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Papers by facundo castro
Journal of Alloys and Compounds
International Journal of Hydrogen Energy, 2021
Magnesium-based wastes were reprocessed by mechanical milling under air atmosphere and used to pr... more Magnesium-based wastes were reprocessed by mechanical milling under air atmosphere and used to produce hydrogen by hydrolysis. The evolution of the material during reprocessing and the generation of hydrogen in a 0.6 M MgCl2 aqueous solution at 24ºC are reported. The morphology, microstructure and phase abundance change with milling time. With mechanical processing particle size and crystallite size reduce, microdeformations accumulate in the material, Al dissolves in (Mg), the amount of Mg17Al12 (β-phase) increases and small quantities of Fe from the milling tools are incorporated in the material. Hydrogen yields in the 70-90% range after 30 minutes of reaction have been obtained, depending on particle size and milling time. Reactants are not exhausted during the hydrolysis reaction in the saline solution, due to the formation of a Mg(OH)2 layer that produces a passivating effect. Higher generation has been observed for larger particles and for materials reprocessed for longer milling times. Reaction kinetics also improves with milling time, with faster rates observed for the smaller particles. The shape of the hydrolysis curves can be fitted with a model that corresponds to a reaction limited by a three dimensional geometric contraction process. Mg17Al12 and Fe favor hydrogen production by acting as micro-galvanic cathodes during the reaction.
Journal of Applied Crystallography, 2020
High-Mn steels attract attention because of their various technological properties. These are mai... more High-Mn steels attract attention because of their various technological properties. These are mainly mechanical and functional, such as the shape-memory effect, high damping capacity, high strength with simultaneous large ductility, the TRIP/TWIP (transformation- and twinning-induced plasticity) effect, low cycle fatigue and high work hardening capacity. All these phenomena are associated with the face-centered cubic (f.c.c.)–hexagonal close-packed (h.c.p.) martensitic transformation which takes place in these alloys. During this phase transition defects are introduced, mainly due to the large volume change between austenite and martensite. Knowing this volume change is key to understanding the mechanical behavior of these metallic systems. In the present article, a full-pattern refinement method is presented. The proposed method uses data obtained by means of conventional X-ray diffraction from regular bulk samples and allows a high-precision calculation of the lattice parameters o...
International Journal of Hydrogen Energy, 2019
The hydrogen absorption and desorption properties of a MgH 2 e 1 mol.% Nb(V) ethoxide mixture are... more The hydrogen absorption and desorption properties of a MgH 2 e 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH 2. Nb ethoxide reacts with MgH 2 during heating, releasing C 2 H 6 and H 2 , and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH 2 without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H 2 to 94 kJ/mol H 2 , and from 176 kJ/mol H 2 to 75 kJ/ mol H 2 , respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 C. The kinetic improvement is attributed to the fine distribution of amorphous/ nanometric NbH x achieved by the dispersion of the liquid additive.
International Journal of Hydrogen Energy, 2018
This paper presents a comparative study of H 2 absorption and desorption in MgH 2 milled with NbF... more This paper presents a comparative study of H 2 absorption and desorption in MgH 2 milled with NbF 5 or NbH 0.9. The addition of NbF 5 or NbH 0.9 greatly improves hydriding and dehydriding kinetics. After 80 h of milling the mixture of MgH 2 with 7 mol.% of NbF 5 absorbs 60% of its hydrogen capacity at 250 C in 30 s, whereas the mixture with 7 mol.% of NbH 0.9 takes up 48%, and MgH 2 milled without additive only absorbs 2%. At the same temperature, hydrogen desorption in the mixture with NbF 5 finishes in 10 min, whereas the mixture with NbH 0.9 only desorbs 50% of its hydrogen content, and MgH 2 without additive practically does not releases hydrogen. The kinetic improvement is attributed to NbH 0.9 , a phase observed in the hydrogen cycled MgH 2 þ NbF 5 and MgH 2 þ NbH 0.9 materials, either hydrided or dehydrided. The better kinetic performance of the NbF 5-added material is attributed to the combination of smaller size and enhanced distribution of NbH 0.9 with more favorable microstructural characteristics. The addition of NbF 5 also produces the formation of Mg(H x F 1-x) 2 solid solutions that limit the practically achievable hydrogen storage capacity of the material. These undesired effects are discussed.
