Ceramics
The word ceramic is derived from the Greek word. The term covers inorganic
non-metallic materials whose formation is due to the action of heat.
Up until
the 1950s or so, the most important of these were the traditional clays, made
into pottery, bricks, tiles and are like, along with cements and glass. The
traditional crafts are described in the article on pottery. A composite material
of ceramic and metal is known as cermet. The word ceramic can be an adjective,
and can also be used as a noun to refer to a ceramic material, or a product of
ceramic manufacture. Ceramics is a singular noun referring to the art of making
things out of ceramic materials.
Many ceramic materials are hard, porous and brittle. The study and
development of ceramics includes methods to mitigate problems associated with
these characteristics, and to accentuate the strengths of the materials as well
as to investigate novel applications.
There are a number of ceramics that are semiconductors. Most of these are
transition metal oxides that are II-VI semiconductors, such as zinc oxide.
While there is talk of making blue LEDs from zinc oxide, ceramicists are most
interested in the electrical properties that show grain boundary effects.
One of the most widely used of these is the varistor. These are devices that
exhibit the property that resistance drops sharply at a certain threshold
voltage. Once the voltage across the device reaches the threshold, there is a
breakdown of the electrical structure in the vicinity of the grain boundaries,
which results in its electrical resistance dropping from several megohms down to
a few hundred ohms. The major advantage of these is that they can dissipate a
lot of energy, and they self reset � after the voltage across the device drops
below the threshold, its resistance returns to being high.
This makes them ideal for surge-protection applications. As there is control
over the threshold voltage and energy tolerance, they find use in all sorts of
applications. The best demonstration of their ability can be found in electrical
substations, where they are employed to protect the infrastructure from
lightning strikes. They have rapid response, are low maintenance, and do not
appreciably degrade from use, making them virtually ideal devices for this
application.
Semiconducting ceramics are also employed as gas sensors. When various gases
are passed over a polycrystalline ceramic, its electrical resistance changes.
With tuning to the possible gas mixtures, very inexpensive devices can be
produced.
Ceramic materials are usually ionic or covalently-bonded materials, and can
be crystalline or amorphous. A material held together by either type of bond
will tend to fracture before any plastic deformation takes place, which results
in poor toughness in these materials. Additionally, because these materials tend
to be porous, the pores and other microscopic imperfections act as stress
concentrators, decreasing the toughness further, and reducing the tensile
strength. These combine to give catastrophic failures, as opposed to the
normally much more gentle failure modes of metals.
These materials do show plastic deformation. However, due to the rigid
structure of the crystalline materials, there are very few available slip
systems for dislocations to move, and so they deform very slowly. With the
non-crystalline (glassy) materials, viscous flow is the dominant source of
plastic deformation, and is also very slow. It is therefore neglected in many
applications of ceramic materials.
Under some conditions, such as extremely low temperature, some ceramics
exhibit superconductivity. The exact reason for this is not known, but there are
two major families of superconducting ceramics.
Piezoelectricity, a link between electrical and mechanical response, is
exhibited by a large number of ceramic materials, including the quartz used to
measure time in watches and other electronics. Such devices use both properties
of piezoelectrics, using electricity to produce a mechanical motion (powering
the device) and then using this mechanical motion to produce electricity
(generating a signal). The unit of time measured is the natural interval
required for electricity to be converted into mechanical energy and back again.
The piezoelectric effect is generally stronger in materials that also exhibit
pyroelectricity, and all pyroelectric materials are also piezoelectric. These
materials can be used to inter convert between thermal, mechanical, and/or
electrical energy; for instance, after synthesis in a furnace, a pyroelectric
crystal allowed to cool under no applied stress generally builds up a static
charge of thousands of volts. Such materials are used in motion sensors, where
the tiny rise in temperature from a warm body entering the room is enough to
produce a measurable voltage in the crystal.
In turn, pyroelectricity is seen most strongly in materials which also
display the ferroelectric effect, in which a stable electric dipole can be
oriented or reversed by applying an electrostatic field. Pyroelectricity is also
a necessary consequence of ferroelectricity. This can be used to store
information in ferroelectric capacitors, elements of ferroelectric RAM.
The most common such materials are lead zirconate titanate and barium
titanate. Aside from the uses mentioned above, their strong piezoelectric
response is exploited in the design of high-frequency loudspeakers, transducers
for sonar, and actuators for atomic force and scanning tunneling microscopes.
Increases in temperature can cause grain boundaries to suddenly become
insulating in some semiconducting ceramic materials, mostly mixtures of heavy
metal titanates. The critical transition temperature can be adjusted over a wide
range by variations in chemistry. In such materials, current will pass through
the material until joule heating brings it to the transition temperature, at
which point the circuit will be broken and current flow will cease. Such
ceramics are used as self-controlled heating elements in, for example, the
rear-window defrost circuits of automobiles.
