Ceramics: An Introduction Comes from the Greek word-keramikos (burnt

stuff) which indicates their desirable properties are achieved through high temperature heat treatment

Compounds between metallic & nonmetallic elements i.e. oxides, nitrides & carbides

Can be classified into clay minerals, cement & glass

Typically insulative to the passage of electricity & heat, & more resistant to high temperatures & harsh environment.

They are hard but very brittle

Classification of Ceramics

Ceramic Bonding Mostly ionic, some covalent. % ionic character increases with difference in

electronegativity

Large vs small ionic bond character:

He -

Ne -

Ar -

Kr -

Xe -

Rn -

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

C 2.5Si 1.8

F 4.0

Ca 1.0

Table of Electronegativities

CaF2: large

SiC: small

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 byCornell University.

Two characteristics of the component ions in crystalline ceramic materials which influence the crystal structure:

The magnitude of the electrical charge on each of the component ions:

The crystal must be balanced by an equal number of anion –ve charges

The relative sizes of cations & anions This involves the sizes or ionic radii, rC & rA respectively

The ratio of rC/rA is less than unity due to cation size that is

small. This is caused by the metallic elements give up electrons when ionized

Stable ceramic crystal structures form when those anions surrounding a cation are all in contact with the cation

The coordination no. is related to rC/rA radius ratio

For a specific coordination no., there is a critical or min rC/rAratio for

which this cation-anion contact is established This ratio maybe determined from pure geometrical considerations The coordination numbers and nearest neighbor for various rC/rA

ratios are presented in the next table.

Example Problem 12.1 Show that the minimum cation-to-anion radius ratio for the

coordination number 3 is 0.155

Solution: The small cation is surrounded by 3 anions to form equilateral triangle.The centers of all four ions are coplanar

AP = rA & AO = rA + rC

Note: the side length ratio AP/AO = cos α

The magnitude of α is 30o, since line AO bisects the 60o angle BAC: AP/AO =rA/rA+rC=30o

=√3/2

The cation-anion radius ratio;

rA/rC=(1- √3/2) / √3/2 =0.155

AX-TYPE CRYSTAL STRUCTURESAX-TYPE CRYSTAL STRUCTURES Some ceramic materials have equal number of cations &

anions These are referred as AX compounds: A-cation & X-anion

Rock Salt StructureRock Salt Structure•A common example for AX crystal structure. Coordination no. is 6 rC/rA ratio between 0.414 & 0.732

•A unit cell is generated from an FCC (Face Centered Cubic) arrangement of anions with one cation situated at the cube center & one at the center of each of 12 cube edges

•An equivalent crystal structure results from a face-centered arrangement of cations

The rock salt crystal is thought of a interpenetrating FCC lattices.

One composed of the cations, the other of anions NaCl, MgO, MnS, LiF, FeO

Cesium Chloride (CsCl) Cesium Chloride (CsCl) StructureStructure•Coordination no. is 8 for both ion types

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

•The cube center is a single cation

•Interchange of anions with cations,vice versa, produce same structure

• Zinc Blende (ZnS) Zinc Blende (ZnS) StructureStructure

• Coordination no. is 4, all ions are tetrahedrally coordinated

• All corner and face positions of the cubic cell are occupied by S atoms

• The Zn atoms fill interior tetrahedral positions

• An equivalent structure results if Zn and S atom positions are reversed

• Most often the atomic bonding is highly covalent in compounds exhibiting this crystal structure

AAmmXXpp TYPE CRYSTAL STRUCTURE TYPE CRYSTAL STRUCTURECharges of cations & anions are not the same, the compound can exist with chemical formula AmXp, m and/or p ≠ 1.

Example:AX2, a common crystal structure found in CaF2

rC/rA is about 0.8 & coordination no. is 8

Ca2+ ions are positioned at the centers of cubes with F- ions at the corners

Half as many Ca2+ ions as F- ions

Only half the center cube positions are occupied by Ca2+ ions

*Note! One unit cell consists of eight cubes as in the figure!

AAmmBBnnXXpp-TYPE CRYSTAL STRUCTURE-TYPE CRYSTAL STRUCTURE

It is possible for ceramic compounds to have more than one type It is possible for ceramic compounds to have more than one type of cation as their chemical formula can be designated as Aof cation as their chemical formula can be designated as AmmBBnnXXpp

Example: Barium Titanate (BaTiOExample: Barium Titanate (BaTiO33), which have both Ba), which have both Ba2+2+ & Ti & Ti4+4+

cationscations Ba2+ ions are situated at all 8 Ba2+ ions are situated at all 8 corners of the cube & a single Ti4+ corners of the cube & a single Ti4+ is at the cube center, with O2- ions is at the cube center, with O2- ions located at the center of each of the located at the center of each of the 6 faces.6 faces.

