Microstructures and Dielectric Properties of Ba 1 x Sr x TiO 3 Ceramics Doped with B 2 O 3 -Li 2 O Glasses for LTCC Technology Applications

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1 J. Mater. Sci. Technol., 212, 28(3), Microstructures and Dielectric Properties of Ba 1 x Sr x TiO 3 Ceramics Doped with B 2 O 3 -Li 2 O Glasses for LTCC Technology Applications Xiujian Chou 1), Zhenyu Zhao 1), Miaoxuan Du 1), Jun Liu 1) and Jiwei Zhai 2) 1) Key Laboratory of Instrumentation Science & Dynamic Measurement, Ministry of Education, National Key Laboratory of Science and Technology on Electronic Test and Measurement (North University of China), Taiyuan 351, China 2) Functional Materials Research Laboratory, Tongji University, Shanghai 292, China [Manuscript received December 27, 21, in revised form April 18, 211] Ba 1 x Sr x TiO 3 ceramics, doped with B 2 O 3 -Li 2 O glasses have been fabricated via a traditional ceramic process at a low sintering temperature of 9 C using liquid-phase sintering aids. The microstructures and dielectric properties of B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 ceramics have been investigated systematically. The temperature dependence dielectric constant and loss reveals that B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 ceramics have diffusion phase transformation characteristics. For 5 wt% B 2 O 3 -Li 2 O glasses doped Ba.55 Sr.45 TiO 3 composites, the tunability is 15.4% under a dc-applied electric field of 3 kv/cm at 1 khz; the dielectric loss can be controlled about.25; and the Q value is 286. These composite ceramics sintered at low temperature with suitable dielectric constant, low dielectric loss, relatively high tunability and high Q value are promising candidates for multilayer low-temperature co-fired ceramics (LTCC) and potential microwave tunable devices applications. KEY WORDS: Sintering; Ferroelectricity; Dielectric properties; Microstructure 1. Introduction In recent years, barium strontium titanate (Ba 1 x Sr x TiO 3 ) with high dielectric nonlinearity and low dissipation factor is one of the most promising materials to realize tunable microwave components. Applications based on barium strontium titanate (BST) with a low-temperature co-fired ceramics (LTCC) module provide a route to more integrated, miniaturized and reconfigurable microwave systems [1,2]. However, the sintering temperature of pure BST is relatively high (>135 C), and not suitable for base metal electrodes [3]. Therefore, reducing the sintering temperature of Ba 1 x Sr x TiO 3 ceramics, in order to be compatible with LTCC technology, has become an attractive challenge. Many researchers [4 6] have already attempted to decrease the sintered temperature of Ba 1 x Sr x TiO 3 ceramics to the vicinity of 9 C through the use Corresponding author. Tel.: ; Fax: ; address: chouxiujian@nuc.edu.cn (X.J. Chou). of various sintering aids. Low-melting glasses were often used to lower the sintering temperature. BST with.5 wt% B 2 O 3 can be sintered at 115 C [7], however, the dielectric properties were degraded with over-doping B 2 O 3 (1. wt%) for the formation of secondary phases. Valant and Suvorov [6] reported that the sintering temperature of BST ceramics can be decreased to 9 C by adding.4 wt% Li 2 O, with dielectric loss.45 and tunability 13.5% under a dcapplied electric field of 3 kv/cm at 1 MHz, meanwhile a few mixed phases were produced. In our present work, 5 wt% B 2 O 3 -Li 2 O glasses was added to Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) prepared by a conventional liquid-state reaction method. The dielectric properties and phase structures of B 2 O 3 -Li 2 O glasses doped ceramics were investigated. The main purpose of this research is to obtain a kind of tunable microwave materials with low dielectric constant and relatively high tunability, which can be sintered at low sintering temperature (about 9 C) for LTCC technology and tunable microwave applications.

