Dielectric tunable properties of low-loss Ba0.4Sr0.6Ti1−yMnyO3 ceramics

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Scripta Materialia 61 (2009) 764–767 www.elsevier.com/locate/scriptamat

Dielectric tunable properties of low-loss Ba0.4Sr0.6Ti1yMnyO3 ceramics Jingji Zhang, Jiwei Zhai* and Xi Yao Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, China Received 29 May 2009; revised 23 June 2009; accepted 23 June 2009 Available online 26 June 2009

Ba0.4Sr0.6Ti1yMnyO3 ceramics showed a bimodal composition of Ba1xSrxTiO3 and 6H-hexagonal BaTiO3. The Curie peaks of samples are markedly suppressed, broadened and shifted to low temperature with increasing Mn content, and the tunability (T) is decreased. Q values of all samples are above 550. The sample with y = 0.10 exhibits good dielectric properties, with e0 of 449, Q of 580 (at 1.906 GHz) and T of 3.1%, which make it a potential candidate for the fabrication of tunable microwave devices in wireless communication applications. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: 6H-hexagonal structure; Tunability; Microwave dielectric properties

The latest research in microwave circuit technology has been devoted to the development of tunable microwave devices operating in the gigahertz frequency range to meet the requirements of increasingly high-frequency wireless telecommunication systems [1,2]. Barium titanate (BaTiO3)-based ferroelectric materials have attracted significant interest due to their electrically tunable dielectric properties for potential application in wireless telecommunications, as tunable mixers, delay lines, filters and phase shifters for steerable antennas [3–6]. Strontium-modified barium titanate (Ba1xSrxTiO3, BST) has been extensively explored for such applications due to its adjustable Curie temperature and promising dielectric properties. Materials with low relative permittivity (e0 ), high tunability (T) and high Q value (Q = 1/tan d) are essential for these applications [7,8]. Generally, the dielectric loss and tunability of ferroelectric materials are proportional to their permittivity [2]. It has been shown that high tunability and low-permittivity, together with a minimal dispersion, can be achieved by introducing low-permittivity nonferroelectric materials, such as MgO, Mg2SiO4, Mg2TiO4 and even air bubbles [4,5,8– 10], or by doping various ions, such as Mg2+ and La3+, [11,12], into BSTs. Our recent studies [10,13] indicate that these methods can reduce the permittivity but increase

* Corresponding author. Tel.: +86 21 65980544; fax: +86 21 65985179; e-mail: [email protected]

microwave loss of BSTs due to the formation of undesired phases. According to the BaO–TiO2 phase diagram [14], 6H-hexagonal BaTiO3 (h-BaTiO3) is thermodynamically stable above 1460 °C [15–17]. Recently, Sinclair and coworkers [18,19] reported that doped h-BaTiO3 ceramics are insulators at room temperature, with e0 of 50–80 and Qfr of 1370–7700 GHz. Wang et al. [20] found that substitution of Ti with Mn is an effective way to improve the dielectric properties of h-BaTiO3. For example, h-Ba(Ti0.65Mn0.35)O3 ceramics possessed a Qfr of as high as 14,300 GHz. It is expected that these results should be extendable to BSTs. In this paper, we present structural and microwave dielectric properties of Ba0.4Sr0.6Ti1yMnyO3 ceramics, in an attempt to create a composite material consisting of a low-loss linear dielectric and a nonlinear dielectric (BST) by means of of Mn doping. Ba0.4Sr0.6Ti1yMnyO3 ceramics were prepared by the conventional solid-state route using high-purity BaCO3 (99.8%), SrCO3 (99.0%), TiO2 (99.9%) and MnCO3 powders. Mixtures based on the composition of Ba0.4Sr0.6Ti1yMnyO3 (with x = 0.00–0.20) were mixed by ball milling with zirconia media in ethanol for 24 h and then dried at 110 °C for 12 h. After drying, the powders were calcined at 1200 °C for 4 h and then remilled for 24 h. The calcined powders, mixed with 8 wt.% polyvinyl alcohol (PVA), were pressed into pellets at 100 MPa. The green pellets were kept at 550 °C for 6 h to remove the solvent and binder and sintered at 1400 °C for 4 h.

