Rate-controlling Process Research Articles

Autoignition behavior of a spherical cluster consisted of a center fine droplet and surrounding twelve fine droplets

Autoignition behavior of spherical clusters of pure droplets was experimentally examined near the ignition limit. The fuel with the melting points little above room temperatures was selected as the test fuel. The three-dimensional droplet clusters having hexagonal closest packing structure were selected as the test clusters, which consisted of a center droplet and surrounding twelve droplets (thirteen droplets in total). The initial diameters of the droplets were much reduced from the authors’ previous studies. The results showed that the ignition delay of the three-dimensional droplet clusters also had a minimum at a certain droplet spacing, which has been predicted by Niioka and coworkers as a result of rate-controlling process transition between reaction and evaporation. Autoignition of the droplet clusters could be achieved for the center fine droplet, which is out of ignition limit when isolated, in combination with finer surrounding droplets.

CO2 adsorption kinetics on mesoporous silica under wide range of pressure and temperature

Abstract Adsorption kinetics is a critical parameter to assess the performance of an adsorbent. Therefore, from a practical standpoint, it is important to characterize the rate-controlling steps associated with an adsorption process. More recently, our research group synthesized a pore-expanded MCM-41 (MCM-41) for CO 2 capture application. In the current work, kinetic measurements of pure CO 2 on MCM-41 at four different temperatures (30, 45, 60, and 75 °C) are performed under wide pressure regime (0.2–11) bar. The experimental data of CO 2 uptake as a function of time at different pressure and temperature conditions are analyzed by both pseudo-first order and the pseudo-second order kinetic models. The type of interaction between CO 2 and MCM-41 as well as adsorption rate behavior of CO 2 on MCM-41 is deduced from the best fitted model. To further investigate the mechanism of CO 2 adsorption on MCM-41, interparticle diffusion, intraparticle diffusion and Boyd’s models are also selected. The CO 2 diffusion mechanism in MCM-41 is elucidated based on the information derived from all the three models. Film diffusion mainly governed the rate-controlling process of CO 2 uptake on MCM-41 at low pressure and low temperature conditions. Interestingly, it is also interpreted that pore diffusion becomes more prominent at higher pressure and higher temperature conditions.

Application of high-pressure torsion to Al-6 %Cu-0.4 %Zr alloy for ultrafine-grain refinement and superplasticity

An Al-6 %Cu-0.4 %Zr alloy was processed by high-pressure torsion to produce an ultrafine-grained structure with a grain size of ~200 nm at the steady-state level where the hardness remains constant with straining. Tensile testing showed that a maximum elongation of ~530 % was attained at 673 K with an initial strain rate of 1 × 10−3 s−1. Evaluation of the strain-rate sensitivity and the activation energy for the deformation confirmed that grain boundary sliding through grain boundary diffusion is the rate-controlling process for the superplastic deformation.

Effect of Adsorption on Solute Dispersion: A Microscopic Stochastic Approach

We report on results obtained with a microscopic modeling approach to Taylor-Aris dispersion in a tube coupled with adsorption-desorption processes at its inner surface. The retention factor of an adsorbed solute is constructed by independent adjustment of the adsorption probability and mean adsorption sojourn time. The presented three-dimensional modeling approach can realize any microscopic model of the adsorption kinetics based on a distribution of adsorption sojourn times expressed in analytical or numerical form. We address the impact of retention factor, adsorption probability, and distribution function for adsorption sojourn times on solute dispersion depending on the average flow velocity. The approach is general and validated at all stages (no sorption; sorption with fast interfacial mass transfer; sorption with slow interfacial mass transfer) using available analytical results for transport in Poiseuille flow through simple geometries. Our results demonstrate that the distribution function for adsorption sojourn times is a key parameter affecting dispersion and show that models of advection-diffusion-sorption cannot describe mass transport without specifying microscopic details of the sorption process. In contrast to previous one-dimensional stochastic models, the presented simulation approach can be applied as well to study systems where diffusion is a rate-controlling process for adsorption.

