A Nautiyal article in JECS

Please cite this article in press as: A. Nautiyal, et al., Local analysis of the grain and grain boundary contributions to the bulk dielec-
tric properties of Ca(Cu3−yMgy)Ti4O12 ceramics: Importance of the potential barrier at the grain boundary, J Eur Ceram Soc (2015),
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.035
ARTICLE IN PRESSG Model
JECS-10438; No.of Pages8
Journal of the European Ceramic Society xxx (2015) xxx–xxx
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Journal of the European Ceramic Society
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Local analysis of the grain and grain boundary contributions to the
bulk dielectric properties of Ca(Cu3−yMgy)Ti4O12 ceramics:
Importance of the potential barrier at the grain boundary
A. Nautiyal∗
, C. Autret∗∗
, C. Honstettre, S. De Almeida-Didry, M. El Amrani, S. Roger,
B. Negulescu, A. Ruyter
GREMAN, UMR 7347-CNRS, F. Rabelais University, Parc de Grandmont, 37200 Tours, France
a r t i c l e i n f o
Article history:
Received 30 June 2015
Received in revised form
19 December 2015
Accepted 22 December 2015
Available online xxx
Keywords:
CCTO ceramics
Substitution
Perovskite ceramics
Ceramic dielectrics
a b s t r a c t
The present work focuses on two aspects related to the synthesis and the dielectric properties of
CaCu3Ti4O12 (CCTO) ceramics. Effective strategies able to synthesize high quality CCTO, with reduced
dielectric losses and high relative permittivity, are achieved by the substitution of Cu ion by Mg one in
the crystal. Ca(Cu3−yMgy)Ti4O12 ceramics have been synthesized by conventional mixed oxide method
involving CCTO and MgO. This approach is found to drastically reduce the synthesis time by lowering
the eutectic point of CCTO as a result of the small fraction (1–3 wt%) of MgO added to the mixture.
Experimental data have been used to successfully conﬁrm this substitution and the only presence of CuO
at grain-boundaries. The role of grain and grain boundary contributions on the dielectric properties of
ceramics are investigated. It is found that samples containing 1% MgO lead to excellent relative permittiv-
ity values of the order of 3.5 × 104
. In these substituted ceramics, extracted relative permittivity values are
found to be extremely stable, with low loss tangent (<0.06), up to 100 kHz at room temperature. From the
various samples investigated, the optimized ceramics are found to contain large grains (≥15 ␮m). Local
intra-grain and inter-grain electrical transport measurements show that the grains and their respective
grain boundaries exhibit metallic and semiconducting behaviors, respectively. This approach conﬁrms
the crucial role of the potential barrier at the grain boundary in determining the dielectric properties of
such Ca(Cu3−yMgy)Ti4O12 ceramics.
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
High relative permittivity has been observed in oxides com-
posed of ACu3Ti4O12 [1] (A—rare earth metals). In particular, CCTO,
which exhibits giant permittivity ε between ≈104–105 over a wide
range of frequencies (typically from DC to 106 Hz range [2]) has
attracted enormous research attention for applications targeting
super capacitors and varistors [3]. Theoretical and experimental
observations have shown that the permittivity of CCTO is almost
temperature independent over a large range, from 100–400 K [4].
A number of investigations, including impedance spectroscopy and
atomic force microscope (AFM) study have focused on the origin
of the high permittivity exhibited by CCTO [5–7]. Extrinsic effect
∗ Corresponding author. Fax: +33 247367121.
∗∗ Corresponding author.
E-mail addresses: anoop.nautiyal@etu.univ-tours.fr (A. Nautiyal),
autret@univ-tours.fr (C. Autret).
associated with thin insulating grain boundaries is generally used
to explain this material property through an Internal Barrier Layer
Capacitor model (IBLC) [8,9].
