Values Of Apparent Activation Energy Research Articles

Oxygen release and reduction kinetics of La0.35Sr0.35Ba0.3Fe1-xCoxO3 as oxygen carriers for chemical looping dry reforming of methane

Chemical looping dry reforming of methane (CL-DRM) is a viable technology by converting CH4 and CO2 to various value-added products to achieve carbon neutrality. However, it is vital for the technology to find suitable oxygen carriers with high oxygen capacity and activity. La0.35Sr0.35Ba0.3Fe1-xCoxO3 perovskite-type oxides were proposed as oxygen carriers for CL-DRM. The oxygen (O2) release property and the kinetics of La0.35Sr0.35Ba0.3Fe1-xCoxO3 reduction by CH4 were investigated via thermogravimetric analysis, O2-temperature programmed desorption and then in a fixed-bed reactor. The O2 release process of La0.35Sr0.35Ba0.3Fe1-xCoxO3 OCs can be divided into two phases. The O2 release process and corresponding rate of OCs were facilitated due to the substitution of Fe with Co in B-site. The total amounts of O2 release for these OCs were enhanced about 1.5 times from 0.445 mmol/gOC to 0.706 mmol/gOC as Co atomic fraction in B-site changes from 0 to 1. The linear correlation for high temperature phase and a volcano-curve for low temperature phase was found for the correlations among total O2 release and Co content during the O2 release process. The reduction kinetics of CH4 over OCs was described using the Avrami–Erofe'ev model (A1.5 or A2). The values of apparent activation energy (Ea) for all OCs were obtained. The results indicated the best substitution proportion of Co in La0.35Sr0.35Ba0.3Fe1-xCoxO3 OCs can be set in the range of 0.2–0.4 to emerge the excellent redox performance. The kinetics models and parameters offer valuable information for CL-DRM reactor design and further development of OCs with different A or/and B-site modifications.

Study of the basic laws of dissolution of gold and copper in solutions with an ultra-low concentration of sodium cyanide

The work sets out to study the basic physicochemical dissolution patterns of gold, copper, and natural copper-containing minerals (chalcopyrite, bornite and azurite) in solutions with an ultra-low concentration of sodium cyanide (from 0.102∙10-3 to 4.08∙10-3 mol/L). The influence of various factors on the rate of dissolution of Au and Cu in solutions with ultra-low NaCN concentrations was studied by the rotating disk method; for natural copper minerals, the powder diffraction method was used. The concentration of gold and copper in solutions was determined by atomic absorption analysis. The chemical composition of the studied copper minerals was determined using the X-ray phase method, while the specific surface of the minerals was detected using a laser granulometer. The process of gold dissolution is shown to proceed in both diffusion and kinetic regions. In the diffusion region, the rate constant was 0.334∙10-6 L∙cm-2∙s-1/2∙rad-1/2; in the kinetic region – 0.919∙10-6 L∙cm-2∙s-1/2. The calculated value of the apparent activation energy for the diffusion region was 22.5 kJ/mol; for the kinetic region – 40.1 kJ/mol. The addition of glycine to a solution with an ultra-low concentration of sodium cyanide is shown to increase the specific dissolution rate of gold by 1.2 times: from 0.692∙10-9 to 0.82∙10-9 mol/cm2∙s. The process of copper dissolution is shown to take place in the diffusion r egion. The rate constant was 0.496∙ 10-6 L∙cm-2∙s-1/2∙rad-1/2 at an activation energy of 17.0 kJ/mol. With a fractional supply of sodium cyanide, the dissolution rate of copper minerals is reduced by 10–30% compared to a single load. The calculated apparent activation energy values for chalcopyrite, bornite, and azurite were 22.03, 24.2, and 24.1 kJ/mol, respectively. Thus, the use of ultra-low concentrations of NaCN in the process of cyanidation of gold and copper has a positive effect, which can be used i n the processing of gold-copper raw materials to significantly reduce the consumption of sodium cyanide.

