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Microwave dielectric properties of temperature stable CoTiNb2O8–CoNb2O6 composite ceramics
Materials Letters 178 (2016) 175–177
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave dielectric properties of temperature stable CoTiNb2O8–CoNb2O6 composite ceramics Yun Zhang, Yingchun Zhang n, Maoqiao Xiang School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 February 2016 Received in revised form 28 April 2016 Accepted 1 May 2016 Available online 3 May 2016
(1 x)CoTiNb2O8–xCoNb2O6 microwave dielectric ceramics were synthesized via conventional solid-state reaction route. Compositionally induced phase formation was investigated by X-ray diffraction. For the CoNb2O6-added compounds, rutile-structured CoTiNb2O8 and columbite phase were coexisted without any secondary phases. The microwave dielectric properties were strongly related to the density, sintering condition, and chemical composition of samples. As expected, CoNb2O6 addition from 0.5 to 0.83 led to a decrease in the εr from 43.0 to 27.2. In contrast, Q f increased considerably, attributing to the CoNb2O6 phase and enhanced densification. When sintered at 1200 °C, the 0.2CoTiNb2O8–0.8CoNb2O6 ceramic possessed excellent microwave dielectric properties with εr 28.9, Q f 36,948 GHz (at 8.1 GHz), and τf 0.5 ppm/°C. & 2016 Elsevier B.V. All rights reserved.
Key words: Microwave dielectric properties Ceramics Sintering
1. Introduction With the rapid development of wireless communication technology operating at microwave frequency, there is an ever-increasing demand for high performance dielectric ceramics [1,2]. These materials are required to possess three major criteria: high dielectric constant (εr 420, to reduce the size of resonators), excellent quality factor (Q f 410,000 GHz, to minimize dielectric loss), and a temperature coefficient of resonant frequency as close to zero as possible (τf 0 ppm/°C, for temperature stability) [3]. Achieving all the above-mentioned characteristics in one material is a formidable task. Recently, ternary compounds with general formula M2 þ M4 þ Nb2O8 (M2 þ ¼Mg, Ca, Mn, Co, Ni, Zn and M4 þ ¼Ti, Zr) were found to be promising candidates for application in microwave devices [4,5]. Amongst the niobates investigated, CoTiNb2O8 with rutile structure was first described by Baumgarte in 1994, and its microwave dielectric properties were reported as εr 64 and Q f 65,300 GHz [5]. In our recent study, the CoTiNb2O8 was characterized by εr ¼63.5 and Q f ¼25,300 GHz after sintering at 1250 °C [6]. However, the large τf ( 86.1 ppm/°C) was poor for practical applications. A common way to prepare temperaturestable ceramics was combining two compatible compounds with opposite τf values. For example, tunability of τf had been done in the (1 x)(Mg0.95Zn0.05)TiO3–x(Ca0.8Sm0.4/3)TiO3 composite [7]. Cobalt niobate CoNb2O6 was a potential candidate for mechanical n
Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.matlet.2016.05.009 0167-577X/& 2016 Elsevier B.V. All rights reserved.
filter coatings and electrical applications. Pullar et al. [8] reported the CoNb2O6 ceramic sintered at 1150 °C exhibited an εr of 22.0 and a high Q f of 41,700 GHz together with a negative τf of 66.6 ppm/ °C. Thus, CoNb2O6 was selected as a τf compensator for CoTiNb2O8 in the present work. Furthermore, the resultant microwave dielectric properties were analyzed based on densification, sintering condition and chemical composition.