2 X.J. Chou et al.: J. Mater. Sci. Technol., 212, 28(3), Experimental The starting raw materials of BaTiO 3 (99.9%) and SrTiO 3 (99.9%) powders were weighed according to the stoichiometric composite of Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) and mixed using alcohol and zirconia milling media for 24 h. After drying, the mixtures were calcined in alumina crucible at 11 C for 4 h in air. Meanwhile, H 3 BO 3 (99.9% purity) and Li 2 CO 3 (99.9% purity) powders were mixed according to the molar ratio of 4:3 and calcined in alumina crucible at 11 C for 3 min. The B 2 O 3 -Li 2 O glasses were produced by the liquid rapid quenching process. Then, B 2 O 3 -Li 2 O glasses powders with 5 wt% was added into the Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) powders and mixed using alcohol and zirconia balls milling media for 24 h. After drying, the obtained powder mixture were pulverized with 8 wt% polyvinyl alcohol (PVA) binder and pressed into diskshaped pellets under 1 MPa. The green pellets of composite ceramics with 5 wt% B 2 O 3 -Li 2 O glasses were sintered at 9 C for 5 h in air. The sintering ceramic samples of B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) are corresponding to samples A D. X-ray diffraction (XRD, Rigaku, Japan) with CuKα radiation was employed to characterize the phase structures. Scanning electron microscopy (SEM, JSM EMP-8) was used to characterize the microstructure. The temperature dependent dielectric constant and loss were measured from 12 to 9 C at frequency of 1 khz using a high-precision LCR meter (HP 4284A) connected with a temperature controlled chamber. The tunability was measured at 1 khz and room temperature up to the maximum bias voltage of 3 kv/cm by a Keithley model 6517A electrometer coupled with a TH2613A LCR meter. The dielectric constant and the Q values at microwave frequency were measured by the Hakki- Coleman dielectric resonator method using a network analyzer (Agilent 8753E) combining a resonating cavity. 3. Results and Discussion The XRD patterns of B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramic samples sintered at 9 C are shown in Fig. 1. As can be seen from Fig. 1, the XRD patterns of B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramic samples are similar to those of pure Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55). The perovskite structure is observed and no obvious secondary phase is found for all samples. This indicates that B 2 O 3 -Li 2 O glasses and Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) can be friendly coexistent in the material system without obvious chemical reactions. The SEM micrographs of pure Ba 1 x Sr x TiO 3 and 5 wt% B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramics are presented in Figs. 2 and 3. All the samples show compact mi- Intensity / a.u. Sample D 9 o C/5 h Pure BST o C/4 h Sample C 9 o C/5 h Pure BST5 135 o C/4 h Sample B 9 o C/5 h Pure BST o C/4 h Sample A 9 o C/5 h Pure BST6 135 o C/4 h / deg. Fig. 1 XRD patterns of pure and 5wt% B 2O 3-Li 2O glass doped Ba 1 xsr xtio 3 (x=.4,.45,.5,.55) composite ceramics crostructures. B 2 O 3 -Li 2 O glasses, as an effective sintering dopant, can realize the LTCC technology at 9 C. Compared with pure Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) sintered at 135 C, the grains size of B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramics sintered at 9 C is decreased greatly. Fig. 4 shows the dielectric constant and dielectric loss as a function of temperature at 1 khz for pure Ba 1 x Sr x TiO 3 and 5 wt% B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55). The dielectric anomalous peaks of ferroelectric-paraelectric phase transition for all the samples are suppressed and broadened. The dielectric constant of 5 wt% B 2 O 3 - Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramic samples is effectively reduced. This result can be interpreted that B 2 O 3 -Li 2 O glasses dielectrics introduced to Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) can dilute the ferroelectricity and originate the diffusion phase transition. The main cause of the suppression of the dielectric constant peak is the dopant of B 2 O 3 -Li 2 O glasses dielectrics and the increase of grain boundary, which is caused by the decrease of Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) grain size. The Curie temperature (T C ) of B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramics shifted to lower temperature, which was caused by the increase of internal stress [8,9]. It is known that the frequency of the low transverse-optic mode of vibration can be expressed as ϖ 2 = (BT K L + K S ) /m = B ( T K ) L K S m B (1) where B is the anharmonic coefficient, T is temperature, K L is long-range electrostatic force constant, and K S is the short-range harmonic restoring force

