Exothermic Transformation Research Articles

Sequential Reactions of Acetylene with the Benzonitrile Radical Cation: New Insights into Structures and Rate Coefficients of the Covalent Ion Products.

Benzonitrile radical cations generated in ionizing environments such as solar nebulae and interstellar clouds can react with neutral molecules such as acetylene to form a variety of nitrogen-containing complex organics. Herein, we present results from mass-selected ion mobility experiments and coupled-cluster and DFT calculations for the sequential reactions of acetylene with the benzonitrile radical cation (C7NH5+•). The results reveal the formation of two covalently bonded adduct ions C9NH7+• and C11NH9+• with individual rate coefficients of 2.1(±0.4) × 10-11 cm3 s-1 and 1.1(±0.9) × 10-11 cm3 s-1, respectively measured at 334.5 K. The direct addition of acetylene onto the N atom of the benzonitrile cation results in the formation of a N-acetylene-benzonitrile+• radical cation with a calculated collision cross-section of 67.5 Å2 in perfect agreement with the measured cross-section of 67.5 Å2 of the C9NH7+• adduct. The measured collision cross-section of the second covalent adduct C11NH9+• (72.2 Å2) is also in excellent agreement with the calculated cross-section (71.2 Å2) of the lowest energy isomer of the C11NH9+• ion corresponding to the 2-phenylpyridine structure. The formation of the bicyclic 2-phenylpyridine radical cation is explained by the rapid conversion of the classical radical cation C11NH9+• into a distonic ion structure that can efficiently cyclize in an exothermic transformation to form the 2-phenylpyridine radical cation. This intriguing mechanism could explain the formation of N-containing complex organics in different regions of outer space. The current results are expected to have direct implications for the search for nitrogen-containing complex organics in space.

Low Temperature Sabatier CO2 Methanation

AbstractThe CO2 methanation reaction, or Sabatier reaction, is experiencing renewed interest in the context of large‐scale recycling of point CO2 emissions, leading to the power‐to‐gas technology. The reaction represents a flexible route to transform CO2 into methane by hydrogenation with (green) dihydrogen. This exothermic transformation takes place at a reasonable rate at temperatures above 200 °C and is directed to the targeted product at low temperatures. The CO2 methanation nevertheless remains kinetically limited due to the chemical stability of CO2 and the high bond dissociation energy for C═O in CO2. Therefore, the current urgent demand is for the development of catalysts and associated processes with superior activity for CO2 activation at low temperatures. This critical review aims to overview the state of the art of this low‐temperature technology using thermal, plasma and photo‐assisted catalysis. We summarize research advances around low‐temperature CO2 methanation, focusing on catalyst formulations (metal, supports and promoters), reaction mechanisms and suitable activation processes. We discuss each of these critical aspects of the technology and identify the main challenges and opportunities for low temperature (≤200 °C) CO2 methanation.

Revealing patterns in reducing the fire-hazardous properties of insulation made from plant raw materials

An issue related to the use of hemp bundle insulation for building structures is to ensure their fire resistance under the action of a low-calorie ignition source. Therefore, the object of research was the change in the properties of the insulation during fire protection with its compositions capable of forming a layer of pinocoke under the influence of high temperature. On the basis of experimental data, it was established that at fire protection of insulation with a fire retardant agent, hydrolytic stable ethers containing phosphorus and nitrogen atoms are formed on the surface of the fibers. According to the temperature values and the shape of peaks on the DTA curve, it was established that at fire protection, the height of the peaks decreases and the width increases, which characterize the flow of exothermic transformations and the destruction of hemp fibers. At the same time, it was established that the formed residue has 13.3 % for impregnation and 26.6 % for coating. It has been proven that in the process of thermal action on the fire-resistant coating, the heat insulation process of the insulation consists in the formation of soot-like products on the surface of the material. So, it was determined that the sample did not catch fire, it was charred in the place of the radiation panel, and the burning was not recorded. On the other hand, for a sample of insulation treated with a coating during thermal action, the formation of a heat-insulating layer of foam coke occurred, which inhibits the penetration of heat, the temperature of flue gases did not exceed 100 °C, and the flammability index was 0. The practical significance is that the results were taken into account when designing buildings and structures. So, there are reasons to assert the possibility of targeted regulation of fire protection processes of insulation by applying coatings that form a protective layer on the surface of the material

ZrB2–MoSi2 ceramics: Kinetics of compaction during hot pressing in СО environment, phase interactions at interfaces and static fatigue

