Isothermal Kinetics Research Articles

Oxidation kinetics and electrical properties of oxide scales formed under exposure to air and Ar–H2-H2O atmospheres on the Crofer 22 H ferritic steel for high-temperature applications such as interconnects in solid oxide cell stacks

A 100 h isothermal oxidation kinetics study for Crofer 22H was conducted in air and the Ar–H2-H2O gas mixture (p(H2)/p(H2O) = 94/6) in the range of 973–1123 K. The parabolic rate constant was independent of oxygen partial pressure in the range from 6.2 × 10−24 to 0.21 atm at 1023 and 1073 K, while at 973 and 1123 K it was higher in air than in Ar–H2-H2O. The scales consisted of Cr2O3 and manganese chromium spinel with an Mn:Cr ratio dependent on the oxidation conditions. Cross-scale resistance was evaluated with regards to the application of the steel in solid oxide fuel cells using a number of methods, including X-ray diffraction, scanning electron microscopy, confocal Raman imaging and area-specific resistance measurements.

Fabrication of N,C-H2TiO3 Ion Sieves for Rapid Adsorption of Lithium Using Rheological Phase Methods

Modifying β-H2TiO3 by elemental doping improves its adsorption capacity and shortens the equilibrium adsorption time, which is of positive significance for its industrial application.Herein, a novel N, C co-doped Li2TiO3 lithium ion sieve precursor N,C-Li2TiO3-3 (noted as N,C-LTO-3) was successfully prepared via the rheological phase method using urea as a vital raw material. Crystal structure of N,C-LTO-3 was characterized via XRD, XPS, FT-IR, and Raman. The morphology of N,C-LTO-3 was analysed by SEM and HR-TEM. The N,C-LTO-3 was pickled and obtained to N,C-H2TiO3-3, namely N,C-HTO-3. The equilibrium adsorption capacity (Qe) of N,C-HTO-3 was as high as 40.6mg⋅g-1 in LiOH solution of 100mg·L-1 at 293K. Ti4+ dissolution loss rate is below 0.5% in the fifth cycle. Adsorption isothermal curve, kinetics, and thermodynamic behaviour were fitted with the according equations. The exchange energy of N,C-HTO-3 for Li+ was calculated by density functional theory (DFT) to assess the feasibility of lithium absorption. Furthermore, N,C-HTO-3 exhibited high regeneration and selectivity for Li+ in lithium-containing brine, making N,C-HTO-3 an ideal candidate for Li+ extraction.

Kinetics of Oxidation of an Al2O3–SiC–C‐Based Blast Furnace Trough Refractory in Air

During tapping of liquid hot‐metal from the blast‐furnace, the blast‐furnace main trough gets exposed to air and the blast‐furnace‐trough refractory becomes prone to oxidation. In this work, isothermal oxidation kinetics of an Al2O3–SiC–C‐based BFT (Blast‐Furnace‐Trough) refractory castable using a high‐temperature vertical tubular furnace equipped with a microbalance is investigated. Herein, it is observed that up to about 800 °C, carbon only oxidizes, resulting in weight loss in the refractory. Around 900 °C, SiC also starts to oxidize via the passive‐oxidation mechanism, leading to the formation of a mullite phase as a result of the reaction between SiO2 generated via passive oxidation and Al2O3 already present in the refractory. This causes a volume expansion and weight gain, both having a beneficial effect on the refractory performance, which is further validated from apparent porosity measurements. Thus, above 900 °C, simultaneous oxidation of C and passive oxidation of SiC occurs, which eventually stabilizes the weight of the refractory. Around 1100 °C, the oxidation process of the refractory is practically complete, with no further increase in the oxidation rate beyond 1100 °C. The activation energy for the oxidation process is obtained as 37.2 ± 5 kJ mol−1 in the temperature range 600–1150 °C.

