STRUCTURES OF CERAMICS

REFF: Materials Science & Engineering; An IntroductionCallister, W. D, Jr, 2007, John Wiley & Sons Fundamental of Ceramics, Barsoum, M. W., 2003, McGraw-HillEngineering Materials 2; An Introduction to Microstructures, Processing and Design, Ashby, M. F and Jones, D. R. H, 1986, Pergamon Press

Introduction• CERAMICS: Greek keramikos = burn stuffsolid compounds formed by heat (&/P)

applications followed by coolingdesirable properties are achieved through high-T

process (firing)Firing causes irreversible transformation resulting

a material that has lost its plasticity & no longer capable to rehydrate

at least 2 elements; 1 is a non-metal, the other may be (a) metal(s) or (an)other non

Ionic Vs Covalent BondingIONIC BONDING• When more than one type of atoms are present in a material,

one atom may donate its valence electrons to a different atom, filling the outer energy shell of the second atom. Both atoms now have filled/emptied outer energy levels, but both have acquired an electrical charge and behave as ions.

• The atom that contributes the electrons is left with a net positive charge and is called a cation, while the atom that accepts the electrons acquires a net negative charge and is called an anion.

• The oppositely charged ions are then attracted to one another and produce the ionic bond.

• Occured by transfer electron; • form between very active metallic & nonmetallic elements• Atomsof a metallic element easily give up their valence

electrons to the nonmetallic atoms• to form AX ionic bonding, A loses e easily, X accepts e without

too much energy input• Ionic bonding is termed nondirectional the magnitude of

the bond is equal in all directions around an ion• It follows that for ionic materials to be stable, all positive ions

must have as nearest neighbors negatively charged ions in a three dimensional scheme, and vice versa

• The predominant bonding in ceramic materials is ionic. • For N valence electrons, an atom can covalently bond with

other atoms 8 – N• For example, N= 7 for chlorine, and 8 - N= 1, which means

that one Cl atom can bond to only one other atom, as in Cl.

COVALENT BONDING• the sharing of covalent bonding electrons between adjacent atoms. • Two atoms will each contribute at least one electron to the bond, and the

shared electrons may be considered to belong to both atoms. • forms when atoms have the same electronegativity combine energies of

bonding electrons of A & X are comparable• If the electron energy of the atoms is different transfer energy (ionic

bonding)• Each instance of sharing represents one covalent bond • e.g: silicon atom, has a valence of four, obtains eight electrons in its outer

energy shell by sharing its electrons with four surrounding silicon atoms each silicon atom is bonded to 4 neighboring atoms by 4 covalent bonds

• Many nonmetallic elemental molecules (H2, Cl2, F2 etc) as well as molecules containing dissimilar atoms, such as CH4,H2O, HNO3, and HF, are covalently

valence• The number of covalent bonds is determined by the

number of valence electrons• The valence of an atom is the number of electrons in

an atom that participate in bonding or chemical reactions.

• The valence of an atom is related to the ability of the atom to enter into chemical combination with other elements

• Usually, the valence is the number of electrons in the outer s and p energy levels.

• Examples of the valence are: Mg: 1s2 2s2 2p6 3s2 valence = 2

• It is possible to have interatomic bonds that are partially ionic and partially covalent

• the degree of either bond type depends on the relative positions of the constituent atoms in the periodic table or the difference in their electronegativities.

• Electronegativity (e greed) the tendency of an atom to gain an electron; the power of atom to attract electrons to itself

• Atoms with almost completely filled outer energy levels—such as chlorine—are strongly electronegative and readily accept electrons.

• However, atom with nearly empty outer levels—such as sodium—readily give up electrons and have low electronegativity.

• The wider the separation (both horizontally—relative toGroup IVA—and vertically) from the lower left to the upper-right-hand corner (i.e.,the greater the difference in electronegativity) the more ionic the bond.

• Conversely, the closer the atoms are together (i.e., the smaller the difference in electronegativity), the greater the degree of covalency.

• If the electronegativity difference between them (x)is large (indicating 1 element is greedier than other), e attracted to the more electronegative element ion attract each other

• x > 1.7 ionic,,,,, x < 1.7 covalent

• Characteristics of ions which affect crystal structure:1. magnitude of electrical charged of each ions

• Crystal electrically neutral• (+) charges must be balanced by an equal number of (–) • chemical formula indicates ratio of + to –• Ex CaF2 calcium ions (+2) & fluoride (-)

2. relative size of + and – ion• Involve size/ionic radii (rc & ra)• Metalic elements give up electrons when ionized cations are smaller

than anions rc/ra <1• Each cation prefers as many neighbour anions, anions also desire a

maximum number of cation.

• Stable structures require that cations and anions are in “touch”

Coordination number• the number of atoms touching a particular atom, or the

number of nearest neighbors for that particular atom.• number of anions neighbors for a cation) related to rc/ra• This is one indication of how tightly and effisiently atoms are

packed together. • For ionic solids, the coordination number of cations is defined

as the number of nearest anions. • The coordination number of anions is the number of nearest

cations.

• Table: Coordination numbers and geometries for various rc/ra

• red cation• whte anion• Common coordination

numbers for ceramic: 4, 6 and 8

• rc/ra>1 coordinate no. 12

The size of an ion depend several factors, e.g:1. coordination number

• Ionic radius increase as the number of opposite charge neighbor ions increases

• ionic radii for (coord no. 4<6<8)

2. charge on an ion • Removing e from atom/ion, the remaining valence

electrons become more tightly bound to the nucleus decrease ionic radius

• Ionic size increases when electrons are added to an atom or ion

• Radii for Fe: Fe2+: Fe3+ = 0.124: 0.077: 0.069

Atom arrangement• 1 unit cell: the smallest group of atoms form a

repetitive pattern in describing crystal structure represent crystal stucture

• Types of atomic or ionic arrangements:1.No Order

These materials randomly fillup whatever space is available to them.In monoatomic gases, such as argon (Ar) atoms or ions have no orderly arrangement.

