Suppression of Phase Transformation in Nb–H Thin Films below Switchover Thickness

Abstract

Hydrogen uptake in metal-hydrogen (M-H) nanosized systems (e.g., thin films, clusters) is both a fundamental and a technologically relevant topic, which is becoming more important due to the recent developments of hydrogen sensors, purification membranes, and hydrogen storage solutions. It was recently shown that hydrogen (H) absorption in nanosized systems adhered to rigid substrates can lead to ultrahigh mechanical stress in the GPa range. About -10 GPa (compressive) stress were reported for hydrogen loaded niobium (Nb) thin films. Such high stresses can be achieved when conventional stress-release channels are closed, e.g., by reducing the system size. In this paper, we demonstrate that the high stress can be used to strongly modify the system's thermodynamics. In particular, a complete suppression of the phase transformation is achieved by reducing the film thickness below a switchover value d. Combined in situ scanning tunneling microscopy (STM) and in situ X-ray diffraction (XRD) measurements serve to determine the switchover thickness of epitaxial Nb/AlO films in the thickness range from 55 to 5 nm. A switchover thickness d = 9 ± 1 nm is found at T = 294 K. This result is supported by complementary methods such as electromotive force (EMF), electrical resistance, and mechanical stress measurements in combination with theoretical modeling.

Defect generation in Pd layers by ‘smart’ films with high H-affinity

“…The chosen frame is representative of the complete film. Similar results were obtained for thicker Nb films 30 , 33 . The perimeter of the border region between the hydride and the α-phase region can be determined by analyzing STM images measured at different loading stages.…”

“…Deposition of Pd ( T Sput ≈ 35 °C) and Nb ( T Sput = 750–800 °C) films on Al 2 O 3 (11–20) sapphire substrate is performed in an ultra-high vacuum (UHV) sputter system by cathode Argon ion-beam sputtering 30 . The thickness of the Pd layer in the Pd/Nb/Al 2 O 3 stack is varied from 2 nm (for STM) to 20 nm (for XRD and ETEM), while the thickness of Nb layer is varied from 5 nm to 55 nm.…”

“…Compared to Pd, the solubility of H in niobium (Nb) is several orders of magnitude higher, reflecting the more negative enthalpy of hydrogen solution of −0.35 eV/atom in Nb 28 compared to +0.1 eV/atom in Pd 29 . Nb-H forms a hydride already at about p H2 ≈1 × 10 −6 mbar at 293 K 30 , whereas the formation pressure of Pd hydride is much higher - p H2 ≈ 18 mbar at 293 K 31 . Thus, in case of combined Pd/Nb - system, preferential hydrogen absorption at p H2 ≪18 mbar occurs first in Nb, at 293 K. If the hydrogen concentration is large, hydrogen absorption in thin metal films grown on rigid substrates can cause high mechanical stresses and strong out-of-plane lattice expansion 27 , 32 – 36 .…”

“…Hydrogen absorption of 1 H/Nb in epitaxial Nb (110) films on rigid substrates results in a strong out-of-plane expansion 30 , 33 , 37 , 38 and a considerable mechanical stress up to 10 GPa 39 . If the initial thickness of the Nb film is d Nb < 5 nm, stress release is not possible and an elastic stress evolution is measured up to a solubility of 1 H/Nb 39 .…”

“…If the initial thickness of the Nb film is d Nb < 5 nm, stress release is not possible and an elastic stress evolution is measured up to a solubility of 1 H/Nb 39 . In films with a maximum initial thickness of d Nb ≅ 9 ± 1 nm, the accumulated mechanical stress results in such a strong destabilization of the hydride phase that the phase transformation is suppressed even at 293 K 30 . Films with supressed phase transformation do not show any hydride-related topography changes.…”