Journal of Solid State Chemistry, 2018
Abstract The structure of κ-Ag 2 Mg 5 has been refined based on X-ray powder diffraction measurem... more Abstract The structure of κ-Ag 2 Mg 5 has been refined based on X-ray powder diffraction measurements (R wp = 0.083). The compound has been prepared by combining mechanical alloying techniques and thermal treatments. The intermetallic presents the prototypical structure of Co 2 Al 5 , an hexagonal crystal with the symmetries of space group P6 3 /mmc , and belongs to the family of kappa-phase structure compounds. The unit cell dimensions are a=8.630(1) A and c=8.914(1) A. Five crystallographically independent sites are occupied, Wyckoff positions 12 k , 6 h and 2 a are filled with Mg, another 6 h site is occupied with Ag, and the 2 c site presents mixed Ag/Mg occupancy. The crystal chemistry of the structure and bonding are briefly discussed in the paper.
Journal of Alloys and Compounds, 2017
Mg(H x F 1Àx) 2 solid solutions have been synthesized by mechanical milling MgF 2 and MgH 2 under... more Mg(H x F 1Àx) 2 solid solutions have been synthesized by mechanical milling MgF 2 and MgH 2 under H 2 atmosphere. Complete solubility has been observed at 475 C. The solid solution presents a rutile-type structure with the anion site randomly occupied by H À or F À and with lattice parameters showing positive deviations from Vegard's law. The solution can be decomposed and reformed reversibly by changing H 2 pressure. For example, Mg(H 0.9 F 0.1) 2 reversibly stores 5.5 wt. % H 2 in less than 3 min at 440 C. During decomposition H 2 is released, and Mg and a solution rich in F appear as products. This process takes place in two clearly marked stages controlled by F À concentration in the material. The decomposition enthalpy of the first one is close to that of MgH 2 for Mg(H 0.9 F 0.1) 2 , and decreases with the H content in the solution.
International Journal of Hydrogen Energy, 2015
ABSTRACT A mixture of MgH2 and (7 mol%) NbF5 has been mechanically milled under Ar atmosphere. Th... more ABSTRACT A mixture of MgH2 and (7 mol%) NbF5 has been mechanically milled under Ar atmosphere. The evolution of the materials has been studied by in situ pressure monitoring, XRD, DSC, TG, TPD, SEM, and isothermal hydrogen absorption and desorption in a volumetric device. During milling, H-rich and F-rich solid solutions MgH2−xFx and MgHyF2−y are produced. After 40 h of milling both solutions merge into a single one with formula MgH1.60F0.40. This solid solution is stable under a thermal treatment of 90 h at 300 °C under 6000 kPa of H2. Hydriding and dehydriding kinetics in the as-milled and cycled materials are considerably faster than in MgH2 milled without additive. Desorption temperature in DSC or TG is lowered 100 °C, and the material modified with NbF5 can be hydrided in less than 4 min at T = 250 °C. A H-rich solution is formed upon rehydriding the material, showing the reversibility of the process. The kinetic improvement seems to be due to a cooperative effect between MgHyF2−y and niobium hydride, the former providing seeding crystals for MgH2 nucleation, and the latter working as a gateway for hydrogen transfer.
Journal of Alloys and Compounds, 2014
ABSTRACT We analyze hydrogen absorption and desorption in Mg and Mg-Ag compounds prepared by mech... more ABSTRACT We analyze hydrogen absorption and desorption in Mg and Mg-Ag compounds prepared by mechanical milling. The materials were obtained by processing mixtures of Mg or MgH2 with different proportions of Ag under Ar or H2 atmosphere. We observe that Ag and AgMg destabilize MgH2. In the first case the products of the reaction of Ag and MgH2 are AgMg and H2, in the second case MgH2 reacts with AgMg to give AgMg4 and H2. The first reaction was only observed in the forward direction, whereas the second one was registered in both directions. The impossibility to reverse the first reaction was attributed to poor kinetics and experimental limitations. The reversible destabilization induced by AgMg is low, MgH2 overall dehydriding enthalpy is reduced from 76.5 kJ/mol H2 to 69.1 kJ/mol H2. This last value allowed us to estimate the formation enthalpy of AgMg4 at -59.5 kJ/mol. Besides destabilization, a slight improvement in hydrogen absorption kinetics and a more noticeable enhancement in desorption kinetics has been observed in samples containing Ag as an additive. During thermal desorption, the dehydriding temperature decreases by 20°C. In isothermal desorption experiments at 325°C, the characteristic induction period of MgH2 without additive practically disappears, and the time to reach 50% of conversion decreases from 17 min to 5.4 min when silver is present. The changes in desorption kinetics could be attributed to AgMg or the interfaces MgH2/AgMg acting as nucleation sites for Mg.