Thermal decomposition features of micro-nano rice husk/polylactic acid blends for 3D printing

The thermal decomposition process of 3D printed filament made from micro-nano rice husk (MNRH)/polylactic acid (PLA) blends was studied by dynamic thermogravimetric analysis. The characteristic temperatures and apparent activation energies of unmodified, single modified, and double modified RH/PLA composites were calculated by Friedman (FD), Flynn-Wall-Ozawa (FWO), Coats-Redfern (CR), and Kissinger (KS) kinetic models. With the modification of MNRH and PLA in the composites, the initial thermal decomposition temperature of the composite increased from 236.3°C to 244°C. At the same time, the thermal degradation degree decreased and the transition temperature interval increased. The apparent activation energy (AAE) values of different modified composites ranged from 90 to 120 kJ/mol, depending on the modification method and calculation method of the material. These four kinetic models provide methods to analyze the thermal stability of composites. It is helpful to known the thermal decomposition behavior of MNRH/PLA composites, and it will contribute to the development of MNRH/PLA filament for 3D printing in the application of automotive interior parts production.

Weak Metal-Support Interaction over CuO/TiO2 Catalyst Governed Low-Temperature Toluene Oxidation.

Regulating the metal-support interaction is essential for obtaining highly efficient catalysts for the catalytic oxidation of volatile organic compounds (VOCs). In this work, CuO-TiO2(coll) and CuO/TiO2(imp) with different metal-support interactions were prepared via colloidal and impregnation methods, respectively. The results demonstrated that CuO/TiO2(imp) has higher low-temperature catalytic activity, with a 50% removal of toluene at 170 °C compared to CuO-TiO2(coll). Additionally, the normalized reaction rate (6.4 × 10-6 mol·g-1·s-1) at 160 °C over CuO/TiO2(imp) was almost four-fold higher than that over CuO-TiO2(coll) (1.5 × 10-6 mol·g-1·s-1), and the apparent activation energy value (27.9 ± 2.9 kJ·mol-1) was lower. Systematic structure and surface analysis results disclosed that abundant Cu2+ active species and numerous small CuO particles were presented over CuO/TiO2(imp). Owing to the weak interaction of CuO and TiO2 in this optimized catalyst, the concentration of reducible oxygen species associated with the superior redox property could be enhanced, thus significantly contributing to its low-temperature catalytic activity for toluene oxidation. This work is helpful in exploring the influence of metal-support interaction on the catalytic oxidation of VOCs and developing low-temperature catalysts for VOCs catalytic oxidation.

Investigation of thermodynamic and kinetic parameters of Albizia lebbeck seed pods using thermogravimetric analysis

Thermodynamic and kinetic studies are very necessary to evaluate the conversion efficiency of biomass to energy. Therefore, this current work reported the thermodynamic and kinetic parameters of Albizia lebbeck seed pods through thermogravimetric analysis, which was carried out at temperatures from 25 °C to 700 °C, and heating rates of 5, 10, 15, and 20 °C/min. Apparent activation energies were determined by applying three iso-conversional model-free methods including Kissinger-Akahira-Sunose (KAS), Ozawa-Flynn-Wall (OFW), and Starink. Resultantly, average apparent activation energy values for the three models of KAS, OFW, and Starink were found to be 155.29, 156.14, and 155.53 kJ/mol, respectively. In addition, thermodynamic triplets such as enthalpy, Gibbs free energy, and entropy were obtained as 151.16 kJ/mol, 150.64 kJ/mol, and −7.57 J/mol·K, respectively. The above results suggest Albizia lebbeck seed pods could become a potential source for bioenergy production aiming to achieve the sustainable goal and waste-to-energy strategy.

