Low-temperature preparation and properties of ceramics with composition (1-x)CaTiO3-xPbF2-xLiF

Article abstract of DOI:10.1016/j.jpcs.2007.01.005

Fluorinated ceramics with initial composition (1-x)CaTiO₃+xPbF₂+xLiF were sintered at 950 °C. The X-ray diffraction (XRD) patterns of the samples showed the formation of a novel solid solution in the initial composition range 0≤x≤0.125. SEM observations were performed on fractured ceramics and DSC analyses were carried out from room temperature up to 600 °C. Three second-order phase transitions were detected for all the samples. Capacitors were prepared from the pre-sintered ceramics then dielectric measurements were performed as a function of temperature in the frequency range 10²-4×10⁷ Hz. The ε′_r-T curves exhibit the profile of dielectrics for class I capacitors, however the values of tan δ are too high (tan δ≥1%).

Full text of DOI:10.1016/j.jpcs.2007.01.005

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1003–1007
www.elsevier.com/locate/jpcs
Low-temperature preparation and properties of ceramics with
composition (1ꢀx)CaTiO ꢀxPbF ꢀxLiF
¨
L. Taıbi-Benziada , H. Cherfouh
3
2
Ã
Faculty of Chemistry, USTHB, P.O. Box 32 El-Alia, 16311 Bab-Ezzouar, Algiers, Algeria
Abstract
Fluorinated ceramics with initial composition (1ꢀx)CaTiO +xPbF +xLiF were sintered at 950 1C. The X-ray diffraction (XRD)
3
2
patterns of the samples showed the formation of a novel solid solution in the initial composition range 0pxp0.125. SEM observations
were performed on fractured ceramics and DSC analyses were carried out from room temperature up to 600 1C. Three second-order
phase transitions were detected for all the samples. Capacitors were prepared from the pre-sintered ceramics then dielectric measurements
2
7
0
were performed as a function of temperature in the frequency range 10 –4 ꢁ 10 Hz. The e
class I capacitors, however the values of tan d are too high (tan dX1%).
r 2007 Elsevier Ltd. All rights reserved.
r
ꢀT curves exhibit the profile of dielectrics for
Keywords: D. Dielectric properties; D. Phase transitions
1
. Introduction
then the investigation of the properties in (Ca,Pb)(Ti,Li)(O,F)3
ceramics sintered at low temperature.
The perovskite CaTiO that is the host material in our
The interest for ABO materials is expanding very fast
3
3
worldwide due to their large range of applications in
several fields and especially in microelectronics. Among
these applications one can cite as examples capacitors,
sensors, resonators, piezoelectric actuators, pyroelectric
infrared detectors, electro-optical modulators and compu-
ter memories [1–5]. Nowadays, ATiO3 ceramics with
perovskite structure like BaTiO and SrTiO became the
study undergoes a sequence of phase transitions. At room
temperature, this compound crystallizes in an orthorhom-
bic lattice with Pbnm space group. Each unit cell of calcium
titanate contains four formula units and the lattice
˚
˚
˚
parameters are: a ¼ 5:385 A, b ¼ 5:445 A, c ¼ 7:646 A.
With increasing temperature, CaTiO transforms first from
3
orthorhombic (Pbnm) to tetragonal (I4/mcm) at 1398 K
then to cubic (Pm3m) at 1523 K [9]. During the last decade,
the structure of CaTiO -based materials has been studied
3
3
heart of smart systems with high level of intelligence. In
the race for manufacturing better computer memories, the
solid solution between barium and strontium titanates
Ba1 Sr TiO (BST) is very attractive for the development
3
extensively as a function of temperature and pressure.
Significant research effort was also directed towards
synthetic routes to prepare calcium titanate and to
optimize its properties owing to a wide range of applica-
tions for this perovskite. The most important property of
ꢀx
x
3
of ferroelectric random access memories (FRAMs) which
could achieve the gigabits density. In the future, dynamic
random access memories (DRAMs) and hard disks (HDD)
in computers will be replaced with FRAMs which are non-
volatile memories [6–8].
CaTiO is its ability to form a lot of solid solutions with
3
lanthanides and actinides and therefore, calcium titanate
could serve as host matrix for the immobilization of high-
level radioactive wastes [10,11]. On the other hand, some
The purpose of our work is first of all the search of new
oxifluoride phases in the chemical system CaTiO –PbF –LiF,
3
2
non-stoichiometric CaTiO -based perovskites with formula
3
Ca1 Sr Ti M O
3ꢀd
(M ¼ Fe, Co, Cr or Ni) could be
ꢀx
used as catalysts for partial oxidation of light hydro-
x
1ꢀy
y
Ã
Corresponding author. Fax: 213 21 24 73 11.
E-mail address: ikra@wissal.dz (L. Taıbi-Benziada).
¨
carbons, thanks to their ability to supply directly oxygen
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.01.005

