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The influence of Cu substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics
Journal of Alloys and Compounds 551 (2013) 463–467
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
The influence of Cu substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics Bin Tang, Shengquan Yu ⇑, Hetuo Chen, Shuren Zhang, Xiaohua Zhou State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a r t i c l e
i n f o
Article history: Received 29 August 2012 Received in revised form 30 October 2012 Accepted 1 November 2012 Available online 10 November 2012 Keywords: Ceramics Sintering Dielectric response Microwave dielectrics
a b s t r a c t BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics were prepared by the conventional solid-state reaction technique and the effects of Cu2+ substitution on the crystal structure, microstructure and microwave dielectric properties of BaZn2Ti4O11 ceramics were investigated. The substitution of Cu2+ for Zn2+ in BaZn2Ti4O11 phase caused the decrease of the lattice parameters, and the adding of CuO formed the liquid-phase sintering mechanism, which effectively improved the densification process and lowered the sintering temperature. More importantly, the decrease of the lattice parameters was likely to make the bonding force and the crystal structure to become stronger, and the substitution of Cu2+ could restrain the lost of oxygen or the formation of Ti3+ ions when sintering BaZn2xCuxTi4O11 ceramics at 1140–1210 °C in air, thereby, both of these two reasons significantly increased the Q f value form 10,600 GHz at x = 0.00–51,400 GHz at x = 0.02. At last, BaZn1.98Cu0.02Ti4O11 was sintered well at 1190 °C for 2.5 h in air and showed good microwave dielectric properties: er = 29.5, Q f = 51,400 GHz and sf = 33.7 ppm/°C. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Microwave dielectric ceramics are a new type of functional materials, which have been rapidly developed in recent years. Because of their low dielectric loss (or high Q f value), high dielectric constant (er) and low temperature coefficient of resonance frequency (sf), they have been widely used in the microwave components such as resonators, filters, antennas and so on [1–3]. The high Q f value enables the outstanding microwave selectivity, which can reduce the risk of cross-talk within a given frequency. The high dielectric constant is very important for realizing the miniaturization of microwave component because the physical length of the dielectric resonator is in proportion to pffiffiffiffi 1= er . And the low temperature coefficient of resonant frequency insures the high stability of the equipments in different temperature environments. So far, kinds of ceramics materials have been reported to be suitable for such applications. BaTi4O9 and Ba2Ti9O20 [4–6] based on the BaO–TiO2 system have superior microwave dielectric properties: high dielectric constant and large Q f value, and have been extensively researched. In order to lower the sintering temperature and adjust dielectric properties, many researchers added appropriate amount of ZnO into BaO–TiO2 system during the raw material stage [7–9].
⇑ Corresponding author. Tel.: +86 28 83208048; fax: +86 28 83202139. E-mail addresses: [email protected] (B. Tang), [email protected], [email protected] (S. Yu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.003
Ceramics based on the BaO–ZnO–TiO2 system have good microwave dielectric properties. The ternary system BaO–ZnO–TiO2 has been found to contain four ternary phases: Ba4ZnTi11O27, BaZn2Ti4 O11, Ba2ZnTi5O13 and BaxZnxTi8xO16-hollandite [10]. Belous et al. [11] synthesized ceramics with composition close to BaZn2Ti4O11 for the purpose of studying the homogeneity range and they found that BaZn2Ti4O11 ceramics had good microwave dielectric properties of er = 30, Q f = 68,000 GHz and sf = 30 ppm/°C. In addition, CuO additive has been proved to show a special function on lowering the sintering temperature and improving the microwave dielectric properties of the ceramics with Zn element, such as Nd(Zn1/2Ti1/2)O3, Zn2TiO4 [12,13]. Furthermore, in our previous work, the doping of CuO had improved the microwave dielectric properties of BaO2–ZnO–4TiO2 ceramics, mainly because of the substitution of Cu2+ ions for Zn2+ ions in BaZn2Ti4O11 phase [9]. Therefore, for a more clear understanding of the effects of Cu2+ substitution on the crystal structure, microstructure and microwave dielectric properties of BaZn2Ti4O11, BaZn2xCuxTi4O11 ceramics were prepared and researched in this paper. 2. Experimental The samples used in this study were prepared by conventional solid-state reaction technique. The starting materials were reagent grade BaCO3, ZnO, CuO and TiO2. The starting materials were weighed according to stoichiometric ratio of BaZn2xCuxTi4O11 (x = 0.00–0.14). Initially, the mixtures of the powders were milled in plastic jars using deionized water and zirconia balls for 20 h. And after drying these powders were calcined at 950 °C for 3 h. Then the calcined powders were re-milled for 6 h. Later, with 6 wt% PVA as binder, the fine powders were pressed
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B. Tang et al. / Journal of Alloys and Compounds 551 (2013) 463–467
Fig. 1. The XRD patterns of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air.