Viscosity of the supercooled liquid and crystallization kinetic analysis in the Fe74B20Nb2Hf2Si2 amorphous alloy

The non-isothermal thermal behavior of Fe74B20Nb2Hf2Si2 bulk metallic glass (BMG) was studied utilizing various thermal analytical methods. The obtained results showed that the viscosity behavior of this alloy in the supercooled region was strong due to the high kinetic fragility parameter (D* = 49.5) and low fragility parameter (m = 20.2). Also, the apparent activation energy (Ep) values for crystallization process were calculated in the range of 188.7–394.6 kJ/mol using the Gao-Wang method. Additionally, the Avrami's exponent (n) values increased from 2 to 4 with the progress of the crystallization process. The two-parameter Šesták-Berggren (SB) model was used to demonstrate the autocatalytic behavior of crystallization. Furthermore, the kinetic analysis revealed that all crystallization stages were controlled by the interface. The phase analysis showed that the Fe23B6, α-Fe, Fe2(Hf, Nb), and Fe2B phases were formed during the crystallization process. As obtained by the microstructural characterization, it was founded that the low dimensional growth (m = 2) is due to the complex structure of the main phase of Fe23B6 in the first crystallization stage. Moreover, the high dimensional growth (m = 3) during the second and third crystallization stages attributed to the Fe2B phase as the heterogeneous nucleation, as well as a significant increase in the volume fraction of the α-Fe phase, and the preferential orientation of the various crystalline phases. Hence, the values of Avrami's exponent (n) in the second and third crystallization stages were calculated equal to 3 and 4, respectively.

Comprehensive assessment of thermal characteristics, kinetics and environmental impacts of municipal solid waste incineration fly ash during thermal treatment

The volatilization of heavy metals and the release of harmful gases during the thermal treatment of municipal solid waste incineration (MSWI) fly ash (FA) can cause secondary pollution. Understanding the thermal characteristics, heavy metal behaviors and environmental emissions in FA roasting is significant for pollution control and heavy metal elimination. In this work, the thermodynamics, kinetics, environmental impacts, carbon emission, phase transition, leaching toxicity, transition behavior of chlorine and heavy metals in FA thermal treatment were investigated comprehensively for the first time through TG-FTIR, XRD, ICP-OES and IC. The apparent activation energy and thermodynamic parameters were obtained according to the kinetic modeling method. The average apparent activation energy values of the first, second, third, and fourth stages were 60.21, 114.91, 190.23, and 226.67 kJ/mol, respectively, and the reaction control models were chemical reaction model (No.3), assumed random nucleation and subsequent growth model (No.21), assumed random nucleation and subsequent growth model (No.14), and nucleation model (No.10). The environmental impacts illustrate that large amounts of gaseous products, including CO2, H2O, NO and SO2, are released during the thermal treatment of FA, and the emission of CO2 sharply increases as the temperature exceeds 550 °C. When treating 1 kg of FA at 1000 °C, the release of CO2 in stages I, II, III and IV is 0.18, 7.06, 116.34 and 177.21 g, respectively, and the total carbon emission is approximately 300 g. The leaching toxicity of heavy metals in FA is higher below 800 °C, and it is mainly caused by Pb. In addition, the volatilization rate of heavy metals and chlorine increased significantly as the calcination temperature exceeded 800 °C, among which, the volatilization rates of Pb, Cu, and Cl can reach 99.87%, 90.25% and 73.85%, respectively. At high temperatures, heavy metals in FA exist primarily in oxidizable and residual states. The obtained findings can provide theoretical guidance for pollution control and process optimization of FA thermal treatment.

Engineering Surface Properties of CuO/Ce0.6Zr0.4O2 Catalysts for Efficient Low-Temperature Toluene Oxidation

The development of superior low-temperature catalytic performance and inexpensive catalysts for the removal of volatile organic compounds (VOCs) is crucial for their industrial application. Herein, CuO/Ce0.6Zr0.4O2 catalysts calcinated at different temperatures (Cu/CZ-X, X represented calcination temperature) were prepared and used to eliminate toluene. It can be found that Cu/CZ-550 presented the highest low-temperature catalytic activity, with the lowest temperature (220 °C) 50% conversion of toluene, the highest normalized reaction rate (3.1 × 10−5 mol·g−1·s−1 at 180 °C) and the lowest apparent activation energy value (86.3 ± 4.7 kJ·mol−1). Systematically, the surface properties analysis results showed that the optimum redox property, abundant oxygen vacancies, and plentiful surface Ce3+ species over Cu/CZ-550 were associated with the strong interaction between Cu and support could significantly favor the adsorption and activation of toluene, thus resulting in its superior catalytic performance.

