- Email: [email protected]
A low-firing Ca5Ni4(VO4)6 ceramic with tunable microwave dielectric properties and chemical compatibility with Ag
Author’s Accepted Manuscript A Low-firing Ca 5Ni4(VO4)6 Ceramic with Tunable Microwave Dielectric Properties and Chemical Compatibility with Ag Dan Wang, Huaicheng Xiang, Ying Tang, Liang Fang, Jibran Khaliq, Chunchun Li www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30918-X http://dx.doi.org/10.1016/j.ceramint.2016.06.085 CERI13101
To appear in: Ceramics International Received date: 18 May 2016 Revised date: 12 June 2016 Accepted date: 13 June 2016 Cite this article as: Dan Wang, Huaicheng Xiang, Ying Tang, Liang Fang, Jibran Khaliq and Chunchun Li, A Low-firing Ca 5Ni4(VO4)6 Ceramic with Tunable Microwave Dielectric Properties and Chemical Compatibility with Ag, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.085 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Low-firing Ca5Ni4(VO4)6 Ceramic with Tunable Microwave Dielectric Properties and Chemical Compatibility with Ag Dan Wang1, Huaicheng Xiang1, Ying Tang1, Liang Fang1, Jibran Khaliq3, Chunchun Li1, 2* 1
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 2
College of Information Science and Engineering, Guilin University of Technology, Guilin, 541004, China
3
Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, Delft 2629 HS, The Netherlands
Abstract A Ca5Ni4(VO4)6 low loss microwave dielectric ceramic with A site deficient garnet structure was prepared via the conventional solid state reaction method. Ca5Ni4(VO4)6 sintered at 980 oC for 4 h to a relative density of 96.2% exhibits favorable microwave dielectric properties such as a permittivity of 10.9, a Q×f value of 96,500 GHz, and a τf value of -63.6 ppm/oC. Its large negative τf could be compensated by forming a solid solution with LiCa2Mg2V3O12, that led to improved properties with a near-zero τf = -3.7 ppm/oC, εr = 10.2, Q×f = 59,300 GHz for 0.8Ca5Ni4(VO4)6-0.2LiCa2Mg2V3O12 after sintering at 955 oC for 4 h. Further on, 0.8Ca5Ni4(VO4)6-0.2LiCa2Mg2V3O12 proved to be chemically compatible with Ag electrodes, so it might be a possible candidate for LTCC applications. Keywords: Microwave dielectric properties; Garnet structure; LTCC; Vanadate
Corresponding Author, [email protected]; [email protected] 1
1. Introduction Microwave dielectric materials have been widely used as dielectric resonators, band pass filters, oscillators and duplexers in communication systems from mobile phones to global positioning system. The more rapidly developing information technology industry requires high integration of circuits and high density of package. Low-temperature co-fired ceramics (LTCC) technology has attracted much attention because of its ability to combine thin layers of ceramic and conducting electrodes to fabricate multilayer LTCC modules.1-5 Advanced substrate materials for LTCC modules should have a low dielectric constant (εr), a high quality factor (Q×f), a near-zero temperature coefficient of resonant frequency (τf), and a low sintering temperature lower than the melting point of Ag (961 oC) or Cu (1080 oC).6-9 Moreover, the chemical compatibility with electrodes must be taken into account. Up to now, a series of garnet vanadates with a general formula A3B2V3O12 were investigated and reported with good microwave dielectric properties. Among them, LiCa3MgV3O12 ceramic sintered at 900 oC has a relative permittivity (εr) ~ 10.5, a quality factor (Q×f) ~ 74,700 GHz and a temperature coefficient of resonant frequency (τf) ~ -61 ppm/oC),10 while the Sr2NaMg2V3O12 ceramic has a εr = 11.7, Q×f = 37,950 GHz and τf = -2.9 ppm/oC when sintered at 900 oC.11 In the garnet structure, three different sites (A, B and C) are available for a wide variety of cations.12 A represents a 8-coordinated position, while B is an octahedral site and C is a tetrahedral site. Moreover, most of the vacancies were developed in the B site and partly in the A site, resulting in the so called cation-deficient garnets. Only a few reports are available for the microwave dielectric properties of the cation-deficient garnet vanadates. Yao et al.13,
14
firstly reported the microwave
dielectric properties of some A-site cation-deficiency garnets in the Ca5M4(VO4)6 (M 2
= Mg, Zn, and Co) system. The crystal structure of Ca5Ni4(VO4)6 was first reported by Lazoryak et al.15 and it belongs to the A-site cation-deficient garnet. Recently, Ca5Ni4(VO4)6 has been investigated for applications in visible-light-driven photocatalyst.16 To the best knowledge of us, however, the microwave dielectric properties of Ca5Ni4(VO4)6 ceramics have not been investigated. In the present work, sintering behavior and microwave dielectric properties of Ca5Ni4(VO4)6 ceramic were studied along with its chemical compatibility with Ag electrodes. 2. Experimental procedure Ca5Ni4(VO4)6 ceramic was prepared by the conventional solid-state reaction method from high-purity (> 99%) powders of CaCO3, NiO, and NH4VO3. The stoichiometric ratios of powders were weighed and ball-milled in alcohol for 6 h using zirconia balls as milling medium. Subsequently, the wet mixture was dried and calcined at 900 oC for 4 h in air. The calcined powder was milled in alcohol medium using zirconia balls for 4 h and after drying, the polyvinyl alcohol (PVA) was added to the powders as binder and then crushed into a fine powder through a sieve with 200 mesh. The obtained powder was pressed into pellets of 10 mm in diameter and 6 mm in height by uniaxial pressing under a pressure of 200 MPa. The pellets were heated at 550 oC for 2h at a heating rate of 1.5 oC/min to remove the PVA and then sintered at 935–995 oC for 4h. The crystal structure and phase composition of the specimens were analyzed with X-ray diffraction (CuKα1, 1.54059 Å, Model X’Pert PRO, PANalytical, Almelo, Holland). The bulk density was measured by the Archimede’s method. The polished and thermally etched surfaces of the samples were observed by scanning electron microscope (Model JSM6380-LV SEM, JEOL, Tokyo, Japan), equipped with dispersive spectrometer (EDS) for elementary analysis. The microwave dielectric 3
