TECHNICAL: This project executes a fundamental, systematic model study of the factors that impact the morphologic evolution and stability of metal particles embedded in an inert oxide matrix. Sapphire provides the inert matrix that surrounds and constrains dispersed metal particles of Pt, a metal chosen for its chemical compatibility with, and close thermal expansion match to, sapphire. Parametric control is exerted over particle size, temperature, annealing time, ambient atmosphere (oxygen partial pressure) and relative crystallographic orientation between sapphire and Pt to allow assessment of their individual effects. The equilibrium shape of a stress-free particle minimizes the total interfacial energy although elastic energy terms can become increasingly important as particle size increases. Ensembles of particles undergo coarsening to reduce total interfacial energy, exploiting any and all available diffusion paths to redistribute chemical constituents. In the nano dimensional domain, this driving force is especially high, while in larger particles, elastic energy effects can impede coarsening. To explore a wide range of particle size and orientation relationships, two complementary synthesis methodologies are employed. Equiaxed nanoscale particles are developed in Pt-ion-implanted sapphire by annealing. Larger, non equilibrium shaped Pt particles are developed by lithographic processing and etching to generate surface cavities that are filled with Pt by thin-film deposition, then encapsulated by from diffusion bonding to a second sapphire substrate. The sapphire orientation and encapsulation cell geometry and crystallography are controlled and varied. The high melting point of Pt allows experiments in a temperature range where diffusion within Pt, within Al2O3, and along their interfaces allows shape changes to occur. Proximity of the particles to the external surfaces allows assessment of the influence of ambient oxygen pressure. The sapphire encapsulation cell has the stable alpha-Al2O3 phase, not a metastable transition phase. Structural and compositional analyses of all samples are conducted with appropriate methods of transmission electron microscopy, including spatially-resolved selected area diffraction, convergent beam electron diffraction, imaging via diffraction contrast and phase contrast methods, and spatially resolved spectrometry, including electron energy loss spectrometry of local bonding configurations coupled to energy-filtered imaging. Equilibrium shapes are characterized, stable facets are identified, and effects of interfacial structure (faceted versus roughened, coherent versus incoherent) on shape relaxation rates are assessed. Interfacial structure and elastic energy effects on particle size distributions and evolution rates are also assessed by in-situ hot stage (1300°C) transmission electron microscopy studies of nanosized particle coarsening. NON-TECHNICAL: Novel synthesis and stabilization strategies are based on understanding the interplay between interfacial structure and kinetic response, and this study provides a platform for investigating more complex systems. The project leverages local programs in the Center for Science and Engineering Education at the Lawrence Berkeley National Laboratory and the Summer Undergraduate Program in Engineering Research at Berkeley (SUPERB) that appeal to students historically underrepresented in science and engineering for reasons of social, cultural, educational, or economic disadvantage, and includes undergraduates and high school students in research. This effort also enhances the scholarly activities of students seeking graduate degrees with a Designated Emphasis in Nanoscale Science & Engineering (NSE) at Berkeley.
