1. resolution near the sample surface. With LIMM we

1.  
Introduction:

Ferroelectric materials possess pyroelectric properties and spontaneous
polarisation. All ferroelectric materials are pyroelectric, however, not all
pyroelectric materials are ferroelectric. Below a transition temperature called
the Curie temperature ferroelectric and pyroelectric materials are polar and
possess a spontaneous polarization or electric dipole moment. Inhomogeneous
polarization is the typical property of ferroelectric materials, particularly
at the surface region. Different mechanisms can cause decrease of polarization
near the surface.1

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

 

Example:         High electric
fields

                        Structural
variation of the sample near the surface

 

Applications for Ferroelectric Materials:

 

Capacitors, Non-volatile memory, Piezo electrics for ultrasound imaging
and actuators, Electro-optic materials for data storage applications,
Thermistors, Switches, Oscillators and filters, Light deflectors, modulators
and display.

Here we are working on PVDF and P (VDF-TrFE) materials. The used P
(VDF-TrFE) compositions are P (VDF-TrFE) 56/44 mol% and P (VDF-TrFE) 70/30
mol%. Bi and multilayer films of P (VDF-TrFE) and PVDF are produced and
measured the polarization distributions using LIMM method.

 

To measure the charge and polarization distributions, various
experimental techniques are available, which are based on the piezoelectric or
on the pyroelectric effect. Pyroelectric effect methods are implemented in the
time or in the frequency domain. The time domain is the thermal pulse method,
while the LIMM uses thermal waves in the frequency domain. Thermal methods are
good enough to achieve high resolution near the sample surface. With LIMM we
can achieve 0.5µm resolution. The results of polarization investigations in bi
and multilayer samples of PVDF and P (VDF-TrFE) are presented.2

1.1 Principle of operation:

 

Charge and polarization distribution in the sample is measured based on
the pyroelectric response to a non uniform variation of temperature. Sample is
prepared in the form of round shape and the film is covered with electrodes on
both sides of surface. Laser light is focused on one side of the surface of the
film and that surface gets heated due to absorption of laser light. Heat goes
inside the sample through the surface it causes change in the temperature
inside the sample.1

 

 

1.2 LIMM (Laser Intensity Modulation
Method):

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1: Laser Intensity Modulation
Method (LIMM)

 

 

 

 

For the Measurement of pyroelectric current a current to voltage
converter, lock in amplifier has been used in computer controlled equipment.
The pyroelectric specimen and the reference photo diode both are connected to
the input of the current to voltage converter. For the measurement of
pyroelectric spectrum laser light is incident on the specimen, for the
measurement of reference spectrum laser light is incident on the photo diode.
Through a fast operational amplifier with a gain band width product of 1.7GHZ
is used in the I-U converter, a significant amount of phase shift can be
avoided in MHZ.1

 

 

 

 

 

 

 

 

 

1.  Theory

2.1
Ferroelectric effects

Solid materials are
classified as ferroelectric if they show two or more orientation states of
spontaneous polarization Ps without
a permanent electric field forcing the crystal in a polarization. Polarization
in this sense is the fixed separation of charges. These charges are arranged
through chemical bonding or through motions of sub lattices 14.

To be ferroelectric
the spontaneous polarization must be switchable in two or more different states
and the states must be stable in zero field. The value of the polarization
measurable at zero field is called remnant polarization Pr. Normally the polarization will be compensated through free
charges in the crystal and the surrounding media. The arrangements of charges
which are polarized in the opposite direction are also compensating the
spontaneous Polarization. This is possible because the whole crystal does not polarize
in the same direction if it’s not been forced through an electric field. Only
certain areas polarize in the same direction this area is called domains 14.

 Furthermore, the spontaneous polarization is
temperature dependent and disappears continually with higher temperature or
disappears suddenly above a certain temperature. This is called pyroelectric
effect. The temperature where the spontaneous polarization disappears
discontinuously is called the Curie temperature TC. Under the TC point the material is ferroelectric. Some
materials also show that the ferroelectric effect disappear under a certain
temperature, so the material is ferroelectric in certain temperature range 14.

Another property is
piezoelectric effect. This means that the material shows a spontaneous
polarization under mechanical stress and if an electric field is applied to
polarize the material, it shows a strain. The effect is linear 14.

In
addition a ferroelectric material shows nonlinear optic effects. It should be
noted that nonlinear optic effects can appear in more than ferroelectric
materials. The effects appearing in ferroelectrics are the spontaneous Kerr
effect (or electro-optic effect) which leads to birefringence and piezo-optic
effect which also occurs in any crystal material. With the change of the
polarization in ferroelectrics the birefringence changes as well 14