Influence of Ti/Hf doping on hydrogen storage performance and mechanical properties of ZrCo compounds: A first principle study

Element doping is an effective way to improve the performance of hydrogen storage. The influences of Ti- and Hf-substituted dopants on the hydrogen storage and mechanical properties in a Zirconium-Cobalt alloy have been investigated by the first principle method. The results show that the Ti and Hf atoms preferentially occupy the Zr atoms to form new Zr₇Co₈Ti and Zr₇Co₈Hf compounds. The cohesive energy of Zr₇Co₈Ti and Zr₇Co₈Hf are ?7.518 eV/atom and ?7.531 eV/atom, thus Zr₇Co₈Hf shows a better structural stability than Zr₇Co₈Ti. The B/G ratio of Zr₇Co₈Ti and Zr₇Co₈Hf are 3.42 and 3.89, respectively, which indicates that Ti doping could increase the ductility of the alloy, thus improving the recycling performance of the alloy. The calculation for the electronic structure, mechanical property and structural stability may provide an effective way and theoretical evidence for the better design and optimization of hydrogen storage materials.     

First-principles investigation on the role of interstitial site preference on the hydrogen-induced disproportionation of ZrCo and its doped alloys

There are two phase structures involved in ZrCo hydrides (ZrCoH_x). When x ≤ 1, the α-phase hydride is generated when hydrogen atoms occupy the 3c and 12i sites. When 1 < x ≤ 3, three interstitial sites of 4c₂, 8f₁, and 8e are occupied by H, and in turn the β-phase hydride is formed. There is a disproportionation reaction in β-phase hydrides during hydrogen discharging process to produce the ZrH₂ phase with higher thermal stability, leading to inferior hydrogen storage performance. In this study, the influence of hydrogen storage capacity on thermodynamic and lattice stabilities of α- and β-phase hydrides for each occupancy position is investigated under the framework of the first-principles study. The results indicate that the binding energy in the 3c site is higher compared with the 12i site under the condition of identical hydrogen storage capacity. Similarly, the binding energy is the largest for the 8e site compared with the other two sites, indicating that there is the least energy released in the reaction process. Thus, the 8e site is proved as the most unfavorable site in β-phase ZrCo hydrides, which is due to its degraded thermodynamic stability. Also, comparisons of mechanical properties and total density of states for each site in two hydride phases are presented to demonstrate that compound lattice stability in the 8e site is the poorest, suggesting that it is more likely to produce disproportionation. Furthermore, the dependence of hydrogen storage performance of β-phase hydrides on Ti/Rh doping is examined as well. It is discovered that there is improved thermodynamic stability and lattice stability in the 8e site for Zr_0.875Ti_0.125Co after Zr is partially substituted by Ti, which significantly enhances the disproportionation resistance. In contrast, when Co is partially replaced by Rh, there is a deterioration in the thermodynamic stability of ZrCo_0.875Rh_0.125 in the 8e site, but its lattice stability is somewhat improved.     

Effects of alloying substitutions on the anti-disproportionation behavior of ZrCo alloy

We perform first-principles calculations to investigate the effects of alloying substitutions (i.e. Ti, Hf, Sc, Fe, Ni and Cu) on hydrogen-induced disproportionation of ZrCo alloy. H at the 8e site of ZrCoH₃ (H(8e)) plays the key role in the disproportionation process. It is found that H(8e) prefers to form strong covalent-like binding with the neighboring Co and its substitute elements, which is distinctly different from H at the 4c₂ and 8f₁ sites. Alloying substitutions can restrain or accelerate the disproportionation by influencing the ZrH(8e) bond length and the size of the 8e site. Judged from this, the anti-disproportionation ability of these alloying substitutions is identified. Our results of Ti, Hf, Sc, Fe and Ni are in good agreement with the previous experimental results. It is also predicated that Cu can accelerate hydrogen-induced disproportionation of ZrCo alloy.     

