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Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics
Progress in Natural Science: Materials International 2013;23(2):152–156 Chinese Materials Research Society
Progress in Natural Science: Materials International www.elsevier.com/locate/pnsmi www.sciencedirect.com
ORIGINAL RESEARCH
Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics Shaochun Lia,n, Qilong Zhangb, Tiejun Zhaoa, Hui Yangb a
School of Civil Engineering, Qingdao Technological University, Qingdao 266033, China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
b
Received 7 September 2012; accepted 10 January 2013 Available online 13 March 2013
KEYWORDS Aqueous tape casting; B2O3; Rheology; Microwave dielectric properties
Abstract The effect of B2O3 addition on the aqueous tape casting, sintering, microstructure and microwave dielectric properties of Li2O–Nb2O5–TiO2 ceramics has been investigated. The tape casting slurries exhibit a typical shear-thinning behavior without thixotropy, but the addition of B2O3 increases the viscosity of the slurries significantly. It was found that doping of B2O3 can decrease the tensile strength, strain to failure and density of the green tapes. The sintering temperature could be lowed down to 900 1C with the addition of 2 wt% B2O3 due to the liquid phase effect. No secondary phase is observed. The addition of B2O3 does not induce much degradation on the microwave dielectric properties. Optimum microwave dielectric properties of er ¼ 67, Q f¼ 6560 GHz are obtained for Li2O–Nb2O5–TiO2 ceramics containing 2 wt% B2O3 sintered at 900 1C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) application. & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
1.
Introduction
Recently, the wireless communication systems such as cellular phones, direct broadcasting satellite and the global positioning n Corresponding author. Tel.: þ86 532 85071276; fax: þ86 532 85071509. E-mail address: [email protected] (S. Li). Peer review under responsibility of Chinese Materials Research Society.
system have been developed. Moreover, low temperature co-fired ceramics (LTCC) is finding increased usage as an interconnect substrate, especially in microwave applications. LTCC multilayer devices consist of alternating microwave dielectric ceramics and internal metallic electrode layers such as Ag, which melting temperature is about 960 1C [1–3]. Therefore, for the fabrication of multilayer devices, it is necessary to develop microwave dielectric ceramics with a low sintering temperature, which can be co-fired with Ag. Recent studies have demonstrated that Li2O–Nb2O5–TiO2 ceramics system exhibited excellent microwave dielectric properties (er ¼68, a tunable tf, Q f ¼9000 GHz), and is promising candidates for use in wireless communication systems [4–7]. However, it is not easy to apply this ceramic for the
1002-0071 & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pnsc.2013.02.007
Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics application of LTCC process since the sintering temperature (1150 1C) is relatively high. Therefore, the addition of sintering aids such as low-melting point oxides and/or glasses is required to reduce the sintering temperature. Several kinds of low-melting point sintering aids have been investigated to reduce the sintering temperature of microwave dielectric materials effectively [8–10]. Typically, B2O3 is the wellknown liquid phase sintering promoter [11–15]. In the present work, the effect of B2O3 addition on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics has been investigated. Tape casting process is the best method for creating large, thin and flat ceramic or metallic parts. In recent years, there is a trend to move away from traditionally solvent-based tape casting towards water-based systems due to the environmental and health aspects. However, in comparison with non-aqueous tape casting slurries, aqueous tape casting systems have a smaller tolerance to minor changes in processing parameters [16–19]. Therefore, it is important to understand and control the effects of the composition on the rheology of the aqueous slurries. In this work, we discuss the effect of B2O3 on the rheological properties of the aqueous tape casting 1.72Li2O– Nb2O5–1.44TiO2 (LNT) ceramics slurries. The influences of B2O3 additions on the sintering temperature, microstructure, green density, and microwave dielectric properties of low-fired LNT ceramics have also been investigated.
2.
