Johnson Mehl Avrami Kolmogorov Research Articles

Investigation of modeling challenges of PEM fuel cells cold start operation

In this study a one-dimensional transient isothermal model was implemented in MATLAB to conduct an in-depth evaluation of available modeling framework for cold start simulation of PEMFC. In this regard, several shortcomings were addressed and a combination of modifications introduced by other researchers and assumptions made in this study was used to improve modeling accuracy; reaction kinetics variation with temperature and ionomer water content was investigated and incorporated in the model; dynamics of nucleation and subcooling was described by Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Classical Nucleation theories; effect of temperature on ECSA loss was examined and assumptions were made regarding electro-osmotic drag in catalyst layer under low load conditions. The results showed high fidelity to experiments in the early stage of cold start and the model was able to capture the effect of all three influential parameters, i.e. temperature, initial ionomer water content, and current density on performance evolution and failing time in isothermal cold starts with acceptable accuracy. Moreover, the linear performance drop at low subcooling and the nonlinear relation between cold start duration and initial ionomer water content were properly reproduced by the model.

Precipitation kinetics of gamma double prime phase during direct aging treatment of Inconel 718 superalloy additively manufactured by selective laser melting

While direct aging of Inconel 718 nickel-based superalloy from the as-built state is known as a promising practice to tune the mechanical properties, the precipitation kinetics of γ″ phase, as the main precipitating phase, has received less attention. Accordingly, in the present work, post-processing in the form of the direct aging treatment of Inconel 718 superalloy manufactured by the laser powder bed fusion additive manufacturing, also known as selective laser melting (SLM), was systematically investigated by microstructural analysis and hardness measurements. It was revealed that the peak aged condition for direct aging can be achieved at much shorter holding times compared to aging after homogenization/solutionizing heat treatment. Moreover, the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis based on both hardness measurements and microstructural analysis were applied to unravel the γ″ precipitation kinetics, which respectively led to the precipitation activation energies of 160 and 157 kJ/mol, which are consistent with the activation energy for the diffusion of Nb in IN718 superalloy manufactured by SLM. Moreover, the Lifshitz–Slyozov–Wagner (LSW) diffusion-controlled growth theory was modified to become applicable for investigating the precipitation kinetics during the evolution of the volume fraction of the precipitating phase, which also led to the activation energy of 157 kJ/mol. These results can also shed light on the procedures used for investigating the kinetics of phase transformations during aging of additively manufactured Inconel 718 superalloy.

Simulating the kinetics of recrystallization in Aluminum alloys

This contribution presents a brief overview of the simulation of recrystallization kinetics in metals. The study focuses on the annealing phenomena in a single-phase 1050 aluminum alloy of technical purity. To reveal the true nature of recrystallization, the kinetics is discussed by the well-establishes Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach, while the constants of this model are related to the energy stored during deformation, nucleation rate, velocity of grain boundary, and grain boundary energy. The listed physical quantities are derived from different models, while the performance of the combined approach was tested for the cases where the diversity of driving forces for recrystallization was ensured by different straining levels. The softening of the material during annealing was evaluated by the microhardness. It was shown that the kinetics of recrystallization is strongly influenced by the stored energy and the process can be simulated by employing the JMAK equation.

Sigma phase kinetics in DSS filler metals: A comparison of sigma phase formation in the as-welded microstructure of super duplex stainless steel and hyper duplex stainless steel

This study investigates and compares the kinetics of sigma phase formation in established Super Duplex Stainless Steel (SDSS) and recently developed Hyper Duplex Stainless Steel (HDSS) filler metal wires. Experimental sigma phase time-temperature-transformation (TTT) maps were developed, revealing nearly equivalent interface area/volume, resulting in similar sigma phase kinetics for 1% volume despite the higher Cr and Mo content in HDSS. However, the growth rate to 5% and 10% sigma phase volumes was slightly higher in HDSS. The sigma phase kinetics in both SDSS and HDSS were analyzed using the exponential Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach based on experimental TTT data. Linearized plots from the JMAK calculations showed a transition in kinetics mechanism from eutectoid decomposition and interface-controlled growth to a diffusion-controlled growth stage in both materials. The higher volumes and morphologies of the sigma phase were found to be diffusion-dependent, mainly occurring during the second kinetics stage. The influence of HDSS's higher Cr and Mo content on the growth rate was observed for these higher volumes. For applications within the 1% sigma phase limit, both wires exhibited equivalent susceptibility to sigma phase formation, regardless of the higher alloying in HDSS.