Remarkable enhancement in durability of ZrCo alloy against hydrogen induced disproportionation by Ti and Nb co-substitution

Considering the thermodynamic stability of various hydrides, a strategy has been employed to improve the hydrogen isotope storage properties of ZrCo alloy which involves partial co-substitution of Zr with Ti and Nb. Herein, alloys of composition Zr_0.8Ti_0.2-xNb_xCo (x = 0.05, 0.1, 0.15) is prepared, characterized and the effect of Ti and Nb doping on hydrogen storage properties of parent ZrCo alloy is investigated. XRD analysis confirmed the formation of desired pure cubic phase of all the synthesized alloys similar to ZrCo phase. The presence of a single plateau in hydrogen desorption pressure-composition isotherms confirms single step hydrogen absorption-desorption behavior in Zr_0.8Ti_0.2-xNb_xCo alloys. The equilibrium pressure of hydrogen desorption decreases marginally with increasing Nb content in Zr_0.8Ti_0.2-xNb_xCo alloys which is further corroborated by differential scanning calorimetry measurements. Investigation of hydrogen induced disproportionation behavior in ITER-simulating condition revealed substantial impact of co-substitution of Ti and Nb on anti-disproportionation properties of ZrCo alloy. These remarkable properties make the Ti and Nb co-substituted quaternary alloys a desirable material for hydrogen isotope storage and delivery application.     

EXAFS and SAXS studies of ZrCo alloy doped with Hf,Sc and Ti atoms

The local structures of ZrCo alloy doped with Hf, Sc and Ti atoms before and after hydrogenation were studied by extended X-ray absorption fine structure (EXAFS) technique. For all the samples, the length of the Co–Zr bond increased and the coordination numbers of Co reduced after hydrogenation. As the Hf, Sc and Ti atoms came into the ZrCo alloy, the lengths of the Co–Zr bond varied. The ingoing Ti atoms had shortened the Co–Zr bond lengths from 2.74 to 2.62 Å in the pre-hydrogenation, whereas all of the doped atoms had no obvious effect in the post-hydrogenation. Also the size distribution of the nanoparticles in the as-cast and hydrogenated Zr–Co alloy was investigated by small-angle X-ray scattering (SAXS) technique.     

Isotope effect on hydrogen storage properties of Ti and Nb co-substituted ZrCo alloy

In our previous study, we showed that the anti-disproportionation properties of Zr_0.8Ti_0.2-xNb_xCo alloys were remarkably improved by the co-substitution of Zr with Ti and Nb. However, the practical application of these alloys in handling of hydrogen isotopes necessitates the first hand knowledge of hydrogen isotope effect. Herein, we discuss the hydrogen isotope effect on storage properties of Zr_0.8Ti_0.2-xNb_xCo alloys. According to PCT measurements on desorption of deuterium from the Zr_0.8Ti_0.2-xNb_xCo deuterides and comparison with corresponding hydrides, the deuterides require relatively lower temperature to achieve the desired equilibrium pressure. DSC measurements reveal a significant decrease in the activation energy for hydrogen/deuterium desorption reactions when Zr is substituted with Ti and Nb. Furthermore, it is observed that the activation energy of deuterium desorption is lower than the desorption of hydrogen from analogous hydride. Isotope effect on isothermal disproportion studies on Zr_0.8Ti_0.2-xNb_xCo-deuterides divulge that Zr_0.8Ti_0.2-xNb_xCo-deuterides have superior anti-disproportionation properties over corresponding hydrides, and further improvement is anticipated for the Zr_0.8Ti_0.2-xNb_xCo-tritides. This study revealed the significant impact of Ti and Nb co-substitution on hydrogen isotope storage properties of Zr_0.8Ti_0.2-xNb_xCo alloys, making them potential candidates for handling hydrogen isotopes.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

Parameter optimization of ZrCo–H system for durable tritium storage and delivery system of ITER

