lecture 8 - non-metals pt1

MATERIALS SCIENCEMASC-210

BEng (Hons) Metallurgy & Materials Engineering Year 2

M.K. Line 2015 MASC-2101

PREPARED BYMOSES KANSIYA LINE

NON-METALS

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SUB TOPICS1. Polymers2. Ceramics


POLYMERS

• Non-metallic materials are broadly of two kinds – Polymers and Ceramics.

• The term "polymer" comes from the Greek words; poly (meaning "many") and mers (meaning "units").

• At the molecular level polymers consist of extremely long, chain-like molecules.

• Polymer molecules are typically made up of thousands of repeating chemical units

• A single mer is called a monomer• Polymers may be natural, such as cellulose or DNA, or synthetic,

such as nylon or polyethylene.


HYDROCARBON MOLECULES• Most polymers are organic, and formed from hydrocarbon

molecules• Each C atom has four e- that participate in bonds, each H atom

has one bonding e-• Examples of saturated (all bonds are single ones) hydrocarbon

molecules include Methane, Ethane, Propane etc• Double and triple bonds can exist between C atoms (sharing of

two or three electron pairs). These bonds are called unsaturated bonds. Unsaturated molecules are more reactive


Saturated hydrocarbons

Unsaturated hydrocarbons

POLYMERISATION• Ethylene (C2H4) is a gas at room temp and pressure. It

transforms to polyethylene (solid) by forming active mers through reactions with an initiator or catalytic radical (R.)

• (.) denotes unpaired electron (active site)


HOMOPOLYMER AND COPOLYMERS• Homopolymers: Polymer chain is made up of single repeating

units. Example: AAAAAAAA• Copolymers: Polymer chains made up of two or more repeating

units.i. Random copolymers: Different monomers randomly

arranged in chains. Eg:- ABBABABBAAAAABAii. Alternating copolymers: Definite ordered alterations of

monomers. Eg:- ABABABABABABiii. Block copolymers: Different monomers arranged in long

blocks. Eg:- AAAAA…….BBBBBBBB……iv. Graft copolymers: One type of monomer grafted to long

chain of another. Eg: AAAAAAAAAAAAAAAAAAA

• Mer units that have 2 active bonds to connect with other mers are called bifunctional.

• Mer units that have 3 active bonds to connect with other mers are called trifunctional. They form 3-d molecular network structures.

BB

BB


MECULAR WEIGHT• Final molecular weight (chain length) is controlled by

relative rates of initiation, propagation, termination steps of polymerization

• Formation of macromolecules during polymerization results in distribution of chain lengths and molecular weights

• The number-average molecular weight, Mn is obtained by dividing the chains into a series of size ranges and then determining the number fraction of chains within each size:


M X Mn i i

wM Mw ii

Where Mi is mean molecular weight of range i, wi is the weight fraction of chains of length i & Xi is the number of fractions of chains of length iAlternative way to express average polymer chain size is degree of

polymerization - the average number of mer units in a chain

MECULAR SHAPE• The angle between the singly bonded carbon atoms is 109o

- carbon atoms form a zigzag pattern in a polymer molecule.

• Random kinks and coils lead to entanglement, like in the spaghetti structure: Moreover, while maintaining the 109o angle between bonds polymer chains can rotate around single C-C bonds (double and triple bonds are very rigid).


For (a), the rightmost atom may lie anywhere on the dashed circle and still subtend a 109 degree angle with the bond between the other two atoms. Straight and twisted chain segments are generated when the backbone atoms are situated as in (b) and (c), respectively

single polymer chain molecule that has numerous random kinks & coils produced by chain bond rotations.

POLYMER MOLECULAR STRUCTURE

• The molecular structure of a fully polymerized polymer can be classified according to one of three major types:-

I. linear PolymersII. branched, or III.cross-linked

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LINEAR POLYMER• are those in which the repeat

units are joined together end to end in single chains. These long chains are flexible and may be thought of as a mass of “spaghetti”.

• there may be extensive van der Waals and hydrogen bonding between thechains.

• Some of the common polymers that form with linear structures are polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate, nylon, & the fluorocarbons

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• in which side-branch chains are connected to the main ones

• The branches, considered to be part of the main-chain molecule, may result from side reactions that occur during the synthesis of the polymer.

• The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density.

• Polymers that form linear structures may also be branched. Forexample, high-density polyethylene (HDPE) is primarily a linear polymer, where as low density polyethylene (LDPE) contains short-chain branches

BRANCHED POLYMER

CROSSED-LINKED POLYMER

• In cross-linked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds.

• The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction.

• Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains.

• Many of the rubber elastic materials are cross-linked; in rubbers, this is called vulcanization

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• Multifunctional monomers forming three or more active covalent bonds make 3-d networks and are termed network polymers.

• Actually, a polymer that is highly cross-linked may also be classified as a network polymer.

• These materials have distinctive mechanical and thermal properties; the epoxies, polyurethanes, and phenol-formaldehyde belong to this group

NETWOK POLYMER

HYDROCARBON MOLECULES• Isomers are molecules that contain the same atoms but in

a different arrangement. An example is butane and isobutane:

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butaneIsobutane

• Two types of isomerism are possible:-Stereoisomerism & geometrical isomerism

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ISOMERISATION

• atoms are linked together in the same order, but can have different spatial arrangement

i. Isotactic configuration: all side groups R are on the same side of the chain.

ii. Syndiotactic configuration: side groups R alternate sides of the chain.

iii. Atactic configuration: random orientations of groups R along the chain.

