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Structure and Microwave dielectric properties of a novel temperature stable low-firing Ba2LaV3O11 ceramic
Journal of the European Ceramic Society 36 (2016) 2143–2148
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Structure and Microwave dielectric properties of a novel temperature stable low-firing Ba2 LaV3 O11 ceramic Jie Li a , Liang Fang a,∗ , Hao Luo a , Ying Tang a , Chunchun Li a,b,∗ a State Key Laboratory Breeding Base of Nonferrous metals and specific Materials Processing, Guangxi universities key laboratory of non-ferrous metal oxide electronic functional materials and devices, College of Material Science and Engineering, Guilin University of Technology, Guilin 541004, China b College of Information Science and Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 8 December 2015 Received in revised form 16 February 2016 Accepted 18 February 2016 Available online 26 February 2016 Keywords: Microwave dielectric properties LTCC Ba2 LaV3 O11
a b s t r a c t A temperature stable low-firing microwave dielectric ceramic Ba2 LaV3 O11 was prepared by the conventional solid state reaction method. Excellent microwave dielectric properties were obtained in Ba2 LaV3 O11 ceramic sintered at 840 ◦ C, with a permittivity of 12.8, a quality factor of 31,800 GHz (at 9.9 GHz), and a temperature coefficient of resonance frequency of −14 ppm/◦ C. Ba2 LaV3 O11 ceramic was found to be chemically compatible with silver at 840 ◦ C based on XRD and EDS analysis of the co-fired sample with 20 wt% silver powders. All the results indicate that the Ba2 LaV3 O11 ceramic might be a promising candidate for low temperature cofired ceramic technology. Further, the effects of ionic polarizability, packing fraction and bond valence on the microwave dielectric properties of Ba2 MV3 O11 (M = La and Bi) ceramics were discussed. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Low-temperature cofired ceramics (LTCC) technology, in the past two decades, has been generating considerable interest due to its advantages over other established packaging technologies like high degree of miniaturization and integration [1–3]. This technology combines many thin layers of ceramic and conducting electrodes resulting in multilayer LTCC modules and has been widely used in microwave modules such as filters, resonators, and capacitors, etc [4]. In order to use the highly conductive and inexpensive metals such as silver or copper, the ceramics need to be sintered at temperatures <960 ◦ C and the chemical compatibility with electrodes must be taken into account [5]. Besides, for microwave substrate applications, the materials should have a low permittivity (εr ) to reduce cross-coupling loss, a high quality factor(Q × f) for better selectivity and a low temperature coefficient of resonant frequency ( f ) for frequency stability [6–8]. More recently, considerable attention has been paid to microwave dielectric ceramics with intrinsic low sintering temperatures [9,10], such as Li2 O-V2 O5 , Bi2 O3 -V2 O5 , WO3 -V2 O5 systems, etc [11,13]. In our previous work, a series of V2 O5 rich compounds
were reported to possess a combination of promising microwave dielectric properties and low firing temperatures [14,15]. Among them, Ba2 BiV3 O11 ceramic was reported to possess a high relative density ∼ 96.8%, a high Q × f value about 68,700 GHz (at 8.7 GHz), a εr of 14.2, and a f of −80.5 ppm/ ◦ C when sintered at 870 ◦ C/4 h [12]. Considering the same valence and similar ionic radius between Bi3+ and La3+ , it is predicted that Ba2 LaV3 O11 may be a potential material for LTCC application. The synthesis and crystal structure of Ba2 LaV3 O11 was first reported by Serkalo, A [16]. Single phase Ba2 LaV3 O11 could be easily synthesized at relatively low temperature (∼ 900 ◦ C) through solid state reaction method [17]. And its application as a host material for phosphors has been reported recently [18]. Up to date, however, there is no report on the microwave dielectric properties of Ba2 LaV3 O11 . In this study, the Ba2 LaV3 O11 ceramic was prepared via the solid state reaction method. The sintering behavior, microwave dielectric properties of Ba2 LaV3 O11 , and its chemical compatibility with Ag were studied in detail. In addition, the packing fraction, bond strength and microwave dielectric properties of Ba2 LaV3 O11 ceramics were investigated. 2. Experimental procedure
∗ Corresponding authors at: No.12, Jiangan Road, Fax: +86 773 5896278. E-mail addresses: [email protected] (L. Fang), [email protected] (C. Li). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.035 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Guilin,
China.