2. Short-Range Order (SRO) A material displays short-range order (SRO) if the special arrangement of the atoms extends only to the atom’s nearest neighbors Amorphous/glassy/non crystalline material; e.g. glass

3. Long-Range Order (LRO) the special atomic arrangement extends repeat periodicity >>bond length over much larger ~>100 nm up to few cm The atoms or ions in these materials form a regular repetitive, gridlike pattern, in three dimension crystalline materials; e.g. ceramics

Non crystalline solid• lack a systematic and regular arrangement• rangement of atoms over relatively large atomic distances. • also called amorphous or supercooled liquids, inasmuch as their

atomic structure resembles that of a liquid.• Whether a crystalline or amorphous solid forms depends on the

ease with which a random atomic structure in the liquid can transform to an ordered state during solidification

• An amorphous condition may be illustrated by comparison of the crystalline and noncrystalline structures of the ceramic compound silicon dioxide (SiO2), which may exist in both states.

Crystal structure• based on the unit cell geometry only. • Within this framework, an x, y, z coordinate system is established with its

origin at one of the unit cell corners; each of the x, y, and z axes coincides with one of the three parallelepiped edges that extend from this corner.

• The unit cell geometry is completely defined in terms of six parameters: the three edge lengths a, b, and c, and the three interaxial angles a, ß, and γ .

• These seven crystal systems are cubic, tetragonal, hexagonal, orthorhombic, rhombohedral (also called trigonal), monoclinic, and triclinic

• The cubic system, for which a = b = c and a = ß = γ = 90 , has the greatest degree of symmetry. Least symmetry is displayed by the triclinic system, since a ≠ b ≠ c and a ≠ ß ≠ γ .

Single crystal• when the periodic and repeated arrangement of atoms is

perfect or extends throughout the entirety of the specimen without interruption, the result is a single crystal.

• All unit cells interlock in the same way and have the same single crystal orientation.

Polycrystalline material• A polycrystalline material is comprised of many crystals with varying

orientations in space. These crystals in a polycrystalline material are known as grains.

• The borders between tiny crystals, where the crystals are in misalignment and are known as grain boundaries.

• Stages in the solidification of a polycrystalline: Initially, small crystals or nuclei form at various positions. These have random crystallographic orientations. The small grains grow by the successive addition from the surrounding liquid of atoms to the structure of each. The extremities of adjacent grains impinge on one another as the solidification process approaches completion.

Type of crystal structure

• AX: structure of NaCl, CsCl, ZnS• AmXp• AmBnXp

AX-type crystal structures

• equal number of A (cation) & X (anion)• Referred as AX• 3 structures: rock salt, CsCl and ZnS • Ionic & or covalent bonding• Ionic MgO; 2 e of A transferred to X, result in Mg2+ & O2- • Covalent ZnS; sharing elektron

Rock salt (NaCl) structure• The most common AX crystal

structure• Electrostatic attraction between

Na+ & Cl- hold the crystal together• Max. electrostatic interaction each Na+ has 6 Cl-, no Na+ neighbours (vice versa)

• Coordination number for both + & - is 6 (octahedral)

• 1 unit cell generated from FCC of anion with 1 cation in cubic center & 1 at centered of each of 12 cube edge

• NaCl, MgO, MnS, LiF and FeO

Cesium cloride (CsCl) stucture

• Coordination number for both ions is 8 (cubic)

• The anions are at each of the corners of a cube

• Single cation is at the cube center

• This structure is possible when the anion and the cation have the same valence

Zinc Blende (ZnS) structure• Coordinate number for both

ions is 4 (tetrahedral)• all corner and face positions

of the cubic cell are occupied by S atoms

• the Zn atoms fill interior tetrahedral positions

• Each Zn atom bonded to 4 S atoms, vice versa

• Most often the atomic bonding is highly covalent

• ZnS, ZnTe, and SiC

AmXp – Type crystal structures

• Charges of + & - are not the same, m ≠ p

• Example: AX2 CaF2• Ca ion at the centers of cube,

F ion in the corner• 1 unit cell consists of 8 cubes

AmBnXp – Type crystal structure

• 2 types of cation, A & B• Chemical formula AxBnXp• Ex. BaTiO3• Ba2+ ions are situated at all 8

corners of the cube, single Ti4+ is at the centre, O2- ions is at the centre of 6 faces

SILICATE CERAMIC

• Silicates are composed primarily of silicon and oxygen, abundant elements in earth’s crush; soil, rock, clay

• Each silicon atom bond strongly to 4 oxygen atom • Basic unit in all silicates tetrahedron (oxygen are situated at

the corners, oxygen is at the centre)

silica• The most simple: silicon

dioxide/silica• Pure silica no metal ions, every

oxygen becomes a bridge between 2 silicon atoms

• Every corner oxygen atom is shared by adjacent tetrahedra

• The material’s electrically neutral, all atoms have stable electronic structures

• Ratio Si to O 1:2 (indicated by chemical formula)

• If tetrahedras are arranged in a regular & order crystalline

• 3 polymorphic: quartz, cristobalite & tridymite

The silicates

Silica Glasses• Noncrystalline solid or glass,

high randomness• Fused silica/vitreous silica• Basic unit tetrahedron (same

as the crystalline)• Pure silica forms glass with

high softening T (1200 C)• Great strength and stability,

low thermal expansion but hard to work with because high in viscosity

• Commercial glasses silica glasses add with other metal oxide to reduce viscosity

• E.g. CaO, Na2O add positive ion to the structure &break up the network network modifiers

• Add 1 Na2O molecules introduces 2 Na+, each attaches to 1 oxygen of tetrahedron non bridging

GLASS & CERAMIC SHAPING & FORMATION

GLASS & GLASS CERAMICSGlass Properties:Upon cooling, a glass continues

to be more viscous with decreasing temperature

no definite temperature at which the liquid transforms to a solid

volume decreases continuously with temperature reduction

TgThe temperature at which the transition in the amorphous regions between the glassy and rubbery state

Slight decrease in slope of curve

Crystal discountinue decrease in vol at Tm

Tg: glass transition T

Tm: melting T for crystalline

Glass Forming• Heat RM to an elevated temperature above which melting

occurs• Most commercial glasses are of the silica–soda–lime variety• silica (quartz)–Na2O(soda ash, Na2CO3)–CaO(limestone,

CaCO3)• Important: homogeneous and pore free• Homogeneity complete melting and mixing of RM• Porosity results from small gas bubbles that are produced

these must be absorbed into the melt or otherwise eliminated, which requires proper adjustment of the viscosity of the molten material

• 4 methods to fabricate glass: pressing, blowing, drawing & fiber forming

pressing

• relatively thick-walled pieces (plates and dishes. • The glass piece is pressed in a graphite-coated cast

iron mold with desired shape• the mold is heated to ensure an even surface.

