Rate-controlling Process Research Articles

Understanding intermetallic compound growth at Ag/Zn interfaces: Kinetics and mechanisms

This study explores the growth behavior and rate-controlling processes of intermetallic compounds at Ag/Zn diffusion couple interfaces. Through experimental observations and quantitative analysis, we identify three distinct intermetallic compounds: β-AgZn, γ-Ag5Zn8, and ε-AgZn3. We analyze the kinetics of intermetallic compound growth to elucidate the mechanisms governing thickness growth in terms of interdiffusion and interface reactions. The rate-controlling process varies with temperature; volume diffusion controls layer growth at 573–593 K, while both volume diffusion and interface reaction contribute at 623–673 K. Additionally, we evaluate the activation enthalpies of parabolic coefficients, providing insights into the temperature dependence of growth kinetics and quantitation. This comprehensive analysis clarifies the intricate mechanisms underlying intermetallic compound formation, offering valuable implications for material performance and processing optimization in various applications.

Electric field/current enhanced grain growth behavior of 8mol% Y2O3 stabilized cubic ZrO2 (8Y-CSZ) polycrystals

The effect of the flash event on grain growth behavior was examined in 8 mol% yttria-stabilized cubic zirconia (8Y-CSZ) polycrystals under direct and alternating electric fields/currents. Even at the same specimen temperature, the rate of the grain growth is accelerated in the flash event than in the heat treatment without electric field/current (0 V), suggesting that the non-thermal effect caused additionally under the flash event contributes to the acceleration of the grain growth. Although the grain growth behavior can be characterized by the empirical equation (dtn –d0n = kt) with the same grain growth exponent of n = 3 irrespective of the electric field/current, the activation energy for the grain growth is lowered under the flash event. This suggests that the non-thermal effect caused by the flash event does not affect the mechanism of the grain growth, but would accelerate the rate-controlling process of the grain growth. The non-thermal effects can be further accelerated in fine-grained specimens under the alternating current flash and/or by increasing the input power density. Since the grain growth is rate-controlled by the grain boundary cation diffusivity, the non-thermal effect under the flash event would accelerate the grain boundary cation diffusivities through the formation of excess oxygen vacancies depending on the polarity and the power density.

Influence of carbon nanotubes on the morphology of Cu6Sn5 in Cu/(Sn–Ag–Cu) solder joints

This study delves into the intricate interaction between multi-walled carbon nanotubes (MWCNTs) and the Sn–Ag–Cu solder system, highlighting its relevance in lead-free soldering applications. Reflow and isothermal aging induce the formation of intermetallic layers at the joint interface, including Cu6Sn5, Cu3Sn, and irregularly shaped Ag3Sn particles. The addition of MWCNTs leads to the flattening of Cu6Sn5 grains, restraining their growth and potentially improving mechanical strength. Notably, MWCNTs exhibit a relatively high affinity with Ag3Sn. Grain size distribution analysis indicates that MWCNTs reduce particle size, effectively suppressing intermetallic compound growth. Over time, grain size increases due to simultaneous coarsening and growth during isothermal aging. In the growth kinetics analysis of intermetallic compounds in SAC305-MWCNTs solder joints, stable behavior is observed for Cu3Sn growth, driven by solid-state diffusion between Cu6Sn5 and Cu. In contrast, Cu6Sn5 exhibits significant variations in growth behavior with temperature changes. Furthermore, the evaluation of activation enthalpy for Cu3Sn growth in the Cu/(SAC305-MWCNTs) and Cu/SAC305 diffusion couples reveals similar n and k values of the power function for the layer growth, indicating a comparable rate-controlling process for Cu3Sn growth. The presence of MWCNTs in the solder does not significantly influence the rate-controlling process or activation enthalpy for Cu3Sn growth, contrasting with the observed effects on Cu6Sn5 growth. This study provides valuable insights into the interactions within the SAC305-MWCNTs solder system, offering significant implications for lead-free soldering applications.

