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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24009号 / エネ博第445号 / 新制||エネ||84(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 馬渕 守, 教授 土井 俊哉, 教授 濵 孝之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

Diffusion is an omnipresent phenomenon in nature. In the world of molecules, it describes their irregular thermal motion. The interplay of diffusion and interaction of molecules with pore walls of nanoporous materials constitutes the benefit of using such materials in applications of separation and catalysis. The need for understanding the rate-limiting mechanisms, further optimization and development of new processes makes this topic subject of continued fundamental research [1].

The workshop program will focus on adsorption measurement techniques and methodologies for the assessment of adsorption properties and textural/structural characterization of novel nanoporous materials including zeolites, carbons, MOFs as well as materials consisting of hierarchically structured pore networks. A major point will be the correlation of textural properties, adsorption behavior, catalytic reaction pathways as well as transport properties with applications in gas and energy storage, separations and catalysis. Within this framework, the workshop will offer a platform for scientific discussions and for a knowledge transfer between various scientific areas where diffusion and transport properties of porous materials are of importance.

With reference to these advantages, the poster goes the other way round and identifies a couple of similarities where, on looking at molecular diffusion in nanoporous materials, one is able to recognize features of spreading, which may occur in quite different fields of research. The examples presented include (i) considering molecular uptake and release with nanoporous particles as a model for, respectively, occupation of a habitat by a new species and, vice versa, for the loss of a species in this habitat [2], (ii) the effect of additional highways on overall mass transfer [3,4], (iii) transport impediment (and enhancement!) by diffusant interference [5], (iv) invader-induced changes in the host system [2] and (v) host-induced changes of the invaders [6].

Successful design and application of nanoporous materials are essentially dependent on the molecular diffusion. Two mechanisms, i.e. surface barriers and intracrystalline diffusion, may dominate the mass transport. In the previous studies, these two mechanisms are difficult to determine with certainty by dual resistance model [1] (DRM). Here, we derive an expression of uptake rate relying solely on surface permeability, which provides a method to directly quantify the surface barriers. Subsequently, the effects of surface barriers and intracrystalline diffusion could be identified separately.

Gold gains many useful properties when it is formed into a nanoporous structure, but it also becomes macroscopically brittle due to flow localization and may therefore be unreliable for many applications. The goal of this work was to establish processing/structure/property relationships of nanoporous gold, discover controllable structure features, and understand the role of structure on flow localization. The nanoporous gold structure, consisting of a 3D network of nanoscale gold ligaments, was quantified with an automatic software developed for this work called AQUAMI, which uses computer vision techniques to make statistically reliable numbers of repeatable and unbiased measurements per image. AQUAMI increased the efficiency and accuracy of characterization in this work, allowed for the conduction of more experiments, and provided better confidence in morphology and size distribution of the complex NPG microstructural features. Nanoporous gold was synthesized while varying numerous processing factors such as dealloying time, annealing time, and mechanical agitation. Through the expanded scope of synthesis experiments and detailed analysis, it was discovered that the curvature of the ligaments and the distribution width of ligament diameters could be controlled through processing. In situ tensile experiments in SEM and TEM revealed that large ligaments arrested crack propagation while curved ligaments increase ductility by straightening in the tensile direction and forming geometrically required defects, which inhibit dislocation activity. Through synthesis and microstructure characterization, two new controllable structure features were discovered experimentally. In situ mechanical testing revealed the role these structures play on the deformation behavior and flow localization of nanoporous gold. / Doctor of Philosophy / Nanoporous gold contains a network of connected pores running through and between at network of solid gold ligaments or struts. It somewhat resembles the structure of coral. The nanoscale pores and ligaments give the material many useful properties. However, this structure also makes the material very fragile and unreliable in many potential application environments. The goal of this research is to investigate how the structure makes the material so fragile and look for ways the material might be made less fragile while still preserving its useful properties. The material properties are controlled through the material’s structure, which in turn is controlled by processing. To control the structure of nanoporous gold, the structure first had to be characterized. A software called AQUAMI was developed, which uses computer vision, to automatically calculate many features of the structure by looking at an image of it. This software was much faster and more accurate than making hundreds of hand measurements on each image. To find new ways to control the structure through processing, nanoporous gold was synthesized in many different conditions and then the structure was analyzed to determine the effect of each synthesis condition. It was discovered that a single specimen could be given a larger variety of ligament thicknesses by making it with a weaker acid or a smaller variety by heating the structure after forming it. Stirring during synthesis resulted in a structure with curvier ligaments. Mechanical tests were performed in electron microscopes to see how these features affected deformation. Large ligaments slowed crack propagation suggesting that a larger variety of ligament diameters could increase strength by having more large ligaments. Curved ligaments deformed more without breaking by straightening during deformation. Through this work, new ways of controlling the nanoporous gold structure were found and mechanical tests suggest that controlling these features may increase the material’s strength making it reliable in more environments

