Pergamon
`
`Materials Research Bulletin 35 (2000) 2445–2456
`
`Low temperature sintering and microwave dielectric
`properties of Ba2Ti9O20 ceramics using glass additions
`Cheng-Liang Huang*, Min-Hung Weng, Ciou-Tizer Lion, Chen-Cher Wu
`Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, ROC
`
`(Refereed)
`Received 21 February 2000; 7 April 2000
`
`Abstract
`The effects of glass additions on the microwave dielectric properties and the microstructures of
`Ba2Ti9O20 ceramics were investigated. The simple glasses B2O3 and SiO2 and the commercial glass
`PbO–B2O3–SiO2 (PBS) were selected as liquid phase sintering aids to lower the sintering temperature
`of Ba2Ti9O20 ceramics. With glass additions, the sintering temperatures can be effectively lowered.
`The dielectric properties of Ba2Ti9O20 ceramics with glass additions are strongly dependent on the
`densification, the microstructure, and the reaction between glasses and ceramics. Ba2Ti9O20 ceramics
`without glass addition sintered at 1380°C has a dielectric constant εr ⫽ 38.8, a quality factor Q ⫽ 7200
`at 6.0 GHz, and a temperature coefficient of resonant frequency ␶f ⫽ 4 ppm/°C. Among these glasses,
`5 wt% B2O3 added Ba2Ti9O20 can be sintered to achieve theoretical densities of 96 and 95% at 1100
`and 1200°C, and gives εr values of 36.5 and 36 together with Q values of 3200 and 6700 (at 6 GHz),
`respectively. Results of XRD analysis and scanning electron microscopy are also presented. © 2001
`Elsevier Science Ltd. All rights reserved.
`
`Keywords: A. Ceramics; A. Oxides; C. X-ray diffraction; D. Dielectric properties
`
`1. Introduction
`
`Miniaturization of dielectric devices for volume efficiency is a major requirement in
`modern microwave telecommunication systems such as portable or automobile telephone
`systems. Multilayer microwave devices are suitable for bandpass filters and antenna duplex-
`
`* Corresponding author.
`E-mail address: huangcl@mail.ncku.edu.tw (C.-L. Huang).
`
`0025-5408/00/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
`PII: S0025-5408(00)00432-3
`
`Exhibit 1035
`IPR2016-00636
`AVX Corporation
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`ers [1]. In the fabrication of multilayer microwave devices, low-firing-microwave dielectric
`materials with high dielectric constants εr, high Q⫻ f values, and small temperature coefficients
`of the resonant frequency ␶f are needed in order to co-fire with low-melting-point conductors.
`Since most of the known commercial dielectric materials for high-frequency applications have
`high sintering temperatures, they are not compatible with the co-fire process. Recently, low-
`melting glass additions, chemical processing, and starting materials with smaller particle sizes
`have been commonly used to reduce the sintering temperature of dielectric ceramics [2–6].
`Among these three methods, low-melting glass additions for liquid-phase sintering is lower
`in cost and easier to process than the other two. Some of the low-temperature sintering
`ceramics using glass additions, such as (Zr,Sn)TiO4 ⫹ glass [2], BaO–TiO2–BaWO4 ⫹ glass
`[3], and BaO–PbO–Nd2O3–TiO2 ⫹ glass [4], have already been investigated.
