Reaction Front Research Articles

Photochemical Reaction Front Propagation and Control in Molecular Crystals.

4-Fluoro-9-anthracenecarboxylic acid (4F-9AC) undergoes a reversible [4 + 4] photodimerization reaction that can generate bending and twisting in microcrystals. Larger crystals resist photomechanical deformation, permitting direct visualization of the [4 + 4] photodimerization reaction dynamics under a variety of conditions. Both birefringence and fluorescence imaging show that the photodimerization reaction starts preferentially at a crystal edge and then sweeps across the crystal as a propagating reaction front. To explain these results, a theory is developed that postulates an exponentially decaying mechanical reaction field that extends from product domain into reactant domains to catalyze the photochemical reaction. Analyzing the data with this theory, we estimate a reaction field penetration distance of ∼20 nm for the 4F-9AC crystal. The propagation velocity can be controlled by varying the excitation intensity or by embedding amorphous barrier regions into the crystal using electron beam lithography. These barriers can channel the reaction flow to select areas of the crystal while other regions remain unreacted.

Effect of Microstructure on Chemo-Mechanical Damage Evolution in Aluminum Foil Anodes for Lithium-Ion Batteries

Aluminum-based foil anodes could enable lithium-ion batteries with high energy density comparable to silicon and lithium metal. However, mechanical pulverization and lithium trapping within aluminum tend to cause capacity fading. The complex interplay between these damage modes is not well understood, as well as the role of microstructure on reaction front evolution. Here, we investigate aluminum foils with different compositions and processing conditions to understand how microstructure influences chemo-mechanical degradation. Backscattered electron imaging of ion-milled foil cross sections is used to visualize reaction front evolution and chemo-mechanical degradation. We find that the shape of the reaction front during lithiation is strongly affected by foil composition, defect distribution, and grain size, which, in turn, affects the evolution of chemo-mechanical damage within the foils. Furthermore, a key finding is that the extent of fracture upon delithiation is inversely related to lithium trapping, with foil compositions that exhibit uniform lithiation, and therefore, less fracturing showing greater lithium trapping. Nonreactive precipitate inclusions within alloy foils are also found to induce void formation. This work shows that material design strategies designed to overcome the inverse fracture-lithium trapping relationship could be useful for improving performance. Published by the American Physical Society 2024

Effects of Anisotropy, Convection, and Relaxation on Nonlinear Reaction-Diffusion Systems

The effects of relaxation, convection, and anisotropy on a two-dimensional, two-equation system of nonlinearly coupled, second-order hyperbolic, advection–reaction–diffusion equations are studied numerically by means of a three-time-level linearized finite difference method. The formulation utilizes a frame-indifferent constitutive equation for the heat and mass diffusion fluxes, taking into account the tensorial character of the thermal diffusivity of heat and mass diffusion. This approach results in a large system of linear algebraic equations at each time level. It is shown that the effects of relaxation are small although they may be noticeable initially if the relaxation times are smaller than the characteristic residence, diffusion, and reaction times. It is also shown that the anisotropy associated with one of the dependent variables does not have an important role in the reaction wave dynamics, whereas the anisotropy of the other dependent variable results in transitions from spiral waves to either large or small curvature reaction fronts. Convection is found to play an important role in the reaction front dynamics depending on the vortex circulation and radius and the anisotropy of the two dependent variables. For clockwise-rotating vortices of large diameter, patterns similar to those observed in planar mixing layers have been found for anisotropic diffusion tensors.

The Kinetics of Polymer Brush Growth in the Frame of the Reaction Diffusion Front Formalism.

