Investigation of hydrogen sorption in magnesium-based sputtered thin films

Yuan, Cao (2020). Investigation of hydrogen sorption in magnesium-based sputtered thin films. University of Birmingham. M.Phil.

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Abstract

Hydrogen sorption into Mg-based thin films prepared by magnetron sputtering was investigated. The hydrogen storage properties and thin films’ physical properties were investigated in the experiments.

Mg thin films were prepared in a Closed Field Unbalanced Magnetron Sputter Ion Plating (CFUBMSIP) system. This system has advantage for homogenous and uniform coatings which is better than conventional coating system. The sputtering parameters are shown to have a significant influence on the microstructure and intrinsic film growth stress in Mg thin films. The intrinsic stress can be examined by X-Ray Diffraction (XRD) during hydrogen absorption, in plane tensile stress of Mg thin films grows due to crystallite coalescence.

The surface roughness and buckling appearance were characterized by Confocal microscopy and Profilometry. Surface roughness has the relationship with the Mg thickness. During hydrogen absorption, high stress accumulates leading to more buckling. While, more buckling indicates high thin film surface roughness.

The effect of Mg layer thickness (lower than 1000 nm) was investigated. Mg thickness can influence the hydrogen sorption behavior. Thinner Mg layers can have a lower hydrogen absorption temperature comparing with thicker films. Film thickness has the effect on Mg grain boundary diffusion which is dominated by trans-granular diffusion. Different film thickness has different Mg grain boundary diffusion rate.

As Pd alloys with Mg at high temperatures, a Ti interlayer can be used to prevent inter-diffusion between Pd and Mg. SEM microscopy was tried to find the grain boundary between Mg and Pd. Si substrate was tried considering weight and volume of glass substrate. Thermodynamics was influenced during hydrogen sorption process with Si substrate.

Mg based thin films were sputtered onto glass substrates. The overall hydrogen storage uptake was reduced due to the volume of glass. Thus, a silicon substrate was used, which has a lower volume and weight comparing to the glass substrate. Hydrogen storage capacity will be modified. In addition, different substrates can influence the adhesion energy between the Mg layer and the substrate. The films growth mode and microstructure are factors which can ultimately influence the thermodynamics of the Mg thin films. Buckling is shown during hydrogen sorption process, which can accumulate stresses. The interface shear stress of Mg thin films can be measured. High stress accumulates with more buckling at the surface. The hydrogenation conditions are relevant with the interface shear stress. Mg thickness, hydrogenation times and hydrogenation temperature are the main hydrogenation conditions. For example, the interface shear stress is around 1.75 GPa of sample Pd (60 nm)-Mg (150 nm) glass substrate after hydrogenation 72h at room temperature. While at the same conditions, sample Pd (60 nm)-Mg (800 nm) glass substrate has the interface shear stress lower than 1 GPa. The interface shear stress is influenced by hydrogenation conditions which may be useful for mechanical hydrogen sensors.

The electrical resistance of Mg based thin film can be measured using two probes and four probes technique. The four probe technique gave more accurate results. 2A 25sccm condition has the lowest electrical resistance among the measuring samples which is around 2Ω.

Finally, a Mg/Y based multilayer system was investigated. Mg/Y based multilayer appears to provide a new method towards MgH2 destabilization by accommodating of lattice mismatch between strained FCC Mg/Y interfaces. There is no alloying phase formation between Y and Mg under high temperature He atmosphere.

Type of Work:

Thesis (Masters by Research > M.Phil.)

Award Type:

Masters by Research > M.Phil.

Supervisor(s):