- Email: [email protected]
Fabrication and characterization of low temperature co-fired cordierite glass–ceramics from potassium feldspar
Journal of Alloys and Compounds 583 (2014) 248–253
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication and characterization of low temperature co-fired cordierite glass–ceramics from potassium feldspar Jianfang Wu, Zhen Li ⇑, Yanqiu Huang, Fei Li, Qiuran Yang Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 8 June 2013 Received in revised form 16 August 2013 Accepted 27 August 2013 Available online 6 September 2013 Keywords: Cordierite Glass–ceramics Potassium feldspar LTCC
a b s t r a c t Cordierite glass–ceramics for low temperature co-fired ceramic (LTCC) substrates were fabricated successfully using potassium feldspar as the main raw material. The sintering and crystallization behaviors of the glass–ceramics were investigated by the differential scanning calorimetry (DSC), X-ray diffraction (XRD), and field emission scanning electron microscope (FESEM). The results indicated that the glass–ceramics could be highly densified at 850 °C and the cordierite was the main crystalline phase precipitated from the glasses in the temperature range between 900 and 925 °C. The study also evaluated the physical properties including dielectric properties, thermal expansion and flexural strength of the glass–ceramics. The glass–ceramics showed low dielectric constants in the range of 6–8 and low dielectric losses in the range of 0.0025–0.01. The coefficients of thermal expansion (CTEs) are between 4.32 and 5.48 106 K1 and flexural strength of the glass–ceramics are 90–130 MPa. All of those qualify the glass–ceramics for further research to be used as potential LTCC substrates in the multilayer electronic substrate field. Additionally, the excess SiO2 acted as a great role in improving the sinterability of the glasses, and the microstructure and dielectric properties of the relevant glass–ceramics. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The considerable progress in the wireless communication and electronics industries has resulted in an increasing demand for light weight, miniaturized, multifunctional electronic devices. Currently, low temperature co-fired ceramic (LTCC) technology with glass–ceramic or glass/ceramic materials has been demonstrated to meet the stringent requirements of the electronic devices for their low dielectric constant and ability to be co-fired with electrodes of high conductivity such as Cu, Ag and Au below 950 °C [1–4]. Furthermore, the ideal LTCC substrate materials should possess several characteristics such as low dielectric constant (below 10) and low dielectric loss, high thermal conductivity, low thermal expansion coefficient (close to Si or GaAs), robustness against environmental stress, and low cost [5]. Commercially available glass–ceramics for LTCC substrates mainly contain MgO–Al2O3–SiO2 [6–13], CaO–Al2O3–SiO2 [14,15] CaO–B2O3–SiO2 [16,17], etc., among which the a-cordierite glass– ceramic has gained a significant attention due to its unique properties. For example, the a-cordierite (Mg2Al4Si5O18) glass– ceramic has very low dielectric constant and dielectric loss, low coefficient of thermal expansion (CTE) and high strength, which are essential for LTCC substrates [6,8,10,11]. The restriction of ⇑ Corresponding author. Tel.: +86 2718971473850. E-mail address: [email protected] (Z. Li). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.187
developing a-cordierite glass–ceramic as the substrate for LTCC is the difficulty to obtain fully densified a-cordierite glass–ceramics below 950 °C, when the chemical composition comes to the stoichiometric ratio of cordierite due to its narrow sintering range and the formation of l-cordierite [18]. Complete densification is required for achieving high mechanical strength and low dielectric loss. However, the formation of l-cordierite worsens mechanical, thermal and dielectric properties of the glass–ceramics. Thus researches about adding sintering aids and adopting non-stoichiometric glasses [7,19–25] have been conducted to improve the sintering capability to obtain fully densified a-cordierite glass– ceramic at low temperature. Significantly, Banjuraizah et al. [26] and Sumi et al. [27] fabricated highly densified a-cordierite glass–ceramics from talc and kaolin below 950 °C successfully, this kind of glass–ceramics from cheap natural raw materials were demonstrated to be promising LTCC substrates with low dielectric constant (in the range 5–6), low dielectric loss (about 0.01), high strength and matched coefficient of thermal expansion (about 3 106 K1) with Si. Some chemical compositions brought into the batches by the natural raw materials were in favor of improving sintering ability of the glass–ceramics [28]. Enlightened by these researches [26–28], we attempted to fabricate glass–ceramics for LTCC substrates using one of the most common minerals in the earth. In our previous work, a forsteritebased glass–ceramic was developed from potassium feldspar for LTCC substrates [29]. In this study, highly densified a-cordierite
249
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253 Table 1 Chemical compositions of potassium feldspar in wt.%. Composition
SiO2
Al2O3
K2O
Na2O
Fe2O3
CaO
SrO
BaO
Others
Potassium feldspar
65.10
17.20
12.15
1.69
0.40
0.90
0.70
0.29
1.57
Table 2 Chemical compositions of the batches. Batches
MAS33 MAS20 MAS30
K2O
MgO
Al2O3
SiO2
Others
wt.%
mol.%
wt.%
mol.%
wt.%
mol.%
wt.%
mol.%
wt.%
4.78 4.68 4.49
0.034 0.033 0.032
11.90 11.64 11.16
0.198 0.193 0.184
30.11 29.45 28.23
0.198 0.193 0.184
51.02 52.08 54.08
0.570 0.580 0.599
2.19 2.14 2.05
glass–ceramics, synthesized in the temperature ranging from 850 to 925 °C using potassium feldspar as the main raw material, showed low dielectric constant, low dielectric loss, low coefficients of thermal expansion, and high flexural strength.