2. Experimental procedure CoTiNb2O8 and CoNb2O6 compounds were individually prepared by conventional solid state reaction method. CoO (99.9%), Nb2O5 (99.99%), and TiO2 (99.9%) were adopted as raw chemicals. Stoichiometric amounts of the oxides were ball-milled in alcohol medium. After drying, the screened powders were calcined at 1100 °C and 1000 °C for 4 h to synthesize CoTiNb2O8 and CoNb2O6, respectively. The calcined powders were then mixed in the desired composition of (1 x)CoTiNb2O8–xCoNb2O6 (x ¼0.5–0.83) and remilled for 6 h. Finally, the fine powders with organic binder (3 wt% PVA) were pressed into pellets of 10 mm in diameter and 6 mm in thickness. These green bodies were sintered at 1150–1275 °C for 4 h with a heating rate of 5 °C/min. Crystal structure was undertaken by X-ray diffraction (XRD, Rigaku, DMAX-RB, Japan) with Cu Kα radiation. Scanning electron microscopy (SEM; JSM-6480LV) and energy dispersive spectroscopy (EDS) were utilized to observe the surface morphology and chemical composition of ceramics, respectively. The relative densities of sintered samples were identified by Archimedes method. Microwave dielectric properties of the ceramics were evaluated by
176
Y. Zhang et al. / Materials Letters 178 (2016) 175–177
the Hakki-Coleman method [9] using a network analyzer (HP8720ES, Hewlett-Packard, Santa, Rosa, CA). Temperature coefficients of the resonant frequencies were obtained in the temperature range from 25 °C to 80 °C.
3. Results and discussions Fig. 1 shows the XRD patterns of (1 x)CoTiNb2O8–xCoNb2O6 ceramics sintered at 1200 °C for 4 h. All the peaks could be indexed as CoTiNb2O8 (JCPDS no. 52-1875) and CoNb2O6 phase (JCPDS no. 32-0304), illustrating that the composite were basically composed of two main phases. Columbite CoNb2O6 had been reported to crystallize in an orthorhombic α-PbO2-type structure
Fig. 1. XRD patterns of (1 x)CoTiNb2O8-xCoNb2O6 ceramics sintered at 1200 °C for 4 h.
[10]. Whereas for CoTiNb2O8, it performed tetragonal rutile structure with space group P42/mnm [4]. The large difference in crystal structure might be responsible for the formation of twophase structure rather than a solid solution. It should be noted that increasing x gradually weakened the intensities of the diffraction peaks of CoTiNb2O8 but strengthened those of CoNb2O6. Furthermore, this enhancement was accompanied with a remarkable peak sharpening (smaller WFHM), probably due to crystallite growth. Systematic analysis is still needed to confirm this conclusion since the XRD peak sharpening can be also related to the reduction in strain or stacking fault annihilation. Fig. 2 illustrates the SEM images of (1 x)CoTiNb2O8–xCoNb2O6 ceramics sintered at 1200 °C for 4 h. For x¼ 0.5, the microstructure depicted terraced-shaped grains, layered atop one another with individual grains. Simultaneously, some typical polygon-like grains were distributed randomly over the observed area. From x¼ 0.67 onwards, the terraced-shaped grains tended to become smaller. On the contrary, the average size of the polygon-like grains increased observably, probably because of the low optimal sintering temperature of CoNb2O6 ( 1150 °C) [8]. As x increased to 0.83, a uniform microstructure with closely packed grains having 2–4 mm size was developed [Fig. 2(e)]. Also, some extremely small grains of 1 mm appeared at the boundary junctions of coarse grains, as seen in Fig. 2(b)–(e). To evaluate the evolution of phase constitution associated with microstructure, EDS analysis was used further on the large terraced-shaped grain (A), small terraced-shaped grain (B), large polygon-like grain (C), and small polygon-like grain (D). As list in Fig. 2(f), the terraced-shaped grains were identified as CoTiNb2O8, and the polygon-like ones were CoNb2O6. The relative density, εr, Q f, and τf values of the (1 x) CoTiNb2O8–xCoNb2O6 ceramics are given in Fig. 3. For each composition, the relative densities first increased with increasing temperature and then declined thereafter. With the increase of x from 0.5 to 0.83, the maximum relative density increased from 96.03–97.73%, accompanied by decreased optimal sintering temperature. As reported earlier, the εr should increase, following
Fig. 2. SEM images of (1 x)CoTiNb2O8–xCoNb2O6 ceramics with (a) x¼ 0.5, (b) x¼ 0.67, (c) x¼ 0.75, (d) x ¼0.8, (e) x ¼0.83 sintered at 1200 °C for 4 h and (f) EDS analysis of 0.2CoTiNb2O8–0.8CoNb2O6 ceramic marked in (d).
