Rate-controlling Process Research Articles

Rate control for MPEG-2 SNR scalability and stream morphing using codeword estimation and overhead modelling

Many encoder architectures, notably for scalable systems such as MPEG-2 SNR scalability and stream morphing, require DCT-domain techniques for the estimation of the rate-distortion characteristics of a video frame for use in the rate control process. This paper develops a CBR rate control framework that can be used for single-layer MPEG-4 Simple Profile as well as two MPEG-4-based scalable systems, one following the structure described in MPEG-2 and also stream morphing. The framework supports perceptual quantization and also low bit rate operation by modelling macroblock overhead as well as the bits required to code non-zero DCT coefficients. Experimental results demonstrate the performance of stream morphing, in both objective and subjective measures, is in almost all cases superior to the MPEG-2 SNR scalability and MPEG-4 Fine Granularity Scalability.

Thermogravimetric and thermokinetic investigation of the dehydroxylation of a hydroxyapatite powder

Abstract The dehydroxylation of HA powder was investigated using thermogravimetric analysis over the range of room temperature to 1250 °C. The kinetic result of dehydroxylation of HA was calculated by means of the Ozawa–Flynn–Wall method. According to calculated activation energy-conversion degree plot, the kinetics of HA dehydroxylation was identified as a comprehensive one, which included four reaction-rate-controlling processes with different activation energies and four successive conversion stages kinetically controlled by different rate-controlling processes. These four reaction-rate-controlling processes were interpreted as OH − -anion diffusion through HA, OH − -anion debonding from HA lattice, the lattice constitution of OA, and 2OH − →H 2 O↑+O 2− at the reaction interface.

The dissolution of organic solvents from Amerlock 400 epoxy paint

The dissolution of organic solvents from containment coatings into the sump water is expected to be a key rate-controlling process, influencing the pH, the steady-state water radiolysis product concentration, and the formation of organic iodides in containment following a postulated reactor accident. The dissolution process is therefore an important component in modelling iodine behaviour in post-accident containment. The rate of release of ethyl benzene and m- and o-xylene from Amerlock 400 epoxy paint in contact with water has been measured. The release rate was found to be comparable to the release rate of methyl isobutyl ketone from various polyurethane, vinyl, and epoxy paints. The pseudo-first-order rate constant for dissolution of the solvents from these containment coatings is dependent only upon paint thickness and temperature, whereas the total amount of solvent released depends upon paint thickness. The solvent-release process is governed by the rate of uptake of water by the coupons, a Fickian diffusion-controlled process.Key words: dissolution, kinetics, organic solvents, paint.

Use of the Flux Ratio Method for Mechanistic Diagnosis in Electroactive Polymer Film Redox Switching

The redox switching of poly(vinylferrocene) (PVF) films was investigated using the electrochemical quartz crystal microbalance in conjunction with cyclic voltammetry (at different voltage scan rates) and reverse potential steps. The mechanism of the redox process was determined using PVF films supported on Au electrodes and exposed to aqueous bathing solutions of 0.1 M sodium hexafluorophosphate. PVF electro-oxidation proceeds via coupled oxidation of uncharged ferrocene sites and entry of counterion and is followed by the entry of water into the film. Structural changes within the polymer may also accompany the latter two steps. Any of these three steps may be the slowest for particular redox conditions. The controlling kinetic step depends on the film’s instantaneous water content, its instantaneous oxidation state, the electrochemical control function, the direction of redox switching, and the associated time scale of the experiment. We describe a new general quantitative approach based upon comparison of the instantaneous fluxes of solvent (water) and counterion during the redox cycle to characterize the rate-controlling process as a function of the extent of film oxidation. This new methodology has the capability to resolve time scale- and potential- (charge-) dependent mechanistic shifts and film relaxation phenomena as they are reflected through the ratio of fluxes of solvent and counterions. © 2003 The Electrochemical Society. All rights reserved.

Oxidation of Boron Carbide-Silicon Carbide Composite at 1073 to 1773 K

The oxidation behavior of B 4 C-(25-60 mol%)SiC composites prepared by arc-melting was investigated in the temperature range of 1073 to 1773K using a thermogravimetric technique. Liquid borosilicate, solid SiO 2 and carbon were identified as oxidation products by X-ray diffraction and Raman spectroscopy. Mass gain was observed during oxidation at 1073 K, while mass loss due to the vaporization of boron oxide in liquid borosilicate was observed at temperatures of 1273 K and higher. In situ Raman spectra of the surface of B 4 C-SiC composites indicated that the silica concentration in the liquid borosilicate increased with increasing SiC content in the composite. Micro-Raman spectroscopy showed that carbon was enriched in the borosilicate layer close to the oxide/composite interface. The parabolic rate constants for B 4 C-50mol%SiC composites at 1073 K were proportional to ambient oxygen partial pressures ranging between 30 and 100kPa. The diffusion of oxygen molecules through the liquid borosilicate layer could be the rate-controlling process. The increase of SiC content in the B 4 C-SiC composites improved the oxidation resistance in both the mass gain and mass loss regions.

