Influence of Zr doping on the dielectric properties of CaCu3Ti4O12

ceramics

Lu Zhang • Yugong Wu • Xiaozan Guo •

Zhiyuan Wang • Yanan Zou

Received: 7 July 2011 / Accepted: 22 August 2011 / Published online: 4 September 2011

� Springer Science+Business Media, LLC 2011

Abstract Pure and Zr-substituted CaCu3(Ti1-xZrx)4O12

(x = 0, 0.01, 0.02, 0.03) ceramics were prepared by the

Pechini method. X-ray powder diffraction analysis indi-

cated the formation of single-phase compound, and all the

diffraction peaks were completely indexed by the body-

centered cubic perovskite-related structure. The effects of

Zr4? ion substituting partially Ti4? ion on the dielectric

properties were investigated in frequency range between

100 Hz and 1 GHz. The low frequency (f B 105 Hz)

dielectric constant decreases with Zr substitution and

the high frequency (f C 107 Hz) dielectric constant is

unchanged. Interestingly, a low-frequency relaxation was

observed at room temperature through Zr substitution. The

observed dielectric properties in Zr-substituted samples

were discussed using the internal barrier layer capacitor

model. A corresponding equivalent circuit was adopted to

explain the dielectric dispersion. The characteristic fre-

quency of low-frequency relaxation rises due to the

decrease of the resistivity of grain boundary with Zr

substitution, which is likely responsible for the large

low-frequency response at room temperature.

1 Introduction

Since the first report by Subramanian et al. [1] in 2000 on

the gigantic permittivity and unusual dielectric behaviors

of CaCu3Ti4O12 (CCTO), there have been lots of reports

about this material. This material in both ceramic and

single-crystalline [2] forms shows giant dielectric response.

The relative dielectric constant er is up to 105 at room

temperature, which is practically frequency independent

between dc and 106 Hz for the temperature range between

100 and 600 K. The dielectric permittivity abruptly drops

down to a value *100 with decreasing the temperature

below 100 K or increasing the frequency above 106 Hz,

accompanying neither phase transitions nor detectable

changes of long-range crystallographic structure. The

decrease in dielectric permittivity demonstrates a typical

Debye-type relaxation behavior. In order to understand this

unique response, many mechanisms have been proposed

including local dipole moments associating with off-center

displacement of Ti ions [1], relaxor-like slowing down of

dipolar fluctuations in nanosize domain [2], collective

ordering of local dipole moments [3], grain boundary

(internal) barrier layer capacitance (IBLC) [4, 5], and

domain boundary barrier layer capacitance [6]. To date, the

origin of the giant dielectric response is still controversial.

It is well known that dielectric properties of CCTO are

very sensitive to the processing condition and doping. For

the doping effects, several researches have reported sub-

stitution of Mn for Cu [7], La, Sr for Ca [8, 9], and Fe, Nb

for Ti [10, 11]. The dielectric properties of the composition

0.1–1.0 wt% ZrO2 ? CaCu3Ti4O12 have been investigated

and both of the dielectric constant and loss can be reduced

[12]. In the above research, the obtained ZrO2-doped

ceramics are nonstoichiometric and the results may not

reveal directly the effects of Zr substituting Ti on the

L. Zhang � Y. Wu (&) � X. Guo � Z. Wang � Y. Zou

School of Electronic and Information Engineering,

Tianjin University, Tianjin 300072,

People’s Republic of China

e-mail: wuyugong@tju.edu.cn

L. Zhang � Y. Wu � X. Guo � Z. Wang � Y. Zou

Key Laboratory of Advanced Ceramics and Machining

Technology, Ministry of Education Tianjin University,

Tianjin 300072, People’s Republic of China

123

J Mater Sci: Mater Electron (2012) 23:865–869

DOI 10.1007/s10854-011-0508-5

dielectric properties. To date, the investigation on dielectric

properties of CCTO with B-site partially occupied by Zr

ion is still not sufficient.

