PIEZOELECTRICITY
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POLARIZATION
When we place a material(dielectric) in the electric field its
becomes polarized, as the result of polarization dipoles are
form
Two charges having same in magnitude separated by the
small distance(less then atomic diameter) form a dipole
Dielectric is an electrical insulator that can be polarized by
an applied electric field
Dielectric has very few free electrons
Although charges in dielectric are not able to move freely,
there may be a displacement of charges when an electric
field is applied which results polarization
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Unpolarized
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Polarized
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d
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Dipole moment
2
PIEZOELECTRICITY
When a mechanical stress is applied on a certain crystal for
example, quartz (𝑆𝑖𝑂2 ) and (𝐵𝑎𝑇𝑖𝑂3 ), becomes polarized.
Charge appears on the surface of crystal. The appearance
of surface charge leads to voltage difference b/w two
surfaces of crystal.
This phenomenon is known as piezoelectricity.
The same effect is also exhibit mechanical strain when
electric field is applied.
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PIEZOELECTRICITY
FORCE
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P=0
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Crystal is strained by
the force that induce
the polarization in the
crystal and generate
surface charges
P≠0
4
CENTRAL SYMMETRY
If we draw a vector from point o to any charge
and then reverse the vector we will find the same
charge this unit cell is said to be central
symmetry
e.g. NaCl
In the absence of applied force the centre of
mass of positive and negative charge is coincide
This condition will no change when the force is
applied
The centre of mass is not change
The net polarization before and after remain zero
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P=0
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P=0
F
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NONCENTRAL SYMMETRY
If we draw a vector from point o to any charge and then reverse the vector we will not find
the same charge this unit cell is said to be noncentral symmetry
e.g. 𝐵𝑎𝑇𝑖𝑂3
In the absence of applied force the centre of mass of positive and negative charge is
coincide
This condition will be change when the force is applied
The centre of mass changes
The net polarization before and after remain is not zero
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