Fundamental Phenomena in the Materials Sciences: Volume 3: Surface Phenomena, Proceedings of the Third Symposium on Fundamental Phenomena in the Materials Sciences Held January 25–26, 1965, at Boston, Mass. PDF
Preview Fundamental Phenomena in the Materials Sciences: Volume 3: Surface Phenomena, Proceedings of the Third Symposium on Fundamental Phenomena in the Materials Sciences Held January 25–26, 1965, at Boston, Mass.
Fundamental Phenomena in the Materials Sciences Volume 3 Surface Phenomena Fundamental Phenomena in the Materials Sciences Volume 3 Surface Phenomena Proceedings of the Third Symposium on Fundamental Phenomena in the Materials Sciences Held January 25-26, 1965, at Boston, Mass. Edited by L. J. Bonis Ilikon Corporation Natick, Massachusetts P. L. de Bruyn Department of Metallurgy Massachusetts Institute of Technology Cambridge, Massachusetts and J. J. Duga Battelle Memorial Institute Columbus, Ohio 9:> PLENUM PRESS· NEW YORK ·1966 ISBN 978-1-4684-8708-4 ISBN 978-1-4684-8706-0 (eBook) DOl 10.1007/978-1-4684-8706-0 Library of Congress Catalog Card No. 64-20752 © 1966 Plenum Press Softcover reprint oft he hardcover 1s t edition 1966 A Division of Plenum Publishing Corporation 227 West 17 Street, New York, N. Y. 10011 All rights reserved No part of this publication may be reproduced in any form without written permission from the publisher Foreword This volume explores in detail the four interrelated branches of the study of surface phenomena-surface thermodynamics, nucleation, diffusion, and fine-particles technology-providing an unusual and comprehensive body of knowledge that will be of interest and practical value to both materials researchers and practising engineers. The growing awareness-since the advent of the space age-among solid-state physicists, metallurgists, ceramists, chemical engineers, and mechanical engineers of the need for a broad interdisciplinary under standing of the fundamental phenomena common to all matedals has led in recent years to the development of a new field of scientific investigation, Materials Science. To help promote interest in and con tributions to this new technology, annual symposia on "Fundamental Phenomena in the Materials Sciences" have been organized by the Ilikon Corporation. The first symposium, reported in Volume 1 of this series, was held in Boston, Massachusetts, on February 1 and 2, 1963; sintering and plastic deformation were the main topics of discussion. The second meeting, also held in Boston, on January 27 and 28, 1964, was exclusively concerned with the general interdisciplinary problems related to surface phenomena, that is, all of those physical and chemical areas that are pertinent to the surface of a solid, or to the interface between a solid and a gas, a solid and a liquid, or a solid and a solid. Ten noted researchers, from major universities and industrial organiza tions, presented papers that dealt with the principles of surface physics and surface chemistry. Surface phenomena were treated, therefore, in a broad sense, with emphasis on structures, electronic configurations, and properties of clean real surfaces, rather than on the generic bases of specific material types. Although unique and broadly rewarding, that second meeting barely "scratched the surface" of what all the participants at the meeting recognized to be a truly fundamental materials technology subject. It was decided, therefore, to continue these surface phenomena investigations at a third meeting. The papers presented at that conference, held January 25 and 26, 1965, in Boston, together with v vi Foreword the panel discussions as they occurred, are presented in this volume. In the first chapter, P. L. de Bruyn (Massachusetts Institute of Tech nology) reviews Gibbs' classical thermodynamics treatment of surfaces and interfaces,giving particular attention to Gibbs' concept of a dividing surface. In this context of the classical thermodynamics equations, the author proceeds in his discussion to point out that, in most thermo dynamic treatments of multiphases, it is assumed that, when the main interest is in the bulk properties, the surface may be completely ignored. Next, he suggests that for the study of the interface between two juxta posed contiguous phases it is convenient to introduce a mathematical "dividing" surface, with respect to which all "excess" surface properties may be defined. This dividing surface and the surface properties defined by it, however, must take into account the fact that there is a definite gradient in properties in the vicinity of any interface. Following a discussion of the implications of this gradient, the author defines and discusses surface tension, arguing that it is always directly propor tional to the pressure differences across the interface boundary. In the second chapter, E. W. Hart (General Electric Research Laboratory) continues the consideration of surface tension by describing the results of the application of the recently developed thermodynamic theory for equilibrium in homogeneous systems to the problem of the surface tension of the interface between two fluid phases. He states that the results of the experimentation were inconsistent with earlier con clusions regarding both the concept of surface tension and the existence of minimum surface tension, proving rather that the surface tension of a planar interface was given solely by the difference of two quantities that are characteristic of the two homogeneous bulk phases. The subject of nucleation is introduced in the third chapter by K. C. Russell (Massachusetts Institute of Technology), who points out that at the present time there is a I-to-7 order-of-magnitude difference between the experimentally determined and the theoretically predicted behavior of homogeneous nucleation and, furthermore, that the best this situation can be improved is by about one order of magni tude-unless more sophisticated techniques are employed. He describes such techniques as those that would focus attention on those additional energy and entropy characteristics that may be meaningful. In the next chapter, J. P. Hirth and K. L. Moazed (Ohio State University) describe different nucleation mechanisms related to deposi tion onto substrates. They demonstrate how through a minimization Foreword vii of the free energy of formation it is possible to determine, in order, the critical nucleus size above which crystal growth can be expected to begin, the volume density of the critical-size nucleus, and the observable nucleation rate. Following this enumeration, the authors describe mechanisms that are dependent principally on the temperature of the substrate. In this chapter they also