We studied the properties of a reaction front that forms in irreversible reaction-diffusion systems with concentration-dependent diffusivities during the synthesis of polymer brushes. A coarse-grained model of the polymerization process during the formation of polymer brushes was designed and investigated for this purpose. In this model, a certain amount of initiator was placed on an impenetrable surface, and the "grafted from" procedure of polymerization was carried out. The system consisted of monomer molecules and growing chains. The obtained brush consisted of linear chains embedded in nodes of a face-centered cubic lattice with excluded volume interactions only. The simulations were carried out for high rafting densities of 0.1, 0.3, and 0.6 and for reaction probabilities of 0.02, 0.002, and 0.0002. Simulations were performed by means of the Monte Carlo method while employing the Dynamic Lattice Liquid model. Some universal behavior was found, i.e., irrespective of reaction rate and grafting density, the width of the reaction front as well as the height of the front show for long times the same scaling with respect to time. During the formation of the polymer layer despite the observed difference in dispersion of chain lengths for different grafting densities and reaction rates at a given layer height, the quality of the polymer layer does not seem to depend on these parameters.

Numerical design in a two-step joint design process of parallel packed bed reactors for CLC of MSW-generated syngas

This paper presents a two-step joint design process for the effective and rational design of the parallel packed bed reactors for chemical looping combustion, fueled by the syngas from municipal solid waste gasification. It integrates phenomenological and numerical design, combining their advantages of low computational cost (the cost is reduced to 5–10 % compared with alone numerical design) and high accuracy. Based on it, the phenomenological design schemes of the 1.5 MWth reactor are firstly obtained, then these schemes are systematically validated and optimized through numerical design. In detail, comprehensive analyses of the distribution and evolution of bed parameters, which are affected by the propagation of the reaction and heat front, are performed, along with an assessment of the reactor’s operational stability. Additionally, the heat/mass regulation strategies for the unit are studied, offering insights into effective heat management. Finally, the design schemes are optimized to achieve a better performance and flexible operational compatibilities (fitting for 2/3 parallel reactors, cocurrent/countercurrent gas feeding mode, inlet gas temperature of 400–600 °C)

Experimental constraints on serpentinite carbonation in the presence of a H2O–CO2–NaCl fluid

Serpentinite carbonation contributes to the deep carbon (C) cycle. Recently, geophysical and numerical studies have inferred considerable hydrothermal alteration in plate bending faults, opening the possibility of significant C storage in the slab mantle. However, there is a lack of quantitative determination of C uptake in serpentinized mantle rocks. Here, we experimentally constrain serpentinite carbonation in H2O–CO2–NaCl fluids to estimate C uptake in hydrated mantle rocks. We find that serpentinite carbonation results in the formation of talc and magnesite along the serpentinite surface. The presence of porous reaction zones (49.2% porosity) promotes the progress of carbonation reactions through a continuous supply of CO2-bearing fluids to the reaction front. Added NaCl effectively decreases the serpentinite carbonation efficiency, particularly at low salinities (< 5.0 wt%), which is likely attributed to the reduction in fluid pH and the transport rate of reactants, and the increase in magnesite solubility. Based on previous and our experiments, we fit an empirical equation for the reaction rate of serpentinite carbonation. Extrapolation of this equation to depths of plate bending fault systems suggests that serpentinite carbonation may contribute to an influx of up to 7.3–28.5 Mt C/yr in subduction zones. Our results provide new insights into serpentinite carbonation in environments with high fluid salinities and potentially contribute to the understanding of the C cycle in subduction zones.

Numerical simulation of detonation propagation and extinction in two-phase gas-droplet ammonia fuel

Numerical simulations of detonation propagation and extinction in ammonia droplet-laden premixed ammonia–oxygen gas are performed in a 2-D planar channel. The sustained propagation and ultimate quenching of detonation with perturbations from ammonia droplets are observed under lower and larger values of initial droplet number density (Nd,0) and diameter (d0), respectively. The detonation always quenches under d0=15μm. The detonation propagation/extinction behaviour and cell structure are dependent on both d0 and Nd,0. The positive correlations between droplet volume fraction, inter-phase mass, momentum, and energy transfers and d0 are more nonlinear than their counterparts of Nd,0. The post-Mach stem region experiences higher-intensity detonative combustion and thus droplet evaporative, accelerative, and heating effects than the post-incident wave region. During the detonation extinction process, the detonation wave degenerates into detonative spots which then decouple into shock and reaction fronts; gaseous pressure, heat release rate, and nitric oxide volume fraction peaks decline.

