Transcript PowerPoint 프레젠테이션

5장 Dielectrics and Insulators
Preface
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‘Ceramic dielectrics and insulators’ is a wideranging and complex topic embracing many types of
ceramic, physical and chemical processes and
applications.
Part I : important ideas relating to their performance
and the wider application of dielectrics and
insulators (capacitors).
Part II : important ceramic types and their
applications.
Capacitative Applications
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5.1 Background
5.2 Dielectric strength
5.2.1 Test conditions
5.2.2 Breakdown mechanisms
(a) Intrinsic breakdown
(b) Thermal breakdown
(c) Discharge breakdown
(d) Long-term effects
Background
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Dielectrics and insulators can be defined as materials with high
electrical resistivities.
Dielectrics’s parameters :  (permittivities) and tan  (dissipation
factors)
Power engineer focus on the loss factor (  ''   ' tan  )
Electronics engineer focus on the dissipation factors ( tan  )
-> Electrical resonance phenomenon
Insulators are used principally to hold conductive elements in
position and to prevent them from coming in contact with one
another -> substrates on circuits.
Ideal insulators :  r =1, tan  =0
Background
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A very large number of types have been developed to meet
particular demands
-> The increase in line voltages as power transmission networks
-> The move towards higher frequencies as
telecommunications systems
The power dissipated in an insulator or a dielectric is
proportional to frequency
Low-loss dielectrics for high-frequency because excessive
power dissipation can lead to unacceptable rises in temperature
and the resonances in tuned circuits become less sharp so that
the precise selection of well-defined frequency bands is not
possible.
Dielectric Strength
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Dielectric strength : the electric field sufficient to initiate
breakdown of the dielectric.
It depends on material homogeneity, specimen geometry,
electrode shape and disposition, stress mode and ambient
conditions at intrinsic breakdown
Thermal breakdown is the most significant mode of failure and is
avoided through experience rather than by application of theory.
Discharge breakdown is important in ceramics because it has its
origins in porosity.
Test Conditions
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Electric strength data are meaningful only if the
test conditions are adequately defined.
DC loading : rate of voltage increase
Pulsed voltage : rise time
AC loading : frequency and waveform
Test Conditions
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Importance of sample geometry
(a) large volume of the specimen is stressed, failure is
likely to be initiated from the electrode edges where the
average electrical stress is magnified by a significant
(b) failure would probably occur at the centre where the
stress is a maximum and known
1. Intrinsic Breakdown
Increasing voltage under well-controlled laboratory conditions.
-> Small current begins to flow which increases to a saturation
value.
-> The voltage is further increased a stage is reached when the
current suddenly rises steeply from the saturation value in a
time.
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When the field is applied the small number of electrons in
thermal equilibrium in the conduction band gain kinetic energy.
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This energy may be sufficient to ionize constituent ions, thus
increasing the number of electrons participating in the process.
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The result may be an electron avalanche and complete failure.
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2. Thermal Breakdown
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Thermal breakdown process can be described in terms of the thermal
properties of the dielectric
The finite DC conductivity of a good dielectric results in Joule
heating; under Ac fields there is additional energy dissipation ->
Rising temperature leads to an increase in conductivity and to
dielectric loss.
Comprehensive theory of thermal breakdown but solution to the
governing differential equation can be found only for the simplest of
geometries.
U b : breakdown voltage
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 d: temperature coefficient of loss factor
1/ 2
Ub  (
 d  tan 
'
) 
 : function of specimen thickness and
heat transfer to the environment
2. Thermal Breakdown
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Ambient temperature is
reached above which
thermal breakdown
caused by joule heating
arising from the
exponentially increasing
ionic conduction in the
glassy phase is dominant
3. Discharge Breakdown
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A ceramics is rarely
homogeneous; A common
inhomogeneity is porosity.
Breakdown can be
initiated at pores and the
occurrence of gas
discharges within pores is
an important factor.
3. Discharge Breakdown
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Disk-shaped cavity with its plane normal to the applied field E,
the field Ec within the cavity
Ec 
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r
E
 rc
 rc Is the relative permittivity of the cavity gas and has a value
close to unity,
 r is the relative permittivity of the dielectric.
3
For a spherical pore, Ec  E ( r  rc )
2
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When the voltage applied to a porous dielectric is increased a
value is reached when a discharge occurs in a particular pore.
3. Discharge Breakdown
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The larger the pore is the
more likely it is to lead to
breakdown.
Under AC conditions
breakdown is more likely
than in the case of applied
DC. -> AC breakdown
voltages are lower than
those for DC
Plot of density against
electric breakdown in fig.
5.4
4. Long-term effects
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In some materials the prolonged application of electric stress at a
level well below that causing breakdown in the normal rapid tests
results in an deterioration in resistivity that may lead to breakdown.
Among the possibilities are the effect of the weather and
atmospheric pollution on the properties to the exposed surfaces of
components. They will become roughened and will absorb
increasing amounts of moisture and conductive impurities.
Local high temperatures and the sputtering of metallic impurities
from attached conductors -> Surface discharge
Dc stress both on the surface and in the bulk of materials. It may
cause silver to migrate over surfaces and along grain boundaries,
thus lowering resistance.