Kuo-Chih Chou

Phase Equilibria, Crystal Structure and Hydriding/Dehydriding Mechanism of Nd4Mg80Ni8 Compound.

In order to find out the optimal composition of novel Nd-Mg-Ni alloys for hydrogen storage, the isothermal section of Nd-Mg-Ni system at 400u2009°C is established by examining the equilibrated alloys. A new ternary compound Nd4Mg80Ni8 is discovered in the Mg-rich corner. It has the crystal structure of space group I41/amd with lattice parameters of au2009=u2009bu2009=u200911.2743(1) Å and cu2009=u200915.9170(2) Å, characterized by the synchrotron powder X-ray diffraction (SR-PXRD). High-resolution transmission electron microscopy (HR-TEM) is used to investigate the microstructure of Nd4Mg80Ni8 and its hydrogen-induced microstructure evolution. The hydrogenation leads to Nd4Mg80Ni8 decomposing into NdH2.61-MgH2-Mg2NiH0.3 nanocomposites, where the high density phase boundaries provide a great deal of hydrogen atoms diffusion channels and nucleation sites of hydrides, which greatly enhances the hydriding/dehydriding (H/D) properties. The Nd4Mg80Ni8 exhibits a good cycle ability. The kinetic mechanisms of H/D reactions are studied by Real Physical Picture (RPP) model. The rate controlling steps are diffusion for hydriding reaction in the temperature range of 100u2009~u2009350u2009°C and surface penetration for dehydriding reaction at 291u2009~u2009347u2009°C. In-situ SR-PXRD results reveal the phase transformations of Mg to MgH2 and Mg2Ni to Mg2NiH4 as functions of hydrogen pressure and hydriding time.

Density functional theory study on hydrogenation mechanism in catalyst-activated Mg(0001) surface

Abstract A small amount of Fe3O4 catalyst is known to substantially improve the adsorption and desorption thermodynamics and kinetics of Mg-based materials. Using density functional theory in combination with nudged elastic band method, the dissociative chemisorptions of hydrogen on both pure and Fe-doped Mg(0001) surfaces were studied. The adsorption energy calculations show that a weakly physisorbed state above pure and Fe-doped Mg surface atoms can serve as a precursor state to dissociative chemisorption. Then, the dissociation pathway of H2 and the relative barrier were investigated. The calculated dissociation barrier (1.08 eV) of hydrogen molecule on a pure Mg(0001) surface is in good agreement with comparable experimental and theoretical studies. For the Fe-doped Mg(0001) surface, the activated barrier decreases to 0.101 eV due to the strong interaction between the s orbital of H and the d orbital of Fe.

Kinetics of hydrogen absorption and desorption of a mechanically milled MgH2＋5at%V nanocomposite

Abstract The experimental data in the MgH2-5at%V composite was summarized and used to investigate the kinetic mechanism of hydrogen absorption and desorption using a new model. The research results indicate that a coincidence of the theoretical calculation values with the experimental data has been reached and the rate-limiting step is hydrogen diffusion through the hydride phase (β phase) with the activation energy of 47.2 kJ per mole H2 for absorption and the diffusion of hydrogen in the α solid solution (α phase) with that of 59.1 kJ per mole H2 for desorption. In addition, the hydriding rate of the MgH2-V composite is 2.9 times faster than that of MgH2 powders when compared with their characteristic absorption time directly.

Grain Refinement Behavior of Al‐Zn‐Si Alloy by Inoculation in Hot‐Dip Coating

Grain refinements were an effective method to improve the mechanical properties of aluminium alloy, and heterogeneous nucleation was widely applied in industry to achieve a grain-refined. In this study, grain refinement behavior of 55%Al-Zn-1.6%Si alloy by Al-Ti-B was investigated. In order to obtain the microstructure in situ, high-temperature liquid quenching by stainless steel pipe was used to capture the sample snapshots. Then, the effect of Al-5Ti-0.2B on the alloy was analyzed by microstructure, solid fraction and grain size. Our results show that the grain size of 55Al-Zn-Si alloy decreased after the Al-5Ti-0.2B addition to some extent, and in order to understand the effect of refinement, it was necessary to analyze the mechanisms of Ti in 55Al-Zn-Si alloy. Meanwhile, during the process of solidification, the solid fraction of 55Al-Zn-Si alloy was further increased with the temperature decreasing, and the result was consistent with our calculation of phase diagram. Finally the reactions were identified using calculation of phase diagram.