into cylindrical samples with 15 mm in diameter and 7 mm in thickness under a pressure of 20 MPa. At last, these samples were sintered in an airtight furnace at 1140–1210 °C for 2.5 h in air. The bulk densities of the sintered samples were measured by the Archimedes method. The phase composition was identified by X-ray diffraction (XRD) using Cu Ka radiation (Phlips x’pert Pro MPD). Microstructure observation was conducted on the polished surface (with a annealing at the temperature with 20 °C lower than its previous sintering temperature for 20 min) of the samples by using scanning electron microscopy (SEM, FEI Inspect F). The valence of Ti ions was identified by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The dielectric characteristics at microwave frequencies were measured by the Hakki– Coleman dielectric resonator method in the TE011 mode using a network analyzer (Agilent Technologies E5071C) [14]. The sf values were determined from the difference between the resonant frequencies obtained at 25 °C and 80 °C using the equation: sf ¼ ðft2 ft1 Þ=ðft1 ðt2 t 1 ÞÞ, where ft1 and ft2 are the resonant frequencies at t1 = 25 °C and t2 = 80 °C, respectively.
3. Results and discussion Fig. 1 illustrates the XRD patterns of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air. It was found that there was only BaZn2Ti4O11 phase and no other phases appeared with increasing CuO content. And after more careful observation, we found there was a movement of the peaks for BaZn2Ti4O11 phase towards high angle direction, so the lattice parameters and unit cell volume for the solid solution BaZn2Ti4O11 phase were calculated and the results are shown in Fig. 2. When increasing the value of x from 0.00 to 0.14, the a-axis, b-axis and c-axis all dropped. The crystal structure of BaZn2Ti4O11 (Orthorhombic, Pbcn) had been reported to consist of a three-dimensional network of distorted, edge-sharing and corner-sharing octahedral with Zn2+ mainly filling tetrahedral interstices, and Ti4+ were found to occupy only octahedral positions [10]. The decrease of the lattice parameters was the result of that Cu2+ (r6 = 0.73 Å) (the superscript number is the coordination number) [15] substituted for Zn2+ (r6 = 0.74 Å) and occupied the tetrahedral interstices, and thus the unit cell volume also declined. To investigate the relationship between the compositional ratio and the microstructure, the SEM photographs of the polished surface for BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air with (a) x = 0.00, 1210 °C, (b)
x = 0.00, 1190 °C, (c) x = 0.02, 1190 °C, (d) x = 0.06, 1160 °C, (e) x = 0.14, 1190 °C and (f) x = 0.14, 1140 °C are shown in Fig. 3. As shown in Fig. 3(a), the dense microstructure for the pure BaZn2Ti4 O11 ceramics was obtained at the sintering temperature 1210 °C. When the sintering temperature was decreased to 1190 °C, a few of pores was appeared in the pure BaZn2Ti4O11 ceramics (Fig. 3(b)), but if a little CuO was added at the same time, the dense microstructure could be obtained too (Fig. 3(c)). It was inferred that the adding of CuO improved the densifying process. The reason for this was that during the sintering at the high temperature CuO caused the formation of liquid phase, which effectively enhanced the sintering ability of BaZn2Ti4O11 ceramics [12].In Fig. 3(c)–(f), the liquid phase, which had crystallized during the annealing process after polishing, could be obviously observed by the grain morphology and it became more and more when CuO was added gradually, as shown by the white arrows. In addition, it was obvious to see that the grain size became small when increasing CuO content, because though the liquid phase could promote the grain growth, the adverse effect, caused by the reduced sintering temperature, played the major role in the sintering process. For the samples with x = 0.14, the oversintering microstructure with the melt of the grains was observed in Fig. 3(e) when the sintering temperature was still at 1190 °C. For the same sample, as shown in Fig. 3(f), the compact microstructure with crystal grains in dense contact could be obtained at 1140 °C. According to the above analysis, the sample with different CuO content had different best sintering temperature, at which the sample sintered would show the maximum Q f value, and the more CuO the sample had, the lower sintering temperature it had. Therefore, in order to avoid the effect of the bad microstructure caused by the unsuitable sintering temperature on the microwave dielectric properties, we lowered the sintering temperature gradually from 1210 °C to 1140 °C when x was increased. Fig. 4 shows the apparent density and