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Effects of Alloying Elements in Steel

Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.

·         Carbon

·         Manganese

·         Chromium

·         Nickel

·         Molybdenum

·         Titanium

·         Phosphorus

·         Sulphur

·         Selenium

·         Niobium

·         Nitrogen

·         Silicon

·         Cobalt

·         Tantalum

·         Copper

Carbon

The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.

Manganese

Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)

Chromium

Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.

Nickel

Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.

Molybdenum

Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.

Titanium

The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.

Phosphorus

Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.

Sulphur

When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.

Selenium

Selenium is added to improve machinability.

Niobium (Columbium)

Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.

Nitrogen

Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.

Silicon

Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels.

Cobalt

Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.

Tantalum

Chemically similar to niobium and has similar effects.

Copper

Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.

MINERAL TREATMENT OF SPODUMENE IN THE CERAMIC INDUSTRY. CHARACTERISATION ANDTECHNOLOGICAL USES

1. INTRODUCTION

In recent years, the branches of ceramics that have undergone the greatest

development in the world have been those of floor and wall tile manufacture. This

expansion has fostered the creation of companies dedicated to the manufacture of frits,

glazes and ceramic pigments, together with marked growth in other already existing

companies. On the other hand, the economic context in which the industrial activities

are being conducted have led manufacturers to seek more efficient production

technologies, among other processes including fast firing, single firing, etc. The focus

has been on reducing costs, mainly for fuels and this requires refractory bodies with

low dilatometric coefficients of expansion. For the manufacture of these refractories, as

well for that of the frits mentioned previously, spodumene offers numerous advantages

if it is included in their composition.

The thermal shock resistance of a ceramic body depends fundamentally on

the following properties: thermal conductivity, coefficient of expansion, mechanical

strength and modulus of elasticity. Of these properties, the one that can vary most

is thermal expansion. This is the cause of the great interest in Li2O·Al2O3·SiO2

systems, which include compounds with very low coefficients of expansion, which

allow making very thermally resistant pieces.

Spodumene is a lithium aluminosilicate LiAl(SiO3)2, which is found in the

natural state as α−spodumene, structurally a monoclinic ‘pyroxene’ that contains

7.9% Li2O and has a density of 3.2 g/cm3. At high temperatures, 900º-1000ºC, this

low temperature form undergoes an irreversible polymorphic transformation,

increasing its volume and converting to β−spodumene, which belongs to the

tetragonal crystalline system. In this form the mineral displays a density of 2.4

g/cm3, and is characterised by exhibiting a very low dilatometric expansion,

corresponding to values below 1.0·10-6 ºC-1.

On the other hand, spodumene has a very pronounced fluxing action, both

of its own and because of the eutectics it forms with other fluxes. This property

is used in the manufacture of frits for ceramic glazes and for earthenware.

It is also very important for reducing the vitrification temperature and the

final porosity of ceramic bodies. Beyond the applications of spodumene in the

ceramic industry, the high percentage of lithium present in the mineral opens up

numerous prospects for industrialisation; thus, lithium is considered worldwide

as a strategic material for its great variety of applications: glass manufacture,

lubricants, alloy obtainment, drug elaboration, operation of cooling systems, and

in the manufacture of batteries.

The spodumene deposits correspond in all the cases, to pegmatites with a

zoned structure, in which the mineral is located in the intermediate bands and the

core. In the Argentine Republic, the main deposits are in the provinces of San Luis

and Catamarca, and they are characterised by a great mineralogical variability. In

all of these, spodumene is associated with: quartz, potassium feldspar, plagioclasesoligoclases

and albite, muscovite and biotite, in addition to amblygonite and

sometimes beryl. In relation to other pegmatites, a little participation of muscovite

is observed and, in contrast, a greater one of albite.

For all these reasons, the objective of this work focuses on optimising the

business operation of spodumene by development of an appropriate infrastructure

and, in addition, to promoting knowledge in the mining field with respect to the

numerous applications that beneficiated spodumene can have.

