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

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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 .

Solid Oxide Fuel Cell

SOLID OXIDE FUEL CELLS

Solid oxide fuel cells (SOFCs) offer a clean, low-pollution technology to electrochemically generate electricity at high efficiencies; since their efficiencies are not limited the way conventional heat engine's is. These fuel cells provide many advantages over traditional energy conversion systems including high efficiency, reliability, modularity, fuel adaptability, and very low levels of polluting emissions. Quiet, vibration-free operation of SOFCs also eliminates noise usually associated with conventional power generation systems.

Up until about six years ago, SOFCs were being developed for operation primarily in the temperature range of 900 to 1000oC (1692 to 1832oF); in addition to the capability of internally reforming hydrocarbon fuels (for example, natural gas), such high temperature SOFCs provide high quality exhaust heat for cogeneration, and when pressurized, can be integrated with a gas turbine to further increase the overall efficiency of the power system. However, reduction of the SOFC operating temperature by 200oC (392oF) or more allows use of a broader set of materials, is less demanding on the seals and the balance-of-plant components, simplifies thermal management, aids in faster start up and cool down, and results in less degradation of cell and stack components. Because of these advantages, activity in the development of SOFCs capable of operating in the temperature range of 650 to 800oC (1202 to 1472oF) has increased dramatically in the last few years. However, at lower temperatures, electrolyte conductivity and electrode kinetics decrease significantly; to overcome these drawbacks, alternative cell materials and designs are being extensively investigated.

 Operating principle of SOFC

Fig. 1. Operating principle of a solid oxide fuel cell.

An SOFC essentially consists of two porous electrodes separated by a dense, oxide ion conducting electrolyte. The operating principle of such a cell is illustrated in Figure 1. Oxygen supplied at the cathode (air electrode) reacts with incoming electrons from the external circuit to form oxide ions, which migrate to the anode (fuel electrode) through the oxide ion conducting electrolyte. At the anode, oxide ions combine with hydrogen (and/or carbon monoxide) in the fuel to form water (and/or carbon dioxide), liberating electrons. Electrons (electricity) flow from the anode through the external circuit to the cathode.

The materials for the cell components are selected based on suitable electrical conducting properties required of these components to perform their intended cell functions; adequate chemical and structural stability at high temperatures encountered during cell operation as well as during cell fabrication; minimal reactivity and interdiffusion among different components; and matching thermal expansion among different components.


Materials and cell designs

  Electrolyte

Yttrium-doped zirconium oxide (YSZ) remains the most widely used material for the electrolyte in SOFCs because of its sufficient ionic conductivity, chemical stability, and mechanical strength. The only drawback of stabilized YSZ is the low ionic conductivity in the lower cell operation temperature regime, below about 750oC (1382oF). Two solutions that have been tried to resolve this problem are to decrease the thickness of the YSZ electrolyte and to find other materials to replace the yttrium. Scandium-doped zirconium oxide has higher conductivity than YSZ but high cost of scandium and detrimental ageing effects in scandium doped zirconium oxide make it less attractive in commercializing SOFCs. Gadolinium- or samarium-doped cerium oxide materials possess higher oxide ion conductivity compared to zirconium based materials. However, cerium oxide based materials, under reducing conditions at high temperatures, exhibit significant electronic conductivity and dimensional change. Operation at temperatures below about 600oC (1112oF) overcomes this problem, and cerium oxide based materials are successfully being used as electrolyte in SOFCs by Ceres Power Limited (UK). In addition to the traditionally used oxides of zirconium and cerium, other mixed oxides also provide an opportunity to develop oxide ion conducting electrolytes. One mixture, containing among others gallium oxide, has attracted attention as an electrolyte. However, it has two drawbacks: uncertain cost of gallium, and uncertain chemical and mechanical stability of the oxide. In spite of these drawbacks, Mitsubishi Materials Corporation (Japan) is using this as the electrolyte in its SOFCs and has successfully built and tested up to 10-kW size SOFC power systems.