STEREOISOMERISMGEOMETRICAL ISOMERISM

• are possible within repeat units having a double bond between chain carbon atoms.

• Can take two forms:-i. CIS Structure: R (CH3)& H

atoms are positioned on the same side of the double bond

ii. TRANS Structure:- R (CH3)& H atoms reside on opposite sides of the double bond

POLYMER CRYSTALLINITY• Atomic arrangement in polymer crystals is more complex than

in metals or ceramics (unit cells are typically large & complex).• Polymer molecules are often partially crystalline

(semicrystalline), with crystalline regions dispersed within amorphous material.

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POLYMER CRYSTALLINITY• Degree of crystallinity is

determined by:-i. Rate of cooling during

solidification: time is necessary for chains to move and align into a crystal structure

ii. Mer complexity: crystallization less likely in complex structures, simple polymers, such as polyethylene, crystallize relatively easily

iii. Chain configuration: linear polymers crystallize relatively easily, branches inhibit crystallization, network polymers almost completely amorphous, cross-linked polymers can be both crystalline and amorphousM.K. Line 2015 MASC-210 16

iv. Isomerism: isotactic, syndiotactic polymers crystallize relatively easily - geometrical regularity allows chains to fit together, atactic difficult to crystallize

v. Copolymerism: easier to crystallize if mer arrangements are more regular - alternating, block cancrystallize more easily as compared to random and graft

More crystallinity: higher density, more strength, higher resistance to dissolution and softening by heating

POLYMER CRYSTALLINITY• Crystalline polymers are denser

than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density

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a) Thin crystalline platelets grown from solution - chains fold back and forth: chain-folded model

a)

b)b) Spherulites: Aggregates of

lamellar crystallites ~ 10 nm thick, separated by amorphous material. Aggregates approximately spherical in shape.

100% c c a

s c a

xcrystallinity

whereρc: Density of perfect crystalline polymerρa: Density of completely amorphous polymerρs: Density of partially crystalline polymer that we are analyzing

CLASSIFICATION OF POLYMERS

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Polymeric materials

Plastics Elastomers

Adhesives Coatings Fibres forms films

Thermoplastics

Thermosetting

Commodity Plastics

Engineering Plastics

Commodity Plastics

Engineering Plastics

PolyethylenePolypropylenePolystyrenePolyvinylchloride

EthenicPolyamidesCellulosicsAcetalsPolycarbonatesPolyimidesPolyesthersetc

PhenolicsUnsaturated PolyestersUreas

SiliconesPolyimidesUrethanesMelaminesEpoxidesetc

CLASSIFICATION OF POLYMERS• There are many different polymeric materials • one way of classifying them is according to their end use.

Within this scheme the various polymer types include plastics, elastomers (or rubbers), fibers, coatings, adhesives, foams, and films.

• Depending on its properties, a particular polymer may be used in two or more of these application categories. For example, a plastic, if cross-linked and used above its glass transition temperature, may make a satisfactory elastomer, or a fiber material may be used as a plastic if it is not drawn into filaments.

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CLASSIFICATION OF POLYMERS• Depending on the response to temperature increase, two types

of polymers can be distinguished:i. Thermoplastic polymers: soften and liquefy when heated,

harden when cooled (reversible). – Molecular structure: linear or branched polymers, with secondary bonding

holding the molecules together. – Easy to fabricate/reshape by application of heat and pressure – Examples: polyethylene, polystyrene, poly(vinylchlodide).

ii. Thermosetting polymers: become permanently hard during their formation, do not soften upon heating.

– Molecular structure: network polymers with a large density of covalent crosslinks between molecular chains (typically, 10-50% of repeat units are crosslinked).

– Harder and stronger than thermoplastics, have better dimensional and thermal stability.

– Examples: vulcanized rubber, epoxies, phenolics, polyester resins.

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CLASSIFICATION OF POLYMERS• Polymers can also be categorised as either Commodity Polymers or

Engineering Polymersi. Commodity polymers:- are basically those polymers which are found

in our daily life usage from low value items such as plastic bags to high value items which doesn’t require precise and high mechanical properties.

– Commodity polymers are utilized for bulk & high-volume ends (like containers and packaging).

– Such polymers exhibit relatively low mechanical properties & are of low cost. – Most of the commodity polymers are made by addition polymerization &

those commodity polymers are thermoplastic polymers. – Examples of commodity polymers are Polyethylene (PE), Polypropylene (PP),

Polystyrene (PS), Poly(vinyl chloride) (PVC), Polytetrafluoroethylene (PTFE), Poly(methyl methacrylate) (PMMA), Poly(ethylene terephthalate) (PET), and more

– The range of products includes Plates, Cups, Carrying Trays, Medical Trays, Containers, Seeding Trays, Printed Material and other disposable items.

– applications such as photographic and magnetic tape, clothing, beverage, trash containers, film for packaging, and a variety of household products where mechanical properties & service environments are not critical

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CLASSIFICATION OF POLYMERSii. Engineering polymers:- are a group of polymers that have better

mechanical and/or thermal properties than the more widely used commodity polymers. These materials have exceptional mechanical properties such as stiffness, toughness, and low creep that make them valuable in the manufacture of structural products like gears, bearings, electronic devices, and auto parts.

– Being more expensive, engineering polymers are produced in lower quantities and tend to be used for smaller objects or low-volume applications (such as mechanical parts), rather than for bulk and high-volume ends (like containers and packaging).

– These plastics normally are not available to the public and frequently are available only to manufacturers in raw material form in order to be melted and molded into end products.

– The term usually refers to thermoplastic materials rather than thermosetting ones.

