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REGULAR PAPER
Dielectric Properties of Y2 O3 and Nb2 O5 Co-Doped Barium Titanate Ceramics Wan Q. Cao, Fang L. Li, Mukhlis M. Ismail, and Gang Xiong
Jpn. J. Appl. Phys. 51 (2012) 041503
# 2012 The Japan Society of Applied Physics
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REGULAR PAPER
Japanese Journal of Applied Physics 51 (2012) 041503 DOI: 10.1143/JJAP.51.041503
Dielectric Properties of Y2 O3 and Nb2 O5 Co-Doped Barium Titanate Ceramics Wan Q. Cao, Fang L. Li, Mukhlis M. Ismail1 , and Gang Xiong2 Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, School of Material Science and Engineering, Hubei University, Wuhan 430062, China 1 School of Applied Sciences, University of Technology, Baghdad 10001, Iraq 2 Department of Electronic and Information Engineering, Xianning College, Xianning 437100, China Received June 11, 2011; accepted October 31, 2011; published online March 30, 2012 Conventional solid-state route method was used to prepare barium titanate ceramics doped with Y2 O3 and Nb2 O5 , Ba(Y0:5 Nb0:5 )x Tið1x Þ O3 (x ¼ 0:02; 0:04; 0:06; 0:08). The X-ray diffraction results confirmed that Y3þ and Nb5þ both substitute for Ti4þ at the B-site of ABO3 perovskite type structure, and second phase appeared for x ¼ 0:04 and 0.06. With the increases of Y3þ and Nb5þ concentrations, peak of dielectric constant of Ba(Y0:5 Nb0:5 )x Tið1xÞ O3 ceramics shifts to lower temperature more rapidly than that of BaZrx Ti1x O3 ceramics as the same x content. It was found that Ba(Y0:5 Nb0:5 )0:06 Ti0:94 O3 ceramics has compact structure, better temperature stability of dielectric constant, larger dielectric tunability under DC electric field. The results showed that, the Y2 O3 and Nb2 O5 doping improved the figure of merit for the BaTiO3 ceramics. # 2012 The Japan Society of Applied Physics
1. Introduction
The materials with a perovskite structure of general formula ABO3 , where A = mono or divalent, B = tri, tetra, or pentavalent ions have been found to be very useful and interesting for different solid-state devices.1–3) There are a large number of lead-free BaTiO3 -base ceramics with different composition, some of which exhibit a relaxor behavior with remarkable change in various characteristics related to the type of ionic substitutions.4–8) Concerning that the dielectric property is mainly dependent on substitution at Ti-sites such as BaZrx Ti1x O3 ,9) coupled heterovalent substitutes at Ti-sites could be a promising route to improve the dielectric and tunable properties. As is well-know, incorporation of rare-earth oxides into barium titanate ceramics is an effective and common method to maximize dielectric properties.2,10) With a trace amount of Y3þ dopant, the dielectric constant of barium zirconate titanate (BZT) ceramics are enhanced remarkably.2) To make multilayer ceramic capacitor Nb-doped barium titanate was used with high dielectric temperature stability,11) and Nb/Co co-doped BaTiO3 -based ceramics has a flat temperature coefficients of capacitance with higher dielectric constant.12) As Y3þ and Nb5þ were valence stable acceptor13) and donor14–16) at Ti-sites, respectively, compensation action of Y/Nb substitution at Ti-sites will influence the dielectric properties of BaTiO3 based dielectric materials. In the present work, Y2 O3 and Nb2 O5 co-doped barium titanate ceramics according to the composition formula Ba(Y0:5 Nb0:5 )x Ti1x O3 (x ¼ 0:02; 0:04; 0:06; 0:08) were fabricated. The dielectric and tunable properties of these ceramics were investigated. 2. Experimental Procedure