Influence of polypropylene and high-density polyethylene on isothermal pyrolytic degradation of discarded bakelite: Kinetic analysis and batch pyrolysis studies

The recycling of thermosetting plastics such as discarded bakelite poses greater challenges than thermoplastic polymers like polypropylene (PP) and high-density polyethylene (HDPE), particularly through pyrolysis requiring specialized reactors. This study examines the isothermal thermal degradation kinetics and batch pyrolysis behaviors of bakelite, along with its blends with PP and HDPE, emphasizing the characterization of pyrolytic oils for designing optimized reactors. For the study on isothermal thermal degradation kinetics, isothermal thermogravimetric analysis was performed at specified temperatures (300, 350, 400, 450, and 500°C), chosen based on the predominant non-isothermal degradation behavior of bakelite. The batch pyrolysis of discarded bakelite and blended bakelite with PP/HDPE is carried out at 450°C. The chemical composition analysis of pyrolytic oils is performed using Fourier transform infrared (FTIR) spectroscopy, with comprehensive compound analysis conducted using Gas Chromatography-Mass Spectrometry (GCMS). Isothermal thermogravimetry at temperatures ranging from 300°C to 500°C reveals increased thermal degradation with rising pyrolytic temperatures, reaching maximum weight losses of 55 % for bakelite, 82.74 % for PP-bakelite blends, and 90.8 % for HDPE-bakelite blends at 500°C. The isothermal kinetics study reveals that bakelite degrades via D1-diffusion, polypropylene-blended bakelite via A2-Avrami-Erofeyev, and high-density polyethylene-blended bakelite via A3-Avrami-Erofeyev, with activation energies of 17.178, 7.193, and 3.550 kJ/mol, and Arrhenius constants of 0.095, 0.042, and 0.017 min.−1, respectively. The highest condensable yield of 58.76 % during PP-blended bakelite co-pyrolysis underscores its potential for resource recovery. FTIR and GC-MS confirm the presence of alkanes, cycloalkanes, alkenes, cycloalkenes, aromatic hydrocarbons, and oxygenated compounds in the pyrolytic oils, providing detailed insights into their chemical composition. These findings offer critical insights into the thermal degradation behavior and kinetics of bakelite and polypropylene/high-density polyethylene-blended bakelite, highlighting opportunities for efficient waste plastic utilization through pyrolysis for resource recovery and energy generation, and providing essential knowledge for designing isothermal pyrolysis reactors.

Microstructure and mechanical properties of a severely cold-rolled and annealed dual-phase compositionally complex alloy (CCA) with an exceptionally deformable Laves phase

A novel CCA was designed by substituting Nb in (FCC + C14 Laves) CoCrFeNi2.1(Nb)0.2 CCA by (Hf + Nb + Ta). The (HfNbTa)0.2 CCA was homogenized, heavily cold-rolled, and isothermally annealed at 800 °C and 1000 °C for different time intervals. The (HfNbTa)0.2 alloy revealed the presence of a Hf and Ni enriched cubic C15 Laves phase. The considerations of site occupancy behavior, formation energy, and highly off-stoichiometric composition stabilized the (Hf, Ni) rich cubic C15 Laves phase. In contrast to the brittle hexagonal C14 Laves phase in (Nb)0.2 CCA, the C15 Laves phase in (HfNbTa)0.2 CCA showed exceptional deformability owing to the high propensity for nano-twin formation. Meanwhile, the FCC matrix developed a deformation-induced nano-lamellar structure with a spacing of ∼45 nm. Annealing resulted in ultrafine recrystallized FCC matrix and precipitation of DO19 structured ε nano-precipitates. The isothermal grain growth kinetics revealed a high grain growth exponent (n) ∼7, which confirmed a Zener-drag mediated process due to the ε nano-precipitates. The Hall-Petch analysis of the hardness data showed relatively high friction stress originating from the dissolution of Hf, Nb, and Ta in the FCC matrix. A high Hall-Petch coefficient indicated increased shear stress for plastic flow across the boundaries, resulting from the elongated Laves phase at the boundaries. The highly deformable Laves phase, ultrafine grain size, and ε nano-precipitates resulted in high yield strength (∼975 MPa) and superior ductility (∼16 %) in the (HfNbTa)0.2 CCA, even surpassing the (Nb)0.2 CCA. It was envisaged that strong yet deformable Laves phases could pave the pathway for developing Laves phase-based CCAs for advanced structural applications.