Cycling stability of ZrCo–H system is extremely important for the long-term operation of the storage and delivery system (SDS) in ITER. Herein, the optimal cycling operation parameters were systematically investigated. It indicates that various parameters, such as hydrogen pressure, temperature, composition, and stoichiometric ratio of H atoms, will all affect the cycling performance of the ZrCo–H system significantly. The decline rate of the hydrogen capacity of the ZrCo–H system is positively correlated with the hydrogen pressure. The experimental result shows that 54% of hydrogen capacity decreases under 28.1 kPa hydrogen pressure, while 30% of attenuation is obtained when the pressure is decreased to 8.1 kPa after 14 cycles. In terms of temperature, the lowest cycling attenuation can be maintained at about 25% after 14 cycles when the dehydrogenation temperature at 550 °C. The effects of doping elements, Hf and Ti, on the cycling stability of ZrCo–H system are also compared. The Zr_0.8Ti_0.2Co sample exhibits higher cycling capacity than ZrCo and Zr_0.8Hf_0.2Co samples. The extremely excellent behavior can be achieved when all ZrCo alloys are continuously evacuated during the hydrogen release process, and the attenuation of only 1.1% is observed for Zr_0.8Ti_0.2Co after 15 cycles. Besides, the cycling attenuation is related to residual stoichiometric ratio of H atoms in ZrCo alloy during the cycling test. When the residual H atoms proportion exceeds 1 in ZrCo during dehydrogenation, hydrogen cycling capacity hardly fades. The XRD results reveal that the disproportionation of ZrCo is directly associated with the cycling degradation, yielding the more stable products of ZrCo₂ and ZrH₂, However, the disproportionation can be avoided during the cycling process by controlling the stoichiometric ratio of H atoms remained in ZrCo above 1. This study demonstrates that the cycling performance of ZrCo can be substantially improved when the operation parameters are properly adjusted, which provides a significant important reference for durable running of SDS in ITER.     

Investigation on synthesis,characterization and hydrogenation behaviour of hydrogen storage material: Fe1−xZrxTi1.3 (x = 0.2)

The present study is aimed at investigations on the structural and microstructural characterization and hydrogenation behaviour of Zr substituted Fe_1−xZr_xTi_1.3 (x = 0.2) alloys. This storage alloy has been synthesized using R.F.induction melting under an argon atmosphere in a previously outgassed graphite crucible. The structural characterization (XRD) has revealed that the as-synthesized sample is multiphasic in nature and embodies the phases FeTi, Fe₂Ti, FeTi₂ and Ti. Microstructural evaluations (SEM) exhibited the presence of interfaces and cracking. The P–C isotherm was determined and it showed the storage capacity of ∼1.20 wt.% at 200°C. The activation as well as desorption kinetics (75% desorption of H₂ in 6 min) were found to be better for Zr-substituted alloy (Fe_0.8Zr_0.2Ti_1.3) as compared to FeTi_1.3. The improved activation and desorption kinetics are thought to be due to presence of several interfaces e.g. Fe(Zr)Ti⧸ Fe(Zr)Ti₂, Fe(Zr)Ti⧸{Fe(Zr)}₂Ti and volume expansion induced cracking of Ti.     

Effect of addition of Zr,Ni, and Zr-Ni alloy on the hydrogen absorption of Body Centred Cubic 52Ti-12V-36Cr alloy

The effects of addition of Zr, Ni and Zr₇Ni₁₀ on the crystal structure, microstructure and hydrogen absorption of Body Centred Cubic (BCC) 52Ti-12V-36Cr were investigated. We found that addition of Zr and/or Ni led to the formation of a Ni/Zr rich secondary phase. This secondary phase is responsible for the much faster first hydrogenation of the alloys with additives compared to the bare BCC alloy. Zirconium addition had positive influence on incubation time and intrinsic hydrogenation kinetics while nickel addition improved the hydrogen capacity. Among the additives tested, Zr₇Ni₁₀ is the best for optimized hydrogenation kinetics and capacity.     