microwave dielectric properties of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140– 1210 °C for 2.5 h in air. As shown in Fig. 4(a), although their sintering temperature declined, the apparent density displayed a continuously growth from 4.92 to 5.05 g/cm3 when increasing CuO content, and this result was consistent with the variation of the density for Cu substituted Zn2TiO4 ceramics [13]. Reasons for this were related to the decrease of the lattice parameters, the suppression of the abnormal grain growth as well as the liquid-phase sintering mechanism. For the dielectric constant (er) as shown in Fig. 4 (b), its variation tendency was closely in accord with the trend between the density and CuO content. It was because both of the density and dielectric constant were influenced much by many of the same reasons such as the phase composition, pores and grain boundaries. The dielectric constant was increased from 29.2 to 30.6 as doping CuO from x = 0.00 to x = 0.14, which should be mainly caused by the improvement of densification. Furthermore, another reason that the substitution of Cu2+ ions with a higher dielectric polarisability a = 2.11 for Zn2+ (a = 2.04) would cause the increase of the total dielectric polarizability might be also possible [16]. For the Q f values of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air as shown in Fig. 4(c), there were three stages: the first stage for the huge increase of Q f values from 10,600 GHz at x = 0.00 to 51,400 GHz at x = 0.02, the second stage for the little decline to 47,700 GHz at x = 0.10 and the third stage for the big drop to 38,000 GHz at x = 0.14. As we know, besides that the lattice vibration modes cause the main microwave dielectric loss, pores, second phases, impurities, lattice defects, crystallizability, cation ordering and inner stress and the average grain size contribute to the microwave dielectric loss, so sometimes it is difficult to determine the key influencing factor [17]. The pure BaZn2Ti4O11 ceramics had a very
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Fig. 2. The lattice parameters and unit cell volume of BaZn2xCuxTi4O11 (x = 0.00–0.14) phase sintered at 1140–1210 °C for 2.5 h in air.
low Q f values mainly because the lost oxygen produced the weakly bound electrons, which was sure to happen in the undoped titanate ceramics when they were sintered in an oxygen-deficient atmosphere [18]. Ti4+ ions attracted these unwanted electrons and transformed into Ti3+ ions. The darkish core observed in our undoped samples could confirm this process. For Cu2+ ions substituted samples, the increase of Q f value could be attributed to the increase of the density and the substitution of Cu2+ for Zn2+ in BaZn2Ti4O11. The substitution could leaded to the reduce of distances between the center cation and surrounding nearest neighbor anions, which was likely to cause the bonding force and the crystal structure to become stronger, thereby, the Q f value was increased [19]. The level of the Q f values in this paper was lower than that in Ref. [11], which might be attributed to the difference in the purity of the raw materials, the preparing technique (twostage process used in reference [11]), the sintering condition (temperature, time and atmosphere) and so on, but it did not influence
our discussion on the effects of Cu substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics. Moreover, X-ray photoelectron spectroscopy of Ti ions in two representative samples is shown in Fig. 5. The main peak was the characteristic peak of Ti4+ ions located at 458.6 eV, and by comparison, it was found that the left of the main peak for the Cu2+-substituted sample was lower than that for the pure sample. This difference suggested that the formation of Ti3+ ions with characteristic peak at about 457.4 eV was restrained in the substituted sample. Butee et al. had also observed near absence of Ti3+ in the Cu-substituted Zn2TiO4 [13]. But, how the substitution restrained the lost of oxygen is not clear now. And the decrease of the Q f value for the sample with more CuO substitution should be related to the increase of liquid-phase in grain boundaries. For the temperature coefficient of resonant frequency (sf) of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140– 1210 °C for 2.5 h in air, there was no obvious variation and they
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Fig. 3. The SEM photographs of the polished surface for BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air with (a) x = 0.00, 1210 °C, (b) x = 0.00, 1190 °C, (c) x = 0.02, 1190 °C, (d) x = 0.06, 1160 °C, (e) x = 0.14, 1190 °C and, (f) x = 0.14, 1140 °C.
