss the dynamics of a membrane interacting hydrodynamically with an active wall that exerts random velocities on the ambient fluid. Solving the hydrodynamic equations of a bound membrane, we first derive a dynamic equation for the membrane fluctuation amplitude in the presence of different types of walls. Membrane two-point correlation functions are calculated for three different cases: (i) a static wall, (ii) an active wall, and (iii) an active wall with an intrinsic time scale. We focus on the mean squared displacement (MSD) of a tagged membrane describing the Brownian motion of a membrane segment. For the static wall case, there are two asymptotic regimes of MSD (∼t^{2/3} and ∼t^{1/3}) when the hydrodynamic decay rate changes monotonically. In the case of an active wall, the MSD grows linearly in time (∼t) in the early stage, which is unusual for a membrane segment. This linear-growth region of the MSD is further extended when the active wall has a finite intrinsic time scale.

Numerical Simulation of the Fluid Flow, Heat Transfer, and Solidification in a Twin-Roll Strip Continuous Casting Machine

In this paper, a three-dimensional enthalpy–porosity mixture solidification model is employed to describe the basic rules of heat transfer and solidification in a twin-roll strip continuous casting machine equipped with a multi-port trough-shaped feeding system, and reasons for the formation of these rules are analyzed. The two-layer zonal turbulence model is used to incorporate the turbulence in fluid flow, and the fluid flow in the wedge-shaped pool region in various conditions is studied. The results show that the fluid flow field in the pool region is quite different with/without the consideration of rotating rolls when the solidification is ignored, which indicates the importance of the consideration of rotating rolls in the water modeling equipment design. There are three aspects including molten steel jet impingement, roll rotating, and backward flow of the mushy zone, which strongly influence the temperature distribution in the pool region. The solidification process in the pool region could be divided into three stages: at the first stage, there exists a fast solidification region which obeys the square root law, and an approximately linear growth region could be found at the second stage, while a parabolic growth region could be found at the last stage. The formation of the first stage and the last stage could be mainly due to the geometry of the feeding system and the backward flow of the mushy zone, respectively. The first stage of solidification could be modified by a better designed feeding system, but the backward flow of mushy zone determined by the properties of metals and casting parameters could be one of the biggest obstacles for commercial production of steel sheets by a twin-roll caster.

In2S3 Atomic Layer Deposition and Its Application as a Sensitizer on TiO2 Nanotube Arrays for Solar Energy Conversion

In2S3 atomic layer deposition (ALD) with indium acetylacetonate (In(acac)3) and H2S was studied with quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Subsequent In2S3 ALD on TiO2 nanotube arrays defined a model semiconductor sensitized solar cell. For In2S3 ALD on initial Al2O3 ALD surfaces, the In2S3 ALD displayed a nucleation period of ∼60−70 cycles followed by a linear growth region. These results were obtained under ALD conditions that were not completely self-limiting for the In(acac)3 exposure because of the low In(acac)3 vapor pressure. The growth per cycle decreased at higher temperature over the temperature range from 130 to 170 °C at these same reactant conditions. The growth per cycle was 0.30−0.35 A per cycle at 150 °C as determined by QCM and XRR measurements at higher In(acac)3 exposures where the surface reactions were self-limiting chemistry versus In(acac)3 and H2S exposures. The FTIR examinations revealed that the...

Activation of Low k Dielectrc Surfaces for ALD Barrier Formation

ABSTRACTAtomic layer deposition (ALD) of ultra-thin barrier layers is a key process for implementation of Cu/low k interconnects. Low k dielectric surfaces are generally characterized by weak surface bonds which have to be properly activated for barrier formation. In this study, we investigated the surface activation of low k dielectrics by atomic hydrogen, nitrogen and ammonia beams for ALD of Ta/TaN barrier layers. In-situ x-ray photoemiision spectroscopy (XPS) together with differential FTIR were used to examine the beam modification of the surface and the subsequent growth on the low k surface. The evolution of the low k surface chemistry revealed an initial transient growth region controlled mainly by the substrate surface chemistry. The initial chemisorption was found to be through formation of Ta-O bonds on organosilicates (OSG) and charge transfer complexes on aromatic SiLK. This led to a significant slower initial nucleation on the OSG low k surface without beam activation. The atomic beams, particularly the nitrogen and amine radials, were found to serve as effective reducing agents in initiating and enhancing the precursor chemisorption. This led to an improvement of the barrier film quality and an increase of the deposition rate at a temperature compatible with low k dielectrics (&lt; 400C). In the subsequent linear growth region, atomic hydrogen species was able to reduce the chlorine content under appropriate temperature and with sufficient purge. The effect of porosity in low k dielectrics on barrier formation will be discussed.