2. Materials and experimental procedure The natural material potassium feldspar was collected from China and its chemical compositions were presented in Table 1. The alkali metal oxides, such as Na2O and K2O, brought into the batches by the potassium feldspar are network modifiers in the glasses, which would seriously deteriorate the structure and properties of residual glassy phase in the glass–ceramics after the cordierite was precipitated. For the consideration of reducing the negative effects of alkali metal oxides, excess SiO2 was added into the batches. The nominal chemical compositions of the glasses were shown in Table 2 and they were named with MAS from MgO, SiO2, and Al2O3. Properly mixed batches of potassium feldspar, MgO, SiO2, and Al2O3 according to Table 2 were melted in corundum crucibles in air at 1550 °C for 6 h to ensure homogeneity with an applied heating rate of 5 °C/min. Glasses in frit form were obtained by quenching of the melts in cold water. The glass powders were prepared by the wet ball-milling technique. To prepare the bulk samples, the obtained glass powders were granulated with 5% poly (vinyl alcohol) (PVA) and then pressed into disks with a diameter in 40 mm under a uniaxial pressure of 15 MPa at room temperature. The bulk samples were sintered in the temperature range of 850–925 °C with a 25 °C temperature interval for 360 min at an applied heating rate of 10 °C/min. After sintering, the samples were cooled to room temperature in the furnace. To investigate the thermal effect of the glasses, the obtained glass powders were characterized by differential scanning calorimetry (DSC) at applied heating rate of 20 °C/min using NETZSCH STA 449 C (NETZSCH, Germany) with matched pairs of platinum–rhodium alloy crucibles. Alumina was used as the reference material. The density of glass–ceramics was calculated by Archimedes method in water. The X-ray diffraction (XRD, D8-FOCUS) was used to determine the crystalline phases of the glass–ceramics. Field Emission Scanning Electron Microscopy (FESEM, SU8010) was also used to characterize the micro-structure of the glass–ceramics at fractured surfaces. Dielectric properties of the glass–ceramics were measured by a precision impedance analysis meter (4294A, Agilent) in the frequency range of 40 Hz–10 MHz. Average coefficient of thermal expansion (CTE), from room temperature to 600 °C, was measured in air at a heating rate of 5 °C/min using a dilatometer (NETZSCH DIL 402C). The flexural strength of the glass–ceramics was finally tested by the three-point bending method.
Fig. 1. DSC curves of the three investigated glasses.
3. Results and discussion The DSC curves of the three glasses are shown in Fig. 1. All the curves exhibit two crystallization peak temperatures (Tp1 and Tp2) and the glass transition temperatures for the three glasses are not evident. The crystallization peak temperature increases slightly with the inclining of SiO2 amount in the glasses, as the SiO2 acts as a glass network former and increases the integrity of the relevant glass resulting in more difficulty for the glasses to be crystallized. So it can be concluded that increasing SiO2 depressed the crystallization of the glass in the low temperature and enhances its sinterability, which can also be deduces from the results of XRD and SEM investigation. The first crystallization peak is due to the precipitation of cordierite (Mg2Al4Si5O18) while the second crystallization peak should be ascribed to the
Fig. 2. XRD patterns of the glass–ceramics prepared at different sintering temperatures.
precipitation of leucite (KAlSi2O6) and cordierite (Mg2Al4Si5O18), which are verified by the XRD investigation. The XRD patterns of the glass–ceramics sintered at different temperatures are shown in Fig. 2. Typical amorphous patterns with rather weak peaks are recorded of the samples sintered at 850 °C for 6 h, indicating only a few micro-crystals produced. With an increase of sintering temperature to 900 °C, the glass–ceramics
250
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253
Fig. 3. SEM images of glass–ceramics prepared from MAS33 (a1, a2 and a3), MAS20 (b1, b2 and b3) and MAS30 (c1, c2 and c3) at different temperatures 850 °C (a1, b1 and c1), 900 °C (a2, b2 and c2) and 925 °C (a3, b3 and c3).
prepared from MAS33 and MAS20 are effectively crystallized, but the sample prepared from MAS30 keeps amorphous. When the sintering temperature rises to 925 °C, the crystallinity of the glass–ceramics is enhanced and crystals are eventually formed in the sample prepared from MAS30. And the crystallinity of the glass–ceramic prepared at 900 °C is higher than that of others. The a-cordierite (indialite), a promising material for LTCC substrate, is the dominated crystalline phase precipitated from the glasses, though in the samples prepared from MAS20 a few other crystalline phases are also formed. As reported in the previous literatures [18,19,30], l-cordierite is always formed in the low temperature and then transformed into a-cordierite at high temperature, but this process is never taken place in this work. The microstructure of the fractured surfaces of the glass–ceramics prepared at different sintering temperatures is investigated and the SEM images are shown in Fig. 3. It is clear that only a few pores
are found in the glass–ceramics indicating that highly densified glass–ceramics are fabricated and the parent glasses are of good sinterability. For the specimens prepared at the same sintering temperature with increasing content of SiO2, the porosity of the glass–ceramics decreases slightly and the reduction is more obviously for the samples sintered at 925 °C. That is to say, increasing SiO2 in the investigated batches improves the densification of the glass–ceramics. And the porosity of the glass–ceramics increases with the rising of sintering temperatures. In the sinter-crystallization process of glass–ceramics, the viscous flow of the glass above the glass transition temperature (Tg) enhances sintering. As a result, the sample gets densified. Sintering by glass viscous flow will be hindered once crystallization is initiated as the crystalline phases sharply increase the viscosity of the system. Consequently, residual porosity is reserved in the glass–ceramics. Since crystallization of MAS30 is more hysteretic than other two batches, it may
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253
251
Fig. 3 (continued)
temperature enhances the crystallization, the density of the glass–ceramic increases with the decreasing content of SiO2 and with increasing temperature. From the result of XRD, the high density of the sample prepared at 900 °C also comes along with higher crystallinity than that of the samples prepared at same sintering temperature. The dielectric properties including dielectric constant and loss are two vital factors on the LTCC components and the performance of the terminal device. Signal propagation is one of the most important aspects in the multilayer electronic devices, which is a direct function of dielectric constant. In the case of glass–ceramic packages, the dielectric constant of the glass–ceramic material over and within the metal lines that are deposited or embedded governs the propagation delay td, which is given by the equation [31]
td ¼ lðer Þ1=2 =c Fig. 4. Density of the glass–ceramics prepared at different sintering temperatures.