`Ba2Ti9O20 ceramics exhibit good microwave dielectric properties with high dielectric
`constant (⬃39.8) and good quality factor Q (⬃5000 at 8 GHz). However, severe degradation
`of dielectric properties during conventional high-temperature sintering (⬃1400°C) leads to
`compositional and structural fluctuations due to the reduction of Ti4⫹ to Ti3⫹ [5]. Several
`researchers have made efforts to stabilize the Ba2Ti9O20 phase and improve the dielectric
`properties, but only a few studies were dedicated to lowering its sintering temperature. The
`sintering temperature of Ba2Ti9O20 ceramics can be effectively lowered to 1200°C by the
`citrate route [5] and sol-gel processing [6]. These two chemical techniques require flexible
`processing and are expensive. There are only a few reports on the effect of glass additions,
`although low-melting glass additions as liquid-phase sintering aids are cheaper and more
`easily lower the firing temperature. In this paper, the influence of glass additions on the
`sintering temperatures and the microwave dielectric properties of Ba2Ti9O20 ceramics have
`been studied. Ba2Ti9O20 ceramics were chosen as the host material to be sintered with simple
`commercial glasses. Silica glass was selected because of its low dielectric loss (tan ␦ ⬍
`0.001) in the microwave region. B2O3 is a well-known liquid promoter. The reason for
`choosing PBS glass is that it exhibits a higher dielectric constant and lower dielectric loss due
`to the high polarizability of Pb and the low dielectric loss of silica glass. The microwave
`dielectric properties of glass sintered Ba2Ti9O20 ceramics were characterized and the rela-
`tionship to the microstructures of Ba2Ti9O20 ceramics was investigated.
`
`2. Experimental procedure
`
`High-purity oxide powders of BaCO3 and TiO2 were used as starting powders. The
`starting materials were mixed according to the desired stoichiometry and ground in distilled
`water for 10 h in a ball mill with grinding agent balls. Mixtures were dried and calcined at
`1150–1250°C for 3 h. Crystalline phases of the calcined powder were identified by X-ray
`diffraction (XRD) pattern analysis (Seimens D5000) to determine the calcining temperature
`of Ba2Ti9O20 ceramics. Calcined powder was then re-milled with 5 wt% different glass flux
`additions (B2O3, SiO2, and PBS) for 5 h. The commercial glass was supplied by Nippon
`Electric Glass Co. Ltd. After drying, polyvinyl alcohol (PVA) was added to the calcined
`powder as a binder. Pellets 11 mm in diameter and 5 mm thick were prepared by uniaxial
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`pressing. After binder removal, these pellets were sintered at temperatures of 1000–1300°C
`for 3 h. Pure Ba2Ti9O20 ceramic disks without glass additions were prepared by a similar
`process and sintered at temperatures of 1200–1380°C for 3 h as standard reference.
`The bulk densities of the sintered pellets were measured by Archimedes method. The
`theoretical densities (D) for glass-sintered Ba2Ti9O20 ceramics were obtained from
`(1)
`D ⫽ 共W1 ⫹ W2兲/共W1/D1 ⫹ W2/D2兲
`where W1 and W2 are the weight percentages of Ba2Ti9O20 ceramics and glass in the
`mixture, respectively. D1 and D2 represent the densities of Ba2Ti9O20 ceramics and glass in
`the mixture, respectively. Eq. (1) is valid only if no interaction takes place between the glass
`and the dielectrics. The dielectric constants εr and the quality factors Q at microwave
`frequencies were measured using the Hakki–Coleman dielectric resonator method [7] as
`modified and improved by Courtney [8]. A cylindrical dielectric resonator was positioned
`between two brass plates connected to the measuring system. A HP8757D network analyzer
`and a HP8350B sweep oscillator were employed in the measurement. Since the shrinkage of
`all the specimens was not uniform, the Q values were measured at different frequencies (6–8
`GHz). Due to the fact that Q ⫻ f is constant in the microwave region, Q values were
`normalized at 6 GHz. The microstructure observation of the sintered surface of ceramics was
`performed by scanning electron microscopy (SEM, JEOL JSM-6400) and energy-dispersive
`spectroscopy (EDS). The crystalline phases of the glass-sintered ceramics were also identi-
`fied from X-ray diffraction patterns.