properties were measured using a network analyzer (Model N5230A, Agilent Co., Palo Alto, Canada) and a temperature chamber (Delta 9039, Delta Design, San Diego, CA). The temperature coefficient of resonant frequency (τf) was measured in the temperature range of 25–85 oC. 3. Results and discussions XRD patterns of the Ca5Ni4(VO4)6 ceramics sintered at 935–995 oC for 4 h are shown in Fig. 1. Within the limitation of XRD equipment, all the observed diffraction peaks could be indexed according to the JCPDS card No. 00-052-0469 for Ca5Ni4(VO4)6 with space group of Ia-3d. Samples with various sintering temperatures exhibited no phase difference. The lattice parameters were refined as a = 12.3567(15) Å, V = 1890.7(4) Å3 with calculated theoretical density of 3.94 g/cm3 (Z = 4). Fig. 2 shows the projection of the structure along the [001] direction. The structure is constructed by [NiO6] octahedra and [VO4] tetrahedra by corner sharing, and each Ca atoms is surrounded by eight oxygen atoms forming a dodecahedron. Fig. 3(a-e) shows the SEM images of polished and thermally etched surfaces. For the ceramic sintered at 935 oC, a porous microstructure with small grains was observed. A uniform and dense microstructure with closely packed grains in range 5~10μm was developed when sintered at 980 oC (Fig. 3 (d)). However, abnormal grain growth with large grains (~ 15μm) appeared along with some porosity and degradation in grain uniformity in the sample sintered at 995 oC. Fig. 3 (f) presents the variation of relative density and bulk density with the sintering temperature. At 935 oC, the bulk density was 3.30 g/cm3 (relative density of 83.8%). With increasing sintering temperature, the density increased, reached a maximum value of 96.2% at 980 oC, and decreased slightly at 995 oC. Fig. 4 presents the variations of the microwave dielectric properties (εr, Q×f, and 4
τf) of Ca5Ni4(VO4)6 ceramics as a function of the sintering temperature. The change in the relative permittivity and quality factor showed a similar trend to that of the densities. In Fig. 4(a), εr increased from 10.7 to 10.9 as the sintering temperature increased from 935 oC to 980 oC, and then slightly decreased. It is reported that the relative permittivity mainly depends on the composition, grain size and the density. The influence of the porosity on the permittivity could be eliminated by applying Bosman and Havinga’s correction:17 εcorrected = εm(1 + 1.5p)
(1)
where, p is the fractional porosity; εcorrected and εm are the corrected and measured values of permittivity, respectively. The εcorrected is about 11.5 for the sample sintered at 980 oC. Furthermore, εr can be interpreted by the sum of ionic polarizability of individual ions ( DT ) and molar volume (Vm) according to Clausius–Mossotti equation:18
1 2b DT Vm r 1 b DT Vm
(2)
where, b = 4π/3. The calculated theoretical permittivity of Ca5Ni4(VO4)6 is 10.8. The relative error of Ca5Ni4(VO4)6 is about 0.9% for the measured value and 6.0% for the porosity corrected value, suggesting there is no other polarization mechanism in the Ca5Ni4(VO4)6 ceramic at microwave region. A strong dependence of the Q×f on the sintering temperature was observed in Fig.4 (b). As the sintering temperature increased, the Q×f value of Ca5Ni4(VO4)6 ceramic increased to a maximum value of ~ 96,500 GHz sintered at 980 oC. Thereafter, the Q×f value decreased with further increase in sintering temperature. Fig.4 (c) represents the change in τf with increasing sintering temperature. As seen, the τf values varied in the range of -74.1 to -63.6 ppm/oC over the sintering region from 5
935 to 980 oC. Comparisons for the microwave dielectric properties of A-site deficient garnet system Ca5M4(VO4)6 (M = Zn, Mg, Co, and Ni) are listed in Table 2. As shown, obvious differences, especially in quality factor, were observed. Since Ca5M4(VO4)6 ceramics are single phases with relative densities > 95%, the effects of such extrinsic factors as density and secondary phase could be neglected. Kim et al.19 firstly proposed a general relationship between the quality factor and the packing fraction of structure. They pointed out that increase in packing fraction can weaken lattice vibrations, and thereby resulting in the decrease in the intrinsic dielectric loss. The higher Q×f value of Ca5Ni4(VO4)6 might be partly explained by its larger packing fraction(~ 60.8%) compared with other Ca5M4(VO4)6 (M= Zn, Mg, and Co) ceramics as shown in Table 2. The large negative τf value of Ca5Ni4(VO4)6 would impede its practical applications to a large extent. More recently, a low-firing garnet ceramic LiCa2Mg2V3O12 with a large positive τf ~ +259.2 ppm/oC was characterized (this work will be published elsewhere). Therefore, LiCa2Mg2V3O12 was chosen to tune the τf of Ca5Ni4(VO4)6 ceramic. A series of (1-x)Ca5Ni4(VO4)6–xLiCa2Mg2V3O12 (0 ≤ x ≤ 0.3) ceramics
were
prepared.