Fig. 5. X-ray photoelectron spectroscopy of Ti ions for the samples: (a) pure BaZn2Ti4O11 sintered at 1210 °C and (b) BaZn1.98Cu0.02Ti4O11 at 1190 °C for 2.5 h in air.
were all between 32.3 and 36.9 ppm/°C, which were at the same level of the sf value for the pure BaZn2Ti4O11 ceramics. At last, BaZn2xCuxTi4O11 ceramics with x = 0.02 showed the best microwave dielectric properties: er = 29.5, Q f = 51,400 GHz and sf = 33.7 ppm/°C. 4. Conclusions
Fig. 4. The apparent density and microwave dielectric properties of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air.
In order to investigate the effects of Cu2+substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics, BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics were prepared by the conventional solid-state reaction technique. The substitution of Cu2+for
B. Tang et al. / Journal of Alloys and Compounds 551 (2013) 463–467
Zn2+could cause the decrease of the lattice parameters of BaZn2Ti4O11 phase, and the adding of CuO caused the liquid-phase sintering mechanism, which effectively improved the densification process and lowered the sintering temperature. More importantly, the decrease of the lattice parameters was likely to make the bonding force and the crystal structure to become stronger, and the substitution of Cu2+could restrain the lost of oxygen or the formation of Ti3+ions when sintering BaZn2xCuxTi4O11 ceramics at 1140–1210 °C in air, thereby, both of these two reasons significantly increased the Q f value form 10,600 GHz at x = 0.00 to 51,400 GHz at x = 0.02. At last, BaZn1.98Cu0.02Ti4O11 was well sintered at 1190 °C for 2.5 h in air and showed good microwave dielectric properties: er = 29.5, Q f = 51,400 GHz and sf = 33.7 ppm/°C. References [1] [2] [3] [4]
E.A. Nenasheva, N.F. Kartenko, J. Eur. Ceram. Soc. 21 (2001) 2697–2701. I.M. Reaney, J. Am. Ceram. Soc. 89 (2006) 2063–2072. R. Freer, F. Azough, J. Eur. Ceram. Soc. 28 (2008) 1433–1441. H.M. O’Bryan, J. Thomson, J.K. Plourde, J. Am. Ceram. Soc. 57 (1974) 450–453.