Isothermal reduction kinetics and reduction prediction for iron ore pellets

Isothermal reduction kinetics and reduction prediction for iron ore pellets

A study on the isothermal decomposition kinetics of energetic cellulose-rich materials using a vacuum stability test

A study on the isothermal decomposition kinetics of energetic cellulose-rich materials using a vacuum stability test

Composition optimization of (Hf, Ta, Zr, Cr)C high‐entropy carbides for good oxidation resistance

AbstractOxidation resistance is crucial to the potential applications of high‐entropy carbides (HECs) at elevated temperatures. Here, we realize the exploration of (Hf, Ta, Zr, Cr)C high‐entropy carbides (HEC‐TM, TM = Hf, Zr, Ta, and Cr) with good oxidation resistance by optimizing their compositions. To be specific, 21 kinds of HEC‐xTM (x = 0–25 mol%) samples are fabricated by a high‐throughput ultrafast high‐temperature sintering technique, followed by oxidation testing at 1673 K for 30 min. Among all the HEC samples, the as‐fabricated HEC‐0Zr samples are proved to possess the best oxidation resistance with an oxidation depth of only 53 µm. Further study on isothermal oxidation kinetics demonstrates that the as‐fabricated HEC‐0Zr samples follow a linear oxidation law. The good oxidation resistance of the as‐fabricated HEC‐0Zr samples is believed to result from the (Ta, Me)2O5 phase with a low melting point, which can promote the densification of the oxide layer. This research opens up a new way for efficiently discovering new HECs for extreme applications.

Exploring adsorption dynamics of heavy metals onto varied commercial microplastic substrates: Isothermal models and kinetics analysis

The increasing presence of plastics in the environment has raised concerns about their potential impact, especially as carriers of heavy metals such as Cd, Ni, and Pb. However, the adsorption mechanism of heavy metals on microplastics remains poorly understood. In this study, we investigated the adsorption behavior of Cd, Ni, and Pb by polystyrene (PS) and polypropylene (PP) microplastics to better comprehend their interaction and potential environmental implications. Our results revealed that equilibrium adsorption of microplastics with different heavy metals was achieved within a 6-h contact time. The FTIR analysis findings, which suggest that physical interactions play a significant role in the adsorption of heavy metals onto microplastics, are further supported by the observed changes in surface morphology after adsorption. We explored the influence of solution pH, contact duration, and initial concentration on the adsorption capacity and found significant effects on the adsorption behavior. To model the adsorption process, we applied Langmuir and Freundlich adsorption isotherm models and observed that the Langmuir model better fit the experimental data. Furthermore, we compared the pseudo-first and pseudo-second-order kinetic models and found that the pseudo-second-order model provided a more accurate description of the adsorption kinetics. Notably, the adsorption percentages varied depending on the type of microplastic and experimental conditions. Overall, this study enhances our understanding of the adsorption mechanism of heavy metals on microplastics and provides valuable insights into their behavior in aquatic environments. These findings have implications for the development of effective strategies for mitigating pollution caused by microplastics and heavy metals in aquatic ecosystems.

Effects of carbon concentration and transformation temperature on incomplete transformation and crystallography of low-carbon bainitic steel

A close relationship exists between the crystallographic characteristics of a bainitic matrix and formation of martensite/austenite (M/A) constituents. In this work, the isothermal transformation kinetics and crystallographic characteristics of low-carbon bainitic (LCB) steel were investigated to elucidate the formation mechanism of M/A constituents and twin-related V1/V2 variant pairs (Σ3 boundaries). The formation of M/A constituents depends on the bainitic transformation kinetics, which are influenced by the carbon concentration and isothermal temperature. An increase in the carbon concentration or transformation temperature promotes the formation of blocky M/A constituents by increasing the activation energy of the grain boundary and autocatalytic nucleation processes. In addition, increasing the carbon concentration or lowering the transformation temperature increases the strength of the austenite phase and promotes autocatalytic nucleation, thereby producing more twin-related V1/V2 variant pairs. Therefore, the increase in the carbon concentration increases the number of twin-related V1/V2 variant pairs while also causing the formation of blocky M/A constituents. To overcome this limitation, a novel heat treatment process design is proposed that can significantly eliminate blocky M/A constituents and generate high-density twin-related V1/V2 variant pairs.

