Inauditus materials processing through surface asymmetries
Surfaces of materials are an enigma, whereas they are both familiar and foreign to the scientific community. Due to its size and metastable nature, analysis of native material surfaces poses a great scientific and technological challenges. Throughout decades of research, incoherencies in the definition of surfaces have led to many debates within scientific communities across the world. Many have overlooked the importance of surfaces in materials, though, when probed properly, surfaces can yield a plethora of unique material properties. Herein, the works shown here highlight the role of surfaces (specifically metal’s oxide layer) in manipulating bulk energy landscape to control different thermodynamic and kinetic events from phase transformation, diffusion and energy/mass propagation.
Thermodynamically, surfaces carry the critical role in maintaining equilibrium between two dissimilar systems or with their surroundings. Due to this nature, surfaces act as a material’s mass and energy dissipation horizon. For example, metallic systems oxidize instantaneously in ambient environments to equilibrate with their surroundings, forming thin layer of passivating oxides. This passivating layer, whilst thin, is highly complex in both composition and energy. In multi-component alloy systems, all species contributes to the formation of this oxide layer, albeit at different amounts and rates. Recent studies have shown the complexity within the sub-surface region where speciation and ordering of the alloy compositions can be seen. This asymmetry in composition and energy acts as a fundamental platform for the works done in this thesis to tip the mass and energy balance of the bulk.
First, based on the induced surface composition speciation, given appropriate stimuli, further surface tunability in composition, thickness, diffusivity can be achieved. By tuning the oxidant flux and stoichiometry design, mass propagation and diffusion directions can be manipulated, leading to various surface oxidation products from thermal oxidative composition inversion (TOCI) and ship-in-a-bottle oxidation can be achieved. Second, since these surface species are hypovalent, energy and chemical potential gradients are present between the bulk and the surface. Depending on the scale of the system, this energy asymmetry, combined with inherent surface tension can be utilized to manipulate the energy landscape of the bulk system. Given that sub-micrometer systems offer high surface area-to-volume ratio, the contribution of these surface asymmetries can be maximized, thus manipulation of the free energy landscape can be achieved. By utilizing oxide passivation and its high non-PV work, frustration of heterogeneous and homogeneous nucleation can be achieved, which brings about stability to the undercooled state in metal alloys. As the stability is achieved through the surface, induced fracturing of the surface provides a novel pathway to achieve room temperature solidification for heat-free metal processing. These works, therefore, provide several examples on the utilization of metal’s surface oxide to access novel material synthesis, metastable states and material processing methods