Review of Scientific Instruments, 2000
We present a new experimental setup designed to perform measurements of thermal desorption spectr... more We present a new experimental setup designed to perform measurements of thermal desorption spectroscopy of hydrogen released from metallic samples. The distinctive features of the proposed arrangement are a compact reactor and the use of a mass flow meter to measure the flux of gas desorbed from the sample. A complete set of hydrogen desorption spectra from Pd samples has been recorded to test the equipment. The spectra show very good reproducibility, a high signal-to-noise ratio, and agree very well with results reported in the literature obtained using more sophisticated equipment. These features encourage the use of this low cost setup to perform a fast and reliable characterization of the processes controling hydrogen desorption from metallic compounds.
Review of Scientific Instruments, 2009
Hydrogen storage materials can form more than one hydride phase. These different phases, in turn,... more Hydrogen storage materials can form more than one hydride phase. These different phases, in turn, display different hydrogen absorption/desorption capacities, kinetics, and stabilities. Studies aimed at characterizing and improving these materials usually need to correlate hydrogen intake with the precise determination of the hydride phase involved in the process. Here, we present a device designed to perform measurements of well known volumetric techniques with simultaneous x-ray diffraction on the material under study. The compact design can stand up to 6000 kPa of internal pressure while the sample can be heated up to 450 °C. The design process was assisted by finite element modeling and by the use of mock-up prototypes in order to optimize the thermal and under load behaviors. We provide two examples of use for this new device: (1) hydride decomposition in LaNi5 at 115 °C and (2) formation of MgCo during the programmed thermal desorption of the Mg2CoH5 hydride.
Journal of Solid State Chemistry, 2007
Hydrogen tungsten bronzes have been synthesized by reactive mechanical alloying monoclinic tungst... more Hydrogen tungsten bronzes have been synthesized by reactive mechanical alloying monoclinic tungsten (VI) oxide under hydrogen atmosphere. Two milling devices with different energy ranges were used. Regardless of the distinct reaction times, a similar phase evolution was observed with both apparatus. The characterization of the materials was performed by XRD, SEM, DSC and total hydrogen content determination. The final product obtained was a mixture of tetragonal H 0.33 WO 3 and H 0.23 WO 3 bronzes.
The Journal of Chemical Physics, 1998
In this work we analyze Thermal Desorption Spectroscopy (TDS) experiments of gases considering si... more In this work we analyze Thermal Desorption Spectroscopy (TDS) experiments of gases considering simultaneously atomic diffusion in the bulk and recombinative reaction on the surface of a solid sample. We perform computer simulations in a three-dimensional cubic lattice, and compare the simulated spectra with numerical results from a simple reaction-diffusion model, and analytical results from models for the fast diffusion and fast reaction regimes. We analyze the effects on the spectra of each relevant parameter: diffusivity, reaction coefficient, heating rate, initial concentration of particles and sample size. We also develop a rate-limiting step identification procedure based on the analysis of peak position as a function of initial concentration. This last procedure applied to the simulations and to experimental TDS spectra produces very good results.
Journal of Alloys and Compounds, 2010
ABSTRACT Recently, we have reported the formation of hydrogen tungsten bronzes by reactive mechan... more ABSTRACT Recently, we have reported the formation of hydrogen tungsten bronzes by reactive mechanical milling monoclinic tungsten (VI) oxide under hydrogen atmosphere. In this work we report the milling of hexagonal WO3 under H2 atmosphere. Our main results are the structural transformation of the hexagonal oxide to the high temperature polymorph of WO3 with orthorhombic structure, and the formation of different hydrogen tungsten bronzes at distinct milling times. The bronzes seem to be formed from the orthorhombic oxide, and compared with the bronzes obtained by milling monoclinic WO3 are rather unstable after short exposure to air. The materials are characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and total hydrogen content determination.
