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A novel low-firing microwave dielectric ceramic Li2ZnGe3O8 with cubic spinel structure
Journal of the European Ceramic Society 37 (2017) 625–629
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Feature article
A novel low-firing microwave dielectric ceramic Li2 ZnGe3 O8 with cubic spinel structure Huaicheng Xiang a , Liang Fang a,b,∗ , Weishuang Fang a , Ying Tang a,b , Chunchun Li a,c,∗ 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 Guangxi Key Laboratory of New Energy and Building Energy Saving, Guilin University of Technology, Guilin 541004, China c 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 5 March 2016 Received in revised form 4 September 2016 Accepted 16 September 2016 Available online 27 September 2016 Keywords: Microwave dielectric properties LTCC Spinel structure Li2 ZnGe3 O8
a b s t r a c t A Li2 ZnGe3 O8 ceramic was investigated as a promising microwave dielectric material for lowtemperature co-fired ceramics applications. Li2 ZnGe3 O8 ceramic was prepared via the conventional solid-state method. X-ray diffraction data shows that Li2 ZnGe3 O8 ceramic crystallized into a cubic spinel structure with a space group of P41 32. Dense ceramic with a relative densities of 96.3% were obtained when sintered at 945 ◦ C for 4 h and exhibited the optimum microwave properties with a relative permittivity (εr ) of 10.3, a quality factor (Q × f) of 47,400 GHz (at 13.3 GHz), and a temperature coefficient of resonance frequency ( f ) of −63.9 ppm/◦ C. The large negative f of Li2 ZnGe3 O8 ceramic could be compensated by rutile TiO2 , and 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramic sintered at 950 ◦ C for 4 h exhibited improved microwave dielectric properties with a near-zero f of −1.6 ppm/◦ C along with εr of 11.3 and a Q × f of 35,800 GHz (11.6 GHz). Moreover, Li2 ZnGe3 O8 was found to be chemically compatible with silver electrode when sintered at 945 ◦ C. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Low-temperature co-fired ceramics (LTCC) technology has been intensively investigated over past decades because of its critical application in the fabrication of miniature multilayer devices for wireless communication [1–3]. Such microwave dielectric ceramics are basically required to have an appropriate relative permittivity (εr ) to reduce the signal transmit delay, a high quality factor (Q × f) for frequency selectivity, a near-zero temperature coefficient of resonant frequency ( f ) for temperature stability, and they should be sintered below 961 ◦ C for being co-fired with Ag electrode [4,5]. The sintering temperatures of the most commonly used microwave dielectric ceramics, unfortunately, are usually higher than 1300 ◦ C such as ATiO3 (A = Ni, Mg, Co, Mn) [6] and Ba(A1/3 Ta2/3 )O3 (A = Zn, Nb, Mg) [7]. Adding glass or oxides with low melting point has been proved to be an effective method to reduce the sintering tem-
∗ Corresponding authors at: 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. E-mail addresses: [email protected] (L. Fang), [email protected] (C. Li). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.09.013 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
perature, but always at the cost of the degradation in microwave dielectric properties [8]. Therefore, searching for new materials with combination of excellent microwave dielectric properties and low sintering temperatures is still an urgent project. Recently, a series of lithium based microwave dielectric ceramics with good microwave dielectric properties have attracted much attention, such as LiCa3 MV3 O12 (M = Mg, Zn) [9,10], and Li2 MTi3 O8 (M = Mg, Zn) [11]. Among them, spinel-type Li2 MTi3 O8 received special attention due to its adequate dielectric properties. For example, it is reported that Li2 ZnTi3 O8 ceramic has excellent microwave dielectric properties (εr = 26.2, Q × f = 62,000 GHz, f = −15 ppm/◦ C) when sintered at 1075 ◦ C [11]; Li2 MgTi3 O8 ceramic sintered at 1075 ◦ C has a relative permittivity (εr ) ∼ 27.2, a quality factor (Q × f) ∼ 42,000 GHz and a temperature coefficient of resonant frequency ( f ) ∼ +3.2 ppm/◦ C [12]. However, the high sintering temperatures (S.T. = 1075 ◦ C) limit their applications in LTCC technology. In order to lower the sintering temperature, glass frits such as BaCu(B2 O5 ), ZnO-B2 O3 -SiO2 , CaO-B2 O3 -SiO2 , and Li2 OB2 O3 -SiO2 were used [13–17]. But the glassy phase with higher dielectric loss could be detrimental to the microwave dielectric properties. Thus, searching for low-firing spinel-type materials is still going on. Li2 ZnGe3 O8 as a member of the spinel family was firstly reported by Grisafe et al. [18] and its structure is characterized by 1:3 cation
