- Email: [email protected]
Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 ceramics with BaCu(B2O5) additions
Materials Chemistry and Physics 113 (2009) 1–5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Review
Low temperature sintering and microwave dielectric properties of Ba3 Ti5 Nb6 O28 ceramics with BaCu(B2 O5 ) additions Huanfu Zhou, Hong Wang ∗ , Yuehua Chen, Kecheng Li, Xi Yao Electronic Materials Research Laboratory, Key Laboratory of the Ministry of the Education, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 4 February 2008 Received in revised form 3 June 2008 Accepted 15 June 2008 Keywords: LTCC Ba3 Ti5 Nb6 O28 ceramics Doping Microwave dielectrics properties
a b s t r a c t The effects of BaCu(B2 O5 ) (BCB) additions on the sintering temperature and microwave dielectric properties of Ba3 Ti5 Nb6 O28 ceramic have been investigated using dilatometer, X-ray diffraction, scanning electron microscopy and dielectric measurement. The pure Ba3 Ti5 Nb6 O28 ceramic shows a high sintering temperature (∼1250 ◦ C) and good microwave dielectric properties as Q × f of 11,400 GHz, εr of 37.0, f of −8 ppm ◦ C−1 . It was found that the addition of BCB to Ba3 Ti5 Nb6 O28 could lower the sintering temperature from 1250 to 925 ◦ C. The reduced sintering temperature was attributed to the BCB liquid phase. The addition of BCB also enhanced the microwave dielectric properties to Q × f of 19,191 GHz, εr of 38.2, f of 12 ppm ◦ C−1 . © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Development of communication systems such as mobile systems requires the miniaturization of device size. Recently, low temperature co-fired ceramic (LTCC) multilayer devices have been investigated to reduce the device size [1–4]. Silver has been widely used as the internal electrode in the multilayer devices because of its high conductivity and low cost. The melting temperature of Ag is low, about 961 ◦ C. Therefore, for the fabrication of the multilayer devices, it is important to develop microwave dielectric ceramics that have low sintering temperatures and can be co-fired with Ag. The dielectric properties of Ba3 Ti5 Nb6 O28 ceramics have been investigated by Sebastian [5]. He reported that Ba3 Ti5 Nb6 O28 ceramic had good microwave dielectric properties with high εr of 41, high Q × f of 4500 GHz (at 5.4 GHz) and f of 8 ppm ◦ C−1 . The
∗ Corresponding author. Tel.: +86 29 8266 8679; fax: +86 29 8266 8794. E-mail address: [email protected] (H. Wang). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.037
1 2 3 4 5 5
ceramic has a high dielectric constant and a fairly high quality factor. But the sintering temperature of Ba3 Ti5 Nb6 O28 ceramic is above 1250 ◦ C, which is too high to be applicable to LTCC. So it is necessary to reduce the sintering temperature. B2 O3 and CuO were then used to decrease the sintering temperatures of Ba(Zn1/3 Ta2/3 )O3 (BZT) and Ba(Zn1/3 Nb2/3 )O3 (BZN) ceramics, and a liquid phase with the composition of BaCu(B2 O5 ) (BCB) was found in them [6,7]. The melting temperature of the BCB ceramic is approximately 850 ◦ C and the BCB-added BZN ceramic was well sintered even at 850 ◦ C with good microwave dielectric properties [8]. The addition of B2 O3 and CuO also reduced the sintering temperature of BST ceramics below 900 ◦ C with a corresponding formation of liquid phase [9]. The composition of the liquid phase was regarded as BCB but without any confirmation. In this work, BaCu(B2 O5 ) additive was made and added to Ba3 Ti5 Nb6 O28 ceramics in order to investigate the possibility of using BCB as a low temperature sintering additive. Furthermore, its effect on sintering temperature, microstructure and microwave dielectric properties of the Ba3 Ti5 Nb6 O28 ceramics was investigated.
2
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
Fig. 1. Shrinkage of the BCB-added Ba3 Ti5 Nb6 O28 ceramics as a function of temperature.
Fig. 2. Bulk density of the Ba3 Ti5 Nb6 O28 samples with 0–5 wt% BCB additions as a function of sintering temperatures.
