الفهرس 
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Abstract
The present work is a study of the electrical and electro-mechanical
properties of PVDF/BaTiO3 ferroelectric composites. The polymer used is
polyvinylidene fluoride (PVDF), while the ferroelectric material used is barium
titanate (BT) in the powder form. PVDF/BaTiO3 composite films were prepared
by mixing PVDF powder with BT powder in certain concentrations and then
stirring at room temperature for several minutes using a droplet of acetone, the
mixture was fused at 473 K for few minutes to get a solid paste. The mixture
paste was sandwiched between two thin aluminium foils and pressed in a hot
press for two minutes at a temperature of 483 ± 2K to obtain a film of polymer
ferroelectric composites.
The films are then quenched in cold water and
aluminium foils were then removed. The samples were coated with silver
conductive paint on the opposite major faces with an area of about 0.2 cm2 to
perform electrical and dielectric measurements.
The properties of these composites were studied by measuring the
dielectric properties e¢, e², the real part (M¢) and imaginary part (M²) of the
electric modulus, Ac conductivity (sac), Dc electrical conductivity (sdc),
thermally stimulated polarization current (TSPC), thermally stimulated
depolarization current (TSDC), x-ray analysis and differential scanning
calorimetry (DSC). The effects of annealing and drawing on some of these
measurements were also investigated.
The temperature dependence of the real part e¢ (T) exhibits a shoulder in
the temperature range (240 – 350 K) while e²(T) shows a sharp peak in the same
temperature range, which is shifted to a higher temperature with increasing
frequency. These features, which are strongly frequency dependent, are similar
to those observed in many polar polymers and are attributed to the micro
Brownian dipolar motion of the main chain in the amorphous region of the
polymer (b- relaxation process). The other peak appeared at a high temperature
above 420 K characterized the a-relaxation process which associated with the
crystalline part of the polymer. The e¢(T) exhibits a sharp minimum at 440 K
corresponding to the dynamic melting temperature Tm of the polymer. The
increase in e¢(T) at lower frequency is ascribed to inhomogeneous conduction
due to interfacial polarization. e¢(T) increases with increasing the BT content in
the sample while e²(T) is only slightly affected by composition.
The behaviour of e¢(f) is dependent on the temperature of measurements
and is independent of composition. There is no relaxation peaks associated with
e²(f) but it decreases with frequency at higher temperatures. On the other hand
e¢(T) peaks are affected by the thermal treatment (annealing) of the samples
especially at lower frequencies where they are shifted to higher temperatures for
slowly cooled samples. The dielectric permittivity e¢(T) increases while e²(T)
decreases for drawn PVDF up to 30% of the original length compared to an
undrawn sample.
The electric modulus representation is used to overcome the problem of
interfacial polarization, which appears in heterogeneous systems at lower
frequencies, i.e. M²(f) shows a peak unlike e²(f). The temperature dependence
of the ac conductivity (sac) is strongly frequency as well as temperature
dependent. The maximum loss peaks (b- and a-relaxations) appeared for all
studied compositions. sac(T) behaves like sdc(T) at high temperatures and lower
frequencies, showing a positive temperature coefficient (PTC), but at higher
frequencies it behaves differently, showing a negative temperature coefficient
(NTC). sac(T) increases slightly by increasing BT-content in the sample. It is
noticed that log sac linearly rises with frequency at low temperatures while it is
frequency independent at high temperatures. sac(f) is found to be sensitive to
annealing in the temperature range of crystalline melting only.
The temperature dependence of sdc shows a positive temperature
coefficient and decreases slightly at high values of the applied voltage. The
effect of a drawing ratio of 40 % gives a slight decrease of sdc(T). The
calculated activation energy for this system indicates that the conduction
mechanism may.
be ionic in nature.
Thermally stimulated polarization current (TSPC) shows a broad peak in
the temperature range of 335 K. The fluctuations in the peak height and position are attributed to space charges in addition the production of displacement
currents from electric dipoles. The behaviour of TSPC is affected by drawing
due to the orientation of molecular chains which enhance crystallinity and
eliminate some of the space charges that reduce the fluctuation in TSPC spectra.
On the other hand, thermally stimulated depolarization current (TSDC) shows a
maximum value in the same temperature range of TSPC (335 K). The peak
height increases by increasing BT-content, and decreases by increasing the
poling temperature, while it is independent of temperature at very high applied
poling voltage (4 kV).
X- ray diffraction analysis is used to show the structure variation that
occurred in the samples. It is found that the structure for pure PVDF and low
BT content belong to orthorhombic a-phase, but adding more BT changes the
structure to orthorhombic b-Phase. The high BT content sample (30 % by wt.,
BT) has a tetragonal structure similar pure BaTiO3 (for T< 393 K). The unit cell
parameters may suggest that slowly cooled samples are in the b-phase because
the strong reflection from (200) is present and the reflections from (110) and
(020) for a-phases are not observed. Differential scanning calorimetry (DSC) shows that the melting
temperature of the samples during the heating process is around 445 K, which is
in agreement with dielectric data. The glass transition temperature obtained from
DSC-scan for the sample containing 20% by wt., BT is 238 K which is
attributed to the glass-transition temperature of PVDF. However the DSCthermogram
for the annealed sample shows a more defined peak where its
position appeared at a higher temperature compared to the other samples. Finally, the electromechanical coupling coefficient coefficient (K2) for the studied samples was determined at different temperatures from the dielectric
data.