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Low-temperature sintering and compatibility with silver electrode of Ba4MgTi11O27 microwave dielectric ceramic
Materials Research Bulletin 45 (2010) 1509–1512
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Low-temperature sintering and compatibility with silver electrode of Ba4MgTi11O27 microwave dielectric ceramic Xiuli Chen a, Huanfu Zhou a,*, Liang Fang a, Laijun Liu a, Changda Li a, Ruli Guo a, Hong Wang b a b
Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, Guilin University of Technology, Jiangan Road 12#, Guilin 541004, China Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 November 2009 Received in revised form 31 May 2010 Accepted 4 June 2010 Available online 14 June 2010
Ba4MgTi11O27 microwave dielectric ceramic was investigated using X-ray diffraction, scanning electron microscopy and dielectric measurement. The pure Ba4MgTi11O27 ceramic shows a high sintering temperature (1275 8C) and good microwave dielectric properties as Q f of 19,630 GHz, er of 36.1, tf of 14.6 ppm/8C. It was found that the addition of BaCu(B2O5) (BCB) can effectively lower the sintering temperature from 1275 to 925 8C, and does not induce much degradation of the microwave dielectric properties. The BCB-doped Ba4MgTi11O27 ceramics can be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Electronic materials C. X-ray diffraction D. Dielectric properties D. Microstructure
1. Introduction Miniaturization of microwave dielectrics is essential for the production of multilayer or chip devices in mobile communications. Particularly for the fabrication of a multichip module (MCM), a simple cofiring process of microwave dielectrics with an internal electrode is highly desirable. For this purpose, low-temperaturecofired ceramics (LTCC) have been widely investigated [1–4]. In LTCC, the firing temperature lower than 950 8C is favorable since Ag with the melting point of 960 8C can be used instead of more expensive electrodes such as Ag–Pd binary or Pt–Pd–Au ternary alloys. Most of the known commercial dielectric materials for the highfrequency applications have good microwave dielectric properties, but they cannot been cofired with Ag electrode because of high sintering temperatures between 1200 and 1500 8C [5–9]. So, how to reduce their sintering temperatures to lower than the melting point of the Ag or Cu electrodes has aroused world-wide interest. Generally, three common methods have been used to reduce the sintering temperature of the dielectric ceramics: low-melting glass addition, chemical processing, and smaller particle sizes of starting materials. Among these methods, low-melting glass additions for liquid-phase sintering are lower in cost and easier to process than the other two. Low-temperature sintering of dielectric materials
* Corresponding author. Tel.: +86 773 5896435; fax: +86 773 5893220. E-mail address: [email protected] (H. Zhou). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.06.012
with glass addition has been successfully achieved by the several microwave dielectric ceramics systems such as MgO–TiO2, Li2O– Nb2O5–TiO2, CaO–Li2O–Nb2O5–TiO2, etc. [10–15]. Within the BaO–MgO–TiO2 system, the crystal structure of Ba4MgTi11O27 has been reported by Kaduk et al. [16]. However, the microwave dielectric properties of Ba4MgTi11O27 ceramic have not been reported. In the present study, the microwave dielectric properties of Ba4MgTi11O27 were investigated. It is well known that BaCu(B2O5) (BCB) addition often makes it possible to decrease the sintering temperature of many materials [17–19]. For example, using 6 mol.% BCB, the Ba(Zn1/3Nb2/3)O3 dielectric can be sintered at 875 8C and good microwave dielectric properties obtained with values of er = 35, Q f = 16,000 GHz and tf = 22.1 ppm/8C [17]. So in order to lower the sintering temperature of Ba4MgTi11O27 ceramic to 900 8C, a small amount of BaCu(B2O5) is added to the ceramic. 2. Experimental procedure Specimens of the Ba4MgTi11O27 ceramics were prepared by a conventional mixed oxide route from the high-purity oxide powders (99.9%, Guo-Yao Co. Ltd., Shanghai, China) of BaCO3, MgO and TiO2. Stoichiometric proportions of the above raw materials were mixed in alcohol medium using zirconia balls for 4 h. The mixtures were dried and calcined at 1150 8C for 4 h. To synthesize the BaCu(B2O5) ceramic powder, Ba(OH)28H2O (>99%, Guo-Yao Co. Ltd., Shanghai, China), CuO (>99%, Guo-Yao Co. Ltd., Shanghai, China) and H3BO3 (>99%, Guo-Yao Co. Ltd., Shanghai,