Joint Transmission Rate Control and Opportunistic Network Coding Over Wireless Networks

Existing opportunistic network coding architectures (e.g., COPE) rely on pseudobroadcast to deliver a coded packet to multiple receivers in a single transmission. Only the primary receiver acknowledges the reception by MAC-layer acknowledgements (synchronous ACKs) and the other receivers receive the coded packet by overhearing and acknowledge the reception by asynchronous ACKs, which are usually piggybacked in outgoing data packets. In realistic wireless networks, this mechanism may cause unnecessary retransmissions if asynchronous ACKs are dropped due to packet losses or arrive late and thus compromise the throughput gain brought by network coding. In this paper, we propose a framework of joint rate control and code selection (ORC) to address this issue, aiming at improving the performance gain of opportunistic network coding in wireless networks. The framework of ORC consists of two mechanisms: (1) Rate control: the optimal transmission rate for coded packets is selected by formulating the rate control process as a Finite Horizon Markov Decision Process. (2) Code selection: based on the results of rate selection, the packet combination for forming the coded packet is determined. Numerical results show that ORC can substantially improve the performance gain of opportunistic network coding compared with COPE.

Selective Detection of Dopamine in the Presence of Uric Acid Using Polymerized Phthalo Blue Film Modified Carbon Paste Electrode

A convenient and useful method for the voltammetric determination of dopamine (DA) and uric acid (UA) based on poly (Phthalo blue) modified carbon paste electrode (PTBMCPE) is reported in this paper. The PTBMCPE exhibits excellent electro-catalytic activities for the oxidationreduction of DA and UA, as well as eliminating the interference. Factors influencing the detection processes are optimized and the kinetic parameters are calculated. The effects of pH, scan rate and concentration of dopamine on the peak current were investigated, and the results indicated that the peak current of dopamine is the highest in 0.2 M pH 7.0 Phosphate buffer solution (PBS) and the electrode reaction corresponds to a rate-controlled process. The proposed method possesses the distinct advantages of simple, appropriate for operation, good reproducibility and highly selective and sensitive.

Thermally activated processes of fatigue crack growth in steels

Fatigue crack growth rates in steels at high and low temperatures have been investigated using Paris curves. The fatigue crack growth rates at high temperatures are quite different from those at low temperatures. Arrhenius plots between fatigue crack growth rate (da/dN) and test temperatures at constant stress intensity factor range (ΔKI) indicate a difference of the rate-controlling process for fatigue crack growth with temperature. Slip deformation at the crack tip governs fatigue crack growth at high temperatures, while hydrogen diffusion is associated with crack growth at low temperatures.

Sorption behavior of nonylphenol (NP) on sewage-irrigated soil: Kinetic and thermodynamic studies

The reuse of wastewater for irrigation of agricultural land is a well established resources management practice but has the disadvantage of inputting various forms of contaminants into the terrestrial environment including nonylphenol (NP), a well known endocrine disrupting substance. To elucidate the environmental fate and transport of NP, the sorption behavior on sewage-irrigated soil was studied by batch experiment. It was found that sorption processes of NP on different sorbents (soil, humic acid (HA) and silica) could be expressed well using two compartment pseudo first-order model, where both surface and intra-particle diffusion were probable rate-controlling processes. Linear model could better express the sorption of NP on soil, black carbon (BC) and mineral (e.g., SiO2) except HA than Freundlich model. The large value of distribution coefficients of normalized organic carbon (Koc) on soils indicated that NP was limited to migrate to deep soil. The higher desorption partition coefficient of NP on soil showed enhanced hysteresis. According to the experimental data, the calculated thermodynamic parameters implied that the sorption reaction on sewage-irrigation was spontaneous, exothermic and entropy decreasing process. The amount of soil organic matter (SOM) dominated the sorption capacity, whereas the sorption behavior of NP on soil showed no significant correlation with ionic strength.

