SIMULATION OF THE MICROSTRUCTURE DISTRIBUTION OF HYDROGEN STORAGE ALLOY AND OPTIMAL DESIGN OF CASTING MOULDS

In this paper,a simulation model for the temperature field in the solidification process and microstructure distribution is presented. Then, the result of simulation for the final microstructure distribution is compared with experiment using 10-Kg ingot of MlNi3.55Al0.3Mn0.4CO0.75 (Ml: Lanthanum-rich Mischmetal) hydrogen stor- age alloy cast in a mould, which agrees with the experiment well. Finally, in order to obtain the expected as-cast microstructure distribution of 15-Kg ingot of MlNi3.55Al0.3Mn0.4Co0.75 alloy, the size of Cu mould is optimized using the model described. The optimized mould is then made and the alloy is cast in it, the expected as-cast microstructure distribution is obtained.

Microstructure and Hot Tearing Sensitivity Simulation and Parameters Optimization for the Centrifugal Casting of Al-Cu Alloy

Four typical theories on the formation of thermal tears:strength,liquid film,intergranular bridging,and solidifica-tion shrinkage compensation theories.From these theories,a number of criteria have been derived for predicting the formation of thermal cracks,such as the stress-based Niyama,Clyne,and RDG(Rapaz-Dreiser-Grimaud)criteria.In this paper,a mathematical model of horizontal centrifugal casting was established,and numerical simulation analysis was conducted for the centrifugal casting process of cylindrical Al-Cu alloy castings to investigate the effect of the centrifugal casting process conditions on the microstructure and hot tearing sensitivity of alloy castings by using the modified RDG hot tearing criterion.Results show that increasing the centrifugal rotation and pouring speeds can refine the microstructure of the alloy but increasing the pouring and mold preheating temperatures can lead to an increase in grain size.The grain size gradually transitions from fine grain on the outer layer to coarse grain on the inner layer.Meanwhile,combined with the modified RDG hot tearing criterion,the overall distribution of the castings’hot tearing sensitivity was analyzed.The analysis results indicate that the porosity in the middle region of the casting was large,and hot tearing defects were prone to occur.The hot tearing tendency on the inner side of the casting was greater than that on the outer side.The effects of centrifugal rotation speed,pouring temperature,and preheating temperature on the thermal sensitivity of Al-Cu alloy castings are summarized in this paper.This study revealed that the tendency of alloy hot cracking decreases with the increase of the centrifugal speed,and the maximum porosity of castings decreases first and then increases with the pouring temperature.As the preheating temperature increases,the overall maximum porosity of castings shows a decreasing trend.

Effects of Strip Casting and Annealing on Electrochemical Properties of LPCNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3) Hydrogen Storage Alloys

The effect of thickness (1 similar to 10 mm) of the ingots on the electrochemical properties of as-cast and annealed strip cast LPCNi3.55Co0.75Mn0.4Al0.3 hydrogen storage alloys was investigated. It is found that the 0.2 C discharge capacity of as-cast LPCNi3.55Co0.75Mn0.4Al0.3 alloy increases with the increase of the thickness of the ingots. As-east alloy with the thickness of 10 mm shows better activation property, higher 1C discharge capacity and better cyclic stability than others. It is mainly contributed to its larger unit cell volume and less internal stress. Annealed LPCNi3.55Co0.75Mn0.4Al0.3 alloy with the thickness of 3 mm shows much better comprehensive electrochemical properties than as-east one; The cyclic. stability of the alloy with the thickness of 6 mm and the activation properties of the alloys with the thickness of 3 similar to 6 mm are improved after annealing. It is mainly owing to the great release of internal stress and the decrease of the segregation of Mn in the alloys.

Electrochemical Properties of Strip Casting LPCNi_(3.55) Co_(0.75) Mn_(0.4) Al_(0.3) Hydrogen Storage Alloys

LPCNi 3.55 Co 0.75 Mn 0.4 Al 0.3 hydrogen storage alloy was investigated, and the effects of thickness of its strip casting ingots(as cast) on the electrochemical performances were discussed. It was found that the 0.2 C discharge capacity increased with the increase of the thickness (from 1 mm to 10 mm) of the ingots, mainly due to the enlargement of the unit cell volume; Among the thickness of the ingots in our study, 10 mm sample showed a better activation property; LPCNi 3.55 Co 0.75 Mn 0.4 Al 0.3 alloy with 10mm showed higher comprehensive properties than those with other thickness under 1C rate.

