Materials physics

gds · 发表于 2010-11-28 16:00:49

Materials physics
P. Chaudhari
IBM Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598
M. S. Dresselhaus
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307
Extraordinary advances in materials physics have occurred over the last century. These advances have
influenced almost every aspect of human endeavor. In this note, the authors sketch some of these
exciting developments in structural, polymeric, and electronic materials. [S0034-6861(99)01502-0]
I. INTRODUCTION
Over the last one hundred years there have been stunning
advances in materials research. At the turn of the
last century we did not know what the atomic structure
of a material was. Today, not only do we know the structure,
but we routinely make artificial structures that require
placement of atoms at specified locations, that mix
atoms to create properties not found in naturally occurring
materials, that have the functionality needed by today’s
technology, and that adjust their properties to a
changing environment (smart materials). Over the last
few years we have begun to manipulate individual atoms
to form structures that enable us to explore scientific
issues, but that will surely lead to profound technological
and social consequences; for example, the manipulation
of nucleotides in a DNA molecule, which is then
correlated with the functioning of, say, a gene and with
its expression in the control of disease.
A hundred years ago there were no electronic devices,
and today there is hardly any electrical appliance without
them. It is anticipated that in the near future there
will be a microprocessor embedded in almost all electrical
appliances and not just in those used for computation
or information storage. These devices, inconceivable a
century ago, could without exception not be made without
the knowledge gained from materials research on
insulators, semiconductors, metals, ceramics, and polymers.
At the end of this century, we have begun a debate
on how far the present devices can continue to develop,
given the limits imposed by the speed of light and
the discrete nature of atoms, a debate that would have
been incomprehensible to scientists and technologists of
a century ago and a debate in which we now discuss the
possibilities of using single electrons to switch a device
on or off.
Our ability to measure temporal phenomena was limited
to fractions-of-a-second resolution a hundred years
ago. Today we can measure changes in properties with a
few femtoseconds’ resolution. Strobelike probes enable
us to measure phenomena ranging over time scales covering
more than ten orders of magnitude. We can, for
example, study the relaxation of electrons in a semiconductor
on a femtosecond time scale, the visible motion
of bacteria in a petri dish, or the slow motion of a sand
dollar on a beach.
Materials research spans the range from basic science,
through engineering, to the factory floor. This has not
changed over the last hundred years or, for that matter,
throughout the history of human civilization. Materials
research came out of the practical needs of mankind.
Eras of civilization were named after materials, so central
has been their role in achieving mankind’s mastery
over nature. The field of materials research can trace its
roots to alchemy, metallurgy, natural philosophy, and
even art, as practiced over many centuries. However, the
field of modern materials research, as represented by
materials physics, is only about sixty years old.
Shifts in materials usage from one type to another are
usually gradual. This is due to the very large investments
associated with products in the materials-related industries,
complex relationships between reliability and functionality,
environmental issues, and energy demands.
However, measured over time, these shifts become quite
perceptible. For example, in automobiles the ratio of
plastic components to iron-based alloys has changed
from less than 3% to more than 15% over the last two
decades. Although the percentage change appears to be
modest, the actual volume of material is large; over 40
million tons of structural materials are used annually in
cars.
The advances of materials research in this century,
which far exceed those of all prior centuries put together,
can be illustrated by three examples: structural,
polymeric, and electronic materials. Our choice of these
three is somewhat, but not completely, arbitrary. It is
out of structural materials, particularly from the fields of
metallurgy and metal physics, that modern materials
physics has evolved. From the crude weaponry of our
forefathers to our mastery of air travel, space flight, surface
transportation, and housing, structural materials
play a role that is unequivocally important. Nature uses
the second category of materials, polymers, in amazing
ways, to perform very complex functions. We humans
are an example of that. Over the last hundred years, we
have begun to understand and develop polymers for
uses in food packaging, fabrics, and structural applications.
We anticipate that polymer research will play an
increasingly important role in biomaterials of the future.
