Effects of ionizational non-equilibrium on the electron density across the non-equilibrium MHD channel

This paper gives the effects of ionizational non-equilibrium of charge carriers on the electron density across the channel of a non-equilibrium MHD generator. The steady and unsteady diffusion equations are solved numerically using an implicit finite difference scheme. It is found that as a result of ionizational non-equilibrium effects, the electron density reduces in the central region and increases near the wall region compared to its value obtained from Saha's equation. The ionizational non-equilibrium effects increase with increasing velocities.     

Effects of Zr addition on electrochemical characteristics of MgTiMnNi hydrogen storage alloys

The electrochemical hydrogen storage properties of 25 h milled Mg_0.80Ti_0.175Mn_0.025Zr_xNi_1-x (x = 0, 0.025, 0.05, 0.1) quinary alloys were investigated. The substitution of Zr for Mg or Ni leads to an increase in structural disorder and amorphization. Thus, the maximum discharge capacity and the cycling stability of MgNi-based alloys can be enhanced. The x-ray diffraction patterns indicate that all additive elements are entirely dissolved in the synthesized alloys, and amorphous structure was successfully obtained by 25 h milling. Among the milled alloys, the Mg_0.80Ti_0.175Mn_0.025Zr_0.10Ni_0.90 alloy exhibited the best discharge capacity of 604 mA h g⁻¹ at the initial charge/discharge cycle. The obtained results demonstrate that using multi-component compositions is beneficial for enhancing the structural and cyclic stability of MgNi-based alloys. Therefore, substituting additive elements for Mg or Ni may offer impressive performance for efficient hydrogen storage applications.     

Finite element modeling of hydrogen atom diffusion and distribution at corrosion defect on aged pipelines transporting hydrogen

Repurposing existing natural gas pipelines for hydrogen transport has attracted wide interests. However, the corrosion defect present on these aged pipelines can affect hydrogen (H) atom accumulation, potentially causing hydrogen embrittlement. In this work, a finite element-based model was developed by coupling solid mechanics and H atom diffusion to investigate the distribution of H atoms at a corrosion defect on a steel pipe segment under applied longitudinal tensile strains. The applied strain causes local stress (both Mises stress and hydrostatic stress) and strain concentrations at the corrosion defect, affecting the H atom diffusion and distribution. In the absence of the tensile strain, the H atoms, once entering the interior of pipe, diffuse uniformly into the pipe body along the radial direction driven by a concentration gradient. When a strain is applied on the pipe, the H atom diffusion is driven by hydrostatic stress. The maximum H atom concentration exceeds the initial concentration of H atoms entering the steel pipe, indicating the H atom accumulation at the corrosion defect. The applied tensile strain also affects the location where the H atoms accumulate. For both internal and external corrosion defects, more H atoms will be concentrated at the defect center when the defect length reduces and the depth increases.     

Effect of rapid solidification on hydrogen solubility in Mg-rich Mg-Ni alloys

The hydrogenation properties of Mg_100−xNi_x alloys (x = 0.5, 1, 2, 5) produced by melt spinning and subsequent high-energy ball milling were studied. The alloys were crystalline and, in addition to Mg matrix, contained finely dispersed particles of Mg₂Ni and metastable Mg₆Ni intermetallic phases. The alloys exhibited excellent hydrogenation kinetics at 300 °C and reversibly absorbed about 6.5 mass fraction (%) of hydrogen. At the same temperature, the as prepared Mg_99.5Ni_0.5 and Mg₉₅Ni₅ powders dissolved about 0.6 mass fraction (%) of hydrogen at the pressures lower than the hydrogen pressure corresponding to the bulk Mg-MgH₂ two-phase equilibrium, exhibiting an extended apparent solubility of hydrogen in Mg-based matrix. The hydrogen solubility returned to its equilibrium value after prolonged hydrogenation testing at 300 °C. We discuss this unusually high solubility of hydrogen in Mg-based matrix in terms of ultrafine dispersion of nanometric MgH₂ precipitates of different size and morphology formed on vacancy clusters and dislocation loops quenched-in during rapid solidification.     