Relating bulk solid caking events to phase change events that happen during crystallization processes

Caking causes significant problems in the storage of bulk solids in process vessels. Caking results in large rathole formation, lumps in product, inability to empty bags, silos, hoppers, and poor flow in feeders. Caking problems reaches all levels of the production process from the large-scale silos to the small single use packages sold in stores.Caking is characterized as a gain of significant bulk strength during storage. It is responsible for many lost production hours and unscheduled downtimes in processes dealing with bulk solids – particularly powders. Caking is also mechanistic in its nature. In this paper we will deal with one of those causes. The goal of this work is to more clearly understand the relationship between the root cause of caking and the kinetics of the gain of strength observed when recrystallization results in powder caking. Detailed strength measurements seem to indicate that the caking of powders induces a gain in strength in accordance with two exponential growth terms. The first strength growth term is proportional to storage time to the 1st power. The second exponential growth term is proportional to storage time to 3rd power. It was found that this behavior was consistent with phase growth kinetics in accordance with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. This equation suggests that within a confined space where crystallization is occurring the fraction of space that transforms into a crystalline material has kinetics consistent with exponential functions that are proportional to time to the 1st power, 2nd power, and 3rd power depending on the dimension of the crystals that form. Rods or fibers correspond to kinetics proportional to time to the 1st power. Plates correspond to kinetics proportional to time to the 2nd power. The formation of spheres or 3D clusters correspond to kinetics proportional to time to the 3rd power. The bulk strength tends to have a one-to-one correspondence to exponential crystal growth to time to the 1st and 3rd powers where the 1st power exponential strength growth happens on a short time frame and the 3rd power exponential growth happens on a longer time frame. This suggests that crystal rods or fibers first form in the pendular volume between particles followed by 3D structure formation in the pendules during bulk solid caking events. Understanding the magnitude and kinetics of these events can give engineers the information to prevent or minimize this issue.

In Situ Observation the Effect of Y on the Solidification Process of 7Mo-SASS under a Low Cooling Rate.

The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm2 for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO3 has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased.

Phase transformation kinetics of high-carbon steel during continuous heating

Effects of heating temperature, heating rate, and holding time on the austenitization process of near-eutectoid high-carbon steel have been investigated. The dilatometric tests were carried out at different heating rates of 1–100 °C/Sec. The Ac1 and Ac3 critical temperatures, the volume fractions of the parent and produced austenite phases, and consequently, the coefficient of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation were obtained from dilatometric diagrams. The activation energy for the transformation was estimated using the Kissinger equation. The optical microscope was used for the microstructural characterization of samples austenitized at different temperatures and durations. The grain growth kinetics of austenite during processing was also characterized. Cellular Automata simulation was used to investigate the nucleation and growth of austenite from the initial full pearlitic structure. Based on the results, the activation energy of 242.6 kJ/mol at low heating rates (1 °C/Sec to 12 °C/Sec) and 436.94 kJ/mol at high heating rates (25 °C/Sec to 100 °C/Sec) were obtained, which indicated the different phase transformation mechanisms. This was attributed to the presence of boron, which postpones nucleation at grain boundaries and helps to activate the shear mechanism at high heating rates. Finally, a continuous heating transformation (CHT) diagram was drawn, which is an indispensable tool to consider the effect of heating rate on the austenitization process and austenite grain size.

