Characterization of hydrogenated niobium interlayer and its application in TiAl/Ti2AlNb diffusion bonding

Hydrogenation of niobium foil was achieved by cathodic charging and its application as the interlayer facilitates a sound joint of TiAl/Ti₂AlNb. The microstructure and mechanical properties of hydrogenated niobium were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), nano-indentation and thermogravimetry (TG) analysis. It is demonstrated that the plasticity of the surface of niobium interlayer is improved and dislocation density in niobium crystal lattice increases significantly, owing to the process of niobium interlayer hydrogenation and subsequent dehydrogenation. To investigate the effect of the hydrogenation of the niobium interlayer on the TiAl/Ti₂AlNb diffusion bonding joint, the pure niobium foil was also applied as the interlayer for comparison. The effect of the bonding parameters on the interfacial microstructure evolution and mechanical properties of the joints were systematically analyzed by SEM coupled with an energy-dispersive spectroscopy (EDS) and shear test. The typical microstructure of the joint is found to be TiAl/B2 phase/Nb/δ phase/β phase/Ti₂AlNb. With the increase of the hydrogen content, bonding temperature, holding time and bonding pressure, the bonding defects decreased and the thickness of the diffusion layer increased correspondingly. The shear strength of joint reached 258.9 MPa at 950 °C for 40 min under a pressure of 5 MPa with the hydrogenated niobium interlayer of 1.0 wt% hydrogen content.     

Boron- and nitrogen-based chemical hydrogen storage materials

Boron- and nitrogen-based chemical hydrides are expected to be potential hydrogen carriers for PEM fuel cells because of their high hydrogen contents. Significant efforts have been devoted to decrease their dehydrogenation and hydrogenation temperatures and enhance the reaction kinetics. This article presents an overview of the boron- and nitrogen-based compounds as hydrogen storage materials.     

Effect of hydrogen on diffusion bonding of TiAl-based intermetallics and Ni-based superalloy using hydrogenated Ti6Al4V interlayer

The diffusion bonding of TiAl-based intermetallics and Ni-based superalloys using hydrogenated Ti₆Al₄V interlayer was carried out. The effect of hydrogen on diffusion bonding at different heating rates and the microstructure of hydrogenated Ti₆Al₄V interlayer after bonding were investigated by using SEM, EPMA, XRD and TG/DSC. It was observed that the fast heating rate promotes improvement of diffusion bonding quality. Specifically, the bonding parameters decreased and the shear strength dramatically increased in comparison with the direct diffusion bonding of the two certain materials. Phase transformation of the hydrogenated Ti₆Al₄V interlayer performed differently when the heating rate changed from 50 °C/min to 1200 °C/min. The addition of hydrogen optimizes the interface contact and facilitated the elements diffusion. However, the hydrogen in hydrogenated Ti₆Al₄V starts to escape before bonding at slow heating rate, which weakens the positive effect of hydrogen addition. The fast heating rate prevents the escape of most hydrogen before achieving the bonding temperature, which facilitates the diffusion bonding process.     

Concentration-dependent hydrogen diffusion in hydrogenation and dehydrogenation of vanadium-coated magnesium nanoblades

We carry out a computational investigation to show how the exponential concentration dependence of hydrogen diffusion, which was recently verified in a combined experimental and analytical study, could affect the characteristics of hydrogenation and dehydrogenation of a vanadium-coated magnesium nanoblade. A reaction model is built that separates hydrogen surface sorption and interior diffusion during the hydrogenation/dehydrogenation process. For the hydrogenation process, the hydrogen surface adsorption is much faster than the hydrogen diffusion, resulting in high hydrogen concentration buildup at the surface at a relatively low temperature. With increasing temperature, the hydrogen diffusion time decreases more rapidly than the hydrogen surface adsorption time. This leads to a relatively low-gradient diffusion field in the nanoblade during most time of the hydrogenation process, and no shell-core structure with a finite hydride layer is observed. However, for the dehydrogenation process, when hydrogen molecules are released at the surface, a hydride core is formed inside the nanoblade and the interface recedes gradually. The receding rate of the hydride core is determined by the hydrogen molecule release rate. In a two-dimensional simulation with decorated vanadium catalyst islands on the surface, isolated interior hydride islands are sometimes observed before the hydride core entirely fades away. The hydride core boundary is sharper at lower temperature when the surface reaction rate is high relative to the interior diffusion rate.     