Journal of Alloys and Compounds, 2002
We present a theoretical study of thermal desorption spectroscopy applied to the characterization... more We present a theoretical study of thermal desorption spectroscopy applied to the characterization of hydrogen desorption kinetics from hydride forming materials. We propose a model that considers bulk and surface processes during desorption in the solid solution1hydride field of a metal-hydrogen system. We consider as possible rate limiting steps: diffusion, phase transformation, bulk to surface passage and two-atom recombination on the surface of the sample.
Journal of Alloys and Compounds, 2002
We studied the influence of Ge on hydrogen desorption from MgH produced by mechanical alloying at... more We studied the influence of Ge on hydrogen desorption from MgH produced by mechanical alloying at room temperature under a 2 hydrogen atmosphere. The structural and morphological properties, and the desorption kinetics of the products were examined by X-ray diffraction, differential scanning calorimetry, thermal desorption spectroscopy and scanning electron microscopy. The mechanical milling of Mg-Ge mixtures under hydrogen leads to the formation of Mg Ge and MgH. The presence of Ge decreases the hydride 2 2 decomposition temperature in a range from 50 to 1508C, depending on the Ge amount. On the contrary, Mg Ge does not show any effect 2 on hydrogen desorption.
Journal of Alloys and Compounds, 2004
The influence of the mechanical grinding (MG) (under reactive atmosphere of hydrogen) conditions ... more The influence of the mechanical grinding (MG) (under reactive atmosphere of hydrogen) conditions on the physico-chemical properties (crystallography, phase composition, morphology and hydrogen sorption properties) of a Mg + 5% Wt.% Cr 2 O 3 mixture are examined. Both a planetary and a vibratory ball miller have been used. In the case of the planetary ball miller, different rotation speeds (200, 250 and 300 rpm), and ball to powder mass ratios were investigated. It appears that the planetary ball miller allows to obtain finer particles and also more amorphous materials. The relationship between crystallinity, morphology and injected power was established.
Journal of Alloys and Compounds, 2005
Equilibria E 4000 Formation, Composition and Stability of Mg-Co Compounds.-As revealed by XRD, DS... more Equilibria E 4000 Formation, Composition and Stability of Mg-Co Compounds.-As revealed by XRD, DSC, and different synthesis methods such as β-Mg2CoH5 dehydration, mechanical alloying of Mg and Co under Ar, mechanical alloying followed by heat treatment, and Mg-Co solid state reaction, only one compound with more than 40 at% Mg exists in the Mg-Co system. MgCo is a stable compound, whose formation process can be greatly influenced by kinetic factors that may suppress its presence in favor of MgCo 2 .-(GENNARI, F. C.; CASTRO*, F.
Journal of Alloys and Compounds, 2002
Mechanochemistry Mechanochemistry H 3000 Synthesis of Mg 2 FeH 6 by Reactive Mechanical Alloying:... more Mechanochemistry Mechanochemistry H 3000 Synthesis of Mg 2 FeH 6 by Reactive Mechanical Alloying: Formation and Decomposition Properties.-The title compound, an attractive material for hydrogen storage, is synthesized by reactive mechanical alloying from Mg and Fe elemental powders in a H 2 atmosphere at room temperature. The samples are characterized by powder XRD, DSC, and SEM. The formation of the title compound involves two steps: MgH 2 formation at shorter milling times, and reaction between MgH 2 and Fe to produce Mg 2 FeH 6 as milling time increases. On the contrary, the decomposition of the title compound occurs in only one step, giving the elemental metals and hydrogen gas as reaction products.-(GENNARI*, F. C.; CASTRO, F.
Journal of Alloys and Compounds, 2004
The hydrogen sorption properties of a mixture of Mg + 10 wt.% WO 3 made by reactive mechanical gr... more The hydrogen sorption properties of a mixture of Mg + 10 wt.% WO 3 made by reactive mechanical grinding at room temperature under 1100 kPa of H 2 were studied and compared with those of elemental Mg subjected to a similar preparation procedure. The milling products were also characterized by XRD, EPMA, and granulometric measurements. It was observed that WO 3 has an important catalytic effect on hydrogen absorption and desorption, more than doubling the rates observed in the samples milled without the additive.
