Thermodesorption Spectroscopy Research Articles

Study of helium diffusion in yttria: A multiscale approach based on the density functional theory and kinetic Monte Carlo, with transmission electron microscopy and thermo-desorption spectroscopy

A known issue for future nuclear reactors is helium accumulation inside the steel structure materials, responsible for structural issues such as embrittlement and cracking. One possible solution is using new types of reinforced steel, such as oxide dispersion strengthened (ODS) steel. It consists of adding oxide nanoparticles to the Fe-based material, especially yttrium oxide (yttria, Y2O3), improving its properties. Therefore, one first step is understanding the helium diffusion inside this system. Very little is known about helium inside yttria, with most studies being theoretical ones. Based on this context, this work proposes a combined theoretical and experimental multiscale approach to investigate helium diffusion inside yttria.The theoretical approach starts with the density functional theory, used to model the atomic yttria cell and determine helium insertion sites. The transitions between the sites were described using the Nudged Elastic Bond (NEB) method. Kinetic Monte Carlo was then employed to obtain the interstitial diffusion coefficient expression for the first time for this material. It showed a limited diffusion at temperatures below 600 K, which may indicate a tendency for He to be blocked in the oxide. Then, the charged vacancies were explored. It showed that the vacancy further reduces helium diffusion. Finally, it was demonstrated that helium spreads across different vacancies and interstitial sites.The experimental part involved implanting helium ions at 50 keV in samples with nanometric or micrometric grains. Then, the specimens were characterised with transmission electron microscopy (TEM) and thermo-desorption spectroscopy (TDS) techniques. TEM did not evidence detectable bubbles even at the highest studied fluence (1 × 1016 cm−2). The TDS highlighted different mechanisms for helium diffusion and the grain size's role, providing a novel model for diffusion coefficient calculation based on interstitial diffusion.

Understanding helium diffusion in iron: a multiscale modelling method based on the DFT and kinetic Monte Carlo, coupled to TEM and thermo-desorption spectroscopy experiments

Helium (He) is known to accumulate in vacancies of the steel structure materials of nuclear reactors, forming bubbles that increase in size and eventually cause structural problems, such as embrittlement. Several studies provided insights into the interstitial diffusion of this gas inside the iron matrix. However, much is still unknown about its behaviour. Based on this context, this study proposes a new combined theoretical and experimental multiscale approach made by the same team in a complementary way. It helps to have a global view of the helium diffusion in iron as a function of He content, temperature and structural defect concentration.Firstly, density functional theory has been used to identify the possible helium insertion sites in iron and the possible transitions between them. Then, the kinetic Monte Carlo method was applied to calculate its interstitial diffusion coefficient. It showed that helium swiftly diffuses interstitially in iron. It also tends to be trapped in iron vacancies and accumulate. The chromium and vacancy effects were also investigated.Experimentally, He was ion-implanted in several pure iron and high-purity Fe10wt%Cr samples. An analysis of He behaviour and its diffusion was carried out with transmission electron microscopy (TEM) and thermo-desorption spectroscopy (TDS) techniques. TEM shows the presence of small (∼0.7 nm radius) He bubbles and no significant difference with the presence of Cr. TDS evidences sequential “burst-type” He releases as T is sequentially increased. Based on these observations and inferred from the literature, we propose that He is detrapped and released from the studied materials as a consequence of the decrease in maximum He capacity of the trapping aggregates as T increases.

