An investigation of hydrogen storage kinetics of melt-spun nanocrystalline and amorphous MgeNi-type alloys

The nanocrystalline and amorphous Mg2Ni-type Mg2–xLaxNi (x=0,0.2) hydrogen storage alloys were synthesized by melt-spinning technique.The as-spun alloy ribbons were obtained.The microstructures of the as-spun ribbons were characterized by X-ray diffraction (XRD),high resolution transmission electronic microscopy (HRTEM) and electron diffraction (ED).The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus,and their electrochemical kinetics were tested by an automatic galvanostatic system.The electrochemical impedance spectrums (EIS) were plotted by an electrochemical workstation (PARSTAT 2273).The hydrogen diffusion coefficients in the alloys were calculated by virtue of potential-step method.The obtained results showed that no amorphous phase was detected in the as-spun La-free alloy,but the as-spun alloys substituted by La held a major amorphous phase,con-firming that the substitution of La for Mg markedly intensified the glass forming ability of the Mg2Ni-type alloy.The substitution of La for Mg notably improved the electrochemical hydrogen storage kinetics of the Mg2Ni-type alloy.Furthermore,the hydrogen storage kinetics of the experimental alloys was evidently ameliorated with the spinning rate growing.     

Electrochemical hydrogen storage performances of the nanocrystalline and amorphous（Mg24 Ni10 Cu2）100-x Ndx（x=0–20） alloys applied to Ni-MH battery

Melt spinning technology was used to prepare the Mg2 Ni-type（Mg24 Ni10 Cu2）100–x Ndx（x=0,5,10,15,20） alloys in order to obtain a nanocrystalline and amorphous structure.The effects of Nd content and spinning rate on the structures and electrochemical hydrogen storage performances of the alloys were investigated.The structure characterizations of X-ray diffraction（XRD）,transmission electron microscopy（TEM） and scanning electron microscopy（SEM） linked with energy dispersive spectroscopy（EDS） revealed that the as-spun Nd-free alloy displayed an entire nanocrystalline structure,whereas the as-spun Nd-added alloys held a nanocrystalline and amorphous structure and the degree of amorphization visibly increased with the rising of Nd content and spinning rate,suggesting that the addition of Nd facilitated the glass forming of the Mg2 Ni-type alloy.The electrochemical measurements indicated that the addition of Nd and melt spinning improved the electrochemical hydrogen storage performances of the alloys significantly.The discharge capacities of the as-cast and spun alloys exhibited maximum values when Nd content was x=10,which were 86.4,200.5,266.3,402.5 and 452.8 mAh/g corresponding to the spinning rate of 0（As-cast was defined as the spinning rate of 0 m/s）,10,20,30 and 40 m/s,respectively.The cycle stability（S20,the capacity maintain rate at 20thcycle） of the as-cast alloy always rose with the increasing of Nd content,and those of the as-spun alloys exhibited the maximum values for Nd content x=10,which were 77.9%,83.4% 89.2% and 89.7%,corresponding to the spinning rate of 10,20,30 and 40 m/s,respectively.     

Gaseous hydrogen storage properties of Mg-Y-Ni-Cu alloys prepared by melt spinning

For purpose of promoting the hydrogen absorption and desorption thermodynamics and kinetics properties of Mg-Ni-based alloys, partially substituting Y and Cu for Mg and Ni respectively and melt spinning technique were applied for getting Mg_(25-x)Y_xNi_9 Cu(χ = 0-7) alloys. Their microstructures and phases were characterized with the help of X-ray diffraction and transmission electron microscopy. Their hydrogen absorbing and desorbing properties were tested by a Sievert apparatus, DSC, and TGA, which were connected with a H_2 detector. In order to estimate the dehydrogenation activation energy of alloy hydride, both Arrhenius and Kissinger methods were applied for calculation. It is found that their hydriding kinetics notably declines, however, their hydrogen desorption kinetics conspicuously improves, with spinning rate and Y content increasing. Their hydrogen desorption activation energy markedly decreases under the same constraint, and it is found that melt spinning and Y substituting Mg improve the real driving force for dehydrogenation. As for the tendency of hydrogen absorption capacity,it presents an elevation firstly and soon after a decline with the rising of spinning rate, however, it always lowers with Y content growing. With Y content and spinning rate increasing, their thermodynamic parameters(△H and △S absolute values) visibly decrease, and the starting hydrogen desorption temperatures of alloy hydrides obviously lower.     

