I&EC REPORTS & COMMENTS Mechanisms and kinetics of nucleation Nucleation from the atmosphere Nucleation from the melt
ABSTRACTS FOR THE NUCLEATION SYMPOSIUM The second symposium in the cooperative venture between the Division of Industrial and Engineering Chemistry of the A C S and IQEC Monthly will be held at A C S headquarters in Washington on 21 and 22, June 1965. Under the chairmanship of Dr. Alan S. Michaels, the symposium this year will be devoted to Nucleation Phenomena. The program and registration information for the symposium appeared in the April issue of 1&EC. MECHANISM AND KINETICS OF NUCLEATION: UNDERLYING THEORY
Bruce Chalmers and D . R. Ullmann, Gordon McKay Laboratory, Harvard University, Cambridge, Mass. Following a brief review of the thermodynamics of interfaces, consideration is given some general features of the nucleation process. The standard thermodynamic and kinetic formalism is developed, emphasizing the fundamental physical concepts ; the results are then applied to specific types of transformations. Some reservations regarding the formalism are discussed and related to experimental findings. The alternative transformation mechanism of spinodal decomposition is considered briefly.
gard to the assumptions made concerning these properties. Predictions of the theory are then compared with nucleation data obtained in cloud chambers, nozzle flows, and molecular beams.
NUCLEATION IN THE ATMOSPHERE
Hirace R. Byers, Dept. of the Geo-
p '2ysical Sciences, The University oj C iicago, Chicago, 111. The large water supersaturations required for cloud condensation from the pure vapor (homogeneous nucleation) are not observed i'n the atmosphere because there are abun-
dant aerosol particles capable of nu-. cleating at or near the vapor-pressure equilibrium of pure, bulk water. Soluble particles, such as sea salt, certain sulfates and nitrates, which lower the aqueous vapor pressure in solution, are prominent nucleating agents which commonly take up the water before insoluble particles or ions become active in this respect. The nucleation of ice requires another set of particles, and those occurring in the natural atmosphere are not very active. As a result, supercooling of IOo to 20" C. or more is often more the rule than the exception. As would be expected, the nucleation occurs on substances providing minimum misfit of the crystalline lattice with that of ice. Clay mineral particles are found as natural ice nuclei, and there is some evidence that meteoric dust particles are active. Silver iodide is most frequently used as an artificial nucleating agent, and a number of other substances have been or are being tried.
PREREGISTRATION F O R M
HOMOGENEOUS NUCLEATION FROM THE VAPOR PHASE
Ronald P. Andres, Department of Chemical Engineering, Princeton University, Princeton, N . J .
Symposium on Nucleation Phenomena ACS Division of Industrial and Engineering Chemistry Washington, D. C. June 21 and 22, 1965 NAME
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In the absence of foreign nuclei, the condensation of a gas takes place via statistical growth of minute clusters of the vapor phase molecules. In large part, detailed understanding of this process rests on our knowledge of the properties of these microscopic species. The classical theories of nucleation are reviewed with re-
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I&EC REPORTS NUCLEATION
FROM THE MELT
Kenneth A . Jackson, Bell Telephont Laboratories, Inc., Murray Hill, N . J. Nucleation experiments designed to examine homogeneous nucleation in melts are difficult because of the abundance of heterogeneous nuclei. The most important single technique has circumvented these difficultiesby subdividing the liquid into droplets. Other techniques involve the removal of nucleating agents from the melt. Experimentally, homogeneous nucleation is observed in most melts at about eight-tenths of the absolute melting temperature. Heterogeneous nucleation is important from a practical standpoint since foreign particles are usually responsible for nucleation during crystal growth. Some rules have been developed to predict whether a
given material will nucleate another. These rules are useful, in general, only as guides. Nucleation of a melt can occur as the result of cavitation produced in an ultrasonic sound field. Cavitation, and hence nucleation, can also be produced mechanically.
are discussed and methods of predicting incipient boiling in forced convection and heat flux cases are suggested. Recent experiments on nucleation in liquid metals are discussed.
NUCLEATION I N POLYMERS NUCLEATION AT HEATINQ SURFACES
Warren H. Rohsenow, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. This paper discusses nucleation associated with boiling a t heated surfaces. After a brief description of the maximum superheat attainable in a pure liquid, the model of nucleation cavities is presented. Experiments verifying cavity nucleation in isothermal and low beat flux cases
U.S. Peroxygen’s FR-222 liquid ketone peroxide requires no yellow label.
Fred Gornick and John D . Hoffman, Institute of Materials Research, National Bureau of Standards, Washington, D. C. This article deals with certain aspects of nucleation controlled processes in homopolymers consisting of flexible linear chains of high molecular weight. The discussion begins with an account of the thermodynamic driving force for nucleation in supercooled polymers. A theoretical foundation based on the Turnbull-Fisher rate theory for nucleation in condensed phases is then adapted for the case of polymers and the crucial role of crystal-liquid interfacial free energies is explained. Homogeneous nucleation in polymers is then treated from both a theoretical and an experimental point of view. The relative roles of heterogeneous and homogeneous nucleation in bulk crystallization are discussed, especially with respect to limits of applicability and the possibility of determination of the rates of such processes. Nucleation controlled growth processes are considered next, with the emphasis on the chain-fold mechanism. The dimensional stability of chain-folded crystals is covered, the rate of approach to equilibrium (i.e., lamellar thickening) being treated in part as a nucleation process. Experimental and theoretical problems of interest to workers in the field are also mentioned.
NUCLEATION IN GLASSES AND MELTS
common PO yester resins.
U. S. PEROXYGEN CORPORATION Iw Morton Avenue. Richmond. Calilornl
J . F. MacDowell, Corning Glass Works, Corning, N. Y. Beginning with the classical theory of Ostwald, Tammann, and Gibbs,
Circle No. I8 PI Rsadsn’ Service C u d
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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the development of the theory and practice of nucleation as it applies to glasses and melts is traced to the present. Evidence is shown to indicate that nucleation of a dispersed glass-inglass phase separation during cooling from the melt is a common precursor to the formation of a uniform dispersion of crystalline nuclei in glass. With the aid of electron micrographs and x-ray data, the sequence of nucleation and crystallization of an aluminosilicate glass is revealed. The fabrication through controlled nucleation of special photosensitive glasses and of a whole range of microcrystalline materials possessing unique properties is also described.
NUCLEATION WITHIN CRYSTALLINE PHASES
J . E. Hilliard, Debartment of M a terials Science, The Technological Institute, Northwestern University, Evanston, Ill.