2. Experimental procedure The Ba3 Ti5 Nb6 O28 powders were prepared using BaCO3 (≥99%), Nb2 O5 (≥99.5%) and TiO2 (≥99%). The powders were ball-milled in a polyethylene bottle with ZrO2 media for 4 h using alcohol as a medium. The mixtures were then rapidly dried and calcined at 1100 ◦ C for 2 h. To synthesize the BCB ceramic powder, BaCO3 (>99%), CuO (>99%) and H3 BO3 (>99%) were mixed for 4 h in a nylon jar with zirconia balls, then dried and calcined at 700 ◦ C for 3 h. After subsequent ball-milling with 0–5.0 wt% BCB, the powders were uniaxially pressed into disks of 8 mm in diameter and 4 mm in thickness under the pressure of about 150 MPa. The pure Ba3 Ti5 Nb6 O28 samples were sintered at 1250 ◦ C for 2 h in air and the ceramic pellets doped with BCB were sintered at 850–950 ◦ C for 2 h in air. Shrinkage of the specimens during heat treatment was measured using a horizontal loading dilatometer with alumina rams and boats (Model DIL402C, Netzsch Instruments, Germany). The bulk densities of sintered specimens were measured by Archimedes method. The crystal structures of ceramics were studied by an Xray diffractometry (Rigaku D/MAX-2400, Japan) with Cu K␣ radiation. The surfaces
of the polished cross-sections of sintered specimens were observed by scanning electron microscopy (SEM) (JSM-6360LV, JEOL, Tokyo, Japan). Dielectric behaviors in microwave frequency were measured by the TE0 1 ı shielded cavity method using a Network Analyzer (8720ES, Agilent, U.S.A.) and a temperature chamber (DELTA 9023, Delta Design, U.S.A.). The temperature coefficients of resonant frequency f values were calculated by the formula as in the following:
f =
fT − f0 f0 (T − T0 )
(1)
where fT , f0 were the resonant frequencies at the measuring temperature T and T0 (25 ◦ C), respectively.
Fig. 3. Scanning electron micrographs of the Ba3 Ti5 Nb6 O28 samples doped with different BCB: (a) 0 wt% sintered at 1250 ◦ C; (b) 3 wt%; (c) 4 wt%; (d) 5 wt% sintered at 925 ◦ C.
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
3
sintering aid to lower the sintering temperature of Ba3 Ti5 Nb6 O28 ceramics. Fig. 2 shows the change in bulk densities of Ba3 Ti5 Nb6 O28 samples with various amounts of BCB additions as a function of sintering temperature. The density of the undoped Ba3 Ti5 Nb6 O28 ceramic is about 5.10 g cm−3 when the sintering temperature is 1250 ◦ C. For the BCB-doped ceramics, the bulk densities increase with the increasing of BCB amount. As shown in Fig. 2, the densities of the samples doped with ≤3.0 wt% BCB remain relatively low, which indicates that the addition of ≤3.0 wt% BCB is not enough to densify the ceramics efficiently at low sintering temperatures. Then, when the BCB amount increased to 5.0 wt%, The bulk densities of Ba3 Ti5 Nb6 O28 samples with BCB additions increase sharply with the increasing of sintering temperature and reach a constant value above 925 ◦ C. The bulk density of Ba3 Ti5 Nb6 O28 sample with 5 wt% BCB addition sintered at 925 ◦ C for 2 h reaches almost 5.05 g cm−3 . The obtained bulk density of 5.05 g cm−3 corresponds to the bulk density (5.10 g cm−3 ) of Ba3 Ti5 Nb6 O28 sample sintered at 1250 ◦ C for 2 h. Therefore, it indicates that significant reduction of the sintering temperatures of Ba3 Ti5 Nb6 O28 samples is possible by BCB addition, while maintaining the high density. The SEM micrographs of the Ba3 Ti5 Nb6 O28 ceramics doped with different amount of BCB are given in Fig. 3. For the pure sample, spherical shaped grains can be observed in the micrograph (Fig. 3(a)). But due to the much lower sintering temperatures, the grain sizes of the BCB-doped samples are smaller than those of pure Ba3 Ti5 Nb6 O28 ceramic. Besides, some densely connected rectangular grains can be found in the samples doped with BCB (Fig. 3(b)–(d)). In order to elucidate the phase composition of the rectangular grains, backscatter image of the polished cross-sections and EDS analysis have also been made (in Fig. 4). From the EDS analysis, it can be seen that the rectangular grains have less Ti and Nb inside than other grains. We approximately calculate the ratio of Ba:Ti:Nb:O, the ratio of spot A is about 3:4:5:25 and the spot B is 3:5:6:28. Fig. 5 shows the XRD patterns of pure Ba3 Ti5 Nb6 O28 and Ba3 Ti5 Nb6 O28 doped with different BCB sintered at 1250 and 925 ◦ C. The main diffraction peaks can be indexed according to the Ba3 Ti5 Nb6 O28 (37–1477). A second phase of Ba3 Ti4 Nb4 O21 (70–1150) was detected for the samples doped with BCB, which is in agreement with the analysis of the EDS. The intensity of the Ba3 Ti4 Nb4 O21 phase peaks slightly increases with the increasing of BCB amount. Combined SEM, EDS and XRD results, it can be seen that the addition of a small amount BCB cannot only enhance the
Fig. 4. Backscatter images of the polished cross-sections and EDS analysis of the Ba3 Ti5 Nb6 O28 samples doped with 4 wt% sintered at 925 ◦ C.
