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Dielectric tunable properties of BaTi4O9-doped Ba0.6Sr0.4TiO3 microwave composite ceramics ZHANG JingJi, ZHAI JiWei†, JIANG HaiTao & YAO Xi Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China
BaTi4O9-doped Ba0.6Sr0.4TiO3 (BST) composite ceramics were prepared by the conventional solid-state reaction and their structure, dielectric nonlinear characteristics and microwave dielectric properties were investigated. The secondary phase of the orthorhombic structure Ba4Ti13O30 is formed among BST composite ceramics with the increase of BaTi4O9. At the same time, a duplex or bimodal grains size distribution shows fine grains in a coarse grain matrix. The degree of frequency dispersion of dielectric permittivity below Tm is increased initially and then decreased with respect to BaTi4O9. As the BaTi4O9 content increases, the tunability of composite ceramics decreases, while the Q value increases. Interestingly, 70 wt% BaTi4O9-doped BST has a tunability ~4.0% (under 30 kV/cm biasing) versus a permittivity ~68 and quality factor ~134.1 (at ~3.2 GHz). tunability, microwave dielectric properties, composite ceramics
1 Introduction In recent years, intensive development efforts have been made in dielectric nonlinear ferroelectric materials due to the potential for substantial miniaturization of microwave components and integration with microelectronic circuits. Barium strontium titanate (BST) ferroelectric materials exhibiting a large tunability under dc-applied electric field have been extensively researched for microwave applications in tunable filters, phase shifters, antennas, etc.[1,2]. Dielectric materials with high tunability, low loss factor, and moderate permittivity are required for such applications. However, pure (Ba1−xSrx)TiO3 with relatively high permittivity and loss tangent have difficulty in satisfying the requirements of tunable microwave devices design[1], which can be overcome by adding to them a low permittivity and loss material. Composite materials such as BST combined with low permittivity and loss nonferroelectric oxide are well known candidates for tunable microwave device. Re― cently, many researchers[3 8] have reported that adding nonferroelectric materials is effective to dilute the permittivity and improve the microwave performance of
ferroelectrics. In this paper, various amounts of microwave dielectric material BaTi4O9, which has low permittivity, loss tangent and low sinter temperature, were added to Ba0.6Sr0.4TiO3 (BST) to synthesize BST composite ceramics. The phase structure, dielectric nonlinear characteristic, and microwave dielectric properties of the BST composite have been investigated. The purpose of this research is to find a composite BST material system, which has not only relatively low permittivity and loss tangent but also high tunability.
2 Experimental The ceramics based on BaTi4O9-doped Ba0.6Sr0.4TiO3 were prepared through the conventional solid-state reac tion. High purity BaTiO3 (99.9%), SrTiO3 (99.9%) and BaCO3 (99.7%), TiO2 (99.9%) were used as the starting Received September 3, 2008; accepted September 11, 2008 doi: 10.1007/s11431-008-0333-0 † Corresponding author (email:
[email protected]) Supported by the Ministry of Science and Technology of China through 973-project (Grant No. 2009CB623302), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (Grant No.707024), Shanghai Committee of Science and Technology (Grant No. 07DZ22302), and Shanghai Foundation Project under 06JC14070
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materials for the synthesis of Ba0.6Sr0.4TiO3 and BaTi4O9 powders at 1200 ℃, respectively. After that, various amounts of BaTi4O9 (30, 40, 60, 70, and 80 wt%) were added to BST powders. The composite materials were milled in polypropylene bottles with zirconia grinding media for 24 h. The obtained powders, added with 8 wt% polyvinyl alcohol (PVA) binder, were pressed into small pellets (10 mm diameter and 1 mm thickness for low frequency dielectric properties measurements, 10 mm diameter and 5 mm thickness, 15 mm diameter and 7 mm thickness, and 15 mm diameter and 7 mm thickness for higher frequency dielectric properties measurements, respectively). The green pellets were burned out at 550℃ for 6 h in air to remove the solvent as well as the binder. Undoped and BaTi4O9-doped BST samples were sintered at 1400℃ and 1300℃ for 4 h in air, respectively. Sintered ceramic samples of 0, 30, 40, 60, 70, and 80 wt% BaTi4O9-doped BST corresponded to Samples A, B, C, D, E, and F, respectively. X-ray diffraction (XRD, Bruker D8 Advanced, Germany) analysis was performed to identify phase structure of the sintered samples. Scanning electron microscope (SEM, JSM EMP-800) with energy dispersive spectroscopy (EDS) was used to characterize microstructure and chemical component elements. Ceramic disks were polished to ~1.0 mm and 0.2 mm thickness and silver paste was fired on both faces of the disks at 600℃ as electrodes. Permittivity and loss tangent as a function of temperature were measured using an HP4284A precision LCR (Agilent, Palo Alto, CA) meter at frequencies from 1 kHz to 1 MHz and in the temperature range of 125 K to 425 K. Room temperature permittivity versus DC bias voltage was measured using Lab-View controlled Keithley 2410 (Cleveland, OH) high voltage source and TH-2816A LCR (Changzhou, China) analyzer at 10 kHz. Permittivity and Q value (where Q=1/tan δ ) at microwave frequencies were measured using the Hakki-Coleman dielectric resonator method [9] by the network analyzer (Agilent 8753E) combining a resonating cavity.
