杏吧原创

Bognor Regis

CERAMIC materials have come a long way since the first pottery objects were
crafted thousands of years ago. At that time, pottery helped the development of
agriculture as sturdy containers were vital to carry and store grains, fruits
and roots. Today, ceramic materials find all sorts of
applications鈥攕culptors, surgeons, computer designers and engineers are all
likely to use ceramics in one form or another.

Ceramic items, such as pottery, china and tiles, are generally produced by
forming clay into the desired shape, and then firing it at a high
temperature鈥攖he word 鈥渃eramic鈥� comes from the Greek keramos,
meaning 鈥渂urnt stuff鈥�. Many building materials such as brick, concrete and glass
are also considered to be ceramics. But the past fifty years have seen the
development of new ceramics, including aluminium oxide, silicon nitride and
tungsten carbide.

All of these materials share some important physical and chemical properties:
they are hard but brittle, they are poor conductors of heat and electricity,
they can survive high temperatures and they are resistant to chemical attack
from strong acids or alkalis. These properties form the basis of the definition
of a ceramic material.

Hard and resistant

The chemical secret

THE PROPERTIES of ceramics are the result of their chemical composition and
structure. Many ceramics are metal oxides and since they are already oxidised,
there is little scope for further chemical reaction. This means they are able to
survive in corrosive environments. The atoms of which they are made up are
closely packed and tightly bonded together to form giant three-dimensional
networks, so that the materials can survive high temperatures without melting or
dissociating. Ceramics have long been used in the production of metals, as
firebricks which line the inside of furnaces and smelting plants. Because of
their resistance to intense heat, they are described as refractory
materials.

Making a ceramic object involves an ancient but simple, two-stage process:
first forming the clay into the desired shape and then firing it in a kiln at a
temperature of several hundred degrees. Clay is a naturally occurring material
consisting of fine-grained sediment, formed by the weathering of rocks, and its
composition and structure are complex and variable. China clay (kaolinite) is
one of the simplest, consisting of fine, plate-like crystals of hydrated
aluminium silicate, Al₂Si₂O₅(OH)₄. The atoms
in these crystals are bonded tightly together in two-dimensional sheets, between
which there are only weak bonds, so they are able to slip over one another. When
surrounded by a thin layer of water, they slide even more easily over one
another so wet clay can be moulded to a desired shape. Leaving the clay to dry
out removes this thin layer of water, and the object retains its shape.

Most clays are more complex than china clay. They are mixtures of several
different minerals (mostly silicates of aluminium and potassium) and are often
mixed together to achieve a desired final product.

To make a clay object durable, it must be fired. At the high temperature
within the kiln鈥攑erhaps 1000 掳C鈥攁ny remaining water around the
crystals rapidly evaporates. Even more water is lost as hydroxyl (-OH) groups
which break away from the crystal structure. At these temperatures, silicates
melt and flow within the material. As a result of surface tension, these liquids
collect between the remaining solid crystals. When the object cools, the liquids
solidify to form glassy bridges between the crystals, gluing them together and
giving the material its greatly increased strength. With china clay, the result
is a ceramic made from crystals of aluminium silicate (3Al₂O₃
.2SiO₂), bonded by silicate glass. This process physically links the
crystals because they melt into each other, a type of bonding known as
sintering.

The density of the ceramic object increases during sintering, as the silicate
liquid helps to fill up the pores which exist in clay. Any remaining pores will
weaken the final product and so, over the years, potters have developed various
additives to reduce porosity. For example, bone china, a low-porosity porcelain,
is made by incorporating bone ash (mainly calcium phosphate), potash feldspar
(potassium aluminium silicate) and quartz into china clay. Defects, such as tiny
cracks or particles of impurities, will weaken the end product, so very pure
starting materials must be used.

Ceramics may be produced by forming and firing, but other techniques have
been developed which can make high-quality ceramics with controllable
properties鈥攐ften called engineering ceramics. Materials such as alumina
(aluminium oxide, Al₂O₃) are often formed by hot pressing. A
powder is put in a die or mould of the desired shape. It is then heated and
simultaneously compressed to a high pressure. The powder does not melt, but
under these conditions, atoms are sufficiently mobile that they can diffuse
across the surface of the grains to form bridges between them. The grains become
joined together and the pores between them are filled up, producing a dense
ceramic object with few defects.