The third category, electronic materials, were not conceived
until quantum mechanics was discovered in this
century. Today we cannot imagine a world without telecommunication,
computers, radio, and television. These
and future devices that will make information available
Reviews of Modern Physics, Vol. 71, No. 2, Centenary 1999 0034-6861/99/71(2)/331(5)/$16.00 ©1999 The American Physical Society S331
instantly are only possible because of advances in the
control of materials structure and processing to achieve
a desired functionality.
II. STRUCTURAL MATERIALS
At the turn of the last century, mankind’s use of structural
materials was limited primarily to metals, particularly
iron and its alloys, ceramics (most notably Portland
cement), and polymers, which were limited to naturally
occurring rubbers, glues, and fibers. Composites, as a
concept were nonexistent even though wood and animals,
each composed of different materials, were used in
a variety of ways. However, the uses of alloying to enhance
the strength of lightweight materials, such as pewter,
or copper additions to aluminum, were established
techniques, known well before this century. This knowledge
was used to build the first dirigibles. The useful
nature of a material was often understood through serendipity
and not through an understanding of its structure
or the relation between structure and properties.
We still cannot predict in any quantitative way the evolution
of structure with deformation or processing of a
material. However, we have come a very long way from
the situation that existed a hundred years ago, thanks to
the contributions of twentieth century science to our understanding
of atomic arrangement and its determination
in a material. Our classification of materials by symmetry
considerations came into existence once atomic
arrangement became known. To the seven crystal systems
and amorphous structures, typified by the glasses
and liquids, we can now add quasicrystals and molecular
phases, such as fullerenes and nanotubes, in a crystalline
solid.
The crystal systems define perfect crystals. At finite
temperatures, the crystals are no longer perfect but contain
defects. It is now understood that these defects are
responsible for atomic transport in solids. In fact, the
structural properties of materials are not only a function
of the inherent strength of a material but also of the
defects that may be present. We know that aluminum is
soft because crystallographic defects, called dislocations,
can be readily generated and moved in this metal. In
contrast, in alumina (Al2O3), dislocation generation and
motion are difficult; hence alumina can be strong but
brittle at room temperature. The addition of copper or
manganese to aluminum creates second-phase precipitates,
which inhibit the motion of dislocations, thus enhancing
its strength-to-weight ratio. Our ability to improve
the strength-to-weight ratio in materials has
increased more than tenfold during the twentieth century.
This is to be compared with a change of less than
ten over the last twenty centuries. Much of the increase
in this century has come from an understanding of the
relationship between the processing of materials and
their structure. The highest strength-to-weight ratios
have been achieved in materials in the form of fibers and
nanotubes. In these structures, dislocations either do not
exist or do not move.
Most structural materials are not single crystals. In
fact, they consist of a large number of crystals joined at
interfaces, which in single-phase materials are called
grain boundaries. These interfaces can, for example, influence
the mechanical and electrical properties of materials.
At temperatures where the grain boundary diffusion
rate is low, a small grain size enhances the strength
of a material. However, when the grain boundary diffusion
rate is high, the material can exhibit very large elongation
under a tensile load (superplastic behavior), or
can exhibit high creep rates under moderate or small
conditions of loading. In demanding high-temperature
environments, such as the engine of a modern aircraft,
grain boundaries are eliminated so that a complex part,
such as a turbine blade, made of a nickel alloy, is a single
crystal. Thus the use of materials for structural purposes
requires an understanding of the behavior of defects in
solids. This is true for metallic, ceramic, and glassy materials.
Both ceramics and glasses were known to ancient civilizations.
