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Material Science:The thermal and optical

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Outline:

The characteristic of thermal

The characteristic of Optical

Fakulti Kejuruteraan dan AlamBina 2

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3

How do materials respond to the application of heat?

How do we define and measure... -- heat capacity? -- thermal expansion? -- thermal conductivity? -- thermal shoc resistance?

How do the thermal properties of ceramics! metals!and polymers differ?

Thermal Properties

" #allister et al.! #hapter$%! pa&e '()

*SS+,S TO /,SS...

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4

0uantitatively: The ener&y re1uired to produce a unit rise intemperature for one mole of a material.

heat capacity234mol-56

ener&y input 234mol6

temperature chan&e 256

Heat Capacit

Two ways to measure heat capacity:C p : Heat capacity at constant pressure.

C v : Heat capacity at constant volume.C p usually 7 C v

Fmoll!Btu

Kmol

"

dT dQ

C

The a8ility of a material to a8sor8 heat

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#

Heat capacity...

-- increases with temperature -- for solids it reaches a limitin& value of 9 R

rom atomic perspective: -- ,ner&y is stored as atomic vi8rations. -- s temperature increases! the avera&e ener&y of

atomic vi8rations increases.

$ependence o% Heat Capacit onTemperature

Adapted %rom Fi&' ()'2*Callister & Rethwisch 8e '

R + &as constant 3R + ,'3( "-mol.K

C v + constant

$e! e temperature/usuall less than T room 0

T /K0

D;

;

C v

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1

Atomic i!rationstomic vi8rations are in the form of lattice waves

or phonons

dapted from i&. $%.$! Callister & Rethwisch 8e .

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i n c r e a s

i n &

c p lass

Metalsluminum

SteelTun&sten>old

$%)$( ;$$';$; ;

%;;@(A$9($)(

c p 2specific heat6: 234 &-56

Material

%@;''(@;

peci%ic Heat: Comparison

C p /heat capacit 0: /"-mol.K0

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,

Thermal 56pansion

Materials chan&e siBe when temperature ischan&ed

l %inal l initial

l initial l /T %inal T initial0

linear coefficient of thermal expansion 2$45 or $4C#6

T initial

T %inal

initial

%inal

T %inal

> T initial

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)

Atomic Perspecti7e: Thermal 56pansion

Adapted %rom Fi&' ()'3* Callister & Rethwisch 8e '

symmetric curve: -- increase temperature!-- increase in interatomic

separation-- thermal expansion

Symmetric curve: -- increase temperature!-- no increase in interatomic

separation-- no thermal expansion

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(8

Coe%%icient o% Thermal 56pansion :Comparison

0:

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((

Thermal 56pansion: 56ample

,x: copper wire $ m lon& is cooled from@; to -%C#. How much chan&e in len&th willit experience?

(1'# 6 (8 1 / o C0 (9 nswer: or #u

mm12m012.0

)]C9(C40[)m15)](C/1(10x5.16[6

0

T

/earran&in& ,1uation $%.98

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(2

The a8ility of a material to transport heat.

temperature&radient

thermal conductivity 234m-5-s6heat flux234m) -s6

tomic perspective : tomic vi8rations and freeelectrons in

hotter re&ions transport ener&y to cooler re&ions.

T 2 T 2 T ( T (

x ( x 2heat flux

Thermal Conducti7it

dx

dT k q

ourierDsEaw

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(3

Thermal Conducti7it :Comparison

i n c r e a s

i n &

k

=olymers=olypropylene ;.$)=olyethylene ;.@A-;. ;=olystyrene ;.$9Teflon ;.)

vi8ration4rotationof chainmolecules

#eramicsMa&nesia 2M&O6 9(

lumina 2 l ) O 96 9%Soda-lime &lass $.'Silica 2cryst. SiO ) 6 $.@

atomic vi8rations

Metalsluminum )@'

Steel )Tun&sten $'(>old 9$

atomic vi8rationsand motion of freeelectrons

k 2

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(4

Occur due to: -- restrained thermal expansion4contraction -- temperature &radients that lead todifferential

dimensional chan&es

Thermal tresses

E /T 8 T f 0 E T Thermal stress

Modulusof

elasticityEinear coefficient of thermalexpansion from ta8le $%.$

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56ample:

Fakulti Kejuruteraan dan AlamBina

8rass rod is to 8e used in an application

re1uirin& its ends to 8e held ri&id. *f the rod isstress free at room temperature F ); o# 2)%9 56G!what is the maximum temperature to which therod may 8e heated without exceedin& acompressive stress of $') M=a? ssume amodulus of elasticity of $;; >=a for 8rass

Modulusof

elasticity

Einear coefficient of thermalexpansion from ta8le $%.$! is); x $; -A 2o#6 -$

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-- 8rass rod is stress-free at room temperature 2);C#6.