Silicon nitride (Si₃N₄) is an extremely strong ceramic
which can be created by reaction bonding. Here, the material is formed by a
chemical reaction as the final object is being produced. A powder of pure
silicon particles is formed into the required shape. It is then heated in an
atmosphere of nitrogen, and a reaction occurs between the powder and the gas.
The silicon nitride occupies more space than the original silicon, and so the
pores between the grains fill up. The size and shape of the object produced by
reaction bonding change little during the process. This is a great advantage,
and means that silicon nitride can be used to make items to very high
tolerances without the need for grinding or machining to finish off the
processing.

The atoms in a ceramic are joined together into giant structures by a mixture
of covalent bonds (in which electrons are shared between the bonding atoms) and
ionic bonds (in which electrons are transferred from one atom to another). If
the ceramic is an oxide, most of its bonds are ionic, while in carbide and
nitride ceramics, most bonds are covalent. Moving one atom or ion past another
would involve the breaking and remaking of these strong, directional bonds, and
this would require high forces. So ceramics are strong, stiff materials. One of
the first scientists to study the strength of ceramic bonding was a German
physicist, Max Born. He showed that a ceramic鈥檚 hardness and resistance to
chemical attack is due to the dense packing of its atoms.

Brittle fracture

The fatal flaw

IN CONTRAST to ceramics, metals consist of regular arrays of closely packed
atoms in a sea of free electrons (see Inside Science 72, Anatomy of a metal, 11
June 1994). Unlike covalent bonding, specific electrons are not shared between
specific atoms: instead, the electrons are free to roam through the metal. Since
these bonds are not directional like covalent and ionic bonds, it is relatively
easy for one plane of atoms to slide past another, so metals are pliable or
ductile.

The consequence of these different types of bonding shows up if you compare
what happens when you stretch a metal and a ceramic and measure their extension.
Plotting a graph of stress against strain, a typical ceramic shows a fairly
straight response, indicating that it is stiffer than the metal鈥攊t has a
higher Young modulus than a metal
(Figure 2). This is a measure of the elastic
stiffness of a material. Beyond a certain point, called the yield point, the
metal starts to stretch significantly with little increase in applied stress,
and it remains permanently stretched when the stress is removed. But ceramics
reach a stress at which they suddenly break, with the catastrophic brittleness
of breaking glass鈥攖his sudden event is known as brittle fracture.FIG-21219502.jpg

A material鈥檚 strength is represented by the greatest stress it can withstand
without breaking or deforming. Here, ceramics perform well compared with metals.
But many ceramics are disappointingly weak compared to their theoretical or
ideal strength, which can be calculated from knowledge of the strengths of the
interatomic bonds. This strength may be ten or more times greater than the
strength determined by stretching or bending tests. Materials scientists who
hope to produce stronger ceramics have put great efforts into understanding why
this should be so.

Ceramics can support a large load which is squashing them, and the
compressive strength of bricks and concrete makes them very useful as
construction materials. Brick buildings can be many storeys high but their
maximum height is limited by the compressive strength of the bricks鈥攖he
lowest courses of bricks must support the weight of all those above them. But on
the other hand, ceramics are much weaker under tension鈥攖hey would fracture
if the same load was used to stretch them.

Tiny flaws in the structure of ceramics are the origins of their weakness
under tension. These flaws were first investigated by Alan Griffith, working at
the Royal Aircraft Establishment, Farnborough, in the 1920s. In a study of glass
fibres and rods, he found that fibres thinner than about 10 micrometres were
much stronger (for their thickness) than rods. When he examined the surface of a
rod, he discovered that it was covered with a myriad of microscopic cracks.
Griffith found that these cracks can result from mechanical or chemical damage
to the surface, so newly prepared glass tends to be stronger than glass which is
hours or days old. Alternatively, the cracks may arise when the material is
heated or cooled rapidly. The surface expands or contracts faster than the
interior of the material, leading to stress and tiny cracks at the surface.
These Griffith cracks were the clue to the weakness of glass.

