Critical Reaction Temperature Research Articles

Facile synthesis of high entropy carbide-nickel based cermets by in-situ carbo-thermal reduction of transition metal oxides

Equimolar and non-equimolar high entropy carbide-nickel based cermets were successfully prepared by in-situ carbo-thermal reduction of constituent metal oxides in a single pressureless vacuum sintering cycle at 1420°C. The carbon content in quinary (Ti0.2V0.2Nb0.2Ta0.2W0.2)Cx-10wt.%Ni and senary (Ti0.2V0.2Nb0.2Ta0.2W0.1Mo0.1)Cx-10wt.% Ni high entropy carbide cermets was varied (x = 0.6–1.0). Two-phase HEC-Ni cermets were formed at x = 0.75–0.85, whereas M6C (η-phase) and graphite phase coexisted at lower and higher carbon contents (x = 0.6, 0.9 and 1.0) in both HEC 5 and HEC 6 systems. In HEC 5, submicrometric WC precipitates were observed, which were attributed to the limited solubility of hexagonal-WC within the in-situ formed HEC 5 phase, contrasting with HEC 6 where no such precipitations were formed across the entire carbon content range. The partial replacement of W with Mo in the (Ti0.2V0.2Nb0.2Ta0.2W0.1Mo0.1)Cx-10 wt% Ni cermets promoted solid solution formation at lower temperature, and the grain size and hardness decreased and the fracture toughness increased. Both HEC-Ni cermets showed a homogeneous distribution of elements within the carbide phase, resulting in (HV30) hardness around 16 GPa and a fracture toughness around 8 MPa.m1/2. The phase evolution was systematically investigated by interrupting the sintering cycle at 600–1400 °C, employing XRD analysis. Thermodynamic calculations were performed to predict the critical reaction temperatures for carbo-thermal reduction. To evaluate the intrinsic mechanical properties of each phase and establish the correlation with microstructural features, micromechanical mapping was performed utilizing the high-speed nano-indentation technique.

The kinetics modeling and reactor simulation of propylene chlorination reaction process

AbstractThe kinetics experiments of fast reaction process of propylene chlorination at low temperature (30–90°C) and high temperature (420–480°C) are respectively conducted, and the corresponding reaction mechanisms and kinetics models are proposed. The radical mechanism at high temperature and the molecular mechanism at low temperature are found to be most likely with the experimental results. Specifically, the kinetics model, firstly considering the reversible reaction step of forming C3H6Cl· and direct hydrogen abstraction of forming C3H5·, shows better agreement with the experimental data. Furthermore, the critical reaction temperature Tcritical is firstly proposed to determine the dominant reaction mechanism in different conditions, and correspondingly the combination method of the high‐temperature and low‐temperature kinetics models has been adopted for tubular reactor simulation, which can reasonably reflect the influence of wide variation range of temperature in the reactor and guide the industrial reactor design in the further work.

Mechanical activation enhanced solid-state synthesis of NaCrO2 cathode material

NaCrO2 has been studied lately as a promising cathode material for Na-ion batteries. Consequently, we have conducted the first investigation on how high-energy ball milling before the high temperature reaction influences the synthesis reaction of NaCrO2 derived from the typical Na2CO3 and Cr2O3 reactants. In-situ synchrotron X-ray diffractometry is employed for the first time to provide a comprehensive understanding of the critical reaction temperatures and reaction pathway. It is found that high-energy ball milling at room temperature can result in significant changes in the synthesis reaction of NaCrO2 when compared to reactants without high-energy ball milling. These changes include a decreased onset temperature for the formation of O3–NaCrO2, an increased reaction kinetics, an alternation of the reaction pathway, and a complete reaction at 900 °C to form thermally-stable O3–NaCrO2 phase. These phenomena have been ascribed to the mechanical activation induced by high-energy ball milling before high temperature reaction. In contrast, without high-energy ball milling the reaction product at 900 °C is a highly impure NaCrO2 with a poor thermal stability. The thermally-stable O3–NaCrO2 powder produced with mechanical activation of the reactants at RT has much higher specific capacity (∼115 mAh/g) than the Na-deficient NaCrO2 powder with unreacted Cr2O3 generated without mechanical activation of the reactants at RT. Furthermore, the thermally-stable O3–NaCrO2 powder exhibits the best capacity retention among all the NaCrO2 without coatings reported so far in the literature.

