Chou Model Research Articles

Exceptional catalytic effect of novel rGO-supported Ni-Nb nanocomposite on the hydrogen storage properties of MgH2

The design of an excellent active catalyst to improve the sluggish kinetic and thermodynamic properties of magnesium hydride (MgH2) remains a great challenge to achieve its practical application. In this study, a novel Ni-Nb/rGO nanocomposite catalyst was successfully prepared by one-spot hydrothermal and subsequent calcination methods. The novel Ni-Nb/rGO nanocomposite exhibits an exceptional catalytic effect on improving MgH2 sorption properties. Specifically, the onset desorption temperature of MgH2 + 10 wt% Ni-Nb/rGO composite is reduced to 198 °C, much lower than that of undoped MgH2 (330 °C). Interestingly, the composite can release 5.0, 5.9, and 6.0 wt% H2 within 10 min at 245, 260, and 275 °C, respectively. Furthermore, the dehydrogenated MgH2 + 10 wt% Ni-Nb/rGO composite starts to absorb hydrogen even at room temperature with approximate 2.75 wt% H2 uptake at 75 °C under 3 MPa H2 pressure within 30 min and exhibits excellent stability by maintaining 6.0 wt% hydrogen content after 20 cycles at 300 °C. Chou's model suggests that the de/hydrogenation kinetics of Ni-Nb/rGO-modified MgH2 switches from surface penetration model to diffusion model at lower temperatures. Additionally, the activation energies (Ea) for the de/hydrogenation of MgH2 + 10 wt% Ni-Nb/rGO are reduced to 57.8 kJ/mol and 33.9 kJ/mol, which are significantly lower than those of undoped MgH2. The work demonstrates that the addition of a novel ternary Ni-Nb/rGO catalyst is an effective strategy to not only boost the sorption kinetics of MgH2 but also maintain its cycling property.

Enhancing Mg-Li alloy hydrogen storage kinetics by adding molecular sieve via friction stir processing

Mg-Li alloy is a lightweight hydrogen storage material with high hydrogen capacity, but its poor kinetics limited its practical applications. In this work, MCM-22 molecular sieve was added to Mg-Li alloy by friction stir processing (FSP) as the catalyst to enhance the kinetic properties of Mg-Li alloy (denoted as Mg-Li-MCM-22). The resulting Mg-Li-MCM-22 possesses the reversible hydrogen storage capacity of ca. 6 wt.% and can release 5.62 wt.% hydrogen within 50 min at 623 K, showing improved kinetics. The Chou model and Johnson–Mehl–Avrami–Kolmogorov (JMAK) model calculations reveal that the lattice defects generated by FSP improve the kinetics of hydrogen adsorption/desorption. The pinning effect of MCM-22 particles produces more grain boundaries and dislocations, thus, increasing the diffusion rate of hydrogen atoms and providing more nucleation sites, therefore, reducing the dehydrogenation activation energy. This work provides a new strategy for the preparation of hydrogen storage materials.

Modification and verification of Miedema model for predicating thermodynamic properties of binary precipitates in multi-element alloys

At present, there is still a certain difference between the calculated and experimental results of the original Miedema formation enthalpy model. Therefore, it is necessary to improve the calculation accuracy of the Miedema model to facilitate the development of the Miedema model and its application in alloy design. To address the problem, a new atomic size factor is introduced to modify the Miedema model in this paper. Then, the accuracy of the modified Miedema model is verified. Firstly, the formation enthalpies of various magnesium-based binary alloys are recalculated using the modified Miedema model. Secondly, the precipitation temperature of MgZn phase in Mg–6Zn-0.6Zr ternary alloy is predicted using the modified Miedema model combined with Chou model, which is compared with the experimental result. Thirdly, the precipitation of binary phases in Mg–Zn–Zr–Y quaternary alloy is analyzed by calculation based on the modified Miedema model. The final results show that the calculated results obtained from the modified Miedema model are in good agreement with the reported results, experimental results and phase diagram results, indicating that the modification is reasonable.