Despite the fact that CCTO ceramics exhibit giant dielectric
permittivity, this material also exhibits high dielectric losses
restraining its use in practical applications [3]. To mitigate these
limitations, some methods such as Cu substitution or insulating
phase doping have been proposed [10–13]. High permittivity and
low dielectric loss values were obtained in CaCu2.9La0.2/3Ti4O12
ceramics (ε ≈ 7500 and tan ı ≈ 0.05) [10]. Other CCTO ceramic
derivatives, such as CaCu3Ti4O11.7F0.3 have also been synthesized,
showing very high permittivity values with relatively low dielec-
tric losses at 100 kHz (∼6000 and <0.075, respectively) [11]. Similar
effects were observed in CaTiO3 and SrTiO3 doped CCTO ceramics
[12,13] however, dielectric losses observed in these studies (>0.15)
are still not low enough while maintaining relative permittivity
above 104. Moreover, in these studies the dielectric losses were
also shown to increase for frequencies above 100 kHz. This loss is
too high to be used in capacitors.
0955-2219/© 2015 Elsevier Ltd. All rights reserved.

2 A. Nautiyal et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
Depending on the specificity of CCTO microstructure, sev-
eral methodologies, able to reduce the dielectric loss have been
adopted. For example, by increasing effective grain boundary resis-
tances [14–18] or via the introduction of new polycrystalline phase
like CaTiO3 and TiO2 [12,19]. To reduce the synthesis time of pure
CCTO, the duration must be reduced without influencing the grain
growth within the CCTO ceramic. MgO has been identified as a
good sintering aid, promoting the densification during the sinter-
ing stage, whilst modifying the microstructure [20]. This in turn,
is expected to significantly influence material dielectric properties.
Previous work on CCTO–MgO composites have shown that MgO
is able to substantially reduce the dielectric loss while maintaining
the high dielectric permittivity [21]. Using this approach, Jumpatam
et al. [22] demonstrated an enhancement of dielectric properties
of Ca2Cu3−xMgxTi4O12. The effect of Mg2+ substitution (in place of
Cu2+) on CCTO properties has been described by Hu et al. [23]. Addi-
tionally, Li et al. [24], Singh et al. [25] and Xu et al. [3] also studied
similar compounds. Li et al. [26] reported that dielectric losses in
CCTO/MgO composites can be reduced to around 0.052 in the fre-
quency range 0–200 kHz. Rahman et al. [27] also studied dielectric
properties of a similar composite at higher frequencies (1 MHz up
to 1 GHz) to assess the effects of Mg substitution on Ca and Cu sites.
Local current voltage (I–V) measurements have already been done
on CCTO in order to find out the nature of grain and grain boundary
resistance [28–30]. Experimental results arising from these stud-
ies were found to be promising, in terms of relative permittivity
as well as dielectric losses however, only at low frequencies below
1 kHz. Moreover, the high relative permittivity was decreased for
both substituted and mixed samples in the majority of these stud-
ies. In addition to this, most of them have seldom shown a clear
relationship between dielectric properties of mixed samples and
their dependence on the grain size and the grain boundary resis-
tance. The present work attempt to address these short falls by
investigating the relationship between grain size and relative per-
mittivity, as well as the relationship between the dielectric losses
and grain boundary resistances.
In case of pure CCTO, long sintering time (∼20 h) is required to
get optimized dielectric properties [1]. So, the aim of this paper
is to find an effective strategy able to: (i) drastically reduce sam-
ple preparation time, (ii) modify the nature of the grains within
the ceramics by introducing small amounts of Mg in CCTO and (iii)
separate grain and grain boundary contributions to bulk dielec-
tric properties. We have greatly reduced this duration due to MgO
addition. All samples investigated in this work were sintered for
3 h only. This approach is shown to simultaneously reduce dielec-
tric losses whilst ensuring that a high relative permittivity value is
maintained up to 10 kHz.
2. Experiment
(1−x) (CaCu3Ti4O12 −(x) MgO (x = 0–3 wt%) ceramics were pre-
pared by a mixed oxide process. Pure CCTO were prepared by an
organic gel-assisted citrate synthesis and sintering process [31].