Investigating possible kinetic limitations to MgB2 hydrogenation

An investigation is reported of possible kinetic limitations to MgB₂ hydrogenation. The role of H–H bond breaking, a necessary first step in the hydrogenation process, is assessed for bulk MgB₂, ball-milled MgB₂, as well as MgB₂ mixed with Pd, Fe and TiF₃ additives. The Pd and Fe additives in the MgB₂ material exist as dispersed metallic particles in the size range ~5–40 nm diameter. In contrast, TiF₃ reacts with MgB₂ to form Ti metal, elemental B and MgF₂, with the Ti and the MgF₂ phases proximate to each other and coating the MgB₂ particulates with a film of thickness ~3 nm. Sieverts studies of hydrogenation kinetics are reported and compared to the rate of H–H bond breaking as measured by H-D exchange studies. The results show that H–H bond dissociation does not limit the rate of hydrogenation of MgB₂ because H–H bond cleavage occurs rapidly compared to the initial MgB₂ hydrogenation. The results also show that surface diffusion of hydrogen atoms cannot be a limiting factor for MgB₂ hydrogenation. Instead, it is speculated that it is the intrinsic stability of the B–B extended hexagonal ring structure in MgB₂ that hinders the hydrogenation of this material. This supposition is supported by B K-edge x-ray absorption measurements of the materials, which showed spectroscopically that the B–B ring was intact in the material systems studied. The TiF₃/MgB₂ system was examined further theoretically with reaction thermodynamics and phase nucleation kinetic calculations to better understand the production of Ti metal when TiB₂ is thermodynamically favored. The results show that there exist physically reasonable ranges for which nucleation kinetics supersede thermodynamics in determining the reactive pathway for the TiF₃/MgB₂ system and perhaps for other additive systems as well.     

Crystal structure of intermetallic compound Y5Co19 and its hydride phases

Y–Co intermetallic compounds and their hydrides were investigated as magnetic materials, but the hydrogenation of these alloys was not observed. We established the existence of Y₅Co₁₉ in the phase diagrams of the Y–Co system. Y₅Co₁₉, an intermetallic compound, was synthesized and its crystal structure was determined by the Rietveld refinement of X-ray diffraction (XRD) data. The structural model of Y₅Co₁₉ is a Ce₅Co₁₉-type model with lattice parameters a = 0.4994 (1) nm and c = 4.800 (1) nm. The crystal structure of the original alloy and its hydride phase is closely related to the hydrogen absorption–desorption property. The hydrogen capacity of Y₅Co₁₉ was 0.63 H/M, while two plateaus were observed in the P–C isotherm. We discovered two hydride phases: Y₅Co₁₉H_3.8 (phase I) and Y₅Co₁₉H_14.9 (phase II), and the structural model of these phases (I and II) was Ce₅Co₁₉-type, which was the same as the original alloy. The unit cell of phase I showed an expansion of 3.9% only along the c-axis from the original alloy, whereas that of phase II indicated an expansion along the a- and c-axes. Additionally, a close isotropic expansion was observed in phase II.     

First-principles study the structural,electronic, vibrational and thermodynamic properties of Zr1−xHfxCoH3

The structural, electronic, vibrational and thermodynamic properties of Zr_1?xHf_xCoH₃ (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) are investigated using first principles approach based on the virtual crystal approximation (VCA). The results indicate the series Zr_1?xHf_xCoH₃ have the similar physical properties. When Hf concentration increases gradually, the lattice parameter reduces and the thermodynamic stability first decreases and then increases, respectively. The calculated results of charge distributions and electron localization function (ELF) suggest that the interactions of HCo and HZr_1?xHf_x are primarily metallic with a small covalent component. The band structure and the corresponding density of states (DOS) around the Fermi level (E_f) indicate the metallicity enhances and the electrical conductivity is better with increasing Hf content. The phonon density of states imply that with the increase of Hf content, the covalent interactions between H(4c₂) and Zr_1?xHf_x are weakened, while the covalent interactions between H(8f₁) and Zr_1?xHf_x basically remained unchanged (H(4c₂) and H(8f₁) represent the hydrogen atoms occupying 4c₂ and 8f₁ site, respectively), which is consistent with the results of charge density. Finally, the thermodynamic properties are obtained and discussed on the base of the obtained vibrational properties.     

Improvement of hydrogen storage properties of TiCrV alloy by Zr substitution for Ti

The effects of Zr substitution for Ti on the hydrogen absorption–desorption characteristics of Ti_1−xZr_xCrV alloys (x = 0, 0.05, 0.1 and 1.0) have been investigated. The crystal structure, maximum hydrogen absorption capacity, kinetics and hydrogen desorption properties have been studied in detail. While TiCrV crystallizes in body centered cubic (BCC) structure, ZrCrV is a C15 cubic Laves phase compound and the intermediate compositions with 5 and 10 at% Zr substitutions for Ti (x = 0.05 and 0.1) show the presence of a small amount of ZrCr₂ Laves phase along with the main BCC phase. The pressure–composition isotherms have been studied at room temperature. TiCrV shows separation of TiH₂ phase on cycling. A small amount of Zr substitution for Ti is found to have advantageous effects on the hydrogen absorption properties of TiCrV as it suppresses TiH₂ phase separation and decreases hysteresis. It is found that the hydrogen absorption capacity of Ti_1−xZr_xCrV decreases as the Zr content increases due to the increased fraction of Laves phase. Temperature-programmed desorption studies have been carried out on the saturated hydrides in order to find the relative desorption temperatures.     

