5? Dielectrics and Insulators

1
5? Dielectrics and Insulators
2
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.

3
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

4
Background

• Dielectrics and insulators can be defined as
  materials with high electrical resistivities.
• Dielectricss parameters (permittivities)
  and (dissipation factors)
• Power engineer focus on the loss factor (
  )
• Electronics engineer focus on the dissipation
  factors ( )
• -gt Electrical resonance phenomenon
• Insulators are used principally to hold
  conductive elements in position and to prevent
  them from coming in contact with one another -gt
  substrates on circuits.
• Ideal insulators 1, 0

5
Background

• A very large number of types have been developed
  to meet particular demands
• -gt The increase in line voltages as power
  transmission networks
• -gt 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.

6
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.

7
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

8
Test Conditions

• 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

9
1. Intrinsic Breakdown

• Increasing voltage under well-controlled
  laboratory conditions.
• -gt Small current begins to flow which increases
  to a saturation value.
• -gt 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.

10
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 -gt 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

11
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

12
3. Discharge Breakdown

• 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.

13
3. Discharge Breakdown

• Disk-shaped cavity with its plane normal to the
  applied field E, the field 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.

14
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. -gt AC breakdown
  voltages are lower than those for DC
• Plot of density against electric breakdown in
  fig. 5.4

15
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 -gt
  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.