The gel formation was based on an auxiliary organic polymer in an
aqueous nitrate solution at 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 where M = Ca, Cu. Titanium
citrate derived from titanium alcoxide was subsequently added
to produce a clear solution stable up to the gel pyrolysis tem-
perature. The solution was gelled via an in-situ formation of an
auxiliary three-dimensional polymeric network. The acrylamide
and N,N -methylenediacrylamide monomers 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 h at 500 ◦C. Following this step, the powder
was ground by mortar-pestle and then ball milled for 2 h. The pow-
der was subsequently dried overnight at 60 ◦C and heated in air for
10 h at 950 ◦C thereafter. After this sintering step, the CCTO powder
was reground once more.
The powder MgO was used as-supplied product (Alfa
product—Magnesium Oxide—99.5% pure) without modification.
CCTO and MgO powders were thoroughly mixed by solid state
mixing (0–3 wt% of MgO) followed by pressing into pellets approx-
imately 10 mm in diameter and 2 mm in thickness. These pellets
were then sintered at 1050 ◦C for 3 h under controlled experimental
conditions (∼3 ◦C/min, both heating and cooling). The temperature
and time of this sintering cycle was especially chosen to achieve
the optimal dielectric properties in our mixed ceramics. It must be
noted that for single phase CCTO, these thermal conditions were
not appropriate for obtaining best dielectric properties, due to the
shorter sintering duration used in the above process. For pure CCTO
the 20 h of sintering is required to have optimized dielectric prop-
erties [1]. As a result of these differences, we referrer to pure CCTO
sample (x = 0) as being “a non-optimized sample”.
Ceramic densities were estimated by measuring the mass and
volume of the pellets and comparing the measured data to the
theoretical X-ray density (see Table 1). To assess the presence of
different extra phases in pure CCTO as well as in mixed phase sam-
ples, X-ray diffraction (XRD) data were collected using a D8 Bruker
instrument ( ≈ 1.54 Å) over a 2Â range of 20–80◦ (in 0.02◦ steps).
Experimental profiles were modelled using a pseudo-Voigt profile
shape function. With respect to the crystallographic structure, such
as lattice parameters and atomic positions, isothermal temperature
factors (Biso) were refined using the FullProf software [32].
Microstructures of as sintered ceramics were observed using a
Hitachi 4160-S scanning electron microscope (SEM). All the SEM
images were acquired under secondary electron imaging mode
(accelerating voltage = 10 kV). The room temperature Raman spec-
tra of sintered samples were measured with a Renishaw Raman
microscope using a green laser ( = 514 nm). For elemental analysis,
a SEM equipped with energy dispersive spectra (EDS) was used.
To determine averaged grain size of pellets, several SEM images
have been analyzed. Generally the Mendelson method is used to
determine averaged grain size but it is not appropriate in our case
because of a bimodal grain size distribution. In this work, we used
two different methods to determine the averaged grain size. The
first one is the averaged grain intercept (AGI) method [33] which
is a technique used to quantify the grain/crystal size for a given
material by drawing a set of randomly positioned line segments
on the micrograph. In order to take into account the bimodal dis-
tribution, we evaluated the averaged grain size for both small and
large grains by AGI method. The other method is an original method
(developed In-house) to exclusively analyze larger grains with the
aid of an Image processing software tool. In this approach the total
area of large grains (≥15 ␮m) was determined and divided by the
number of large grains. Following this procedure, we estimated the
averaged size of large grains by approximating the grain shape to a
circle, where the diameter simply represents grain size diameter.
Well-polished pellets having sputtered Ag films on both sides
(∼300 nm thick) were used for the dielectric measurements, per-
formed by an Agilent 4294A frequency-response analyzer at room
temperature and in the frequency range of 100 Hz–1 MHz. The elec-
tric field dependence resistivity of the pellets was obtained by I–V
measurements carried out on an Agilent B2911A source meter.
Before analyzing the dielectric properties of our samples as a func-
tion of the Mg content, we have also checked the influence of the
pellet’s thickness on dielectric and resistivity measurements.
Inter and intra grain I–V measurements were performed using
the same digital source meter (Agilent B2911A unit). For this type
of measurement, samples were polished and circular aluminum

A. Nautiyal et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx 3
Table 1
Comparison of bulk sample density and averaged grain size of Ca(Cu3−yMgy)Ti4O12 ceramics with MgO weight%.