3. Results and discussion To identify whether the BCB additive would be effective on the low-temperature firing of Ba3 Ti5 Nb6 O28 , the linear thermal shrinkages of as-pressed pellets as a function of temperature were firstly measured. The results are shown in Fig. 1. The results indicate that the onset temperature of shrinkage is lowered by the addition of a small amount of BCB. The shrinkage of Ba3 Ti5 Nb6 O28 without BCB does not occur as rapidly as that of Ba3 Ti5 Nb6 O28 doped with the BCB. It is noteworthy that the densification of Ba3 Ti5 Nb6 O28 with 3–5 wt% of BCB addition begins below 800 ◦ C and that the shrinkage reaches maximum at about 950 ◦ C. For the Ba3 Ti5 Nb6 O28 samples doped with 8 wt% BCB, the shrinkage reaches a maximum value at approximately 900 ◦ C. All of these imply that the BCB acts as a good
Fig. 5. XRD patterns of the Ba3 Ti5 Nb6 O28 samples with x wt% BCB addition: (a) 0 wt% sintering at 1250 ◦ C; (b) 2 wt%; (c) 3 wt%; (d) 4 wt%; (e) 5 wt% sintering at 925 ◦ C.
4
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
Fig. 6. Dielectric constant of Ba3Ti5 Nb6 O28 samples doped with different BCB as a function of sintering temperature.
sinterability, but also results a reaction with Ba3 Ti5 Nb6 O28 to form Ba3 Ti4 Nb4 O21 primarily. However, the secondary phase of TiO2 or Nb2 O5 after the decomposition of Ba3 Ti5 Nb6 O28 ceramics has not been detected in present work. Fig. 6 shows the change in relative permittivities of Ba3 Ti5 Nb6 O28 ceramics with the increasing of BCB addition at different sintering temperatures, dots on the vertical axes show the properties of pure Ba3 Ti5 Nb6 O28 ceramic sintered at 1250 ◦ C. The relationship between εr values of BCB-doped ceramics and sintering temperatures presents a trend similar to that between densities and sintering temperatures since higher density means lower porosity. The dielectric constant increased with the increasing of sintering temperature. Moreover, the addition of BCB increased εr values can be explained as the results of the increase of the second phase of Ba3 Ti4 Nb4 O21 which owns a high εr value about 55 [5]. Fig. 7 presents the Q × f values of the Ba3 Ti5 Nb6 O28 ceramics with various BCB additions as a function of sintering temperature. The Q × f values of the BCB-doped Ba3 Ti5 Nb6 O28 ceramics are strongly dependent on the sintering temperature and the amount of BCB addition. As we know, the microwave dielectric loss includes not only intrinsic losses which were mainly contributed by the lattice vibrational modes but also extrinsic losses caused by densifi-
Fig. 8. Temperature coefficient of resonant frequency ( f ) of Ba3 Ti5 Nb6 O28 samples as a function of BCB additions.
