Table Of ContentFundamentals of
Inorganic Glasses
Arun K. Varshneya
New York State College of Ceramics
Alfred University
Alfred, New York
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Library of Congress Cataloging-in-Publication Data
Varshneya, Arun K.
Fundamentals of inorganic glasses / Arun K. Varshneya.
p. cm.
Includes bibliographical references and index.
ISBN 0-12-714970-8 (acid-free paper)
1. Glass. I. Title.
TP857.V37 1993
620.1'44—dc20 93-16591
CIP
Printed in the United States of America
93 94 95 96 97 BB 9 8 7 6 5 4 3 2 1
In memory of
my father
Nathi Lai Varshneya
Preface
"What, a glass scientist?" "What's that?" These have often been the typical
responses to my indicated profession in social circles. Clearly, steel's impact
on society has been powerful enough for the term "metallurgist" to be
recognizable as a profession. Glass has yet to graduate to this level of
recognition despite the fact that indulgence in drinking fluids out of a glass
vessel, and looking at the world through a pair of eyeglasses and through a
room window have been around for quite some time. Presumably, such
aberrations will be corrected in the now long-overdue materials age when,
along with crystalline ceramics such as ceramic superconductors, glass fiber
for communication links will be a part of the common household vocabu
lary. As it happens, my father never had any confusion between a "metal
lurgist" and a "glass scientist." He was a laboratory glass supplier in India
and knew some 30 years ago that a future for glass professionals existed.
And so, there I was... headed toward becoming a glass scientist. (Thanks,
Dad, for that remarkable foresight!) As such, one of my primary purposes
in writing this book is to convey that feeling of "identity" to the young that
a glass professional (scientist, engineer, or technologist) does belong to a
reputable caste. The day is not far, probably, when some education about
glass will find its way through every college-level engineering and science
curriculum.
A second purpose is to bring together a host of fine quality books on glass
into a single book which has the flavor of being a textbook for an
undergraduate student—comprehensive, yet confining itself to a general
understanding of the topics. Trying to strike a balance between the depth
and the breadth has always been my aim. Unfortunately, I did have to set
limits on the coverage. This book is about inorganic glasses, and mostly
about their science. Glasses based upon the carbon chains and macro-
molecules have not been included. Also, details of the technology and
XV
xvi Preface
engineering of glass and glass product manufacture have been spared for a
later date.
The book is intended to be a textbook on glass science suitable for teaching
at a junior/senior level in a materials curriculum. Emphasis has been placed
upon developing the fundamental concepts, whether they were ultimately
proven wrong or not. As such, the book may also be useful to industrial
scientists and engineers who are attempting to acquire a basic knowledge in
glass. While all efforts have been made to avoid deep scientific discussions
and heavy mathematics, there are places where such was unavoidable.
Because of the size of Chapter 13, a summary has been written at the end.
Some topics in phase separation (Chapter 4), much of the glass transforma
tion range behavior (Chapter 13), and some topics in dielectric properties
(Chapter 15), electronic conduction (Chapter 16) and optical properties
(Chapter 19) could be spared for a second-time reading or, perhaps, for
graduate-level instructions.
In writing the book, I have taken a teacher's point of view. The organiza
tion of the chapters is almost the way I like to teach "Introduction to Glass
Science" to our students with one exception; Chapter 20 (Fundamentals of
Inorganic Glassmaking) is taught after Chapter 5, primarily because the
students get a bit "itchy" to learn some technology after a load of structures.
Several key ideas have been set in italics: many key words are set in bold
lettering. Occasionally, it may appear as if I am leading the reader by the
hand—please forgive me for this audacity on my part.
I strongly recommend that students practice the drawing of glass networks.
(One picture is worth a thousand words.) Likewise, I urge them to attack at
least some of the questions posed at the end of most chapters. Answers to
a few are provided. Further consultation of "Suggested Reading" is always
encouraged.
I am sure that many errors have slipped by in this first attempt. Please
drop me a note if you can help bring even the smallest of corrections or
improvements to this book.
June 30, 1993 Arun K. Varshneya
New York State College of Ceramics
Alfred University
Alfred, NY 14802
Acknowledgments
I am forever grateful to my own teacher, Professor Alfred R. Cooper, Jr., of
Case Western Reserve University, for several wonderful years of association.
