材料物理性能热学

Prof.

Xiaoqin

Yanxqyan@2012.08ContentsOptical

PropertiesElectrical

PropertiesThermal

PropertiesMagnetic

PropertiesOther

Properties

of

Materials

(For

Self-Study)Electrical

Properties：Thermoelectric,Pyroelectric

and

Magnetoelectric

Property(热电、热释电、磁电性能)Magnetic

Properties：Magnetostriction

and

Magnetoresistance(磁致伸缩、磁电阻)Optical

Properties:Electrooptic,Photoelectric

and

Magnetooptic

Property(电光、光电、磁光性能)Acoustic

Properties:Propagation,Absorption

and

Electroacoustic

Property(声音的传播、吸收、电声性能)Elasticity:Anelasticity

and

Internal

Friction(滞弹性与内耗)3.

Thermal

PropertiesHEAT

CAPACITY

Heat

capacity(热容)C:

ratio

of

energy

change(energy

gained

or

lostthe

resulting

temperature

change.C

=

dQ/dT

(3.1)Specific

heat(比热)c:

the

heat

capacity

per

unit

mass.

Cv:

maintaining

the

specimen

volume

constant.

Cp:

constant

externalpressure.Vibrational

Heat

Capacity

The

vibrations

may

be

thought

of

as

elasticwaves

or

sound

waves,

having

shortwavelengths

and

very

high

frequencies,which

propagate

through

the

crystal

at

thevelocity

of

sound.Only

certain

energy

values

are

allowed

(to

bequantized),

and

a

single

quantum

ofvibrational

energy

is

called

a

phonon(声子Lat)t.ice

waves

in

a

crystal

bymeans

of

atomic

vibrationsThe

temperature

dependence

of

theheat

capacity

at

constant

volumeTemperature

Dependence

of

the

Heat

Capacity

Dependence

of

heat

capacity

(at

constant

volume)

on

temperature,

at

ltemperatures

(near

0

K):Cv

=

AT3

(3.2)

Above

the

Debye

temperature(德拜温度,θD):Cv

≈3R

(R

is

the

gasconstant).Other

Heat

Capacity

ContributionsElectrons

absorb

energy

by

increasingtheir

kinetic

(for

example,

freeelectrons

excited

from

filled

states

tempty

states

above

the

Fermi

energy).

Energy-absorptive

processes

occur

atspecific

temperatures

(for

example,

trandomization

of

electron

spins

in

aferromagnetic

material

as

it

is

heatedthrough

its

Curie

temperature).

In

most

instances,

these

are

minorrelative

to

the

vibrational

contribut3.2

THERMAL

EXPANSIONMost

solid

materials

expand

upon

heating

and

contract

when

cooled.(lf

-

l0)

/

l0

=

αl

(Tf

-

T0)

or

Δl

/

l0

=

αl

ΔT(3.3)αl

is

the

linear

coefficient

of

thermal

expansion.ΔV

/

V0

=

αvΔT(3.4)αvsymbolizes

the

volume

coefficient

of

thermal

expansion.

From

an

atomic

perspective,

thermal

expansion

is

reflected

by

an

increaverage

distance

between

the

atoms.

Thermal

expansion

is

really

due

tothe

asymmetric

curvature

of

this

poenergy

trough,

rather

than

the

increased

atomic

vibrational

amplituderising

temperature.Potential

energy

versus

interatdistance,

demonstrating

the

increainteratomic

separation

with

risingtemperature.

(b)

For

a

symmetricpotential

-

energy

versus

-

interatdistance

curve,

there

is

no

increasinteratomic

separation

with

risingtemperature

For

each

class

of

materials

(metals,

ceramics,

and

polymers),

the

greaatomic

bonding

energy,

the

deeper

and

more

narrow

this

potential

energtrough.

The

increase

in

interatomic

separation

with

a

given

rise

in

temwill

be

lower,

yielding

a

smaller

value

of

αl.

Metals:

linear

coefficients

of

thermal

expansion

for

the

common

metalbetween

about

5×10-6

and

25×10-6;

these

values

are

intermediate

betwefor

ceramic

and

polymeric

materials.

Ceramics:

comparatively

low

coefficients

of

thermal

expansion;

valutypically

range

between

about

0.5×10-6

and

15×10-6

.