2. Methedology

The purpose of this work is to transmit the experience developed in an

Argentine industry, in the mineral treatment of a spodumene-bearing pegmatite,

for use in the manufacture of ceramic materials. The beneficiation of the mineral

was done through a heat treatment and subsequent refining, producing the

irreversible the a-b polymorphic transformation of spodumene, which is why

its contribution to the microstructure of a ceramic body will increase its thermal

shock resistance. Comparative physical, chemical and mineralogical studies were

conducted with samples of natural lithium pegmatite from the El Alto area in the

Province of Catamarca and treated spodumene. Finally the effect of the spodumene

addition to an earthenware body has been compared.

5. CONCLUSIONS

ü The thermal transformation of a-b spodumene with a change of density from

3.2 g/cm3 to 2.4 g/cm3, facilitates its separation from quartz and feldspar

by selective sieving, since b-spodumene is concentrated in the finer sieving

fractions.

ü The coefficient of expansion of an earthenware body decreases strongly by

adding b-spodumene to its composition.

6. Acknowledgements

The author wishes to thank Mr. Alfredo Inocencio, Mrs. Gloria Brunetti, Mrs.

Ana Rodriguez Velo and Mr. Diego Pucciarelli, for their help in this work; thanks

also go to the Project for the ‘Improvement of the Efficiency and Competitiveness of

the Argentine Economy’, a Community (E.U.) cooperation project with the Argentine

Republic.

REFERENCES

[1] Botto, I., Baran, E., Cohen Arazi, S., Krenkel, T. Aspectos estructurales de la espodumena en relación con el

método de tratamiento ácido. Bol. Soc. Esp. Cer. y Vid., V15, Nº3, May/June 1976.

[2] Fishwick, J. Spodumene aids fast firing, improves bodies. Ceramic Industry Magazine p34, May 1967.

[3] González Peña, J.M. Materiales cerámicos de bajo coeficiente de dilatación a base de silicoaluminato de litio.

Bol. Soc. Esp. Cer. y Vid., V7, Nº3, 1968.

[4] Hummel, F.A review of thermal expansion data of ceramic materials. Especially ultra-low expansion

compositions. Interceram Nº6, p 27-30, 1984.

[5] Mc Cracken, D. Lithium minerals review 1993. Industrial minerals, p85-89, set/1994.

[6] Luis Sanchez-Muñoz, J.B. Carda Castelló. Enciclopedia cerámica: TOMOS 2.1 y 2.2. “Materias primas y

aditivos cerámicos”. 2002.

[7] Ginés F, Orenga A., Sheth A., Thiery D., “Ejemplo del espodumeno para fabricar pastas de gres porcelánico

técnico” QUALICER 2004, Pág. 233-236.

How Whitewares are Processed?

The forming and firing processes employed in the manufacture of whiteware products are outlined in the article industrial ceramics. Typically, pressing is employed in the forming of tiles, chemical ware, and technical porcelains; extrusion in the forming of tiles and sanitary ware (including pipe); and slipcasting in the forming of plumbing fixtures and some tableware. In addition to these standard processes, jiggering is employed in the manufacture of tableware. Jiggering involves the mixing of a plastic mass and turning it on a wheel beneath a template to a specified size and shape.

Most whitewares are fired in continuous tunnel kilns. The porous varieties are fired at lower temperatures (1,100–1,250 °C, or approximately 2,000–2,300 °F), whereas china and true porcelains are fired at 1,250 to 1,300 °C (2,300 to 2,400 °F). Porous and semivitreous whitewares may be glazed in a second firing to produce an impermeable glass coating for decorative or functional purposes. One of the great advantages of the triaxial composition of whitewares is that it makes the formed piece relatively insensitive to minor changes in composition and in firing time or temperature. This stability is a result of the wide range of temperatures over which the three ingredients melt to form glass. As an example, in a typical feldspar-clay-silica composition for porcelain, a whiteware with a particularly high glassy component, small grains of feldspar would begin to form liquid at temperatures as low as 990 °C (1,810 °F), and large feldspar grains would be molten by 1,140 °C (2,080 °F). Because of the high viscosity of the liquid formed, there would be no change in the shape of the ceramic piece until approximately 1,200 °C (2,200 °F). Above this temperature the feldspar grains would react with surrounding clay particles to form glass, and “needles” of mullite (a crystalline aluminosilicate mineral formed during the firing of clay-silica mixtures) would grow into the liquid regions. In addition, the surfaces of silica particles would begin to dissolve and form solution rims, or envelopes of glass surrounding the crystalline particle. As more and more of the silica particles dissolved, the resulting glass would become increasingly viscous, helping to maintain the integrity of the piece.