Ceramics were used extensively in pottery and
art. The widespread use of ceramics for structural purposes
is largely limited by their brittle behavior. This is
now well understood, and schemes have been proposed
to overcome brittleness by controlling the propagation
of cracks. In metals, dislocations provide the microscopic
mechanism that carries energy away from the tip
of a crack, thereby blunting it. In ceramics, the use of
phase transformations induced at the tip of a propagating
crack is one analog of dislocations in metals. Other
schemes involve the use of bridging elements across
cracks so as to inhibit their opening and hence their
propagation. Still another scheme is to use the frictional
dissipation of a sliding fiber embedded in a matrix not
only to dissipate the energy of crack propagation, but
also, if the crack propagates through the material, to
provide structural integrity. Use of these so-called faulttolerant
materials requires both an understanding of mechanical
properties and control over the properties of
interfaces to enable some sliding between the fiber and
the matrix without loss of adhesion between them. Such
schemes rely either on composite materials or on microstructures
that are very well controlled.
The widespread use of silicate glasses, ranging from
windows to laptop displays, is only possible through the
elimination of flaws, which are introduced, for example,
by inhomogeneous cooling. These flaws, which are
minute cracks, are eliminated during processing by controlling
the cooling conditions, as in a tempered glass,
and also by introducing compressive strains through
composition modulations.
There are a number of fibers that are available for use
with ceramics, polymers, and metals to form composite
materials with specific applications; these include carbon
fibers, well known for their use in golf clubs and fishing
rods, and silicon carbide or nitride fibers. Optical fibers,
which are replacing copper wires in communication
technologies, owe their widespread use not only to their
optical transparency, but also to improvements in their
S332 P. Chaudhari and M. S. Dresselhaus: Materials physics
Rev. Mod. Phys., Vol. 71, No. 2, Centenary 1999
structural properties. Fibers must withstand mechanical
strains introduced during their installation and operation.
The use of composite materials in today’s civilization
is quite widespread, and we expect it to continue as new
applications and ‘‘smart’’ materials are developed. An
outstanding example of a functional composite product
comes from the electronics industry. This is a substrate,
called a package, which carries electronic devices. Substrates
are complicated three-dimensionally designed
structures, consisting of ceramics, polymers, metals,
semiconductors, and insulators. These packages must
satisfy not only structural needs but also electrical requirements.
Although we have made great progress over the last
hundred years in materials physics, our microscopic understanding
of the physics of deformation (particularly
in noncrystalline solids), fracture, wear, and the role of
internal interfaces is still far from complete. There has
been considerable progress in computer simulation of
some of these issues. For example, there is now a concerted
effort to model the motion of dislocations, during
deformation, in simulations of simple metallic systems.
We anticipate that within the next decade, as computational
power continues to increase, many of these problems
will become tractable. The ultimate goal is to design
a structural component for a set of specified
environmental conditions and for a predictable lifetime.
III. POLYMERS
Polymers, also known as macromolecules, are longchain
molecules in which a molecular unit repeats itself
along the length of the chain. The word polymer was
coined approximately 165 years ago (from the Greek
polys, meaning many, and meros, parts). However, the
verification of the structure of polymers, by diffraction
and other methods, had to wait, approximately, another
100 years. We now know that the DNA molecule, proteins,
cellulose, silk, wool, and rubber are some of the
naturally occurring polymers. Synthetic polymers, derived
mainly from petroleum and natural gas, became a
commodity starting approximately 50 years ago. Polymers
became widely known to the public when nylon
was introduced as a substitute for silk and, later, when
Teflon-coated pans became commercially available.
Polymers are now widely used in numerous household
applications. Their industrial use is even more widespread.
Most of the applications associated with polymers
have been as structural materials. Since the 1970s it was
realized that with suitable doping of the polymers, a
wide variety of physical properties could be achieved,
resulting in products ranging from photosensitive materials
to superconductors. The field of materials physics
of polymers has grown rapidly from this period onwards.
Polymers are a remarkably flexible class of materials,
whose chemical and physical properties can be modified
by molecular design. By substitution of atoms, by adding
side groups, or by combining (blending) different polymers,
chemists have created a myriad of materials with
remarkable, wide ranging, and useful properties. This
research is largely driven by the potential applications of
these materials in many diverse areas, ranging from cosmetics
to electronics. Compared to most other materials,
polymers offer vast degrees of freedom through blending
and are generally inexpensive to fabricate in large
volumes. They are light weight and can have very good
strength-to-weight ratios.
