Layer Diffusion Research Articles

Granulation of Bismuth Oxide by Alginate for Efficient Removal of Iodide in Water.

The granulation of bismuth oxide (BO) by alginate (Alg) and the iodide adsorption efficacy of Alg–BO for different initial iodide concentrations and contact time values were examined. The optimal conditions for Alg–BO granulation were identified by controlling the weight ratio between Alg and BO. According to the batch iodide adsorption experiment, the Alg:BO weight ratio of 1:20 was appropriate, as it yielded a uniform spherical shape. According to iodide adsorption isotherm experiments and isotherm model fitting, the maximum sorption capacity (qm) was calculated to be 111.8 mg/g based on the Langmuir isotherm, and this value did not plateau even at an initial iodide concentration of 1000 mg/L. Furthermore, iodide adsorption by Alg–BO occurred as monolayer adsorption by the chemical interaction and precipitation between bismuth and iodide, followed by physical multilayer adsorption at a very high concentration of iodide in solution. The iodide adsorption over time was fitted using the intraparticle diffusion model. The results indicated that iodide adsorption was proceeded by boundary layer diffusion during 480 min and reached the plateau from 1440 min to 5760 min by intraparticle diffusion. According to the images obtained using cross-section scanning electron microscopy assisted by energy-dispersive spectroscopy, the adsorbed iodide interacted with the BO in Alg–BO through Bi–O–I complexation. This research shows that Alg–BO is a promising iodide adsorbent owing to its high adsorption capacity, stability, convenience, and ability to prevent secondary pollution.

(Digital Presentation) Evaluating the Performance of Multi-Walled Carbon Nanotube Composite Microporous Layers Deposited on Carbon Felt Gas Diffusion Layers

Electrochemical energy storage devices (EESDs) such as metal-air batteries (MABs) and alkaline electrolyte membrane fuel cells (AEMFCs) have begun to gain vast amounts of attention due to their high theoretical energy densities and open-circuit voltage values. Although these devices present promising aspects for the environment and sustainability, they suffer from the slow reaction kinetics and degradation effects that occur at the catalyst layer interface of the air electrode. Typically, these effects are due to the accumulation of molecular species such as water or oxygen at catalyst active sites because of poor electrode hydrophobicity and fluid management. Herein, we perform experimentation to mitigate these effects by implementing mixed carbon composite microporous layers onto porous carbon felt based air electrodes. This adaptation of the air cathode was predominantly based on the aforementioned downfalls and the lack of availability of commercial carbon felts that have adequate wet proofing and conductivity like some of the related carbon paper gas diffusion layers. Microporous layers (MPLs) enhance electrical conductivity, surface area, and adhesion at the catalyst layer and gas diffusion layer interface. These layers typically consist of a hydrophobic polymer such as polytetrafluoroethylene (PTFE) and a conductive carbon black substrate such as Ketjen Black 600JD (KJB). The incorporation of multi-walled carbon nanotube (MWCNT) and carbon black composites into the structure of the MPLs drastically decreases ohmic polarization with minimal increases in specific resistance. The electrical resistance of the synthesized carbon substrate decreases at high rates of MWCNT dispersion, which corresponds directly to a low MWCNT weight percentage within the composite. The MPL increases the mass transport through the electrode by mitigating flooding effects that typically occur within the pores near the surface of the catalyst layer, ultimately aiding in charge transfer between the gas diffusion layer and catalyst active sites. Preliminary data from AEMFC symmetric cell tests suggests that a microporous layer consisting of a 10 wt.% MWCNT/KJB (m/m) substrate is the optimal loading of carbon nanotubes and leads to better cell performance. There is an increase in current density of 60 mA/cm2 compared to an air electrode with no MPL within the ohmic region of the polarization curve corresponding to approximately 0.7 V, while only an increase of 18 mA/cm2 is seen for an MPL consisting of a 30 wt.% MWCNT/KJB (m/m) carbon substrate. The increase of charge transfer by the 10 wt.% synthesized carbon substrate MPL was attributed to the ability of the nanotubes at high rates of dispersion to act as molecular wires within the electrode. Additionally, zinc-air battery tests using the optimally configured air electrode, determined from previous experiments, show that the current density at an operating voltage of 1.1 V was increased by 2.3 times that of an electrode without an MPL using the same catalyst material and loading. Significantly, this research will lead to a better understanding of optimization of the air electrode used within electrochemical devices and how to eliminate some of the adverse effects associated with it. Figure 1