Sigma phase precipitation and growth in solution annealed 321 stainless steel during thermal aging: Effects on toughness & intergranular corrosion

Sigma (σ) phase is considered as one of the most deleterious phases in stainless steels (SS) due to its detrimental effects on mechanical and corrosion properties. In this work, σ phase formation and growth in annealed SS 321 (without δ-ferrite) and its effects on impact toughness and intergranular corrosion (IGC) were investigated, following thermal aging at 750 °C up to 5500 h. Microstructural characterization using X-ray diffraction and electron microscopy techniques revealed (a) precipitation of intergranular Cr-rich M23C6 and fine Ti-rich intra & intergranular carbides after short aging time of 27 h, (b) σ phase nucleation at the Cr-rich M23C6 - austenite interfaces after 100 h of aging and (c) σ phase at the austenite grain boundaries (GBs) beyond 700 h of aging. The experimental results, combined with thermodynamic calculations (ThermoCalc) showed that the meta stable Cr-rich carbide is the source of σ phase nucleation. σ phase started to grow at the expense of Cr-rich M23C6, and further, its size increased preferentially along the GB and into one of the austenite grains with increase of aging duration. Hardness remained in the range of 190–200 HV for all the specimens aged beyond 100 h and is shown to be due to stable Ti-rich intragranular carbides of 7–20 nm size. Impact toughness values decreased with the growth of σ phase, and up to 50% reduction in toughness was found by about 1% σ phase, with a ductile-brittle cleavage fracture. The normalized values of σ phase fraction, determined from its size and the toughness values, with aging duration followed Kolmogorov-Johnson-Mehl-Avrami (KJMA) model and both yielded the same empirical rate constant (k) of 2.7 × 10−7/s and the n value of ∼1. The lower k value and the n value of 1 suggest slower kinetics and one-dimensional growth of σ phase, which is in well agreement with microstructural observations. Susceptibility to IGC was seen in specimens with sub-microscopic σ phase (aged up to 100 h). However, aging beyond 700 h decreased resistance to uniform corrosion due to σ phase growth.

Insights into the Sodiation Kinetics of Si and Ge Anodes for Sodium-Ion Batteries

Group IVA elements exhibit interesting Na storage capabilities due to the success of their Li alloy analogues. However, beyond hard carbon, they remain poorly understood as anodes for sodium-ion batteries (SIBs). Here, kinetic investigations of the electrochemical sodiation of Si and Ge are conducted using liquid electrolytes and half-cell configurations. Sodiation of Ge is found to be kinetically limited rather than thermodynamically limited. Either increasing temperature or decreasing sodiation rate can facilitate easier transformations from Ge to Na-Ge phases. A critical temperature seems to exist between 50 °C and 60 °C, beyond which a higher sodiation capacity is evident. The phase transformations are analyzed using Kolmogorov–Johnson–Mehl–Avrami theory. Following a one-dimensional growth, the Ge to NaGe4 is determined to be diffusion limited whereas NaGe4 to Na1+x Ge is controlled by reaction speed. Moreover, the Arrhenius equation is employed to investigate the temperature dependence on both phase transformations, giving activation energies of ∼50 kJ·mol−1 and ∼70 kJ·mol−1, respectively. Schematic models are proposed to elucidate the sodiation mechanisms, potentially influencing sought-after advancements in cell formats and classifications. Not only does this work lay the foundation for efforts on the Ge-based anodes, but also provides analogous kinetic information to Si/Sn-based ones for SIBs.

Kinetic analysis of 2D Mo2C crystal growth via CVD

We investigated the growth mechanism of 2D Mo2C crystals by chemical vapor deposition (CVD) under various time and temperature conditions. The growth kinetics and mechanism of Mo2C on Cu via chemical vapor deposition (CVD) were investigated using a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. To analyze the surface coverage, we employed scanning electron microscopy (SEM) and applied the modified JMAK model to determine the growth rate and activation energy. The growth rate of Mo2C exhibited temperature-dependent behavior, described by the Arrhenius relationship, with an apparent activation energy of 4 eV. The Avrami plot exhibited an exponent of 3 indicating a complex process with nucleation and growth.