Innovative Recovery of Gallium and Zinc from Corundum Flue Dust by Ultrasound-Assisted H2SO4 Leaching

ABSTRACT Rare metal gallium is classified as a strategic material because of continued high demand, uncertain supply, and low reserves. Corundum flue dust (CFD) is generated during bauxite smelting to produce brown corundum. Gallium (Ga), Zinc (Zn), and potassium in the bauxite could be enriched with CFD through this process, so CFD is regarded as one of the potential sources of Ga and Zn. A novel that utilizes ultrasound to enhance the H2SO4 leaching efficiency of Ga and Zn from CFD was developed from CFD. The effects of ultrasound power, leaching time, sulfuric acid concentration, and leaching temperature on the leaching behavior of Ga and Zn were studied, and the corresponding kinetics model was established. It was found that the maximum Ga and Zn leaching efficiencies of 81.22% and 96.72% were obtained at optimum leaching conditions: the leaching temperature of 90°C, the leaching time of 60 min, 3 M H2SO4, the solid-to-liquid ratio of 1:5 (g/mL), and the ultrasound power of 750 W. In all cases, the leaching effect of Zn is not significantly improved by ultrasonic treatment, but the leaching efficiency of Ga has a positive effect (more than an 18% increase). Kinetic analysis shows that the leaching process of Ga and Zn were controlled by chemical reaction and diffusion, respectively. The apparent activation energy values are 20.39 kJ/mol for Ga and 13.44 kJ/mol for Zn. The characterization of the leaching residues reveals that the ultrasound effected to enable the whole block to be fractured or dispersed into fragments, accelerating the penetration of H2SO4 into the pores and cracks at the solid, which further prompts the dissolution of the encapsulated gallium and achieves the conspicuous improvement to Ga leaching efficiency.

A Novel Hydro-Thermal Synthesis of Nano-Structured Molybdenum-Iron Intermetallic Alloys at Relatively Low Temperatures

Nano-structured Mo/Fe intermetallics were synthesized from precursors that contained 72/28% and 30/70% molar ratios of Mo/Fe, which were given as precursors A and B, respectively. These precursors were prepared from the co-precipitation of aqueous hot solutions of ammonium heptamolybdate tetrahydrate (AHM) and ferrous oxalate. The dry precipitates were thermally treated using TG-DSC to follow up their behavior during roasting, in an Ar atmosphere of up to 700 °C (10° K/min). The TG profile showed that 32.5% and 55.5% weight losses were measured from the thermal treatment of precursors A and B, respectively. The DSC heat flow profile showed the presence of endothermic peaks at 196.9 and 392.5-400 °C during the thermal decomposition of the AHM and ferrous oxalate, respectively. The exothermic peak that was detected at 427.5 °C was due to the production of nano-sized iron molybdate [Fe2(MoO4)3]. An XRD phase analysis indicated that iron molybdate was the only phase that was identified in precursor A, while iron molybdate and Fe2O3 were produced in precursor B. Compacts were made from the pressing of the nano-sized precursors, which were roasted at 500 °C for 3 h. The roasted compacts were isothermally reduced in H2 at 600-850 °C using microbalance, and the O2 weight loss that resulted from the reduction reactions was continuously recorded as a function of time. The influence of the reduction temperature and precursor composition on the reduction behavior of the precursors was studied and discussed. The partially and completely reduced compacts were examined with X-ray powder diffraction (XRD), a reflected light microscope (RLM), and a scanning electron microscope (SEM-EDS). Depending on the precursor composition, the reduction reactions of the [Fe2(MoO4)3] and Fe2O3 proceeded through the formation of intermediate lower oxides, prior to the production of the MO/Fe intermetallic alloys. Based on the intermediate phases that were identified and characterized at the early, intermediate, and final reduction degrees, chemical reaction equations were given to follow up the formation of the MoFe and MoFe3 intermetallic alloys. The mechanism of the reduction reactions was predicted from the apparent activation energy values (Ea) that were computed at the different reduction degrees. Moreover, mathematical formulations that were derived from the gas-solid reaction model were applied to confirm the reduction mechanisms, which were greatly dependent on the precursor composition and reduction temperature. However, it can be reported that nano-structured MoFe and MoFe3 intermetallic alloys can be successfully fabricated via a gas-solid reaction technique at lower temperatures.