Explosion risks: Variety of deflagration-to-detonation transition scenarios in smooth tubes

In the framework of comprehensive assessment of explosion risks on board of spacecrafts and on the facilities of launch places, the paper is focused on the detailed analysis of particular scenarios of deflagration-to-detonation transition taking place in smooth tubes filled with acetylene-oxygen mixtures of different compositions. By means of precise numerical simulation it is demonstrated that various scenarios of detonation onset can take place depending on the mixture composition and its initial thermodynamic state. It is demonstrated that independent on the particular scenario always the basic mechanism of detonation onset via the formation of strong enough shock wave takes place. In more reactive mixtures the strong shock originates from the self-sustained process of joint pressure build up and reaction intensification exactly at the flame front. In less reactive mixtures the transient flow behavior leads to the shock waves generation and interaction. As a result, a brand new reaction kernel could arise in the area of shock waves interaction. In number of cases, that leads to the coupling between the shock wave and the newborn reaction front and results in the strong shock formation and further detonation onset.

Correlative electron microscopy of discontinuous precipitation

This paper presents a comprehensive study on discontinuous precipitation (DP) in Al-22 at.% Zn alloy, leveraging the correlative and synergic effects of advanced microscopy and modelling techniques. The study is structured into three pivotal steps: in-situ scanning electron microscopy combined with an energy backscattered diffraction analysis for real-time monitoring and an analysis of DP reaction kinetics and grain boundary misorientations; transmission electron microscopy and an energy X-ray dispersive microanalysis for detailed observation of solute-depleted α and solute-rich β lamellae, offering insights into the kinetics of the reaction front (RF) and solute concentration profiles; and a sophisticated modelling approach addressing the limitations of direct observation methods by simulating solute concentration changes within αphase lamellae during RF go-and-stop cycles. This multifaceted approach elucidates the nuances of DP mechanisms, from the nucleation and growth of precipitates to the estimation of instantaneous RF velocity and the impact of grain boundary misorientations. The integration of these methodologies enhances our understanding of DP kinetics and morphology at both meso- and nanoscales, highlighting the significant role of correlative microscopy in metallurgical research. The findings not only deepen our grasp of the fundamental mechanisms driving DP reactions but also provide a valuable comparison framework for experimental data, potentially guiding the development and optimization of metallurgical processes.

The process optimization and exergy efficiency analysis for biogas to renewable hydrogen by chemical looping technology

The study on biogas-Chemical Looping Hydrogen Production (biogas-CLHP) highlights its potential to produce high-purity hydrogen with recycling waste bioenergy. It delves into the gas-solid reaction characteristics and optimizes crucial process parameters for the fixed-bed CLHP system. Movement rate equations for the reaction fronts of Fe2O3-Fe3O4 (Rf1), Fe3O4-FeO (Rf2), and FeO-Fe (Rf3) are formulated, aiding in clarifying CO breakthrough curves. The identified convergence temperatures (Tm) under CO and H2 atmospheres are 916 °C and 907 °C, respectively. Rf1 and Rf2 converged above Tm, which leads to the disappearance of the Fe3O4 region in oxygen carrier (OC) beds. A theoretical model for calculating the reduction solid conversion (Xaver) is established, accurately characterizing Xaver under varying reaction times, H2/CO ratios, and temperatures. By using the Particle Swarm Optimization (PSO) algorithm and thermodynamic analysis, the study determines the optimal length-radius ratio (L/r), reduction temperature (T), and H2/CO ratio (α) for the OC bed as 12.5, 916 °C, and 0.72, respectively, at which a maximum conversion ratio of 43.55 % is achieved. Based on optimal parameters, the results of Aspen simulation reveal energy and exergy efficiencies of 74.9 % and 70.8 %. Generally, this research comprehensively outlines the process dynamics, optimizes key parameters, and evaluates exergy efficiency of the biogas-CLHP system.