-- *t is heated up! 8ut prevented from len&thenin&. -- t what temperature does the stress reach -$') M=a?

56ample Pro!lem

T 8

8

Solution:Ori&inal conditions

roomthermal /T f T 8 0

T f

Step $: ssume unconstrained thermal expansion

8

Step ): #ompress specimen 8ac to ori&inal len&th

8

compress

roomthermal

(1

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(

56ample Pro!lem /cont'0 8

The thermal stress can !e directlcalculated as

E / compress 0

E / thermal 0 E /T f T 8 0 E /T 8 T % 0

;otin& that compress + . thermal and su!stitutin& &i7es

28 6 (8 .1 -

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(,

Occurs due to: nonuniform heatin&4coolin&

,x: ssume top thin layer is rapidly cooled from T 1 to T 2

Tension develops at surface

E /T ( T 2 0

#ritical temperature differencefor fracture 2set s s f 6

/T ( T 2 0%racture

f E

set e ual

9 ar&e TSR =hen is lar&e

f k

E

Thermal hock ?esistance

Temperature difference thatcan 8e produced 8y coolin&:

k T T

rate uench0/ 2(

rapid 1uench

resists contraction

tries to contract durin& coolin& T 2

T 1

/Auench rate0 %or %racture Thermal hock ?esistance / TSR 0

f k E

9

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The thermal properties o% materials include: 9 Heat capacit : .. ener& re uired to increase a mole o% material ! a unit T .. ener& is stored as atomic 7i!rations9 Coe%%icient o% thermal e6pansion :

.. the si e o% a material chan&es =ith a chan&e in temperatur .. pol mers ha7e the lar&est 7alues9 Thermal conducti7it : .. the a!ilit o% a material to transport heat .. metals ha7e the lar&est 7alues9 Thermal shock resistance : .. the a!ilit o% a material to !e rapidl cooled and not %racture .. is proportional to

()

ummar

f k E

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*SS+,S TO /,SS...

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2(

Optical PropertiesEi&ht has 8oth particulate and waveli e characteristics

=hoton - a 1uantum unit of li&ht

E h

hc

m-s0(86/3'887acuumainli&hto% speed

0s"(8612'1/constantsPlanckD

radiationo% %reAuencradiationo% =a7elen&th

photonao% ener&

,

34

c

h

E

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?e%raction

medium0inli&hto% /7elocit7acuum0inli&hto% /7elocit

v c

Transmitted li&ht distorts electron clouds.

The velocity of li&ht in a material is lower than in avacuum.

n index ofrefraction

Eno

transmittedli&ht

transmittedli&ht E

electronclouddistorts

.. Addin& lar&e ions /e'&'* lead 0 to &lass decreases the speed o% li&ht in the &lass'.. i&ht can !e !entG as it passes throu&h a

transparent prism

elected 7alues %rom Ta!le 2('(*Callister & Rethwisch 8e '

T pical &lasses ca' ('# .('Plastics ('3 .('1P!O / ithar&e0 2'1$iamond 2'4(

Material

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Total nternal ?e%lectance

(

2

sin 2sin (

2

(

2 I 1

(

c

2

9 i8er optic ca8les are clad in low n material so that li&htwill experience total internal reflectance and not escapefrom the optical fi8er.

( + incident an&le

2 + re%racted an&le

c + critical an&le

c e6ists =hen 2 + )8J

For ( c li&ht is internall re%lected

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56ample: $iamond inair 9

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2#

*ncident li&ht is reflected! a8sor8ed! scattered! and4or

transmitted: I 8 I T I ! I R I S

i&ht nteractions =ith olids

Optical classification of materials:

Adapted %rom Fi&' 2('(8*Callister "e ' /Fi&' 2('(8 is !

"' Tel%ord* =ith specimenpreparation ! P'A' essin&'0

sin&lecrystal

polycrystalline dense

polycrystalline porous

TransparentTranslucent

Opa1ue

ncident: I 8

A!sor!ed: I !Transmitted: I T

cattered: I S

?e%lected : I R

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istin&uish 8etween materials that areopa1ue! translucent! and transparent interms of their appearance and li&httransmittance

#onceptchec :

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2

8sorption of photons 8y electron transitions:

+nfilled electron states are adIacent to filled states Jear-surface electrons a8sor8 visi8le li&ht.