STANFORD BIOENGINEERS REDESIGN PROTEIN MOTORS TO CREATE NOVEL NANOMACHINES

Stanford scientists genetically engineer versions of myosin proteins that transport biological materials in cells to illuminate design features that keep these protein motors on track.

Matt Davenport | Stanford Engineering

Inside our cells, proteins known as myosins can act as a delivery service for biological materials. To better understand how molecular motors move, Stanford bioengineers have built experimental versions of the proteins, changing the way these transporters get around.

Led by Zev Bryant, an assistant professor of bioengineering at Stanford, a team of researchers has genetically engineered “mutant” myosins with new features such as gearshifts and improved traction. The group’s most recent findings are published in the January issue of Nature Nanotechnology, where they are highlighted alongside other studies of molecular motors.

“You look at biology, and you see motors that have diverse mechanical properties, and you want to understand how these arise,” Bryant said. “You test your understanding by trying to build something new.”

Molecular motors are a class of proteins that make up the moving machinery of cells. Myosins are one family of molecular motors. Some of them can shuttle biomolecules from one region of the cell to another.

These myosins move along microscopic filaments made of the protein known as actin. These actin filaments are one component of the cytoskeleton, or internal support structure of the cell.

Bryant wanted to test his understanding of how evolution has designed these myosin proteins to shuttle cellular freight. Funded by an NIH “New Innovator” Award, members of the group launched a series of experiments in 2008 that steered their myosin research in a new direction. They began engineering myosins with extra parts to give natural myosins new capabilities.

Natural myosins, for example, see actin filaments as one-way tracks. To better understand this one-directional motion, Bryant challenged his group to design mutant myosins that could move forward and backward on command.

The researchers engineered myosin motors with extra components that behaved like a molecular gearshift. In a 2012 Nature Nanotechnology report, the researchers showed that they could shift their mutant myosin motion between forward and reverse.

However, these two-way myosins had trouble hanging onto their actin tracks.

“When we engineer motors to have new capabilities, we often sacrifice some capabilities that they already had,” Bryant said.

So the group focused on creating motors that excelled at hanging on.

This artist’s whimsical rendition depicts the Bryant group’s bidirectional myosin motors, shown with orange arms and hands. Natural myosin proteins that shuttle molecules around cells can only move one way along their tracks (the actin filaments shown in blue). Bryant’s team engineered gearshifts into a new version of myosin that could be toggled between forward and reverse. (Lu Chen and XVIVO)

Natural myosins that shuttle molecules in cells – and the Bryant lab’s original bidirectional mutants – have two molecular arms. These arms terminate in two molecular hands. These hands attach to the actin filaments.

Myosin proteins can move along these filaments, hand over hand, like a person on monkey bars. The Stanford team designed three- and four-handed versions of myosins, hypothesizing that adding extra hands would help stabilize the mutant motors and enable them to better grip the monkey bars.

But, “we didn’t know if adding those extra elements would muck everything up,” said doctoral student Tony Schindler, first author of the January Nature Nanotechnology report.

The team members made these many-fisted myosins using common practices in molecular biology. They assembled the DNA segments that instruct cells to make myosin proteins. They modified these DNA sequences with instructions to add extra hands. The team members injected these modified DNA segments into insect cells that they used as factories. Following the directions coded into the DNA, the insect cells produced the mutant myosins.