Journal of Alloys and Compounds
International Journal of Hydrogen Energy, 2021
Magnesium-based wastes were reprocessed by mechanical milling under air atmosphere and used to pr... more Magnesium-based wastes were reprocessed by mechanical milling under air atmosphere and used to produce hydrogen by hydrolysis. The evolution of the material during reprocessing and the generation of hydrogen in a 0.6 M MgCl2 aqueous solution at 24ºC are reported. The morphology, microstructure and phase abundance change with milling time. With mechanical processing particle size and crystallite size reduce, microdeformations accumulate in the material, Al dissolves in (Mg), the amount of Mg17Al12 (β-phase) increases and small quantities of Fe from the milling tools are incorporated in the material. Hydrogen yields in the 70-90% range after 30 minutes of reaction have been obtained, depending on particle size and milling time. Reactants are not exhausted during the hydrolysis reaction in the saline solution, due to the formation of a Mg(OH)2 layer that produces a passivating effect. Higher generation has been observed for larger particles and for materials reprocessed for longer milling times. Reaction kinetics also improves with milling time, with faster rates observed for the smaller particles. The shape of the hydrolysis curves can be fitted with a model that corresponds to a reaction limited by a three dimensional geometric contraction process. Mg17Al12 and Fe favor hydrogen production by acting as micro-galvanic cathodes during the reaction.
Journal of Applied Crystallography, 2020
High-Mn steels attract attention because of their various technological properties. These are mai... more High-Mn steels attract attention because of their various technological properties. These are mainly mechanical and functional, such as the shape-memory effect, high damping capacity, high strength with simultaneous large ductility, the TRIP/TWIP (transformation- and twinning-induced plasticity) effect, low cycle fatigue and high work hardening capacity. All these phenomena are associated with the face-centered cubic (f.c.c.)–hexagonal close-packed (h.c.p.) martensitic transformation which takes place in these alloys. During this phase transition defects are introduced, mainly due to the large volume change between austenite and martensite. Knowing this volume change is key to understanding the mechanical behavior of these metallic systems. In the present article, a full-pattern refinement method is presented. The proposed method uses data obtained by means of conventional X-ray diffraction from regular bulk samples and allows a high-precision calculation of the lattice parameters o...
International Journal of Hydrogen Energy, 2019
The hydrogen absorption and desorption properties of a MgH 2 e 1 mol.% Nb(V) ethoxide mixture are... more The hydrogen absorption and desorption properties of a MgH 2 e 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH 2. Nb ethoxide reacts with MgH 2 during heating, releasing C 2 H 6 and H 2 , and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH 2 without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H 2 to 94 kJ/mol H 2 , and from 176 kJ/mol H 2 to 75 kJ/ mol H 2 , respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 C. The kinetic improvement is attributed to the fine distribution of amorphous/ nanometric NbH x achieved by the dispersion of the liquid additive.
International Journal of Hydrogen Energy, 2018
This paper presents a comparative study of H 2 absorption and desorption in MgH 2 milled with NbF... more This paper presents a comparative study of H 2 absorption and desorption in MgH 2 milled with NbF 5 or NbH 0.9. The addition of NbF 5 or NbH 0.9 greatly improves hydriding and dehydriding kinetics. After 80 h of milling the mixture of MgH 2 with 7 mol.% of NbF 5 absorbs 60% of its hydrogen capacity at 250 C in 30 s, whereas the mixture with 7 mol.% of NbH 0.9 takes up 48%, and MgH 2 milled without additive only absorbs 2%. At the same temperature, hydrogen desorption in the mixture with NbF 5 finishes in 10 min, whereas the mixture with NbH 0.9 only desorbs 50% of its hydrogen content, and MgH 2 without additive practically does not releases hydrogen. The kinetic improvement is attributed to NbH 0.9 , a phase observed in the hydrogen cycled MgH 2 þ NbF 5 and MgH 2 þ NbH 0.9 materials, either hydrided or dehydrided. The better kinetic performance of the NbF 5-added material is attributed to the combination of smaller size and enhanced distribution of NbH 0.9 with more favorable microstructural characteristics. The addition of NbF 5 also produces the formation of Mg(H x F 1-x) 2 solid solutions that limit the practically achievable hydrogen storage capacity of the material. These undesired effects are discussed.