[(Fig._2)TD$IG]
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China) were mixed for 4 h in a nylon jar with zirconia balls, then dried and calcined at 800 8C for 4 h. After subsequent ball-milling with 4–8.0 wt% BCB, the powders were uniaxially pressed under the pressure of about 150 MPa into disks of 10 mm in diameter and 6 mm in thickness. The pure Ba4MgTi11O27 pellets were sintered at 1225–1325 8C for 0.5–8 h in air and the ceramic pellets doped with BCB were sintered at 850–950 8C for 2 h in air. The sintered samples were naturally cooled in the furnace from the sintering temperature. The crystal structures of the specimens were analyzed by an X-ray diffractometer (Rigaku D/MAX-2400, Japan) with Cu Ka radiation generated at 40 kV and 100 mA. The bulk densities of the sintered samples were measured by the Archimedes method. Samples were then thermally etched for 1 h at 100 8C lower than the sintering temperature and then Au coated to prevent surface charging in the microscope. The microstructure observation of the samples was performed using scanning electron microscopy (JEOL JSM-6460LV, Japan). Dielectric behaviors in microwave frequency were measured in the range of 6–7 GHz by the TE01d shielded cavity method using a Network Analyzer (8720ES, Agilent, U.S.A.) and a temperature chamber (DELTA 9023, Delta Design, U.S.A.). The specimens were placed on a low-loss quartz spacer inside a copper cavity, whose inner side was silver-plated. The use of low-loss single-crystal quartz spacer reduces the effect of losses due to the surface resistivity of the cavity. The diameter of the cavity was about 4 times larger than that of the sample for better isolation of the excited TE01d mode. The temperature coefficients of resonant frequency tf values were calculated by the formula as follows:
tf ¼
fT f0 f 0 ðT T 0 Þ
(1)
Fig. 2. SEM photographs of Ba4MgTi11O27 samples sintered at 1275 8C for 2 h.
Z = 4 (Z is the number of formula units per unit cell), which agrees well with that reported by Kaduk et al. [16]. The theoretical density of the Ba4MgTi11O27 ceramic calculated from XRD data is about 4.78 g/ cm3. The SEM micrographs of the surfaces of Ba4MgTi11O27 ceramics are shown in Fig. 2. The dense microstructure of Ba4MgTi11O27 ceramics sintered at 1275 8C for 2 h with only little pores existing can be confirmed by the SEM result. Fig. 3 shows the relative density, permittivity, Q f and tf values of the Ba4MgTi11O27 ceramics sintered at various temperatures for 2 h. The relative density of the specimen sintered below 1200 8C was very low at approximately 87% and increased to 94% for the specimen sintered at 1225 8C for 2 h, but saturated when the sintering temperature exceeded 1275 8C for electronic applications.
[(Fig._3)TD$IG]
where T, T0 were the measuring temperature and room temperature T0 (25 8C), fT, f0 were the resonant frequencies at T and T0 respectively. 3. Results and discussion The room-temperature XRD patterns recorded for the powder calcined at 1150 8C and the ceramics sintered at 1275 8C for 2 h are shown in Fig. 1. A single-phase Ba4MgTi11O27 structure (JCPDS File No. 49-0580) was formed and no secondary phase could be observed in the diffraction patterns. Its pattern could be indexed with a C2/m monoclinic cell with a = 19.85494 0.00008 A˚, b = 11.45173 0.0003 A˚, c = 9.91265 0.004 A˚, V = 2129.27 A˚3, and
[(Fig._1)TD$IG]
Fig. 1. X-ray diffraction patterns (XRD) of Ba4MgTi11O27 (a) powder calcined at 1150 8C and (b) samples sintered at 1275 8C for 2 h.