Characteristic and kinetics of hydrogen absorption during the heat preservation stage and the cooling stage of TC21 alloy

TC21 alloy is hydrogenated under different initial hydrogen pressures at hydrogenation temperatures in the range of 450 °C–850 °C. Hydrogen absorption characteristic and kinetics during the heat preservation stage and cooling stage, hydrogen content and activation energy are investigated. The hydrogen absorption reaches equilibrium first at higher hydrogenation temperature and initial hydrogen pressure during the heat preservation stage. The hydrogen absorption reaches equilibrium first at lower hydrogenation temperature and initial hydrogen pressure during the cooling stage. Mechanisms of hydrogen absorption are analyzed during the heat preservation stage and the cooling stage. Phase compositions of the hydrogenated TC21 alloys are analyzed by XRD. Hydrogen content increases first and then decreases, then increases slightly, and finally decreases with the increase of hydrogenation temperature. Hydrogen content increases gradually with the increase of initial hydrogen pressure. The activation energy of hydrogen absorption in TC21 alloy is about 18.304 kJ/mol.     

Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

Effects of hydrogen on diffusion bonding of TiAl-based intermetallics using hydrogenated Ti6Al4V interlayer

The diffusion bonding of TiAl-based intermetallics using the hydrogenated Ti₆Al₄V interlayer was carried out. The results were investigated by SEM, EPMA, XRD and TG/DSC. According to the experimental observations, the reaction layer of diffusion bonding using the hydrogenated Ti₆Al₄V interlayer containing 0.5 wt% hydrogen formed at 850 °C for 15 min under a pressure of 15 MPa, and the shear strength of the joint was up to 290 MPa. Compared with the direct diffusion bonding of TiAl-based intermetallics, the bonding parameters obviously decreased. Particularly, the bonding temperature decreased by 350 °C, the holding time decreased by 45 min, and the bonding pressure decreased by 15 Mpa. The diffusion bonding of TiAl-based intermetallics at a low temperature was achieved. The dehydrogenated process and the reaction mechanism were also discussed.     

Hydrogen in polycrystalline silicon solar cell material: Its role and characteristics

The role and the characteristics of hydrogen in different polycrystalline silicon material are studied in this paper. The hydrogen diffusion through the grains and the grain boundaries, and its diffusion coefficient, are investigated. Moreover, the influence of the oxygen and the carbon contents on the diffusion mechanism of hydrogen in silicon is addressed. Also, it is found that the minority carrier diffusion length of polycrystalline silicon wafers considerably improves when annealed in hydrogen at high temperatures (>1000°C). Consequently, a significant amount of oxygen is out-diffused from the bulk of the wafers.     

Aluminum-silicon hydride clusters for prospective hydrogen storage

The structures and bonding properties of Al₄Si₂H_2n (n = 0–10) clusters are systematically studied by using the evolutionary algorithm combined with ab initio computations. While the H atoms are bond on the terminal sites of the clusters at low H contents, the Al atoms are combined together by double H-bridges and the Al/Si atoms are tetrahedrally coordinated at high H contents. The Al₄Si₂H_2n clusters break into a few fragments for n = 9,10. Analysis on the bonding natures shows that the Al–H bonds are strongly polarized and the Si atoms balance the charge states of the Al/H atoms according to the hydrogen concentrations. The hydrogen storage capacity in Al₄Si₂H₁₆ cluster reaches 8.9 wt%, and the estimated strength of the hydrogen bonding is about −0.55 eV per H₂, which falls in the ideal window for reversible hydrogen storage at ambient temperatures. The high hydrogen capacity and moderate bonding strength suggest that Al–Si hydrides can be promising candidates for hydrogen storage.     