Development of Strip Casting Technology in Rare Earth Permanent Magnet Alloys and Hydrogen Storage Alloys in China

The SC technique is now being applied widely in material preparation, especially in rare earth functional materials in virtue of its advanced process and high performance product. The applications of SC technique in rare earth permanent magnet alloys and hydrogen storage alloys were analyzed integrative, on the basis of summary of SC technique development in this paper. The paper mainly includes development history of SC technology, effect of SC technology on alloy microstructure, application of SC technology in RE storage hydrogen alloy and sintered Nd-Fe-B alloy, development of SC equipment and SC product industry. At the same time, the paper points out the existing problem of SC products.

Electrode properties and phase composition of Ti_(0.5)Ni_(0.25)Al_(0.25) hydrogen storage alloys and theoretical simulation

Ti0.5Al0.25Ni0.25 alloy prepared by vacuum induction melting was studied.The phase composition was analyzed with X-ray technique and EDS analysis,and its electrochemical properties were investigated at various temperatures.Electrochemical reaction kinetic parameters were also studied with proper electrochemical techniques.The influence of the secondary corrosion reaction on the anodic linear polarization measurement was also analyzed by theoretical simulation.The results show that,proper ball-milling with nickel powders is beneficial to electrochemical performance.The theoretical simulation proves that,the existence of the side reaction can disturb the measurement of electrochemical reaction kinetic parameters.

Solidification microstructure of Ti-43Al alloy by twin-roll strip casting

As a near-net-shape technology,the twin-roll strip casting(TRC)process can be considered to apply to the fabrication of TiAl alloy sheets.However,the control of the grain distribution is very important in strip casting because the mechanical properties of strips are directly determined by the solidification microstructure.A three-dimensional(3D)cellular automation finite-element(CAFE)model based on ProCAST software was established to simulate the solidification microstructure of Ti-43Al alloy.Then,the influence of casting temperature and the maximum nucleation density(nmax)on the solidification microstructure was investigated in detail.The simulation results provide a good explanation and prediction for the solidification microstructure in the molten pool before leaving the kissing point.Experimental and simulated microstructure show the common texture<001>orientation in the columnar grains zone.Finally,the microstructure evolution of the Ti-43Al alloy was analyzed and the solidification phase transformation path during the TSC process was determined,i.e.,L→L+β→β→β+α→α+γ+β/B2 phase under a faster cooling rate and L→L+β→β→β+α→γ+lamellar(α_(2)+γ)+β/B2 phase under a slower cooling rate.

Effects of annealing treatment on the microstructure and electrochemical properties of low-Co hydrogen storage alloys containing Cu and Fe

The effects of annealing treatment on the microstructure and electrochemical properties of low-Co LaNi 3.55 Mn 0.35 Co 0.20 Al 0.20 Cu 0.75 Fe 0.10 hydrogen storage alloys were investigated. X-ray diffraction （XRD） analysis indicated that annealing treatment remarkably reduced the lattice strain and defects, and increased the unit-cell volume. The optical microscope analysis showed that the as-cast alloy had a crass dendrite microstructure with noticeable composition segregation, which gradually disappeared with increasing annealing temperature, and the micro-structure changed to an equiaxed structure after annealing the alloy at 1233 K. The electrochemical tests indicated that the annealed alloys demonstrated much better cycling stability compared with the as-cast one. The capacity retention at the 100th cycle increased from 90.0% （as-cast） to 94.7% （1273 K）. The annealing treatment also improved the discharge capacity. However, the high rate dischargeability （HRD） value of the annealed alloy slightly dropped, which was believed to be ascribed to the decreased exchange current density and the hydrogen diffusion coefficient in alloy bulk.