Experimental procedure
Reagent-grade raw materials of Li2CO3, Nb2O5, TiO2 with purities higher than 99% were used as starting materials. These powders were weighted according to the mol ratio of Li2CO3:Nb2O5:TiO2 ¼1.72:1:1.4, and then ground in ethanol for 12 h in a balling mill with ZrO2 balls. The mixtures were dried and calcined at 750 1C for 3 h. The calcined powders were mixed with different amounts of B2O3 additions and re-milled for 4 h. The milled powders were then tape casted to form green tapes. Table 1 lists the optimal composition of tape casting slurries. In aqueous tape casting process, an aqueous emulsion (WB4101) with solid content 50 wt% was used as binder. A commercial ammonium polyacrylate (NH4 PA) solution with molecular weight of 5000 g/mol was choosing as dispersant. Rheological measurements of the slurries were performed using a rotating cylinder viscometer (Haake VT550, Germany) at 25 1C. The slurries were cast on a glass plate using a tape casting machine. The tapes were then dried in an Table 1 Slurry formulation of aqueous tape casting LNT ceramics. Materials
Function
Content (wt%)
LNT B2O3 Water NH4 PA WB4101 PL001 DF001
Ceramic powders Sintering aid Solvent Dispersant Binder Plasticizer Defoamer
50, 49, 48, 47 0, 1, 2, 3 29 0.8 15 5 0.2
153
oven at 60 1C for 4 h. The tapes were weighted and measured to determine the green density. To obtain the tensile strength and strain-to-failure values, the tapes were measured using an Instron universal testing instrument (AG-1 Instron tensile tester, Japan) at a constant load speed of 2.5 mm/min. The green tapes with silver electrode were stacked and pressed to prepare multilayer structure ceramic sheets. The sheets were heated up to 500 1C at a heating rate of 1 1C/min to burn out organic additives. It was then heated up at 5 1C/min to the sintering temperature of 850–1150 1C and held for 2 h. The densities of the sintered tapes were measured using the Archimedes principle. The crystalline phases were analyzed on a Rigaku D/max-RA diffractometer using CuKa radiation. Scanning electron microscopy (Hitachi S-570, Japan) was used to examine the microstructures of both the green and sintered tapes. Microwave dielectric constants er and the quality values Q f were measured at 1.5–2.5 GHz by Hakki–Coleman dielectric resonator method using an Agilent 8719 ET (50 MHz–13.5 GHz) Network Analyzer.
3. 3.1.
Results and discussion Rheological properties
The flow curves of LNT aqueous tape casting slurries with different B2O3 contents are shown in Fig. 1. As it can be observed from Fig. 1(a), a shear thinning effect is found in all cases, indicating that the slurries are not fully deflocculated. Therefore, there is a net attractive inter-particle force that gives rise to the formation of three-dimensional networks of particles. It is necessary to break these networks in order to release the liquid inside the aggregates and let the suspension began to flow. The shear thinning behavior is desired for the tape casting process. This enables structure decomposition when the slurry passes under the blade and its level out, and the structure regeneration after padding the blade, avoiding particles sedimentation and unwanted post-casting flows [20]. As the concentration of B2O3 increases, slurries viscosity increases dramatically. It is believed that the increasing trend of viscosity is caused by the formation of B(OH) 4 , which is generated by the dissolution of B2O3 in the aqueous solution. Moreover, it can be seen from Fig. 1(b) that no thixotropic hysteresis cycle is found in all cases over the applied shear-rate since the increasing and decreasing shear stress curves coincide fairly well. 3.2.
Characterization of green tapes
Fig. 2 shows the tensile strength and the strain to failure values of green tapes, respectively, as a function of the B2O3 content. It can be seen that the strength and the strain to failure of the green tapes decreased sharply with amount of B2O3. The tensile strength was 1.73 MPa for undoped tape, and in the range of 1.47–0.75 MPa for 1–3 wt% B2O3. This is mainly due to the non-homogeneous structures in the aqueous slurries with high viscosity. Obviously, B2O3 addition played an important role in the mechanical properties of the aqueous green tape. However, these mechanical properties produced sufficient handling of the green tapes for further processing such as binder burnout and sintering stages.
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Fig. 3 Green density of LNT tapes as a function of the B2O3 content.
Fig. 1 Rheological properties of the slurries with different B2O3 contents: (a) shear rate vs. viscosity curves; (b) shear rate vs. shear stress curves. Fig. 4 X-ray diffraction patterns of (a) the undoped LNT ceramic sintered at 1100 1C, and the ceramics sintered at 900 1C with (b) 1 wt%, (c) 2 wt%, and (d) 3 wt% B2O3 addition.
3.3.
Fig. 2 Tensile strength and strain to failure of green tapes as a function of the B2O3 content.