Modeling the Combustion-Deflagration-Detonation Transition (CoDDT) in a porous high explosive, comparison with experiments

ABSTRACT A reactive multiphase model stemming from the Baer and Nunziato model was developed and extended to three phases, one solid and two gaseous respectively representing the solid explosive, the initial porosity and the reaction products. It was used to model the transition from deflagration to detonation of a highly confined porous HMX 150–200 explosive thermally initiated, for several initial densities, ranging from 65% to 88% of the material's Theoretical Maximum Density (TMD). The influence of the deconsolidative burning in the critical acceleration of the reactive front was implemented through a fluidization parameter, allowing to switch from pore to grain burning. Other microstructural parameters such as the pores and grains size were determined by careful experimental characterizations presented in Bouchet et al. (2022). The model then succeeded in reproducing the physical mechanisms that were previously experimentally observed, such as the convective burning, the plug formation, and the transition to detonation at the distances that were experimentally measured.

Formation of PGE- and sulfide-bearing chromitites and associated anorthositic rocks in layered intrusions by the infiltration of reactive, Cl-rich fluid

The chromatographic model for chromitite formation previously presented is updated to include sulfide as a possible participating phase. In the model, chromite is precipitated at an upward-moving reaction front as a Cr-bearing (gabbro)norite protolith reacts with a Cl-rich fluid that becomes progressively undersaturated in pyroxene as it rises into hotter parts of the crystal pile, leaving an anorthositic residue. The liberation of Fe during the dissolution of orthopyroxene can drive concurrent sulfide saturation. If the reaction front encounters an orthopyroxenite the model produces a first-order approximation for the Merensky Reef section of anorthosite–chromite–orthopyroxenite. In addition, MELTS modelling shows that the isothermal addition of Cr 2 O 3 alone to a nearly solid (gabbro)norite assemblage increases the amount of both chromite and liquid at the expense of other silicate minerals owing to the liberation of Ca, Na and Si, the latter two acting as fluxing agents. A generalized constitutional zone refining model is presented in which a variety of spinel–silicate layering types (e.g. Bushveld magnetite layers and Skaergaard trough layering) can develop over time.

Radiative heat recuperation in combustion of layered solid fuel

In this work, we investigate the properties and stability of the combustion waves propagating in a system of layered solid fuel material in which the neighbouring fuel sheets can recuperate heat via thermal electromagnetic radiation. The analysis is undertaken within the flame sheet model, which allows to determine the combustion wave speed and structure and derive the dispersion relation characterising the stability of the reaction front. The proper choice of parameters leads to superadiabatic combustion temperature and significant increase of the combustion wave speed. In addition, the stable steady regimes of combustion can be substantially extended in the space of parameters. Such an enhancement of stability and ability to control the characteristics of the combustion wave propagation can be very attractive for the combustion synthesis of new layered materials, similarly to the chemical furnace technology proposed earlier.

The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction.

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S2O82-), and the strong reductant, oxalate (C2O42-), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S2O82- + C2O42- → 2 SO42- + 2 CO2 (ΔG0 = -490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the time scale of a day due to its inherently slow kinetics. We demonstrate that the S2O82-/C2O42- redox reaction occurs rapidly, once initiated by the Ru(NH3)62+-mediated 1e- reduction of S2O82- to form S2O83•-, which rapidly undergoes bond cleavage to form SO42- and the highly oxidizing radical SO4•-. Theoretically, the mediated electrochemical generation of a single molecule of S2O83•- can initiate an autocatalytic cycle that consumes both S2O82- and C2O42- in bulk solution. Several experimental demonstrations of S2O82-/C2O42- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO2 is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S2O82- in solutions containing excess C2O42- is initiated by electrogeneration of immeasurably small quantities of S2O83•-. Capture of CO2 as BaCO3 during electrolysis additionally confirms the autocatalytic generation of CO2. First-principles density functional theory calculations, ab initio molecular dynamics simulations, and finite difference simulations of cyclic voltammetric responses are presented that support and provide additional insights into the initiation and mechanism of S2O82-/C2O42- autocatalysis. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.

