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Effect of glass fluxing on densification and microwave dielectric properties of LiInSiO4 ceramic
Journal of Alloys and Compounds 552 (2013) 83–87
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of glass fluxing on densification and microwave dielectric properties of LiInSiO4 ceramic Annrose Sunny, Kokken Anlin Lazer, Kurusaroor Mana Manu, Kuzhichalil Peethambharan Surendran, Mailadil Thomas Sebastian ⇑ Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvananthapuram 695019, India
a r t i c l e
i n f o
Article history: Received 29 June 2012 Received in revised form 10 October 2012 Accepted 11 October 2012 Available online 23 October 2012 Keywords: Ceramics Sintering Solid state reaction Grain boundaries
a b s t r a c t A novel microwave substrate ceramic LiInSiO4 (LIS) was synthesized by solid-state ceramic route. LIS ceramic sintered at 1150 °C for 4 h showed a densification of 91% with er = 8.2, Qu f = 12,600 GHz and sf = 55 ppm/°C. Its thermal conductivity at room temperature (Tc) was found to be 6.0 Wm1 K1 and coefficient of thermal expansion (CTE) was 6.0 ppm/°C. For improving the relative density and dielectric properties of LIS ceramic, B2O3 and Lithium Magnesium Zinc Borosilicate (LMZBS) glasses were added. LIS + 1 wt.% LMZBS glass sintered at 1100 °C for 4 h showed a relatively good densification of 95% with moderate dielectric properties (er = 8.4, Qu f = 22,000 GHz, sf = 45 ppm/°C). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent explosive growth of microwave integrated circuits (MICs) has increased the demand of low loss ceramic materials which find a wide range of applications especially in cell phones, ultra high speed WLAN, optical communications, global positioning systems, antennas, packages, substrates, capacitors, filters etc. [1–4]. The relative permittivity (er) of the ceramic has a major role in determining the function of a ceramic in a microwave communication system. For the ceramic to be used as a substrate, it should have a low value of relative permittivity (er < 15) for faster signal transmission [4,5]. In microwave circuits, the substrate acts as a guided medium for the signal propagation. Hence the dielectric loss (tand) (equivalently high quality factor (Qu f)) of the substrate must be low enough to reduce the signal attenuation [4]. Several semiconductor components are attached to the substrate and a large mismatch in their thermal expansion characteristics may lead to the de-lamination of the components. The commonly used semiconductors in microwave devices are Si, GaAs which have coefficient of thermal expansions 4.2 and 6.5 ppm/°C, respectively [6]. Hence the ceramic substrates with coefficient of thermal expansion matching with these values are required for microwave applications. High thermal conductivity is also desirable to conduct and dissipate the excess heat generated in microwave circuits [4].
⇑ Corresponding author. Tel.: +91 4712515294. E-mail address: [email protected] (M.T. Sebastian). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.064
Several dielectric ceramics meeting these requirements have been developed so far [7,8]. In search of new high performance substrates, phosphates and silicates are proposed to be promising candidates because of their low relative permittivity and dielectric loss [9–11]. The silicates such as magnesium silicate (MgSiO3), forsterite (Mg2SiO4), zinc silicate (Zn2SiO4), wollastonite (CaSiO3), and rare earth based silicates are reported to have low relative permittivities and high quality factors [8,11–14]. In general, most silicates show low relative permittivity due to the covalent nature of Si–O bond in SiO4 tetrahedra [9]. Song et al. reported that Mg2SiO4 have a strong processing sensitivity [15]. Zn2SiO4 is another useful candidate which can be easily synthesized through solid state ceramic route [16]. Ternary silicates such as Li2MgSiO4, Sr2ZnSi2O7 reported in the literature also showed good microwave dielectric properties [17]. However, investigations on the development of new high performance silicates are still continuing for substrate application. Compounds having general formula LiInX (X = SiO4, GeO4, PO4, etc.) belongs to the olivine type structure [18]. The structural and optical studies of LIS ceramic are well documented [18,19]. The microwave dielectric property of a ceramic material is extrinsically related to its relative density [20] and therefore electronic application requires materials having high densification. Some silicate based ceramics are reported to have relatively low densification [21]. Tailoring the densification is important in this context. In ceramics, liquid phase sintering is the best method for the enhancement of densification. Recently Li et al. reported that calcium borosilicate glass improved the densification of (ZnMg)2
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A. Sunny et al. / Journal of Alloys and Compounds 552 (2013) 83–87
SiO4 via Sol–gel method [22,23]. The present paper, report the microwave dielectric properties of novel LiInSiO4 ceramic and tailoring its properties by the addition of B2O3 and LMZBS glasses. 2. Experimental procedures The stoichiometric amount of Li2CO3, In2O3 and SiO2 were taken as the precursors for the preparation of LiInSiO4 (LIS) and was prepared through the solid state ceramic route. The following equation represents the chemical equation for the preparation of LiInSiO4 (LIS) ceramic.
