Initial Stage Of Reaction Research Articles

Interfacial nanobubbles’ growth at the initial stage of electrocatalytic hydrogen evolution

The growth process of interfacial nanobubbles during the initial stage of the hydrogen evolution reaction, and their influence on the overpotential were revealed byin situelectrochemical surface plasmon resonance imaging combined with atomic force microscopy.

Applicability and antibacterial activity of polycyclic aromatic compound derivatives used as photocatalysts for water oxidation

Water is oxidized using 5,8-dicyano[5]phenacene, a thin-film catalyst supported on a silica plate by vacuum deposition; hydrogen peroxide is produced under visible-light irradiation. Despite the high hydrogen peroxide production rate during the initial reaction stage, the rate decreases after 12 h, and the reaction is almost terminated after 24 h, possibly because hydrogen peroxide undergoes self-decomposition. The self-decomposition rate of hydrogen peroxide increases with increasing concentration, and attains equilibrium. Additionally, Escherichia coli is efficiently sterilized using this catalyst under visible-light; the antibacterial effect is exhibited even under indoor light. Among several polycyclic aromatic derivatives, 9,10-dicyanoanthracene exhibits the highest hydrogen peroxide production rate and antibacterial activity against E. coli. Its antibacterial efficiency is approximately 16 times higher than that of 5,8-dicyano[5]phenacene. The oxygen species causing these antibacterial effects is investigated using trapping reagents. superoxide anion radicals and hydroxyl radicals induce antibacterial effects; hydroxyl radicals are particularly effective.

Enhanced degradation performance and mechanisms of decabromodiphenylether in homogeneous system of Fe3+ activated dithionite

Brominated flame retardant (BFR) generally could be debrominated and degraded effectively by photolysis with high energy demand and waste production. However, in this study, an innovatively non-photolysis system based on dithionite (DTN) has been established for BFR treatment in water to meet the requirement of low-carbon and sustainable development. Fe3+-activated dithionite (DTN), a homogeneous non-photolysis coupled system, was innovatively used for decabromodiphenylether (BDE-209, a frequently detected priority contaminant in several environmental matrices) degradation effectively in 30 min with the simultaneous debromination and mineralization. Based on scavenging experiment and ESR analysis, SO4•-, •OH, and SO3•- were confirmed to be responsible for BDE-209 removal in Fe3+/DTN system. And the contribution rates of SO4•-, ·OH, and other species were about 47.5%, 30.8%, and 21.7% for BDE-209 degradation, respectively. Moreover, based on central composite design (CCD) in response surface methodology (RSM), a satisfactory quadratic model with low probabilities (<0.0001) at a confidence level of 95% was established to predict BDE-209 degradation in Fe3+/DTN system (R2 = 0.9691, R2-adj = 0.9413). Furthermore, the predicted BDE-209 degradation efficiency (C/C0 ≈ 0.11) was achieved with the optimum Fe3+ dosage, DTN dosage and initial pH of 81.78 μM, 173.26 μM, and 6.27, respectively. Furthermore, debromination stepwise, hydroxylation, and mineralization were involved for BDE-209 degradation in Fe3+/DTN system according to the identified intermediates by gas chromatography–mass spectrometry (GC–MS). And due to the generated mixture including BDE-209 derivates, intermediates and bromide ion, the toxicity effect on Photobacterium phosphoreum growth was increasing in the initial BDE-209 reaction stage and then decreasing to be lower than that of BDE-209 itself. This study has revealed a promising system of DTN activated by Fe3+ for BFRs in-situ degradation and detoxification with the synergetic debromination and mineralization.