Hydrogen diffusion and the effect of hydrogen on structural transformations in binary TiNi based alloys

The paper analyzes the effect of electrolytic hydrogenation on martensite transformation temperatures in binary TiNi alloys. The analysis shows that this effect can be strong or weak depending on the phase state of TiNi. Research data are presented on the diffusivity of hydrogen in binary TiNi alloys in martensite and austenite states. The diffusion coefficient of hydrogen is estimated from its distribution measured by glow discharge spectroscopy in TiNi after hydrogenation. The experimental results about the formation of TiNiH hydride in the martensitic and austenitic state in binary TiNi based alloys are also presented.     

Study of cyclic performance of V-Ti-Cr alloys employed for hydrogen compressor

The development of a suitable hydrogen compressor plays one of the key roles to realize the fuel cell vehicle as well as for many other stationary and mobile applications of hydrogen. V-Ti-Cr BCC alloys are considered as promising candidates for effective hydrogen storage. The cyclic durability of hydrogen absorption and desorption is very important for these alloys to be realized as practical options. In connection to this, two alloys of V-Ti-Cr, (1) V₄₀Ti_21.5Cr_38.5 and (2) V₂₀Ti₃₂Cr₄₈, were selected and their cyclic hydrogen absorption-desorption performance was evaluated up to 100 cycles for temperature and pressure ranges of 20–300 °C and 5–20 MPa, respectively. It has been found that the cyclic hydrogen storage capacity continuously decreased for one composition while it was stable after 10 cycles for another composition. This performance difference of the alloys was studied in terms of their structural and microscopic properties and the results are presented in this paper.     

Interactions of Y and Cu on Mg2Ni type hydrogen storage alloys: A study based on experiments and density functional theory calculation

Evolution of microstructure and hydrogen storage performances were studied in a Y substituted Mg₂₄Ni₁₀Cu₂ hydrogen storage alloy. Interactions of Y and Cu on the phase structure and hydrogen storage properties were explore. Substitution by Y refined the microstructure and yield existence of YMgNi₄. Furthermore, Y addition promoted the replacement of Cu for Ni in the Mg₂Ni.The study of the alloy's dehydrogenation performance and mechanism showed that the addition of Y did not alter the mechanism of random nucleation and subsequent growth, but reduced the activation energy of the dehydrogenation of the alloy from 77.4 kJ/mol to 67.6 kJ/mol. The thermodynamic energy of the dehydrogenation was also improved, and the enthalpy change (ΔH) and entropy change (ΔS) of the Mg₂NiH₄ phase decreased from 67.1 J/K/mol H₂ and 123.1 J/K/mol H₂ to 61.1 J/K/mol H₂ and 115.4 J/K/mol H₂, respectively. Furthermore, the density functional theory calculation showed that the addition of Y promoted the substitution of Cu for Ni, further reduced the stability of the main hydride Mg₂NiH₄, facilitated the release of hydrogen, and reduced the ΔH and ΔS of the hydride dehydrogenation.     

Investigation on microstructure and hydrogen generation performance of Al-rich alloys

In order to study the effect of cooling rate on the microstructures and hydrogen generation performance of Al alloys, two ingots (20 g and 45 g) with a composition of 94 Al, 3.8 Ga, 1.5 In and 0.7 Sn (in mass%) were prepared by arc melting under high purity argon atmosphere, and a rod (10 g) with the same composition was cast in a vacuum chamber. The microstructures and phase compositions of the three samples were investigated by means of X-ray diffraction and scanning electron microscope with energy dispersed X-ray. The melting point of the grain boundary phase was measured using differential scanning calorimeter. Based on the structural analysis, samples with different but uniform grain sizes were cut from these alloys for H₂ generation. The reactions of Al alloys with water were measured at different temperatures. The measured H₂ generation rates were found to increase rapidly once the grain size was reduced below 50 μm. An isothermal kinetic model was employed to analyze the measured kinetic data so as to obtain kinetic parameters of reactions. The reaction order (n) for these alloys was found to be about 0.7. The activation energy (E_a) decreases with grain size d, i.e., 30% reduction of E_a as d was reduced from 258 to 23 μm. A mechanism of Al alloy corrosion in water was proposed.     