`
`3. Results and discussion
`
`3.1. Properties of pure ceramics and glass
`
`Since there are several thermodynamically stable compounds in the vicinity of the desired
`composition of TiO2-rich BaO–TiO2 system, the stoichiometry must be precisely controlled
`for the preparation of single phase Ba2Ti9O20 from TiO2 and BaCO3 oxide powders by
`conventional solid-state reaction. Fig. 1 shows the X-ray diffraction patterns of Ba2Ti9O20
`powders calcined at 1150–1250°C. At the calcining temperature (Tc) ⫽ 1150°C, BaTi4O9
`was the major phase. However, Ba2Ti9O20 appeared as the main crystalline phase associated
`with minor amounts of BaTi4O9 at Tc ⫽ 1200°C. At 1250°C, the Ba2Ti9O20 phase was
`completely formed. The relative proportion of Ba2Ti9O20 phase was defined as the ratio of
`the most-intense XRD peak height of Ba2Ti9O20 to the sum of the most-intense peak heights
`of Ba2Ti9O20 and BaTi4O9. The ratios were found to be 0.1, 0.7, and 0.92 at Tc ⫽ 1150,
`1200, and 1250°C, respectively. After calcination of the Ba2Ti9O20 powder, some Ba2Ti9O20
`phases were needed for obtaining the single phase Ba2Ti9O20 after sintering [9]. The ceramic
`powder calcined at 1200°C was selected for sintering with the glass additions.
`Table 1 shows the densification and the microwave dielectric properties of pure Ba2Ti9O20
`ceramics sintered at different temperatures as standards. The density of pure Ba2Ti9O20 ceramics
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`Fig. 1. X-ray diffraction patterns of Ba2Ti9O20 powders calcined at different temperature for 3 h (E, Ba2Ti9O20;
`⫻, BaTi4O9).
`
`steadily increased from 65 to 96% of theoretical densities (TDs) as the sintering temperature
`increased from 1100 to 1380°C. The εr and Q values increased with increasing sintering
`temperature and exhibited the same trend as the density. At 1380°C, as the density reached its
`maximum value, the εr and Q values were 38.8 and 7200 (at 6 GHz), respectively. For lower
`densities, the Q values rapidly decreased since interfacial polarization is thought to play a role in
`porous materials. Furthermore, the Q values varied with the measuring conditions, especially
`humidity, at lower density. The properties of different glasses are shown in Table 2. XRD
`analysis revealed that the structure of the PBS glass disk remained amorphous.
`
`Table 1
`Dielectric properties of pure Ba2Ti9O20 ceramics at different sintering temperature
`εr
`Material
`Sintering
`Density
`temperature (°C)
`(g/cm3)
`1200
`3.43
`1300
`4.37
`1380
`4.42
`
`Ba2Ti9O20
`Ba2Ti9O20
`Ba2Ti9O20
`
`23.8
`36.3
`38.8
`
`Q value
`(at 6 GHz)
`1580
`4530
`7200
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`Table 2
`Physical properties of glass used as liquid phase sintering aids
`Material
`Melting
`Density
`temperature (°C)
`(g/cm3)
`1710
`2.30
`450
`2.46
`486
`5.38
`
`SiO2
`B2O3
`PBS
`
`εr
`
`—
`—
`11.7
`
`Tan ␦ ⫽(Q ⫺1)
`(10⫺4 at 6 GHz)
`⬍10
`—
`26
`
`3.2. Densification of glass-sintered ceramics
`
`The relative densities of glass-sintered Ba2Ti9O20 ceramics as a function of their sintering
`temperatures are indicated in Fig. 2. It showed that only 74 and 93% of TDs were obtained for
`undoped Ba2Ti9O20 ceramics at 1200 and 1300°C, respectively. The estimated relative densities
`were 4.46, 4.43, and 4.64 g/cm3 for 5 wt% SiO2-, B2O3-, and PBS-sintered Ba2Ti9O20 ceramics,
`respectively. The TDs of B2O3-sintered Ba2Ti9O20 ceramics increased from 80 to 96% as the
`sintering temperature increased from 1000 to 1200°C. The PBS-sintered Ba2Ti9O20 ceramics
`showed similar results. After reaching a maximum value (95.3%) at 1200°C, TDs of PBS-
`
`Fig. 2. Densities of glass-sintered Ba2Ti9O20 ceramics as functions of their sintering temperatures.
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`Fig. 3. SEM micrographs of Ba2Ti9O20 ceramics at (a) 1200 and (b) 1300°C, with 5 wt% SiO2 at (c) 1100 and
`(d) 1200°C, with 5 wt% B2O3 at (e) 1100 and (f) 1200°C, and with 5 wt% PBS at (g) 1100 and (h) 1200°C.
