my paper-icanm-

Dielectric properties of CCTO/MgTiO3 composites: A new approach for Capacitor application
Anoop Nautiyal, Cécile Autret, Christophe Honstettre, Sonia Didry, Mohamed El Amrani, Sylvain Roger
and Antoine Ruyter
GREMAN, UMR 7347-CNRS, F. Rabelais University, UFR Sciences, Parc de Grandmont, 37200 Tours, France
Abstract : In this work, first, single phase CaCu3Ti4O12 (CCTO) and MgTiO3 were synthesized by sol gel
and solid state method respectively and then (1-x) CaCu3Ti4O12 - x MgTiO3 (x = 0, 1,3 and 5 weight%)
composites were prepared by a conventional mixed oxide method using CCTO and MgTiO3 single phase
powders. The characterization for phase identification was done by X-Ray diffraction, for surface
morphology, scanning electron microscope was used. Raman measurements were done to identify the
nature of grain and grain boundaries. CuO was identified as a main component of grain boundary. The
dielectric and resistivity properties of CaCu3Ti4O12/ MgTiO3 composites were investigated. The results
revealed that mixed phase samples can improve both dielectric and resistivity properties. The low
frequency (<105
Hz) loss tangent was greatly reduced(<0.2 for 1 % mixed sample); while, the dielectric
constant was found to be higher than 3.0×106
at room temperature (RT). The nature of CCTO/MgTiO3
composite’s grains and grain boundaries, varies according to the weight % of MgTiO3 in CCTO. More,
the synthesis duration is drastically reduced and it turns out that a small amount of MgTiO3 is good to
improve both dielectric and resistivity properties
Introduction : CCTO has attracted considerable interest in view of its giant permittivity (104
to 105
) in the
frequency range from dc to 105
Hz [1] which suggests the potential application in capacitor based devices.
More, its giant permittivity is independent of temperature within a temperature range of 100–400K
[1][2][3]. A large number of investigations have focused on the origin of the giant permittivity for CCTO
[4][5][6]. Generally, the extrinsic effect has now been accepted to be responsible for the giant permittivity.
For CCTO ceramics, the origin of the giant permittivity is attributed to an internal barrier layer capacitor
(IBLC). Nowadays, a lot of investigations suggest that IBLC is associated with semiconducting grains and
“insulating” grain boundaries. However, using local current probing with atomic force microscopy, Fu et
al. [5] clarify that the grain boundary displays semiconducting and the grain consists of semiconducting
regions and insulating regions. There is much debate on the origin of IBLC mechanism in CCTO
ceramics. The IBLC model shows a nature of Maxwell–Wagner relaxation in CCTO ceramics.
Unfortunately, the CCTO ceramics with giant permittivity exhibits higher dielectric loss and conductivity,
which limits its practical applications. Therefore, some methods such as the substitutions and insulating
phases doping have been conducted to decrease the dielectric loss. Some investigations show that the high
permittivity and low loss have been achieved, at some certain frequencies, through substitutions such as
CaCu2.9La0.2/3Ti4O12 [7] ceramics with a high permittivity of 7500 and low dielectric loss (less than 0.05).
CaCu3Ti4O11.7F0.3 [8] ceramics exhibits a giant permittivity (over 6000) and low dielectric loss (below
0.10). The similar effects have been observed in CaTiO3, SrTiO3 doped CCTO ceramics[9][10]. However,
the dielectric loss is still not below 10−1
, while maintaining the high permittivity (>104
) and obviously
dielectric loss increases with the frequency above 100 kHz. So, it is too high for capacitor applications.
Quite recently, some research has been done on addition of Magnesium Titanate (MgTiO3) to (Ba,Sr)TiO3
[11]. It shows significant improvement in reducing dielectric loss. The reduction in dielectric loss was
suggested to be related to the decrease of grains size which affects grain boundaries resistance.
Rajmi et al[12] studied the effect of MgTiO3 addition to the dielectric properties of CCTO and they have
reported that the dielectric constant as well as dielectric loss was decreased due to addition of MgTiO3(10-