Polymers have traditionally been divided into five
classes:
(1) Plastics are materials that are molded and shaped
by heat and pressure to produce low-density, transparent,
and often tough products, for uses ranging from
beverage bottles to shatterproof windows.
(2) Elastomers are chemically cross-linked or entangled
polymers in which the chains form irregular coils
that straighten out during strain (above their glass transition
temperatures), thus providing large elongations,
as in natural and synthetic rubbers.
(3) Fibers, which are spun and woven, are used primarily
in fabrics. About fifty million tons of fibers are produced
annually for uses ranging from clothing to drapes.
Apart from naturally occurring fibers such as silk and
wool, there are regenerated fibers made from cellulose
polymers that make up wood (rayon) and synthetic fibers,
comprising molecules not found in nature (nylon).
(4) Organic adhesives have been known since antiquity.
However, with demanding environments and performance
requirements, synthetic adhesives and glues
have largely replaced natural ones. The microscopic
mechanisms of adhesion and the toughness of joints are
still debated. There is an increasing trend to use UV
radiation to promote polymerization in adhesives and,
more generally, as a method of polymerization and cross
linking in polymers.
(5) Finally, polymers, frequently with additives, are
used as protective films, such as those found in paints or
varnishes.
Physicists have played a significant role in explaining
the physical properties of polymeric materials. However,
the interest of physicists in polymers accelerated when it
was discovered that polyacetylene could be made conductive
by doping. This development was noteworthy
for it opened the possibility of deliberately controlling
conductivity in materials that are generally regarded as
good insulators. The structure of all conjugated polymers,
as these materials are known, is characterized by a
relatively easily delocalized p bond, which, with suitable
doping, results in effective charge motion by solitons,
polarons, or bipolarons. Since the discovery that polymers
could be electrical conductors, active research areas
have developed on the physics of polymer superconductors,
ferro- and ferri-magnets, piezoelectrics,
ferroelectrics, and pyroelectrics. Within the field of
doped polymers, devices have been built to demonstrate
light-emitting diodes, photovoltaic cells, and transistors.
Conjugated polymers have also been investigated extensively
for their large nonlinear, third-order polarizability,
which is of interest to the field of nonlinear op-
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Rev. Mod. Phys., Vol. 71, No. 2, Centenary 1999
tics. Large nonlinearities are associated with the strong
polarizability of the individual molecules that make up
the building blocks of the polymer. Furthermore, the
flexibility of polymer chemistry has allowed the optical
response of polymers to be tailored by controlling their
molecular structure, through the selective addition of
photoactive molecules. Hence these materials have been
widely investigated by physicists and engineers for optical
applications, such as in holographic displays (dichromated
gelatin), diffraction gratings, optocouplers, and
wave guides.
Polymers have long interested physicists for their conformational
and topological properties. This interest has
shifted from the conformational behavior of individual
molecules to that of a macromolecular assembly, phase
behavior, and a search for universal classes. Block copolymers,
consisting of two or more polymers, can give
rise to nanoscale phases, which may, for example, be
present as spheres, rods, or parallel lamallae. The distribution
of these phases and their topologies are of current
theoretical and practical interest. Block copolymer
morphologies are also being used as nanoscale templates
for production of ceramics of unique properties having
the same morphology.
Block copolymers are also of interest as biomaterials.
Proteins are an example of block copolymers, in which
the two phases form helical coils and sheets. Attempts to
mimic the hierarchical structure present in natural polymers
have only been partly successful. The principal difficulty
has been to control the length of the polymer
chains to the precision that Nature demands. Significant
progress has been made in controlling polymer morphologies
with the use of new catalysts. For example,
metallocenes have been used as catalysts to control
branched polymers and organonickel initiators to suppress
chain transfer and termination, so that polypeptides
with well-defined sequences and with potential for
applications in tissue engineering could be made. The
growth of well-controlled polymer chains is an example
of ‘‘living’’ polymerization.
