ch-6 (Lect.-21)

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STRUCTURE AND PROPERTIES OF CERAMICS

How do ceramics differ from metals ?

Keramikos ~ burnt stuffHeat treatment is necessary

Usually a compound between a metal and a non-metalBonding displays a mixture of ionic and covalent

1

c arac er Generally hard and brittle, have high melting temperatureWhy ?

Generally thermally and electrically insulating Can be opaque, semi-transparent or transparent Traditional ceramics ~ based on clay (china, porcelain,

bricks, tiles) and glasses Hi-tech ceramics => electronic, communication, computer

hardware, aerospace industries

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Bonding:--Mostly ionic, some covalent.

--% ionic character increases with difference in

electronegativity. What is electronegativity ?

HeH CaF : lar e

Large vs small ionic bond character:

CERAMIC BONDING

2

-

Ne-

Ar-

Kr-

Xe-

Rn-

Cl3.0

Br2.8

I2.5

At2.2

Li1.0

Na0.9

K0.8

Rb0.8

Cs0.7

Fr0.7

2.1

Be1.5

Mg1.2

Sr1.0

Ba0.9

Ra0.9

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

C2.5

Si1.8

F4.0

Ca1.0

Table of Electronegativities

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 by

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Crystal Structure of Ionicly Bonded Ceramics

Crystal structure is defined by 2 criterions

1. Magnitude of the electrical charge on each ion. Chargebalance dictates chemical formula (Ca2+ and F- form CaF2).

2. Relative sizes of the cations and anions. Cations wantsmaximum ossible number of anion nearest nei hbors

and vice-versa.

Stable ceramic crystal structures require anions surroundinga cation to be all in contact with that cation.

For a specific coordination number there is a critical orminimum cation/anion radius ratio rC/rA for which thiscontact can be maintained. Pure geometrical consideration

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1. Charge Neutrality:

--Net charge in the

crystal structure

should be zero.

--General form:A X

CaF2:

Ca2+

cationF-

F-

anions+

IONIC BONDING & CRYSTAL STRUCTURE

3

m, p determined by charge neutrality

2. Maximize the # of nearest oppositely charged neighbors--stable structures:

Adapted from Fig. 12.1, Callister

6e.

- -

- -+

unstable

- -

- -+

stable

- -

- -+

stable

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Coordination # increases withrcationranion

rcationranion

Coord #

< .155

ZnS(zincblende)

2 Adapted from Fig. 12.4,

COORDINATION # AND IONIC RADII

4

.155-.225

.225-.414

.414-.732

.732-1.0

NaCl(sodium

chloride)

CsCl(cesiumchloride)

3

4

6

8

Adapted from Table

12.2, Callister 6e.

Adapted from Fig. 12.2,

Callister 6e.


Callister 6e.

.

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Home work

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On the basis of ionic radii, what crystal structure

would you predict for FeO?

Cation

Al3+

Fe2++

Ionic radius (nm)

0.053

0.077

Answer:

rcation

ranion=0.077

0.140

= 0.550based on this ratio,

EX1: PREDICTING STRUCTURE OF FeO

5

Ca2+

Anion

O2-

Cl-

F-

.

0.100

0.140

0.181

0.133

--coor =

--structure = NaCl (rocksalt)

Data from Table 12.3,

Callister 6e.

Two penetrating FCC units; otherexamples are MgO, MnS, LiF.

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Consider CaF2 :rcationranion

=0.100

0.133 0.8

Based on this ratio, coord # = 8 and structure = CsCl.

Result: CsCl structure w/only half the cation sites

occupied.

EX2: AmXp STRUCTURES

6

are occupied since#Ca2+ ions = 1/2 # F- ions.


Callister 6e.

Empty

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Ceramic Density Computations

n: number of formula units in unit cell (all ions that are includedin the chemical formula of the compound = formula unit)

AC: sum of atomic weights of cations in the formula unit

AA: sum of atomic weights of anions in the formula unit

VC: volume of the unit cell

NA: Avogadros number, 6.023 X 1023 (formula units)/mol

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EX4: NaCl density

a

n = 4 in FCC lattice

AC= ANa= 22.99 g/mol

AA= ACl= 35.45 g/mol

VC= a3=[2 (rNa + rCl)]3

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Silicate Ceramics

Composed mainly of silicon and oxygen, the twomost abundant elements in earths crust (rocks, soils,clays and sand- SiO2 silica)

Basic building block: SiO44- tetrahedron: Si-O bonding is largely covalent, but overall SiO4

block has charge of 4

Various silicate structures different ways toarrange SiO4

4- blocks

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EX: Crystalline form of SiO2Three polymorphs of SiO2 :

Quartz, Crystobalite, Tridymite

Not a very closed pack structure lowdensity ~ 2.65 g/cm3

3D networks of SiO44- tetrahedra

Each O atom is shared by an

adjacent tetrahedron

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Window Glass

Still SiO44- tetrahedra are the basic

building block.

Most common window glasses are

produced by adding other oxides (e.g.CaO, Na2O, B2O3, etc) whose cationsare incorporated within SiO4 network.

These cations break the tetrahedralne wor an g asses me a owertemperature than pure amorphousSiO2 .