– Examples of engineering plastics include acrylonitrile butadiene styrene (ABS), used for car bumpers, dashboard trim and Lego bricks; polycarbonates, used in motorcycle helmets; and polyamides (nylons), used for skis and ski boots.

– Engineering polymers have gradually replaced traditional engineering materials such as wood or metal in many applications. Besides equaling or surpassing them in weight/strength and other properties, engineering polymers are much easier to manufacture, especially in complicated shapes.

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PLASTICS• Plastics are materials that have some structural rigidity under

load & are used in general-purpose applications. • Polyethylene, polypropylene, poly(vinyl chloride), polystyrene,

and the fluorocarbons, epoxies, phenolics, and polyesters may all be classified as plastics. They have a wide variety of combinations of properties.

• Some plastics are very rigid and brittle. Others are flexible, exhibiting both elastic and plastic deformations when stressed and sometimes experiencing considerable deformation before fracture

• Polymers falling within this classification may have any degree of crystallinity, and all molecular structures and configurations (linear, branched, isotactic, etc.) are possible.

• Plastic materials may be either thermoplastic or thermosetting• linear or branched plastic polymers must be used below their

glass transition temperatures (if amorphous) or below their melting temperatures (if semi-crystalline), or they must be cross-linked enough to maintain their shape

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PLASTICS

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PLASTICS

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ELASTOMERS• The typical properties of elastomers depend on the degree of

vulcanization & on whether any reinforcement is used. • Natural rubber is still used to a large degree because it has an

outstanding combination of desirable properties. However, the most important synthetic elastomer is SBR, which is used predominantly in automobile tires, reinforced with carbon black.

• NBR, which is highly resistant to degradation and swelling, is another common synthetic elastomer.

• For many applications (e.g., automobile tires), the mechanical properties of even vulcanized rubbers are not satisfactory in terms of tensile strength, abrasion and tear resistance, and stiffness. These characteristics may be further improved by additives such as carbon black

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ELASTOMERS

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FIBERS• Fiber polymers are capable of being drawn into long filaments having

at least a 100:1length-to-diameter ratio. • Most commercial fiber polymers are used in the textile industry, being

woven or knit into cloth or fabric. • In addition, the aramid fibers are employed in composite materials • To be useful as a textile material, a fiber polymer must have a host of

rather restrictive physical and chemical properties. • While in use, fibers may be subjected to a variety of mechanical

deformations—stretching, twisting, shearing, and abrasion. Consequently, they must have a high tensile strength (over a relatively wide temperature range) and a high modulus of elasticity, as well as abrasion resistance.

• Convenience in washing and maintaining clothing depends primarily on the thermal properties of the fiber polymer, that is, its melting and glass transition temperatures.

• Furthermore, fiber polymers must exhibit chemical stability to a rather extensive variety of environments, including acids, bases, bleaches, dry-cleaning solvents, and sunlight.

• In addition, they must be relatively nonflammable and amenable to drying

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COATINGS• Coatings are frequently applied to the surface of materials to

serve one or more of the following functions: (1) to protect the item from the environment, which may produce corrosive or deteriorative reactions; (2) to improve the item’s appearance; and (3) to provide electrical insulation.

• Many of the ingredients in coating materials are polymers, most of which are organic in origin. These organic coatings fall into several different classifications: paint, varnish, enamel, lacquer, & shellac.

• Many common coatings are latexes. A latex is a stable suspension of small, insoluble polymer particles dispersed in water.

• they have low volatile organic compound (VOC) emissions

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ADHESIVES• An adhesive is a substance used to bond together the surfaces of two

solid materials (termed adherends). • There are two types of bonding mechanisms: mechanical & chemical. • In mechanical bonding there is actual penetration of the adhesive into

surface pores and crevices. • Chemical bonding involves intermolecular forces between the adhesive

and adherend, which forces may be covalent and/or van der Waals; the degree of van der Waals bonding is enhanced when the adhesive material contains polar groups

• Can be natural (animal glue, casein, starch, and rosin) or synthetic (polyurethanes, polysiloxanes (silicones), epoxies, polyimides, acrylics, and rubber materials.)

• the choice of which adhesive to use will depend on such factors asi. the materials to be bonded & their porosities;ii. the required adhesive properties (i.e., whether the bond is to be

temporary or permanent);iii. maximum/minimum exposure temperatures;iv. processing conditions.

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FILMS• Polymers used in the form of thin films. • Films having thicknesses between 0.025 and 0.125 mm are fabricated

and used extensively as bags for packaging food products and other merchandise, as textile products

• Important characteristics of the materials produced & used as films include low density, a high degree of flexibility, high tensile & tear strengths, resistance to attack by moisture & other chemicals, & low permeability to some gases, especially water vapor

• Some of the polymers that meet these criteria & are manufactured in film form are polyethylene, polypropylene, cellophane, & cellulose acetate.

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FOAMS• Foams are plastic materials that contain a relatively high volume

percentage of small pores and trapped gas bubbles. • Both thermoplastic & thermosetting materials are used as foams; these

include polyurethane, rubber, polystyrene, and poly(vinyl chloride). • Foams are commonly used as cushions in automobiles and furniture, as

well as in packaging and thermal insulation. • The foaming process is often carried out by incorporating into the

batch of material a blowing agent that, upon heating, decomposes with the liberation of a gas. Gas bubbles are generated throughout the now-fluid mass, which remain in the solid upon cooling and give rise to a sponge-like structure. The same effect is produced by dissolving an inert gas into a molten polymer under high pressure. When the pressure is rapidly reduced, the gas comes out of solution and forms bubbles & pores that remain in the solid as it cools.