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[5] J.K. Plourde, D.F. Linn, H.M. O’Bryan, J. Thomson, J. Am. Ceram. Soc. 58 (1975) 418–420. [6] S.G. Mhaisalker, D.W. Readey, S.A. Akbar, J. Am. Ceram. Soc. 74 (1991) 1894– 1898. [7] Y.I. Gormikov, Z.Y. Makarova, A.G. Belous, L.G. Gavrilova, V.M. Paskov, V.P. Chalyi, Sov. Prog. Chem. 50 (1984) 11–12. [8] S.Q. Yu, B. Tang, S.R. Zhang, X.H. Zhou, J. Alloys Comp. 505 (2010) 814–817. [9] S.Q. Yu, B. Tang, X. Zhang, S.R. Zhang, X.H. Zhou, J. Am. Ceram. Soc. 95 (2012) 1939–1943. [10] R.S. Roth, C.J. Rawn, C.G. Lindsay, J. Solid Chem. 104 (1993) 99–118. [11] A.G. Belous, O.V. Ovchar, M.K. Marjeta, M. Valant, J. Eur. Ceram. Soc. 26 (2006) 3733–3739. [12] C.L. Huang, W.R. Yang, Mater. Lett. 63 (2009) 103–105. [13] S. Butee, A.R. Kulkarni, O. Prakash, R.P.R.C. Aiyar, I. Wattamwar, D. Bais, K. Sudheendran, K.C.J. Raju, Mater. Sci. Eng. B 176 (2011) 567–572. [14] B.W. Hakki, P.D. Coleman, IEEE Trans. Microwave Theory Tech. MTT8 (1960) 402–410. [15] M.T. Sebastian, Dielectric Materials for Wireless Communication, Elsevier Ltd, oxford, 2008. [16] R.D. Shannon, J. Appl. Phys. 73 (1993) 348–366. [17] W. Lei, W.Z. Lu, X.H. Wang, F. Liang, J. Wang, J. Am. Ceram. Soc. 94 (2011) 20– 23. [18] M.J. Lee, B.D. You, D.S. King, J. Mater. Sci. Mater. El. 6 (1995) 165–172. [19] H.T. Kim, S.H. Kim, S. Nahm, J.D. Byun, Y. Kim, J. Am. Ceram. Soc. 82 (1999) 3043–3048.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
The influence of Cu substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics Bin Tang, Shengquan Yu ⇑, Hetuo Chen, Shuren Zhang, Xiaohua Zhou State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a r t i c l e
i n f o
Article history: Received 29 August 2012 Received in revised form 30 October 2012 Accepted 1 November 2012 Available online 10 November 2012 Keywords: Ceramics Sintering Dielectric response Microwave dielectrics
a b s t r a c t BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics were prepared by the conventional solid-state reaction technique and the effects of Cu2+ substitution on the crystal structure, microstructure and microwave dielectric properties of BaZn2Ti4O11 ceramics were investigated. The substitution of Cu2+ for Zn2+ in BaZn2Ti4O11 phase caused the decrease of the lattice parameters, and the adding of CuO formed the liquid-phase sintering mechanism, which effectively improved the densification process and lowered the sintering temperature. More importantly, the decrease of the lattice parameters was likely to make the bonding force and the crystal structure to become stronger, and the substitution of Cu2+ could restrain the lost of oxygen or the formation of Ti3+ ions when sintering BaZn2xCuxTi4O11 ceramics at 1140–1210 °C in air, thereby, both of these two reasons significantly increased the Q f value form 10,600 GHz at x = 0.00–51,400 GHz at x = 0.02. At last, BaZn1.98Cu0.02Ti4O11 was sintered well at 1190 °C for 2.5 h in air and showed good microwave dielectric properties: er = 29.5, Q f = 51,400 GHz and sf = 33.7 ppm/°C. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Microwave dielectric ceramics are a new type of functional materials, which have been rapidly developed in recent years. Because of their low dielectric loss (or high Q f value), high dielectric constant (er) and low temperature coefficient of resonance frequency (sf), they have been widely used in the microwave components such as resonators, filters, antennas and so on [1–3]. The high Q f value enables the outstanding microwave selectivity, which can reduce the risk of cross-talk within a given frequency. The high dielectric constant is very important for realizing the miniaturization of microwave component because the physical length of the dielectric resonator is in proportion to pffiffiffiffi 1= er . And the low temperature coefficient of resonant frequency insures the high stability of the equipments in different temperature environments. So far, kinds of ceramics materials have been reported to be suitable for such applications. BaTi4O9 and Ba2Ti9O20 [4–6] based on the BaO–TiO2 system have superior microwave dielectric properties: high dielectric constant and large Q f value, and have been extensively researched. In order to lower the sintering temperature and adjust dielectric properties, many researchers added appropriate amount of ZnO into BaO–TiO2 system during the raw material stage [7–9].