At the transition temperature, the material's dielectric response becomes
theoretically infinite. While a lack of temperature control would rule out any
practical use of the material near its critical temperature, the dielectric
effect remains exceptionally strong even at much higher temperatures. Titanates
with critical temperatures far below room temperature have become synonymous
with "ceramic" in the context of ceramic capacitors for just this reason.
Non-crystalline ceramics: Non-crystalline ceramics, being glasses, tend to be
formed from melts. The glass is shaped when either fully molten, by casting, or
when in a state of toffee-like viscosity, by methods such as blowing to a mold.
If later heat-treatments cause this class to become partly crystalline, the
resulting material is known as a glass-ceramic.
Crystalline ceramics: Crystalline ceramic materials are not amenable to a
great range of processing. Methods for dealing with them tend to fall into one
of two categories - either make the ceramic in the desired shape, by reaction in
situ, or by "forming" powders into the desired shape, and then sintering to form
a solid body. Ceramic forming techniques include shaping by hand (sometimes
including a rotation process called "throwing"), slip casting, tape casting
(used for making very thin ceramic capacitors, etc.), injection molding, dry
pressing, and other variations. (See also Ceramic forming techniques. Details of
these processes are described in the two books listed below.) A few methods use
a hybrid between the two approaches.
In the early 1980s, Toyota researched production of an adiabatic ceramic
engine which can run at a temperature of over 6000 �F (3300 �C). Ceramic engines
do not require a cooling system and hence allow a major weight reduction and
therefore greater fuel efficiency. Fuel efficiency of the engine is also higher
at high temperature, as shown by Carnot's theorem. In a conventional metallic
engine, much of the energy released from the fuel must be dissipated as waste
heat in order to prevent a meltdown of the metallic parts.
Despite all of these desirable properties, such engines are not in production
because the manufacturing of ceramic parts in the requisite precision and
durability is difficult. Imperfection in the ceramic leads to cracks, which can
lead to potentially dangerous equipment failure. Such engines are possible in
laboratory settings, but mass-production is unfeasible with current technology.
Work is being done in developing ceramic parts for gas turbine engines.
Currently, even blades made of advanced metal alloys used in the engines' hot
section require cooling and careful limiting of operating temperatures. Turbine
engines made with ceramics could operate more efficiently, giving aircraft
greater range and payload for a set amount of fuel.
Ceramics are used in the manufacture of knives. The blade of the ceramic
knife will stay sharp for much longer than that of a steel knife, although it is
more brittle and can be snapped by dropping it on a hard surface.
Since the late 1990s, highly specialized ceramics, usually based on boron
carbide, formed into plates and lined with Spectra, have been used in ballistic
armored vests to repel large-caliber rifle fire. Such plates are known commonly
as small-arms protective inserts (SAPI). Very similar technology is used to
protect cockpits of some military airplanes, because of the low weight of the
material.
Recently, there have been advances in ceramics which include bio-ceramics,
such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral
component of bone, has been made synthetically from a number of biological and
chemical sources and can be formed into ceramic materials. Orthopedic implants
made from these materials bond readily to bone and other tissues in the body
without rejection or inflammatory reactions. Because of this, they are of great
interest for gene delivery and tissue engineering scaffolds. Most hydroxy
apatite ceramics are very porous and lack mechanical strength and are used to
coat metal orthopedic devices to aid in forming a bond to bone or as bone
fillers. They are also used as fillers for orthopedic plastic screws to aid in
reducing the inflammation and increase absorption of these plastic materials.
Work is being done to make strong-fully dense nano crystalline hydroxapatite
ceramic materials for orthopedic weight bearing devices, replacing foreign metal
and plastic orthopedic materials with a synthetic natural bone mineral.
Ultimately these ceramic materials may be used as bone replacements or with the
incorporation of protein collagens, synthetic bones.
Liquid crystals
Liquid crystals are substances that exhibit a phase of matter that has
properties between those of a conventional liquid, and those of a solid crystal.
For instance, a liquid crystal (LC) may flow like a liquid, but have the
molecules in the liquid arranged and/or oriented in a crystal-like way. There
are many different types of LC phases, which can be distinguished based on their
different optical properties (such as birefringence). When viewed under a
microscope using a polarized light source, different liquid crystal phases will
appear to have a distinct texture. Each "patch" in the texture corresponds to a
domain where the LC molecules are oriented in a different direction. Within a
domain, however, the molecules are well ordered. Liquid crystal materials may
not always be in an LC phase (just as water is not always in the liquid phase:
it may also be found in the solid or gas phase). Liquid crystals can be divided
into thermotropic and lyotropic LCs. Thermotropic LCs exhibit a phase transition
into the LC phase as temperature is changed, whereas lyotropic LCs exhibit phase
transitions as a function of concentration of the mesogen in a solvent
(typically water) as well as temperature.