Hydriding/dehydriding behaviors of La2Mg17-10 wt.%Ni composite prepared by mechanical milling

The structure and hydriding/dehydriding behaviors of La2Mg17-10 wt.%Ni composite prepared by mechanical milling were investigated. Compared with the un-milled sample, the as-milled alloys were ready to be activated and the kinetics of hydrogen absorption was relatively fast even at environmental temperature. The composite milled for 10 h absorbed 3.16 wt.% hydrogen within 100 s at 290 K. The kinetic mechanisms of hydriding/dehydriding reactions were analyzed by using a new model. The results showed that hydrogenation processes for all composites were controlled by hydrogen diffusion and the minimum activation energy was 15.3 kJ/mol H2 for the composite milled for 10 h. Mechanical milling changed the dehydriding reaction rate-controlling step from surface penetration to diffusion and reduced the activation energy from 204.6 to 87.4 kJ/mol H2. The optimum milled duration was 5 h for desorption in our trials.     

Hydrogen Storage Performances of REMg11Ni (RE = Sm,Y) Alloys Prepared by Mechanical Milling

This study adopted mechanical milling to prepare Mg-based REMg₁₁Ni (RE = Sm, Y) hydrogen storage alloys. The alloy structures were examined by X-ray diffraction and transmission electron microscopy. The isothermal hydrogenation thermodynamics and kinetics were determined by an automatic Sievert apparatus. The non-isothermal dehydrogenation performance of the alloys was tested by differential scanning calorimetry and thermogravimetry at different heating rates. The results showed a nanocrystalline and amorphous tendency for the alloys. The YMg₁₁Ni alloy exhibited a larger hydrogen absorption capacity, faster hydriding rate, and lower temperature of onset hydrogen desorption than the SmMg₁₁Ni alloy. The hydrogen desorption temperatures of the REMg₁₁Ni (RE = Sm, Y) alloys were 557.6 K and 549.8 K (284.6 °C and 276.8 °C), respectively. The hydrogen desorption property of the RE = Y alloy was found superior to the RE = Sm alloy based on the time required to absorb 3 wt pct H₂, i.e., the time needed by the RE = Y alloy was reduced to 1106, 456, 363, and 180 s, respectively, corresponding to the hydrogen desorption temperatures of 593 K, 613 K, 633 K, and 653 K (320 °C, 340 °C, 360 °C, and 380 °C), compared to 1488, 574, 390, and 192 s for the RE = Sm alloy under identical conditions. The dehydrogenation activation energies were 100.31 and 98.01 kJ/mol for the REMg₁₁Ni (RE = Sm, Y) alloys, respectively, which agreed with those of the RE = Y alloy showing a superior hydrogen desorption property.

     

Analysis of Fluxless,Reactive Brazing of Al Alloys Using Differential Scanning Calorimetry

During this investigation, a technique was developed, using differential scanning calorimetry (DSC), to quantitatively analyze the influence of a Ni-based electrolytic braze promotor surface deposit on the furnace brazing of aluminum alloys. The purpose of this braze promoter was to induce a large exothermic surface reaction capable of disrupting a tenacious oxide present on the aluminum braze sheet faying surface. A cyclic DSC methodology was developed which was capable of a quantitative determination of the exothermic reaction (ΔH _exo) induced by the Ni plating. Samples with a small quantity of Ni plating exhibited significant “pre-reaction” between Ni and Al in the solid state which resulted in very low ΔH _exo values. Samples with higher quantities of Ni plating exhibited large ΔH _exo values up to 85 kJ/mole.     