Microstructure evolution and strengthening mechanisms of high strength Al-Zn-Mg-Cu alloy via pre-hardening forming

The precipitating and strengthening behavior of the pre-hardening forming (PHF) process for a high-strength Al-Zn-Mg-Cu alloy is studied. The PHF process obviates the need for artificial aging (120 °C for 24 h) during hot forming. Despite the considerably lower volume fraction of η' (with a maximum of 0.54 %) in the PHF samples compared to that of the T6 samples, the maximum yield strength of the 12 h-5 min tensile specimen is 496 MPa, similar to that of the T6 alloy (490 MPa). It is assumed that the GPII zones have compensated for the insufficient yield strength in PHF samples. Through the integration of small-angle X-ray scattering (SAXS) experiments and the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equations, the strengthening effects attributable to GPII zones and η' have been discerned. Intriguingly, the contribution of GPII zones to yield strength (219 MPa in the 12 h-5 min sample) equals that of η' (167 MPa in the 12 h-5 min sample). A comprehensive yield strength model is subsequently established, including both GPII zones and η' phase, exhibiting a deviation between fitting and experimental results of less than 5 %.

(Invited) Operando Observation of Fast Charging in Li-Ion Cells

Fast charging (<15 min) of lithium-ion batteries (LIBs) for electrical vehicles (EVs) is widely seen as the key factor that will greatly stimulate the EV markets, and its realization is mainly hindered by the sluggish diffusion of Li+. To have a mechanistic understanding of Li+ diffusion within LIBs, in this study, structural evolutions of electrodes for a Ni-rich LiNi0.6Mn0.2Co0.2O2 (NMC622) || graphite cylindrical cell with high areal loading (2.78 mAh cm−2) are developed for operando neutron powder diffraction study at different charging rates. Via sequential Rietveld refinements, changes in structures of NMC622 and Li x C6 are obtained during moderate and fast charging (from 0.27 C to 4.4 C). NMC622 exhibits the same structural evolution regardless of C-rates. For phase transitions of Li x C6, the stage I (LiC6) phase emerges earlier during the stepwise intercalation at a lower state of charge when charging rate is increased. It is also found that the stage II (LiC12) → stage I (LiC6) transition is the rate-limiting step during fast charging. The LiC12 → LiC6 transition mechanism is further analyzed using the Johnson–Mehl–Avrami–Kolmogorov model. It is concluded as a diffusion-controlled, 1D phase transition with decreasing nucleation kinetics under increasing charging rates.

The shelf life of ASDs: 2. Predicting the shelf life at storage conditions

Amorphous solid dispersions (ASDs) are a widely used formulation technology for poorly water-soluble active pharmaceutical ingredients (API). Depending on the API-polymer combination and API load in the ASD, the amorphous API might be thermodynamically metastable and crystallize over time. The crystallization onset is one critical factor that can define the shelf life of the ASD. Thus, for ASD formulations, long-term stability against crystallization of the API is of particular interest. This work presents a method for predicting the long-term physical stability of ASDs (crystallization onset time). The new approach combines the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation with classical nucleation theory. The shelf life predicted using the new approach depends on supersaturation (determined with PC-SAFT), viscosity (determined with WLF equation or Arrhenius equation) and two specific model parameters k’ and B. The latter were fitted to a few fast crystallization-kinetics measurements above the glass transition of the ASD. An additional crystallization-kinetics measurement below the glass-transition temperature of the ASD was used to determine the Arrhenius parameters. Once all parameters are determined for a given API/polymer combination and manufacturing method, they are valid for any API load, temperature, and RH. The proposed approach allows predicting the shelf life (crystallization onset) of a potential ASD in early stage of development within a few days. It was successfully verified for ASDs stored at 25 °C and 10% RH or 60% RH.

Investigation of Recrystallization Kinetics in 1050 Al Alloy by Experimental Evidence and Modeling Approach.