discuss surface imperfections and the phenomenon of epitaxy. F. P. Price (General Electric Research Laboratory) discusses, in the fifth chapter, the nucleation and growth of single-crystal organic polymers, presenting evidence that classical nucleation theory applies equally well to polymer crystals and low-molecular-weight materials. He shows that the thermal history of organic solids controls the size of crystalline spherulites and that for polychlorotrifluoroethylene there is correlation between spherulite size and fatigue life. In the sixth chapter, P. G. Shewmon (Carnegie Institute of Technology) introduces the subject of surface diffusion by examining the experimental techniques used to determine the surface diffusion coefficient Ds . Then he argues that surface diffusion rather than volume is the major factor in the process of fine powder sintering. He describes recent computer studies in support of this minority opinion. Consideration of the relation of surface diffusion to the sintering process is continued in the seventh chapter, where C. E. Birchenall and J. M. Williams (University of Delaware) express the view that any consideration of surface diffusion must take into account the additional factors of surface impurity size and distribution, crystalline anisotropies, divacancy migration through the surface, the long mean free paths at high temperatures, and the chemical effects that "pin" atoms to the adsorbed species. They discuss these factors in some detail and then proceed to comment on the sintering process itself, expressing the opinion-contrary to the usual-that neck size is more important than particle size. In the eighth chapter, after commenting on the formation of surface point defects on ionic crystals, authors C. Y. Li and J. M. Blakely (Cornell University) question the legitimacy of utilizing the bulk dielectric constant in the neighborhood of a surface, pointing out that a correction in this constant would make it more difficult to create a surface pair, for any such correction would result in an increase of the polarization component of the removal energy. In the discussion of surface-initiated failures in structural materials presented in the ninth chapter, I. R. Kramer (Martin Company) points viii Foreword out that the surface of any material has a significant effect on the mechanical behavior of the material and that, in fact, suitable changes in the surface can result in the altering of all of the usual mechanical properties of the material-tensile behavior, fatigue, creep, and stress rupture. Ultrafine particles in gases is the subject of the final chapter. One of the principal suggestions in this broad, comprehensive discus sion by J. Turkevich (Princeton University) is that the formation as well as the texture of these particles are the result of a degree of "memory" residing in an organized aggregate of the materials. Another suggestion is that in the growth of these particles there are none of the growth-promoting driving forces found in crystallites and that these particles are so nearly perfect that the "growth sites" associated with dislocations do not exist. In addition to these ideas, the author describes in detail some of the problems associated with the formation of these ultrafine particles. Great efforts were put into trying to preserve the flavor of the meeting in this book, examples of which are shown in the two sections where selected discussions are presented. It is not without pride that we view the ever-mounting interest in these meetings, viewed not only from the number of people attending (because of this, attendance is now limited to "by invitation only"), but also from the high caliber of the participants. L. J. Bonis Contents Some Aspects of Classical Surface Thermodynamics P. L. de Bruyn, Massachusetts Institute of Technology. Modern Theory of Fluid Surface Tension Edward W. Hart, General Electric Research Laboratory. 37 Nucleation in Homogeneous Vapors K. C. Russell, Massachusetts Institute of Technology. 43 Nucleation Processes in Deposition onto Substrates J. P. Hirth and K. L. Moazed, Ohio State University. 63 Nucleation and Condensation in Polymer Systems Fraser P. Price, General Electric Research Laboratory. 85 Panel Discussion: Session I. . . . . . . . . 105 Surface Self-Diffusion at High Temperatures P. G. Shewmon, Carnegie Institute of Technology. 111 Self-Diffusion on Nearly Pure Metallic Surfaces C. E. Birchenall and J. M. Williams, University of Delaware 133 Defects Near Ionic Crystal Surfaces Che-Yu Li and J. M. Blakely, Cornell University. 153 Effect of Surfaces on Mechanical Behavior of Metals 1. R. Kramer, Martin Company. 171 Ultrafine Particles in the Gas Phase John Turkevich, Princeton University. 195 Panel Discussion: Session II. 213 Index .......... . 223 ix Some Aspects of Classical Surface Thermodynamics P. L. de Bruyn Department of Metallurgy Massachusetts Institute of Technology Cambridge, Massachusetts INTRODUCTION The special contribution of matter lying in and around phase boundaries to the total energy of heterogeneous systems is normally ignored in thermodynamic treatises. This neglect implies that the density of energy of a given phase remains uniform up to a mathematical surface separating it from contiguous phases. However, because of the finite, although short, range of action of atomic forces, the assumed sharp phase boundaries should actually be replaced by an interphasal region of finite thickness across which the density of energy or of any other thermodynamic property changes much less abruptly. It is reasonable to expect the normal thickness of this transition region to be of the order of a few molecular diameters; unless the system has a relatively high surface to volume ratio, as would be the case in colloidal systems, the additional contribution of the "surface" atoms to the total energy content may justifiably be ignored. However, a solid or liquid surface, no matter how small in extent, will be the seat of special physical and chemical phenomena because those atoms or molecules which terminate the condensed phase are subjected to unsymmetrical forces. Adsorption from the gaseous or liquid phase onto solid surfaces, wetting and spreading of liquids on solids, and capillarity are examples of industrially important surface phenomena. Surfaces also play an important, although not dominant, role in such physical and chemical processes as phase transformations, nucleation and growth, electrode reactions, sintering, and electron emission-to name but a few. The colloid chemist has long been aware of the role of surfaces in colloidal phenomena because he has 1
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