Novel oxymagnesite/green rust nanohybrids for selective removal and slow release of phosphate in water

The new paradigm in wastewater treatment demands to change traditional pollutants removal into resource recovery, especially for non-renewable P resources, effectively recovering phosphate from wastewater and reutilizing it as a nutrient is crucial to P sustainable utilization and P-related pollution control. The nanomaterial-based adsorption technology for P recovery from wastewater is becoming a research hotspot due to its high efficiency and selectivity. Herein, to recover aqueous phosphate, we developed novel oxymagnesite/green rust (OMGR) nanohybrids by a one-pot hydrothermal method. Green rust nanoparticles dispersed on the highly reactive oxymagnesite (MgO2MgCO3) nanosheets could achieve efficient recovery and reuse of P. The volume ratio of water to ethylene glycol played an important role in the preparation of OMGR. The OMGR possessed an excellent selectivity of phosphate removal in the presence of multi-anions and wide pH adaptability in 4.0–10.0. The formation of MgP nanocrystals and the inner-sphere FeOP complexes via ligand exchange contributed to the selective removal of P by OMGR, and the removal capacity reached 141 mg P.g−1. The process of phosphate removal by OMGR was spontaneously endothermic and controlled by the intraparticle and boundary layer diffusion. Most importantly, the high bioavailable P (127 mg.g−1) of P-loaded OMGR had a persistent release behavior regulated by dissolution and diffusion, indicating that the P-loaded OMGR can act as a slow-release P-fertilizer. The findings provide a green and eco-friendly approach to realizing P resource recovery and reuse for phosphate-containing wastewaters.

Fast arsenate As(V) adsorption and removal from water using aluminium Al(III) fixed on Kapok fibres

Arsenic (As) is among the most dangerous metalloids and is harmful to human wellbeing. In this laboratory study, Al(III)-modified kapok fibres (Al-Kapok) were used to remove As(V) from water. The sorbent was characterised using Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX). Batch experiments were performed to observe the performance of Al-Kapok in the removal of As(V) and to examine the effects of pH, temperature, adsorbent dose, and coexisting ions on the adsorption process. The surface of the sorbent changed after aluminium modification, and the results of the batch experiments showed that the adsorption of As(V) occurred mainly via endothermic-spontaneous chemisorption at the solution and solid interface of Al-Kapok. The As(V) removal efficiency was approximately 76%–84%, and it was slightly affected at pH levels below 8.0. Further study showed that the maximum adsorption capacity of Al-Kapok for As(V) was 118 μg/g at 30 °C and pH 6, and notable adverse effects were caused by the presence of SO42−and PO43−. It was also found that the boundary layer and film diffusion contributed more to As(V) adsorption. After five adsorption/desorption cycles, regeneration recovered approximately 92% of the adsorption capacity of Al-Kapok used. Overall, Al-Kapok appears to be a suitable adsorbent material for the purification of As-contaminated water.

Development of a predictive kinetic model with statistically analyzed parameters for Donnan Dialysis process

Donnan Dialysis (DD) utilizes ion exchange membranes that allow the selective transport of target ions from a feed solution to a concentrated receiver solution. The kinetics of DD involves boundary layer diffusion (BLD), diffusional membrane transport (MD), and a mixed regime where both mechanisms are active. The objective of the research was to develop an automated computational model that can predict nitrate, bicarbonate, and sulfate concentration profiles and transport mechanisms in DD processes. Experiments were conducted in a batch dialyzer in sole- and multi-component feed solutions and NaCl receiver solutions.Training of the model included numerical extraction of the BLD and MD coefficients from the experimental data; determination of the statistically significant parameters that affect the DD transport; development of correlations between these parameters and the kinetic coefficients; and mapping of the ratio between the boundary layer resistance and the membrane resistance to determine the transport mechanism.The predictive model, included the kinetic equations, i.e., the Nernst-Planck formulation, mass balances, and electro-neutrality, the correlations, and the mapping of the transport mechanism. The model was verified by the experimental data used for the model training and validated by independent datasets. Very good fits between the predicted and experimental concentration profiles were obtained, and the ability to determine the transport mechanism was demonstrated.

Polyvinyl Chloride Microplastics Leach Phthalates into the Aquatic Environment over Decades.