3 282 X.J. Chou et al.: J. Mater. Sci. Technol., 212, 28(3), Fig. 2 SEM of pure Ba 1 xsr xtio 3 (x=.4,.45,.5,.55) composite ceramics: (a) x=.4, (b) x=.5, (c) x=.5, (d) x=.55 Fig. 3 SEM micrographs of 5 wt% B 2O 3-Li 2O glass doped Ba 1 xsr xtio 3 (x=.4,.45,.5,.55) composite ceramics: (a) x=.4, (b) x=.45, (c) x=.5, (d) x=.55 constant, m is the mass of the ion, and (K L K S )/B here is just the Curie temperature T C. The lattice constant of B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) samples is decreased. It is possible because the Li + ion incorporates into the crystal lattice of Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) and occupies the A-site ions. The ionic radius of Li + (.76 nm) is much smaller than that of Ba 2+ (.161 nm). Therefore, the space between ions (r) in the lattice structure is decreased and K L and K S of the composites increased with the addition of B 2 O 3 - Li 2 O glasses. However, the K L varies directly with the r 3 and the K S varies directly with the r n, here the power n (about 1 11) is more than 3 for displacive ferroelectrics. So the increasing amplitude of K S is larger than that of K L. From the equation (K L K S )/B, it is clear that the T C of B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) samples is shifted to lower temperature. In addition, the dielectric loss of the 5 wt% B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) samples is increased compared with pure Ba 1 x Sr x TiO 3, but

4 khz X.J. Chou et al.: J. Mater. Sci. Technol., 212, 28(3), (a) Pure BST6 135 o C/4 h 28 BST6 with 5 wt% B-L glass 925 o C/5 h khz Pure BST5 135 o C/4 h 245 BST5 with 5 wt% B-L glass 925 o C/5 h (c) BST55 with 5 wt% B-L glass 925 o C/5 h khz (b) Pure BST o C/4 h BST45 with 5 wt% B-L glass 925 o C/5 h khz (d) Pure BST o C/4 h Fig. 4 Temperature dependence of dielectric constant and loss of pure and 5 wt% B 2O 3-Li 2O glass doped Ba 1 xsr xtio 3 composite ceramics: (a) x=.4, (b) x=.45, (c) x=.5, (d) x= still keeps below.4 under the measurement of 1 khz. With the results above we conclude that B 2 O 3 -Li 2 O glasses have certain effects on the dielectric properties of Ba 1 x Sr x TiO 3, and hence it is an effective way to optimize the dielectric properties by manipulating the addition of B 2 O 3 -Li 2 O glasses dielectrics into Ba 1 x Sr x TiO 3 ceramics. The dielectric constant vs electric field characteristics as a function of 5 wt% B 2 O 3 -Li 2 O doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramic samples at 9 C for 5 h are presented in Fig. 5. The dielectric properties and the calculated tunability can be defined as T = ε r() ε r(e) εr() 1% (2) where ε r() is the zero-field dielectric constant and ε r(e) is the dielectric constant that results from an applied electric field of E. The reduction of tunability of the ceramic samples can be interpreted as a decrease in the anharmonic interactions of Ti 4+ ions due to the decrease of the quantity and size of cluster. For samples A, B, and C, the tunability reduced compared with pure Ba 1 x Sr x TiO 3. However, the tunability of sample D is 15.4%, which is higher than that of pure Ba.45 Sr.55 TiO 3 (the tunability is 12.3%). The existence of fine-grained microstructures of all the ceramic samples together with the continuously connected chains of Ti O Ti bonds contributes to the relatively high tunability [1,11]. The dielectric constant of all the ceramic samples was effectively reduced. Especially for sample D, the dielectric constant was tailored from 1372 to 635 at the measurement frequency of 1 khz. For paraelectric material system, the change of dielectric constant under dc applied electric field is associated with the anharmonic interactions of Ti 4+ ions, which exist in clusters and microdomains [12]. Meanwhile, loss tangent measurements as a function of bias voltage give curves of similar shape to the tuning curves. The microwave properties are very critical to realize the microwave tunable device applications for these ceramics. Table 1 presents the microwave properties of 5 wt% B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) ceramic samples, which were measured using the Hakki- Coleman dielectric resonator method. It can be seen from Table 1 that the dielectric constant of the ceramic samples is slightly decreased at microwave frequencies compared with those at low frequencies (under 1 MHz). This phenomenon can be illustrated by the fact that Ba 1 x Sr x TiO 3 -based composite ceramics show typical ferroelectric relaxor behavior at microwave frequencies between Hz, which is associated with the freeze of domain shall movement.