Fatty acid chain modification of loxenatide and its kinetics in a continuous flow microchannel reactor

In this study, a continuous flow microchannel reactor was designed and constructed for continuous and efficient fatty acid chain modification (FACylation) of polypeptides to achieve higher production capacity than that obtained in a batch reactor. In this microreactor, loxenatide (LOX) was site-specifically modified by N-tetradecylmaleimide (C14-MAL) to continuously synthesize the target product, N-tetradecyl-loxenatide (C14-LOX). Based on the experimental data under different conditions, a kinetic model of FACylation was established to describe the initial stage of the continuous reaction in the microreactor. Under the optimal conditions obtained by the kinetic model, the reaction time in the microreactor could be reduced to 1/3 that of the batch reactor at the same yield, and the production capacity of the microreactor was at least 3.78 times that of the batch reactor within the same time (40 h). These results would contribute to the development of microreactors in the biopharmaceutical industry.

Study on the inhibitory mechanism of dehydrogenated antioxidants on coal spontaneous combustion

In order to comprehensively and systematically analyze the reasons why antioxidant inhibitors can scavenge free radicals in coal and inhibit coal spontaneous combustion, this paper studies the effects of VC, TBHQ, EGCG and BHT on coal spontaneous combustion by means of coal spontaneous combustion characteristics experiments and quantum chemical simulation methods. The low-temperature oxidation characteristics of coal were studied through temperature-programmed experiments. The results showed that the CO emission of coal samples with antioxidants was significantly lower than that of raw coal. At 170°C, the maximum decrease was 37.74%. Fourier infrared test showed that compared with the coal samples without antioxidant treatment, the adsorption strength of hydroxyl structure and oxygen-containing functional groups of the treated coal samples was significantly reduced. The area percentages of hydroxyl and methylene changed significantly, decreased by 7.14% and 6.46%, respectively. Subsequently, molecular models of four antioxidants were constructed using quantum chemical theory, and their Mulliken charges, BDE values ​and frontier orbitals were calculated according to density functional theory (DFT), and the active sites and inhibition mechanisms of antioxidants were discussed. The results showed that H9 of VC, H33 of EGCG, H1 of TBHQ and H40 of BHT all had strong ability to scavenge oxygen-containing free radicals, and their order of strength was TBHQ > BHT > EGCG > VC. Antioxidant inhibitors mainly reduce the number of active free radicals by removing the peroxide groups in the initial stage of the coal oxygen reaction, and remove the hydroxyl groups to prevent the further spontaneous combustion of coal and inhibit the low temperature oxidation process of coal.

Effect of oxygen on thermal behaviors and kinetic characteristics of biomass during slow and flash pyrolysis processes

Pyrolysis is usually considered as the initial stage of solid fuel combustion and gasification, and the pyrolysis characteristics in oxidizing atmosphere greatly affect the ignition behavior, flame stability and product distribution. However, the effect of oxygen on thermal behaviors and kinetic characteristics of biomass pyrolysis was poorly understood and has been studied only based on thermogravimetric analysis (TGA). TGA has heat transfer limitations, resulting in a significant difference between the calculated kinetic results and actual ones. In this work, a bubbling fluidized bed with an on-line analysis system was developed to investigate the real-time gas releasing behaviors of biomass flash pyrolysis in inert and oxidizing atmospheres. The slow pyrolysis characteristics were investigated in TGA for comparison. The effects of oxygen on the kinetics of flash and slow pyrolysis were investigated by combining the distributed activation energy model (DAEM) isothermal model and non-isothermal models including Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Starink. Under slow heating conditions, the presence of oxygen led to a higher maximum pyrolysis rate in a smaller temperature interval, and the maximum pyrolysis rate shifted to the lower temperature. In oxidizing atmosphere, the pore volume and specific surface of char were about 4–5 times higher than that of the char prepared in inert atmosphere. The values of apparent activation energy (Ea) at the initial stage of pyrolysis reaction in oxidizing atmosphere were much higher than those in inert atmosphere. As the conversion rate reached 0.4–0.5, the presence of oxygen led to char gasification and combustion, as well as a considerable change in Ea values. In addition, the Ea values of biomass flash pyrolysis obtained by the fluidized bed were about 4–5 times smaller than those of slow pyrolysis obtained by TGA.