Phthalic acid esters (phthalates) have been detected everywhere in the environment, but data on leaching kinetics and the governing mass transfer process into aqueous systems remain largely unknown. In this study, we experimentally determined time-dependent leaching curves for three phthalates di(2-ethylhexyl) phthalate, di(2-ethylhexyl) terephthalate, and diisononyl phthalate from polyvinyl chloride (PVC) microplastics and thereby enabled a better understanding of their leaching kinetics. This is essential for exposure assessment and to predict microplastic-bound environmental concentrations of phthalates. Leaching curves were analyzed using models for intraparticle diffusion (IPD) and aqueous boundary layer diffusion (ABLD). We show that ABLD is the governing diffusion process for the continuous leaching of phthalates because phthalates are very hydrophobic (partitioning coefficients between PVC and water log KPVC/W were higher than 8.6), slowing down the diffusion through the ABL. Also, the diffusion coefficient in the polymer DPVC is relatively high (∼8 × 10–14 m2 s–1) and thus enhances IPD. Desorption half-lives of the studied PVC microplastics are greater than 500 years but can be strongly influenced by environmental factors. By combining leaching experiments and modeling, our results reveal that PVC microplastics are a long-term source of phthalates in the environment.

Magnetized vermiculite loaded with waste polishing powder to recover phosphorus in waste water

In view of the phosphorus pollution in water bodies and the shortage of phosphate rock resources, the preparation of cheap, accessible and easy-to-recycle adsorbents for the recovery of phosphorus in sewage has attracted much attention. In this study, waste rare earth polishing powder (WP) was doped with magnetized vermiculite (Mev) at a mass ratio of 1:1 through acid leaching and advection precipitation method to prepare the magnetic Mev-WP composite material for adsorbing and recovering phosphorus in the water. The results show that Mev-WP is easy to be separated from water by an external magnetic field and the main reason why Mev-WP specificity can adsorb phosphorus is that the rare earth elements of La and Ce in the polishing powder are loaded on the flake magnetized vermiculite in the form of La (OH)3 and Ce(OH)3. The phosphorus adsorption process of Mev-WP is an endothermic mono-layer chemical adsorption that includes boundary layer diffusion and internal diffusion, of which the boundary layer diffusion is the rate-limiting step, which can complete rapid adsorption within 30 min. Except for HCO3− ions, other coexisting anions in the water have no significant effect on adsorption, but the adsorption capacity is greatly affected by pH. At pH 11 to 3, the material is a stable and environmentally-friendly adsorbent, and with the decrease of pH, the adsorption capacity increases significantly. When the pH is 3, at 25 °C, the adsorption capacity can reach up to 63.79 mg/g. The phosphorus adsorption mechanism of Mev-WP is electrostatic attraction and ligand exchange between -OH on La-OH and Ce-OH and phosphate groups in the water to generate La-O-P and Ce-O-P coordination complexes to remove phosphorus. After 5 times desorption and recycling, the phosphorus recovery rate of the material reaches 65 %, and the deep phosphorus removal verification in urban sewage is effective. Studies have shown that Mev-WP is a promising specific phosphorus recovery agent.

Performance and mechanism of CO2 absorption during the simultaneous removal of SO2 and NOx by wet scrubbing process

The co-removal of CO2 while removing SO2 and NOx from industrial flue gas has great potential of carbon emission reduction but related research is lacking. In this study, a wet scrubbing process with various urea solutions for desulfurization and denitrification was explored for the possibility of CO2 absorption. The results showed that the urea-additive solutions were efficient for NOx and SO2 abatement, but delivered < 10% CO2 absorption efficiency. The addition of Ca(OH)2 dramatically enhanced the CO2 absorption, remained the desulfurization efficiency, unfortunately restricted the denitrification efficiency. Among various operating parameters, pH of solution played a determining role during the absorption. The contradictory pH demands of CO2 absorption and denitrification were observed and discussed in detail. A higher pH of solution than 10 was favorable for CO2 absorption, while the oxidizing of NO to NO2, NO2− or NO3− by NaClO2 was inhibited in this condition. When 7 < pH < 10, it was favorable for the conversion and absorption of NO and NOx. However, the conversion of HCO3− to CO32− was significantly inhibited, hence preventing the absorption of CO2. Large part of Ca(OH)2 became CaCO3 with a finer particle size, which covered the unreacted Ca(OH)2 surface after the reaction. Kinetic analysis showed that the CO2 absorption in urea-NaClO2-Ca(OH)2 absorbent was controlled by chemical reaction in early stage, then by ash layer diffusion in later stage.