Phase precipitation and magnetic properties of melt-spun ternary Gd2Fe14B alloy and advantages of gadolinium substitution in Y2Fe14B alloy

To take the advantage of gadolinium(Gd) in developing and manufacturing RE-permanent magnets,the magnetic properties and phase precipitation behavior of Gd₂Fe₁₄B alloys prepared by melt spinning were investigated in this work.The results show that optimally direct quenched nanocrystalline Gd₂Fe₁₄B alloy exhibits the magnetic properties with remanence J_r of 0.51 T,coercivity H_c of 187 kA/m,and maximum energy product(BH)_max     

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

The Mg₁₇Ni_1.5Ce_0.5 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 (~25 ??m) 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 Mg₁₇Ni_1.5Ce_0.5 with 5 wt pct graphite were 5.34 and 5.30 wt pct H₂ at 573 K (300 °C), its onset temperature of dehydriding reaction was 511 K (238 °C), and its activation energies of H/D reaction were 40.9 and 54.5 kJ/mol H₂, respectively. The kinetic mechanisms of the H/D reaction are also discussed.     

快淬纳米晶/非晶Mg2-xLaxNi (x=0～0.6)合金的贮氢动力学

为了改善Mg2Ni型合金气态及电化学贮氢动力学性能,用La部分替代合金中的Mg,用快淬技术制备了Mg2-xLaxNi(x=0,0.2,0.4,0.6)合金,用XRD,SEM,HRTEM分析了铸态及快淬态合金的微观结构;用自动控制的Sieverts设备测试了合金的气态贮氢动力学性能,用程控电池测试仪测试了合金的电化学贮氢动力学.结果发现,快淬无La合金具有典型的纳米晶结构,而快淬含La合金显示了以非晶相为主的结构,表明La替代Mg提高Mg2Ni型合金的非晶形成能力.La替代Mg明显地改变Mg2Ni型合金的相组成.当La替代量x=0.4时,合金的主相改变为(La,Mg) Ni3+ LaMg3.合金的气态及电化学吸放氢动力学对La含量及快淬工艺敏感,La替代使合金的吸氢动力学降低,但适量的La替代可以明显改善合金的放氢动力学及高倍率放电能力.适当的快淬处理可以提高合金的气态及电化学贮氢动力学,但获得最佳贮氢动力学的快淬工艺与合金的成分密切相关.     

Hydriding and dehydriding characteristics of nanocrystalline and amorphous Mg20-xLaxNi10（X=0-6） alloys prepared by melt-spinning

In order to improve the hydrogenation and dehydrogenation performances of the Mg2Ni-type alloys, Mg was partially substituted by La in the alloy, and melt spinning technology was used for the preparation of the Mg20-xLaxNi10 (x=0, 2, 4, 6) hydrogen storage alloys. The structures of the alloys were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). It was found that no amorphous phase formed in the as-spun La-free alloy, but the as-spun alloys containing La held a major amorphous phase. When La content x≤2, the major phase in the as-cast alloys was Mg2Ni phase, but with further increase of La content, the major phase of the as-cast alloys changed into LaNi5+LaMg3 phase. Thermal stability of the as-spun alloys was studied by differential scanning calorimetry (DSC), showing that spinning rate was a negligible factor on the crystallization temperature of the amor-phous phase. The hydrogen absorption and desorption kinetics of the as-cast and as-spun alloys were measured using an automatically con-trolled Sieverts apparatus, confirming that the hydrogen absorption and desorption capacities and kinetics of the as-cast alloys clearly in-creased with rising La content. For La content x=2, the as-spun alloy displayed optimal hydrogen desorption kinetics at 200 ℃.     

Facile synthesis and highly efficient selective adsorption properties of Y2Mo4O15 for methylene blue: Kinetics,thermodynamics and mechanical analyses

Y_2 Mo_4 O_(15) particles were prepared using a simple solution method(SSM) and used as a highly efficient selective adsorbent for methylene blue(MB) in aqueous solutions. The maximum adsorption capacity of the samples was determined based on the adsorption isotherms with different adsorbent doses at 298,318 and 338 K. The fittings of the temperature-dependent isotherms yield ΔrG_m~θ =-34.1 kJ/mol,Δ_rH_m~θ=-36.9 kJ/mol and Δ_rS_m~θ=-9.67 J/mol·K. The as-prepared Y_2 Mo_4 O_(15) has a very large maximum adsorption capacity(i.e., 198 mg/g) for MB at room temperature, and this value is only less than that of amorphous hardwood powder. Notably, 80 mg of adsorbent is able to completely decolorize 250 mL of30 mg/L MB aqueous solution. The kinetic parameters of the adsorption process were obtained from the temperature-dependent adsorption isotherm(i.e., E_1=26.9 kJ/mol and E_1 = 63.8 kJ/mol). The results of adsorption kinetics show that it is a pseudo-second-order reaction. The mechanism of the high selectivity and the large adsorption capacity is discussed based on competitive ion(CI) experiments and coordination theory.     

