- Email: [email protected]
Effects of CaF2 addition on sintering behavior and microwave dielectric properties of ZnTa2O6 ceramics
Materials Letters 65 (2011) 3317–3319
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Effects of CaF2 addition on sintering behavior and microwave dielectric properties of ZnTa2O6 ceramics Wang-Suo Xia, Ling-Xia Li ⁎, Ping Zhang, Ping-Fan Ning School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 1 April 2011 Accepted 18 July 2011 Available online 22 July 2011 Keywords: Microwave dielectric properties ZnTa2O6 ceramics Microstructure Sintering
a b s t r a c t Microwave dielectric ceramics ZnTa2O6 were prepared by conventional mixed oxide route. The effects of CaF2 addition on the microstructures and microwave dielectric properties of ZnTa2O6 ceramics were investigated. Formation of second phase can be detected at the high addition of CaF2 (0.5–1.0 wt.%). Variation of grain shapes were observed with CaF2 content increasing. The sintering temperature of CaF2-doped ZnTa2O6 ceramics can be effectively lowered from 1400 °C to 1225 °C due to liquid phase effect. The microwave dielectric properties were affected by the amount of CaF2 addition. At 1225 °C for 4 h, ZnTa2O6 ceramics with 0.25 wt.% CaF2 possesses excellent microwave dielectric properties: εr = 31.32, Q × ƒ = 73600 GHz(6.8 GHz) and τƒ = − 6.97 ppm/°C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction With the rapid progress of the electronic industry, several types of microwave dielectric materials have been investigated to meet the requirements of microwave application during the last decades [1,2]. In general, these dielectric materials for microwave application required high dielectric constant, low dielectric loss and a near-zero temperature coefficient of resonant frequency. Among these different materials, ZnTa2O6 ceramics have been found to be promising candidate materials for dielectric resonators or filters at microwave frequency. ZnTa2O6 ceramics exhibited excellent microwave dielectric properties with a εr value of 33.7, a Q × ƒ value of 79308 GHz and a τƒ value of 8.4 ppm/°C [3]. However, the sintering temperature of ZnTa2O6 ceramics was higher than 1350 °C without sintering aid if the powder was synthesized by conventional solid-state reaction methods [2,3]. Therefore, there is considerable interest in the development of ZnTa2O6 ceramics with low sintering temperature. Recently, much attention has been paid to developing low-temperature sintering for ZnTa2O6 ceramics. For example, the sintering characteristics of ZnTa2O6 ceramics with CuO addition as sintering aid were reported by Cheng-liang Huang et al. [4]. Moreover, ZnTa2O6 nano-powders were prepared by sol–gel method by Ying-chun Zhang et al. [5]. In addition, fluorides, such as LiF, MgF2 and CaF2 with low melting point, are the well-known sintering aids for microwave ceramics to lower the sintering temperature [6–8]. However, few attempts have been made to ZnTa2O6 ceramics with tri-α-PbO2 structure. In this
⁎ Corresponding author. Tel./fax: + 86 22 27402838. E-mail address: [email protected] (L.-X. Li). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.044
paper, CaF2 was chosen to be the liquid-phase sintering aid to lower the sintering temperature of ZnTa2O6 ceramics. Moreover, the variations of grain shapes, phase composition and microwave dielectric properties of the CaF2-added ZnTa2O6 ceramics were also investigated. 