Effect of rapid quenching on the microstructure and electrochemical characteristics of La_(0.6)Ce_(0.4)Ni_(3.6)Co_(0.65)Mn_(0.4)Al_(0.2)Ti_(0.05_(FeB)_(0.1) hydrogen storage alloy

In order to improve the cycling stability of AB5 type alloy electrodes,rapid quenching technology and new alloy composition design were employed.A hydrogen storage alloy with nominal composition La0.6Ce0.4Ni3.6Co0.65Mn0.4Al0.2Ti0.05（FeB）0.1 was prepared by vacuum magnetic levitation melting under high purity argon atmosphere,followed by rapid quenching at different cooling rates.XRD results show that all alloys exhibit the single-phase CaCu5-type structure.Electrochemical tests indicate that rapid quenching can slightly improve the cycling life of the alloy.Nevertheless,the high-rate dischargeability of the quenched alloys is lower than that of the as-cast alloy.

Key technology and application of AB_(2) hydrogen storage alloy in fuel cell hydrogen supply system

At present,there is limited research on the application of fuel cell power generation system technology using solid hydrogen storage materials,especially in hydrogen-assisted two-wheelers.Considering the disadvantages of low hydrogen storage capacity and poor kinetics of hydrogen storage materials,our primary focus is to achieve smooth hydrogen ab-/desorption over a wide temperature range to meet the requirements of fuel cells and their integrated power generation systems.In this paper,the Ti_(0.9)Zr_(0.1)Mn_(1.45)V_(0.4)Fe_(0.15) hydrogen storage alloy was successfully prepared by arc melting.The maximum hydrogen storage capacity reaches 1.89 wt% at 318 K.The alloy has the capability to absorb 90% of hydrogen storage capacity within 50 s at 7 MPa and release 90% of hydrogen within 220 s.Comsol Multiphysics 6.0 software was used to simulate the hydrogen ab-/desorption processes of the tank.The flow rate of cooling water during hydrogen absorption varied in a gradient of(0.02 t x)m s^(-1)(x=0,0.02,0.04,0.06,0.08,0.1,0.12).Cooling water flow rate is positively correlated with the hydrogen absorption rate but negatively correlated with the cost.When the cooling rate is 0.06 m s^(-1),both simulation and experimentation have shown that the hydrogen storage tank is capable of steady hydrogen desorption for over 6 h at a flow rate of 2 L min^(-1).Based on the above conclusions,we have successfully developed a hydrogen-assisted two-wheeler with a range of 80 km and achieved regional demonstration operations in Changzhou and Shaoguan.This paper highlights the achievements of our team in the technological development of fuel cell power generation systems using solid hydrogen storage materials as hydrogen storage carriers and their application in twowheelers in recent years.

Microstructure and electrochemical properties of melt-spun Nd_(0.8)Mg_(0.2)(Ni_(0.8)Co_(0.2))_(3.8) hydrogen storage alloy

The effects of rapid solidification on the microstructure and electrochemical properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloy were systematically investigated.The microstructure of alloys was characterized by scanning electron microscopy(SEM),X-ray diffractometer(XRD) and transmission electron microscopy(TEM).It was found that the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons became thinner and the average grain size of the ribbons became smaller with increasing wheel speed.A fraction of amorphous phase was obs...

Microstructural evolution of melt-spun Mg-10Ni-2Mm hydrogen storage alloy

The microstructural evolution of a Mg-10Ni-2Mm（molar fraction,%）（Mm=Ce,La-rich mischmetal） hydrogen storage alloys applied with various solidification rates was studied.The results show that the grain size of melt-spun ribbon is remarkably reduced by increasing the solidification rate.The microcrystalline,nanocrystalline and amorphous microstructures are obtained by applying the surface velocities of the graphite wheel of 3.1,10.5 and 20.9 m/s,respectively.By applying the surface velocity of the graphite wheel of 3.1 m/s,the melt-spun specimen obtains full crystalline with a considerable amount of coarse microcrystalline Mg and Mg2Ni except for some Mm-rich particles.The amount of nanocrystalline phases significantly increases with increasing the surface velocity of the wheel to 10.5 m/s,and the microstructure is composed of a large amount of nanocrystalline phases of Mg and Mg2Ni particles.A mixed microstructure containing amorphous and nanocrystalline phases is obtained at a surface velocity of the wheel of 20.9 m/s.The optimal microstructure with a considerable amount of nanocrystalline Mg and Mg2Ni in an amorphous matrix is expected to have the maximum hydrogen absorption capacity and excellent hydrogenation kinetics.