 Cathode

The oxidant gas is air or oxygen at the SOFC cathode, and the electrochemical reduction of oxygen requires a series of elementary reactions and involves the transfer of multiple electrons. The SOFC cathode must meet the requirements of high catalytic activity for oxygen molecule dissociation and oxygen reduction, high electronic conductivity, chemical and dimensional stability in environments encountered during cell fabrication and cell operation, thermal expansion match with other cell components, and compatibility and minimum reactivity with the electrolyte and the interconnection. Finally, the cathode must have a stable, porous microstructure so that gaseous oxygen can readily diffuse through the cathode to the cathode/electrolyte interface. These stringent electrochemical and mechanical requirements greatly restrict the number of suitable candidate materials. Lanthanum manganite, which, when substituted with low valence elements such as calcium or strontium, has good electronic conduction. Moreover, it possesses adequate electrocatalytic activity, a reasonable thermal expansion match to YSZ, and stability in the SOFC cathode operating environment.
For SOFCs operating at substantially lower temperatures, such as 650 to 800oC (1202 to 1472oF), alternative cathode materials, typically containing transition metals such as cobalt, iron, and/or nickel, have been developed and optimized for better performance. In general, these materials offer higher oxide ion diffusion rates and exhibit faster oxygen reduction kinetics at the cathode/electrolyte interface compared with lanthanum manganite. However, the thermal expansion coefficient of cobaltites is much higher than that of the YSZ electrolyte, and the electrical conductivities of ferrites and nickelites are low. Nevertheless, promising results have been reported using these materials, though in many cases the improved cathodic performance is found to decrease during the cell lifetime as a result of chemical or microstructural instability. Minimization of cathodic polarization losses is one of the biggest challenges to be overcome in obtaining high, stable power densities from lower temperature SOFCs. However, these materials are very reactive toward YSZ. Therefore, a thin layer, generally of a cerium oxide based material, is used to reduce the chemical reaction between the cathode and YSZ. Microstructure also plays a major role in the cathode polarization; this is particularly true when a composite cathode, which shows a better performance compared to a single composition cathode, is used. It has been shown that polarization resistance depends upon the grain size of the ionic conductor in the composite electrode and the volume fraction of porosity.

  Anode

The anode must be an excellent catalyst for the oxidation of fuel (hydrogen, carbon dioxide), stable in the reducing environment of the fuel, electronically conducting, and must have sufficient porosity to allow the transport of the fuel to and the transport of the products of fuel oxidation away from the electrolyte/anode interface where the fuel oxidation reaction takes place. The other requirements include matching of its thermal expansion coefficient with that of the electrolyte and interconnect; integrity of porosity for gas permeation; chemical stability with the electrolyte and interconnect; and applicability to use with versatile fuels and impurities. In addition, cost effectiveness is always a factor for commercialization.
Nickel-YSZ composites are the most commonly used anode materials for SOFCs. Nickel is an excellent catalyst for fuel oxidation; however, it possesses a high thermal expansion coefficient, and exhibits coarsening of microstructure due to metal aggregation through grain growth at cell operation temperatures. YSZ in the anode constrains nickel aggregation and prevents sintering of the nickel particles, decreases the effective thermal expansion coefficient bringing it closer to that of the electrolyte, and provides better adhesion of the anode with the electrolyte. In these anodes, nickel has dual roles of the catalyst for hydrogen oxidation and the electrical current conductor. In addition, it is also highly active for the steam reforming of methane. This catalytic property is exploited in the so-called internal reforming SOFCs that can operate on fuels composed of mixtures of methane and water. Although nickel is an excellent hydrogen oxidation and methane-steam reforming catalyst, it also catalyzes the formation of carbon from hydrocarbons under reducing conditions. Unless sufficient amounts of steam are present along with the hydrocarbon to remove carbon from the nickel surface, the anode may be destroyed. As a result, even when using methane as the fuel, relatively high steam-to-carbon ratios are needed to suppress this deleterious reaction. Unfortunately, due to the high catalytic activity of nickel for hydrocarbon cracking, this approach does not work for higher hydrocarbons, and it is generally not possible to operate nickel-based anodes on higher hydrocarbon-containing fuels without pre-reforming with steam or oxygen. In spite of this drawback, nickel-YSZ composite remains the most commonly utilized anode material for SOFCs and is satisfactory for cells operating on clean, reformed fuel.

