Influence of Limited Efficiency and Competition of Vacancy Sinks/Sources on the Diffusion-Controlled Intermediate Phase Growth

2014 ◽  
Vol 2 ◽  
pp. 141-158 ◽  
Author(s):  
Andriy Gusak ◽  
Semen Kornienko ◽  
Nadiya Storozhuk ◽  
Tetyana Zaporozhets
Keyword(s):  
Intermediate Phase ◽  
Vacancy Concentration ◽  
Structural Evolution ◽  
Reactive Diffusion ◽  
Concentration Gradients ◽  
Phase Layer ◽  
Moving Interfaces ◽  
Diffusion Controlled ◽  
Nonequilibrium Vacancy ◽  
Isothermal Regime

Kinetics and structural evolution of the intermediate phase layer formation during reactive diffusion is revisited. Main new input is an account of limited efficiency of vacancy sinks/sources at the moving interfaces and in the bulk, leading to nonequilibrium vacancy concentration gradients. Competition of the two types of vacancy sinks/sources is studied – K-sinks (providing Kirkendall lattice shift) and F-sinks (Frenkel-Kirkendall voids formation). Reactive diffusion with limited vacancy sources/sinks power in isothermal regime as well as in adiabatic SHS-regime is considered.

Entropy of Nanostructures

2017 ◽  
pp. 600-614
Author(s):  
Ottorino Ori ◽  
Franco Cataldo ◽  
Mihai V. Putz
Keyword(s):  
Mechanical Properties ◽  
Free Energy ◽  
Gibbs Free Energy ◽  
Defect Formation ◽  
Vacancy Concentration ◽  
Structural Evolution ◽  
Point Of View ◽  
Structural Defects ◽  
Formation Mechanisms ◽  
Topological Invariants

Recent advances in graphene studies deal with the influence of structural defects on graphene chemical, electrical, magnetic and mechanical properties. Here the complex mechanisms leading to the formation of clusters of vacancies in 2D honeycomb HD lattices are described by a pure topological point of view, aiming to correlate the variation of specific topological invariants, sensible to vacancy concentration, to the structural evolution of the defective networks driven by the topo-thermodynamical Gibbs free energy. Interesting predictions on defect formation mechanisms add details on the topological mechanisms featured by the graphenic structures with defects. Future roles of bondonic particles in defective HD materials are also envisaged.

Model of Lateral Growth Stage during Reactive Phase Formation

2008 ◽  
Vol 277 ◽  
pp. 47-52 ◽  
Author(s):  
Mykola Pasichnyy ◽  
Andriy Gusak
Keyword(s):  
Phase Formation ◽  
Driving Force ◽  
Growth Process ◽  
Growth Stage ◽  
Intermediate Phase ◽  
Reactive Diffusion ◽  
Composition Gradient ◽  
Lateral Growth ◽  
Proposed Model ◽  
Asymmetric Case

Lateral growth of intermediate phase during reactive diffusion was analyzed. Proposed model is based on the assumption that the main driving force of the lateral growth process is the chemical one (proportional to composition gradient along the interface). Asymmetric case of phase formation taking into account the curvature of all three interfaces at the triple joint is considered.

The solid state dehydration of d lithium potassium tartrate monohydrate is complete in two rate processes I. The deceleratory diffusion-controlled first reaction

1994 ◽  
Vol 347 (1682) ◽  
pp. 139-156 ◽  
Keyword(s):  
Activation Energy ◽  
Vacancy Concentration ◽  
Nucleation And Growth ◽  
Crystalline Hydrate ◽  
Rate Equations ◽  
Mechanistic Study ◽  
Crystal Thickness ◽  
Potassium Tartrate ◽  
Diffusion Controlled ◽  
Rate Processes