Stretching a glass rod tends to pull a surface crack open
(Figure 3). Stress
is concentrated at the tip of the crack and, as the force on each end of the rod
increases, the stress becomes greater until it is sufficient to start breaking
bonds between atoms in the material. Now the crack can continue to propagate
through the glass, and the rod snaps in a fraction of a second. The smooth
surface produced when glass fractures shows that, once the crack starts to
spread, there is nothing to impede its progress. Griffith showed that the
smaller the crack, the higher the stress needed to make it spread spontaneously.
Thin fibres are strong simply because there can only be very fine cracks in
their surfaces.FIG-21219503.jpg

Reducing brittleness

Controlling the cracks

IN CONTRAST, compressive forces tend to close cracks and prevent them from
extending. The compressive strength of ceramics is limited only by the strength
of the interatomic bonds, rather than by the presence of surface flaws.

Several techniques have been developed to reduce the effects of surface flaws
in ceramics. Glass can be strengthened by thermal toughening, which is often
used for strengthening car windscreens. As the glass cools, cold air is blasted
onto it so the surface layers solidify rapidly, so fast that there is no time
for atomic rearrangement to occur. The interior of the glass cools more slowly,
the atoms are able to alter their positions, and the material of the interior
can contract more than the surface. Consequently, the surface is artificially
expanded, and the interior pulls on it, closing any microscopic cracks at the
surface.

A second technique used with glass is chemical toughening, in which an
object鈥攁 bottle or jar, for example, is placed in a hot ionic solution.
Large ions, such as Na⁺ from the solution, replace smaller ions, such as Li⁺,
in the surface of the glass and the result is a surface layer which cannot
shrink as much as the interior when the glass cools. The surface is thus in
compression, and the glass will not break until sufficiently high tensile forces
are applied to overcome this compression.

Toughened glass artefacts have made the storage and transport of substances
such as milk, beer and oil much easier. Toughened glass sheet is also much safer
than traditional glass. Traditional ceramics can also be toughened by coating
them with a glaze before firing. When cool, the glaze forms a glassy outer layer
which expands less than the ceramic beneath, putting the ceramic鈥檚 surface under
a compressive stress, closing any cracks and increasing the object鈥檚 strength as
much as threefold.

Pre-stressed concrete, reinforced with metal wires, is widely used in
buildings and bridges. These wires are kept in tension as the concrete
solidifies. When it has solidified and the tension is released, the wires pull
inwards on the concrete, compressing it and closing any surface cracks.

Modern materials have been developed to make the best use of some of the
properties of ceramics. Often they are chemically simpler than the traditional
clay ceramics, so their structure is easier to control and many of the problems
associated with flaws and non-uniformities may be overcome by careful
processing.

Many ceramics show excellent creep resistance. Creep is the phenomenon where
a material gradually stretches over a period of time when kept under tension,
particularly at higher temperatures. For example, lead sheeting will slowly
creep down a church roof under its own weight. Metals, held together by metallic
bonding, are particularly prone to creep as their ductility increases at high
temperature and their atoms can easily slip past one another. In ceramics, where
the atoms are held together by strong, directional bonds, movement of one atom
past another is much more difficult.

Rotor blades in gas turbine engines spin at high speed, and run at
temperatures approaching 1000 掳C. If they are made of metal, creep becomes a
big problem, and engine temperatures must be kept low to prevent this. But when
an engine鈥檚 temperature is raised, it uses less fuel and becomes more efficient.
Ceramic rotor blades have been developed which can raise the operating
temperature of engines by 300 掳C or more, increasing their efficiency by as
much as half. One suitable material for this is silicon nitride, which has low
creep and high thermal shock resistance鈥攖hat is, it can withstand rapid
changes in temperature. Many ceramic materials break when they are suddenly
heated or cooled, because their surface expands or contracts more rapidly than
their interior (because they are such poor conductors of heat). This sets up
great stresses. This is less of a problem with silicon nitride because its
thermal expansivity is very low. Many commercially produced vehicles now have
diesel engines with ceramic components. An additional benefit is that the engine
is lighter, resulting in further improvements to fuel efficiency.

