Boundary conductance in macroscopic bismuth crystals

The interface between a solid and vacuum can become electronically distinct from the bulk. This feature, encountered in the case of quantum Hall effect, has a manifestation in insulators with topologically protected metallic surface states. Non-trivial Berry curvature of the Bloch waves or periodically driven perturbation are known to generate it. Here, by studying the angle-dependent magnetoresistance in prismatic bismuth crystals of different shapes, we detect a robust surface contribution to electric conductivity when the magnetic field is aligned parallel to a two-dimensional boundary between the three-dimensional crystal and vacuum. The effect is absent in antimony, which has an identical crystal symmetry, a similar Fermi surface structure and equally ballistic carriers, but an inverted band symmetry and a topological invariant of opposite sign. Our observation confirms that the boundary interrupting the cyclotron orbits remains metallic in bismuth, which is in agreement with what was predicted by Azbel decades ago. However, the absence of the effect in antimony indicates an intimate link between band symmetry and this boundary conductance.

A metallic anti-biofouling surface with a hierarchical topography containing nanostructures on curved micro-riblets

Metallic surface finishes have been used in the anti-biofouling, but it is very difficult to produce surfaces with hierarchically ordered structures. In the present study, anti-biofouling metallic surfaces with nanostructures superimposed on curved micro-riblets were produced via top-down fabrication. According to the attachment theory, these surfaces feature few attachment points for organisms, the nanostructures prevent the attachment of bacteria and algal zoospores, while the micro-riblets prohibit the settlement of macrofoulers. Anodic oxidation was performed to induce superhydrophilicity. It forms a hydration layer on the surface, which physically blocks foulant adsorption along with the anti-biofouling topography. We characterized the surfaces via scanning electron and atomic force microscopy, contact-angle measurement, and wear-resistance testing. The contact angle of the hierarchical structures was less than 1°. Laboratory settlement assays verified that bacterial attachment was dramatically reduced by the nanostructures and/or the hydration layer, attributable to superhydrophilicity. The micro-riblets prohibited the settlement of macrofoulers. Over 77 days of static immersion in the sea during summer, the metallic surface showed significantly less biofouling compared to a surface painted with an anticorrosive coating.

Estimation of impurity release from planar liquid surface in plasma

A liquid metallic surface exposed to a plasma interacts with ions and electrons numerously. The surface becomes charged. Subsequently, the electric field is formed via the balance of incoming ion and electron currents. The kinetic energies and the momentum of the incoming ions and electrons are transferred to the surface. This is enhanced by the self-generated electric field and gives rise to notable pressures and energy fluxes on the surface. By adopting several plasma and liquid parameters to the physical models, the time evolution of the surface temperature can be characterized with the net energy flux, the net pressure, and the net impurity outgoing flux. The study suggests that electron temperature and ion mass significantly govern, but the ion to electron temperature ratio does not, the trend of the surface temperature. Besides, it is found that at long elapsed time at which evaporation becomes strong, the energy and impurity fluxes reduce in conjunction with the rate of change of the surface temperature lessens.

Experimental Study on the Corrosion of Carbon Steel and Aluminum Alloy in Firefighting Protein Foam Concentrates

The corrosion of mild steel and Al alloy in Fomtec P 6% and 6% P Profoam 806 protein-based foam concentrates was investigated. Weight-loss data for steel showed corrosion penetration of 0.745 mipy in Fomtec and 2.269 mipy in Profoam, whereas for Al alloy the penetration levels were 0.474 and 1.093 mipy, respectively. Scanning electron microscopy and energy dispersive X-ray spectroscopy allowed characterization of the metallic surface covered or free from corrosion products. Values of corrosion potential, corrosion current density and corrosion penetration were calculated by using potentiodynamic polarization curves. Electrochemical impedance spectra illustrated the change in polarization resistance during anodic polarization. Data obtained by accelerated electrochemical methods confirm the greater aggressiveness of the Profoam concentrate compared to Fomtec concentrate.