Understanding the reaction kinetics of diesel exhaust soot during oxidation process

The purpose of the present study is to better understand the reaction kinetics of diesel exhaust soot during oxidation process. A thermogravimetric analyzer was used to oxidize real diesel exhaust soot generated from a Euro VI diesel engine under non-isothermal conditions. The Friedman-Reich-Levi method and the Sestak-Berggren model were used to determine the oxidation kinetics. Raman spectroscopy and high-resolution transmission electron microscopy were employed to follow the changes of the soot structure during oxidation. The activation energy gradually increased with increasing conversion level during soot oxidation. The oxidation process of diesel exhaust soot could be described as three-step kinetics, and the calculated conversions fitted the experimental results very well. The kinetic predictions of diesel soot oxidation that were obtained using the proposed kinetic models were more accurate and precise than those with the common first-order model. The structural order increased as oxidation progressed, which was responsible for the increased activation energy. The structural ordering was principally caused by the preferential oxidation of the disordered fraction in the diesel soot, especially for the amorphous carbon, which was oxidized in the initial stage of the oxidation reaction.

Hydrogen/PRF skeletal mechanism study based on shock tube experiments and kinetic analysis

In the present work, the experiments of measuring ignition delay times (IDTs) of the hydrogen/n-heptane mixture at high temperature (1130–1485 K) and low pressure (1.25 atm) by a shock tube is implemented. The results show that mixtures with higher hydrogen proportion and lower equivalence ratio present shorter IDT. In the case of high hydrogen proportion, the n-heptane addition causes an obvious inhibition effect on the ignition of the mixture. Whereas, as the hydrogen proportion decreases, such inhibition effect weakens. As the experimental values are consistent with the predicted IDTs by NUI (National University of Ireland) mechanism, the NUI mechanism was selected to conduct the kinetic study. The consumption of n-heptane at the initial stage of reaction is earlier than that of hydrogen because the activities of n-heptane’s H-abstraction and decomposition reactions are relatively high. With shock tube experimental data as the calibration basis, a skeletal hydrogen/PRF (Primary Reference Fuel) mechanism, was updated from a skeletal PRF mechanism. The skeletal hydrogen/PRF mechanism, consisting of 49 species and 162 reactions had been validated against shock tube experiment of hydrogen, n-heptane, isooctane pure fuel, PRF, and hydrogen/n-heptane mixture.

Experimental and simulation study on the delayed release of CO in the initial stage of the low-temperature oxidation of coal

To investigate the delayed release characteristics of CO gas in the initial stage of the low-temperature oxidation of coal, closed oxygen consumption experiments were conducted on coal samples taken from the Hongqingliang coal mine, and the corresponding relationship between the CO concentration and time in the initial stage of the experimental reaction was analyzed. A physical adsorption model of the macromolecules in coal for O2 and CO was established, and the difference in the competitive adsorption between the CO and O2 gas molecules on the coal surface was analyzed from a microscopic perspective using the grand canonical ensemble Monte Carlo simulation. The results showed a delayed CO release phenomenon in the initial stage of the reaction in all the experiments, and the delayed time of CO release was negatively correlated with the temperature; the relationship between the adsorption amounts of CO and O2 in the molecular structure model of coal was CO > O2. With increasing temperature, the adsorption capacity of the two gases decreased. Under the same conditions, there was competitive adsorption of the mixture of CO and O2 by coal, with the adsorption capacity of CO being much greater than that of O2. The adsorption of CO gas molecules by coal played an inhibitory role in the release of CO gas in the initial oxidation stage. The study results are expected to help understand the CO generation characteristics in the goaf of coal seam working faces and thus prevent coal mine disasters.