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ADVANCED POLYMERIC MATERIALS

• new polymers having unique and desirable combinations of properties have been developed over the past several years

• Some of these include:-i. ultra-high-molecular-weight polyethylene, ii. liquid crystal polymers, &iii. thermoplastic elastomers

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ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE (UHMWPE)

• is a linear polyethylene that has an extremely high molecular weight of approximately 4x106 g/mol,

• In fiber form, UHMWPE is highly aligned and has the trade name Spectra. Some of the extraordinary characteristics of this material are as follows:i. An extremely high impact resistanceii. Outstanding resistance to wear and abrasioniii. A very low coefficient of frictioniv. A self-lubricating and nonstick surfacev. Very good chemical resistance to normally encountered solventsvi. Excellent low-temperature propertiesvii. Outstanding sound damping and energy absorption characteristicsviii. Electrically insulating and excellent dielectric properties

• However, because this material has a relatively low melting temperature, its mechanical properties deteriorate rapidly with increasing temperature.

• This unusual combination of properties leads to numerous and diverse applications including bulletproof vests, composite military helmets, fishing line, ski-bottom surfaces, golf-ball cores, bowling alley and ice-skating rink surfaces, biomedical prostheses, blood filters, marking-pen nibs, bulk material handling equipment (for coal, grain, cement, gravel, etc.), bushings, pump impellers, and valve gaskets.M.K. Line 2015 MASC-210 34

LIQUID CRYSTAL POLYMERS (LCPs)

• LCPs are a group of chemically complex and structurally distinct materials that have unique properties and are used in diverse applications.

• Are composed of extended, rod-shaped, and rigid molecules. • In terms of molecular arrangement, these materials do not fall within any of

conventional liquid, amorphous, crystalline, or semicrystalline classifications but may be considered a new state of matter—the liquid crystalline state, being neither crystalline nor liquid.

• In the melt (or liquid) condition, whereas other polymer molecules are randomly oriented, LCP molecules can become aligned in highly ordered configurations. As solids, this molecular alignment remains, &, in addition, the molecules form in domain structures having characteristic intermolecular spacings.

• Three types of liquid crystals, based on orientation and positional ordering are smectic, nematic, & cholesteric;

• The principal use of liquid crystal polymers is in liquid crystal displays (LCDs) on digital watches, flat-panel computer monitors and televisions, and other digital displays.

• Also used extensively by the electronics industry (in interconnect devices, relay and capacitor housings, brackets, etc.), by the medical equipment industry (in components that are sterilized repeatedly), and in photocopiers and fiber-optic components.

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LIQUID CRYSTAL POLYMERS (LCPs)

• these materials exhibit the following behaviors:i. Excellent thermal stability; they may be used to temperatures

as high as 2300C ii. Stiffness and strength; their tensile moduli range between 10 and 24

GPa & their tensile strengths are from 125 to 255 MPaiii. High impact strengths, which are retained upon cooling to relatively

low temperatures.iv. Chemical inertness to a wide variety of acids, solvents, bleaches, and

so on.v. Inherent flame resistance and combustion products that are

relatively nontoxic.• The following may are their processing and fabrication characteristics:i. All conventional processing techniques available for thermoplastic materials may be

used.ii. Extremely low shrinkage and warpage take place during molding.iii. There is exceptional dimensional repeatability from part to part.iv. Melt viscosity is low, which permits molding of thin sections and/or complex shapes.v. Heats of fusion are low; this results in rapid melting and subsequent cooling, which

shortens molding cycle times.vi.They have anisotropic finished-part properties; molecular orientation effects are

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POLYMER ADDITIVES• Foreign substances called additives are intentionally introduced to

enhance or modify many of these properties and thus render a polymer more serviceable.

• Typical additives include filler materials, plasticizers, stabilizers, colorants, and flame retardants.

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• are most often added to polymers to improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties.

• Materials used as particulate fillers include wood flour (finely powdered sawdust), silica flour and sand, glass, clay, talc, limestone, and even some synthetic polymers.

• Polymers that contain fillers may also be classified as composite materials, • Often the fillers are inexpensive materials that replace some volume of the more

expensive polymer, reducing the cost of the final product.

Fillers

PLASTICISERS• The flexibility, ductility, and toughness of polymers may be improved with the

aid of additives called plasticizers. Their presence also produces reductions in hardness & stiffness.

• Plasticizers are generally liquids with low vapor pressures and low molecular weights.

• The small plasticizer molecules occupy positions between the large polymer chains, effectively increasing the interchain distance with a reduction in the secondary intermolecular bonding.

• Plasticizers are commonly used in polymers that are intrinsically brittle at room temperature, such as poly(vinyl chloride) and some of the acetate copolymers.

• The plasticizer lowers the glass transition temperature, so that at ambient conditions the polymers may be used in applications requiring some degree of pliability & ductility. These applications include thin sheets or films, tubing, raincoats, and curtains

POLYMER ADDITIVES

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• Additives that counteract deteriorative processes are called stabilizers.• One common form of deterioration results from exposure to light. There are two

primary approaches to UV stabilization. The first is to add a UV absorbent material, often as a thin layer at the surface. This essentially acts as a sunscreen & blocks out the UV radiation before it can penetrate into & damage the polymer. The second approach is to add materials that react with the bonds broken by UV radiation before they can participate in other reactions that lead to additional polymer damage.

• Another important type of deterioration is oxidation It is a consequence of the chemical interaction between oxygen [as either diatomic oxygen (O2) or ozone (O3)] & the polymer molecules. Stabilizers that protect against oxidation consume oxygen before it reaches the polymer &/or prevent the occurrence of oxidation reactions that would further damage the material.