Ba(Y0:5 Nb0:5 )x Tið1xÞ O3 (BYNT; x ¼ 0:02, 0.04, 0.06, and 0.08) ceramics were prepared by the conventional solidstate reaction method with starting powders BaCO3 (99%), TiO2 (98%), Y2 O3 (99.99%), Nb2 O5 (99%) as the following composition formula: 4BaCO3 þ xY2 O3 þ xNb2 O5 þ 4ð1 xÞ TiO2 ! 4BaðY0:5 Nb0:5 Þx Tið1xÞ O3 þ 4CO2 ":
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The mixtures were milled for 4 h. After drying, the mixtures were calcined at 1150 C for 2 h, then re-milled, dried, and pressed to pellets with 10 mm on diameter and 0.2–0.3 mm thick. The pellets were sintered at 1250 C for 4 h in air. Silver slurry was pasted on the two sides of sintered pellets, and the samples were heated at 550 C for testing dielectric properties. Microstructures of the samples were examined by scanning electron microscope (SEM). Crystal structures of ceramic samples were analyzed by X-ray diffraction (D/Max-RP XRD-6000) at room temperature using Cu K ( ¼ 15:406 nm) radiation in the 2 range of 20–80 . The capacitances of the samples were measured using a HP 4192A impedance analyzer at 1, 10, and 100 kHz in temperature range of 40 to 120 C. 3. Result and Discussion
SEM results of samples Ba(Y0:5 Nb0:5 )x Tið1xÞ O3 sintered at 1250 C are illustrated in Fig. 1. From Fig. 1, the sample exhibits a uniform grain size distribution, homogeneous and dense structure with very few white points. Porous, loose structure, small crystals, and lot of small white granules on the crystal surface were formed at x ¼ 0:04 [Fig. 1(b)]. The heterogeneity can be observed from the shape and size of the ceramic crystals at x ¼ 0:06 [Fig. 1(c)]. The sample of x ¼ 0:08 exhibits dense and homogeneous ceramics in Fig. 1(d). It is found that, the average grain size increases with the increases of Y and Nb concentrations. XRD patterns of BYNT ceramics are given in Fig. 2. As the x increased (x ¼ 0:04), a Ba2 YNbO6 (JCPDS 24-1042) as a second phase was appeared, which corresponding to the white granules as shown in Figs. 1(b) and 1(c). The diffraction peaks of samples shift to the lower angle with increase of Y2 O3 and Nb2 O5 content. The corresponding lattice constants of four samples were calculated and listed in Table I. The increase of the lattice constant is due to the substitution of smaller Ti4þ cation by larger Y3þ and/or Nb5þ cations. At x increases to 0.08, the peak corresponding to Ba2 YNbO6 was not detectable and white granules disappeared as shown in Fig. 1(d). The diffraction peaks of samples shift to the lower angle with increase of Y2 O3 and Nb2 O5 content. This phenomenon indicates that the lattice constant of the samples increase with the increase of Y2 O3 and Nb2 O5 -doped concentrations. The reason to this shift is due to the substitution of smaller Ti4þ cation by larger Y3þ
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Jpn. J. Appl. Phys. 51 (2012) 041503
W. Q. Cao et al.
Fig. 1. Section SEM images of Ba(Y0:5 Nb0:5 )x Ti1x O3 ceramics for x ¼ 0:02 (a), 0.04 (b), 0.06 (c), and 0.08 (d).
(101)
Intensity (arb unit)
(111) (002)
(211) (202)
(210)
x=0.08 x=0.06 x=0.04 x=0.02
[83-1880]: BaTiO3(T) [89-2475]: BaTiO3(C)
20
30
40
50
60
70
2 θ ( °) Fig. 2. (Color online) XRD patterns of Ba(Y0:5 Nb0:5 )x Ti1x O3 ceramics with x ¼ 0:02, 0.04, 0.06, and 0.08.
and/or Nb5þ cations. As the sample Ba(Y0:5 Nb0:5 )0:06 Ti0:94 O3 has the most broadened dielectric peak and the largest extra-small peaks on XRD result, and the main dielectric peak-temperatures of four samples are determined by total amount of Y and Nb, the impurity phase Ba2 YNbO6 is important in enhancement of temperature stability of dielectric constant. The reason would be aggregated BaYNbO6 phase mixes inside the ceramics, and resulting in the tilt of neighbor cells of a double cell to decrease dielectric constant of the main peak and to increase the nearby peaks.
