材料性能 Ch4-2_Polarization & dielectric Properties of Materials

Vac, εr = 1
Parallel plate Capacitance
P-n junction Capacitance
Polarization/介质的极化
An external electric field causes a small separation of the centers of the charges electron cloud and the positive/negative ion core )so that each infinitesimal element of volume behaves as an electric dipole. Non-Polar: which behave as above Polar : in which the molecules or atoms possess a permanent dipole moment which is ordinarily randomly oriented, but which become more or less oriented by the application of an external electric field!
Electronic/电子极化:
a displacement of the electronic cloud w.r.t the nucleus.
Ionic/离子极化:
separation of +ve and -ve ions in the crystal.
Orientational/分子极化:
Note they may perform different with each in terms of response rate (frequency of alternating voltage)!
The Electric Polarization P
is the dipole moment per unit volume at a given point. P = Np where p is the average dipole moment per molecule 1. N is the number of molecules per unit volume.
Origins of Polarization
Where do the electric dipoles come from? Usually,
– Electronic Polarization: Displacement of negative
electron “clouds” with respect to positive nucleus.
alignment of permanent dipoles (molecules).
Space-charge/内界面极化:
free electrons are present, but are prevented from moving by barriers such as grain boundaries - the electrons "pile up".
Capacitance/capacitor
Capacitance is the ability to store charge across a potential difference.
• Examples: parallel conducting plates, semiconductor p-n junction • Magnitude of the capacitance, C: C=Q/V Units: Farads • Parallel- plate capacitor, C depends on geometry of plates and material between plates C = εr εo A / L A : Plate Area; L : Plate Separation εo : Permittivity of Free Space (8.85x10-12 F/m2) εr : Relative permittivity, εr = ε / εo
Requires applied electric field. Occurs in all materials.
– Ionic Polarization: In ionic materials, applied electric
field displaces cations and anions in opposite directions
– Orientation Polarization: Some materials possess
permanent electric dipoles, due to distribution of charge in their unit cells. In absence of electric field, dipoles
When an electric field is applied, these dipoles align to the field, causing a net dipole moment that affects the material properties.
Dielectrics are materials which have no free charges; all electrons are bound and associated with the nearest atoms. Free charges in a conductor will respond to exactly cancel an applied field. The situation in an insulator is more complicated, however, since a molecule in the insulator will also experience a field due to the response of the insulator.
Polarizabilty/ 极化率
p = aE where a is the polarizabilty
q l
The Susceptibility/极化强度
Provided that P ∝E , which it is in practice for moderate fields The Electric Displacement D, or surface density charge
Properties of Materials MP, SWJTU, GJ WAN Professor
材料性能 西南交通大学 万国江 教授 2012 双语 教程首页 本章首页 上一页
下一页 结束放映
Dielectric Materials
A dielectric material is an insulator which contains electric dipoles, that is where positive and negative charge are separated on an atomic or molecular level
There is a reaction field due to the response of the medium to charges on the molecule and there is a local field due to polarization of the solvent in the applied field. Here we shall demonstrate the role of the dielectric constant (also called the relative permittivity) as a factor that relates the polarization of an insulator to an applied electric field .
The Permittivity/介电常数（或电容率）
The Electric Displacement D
Dielectric constant/介电常数
Increase in capacitance in dielectrpolarization of electric dipoles in dielectric.
•In absence of applied field (b), these are oriented randomly •In applied field these align according to field (c) In vacuum Co=Qo/V ＝ε0 A/ l In dielectric C＝Q/V=εA/l 0，8．85xl0-12 F／m permittivity r relative permittivity= / 0>1 dimensionless
q l
极化就是介质内质点(原子、分子、离子)正负电荷重心的分离。
The induced dipole field opposes the applied field. In the diagram shown opposite the volume element indicated could represent an atom, a molecule, or a small region.
Any molecule develops a dipole moment which is proportional to the applied field
Magnitude of electric dipole moment from one dipole: p=qd