Samples were prepared by conventional solid state route using reagent-grade raw materials of Ba2 CO3 (>99%, Guo-Yao Co., Ltd., Shanghai, China), La2 O3 (>99.99%, Guo-Yao Co., Ltd., Shanghai, China) and NH4 VO3 (>99%, Guo-Yao Co, Ltd., Shanghai, China). After
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weighing according to the stoichiometry, ball milled was carried out in alcohol medium for 4 h in a plastic bottle using zirconia balls as a grinding medium. Then the mixture was dried and calcined at 800 ◦ C for 4 h in air, followed by remilling for 6 h. The calcination temperature was firstly selected at 800 ◦ C since Ba2 LaV3 O11 was reported to easily synthesized at 900 ◦ C [17] and the calcination temperature of Ba2 BiV3 O11 ceramic is 800 ◦ C [12]. Polyvinyl alcohol (PVA) was added as a binder to the powders, then dried and ground. The powders were pressed into cylindrical pellets of 12 mm diameter and 7 mm thickness under a pressure of 200 MPa. These pellets were heated to 550 ◦ C for 2 h to remove the organic binder, and then sintered at different temperatures for 4 h in air. The optimum sintering temperature of Ba2 LaV3 O11 is 870 ◦ C [12] which supposed that the sintering temperature of Ba2 LaV3 O11 might be above 870 ◦ C, however, it was found that the optimum sintering temperature of Ba2 LaV3 O11 is 840 ◦ C and the ceramic sintered at 800 ◦ C exhibits high relative density around 95%, then the range of sintering temperature was adjusted to 780–860 ◦ C. To investigate the chemical compatibility of these compounds with electrode metal powders, 20 wt% Ag was mixed with Ba2 LaV3 O11 compounds and sintered at 840 ◦ C for 4 h. The phase composition and crystal structure were examined by X-ray diffraction (XRD) with CuK␣1 radiation (Model X’Pert PRO, PANalytical, Almelo, The Netherlands). The bulk densities were measured using the Archimedes method. The microstructures of the polished and thermally etched surfaces of sintered disks were observed by a scanning electron microscope (SEM, Model JSM6380LV, JEOL, Tokyo, Japan). The thermally etching was carried out at temperatures 50 ◦ C lower than the respective optimum sintering temperatures for 30 min. The microwave dielectric properties were measured by a network analyzer (Model N5230A, Agilent Co., Palo Alto, California) and a temperature chamber (Delta 9039, Delta Design, San Diego, California). The temperature coefficients of resonant frequency f values were calculated by the equation: f =
f 85 − f 25 60 × f 25
Fig 1. XRD patterns of the Ba2 LaV3 O11 powders calcined at 800 ◦ C for 4 h, sintered at 840 ◦ C for 4 h and co-fired ceramics with 20 wt% Ag at 840 ◦ C for 4 h.
(1)
where f85 and f25 were the resonant frequencies at 85 ◦ C and 25 ◦ C, respectively. 3. Results and discussion Fig. 1 shows the room-temperature XRD patterns of the calcined Ba2 LaV3 O11 powders at 800 ◦ C for 4 h, the sintered and the co-fired ceramics sintered at 840 ◦ C for 4 h. All the observed diffraction peaks of the calcined powders could be indexed to Ba2 LaV3 O11 by JCPDS No. 040–0105. No additional peaks were detected, indicating the formation of single phase. By comparison, no significant difference in the XRD pattern of the calcined and sintered samples was observed. In the co-fired samples of Ba2 LaV3 O11 with Ag, only the Ba2 LaV3 O11 and silver phase (JCPDS No. 04–0783) were identified, indicating that Ba2 LaV3 O11 did not react with silver at the sintering temperature 840 ◦ C for 4 h. To clarify the structure details and the local crystal environment of Ba2 LaV3 O11 , Rietveld refinement on the XRD patterns was carried out using Topas software. Fig. 2 shows the experimental and refined plots and the schematic crystal structure of the Ba2 LaV3 O11 . The low final agreement factors (Rp = 3.9%, Rwp = 5.2%, and Rexp = 3.18%) in Table 1 suggests that Ba2 LaV3 O11 crystallized into the monoclinic P21 /c structure. The refined lattice parameters were a = 12.45 Å, b = 7.79 Å, c = 11.28 Å, ˇ = 103.09 ◦ and V = 1066.80 Å3 , which agrees well with those reported in previous work [19]. The structure of Ba2 LaV3 O11 shows that La atoms are six-coordinated in a distorted octahedral environment, while V5+ atoms coordinate with four oxygen forming (VO4 )3− tetrahedra.
Fig. 2. Observed and calculated X-ray power diffraction profiles for Ba2 LaV3 O11 sample.
Fig. 3(a–e) shows the SEM images of the Ba2 LaV3 O11 ceramics sintered at different temperatures for 4 h. As shown, the sample sintered at 780 ◦ C exhibited a relatively dense microstructure with a small amount of porosity and with small grains varying from 0.4–1 m. As the sintering temperature was increased, a increase in grain size along with decrease in the degree of porosity was observed. For sample sintered at 840 ◦ C, a dense microstructure with closely packed grains (1–2 m) was developed. However, exaggerated grain growth with large grains (>4 m) was observed in the ceramic sintered at 860 ◦ C. SEM images and energy dispersive spectrometer (EDS) analysis of the co-fired samples with 20 wt% Ag are shown in Fig. 4. Two different kinds of grains marked as A and B were detected. From the EDS analysis, the grains marked A were rich in Ag element, while the B-type grains were rich in Ba, La, and V elements and the
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Table 1 Structural parameters, refined atomic fractional coordinates, and profile R-factors of Ba2 LaV3 O11 sample from Rietveld refinement. Atom
Site
x/a
y/b
z/c
Biso.