Fig.5
shows
XRD
pattern
of
the
0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 ceramic sintered at 955 oC/4h. As shown, all the peaks could be well indexed according to the cubic garnet structure. The microwave dielectric properties of (1-x)Ca5Ni4(VO4)6–xLiCa2Mg2V3O12 (0 ≤ x ≤ 0.3) ceramics sintered at their optimum temperatures are listed in Table 1. A near-zero τf could be achieved at x = 0.2. The 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 ceramics fired at 955 o
C for 4 h exhibited excellent microwave dielectric properties of εr = 10.2, Q×f = 6
59,300 GHz (10.6 GHz) and τf = -3.7 ppm/oC. To evaluate the chemical compatibility with silver electrode, 20 wt% silver was mixed with 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 and co-fired at 955 oC for 4 h. X-ray diffraction pattern, backscattered electron image (BSE) and energy dispersive spectrometer (EDS) analysis of the co-fired sample are presented in Fig.6. Only the peaks of Ca5Ni4(VO4)6 and Ag (JCPDS No. 01-087-0717) were observed in the XRD, implying that 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 did not react with Ag electrode. Two types of grains with different grain sizes and distinct element contrast can be clearly distinguished in BSE image. EDS analysis revealed that the larger bright grains are pure silver. This further confirms that 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 is chemically compatible with Ag electrodes. 4. Conclusions Ca5Ni4(VO4)6 ceramic with a cubic garnet structure was prepared by a conventional solid-state reaction method. The optimum microwave dielectric properties were obtained in Ca5Ni4(VO4)6 ceramic sintered at 980 oC for 4 h, with a permittivity of 10.9, a Q×f value of 96,500 GHz, and a negative τf of -63.6 ppm/oC. Ca5Ni4(VO4)6 is found to have chemical compatibility with Ag electrodes. The large negative τf of Ca5Ni4(VO4)6 ceramic could be tuned by forming solid-solution with LiCa2Mg2V3O12, and the 0.8Ca5Ni4(VO4)6-0.2LiCa2Mg2V3O12 ceramic sintered at 955 o
C for 4 h exhibited a near-zero τf value of -3.7 ppm/oC along with εr of 10.2 and Q×f
of 59,300 GHz. These merits make Ca5Ni4(VO4)6 a possible candidate for LTCC application. Acknowledgments This research was supported by Natural Science Foundation of China (Nos. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi 7
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.
KY2015YB122).
8
YB2014160,
KY2015YB341,
and
References 1
Y. Higuchi, Y. Sugimoto, J. Harada, and H. Tamura, “LTCC system with new high-εr and high-Q material co-fired with conventional low-εr base material for wireless communications,” J. Eur. Ceram. Soc., 27, 2785–8 (2007).
2
C.R. Chang, J.H. Jean, “Effects of silver-paste formulation on camber development during the cofiring of a silver-based, low-temperature-cofired ceramic package,” J. Am. Ceram. Soc., 81, 2805–14 (1998).
3
M.T. Sebastian and H. Jantunen, “Low loss dielectric materials for LTCC applications: a review,” Int. Mater. Rev., 53, 57–90 (2008).
4
D. Zhou, L.X. Pang, J. Guo, Z.M. Qi, T. Shao, Q.P. Wang, H.D. Xie, X. Yao, and C.A. Randall, “Influence of Ce substitution for Bi in BiVO4 and the impact on the phase evolution and microwave dielectric properties,” Inorg. Chem., 53, 1048–1055 (2014).
5
D. Zhou, C.A. Randall, H. Wang, and X. Yao, “Microwave dielectric properties of Li2WO4 ceramic with ultra-low sintering temperature,” J. Am. Ceram. Soc., 94 348–50 (2011).
6
I.M. Reaney and D. Iddles, “Mcrowave dielectric ceramics for resonators and filters in mobile phone networks,” J. Am. Ceram. Soc., 89, 2063–72(2006).
7
D. Zhou, L.X. Pang, H. Wang, and X. Yao, “Low temperature firing microwave dielectric ceramics (K0.5Ln0.5)MoO4 (Ln = Nd and Sm) with low dielectric loss,” J. Eur. Ceram. Soc., 31, 2749–52 (2011).