Mechanical properties and rate-sensitive deformation of AA6063 aluminum alloys at 298 K, 78 K, and 4 K

The mechanical properties and rate-sensitive deformation of T4 and T6 AA6063 aluminum alloys have been analyzed at 298 K, 78 K, and 4 K. The strength, ductility, and work hardening capacity of two alloys increase with decreasing deformation temperature. Plastic deformation of AA6063-T4 is accompanied by adiabatic flow stress instabilities at 4 K and the Portevin-Le Chatelier (PLC) effect at 298 K. The alloy shows a better synergy of flow stress and work hardening than the T6 condition, determined by the underlying microstructure. The limited ability of dislocation storage in T6 alloy arises from shearing β″ particles and the enhanced dislocation annihilation occurring in pure Al matrix. The failure of the alloys is promoted by void nucleation at intermetallic particles before the Considére criterion is fulfilled. AA6063-T6 exhibits positive strain rate sensitivity (SRS) at three temperatures. Dislocations-β″ particle interactions determine its SRS and are a rate-controlling process at 298 K. AA6063-T4 shows negative strain rate sensitivity (nSRS) at 298 K controlled by dislocation-solute interactions. Data for SRS and thermodynamic deformation parameters characterizing different mechanisms in T4 and T6 alloys between 4 K and 298 K are discussed. The results indicate that plastic deformation produces vacancies by jogs' dragging at 298 K, but this process does not occur at 78 K and 4 K.

Camellia oleifera shell–reduced graphene oxide for adsorption of copper(II)

Copper pollution in aquatic environments poses a significant health challenge due to its non-biodegradable nature. This study introduces a novel and environmentally friendly approach to preparing Camellia oleifera shell extract reduced graphene oxide (COS-rGO) for the effective removal of Cu(II) from aqueous solutions. By using a 0.5 g/L dosage of COS-rGO at a pH of 5.1 and a temperature of 20 °C, an 85.8 % removal efficiency (18.7 mg/g) of Cu(II) was achieved at an initial concentration of 10 mg/L. Various characterization techniques, including Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), were employed to examine the synthesized COS-rGO. Additionally, thermodynamic and kinetic studies were conducted to investigate the mechanism of Cu(II) adsorption on COS-rGO. The results demonstrate that the adsorption of Cu(II) on COS-rGO follows pseudo-second-order kinetics, representing a spontaneous exothermic reaction. Our findings further support that the removal of Cu(II) by COS-rGO is driven by chemical adsorption in accordance with the rate-controlled process based on the Langmuir adsorption model. Overall, this study presents the green synthesis of COS-rGO as a promising technique for producing a cost-effective adsorbent from waste materials, which can be utilized for the mitigation of heavy metals in water.

Gallium and germanium static and kinetic sorption and desorption studies on cerium dioxide nanoparticles

Positron-emitting 68Ga has become an integral part of nuclear medicine. It is a radionuclide obtained from 68Ge/68Ga radionuclide generators. Apart from other qualities, it is a feature that have caused its growing popularity in medical practice. Nevertheless, the increasing demand for 68Ga supply results in the need of new and improved separation systems to obtain it. Hydrous oxides such as titanium or zirconium dioxide are usually the preferred separation materials, but other oxidic materials are under study. Thorough testing of the sorption behaviour is often overlooked in these studies, though it is indispensable for the applicability and suitability assessment. This work is focused on the sorption behaviour description of cerium dioxide prepared by calcination of ceric nitrate. Both batch and kinetic sorption experiments were conducted. Such a thorough study has never been carried out for this material. In batch experiments, the optimal conditions for separation of 68Ga and 68Ge were found. Kinetic experimental data were used for mathematical modelling. Six kinetic models derived from various rate-controlling processes were used and their applicability was demonstrated. The diffusion in an inert layer is the rate-controlling process of both gallium and germanium sorption and desorption. This model description of sorption kinetics will further enable us to optimise the sorption and desorption processes numerically. Thus far, it was found out that the prepared cerium dioxide can quantitatively adsorb germanium in very short time of 10 min from 1mM hydrochloric acid and the most convenient medium for 68Ge and 68Ga separation is 0.1 M hydrochloric acid.