Mg3Cd: A model alloy for studying the destabilization of magnesium hydride

We prepared an ordered Mg₃Cd alloy by high energy ball milling of elemental powders. The synthesized alloy exhibited good hydrogenation kinetics and reversibly absorbed about 2.8 wt. % of hydrogen. The temperature dependence of hydrogenation kinetics of the alloy measured in the range of temperatures covering the order-disorder phase transformations in the Mg₃Cd and MgCd phases did not exhibit any anomalies and could be fitted with a single Arrhenius line. The measured apparent activation energy (69 ± 2 kJ/mol) hinted that hydrogenation process was controlled by diffusion of Cd in metallic phase. The pressure-composition isotherms exhibited negligible pressure hysteresis and sloping pressure plateau. Based on microstructural evidence obtained with the aid of X-ray diffraction and scanning electron microscopy, we built a thermodynamic model predicting the plateau hydrogen pressure for partially hydrogenated alloy. The predictions of the model were in a good agreement with the experimental data. Finally, we discussed the origins and the growth mechanisms of Cd whiskers observed in the alloys after full hydrogenation cycle.     

Improved desorption temperature of magnesium hydride via multi-layering Mg/Fe thin film

Hydrogen sorption property of Mg in Pd-capped thin film nanoconfined with Fe is investigated. Two methods of depositing the thin films were utilised, i.e., resistive heating method and pulsed laser deposition (PLD) method. In the thinnest Mg film prepared by resistive heating, hydrogen content was observed to be the highest among all samples and the hydrogen desorption temperature is 230 °C. Using pulsed laser deposition method, Mg/Fe nanoconfined multilayers are easily prepared. The hydrogen desorption temperature of Mg film with 12 Mg/Fe layers prepared via PLD method was significantly reduced to 155 °C, and the hydrogen storage capacity is improved as compared to the Mg/Fe with only several layers of same overall thickness. This study showed that the desorption temperatures correlate with the film thickness, thinner films react with hydrogen at lower temperatures. In addition, multi-layering Mg with Fe improves the desorption temperatures and hydrogen capacity, due to the higher grain boundary density, which acts as diffusion pathways for Pd in hydrogenation and dehydrogenation process.     

Seasonal storage of electricity by hydrogen in benzene–water system

The use of hydrogen in benzene–water system which combines water electrolysis and hydrogenation in a polymer electrolyte cell was carried out as a means for seasonal storage of electricity. Gas diffusion electrodes were effective in improving coupled reactions of electrochemical benzene hydrogenation and water electrolysis. The reaction kinetics for the electrochemical hydrogenation process using gas diffusion electrodes was investigated by evaluating current efficiency and reaction rate. The results showed that the rate of hydrogen evolution was higher than the rate of benzene hydrogenation and the apparent activation energy of hydrogen evolution was lower than that of benzene hydrogenation. As the electrode potential increased, the hydrogen evolution rate increased. The benzene hydrogenation reaction rate reached a maximum at −0.8 V electrode potential, then decreased slightly. The current efficiency, however, reached its maximum at −0.7 V. Modifying electrodes by adding 0.2 wt% polyethylene glycol (PEG6000) reduced the mass transfer resistance of organic phase (cyclohexane/benzene) and improved the hydrogenation reaction rate.     