8
D.K. Kwon, M.T. Lanagan, and T.R. Shrout, “Microwave dielectric properties and 9
low-temperature cofiring of BaTe4O9 with aluminum metal electrode,” J. Am. Ceram. Soc., 88, 3419–22 (2005). 9
D. Zhou, D. Guo, W.B. Li, L.X. Pang, X. Yao, D.W. Wang, Ian M. Reaney, “Novel temperature stable high-εr microwave dielectrics in the Bi2O3–TiO2–V2O5 system”, J. Mater. Chem. C, 4, 5357–5362 (2016).
10
L. Fang, C.X. Su, H.F. Zhou, Z.H. Wei, and H. Zhang, “Novel low-firing microwave dielectric ceramic LiCa3MgV3O12 with low dielectric loss,” J. Am. Ceram. Soc., 96, [3] 688–690 (2013).
11
H.C. Xiang, L. Fang, X.W. Jiang, Y. Tang, and C.C. Li, A Novel Temperature Stable Microwave Dielectric Ceramic with Garnet Structure: Sr2NaMg2V3O12, J. Am. Ceram. Soc., 99 [2]: 399–401 (2016).
12
S. Geller, “Crystal chemistry of the garnets,” Z. Kristallogr., 125, 1–47 (1967).
13
G. G. Yao, P. Liu, and H. W. Zhang, “Novel series of low-firing microwave dielectric ceramics: Ca5A4(VO4)6 (A+2 = Mg, Zn),” J. Am. Ceram. Soc., 96 [6] 1691–1693 (2013).
14
G. G. Yao, P. Liu, X. G. Zhao, J. P. Zhou, and H. W. Zhang, “Low-temperature sintering and microwave dielectric properties of Ca5Co4(VO4)6 ceramics,” J. Eur. Ceram. Soc., 34, 2983–2987 (2014).
15
B.I. Lazoryak, V.A. Morozov, and A.A. Belik, “Crystal structures and characterization of Ca9Fe(PO4)7 and Ca9FeH0.9(PO4)7,” J. Solid State Chem., 122, 15–21 (1996).
16
Y.T. Lu, L.Y. Chen, Y.L. Huang, H. Cheng, H.J. Seo, “Photocatalytic ability of 10
vanadate garnet Ca5Ni4(VO4)6 under visible-light irradiation,” J. Phys. D: Appl. Phys., DOI: 10.1088/0022-3727/48/30/305107. 17
A.J. Bosman and E.E. Havinga, “Temperature dependence of dielectric constants of cubic ionic compounds,” Phys. Rev., 129, 1593–600 (1963).
18
R.D. Shannon, “Dielectric polarizabilities of ions in oxides and fluorides,” J. Appl. Phys., 73, 348–66 (1993).
19
Eung Soo Kim, Byung Sam Chun, Robert Freer, Robert J. Cernik, “Effects of packing fraction and bond valence on microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics,” J. Eur. Ceram. Soc., 30, 1731–1736 (2010).
11
Table
1
Microwave
Dielectric
Properties
of
(1-x)Ca5Ni4(VO4)6-xLiCa2Mg2V3O12 (0 ≤ x ≤ 0.3) ceramics x
S.T. (oC)
0 0.1 0.2 0.3
980 965 955 945
Lattice parameters (Ǻ) a=b=c 12.3657 12.3790 12.3880 12.3901
εr
Q×f (GHz)
τ f (ppm/ ℃)
10.9 10.5 10.2 10.0
96,500 72,600 59,300 43,520
-63.6 -14.1 -3.7 +16.2
Table 2 Microwave dielectric properties of Ca5M4(VO4)6 (M = Zn, Mg, Co, and Ni) ceramics sintered at each optimized temperature (Reference 14, 15) Relatively Q×f τf Packing M S. T. (°C) εr εtheo o density (%) (GHz) (ppm/ C) fraction (%) Zn 725 96.6 11.7 11.4 49,400 -83 59.8 Mg 800 93.7 9.2 10.2 53,300 -50 59.9 Co 875 95.7 10.1 10.9 95,200 -63 60.4 Ni 980 96.2 10.9 10.8 96,500 -63.6 60.8
12
Figure Captions: Fig.1 XRD patterns of Ca5Ni4(VO4)6 ceramics sintered at different temperatures: (a) 935 oC, (b) 950 oC, (c) 965 oC, (d) 980 oC, and (e) 995 oC. Fig.2 The structure framework of Ca5Ni4(VO4)6 viewed on the ab-plane. Fig.3 The densities and SEM images of polished and thermally etched surface of Ca5Ni4(VO4)6 ceramics sintered at different temperatures: (a) 935 oC, (b) 950 oC, (c) 965 oC, (d) 980 oC, (e) 995 oC, and (f) the densities. Fig.4 The microwave dielectric properties (εr, Q×f, and τf) of Ca5Ni4(VO4)6 ceramics as a function of sintering temperatures. Fig.5 XRD patterns of 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 ceramics sintered at 955 o
C for 4 h in air.
Fig.6 XRD, SEM, and EDS analysis of the co-fired Ca5Ni4(VO4)6 ceramic with 20 wt% silver sintered at 955 oC for 4 h.