Wet chemical methods have been used to prepare CCTO

ceramics [13–15]. In comparison with the traditional solid-

state method, wet chemical method has shown consider-

able advantages such as excellent chemical stoichiometry,

compositional homogeneity, and dopant distribution. In

this work, single-phase, stoichiometric CaCu3(Ti1-x

Zrx)4O12 (x = 0, 0.01, 0.02, 0.03) ceramics were prepared

by the Pechini method (one of wet chemical methods [16]).

The frequency-dependent dielectric properties and com-

plex impedance spectra were investigated at room tem-

perature and above. The obtained dielectric properties in

the Zr-substituted CCTO were discussed in terms of the

IBLC model.

2 Experimental

CaCu3(Ti1-xZrx)4O12 (x = 0, 0.01, 0.02, 0.03) powders

were prepared by the Pechini method using CaCO3 (99%,

Tianjin NO. 3 Chemical Reagent Factory, China),

Cu(CH3COO)2�H2O (99%, Tianjin Jiangtian Chemical

Technology Corporation Ltd., China), ZrOCl2�8H2O (99%,

Tianjin Jiangtian Chemical Technology Corporation Ltd.,

China), Ti(OC4H9)4 (98%, China National Pharmaceutical

Industry Corporation Ltd., China), citric acid (CA, 99.5%,

Tianjin Jiangtian Chemical Technology Corporation Ltd.,

China), and ethylene glycol (EG, 99%, Tianjin Jiangtian

Chemical Technology Corporation Ltd., China) as starting

raw materials. At first, stoichiometrical Ti(OC4H9)4 and

ZrOCl2�8H2O were dissolved in EG with continuous stir-

ring on a magnetic stirrer at room temperature. Secondly,

CA was slowly added into the solution and the temperature

rose to 60 �C with continuous stirring so as to dissolve the

added solids completely. The amount of CA and EG were

decided by the mole ratio of total metal ion, CA and EG

which is 1:1:4. After the solution was clear, stoichiomet-

rical CaCO3 and Cu(CH3COO)2�H2O were added into the

solution with continuous stirring and the temperature were

increased further to 80 �C so as to dissolve the entire

amounts of solids added. The solution was then aged in an

oven at 90 �C for a week. The dry gel was calcined at

1,000 �C in air for 2 h. The calcined powder was pressed

into pellets of 10 mm diameter and *1.6 mm thickness.

All of the pellets were sintered at a temperature of

1,100 �C in air for 2 h and then furnace-cooled to room

temperature.

The phase composition of powder was analyzed by

XRD (Rigaku D/MAX 25000) using a Cu Ka radiation for

a range of Bragg angles 2h (10� B 2h B 90�) at room

temperature. The final density of each sintered pellet was

determined following the Archimedes method. The

microstructure of the ceramic samples was investigated by

scanning electron microscope (SEM, HITACHI X-650).

Silver paste was coated on each sample face and fired as

the electrodes at 800 �C. Dielectric properties were mea-

sured using a LCR meter (Tonghui TH2816) in the fre-

quency range 100–150 kHz and an impedance analyzer

(Agilent 4991) in the frequency range of 1 MHz–1 GHz.

3 Results and discussion

XRD patterns for CaCu3(Ti1-xZrx)4O12 powder (x = 0,

0.01, 0.02, 0.03 and 0.04) prepared by the Pechini method

calcined at 1,000 �C for 2 h are shown in Fig. 1. As for

x B 0.03 samples, all the diffraction peaks were completely

indexed by the body-centered cubic perovskite-related

structure. No secondary phase was observed in x B 0.03

samples. Moreover, compared with pure CCTO, the shift of

the peak positions was hardly observed in Zr-substituted

samples. When Zr substitution was increased to x = 0.04,

Ca2Zr5Ti2O16 and CuO phases were found in XRD pattern.