However, advanced SOFC designs place additional constraints on the anode, such as tolerance of oxidizing environments and/or the ability to tolerate significant quantities of sulphur and/or hydrocarbon species in the fuel stream. Alternative materials, such as cerium oxide or strontium titanate/cerium oxide mixtures, have yielded some promising results in these designs, but the benefits obtained in terms of sulphur, hydrocarbon and/or redox tolerance are counterbalanced by other limitations (such as the difficulty of integrating such materials with existing cell and stack fabrication processes and materials). Copper based anodes have also been proposed for intermediate temperature (<800oC; <1472oF) SOFCs intended to operate directly on hydrocarbon fuels without prior reformation, but the lack of catalytic activity for oxidation of fuel in copper and sintering of copper at the cell operating temperatures have limited their use in practical SOFCs.

  Interconnect

Since a single cell only produces voltage less than 1 V and power around 1 W/cm2, many cells are electrically connected together in a cell stack to obtain higher voltage and power. To connect multiple cells together, an interconnection is used in SOFC stacks. The requirements of the interconnection are the most severe of all cell components and include: nearly 100 percent electronic conductivity; stability in both oxidizing and reducing atmospheres at the cell operating temperature since it is exposed to air (or oxygen) on the cathode side and fuel on the anode side; low permeability for oxygen and hydrogen to minimize direct combination of oxidant and fuel during cell operation; a thermal expansion coefficient close to that of the cathode and the electrolyte; and non-reactivity with other cell materials. To satisfy these requirements, doped lanthanum chromite is used as the interconnection for cells intended for operation at about 1000oC (1832oF).
In cells intended for operation at lower temperatures (<800oC; <1412oF), it is possible to use oxidation-resistant metallic materials for the interconnection. Compared to lanthanum chromite ceramic interconnects, metallic alloys offer advantages such as improved manufacturability, significantly lower raw material and fabrication costs, and higher electrical and thermal conductivity. But to be useful for the interconnect application, the metallic alloys must satisfy additional requirements, including resistance to surface oxidation and corrosion in a dual atmosphere (simultaneous exposure to oxidizing and reducing atmospheres), thermal expansion matching to other stack components (particularly for stacks using a rigid seal design), chemical compatibility with other materials in contact with the interconnect, such as seals and cell materials, high electrical conductivity not only through the bulk material but also in in-situ-formed oxide scales, mechanical reliability and durability at the cell operating temperature, and strong adhesion between the as-formed oxide scale and the underlying alloy substrate. Ferritic stainless steels are the most promising candidates, owing to the fact that some alloys in this family offer a protective and conductive chromium-based oxide scale, appropriate thermal expansion behavior, ease of manufacturing and low cost. Several new ferritic stainless steels have been developed specifically for SOFC interconnects. Although these alloys demonstrate improved performance over traditional compositions, several critical issues remain; among these are chromium oxide scale evaporation and subsequent poisoning of cathodes; scale electrical resistivity in the long term; corrosion and spalling under interconnect exposure conditions; and compatibility with the adjacent components such as seals and electrical contact layers. To overcome some of these problems, some surface coatings can be applied onto metallic interconnects to minimize scale growth, electrical resistance and chromium volatility.

  Stack design

Fig. 2. Illustration of planar cell stacks (top) and a tubular cell bundle (bottom).