Effect of Ti additive and preparation procedure on the temperature and decomposition kinetics of the MgH2 phase in the synthesized mechanicals alloys

The mechanical alloy Mg + 10 wt.% Ti and two mechanical alloys MgH2 + 10 wt.% Ti were synthesized by a reactive mechanical alloying (RMA) method, but in different ways. The influence of doping and the preparation procedure of mechanical alloys (MAs) on thehydrogen sorption properties, temperature and decomposition kinetics of their MgH2 phase was investigated by isobaric thermodesorption spectroscopy at a constant hydrogen pressure of 0.1 MPa. The desorption kinetics of the milled MgH2 + 10 wt.% Ti are faster than in the case of the ball milled Mg + 10 wt.% Ti. It was found that the manner of obtaining a mechanical alloys by reactive milling of MgH2 +10 wt. % Ti powder mixture rather than Mg +10 wt. % Ti powder mixture to improve the kinetic characteristics. The role of Ti as an alloying element in improving the hydrogen desorption kinetics of МAs obtained by various preparation procedures was studied. The stabilizing effect of Ti on the nanocrystalline structure and growth of the crystallites (grains) of the MgH2 phase during the cycling was also evaluated. It has been established the practical absence of the influence of Ti additive and the manner of obtaining MAs on the its thermodynamic stability.

Structural characterization and hydrogen storage properties of the Ti31V26Nb26Zr12M5 (M = Fe, Co, or Ni) multi-phase multicomponent alloys

Structure and hydrogen storage properties of three Ti31V26Nb26Zr12M5 multicomponent alloys with M = Fe, Co and Ni are investigated. The alloys synthesized by arc melting are characterized via X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The as-cast ingots present multi-phase dendritic structures composed mainly of BCC phases and small amounts of C14 Laves phases. Upon hydrogenation, each alloy absorbs around 1.9 H/M (number of hydrogen atoms per metal atoms) at room temperature. XRD of fully hydrogenated samples shows the formation of multi-phase structures composed of FCC and C14 hydrides. Thermo Desorption Spectroscopy (TDS) shows that the hydrogenated alloys present multi-step desorption processes with wide temperature ranges and low onset temperatures. XRD of partially hydrogenated samples indicate the presence of intermediate BCC hydrides. XRD of desorbed samples suggest reversible reactions of absorption/desorption: BCC + C14 alloy ↔ intermediate BCC hydride + C14 hydride ↔ FCC + C14 hydrides.

Фотокаталитическая реакция NO+COhν(CO-=SUB=-2-=/SUB=-)-=SUB=-ads-=/SUB=-+1/2N-=SUB=-2-=/SUB=-, активируемая на ZnO/ZnO-=SUB=-1-x-=/SUB=-/O-=SUP=---=/SUP=- при возбуждении экситонного резонанса

The exciton channel of ZnO photoactivation was used for the first time to carry out the environmentally important photocatalytic reaction NO+CO+hν→(CO2)ads+1/2N2 (1) on ZnO. When the excitation energy is delivered to the surface by an electrically neutral exciton, there are no losses in recombination and in overcoming the potential surface barrier, which are the main losses in the transfer of photo-generated e-/h+ pairs. To suppress the undesirable radiative decay of an exciton, the ZnO/ZnO(1-x)/O- 2D structure was created on the ZnO surface, in which the exciton decays nonradiatively into a pair of long-lived local states of an electron and a hole, on which the reactionary acts occur. For the redox reaction (1), the obtained values of the quantum yield Y (the number of reaction acts per one absorbed quantum) and the efficiency E (the number of acts per one incident quantum) upon exciton excitation at λ = 382 nm are 5–7 times greater than when excited in the interband absorption region at λ = 365 nm. The methods of mass-spectrometry (MS) and thermodesorption spectroscopy (TDS) were used to study the initial, intermediate and final products of reaction (1) in the gas and adsorbed phases, and the features of the influence of the initial products NO and CO on each other during the interaction of their mixture with ZnO in the dark and under irradiation at λ = 382 nm.