Ceramics in the ZrB2–MoSi2 system pass through different stages during hot pressing in CO-CO2 environment. The first stage follows a smooth parabolic curve up to 1600 °C, then, a sharp compaction to full density occurs by transient liquid-phase sintering in the 1610–1850 °C temperature range. Ceramics sintered in this range have the primary compounds, ZrB2 and MoSi2, disappeared and new phases, such as (ZrB2)SS, SiC, B4C and ZrC, formed. The high sintering rate are associated to exothermic phase transformations, which significantly increase the temperature in the diffusion zones. In the 1850–2150 °C range, complete homogenization of the solid solution based on ZrB2 occurs and the sintering curve follows that typical of solid-state densification, i.e. increases continuously and slowly. Materials sintered at 1370–1850 °C have matrix grain size around 0.6–1.8 µm, while those sintered at 1940–2150 °C have grain size around 8–10 µm. Hot compression tests showed that the ZrB2-MoSi2 ceramic sintered at 1870 °C had the highest creep resistance compared with a ZrB2-SiC baseline.

Factors Affecting the Energy Efficiency of Exothermic Transformations during a Controlled High-Voltage Electrochemical Explosion

Factors Affecting the Energy Efficiency of Exothermic Transformations during a Controlled High-Voltage Electrochemical Explosion

Silver Ion-Induced Formation of Unprecedented Thorium Nonamer Clusters via Lacuna-Construction Strategy

Silver Ion-Induced Formation of Unprecedented Thorium Nonamer Clusters via Lacuna-Construction Strategy

The stability of expanding reactive shocks in a van der Waals fluid

Despite the extensive literature accumulated since the pioneering works of D'yakov and Kontorovich in the 1950s, the stability of steady shocks is still an open question when realistic boundary conditions are accounted. The consideration of a supporting mechanism, which is indeed a necessary condition for shock steadiness, modifies the perturbation shock dynamics in the unstable range. The Noh problem is a suitable example to form steady expanding shocks. This configuration is of great interest to the high-energy-density-physics community because of its direct application to inertial confinement fusion and astrophysics, for which the stagnation of a supersonically converging material via an accretion shock front is ubiquitous. In this work, we extend the generalized Noh problem, both base-flow solution and linear stability analysis, to conditions where endothermic or exothermic transformations undergo across the shock. Within the spontaneous acoustic emission conditions found for a van der Waals gas [J. W. Bates and D. C. Montgomery, “The D'yakov-Kontorovich instability of shock waves in real gases,” Phys. Rev. Lett. 84, 1180 (2000)], we find that cylindrical and spherical expanding shocks become literally unstable for sufficiently high mode numbers. Counterintuitively, the effect of exothermicity or endothermicity across the shock is found to be stabilizing or destabilizing, respectively.

Factors of Influence on the Energy Efficiency of Exothermal Transformations in a Controlled High-Voltage Electrochemical Explosion

The analysis of the results of experimental studies of the influence of changes in the initial conditions on the energy characteristics of a controlled high-voltage electrochemical explosion (HVECE) is carried out, the main factors affecting the specific energy efficiency of chemical transformations of a water-filled exothermic medium (WFEM) in its discharge channel are determined. The range of rational changes in the main factors has been established in which an increase in the specific energy efficiency of exothermic transformations of the WFEM in the channel of the controlled HVECE is ensured. This will make it possible to reduce the weight and size characteristics and power consumption of the pulse current generator to ensure the necessary energy characteristics of the controlled HVECE, specified by a specific discharge-pulse technology.

Microstructural and thermodynamic study of γ-Ga2O3

Abstract The metastable nanocrystalline γ form of gallium oxide has been prepared and its microstructure and thermochemistry have been studied for the first time by employing X-ray and electron diffraction, high-resolution transmission electron microscopy, adiabatic and differential scanning calorimetry. The randomly oriented crystallites of maximum 5 nm in size have been observed. The sponge-like morphology of γ-Ga2O3 particles may explain the high specific surface area, previously reported for this material. The defect spinel-type structure of γ-Ga2O3 is similar to that of γ and η-Al2O3. Up to 5.7 wt.% of water can be stored in γ-Ga2O3 and subsequently released at elevated temperatures. Dry γ-Ga2O3 specifically absorbs atmospheric water at room temperature. The transformation of γ-Ga2O3 into stable β-Ga2O3 occurs in two steps. In the range 650–800 K, γ'-Ga2O3 is formed in the course of a reversible higher-order phase transition. The latter irreversibly transforms into β-form above 873 K. The enthalpy of this exothermic transformation is determined as –19.3 ± 0.4 kJ · mol–1. The coefficients of the Gibbs energy equation for γ and γ'-Ga2O3 have been assessed.