MgO weight (%) Sample relative density (%) AGI method (averaged grain size in ␮m) Our method (only averaged large grain size in ␮m)
Large Small Bulk
0 94.4 ######### ######## 2.8 #########
1 94.2 32.3 3.0 17.7 47
2 93.6 29.2 3.3 16.3 40
3 93.8 23.4 3.7 13.6 30
Fig. 1. XRD Pattern of CCTO.
electrodes of 20 ␮m in diameter and 20 ␮m in separation were
patterned on ceramic surfaces by conventional optical lithogra-
phy. Micro-contact measurements were performed using tungsten
probes (10 ␮m in radius).
3. Results and discussions
Powder XRD patterns obtained at room temperature confirmed
that non-optimized sample CCTO was indeed a pure polycrystalline
single phase compound. Measured and refined patterns for CCTO
are shown in Fig. 1 and confirm the Im-3 cubic symmetry. The CCTO
lattice parameter of this sample was determined as a = 7.39(1) Å, in
accordance with the results reported by Subramanian et al. [1]. The
unit cell volume V was deduced to be ∼403.08 Å [3]. The reliability
factors Rwp, Rbragg and 2 were deduced to be approximately 7.18,
2.95 and 5.09 respectively.
Fig. 2 shows the X-ray diffraction patterns for all mixed sam-
ples. As shown in Fig. 2a, it is clear that, in addition to main phase
CCTO, extra peaks appear for the composite samples. These peaks
have been assigned to CuO and TiO2 secondary phases. The appear-
ance of these peaks may be attributed to the sintering process
employed, in which liquid phase CuO–TiO2 may form [34]. Fig. 2b
shows a magnified XRD image of the TiO2 phase. From these XRD
images, it can clearly be seen that no magnesium based compounds
were detected probably because the MgO content is well below the
sensitivity of the detector. XRD data for all investigated samples
were refined, revealing negligible change in lattice parameters of
the primary phase for either sample types (CCTO and mixed sam-
ples ∼7.39(1) Å). This observation may be sufficiently explained by
probable substitution of Cu2+ by Mg2+ (both exhibit comparable
ionic radii).
Almost alike relative bulk density (calculated by measuring
mass and volume of pellets) was found for all the samples (see
Table 1). Based on this experimental observation, it may be con-
cluded that the bulk density is almost constant and does not
Fig. 2. (a) XRD patterns of pure CCTO and CCTO/MgO ceramics. (b) Zoom of the TiO2
phase peak.
contribute significantly to dielectric properties of the different sam-
ples.
Fig. 3 shows representative scanning electron micrographs.
From these experimental data, it can clearly be seen that the addi-
tion minute fractions of MgO to the CCTO ceramic greatly enhances
grain growth, even for short sintering time (3 h).
To determine averaged grain size, several SEM images have been
analyzed. Contrarily to the pure CCTO sample, which shows uni-
form grains of size around 2.8 ␮m, a bimodal grain growth is clearly
observed for all mixed samples. Such growth-type was reported
recently for CCTO [35]. As explained before, two methods were used
to calculate the averaged grain size: AGI and In house method. Both
methods lead to the same qualitative dependence of the averaged
grain size vs. Mg content.
According to IBLC model, the effective permittivity is directly
related to the microstructure and varies as ε ∼(tg/tgb) εgb, where
tg and tgb are the averaged grain size and thickness of grain-
boundaries, respectively, and εgb is the grain boundary permittivity
[36]. From the above expression and in later sections of this present
work, it becomes apparent that the relative permittivity is propor-
tional to grain size [31].

Fig. 3. SEM images of surface’s morphology for pure CCTO and CCTO/MgO ceramics.
Table 2
EDX analysis on grains and grain boundaries.
MgO weight (%) Probable grain composition Probable grain boundary composition
0 <Ca1.0Cu3.2Mg0.0Ti4.3Oy> ############################
1 Ca1Cu3.0Mg0.1Ti4.2Oy 97% CuO
2 Ca1Cu2.9Mg0.2Ti4.2Oy 25% CuO + CCTO
3 Ca1Cu2.8Mg0.3Ti4.2Oy 12% CuO + C(Cu, Mg)TO
Both grain and grain boundary compositions have been inves-
tigated using statistical EDX measurements (see Table 2). These
measurements revealed the incorporation of Mg replacing Cu on
grain sites and that Mg content varies linearly with Mg initial ratio.
