关键词： Subcooled flow boiling   Bubble sliding   Wall heat flux partitioning model   Bubble dynamics  

摘要： This paper aimed to develop a mechanistic model for wall heat flux partitioning based on bubble dynamics in horizontal subcooled flow boiling. The model was established from two dimensions of space and time. In the space dimension, the heating surface was divided into three regions based on the motion characteristics of bubbles, including bubble nucleation region, bubble sliding region, and non-boiling region. In the time dimension, the heat transfer mechanism on the heating surface of the bubble nucleation region and bubble sliding region could be divided into evaporation, condensation, and transient heat transfer. An evaporation model for subcooled boiling was developed based on condensation at the top of the bubble. A transient quench heat transfer model for subcooled flow boiling was modified. The heat flux of each region was obtained by the sum of the time-averaged heat fluxes of these heat transfer mechanisms during a bubble periodic time. The total wall heat flux was obtained by the sum of the area weights of the heat fluxes in these three regions. The new wall heat flux partitioning model and the RPI model were used to predict the experimental values at a variety of flow conditions. The results showed that the new model could well predict the experimental values and the deviation between the predicted heat flux and the experimental results was within 25%. The study also found that it was necessary to consider the condensation at the top of the bubble and the heat transfer mechanism caused by bubble sliding. (C) 2021 Elsevier Ltd. All rights reserved.

关键词： Hydrogen charging kinetics   Mg-10 wt% Fe2O3   Rate-determining steps   Dissociative chemisorption   Hydrogen diffusion  

摘要： The rate-determining steps in the hydrogen charging of Mg-10Fe(2)O(3), prepared by mechanical milling in hydrogen (MgH2-forming mechanical milling), were analyzed by comparing dependences of hydrogen charging rate on hydrogen pressure and temperature with those of previously established theoretical rate equations. Hydrogen charging of the activated Mg-10Fe(2)O(3) can be classified into dissociative chemisorption of hydrogen molecules on particle surfaces, diffusion of hydrogen atoms through a thickening Mg hydride layer, and chemical reaction of Mg with hydrogen at the interface. The dependences of the initial hydrogen charging rates on hydrogen pressure (linear) and temperature (Arrhenius equation / T-1/2) showed that the rate-determining step of the hydrogen charging of Mg-10Fe(2)O(3) was the dissociative chemisorption of hydrogen molecules on the surfaces of Mg and/or Fe in the initial hydrogen charging stage. In the later stage, the diffusion of hydrogen atoms through the thickening Mg hydride layer determined the hydrogen charging rate of Mg-10Fe(2)O(3).

关键词： high-ordered nanotwins   rapid solidification   TiAl-based alloys   undercooling  

摘要： The strength and ductility trade-off dilemma has limited the wide application of TiAl-based alloys. Here, a new insight into the potential for increasing the strength and ductility of a hypoperitectic Ti-48 at% Al-8 at% Nb alloy is accomplished by the electromagnetic levitation (EML) technique. Moreover, a systematic analysis of the primary and subsequent peritectic solidification kinetics is conducted in the undercooling range of 308 K. Assisted by a high-speed camera, in situ observation of the liquid-solid (primary beta-Ti phase and peritectic alpha-Ti phase) interface migration is accomplished. When the alloy melt is undercooled to 240 K, high-ordered nanotwins are observed in the Ti-48 at% Al-8 at% Nb alloy, which form a unique hierarchical microstructure. Upon further increasing the undercooling, the density of these nanotwins is significantly enhanced. The room-temperature compression results reveal that the strength and ductility increase up to 140% and 150%, respectively. This is mainly ascribed to the remarkable grain refinement, formation of nanotwins with various orientations, accumulation of dislocations and stacking faults, and retention of the metastable gamma-phase. The superior combination of strength and ductility indicates the possibility to fabricate high-ordered nanotwins via rapid solidification, thus improving the performance of gamma-TiAl-based alloys.