Exploration of Ti substitution in AB2-type YZrFe based hydrogen storage alloys

To improve the hydrogen storage properties of YZrFe alloys, the alloying with Ti was carried out to obtain Y_0.7Zr_(0.3-x)Ti_xFe₂ (x = 0.03, 0.09, 0.1, 0.2) alloys by different processes. It was expected that Ti would substitute Zr and decrease the lattice constant of YFe₂-based C15 Laves phase. All YZrTiFe quaternary alloys consist of the main Y(Zr)Fe₂ phase and the minor YFe₃ phase. Despite the large solubility of Ti in Zr or Zr in Y, the Ti incorporation into YZrFe alloys results in the inhomogeneity of Y and the segregation of Ti, and thus decreases the hydrogen storage capacity. Only the alloy Y_0.7Zr_0.27Ti_0.03Fe₂ containing very few Ti shows the substitution of Ti to Zr and the resultant improvement in the dehydriding equilibrium pressure.     

Effects of hydrogen injection strategy on the hydrogen mixture distribution and combustion of a gasoline/hydrogen SI engine under lean burn condition

Hydrogen is a carbon free energy carrier with high diffusivity and reactivity, it has been proved to be a kind of suitable blending fuel of spark ignition (SI) engine to achieve better efficiency and emissions. Hydrogen injection strategy affects the engine performance obviously. To optimize the combustion and emissions, a comparative study on the effects of the hydrogen injection strategy on the hydrogen mixture distribution, combustion and emission was investigated at a SI engine with gasoline intake port injection and four hydrogen injection strategies, hydrogen direct injection (HDI) with stratified hydrogen mixture distribution (SHMD), hydrogen intake port injection with premixed hydrogen mixture distribution (PHMD), split hydrogen direct injection (SHDI) with partially premixed hydrogen mixture distribution (PPHMD) and no hydrogen addition. Results showed that different hydrogen injection strategy formed different kinds of hydrogen mixture distribution (HMD). The ignition and combustion rate played an important role on engine efficiency. Since the SHDI could use two hydrogen injection to organize the HMD, the ignition and combustion rate with the PPHMD was the fastest. With the PPHMD, the brake thermal efficiency of the engine was the highest and the emissions were slight more than that with the PHMD. PHMD achieve the optimum emission performance by its homogeneous hydrogen. The engine combustion and emission performance can be optimized by adjusting the hydrogen injection strategy.     

Compositional effects on the hydrogen cycling stability of multicomponent Ti-Mn based alloys

Intermetallic alloys such as AB, AB₂, and AB₅ type have been studied due to their capability to reversibly store hydrogen. These alloys exhibit varying hydrogen storage properties depending on the crystal structure and composition. Compositional modification is commonly known as an effective method to modify the alloys thermodynamic and kinetics for various applications such as metal hydride batteries, metal hydrides hydrogen storage and compression. However, the effects of the compositional modification on the cyclic stability of these alloys are not usually well studied.Here, the hydrogen cycling stabilities of Ti-Mn based alloys with C14 type structure are studied. Hyper-stoichiometry, stoichiometry and hypo-stoichiometry alloys were prepared accordingly: Ti_30.6V_16.4Mn_48.7 (Zr_0.7Cr_0.8Fe_2.8) (B/A = 2.19), Ti_32.8V_15.1Mn_47.1 (Zr_0.9Cr_1.2Fe_2.9) (B/A = 1.97) and Ti_34.5V_15.4Mn_44.7 (Zr_0.9Cr_1.3Fe_3.2) (B/A = 1.87). Whilst the hyper-stoichiometry alloy showed almost a stable (about 9% capacity reduction) hydrogen capacity after 1000 cycles of hydrogenation and dehydrogenation, the stoichiometry and hypo-stoichiometry alloys failed to hydrogenate after about 950 and 500 cycles respectively. A limited reduction in the calculated crystalline size of the alloys was observed before and after the hydrogen cycling, denoting that pulverisation plays a less significant role on the observed hydrogen capacity loss. In addition, a reduction in the B/A ratio from 2.19 to 1.82 (hyper to hypo-stoichiometry) encouraged the formation of more stable hydride and a higher level of heterogeneous lattice strain. Whilst a small loss of hydrogen capacity (9%) in the hyper-stoichiometry alloy was attributed to the trapped hydrogen, the complete loss of hydrogen capacity in the stoichiometry and hypo-stoichiometry alloys seemed to originate from the formation of stable hydride and the lattice distortion.     