Nanostructural and nanoelemental analysis of metastable M 3C-type carbides with alloy-rich layer in heat resistant 2Cr-martensitic steel

A combination of structural analysis and analytical investigation by transmission electron microscopy was employed to investigate precipitated carbides on 2Cr-steel. Spectral and image analysis by energy dispersive X-ray spectroscopy (EDS), and energy-filtered images in a transmission electron microscopy revealed compositions of precipitated carbides from both boundaries. The presence of concentration gradient of alloy elements within the precipitated carbide, were shown by EDS and energy-filtering transmission electron microscopy. The results indicate that the rate-controlling process should be governed by not only the diffusion of depletion layer but also that of alloy elements within the carbides. It is therefore necessary to consider these phenomena to model the rate-controlling process, for further discussion of the growth mechanisms of the carbides in steels.

Effect of the fluoride additives on the oxidation of AlN

The oxidation of AlN powder added by the fluorides at temperatures below 700°C in air was discovered in this study. The obvious onset of oxidation of AlN with cryolite and YF 3 additions is below 700°C with the product of α-Al 2O 3 phase, which usually occurs in single AlN powder above 1100°C. The changes on the weight and the FTIR spectra of the AlN powder fired at temperatures lower than 700°C show that cryolite and YF 3 greatly promote the oxidation of AlN powder at these temperatures. Different from the action of cryolite and YF 3 powder, CaF 2 has no obvious effect on the oxidation of AlN. A possible oxidation process, in part corroborated by FTIR and XRF, was proposed to explain the results in the experiments. The oxidation kinetics of AlN in the presence of cryolite were also discussed at the temperatures ranging from 550 to 700°C from the data of the weight gains in this region. The result shows that the oxidation follows a linear law, which implies a reaction rate-controlled process. The considerably low activation energy of 67 kJ mol −1, which is associated with the quick oxidation and the formation of α-Al 2O 3 at temperatures below 700°C, was determined from the slope of the line fit.

Hydrogen transport in palladium membranes

The hydrogen flux in palladium membranes acquired from different vendors has been measured as a function of membrane thickness, temperature and feed (pure H 2) pressure p 1, with permeate (pure H 2) pressure p 2 fixed at 1 atm. Palladium foils 12.5, 25 and 47 μm thick were studied at 75–500°C and p 1 values of 3.4–6.8 atm. Arrhenius plots of flux, measured with p 1 fixed, include a peak within an intermediate temperature range, and different activation energies in the low and high temperature ranges. The peak may be explained by palladium hydride's α—β phase transition, which, at intermediate temperatures, takes place at hydrogen activities between those at the respective membrane surfaces. The two activation energies apparently reflect different rate-controlling processes: bulk diffusion at high temperatures and surface phenomena at low temperatures.

Oxidation of Silicon and Silicon Carbide in Ozone‐Containing Atmospheres at 973 K

The oxidation behavior of a silicon wafer, chemically vapor‐deposited SiC, and single‐crystal SiC was investigated in an oxygen—2%–7% ozone gas mixture at 973 K. The thickness of the oxide film that formed during oxidation was measured by ellipsometry. The oxidation rates in the ozone‐containing atmosphere were much higher than those in a pure oxygen atmosphere. The parabolic oxidation kinetics were observed for both silicon and SiC. The parabolic rate constants varied linearly with the ozone‐gas partial pressure. Inward diffusion of atomic oxygen formed by the dissociation of ozone gas through the SiO2 film apparently was the rate‐controlling process.

Environmentally assisted, sustained-load crack growth in powder metallurgy nickel-based superalloys

To examine the influence of niobium (Nb) on sustained-load crack growth (SLCG) in oxygen, three powder metallurgy (P/M) nickel-based superalloys, with nominal compositions similar to IN100, but with 0, 2.5, and 5 wt pct of Nb, are used. These alloys are gamma-prime (γ’) strengthened and have comparable volume fractions (53 vol pct) of γ’ precipitates. The SLCG experiments are conducted in high-purity oxygen and argon at 873, 923, and 973 K. The environmental cracking sensitivity (ECS) for the alloys with 2.5 and 5 wt pct of Nb is consistent with that of INCONEL 718 and supports the previously identified role of Nb-rich carbides in enhancing crack growth. The susceptibility of the Nb-free alloy to oxygen, however, is much greater than expected. The apparent activation energy for crack growth in oxygen was found to depend on stress-intensity-factor (K) levels for the Nb-containing alloys and ranged from about 320 to 260 kJ/mol for K levels of 35 to 60 MPa√m. It was nearly independent of K at about 250 kJ/mol for the Nb-free alloy. The results are discussed in terms of the rate-controlling process and of the mechanism for crack-growth enhancement.

