CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
EFFECTS OF INITIAL POROSITY ON THE SHOCK RESPONSE OF
NORMALLY POLED PZT 95/5
R. E. Setchell, B. A. Tuttle, J. A. Voigt, and E. L. Venturini
Sandia National Laboratories, Albuquerque, NM, 87185
Abstract. Early studies of the ferroelectric ceramic PZT 95/5 indicated that the initial porosity could
have strong effect on material response under shock loading. The porosity can be modified through the
addition of organic pore formers that are pyrolyzed during ceramic firing. In the present study, the
shock response of PZT 95/5 materials having different porous microstructures has been examined.
Pore formers were added in quantities from 0.45 to 4.0 weight percent, resulting in sample densities
from 7.65 to 6.98 g/cm3 and corresponding void volume fractions of 4% to 13%. Gas gun experiments
were conducted to examine the response of these materials under uniaxial strain conditions.
Transmitted wave profiles in unpoled and normally poled samples were obtained using laser
interferometry (VISAR), and depoling currents into short-circuit loads were measured with normally
poled samples. The results to date show that both mechanical and electrical properties of PZT 95/5 are
quite sensitive to the level of porosity, but not the pore morphology.
organic materials are pyrolyzed during the bisque
firing, leaving voids that reflect the morphology of
the pore former. One pore former that has been
used is Lucite® (PMMA) in the form of spheres 50100 um in diameter. A second pore former is
Avicel® (microcrystalline cellulose) in the form of
rods 5-15 urn in diameter and > 20 (im long. The
actual density used in an application depends on
requirements such as mechanical strength and
dielectric-breakdown strength. Recent studies (3,4)
have examined PZT 95/5 having a density of 7.30
g/cm3, corresponding to a porosity of 9%.
A new experimental program is carefully reexamining the effects of porosity in this material.
The goal of this program is to gain specific insights
into how the porous microstructure affects the
mechanical and electrical behavior of normally
poled PZT 95/5 under shock loading. Materials for
this study have been prepared over a range of
densities by varying the amount of added pore
formers. Static mechanical and electrical properties
and phase boundaries have been characterized for
these materials, and their dynamic response under
shock loading has been examined in planar-impact
INTRODUCTION
Ferroelectric ceramics exhibit a remanent
polarization, and the release of bound charge
through shock-induced depoling has been utilized
in pulsed power applications for a number of years.
Most applications have used a lead zirconate
titanate ceramic having a Zr:Ti ratio of 95:5 and
modified with 2% niobium, subsequently referred to
as PZT 95/5. The nominal state of this material is
ferroelectric (FE), but it is near an antiferroelectric
(AFE) phase boundary. Bound charge is released
through shock compression into the AFE phase.
The maximum density of PZT 95/5 is 8.00 g/cm3,
but normal processing yields material with a density
of 7.65-7.70 g/cm3, corresponding to a void volume
fraction (porosity) of about 4%. An early PZT 95/5
study by Doran (1) found that the position of a cusp
in the Hugoniot curve, presumed to be the Hugoniot
elastic limit, was sensitive to small density
variations. Later studies investigated the properties
of PZT 95/5 materials made at lower densities by
adding organic pore formers to powders prior to
pressing, bisque firing, and sintering steps (2). The
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Z-CUT SAPPHIRE OR ALUMINA-FILLED EPOXY (ALOX)
experiments conducted on a gas gun facility. This
paper will summarize the shock experiments that
have been performed to date.
Z-CUT SAPPHIRE
GROUND RETURN
CURRENT
PZT 95/5 MATERIALS
The response of normally poled PZT 95/5 under
shock loading was first examined more than twentyfive years ago (5). More recently, the shock
response of a particular PZT 95/5 material has been
extensively characterized (3,4). This material has a
density of 7.30±0.02 g/cm3, which is achieved
through the addition of approximately 1.0% by
weight of Lucite pore formers. The results obtained
with this material serve as a baseline for
comparisons in the present study. The new
materials have been prepared using Avicel pore
formers, with additions by weight ranging from
0.45% to 4.0%. The corresponding range in
densities is from 7.65 g/cm3 (4% porosity) to 6.98
g/cm3 (13% porosity). Both unpoled and normally
poled samples have been examined.
