Dielectric and tunable characteristics of Ba0.4Sr0.6TiO3–BaWO4 composite ceramics for microwave applications

Materials Research Bulletin 46 (2011) 1045–1050

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Dielectric and tunable characteristics of Ba0.4Sr0.6TiO3–BaWO4 composite ceramics for microwave applications Mingwei Zhang, Jiwei Zhai *, Bo Shen, Xi Yao Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 October 2010 Received in revised form 10 December 2010 Accepted 11 March 2011 Available online 21 March 2011

Phase compositions, microstructure and microwave dielectric properties, of BaWO4 (BW)–Ba0.4Sr0.6TiO3 (BST) composite ceramics, prepared by the traditional solid-state route, were systematically characterized. Meanwhile, mechanism of dielectric tunability of those materials was discussed. Dielectric properties of the BW–BST composites at a DC bias field near the phase transition temperature could be interpreted by using Johnson’s phenomenological equation. The sample with x = 0.60 exhibited a tunability of 29.5%, a dielectric permittivity of 192 and a Q value of 231 (at 2.700 GHz), which make it a promising candidate for applications in electrically tunable microwave devices. ß 2011 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics A. Composites B. X-ray diffraction D. Dielectric properties

1. Introduction Barium strontium titanate (Ba1xSrxTiO3 or BST) ferroelectric materials have attracted significant interests due to their electrically tunable dielectric properties for potential application in tunable microwave devices, such as oscillators, tunable mixers, delay lines, steerable antennas, parametric amplifiers, capacitors, varactors, resonators, voltage-controlled oscillators, tunable filters and phase shifters [1–8]. All these applications require materials with low dielectric loss (tan d), high tunability (T) and appropriate value of dielectric permittivity (<500) [9]. BST is one of the most promising candidates owing to its high tunability and low loss [10,11]. However, pure BST has relatively high dielectric permittivity (e) and thus is not suitable for practical tunable microwave device applications. It has been shown that adding low dielectric permittivity nonferroelectric phase (such as MgO, Al2O3, Mg2SiO4, and Mg2TiO4) [11–19] into ferroelectric material is an effective method to reduce its dielectric permittivity and loss while maintaining acceptable dielectric tunabilities. In most cases, low dielectric permittivity is readily achievable, but microwave loss is usually increased while tunability is decreased. Addition of MgO was found to suppress dielectric permittivity, losses and tunability of BST ceramics. Wu et al. [17] reported that 1 wt% Al2O3-doped Ba0.6Sr0.4TiO3 ceramics had very low loss and relatively high tunability, but their dielectric

* Corresponding author. Tel.: +86 216 598 0544; fax: +86 216 598 5179. E-mail address: [email protected] (J. Zhai). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.03.009

permittivity was still higher than the requirement of real applications. BST–Mg2SiO4 composite ceramics displayed relatively high tunability, adjustable dielectric permittivity and relatively low dielectric loss. However, these properties were still not satisfactory for microwave applications [18]. Similarly, Ba1xSrxTiO3–Mg2TiO4 based composite ceramics also had similar problems for microwave applications due to their relatively high loss tangent at microwave frequencies. Introduction of MgO into BST– Mg2SiO4 binary composites resulted in BST–Mg2SiO4–MgO ternary composites, which had readily adjustable dielectric permittivity without decreasing their tunability but increasing their dielectric loss at microwave frequencies [13,19]. The reduced dielectric permittivities of BST as a result of the addition of low dielectric permittivity non-ferroelectric phases were at the expense of increase in dielectric losses as well as decrease in dielectric tunability. In this study, we tried to further improve microwave dielectric properties of BST-based composite ceramics. BaWO4 microwave dielectric material [20] with low dielectric permittivity (e = 8.27) and high quality factor (Q  f = 30,229) was used to form composites with Ba0.4Sr0.6TiO3 ferroelectric ceramics. The effect of BaWO4 on the dielectric properties of BST40 ceramics was systematically investigated to evaluate their suitability for microwave device applications. This study was aimed at finding a composite BW–BST system that has high Q value, high dielectric tunability and appropriate level of dielectric permittivity. At the same time, mechanisms that govern dielectric properties of the BW–BST composite ceramics will be discussed in detail.