1 mol Li2 CO3 þ 1 mol In2 O3 þ 2 mol SiO2 ! 2 mol LiInSiO4 þ 1 mol CO2 High purity powders of Li2CO3 (99.9%, Sigma–Aldrich, Inc., Milwaukee, WI, USA), In2O3 (99.5% HPLC, Bombay), and SiO2 (99.6% Sigma–Aldrich, Inc., St. Louis, MO 63103, USA) were weighed and ball milled using zirconia balls in ethanol for 24 h. The ceramic powders were calcined at 950 °C for 4 h. Different weight percentage of (20Li2O–20MgO–20ZnO–20B2O3–20SiO2, all from Sigma–Aldrich Chemical Company, 99%) LMZBS and B2O3 glasses were added to the fine powder of calcined LIS, 4 wt.% Poly Vinyl Alcohol (PVA) (molecular weight 22,000, BDH lab Suppliers, Poole, UK) solution was then added to it and mixed. It was then dried, ground well and pressed into cylindrical disks of about 11 mm diameter and 3–4 mm thickness under an uniaxial stress of about 100 M Pa. The compacts were muffled by LIS powder of the same composition during sintering, in order to prevent the loss of volatile lithium at elevated temperatures [24]. The sintering temperature of LIS ceramic was optimized in the range 1050–1150 °C for 4 h. The sintered powdered samples were used to analyze the crystalline structure and phase purity by the X-ray diffraction technique using Ni filtered Cu Ka radiation (PAN analytical X0 Pert PRO Diffractometer, Netherlands). The microstructures of the sintered samples were studied using scanning electron microscope (JEOL-SEM 5601v, Tokyo, Japan). The sintered densities of the specimens were measured by the Archimedes method. The microwave dielectric properties were measured in the frequency range of 6–14 GHz using a vector network analyzer (E5071C ENA series Network Analyzer, Agilent Technologies, USA). The relative permittivity of the material was measured using Hakki–Coleman technique modified by Courtney. The TE011 mode was used for this measurement [25]. The unloaded quality factor (Qu) was measured using TE01º mode by the cavity method [26]. The temperature variation of resonant frequency (sf) was measured by noting the temperature at regular intervals in the course of heating and was calculated using the equation [4].
1 f
sf ¼
Df DT
ð1Þ
where f is the resonant frequency at room temperature, Df is the variation in resonant frequency and Df is the corresponding difference in temperatures. The linear coefficient of thermal expansion (CTE) of the LIS ceramic sintered at 1150 °C was measured using dilatometer (DIL 402 PC, NETZSCH, Selb, Germany) in the temperature range 30–500 °C and the CTE was calculated using the equation [27].
aL ¼
1 dL L dT
ð2Þ
(aL is the linear coefficient of thermal expansion (CTE), L is the length of the dL material, dT is the fractional change in length per unit change in temperature). The thermal conductivity (Tc) was measured in the temperature range 25–200 °C using a laser thermal properties analyzer (Flash Line 2000, Anter Corporation, Pittsburgh, USA). For thermal conductivity (Tc) measurement, the specific heat capacity of reference sample (alumina) was taken from the literature [28] and the Tc was calculated using the equation [27]
T c ¼ aqC p
Fig. 1. Variation of relative density and relative permittivity as a function of sintering temperature of LiInSiO4 (LIS) ceramic.
The crystal structure of LiInSiO4 (lithium indium silicate) is isotopic with olivine Mg2SiO4 (forsterite) which is a well known low loss dielectric. The principal differences between lithium indium silicate and forsterite are found in the bond lengths and bond angles opposite to common edges between the tetrahedron and the Li+ and In3+ ion sites. The distorted tetrahedron shares one common edge with the Li+ site and two common edges with the In3+ site [18]. In 2003, Redhammer and Roth reported that LiInSiO4 crystallizes in orthorhombic structure with Pnma [No. 62] space group, with unit cell parameters a = 4.845 Å, b = 10.504 Å and c = 6.063 Å. Fig. 2(a) shows the X-ray diffraction (XRD) pattern of LiInSiO4 (LIS) ceramic sintered at 1150 °C for 4 h. All the recognizable peaks were indexed based on the ICDD File No. 34-0067. The lattice parameters a = 10.478 Å, b = 6.063 Å, and c = 4.849 Å were calculated [29], which are in good agreement with the previously reported values [18]. Fig. 2(b) and (c) are the XRD patterns of LIS + 1 wt.% LMZBS glass and LIS + 1 wt.% B2O3 glass sintered at 1100 °C for 4 h, respectively. It is observed that there are no secondary phases with the addition of LMZBS and B2O3 glass up to 1.5 wt.%. The microwave dielectric properties of a ceramic have direct relation to its microstructure [4]. Fig. 3(a)–(c) represent the scanning electron micrographs of thermally etched LIS ceramic,
ð3Þ
where Cp is the specific heat capacity, a is the thermal diffusivity, q is the density of the material.
3. Results and discussion Fig. 1 shows the variation of relative density and relative permittivity as a function of sintering temperature of LIS ceramic. The relative density is found to be increasing with increase in sintering temperature and reaches a maximum value at 1150 °C for 4 h. Further increase of sintering temperature resulted in melting of the sample. The relative permittivity of LIS ceramic varies in a similar manner as that of relative density. The lower value of relative permittivity at low temperatures may be due to the low densification and porosity of LIS ceramic [4]. The sintering temperature optimization of all glass added compositions of LIS ceramic were done, to obtain highest density and best dielectric properties.