13
Fig. 1
14
Fig. 2
15
Fig. 3
16
Fig. 4
17
Fig. 5
18
Fig. 6
19
PII: DOI: Reference:
S0272-8842(16)30918-X http://dx.doi.org/10.1016/j.ceramint.2016.06.085 CERI13101
To appear in: Ceramics International Received date: 18 May 2016 Revised date: 12 June 2016 Accepted date: 13 June 2016 Cite this article as: Dan Wang, Huaicheng Xiang, Ying Tang, Liang Fang, Jibran Khaliq and Chunchun Li, A Low-firing Ca 5Ni4(VO4)6 Ceramic with Tunable Microwave Dielectric Properties and Chemical Compatibility with Ag, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.085 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Low-firing Ca5Ni4(VO4)6 Ceramic with Tunable Microwave Dielectric Properties and Chemical Compatibility with Ag Dan Wang1, Huaicheng Xiang1, Ying Tang1, Liang Fang1, Jibran Khaliq3, Chunchun Li1, 2* 1
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 2
College of Information Science and Engineering, Guilin University of Technology, Guilin, 541004, China
3
Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, Delft 2629 HS, The Netherlands
Abstract A Ca5Ni4(VO4)6 low loss microwave dielectric ceramic with A site deficient garnet structure was prepared via the conventional solid state reaction method. Ca5Ni4(VO4)6 sintered at 980 oC for 4 h to a relative density of 96.2% exhibits favorable microwave dielectric properties such as a permittivity of 10.9, a Q×f value of 96,500 GHz, and a τf value of -63.6 ppm/oC. Its large negative τf could be compensated by forming a solid solution with LiCa2Mg2V3O12, that led to improved properties with a near-zero τf = -3.7 ppm/oC, εr = 10.2, Q×f = 59,300 GHz for 0.8Ca5Ni4(VO4)6-0.2LiCa2Mg2V3O12 after sintering at 955 oC for 4 h. Further on, 0.8Ca5Ni4(VO4)6-0.2LiCa2Mg2V3O12 proved to be chemically compatible with Ag electrodes, so it might be a possible candidate for LTCC applications. Keywords: Microwave dielectric properties; Garnet structure; LTCC; Vanadate
Corresponding Author, [email protected]; [email protected] 1
1. Introduction Microwave dielectric materials have been widely used as dielectric resonators, band pass filters, oscillators and duplexers in communication systems from mobile phones to global positioning system. The more rapidly developing information technology industry requires high integration of circuits and high density of package. Low-temperature co-fired ceramics (LTCC) technology has attracted much attention because of its ability to combine thin layers of ceramic and conducting electrodes to fabricate multilayer LTCC modules.1-5 Advanced substrate materials for LTCC modules should have a low dielectric constant (εr), a high quality factor (Q×f), a near-zero temperature coefficient of resonant frequency (τf), and a low sintering temperature lower than the melting point of Ag (961 oC) or Cu (1080 oC).6-9 Moreover, the chemical compatibility with electrodes must be taken into account. Up to now, a series of garnet vanadates with a general formula A3B2V3O12 were investigated and reported with good microwave dielectric properties. Among them, LiCa3MgV3O12 ceramic sintered at 900 oC has a relative permittivity (εr) ~ 10.5, a quality factor (Q×f) ~ 74,700 GHz and a temperature coefficient of resonant frequency (τf) ~ -61 ppm/oC),10 while the Sr2NaMg2V3O12 ceramic has a εr = 11.7, Q×f = 37,950 GHz and τf = -2.9 ppm/oC when sintered at 900 oC.11 In the garnet structure, three different sites (A, B and C) are available for a wide variety of cations.12 A represents a 8-coordinated position, while B is an octahedral site and C is a tetrahedral site. Moreover, most of the vacancies were developed in the B site and partly in the A site, resulting in the so called cation-deficient garnets. Only a few reports are available for the microwave dielectric properties of the cation-deficient garnet vanadates. Yao et al.13,
14
firstly reported the microwave
dielectric properties of some A-site cation-deficiency garnets in the Ca5M4(VO4)6 (M 2
= Mg, Zn, and Co) system. The crystal structure of Ca5Ni4(VO4)6 was first reported by Lazoryak et al.15 and it belongs to the A-site cation-deficient garnet. Recently, Ca5Ni4(VO4)6 has been investigated for applications in visible-light-driven photocatalyst.16 To the best knowledge of us, however, the microwave dielectric properties of Ca5Ni4(VO4)6 ceramics have not been investigated. In the present work, sintering behavior and microwave dielectric properties of Ca5Ni4(VO4)6 ceramic were studied along with its chemical compatibility with Ag electrodes. 2. Experimental procedure Ca5Ni4(VO4)6 ceramic was prepared by the conventional solid-state reaction method from high-purity (> 99%) powders of CaCO3, NiO, and NH4VO3. The stoichiometric ratios of powders were weighed and ball-milled in alcohol for 6 h using zirconia balls as milling medium. Subsequently, the wet mixture was dried and calcined at 900 oC for 4 h in air. The calcined powder was milled in alcohol medium using zirconia balls for 4 h and after drying, the polyvinyl alcohol (PVA) was added to the powders as binder and then crushed into a fine powder through a sieve with 200 mesh. The obtained powder was pressed into pellets of 10 mm in diameter and 6 mm in height by uniaxial pressing under a pressure of 200 MPa. The pellets were heated at 550 oC for 2h at a heating rate of 1.5 oC/min to remove the PVA and then sintered at 935–995 oC for 4h. The crystal structure and phase composition of the specimens were analyzed with X-ray diffraction (CuKα1, 1.54059 Å, Model X’Pert PRO, PANalytical, Almelo, Holland). The bulk density was measured by the Archimede’s method. The polished and thermally etched surfaces of the samples were observed by scanning electron microscope (Model JSM6380-LV SEM, JEOL, Tokyo, Japan), equipped with dispersive spectrometer (EDS) for elementary analysis. The microwave dielectric 3