Hydrogenation of Pd/Mg films: A quantitative assessment of transport coefficients

Due to increasing energy demands, interest in hydrogen storage is substantial. Magnesium is one of the most attractive systems, yet, development for practical application remains challenging. By combination of X-ray diffraction, electron microscopy and in-situ measurements of resistivity we determine the diffusion coefficient of hydrogen in MgH₂ at technically relevant pressure (20 bar). Pd coated thin films of well-defined thickness enable a quantitative evaluation of the hydrogenation rate. From this, we detect linear to parabolic kinetic transition and obtain the diffusion coefficient of hydrogen in MgH₂. Measurements at different temperatures (RT-300 °C) demonstrate an Arrhenius behaviour with an activation energy E_a = 28.1 kJ mol⁻¹. This low value and the transformation into a nanocrystalline microstructure upon hydrogenation indicate grain boundary diffusion as the essential mechanism. In completion, the interface Pd/Mg is studied. Mg₅Pd₂ and Mg₆Pd form at elevated temperatures required for dehydrogenation. These phases affect, but do not prevent, further hydrogen loading.     

Hydrogen diffusion and vacancy clusterization in iron

Molecular statics and molecular dynamics simulations were performed to study hydrogen diffusion and vacancy clustering in alpha iron. In particular, it was found that hydrogen atom binds very strongly with vacancies, rather than other hydrogen atoms. The monovacancies were inclined to form the VH4, VH3, VH2 and VH1 complexes, rather than VH6 in the range of our simulated temperatures. The rate of hydrogen diffusion was apparently reduced in the presence of vacancies, while the vacancy trap effect was gradually weakened with increasing temperature. The presence of vacancies changes the diffusion mechanism of H atoms. Moreover, we found that vacancy clusters tended to be formed at the moderate range of temperatures, and fewer clusters were observed at either low or high temperatures. The number of vacancy clusters reduced, while hydrogen-vacancy clusters were gradually created with the increase of hydrogen concentration.     

Hydrogen storage properties of ultrahigh pressure Mg12NiY alloys with a superfine LPSO structure

Ultrahigh pressure (UP) plays a crucial role in modifying structures and properties of functional materials. The effects of UP treatment (4 GPa) on phase transition and hydrogen storage properties of Mg₁₂NiY alloys has been investigated at the temperature range of 800–1300 °C. The results show that the dimension of 18R-type long period stacking ordered (LPSO) structure in the Mg₁₂NiY sample after UP treatment at 1300 °C is two orders of magnitude smaller than that in the as-cast sample. The hydrogen storage capacity, kinetics and cycle properties of Mg₁₂NiY alloys are concurrently improved after UP treatment. The two-step reaction process is confirmed during hydrogenation process by combining cycle testing and in-situ transmission electron microscopy (TEM) observations. The reasons for high hydrogen storage properties are mainly related to three aspects: the increased volume fraction of high angle interfaces between LPSO phase and matrix, the reduction of hydrogen diffusion distance, and the low energy barrier of hydrogen diffusion in the interior of superfine LPSO structures.     

Effect of hydrogen on diffusion bonding of commercially pure titanium and hydrogenated Ti6Al4V alloys

The diffusion bonding of commercially pure titanium and hydrogenated Ti6Al4V alloys was carried out, and the effect of hydrogen was investigated by SEM, XRD, TEM and TG/DSC. The β_H phase increased with the hydrogen content increasing in hydrogenated alloys, and the δ titanium hydride and α′ martensite were found in high hydrogen content. The TG curves of hydrogenated alloys descended between 600 °C and 950 °C, and the DSC curves represented a large endothermic peak correspondingly. Moreover, some voids were observed at the diffusion bonding interface. The amount of voids decreased and diffusion bonding quality improved gradually with the increase of hydrogen content, which was attributed to the increase of soft β_H phase and release speed of hydrogen as well as the occurrence of more dehydrogenation reaction.     