Interfacial microstructure and strengthening mechanism of stainless steel/carbon steel laminated composite fabricated by liquid-solid bonding and hot rolling

Stainless steel/carbon steel (SS/CS) laminated composites were prepared by liquid-solid (L-S) bonding followed by hot rolling under different reduction rates, and the microstructure characteristics at the cladding interface zone and their influences on the interfacial bonding strength of the SS/CS laminated composites were analyzed. A metallurgical bonding interface was obtained by the L-S bonding method, and coarse equiaxed grains, decarburization layer and oxides formed at the cladding interface zone. During hot rolling, heterodeformation occurred in the SS/CS laminated composites, resulting in asynchronous dynamic recrystallization and the refinement of grains in both SS and CS. The thickness of the decarburization layer and elements diffusion distance obviously decreased after hot rolling, while the ratio of the diffusion distances of Cr and Ni elements to the width of the decarburization layer increased with increasing reduction rates. Additionally, the oxides at the cladding interface zone transformed into hybrid inclusions after hot rolling, and the maximum equivalent diameter and volume fraction of the inclusions decreased with increasing reduction rates. The interfacial shear strength of the L&R bonded SS/CS laminated composites increased with increasing reduction rates, and a maximum value of 400 MPa was obtained at a reduction rate of 75%, which is 34% higher than that of the L-S bonded SS/CS laminated composite due to the grain refinement, improved relative diffusion distance of Cr and Ni elements, reduced and refined inclusions at the cladding interface zone.

Thermodynamics, Kinetics and Isotherms Studies for Sorption of Direct and Disperse Dyes onto Eco-friendly Pre-treated Cellulose Acetate Fabric using Ultraviolet Irradiation

Introduction: Owing to the restoration of hydroxyl groups, cellulose acetate fibers can be dyed with direct dyes. There are some drawbacks in the conventional deacetylation process of cellulose acetate from environmental point of view. Methods: This process involves high temperature, alkalinity and large volume of effluent. The goal of this work is to improve the dyeing properties of cellulose acetate fabric using an eco-friendly treatment process. In this paper, cellulose acetate fabric was treated with ultraviolet light (UVB) at an air pressure of 1 atm to improve dyeability. Then, the untreated and UV treated fabrics were dyed with direct and disperse dyes. UV treated cellulose acetate fabric showed higher dye adsorption compare to that of untreated cellulose acetate fabric. Five adsorption isotherm models including sold solution, Langmuir, Freundlich, Temkin and BET were applied to determine the adsorption behavior. At all temperatures studied, experimental data were better fitted with the Freundlich and Nernst models for direct and disperse dyes respectively. Thermodynamic parameters such as change in free energy (ΔG0), the enthalpy (ΔH0), and the entropy (ΔS0) were also evaluated. Results: The calculated thermodynamic values showed that the adsorption of these dyes onto the cellulose acetate fabric was a physical adsorption process and endothermic in nature. These data also implied that the adsorption of direct dye onto cellulose acetate fabric was spontaneous at the experimental temperature range and adsorption of disperse dyes can be spontaneous at higher temperatures. Moreover, the ΔG0 values for the adsorption of disperse dyes onto the UV-treated fabrics were less than those for untreated fabrics suggesting that UV treated fabrics require less external energy. Conclusion: Among the kinetic models studied, it was found that the pseudo second-order kinetic model was the best model to describe the dye sorption process on the UV treated and untreated cellulose acetate fabrics. The UV treatment led to an improvement in the boundary layer diffusion effect.

Cost-Effective and High Purity Valuable Metals Extraction from Water Leaching Solid Residues Obtained as a By-Product from Processing the Egyptian Boiler Ash