Fig. 2. X-ray diffraction pattern of (a) LiInSiO4 (LIS) ceramic sintered at 1150 °C/4 h, (b) LIS + 1 wt.% LMZBS glass sintered at1100 °C/4 h and (c) LIS + 1 wt.% B2O3 glass sintered at 1100 °C/4 h.
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Fig. 3. SEM images of (a) LIS sintered at 1150 °C/4 h and (b) LIS + 1 wt.% B2O3 sintered at 1100 °C (c)LIS + 1 wt.% LMZBS sintered at 1100 °C/4 h.
LIS + 1 wt.% B2O3 glass and LIS + 1 wt.% LMZBS glass at their optimized sintering temperatures. Microstructure with variable grain size distribution along with pores is a clear indication of low relative density of pure LIS ceramic compared to LIS + 1 wt.% LMZBS glass added composition. The relative density of a ceramic is directly related to its microwave dielectric properties [30]. The presence of porosity in ceramic materials decreases its dielectric performance due to the entrapment of moisture. Hence the improvement of densification by the addition of suitable amount of sintering aids is necessary for better performance. In the present study, we have employed two glasses B2O3 and LMZBS with the objective of improving the sinterability of LIS ceramic. Fig. 4(a) and (b) shows the variation of relative density and relative permittivity for LIS ceramic with different wt.% of B2O3 and LMZBS glasses. In glass added ceramic materials, the effectiveness of sintering aids depend on several factors such as sintering condition, viscosity, sinterability and glass wettability [4,31]. It is clear from Fig. 4(a) that the densification is found to be 94% with the addition of 1 wt.% B2O3 glass at 1100 °C/4 h. In an earlier report [32] on the effect of B2O3 glass on low loss Nd (Co1/2Ti1/2) O3 ceramics. It was demonstrated that the glassy liquid phase at the grain boundary effectively eliminate the pores and
thereby increases the relative density for smaller volume fraction of B2O3 [33,34]. Further addition of glass above 1 wt.% reduces the relative density, which may be due to the trapped porosity associated with grain growth and formation of pores by the evaporation of excess glass components [21]. Fig. 4(b) shows similar effect and LIS + 1 wt.% LMZBS glass sintered at 1100 °C/4 h gives a densification of 95%. The LMZBS glass added compositions of LIS ceramic show same trend as B2O3 glass added composite. The variations of relative permittivity with different concentration of (B2O3 and LMZBS) glass are also shown in Fig. 4(a) and (b) respectively. The permittivity values are in the range of 8–8.4 for both LISB2O3 and LIS-LMZBS ceramic glass composites. Moreover the variation of relative permittivity shows similar trend as that of densification. Various intrinsic and extrinsic factors affect the microwave dielectric properties of a ceramic. The intrinsic factors are mainly caused by the lattice vibration modes, while the extrinsic factors are dominated by the defects in ceramics, secondary phases, grain size, and porosity [35]. The variation of Qu f and sf as a function of different weight percentage of B2O3 and LMZBS glass are shown in Fig. 5(a) and (b). The Qu f value increases with amount of B2O3 and LMZBS glasses. In the case of LIS + 1 wt.% B2O3 ceramic-glass
Fig. 4. (a) Variation of relative permittivity and relative density of LiInSiO4 (LIS) ceramic as a function of B2O3 (wt.%) glass and (b) variation of relative permittivity and relative density of LiInSiO4 (LIS) ceramic as a function of LMZBS (wt.%) glass.
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Fig. 5. (a) Variation of Qu f and sf of LIS ceramic at different wt.% of B2O3 glass and (b) variation of Qu f and sf of LIS ceramic at different wt.% of LMZBS glass.
composite, the maximum Qu f value obtained is 16,700 GHz sintered at 1100 °C for 4 h. It is believed that any effort to improve densification would eventually lead to an increase in the quality factor, since the moisture, volatile impurities and secondary phases associated with porosity eventually aggravate loss factor [32,36]. The improvement in densification for 1 wt.% B2O3 glass is reflected from its increase in quality factor. Further increase in wt.% of B2O3 glass decreases the Qu f value. This may be due to the porosity associated with grain growth and the evaporation of excess glass components [11,37]. The relatively high dielectric loss of glass also affects the Qu f value at higher glass fluxing [11]. Fig. 5(b), the amount of LMZBS glass with Qu f also shows a similar trend. The LMZBS glass added composition shows relatively better dielectric properties [38]. For example among the samples investigated in this study, 1 wt.% LMZBS glass added lithium indium silicate sintered at 1100 °C for 4 h shows best densification (95%) and dielectric properties (er = 8.4, Qu f = 22,000 GHz). Earlier investigations on glass added ceramics revealed that multicomponent boron rich glasses are effective in liquid phase sintering without much degrading the microwave dielectric properties of a ceramic [11,38], which could be the reason for better Qu f value of LMZBS glass added composite. The temperature variation of resonant frequencies of all the compositions is also shown in Fig. 5(a) and (b). It is well known that the sf value depends on the composition, additives and second phase of the material [32]. Moreover the sf value of all ceramic glass composites is in the range 30 ppm/°C to 65 ppm/°C. The substrate material that facilitate connection between the silicon based IC chip and board should have lower coefficient of thermal expansion (CTE), since CTE much higher than that of silicon not only leads to warpage and residual stress but also contributes the substrate cracking and delamination problems. Generally silicates show relatively low CTE value, this is due to the adjustment of bond angles by the absorption of vibrational energy in the transverse vibration modes [27]. Fig. 6 shows the variation of fractional change in length with temperature of LIS and LIS + 1 wt.% LMZBS ceramic glass composite. The average CTE value of LIS ceramic is 6.0 ppm/°C, LIS + 1 wt.% LMZBS is 5.0 ppm/°C. The CTE values are found to be relatively low and this value is comparable to the CTE of alumina, mullite and several other well investigated ceramic substrates [27]. Heat dissipation is a major problem encountered by ceramic materials in microelectronic industry. Fig. 7 shows the temperature dependence of thermal conductivity of LIS ceramic. The thermal conductivity gradually decreases with increase of temperature. In ceramic materials the high thermal conductivity at low temperature corresponds to an increased value of mean free path resulting from the lower amplitude and greater harmonicity
Fig. 6. Variation of fractional change in length with temperature of (a) LiInSiO4 (LIS) ceramic and (b) LIS + 1 wt.% LMZBS glass composite.