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ordering of Li and Ge at octahedral sites and tetrahedral sites occupied by a disordered mixture of Li and Zn [19,20]. In recent years, Li2 ZnGe3 O8 has been widely investigated for applications in lithium batteries [21–23]. To the best knowledge of us, however, the microwave dielectric properties of Li2 ZnGe3 O8 ceramics have not been investigated. With a purpose to develop LTCC materials, the sintering behavior and microwave dielectric properties of Li2 ZnGe3 O8 ceramic and its chemical compatibility with silver electrode were studied in the present work. 2. Experimental procedure Li2 ZnGe3 O8 ceramic was prepared via a conventional solid-state reaction of Li2 CO3 (99.99%, Guo-Yao Co. Ltd., Shanghai, China), ZnO (99.99%, Guo-Yao Co. Ltd., Shanghai, China), and GeO2 (99.999%, Guo-Yao Co. Ltd., Shanghai, China). After weighing according to the stoichiometry, ball milled was carried out in alcohol medium for 6 h in a plastic bottle using zirconia balls as a grinding medium. Subsequently, the wet mixtures were dried and calcined at 900 ◦ C for 4 h in air. The calcined powders were re-milled for 4 h and after drying, the polyvinyl alcohol (PVA, 10 vol%) was added to the powders as binder. Then the powders were pressed into cylinders with 12 mm in diameter and 7 mm in height under a pressure of 200 MPa. The samples were heated to 550 ◦ C for 2 h at a heating rate of 1.5 ◦ C/min to remove the organic binder and sintered from 915 ◦ C to 960 ◦ C for 4 h with a heating rate of 5 ◦ C/min. In addition, to tune the negative value of Li2 ZnGe3 O8 , composite ceramics (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) were prepared. The calcined Li2 ZnGe3 O8 powders were mixed with TiO2 , ball milled for 4 h, and then pressed into pellets and sintered. In order to explore the chemical compatibility of Li2 ZnGe3 O8 ceramic with Ag, the calcined powders were mixed with 20 wt% Ag powders ball milled and dried. Then the mixtures were pressed into cylinders and sintered at 945 ◦ C for 4 h in air. Crystalline phases of the of the specimens were studied using an X-ray diffraction measurement (CuK˛1, 1.54059 Å, Model X’Pert PRO, PANalytical, Almelo, Holland) with CuK˛ radiation and a monochromator in the 2 range of 10–80◦ . The apparent densities of the sintered ceramics were measured using the Archimedes method. Microstructures of the samples were observed by scanning electron microscope (FE-SEM, Model S4800, Hitachi, Japan). The microwave dielectric properties were analyzed using a network analyzer (Model N5230A, Agilent Co., Palo Alto, California) and a temperature chamber (Delta 9039, Delta Design, San Diego, CA). The temperature coefficient of resonant frequency ( f ) was obtained by noting the temperature variations of the TE011 resonance scope from 25 ◦ C to 85 ◦ C. The f value was calculated as the following relationship: f =
f85 − f25 (85 − 25) × f25
(1)
where, f85 and f25 were the resonant frequencies of the dielectric resonator at temperature 85 ◦ C and 25 ◦ C, respectively. 3. Results and discussions All Li2 ZnGe3 O8 ceramics sintered at different temperatures from 915 ◦ C to 960 ◦ C for 4 h crystallized into a single spinel phase with a space group of P41 32 (213). To obtain the lattice parameters of the as-sintered ceramics, Rietveld refinement was employed. As a representative, the observed and calculated XRD patterns of Li2 ZnGe3 O8 sample sintered at 945 ◦ C for 4 h are shown in Fig. 1 with residual factors Rwp = 10.97% and Rp = 5.82%. The refined lattice parameter and unit cell volume of Li2 ZnGe3 O8 sample were 8.1934(1) Å and 550.0(4) Å3 , respectively. SEM image of the polished and thermally etched surface of the ceramic sintered at 945 ◦ C is shown in the insert of Fig. 1. As seen, a uniform and dense
Fig. 1. Rietveld refinement patterns of Li2 ZnGe3 O8 ceramic sintered at 945 ◦ C for 4 h. Inset: the optimal SEM photograph of Li2 ZnGe3 O8 ceramic sintered at 945 ◦ C.
Fig. 2. Raman spectra of Li2 ZnGe3 O8 ceramic sintered in the temperature range 915–960 ◦ C for 4 h.
microstructure with closely packed grains (∼5 m in average grain size) was developed. The room-temperature Raman spectra of Li2 ZnGe3 O8 ceramics are shown in Fig. 2. All spectra showed main Raman bands at around 131 cm−1 , 195 cm−1 , 230 cm−1 , 271 cm−1 , 394 cm−1 , 456 cm−1 , 524 cm−1 , 619 cm−1 , 670 cm−1 ,714 cm−1 , and 760 cm−1 . The Raman spectra are similar to that of Li2 ZnTi3 O8 reported by Lu et al. [24]. For wavenumbers between 190 cm−1 and 390 cm−1 , the bands were principally due to bending of O–cation–O. The origin of the bands with wavenumber below 190 cm−1 is lattice vibrations, mainly associated with cations [25]. The peaks observed at 394 cm−1 and 524 cm−1 were assigned to stretching vibration of [Li/ZnO4 ] and [LiO6 ], respectively, while the peak at 760 cm−1 arises from symmetric breathing of [GeO6 ] octahedron. As observed, with increasing sintering temperature, no other Raman peak appeared, indicating that Li2 ZnGe3 O8 ceramics with a relatively wide sintering temperature range and can remain stable in the temperature range 915–960 ◦ C. Fig. 3 presents the variations in bulk density, relative density and microwave dielectric properties of Li2 ZnGe3 O8 ceramics as a function of the sintering temperature. In Fig. 3(a), with increasing sintering temperature the relative density firstly increased from 93.3% at 915 ◦ C, reached a maximum value of 96.3% at 945 ◦ C, and then decreased slightly to 95.3% at 960 ◦ C. The relative permittivity (εr ) increased from 9.3 to 10.5 as the sintering temperature increased from 915 ◦ C to 960 ◦ C, and then slightly decreased. The
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Table 1 Microwave dielectric properties of (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) ceramics.