`
`sintered Ba2Ti9O20 ceramics decreased. The TDs of B2O3- and PBS-sintered Ba2Ti9O20 ceramics
`at 1200°C were 96 and 95.3%, respectively. Each of these TDs was more than 20% higher than
`that of pure Ba2Ti9O20 ceramics at the same sintering temperature. Although SiO2-sintered
`Ba2Ti9O20 ceramics showed the worst sinterability, it was still better than that of pure Ba2Ti9O20.
`The main requirement for liquid phase sintering is that the liquid phase must wet the Ba2Ti9O20
`grains. It was believed that the grain-wetting ability of B2O3 and PBS additions were better than
`that of SiO2 addition in Ba2Ti9O20 ceramics.
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`Fig. 4. XRD patterns of Ba2Ti9O20 ceramics with and without glass additions sintered at 1100°C.
`
`3.3. Microstructures of glass-sintered ceramics
`
`To investigate the change of densification and grain size of the samples, the surfaces
`of the sintered specimens were examined. The SEM micrographs of Ba2Ti9O20 ceramics
`with and without glass additions at different sintering temperatures are presented in Fig.
`3. The surfaces of pure Ba2Ti9O20 ceramics sintered at 1200°C were porous and the
`grains had not grown. However, uniform grain growth and a reduction in porosity was
`observed for glass-sintered Ba2Ti9O20 ceramics at 1100°C. The effect of glass doping
`was demonstrated not only by the decrease of the sintering temperature, but also by the
`improved grain growth of the glass-doped Ba2Ti9O20 ceramics. The grain growth rate
`was higher for B2O3- and PBS-sintered Ba2Ti9O20 ceramics (more liquid phase existed),
`but grain growth saturated to a size of 5 ␮m at 1200°C. Due to an inhomogeneous liquid
`phase distribution, a heterogeneous microstructure formed during sintering, as shown in
`Fig. 3.
`The crystalline phases of glass-sintered Ba2Ti9O20 ceramics at 1100°C for 3 h were
`identified. The XRD diffraction patterns are shown in Fig. 4. The Ba2Ti9O20 phases
`existed as the main crystalline phases and no other crystalline phase was formed for
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`Fig. 5. EDS analysis of B2O3-sintered Ba2Ti9O20 ceramics at 1200°C.
`
`PBS-sintered ceramics. However, a small amount of secondary phases such as BaTi4O9
`and TiO2 were detected for B2O3-sintered Ba2Ti9O20 ceramics, although some of the
`peaks overlapped the main phase and were difficult to identify. To exclude the existence
`of secondary phases, EDS analysis of B2O3-sintered Ba2Ti9O20 ceramics was performed;
`the results are given in Fig. 5. Since the grain morphologies were similar, the grains of
`different phases were distinguished by back electron imaging (BEI). After many trials,
`Spot A, in which Ba and Ti coexisted, was the principal type of grain in the matrix, while
`spot B showed Ti only. This confirmed the existence of TiO2 in B2O3-sintered Ba2Ti9O20
`ceramics.
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`Fig. 6. Dielectric constants of pure and glass-sintered Ba2Ti9O20 ceramics as functions of the sintering
`temperatures.
`
`3.4. Microwave dielectric properties of glass-sintered ceramics
`
`The dielectric constants of glass-sintered Ba2Ti9O20 ceramics as a function of their
`sintering temperatures are shown in Fig. 6. The relationship between εr values and sintering
`temperatures reveals the same trend as that between densities and sintering temperatures. The
`εr values increased with increasing sintering temperatures, due to the increase of densities.
`The εr values of well-sintered (at 1200°C) SiO2-, B2O3-, and PBS-Ba2Ti9O20 ceramics were
`35, 36.5, and 37.2, respectively. The decrease of the εr values of glass-sintered ceramics was
`expected due to the mixture of high εr value dielectric ceramics and lower εr value glass, and
`can be explained by the mixing rules [10].
`Fig. 7 illustrates the plots of Q values of glass-sintered Ba2Ti9O20 ceramics vs. their
`sintering temperatures. The Q values increased with increasing sintering temperature, fol-
`lowing the trend with density. SiO2-sintered Ba2Ti9O20 ceramics showed the lowest Q value.