Point Cloud Resampling by Simulating Electric Charges on Metallic Surfaces

3D point cloud resampling based on computational geometry is still a challenging problem. In this paper, we propose a point cloud resampling algorithm inspired by the physical characteristics of the repulsion forces between point electrons. The points in the point cloud are considered as electrons that reside on a virtual metallic surface. We iteratively update the positions of the points by simulating the electromagnetic forces between them. Intuitively, the input point cloud becomes evenly distributed by the repulsive forces. We further adopt an acceleration and damping terms in our simulation. This system can be viewed as a momentum method in mathematical optimization and thus increases the convergence stability and uniformity performance. The net force of the repulsion forces may contain a normal directional force with respect to the local surface, which can make the point diverge from the surface. To prevent this, we introduce a simple restriction method that limits the repulsion forces between the points to an approximated local plane. This approach mimics the natural phenomenon in which positive electrons cannot escape from the metallic surface. However, this is still an approximation because the surfaces are often curved rather than being strict planes. Therefore, we project the points to the nearest local surface after the movement. In addition, we approximate the net repulsion force using the K-nearest neighbor to accelerate our algorithm. Furthermore, we propose a new measurement criterion that evaluates the uniformity of the resampled point cloud to compare the proposed algorithm with baselines. In experiments, our algorithm demonstrates superior performance in terms of uniformization, convergence, and run-time.

Computational Methods for Light Scattering by Metallic Nanoparticles

<p>The unifying theme of this thesis is that of light scattering by particles, using computational approaches. This contributions here are separated into two main areas. The first consists of examining the behaviour of the extended boundary-condition method and T-matrix method, and providing a modified set of equations to use to calculate the relevant integrals. From this, some linear relations between integrals were found, which hint at the possibility of a more efficient means of performing these calculations. As well as this, the severe numerical problems associated with this method were investigated, and the primary source of these problems was identified in the case of two commonly-used shapes, spheroids and offset spheres. The cause of these numerical problems is that dominant, leading terms in the power series expansion of the integrands integrate identically to zero, but in practice, numerical calculations have insufficient precision to compute this exactly, and the overwhelming errors from this lead to drastically incorrect results. Following this identification, a new formulation of the integrals for spheroids is presented, which allows the much easier treatment of spheroids, approaching the level of ease of calculations for spheres in Mie theory. This formulation replaces some terms in the integrands with modified terms, that do not contain the parts of the power series that cause problems. As these should integrate to zero, we are able to remove them from the integrand without affecting the correct result. The second area of this thesis is concerned with calculations of the near-field for systems of interest in plasmonics, and specifically in surface-enhanced Raman spectroscopy. Here, the enhancement of the electric field in the vicinity of a metallic surface has a large effect on measured signals. The contribution of this thesis is to study the geometric parameters that influence the distribution of the field enhancement at the particle's resonance, specifically focusing on different effects caused by the overall shape of the particle, as opposed to those effects due to the local shape of the particle in regions of high enhancement. It is shown that the overall shape determines the location of the resonance, while the local shape determines how strongly the enhancement is localised. Understanding the factors that influence the enhancement localisation will help in guiding the design of suitable plasmonic substrates.</p>

Computational Methods for Light Scattering by Metallic Nanoparticles

<p>The unifying theme of this thesis is that of light scattering by particles, using computational approaches. This contributions here are separated into two main areas. The first consists of examining the behaviour of the extended boundary-condition method and T-matrix method, and providing a modified set of equations to use to calculate the relevant integrals. From this, some linear relations between integrals were found, which hint at the possibility of a more efficient means of performing these calculations. As well as this, the severe numerical problems associated with this method were investigated, and the primary source of these problems was identified in the case of two commonly-used shapes, spheroids and offset spheres. The cause of these numerical problems is that dominant, leading terms in the power series expansion of the integrands integrate identically to zero, but in practice, numerical calculations have insufficient precision to compute this exactly, and the overwhelming errors from this lead to drastically incorrect results. Following this identification, a new formulation of the integrals for spheroids is presented, which allows the much easier treatment of spheroids, approaching the level of ease of calculations for spheres in Mie theory. This formulation replaces some terms in the integrands with modified terms, that do not contain the parts of the power series that cause problems. As these should integrate to zero, we are able to remove them from the integrand without affecting the correct result. The second area of this thesis is concerned with calculations of the near-field for systems of interest in plasmonics, and specifically in surface-enhanced Raman spectroscopy. Here, the enhancement of the electric field in the vicinity of a metallic surface has a large effect on measured signals. The contribution of this thesis is to study the geometric parameters that influence the distribution of the field enhancement at the particle's resonance, specifically focusing on different effects caused by the overall shape of the particle, as opposed to those effects due to the local shape of the particle in regions of high enhancement. It is shown that the overall shape determines the location of the resonance, while the local shape determines how strongly the enhancement is localised. Understanding the factors that influence the enhancement localisation will help in guiding the design of suitable plasmonic substrates.</p>