30 %) to CCTO. The dielectric properties stability with frequency was also reported but the small (1-5 %)
addition of MgTiO3 to CCTO has not been reported much and it remains to be seen if this small addition
of MgTiO3 to CCTO could modify its microstructure and grain boundaries which could improve dielectric
properties of CCTO. So we mixed MgTiO3 to CCTO in small amount (1-5 %) and results were really
impressive. Even for pure CCTO, we have two order more dielectric constant values as compared to the
Rajmi et al paper due to the different CCTO synthesis techniques. There is clear difference in grains
growth for small MgTiO3 mixing as compared to previously reported by Rajmi et al. The mixing method
adopted by Rajmi et al. and us is different. We applied mixed oxide technique while Rajmi et al applied
solid state techniques. For mixed samples, dielectric constant values increases by two order and we got
moderately lower dielectric losses. We have also studied grain and grain boundaries by using Raman
spectroscopy which was not done by Rajmi et al[12]. The resistivity values were quite high and stable (at
electric fields E <102
V/cm) for 1 % MgTiO3 mixed sample. Since there are very limited studies on
MgTiO3 -CCTO ceramics, therefore it is interesting to further investigate the effect of small amount of
MgTiO3 addition on the dielectric properties of CCTO ceramics at room temperature.
Experiment : (1-x) CaCu3Ti4O12 – x MgTiO3 (x=0, 0.01, 0.03 and 0.05) ceramics were prepared by using
mixing oxide technique. In the first stage, pure CCTO were prepared by an organic gel-assisted citrate
synthesis and sintering process. It consists of the gel formation by an auxiliary organic polymer of an
aqueous nitrate solution in stoichiometric ratios. CCTO single phase was obtained by the following
method.
First, triammonia citrate as a chelating agent was added to an aqueous solution of metal (M) nitrates with
M = Ca, Cu. Titanium citrate formed from titanium alcoxide was then added and a clear solution stable up
to the gel pyrolysis was obtained. The solution was gelled by in situ formation of an auxiliary three-
dimensional polymeric network. The monomers acrylamide and N,N’- methylenediacrylamide were
dissolved and co-polymerized by heating at 150°C with azobisisobutyronitrile (AIBN) as a radical
polymerization initiator. The aqueous gel was then calcined for 20 hours at 500°C. After this the powder
was grinded by mortar-pestle and then ball milled for 2 hours. In the next step, we put the powder for
drying overnight and then sintered in air at 950°C for 10 h. This sintering cycle was chosen in order to get
single phase CCTO. Finally, the sintered CCTO powder was reground.
In earlier reports on synthesis of MgTiO3 by the solid-state method using oxides of magnesium and
titanium, a secondary phase of MgTi2O5 was obtained which was very difficult to remove from the
reaction product [13]. Dielectric losses are influenced by the presence of lattice defects (vacancies,
dislocations, impurities), secondary phase and porosity [14][15]. To reduce the dielectric loss, it is
therefore, important to prepare single phase magnesium titanate so that we can properly understand the
dielectric properties. To make single phase MgTiO3, stoichiometric ratio of Mg(NO3)2·6H2O and TiO2
were taken for the preparation of magnesium titanate by solid-state method. The mixture was properly
homogenized in an agate mortar. The homogenized mixture was loaded in an alumina crucible and kept in
the programmable furnace for heating at 900°C for 12 h. The sample was further heated at 1025°C for 19
h and at 1100°C for 10 h.
For both single phase samples (CCTO and MgTiO3), X-ray diffraction (XRD) data were collected using a
D8 Bruker (λ ≈ 1.540 Å) over a 2θ range of 20° – 80° with a step size of 0.02°, and a step time 12 s.
Experimental profiles were modelled using a pseudo-Voigt profile shape function. With respect to the
crystallographic structure, the lattice parameters, atomic positions, isothermal temperature factors (Biso),
and site occupancies were refined using the FullProf software [16].
Once single phase CCTO and MgTiO3 powders were prepared, we mixed both of them according to
stoichiometric ratios (1, 3 and 5 percent of weight of MgTiO3). The mixed powders were then ground and
pressed into pellets of about 10 mm in diameter and 2 mm in thickness. Finally, pellets were sintered at
1060°C for 3 hours. The temperature and time of sintering cycle was chosen in order to have maximum
dielectric constant and lowest loss. It must be noticed that, for single phase CCTO, these thermal
conditions are not appropriate to obtain the best dielectric properties because the optimal sinter time for

