A.R. James et al. / Journal of Alloys and Compounds 496 (2010) 624–627
625
2. Experimental details
Lead lanthanum zirconate titanate with nominal composition [(Pb0.92La0.08)
Zr0.60TiO0.40)O3] was prepared by the sol–gel method as described in the follow-
ing lines. Stoichiometric amounts of nitrates of lead, lanthanum and zirconium i.e.,
Pb(NO3)2], [La(NO3)3], [Zr(ONO3)2] of purity >99% were taken in aqueous solution
(
[
and was designated as solution A. Stoichiometric amount of titanium dioxide [TiO2]
was dissolved in hydrofluoric acid (HF). Then ammonia solution was added drop
wise to get a precipitate of titanium hydroxide. This precipitate was washed with
double-distilled water to remove excess HF. The resultant precipitate was added to
solution A. Then citric acid was added to this solution such that the mole ratio of citric
acid: metal ion is 2:1. At this stage metal citrates were believed to be formed. The pH
of the resultant metal citrate solution was adjusted to 6–7 by adding dilute ammonia
solution drop wise. The solution was then slowly evaporated on a hot plate till a vis-
cous liquid was obtained. At this stage ethylene glycol (gelating reagent) was added
such that the molar ratio of citric acid to ethylene glycol was 1:1.2. This mixture was
◦
heated at 100 C for 2–3 h with constant stirring. The temperature was increased to
◦
1
60–180 C at the onset of solidification to obtain dry gel. The dried gel powders
were subjected to X-ray diffraction studies using a Philips PW-3020 diffractometer
◦
−1
to confirm phase formation. A scan rate of 2 min was used. The average size distri-
bution of the particles can be obtained by using small angle X-ray scattering (SAXS)
technique. This was done using a PW-3830 X-ray generator (Anton-Paar, Austria)
operated at 40 kV and 50 mA with a Cu target. The scattering data collected were
used to calculate the particle size, shape and distribution of the PLZT nano-particles.
Initially the powders were dispersed in a solvent (toluene) and then this solution is
subjected to ultrasonic vibration in order to separate the agglomeration of particles
Fig. 1. X-ray diffractogram of (a) dried gel powder and (b) sintered PLZT.
3
. Results and discussion
(
since nano-particles are chemically highly reactive). Thereafter the powders were
allowed to remain in suspension for about 24 h following which with the help of a
syringe a part of the top of the solution was siphoned off and then injected into a
quartz capillary tube for measurement. In addition, to estimate the particle size of
the powders, the powders were examined via scanning electron microscopy.
Thermal analysis for the as-dried powders was carried out using a TG–DTA ana-
Fig. 1(a) shows the XRD pattern of the sol–gel derived PLZT pow-
ders and Fig. 1(b) the sintered compact, indicating the formation of
the PLZT in the single phase. It can be seen that Fig. 1(a) exhibits
some peaks of crystalline PbO, in addition to the PLZT peaks. This
indicates that some of the constituent oxides are still un-reacted.
Besides the fact the sol–gel process resulted in a partial phase for-
mation of PLZT (without any heat treatment). The strongest peak,
◦
lyzer (TA Instruments, SDT 2960) in air between 30 and 1000 C using a heating rate
◦
−1
of 10 C min . In order to obtain further information about the reaction kinetics in
the dried powders, differential scanning calorimetric (DSC) measurements of the
as-dried powders were carried out using a DSC analyzer (TA Instruments, DS2920)
◦
◦
−1
between 30 and 600 C with a heating rate of 10 C min in air.