ARTICLE IN PRESS
L. Ta ı¨ bi-Benziada, H. Cherfouh / Journal of Physics and Chemistry of Solids 68 (2007) 1003–1007
1004
from the oxides to surface-adsorbed species [12]. The
+
microscopy (SEM), differential scanning calorimetry
(DSC) and dielectric measurements (DE). The XRD
patterns were recorded at room temperature using a
PHILIPS PW 1710 diffractometer with the Cu Ka1
IV
+V
substitution of Ti
by Nb
induces a strong change in
the electrical and optical properties of Ca(Ti,Nb)O3 or
CNTO phases that make them of interest in the fabrication
of multilayer optical devices [13]. The addition of Ga to
˚
monochromatic radiation (l ¼ 1:5406 A) by step scanning
CaTiO :Pr perovskite enhances the luminescence intensity
3
of 0.021 in the 2y range 5–801 and a counting time of 0.5 s
per step. A PHILIPS LX 30 SEM operating at accelerating
voltage of 20 kV and a magnification of 8000 times was
used to probe the sample microstructure. Prior to SEM
observations, the ceramics were fractured and then coated
with a carbon (C) film in order to increase the sample
conductivity. The DSC analyses were performed on
heating under nitrogen gas between room temperature
and the obtained compounds could find an application in
field emission displays (FEDs) [14]. Moreover, CaTiO has
3
a large positive temperature coefficient of the resonant
frequency t and forms solid solutions with other com-
f
pounds with negative t yielding materials with an almost
f
zero t [15,16]. Therefore, CaTiO is a promising material
f
3
for communications equipment operating at microwave
frequencies (UHF and SHF). The objective of our study is
to prepare new dielectric ceramics related to CaTiO at a
3
temperature lower than 1000 1C, which could be used in the
fabrication of future capacitors.
ꢀ
1
and 600 1C with a rate of 10 1C min using a Perklin-
Elmer machine. Two kinds of DE measurements were
carried out under helium gas. In the first ones, the relative
0
00
permittivities e (real part) and e (imaginary component)
r
r
were measured on heating (25 1CpTp550 1C) at various
2
2
3
3
4
4
5
frequencies (10 , 5 ꢁ 10 , 10 , 5 ꢁ 10 , 10 , 5 ꢁ 10 , 10 ,
2
. Experimental procedures
5
2
ꢁ 10 Hz) by means of a Wayne-Kerr capacitance
0
00
bridge. In the second ones, e and e were collected
r
r
Commercial powders of CaCO , TiO (rutile), PbF and
2
3
2
at room temperature as
5
a
function of frequency
LiF were used in our study. All these starting materials
were MERCK products of high-purity (99.99%) or super-
pure quality. Calcium carbonate and titanium oxide were
dried up in an oven at 300 1C for 72 h to eliminate
moisture. These powders were then weighed in stoichio-
metric composition (Ca=Ti ¼ 1), mixed thoroughly and
milled in an agate mortar. The obtained mixture was
calcined at 850 1C for 8 h in a platinium capsule. After
cooling, the powder was remilled and annealed at the same
temperature for 2 h. The fluorides PbF2 and LiF were
dessicated under vacuum at 150 1C for 4 h to avoid their
hydrolysis during the sintering. Ceramics with initial
composition (1ꢀx)CaTiO +xPbF +xLiF were prepared
7
(
10 Hzpfp4 ꢁ 10 Hz) using a Hewlett-Packard (HP
4194A) analyzer. Before each measurement, the circular
faces of the ceramic were electroded by sputter deposition
of gold (Au) to make a capacitor, then heated in the cell
under vacuum at 150 1C for 2 h to prevent any contribution
of water vapor in the dielectric characteristics.
3. Results and discussion
The loss in ceramic’s weight is negligible and, therefore,
we can deduce that the final and the initial compositions of
the ceramics are close to one another. CaTiO3 and
(Ca,Pb)(Ti, Li)(O,F)3 samples behave differently during
3
2
using raw CaTiO (distribution grain size 50–500 mm).
3
The appropriate amounts of calcium titanate, lead and
lithium fluorides were homogenized in an agate mortar and
dry ground for 30 min in a glove box. The powders thus
the sintering process. Pure CaTiO ceramics are very brittle
3
even when sintered at 1200 1C. On the other hand, the
fluorinated ceramics are very hard and the relative density
reaches 95%. The relative shrinkages on ceramic diameter
DF/F of the oxifluorides are in the range 14–17.5%,
whereas the shrinkage of CaTiO sample sintered at 950 1C
is about 2% and does not exceed 5% after sintering at
1200 1C (Table 1). Larger is the amount of PbF +LiF
obtained were compacted into tablets with a diameter of
1
2
3 mm under a pressure of 10 MPa. Finally, the pellets
were placed on platinium plates and sintered by the
conventional route at 950 1C for 4 h in an electrical furnace
outside the box. The pellets were weighed before and after
sintering.
3
2
The oxifluorides obtained were investigated by several
methods: X-ray diffraction (XRD), scanning electron
additive and greater is the shrinkage. The greatest value of
DF/F is obtained for x ¼ 12:5%.
Table 1
Shrinkage coefficients, lattice parameters and volume for ceramics with initial composition (1ꢀx)CaTiO
+xPbF +xLiF
3 2
3
V (A )
˚
a (A)
˚
b (A)
˚
c (A)
˚
X
DF/F (%)
0
1.9
14
5.443(7)
5.456(1)
5.446(8)
5.462(3)
5.454(7)
5.451(5)
7.653(1)
7.651(1)
7.654(1)
7.653(5)
7.672(1)
7.672(6)
5.376(8)
5.370(1)
6.379(8)
5.371(3)
5.373(7)
5.380(4)
223.938
224.164
224.216
224.511
224.822
224.991
0
0
0
0
0
.025
.05
14.6
16.5
16.8
17.5
.075
.10
.125