Evolution of Hydrogen Storage Alloys Prepared by Special Methods

Microstructure characteristics and electrochemical properties of hydrogen storage alloys prepared by gas atomization, melt spinning and strip casting respectively were outlined.The advantages, disadvantages and research development of the above methods for preparing hydrogen storage alloys were explained.The strip casting is a new special means for preparing AB5 rare earth hydrogen storage alloys of high performance and low cost, and the study of the strip casting for preparing hydrogen storage alloys is presented specially.

Multidimensional regulation of Ti-Zr-Cr-Mn hydrogen storage alloys via Y partial substitution

High density and safe storage of hydrogen are the preconditions for the large-scale application of hydrogen energy.Herein,the hydrogen storage properties of Ti_(0.6)Zr_(0.4)Cr_(0.6)Mn_(1.4) alloys are systematically studied by introducing Y element instead of Ti element through vacuum arc melting.After the partial substitution of Y,a second phase of rare earth oxide is added in addition to the main suction hydrogen phase,C14 Laves phase.Thanks to the unique properties of rare earth elements,the partial substitution of Y can not only improve the activation properties and plateau pressure of the alloys,but also increase the effective hydrogen storage capacity of the alloys.The comprehensive properties of hydrogen storage alloys are improved by multidimensional regulation of rare earth elements.Among them,Ti_(0.552)Y_(0.048)Zr_(0.4)Cr_(0.6)Mn_(1.4) has the best comprehensive performance.The alloy can absorb hydrogen without activation at room temperature and 5 MPa,with a maximum hydrogen storage capacity of 1.98 wt.%.At the same time,it reduces the stability of the hydride and the enthalpy change value,making it easier to release hydrogen.Through theoretical analysis and first-principle simulation,the results show that the substitution of Y element reduces the migration energy barrier of hydrogen and the structural stability of the system,which is conducive to hydrogen evolution.The alloy has superior durability compared to the original alloy,and the capacity retention rate was 96.79%after 100 hydrogen absorption/desorption cycles.

Modeling of the Microstructure Evolution in Continuously Cast Al-pb Alloys

A numerical model is presented describing the microstructure evolution of an immiscible alloy under the continuous casting conditions. Calculations are carried out to investigate the microstructure evolution in a vertical strip cast sample of Al+5wt pct Pb alloy. The numerical results show that there exists a peak value for the supersaturation in front of the solid祃iquid interface, and the minority phase droplets are nucleated in a region around this peak. Under strip casting conditions the Marangoni migration dominates the motion of droplets. This leads to an accumulation of the minority phase droplets in front of the solid祃iquid interface.

Influence of Cr or Co doping on electrode properties of V_(2.1) TiNi_(0.5) Hf_(0.05) hydrogen storage alloy

The microstructures and electrode properties of Cr or Co-doped V2.1 TiNi0.5 Hf0.05M 0.113 （M=Cr, Co） hydrogen storage alloys were investigated. It is found that all alloys consist of a main phase of V-based solid solution with b.c.c. structure and a secondary phase of C14-type Laves phase with a three-dimensional network structure. After the doping of Cr or Co into V 2.1TiNi 0.5Hf 0.05, the abundance and unit cell volume of the main phase decrease, and those of the secondary phase increase. The electrochemical measurements show that the discharge capacities of Cr-doped V2.1 TiNi0.5 Hf0.05Cr 0.113 alloy and Co-doped V2.1 TiNi0.5 Hf0.05Co 0.113 alloy are less than that of V2.1 TiNi0.5 Hf0.05 alloy, but their cycle stability and high-rate dischargeability are improved markedly. The results show that Cr or Co doping into V2.1 TiNi0.5 Hf0.05 alloy is significantly beneficial for the cycling stability.

Influence of Annealing Treatment on Microstructure and Cycling Stability of La-Rich Ml(NiCoMnAl)_5 Alloy Electrode for Ni/MH Batteries

The crystallographic structure, microstructure, composition homogeneity and electrode charge-discharge cycling stability were investigated of the as-cast and annealed La-rich mischmetal (designating Ml)-based hydrogen storage alloy with a composition of MlNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3). X-ray diffraction analysis shows that the MlNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3) alloy is composed of the dominant phase with a CaCu_5-type hexagonal structure and small amounts of the second phase with a La_2Ni_7-type structure. The annealing heat treatment conducted at 1273 K for 10 h results in decrease of the crystal lattice strain and composition segregation, disappearance of the dendrite structure and growth of the crystal grain of the MlNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3) alloy. The annealing causes the cycle life to be increased by about 30% over the as-cast alloy electrode. The cycling stability of the alloy electrode is improved significantly upon annealing. The cause of the improvement in the cycling stability was discussed based on the alloy composition distribution and microstructure changes due to annealing.