Journal of Solid State Chemistry, 2018
Abstract The structure of κ-Ag 2 Mg 5 has been refined based on X-ray powder diffraction measurem... more Abstract The structure of κ-Ag 2 Mg 5 has been refined based on X-ray powder diffraction measurements (R wp = 0.083). The compound has been prepared by combining mechanical alloying techniques and thermal treatments. The intermetallic presents the prototypical structure of Co 2 Al 5 , an hexagonal crystal with the symmetries of space group P6 3 /mmc , and belongs to the family of kappa-phase structure compounds. The unit cell dimensions are a=8.630(1) A and c=8.914(1) A. Five crystallographically independent sites are occupied, Wyckoff positions 12 k , 6 h and 2 a are filled with Mg, another 6 h site is occupied with Ag, and the 2 c site presents mixed Ag/Mg occupancy. The crystal chemistry of the structure and bonding are briefly discussed in the paper.
Journal of Alloys and Compounds, 2017
Mg(H x F 1Àx) 2 solid solutions have been synthesized by mechanical milling MgF 2 and MgH 2 under... more Mg(H x F 1Àx) 2 solid solutions have been synthesized by mechanical milling MgF 2 and MgH 2 under H 2 atmosphere. Complete solubility has been observed at 475 C. The solid solution presents a rutile-type structure with the anion site randomly occupied by H À or F À and with lattice parameters showing positive deviations from Vegard's law. The solution can be decomposed and reformed reversibly by changing H 2 pressure. For example, Mg(H 0.9 F 0.1) 2 reversibly stores 5.5 wt. % H 2 in less than 3 min at 440 C. During decomposition H 2 is released, and Mg and a solution rich in F appear as products. This process takes place in two clearly marked stages controlled by F À concentration in the material. The decomposition enthalpy of the first one is close to that of MgH 2 for Mg(H 0.9 F 0.1) 2 , and decreases with the H content in the solution.
International Journal of Hydrogen Energy, 2015
ABSTRACT A mixture of MgH2 and (7 mol%) NbF5 has been mechanically milled under Ar atmosphere. Th... more ABSTRACT A mixture of MgH2 and (7 mol%) NbF5 has been mechanically milled under Ar atmosphere. The evolution of the materials has been studied by in situ pressure monitoring, XRD, DSC, TG, TPD, SEM, and isothermal hydrogen absorption and desorption in a volumetric device. During milling, H-rich and F-rich solid solutions MgH2−xFx and MgHyF2−y are produced. After 40 h of milling both solutions merge into a single one with formula MgH1.60F0.40. This solid solution is stable under a thermal treatment of 90 h at 300 °C under 6000 kPa of H2. Hydriding and dehydriding kinetics in the as-milled and cycled materials are considerably faster than in MgH2 milled without additive. Desorption temperature in DSC or TG is lowered 100 °C, and the material modified with NbF5 can be hydrided in less than 4 min at T = 250 °C. A H-rich solution is formed upon rehydriding the material, showing the reversibility of the process. The kinetic improvement seems to be due to a cooperative effect between MgHyF2−y and niobium hydride, the former providing seeding crystals for MgH2 nucleation, and the latter working as a gateway for hydrogen transfer.
Journal of Alloys and Compounds, 2014
ABSTRACT We analyze hydrogen absorption and desorption in Mg and Mg-Ag compounds prepared by mech... more ABSTRACT We analyze hydrogen absorption and desorption in Mg and Mg-Ag compounds prepared by mechanical milling. The materials were obtained by processing mixtures of Mg or MgH2 with different proportions of Ag under Ar or H2 atmosphere. We observe that Ag and AgMg destabilize MgH2. In the first case the products of the reaction of Ag and MgH2 are AgMg and H2, in the second case MgH2 reacts with AgMg to give AgMg4 and H2. The first reaction was only observed in the forward direction, whereas the second one was registered in both directions. The impossibility to reverse the first reaction was attributed to poor kinetics and experimental limitations. The reversible destabilization induced by AgMg is low, MgH2 overall dehydriding enthalpy is reduced from 76.5 kJ/mol H2 to 69.1 kJ/mol H2. This last value allowed us to estimate the formation enthalpy of AgMg4 at -59.5 kJ/mol. Besides destabilization, a slight improvement in hydrogen absorption kinetics and a more noticeable enhancement in desorption kinetics has been observed in samples containing Ag as an additive. During thermal desorption, the dehydriding temperature decreases by 20°C. In isothermal desorption experiments at 325°C, the characteristic induction period of MgH2 without additive practically disappears, and the time to reach 50% of conversion decreases from 17 min to 5.4 min when silver is present. The changes in desorption kinetics could be attributed to AgMg or the interfaces MgH2/AgMg acting as nucleation sites for Mg.