⇑ Corresponding author. Tel.: +86 28 83208048; fax: +86 28 83202139. E-mail addresses: [email protected] (B. Tang), [email protected], [email protected] (S. Yu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.003
Ceramics based on the BaO–ZnO–TiO2 system have good microwave dielectric properties. The ternary system BaO–ZnO–TiO2 has been found to contain four ternary phases: Ba4ZnTi11O27, BaZn2Ti4 O11, Ba2ZnTi5O13 and BaxZnxTi8xO16-hollandite [10]. Belous et al. [11] synthesized ceramics with composition close to BaZn2Ti4O11 for the purpose of studying the homogeneity range and they found that BaZn2Ti4O11 ceramics had good microwave dielectric properties of er = 30, Q f = 68,000 GHz and sf = 30 ppm/°C. In addition, CuO additive has been proved to show a special function on lowering the sintering temperature and improving the microwave dielectric properties of the ceramics with Zn element, such as Nd(Zn1/2Ti1/2)O3, Zn2TiO4 [12,13]. Furthermore, in our previous work, the doping of CuO had improved the microwave dielectric properties of BaO2–ZnO–4TiO2 ceramics, mainly because of the substitution of Cu2+ ions for Zn2+ ions in BaZn2Ti4O11 phase [9]. Therefore, for a more clear understanding of the effects of Cu2+ substitution on the crystal structure, microstructure and microwave dielectric properties of BaZn2Ti4O11, BaZn2xCuxTi4O11 ceramics were prepared and researched in this paper. 2. Experimental The samples used in this study were prepared by conventional solid-state reaction technique. The starting materials were reagent grade BaCO3, ZnO, CuO and TiO2. The starting materials were weighed according to stoichiometric ratio of BaZn2xCuxTi4O11 (x = 0.00–0.14). Initially, the mixtures of the powders were milled in plastic jars using deionized water and zirconia balls for 20 h. And after drying these powders were calcined at 950 °C for 3 h. Then the calcined powders were re-milled for 6 h. Later, with 6 wt% PVA as binder, the fine powders were pressed
464
B. Tang et al. / Journal of Alloys and Compounds 551 (2013) 463–467
Fig. 1. The XRD patterns of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air.
into cylindrical samples with 15 mm in diameter and 7 mm in thickness under a pressure of 20 MPa. At last, these samples were sintered in an airtight furnace at 1140–1210 °C for 2.5 h in air. The bulk densities of the sintered samples were measured by the Archimedes method. The phase composition was identified by X-ray diffraction (XRD) using Cu Ka radiation (Phlips x’pert Pro MPD). Microstructure observation was conducted on the polished surface (with a annealing at the temperature with 20 °C lower than its previous sintering temperature for 20 min) of the samples by using scanning electron microscopy (SEM, FEI Inspect F). The valence of Ti ions was identified by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The dielectric characteristics at microwave frequencies were measured by the Hakki– Coleman dielectric resonator method in the TE011 mode using a network analyzer (Agilent Technologies E5071C) [14]. The sf values were determined from the difference between the resonant frequencies obtained at 25 °C and 80 °C using the equation: sf ¼ ðft2 ft1 Þ=ðft1 ðt2 t 1 ÞÞ, where ft1 and ft2 are the resonant frequencies at t1 = 25 °C and t2 = 80 °C, respectively.