Fig. 3. The relative density, permittivity, Q f and temperature coefficient of resonant frequency values of Ba4MgTi11O27 ceramics as a function of sintering temperature.
[(Fig._4)TD$IG]
X. Chen et al. / Materials Research Bulletin 45 (2010) 1509–1512
Fig. 4. The relative density, permittivity, Q f and temperature coefficient of resonant frequency values of Ba4MgTi11O27 ceramics as a function of soak time.
Therefore, the optimum sintering temperature for the Ba4MgTi11O27 ceramics was considered to be 1275 8C. The permittivity and Q f values versus sintering temperature of Ba4MgTi11O27 ceramics have a trend similar to that of the relative density. When the sintering temperature increases to 1275 8C, the permittivity reaches a saturated value of 36.1 and the Q f value of Ba4MgTi11O27 ceramics reaches the maximum with a value of 19630 GHz (at 6.2 GHz). Due to frozen-in point defects caused by high-temperature reduction
[(Fig._5)TD$IG]
1511
during sintering, the Q f values decrease as the sintering temperature further increasing. The tf value of the Ba4MgTi11O27 ceramic sintered at 1275 8C was approximately 15 ppm/8C and its variation with the sintering temperature was not large, ranging between 14.5 and 16.0 ppm/8C. The Ba4MgTi11O27 ceramics are sintered at 1275 8C for various durations of time and their relative density, permittivity, Q f, and tf values were measured, as shown in Fig. 4. The relative density, permittivity, Q f values of the Ba4MgTi11O27 ceramics sintered for 0.5 h were somewhat low. However, a high relative density, permittivity and Q f value were obtained for the specimens sintered for 2 h, and their variation with the sintering time was not significant when the sintering time was over 2 h. These results suggest that sintering at 1275 8C for 2 h is sufficient for the densification of specimens. The tf value of the Ba4MgTi11O27 ceramic sintered at 1275 8C for 2 h was approximately 15 ppm/8C and its variation with the soak time was not large, ranging between 13 and 17.0 ppm/8C. In general, the Ba4MgTi11O27 ceramics sintered at 1275 8C have good microwave dielectric properties of er = 36.1, Q f = 19,630 GHz, tf = 14.6 ppm/8C [20]. Comparing with Ba2Ti9O20 and ZTS systems, the tf value of the Ba4MgTi11O27 ceramic is relatively high for resonators; however, through adding other low melting point materials having a negative tf value, it can be adjusted to zero, which indicated this material will be applied in LTCC components. Comparing with other glasses added to reduce the sintering temperature of the materials, BaCu(B2O5) not only has low sintering temperature (810 8C) and low melting point (850 8C), but also exhibits excellent microwave dielectric properties with permittivity of 7.4, Q f values of 50,000 GHz and tf values of 32 ppm/8C [22]. To further decrease the sintering temperature of this microwave dielectric ceramic, a small amount of BaCu(B2O5) (BCB) has been added into the samples. Due to the liquid-phase effect, the addition of BCB to Ba4MgTi11O27 ceramics can efficiently lower the sintering temperature from 1275 to 925 8C. This result would be in agreement with the XRD analysis. From Fig. 5, it can be seen that the BCB phase will be existed in the BCB-doped Ba4MgTi11O27 ceramics. Table 1 shows bulk density, relative density and microwave dielectric properties of Ba4MgTi11O27 ceramics doped with different amounts of BCB. The relative density increase with increasing the addition of BCB, reaches a maximum when the addition of BCB is 6 wt%. The er and Q f values present a trend similar to that between densities and sintering temperatures since higher density means lower porosity.
Fig. 5. X-ray diffraction patterns (XRD) of (a) BCB powders sintered at 800 8C for 4 h, (b) 4 wt%, (c) 5 wt%, (d) 6 wt% and (e) 8 wt% BCB-doped Ba4MgTi11O27 ceramics sintered at 925 8C for 2 h.