The temperature dependence of dielectric constant and dielectric loss for BYNT samples are shown in Fig. 3. It shown that, the frequency dispersion of all samples is observed around the maximum value of the dielectric constant peak Tm . The transition temperature (Tm ) shifts to the low temperature with the increase of Y2 O3 and Nb2 O5 concentration and moved toward higher temperature with increasing frequency for all samples. The (Tm ) of the sample of x ¼ 0:06 is located at the vicinity of room temperature. The reduction of dielectric loss leads to increasing dielectric constant at T < Tm , while at T > Tm it leads to decreasing the dielectric constant as shown in Fig. 3. The suppression peak effect is stronger by the co-doping of Y2 O3 and Nb2 O5 than by the doping of ZrO2 in barium titanate ceramics.17) For the x ¼ 0:06 sample two shoulders around a peak at about 25 C shows small change of dielectric constant. The broadened peak indicates that the diffuse phase transition (DPT) is attributed to the disorder in the arrangement of cations at the B-site, which is occupied by Ti4þ with Y3þ and Nb5þ leading to a microscopic heterogeneity in the composition, or core-shell structure with rich Y3þ and Nb5þ ions on the grain boundary. For the samples of x ¼ 0:02 to 0.06, dielectric loss is very low at temperatures higher than 0 C, but a dielectric peak appearing at 10 C gives rise to the increase of dielectric loss. Figure 4 is the relationship between transition temperature (Tm ) of BYNT samples and the additive compositions. As a comparison, the result of Ba(Zrx Ti1x )O3 (BZT) ceramics was also depicted from ref. 18. Figure 4 shows that the
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Jpn. J. Appl. Phys. 51 (2012) 041503
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Table I. Dielectric parameters of the YBNT ceramic samples measured at 1 kHz.
a (A)
Sample
c (A)
c=a
"m
Dielectric loss "m
" (at RT)
"="m % (20 C)
Figure of merit
K (%)
x ¼ 0:02
3.9814
4.0507
1.0174
10737
0.03
8500
8.70
1.43
28.0
x ¼ 0:04
3.9926
4.0507
1.0146
7809
0.02
6400
7.86
1.66
33.0
9.33 16.5
x ¼ 0:06
3.9870
4.0440
1.0143
6005
0.015
6000
2.60
1.88
32.5
21.66
x ¼ 0:08
3.9953
4.0469
1.0129
6620
0.025
6450
3.15
1.04
27.7
11.08
0.20
0.20
6000
8000 1 kHz 10 kHz 100 kHz
6000
0.10
0.05 4000
0.00
2000 -40
-20
0
20
40
60
80
100
Dielectric constant
0.15 5000 1 kHz 10 kHz 100 kHz
0.05
-40
-20
0
20
40
60
80
100
120
Temperature (°C) 0.20
7000
d 0.15
6000 1 kHz 10 kHz 100 kHz
0.10
4000 0.05
Dielectric constant
b Dielectric loss
Dielectric constant
0.00
3000
120
0.20
-20
0
20
40
60
80
100
0.15
6000
0.10
1 kHz 10 kHz 100 kHz
5000
0.05
4000
0.00
2000 -40
0.10
4000
Temperature (°C) 8000
c
0.00
3000
120
-40
Temperature (°C)
Dielectric loss
Dielectric constant
0.15
Dielectric loss
a 10000
Dielectric loss
12000
-20
0
20
40
60
80
100
120
Temperature (°C)
Fig. 3. (Color online) Temperature dependence of dielectric constants and loss of Ba(Y0:5 Nb0:5 )x Ti1x O3 ceramics with x ¼ 0:02 (a), 0.04 (b), 0.06 (c), and 0.08 (d).
140
The relative variation of dielectric constant can be expressed by "="m in some temperature region T . If T ¼ 20 C, the samples of x ¼ 0:06 has the smallest value of "="m % listed in Table I. In order to characterize the dielectric dispersion and diffuseness of the phase transition, a modified Curie–Weiss law is followed by2,3)
120
Tm (°C)
100 80 60 40
0 -20
1 1 ðT Tm Þ ¼ ; " "m C0
Ba(Y0.5Nb0.5)xTi1-xO3 Ba(ZrxTi1-x)O3
20
0
2
4
6
8
x (mol % ) Fig. 4. (Color online) Temperature dependences of peak dielectric constant for the BYNT and the BaZrx Ti1x O3 ceramics at 1 kHz.
dielectric constant peak drops more dramatic than that of BZT, which indicates the high heterogeneity at the B-site by co-doping of Y2 O3 and Nb2 O5 .