3 Results and discussion Figure 1 shows the XRD patterns of different amounts of BaTi4O9-doped BST composite ceramic samples. For the undoped BST sample, only a cubic perovskite structure BST is found. As the content of BaTi4O9 increases
Figure 1 X-ray diffraction (XRD) patterns of BaTi4O9-doped BST composite ceramic samples. The inset presents the (110) peak of BaTi4O9-doped BST composite ceramics.
from 30 to 80 wt%, the secondary phase of the orthorhombic structure Ba4Ti13O30 is obviously observed and the relative content is increased with respect to BaTi4O9. To further discuss the doping mechanism of BaTi4O9 in BST matrix, (110) peaks of BaTi4O9-doped BST are presented in the inset of Figure 1. As shown in the inset of Figure 1, the (110) peak of BST is evidently shifted towards a high angle and the structural evolution can be clearly observed from tetragonal to cubic symmetry. The formation of the secondary phase Ba4Ti13O30 can be described by the following reaction: Ba0.6Sr0.4TiO3+BaTi4O9 → Ba1−xSrxTiO3+Ba4Ti13O30 (x≥0.4) where x increases with increasing BaTi4O9. For example, for 70 wt% BaTi4O9-doped BST composite ceramic, the mol ratio of BST/BaTi4O9 is 0.140/0.148. Therefore, the above reaction can be described as 3 Ba0.6Sr0.4TiO3+3 BaTi4O9 → 2 Ba0.4Sr0.6TiO3+ Ba4Ti13O30. It can be interpreted that the reaction between Ba2+ in BST matrix and BaTi4O9 can form the Ba4Ti13O30 phase at around 1300℃, which results in the reduction of Ba/Sr ratio and then leads to the constriction of BST crystal cells. Nevertheless, this is out of accord with the result reported by Yan et al.[10] that both Ba0.1Sr0.9TiO3 and BaTi4O9 phases were detected in Ba0.1Sr0.9TiO3BaTi4O9 composite thin films deposited on (100) LaAlO3 single crystal substrates by pulsed laser deposition at a substrate temperature of 720℃, which is due to no reaction between Ba2+ in BST matrix and BaTi4O9 at lower temperature.
