Transcript Determination of Piezoelectric and Pyroelectric

Lead zirconate titanate/polyurethane(PZT/PU)
composite for acoustic emission sensors
W.K Sakamoto,P.Marin-Franch, D.Tunicliffe and D.K Das-Gupta
Universidade Estadual Paulista-UNESP/Ilha Splteira, Sao Paulo-Brazil
University of ales Bangor-UWB, School of Informatics
BAE SYSTEM, Advanced Technology Centre-Sowerby
Abstract

Piezoelectric composite, made from Ferroelectric ceramic lead
zirconate titanate(PZT) and vegetable based polyurethane(PU)
polymer was doped with a semiconductor filer, graphite.

Resulting composite(PZT/PU):49/1/50 vol.% composition poled
at lower field and shorter time due to the increased conductivity
of the polymer phase following the introduction of graphite.

The PZT/C/PU composite showed higher pyroelectric coefficient in
comparison with the undoped PZT/PU composite with 50/50-vol.%
composition.

PZT/C/PU composite has shown the ability to detect both
extensional and flexural modes of simulated acoustic emission at a
distance up to 8.0m from the source, thus indicating that it may be
used for detection of structural damages.
1. Introduction






Transducer senses a dynamics stress waves propagating through a
structure which was generated by the release of energy due to a
failure mechanism.
The elastic waves produced by an AE source is converted to a voltage
signal by a resonant transducer and parameters such as peak
amplitude or energy can be recorded.
These parameters are related to load, strain and temperature.
AE sensor can facilitate the continuous and non-destructive
monitoring of the structure health.
Ferroelectric ceramic/polymer composites can combine mechanical
strength and flexibility of polymer with the high piezo and pyroelectric
activities of ceramic
The difficulty with ceramic/polymer composite is in achieving an
efficient polarization of ceramic dipoles, because most of the applied
voltage drops across the polymer phase.
1. Introduction
 By adding a small amount of semiconductor filer a resistance
in parallel with that of polymer is introduced, thus the
resistance of the polymer phase and the poling process
becomes effective.
 The present paper reports some results of dielectric and
electroactive characteristation of both PZT/PU and PZT/C/PU
composite to detect structural damages.
2. Sample Preparation
 diameter from 3um to 10um and fine-grained graphite were used as
ferroelectric ceramic and semiconductor filer.
 Vegetable-based polyurethane was added to both powders and mixed
manually.
 The mixture was pressed at 20Mpa at room temperature between two
greased alumimium foil.
 Aluminium electrodes with 1.0cm of diameter were vacuum evaporated
onto both surface of the sample.
 The PZT/C/PU composites 49/1/50-vol.%,in the thickness range of
200um to 350um were poled at 5*106V/m DC field for 30 minute at
373K in silicone oil bath.
 The PZT/PU(50/50vol.%)composites in the thickness range of 150um
to 250um were poled at same temperature at 1*107V/m for 1 hour.
3. Measurements
 The pyroelectric currents were measured using a direct method
in which a linear heating rate of 1 deg/min was applied to the
polatised samples
 Piezo d33 Tester was used to measure the d33 coefficient
 The composite was stretched in the thickness direction and the
electrical potential from piezoelectricity is compared with the
value obtained from the standard ceramic sample.
 The electromechanical coupling factor kt was obtained from the
measurement of the complex impedance around the peak of
the composite acting as a free resonator.
 Using two simulated acoustic emission sources, ball bearing
drop and pencil lead break, AE tests were carried out.
 PZT/PU and PZT/C/PU were surface mounted on a 56*56cm
fibreglass reinforced board(FRB)
4. Results and Discussion 1
 In the direct method of measuring pyroelectric coefficient,a
polarised sample is heated in a chamber at a reduce
pressure(3*10-2 torr) at a constant rate(1.0 deg/min) with its
electrodes shorted and the current is monitored with an
electrometer.
 Pyroelectric coefficient
p(T ) 
1 Ip
A dT
dt
(1)
Ip: pyroelectric current, A: sample electrode area
dT/dt: constant heating rate
4. Results and Discussion 2
 Figure 1 shows the nature of the reversible pyroelectric current in the
range of temperature of 300K to 353K in both PZT/C/PU and PZT/PU
composites poled as described earlier.
 The values of p(T):5.6pC/m2K for PZT/PU
and 10.7pC/m2 for PZT/C/PU at 303K.
 The increased value of p(T) for
PZT/C/PU composite can be
attributed to the efficiency of poling
process in the composite doped with
small amount of graphite(1.0vol.%)
Fig 1-Reversible pyrorlrctric current for
 The semiconductor filler can create
PZT/C/PU(poled at 373K in E=5MV/m for
30min)and PZT/PU(poled at 373K in
a continuous electric flux path in
E=10MV/m for 1 h) composite
the polymer phase, thus reducing
the voltage drop across the polymer phase.
4. Results and Discussion 3



