Volume 22 Issue 3 - May 11, 2012 PDF
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Dielectric properties and microstructure of Nb-Co codoped BaTiO3-(Bi0.5Na0.5)TiO3 ceramics
Hsing-I Hsiang*, Li-Then Mei, Yi-Jih Chun
Department of Resources Engineering, College of Engineering, National Cheng Kung University
 
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Abstract
X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and an impedance analyzer were used to examine the Nb-Co co-doping effects on the densification, crystalline phase, microstructure development and dielectric-temperature characteristics of BaTiO3-(Bi0.5Na0.5)TiO3 ceramics. The results indicate that the Curie temperature shifted to a higher temperature (above 140℃) by adding BNT. The dielectric constant-temperature (ε-T) curve broadened at the Curie temperature due to the small grain size (0.3-0.4μm). A core-shell structure was developed, which is helpful to flatten the ε-T curve of BaTiO3 ceramic at high temperatures.

I.Introduction
The multilayer ceramic capacitor (MLCC) has recently been widely used in automotive electronic systems such as ABS and ECU. The dielectric materials used in MLCC, which can meet the EIA X8R specification (the dielectric constant change is less than ±15% from its 25℃ value over the temperature range of -55 to 150℃) have attracted much attention. The BaTiO3-Nb2O5-Co3O4 system is widely used for commercial MLCC due to the high permittivity and temperature stability. 1-3 However, for the BaTiO3-Nb2O5-Co3O4 system, the dielectric constant increases with increasing temperature and reaches a maximum value at near 125℃ (Tc, Curie temperature). The dielectric constant then decreases rapidly as the temperature exceeds Tc. As a result these materials have difficulty satisfying the EIA X8R specification near 150℃. 1-3 It is therefore necessary to improve the dielectric-temperature (ε-T) characteristics at high temperatures by shifting Tc of BaTiO3-based ceramics to a higher temperature. Incorporating the (Bi0.5Na0.5)TiO3 phase into BaTiO3-based ceramics is considered an effective method for shifting Tc to a high temperature. 4-5 This study produced BaTiO3-(Bi0.5Na0.5)TiO3 ceramics with Tc near 150℃ as dielectric materials. The Nb-Co co-doping effects on the densification, crystalline phase, microstructure development and ε-T characteristics of BaTiO3-(Bi0.5Na0.5)TiO3 ceramics are investigated.

II.Experimental Procedure
The starting materials were commercial BaTiO3 (Palceram BT-4; Nippon Chemical, Tokyo, Japan), high purity Nb2O5 (99.9985%, Alfa Aesar, Ward Hill, MA), Co3O4 (99.9985%, Alfa Aesar), Bi2O3 (99.975%, Alfa Aesar), Na2CO3 (99.5%, Showa, Tokyo, Japan), and TiO2 (99.99%, Alfa Aesar). The (Bi0.5Na0.5)TiO3 ceramic was prepared from Bi2O3, Na2CO3, and TiO2, mixed and then calcined at 900℃ for 2 h. The BaTiO3 added with 5 mol% (Bi0.5Na0.5)TiO3 powder was mixed and calcined at 900℃ for 2 h and denoted as BT-5BNT. The Curie temperature of the BT-5BNT sample was determined using differential scanning calorimetry (DSC-7, Perkin Elmer, Shelton, CT) near 150℃. The BT-5BNT (98 mol%) powder was ball milled with 1.5 mol% NbO2.5 and 0.5 mol% CoO4/3 for 12 h, dried in an oven and denoted as BT-5BNT-NC. The powders were dry-pressed at 180 MPa into pellets. These specimens were sintered at the temperature range of 1250-1350℃ for different periods.

The crystalline-phase identification was determined using X-ray diffractometry (XRD) (Siemens, D5000, Karlsruhe, Germany) with CuKa radiation. The densities of the sintered samples were determined using the Archimedean method. The true densities were calculated using the lattice parameter obtained from XRD. The microstructure was observed using scanning electron microscopy (Hitachi S4100, Tokyo, Japan). The TEM (JEOL, JEM-3010,Tokyo, Japan) was used to observe the specimen grain size and morphology. The electron diffraction patterns of the crystalline species were also obtained using the TEM with the camera constant at 80 cm. Semi-quantitative determination of the element content was detected using the EDS (Noran, Voyager 1000, Waltham, MA) attached to the TEM. Dielectric properties (relative dielectric constant (K) and tanδ) were measured using an LCR meter (HP 4284A, HP Co. Ltd., Hyogo, Japan).