ARTICLE IN PRESS
L. Ta ı¨ bi-Benziada, H. Cherfouh / Journal of Physics and Chemistry of Solids 68 (2007) 1003–1007
1005
As results of the XRD study, a solid solution was formed
by the addition of PbF and LiF to CaTiO . It occurs in the
0pxp0.125 initial composition range. Beyond x ¼ 0:125
the ceramics are polyphase. The room-temperature XRD
graph is not smooth grained. The oxifluoride is composed
of round and rectangular particles, which are clearly visible
with a grain size of about 1–2 mm.
At this stage of investigation by taking into account the
high values of shrinkages (Table 1), the low porosity and
the XRD data, we can conclude that the additive
2
3
spectra of both CaTiO and the sample corresponding to
3
x ¼ 0:05 are reported in Fig. 1. As seen, these patterns are
similar to each other and the isomorphism between the two
samples is evident. The peaks were indexed in an
orthorhombic symmetry and Table 1 gives the lattice
parameters (a, b, c) and volume (V) of the oxifluorides. As
we can take it from this table, the variation of V with
increasing PbF2 and LiF content is negligible and the
PbF +LiF plays simultaneously a double role: as a
2
substituent which enters the host lattice to form the
(Ca,Pb)(Ti,Li)(O,F) phases and as a sintering chemical
3
agent to improve the compactness and the density of the
ceramics.
The DSC analyses reveal three thermal phenomena for
each sample. These transformations are accompanied by
small changes in enthalpy and, therefore, we can conclude
orthorhombic perovskite structure of pure CaTiO is not
3
disturbed at all when calcium, titanium and oxygen ions
2
+
+
ꢀ
are partially substituted by Pb , Li and F , respectively.
Concerning the SEM observations, the microstructural
analyses confirmed the results obtained by the XRD study.
The ceramics are single phases with a residual intergranular
porosity varying between 5% and 8% as x decreases from
that the CaTiO structure does not change so much. These
3
phenomena are ascribed to second-order phase transitions
with structural changes in space group without any
disturbance in the orthorhombic lattice of CaTiO . Table
3
2 reports the values of the phase transition temperatures
(T , T , T ) and the heat capacities (C , C , C ) at,
12.5% to 2.5%. Fig. 2 reports the scanning electron
micrograph of the fracture for sample with initial
1
2
3
p1
p2
p3
respectively, T , T and T .
1 2 3
composition (1ꢀx)CaTiO +xPbF +xLiF. The micro-
The dielectric measurements display the same profile for
the temperature dependence of the dielectric permittivities
3
2
0
00
0
00
e
CaTiO sintered with the aid of 5 mol% of PbF +LiF is
and e . The variation of e and e for a ceramic of
r
r
r
r
CaTiO3
3
2
1
1
200
000
illustrated in Fig. 3 from room temperature up to 550 1C.
0
00
In the temperature range 25–250 1C, e and e are
r
r
frequency independent. The real component decreases
progressively whereas the imaginary part of the permittiv-
ity is very stable with increasing temperature. Between 250
8
6
4
2
00
00
00
00
0
0
and 450 1C, the e’ ꢀT and e ꢀT curves exhibit a very
r
r
broad peak. The maximum occurs around 400 1C and the
0
00
r
values of e and e decrease as the frequency increases,
r
wihout any displacement of the maximum toward high or
0
00
low temperature. Beyond 450 1C, an increase of e and e
r
r
with temperature is observed due especially to the
+
contribution of the electrical conductivity of Li ion on
the dielectric characteristics.
0
.0
0
20
40
60
80
[°2 θ]
Ca0.95Pb0.05(Ti0.95Li0.05)O2.85F0.15
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
.0
0
20
40
60
80
[
°2 θ]
Fig. 1. XRD patterns of CaTiO
3
and 0.95CaTiO
3
+0.05PbF
2
+0.05LiF
Fig. 2. Scanning electron micrograph of the fracture of 0.95Ca-
sample sintered at 950 1C.
3 2
TiO +0.05PbF +0.05LiF ceramic.

