Advances in Structural Analysis

396 Pages·1990·20.44 MB·English

by  D.R. UHLMANN and N.J. KREIDL (Eds.)

Contributors
Philip J. Bray
G. N. Greaves
D. L. Griscom
C. R. Masson
Phillip E. Stallworth
G. Tomandl
J. Zarzycki
GLASS
SCIENCE AND TECHNOLOGY
Edited by D. R. UHLMANN
DEPARTMENT OF MATERIALS SCIENCE
AND ENGINEERING
UNIVERSITY OF ARIZONA
TUCSON, ARIZONA
N. J. KREIDL
SANTE FE, NEW MEXICO
VOLUME 4B
Advances in Structural Analysis
1990
ACADEMIC PRESS, INC.
Harcourt Brace Jovanovich, Publishers
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COPYRIGHT © 1990 BY ACADEMIC PRESS, INC.
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Library of Congress Cataloging-in-Publication Data
(Revised for vol. 4A-4B)
Glass—science and technology.
Includes bibliographies and indexes.
Contents: 1. Glass-forming systems—v. 2, pt. 1.
Processing—[etc.]—v. 4A. Structure,
microstructure, and properties, v. 4B. Advances in
structural analysis.
1. Glass—Collected works. I. Uhlmann, D. R.
(Donald Robert) II. Kreidl, N. J.
TP848.G56 666M 80-51
ISBN 0-12-706704-3 (vol. 4A)
ISBN 0-12-706707-8 (vol. 4B)
PRINTED IN THE UNITED STATES OF AMERICA
89 90 91 92 9 8 7 6 5 4 3 2 1
To the memory of
Dr. C. R. Masson
Contents of Volume 4A
Chapter 1 Overview, E. A. Porai-Koshits
Chapter 2 Properties and Structures of Glasses and Melts versus
Preparation, Rolf Bruckner
Chapter 3 Structure and Electrical Properties, A. Feltz
Chapter 4 Structure of Sol-Gel Derived Glasses, C. J. Brinker
Chapter 5 Small-Angle X-Ray Scattering, E. A. Porai-Koshits
Chapter 6 Stochastic and Molecular Dynamic Models of Glass Structure,
Thomas F. Soules
IX
List of Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
PHILIP J. BRAY (77), Department of Physics, Brown University, Providence,
Rhode Island 02912
G. N. GREAVES (1), SERC Daresbury Laboratory, Warrington WA4 4AD,
United Kingdom
D. L. GRISCOM (151), Optical Sciences Division, Naval Research Laboratory,
Washington, DC 20375
C. R. MASSON (313), Atlantic Research Laboratory, National Research
Council of Canada, Halifax, N. S. B3H 3Z1 Canada
PHILLIP E. STALLWORTH, (77), Department of Physics, Brown University,
Providence, Rhode Island 02912
G. TOMANDL, (273), Institut fur Werkstoffwissenschaften III (Glas und
Keramik), Universitat Erlangen, West Germany
J. ZARZYCKI (253), Laboratory of Science of Vitreous Materials, University of
Montpellier, France
xi
Preface
Contemporary Concepts in Structure
The present volumes offer the thoughtful comments of leading workers in
the field of glass structural science. The respective chapters cover, with two
notable exceptions, the principal methods used to obtain experimental data
on glass structure.
The first of these exceptions, wide angle x-ray scattering (WAXS), has been
well summarized in a recent work by Wright (1989) and is embraced to
differing extents in several of the present chapters. While WAXS may well be
regarded as the cornerstone of structural investigations, the work by Wright
is so cogent and comprehensive that a presentation at this time would likely
represent in large measure a restatement or a brief update.
The second group of important structural characterization methods,
which will not be found in the present volumes, is that of infrared and Raman
spectroscopy. These methods yield information about the optical character-
istics of materials as well as their structure. After lengthy consideration, it was
decided that they would best be included in a forthcoming volume on optical
properties (Uhlmann, to be published).