be easier for this batch to obtain densified glass–ceramics in a broad temperature interval. The fast crystallization rate at high sintering temperature is the main reason for the pores observed in the glass–ceramics prepared at high temperatures. The variation of the glass–ceramics’ density versus sintering temperature is shown in Fig 4. Density of the glass–ceramics is in the range between 2.5 and 2.6 g/cm3, which inclines with the decreasing content of SiO2 and with the temperature, except for the glass–ceramic prepared at 900 °C of which the density is higher than that of others. The density of a glass–ceramic is related to their components and microstructure which can be determined by chemical composition of batches, sintering and crystallization processes. The density of glass–ceramics will decrease with porosity, though increase with the crystallinity as the crystals always own a more compacted structure than the glassy phase with the same chemical compositions. In this work, the crystallinity of glass–ceramic is the main reason for the variation of density, as increasing content of SiO2 depresses the crystallization and high
where l is the line length, er is the relative dielectric constant of the substrate and c is the speed of light. Thus substrates with low dielectric constant are required to increase the speed of the signal [1]. Dielectric loss is described by loss angle tan d, which is always related to the energy transformation from electrical power to heat. High dielectric loss will lead to overheated of the terminal devices and shorten the service time of the terminal devices. The dielectric loss comes from two groups; intrinsic and extrinsic. Intrinsic losses depend on crystal structure and express the interaction of the crystal lattice with the external electric field. Extrinsic losses relate to materials’ microstructure, e.g. the presence of microstructural defects, porosity, micro cracks and impurities [32]. The dielectric constants (a) and losses (b) of glass–ceramics prepared at different sintering temperatures as a function of frequency in the range 40 Hz–10 MHz were shown in Fig. 5. The dielectric constants are in the range 6.0–8.0 and decrease with the frequency from 40 Hz to 10 MHz for all samples. The dielectric constants of the glass–ceramics vary with the sintering temperature as well as the content of SiO2. For the glass–ceramics prepared from MAS33 and MAS30, the dielectric constant decreases with an increase of sintering temperature, while for the glass–ceramics from MAS20, the dielectric constant increases firstly and then
252
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253
Fig. 6. Thermal expansion behavior of the glass–ceramics prepared at different sintering temperatures.
Fig. 5. Dielectric constants (a) and losses (b) of glass–ceramics prepared at different sintering temperatures versus frequency in the range 40 Hz–10 MHz.
decreases. With an increase of SiO2, the dielectric constants of the glass–ceramics increase firstly and then decrease, and the glass– ceramics prepared from MAS30 own the lowest dielectric constant. Many factors influence the dielectric constant of the materials [33– 35], mainly the crystalline phases, crystallinity, chemical composition of the residual glassy phase, grain size, porosity, etc. In this work, the crystalline phase, chemical composition of the residual glassy phase and the porosity are the aspects determining the dielectric constant of the glass–ceramics. On one hand, the dielectric constant of the glass–ceramics with a high porosity is low, which may be the main reason for the low dielectric constant of the glass–ceramics prepared from MAS33. Even though the high SiO2 content densified the glass–ceramics, the SiO2 owns extremely low dielectric constant, so the dielectric constant of glass– ceramics prepared from MAS30 is also low. On the other hand, the dielectric constant of the glass–ceramics was determined by the crystalline phases but also the residual glassy phase, the alkali oxides remaining in the glassy phase after precipitation of cordierite deteriorates the dielectric constant which is the main reason for the high dielectric constant of the glass–ceramic prepared at 900 °C from MAS20. The dielectric losses of the prepared glass– ceramics are in 0.0025–0.01, which are higher than the cordierite glass–ceramics from chemical pure oxides [6,7,13] because of the impurities brought into batches by the potassium feldspar but are lower than the glass–ceramics fabricated in previous literatures [26,27]. Increasing sintering temperature deteriorates the losses of the glass–ceramics, which shows a similar tendency as the variation of porosity in the glass–ceramics, and this phenomenon is more evidently for the glass–ceramics prepared from batches with low SiO2, which should be ascribed to the intrinsic low loss, densification effect on the glass–ceramics and the
enhanced effect on the integrity of the residual glassy phase of SiO2. Generally speaking, each constituent in the glass is crucial to the crystallization, densification, and thus dielectric properties. And their effects are intercorrelated and sometimes, unable to distinguish the contributions from an individual constituent. However, the SiO2 plays a more important role in improving the dielectric properties of the glass–ceramics in this work. For the design of muti-layer electronic substrates, the LTCC substrates are always attached to an active device such as Si (3– 5 106 K1) [36] or GaAs (5.7 106 K1), and the CTE of the LTCC substrate should be matchable to that of the active device to improve the reliability of the terminal devices [5]. The thermal expansion behaviors of glass–ceramics in the temperature range 0–600 °C are shown in Fig. 7. The linear CTEs are deduced from the slopes of the curves of dL/L0 versus temperature as 5.24, 5.41, 5.41, 5.21, 4.32 and 5.49 106 K1 for the samples prepared from MAS33, MAS20 and MAS30 at 900 and 925 °C respectively, which matches to the fore-mentioned active devices. The CTEs of the glass–ceramics shows little difference, except for the glass–ceramic prepared from MAS20 at 925 °C whose CTE is rather lower than that of others ascribed to its higher crystallinity. The CTEs of the glass–ceramics in this work are higher than that of the
Fig. 7. Flexural strength of the glass–ceramics prepared at different sintering temperatures.
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253 Table 3 Comparison between glass–ceramics in this work and the commercial LTCC substrates.
Sintering temperature (°C) Coefficient of thermal expansion (CTE, ppm) Flexural strength (MPa) Dielectric constant Dielectric loss
MAS30
MAS20
Commercial LTCC materials
925 5.49
900 5.41
<1000 4.5–7.5
114 6.1 (10 MHz) 0.0069 (10 MHz)
116 7.5 (10 MHz) 0.0057 (10 MHz)
116–320 3.8–9.2 (1 MHz) 0.0007–0.006 (1 MHz)
glass–ceramics prepared from chemical pure oxides, and it is because of the alkali metal oxides in the batches. The flexural strength of glass–ceramics sintered at different sintering temperatures is shown in Fig. 6. The values of the flexural strength of the glass–ceramics are in the range 90–130 MPa. The flexural strength of the glass–ceramics decreases with an increase of sintering and it increases along with SiO2 ratio in the batches, and this phenomenon can be explained by the variation of porosity in the glass–ceramics. As many LTCC integrated devices has to operate under harsh environmental conditions, the mechanical strength of the substrate materials is a critical issue regarding prolonging the lifetime of the device, and high values of flexural strength of the substrate materials are required. The low temperature co-fired cordierite glass–ceramics in this work show appropriate strength for the LTCC substrate application. The property parameters of typical glass–ceramics fabricated in this work and the commercial LTCC substrate materials [5] are summarized in Table 3. It indicates that the glass–ceramics fabricated in this work exhibits excellent properties and great potential for the application as LTCC substrates.