Photocatalytic Reaction NO+COhν(CO-=SUB=-2-=/SUB=-)-=SUB=-ads-=/SUB=-+1/2N-=SUB=-2-=/SUB=-, activated on ZnO/ZnO-=SUB=-1-x-=/SUB=-/O-=SUP=---=/SUP=- under exciton resonance excitation

The exciton channel of ZnO photoactivation was used for the first time to carry out the environmentally important photocatalytic reaction NO+COhν(CO2)ads+1/2N2 on ZnO. When the excitation energy is delivered to the surface by an electrically neutral exciton, there are no losses in recombination and in overcoming the potential surface barrier, which are the main losses in the transfer of photo-generated e-/h+ pairs. To suppress the undesirable radiative decay of an exciton, the ZnO/ZnO1-x/O- 2D structure was created on the ZnO surface, in which the exciton decays nonradiatively into a pair of long-lived local states of an electron and a hole, on which the reactionary acts occur. For the redox reaction (1), the obtained values of the quantum yield Y (the number of reaction acts per one absorbed quantum) and the efficiency E (the number of acts per one incident quantum) upon exciton excitation at λ=382 nm are 5-7 times greater than when excited in the interband absorption region at λ=365 nm. The methods of mass-spectrometry (MS) and thermodesorption spectroscopy (TDS) were used to study the initial, intermediate and final products of reaction (1) in the gas and adsorbed phases, and the features of the influence of the initial products NO and CO on each other during the interaction of their mixture with ZnO in the dark and under irradiation at λ=382 nm. Keywords: photocatalysis, exciton, 2D structure ZnO/ZnO1-x/O-, long-lived active centers, redox reaction, air purification.

Design of TiVNb-(Cr, Ni or Co) multicomponent alloys with the same valence electron concentration for hydrogen storage

Recently, it has been hypothesized that the hydrogen sorption properties of body-centered cubic (BCC) multicomponent alloys might be related to the alloy’s valence electron concentration (VEC). In this work, we employed CALPHAD method to design three BCC alloys with the same VEC to evaluate this hypothesis. The (TiVNb)85Cr15, (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 multicomponent alloys with VEC = 4.87 were produced by arc melting. Although some amount of segregation could not be avoided, the as-cast microstructures were composed of a BCC phase with highly homogeneous chemical composition. The alloys quickly absorb hydrogen at room temperature and formed FCC hydrides with high capacities of 2 H/M (3.1–3.2 wt%). For the three alloys, it was observed a two-step hydrogenation sequence indicating the formation of an intermediate BCC monohydride prior to the formation of the FCC dihydride. Moreover, the three alloys presented very similar values of enthalpy and entropy of hydrogenation, showing that the initial hypothesis seems to hold for the thermodynamic properties. The alloys hydrogenation/dehydrogenation cycling behavior were also evaluated. (TiVNb)85Cr15 had a small and continuous drop in capacity over cycling that was addressed to incomplete transformation of the BCC monohydride to the FCC dihydride, whereas (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 showed a more stable behavior achieving a reversible capacity of 1.76 H/M (2.77 wt%) after 20 cycles. The desorption is strongly affected by absorption/desorption cycling for all alloys, as revealed by thermo desorption spectroscopy (TDS). The onset temperature of desorption depends on the alloys’ composition as well as the number of hydrogenation/dehydrogenation cycles. The results suggest that cycling and desorption properties of the alloys are less related to their VEC.

ВПЛИВ Fe ТА Y НА ВОДЕНЬСОРБЦІЙНІ ВЛАСТИВОСТІ, ТЕРМІЧНУ СТІЙКІСТЬ ТА КІНЕТИКУ ДЕСОРБЦІЇ ВОДНЮ З ГІДРИДНОЇ ФАЗИ MgH2 МЕХАНІЧНОГО СПЛАВУ Mg + 10%ваг.Fe + 5% ваг.Y