Analysis of the interaction between moving α/γ interfaces and interphase precipitated carbides during cyclic phase transformations in a Nb-containing Fe-C-Mn alloy

The interaction between moving α/γ interfaces and interphase precipitated (IPd) carbides during the austenite (γ) to ferrite (α) and the ferrite (α) to austenite (γ) transformation has been systematically investigated through cyclic phase transformation experiments for a 0.1C-1.5Mn alloy containing 0.1 wt% Niobium (Nb) and its Nb-free counterpart. Shifts in the critical reaction temperatures during continuous heating and cooling are observed, which are attributed to the pinning force (PF) originating from the IPd carbides present. By applying the Gibbs energy balance (GEB) model to analyze experimental results, the PF was derived to be about 15 J/mol for the α→γ transformation and about 5 J/mol for the γ→α transformation, respectively, both of which are quite small compared to chemical driving force of phase transformations. Moreover, various modified Zener pinning equations have also been used to predict the PF, and it was found that these values are comparable with those obtained from experiments, which suggests that the classical Zener theory still has promising potential for carbide-interface interaction analysis.

CO2 decomposition using metal ferrites prepared by co-precipitation method

To catalytically decompose the greenhouse gas, CO2, spinel structure M-ferrites (M=Co, Ni, Cu, Zn) were synthesized by chemical co-precipitation using metal salts and sodium hydroxide as starting materials. The crystallite size of the newly-prepared M-ferrites increased and the BET surface area decreased with increasing calcination temperature. A thermal analysis of the reduction and reoxidation of M-ferrites indicated that substitution of divalent transition metals (i.e., Cu, Ni and Co) into Fe3O4 improved the reduction kinetics in the order of Cu>Ni>Co. ZnFe2O4 was the most difficult compound to completely reduce due to its stable structure. Commercial samples of the reduced Fe3O4, CoFe2O4 and ZnFe2O4 showed an increase in mass through the reoxidation process, but it was much more difficult for oxygen atoms to enter the structure of the reduced samples of NiFe2O4 and CuFe2O4. The M-ferrites in a batch type reactor showed better efficiency than the commercial Fe3O4. Also found was that CoFe2O4 showed a high regeneration potential, although it required a higher critical reaction temperature. NiFe2O4 and CuFe2O4 were excellent candidate materials for CO2 decomposition at lower temperatures.

Novel antioxidative peptides from the protein hydrolysate of oysters (Crassostrea talienwhanensis)

The antioxidative activity of hydrolysate peptides from oysters (Crassostrea talienwhanensis) was investigated. After hydrolysis with subtilisin, the yields of the peptides that were soluble in trichloroacetic acid (TCA-soluble) and the antioxidant activities of the resulting hydrolysate were determined using an orthogonal design and a hydroxyl radical scavenging reaction. The hydrolysate was fractionated using Sephadex G-15 gel filtration chromatography, and the two resulting bioactive peptides were subsequently purified by RP-HPLC with a Kromasil C18 (ODS) column. The amino acid sequences were analyzed by nano-ESI-MS/MS.The critical reaction temperature, pH, hydrolysis time and enzyme-to-substrate (E/S) ratio were determined for the optimum hydrolysis with subtilisin, and the E/S ratio was found to be the most critical reaction condition. The amino acid sequences of the peptides (518 and 440Da) were proline-valine-methionine-glycine-aspartic acid (PVMGA) and glutamine-histidine-glycine-valine (QHGV), respectively. These two novel peptides exhibited high antioxidative actions based on their hydroxyl and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities.

Microwave absorption enhancement, magnetic coupling and ab initio electronic structure of monodispersed (Mn1−xCox)3O4 nanoparticles

Monodispersed manganese oxide (Mn1-xCox)3O4 (0 ≤ x ≤ 0.5) nanoparticles, less than 10 nm size, are respectively synthesized via a facile thermolysis method at a rather low temperature, ranging from 90 to 100 °C, without any inertia gas for protection. The influences of the Co dopant content on the critical reaction temperature required for the nanoparticle formation, electronic band structures, magnetic properties, and the microwave absorption capability of (Mn1-xCox)3O4 are comprehensively investigated by means of both experimental and theoretical approaches including powder X-ray diffraction (XRD), electron energy loss spectroscopy (EELS), super conductivity quantum interference device (SQUID) examination, and first-principle simulations. Co is successfully doped into the Mn atomic sites of the (Mn1-xCox)3O4 lattice, which is further confirmed by EELS data acquired from one individual nanoparticle. Therefore, continuous solid solutions of well-crystallized (Mn1-xCox)3O4 products are achieved without any impurity phase or phase separation. With increases in the Co dopant concentration x from 0 to 0.5, the lattice parameters change systemically, where the overall saturation magnetization at 30 K increases due to the more intense coupling of the 3d electrons between Mn and Co, as revealed by simulations. The microwave absorption properties of the (Mn1-xCox)3O4 nanoparticles are examined between 2 and 18 GHz. The maximum absorption peak -11.0 dB of the x = 0 sample is enhanced to -11.5 dB for x = 0.2, -12.7 dB for x = 0.25, -15.6 dB for x = 0.33, and -24.0 dB for x = 0.5 respectively, suggesting the Co doping effects. Our results might provide novel insights into the understanding of the influences of metallic ion doping on the electromagnetic properties of metallic oxide nanomaterials.