LOAD RESISTOR
NORMALLY POLED PZT 95/5
FIGURE 1. Configuration used for shock experiments on normally
poled PZT 95/5.
similar but shorted samples that provide a shockimpedance match at their boundaries. The end
faces of the active sample (shown within the plane
of Fig. 1) are in contact with an alumina-filled
epoxy encapsulant, which has lower shock
impedance. This boundary results in a small
fraction of the active sample experiencing multidimensional strain during shock propagation. A
detailed description of this configuration has been
given previously (3). In the present study, the
available samples did not always have the same
dimensions. In order to compare results, scaling
was done using a simple model for shock-induced
depoling (4). This scaling did not take into account
any differences in the fraction of the samples
experiencing multi-dimensional strain.
EXPERIMENTAL CONFIGURATIONS
Several different experimental configurations
have been used in the present study. To determine
Hugoniot states, reverse-impact experiments with
unpoled PZT 95/5 samples have been conducted. In
these experiments, a sample mounted on the face of
a projectile is impacted into a stationary fused silica
window. Laser velocity interferometry (VISAR) is
used to record the particle velocity history of the
impact interface. A Hugoniot state is found from
the interface velocity, the impact velocity, and the
Hugoniot properties of the window. To observe the
structure in waves propagating through unpoled
samples, a fused silica disc is impacted into a PZT
95/5 sample. VISAR is used to record the particle
velocity history when the wave propagating through
the sample arrives at an interface with a sapphire
window. The configuration used for examining the
shock response of normally poled samples is more
complex, and is shown in Fig. 1. The active PZT
95/5 sample is poled normal to the direction of
shock motion, and is sandwiched between two
RESULTS
Figure 2 shows Hugoniot states determined in
reverse-impact experiments at two different impact
velocities. The material made with 1.8% Avicel has
nearly the same density as the baseline material
(with 1.0% Lucite), and its states are close to the
baseline curve. The high and low density cases
appear to have Hugoniot curves that deviate rapidly
from the baseline curve at shock pressures above 3.0
GPa. The degree of deviation appears quite large
for materials whose density varies by only ± 5%.
Figure 3 shows waveforms recorded after 2.5 GPa
shocks have propagated 4.0 mm through normally
poled samples having the same density, but made
210
-J\
30
25
HUGONIOT CURVE ESTABLISHED FOR
PZT 95/5 WrTH1.0%LUCrTE
i,.
V)
CURRENT FROM 1.8% AVICEL SAMPLE
SCALED BYX1.06 TO ACCOUNT FOR
DIFFERENCES N SAMPLE SIZE
A -0.45%AVCEL
• -1.80% AVICEL
4-4.0% AVICEL
0.10
0.15
0.25
0.20
-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30
PARTICLE VELOCITY- km/s
TIME - MICROSECONDS
FIGURE 4. Short-circuit currents generated during 2.5 GPa shock
transit through 4.0 mm, normally poled samples at a common
density.
FIGURE 2. Hugoniot states obtained in reverse-impact
experiments.
0.07
0.06
1 0.05
O 0-04
2
> 0.03
I 0.02
1.8% AVICEL SAMPLE 4.0 mm THICK
(WAVEFORM SHIFTED N TIME)
0.01
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
0.65
TIME - MICROSECONDS
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
TIME-MICROSECONDS
FIGURE 3. Observed waveforms after 2.5 GPa shock propagation
through 4.0 mm, normally poled samples at a common density.
FIGURE 5. Observed waveforms after 2.5 GPa shock propagation
through 3.0 mm, normally poled samples at different densities.
with different pore formers. Only a small difference
in the initial wave amplitude is apparent. Figure 4
shows the short-circuit currents generated during
these experiments, and the differences are again
quite small. Under these conditions, it appears that
the pore morphology is not a significant factor.
Figure 5 shows waveforms recorded after 2.5 GPa
shocks have propagated 3.0 mm through normally
poled samples having different densities. The
initial portion of the waveform is similar in all cases
except for the highest density. The plateau reached
later shows a substantial spread in amplitudes.