Ceramics at work

Tough tasks

THE MECHANICAL properties of modern ceramics mean they are also useful as
biomaterials, direct replacements for natural materials in the body, such as
bone. The extreme hardness of silicon carbide makes it suitable for hip
replacements, and other ceramics are used for dentures. Again, their low density
and wear resistance are very useful. It is also possible to control the porosity
of ceramics during the sintering process by altering the temperature and
pressure, and artificial bone replacements are now deliberately made to be
porous, so that the patient鈥檚 regenerating bone can grow into and bond with the
ceramic implant.

Ceramics are often incorporated into composite materials鈥攃ombinations
of two or more materials鈥攚hich are designed to have better mechanical
properties than either material separately
(Figure 4). Because of their high
strength, ceramic fibres have been popular for inclusion in composites. Glass
fibres are often added to a polymer resin matrix and this composite is widely
used in boats or caravans as glass-fibre reinforced plastic (GFRP). Ceramic
fibres can also be embedded in a metal matrix (metal matrix composites) or in
another ceramic for more demanding applications. Most recently, materials have
been developed in which two crystalline ceramics interpenetrate one another.
Such materials are found to be both strong and ductile at very high
temperatures. This suggests that they may be formed into useful shapes while
retaining their strength, auguring well for their use in advanced engineering
applications, such as inside jet engines.FIG-21219504.jpg

Many ceramics also have interesting electrical properties. Semiconductors,
which are at the heart of the modern microelectronics industry, are ceramics. So
too is barium titanate, a material with one of the highest known values of
dielectric permittivity, which is a measure of how well a material transmits an
electric field. The high dielectric permittivity of barium titanate makes it
suitable for insulating layers inside electrical components such as capacitors.
Capacitors require insulating layers with high dielectric permittivity if they
are to be miniaturised for use inside lightweight computers.

Some ceramics, such as lead zirconate titanate (PZT), are piezoelectric
materials. These can directly convert electricity into movement, and vice versa.
Squeezing or stretching a piezoelectric material鈥檚 lattice generates a small
voltage across the crystal, so they are often used in gas ignition systems.
Squeeze a piezoelectric and the resulting voltage produces a spark, which lights
the gas. Piezoelectrics can also be used as sensors for detecting vibrations or
movement of, for instance, a window pane. Vibrations caused by an intruder
produce a rapidly varying voltage across a piezoelectric, and this can be
amplified and used to trigger an alarm. Since piezoelectrics can work in
reverse, a varying voltage applied across the material makes it vibrate, and
this can be used to build small loudspeakers and earphones.

A new range of high-temperature superconductors are ceramics (see Inside
Science No. 97, Superconductivity, 18 January 1997). Superconductors lose all
their electrical resistance below a certain, critical temperature, but until
now, this temperature has been so low that the materials were of no practical
use. Ceramic superconductors might one day function at or very close to room
temperature and allow electrical engineers to improve the efficiency of motors
and transformers or even to design completely new electrical devices.

Today鈥檚 advanced ceramics may seem unrelated to fragments of ancient pottery
often seen in museums. But their basic chemistry is the same, and an improved
grasp of this has helped researchers develop entirely new kinds of harder,
tougher ceramics which can be used in ever more demanding applications.

* * *

Cracking up: why some do and some don鈥檛

It is not only glass and other ceramics whose strengths are limited by flaws.
Most materials are found to be weaker than their ideal or theoretical strength.
Metals, for example, are weakened in two ways. Firstly, they are made from many
tiny crystals (for this reason they are described as polycrystalline), and the
boundaries between these tiny crystals can be a source of weakness, helping
cracks to spread through the material. They are also likely to contain large
numbers of dislocations鈥攈iccups in the atomic structure. These defects in
the crystalline structure allow planes of atoms to slide easily over one
another. This explains how metals can stretch under an applied stress鈥攖hey
are ductile materials. Surface cracks are less of a problem in metals than in
ceramics, because of this ductility. At the tip of a surface crack, atoms can
move under the high stress to blunt the tip and reduce the local stress,
preventing the crack from extending into the bulk of the material.