Completely Elastic Deformation of Hydrogenated Ta Thin Films

Abstract

Nanostructured metal hydrides could play a key role in a hydrogen economy. The nanostructuring or confinement of these materials as, e.g., thin films significantly affects the structural and functional properties. For tantalum hydride, a versatile hydrogen sensing material, we show that the confinement of tantalum as a thin film extends the solubility limit by suppressing the phase transition observed in bulk upon hydrogenation. Different from bulk, the body centered cubic unit cell continuously deforms with unequal lattice constants and angles between lattice vectors. This deformation ensures that the volumetric expansion is realized in the out-of-plane direction, and surprisingly, completely elastic in nature. The first-order phase transition suppression combined with the continuous elastic deformation of the tantalum unit cell over an extraordinary wide solubility range ensures the superb performance of tantalum and its alloys as a hysteresis-free optical hydrogen sensing range over a hydrogen pressure/concentration range of over 7 orders of magnitude.

Structural and Optical Properties of Thin Film β-Ta upon Exposure to Hydrogen to Asses Its Applicability as Hydrogen Sensing Material

“…In addition, this proportionality is the same for both α and β-Ta, indicating that the volume the Ta host lattice expands for a given hydrogen-to-metal ratio x is irrespective of the crystal structure. This observation is thus consistent with an elastic deformation of the Ta unit cell …”

“…In situ out-of-plane and in-plane X-ray diffraction results at T = 25 °C of a thin film of 40 nm β-Ta capped with a 10 nm Pd 0.6 Au 0.4 layer. Data on α-Ta is adapted from ref (a) Out-of-plane X-ray diffraction measurements around the (002) reflection as measured under the partial hydrogen pressures indicated and by stepwise increasing the pressure. The continuous lines represent a fit of a pseudo-Voigt function to the data to deduce the d -spacing.…”

“…The in situ neutron reflectometry measurements were performed at the time-of-flight neutron reflectometer ROG, located at the 2.3 MW HOR reactor of the Delft University of Technology, Delft, The Netherlands . For these measurements, the same settings as the measurements on α-Ta reported in ref were used. These settings yield a footprint of 65/80 × 40 mm 2 (umbra/penumbra) and a resolution of Δ Q / Q ≈ 5%.…”

“…This observation is thus consistent with an elastic deformation of the Ta unit cell. 9 3.1.3. In Situ X-ray Diffraction.…”

“…Tantalum and alloys thereof have recently been studied for hydrogen purification membranes, − and optical hydrogen sensing applications. − Optical hydrogen sensors rely on materials that, when exposed to a hydrogen-containing environment, absorb hydrogen and, in turn, change their optical properties. These optical properties can be probed by, e.g., measuring the optical reflectivity or transmission of the material, which is directly related to the hydrogen concentration around the sensor.…”