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Table 1 Relative density and microwave dielectric properties of Ba4MgTi11O27 ceramics doped with different amounts of BCB. Ba4MgTi11O27 + x wt% BCB
Sintering temperature (8C)
Relative density (%)
er
Q f (GHz)
tf (ppm/8C)
0 4 5 6 8
1275 925 925 925 925
97.4 92 95 97 96
36.1 27.3 29.3 30.2 30.6
19,630 11,200 12,890 13,450 12,560
14.6 9 6 4 3
[(Fig._6)TD$IG] a LTCC component and the corresponding Ag distribution. It is obvious that the ceramic layer and the electrode layer are compatible and almost no crack exists at the interface between them. The element distribution of Ag shows that Ag is distributed in the central conductor region, and does not diffuse into the ceramic region. Overall, it is concluded that the BCB added Ba4MgTi11O27 ceramic is able to match with Ag electrode well. Acknowledgements This work was supported by Research start-up funds Doctor of Guilin University of Technology (Nos. 000788and 000787), Natural Science Foundation of China (Nos. 50962004 and 50762002), Natural Science Foundation of Guangxi (Nos. 0832003Z and 0832001), and Program for NCET-06-0656, MOE, China References
Fig. 6. The backscattered electron image and EDS analysis of the interfacial microstructure of (6 wt% BCB-doped Ba4MgTi11O27)/Ag cofired at 875 8C in a LTCC component.
The temperature coefficient of the resonant frequency (tf) was relevant to the composition and the phases existing in the ceramics. The higher the contents of BCB addition is, the lower the tf of the low-fired Ba4MgTi11O27 samples is. BCB liquid phases having a low tf would contribute to the slight decrease in tf of the BCB added Ba4MgTi11O27 system. For the 6 wt% BCB-doped ceramic, a high density of 4.64 g/cm3 and relatively good microwave dielectric properties of er = 30.2, Q f = 13,450 (6.18 GHz) GHz and tf = 4 ppm/8C have been obtained by sintering at 925 8C. For chemical compatibility tests, the green tapes of Ba4MgTi11O27 powders with 6 wt% BCB additives were obtained by the tape-casting process [21]. Ceramic sheets and Ag electrodes printed on it were cofired and analyzed to detect interaction between the low-fired ceramics and electrodes. Fig. 6 exhibits backscattered electron image and EDS analysis of the interface of 8 wt% BCB added Ba4MgTi11O27 ceramic sheet with Ag electrode in
[1] T.K. Gupta, J.H. Jean, J. Mater. Res. 11 (1996) 243–246. [2] T. Takada, S.F. Wang, S. Yoshikawa, S.J. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (1994) 1909–1916. [3] C.R. Chang, J.H. Jean, J. Am. Ceram. Soc. 81 (1998) 2805–2814. [4] S.H. Wee, D.W. Kim, S.I. Yoo, J. Am. Ceram. Soc. 87 (2004) 871–874. [5] H. Tamura, T. Konoike, Y. Sakabe, K. Wakino, J. Am. Ceram. Soc. 67 (1984) c59–c61. [6] S.B. Cohn, IEEE Trans. Microw. Theory Tech. 16 (1968) 218–227. [7] D. Kolar, S. Gabrscek, B. Volavsek, H.S. Parker, R.S. Roth, J. Solid State Chem. 38 (1981) 158–164. [8] A. Ioachim, M.I. Toacsan, M.G. Banciu, L. Nedelcu, C.A. Dutu, H.V. Alexandru, S. Antohe, E. Andronescu, S. Jinga, P. Nita, Thin Solid Films 516 (2008) 1558–1562. [9] D.A. Sagala, S. Nambu, J. Am. Ceram. Soc. 75 (1992) 2573–2575. [10] Q.L. Zhang, H. Yang, J.X. Tong, Mater. Lett. 60 (2006) 1188–1191. [11] H. Jantunen, R. Rautioaho, A. Uusima¨ki, S. Leppa¨vuori, J. Eur. Ceram. Soc. 20 (2000) 2331–2336. [12] Q. Zeng, W. Li, J. Shi, X. Dong, J. Guo, Mater. Res. Bull. 43 (2008) 411–417. [13] H.F. Zhou, H. Wang, D. Zhou, L.X. Pang, X. Yao, Mater. Chem. Phys. 109 (2008) 510– 514. [14] S. George, M.T. Sebastian, J. Alloys Compd. 