STABLISERS

FLAME RETARDANTS

• The flammability resistance of the remaining combustible polymers may be enhanced by additives called flame retardants

• Most polymers are flammable in their pure form; exceptions include those containing significant contents of chlorine and/or fluorine, such as PVC & PTFE

• The retardants may function by interfering with the combustion process through the gas phase or by initiating a different combustion reaction that generates less heat, thereby reducing the temperature; this causes a slowing or cessation of burning

POLYMER ADDITIVES

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• Colorants impart a specific color to a polymer• They may be added in the form of dyes or pigments. • The molecules in a dye actually dissolve in the

polymer. Pigments are filler materials that do not dissolve but remain as a separate phase; normally, they have a small particle size and a refractive index near that of the parent polymer.

• Others may impart opacity as well as color to the polymer

COLORANTS

CERAMICS• The term ceramic comes from the Greek word keramikos, which

means “burnt stuff,” indicating that desirable properties of these materials are normally achieved through a high-temperature heat treatment process called firing

• Usually a compound between metallic and nonmetallic elements• Bonds are partially or totally ionic, and can have combination of

ionic and covalent bonding• Always composed of more than one element (e.g.,Al2O3, NaCl,

SiC, SiO2)• Generally hard and brittle• Generally electrical and thermal insulators• Can be optically opaque, semi-transparent, or transparent

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CERAMIC BONDING• The atomic bonding in these materials ranges from purely ionic

to totally covalent; many ceramics exhibit a combination of these two bonding types.

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NaClMgOCaF2CsCl

This type of bonding gives the following properties:-BrittleHigh TmPoor conductor of heat & electricity

CERAMIC STRUCTURES• Crystal structures for ceramics are generally more complex than

those for metals. • Ceramics that are predominantly ionic in nature have crystal

structures comprised of charged ions, where positively-charged (metal) ions are called cations, and negatively-charged (non-metal) ions are called anions

• The crystal structure for a given ceramic depends upon two characteristics:-i. The magnitude of electrical charge on each component ion,

recognizing that the overall structure must be electrically neutral

ii. The relative size of the cation(s) and anion(s),which determines the type of interstitial site(s) for the cation(s) in an anion lattice

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CaF2Ca2+

cationF-

F-anions+

• Charge Neutrality: --Net charge in the structure should be zero.

SiO2, MgO, SiC, Al2O3

(i)(ii)

CN IN CERAMICS• Coordination Number is the number of adjacent atoms (ions)

surrounding a reference atom (ion) without overlap of electron orbitals. Also called ligancy

• Calculated by considering the greatest number of larger ions (radius R) that can be in contact with the smaller one (radius r).

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AX-TYPE CRYSTAL STRUCTURES IN CERAMICS

• are those in which there are equal numbers of cations and anions. These are often referred to as AX compounds, where A denotes the cation and X the anion.

• There are several different crystal structures for AX compounds; each is typically named after a common material that assumes the particular structure

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A unit cell for the cesium chloride (CsCl) crystal structure.

A unit cell for the rock salt, or sodium chloride (NaCl), crystal structure. (Two interpenetrating FCC latticesNaCl, MgO, LiF, FeO have this crystal structure)

A unit cell for the zinc blende (ZnS) crystal structure.

AmXp & AmBnXp -TYPE CRYSTAL STRUCTURES IN CERAMICS

• If the charges on the cations and anions are not the same, a compound can exist with the chemical formula AmXp, where m and/or p ≠1. An example is AX2, for which a common crystal structure is found in fluorite (CaF2). The ionic radii ratio rC/rA for CaF2 is about 0.8, which gives a coordination number of 8

• It is also possible for ceramic compounds to have more than one type of cation; for two types of cations (represented by A and B), their chemical formula may be designated as AmBnXp. Barium titanate (BaTiO3), having both Ba2+ & Ti4+cations, falls into this classification.

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A unit cell for the perovskite crystal structure..

A unit cell for the fluorite (CaF2) crystal structure

DENSITY COMPUTATIONS IN CERAMICS

• For a crystalline ceramic material theoretical density can be computed from unit cell data in a manner similar to that for metals.

• In this case the density, ρ may be determined using the equation as follows

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AMORPHOUS SILICA• Silica gels - amorphous SiO2

– Si4+ and O2- not in well-ordered lattice

– Charge balanced by H+ (to form OH-) at “dangling” bonds

– SiO2 is quite stable, therefore un-reactive to makes good catalyst support

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SILICA GLASS• Dense form of amorphous

silica– Charge imbalance

corrected with “counter cations” such as Na+

– Borosilicate glass is the pyrex glass used in labs

• better temperature stability & less brittle than sodium glassSi, B - Network former

Other Cations - Network modifier

SILICATE CERAMICSMost common elements on earth

are Si & O• SiO2 (silica) structures are

quartz, crystobalite, & tridymite

• The strong Si-O bond leads to a strong, high melting material (1710ºC)

Si4+ O2-

crystobalite

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• Combine SiO44- tetrahedra by

having them share corners, edges, or faces

• Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding

Mg2SiO4 Ca2MgSi2O7

SILICATE ELEMENTS

LAYERED SILICATES• Layered silicates (clay

silicates)– SiO4 tetrahedra

connected together to form 2-D plane

• (Si2O5)2-

• So need cations to balance charge

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• Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)4

2+ layer

LAYERED SILICATES

Note: these sheets loosely bound by van der Waal’s forces

• Frenkel Defect --a cation is out of place.

