PhysicsLettersA 181 (1993) 245—250 PHYSICS LETTERS A North-Holland Study of the hydrogen diffusion in superlattices by grazing angle neutron reflectometry M. Mâaza, B. Farnoux Laboratoire Leon Brillouin, Coininissariat a l’EnergieAtomique — Centre National de la Recherche Scientifique, Bat. 563, Centre d’Etudes Nucléaires de Saclay, 91191 Gjjsur-Yvette, France and F. Samuel Compagnie Industrielle des Lasers, route de Nozay B.P. 27, 91460 Marcoussis, France Received 3 March 1993; accepted for publication 10 August 1993 Communicated by J.P. Vigier This paper deals with the possibility of studying H diffusion in a small depth range (~ 80 A) with grazing angle neutron reflec- tometry. This new optical and nondestructive technique allowed us to follow the evolution ofH in small period superlattices with a small annealing temperature. To illustrate this, a superlattice of 10(Ni—TiH~,) sputtered on float-glass is studied. Variations in both neutron refractive indices of Ni and TiH~, layers, the period and the neutron reflectivity of the stratified medium were observed. 1. Introduction fusion and its isotopes, namely, electrochemical methods [4], nuclear reaction profiling [5], the The diffusion of hydrogen in metals has attracted tracer method [6], nuclear magnetic resonance [7], special attention for several reasons [1]. These have quasi-elastic neutron scattering [8] and the Moss- been motivated from the points of view of both basic bauer effect [91. In general, all the previous methods as well as applied research. The diffusivity of hy- require a bulk material or at least a thick layer. The drogen in metals is extremely high and exceeds that present work is undertaken to show the possibility to of heavy interstitials such as oxygen and nitrogen by study H diffusion with grazing angle neutron reflec- 15—20 orders of magnitude. Phenomenologically, this tometry (GANR) in very thin metal—metal bilayers high diffusion is a consequence of the low activation in a stratified medium configuration. The studied energy for hydrogen. So hydrogen diffusion can be sample is a sputtered Ni—TiH~superlattice. observed at low temperatures. The TiHX layers were obtained successfully by hy- The technical interest in hydrogen—metal systems drogenation of Ti layers during the sputtering pro- extends into many areas: hydrogen—metal alloys as cess. The annealing treatment of the superlattice in- moderators in reactors [2], storage of hydrogen as duces changes in the neutron reflectivity profiles. metal—hydrogen alloys. In neutron optics, the hy- These changes are linked to H diffusion in the strat- drogenation of Ti in Ni—Ti superlattices was used to ified medium. The method used to follow this H dif- improve the neutron reflectivity of these devices [3]. fusion is to built a model of the profile of the nuclear As these neutron optics devices will be submitted to scattering length density, simulate the reflectivity and a high neutron flux, a study of H diffusion with ther- finally compare the calculated neutron reflectivity mal annealing is required. profile with the experimental data. There are various techniques used to study H dif- Elsevier Science Publishers B.V. 245