cation/porosity, secondary phases, grain sizes and oxygen vacancies [10]. Some investigations also reported that the Q × f value is independent of the density and the porosity for a theoretical density higher than 90%. The relative density plays an important role in controlling dielectric loss, as has been shown for other microwave dielectric materials [11]. With ≤3.0 wt% BCB addition, the Q × f value was relative low. Since porous samples were obtained after sintering at low temperatures, the presence of the pores may have caused the Q × f value to be diminished. The Q × f values decreased with the increasing of BCB doping amount. High BCB amount would degrade the Q × f value of Ba3 Ti5 Nb6 O28 ceramics because the grain boundary phases are prominent products at higher sintering temperatures. Fig. 8 shows the temperature coefficient of resonant frequency ( f ) values of the BCB-doped ceramics sintered at 900 ◦ C as a function of the amount of BCB addition. From Fig. 8, it can be found that the f value increases with the increasing of BCB amount. From Figs. 5 and 8, the similar trends of the variation of f values and the change of XRD peak can be observed. With the BCB addition, the second phase peaks of Ba3 Ti4 Nb4 O21 appear, and the XRD intensity increase with the increasing of BCB addition. The f value of Ba3 Ti4 Nb4 O21 is about 100, thus the f value of Ba3 Ti5 Nb6 O28 samples doped with BCB increase. In general, with 5.0 wt% BCB addition, the Ba3 Ti5 Nb6 O28 ceramics sintered at 925 ◦ C have better microwave dielectric properties of εr = 38.2, Q × f = 19,191 GHz, f = 12 ppm ◦ C−1 . The pure Ba3 Ti5 Nb6 O28 ceramic reported by Sebastian [5] have the microwave properties of εr = 41, Q × f = 4500 GHz, f = 8 ppm ◦ C−1 . In this work, the pure Ba3 Ti5 Nb6 O28 ceramic shows a better microwave dielectric property of εr = 37, Q × f = 11400 GHz, f = −8 ppm ◦ C−1 than Sebastian’s work, and is in agreement with the Kim’s work [12]. As we know, there are a lot factors, such as raw powders [13], preparation procedure [14], measure method [15,16], etc., would make a great influence on the microwave dielectric properties of microwave ceramics. Thus it is acceptable to the differences of the pure Ba3 Ti5 Nb6 O28 ceramics in our work and Sebastian’s. 4. Conclusion
Fig. 7. Q × f values of Ba3 Ti5 Nb6 O28 samples doped with different BCB as a function of sintering temperature.
The sintering behaviors, phase evolution and microwave dielectric properties of Ba3 Ti5 Nb6 O28 ceramics were investigated as a function of BCB content. It was found that the proper additions of BCB to Ba3 Ti5 Nb6 O28 ceramics enabled a reduction in sintering
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
temperature from 1250 to 925 ◦ C. The reduced sintering temperature was attributed to the BCB liquid phase. The addition of BCB also enhanced the microwave dielectric properties to Q × f = 19,191 GHz, εr = 38.2, f = 12 ppm ◦ C−1 . The results show that the Ba3 Ti5 Nb6 O28 dielectric materials are good candidates for LTCC applications. Acknowledgements This work was supported by National 863-project of China (2006AA03Z0429), National 973-project of China (2002CB613302) and NCET-05-0840. References [1] M. Abe, T. Nanataki, S. Yano, U.S. Patent 5,292,694 (1994). [2] M. Abe, T. Sugiura, T. Nanataki, S. Yano, U.S. Patent 5,332,984 (1994). [3] M. Abe, T. Nanataki, S. Yano, U.S. Patent 5,350,721 (1994).
5
[4] J.B. Lim, K.H. Cho, S. Nahm, J.H. Paik, J.H. Kim, Mater. Res. Bull. 41 (2006) 1868. [5] M.T. Sebastian, J. Mater. Sci. Mater. Electron. 10 (1999) 475. [6] M.H. Kim, S. Nahm, W.S. Lee, M.J. Yoo, N.K. Kang, H.T. Kim, H.J. Lee, Jpn. J. Appl. Phys. 44 (2005) 3091. [7] M.H. Kim, Y.H. Jeong, S. Nahm, H.T. Kim, H.J. Lee, J. Eur. Ceram. Soc. 26 (2006) 2139. [8] M.H. Kim, J.B. Lim, J.C. Kim, S. Nahm, J.H. Paik, J.H. Kim, J. Am. Ceram. Soc. 89 (2006) 3124. [9] K.H. Cho, J.B. Lim, S. Nahm, H.T. Kim, J.H. Kim, J.H. Paik, H.J. Lee, J. Eur. Ceram. Soc. 27 (2007) 1053. [10] C.L. Huang, C.F. Tseng, J. Am. Ceram. Soc. 90 (2007) 3409. [11] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 5367. [12] J.R. Kim, D.W. Kim, I.S. Cho, B.S. Kim, J.S. An, K.S. Hong, J. Eur. Ceram. Soc. 27 (2007) 3075. [13] M.R. Varma, H. Sreemoolanadhan, V. Chanderasekaran, J. Mater. Sci. Mater. Electron. 15 (2004) 345. [14] Z.Q. Tian, H. Wang, W.J. Huang, C.Y. Zhang, J. Mater. Sci. Mater. Electron. 19 (2008) 227. [15] C. Vineis, P.K. Davis, T. Negas, S. Bell, Mater. Res. Bull. 31 (1996) 431. [16] R. Ratheesh, M.T. Sebastian, P. Mohanan, M.E. Tobar, J. Hartnett, R. Woode, D.G. Blair, Mater. Lett. 45 (2000) 279.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Review