The static and dynamic arrangement of atoms on the
surfaces and interfaces of polymers is another area of
active investigation. For example, thin films of polymers,
in which the chain lengths are long compared to the
thickness of a film, show unusual physical properties: the
glass transition temperature for a thin-film polymer decreases
significantly, but between solid surfaces polymer
liquids solidify.
Even though we have some way to go in making tailored
proteinlike structures, polymer research has
played a significant role in the class of materials called
biomaterials. Polymers have been used, for example, to
produce artificial skin, for dental fillings that are polymerized
in situ by a portable UV lamp, and for highdensity
polyethylene used in knee prostheses. Physicists
play a significant role in these developments, not only
for their interest in the materials, but also because of
their familiarity with physical processes that can be used
to tailor the properties of polymers. A particularly good
example of this interplay is the recent and rapidly growing
use of excimer laser radiation to correct corneal abnormalities;
using a technology developed from studies
of the ablation of polymeric materials for applications in
the electronics industry, physicists realized that the
small, yet precisely controlled, ablation of a polymeric
surface might be useful in shaping the surface of an eye.
IV. ELECTRONIC MATERIALS
The roots of the electronic materials field can be
traced back to Europe in the 1920s, with the advent of
quantum mechanics and its application to periodic structures
like those occurring in crystals. The early experimental
focus was on alkali halides, because these materials
could be prepared in a controlled way from both a
structural and a compositional standpoint. The creation
of a strong academic program in solid-state physics at
the University of Illinois in the 1930s had an important
impact on the early history of the electronic materials
field in the United States. This knowledgeable human
resource played a significant role in mobilizing the national
materials program during World War II, especially
in the development of semiconducting materials
with enhanced purity, suitable for use in diode detectors
at microwave frequencies for communications applications.
The availability of these new semiconducting materials
in purified, crystalline form soon led to the discovery
of the transistor, which ushered in the modern
era of electronics, computers, and communications,
which is now simply called the ‘‘information age.’’
Semiconductors have been a central focus for electronic
materials. Quantum-mechanical treatments of a
periodic lattice were successful in laying the groundwork
for describing the electronic band structure, which could
account for electrical conduction by electrons and holes,
carrier transport under the action of forces and fields,
and the behavior of early electronic devices. Because of
the interest of industrial laboratories and the Defense
Department in the newly emerging field of semiconductor
electronics, semiconductor physics developed rapidly,
and this focus soon led to the development of the
integrated circuit and the semiconductor laser.
The strong interplay between technological advances
and basic scientific discovery has greatly energized semiconductor
physics, by raising challenging fundamental
questions and by providing new, better materials and
devices, which in turn opened up new research areas.
For example, the development of molecular-beam epitaxy
in the 1960s and 1970s led to the ability to control
layer-by-layer growth of semiconductor quantum wells
and superlattices. The use of modulation doping of the
quantum wells, whereby the dopants are introduced
only in the barrier regions, led to the possibility of preparing
semiconductors with low-temperature carrier mobilities,
orders of magnitude greater than in the best
bulk semiconductors. These technological advances
soon led to the discovery of the quantum Hall effect, the
fractional quantum Hall effect, and a host of new phenomena,
such as Wigner crystallization, which continue
to challenge experimentalists and theorists. Lithographic
S334 P. Chaudhari and M. S. Dresselhaus: Materials physics
Rev. Mod. Phys., Vol. 71, No. 2, Centenary 1999
and patterning technologies developed for the semiconductor
industry have led to the discovery of the quantized
conductance for one-dimensional semiconductors
and to the fabrication of specially designed semiconductor
devices, in which the transport of a single electron
can be controlled and studied. The ever decreasing size
of electronic devices (now less than 0.2 microns in the
semiconductor industry) is greatly stimulating the study
of mesoscopic physics, in which carriers can be transported
ballistically without scattering and the effect of
the electrical leads must be considered as part of the
electronic system. New materials, such as carbon nanotubes
with diameters of 1 nm, have recently been discovered,
and junctions between such nanotubes are being
considered for possible future electronics applications
on the nm scale, utilizing their unique one-dimensional
characteristics.
