Annealing of sintered Pb0.9175La0.055Zr0.975Ti0.025O3 in air

Article abstract of DOI:10.1111/j.1551-2916.2007.01805.x

Annealing of a sintered pellet of Pb0.9175La0.055Zr0.975Ti0.025O3 in air without any precautions against PbO evaporation results in a core-shell microstructure with a solid PLZT core and a porous ZrO2-rich shell.

Full text of DOI:10.1111/j.1551-2916.2007.01805.x

J. Am. Ceram. Soc., 90 [8] 2664–2666 (2007)
DOI: 10.1111/j.1551-2916.2007.01805.x
r 2007 The American Ceramic Society
ournal
J
Annealing of Sintered Pb0.9175La0.055Zr0.975Ti0.025O3 in Air
Else Breval, Maria Klimkiewicz, Chiping Wang, and Joseph P. Dougherty*,w
Materials Research Institute, Penn State University, University Park, Pennsylvania 16802
Ann Crespi
Medtronic Energy & Component Center, 6700 Shingle Creek Parkway, Brooklyn Center, Minnesota 55430
Annealing of a sintered pellet of Pb0.9175La0.055Zr0.975Ti0.025O3
in air without any precautions against PbO evaporation results
in a core–shell microstructure with a solid PLZT core and a
porous ZrO2-rich shell.
against PbO evaporation. The PLZT pellets (2 mm  12.5 mm)
were sintered at 13001C for 2 h, with PbO powder to protect
against PbO evaporation, as described earlier.1 Surface mor-
phology and element identification and distribution of a frac-
tured cross section were carried out using scanning electron
microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS)
techniques (HITACHI S-3500N-PGT-EDS, Tokyo, Japan).
Phase determination on the face of a pellet annealed for 1 h
was carried out using an automated X-ray powder diffractometer
(PAD V, Scintag, Santa Clara, CA) using CuKa radiation and
I. Introduction
NTIFERROELECTRIC (Pb,La)(Zr,Ti)O3 PLZT with a 5.5/97.5/
2.5 composition was previously studied to investigate sinte-
ring in the presence of PbO powder. The effects of sintering
temperature and time on the microstructure of a PLZT compo-
sition were reported.1
A
˚
l5 1.54056 A and a scan of 101–801 2Y with a speed of 21/min.
Attempts have been made to anneal a sintered lead zirconate
structure by heating in air for a short time and at a temperature
just high enough to remove free PbO in the grain boundaries.2,3
The intention was to evaporate the free PbO and thereby allow
the perovskite grains to grow together with a grain boundary
that is close to being ‘‘clean.’’ An increase in the dielectric loss
can occur if PbO is left in the grain boundaries after the sinte-
ring. Therefore, it is critical that both the sintering and the an-
nealing processes are carried out in an optimized manner.
During sintering, it is important that the temperature is just
high enough to allow the free PbO to evaporate without losing
PbO from the PZT structure.2,3 However, in an effort to prevent
PbO loss, sometimes, the PbO vapor pressure in the surrounding
atmosphere during sintering can be too high. This was pointed
out by Xia and Yao,2 and by Kingon and Clark.4,5 They found
lower densities in PZT structures when using a high PbO vapor
pressure in the sintering atmosphere. The annealing process has
mainly been used for rapid annealing of thin films of ferroelec-
tric PLZT compositions.3,6 These compositions contain more
Ti than the antiferroelectric PLZT, and therefore the melting
point of the PbO-rich grain boundary phase will be lower and
the solubility of PLZT material in the PbO phase will be in-
creased.7–9 Thus, it will be easier for the creation of a PbO-free
‘‘clean’’ PLZT grain boundary phase in the titanium-rich PLZT
compositions.
III. Results and Discussions
Figure 1 shows an intergranular fractured surface of a sintered
pellet before annealing. Annealing a sintered pellet for 1 h
resulted in the formation of a well-defined outer layer. The
pellet was fractured and the interior and the surface layers were
studied using SEM/EDS. The interior showed an intergranular
fracture similar to that found for an unannealed sample. The
outer layer was on the order of 6-mm thick (Fig. 2) and had a