Fig. 7. Thermal Conductivity variation of LiInSiO4 (LIS) ceramic as a function of temperature.
of lattice vibrations. At high temperatures, the mean free path decreases or it is fixed by the inter-atomic separation and by the porosity [27]. In the present investigation, the thermal conductivity
A. Sunny et al. / Journal of Alloys and Compounds 552 (2013) 83–87
of LIS ceramic is found to be in the range (3–6) Wm1 K1 which is comparable with the thermal conductivity of well known substrate materials such as forsterite, mullite, zircon etc. [27]. In conclusion the thermal and dielectric properties of LiInSiO4 are good enough to be qualified as a novel substrate. 4. Conclusions The LIS sintered at 1150 °C for 4 h showed a densification of 91% with er = 8.2, Qu f = 12,600 GHz and sf = -55 ppm/°C, thermal conductivity at room temperature (Tc) 6.0 Wm1 K1 and coefficient of thermal expansion (CTE) 6.0 ppm/°C. The effects of LMZBS and B2O3 glass on the densification and dielectric properties of LiInSiO4 (LIS) for substrate application were studied. The unloaded quality factor and relative density of LIS ceramic were effectively promoted by the addition of LMZBS glass which acts as a good sintering aid. LIS + 1 wt.% LMZBS glass sintered at 1100 °C for 4 h showed a relatively good densification of 95% with moderate dielectric properties (er = 8.4, Qu f = 22,000 GHz, sf = 45 ppm/°C). The physical and microwave dielectric properties make this composite a possible candidate for microwave substrate application. Acknowledgements The authors acknowledge the financial support from Ministry of Human Resource Development, Government of India, New Delhi. Dr. Prabhakar Rao and M.R. Chandran are also thankfully acknowledged for extending XRD and SEM facilities. References [1] [2] [3] [4]
N.K. Singh, P. Kumar, Adv. Mat. Lett. 2 (2011) 239–244. I.M. Reaney, D. Iddles, J. Am. Ceram. Soc. 89 (2006) 2063–2072. N.K. Singh, P. Kumar, R. Rai, Adv. Mater. Lett. 2 (2011) 200–205. M.T. Sebastian, Dielectric Materials for Wireless Communication, Elsevier, Oxford, UK, 2008.
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[5] T. Kolodiazhnyi, J. Eur. Ceram. Soc. 32 (2012) 4305–4309. [6] M.A. Occhionero, R.A. Hay, R.W. Adams, K.P. Fennessy, Ceramic Process Systems Corp., 1999. [7] Y. Hu, D. Wei, Q. Fu, J. Zhao, D. Zhou, J. Eur. Ceram. Soc. 32 (2012) 521–524. [8] T.S. Sasikala, M.N. Suma, P. Mohanan, C. Pavithran, M.T. Sebastian, J. Alloys Comp. 461 (2008) 555–559. [9] L. Pauling, Am. Mineral. 65 (1980) 321–323. [10] D. Thomas, K.T. Rethika, M.T. Sebastian, J. Mater. Sci.: Mater. Electron. 23 (2012) 1268–1271. [11] S. George, P.S. Anjana, V. Deepu, P. Mohanan, M.T. Sebastian, J. Am. Ceram. Soc. 92 (2009) 1244–1249. [12] H.P. Wang, Q.L. Zhang, H. Yang, H.P. Sun, Ceram. Int. 34 (2008) 1405–1408. [13] S.P. Wu, D.F. Chen, Y.X. Mei, Q. Ma, J. Alloys Comp. 521 (2012) 8–11. [14] K.X. Song, X.M. Chen, J. Mater. Lett. 62 (2008) 520–522. [15] K.X. Song, X.M. Chen, X.C. Fan, J. Am. Ceram. Soc. 90 (2007) 1808–1811. [16] N.H. Nguyen, J.B. Lim, S. Nahm, J. Am. Ceram. Soc. 90 (2007) 3127–3130. [17] T. Joseph, M.T. Sebastian, Int. J. Appl. Ceram. Technol. 8 (2011) 854–864. [18] G.J. Redhammer, G. Roth, Acta Crystallogr. Sect. C: Cryst. Struct. Commun. C59 (2003) i38–i40. [19] Y. Zhuang, J. Luo, Y. Teng, Y. Song, B. Zhu, Q. Jianrong, J. Mater. Sci. 25 (2010) 224–228. [20] M.T. Sebastian, H. Jantunen, Int. Mater. Rev. 53 (2008) 57–90. [21] K.M. Manu, P.S. Anjana, M.T. Sebastian, J. Mater. Lett. 65 (2011) 565–567. [22] B. Li, B. Tang, S.R. Zhang, H.M. Jiang, Ceram. Silikathy 55 (2011) 14–19. [23] Y.W. Kim, K.Y. Lim, K.J. Kim, J. Eur. Ceram. Soc. 32 (2012) 4427–4434. [24] S. George, M.T. Sebastian, J. Alloys Comp. 473 (2009) 336–340. [25] B.W. Hakki, P.D. Coleman, IEEE Trans. Microw. Theory Tech. 8 (1960) 402–410. [26] J. Krupka, Meas. Sci. Technol. 