dielectrics_1
DESCRIPTION
Dielectric studies of materialsTRANSCRIPT
Dielectrics
Prof. V. KrishnakumarProfessor and Head
Department of PhysicsPeriyar UniversitySalem – 636 011
IntroductionIntroduction• Dielectric materials: high electrical resistivities, but
an efficient supporter of electrostatic fields.• Can store energy/charge.• Able to support an electrostatic field while
dissipating minimal energy in the form of heat.• The lower the dielectric loss (proportion of energy
lost as heat), the more effective is a dielectric material.
• Another consideration is the dielectric constant, the extent to which a substance concentrates the electrostatic lines of flux.
Capacitance• Two electrodes separated by a gap
define a capacitor.• When a bias is applied across the
capacitor plates, one charges positively, the other negatively.
• The amount of charge that the capacitor can store (Q) is proportional to the bias (V) times how good the capacitor is, the ‘capacitance’ (C).
• The capacitance is related to the area of the plates (A), their separation (d), and the Dielectric Constant (εεo) of the dielectric between the plates
• Dielectric constant of vacuum; εo = 8.85x10-12 F/m=55.2 Me/(V*m)
d
AC o
eVm
mV
d
AQ mV
eo *
* 2*
Why does charge built up?There is generally not a built-in electric field between the plates of an unbiased capacitor.
When an electric field is applied, any charged carriers or species within the material will respond.
For a conductor or semiconductor, e- will flow to the + plate, and possibly also holes will flow to the - plate. Current is carried=no charge buildup.
For an insulator, there aren’t a significant number of free carriers. There are highly ionic species, however, but they aren’t very mobile at low temperatures. No appreciable current is carried=charge buildup.
Polarization in InsulatorsPositively charged species in insulators shift/rotate/align toward the negative electrode and negatively charged species shift/rotate/align towards the positive electrode; creating dipoles. The dipole moment density is termed the Polarization (P) and has the units of C/m2.
+
-
Electron CloudElectron Cloud
+
E
Electronic polarization, occurs in all insulators
-
+ +
+-
+ +
E
Ionic polarization occurs in all ionic solids: NaCl,
MgO…
-
- -
-
+ -
-
+
+
-
++
E
Molecular polarization, occurs in all insulating molecules;
oils, polymers, H2O…
A
q
V
pP
Electric Dipole Moment
Polarization
xqp
Dielectric Effects
Metal plates
Dielectric
d
AC
What makes different from 0?
POLARIZATION
1
0
r
r
In electrostatics, the CONSTITUITIVE RELATION is
EP
PEED
0
0
Polarization
Susceptibility
Dielectric Effects
POLARIZATION arises from charge shifts in the material—there is a macroscopic separation of positive charge (e.g., the ions) and negative charge (e.g., the BONDING ELECTRONS).
Induced DIPOLE MOMENT
POLARIZATION is then
There are many sources of dipoles.
0xqdi Amount of charge shift
idipolesdNP
Definitions•Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium and is determined by the ability of a material to polarize in response to an applied electric field, and thereby to cancel, partially, the field inside the material. Permittivity relates therefore to a material's ability to transmit (or "permit") an electric field…The permittivity of a material is usually given relative to that of vacuum, as a relative permittivity, (also called dielectric constant in some cases)….- Wikipedia
DkDkDfDf
'r'r "
r"r
Permittivity and Permeability Definitions
•interaction of a material in the presence of an external electric field.
"'
0
rrrj
Permittivity (Dielectric Constant)
Permittivity and Permeability Definitions
•interaction of a material in the presence of an external electric field.
"'
0
rrrj
Permittivity (Dielectric Constant)
DkDk
Permittivity and Permeability Definitions
•interaction of a material in the presence of an external electric field.
"'
0
rrrj
"'
0rr j
interaction of a material in the presence of an external magnetic field.
Permittivity (Dielectric Constant)
Permeability
DkDk
Permittivity and Permeability Definitions
•interaction of a material in the presence of an external electric field.
"'
0
rrrj
"'
0rr j
interaction of a material in the presence of an external magnetic field.
Permittivity (Dielectric Constant)
Permeability
DkDk
"'rrr j "'
rrr j
Electromagnetic Field Interaction
Electric Magnetic
Permittivity Permeability
FieldsFields
STORAGE
MUT
STORAGE
"'rrr j "'
rrr j
Electromagnetic Field Interaction
Electric Magnetic
Permittivity Permeability
FieldsFields
STORAGE
LOSS
MUT
STORAGE
LOSS
Loss Tangent
'
"
tanr
r
'
"
tanr
r
CycleperStoredEnergy
CycleperLostEnergy
QD
1tan
CycleperStoredEnergy
CycleperLostEnergy
QD
1tan
Dissipation FactorDD
Quality FactorQQ
r
'r
''r
DfDf
Relaxation Constant
= Time required for 1/e of an aligned system to return to equilibrium or random state, in seconds.
cc f
2
11
cc f
2
11
11
10
100
10 100
Water at 20o C
f, GHz
most energy is lost at 1/
'r'r
"r"r
j
s
1
)( :equation Debyej
s
1
)( :equation Debye
Dielectric Effects
ln()0
statics optical
PLO10-15 eV 30-50 meV
visibleinfrared
Major source of POLARIZATION is distortion of the bonding electrons around atoms. This leads to the normal semiconductor dielectric constant.
In POLAR materials, likeGaAs and SiC, the different charge on the A and B atoms can be polarized as well, leading to a difference between the optical and the static dielectric constants.
In Appendix C, the two values for GaAs are reversed!
Definition:
A photonic crystal is a periodic arrangementof a dielectric materialthat exhibits strong interaction with light
Piezoelectric Effect
In materials with NO REFLECTION SYMMETRY (like GaAs or many molecular species) the applied electric field produces a DISTORTION OF THE LATTICE (size change) and vice versa.
FORCE
ELECTRIC FIELD
A common piezoelectric is Poly-Vinylidene Flouride, which is used in a variety of stereo headsets. The most common is crystalline quartz used as frequency control crystals—pressure applied to the quartz has a resonance which can be used in a feedback loop to create a highly-stable oscillator—the quartz crystal oscillator.
Phase sensitive multimeter interfaced with impendence analyzer for dielectric
measurements