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|Title:||Ion Beam Mixing and Electrocatalytic Characteristics of Thin Film Ni/Pd Surface Alloys|
|Authors:||Akano, Gbadebo Usman|
|Department:||Materials Science and Engineering|
|Keywords:||Materials Science and Engineering;Materials Science and Engineering|
|Abstract:||<p>Atomic mixing resulting from heavy-ion bombardment of thin-film Ni/Pd bilayers and thin Pd markets sandwiched between two Ni layers has been investigated. Experiments were carried out using 120 keV Ar⁺ and 145 keV Kr⁺ ions for doses 0-4x10¹⁶cm⁻² over a wide temperature range (40-673K). The resulting interdiffusion was measured using 2 MeV ⁴He⁺ ion backscattering in-situ. In some cases, additional depth analysis was undertaken using Auger Electron Spectroscopy (AES) with sputter depth profiling. The influence of film microstructure and bombardment-induced microstructural changes on the mixing have been assessed with transmission electron microscopy (TEM). The electrocatalytic behavior of the ion-mixed bilayer samples was determined for the hydrogen evolution reaction in strong KOH solution, using potentiostatic polarization techniques. The amount of ion beam mixing is found to vary with the square root of the ion dose for temperatures ≤ 573K. Also, at 40K, where it is assumed that no significant thermally activated process contributes to the mixing, it is found that the mixing is dependent on the damage energy, FD(subscript, see original file), deposited at the interface region. Between 40 and ~400K, the mixing parameter shows a weak temperature dependence with an apparent activation energy Ea ~ 0.1eV. This suggests that some interstitial diffusion contributes to the inter-mixing in this temperature regime. Above ~ 473K, the mixing (at each temperature) increases rapidly with ion dose for ir radiation doses ≤ 5x10¹⁵cm⁻² with high initial mixing efficiencies of 6-35 atoms/ion between 573-673K. This initial rapid mixing level eventually slows to a less rapid (~1.2 atoms/ion at 673K) rate for doses ≥ 5x10¹⁵cm⁻². TEM observations of the irradiated and unirradiated films suggests that the change to a less rapid mixing rate is correlated with an increase in the average grain diameter (10-80nm) in the film following ion bombardment, and therefore with a reduction in the contribution of grain boundary diffusion to the intermixing. For samples annealed, without irradiation, below ~ 473K, no measurable interdiffusion of the Ni/Pd layers was observed. For samples annealed above ~473K, Whipple analyses of the interdiffusion show two distinct regions - lattice diffusion-dominated region near the original interface and grain boundary diffusion dominated region further away from the interface. The lattice diffusivities, Dℓ(subscript), under thermal annealing conditions were evaluated and ranged from ~ 0.22 to 52x16⁻¹⁶cm⁻²-sec⁻¹ for Pd diffusing in Ni, and 0.33 to 56.6x10⁻¹⁶cm²-sec⁻¹ for Ni diffusing in Pd between 523-673K. These lattice diffusivities are high because of enhancement of diffusion by defects present in the as-prepared films. The grain boundary diffusion coefficients (for Pd diffusion in Ni boundaries) DB(subscript) varied from ~ 5.4x10⁻¹⁴cm²-sec⁻¹ at 523K to ~ 3x10⁻¹¹cm²-sec⁻¹ at 623K, with an apparent activation energy EB(subscript) ~ 1.35 eV and a prefactor D⁰B(subscript) ~ 0.75 cm²-sec⁻¹. Under irradiation conditions, ion irradiation results in an enhancement of the mixing especially below ~ 400K where almost an order of magnitude enhancement over thermal anneal only, was observed. At ~ 673K, the contribution of equilibrium defects to interdiffusion over-whelms the irradiation effects. The diffusion coefficients under irradiation, Drad(subscript), were also evaluated and ranged from ~4 to 70x10⁻¹⁶cm²-sec⁻¹ at 523 and 673K with apparent activation energy Ea ~ 0.57 eV. For Pd films deposited on large-grain Ni substrates, thermal annealing and/or bombardment result in mixing levels that are almost two orders of magnitude lower than observed from small-grained Ni/Pd couples, though under irradiation conditions, radiation enhancement of the mixing was also observed. As potential electrocatalysts for the H₂ evolution reaction in strong KOH solution, the ion-beam mixed surface alloys (in the over-potential range ≤ 0.4 V) show superior performance over smooth Ni, and Ni coated with evaporated Pd.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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