The recrystallization (RX) kinetics of commercially pure Al alloy is studied under the scope of annealing temperature, time, and degree of deformation. To examine the distribution of recrystallization, Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory is employed, where the path of microstructural transformation from the deformed state to the fully recovered one is studied as a function of the volume fraction of recrystallized grains (XV) and annealing time. The drop in hardness is recorded for the samples at various stages of annealing with a corresponding decrease in stored energy as the annealing time increases. The stored energy obtained from the hardness results and Orientation Imaging Microscopy (OIM)-based method is found to be in good agreement with each other, proving the efficiency of both techniques. To determine the volume fraction of the recrystallized microstructure, data obtained from Vickers hardness measurements are used. Various parameters associated with recrystallization statistics such as the critical radius of nuclei, the incubation period, and the mobility of High-Angle Grain Boundaries (HAGB) were derived from the experimental evidence. The experimental data also suggest a sharp drop in the velocity of HAGB as the RX transformation process approaches its completion, which is found to be a direct result of a drop in stored energy. A softening window between 42 s and 55 s is identified for our experimental data where the hardness, stored energy, and velocity of HAGB drops very sharply, and the maximum fraction of deformed grains is expected to be converted to the recrystallized ones. Along with experimental observations, an analytical model was developed, which helps to approximate the kinetics of RX and corresponding parameters for various annealing temperatures and strains while revealing the characteristic feature of Avrami exponent n. Both experimental evidence and model data reveal a very strong dependency of recrystallization behavior on the stored energy.

Variable-Temperature Photocyclization Kinetics in a Metal–Organic Framework (MOF): A Comparison of the Johnson–Mehl–Avrami–Kolmogorov and Finke–Watzky Models

Variable-Temperature Photocyclization Kinetics in a Metal–Organic Framework (MOF): A Comparison of the Johnson–Mehl–Avrami–Kolmogorov and Finke–Watzky Models

Modelling crystallization: When the normal growth velocity depends on the supersaturation

The crystallization proceeds by the advance of the crystal faces into the disordered phase at the expense of the material excess, the supersaturation. Using a conservation constraint for the transformation ratio α∈[0,1] as complementing the rescaled supersaturation to 1 and a kinetic law for the normal growth velocity as function of the supersaturation raised to power g, the growth order, we derive an equation for the rate of transformation dα/dt. We integrate it for the six combinations of the three spatial dimensions D = 1, 2, 3 and the two canonical values of g = 1, 2 towards obtaining expressions for αDg. The same equation, with g = 1 and D = n (n is the so called Avrami exponent) is obtained when taking only the linear in α term from the Taylor's expansion around α = 0 of the model equation of Johnson-Mehl-Avrami-Kolmogorov (JMAK). We verify our model by fitting datasets of α21 and α31 (from α = 0 to αupper = 0.999) with JMAK to obtain from the fit n = 1.725, 2.43, resp. We show further how the values of n depend on the value of αupper to which the fit is performed starting always from 0. Towards building a validation protocol, we start with validating α21 with published results.

Atomic structure, thermal stability and isothermal crystallization kinetics of novel Co-based metallic glasses with excellent soft magnetic properties