Ba1 Ba2 La1 V1 V2 V3 O1 O2
4e 4e 4e 4e 4e 4e 4e 4e
0.2357(2) 0.5550(2) 0.0011(4) 0.0921(6) 0.3122(7) 0.3277(7) 0.003(3) 0.056(3)
0.7407(6) 0.2524(6) 0.5157(6) 0.2671(1) 0.0089(1) 0.4532(1) 0.177(4) 0.301(4)
0.0602(5) 0.0999(5) 0.2872(3) −0.0013(7) 0.3343(1) 0.3283(1) 0.337(2) 0.119(3)
1.6816(9) 1.6816(9) 0.7665(0) 0.4793(0) 0.4793(0) 0.4793(0) 1 1
Rp = 3.9%, Rwp = 5.2%, and Rexp = 3.18%.
Fig. 3. SEM micrographs of the thermal etched surfaces of Ba2 LaV3 O11 ceramics sintered at different temperatures (a) 780 ◦ C; (b) 800 ◦ C; (c) 820 ◦ C; (d) 840 ◦ C; (e) 860 ◦ C.
corresponding ratio of Ba:La:V was about 2:1:3. These results suggest that the grains marked B belonged to the Ba2 LaV3 O11 phases, whereas the grains marked A were silver. Combined with the XRD analysis of the cofired sample shown in Fig. 1(c), it is concluded that Ba2 LaV3 O11 phase did not react with Ag when co-fired at 840 ◦ C for 4 h. The variations of the density and microwave dielectric properties (εr , Q × f, and f ) of Ba2 LaV3 O11 ceramics as a function of the sintering temperature are shown in Fig. 5. As sintering temperature increased from 780 ◦ C to 840 ◦ C, the bulk density increased from 4.40 g/cm3 to a maximum value of 4.450 g/cm3 (∼96.3% of the calculated theoretical density 4.62 g/cm3 ). With further increase in sintering temperature, the bulk density and relative density
decreased slightly, which might be due to the exaggerated grain growth. Similarly, the relative permittivity (εr ) of the sintered ceramics exhibited a obvious dependence on the sintering temperature. As shown in Fig. 5(b), the variation trend in εr values with increasing sintering temperature is similar to that of the relative density. A saturated value ∼12.8 was achieved when sintered at 840 ◦ C. The porosity corrected values were calculated by applying Bosman and Having’s correction [20]: εcorr = r (1 + 1.5p)
(2)
where, εcorr and εr are the porosity corrected and experimentally measured values of permittivity, respectively. p is the fractional
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Fig. 5. Bulk densities, relative densities and microwave dielectric properties (εr , Q × f, and f values) of the Ba2 LaV3 O11 ceramics as a function of sintering temperature.
Fig. 4. SEM images (a) and EDS analysis (b) and (c) of the Ba2 LaV3 O11 ceramics co-fired with 20 wt% Ag powder.
porosity. The porosity corrected εr values varied from 13.2 to 13.6, slightly higher than the experimentally measured values. Fig. 5(c) shows the variation in quality factor of the sintered ceramics as a function of the sintering temperature. Q × f value of Ba2 LaV3 O11 ceramics sintered at 780 ◦ C was relatively low (∼9,900 GHz at 9.5 GHz). It increased obviously to 22,700 GHz when sintered at 820 ◦ C, reached a maximum value of ∼31,800 GHz (at 9.9 GHz) at 840 ◦ C, and subsequently decreased to 16,500 GHz as the sintering temperature increased to 860 ◦ C. It is well known that some factors such as ionic polarizability, bulk density, microstructure, defects, and grain sizes play crucial roles in affecting the relative permittivity and quality factor [21–26]. In the present work, high densification and homogeneous microstructure lead to a large relative permittivity and a high Q × f value. The variation in f values of Ba2 LaV3 O11 ceramics with sintering temperature was not significant and f remained stable at about −14 ppm/◦ C. Excellent microwave dielectric properties were obtained in Ba2 LaV3 O11 ceramic sintered at 840 ◦ C, with a permittivity value of 12.8, a Q × f value of 31,800 GHz (at 9.9 GHz), and a f value of −14 ppm/ ◦ C.
