Coherent-to-semi-coherent transition of precipitates in niobium-hydrogen thin films

Abstract

Nucleation and growth in thin films are studied by using niobium-hydrogen ͑Nb-H͒ as model system. Hydride precipitation in thin films results in local surface topography changes that can be monitored by scanning tunneling microscopy. The local film expansion can be used to detect hydride precipitates, to study their growth, to gain information about their shape and their lattice coherency. With the help of theoretical calculations, it will be shown that cylindrical Nb-H precipitates evolve in early stages. These precipitates are coherent to the matrix as long as the film is thin and the hydride size is below a critical volume. Above this critical volume, a coherent-to-semi-coherent transition occurs. The critical size is controlled by the balance between the elastic energy stored in the coherent precipitate and the energy needed for the formation of dislocations. Consequently, films below 26 nm thickness keep coherency for all hydride precipitate volumes and never get semi-coherent.

Mechanical stress and stress release channels in 10–350 nm palladium hydrogen thin films with different micro-structures

“…Its onset is marked by Ds plast in Table 1. Plastic deformation includes the emergence of new misfit dislocations finally appearing at the film-substrate-interface [3,6,11,21] or the emission of dislocation loops around hydride precipitates [17]. The emission of new dislocations is directly visible with in-situ STM.…”

“…[29]. For epitaxial Nb-H thin films N€ orthemann et al [17] have proposed the emergence of misfit dislocation loops around semi-coherent hydride precipitates. Interestingly, other possible mechanisms like the migration of twin boundaries within grains have not been observed in deformed nanocrystalline Pd e.g.…”

Double-locked nucleation and growth kinetics in Nb-H thin films

“…27 A phase-transition pressure of 6 × 10 −5 Pa was determined for epitaxial Nb films, which was found to be independent of the film thickness. 28 To summarize, in thin films both the hydrogen concentration and the lattice expansion increase with increasing hydrogen gas pressure. Thus, in situ studies using the same Nb-H sample can be performed by controlling the hydrogen partial pressure.…”

“…28 Because of the strong out-of-plane expansion during hydride formation in thin films, hydride precipitation results in local surface topography changes. As was recently shown, the lateral extension of these topographies correlates to the hydride's lateral size.…”

“…As was recently shown, the lateral extension of these topographies correlates to the hydride's lateral size. 28 Furthermore, the topography's height is generated by the hydride shape and vertical extension. 28 This paper focuses on the nucleation and growth of hydrides in epitaxial Nb thin films.…”