His knowledge, insight, and objective thinking about glass problems were a
model for me.
I am indebted to Harold Rawson of Sheffield (U.K.), Prabhat Gupta of
Ohio State University, George Scherer of Du Pont Company, Joe Simmons
of the University of Florida, and Alastair Cormack of Alfred University, who
read parts of this book (voluntarily). Their constructive criticism helped the
content of this book immensely.
I would like to express my sincere appreciation to several of my colleagues
and members of administration at the New York State College of Ceramics
for their sustained colleagueship, comradery, and constant encouragement.
Of these, I owe special thanks to Bill LaCourse: besides all the stimulating
technical discussions in the hallways and the no-charge book loans, he almost
always had a medicine for the various computer hiccups such that my
floppies rarely needed to see the trash.
Frequent technical discussions with Oleg Mazurin of the Russian Academy
of Sciences, St. Petersburg (Russia), were quite useful. Thanks are also due
to Tony DiGaudio of Williamsville, New York, for help with computer-
graphics.
The patience, understanding, encouragement, and continuing love ex
pressed by my wife, Darshana, and daughters, Pooja, Kajal, and Rupal,
helped me endure the pains of writing this book.
June 30, 1993 Arun K. Varshneya
Alfred, New York
xvii
Chapter 1
Introduction
1.1. Brief History
The word glass is derived from a late-Latin term glœsum, used to refer to
a lustrous and transparent material. Another word often used to refer to
glassy substances is vitreous, originating from the Latin word vitrum. Luster,
or shine, and in particular its durability when exposed to the elements of
nature, were probably the most significant properties of glass recognized by
early civilizations. Glazed stone beads from Egypt date back to 12,000 B.C.
Several of the artifacts unearthed from the tombs of the pharaohs exhibit
excellent glass inlay work in a variety of colors. As independent objects,
glassware perhaps existed roughly five to six thousand years ago. The
technology of the glass window exploiting the property of transparency had
developed around the birth of Christ and was developed to new heights of
artistry by the Christian Church during the Middle Ages. Many of these
beautifully stained windows, which can still be viewed in a number of
churches over the European continent, show the deep commitment of the
church to preserve the history of mankind and religious teachings through
the medium of glass.
Many of the uses of glass in the modern world continue to exploit the
transparency, luster, and durability properties of glass. Containers, windows,
lighting, insulation, fiber, stemware, and other hand-crafted art objects are
1
2 Fundamentals of Inorganic Glasses
typical of these traditional uses. At this point, it is worth noting that for a
material to be used in a product it must have certain desirable properties
that determine its use. Later on in our discussion, it will become clear that
the properties of transparency, luster, and durability are neither sufficient nor
necessary to describe "glass." Through the application of basic sciences to
the study of glass, newer properties of glasses have been developed, and
hence, newer products have been conceived.
As may be expected, much of the glass science developed on the basis of
the major commercial uses of glass. More than 99% of the commercial
tonnage consists of glass compositions that are oxides. A large percentage
of these are silica-based. This includes even the highly specialized application
of glass to microelectronic packaging where the annual volume of sale may
be low but glass is the "value-adding" component, i.e., the application of
glass enhances the value of the assembly after the incorporating process. It
is not surprising that when the term "glass" is used in scientific conversation,
oxide glasses are usually implied. Over the past two to three decades,
however, the possibility of some exotic uses of glass such as repeaterless
transoceanic or transcontinental telecommunication lines and the delivery
of C0 laser power to perform microsurgery has triggered a great many
2
studies of non-oxide glasses. It is well, therefore, to review our thoughts on
the various families of glasses, their compositions, and their uses before we
delve into the science of glass.
1.2. Glass Families of Interest
Table 1-1 presents a summary of the various inorganic glass families that
are of commercial interest. All the glasses listed here are silica-based.
One may note that, besides silica, other major constituents usually are the
alkalis, the alkaline earths, alumina, boric oxide, and lead oxide. Compounds
such as arsenic and sulfur are added as traces (minor constituents) intention
ally. Many of the reasons for the major component additions should become
clear as we continue. The various glass families are discussed in what follows.