In addition to giving these myosins extra hands, the engineers found that they could help the motors stay on track by inserting flexible regions into the arms. This innovation seemed to boost the dexterity of the mutant myosins, helping their hands grip the actin rails and avoid falling off.

To verify their findings, the team members put their new myosins to the test. First, they combined these multi-handed molecular upgrades with their earlier bidirectional add-ons.

Without the extra arms, bidirectional mutant myosins frequently fell off their actin tracks. With the additional arms, the bidirectional motors held on tight. As Bryant put it, the team had bioengineered a “four-wheel drive version” of a myosin that could be directed to go forward or backward and remain on the track.

Bryant and his team watched their mutant myosins move underneath a microscope. While their natural counterparts shuttle biological molecules inside cells, these motors carry fluorescent dye molecules along actin tracks (shown in red) set up on a microscope slide, allowing the Bryant group to track their motion. The green dots are four-handed myosins motoring along actin tracks. Once a myosin attaches, it hangs on tight and stays on the track for micrometers — the length scale of cells. (Used with permission from Macmillan Publishers Ltd: Nature Nanotechnology 9, 33–38 (2014), copyright 2013)

In a final test, the researchers tried pushing one-directional myosins with multiple limbs to high speeds. They grafted fast-moving hands to their three- and four-armed mutants to see whether the increased hand-over-hand velocities would upset the newly improved grip.

Not only did the mutant myosins stay on track at these higher speeds, but the group also built a four-handed version of myosin that moves faster than any natural hand-over-hand motor.

“It’s a design-driven approach,” Bryant said. “We want to understand the structure-function rules that underlie these machines that fascinate us and have been constructed by evolution.”

In the future, researchers may put these engineered motors back inside cells to change the way molecules move around. Scientists may also incorporate customized molecular motors into medical diagnostic devices that use these nanomachines to sort molecules and move them to sensors, imitating the molecular traffic found in living cells.

Graduate students Lu Chen, Paul Lebel and Muneaki Nakamura also contributed to the January article.

Matt Davenport is a science writing intern with Stanford Engineering.

Monday, January 6, 2014

Composite Materials

Composite materials (also called composition materials or shortened to composites) are materials made from two or more constituent materials with significantly different physical or chemical properties, that when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure. The new material may be preferred for many reasons: common examples include materials which are stronger, lighter or less expensive when compared to traditional materials.
Typical engineered composite materials include:

Composite building materials such as cements, concrete
Reinforced plastics such as fiber-reinforced polymer
Metal Composites
Ceramic Composites (composite ceramic and metal matrices)

Composite materials are generally used for buildings, bridges and structures such as boat hulls, swimming pool panels, race car bodies, shower stalls, bathtubs, storage tanks, imitation granite and cultured marble sinks and counter tops. The most advanced examples perform routinely on spacecraft in demanding environments.
Examples
Concrete is the most common artificial composite material of all and typically consists of loose stones (aggregate) held with a matrix of cement. Concrete is a very robust material, much more robust than cement, and will not compress or shatter even under quite a large compressive force. However, concrete cannot survive tensile loading (i.e. if stretched it will quickly break apart). Therefore to give concrete the ability to resist being stretched, steel bars, which can resist high stretching forces, are often added to concrete to form reinforced concrete.
Fibre-reinforced polymers or FRPs include carbon-fibre reinforced plastic or CFRP, and glass-reinforced plastic or GRP. If classified by matrix then there are thermoplastic composites, short fibre thermoplastics, long fibre thermoplastics or long fibre-reinforced thermoplastics. There are numerous thermoset composites, but advanced systems usually incorporate aramid fibre and carbon fibre in an epoxy resin matrix.
Shape memory polymer composites are high-performance composites, formulated using fibre or fabric reinforcement and shape memory polymer resin as the matrix. Since a shape memory polymer resin is used as the matrix, these composites have the ability to be easily manipulated into various configurations when they are heated above their activation temperatures and will exhibit high strength and stiffness at lower temperatures. They can also be reheated and reshaped repeatedly without losing their material properties. These composites are ideal for applications such as lightweight, rigid, deployable structures; rapid manufacturing; and dynamic reinforcement.
Composites can also use metal fibres reinforcing other metals, as in metal matrix composites (MMC) or ceramic matrix composites (CMC), which includes bone (hydroxyapatite reinforced with collagen fibres), cermet (ceramic and metal) and concrete. Ceramic matrix composites are built primarily for fracture toughness, not for strength. Organic matrix/ceramic aggregate composites include asphalt concrete, mastic asphalt, mastic roller hybrid, dental composite, syntactic foam and mother of pearl. Chobham armour is a special type of composite armour used in military applications.
Additionally, thermoplastic composite materials can be formulated with specific metal powders resulting in materials with a density range from 2 g/cm³ to 11 g/cm³ (same density as lead). The most common name for this type of material is "high gravity compound" (HGC), although "lead replacement" is also used. These materials can be used in place of traditional materials such as aluminium, stainless steel, brass, bronze, copper, lead, and even tungsten in weighting, balancing (for example, modifying the centre of gravity of a tennis racquet), vibration damping, and radiation shielding applications. High density composites are an economically viable option when certain materials are deemed hazardous and are banned (such as lead) or when secondary operations costs (such as machining, finishing, or coating) are a factor.
A sandwich-structured composite is a special class of composite material that is fabricated by attaching two thin but stiff skins to a lightweight but thick core. The core material is normally low strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density.