Y. Zhang et al. / Materials Letters 178 (2016) 175–177
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the Lichtenecker empirical rule for multiphase dielectrics [13]. It was well known that the τf was governed by the composition, the additives, and the second phase of the material [14]. As shown in Fig. 3(d), τf decreased almost linearly to 2.2 ppm/°C after the addition of CoNb2O6. Particularly, a near-zero τf value of 0.5 ppm/°C was obtained at x ¼0.8.
4. Conclusion Microwave dielectric properties of (1 x)CoTiNb2O8–xCoNb2O6 samples with controlled temperature coefficient were studied in this article. X-ray diffraction patterns revealed that CoTiNb2O8 phase coexisted with columbite phase in the entire composition range. With the introduction of CoNb2O6, the sintering behavior of the composite ceramics was improved significantly. As x increased, the εr values presented a rapid drop. On the contrary, the Q f increased gradually, owing to the CoNb2O6 phase with high Q f value. A near-zero τf was achieved by adjusting the x value of the system. Specifically, 0.2CoTiNb2O8–0.8CoNb2O6 ceramics exhibited a well-sintered microstructure with εr ¼ 28.9, Q f ¼36,948 GHz, and τf ¼0.5 ppm/°C at Ts ¼ 1200 °C.
Acknowledgments This work has been sponsored by the National Natural Science Foundation of China, China (Nos. 51372017 and 51172019). Fig. 3. Microwave dielectric properties of (1 x)CoTiNb2O8–xCoNb2O6 ceramics: (a) relative density, (b) εr, (c) Q f vs. sintering temperature, and (d) τf vs. CoNb2O6 content.
References
somewhat similar trend as that of relative density [11]. However, the εr decreased with increasing content of CoNb2O6 in the present study. This might result from the lower dielectric constant of CoNb2O6 (εr ¼ 22.0) than that of CoTiNb2O8 (εr ¼ 63.5) [6,8]. In contrast to εr, Q f increased continuously with the increase of x. The microwave dielectric loss was caused not only by the lattice vibrational modes, but also by pores, secondary phases, and impurities or lattice defects [12]. Here, the increment in Q f values was mainly attributed to the enhanced densification, as well as the CoNb2O6 phase with high Q f value. The microwave dielectric properties of the composite varied as expected in consideration of
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
R. Muhammad, Y. Iqbal, Mater. Lett. 153 (2015) 121–123. A. Tumuluri, P.M.S. Raju, K.C.J. Raju, et al., Mater. Lett. 154 (2015) 128–131. H.D. Xie, C. Chen, B.B. Su, H.H. Xi, Mater. Lett. 166 (2016) 167–170. A. Baumgarte, R. Blachnik, J. Alloy. Compd. 215 (1994) 117–120. C.F. Tseng, J. Eur. Ceram. Soc. 34 (2014) 3641–3648. Y. Zhang, Y.C. Zhang, M.Q. Xiang, et al., Ceram. Int. 42 (2016) 3542–3547. Y.D. Zhang, D. Zhou, J. Guo, H.H. Xi, B. He, Mater. Lett. 132 (2014) 200–202. R.C. Pullar, J.D. Breeze, N.M. Alford, J. Am. Ceram. Soc. 88 (2005) 2466–2471. B.W. Hakki, P.D. Coleman, IRE Trans. Microw. Theory Tech. 8 (1960) 402–410. R.C. Pullar, J. Am. Ceram. Soc. 92 (2009) 563–577. L.X. Li, H. Sun, X.S. Lv, S. Li, Mater. Lett. 160 (2015) 363–365. C.L. Huang, M.H. Weng, Mater. Res. Bull. 35 (2000) 1881–1888. Y.G. Wu, X.H. Zhao, F. Li, Z.G. Fan, J. Electroceram. 11 (2003) 227–239. Y.C. Chen, Y.W. Zeng, J. Alloy. Compd. 481 (2009) 369–372.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave dielectric properties of temperature stable CoTiNb2O8–CoNb2O6 composite ceramics Yun Zhang, Yingchun Zhang n, Maoqiao Xiang School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 February 2016 Received in revised form 28 April 2016 Accepted 1 May 2016 Available online 3 May 2016