2. Experimental procedure High-pure (N99%) oxide powders of ZnO and Ta2O5 were used as raw materials to prepare the ZnTa2O6 ceramics. They were mixed according to desired stoichiometry ZnTa2O6 ceramics and milled with zirconia balls for 12 h in distilled water. All the slurries were dried and calcined at 1000 °C for 2 h. The calcined powders with different amount of CaF2 additions as sintering aid were re-milled for 24 h with PVA solution as a binder. After drying, the powders were pressed into 10 mm diameter and 5 mm thickness pellets. Then these pellets were sintered at temperatures of 1100–1275 °C for 4 h. The crystalline phases of the sintered samples were identified by X-ray diffraction using Cu Kα radiation. The microstructure observation on ceramic surfaces of samples was performed and analyzed by a scanning electron microscopy (SEM). The densities of the specimens were measured by Archimedes method. The dielectric properties at microwave frequency were measured by a HP8720ES network analyzer using Hakki–Coleman's dielectric resonator method [9,10]. The temperature coefficient of resonant frequency (τƒ) was measured in the temperature range from 25 °C to 85 °C. The τƒ (ppm/°C) can be calculated by noting the change in resonant frequency (Δƒ)
τf =
f2 −f1 f1 ðT2 −T1 Þ
ð1Þ
3318
W.-S. Xia et al. / Materials Letters 65 (2011) 3317–3319
where ƒ1 is resonant frequency at T1 and ƒ2 is the resonant frequency at T2. 3. Result and discussion The X-ray diffraction patterns of the ZnTa2O6-x wt.% CaF2 ceramics with 0.25 ≤ x ≤ 5.0 sintered at 1225 °C for 4 h were shown in Fig. 1. The main crystal phase of the samples could be indexed by tri-α-PbO2 crystal structure and the second phase was observed when x ≥ 0.5. It was also shown that the peak intensities of the second phase increased with increasing CaF2 addition. The variation in phase composition with the content of the CaF2 addition was due to the formation of second phase which was indexed by CaTa2O6 crystal structure as aeschynite. Trend of the peak intensities of CaTa2O6 phase suggested that the proportion of CaTa2O6 was increased with increasing CaF2 content. With 0.25 wt.% CaF2 addition, no second phase was detected as compared to the XRD spectra of pure ZnTa2O6 phase. It indicated that ZnTa2O6 ceramics sintered at 1225 °C with CaF2 addition (0.25 wt.%) exhibited almost a single phase. Fig. 2 illustrated the SEM images of the ZnTa2O6-x wt.% CaF2 ceramics with 0.25 ≤ x ≤ 5.0 sintered at 1225 °C for 4 h. The result indicated that well-developed microstructures of ZnTa2O6 ceramics could be achieved at suitable sintering temperature (1225 °C). In addition, the average grain size of the specimen increased from 1.40 μm to 2.55 μm with an increase in CaF2 content [11]. It suggested that the added CaF2 may accelerate the grain growth due to the liquid phase effect during sintering. However, more uniform morphologies were only revealed for specimen with 0.25 wt.% CaF2 addition. Moreover, with CaF2 content increasing, it was important to note that the grain shape tended to be ruleless, specifically as was shown in Fig. 2e, in which two kinds of shapes were observed, indicating that exceeded CaF2-liquid phase had no benefits to the densification of the ZnTa2O6 ceramics. In order to confirm the content of Ca2+ ionic that solubilized in ZnTa2O6 ceramics, the elemental analysis for marked spots was taken from the well-developed specimens, as illustrated in Fig. 2e. All the EDS results such as a ratio Zn:Ca:Ta = 17.51:0.34:36.31 at.% for spot A, together with the XRD analysis, suggested that the solubility of Ca 2+ ionic in ZnTa2O6 structure was low (~0.3%). Fig. 3 showed the apparent densities and dielectric constant of ZnTa2O6-x wt.% CaF2 ceramics with 0.25 ≤ x ≤ 1.0 at different sintering temperatures for 4 h. With increasing sintering temperature, the apparent density of the specimen increased to a maximum value and thereafter it slightly decreased. The apparent density of the ZnTa2O6
Fig. 1. XRD patterns of ZnTa2O6 ceramics with CaF2 addition sintered at 1225 °C as a function of the CaF2 content.