Investigation of the rate-controlling process of intermetallic layer growth at the interface between ferrous metal and molten Al–Mg–Si alloy

In this study, we focused on unraveling the intricate world of intermetallic compound formation at the interface of solid Fe and liquid Al–Mg–Si alloy. Our primary aim was to shed light on the phenomenon of Fe degradation in molten Al alloys. Our investigation led us to discover two distinct intermetallic phases, namely Fe2Al5 and Fe4Al13, each exhibiting unique growth dynamics influenced by the interplay of volume diffusion and interface reactions. Moreover, delving deeper into the mechanics of these processes, we meticulously analyzed the rate-controlling mechanisms governing layer growth, exploring both volume diffusion and interface reactions. Furthermore, we quantified the activation enthalpy associated with these phenomena, enriching our understanding of the energy barriers involved in atom or molecule diffusion across solid-liquid interfaces. The implications of our findings extend far beyond the confines of the laboratory, holding significant promise for real-world applications. These insights into Fe degradation prevention and the enhancement of Al alloy performance when in contact with Fe materials offer exciting prospects for materials design and process optimization across diverse engineering and industrial domains.

In-situ formation of uniform Mg2Ni catalyst and hydrogen storage properties of Mg[sbnd]41Ni alloys with varied grain sizes and morphologies

In order to clarify the cyclic microstructure evolution process and the influence of grain size on the hydrogen de−/absorption properties of magnesium-based alloys. Mg-41 wt%Ni (Mg41Ni) alloys with varying grain sizes were prepared using different solidification rates. By analyzing the cross sections of ball-milled particles, it is found that the amorphous layer generated during ball milling is not conducive to the activation stage. Additionally, the coarse primary Mg2Ni and the lamellar Mg2Ni in the eutectic break into smaller Mg2Ni particles during the hydrogen absorption and release cycles. Thus, the fast solidified alloys can absorb 2.1 wt% of hydrogen gas at 125 °C, and complete decomposition can be achieved in 2.4 min at 300 °C. The activation energy of dehydrogenation decreases from 93.2 kJ/mol for slow solidification to 85.9 kJ/mol for fast solidification. Different kinetic models are used to fit and investigate the rate-controlling process of hydrogen desorption. The activated sample consists of the Mg matrix and uniformly distributed fine Mg2Ni particles due to the self-refinement, which significantly shortens the diffusion distance of H atoms. Besides, the multitude of pores generated inside the particles also facilitate the ingress and outflow of hydrogen.

Characterization of the micro-interfacial interactions of heterogeneous particulate matter (fine-grained sediment and microplastics) with copper ions

Particulate matter, such as sediment and microplastics (MPs), in aqueous environments has comparable micro-interfacial characteristics. However, the micro-interfacial characteristics of heterogeneous particulate matter are complex and are not fully understood. In this study, micro-interfacial kinetics and the interactions governing copper (Cu) ion adsorption to PP–sediment were investigated using micro-interfacial characterization experiments and kinetic experiments for Cu ion absorption to sediment, polypropylene (PP), and PP–sediment particles. The results show that (1) a pseudo-second-order model can be used to accurately describe the micro-interfacial kinetics of Cu ion adsorption to PP–sediment, indicating that chemical adsorption dominates the process. For Cu, the adsorption capacity, qe, of PP–sediment is approximately 0.4808 mg/g, between that of sediment and PP. (2) Film diffusion is the rate-controlling process in the early adsorption stage. Intraparticle diffusion gradually becomes the controlling process as the film diffusion constant (kfd) decreases and the film diffusion model becomes less applicable, but intraparticle diffusion is weaker in the PP–sediment system than in the sediment-only system. (3) The applicability of the intraparticle diffusion model for Cu ion adsorption to PP–sediment in the later stage of adsorption is between that of the sediment and PP systems. Furthermore, the heterogeneous aggregation of sediment and PP affects the micro-interfacial interactions of both types of particulate matter. The current study contributes to a better understanding of the micro-interfacial interactions of heterogeneous particulate matter (such as fine-grained sediment and MPs) with heavy metal ions in aqueous environments.