Another name for this structure is Another name for this structure is perovskite crystal structureperovskite crystal structure

Crystal Structures From the Close Packing of Anions

•A number of ceramic crystal structures maybe considered in terms of closed-packed planes of ions, as well as unit cells

•Closed packed planes are composed of large anions

•These planes are stacked atop each other, small interstitial sites are created between them, cations may reside between them

• 4 atoms (3 in 1 plane,& a single one in the adjacent plane) surround one type, labeled T-tetrahedral position

• 6 join spheres, 3 in each of 2 planes, denoted as 0

• Because an octahedron is produced by joining these 6 sphere centers-octahedral position

• Coordination numbers for cations filling tetrahedral & octahedral are 4 & 6 respectively

Ceramic crystal structure depends on 2 factors: The stacking of the close-packed anion layers (both FCC & HCP

arrangements are possible) The manner in which the interstitial sites are filled with cation

Example:Example:

The unit cell has cubic symmetry & each The unit cell has cubic symmetry & each cation (Nacation (Na++) at the center has 6 Cl) at the center has 6 Cl-- ion ion nearest neighbor that reside at the centers of nearest neighbor that reside at the centers of each of the cube faceseach of the cube faces

The crystal structure having cubic symmetry The crystal structure having cubic symmetry is considered in an FCC array of close-is considered in an FCC array of close-packed planes of anions & all planes are packed planes of anions & all planes are {111} type{111} type

The cations reside in octahedral positions The cations reside in octahedral positions because they have as nearest neighbors six because they have as nearest neighbors six anionsanions

All octahedral positions are filled, since there All octahedral positions are filled, since there is a single octahedral site per anion and the is a single octahedral site per anion and the ratio of anions to cations is 1:1ratio of anions to cations is 1:1

Question:Question: On the basic of ionic radii, what crystal structure would you On the basic of ionic radii, what crystal structure would you predict for FeO?predict for FeO?

Solution:Solution: FeO is an AX-type compound. Determine cation-anion FeO is an AX-type compound. Determine cation-anion radius ratio (refer to table 3.4),radius ratio (refer to table 3.4),

rrFe2+Fe2+/r/rO2- O2-

= 0.077nm/0.140 nm= 0.077nm/0.140 nm

=0.550=0.550

The coordination no. for Fe2+ ion is 6; also the coordination no. for O2-The coordination no. for Fe2+ ion is 6; also the coordination no. for O2-

The predicted crystal structure will be rock salt, which is AX crsytal The predicted crystal structure will be rock salt, which is AX crsytal structure having a coordination no. as 6.structure having a coordination no. as 6.

Density Computations- CeramicsDensity Computations- Ceramics This is the alternative way to compute the This is the alternative way to compute the

theoretical density of a crystalline ceramic material.theoretical density of a crystalline ceramic material. The density, The density, ρρ is determined as follows: is determined as follows:

ρρ= n’ (= n’ (ΣΣAACC + + ΣΣAAAA))

VVCCNNAA

n’ = the number of formula units within the unit celln’ = the number of formula units within the unit cell

ΣΣAACC= the sum of atomic weights of all cations in the = the sum of atomic weights of all cations in the

formula unitformula unit

ΣΣAAAA= the sum of atomic weights of all anions in the = the sum of atomic weights of all anions in the

formula unitformula unit

VVCC= the unit cell volume= the unit cell volume

NNAA= Avogadro no., 6.023× 10= Avogadro no., 6.023× 102323 formula units/mol formula units/mol

Question: On the basis of crystal structure, compute the Question: On the basis of crystal structure, compute the theoretical density for sodium chloride. How does this compare theoretical density for sodium chloride. How does this compare with its measured density?with its measured density?

Solution: The theoretical density can be determined using:Solution: The theoretical density can be determined using:

ρρ= n’ (= n’ (ΣΣAACC + + ΣΣAAAA)) VVCCNNAA

Where Where

nn’, the no. of NaCl units per unit cell = 4, ’, the no. of NaCl units per unit cell = 4,

(both sodium & chloride ions form FCC lattices)(both sodium & chloride ions form FCC lattices)

ΣΣAACC=A=ANaNa = 22.99 g/mol = 22.99 g/mol

ΣΣAAAA=A=AClCl= 35.45 g/mol= 35.45 g/mol

VVCC= a= a33, & a=2r, & a=2rNa+Na+ + 2r + 2rCl- Cl- , r, rNa+Na+=0.102 nm & r=0.102 nm & rCl-Cl-=0.181 nm=0.181 nm