Low temperature sintering and microwave dielectric properties of Ba3 Ti5 Nb6 O28 ceramics with BaCu(B2 O5 ) additions Huanfu Zhou, Hong Wang ∗ , Yuehua Chen, Kecheng Li, Xi Yao Electronic Materials Research Laboratory, Key Laboratory of the Ministry of the Education, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 4 February 2008 Received in revised form 3 June 2008 Accepted 15 June 2008 Keywords: LTCC Ba3 Ti5 Nb6 O28 ceramics Doping Microwave dielectrics properties
a b s t r a c t The effects of BaCu(B2 O5 ) (BCB) additions on the sintering temperature and microwave dielectric properties of Ba3 Ti5 Nb6 O28 ceramic have been investigated using dilatometer, X-ray diffraction, scanning electron microscopy and dielectric measurement. The pure Ba3 Ti5 Nb6 O28 ceramic shows a high sintering temperature (∼1250 ◦ C) and good microwave dielectric properties as Q × f of 11,400 GHz, εr of 37.0, f of −8 ppm ◦ C−1 . It was found that the addition of BCB to Ba3 Ti5 Nb6 O28 could lower the sintering temperature from 1250 to 925 ◦ C. The reduced sintering temperature was attributed to the BCB liquid phase. The addition of BCB also enhanced the microwave dielectric properties to Q × f of 19,191 GHz, εr of 38.2, f of 12 ppm ◦ C−1 . © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Development of communication systems such as mobile systems requires the miniaturization of device size. Recently, low temperature co-fired ceramic (LTCC) multilayer devices have been investigated to reduce the device size [1–4]. Silver has been widely used as the internal electrode in the multilayer devices because of its high conductivity and low cost. The melting temperature of Ag is low, about 961 ◦ C. Therefore, for the fabrication of the multilayer devices, it is important to develop microwave dielectric ceramics that have low sintering temperatures and can be co-fired with Ag. The dielectric properties of Ba3 Ti5 Nb6 O28 ceramics have been investigated by Sebastian [5]. He reported that Ba3 Ti5 Nb6 O28 ceramic had good microwave dielectric properties with high εr of 41, high Q × f of 4500 GHz (at 5.4 GHz) and f of 8 ppm ◦ C−1 . The
∗ Corresponding author. Tel.: +86 29 8266 8679; fax: +86 29 8266 8794. E-mail address: [email protected] (H. Wang). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.037
1 2 3 4 5 5
ceramic has a high dielectric constant and a fairly high quality factor. But the sintering temperature of Ba3 Ti5 Nb6 O28 ceramic is above 1250 ◦ C, which is too high to be applicable to LTCC. So it is necessary to reduce the sintering temperature. B2 O3 and CuO were then used to decrease the sintering temperatures of Ba(Zn1/3 Ta2/3 )O3 (BZT) and Ba(Zn1/3 Nb2/3 )O3 (BZN) ceramics, and a liquid phase with the composition of BaCu(B2 O5 ) (BCB) was found in them [6,7]. The melting temperature of the BCB ceramic is approximately 850 ◦ C and the BCB-added BZN ceramic was well sintered even at 850 ◦ C with good microwave dielectric properties [8]. The addition of B2 O3 and CuO also reduced the sintering temperature of BST ceramics below 900 ◦ C with a corresponding formation of liquid phase [9]. The composition of the liquid phase was regarded as BCB but without any confirmation. In this work, BaCu(B2 O5 ) additive was made and added to Ba3 Ti5 Nb6 O28 ceramics in order to investigate the possibility of using BCB as a low temperature sintering additive. Furthermore, its effect on sintering temperature, microstructure and microwave dielectric properties of the Ba3 Ti5 Nb6 O28 ceramics was investigated.
2
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
Fig. 1. Shrinkage of the BCB-added Ba3 Ti5 Nb6 O28 ceramics as a function of temperature.
Fig. 2. Bulk density of the Ba3 Ti5 Nb6 O28 samples with 0–5 wt% BCB additions as a function of sintering temperatures.