Calorimetry Investigation of Phase Stability and Thermal Property in Ti-5 % Ta Alloy

An accurate measurement of enthalpy increment H _T–H _298.15 has been made for Ti-5 % Ta alloy in the temperature range of 463–1,257 K using drop calorimetry. This temperature interval covers the low temperature α + β two phase and also the α(hcp) → β(bcc) transformation domain (1,083 K ≤ T ≤ 1,183 K), in which the enthalpy versus temperature variation exhibited a clearly delineated inflection. The drop calorimetry data has been phenomenologically modelled to obtain the transformation enthalpy, Δ°H _tr ^α→β as 66 J g^?1. Further, in the α → β diffusive transformation zone the transformation kinetics has been quantitatively modelled in terms of Kolmogorov–Johnson–Mehl–Avrami model of diffusion limited phase transformation, to obtain the effective activation energy as 284 ± 10 kJ mol^?1.     

Microstructure and High Temperature Impression Creep Properties of Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) Alloys

The current study has investigated the influence of zirconium (Zr) addition to Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) alloys prepared using argon arc melting on the microstructure and impression properties at 448–498 K under constant stress of 380 MPa. Microstructural analysis of as-cast Mg–3Ca–xZr alloys showed grain refinement with Zr addition. The observed grain refinement was attributed to the growth restriction effect of Zr in hypoperitectic Mg–3Ca–0.3 wt% Zr alloys. Heterogeneous nucleation of α-Mg in properitectic Zr during solidification resulted in grain refinement of hyperperitectic Mg–3Ca–0.6 wt% Zr and Mg–3Ca–0.9 wt% Zr alloys. The hardness of Mg–3Ca–xZr alloys increased as the amount of Zr increased due to grain refinement and solid solution strengthening of α-Mg by Zr. Creep resistance of Mg–3Ca–xZr alloys increased with the addition of Zr due to solid solution strengthening of α-Mg by Zr. The calculated activation energy (Q_a) for Mg–3Ca samples (131.49 kJ/mol) was the highest among all alloy compositions. The Q_a values for 0.3, 0.6 and 0.9 wt% Zr containing Mg–3Ca alloys were 107.22, 118.18 and 115.24 kJ/mol, respectively.     

Effect of scandium and zirconium alloying on microstructure and gaseous hydrogen storage properties of YFe3

RM₃ compounds (R = rare earth metals, M = transition metals) have rarely been studied for gaseous hydrogen storage applications because of unfavorable thermodynamics. In this work, the hydrogen storage properties of a single-phase YFe₃ alloy were improved by non-stoichiometric composition and alloying with Sc and Zr. Only the Y_1.1–ySc_yFe₃ (y = 0.22, 0.33) alloys consist of a single rhombohedral phase. The Sc substitution for Y leads to the reduction in the unit cell volume of the YFe₃ phase, and thus significantly increases the dehydriding equilibrium pressure and decreases the dehydrogenation temperature. The alloy Y_0.77Sc_0.33Fe₃ delivers a decomposition enthalpy change of 33.54 kJ/mol and a lowest dehydrogenation temperature of 135 °C, in comparison with 38.99 kJ/mol and 165 °C for the alloy Y_1.1Fe₃. The Zr substitution causes a similar thermodynamic destabilization effect, but the composition and microstructure of Y–Zr–Fe alloys need to be further optimized.     

Enthalpies of formation for boron silicides

We have studied silicides of the system B-Si formed upon vaporization using mass spectrometry. We determined the heats of formation: Δ_fH⁰(298.15 K), B₄Si=?90.8 kJ/mole, Δ_fH⁰(298.15 K), B₆Si=?122.2 kJ/mole.     