New hydrides of ternary intermetallics based on Zr with Fe,Co, Ni and Sn or Sb

A series of hydrides based on the alloys Zr₆M′ _1.5X_1.5 (M′ = Fe, Co, Ni, X = Sn, Sb) and Zr₆NiAl₂ have been synthesized (conditions of hydrogenation: T = 130°C, P_H2 = 20 MPa) and their crystal structures have been determined using both X-ray and neutron diffraction. During hydrogenation the crystal structure of Zr₆M′ _1.5Sb_1.5 and Zr₆Fe_1.5Sn_1.5 changes from the Fe₂P type to the Ni₃P type (Zr₅M′X₂H_∼11). The hydrogen atoms occupy eight sites, six of those have a tetrahedral coordination whereas two H atoms occupy trigonal bipyramids. The structure of Zr₆NiAl₂D_7.5 (and Zr₆NiAl₂H_9.6) has a doubled c cell parameter reference to the starting alloy; it belongs to the Zr₆FeAl₂D₁₀ structure type.     

Modification of nano-eutectic structure and the relation on hydrogen storage properties: A novel Ti–V–Zr medium entropy alloy

TiV-based alloys present desirable hydrogen storage properties owing to the formation of Body-centered cubic (BCC) solid solutions. However, the nanostructure that helps hydrogen absorption and desorption is hard to be designed and prepared in these alloys. In this study, Ti₄₀Zr_60-xV_x (x = 20, 25, 30) alloys with hyperfine nano-eutectic structures of 50–500 nm in lamellar space are prepared, and the nano-eutectic structures can be refined by increasing Zr content. Ti₄₀Zr_60-xV_x alloy powder exhibits excellent activation and hydrogenation properties. The phase separation and nano-eutectic structure are formed due to the differences of atomic size in Ti₄₀Zr_60-xV_x alloys. The highest total hydrogenation capacity of 2.4 wt% is obtained within 10 min at 200 °C under 1 MPa H₂ by Ti₄₀V₃₅Zr₂₅ alloy, surpassing that of Ti₄₀Zr₄₀V₂₀ and Ti₄₀Zr₃₀V₃₀ alloys of 2.2 wt% in 20min. Based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, lower energy is required for the hydrogenation of Ti₄₀V₃₅Zr₂₅ alloy. Due to the formation of some stable hydrides, the Ti₄₀Zr_60-xV_x alloys show lower reversible hydrogenation capacities. The spinodal decomposition in Ti₄₀V₃₅Zr₂₅ alloy facilitates the formation of reticular eutectics, which provide high-density phase interfaces and produce “synergistic effect”. As a result, the hydrogenation kinetic and capacity are enhanced significantly.     

Systematical study on the roles of transition alloying substitutions on anti-disproportionation reaction of ZrCo during charging and releasing hydrogen

Intermetallic alloy ZrCo is believed to be a good substitution for uranium to store tritium. Nevertheless, disproportionation reaction often happens during the hydriding and dehydriding processes, and hydrogen storage property of ZrCo is therefore degraded. Alloying elements are often used to substitute Zr or Co in ZrCo to restrain disproportionation reaction. However, many experimental results do not agree with each other, and it lacks overall tendency for all transition metal elements. In this work, systematical ab initio calculations are performed to study more than 20 transition alloying elements to substitute Co and Zr in ZrCoH₃ to study the anti-disproportionation effects. It is found that substitution of Co by transition metal elements on anti-disproportionation reaction is unconspicuous, and only Ni can enlarge Zr–H bond length and decrease the volume of 8e site, presenting anti-disproportionation effect, which qualitatively agrees with the previous experiments. In contrast, all transition alloying elements considered except Fe, Co, Ni, Ru, Rh, Pd, Os and Ir replacing Zr can both enlarge the length of Zr–H bond and decrease the volume of 8e site, and thus restrain the disproportionation effects. At last, two-dimensional charge density and density of states are calculated to analyze the underlying mico-mechanisms affecting the effects of transition alloying elements on anti-disproportionation reaction.     