It is in agreement with XRD results which showed no diffraction
peak of MgO phase. It must also be noted that grain boundary anal-
ysis revealed no definitive evidence except the fact that it mainly
contains copper oxide.
Due to complex nature of grain boundaries which can be clearly
seen from SEM microstructures and EDX data (see Table 2), we per-
formed Raman spectroscopy on grains and grain boundaries. Due to
the relatively smaller grain boundary thickness exhibited by pure
CCTO pellets, it was challenging to focus the laser on grain bound-
ary during the Raman spectra studies for such samples. As a result,
for this sample, measurement represented both grains and grain
boundaries. Raman spectra of all other samples confirmed that
grains consisted of CCTO and TiO2 phase. The main vibration modes,
corresponding to CCTO modes (at 444 cm−1, 510 cm−1, 576 cm−1)
and TiO2 ones (144 cm−1) were in close agreement with experi-
mental data shown in Fig. 4a. For grain boundaries (see Fig. 4b), the
main contribution is that of the CuO phase (at 298 cm−1, 346 cm−1,
631 cm−1). Additionally, we also observed peaks for CCTO phase
however, this was due to the spot size of the laser, which made dif-
ferentiation between grains and grain size difficult to assess. These
results further confirm phases obtained in the XRD data. This can be
explained by the substitution of Cu by Mg as well as the subsequent
migration of Cu toward grain boundaries. As we will show in sub-
sequent sections of this present work using experimental I–V data,
the nature of grain-boundaries profoundly influences the dielectric
properties of CCTO ceramics.
Measurements of dielectric properties were carried out at room
temperature from 100 Hz to 1 MHz on polished samples. Depen-
dence of sample thickness on the dielectric/electrical properties
is shown in Fig. 5. From data, it must be noted that the mea-
sured resistivity of all samples remained fairly constant for different
applied fields. Concerning the dielectric properties, a small differ-
ence appears at high frequency (>100 kHz) and only for sample
thicknesses below 1 mm. This could be explained by the fact that
a very thin sample may easily promote percolation path for cur-
rent transport, resulting in an increase of leakage current. Although,
such difference was not considered to be significant, compared
to those due to substitution effects. Moreover, our aim is just to
compare results from one composition to another. As such, all mea-
surements have been done using 2 mm thickness samples only.
Since Internal Barrier Layer Capacitance (IBLC) model assumes
that CCTO ceramics consist of semiconducting grains and insulat-
ing grain boundaries, it is very important to separate the grain (Rg,
Cg) and the grain boundary (Rgb, Cgb) contributions [37,38]. It is to
be mentioned that the grain boundary contribution is more rele-
vant at lower frequencies, while the grain contribution plays a more
important role at higher ones. We used an equivalent circuit model,
consisting of two parallel RC elements connected in series and
assuming negligible pure Ohmic resistance of the electrical probes
(Rc). Instead of a simple capacitance model, we used this one with
a Constant Phase Element (CPE) to correctly fit the experimental
data. One RC element represents semiconducting grains, whilst the

Fig. 5. Thickness dependence of dielectric/electric measurements for 1% sample (inset: thickness dependence of resistivity).
Fig. 4. Raman spectra on grains (a) and on grain boundaries (b).
Fig. 6. Equivalent circuit used for the IBLC model.
other represents insulating grain boundary regions (Fig. 6). Taking
into account the frequency dependence of these two contributions,
we analyzed the frequency dependences of both the relative per-
mittivity and the loss tangent. Note that the fitting has not been
done for pure CCTO sample because it was non-optimized. Fitting
and experimental data for the mixed samples can be found in Fig. 7.