Fig. 3. The densities and the relationship between A1g(1) mode and microwave dielectric properties of Li2 ZnGe3 O8 ceramic sintered at different temperatures (915–960 ◦ C) for 4 h.
change in εr as a function of sintering temperature is similar to that of the density, indicating that the higher density, the higher dielectric constant. The lower permittivity at low temperature could be attributed to the existence of pores. The influence of the porosity on εr could be eliminated by applying Bosman and Having’s correction [26,27]: εcorrected = εm (1 + 1.5p)
(4)
where, p is the fractional porosity; εcorrected and εm are the corrected and measured values of permittivity, respectively. The εcorrected is about 11.0 for Li2 ZnGe3 O8 sintered at 945 ◦ C. Furthermore, εr can be interpreted by the sum of ionic polarizability of individual ions (˛TD ) and molar volume (Vm ) according to Clausius–Mossotti equation [28,29]: εr =
1 + 2b˛TD /Vm 1 − b˛TD /Vm
(5)
where, b = 4/3. The sum polarizability of Li2 ZnGe3 O8 could be calculated with the additive rule: ˛TD = 2˛(Li+ ) + ˛(Zn2+ ) + 3˛(Ge4+ ) + 8˛(O2− )
(6)
where, ␣(Li+ ), ␣(Zn2+ ), ␣(Ge4+ ) and ␣(O2− ) are 1.20 Å3 , 2.04 Å3 , 1.63 Å3 and 2.01 Å3 , respectively [30]. The calculated theoretical permittivity of Li2 ZnGe3 O8 is 10.7. The relative error of Li2 ZnGe3 O8 is about 1.9% for the measured value and 2.7% for the porosity corrected value, which means that there is no other polarization mechanism in the Li2 ZnGe3 O8 ceramic at microwave region. Shannon [28] highlights that substantial discrepancy between the measured and estimated values is an indication of other polarization mechanism or the presence of dipolar impurities. Similar results were also reported in W-based ceramic systems [31,32]. Fig. 3(c) and (d) shows the Q × f values and the temperature coefficient of the resonant frequency ( f ) of Li2 ZnGe3 O8 ceramic sintered at 915–960 ◦ C for 4 h, respectively. The variation in Q × f values with sintering temperature presented similar behavior to that of density and relative permittivity. As the sintering temperature increased, a maximum value of 47,400 GHz was reached in the sample sintered at 945 ◦ C for 4 h. Generally, the Q × f value can be affected by many factors including intrinsic and extrinsic factors such as densification, impurity, second phases, and the intrinsic parameters such as packing fraction and crystal structure [33,34]. As shown in Fig. 3(d), the f value of Li2 ZnGe3 O8 ceramics varied
x
S.T. (◦ C)
εr
Q × f (GHz)
f (ppm/◦ C)
0 0.08 0.1 0.12
945 940 950 960
10.5 ± 0.03 11.0 ± 0.04 11.3 ± 0.03 11.5 ± 0.04
47,400 ± 2000 37,220 ± 1800 35,800 ± 2200 32,820 ± 2700
−63.9 ± 1.9 −6.4 ± 0.7 −1.6 ± 0.3 +7.3 ± 0.7
from −78.9 to −63.9 ppm/◦ C over the sintering temperature range 915–960 ◦ C. It is well known that microwave dielectric properties are strongly dependent on the structural characteristics. Raman spectroscopy is a useful tool to investigate the local crystal structure, short range ordering and cation occupancy. Many studies have correlated the vibrational characteristics to microwave dielectric properties through the information provided by Raman analysis [35]. The stretch mode (A1g(1) with wavenumber around 760 cm−1 ) of the oxygen octahedrons play the strongest influence on microwave dielectric properties [36–38]. As shown in Fig. 3(b), with increasing sintering temperature the Raman peak of A1g(1) mode shifts towards higher frequency side, opposite to that of the relative permittivity (εr ). This can be explained by the fact that as temperature increased the A1g(1) mode at higher wavenumber corresponds to higher vibration energy of oxygen octahedron, giving rise to a lower dielectric constant. On the other hand, the variation in FWHM of A1g(1) mode also displays the opposite trend compared with the Q × f value as the sintering temperature increased. This phenomenon is mainly due to the space of the lattice vibrations reduce and the anharmonic vibrations decrease, thereby the intrinsic dielectric loss decreased. In contrast, with decrease of the A1g(1) FWHM, the coherence and damping behavior of the A1g(1) stretch vibration weaken, and the intrinsic Q × f value increases inversely [24,25]. Li2 ZnGe3 O8 ceramic sintered at 945 ◦ C for 4 h showed excellent microwave dielectric properties (εr = 10.5, Q × f = 47,400 GHz and f = −63.9 ppm/◦ C). Table 2 lists the sintering temperature and microwave dielectric properties of some Ge-containing ceramics, such as Zn2 GeO4 and Mg2 GeO4 . By comparison, it is found that the relative permittivity of Li2 ZnGe3 O8 is higher than those of others while the quality factor is inferior. It is worth