Microstructure evolution at the interface between Cu and eutectic Sn–Bi alloy with the addition of Ag or Ni

The study investigated the growth kinetics and rate-controlling processes of intermetallic layers formed at the interface between Cu and eutectic Sn–Bi alloys with Ag or Ni addition at solid-state temperatures. Isothermal sections of the equilibrium phase diagram were calculated to understand phase stability at the interface. The thicknesses of the intermetallic layers were plotted against annealing time, and a power function equation with an exponent slightly smaller than 0.25 described their relationship, indicating boundary diffusion with grain growth as the predominant control mechanism. The relationship between the exponent and annealing temperature suggested that the grain growth in the intermetallic layer followed an approximate parabolic law. The eutectic microstructure in Sn–57Bi–1Ag and Sn–57Bi–1Ni alloys changed with annealing, with differences in the Bi region width. However, the contact surface area between the Bi region and Cu6Sn5 compound was similar in both alloys. Activation enthalpies for the rate-controlling process of intermetallic layer growth were evaluated, providing insights into the energy barrier for atomic diffusion or transport across the interface.

Growth behavior of intermetallic layers at the interface between Cu and eutectic Sn–Bi by grain boundary diffusion with the grain growth at solid-state temperatures

The growth kinetics and rate-controlling processes of intermetallic layers formed in Cu/(Sn-58 wt% Bi) diffusion couples were investigated. Isothermal annealing of diffusion couples was conducted in the temperature range of 363–393 K for various times up to 384 h. In the diffusion couple, an intermetallic layer composed of Cu6Sn5 with irregular scallop shapes and Cu3Sn with relatively uniform thickness forms at the Cu/(Sn–Bi) interface. Isothermal sections of the equilibrium phase diagram in the ternary Bi–Cu–Sn system were calculated at the isothermal annealing temperatures and the peak temperature of the reflow process by a CALPHAD (calculation of phase diagrams) method. The diffusion path passes through the three-phase tie-triangle, including Cu6Sn5 and Cu3Sn. The obtained thicknesses were plotted against the annealing time, and a power function equation was found to describe their relationship. The values of the exponent in the power function were close to 0.25, indicating that the layer growth was predominantly controlled by boundary diffusion with the grain growth. Furthermore, the exponent values were relatively insensitive to the annealing temperature. The relationship between the exponent and the annealing temperature was analyzed, and it was concluded that the grain growth in the intermetallic layer followed a parabolic law. The growth and morphology evolution of the intermetallic compound and Sn–Bi eutectic microstructure at the interface were depicted, shedding light on the widening of the eutectic microstructure. Additionally, the activation enthalpy for the rate-controlling process of intermetallic layer growth at solid-state temperatures was evaluated using a least-squares method, providing insights into the energy barrier for the diffusion or transport of atoms across the interface. This study contributes to a better understanding of intermetallic layer formation and growth in Cu/(eutectic Sn–Bi) diffusion couples, offering valuable information for developing reliable solder joints and high-temperature solders.

Cyclic Solid-Liquid Phase Transformations in the text {CaO}–text {SiO}_2 System—Experiments and Modelling

The cyclic growth and shrinkage of solid oxides (i.e. wollastonite) in text {CaO}–text {SiO}_2-based slags is investigated in-situ by means of High-Temperature Confocal Scanning Laser Microscopy (HT-CSLM). The compositions of the slags are carefully selected to induce different phase transformation conditions, i.e. congruent and incongruent melting/solidification. To complement the experimental results, the kinetics of growth and shrinkage of oxide crystals is investigated by means of a sharp interface model where the interfacial reaction and diffusion in the liquid bulk are considered as possible rate-controlling processes. The modelling approach combined with data analysis from key experiments reveals the underlying dissipative processes of solidification and melting phenomena, here, diffusion in the liquid bulk material and/or the interfacial reactions. The modelling approach is likely to be applicable for future materials design and processing problems from this perspective.