Investigation of Ti Addition Effects on the Thickness of 55 pct Al-Zn-1.6 pct Si Coating by First-Principles Calculation

First-principles method was performed to predict the effect of Ti addition on thickness and adhesion of 55xa0pct Al-Zn-1.6xa0pct Si coating. The results of optimized geometric configurations, total energy, and charge distributions for the Ti substitution in Fe2Al5 and FeAl3 phases indicated Ti will grab electronic charges from Al atoms, form bonds with neighboring Al, which will reduce the growth of Fe-Al intermetallic layers, and finally enhance the adhesion of the coating/substrate. Furthermore, experiments were performed to validate the prediction results of first-principles successfully.

Kinetics of Synthesis of Ba1.0Co0.7Fe0.2Nb0.1O3–δ through Solid-Solid Reaction

Abstract The mechanism of solid-solid reaction between BaCO 3 and Co 3 O 4 /Fe 2 O 3 /Nb 2 O 5 has been investigated by means of non-isothermal thermogravimetry and differential scanning calorimetry (DSC) under flowing air gas conditions at atmospheric pressure with a new solid-solid reaction model. The effects of high speed agitating mixing and ball-milling mixing processes on the synthesis kinetics were also studied. The synthesis kinetics of Ba 1.0 Co 0.7 Fe 0.2 Nb 0.1 O 3–δ from the BaCO 3 and Co 3 O 4 /Fe 2 O 3 /Nb 2 O 5 particles was calculated by applying the modified model. The results indicated that the overall reaction process was considered involving two stages: addition reaction between BaCO 3 and Co 3 O 4 /Fe 2 O 3 /Nb 2 O 5 particles in the first stage and solution reaction between BaCoO 3 , BaFeO 3 , and BaNbO 3 to form a homogeneous Ba 1.0 Co 0.7 Fe 0.2 Nb 0.1 O 3–δ phase in the second stage. The new model matched well with the experimental data. The apparent activation energy of addition reaction stage of the high speed agitating mixing sample was estimated to be 376.76 kJ·mol −1 , which was only 3/4 of that of the ball-milling mixing sample (494.76 kJ·mol −1 ). These results indicated that the high-speed agitating process could enhance atomic diffusion and facilitate the subsequent reaction, thus it is believed as a more effective, energy saving, and environmentally benign mixing process.依据新的固相反应模型,采用非等温热重和差示扫描量热法研究了由BaCO3和Co3O4、Fe2O3、Nb2O5粉末固相反应合成Ba1.0Co0.7Fe0.2Nb0.1O3-δ的动力学.考察了高速机械搅拌方式混料和球磨方式混料对合成动力学的影响.结果表明,反应过程分为两个阶段：第一阶段为BaCO3和Co3O4、Fe2O3、Nb2O5之间的加成反应;第二阶段为加成反应生成的BaCoO3、BaFeO3和BaNbO3三相之间固溶生成均相的Ba1.0Co0.7Fe0.2Nb0.1O3-δ,此过程中伴随有氧的脱出.应用修正的模型对实验结果进行了拟合,实验数据和理论模型符合良好.高速机械搅拌样品加成反应阶段的活化能为376.76kJ·mol-1,仅为球磨样品加成反应阶段活化能494.76kJ·mol-1的3/4.高速机械搅拌工艺促进了离子的扩散,有利于后续反应的进行,是更为有效、节能、环保的混料方式.