Fig. 3 shows the green density of the tapes versus the B2O3 content. A linear correlation between the green density and the B2O3 content was found. Here, the increased of B2O3 content seems to lower the particle dispersion and increase the viscosity of the slurries, resulting in lower green density.
Phase and microstructures
The X-ray diffraction patterns of the samples with different amounts of B2O3 additive are shown in Fig. 4, where the undoped ceramic was sintered at 1100 1C, and the B2O3-doped ceramics were sintered at 900 1C. It is observed that all the compounds exhibited a single LNT phase, and no secondary phases could be detected because of its low mass fraction. Compared with the theoretic XRD pattern of the LNT structure, there was a peak shift to lower 2y angles for the B2O3-doped ceramics. Fig. 5 shows the SEM microstructure of the green tapes obtained from 2 wt%-B2O3-doped LNT ceramics. A fine microstructure with extremely small pores distributed homogenously was found in the green body. The average particle size was 0.5–1 mm. There is no apparent agglomerate in the tape. It can be seen that a uniformly green density was achieved by the aqueous tape-casting process. The SEM micrographs of LNT tapes doped with different amounts of B2O3 additions sintered at different temperatures
Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics are shown in Fig. 6. For un-doped LNT tape some pores were easily observed (Fig. 6(a)), even sintering at 1100 1C. The grains are plate-like which a typical feature of LNT is. For LNT tapes with 2 wt% B2O3 addition, the homogeneously fine microstructures with almost no pores are observed in Fig. 6(b). However, further increasing the amount of B2O3 addition, such as 3 wt%-B2O3-doped LNT tape, some pores appear again due to the evaporation of liquid phase illustrated in Fig. 6(c). 3.4.
Densification and microwave dielectric properties
Fig. 7 shows the bulk density of the LNT tapes with various amounts of B2O3 additions as a function of the sintering temperature. It can be seen that the densities of the B2O3doped LNT ceramic tapes increase steady with increasing sintering temperature, after reaching a maximum at 900– 950 1C, then decreased slightly. The ceramic with 1 wt% B2O3 addition can be sintered to the density of 4.26 g/cm3 at 950 1C. When the B2O3 content increased to 2 wt%, the ceramic tapes could reach the densities of 4.3 g/cm3 at 950 1C. The results show that B2O3 is effective in enhancing the sintering ability of the LNT ceramic. However, the bulk densities of LNT tapes decreased when the amount of B2O3 additions up to 3 wt%. It indicated that large amount of B2O3 additions is not needed. The microwave dielectric properties of the tapes sintered at 900 1C with different B2O3 content are illustrated in Table 2. The relationship between the er values and B2O3 doped contents presents a trend similar to that between densities and B2O3 contents. Owing to the higher densities, the er values
155
of the B2O3-doped LNT tapes are higher than the un-doped tape. Furthermore, for LNT ceramics with 1–3 wt% additions of B2O3, the er values increased with B2O3 addition and then decreased. Density plays an important role in controlling the dielectric loss and has been shown for other microwave dielectric materials. With increasing B2O3 contents from 0 to 2 wt%, the Q f value was found to increase, which was consistent with the variation of density. The increases in Q f values were attributed to the increases in densities and grain growth. However, the Q f value was decreased with further increase B2O3 contents to 3 wt%. It has been reported that the microwave dielectric loss is mainly caused not only by the lattice vibrational modes but also by the pores and the grain morphology. The abnormal grain growth was reduce the quality factor of microwave dielectric ceramics with more than 90% relative density. Maximum er and Q f values measured were about 67 and 6560 GHz for 2 wt% B2O3doped ceramics.
Fig. 7 Bulk densities of LNT tapes with different amounts of B2O3 additions as a function of sintering temperature.
Table 2 Microwave dielectric properties of the LNT tapes sintered at 900 1C with different B2O3 contents. Item
er Q f (GHz)
B2O3 contents 0 wt%
1 wt%
2 wt%
3 wt%
41 3200
58 6170
67 6560
61 6220
Fig. 5 SEM image of the LNT green tapes.
Fig. 6 SEM micrographs of the LNT tapes with: (a) 0 wt% B2O3 additions sintered at 1100 1C, (b) 2 wt% and (c) 3 wt% B2O3 additions sintered at 900 1C.
156 4.