Efficient hydrogen absorption and dehydrogenation of synergistically catalyzed Mg10Fe by MoS2 and Y2O3 via one-step synthesis and multistage regulation

To effectively deal with the long preparation period and low hydrogen absorption capacity issue of Mg-based hydrogen storage materials. One-step multistage control strategy and high-pressure hydrogenation have been employed to prepare Mg10Fe-M (M = MoS2 or Y2O3 or both) composites. The phase composition, microstructure, morphology, non-isothermal dehydrogenation thermodynamics and isothermal dehydrogenation kinetics at different milling time (2, 4, 8 h) were explored, and the catalytic hydrogen absorption and discharge mechanism of Mg10Fe–MoS2–Y2O3 was proposed. The addition of MoS2 nanoflowers and Y2O3 powder can synergistically regulate and improve the hydrogenation capacity and dehydrogenation temperature of Mg10Fe. The hydrogen absorption and dehydrogenation of Mg10Fe–MoS2–Y2O3 in short time milling for 2 h are 3.64 wt% and 3.27 wt% respectively. After extending the milling time to 8 h, the thermodynamic and kinetic behavior of Mg10Fe-M (M = MoS2 or Y2O3 or both) is greatly improved. Kinetically, the hydrogenation or dehydrogenation capacity increases significantly, Mg10Fe–MoS2 doesn't need activation, and the hydrogenation capacity reaches 5.42 wt% at 623 K, with a yield of about 85.9 % in 60 min. Thermodynamically, the initial dehydrogenation peak temperature drops to about 310 °C, and the apparent activation energy also decreases significantly, among which Mg10Fe–MoS2–Y2O3 was 91.87 kJ/mol. These are attributed to the fact that MoS2 nanoflowers provide many active sites for H2 adsorption and H atom dissociation. Synchro-efficient synthesis and regulation of Mg-rich composite hydrogen storage materials are expected to shorten the synthesis cycle, improve the hydrogen storage performance, and lay a material foundation for the design and development of solid hydrogen storage devices.

Non-isothermal and isothermal oxidation behavior and kinetics of shale oil during air injection

Non-isothermal and isothermal oxidation behavior and kinetics of shale oil during air injection

Facile Characterization of Isothermal Crystallization and Microphase Separation Kinetics of Polyamide 6 (PA6)‐Based Thermoplastic Elastomers

AbstractThe facile characterization of isothermal microphase separation kinetics in polyamide 6 (PA6)‐based thermoplastic elastomers (TPAE‐6) has long posed a challenge for the development of suitable melt spinning processes. In this study, this challenge is addressed through differential scanning calorimetry (DSC) measurements. It is assumed that the enthalpy changes of TPAE‐6 during the isothermal process are a linear superposition of enthalpy changes associated with microphase separation and crystallization of PA6 in hard phases, resembling that of TPAE‐6 without soft segments (TPAE‐6‐0). The study reveals that, as the concentration of soft segments in TPAE‐6 increases, the accelerated dynamics of phase separation become stronger than the dilution of soft segments to PA6 segments during isothermal process, resulting in an increase in the microphase separation rate of TPAE‐6. Furthermore, despite microphase separation, the overall crystallization rate of TPAE‐6 decreases with rising isothermal temperature and varies with increasing soft segment content at different temperatures. Additionally, the crystallization mode of TPAE‐6 follows two‐dimensional, three‐dimensional, or a combination of both crystal growth mechanisms, accompanied by a heterogeneous nucleation mechanism.

Isothermal calcination kinetics of boric acid and its use for nano boron oxide production under cryogenic conditions

The isothermal calcination kinetics of H3BO3 to B2O3 was firstly investigated as a function of time at 160, 200, 240 and 280 °C using heterogeneous and homogeneous reaction models and were characterized by XRD, FTIR-ATR, SEM and DTA/TG devices. Peaks in XRD patterns and bands in FTIR-ATR spectrum confirmed the structure of H3BO3 and B2O3. SEM showed that H3BO3 had a rather clean, flaky, smooth and plate-shaped surface while B2O3 had a rough surface. TG analysis confirmed that H3BO3 was degraded in two steps. Avrami model fitted a very good agreement with the experimental data. The activation energy was calculated as 14.2 kJ/mol. The smallest particle size for nano B2O3 was measured as 33 nm with Zetasizer NanoS. TEM images also confirmed that the particle size of B2O3 was measured to be approximately 50 nm. Additionally, the BET surface area of nano B2O3 was determined as 10.8 m2/g.