III. Results and discussion
The relative densities of the samples BT-5BNT and BT-5BNT-NC sintered at various temperatures are shown in Fig.1. In the BT-5BNT sample case the relative density was not above 90% until the sintering temperature was increased to 1350℃. However, for the BT-5BNT-NC sample the relative density increased slightly with increasing sintering temperature and reached above 95% at 1275℃.  At sintering temperatures below 1300℃, the BT-5BNT-NC sample exhibited higher relative densities compared with sample BT-5BNT, indicating that Co-Nb codoping can promote BT-5BNT ceramic densification. This is in good agreement with the fact that Chozono et al. 1 found that densification was promoted in Co-Nb codoped BaTiO3 compared with pure BaTiO3. Figures 2(a) and (b) show a TEM bright field image and corresponding selected area electron diffraction pattern (SAEDP) at region I for sample BT-5BNT-NC sintered at 1250℃. Moreover, the chemical composition of regions I and II, as confirmed by EDS are shown in Table I. The crystalline phase at region I can be identified as a perovskite structure on the basis of its diffraction pattern and EDS analysis. The second phase at region II can be assigned as the Ba6Ti17O40 inferred from EDS analysis. This is supported by the findings observed by Xu 6 and Chazono et al. 1 who reported that Ba6Ti17O40 second phase was observed for BaTiO3 ceramics doped with Nb. In BT-5BNT-NC system, Nb+5 and Co+3 ions substituted for Ti+4 ions in the perovskite structure via solid state diffusion, which resulted in the segregation of Ti+4 and hence forming Ba6Ti17O40 second phase. Figure 2(c) shows the Nb and Co distribution profiles across region I in Fig. 2(a) as determined using EDS attached to the TEM. This indicates that the Nb and Co concentration gradients occurred in the BaTiO3 grain and the core region was free from Co, which may result from the solid state diffusion of Nb+5 and Co+3 ions from the shell into the core region.
Fig.1 Relative densities of the samples BT-5BNT and BT-5BNT-NC sintered at various temperatures.

Fig.2 (a) TEM bright field image and (b) corresponding selected area electron diffraction pattern (SAEDP) at region I (c) Nb and Co distribution profiles across region I for sample BT-5BNT-NC sintered at 1250℃.

Table I. Chemical composition of regions I and II determined by EDS.
Region / Atom%
Ti
Ba
Nb
Co
I
49.13
50.43
0.44
-
II
68.54
30.2
0
1.26

Figure 3 shows the XRD patterns of the sample BT-5BNT-NC sintered at various temperatures. For all samples, a well defined pseudo cubic perovskite structure, as evidenced by the overlap of the (200) and (002) peaks near 2θ~45o, with a small amount of second phase, Ba6Ti17O40, (occurred at 2θ~28.8o) was observed. The result is supported by the finding observed by Arlt et al. 7 who reported that at the grain sizes below 0.7μm, the tetragonal distortion of the BaTiO3 unit call decreased and a change from the tetragonal to a pseudo-cubic structure occurred. Table II shows the variation of the full width at half maximum (FWHM) of (200) X-ray diffraction peak for different samples. Comparison of the BT-5BNT to BT-5BNT-NC samples shows that Co-Nb codoping caused the x-ray diffraction peak to broaden which may result from the chemical inhomogeneity of Co and Nb as inferred from the TEM/EDS analysis.
Fig.3 XRD patterns of the sample BT-5BNT-NC sintered at various temperatures (a) 1250℃, (b) 1275℃, (c) 1300℃.

