Cluster Assembly of Hierarchical Nanostructures
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CLUSTER ASSEMBLY OF HIERARCHICAL NANOSTRUCTURES
RICHARD W. SIEGEL Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
ABSTRACT In the past few years, atom clusters with diameters in the range of 2-20 nm of a variety of materials, including both metals and ceramics, have been synthesized by evaporation and condensation in high-purity gases and subsequently consolidated in situ under ultrahigh vacuum conditions to create nanophase materials. These new ultrafine-grained materials have properties that are often significantly different and considerably improved relative to those of their coarser-grained counterparts owing to both their small grain-size scale and the large percentage of their atoms in grain boundary environments. Since their properties can be engineered during the synthesis and processing steps, cluster-assembled materials appear to have significant potential for the introduction of a hierarchy of both structure and properties. Some of the recent research on nanophase materials related to properties and scale are reviewed and some of the possibilities for synthesizing hierarchical nanostructures via cluster assembly are considered. INTRODUCTION Hierarchical materials are ensembles of structure and function in which scale, interaction, and architecture play a dominant role. Cluster-assembled nanostructures already exhibit simple forms of hierarchy, and more complex hierarchical materials should be possible to synthesize by cluster assembly in the future, but much work remains before this can happen. Nanophase materials appear to have existed since the beginning of time, even before the advent of the beautiful and complex biological hierarchies cited in this Symposium. Evidence from the earliest meteorites suggests that primordial materials with nanometer-scale phase structures condensed from our solar nebula [1-3]. Billions of years later, the man-made synthesis of nanometer size atomic clusters of metals and ceramics by means of the gas-condensation method, followed by their in situ consolidation under high-vacuum conditions, has resulted in a new class of synthetic ultrafinegrained materials where sophisticated control of scale, interaction, and architecture may become possible. For now, such hierarchy is in its infancy. Increasing interest has focused on a variety of synthetic nanostructured materials, with average grain sizes below 100 nm, during the past several years with the anticipation that their properties will be different from, and often superior to, those of conventional materials that have phase or grain structures on a coarser size scale [4]. This interest has been stimulated not only by the considerable recent effort and success in synthesizing a variety of one-dimensionally modulated, multilayered materials with nanometer scale modulations, but also by the exciting possibility of synthesizing three-dimensionally analogous, bulk nanophase materials via the assembly of clusters of atoms [5]. Atom clusters in the nanometer size regime, containing hundreds to te
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