Ultrasound enhances the removal of V5+ in wastewater by ball-milled zero-valent iron: Effects and mechanisms

Vanadium is considered to be a harmful industrial pollutant that poses a serious threat to health. In this study, the effect of ultrasonic-enhanced ball milling zero-valent iron removal of vanadium in wastewater was investigated and the basic mechanism of this process was elucidated. The results of the study confirmed that the ultrasonic intervention significantly accelerated the vanadium removal rate and greatly shortened the time required to achieve equilibrium. Under certain experimental conditions, the removal efficiency of vanadium under ultrasonic conditions is 99.1 %, which is 1.4 times higher than that of the traditional constant-temperature water bath device (70.86 %). Analytical techniques such as electron paramagnetic resonance (EPR) and ultraviolet (UV) spectroscopy have confirmed that ultrasonic-induced cavitation and microjet phenomena actively stimulate the formation of more H radicals and Fe2+. The key role of Fe2+ in the removal reaction was further explored and confirmed by a quenching experiment. Through fitting the experimental data of different isothermal adsorption models and reaction kinetics, it is concluded that the adsorption process conforms to the pseudo-second-order kinetics and the Langmuir isothermal model. The adsorption process is dominated by chemisorption, and the maximum adsorption capacity is 153.033 mg·g−1. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and other analyses show that under the action of ultrasonic waves, V5+ is rapidly reduced to its lower oxidation state through reduction within the first 10 min of the reaction. Secondly, as the reaction progresses and the pH value increases, electrostatic adsorption and coprecipitation are involved in the removal reaction.

Improved dehydrogenation performance of LiAlH4 doped with BaMnO3

Due to its high gravimetric capacity of hydrogen (10.5 wt%), LiAlH4 has been regarded as a promising material for solid-state hydrogen storage material for onboard usage. However, high decomposition temperature, poor kinetics and irreversibility retard its application. To counter this problem, various weight percentages of BaMnO3 are introduced into the LiAlH4 system as an additive in this work. As a result, the starting hydrogen release of LiAlH4 was reduced to 109−115 °C and the second desorption temperature occurred at around 134−158 °C, much lower than pure LiAlH4. The isothermal desorption kinetics also proved that faster desorption kinetics can be observed at 90 °C for 80 min. About 2.00−2.60 wt% of H2 could be desorbed by the composite, whereas only <1.00 wt% of H2 was desorbed by undoped LiAlH4. Additionally, adding BaMnO3 reduced the activation energies by 30 kJ/mol for the first stages and 34 kJ/mol for the second stages. Based on the X-ray diffraction result, the active species formed of MnO2 and Ba or Ba−containing materials are believed to be responsible for the noticeable enhancement in the desorption properties of LiAlH4.

Effect of applied stress on bainite transformation, microstructure, and properties of 15CrMo steel

The effect of external stress on the isothermal bainite transformation kinetics and microstructure of 15CrMo steel was studied using dilatometer, optical microscopes (OM), electron back-scatter diffraction (EBSD), scanning electron microscopes (SEM) and X-ray diffraction (XRD). The results show that the additional mechanical driving force provided by the external stress promotes the transformation of bainite, resulting an increase volume fraction of bainite-ferrite (BF) and causing the BF laths to become slender. The application of external stress can also induce selective orientation during the bainite transformation process, and the orientation relationship between the prior austenite and the BF under stress is closer to the Kurdjumov-Sachs (K-S) relationship. There is also an incomplete transformation of bainite transformation under stress. At the same time, the Vickers hardness of 15CrMo steel would increase after applying external stress. The mechanical driving forces calculations demonstrate that when tensile stress and compressive stress are of similar magnitudes, they provide comparable mechanical driving forces. As a result, they exert similar effects on the kinetics of bainite transformation, leading to no discernible differences in the resulting bainite microstructure.