Wood is a naturally occurring composite comprising cellulose fibres in a lignin and hemicellulose matrix. Engineered wood includes a wide variety of different products such as wood fibre board, plywood, oriented strand board, wood plastic composite (recycled wood fibre in polyethylene matrix), Pykrete (sawdust in ice matrix), Plastic-impregnated or laminated paper or textiles, Arborite, Formica (plastic) and Micarta. Other engineered laminate composites, such as Mallite, use a central core of end grain balsa wood, bonded to surface skins of light alloy or GRP. These generate low-weight, high rigidity materials.

Physical Properties:
The physical properties of composite materials are generally not isotropic (independent of direction of applied force) in nature, but rather are typically anisotropic (different depending on the direction of the applied force or load). For instance, the stiffness of a composite panel will often depend upon the orientation of the applied forces and/or moments. Panel stiffness is also dependent on the design of the panel. For instance, the fibre reinforcement and matrix used, the method of panel build, thermoset versus thermoplastic, type of weave, and orientation of fibre axis to the primary force.
In contrast, isotropic materials (for example, aluminium or steel), in standard wrought forms, typically have the same stiffness regardless of the directional orientation of the applied forces and/or moments.
The relationship between forces/moments and strains/curvatures for an isotropic material can be described with the following material properties: Young's Modulus, the shear Modulus and the Poisson's ratio, in relatively simple mathematical relationships. For the anisotropic material, it requires the mathematics of a second order tensor and up to 21 material property constants. For the special case of orthogonal isotropy, there are three different material property constants for each of Young's Modulus, Shear Modulus and Poisson's ratio—a total of 9 constants to describe the relationship between forces/moments and strains/curvatures.
Techniques that take advantage of the anisotropic properties of the materials include mortise and tenon joints (in natural composites such as wood) and Pi Joints in synthetic composites.

