H in thin films: size and stress effects on the system thermodynamics and kinetics

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Due to its high diffusivity hydrogen atoms alloy with metals even at room temperature. At this temperature, the materials microstructure remains rather stable. When the system size is reduced to the nano-scale, microstructural defects as well as mechanical stress significantly affect the thermodynamics and kinetics properties of the system.[1-6] Effects will be demonstrated on Niobium-H and Palladium-H thin films.
Hydrogen absorption in metal systems commonly leads to lattice expansion. The lateral expansion is hindered when the metal adheres to a rigid substrate, as for thin films. Consequently, high mechanical stresses arise upon hydrogen uptake. In theory, these stresses can reach about -10 GPa for 1 H/M. Usually, metals cannot yield such high stresses and deform plastically. Thereby, maximum compressive mechanical stress of -2 to -3 GPa is commonly measured for 100 nm Nb thin films adhered to Sapphire substrates.
It will be shown that phase transformations change in the coherency state upon film thickness reduction. The coherency state affects the nucleation and growth behaviour of the hydride phase as well as the kinetics of the phase transformation.[1] It will be further demonstrated that plastic deformation can be hindered and even suppressed upon film thickness reduction. In this case the system behaves purely elastic and ultra-high stress of about -10 GPa can be experimentally reached.[2] These high mechanical stresses result in changes of the materials thermodynamics. In the case of Nb-H thin films of less than 8 nm thickness, the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed, at 300 K.[3,4,5] The experimental results go in line with the σDOS model that includes microstructural and mechanical stress effects on the chemical potential [6].

[1] V. Burlaka, K. Nörthemann, A. Pundt, „Nb-H Thin Films: On Phase Transformation Kinetics“, Def. Diff. Forum 371 (2017) 160.
[2] M. Hamm, V. Burlaka, S. Wagner, A. Pundt, “Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films”, Appl. Phys. Letters 106 (2015) 243108
[3] S. Wagner, A. Pundt, “Quasi-thermodynamic model on hydride formation in palladium-hydrogen thin films: Impact of elastic and microstructural constraints “, Int. J. Hydrog. Energy 41 (2016) 2727.
[4] V. Burlaka, S. Wagner, M. Hamm, A. Pundt, “Suppression of phase transformation in Nb-H thin films below switchover-thickness”, Nano Letters 16 (2016) 6207.
[5] S. Wagner, P. Klose, V. Burlaka, K: Nörthemann, M. Hamm, A. Pundt, Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress, Phase Equilibria and Critical Temperatures, Chem. Phys. Chem. 20 (2019) 1890–1904.
[6] S. Wagner, A. Pundt, Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films, AIMS Materials Science 7 (2020), 399–419.