Comparison of interfacial reactions and isothermal aging of cone Ni-P and flat Ni-P with Sn3.5Ag solders

Electroless Ni-P plating has good prospects for application as a diffusion barrier layer for interfacial reactions. In this study, the interfacial reaction between the microcone-structured Ni-3.5%P coating with high crystallinity and the Sn3.5Ag solder was investigated and compared with Sn3.5Ag/flat-Ni-(2%, 5%, 10%)P couples. Wettability experiments of Sn3.5Ag on four types of Ni-P coatings were carried out. Reflow at 250 °C for various minutes was performed, as well as isothermal aging experiments at 175 °C for various durations. The results showed that the Sn3.5Ag achieved the best wettability on cone Ni-3.5%P due to the pyramidal morphology that provides an additional driving force for wetting. During isothermal aging, the growth of intermetallics (IMCs) in the Sn3.5Ag/cone-Ni-3.5%P couple was attributed to the contribution of boundary diffusion mechanism to the rate control process, while that in the remaining three couples was a grain boundary diffusion-limited process. After reflow and isothermal aging, fewer microvoids and no harmful Ni-Sn-P layers were detected in the interface of Sn3.5Ag/cone-Ni-3.5%P couples, while they were detected in the other three couples, indicating that the cone Ni-3.5%P couple could inhibit the formation of Ni-Sn-P layer and microvoids. As a result, Sn3.5Ag/cone Ni-3.5% couples exhibited better shear resistance.

Extraction characteristics and radiation stability of the CHALMEX process for separation of Am(III) and Eu(III) at different temperatures

The behavior of the CHALMEX system comprising CyMe4-BTBP extractant in 30% tri-n-butyl phosphate and 70% phenyl trifluoromethyl sulfone diluents was characterized. The kinetic model based on mass transfer as the rate-controlling process was found to best describe the kinetics of Am(III) and Eu(III) extraction; the overall mass–transfer coefficients were determined. The dependence of distribution coefficients on temperature allowed change in free enthalpy and entropy of the system to be calculated. An investigation into the extraction properties of various CHALMEX systems combinations after irradiation with accelerated electrons up to an absorbed dose of 500 kGy at different temperatures up to 45 °C was carried out together with the characterisation of radiolytic products and their quantification using LC-MS and 1H NMR. The results obtained suggest that some of the radiolytic degradation products/adducts are efficient Am(III)/Eu(III) extractants, extracting Am and Eu by the ion-exchange mechanism. Adducts formed by hydroxylation or transfer of the CF3 moiety to the CyMe4-BTBP molecule are formed under irradiation together with products formed by elimination of a triazine ring or a cleavage of triazine ring(s). The results obtained now allow for a scheme of the main pathways of the CyMe4-BTBP molecule transformation under irradiation to be proposed.

Room-Temperature Strengthening, Portevin-Le Chatelier Effect, High-Temperature Tensile Deformation Behavior, and Constitutive Modeling in a Lightweight Mg-Gd-Al-Zn Alloy