Table 2 lists the sintering temperature, relative density, and microwave dielectric properties of Ba2 MV3 O11 (M = La, Bi) ceramics. By comparison, it is found that both εr and Q × f values of Ba2 LaV3 O11 are lower than those of Ba2 BiV3 O11 as well as the sintering temperature. It must be noted that the temperature coefficient of resonance frequency of Ba2 LaV3 O11 is much smaller than Ba2 BiV3 O11 . As stated above, there are a series of factors affecting the microwave dielectric properties. For single phase and dense ceramics with relative densities >95%, the effects of such extrinsic factors as density and secondary phase could be neglected. Thus, ionic polarizability, packing fraction, and bond valences theories were used to explain the distinct difference in microwave dielectric properties of Ba2 LaV3 O11 and its Ba2 BiV3 O11 . The theoretical permittivity could be determined by the Clausius–Mosotti equation: εr =
1 + 2b˛TD /Vm
(3)
1 − b˛TD /Vm
where, b = 4/3, Vm is the cell volume of Ba2 LaV3 O11 . Shannon [27] suggested that the total polarizability of complex substances could be estimated by the additive rule. Then the polarizability of a Ba2 LaV3 O11 could be calculated as following: ˛TD = 2˛(Ba2+ ) + ˛(La3+ ) + 3˛(V5+ ) + 11˛(O2− ) ␣(Ba2+ ),
␣(La3+ ),
␣(V5+ )
(4)
␣(O2− )
where, and are the polarizability of Ba2+ (6.40 Å3 ), La3+ (6.07 Å3 ), V5+ (2.92 Å3 ) and O2− (2.01 Å3 )
Table 2 Microwave dielectric properties of Ba2 MV3 O11 (M = La, Bi) ceramics sintered at each optimized temperature. M
S. T. (◦ C)
Relatively density (%)
εr
εtheo
Q × f (GHz)
f (ppm/ ◦ C)
La Bi
840 870
96.3 ± 0.4 96.8 ± 0.5
12.8 ± 0.03 14.2 ± 0.03
11.7 12.6
31,800 ± 2,700 68,700 ± 3,600
–14 ± 0.4 –81 ± 1.7
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Table 3 Packing fraction of Ba2 MV3 O11 (M = La, Bi) ceramics. M 1-rBa (Å) (CN = 10)
1-rBa (Å) (CN = 11)
1-rLn (Å) (CN = 6)
3-rV (Å) (CN = 4)
9-rO (Å) (CN = 4)
1-rO (Å) (CN = 2)
1-rO (Å) (CN = 3)
Unit cell volume (Å3 )
Z Packing fraction (%)
La 1.52 Bi 1.52
1.57 1.57
1.032 1.03
0.355 0.355
1.38 1.38
1.35 1.35
1.36 1.36
1066.8 1049.4
4 58.5 4 59.5
Table 4 Bond valence of Ba2 MV3 O11 (M = La, Bi) ceramics. M
RM (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
b (Å)
VM–O
La Bi
2.172 2.09
2.647 2.302
2.287 2.262
2.708 2.211
2.248 2.681
2.355 2.211
2.249 2.611
0.37 0.37
3.481 3.081
[28].The theoretical permittivity of Ba2 LaV3 O11 ceramic is 11.7 that is slightly smaller than that of Ba2 BiV3 O11 (∼12.6). Thus, the smaller relative permittivity of Ba2 LaV3 O11 could partly be explained by its smaller ionic polarizability of La3+ (6.07 Å) relative to Bi3+ (6.12 Å). Besides, the theoretical permittivity of Ba2 LaV3 O11 ceramic is a little smaller than the measured value. The deviation is 8.6%, which is considered acceptable considering the simplicity of additive rule [29]. Kim et al. [30] proposed a general relationship between the quality factor and the packing fraction of structure, that is, the Q × f value increases with the increase of packing fraction. Based on crystal structural considerations, the packing fraction could be calculated as following: Packing fraction (%) =
volume of packed ions ×Z volume of unit cell
(5)
As shown in Table 3, the calculated packing fractions for Ba2 MV3 O11 (M = La, Bi) ceramics are 58.5% and 59.5%, respectively. Ba2 BiV3 O11 has a higher Q × f value than Ba2 LaV3 O11 due to the larger packing fraction. It is generally accepted that f has close relationship with the bond valence. Park et al. [28] demonstrated the relationship between f value and bond valance for perovskites. The M-site bond valences were calculated using the following equations: Vi =
vij
(6)
j
vij = exp
Rij − dij b
(7)
where Rij is the bond valence parameter, dij is the length of a bond between atoms i and j, and b is a universal constant (0.37 Å) [31]. According to the bond valence theory [32], the shorter the bond length is, the stronger the bond energy becomes, thereby resulting in increase in the restoring force for recovering the tilting of oxygen octahedra, and f value would thereupon decreased. Therefore, as given in Table 4, it is believed that the decrease in f value of Ba2 LaV3 O11 might be related to the larger bond valences VM–O . 4. Conclusions A novel low temperature firing microwave dielectric ceramic Ba2 LaV3 O11 was prepared by the conventional solid state reaction method. High performance of microwave dielectric properties could be obtained in the ceramic sintered at 840 ◦ C for 4 h with a permittivity value of 12.8, a Q × f value of 31,800 GHz (at 9.9 GHz), and a f value of −14 ppm/ ◦ C. From the XRD analysis and EDS result, the Ba2 LaV3 O11 ceramic was found to be chemically compatible with silver powders at 840 ◦ C. All the results indicated that the Ba2 LaV3 O11 ceramic might be a promising candidate for low
temperature cofired ceramic technology. Ionic polarizability, packing fraction, and bond valences theories were applied to explain the distinct difference between Ba2 LaV3 O11 and its Ba2 BiV3 O11 .
Acknowledgments This work was supported by Natural Science Foundation of China (No. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (No. 2015GXNSFBA139234, and 2015GXNSFFA139003), Project of Department of Science and Technology of Guangxi (No. 114122005–28), and Projects of Education Department of Guangxi Zhuang Autonomous Region (No. YB2014160, KY2015YB341, and KY2015YB122).
