Fabrication of Hollow AlAu2 Nanoparticles by Solid State Dewetting and Oxidation of Al on Sapphire Substrate

The Al-Au binary diffusion couple is a classic example of the system exhibiting Kirkendall voiding during interdiffusion. We demonstrate that this effect, which is a major reason for failures of the wire bonds in microelectronics, can be utilized for producing hollow AlAu2 nanoparticles attached to sapphire substrate. To this end, we produced the core-shell Al-Au nanoparticles by performing a solid state dewetting treatment of Al thin film deposited on sapphire substrate, followed by the deposition of thin Au layer on the top of dewetted sample. Annealing of the core-shell nanoparticles in air resulted in outdiffusion of Al from the particles, formation of pores, and growth of the AlAu2 intermetallic phase in the particles. We demonstrated that the driving force for hollowing is the oxidation reaction of the Al atoms at the Au-sapphire interface, leading to the homoepitaxial growth of newly formed alumina at the interface. We developed a kinetic model of hollowing controlled by diffusion of oxygen through the Au thin film, and estimated the solubility of oxygen in solid Au. Our work demonstrates that the core-shell nanoparticles attached to the substrate can be hollowed by the Kirkendall effect in the thin film spatially separated from the particles.Comment: 27 pages, 8 figure

Annealing-induced recovery of indents in thin Au(Fe) bilayer films

We employed depth-sensing nanoindentation to produce ordered arrays of indents on the surface of 50 nm-thick Au(Fe) films deposited on sapphire substrates. The maximum depth of the indents was approximately one-half of the film thickness. The indented films were annealed at a temperature of 700 °C in a forming gas atmosphere. While the onset of solid-state dewetting was observed in the unperturbed regions of the film, no holes to the substrate were observed in the indented regions. Instead, the film annealing resulted in the formation of hillocks at the indent locations, followed by their dissipation and the formation of shallow depressions nearby after subsequent annealing treatments. This annealing-induced evolution of nanoindents was interpreted in terms of annihilation of dislocation loops generated during indentation, accompanied by the formation of nanopores at the grain boundaries and their subsequent dissolution. The application of the processes uncovered in this work show great potential for the patterning of thin films

Self-Healing and Shape Memory Effects in Gold Microparticles through the Defects-Mediated Diffusion

Some metal alloys subjected to irreversible plastic deformation can repair the inflicted damage and/or recover their original shape upon heating. The conventional shape memory effect in metallic alloys relies on collective, or “military” phase transformations. This work demonstrates a new and fundamentally different type of self-healing and shape memory in single crystalline faceted nano and microparticles of pure gold, which are plastically deformed with an atomic force microscope tip. It is shown that annealing of the deformed particles at elevated temperatures leads to nearly full recovery of their initial asymmetric polyhedral shape, which does not correspond to global energy minimum shape. The atomistic molecular dynamic simulations demonstrate that the shape recovery of the particles is controlled by the self-diffusion of gold atoms along the terrace ledges formed during the particles indentation. This ledge-guided diffusion leads to shape recovery by the irreversible diffusion process. A semiquantitative model of healing developed in this work demonstrates a good agreement with the experimental data

Thermal Stability of Thin Au Films Deposited on Salt Whiskers

Thin metal films deposited on patterned or rough substrates play an increasing role in microelectronics, sensing, catalysis, and other areas of nanotechnology. However, the thermal stability and solid state dewetting of thin metal films with complex three-dimensional architecture is still poorly understood. In this work we employed a model system of nanocrystalline Au thin films deposited on prismatic single crystalline KCl whiskers to study the solid state dewetting of thin films in a three-dimensional setting. The arrays of KCl whiskers were grown on porous substrates under well-defined humidity and temperature conditions. Single crystalline prismatic KCl whiskers with a very high aspect ratio, [001] axis and {100} side facets were obtained. The whiskers were coated with thin conformal Au films of 20-30 nm in thickness. The annealing of these core-shell whiskers at the temperature of 350oC resulted in solid state dewetting of the Au film, with the dewetting processes occurring much faster along the whisker edges than on the side facets. The orientation relationships between Au and KCl were determined by employing similarly prepared thin Au films deposited on the flat KCl (100) substrates. Inspired by our experimental results, we developed a numerical model describing the curvature-gradient driven and surface diffusion-controlled growth of a hole in the thin film deposited on a curved substrate. The model predicted the growth of anisotropic elliptical holes elongated along the whisker axis. We discuss the experimental results in terms of the proposed model, indicating the importance of the change in orientation relationship between the Au grains and KCl whisker along the whisker edges