`The Q value (6 GHz) of PBS-sintered Ba2Ti9O20 ceramics was lower than that of pure
`Ba2Ti9O20 although the dielectric constant remained high. The quality factors Q can be
`estimated from the following relation [10]:
`⫺1
`⫺1 ⫹ v2Q2
`Q⫺1 ⫽ v1Q1
`(2)
`where vi is the volume fraction of the ceramics (i ⫽ 1) and the glass (i ⫽ 2). However, the
`measured dielectric loss tan ␦ was higher than the estimated values. Many factors can affect
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`Fig. 7. Q values of pure and glass-sintered Ba2Ti9O20 ceramics as functions of the sintering temperatures.
`
`the microwave dielectric loss of dielectric resonators, such as lattice vibrational modes, the
`porosity, and secondary phases [11]. For Ba2Ti9O20 ceramics, grain size and phase control
`are often responsible [9]. Generally, larger grain size leads to a reduction in dielectric loss
`[11]. However, the experimental results indicated that the direct influence of grain bound-
`aries on dielectric loss was small (compare Fig. 3 to Fig. 7). In fact, the effect of glass
`addition on microwave dielectric loss was dependent upon the chemistry of the glass, the
`chemical reactions, the phase changes during sintering, and the final density. There are three
`types of dielectric loss for glasses: resonance-type vibration losses, migration losses, and
`deformation losses. Resonance-type vibration losses are particularly important in the micro-
`wave region. It seemed that the dielectric loss of glass was important in PBS-Ba2Ti9O20
`ceramics. For B2O3-sintered Ba2Ti9O20 ceramic, the Q values still maintained good prop-
`erties even at low sintering temperature. The Q values reached a maximum of 6500 (at 6
`GHz) at 1200°C. The high Q values of B2O3-sintered Ba2Ti9O20 ceramics were caused by
`the high densification and the existence of second phase TiO2 (εr ⬃104 and Q ⬃14000 at 3
`GHz ) [12], as shown in Figs. 4 and 5. The result was similar to that observed in
`B2O3-sintered BaO-TiO2-BaWO4 ceramics [3].
`Fig. 8 shows the ␶f values of glass-sintered Ba2Ti9O20 ceramics as functions of the
`sintering temperatures. The ␶f values of SiO2-sintered Ba2Ti9O20 ceramics were not shown
`due to the poor quality values. For PBS-sintered Ba2Ti9O20 ceramics, the ␶f showed small
`positive values. However, the ␶f values were 32 and 38 ppm/°C for B2O3-Ba2Ti9O20
`ceramics sintered at 1100 and 1200°C, respectively. It is believed that the large positive ␶f
`values were caused by the existence of TiO2 phase (␶f ⬃400 ppm/°C).
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`Fig. 8. ␶f values of pure and glass-sintered Ba2Ti9O20 ceramics as functions of the sintering temperatures.
`
`4. Conclusions
`
`In this study, several glass fluxes (B2O3, SiO2, and PbO–B2O3–SiO2 ) are employed as
`liquid phase aids in lowering the sintering temperature of Ba2Ti9O20 ceramics. With B2O3
`or PbO–B2O3–SiO2 additions, Ba2Ti9O20 ceramics can be well sintered to achieve more than
`95% theoretical densities below 1200°C. Single phase Ba2Ti9O20 can be obtained for
`PBS-Ba2Ti9O20 ceramics at 1200°C. The dielectric properties of Ba2Ti9O20 ceramics with
`glass additions are strongly dependent on the densification, the microstructure, and the
`reaction between glasses and ceramics. Dielectric constants of 36.5 and 37.2, Q values of
`6700 and 1650 (at 6 GHz), and ␶f values of 38 and 9 ppm/°C were obtained for Ba2Ti9O20
`ceramics sintered at 1200°C with 5 wt% B2O3 and PbO–B2O3–SiO2 additions, respectively.
`
`Acknowledgments
`
`This work was supported by the National Science Council of the Republic of China under
`Grant NSC-87-2213-E-006-027.
`
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