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Structure and Microwave dielectric properties of a novel temperature stable low-firing Ba2 LaV3 O11 ceramic Jie Li a , Liang Fang a,∗ , Hao Luo a , Ying Tang a , Chunchun Li a,b,∗ a State Key Laboratory Breeding Base of Nonferrous metals and specific Materials Processing, Guangxi universities key laboratory of non-ferrous metal oxide electronic functional materials and devices, College of Material Science and Engineering, Guilin University of Technology, Guilin 541004, China b College of Information Science and Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 8 December 2015 Received in revised form 16 February 2016 Accepted 18 February 2016 Available online 26 February 2016 Keywords: Microwave dielectric properties LTCC Ba2 LaV3 O11
a b s t r a c t A temperature stable low-firing microwave dielectric ceramic Ba2 LaV3 O11 was prepared by the conventional solid state reaction method. Excellent microwave dielectric properties were obtained in Ba2 LaV3 O11 ceramic sintered at 840 ◦ C, with a permittivity of 12.8, a quality factor of 31,800 GHz (at 9.9 GHz), and a temperature coefficient of resonance frequency of −14 ppm/◦ C. Ba2 LaV3 O11 ceramic was found to be chemically compatible with silver at 840 ◦ C based on XRD and EDS analysis of the co-fired sample with 20 wt% silver powders. All the results indicate that the Ba2 LaV3 O11 ceramic might be a promising candidate for low temperature cofired ceramic technology. Further, the effects of ionic polarizability, packing fraction and bond valence on the microwave dielectric properties of Ba2 MV3 O11 (M = La and Bi) ceramics were discussed. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Low-temperature cofired ceramics (LTCC) technology, in the past two decades, has been generating considerable interest due to its advantages over other established packaging technologies like high degree of miniaturization and integration [1–3]. This technology combines many thin layers of ceramic and conducting electrodes resulting in multilayer LTCC modules and has been widely used in microwave modules such as filters, resonators, and capacitors, etc [4]. In order to use the highly conductive and inexpensive metals such as silver or copper, the ceramics need to be sintered at temperatures <960 ◦ C and the chemical compatibility with electrodes must be taken into account [5]. Besides, for microwave substrate applications, the materials should have a low permittivity (εr ) to reduce cross-coupling loss, a high quality factor(Q × f) for better selectivity and a low temperature coefficient of resonant frequency ( f ) for frequency stability [6–8]. More recently, considerable attention has been paid to microwave dielectric ceramics with intrinsic low sintering temperatures [9,10], such as Li2 O-V2 O5 , Bi2 O3 -V2 O5 , WO3 -V2 O5 systems, etc [11,13]. In our previous work, a series of V2 O5 rich compounds
were reported to possess a combination of promising microwave dielectric properties and low firing temperatures [14,15]. Among them, Ba2 BiV3 O11 ceramic was reported to possess a high relative density ∼ 96.8%, a high Q × f value about 68,700 GHz (at 8.7 GHz), a εr of 14.2, and a f of −80.5 ppm/ ◦ C when sintered at 870 ◦ C/4 h [12]. Considering the same valence and similar ionic radius between Bi3+ and La3+ , it is predicted that Ba2 LaV3 O11 may be a potential material for LTCC application. The synthesis and crystal structure of Ba2 LaV3 O11 was first reported by Serkalo, A [16]. Single phase Ba2 LaV3 O11 could be easily synthesized at relatively low temperature (∼ 900 ◦ C) through solid state reaction method [17]. And its application as a host material for phosphors has been reported recently [18]. Up to date, however, there is no report on the microwave dielectric properties of Ba2 LaV3 O11 . In this study, the Ba2 LaV3 O11 ceramic was prepared via the solid state reaction method. The sintering behavior, microwave dielectric properties of Ba2 LaV3 O11 , and its chemical compatibility with Ag were studied in detail. In addition, the packing fraction, bond strength and microwave dielectric properties of Ba2 LaV3 O11 ceramics were investigated. 2. Experimental procedure
∗ Corresponding authors at: No.12, Jiangan Road, Fax: +86 773 5896278. E-mail addresses: [email protected] (L. Fang), [email protected] (C. Li). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.035 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Guilin,
China.