Effects of surface chemistry on ALD Ta3N5 barrier formation on low-k dielectrics

In situ x-ray photoelectron spectroscopy (XPS) was employed to study the effects of the substrate surface chemistry on atomic layer deposition (ALD) of Ta-based barrier layers on various low k surfaces. The initial chemisorption was found to be through formation of Ta–O bonding on organosilicates (OSG) and charge transfer complexes on aromatic SiLK™. ALD process nucleated very slowly on the OSG low-k films, in distinct contrast with its ready nucleation on SiLK™ films. The evolution of the low-k surface chemistry revealed an initial transient growth region controlled mainly by the substrate surface chemistry. Pretreatment of the low-k surfaces with nitrogen and hydrogen radical beams, particularly with nitrogen radicals, was observed to enhance significantly the chemisorption of the TaCl5 precursor on the OSG surfaces. The enhancement was attributed to dissociation of weakly bonded methyl groups from the low-k surface followed by nitridation with the nitrogen radicals. In the subsequent linear growth region, atomic hydrogen species was able to reduce the chlorine content under appropriate temperature and with sufficient purge. The role of the atomic hydrogen in this process enhancement is discussed.

In-situ XPS Study of ALD Ta(N) Barrier Formation on Organosilicate Dielectric Surface

AbstractBeams of nitrogen and hydrogen radicals were investigated as surface pre-treatment and process enhancement techniques for atomic layer deposition (ALD) of tantalum nitride barrier layer on a dense organosilicate (OSG) low k film. In-situ x-ray photoelectron spectroscopy (XPS) studies of the evolution of the low k surface chemistry revealed an initial transient growth region controlled mainly by the substrate surface chemistry. Pre-treatment of the low k surface with radical beams, particularly with nitrogen radicals, was found to enhance significantly the chemisorption of the TaCl5 precursor on the OSG surfaces. The enhancement was attributed to the dissociation of the weakly bonded methyl groups from the low k surface followed by nitridation with the nitrogen radicals. In the subsequent linear growth region, atomic hydrogen species was able to reduce the chlorine content under appropriate temperature and with sufficient purge. The role of the atomic hydrogen in this process enhancement is discussed.

Structure and Growth Kinetics of Films Formed by the Thermal Decomposition of CCl4on Iron Surfaces

Carbon tetrachloride thermally decomposes on iron in the temperature range 500−700 K to form films that consist predominantly of iron chloride. Two growth regimes are found: (1) parabolic, where the film thickness (X) varies as a function of time as X2 ∼ t and where the growth rate is independent of pressure, and (2) a linear growth region (X ∼ t), where the growth kinetics are first order in CCl4 pressure. The kinetics are analyzed by a model which assumes that growth is controlled either by CCl4 thermally decomposing at the growing gas−film interface or by diffusion through the film. Analysis of the experimental data gives an activation energy for diffusion of 21.5 ± 0.3 kcal/mol and an activation energy for CCl4 decomposition of 18.3 ± 0.5 kcal/mol. In contrast to the behavior found for methylene chloride and chloroform, where small particles of carbon were identified in the film using Raman spectroscopy, no carbon particles are found in films formed from carbon tetrachloride decomposition below ∼700 K although carbon is found in the film using X-ray photoelectron spectroscopy. XPS results also suggest that, in addition to completely thermally decomposing to yield carbon and chlorine, a small portion of the CCl4 can react to form adsorbed CCl2 species.

Effect of Velocity Ratio on the Control of Large-Scale Structures in Mixing Layers.