• Shottky Defect --a paired set of cation and anion vacancies.

DEFECTS IN CERAMIC STRUCTURES

Shottky Defect:

Frenkel Defect

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• Impurities must also satisfy charge balance = Electroneutrality

• Ex: NaCl

• Substitutional cation impurity

DEFECTS IN CERAMIC STRUCTURES - Impurities

Na + Cl -

initial geometry Ca2+ impurity resulting geometry

Ca2+

Na+

Na+Ca2+

cation vacancy

• Substitutional anion impurity

initial geometry O2- impurity

O2-

Cl-

anion vacancy

Cl-

resulting geometryM.K. Line 2015 MASC-210 51

CERAMIC PHASE DIAGRAMS

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Ceramics Vs Metals

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Property Ceramic Metal Polymer

Hardness Very High Low Very Low

Elastic modulus Very High High Low

Thermal expansion High Low Very Low

Wear resistance High Low Low

Corrosion resistance High Low Low

Ductility Low High High

Density Low High Very Low

Electrical conductivity Depends High Low on material

Thermal conductivity Depends High Low on material

Magnetic Depends High Very Low on material

CLASSIFICATION OF CERAMICS• Based on their

engineering applications, ceramics are classified into two groups as:- i. traditional &ii. engineering

ceramics.• Based on their specific

applications, ceramics are classified as:-i. Glassesii. Clay productsiii. Refractoriesiv. Abrasivesv. Cementsvi. Carbonsvii. Advanced ceramics

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CLASSIFICATION OF CERAMICS• Based on their composition, ceramics are:

i. Oxidesii. Carbidesiii. Nitridesiv. Sulfidesv. Fluorides

• Traditional ceramics – the older and more generally known types (porcelain, brick, earthenware, etc.). Based primarily on natural raw materials of clay and silicates

• Engineering ceramics – Include artificial ceramic raw materials, exhibit specialized properties, require more sophisticated processing . Applied as thermal barrier coatings to protect metal structures, wearing surfaces. Engine applications (silicon nitride (Si3N4), silicon carbide (SiC), Zirconia (ZrO2), Alumina (Al2O3)

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CLASSIFICATION OF CERAMICS

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GLASS• These are noncrystalline silicates containing other oxides,

notably CaO, Na2O, K2O, and Al2O3,which influence the glass properties.

• A typical soda-lime glass consists of approximately 70 wt% SiO2, the balance being mainly Na2O (soda) and CaO (lime).

• Possibly the two prime attractive properties of these materials are their optical transparency and the relative ease of fabrication.

• Typical applications include containers, lenses, and fiberglass

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GLASS-CERAMICS• Inorganic glasses are transformed from a noncrystalline

state into a crystalline state by proper high-temperature heat treatment called crystallization, & the product is a fine-grained polycrystalline material, often called a glass–ceramic

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Continuous-cooling transformation diagram for the crystallization of a lunar glass (35.5 wt% SiO2, 14.3 wt% TiO2, 3.7 wt% Al2O3, 23.5 wt% FeO, 11.6 wt% MgO, 11.1 wt% CaO, and0.2 wt% Na2O). Superimposed on this plot are two cooling curves, labelled 1 and 2

• Glass–ceramic materials have been designed to have the following characteristics:

relatively high mechanical strengths; low coefficients of thermal expansion (to

avoid thermal shock); good high-temperature capabilities; good dielectric properties (for electronic

packaging applications); good biological compatibility.

• Some glass–ceramics may be made optically transparent; others are opaque.

• Possibly the most attractive attribute of this class of materials is the ease with which they may be fabricated; conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware.

GLASS-CERAMICS• Glass-ceramics are manufactured commercially under the trade

names of Pyroceram, CorningWare, Cercor, and Vision. • Most common uses for glass-ceramics are as ovenware,

tableware, oven windows, and range tops—primarily because of their strength and excellent resistance to thermal shock.

• They also serve as electrical insulators & as substrates for printed circuit boards

• They are also used for architectural cladding & for heat exchangers & regenerators

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CLAY PRODUCTS• Clay is the most widely used ceramic raw material. • Is an inexpensive ingredient, found naturally in great abundance & can be

used as mined • Another reason for its popularity lies in the ease with which clay products

may be formed• When mixed in the proper proportions, clay and water form a plastic mass

that is very amenable to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength

• Most clay-based products fall within two broad classifications: i. the structural clay products - include building bricks, tiles, &

sewer pipes (applications in which structural integrity is important). ii.Whiteware ceramics - become white after high-temperature firing -

includes porcelain, pottery, tableware, china, and plumbing fixtures (sanitary ware). In addition to clay,

• These products may contain nonplastic ingredients, which influence the changes that take place during the drying and firing processes and the characteristics of the finished piece

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REFRACTORY CERAMICS• They have capacity to withstand high temperatures without

melting or decomposing & the capacity to remain unreactive and inert when exposed to severe environments.

• These provide thermal insulation which is often an important consideration.

• Refractory materials are marketed in a variety of forms, but bricks are the most common.

• Typical applications include furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, & power generation

• Their depends to a large degree on its composition & based on their compositions, they are classified asi. fireclay,ii. silica,iii. basic, &iv. special refractories.