Thermal Stability of Thin Au Films Deposited on Salt Whiskers

Thin metal films deposited on patterned or rough substrates play an increasing role in microelectronics, sensing, catalysis, and other areas of nanotechnology. However, the thermal stability and solid state dewetting of thin metal films with complex three-dimensional architecture is still poorly understood. In this work we employed a model system of nanocrystalline Au thin films deposited on prismatic single crystalline KCl whiskers to study the solid state dewetting of thin films in a three-dimensional setting. The arrays of KCl whiskers were grown on porous substrates under well-defined humidity and temperature conditions. Single crystalline prismatic KCl whiskers with a very high aspect ratio, [001] axis and {100} side facets were obtained. The whiskers were coated with thin conformal Au films of 20-30 nm in thickness. The annealing of these core-shell whiskers at the temperature of 350oC resulted in solid state dewetting of the Au film, with the dewetting processes occurring much faster along the whisker edges than on the side facets. The orientation relationships between Au and KCl were determined by employing similarly prepared thin Au films deposited on the flat KCl (100) substrates. Inspired by our experimental results, we developed a numerical model describing the curvature-gradient driven and surface diffusion-controlled growth of a hole in the thin film deposited on a curved substrate. The model predicted the growth of anisotropic elliptical holes elongated along the whisker axis. We discuss the experimental results in terms of the proposed model, indicating the importance of the change in orientation relationship between the Au grains and KCl whisker along the whisker edges

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Structure Refinement and Fragmentation of Precipitates under Severe Plastic Deformation: A Review

During severe plastic deformation (SPD), the processes of lattice defect formation as well as their relaxation (annihilation) compete with each other. As a result, a dynamic equilibrium is established, and a steady state is reached after a certain strain value. Simultaneously, other kinetic processes act in opposite directions and also compete with each other during SPD, such as grain refinement/growth, mechanical strengthening/softening, formation/decomposition of solid solution, etc. These competing processes also lead to dynamic equilibrium and result in a steady state (saturation), albeit after different strains. Among these steady-state phenomena, particle fragmentation during the second phase of SPD has received little attention. Available data indicate that precipitate fragmentation slows down with increasing strain, though saturation is achieved at higher strains than in the case of hardness or grain size. Moreover, one can consider the SPD-driven nanocrystallization in the amorphous phase as a process that is opposite to the fragmentation of precipitates. The size of these crystalline nanoprecipitates also saturates after a certain strain. The fragmentation of precipitates during SPD is the topic of this review

Grain Boundary Wetting Phenomena in High Entropy Alloys Containing Nitrides, Carbides, Borides, Silicides, and Hydrogen: A Review

In this review, we analyze the structure of multicomponent alloys without principal components (they are also called high entropy alloys-HEAs), containing not only metals but also hydrogen, nitrogen, carbon, boron, or silicon. In particular, we discuss the phenomenon of grain boundary (GB) wetting by the melt or solid phase. The GB wetting can be complete or incomplete (partial). In the former case, the grains of the matrix are completely separated by the continuous layer of the second phase (solid or liquid). In the latter case of partial GB wetting, the second solid phase forms, between the matrix grains, a chain of (usually lenticular) precipitates or droplets with a non-zero value of the contact angle. To deal with the morphology of GBs, the new GB tie-lines are used, which can be constructed in the two- or multiphase areas of the multidimensional HEAs phase diagrams. The GBs in HEAs in the case of complete or partial wetting can also contain hydrides, nitrides, carbides, borides, or silicides. Thus, GB wetting by the hydrides, nitrides, carbides, borides, or silicides can be used in the so-called grain boundary chemical engineering in order to improve the properties of respective HEAs.This research was funded by the Russian Ministry of Science and Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013) Support from the University of the Basque Country under the GIU19/019 project is also acknowledged