INFLUENCE OF TEMPERATURE AND PRESSURE ON THE KINETICS OF Mg-6mol%LaNi PREPARED BY HYDRIDING COMBUSTION SYNTHESIS

A new model to study the hydriding/dehydriding (H/D) kinetic mechanism has been applied in the two-phase (α-β) region of the Mg-6mol%LaNi composite at temperature and pressure ranging from 523 to 623K and 0.256 to 0.992MPa H 2 , respectively. The coincidence of the theoretical calculation with the experimental data indicates that the rate-limiting step is hydrogen diffusion in the β phase for hydriding process and the diffusion of hydrogen in the α solid solution for hydrogen desorption with activation energies 89500 and 87900J/mol H 2 for H/D processes, respectively, which were much smaller than those of MgH 2 and can be attributed to the La and Ni additions.

Hydrogen Storage Properties of Graphite-Modified Mg-Ni-Ce Composites Prepared by Mechanical Milling Followed by Microwave Sintering

The Mg17Ni1.5Ce0.5 hydrogen storage composites with different contents of graphite were prepared by a new method of mechanical milling and subsequent microwave sintering. The small particle size (~25xa0μm) and the low echo ratio of power indicate that graphite plays an important role not only as a lubricant during mechanical milling but also as a supplementary heating material during microwave sintering. As a catalyst in the hydriding/dehydriding (H/D) reaction, graphite also improved the hydrogen storage properties of the composites. The hydrogen absorption and desorption capacities of Mg17Ni1.5Ce0.5 with 5xa0wtxa0pct graphite were 5.34 and 5.30xa0wtxa0pct H2 at 573xa0K (300xa0°C), its onset temperature of dehydriding reaction was 511xa0K (238xa0°C), and its activation energies of H/D reaction were 40.9 and 54.5xa0kJ/mol H2, respectively. The kinetic mechanisms of the H/D reaction are also discussed.

The Mechanism of Dross Formation during Hot-dip Al-Zn Alloy Coating Process

Abstract The dross formation is closely related with the solubility of Fe in the molten bath. When the Fe level exceeds the saturation solubility of liquid, the dross precipitates in the form of intermetallic compounds. Inductively coupled plasma atomic emission spectrometry (ICP) was used to determine the content of Fe in molten bath. The morphology of the bottom dross was observed by scanning electron microscopy (SEM). Based on the experimental results and the calculated phase diagram of the Al-Zn-Si-Fe system, the effects of bath temperature and Si content in the molten bath on the Fe solubility were analyzed, and then the mechanism of dross formation was investigated. The results showed that when the bath temperature decreased from 630 °C to 590 °C, the solubility of Fe in liquid reduced from 0.12 wt.% to 0.06 wt.% and 0.17 wt.% FeAl3 would precipitate from the liquid. Compared with the Fe dissolution lines at different Si content, it can be seen that the solubility of Fe in Al-Zn melt increased with the increasing of Si content from 0.5 wt.% to 1.5 wt.%. However, the amount of precipitated FeAl3 in 55wt.%Al-Zn-1.5Si molten alloy (0.17 wt.%) was greater than that in 55wt.%Al-Zn-0.5Si molten alloy (0.14 wt.%) in the same temperature range. It means that the more the Si contents in the liquid, the greater the amount of dross formation.

First principles application for mechanical properties of Ti-doped W particles enhanced U matrix composite

The stability, bonding, work of adhesion and electronic structure of the U/W interface with and without Ti were investigated by first principles to explore the mechanical properties of W particles enhanced U–Ti alloy matrix composite as a construction material. The calculated results indicate that the preferable orientation of the U/W interfacial structure is (001)U/(110)W crystallographic plane, Ti atoms originating from U slab are prone to diffuse into W slab through the interface, and additional Ti in U matrix is the stronger adhesion to W, with an ideal work of adhesion of 6.93xa0J·m−2 for U–Ti/W interface, relative to the value of 6.72xa0J·m−2 for clean U/W interface. The stronger adhesion performance is due to the increase in valence electron hybridization for U–Ti/W compared with U/W interface, as evidenced by the characteristic of the local density of states for the interfacial atoms.