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Compositions of Five Common Ceramic Refractory Materials

ABRASIVE CERAMICS • These are used to wear, grind, or cut away other material. Thus, the prime requisite

for this group of materials is hardness or wear resistance; • In addition, a high degree of toughness is essential to ensure that the abrasive

particles do not easily fracture. • Some refractoriness is also desirable ass high temperatures may be produced from

abrasive frictional forces• Diamonds, both natural and synthetic, are used as abrasives; however, they are

relatively expensive. • The more common ceramic abrasives include silicon carbide, tungsten carbide (WC),

aluminum oxide (or corundum), & silica sand.• Abrasives are used in several forms:

i. bonded to grinding wheels - the abrasive particles are bonded to a wheel by means of a glassy ceramic or an organic resin. The surface structure should contain some porosity; a continual flow of air currents or liquid coolants within the pores that surround the refractory grains prevents excessive heating.

ii. as coated abrasives - those in which an abrasive powder is coated on some type of paper or cloth material; sandpaper is probably the most familiar example. Wood, metals, ceramics, and plastics are all frequently ground and polished using this form of abrasive.

iii. as loose grains - are delivered in some type of oil- or water-based vehicle. Grinding, lapping, and polishing wheels often employ loose abrasive grains. Diamonds, corundum, silicon carbide, & rouge (an iron oxide) are used in loose form over a variety of grain size rangesM.K. Line 2015 MASC-210 65

CEMENTS• The inorganic ceramic material cement, plaster of Paris, & lime, as

a group, are produced in extremely large quantities. • The characteristic feature of these materials is that when mixed

with water, they form a paste that subsequently sets and hardens. • Some of these materials act as a bonding phase that chemically

binds particulate aggregates into a single cohesive structure• The role of the cement is similar to that of the glassy bonding

phase that forms when clay products and some refractory bricks are fired. One important difference, however, is that the cementitious bond develops at room temperature.

• Of this group of materials, Portland cement is consumed in the largest tonnages.

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CEMENTS – PORTLAND CEMENT• is produced by grinding & intimately mixing clay & lime-bearing

mineralsin the proper proportions & then heating the mixture to about 1400oC (2550oF) in a rotary kiln; this process, sometimes called calcination, produces physical & chemical changes in the raw materials. The resulting “clinker” product is then ground into a very fine powder, to which is added a small amount of gypsum (CaSO4–2H2O) to retard the setting process (producing a product known as Portland cement).

• The properties of Portland cement, including setting time & final strength, to a large degree depend on its composition.

• Several different constituents are found in Portland cement, the principal onesbeing tricalcium silicate (3CaO–SiO2) and dicalcium silicate (2CaO–SiO2).

• The setting & hardening of this material result from relatively complicated hydration reactions that occur among the various cement constituents and the water that is added. For example, one hydration reaction involving dicalcium silicate is as follows:2CaO-SiO2 + xH2O 2CaO-SiO2-xH2Owhere x is variable that depends on how much water is available.

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CARBONS• In terms of crystal structures there are two polymorphic forms of

carbon:-i. diamond & ii. graphite.

• Furthermore, fibers are made of carbon materials that have other structures.

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DIAMOND• Have extra ordinary physical properties • Chemically, it is very inert & resistant to attack. • Of all known bulk materials, diamond is the hardest as a result of its extremely

strong interatomic sp3 bonds. • Of all solids, it has the lowest sliding coefficient of friction. • Extremely high thermal conductivity , • optically, it is transparent in the visible & infrared regions of the electromagnetic

spectrum - has the widest spectral transmission range of all materials. The high index of refraction and optical brilliance of single crystals makes diamond a most highly valued gemstone.

• High-pressure high-temperature (HPHT) techniques to produce synthetic diamonds were developed in mid-1950s. These have been refined that today a large proportion of industrial-quality diamonds are synthetic, as are some of those of gem quality.

• Industrial-grade diamonds are used for diamond-tipped drill bits & saws, dies for wire drawing, & as abrasives used in cutting, grinding, & polishing equipment

GRAPHITE• Is highly anisotropic (property values depend on crystallographic direction

along which they are measured)• Have weak interplanar van der Waals bonds, relatively easy for planes to

slide past one another hence have excellent lubricative properties• When compared to diamond, graphite is very soft & flaky & has a

significantly smaller modulus of elasticity; Its in-plane electrical conductivity is higher than that of diamond; thermal conductivities are approximately the same; coefficient of thermal expansion for diamond is relatively small and positive while graphite’s in-plane value is small & negative, & the plane-perpendicular coefficient is positive and relatively large.

• Graphite is optically opaque with a black–silver color. • Other desirable properties of graphite include good chemical stability at

elevated temperatures & in nonoxidizing atmospheres, high resistance to thermal shock, high adsorption of gases, & good machinability

• Applications for graphite include lubricants, pencils, battery electrodes, friction materials (e.g., brake shoes), heating elements for electric furnaces, welding electrodes, metallurgical crucibles, high-temperature refractories and insulations, rocket nozzles, chemical reactor vessels, electrical contacts (e.g., brushes), & air purification devices.

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CARBON FIBERS• These are small-diameter, high-strength, & high-modulus fibers composed

of carbon• Used as reinforcements in polymer-matrix composites• Carbon is in the form of graphene layers. However, depending on precursor

(i.e., material from which the fibers are made) and heat treatment, different structural arrangements of these graphene layers exist. These include:-

i. graphitic carbon fibers - the graphene layers assume the ordered structure of graphite & planes are parallel to one another having relatively weak van der Waals interplanar bonds.

ii. turbostratic carbon - a more disordered structure results when, during fabrication, graphene sheets become randomly folded, tilted, and crumpled

iii. Hybrid graphitic-turbostratic fibers - composed of regions of both structure types, may also be synthesized

• Because most of these fibers are composed of both graphitic and turbostratic forms, the term carbon rather than graphite is used to denote these fibers

• Of the three most common reinforcing fiber types used for polymer-reinforced composites (carbon, glass, and aramid), carbon fibers have the highest modulus of elasticity & strength & are the most expensive.