17 (2006) R55–R70. [27] R.C. Buchanan, Ceramic Materials for Electronics, Marcel Dekker, Inc., New York, 2004. [28] T.B. Douglas, R.E. McCoskey, D.C. Ginnings, J. Res. Nat. Bur. Stand. 57 (1956) 67–82. [29] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley Publishing Company, USA, 1978. [30] S.J. Penn, N.M. Alford, J. Appl. Phys. 80 (1996) 5895–5898. [31] C.L. Huang, S.H. Lin, Y.B. Chen, J. Alloys Comp. 489 (2010) 719–721. [32] C.F. Tseng, C.L. Huang, Mater. Res. Bull. 42 (2007) 9–16. [33] C.L. Huang, C.L. Pan, S.J. Shium, Mater. Chem. Phys. 78 (2003) 111–115. [34] W.S. Xia, L.X. Li, P.F. Ning, Q.W. Liao, J. Am. Ceram. Soc. 95 (2012) 2587–2592. [35] S. George, M.T. Sebastian, Int. J. Appl. Ceram. Technol. 8 (2011) 1400–1407. [36] K.H. Lin, C.L. Liao, S.T. Lin, Ceram. Int. 36 (2010) 2365–2374. [37] C.L. Huang, C.L. Pan, W.C. Lee, J. Alloys Comp. 462 (2008) L5–L8. [38] D.X. Zhou, F. Sun, Y.X. Hu, Q.Y. Fu, G. Dou, J. Mater. Sci.: Mater. Electron. 23 (2012) 981–989.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of glass fluxing on densification and microwave dielectric properties of LiInSiO4 ceramic Annrose Sunny, Kokken Anlin Lazer, Kurusaroor Mana Manu, Kuzhichalil Peethambharan Surendran, Mailadil Thomas Sebastian ⇑ Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvananthapuram 695019, India
a r t i c l e
i n f o
Article history: Received 29 June 2012 Received in revised form 10 October 2012 Accepted 11 October 2012 Available online 23 October 2012 Keywords: Ceramics Sintering Solid state reaction Grain boundaries
a b s t r a c t A novel microwave substrate ceramic LiInSiO4 (LIS) was synthesized by solid-state ceramic route. LIS ceramic sintered at 1150 °C for 4 h showed a densification of 91% with er = 8.2, Qu f = 12,600 GHz and sf = 55 ppm/°C. Its thermal conductivity at room temperature (Tc) was found to be 6.0 Wm1 K1 and coefficient of thermal expansion (CTE) was 6.0 ppm/°C. For improving the relative density and dielectric properties of LIS ceramic, B2O3 and Lithium Magnesium Zinc Borosilicate (LMZBS) glasses were added. LIS + 1 wt.% LMZBS glass sintered at 1100 °C for 4 h showed a relatively good densification of 95% with moderate dielectric properties (er = 8.4, Qu f = 22,000 GHz, sf = 45 ppm/°C). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent explosive growth of microwave integrated circuits (MICs) has increased the demand of low loss ceramic materials which find a wide range of applications especially in cell phones, ultra high speed WLAN, optical communications, global positioning systems, antennas, packages, substrates, capacitors, filters etc. [1–4]. The relative permittivity (er) of the ceramic has a major role in determining the function of a ceramic in a microwave communication system. For the ceramic to be used as a substrate, it should have a low value of relative permittivity (er < 15) for faster signal transmission [4,5]. In microwave circuits, the substrate acts as a guided medium for the signal propagation. Hence the dielectric loss (tand) (equivalently high quality factor (Qu f)) of the substrate must be low enough to reduce the signal attenuation [4]. Several semiconductor components are attached to the substrate and a large mismatch in their thermal expansion characteristics may lead to the de-lamination of the components. The commonly used semiconductors in microwave devices are Si, GaAs which have coefficient of thermal expansions 4.2 and 6.5 ppm/°C, respectively [6]. Hence the ceramic substrates with coefficient of thermal expansion matching with these values are required for microwave applications. High thermal conductivity is also desirable to conduct and dissipate the excess heat generated in microwave circuits [4].