A kinetic and mechanistic study of the dehydration of d lithium potassium tartrate monohydrate has been undertaken. Water evolution is completed through two separate rate processes. The first reaction is the deceleratory, diffusion-controlled release of water from the superficial zones of the reactant crystals. The yield of this process corresponds to the dehydration of a superficial layer of crystal, thickness 10 µm. About 4% of the constituent water was evolved from the single crystals studied, rising to 50% from crushed powder reactants. The second reaction, reported in Part II, is a nucleation and growth process yielding the crystalline anhydrous salt. Gravimetric measurements for the first reaction identified three distinct dehydration processes. The first step was the rapid release of loosely bonded superficial water. The subsequent two deceleratory stages are characterized as diffusive loss of H 2 O molecules from a crystal zone that is at first ordered but later becomes disordered as the water-site vacancy concentration increases. Rate measurements based on water evolution measured the activation energy of this third step as 153 + 4 kJ mol -1 . Irreproducibility of rate data is ascribed to variations in numbers and distributions of imperfections between individual crystals. The extent and rate of the first reaction increased when initiated in small pressures of water vapour. Electron microscope observations identified a structural discontinuity ca. 1 µm below reacted crystal faces, evidence of superficial retexturing of the reactant. Rates of powder dehydrations were more reproducible than those of crystals but the kinetic behaviour was similar. The same rate equations were obeyed and the activation energy was unaltered. Water loss during the first reaction of this crystalline hydrate gives a comprehensive layer of extensively dehydrated material across all surfaces. Subsequently, in or under this water depleted layer, salt is recrystallized and dehydration continues as a nucleation and growth reaction (part II, following paper).

scholarly journals Construction of K+ ions gradient in crystalline carbon nitride to accelerate charge separation for efficient visible light H2 production

2021 ◽  
Author(s):  
Guoqiang Zhang ◽  
Yangsen Xu ◽  
Chuan-Xin He ◽  
Yongliang Li ◽  
Xiangzhong Ren ◽  
...  
Keyword(s):  
Visible Light ◽  
Driving Force ◽  
Carbon Nitride ◽  
Photocatalytic Performance ◽  
H2 Production ◽  
H2 Evolution ◽  
Concentration Gradients ◽  
Diffusion Controlled ◽  
Activity Enhancement ◽  
First Time

Abstract Like most of the recent reported semiconductor photocatalysts, the sluggish dynamic charges transfer and separation caused by weak driving force still restricts the further improvement of photocatalytic performance in crystalline carbon nitride (CCN). Here, we successfully prepared a series of heptazine-based K+ implanted CCN (KCN) for the first time, where the K+ ions concentration was gradiently inserted through controlling its diffusion from the surface to bulk in carbon nitride (CN). As a powerful driving force, the built-in electric field (BIEF) induced by this concentration gradient, greatly accelerates the drift movement and the transport from bulk to the surface, as well as the separation of photogenerated carriers. Consequently, the KCN with optimized BIEF displays a ～34 times promotion than original CN for visible-light H2 evolution. Such a high activity enhancement factor is at a relatively good level in reported CCN. Our proposed strategy to induce BIEF production by constructing concentration gradients through thermodynamically feasible diffusion controlled solid-state reaction, can be adopted to build other efficient photocatalytic systems.

scholarly journals On the question of two-step nucleation in protein crystallization

2015 ◽  
Vol 179 ◽  
pp. 41-58 ◽  
Author(s):  
Andrea Sauter ◽  
Felix Roosen-Runge ◽  
Fajun Zhang ◽  
Gudrun Lotze ◽  
Artem Feoktystov ◽  
...  
Keyword(s):  
Real Time ◽  
Intermediate Phase ◽  
Early Stage ◽  
Protein Crystallization ◽  
Structural Evolution ◽  
Equation Model ◽  
Nucleation Mechanism ◽  
Crystal Nucleation ◽  
Time Study ◽  
Nucleation Kinetics

We report a real-time study on protein crystallization in the presence of multivalent salts using small angle X-ray scattering (SAXS) and optical microscopy, focusing particularly on the nucleation mechanism as well as on the role of the metastable intermediate phase (MIP). Using bovine beta-lactoglobulin as a model system in the presence of the divalent salt CdCl2, we have monitored the early stage of crystallization kinetics which demonstrates a two-step nucleation mechanism: protein aggregates form a MIP, which is followed by the nucleation of crystals within the MIP. Here we focus on characterizing and tuning the structure of the MIP using salt and the related effects on the two-step nucleation kinetics. The results suggest that increasing the salt concentration near the transition zonepseudo-c** enhances the energy barrier for both MIPs and crystal nucleation, leading to slow growth. The structural evolution of the MIP and its effect on subsequent nucleation is discussed based on the growth kinetics. The observed kinetics can be well described, using a rate-equation model based on a clear physical two-step picture. This real-time study not only provides evidence for a two-step nucleation process for protein crystallization, but also elucidates the role and the structural signature of the MIPs in the nonclassical process of protein crystallization.

