Wayne D. Kaplan

Oscillatory Mass Transport in Vapor-Liquid-Solid Growth of Sapphire Nanowires

Growing Nanowires In vapor-liquid-solid (VLS) growth of nanowires, the liquid phase acts as a transporter to bring material from the gas phase to the growing solid. By heating a single crystal of sapphire in a high-resolution transmission microscope, Oh et al. (p. 489) monitored the growth of sapphire (α-Al2O3) nanowires out of an aluminum droplet. The liquid aluminum brings oxygen to the growing wire surface, in alternating growth and dissolution reactions at the edge of the wire. The oscillation created an optimum face at the self-catalytic site for atomic stacking and regenerated the junction between the VLS phases, allowing growth of the nanowire. High-resolution transmission electron microscopy is used to identify oscillatory growth of a sapphire nanowire. In vapor-liquid-solid (VLS) growth, the liquid phase plays a pivotal role in mediating mass transport from the vapor source to the growth front of a nanowire. Such transport often takes place through the liquid phase. However, we observed by in situ transmission electron microscopy a different behavior for self-catalytic VLS growth of sapphire nanowires. The growth occurs in a layer-by-layer fashion and is accomplished by interfacial diffusion of oxygen through the ordered liquid aluminum atoms. Oscillatory growth and dissolution reactions at the top rim of the nanowires occur and supply the oxygen required to grow a new (0006) sapphire layer. A periodic modulation of the VLS triple-junction configuration accompanies these oscillatory reactions.

Nanometer-Thick Equilibrium Films: The Interface Between Thermodynamics and Atomistics

Model experiments show that nanometer-thick films at interfaces reduce interface energy and form an equilibrium state. Nanometer-thick films at interfaces and surfaces exist in various materials and can substantially influence their properties. Whether these films are an equilibrium or transient state is debated. To address this question, we equilibrated 1.2-nanometer-thick films at gold-sapphire interfaces in the presence of anorthite glass and measured the solid-solid interface energy. The equilibrated film significantly reduced the interfacial energy and could be described by the Gibbs adsorption isotherm expanded to include structure in addition to chemical excess. Unlike artificially made conventional thin films, these films do not break up during equilibration and offer an alternative design criterion for thin-film technology. These results demonstrate that nanometer-thick films at interfaces and surfaces can be an equilibrium state and included in phase diagrams with dedicated tie-lines.

Aluminium-alumina interface morphology and thermodynamics from dewetting experiments

Abstract This work presents the morphology of micron-sized metallic drops (Al) equilibrated on ceramic substrates (sapphire), with emphasis on the interfacial region. The samples were made by controlled dewetting of a thin Al film on the basal surface of sapphire substrates, at temperatures above the melting point of Al. Morphological inspections showed that evaporation of the sapphire substrate atoms, which diffuse at the interface, has an important role in the evolution of the Al-sapphire interface morphology. The absolute value of the sapphire basal surface energy was extracted (1279 ± 78 mJ/m 2 at 900 °C). The values of the thermodynamic parameters determined from the interface morphology agree well with those from experiments made under identical conditions but using millimetre-sized drops. This confirms the validity of Young’s equation for large sessile drops in systems undergoing limited morphological changes (ridging) at the interface.

Oxygen induced interfacial phenomena during wetting of alumina by liquid aluminium

Abstract Sessile drop experiments of liquid Al on sapphire (α-Al2O3) were conducted under a low pressure (10−3 Torr) controlled Ar atmosphere as a function of oxygen partial pressure, temperature and/or time. Microstructural investigations of the samples from the experiments indicated that two different dominant processes occur at the liquid Al-sapphire interface: epitaxial growth of new α-Al2O3 layers on the sapphire substrate at temperatures below ≈1100°C and dissolution of the sapphire substrate at temperatures above ≈1100°C. Possible mechanisms by which oxygen affects wetting and adhesion of liquid Al on sapphire are examined. The non-wetting to wetting transition in this system may be explained by formation of an oxygen-rich interphase at the liquid Al-sapphire interface. It is concluded that dissolution of sapphire under Al oxidation conditions is capillary driven.

