Computer simulations of interactions between ultrafine alumina particles produced by an arc discharge
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Computer simulations of interactions between ultrafine alumina particles produced by an arc discharge M. H. Teng, L. D. Marks, and D. L. Johnson Department of Materials Science and Engineering, Northwestern University, 2225 N. Campus Dr., Evanston, Illinois 60208 (Received 25 September 1995; accepted 24 August 1996)
We wrote two computer programs, 3D and BUMP, to interpret transmission electron microscope (TEM) micrographs made during a study of the initial stage sintering of ultrafine alumina particles (UFP’s, 20–50 nm in diameter). The first simulated the 3D geometric relationships of particles, from which we concluded that surface diffusion was the predominant sintering mechanism because no shrinkage occurred. BUMP simulated random contact of two particles and showed that the particle chains that formed before sintering were not formed purely by chance. Instead the particles experienced a rearrangement process (rotation and sliding) which reduced the total surface energy.
I. INTRODUCTION
Traditional sintering models assume spherical particle shape, and isotropic surface energy and diffusion coefficient.1–17 The models break down when applied to submicrometer particle sizes and materials with anisotropic surface properties. To explore this new area of sintering science, a unique experiment was carried out. Ultrafine alumina particles were produced by an arc-discharge method, sintered in flight through a furnace, and collected and observed in a clean UHV environment.18–22 The experimental results were recorded on several hundred micrographs taken by a Hitachi UHV-H9000 TEM. Although several studies related to sintering of ultrafine alumina have been reported, none was done in a well-controlled UHV environment.23–26 The information about the 3D arrangement of the particles was critical to the successful deduction of the sintering mechanism. Unfortunately, micrographs show only projected outlines of particles (Fig. 1). In some cases when the particles are aligned well (in a known crystal axis direction) a simple measurement of the distances between each parallel pair of outline facets would be enough and may determine their geometric center and shape.27,28 In most cases the particles, which were in chains, were randomly oriented and their 3D relationships were impossible to determine by inspection. Electron and x-ray diffraction revealed that the alumina particles had a spinel structure18,20 and nearly uniform shape (see Sec. II), with only minor variations. These facts facilitated the development of simulation programs. This paper consists of three parts. In Sec. II we define two parameters (GR and V) to describe the geometric shapes of particles. Each of the following two sections J. Mater. Res., Vol. 12, No. 1, Jan 1997
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describes a simulation program that was used as a tool to solve one specific problem related to the study. In Sec. III, the first computer program, named 3D, is described and its interactive op
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