Hydrogen absorption kinetics of V4Cr4Ti alloy prepared by aluminothermy

The hydrogen absorption kinetics of V4Cr4Ti alloy, synthesized by aluminothermy process has been investigated in the temperature range of 373–773 K. The obtained hydrogen absorption kinetic curves were linearly fitted using a series of mechanism function to reveal the kinetics parameter and reaction mechanism. Nucleation and growth, one dimensional diffusion and three-dimensional diffusion processes are the intrinsic rate limiting steps of hydrogen absorption at 373 K. It was found that nucleation and growth processes disappear between 413 K–473 K. However at higher temperatures (>473 K), nucleation and growth as well as one dimensional diffusion process disappear. In the temperature ranges investigated (473 K–773 K), three-dimensional diffusion process was the intrinsic rate limiting step. The apparent activation energy was calculated using Arrhenius equation and found to be 6.1 kJ/mol. This value appears to be relatively higher which can be attributed to the presence of aluminium, which has blocked the absorption sites and increased the activation energy.     

天津石化炼油节能优化措施及潜力分析

天津石化1000×104t/a炼油工程由3号常减压、2号加氢裂化、重整抽提、2号延迟焦化、2号柴油加氢、蜡油加氢、航煤加氢、2号硫磺回收等装置及储运系统和公用工程系统组成。炼油新区在设计中进行了能量综合优化,采取了一系列节能措施。在流程设置上,加氢装置采用热高压分离器流程和循环氢脱硫流程,一些装置采用热直供料,2号柴油加氢装置与航煤加氢装置实现了热联合。在低温热利用方面,设立高温热媒水系统,回收新区加氢装置低温热,用来加热热电部除盐水。设立低温热媒水系统,回收2号延迟焦化装置的低温热,冬季为新区装置采暖伴热提供热源,夏季为溴化锂机组供热。炼油新区各装置实施了节能优化,主要项目有:重整抽提装置蒸汽凝结水热能利用,2号延迟焦化和重整抽提装置部分蒸汽伴热改为水伴热,2号延迟焦化装置热出料流程优化。针对炼油新区在低压蒸汽平衡、中压蒸汽管网运行方面存在的问题,提出优化措施。     

Complex hydrides for energy storage

In the past decades, complex hydrides and complex hydrides-based materials have been thoroughly investigated as materials for energy storage, owing to their very high gravimetric and volumetric hydrogen capacities and interesting cation and hydrogen diffusion properties. Concerning hydrogen storage, the main limitations of this class of materials are the high working temperatures and pressures, the low hydrogen absorption and desorption rates and the poor cyclability. In the past years, research in this field has been focused on understanding the hydrogen release and uptake mechanism of the pristine and catalyzed materials and on the characterization of the thermodynamic aspects, in order to rationally choose the composition and the stoichiometry of the systems in terms of hydrogen active phases and catalysts/destabilizing agents. Moreover, new materials have been discovered and characterized in an attempt to find systems with properties suitable for practical on-board and stationary applications. A significant part of this rich and productive activity has been performed by the research groups led by the Experts of the International Energy Agreement Task 32, often in collaborative research projects. The most recent findings of these joint activities and other noteworthy recent results in the field are reported in this paper.     

A first-principles study on the hydrogenation of acetone on HxMoO3 surface

Hydrogenation of acetone on the (010) surface of hydrogen molybdenum bronzes was investigated by density functional theory (DFT) calculations with periodic slab models. The formation of H-bond between the carbonyl oxygen of acetone and the terminal OH group of the surface leads to a stable adsorption of acetone. The effect of hydrogen concentration in the bronzes on the hydrogenation of acetone was systematically investigated, indicating the hydrogenation reaction is a one-step concerted and exothermic process regardless of the hydrogen contents in the bronzes surface. The 8H surface with increased H-content shows a significantly exothermic reaction process and exhibits the smallest kinetic barrier compared with 4H or 6H surfaces. Additionally, the selectivity for hydrogenation acetone could increase owing the absence of CC bond activation. The findings in this study can help with designing of high-efficient and low-cost metal oxide catalysts for hydrogenation of unsaturated substances.