4. Conclusions Highly densified cordierite glass–ceramics were successfully fabricated using potassium feldspar as the main raw material in the low temperature range 850–925 °C. The sintering and crystallization behaviors and properties depended on the chemical compositions as well as the sintering temperatures. For the glass–ceramics prepared from batch with low content of SiO2, the tendency of worsening the dielectric properties was more observable by increasing the sintering temperature, and excess SiO2 optimized the sinterability of the glasses, and the structure and properties of the glass–ceramics. Especially, the obtained cordierite glass–ceramics were of low dielectric constants in the range of 6–8, low dielectric losses in the range of 0.0025–0.01, matchable thermal expansion behavior to that of the active devices and appropriate flexural strength in the range of 90–130 MPa. The batch MAS20 and MAS30 appeared to be of the best sinterability and the relevant glass–ceramics exhibited excellent properties, which were potential candidates as LTCC substrates for the application.
253
Acknowledgement The work was financial supported by the National Key Technology R&D Program of Peoples Republic of China (No. 2011BAB03B01). References [1] R.R. Tummala, Journal of the American Ceramic Society 74 (1991) 895–908. [2] J.U. Knickerbocker, S.H. Knickerbocker, Low-temperature co-fired ceramic chip carriers, in: J.K. Wessel (Ed.), Handbook of Advanced Materials, John Wiley & Sons Ltd, American, 2004. [3] S. Yuzo, K. Yoshinobu, K. Keiichiro, K. Masayuki, T. Hideo, IEEE Transactions on Components, Hybrids, and Manufacturing Technology 13 (1990) 751–758. [4] Y. Imanaka, Multilayered low temperature cofired ceramics (LTCC), technology, 2005. [5] M.T. Sebastian, H. Jantunen, International Materials Reviews 53 (2008) 57–90. [6] A.H. Kumar, S. Knickerbocker, R.R. Tummala, Sinterable glass–ceramics for high-performance substrates, in: 42nd Electronic Components and Technology Conference, San Dieg, 1992, pp. 678–681. [7] G. Chen, Journal of Alloys and Compounds 455 (2008) 298–302. [8] G. Chen, X. Liu, Journal of Alloys and Compounds 431 (2007) 282–286. [9] S. Kemethmuller, A. Roosen, F. Goetz-Neunhoeffer, J. Neubauer, Journal of the American Ceramic Society 89 (2006) 2632–2637. [10] Z. Yue, J. Zhou, H. Hang, Z. Gui, L. Li, Journal of Materials Science Letters 19 (2000) 213–215. [11] S. Wang, H. Lu, Z. Hou, Ceramics International 39 (2013) 991–997. [12] E.M. Hamzawy, A.A. El-Kheshen, M.F. Zawrah, Ceramics International 31 (2005) 383–389. [13] M. Majumder, S. Mukhopadhyay, O. Parkash, D. Kumar, Ceramics International 30 (2004) 1067–1070. [14] C.L. Lo, J.G. Duh, B.S. Chiou, W.H. Lee, Journal of the American Ceramic Society 85 (2002) 2230–2235. [15] C.J. Dileep Kumar, E.K. Sunny, N. Raghu, N. Venkataramani, A.R. Kulkarni, Journal of the American Ceramic Society 91 (2008) 652–655. [16] C. Chang, J. Jean, Journal of the American Ceramic Society 82 (1999) 1725– 1732. [17] H. Shao, T. Wang, Q. Zhang, Journal of Alloys and Compounds 484 (2009) 2–5. [18] S. Hwang, J. Wu, Journal of the American ceramic Society 84 (2001) 1108– 1112. [19] G. Chen, Journal of Materials Science. Materials in Electronics 18 (2007) 1253– 1257. [20] R.W. Dupon, R.L. Mcconville, D.J. Musolf, A.C. Tanous, M.S. Thompson, Journal of the American Ceramic Society 73 (1990) 335–339. [21] B.H. Kim, K.H. Lee, Journal of Materials Science 29 (1994) 6592–6598. [22] Y. Sung, Journal of Materials Science 31 (1996) 5421–5427. [23] S. Wang, H. Zhou, L. Luo, Materials Research Bulletin 38 (2003) 1367–1374. [24] G. Chen, X. Liu, Journal of Materials Science. Materials in Electronics 15 (2004) 595–600. [25] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Journal of the American Ceramic Society 94 (2011) 687–694. [26] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Journal of Alloys and Compounds 482 (2009) 429–436. [27] K. Sumi, Y. Kobayashi, E. Kato, Journal of the American Ceramic Society 82 (1999) 783–785. [28] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Journal of Alloys and Compounds 509 (2011) 7645–7651. [29] J. Wu, Z. Li, H. He, Y. Huang, H. Wu, Journal of Materials Science. Materials in Electronics (2013), http://dx.doi.org/10.1007/s10854-013-1089-2. [30] N.J. Azín, M.A. Camerucci, A.L. Cavalieri, Ceramics International 31 (2005) 189– 195. [31] B. Schwartz, American Ceramic Society Bulletin 63 (1984) 577–581. [32] S.J. Penn, N.M.N. Alford, A. Templeton, X. Wang, M. Xu, M. Reece, K. Schrapel, Journal of the American Ceramic Society 80 (1997) 1885–1888. [33] J. Chen, H. Wang, S. Feng, H. Ma, D. Deng, S. Xu, Ceramics International 37 (2011) 989–993. [34] Y. Fang, L. Li, Q. Xiao, X.M. Chen, Ceramics International 38 (2012) 4511–4515. [35] S.O. Yoon, S.H. Shim, K.S. Kim, J.G. Park, S. Kim, Ceramics International 35 (2009) 1271–1275. [36] Y. Shimada, Y. Yamashita, H. Takamizawa, IEEE Transactions on Components, Hybrids, and Manufacturing Technology 11 (1988) 163–170.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication and characterization of low temperature co-fired cordierite glass–ceramics from potassium feldspar Jianfang Wu, Zhen Li ⇑, Yanqiu Huang, Fei Li, Qiuran Yang Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 8 June 2013 Received in revised form 16 August 2013 Accepted 27 August 2013 Available online 6 September 2013 Keywords: Cordierite Glass–ceramics Potassium feldspar LTCC