473 (2009) 336–340. [15] P. Liu, E.S. Kim, S.G. Kang, H.S. Jang, Mater. Chem. Phys. 79 (2003) 270–272. [16] J. Kaduk, B. Toby, W. Wong-Ng, W. Greenwood, Powder Diffraction 13 (1998) 178–185. [17] J.B. Lim, K.H. Cho, S. Nahm, J.H. Paik, J.H. Kim, Mater. Res. Bull. 41 (2006) 1868– 1874. [18] J.B. Lim, Y.H. Jeong, N.H. Nguyen, S. Nahm, J.H. Paik, J.H. Kim, H.J. Lee, J. Eur. Ceram. Soc. 27 (2007) 2875–2879. [19] H.F. Zhou, H. Wang, Y.H. Chen, K.C. Li, X. Yao, Mater. Chem. Phys. 113 (2009) 1–5. [20] W. Wersing, Curr. Opin. Solid State Mater. Sci. 1 (1996) 715–731. [21] J.J. Choi, J.H. Lee, B.D. Hahn, W.H. Yoon, D.S. Park, Mater. Res. Bull. 43 (2008) 483–490. [22] M.H. Kim, J.B. Lim, J.C. Kim, S. Nahm, J. Am. Ceram. Soc. 89 (2006) 3124–3128.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Low-temperature sintering and compatibility with silver electrode of Ba4MgTi11O27 microwave dielectric ceramic Xiuli Chen a, Huanfu Zhou a,*, Liang Fang a, Laijun Liu a, Changda Li a, Ruli Guo a, Hong Wang b a b
Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, Guilin University of Technology, Jiangan Road 12#, Guilin 541004, China Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 November 2009 Received in revised form 31 May 2010 Accepted 4 June 2010 Available online 14 June 2010
Ba4MgTi11O27 microwave dielectric ceramic was investigated using X-ray diffraction, scanning electron microscopy and dielectric measurement. The pure Ba4MgTi11O27 ceramic shows a high sintering temperature (1275 8C) and good microwave dielectric properties as Q f of 19,630 GHz, er of 36.1, tf of 14.6 ppm/8C. It was found that the addition of BaCu(B2O5) (BCB) can effectively lower the sintering temperature from 1275 to 925 8C, and does not induce much degradation of the microwave dielectric properties. The BCB-doped Ba4MgTi11O27 ceramics can be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Electronic materials C. X-ray diffraction D. Dielectric properties D. Microstructure
1. Introduction Miniaturization of microwave dielectrics is essential for the production of multilayer or chip devices in mobile communications. Particularly for the fabrication of a multichip module (MCM), a simple cofiring process of microwave dielectrics with an internal electrode is highly desirable. For this purpose, low-temperaturecofired ceramics (LTCC) have been widely investigated [1–4]. In LTCC, the firing temperature lower than 950 8C is favorable since Ag with the melting point of 960 8C can be used instead of more expensive electrodes such as Ag–Pd binary or Pt–Pd–Au ternary alloys. Most of the known commercial dielectric materials for the highfrequency applications have good microwave dielectric properties, but they cannot been cofired with Ag electrode because of high sintering temperatures between 1200 and 1500 8C [5–9]. So, how to reduce their sintering temperatures to lower than the melting point of the Ag or Cu electrodes has aroused world-wide interest. Generally, three common methods have been used to reduce the sintering temperature of the dielectric ceramics: low-melting glass addition, chemical processing, and smaller particle sizes of starting materials. Among these methods, low-melting glass additions for liquid-phase sintering are lower in cost and easier to process than the other two. Low-temperature sintering of dielectric materials
* Corresponding author. Tel.: +86 773 5896435; fax: +86 773 5893220. E-mail address: [email protected] (H. Zhou). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.06.012