2. Experimental procedure The Ba3 Ti5 Nb6 O28 powders were prepared using BaCO3 (≥99%), Nb2 O5 (≥99.5%) and TiO2 (≥99%). The powders were ball-milled in a polyethylene bottle with ZrO2 media for 4 h using alcohol as a medium. The mixtures were then rapidly dried and calcined at 1100 ◦ C for 2 h. To synthesize the BCB ceramic powder, BaCO3 (>99%), CuO (>99%) and H3 BO3 (>99%) were mixed for 4 h in a nylon jar with zirconia balls, then dried and calcined at 700 ◦ C for 3 h. After subsequent ball-milling with 0–5.0 wt% BCB, the powders were uniaxially pressed into disks of 8 mm in diameter and 4 mm in thickness under the pressure of about 150 MPa. The pure Ba3 Ti5 Nb6 O28 samples were sintered at 1250 ◦ C for 2 h in air and the ceramic pellets doped with BCB were sintered at 850–950 ◦ C for 2 h in air. Shrinkage of the specimens during heat treatment was measured using a horizontal loading dilatometer with alumina rams and boats (Model DIL402C, Netzsch Instruments, Germany). The bulk densities of sintered specimens were measured by Archimedes method. The crystal structures of ceramics were studied by an Xray diffractometry (Rigaku D/MAX-2400, Japan) with Cu K␣ radiation. The surfaces
of the polished cross-sections of sintered specimens were observed by scanning electron microscopy (SEM) (JSM-6360LV, JEOL, Tokyo, Japan). Dielectric behaviors in microwave frequency were measured by the TE0 1 ı shielded cavity method using a Network Analyzer (8720ES, Agilent, U.S.A.) and a temperature chamber (DELTA 9023, Delta Design, U.S.A.). The temperature coefficients of resonant frequency f values were calculated by the formula as in the following:
f =
fT − f0 f0 (T − T0 )
(1)
where fT , f0 were the resonant frequencies at the measuring temperature T and T0 (25 ◦ C), respectively.
Fig. 3. Scanning electron micrographs of the Ba3 Ti5 Nb6 O28 samples doped with different BCB: (a) 0 wt% sintered at 1250 ◦ C; (b) 3 wt%; (c) 4 wt%; (d) 5 wt% sintered at 925 ◦ C.
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
3
sintering aid to lower the sintering temperature of Ba3 Ti5 Nb6 O28 ceramics. Fig. 2 shows the change in bulk densities of Ba3 Ti5 Nb6 O28 samples with various amounts of BCB additions as a function of sintering temperature. The density of the undoped Ba3 Ti5 Nb6 O28 ceramic is about 5.10 g cm−3 when the sintering temperature is 1250 ◦ C. For the BCB-doped ceramics, the bulk densities increase with the increasing of BCB amount. As shown in Fig. 2, the densities of the samples doped with ≤3.0 wt% BCB remain relatively low, which indicates that the addition of ≤3.0 wt% BCB is not enough to densify the ceramics efficiently at low sintering temperatures. Then, when the BCB amount increased to 5.0 wt%, The bulk densities of Ba3 Ti5 Nb6 O28 samples with BCB additions increase sharply with the increasing of sintering temperature and reach a constant value above 925 ◦ C. The bulk density of Ba3 Ti5 Nb6 O28 sample with 5 wt% BCB addition sintered at 925 ◦ C for 2 h reaches almost 5.05 g cm−3 . The obtained bulk density of 5.05 g cm−3 corresponds to the bulk density (5.10 g cm−3 ) of Ba3 Ti5 Nb6 O28 sample sintered at 1250 ◦ C for 2 h. Therefore, it indicates that significant reduction of the sintering temperatures of Ba3 Ti5 Nb6 O28 samples is possible by BCB addition, while maintaining the high density. The SEM micrographs of the Ba3 Ti5 Nb6 O28 ceramics doped with different amount of BCB are given in Fig. 3. For the pure sample, spherical shaped grains can be observed in the micrograph (Fig. 3(a)). But due to the much lower sintering temperatures, the grain sizes of the BCB-doped samples are smaller than those of pure Ba3 Ti5 Nb6 O28 ceramic. Besides, some densely connected rectangular grains can be found in the samples doped with BCB (Fig. 3(b)–(d)). In order to elucidate the phase composition of the rectangular grains, backscatter image of the polished cross-sections and EDS analysis have also been made (in Fig. 4). From the EDS analysis, it can be seen that the rectangular grains have less Ti and Nb inside than other grains. We approximately calculate the ratio of Ba:Ti:Nb:O, the ratio of spot A is about 3:4:5:25 and the spot B is 3:5:6:28. Fig. 5 shows the XRD patterns of pure Ba3 Ti5 Nb6 O28 and Ba3 Ti5 Nb6 O28 doped with different BCB sintered at 1250 and 925 ◦ C. The main diffraction peaks can be indexed according to the Ba3 Ti5 Nb6 O28 (37–1477). A second phase of Ba3 Ti4 Nb4 O21 (70–1150) was detected for the samples doped with BCB, which is in agreement with the analysis of the EDS. The intensity of the Ba3 Ti4 Nb4 O21 phase peaks slightly increases with the increasing of BCB amount. Combined SEM, EDS and XRD results, it can be seen that the addition of a small amount BCB cannot only enhance the
Fig. 4. Backscatter images of the polished cross-sections and EDS analysis of the Ba3 Ti5 Nb6 O28 samples doped with 4 wt% sintered at 925 ◦ C.