a b s t r a c t Cordierite glass–ceramics for low temperature co-fired ceramic (LTCC) substrates were fabricated successfully using potassium feldspar as the main raw material. The sintering and crystallization behaviors of the glass–ceramics were investigated by the differential scanning calorimetry (DSC), X-ray diffraction (XRD), and field emission scanning electron microscope (FESEM). The results indicated that the glass–ceramics could be highly densified at 850 °C and the cordierite was the main crystalline phase precipitated from the glasses in the temperature range between 900 and 925 °C. The study also evaluated the physical properties including dielectric properties, thermal expansion and flexural strength of the glass–ceramics. The glass–ceramics showed low dielectric constants in the range of 6–8 and low dielectric losses in the range of 0.0025–0.01. The coefficients of thermal expansion (CTEs) are between 4.32 and 5.48 106 K1 and flexural strength of the glass–ceramics are 90–130 MPa. All of those qualify the glass–ceramics for further research to be used as potential LTCC substrates in the multilayer electronic substrate field. Additionally, the excess SiO2 acted as a great role in improving the sinterability of the glasses, and the microstructure and dielectric properties of the relevant glass–ceramics. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The considerable progress in the wireless communication and electronics industries has resulted in an increasing demand for light weight, miniaturized, multifunctional electronic devices. Currently, low temperature co-fired ceramic (LTCC) technology with glass–ceramic or glass/ceramic materials has been demonstrated to meet the stringent requirements of the electronic devices for their low dielectric constant and ability to be co-fired with electrodes of high conductivity such as Cu, Ag and Au below 950 °C [1–4]. Furthermore, the ideal LTCC substrate materials should possess several characteristics such as low dielectric constant (below 10) and low dielectric loss, high thermal conductivity, low thermal expansion coefficient (close to Si or GaAs), robustness against environmental stress, and low cost [5]. Commercially available glass–ceramics for LTCC substrates mainly contain MgO–Al2O3–SiO2 [6–13], CaO–Al2O3–SiO2 [14,15] CaO–B2O3–SiO2 [16,17], etc., among which the a-cordierite glass– ceramic has gained a significant attention due to its unique properties. For example, the a-cordierite (Mg2Al4Si5O18) glass– ceramic has very low dielectric constant and dielectric loss, low coefficient of thermal expansion (CTE) and high strength, which are essential for LTCC substrates [6,8,10,11]. The restriction of ⇑ Corresponding author. Tel.: +86 2718971473850. E-mail address: [email protected] (Z. Li). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.187
developing a-cordierite glass–ceramic as the substrate for LTCC is the difficulty to obtain fully densified a-cordierite glass–ceramics below 950 °C, when the chemical composition comes to the stoichiometric ratio of cordierite due to its narrow sintering range and the formation of l-cordierite [18]. Complete densification is required for achieving high mechanical strength and low dielectric loss. However, the formation of l-cordierite worsens mechanical, thermal and dielectric properties of the glass–ceramics. Thus researches about adding sintering aids and adopting non-stoichiometric glasses [7,19–25] have been conducted to improve the sintering capability to obtain fully densified a-cordierite glass– ceramic at low temperature. Significantly, Banjuraizah et al. [26] and Sumi et al. [27] fabricated highly densified a-cordierite glass–ceramics from talc and kaolin below 950 °C successfully, this kind of glass–ceramics from cheap natural raw materials were demonstrated to be promising LTCC substrates with low dielectric constant (in the range 5–6), low dielectric loss (about 0.01), high strength and matched coefficient of thermal expansion (about 3 106 K1) with Si. Some chemical compositions brought into the batches by the natural raw materials were in favor of improving sintering ability of the glass–ceramics [28]. Enlightened by these researches [26–28], we attempted to fabricate glass–ceramics for LTCC substrates using one of the most common minerals in the earth. In our previous work, a forsteritebased glass–ceramic was developed from potassium feldspar for LTCC substrates [29]. In this study, highly densified a-cordierite
249
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253 Table 1 Chemical compositions of potassium feldspar in wt.%. Composition
SiO2
Al2O3
K2O
Na2O
Fe2O3
CaO
SrO
BaO
Others
Potassium feldspar
65.10
17.20
12.15
1.69
0.40
0.90
0.70
0.29
1.57
Table 2 Chemical compositions of the batches. Batches
MAS33 MAS20 MAS30
K2O
MgO
Al2O3
SiO2
Others
wt.%
mol.%
wt.%
mol.%
wt.%
mol.%
wt.%
mol.%
wt.%
4.78 4.68 4.49
0.034 0.033 0.032
11.90 11.64 11.16
0.198 0.193 0.184
30.11 29.45 28.23
0.198 0.193 0.184
51.02 52.08 54.08
0.570 0.580 0.599
2.19 2.14 2.05
glass–ceramics, synthesized in the temperature ranging from 850 to 925 °C using potassium feldspar as the main raw material, showed low dielectric constant, low dielectric loss, low coefficients of thermal expansion, and high flexural strength.