As a framework for any consideration of glass structure, it is useful to
recall an underlying principle that was enunciated long ago by Warren.
Specifically, it is unreasonable to demand that an amorphous structure be
uniquely established by a particular structural investigation. Rather, such
investigations should be viewed as providing data with which any proposed
structural model must be consistent. This perspective is shared by other
distinguished contributors to the science of amorphous structure, such as
Professor Porai-Koshits, and underlies the recent comment of Wright (1989)
to the effect that, "Frequently, however, comparisons between models and
experiments are at a very superficial level and hence the literature contains a
wide range of structural models all claiming to be in 'good agreement' with
the experiments with the result that the structures of even the simplest
amorphous solids are not well established beyond the first few co-ordination
shells." Again, "the greatest barrier to progress in understanding the structure
of amorphous solids lies with the development of adequate modeling
techniques rather than in the improvement of the diffraction data
themselves."
xiii
xiv PREFACE
From Zarchariasen's celebrated remark of 1932, "It must be frankly
admitted that we know practically nothing about the atomic arrangement in
glasses," it is not a great leap to Professor Bishay's comment at the Kreidl
Symposium of 1984 that, "It is a pleasure to return to a meeting on glass
science after a multiyear absence and see many familiar faces discussing many
of the same problems, such as 'What is the structure of glass'? (Bishay, oral
comm., 1984)." Yet definite progress has been made. The data of today are
much more detailed and more extensive than those of few decades ago, and,
even more important, data are available based on a wider range of methods.
In the opinion of the present authors, one of the principal challenges of the
coming decade will be the integration of data provided by a variety of
techniques combined with more critical evaluations of the consistency of such
data with structural models. The effective response to this challenge will
require scientists who are conversant in detail with the capabilities and
limitations of a range of experimental methods, the likely accuracy (or
inaccuracy) of data provided by each method, the techniques of data
manipulation and refinement used with each method, and the limits within
which the various types of data can be used to distinguish among structural
models.
A second notable challenge for the coming decade is the development of
models and data that provide improved and more detailed descriptions of
structural features on various scales of structure. Of particular importance in
this regard is the elucidation of features in the intermediate range of
structure—i.e., from 5-8 A out to 20-30 A. This represents a formidable
challenge for theoreticians as well as experimentalists, and must be accom-
panied by critical assessments of the utility of various methods for providing
the desired insight (hard insight rather than "pie in the sky"). It seems likely
that meeting this challenge will require the development of novel
methods/approaches coupled with novel extensions of existing methods.
A third notable challenge is the use of structural models to predict the
properties or responses of materials. Most individuals feel that it does matter
whether a glass is a random network (at some level) or an array of
paracrystals, but they would be hard-pressed to specify how such structural
differences would influence properties or processing. To see how effective
such predictions of properties based on structural models can be, one need
only turn to the field of polymeric materials. Thanks to the pioneering work
of Flory and his successors, the implications of the random coil model for a
variety of properties of amorphous polymers have been explored. In fact,
when that model was challenged by reports of nodular features in a number
of amorphous polymers, the agreement between predicted and measured
properties was used to support the random coil model and to cast doubt on
the reports of the nodular features (a conclusion which was later supported
PREFACE XV
by direct structural investigations). The time is long overdue for developing
predictive structure-property relations for amorphous materials other than
polymers.
A fourth notable challenge for the coming decade is the elucidation of the
dependence of glass structure on the mode of formation of the glass. Even for
melt-derived glasses, relatively little detailed information exists on the effects
of cooling rate on structure; and the situation is much worse when one
includes quite different methods of forming glasses (e.g., sol-gel methods,
vapor deposition methods, and electrochemical methods). Structural dif-
ferences between amorphous coatings and bulk glasses provide further
examples of this type of investigation. In several instances, notable changes in
structure and defects with mode or conditions of glass formation have been
suggested, but the issue requires much more extensive and critical explora-
tion. Such exploration has been the subject of recent conferences organized
by Weeks (1985).