1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.06.027

h(214)

h(107)

h(006)

h(203) h(204)

h(104)

h(103)

J. Zhang et al. / Scripta Materialia 61 (2009) 764–767 Raw materal c-BST

y=0.20 y=0.10 y=0.05 y=0.01 y=0.00

20 25 30 35 40 45 50 55 60 65 70 75 80 2 Theta(degrees)

Figure 1. XRD patterns of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.00, 0.01, 0.05, 0.10 and 0.20) ceramics.

Phase compositions of the samples were examined by X-ray diffraction (XRD, Bruker D8 Advanced, Germany) with Cu Ka radiation. Scanning electron microscopy (SEM, JSM EMP-800) was used to characterize the samples’ microstructures. Temperature-dependent permittivity (e0 ) and loss tangent (tan d) were measured over 130 to 140 °C at 10 kHz, using a HP4284A precision LCR meter (Agilent, Palo Alto, CA). Room-temperature permittivity vs. DC bias voltage was measured at 10 kHz by using a Keithley model 2410 (Cleveland, OH) high-voltage source coupled with a TH2816A LCR meter (Changzhou, China). e0 and tan d at microwave frequencies were measured by the resonance method [21] using a vector network analyzer (Agilent E5071C). XRD patterns of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.00, 0.01, 0.05, 0.10, 0.20) ceramics are shown in Figure 1. The samples with low Mn levels (y 6 0.10) are all single-phase

765

c-BST, whereas those with high Mn level (y = 0.20) have additional diffraction lines, which can be assigned to hBa(Ti1xMnx)O3 as reported by Wang et al. [20]. This observation means that a high level of Mn doping resulted in samples with two phases of c-BST and h-BaTiO3 (PDF Card No. 15-0239), i.e. c-Ba0.4Sr0.6Ti1yMnyO3 ? c-Ba1xSr xTi 1yMnyO 3 + h-Ba(Ti 1yMny )O 3. Wang et al. [20] reported that pure hexagonal phase Ba(Ti1xMnx)O3 can be formed at 1250–1450 °C with x = 0.04–0.20. However, no hexagonal phase of Ba1xSrxTi1yMnyO3 (y 6 0.10) is found in our present study, possibly due to the presence of Sr which suppressed the formation of 6H-hexagonal phase. In addition, it can be seen that all diffraction peaks of perovskite obviously shifted toward high angle with increasing y, which can be readily ascribed to the formation of oxygen vacancies [20] and the differences in ion radius of Mn4+,Ti4+ and Sr 2+,Ba2+. On one hand, it has been reported that Mn ions are likely to exhibit a high oxidation state (Mn4+) in air-sintered Ba(Ti1xMnx)O3 ˚ ) in 6ceramics [22]. The ionic radius of Mn4+ (0.530 A ˚) fold coordination is smaller than that of Ti4+ (0.605 A [23], which led to shrinkage of the crystal cells. On the other hand, the formation of Ba(Ti1xMnx)O3 causes a reduction in the Ba/Sr ratio – the smaller Sr2+ ion ˚ ) as compared to Ba2+ (1.61 A ˚ ) is also responsible (1.44 A for lattice shrinkage [23]. SEM images of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.01, 0.05, 0.10, 0.20) ceramics are shown in Figure 2. All samples exhibit very dense microstructures. The sample with y = 0.01 has a homogeneous microstructure with well-grown grains of 5.0–20 lm. With increasing doping level of Mn (y P 0.05), the sample’s microstructure becomes bimodal, consisting of vermiculate-shaped grains with size of 5.0 lm and fine grains of 0.5–1.0 lm. In this study, the absence of needle-shaped grains, like those reported by Wang et al. [20], is probably due to

Figure 2. SEM images of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.01, 0.05, 0.10 and 0.20) ceramics.

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J. Zhang et al. / Scripta Materialia 61 (2009) 764–767 0.12

25000

y=0.00 y=0.01 y=0.05

20000

0.10 0.08

15000

0.06 10000

Measured at 10 kHz

0.04 0.02

0

Loss tangent

Dielectric Permittivity

5000

0.00

2500

2.0

2000

y=0.10 y=0.20

1500

1.5

1000

1.0

500

0.5

0 -150

-100

-50

0

50

0.0 150

100

Temperature(oC)

Dielectric permittivity

Figure 3. Temperature dependence of the dielectric permittivity and loss tangent of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.00, 0.01, 0.05, 0.10 and 0.20) ceramics measured at 10 kHz.