In terms of stack design, most development has focused on planar and tubular design cells, each of these designs having a number of interesting variants; for example, the planar SOFC may be in the form of a circular disk fed with fuel from the central axis, or it may be in the form of a square plate fed from the edges. The tubular SOFC may be of a large diameter (>15 mm), or of much smaller diameter (<5 mm), the so-called microtubular cells. Also, the tubes may be flat and joined together to give higher power density and easily printable surfaces for depositing the electrode layers. Figure 2 illustrates typical planar cell stacks and a tubular cell bundle. One of the inherent advantages of tubular cell bundles is that the air and the fuel are naturally isolated because the tubes are closed at one end.
However, in the case of planar cell stacks, an effective seal must be provided to isolate air from the fuel. The seal must have a thermal expansion match to the fuel cell components, must be electrically insulating and must be chemically stable under the operational conditions of the stack. Also, the seal should exhibit no deleterious interfacial reactions with other cell components, should be stable under both the high temperature oxidizing and reducing operational conditions, should be created at a low enough temperature to avoid damaging cell components (under 850oC or 1562oF for some materials), and should not migrate or flow from the designated sealing region during sealing or cell operation. In addition, the sealing system should be able to withstand thermal cycling between the cell operation temperature and room temperature. A number of different sealing approaches are under development, including rigid, bonded seals (for example, glass-ceramics and brazes), compliant seals (for example viscous glasses) and compressive seals (for example, mica-based composites); multiple sealants may also be used in any given stack design between different cell components. Successful development of sealing materials and concepts for planar SOFCs is probably the most important issue for the long-term performance stability and lifetime of planar SOFC stacks and hence for their eventual commercialization at competitive costs.

 Alternate tubular cells

 

Fig. 3. Alternate tubular geometry cells being developed by Siemens.

The single biggest advantage of tubular cells over planar cells is that they do not require any high temperature seals to isolate oxidant from the fuel, and this makes performance of tubular cell stacks very stable over long periods of times (several years). However, their areal power density is much lower (about 0.2 W/cm2) compared to planar cells (up to 2 W/cm2 for single cells and at least 0.5 W/cm2 for stacks) and manufacturing costs higher. The volumetric power density is also lower for tubular cells than for planar cells. To increase the power density and reduce the physical size and cost of tubular SOFC stacks, alternate tubular geometry cells, as illustrated in Figure 3, are under development by Siemens. Such alternate geometry cells combine all the advantages of the tubular SOFCs, such as not requiring high temperature seals, while providing higher areal and volumetric power densities. The performance of these new design cells is higher than that of cylindrical tubular cells, but still lower than that of anode-supported planar cells.


Applications

SOFC power system

Fig. 4. A Siemens Westinghouse 100 kilowatt SOFC power system.

Using planar SOFCs, stationary power generation systems of from 1-kW to 25-kW size have been fabricated and tested by several organizations. Several hundred 1-kW size combined heat and power units for residential application were field tested by Sulzer Hexis; however, their cost and performance degradation was high and stack lifetime too short. With improved sealing materials and sealing concepts, planar SOFC prototype systems in the 1- to 5-kW sizes have recently been developed and are being tested by various organizations with greater success. Using tubular (cylindrical) SOFCs, Siemens/ Westinghouse fabricated a 100-kW atmospheric power generation system (Figure 4). The system was successfully operated for two years in the Netherlands on desulfurized natural gas without any detectable performance degradation. It provided up to 108 kW of ac electricity at an efficiency of 46% to the Dutch grid and approximately 85 kW of hot water for the local district heating system. At the conclusion of the operation in The Netherlands, the system was moved to a German utility site in Essen, Germany, where it operated successfully for another 4,000 hours. After replacing some cells, the system was then installed and operated in Italy, for over two years, again with very stable performance. Siemens/Westinghouse tubular cells have also been used to fabricate and field test over a dozen 5-kW size combined heat and power units, each about the size of a refrigerator. These units gave excellent performance and performance stability on a variety of hydrocarbon fuels. However, at present, their cost is high; future such units are expected to use higher power density alternate tubular geometry cells to drive down the cost.
Another application of SOFC systems is in the transportation sector. The polymer electrolyte membrane fuel cell is generally regarded as the fuel cell of choice for transportation applications. These fuel cells require pure hydrogen, with no carbon monoxide, as the fuel to operate successfully. However, presently no hydrogen infrastructure exists, and on-board reformer systems to produce hydrogen from existing fuel base (gasoline, diesel) are technically challenging, complex, and expensive. Furthermore, it is difficult to eliminate the carbon monoxide entirely from the reformate stream. In contrast, SOFCs can use carbon monoxide along with hydrogen as fuel, and their higher operating temperature and availability of water on the anode side makes on-cell or in-stack reformation of hydrocarbon fuels feasible. Also, no noble metal catalysts are used in SOFCs reducing cost of the cells. The initial application of SOFCs in the transportation sector will be for on-board auxiliary power units. Such auxiliary power units, operating on existing fuel base, will supply the ever increasing electrical power demands of luxury automobiles, recreational vehicles, and heavy-duty trucks. Delphi Corporation has developed a 5-kW auxiliary power unit using anode-supported planar SOFCs. This unit is intended to operate on gasoline or diesel, which is reformed through catalytic partial oxidation. The building blocks of such an auxiliary power unit consist of an SOFC stack, fuel reformation system, waste energy recovery system, thermal management system, process air supply system, control system, and power electronics and energy storage (battery) system. Delphi has reduced the mass and volume in successive generation auxiliary power units to meet the stringent automotive requirements; the remaining issues of start up time and tolerance to thermal cycling are presently being worked on.