3. Results and discussion Fig. 1 illustrates the XRD patterns of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air. It was found that there was only BaZn2Ti4O11 phase and no other phases appeared with increasing CuO content. And after more careful observation, we found there was a movement of the peaks for BaZn2Ti4O11 phase towards high angle direction, so the lattice parameters and unit cell volume for the solid solution BaZn2Ti4O11 phase were calculated and the results are shown in Fig. 2. When increasing the value of x from 0.00 to 0.14, the a-axis, b-axis and c-axis all dropped. The crystal structure of BaZn2Ti4O11 (Orthorhombic, Pbcn) had been reported to consist of a three-dimensional network of distorted, edge-sharing and corner-sharing octahedral with Zn2+ mainly filling tetrahedral interstices, and Ti4+ were found to occupy only octahedral positions [10]. The decrease of the lattice parameters was the result of that Cu2+ (r6 = 0.73 Å) (the superscript number is the coordination number) [15] substituted for Zn2+ (r6 = 0.74 Å) and occupied the tetrahedral interstices, and thus the unit cell volume also declined. To investigate the relationship between the compositional ratio and the microstructure, the SEM photographs of the polished surface for BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air with (a) x = 0.00, 1210 °C, (b)
x = 0.00, 1190 °C, (c) x = 0.02, 1190 °C, (d) x = 0.06, 1160 °C, (e) x = 0.14, 1190 °C and (f) x = 0.14, 1140 °C are shown in Fig. 3. As shown in Fig. 3(a), the dense microstructure for the pure BaZn2Ti4 O11 ceramics was obtained at the sintering temperature 1210 °C. When the sintering temperature was decreased to 1190 °C, a few of pores was appeared in the pure BaZn2Ti4O11 ceramics (Fig. 3(b)), but if a little CuO was added at the same time, the dense microstructure could be obtained too (Fig. 3(c)). It was inferred that the adding of CuO improved the densifying process. The reason for this was that during the sintering at the high temperature CuO caused the formation of liquid phase, which effectively enhanced the sintering ability of BaZn2Ti4O11 ceramics [12].In Fig. 3(c)–(f), the liquid phase, which had crystallized during the annealing process after polishing, could be obviously observed by the grain morphology and it became more and more when CuO was added gradually, as shown by the white arrows. In addition, it was obvious to see that the grain size became small when increasing CuO content, because though the liquid phase could promote the grain growth, the adverse effect, caused by the reduced sintering temperature, played the major role in the sintering process. For the samples with x = 0.14, the oversintering microstructure with the melt of the grains was observed in Fig. 3(e) when the sintering temperature was still at 1190 °C. For the same sample, as shown in Fig. 3(f), the compact microstructure with crystal grains in dense contact could be obtained at 1140 °C. According to the above analysis, the sample with different CuO content had different best sintering temperature, at which the sample sintered would show the maximum Q f value, and the more CuO the sample had, the lower sintering temperature it had. Therefore, in order to avoid the effect of the bad microstructure caused by the unsuitable sintering temperature on the microwave dielectric properties, we lowered the sintering temperature gradually from 1210 °C to 1140 °C when x was increased. Fig. 4 shows the apparent density and microwave dielectric properties of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140– 1210 °C for 2.5 h in air. As shown in Fig. 4(a), although their sintering temperature declined, the apparent density displayed a continuously growth from 4.92 to 5.05 g/cm3 when increasing CuO content, and this result was consistent with the variation of the density for Cu substituted Zn2TiO4 ceramics [13]. Reasons for this were related to the decrease of the lattice parameters, the suppression of the abnormal grain growth as well as the liquid-phase sintering mechanism. For the dielectric constant (er) as shown in Fig. 4 (b), its variation tendency was closely in accord with the trend between the density and CuO content. It was because both of the density and dielectric constant were influenced much by many of the same reasons such as the phase composition, pores and grain boundaries. The dielectric constant was increased from 29.2 to 30.6 as doping CuO from x = 0.00 to x = 0.14, which should be mainly caused by the improvement of densification. Furthermore, another reason that the substitution of Cu2+ ions with a higher dielectric polarisability a = 2.11 for Zn2+ (a = 2.04) would cause the increase of the total dielectric polarizability might be also possible [16]. For the Q f values of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air as shown in Fig. 4(c), there were three stages: the first stage for the huge increase of Q f values from 10,600 GHz at x = 0.00 to 51,400 GHz at x = 0.02, the second stage for the little decline to 47,700 GHz at x = 0.10 and the third stage for the big drop to 38,000 GHz at x = 0.14. As we know, besides that the lattice vibration modes cause the main microwave dielectric loss, pores, second phases, impurities, lattice defects, crystallizability, cation ordering and inner stress and the average grain size contribute to the microwave dielectric loss, so sometimes it is difficult to determine the key influencing factor [17]. The pure BaZn2Ti4O11 ceramics had a very
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Fig. 2. The lattice parameters and unit cell volume of BaZn2xCuxTi4O11 (x = 0.00–0.14) phase sintered at 1140–1210 °C for 2.5 h in air.