Effect of Sn substitution for Co on microstructure and electrochemical performance of AB_5 type La_(0.7)Mg_(0.3)Al_(0.3)Mn_(0.4)Co_(0.5-x)Sn_xNi_(3.8)(x=0-0.5) alloys

The effects of substitution of Sn for Co on the microstructure, hydrogen storage and electrochemical discharge capacity of La0.7Mg0.3Al0.3Mn0.4Co0.5-xSnxNi3.8 （x=0, 0.1, 0.2, 0.3 and 0.5） alloys were investigated using X-ray diffraction （XRD）, pressure composition isotherm （PCT） and electrochemical discharge cycle. XRD, scanning electron microscopy （SEM） and energy dispersive spectroscopy （EDS） tests showed that all of alloys are mainly composed of LaNi5 and MgNi2 phases, but when increasing the content of Sn in alloys, the LaNiSn phase appears and microstructure is refined. The PCT showed that increasing substitution of Sn for Co results in decrease of the maximum hydrogen storage capacity from 1.48% （x=0） to 0.85% （x=0.5）. The electrochemical tests indicated that the maximum discharge capacity decreases from 337.1 mA-h/g （x=0） to 239.8 mA.h/g （x=0.5）; however, the discharge capacity retention at the 100th cycle increases from 70.2% （x=0） to 78.0% （x=0.5）.

Effect of CeH_(2.29) on the microstructures and hydrogen properties of LiBH_4-Mg_2NiH_4 composites

A composite of LiBH4-Mg2NiH4 doped with 10wt% CEH2.29 was prepared by ball milling followed by dynamic interspace vac- uum treatment at 573 K. The introduction of CEH2.29 caused a transformation in the morphology of Mg from complex spongy and lamellar to uniformly spongy, resulting in refined particle size and abundant H diffusion pathways. This LiBH4-Mg2NiH4 ＋ 10wt% CEH2.29 composite exhibited excellent hydrogen storage properties. The starting temperature of rapid H absorption decreased to 375 K in the doped composite from 452 K for the unmodified material, and the onset decomposition temperature of its hydride was reduced from 536 K to 517 K. In addi- tion, the time required for a hydrogen release of 1.5wt% （at 598 K） was 87 s less than that of the un-doped composite.

A review of classical hydrogen isotopes storage materials

Hydrogen storage alloys(HSAs)are attracting widespread interest in the nuclear industry because of the generation of stable metal hydrides after tritium absorption,thus effectively preventing the leakage of radioactive tritium.Commonly used HSAs in the hydrogen isotopes field are Zr2M(M=Co,Ni,Fe)alloys,metallic Pd,depleted U,and ZrCo alloy.Specifically,Zr2M(M=Co,Ni,Fe)alloys are considered promising tritium-getter materials,and metallic Pd is utilized to separate and purify hydrogen isotopes.Furthermore,depleted U and ZrCo alloy are well suited for storing and delivering hydrogen isotopes.Notably,all the aforementioned HSAs need to modulate their hydrogen storage properties for complex operating conditions.In this review,we present a comprehensive overview of the reported modification methods applied to the above alloys.Alloying is an effective amelioration method that mainly modulates the properties of HSAs by altering their local geometrical/electronic structures.Besides,microstructural modifications such as nano-sizing and nanopores have been used to increase the specific surface area and active sites of metallic Pd and ZrCo alloys for enhancing de-/hydrogenation kinetics.The combination of metallic Pd with support materials can significantly reduce the cost and enhance the pulverization resistance.Moreover,the poisoning resistance of ZrCo alloy is improved by constructing active surfaces with selective permeability.Overall,the review is constructive for better understanding the properties and mechanisms of hydrogen isotope storage alloys and provides effective guidance for future modification research.