noting that the sintering temperature of Li2 ZnGe3 O8 ceramic is much lower than other Ge-containing ceramics. The low sintering temperature and favorable microwave dielectric properties make Li2 ZnGe3 O8 a possible candidate for LTCC application. From the practical application point of view, however, the microwave dielectric materials should have a near-zero f for thermally stable electronic devices. The large negative f value would impede the practical applications. It is reported that f value can be turned by formation of solid solution or mixtures of dielectrics with opposite f values [39]. In our previous report [40], near-zero f values could be achieved by compensating the large negative f with rutile TiO2 having a positive one (∼+465 ppm/◦ C). Therefore, in this work rutile TiO2 was chosen to compensate the f value of Li2 ZnGe3 O8 ceramic. A series of composite ceramics in (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) system were prepared. Fig. 4 shows XRD pattern of the 0·9Li2 ZnGe3 O8 –0·1TiO2 ceramic sintered at 950 ◦ C for 4 h and XRD patterns of Li2 ZnGe3 O8 and TiO2 are also given for comparison. Only the peaks belonging to Li2 ZnGe3 O8 and TiO2 could be observed without secondary phase detected. In addition, the microwave dielectric properties of (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) ceramics sintered at their optimum temperatures are listed in Table 1. With increase in x value, the εr increased from 10.5 to 11.5 and f value increased from −63.9 to −1.6 ppm/◦ C. Although Q × f was slightly decreased, a near-zero f could be achieved. When x = 0.1, the 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramics fired at 950 ◦ C
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4. Conclusions In this study, a microwave dielectric ceramic Li2 ZnGe3 O8 with cubic spinel structure was prepared by a conventional solid-state reaction method. The dense microstructure with a relative density of 96.3% and excellent microwave dielectric properties with a permittivity of 10.5, a Q × f value of 47,400 GHz (at 13.3 GHz), and a negative f value of −63.9 ppm/◦ C could be achieved when sintered at 945 ◦ C for 4 h. Li2 ZnGe3 O8 shows good chemical compatibility with the Ag electrodes. Moreover, the large negative f of Li2 ZnGe3 O8 ceramic could be modified by adding rutile TiO2 , and the 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramic sintered at 950 ◦ C for 4 h exhibited favorable dielectric properties with a εr of 11.3, a Q × f value of 35,800 GHz (11.6 GHz) and a near-zero f value of −1.6 ppm/◦ C. All above merits make Li2 ZnGe3 O8 a promising candidate for LTCC application. Fig. 4. X-ray diffraction patterns of 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramics sintered at 950 ◦ C for 4 h in air. Table 2 Comparison of the sintering temperature and microwave dielectric properties of some Ge-containing ceramics. Composition
S.T. (◦ C) εr
Q × f (GHz) f (ppm/◦ C) Ref.
Li2 ZnGe3 O8 Zn2 GeO4 (Zn0.6 Mg0.4 )1.918 GeO3.918 Mg1.918 GeO3.918 Mg2 GeO4 Mg2 GeO4 -B2 O3
945 1300 1300 1300 1450 1250
47,400 102,700 275,000 170,000 11,037 95,000
10.5 6.87 6.7 7.6 5.48 6.76
−63.9 −32.4 −42.5 −55 −27.61 −28.7
This work [41] [35] [35] [42] [42]
Acknowledgments This research was supported by Natural Science Foundation of China (Nos. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (Nos. 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 (Nos. YB2014160, KY2015YB341, and KY2015YB122), the Foundation of Guangxi Key Laboratory of New Energy and Building Energy Saving (No. 15-J-21-13).
References
Fig. 5. XRD patterns of unfired Li2 ZnGe3 O8 ceramic with 20 wt% silver and the cofired sample at 930 ◦ C. The inset shows backscattered electron image micrograph of the Li2 ZnGe3 O8 ceramic with silver powders co-fired at 930 ◦ C.
for 4 h exhibited excellent microwave dielectric properties with εr = 11.3, Q × f = 35,800 GHz (11.6 GHz) and f = −1.6 ppm/◦ C. To evaluate the chemical compatibility of Li2 ZnGe3 O8 ceramic with silver electrode, Li2 ZnGe3 O8 was co-fired with 20 wt% silver at 945 ◦ C for 4 h. XRD patterns of the unfired mixture of Li2 ZnGe3 O8 and 20 wt% Ag powders and the co-fired sample are shown in Fig. 5. By comparison, it is noted that the relative intensities of the peaks belonging to Li2 ZnGe3 O8 and Ag are almost the same before and after cofiring, suggesting no reaction between Li2 ZnGe3 O8 and Ag. This was further confirmed from the backscattered electron image (BSE) of Li2 ZnGe3 O8 ceramic with silver powders co-fired at 945 ◦ C, as shown in the inset of Fig. 5. Two distinct grains could be observed and the larger bright grains were detected to be Ag.