In this work, Co66-xFexHf6.5B27.5 (x = 0, 10, 15, 20) metallic glasses (MGs) have been prepared by melt-spinning, and a comprehensive qualitative study including the atomic structure, thermal behavior, crystallization kinetics, and magnetic properties of the MGs is presented. The addition of Fe notably improves the thermal stability of the prepared glasses by increasing the width of the supercooled liquid region (SLR) from 26 K to 55 K. Analysis of the structure factor and the reduced-pair distribution function (PDF) reveal subtle variations in short-range ordering (SRO) and an increase of the atomic packing density of the glassy phase by 20 at% Fe addition. Notable rises in saturation polarization, Js, (from 0.21 T to 0.67 T), Curie point, Tc, (from 315 K to 530 K) and coercivity, Hc, (from 0.5 A/m to 2.5 A/m) are found after Fe alloying. The isothermal crystallization kinetics determined by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach demonstrate that the novel Co46Fe20Hf6.5B27.5 MG exhibits a remarkably higher activation energy and a significantly longer incubation time (about 16 min) before crystallization compared to existing Fe-free (x = 0) as well as most other Fe/Co-based MGs. Slight increases in Js (up to 5%) and Tc (about 4.7%), and a remarkable drop in Hc (up to 80%) are achieved for the Co46Fe20Hf6.5B27.5 MG upon isothermal annealing for a period of 3600 s. The attractive features such as high thermal stability, low coercivity of 0.5 A/m, and higher Js (up 0.7 T) compared to many Co-based glasses, make Co46Fe20Hf6.5B27.5 MG a promising soft magnetic material for future applications in power electronics, electric machines, and fabrication of large-size glassy samples by hot consolidation of MG particles.

Thermodynamics and Kinetics Analysis of Hydrogen Absorption by Zr0.8Ti0.2Co Alloy

The ZrCo-based alloy is considered one of the most promising materials for hydrogen isotope storage in the conceptual design of a fusion reactor. However, there are few systematic studies on the thermodynamic and kinetic models of hydrogen absorption in the new Zr0.8Ti0.2Co alloy. The aim of this study is to computationally derive the general mathematical equations for the thermodynamics and kinetics of hydrogen absorption by Zr0.8Ti0.2Co. In order to obtain the thermodynamic and kinetic data quickly, a constant-flow hydrogen absorption test was used in this study. The thermodynamic performance test revealed that the Zr0.8Ti0.2Co hydrogen absorption transition process was switched from ZrCo to ZrCoHx (metastable phase) and then to ZrCoH3 with an enthalpy of hydrogenation (ΔH) of 66.59 kJ·mol−1 H2, which was obviously lower than that of the ZrCo-based alloy due to the metastable phase. A mathematical model of the hydrogen absorption coupled with the kinetic equations was established by kinetic process analysis. The hydrogen absorption process was divided into two stages, and the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model could fit the two stages of the Zr0.8Ti0.2Co hydrogen absorption well. In the first stage, the JMAK index was n1 = 1.04, activation energy Ea1 = 7594.6 J/mol, and rate coefficient of reaction k01 = 1.958E-4 s−1. While in the second stage, it was n2 = 1.39, Ea2 = 5221 J/mol, and k02 = 9.938E-5 s−1. Based on the range of n values, it can be inferred that both the nucleation and growth mechanisms or the diffusion mechanism were expressed as the rate-limiting steps. Combined with the simulation software, metal hydride bed performance could be better investigated and the structural design could be guided by the obtained mathematical equation of Zr0.8Ti0.2Co hydriding.

Precipitation kinetics of niobium carbide (NbC) during homogenization heat treatment of additively manufactured inconel 718 superalloy

High cooling rate of the laser powder bed fusion (L-PBF) additive manufacturing can suppress the formation of niobium carbide (NbC) phase in the fabricated Inconel 718 nickel-based superalloy parts. However, it might precipitate during elevated temperature exposure. Accordingly, in the present work, the precipitation kinetics of the NbC carbide was investigated during homogenization heat treatment at temperatures in the range of 1000–1150 °C for soaking times of 15 min to 10 h. It was observed that increasing the homogenization temperature leads to the faster kinetics of NbC formation. The precipitation kinetics was modeled by the general phase transformation kinetics formula (based on the transformed fraction of 0.5 or 50%, t0.5) and Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, resulting in the precipitation activation energies of 159.2 and 157.4 kJ/mol, respectively. It was concluded that the diffusion of both C and Nb atoms in the Ni matrix are the underlying mechanisms for the NbC carbide formation during homogenization heat treatment. The relationship of 1/t0.5∝exp(0.0097T) was also obtained for the precipitation rate as a function of temperature.

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.