with glass addition has been successfully achieved by the several microwave dielectric ceramics systems such as MgO–TiO2, Li2O– Nb2O5–TiO2, CaO–Li2O–Nb2O5–TiO2, etc. [10–15]. Within the BaO–MgO–TiO2 system, the crystal structure of Ba4MgTi11O27 has been reported by Kaduk et al. [16]. However, the microwave dielectric properties of Ba4MgTi11O27 ceramic have not been reported. In the present study, the microwave dielectric properties of Ba4MgTi11O27 were investigated. It is well known that BaCu(B2O5) (BCB) addition often makes it possible to decrease the sintering temperature of many materials [17–19]. For example, using 6 mol.% BCB, the Ba(Zn1/3Nb2/3)O3 dielectric can be sintered at 875 8C and good microwave dielectric properties obtained with values of er = 35, Q f = 16,000 GHz and tf = 22.1 ppm/8C [17]. So in order to lower the sintering temperature of Ba4MgTi11O27 ceramic to 900 8C, a small amount of BaCu(B2O5) is added to the ceramic. 2. Experimental procedure Specimens of the Ba4MgTi11O27 ceramics were prepared by a conventional mixed oxide route from the high-purity oxide powders (99.9%, Guo-Yao Co. Ltd., Shanghai, China) of BaCO3, MgO and TiO2. Stoichiometric proportions of the above raw materials were mixed in alcohol medium using zirconia balls for 4 h. The mixtures were dried and calcined at 1150 8C for 4 h. To synthesize the BaCu(B2O5) ceramic powder, Ba(OH)28H2O (>99%, Guo-Yao Co. Ltd., Shanghai, China), CuO (>99%, Guo-Yao Co. Ltd., Shanghai, China) and H3BO3 (>99%, Guo-Yao Co. Ltd., Shanghai,
[(Fig._2)TD$IG]
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China) were mixed for 4 h in a nylon jar with zirconia balls, then dried and calcined at 800 8C for 4 h. After subsequent ball-milling with 4–8.0 wt% BCB, the powders were uniaxially pressed under the pressure of about 150 MPa into disks of 10 mm in diameter and 6 mm in thickness. The pure Ba4MgTi11O27 pellets were sintered at 1225–1325 8C for 0.5–8 h in air and the ceramic pellets doped with BCB were sintered at 850–950 8C for 2 h in air. The sintered samples were naturally cooled in the furnace from the sintering temperature. The crystal structures of the specimens were analyzed by an X-ray diffractometer (Rigaku D/MAX-2400, Japan) with Cu Ka radiation generated at 40 kV and 100 mA. The bulk densities of the sintered samples were measured by the Archimedes method. Samples were then thermally etched for 1 h at 100 8C lower than the sintering temperature and then Au coated to prevent surface charging in the microscope. The microstructure observation of the samples was performed using scanning electron microscopy (JEOL JSM-6460LV, Japan). Dielectric behaviors in microwave frequency were measured in the range of 6–7 GHz by the TE01d shielded cavity method using a Network Analyzer (8720ES, Agilent, U.S.A.) and a temperature chamber (DELTA 9023, Delta Design, U.S.A.). The specimens were placed on a low-loss quartz spacer inside a copper cavity, whose inner side was silver-plated. The use of low-loss single-crystal quartz spacer reduces the effect of losses due to the surface resistivity of the cavity. The diameter of the cavity was about 4 times larger than that of the sample for better isolation of the excited TE01d mode. The temperature coefficients of resonant frequency tf values were calculated by the formula as follows:
tf ¼
fT f0 f 0 ðT T 0 Þ
(1)
Fig. 2. SEM photographs of Ba4MgTi11O27 samples sintered at 1275 8C for 2 h.