Table II. Variation of the full width at half maximum (FWHM) of (200) X-ray diffraction peak for different samples.
Sample
FWHM (radian)
BT-5BNT
0.194
BT-5BNT-NC, 1250℃/2h
0.313
BT-5BNT-NC, 1275℃/2h
0.293
BT-5BNT-NC, 1300℃/2h
0.322

Figures 4 and 5 show the temperature dependence of dielectric constant and dielectric loss for sample BT-5BNT-NC sintered at various temperatures, respectively. The results show that the dielectric loss decreases with increasing temperature and no obvious difference in the dielectric loss for all samples. For the samples sintered at 1250℃ for 1.5 h and 1275℃ for 1 h, the ε-T curves exhibit two peaks, the high temperature peak at 140℃ and the second peak at about 20℃. Comparison of the ε-T curve of the BT-5BNT to BT-5BNT-NC samples shows that the high temperature peak, Tc, was shifted from 150℃ to 140℃, the ε-T curve was broadened at Tc and the dielectric constant at room temperature increased by codoping with Co-Nb. With increasing sintering temperature or soaking time, an additional peak at 80-95℃ was observed and the dielectric constant at room temperature increased for sample BT-5BNT-NC. These results can be accounted for by the internal stress model as suggested by Arlt et al. 7 For BaTiO3-BNT in the core region, the volume change involved in the cubic to tetragonal phase transformation will be obstructed by the surrounding shell constituents, thus, an internal stress is created. Buessem et al. 8 reported that the value of dielectric constant at room temperature in BaTiO3 ceramics increased with the increase in internal stress. Park et al. 9 investigated the effect of stress on the ε-T characteristics of core-shell grain structure and observed that the internal stress was proportional to the volume fraction of the shell region and shifted Tc to a lower value. Based on the TEM/EDS measurements and XRD results, it can be inferred that the core region was composed of BaTiO3-BNT free from the dopants and the shell was formed by the substitution of Nb and Co into BaTiO3-BNT grain. Therefore, Tc was lowered, the ε-T curve was broadened at Tc and the dielectric constant at room temperature was increased due to the internal stress created by the core-shell structure development for BT-5BNT-NC compared with sample BT-5BNT. Moreover, the volume fraction of the shell region increased with increasing sintering temperature and soaking time due to the solid state diffusion of more Nb-Co toward core region resulted in the higher internal stress existing in the core region, thereby increasing the dielectric constant at room temperature. The dielectric constant peak occurred at 80-95℃ for the sample BT-5BNT-NC sintered at higher temperature or longer soaking time may be due to the shell formation containing a small amount of Nb-Co.1 Compared with the BaTiO3 codoped with Co-Nb system reported by previous researches 1,3 , the ε-T characteristic of the sample BT-5BNT-NC at near 150℃ was significantly improved. For BaTiO3-based ceramics, the dielectric constant increases with increasing temperature and reaches a maximum value at near 125℃ (Tc) and then decreases rapidly as the temperature exceeds Tc, which results in these materials are difficult to satisfy EIA X8R specification at near 150℃. In this study, Tc was shifted to a higher temperature (above 140℃) by adding BNT and the ε-T curve was broadened at Tc due to the small grain size (0.3-0.4μm) and the core-shell structure developed by codoping Nb-Co, which are helpful to flatten the ε-T curve at high temperature.
Fig.4 Temperature dependence of dielectric constant for sample BT-5BNT-NC sintered at various temperatures.

Fig.5 Temperature dependence of dielectric loss for sample BT-5BNT-NC sintered at various temperatures.

IV. Conclusions
(1)In BT-5BNT-NC system, Nb+5 and Co+3 ions substituted for Ti+4 ions in the perovskite structure via solid state diffusion, which resulted in Ti+4 segregation and hence forming a Ba6Ti17O40 second phase which suppressed BaTiO3 grain growth due to the pinning effect.
(2)Tc of BaTiO3 ceramics was shifted to a higher temperature (above 140℃) with the addition of BNT. The ε-T curve was broadened at Tc due to the small grain size (0.3-0.4μm) and core-shell structure developed by codoping Nb-Co. This structural change is helpful in flattening the ε-T curve at high temperatures.

References
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  8. W.R. Buessem, L.E. Cross, and A.K. Goswami, “Phenomenological Theory of High Permittivity in Fine-Grained Barium Titanate,” J. Am. Ceram. Soc., 49, 33-36 (1966).
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