Ефективний і безпечний спосіб зберігання водню - це його хімічне зв'язування в металогідридах. Незважаючи на те, що вчені приділяють велику увагу гідриду магнію, він ще не знайшов широке застосування в якості акумулятора водню для автомобільної промисловості через високу температуру (300 °C при 0,1 МПа H2) і повільну кінетику дисоціації. У даній роботі досліджена можливість зниження температури, поліпшення кінетики розкладу стехіометричного гідриду MgH2 за рахунок його комплексного легування Fe і Y з застосуванням методу реактивного механохімічного сплавлення (РМС). Були синтезовані механічні сплави Mg + 10% ваг. Fe + 5% ваг. Y (МС1) і Mg + 10% ваг. Fe (МС2) і досліджено їх фазовий склад, мікроструктуру, водородосорбційні властивості, термічну стабільність і кінетику десорбції водню з використанням методів рентгенівської дифракції (XRD), скануючої електронної мікроскопії (SEM) і термодесорбційної спектроскопії (TDS). Для оцінки впливу комплексного легування на температуру розкладу і термостабільність фази MgH2 отримані ізобари десорбції водню при першому нагріванні після синтезу РМС зразків механічних сплавів і після подальшого їх циклічного гідрування з газової фази. Всі ізобари отримані при тиску водню в реакторі 0,1 МПа і швидкості нагріву зразка 30/ хв. і використані для визначення як температури початку десорбції водню (Тпоч.) з гідридної фази MgH2 МС, так і температури Тмакс., що відповідає максимальній швидкості виділення водню. Кінетичні криві десорбції водню з механічних сплавів-композитів отримані при постійному тиску водню 0,1 МПа в реакторі і температурах 310 і 330 °С і були використані для визначення як часу виділення половини кількості водню (τ1/2), так і загальної кількості виходу водню (τп) з МС. Встановлено, що додавання Fe і Y до магнію призводить до значного поліпшення кінетики десорбції водню з гидридної фази MgH2, про що свідчить значне зменшення (в 15 і 6 разів) часу виділення з неї половини і всього водню при 330 0С. Розроблені матеріали можуть знайти практичне використання в стаціонарних умовах їх застосування.Бібл. 48, табл.2, рис. 9.

Effect of aging treatment on apparent hydrogen solubility and trapping in a new generation maraging steel

New generation maraging steel developed by the aeronautical industry meet the highest mechanical standards thanks to the controlled growth of M2C carbides and B2-NiAl precipitates during aging treatment at around 500 °C. However, the literature has reported a low resistance of precipitation-hardened steel to hydrogen embrittlement that is associated with aging treatment and trapping at precipitates or reverted austenite. Thus, in relation to the complex microstructure of X23NiCoCr 13-6-3 maraging steel, hydrogen solubility and trapping was investigated using Thermodesorption Spectroscopy. A high apparent solubility of hydrogen was observed after aging that was attributed to reversible trapping at coherent B2-NiAl precipitates.

Experimental determination of intragranular helium diffusion rates in boron carbide (B4C)

Boron carbide, B4C, is widely used as a neutron absorber in nuclear reactors. Since neutron absorption by 10B leads to 4He production, it appears necessary to study He behaviour and its possible effects on the B4C ceramic. In this study, the diffusion characteristics of ion-implanted He in B4C (500 keV, fluences from 1 x 10+13 to 2 x 10+15 He.cm−2) were investigated mainly by Thermo-Desorption Spectroscopy (TDS) from 600 °C up to B4C melting point. The experiments were done on dense B4C samples having large grains (30–60 μm) to render grain boundaries effects on He outward diffusion ineffective and thus to access intragranular He diffusion kinetics. From controlled temperature ramp experiments, it was notably observed that He release was realized in two main stages. A first He population was able to exit the material at moderate temperatures by interstitial diffusion. Then a second population was quantitatively released only over 1150 °C. This was attributed to He atoms that, in their initial interstitial diffusion course at moderate temperatures from their implantation sites, got trapped in defect aggregates and/or He bubbles. As the nucleations of both these traps are expected to be related to helium and irradiation defect concentrations, the ratio of the two He populations was indeed found to be correlated with the implantation fluence. From the obtained He release curves, the apparent activation energies (Ea) of He intragranular diffusion in B4C was determined (2.6–3.1 eV) in the 800–1100 °C temperature range. This value appears slightly higher than the ones determined at lower temperatures, hinting that a change in diffusion mechanism may occur around 800 °C. The apparent Ea of He detrapping from He bubbles (∼2.5 eV) and from defect aggregates (∼4 eV) were also determined for temperatures within 1200–1500 °C.