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2. Experimental xBaWO4(BW)–(1  x)Ba0.4Sr0.6TiO3(BST) (x = 15, 30, 45, 60 wt%) composite ceramics were prepared by the conventional solid-state reaction route. High purity BaCO3 (99.8%), SrCO3 (99.0%), TiO2 (99.9%) and WO3 (99.8%) were used as the starting materials. BST and BaWO4 powers were synthesized at 1200 8C and 1100 8C,

[()TD$FIG]

respectively. Then, the two powders were mixed according to the stoichiometric ratios of xBW–(1  x)BST and ball-milled with zirconia media in ethanol for 24 h. The dried powders, mixed with 8 wt% polyvinyl alcohol (PVA), were pressed into pellets. Samples for low frequency dielectric measurement are 10 mm in diameter and 1 mm in thickness, while those for microwave frequency measurements have dimensions of 10/5 mm, 12/6 mm, 15/7 mm

Fig. 1. BEI micrographs of the xBaWO4–(1  x)Ba0.4Sr0.6TiO3 composite ceramics with (a) x = 15, (b) x = 30, (c) x = 45 and (d) x = 60 wt% and EDS spectra of the sample (d).

M. Zhang et al. / Materials Research Bulletin 46 (2011) 1045–1050

and 17/8 mm in diameter/thickness, respectively. The green pellets were kept at 550 8C for 6 h in air to remove the solvent and the binder and then sintered at 1320–1340 8C in air for 4 h. Phase identification of the sintered ceramics was conducted by using X-ray diffraction (XRD, Bruker D8 Advanced, Germany) with Cu Ka radiation. Back electron image (BEI, JSM EMP-800) was used to characterize microstructures of the samples. Temperature dependent dielectric permittivity (e) and loss tangent (tan d) of the ceramics were measured over 150–400 K at 10 kHz by using a HP4284A precision LCR meter (Agilent, Palo Alto, CA). Room temperature dielectric permittivity versus DC bias voltage was measured at 10 kHz by using a Keithley model 2410 (Cleveland, OH) high voltage source coupled with TH2816A LCR meter (Changzhou, China). Microwave dielectric permittivity and loss of the samples were measured by using the resonance method [21] with a vector network analyzer (Agilent E5071C). 3. Results and discussion BEI micrographs of natural surfaces of the BW–BST40 ceramics are shown in Fig. 1. It is demonstrated that BW–BST ceramics have

[()TD$FIG]

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all a dense and uniform microstructure. The two component phases can be observed distinctively in the composite ceramics. Energy dispersive spectroscopy (EDS) result of the sample with x = 60 wt% is also shown in Fig. 1. The dark phase consists of primarily Ba, Sr, Ti and O, while the light phase consists of primarily Ba, W, O and small amount of Sr. It suggests that the dark grains are BST (marked as ‘‘1’’) while the light grains are BW (marked as ‘‘2’’). This EDS result indicated that inter-diffusion between BST and BW occurred during the sintering process. BST grains of the composite ceramics are smaller than those of pure BST40 (not shown here). The grain sizes of pure BST40 are more than 15 mm, however, the grain size in BST–BW composite is about 1 mm. This indicates that the addition of BW reduced the grain sizes of BST. The grain size of the BW phase increased with increasing content of BW and its distribution became more homogeneous, but grain size of the BST phase kept almost unchanged. XRD patterns of the BW–BST composite ceramics are shown in Fig. 2. All samples can be indexed as two phases of cubic perovskite BST and wolframite BaWO4. The absence of secondary crystalline phases implies no obvious chemical reaction between BST and BW phase. The (2 1 1) and (2 2 0) peaks of pure BST and the BW–BST are enlarged in order to observe the shift of diffraction peaks as shown in Fig. 2(b). Compared with pure BST, both (2 1 1) and (2 2 0) peaks shift toward lower angle with increasing BW content, indicating a lattice expansion of the BST. The variation of the cell parameter obtained from XRD analysis by Jade 5.0 is plotted in Fig. 3. This lattice expansion can be attributed to an increase of Ba/Sr ratio in the BST phase due to the inter-diffusion, as

[()TD$FIG]

Fig. 3. Lattice parameter of the xBaWO4–(1  x)Ba0.4Sr0.6TiO3 (x = 15, 30, 45 and 60 wt%) composite ceramics.