Thus,

VVcc= a= a33= (2r= (2rNa+Na+ + 2r + 2rCl-Cl-))33,,

Finally,

ρρ= n’ (A= n’ (ANaNa + A + AClCl))

(2r(2rNa+Na+ + 2r + 2rCl-Cl-))33NaNa

=4(22.99 + 35.49)=4(22.99 + 35.49)[2(0.102×10[2(0.102×10-7-7) + 2(0.181×10) + 2(0.181×10-7-7)])]3 3

(6.023×10(6.023×102323))

= 2.14 g/cm= 2.14 g/cm33

Silicate Ceramics: Introduction Composed primarily of silicon & oxygen It is more convenient to characterized these materials

in terms of various arrangement of SiO44-

4 oxygen atoms at tetrahedron corners, a silicon atom at the center. Usually treated as a –ve charged entity

Si-O bonds are covalently bond which are directional and relatively strong.

A silicon-oxygen tetrahedron

Silica/ Silicon Dioxide (SiO2) The most simple silicate material A three dimensional network. Generated when every

corner O atom is shared by adjacent tetrahedra. Electrically neutrally & all atoms have stable

electronic structures Three primary polymorphic crystalline forms: quartz,

cristobalite & tridymite Have a relatively complicated structure & the atoms

are not closely packed together This results in relatively low densities The melting point is high:1710o due to strong Si-O

interatomic bond

Silica Glasses Fused/vitreous silica-a noncrystalline solid/glass, high

degree of atomic randomness (character of liquid) SiO4

4- tetrahedron is the basic unit as with crystalline silica. Beyond this structure, considerable disorder exists

The common inorganic glasses that are used for containers, windows are silica glasses. Other oxides i.e. CaO & Na2O

These oxides don't form polyhedral networks Their cations are incorporated within & modify SiO4

4- network; these oxide additives- network modifiers

Intermediates i.e. oxides like TiO2 & Al2O3 are not network former, substitute for Si & become part & stabilize the network

Addition of modifiers & intermediates lowers melting point & viscosity of glass

The Silicates For various silicate minerals, the corner oxygen atoms

of the SiO44- tetrahedra are shared by other tetrahedra

to form complex structures (some represented below):

Positively charged cations i.e. Ca2+, Mg2+ & Al3+ serve to neutralize –ve charges from SiO4

4- units & bonding the SiO4

4-tetrahedra together

Simple Silicates Include the most structurally simple ones involve

isolated tetrahedra For ex.; forsterite (Mg2SiO4) has the equivalent of two

Mg2+ ions associated with each tetrahedron in such a way that every Mg2+ ion has 6 oxygen nearest neighbor

Si2O76- ion is formed when two tetrahedra share a

common oxygen atom Akermanite (Ca2MgSi2O7) is a mineral having the

equivalent of two Ca2+ ions & one Mg2+ ion bonded to each Si2O7

6-

Layered Silicates A 2D sheet or layered structure can be produced by

sharing 3 oxygen ions in each tetrahedra The repeating unit formula represented by (Si2O5)2-

The net negative charge is associated with the unbonded oxygen atoms projecting out of the plane of the page

Electroneutrality is ordinarily established by a 2nd planar sheet structure having an excess of cations, which bond to these unbonded oxygen atoms from the Si2O5 sheet

Such materials are called the sheet or layered silicates & their basic structure is characteristic of the clays & other minerals.

Kaolinite {Al2(Si2O5)(OH)4}as a relatively simple 2 layer silicate sheet structure. The silica tetrahedral layer represented by (Si2O5)2- is made neutral by Al2 (OH)4

2+

The bonding within this 2 layered sheet is strong & intermediate ionic-covalent. Adjacent sheets are loosely bound by weak Van der Waals forces

Carbon Diamond

Metastable carbon polymorph at room temperature & atmospheric pressure

Its crystal structure is a variant of zinc blende,carbon atoms occupy all positions

Each carbon bonds to 4 other carbons. The bond is totally covalent. This crystal structure-diamond cubic crystal structure

Physical properties of diamond: hardest known material, very low electrical conductivity attributed to its crystal structure & strong interatomic covalent bonds

Other properties: high thermal conductivity, optically transparent in the visible & infrared light, high index of refraction

Applications: gem stones, grinding/cutting softer materials in industry (mostly man-made)

Latest: diamond thin films has been produced

Graphite is another polymorph carbon; it has different crystal structure than diamond. More stable than diamond at ambient temperature & pressure