properties were measured using a network analyzer (Model N5230A, Agilent Co., Palo Alto, Canada) and a temperature chamber (Delta 9039, Delta Design, San Diego, CA). The temperature coefficient of resonant frequency (τf) was measured in the temperature range of 25–85 oC. 3. Results and discussions XRD patterns of the Ca5Ni4(VO4)6 ceramics sintered at 935–995 oC for 4 h are shown in Fig. 1. Within the limitation of XRD equipment, all the observed diffraction peaks could be indexed according to the JCPDS card No. 00-052-0469 for Ca5Ni4(VO4)6 with space group of Ia-3d. Samples with various sintering temperatures exhibited no phase difference. The lattice parameters were refined as a = 12.3567(15) Å, V = 1890.7(4) Å3 with calculated theoretical density of 3.94 g/cm3 (Z = 4). Fig. 2 shows the projection of the structure along the [001] direction. The structure is constructed by [NiO6] octahedra and [VO4] tetrahedra by corner sharing, and each Ca atoms is surrounded by eight oxygen atoms forming a dodecahedron. Fig. 3(a-e) shows the SEM images of polished and thermally etched surfaces. For the ceramic sintered at 935 oC, a porous microstructure with small grains was observed. A uniform and dense microstructure with closely packed grains in range 5~10μm was developed when sintered at 980 oC (Fig. 3 (d)). However, abnormal grain growth with large grains (~ 15μm) appeared along with some porosity and degradation in grain uniformity in the sample sintered at 995 oC. Fig. 3 (f) presents the variation of relative density and bulk density with the sintering temperature. At 935 oC, the bulk density was 3.30 g/cm3 (relative density of 83.8%). With increasing sintering temperature, the density increased, reached a maximum value of 96.2% at 980 oC, and decreased slightly at 995 oC. Fig. 4 presents the variations of the microwave dielectric properties (εr, Q×f, and 4
τf) of Ca5Ni4(VO4)6 ceramics as a function of the sintering temperature. The change in the relative permittivity and quality factor showed a similar trend to that of the densities. In Fig. 4(a), εr increased from 10.7 to 10.9 as the sintering temperature increased from 935 oC to 980 oC, and then slightly decreased. It is reported that the relative permittivity mainly depends on the composition, grain size and the density. The influence of the porosity on the permittivity could be eliminated by applying Bosman and Havinga’s correction:17 εcorrected = εm(1 + 1.5p)
(1)
where, p is the fractional porosity; εcorrected and εm are the corrected and measured values of permittivity, respectively. The εcorrected is about 11.5 for the sample sintered at 980 oC. Furthermore, εr can be interpreted by the sum of ionic polarizability of individual ions ( DT ) and molar volume (Vm) according to Clausius–Mossotti equation:18
1 2b DT Vm r 1 b DT Vm
(2)
where, b = 4π/3. The calculated theoretical permittivity of Ca5Ni4(VO4)6 is 10.8. The relative error of Ca5Ni4(VO4)6 is about 0.9% for the measured value and 6.0% for the porosity corrected value, suggesting there is no other polarization mechanism in the Ca5Ni4(VO4)6 ceramic at microwave region. A strong dependence of the Q×f on the sintering temperature was observed in Fig.4 (b). As the sintering temperature increased, the Q×f value of Ca5Ni4(VO4)6 ceramic increased to a maximum value of ~ 96,500 GHz sintered at 980 oC. Thereafter, the Q×f value decreased with further increase in sintering temperature. Fig.4 (c) represents the change in τf with increasing sintering temperature. As seen, the τf values varied in the range of -74.1 to -63.6 ppm/oC over the sintering region from 5
935 to 980 oC. Comparisons for the microwave dielectric properties of A-site deficient garnet system Ca5M4(VO4)6 (M = Zn, Mg, Co, and Ni) are listed in Table 2. As shown, obvious differences, especially in quality factor, were observed. Since Ca5M4(VO4)6 ceramics are single phases with relative densities > 95%, the effects of such extrinsic factors as density and secondary phase could be neglected. Kim et al.19 firstly proposed a general relationship between the quality factor and the packing fraction of structure. They pointed out that increase in packing fraction can weaken lattice vibrations, and thereby resulting in the decrease in the intrinsic dielectric loss. The higher Q×f value of Ca5Ni4(VO4)6 might be partly explained by its larger packing fraction(~ 60.8%) compared with other Ca5M4(VO4)6 (M= Zn, Mg, and Co) ceramics as shown in Table 2. The large negative τf value of Ca5Ni4(VO4)6 would impede its practical applications to a large extent. More recently, a low-firing garnet ceramic LiCa2Mg2V3O12 with a large positive τf ~ +259.2 ppm/oC was characterized (this work will be published elsewhere). Therefore, LiCa2Mg2V3O12 was chosen to tune the τf of Ca5Ni4(VO4)6 ceramic. A series of (1-x)Ca5Ni4(VO4)6–xLiCa2Mg2V3O12 (0 ≤ x ≤ 0.3) ceramics
were
prepared.