BLOWING some glass blowing is done

by hand art object RM press in mold parison (temporary shape); place into finishing or blow mold & forced to conform to the mold contours by the pressure created from a blast of air

Glass bottle, jar, light bulb

DRAWING

Form long glass pieces (sheet, tubing, rod etc)hot rolling may appliedFlatness & surface finish may be improved by floating the

sheet on a bath of molten tin at high T followed slowly cooled and subsequently heat treated

Clay Products• Clay are aluminosilicates Al2O3 & SiO3 contain

chemically bound water• Broad in physical characteristic, chemical

composition, structure• Impurities: oxide of Ba, Ca, Na, K, Fe• May contain nonplastic ingredient• Nonclay minerals: flint, quartz, feldspar• Quartzrelatively hard, little change in high T, ability

to form glass

Composition• contain mineral• Clay minerals play 2 roles:1.When added water, form hydroplasticity 2.Fuse or melt over a range T dense & strong

ceramic during firing without complete melting; desired shape

• Common clay mineral: kaolinite (Al2(Si2O5)(OH)4

• Most prevailing structure layer structure• When water is added, the water molecules

fit in between these layered sheets and form a thin film around the clay particles.

• RM milling & grinding screening & sizing mixing all RM shaping

Binder & plastiziser• Binder a component that is added to hold the powder

together while shaping the body • 2 functions of binder: 1) provide plasticity necessary for

forming & 2) provides the dry (green) shape with strength sufficient to survive the handling process between shaping and sintering

• requirements for the binder is able to be eliminated from the compact during the firing process without any disruptive effect water, polymers

• Poly (vinyl alcohol) (PVA high green strength) and poly (ethylene glycol) (PEGhigh green density) are the two of the most popular binders for dry pressing ceramics

Plastic forming• Plasticizer is the component of a binder that keeps it soft or

pliable; it improves the rheological properties• Mixing ceramic powder with large vol of liquid to produce a mass

that deformable/plastic under P• The binder: water, organic liquid, complex comp to achieve

required viscosity & properties

NOTE Slurry a suspension of ceramic particles in a liquid• Green state ceramic compact that is strong enough to be

handled and machined but is not fully dense and the bonds between the grains are not strong.

• represents a transition state between the loose powder and the high-density sintered product ; ceramic before fired

Slip & slurry

• slip consists of fine (<10 µm) cceramic powder particles that are suspended in a fluid usually water.

• a solid content up to ~60 vol%. • Deflocculents are added to the slip to modify the electrical

properties of each particle (the particles repel each other)• deflocculation process by which floccules present in a liquid

break up into fine particles producing a dispersion• Defloculation >< coagulation• a deflocculant an additive that causes this process.

CERAMIC SHAPING3 methods in ceramic shaping: Powder compaction: dry pressing, hot pressing, cold

isostatic pressing, etc casting: using a mold with the ceramic as or containing of

liquid or slurry Plastic forming: using pressure to shape the green ceramic;

extrusion, injection molding etc.

Powder compaction• Pressing of free flowing powder• Pressure application depends on final product • 2 types: dry pressed (i.e. without addition of binder) &

pressed with the addition of suitable binder• uniaxially simple shape, or isostaticcally complex shape

Dry pressing• three basic steps: filling the die, compacting the contents, and

ejecting the pressed solid• A particle size 20 and 200 µm; a high volume fraction of small

particles s problems with flow and sticking of the punches. • During pressing the powder particles must flow between the

punches uniformly filled.• In a double-action press top and bottom punches are movable.

• bottom punch is in the low position a cavity the cavity is filled with free flowing powder ; the powder is struck off level with the top of the die. The top punch descends and compresses the powder either to a predetermined volume/set pressure.

• After pressing, both punches move upward The compact is then ejected

Hot pressing• Pressing performed at high temperatures• The die assembly is contained within a high temperature furnace• During hot pressing the ceramic powders may sinter together to

form a high-density component.• the ADVANTAGES of this process:1 The powder does not have to be of the highest quality.2 Large pores that are caused by non uniform mixing are easily

removed.3 densify at temperatures lower (typically half the melting

temperature of the material) than those needed for conventional pressure less sintering.

4 densify covalently bonded materials such as B4C, SiC, and Si3N4 without additives

• DISADVANTAGE :1 Die for high T is expensive

and do not generally last long.

2 limited simple shape: flat plates, block, cylinder

• Graphite is the most widely used die material (up to 2200°C, 10 - 30 Mpa)

• Graphite properties:1 easy to machine2 cheap3 strength increase with T4 good creep resistance5 excellent thermal conductivity6 low coeff of thermal

expansion

Cold isostatic pressing-CIP• apply of hydrostatic pressure to a powder in a

flexible container.• The advantage of applying pressure in all directions

more uniform compaction of the powder and more complex shapes

• Can be performed either with or without applied heat.

WET BAG CIP PROCESSADVANTAGES:• Wide range of shapes and sizes can be produced• Uniform density of the pressed product• Low tooling costsDISADVANTAGES:• Poor shape and dimensional control (particularly for

complex shapes) • Long cycle times (typically between 5 and 60

minutes) low production rates

Casting ceramic

• In Tr, require slurry ceramic powder particles to be suspended in a liquid

• 2 type: slip casting & tape casting• S lip casting slurry is poured into porous mold that

remove the liquid, leave a particulate compact in the mold

• Tape casting to make thick film/sheet

Slip casting• The slip is poured into a mold (usually plaster of

Paris-2CaSO.4H2O) that has been made by casting round a model of the required shape

• The mold allow for the shrinkage of the cast ceramic on drying and sintering.

• The fineness of the powder (in the slip) and the consequent high surface area ensure that settling does not occur.