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Feature article
A novel low-firing microwave dielectric ceramic Li2 ZnGe3 O8 with cubic spinel structure Huaicheng Xiang a , Liang Fang a,b,∗ , Weishuang Fang a , Ying Tang a,b , Chunchun Li a,c,∗ 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 Guangxi Key Laboratory of New Energy and Building Energy Saving, Guilin University of Technology, Guilin 541004, China c 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 5 March 2016 Received in revised form 4 September 2016 Accepted 16 September 2016 Available online 27 September 2016 Keywords: Microwave dielectric properties LTCC Spinel structure Li2 ZnGe3 O8
a b s t r a c t A Li2 ZnGe3 O8 ceramic was investigated as a promising microwave dielectric material for lowtemperature co-fired ceramics applications. Li2 ZnGe3 O8 ceramic was prepared via the conventional solid-state method. X-ray diffraction data shows that Li2 ZnGe3 O8 ceramic crystallized into a cubic spinel structure with a space group of P41 32. Dense ceramic with a relative densities of 96.3% were obtained when sintered at 945 ◦ C for 4 h and exhibited the optimum microwave properties with a relative permittivity (εr ) of 10.3, a quality factor (Q × f) of 47,400 GHz (at 13.3 GHz), and a temperature coefficient of resonance frequency ( f ) of −63.9 ppm/◦ C. The large negative f of Li2 ZnGe3 O8 ceramic could be compensated by rutile TiO2 , and 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramic sintered at 950 ◦ C for 4 h exhibited improved microwave dielectric properties with a near-zero f of −1.6 ppm/◦ C along with εr of 11.3 and a Q × f of 35,800 GHz (11.6 GHz). Moreover, Li2 ZnGe3 O8 was found to be chemically compatible with silver electrode when sintered at 945 ◦ C. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Low-temperature co-fired ceramics (LTCC) technology has been intensively investigated over past decades because of its critical application in the fabrication of miniature multilayer devices for wireless communication [1–3]. Such microwave dielectric ceramics are basically required to have an appropriate relative permittivity (εr ) to reduce the signal transmit delay, a high quality factor (Q × f) for frequency selectivity, a near-zero temperature coefficient of resonant frequency ( f ) for temperature stability, and they should be sintered below 961 ◦ C for being co-fired with Ag electrode [4,5]. The sintering temperatures of the most commonly used microwave dielectric ceramics, unfortunately, are usually higher than 1300 ◦ C such as ATiO3 (A = Ni, Mg, Co, Mn) [6] and Ba(A1/3 Ta2/3 )O3 (A = Zn, Nb, Mg) [7]. Adding glass or oxides with low melting point has been proved to be an effective method to reduce the sintering tem-
∗ Corresponding authors at: 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. E-mail addresses: [email protected] (L. Fang), [email protected] (C. Li). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.09.013 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
perature, but always at the cost of the degradation in microwave dielectric properties [8]. Therefore, searching for new materials with combination of excellent microwave dielectric properties and low sintering temperatures is still an urgent project. Recently, a series of lithium based microwave dielectric ceramics with good microwave dielectric properties have attracted much attention, such as LiCa3 MV3 O12 (M = Mg, Zn) [9,10], and Li2 MTi3 O8 (M = Mg, Zn) [11]. Among them, spinel-type Li2 MTi3 O8 received special attention due to its adequate dielectric properties. For example, it is reported that Li2 ZnTi3 O8 ceramic has excellent microwave dielectric properties (εr = 26.2, Q × f = 62,000 GHz, f = −15 ppm/◦ C) when sintered at 1075 ◦ C [11]; Li2 MgTi3 O8 ceramic sintered at 1075 ◦ C has a relative permittivity (εr ) ∼ 27.2, a quality factor (Q × f) ∼ 42,000 GHz and a temperature coefficient of resonant frequency ( f ) ∼ +3.2 ppm/◦ C [12]. However, the high sintering temperatures (S.T. = 1075 ◦ C) limit their applications in LTCC technology. In order to lower the sintering temperature, glass frits such as BaCu(B2 O5 ), ZnO-B2 O3 -SiO2 , CaO-B2 O3 -SiO2 , and Li2 OB2 O3 -SiO2 were used [13–17]. But the glassy phase with higher dielectric loss could be detrimental to the microwave dielectric properties. Thus, searching for low-firing spinel-type materials is still going on. Li2 ZnGe3 O8 as a member of the spinel family was firstly reported by Grisafe et al. [18] and its structure is characterized by 1:3 cation