Clays Minerals

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

Clay minerals form in the presence of water and have been important to life, and many theories of abiogensis involve them. They have been useful to humans since ancient times in agriculture and manufacturing.

Properties

Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone, and siltstone and in fine grained metamorphic slate and phyllite.

Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical techniques for their identification/study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, and SEM-EDS or automated mineralogy solutions. These methods can be augmented by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.

Occurance

Given the requirement of water, clay minerals are relatively rare in the Solar System, though occur extensively on Earth where water has interacted with other minerals and organic matter. Clay minerals have been detected at several locations on Mars including Echus Chasma and Mawrth Vallis and the Memnonia quadrangle and the Elysium quadrangle. Spectography has confirmed their presence on Ceres and Europa.

Classifications:

Clay minerals can be classified as 1:1 or 2:1, this originates from the fact that they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in the structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite and montmorillonite.
Clay minerals include the following groups:
Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al2Si2O5(OH)4).
Some sources include the kaolinite-serpentine group due to structural similarities (Bailey 1980).
Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite. In 2013, analytical tests by the Curiosity rover found results consistent with the presence of smectite clay minerals on the planet Mars.
Illite group which includes the clay-micas. Illite is the only common mineral.
Chlorite group includes a wide variety of similar minerals with considerable chemical variation.
Other 2:1 clay types exist such as sepiolite or attapulgite, clays with long water channels internal to their structure.

Mixed layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach. Literature articles will refer to a R1 ordered illite-smectite, for example. This type would be ordered in an ISISIS fashion. R0 on the other hand describes random ordering, and other advanced ordering types are also found (R3, etc.). Mixed layer clay minerals which are perfect R1 types often get their own names. R1 ordered chlorite-smectite is known as corrensite, R1 illite-smectite is rectorite.
Structure:
Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO4 tetrahedra and/or AlO4 octahedra. The sheet units have the chemical composition (Al,Si)3O4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction; i.e. all of the unshared vertices are on the same side of the sheet.
In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedral. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.
Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite.

Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.

Why is glass fiber stronger than normal glass?

Flexibility!

Glass Fiber can dissipate energy along the fiber, normal glass does not have a structure to dissipate the energy in an even form and proves easier to crack as a result. This is also due to the structural flaws in Normal Glass.

Similarly the lifting ropes of cranes are actually coiled metal fibres and not solid chain link, the metal fibres dissipate the weight better than the chain.

What is Glass?