Dynamic Properties of Magnesium Alloys

With the rapidly expanding use of automobiles globally, the application of lighter structural materials in the form of magnesium alloys has become an urgent necessity to improve fuel efficiency and reduce the environmental impact. Within automobiles, magnesium alloys are used for components such as steering wheel frames and wheel rims, which are subjected to not only quasi-static but also dynamic strains. Indeed, catastrophic failure of a component by dynamic strain effects is a critical consideration, such as that of a wheel rim by braking at a high speed of 320 km/h. A recent consideration of magnesium for crash rail applications was rapidly brought to an end by the poor performance of initial crash test results. Dynamic strain rate testing is also important because of the emergence of magnesium alloys for armor with good ballistic properties. Just as with quasi-static strain, the mechanical behavior at dynamic strain rates depends on the microstructure. Deformation at dynamic strain rates produces more uniform distribution of dislocations and hinders the formation of discrete dislocation cells. Suppression of thermally activated dislocation processes occurs in this regime, which can lead to stresses high enough to activate deformation twins even in face-centered cubic (fcc) alloys with the high-stacking-fault energy. Among the fcc, body-centered cubic (bcc), and hexagonal closepacked (hcp) metals and alloys, high-purity fcc metals exhibit yielding that is nearly independent of the strain rate, while the strain hardening after yielding is strongly dependent on strain rate, which is due to the suppression of dynamic recovery process. In the case of bcc metals and alloys, yielding is strongly dependent on strain rate, while strain hardening is independent of the strain rate. For hcp metals, both the yielding and the strain hardening are dependent on strain rate. In hcp metals and alloys, twins play a very important role in deformation, especially for magnesium alloys in which nonbasal slips are not easily activated. The texture and c/a ratio greatly influence the deformation behavior of hcp metals. The effects of the polarity of twinning are similar to that at quasi-static tests. During in-plane testing of rolled plates with strong basal texture, in which the basal planes are parallel to the loading direction, it is observed that the twinning propensity increases with increasing strain rate. Through-thickness dynamic testing, in which the loading direction is perpendicular to the basal planes, reveals that the main deformation twinning mode, 1012 extension twinning, may not be activated. As a result, the work-hardening response in in-plane loading is more sensitive to strain rate than through-thickness loading. In this issue, we have four contributions on the dynamic behavior of magnesium alloys. The first is titled, ‘‘Dynamic Behavior of a Rare-Earth-Containing Mg Alloy WE43B-T5 Plate with Comparison to Conventional Alloy AM30-F’’ by S.R. Agnew, W. Wittington, A. Oppedal, H. El Kadiri, M. Shaeffer, K.T. Ramesh, J.J. Bhattacharyya, R. DeLorme, and B. Davis, which compares the dynamic behavior of two wrought alloys, a rolled WE43B alloy in peakaged condition, and an extruded AM30 alloy. The WE43B plate exhibited a relative isotropy of yield strength at quasi-static and dynamic strain rates, although it did show tension–compression strength asymmetry and anisotropy in strain hardening, which is consistent with the texture. The extruded AM30 alloy showed stronger anisotropy in strainhardening behavior related to the stronger texture in this material. At larger strain levels, the rate sensitivities along different directions converge in both the alloys. The authors also report that the yield strength is insensitive to strain rates along loading directions in which the rate-controlling process is 1012 type twinning, which is known to be relatively rate insensitive. The second contribution titled, ‘‘Dynamic Behavior of Some Magnesium Alloys’’ by E.S. Prasad, B. Li, N. Dixit, M. Shaefer, S.N. Mathaudhu, and K.T. Ramesh investigates the two important alloys, AZ31B and ZK60, at fine grain and ultrafine grain JOM, Vol. 66, No. 2, 2014

Mechanochemical approach to get layered double hydroxides: Mechanism explore on crystallite growth