• Na silicate (or soda ash) is added to the slip to deflocculate the particles. The water passes, via capillary action, into the porous plaster leaving a layer of the solid on the wall of the mold. Once a sufficient thickness has been cast, the surplus slip is poured out and the mold and cast are allowed to dry.

• Drain slip casting the process’s terminated when desired thickness reached, pour out the excess slip.

• After dried, the mold is disassembled• Characteristic the Slip high specific gravity, very

fluid & pourable• sanitary lavatory ware, art objects

• Slip casting is a low cost way to produce complex shapes • method for the production of teapots, jugs, and large articles

(whitewares)• One of the t signs of slip casting ceramic is that it is hollow. • Another variant of the slip casting process is solid/tape casting

slip is continually added until a solid cast is made. These items will not be hollow—relatively, they will be heavier.

• Characteristic slip: high specific gravity, very fluid, pourable (depend on solid to water ratio), free of bubble, low drying shrinkage & high strength

• Mold properties quality of casting• Usually plaster of paris ecomonical, easy to fabricate, reusable• Solid casting water from slip is absorbed

into the mold when poured, leaving solid layer on the mold wall (thickness, f=(t))

• May continue until entire mold cavity becomes solid

extrusion

• Extrusion involves forcing a deformable mass through a die orifice (like toothpaste from a tube)

• widely used to produce ceramic components having a uniform cross section and a large length-to-diameter ratio such as ceramic tubes and rods

• Extrusion is also used to produce the alumina shells for sodium vapor lamps and the honeycomb-shaped catalyst supports for automotive emission-control devices

Injection molding• Used for ceramic powder which is added to a thermoplastic polymer. • the polymer is usually referred to as the binder • The ceramic powder is added to the binder and other organic materials to

provide a mass that has the desired rheological properties• The plastic mass is first heated, at which point the thermoplastic polymer

becomes soft and is then forced into a mold cavity. The heated mixture is very fluid and is not self-supporting (this is different from the situation encountered in extrusion). The mixture is allowed to cool in the mold during which time the thermoplastic polymer hardens.

• complex shapes are retained with very little distortion during sintering since the densities, although low, are uniform.

• ADVANTAGES: used to fabricate ceramic components with complex shapes; cycle times can be rapid high-volume process.

• DISADVANTAGES: initial tooling costs is quite high

Drying & Firing

• Ceramic formed hydroplastically/slip casting significant porosity & insufficient strength, contain some liquid added in previous operation

• Remove by drying “green body”• Continued with firing• Defect may be introduced results of nonuniform

shrinkage

drying• early stage, clay particles surrounded & separated by thin

water film• Drying: remove some liquid remain interparticle separation

decrease (shrinkage); • Critical to control the rate of water removal• Drying interior is accomplished by diffusion of water

molecules to surface where evaporation occurs• Rate of evap>diffusion surface will dry faster than interior

shrink• Nonuniform shrinkage & defect formation thick >thin pieces• More water content more extensive the shrinkage; kept as

low as possible• Particle size decrease shrinkage increase

Binder burnout• removal of water from the shaped clay. • The rest of the firing process causes structure changes and

transformations in the silicate itself.• Expected remove binder without cracking or distorting the ceramic

compact.• Binder burnout forms defects in the processing of a ceramic: macroscopic

defects, such as cracks and blisters affect the mechanical strength and other properties.

• In commercial ceramic which often consists of several components challenging to be controlled, different boiling points and decomposition temperatures.

• The components with low boiling points (e.g., waxes)may be removed by evaporation at fairly low temperatures.

• The process of binder removal is kept slow to redue possibility of macrodefects being produced.

firing• Fire between 900-1400 C (RM composition &

desire properties)• During firing operation density increase

(porosity decrease) & mechanical strength enhance

• Complex reactions occured• vitrification: gradual liquid glass formation that

flows into & fills pore volume ; f=(T, t, composition); accompanied by shrinkage

• Degree of vitrification controls ceramic properties (strength, durability & density)

• Addition of fluxing agent reduce T of liquid phase

• Upon cooling, this fused phase forms in a dense, strong body

• Complete vitricifation is avoided body become too soft, may collapse

IMPERFECTIONS & APPLICATION OF CERAMIC

• Point defects are localized disruptions in an otherwise perfect atomic or ionic arrangements in a crystal structure.

• may be introduced by movement of the atoms or ions when they gain energy by heating, during processing of the material or by introduction of other atoms.

• The distinction between an impurity and a dopant :• Impurities element/compounds that are present from raw materials or

processing (e.g. Si single crystals contain oxygen as an impurity)• Dopants elements/compounds that are deliberately added, in known

concentrations, at specific locations in the microstructure, with an intended beneficial effect on properties or processing (e.g. P & B are added to Si crystals to improve or alter the electrical properties of pure silicon

• the effect of impurities is deleterious, whereas the effect of dopants on the properties of materials is useful.

Point defect

• crystalline defect a lattice irregularity having one or more of its dimensions on the order of an atomic diameter

• The simplest of the point defects is a vacancy, or vacant lattice site

• A vacancy is produced when an atom or an ion is missing from its normal site in the crystal structure

Imperfections in ceramics

• An interstitial defect is formed when an extra atom or ion is inserted into the crystal structure at a normally unoccupied position

• Interstitial atoms or ions, although much smaller than the atoms or ions located at the lattice points, are still larger than the interstitial sites that they occupy.

• Consequently, the surrounding crystal region is compressed and distorted.

• Interstitial atoms such as hydrogen are often present as impurities; whereas carbon atoms are intentionally added to iron to produce steel.

• Unlike vacancies, once introduced, the number of interstitial atoms or ions in the structure remains nearly constant, even when the temperature is changed.

Interstitial defect

• introduced when one atom or ion is replaced by a different type of atom or ion

• occupy the normal lattice sites. • may either be larger than the normal atoms or ions, in which case

the surrounding interatomic spacings are reduced, or smaller causing the surrounding atoms to have larger interatomic spacings.