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SEM micrographs of various amounts of BaTi4O9doped BST composite ceramics are displayed in Figure 2. The BaTi4O9-doped BST composite ceramic body is porous and a complicated mixture of grains of different sizes occurs. As the BaTi4O9 content increases, the amount of fine grains gradually becomes smaller and fine grains are formed in a coarse grain matrix. Based on energy-dispersive spectroscopy (EDS) and XRD analysis results, the fine grains marked as “A” are BST and the coarse grains marked as “B” are Ba4Ti13O30. Porous morphology in the ceramic body originates from the exaggerated grain growth at sintering temperature
~1300℃, which is associated with the liquid phase of Ba4Ti13O30 at about 1350℃[11]. Similar results were obtained by Kumar et al.[11] Temperature dependence of the permittivity (ε) and loss tangent (tan δ ) for BaTi4O9-doped BST composite ceramics measured at the frequencies, varying from 1 kHz to 1 MHz, are given in Figure 3. By combining Figure 3 and Table 1, it can be seen that the dielectric anomalous peaks of ferroelectric-paraelectric phase transition for composite ceramics are suppressed with the increase of the BaTi4O9 content. The main cause of the permittivity reduction is that Ba4Ti13O30[12] (ε =41),
Figure 2 Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) micrographs of BaTi4O9-doped BST composite ceramics. (a) Sample A; (b) Sample B; (c) Sample C; (d) Sample D; (e) Sample E; and (f) Sample F. 118
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Figure 3 Temperature dependence of the permittivity and loss tangent of BaTi4O9-doped BST composite ceramics. (a) Sample A; (b) Sample B; (c) Sample C; (d) Sample D; (e) Sample E; and (f) Sample F. Microwave and dielectric properties of BaTi4O9-doped Ba0.6Sr0.4TiO3 (BST) composite ceramic samples Dielectric properties (at 10 kHz) Microwave properties At about 25℃ Sample Tunability Resonant εγ (at resonant TC (℃) Q value(1/ tanδ) (30 kV/cm)(%) frequency(GHz) frequency) tan δ εγ A 14.0 9890 0.0065 56.2 B 6.2 1210 0.0059 38.6 1256.9 865 50.1 C −8.3 702 0.0096 18.9 1904.4 557 89.0 D −20.7 247 0.0035 15.2 2589.5 276 81.1 E −81.5 88 0.0055 4.0 3209.5 68 134.1 F −88.0 54 0.0001 2865.4 48 755.7 Sample A, undoped BST; Sample B, BST with 30 wt%; Sample C, BST with 40 wt%; Sample D, BST with 60 wt%; Sample E, BST with 70 wt%; Sample F, BST with 80 wt%.
Table 1
which is also a kind of low permittivity material like BaTi4O9, dilutes the BST ferroelectricity, increasing with the increase of BaTi4O9. It can be also seen that the Curie temperature (TC) of BaTi4O9-doped BST samples is gradually shifted toward a low temperature with the addition of BaTi4O9, due to the reduction of Ba/Sr ratio in BST matrix. The variation tendency of TC is consis-
tent with the XRD analysis results. An evident diffuse dielectric represented by extremely high permittivity peak, reflecting a strong frequency dispersion feature, is also observed in Figures 3(b)―3(f). It can be seen that the spectral features closely depend on the composition. The degree of frequency dispersion of permittivity below Tm is increased initially and then
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decreased with respect to BaTi4O9. For Sample F, the dielectric peak is relatively flat at TC= −88.0℃ with no frequency dispersion. A small shift of Tm toward a high temperature with increasing frequency is observed for Samples B, C, D, and E. In order to further discuss the diffuseness of the phase transition of composite ceramics, an empirical relation is proposed to describe the high-temperature slope of the dielectric peak[13], i.e. 1
ε
=
1
ε max
+
(T − Tmax )γ 2ε maxδ 2
,
(1)
where Tmax is the temperature corresponding to εmax; and δ is a measure of the degree of diffuseness of the peak; γ is a constant which characters the degree of relaxation: for γ =1, the material follows an ideal Curie-
Weiss law which shows the validity in case of the normal ferroelectrics, whereas γ =2 indicates a typical ferroelectric relaxation response. We fitted our data to eq. (1) as indicated by the solid line in Figure 4. The value of δ obtained here using the fitting increases from 1.31 for Sample A (undoped BST) to 1.93 for Sample C, and then decreases to 1.5 for Sample F. Similar results were reported by our previous study. This phenomenon can be explained as follows. On the one hand, BaTi4O9doped BST results in the compositional inhomogeneity of BST composite ceramics and the ferroelectric relaxation behavior of samples is enhanced. On the other hand, the relative content of the second-phase Ba4Ti13O30 is increased and that of BST in composite ceramics is decreased with respect to BaTi4O9. The ferroelectric