S. Li et al. Conclusions
In this study, the effects of B2O3 addition on the aqueous tape casting and microwave dielectric properties of Li2O–Nb2O5– TiO2 ceramics were investigated. A typical shear-thinning with no time dependent behavior of the slurries were observed, which is desirable in tape casting process. However, the addition of B2O3 increased the viscosity of the slurries sharply. The tensile strength, strain to failure and density of the green tapes decreased with B2O3 content. Aqueous casting LNT tapes with proper B2O3 content can be sintered at 900 1C. The microwave dielectric properties are found to strongly correlate with the amount of B2O3 addition. At 900 1C, 2 wt% B2O3doped LNT tapes gave excellent microwave dielectric properties: er ¼67, Q f ¼6560 GHz. This results show that Li2O– Nb2O5–TiO2 dielectric ceramic is a good candidate for LTCC application. Acknowledgment This study was supported by the National Basic Research Program of China (‘‘973’’ Project) (No. 2009CB623203), the National Natural Science Foundation of China (Contract no. 50739001), and the University Scientific and Technological Project of Shandong Province (Contract no. J09LD14). References [1] K. Malecha, D.G. Pijanowska, L.J. Golonka, P. Kurek, Low temperature co-fired ceramic (LTCC)-based biosensor for continuous glucose monitoring, Sensors and Actuators B: Chemical 155 (2011) 923–929. [2] A. Bittner, H. Seidel, U. Schmid, Electromigration resistance and long term stability of textured silver thin films on LTCC, Microelectronic Engineering 88 (2011) 127–130. [3] P. Bembnowicz, L.J. Golonka, Integration of transparent glass window with LTCC technology for mTAS application, Journal of the European Ceramic Society 30 (2010) 743–749. [4] A. Borisevich, P.K. Davies, Microwave dielectric properties of Li1þxyM1x3yTixþ4yO3 (M¼Nb5þ, Ta5þ) solid solutions, Journal of the European Ceramic Society 21 (2001) 1719–1722. [5] Q. Zeng, W. Li, J.L. Shi, X.L. Dong, J.K. Guo, The Li2O– Nb2O5–TiO2 composite microwave dielectric ceramics with adjustable permittivities, Journal of the American Ceramic Society 91 (2008) 644–647. [6] S.C. Li, Y.J. Geng, Q.L. Zhang, H. Yang, Aqueous processing of Li1.075Nb0.625Ti0.45O3 green tapes, Journal of Ceramic Processing Research 12 (2011) 155–159. [7] S.C. Li, Q.L. Zhang, H. Yang, Co-firing of LNT/Ag multilayer sheets prepared by aqueous tape casting, Journal of Materials Sciences & Technology 25 (2009) 269–272.
[8] D.H. Kang, K.C. Nam, H.J. Cha, Effect of Li2O–V2O5 on the low temperature sintering and microwave dielectric properties of Li1.0Nb0.6Ti0.5O3 ceramics, Journal of the European Ceramic Society 26 (2006) 2117–2121. [9] H.F. Zhou, H. Wang, D. Zhou, L.X. Pang, X. Yao, Effect of ZnO and B2O3 on the sintering temperature and microwave dielectric properties of LiNb0.6Ti0.5O3 ceramics, Materials Chemistry and Physics 109 (2008) 510–514. [10] Q. Zeng, W. Li, J.L. Shi, J.K. Guo, H. Chen, M.L. Liu, Effect of B2O3 on the sintering and microwave dielectric properties of M-phase LiNb0.6Ti0.5O3 ceramics, Journal of the European Ceramic Society 27 (2007) 261–265. [11] J.S. Kim, M.E. Song, M.R. Joung, J.H. Choi, S. Nahm, S.I. Gu, J.H. Paik, B.H. Choi, Effect of B2O3 addition on the sintering temperature and microwave dielectric properties of Zn2SiO4 ceramics, Journal of the European Ceramic Society 30 (2010) 375–379. [12] Y.C. Chen, K.C. Chen, K.H. Chen, Effect of B2O3 additions and sintering temperature on the microwave dielectric properties of 0.7La(Mg0.5Sn0.5)O3–0.3(Sr0.8Ca0.2)3Ti2O7 