Melting Behavior in Granular Metal Thin Films
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MELTING BEHAVIOR IN GRANULAR METAL THIN FIlMS KARL M. UNRUH,* B.M. PATTERSON,* AND S.I. SHAH** *Department of Physics and Astronomy, University of Delaware, Newark, DE 19716 **Central Research and Development, Experimental Station, E.I. du Pont de Nemours and Co., Wilmington, DE 19880 ABSTRACT Granular metal films consisting of small Sn, Bi, and Pb particles, typically from less than 100 A to several 1000 A in size and embedded in a SiOz matrix, have been fabricated over a range of metal compositions by RF sputter deposition. Two different film geometries have been prepared, homogeneous films and multilayer films consisting of alternating layers of granular metal and SiO2. These films have been characterized by x-ray diffraction and transmission electron microscopy and their melting behavior studied by differential scanning calorimetry. As the concentration of the metal component is decreased, the average particle size decreases and the particle size distribution becomes more narrow. When the solid-liquid transition is studied, the melting temperature has been found to be increasingly depressed as the particle size is reduced. In the smallest particles the relative reduction in the melting temperature is greater than 10 percent. No strong evidence for melting point enhancements, due to pressure effects arising from the different thermal expansions of the metal particles and the SiO2 matrix, has been observed in either the homogeneous or multilayer films. INTRODUCTION Finely divided matter exhibits a number of size dependent physical properties arising from intrinsic size effects, as well as from the relatively large influence of surface properties when surface to volume ratios are large. A number of chemical, mechanical, and vapor deposition methods can be used to prepare particles sufficiently small for the study of these effects.' Chemical methods have been used to obtain narrow particle size distributions and various particle morphologies; typical particle sizes, however, are rarely less than about 1000 A.2 Mechanical methods, particularly high energy ball milling, can be used to obtain small particles by the repeated fracturing and cold welding of immiscible powders. Although particles as small as about 100 A have been3 obtained, this technique tends to produce particles with high defect densities. Finally, particles as small as several 10's of A, both in the form of discontinuous thin films and granular metal films, have been prepared by several vapor deposition methods.A Vapor deposited granular metal films offer a number of advantages for the study of the physical properties of small particles. These include ease and flexibility of fabrication, automatic environmental protection for the small particles, particularly at small metal concentrations, due to the surrounding insulating matrix, and the opportunity for studying various effects arising from the interaction of the particles with the constraining matrix. In this work we report the results of a study of the size dependent thermal properties of small met
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