Microstructural characterisation and thermal stability of a metastable Mg-8.6 wt.% Zr alloy produced by physical vapour deposition

Abstract The thermal stability of a Mg-8.6 wt.% Zr alloy processed by physical vapour deposition has been studied using differential scanning calorimetry and transmission electron microscopy. The alloy, in the as-deposited condition, is a solid solution of Zr in the Mg matrix. The microstructure is characterised by elongated grains oriented with their [0001] direction parallel to the growth direction. After DSC experiments, three exothermic transformations have been observed. The first one takes place above the deposition temperature and is related to the recovery phenomena due to stress relaxation generated during the growth process. The second transformation corresponds to the breakdown of the solid solution by the precipitation of Zr in the Mg matrix. This reaction takes place in two stages: (i) Zr plates are formed in the basal plane and (ii) these initial precipitates grow along the [0001] direction. An epitaxial relationship between those Zr precipitates and the Mg matrix of [0001]Zr || [0001]Mg and [1120]Zr || [1120]Mg has been obtained. The third exothermic transformation appears above 720 K and is caused by the strong oxidation of the deposit.

Polymorphic transformations in iodine titanium

Based on the analysis of differential scanning calorimetry data, the possibility of classifying the observed endothermic or exothermic transformations as phase transformations of the first oder is considered. Two approaches have been implemented. The first is based on the correspondence between the temperatures of the maximum conversion rate and the temperatures of the extrema on the second derivative of the differential scanning calorimetry signal with respect to temperature. In the second approach, the phase transformation is considered as a kind of kinetic reaction of a chemical process with the determination of some parameters included in the kinetic equations. In this case, the order parameter of such reaction n is obtained from the analysis of the differential scanning calorimetry signal shape in the region of phase transformation registration temperatures. Using the example of experiments carried out during thermal cycling of titanium iodide samples, it is shown that both the first and second approaches make it possible to fairly adequately attribute the processes that cause calorimetric effects on the dependences of differential scanning calorimetry to first-order phase transitions. In particular, the obtained results of differential scanning calorimetry during heating and cooling of iodide titanium show that polymorphic transformations in it are realized by various mechanisms depending on the rate of thermal cycling and the thermal history of the metal. Keywords: activation energy, titanium, calorimetry, polymorphism, structure, approximation.

Полиморфные превращения в йодидном титане

Based on the analysis of differential scanning calorimetry data, the possibility of classifying the observed endothermic or exothermic transformations as phase transformations of the first oder is considered. Two approaches have been implemented. The first is based on the correspondence between the temperatures of the maximum conversion rate and the temperatures of the extrema on the second derivative of the differential scanning calorimetry signal with respect to temperature. In the second approach, the phase transformation is considered as a kind of kinetic reaction of a chemical process with the determination of some parameters included in the kinetic equations. In this case, the order parameter of such reaction n is obtained from the analysis of the differential scanning calorimetry signal shape in the region of phase transformation registration temperatures. Using the example of experiments carried out during thermal cycling of titanium iodide samples, it is shown that both the first and second approaches make it possible to fairly adequately attribute the processes that cause calorimetric effects on the dependences of differential scanning calorimetry to first-order phase transitions. In particular, the obtained results of differential scanning calorimetry during heating and cooling of iodide titanium show that polymorphic transformations in it are realized by various mechanisms depending on the rate of thermal cycling and the thermal history of the metal.

Effects of strain rate on strain-induced martensite nucleation and growth in 301LN metastable austenitic steel