Study of the Kinetics of Reduction of IrO2 on TiO2 (Anatase) by Temperature-Programmed Reduction

The interaction between IrO2 and TiO2 (anatase) in non-isothermal reduction conditions has been studied by the temperature programmed reduction technique. IrO2 clusters are of sizes between 0.5 and 0.9 nm as determined from High Resolution Transmission Electron Microscopy (HRTEM). Largely, two main regions for reduction were found and modeled at ca. 100 and 230 °C. The first region is attributed to the partial reduction of IrO2 clusters, while the second one is due to reduction of the formed crystalline (rutile IrO2), during TPR, to Ir metal. Two methods for calculating kinetic parameters were tested. First, by applying different ramping rates on a 3.5 wt.% IrO2/TiO2 using Kissinger’s method. The apparent activation energy values for the first and second reduction regions were found to be ca. 35 and 100 kJ/mol, respectively. The second method was based on fitting different kinetic models for the experimental results in order to extract qualitative information on the nature of interaction during the reduction process. It was found that the first reduction is largely due to the amount of IrO2 (reactant concentration) while the second one involved phase boundary effect as well as nucleation.

Catalytic hydrogen production in ammonia borane by lanthanum-added nonprecious metal high-entropy alloys

As excellent materials, high-entropy alloys have excellent mechanical properties, wear resistance, oxidation resistance, corrosion resistance, physical properties, and radiation resistance. This work presents the design and preparation of a novel high-entropy alloy based on the BCC structure, added with lanthanum, and shows excellent catalytic performance in the hydrolysis of ammonia borane. The high-entropy alloy catalyst being able to catalyse the complete release of hydrogen in 1.3 min at 323 K, and the value of turnover frequency (TOF) value as high as 93.96 min−1. The value of the apparent activation energy (Ea) can reach 26.98 kJ/mol. It retains good catalytic performance after 10 cycles of catalytic hydrogen production from equivalent amounts of boron ammonia at 303 K. In this work, a new strategy is presented to improve the performance of transition group high-entropy alloys and the potential energy and environmental benefits of these alloys are demonstrated.

Microstructure evolution, deformation behavior and manufacture design of TiAl matrix composites reinforced with in-situ borides precipitation

Phase transformation and borides precipitation caused by boron addition influence microstructure evolution and deformation behavior of in-situ TiAl matrix composites. Dynamic recrystallization mechanism and dynamic recovery mechanism were investigated by isothermal compression tests, scanning electron microscopy and transmission electron microscopy, etc. The value of apparent deformation activation energy (Q) of present composites is calculated to be 691.506 kJ/mol. In the temperature range of 1100−1200 °C, nucleation and growth of recrystallized γ and α grains promoted by TiB obstacles dominate the deformation below or above Tα2→α. The dynamic recovery of α phase dominates the deformation in condition with low strain rates at 1250 °C. Boron addition increases the fraction of α phase and decreases the transformation temperatures of γ→α and α2→α, which promotes the nucleation and growth of recovered α grains during the loading. Deformation mechanisms and processing performance were also clarified based on the reestablished constitutive model.

Conversion of Levulinic Acid over Ru/SrZrO3 and Ru/BaZrO3 Supported Basic Catalysts

AbstractThe catalytic performance of Ru impregnated perovskites‐based catalysts (Ru/SrZrO3 and Ru/BaZrO3) in the conversion of levulinic acid (LA) into higher valuable chemicals was carried out at different reaction temperatures. The Ru/SrZrO3 and Ru/BaZrO3 catalysts, with a loading of 2.0 Ru atoms nm−2, were prepared by incipient impregnation, and characterized by different techniques. The characterization results indicate presence of dispersed Ru catalysts, with a crystalline structure, with absence of carbonate species for Ru/SrZrO3 and larger carbonates as segregated phase and with stronger basic sites for Ru/BaZrO3. At higher reaction temperatures (200 °C – 250 °C), the higher strength of the basic sites of Ru/BaZrO3 had a direct influence on the levulinic acid (LA) conversion reaction. Differences in the apparent activation energy (Eap) values follow the same trend. The catalysts produce a 100 % of selectivity towards GVL. The formation of GVL was proposed to proceed via the intermediacy of 4‐hydroxypentanoic acid due to the presence of basic surface sites.