2. Materials and experimental procedure The natural material potassium feldspar was collected from China and its chemical compositions were presented in Table 1. The alkali metal oxides, such as Na2O and K2O, brought into the batches by the potassium feldspar are network modifiers in the glasses, which would seriously deteriorate the structure and properties of residual glassy phase in the glass–ceramics after the cordierite was precipitated. For the consideration of reducing the negative effects of alkali metal oxides, excess SiO2 was added into the batches. The nominal chemical compositions of the glasses were shown in Table 2 and they were named with MAS from MgO, SiO2, and Al2O3. Properly mixed batches of potassium feldspar, MgO, SiO2, and Al2O3 according to Table 2 were melted in corundum crucibles in air at 1550 °C for 6 h to ensure homogeneity with an applied heating rate of 5 °C/min. Glasses in frit form were obtained by quenching of the melts in cold water. The glass powders were prepared by the wet ball-milling technique. To prepare the bulk samples, the obtained glass powders were granulated with 5% poly (vinyl alcohol) (PVA) and then pressed into disks with a diameter in 40 mm under a uniaxial pressure of 15 MPa at room temperature. The bulk samples were sintered in the temperature range of 850–925 °C with a 25 °C temperature interval for 360 min at an applied heating rate of 10 °C/min. After sintering, the samples were cooled to room temperature in the furnace. To investigate the thermal effect of the glasses, the obtained glass powders were characterized by differential scanning calorimetry (DSC) at applied heating rate of 20 °C/min using NETZSCH STA 449 C (NETZSCH, Germany) with matched pairs of platinum–rhodium alloy crucibles. Alumina was used as the reference material. The density of glass–ceramics was calculated by Archimedes method in water. The X-ray diffraction (XRD, D8-FOCUS) was used to determine the crystalline phases of the glass–ceramics. Field Emission Scanning Electron Microscopy (FESEM, SU8010) was also used to characterize the micro-structure of the glass–ceramics at fractured surfaces. Dielectric properties of the glass–ceramics were measured by a precision impedance analysis meter (4294A, Agilent) in the frequency range of 40 Hz–10 MHz. Average coefficient of thermal expansion (CTE), from room temperature to 600 °C, was measured in air at a heating rate of 5 °C/min using a dilatometer (NETZSCH DIL 402C). The flexural strength of the glass–ceramics was finally tested by the three-point bending method.
Fig. 1. DSC curves of the three investigated glasses.
3. Results and discussion The DSC curves of the three glasses are shown in Fig. 1. All the curves exhibit two crystallization peak temperatures (Tp1 and Tp2) and the glass transition temperatures for the three glasses are not evident. The crystallization peak temperature increases slightly with the inclining of SiO2 amount in the glasses, as the SiO2 acts as a glass network former and increases the integrity of the relevant glass resulting in more difficulty for the glasses to be crystallized. So it can be concluded that increasing SiO2 depressed the crystallization of the glass in the low temperature and enhances its sinterability, which can also be deduces from the results of XRD and SEM investigation. The first crystallization peak is due to the precipitation of cordierite (Mg2Al4Si5O18) while the second crystallization peak should be ascribed to the
Fig. 2. XRD patterns of the glass–ceramics prepared at different sintering temperatures.
precipitation of leucite (KAlSi2O6) and cordierite (Mg2Al4Si5O18), which are verified by the XRD investigation. The XRD patterns of the glass–ceramics sintered at different temperatures are shown in Fig. 2. Typical amorphous patterns with rather weak peaks are recorded of the samples sintered at 850 °C for 6 h, indicating only a few micro-crystals produced. With an increase of sintering temperature to 900 °C, the glass–ceramics
250
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253
Fig. 3. SEM images of glass–ceramics prepared from MAS33 (a1, a2 and a3), MAS20 (b1, b2 and b3) and MAS30 (c1, c2 and c3) at different temperatures 850 °C (a1, b1 and c1), 900 °C (a2, b2 and c2) and 925 °C (a3, b3 and c3).
prepared from MAS33 and MAS20 are effectively crystallized, but the sample prepared from MAS30 keeps amorphous. When the sintering temperature rises to 925 °C, the crystallinity of the glass–ceramics is enhanced and crystals are eventually formed in the sample prepared from MAS30. And the crystallinity of the glass–ceramic prepared at 900 °C is higher than that of others. The a-cordierite (indialite), a promising material for LTCC substrate, is the dominated crystalline phase precipitated from the glasses, though in the samples prepared from MAS20 a few other crystalline phases are also formed. As reported in the previous literatures [18,19,30], l-cordierite is always formed in the low temperature and then transformed into a-cordierite at high temperature, but this process is never taken place in this work. The microstructure of the fractured surfaces of the glass–ceramics prepared at different sintering temperatures is investigated and the SEM images are shown in Fig. 3. It is clear that only a few pores
are found in the glass–ceramics indicating that highly densified glass–ceramics are fabricated and the parent glasses are of good sinterability. For the specimens prepared at the same sintering temperature with increasing content of SiO2, the porosity of the glass–ceramics decreases slightly and the reduction is more obviously for the samples sintered at 925 °C. That is to say, increasing SiO2 in the investigated batches improves the densification of the glass–ceramics. And the porosity of the glass–ceramics increases with the rising of sintering temperatures. In the sinter-crystallization process of glass–ceramics, the viscous flow of the glass above the glass transition temperature (Tg) enhances sintering. As a result, the sample gets densified. Sintering by glass viscous flow will be hindered once crystallization is initiated as the crystalline phases sharply increase the viscosity of the system. Consequently, residual porosity is reserved in the glass–ceramics. Since crystallization of MAS30 is more hysteretic than other two batches, it may
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253
251
Fig. 3 (continued)
temperature enhances the crystallization, the density of the glass–ceramic increases with the decreasing content of SiO2 and with increasing temperature. From the result of XRD, the high density of the sample prepared at 900 °C also comes along with higher crystallinity than that of the samples prepared at same sintering temperature. The dielectric properties including dielectric constant and loss are two vital factors on the LTCC components and the performance of the terminal device. Signal propagation is one of the most important aspects in the multilayer electronic devices, which is a direct function of dielectric constant. In the case of glass–ceramic packages, the dielectric constant of the glass–ceramic material over and within the metal lines that are deposited or embedded governs the propagation delay td, which is given by the equation [31]
td ¼ lðer Þ1=2 =c Fig. 4. Density of the glass–ceramics prepared at different sintering temperatures.