Light Metal Alloys Local Reactivity, from AFM Based Scanning Kelvin Probe Force Microscopy (AFM-SKPFM) to Scanning Electrochemical Nanocapillary (SEN)

Materials surface heterogeneities obviously plays a crucial role in reactivity issues and generate various types of detrimental localized corrosion attacks. In corrosion science, a first question to be addressed is how these various materials microstructure affect oxidation / passivation and localized corrosion initiation mechanisms. A methodology for microscale characterization of initial stage of surface reactions has been developed based on the Atomic Force Microscopy (AFM) Scanning Kelvin Probe Force Microscopy (AFM-SKPFM). In very early work, potential mapping with micrometer lateral resolution (lateral detection limit of materials surface heterogeneities is better but the signal has to be deconvoluted) allowed to clearly identify "cathodic" and anodic sites on Al- alloys [1]. Careful potential calibration and surface conditioning with solution exposure generating the formation of an electrochemical double layer allow obtaining local thermodynamic information closely linked to "practical electrochemical potential series" measured in bulk solution conditions. This contribution will start by revisiting some "historical" examples of AFM-SKPFM characterization (a tribute to Jerry Frankel's activities) of aluminum and magnesium alloys [1,2]. These materials surfaces will further be used to show how reaction kinetic information on corrosion processes can be obtained with similar lateral resolution by means of the newly developed Scanning Electrochemical Nanocapillary (SEN) technique. After introducing the SEN technique and methodology, the presentation will focus on showing how the combination of these two approaches allows refining the local corrosion mechanism assessment of heterogeneous materials. The SEN technique is very versatile and allows various types of electrochemical measurements to be performed with constant tracking of the topography. The technique is based on a nanocapillary glass tip (< 100 nm) excited laterally by a piezoelectric element. A control of the capillary vibration damping when approaching the surface and a feedback loop (like the tapping mode in AFM) allows a very precise control of the approach and electrolyte contact on the surface. The glass capillary filled with the electrolyte of interest integrates a reference and Pt counter electrodes to perform electrochemical measurements exposing only the area of interest to aggressive electrolytes. Using its "hopping" mode (a scan with electrochemical characterization of successive individual nanoscale areas), corrosion initiation susceptibility can be addressed. This will be illustrated through the surface reactivity study of model Al-Cu-Mg microscale intermetallics and Mg-based alloys, using SEN OCP linescans. In addition information about extend of the attack can be subsequently retrieved from SEM observations. Using the precise positioning ability of the SEN setup with X, Y and Z direction piezos, potentiodynamic polarization measurements can furthermore be performed on selected area of interest. The example of an Mg-Fe composite material will be used to discuss its local electrochemical behavior.After the presentation of the SEN characterization results, additional aspects of the measured potential by AFM-SKPFM will also be discussed as adsorbed species, especially strong dipolar molecules, can significantly contribute to the measured signal [3]. Modification of the AFM-SKPFM setup towards atmospheric corrosion investigation will finally be presented. This environmental AFM/SKPFM bridges the gap between the full electrochemical local characterizations that can be offered by the SEN technique and an "electrochemical reaction" diagnostic obtained by SKPFM in thin water layer. The example of a very reactive WC-Co composite will be presented to demonstrate the reactivity issue, galvanic coupling controlling local dissolution processes [4].AcknowledgementsGerald Frankel for the great time I spent at Ohio State University in the Fontana Corrosion Center developing the AFM-based SKPFM methodology. Empa for internal project financial support of the SEN instrument.