single phase CCTO has been determined to be 20 hours [1]. Our approach proves to be better for industrial
applications because, just in 3hours sintering, we have better dielectric properties than pure CCTO.
All the mixed phase samples were characterized by X-ray diffraction with the same instrument like
previously mentioned. The microstructures of the ceramics were recorded using Hitachi 4160-F scanning
electron microscope (SEM). The room temperature Raman spectra of the sintered samples were measured
with Renishaw Raman microscope using green laser (514nm). The laser power was 5 %, numbers of
accumulations were 10 and exposure time was 10 seconds.
For dielectric measurements, the pellets were polished in order to obtain smoother surfaces and silver
sputtered contacts having ~300 nm thickness were applied to both sides of pellets. Dielectric
spectroscopies were performed at room temperature in a frequency range from 100 Hz up to 1 MHz and
using a Agilent B2911A frequency-response analyzer. For resistivity, current voltage measurements were
carried out by the same source-measure unit.
Results and Discussion: Powder XRD patterns registered at room temperature confirmed that both
samples CCTO and MgTiO3 are single phase. Observed and simulate patterns for CCTO are given in
20 30 40 50 60 70 80
Intensity(a.u.)
2
Yobs
Ycalc
Yobs-Ycalc
(211)
(220)(130)
(222)
(231)
(400)
(422)
(440)
(350)
CaCu3
Ti4
O12
Figure 1a. XRD Pattern of CCTO
Figure 1a. The structure was refined and fit confirms the structure of cubic symmetry Im-3. The cell
parameter is close to a  7.38693(9) Å and the unit cell volume V is about 403.081(9) Å3
. The reliability
factors are RBragg = 2.98 % and 2
= 6.7. Figure 1b shows the refined XRD pattern of MgTiO3. All peaks
are indexed and no impurity was detected. The reflections are indexed in a rhombohedral structure with
the space group R-3c with following reliability factors - RBragg and 2
close to 7.09 and 2.90 respectively.
The refined cell parameters are a = 5.0540(2) Å, c = 13.8956(9) Å and V = 307.38(3) Å3
. In Figure 2,
which shows the XRD patterns of (1-x)CCTO-xMgTiO3 composites, together with the reflections
associated to CCTO phase and additional phases. They show the presence of CCTO single phase for 0, 1
and 3%.
Additional peaks appear on the “5%” pattern corresponding to intermediate phases. It can be noted that the
percent of MgTiO3 is very weak and difficult to see. Only for 5 percent, which is much greater height than
the limit of XRD detection, two supplementary phases were detected and identified as CuO and MgTi2O5.

10 20 30 40 50 60 70
Intensity(a.u.)
2
Yobs
Ycalc
Yobs-Ycalc
MgTiO3
(116)
(211)
(300)
(214)
(018)
(024)
(113)
(110)
(104)
(012)
(101)
(003)
Figure 1b. XRD Pattern of MgTiO3
20 30 40 50 60 70 80
  