Z = 4 (Z is the number of formula units per unit cell), which agrees well with that reported by Kaduk et al. [16]. The theoretical density of the Ba4MgTi11O27 ceramic calculated from XRD data is about 4.78 g/ cm3. The SEM micrographs of the surfaces of Ba4MgTi11O27 ceramics are shown in Fig. 2. The dense microstructure of Ba4MgTi11O27 ceramics sintered at 1275 8C for 2 h with only little pores existing can be confirmed by the SEM result. Fig. 3 shows the relative density, permittivity, Q f and tf values of the Ba4MgTi11O27 ceramics sintered at various temperatures for 2 h. The relative density of the specimen sintered below 1200 8C was very low at approximately 87% and increased to 94% for the specimen sintered at 1225 8C for 2 h, but saturated when the sintering temperature exceeded 1275 8C for electronic applications.
[(Fig._3)TD$IG]
where T, T0 were the measuring temperature and room temperature T0 (25 8C), fT, f0 were the resonant frequencies at T and T0 respectively. 3. Results and discussion The room-temperature XRD patterns recorded for the powder calcined at 1150 8C and the ceramics sintered at 1275 8C for 2 h are shown in Fig. 1. A single-phase Ba4MgTi11O27 structure (JCPDS File No. 49-0580) was formed and no secondary phase could be observed in the diffraction patterns. Its pattern could be indexed with a C2/m monoclinic cell with a = 19.85494 0.00008 A˚, b = 11.45173 0.0003 A˚, c = 9.91265 0.004 A˚, V = 2129.27 A˚3, and
[(Fig._1)TD$IG]
Fig. 1. X-ray diffraction patterns (XRD) of Ba4MgTi11O27 (a) powder calcined at 1150 8C and (b) samples sintered at 1275 8C for 2 h.
Fig. 3. The relative density, permittivity, Q f and temperature coefficient of resonant frequency values of Ba4MgTi11O27 ceramics as a function of sintering temperature.
[(Fig._4)TD$IG]
X. Chen et al. / Materials Research Bulletin 45 (2010) 1509–1512
Fig. 4. The relative density, permittivity, Q f and temperature coefficient of resonant frequency values of Ba4MgTi11O27 ceramics as a function of soak time.
Therefore, the optimum sintering temperature for the Ba4MgTi11O27 ceramics was considered to be 1275 8C. The permittivity and Q f values versus sintering temperature of Ba4MgTi11O27 ceramics have a trend similar to that of the relative density. When the sintering temperature increases to 1275 8C, the permittivity reaches a saturated value of 36.1 and the Q f value of Ba4MgTi11O27 ceramics reaches the maximum with a value of 19630 GHz (at 6.2 GHz). Due to frozen-in point defects caused by high-temperature reduction
[(Fig._5)TD$IG]
1511
during sintering, the Q f values decrease as the sintering temperature further increasing. The tf value of the Ba4MgTi11O27 ceramic sintered at 1275 8C was approximately 15 ppm/8C and its variation with the sintering temperature was not large, ranging between 14.5 and 16.0 ppm/8C. The Ba4MgTi11O27 ceramics are sintered at 1275 8C for various durations of time and their relative density, permittivity, Q f, and tf values were measured, as shown in Fig. 4. The relative density, permittivity, Q f values of the Ba4MgTi11O27 ceramics sintered for 0.5 h were somewhat low. However, a high relative density, permittivity and Q f value were obtained for the specimens sintered for 2 h, and their variation with the sintering time was not significant when the sintering time was over 2 h. These results suggest that sintering at 1275 8C for 2 h is sufficient for the densification of specimens. The tf value of the Ba4MgTi11O27 ceramic sintered at 1275 8C for 2 h was approximately 15 ppm/8C and its variation with the soak time was not large, ranging between 13 and 17.0 ppm/8C. In general, the Ba4MgTi11O27 ceramics sintered at 1275 8C have good microwave dielectric properties of er = 36.1, Q f = 19,630 GHz, tf = 14.6 ppm/8C [20]. Comparing with Ba2Ti9O20 and ZTS systems, the tf value of the Ba4MgTi11O27 ceramic is relatively high for resonators; however, through adding other low melting point materials having a negative tf value, it can be adjusted to zero, which indicated this material will be applied in LTCC components. Comparing with other glasses added to reduce the sintering temperature of the materials, BaCu(B2O5) not only has low sintering temperature (810 8C) and low melting point (850 8C), but also exhibits excellent microwave dielectric properties with permittivity of 7.4, Q f values of 50,000 GHz and tf values of 32 ppm/8C [22]. To further decrease the sintering temperature of this microwave dielectric ceramic, a small amount of BaCu(B2O5) (BCB) has been added into the samples. Due to the liquid-phase effect, the addition of BCB to Ba4MgTi11O27 ceramics can efficiently lower the sintering temperature from 1275 to 925 8C. This result would be in agreement with the XRD analysis. From Fig. 5, it can be seen that the BCB phase will be existed in the BCB-doped Ba4MgTi11O27 ceramics. Table 1 shows bulk density, relative density and microwave dielectric properties of Ba4MgTi11O27 ceramics doped with different amounts of BCB. The relative density increase with increasing the addition of BCB, reaches a maximum when the addition of BCB is 6 wt%. The er and Q f values present a trend similar to that between densities and sintering temperatures since higher density means lower porosity.