ceramics, Ferroelectrics Letters 38 (2011) 59–68. [13] H.I. Hsiang, S.H. Chung, Effects of B2O3 addition on the microstructure and microwave dielectric properties of La4Ba2Ti5O18, Journal of Alloys and Compounds 465 (2008) 356–360. [14] M.R. Joung, J.S. Kim, M.E. Song, D.S. Paik, S. Nahm, J.H. Paik, B.H. Choi, Effect of B2O3 on the sintering condition and microwave dielectric properties of Bi4(SiO4)3 ceramics, Journal of the American Ceramic Society 91 (2008) 4165–4167. [15] H.T. Wu, L.X. Li, B2O3 additives on sintering and microwave dielectric behaviors of Mg4Nb2O9 ceramics synthesized through the aqueous sol–gel process, Journal of Sol-Gel Science and Technology 58 (2011) 48–55. [16] C.J. Fu, S.H. Chan, Q.L. Liu, X.M. Ge, G. Pasciak, Fabrication and evaluation of Ni-GDC composite anode prepared by aqueous-based tape casting method for low-temperature solid oxide fuel cell, International Journal of Hydrogen Energy 35 (2010) 301–307. [17] C.C. Wang, L.H. Luo, Y.F. Wu, B.X. Hou, L.L. Sun, A novel multilayer aqueous tape casting method for anode-supported planar solid oxide fuel cell, Materials Letters 65 (2011) 2251– 2253. [18] Z.H. Lv, D.L. Jiang, J.X. Zhang, Q.L. Lin, Aqueous tape casting of zirconium diboride, Journal of the American Ceramic Society 92 (2009) 2212–2217. [19] K. Prabhakaran, E.M. Jayasingh, S. Raghunath, C. Durgaprasad, S.C. Sharma, Aqueous tape casting of PMN-PT powder prepared by the partial oxalate route, Journal of Materials Processing Technology 209 (2009) 4217–4221. [20] S.C. Li, Q.L. Zhang, H. Yang, D. Zou, Fabrication and characterization of Li1þxyNb1x-3yTixþ4yO3 substrates using aqueous tape casting process, Ceramics International 35 (2009) 421–426.
Progress in Natural Science: Materials International www.elsevier.com/locate/pnsmi www.sciencedirect.com
ORIGINAL RESEARCH
Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics Shaochun Lia,n, Qilong Zhangb, Tiejun Zhaoa, Hui Yangb a
School of Civil Engineering, Qingdao Technological University, Qingdao 266033, China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
b
Received 7 September 2012; accepted 10 January 2013 Available online 13 March 2013
KEYWORDS Aqueous tape casting; B2O3; Rheology; Microwave dielectric properties
Abstract The effect of B2O3 addition on the aqueous tape casting, sintering, microstructure and microwave dielectric properties of Li2O–Nb2O5–TiO2 ceramics has been investigated. The tape casting slurries exhibit a typical shear-thinning behavior without thixotropy, but the addition of B2O3 increases the viscosity of the slurries significantly. It was found that doping of B2O3 can decrease the tensile strength, strain to failure and density of the green tapes. The sintering temperature could be lowed down to 900 1C with the addition of 2 wt% B2O3 due to the liquid phase effect. No secondary phase is observed. The addition of B2O3 does not induce much degradation on the microwave dielectric properties. Optimum microwave dielectric properties of er ¼ 67, Q f¼ 6560 GHz are obtained for Li2O–Nb2O5–TiO2 ceramics containing 2 wt% B2O3 sintered at 900 1C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) application. & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
1.
Introduction
Recently, the wireless communication systems such as cellular phones, direct broadcasting satellite and the global positioning n Corresponding author. Tel.: þ86 532 85071276; fax: þ86 532 85071509. E-mail address: [email protected] (S. Li). Peer review under responsibility of Chinese Materials Research Society.