[()TD$FIG]

Fig. 2. XRD patterns of the xBaWO4–(1  x)Ba0.4Sr0.6TiO3 composite ceramics.

Fig. 4. Curie temperature (TC) of Ba1xSrxTiO3 as a function of x.

[()TD$FIG]

[()TD$FIG]

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Fig. 5. Temperature dependences of dielectric permittivity and dielectric loss of the xBaWO4–(1  x)Ba0.4Sr0.6TiO3 composite ceramics.

demonstrated by the above EDS result, because the radius of Sr2+ ion (coordination number N = 12, ionic radius = 1.44 A˚) is smaller than that of Ba2+ ion (coordination number N = 12, ionic radius = 1.61 A˚) [22]. Fig. 3 also shows that the lattice expansion tended to saturate at x = 45 wt%, which means that there has a limit in ion s inter-diffusion at this composition. In our previous work, we got a relationship between the value of Ba1xSrxTiO3 and TC, as shown in Fig. 4. According to the fitting formula we can get a rough Ba/Sr ratio of the samples as follows: 0.422/0.578, 0.433/0.567, 0.441/0.549, and 0.473/0.527. Temperature dependences of dielectric permittivity of the BW– BST composite ceramics are shown in Fig. 5. Microwave dielectric properties of all samples measured at room temperature are listed in Table 1. Dielectric peaks of the composite ceramics were broadened and suppressed with increasing content of BW. Meanwhile, Curie temperature (TC) of the composites increased gradually. The decrease in dielectric permittivity can be readily attributed to a dilution effect caused by BW and the diffuse phase transition of the samples [10]. For example, room temperature dielectric permittivity (10 kHz) is decreased from 1098 for pure BST to 212 for the sample with x = 0.6. On the other hands, the increase in fraction of grain boundary which is caused by the decrease in BST grain size could be an additional contributor to the reduction in dielectric permittivity with increasing content of BW. Temperature dependent dielectric losses of all samples are also presented in Fig. 5. The composite ceramics have significantly low dielectric losses as a result of the addition of the low loss nonferroelectric BW [23]. Dielectric losses of all samples increase with increasing temperature above 250 K, which may be attributed to the increase of conduction loss at high temperatures [24].

Fig. 6. DC bias field dependences of dielectric permittivity of the xBaWO4– (1  x)Ba0.4Sr0.6TiO3 composite ceramics measured at 10 kHz and room temperature.

Furthermore, TC increases with increasing content of BW from 15 to 60 wt%, which should be attributed to the increase of Ba/Sr ratio originating from the inter-diffusion as discussed above [25]. Fig. 6 shows DC field dependences of dielectric permittivity of all samples measured at 10 kHz and 20 8C. Loss tangents of the samples were also measured at 10 kHz and room temperature (not shown here). The BW–BST composite ceramics have higher tunability than pure BST, which could be related to their relatively high Ba/Sr ratio. Tunabilities of the samples decrease from 23.2 to 16.0% (at 30 kV) with increasing content of BW from x = 15 wt% to x = 60 wt%, which is attributed to the increase of the nonferroelectric content. However, our BW–BST composite ceramics still have sufficiently high tunabilities. The inter-diffusion between Ba ions (in BW) and Sr ions (in BST) drove TC upwards and thus led to high tunability at room temperature. Their high tunabilities might also be related to their homogeneous microstructures [26,27]. It is well known that tunability in a ferroelectric state is larger than that in a paraelectric state and the maximum tunability appears at the vicinity of phase transition temperature [28,29]. Specifically, the sample with x = 60 wt% has a dielectric permittivity of 249 and yet a tunability of 16% (at 30 kV/cm and 20 8C). This result is very encouraging as compared with those of other BSTbased composites [30]. For instance, the tunability of MgO–BST decreased abruptly with increasing content of MgO. The high tunability and low dielectric permittivity make our BW–BST composite ceramics attractive materials for tunable microwave device applications. It is well known that the effect on dielectric permittivity by DC field of BST in paraelectric state originates from the anharmonic