Pore development and mechanical properties of iced concrete during hydration

High geothermal temperature is a key problem that restricts the mining of mineral resources. In order to maintain the normal production of deep mines, we must seek a new stope cooling method that is more green, economic and efficient. Based on this, this paper proposes a iced concrete technology applied to the cooling of deep mine mining, and focused on the mechanical strength, pore structure, heat transfer evolution law, hydration reaction process and other change characteristics of concrete with different initial ice content (0 %, 10 %, 20 %, 30 %, 40 %, 50 %, 60 %). Finally, the pore structure and mechanical properties of iced concrete in the hydration process were analyzed and studied. The results demonstrate that (1) for concrete specimens cured for 1 d, the mechanical strength of iced concrete decreases slightly with an increase in initial ice content. After 28 d of curing, the mechanical strength is positively correlated with the initial ice content; (2) the porosity of macropores and mesopores of iced concrete is negatively correlated with the compressive strength, whereas the microporous porosity of iced concrete is positively correlated with the compressive strength because macropores and mesopores primarily undertake compression fracture penetration during dissipation, and micropores can be conducive to elastic strain energy accumulation; (3) the temperature change trend of iced concrete rises first and then decreases with the inflection point at 24 h. With an increase in the ice content in iced concrete, the initial temperature drops and the heating time is delayed. Therefore, in the initial hydration reaction stage, owing to the cooling effect of ice particles, an increase in the initial ice content decreases the content of hydration reaction products of concrete, increases the content of harmful pores such as macropores and mesopores, and relatively reduces the content of micropores; at 28 d, because the ice particles enhance the water-storage capacity of the concrete, an increase in the initial ice content leads to a more intense hydration reaction at the later stage and better pore structure distribution, which improves the final mechanical strength of the concrete.

A study on the catalytic role of Ce-Co species and coke deposited on ordered mesoporous Al2O3 in N2O-assisted oxidative dehydrogenation of ethylbenzene

The ordered mesoporous Al2O3 material was employed as support to prepare the CeO2-Co3O4 binary oxide catalysts for N2O-assisted oxidative dehydrogenation of ethylbenzene (EB), which was treated both as an effective method of styrene (ST) production and a high value-added utilization of the greenhouse gas. The catalytic performance was observed for the optimal 0.3Ce-7Co/OMA catalyst, which achieved 52.1% EB conversion with 88.7% selectivity toward ST at temperature as low as 500 ºC, far exceeding any other reported catalyst systems. Characterization results showed that an increase in the Co content led to a partial destruction of the order degree of pores, but maintained the mesoporous structure. The Ce modification not only decreased the particle size (<12 nm), considerably boosting the specific surface area, but also weakened the Co-O bond, making more O species available for the EB conversion. Studies on the carbon deposition formed during the catalytic runs revealed that the carbon deposited at the initial stage of reaction acted directly as the catalytically active sites enhanced the EB conversion. As the reaction progressed, the dynamic equilibrium between carbon deposition and burning relieved the loss of active sites and maintained the stability of catalyst activity. In addition, the excellent regeneration property of 0.3Ce-7Co/OMA catalyst indicated that N2O, which was used as an oxidizing agent, protected the catalyst against the formation of inactive carbonaceous deposition.

Unveiling the role of counter-anions in amorphous transition metal-based oxygen evolution electrocatalysts

At the initial stage of the oxygen evolution reaction (OER) most electrocatalysts undergo structural and chemical surface reconstruction. While this reconstruction strongly influences their performance, it is frequently overlooked. Herein, we analyze the role of the oxidized anions, which is particularly neglected in most previous works. We introduce a range of different anionic groups (Cl-, CH3COO-, NO3-, SO42-) on the surface of an amorphous ZnCoxNiyOz catalyst by a facile proton etching and ion exchange method from a ZIF-8 self-sacrificial template. The structural and chemical properties of the obtained set of materials are thoroughly analysed and correlated with their electrocatalytic performance to study the effect of surface anionic groups, phase transition, metal leaching and defect generation on OER activity. Exploiting the control possibilities provided by the synthesis method here described and employing the uncovered property-performance correlations, the electrocatalyst is optimized. As a result, we produce ZnCo1.25Ni0.73Ox-SO4 catalysts with outstanding OER performances, including a low overpotential of 252 mV at 10 mA cm−2 with a small Tafel slope of 41.6 mV dec−1. Furthermore, this catalyst exhibits remarkable stability with negligible overpotential variation for 100 h. The excellent catalytic properties are rationalized using density functional theory calculations, showing that the surface-adsorbed anions, particularly SO42−, can stabilize the OOH* intermediate, thus enhancing the OER activity. This work offers new insight into the roles of metal leaching and surface-adsorbed anions in the OER progress and facilitates the rational design of highly-efficient electrocatalysts for OER.