• alter the interatomic distances in the surrounding crystal. • introduced either as an impurity or as a deliberate addition• once introduced, the defects is relatively temp. independent• Examples dopants such a (P) or boron (B) into Si. • Whether atoms or ions go into interstitial or substitutional sites

depends upon the size and valence of guest atoms or ions compared to the size and valence of host ions.

• The size of the available sites also plays a role in this.

Substitutional Defects

Atomic defects involving host atoms for each ion vacancies & interstitial are possible since ceramic materials contain ions of at least two kinds (+

& -), defects for each ion type may occur NaCl : Na & Cl each interstitials & vacancies highly improbable that there would be appreciable

concentrations of anion (Cl) interstitials. The anion is relatively large, and to fit into a small interstitial

position, substantial strains on the surrounding ions must be introduced.

• Because the atoms exist as charged ions, when defect structures are considered, conditions of electroneutrality must be maintained.

• Electroneutrality the state when there are equal numbers of positive and negative charges from the ions.

• consequently, defects in ceramics do not occur alone• in AX materials, defect is a cation vacancy –anion vacancy pair

known as a Schottky defect• created by removing one cation and one anion from the

interior of the crystal and then placing them both at an external surface.

• Since both cations and anions have the same charge, and since for every anion vacancy there exists a cation vacancy, the charge neutrality of the crystal is maintained

• Defect of a cation–vacancy and a cation–interstitial pair a Frenkel defect

• formed by a cation leaving its normal position vacant and moving into an interstitial site.

• There is no change in charge because the cation maintains the same positive charge as an interstitial.

The ratio of cations to anions is not altered by the formation of either a Frenkel or a Schottky defect

If no other defects are present, the material is said to be stoichiometric.

Stoichiometry as a state for ionic compounds wherein there is the exact ratio of cations to anions predicted by the chemical formula.

For example, NaCl is stoichiometric if the ratio of Na+ ions to Cl− ions is exactly 1:1.

A ceramic compound is nonstoichiometric if there is any deviation from this exact ratio

Nonstoichiometry may occur for some ceramic materials in which two valence (or ionic) states exist for one of the ion types.

Iron oxide (FeO) can be present in both Fe2+ and Fe3+ states; depends on temperature and the ambient oxygen pressure.

The formation of an Fe3+ ion disrupts the electroneutrality of the crystal by introducing an excess +1 charge, which must be offset by some type of defect.

This may be accomplished by the formation of one Fe2+ vacancy (or the removal of two positive charges) for every two Fe3+ ions that are formed

The crystal is no longer stoichiometric because there is one more O ion than Fe ion; however, the crystal remains electrically neutral.

• Type: solid solutions of both substitutional and interstitial• For an interstitial, the ionic radius of the impurity must

be relatively small in comparison to the anion. • A substitutional impurity will substitute for the host ion

(c/a) to which it is most similar in an electrical sense: • if the impurity atom normally forms a cation in a ceramic

material, it most probably will substitute for a host cation.

• For example, in NaCl, impurity Ca2+ and O2− ions would most likely substitute for Na+ and Cl− ions, respectively.

Impurities in ceramics

To achieve any appreciable solid solubility of substituting impurity atoms, the ionic size and charge must be very nearly the same as those of one of the host ions

For an impurity ion having a charge different from the host ion for which it substitutes, the crystal must compensate for this difference in charge so that electroneutrality is maintained with the solid.

One way this is accomplished is by the formation vacancies or interstitials of both ion types

APPLICATIONS

Glasses• The glasses are a familiar group of

ceramics; e.g. containers, lenses, window and fiberglass

• Non crystalline silicates with other oxide (e.g. CaO, Na2O, K2O & Al2O3) influence the glass properties.

• A typical soda–lime glass consists of approximately 70 wt% SiO3, the balance being mainly NaO (soda) and CaO (lime)

• Optical transparency & relative ease to fabricated

• Glass can be transformed to crystalline by high T heat treatment crystallisation

• Product: glass-ceramics (fine-grained polycrystalline)• Process involves nucleation & growth stage• A nucleation agent (frequently TiO2) is addded to

promote crystallization; • Commonly used as ovenware, tableware, oven

window etc

Glass Ceramics

• Very popular products (abundant, inexpensive, easy to be formed)

• Contain nonplastic ingredient which affect the change that take place during the drying and firing processes the characteristics of the finished piece

• two broad classifications: the structural clay products and the whitewares.

• Structural clay products include bricks, tiles, and sewer pipes—applications in which structural integrity is important.

• whiteware ceramics become white after the high-temperature firing group are porcelain, pottery, tableware, china, and plumbing fixtures, sanitary ware

Clay Products

Refractories• Properties: the capacity to withstand high temperatures

without melting or decomposing, endure at high T, capacity to remain inert in severe enviroment, provide thermal insulation

• Common product bricks• Application: metal refining, glass manufacturing, metallurgical

heat treatment, power generation• the performance of a refractory ceramic, to a large degree,

depends on composition. • there are several classifications:

• For many commercial materials, the raw ingredients consist of both large and fine particles, which may have different compositions.

• Upon firing, the fine particles normally are involved in the formation of a bonding phase, which is responsible for the increased strength of the brick

• The service temperature is normally below that at which the refractory piece was fired.

• the optimum porosity depends on the conditions of service.

• Porosity is one microstructural variable that must be controlled to produce a suitable refractory brick. Strength, load-bearing capacity, and resistance to attack by

• corrosive materials all increase with porosity reduction.

• At the same time, thermal insulation characteristics and resistance to thermal shock are diminished.

Abrasives• used to wear, grind, or cut away other

material, which necessarily is softer. • Properties: hardness/wear resistance,

tough• Diamond; silicon carbide, tungsten carbide,

aluminium oxide, silica sand, diamonds• used in several forms: 1) bonded to

grinding wheels, 2) as coated abrasives, and 3) as loose grains.

• In the first case, the abrasive particles are bonded to a wheel by means of a glassy ceramic or an organic resin

• Coated abrasives are 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.

• Grinding, lapping, and polishing wheels often employ loose abrasive grains that are delivered in some type of oil- or water-based vehicle.

Cements• cement, plaster cement of paris, and lime• Characteristic: form paste when mixed with

water subsequently set & hardens• act as a bonding phase that chemically binds

particulate aggregates into a single cohesive structure at Tr

• 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

• The properties of portland cement, including setting time and final strength, to a large degree depend

• on its composition.