Lattice parameters of CaCu3(Ti1-xZrx)4O12 ceramics sin-

tered at 1,100 �C for 2 h are displayed in Table 1. The

relative density is the ratio of ceramic density and theoret-

ical density which is calculated using the lattice parameter

of CaCu3(Ti1-xZrx)4O12 sample. Compared with pure

CCTO sample, the effects of Zr substitution on the lattice

parameter and porosity of ceramics are very weak. It costs

much reaction time to prepare single-phase CCTO powder

by traditional solid-state method. The synthesis of ZrO2-

doped CCTO powder, for example, needs a prolonged total

reaction time of 24 h [12]. A much shorter calcination time

of 2 h is enough to yield single-phase Zr-substituted CCTO

powder in our Pechini method. Moreover, the Pechini

method improved the Zr-doping level in CCTO to 3 mol%.

Fig. 1 XRD patterns of CaCu3(Ti1-xZrx)4O12 powder synthesized by

Pechini method

866 J Mater Sci: Mater Electron (2012) 23:865–869

123

In former report, the Zr-doping level in CCTO is 1.2 mol%

(1 wt% ZrO2) [12].

Figure 2 shows the SEM images of the fractured sur-

face. Abnormal grain growth occurs in pure CCTO

ceramics and the grain size distribution is non-uniform as

shown in Fig. 2a. In the Zr-substituted CCTO ceramics

abnormal grain growth is rarely observed and the grain size

uniformity is enhanced via Zr substitution (Fig. 2b–d).

Figure 3 shows the frequency dependence of the

dielectric constant and loss measured at room temperature.

As can be seen, all the samples show giant dielectric

constant of er C 5,000 in a broad frequency range lower

than 106 Hz. The remarkable dielectric relaxation (the

high-frequency relaxation) in the frequency range higher

than 106 Hz is observed in all samples. The dielectric

constant values in the frequency range lower than 106 Hz

decrease with Zr substitution, and the dielectric constant

values of all samples in the frequency range higher than

108 Hz are *100 as shown in Fig. 3a. For the x = 0.01,

0.02, and 0.03 Zr-substituted samples, apart from the one

mentioned above in the frequency range higher than 106

Hz, there is another relaxation (the low-frequency relaxa-

tion) in the frequency range lower than 103 Hz. Usually,

only high-frequency relaxation can be observed in pure

CCTO at room temperature. For the polished CCTO, both

of relaxations can be measured [17]. Figure 4 illustrates the

frequency dependence of the dielectric dispersion for the

x = 0.03 Zr-substituted sample at different temperatures.

The low-frequency relaxation becomes obvious with

increasing the temperature, as shown in Fig. 4a. The cor-

responding peaks in the imaginary parts of dielectric

spectra are displayed in Fig. 4b.

The complex impedance spectroscopy of all samples at

room temperature is shown in Fig. 5. For all samples, only

a semi-circular arc was observed with a non-zero intercept

on the real axis at a high frequency. According to the

equivalent circuit analysis, the high frequency, non-zero

intercept is associated with semiconducting grains and the

low-frequency, extrapolated intercept is attributed to the

insulating grain boundary [7]. The resistivity of grain

remains unchanged with Zr substitution at 20 X cm as

shown in the inset of Fig. 5. On the other hand, the resis-

tivity of grain boundary is reduced by approximately one

order of magnitude with Zr substitution from 3.06 MX cm

(pure) to 437 kX cm (x = 0.03 Zr).