Dealloying is a corrosion process in which one or more elements are selectively removed from an alloy leading to a 3-dimensional porous structure of the more noble element(s). These porous structures have been known to cause stress corrosion cracking in noble metal alloy systems but more recent interest in using the corrosion process to produce porous metals has developed. Applications for these structures range from high surface area electrodes for biomedical sensors to use as skeletal structures for fundamental studies (e.g. low temperature heat exchangers or sensitivity of surface diffusivity to chemical environment). In this work we will review our current understanding of alloy corrosion including our most recent results demonstrating a more accurate method for calculating alloy critical potential based on potential hold experiments. The critical potentials calculated through the potential hold method were â 0.030VMSE, 0.110VMSE, and 0.175VMSE for Cu80Pt20, Cu75Pt25 Cu71Pt29 respectively. We will present the use of porous metals for making surface diffusivity measurements in the Pt systems as a function of chemical environment. A review of the use of small angle neutron scattering to make accurate measurements of pore size is presented and the sensitivity of pore size to electrolyte, electrolyte composition, applied potential and temperature will be explained. The production of porous Pt with pore sizes ranging from 2-200nm is demonstrated. / Ph. D.

In this dissertation, simulation techniques are used to understand the role of surfaces, interfaces, and capillary forces on the deformation response of bicontinuous metallic composites and porous materials. This research utilizes atomistic scale modeling to study nanoscale deformation phenomena with time and spatial resolution not available in experimental testing. Molecular dynamics techniques are used to understand plastic deformation of metallic bicontinuous lattices with varying solid volume fraction, connectivity, size, surface stress, loading procedures, and solid density. Strain localization and yield response on nanoporous gold lattices as a function of their solid volume fraction are investigated in axially strained periodic samples with constant average ligament diameter. Simulation stress results revealed that yield response was significantly lower than what can be expected form the Gibson-Ashby formalism for predicting the yield response of macro scale foams. It was found that the number of fully connected ligaments contributing to the overall load bearing structure decreased as a function of solid volume fraction. Correcting for this with a scaling factor that corrects the total volume fraction to "connected, load bearing" solid fraction makes the predictions from the scaling equations more realistic. The effects of ligament diameter in nanoporous lattices on yield and elastic response in both compressive and tensile loading states are reported. Yield response in compression and tension is found to converge for the two deformation modes with increasing ligament diameter, with the samples consistently being stronger in tension, but weaker in compression. The plastic response results are fit to a predictive model that depends on ligament size and surface parameter (f). A modification is made to the model to be in terms of surface area to volume ratio (S/V) rather than ligament diameter (1/d) and the response from capillary forces seems to be more closely modeled with the full surface stress parameter rather than surface energy. Fracture response of a nanoporous gold structure is also studied, using the stress intensity-controlled equations for deformation from linear elastic fracture mechanics in combination with a box of atoms, whose interior is governed by the molecular dynamics formalism. Mechanisms of failure and propagation, propagation rate, and ligament-by-ligament deformation mechanisms such as dislocations and twin boundaries are studied and compared to a corresponding experimental nanoporous gold sample investigated via HRTEM microscopy. Stress state and deformation behavior of individual ligaments are compared to tensile tests of cylinder and hyperboloid nanowires with varying orientations. The information gathered here is used to successfully predict when and how ligaments ahead of the crack tip will fracture. The effects of the addition of silver on the mechanical response of a nanoporous lattice in uniaxial tension and compression is also reported. Samples with identical morphology to the study of the effects of ligament diameter are used, with varying random placement concentrations of silver atoms. A Monte Carlo scheme is used to study the degree of surface segregation after equilibration in a mixed lattice. Dislocation behavior and deformation response for all samples in compression and tension are studied, and yield response specifically is put in the context of a surface effect model. Finally, a novel bicontiuous fully phase separated Cu-Mo structure is investigated, and compared to a morphologically similar experimental sample. Composite interfacial energy and interface orientation structure are studied and compared to corresponding experimental results. The effect of ligament diameter on mechanical response in compressive stress is investigated for a singular morphology, stress distribution by phase is investigated in the context of elastic moduli calculated from the full elastic tensor and pure elemental deformation tests. Dislocation evolution and its effects on strain hardening are put in the context of elastic strain, and plastic response is investigated in the context of a confined layer slip model for emission of a glide loop. The structure is shown to be an excellent, low interface energy model that can arrest slip plane formation while maintaining strength close to the theoretical prediction. Dislocation content in all samples was quantified via the dislocation extraction algorithm. All visualization, phase dependent stress analysis, and structural/property analysis was conducted with the OVITO software package, and its included python editor. All simulations were conducted using the LAMMPS molecular dynamics simulation package. Overall, this dissertation presents insights into plastic deformation phenomena for nano-scale bicontinuous metallic lattices using a combination of experimentation and simulation. A more holistic understanding of the mechanical response of these materials is obtained and an addition to the theory concerning their mechanical response is presented. / Doctor of Philosophy / Crystalline metals can be synthesized to have a sponge-like structure of interconnected ligaments and pores which can drastically change the way that the material chemically interacts with its environment, such as how readily it can absorb oxygen and hydrogen ions. This makes it attractive as a catalyst material for speeding up or altering chemical reactions. The change in structure can also drastically change how the material responds when deformed by pressing, pulling, tearing or shearing, which are important phenomena to understand when engineering new technology. High surface or interface area to volume ratios can cause a massive surface-governed capillary force (the same force that causes droplets of water to bead up on rain coat) and lead to a higher pressure within the material. The effect that both structure and capillary forces have on the way these materials react when deformed has not been established in the context of capillary force theory or crystalline material plasticity theory. For this reason, we investigate these materials using simulation methods at the atomic level, which can give accurate and detailed data on the stress and forces felt atom-by-atom in a material, as well as defects in the material, such as dislocations and vacancies, which are the primary mechanisms that cause the crystal lattice to permanently deform and ultimately break. A series of parameters are varied for multiple model systems to understand the effects of various scenarios, and the understanding provided by these tests is presented.