3. Results and discussion To identify whether the BCB additive would be effective on the low-temperature firing of Ba3 Ti5 Nb6 O28 , the linear thermal shrinkages of as-pressed pellets as a function of temperature were firstly measured. The results are shown in Fig. 1. The results indicate that the onset temperature of shrinkage is lowered by the addition of a small amount of BCB. The shrinkage of Ba3 Ti5 Nb6 O28 without BCB does not occur as rapidly as that of Ba3 Ti5 Nb6 O28 doped with the BCB. It is noteworthy that the densification of Ba3 Ti5 Nb6 O28 with 3–5 wt% of BCB addition begins below 800 ◦ C and that the shrinkage reaches maximum at about 950 ◦ C. For the Ba3 Ti5 Nb6 O28 samples doped with 8 wt% BCB, the shrinkage reaches a maximum value at approximately 900 ◦ C. All of these imply that the BCB acts as a good
Fig. 5. XRD patterns of the Ba3 Ti5 Nb6 O28 samples with x wt% BCB addition: (a) 0 wt% sintering at 1250 ◦ C; (b) 2 wt%; (c) 3 wt%; (d) 4 wt%; (e) 5 wt% sintering at 925 ◦ C.
4
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
Fig. 6. Dielectric constant of Ba3Ti5 Nb6 O28 samples doped with different BCB as a function of sintering temperature.
sinterability, but also results a reaction with Ba3 Ti5 Nb6 O28 to form Ba3 Ti4 Nb4 O21 primarily. However, the secondary phase of TiO2 or Nb2 O5 after the decomposition of Ba3 Ti5 Nb6 O28 ceramics has not been detected in present work. Fig. 6 shows the change in relative permittivities of Ba3 Ti5 Nb6 O28 ceramics with the increasing of BCB addition at different sintering temperatures, dots on the vertical axes show the properties of pure Ba3 Ti5 Nb6 O28 ceramic sintered at 1250 ◦ C. The relationship between εr values of BCB-doped ceramics and sintering temperatures presents a trend similar to that between densities and sintering temperatures since higher density means lower porosity. The dielectric constant increased with the increasing of sintering temperature. Moreover, the addition of BCB increased εr values can be explained as the results of the increase of the second phase of Ba3 Ti4 Nb4 O21 which owns a high εr value about 55 [5]. Fig. 7 presents the Q × f values of the Ba3 Ti5 Nb6 O28 ceramics with various BCB additions as a function of sintering temperature. The Q × f values of the BCB-doped Ba3 Ti5 Nb6 O28 ceramics are strongly dependent on the sintering temperature and the amount of BCB addition. As we know, the microwave dielectric loss includes not only intrinsic losses which were mainly contributed by the lattice vibrational modes but also extrinsic losses caused by densifi-
Fig. 8. Temperature coefficient of resonant frequency ( f ) of Ba3 Ti5 Nb6 O28 samples as a function of BCB additions.
cation/porosity, secondary phases, grain sizes and oxygen vacancies [10]. Some investigations also reported that the Q × f value is independent of the density and the porosity for a theoretical density higher than 90%. The relative density plays an important role in controlling dielectric loss, as has been shown for other microwave dielectric materials [11]. With ≤3.0 wt% BCB addition, the Q × f value was relative low. Since porous samples were obtained after sintering at low temperatures, the presence of the pores may have caused the Q × f value to be diminished. The Q × f values decreased with the increasing of BCB doping amount. High BCB amount would degrade the Q × f value of Ba3 Ti5 Nb6 O28 ceramics because the grain boundary phases are prominent products at higher sintering temperatures. Fig. 8 shows the temperature coefficient of resonant frequency ( f ) values of the BCB-doped ceramics sintered at 900 ◦ C as a function of the amount of BCB addition. From Fig. 8, it can be found that the f value increases with the increasing of BCB amount. From Figs. 5 and 8, the similar trends of the variation of f values and the change of XRD peak can be observed. With the BCB addition, the second phase peaks of Ba3 Ti4 Nb4 O21 appear, and the XRD intensity increase with the increasing of BCB addition. The f value of Ba3 Ti4 Nb4 O21 is about 100, thus the f value of Ba3 Ti5 Nb6 O28 samples doped with BCB increase. In general, with 5.0 wt% BCB addition, the Ba3 Ti5 Nb6 O28 ceramics sintered at 925 ◦ C have better microwave dielectric properties of εr = 38.2, Q × f = 19,191 GHz, f = 12 ppm ◦ C−1 . The pure Ba3 Ti5 Nb6 O28 ceramic reported by Sebastian [5] have the microwave properties of εr = 41, Q × f = 4500 GHz, f = 8 ppm ◦ C−1 . In this work, the pure Ba3 Ti5 Nb6 O28 ceramic shows a better microwave dielectric property of εr = 37, Q × f = 11400 GHz, f = −8 ppm ◦ C−1 than Sebastian’s work, and is in agreement with the Kim’s work [12]. As we know, there are a lot factors, such as raw powders [13], preparation procedure [14], measure method [15,16], etc., would make a great influence on the microwave dielectric properties of microwave ceramics. Thus it is acceptable to the differences of the pure Ba3 Ti5 Nb6 O28 ceramics in our work and Sebastian’s. 4. Conclusion
Fig. 7. Q × f values of Ba3 Ti5 Nb6 O28 samples doped with different BCB as a function of sintering temperature.