system have been developed. Moreover, low temperature co-fired ceramics (LTCC) is finding increased usage as an interconnect substrate, especially in microwave applications. LTCC multilayer devices consist of alternating microwave dielectric ceramics and internal metallic electrode layers such as Ag, which melting temperature is about 960 1C [1–3]. Therefore, for the fabrication of multilayer devices, it is necessary to develop microwave dielectric ceramics with a low sintering temperature, which can be co-fired with Ag. Recent studies have demonstrated that Li2O–Nb2O5–TiO2 ceramics system exhibited excellent microwave dielectric properties (er ¼68, a tunable tf, Q f ¼9000 GHz), and is promising candidates for use in wireless communication systems [4–7]. However, it is not easy to apply this ceramic for the
1002-0071 & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pnsc.2013.02.007
Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics application of LTCC process since the sintering temperature (1150 1C) is relatively high. Therefore, the addition of sintering aids such as low-melting point oxides and/or glasses is required to reduce the sintering temperature. Several kinds of low-melting point sintering aids have been investigated to reduce the sintering temperature of microwave dielectric materials effectively [8–10]. Typically, B2O3 is the wellknown liquid phase sintering promoter [11–15]. In the present work, the effect of B2O3 addition on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics has been investigated. Tape casting process is the best method for creating large, thin and flat ceramic or metallic parts. In recent years, there is a trend to move away from traditionally solvent-based tape casting towards water-based systems due to the environmental and health aspects. However, in comparison with non-aqueous tape casting slurries, aqueous tape casting systems have a smaller tolerance to minor changes in processing parameters [16–19]. Therefore, it is important to understand and control the effects of the composition on the rheology of the aqueous slurries. In this work, we discuss the effect of B2O3 on the rheological properties of the aqueous tape casting 1.72Li2O– Nb2O5–1.44TiO2 (LNT) ceramics slurries. The influences of B2O3 additions on the sintering temperature, microstructure, green density, and microwave dielectric properties of low-fired LNT ceramics have also been investigated.
2.
Experimental procedure
Reagent-grade raw materials of Li2CO3, Nb2O5, TiO2 with purities higher than 99% were used as starting materials. These powders were weighted according to the mol ratio of Li2CO3:Nb2O5:TiO2 ¼1.72:1:1.4, and then ground in ethanol for 12 h in a balling mill with ZrO2 balls. The mixtures were dried and calcined at 750 1C for 3 h. The calcined powders were mixed with different amounts of B2O3 additions and re-milled for 4 h. The milled powders were then tape casted to form green tapes. Table 1 lists the optimal composition of tape casting slurries. In aqueous tape casting process, an aqueous emulsion (WB4101) with solid content 50 wt% was used as binder. A commercial ammonium polyacrylate (NH4 PA) solution with molecular weight of 5000 g/mol was choosing as dispersant. Rheological measurements of the slurries were performed using a rotating cylinder viscometer (Haake VT550, Germany) at 25 1C. The slurries were cast on a glass plate using a tape casting machine. The tapes were then dried in an Table 1 Slurry formulation of aqueous tape casting LNT ceramics. Materials
Function
Content (wt%)
LNT B2O3 Water NH4 PA WB4101 PL001 DF001
Ceramic powders Sintering aid Solvent Dispersant Binder Plasticizer Defoamer
50, 49, 48, 47 0, 1, 2, 3 29 0.8 15 5 0.2
153
oven at 60 1C for 4 h. The tapes were weighted and measured to determine the green density. To obtain the tensile strength and strain-to-failure values, the tapes were measured using an Instron universal testing instrument (AG-1 Instron tensile tester, Japan) at a constant load speed of 2.5 mm/min. The green tapes with silver electrode were stacked and pressed to prepare multilayer structure ceramic sheets. The sheets were heated up to 500 1C at a heating rate of 1 1C/min to burn out organic additives. It was then heated up at 5 1C/min to the sintering temperature of 850–1150 1C and held for 2 h. The densities of the sintered tapes were measured using the Archimedes principle. The crystalline phases were analyzed on a Rigaku D/max-RA diffractometer using CuKa radiation. Scanning electron microscopy (Hitachi S-570, Japan) was used to examine the microstructures of both the green and sintered tapes. Microwave dielectric constants er and the quality values Q f were measured at 1.5–2.5 GHz by Hakki–Coleman dielectric resonator method using an Agilent 8719 ET (50 MHz–13.5 GHz) Network Analyzer.
3. 3.1.
Results and discussion Rheological properties
The flow curves of LNT aqueous tape casting slurries with different B2O3 contents are shown in Fig. 1. As it can be observed from Fig. 1(a), a shear thinning effect is found in all cases, indicating that the slurries are not fully deflocculated. Therefore, there is a net attractive inter-particle force that gives rise to the formation of three-dimensional networks of particles. It is necessary to break these networks in order to release the liquid inside the aggregates and let the suspension began to flow. The shear thinning behavior is desired for the tape casting process. This enables structure decomposition when the slurry passes under the blade and its level out, and the structure regeneration after padding the blade, avoiding particles sedimentation and unwanted post-casting flows [20]. As the concentration of B2O3 increases, slurries viscosity increases dramatically. It is believed that the increasing trend of viscosity is caused by the formation of B(OH) 4 , which is generated by the dissolution of B2O3 in the aqueous solution. Moreover, it can be seen from Fig. 1(b) that no thixotropic hysteresis cycle is found in all cases over the applied shear-rate since the increasing and decreasing shear stress curves coincide fairly well. 3.2.