Table 1 Microwave and dielectric properties of all the samples with different contents of BaWO4. x

0 15 30 45 60

TC (K)

212.2 217.9 221.8 228.3 236.0

Dielectric properties (at 10 kHz)

Microwave properties

At about 20 8C

Resonant frequency (GHz)

e (at resonant frequency)

Q (1/tand)

1.409 1.520 1.478 1.755 2.700

1021 674 598 481 192

935 52 126 221 231

e0

tan d

Tunability (%) at 30 kV/cm bias

1098 869 759 669 259

0.0009 0.0162 0.0077 0.0030 0.0010

8.8 23.2 20.1 17.4 16.0

[()TD$FIG]

[()TD$FIG]

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Fig. 7. Electric field dependences of dielectric permittivity of the xBaWO4– (1  x)Ba0.4Sr0.6TiO3 composite ceramics: (a) x = 15, (b) x = 30, (c) x = 45 and (d) x = 60 wt%, measured at 10 kHz and room temperature.

Fig. 8. Tunability of the xBaWO4–(1  x)Ba0.4Sr0.6TiO3 composite ceramics: (a) x = 15, (b) x = 30, (c) x = 45 and (d) x = 60 wt%, measured at 10 kHz and room temperature.

interaction of Ti4+ ions. An anharmonic interaction is attributed to the crystal field exerted in a material. An anharmonic potential of Ti4+ is generated in a material with perovskite structure due to the symmetry group. The anharmonic interaction can be visualized as an inelastic feature of the covalent bond constructed by Ti4+ and O2. Therefore, the magnitude of the dipole moment will be suppressed at DC biasing [31], which causes the reduction of dielectric permittivity. As is well known, dielectric tunability of a given material is correlative with strength of the external field. Generally, the higher the external field is applied, the higher the tunability will be. As shown in Fig. 6, dielectric permittivity of the BST–BW composite ceramics decrease with increasing external field, which is attributed to the anharmonic interaction of Ti4+ ions. It can be explained by the phenomenological theory of Devonshire [32,33]. Johnson proposed an expression for permittivity under DC field as:

which leads to a reduction in dielectric permittivity. The congelation and consolidation of polar micro-regions tend to saturate with further increasing DC field, whose contribution to tunability is dying down. For the samples with x = 0.45 and 0.60, since TC is getting closer to measuring temperature, polar microregion contribution to the change of dielectric permittivity is increasing, which is the reason why the slope dose not depress with increasing DC field. In Fig. 7, the decline slope means a decrease in tunability with increasing content of BW, which can be attributed to the increase of the non-ferroelectric content. Shown in Fig. 8, we measured tunabilities at different DC biases (7 points). An increase in tenability with DC bias can also be observed, which is consistent with the conclusion drawn above. The change in slopes of the samples with x = 15 and x = 30 means a reduction in change rate of their dielectric permittivity as a function of DC electric field. Slopes of the samples with x = 45 and x = 60 are constant, indicating a steady change rate of decrease in their dielectric permittivity. Microwave dielectric properties of all samples measured at room temperature are listed in Table 1. Dielectric permittivities of the composite ceramics are evidently decreased at microwave frequencies as compared with those at low frequencies (below 1 MHz). Q value decreases with the content of BW at first, reaches a minimum vale at x = 15 wt% and then increases with increasing content of BW. This variation can be understood by taking into account both intrinsic and extrinsic dielectric responses [36,37]. Extrinsic loss contributors include charged defect, residual ferroelectric polarization, and local polar-regions. The high microwave losses of the samples with low contents of BW could be attributed to the main extrinsic loss due to their poor microstructure homogeneities [37]. At high concentrations of BW, the high Q value of BW became dominate factor. On the other hand, good compositional homogeneity of the samples with high contents of BW also favored to increase their Q factors [38]. It is worth mentioning that the sample with x = 0.6 has an acceptable level of Q value of 231 at 2.700 GHz, a high tunability (29.5% at 60 kV/cm) and an appropriate value of dielectric permittivity (192 at zero biasing), which makes this composite a promising candidate for applications in tunable microwave devices.