The sintering behaviors, phase evolution and microwave dielectric properties of Ba3 Ti5 Nb6 O28 ceramics were investigated as a function of BCB content. It was found that the proper additions of BCB to Ba3 Ti5 Nb6 O28 ceramics enabled a reduction in sintering
H. Zhou et al. / Materials Chemistry and Physics 113 (2009) 1–5
temperature from 1250 to 925 ◦ C. The reduced sintering temperature was attributed to the BCB liquid phase. The addition of BCB also enhanced the microwave dielectric properties to Q × f = 19,191 GHz, εr = 38.2, f = 12 ppm ◦ C−1 . The results show that the Ba3 Ti5 Nb6 O28 dielectric materials are good candidates for LTCC applications. Acknowledgements This work was supported by National 863-project of China (2006AA03Z0429), National 973-project of China (2002CB613302) and NCET-05-0840. References [1] M. Abe, T. Nanataki, S. Yano, U.S. Patent 5,292,694 (1994). [2] M. Abe, T. Sugiura, T. Nanataki, S. Yano, U.S. Patent 5,332,984 (1994). [3] M. Abe, T. Nanataki, S. Yano, U.S. Patent 5,350,721 (1994).
5
[4] J.B. Lim, K.H. Cho, S. Nahm, J.H. Paik, J.H. Kim, Mater. Res. Bull. 41 (2006) 1868. [5] M.T. Sebastian, J. Mater. Sci. Mater. Electron. 10 (1999) 475. [6] M.H. Kim, S. Nahm, W.S. Lee, M.J. Yoo, N.K. Kang, H.T. Kim, H.J. Lee, Jpn. J. Appl. Phys. 44 (2005) 3091. [7] M.H. Kim, Y.H. Jeong, S. Nahm, H.T. Kim, H.J. Lee, J. Eur. Ceram. Soc. 26 (2006) 2139. [8] M.H. Kim, J.B. Lim, J.C. Kim, S. Nahm, J.H. Paik, J.H. Kim, J. Am. Ceram. Soc. 89 (2006) 3124. [9] K.H. Cho, J.B. Lim, S. Nahm, H.T. Kim, J.H. Kim, J.H. Paik, H.J. Lee, J. Eur. Ceram. Soc. 27 (2007) 1053. [10] C.L. Huang, C.F. Tseng, J. Am. Ceram. Soc. 90 (2007) 3409. [11] W.S. Kim, T.H. Hong, E.S. Kim, K.H. Yoon, Jpn. J. Appl. Phys. 37 (1998) 5367. [12] J.R. Kim, D.W. Kim, I.S. Cho, B.S. Kim, J.S. An, K.S. Hong, J. Eur. Ceram. Soc. 27 (2007) 3075. [13] M.R. Varma, H. Sreemoolanadhan, V. Chanderasekaran, J. Mater. Sci. Mater. Electron. 15 (2004) 345. [14] Z.Q. Tian, H. Wang, W.J. Huang, C.Y. Zhang, J. Mater. Sci. Mater. Electron. 19 (2008) 227. [15] C. Vineis, P.K. Davis, T. Negas, S. Bell, Mater. Res. Bull. 31 (1996) 431. [16] R. Ratheesh, M.T. Sebastian, P. Mohanan, M.E. Tobar, J. Hartnett, R. Woode, D.G. Blair, Mater. Lett. 45 (2000) 279.