To explore room-temperature strengthening and high-temperature ductility, a lightweight novel Mg-1.85Gd-0.64Al-0.62Zn alloy was fabricated by innovative multidirectional forging and a hot-rolling technique. Microstructures and mechanical properties were studied at room and elevated temperatures with an optical microscope, an X-ray diffractometer, and a tensile tester. An ultimate tensile strength of 260 MPa, yield strength of 171 MPa, and elongation of 20.4% were demonstrated at room temperature. The room-temperature strengthening mechanisms were identified by strengthening the model estimation. A type C Portevin-Le Chatelier effect was discovered and elucidated in this alloy. X-ray diffraction analysis revealed that the phase composition is α-Mg solid solution and (Mg, Al)3Gd, Al7Zn3, and Al2Gd intermetallic compounds. Examination of the microstructure at elevated temperatures revealed that dynamic recrystallization and dynamic grain growth occur. In particular, it was discovered that bimodal microstructures or incomplete dynamic recrystallization microstructures exist in high-temperature deformation. A maximum quasi-superplasticity of 228.4% was demonstrated in this alloy at 673 K and 5.0 × 10-4 s-1. Flow stress curves showed that the present alloy exhibits Sotoudeh-Bate curves or a long intermediate strain-hardening stage followed by a strain-softening stage. A modified Zerilli-Armstrong constitutive equation incorporating the number of dislocations was established. The power-law constitutive equation was established to identify the deformation mechanism. Both constitutive models had good predictability. At 673 K and 5.0 × 10-4 s-1, the stress exponent was 4, and the average deformation activation energy was 104.42 kJ/mol. The number of dislocations inside a grain was 146. This characteristic evidence confirmed that dislocation motion controlled by pipe diffusion dominates the rate-controlling process under this condition.

SUR-Driven Video Coding Rate Control for Jointly Optimizing Perceptual Quality and Buffer Control.

Rate control plays an important role in video coding and has attracted lots of attention from researchers. However, the problems of human visual experience and buffer stability still remain. For scenes with drastic motions, parts of distortions can be masked due to the limitation of the Human Visual System (HVS), while buffers tend to suffer more overflow and underflow cases from the fluctuating bits. In this paper, we propose a novel joint rate control scheme, which is composed of the proposed SUR-based perception modeling and the proposed SUR-based Perception-Buffer Rate Control (PBRC), for HEVC to maximize human visual perception quality while preventing the underflow and overflow of buffers. First of all, to effectively model human visual quality, we introduce the perception-related Satisfied-User-Ratio (SUR) metric into the rate control process. Secondly, a time-efficient video quality prediction method called Fast Visual Multimethod Assessment Fusion (VMAF) Quality Prediction (FVQP) is designed for the generation of SUR curves within an affordable computational complexity. Thirdly, a dual-objective optimization framework is established. By jointly conducting perception modeling and PBRC, we can flexibly adjust the optimization priority between human visual quality and buffer stability, and thus the quality of achieved reconstructed videos can be effectively improved because of the decrease in frame skipping. Experimental results demonstrate that the proposed joint rate control scheme improves the human visual experience when considering frame skipping and more effectively stabilizes buffer stability than existing methods.

Rate-Controlling Microplastic Processes during Plastic Flow in FCC Metals: Origin of the Variation of Strain Rate Sensitivity in Aluminum from 78 to 300 K

The thermodynamic response of dislocation intersections with forest dislocations and other deformation products is recorded using the Eyring rate relation wherein the application of shear stress increases the probability of activation at a given strain rate and temperature. The inverse activation volume, 1/ν, can be directly determined by instantaneous strain-rate change and its dependence on flow stress, τ, defines the strain-rate sensitivity, S, through the Haasen plot slope. A linear slope over a large strain interval is observed even for a heterogeneous distribution of obstacles that could be of more than one type of obstacles encountered by the gliding dislocation. It was deduced that ν and τ at each activation site are coordinated by the internal stress resulting in constant activation work (k/S). The stress changes from down-rate changes become larger than that from up-rate changes due to the formation of weaker obstacles, resulting in a composite S, whereas only forest dislocations are detected by the up-change. The additivity of 1/ν was used to separate obstacle species in specially prepared AA1100 and super-pure aluminum from 78 to 300 K. The deduction that repulsive intersection is the rate-controlling process and creates vacancies at each intersection site depending on temperature was validated by observing the pinning and depinning of dislocations via pipe diffusion above 125 K. A new method to separate S for dislocation-dislocation intersections from the intersections with other obstacles and their temperature dependence is presented and validated.