Samples were prepared by conventional solid state route using reagent-grade raw materials of Ba2 CO3 (>99%, Guo-Yao Co., Ltd., Shanghai, China), La2 O3 (>99.99%, Guo-Yao Co., Ltd., Shanghai, China) and NH4 VO3 (>99%, Guo-Yao Co, Ltd., Shanghai, China). After
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weighing according to the stoichiometry, ball milled was carried out in alcohol medium for 4 h in a plastic bottle using zirconia balls as a grinding medium. Then the mixture was dried and calcined at 800 ◦ C for 4 h in air, followed by remilling for 6 h. The calcination temperature was firstly selected at 800 ◦ C since Ba2 LaV3 O11 was reported to easily synthesized at 900 ◦ C [17] and the calcination temperature of Ba2 BiV3 O11 ceramic is 800 ◦ C [12]. Polyvinyl alcohol (PVA) was added as a binder to the powders, then dried and ground. The powders were pressed into cylindrical pellets of 12 mm diameter and 7 mm thickness under a pressure of 200 MPa. These pellets were heated to 550 ◦ C for 2 h to remove the organic binder, and then sintered at different temperatures for 4 h in air. The optimum sintering temperature of Ba2 LaV3 O11 is 870 ◦ C [12] which supposed that the sintering temperature of Ba2 LaV3 O11 might be above 870 ◦ C, however, it was found that the optimum sintering temperature of Ba2 LaV3 O11 is 840 ◦ C and the ceramic sintered at 800 ◦ C exhibits high relative density around 95%, then the range of sintering temperature was adjusted to 780–860 ◦ C. To investigate the chemical compatibility of these compounds with electrode metal powders, 20 wt% Ag was mixed with Ba2 LaV3 O11 compounds and sintered at 840 ◦ C for 4 h. The phase composition and crystal structure were examined by X-ray diffraction (XRD) with CuK␣1 radiation (Model X’Pert PRO, PANalytical, Almelo, The Netherlands). The bulk densities were measured using the Archimedes method. The microstructures of the polished and thermally etched surfaces of sintered disks were observed by a scanning electron microscope (SEM, Model JSM6380LV, JEOL, Tokyo, Japan). The thermally etching was carried out at temperatures 50 ◦ C lower than the respective optimum sintering temperatures for 30 min. The microwave dielectric properties were measured by a network analyzer (Model N5230A, Agilent Co., Palo Alto, California) and a temperature chamber (Delta 9039, Delta Design, San Diego, California). The temperature coefficients of resonant frequency f values were calculated by the equation: f =
f 85 − f 25 60 × f 25
Fig 1. XRD patterns of the Ba2 LaV3 O11 powders calcined at 800 ◦ C for 4 h, sintered at 840 ◦ C for 4 h and co-fired ceramics with 20 wt% Ag at 840 ◦ C for 4 h.
(1)
where f85 and f25 were the resonant frequencies at 85 ◦ C and 25 ◦ C, respectively. 3. Results and discussion Fig. 1 shows the room-temperature XRD patterns of the calcined Ba2 LaV3 O11 powders at 800 ◦ C for 4 h, the sintered and the co-fired ceramics sintered at 840 ◦ C for 4 h. All the observed diffraction peaks of the calcined powders could be indexed to Ba2 LaV3 O11 by JCPDS No. 040–0105. No additional peaks were detected, indicating the formation of single phase. By comparison, no significant difference in the XRD pattern of the calcined and sintered samples was observed. In the co-fired samples of Ba2 LaV3 O11 with Ag, only the Ba2 LaV3 O11 and silver phase (JCPDS No. 04–0783) were identified, indicating that Ba2 LaV3 O11 did not react with silver at the sintering temperature 840 ◦ C for 4 h. To clarify the structure details and the local crystal environment of Ba2 LaV3 O11 , Rietveld refinement on the XRD patterns was carried out using Topas software. Fig. 2 shows the experimental and refined plots and the schematic crystal structure of the Ba2 LaV3 O11 . The low final agreement factors (Rp = 3.9%, Rwp = 5.2%, and Rexp = 3.18%) in Table 1 suggests that Ba2 LaV3 O11 crystallized into the monoclinic P21 /c structure. The refined lattice parameters were a = 12.45 Å, b = 7.79 Å, c = 11.28 Å, ˇ = 103.09 ◦ and V = 1066.80 Å3 , which agrees well with those reported in previous work [19]. The structure of Ba2 LaV3 O11 shows that La atoms are six-coordinated in a distorted octahedral environment, while V5+ atoms coordinate with four oxygen forming (VO4 )3− tetrahedra.
Fig. 2. Observed and calculated X-ray power diffraction profiles for Ba2 LaV3 O11 sample.
Fig. 3(a–e) shows the SEM images of the Ba2 LaV3 O11 ceramics sintered at different temperatures for 4 h. As shown, the sample sintered at 780 ◦ C exhibited a relatively dense microstructure with a small amount of porosity and with small grains varying from 0.4–1 m. As the sintering temperature was increased, a increase in grain size along with decrease in the degree of porosity was observed. For sample sintered at 840 ◦ C, a dense microstructure with closely packed grains (1–2 m) was developed. However, exaggerated grain growth with large grains (>4 m) was observed in the ceramic sintered at 860 ◦ C. SEM images and energy dispersive spectrometer (EDS) analysis of the co-fired samples with 20 wt% Ag are shown in Fig. 4. Two different kinds of grains marked as A and B were detected. From the EDS analysis, the grains marked A were rich in Ag element, while the B-type grains were rich in Ba, La, and V elements and the
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Table 1 Structural parameters, refined atomic fractional coordinates, and profile R-factors of Ba2 LaV3 O11 sample from Rietveld refinement. Atom
Site
x/a
y/b
z/c
Biso.