In this paper, the mechanochemical approach, which includes solid state reactions and hydrothermal treatment, has been proposed to synthesize magnesium–aluminum–layered double hydroxides (Mg–Al–LDHs). Specially, the reaction process of solid state reactions has been explored, and it presents that crystallite growth is the rate-controlling process. The hydrothermal treatment is performed after solid state reactions, on one hand, the crystallinity and monodispersity of final LDHs particles can be improved, on the other hand, such treatment can tailor the particle size efficiently. Furthermore, the relationship between particle size and hydrothermal conditions (time and temperature) has been systematically investigated, which indicates that the particle size and monodispersity can be effectively controlled. The crystallite growth along a–b plane and c-axis has been emphatically discussed, and the results show that under relatively low temperatures such as 100°C, the gradual growth along c-axis has been found in the range of 48h, and high temperatures will hider its growth on the contrary. Crystal growth along a–b plane could be accelerated by higher hydrothermal temperature and longer treatment time. Our studies also show that during the hydrothermal treatment, such events as aggregation, disaggregation and particle growth, occur in series or in parallel with time. At last, the Mg–Al–CO3–LDHs samples (synthesized at 100°C for 24, 36 and 48h) which were acid activated by HCOOH were used to adsorb fluoride ions present in aqueous solution.

Atomic simulations of , twinning and detwinning in magnesium

and twinning and detwinning for Mg were investigated from the viewpoint of mobility of twinning dislocations and atomic shuffling. First-principles calculations suggested that the twinning dislocations glide more readily for the twinning than for the twinning. However, this conflicts with the experimental fact of easier twin formation. On the other hand, molecular dynamics simulations showed that the atomic shuffling was more activated for the twinning than for the twinning, which corresponds to the experimental fact. Therefore, it is suggested that the rate-controlling process for the twin formation is the atomic shuffling. Moreover, the calculations and simulations showed that the twinning dislocations glide more readily for the detwinning than for the twinning, whereas the atomic shuffling is less activated for the detwinning, suggesting that the detwinning occurs easily but is unstable, resulting in easy repetition of twinning–detwinning.

Study on thermal degradation kinetics of cellulose-graft-poly(l-lactic acid) by thermogravimetric analysis

Abstract The thermal degradation of cellulose- graft -poly( l -lactic acid) at different heating rates in nitrogen was studied by thermogravimetric analysis (TGA) in the temperature range 20–550 °C. Nuclear magnetic resonance (NMR), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC) analyses were utilized to determine the microstructure and glass transition temperature of cellulose- graft -poly( l -lactic acid). The kinetic parameters were determined by using Friedman, Flynn–Wall–Ozawa, and Kissinger methods. Coats–Redfern method was used to investigate the probable degradation mechanism. The results indicated that the trend of activation energy of cellulose- graft -poly( l -lactic acid) increases with increasing the content of the poly( l -lactic acid) and the process of the non-spontaneous degradation stage, goes to a random nucleation with two nuclei on the individual particle mechanism, whose rate-controlling process could be described by conversion rate ( α ) (integral form 1/(1 − α )).

Kinetics of formation and coarsening of nanoporous α-titanium dealloyed with Mg melt

The transition behavior from a Ti–Cu alloy to nanoporous α-Ti, the kinetics of the dealloying reaction, and the coarsening of nanoporous α-Ti were studied, and the mechanism of dealloying with a metallic melt was investigated. When a Ti50Cu50 alloy disk was immersed in a Mg melt, the Cu content decreased from the surface of the disk, and transition layers of α-Ti and Ti2Cu were formed. The growth of the transition layers followed a parabolic law and the activation energy was close to those reported for the diffusion of solute atoms in liquids, suggesting that diffusion of Cu in the Mg melt mainly contributed to the rate-controlling process for the dealloying reaction. The temperature and time dependences of the ligament size followed a power law with an exponent of 4, suggesting that surface diffusion plays an important role in coarsening. All of these results are consistent with those reported for dealloying in aqueous solution, indicating that the underlying dealloying mechanism in a metallic melt is similar to that in aqueous solution.

Towards a unified treatment of fully flashing sprays

This paper presents a systematic study on flashing atomisation, which includes both standards and retrograde fluids. A novel data reduction method is proposed in terms of the controlling parameters for (bubble) nucleation. The analysis indicates that bubble nucleation is the rate-controlling process for both the transition to fully flashing and for the spray lateral spreading. Specifically, the onset condition coincides with the surmount of the energy barrier to nucleation. The spray lateral spreading, instead, is directly linked to the population of bubble clusters: the larger the population the wider the spray angle. Theoretical aspects of bubble nucleation theory are also reviewed. An interesting conclusion of the analysis is that the experimental trends observed in fully flashing jets are compliant with recent advances in nucleation theory. At very high initial superheat, a complex shock wave structure appears around the flashing jets. The novel aspect of this work is that such shock-systems are observed consistently in both standard and retrograde substances. This similarity indirectly confirms that, far from the critical temperature, the phase transition mechanism is the same for all substances, independently from their degree of retrogradicity.