Fig. 2. Micrographs of ZnTa2O6 ceramics with CaF2 addition at (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, (d) 1.00 wt.% and (e) 5.00 wt.% at 1225 °C.
ceramics with 0.25 wt.% CaF2 content reached the maximum at the lower temperature (1225 °C) due to the single phase. Allow for the variation of grain shapes in SEM images and the XRD results, excessive CaF2 doping would have no benefits to the densification of ZnTa2O6 ceramics. The sintering temperature of pure ZnTa2O6 ceramics was about 1400 °C but it suggested 1225 °C was a suitable temperature to achieve a well-sintered specimen when CaF2 was added. The results suggested that CaF2 was an effective sintering aid for ZnTa2O6 ceramics. It was known that higher density would lead to higher dielectric constant owing to lower porosity. The variation of εr was consistent with that of the density and a maximum εr of 31.84 was obtained for ZnTa2O6 ceramics sintered at 1250 °C for 4 h. Compared with the dielectric constant of pure ZnTa2O6 reported in Ref. [3], the
Fig. 3. Apparent densities and dielectric constants of ZnTa2O6-x wt.% CaF2 ceramics as a function of the sintering temperature.
W.-S. Xia et al. / Materials Letters 65 (2011) 3317–3319
3319
4. Conclusion CaF2 lowered the sintering temperature of ZnTa2O6 ceramics effectively from 1400 °C to 1225 °C due to liquid-phase effect. The ceramics could be well sintered to achieve a relative density as high as 93.4%. The phase composition, grain shape, sintering density and dielectric properties were found to strongly correlate with the amount of CaF2. The CaTa2O6 phase and changes of grain shapes were observed with the addition of CaF2. With increasing the CaF2 content, the Q × ƒ value of ZnTa2O6 ceramics decreased and the τƒ shifted to a negative value. At 1225 °C, 0.25 wt.% CaF2 doped ZnTa2O6 ceramics possesses excellent microwave dielectric properties with an εr value of 31.32, a Q × ƒ value of 73600 GHz and τƒ value of − 6.97 ppm/°C.
Acknowledgments Fig. 4. Q × ƒ and τƒ values of ZnTa2O6 ceramics with CaF2 addition sintered at 1225 °C as a function of the CaF2 content.
lower εr was caused by the CaTa2O6 with εr = 21.2 [2] according to the Lichtenecker logarithmic mixing rule. The Q × ƒ and τƒ values of ZnTa2O6 ceramics with CaF2 addition sintered at 1225 °C were demonstrated in Fig. 4. The Q × ƒ decreased with increasing the CaF2 content, together with the XRD results, suggesting that the variation of Q × ƒ was mainly related to its phase composition. The Q × ƒ with 0.25 wt.% CaF2 addition showed a maximum value of 73600 GHz(6.8 GHz) due to its single phase. The lowing Q × ƒ was due to the formation of the CaTa2O6 second phase. It was well known that the τƒ value was governed by the composition, the additive, and the second phase of the materials. The τƒ tended to be shifted to negative region and varied from −6.97 ppm/°C to −21.1 ppm/°C due to the increase of CaF2.