be easier for this batch to obtain densified glass–ceramics in a broad temperature interval. The fast crystallization rate at high sintering temperature is the main reason for the pores observed in the glass–ceramics prepared at high temperatures. The variation of the glass–ceramics’ density versus sintering temperature is shown in Fig 4. Density of the glass–ceramics is in the range between 2.5 and 2.6 g/cm3, which inclines with the decreasing content of SiO2 and with the temperature, except for the glass–ceramic prepared at 900 °C of which the density is higher than that of others. The density of a glass–ceramic is related to their components and microstructure which can be determined by chemical composition of batches, sintering and crystallization processes. The density of glass–ceramics will decrease with porosity, though increase with the crystallinity as the crystals always own a more compacted structure than the glassy phase with the same chemical compositions. In this work, the crystallinity of glass–ceramic is the main reason for the variation of density, as increasing content of SiO2 depresses the crystallization and high
where l is the line length, er is the relative dielectric constant of the substrate and c is the speed of light. Thus substrates with low dielectric constant are required to increase the speed of the signal [1]. Dielectric loss is described by loss angle tan d, which is always related to the energy transformation from electrical power to heat. High dielectric loss will lead to overheated of the terminal devices and shorten the service time of the terminal devices. The dielectric loss comes from two groups; intrinsic and extrinsic. Intrinsic losses depend on crystal structure and express the interaction of the crystal lattice with the external electric field. Extrinsic losses relate to materials’ microstructure, e.g. the presence of microstructural defects, porosity, micro cracks and impurities [32]. The dielectric constants (a) and losses (b) of glass–ceramics prepared at different sintering temperatures as a function of frequency in the range 40 Hz–10 MHz were shown in Fig. 5. The dielectric constants are in the range 6.0–8.0 and decrease with the frequency from 40 Hz to 10 MHz for all samples. The dielectric constants of the glass–ceramics vary with the sintering temperature as well as the content of SiO2. For the glass–ceramics prepared from MAS33 and MAS30, the dielectric constant decreases with an increase of sintering temperature, while for the glass–ceramics from MAS20, the dielectric constant increases firstly and then
252
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253
Fig. 6. Thermal expansion behavior of the glass–ceramics prepared at different sintering temperatures.
Fig. 5. Dielectric constants (a) and losses (b) of glass–ceramics prepared at different sintering temperatures versus frequency in the range 40 Hz–10 MHz.
decreases. With an increase of SiO2, the dielectric constants of the glass–ceramics increase firstly and then decrease, and the glass– ceramics prepared from MAS30 own the lowest dielectric constant. Many factors influence the dielectric constant of the materials [33– 35], mainly the crystalline phases, crystallinity, chemical composition of the residual glassy phase, grain size, porosity, etc. In this work, the crystalline phase, chemical composition of the residual glassy phase and the porosity are the aspects determining the dielectric constant of the glass–ceramics. On one hand, the dielectric constant of the glass–ceramics with a high porosity is low, which may be the main reason for the low dielectric constant of the glass–ceramics prepared from MAS33. Even though the high SiO2 content densified the glass–ceramics, the SiO2 owns extremely low dielectric constant, so the dielectric constant of glass– ceramics prepared from MAS30 is also low. On the other hand, the dielectric constant of the glass–ceramics was determined by the crystalline phases but also the residual glassy phase, the alkali oxides remaining in the glassy phase after precipitation of cordierite deteriorates the dielectric constant which is the main reason for the high dielectric constant of the glass–ceramic prepared at 900 °C from MAS20. The dielectric losses of the prepared glass– ceramics are in 0.0025–0.01, which are higher than the cordierite glass–ceramics from chemical pure oxides [6,7,13] because of the impurities brought into batches by the potassium feldspar but are lower than the glass–ceramics fabricated in previous literatures [26,27]. Increasing sintering temperature deteriorates the losses of the glass–ceramics, which shows a similar tendency as the variation of porosity in the glass–ceramics, and this phenomenon is more evidently for the glass–ceramics prepared from batches with low SiO2, which should be ascribed to the intrinsic low loss, densification effect on the glass–ceramics and the
enhanced effect on the integrity of the residual glassy phase of SiO2. Generally speaking, each constituent in the glass is crucial to the crystallization, densification, and thus dielectric properties. And their effects are intercorrelated and sometimes, unable to distinguish the contributions from an individual constituent. However, the SiO2 plays a more important role in improving the dielectric properties of the glass–ceramics in this work. For the design of muti-layer electronic substrates, the LTCC substrates are always attached to an active device such as Si (3– 5 106 K1) [36] or GaAs (5.7 106 K1), and the CTE of the LTCC substrate should be matchable to that of the active device to improve the reliability of the terminal devices [5]. The thermal expansion behaviors of glass–ceramics in the temperature range 0–600 °C are shown in Fig. 7. The linear CTEs are deduced from the slopes of the curves of dL/L0 versus temperature as 5.24, 5.41, 5.41, 5.21, 4.32 and 5.49 106 K1 for the samples prepared from MAS33, MAS20 and MAS30 at 900 and 925 °C respectively, which matches to the fore-mentioned active devices. The CTEs of the glass–ceramics shows little difference, except for the glass–ceramic prepared from MAS20 at 925 °C whose CTE is rather lower than that of others ascribed to its higher crystallinity. The CTEs of the glass–ceramics in this work are higher than that of the
Fig. 7. Flexural strength of the glass–ceramics prepared at different sintering temperatures.
J. Wu et al. / Journal of Alloys and Compounds 583 (2014) 248–253 Table 3 Comparison between glass–ceramics in this work and the commercial LTCC substrates.
Sintering temperature (°C) Coefficient of thermal expansion (CTE, ppm) Flexural strength (MPa) Dielectric constant Dielectric loss
MAS30
MAS20
Commercial LTCC materials
925 5.49
900 5.41
<1000 4.5–7.5
114 6.1 (10 MHz) 0.0069 (10 MHz)
116 7.5 (10 MHz) 0.0057 (10 MHz)
116–320 3.8–9.2 (1 MHz) 0.0007–0.006 (1 MHz)
glass–ceramics prepared from chemical pure oxides, and it is because of the alkali metal oxides in the batches. The flexural strength of glass–ceramics sintered at different sintering temperatures is shown in Fig. 6. The values of the flexural strength of the glass–ceramics are in the range 90–130 MPa. The flexural strength of the glass–ceramics decreases with an increase of sintering and it increases along with SiO2 ratio in the batches, and this phenomenon can be explained by the variation of porosity in the glass–ceramics. As many LTCC integrated devices has to operate under harsh environmental conditions, the mechanical strength of the substrate materials is a critical issue regarding prolonging the lifetime of the device, and high values of flexural strength of the substrate materials are required. The low temperature co-fired cordierite glass–ceramics in this work show appropriate strength for the LTCC substrate application. The property parameters of typical glass–ceramics fabricated in this work and the commercial LTCC substrate materials [5] are summarized in Table 3. It indicates that the glass–ceramics fabricated in this work exhibits excellent properties and great potential for the application as LTCC substrates.