erða p pÞ 1 ¼ 1=3 erð0Þ ½1 þ ae3rð0Þ E2 

(1)

where er(0) and er(app) are the dielectric permittivity at zero DC field and an applied field, respectively, and a is the anharmonic coefficient. a is determined by both the anharmonic interaction and the dielectric permittivity of the sample. Larger dielectric permittivity and stronger anharmonic interaction would yield a higher value of field coefficient. In this case, DC field will be more effective in reducing the dielectric permittivity. Eq. (1) can be simplified by using Eq. (2): y ¼ 1 þ bx

(2)

where y = (er(0)/er(app))3, x = E2 and b ¼ ae30 . It was reported that all experimental curves in the intermediate range of electric fields can be well fitted by the bias equation [34]. Fig. 7 shows linear fittings of experimental data of the BW–BST composite ceramics. The samples with x = 15 wt% and x = 30 wt% have two linear regions, with a large slope over 0–16 kV/cm1 and small slope over 16–30 kV/cm1. It means that the dielectric responses of the composite materials at external DC fields cannot be solely described by the phenomenological theory at different effective Ba concentrations in BST, which result in the difference in TC. This deviation could have other extrinsic contributions, such as polar clusters, microcluster boundaries, defects, and space charge polarization [35]. At a strong DC field, polar micro-domains in nanoscale polar micro-regions would grow up, resulting in a congelation and consolidation of polar micro-regions and a reduction in the number of ferroelectric–paraelectric domain walls. The contribution of reversible polarizations is reduced,

4. Conclusions xBaWO4–(1  x)Ba0.4Sr0.6TiO3 composite ceramics were fabricated and characterized. All samples consisted of Ba0.4Sr0.6TiO3 and BaWO4 phases. TC peaks of the composite ceramics were suppressed, broadened and shifted to higher temperatures with increasing content of BW. Dielectric tunability properties of the

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BW–BST composites at a DC bias field near the phase transition temperature were interpreted using Johnson’s phenomenological equation. The sample with x = 0.60 had a dielectric permittivity of 192, a tunability of 29.5% and a Q factor of 231 (at 2.700 GHz). The low dielectric permittivity composite ceramics with high tunability and good microwave properties are promising candidates in tunable device applications. Acknowledgements This work was supported by the Ministry of Sciences and Technology of China through 973-project under Grant 2009CB623302, the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 707024) and the Shanghai Committee of Science and Technology (No. 07DZ22302). References [1] L.B. Kong, S. Li, T.S. Zhang, J.W. Zhai, F.Y.C. Boey, J. Ma, Prog. Mater. Sci. 55 (2010) 840. [2] X.X. Xi, H.C. Li, W.D. Si, I.A. Akimov, J.R. Fox, A.M. Clark, J.H. Hao, J. Electroceram. 4 (2000) 393. [3] R.E. Treece, J.B. Thompson, C.H. Mueller, T. Rivkin, M.W. Cromar, Appl. Supercond. 7 (1997) 2363. [4] H.C. Li, W.D. Si, A.D. West, X.X. Xi, Appl. Phys. Lett. 73 (1998) 190. [5] A.B. Kozyrev, T.B. Samoilova, A.A. Golovkov, E.K. Hollmann, D.A. Kalinikos, V.E. Loginov, A.M. Prudan, O.I. Soldatenkov, D. Galt, C.H. Mueller, T.V. Rivkin, G.A. Koepf, J. Appl. Phys. 84 (1998) 3326. [6] C.L. Chen, H.H. Feng, Z. Zhang, A. Brazdeikis, Z.J. Huang, W.K. Chu, C.W. Chu, F.A. Miranda, F.W. Van Keuls, R.R. Romanofsky, Appl. Phys. Lett. 75 (1999) 412. [7] A. Outzourhit, J.U. Trefny, T. Kito, B. Yarar, J. Mater. Res. 10 (1995) 1411.