Adapted %rom Fi&' 2('4/a0*Callister & Rethwisch 8e '

Optical Properties o% @etals: A!sorption

5ner& o% electron

H n c i d e n t p h

o t o n

PlanckDs constant

/1'13 6 (8.34

"-s0

%re 'o%incidentli&ht

%illed states

un%illed states

E + h re uired

o % e n e r

& h

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2,

i&ht A!sorption

e8 I I T

The amount of li&ht a8sor8ed 8y a material is calculatedusin& KeerDs Eaw

+ a!sorption coe%%icient* cm .( + sample thickness* cm + incident li&ht intensit

+ transmitted li&ht intensit

8 I

T I

ln8 I

I T

/earran&in& and ta in& the natural lo& of 8oth sides ofthe e1uation leads to

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56ample 2('(


The fraction of nonreflected li&ht that istransmitted throu&h a );;-mm thic ness of &lassis ;.%(. calculate the a8sorption coefficient ofthis material

e8 I I T

ln8

I

I T

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Adapted %rom Fi&' 2('4/!0*Callister & Rethwisch 8e '

?e%lection o% i&ht %or @etals

,lectron transition from an excited stateproduces a photon.

photon emitted%rom metalsur%ace

5ner& o% electron

%illed states

un%illed states

5lectron transition

I R conductin&G electron

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3(

/eflectivity + # R -# 8 is !et=een 8')8 and 8')#'@etal sur%aces appear shin@ost o% a!sor!ed li&ht is re%lected at the same=a7elen&th

mall %raction o% li&ht ma !e a!sor!edColor o% re%lected li&ht depends on =a7elen&thdistri!ution

56ample: The metals copper and &old a!sor!li&ht in 8lue and &reen + re%lected li&ht has&old color

?e%lection o% i&ht %or @etals/cotd0

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?e%lection


I 0 intensities of incident8eamsI R *ntensities of reflected8eams

n $ and n ) are the indices ofrefraction of the two media

*f the li&ht is normal2or perpendicular6 to theinterface! then:

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?e%lecti7it o% ;onmetalsor normal incidence and li&ht passin& into a

solid havin& an index of refraction n :

R re%lecti7it ((

2

(3'8(4('2(4('2

2

R

re%lectedisli&hto% L(

,xample: or iamond n ).@$

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Pro!lem:


The index of refraction of corundum 2 l ) O 96 isanisotropic. Suppose that visi8le li&ht ispassin& from one &rain to another of differentcrystallo&raphic orientation and at normal

incidence to the &rain 8oundary. #alculate thereflectivity at the 8oundary if the indices ofrefraction for the two &rains are $;.$) and (.)in the direction of li&ht propa&ation

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Transmission


=ro8lemLerive ,1uation

)$.$%! startin& fromother expressions&iven in the chapter

2e1. )$.$%6

*ncident 8eam !I o

/eflected8eam ! I R = I oR

Transmitted8eam

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catterin& o% i&ht in Pol mers

or hi&hly amorphous and pore-free polymersEittle or no scatterin&These materials are transparent

Semicrystalline polymers

ifferent indices of refraction for amorphous andcrystalline re&ionsScatterin& of li&ht at 8oundariesHi&hly crystalline polymers may 8e opa1ue

,xamples:=olystyrene 2amorphous6 clear and transparentEow-density polyethylene mil cartons opa1ue

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3

8sorption of li&ht of fre1uency 8y 8y electron transition

occurs ifh n 7 E &ap

*f E &ap N $.( e ! all li&ht a8sor8edP material is opa1ue 2e.&.! Si! >a s6 *f E &ap 7 9.$ e ! no li&ht a8sorptionP material is transparent and

colorless 2e.&.! diamond6

elected i&ht A!sorption inemiconductors

*f $.( e N E &ap N 9.$ e ! partial li&ht a8sorptionP material is colored

Adapted %rom Fi&' 2('#/a0*Callister & Rethwisch 8e '

8lue li&ht: h 9.$ ered li&ht: h $.( e

incident photonener&y h n

,ner&y of electron

filled states

unfilled states

E &ap

,xamples of photonener&ies:

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3,

>e /min0

hc

E $ />e0

/1'13 6 (8 34 " s0/3 6 (8 , m-s0

/8'1 e 0/('18 6 (8()

"-e 0

Computations o% @inimumMa7elen&th A!sor!ed

;ote: the presence o% donor and-or acceptor states allo=s %orli&ht a!sorption at other =a7elen&ths'

Solution:

2a6 e! for which E g ;.A' e ?