Equilibrium and kinetic modeling of char reduction reactions in a downdraft biomass gasifier: A comparison

The thermodynamic and kinetic modeling of char reduction reactions in a downdraft (biomass) gasifier has been presented. Mass and energy balance are coupled with equilibrium relations or kinetic rate parameters (using varying char reactivity factor) in order to predict status of un-converted char in addition to gas composition, calorific value, conversion efficiency, exit gas temperature, endothermic heat absorption rate and gasifier power output. Both modeling predictions are compared against experimental data for their validity. The influence of char bed length and reaction temperature in reduction zone has been examined. CO and H 2 component, calorific value of product gas and the endothermic heat absorption rate in reduction zone are found to be sensitive with reaction temperature, while char bed length is less sensitive to equilibrium predictions. For present case, all char conversion takes place at critical reaction temperature of 932 K for equilibrium, while for kinetic modeling critical reaction temperature and critical char bed length of 950 K and ∼25 cm have been identified, comparing the predictions. The critical reaction temperatures and critical char bed length also depend on inlet components composition and initial temperature supplied to the reduction reaction zone model.

Enhancement of thermal uniformity for a microthermal cycler and its application for polymerase chain reaction☆

Thermal uniformity is essentially important for micro reactors which require precise control of critical reaction temperatures. Accordingly, we report a new approach to increase the temperature uniformity inside a microthermal cycler, especially for polymerase chain reaction (PCR). It enhances the thermal uniformity in the reaction region of a PCR chip by using new array-type microheaters with active compensation (AC) units. With this approach, the edges of the microthermal cyclers which commonly have significant temperature gradients can be compensated. Significantly, the array-type microheaters provide higher uniformity than conventional block-type microheaters. Besides, experimental data from infrared (IR) images show that the percentages of the uniformity area with a thermal variation of less than 1 °C are 63.6%, 96.6% and 79.6% for three PCR operating temperatures (94, 57 and 72 °C, respectively) for the new microheaters. These values are significantly better than the conventional block-type microheaters. Finally, the performance of this proposed microthermal cycler is successfully demonstrated by amplifying a detection gene associated with Streptococcus Pneumoniae (S. Pneumoniae). The PCR efficiency of the new microthermal cycler is statistically higher than the block-type microheaters. Therefore, the proposed microthermal cycler is suitable for DNA amplification which requires a high temperature uniformity and is crucial for micro reactors with critical thermal constraints.

Mechanism and kinetics of transesterification in poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalene dicarboxylate) polymer blends

In a previous work [L. Alexandrova, A. Cabrera, M.A. Hernández, M.J. Cruz, M.J.M. Abadie, O. Manero, D. Likhatchev, Polymer 43 (2002) 5397. [1]], a model compounds study on the kinetics of a transesterification reaction in poly(ethylene terphthalate)–poly(ethylene naphthalene 2,6-dicarboxylate), PET–PEN blends, resulting from melt processing, was simulated using model compounds of ethylene dibenzoate (BEB) and ethylene dinaphthoate (NEN). A first-order kinetics was established under pseudo first-order conditions for both reactants, and thus the overall transesterification reaction was second-order reversible. Direct ester–ester exchange was deduced as a prevailing mechanism for the transesterification reaction under the conditions studied.In this work, the actual PET–PEN system was melt processed by mixing the polymers below the critical reaction temperature in a twin-screw extruder. Thereafter, the reaction was induced by temperature in open glass ampoules. A second order reversible kinetics was measured, in agreement with the kinetics established in the previous model compounds study. The equilibrium constant value corresponds to a forward rate constant which is four times larger than the reverse rate constant. The activation thermodynamic parameters confirmed the direct ester–ester exchange mechanism for the reaction.

Mechanically activated reduction of nickel oxide with graphite

The reduction of nickel oxide with graphite during ball milling at both ambient and elevated temperatures was investigated using X-ray diffraction (XRD), simultaneous thermogravimetry and differential thermal analysis (TG/DTA), and electron microscopy. It was found that milling at ambient temperature did not result in the reduction of nickel oxide to nickel. However, milling significantly reduced the critical reaction temperature for the reduction, from 1350 K for the unmilled sample to ∼650 K for samples milled for 12 hours or longer. This reduction in reaction temperature is rationalized in terms of the microstructural refinement observed in the milled samples. The reduction of nickel oxide to nickel was observed to occur at elevated temperatures during milling. The thermodynamics and kinetics of the reduction reaction are discussed.