Fig.5
shows
XRD
pattern
of
the
0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 ceramic sintered at 955 oC/4h. As shown, all the peaks could be well indexed according to the cubic garnet structure. The microwave dielectric properties of (1-x)Ca5Ni4(VO4)6–xLiCa2Mg2V3O12 (0 ≤ x ≤ 0.3) ceramics sintered at their optimum temperatures are listed in Table 1. A near-zero τf could be achieved at x = 0.2. The 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 ceramics fired at 955 o
C for 4 h exhibited excellent microwave dielectric properties of εr = 10.2, Q×f = 6
59,300 GHz (10.6 GHz) and τf = -3.7 ppm/oC. To evaluate the chemical compatibility with silver electrode, 20 wt% silver was mixed with 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 and co-fired at 955 oC for 4 h. X-ray diffraction pattern, backscattered electron image (BSE) and energy dispersive spectrometer (EDS) analysis of the co-fired sample are presented in Fig.6. Only the peaks of Ca5Ni4(VO4)6 and Ag (JCPDS No. 01-087-0717) were observed in the XRD, implying that 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 did not react with Ag electrode. Two types of grains with different grain sizes and distinct element contrast can be clearly distinguished in BSE image. EDS analysis revealed that the larger bright grains are pure silver. This further confirms that 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 is chemically compatible with Ag electrodes. 4. Conclusions Ca5Ni4(VO4)6 ceramic with a cubic garnet structure was prepared by a conventional solid-state reaction method. The optimum microwave dielectric properties were obtained in Ca5Ni4(VO4)6 ceramic sintered at 980 oC for 4 h, with a permittivity of 10.9, a Q×f value of 96,500 GHz, and a negative τf of -63.6 ppm/oC. Ca5Ni4(VO4)6 is found to have chemical compatibility with Ag electrodes. The large negative τf of Ca5Ni4(VO4)6 ceramic could be tuned by forming solid-solution with LiCa2Mg2V3O12, and the 0.8Ca5Ni4(VO4)6-0.2LiCa2Mg2V3O12 ceramic sintered at 955 o
C for 4 h exhibited a near-zero τf value of -3.7 ppm/oC along with εr of 10.2 and Q×f
of 59,300 GHz. These merits make Ca5Ni4(VO4)6 a possible candidate for LTCC application. Acknowledgments This research was supported by Natural Science Foundation of China (Nos. 21261007, 21561008, and 51502047), the Natural Science Foundation of Guangxi 7
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.
KY2015YB122).
8
YB2014160,
KY2015YB341,
and
References 1
Y. Higuchi, Y. Sugimoto, J. Harada, and H. Tamura, “LTCC system with new high-εr and high-Q material co-fired with conventional low-εr base material for wireless communications,” J. Eur. Ceram. Soc., 27, 2785–8 (2007).
2
C.R. Chang, J.H. Jean, “Effects of silver-paste formulation on camber development during the cofiring of a silver-based, low-temperature-cofired ceramic package,” J. Am. Ceram. Soc., 81, 2805–14 (1998).
3
M.T. Sebastian and H. Jantunen, “Low loss dielectric materials for LTCC applications: a review,” Int. Mater. Rev., 53, 57–90 (2008).
4
D. Zhou, L.X. Pang, J. Guo, Z.M. Qi, T. Shao, Q.P. Wang, H.D. Xie, X. Yao, and C.A. Randall, “Influence of Ce substitution for Bi in BiVO4 and the impact on the phase evolution and microwave dielectric properties,” Inorg. Chem., 53, 1048–1055 (2014).
5
D. Zhou, C.A. Randall, H. Wang, and X. Yao, “Microwave dielectric properties of Li2WO4 ceramic with ultra-low sintering temperature,” J. Am. Ceram. Soc., 94 348–50 (2011).
6
I.M. Reaney and D. Iddles, “Mcrowave dielectric ceramics for resonators and filters in mobile phone networks,” J. Am. Ceram. Soc., 89, 2063–72(2006).
7
D. Zhou, L.X. Pang, H. Wang, and X. Yao, “Low temperature firing microwave dielectric ceramics (K0.5Ln0.5)MoO4 (Ln = Nd and Sm) with low dielectric loss,” J. Eur. Ceram. Soc., 31, 2749–52 (2011).