Modelling the Melting of Gallium Clusters: A Path to Understanding Molecular Solids

<p>Gallium is a molecular solid with many unique properties. Comprised of Ga2 dimers but exhibiting metal-like electronic characteristics, gallium may be deemed a molecular metal. The role of this dual covalent-metallic nature may explain gallium’s fascinating thermodynamic behaviour. While bulk gallium melts at 303 K, clusters with only 10’s of atoms melt at temperatures between 500 and 800 K, according to experiment. The measured specific heat curves exhibit a strong size-sensitivity, where the addition of a single atom can alter the melting temperature by up to 100 K. This research addresses the relationship of electronic structure to the melting behaviour in small gallium clusters through a parallel tempering implementation of first-principles molecular dynamics simulations. These simulations cover 11 cluster sizes and two charge states, including neutral clusters sized 7-12 atoms and cationic clusters sized 32-35 atoms. The modelling of small clusters sets a lower size limit for melting and illustrates that greater-than-bulk melting is not universal for small gallium clusters. The larger cluster sizes allow for a direct comparison to experimental data. Each simulation reveals that the clusters have a non-covalent nature more similar to the metallic surface structure of bulk gallium than its covalently bonded interior. The dramatic lowering of melting temperatures and cluster stabilities with single atom additions supports the conclusion that the difference in the nature of bonding between bulk and clusters accounts for the melting temperature discrepancy. Finally, in order to gain additional insight into the nature of bonding in molecular solids, the cohesive energies of the solid halogens are calculated by the method of increments. These calculations investigate the relative N-body correlation energy contributions to the total cohesive energy for solid Cl2, Br2 and I2.</p>

Modelling the Melting of Gallium Clusters: A Path to Understanding Molecular Solids

<p>Gallium is a molecular solid with many unique properties. Comprised of Ga2 dimers but exhibiting metal-like electronic characteristics, gallium may be deemed a molecular metal. The role of this dual covalent-metallic nature may explain gallium’s fascinating thermodynamic behaviour. While bulk gallium melts at 303 K, clusters with only 10’s of atoms melt at temperatures between 500 and 800 K, according to experiment. The measured specific heat curves exhibit a strong size-sensitivity, where the addition of a single atom can alter the melting temperature by up to 100 K. This research addresses the relationship of electronic structure to the melting behaviour in small gallium clusters through a parallel tempering implementation of first-principles molecular dynamics simulations. These simulations cover 11 cluster sizes and two charge states, including neutral clusters sized 7-12 atoms and cationic clusters sized 32-35 atoms. The modelling of small clusters sets a lower size limit for melting and illustrates that greater-than-bulk melting is not universal for small gallium clusters. The larger cluster sizes allow for a direct comparison to experimental data. Each simulation reveals that the clusters have a non-covalent nature more similar to the metallic surface structure of bulk gallium than its covalently bonded interior. The dramatic lowering of melting temperatures and cluster stabilities with single atom additions supports the conclusion that the difference in the nature of bonding between bulk and clusters accounts for the melting temperature discrepancy. Finally, in order to gain additional insight into the nature of bonding in molecular solids, the cohesive energies of the solid halogens are calculated by the method of increments. These calculations investigate the relative N-body correlation energy contributions to the total cohesive energy for solid Cl2, Br2 and I2.</p>