Kinetics of Reactive Diffusion in the (Pd-Cu)/Sn System at Solid-State Temperatures

The kinetics of solid-state reactive diffusion in the (Pd-Cu)/Sn system was experimentally observed to examine how adding Cu into Pd influenced the growth behavior of compounds at the junction between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating. In our experiment, Sn/(Pd-Cu)/Sn diffusion couples with Cu mole fractions (y) of 0.243, 0.497, and 0.735 were isothermally annealed at temperatures of 433 K to 473 K for various times up to 385 h. Annealing caused an intermetallic layer of rather uniform thickness to form between the Sn and Pd-Cu specimens in the diffusion couple. The intermetallic layer consisted of PdSn4, PdSn3, PdSn2, and Cu6Sn5 for y = 0. 243. However, PdSn3 and PdSn2 were not clearly detected for y = 0.497 or 0.735. The total thickness of the intermetallic layer was proportional to a power function of the annealing time, with the exponent of the power function being 0.33 to 0.53. Cases where the exponent was smaller than 0.5 indicated that boundary diffusion controlled the growth of the intermetallic layer. Cases where the exponent was equal to 0.5 implied that volume diffusion was the rate-controlling process. Even for exponents close to 0.5, however, the grain growth behavior of the intermetallic layer suggested that the layer growth was governed by boundary diffusion. For the annealing times tested in this work, the overall growth rate of the intermetallic layer was insensitive to the Cu concentration at y 0.75. As a consequence, the growth of compounds at the Pd junction in the multilayer Pd/Ni/Cu conductor was markedly decelerated by adding Cu when y > 0.75.

Solid state diffusion between Sn and Cu microcones on Cu microcones

Abstract Theoretical study of solid state diffusion mechanism in low-temperature bonding method based on Cu microcones is reported. A Sn layer was electrodeposited on Cu microcones as well as flat Cu substrate then annealed at bonding temperature 463 K. IMC grains of Sn/Cu-microcones diffusion samples grew with a relatively fine and uniform size. Cu 6 Sn 5 crystal orientation in Sn/Cu-microcones and Sn/flat-Cu diffusion samples were different according to X-ray diffusion patterns. Nucleation was more likely to occur near the pinpoints of the Cu microcones. The kinetic factors showed that the special morphology of Cu microcones was beneficial for the growth of intermetallic layer (Cu 6 Sn 5 ) in short annealing time and the grain boundary diffusion partially contributes to the rate-controlling process.

Accurate hydrodynamic models for the prediction of tracer diffusivities in supercritical carbon dioxide

The tracer diffusion coefficients, D12, are fundamental properties for the design and simulation of rate-controlled processes. Nowadays, under the scope of the biorefinery concept and strict environmental legislation, the D12 values are increasingly necessary for extractions, reactions, and chromatographic separations carried out at supercritical conditions, particularly using carbon dioxide. Hence, the main objective of this work is the development of accurate and simple models for the pure prediction of D12 values in supercritical CO2. Two modified Stokes–Einstein equations (mSE1 and mSE2) are proposed and validated using a large database comprehending extremely distinct molecules in terms of size, molecular weight, polarity and sphericity. The global deviations achieved by the mSE1 (Eqs. (2) and (13)) and mSE2 (Eqs. (5), (13), (3), (4)) models are only 6.38% and 6.75%, respectively, in contrast to the significant errors provided by well known predictive correlations available in the literature: Wilke–Chang, 12.17%; Tyn–Calus, 17.01%; Scheibel, 19.04%; Lusis–Ratcliff, 27.32%; Reddy–Doraiswamy, 79.34%; Lai–Tan, 25.82%. Furthermore, the minimum and maximum deviations achieved by the new models are much smaller than those of the reference equations adopted for comparison. In conclusion, our mSE1 and mSE2 models can be recommended for the prediction of tracer diffusivities in supercritical CO2.