Figure 4 Temperature dependence of the permittivity for BaTi4O9-doped BST composite ceramics at 10 kHz. The symbols indicate the experimental data, and the solid lines represent fitting to the relation (1). 120
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relaxation behavior of BaTi4O9-doped BST composite ceramics is weakened due to the decrease of BST and the increase of Ba4Ti13O30. In addition, for Samples E and F, an obvious deviation from the fitting line above higher temperature is also observed in Figures 2(e) and 2(f), due to the decrease of the relative content of BST in composite ceramics resulting in the weak connectivity between BST-BST grains. The loss tangent of samples at room temperature is slightly fluctuated with the addition of BaTi4O9. Figure 5 shows the permittivity versus DC bias field characteristics as a function of BaTi4O9-doped BST composite ceramic samples at 10 kHz and room temperature. The loss tangent (< 0.001) of the samples is also measured at 10 kHz and room temperature (not shown here) and tan δ as a function of bias voltage gives curves of similar shape to the tuning curves. The tunability of all samples is calculated by [ε (E0) − ε (E)]/ε (E0), where ε (E0) is the zero-field permittivity and ε (E) is the maximum applied-field permittivity. The tunability of composite ceramics under 30 kV/cm biasing at room temperature is calculated and presented in Table 1. The tunability of BaTi4O9-doped BST composite ceramics, which decreases with increasing BaTi4O9, is not more than that (56.2%) of pure BST sample. It is well known that the change of permittivity under the applied electric field is associated with the anharmonic interaction of Ti4+ ions for paraelectric BaTiO3-based material system. In our present work, the tunability is reduced because the non-ferroelectric phase Ba4Ti13O30 forms and the connectivity between BST-BST grains
weakens with the increase of BaTi4O9 resulting from the continuous decrease of the relative content of ferroelectric phase BST. In addition, the reduction of Curie temperature also causes the decrease of tunability. The BaTi4O9 is introduced into BST material system to result in the formation of non-ferroelectric phases Ba4Ti13O30 and the decrease of Ba/Sr ratio in BST. Curie temperature decreases as the Ba/Sr ratio decreases and causes the absence of the polar clusters in paraelectric phase, which causes the decrease of tunability. The microwave properties are very crucial to realize the microwave tunable device applications for this kind of composite ceramics. The microwave dielectric data measured at room temperature are presented in Table 1. For Samples B and C, the permittivity of composite ceramics is decreased at microwave frequencies compared with those at low frequencies (under 1 MHz), and for Sample D, the permittivity at microwave frequencies is higher than that at low frequencies (under 1 MHz), which can be attributed to the ferroelectric relaxor behavior of composite ceramics[12]. The Q value of composite ceramics increases with the increasing BaTi4O9 content. This is because of the formation of low permittivity Ba4Ti13O30[14] and the higher relative content of Ba4Ti13O30 with the increase of BaTi4O9. Although no relative research reports the Q value of Ba4Ti13O30, it is believed that low permittivity possesses high Q value in microwave frequencies. In particular, the Q value is up to 134.1 at ~3.2 GHz for Sample E. At the same time, Sample E has a tenability ~4.0% (under 30 kV/cm biasing) versus a permittivity ~ 88 (under zero biasing).
4
Figure 5 ε-E characteristics of BaTi4O9-doped BST composite ceramics (at 10 kHz).
Conclusion
BaTi4O9-doped Ba0.6Sr0.4TiO3 (BST) composite ceramics prepared by the mixed oxide route display two crystalline phases, a cubic structure BST and the orthorhombic structure Ba4Ti13O30. The diffraction peaks of BST are evidently shifted towards a high angle with increasing BaTi4O9. A complicated mixture of grains of different sizes occurs and fine grains are formed in a coarse grain matrix. The degree of frequency dispersion of permittivity below Tm is increased initially and then decreased with respect to BaTi4O9. In addition, a pronounced deviation from the fitting line above high temperature is also observed for Samples E and F. As the BaTi4O9 content increases, the tunability of composite ceramics decreases, as similarly to the variation ten-
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dency of the TC, while the Q value increases. Interestingly, 70 wt% BaTi4O9-doped BST has a tunability ~4.0% (under 30 kV/cm biasing) versus a permittivity ~68 and quality factor ~134.1 (at ~3.2 GHz).
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