Characterization of green tapes
Fig. 2 shows the tensile strength and the strain to failure values of green tapes, respectively, as a function of the B2O3 content. It can be seen that the strength and the strain to failure of the green tapes decreased sharply with amount of B2O3. The tensile strength was 1.73 MPa for undoped tape, and in the range of 1.47–0.75 MPa for 1–3 wt% B2O3. This is mainly due to the non-homogeneous structures in the aqueous slurries with high viscosity. Obviously, B2O3 addition played an important role in the mechanical properties of the aqueous green tape. However, these mechanical properties produced sufficient handling of the green tapes for further processing such as binder burnout and sintering stages.
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Fig. 3 Green density of LNT tapes as a function of the B2O3 content.
Fig. 1 Rheological properties of the slurries with different B2O3 contents: (a) shear rate vs. viscosity curves; (b) shear rate vs. shear stress curves. Fig. 4 X-ray diffraction patterns of (a) the undoped LNT ceramic sintered at 1100 1C, and the ceramics sintered at 900 1C with (b) 1 wt%, (c) 2 wt%, and (d) 3 wt% B2O3 addition.
3.3.
Fig. 2 Tensile strength and strain to failure of green tapes as a function of the B2O3 content.
Fig. 3 shows the green density of the tapes versus the B2O3 content. A linear correlation between the green density and the B2O3 content was found. Here, the increased of B2O3 content seems to lower the particle dispersion and increase the viscosity of the slurries, resulting in lower green density.
Phase and microstructures
The X-ray diffraction patterns of the samples with different amounts of B2O3 additive are shown in Fig. 4, where the undoped ceramic was sintered at 1100 1C, and the B2O3-doped ceramics were sintered at 900 1C. It is observed that all the compounds exhibited a single LNT phase, and no secondary phases could be detected because of its low mass fraction. Compared with the theoretic XRD pattern of the LNT structure, there was a peak shift to lower 2y angles for the B2O3-doped ceramics. Fig. 5 shows the SEM microstructure of the green tapes obtained from 2 wt%-B2O3-doped LNT ceramics. A fine microstructure with extremely small pores distributed homogenously was found in the green body. The average particle size was 0.5–1 mm. There is no apparent agglomerate in the tape. It can be seen that a uniformly green density was achieved by the aqueous tape-casting process. The SEM micrographs of LNT tapes doped with different amounts of B2O3 additions sintered at different temperatures
Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O–Nb2O5–TiO2 ceramics are shown in Fig. 6. For un-doped LNT tape some pores were easily observed (Fig. 6(a)), even sintering at 1100 1C. The grains are plate-like which a typical feature of LNT is. For LNT tapes with 2 wt% B2O3 addition, the homogeneously fine microstructures with almost no pores are observed in Fig. 6(b). However, further increasing the amount of B2O3 addition, such as 3 wt%-B2O3-doped LNT tape, some pores appear again due to the evaporation of liquid phase illustrated in Fig. 6(c). 3.4.