Review of Scientific Instruments, 2000
We present a new experimental setup designed to perform measurements of thermal desorption spectr... more We present a new experimental setup designed to perform measurements of thermal desorption spectroscopy of hydrogen released from metallic samples. The distinctive features of the proposed arrangement are a compact reactor and the use of a mass flow meter to measure the flux of gas desorbed from the sample. A complete set of hydrogen desorption spectra from Pd samples has been recorded to test the equipment. The spectra show very good reproducibility, a high signal-to-noise ratio, and agree very well with results reported in the literature obtained using more sophisticated equipment. These features encourage the use of this low cost setup to perform a fast and reliable characterization of the processes controling hydrogen desorption from metallic compounds.
Review of Scientific Instruments, 2009
Hydrogen storage materials can form more than one hydride phase. These different phases, in turn,... more Hydrogen storage materials can form more than one hydride phase. These different phases, in turn, display different hydrogen absorption/desorption capacities, kinetics, and stabilities. Studies aimed at characterizing and improving these materials usually need to correlate hydrogen intake with the precise determination of the hydride phase involved in the process. Here, we present a device designed to perform measurements of well known volumetric techniques with simultaneous x-ray diffraction on the material under study. The compact design can stand up to 6000 kPa of internal pressure while the sample can be heated up to 450 °C. The design process was assisted by finite element modeling and by the use of mock-up prototypes in order to optimize the thermal and under load behaviors. We provide two examples of use for this new device: (1) hydride decomposition in LaNi5 at 115 °C and (2) formation of MgCo during the programmed thermal desorption of the Mg2CoH5 hydride.
Journal of Solid State Chemistry, 2007
Hydrogen tungsten bronzes have been synthesized by reactive mechanical alloying monoclinic tungst... more Hydrogen tungsten bronzes have been synthesized by reactive mechanical alloying monoclinic tungsten (VI) oxide under hydrogen atmosphere. Two milling devices with different energy ranges were used. Regardless of the distinct reaction times, a similar phase evolution was observed with both apparatus. The characterization of the materials was performed by XRD, SEM, DSC and total hydrogen content determination. The final product obtained was a mixture of tetragonal H 0.33 WO 3 and H 0.23 WO 3 bronzes.
The Journal of Chemical Physics, 1998
In this work we analyze Thermal Desorption Spectroscopy (TDS) experiments of gases considering si... more In this work we analyze Thermal Desorption Spectroscopy (TDS) experiments of gases considering simultaneously atomic diffusion in the bulk and recombinative reaction on the surface of a solid sample. We perform computer simulations in a three-dimensional cubic lattice, and compare the simulated spectra with numerical results from a simple reaction-diffusion model, and analytical results from models for the fast diffusion and fast reaction regimes. We analyze the effects on the spectra of each relevant parameter: diffusivity, reaction coefficient, heating rate, initial concentration of particles and sample size. We also develop a rate-limiting step identification procedure based on the analysis of peak position as a function of initial concentration. This last procedure applied to the simulations and to experimental TDS spectra produces very good results.
Journal of Alloys and Compounds, 2010
ABSTRACT Recently, we have reported the formation of hydrogen tungsten bronzes by reactive mechan... more ABSTRACT Recently, we have reported the formation of hydrogen tungsten bronzes by reactive mechanical milling monoclinic tungsten (VI) oxide under hydrogen atmosphere. In this work we report the milling of hexagonal WO3 under H2 atmosphere. Our main results are the structural transformation of the hexagonal oxide to the high temperature polymorph of WO3 with orthorhombic structure, and the formation of different hydrogen tungsten bronzes at distinct milling times. The bronzes seem to be formed from the orthorhombic oxide, and compared with the bronzes obtained by milling monoclinic WO3 are rather unstable after short exposure to air. The materials are characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and total hydrogen content determination.