(1 x)CoTiNb2O8–xCoNb2O6 microwave dielectric ceramics were synthesized via conventional solid-state reaction route. Compositionally induced phase formation was investigated by X-ray diffraction. For the CoNb2O6-added compounds, rutile-structured CoTiNb2O8 and columbite phase were coexisted without any secondary phases. The microwave dielectric properties were strongly related to the density, sintering condition, and chemical composition of samples. As expected, CoNb2O6 addition from 0.5 to 0.83 led to a decrease in the εr from 43.0 to 27.2. In contrast, Q f increased considerably, attributing to the CoNb2O6 phase and enhanced densification. When sintered at 1200 °C, the 0.2CoTiNb2O8–0.8CoNb2O6 ceramic possessed excellent microwave dielectric properties with εr 28.9, Q f 36,948 GHz (at 8.1 GHz), and τf 0.5 ppm/°C. & 2016 Elsevier B.V. All rights reserved.
Key words: Microwave dielectric properties Ceramics Sintering
1. Introduction With the rapid development of wireless communication technology operating at microwave frequency, there is an ever-increasing demand for high performance dielectric ceramics [1,2]. These materials are required to possess three major criteria: high dielectric constant (εr 420, to reduce the size of resonators), excellent quality factor (Q f 410,000 GHz, to minimize dielectric loss), and a temperature coefficient of resonant frequency as close to zero as possible (τf 0 ppm/°C, for temperature stability) [3]. Achieving all the above-mentioned characteristics in one material is a formidable task. Recently, ternary compounds with general formula M2 þ M4 þ Nb2O8 (M2 þ ¼Mg, Ca, Mn, Co, Ni, Zn and M4 þ ¼Ti, Zr) were found to be promising candidates for application in microwave devices [4,5]. Amongst the niobates investigated, CoTiNb2O8 with rutile structure was first described by Baumgarte in 1994, and its microwave dielectric properties were reported as εr 64 and Q f 65,300 GHz [5]. In our recent study, the CoTiNb2O8 was characterized by εr ¼63.5 and Q f ¼25,300 GHz after sintering at 1250 °C [6]. However, the large τf ( 86.1 ppm/°C) was poor for practical applications. A common way to prepare temperaturestable ceramics was combining two compatible compounds with opposite τf values. For example, tunability of τf had been done in the (1 x)(Mg0.95Zn0.05)TiO3–x(Ca0.8Sm0.4/3)TiO3 composite [7]. Cobalt niobate CoNb2O6 was a potential candidate for mechanical n
Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.matlet.2016.05.009 0167-577X/& 2016 Elsevier B.V. All rights reserved.
filter coatings and electrical applications. Pullar et al. [8] reported the CoNb2O6 ceramic sintered at 1150 °C exhibited an εr of 22.0 and a high Q f of 41,700 GHz together with a negative τf of 66.6 ppm/ °C. Thus, CoNb2O6 was selected as a τf compensator for CoTiNb2O8 in the present work. Furthermore, the resultant microwave dielectric properties were analyzed based on densification, sintering condition and chemical composition.
