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Introduction
Research in the area of Ferroelectrics is drive by the market potential of next generation memories and transducers. Thin films of ferroelectrics and dielectrics are rapidly emerging in the fields of MEMS application. Ultrasonic micro-motors utilizing PZT thin films and pyroelectric sensors using micro-machined structures have been fabricated. Wide spread use of such sensors and actuators have made Hubble telescope a great success story. Characterization is an important step in the development of different types of new materials. This experiment is aimed to expose the young students to ferroelectric hysteresis and ferroelectric phase transition in Ferroelectric Ceramics.
FERROELECTRICITY
Ferroelectricity is the phenomenon, by virtue of which some materials exhibit spontaneous electric polarization, even in the absence of any externally applied field. Ferroelectric crystals possess regions with uniform polarization called ferroelectric domains. Within a domain, all the electric dipoles are aligned in the same direction. There may be many domains in a crystal separated by interfaces called domain walls. A ferroelectric single crystal, when grown, has multiple ferroelectric domains. A single domain can be obtained by domain wall motion made possible by the application of an appropriate electric field. A very strong field could lead to the reversal of the polarization in the domain, known as domain switching.
The polarization reversal can be observed by measuring the ferroelectric hysteresis as shown in Figure 1.
The properties of the ferroelectric ceramics may be summarized as follows:-
1) They exhibit dielectric hysteresis loop below a certain temperature (called Curie temperature) indicating the existence of spontaneous polarization.
2) They have a high dielectric constant, which rises to a peak value at the Curie temperature and fall thereafter in accordance with Curie-Weiss law.
3) The curie temperature can be raised by application of biasing filed or hydrostatic pressure.
4) They show piezoelectric properties below Curie temperature.
5) Most unique is the capability to exhibit hysteresis loop.
The following studies can be done with the apparatus:
1) Determination of spontaneous polarization (Ps)
2) Determination of Remnant polarization (Pr)
3) Determination of coercive field (Ps)
System Description
The block diagram of the experimental unit is shown in Figure 2. A high voltage oscillator produces an adjustable sinusoidal output from 0 to 350V (r.m.s.). The frequency is variable in a small range around 30Hz. The oscillator output is applied to the dielectric sample kept in a temperature-controlled oven. Due to the large amplitude of the alternating source, the dielectric material exhibits a hysteresis loop similar to that observed in a ferro-magnetic material under the influence of an alternating magnetic field.
The loop can be displayed on a CRO with the help of a circuit, which basically monitors the charge flow through the sample and plots it against the voltage applied on it. This is shown as the loop display block, the outputs of which are connected to the X and Y plates of a cathode ray oscilloscope. Provision also exists to cancel out the conductive component of the charge through the sample. Appropriate amplitude of the oscillator output is required to ensure saturation of the sample which may be observed on the CRO.
When the temperature is raised slowly a point is reached at which the hysteresis loop collapses and a straight line is seen on the screen. This temperature is a called Curie Temperature (Tc) and is a characteristics of the dielectric material.
Description of the Experimental Set-up
1. Probes Arrangement
It has two individually spring loaded probes. The probes arrangement is mounted in a suitable stand, which also holds the sample plate. To ensure the correct measurement of sample temperature, the RTD is embedded in the sample plate just below the sample. This stand also serves as the lid of temperature controlled oven. Proper leads are provided for connection to Capacitance Meter and Temperature Controller.
2. Sample
Barium Titanate (BaTiO3)
3. Oven
This is a high quality temperature controlled oven. The oven has been designed for fast heating and cooling rates, which enhances the effectiveness of the controller.
4. Main Units
Besides the main circuit, two other units are also housed in the samne cabinet.
(i) High Voltage Oscillator
Amplitude : 0-350V rms, 1000V (p-p)
Frequency : 28-34Hz (sine)
Waveform : Low distortion sine wave
(ii) Oven Controller
Platinum RTD (A class) has been used for sensing the temperature. A Wheatstone bridge and an instrumentation amplifier are used for signal conditioning. Feedback circuit ensures offset and linearity trimming and a fast accurate control of the oven temperature.
Temperature Range : Ambient to 200°C
Display : 3½ digit, 7 segment LED with autopolarity & decimal indication
Resolution : 0.1°C
Accuracy : ?0.5°C (typical)
Stability : ?0.1°C
Power : 150W |
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