Ultrafine Metal Particles in Porous and Dense Silica Gels
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Ultrafine Metal Particles in Porous and Dense Silica Gels* Edward J.A. Pope and J.D. Mackenzie Introduction The present trend of developing electronic devices with increasingly fine dimensions borders on a number of fundamental scientific questions about the very nature of how materials at ultrafine d i m e n s i o n s b e h a v e . This article addresses some of these questions. The fabrication of discrete metallic phases in porous and nonporous glassy matrices presents a number of exciting device possibilities. Methods of fabricating ultrafine metallic phases in silica via the sol-gel route are presented. In attempting to fabricate materials with ultrafine physical dimensions for a wide variety of applications, several fundamental questions arise about the nature of materials behavior. For example, how many metal atoms are necessary to form a cluster exhibiting "metallic" properties? Moreover, does the number of atoms necessary depend upon which metallic property is examined? This question has been partly addressed by D.C. Johnson and coworkers with regard to magnetism in osmium clusters.1 Their results show a threefold increase in magnetic susceptibility between clusters containing 3-10 osmium atoms. Another important question, especially when considering device applications, is how the relative contributions of surface and bulk thermodynamics affect such properties as phase transformations. In addition, ultrafine phase dimensions interact with the fundamental unit lengths of a wide range of processes, including the wavelength of visi-
ble light, the mean free path lengths of conduction processes, the wavelengths of phonon vibrations, etc. How do these interactions affect optical, thermal, and electronic properties? Fabrication of ultrafine metallic particles in porous and nonporous matrices may lead to many possible device applications including heterogeneous cata-
Phase Transformations Various geometries are possible for ultrafine phase interactions. Table I presents the number of dimensions available for interaction and the surface-areato-volume (SA/V) ratio dependencies for thin films, filaments, and spherical particles. The spherical particle geometry has the highest number of dimensions available for interaction and SA/V ratio dependency. For example, a particle of lOA radius has a SA/V ratio five orders of magnitude greater than a particle with a 10 micron radius. The effect of this change in SA/V ratio on the free energy change for a transformational process is described mathematically by the relationship AG, = (47rr3/3)AGt, + (4m2) A-y+(W/3)Ae where: AG„ = volume free energy change Ay = surface free energy change Ae = strain energy change. From this relationship, it is evident that
Table I: Geometries and Dimensions Available for Ultrafine Interactions
Geometry of
Sample
Dimensions Available for . Interaction
Surface Area to Volume Ratio Dependency
Thin Film
2/d,
4/di
6/d,
*This paper was presented at the symposium on Multicomponent Ultrafine Microstructures at an MRS Meeting. 20
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