Water Uptake and Transport Properties of La1-xCaxScO3-α Proton-Conducting Oxides.

In this study, oxide materials La1−xCaxScO3−α (x = 0.03, 0.05 and 0.10) were synthesized by the citric-nitrate combustion method. Single-phase solid solutions were obtained in the case of calcium content x = 0.03 and 0.05, whereas a calcium-enriched impurity phase was found at x = 0.10. Water uptake and release were studied by means of thermogravimetric analysis, thermodesorption spectroscopy and dilatometry. It was shown that lower calcium content in the main phase leads to a decrease in the water uptake. Conductivity was measured by four-probe direct current (DC) and two-probe ascension current (AC) methods at different temperatures, pO2 and pH2O. The effects of phase composition, microstructure and defect structure on electrical conductivity, as well as correlation between conductivity and water uptake experiments, were discussed. The contribution of ionic conductivity of La1−xCaxScO3−α rises with decreasing temperature and increasing humidity. The domination of proton conductivity at temperatures below 500 °C under oxidizing and reducing atmospheres is exhibited. Water uptake and release as well as transport properties of La1−xCaxScO3−α are compared with the properties of similar proton electrolytes, La1−xSrxScO3−α, and the possible reasons for their differences were discussed.

Investigation of the local structure of nanosized rhodium hydride

The local structure and the thermal stability of small and well-dispersed RhHx nanoparticles (average size of 1.4 nm) were studied by in situ X-ray Absorption Spectroscopy. The RhHx nanoparticles are stable at room temperature and undergo a structural transition from hydride (fcc) to metal phase (fcc) with a shrinking of the lattice volume due to the desorption of hydrogen. This phase transition occurs in the temperature range of 150–180 °C, in good agreement with the results from thermo-desorption spectroscopy. Above 180 °C, the desorbed nanoparticles undertake important coalescence. In situ transmission electron microscopy performed up to 300 °C proves that this process cannot be only thermal, thus it may be ascribed to a X-ray beam effect.

Deuterium atom loading of self-damaged tungsten at different sample temperatures

The influence of surface parameters on hydrogen isotope atom absorption into tungsten material was studied. For this purpose a series of experiments was performed, exposing tungsten pre-damaged by tungsten ions, the so-called self-damaged W, to low energy deuterium atoms with the flux density of 4.2×1018D/m2s for 121 h. Exposures were performed at four sample temperatures between 450 K and 600 K. Deuterium concentration was measured in situ and in real time during the exposure by nuclear reaction analysis. After the exposure, thermodesorption spectroscopy was performed on the samples. We have modeled the experimental data using a 1-D rate equation model and determined the detrapping energies and values of modeling parameters, which are describing deuterium atom adsorption on the surface of tungsten and migration of atoms from the surface to the bulk of the material. Assuming two adsorption site types, the determined surface adsorption energies are (0.68±0.02)eV and (0.71±0.02)eV and the activation energies for migration from the surface to the bulk are (1.41±0.02)eV and (1.44±0.02)eV, respectively. Modeling a low temperature plasma loading with the derived parameter set reveals that plasma loading experimental data could not be reproduced. We discuss possible reasons for the discrepancy between atom and plasma loading.