The water leaching solid residues (WLSR) obtained from salt-roasting Egyptian boiler ash are considered an essential secondary resource for (13%) nickel and (5.6%) zinc extraction. Hence, the current study aims for the cost-effective and high purity Ni, Zn, Fe and Mg metal ion extraction from (WLSR) using a sulfuric acid leaching process. The factors affecting the percentage recovery of Ni, Zn, Fe and Mg from WLSR, including leaching temperature, time, acid concentration and solid/liquid ratio, have been investigated. The obtained leaching solutions were analyzed chemically using ICP, and the different precipitates were analyzed mineralogically using XRD and EDX analysis and chemically using XRF. The maximum percentage recovery of Ni, Zn, Fe and Mg was 95.02%, 90.13%, 66.29% and 75.73%, which was obtained under the optimum leaching conditions of 8% H2SO4 concentration and 1/15 solid/liquid ratio at 85 °C for 240 min. The effect of pH, Fe2O3 dosage as nucleating agent and the precipitation duration on iron removal and Ni and Zn loss have been thoroughly studied. It has been found that &gt;95% of the contained iron impurity can be removed, while nickel and zinc losses are around 4.2% and 3.8%, respectively. Additionally, a pH of 6 and 0.45 mol/L concentration of H2C2O4 was utilized to precipitate Mg as MgC2O4.2H2O, demonstrating that the precipitation efficiency of Mg reaches 96.9%. Nickel and zinc precipitation efficiency was 92.25% and 85.51%, respectively, by raising the solution pH to approximately 9. The kinetic of Ni and Zn dissolution has been investigated to explain the mechanism prevalent and the factors influencing the leaching process. It has been found that the nickel leaching kinetic is controlled by both diffusion through an inert porous layer and by chemical reaction with an activation energy of 20.25 kJ.mol−1. Meanwhile, the kinetic of zinc leaching is controlled by solid product layer diffusion with an activation energy of 11.67 kJ mol−1.

Thermodynamics as a Driving Factor of LiCoO2 Grain Growth on Nanocrystalline Ta-LLZO Thin Films for All-Solid-State Batteries.

All-solid-state batteries primarily focus on macrocrystalline solid electrolyte/cathode interfaces, and little is explored on the growth and stability of nanograined Li-garnet and cathode ones. In this work, a thin (∼500 nm) film of LiCoO2 (LCO) has been grown on top of the polycrystalline layer of Ta-doped Li7La3Zr2O12 (Ta-LLZO) solid electrolyte using the pulsed laser deposition (PLD) technique. Scanning transmission electron microscopy, electron diffraction, and electron tomography demonstrated that the LCO film is formed by columnar elements with the shape of inverted cones. The film appears to be highly textured, with the (003) LCO crystal planes parallel to the LCO/Ta-LLZO interface and with internal pores shaped by the {104} and {102} planes. According to density functional theory (DFT) calculations, this specific microstructure is governed by a competition between free energies of the corresponding crystal planes, which in turn depends on the oxygen and lithium chemical potentials during the deposition, indicating that thermodynamics plays an important role in the resulting LCO microstructure even under nonequilibrium PLD conditions. Based on the thermodynamic estimates, the experimental conditions within the LCO stability domain are proposed for the preferential {104} LCO orientation, which is considered favorable for enhanced Li diffusion in the positive electrode layers of all-solid-state batteries.

Investigating Comproportionation in Multielectron Transfers via UV-Visible Spectroelectrochemistry: The Electroreduction of Anthraquinone-2-sulfonate in Aqueous Media.

UV-vis spectroelectrochemistry is assessed as a tool for the diagnosis and quantitative in situ investigation of the incidence of comproportionation in multielectron transfer processes. Thus, the sensitivity of the limiting current chronoabsorptometric signals related to the different redox states to the comproportionation kinetics is studied theoretically for different working modes (normal and parallel light beam arrangements) and mass transport regimes (from semi-infinite to thin layer diffusion). The theoretical results are applied to the spectroelectrochemical study of the two-electron reduction of the anthraquinone-2-sulfonate in alkaline aqueous solution, tuning the thermodynamic favorability of the comproportionation reaction through the electrolyte cation. The quantitative analysis of the experimental results reveals the occurrence of comproportionation in the three media examined, showing different kinetics depending on the cationic species in solution.

Effective Extraction of the Al Element from Secondary Aluminum Dross Using a Combined Dry Pressing and Alkaline Roasting Process.