*
*
* *
*
*
***
 5%MgTO
3%MgTO
1%MgTO
Intensity(u.a.)
2 (°)
CCTO
*
*CCTO
 CuO
 MgTi2
O5
Figure 2. XRD Pattern of pure CCTO and CCTO/ MgTiO3 ceramics
Figure 3 shows the scanning electron micrographs of pure phase CCTO and MgTiO3 mixed CCTO
samples. As observed, the average grain size (tg) of MgTiO3 mixed CCTO composites increases

significantly as compared to the pure phase sample. The grain growth is totally different as compared to
the previous study done by Rajmi et al [12] due to the different mixing techniques. We applied mixed
oxide technique while Rajmi et al. adopted solid state technique. They reported that the grain size
decreases for all the MgTiO3 doped (10, 20 and 30 %) samples as compared to pure sample(page 408
Rajmi et al. [12]). In our case, for all the mixed phase samples, grain size is bigger than the pure CCTO.
So the difference of MgTiO3 weight % addition to CCTO, seems to play an important role in dielectric
properties. We studied just 0 to 5 % MgTiO3 addition in CCTO and the dielectric properties and
resistivity values were improved. For pure CCTO sample, it consists of small grains with some residual
porosity observed among micro-sized loosely connected grains, showing however that the sintering
process is not optimal (Brize et al. have shown that the best properties have been obtained with a thermal
treatment at1050°C during 24h [17] [18]. The arrows represent the different kind of grain boundaries for
mixed phase samples. It is clear that, small amount of MgTiO3 is able to drastically change the grain size
Figure 3. SEM images of surface morphologies for CCTO/ MgTiO3 ceramics
even the sintering time was just 3 hours. In other words, it is clearly a way to decrease the CCTO
processing time. Although early studies addressed the potentially intrinsic character of the phenomenon

[19], it became progressively clear that an extrinsic cause was responsible for the giant permittivity.
According to impedance spectroscopy measurements of CCTO ceramics, Sinclair et al. [20] suggested that
these ceramics contain semiconducting grains and insulating grain boundaries. They explained that the
high dielectric constant is due to the Internal Barrier Layer Capacitance (IBLC) effect from grain
boundaries. Furthermore, Fang et al.[21] published a detailed investigation on the role of boundary barrier
layers in CCTO ceramics. Their results are consistent with the IBLC model suggesting that both grains
and grain boundaries are responsible for dielectric response of CCTO.
The difference in dielectric constant values for the pure CCTO reported by Rajmi et al[12] and our pure
CCTO, can be attributed to the different synthesizing techniques [22] for CCTO. Rajmi et al adopted solid
state reaction method but we applied sol-gel method for CCTO synthesis. Dielectric properties are directly
related to the microstructure, the effective permittivity ε has been shown to be proportional to tg/tgb where
tg and tgb are the average grain size and thickness of the grain-boundary regions, respectively (Amaral et al
[23]). Rajmi et al [12] had shown smaller grain size after mixing, so the dielectric constant value is lesser
as compared to our mixed phase samples. So, it is a well known fact that, as grain size increases, the
dielectric constant also increases but at the same time it is very important to consider the role/ nature of
grain boundaries and the role of different phases present in the sample.
For CCTO ceramic, the particular microstructure is observed by many researchers, which is associated
with the IBLC [24]. The melting phase, identified as CuO rich phase, has been observed in the grain
boundary of CCTO ceramics. This suggests that the CuO rich liquid phase appears during the sintering of
CCTO based ceramics, which facilitates the grain growth [25]. Due to the melting point of CuO beyond
1100°C, the liquid phase may be the CuO–TiO2 eutectic phase which exhibits a low melting temperature
of 1020°C in air. If it is true, the tuning of Cu and/or Ti stoichiometry will result in a significant variation
of microstructure of CCTO.
One big advantage of this mixed phase approach is that we can easily separate grain and grain boundary
contributions to study them separately. Table 1 shows the optical microscopy measurement of average
grain and grain boundary sizes for all samples. Measurements were done on several grains and grain
boundaries. For 1 % MgTiO3 mixed phase sample, average grain size is the highest and average grain
boundary thickness is the lowest among all. The ratio tg/tgb is maximum for 1 % and it decreases for 3 and
5 %. For our pure CCTO, the value is intermediate. To compare with the optimally sintered (24 hours at
1050O
C) sample, we have measured the same ratio. We will see further that this ratio is in accordance
with the dielectric constant values of all the mixed phase pellets.
Table 1: Comparison of average grain and grain boundary size for CCTO/MgTiO3 composites
MgTiO3
weight %
Average Grain size(tg) in
micron
Average Grain boundary
size(tgb) in micron
Ratio(tg/tgb)
Pure CCTO 10.85 0.53 20.47
1 % MgTiO3 138.74 5.47 25.34
3 % MgTiO3 105.29 13.18 7.98
5 % MgTiO3 103.46 18.85 5.48
Optimal CCTO 58.62 3.94 14.87
To have more clear idea about the grains and grain boundaries compositions and structures, we performed
Raman spectra measurement for all the mixed phase samples and different measurements were done on