Gas Souring by Thermochemical Sulfate Reduction  at 140¡C: Reply

Natural gas in the Permian-Triassic Khuff Formation of Abu Dhabi contains variable amounts of H2S. Gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases. Sour gas is observed only in reservoirs hotter than a critical reaction temperature: 140°C. Petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m. Gas composition data show that only reservoirs deeper than 4300 m contain large quantities of H2S (i.e., >10%). At present-day geothermal gradients, 4300 m is equivalent to 140°C. Fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140°C. Thus, three independent indicators all show that 140°C is the critical temperature above which gas souring by thermochemical sulfate reduction begins. The previously suggested lower temperature thresholds for other sour gas provinces (80-130°C) derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps. Conversely, published proposals for higher threshold temperature (180-200°C) derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales. Therefore, the temperature of 140°C derived from our study of the Khuff Formation may be th best estimate of temperature required for in-situ thermochemical sulfate reduction to produce the high H2S concentrations encountered in deep carbonate gas reservoirs.

Effect of initiator characteristics on high-pressure ethylene polymerization in autoclave reactors

The steady-state and transient behaviors of a high-pressure ethylene polymerization reactor have been analyzed for a continuous stirred autoclave reactor consisting of two reaction compartments. The nonlinear temperature dependence of the specific initiator consumption rate is modeled using the initiator efficiency factor which varies with reaction temperature. It is shown that there exists a critical reaction temperature at which the steady-state process gain (reactor temperature change/initiator feed rate change) changes its sign. When only the first reaction zone temperature is controlled by regulating the initiator injection rate, the second zone temperature is strongly affected by the first zone temperature for certain initiator types

Combustion Reaction of Zinc Oxide with Magnesium during Mechanical Milling

Combustion in the solid-state displacement reaction between zinc oxide and magnesium during mechanical milling was investigated using X-ray powder diffraction, electron microscopy, and thermal analysis. It was found that mechanical milling decreases the critical reaction temperature at which the combustion reaction occurs. Under the milling conditions employed in this study, the temperature of the powder mixture during ball/powder collisions is estimated to reach a value of 640 K before combustion.

Anomalous combustion effects during mechanical alloying

Combustion in solid-state displacement reactions during mechanical alloying has been investigated. It was found that mechanical alloying decreases the critical reaction temperature at which unstable combustion reactions propagate. Some reactions exhibited an unusual “interrupted combustion” effect, whereby reactant powders which had been milled for subcombustion times and left to age at room temperature combusted on the immediate resumption of milling. For one reaction, the reduction of ZnO by Ti, combustion was only observed in aged powders. Interrupted combustion is associated with the diffusion of reactant species during aging and with a decrease in the temperature required for combustion.

Entropy-driven loss of gas phase group V species from gold/III-V compound semiconductor systems

Temperature-dependent chemical interactions between Au and nine III-V compound semiconductors (III = Al,Ga,In and V = P,As,Sb) have been calculated using bulk thermodynamic properties. Enthalpic considerations alone are insufficient to predict metal/ compound semiconductor reactivities. The entropy of vaporization of the group V elements is shown to be an extremely important driving force for chemical reactions involving the III-V's, since it enables several endothermic reactions to occur spontaneously under certain temperature and pressure conditions. Plots of either Gibbs' free energies of reaction or equilibrium vapor pressure of the group V element versus temperature are used to predict critical reaction temperatures for each of the systems studied. These plots agree extremely well with previous experimental observations of thin film reactions of Au on GaAs.

Some characteristics of chemical reactions in vaporizing hybrid systems

The author's method [1] of analyzing transport processes associated with the injection of a fuel-oxidizer mixture into a laminar boundary layer through a porous plate can be extended to the case of sublimation of a surface in a high-temperature gas flow. Such conditions may develop in hybrid (combined) systems in which a mixture of fuel and oxidizer from the vaporizing element must mix and react with an external oxidizer flow. Preliminary results [2–4] indicate that in “pure” systems (in which oxidizer is not mixed with the solid propellant) the reaction kinetics have secondary significance as compared with the transport processes in the boundary layer and in the majority of cases can be ignored for all practical purposes. However, in other hybrid systems and, in particular, where a certain amount of oxidizer is mixed with the solid propellant, the reaction kinetics may acquire considerable importance. The methods of analyzing transport processes in hybrid systems proposed in [3,4], although they supplement one another, do not give a sufficient description. The present method of theoretical investigation of hybrid systems, although based on a diffusion interaction mechanism, permits a somewhat better understanding of the kinetic laws thanks to the assumption of two reaction fronts. As shown in [1], this is conditioned by the fact that in the case of simultaneous vaporization of the fuel and oxidizer components the mixture temperature may reach the critical reaction temperature T* (condition of formation of the first front) before complete stoichiometry is achieved. If the mass velocity of the fuel component is greater than the stoichiometric value corresponding to the presence of oxidizer in the mixture, the occurrence of a second reaction front will be determined by attainment of the full stoiehiometric relation between the mass flow of fuel unreacted in the first front and the external oxidizer flow.