Double flame dynamics in hotspot ignition

Hotspot ignition is a key problem in fundamental flame initiation, combustion control and prevention of abnormal combustion phenomena. Depending on the characteristics of the hotspot and the thermochemical condition of the reactive mixture, subsequent reaction front initiated from a hotspot can vary substantially. The current paper focus on the dynamics and chemistry for one of the most complicated scenarios - hotspot induced double flames, which include a cool and a hot flame segment. This work adopts a recently developed compressible reacting flow simulation code COGNAC to investigate the initiation and propagation of double flame dynamics from hotspot ignition in a one-dimensional spherical coordinate. The results have shown that under atmospheric pressure, double flame initiation is not feasible for sufficiently small hotspot. Under elevated pressures, double flame can be initiated within a much wider hotspot window, exhibiting complex flame dynamics, such as variations in cool and hot flame bifurcation behaviors and their initiation locations. Analysis on thermochemical structure and dynamics of double flame has shown that the interaction of cool and hot flames in a double flame structure is inherently intermutual, in that the leading cool flame enhances trailing hot flame speed through reform of the unburnt mixture, while the trailing hot flame initiation affects the leading cool flame via thermal expansion effects.

The influence of heat feedback and thermal conductivity on the burn rate of thermite composites

In this paper we clarify some concepts related to heat transfer to the reaction front in composite burning. Here, 2.5 wt% carbon fiber (CF) is added to various thermite compositions to assess the effect of thermal conductivity on burn rate. We observe that in some cases the burn rate is enhanced and in others decreased. Upon further analysis we find that this behavior depends on the phase of the reaction product (dispersed vs. continuous). Adding carbon fiber increases the burn rate of composites with a dispersed phase product, which includes Al/MnO2, Ti/MnO2 and B/Bi2O3. However, CF added to composites with continuous phase product like B/MnO2, B/Fe2O3 and B/Fe3O4, the burn rate is reduced. A simple thermal analysis shows that for solid composites with continuous product, increase in thermal conductivity should actually lead to a decrease in burn rate, as any mechanism that bleads energy away for the reaction front will decrease overall reaction velocity. In contrast, for cases where the products are dispersed aerosol droplets, the fibers serve to capture the droplets as they lift off the surface, and thus offers an additional heat feedback mechanism that enhances the burn rate. This study shows that micro-engineering methods to couple heat to the reaction front with minimal loss of heat to the pre-heat zone will result in an increase in power.

Tuned Reactivity at the Lithium Metal – Argyrodite Solid State Electrolyte Interphase

Thin intermetallic Li2Te–LiTe3 bilayer (0.75 mm) derived from 2D tellurene stabilizes solid electrolyte interphase (SEI) of lithium metal and argyrodite (LPSCl, Li6PS5Cl) solid-state electrolyte (SSE). Tellurene is loaded onto standard battery separator and reacted with lithium through single-pass mechanical rolling, or transferred directly to SSE surface by pressing. State-of-the-art electrochemical performance is achieved, e.g. symmetric cell stable for 300 cycles (1800 hours) at 1 mA cm-2 and 3 mAh cm-2 (25% DOD, 60 mm foil). Cryo-FIB sectioning and Raman mapping demonstrate that Li2Te–LiTe3 bilayer impedes SSE decomposition. The unmodified Li–LPSCl interphase is electrochemically unstable with geometrically heterogeneous reduction decomposition reaction front that extends deep into the SSE. Decomposition drives voiding in Li metal due to its high flux to the reaction front, as well as voiding in the SSE due to the associated volume changes. Analysis of cycled SSEs found no evidence for pristine (unreacted) lithium metal filaments/dendrites, implying failure driven by decomposition phases with sufficient electrical conductivity that span electrolyte thickness. Density Functional Theory (DFT) calculations clarify thermodynamic stability, interfacial adhesion, and electronic transport properties of interphases, while mesoscale modeling examines interrelations between reaction front heterogeneity (SEI heterogeneity), current distribution and localized chemo-mechanical stresses.