Secondary aluminum dross (SAD) is a hazardous solid waste discharged from aluminum electrolysis and processing and the secondary aluminum industries, which causes severe environmental pollution and public health disasters. The stable presence of the α-Al2O3 and MgAl2O4 phases in SAD makes it difficult for it to be efficiently utilized. A combined dry pressing and alkaline roasting process was proposed for extracting the valuable Al element from SAD. Two alkaline additives (NaOH and Na2CO3) were selected as a sodium source for extracting the aluminum source from SAD in order to perform the thermodynamic analysis and roasting experiments. The phase transition behavior and the leaching performance tests were conducted using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, leaching kinetics and thermal analysis. The recovery of Al and Na reached the values of 90.79% and 92.03%, respectively, under the optimal conditions (roasting temperature of 1150 °C, Na2CO3/Al2O3 molar ratio of 1.3, roasting time of 1 h, leaching temperature of 90 °C, L/S ratio of 10 mL·g−1 and leaching time of 30 min). Meanwhile, the removal efficiency of N and Cl reached 98.93% and 97.14%, respectively. The leaching kinetics indicated that the dissolution of NaAlO2 clinkers was a first-order reaction and controlled by layer diffusion process. The green detoxification and effective extraction of the Al element from SAD were simultaneously achieved without any pretreatments.

Hydrolysis Behavior and Kinetics of AlN in Aluminum Dross during the Hydrometallurgical Process.

In this study, the hydrolysis behavior and kinetics of AlN in aluminum dross (AD) were investigated in order to better identify the steps controlling the AlN hydrolysis reaction and the factors influencing the hydrolysis rate to enhance the removal efficiency of AlN. The hydrolysis behavior of AlN, including AlN content, phase composition, chemical composition, microstructure, and element distribution, was determined by a leaching test, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and energy dispersive spectroscopy, respectively. The results showed that increasing the leaching liquid–solid ratio as well as the temperature was helpful for the removal efficiency of AlN. When the liquid-–solid ratio was 4:1, temperature was 90 °C, and leaching time was 300 min, the removal efficiency of AlN reached 89.05%. The kinetics were described using the unreacted core model, and when the temperature was 30–40, 50–70, and 80–90 °C, the hydrolysis reaction rate of AlN was controlled by boundary layer diffusion, chemical reaction control, and product layer diffusion control, respectively.

Generalized expression for the self-normalized signal in photothermal experiments for multilayered materials in the frequency domain

The analytical solution for diffusion of heat for a four-layered material, in a surface absorption model, is used to describe a simple diagram-based construction methodology for obtaining the analytical solution for a general photothermal signal in the transmission and front photothermal configurations. The general expressions derived can be used for analysis during the application of self-normalized photothermal experimental techniques to thermal diffusivity measurements in the frequency domain. The thermally thick limit of these equations can be used to obtain an effective thermal diffusivity expression for multilayered systems that can be used to retrieve the thermal diffusivities of buried layers. If such a system consists of alternating layers of two different materials, as is the case for some heterostructures and superlattices, it is possible to obtain the thermal impedances at the interfaces of the consecutive layers. Although the resulting self-normalized expressions are complex, their thermally thick limit reduces them to linear equations for the photothermal phase and amplitude (amplitude in semi-log scale). This simplifies the analysis required to obtain quantitative results.

Laser additive manufacturing of a 316L/CuSn10 multimaterial coaxial nozzle to alleviate spattering adhesion and burning effect in directed energy deposition

Multimaterial additive manufacturing is aimed at the integration of functionality and mechanical performance of dissimilar materials for various applications. In this work, a 316L/CuSn10 multimaterial coaxial nozzle was designed and additively manufactured by laser powder bed fusion (LPBF) for alleviating spattering adhesion and burning effect in directed energy deposition. The applicability of the 316L/CuSn10 nozzle was then validated in the laser-based directed energy deposition (L-DED) process. The interfacial diffusion zone of the 316L/CuSn10 structure was measured with a length of ~400 μm, indicating a well bonding between 316L and CuSn10. A high laser power (P = 230 W) and a low scanning speed (v = 300 mm/s) adopted for printing CuSn10 on 316L could lead to “keyhole mode”, resulting in a large penetration depth of the pre-solidified layer and wide diffusion zone at the interface. The spattering behavior of the 316L/CuSn10 nozzle applied in the L-DED process showed that the heterogeneity of dissimilar materials could reduce the adverse effect of the adhesion of spattering droplets. High laser reflection and high thermal conductivity of CuSn10 could relieve the burning effect of the reflected laser beam on the tip of the 316L/CuSn10 nozzle. Therefore, the utilization of CuSn10 on the nozzle tip could improve its service life. These findings may serve as a new way for low-cost and rapid mass production of the coaxial nozzles that can be applied in the L-DED technique.

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.

