Anomalous Correlation between Dielectric Constant and Tunability in ...

J. Am. Ceram. Soc., 96 [4] 1203–1208 (2013) DOI: 10.1111/jace.12167 © 2013 The American Ceramic Society

Journal

Anomalous Correlation between Dielectric Constant and Tunability in (Ba, Sr) TiO3–MgO–Mg2SiO4 Composite Ceramics Yanyan He, Jingyuan Zhao, Yebin Xu,† and Changnian Li School of Optics and Electronics Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

(Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics were prepared by a solid-state reaction method. The microstructures, microwave dielectric characteristics, and tunability of composite ceramics were investigated. An anomalous correlation between tunability and dielectric constant was observed: with the increase in Mg2SiO4 content and the decrease in MgO content, the dielectric constant of (Ba, Sr)TiO3– MgO–Mg2SiO4 composite ceramics decreases, but the tunability increases. The anomalous increased tunability is beneficial for tunable microwave applications and can be attributed to the redistribution of the electric field. For 50Ba0.5Sr0.5TiO3– (50
x)MgO–xMg2SiO4, the dielectric constant was decreased from 164.2 to 126.5 by increasing Mg2SiO4 content from 5 to 45 wt% and the tunability at 3.9 kV/mm increased from 11.5% to 15.2%.

I.

system, with the increase in Mg2TiO4 content from 50 to 80 wt%, the dielectric constant at 10 kHz decreased from 335 to 35, and the tunability at 3 kV/mm decreased from 18.4% to 10.8%.13 More recently, Kozyrev et al.14 reported that increasing Mg2TiO4 content from 8.3 to 44.4 wt% led to the decrease in dielectric constant of BST–Mg2TiO4 from 810 to 260, but the tunability increased anomalously. The fact that the tunability increases with the decrease in dielectric constant is just contrary to that observed in most ferroelectric–dielectric composites and is beneficial for tunable microwave application. The seemingly contradictory results between Ref. [13] and [14] maybe results from different Mg2TiO4 content range. To date, only Kozyrev et al.14 reported the anomalous correlation between dielectric constant and tunability in the BST– Mg2TiO4 system. Therefore, it is meaningful to seek for new ferroelectric–dielectrics system where tunability increases with the decrease in dielectric constant. In this article, the microstructures and dielectric tunable properties of Ba1
xSrxTiO3 (x = 0.45 and 0.5)–MgO– Mg2SiO4 composite ceramics were investigated. An anomalous correlation between dielectric constant and tunability in BST–MgO–Mg2SiO4 system was observed: with increasing the Mg2SiO4 content and decreasing the MgO content, the dielectric constant of composite ceramics decreased, but the tunability increased.

Introduction

B

ARIUM strontium titanate (BST) exhibits a large dielectric constant change with an applied DC electric field. The DC electric field-dependent dielectric constant can be used to develop the phase shifting elements in phased array antennas and tuning elements in devices operating at microwave frequencies.1–3 Recently, the accelerator applications of BST-based ferroelectrics were also reported: bulk BST ferroelectrics can be used as active elements of electrically controlled switches and phase shifters in pulse compressors or power distribution circuits of future linear colliders as well as tuning layers for the dielectric-based accelerating structures.4–7 The main requirement for the electrical properties of ceramic materials for tunable microwave applications is a combination of relatively low dielectric constant, low loss tangents, and high dielectric tunability.8,9 However, the high inherent material loss and high dielectric constant of pure BST has restricted its application in tunable microwave devices. Various methods have been investigated to lower the dielectric constant and loss tangent of BST. It is reported that forming a ferroelectric–dielectric composite is an efficient method to reduce material dielectric constant, loss tangent, and maintain tunability at a sufficiently high level.2,8–14 Some linear dielectrics with low dielectric constant were added into BST and it is found that a BST–MgO composite shows better dielectric properties.2,8,10–12 In general, increasing the content of dielectrics, such as MgO and Mg2TiO4, leads to the decrease in both the dielectric constant and the tunability of the ferroelectrics.2 Increasing MgO content from 10 to 60 wt%, the dielectric constant of Ba0.6Sr0.4TiO3–MgO decreased from 1431 to 118, and the tunability at 2 kV/mm decreased from 16.6% to 10%.10 In the Ba0.5Sr0.5TiO3–Mg2TiO4

II.