In the IBLC model, the effective dielectric constant is

directly proportional to the ratio of the grain size (tg) to

thickness of the insulating layer (grain boundary) (tgb),

assuming that the dielectric constants for the grain and

grain boundary are the same and the resistivity of grain

boundary is much larger than that of the grain [18]. On the

condition that the grain boundary thickness is relatively

Table 1 Lattice parameters of

samples sintered at 1,100 �C for

2 h

CaCu3(Ti1-xZrx)4O12 x = 0 x = 0.01 x = 0.02 x = 0.03

a (A) 7.399 7.401 7.397 7.406

Density (g/cm3) 4.84 4.82 4.83 4.84

Relative density (%) 96 96 95 96

Fig. 2 SEM images of x = a 0,

b 0.01, c 0.02, and d 0.03

CaCu3(Ti1-xZrx)4O12 ceramics

sintered at 1,100 �C for 2 h

J Mater Sci: Mater Electron (2012) 23:865–869 867

123

unchanged with grain size, a higher effective dielectric

constant is attributed to a larger grain [18]. As can be seen

in Fig. 2, the large grain growth is constrained by Zr

substitution. Thus the decrease of low-frequency dielectric

constant for Zr-substituted samples could be attributed to

small grain size. Base on the results mentioned above, we

adopt an equivalent circuit [19] as shown in Fig. 6 to

interpret these two dielectric relaxations in Zr-substituted

samples. It contains three RC elements (RgCg, RgbCgb, and

RxCx, respectively) and a frequency-dependent term ZUDR.

The element RgCg is used to describe the effect of the

grains, and ZUDR is employed to represent the effect of

hopping conduction of localized charge carriers [20]. The

RgbCgb element is used to delineate the effects of grain

boundaries. The RxCx element is employed to describe the

effect of domain boundary [20]. The capacitances, Cx, Cgb,

and Cg, show little temperature dependence and the resis-

tances, Rx, Rgb, Rg, exhibit the thermally activated

behaviors. The characteristic frequencies of high-frequency

and low-frequency relaxation are essentially determined by

CgbRg and CxRgb, respectively. For the pure CCTO, the

low-frequency relaxation can not be observed at room

temperature, because its characteristic frequency at room

temperature is very low. As the measuring temperature is

raised, Rgb decreases and causes an increase in the

characteristic frequency of the low-frequency relaxation.

Consequently, the low-frequency relaxation for the pure

CCTO can be observed only at high measuring temperature.

The resistance of grain boundary Rgb decreases dramati-

cally with Zr substitution compared with pure CCTO from

the complex impedance spectroscopy in Fig. 5. There-

fore, the characteristic frequencies of the low-frequency

Fig. 3 Frequency dependence of a the dielectric constant and b loss

in CaCu3(Ti1-xZrx)4O12 ceramics measured at room temperature

(300 K)

Fig. 4 Frequency dependence of the a real part (e0) and b imaginary

parts (e00) of the dielectric constant for x = 0.03 Zr-substituted sample

at different measuring temperature

Fig. 5 Impedance complex plane plots for the CaCu3(Ti1-xZrx)4O12

ceramics at room temperature (300 K); The inset shows an expanded

view of the high-frequency data close to the origin. Filled symbolsindicate selected frequency

868 J Mater Sci: Mater Electron (2012) 23:865–869

123

relaxation for Zr-substituted samples raise and the low-

frequency relaxation can be observed at room temperature.

The low-frequency relaxation becomes obvious with

increasing the measuring temperature.

4 Conclusions

CaCu3(Ti1-xZrx)4O12 (x = 0, 0.01, 0.02, 0.03) ceramics

have been successfully prepared using the Pechini method.

The total reaction time to prepare single-phase powder can

be reduced and the Zr-doping level in CCTO can be

enhanced through this method. The crystal structure,

dielectric properties and complex impedances are investi-

gated. The low-frequency dielectric constant decreases

with Zr-substitution. The room temperature impedance

spectroscopy analysis shows that the resistivity of grain

boundary is reduced dramatically through Zr substitution.

As a result, the low-frequency relaxation can be observed

for Zr-substituted samples at room temperature.

Acknowledgments Project supported by the open foundation of

Key Laboratory of Advanced Ceramics and Machining Technology,

Ministry of Education Tianjin University.

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Fig. 6 An equivalent circuit model to delineate the electrical

properties of Zr-substituted samples

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