A Mechanism for Particle Coalescence, Grain Growth and Twin Formation during Annealing of Gold Particles in an Amorphous
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A MECHANISM FOR PARTICLE COALESCENCE, GRAIN GROWTH AND TWIN FORMATION DURING ANNEALING OF GOLD PARTICLES IN AN AMORPHOUS SILICA MATRIX J. McGinnI, V.A. Greenhut 2, and T. Tsakalakos2 1 David Sarnoff Research Center, RCA Laboratories, Princeton, 2 Rutgers University, College of Engineering, Piscataway, NJ
NJ 08540 08854
ABSTRACT Direct evidence was obtained for the coalescence of discrete gold particles (%lO0 nm) surrounded by a continuous 0 amorphous silica matrix during annealing at 900 C. The particles were found to form high angle boundaries as coalescence occurred. Subsequent annealing revealed that grain growth occurred with relatively rapid elimination of high angle grain boundaries. The coalesced particles became approximately spherical in shape and seven particle morphologies could be distinguished. A majority of particles showed either no defect structure or conventional f.c.c. twins. In more limited cases, a small area of high angle grain boundary, usually of coincidence lattice type, was observed which accommodated intersecting twins. A single self-consistent mechanism can explain the elimination of high angle boundaries and all the various twin structures observed. The mechanism is based on Gleiter's [l] model for grain boundary migration with twin structures resulting from growth accidents on the migrating high angle boundary.
INTRODUCTION A variety of mechanisms have been proposed for grain growth and formation of growth twins in f.c.c. metals, but rather limited experimental evidence exists with which to examine these models. An ideal system for this purpose is that of gold particles within an amorphous silica matrix. Such a cermet system can be prepared conveniently by sputtering techniques, yielding films suitable for TEM or X-ray analysis. Spherical gold particles form, completely surrounded or "encapsulated" in a continuous matrix of amorphous silica. Amongst the advantages of this system are: (1) A small particle size which drives structures to quasi-stability rapidly (2) A highly activated gold particle is held within a rather stable, elastic solid matrix of silica at annealing temperatures (3) A boundary between particles and matrix which is free of the complexities of coherency effects (4) A minimal chemical interaction between gold and silica (5) An absence of matrix-imposed orientation effects on particles (6) A high atomic number for the gold particles and high atomic number difference with the silica matrix for good TEM and X-ray diffraction contrast (7) An amorphous matrix to facilitate observation of crystalline particles The gold-amorphous silica system may indeed prove useful as a model system for the study of various particle growth phenomena upon annealing. It is a useful model material for examining the growth kinetics of electronic thin film cermets. The effects of volume fraction, initial particle size distribution, time, temperature, purity, etc. can be conveniently examined. A preliminary study [2] has shown that the kinetics of particle growth 0 upon annealing of the gold-s
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