Ozone-assisted low temperature oxidation of methanol and ethanol

Ozone-assisted combustion is an advanced combustion technology that is considered a promising means for the active control of combustion. Ozone addition can enhance combustion processes and extend the experimental parameters to extreme conditions. In this work, ozone-assisted low temperature oxidation of methanol and ethanol in a jet-stirred reactor (JSR) was carried out at atmospheric pressure, with a fuel initial mole fraction of 0.005, an equivalence ratio of 0.28, and in the temperature range of 330–800 K. Significant conversions of methanol and ethanol were observed in the low temperature region when ozone was added. Numerous intermediate species were analyzed and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography, which are crucially important for unraveling the low temperature oxidation reaction network. The kinetic model of NUIGMech1.1 is tentatively updated to better explain the measured results, although the improvements of the C1–C2 core mechanisms are difficult. Ozone addition enhances the action of the reaction channel of related radicals. Model analysis shows that the kinetics of Ö/ȮH/HȮ2 atoms/radicals are crucial for the overall reaction progress, as well as the reaction of HȮ2 radicals with ozone, which mainly contributes to the formation of ȮH radicals during the initial reaction stage. Inconsistency between measurements and model predictions highlights the considerable uncertainty in the C1-C2 alcohols subchemistry at lower temperatures. The experimental results obtained in this work and model development will motivate further study of the low temperature oxidation chemistry of methanol and ethanol.

Synthesis and Characterization of Fly Ash-Based Geopolymers Activated with Spent Caustic.

The spent caustic with strong alkali first replaced the alkali activator to prepare the geopolymer. The influence of spent caustic to the geopolymer was characterized through compressive strength measurement, XRD, MIP analysis and NMR, and the immobilization efficiency of organic in geopolymer was evaluated through the measurement of total organic carbon (TOC). The results show that the spent caustic can partially replace the alkali activator to prepare the geopolymer, and it shows a better performance than that which was activated with pure NaOH solution when the alkalinity is between 4 mol and 14 mol. The organic matter in the spent alkali can be effectively fixed in the geopolymer, which will hinder the geopolymerization in the initial stage of the polymerization reaction but has little effect on the chemical structure and mechanical properties of the final product. With the degree of alkalinity increasing, the immobilization efficiency is improved, and the maximum can reach 84.5%. The organics in the spent caustic will hinder geopolymerization at the initial stage but has little effect on the chemical structure and mechanical property of the final product. This study proposes a new method for the recycling of spent caustic, which also reduces the preparation cost of geopolymers.

Tuning pyrolysis temperature to improve the in-line steam reforming catalyst activity and stability

This study analyzes the two-step process of biomass pyrolysis and in-line steam reforming for the production of H2. In order to evaluate the effect of the volatile composition on the commercial Ni/Al2O3 catalyst performance and stability, biomass pyrolysis step was conducted at different temperatures (500–800 °C). The analysis of the deactivated catalysts has also allowed identifying the main bio-oil compounds responsible for catalyst decay (coke precursors). Pyrolysis temperature allows modifying the composition of the volatile stream that is subsequently reformed at 600 °C. An increase in pyrolysis temperature to 800 °C improves considerably the production of both H2 and gaseous stream at the initial reaction stages, reaching values of 12.95 wt% and 2.23 Nm3 kg−1, respectively. Catalyst stability is also considerably improved when pyrolysis temperature is increased due to the lower bio-oil yield and its different composition at high temperatures. Coke was the main cause of catalyst deactivation. Besides, the nature of the coke deposited is influenced by the composition of the pyrolysis volatiles, with encapsulating coke being formed by the adsorption and subsequent condensation of all hydrocarbons (oxygenated and non-oxygenated ones) preferably at low temperatures, whereas filamentous coke is formed when the concentrations of CO and light hydrocarbons in the volatile stream are increased at 800 °C.