Journal of Alloys and Compounds, 2002
We present a theoretical study of thermal desorption spectroscopy applied to the characterization... more We present a theoretical study of thermal desorption spectroscopy applied to the characterization of hydrogen desorption kinetics from hydride forming materials. We propose a model that considers bulk and surface processes during desorption in the solid solution1hydride field of a metal-hydrogen system. We consider as possible rate limiting steps: diffusion, phase transformation, bulk to surface passage and two-atom recombination on the surface of the sample.
Journal of Alloys and Compounds, 2002
We studied the influence of Ge on hydrogen desorption from MgH produced by mechanical alloying at... more We studied the influence of Ge on hydrogen desorption from MgH produced by mechanical alloying at room temperature under a 2 hydrogen atmosphere. The structural and morphological properties, and the desorption kinetics of the products were examined by X-ray diffraction, differential scanning calorimetry, thermal desorption spectroscopy and scanning electron microscopy. The mechanical milling of Mg-Ge mixtures under hydrogen leads to the formation of Mg Ge and MgH. The presence of Ge decreases the hydride 2 2 decomposition temperature in a range from 50 to 1508C, depending on the Ge amount. On the contrary, Mg Ge does not show any effect 2 on hydrogen desorption.
Journal of Alloys and Compounds, 2004
The influence of the mechanical grinding (MG) (under reactive atmosphere of hydrogen) conditions ... more The influence of the mechanical grinding (MG) (under reactive atmosphere of hydrogen) conditions on the physico-chemical properties (crystallography, phase composition, morphology and hydrogen sorption properties) of a Mg + 5% Wt.% Cr 2 O 3 mixture are examined. Both a planetary and a vibratory ball miller have been used. In the case of the planetary ball miller, different rotation speeds (200, 250 and 300 rpm), and ball to powder mass ratios were investigated. It appears that the planetary ball miller allows to obtain finer particles and also more amorphous materials. The relationship between crystallinity, morphology and injected power was established.
Journal of Alloys and Compounds, 2005
Equilibria E 4000 Formation, Composition and Stability of Mg-Co Compounds.-As revealed by XRD, DS... more Equilibria E 4000 Formation, Composition and Stability of Mg-Co Compounds.-As revealed by XRD, DSC, and different synthesis methods such as β-Mg2CoH5 dehydration, mechanical alloying of Mg and Co under Ar, mechanical alloying followed by heat treatment, and Mg-Co solid state reaction, only one compound with more than 40 at% Mg exists in the Mg-Co system. MgCo is a stable compound, whose formation process can be greatly influenced by kinetic factors that may suppress its presence in favor of MgCo 2 .-(GENNARI, F. C.; CASTRO*, F.
Journal of Alloys and Compounds, 2002
Mechanochemistry Mechanochemistry H 3000 Synthesis of Mg 2 FeH 6 by Reactive Mechanical Alloying:... more Mechanochemistry Mechanochemistry H 3000 Synthesis of Mg 2 FeH 6 by Reactive Mechanical Alloying: Formation and Decomposition Properties.-The title compound, an attractive material for hydrogen storage, is synthesized by reactive mechanical alloying from Mg and Fe elemental powders in a H 2 atmosphere at room temperature. The samples are characterized by powder XRD, DSC, and SEM. The formation of the title compound involves two steps: MgH 2 formation at shorter milling times, and reaction between MgH 2 and Fe to produce Mg 2 FeH 6 as milling time increases. On the contrary, the decomposition of the title compound occurs in only one step, giving the elemental metals and hydrogen gas as reaction products.-(GENNARI*, F. C.; CASTRO, F.
Journal of Alloys and Compounds, 2004
The hydrogen sorption properties of a mixture of Mg + 10 wt.% WO 3 made by reactive mechanical gr... more The hydrogen sorption properties of a mixture of Mg + 10 wt.% WO 3 made by reactive mechanical grinding at room temperature under 1100 kPa of H 2 were studied and compared with those of elemental Mg subjected to a similar preparation procedure. The milling products were also characterized by XRD, EPMA, and granulometric measurements. It was observed that WO 3 has an important catalytic effect on hydrogen absorption and desorption, more than doubling the rates observed in the samples milled without the additive.