关键词： Rubber   Carbon nanotubes   Functionalization   Mechanical properties   Tribological properties   Molecular dynamics  

摘要： In this research, the effects of the functional group type, degree of sidewall functionalization, and different amido-amines modification of carbon nanotubes (CNTs) on the mechanical and tribological properties of nitrile rubber (NBR)/CNT nanocomposites were comprehensively studied using molecular dynamics (MD) simulations. The Young's modulus, shear modulus, and bulk modulus of the nanocomposites were estimated by the constant strain method. Three-layer friction models were established to study the coefficient of friction and abrasion rate of CNT/NBR composites under a normal loading. It is found that among these different surface-modified CNTs (i. e., carboxyl, hydrazide, and ethylenediamine), the amide-functionalized CNT behaves the best reinforcing efficiency on the mechanical and tribological capacities of NBR nanocomposites due to the formation of strong dipole-dipole interactions, hydrogen bonding interactions, van der Waals (vdW) interactions, and good interfacial compatibility between CNTs and NBR matrices. Comparing the results obtained with different degree of sidewall functionalization, the vdW force between the amide-functionalized CNT and the NBR matrix plays a major role in determining the interfacial bonding strength. Meanwhile, higher degree of sidewall functionalization (>10%) will lower the enhancement effect by destroying the perfect structure of CNTs. Finally, the reinforcement mechanisms for the interfacial interactions between the functionalized CNTs and NBR matrices were elucidated from an atomic perspective.

关键词： Mortar   Alfa fibers   Morphology   Workability   Porosity   Mechanical strength   Hydration kinetics  

摘要： The Alfa plant is indigenous and spans a large zone throughout North Africa. Recently, Alfa plant has been used in building materials, notably as a reinforcement. This study aims to valorize the Alfa plant into fiber-reinforced mortar based on a cement-glass composite mix design. Two Alfa fiber morphologies are considered herein: different cutting lengths (10, 20 and 30 mm), and ground using a knife mill (shorter than 2 mm). These fibers were added to mortars at various volume ratios: 1, 2 and 3%. The workability and density of these distinct formulations were then measured in their fresh state. In comparison with the control mortar (CM), the introduction of Alfa fibers (either cut or ground), with ratios below 2% vol., leads to a slight decrease in mixture workability. However, Alfa fibers do not significantly alter the fresh material density. It was recorded a delay in setting time of the composites as a function of fiber addition rate. In the hardened state, an improvement in flexural strength is noticed for mortars reinforced with 1% vol. of fibers compared to the CM. However, results show that the increase in either type of fiber (cut or ground) generates a decrease in compressive strength compared to the CM. This outcome can be confirmed by both the decrease in heat released during the hydration reaction and water-accessible porosity inside the matrix. (c) 2021 Elsevier Ltd. All rights reserved.

关键词： Composite hydrogel   Cellulose nanofibrils   Sensitivity   Generator  

摘要： It is still a challenge to integrate high sensitivity, mechanical adaptability, and self-powered properties for hydrogels. Herein, we report a conductive polyvinyl alcohol (PVA) hydrogel based on natural nanoclay and cellulose nanofibrils (CNFs). The CNFs and PVA chains could construct a double network structure, resulting in a high mechanical composite hydrogel. Meanwhile, the nanoclay could be well dispersed and immobilized in the network of the hydrogel, thus improving mechanical adaptability of the hydrogel for curved and dynamic surfaces. Moreover, the conductive ions (Al3+) imparted the hydrogel with high conductivity (6.67 S m-1) and gauge factor (1.17). Therefore, the composite hydrogel exhibited high sensitivity to tiny pressure changes, enabling recognition of the complicated sounding and handwriting. More importantly, the composite hydrogel possessed self-powered property, which could generate an output voltage of up to 78 mV. In summary, the multifunctional composite hydrogel may have giant applications in artificial electronic skins or wearable devices.

关键词： Molecular dynamics   Aluminosilicate   Glass   Mechanical   Deformation  

摘要： The Al2O3.2SiO2 glass sample was constructed by using the molecular dynamics method. The structural characteristics were analyzed by the pair radial distribution function, bond angle distribution, coordination number, simplex analysis and local configuration entropy responsible for their mechanical behavior. The AS2 glass exhibits brittle behavior, at which most SiO4 units are maintained and the breakages of Al-O bonds cause the transition of AlOy units from higher- to lower-fold Al coordination. The big simplexes, with the radii >= 2.6 angstrom, tend to gather to form the clusters, where the large pores locate. Big simplexes are formed under the tensile deformation. Most atoms belonging to these big simplexes have the highest configurational entropy, which are considered as liquid-like atoms. These atoms play important roles in the evolution and coalescence of the big simplexes causing the crack propagation. Most of them are O atoms, which belong to the big simplexes.