Ceramics

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Ceramics can be defined as heat-resistant, nonmetallic, inorganic solids that are (generally) made up of compounds formed from metallic and nonmetallic elements. Although different types of ceramics can have very different properties, in general ceramics are corrosion-resistant and hard, but brittle. Most ceramics are also good insulators and can withstand high temperatures. These properties have led to their use in virtually every aspect of modern life.
The two main categories of ceramics are traditional and advanced. Traditional ceramics include objects made of clay and cements that have been hardened by heating at high temperatures. Traditional ceramics are used in dishes, crockery, flowerpots, and roof and wall tiles. Advanced ceramics include carbides, such as silicon carbide, SiC; oxides, such as aluminum oxide, Al ₂ O ₃ ; nitrides, such as silicon nitride, Si ₃ N ₄ ; and many other materials, including the mixed oxide ceramics that can act as superconductors. Advanced ceramics require modern processing techniques, and the development of these techniques has led to advances in medicine and engineering.
Glass is sometimes considered a type of ceramic. However, glasses and ceramics differ in that ceramics have a crystalline structure while glasses contain impurities that prevent crystallization . The structure of glasses is amorphous, like that of liquids. Ceramics tend to have high, well-defined melting points, while glasses tend to soften over a range of temperatures before becoming liquids. In addition, most ceramics are opaque to visible light, and glasses tend to be translucent. Glass ceramics have a structure that consists of many tiny crystalline regions within a noncrystalline matrix. This structure gives them some properties of ceramics and some of glasses. In general, glass ceramics expand less when heated than most glasses, making them useful in windows, for wood stoves, or as radiant glass-ceramic cooktop surfaces.

Composition

Some ceramics are composed of only two elements. For example, alumina is aluminum oxide, Al ₂ O ₃ ; zirconia is zirconium oxide, ZrO ₂ ; and quartz is

Ceramics are good insulators and can withstand high temperatures. A popular use of ceramics is in artwork.

silicon dioxide, SiO ₂ . Other ceramic materials, including many minerals, have complex and even variable compositions. For example, the ceramic mineral feldspar, one of the components of granite, has the formula KAlSi ₃ O ₈ .
The chemical bonds in ceramics can be covalent, ionic, or polar covalent, depending on the chemical composition of the ceramic. When the components of the ceramic are a metal and a nonmetal, the bonding is primarily ionic; examples are magnesium oxide (magnesia), MgO, and barium titanate, BaTiO ₃ . In ceramics composed of ametalloid and a nonmetal, bonding is primarily covalent; examples are boron nitride, BN, and silicon carbide, SiC. Most ceramics have a highly crystalline structure, in which a three-dimensional unit, called a unit cell, is repeated throughout the material. For example, magnesium oxide crystallizes in the rock salt structure. In this structure, Mg ²⁺ ions alternate with O ²⁻ ions along each perpendicular axis.

Manufacture of Traditional Ceramics

Traditional ceramics are made from natural materials such as clay that have been hardened by heating at high temperatures (driving out water and allowing strong chemical bonds to form between the flakes of clay). In fact, the word "ceramic" comes from the Greek keramos , whose original meaning was "burnt earth." When artists make ceramic works of art, they first mold clay, often mixed with other raw materials, into the desired shape. Special ovens called kilns are used to "fire" (heat) the shaped object until it hardens.
Clay consists of a large number of very tiny flat plates, stacked together but separated by thin layers of water. The water allows the plates to cling together, but also acts as a lubricant, allowing the plates to slide past one another. As a result, clay is easily molded into shapes. High temperatures drive out water and allow bonds to form between plates, holding them in place and promoting the formation of a hard solid. Binders such as bone ash are sometimes added to the clay to promote strong bond formation, which makes the ceramic resistant to breakage. The common clay used to make flowerpots and roof tiles is usually red-orange because of the presence of iron oxides. White ceramics are made from rarer (and thus more expensive) white clays, primarily kaolin.
The oldest known ceramics made by humans are figurines found in the former Czechoslovakia that are thought to date from around 27,000 B.C.E. It was determined that the figurines were made by mixing clay with bone, animal fat, earth, and bone ash (the ash that results when animal bones are heated to a high temperature), molding the mixture into a desired shape, and heating it in a domed pit. The manufacture of functional objects such as pots, dishes, and storage vessels, was developed in ancient Greece and Egypt during the period 9000 to 6000 B.C.E.
An important advance was the development of white porcelain. Porcelain is a hard, tough ceramic that is less brittle than the ceramics that preceded it. Its strength allows it to be fashioned into beautiful vessels with walls so thin they can even be translucent. It is made from kaolin mixed with china stone, and the mixture is heated to a very high temperature (1,300°C, or 2,372°F). Porcelain was developed in China around C.E. 600 during the T'ang dynasty and was perfected during the Ming dynasty, famous for its blue and white porcelain. The porcelain process was introduced to the Arab world in the ninth century; later Arabs brought porcelain to Spain, from where the process spread throughout Europe.
Bone china has a composition similar to that of porcelain, but at least 50 percent of the material is finely powdered bone ash. Like porcelain, bone china is strong and can be formed into dishes with very thin, translucent walls. Stoneware is a dense, hard, gray or tan ceramic that is less expensive than bone china and porcelain, but it is not as strong. As a result, stoneware dishes are usually thicker and heavier than bone china or porcelain dishes.