To clarify the effect of velocity ratio on the control of large-scale structures in mixing layers, two-dimensional spatially developing mixing layers perturbed periodically were studied using direct numerical simulation. The perturbation consists of fundamental and subharmonic modes of the most unstable wave based on linear stability theory. The results show that it is possible to control the development of large-scale structures by controlling the initial phase difference between two modes, and that there exists a certain initial phase difference which suppresses vortex pairing. This initial phase difference varies with the velocity ratio. The downstream variations of the phase difference between fundamental and subharmonic modes of streamwise velocity fluctuation were also investigated. In the linear growth region, the local phase difference increases at a constant rate which varies with the velocity ratio. This linear increase of phase difference is explained by the dependence of phase velocity on the velocity ratio, which is derived from the linear stability theory. Because of this linear increase of phase difference, the initial phase difference which suppresses the pairing varies with the velocity ratio.

Theory for the plasma anodization of silicon under constant voltage and constant current conditions

A theoretical model is presented for the anodization of silicon in an oxygen plasma. The model is based on the space-charge-limited drift of negative oxygen ions through the growing oxide film to react at the silicon-silicon dioxide interface. In the initial linear growth region the supply of ions from the plasma determines the growth, whilst for longer periods the drift of ions through the oxide gives rise to parabolic growth kinetics. The theory is consistent with the lack of orientation dependence observed for the plasma oxidation of silicon. It is applicable over the whole of the growth kinetics, for anodization in both constant voltage and constant current modes. The dependence of current and voltage with time during anodization may also be predicted. The theory is shown to be applicable to oxide growth occurring in both an inductively coupled, rf-stimulated plasma and a microwave-sustained oxygen plasma over a range of experimental conditions.

Development of the turbulent near wake of a tapered thick flat plate

The velocity field and turbulence structure in the near wake of a thick flat plate with a tapered trailing-edge geometry are examined using both hydrogen-bubble flow visualization and hot-film anemometry measurements. Tests were conducted for Re1 = 8.5 × 105 in the region 0 &lt; x+ &lt; 6400 behind the trailing edge. The probe and visualization results indicate a similarity between both (i) velocity and turbulence structure variations wih x+ in the near wake, and (ii) the corresponding changes in similar flow characteristics with y+ within a turbulent boundary layer. In particular, visualization data in the vicinity of the wake centreline reveal the existence of strong streamwise flow structures in the region close (x+ &lt; 270) to the trailing edge. The streamwise orientation of the observed structures diminishes as x+ increases. From hot-film measurements, two separate regions along the wake centreline can be distinguished: (i) a linear growth region which extends over 0 &lt; x+ &lt; 100, wherein the centreline velocity varies linearly with x+; and (ii) a logarithmic growth region for x+ &gt; 270, wherein the centreline velocity varies as log x+. The similarity in behaviour between these regions and the comparable wall region of a turbulent boundary layer suggests the existence of a common functionality. This similarity is demonstrated by a simple linear relationship of the form y+ = Kx+, which is shown to approximately collapse the velocity behaviour both across a turbulent boundary layer and along the wake centreline to a unified set of empirical relationships.

An experimental study of the linear growth region of ultrashort-pulse generation in a mode-locked Nd : glass laser

An experiment is performed to measure τ1, the duration of the linear growth stage of ultrashort-pulse (USP) generation, in a mode-locked Nd : glass laser. A linear relationship between USP pulsewidth and τ1 is derived and borne out experimentally. From the linear gradient τ1 is estimated to be about 100 μsec. Some precursor spikes are observed in most shots and might account for the long τ1.

Batch kinetics and oxygen consumption of Candida lipolytica 4-1 on n-alkanes.

Detailed batch kinetics ofCandida lipolytica 4-1 onn-hexadecane for varying dispersed phase volume from 0.5 to 5% v/v is presented. All batch growth curves exhibited a linear growth region, indicating a substrate uptake limit. System productivities derived from the linear region were correlated to the dispersed phase volume. The correlation coefficient was identical with that obtained on gas oil. This implies that a correlation coefficient of interfacial area to the dispersed phase volume is identical for both systems. Dissolved oxygen profile and uptake of oxygen from gas phase were also measured. The oxygen uptake rate, volumetric oxygen transfer rate and oxygen demand (requirement) were calculated by means of the balance method. Under limiting dissolved oxygen concentration the maximal oxygen transfer of the fermenter was assessed.