ARTICLE IN PRESS
L. Ta ı¨ bi-Benziada, H. Cherfouh / Journal of Physics and Chemistry of Solids 68 (2007) 1003–1007
1006
Table 2
Phase transition temperatures and heat capacities for (1ꢀx)CaTiO
3
2
+xPbF +xLiF samples sintered at 950 1C
ꢀ1
ꢀ1
ꢀ1 ꢀ1
ꢀ1 ꢀ1
p3 (J mol K )
x
T
1
(K)
T
2
(K)
T
3
(K)
C
p1 (J mol
K
)
C
p2 (J mol
K
)
C
0
—
702
678
605
565
512
—
748
715
705
634
630
—
805
742
748
778
813
—
—
—
0
0
0
0
0
.025
.05
8.2787
2.2338
4.1393
16.7230
2.2209
1.5846
7.9188
22.2974
24.8398
7.0113
13.5317
39.4514
38.8688
11.5114
.075
.10
.125
10.7470
a
140
120
14
2
0 Hz
12
10
8
1
5
1
5
1
1
000
2
x10 Hz
100
3
0 Hz
8
6
4
2
00
00
00
00
0
3
x10 Hz
80
4
0 Hz
4
5x10 Hz
60
40
6
5
0 Hz
1
5
x10 Hz
4
2
20
2
0
0
5
.0
5.5
6.0
6.5
7.0
log f (Hz)
7.5
8.0
100
200
300
400
500
600
Fig. 4. Variation of the real and the imaginary permittivity as a function
of frequency at room temperature of 0.95CaTiO
ceramic.
T (°C)
3 2
+0.05PbF +0.05LiF
b
2
1
1
1
1
1
000
800
600
400
200
000
2
10 Hz
frequency, the real permittivity is practically constant with
a value of ꢂ130 while the imaginary one decreases slightly
2
x10 Hz
T2
5
1
5
3
0 Hz
T1
T3
5
6
3
x10 Hz
from ꢂ3 to ꢂ1 in the frequency range 10 ꢀ3 ꢁ 10 Hz then
6
4
10 Hz
increases strongly beyond 3 ꢁ 10 Hz.
4
x10 Hz
5
1
2
5
0 Hz
8
6
4
2
00
00
00
00
0
5
x10 Hz
4. Conclusion
The fluoride mixture of PbF2 and LiF is a suitable
chemical agent to sinter CaTiO3 at low temperature
(
Tsint: ¼ 950 1C), the relative density of the deriving
ceramics reaches 95%. The addition of lead and lithium
fluorides to calcium titanate allowed us to obtain new
oxyfluorides (Ca,Pb)(Ti,Li)(O,F) . The triple substitution
1
00
200
300
400
T (°C)
500
600
3
Ca–Pb, Ti–Li and O–F occurs in the 0pxp0.125 initial
composition range and induces no significant change in the
CaTiO3 cell which remains orthorhombic whatever the
value of x is. Three second-order phase transitions are
detected by DSC and confirmed by dielectric measure-
ments. These materials could be of interest in several
technological applications and particularly in the produc-
tion of capacitors.
Fig. 3. Variation of permittivity as a function of temperature at various
frequencies: (a) real component and (b) imaginary part for ceramic
corresponding to x ¼ 0:05.
The very wide peak is probably due to the overlap of the
three thermal phenomena detected by DSC. The maximum
could be attributed to the phase transition at T while the
2
phase transitions at T and T appear as inflection points,
1
3
00
which are more visible on e ꢀT curve, before and after the
r
maximum at T .
2
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0
00
r
Fig. 4 depicts the e ꢀlog f and e ꢀlog f curves at room
r
temperature for the sample corresponding to x ¼ 0:05 and
3
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