DFT study of crystal structure and electronic properties of metal-doped AlH3 polymorphs

AlH₃ has been considered for a long time as a hydrogen storage material with suitable gravimetric and volumetric density for practical applications. Among eight AlH₃ polymorphs observed so far, in this work we focus our attention on an investigation of the effects of various metal dopants in α- and β-AlH₃, to perceive a way of enhancing them. Substitutional incorporation of the metal dopants (Li, Sc, Ti, Cu, Cr, Fe, Nb, Mo, Zn, or Zr) is considered, as well as interstitial doping with Li, Sc, Ti, Cu, and Zr. The density functional theory (DFT) (using GGA-PW91) approach is used to address the crystal structure, bonding, dopant stability, and changes in hydrogen desorption energy. In addition, the kinetics of hydrogen desorption is also considered for several interstitially doped cases, by calculating the stability of native point defects. Promising results are presented for Zr, Ti, and Sc – doped hydrides. Doped hydrides, here studied, are considered as n- or p-type semiconducting materials, enabling wider application overcoming hydrogen storage scope.     

Formation and properties of titanium-manganese alloy hydrides

Hydride formation and hydriding properties of Ti-Mn alloy systems, which have a hexagonal structure of MgZn₂(C14)-type known as the Laves phase, were studied by measuring pressure-composition isotherms in the temperature range 0–80°C. It was found that the Ti-Mn binary alloys whose Ti contents were less than 36 at % absorbed almost no hydrogen (P ? 4.5 MPa), but the alloys containing more Ti did react readily with hydrogen at room temperature without any activation treatment. The maximum absorbed hydrogen content of every Ti-Mn alloy was H/M ～ 1.The TiMn_1.5 hydride showed the most desirable properties of all the Ti-Mn binary alloy hydrides; the dissociation plateau pressure is approximately 0.7 MPa, the maximum amount of absorbed hydrogen is 228 ml g^?1 the maximum amount of released hydrogen is 190 ml g^?1 at 20°C, and ΔHΔH is the molar enthalpy change of hydrogen (i.e. the heat of formation).= ?28.7 kJ(mol H₂)^?1. Also, hydriding properties of TiMn₂ based Ti-Mn multi-component alloys containing other transition metals, such as Zr, V, Cr, Fe, Co, Ni, Cu, Nb, Mo, Ta, La and Ce, were studied. The dissociation plateau pressure at 20°C was obtained in a range from 0.01 MPa (for Ti_0.5Zr_0.5 Mn₂-H) to 1 MPa (for Ti_0.9Zr_0.1Mn_1.4V_0.2Cr_0.4-H).     

Hydrogen storage properties of Zr2Co crystalline and amorphous alloys

To further explore the application feasibility of Zr₂Co alloy in tritium-related fields, hydrogenation/dehydrogenation properties of this material of crystalline or amorphous structure, prepared by arc melting or melt spinning, were studied by pressure-composition temperature measurement, X-ray diffraction, differential scanning calorimeter, thermal desorption spectroscopy. It was found that the two kinds of Zr₂Co alloys can absorb hydrogen in a close full concentration of ~9 mmol/g, and may have similar equilibrium hydrogen pressure in the order of 10^?6 Pa at room temperature. In their hydrogenated samples various hydrides were observed to form, including ZrH₂, Zr₂CoH₅, ZrCoH₃ and an amorphous one with gradually decreasing general thermostability. The amorphous alloy exhibited easier hydrogen induced disproportionation caused by highly stable ZrH₂ and much slower hydrogen absorption kinetics. This disproportionation behavior of the crystalline alloy was found to be entirely suppressed by changing heating process. The results firmly indicate that crystalline Zr₂Co alloy could be more favorable for tritium treatment due to very low equilibrium pressure and the feasibility of eliminating the disproportionation.