The efficiency of poling process in graphite doped composite was observed
also with the d33 values of the piezoelectric coefficient.
For both composites poled as described earlier(E=107V/m for PZT/PU and
E=5*106V/m for PZT/C/PU) the d33 value obtained was 13.0pC/N
The electromechanical copuling factor kt was obtained by fitting the
experimentally measured impedance using the equation.
1 2

2
1/ 2
t   R maxf0 C0 (  4) thanh( )
2
4
k

2
(2)
C0:capacitance, Ψ :mechanical loss, f0: resonance frequency
Rmax is the amount of the real part of the impedance in the resonance
frequency from the base line.
 Ψ :0.13, f0:6.0MHz, Rmax:3, C0:106 pF
 Used in equation 2 gave kt=0.04
4. Results and Discussion 4



Figure 2 shows the fitting of
the real impedance of the
composite.
To characterise the composite
as AE detector, the FRB panel
was exited using two different
simulated source.
Ball bearing drop produces
relatively large amplitude and
low frequency stress waves
while the pencil lead break
produces stress waves with
Figure 2-Experimental and theortical real
low amplitude and higher
impedance of PZT/PU 50/50
frequency.
composite around resonant peak
4. Results and Discussion 5
 Fig 3 shows the response of
the both sensors to
increasing energy impacts.
 Taking a noise level of
2.0mV, it can be predicted
that the lowest energy level
detectable for PZT/PU and
PZT/C/PU are 4.0*10-6 and
3.0*10-6 .
Figure 3-Response of the sensors to a
ball-bearing drop test
4. Results and Discussion 5




The sensors were also
compared for their ability to
detect AE at different distance.
Ball bearing drops of fixed
height(5cm) were used.
Figure 4 shows that the sensor
response follow an inverse law
to distance.
The maximum distance in
which the 0.25mJ ball bearing
impact can be detected are
615cm and 821 for undoped
Figure 4-Response of the sensor to a
and graphite-doped composite.
different distance AE sources
4. Results and Discussion 6
 The pencil lead break
experiment can show the
ability of the sensor to
detect the composite of a
plate wave.
 Figure 5 shows the time
response of the PZT/C/PU
sensor in which the
extensional and flexural
modes can be clearly
observed.
Figure 5-Response of the
PZT/C/PU(49/1/50 vol.%) composite
to pencil lead break experiment
4. Results and Discussion 7
 The fast Fourier
Transform(FFT) of the data
of figure 5 is shown in
figure 6.
 A peak occuring at 5kHz
due to the flexural mode
and the peak at 75kHz may
be attributed to the
extensional mode.
Figure-6 FFT of the response of
the PZT/C/PU composite to a
lead break
4. Results and Discussion 8
 Adding small amount of semiconductor filler made PZT/C/PU
composite better sensor for AE detection because the poling
process became more efficient.
 The sensitivity of the graphite-doped sensor was increased by a
factor of 25% in comparison with the undoped sensor.
 Further work is in progress on AE detection to obtain the response
of the embedded sensor.