First principles investigations of hydrogen interaction with vacancy-oxygen complexes in vanadium alloys

We reported the hydrogen (H) interacting with the vacancy-oxygen (V–O) pairs complexes and its effect on H accumulation behavior in bcc vanadium in comparison with alone vacancies using first-principles calculations. The H interaction with interstitial O atom is weak repulsion and all interstitial sites near O atom are unstable for H. The alone mono-, di- and trivacancy defects can trap 6, 8–11 and 13 H atoms, while one V–O pair, two V–O pairs, and three V–O pairs complexes trap approximately same about 4–6 H atoms, respectively. The formation of O-vacancy pairs weakens the binding energy of the V_mH_n clusters, the number of trapped H atoms is drastically reduced to form nearly sized H clusters. We presented the stable configurations of various V_mH_n and V_mO_mH_n complexes (with m = 1–3 and n = 1–15), the most stable configurations are the V₁H₂/V₂H₂/V₃H₂ clusters in alone vacancies, while there are the V₁O₁H/V₂O₂H/V₃O₃H clusters in the V–O complexes. In presence of alloying Ti near (V–O) pair, one Ti(V–O) pair complex still trap four H atoms, but the H binding strength has some changes. These results demonstrated that the existence of the multi-V-O complexes significantly reduce the growth and swelling of the H clusters.     

Effect of the precipitates on the hydrogen desorption kinetics from zirconium-niobium alloys

This study aims at identifying and quantifying the rate-limiting steps of the hydrogen desorption process from unoxidized M5_Framatome alloy. Gaseous deuterium charging, Thermal Desorption Spectrometry (TDS), Differential Scanning Calorimetry (DSC) and finite elements simulations of TDS results reveal that hydrogen desorption kinetics from the metal is limited by the surface molecular recombination (similarly to Zircaloy-4) and that part of the hydrogen originated from the elaboration process of M5_Framatome and Zr-2.5%Nb is trapped by the Nb-rich precipitates. In the studied conditions and regarding M5_Framatome, the desorption flux corresponding to this initially trapped hydrogen reaches its maximum after the total dissolution of the precipitates, which releases hydrogen into solid solution. The kinetic constant corresponding to surface recombination identified on M5_Framatome was identical to the one previously determined on Zircaloy-4.     

Design of hydrogen storage alloys in view of chemical bond between atoms

The chemical interactions operating in hydrogen storage alloys are simulated by the DV-Xα molecular orbital method, using tetrahedral or octahedral model clusters. It is found that hydrogen interacts more strongly with hydride non-forming elements, B (e.g., Ni, Fe) than hydride forming elements, A (e.g., La, Ti, Mg), in agreement with our previous calculations of hydrogen storage alloys (e.g., LaNi₅, TiFe, Mg₂Ni). However, it is noted that such a B–H interaction is not dominant unless A elements exist in the neighborhood, so that both A and B are indeed essential elements for hydrogen storage alloys. Also, it is shown that the A/B compositional ratio of hydrogen storage alloys can be understood in terms of a simple parameter, 2 Bo(A–B)/[Bo(A–A)+Bo(B–B)], where the Bo(A–B), Bo(A–A) and the Bo(B–B) are the bond strengths between atoms given in the parentheses.     