STRUCTURE OF POLYMERS

• Poly & mers Greek ; meros=part; polymer=many parts• Natural polymer derived from animals & plants wood, rubber,

cotton, wool, leather, and silk• Other natural polymers such as proteins, enzymes, starches,

and cellulose • this group of materials and the development of numerous

polymers synthesized from small organic molecules. • Many of our useful plastics, rubbers, and fiber materials are

synthetic polymers. • It can be produced inexpensively, and their properties can be

managed to the degree that many are superior to their natural counterparts.

• In some applications metal and wood parts have been replaced by plastics that have satisfactory properties and may be produced at a lower cost.

• Most of polymers are organic in origin & based on hydrocarbon (H & C)

Hydrocarbon - HC• HC Intramolecular bonds are covalent• Each C atom has 4 e to participate in covalent

bonding, every H has 1 bonding e• Single covalent bondeach of 2 bonding atoms

contributes 1 e; CH4• Double & triple bond 2 C atoms share 2 & 3 pairs of

e; C2H4• Saturated HC all single bond• No new atom may be joined without removal of

atoms that are already bonded • Double & triple covalent bonds unsaturated; • each C is not bonded to max atoms other atoms are

possible to be bonded to the molecule

• Some of the simple hydrocarbons belong to the paraffin family;

• the chainlike paraffin molecules include methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10)

• The covalent bonds in each molecule are strong, but only weak hydrogen and van der Waals bonds exist between molecules, and thus these hydrocarbons

• have relatively low melting and boiling points.

• HC comp with same composition but different arrangement isomerism; affect the properties

• E.g. N-buthane & isobuthane

Polymer molecules• Large molecule built up by repetition of small, simple

chemical units• Because of their size macromolecules• Atoms’re bound by covalent bonds• For C polymer C the backbone • Many times each carbon atom singly bonds to two

adjacent carbons atoms on either• side• 2 remaining valence of C may involve in side-bonding

with atoms/radical that are positioned adjacent to the chain

• C2H4-ethylene (P&Tr) gas• If the ethylene gas is reacted under appropriate conditions,

it will transform to polyethylene (PE), which is a solid polymeric material.

• This process begins when an active center is formed by the reaction between an initiator or catalyst species (R*) the ethylene monomer, as follows:R*+C2H4 R-C*2H4

• The polymer chain then forms by the sequential addition of monomer units to this active growing chain molecule. The active site, or unpaired electron (denoted by *), is transferred to each successive end monomer as it is linked to the chain. This may be represented schematically as follows:

• The final result, after the addition of many ethylene monomer units, is the polyethylene molecule;

• This polyethylene chain structure can also be represented as

• Here the repeat units are enclosed in parentheses, and the subscript n indicates the number of times it repeats.

• The vinyl chloride monomer is a slight variant of that for ethylene, in which one of the four H atoms is replaced with a Cl atom.

• Its polymerization is represented as

• Some polymers may be represented using the following generalized form:

• where the “R” depicts either an atom [i.e., H or Cl, for polyethylene or poly(vinylchloride), respectively], or an organic group such as CH3,C2H5, and C (methyl, ethyl, and phenyl). For example, when R represents a CH

• group, the polymer isvpolypropylene (PP).

• The molecules are composed of structure called repeat units (Mers)

• Monomer: small molecule from which a polymer is synthesized

• When all of the repeating units are the same homopolymer

• Chain may be composed of 2 or more different repeat units copolymer

• Ex: General N-alkane HC• Higher MW increase viscosity

MOLECULAR WEIGHT• Extremely large molecular weights are observed in polymers with very

long chains.• During polymerization process, not all polymer chains grow the same

length• Result in distribution of chain length/MW length• Ordinarily,an average molecular weight is specified• the melting or softening temperature increases with increasing

molecular weight• At Tr polymers with very short chains (M ~100 g/mol) liquid; ~ 1000

g/mol are waxy solids (such as paraffin wax) and soft resins; Solid polymers (sometimes termed high polymers), commonly have M ranging 10,000 - several million g/mol)

• Thus, the same polymer material can have quite different properties if it is produced with a different molecular weight.

• There are several ways of defining average molecular weight: 1) the number-ave MW, 2) weight-ave MW, and 3) degree of polymerisation

1)The number-average MW• Dividing the chains into series of

size range • then determining the number

fraction of chain within each size range.

• Expressed as:

• Mi=mean/middle MW of size range i

• xi = fraction of total number of chain within the corresponding size range

2) The weight-average MW• weight fraction of molecules within

various size ranges.Calculated as:

• Mi=mean MW within size range i• wi =weight fraction of molecules within

the same size interval• = degree of distribution of MW

• A typical molecular weight distribution along with these molecular weight averages

3)Degree of polymerization

• DP Average chain size of polymer • DP average number of repeat units (mers) in a

chain• Can be expressed as :

• Mn & m = number average MW & repeat unit (mer) MW

Example• Figures of MW

distribution are for PVC. Calculate a) number-average MW b) weight-average MW & c) degree of polymerisation

a) Table for number-average MW 21,150 g/mol

• b) Table for weight-average MW 23,200 g/mol

c) PVC 2 C, 3 H & 1 Cl

Molecular Structure• Linear, branced, crosslinked,

networkLINIER POLYMERS• repeat units are joined end to

end in single chains• each circle represents a repeat

unit• Melt on heating• Mechanical strength increases

with entangle chain

Example ofLinier Polymer

• Polyethylene HDPE• PVC• Polystyrene• Nylon• fluorocarbon

BRANCHED POLYMERS• The branch considered to be part of the main chain molecules• side-branch chains are connected to the main one• May result from side reactions that occur during the synthesis• The chain packing efficiency reduces with formation of side

branches lowering polymer density• By changing T, the branched polymer can be hardened or

softened• Those polymers that form linear structures may also be

branched. • E.g. HDPE (primarily a linear Polymer), while LDPE contains

short chain branches.