⇑ Corresponding author. Tel.: +91 4712515294. E-mail address: [email protected] (M.T. Sebastian). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.064
Several dielectric ceramics meeting these requirements have been developed so far [7,8]. In search of new high performance substrates, phosphates and silicates are proposed to be promising candidates because of their low relative permittivity and dielectric loss [9–11]. The silicates such as magnesium silicate (MgSiO3), forsterite (Mg2SiO4), zinc silicate (Zn2SiO4), wollastonite (CaSiO3), and rare earth based silicates are reported to have low relative permittivities and high quality factors [8,11–14]. In general, most silicates show low relative permittivity due to the covalent nature of Si–O bond in SiO4 tetrahedra [9]. Song et al. reported that Mg2SiO4 have a strong processing sensitivity [15]. Zn2SiO4 is another useful candidate which can be easily synthesized through solid state ceramic route [16]. Ternary silicates such as Li2MgSiO4, Sr2ZnSi2O7 reported in the literature also showed good microwave dielectric properties [17]. However, investigations on the development of new high performance silicates are still continuing for substrate application. Compounds having general formula LiInX (X = SiO4, GeO4, PO4, etc.) belongs to the olivine type structure [18]. The structural and optical studies of LIS ceramic are well documented [18,19]. The microwave dielectric property of a ceramic material is extrinsically related to its relative density [20] and therefore electronic application requires materials having high densification. Some silicate based ceramics are reported to have relatively low densification [21]. Tailoring the densification is important in this context. In ceramics, liquid phase sintering is the best method for the enhancement of densification. Recently Li et al. reported that calcium borosilicate glass improved the densification of (ZnMg)2
84
A. Sunny et al. / Journal of Alloys and Compounds 552 (2013) 83–87
SiO4 via Sol–gel method [22,23]. The present paper, report the microwave dielectric properties of novel LiInSiO4 ceramic and tailoring its properties by the addition of B2O3 and LMZBS glasses. 2. Experimental procedures The stoichiometric amount of Li2CO3, In2O3 and SiO2 were taken as the precursors for the preparation of LiInSiO4 (LIS) and was prepared through the solid state ceramic route. The following equation represents the chemical equation for the preparation of LiInSiO4 (LIS) ceramic.
1 mol Li2 CO3 þ 1 mol In2 O3 þ 2 mol SiO2 ! 2 mol LiInSiO4 þ 1 mol CO2 High purity powders of Li2CO3 (99.9%, Sigma–Aldrich, Inc., Milwaukee, WI, USA), In2O3 (99.5% HPLC, Bombay), and SiO2 (99.6% Sigma–Aldrich, Inc., St. Louis, MO 63103, USA) were weighed and ball milled using zirconia balls in ethanol for 24 h. The ceramic powders were calcined at 950 °C for 4 h. Different weight percentage of (20Li2O–20MgO–20ZnO–20B2O3–20SiO2, all from Sigma–Aldrich Chemical Company, 99%) LMZBS and B2O3 glasses were added to the fine powder of calcined LIS, 4 wt.% Poly Vinyl Alcohol (PVA) (molecular weight 22,000, BDH lab Suppliers, Poole, UK) solution was then added to it and mixed. It was then dried, ground well and pressed into cylindrical disks of about 11 mm diameter and 3–4 mm thickness under an uniaxial stress of about 100 M Pa. The compacts were muffled by LIS powder of the same composition during sintering, in order to prevent the loss of volatile lithium at elevated temperatures [24]. The sintering temperature of LIS ceramic was optimized in the range 1050–1150 °C for 4 h. The sintered powdered samples were used to analyze the crystalline structure and phase purity by the X-ray diffraction technique using Ni filtered Cu Ka radiation (PAN analytical X0 Pert PRO Diffractometer, Netherlands). The microstructures of the sintered samples were studied using scanning electron microscope (JEOL-SEM 5601v, Tokyo, Japan). The sintered densities of the specimens were measured by the Archimedes method. The microwave dielectric properties were measured in the frequency range of 6–14 GHz using a vector network analyzer (E5071C ENA series Network Analyzer, Agilent Technologies, USA). The relative permittivity of the material was measured using Hakki–Coleman technique modified by Courtney. The TE011 mode was used for this measurement [25]. The unloaded quality factor (Qu) was measured using TE01º mode by the cavity method [26]. The temperature variation of resonant frequency (sf) was measured by noting the temperature at regular intervals in the course of heating and was calculated using the equation [4].
1 f
sf ¼
Df DT
ð1Þ
where f is the resonant frequency at room temperature, Df is the variation in resonant frequency and Df is the corresponding difference in temperatures. The linear coefficient of thermal expansion (CTE) of the LIS ceramic sintered at 1150 °C was measured using dilatometer (DIL 402 PC, NETZSCH, Selb, Germany) in the temperature range 30–500 °C and the CTE was calculated using the equation [27].
aL ¼
1 dL L dT
ð2Þ
(aL is the linear coefficient of thermal expansion (CTE), L is the length of the dL material, dT is the fractional change in length per unit change in temperature). The thermal conductivity (Tc) was measured in the temperature range 25–200 °C using a laser thermal properties analyzer (Flash Line 2000, Anter Corporation, Pittsburgh, USA). For thermal conductivity (Tc) measurement, the specific heat capacity of reference sample (alumina) was taken from the literature [28] and the Tc was calculated using the equation [27]
T c ¼ aqC p
Fig. 1. Variation of relative density and relative permittivity as a function of sintering temperature of LiInSiO4 (LIS) ceramic.