Ba1 Ba2 La1 V1 V2 V3 O1 O2
4e 4e 4e 4e 4e 4e 4e 4e
0.2357(2) 0.5550(2) 0.0011(4) 0.0921(6) 0.3122(7) 0.3277(7) 0.003(3) 0.056(3)
0.7407(6) 0.2524(6) 0.5157(6) 0.2671(1) 0.0089(1) 0.4532(1) 0.177(4) 0.301(4)
0.0602(5) 0.0999(5) 0.2872(3) −0.0013(7) 0.3343(1) 0.3283(1) 0.337(2) 0.119(3)
1.6816(9) 1.6816(9) 0.7665(0) 0.4793(0) 0.4793(0) 0.4793(0) 1 1
Rp = 3.9%, Rwp = 5.2%, and Rexp = 3.18%.
Fig. 3. SEM micrographs of the thermal etched surfaces of Ba2 LaV3 O11 ceramics sintered at different temperatures (a) 780 ◦ C; (b) 800 ◦ C; (c) 820 ◦ C; (d) 840 ◦ C; (e) 860 ◦ C.
corresponding ratio of Ba:La:V was about 2:1:3. These results suggest that the grains marked B belonged to the Ba2 LaV3 O11 phases, whereas the grains marked A were silver. Combined with the XRD analysis of the cofired sample shown in Fig. 1(c), it is concluded that Ba2 LaV3 O11 phase did not react with Ag when co-fired at 840 ◦ C for 4 h. The variations of the density and microwave dielectric properties (εr , Q × f, and f ) of Ba2 LaV3 O11 ceramics as a function of the sintering temperature are shown in Fig. 5. As sintering temperature increased from 780 ◦ C to 840 ◦ C, the bulk density increased from 4.40 g/cm3 to a maximum value of 4.450 g/cm3 (∼96.3% of the calculated theoretical density 4.62 g/cm3 ). With further increase in sintering temperature, the bulk density and relative density
decreased slightly, which might be due to the exaggerated grain growth. Similarly, the relative permittivity (εr ) of the sintered ceramics exhibited a obvious dependence on the sintering temperature. As shown in Fig. 5(b), the variation trend in εr values with increasing sintering temperature is similar to that of the relative density. A saturated value ∼12.8 was achieved when sintered at 840 ◦ C. The porosity corrected values were calculated by applying Bosman and Having’s correction [20]: εcorr = r (1 + 1.5p)
(2)
where, εcorr and εr are the porosity corrected and experimentally measured values of permittivity, respectively. p is the fractional
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Fig. 5. Bulk densities, relative densities and microwave dielectric properties (εr , Q × f, and f values) of the Ba2 LaV3 O11 ceramics as a function of sintering temperature.
Fig. 4. SEM images (a) and EDS analysis (b) and (c) of the Ba2 LaV3 O11 ceramics co-fired with 20 wt% Ag powder.
porosity. The porosity corrected εr values varied from 13.2 to 13.6, slightly higher than the experimentally measured values. Fig. 5(c) shows the variation in quality factor of the sintered ceramics as a function of the sintering temperature. Q × f value of Ba2 LaV3 O11 ceramics sintered at 780 ◦ C was relatively low (∼9,900 GHz at 9.5 GHz). It increased obviously to 22,700 GHz when sintered at 820 ◦ C, reached a maximum value of ∼31,800 GHz (at 9.9 GHz) at 840 ◦ C, and subsequently decreased to 16,500 GHz as the sintering temperature increased to 860 ◦ C. It is well known that some factors such as ionic polarizability, bulk density, microstructure, defects, and grain sizes play crucial roles in affecting the relative permittivity and quality factor [21–26]. In the present work, high densification and homogeneous microstructure lead to a large relative permittivity and a high Q × f value. The variation in f values of Ba2 LaV3 O11 ceramics with sintering temperature was not significant and f remained stable at about −14 ppm/◦ C. Excellent microwave dielectric properties were obtained in Ba2 LaV3 O11 ceramic sintered at 840 ◦ C, with a permittivity value of 12.8, a Q × f value of 31,800 GHz (at 9.9 GHz), and a f value of −14 ppm/ ◦ C.