The effects of strain rate on strain-induced α′-martensite nucleation and growth were analyzed in this work. Tension tests were performed at room temperature at strain rates of 2×10−4 s−1 and 0.5 s−1 using small polished specimens that fit inside a scanning electron microscope. The specimens were deformed incrementally, and microstructural evolution was tracked carefully at a specific location on the specimen surface. This approach allows the analysis not only of the spatial but also of the temporal evolution of the α′-martensite. Optical microscopy images and electron backscatter diffraction (EBSD) measurements were taken for each plastic deformation increment. The size and number of α′-martensite particles were evaluated from the EBSD images, whereas local microlevel strains were obtained using Digital Image Correlation (DIC). According to the results, the number of nucleation sites for α′-martensite does not seem to be affected much by the strain rate. However, there is a notable strain rate effect on how the transformation proceeds in the neighborhood of freshly formed α′-martensite particles. At a low strain rate, repeated nucleation and coalescence leads to the notable growth of α′-martensite particles, whereas at a high strain rate, once nucleated α′-martensite particles remain as small isolated islands that do not markedly grow with further plastic strain. This phenomenon can be attributed to local microstructure-level heating caused by plastic deformation and exothermic phase transformation. This reduces the local growth rate of the α′-martensite particles in the vicinity of the above-mentioned islands, thus leading to a lower bulk transformation rate at higher strain rates.

A sequential Box-Behnken Design (BBD) and Response Surface Methodology (RSM) to optimize SAPO-34 synthesis from kaolin waste

Kaolin waste from an industry in the Amazon region was selected as an economic source of Si and Al to optimize the synthesis of SAPO-34 molecular sieve. A sequential BBD, RSM and global desirability function (D) was used. The factors studied for optimization were the SiO2/Al2O3 molar ratio, the concentration of the structure-directing agent (SDA; morpholine) and the crystallization time. The responses evaluated in the synthesized SAPO-34 were the relative crystallinity (%), the enthalpy of formation of transition alumina (J.g−1) and the mass loss (%). The synthesis as a function of time and its products post-DSC and calcined at 1000 °C was performed to relate the enthalpy of formation of the transition alumina with the SAPO-34 formation. Prediction models and response surfaces were corroborated by the level values optimized by the desirability function. The ideal values based on the data used in this study included a crystallization time of 20 h, a xSiO2 value of 1.3 and a ySDA value of 1.6. The experimental validation based on the ideal values was within the established confidence level (±95%). The formation of the transition alumina at 985 °C influences the enthalpy of exothermic transformation identified at this temperature. Low values of this enthalpy are related to the effective formation of SAPO-34.

Chemical Oxidative Polymerization of Methylene Blue: Reaction Mechanism and Aspects of Chain Structure.

The kinetic regularities of the initial stage of chemical oxidative polymerization of methylene blue under the action of ammonium peroxodisulfate in an aqueous medium have been established by the method of potentiometry. It was shown that the methylene blue polymerization mechanism includes the stages of chain initiation and growth. It was found that the rate of the initial stage of the reaction obeys the kinetic equation of the first order with the activation energy 49 kJ · mol−1. Based on the proposed mechanism of oxidative polymerization of methylene blue and the data of MALDI, EPR, and IR spectroscopy methods, the structure of the polymethylene blue chain is proposed. It has been shown that polymethylene blue has a metallic luster, and its electrical conductivity is probably the result of conjugation over extended chain sections and the formation of charge transfer complexes. It was found that polymethylene blue is resistant to heating up to a temperature of 440 K and then enters into exothermic transformations without significant weight loss. When the temperature rises above 480 K, polymethylene blue is subject to endothermic degradation and retains 75% of its mass up to 1000 K.

Local Temperature as a Chemical Reactivity Descriptor.

Using the electron density and its associated quantities in a molecular system to quantify chemical reactivity in density functional theory is of considerable recent interest. Local temperature based on the kinetic energy density is an intrinsic property of a molecular system, which can be employed for this purpose. In this work, we explore such a possibility. To this end, we examine the local behavior of local temperature with a few choices of the kinetic energy density, apply it to determine regioselectivity of nucleophilic and electrophilic compounds, and then investigate its performance in appreciating reactions along the intrinsic reaction pathway for exothermic, endothermic, and thermoneutral transformations. Our results confirm that local temperature can be used as an effective descriptor of molecular reactivity.

Effects of Strain Rate on the Strain Induced Martensite Nucleation and Growth in 301LN Metastable Austenitic Steel