>e /min0 (',1 6 (8.1 m (',1 m

286 /edoin& this computation for Si which has a 8and&ap

of $.$ e i /min0 ('(3 m

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3)

uminescenceEuminescence reemission of li&ht 8y a material

Material a8sor8s li&ht at one fre1uency and reemits it atanother 2lower6 fre1uency.Trapped 2donor4acceptor6 states introduced 8yimpurities4 defects

acti7atorle7el

alence !and

Conduction !and

trappedstatesE $

E emissi%

9 *f residence time in trapped stateis relatively lon& 27 $; -( s6-- phosphorescence

9 or short residence times 2N $; -(

s6-- fluorescence

,xample: Toys that &low in thedar . #har&e toys 8y exposin&

them to li&ht. /eemission ofli&ht over timeQ

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Photoluminescence

rc 8etween electrodes excites electrons in mercury atoms in thelamp to hi&her ener&y levels.

s electron falls 8ac into their &round states! + li&ht is emittede.&.! suntan lamp

*nside surface of tu8e lined with material that a8sor8s + andreemits visi8le li&ht

,.&.! #a $; ) = AO )@ with );R of - replaced 8y #l -

dIust color 8y dopin& with metal cations S8 9 8lue

Mn ) oran&e-red

H& atom

N li&ht

electrode electrode

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4(

Cathodoluminescence+sed in cathode-ray tu8e devices 2e.&.! T s! computer monitors6

*nside of tu8e is coated with a phosphor material=hosphor material 8om8arded with electrons,lectrons in phosphor atoms excited to hi&her state=hoton 2visi8le li&ht6 emitted as electrons drop 8ac into&round states#olor of emitted li&ht 2i.e.! photon wavelen&th6 depends oncomposition of phosphor

nS 2 & U #l -6 8lue2 n! #d6 S 2#u l 9 6 &reen

V ) O ) S 9R ,u red

Jote: li&ht emitted is random in phase U directioni.e.! is noncoherent

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The A 5?

The laser &enerates li&ht waves that are in phase2coherent6 and that travel parallel to one another

E S,/Ei&ht

mplification 8yS timulated, mission of / adiation

Operation of laser involves a population inversion ofener&y states process

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Operation o% the ?u! aser WpumpX electrons in the lasin& material toexcited states

e.&.! 8y flash lamp 2incoherent li&ht6.

irect electron decay transitions Q produce incoheren

Fi&' 2('(3* Callister & Rethwisch 8e '

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emiconductor aser Applications

#ompact dis 2# 6 player +se red li&ht

Hi&h resolution players+se 8lue li&htKlue li&ht is a shorter wavelen&th than red li&htso it produces hi&her stora&e density

#ommunications usin& optical fi8ers

i8ers often tuned to a specific fre1uencyKan s of semiconductor lasers are used as flashlamps to pump other lasers

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Other Applications . olar Cells p-n Iunction: Operation:

-- incident photon of li&ht produces elec.-holepair. -- typical potential of ;. produced across

Iunction -- current increases w4li&ht intensity.

Solar powered weather station:

polycrystalline Sios Alamos Hi&h chool =eather

station /photo courtesP'@' Anderson0

n -type Si

p -type Si p-n Iunction

K-doped Si

i

i

i iB

hole

P

i

i

i i

conductanceelectron

=-doped Si

.t pe i

p .t pe i p junction

li&ht

E.

EE E

...

creation o%hole.electronpair

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4

Optical Fi!ers /cont'0

fi8ers have diameters of $) m or lessplastic claddin& A; m thic is applied to fi8ers

Fi&' 2('28* Callister &Rethwisch 8e '

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Ei&ht radiation impin&in& on a material may 8e reflected

from! a8sor8ed within! and4or transmitted throu&h Ei&ht transmission characteristics: -- transparent ! translucent ! opa1ue Optical properties of metals: -- opa1ue and hi&hly reflective due to electron ener&y 8and

structure. Optical properties of non-Metals: -- for E gap N $.( e ! a8sorption of all wavelen&ths of li&htradiation

-- for E gap 7 9.$ e ! no a8sorption of visi8le li&ht radiation -- for $.( e N E gap N 9.$ e ! a8sorption of some ran&e ofli&ht

radiation wavelen&ths -- color determined 8y wavelen&th distri8ution of

N@@A?

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