In this study, samples of nanoscale structures of nickel (Ni) nanorods were prepared using the glancing angle deposition (GLAD) technique. Annealing was done using a split- top tube furnace at high vacuum chamber pressure. The pre-melting of the nanorods was maintained at 500 °C for 30 minutes in all the samples. Using the samples with 90 minutes of GLAD time, the annealing behavior of the nanorods was studied at 300 °C, 400 °C, 500 °C and 600 °C. The nanorods were then imaged using scanning electron microscopy. Using X-ray diffraction, the crystalline microstructures of the nanorods were studied. It was found that with increasing annealing temperatures, the intensity of peaks for both Ni (111) and Ni (200) increased, which indicates that better crystals were formed. The results indicate that re-crystallization occurs after annealing, leading to the formation of larger grain sizes compared to as-deposited grain sizes. Annealing substantially changed the structure of the nanorods, leading to different smoother, more connected crystal structures for the annealed nanorods compared to as-deposited ones.

Ag nanoparticle (NP) paste was fabricated and used to bond Cu wire to Cu foil at low temperatures down to 433 K. The relatively low bonding temperature promotes this method to be used in polymer-based flexible electronics, which cannot withstand high bonding temperatures due the possible melting of the polymer substrate. Unlike low-temperature soldering techniquies, bonds formed by this method was proved to withstand temperatures higher than the bonding temperature, which also promotes it to be used in electronics that operate at high temperatures. The Ag NP paste was developed by increasing the concentration of 50 nm Ag NP sol from 0.001 vol.% to 0.1 vol.% by centrifugation. The 0.001 vol.% Ag NP sol was fabricated in water by reducing silver nitrate (AgNO3) using sodium citrate dihydrate (Na3C6H5O7.2H2O). The bond was formed by solid state sintering among the individual Ag NPs and solid state bonding of these Ag NPs onto both Cu wire and foil. Metallurgical bonds between Ag NPs and Cu were confirmed by transmission electron microscopy (TEM). The Ag NPs were coated with an organic shell to prevent sintering at room temperature. It was found that the organic shell decomposed at 433 K, defining the lowest temperature at which a bond could be formed. Shear tests showed that the joint strength increased as the bonding temperature increased due to enhanced sintering of Ag NPs at higher temperatures. For better understanding of the melting and the sintering kinetics of Ag NPs, a molecular dynamics (MD) simulation based on the embedded atom method (EAM) was conducted to different sizes of Ag NPs with diameters between 4 nm and 20 nm. Programmed heating of an equal rate was applied to all sizes of NPs to find the complete melting and surface premelting points and sintering kinetics of the Ag NPs. The initial structural configuration of the Ag NPs was FCC truncated octahedral, which found to be stable for this size range of NPs. As a first step toward drawing a phase map of stable solid phases of Ag NPs at different temperatures and sizes of Ag NPs, the stability of the FCC truncated octahedral was studied for Ag NPs in size range of 1 nm to 4 nm. The smallest Ag NPs at which this configuration is stable was determined as 1.8 nm. Unlike the previous theoretical models, this MD model predicted both complete melting and surface premelting points for a wider size range of NPs. Melting kinetics showed three different trends that are, respectively, associated with NPs in the size ranges of 4 nm to 7 nm, 8 nm to 10 nm, and 12 nm to 20 nm. Ag NPs in the first range melted at a single temperature without passing through a surface premelting stage. Melting of the second range started by forming a quasi-liquid layer that expanded to the core, followed by the formation of a liquid layer of 1.8 nm thickness that also subsequently expanded to the core with increasing temperature, completing the melting process. For particles in the third range, the 1.8 nm liquid layer was formed once the thickness of the quasi-liquid layer reached 5 nm. The liquid layer expanded to the core and formed thicker stable liquid layers as the temperature increased toward the complete melting point. The ratio of the quasi-liquid layer thickness to the NP radius showed a linear relationship with temperature. Sintering kinetics of two Ag NPs in the size range of 4 nm to 20 nm, and sintering of three and four Ag NPs of 4 nm diameter was also studied by MD simulation. The sintering process passed through three main stages. The first was the neck formation followed by a rapid increase of the neck radius to particle radius ratio at 50 K for 20 nm particles and at 10 K for smaller NPs. The second was characterized by a gradual linear increase of the neck radius to particle radius ratio as the temperature of the sintered structure was increased to the surface premelting point. A twin boundary was formed during the second stage that relaxed the sintered structure and decreased the average potential energy (PE) of all atoms. The third stage of sintering was a rapid shrinkage during surface premelting of the sintered structure. Based on pore geometry, densification occurred during the first stage for three 4 nm particles and during the second stage for four 4 nm particles. Sintering rates obtained here were higher than those obtained by theoretical models generally used for predicting sintering rates of micro-particles.

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<p>A framework is described using the phase-field crystal model for the study of premelting in binary alloys through short-range interfacial interactions that arise from the structure of grain boundaries. A nonconserved model A formulation of PFC was used to model grain boundaries in two dimensions for several different angles of misorientation: 27.8, 21.8, 17.8, 13.2, and 5 degrees. The character of the premelting transition, whereby a liquid-like film develops at a defect at temperatures below the melting point, changed with misorientation angle. An excess mass over the grain boundary can be defined as an analog to the liquid layer thickness due to premelting. It is found that low-angle grain boundaries remain at a relatively constant value of excess mass, and indeed can remain solid above the melting point. High-angle grain boundaries have a logarithmically increasing width that diverges at the melting point. A width-dependent energy can be defined called the disjoining potential that takes into account structure, interfacial and bulk energies to describe the liquid-layer width. The form of this disjoinging potential was found to be exponential and monotonically decreased as width increased for high angles and produced an attractive minimum for low angles. The results of this work were compared to a pure material from a previous study.</p> / Master of Applied Science (MASc)

<p>Premelting is the formation of a thin, thermodynamically stable, liquid-like film at an interface for temperatures below the equilibrium melting temperature. Using a Monte Carlo technique, the underlying short range structural forces for premelting at the grain boundary can be directly calculated. This technique is applied to a (i) Σ9 ⟨115⟩ 120^o twist boundary and a (ii) Σ9 ⟨011⟩ {411} symmetric tilt boundary in an embedded atom model of Aluminum-Magnesium alloy. Both grain boundaries exhibit disordered structures near the melting point that depend on the concentration of Magnesium. The behavior is described quantitatively with sharp interface thermodynamics, involving an interfacial free energy that depends on width of the grain boundary, referred to as the disjoining potential. The disjoining potential calculated for boundary (i) displays a decreasing exponential dependence on width of the grain boundary, while the disjoining potential of (ii) features a weak attractive minimum. This work is discussed in relation to a previous study using pure Nickel, results of which can be useful to the theoretical study of thermodynamic forces underlying grain boundary premelting in an alloy.</p> / Master of Science (MSc)