The effects of strain rate on the strain induced α’-martensite nucleation and growth were analysed in this work. Tension tests were performed at room temperature at strain rates of 2×10-4 s-1 and 0.5 s -1 using small polished specimens, which fit inside a scanning electron microscope. The specimens were deformed incrementally, and microstructural evolution was tracked carefully at the same location. This approach allows analysing not only the spatial but also the temporal evolution of the α’-martensite. Optical microscopy images and electron backscatter diffraction (EBSD) measurements were taken for each plastic deformation increment. The size and number of α’-martensite particles were evaluated from the EBSD images, whereas the local microlevel plastic strains were obtained with Digital Image Correlation (DIC). According to the results, the number of nucleation sites for α’-martensite does not seem to be affected much by the strain rate. However, there is a notable strain rate effect on how the transformation proceeds in the neighbourhood of freshly formed α’-martensite particles. At low strain rate, repeated nucleation and coalescence leads to notable growth of the α’-martensite particles, whereas at high strain rate, once nucleated α’-martensite particles remain as small isolated islands which do not markedly grow with further plastic strain. This phenomenon can be attributed to local microstructure level heating caused by plastic deformation and the exothermic phase transformation. This reduces the local growth rate of the α’-martensite particles in the vicinity of the above-mentioned islands, thus leading to a lower bulk transformation rate at higher strain rates.

Thermostructural shape memory effect observations of ductile Cu-Al-Mn smart alloy

The Cu-Al-Mn shape memory alloy (SMA) with a new different composition was fabricated by arc melting method. The characteristic shape memory effect (SME) property of Cu-Al based SMA was revealed by performing thermostructural measurements. The differential scanning calorimetry (DSC) tests were taken to observe the reversible martensitic phase transformation peaks of the alloy as evidence of SME property of the alloy. To determine the basic thermodynamical parameters of the alloy, these endothermic and exothermic transformation peaks were analyzed by the tangent differentiation method that was performed automatically by the DSC analyzing program over a manually selected part on the DSC curve and by these analyses the characteristic martensitic transformation temperatures (working temperatures) that found below 100°C and the enthalpy change values of the alloy were directly obtained. The other kinetic transformation parameters of the alloy - the entropy change, hysteresis, and equilibrium temperature - were also determined. The common high-temperature behavior of the Cu-Al based Heusler alloys was detected by differential thermal analysis (DTA) measurement. The XRD and metallography tests that were conducted at room temperature showed the presence of M18R and the dominant 2H martensite structures that formed in the alloy and this dual martensitic structure was also prescribed by determining the theoretical e/a ratio of the alloy. Furthermore, the microhardness tests on the alloy demonstrated the high ductility feature of the alloy. All results demonstrated that the CuAlMn alloy exhibiting a shape memory effect property can be useful in smart alloy applications.

Development of a technology of isothermal annealing with the use of the forging heat for chromium‑molybdenum steel

The article discusses the results of investigations performed during a thermo-mechanical treatment of forgings made of chromium‑molybdenum 42CrMo4 grade steel. The treatment was realized during a regular series production. The forging process was combined with a heat treatment carried out directly after forging on a specially adapted station. Such a production technology will make it possible to eliminate the step of repeated heating of the forgings. On the example of an element of a steering gear, it was demonstrated how it is possible to perform an isothermal annealing process starting from the temperature at which the trimming of the forgings ends. During the cooling of the forgings, it is enough to maintain the temperature at the proper level in order for the exothermal phase transformation of austenite into pearlite to take place. The test results show that, with the properly selected temperature of isothermal annealing, it is possible to obtain an equilibrial ferritic-pearlitic structure in the required hardness scope. Introducing such a solution into the industrial practice would allow savings more than 80% of the energy used for the heat treatment of forging in this case.

Confinement of molecular materials using a solid-state loading method: a route for exploring new physical states and their subsequent transformation highlighted by caffeine confined to SBA-15 pores.

Using the innovative solid-state loading (milling-assisted loading, MAL) method to confine caffeine to cylindrical pores (SBA-15, ∅ = 6 nm) gives the opportunity to explore the original physical states of caffeine and their subsequent transformation using low-frequency Raman spectroscopy, powder X-ray diffraction and microcalorimetry investigations. It was shown that MAL makes possible the loading of the selected form in the polymorphism of caffeine. While form II has similar structural and dynamics properties in confined and bulk forms, the confined rotator phase (form I) exhibits clear differences with the bulk form inherent to its orientational disorder. Interestingly, the two confined forms of caffeine undergo an exothermic disordering transformation upon heating into a physical state at the border between a nanocrystallized orientationally disordered phase and an amorphous state, not existing in the bulk form. The melting of this new physical state was observed at 150 °C, i.e. 85 degrees below the melting temperature of the bulk form I, thus demonstrating the confinement of caffeine. It was also found that the liquid confined to pores of 6 nm mean diameter recrystallizes upon cooling, which can be explained by the very disordered nature of the recrystallized state.