1150 1100 1050 1000 950 900 700

T=12.1% Measured at 10 kHz & RT

T=8.7% T=5.6%

650

y=0.00 y=0.01 y=0.05 y=0.10

600 550 500

T=3.1%

450 -30

-20

-10 0 10 20 DC bias field(kV/cm)

30

Figure 4. DC bias field dependence of the dielectric permittivity of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.00, 0.01, 0.05, 0.10 and 0.20) ceramics measured at 10 kHz and room temperature.

the fact that it would require different amount of Sr in the h-Ba(Ti1yMny)O3 phase. Nevertheless, the multiphase characteristic of the samples with high levels of

Mn is further evidenced by the SEM image of the sample with y = 0.20, as shown in Figure 2d. Figure 3 shows the temperature dependence of e0 and tan d of all samples measured at 10 kHz. Compared with pure BST ceramics, the dielectric anomalous peaks of the Mn-doped BST ceramics, corresponding to the cubic–tetragonal phase transitions, are all markedly suppressed and broadened. The suppression and broadening become increasingly pronounced as the concentration of Mn increases. Meanwhile, the maximum permittivity temperatures Tm (TC) of the Ba0.4Sr0.6Ti1yMnyO3 ceramics are decreased. At low Mn concentration (y 6 0.05), the decrease in TC may be caused by an increase in internal stress due to the reduction in the grain size [9] and the deterioration of the ferroelectric long-range order due to the replacement of Ti by Mn [24]. However, for the samples with y > 0.05, the decrease in TC may be caused by the reduction in the Ba/Sr ratio due to the formation of h-Ba(Ti1yMny)O3. Figure 4 shows the DC bias-field-dependent e0 of the samples with y = 0.00, 0.01, 0.05 and 0.10 at 20 °C and 10 kHz. Their dielectric properties and calculated tunabilities are summarized in Table 1. Compared with pure BST ceramics, Mn doping lowered the tunability. The degree of reduction in tunability increases with increasing Mn concentration, which is similar behavior to that of TC. This observation is probably related to the presence of residual polar clusters as a result of the addition of Mn [13]. It can be seen from Table 1 that the sample with y = 0.20 actually has no nonlinear dielectric behavior, which is possibly due to the fact that the TC (<<130 °C) of the sample is too low and the relative content of ferroelectric phase BST is too small. Microwave dielectric parameters measured at room temperature are listed in Table 1. The e0 of Ba0.4Sr0.6Ti1yMnyO3 ceramics slightly decreased at microwave frequencies as compared to that at low frequencies (below 1 MHz). Q values of the Mn-doped BST ceramics are at the same level of pure BST. As shown in Table 1, the Q value decreases from 721 for y = 0.00 to a minimum value of 552 for y = 0.05, and then starts to increase with further increase in Mn content. The reduction in Q values of the samples with y 6 0.05 may be attributed to the deterioration of ferroelectric long-range order resulting from the replacement of Ti by Mn [13], whereas the increase in Q values at y > 0.05 is due to the formation of h-Ba(Ti1yMny)O3 [20]. In conclusion, Ba0.4Sr0.6Ti1yMnyO3 with y = 0.20 has a bimodal composition of BST and h-BaTiO3. TC

Table 1. Microwave and dielectric properties of the Ba0.4Sr0.6Ti1yMnyO3 (y = 0.00, 0.01, 0.05, 0.10 and 0.20) ceramics. y

Dielectric properties (at 10 kHz) TC (°C)

At about 20 °C 0

0.00 0.01 0.05 0.10 0.20

61 72 112 <130 <<130

e

tan d

1118 1017 728 485 265

0.0006 0.0003 0.0014 0.0093 0.0215

Microwave properties

Tunability (30 kV cm1) (%)

Resonant frequency (GHz)

e0 (at resonant frequency)

Q value

12.1 8.7 5.6 3.1 –

1.131 1.205 1.476 1.906 2.380

1032 969 626 449 243

721 615 552 580 779

J. Zhang et al. / Scripta Materialia 61 (2009) 764–767

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