Densification and microwave dielectric properties
Fig. 7 shows the bulk density of the LNT tapes with various amounts of B2O3 additions as a function of the sintering temperature. It can be seen that the densities of the B2O3doped LNT ceramic tapes increase steady with increasing sintering temperature, after reaching a maximum at 900– 950 1C, then decreased slightly. The ceramic with 1 wt% B2O3 addition can be sintered to the density of 4.26 g/cm3 at 950 1C. When the B2O3 content increased to 2 wt%, the ceramic tapes could reach the densities of 4.3 g/cm3 at 950 1C. The results show that B2O3 is effective in enhancing the sintering ability of the LNT ceramic. However, the bulk densities of LNT tapes decreased when the amount of B2O3 additions up to 3 wt%. It indicated that large amount of B2O3 additions is not needed. The microwave dielectric properties of the tapes sintered at 900 1C with different B2O3 content are illustrated in Table 2. The relationship between the er values and B2O3 doped contents presents a trend similar to that between densities and B2O3 contents. Owing to the higher densities, the er values
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of the B2O3-doped LNT tapes are higher than the un-doped tape. Furthermore, for LNT ceramics with 1–3 wt% additions of B2O3, the er values increased with B2O3 addition and then decreased. Density plays an important role in controlling the dielectric loss and has been shown for other microwave dielectric materials. With increasing B2O3 contents from 0 to 2 wt%, the Q f value was found to increase, which was consistent with the variation of density. The increases in Q f values were attributed to the increases in densities and grain growth. However, the Q f value was decreased with further increase B2O3 contents to 3 wt%. It has been reported that the microwave dielectric loss is mainly caused not only by the lattice vibrational modes but also by the pores and the grain morphology. The abnormal grain growth was reduce the quality factor of microwave dielectric ceramics with more than 90% relative density. Maximum er and Q f values measured were about 67 and 6560 GHz for 2 wt% B2O3doped ceramics.
Fig. 7 Bulk densities of LNT tapes with different amounts of B2O3 additions as a function of sintering temperature.
Table 2 Microwave dielectric properties of the LNT tapes sintered at 900 1C with different B2O3 contents. Item
er Q f (GHz)
B2O3 contents 0 wt%
1 wt%
2 wt%
3 wt%
41 3200
58 6170
67 6560
61 6220
Fig. 5 SEM image of the LNT green tapes.
Fig. 6 SEM micrographs of the LNT tapes with: (a) 0 wt% B2O3 additions sintered at 1100 1C, (b) 2 wt% and (c) 3 wt% B2O3 additions sintered at 900 1C.
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S. Li et al. Conclusions
In this study, the effects of B2O3 addition on the aqueous tape casting and microwave dielectric properties of Li2O–Nb2O5– TiO2 ceramics were investigated. A typical shear-thinning with no time dependent behavior of the slurries were observed, which is desirable in tape casting process. However, the addition of B2O3 increased the viscosity of the slurries sharply. The tensile strength, strain to failure and density of the green tapes decreased with B2O3 content. Aqueous casting LNT tapes with proper B2O3 content can be sintered at 900 1C. The microwave dielectric properties are found to strongly correlate with the amount of B2O3 addition. At 900 1C, 2 wt% B2O3doped LNT tapes gave excellent microwave dielectric properties: er ¼67, Q f ¼6560 GHz. This results show that Li2O– Nb2O5–TiO2 dielectric ceramic is a good candidate for LTCC application. Acknowledgment This study was supported by the National Basic Research Program of China (‘‘973’’ Project) (No. 2009CB623203), the National Natural Science Foundation of China (Contract no. 50739001), and the University Scientific and Technological Project of Shandong Province (Contract no. J09LD14). References [1] K. Malecha, D.G. Pijanowska, L.J. Golonka, P. Kurek, Low temperature co-fired ceramic (LTCC)-based biosensor for continuous glucose monitoring, Sensors and Actuators B: Chemical 155 (2011) 923–929. [2] A. Bittner, H. Seidel, U. Schmid, Electromigration resistance and long term stability of textured silver thin films on LTCC, Microelectronic Engineering 88 (2011) 127–130. [3] P. Bembnowicz, L.J. Golonka, Integration of transparent glass window with LTCC technology for mTAS application, Journal of the European Ceramic Society 30 (2010) 743–749. [4] A. Borisevich, P.K. Davies, Microwave dielectric properties of Li1þxyM1x3yTixþ4yO3 (M¼Nb5þ, Ta5þ) solid solutions, Journal of the European Ceramic Society 21 (2001) 1719–1722. [5] Q. Zeng, W. Li, J.L. Shi, X.L. Dong, J.K. Guo, The Li2O– Nb2O5–TiO2 composite microwave dielectric ceramics with adjustable permittivities, Journal of the American Ceramic Society 91 (2008) 644–647. [6] S.C. Li, Y.J. Geng, Q.L. Zhang, H. Yang, Aqueous processing of Li1.075Nb0.625Ti0.45O3 green tapes, Journal of Ceramic Processing Research 12 (2011) 155–159. [7] S.C. Li, Q.L. Zhang, H. Yang, Co-firing of LNT/Ag multilayer sheets prepared by aqueous tape casting, Journal of Materials Sciences & Technology 25 (2009) 269–272.
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