The nickel hydrogenation in situ investigation by X-ray diffraction using synchrotron radiation
- Prof. Boris TOLOCHKO
- Mr. Marat SHARAFUTDINOV
- Prof. Boris TOLOCHKO (Institute of solid state chemistry and mechanochemistry)
The process of nickel hydrogenation was studied in situ by X-ray diffraction using synchrotron radiation. It is generally believed that the occurrence of defects in the metal during hydrogenation is due to the fact that hydrogen, meeting pores, cracks and other discontinuities, gathers in them, recombining into a molecular form. As a result of a constant influx of hydrogen, a high pressure is created in the cavities, leading to deformation and local destruction of the lattice of the crystal. Almost simultaneously with the beginning of the change in the X-ray diffraction pattern of nickel, the formation of the metastable phase of nickel hydride begins - reflexes appear from the planes (111), (200), (220), (311), (222), (400). Reflexes of nickel hydride are shifted towards smaller diffraction angles with respect to nickel reflexes, since the lattice parameter of the new phase is 6% larger. At the initial stage of the formation of the new phase, the reflexes of nickel hydride are strongly broadened, which probably indicates the small size of the nuclei of the new phase in the nickel matrix. In the process of hydrogenation, nickel hydride nuclei increase in size, and approach L = 200 Ǻ. At the same time, it was found that at the initial stages of the formation of a new phase, the lattice parameter of the nickel hydride differs from the lattice parameter of the parent metal by only 3%, and not by 6%, as measured after the cathodic hydrogenation process was completed. Analyzing the data on the change in the lattice parameter of nickel and the dynamics of the formation of nickel hydride, it can be concluded that the lattice parameter changes faster than a new phase is formed. This indicates that hydrogen first penetrates into the interstices of the crystal lattice, creating an excessive concentration there (it is known that the concentration of hydrogen in the interstice of the matrix of the metal is linearly related to the change in the lattice parameter), and only then the formation of nickel hydride begins. Thus, during the first cycle of hydrogenation in 40 minutes the concentration of hydrogen in the matrix reaches 90% of its limit value, while nickel hydride forms only 69% of its limit value.