0 200 400 600 800 1000
Intensity(Normalized)
Raman Shift (cm-1)
0 %
1 %
3 %
5 %
Figure 4a. Raman spectra on grains of CCTO/ MgTiO3 (Corresponding to CCTO modes)
numerous grains and grain boundaries. For pure phase CCTO, grain and grain boundary sizes are so small
that we only checked that the average structure is CCTO one. The results are shown in Figure 4a and 4b
0 200 400 600 800 1000
Intensity(Normalized)
Raman Shift(cm-1)
1 %
3 %
5 %
Figure 4b. Raman spectra on grain boundaries of CCTO/ MgTiO3 (Corresponding to CuO modes)
for grain and grain boundaries respectively for mixed phase samples. For grains, the main vibration modes
corresponding to modes of CCTO are shown in the Figure 4a at 444 cm-1
, 510 cm-1
, 576cm-1
. The
measurements on grain boundaries show three modes of CuO (Figure 4b) at 298 cm-1
, 346 cm-1
and 631
cm-1
. So from these measurements, it is clear that the CuO is the main composition of grain boundary and
it plays a big role in CCTO dielectric properties.

10
2
10
3
10
4
10
5
10
6
10
4
10
5
10
6
10
7
EpsilonR
Frequency (Hz)
0 %
1 %
3 %
5 %
Optimal Sample
Figure 5a. Frequency dependence of dielectric constant at RT for pure CCTO and CCTO/ MgTiO3
ceramics
Measurements of dielectric properties for all samples, pure and MgTiO3 mixed, were carried out on disks
of thickness ~ 1mm at room temperature from 100 Hz to 1 MHz. Figure 5a shows the dielectric constant
measurements for all the pellets as well as, for optimal sample. The dotted vertical line is a guide for eyes
and shows that dielectric constant values for mixed phase samples are very stable for frequency <105
Hz. It
can be seen clearly that all samples have almost same order of dielectric constant values (>105
). The
sample mixed with 1 % MgTiO3 exhibits high dielectric constant of 4.34×106
(at room temperature and at
1 kHz) which is ~22 % larger than 3.55 ×106
for pure sample. Since the ratio(tg/tgb) is less for optimal
sample as compared to our pure sample so, dielectric constant for our sample is more. This sample also
shows very flat curve for frequency <105
Hz. High value of dielectric constant can be attributed to large
grains and thin grain boundaries.
10
2
10
3
10
4
10
5
10
6
0,0
0,2
0,4
0,6
0,8
1,0
LossTangent
Frequency (Hz)
0 %
1 %
3 %
5 %
Optimal Sample
Figure 5b. Frequency dependence of dielectric loss at RT for pure CCTO and CCTO/ MgTiO3
ceramics

As far as the loss part is concerned (Figure 5b), all the mixed phase samples show relatively low loss as
compared to pure CCTO (at RT and frequency <100 kHz). It can be seen clearly in Figure 5b that the
0 1 2 3 4 5
0
1x10
6
2x10
6
3x10
6
4x10
6
5x10
6
EpsilonR
Doping %
10
2
Hz
10
3
Hz
10
4
Hz
10
5
Hz
Figure 6a. Frequency dependence of dielectric constant as a function of MgTiO3 weight % at RT for
pure CCTO and CCTO/ MgTiO3 ceramics
mixed phase sample with 1 % MgTiO3 shows minimum loss (<0.08) at room temperature and at 1 kHz
frequency. Again, this sample shows very stable low value of loss for frequency <100 kHz. In order to
0 1 2 3 4 5
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
LossTangent
Doping %
10
2
Hz
10
3
Hz
10
4
Hz
10
5
Hz
Figure 6b. Frequency dependence of dielectric loss as a function of MgTiO3 weight % at RT for
pure CCTO and CCTO/ MgTiO3 ceramics
determine the optimal MgTiO3 weight % composition, the dielectric constant and loss were plotted as a
function of MgTiO3 weight % in Figure 6a and 6b. It is evident from this plots that 1 % MgTiO3 is better