Liquid crystals find wide use in liquid crystal displays, which rely on the
optical properties of certain liquid crystalline molecules in the presence or
absence of an electric field. In a typical device, a liquid crystal layer sits
between two polarizers that are crossed (oriented at 90� to one another). The
liquid crystal is chosen so that its relaxed phase is a twisted one. This
twisted phase reorients light that has passed through the first polarizer,
allowing it to be transmitted through the second polarizer and reflected back to
the observer. The device thus appears clear. When an electric field is applied
to the LC layer, all the mesogens align (and are no longer twisting). In this
aligned state, the mesogens do not reorient light, so the light polarized at the
first polarizer is absorbed at the second polarizer, and the entire device
appears dark. In this way, the electric field can be used to make a pixel switch
between clear or dark on command. Color LCD systems use the same technique, with
color filters used to generate red, green, and blue pixels. Similar principles
can be used to make other liquid crystal based optical devices.
Thermotropic chiral LCs whose pitch varies strongly with temperature can be
used as crude thermometers, since the color of the material will change as the
pitch is changed. Liquid crystal color transitions are used on many aquarium and
pool thermometers. Other liquid crystal materials change color when stretched or
stressed. Thus, liquid crystal sheets are often used in industry to look for hot
spots, map heat flow, measure stress distribution patterns, and so on. Liquid
crystal in fluid form is used to detect electrically generated hot spots for
failure analysis in the semiconductor industry. Liquid crystal memory units with
extensive capacity were used in Space Shuttle navigation equipment.
It is also worth noting that many common fluids are in fact liquid crystals.
Soap, for instance, is a liquid crystal, and forms a variety of LC phases
depending on its concentration in water.
Thermochromics
Thermochromism is the ability of substance to change colour due to a change
in temperature. A mood ring is an excellent example of this, but it has many
other uses. Thermochromism is one of several types of chromism.
The two basic approaches are based on liquid crystals and leuco dyes. Liquid
crystals are used in precision applications, as their responses can be
engineered to accurate temperatures, but their color range is limited by their
principle of operation. Leuco dyes allow wider range of colors to be used, but
their response temperatures are more difficult to set with accuracy.
Some liquid crystals are capable of displaying different colors at different
temperatures. This change is dependent on selective reflection of certain
wavelengths by the crystallic structure of the material, as it changes between
the low-temperature crystallic phase, through anisotropic chiral or twisted
nematic phase, to the high-temperature isotropic liquid phase. Only the nematic
mesophase has thermochromic properties; this restricts the effective temperature
range of the material.
The twisted nematic phase has the molecules oriented in layers with regularly
changing orientation, which gives them periodic spacing. The light passing the
crystal undergoes Bragg diffraction on these layers, and the wavelength with the
greatest constructive interference is reflected back, which is perceived as a
spectral color. As the crystal undergoes changes in temperature, thermal
expansion occurs, resulting in change of spacing between the layers, and
therefore in the reflected wavelength. The color of the thermochromic liquid
crystal can therefore continuously range from black through the spectral colors
to black again, depending on the temperature.
Some such materials are cholesteryl nonanoate or cyanobiphenyls.
Liquid crystals used in dyes and inks often come microencapsulated, in the
form of suspension.
Liquid crystals are used in applications where the color change has to be
accurately defined. They find applications in thermometers for room,
refrigerator, aquarium, and medical use, and in indicators of level of propane
in tanks.
Liquid crystals are difficult to work with and require specialized printing
equipment. The material itself is also typically more expensive than alternative
technologies. High temperatures, ultraviolet radiation, some chemicals and/or
solvents have a negative impact on their lifespan.
Thermochromic paint is a relatively recent development in the area of
color-changing pigments. It involves the use of liquid crystal or leuco dye
technology. After absorbing a certain amount of light or heat, the crystallic or
molecular structure of the pigment reversibly changes in such a way that it
absorbs and emits light at a different wavelength than at lower temperatures.
Thermochromic paints are seen quite often as a coating on coffee mugs, whereby
once hot coffee is poured into the mugs, the thermochromic paint absorbs the
heat and becomes colored or transparent, therefore changing the appearance of
the mug.
Crystallography
Crystallography is the experimental science of determining the arrangement of
atoms in solids. In older usage, it is the scientific study of crystals.