Polymeric

materials:

very

large

thermal

expansions

upon

heating,

asby

coefficients

that

range

from

approximately

50×10-6

to

400×10-6.αl

is

isotropic

or

anisotropic.3.3

THERMAL

CONDUCTIVITY

Thermal

conduction

is

the

phenomenon

by

which

heat

is

transported

frohigh-

to

low-temperature

regions

of

a

substance.q

=

-

k

dT/dx

（3.5）q

denotes

the

heat

flux,or

heat

flow,per

unit

time

per

unit

area,

thermal

conductivity(热导率),and

dT/dx

is

the

temperature

gradient

Mechanisms

of

Heat

Conduction:

Heat

is

transported

in

solid

materialboth

lattice

vibration

waves

(phonons)

and

free

electrons,

usually

onother

predominates.k

=

kl

+

ke

(3.6)Metals:

In

high-purity

metals,

the

electron

mechanism

is

much

more

efficientbecause

electrons

are

not

as

easily

scattered

as

phonons

and

have

highvelocities.

Metals

are

extremely

good

conductors

of

heat

because

relatively

largnumbers

of

free

electrons

participating

in

thermal

conduction.

The

thconductivities

generally

range

between

about

20

and

400

W/m

K.

Free

electrons

are

responsible

for

both

electrical

and

thermal

condupure

metals,

so

Wiedemann–Franz

law:L

=

k

/

(σT)(3.7)σ

is

the

electrical

conductivity,

T

is

the

absolute

temperature,

an

constant.

The

theoretical

value

of

L,

2.44×10-8

Ω

W/(K)2.

Alloying

metals

with

impurities

results

in

a

reduction

in

the

thermal

conductivity,

for

the

same

reason

that

the

electrical

conductivity

is

diminished.Thermal

conductivity

versuscomposition

for

copper–zinc

alloysCeramics:

Nonmetallic

materials

are

thermal

insulators

inasmuch

as

they

lack

lanumbers

of

free

electrons.

Thus

the

phonons

are

primarily

responsiblethermal

conduction.

Room-temperature

thermal

conductivities

rangebetween

approximately

2

and

50

W/m

K.

The

phonons

are

not

as

effective

asfree

electrons

in

the

transport

ofheat

energy

as

a

result

of

the

veryefficient

phonon

scattering

bylattice

imperfections.

Glass

and

other

amorphous

ceramics

have

lower

conductivitiesthan

crystalline

ceramics,

becausethe

phonon

scattering

is

much

moreeffective

when

the

atomic

structureis

highly

disordered

and

irregular.Thermal

conductivity

on

temperaturefor

several

ceramic

materials

The

scattering

of

lattice

vibrations

becomes

more

pronounced

with

ristemperature;

hence,

the

thermal

conductivity

of

most

ceramic

materialnormally

diminishes

with

increasing

temperature.

The

conductivity

begins

to

increase

at

higher

temperatures,

which

isradiant

heat

transfer.

Porosity

in

ceramic

materials

may

have

a

dramatic

influence

on

therma

conductivity;

increasing

the

pore

volume

will,

under

most

circumstanc

result

in

a

reduction

of

the

thermal

conductivity.Polymers:Thermal

conductivities

for

most

polymers

are

on

the

order

of

0.3

W/m

K

Energy

transfer

is

accomplished

by

the

vibration

and

rotation

of

the

cmolecules.

The

thermal

conductivity

depends

on

the

degree

of

crystallinity;

a

powith

a

highly

crystalline

and

ordered

structure

will

have

a

greaterconductivity

than

the

equivalent

amorphous

material.

Polymers

are

often

used

as

thermal

insulators,

and

their

insulative

pmay

be

further

enhanced

by

the

introduction

of

small

pores.3.4

THERMAL

STRESSES

Thermal

stresses

are

stresses

induced

in

a

body

as

a

result

of

changes

itemperature.Stresses

Resultingfrom

Restrained

Thermal

Expansion

and

Contract

Dependence

of

thermal

stress

on

elastic

modulus

(E),

linear

coefficienthermal

expansion

(αl

),

and

temperature

change:σ

=

Eαl

(T0

-

Tf)

=

Eαl

ΔT(3.8)Stresses

Resulting

from

Temperature

Gradients

Temperature

gradients

are

caused

by

rapid

heating

or

cooling,

in

that

tchanges

temperature

more

rapidly

than

the

interior;

differential

dimenchanges

restrain

the

free

expansion

or

contraction

of

adjacent

volume

ewithin

the

piece.Thermal

Shock