Structure refinement of titanium carbonitride (TiCN)

Rietveld structure refinement was conducted on X-ray powder diffraction data of TiCN. The existing structural model for TiCN, based on a cubic lattice with random occupation of sites by Ti, C and N, was found to be incorrect. The TiCN structure is better described based on the TiN structure (NaCl type) with substitution of N by C. Rietveld refinement showed that only a low concentration of vacancies exists in the nonmetallic sublattice.

Processing and properties of Al2O3 nanocomposites reinforced with sub-micron Ni and NiAl2O4

Abstract A new process has been developed to produce sintered α-Al2O3 reinforced with sub-micron Ni particles. The process is based on the infiltration of ceramic preforms with liquid salts, which are reduced during pressureless sintering to form metal particles. The reduction occurs within the open pores of the ceramic preform, after evaporation of the metal salts into the gas phase. By controlling the partial pressure of oxygen during the sintering process, a reaction between the Ni particles and the alumina matrix can be invoked to form Ni-spinel (NiAl2O4) particles. Three-point bending experiments show an increase in strength for the nickel-reinforced alumina and a significant increase for the spinel-reinforced alumina.

Residual stresses in alumina-SiC nanocomposites

Abstract Residual stresses in an alumina-SiC particulate composite were studied as a function of SiC content by X-ray diffraction. The average microstresses in each phase and the stress fluctuations in the matrix were evaluated from a combination of X-ray reflection shift and line broadening analysis. The measured average microstresses show good agreement with those calculated theoretically from a simple model. The average microstresses in the matrix increases with the SiC content while the fluctuations of the stress field decrease. In effect, the mean dislocation density in the alumina matrix increases with the SiC volume fraction with more dislocations forming networks and subboundaries, as was confirmed by transmission electron microscopy.

Quantitative HRTEM analysis of FIB prepared specimens.

The preparation of good transmission electron microscopy specimens with minimum milling damage can be very complicated, especially from a specific area in a sample. Therefore, a novel approach for transmission electron microscopy specimen preparation using a focused ion beam system is proposed, based on the use of low energy (5 kV)Ga ions and a low incident ion angle (∼1°) from a thickness of ∼500 nm until the sample is electron transparent. Transmission electron microscopy specimens prepared by this method have significantly less irradiation damage, demonstrated by successful quantitative high‐resolution transmission electron microscopy conducted on sapphire from data acquired using an aberration‐corrected field emission gun transmission electron microscopy. Quantitative analysis was conducted by iterative digital image matching. The accuracy and sensitivity of the matching process is discussed.

Atomistic study of structural correlations at a liquid–solid interface

Abstract Structural correlations at a liquid–solid interface were explored with molecular dynamics simulations of a model aluminium system using the Ercolessi–Adams potential and up to 4320 atoms. Substrate atoms were pinned to their equilibrium crystalline positions while liquid atoms were free to move. The density profile at the interface was investigated for different substrate crystallographic orientations and temperatures. An exponential decay of the density profile was observed, ρ ( z )∼e − κz , leading to the definition of κ as a quantitative measure of the ordering at the liquid solid interface. A direct correlation between the amount of ordering in the liquid phase and the underlying substrate orientation was found.

Structure of Electrodeposited Cobalt

The structure of cobalt formed by electrodeposition and the influence of the pH of the plating solution and the cathode potential was studied by potentiodynamic measurements and X-ray diffraction. It was found that the level of overpotential significantly affects the structure of the formed cobalt. When electrodeposition is performed far from equilibrium conditions, i.e., at a high overpotential, face-centered cubic (fcc) cobalt is deposited while at low overpotential hexagonal close packed Co is formed with a lower rate of hydrogen evolution. A higher overpotential is needed in a neutral compared to acidic solution in order to enhance the evolution of hydrogen that is required for the formation of fcc cobalt.