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It gives basic knowledge of Dielectric and Ferroelectric materials
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Prof.P. Ravindran,
Dielectric and Ferroelectric Properties of Materials 1
Dielectric means a non-conductor or poor conductor of electricity. Dielectric means a material that presents electric polarization. The dielectric is an insulating material or a very poor conductor of electric current. When dielectrics are placed in an electric field, practically no current flows in them because, unlike metals, they have no loosely bound, or free, electrons that may drift through the material. Instead, electric polarization occurs. The positive charges within the dielectric are displaced minutely in the direction of the electric field, and the negative charges are displaced minutely in the direction opposite to the electric field. This slight separation of charge, or polarization, reduces the electric field within the dielectric.
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Polarization in dielectrics
Dielectric strength
Polarization mechanisms in materials:
boundaries.
Dielectric Constant ļ®If you apply an electric field, E, across a material the charges in the material will respond in such a way as to reduce (shield) the field experienced within the material, D (electric displacement)
0
0
0
e
0
e
ļ®where e 0 is the dielctric permitivity of free space (8.85 x 10 12 C 2 /N-m 2 ), P is the polarization of the material, and ce is the electric susceptibility. The relative permitivity or dielectric constant of a material is defined as:
r
0
e ļ®When evaluating the dielectric properties of materials it is this quantity we will use to quantify the response of a material to an applied electric field.
Electronic Polarizability ļ® Letās limit our discussion to insulating extended solids. In the absence of charge carriers (ions or electrons) or molecules, we only need to consider the electronic and ionic polarizabilities. The presence of an electric field polarizes the electron distribution about an atom creating a dipole moment,
The dipole moment per unit volume, P, is then given by
where nm is the number of atoms per unit volume.
Frequency Dependence ļ®Reorientation of the dipoles in response to an electric field is characterized by a relaxation time, t. The relaxation time varies for each of the various contributions to the polarizability:
1. Electronic Polarizability ( a e) Response is fast, t is small 2. Ionic Polarizability ( a i) Response is slower 3. Dipolar Polarizability ( a d) Response is still slower 4. Space Charge Polarizability ( a s) Response is quite slow, t is large Audiofrequencies (~ 10 3 Hz) a = a e+ a i+ a d+ a s Radiofrequencies (~ 10 6 Hz) (a s ļ® 0) a = a e+ a i+ a d Microwave frequencies (~ 10 9 Hz) (a s, a d ļ® 0) a = a e+ a i Visible/UV frequencies (~ 10 12 Hz) (a s, a d, a i ļ® 0) a = a e
Frequency Dependence
Microwaves
tan d (Loss) e r (Dielectric Const.)
Ionic Polarization and Ferroelectricity ļ®Most dielectric materials are insulating (no conductivity of either electrons or ions) dense solids (no molecules that can reorient). Therefore, the polarizability must come from either ionic and electronic polarizability. Of these two ionic polarizability can make the largest contribution, particularly in a class of solids called ferroelectrics. The ionic polarizability will be large, and a ferroelectric material will result, when the following two conditions are met:
Ferroelectricity
Ferroelectrics:
Ferroelectricity ļ® Two stimuli that will change the lattice dimensions of a material are force and temperature. ļ® The generation of a current in response to the application of a force to a capacitor is called piezoelectricity. ļ® The generation of current in response to a change in temperature is called pyroelectricity.
Ferroelectricity