Before the development of X-ray diffraction crystallography (see below), the
study of crystals was based on the geometry of the crystals. This involves
measuring the angles of crystal faces relative to theoretical reference axes
(crystallographic axes), and establishing the symmetry of the crystal in
question. The former is carried out using a goniometer. The position in 3D space
of each crystal face is plotted on a stereographic net, e.g. Wolff net or
Lambert net. In fact, the pole to each face is plotted on the net. Each point is
labelled with its Miller index. The final plot allows the symmetry of the
crystal to be established.
Crystallographic methods now depend on the analysis of the diffraction
patterns that emerge from a sample that is targeted by a beam of some type. The
beam is not always electromagnetic radiation, even though X-rays are the most
common choice. For some purposes electrons or neutrons are used, which is
possible due to the wave properties of the particles. Crystallographers often
explicitly state the type of illumination used when referring to a method, as
with the terms X-ray diffraction, neutron diffraction and electron diffraction.
These three types of radiation interact with the specimen in different ways.
X-rays interact with the spatial distribution of the valence electrons, while
electrons are charged particles and therefore feel the total charge distribution
of both the atomic nuclei and the surrounding electrons. Neutrons are scattered
by the atomic nuclei through the strong nuclear forces, but in addition, the
magnetic moment of neutrons is non-zero. They are therefore also scattered by
magnetic fields. Because of these different forms of interaction, the three
types of radiation are suitable for different crystallographic studies.
In several cases, an image of a microscopic object is generated by focusing
the rays of the visible spectrum using a lens as in light microscopy. However,
because the wavelength of visible light is long compared to atomic bond lengths
and atoms themselves, it is necessary to use radiation with shorter wavelengths,
such as X-rays. Employing shorter wavelengths implies abandoning microscopy and
true imaging, however, because there exists no material from which a lens
capable of focusing this type of radiation can be created. (That said,
scientists have had some success focusing X-rays with microscopic Fresnel zone
plates made from gold). Generally, in diffraction-based imaging, the only
wavelengths used are those that are too short to be focused. This difficulty is
the reason that crystals must be used.
Because of their highly ordered and repetitive structure, crystals are an
ideal material for analyzing the structure of solids. To use X-ray diffraction
as an example, a single X-ray photon diffracting off of one electron cloud will
not generate a strong enough signal for the equipment to detect. However, many
X-rays diffracting off many electron clouds in approximately the same relative
position and orientation throughout the crystal will result in constructive
interference and hence a detectable signal.
Crystallography is a tool that is often employed by materials scientists. In
single crystals, the effects of the crystalline arrangement of atoms is often
easy to see macroscopically, because the natural shapes of crystals reflect the
atomic structure. In addition, physical properties are often controlled by
crystalline defects. The understanding of crystal structures is an important
prerequisite for understanding crystallographic defects. Mostly, materials do
not occur in a syngle crystalline, but poly-crystalline form, such that the
powder diffraction method plays a most important role in structural
determination.
A number of other physical properties are linked to crystallography. For
example, the minerals in clay form small, flat, platelike structures. Clay can
be easily deformed because the platelike particles can slip along each other in
the plane of the plates, yet remain strongly connected in the direction
perpendicular to the plates. Such mechanisms can be studied by crystallographic
texture measurements.
In another example, iron transforms from a body-centered cubic (bcc)
structure to a face-centered cubic (fcc) structure called austenite when it is
heated. The fcc structure is a close-packed structure, and the bcc structure is
not, which explains why the volume of the iron decreases when this
transformation occurs.
Crystallography is useful in phase identification: That is, when performing
some kind of processing on a material, it is often desired to find out what
compounds and what phases are present in the material. Each phase has a
characteristic arrangement of atoms. Techniques like X-ray diffraction can be
used to identify which patterns are present in the material, and thus which
compounds are present (note: the determination of the "phases" within a material
should not be confused with the more general problem of "phase determination,"
which refers to the phase of waves as they diffract from planes within a
crystal, and which is a necessary step in the interpretation of complicated
diffraction patterns).
Crystallography covers the enumeration of the symmetry patterns which can be
formed by atoms in a crystal and for this reason has a relation to group theory
and geometry. See symmetry group.
X-ray crystallography is the primary method for determining the molecular
conformations of biological macromolecules, particularly protein and nucleic
acids such as DNA and RNA. In fact, the double-helical structure of DNA was
deduced from crystallographic data. The first crystal structure of a
macromolecule was solved in 1958 (Kendrew, J.C. et al. (1958) A
three-dimensional model of the myoglobin molecule obtained by X-ray analysis
(Nature 181, 662�666). The Protein Data Bank (PDB) is a freely accessible
repository for the structures of proteins and other biological macromolecules.
RasMol can be used to visualize biological molecular structures.
Electron crystallography has been used to determine some protein structures,
most notably membrane proteins and viral capsids.
|