关键词： Holy Basil leaf extract   Nanocomposite   Wastewater treatment   Phenanthrene   Kinetic studies  

摘要： In the present study, calcium oxide supported on activated carbon (CaO@AC) nanocomposite was synthesized using Basil leaf extract as a promoter and used to remove phenanthrene, an environmental pollutant, from aqueous solution. The activated carbon (AC) was prepared by the carbonization of Palm shells under pyrolytic conditions. The CaO@AC nanocomposite was characterized by FTIR, SEM-EDX, BET, and PXRD. The characterized CaO@AC nanocomposite was employed as an adsorbent for selective removal of phenanthrene from wastewater, maintaining the optimized conditions at initial phenanthrene concentration (5 mg/L), catalyst dosage (1 g), temperature (30 degrees C), and pH (7.6) for all batches. The adsorption isotherm and the kinetic studies for regression analysis were well fitted for the Freundlich model (R-2 = 0.9956) and non-linear Pseudo (II order) mechanism (R-2 = 0.9942). The results showed that the type IV linear form of pseudo-II order kinetic expression was inadequate for the kinetic rate parameters compared to the type I - III models. The CaO@AC was demonstrated as an inexpensive, scalable, recyclable, and eco-friendly adsorbent material for removing phenanthrene from wastewater. (C) 2021 Elsevier B.V. All rights reserved.

关键词： Bio-char   Re-heating   Heat transport   Shrinking   Compensation effect   Competitive steps  

摘要： This paper provides insights into understanding the physical and chemical changes that take place during reheating process of Poplar fluff bio-char derived by carbonization at high operating temperature (850 ?C). Biochar re-heating experiments were performed by simultaneous TGA-DTG-DSC techniques in N2 atmosphere at 5, 10, 15 and 20 ?C min- 1. Kinetic modeling of the process was done through implementation of model-free and model-based approaches. It was identified that re-heating process contains two stages. The first was described by single consecutive reaction step (up to 200 ?C) which is attributed to removal of physisorbed moisture at surface and in pores of carbonized material, while second was described by multiple reactions, including one consecutive step and two competitive steps. In the last, consecutive pathway includes reduction of PAH compounds (400?600 ?C) into smaller ones, changing degree of condensation of carbon structure continuously. Competitive steps encompass competition between C10H8 (naphthalene) (700?780 ?C) and C9H8 (indene) (700?750 ?C) molecules decomposition. It was found that used heating rates have great influence on regulating the amount of gaseous (combustible) products from these reactions. It was established that internal heat and mass transfer processes control the release of products. Changing the pyrolysis mechanism with temperature of carbonized biochar is the main factor for occurrence of true kinetic compensation effect, which is also discussed by applied isothermal life-time analysis.

关键词： Free energy  

摘要： Ab initio molecular dynamics was used to estimate the response to constant imposed strain on a short polyethylene (PE) chain and a radical chain with a removed hydrogen atom. Two independent types of simulations were run. In the first case, the chains were strained by expanding a periodic cell, restraining the length but allowing the internal degrees of freedom to reach equilibrium. From these simulations, the average force on the chain was computed, and the resulting force was integrated to determine the Helmholtz free energy for chain stretching. In the second set of simulations, chains were constrained to various lengths, while a bond was restrained at various bond lengths using umbrella sampling. This provided free energy of bond scission for various chain strains. The sum of the two free energy functions results in an approximation of the free energy of chain scission under various strains and gives a realistic and new picture of the effect of chain strain on bond breaking. Unimolecular scission rates for each chain type were examined as a function of chain strain. The scission rate for the radical chain is several orders of magnitude larger than that of the pristine chain at smaller strains and at equilibrium. This highlights the importance of radical formation in PE rupture and is consistent with experiments. Constant strain results were used to derive a constant-force model for the radical chain that demonstrates a roll over in rate similar to the "catch-bond" behavior observed in protein membrane detachment experiments. Published under an exclusive license by AIP Publishing.