CROSSLINKED POLYMERS• Adjacent linear chains are joined one to another at various

positions by covalent bonds• increase strength, reduce plasticity• Achieved during synthesis or by nonreversible chemical

reaction• Often, accomplish by additive atom/molecules that are

covalently bonded to the chains• The movement of adjacent chains is greatly restricted, affected

the mechanical properties to a great extent• E.g. rubber elastic material

NETWORKING POLYMER

• Multifunctional monomers forming three or more• a polymer that is highly crosslinked 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.• Polymers are not usually of only one distinctive structural type.

For example, a predominantly linear polymer might have limited branching and crosslinking.

Thermoplastic Polymers• Soften when heated (eventually liquefy), harden when cooled

reversible & may be repeated• Plastic & flexible properties• Formed at high T, cooled, remelted & reformed into different

shape without changing properties• Overheat material decomposes, irreversible degradation• Most linear, some branches polymer• Fabricated by simultaneous heat & pressure • Example: polyethylene, polystyrene, PVC, poly(ethylene

terephthalate)

Thermosetting• Network polymers• Strong bonds, often formed by condensation• Permanently hard during formation when heat

applied• Do not softened/reshaped upon subsequent

heating loss of part of the molecule• Further heat burn/decompose • Generally harder, stronger & better stability than

thermoplastic• Most crosslinked, in that 10 to 50% of the chain

repeat units are crosslinked.• Only heating to excessive temperatures causes

severance of these crosslink bonds and polymer degradation.

• Ex: phenolic, vulcanized rubber, epoxies

Copolymers• Polymers with more than 1 repeat unit• Different type depends on method synthesis & repeat unit

type• Sequencing arrangement: random, alternating, block & graft

copolymer• 1) Random copolymer random distribution of various mers• E.g nitrile rubber

• 2) Alternating copolymer 2 mer units alternate chain position

• 3) Block copolymer identical repeat units are clustered in blocks along the chain

4) Grafted copolymer homopolymer side branches of one type may be grafted to homopolymer main chain that are composed of different mer

Synthetic & processing

Of polymers

SYNTHESISstages in polymer synthesis: 1)polymerisation,

2)additive materials, & 3) finishing pieces

• POLYMERISATION monomers are linked together to generate long chains

composed of repeat units raw material: derived from coal, natural gas,

petroleum 2 classifications according reaction mechanism:

addition & condensation polymer

1) Addition polymerization• Chain reaction polymerization through free radical

polymerisation• Free radical/ unpaired electron highly reactive, tend to rip

electron from other• Free radicala are created by the division of initiator into two

fragment• Stability of free radical varies depending on the properties of

molecule• monomer units are attached one by one to an active site to form

a linear macromolecules -Result in exact multiple of original monomer

• synthesis of polyethylene, polypropylene, PVC• 3 stages: initiation, propagation & termination affect MW• Stages are controlled to ensure degree of polymerization of

product

i) Initiation • Begins when an initiator decomposes into free radical in the present

of monomer.• The instability of C-C double bond in monomer makes them

susceptible to reaction with the unpaired electron in the radical• active centre(location of unpaired electron) is formed by reaction

between an initiator (catalyst) species & monomer unit

R•active initiator; •unpaired electron* In free radical polymerisation the radical attacks one monomer, and

the electron migrates to another part. This newly radical attack another monomer & the process repeat.

ii) Propagation linear growth of polymer chain by sequential addition of monomer unit to the active growing chain molecule

• 1000 repeat units in 10-2 -10-3 s• In theory, the propagation continue until the supply of monomers

exhausted• However, most often the growth of the chain is stopped by the

termination reaction

iii) Termination ,there are 2 ways:a) combination

active end of free electron from 2 growing propagation chain may link together form 1 molecule

b) disproportionation free radical strips a hydrogen atom from an active chain

A C-C double bond takes the place of the missing hydrogen

2) Condensation polimerization• Stepwise intermolecular chemical reaction that may involve

more than 1 monomer species• By product: small MW (water) that is eliminated (condensed)• t condensation > t addition polymerisation• Thermosetting polyester & phenol-formaldehyde, nylons,

polycarbonates• No reactant species has chemical formula in the repeat unit;

intermolecular reaction occurs everytime a repeat unit is formed

POLYMER ADDITIVES

Additives substances introduce to enhance/modify properties, thus more serviceable: plasticizers, stabilizers, colorants, flame retardants, fillers

FILLERS# to improve tensile & compressive strength, abrasive resistance,

toughness, dimensional, thermal stability etc# inexpensive materials that replace some vol of more expensive

polymer reduce cost# wood flour (sawdust), silica flour, sand, glass, clay etc

PLASTISIZER

- Improve flexibility, toughness; reduce hardness & stiffness- Plasticizers are usually selected to be nonvolatile materials

and have good compatibility with the desired polymer. - Small plasticizer molecules occupy positions between large

polymer chain, increase interchain distance with reduction intermolecular bonding

- Used in brittle materials (Tr): PVC - Lower Tg at Tr, polymer may be used with some pliability &

ductility- (Liquids having low vapor P & low MW) phtalate ester,

adipate- Application: PVC, thin sheet, film, tubing, raincoats

STABILIZERS

- Additive to counteract deterioration in some polymers under normal environment expose to light-UV & oxidation

# Oxidation chemical interaction between oxygen with polymer; Stabilizer consume oxygen before it reaches polymer &/

prevent the oxidation reaction that would further damage

# Prevent photochemical degradation C black

# UV radiation interact with & cause severe in covalent bond & molecular

chain- Approaches to UV stabilization: add UV absorbent & add

material react with broken bond1) Add UV absorbent material

- layer at surface as sunscreen- to block out the radiation before penetrating into & damage the polymer

2) Add materials that react with bond broken by radiation before they participate in other reactions lead more damage

Colorants

- give color to polymer- added as dyes- molecules dyes dissolve in polymer-added as pigment filler materials that do- not dissolve, remain separate phase- small size & refractive index near the parent polymers

Flame retardants

- Most all pure polymer are flammable

- used in textile & toys- interfere the combustion

process by initiating different combustion reaction generate less heat, reduce T slowing burning

Polymer processing• There are a number of methods for producing polymer

shapes, including molding, extrusion, and manufacture of films and fibers.