2. Experimental procedure CoTiNb2O8 and CoNb2O6 compounds were individually prepared by conventional solid state reaction method. CoO (99.9%), Nb2O5 (99.99%), and TiO2 (99.9%) were adopted as raw chemicals. Stoichiometric amounts of the oxides were ball-milled in alcohol medium. After drying, the screened powders were calcined at 1100 °C and 1000 °C for 4 h to synthesize CoTiNb2O8 and CoNb2O6, respectively. The calcined powders were then mixed in the desired composition of (1 x)CoTiNb2O8–xCoNb2O6 (x ¼0.5–0.83) and remilled for 6 h. Finally, the fine powders with organic binder (3 wt% PVA) were pressed into pellets of 10 mm in diameter and 6 mm in thickness. These green bodies were sintered at 1150–1275 °C for 4 h with a heating rate of 5 °C/min. Crystal structure was undertaken by X-ray diffraction (XRD, Rigaku, DMAX-RB, Japan) with Cu Kα radiation. Scanning electron microscopy (SEM; JSM-6480LV) and energy dispersive spectroscopy (EDS) were utilized to observe the surface morphology and chemical composition of ceramics, respectively. The relative densities of sintered samples were identified by Archimedes method. Microwave dielectric properties of the ceramics were evaluated by
176
Y. Zhang et al. / Materials Letters 178 (2016) 175–177
the Hakki-Coleman method [9] using a network analyzer (HP8720ES, Hewlett-Packard, Santa, Rosa, CA). Temperature coefficients of the resonant frequencies were obtained in the temperature range from 25 °C to 80 °C.
3. Results and discussions Fig. 1 shows the XRD patterns of (1 x)CoTiNb2O8–xCoNb2O6 ceramics sintered at 1200 °C for 4 h. All the peaks could be indexed as CoTiNb2O8 (JCPDS no. 52-1875) and CoNb2O6 phase (JCPDS no. 32-0304), illustrating that the composite were basically composed of two main phases. Columbite CoNb2O6 had been reported to crystallize in an orthorhombic α-PbO2-type structure
Fig. 1. XRD patterns of (1 x)CoTiNb2O8-xCoNb2O6 ceramics sintered at 1200 °C for 4 h.
[10]. Whereas for CoTiNb2O8, it performed tetragonal rutile structure with space group P42/mnm [4]. The large difference in crystal structure might be responsible for the formation of twophase structure rather than a solid solution. It should be noted that increasing x gradually weakened the intensities of the diffraction peaks of CoTiNb2O8 but strengthened those of CoNb2O6. Furthermore, this enhancement was accompanied with a remarkable peak sharpening (smaller WFHM), probably due to crystallite growth. Systematic analysis is still needed to confirm this conclusion since the XRD peak sharpening can be also related to the reduction in strain or stacking fault annihilation. Fig. 2 illustrates the SEM images of (1 x)CoTiNb2O8–xCoNb2O6 ceramics sintered at 1200 °C for 4 h. For x¼ 0.5, the microstructure depicted terraced-shaped grains, layered atop one another with individual grains. Simultaneously, some typical polygon-like grains were distributed randomly over the observed area. From x¼ 0.67 onwards, the terraced-shaped grains tended to become smaller. On the contrary, the average size of the polygon-like grains increased observably, probably because of the low optimal sintering temperature of CoNb2O6 ( 1150 °C) [8]. As x increased to 0.83, a uniform microstructure with closely packed grains having 2–4 mm size was developed [Fig. 2(e)]. Also, some extremely small grains of 1 mm appeared at the boundary junctions of coarse grains, as seen in Fig. 2(b)–(e). To evaluate the evolution of phase constitution associated with microstructure, EDS analysis was used further on the large terraced-shaped grain (A), small terraced-shaped grain (B), large polygon-like grain (C), and small polygon-like grain (D). As list in Fig. 2(f), the terraced-shaped grains were identified as CoTiNb2O8, and the polygon-like ones were CoNb2O6. The relative density, εr, Q f, and τf values of the (1 x) CoTiNb2O8–xCoNb2O6 ceramics are given in Fig. 3. For each composition, the relative densities first increased with increasing temperature and then declined thereafter. With the increase of x from 0.5 to 0.83, the maximum relative density increased from 96.03–97.73%, accompanied by decreased optimal sintering temperature. As reported earlier, the εr should increase, following
Fig. 2. SEM images of (1 x)CoTiNb2O8–xCoNb2O6 ceramics with (a) x¼ 0.5, (b) x¼ 0.67, (c) x¼ 0.75, (d) x ¼0.8, (e) x ¼0.83 sintered at 1200 °C for 4 h and (f) EDS analysis of 0.2CoTiNb2O8–0.8CoNb2O6 ceramic marked in (d).