8
D.K. Kwon, M.T. Lanagan, and T.R. Shrout, “Microwave dielectric properties and 9
low-temperature cofiring of BaTe4O9 with aluminum metal electrode,” J. Am. Ceram. Soc., 88, 3419–22 (2005). 9
D. Zhou, D. Guo, W.B. Li, L.X. Pang, X. Yao, D.W. Wang, Ian M. Reaney, “Novel temperature stable high-εr microwave dielectrics in the Bi2O3–TiO2–V2O5 system”, J. Mater. Chem. C, 4, 5357–5362 (2016).
10
L. Fang, C.X. Su, H.F. Zhou, Z.H. Wei, and H. Zhang, “Novel low-firing microwave dielectric ceramic LiCa3MgV3O12 with low dielectric loss,” J. Am. Ceram. Soc., 96, [3] 688–690 (2013).
11
H.C. Xiang, L. Fang, X.W. Jiang, Y. Tang, and C.C. Li, A Novel Temperature Stable Microwave Dielectric Ceramic with Garnet Structure: Sr2NaMg2V3O12, J. Am. Ceram. Soc., 99 [2]: 399–401 (2016).
12
S. Geller, “Crystal chemistry of the garnets,” Z. Kristallogr., 125, 1–47 (1967).
13
G. G. Yao, P. Liu, and H. W. Zhang, “Novel series of low-firing microwave dielectric ceramics: Ca5A4(VO4)6 (A+2 = Mg, Zn),” J. Am. Ceram. Soc., 96 [6] 1691–1693 (2013).
14
G. G. Yao, P. Liu, X. G. Zhao, J. P. Zhou, and H. W. Zhang, “Low-temperature sintering and microwave dielectric properties of Ca5Co4(VO4)6 ceramics,” J. Eur. Ceram. Soc., 34, 2983–2987 (2014).
15
B.I. Lazoryak, V.A. Morozov, and A.A. Belik, “Crystal structures and characterization of Ca9Fe(PO4)7 and Ca9FeH0.9(PO4)7,” J. Solid State Chem., 122, 15–21 (1996).
16
Y.T. Lu, L.Y. Chen, Y.L. Huang, H. Cheng, H.J. Seo, “Photocatalytic ability of 10
vanadate garnet Ca5Ni4(VO4)6 under visible-light irradiation,” J. Phys. D: Appl. Phys., DOI: 10.1088/0022-3727/48/30/305107. 17
A.J. Bosman and E.E. Havinga, “Temperature dependence of dielectric constants of cubic ionic compounds,” Phys. Rev., 129, 1593–600 (1963).
18
R.D. Shannon, “Dielectric polarizabilities of ions in oxides and fluorides,” J. Appl. Phys., 73, 348–66 (1993).
19
Eung Soo Kim, Byung Sam Chun, Robert Freer, Robert J. Cernik, “Effects of packing fraction and bond valence on microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics,” J. Eur. Ceram. Soc., 30, 1731–1736 (2010).
11
Table
1
Microwave
Dielectric
Properties
of
(1-x)Ca5Ni4(VO4)6-xLiCa2Mg2V3O12 (0 ≤ x ≤ 0.3) ceramics x
S.T. (oC)
0 0.1 0.2 0.3
980 965 955 945
Lattice parameters (Ǻ) a=b=c 12.3657 12.3790 12.3880 12.3901
εr
Q×f (GHz)
τ f (ppm/ ℃)
10.9 10.5 10.2 10.0
96,500 72,600 59,300 43,520
-63.6 -14.1 -3.7 +16.2
Table 2 Microwave dielectric properties of Ca5M4(VO4)6 (M = Zn, Mg, Co, and Ni) ceramics sintered at each optimized temperature (Reference 14, 15) Relatively Q×f τf Packing M S. T. (°C) εr εtheo o density (%) (GHz) (ppm/ C) fraction (%) Zn 725 96.6 11.7 11.4 49,400 -83 59.8 Mg 800 93.7 9.2 10.2 53,300 -50 59.9 Co 875 95.7 10.1 10.9 95,200 -63 60.4 Ni 980 96.2 10.9 10.8 96,500 -63.6 60.8
12
Figure Captions: Fig.1 XRD patterns of Ca5Ni4(VO4)6 ceramics sintered at different temperatures: (a) 935 oC, (b) 950 oC, (c) 965 oC, (d) 980 oC, and (e) 995 oC. Fig.2 The structure framework of Ca5Ni4(VO4)6 viewed on the ab-plane. Fig.3 The densities and SEM images of polished and thermally etched surface of Ca5Ni4(VO4)6 ceramics sintered at different temperatures: (a) 935 oC, (b) 950 oC, (c) 965 oC, (d) 980 oC, (e) 995 oC, and (f) the densities. Fig.4 The microwave dielectric properties (εr, Q×f, and τf) of Ca5Ni4(VO4)6 ceramics as a function of sintering temperatures. Fig.5 XRD patterns of 0.8Ca5Ni4(VO4)6–0.2LiCa2Mg2V3O12 ceramics sintered at 955 o
C for 4 h in air.
Fig.6 XRD, SEM, and EDS analysis of the co-fired Ca5Ni4(VO4)6 ceramic with 20 wt% silver sintered at 955 oC for 4 h.
13
Fig. 1
14
Fig. 2
15
Fig. 3
16
Fig. 4
17
Fig. 5
18
Fig. 6
19