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ordering of Li and Ge at octahedral sites and tetrahedral sites occupied by a disordered mixture of Li and Zn [19,20]. In recent years, Li2 ZnGe3 O8 has been widely investigated for applications in lithium batteries [21–23]. To the best knowledge of us, however, the microwave dielectric properties of Li2 ZnGe3 O8 ceramics have not been investigated. With a purpose to develop LTCC materials, the sintering behavior and microwave dielectric properties of Li2 ZnGe3 O8 ceramic and its chemical compatibility with silver electrode were studied in the present work. 2. Experimental procedure Li2 ZnGe3 O8 ceramic was prepared via a conventional solid-state reaction of Li2 CO3 (99.99%, Guo-Yao Co. Ltd., Shanghai, China), ZnO (99.99%, Guo-Yao Co. Ltd., Shanghai, China), and GeO2 (99.999%, Guo-Yao Co. Ltd., Shanghai, China). After weighing according to the stoichiometry, ball milled was carried out in alcohol medium for 6 h in a plastic bottle using zirconia balls as a grinding medium. Subsequently, the wet mixtures were dried and calcined at 900 ◦ C for 4 h in air. The calcined powders were re-milled for 4 h and after drying, the polyvinyl alcohol (PVA, 10 vol%) was added to the powders as binder. Then the powders were pressed into cylinders with 12 mm in diameter and 7 mm in height under a pressure of 200 MPa. The samples were heated to 550 ◦ C for 2 h at a heating rate of 1.5 ◦ C/min to remove the organic binder and sintered from 915 ◦ C to 960 ◦ C for 4 h with a heating rate of 5 ◦ C/min. In addition, to tune the negative value of Li2 ZnGe3 O8 , composite ceramics (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) were prepared. The calcined Li2 ZnGe3 O8 powders were mixed with TiO2 , ball milled for 4 h, and then pressed into pellets and sintered. In order to explore the chemical compatibility of Li2 ZnGe3 O8 ceramic with Ag, the calcined powders were mixed with 20 wt% Ag powders ball milled and dried. Then the mixtures were pressed into cylinders and sintered at 945 ◦ C for 4 h in air. Crystalline phases of the of the specimens were studied using an X-ray diffraction measurement (CuK˛1, 1.54059 Å, Model X’Pert PRO, PANalytical, Almelo, Holland) with CuK˛ radiation and a monochromator in the 2 range of 10–80◦ . The apparent densities of the sintered ceramics were measured using the Archimedes method. Microstructures of the samples were observed by scanning electron microscope (FE-SEM, Model S4800, Hitachi, Japan). The microwave dielectric properties were analyzed using a network analyzer (Model N5230A, Agilent Co., Palo Alto, California) and a temperature chamber (Delta 9039, Delta Design, San Diego, CA). The temperature coefficient of resonant frequency ( f ) was obtained by noting the temperature variations of the TE011 resonance scope from 25 ◦ C to 85 ◦ C. The f value was calculated as the following relationship: f =
f85 − f25 (85 − 25) × f25
(1)
where, f85 and f25 were the resonant frequencies of the dielectric resonator at temperature 85 ◦ C and 25 ◦ C, respectively. 3. Results and discussions All Li2 ZnGe3 O8 ceramics sintered at different temperatures from 915 ◦ C to 960 ◦ C for 4 h crystallized into a single spinel phase with a space group of P41 32 (213). To obtain the lattice parameters of the as-sintered ceramics, Rietveld refinement was employed. As a representative, the observed and calculated XRD patterns of Li2 ZnGe3 O8 sample sintered at 945 ◦ C for 4 h are shown in Fig. 1 with residual factors Rwp = 10.97% and Rp = 5.82%. The refined lattice parameter and unit cell volume of Li2 ZnGe3 O8 sample were 8.1934(1) Å and 550.0(4) Å3 , respectively. SEM image of the polished and thermally etched surface of the ceramic sintered at 945 ◦ C is shown in the insert of Fig. 1. As seen, a uniform and dense
Fig. 1. Rietveld refinement patterns of Li2 ZnGe3 O8 ceramic sintered at 945 ◦ C for 4 h. Inset: the optimal SEM photograph of Li2 ZnGe3 O8 ceramic sintered at 945 ◦ C.
Fig. 2. Raman spectra of Li2 ZnGe3 O8 ceramic sintered in the temperature range 915–960 ◦ C for 4 h.
microstructure with closely packed grains (∼5 m in average grain size) was developed. The room-temperature Raman spectra of Li2 ZnGe3 O8 ceramics are shown in Fig. 2. All spectra showed main Raman bands at around 131 cm−1 , 195 cm−1 , 230 cm−1 , 271 cm−1 , 394 cm−1 , 456 cm−1 , 524 cm−1 , 619 cm−1 , 670 cm−1 ,714 cm−1 , and 760 cm−1 . The Raman spectra are similar to that of Li2 ZnTi3 O8 reported by Lu et al. [24]. For wavenumbers between 190 cm−1 and 390 cm−1 , the bands were principally due to bending of O–cation–O. The origin of the bands with wavenumber below 190 cm−1 is lattice vibrations, mainly associated with cations [25]. The peaks observed at 394 cm−1 and 524 cm−1 were assigned to stretching vibration of [Li/ZnO4 ] and [LiO6 ], respectively, while the peak at 760 cm−1 arises from symmetric breathing of [GeO6 ] octahedron. As observed, with increasing sintering temperature, no other Raman peak appeared, indicating that Li2 ZnGe3 O8 ceramics with a relatively wide sintering temperature range and can remain stable in the temperature range 915–960 ◦ C. Fig. 3 presents the variations in bulk density, relative density and microwave dielectric properties of Li2 ZnGe3 O8 ceramics as a function of the sintering temperature. In Fig. 3(a), with increasing sintering temperature the relative density firstly increased from 93.3% at 915 ◦ C, reached a maximum value of 96.3% at 945 ◦ C, and then decreased slightly to 95.3% at 960 ◦ C. The relative permittivity (εr ) increased from 9.3 to 10.5 as the sintering temperature increased from 915 ◦ C to 960 ◦ C, and then slightly decreased. The
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Table 1 Microwave dielectric properties of (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) ceramics.