XPS study of influence of exposure to air on thermal stability and kinetics of hydrogen decomposition of MgH2 films obtained by direct hydrogenation from gaseous phase of metallic Mg

Mechanism of influence of exposure to air on thermal stability of MgH2 obtained by direct hydrogenation from the gas phase, the nature of the hydride sensitivity to the negative impact of air and the role of its surface chemical state have not been studied enough. The present article presents data of X-ray photoelectron spectroscopy (XPS) measurements of the Mg 2s, O 1s, C 1s core-level spectra of surface of hydride MgH2 films derived by gas phase hydrogenation of model samples of metallic Mg, and the evolution of changes in the chemical state of the surface of the hydride films depending on the time of exposure to air and formation conditions (hydrogen pressure and hydrogenation regime). Based on results of XPS, X-ray diffraction (XRD), and thermodesorption spectroscopy (TDS), the existence of a relationship (correlation) between chemical surface condition of hydride MgH2 films obtained at different hydrogen pressures (3.0MPa and 11.5MPa) and their thermal stableness and temperature of the beginning of hydride decomposition has been established.

Influence of Titanium and Iron Additives to Magnesium on Hydrogen-Sorption Properties, Thermal Stability, and Kinetics of Hydrogen Desorption from MgH2 Phase of Mechanical Alloy

We report on studies of the process of cyclic hydriding–dehydriding (eight cycles) of a mechanical alloy synthesized in a ball mill by grinding powder mixture Mg + 10 wt.% TiH2 + 10 wt.% Fe in argon medium. Hydrogen-sorption properties and thermal stability of the mechanical alloy are studied employing thermodesorption spectroscopy at hydrogen pressure of 0.1 MPa. Hydrogen capacity of the mechanical alloy is determined to be 4.9 wt.%, and onset temperature of hydrogen desorption from the alloy under this study equals 220°C. Kinetics of the process of hydrogen desorption from the hydride phases of the mechanical alloy is examined. The present studies have revealed that mechanical treatment of the magnesium powder in Ar medium with both Fe and Ti additives causes a greater effect on improving the kinetics of hydrogen desorption from hydride phase MgH2 of the alloy and decreasing its thermal stability in comparison with that caused by mechanical treatment of the same powder with either Fe or Ti additives in hydrogen medium.

Isotopic transient kinetic study of CO oxidation on Pd(1 1 0)

Thermodesorption spectroscopy (TDS) and temperature-programmed reaction (TPR) using a beam-forming gas doser and oxygen isotopes 18O2 and 16O2 have been applied for studying the titration of adsorbed Оads and subsurface oxygen Оsubs on a reconstructed Pd(110) surface. Subsurface oxygen appears to be non-reactive. However, as the reaction proceeds, it comes onto the surface and interacts with adsorbed CO. Numerical simulations have been applied to investigate the Оsubs→Оads transfer kinetics. On the reconstructed surface the stage Оsubs→Оads is reversible, and its rate is linearly dependent on the concentration of surface sites freed during the reaction Оads+СОads→СО2. The temperature dependencies of direct and reversed reaction rate constants have been determined. As the adsorbed oxygen concentration decreases to some threshold value, a self-accelerated irreversible subsurface oxygen outcome occurs caused by a surface reconstruction to the structure (1×1). The kinetic equation for the process has been suggested. The temperature dependence of the critical surface oxygen coverage initiating the surface reverse reconstruction has been determined.

The Process of Dispersing Bismuth Atoms into Iridium via an Intermediate Monolayer of Graphite

It is first revealed by thermodesorption spectroscopy that when bismuth atoms diffuse the indium atoms through an intermediate graphite layer, they show certain characteristic features. In the Ir (111)-C system at 1400 K δ = 0.4 ± 0.05 and diffusion δBi = Ndif/N = 2.5 × 10-3 bismuth atoms in the Ir(111)-C system. The amount of intercalated and diffused bismuth depends on the electrical field, i.e., on the positive potential in the Ir(111)-C system. It has been noted that a considerable diffusion of bismuth into iridium started at 180 V and increased up to 3000 V. The activation energies for bismuth diffusion into and from iridium were calculated to be En1 = 6.05 ± 0.05 eV and E1n = 6.3 ± 0.1 eV, respectively.