Periodic Pattern Formation in Metal-Ceramic Reactions

2006 ◽  
Vol 46 ◽  
pp. 136-145 ◽  
Author(s):  
A.A. Kodentsov ◽  
F.J.J. van Loo
Keyword(s):  
Pattern Formation ◽  
Reaction Zone ◽  
Diffusion Zone ◽  
Kirkendall Effect ◽  
Periodic Pattern ◽  
Time And Space ◽  
Phase Layer ◽  
Diffusion Controlled ◽  
Metal Ceramic ◽  
Patterned Microstructure

Formation of diffusion zone morphologies periodic in time and space during metalceramic reactions is considered as a manifestation of the Kirkendall effect. In a diffusion-controlled interaction, the Kirkendall marker plane can bifurcate, which is attributed to diverging vacancies fluxes in the reaction zone. When the Kirkendall plane is present in a phase layer, it attracts in situproduced inclusions of “secondary-formed phase”, which, in turn, can result in a highly patterned microstructure.

Thermosolutal Convection in an Inclined Rectangular Enclosure with a Partition

2004 ◽  
Vol 20 (3) ◽  
pp. 233-239 ◽  
Author(s):  
L. W. Wang ◽  
Y. C. Kung ◽  
C. Y. Wu ◽  
M. F. Kang ◽  
S. L. Wang
Keyword(s):  
Transfer Rate ◽  
Mass Transfer Rate ◽  
Electrode Reaction ◽  
Partition Ratio ◽  
Thermosolutal Convection ◽  
Limiting Current ◽  
Working Fluid ◽  
Concentration Gradients ◽  
Rectangular Enclosure ◽  
Diffusion Controlled

AbstractAn experimental study of thermosolutal convection in an inclined rectangular enclosure with a partition is presented in this article. Aspect ratio, partition ratio, and inclination angle were kept constant at Ar =0.5, Ap = 0.25 and φ = 30°, respectively. The convective flow is generated by both inclined temperature and concentration gradients under limiting current condition. Both the thermal and solutal buoyancies, which either cooperated or opposed one another, were induced from the copper plates. The temperature gradient was maintained and controlled using two separate constant temperature baths that circulated heated or cooled water through a heat exchanger. We used copper sulphate-sulfuric acid solution as both the working fluid and the electrolyte. An electrochemical method based on a diffusion-controlled electrode reaction was employed to create the concentration gradient. We used the shadowgraph recording technique to visualize and analyze the flow field phenomenon. Thermal Grashof numbers ranging from 8.16 × 105 to 16.32 × 105 and a solutal Grashof number Grm = 4.36 × 106 were investigated. It is demonstrated that the mass transfer rate increases with the increasing thermal Grashof numbers within our experimental ranges. Multilayer structures are found in the cooperating case or the opposing case.

Formation kinetics of nanocrystalline Fe–4 wt.% Al solid solution during ball milling

1997 ◽  
Vol 12 (6) ◽  
pp. 1429-1432 ◽  
Author(s):  
H. G. Jiang ◽  
R. J. Perez ◽  
M. L. Lau ◽  
E. J. Lavernia
Keyword(s):  
Solid Solution ◽  
Ball Milling ◽  
Structural Evolution ◽  
Milling Process ◽  
Avrami Equation ◽  
Scanning Calorimetry ◽  
Diffusion Controlled ◽  
Interphase Boundaries ◽  
Kinetics Of Formation ◽  
Kinetics Of

Formation of nanocrystalline Fe–4 wt.% Al solid solution has been achieved through SPEX ball milling of blended elemental Fe and Al powders. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) have been employed to follow the structural evolution during the ball-milling process. Exothermic peaks exhibited in DSC diagrams of the powders milled for 10 to 60 min yielded thermal enthalpies corresponding to the formation of Fe–4 wt.% Al solid solution, from which the kinetics of formation were found to follow the Johnson–Mehl–Avrami equation. Assessment of the kinetic parameter n reveals a diffusion controlled mechanism, in which grain and interphase boundaries may play a crucial role, during the solid solution formation of Fe–4 wt.% Al.

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