Thermal degradation and kinetic studies of redwood (Pinus sylvestris L.)

AbstractIn this scientific paper, thermochemical conversion of redwood (RW) was studied. Using the thermogravimetric analysis' technique (TGA), the thermal behavior of RW samples was examined at four heating rates ranging from 5 to 20 K min−1 in inert atmosphere between 300 and 900 K. Two main objectives have been set for this study; the first one was the determination of the kinetic decomposition parameters of RW (Pinus sylvestris L.), and the second one was the study of the variation of characteristic parameters from the TG-DTG curves of the main RW's components, such as; cellulose, hemicellulose and lignin. The kinetic analysis was performed using three isoconversional methods (Vyazovkin (VYA), Friedman (FR) and Flynn-Wall-Ozawa (FWO)), Avrami theory method and the Integral master-plots (Z(x)/Z(0.5)) method to estimate activation energy (Ea), reaction order (n), pre-exponential factor (A) and model kinetic (f(x)) for the thermal decomposition of cellulose, hemicellulose and lignin components.The DTG and TG curves showed that three stages identify the thermal decomposition of RW, the first stage corresponds to the decomposition of hemicellulose and the second stage corresponds to the cellulose, while the third stage corresponds to the lignin's decomposition. For the range of conversion degree (x) investigated (0.1 ≤ x ≤ 0.7), the mean values of apparent activation energies for RW biomass were 127.60–130.65 KJ mol−1, 173.74–176.48 KJ mol−1 and 197.21–200.36 KJ mol−1 for hemicellulose, cellulose and lignin, respectively. Through varied temperatures from 550 to 600 K for hemicellulose, from 600 to 650 K for cellulose and from 750 to 800 K for lignin, the corresponding mean values of reaction order (n) were 0.200 for hemicellulose, 0.209 for cellulose and 0.047 for lignin. The pre-exponential factor's average values for three components of RW ranges from 0.08 × 1012 s−1 to 2.5 × 1012 s−1 (Ahemicellulose = 1.09 × 1012 s−1), 0.10 × 1014 s−1 to 0.28 × 1014 s−1 (Acellulose = 0.17 × 1014 s−1) and 3.07 × 1016 s−1 to 3.69 × 1016 s−1 (Alignin = 3.33 × 1016 s−1), respectively. The experimental data of RW had overlapped the D4, D2 and F3 in the conversion degree of 10–30%, 30–55% and 55–70% for the three components, respectively.

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.

High temperature flow behavior and deformation mechanism of Nb-22.5Cr-2.5Mo alloy

The high temperature compression is performed by isothermal compression tests using Gleeble 3500 thermo-mechanical simulator at a constant strain rate for the hot-pressed Nb-22.5Cr-2.5Mo alloy. The deformation temperature ranges from 800 to 1200 °C and the strain rate ranges from 0.001 to 0.1 s−1. The high temperature flow behavior and deformation mechanism of the alloy are investigated based on the compression test data and TEM observation of the deformed microstructure. The results show that the ductile–brittle transition temperature of the alloy is about 800 °C, and the flow stress is sensitive to the deformation temperature and strain rate, in which the plastic deformation ability increases obviously with the increase of deformation temperature and the decrease of strain rate. Besides, the hyperbolic sine Arrhenius model with stress exponent of 3.131 and apparent activation energy of 402 kJ/mol is established for the peak flow stress. The value of apparent activation energy is higher than that of Nb metal (246 kJ/mol), which implies the existence of Laves phase NbCr2 makes the deformation of the alloy more difficult. Moreover, it is found that the deformation mechanisms in the Nbss matrix are mainly DRV controlled by the glide and climb of the dislocations and DRX formed by bulging nucleation mechanism, while the deformation mechanisms in the Laves phase NbCr2 particles are mainly twinning and synchroshear of the Shockley partial dislocation.