The structure is composed of hexagonal layers arranged carbon atoms

Within the layers, each carbon atom is bonded to 3 coplanar neighbor atoms by strong covalent bonds

The fourth bonding electron participates in a weak van der Waals type of bond between layers

Interplanar cleavage is facile, which gives rise to the excellent lubricative properties of graphite

The electrical conductivity is relatively high in crystallographic directions parallel the hexagonal sheets

Properties of graphite: High strength & good chemical stability at elevated temperatures

& non oxidizing atmospheres High thermal conductivity, low coefficient of thermal expansion &

high resistance to thermal shock High adsorption of gases & good machinability Application:

Heating element for electric furnace Electrodes for arc welding Casting molds for metal alloys and ceramics High temperature refractories and insulations Brushes, resistors

Fullerenes & Carbon Nanotubes Exist in discrete molecular form & consists of a hollow

spherical cluster of 60 carbon atoms (C60)

Each molecule is composed of carbon atoms that are bonded to one another to form both hexagon and pentagon geometrical configuration

The molecular surface exhibits symmetry of a soccer ball

Carbon atoms in C60 (buckminsterfullerene) bond together to form spherical molecules

In solid state, C60 unit form crystalline structure & packed together in a face centered cubic array

As a pure crystalline solid, the material is electrically insulating

Can be highly conductive and semi conductive if impurity is added

Carbon Nanotubes

•Its structure consists of a single sheet of graphite rolled into a tube, both ends are capped with fullerene hemisphere

•The tube diameters can be 100 nm or less

•Each nanotube is a single molecule composed of millions of atoms; the length of the molecule might be thousand times greater than its diameter

•Carbon nanotubes are extremely strong, stiff, relatively ductile, & have low densities. It may behave electrically as metal or semiconductor

IMPERFECTIONS IN CERAMICSIMPERFECTIONS IN CERAMICS Atomic Point DefectsAtomic Point Defects

The expression defect structuredefect structure is used to designate types & concentrations of atomic defects in ceramics

ElectroneutralityElectroneutrality is the state that exists when there are equal no. of +ve & -ve charges from ions

Frenkel defect Frenkel defect a defect which involves cation-vacancy and cation-interstitial pair.

This is formed by a cation leaving its normal position & moving into an interstitial site. There is no change in charge because the cation maintains the same positive charge as an interstitial.

Schottky defectSchottky defect found in AX materials is a cation vacancy-anion vacancy pair

This defect might be created by removing one cation & one anion from the interior of the crystal & then placing them both at an external surface

Since both cations & anions have the same charge, & since anion vacancy there exists a cation vacancy, the charge neutrality o the crystal is maintained.

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

StoichiometryStoichiometry- a state for ionic compounds where is the exact ratio of cations to anions as predicted by chemical formula.

Non stoichiometryNon stoichiometry exists in which two valence or ionic states exist for one of the ion types.

Impurities in CeramicsImpurities in Ceramics Impurity atoms can form solid solutions in ceramic

materials from both substitutional &interstitial types

Ceramics Application in Biomedical EngineeringCeramics Application in Biomedical Engineering Maybe divided into 3 classes according to their chemical

reactivity with the environment Nearly inert (alumina, carbons) Surface reactive (bioglass) Completely resorbable (hydroxyapatite)

Nearly inert ceramic show little chemical reactivity of long hours of exposure to physiological pH & show minimal interfacial bonds with living tissue. Fibrous capsule adjacent to implant is few cells thick

Surface reactive ceramic show intermediate behavior which bond the soft tissue and cell membrane, producing tissue adherence

Reactive material release ions from the surface & provide protein bond site over some time

Carbons Carbon coating-applications in heart valves,

blood vessel grafts, knee prosthesis

Knee prosthesis featuring diamond-like carbon coating From-www.azom.com/details.asp?ArticleID=2568

Two glassy polymeric carbon ( GPC ) heart valves. From-http://cim.aamu.edu/Activities/df.html

Alumina Applications-hip prostheses & dental implants

Dental implants From-http://www.bicon.com/tech/t_acc06.html

Alumina acetabular cup

From

http://www.wmt.com

Surface Reactive Ceramics- Bioglass Usually used as coatings on implant

SEM image of Bioglass 45S5 after incubation in SBF. Brunner al, 2006

Resorbable Ceramics Resorbable biomaterials commonly used are

hydroxyapatite & β-tricalcium phosphate Artificial bone & dental implants

Rootform implant

From-http://www.dentalinsurance.co.uk/implants/HAScrew.jpg/

Blade implant From-http://www.dentalinsurance.co.uk/implants/MandBlade.jpg