Fig. 3. The densities and the relationship between A1g(1) mode and microwave dielectric properties of Li2 ZnGe3 O8 ceramic sintered at different temperatures (915–960 ◦ C) for 4 h.
change in εr as a function of sintering temperature is similar to that of the density, indicating that the higher density, the higher dielectric constant. The lower permittivity at low temperature could be attributed to the existence of pores. The influence of the porosity on εr could be eliminated by applying Bosman and Having’s correction [26,27]: εcorrected = εm (1 + 1.5p)
(4)
where, p is the fractional porosity; εcorrected and εm are the corrected and measured values of permittivity, respectively. The εcorrected is about 11.0 for Li2 ZnGe3 O8 sintered at 945 ◦ C. Furthermore, εr can be interpreted by the sum of ionic polarizability of individual ions (˛TD ) and molar volume (Vm ) according to Clausius–Mossotti equation [28,29]: εr =
1 + 2b˛TD /Vm 1 − b˛TD /Vm
(5)
where, b = 4/3. The sum polarizability of Li2 ZnGe3 O8 could be calculated with the additive rule: ˛TD = 2˛(Li+ ) + ˛(Zn2+ ) + 3˛(Ge4+ ) + 8˛(O2− )
(6)
where, ␣(Li+ ), ␣(Zn2+ ), ␣(Ge4+ ) and ␣(O2− ) are 1.20 Å3 , 2.04 Å3 , 1.63 Å3 and 2.01 Å3 , respectively [30]. The calculated theoretical permittivity of Li2 ZnGe3 O8 is 10.7. The relative error of Li2 ZnGe3 O8 is about 1.9% for the measured value and 2.7% for the porosity corrected value, which means that there is no other polarization mechanism in the Li2 ZnGe3 O8 ceramic at microwave region. Shannon [28] highlights that substantial discrepancy between the measured and estimated values is an indication of other polarization mechanism or the presence of dipolar impurities. Similar results were also reported in W-based ceramic systems [31,32]. Fig. 3(c) and (d) shows the Q × f values and the temperature coefficient of the resonant frequency ( f ) of Li2 ZnGe3 O8 ceramic sintered at 915–960 ◦ C for 4 h, respectively. The variation in Q × f values with sintering temperature presented similar behavior to that of density and relative permittivity. As the sintering temperature increased, a maximum value of 47,400 GHz was reached in the sample sintered at 945 ◦ C for 4 h. Generally, the Q × f value can be affected by many factors including intrinsic and extrinsic factors such as densification, impurity, second phases, and the intrinsic parameters such as packing fraction and crystal structure [33,34]. As shown in Fig. 3(d), the f value of Li2 ZnGe3 O8 ceramics varied
x
S.T. (◦ C)
εr
Q × f (GHz)
f (ppm/◦ C)
0 0.08 0.1 0.12
945 940 950 960
10.5 ± 0.03 11.0 ± 0.04 11.3 ± 0.03 11.5 ± 0.04
47,400 ± 2000 37,220 ± 1800 35,800 ± 2200 32,820 ± 2700
−63.9 ± 1.9 −6.4 ± 0.7 −1.6 ± 0.3 +7.3 ± 0.7
from −78.9 to −63.9 ppm/◦ C over the sintering temperature range 915–960 ◦ C. It is well known that microwave dielectric properties are strongly dependent on the structural characteristics. Raman spectroscopy is a useful tool to investigate the local crystal structure, short range ordering and cation occupancy. Many studies have correlated the vibrational characteristics to microwave dielectric properties through the information provided by Raman analysis [35]. The stretch mode (A1g(1) with wavenumber around 760 cm−1 ) of the oxygen octahedrons play the strongest influence on microwave dielectric properties [36–38]. As shown in Fig. 3(b), with increasing sintering temperature the Raman peak of A1g(1) mode shifts towards higher frequency side, opposite to that of the relative permittivity (εr ). This can be explained by the fact that as temperature increased the A1g(1) mode at higher wavenumber corresponds to higher vibration energy of oxygen octahedron, giving rise to a lower dielectric constant. On the other hand, the variation in FWHM of A1g(1) mode also displays the opposite trend compared with the Q × f value as the sintering temperature increased. This phenomenon is mainly due to the space of the lattice vibrations reduce and the anharmonic vibrations decrease, thereby the intrinsic dielectric loss decreased. In contrast, with decrease of the A1g(1) FWHM, the coherence and damping behavior of the A1g(1) stretch vibration weaken, and the intrinsic Q × f value increases inversely [24,25]. Li2 ZnGe3 O8 ceramic sintered at 945 ◦ C for 4 h showed excellent microwave dielectric properties (εr = 10.5, Q × f = 47,400 GHz and f = −63.9 ppm/◦ C). Table 2 lists the sintering temperature and microwave dielectric properties of some Ge-containing ceramics, such as Zn2 GeO4 and Mg2 GeO4 . By comparison, it is found that the relative permittivity of Li2 ZnGe3 O8 is higher than those of others while the quality factor is inferior. It is worth noting that