Inhibition mechanisms of Fe2+/Fe3+ and Mn2+ on fungal laccase-enabled bisphenol a polyreaction

Bisphenol A (BPA) is regarded as an endocrine disruptor associated with negative health effects in animals and humans. Laccase from white-rot fungus can enable BPA oxidation and auto-polymerization to circumvent its biotoxicity, but the work concerning the effect mechanisms of divalent and trivalent metal ions (MIs) on BPA polyreaction have rarely been reported. Herein, Trametes versicolor laccase-started BPA conversion within 1 h followed pseudo-first order kinetics, and the rate constant (kprcs) and half-life were respectively 0.61 h−1 and 1.14 h. The presence of Ca2+, Mg2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+ exhibited insignificant impact on BPA removal, whereas Fe2+, Fe3+ and Mn2+ had a strong inhibiting effect. Compared with MI-free, the kprcs values of BPA respectively lowered 34.4%, 44.3% and 98.4% in the presence of Fe2+, Fe3+ and Mn2+. Enzymatic activity and differential absorption spectrum disclosed that the inhibitory actions were accomplished by two different mechanisms. One is Fe2+ was preferentially oxidized into Fe3+ that restrained laccase activity at the initial stage of reaction, and subsequently, the formed Fe3+ complex bound with laccase T1-Cu site and thus impeded the single-electron transfer system. The other is Mn2+ was instantly oxidized by laccase to generate Mn3+-citrate complex, which completely consumed the dissolved O2 in solution and consequently terminated BPA removal. Considering environmental bioremediation, T. versicolor laccase-enabled auto-polymerization is a simple and convenient candidate to eliminate BPA in enzymatic wastewater treatment, however the effects of Fe2+/Fe3+ and Mn2+ on BPA decontamination should be cautiously assessed.

Combustion crack-network reaction evolution model for highly-confined explosives

The evolution behavior of combustion crack reaction of highly confined solid explosives after non-shock ignition is governed by multiple dynamic processes, including intrinsic combustion of explosives, crack propagation, and rapid growth of combustion surface area. Here, the pressure increase can accelerate the combustion rate of explosives, and the crack propagation can enlarge the combustion surface area. The coupling between these two effects leads to the self-enhanced combustion of explosive charge system, which is the key mechanism for the reaction development after ignition. In this study, combustion crack-network (CCN) model is established to describe the evolution of combustion crack reaction of highly confined solid explosives after non-shock ignition and quantify the reaction violence. The feasibility of the model is verified by comparing the computational and experimental results. The results reveal that an increase in charge structure size causes an increase in the time of crack pressurization and extension of cracks due to the high temperature-generated gas flow and surface combustion during the initial stage of explosive reaction, but when the casing is fractured, the larger the charge structure, the more violent the late reaction and the larger the charge reaction degree. The input pressure has no obvious influence on the final reaction violence. Further, a larger venting hole area leads to better pressure relief effect, which causes slower pressure growth inside casing. Larger reserved ullage volume causes longer low-pressure induction stage, which further restrains the internal pressure growth. Furthermore, the stronger the casing constraint, the more rapid the self-enhanced combustion of the high temperature-generated gas, which results in more violent charge reaction and larger charge reaction degree during casing break. Overall, the proposed model can clarify the effects of intrinsic combustion rate of explosives, charge structure size, input pressure, relief area, ullage volume, and constraint strength on the reaction evolution, which can provide theoretical basis for violence evaluation and safety design for ammunition under accident stimulus.