The influence of nitrogen co-deposition in mixed layers on deuterium retention and thermal desorption

The influence of nitrogen co-deposition in ITER relevant mixed layers on retention and release dynamics of deuterium was studied in situ by Nuclear Reaction Analysis (NRA) and thermodesorption spectroscopy. W:Al and W:Be mixed layers deposited by thermionic vacuum arc (TVA) method were used in order to verify the possibility of Al being used as proxy material for Be in experiments regarding D uptake. Samples were exposed to neutral deuterium atom beam with fluence of 3.24 × 1019 D/cm2 (flux 4.5 × 1014 D/cm2s) at 390 K and NRA with 3He ions was used for depth profile analysis of deuterium content after the exposure. For the investigation of deuterium release dynamics the samples were linearly heated to around 1000 K and during this process NRA spectra at single energy were collected every minute. Complementary to NRA a quadrupole mass spectrometer was used, following masses 2, 3 and 4 in the background vacuum. The numeric simulation of deuterium thermal desorption was performed and desorption energies of desorption sites were calculated. Same experiments were performed with mixed material samples produced by TVA in the presence of nitrogen atmosphere. Nitrogen co-deposition in the mixed layers was found to have an important influence on deuterium retention. The concentration of deuterium in the sample increased by a factor of 4.8 in the presence of nitrogen in the W:Al and by a factor of 1.8 in the W:Be layer compared to the nitrogen-free sample.

The effect of Al on thermal stability and kinetics of decomposition of MgH2 prepared by mechanochemical reaction at different conditions

This work is directed at decreasing decomposition temperature of MgH2. In accordance with the theoretical predictions, hydride of solid solution of Al in Mg should decompose at lower temperature compared to that of MgH2, and numerous experiments indicate that thermodynamic stability of the hydride depends on methods and conditions of its synthesis. Therefore, the goal of this article is to study the possibility of obtaining the hydride of solid solution of aluminum in magnesium by different technological approaches. Three mechanical composite alloys derived by the different approaches are studied employing the X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermodesorption spectroscopy (TDS) methods. XRD Rietveld analysis reveals that the hydrogenation of a Mg + 10 wt% Al powder by means of reactive milling in the hydrogen atmosphere as well as hydrogenation of the powder from the gaseous phase after milling the powder for 20 h in the argon atmosphere leads to formation of aluminum-free MgH2 hydride and the Mg17Al12 phase. Our TDS studies (at PH2 = 0.1 MPa) indicate that additives of 10 wt% Al to magnesium do not lead to decreasing the decomposition temperature of MgH2 prepared by mechanochemical reaction at the different conditions.

Influence of Exposure to Air and Mechanical Dispersion upon Thermal Stability and Decomposition Temperature of β-MgH<sub>2</sub> Phase, a Component of Composites Derived by Different Methods

Influence of mechanical dispersion and exposure to air on hydrogen-sorption properties and thermal stability of β-MgH2 synthesized by two different methods, namely through direct hydriding in gaseous medium and by reactive mechanical alloying, is studied employing thermodesorption spectroscopy and X-ray photoelectron spectroscopy. It is established that the temperature of the beginning of desorption of the β-MgH2 phase of the composite synthesized through direct hydriding in gaseous medium depends upon exposure to air after its synthesis. Exposure of the sample to air for 20 min (for more than 5 h) increases the temperature of decomposition of the β-MgH2 phase from 360 to 375 oC (from 360 to 440 oC). Following dispersion of the composite in the ball mill decreases the decomposition temperature of the hydride phase from 440 to 340 oC. Exposure of the dispersed composite to air for 5 h and even for several days does not increase the decomposition temperature of the MgH2 phase. Significant role of the surface state in increase of the decomposition temperature (and thermal stability) due to exposure to air of MgH2 hydride derived by hydriding of magnesium powder in gaseous medium is revealed. It is found that dispersion of the hydride in the ball mill reduces the temperature of decomposition of the MgH2 phase.