10
0
10
1
10
2
10
3
10
6
10
7
10
8
Resistivity(Ohm-cm)
Electric Field(V/cm)
0 %
1 %
3 %
5 %
Figure 7. Resistivity measurements as a function of electric field at RT for pure CCTO and CCTO/
MgTiO3 ceramics
in terms of properties, the high dielectric constant as well as the low dielectric loss. Again, the loss values
are also quite stable for frequency <105
Hz. Since the loss part is related to sample resistivity [26], we
performed resistivity measurements for all pellets (See Figure 7) and the resistivity of all the mixed phase
samples was found better than the pure CCTO sample. The two dotted lines show the two different
regions of the plots. If we consider electric fields E lower than 102
V/cm, resistivity values were one order
higher for mixed phase samples than the value measured for pure phase CCTO. More, if we reduce the
value of electric field (~10 V/cm), resistivity values for mixed phase pellets are really stable and very
high. So, the performance of the mixed phase samples is excellent if the electric field is not more than 10
V/cm.
So, the mixed phase samples are better as compared to pure phase CCTO in terms of dielectric properties
and resistivity measurements. The reason behind the better properties of mixed samples at this point seems
to be the contribution of the grain/grain boundaries and the new mixed phases. The CuO plays important
role in determining CCTO dielectric properties. Also the thickness of the grain boundaries, as we can see
in SEM images, is thinner for 1% mixed sample.
The study of temperature dependence of dielectric properties is very important. It has not been included
here because it was out of the scope of this article. Rajmi et al.[12] have done it in their study.
Conclusions : We studied the effect of small MgTiO3 mixing on dielectric and electrical properties of
CCTO ceramics. All pellets were prepared using CCTO and MgTiO3 powders in appropriate ratio [(1-x)
CCTO- x MgTiO3 (where x= 0, 1, 3, 5 weight %)). Mixed oxide technique was adopted instead of solid
sate technique, which was earlier applied by Rajmi et al. The sintering time was significantly reduced
from 20 hours to just 3 hours without reducing dielectric constant values. Rajmi et al. [12] also used the
same sintering cycle but dielectric constant values for their samples are two order less than our samples.
Rajmi et al.[12] samples are good for frequency >105
Hz because dielectric constant and loss values are
more stable with frequency. Raman spectroscopy was used to identify CuO as a main grain boundary
component. Large grains size and clear grain boundaries were observed for all the MgTiO3 mixed
samples. All MgTiO3 mixed pellets show better dielectric and resistivity properties as compared to the
pure CCTO pellet. For 1 % MgTiO3, we got excellent and stable dielectric constant as well as dielectric
loss values, specially for frequency <100 kHz. At room temperature and at 1 kHz, very high values of
dielectric constant (3×106
) and very low values of dielectric loss (<0.08) has been obtained in this study

for this sample(1 % MgTiO3 mixed). The resistivity of this sample was very high for low electric field
values (~102
V/cm). This suggests that small amount of MgTiO3 in CCTO is good to improve dielectric
properties. Specially, CCTO mixed with 1 % MgTiO3 shows the best dielectric and resistivity properties
among all the mixed phase or pure samples. So we can use this composite to improve the CCTO ceramic
performance. The results of this study are helpful to promote the MgTiO3 mixed CCTO material for
industrial and practical capacitor applications.
Acknowledgement : We thank Region Centre for financial support.
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