Search

We study the process of hydrogen release from magnesium hydride under the conditions of uniaxial pressing in a vacuum at room temperature up to pressures of about 2.4 ton/cm2. It is shown that the desorption of hydrogen occurs under loading. The amount of hydrogen almost linearly depends on the applied pressure. The measured coefficient is 5 ·10– 3 wt.%· cm2 /ton.

Speed-up of thermal decomposition of magnesium hydride due to uniaxial pressing, including pressing in presence of nickel catalyst, was studied by barometry, SEM, DSC, XRD methods, and using mathematical modeling. Pressing even with no catalyst is shown to hasten the hydrogen desorption. The most probable reason for this is the formation of multiple defects in crystal lattice. They can serve as nucleation centers: metal nuclei significantly hasten hydrogen desorption. Besides, metal magnesium not converted to the hydride during the synthesis stage can possibly appear at surface. Adding nickel powder before pressing hastens dehydriding process even more. Comparing hydrogen evolution curves for samples with different amount of nickel allowed to propose the probable mechanism of hydrogen evolution. It is based on description of reactions of hydrogen desorption and hydride decomposition and of the metal-hydride phase morphology change due to these reactions. We develop a mathematical model in form of ordinary differential equations that fits the experimental data well. The model is based on conservation and symmetry assumptions. The fitting allowed to evaluate rate parameters of hydride phase decomposition.

We have studied the activation of magnesium hydride decomposition by means of its pressing with a catalyst. It is established that pressing leads to the formation of metal nuclei, which favor a decrease in the temperature threshold of magnesium hydride decomposition. The introduction of catalytic additives also reduces the temperature of dehydrogenation. The most effective in this respect was found to be the addition of nickel powder.

The synthesis procedure of a thin aluminum-hydrogen film on a silicon substrate is described and its result thoroughly investigated by a number of experimental methods. The reactive sputtering deposition was carried out to obtain a structure containing aluminum hydride. The resulting film has characteristic non-metallic properties, though according to thermal desorption studies its hydrogen content is close to AlH1.1, which is lower than stoichiometric aluminum hydride AlH3. Thermal desorption of hydrogen differs significantly from that of AlH3 powder as it has not one but several peaks. According to transmission electron microscopy the film is mostly amorphous but contains crystalline phase. Our interpretation of the experimental data suggests that some hydride phase microcrystals were formed as the film was deposited, but most hydrogen was stored inside the film without forming a crystalline structure in both bounded and unbounded states. The luminescent properties of the synthesized film are similar to aluminum hydride, and it can be concluded that the amorphous Al-H structure of the synthesized film shows resemblance with AlH3 crystals populated with hydrogen vacancies.

We studied cathodoluminescence and photoluminescence of α-AlH3 – a likely candidate for use as possible hydrogen carrier in hydrogen-fueled vehicles. Luminescence properties of original α-AlH3 and α-AlH3 irradiated with ultraviolet were compared. The latter procedure leads to activation of thermal decomposition of α-AlH3 and thus has a practical implementation. We showed that the original and UV-modified aluminum hydride contain luminescence centers ‐ structural defects of the same type, presumably hydrogen vacancies, characterized by a single set of characteristic bands of radiation. The observed luminescence is the result of radiative intracenter relaxation of the luminescence center (hydrogen vacancy) excited by electrons or photons, and its intensity is defined by the concentration of vacancies, and the area of their possible excitation. UV-activation of the dehydrogenation process of aluminum hydride leads to changes in the spatial distribution of the luminescence centers. For short times of exposure their concentration increases mainly in the surface regions of the crystals. At high exposures, this process extends to the bulk of the aluminum hydride and ends with a decrease in concentration of luminescence centers in the surface region.

We investigated the mechanism of activation of α-AlH 3 powder decomposition by irradiation with ultraviolet (UV) light using barometry, photoluminescence (PL), cathode luminescence (CL) and X-ray diffraction (XRD) methods. Exposing the samples under a mercury lamp through the filters we have shown that only the line, corresponding to the energy of 4.88 eV, leads to activation, whereas the spectral lines of lower energy are ineffective. XRD shows that after continued UV activation the aluminum clusters with metallic properties are formed on the surface of the AlH 3 powder particles at room temperature. The analysis of results of performed calculations of the absorption spectrum of the perfect crystal α-AlH 3 using DFT method showed that hydrogen vacancies are involved in activation by UV light. Studying of dynamics of CL and PL spectra at different UV expositions permitted to suggest the following mechanism of activation. Absorption of a photon implies that vacancy captures an electron from a neighboring hydrogen atom. Hydrogen atom then can leave its regular position in the lattice and form a new vacancy next to the first. Clustering of vacancies occurs. Continued UV activation is accompanied by merging of vacancies, which deprives the aluminum atoms of hydrogen bonds connecting them. Thus the aluminum clusters with metallic properties are formed.

We investigated activation of thermal dehydriding of α-AlH3 crystal by preliminary irradiation by ultraviolet light using thermal desorption spectroscopy, barometry, and cathode luminescence methods. It is shown that hydrogen vacancies appear due to irradiation; they serve as points where metal nuclei probably appear, so dehydriding becomes significantly faster. Possible explanation of transformation of hydrogen vacancies to metal phase nuclei is suggested: new vacancies are more likely to appear near the first one compared to remote places. Using density functional theory method we calculated the electronic structure of stoichiometric α-AlH3 and α-AlH3 with a hydrogen atom removed from a regular lattice site with a vacancy in place of it. It is suggested that an appearance of a new vacancy near the first vacancy needs less energy compared to the first one. From cathode luminescence data we see that appearance of vacancies can also be activated thermally. The model of hydrogen desorption from α-AlH3 activated by UV light is suggested and kinetic parameters of desorption are evaluated.

Processes occurring in the phase of AlH3 dehydrogenation incubation that precedes the active decomposition of the hydride and is evidently accompanied by a change in its material properties are investigated by thermal desorption spectroscopy and barometry. The electronic structures of α-AlH3 and α-AlH3:V(H0) (i.e., aluminum hydride with a neutral hydrogen atom removed) are calculated by the density functional method. It is shown that hydrogen vacancies are the source of nuclei for the metallic phase, and their emergence could be thermally activated. It is established that UV irradiation also leads to the formation of hydrogen vacancies in α-AlH3. A description of the probable mechanism for the accumulation of hydrogen vacancies at elevated temperatures and finally to the appearance of metallic phase nuclei is offered. It is shown that UV irradiation allows us to lower the temperature of the dehydrogenation of α-AlH3 crystals.

Process of the solid-phase diffusion of manganese into single crystals ZnGeP2 are investigated by electron paramagnetic resonance and X-ray element microanalysis. Conditions for formation of a solid solution with Mn2+ concentration of 1018 cm-3 were found. A diffusion coefficient of manganese in single crystal of ZnGeP2 at a diffusion annealing temperature was determined.