The crystal structure of LiInSiO4 (lithium indium silicate) is isotopic with olivine Mg2SiO4 (forsterite) which is a well known low loss dielectric. The principal differences between lithium indium silicate and forsterite are found in the bond lengths and bond angles opposite to common edges between the tetrahedron and the Li+ and In3+ ion sites. The distorted tetrahedron shares one common edge with the Li+ site and two common edges with the In3+ site [18]. In 2003, Redhammer and Roth reported that LiInSiO4 crystallizes in orthorhombic structure with Pnma [No. 62] space group, with unit cell parameters a = 4.845 Å, b = 10.504 Å and c = 6.063 Å. Fig. 2(a) shows the X-ray diffraction (XRD) pattern of LiInSiO4 (LIS) ceramic sintered at 1150 °C for 4 h. All the recognizable peaks were indexed based on the ICDD File No. 34-0067. The lattice parameters a = 10.478 Å, b = 6.063 Å, and c = 4.849 Å were calculated [29], which are in good agreement with the previously reported values [18]. Fig. 2(b) and (c) are the XRD patterns of LIS + 1 wt.% LMZBS glass and LIS + 1 wt.% B2O3 glass sintered at 1100 °C for 4 h, respectively. It is observed that there are no secondary phases with the addition of LMZBS and B2O3 glass up to 1.5 wt.%. The microwave dielectric properties of a ceramic have direct relation to its microstructure [4]. Fig. 3(a)–(c) represent the scanning electron micrographs of thermally etched LIS ceramic,
ð3Þ
where Cp is the specific heat capacity, a is the thermal diffusivity, q is the density of the material.
3. Results and discussion Fig. 1 shows the variation of relative density and relative permittivity as a function of sintering temperature of LIS ceramic. The relative density is found to be increasing with increase in sintering temperature and reaches a maximum value at 1150 °C for 4 h. Further increase of sintering temperature resulted in melting of the sample. The relative permittivity of LIS ceramic varies in a similar manner as that of relative density. The lower value of relative permittivity at low temperatures may be due to the low densification and porosity of LIS ceramic [4]. The sintering temperature optimization of all glass added compositions of LIS ceramic were done, to obtain highest density and best dielectric properties.
Fig. 2. X-ray diffraction pattern of (a) LiInSiO4 (LIS) ceramic sintered at 1150 °C/4 h, (b) LIS + 1 wt.% LMZBS glass sintered at1100 °C/4 h and (c) LIS + 1 wt.% B2O3 glass sintered at 1100 °C/4 h.
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Fig. 3. SEM images of (a) LIS sintered at 1150 °C/4 h and (b) LIS + 1 wt.% B2O3 sintered at 1100 °C (c)LIS + 1 wt.% LMZBS sintered at 1100 °C/4 h.
LIS + 1 wt.% B2O3 glass and LIS + 1 wt.% LMZBS glass at their optimized sintering temperatures. Microstructure with variable grain size distribution along with pores is a clear indication of low relative density of pure LIS ceramic compared to LIS + 1 wt.% LMZBS glass added composition. The relative density of a ceramic is directly related to its microwave dielectric properties [30]. The presence of porosity in ceramic materials decreases its dielectric performance due to the entrapment of moisture. Hence the improvement of densification by the addition of suitable amount of sintering aids is necessary for better performance. In the present study, we have employed two glasses B2O3 and LMZBS with the objective of improving the sinterability of LIS ceramic. Fig. 4(a) and (b) shows the variation of relative density and relative permittivity for LIS ceramic with different wt.% of B2O3 and LMZBS glasses. In glass added ceramic materials, the effectiveness of sintering aids depend on several factors such as sintering condition, viscosity, sinterability and glass wettability [4,31]. It is clear from Fig. 4(a) that the densification is found to be 94% with the addition of 1 wt.% B2O3 glass at 1100 °C/4 h. In an earlier report [32] on the effect of B2O3 glass on low loss Nd (Co1/2Ti1/2) O3 ceramics. It was demonstrated that the glassy liquid phase at the grain boundary effectively eliminate the pores and
thereby increases the relative density for smaller volume fraction of B2O3 [33,34]. Further addition of glass above 1 wt.% reduces the relative density, which may be due to the trapped porosity associated with grain growth and formation of pores by the evaporation of excess glass components [21]. Fig. 4(b) shows similar effect and LIS + 1 wt.% LMZBS glass sintered at 1100 °C/4 h gives a densification of 95%. The LMZBS glass added compositions of LIS ceramic show same trend as B2O3 glass added composite. The variations of relative permittivity with different concentration of (B2O3 and LMZBS) glass are also shown in Fig. 4(a) and (b) respectively. The permittivity values are in the range of 8–8.4 for both LISB2O3 and LIS-LMZBS ceramic glass composites. Moreover the variation of relative permittivity shows similar trend as that of densification. Various intrinsic and extrinsic factors affect the microwave dielectric properties of a ceramic. The intrinsic factors are mainly caused by the lattice vibration modes, while the extrinsic factors are dominated by the defects in ceramics, secondary phases, grain size, and porosity [35]. The variation of Qu f and sf as a function of different weight percentage of B2O3 and LMZBS glass are shown in Fig. 5(a) and (b). The Qu f value increases with amount of B2O3 and LMZBS glasses. In the case of LIS + 1 wt.% B2O3 ceramic-glass
Fig. 4. (a) Variation of relative permittivity and relative density of LiInSiO4 (LIS) ceramic as a function of B2O3 (wt.%) glass and (b) variation of relative permittivity and relative density of LiInSiO4 (LIS) ceramic as a function of LMZBS (wt.%) glass.