This model can be described by the following equation:
∗
(ω) − ∞ =
( s − ∞)
1+(ω )␣
where * (␻) is the complex dielectric constant, s and ∞ are the
dielectric constants at “static” and “infinite” frequencies respec-
tively, ω is the angular frequency and is a time constant linked
to relaxation phenomena. The exponent ˛, which takes a value
between 0 and 1, allows the description of different spectral shapes
as a result of different relaxation times (DRT). Note that, for the
Cole–Cole model, which reduces to the Debye one when ˛ = 1, two
semicircles related to grains (Rg, Cg) and grain boundaries (Rgb, Cgb)
are expected to apparent in complex impedance plots. The radius of
each semicircle represents the bulk grain and the bulk grain bound-
ary resistances. In our case, it is better to consider the experimental
and fitted frequency dependence of both, the dielectric constant
and the loss tangent because we did not observe two semicircles in
the Cole–Cole plot. This is possibly due to big difference in between
grain and grain boundary resistances (as presented in the Table 3).
All parameters obtained by fitting our data show a variation of
grain and grain boundary resistance with the amount of MgO used
during ceramic synthesis. It is clear that the MgO mixing amount

Fig. 7. Relative permittivity and loss tangent frequency dependence with fits for pure CCTO and CCTO/MgO ceramics.
Table 3
Data from RC element model fitting.
MgO (%) Grain
boundary
resistance
(Rgb) ( )
Grain
resistance
(Rg) ( )
Grain
boundary
capacitance
(Cgb) (F)
Grain
capacitance
(Cg) (F)
˛gb ˛g Relative
permittiv-
ity at
100 Hz
Loss tangent at 100 Hz
1 5.00 × 107
98 2.25 × 10−8
1.70 × 10−7
0.97 1 3.97 × 104
0.05
2 1.00 × 107
125 2.15 × 10−8
1.40 × 10−7
0.95 1 3.48 × 104
0.09
3 0.20 × 107
150 1.85 × 10−8
1.10 × 10−7
0.94 1 2.84 × 104
0.16
has an opposite effect on grain and grain boundary resistances. The
grain resistance increases with the MgO mixing amount, while the
grain boundary resistance decreases.
The frequency dependence of relative permittivity and dielec-
tric loss attained by Ca(Cu3−yMgy)Ti4O12 ceramics are plotted in
Fig. 7. First, it may be noticed that all mixed samples show com-
paratively higher relative permittivity than the pure CCTO sample
(>104 for frequencies between 102 Hz–3.105 Hz), due to the much
larger average grain size (>15 ␮m). This confirms (based on the IBLC
model) that the relative permittivity value strongly depends on the
grain size. Moreover, the sample prepared with 1% MgO exhibited
the highest relative permittivity (ε ≈ 3.6 × 104 at room tempera-
ture and at 1 kHz as compared to 2.2 × 104 for pure sample—see
Fig. 7). Concerning the loss tangent (at RT and for frequencies lower
than 50 kHz), all mixed samples were found to show relatively low
loss values (<0.15) as compared to pure CCTO. Again, the 1% sam-
ple appears to be the best compound as confirmed by the lowest
value of the loss tangent for frequencies up to 20 kHz. Such low
loss values (<0.06 at 1 kHz) and high relative permittivity may be
attributed to larger grains favored by the excess of CuO [26,28].
These results are in agreement with the Mg content dependence of
the averaged grain size (see Table 1). Moreover, for mixed samples,
additional TiO2 phase play an important role in reducing losses,
whilst increasing sample resistivity [19,39]. Since the dielectric loss
part of CCTO is directly related to the bulk sample resistivity [18,40],
we performed these measurements for all samples. From this inves-
tigation, the measured resistivity was found to be higher in mixed
samples than the pure CCTO samples. It can be attributed to the
much higher grain boundary resistance in the former. From the
experimental bulk resistivity vs. Electric field plot (Fig. 8), it is clear
that samples containing 1 and 2% show higher bulk resistivity val-
ues over the entire range of electric field (even if the resistivity
Fig. 8. Resistivity measurements as a function of the electric field at RT for pure
CCTO and CCTO/MgO ceramics.
of the 2% sample decreases more rapidly with increasing electric
field).