Characteristics of adsorption behavior of potentially toxic metals by biochar derived from fallen leaves (Platanus) and its mechanism

In this study, we aimed to evaluate the properties and mechanisms of heavy metal adsorption by biochar derived from fallen leaves (FLB). The optimum manufacturing temperature of FLB for heavy metal removal was 650 °C, which resulted in a high carbon content and specific surface area. The maximum adsorption capacities of Cd and Pb by FLB650 were 86.2 and 135.1 mg/g, respectively; their adsorption was achieved via chemisorption. The results from chemisorption agreed with the Langmuir isotherm and pseudo-second order models. We confirmed that the adsorption of Pb by FBL was dominantly affected by the surface boundary layer; however, the adsorption of Cd occurred in cooperation with the boundary layer and pore diffusion; which was accomplished by employing the intraparticle diffusion model. After heavy metal adsorption, the distribution characteristics and mechanisms of each heavy metal in the FLB were noticeably observed through SEM-EDS, FTIR, and XRD. In addition, the content of exchangeable cations released from the FLB during the heavy metal adsorption process corresponded to 52% for Cd and 34% for Pb of the total amount of adsorbed heavy metals. We determined that FLB exhibits high affinity for heavy metals and its adsorption occurs via an adsorption mechanism based on the properties of FLB.

Oxygen-Deficient (Ln,Sr)2NiO4- δ Nickelates for Oxygen Electrodes of Solid Oxide Fuel and Electrolysis Cells: Anisotropic Thermochemical Expansion and Thermomechanical Constraints

Perovskite-like and perovskite-related oxides based on variable-valence transition metals may exhibit substantial changes in oxygen content in the course of temperature cycling or under variation of oxygen chemical potential at elevated temperatures. This gives rise to a chemical contribution to the thermal expansion (chemical expansion/contraction or chemical strain) originating mainly from the accompanying changes in the average oxidation state of transition metal cations and, consequently, their ionic radii. Excessive dimensional changes caused by the chemical expansion impose constraints on the applicability of functional oxides at elevated temperatures and under variable redox conditions (due to thermomechanical instability or incompatibility with other materials) and complicate the processing of oxide ceramics at high temperatures. Another unfavorable phenomenon is microcracking caused by a strongly anisotropic expansion of crystal lattice, with effects on the microstructural and mechanical integrity and electrical and electrochemical properties. The present work is a representative study case that focuses on layered Ln2NiO4-based ceramic materials for oxygen electrodes of solid oxide fuel/electrolysis cells (SOFC/SOEC).Ln2NiO4+δ–based oxides (Ln = La, Pr, Nd) with perovskite-related K2NiF4-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity and are considered, therefore, as attractive mixed ionic-electronic conductors for oxygen electrodes of high-temperature SOFC/SOEC [1]. Acceptor-type substitution of Ln3+ by Sr2+ in Ln2NiO4+ δ is compensated by comparatively high oxygen deficiency in Sr-rich Ln2-xSrxNiO4-δ phases at elevated temperatures [2,3]. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change in the ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers.Ceramic powders of Ln2-xSrxNiO4-δ (Ln = La, Nd, Pr, x = 1.0-1.6) were prepared by the Pechini method with final calcination steps at 1150-1200°C in oxygen. Thermogravimetric studies confirmed that acceptor-type substitution by strontium is compensated by the formation of oxygen vacancies and electron-holes and progressively increases high-temperature oxygen nonstoichiometry (Fig.1A), which reaches as high as δ = 0.36-0.40 for x = 1.6 at 900°C in air. High-temperature XRD studies demonstrated that Ln2-xSrxNiO4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating (Fig.1B). Highly anisotropic expansion induces microcracking phenomenon, which, in turn, results in a strong hysteresis in dimensional changes (Fig.1C) and also in variations of electrical conductivity (Fig.1D) on cycling in air for the ceramic samples sintered at 1200-1300°C.An appropriate approach to minimize or neglect the undesirable effects of anisotropic expansion and microcracking is to preserve the grain size in Ln2-xSrxNiO4-δ ceramics or porous electrodes at ~1 micron or submicron level. In particular, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce mechanically stable ceramics with smooth reversible thermochemical expansion and enhanced electrical properties. Acknowledgments: This work was supported by the FCT/MCTES/FEDER (projects CARBOSTEAM (POCI-01-0145-FEDER-032295) and CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), and PhD grant SFRH/BD/138773/2018).