This work was supported by Program for New Century Excellent Talents in University (NCET) and 863 program (2007AA03Z423) and China Postdoctoral Science Foundation.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Maeda M, Yamamura T, Ikeda T. Jpn J Appl Phys 1987;26:76–9. Lee HJ, Kim IT, Hong KS. Jpn J Appl Phys 1997;36:1318–20. Kan A, Ogawa H, Ohsato H. J Alloy Compd 2002;337:303–8. Huang CL, Chiang KH. Mater Res Bull 2004;39:1701–8. Zhang YC, Fu BJ, Wang X. J Alloy Compd 2009;478:498–500. Pollet M, Marinel S. J Eur Ceram Soc 2003;23:1925–33. Benziada L, Kermoun H. J Fluorine Chem 1999;96:25–9. Cao LF, Li LX, Zhang P, Wu HT. Rare Metals 2010;29:50–4. Hakki BW, Coleman PD. IEEE Trans Microwave Theory Technol 1960;8:402–10. Courtney WE. IEEE Trans Microwave Theory Technol 1970;18:476–85. Mendelson MI. J Ameri Ceram Soc 1969;52:443–6.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Effects of CaF2 addition on sintering behavior and microwave dielectric properties of ZnTa2O6 ceramics Wang-Suo Xia, Ling-Xia Li ⁎, Ping Zhang, Ping-Fan Ning School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 1 April 2011 Accepted 18 July 2011 Available online 22 July 2011 Keywords: Microwave dielectric properties ZnTa2O6 ceramics Microstructure Sintering
a b s t r a c t Microwave dielectric ceramics ZnTa2O6 were prepared by conventional mixed oxide route. The effects of CaF2 addition on the microstructures and microwave dielectric properties of ZnTa2O6 ceramics were investigated. Formation of second phase can be detected at the high addition of CaF2 (0.5–1.0 wt.%). Variation of grain shapes were observed with CaF2 content increasing. The sintering temperature of CaF2-doped ZnTa2O6 ceramics can be effectively lowered from 1400 °C to 1225 °C due to liquid phase effect. The microwave dielectric properties were affected by the amount of CaF2 addition. At 1225 °C for 4 h, ZnTa2O6 ceramics with 0.25 wt.% CaF2 possesses excellent microwave dielectric properties: εr = 31.32, Q × ƒ = 73600 GHz(6.8 GHz) and τƒ = − 6.97 ppm/°C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction With the rapid progress of the electronic industry, several types of microwave dielectric materials have been investigated to meet the requirements of microwave application during the last decades [1,2]. In general, these dielectric materials for microwave application required high dielectric constant, low dielectric loss and a near-zero temperature coefficient of resonant frequency. Among these different materials, ZnTa2O6 ceramics have been found to be promising candidate materials for dielectric resonators or filters at microwave frequency. ZnTa2O6 ceramics exhibited excellent microwave dielectric properties with a εr value of 33.7, a Q × ƒ value of 79308 GHz and a τƒ value of 8.4 ppm/°C [3]. However, the sintering temperature of ZnTa2O6 ceramics was higher than 1350 °C without sintering aid if the powder was synthesized by conventional solid-state reaction methods [2,3]. Therefore, there is considerable interest in the development of ZnTa2O6 ceramics with low sintering temperature. Recently, much attention has been paid to developing low-temperature sintering for ZnTa2O6 ceramics. For example, the sintering characteristics of ZnTa2O6 ceramics with CuO addition as sintering aid were reported by Cheng-liang Huang et al. [4]. Moreover, ZnTa2O6 nano-powders were prepared by sol–gel method by Ying-chun Zhang et al. [5]. In addition, fluorides, such as LiF, MgF2 and CaF2 with low melting point, are the well-known sintering aids for microwave ceramics to lower the sintering temperature [6–8]. However, few attempts have been made to ZnTa2O6 ceramics with tri-α-PbO2 structure. In this
⁎ Corresponding author. Tel./fax: + 86 22 27402838. E-mail address: [email protected] (L.-X. Li). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.044