Effect of rare earth (RE) elements on V-based hydrogen storage alloys

In this work, the effect of RE additives on the properties of _{V55Ti22.5Cr16.1Fe6.4}${\mathrm{V}}_{55}{\mathrm{Ti}}_{22.5}{\mathrm{Cr}}_{16.1}{\mathrm{Fe}}_{6.4}$ alloy (RE=La$\mathrm{RE}=\mathrm{La}$, Pr, Ce and Nd, separately) was discussed. It was demonstrated that RE additives improve the activation property rather than kinetics during cycling, absorption capacity and the plateau pressure. Two phases, including BCC main phase and Ce second one, were found in Ce-containing alloy. It is inferred that RE element offers a route for hydrogen to enter the alloys more easily, which leads to the improvement of activation property of the alloys.     

Hydrolysis of Mg-based alloys and their hydrides for efficient hydrogen generation

The hydrolysis of Mg-based alloys and their hydrides with high abundance on the earth and low cost could produce hydrogen with high theoretical capacity and the formation of by-products that have no pollution to the environment. Hence, it has been regarded as one of the most promising way for hydrogen generation. Particularly, a gravimetric capacity of 6.4 wt% and 3.4 wt% H₂ could be produced from the hydrolysis of pure Mg and MgH₂, respectively, even when stoichiometric water is included for calculation. The formation of passive magnesium hydroxides with dense structure, however, could immediately interrupt the hydrolysis reaction of Mg/MgH₂, which leads to ultralow yield and sluggish hydrogen generation rate. Recent studies have demonstrated that the hydrolysis reaction of Mg/MgH₂ could be effectively enhanced in terms of both yield and kinetics by the formation of Mg-based alloys and their hydrides. This review aims to summarize the recent progress in the hydrolysis of Mg-based alloys and their hydrides and the involved hydrolysis mechanisms.     

Compatibility of materials with hydrogen. Particular case: Hydrogen embrittlement of titanium alloys

A review of the effect of hydrogen on materials is addressed in this paper. General aspects of the interaction of hydrogen and materials, hydrogen embrittlement, low temperature effects, material suitability for hydrogen service and materials testing are the main subjects considered in the first part of the paper. As a particular case of the effect of hydrogen in materials, the hydride formation of titanium alloys is considered. Hydrogen absorption and the possible associated problems must be taken into account when considering titanium as a candidate material for high responsibility applications. The sensitivity of three different titanium alloys to the Hydrogen Assisted Stress Cracking phenomena has been studied by means of the Slow Strain Rate Technique (SSRT). The testing media have been sea water and hydrogen has been produced on the specimen surface during the test by cathodic polarization. Tested specimens have been characterized by metallography and scanning electron microscopy. Results obtained show that the microstructure of the materials, particularly the β-phase content, plays an important role on the sensitivity of the studied alloys to the Hydrogen Assisted Stress Cracking Phenomena.     

Role of atomic hydrogen supply on the onset of CO2 methanation over La–Ni based hydrogen storage alloys studied by in-situ approach

Mechanochemical CO₂ methanation reactions using LaNi₅ and LaNi_4.6Al_0.4 hydrogen storage alloy powders were investigated by the in-situ monitoring of the gas pressure change during ball-milling. Methane generation begins when the H₂ partial pressure drops due to the H-uptake by the powder. Phase transition occurred in the sample after milling for 15 min and 224 min, with separate metallic Ni, La-oxide and La-hydroxide phases observed. Methane generation continued even after this phase separation. Our results imply that the formation of La-hydroxide at the surface and sub-surface contributed to methane generation during ball-milling. A comparison of LaNi₅ and LaNi_4.6Al_0.4 suggests the amount of hydrogen stored in the hydrogen storage powder dominates the timing of the onset of the methane generation.     

Radiation and ionizational nonequilibrium effects on the electron density in a nonequilibrium MHD generator duct

This paper presents an investigation of the radiation and ionizational nonequilibrium effects of charge carriers on the electron density across a nonequilibrium MHD channel. Using the generated and empirical velocity and temperature profiles, the steady and unsteady diffusion equations are solved numerically using an implicit finite difference technique. It is found that, as a result of the radiation effects during ionizational nonequilibrium processes, the electron density is more, in comparison with its equilibrium value, in the boundary layer region, whereas it is less than its equilibrium value in the core.