The electronic materials field today is highly focused
on the development of new materials with special properties
to meet specific needs. Advances in condensedmatter
physics offer the possibility of new materials
properties. In photonics, new materials are providing increased
spectral range for light-emitting diodes, smaller
and more functional semiconducting lasers, new and improved
display materials. The new field of photonic
band-gap crystals, based on structures with periodic
variations in the dielectric constant, is just now emerging.
Research on optoelectronic materials has been
greatly stimulated by the optical communications industry,
which was launched by the development of low-loss
optical fibers, amplifiers, and lasers.
Ferroelectrics have become important for use as capacitors
and actuators, which are needed in modern robotics
applications, as are also piezoelectric materials,
which are critical to the operation of scanning tunneling
probes that provide information at the atomic level on
structure, stoichiometry, and electronic structure. The
technological development of microelectromechanical
systems (MEMS), based on silicon and other materials,
is making possible the use of miniature motors and actuators
at the micrometer level of integrated circuits.
Some of these have already found applications, such as
the triggering mechanism for the release of airbags in
automobiles. Such developments are not only important
to the electronics industry, but are also having great impact
on fields such as astronomy and space science,
which are dependent on small, light-weight instruments
with enhanced capabilities to gather signals at ever increasing
data rates and from ever increasing distances
from Earth. The developments in new materials and
low-dimensional fabrication techniques have recently rejuvenated
the field of thermoelectricity, where there is
now renewed hope for enhanced thermoelectric performance
over a wider temperature range.
Research on magnetic materials has been strongly influenced
by applications ranging from the development
of soft magnetic materials (by the utilization of rapid
solidification techniques) to hard magnetic materials
such as neodymium-iron-boron for use in permanent
magnets. In the 1980s efforts focused on the development
of small magnetic particles for magnetic memory
storage applications. New magnetic materials, especially
magnetic nanostructures, are now an extremely active
research field, where the discovery of new phenomena
such as giant magnetoresistance and colossal magnetoresistance
are now being developed for computer memory
applications.
The strong interplay between fundamental materials
physics and applications is also evident in the area of
superconducting materials. Early use of superconducting
materials was in the fabrication of superconducting magnets,
which in turn promoted understanding of type-II
superconductors, flux dynamics, and flux pinning phenomena.
The discovery of the Josephson tunneling effect
led to the development of the SQUID (superconducting
quantum interference device), which has
become a standard laboratory tool for materials characterization
and for the sensitive measurement of extremely
small magnetic fields, such as the fields associated
with brain stimuli. The discovery of high-Tc
superconductivity in 1986 has revolutionized this field,
with much effort being devoted to studies of the mechanism
for high-Tc superconductivity, along with efforts to
discover materials with yet higher Tc and critical current
values, to improve synthesis methods for the cuprate superconductors,
and to develop applications for these
materials to electronics, energy storage, and highmagnetic-
field generation.
When viewed from the perspective of time, the developments
in electronic materials have been truly remarkable.
They have generated businesses that approach a
trillion dollars, have provided employment to millions of
workers, either directly and indirectly associated with
these industries, and have enabled us, as humans, to extend
our abilities, for example, in information gathering,
communication, and computational capabilities. Science
has been the key to these marvellous developments, and
in turn these developments have enabled us, as scientists,
to explore and understand the subtleties of nature.
V. SUMMARY
In this very brief note, we have only touched on some
of the advances made in structural, polymeric, and electronic
materials over the last century, showing how materials
physics has played a central role in connecting
science to technology and, in the process, revolutionized
our lives.
P. Chaudhari and M. S. Dresselhaus: Materials physics S335
Rev. Mod. Phys., Vol. 71, No. 2, Centenary 1999

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