Table 2 lists the sintering temperature, relative density, and microwave dielectric properties of Ba2 MV3 O11 (M = La, Bi) ceramics. By comparison, it is found that both εr and Q × f values of Ba2 LaV3 O11 are lower than those of Ba2 BiV3 O11 as well as the sintering temperature. It must be noted that the temperature coefficient of resonance frequency of Ba2 LaV3 O11 is much smaller than Ba2 BiV3 O11 . As stated above, there are a series of factors affecting the microwave dielectric properties. For single phase and dense ceramics with relative densities >95%, the effects of such extrinsic factors as density and secondary phase could be neglected. Thus, ionic polarizability, packing fraction, and bond valences theories were used to explain the distinct difference in microwave dielectric properties of Ba2 LaV3 O11 and its Ba2 BiV3 O11 . The theoretical permittivity could be determined by the Clausius–Mosotti equation: εr =
1 + 2b˛TD /Vm
(3)
1 − b˛TD /Vm
where, b = 4/3, Vm is the cell volume of Ba2 LaV3 O11 . Shannon [27] suggested that the total polarizability of complex substances could be estimated by the additive rule. Then the polarizability of a Ba2 LaV3 O11 could be calculated as following: ˛TD = 2˛(Ba2+ ) + ˛(La3+ ) + 3˛(V5+ ) + 11˛(O2− ) ␣(Ba2+ ),
␣(La3+ ),
␣(V5+ )
(4)
␣(O2− )
where, and are the polarizability of Ba2+ (6.40 Å3 ), La3+ (6.07 Å3 ), V5+ (2.92 Å3 ) and O2− (2.01 Å3 )
Table 2 Microwave dielectric properties of Ba2 MV3 O11 (M = La, Bi) ceramics sintered at each optimized temperature. M
S. T. (◦ C)
Relatively density (%)
εr
εtheo
Q × f (GHz)
f (ppm/ ◦ C)
La Bi
840 870
96.3 ± 0.4 96.8 ± 0.5
12.8 ± 0.03 14.2 ± 0.03
11.7 12.6
31,800 ± 2,700 68,700 ± 3,600
–14 ± 0.4 –81 ± 1.7
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Table 3 Packing fraction of Ba2 MV3 O11 (M = La, Bi) ceramics. M 1-rBa (Å) (CN = 10)
1-rBa (Å) (CN = 11)
1-rLn (Å) (CN = 6)
3-rV (Å) (CN = 4)
9-rO (Å) (CN = 4)
1-rO (Å) (CN = 2)
1-rO (Å) (CN = 3)
Unit cell volume (Å3 )
Z Packing fraction (%)
La 1.52 Bi 1.52
1.57 1.57
1.032 1.03
0.355 0.355
1.38 1.38
1.35 1.35
1.36 1.36
1066.8 1049.4
4 58.5 4 59.5
Table 4 Bond valence of Ba2 MV3 O11 (M = La, Bi) ceramics. M
RM (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
dM–O (Å)
b (Å)
VM–O
La Bi
2.172 2.09
2.647 2.302
2.287 2.262
2.708 2.211
2.248 2.681
2.355 2.211
2.249 2.611
0.37 0.37
3.481 3.081
[28].The theoretical permittivity of Ba2 LaV3 O11 ceramic is 11.7 that is slightly smaller than that of Ba2 BiV3 O11 (∼12.6). Thus, the smaller relative permittivity of Ba2 LaV3 O11 could partly be explained by its smaller ionic polarizability of La3+ (6.07 Å) relative to Bi3+ (6.12 Å). Besides, the theoretical permittivity of Ba2 LaV3 O11 ceramic is a little smaller than the measured value. The deviation is 8.6%, which is considered acceptable considering the simplicity of additive rule [29]. Kim et al. [30] proposed a general relationship between the quality factor and the packing fraction of structure, that is, the Q × f value increases with the increase of packing fraction. Based on crystal structural considerations, the packing fraction could be calculated as following: Packing fraction (%) =
volume of packed ions ×Z volume of unit cell
(5)
As shown in Table 3, the calculated packing fractions for Ba2 MV3 O11 (M = La, Bi) ceramics are 58.5% and 59.5%, respectively. Ba2 BiV3 O11 has a higher Q × f value than Ba2 LaV3 O11 due to the larger packing fraction. It is generally accepted that f has close relationship with the bond valence. Park et al. [28] demonstrated the relationship between f value and bond valance for perovskites. The M-site bond valences were calculated using the following equations: Vi =
vij
(6)
j
vij = exp
Rij − dij b
(7)
where Rij is the bond valence parameter, dij is the length of a bond between atoms i and j, and b is a universal constant (0.37 Å) [31]. According to the bond valence theory [32], the shorter the bond length is, the stronger the bond energy becomes, thereby resulting in increase in the restoring force for recovering the tilting of oxygen octahedra, and f value would thereupon decreased. Therefore, as given in Table 4, it is believed that the decrease in f value of Ba2 LaV3 O11 might be related to the larger bond valences VM–O . 4. Conclusions A novel low temperature firing microwave dielectric ceramic Ba2 LaV3 O11 was prepared by the conventional solid state reaction method. High performance of microwave dielectric properties could be obtained in the ceramic sintered at 840 ◦ C for 4 h with a permittivity value of 12.8, a Q × f value of 31,800 GHz (at 9.9 GHz), and a f value of −14 ppm/ ◦ C. From the XRD analysis and EDS result, the Ba2 LaV3 O11 ceramic was found to be chemically compatible with silver powders at 840 ◦ C. All the results indicated that the Ba2 LaV3 O11 ceramic might be a promising candidate for low
temperature cofired ceramic technology. Ionic polarizability, packing fraction, and bond valences theories were applied to explain the distinct difference between Ba2 LaV3 O11 and its Ba2 BiV3 O11 .
Acknowledgments This work was supported by Natural Science Foundation of China (No. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (No. 2015GXNSFBA139234, and 2015GXNSFFA139003), Project of Department of Science and Technology of Guangxi (No. 114122005–28), and Projects of Education Department of Guangxi Zhuang Autonomous Region (No. YB2014160, KY2015YB341, and KY2015YB122).
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