Experimental Procedure

The starting raw chemicals were BaTiO3, SrTiO3, MgO, and SiO2. BaTiO3 and SrTiO3 powders were mixed to achieve Ba0.55Sr0.45TiO3 and Ba0.5Sr0.5TiO3 and ball-milled in deionized water using agate media for 6 h. The dried powder was then calcined at 1000°C for 3 h. Mg2SiO4 was synthesized by the same method using MgO and SiO2 powders at 1200°C. The Ba0.55Sr0.45TiO3 or Ba0.5Sr0.5TiO3 powders were mixed with Mg2SiO4 and MgO according to the formula 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4 or 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4 and milled with agate balls for 6 h. After adding the binder (5% polyvinyl alcohol solution) the composite powders were pressed uniaxially into pellets at a pressure of 150 MPa and subsequently sintered at 1350°C for 3 h. Silver paste was coated to form electrodes on both sides of the sintered ceramic samples for dielectric measurements. The phase compositions of the sintered samples were determined by powder X-ray diffraction (XRD) using CuKa radiation (X’Pert PRO; PANalytical B.V., Almelo, The Netherlands) after crushing and grinding. The diffractometer operated at a voltage of 40 kV and current of 40 mA. A Hitachi S-4800 field emission scanning electron microscope (Hitachi Ltd., Tokyo, Japan) with an energy dispersive X-ray analysis system (EMAX Energy EX-350; Horiba, Kyoto, Japan) was used to characterize the microstructure and chemical component elements. The dielectric constant and loss tangent were measured using Agilent 4294A precision impedance analyzer (Agilent Technologies, Santa Clara, CA). Temperature-dependent

C. Randall—contributing editor

Manuscript No. 31805. Received July 25, 2012; approved December 13, 2012. † Author to whom correspondence should be addressed. e-mail: xuyebin@yahoo. com

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dielectric constant and loss tangent were measured at 10 kHz with a heating rate of 2°C/min in a cryogenic system. The tunability was measured at 10 kHz with a Novocontrol dielectric/ impedance spectrometer (BDS 80; Novocontrol Technologies, Hundsangen, Germany) at biases up to 2000 V DC via an external power supply (High Voltage Booster 4000; Novocontrol Technologies). The dielectric constants and unloaded Q values at microwave frequency were measured in the TE01d dielectric resonator mode using the Hakki and Coleman method15 by the network analyzer (HP Agilent 8756A; Agilent Technologies).

III.

Results and Discussion

Figure 1 shows the XRD patterns of (Ba, Sr)TiO3–MgO– Mg2SiO4 composite ceramics. A cubic paraelectric (Ba, Sr) TiO3, a cubic MgO and an orthorhombic Mg2SiO4 are clearly observed without other phases being detected for the composite ceramics. With the decrease in MgO content and the increase in Mg2SiO4 content, the diffraction peaks from MgO decreases gradually and the diffraction peaks from Mg2SiO4 increases. From the XRD study, it is apparent that (Ba, Sr)TiO3, MgO, and Mg2SiO4 are distributed as individual phases in the final composite bulk ceramics. Figure 2 shows the representative FESEM images of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics. The FESEM image and the energy dispersive spectrometer (EDS) spectra for 40Ba0.55Sr0.45TiO3–12MgO–48Mg2SiO4 composite ceramics are shown in Fig. 3. The images exhibit quite dense microstructures and three kind of different grains are distinctively observed. Based on the EDS analyses, the larger dark granular grain marked B is MgO, the grain marked A is Ba0.55Sr0.45TiO3, and C is Mg2SiO4. The FESEM and EDS observations are consistent with the XRD results. Figure 4 shows the dielectric constant and loss tangent of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics measured at 10 kHz. With the increase in Mg2SiO4 content and the decrease in MgO content, the dielectric constant of composite ceramics decreases. The loss tangent did not change and remained at ~6 9 10
4 (for Ba0.55Sr0.45TiO3–MgO–Mg2SiO4) or 5 9 10
4 (for Ba0.5Sr0.5TiO3–MgO–Mg2SiO4). Decreasing Ba content in (Ba, Sr)TiO3 or increasing MgO–Mg2SiO4 content can decrease dielectric constant evidently. Although the dielectric constant of Ba0.5Sr0.5TiO3 is smaller than that of Ba0.55Sr0.45TiO3, the dielectric constant of 50Ba0.5Sr0.5TiO3– (50
x)MgO–xMg2SiO4 is higher than that of 40Ba0.55Sr0.45-