Determination of Oxygen Vacancy Concentration in a Thin Film of La0.6Sr0.4CoO3 − δ by an Electrochemical Method

Equivalent circuit analysis was undertaken on a mixed conductor electrode/solid oxide electrolyte system. In a limited case where surface reaction is the predominant rate-controlling process, the equivalent circuit was simplified to parallel connection of a resistor (surface reaction resistance) and a capacitor (chemical capacitance due to oxygen nonstoichiometry). Equilibrium oxygen vacancy concentration was correlated with the chemical capacitance. The model was applied to a dense film of deposited on a sintered plate of by a laser ablation method. Frequency response of the electrochemical impedance was measured under dc bias at 873-1073 K in gas mixtures. The observed capacitance was extremely large, e.g., around 0.1 to 1 for the 1.5 μm thick film. The oxygen vacancy concentration in the film was calculated from the capacitance and compared with the literature data measured by thermogravimetry. The film was found to show smaller oxygen nonstoichiometry. The enthalpy for oxygen vacancy formation in the film was about 40 kJ mol larger than the bulk. © 2002 The Electrochemical Society. All rights reserved.

Experimental investigation of the kinetics of the reaction wollastonite + calcite + anorthite = grossular + CO2

Mechanisms and kinetics of the reaction wollastonite + calcite + anorthite ⇒ grossular + carbon dioxide were studied in powder experiments in the presence of a H₂O-CO₂ fluid with 8 to 13 mole percent CO₂ at 665° to 750°C and 400 MPa. The reaction was studied in terms of overall reaction rates, nucleation rates, and crystal growth rates. Grossular was the only solid reaction product. The crystals were grown around minute andradite-rich cores, which enriched the grossular with traces of iron. At a given temperature, the conversion to grossular + CO₂ increased linearly with time at least up to 25 percent. Reaction rates increased in the temperature range from 710° to 750°C by a factor of 22. At the constant temperature of 730°C the reaction rate increased by a factor of 2.4 when overstepping of the equilibrium temperature was increased from 55° to 95°C due to a decrease in the CO₂-concentration in the fluid. Nucleation and growth rates were measured at 730°C as a function of time. There was a sharp nucleation rate maximum during the second day of the experiments followed by a steady state nucleation rate at a much lower level during the rest of the experiment. Crystal growth rates permanently decreased either with respect to diameter, surface area, or volume. In experiments with varied surface area for either the reactants or for the grossular, no consistent relationship between surface area and reaction rate was found and thus no single rate-limiting step could be identified. For the rate-controlling process of the overall reaction a model similar to competitive, diffusion-controlled growth of garnet porphyroblasts is suggested. In this model each grossular crystal is surrounded by a diffusion halo where nutrients deplete with time. Once the diffusion halos impinge throughout the reactant mixture, the diffusion gradients are leveled. Consequently, the nucleation rate drops down to a steady state level and crystal growth rates decrease. Nucleation never totally ceases because the nucleation barrier is overcome by enrichment of andradite component in the garnet cores. Analog enrichment of trace elements during nucleation might be common in metamorphic crystallization. The results achieved will serve to better understand garnet formation in porphyroblastic rocks.

Compaction creep of quartz sand at 400–600°C: experimental evidence for dissolution-controlled pressure solution

Intergranular pressure solution (IPS) is an important compaction and deformation mechanism in quartzose rocks, but the kinetics and rate-controlling process remain unclear. The aim of the present study is to test microphysical models for compaction creep by IPS against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution (confining pressure 300 MPa, pore water pressure 150–250 MPa, temperature 400–600°C). Microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions. For porosities down to 15%, the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz, suggesting that the rate-limiting mechanism of IPS was dissolution. The model also predicts IPS rates within one order of magnitude of those measured in previous experiments at 150–350°C, and thus seem robust enough to model sandstone compaction in nature. Such applications may not be straightforward, however, as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence IPS rates in sandstones.

Activation energy of self-heating process Studied by DSC

In order to identify the kinetic process of self-heating in DSC experiment for Ti+3Al→TiAl3 reaction, two approaches, linear-fitting approach developed from Semenov"s theory of spontaneous ignition and variation of Friedman method, were carried out with cylindrical Ti-75 at% Al samples. Following these approaches, two identical activation energies are obtained as 169±15 kJ mol-1 and 170±5 kJ mol-1, respectively. Compared with the activation energies of reactions and interdiffusions between Ti and Al, the possible rate-controlling process of self-heating in DSC experiment for Ti+3Al→TiAl3 reaction is the interdiffusion between Ti and Al through TiAl3-layer.