Novel phenylalanine-modified magnetic ferroferric oxide nanoparticles for ciprofloxacin removal from aqueous solution

The pollution of natural water bodies by pharmaceutical compounds has led to serious concerns regarding ecological and public health safety. In this study, novel recyclable phenylalanine (Phe)-modified magnetic ferroferric oxide nanoparticles (Fe3O4@Phe NPs) were successfully synthesized for the first time using a simple one-pot method to remove ciprofloxacin (CIP) from aqueous solutions. Fe3O4 and Fe3O4@Phe NPs were characterized using different techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), Turbiscan analysis and vibrating-sample magnetometry (VSM). The results show that Fe3O4 NPs are fully encapsulated by Phe, exhibiting an average diameter of 200 nm, a high specific surface area (35.79 m2 g-1), good dispersion and superparamagnetic properties. The effects of Phe content, initial pH and ionic strength on CIP adsorption onto Fe3O4 and Fe3O4@Phe NPs are investigated. The maximal adsorption capacity of CIP onto Fe3O4@Phe NPs is determined to be 49.27 mg g-1. The adsorption kinetics and isotherms show that the adsorption process follows the pseudo-second-order-kinetic and Langmuir isotherm models, respectively. This indicates that the adsorption involves a rate-controlled monolayer chemisorption process. The regeneration experiments show that Fe3O4 and Fe3O4@Phe NPs exhibit good reusability for CIP adsorption. Adsorption mechanisms include electrostatic interactions, hydrogen bonding, hydrophobicity and π-π interactions. This study presents a promising strategy for the design and preparation of multifunctional nanoparticles to remove contaminants from the environment.

Kinetic analysis of the reduction of hematite fines by cold hydrogen plasma

This study deals with the reduction of hematite fines by cold hydrogen plasma in the temperature range of 360–450 °C. It aims to examine the rate-controlling processes of fine hematite reduction by hydrogen plasma. The gaseous reduction was also performed for comparison. The reduction experiments were performed in a DC pulsed plasma reactor and the phases were characterized by X-ray diffraction. Reacted fraction curves were obtained by using the gravimetric technique. The kinetic analysis suggests that the controlling step of the hematite reduction process by cold hydrogen plasma was the nucleation and growth of iron on the magnetite substrate. The controlling step for the gaseous reduction process was found to be the chemical reaction at the magnetite–gas interface. The activation energy values obtained for cold plasma and gaseous hydrogen reduction processes were 68.5 and 87.2 kJ/mol.

Kinetic modelling and experimental validation of single large particle combustion of coal char

Understanding apparent kinetics of single large fuel particle combustion is of significance to the design and optimization of grate-firing and circulating fluidized bed boilers. Based on the concept of finite reaction zone approximation, a simple heterogeneous single particle model was formulated to consider the effects of external gas film, ash layer and chemical reaction simultaneously. To validate the proposed model and gain insight into the prevailing rate-controlling mechanism during the single particle combustion process at different combustion temperatures and particle sizes, the experiments on the combustion of coal char powder and single large char particles were carried out in a thermal gravimetric analyzer and a bench scale fixed-bed reactor, respectively. The intrinsic and apparent kinetics as well as the effective reacting zone thickness of single large particle combustion were quantified by combining theoretical analyses and experimental data. Both the bulk flow temperature and particle size have a remarkable influence on the global reactivity. The rate-controlling process was found to shift from the intrinsic chemical reaction to ash layer diffusion and return again to the intrinsic kinetics at the burnout stage. Particularly, an external effectiveness factor was defined as a function of conversion degree to better describe the ash diffusion effect on the apparent reactivity of large particles. The proposed model is physically general but simple enough to be incorporated into the computational fluid dynamic simulation of large-scale grate-firing and fluidized bed boilers.