A further notable challenge involves the characterization of defects,
including but not restricted to point defects and composition-related defects,
in glass structure. Thanks to the initiative of Weeks (1980) and Griscom
(1980), this area has begun to receive increased attention in recent years. Such
attention has served to clarify a number of outstanding questions, but it has
also led to the posing of new questions, and has emphasized the need for
improved structural models. Improved understanding of intermediate-range
structure will undoubtedly lead to important new questions concerning
defects at this level of structure. In addition, there exists a particular need for
improved models that relate defects to chemistry, melting conditions, heat
treatment conditions, radiation exposures, etc.—as well as models that
represent the relationships between defects of different types and the
properties of the glasses.
An additional notable challenge involves the elucidation of structural
features, including defect structures, in a broader range of composition
including complex compositions. Even in the case of simple glasses, there
remain important unresolved questions concerning structure, but these are
amplified when complex multicomponent compositions are considered.
Systematic investigations employing combinations of experimental tech-
niques will almost surely yield critical new insight into structural issues,
including insight into the structure of end-member glasses and the
generality/validity of structural models.
The issue of models for glass structure seems deserving of particular
comment. Such models have considerable value as guides for thought, even
when they are wrong in detail. The early disputes between proponents of
random network and crystallite models applied to the classic oxide glasses
were effectively resolved in favor of the random network concept, based
XVI PREFACE
largely on the arguments of Warren concerning the size of the crystallites
(estimated from line broadening) and the absence of intense small-angle
scattering. This does not exclude deviations from an overall random
arrangement in the form of either disordered local variations in concen-
tration and/or structure as well as defects. On the contrary, such deviations
have been suggested even in simple, single-component glasses such as Si0 .
2
Subsequent small angle x-ray scattering (SAXS) studies of a number of
simple oxide glasses indicated an intensity of asymptotic scattering that is
very similar to that expected from thermodynamic fluctuation theory,
assuming that the fluctuations present at the glass transition temperature are
frozen-in as the glass is formed. The observations suggest that the glasses are
substantially homogeneous and seem inconsistent with models that are based
on distinct heterogeneities that differ to any considerable extent in electron
density from the intermediate "glue."
SAXS studies on simple oxide glasses, carried out as a function of
temperature, have indicated a level of fluctuation scattering that is sub-
stantially independent of temperature up to the glass transition (T \ and then
g
increases with increasing temperature above T in accordance with pre-
g
dictions of fluctuation theory. In the case of amorphous polymers, the
observed asymptotic SAXS intensity is again consistent with the predictions
of thermodynamic fluctuation theory, and again the intensity of such
scattering increases with increasing temperature above T . With polymers,
g
however, the intensity of the scattering—and hence the magnitude of density
fluctuations—decreases with falling temperature over a range of temperature
below T (down to the secondary transition temperature). The temperature
g
dependence of the fluctuation scattering below T is markedly smaller than
g
that observed above T . The results suggest that portions of the chains retain
g
mobility over a range of temperature below that at which large-scale chain
mobility is frozen-in.
The picture that emerges from such studies is one of glasses as fluctuated
systems, with a level of fluctuations characteristic of T (in the case of oxide
g
glasses) and of a temperature somewhat below T (in the case of polymer
g
glasses). The observed fluctuation scattering is of a type that would be
expected on the basis of thermodynamic fluctuation theory for even the
simplest fluids.
While SAXS studies of glasses of different types consistently indicate a
level of asymptotic scattering consistent with thermodynamic fluctuations, a
number of electron microscope studies of both oxide glasses and polymer
glasses have indicated the presence of structural heterogeneities on a scale of
tens to hundreds of angstroms. These heterogeneities have been designated as
micelles or nodules and have been interpreted in various ways. All of the
interpretations involve the suggestion that the glass contains regions of