• The techniques depend to a large extent on the nature of the polymer

• The greatest variety of techniques are used to form the thermoplastics.

• The polymer is heated to near or above the melting temperature so that it becomes rubbery or liquid.then formed in a mold or die to produce the required shape.

BLOW MOLDING• A hollow preform of a thermoplastic called a parison is

introduced into a die by gas pressure and expanded against the walls of the die.

• This process is used to produce plastic bottles, containers, automotive fuel tanks, and other hollow shapes.

EXTRUSION• This is the most widely used technique for processing thermoplastics. • Extrusion can serve two purposes: 1) it provides a way to form certain

simple shapes continuously, and 2) extrusion provides an excellent mixer for additives (e.g., carbon black, fillers, etc.)

• A screw mechanism consisting of one or a pair of screws (twin screw) forces heated thermoplastic (either solid or liquid) and additives through a die opening to produce solid shapes, films, sheets, tubes, pipes, and even plastic bags.

• The extruder consist of di¤erent heating or cooling zones. Extrusion can be used to make film, coat wires and cables with either thermoplastics or elastomers.

INJECTION MOLD• Thermoplastics heated above the melting temperature using

an extruder are forced into a closed die to produce a molding.• This process is similar to die casting of molten metals. • A plunger or a special screw mechanism applies pressure to

force the hot polymer into the die. • A wide variety of products, ranging from cups, combs, and

gears to garbage cans, can be produced in this manner.

THERMOFORMING• Thermoplastic polymer sheets heated to the plastic region can

be formed over a die to produce such diverse products as egg cartons and decorative panels.

• The forming can be done using matching dies, a vacuum, or air pressure.

CALENDARING• molten plastic is poured into a set of rolls with a small

opening. • The rolls, which may be embossed with a pattern, squeeze out

a thin sheet of the polymer—often, polyvinyl chloride. Typical products include vinyl floor tile and shower curtains.

SPINNING• Filaments, fibers, and yarns may be produced by spinning. • The molten thermoplastic polymer is forced through a die

containing many tiny holes. • The die, called a spinnerette, can rotate and produce a yarn. • For some materials, including nylon, the fiber may

subsequently be stretched to align the chains parallel to the axis of the fiber; this process increases the strength of the fibers.

CASTING • Many polymers can be cast into molds and permitted to

solidify.• The molds may be plate glass for producing individual thick

plastic sheets or moving stainless steel belts for continuous casting of thinner sheets.

• Rotational molding is a special casting process in which molten polymer is poured into a mold rotating about two axes.

COMPRESSION MOLDING• placing the solid material before cross-linking into a heated

die • Application of high pressure and temperature causes the

polymer to melt, fill the die, and immediately begin to harden• Small electrical housings as well as fenders, hoods, and side

panels for automobiles can be produced by this process

TRANSFER MOLDING• A double chamber is used in the transfer molding of

thermosetting polymers. • The polymer is heated under pressure in one chamber.• After melting, the polymer is injected into the adjoining die

cavity. • This process permits some of the advantages of injection

molding to be used for thermosetting polymers

Polymer applicationClassification is based on end-use: plastics, elastomers, fibers,

coating, adhesive, foams & films. Particular polymers may be used in more than 1 applications.

1) PLASTICSmaterials that have structural rigidity under load & use for

general purpose applicationPolyethylene, polypropylene, PVC, polystyrene, epoxiesSome are very rigid, other are flexible exhibit elastic &

plastic deformation when stress, sometimes experiencing deformation before fracture

Linear/branched must be used below glass transition T (if amorphous) or below melting T (semicryatalline)

2) ElastomersTo increase tensile strength, abrasion, tear resistance &

stiffness additives (C black)Synthetic elastomers SBR (styrene butadiene copolymers),

reinforced with C back tyresCrosslink structure• Silicon elastomers:have a high degree of flexibility at low T (~90 C); stable at T as high as 250 CResistant to weathering & lubricant oil (used in automobile

engine compartments)Biocompatible (blood tubing)

• ELASTOMERS (rubber)

3) Fibers• long filament (L/D)=100:1 • Mostly used in textile industry; woven or knit• Subjected to stretching, twisting, shearing & abrasion • high tensile strength, high modulus elastisity & abrasion

resistancehigh MW to be a strong material; will not break during drawing• linear structure with regular order • convenience in washing thermal properties• stable in various environments: acids, base, bleaches, dry

cleaning solvent & sunlight• relative nonflammable and amenable to drying

Miscellaneous ApplicationsCOATINGFunction:1) Protect from corrosive/deteriorative

reaction2) Improve appearance3) Provide electrical insulation Organic coaating:paint, varnish, enamel Many coating are latex (stable

suspension of small insoluble polymer particle disperse in water); less organic solvent, less VOC emission smog

• substances used to bond 2 surfaces of solid material (adherends)

• 2 types of bond: mechanical & chemical

*Mechanical actual penetration of adhhesive into surface pores & crevices

*Chemical intermolecular forces between adhesive & adherend

• Natural: casein, starch• Synthetic: polyurethanes,

polysiloxanes, epoxies acrylicFactors to choose adhesive:1. Materials & porosities to be bond2. Requires adhesive properties3. T exposure environment4. Processing conditions

ADHESIVES

• Adhesive advantage over other joining technologies: 1. join dissimilar materials & thin components2. lighter weight3. better fatigue resistance4. lower manufacturing cost

• DrawbacksT limitation; maintain mechanical integrity only at low T, strength decreases with increase in T

• Used in aerospace, automotive, construction etc

FILMS• Thin layer, 0.025-0.125 mm thickness• Packaging, tape• Characteristic: low density, high flexibility, high tensile

& tear strength, resistance to moisture & chemical, low -permeability to some gases (water vapor)

• polytethylene, polypropylene, cellulose acetate

FOAMS• plastic material with high vol of small pores & trapped

gas bubble• Thermoplastic & thermosetting: polyurethane, rubber,

polystyrene• Cushion, thermal insulation• Bubbles are generated by dissolving an inert into

molten polymer (high P); when P reduce rapidly, the gas comes out of solution & forms bubbles & pores that remain in the solid as it cools