[8] W.T. Chang, L. Sengupta, J. Appl. Phys. 92 (2002) 3941. [9] M.W. Cole, P.C. Joshi, M.H. Ervin, J. Appl. Phys. 9 (2001) 6336. [10] J.J. Zhang, J.W. Zhai, M.W. Zhang, P. Qi, X. Yu, X. Yao, J. Phys. D: Appl. Phys. 42 (7) (2009) 075414. [11] R.H. Liang, X.L. Dong, Y. Chen, F. Cao, Mater. Chem. Phys. 95 (2006) 222. [12] U.C. Chung, C. Elissalde, M. Maglione, C. Estournes, M. Pate, J.P. Ganne, Appl. Phys. Lett. 92 (4) (2008) 042902. [13] Y. Chen, X.L. Dong, R.H. Liang, J.T. Li, Y.L. Wang, J. Appl. Phys. 98 (2005) 064107. [14] X.J. Chou, J.W. Zhai, X. Yao, Appl. Phys. Lett. 91 (2007) 122908. [15] X.H. Wang, W.Z. Lu, J. Liu, Y.L. Zhou, D.X. Zhou, J. Eur. Ceram. Soc. 26 (10-11) (2006) 1981. [16] J.J. Zhang, J.W. Zhai, H.T. Jiang, X. Yao, J. Appl. Phys. 104 (8) (2008) 084102. [17] L. Wu, Y.C. Chen, Y.P. Chou, Y.T. Tsai, S.Y. Chu, Jpn. J. Appl. Phys. 38 (1999) 5154. [18] L.H. Chiu, X. Zhang, L.C. Sengupta, US Patent No. 6,514, 895 B1 (2003). [19] L.C. Sengupta, X. Zhang, L.H. Chiu, US Patent No. 0,145,647 A1 (2007). [20] K. Yang, Z.F. Gao, J.J. Bian, J. Chin. Ceram. Soc. 34 (2006) 251. [21] B.W. Hakki, P.D. Coleman, IRE Trans. Microwave Theory Tech. 8 (1960) 402. [22] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [23] Q. Xu, X.F. Zhang, Y.H. Huang, W. Chen, H.X. Liu, M. Chen, B.H. Kim, J. Alloys Compd. 48 (2009) 448. [24] Y. Chen, Y.Y. Zhang, X.L. Dong, G.S. Wang, F. Cao, J. Am. Ceram. Soc. 93 (2010) 161. [25] P. Liu, J.L. Ma, L. Meng, J. Li, L.F. Ding, J.L. Wang, H.W. Zhang, Mater. Chem. Phys. 114 (2009) 624. [26] R. Waser, Ferroelectrics 133 (1992) 109. [27] A. Feteira, D.C. Sinclair, I.M. Reaney, J. Am. Ceram. Soc. 87 (2004) 1082. [28] T. Maiti, R. Guo, A.S. Bhalla, Appl. Phys. Lett. 89 (2006) 122909. [29] X. Wei, X. Yao, Mater. Sci. Eng. B 99 (2003) 74. [30] L.C. Sengupta, S. Sengupta, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 44 (1997) 792. [31] S.G. Lee, D.S. Kang, Mater. Lett. 57 (2003) 1629. [32] K.M. Johnson, J. Appl. Phys. 33 (1962) 2826. [33] H. Diamond, J. Appl. Phys. 32 (1961) 909. [34] J.W. Liou, B.S. Chiou, J. Mater. Sci.: Mater. Electron. 11 (2000) 645. [35] A. Chen, Y. Zhi, Phys. Rev. B 69 (2004) 174109. [36] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Electroceram. 11 (2003) 199. [37] R.H. Liang, X.L. Dong, Y. Chen, F. Cao, J. Am. Ceram. Soc. 89 (2006) 3273. [38] X.H. Wang, W.Z. Lv, J. Liu, F. Liang, D.X. Zhou, J. Chin. Ceram. Soc. 32 (2004) 738.