Dynamics of Cu isotope fractionation during the reactions of pyrite with Cu(I)-bearing hydrothermal fluids

Experimental investigation of the fractionation behavior of Cu isotopes during the replacement of pyrite by Cu-bearing sulfides (chalcopyrite, bornite, and chalcocite) was conducted under hydrothermal conditions. At the initial stages of the replacement reaction, small mounds of product formed on the pyrite surface; these mounds then grew and coalesced, progressively covering the pyrite grains as the reaction proceeded. Chalcopyrite, bornite, and chalcocite formed sequential monomineralic zones from the pyrite reaction front towards the solution. During the early stage of the reaction, Cu isotope fractionation between the solids and the aqueous solution (Δ65Cusolid-aqueous, Δ65Cusolid-aqueous = δ65Cusolid – δ65Cuaqueous) was approximately –0.6 ‰, similar to the calculated equilibrium Cu isotope fractionation factors between Cu-bearing sulfides (chalcopyrite, bornite, and chalcocite) and CuCl2–. However, the Δ65Cusolid-aqueous moved progressively further from equilibrium with prolonged reaction time, with fractionation factors ranging from approximately –0.6 ‰ to –1.2 ‰. We found that significant fractionation of Cu isotopes between the solid products and aqueous solution was mainly controlled by the amount of chalcopyrite that formed in the product layers. We interpret that the Δ65Cusolid-aqueous was controlled by the diffusion of aqueous Cu(I) species among the product layers and the changes in Cu redox state during the precipitation of chalcopyrite. These results imply that the δ65Cu values measured in sulfides from low-temperature hydrothermal deposits may be controlled by the local chemistry of the fluids in product layers that precipitate sulfides. This study highlights the importance of local fluid characteristics, such as redox conditions, metal speciation, and diffusion, in controlling isotope fractionation pathways during non-equilibrium mineral replacement.

Role of Surface Tension on Heat Feedback and Power from Energetic Composites.

Heat feedback to the unburned reaction interface is an important controlling factor of the velocity of the reaction front and power delivery. In this paper, we investigate the effect of agglomerate surface tension and its relationship to surface residence time and heat feedback on the combustion characteristics by Si addition to an Al/KClO4 composite. Macroscopic imaging demonstrates a significant increase in burn rate with the addition of Si despite the fact that Si/KClO4 has a slightly lower energy density than Al/KClO4. Microscopic imaging coupled with three-color pyrometry reveals that molten liquid forms and evolves into spherical droplets on the burning surface, which are subsequently ejected from the surface. We find that the addition of Si results in a small increase in droplet size and a negligible impact on droplet temperature. However, the droplet formation rate on the surface is slower, leading to a significantly longer surface residence time. This leads to enhanced conductive heat feedback to the unburnt materials, thereby increasing the burn rate and energy release rate. We attribute the decreased droplet growth rate to the lowered surface tension of the liquid mixture with Si addition. This study highlights the crucial role of agglomerate physical property (e.g., surface tension) in influencing the combustion behavior of energetic composites.

Transition fronts of combustion reaction–diffusion equations around an obstacle

Transition fronts of combustion reaction–diffusion equations around an obstacle