Thermodynamic, surface and transport properties of ternary Al–Sn–Zn liquid alloy and its sub binaries

ABSTRACT The excess free energies of sub-binary Al–Sn, Sn–Zn and Zn–Al alloys and ternary Al–Sn–Zn alloy were computed using the optimised exponential parameters of R–K polynomial at 973, 1073, 1173 and 1273 K. The computed values of free energy of mixing, enthalpy of mixing and activity of the ternary alloy were compared with the available experimental data. The surface concentrations of the monomers of the ternary Al–Sn–Zn alloy were computed using Butler model. The surface tension of the ternary liquid alloy was calculated in the framework of Chou, Kohler, Toop and Bulter models using optimised parameters for the excess surface tension. The viscosity of the sub-binary systems have been computed using Kaptay equation with the help of calculated results of enthalpy of mixing, and the viscosity of the ternary alloy were computed in the framework of Chou, Kohler and Toop model using optimised parameter for the excess viscosity.

Estimation of Viscosity of Alloys Using Gibbs Free Energy of Mixing and Geometric Model

In the present work, using mixing Gibbs free energies and Chou’s general solution model (GSM), by considering the excess activation energies from the binary subsystems, the viscosities of the simple ternary Au–Ag–Cu, Al–Cu–Si, and Fe–Ni–Co and liquid alloys of binary subsystems have been evaluated via well known Chou model and physical models, such as Kaptay, Kozlov–Romanov–Petrov (KRP), and Schick et al. at temperatures 1373, 1375, and 1873 K. A comparison between the evaluated results and experimental values of the Au–Ag–Cu, Al–Cu–Si, and Fe–Ni–Co ternary alloys was carried out. In this study, the success of the application of the aforementioned geometric and physical models to the viscosity calculations of the alloys discussed and the viscosite data are presented to the literature. In order to determine the applicability success, the mean square deviation analysis was performed. According to the values in this table, Schick et al. and KRP models which are derived from the physical quantities among the models discussed provide best description of the viscosity for the Al–Cu–Si and Au–Ag–Cu alloys, respectively.

Modelling the Strength of Cellulose Nanofiber-Filled Rigid Low-Density PU Foams

Rigid low-density closed-cell polyurethane (PU) foams are used primarily as a thermal insulation material. The foams have to possess a sufficient strength and stiffness in order to ensure their mechanical integrity and dimensional stability in service. The mechanical characteristics of foams are enhanced by adding cellulose nanofibers to the polyol system, which both modify the foaming process and act as a reinforcement of cell struts and walls. A model of composite foam strength is developed based on a regular unit cell and assuming the onset of strut failure as the foam fracture criterion. The load-bearing capacity of foam struts is estimated by the modified Fukuda and Chou model considering the orientation of nanofibers along the strut axis. The model developed is shown to provide a reasonably accurate prediction for the nanofiber loading effect on the strength of composite foams.

Effects of Zr Addition on Thermodynamic and Kinetic Properties of Liquid Mg-6Zn-xZr Alloys

Mg-6Zn-xZr (ZK60) alloys are precipitation strengthened by Mg-Zn intermetallics. Therefore, it is important to investigate the thermodynamic and kinetic effects of Zr addition on formations of these reinforcement phases (Mg7Zn3, MgZn2, and MgZn) in Mg-6Zn-xZr melts. Because it is difficult to gain thermodynamic and kinetic data in melts by experiment, obtaining these data points still depends on a theoretical calculation. Based on the Miedema formation enthalpy model and the Chou model, the thermodynamic properties (the mixing enthalpies, the Gibbs free energies, and the component activities) of Mg-6Zn-xZr ternary alloys and their constitutive binary alloys are calculated. The thermodynamic effects of Zr addition on formations of Mg7Zn3, MgZn2, and MgZn are predicted. Using a ternary model for predicting diffusion coefficients, the diffusion coefficients of Zn and Zr in liquid Mg-6Zn-xZr alloys are calculated and the kinetic effects of Zr addition on the diffusion coefficient of Zn is discussed. The results show that the Zr addition can hinder the formations of Mg7Zn3, MgZn2, and MgZn inter-metallics in thermodynamics and kinetics.