surplus of Zr and a deficiency of Pb as shown in the EDS
element X-ray map (Fig. 3). Figure 4 shows that the layer is very
porous with 50–100 nm-sized grains of ZrO2. The composition
was determined by X-ray diffraction. The ZrO2 X-ray peaks
were broader than the peaks found for the very fine-grained
ZrO2 raw material used for fabrication of PLZT pellets. This
relates well with the nm grain size of the formed ZrO2 as ob-
served in the SEM of the outer surface.
Annealing for 1 h creates a sharp boundary between unal-
tered PLZT and the ZrO2-rich layer. This suggests that the first
part of the removal of the PbO from the PLZT is a fast process
as PbO vapor is transported through the porous material. The
II. Experimental Procedure
It is important to investigate which reactions occur when an-
nealing sintered samples of Zr-rich antiferroelectric PLZT. The
present study comprises both short-time (2.5 min) and long-time
(1 h) annealing of a sintered antiferroelectric PLZT pellet in a
preheated furnace at 10001C, in still air, without any precautions
E. Suvaci—contributing editor
Manuscript No. 22596. Received December 18, 2006; approved April 7, 2007.
Supported by Medtronic, Energy & Component Division, Brooklyn Center, MN 55430.
*Member, American Ceramic Society.
Fig. 1. Scanning electron microscopy image of a fractured surface of
sintered un-annealed PLZT 5.5/97.5/2.5.
wAuthor to whom correspondence should be addressed. e-mail: jxd6@psu.edu
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August 2007
Communications of the American Ceramic Society
2665
Fig. 2. Scanning electron microscopy image of a fractured surface of a
sintered PLZT 5.5/97.5/2.5 after 1 h annealing at 10001C in air. The
image shows the top surface to the left.
Fig. 4. Close-up of the region seen in Fig. 2. There is a sharp boundary
between the sintered PLZT material and the fine-grained ZrO2-rich,
PbO-depleted surface material.
second process is a bulk material diffusion of PbO along the
grain boundaries through the condensed material in the grain
boundaries. Even though, at 10001C, PbO is above its nominal
melting temperature, the diffusion is much slower than the gas-
eous diffusion, and some of the PbO in the grain boundary
phase may remain in place. The fracture surface of the interior
(Fig. 2) is intergranular and appears similar to the fractured
surface of an unannealed sample (Fig. 1), suggesting that there
may still be free PbO left in the grain boundaries after the
annealing.2 Only with further PbO depletion, as explained ear-
lier,1 will the PbO grain boundary phase completely disappear,
resulting in an intragranular fracture.
min. In order to ensure a fresh PLZT surface a fractured piece
was used. Figure 5 shows that some small grains, presumably
ZrO2, were formed. The grains are of the order of 50–100 nm,
and the thickness of the layer cannot be determined, as only a
few new grains have nucleated on the surface.
These results show that, when a sintered antiferroelectric
PLZT pellet is annealed in a preheated furnace at 10001C in
air, and without any precautions against PbO evaporation, the
first process is a rapid diffusion of PbO into the air. This results
in the formation of a porous PbO depleted outer layer that con-
sists of nonometer-sized ZrO2. There were no detectable changes
in the interior. These results suggest the much slower process of
diffusion of free PbO in the interior through solid material to-
ward the outer surface or the porous outer layer.
The short-time annealing process was studied with an exper-
iment similar to the 1-h annealing, but was heated for only 2.5
Fig. 3. Sintered PLZT 5.5/97.5/2.5 after 1 h annealing at 10001C in air. Element X-ray dot-map from the area seen in Fig. 2. The surface layer is Zr rich
and Pb deficient.

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Communications of the American Ceramic Society
Vol. 90, No. 8
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