4. Conclusions Highly densified cordierite glass–ceramics were successfully fabricated using potassium feldspar as the main raw material in the low temperature range 850–925 °C. The sintering and crystallization behaviors and properties depended on the chemical compositions as well as the sintering temperatures. For the glass–ceramics prepared from batch with low content of SiO2, the tendency of worsening the dielectric properties was more observable by increasing the sintering temperature, and excess SiO2 optimized the sinterability of the glasses, and the structure and properties of the glass–ceramics. Especially, the obtained cordierite glass–ceramics were of low dielectric constants in the range of 6–8, low dielectric losses in the range of 0.0025–0.01, matchable thermal expansion behavior to that of the active devices and appropriate flexural strength in the range of 90–130 MPa. The batch MAS20 and MAS30 appeared to be of the best sinterability and the relevant glass–ceramics exhibited excellent properties, which were potential candidates as LTCC substrates for the application.
253
Acknowledgement The work was financial supported by the National Key Technology R&D Program of Peoples Republic of China (No. 2011BAB03B01). References [1] R.R. Tummala, Journal of the American Ceramic Society 74 (1991) 895–908. [2] J.U. Knickerbocker, S.H. Knickerbocker, Low-temperature co-fired ceramic chip carriers, in: J.K. Wessel (Ed.), Handbook of Advanced Materials, John Wiley & Sons Ltd, American, 2004. [3] S. Yuzo, K. Yoshinobu, K. Keiichiro, K. Masayuki, T. Hideo, IEEE Transactions on Components, Hybrids, and Manufacturing Technology 13 (1990) 751–758. [4] Y. Imanaka, Multilayered low temperature cofired ceramics (LTCC), technology, 2005. [5] M.T. Sebastian, H. Jantunen, International Materials Reviews 53 (2008) 57–90. [6] A.H. Kumar, S. Knickerbocker, R.R. Tummala, Sinterable glass–ceramics for high-performance substrates, in: 42nd Electronic Components and Technology Conference, San Dieg, 1992, pp. 678–681. [7] G. Chen, Journal of Alloys and Compounds 455 (2008) 298–302. [8] G. Chen, X. Liu, Journal of Alloys and Compounds 431 (2007) 282–286. [9] S. Kemethmuller, A. Roosen, F. Goetz-Neunhoeffer, J. Neubauer, Journal of the American Ceramic Society 89 (2006) 2632–2637. [10] Z. Yue, J. Zhou, H. Hang, Z. Gui, L. Li, Journal of Materials Science Letters 19 (2000) 213–215. [11] S. Wang, H. Lu, Z. Hou, Ceramics International 39 (2013) 991–997. [12] E.M. Hamzawy, A.A. El-Kheshen, M.F. Zawrah, Ceramics International 31 (2005) 383–389. [13] M. Majumder, S. Mukhopadhyay, O. Parkash, D. Kumar, Ceramics International 30 (2004) 1067–1070. [14] C.L. Lo, J.G. Duh, B.S. Chiou, W.H. Lee, Journal of the American Ceramic Society 85 (2002) 2230–2235. [15] C.J. Dileep Kumar, E.K. Sunny, N. Raghu, N. Venkataramani, A.R. Kulkarni, Journal of the American Ceramic Society 91 (2008) 652–655. [16] C. Chang, J. Jean, Journal of the American Ceramic Society 82 (1999) 1725– 1732. [17] H. Shao, T. Wang, Q. Zhang, Journal of Alloys and Compounds 484 (2009) 2–5. [18] S. Hwang, J. Wu, Journal of the American ceramic Society 84 (2001) 1108– 1112. [19] G. Chen, Journal of Materials Science. Materials in Electronics 18 (2007) 1253– 1257. [20] R.W. Dupon, R.L. Mcconville, D.J. Musolf, A.C. Tanous, M.S. Thompson, Journal of the American Ceramic Society 73 (1990) 335–339. [21] B.H. Kim, K.H. Lee, Journal of Materials Science 29 (1994) 6592–6598. [22] Y. Sung, Journal of Materials Science 31 (1996) 5421–5427. [23] S. Wang, H. Zhou, L. Luo, Materials Research Bulletin 38 (2003) 1367–1374. [24] G. Chen, X. Liu, Journal of Materials Science. Materials in Electronics 15 (2004) 595–600. [25] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Journal of the American Ceramic Society 94 (2011) 687–694. [26] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Journal of Alloys and Compounds 482 (2009) 429–436. [27] K. Sumi, Y. Kobayashi, E. Kato, Journal of the American Ceramic Society 82 (1999) 783–785. [28] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Journal of Alloys and Compounds 509 (2011) 7645–7651. [29] J. Wu, Z. Li, H. He, Y. Huang, H. Wu, Journal of Materials Science. Materials in Electronics (2013), http://dx.doi.org/10.1007/s10854-013-1089-2. [30] N.J. Azín, M.A. Camerucci, A.L. Cavalieri, Ceramics International 31 (2005) 189– 195. [31] B. Schwartz, American Ceramic Society Bulletin 63 (1984) 577–581. [32] S.J. Penn, N.M.N. Alford, A. Templeton, X. Wang, M. Xu, M. Reece, K. Schrapel, Journal of the American Ceramic Society 80 (1997) 1885–1888. [33] J. Chen, H. Wang, S. Feng, H. Ma, D. Deng, S. Xu, Ceramics International 37 (2011) 989–993. [34] Y. Fang, L. Li, Q. Xiao, X.M. Chen, Ceramics International 38 (2012) 4511–4515. [35] S.O. Yoon, S.H. Shim, K.S. Kim, J.G. Park, S. Kim, Ceramics International 35 (2009) 1271–1275. [36] Y. Shimada, Y. Yamashita, H. Takamizawa, IEEE Transactions on Components, Hybrids, and Manufacturing Technology 11 (1988) 163–170.