These levels are somewhat less than predictions
based on Hugoniot properties, and the waveforms
all rise slightly if shown on an extended time scale.
These features are more apparent in the profiles
shown in Fig. 6. These are transmitted waveforms
in unpoled samples from impact conditions
intended to match the highest Hugoniot states
0.14
0.12
0.10
0.08
j 0.06
0.04
0.02
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
0.9
1.0
1.1
1.2
TIME - MICROSECONDS
FIGURE 6. Observed waveforms after high stress (>4.0 GPa)
shock propagation through 4.0 mm, unpoled samples at different
densities.
shown in Fig. 2. Also shown on this figure are
estimates of the axial stress levels in PZT 95/5
corresponding to the initial plateaus.
These
211
plateaus represent the onset of pore collapse in the
material (4). The profiles slowly rise from these
levels at a rate that depends on the initial density.
Final states predicted by Hugoniot calculations are
not reached within the available test time.
Figure 7 shows short-circuit currents generated
during 2.5 GPa shock propagation through samples
having different densities. The average current
level should be proportional to the product of the
shock velocity and the remanent polarization (4). If
these currents are integrated over time, the total
charge released during shock transit is obtained.
Figure 8 shows the released charge divided by the
electrode area for the cases shown in Fig. 7. The
final value is an accurate measure of the remanent
polarization, provided that the shock pressure is
high enough to ensure complete depoling. The
insert in Fig. 8 shows the final values plotted
against porosity. If the remanent polarization
resulting from the fixed poling field was
proportional to density, the data would follow the
linear curve shown in the insert. A faster-thanlinear decrease is shown, as observed in hydrostatic
depoling of similar PZT 95/5 materials (6).
14.0
1.8% AVICEL CASE SCALED
FOR DIFFERENT SAMPLE
DIMENSIONS
TIME - MCROSECONDS
FIGURE 8. Total charge per unit area released during shock
transit, showing a faster-than-linear decrease with increasing
porosity.
significant change in the threshold stress for the
onset of pore collapse. This threshold corresponds
to the Hugoniot cusp first identified by Doran (1).
All of these porosity-sensitive features will continue
to be examined in the current study.
ACKNOWLEDGMENTS
The authors would like to thank David E. Cox for
skillfully
preparing and
conducting the
experiments. Sandia is a multiprogram laboratory
operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of
Energy under Contract DE-AC04-94AL85000.
0.45%AVICEL
13.0
12.0
11.0
, 10.0
i
'
9.0
8.0
7
i; 6.0
-°
J
REFERENCES
5.0
1.8% AVICEL CASE SCALED FOR
DIFFERENT SAMPLE DIMENSIONS
4.0
3.0
1. Doran, D. G., J. Appl. Phys. 39, 40-47 (1968).
2. Dungan, R. H., and Storz, L. J., J. Am. Ceram. Soc. 68,
530-533(1985).
3. Setchell, R. E. et al., "The Effects of Shock Stress and
Field Strength on Shock-Induced Depoling of Normally
Poled PZT 95/5," in Shock Compression of Condensed
Matter - 1999, edited by M. D. Furnish et al., AIP
Conference Proceedings 505, New York, 2000, pp.
979-982.
4. Setchell, R. E., "Recent Progress in Understanding the
Shock Response of Ferroelectric Ceramics" this
volume.
5. Lysne, P. C., and Percival, C. M., J. Appl. Phys. 46,
1519-1525(1975).
6. Tuttle, B. A. et al., J. Am. Ceram. Soc. 84, 1260-1264
(2001).
2.0
1.0
0.00
0.10
0.20
0.30
0.40 0.50 0.60 0.70 0.80
TIME-MICROSECONDS
0.90
1.00
1.10
1.20
FIGURE 7. Short-circuit currents generated during 2.5 GPa shock
transit through 3.0 mm, normally poled samples.
SUMMARY
Experiments to date with materials having
different pore morphologies but common densities
do not show significant differences in wave
structure or depoling rates. Varying porosity from
4% to 13%, however, resulted in large differences
in Hugoniot states, a faster-than-linear decrease in
total charge release with increasing porosity, and a
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