In-situ observation of peritectic solidification of Fe-Mn-Al-C steel with medium manganese

The design of the continuous casting process for the third generation of Fe-Mn-Al-C steel with medium manganese should be based on the understanding of the peritectic solidification mechanism. So, in the present study, Fe-5.56Mn-0.078Al-0.157C (wt%) steel was taken as the research object. By means of high-temperature confocal scanning laser microscopy (HT-CSLM), Thermo-Calc thermodynamic software and optical microscopy, the peritectic solidification process and microstructure characteristics of the medium manganese steel at different cooling rates were studied and revealed. And thus, the influence of the cooling rate on the peritectic solidification mechanism was clarified. Meanwhile, the kinetics of the γ-Fe grain wrapping the δ-Fe grain and the subsequent growth were determined. The results show that at low cooling rate of 10 °C/min, the initial δ-Fe phase is cellular. The γ-Fe phase firstly nucleates at the L/δ interface, and then grows along the δ/L interface until the L and δ-Fe phases are separated. Subsequently, the peritectic transformation occurs, that is the L and δ-Fe phases are directly transformed into the γ-Fe phase. Under the conditions with higher cooling rates of 20 and 30 °C/min, when the γ-Fe phase separates the L and δ-Fe phases completely, multiple γ-Fe grains form in a single δ-Fe grain at the same time, and the whole δ-Fe grain is covered within a very short time, which is the typical massive-like transformation. In the initial stage of the peritectic reaction, the linear velocity of the γ-Fe phase wrapping the δ-Fe phase is inversely proportional to the cooling rate, which is closely related to the size and morphology of δ-Fe grains. At cooling rates of 10, 20 and 30 °C/min, the initial migration rates of the δ/γ interface are 45.9, 8.9 and 12.5 µm/s, respectively. At higher cooling rates, the occurrence of the massive-like transformation greatly promotes the peritectic transformation, and the migration rate of the δ/γ interface increases to 16.7 and 17.8 µm/s. The initial migration rates of the γ/L interface are 6.0, 1.8 and 6.5 µm/s, respectively. It is found that the migration rate of the δ/γ interface is much larger than that of the γ/L interface. Dendrite etching experiment in the top, bottom and longitudinal sections of the medium manganese steel samples shows that the larger the cooling rate, the finer the solidification structure. • The δ/L interface of Fe-5.56Mn-0.078Al-0.157C (wt%) steel becomes more unstable with the increase of the cooling rate. • The mechanisms of the peritectic solidification of the medium Mn steel at different cooling rates are revealed. • Kinetics of the γ-Fe phase wrapping δ-Fe phase and then growing into δ-Fe and L phases are measured for the medium Mn steel. • The relationships between the dendrites arm spacings of the medium manganese steel and cooling rates are presented.

Atomic Scale Insights into the First Reaction Stages Prior to Al/CuO Nanothermite Ignition: Influence of Porosity

This theoretical work aims to understand the influence of nanopores at CuO-Al nanothermite interfaces on the initial stage of thermite reaction. ReaxFF molecular dynamics simulations were run to investigate the chemical and structural evolution of the reacting interface between the fuel, Al, and oxidizer, CuO, between 400 and 900 K and considering interfaces with and without a pore. Results show that the initial alumina layer becomes enriched with Al and grows primarily into the Al metal at higher temperatures. The modification of alumina is driven by simultaneous Al and O migration between metallic Al and the native amorphous Al2O3 layer. However, the presence of a pore significantly affects the growth kinetics and the composition of this alumina layer at temperatures exceeding 600 K, which impacts the initiation properties of the nanothermite. In the system without a pore, where Al is in direct contact with CuO, a ternary aluminate layer, a mixture of Al, O, and Cu, is formed at 800 K, which slows Al and O diffusion, thus compromising the nanothermite reactivity in fully dense Al/CuO composites. Conversely, the presence of a pore between Al and CuO promotes Al enrichment of the alumina layer above 600 K. At that temperature, any free oxygen molecules in the pore become attached to the reactive alumina surface resulting in a rapid oxygen pressure drop in the pore. This is expected to accelerate the reduction of the adjacent CuO as observed in experiments with Al/CuO composites with porosity at the CuO-Al interfaces.