5 284 X.J. Chou et al.: J. Mater. Sci. Technol., 212, 28(3), Table 1 Microwave and dielectric properties of pure and 5 wt% B 2O 3-Li 2O glass doped Ba 1 xsr xtio 3 (x=.4,.45,.5,.55) Dielectric properties (at 1 khz) Microwave properties Ceramic samples T C/ C at about 2 C Tunability Resonant frequency ε r, at resonant Q value/(tanδ) ε r tanδ 3 kv/cm /MHz frequency BST % /(.17) BL-BST % /(.35) BST % /(.24) BL-BST % /(.38) BST % /(.31) BL-BST % /(.65) BST % /(.12) BL-BST % /(.154) Notes: ε r: dielectric loss, ε r: dielectric constant, Q value/(tanδ): unloaded quality factor Dielectric constant khz / 2 o C Sample A Sample B Sample C Sample D T=33.4% T=29.9% T=19.5% T=15.4% DC electric field / (kv/cm) Fig. 5 DC electric field dependent dielectric constant of 5 wt% B 2O 3-Li 2O glass doped Ba 1 xsr xtio 3 (x=.4,.45,.5,.55) composite ceramics There are two kinds of microwave dielectric loss, including intrinsic loss and extrinsic loss [13]. When doped with 5 wt% B 2 O 3 -Li 2 O glasses, the Q value of the composites decreases at microwave frequency and extrinsic loss dominates the loss mechanism. In addition, the microwave properties of Ba 1 x Sr x TiO 3 material system were decreased to some extent owing to the dopant of B 2 O 3 -Li 2 O glasses. 4. Conclusion With sintering aids based on the Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) material system the sintering behavior has been investigated. The microstructures and the dielectric properties have been explored. With small amount of such sintering aids, the sintering temperature of Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) composites doped with 5 wt% B 2 O 3 -Li 2 O glasses is effectively reduced to 9 C. The dielectric anomalous peaks of the ceramic samples, corresponding to ferroelectric-paraelectric phase transition, are suppressed and broadened. Also, the dielectric loss keeps around.4 with relatively high tunability. For sample D, the dielectric constant is significantly tailored from 1372 to 635, and the tunability is 15.4% under 1 khz. The Q value of all the ceramic samples is reduced compared with pure Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55). These B 2 O 3 -Li 2 O glasses doped Ba 1 x Sr x TiO 3 (x=.4,.45,.5,.55) material system sintered at 9 C with proper dielectric properties and relatively high tunability, can be one of the most promising LTCC candidates for microwave tunable applications. Acknowledgements The authors acknowledge the support from Functional Materials Research Laboratory, Tongji University, China. This work was funded as part of the Ministry of Sciences and Technology of China through 973-project under Grant No.29CB62332, the National Natural Science Foundation of China under Grant No and Program for the Outstanding Innovative Teams of High Learning Institutions of Shanxi. REFERENCES [1 ] J.F. Scott: Integr. Ferroelectr., 1998, 2, 15. [2 ] S.J. Fiedziuszko, I.C. Hunter, T. Itoh, Y. Kobayashi, T. Nishikawa, S.N. Stitzer and K. Wakino: Microw. Theory Tech. IEEE Trans., 22, 5, 76. [3 ] J.H. Jeon, Y.D. Hahn and H.D. Kim: J. Eur. Ceram. Soc., , [4 ] T. Hu, H. Jantunen, A. Uusimäki and S. Leppävuori; Mater. Sci. Semicon. Proc., 22, 5, 215. [5 ] T. Hu T, T.J. Price, D.M. Iddles, A. Uusimäki and H. Jantunen: J. Eur. Ceram. Soc., 25, 25, [6 ] M. Valant and D. Suvorov: J. Am. Ceram. Soc., 24, 87, [7 ] S.M. Rhim, S. Hong, H. Bak and O.K. Kim: J. Am. Ceram. Soc., 2, 83, [8 ] Y. Chen, X.L. Dong, R.H. Liang, J.T. Li and Y.L. Wang: J. Appl. Phys., 25, 98, [9 ] K.M. Johnson: J. Appl. Phys., 1962, 33, [1] R. Waser: Ferroelectrics, 1992, 133(1), 19. [11] A. Feteira, D.C. Sinclair, I.M. Reaney, Y. Somiya and M.T. Lanagan: J. Am. Ceram. Soc., 24, 87, 182. [12] W.L. Zhong: Physics of Ferroelectrics, Science Press, Beijing, 2, (in Chinese) [13] C.L. Huang, M.H. Weng and H.L. Chen: Mater. Chem. Phys., 21, 71, 17.