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Prof.
Xiaoqin
Yanxqyan@2012.08ContentsOptical
PropertiesElectrical
PropertiesThermal
PropertiesMagnetic
PropertiesOther
Properties
of
Materials
(For
Self-Study)Electrical
Properties:Thermoelectric,Pyroelectric
and
Magnetoelectric
Property(熱電、熱釋電、磁電性能)Magnetic
Properties:Magnetostriction
and
Magnetoresistance(磁致伸縮、磁電阻)Optical
Properties:Electrooptic,Photoelectric
and
Magnetooptic
Property(電光、光電、磁光性能)Acoustic
Properties:Propagation,Absorption
and
Electroacoustic
Property(聲音的傳播、吸收、電聲性能)Elasticity:Anelasticity
and
Internal
Friction(滯彈性與內(nèi)耗)3.
Thermal
PropertiesHEAT
CAPACITY
Heat
capacity(熱容)C:
ratio
of
energy
change(energy
gained
or
lostthe
resulting
temperature
change.C
=
dQ/dT
(3.1)Specific
heat(比熱)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(聲子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(德拜溫度,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(熱導(dǎo)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
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