Fig. 5. X-ray diffraction patterns (XRD) of (a) BCB powders sintered at 800 8C for 4 h, (b) 4 wt%, (c) 5 wt%, (d) 6 wt% and (e) 8 wt% BCB-doped Ba4MgTi11O27 ceramics sintered at 925 8C for 2 h.
X. Chen et al. / Materials Research Bulletin 45 (2010) 1509–1512
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Table 1 Relative density and microwave dielectric properties of Ba4MgTi11O27 ceramics doped with different amounts of BCB. Ba4MgTi11O27 + x wt% BCB
Sintering temperature (8C)
Relative density (%)
er
Q f (GHz)
tf (ppm/8C)
0 4 5 6 8
1275 925 925 925 925
97.4 92 95 97 96
36.1 27.3 29.3 30.2 30.6
19,630 11,200 12,890 13,450 12,560
14.6 9 6 4 3
[(Fig._6)TD$IG] a LTCC component and the corresponding Ag distribution. It is obvious that the ceramic layer and the electrode layer are compatible and almost no crack exists at the interface between them. The element distribution of Ag shows that Ag is distributed in the central conductor region, and does not diffuse into the ceramic region. Overall, it is concluded that the BCB added Ba4MgTi11O27 ceramic is able to match with Ag electrode well. Acknowledgements This work was supported by Research start-up funds Doctor of Guilin University of Technology (Nos. 000788and 000787), Natural Science Foundation of China (Nos. 50962004 and 50762002), Natural Science Foundation of Guangxi (Nos. 0832003Z and 0832001), and Program for NCET-06-0656, MOE, China References
Fig. 6. The backscattered electron image and EDS analysis of the interfacial microstructure of (6 wt% BCB-doped Ba4MgTi11O27)/Ag cofired at 875 8C in a LTCC component.
The temperature coefficient of the resonant frequency (tf) was relevant to the composition and the phases existing in the ceramics. The higher the contents of BCB addition is, the lower the tf of the low-fired Ba4MgTi11O27 samples is. BCB liquid phases having a low tf would contribute to the slight decrease in tf of the BCB added Ba4MgTi11O27 system. For the 6 wt% BCB-doped ceramic, a high density of 4.64 g/cm3 and relatively good microwave dielectric properties of er = 30.2, Q f = 13,450 (6.18 GHz) GHz and tf = 4 ppm/8C have been obtained by sintering at 925 8C. For chemical compatibility tests, the green tapes of Ba4MgTi11O27 powders with 6 wt% BCB additives were obtained by the tape-casting process [21]. Ceramic sheets and Ag electrodes printed on it were cofired and analyzed to detect interaction between the low-fired ceramics and electrodes. Fig. 6 exhibits backscattered electron image and EDS analysis of the interface of 8 wt% BCB added Ba4MgTi11O27 ceramic sheet with Ag electrode in
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