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TiO3–(60
x)MgO–xMg2SiO4 due to lower MgO–Mg2SiO4 content. Figure 5 shows the temperature dependences of the dielectric constants and the loss tangent of (Ba, Sr)TiO3–MgO– Mg2SiO4 composite ceramics measured at 10 kHz. Mg2SiO4 content has obvious effect on the dielectric properties of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics. With increasing Mg2SiO4 content, the dielectric constant of (Ba, Sr)TiO3–MgO –Mg2SiO4 composite ceramics tends to decrease although the linear dielectrics content remain unchanged. The dielectric peaks of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics were broadened and suppressed with increasing Mg2SiO4 content. emax of 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4 ceramics decreases from 249.2 (x = 6 wt%) to 124.0 (x = 54 wt%) [Fig. 5(a)]. With the increase in Mg2SiO4 content from 5 to 45 wt%, emax of 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4 ceramics decreases from 456.2 to 311.6 [Fig. 5(b)]. Meanwhile, Mg2SiO4 content has different effect on the Curie temperature Tc of 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4 and 50Ba0.5 Sr0.5TiO3–(50
x)MgO–xMg2SiO4. For 40Ba0.55Sr0.45TiO3– (60
x)MgO–xMg2SiO4, Tc fluctuates around ~
90°C [Fig. 5(a)]. With x = 48 and 54 wt%, Tc becomes a temperature range:
92.3°C to
90.7°C and
88.7°C to
89.8°C, respectively. On the other hand, it seems that there is another smaller dielectric constant peak (shoulder) at ~
30°C for all five compositions. The second peak dielectric constant becomes comparable with the emax for samples with x = 48 and 54 wt %. Therefore, the dielectric constant-temperature stability is improved with the increase in Mg2SiO4 content. As we know, Tc is the cubic-tetragonal phase transition temperature,2 but it is difficult for us to determine the physical origin of the second dielectric peak. For 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4, Tc is observed to increase with the increase in Mg2SiO4 content monotonously: Tc increases from
106.3°C for x = 5 wt% to
90.4°C for x = 45 wt% [Fig. 5(b)]. With the decrease in temperature, the loss tangent of all samples increases. The tunability is defined as the change in the dielectric constant under a DC-bias electric field relative to the initial unbiased value, i.e.: T ¼ ½eð0Þ
eðEÞ=eð0Þ ¼ ½Cð0Þ
CðEÞ=Cð0Þ where C is the capacitance and E is the applied DC electric field. Figure 6 shows the effect of applied field on the tunability of the (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics at 10 kHz. It shows that the tunability increases with the

(b)

Fig. 1. The XRD patterns of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics. (a) 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4, (b) 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4.

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Permittivity and Tunability in BST (a)

(b)

(c)

(d)

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(e)

Fig. 2. FESEM images of (Ba, Sr)TiO3–MgO–Mg2SiO4 composites ceramics. From (a) to (d), 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4, x = 12, 30, 48, and 54 wt%, respectively.(e) 50Ba0.5Sr0.5TiO3–40MgO–10Mg2SiO4.

increase in the applied field. With the increase in Mg2SiO4 content, the tunability of composite ceramics tends to increase. For example, the tunability of 40Ba0.55Sr0.45TiO3– (60
x)MgO–xMg2SiO4 at 3.9 kV/mm increases from 13.7% for x = 6 wt% to 15.8 for x = 54 wt%, although the dielectric constant decreases from 114.9 to 73.3. 50Ba0.5Sr0.5TiO3– (50
x)MgO–xMg2SiO4 exhibits the same trend: the tunability at 3.9 kV/mm increases from 11.5% for x = 5 wt% to 15.2% for x = 45 wt% and the dielectric constant decreases from 164.2 to 126.5. Therefore, the tunability increases anomalously with the decrease in dielectric constant in (Ba, Sr)TiO3 –MgO–Mg2SiO4 composite ceramics. The clearest evidence of this phenomenon can be seen in Fig. 7, where the tunability is presented as a function of dielectric constant. With the increase in Mg2SiO4 content, the dielectric constant of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics decreases, but tunability increases anomalously. As we mentioned previously, linear dielectrics, such as MgO and Mg2TiO4, was usually added to ferroelectrics to form ferroelectric–dielectric composites so that the dielectric constant can be reduced. In general, the dielectric constant decreases with the increase in linear dielectrics content, at the meantime, the tunability decreases inevitably.2,13 Therefore, the anomalous correlation between dielectric constant and tunability observed in (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics is beneficial for tunable microwave application. Up to now, only Kozyrev et al.14 and Nenasheva et al.9 reported similar anomalous phenomenon in (Ba, Sr)TiO3-