Thermal stability of nanostructured uranium within a surface layer processed using shot peening

A nanostructured α‑uranium surface layer with an average subgrain/grain size of 140 nm was obtained by using shot peening treatment (SPT). While, the nanostructures might be unstable in thermal condition due to dense lattice defects, high density of dislocations and ultrafine grains induced by severe plastic deformation. In this work, the thermal stability of the nanostructured surface layer was investigated using quasi-in situ electron backscatter diffraction (EBSD) after annealing treatments at temperatures ranging from 400 °C to 500 °C. Results show that the nanostructured surface layer has reasonably good thermal stability below 400 °C. Recrystallization and subsequent growth occurred when the temperature increased to 450 °C. The recrystallization nuclei are rooted in the initial nanostructured sites in the top surface layer and from dense dislocation walls in the subsurface consisting of high density of twin boundaries. At 450 °C, the exponent of recrystallization subsequent growth is determined to be 2.86, and the apparent activation energy value is 112.24 kJ/mol. Recrystallization occurs predominantly in a continuous manner. Furthermore, the evolution of dislocation density and grain size accounts for the decaying of surface mechanical properties.

Kinetics of Low-Grade Scheelite Leaching with a Mixture of Sodium Phosphate and Sodium Fluoride

The current technology of leaching low-grade scheelite with sodium hydroxide or sodium carbonate has the disadvantages of large leaching reagent dosage and low leaching efficiency of tungsten. In order to extract scheelite efficiently, the kinetics of low-grade scheelite leaching with a mixture of sodium phosphate and sodium fluoride was investigated. In this study, the effects of temperature, phosphate concentration, and fluoride ion concentration on the leaching rate of tungsten were investigated. Our results showed that the leaching rate of tungsten was greatly influenced by the temperature and less affected by the concentration of phosphate and fluorine ions. The leaching process was controlled by a chemical reaction with an apparent activation energy value of 51 ± 0.2 kJ/mol. The apparent reaction orders with respect to phosphate and fluorine ions were 0.49 and 0.11, respectively. The reaction product calcium fluorophosphate was a loose, rod-like crystal, which would not tightly wrap on the surface of scheelite to prevent the diffusion process. The leaching kinetics of low-grade scheelite was in accordance with the shrinking core model, and the corresponding kinetic equation was also established.

Evaluation of the kinetic and thermodynamic parameters in catalytic pyrolysis process of sunflower oil using Al-MCM-41 and zeolite H-ZSM-5

The sunflower oil pyrolysis process was evaluated by thermogravimetry coupled to a Fourier transform infrared spectrophotometer (TGA-FTIR), in the presence of microporous (H-ZSM-5) and mesoporous (Al-MCM-41) catalysts in different proportions, to qualitatively evaluate the effect of deoxygenation of pure oil in obtaining renewable hydrocarbons. The catalysts were characterized by techniques and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and determination of surface acidity. Through these techniques, the catalysts present characteristics consistent with the literature, where they show good structural organization. The reaction kinetics was evaluated using isoconversional methods, e.g., Flynn Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Vyazovkin (VY) at three heating rates (5, 10 and 20 °C min−1) in the conversion range between 20 % and 80 %. The methods present a level of confidence, as they numerically present relatively close Ea values. The apparent activation energies for 50 % conversion of sunflower oil in the absence of catalysts using the FWO, KAS and Vyazovkin methods were 262.56, 255.35 and 234.85 kJ mol−1, respectively; on the other hand, the (50 % wt/wt) H-ZSM-5 and (50 % wt/wt) Al-MCM-41 – OGMH2 mixture showed Ea values corresponding to 152.23, 143.18 and 142.72 kJ mol−1 for the same 50 % conversion, respectively for heating rate of 10 °C min−1. The thermodynamic parameters, viz. enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS), were calculated with the values of apparent activation energy from the kinetic methods studied.