Study on kinetics of hydrogen dissolution and hydrogen solubility in oxides using imaging plate technique

Using a tritium imaging plate technique, kinetics of tritium dissolution and its solubility in several oxides were examined. Mirror-polished single crystals of alumina, spinel and zirconia were used as specimens, which were exposed to 133Pa of a tritium(T)–deuterium(D) gas mixture (T/(T+D)∼0.17) at temperatures ranging from 673 to 973K for 1–5h. The T surface activity on the specimens increased with increasing temperature and exposure time, it almost saturated at 873K and reached 2×105Bq/cm2 (1×1014T/cm2), and no clear difference appeared among the types of specimens. The T activity in the oxide bulk also increased with temperature, in which there was a trend for the oxides: spinel≧zirconia≧alumina. In the T dissolution process for all oxides, the concentration gradient due to its diffusion was not observed even for short exposure times: the T density was almost uniform over the specimens in transition states and increased with exposure time up to the saturated value. These experimental results suggested that the rate-controlling process of T dissolution in the temperature region should be not its diffusion in the oxides but dissociation of hydrogen molecules (T–D mixture in this case) into atoms, its adsorption on the surface and/or T penetration from the surface into the bulk.

Creep behavior of a zirconium diboride–silicon carbide composite

Abstract Flexural creep studies of ZrB 2 –20 vol% SiC ultra-high temperature ceramic were conducted over the range of 1400–1820 °C in an argon shielded testing apparatus. A two decade increase in creep rate, between 1500 and 1600 °C, suggests a clear transition between two distinct creep mechanisms. Low temperature deformation (1400–1500 °C) is dominated by ZrB 2 grain or ZrB 2 –SiC interphase boundary and ZrB 2 lattice diffusion having an activation energy of 364 ± 93 kJ/mol and a stress exponent of unity. At high temperatures (>1600 °C) the rate-controlling processes include ZrB 2 –ZrB 2 and/or ZrB 2 –SiC boundary sliding with an activation energy of 639 ± 1 kJ/mol and stress exponents of 1.7 n n = 2.2. Microstructure observations show cavitation may partially accommodate grain boundary sliding, but of most significance, we find evidence of approximately 5% contribution to the accumulated creep strain.

Kinetics and modeling studies of the CaH2/LiBH4, MgH2/LiBH4, Ca(BH4)2 and Mg(BH4)2 systems

Borohydrides of alkali and alkaline-earth elements are of interest because of their high hydrogen holding capacities. However, their usefulness for hydrogen storage applications is often limited by high thermodynamic stabilities and slow kinetics. In this study comparisons have been made of the physical and chemical characteristics of Mg(BH4)2, Ca(BH4)2, and mixtures of LiBH4/MgH2 and LiBH4/CaH2. Temperature programmed desorption, TPD, analyses showed that samples containing Mg generally had lower onset temperatures than the corresponding samples containing Ca. Pressure Composition Temperature, PCT, isotherms showed that these materials displayed well defined two phase plateau regions. Therefore, kinetics measurements were done to determine the hydrogen desorption rates in the two phase plateau region. All kinetics determinations were done using constant pressure thermodynamic forces in which the ratios of the plateau pressure to the applied hydrogen pressure were kept the same for all samples. In all these measurements the pressure ratio was set at 3 and the temperature was 450°C. Under these conditions the reaction rates were in the order: Mg(BH4)2>Ca(BH4)2>LiBH4/MgH2>LiBH4/CaH2. An attempt was also made to determine the rate-controlling process in these samples by doing kinetic modeling. The results showed that diffusion was the rate-controlling process in Mg(BH4)2 whereas reaction at the phase boundary controlled reaction rates in Ca(BH4)2. In LiBH4/MgH2 and LiBH4/CaH2, reaction at the phase boundary controlled reaction rates initially whereas diffusion controlled rates in the latter stages.