Nanostructured porous materials generally, and nanoporous noble metals specifically, have received considerable attention due to their superior chemical and physical properties over nanoparticles and bulk counterparts. This dissertation work aims to develop well-established strategies for the preparation of multifunctional nanostructured porous materials based on the combination of inorganic-chemistry, organic-chemistry and electrochemistry. The preparation strategies involved one or more of the following processes: sol-gel synthesis, co-electrodeposition, metal ions reduction, electropolymerization and dealloying or chemical etching. The study did not stop at the preparation limits but extended to investigate the reaction mechanism behind the formation of these multifunctional nanoporous structures in order to determine the different factors controlling the nanoporous structures formation. First, gold-silica nanocomposites were prepared and used as a building blocks for the fabrication of high surface area gold coral electrodes. Well-controlled surface area enhancement, film thickness and morphology were achieved. An enhancement in the electrode’s surface area up to 57 times relative to the geometric area was achieved. A critical sol-gel monomer concentration was also noted at which the deposited silica around the gold coral was able to stabilize the gold corals and below which the deposited coral structures are not stable. Second, free-standing and transferable strata-like 3D porous polypyrrole nanostructures were obtained from chemical etching of the electrodeposited polypyrrole-silica nanocomposite films. A new reaction mechanism was developed and a new structural directing factor has been discovered for the first time. Finally, silver-rich platinum alloys were prepared and dealloyed in acidic medium to produce 3D bicontinuous nanoporous platinum nanorods and films with a nanoporous gold-like structure. The 3D-BC-NP-Pt displayed high surface area, typical electrochemical sensing properties in an aqueous medium, and exceptional electrochemical sensing capability in a complex biofouling environment containing fibrinogen. The 3D-BC-NP-Pt displayed high catalytic activity toward the methanol electro-oxidation that is 30 times higher that of planar platinum and high volumetric capacitance of 400 F/cm3. These findings will pave the way toward the development of high performance and reliable electrodes for catalysis, sensing, high power outputs fuel cells, battery-like supercapacitors and miniaturized device applications.