Magnesium and magnesium alloy hydrides

Hydrogen is stored atomically in metal hydrides. Dissociative chemisorption and associative desorption are therefore important steps in the hydrogen absorption and desorption processes. We summarize the theoretical and experimental knowledge of hydrogen chemisorption on clean and precovered metal surfaces and correlate it with the techniques for preparing metal hydrides. At the surface of hydride-forming intermetallics, precipitates of d metals and a metallic subsurface are produced by surface segregation and decomposition. The subsurface and the precipitates are able to dissociate H2. Our recent work on the surface analysis of LaNi5, FeTi, Mg2Ni and ErFe2 is reviewed.

Surface effects and the formation of metal hydrides

The proposition that the apparent differences between the reactivities of magnesium and hydrogen are partially due to a lack of appreciation of the effects of various parameters on the kinetics is developed. The results of investigations of the formation-decomposition kinetics and morphology of CeMg12 and Mg-10wt.%Ni alloys performed at pressures and temperatures where only the formation of the MgH2 hydride is reversible support the hypothesis that the high reactivity of such alloys with hydrogen is due, at least partially, to the effects of morphological parameters such as mechanical stresses and strains and incoherent grain boundaries which increase hydrogen diffusion and hydride nucleation and which are produced by the formation of finely divided precipitates of CeH3 or Mg2Ni. The decrease in the hydrogen capacity on cycling is interpreted as a consequence of the removal of defects.

The mechanism and kinetics of hydride formation in Mg-10wt.%Ni and CeMg12

Hydriding and dehydriding characteristics of Mg-LaNi5 composite materials prepared by mechanical alloying

Ribbons of hexagonal material are produced when silicon is indented in the temperature range 400–650°C. This series of three papers, based on two previous preliminary reports [P. Pirouz, R. Chaim and J. Samuels, Proc. 5th Int. Congr. Structure and Properties of Dislocations in Semiconductors , Moscow, U.S.S.R., 1986. Izvestiya Nauk Akademiya S.S.S.R. 51 , 753 (1987); P. Pirouz, R. Chaim and U. Dahmen, Mater. Res. Soc. Symp. Proc. 104 , 133 (1988)], is a detailed account of an investigation of this phase. In the first paper, the details of the experimental investigation are described. Vickers microindentations have been made in float-zone silicon in the temperature range of 400–500°C. The ribbons of hexagonal silicon are then studied by different techniques of transmission electron microscopy (TEM). Conventional microscopy using strain contrast, selected area diffraction technique, and high resolution electron microscopy (HREM) have been employed to investigate the structure of the hexagonal phase. Using the latter technique, the ribbons are investigated to within a resolution of 0.18 nm and HREM images are compared with simulated ones. HREM is specially useful for studying the faulting within the hexagonal phase and the cubic/hexagonal interface. Analytical microscopy using Electron Energy Loss Spectroscopy (EELS) has been used to show that no oxygen is present in the ribbons within the resolution of this technique. Throughout the paper, it has been attempted to show that the present hexagonal Si is not a thermodynamically stable high pressure phase, but is related to stress relief when twins interact in silicon. In Part II of this series [U. Dahmen, K. H. Westmacott, P. Pirouz and R. Chaim, Acta metall. mater. 38 323 (1990)], a mechanism for the formation of hexagonal silicon by double twinning is presented through a crystallographic analysis. Finally, in Part III [P. Pirouz, U. Dahmen, K. H. Westmacott and R. Chaim, Acta metall. mater. 38 , 329 (1990)], a discussion of the results in the context of martensitic transformations and dislocation mechanisms is given and other recent reports of this phase in silicon which has undergone different treatments are discussed.

Th. von Waldkirch

Papers

Mg-based hydrogen storage materials: Surface segregation in Mg2Cu and related catalytic effects