5 장 Dielectrics and Insulators

Preface ‘Ceramic dielectrics and insulators’ is a wide-

ranging 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 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 Dielectrics and insulators can be defined as materials with high electrica

l resistivities. Dielectrics’s parameters : (permittivities) and (dissipation facto

rs) Power engineer focus on the loss factor ( ) Electronics engineer focus on the dissipation factors ( ) -> Electrical resonance phenomenon Insulators are used principally to hold conductive elements in position a

nd to prevent them from coming in contact with one another -> substrates on circuits.

Ideal insulators : =1, =0

tan

tan''' tan

r tan

Background 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 syst

ems The power dissipated in an insulator or a dielectric is proportional to

frequency Low-loss dielectrics for high-frequency because excessive power diss

ipation 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 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

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 Importance of sample geometry (a) large volume of the specimen is stressed, failure is likely t

o 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.

When the field is applied the small number of electrons in thermal equilibrium in the conduction band gain kinetic energy.

This energy may be sufficient to ionize constituent ions, thus increasing the number of electrons participating in the process.

The result may be an electron avalanche and complete failure.

2. Thermal Breakdown 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.

: breakdown voltage : temperature coefficient of loss factor : function of specimen thickness and heat transfer to the environment

2/1' )tan

(d

bU bUd

2. Thermal Breakdown 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 A ceramics is rarely homoge

neous; 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 Disk-shaped cavity with its plane normal to the applied field E, the fie

ld within the cavity

Is the relative permittivity of the cavity gas and has a value close to unity, is the relative permittivity of the dielectric.

For a spherical pore,

When the voltage applied to a porous dielectric is increased a value is reached when a discharge occurs in a particular pore.

cE

EErc

rc

rcr

EEc 23

)( rcr

3. Discharge Breakdown 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 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.