Y. Zhang et al. / Materials Letters 178 (2016) 175–177
177
the Lichtenecker empirical rule for multiphase dielectrics [13]. It was well known that the τf was governed by the composition, the additives, and the second phase of the material [14]. As shown in Fig. 3(d), τf decreased almost linearly to 2.2 ppm/°C after the addition of CoNb2O6. Particularly, a near-zero τf value of 0.5 ppm/°C was obtained at x ¼0.8.
4. Conclusion Microwave dielectric properties of (1 x)CoTiNb2O8–xCoNb2O6 samples with controlled temperature coefficient were studied in this article. X-ray diffraction patterns revealed that CoTiNb2O8 phase coexisted with columbite phase in the entire composition range. With the introduction of CoNb2O6, the sintering behavior of the composite ceramics was improved significantly. As x increased, the εr values presented a rapid drop. On the contrary, the Q f increased gradually, owing to the CoNb2O6 phase with high Q f value. A near-zero τf was achieved by adjusting the x value of the system. Specifically, 0.2CoTiNb2O8–0.8CoNb2O6 ceramics exhibited a well-sintered microstructure with εr ¼ 28.9, Q f ¼36,948 GHz, and τf ¼0.5 ppm/°C at Ts ¼ 1200 °C.
Acknowledgments This work has been sponsored by the National Natural Science Foundation of China, China (Nos. 51372017 and 51172019). Fig. 3. Microwave dielectric properties of (1 x)CoTiNb2O8–xCoNb2O6 ceramics: (a) relative density, (b) εr, (c) Q f vs. sintering temperature, and (d) τf vs. CoNb2O6 content.
References
somewhat similar trend as that of relative density [11]. However, the εr decreased with increasing content of CoNb2O6 in the present study. This might result from the lower dielectric constant of CoNb2O6 (εr ¼ 22.0) than that of CoTiNb2O8 (εr ¼ 63.5) [6,8]. In contrast to εr, Q f increased continuously with the increase of x. The microwave dielectric loss was caused not only by the lattice vibrational modes, but also by pores, secondary phases, and impurities or lattice defects [12]. Here, the increment in Q f values was mainly attributed to the enhanced densification, as well as the CoNb2O6 phase with high Q f value. The microwave dielectric properties of the composite varied as expected in consideration of
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
R. Muhammad, Y. Iqbal, Mater. Lett. 153 (2015) 121–123. A. Tumuluri, P.M.S. Raju, K.C.J. Raju, et al., Mater. Lett. 154 (2015) 128–131. H.D. Xie, C. Chen, B.B. Su, H.H. Xi, Mater. Lett. 166 (2016) 167–170. A. Baumgarte, R. Blachnik, J. Alloy. Compd. 215 (1994) 117–120. C.F. Tseng, J. Eur. Ceram. Soc. 34 (2014) 3641–3648. Y. Zhang, Y.C. Zhang, M.Q. Xiang, et al., Ceram. Int. 42 (2016) 3542–3547. Y.D. Zhang, D. Zhou, J. Guo, H.H. Xi, B. He, Mater. Lett. 132 (2014) 200–202. R.C. Pullar, J.D. Breeze, N.M. Alford, J. Am. Ceram. Soc. 88 (2005) 2466–2471. B.W. Hakki, P.D. Coleman, IRE Trans. Microw. Theory Tech. 8 (1960) 402–410. R.C. Pullar, J. Am. Ceram. Soc. 92 (2009) 563–577. L.X. Li, H. Sun, X.S. Lv, S. Li, Mater. Lett. 160 (2015) 363–365. C.L. Huang, M.H. Weng, Mater. Res. Bull. 35 (2000) 1881–1888. Y.G. Wu, X.H. Zhao, F. Li, Z.G. Fan, J. Electroceram. 11 (2003) 227–239. Y.C. Chen, Y.W. Zeng, J. Alloy. Compd. 481 (2009) 369–372.