It must be noted that the pure CCTO pellet has not been opti-
mized. As a result, the grain boundary structure is not well defined
(see Fig. 3). Using IBLC model, it appears that the grain boundary
resistance for the doped samples is very high as compared to grain
resistance (see Table 3). Based on this observation, we can sum-
marize that the resistance of grain boundaries contribute more to
the bulk resistivity. Moreover, the modification of grain boundaries

Fig. 9. I–V characteristics for both intra-grain and inter-grains measurements.
lead to higher bulk resistivity in doped samples than in pure CCTO
one. Taking the frequency dependence of the loss tangent variation
into account, we can conclude that the 1% sample yielded the best
dielectric and resistive properties in this work. As we will show in
later sections that it could be due to the high resistivity of grain
boundaries.
Despite a low Rg value, the 1% sample exhibits the best compro-
mise, leading to the highest relative permittivity and the lowest
loss tangent. In order to clarify the role of grains and grain bound-
aries on governing dielectric properties, we performed intra-grain
and inter-grains electrical transport measurements. Intra- and
inter-grains both exhibit different type of electrical properties
(metal-like and semiconductor-like, respectively). From Fig. 9, it
can be seen that conductivity consistently varies with the MgO
weight%. Moreover, the inter-grain resistance is almost three orders
of magnitude higher than that of the intra-grain. This confirms
that the bulk resistance originates mainly from the grain bound-
ary resistance. It can be seen that inter-grain conductivity for 1%
sample is minimum, as shown in Table 3. Such kind of feature has
already been observed in previous reports by De Almeida-Didry
et al. [14], whereby the bulk sample resistance and loss values
are mainly related to grain boundaries, with little influence due to
grains. These results are coherent with the IBLC model. However, it
must be noted in our case that, grains and grain boundaries behave
metallic and semiconducting like, respectively. The best compro-
mise is still found for the 1% sample for which grain boundaries
are nearly always composed of pure CuO. Moreover, it confirms
the important role played by grain boundaries in bulk dielectric
properties of samples. We acknowledge that additional investiga-
tions on the role of the grain boundaries are still needed to fully
understand bulk properties of such ceramics.
4. Conclusions
We successfully separated grain and grain boundary contribu-
tions to dielectric properties of substituted Ca(Cu3−yMgy)Ti4O12
ceramics by changing the nature of the grain (composition and
size) and the grain boundary conductivity. MgO addition decreases
the eutectic temperature of CCTO and, in consequence, strongly
decreases the total processing time as compared to the one usually
used for pure CCTO. More, it also leads to good dielectric proper-
ties due to a larger averaged “Bulk” grain size. Our analysis (XRD
and EDX) confirm the “natural” Cu substitution and the presence
of CuO at the grain boundary. It confirms that this approach (mix-
ing two phases as MgO and CCTO) may promote this material for
industrial and practical capacitor applications. EDX analysis and
Raman spectroscopy were used to determine both grain and grain
boundary compositions. All substituted Ca(Cu3−yMgy)Ti4O12 pel-
lets show better bulk relative permittivity and lower loss tangent
as compared to pure CCTO sample which is not optimized for such a
thermal treatment. RC element fitting model was used to separate
grain and grain boundary contributions to the bulk relative permit-
tivity and loss values. Our results revealed that both grain and grain
boundary capacitances decrease as the Mg content increases. Con-
cerning resistances, it appears that they increase and decrease for
grain and grain boundary, respectively. It confirms the importance
of the potential barrier at the grain boundary observed using local
I–V measurements. Our results clearly show that grain-boundaries,
mainly composed of a CuO, give a better bulk relative permittivity
and significantly decrease the loss tangent value. The frequency
dependence of dielectric properties was found to be remarkably
constant up to 10 kHz. Especially, for 1% MgO, we got an excellent
and stable relative permittivity as well as a very low loss tangent
up to 100 kHz due to its largest bulk averaged grain size. At room
temperature and at 1 kHz, very high values of relative permittivity
(3.35 × 104) and very low values of dielectric loss (<0.06) have been
obtained for this sample (1% MgO mixed). More, the bulk resistiv-
ity of this sample is very high (∼108 -cm) for electric field values
(<102 V/cm).
Acknowledgments
We thank Joe Sakai for very helpful discussions, and Region
Centre (France) for financial support.
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