Manufacture of Advanced Ceramics

The preparation of an advanced ceramic material usually begins with a finely divided powder that is mixed with an organic binder to help the powder consolidate, so that it can be molded into the desired shape. Before it is fired, the ceramic body is called "green." The green body is first heated at a low temperature in order to decompose or oxidize the binder. It is then heated to a high temperature until it is "sintered," or hardened, into a dense, strong ceramic. At this time, individual particles of the original powder fuse together as chemical bonds form between them. During sintering the ceramic may shrink by as much as 10 to 40 percent. Because shrinkage is not uniform, additional machining of the ceramic may be required in order to obtain a precise shape.
Sol-gel technology allows better mixing of the ceramic components at the molecular level, and hence yields more homogeneous ceramics, because the ions are mixed while in solution. In the sol-gel process, a solution of an organometallic compound is hydrolyzed to produce a "sol," a colloidal suspension of a solid in a liquid. Typically the solution is a metal alkoxide such as tetramethoxysilane in an alcohol solvent. The sol forms when the individual formula units polymerize (link together to form chains and networks). The sol can then be spread into a thin film, precipitated into tiny uniform spheres called microspheres, or further processed to form a gel inside a mold that will yield a final ceramic object in the desired shape. The many crosslinks between the formula units result in a ceramic that is less brittle than typical ceramics.
Although the sol-gel process is very expensive, it has many advantages, including low temperature requirements; the ceramist's ability to control porosity and to form films, spheres, and other structures that are difficult to form in molds; and the attainment of specialized ceramic compositions and high product purity.
Porous ceramics are made by the sol-gel process. These ceramics have spongelike structures, with many porelike lacunae, or openings, that can make up from 25 to 70 percent of the volume. The pore size can be large, or as small as 50 nanometers (2 × 10 ⁻⁶ inches) in diameter. Because of the large number of pores, porous ceramics have enormous surface areas (up to 500 square meters, or 5,382 square feet, per gram of ceramic), and so can make excellent catalysts. For example, zirconium oxide is a ceramic oxygen sensor that monitors the air-to-fuel ratio in the exhaust systems of automobiles.
Aerogels are solid foams prepared by removing the liquid from the gel during a sol-gel process at high temperatures and low pressures. Because aerogels are good insulators, have very low densities, and do not melt at high temperatures, they are attractive for use in spacecraft.