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CARBON FIBERS

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CARBONS

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ADVANCED CERAMICS• These are unique ceramics having superlative combination of

properties such as electrical, magnetic, & optical exploited in a host of new products

• Advanced ceramics include materials used in microelectromechanical systems (MEMS) as well as the nanocarbons (fullerenes, carbon nanotubes, and graphene)

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MICROELECTROMECHANICAL SYSTEMS (MEMS)• are miniature “smart” systems consisting of a multitude of mechanical

devices that are integrated with large numbers of electrical elements on a substrate of silicon.

• The mechanical components are microsensors & microactuators. • Microsensors collect environmental information by measuring mechanical,

thermal, chemical, optical, and/or magnetic phenomena. The microelectronic components then process this sensory input and subsequently render decisions that direct responses from the microactuator devices - devices that perform such responses as positioning, moving, pumping, regulating, and filtering.

• These actuating devices include beams, gears, motors, membranes, etc. which are of microscopic dimensions, on the order of microns in size.

• The processing of MEMS is virtually the same as that used for the production of silicon-based integrated circuits; this includes photolithographic, ion implantation, etching, and deposition technologies. In addition, some mechanical components are fabricated using micromachining techniques.

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MICROELECTROMECHANICAL SYSTEMS (MEMS)

• MEMS components are very sophisticated, reliable, and minuscule in size. Furthermore, because the preceding fabrication techniques involve batch operations, the MEMS technology is very economical and cost effective

• There are some limitations to the use of silicon in MEMS. Silicon has a low fracture toughness (~0.90 MPam1/2) & a relatively low softening temperature (600oC) & is highly active to the presence of water & oxygen. Hence research is being conducted into using ceramic materials which are tougher, more refractory, & more inert—for some MEMS components, especially high-speed devices and nanoturbines. The ceramic materials being considered are amorphous silicon carbonitrides (siliconcarbide–silicon nitride alloys).

• One example of a practical MEMS application is an accelerometer (accelerator/decelerator sensor) that is used in the deployment of air-bag systems in automobile crashes

MICROELECTROMECHANICAL SYSTEMS (MEMS)

• Potential MEMS applications include electronic displays, data storage units, energy conversion devices, chemical detectors (for hazardous chemical and biological agents & drug screening), & microsystems for DNA amplification and identification

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Scanning electron micrograph showinga linear rack gear reduction drive MEMS. This gear chain converts rotational motion from the top-left gear to linear motion to drive the linear track (lower right). Approximately 100X.

NANOCARBONS• The “nano” prefix denotes that the particle size is less than about

100 nanometers. In addition, the carbon atoms in each nanoparticle are bonded to one another through hybrid sp2 orbitals.

• They have novel and exceptional properties & are currently being used in some cutting-edge technologies

• Three nanocarbons that belong to this class are:-i. fullerenes, ii. carbon nanotubes, & iii. graphene.

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NANOTUBES• Consists of a single sheet of graphite (i.e.,

graphene) that is rolled into a tube• The term single-walled carbon nanotube

(abbreviated SWCNT) is used to denote this structure.

• Each nanotube is a single molecule composed of millions of atoms; the length of this molecule is much greater (on the order of thousands of times greater) than its diameter.

• Multiple-walled carbon nanotubes (MWCNTs) consisting of concentric cylinders also exist

• Nanotubes are extremely strong, stiff & relatively ductile

• Carbon nanotubes have the potential to be used in structural applications. Most current applications, however, are limited to the use of bulk nanotubes - collections of unorganized tube segments

• Bulk nanotubes are currently being used as reinforcements in polymer-matrix nanocomposites to improve not only mechanical strength, but also thermal & electrical properties.

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The structure of a single-walled carbon nanotube (schematic).

NANOTUBES• Carbon nanotubes also have unique & structure-sensitive electrical

characteristics.• the nanotube may behave electrically as either a metal or a

semiconductor. As a metal, they have the potential for use as wiring for small-scale circuits. In the semiconducting state they may be used for transistors & diodes

• nanotubes are excellent electric field emitters. As such, they can be used for flat-screen displays (e.g., television screens and computer monitors).

• Other potential applications are varied and numerous, and include the following:-

More efficient solar cells Better capacitors to replace batteries Heat removal applications Cancer treatments (target and destroy cancer cells) Biomaterial applications (e.g., artificial skin, monitor and

evaluate engineered tissues) Body armor Municipal water-treatment plants (more efficient removal of

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GRAPHENE• is a single-atomic-layer of graphite, composed of

hexagonally sp2 bonded carbon atoms• These bonds are extremely strong, yet flexible,

which allows the sheets to bend. • Two characteristics of graphene make it an

exceptional material:-i. The perfect order found in its sheets: no

atomic defects such as vacancies exist; also these sheets are extremely pure—only carbon atoms are present.

ii. The second characteristic relates to the nature of the unbonded electrons: at room temperature, they move much faster than conducting electrons in ordinary metals & semiconducting materials.

• It is the strongest known material (~130 GPa), the best thermal conductor (~5000 W/m.K), & has the lowest electrical resistivity (is the best electrical conductor). It is transparent, chemically inert, & has a modulus of elasticity comparable to the other nanocarbons (~1 TPa).

• Economical & reliable methods of mass production not yet revolutionised

• Has potential applications in electronics, energy, transportation, medicine/biotechnology, & aeronautics.

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The structure of a graphene layer (schematic).

lecture 8 - non-metals pt1

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