The identification of hydrogen trapping states in an Al-Li-Cu-Zr alloy using thermal desorption spectroscopy

Thermal desorption spectroscopy (TDS) was utilized to identify several metallurgical states in an Al -2Li - 2Cu-0.1Zr (wt pct) alloy, which trap absorbed hydrogen. Six distinct metallurgical desorption states for hydrogen were observed for tempers varying from the T3 to peakaged condition. Lower energy thermal desorption states were correlated with interstitial sites, lithium in solid solution, and δ′ (Al₃Li) precipitates. These states have trap-binding energies ≤25.2 kJ/mol. Under the charging conditions utilized, approximately 4 pct of the total (e.g., trapped and lattice) hydrogen content was associated with interstitial sites, consistent with the view that the intrinsic lattice solubility of hydrogen in aluminum is very low. In contrast, dislocations, grain boundaries, and T₁ (Al₂CuLi) particles were found to be higher energy-trap states with trap-binding energies ≥31.7 kJ/mol. Approximately 78 pct of all absorbed hydrogen occupied these states. Moreover, greater than 13 pct of the available trap sites at grain boundaries were occupied. Such a high hydrogen coverage at grain-boundary sites supports the notion that hydrogen contributes to grain-boundary environmental cracking in Al-Li-Cu-Zr alloys. Also, it points out the error in assuming that hydrogen cannot play a major role in cracking of Al-based alloys due to the low lattice solubility.     

Fabrication of Porous Ti-rich Ti<Subscript>51</Subscript>Ni<Subscript>49</Subscript> by Evaporating NaCl Space Holder

Net-shaped porous Ti-rich Ti₅₁Ni₄₉ alloy with well-controlled porosity, pore size, and pore shape are fabricated by pressing-and-sintering compacts containing fine Ti and Ni powders and coarse NaCl powders. After sintering at 1323 K (1050 °C) for 30 minutes in a high vacuum, the NaCl space holder is removed by evaporation, and the remaining Ti and Ni powders are sintered with about 2.3 vol pct liquid phase. The sintered Ti₅₁Ni₄₉ compacts have porosities of 26, 64, 70, 78, and 85 pct, and no distortion is observed. DSC tests show that the M _S temperature and ΔH are about 347 K (74 °C) and 28 J/g, respectively, and that they are almost independent of the porosity and close to those of wrought Ti-rich TiNi alloys. These porous Ti₅₁Ni₄₉ compacts exhibit a homogeneous microstructure, and the compressive properties and porosity are close to those of human bones.     

Thermochemical Properties of Liquid Alloys in Fe ― O and Fe ― O ― Metal Systems

An isoperibolic calorimeter has been used to determine the partial and integral enthalpies of mixing for liquid alloys in the binary Fe ― O system and ternary Fe ― O ― M ones at 1915 K. The enthalpies of mixing in these systems are high exothermic quantities, which agree with published data. It is found that the first partial enthalpy of mixing for oxygen is _280 kJ/mole. The enthalpies of mixing have been calculated from the standard enthalpies of formation Δ_f H ^º ₂₉₈ for iron oxides throughout the concentration range. The enthalpies of mixing have been calculated and measured for binary alloys in the Fe ― O system, which are correlated one with the other.     

On the dilute solutions of gold in Bi-Sn alloys

The relative partial gram-atomic enthalpy of gold Δ?H_Au at infinite dilution in bismuth, tin, and six Bi-Sn alloys has been measured at 623°K. The composition dependence (dΔ?H_Au/dx _Au)_xAu→0 in bismuth and two Bi-Sn alloys and the temperature coefficient (dΔ?H_Au,_xAu→0)/dT in three Bi-Sn alloys have also been measured. An analysis of the results based on the quasichemical theory shows that (dΔ?H_Au/dx _Au)xAu→0) is very sensitive and Δ?H_Au,xAu→0 is insensitive to nonzero values of the relative energy of solute-solute bonds; the analysis also shows that a decrease in the coordination number has a small effect on (dΔ?H_Au/dx _Au)_xAu→0 but has an appreciable effect on Δ?H_Au,xAu→0