Analysis, modeling and experimental validation of temperature-changing effect on mechanical properties of pneumatic artificial muscle

ABSTRACTThis paper focuses on quantify how temperature affects the mechanical properties of pneumatic artificial muscle (PAM). The influence of temperature variations on PAM’s structure size and internal friction was analyzed firstly, based on the thermal characteristics of rubber diaphragm and fiber mesh, then a novel model considering the influence of temperature variation is proposed, without any fitting parameter or any parameter with uncertain physical meanings. In order to verify the proposed model, temperature-rising experiments using heating device and continuous operation were both carried out separately. Through the comparison of characteristic curves under different temperatures, PAM’s characteristics were proven to be affected by temperature variations. The results show that inside temperature changing of PAM is why there are drifting and time-varying phenomenon of their mechanical characteristics in long-term operation, and the proposed model can predict the variations caused by temperature changing well. Meanwhile, the accuracy of the new model is higher than Chou model and lower than Andrikopoulos model; it is understandable because our new model has no fitting parameters, but the advantage of new model is that it takes temperature as independent variables and, therefore, can be used at different temperatures directly without any calibration.

Interpretation of negative temperature dependence of hydriding reaction in LaNi5-Mg alloys by modified Chou model

A modified Chou model with parameters of temperature (T), initial hydrogen pressure (P0), enthalpy change (ΔH) and entropy change (ΔS) is proposed in consideration of thermodynamic driving force. The meaningful activation energy Ev can be calculated for the negative temperature dependence of hydriding reaction in hydrogen storage materials. This model is verified by the hydriding/dehydriding experimental data of (LaNi5)1−xMgx (x=0, 0.018, 0.041, 0.063). The calculated results indicate that the hydriding kinetics of (LaNi5)1−xMgx alloys are controlled by diffusion of hydrogen atom in hydride. The addition of small amount of magnesium improves the kinetic performance of LaNi5 slightly. Furthermore, the modified Chou model was compared with the previous model that successfully explain the negative temperature depending hydrogenation reaction of Mg-based alloys. The fitting results show that the modified Chou model can describe the hydrogen absorption kinetic mechanism of negative temperature dependent materials more accurately.

Kinetic mechanisms of hydriding and dehydriding reactions in La–Mg–Ni alloys investigated by the modified Chou model

The phenomenon of volume change is very important in the hydriding and dehydriding reactions to clarify their kinetic mechanisms. However, few of the theoretical models take this factor into account in the study of kinetic mechanisms. In the present work, the modified Chou model with considering the volume change by Pilling-Bedworth Ratio (PBR) β is applied to interpret the kinetic mechanisms of hydriding and dehydriding reactions in La–Mg–Ni ternary hydrogen storage alloys. The calculation results agree well with the experimental data, indicating that the modified Chou model can well describe the kinetic behaviors of La–Mg–Ni alloys. Diffusion is determined to be the rate-controlling step of hydriding and dehydriding for the examples in this work. The activation energies are calculated to be 39.4 kJ/mol for hydriding reaction (β = 1.29) and 93.1 kJ/mol for dehydriding reaction (β = 0.78) of Mg–10.6La–3.5Ni nanoparticles at 453–623 K. For the hydrogen absorption kinetics of La2Mg16Ni alloy under 0.5–3 MPa at 598 K, β is 1.25 and the equilibrium hydrogen pressure is calculated to be 0.332 MPa. For the hydrogen desorption kinetics of Mgx(LaNi3)100−x alloys at 623 K, the characteristic times increase in the order of tc(x = 70,β= 0.79) < tc(x= 60, β= 0.79) < tc(x= 40, β= 0.97) < tc(x= 50, β = 0.85). The value of β is 0.77 and the hydrogen desorption rate of Mg–5La–10Ni alloy prepared by the ultrahigh pressure (UHP) method is faster than that of the alloy prepared by the as-cast method, whose characteristic times are 7428 s and 4651 s, respectively. Furthermore, the calculation accuracy can also be improved by the modified Chou model compared with the original Chou model after taking PBR into consideration.