paper, CaF2 was chosen to be the liquid-phase sintering aid to lower the sintering temperature of ZnTa2O6 ceramics. Moreover, the variations of grain shapes, phase composition and microwave dielectric properties of the CaF2-added ZnTa2O6 ceramics were also investigated. 2. Experimental procedure High-pure (N99%) oxide powders of ZnO and Ta2O5 were used as raw materials to prepare the ZnTa2O6 ceramics. They were mixed according to desired stoichiometry ZnTa2O6 ceramics and milled with zirconia balls for 12 h in distilled water. All the slurries were dried and calcined at 1000 °C for 2 h. The calcined powders with different amount of CaF2 additions as sintering aid were re-milled for 24 h with PVA solution as a binder. After drying, the powders were pressed into 10 mm diameter and 5 mm thickness pellets. Then these pellets were sintered at temperatures of 1100–1275 °C for 4 h. The crystalline phases of the sintered samples were identified by X-ray diffraction using Cu Kα radiation. The microstructure observation on ceramic surfaces of samples was performed and analyzed by a scanning electron microscopy (SEM). The densities of the specimens were measured by Archimedes method. The dielectric properties at microwave frequency were measured by a HP8720ES network analyzer using Hakki–Coleman's dielectric resonator method [9,10]. The temperature coefficient of resonant frequency (τƒ) was measured in the temperature range from 25 °C to 85 °C. The τƒ (ppm/°C) can be calculated by noting the change in resonant frequency (Δƒ)
τf =
f2 −f1 f1 ðT2 −T1 Þ
ð1Þ
3318
W.-S. Xia et al. / Materials Letters 65 (2011) 3317–3319
where ƒ1 is resonant frequency at T1 and ƒ2 is the resonant frequency at T2. 3. Result and discussion The X-ray diffraction patterns of the ZnTa2O6-x wt.% CaF2 ceramics with 0.25 ≤ x ≤ 5.0 sintered at 1225 °C for 4 h were shown in Fig. 1. The main crystal phase of the samples could be indexed by tri-α-PbO2 crystal structure and the second phase was observed when x ≥ 0.5. It was also shown that the peak intensities of the second phase increased with increasing CaF2 addition. The variation in phase composition with the content of the CaF2 addition was due to the formation of second phase which was indexed by CaTa2O6 crystal structure as aeschynite. Trend of the peak intensities of CaTa2O6 phase suggested that the proportion of CaTa2O6 was increased with increasing CaF2 content. With 0.25 wt.% CaF2 addition, no second phase was detected as compared to the XRD spectra of pure ZnTa2O6 phase. It indicated that ZnTa2O6 ceramics sintered at 1225 °C with CaF2 addition (0.25 wt.%) exhibited almost a single phase. Fig. 2 illustrated the SEM images of the ZnTa2O6-x wt.% CaF2 ceramics with 0.25 ≤ x ≤ 5.0 sintered at 1225 °C for 4 h. The result indicated that well-developed microstructures of ZnTa2O6 ceramics could be achieved at suitable sintering temperature (1225 °C). In addition, the average grain size of the specimen increased from 1.40 μm to 2.55 μm with an increase in CaF2 content [11]. It suggested that the added CaF2 may accelerate the grain growth due to the liquid phase effect during sintering. However, more uniform morphologies were only revealed for specimen with 0.25 wt.% CaF2 addition. Moreover, with CaF2 content increasing, it was important to note that the grain shape tended to be ruleless, specifically as was shown in Fig. 2e, in which two kinds of shapes were observed, indicating that exceeded CaF2-liquid phase had no benefits to the densification of the ZnTa2O6 ceramics. In order to confirm the content of Ca2+ ionic that solubilized in ZnTa2O6 ceramics, the elemental analysis for marked spots was taken from the well-developed specimens, as illustrated in Fig. 2e. All the EDS results such as a ratio Zn:Ca:Ta = 17.51:0.34:36.31 at.% for spot A, together with the XRD analysis, suggested that the solubility of Ca 2+ ionic in ZnTa2O6 structure was low (~0.3%). Fig. 3 showed the apparent densities and dielectric constant of ZnTa2O6-x wt.% CaF2 ceramics with 0.25 ≤ x ≤ 1.0 at different sintering temperatures for 4 h. With increasing sintering temperature, the apparent density of the specimen increased to a maximum value and thereafter it slightly decreased. The apparent density of the ZnTa2O6
Fig. 1. XRD patterns of ZnTa2O6 ceramics with CaF2 addition sintered at 1225 °C as a function of the CaF2 content.