Oxidation kinetics of typical high FeO ferrous spinels

In this study, the isothermal oxidation kinetics of magnetite (OM), high-Mg magnetite (MM), titanomagnetite (TM), and chromite (CM) were investigated by applying thermogravimetry (TG) analysis at temperatures ranging from 1073 K to 1223 K. The results show that different high-FeO spinels possess distinct oxidizability. The oxidation process of OM in the temperature range from 1073 K to 1223 K is faster than others, followed by MM and TM. While CM exhibits the poorest oxidizability, and generally undergoes complex phase transitions. In the initial stage of oxidation, high FeO spinels have a higher oxidation rate due to the surface oxidation of spinel particles. However, the oxidation rate gradually declines in the later stages of oxidation due to increased internal diffusion resistance. The results of oxidation kinetics indicate that the initial oxidation stage of four spinels can be described as random nucleation and subsequent growth mechanism. The average apparent activation energies of the initial oxidation stage of OM, MM, TM, and CM are 25.09 kJ/mol, 32.39 kJ/mol, 58.10 kJ/mol, and 82.42 kJ/mol, respectively.

Adsorption, Kinetic and Thermodynamic Studies for the Adsorption of Cadmium onto Combination of Chitosan and Coffee Ground Activated Carbon

The presence of cadmium in water due to its natural mobility can cause the nature of the water to become toxic and threaten the surrounding ecosystem when it accumulates in the food chain. The aim of this study was to investigate the maximum adsorption capacity using an isothermal model, to determine the rate of adsorption kinetics using chitosan and coffee powder adsorbents in reducing cadmium concentrations in industrial wastewater, and to investigate its thermodynamic magnitude. The research method was applied with laboratory experiments followed by quantitative data analysis to determine the isothermal model and adsorption kinetics. The results showed that the adsorption isotherm follows the Langmuir isotherm model with a correlation coefficient of 0.9970 and a maximum adsorption capacity of 0.7546 mg.g-1, which indicates that the chemical adsorption occurs in a monolayer, where the adsorption sites are homogeneously distributed with the adsorption energy. Permanent and negligible interactions between cadmium molecules (adsorbate) and adsorbent. Thus the pseudo second order kinetic model is a better way to explain the reaction rate for cadmium in combination chitosan and coffee ground activated carbon. Negative ΔGo values indicate that the adsorption reaction takes place spontaneously, ΔHo of 0.3467 indicates an endothermic reaction, and ΔSo of 3.5296 indicates an increase in the randomness of the adsorption process at the adsorbent interface and cadmium during adsorption.

Melt-crystallization and self-nucleation of UHMWPE/bi-HDPE blends: The combined role of composition and molecular weight

Blends of bi-modal high-density polyethylene (bi-HDPE) and ultra-high molecular weight polyethylene (UHMWPE) with varying content (0–30 wt%.) and different molecular weights (MWs) (2.2, 3.5, and 4.5 M g/mol) of UHMWPE were prepared. The effect of UHMWPE content, MW, and their interplay on the microstructure, nucleation, and crystallization behavior of bi-HDPE was examined through rheology, scanning electron microscopy, and differential scanning calorimetry. The results revealed that UHMWPE serves as a partially miscible nucleating agent, effectively dispersing in the bi-HDPE matrix up to 5 wt%, thereby promoting the transition from randomly dispersed lamellae to a spherulitic morphology. The nucleating effect saturated when the UHMWPE content reached 10 wt%, leading to a diminished spherulitic structure. Beyond 10 wt% of UHMWPE, an anti-nucleation effect with segregated crystalline regions emerged. Isothermal kinetics analysis also revealed the highest crystallization rate at the optimum UHMWPE content of 5 wt%, where the critical nucleation energy barrier and fold-surface free energy were minimized. Apart from the UHMWPE content, signs of nucleation enhancement were slightly more pronounced in the bi-HDPE blend with the lowest molecular weight of UHMWPE. This phenomenon results from increased concentration fluctuations due to the larger interfacial area, as postulated from entanglement density measurements. For the first time, self-nucleation (SN) analysis revealed that the incorporation of 5 wt% of UHMWPE into bi-HDPE resulted in a wider Domain II width, indicating an increased memory effect. Notably, the crystalline memory was more pronounced in the blend containing UHMWPE with the MW of 2.2 M g/mol, exhibiting the super-nucleating effect with the highest nucleation efficiency of 218 %.