A lower melting point makes it easy toform glass to make, for instance,

bottles. Some other oxides (TiO2, Al2O3)

substitute for silicon and become partof the network

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Carbon/Diamond/Fullerenes/ Nanotubes

http://www.nas.nasa.gov/Groups/SciTech/nano/

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Mechanical Properties of Ceramics

Ceramics are very brittle. (Fracture Toughness) For brittle materials fracture stress concentrators are very

important. (Chapter 8: measured fracture strengths aresignificantly smaller than theoretical predictions for perfect

materials due to the stress risers) Fracture strength of ceramic may be greatly enhanced by creating

compressive stresses in the surface region (similar to shotpeening, case hardening in metals, chapter 8)

strength. This makes ceramics good structural materialsunder compression (e.g., cement, bricks in buildingapartments, stone blocks in the pyramids).

Generally, tensile test is not used

Hard to machine, grippers may break the piece, fail after 0.1%strain.

Size is important due impact of # of cracks on strength, why ?

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Elevated Temperature Tensile Test (T > 0.4 Tmelt).

creep test

x

slope = ss = steady-state creep rate.

MEASURING ELEVATED T RESPONSE

11

Generally,

time

ssceramics

< ssmetals

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Ceramic materials have mostly covalent & some

ionic bonding. Structures are based on:

--charge neutrality--maximizing # of nearest oppositely charged neighbors.

Structures may be predicted based on:

--ratio of the cation and anion radii.

SUMMARY

12

Defects--must preserve charge neutrality

--have a concentration that varies exponentially w/T.

Room T mechanical response is elastic, but fracture

brittle, with negligible ductility. Elevated T creep properties are generally superior to

those of metals (and polymers).

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Nano Materials

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Definition

Nanotechnology is the understanding and control ofmatter at dimensions of roughly 1 to 100nanometers, where unique phenomena enable novelapplications.

Encompassing nanoscale science, engineering andtechnology, nanotechnology involves imaging,measuring, modeling, and manipulating matter at

this length scale.

National Nanotechnology Initiative, 2007

7

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Scale of ThingsNanometers

Figure 1.5: National Nanotechnology Initiative.

8

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Nanoscience and nanotechnology concerns

objects that are extremely small.

How small?

What is Nanotechnology?

Bigger than atoms, but smaller than you can

see with a light microscope.

1 100 nanometers

(3-4 atoms side by side = 1 nm)

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How Big is 1 Nanometer -- Length Scale?Soccer: 22cm

Carbon 60: 0.7nm

Pet Flea: 1mm Virus: 150nmHair: 80m Red Cell: 7m

.

DNA: 2nm

TiOx Particles: 13nm IBM Logo: 5nm

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One Nanometer

Water (H20)

DNA

Small

Protein This slide is adapted from the lecture notes p

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One Nanometer

Water (H20)

Quantum

Dot

Carbon

Nanotube

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Tiny machines in

What is Nanotechnology?

cancer?

http://smalley.rice.edu/emplibrary/SA285-

76.pdf

This slide is adapted from the lecture notes p

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This slide is adapted from the lecture no

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Making Small Objects

Self-assembly of nanoparticles,

25 um

Optical lithography

Electron beam lithographyTop-down approach

n v ua a oms, mo ecu es

Bottom-up approach

Chemical deposition

Bottom-up approach

IBM Research

This slide is adapted from the presentation on An Introductio

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Manipulating Small Objects

In 1989, Don Eigler arranged these xenon atoms, one by one, on a nickel surface to spell

out the name of his company. (using a Scanning Tunneling Microscope)


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Current Applications: Transportation

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Current Applications: TransportationPolyurethane nanocomposites for tires?

Fuel emissions: fuel efficiency may

increase by more than 3%

Road noise: reduced volume of air,

decreasing noise pollution

Weight: total weight of four tires +

insert. Less than five standard tires

(no spare needed)

(The first cars carried up to 6 spare tires)!

Lightweight nanoreinforced polyurethanes will provide the

next generation materials - towards an all PU tire?

(source Goodyear)

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Which one are actual nano-products?


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Which one are actual nano-products?


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Nano-products

Display ScreensMotorola (NTs) Nano SilverSeal

Refrigerator

Cars - HummerGM (Nanocomposites)

Nano-Products on the Market Now

Samsung nanopartic e-coate

Tennis RacketsWilson (C fibers)

This slide is adapted from the present

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Nano-products

Shemen Industriescanola oil by NutraLease, an

Israeli startup, using 30 nmcapsules

Nano-Care fabric

wrinkle-resistant, stain-repellent

Plenitude Revitalift

Loreal

(Eddie Bauer, Lee, Old Navy, TigerWoods, Bass, Nike)

Superhydrophobic nanoscalecoating applied to fabric

This slide is adapted from the present

Claynanocomposite

barrier coating

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16.8 GB

Nanodevices - Magnetic StorageThe hard drive in your computer uses a

nanotechnology innovation called giant

magnetoresistance.Giant magnetoresistance is an effect

where small magnetic fields can be

detected as a change in resistance.


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Nanomaterials - UV Protection

Advanced Powder

Small =

Transparent


ec no ogy ty t

90 nm

25 nm

250 nm

Zinc

Oxide

Zinclear

in

Wet Dreams

sunscreen

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Nanomaterials - CatalysisYour car has

nanoparticlesin it!

Gold nanoparticles can

turn carbon monoxide

into relatively

innocuous carbon

dioxide at temperatures

as low as -107F.


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ch-6 (Lect.-21)