◦
at 2ꢂ≈ 29.17 , corresponds to the PbO (1 1 1) peak, which disap-
The sol–gel derived powders were stacked in crucible and calcined isothermally
◦
pears after heat treatment. Fig. 1(b) is the diffraction pattern for
the sintered PLZT pellet.
in air at 800 C for 4 h and then cooled. The calcined powder was crushed and pressed
into circular pellets of diameter 1 cm and thickness 1 mm using 2 mol% polyvinyl
alcohol at a pressure of ∼4 MPa using a hydraulic press. The pellets were sintered
The results of the SAXS experiments are discussed in the ensuing
paragraphs. The experimental scattering function I(q) was obtained
for the samples are shown in Fig. 2(a). From the figure it is clear that
samples are poly disperse in nature [12]. Fig. 2(b) shows the pair
distances distribution functions as a function of r. It can be inferred
from Fig. 2(b) that the PLZT (8/60/40) samples prepared via the
sol–gel route have an average particle size of around 30 nm.
In the sol gel method, the particles obtained generally are spher-
ical in shape whereas in other processes such as high energy ball
milling, the shape of the particles deviates from sphericity, result-
ing in flaked or elliptically shaped particles. The particles seem
to be free from agglomeration since only top solution was taken
and injected into the quartz capillary tube. Whereas SAXS gives
the average size of the particles, in SEM the estimation of particle
size is rather localized to the region being seen under the micro-
scope. Fig. 3(a) shows the SEM micrograph of the PLZT powders.
Due to the very fine scale nature of the powders, agglomeration
took place resulting in tightly packed agglomerates of size around
450 nm. The average particle size was found to be in good agree-
ment with the SAXS result. The result of SEM and SAXS together
confirm that the particles are nano-sized. The particles are spheri-
cally shaped as expected from the sol gel technique. Fig. 3(b) is the
SEM micrograph of a fractured surface of sintered PLZT. The grain
size of the PLZT ceramics was found to be 1–3 m. The ceramics
show a well-sintered dense microstructure.
◦
at 1200 C for 4 h. During sintering, in order to prevent PbO loss, particularly at high
temperatures, asmall crucible containing PbZrO3 with10 wt%excessPbOwasplaced
in a double crucible configuration. The entire assembly consisted of a doubly sealed
alumina crucible with lids sealed with alumina powder as cement. All samples were
coated with silver paste on the larger faces to make proper electrical contact. The
sample geometries for measurement of the material properties were in conformity
−
1
with the IEEE standards. Samples were electrically poled at a field of 15 kV cm for
5 min at 100 ◦C immersing the samples in a silicone oil (Dow Corning 704®) bath.
1
Resonance data were acquired using an Agilent-E-4980 Precision LCR Meter, in the
frequency range from 20 Hz to 2 MHz, on poled samples.
The elastic compliance sE and coupling factor k31 can be expressed as follows:
11
1
=
4ꢀfr2l2
(1)
E
s
11
where fr is the resonant frequency, l is the sample thickness and ꢀis the density of
the specimen (in meters).
Similarly,
D
2
E
s
= (1 − k )s
(2)
(3)
11
31 11
2
3
k
=
1
1
+
where
ꢀ
ꢁ
ꢀ
ꢁ
ꢁ
fa − fr
ꢁ(fa − fr
2fr
= 2
1 +
tan
(4)
fr
The frequency constant of the disks (radial mode disk) were also computed from
resonance data using the relation:
Fig. 4(a) shows the thermogravimetric analysis (TGA) and differ-
ential thermal analysis (DTA). Curves for the sol–gel derived PLZT
derived powders taken simultaneously in the temperature range
Nt = fr × D
(5)
where D is the diameter of the disk. Piezoelectric constants are defined as partial
derivatives evaluated at constant stress (T), constant field (E), constant displacement
◦
of 30–1000 C. The weight loss observed in TGA curves between
(D) and constant strain (S) and these boundary conditions can be thought as free,
◦
2
00 and 400 C can be attributed to the removal of water and other
short circuit, open circuit and clamped response, respectively.
The dielectric polarization versus electric field measurements (P–E hysteresis
loops) were conducted using a modified Sawyer–Tower circuit. P–E measurements
were traced using a triangular waveform.
◦
volatiles. The TGA curve shows a sharp weight loss peak at 580 C;
this is due to the release of volatiles and onset of lead loss from the
powders. The lead loss from the powders continued even beyond
James