low Q f values mainly because the lost oxygen produced the weakly bound electrons, which was sure to happen in the undoped titanate ceramics when they were sintered in an oxygen-deficient atmosphere [18]. Ti4+ ions attracted these unwanted electrons and transformed into Ti3+ ions. The darkish core observed in our undoped samples could confirm this process. For Cu2+ ions substituted samples, the increase of Q f value could be attributed to the increase of the density and the substitution of Cu2+ for Zn2+ in BaZn2Ti4O11. The substitution could leaded to the reduce of distances between the center cation and surrounding nearest neighbor anions, which was likely to cause the bonding force and the crystal structure to become stronger, thereby, the Q f value was increased [19]. The level of the Q f values in this paper was lower than that in Ref. [11], which might be attributed to the difference in the purity of the raw materials, the preparing technique (twostage process used in reference [11]), the sintering condition (temperature, time and atmosphere) and so on, but it did not influence
our discussion on the effects of Cu substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics. Moreover, X-ray photoelectron spectroscopy of Ti ions in two representative samples is shown in Fig. 5. The main peak was the characteristic peak of Ti4+ ions located at 458.6 eV, and by comparison, it was found that the left of the main peak for the Cu2+-substituted sample was lower than that for the pure sample. This difference suggested that the formation of Ti3+ ions with characteristic peak at about 457.4 eV was restrained in the substituted sample. Butee et al. had also observed near absence of Ti3+ in the Cu-substituted Zn2TiO4 [13]. But, how the substitution restrained the lost of oxygen is not clear now. And the decrease of the Q f value for the sample with more CuO substitution should be related to the increase of liquid-phase in grain boundaries. For the temperature coefficient of resonant frequency (sf) of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140– 1210 °C for 2.5 h in air, there was no obvious variation and they
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Fig. 3. The SEM photographs of the polished surface for BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air with (a) x = 0.00, 1210 °C, (b) x = 0.00, 1190 °C, (c) x = 0.02, 1190 °C, (d) x = 0.06, 1160 °C, (e) x = 0.14, 1190 °C and, (f) x = 0.14, 1140 °C.
Fig. 5. X-ray photoelectron spectroscopy of Ti ions for the samples: (a) pure BaZn2Ti4O11 sintered at 1210 °C and (b) BaZn1.98Cu0.02Ti4O11 at 1190 °C for 2.5 h in air.
were all between 32.3 and 36.9 ppm/°C, which were at the same level of the sf value for the pure BaZn2Ti4O11 ceramics. At last, BaZn2xCuxTi4O11 ceramics with x = 0.02 showed the best microwave dielectric properties: er = 29.5, Q f = 51,400 GHz and sf = 33.7 ppm/°C. 4. Conclusions
Fig. 4. The apparent density and microwave dielectric properties of BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics sintered at 1140–1210 °C for 2.5 h in air.
In order to investigate the effects of Cu2+substitution on the microwave dielectric properties of BaZn2Ti4O11 ceramics, BaZn2xCuxTi4O11 (x = 0.00–0.14) ceramics were prepared by the conventional solid-state reaction technique. The substitution of Cu2+for
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Zn2+could cause the decrease of the lattice parameters of BaZn2Ti4O11 phase, and the adding of CuO caused the liquid-phase sintering mechanism, which effectively improved the densification process and lowered the sintering temperature. More importantly, the decrease of the lattice parameters was likely to make the bonding force and the crystal structure to become stronger, and the substitution of Cu2+could restrain the lost of oxygen or the formation of Ti3+ions when sintering BaZn2xCuxTi4O11 ceramics at 1140–1210 °C in air, thereby, both of these two reasons significantly increased the Q f value form 10,600 GHz at x = 0.00 to 51,400 GHz at x = 0.02. At last, BaZn1.98Cu0.02Ti4O11 was well sintered at 1190 °C for 2.5 h in air and showed good microwave dielectric properties: er = 29.5, Q f = 51,400 GHz and sf = 33.7 ppm/°C. References [1] [2] [3] [4]
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