Fig. 2. Micrographs of ZnTa2O6 ceramics with CaF2 addition at (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, (d) 1.00 wt.% and (e) 5.00 wt.% at 1225 °C.
ceramics with 0.25 wt.% CaF2 content reached the maximum at the lower temperature (1225 °C) due to the single phase. Allow for the variation of grain shapes in SEM images and the XRD results, excessive CaF2 doping would have no benefits to the densification of ZnTa2O6 ceramics. The sintering temperature of pure ZnTa2O6 ceramics was about 1400 °C but it suggested 1225 °C was a suitable temperature to achieve a well-sintered specimen when CaF2 was added. The results suggested that CaF2 was an effective sintering aid for ZnTa2O6 ceramics. It was known that higher density would lead to higher dielectric constant owing to lower porosity. The variation of εr was consistent with that of the density and a maximum εr of 31.84 was obtained for ZnTa2O6 ceramics sintered at 1250 °C for 4 h. Compared with the dielectric constant of pure ZnTa2O6 reported in Ref. [3], the
Fig. 3. Apparent densities and dielectric constants of ZnTa2O6-x wt.% CaF2 ceramics as a function of the sintering temperature.
W.-S. Xia et al. / Materials Letters 65 (2011) 3317–3319
3319
4. Conclusion CaF2 lowered the sintering temperature of ZnTa2O6 ceramics effectively from 1400 °C to 1225 °C due to liquid-phase effect. The ceramics could be well sintered to achieve a relative density as high as 93.4%. The phase composition, grain shape, sintering density and dielectric properties were found to strongly correlate with the amount of CaF2. The CaTa2O6 phase and changes of grain shapes were observed with the addition of CaF2. With increasing the CaF2 content, the Q × ƒ value of ZnTa2O6 ceramics decreased and the τƒ shifted to a negative value. At 1225 °C, 0.25 wt.% CaF2 doped ZnTa2O6 ceramics possesses excellent microwave dielectric properties with an εr value of 31.32, a Q × ƒ value of 73600 GHz and τƒ value of − 6.97 ppm/°C.
Acknowledgments Fig. 4. Q × ƒ and τƒ values of ZnTa2O6 ceramics with CaF2 addition sintered at 1225 °C as a function of the CaF2 content.
lower εr was caused by the CaTa2O6 with εr = 21.2 [2] according to the Lichtenecker logarithmic mixing rule. The Q × ƒ and τƒ values of ZnTa2O6 ceramics with CaF2 addition sintered at 1225 °C were demonstrated in Fig. 4. The Q × ƒ decreased with increasing the CaF2 content, together with the XRD results, suggesting that the variation of Q × ƒ was mainly related to its phase composition. The Q × ƒ with 0.25 wt.% CaF2 addition showed a maximum value of 73600 GHz(6.8 GHz) due to its single phase. The lowing Q × ƒ was due to the formation of the CaTa2O6 second phase. It was well known that the τƒ value was governed by the composition, the additive, and the second phase of the materials. The τƒ tended to be shifted to negative region and varied from −6.97 ppm/°C to −21.1 ppm/°C due to the increase of CaF2.
This work was supported by Program for New Century Excellent Talents in University (NCET) and 863 program (2007AA03Z423) and China Postdoctoral Science Foundation.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Maeda M, Yamamura T, Ikeda T. Jpn J Appl Phys 1987;26:76–9. Lee HJ, Kim IT, Hong KS. Jpn J Appl Phys 1997;36:1318–20. Kan A, Ogawa H, Ohsato H. J Alloy Compd 2002;337:303–8. Huang CL, Chiang KH. Mater Res Bull 2004;39:1701–8. Zhang YC, Fu BJ, Wang X. J Alloy Compd 2009;478:498–500. Pollet M, Marinel S. J Eur Ceram Soc 2003;23:1925–33. Benziada L, Kermoun H. J Fluorine Chem 1999;96:25–9. Cao LF, Li LX, Zhang P, Wu HT. Rare Metals 2010;29:50–4. Hakki BW, Coleman PD. IEEE Trans Microwave Theory Technol 1960;8:402–10. Courtney WE. IEEE Trans Microwave Theory Technol 1970;18:476–85. Mendelson MI. J Ameri Ceram Soc 1969;52:443–6.