The word ceramic is derived from the Greek word
(the name of a suburb of Athens), and in its strictest sense refers
to clay in all its forms. However, modern usage of the term broadens
the meaning to include all inorganic non-metallic materials. Up
until the 1950s or so, the most important of these were the traditional
clays, made into pottery, bricks, tiles and the 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
The Venus of Dolni Vestonice is the oldest known
ceramic in the world. Historically, ceramic products have been hard,
porous and brittle. The study of ceramics consists to a large extent
of methods to mitigate these problems, and accentuate the strengths
of the materials, as well as to offer up unusual uses for these
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.
Whilst 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 unusual property of negative
resistance. Once the voltage across the device reaches a certain
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 megaohms 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
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.
Processing of Ceramic Materials
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 mould. If later heat-treatments cause this class to
become partly crystalline, the resulting material is known as a
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. A few
methods use a hybrid between the two approaches.
In situ manufacturing
The most common use of this method is in the production of cement
and concrete. Here, the dehydrated powders are mixed with water.
This starts hydration reactions, which result in long, interlocking
crystals forming around the aggregates. Over time, these result
in a solid ceramic.
The biggest problem with this method is that most
reactions are so fast that good mixing is not possible, which tends
to prevent large-scale construction. However, small-scale systems
can be made by deposition techniques, where the various materials
are introduced above a substrate, and react and form the ceramic
on the substrate. This borrows techniques from the semiconductor
industry, such as chemical vapour deposition, and is very useful
These tend to produce very dense ceramics, but
do so slowly.
Other applications of ceramics
A couple of decades ago, 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. 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 infeasible 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.
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 for armoring of
cockpits of some military airplanes, because of the low weight of