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Fig. 5. (a) Variation of Qu f and sf of LIS ceramic at different wt.% of B2O3 glass and (b) variation of Qu f and sf of LIS ceramic at different wt.% of LMZBS glass.
composite, the maximum Qu f value obtained is 16,700 GHz sintered at 1100 °C for 4 h. It is believed that any effort to improve densification would eventually lead to an increase in the quality factor, since the moisture, volatile impurities and secondary phases associated with porosity eventually aggravate loss factor [32,36]. The improvement in densification for 1 wt.% B2O3 glass is reflected from its increase in quality factor. Further increase in wt.% of B2O3 glass decreases the Qu f value. This may be due to the porosity associated with grain growth and the evaporation of excess glass components [11,37]. The relatively high dielectric loss of glass also affects the Qu f value at higher glass fluxing [11]. Fig. 5(b), the amount of LMZBS glass with Qu f also shows a similar trend. The LMZBS glass added composition shows relatively better dielectric properties [38]. For example among the samples investigated in this study, 1 wt.% LMZBS glass added lithium indium silicate sintered at 1100 °C for 4 h shows best densification (95%) and dielectric properties (er = 8.4, Qu f = 22,000 GHz). Earlier investigations on glass added ceramics revealed that multicomponent boron rich glasses are effective in liquid phase sintering without much degrading the microwave dielectric properties of a ceramic [11,38], which could be the reason for better Qu f value of LMZBS glass added composite. The temperature variation of resonant frequencies of all the compositions is also shown in Fig. 5(a) and (b). It is well known that the sf value depends on the composition, additives and second phase of the material [32]. Moreover the sf value of all ceramic glass composites is in the range 30 ppm/°C to 65 ppm/°C. The substrate material that facilitate connection between the silicon based IC chip and board should have lower coefficient of thermal expansion (CTE), since CTE much higher than that of silicon not only leads to warpage and residual stress but also contributes the substrate cracking and delamination problems. Generally silicates show relatively low CTE value, this is due to the adjustment of bond angles by the absorption of vibrational energy in the transverse vibration modes [27]. Fig. 6 shows the variation of fractional change in length with temperature of LIS and LIS + 1 wt.% LMZBS ceramic glass composite. The average CTE value of LIS ceramic is 6.0 ppm/°C, LIS + 1 wt.% LMZBS is 5.0 ppm/°C. The CTE values are found to be relatively low and this value is comparable to the CTE of alumina, mullite and several other well investigated ceramic substrates [27]. Heat dissipation is a major problem encountered by ceramic materials in microelectronic industry. Fig. 7 shows the temperature dependence of thermal conductivity of LIS ceramic. The thermal conductivity gradually decreases with increase of temperature. In ceramic materials the high thermal conductivity at low temperature corresponds to an increased value of mean free path resulting from the lower amplitude and greater harmonicity
Fig. 6. Variation of fractional change in length with temperature of (a) LiInSiO4 (LIS) ceramic and (b) LIS + 1 wt.% LMZBS glass composite.
Fig. 7. Thermal Conductivity variation of LiInSiO4 (LIS) ceramic as a function of temperature.
of lattice vibrations. At high temperatures, the mean free path decreases or it is fixed by the inter-atomic separation and by the porosity [27]. In the present investigation, the thermal conductivity
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of LIS ceramic is found to be in the range (3–6) Wm1 K1 which is comparable with the thermal conductivity of well known substrate materials such as forsterite, mullite, zircon etc. [27]. In conclusion the thermal and dielectric properties of LiInSiO4 are good enough to be qualified as a novel substrate. 4. Conclusions The LIS sintered at 1150 °C for 4 h showed a densification of 91% with er = 8.2, Qu f = 12,600 GHz and sf = -55 ppm/°C, thermal conductivity at room temperature (Tc) 6.0 Wm1 K1 and coefficient of thermal expansion (CTE) 6.0 ppm/°C. The effects of LMZBS and B2O3 glass on the densification and dielectric properties of LiInSiO4 (LIS) for substrate application were studied. The unloaded quality factor and relative density of LIS ceramic were effectively promoted by the addition of LMZBS glass which acts as a good sintering aid. LIS + 1 wt.% LMZBS glass sintered at 1100 °C for 4 h showed a relatively good densification of 95% with moderate dielectric properties (er = 8.4, Qu f = 22,000 GHz, sf = 45 ppm/°C). The physical and microwave dielectric properties make this composite a possible candidate for microwave substrate application. Acknowledgements The authors acknowledge the financial support from Ministry of Human Resource Development, Government of India, New Delhi. Dr. Prabhakar Rao and M.R. Chandran are also thankfully acknowledged for extending XRD and SEM facilities. References [1] [2] [3] [4]
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