Properties and Uses

For centuries ceramics were used by those who had little knowledge of their structure. Today, understanding of the structure and properties of ceramics is making it possible to design and engineer new kinds of ceramics.
Most ceramics are hard, chemically inert , refractory (can withstand very high heat without deformation), and poor conductors of heat and electricity. Ceramics also have low densities. These properties make ceramics attractive for many applications. Ceramics are used as refractories in furnaces and as durable building materials (in the form of bricks, tiles, cinder blocks, and other hard, strong solids). They are also used as common electrical and thermal insulators in the manufacture of spark plugs, telephone poles, electronic devices, and the nose cones of spacecraft. However, ceramics also tend to be brittle. A major difficulty with the use of ceramics is their tendency to acquire tiny cracks that slowly become larger until the material falls apart. To prevent ceramic materials from cracking, they are often applied as coatings on inexpensive materials that are resistant to cracks. For example, engine parts are sometimes coated with ceramics to reduce heat transfer.
Composite materials that contain ceramic fibers embedded in polymer matrices possess many of the properties of ceramics; these materials have low densities and are resistant to corrosion, yet are tough and flexible rather than brittle. They are used in tennis rackets, bicycles, and automobiles. Ceramic composites may also be made from two distinct ceramic materials that exist as two separate ceramic phases in the composite material. Cracks generated in one phase will not be transferred to the other. As a result, the resistance of the composite material to cracking is considerable. Composite ceramics made from diborides and/or carbides of zirconium and hafnium mixed with silicon carbide are used to create the nose cones of spacecraft. Break-resistant cookware (with outstanding thermal shock resistance) is also made from ceramic composites.
Although most ceramics are thermal and electrical insulators, some, such as cubic boron nitride, are good conductors of heat, and others, such as rhenium oxide, conduct electricity as well as metals. Indium tin oxide is a transparent ceramic that conducts electricity and is used to make liquid crystal calculator displays. Some ceramics are semiconductors, with conductivities that become enhanced as the temperature increases. For example, silicon carbide, SiC, is used as a semiconductor material in high temperature applications.
High temperature superconductors are ceramic materials consisting of complex ionic oxides that become superconducting when cooled by liquid nitrogen. That is, they lose all resistance to electrical current. One example is the material YBa ₂ Cu ₃ O _7− x , which crystallizes to form "sheets" of copper and oxygen atoms that can carry electrical current in the planes of the sheets.
Some ceramics, such as barium ferrite or nickel zinc ferrites, are magnetic materials that provide stronger magnetic fields, weigh less, and cost less than metal magnets. They are made by heating powdered ferrite in a magnetic field under high pressure until it hardens. Ceramic magnets are brittle, but are often used in computers and microwave devices.
The properties of piezoelectric ceramics are modified when voltage is applied to them, making them useful as sensors and buzzers. For example, lead zirconium titanate is a piezoelectric ceramic used to provide "muscle action" in robot limbs in response to electrical signals.
Some ceramics are transparent to light of specific frequencies. These optical ceramics are used as windows for infrared and ultraviolet sensors and in radar installations. However, optical ceramics are not as widely used as glass materials in applications in which visible light must be transmitted. An electro-optic ceramic such as lead lanthanum zirconate titanate is a material whose ability to transmit light is altered by an applied voltage. These electro-optic materials are used in color filters and protective goggles, as well as in memory-storage devices.
Still other ceramics are important in medicine. For example, they are used to fabricate artificial bones and to crown damaged teeth. The fact that many ceramics can be easily sterilized and are chemically inert makes ceramic microspheres made of these materials useful as biosensors. Drugs and other chemicals can be carried within microsphere pores to desired sites in the body.

Loretta L. Jones

Bibliography

Ball, Philip (1997). Made to Measure: New Materials for the Twenty-First Century. Princeton, NJ: Princeton University Press.
Barsoum, Michael W. (1996). Fundamentals of Ceramics. New York: McGraw-Hill.
Brinker, C. Jeffrey, and Scherer, George W. (1990). Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Boston: Academic Press.
Calvert, Paul (2000). "Advanced Materials." In The New Chemistry , ed. Nina Hall. New York: Cambridge University Press.
Kingery, W. D.; Bowen, H. K.; and Uhlmann, D. R. (1976). Introduction to Ceramics , 2nd edition. New York: Wiley.
Richerson, David W. (1992). Modern Ceramic Engineering: Properties, Processes, and Use in Design , 2nd edition, revised and expanded. New York: Marcel Dekker. Richerson, David W. (2000). The Magic of Ceramics. Westerville, OH: American Ceramic Society.
Shackleford, James F., ed. (1998). Bioceramics: Applications of Glass and Ceramic Materials in Medicine. Zurich: Trans-Tech Publications.
Wachtman, John B., Jr., ed. (1999). Ceramic Innovations in the 20th Century. Westerville, OH: American Ceramic Society.

Internet Resources

"About Ceramics." American Ceramic Society. Available from http://www.ceramics.org .