ðT > Tm Þ;
ð1Þ
where and C0 are the critical exponent and diffuseness parameter, respectively. The equation is derived on the basis of a Gaussian distribution, ¼ 2, of the Curie temperature. The experimental results fit well with the theoretical curve of the modified Curie–Weiss law shown in Fig. 5, and the fitted values were also listed in Table I. With x from 0.02 to 0.06, the values of three samples increase (toward to become a complete DFT), but for x ¼ 0:08, the is only 1.04, approaching the Curie–Weiss law.
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of x ¼ 0:04 has loose structure corresponding to larger tunability. For the sample of x ¼ 0:08, densification will increases internal stress, though the increase of the grain size would decreases internal stress. The reduction of dielectric loss leads to increase the tunability up to x ¼ 0:06 and at 0.08 the tunability decreases. In tunable microwave device applications, the figure of merit (F) is usually used to compare the quality of ferroelectric materials. The F can be define as
-11
ln(1/ε-1/ε m)
-12
-13
x=0.02 x=0.04 x=0.06 x=0.08
-14
-15
F¼ 1.5
2.0
2.5
3.0
3.5
ln (T-T m)
Fig. 5. (Color online) Plot of lnð1=" 1="m Þ as a function of lnðT Tm Þ at 1 kHz for Ba(Y0:5 Nb0:5 )x Ti1x O3 samples [symbols: experimental data; the solid line: fitting to eq. (1)].
K : dielectric loss
ð3Þ
It is noticed that, the F factor is proportional to x content as shown in Table I. The K factor showed that the Y2 O3 and Nb2 O5 doping succeeded in improving the dielectric properties of BYNT ceramics. 4. Conclusions
Ba(Y0:5 Nb0:5 )x Tið1xÞ O3 ceramics are fabricated via conventional solid-state route. Y2 O3 and Nb2 O5 co-doped BaTiO3 ceramics exhibit many interesting characterizes, such as the strong frequency dispersion, broadened dielectric constant peak with heterogeneity in the composition, and the rapid shift of Curie point to low temperature with the increase of Y2 O3 and Nb2 O5 concentration. The increase of indicated that the dispersion degree increases with contents of Y and Nb. The sample of x ¼ 0:06 shows compact structure, heterogeneity with second phase, better temperature stability of dielectric constant and larger dielectric tunability under DC electric field. This new type of the lead-free ferroelectric Ba(Y0:5 Nb0:5 )x Tið1xÞ O3 ceramics with suitable dielectric constant, low dielectric loss could be attractive to the multilayer ceramic capacitors.
7000
x = 0.08
6000
dielectric constant, ε '
5000 4000 6000
x = 0.06
5000 4000
x = 0.04
6000 5000 4000
x = 0.02
8000 7000 6000 5000 -10
-5
0
5
10
DC Electric Field (kV/cm)
Acknowledgements
Fig. 6. (Color online) The dielectric constant of the YBNT samples on the DC biasing electric field measured at 1 kHz.
The dielectric constants for BYNT samples were studied on the DC biasing field at room temperature and 1 kHz and the results are shown in Fig. 6. It is evident that, the dielectric constants for BYNT samples behaviors have been observed a strong response to the applied electric field. The nonlinearity of the dielectric constant with the DC biasing electric field results from the anharmonic interaction of Ti ions in the perovskite cubic structure. This can be explained by Johnson’s theory.19) The tunability K is defined by the percentage change in capacitance, or dielectric constant, with applied DC biasing electric field relative to zero-bias capacitance20) as K¼
"ð0Þ "ðEÞ 100%; "ð0Þ
Authors thank the support from the Research Foundation of Science and Technology Bureau of Hubei Province, China (Grant Nos. D 2009CDB027 and 2009CDB020).
1) M. T. Buscaglia, V. Buscaglia, M. Viviani, P. Nanni, and M. Hanuskova:
21)
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ð2Þ
where the "ð0Þ and "ðEÞ represent the dielectric constant value at a zero applied electric field and maximum applied electric field respectively. The tunability values of four YBNT samples were listed in Table I. The samples of x ¼ 0:04 and 0.06 have larger tunability and x ¼ 0:08 has the smallest one. This is due to the fact that the internal stress among grains can decrease tunability.21) The sample
2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)