Phase equilibria of Ce–Mg–Ni ternary system at 673 K and hydrogen storage properties of selected alloy

The isothermal section of Ce–Mg–Ni ternary system at 673 K was constructed through thermodynamic calculation coupled with experimental verification. Under guidance of the phase diagram in Mg-rich corner, Ce2.92Mg91.84Ni5.24 was designed to investigate its hydrogen storage properties. Moreover, the mechanism of α-to-β (α: diluted phase and β: hydride) phase transformation in the Ce2.92Mg91.84Ni5.24–H2 system was elucidated by in situ synchrotron powder X-ray diffraction (SR-PXRD). The alloy showed a good activated ability and displayed two plateaus in the pressure–composition–temperature curves. The kinetic mechanisms of hydriding and dehydriding reactions were analyzed by Chou model. The rate-controlling steps were the diffusion for the initial stage in hydriding reaction and the surface penetration for dehydriding reaction. SR-PXRD patterns indicated that the activated Ce2.92Mg91.84Ni5.24 gradually transformed into CeH2.51, (Mg) (with solid solution of H atoms) and Mg2NiH0.3, finally transformed into CeH2.51, MgH2 and Mg2NiH4 during the first hydriding process.

Viscosity evaluation of Fe–Ni–Co ternary alloy from the measured binary systems

The iso-viscosity curves of liquid Fe–Ni–Co ternary alloys at 1600°C were investigated with considering the excess viscosity of ternary alloy by the means of three models (Kohler, Toop and Chou) from the measured sub-binary data in this work. Excess viscosities were used instead of excess thermodynamic properties in geometrical models, and the excess viscosities of the sub-binary systems were fit by using the 3rd degree (n=3) Redlich–Kister polynomial. The increase of Ni content results in the decrease of the viscosity of the ternary alloy, while Fe has opposite effect. When the molar content of Ni within the alloy does not exceed 50%, the increase of Co lowers the viscosity. Once its content is above 50%, Co can promote the increase of the viscosity of the ternary alloy. Similarity coefficients of the Fe–Ni, Fe–Co and Ni–Co three binary systems mentioned in the Chou model have been calculated and their values are 0.82, 0.20 and 0.47, respectively, showing that the Fe–Ni–Co ternary system is not exactly the “Kohler model” or “Toop model”, so both Kohler model and Toop model cannot obtain the accurate predicted values. The predicted iso-viscosity curves calculated by the Chou model should be recommended. Some activation energies of the ternary alloy were calculated by the three models and compared with three sub-binary systems, and the evaluated activation energies calculated by the Chou model should be recommended. Except the sample AE3 and AE6, other activation energy values predicted by three different models are similar by a narrow margin. The activation energies for AE3 and AE6 are the lowest and highest, respectively, because of the mix effect of the ternary alloy.

Oxidation behavior and kinetics of Al2O3–SiC–SiO2–C composite in air

Al2O3–SiC–SiO2–C composites are widely used in ironmaking operations due to their favorable erosion-resistance. In the present paper, the oxidation behavior and kinetics of Al2O3–SiC–SiO2–C composites in air are investigated in terms of a theoretical analysis associated with the experimental data. Furthermore, the effects of temperature on the oxidation reaction are discussed. The results show that the oxidation of Al2O3–SiC–SiO2–C composites is mainly caused by atmospheric oxygen reacting with C and SiC in the materials. At 1200°C, a protective layer can be found on the material surface due to the formation of the mullite phase. Predictions from the Chou model are in agreement with the experimental data. The characteristic oxidation time which indicates anti-oxidation properties for Al2O3–SiC–SiO2–C composites at 1000°C and 1200°C are 142.41min and 264.89min, respectively, indicating that the material at 1200°C is more resistant to oxidation due to the formation of a protective layer on the surface.

Improvement of the Blast Furnace Viscosity Prediction Model Based on Discrete Points Data

Viscosity is considered to be a significant indicator of the metallurgical property of blast furnace slag. An improved model for viscosity prediction based on the Chou model was presented in this article. The updated model has optimized the selection strategy of distance algorithm and negative weights at the reference points. Therefore, the extensionality prediction disadvantage in the original model was ameliorated by this approach. The model prediction was compared with viscosity data of slags of compositions typical to BF operations obtained from a domestic steel plant. The results show that the approach can predict the viscosity with average error of 9.23 pct and mean standard deviation of 0.046 Pa s.