Glass is an amorphous (non-crystalline) solid material that exhibits a glass transition, which is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or rubber-like state. Glasses are typically brittle and can be optically transparent. The most familiar type of glass is soda-lime glass, which is composed of about 75% silicon dioxide (SiO2), sodium oxide (Na2O) from soda ash, lime (CaO), and several minor additives. The term glass is often used to refer only to this specific use.
Silicate glass generally has the property of being transparent. Because of this, it has a great many applications. One of its primary uses is as a building material, traditionally as small panes set into window openings in walls, but in the 20th-century often as the major cladding material of many large buildings. Because glass can be formed or moulded into any shape, and also because it is a sterile product, it has been traditionally used for vessels: bowls, vases, bottles, jars and glasses. In its most solid forms it has also been used for paperweights, marbles, and beads. Glass is both reflective and refractive of light, and these qualities can be enhanced by cutting and polishing in order to make optical lenses, prisms and fine glassware. Glass can be coloured by the addition of metallic salts, and can also be painted. These qualities have led to the extensive use of glass in the manufacturing of art objects and in particular, stained glass windows. Although brittle, glass is extremely durable, and many examples of glass fragments exist from early glass-making cultures.
In science, however, the term glass is defined in a broader sense, encompassing every solid that possesses a non-crystalline (i.e. amorphous) structure and exhibits a glass transition when heated towards the liquid state. These sorts of glasses can be made of quite different kinds of materials: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications (bottles, eyewear) polymer glasses (acrylic glass, polycarbonate, polyethylene terephthalate) are a lighter alternative to traditional silica glasses.
What is Safety Glass?
Safety glass is glass with additional safety features that make it less likely to break, or less likely to pose a threat when broken. Common designs include toughened glass (also known as tempered glass), laminated glass, and wire mesh glass (also known as wired glass).

What is Reinforced Concrete?

Reinforced concrete is a composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and/or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets. Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/or structural failure. Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite material in conjunction with rebar or not. Reinforced concrete may also be permanently stressed (in compression), so as to improve the behaviour of the final structure under working loads. In the United States, the most common methods of doing this are known as pre-tensioning and post-tensioning.
For a strong, ductile and durable construction the reinforcement needs to have the following properties at least:
High relative strength
High toleration of tensile strain
Good bond to the concrete, irrespective of pH, moisture, and similar factors
Thermal compatibility, not causing unacceptable stresses in response to changing temperatures.
Durability in the concrete environment, irrespective of corrosion or sustained stress for example.
Behavior Of Concrete:

Materials:

Concrete is a mixture of coarse (stone or brick chips) and fine (generally sand or crushed stone) aggregates with a paste of binder material (usually Portland cement) and water. When cement is mixed with a small amount of water, it hydrates to form microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid structure. The aggregates used for making concrete should be free from harmful substances like organic impurities, silt, clay, lignite etc. Typical concrete mixes have high resistance to compressive stresses (about 4,000 psi (28 MPa)); however, any appreciable tension (e.g., due to bending) will break the microscopic rigid lattice, resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.

If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists not only compression but also bending and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel resists the tension can be made into almost any shape and size for the construction industry.

Characteristics of Concrete:

Three physical characteristics give reinforced concrete its special properties:

1) The coefficient of thermal expansion of concrete is similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.

2) When the cement paste within the concrete hardens, this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.

3) The alkaline chemical environment provided by the alkali reserve (KOH, NaOH) and the portlandite (calcium hydroxide) contained in the hardened cement paste causes a passivating film to form on the surface of the steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions. When the cement paste exposed to the air and meteoric water reacts with the atmospheric CO2, portlandite and the Calcium Silicate Hydrate (CSH) of the hardened cement paste become progressively carbonated and the high pH gradually decreases from 13.5 – 12.5 to 8.5, the pH of water in equilibrium with calcite (calcium carbonate) and the steel is no longer passivated.

As a rule of thumb, only to give an idea on orders of magnitude, steel is protected at pH above ~11 but starts to corrode below ~10 depending on steel characteristics and local physico-chemical conditions when concrete becomes carbonated. Carbonatation of concrete along with chloride ingress are amongst the chief reasons for the failure of reinforcement bars in concrete.

The relative cross-sectional area of steel required for typical reinforced concrete is usually quite small and varies from 1% for most beams and slabs to 6% for some columns. Reinforcing bars are normally round in cross-section and vary in diameter. Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture & humidity.

Distribution of concrete (in spite of reinforcement) strength characteristics along the cross-section of vertical reinforced concrete elements is inhomogeneous.