Indentation creep of β-Sn and Sn–Pb eutectic alloy

This study has been conducted to determine whether or not the indentation creep test provides results equivalent to those of the conventional uniaxial creep tests in the dislocation creep regime. When a conical indenter is pressed into the test surface, the plastic region beneath the indenter extends while maintaining its geometrical self-similarity as indentation proceeds. The occurrence of self-similar indentation leads to a constitutive equation from which the power-law creep exponent, n, and the activation energy for creep, Q, can be derived. The values of Q and n so obtained are in approximate agreement with the conventional uniaxial creep test results, drawn from some sources. The results indicate that two independent rate-controlling processes take place during the creep deformation of β-Sn and fine-grained Sn–Pb eutectic alloy (grain size: ∼10 μm) regardless of the temperature employed.

Formation process of calcium carbonate from highly supersaturated solution

The precipitation process of calcium carbonate from highly supersaturated solutions was observed in situ by mixing CaCl 2 and Na 2CO 3 aqueous solutions at about 20°C under an optical microscope and an infrared microspectroscope. After an amorphous phase forms, spherulitic vaterite and calcite in a rhombohedral shape nucleate simultaneously but separately, and grow by forming the precipitate-free zone around them. This transformation is solvent-mediated, and the measurement of the growth rate suggests that the rate-controlling process is the diffusion of elements in the earlier stage of this process, which then changes to surface kinetics.

Water Vapor Adsorption on Basic Anion Exchangers

The adsorption process of water vapor on one strongly basic and two weakly basic macroreticular anion-exchange resins was examined by using a batch setting. Experiments were performed under a wide range of water activities (0.05-0.8) and at three different temperatures (298, 305, and 313 K). Applying Flory-Huggins theory, it was concluded that the strongly basic anion-exchange resin showed much stronger site binding of water molecules at low water activities than both weakly basic resins. No tendency of the adsorbed water to cluster could be observed for all three adsorbents in the activity range under study. Diffusion in the microparticles of the resins was found to be the rate-controlling transport process. The obtained effective intraparticle diffusivities were extremely small. An exponential decrease of the diffusion coefficients with increasing water concentration could be observed and was attributed to association of few water molecules in hydration shells around the functional units of the resins, hindering further solvent transport. Temperature change had no effect on the kinetics of the process.

Grain size-dependent viscosity convection and the thermal evolution of the Earth

Scaling relationships for grain size-dependent viscosity convection are derived assuming that eutectoid-like transformations at 660 km depth produce very small grains and that the grain size is mainly controlled by a coupled Ostwald ripening of Mg-perovskite, Ca-perovskite and magnesiowüstite. Parameterized convection calculations based on these relationships show that the thermal evolution of the Earth strongly depends on the grain growth parameters. The Earth can either quickly forget the initial conditions with temperature and heat loss following the decaying radiogenic heat production (Tozer-type evolution) or the initial conditions can essentially determine the temperature and heat loss (Christensen-type evolution). Christensen-type models can easily satisfy both the present-day heat flux and geochemical constraints on abundances of radiogenic isotopes. For example, this happens if the rate-controlling process for both grain growth and viscous creep is volume diffusion and the activation enthalpy for grain growth is about 50% higher than that for viscous creep.

Mechanistic study of active transport of copper(II) from ammoniacal/ammonium carbonate medium using LIX 973N as a carrier across a liquid membrane

The permeation of copper(II) through a supported liquid membrane impregnated with LIX 973N (oxime derivative) in Iberfluid has been studied. A model is proposed describing the transport mechanism, which consists of diffusion process through the source aqueous diffusion layer, fast interfacial chemical reaction and diffusion through the membrane. The experimental results are explained by equations describing the rate of transport. Various rate-controlling processes take place as the metal transport occurs.

Vapor diffusion of silver in cofired silver/palladium–ferroelectric ceramic multilayer

The mechanism of silver interfacial migration was investigated during cofiring multilayer structures of Pb-based relaxor ferroelectric ceramics and Ag/Pd alloy electrode. A several-micron depth of silver diffusion could be confirmed by energy dispersive X-ray spectroscopy measurements. Scanning electron microscopy and transmission electron microscopy observations indicated that the silver migration was not attributed to a single factor. This involved the electrode composition, the sintering characteristic of the ceramics, and the related sintering procedure. All these results have demonstrated that the vapor-phase diffusion via open pores on the surface of the ceramics at the early period of sintering was the most important reason for the silver migration even though the migration along grain boundaries or into grains occurs at the final stage of sintering. Based on an evaporation–condensation vapor-phase mass transport mechanism, a rate-controlled sintering process or a low-temperature sintering route of the materials was suggested to effectively inhibit the interfacial diffusion of silver.