the sintering temperature of Li2 ZnGe3 O8 ceramic is much lower than other Ge-containing ceramics. The low sintering temperature and favorable microwave dielectric properties make Li2 ZnGe3 O8 a possible candidate for LTCC application. From the practical application point of view, however, the microwave dielectric materials should have a near-zero f for thermally stable electronic devices. The large negative f value would impede the practical applications. It is reported that f value can be turned by formation of solid solution or mixtures of dielectrics with opposite f values [39]. In our previous report [40], near-zero f values could be achieved by compensating the large negative f with rutile TiO2 having a positive one (∼+465 ppm/◦ C). Therefore, in this work rutile TiO2 was chosen to compensate the f value of Li2 ZnGe3 O8 ceramic. A series of composite ceramics in (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) system were prepared. Fig. 4 shows XRD pattern of the 0·9Li2 ZnGe3 O8 –0·1TiO2 ceramic sintered at 950 ◦ C for 4 h and XRD patterns of Li2 ZnGe3 O8 and TiO2 are also given for comparison. Only the peaks belonging to Li2 ZnGe3 O8 and TiO2 could be observed without secondary phase detected. In addition, the microwave dielectric properties of (1-x)Li2 ZnGe3 O8 –xTiO2 (0 ≤ x ≤ 0.12) ceramics sintered at their optimum temperatures are listed in Table 1. With increase in x value, the εr increased from 10.5 to 11.5 and f value increased from −63.9 to −1.6 ppm/◦ C. Although Q × f was slightly decreased, a near-zero f could be achieved. When x = 0.1, the 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramics fired at 950 ◦ C
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4. Conclusions In this study, a microwave dielectric ceramic Li2 ZnGe3 O8 with cubic spinel structure was prepared by a conventional solid-state reaction method. The dense microstructure with a relative density of 96.3% and excellent microwave dielectric properties with a permittivity of 10.5, a Q × f value of 47,400 GHz (at 13.3 GHz), and a negative f value of −63.9 ppm/◦ C could be achieved when sintered at 945 ◦ C for 4 h. Li2 ZnGe3 O8 shows good chemical compatibility with the Ag electrodes. Moreover, the large negative f of Li2 ZnGe3 O8 ceramic could be modified by adding rutile TiO2 , and the 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramic sintered at 950 ◦ C for 4 h exhibited favorable dielectric properties with a εr of 11.3, a Q × f value of 35,800 GHz (11.6 GHz) and a near-zero f value of −1.6 ppm/◦ C. All above merits make Li2 ZnGe3 O8 a promising candidate for LTCC application. Fig. 4. X-ray diffraction patterns of 0.9Li2 ZnGe3 O8 –0.1TiO2 0·1TiO2 ceramics sintered at 950 ◦ C for 4 h in air. Table 2 Comparison of the sintering temperature and microwave dielectric properties of some Ge-containing ceramics. Composition
S.T. (◦ C) εr
Q × f (GHz) f (ppm/◦ C) Ref.
Li2 ZnGe3 O8 Zn2 GeO4 (Zn0.6 Mg0.4 )1.918 GeO3.918 Mg1.918 GeO3.918 Mg2 GeO4 Mg2 GeO4 -B2 O3
945 1300 1300 1300 1450 1250
47,400 102,700 275,000 170,000 11,037 95,000
10.5 6.87 6.7 7.6 5.48 6.76
−63.9 −32.4 −42.5 −55 −27.61 −28.7
This work [41] [35] [35] [42] [42]
Acknowledgments This research was supported by Natural Science Foundation of China (Nos. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (Nos. 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 (Nos. YB2014160, KY2015YB341, and KY2015YB122), the Foundation of Guangxi Key Laboratory of New Energy and Building Energy Saving (No. 15-J-21-13).
References
Fig. 5. XRD patterns of unfired Li2 ZnGe3 O8 ceramic with 20 wt% silver and the cofired sample at 930 ◦ C. The inset shows backscattered electron image micrograph of the Li2 ZnGe3 O8 ceramic with silver powders co-fired at 930 ◦ C.
for 4 h exhibited excellent microwave dielectric properties with εr = 11.3, Q × f = 35,800 GHz (11.6 GHz) and f = −1.6 ppm/◦ C. To evaluate the chemical compatibility of Li2 ZnGe3 O8 ceramic with silver electrode, Li2 ZnGe3 O8 was co-fired with 20 wt% silver at 945 ◦ C for 4 h. XRD patterns of the unfired mixture of Li2 ZnGe3 O8 and 20 wt% Ag powders and the co-fired sample are shown in Fig. 5. By comparison, it is noted that the relative intensities of the peaks belonging to Li2 ZnGe3 O8 and Ag are almost the same before and after cofiring, suggesting no reaction between Li2 ZnGe3 O8 and Ag. This was further confirmed from the backscattered electron image (BSE) of Li2 ZnGe3 O8 ceramic with silver powders co-fired at 945 ◦ C, as shown in the inset of Fig. 5. Two distinct grains could be observed and the larger bright grains were detected to be Ag.
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