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H2 in 600 s and desorb 3.9 wt.% H2 in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H2, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.

Effect of Temperature on Reaction of Hexagonal BN Powder in Wet Air Between 1073 and 1373 K

The reaction of hexagonal BN powder in wet air at 1073–1373 K for 18 h was investigated using TG, XRD, and SEM analysis. Initially, the oxidation occurred rapidly with simultaneous volatile reaction. At long times, a steady state was achieved and the degradation rate was determined by the volatilization reaction. Temperature affected the reaction behavior. At 1073 K, it consisted of two linear processes. With temperature increasing, initially, the paralinear kinetics was observed and then followed the linear rate. The sample shape also affected the reaction behavior. The reaction kinetics was dealt with Chou's model and gotten good agreement.

Oxidation kinetics of TiN-containing composites

Abstract The oxidation of TiN-containing composites has been discussed from theoretical aspect based on the experimental data available in the literature. Although the experimental results show that the oxidation behavior is a complicated process involving different phases and oxidation mechanisms, it is diffusion controlled essentially. Therefore Chou's model has been employed to deal with the oxidation of TiN-containing composites. To describe more closely the oxidation behavior of TiN-containing composites, the factor of the sample shape has been taken into consideration. A good agreement has been found between the experimental results and theoretical calculation. In addition, a comparison of calculation errors has been given between our new model and the traditional model used in the literature.

Modeling Activity and Interaction Coefficients of Components of Multicomponent Alloy Melts: An Example of Iron Melt

AbstractAs a popular thermodynamic calculation method for binary alloys, Miedema's model has been applied in many fields. Chou's Model, a new generation of geometric model for ternary and multicomponent alloy systems, overcomes the intrinsic theoretical defects (including symmetric and asymmetric) existing in some original geometric models. Here, by means of combining Miedema's model and Chou's model as well as including the consideration of the excess entropy we attempted to build the new thermodynamic model to evaluate thermodynamic properties of ternary and multicomponent alloying systems in terms of the physical parameters (molar volume, electronegativity, electronic density and melting point) of constituents. Moreover, the activity and interaction coefficients of a wide of components in iron melt have been discussed in details.

The Reaction Mechanism and Kinetics of α‐BN Powder in Wet Air at 1273 K

The reaction behavior of α‐Boron nitride (BN) powder in wet air was investigated at 1273 K using thermogravimetric (TG) analysis, X‐ray diffraction (XRD), and mass spectrometry. Scanning electron microscopy and electron probe X‐ray microanalysis (EPMA) were used to analyze the morphological and elemental development of the sample. The results show that the reaction process consists of two stages: the oxidation of BN powder to form B2O3 and the further reaction of B2O3 with H2O. In the first stage, the oxidation reaction occurs very quickly and can be described by Chou's model. In the second stage, the reaction rate followed the linear rate law.

Hydrogen storage characterization of Mg17Ni1.5Ce0.5/5 wt.% Graphite synthesized by mechanical milling and subsequent microwave sintering

SUMMARY Mg17Ni1.5Ce0.5/5 wt.% Graphite was synthesized by microwave sintering in the present work. Its pressure-composition-isotherm curves exhibit two plateaus corresponding to MgH2 and Mg2NiH4 from 563 K to 623 K, and the hydrogen storage capacity is about 5.6 wt.% at 593 K. Differential scanning calorimeter results reveal that its onset temperature and activation energy of desorption are 654.6 K and 107.41 kJ/mol, respectively, which are lower than those of MgH2. Its rate-controlling step of the dehydriding reaction is the diffusion of hydrogen at lower temperature according to Chou model. As the catalyst and microwave absorbing agent, the addition of graphite is in favor of improving the hydrogen storage properties of the sample. Copyright © 2012 John Wiley & Sons, Ltd.