1.2.1. Vitreous Silica
Vitreous silica is the most refractory glass in commercial use. In addition to
its refractoriness, it has a high chemical resistance to corrosion (particularly
to acids), a very low electrical conductivity, a near-zero (~5.5 x 10"7/°Q
coefficient of thermal expansion, and good UV transparency. Because of the
high cost of manufacture, the uses of vitreous silica are mostly limited to
Optical flint 49.8 0.1 13.4 18.7 1.2 8.2 8.0 0.4
s
S glas 65.0 25.0 10.0
E glass 52.9 14.5 9.2 17.4 4.4 1.0
Glass halogen lamp 60.0 14.3 0.3 6.5 18.3 0.01 Tr.
Weight Lead bleware 67.0 0.4 17.0 6.0 9.6 Tr.
y ta
Per Cent b Borosilicate crown 69.6 9.9 2.5 8.4 8.4 0.3
Oxides Therometer 72.9 6.2 10.4 0.4 0.2 9.8 0.1 Tr.
n m
Compositions i Lime Pyrex bleware type 74.0 81.0 0.5 2.0 12.0 7.5 18.0 4.5 Tr.
a
s t
Glas bing 2.1 1.6 5.6 3.4 6.3 1.0
al Tu 7 1
merci Bulb 73.6 1.0 5.2 3.6 16.0 0.6 Tr.
Table 1-1. Com Window Bottle or container 72.0 74.0 0.6 1.0 Tr. 0.7 5.4 10.0 3.7 2.5 Tr. 14.2 15.3 0.6 Tr. Tr.
Plate 72.7 0.5 0.5 13.0 13.2 Tr.
or 0 0 0
Vyc 94. 5. 1.
Vitreous silica 100.0
Si02 Al023 B023 S03 CaO MgO BaO PbO Na0 2κο 2ZnO As025
W
4 Fundamentals of Inorganic Glasses
astronomical mirrors, optical fibers, crucibles for melting high-purity silicon,
and high-efficacy lamp envelopes. In one technique, the glass is obtained
by melting high-purity quartz crystals or beneficiated sand at temperatures
in excess of 2,000°C. In a second technique, SiCl is sprayed into an
4
oxy-hydrogen flame or water-vapour-free oxygen plasma. Silica vapors
deposit on a substrate and are consolidated subsequently at ~ 1,800°C.
1.2.2. Soda-Lime Glass
Soda-lime glass or soda-lime-silicate glass is perhaps the least expensive and
the most widely used of all the glasses made commercially. Most of the
beverage containers, glass windows, and incandescent and fluorescent lamp
envelopes are made from soda-lime glass. It has good chemical durability,
high electrical resistivity, and good spectral transmission in the visible region.
Because of its relatively high coefficient of thermal expansion (~100 x
10_7/°C), it is prone to thermal shock failure, and this prevents its use in a
number of applications. Large-scale continuous melting of inexpensive batch
materials such as soda ash (Na C0 ), limestone (CaC0 ), and sand at
2 3 3
1,400-1,500°C makes it possible to form the products at high speeds
inexpensively.
1.2.3. Borosilicate Glass
Small amounts of alkali added to silica and boron oxide make a family
of glasses which are utilized for their low thermal expansion coefficient
(~ 30-60 x 10_7/°Q and a high resistance to chemical attack. Laboratory
glassware, household cooking utensils, and automobile headlamps are prime
examples of their usage. Glasses can be made commercially in a manner
similar to the soda-lime glasses, but require slightly higher temperatures
(~ 1,550-1,600°C). The high cost of B 0 makes them much less competitive
2 3
compared to the soda-lime glasses for common products.
1.2.4. Lead Silicate Glass
This family of glasses contains PbO and Si0 as the principal components
2
with small amounts of soda or potash. These glasses are utilized for their
high degree of brilliance (as stemware or "crystal"), large working range
(useful to make art objects and intricate shapes without frequently reheating
the glass), and high electrical resistivity (e.g., for electrical feedthrough
components). PbO additions increase the fluidity of glass and its wettability
to oxide ceramics. Hence, high lead borosilicate glasses (generally without