Mg2TiO4 system: the reduction in the dielectric constant resulted in a increase in the tunability. A different trend was reported by Chou et al.13: with the increase in Mg2TiO4 content from 50 to 80 wt%, both the dielectric constant and tunability of Ba0.5Sr0.5TiO3–Mg2TiO4 decreases. It seems that the difference between Ref. [13] and [14] can be attributed to the different Mg2TiO4 content range. In the present work, linear dielectrics content are 50 and 60 wt%, and anomalous phenomenon is observed in both groups of samples. The density of Mg2SiO4 is smaller than that of MgO, with the increase in Mg2SiO4 content and the decrease in MgO content, the volume of ferroelectrics (Ba, Sr)TiO3 will decrease although the weight of (Ba, Sr)TiO3 remains unchanged. The ferroelectricity will be diluted further by linear dielectrics and the tunability should decrease further, as in the BST–MgO system.2 The opposite trend is observed in (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics. Maybe, the increased tunability in 50Ba0.5Sr0.5TiO3–(50
x)MgO– xMg2SiO4 can be attributed to increased Tc toward room temperature: the nearer Tc is to room temperature, the higher the tunability is. The Tc change cannot explain the same trend observed in 40Ba0.55Sr0.45TiO3–(60
x)MgO– xMg2SiO4 ceramics because the Tc is not increased monotonously. Therefore, the increased tunability should come from the other resource. The dielectric response of ferroelectric–dielectric composites is theoretically addressed by Sherman et al.16 In spherical inclusion model, the redistribution of the electric field sur-

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(a)

(c)

(d)

Fig. 3. The FESEM image (a) and energy dispersive spectrometer (EDS) spectra (b), (c), (d) of 40Ba0.55Sr0.45TiO3–12MgO–48Mg2SiO4 composite ceramics.

(a)

(a)

(b)

(b)

Fig. 4. The dielectric constant (solid) and loss tangent (open) for (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics measured at 10 kHz. (a) 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4, (b) 50Ba0.5Sr0.5TiO3– (50
x)MgO–xMg2SiO4.

Fig. 5. Variation in dielectric constant (solid) and loss tangent (open) for (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics with temperature measured at 10 kHz. (a) 40Ba0.55Sr0.45TiO3–(60
x) MgO–xMg2SiO4, (b) 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4.

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Permittivity and Tunability in BST

(a)

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(a)

(b)

(b)

Fig. 7. The tunability as a function of dielectric constant for (Ba, Sr) TiO3–MgO–Mg2SiO4 composites ceramics. (a) 40Ba0.55Sr0.45TiO3– (60
x)MgO–xMg2SiO4 at 10 kHz, 2.4 kV/mm, (b) 50Ba0.5Sr0.5TiO3– (50
x)MgO–xMg2SiO4 at 10 kHz, 2.5 kV/mm.

Fig. 6. The tunability of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics at 10 kHz. (a) 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4, (b) 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4.

rounding the inclusion (linear dielectrics) will affect the local tuning of the ferroelectric and leads to the increase in tunability of composites.16 Although their model considered only small concentration of dielectrics, it can be used to explain our results qualitatively. In (Ba, Sr)TiO3–MgO– Mg2SiO4 composite ceramics, the big contrast in the values of dielectric constants of the spherical inclusion (MgO– Mg2SiO4) and the ferroelectric [(Ba, Sr)TiO3] affects the redistribution of the electric field around the inclusion. The dielectric constant of the ferroelectric under applied electric field becomes inhomogeneously distributed over the volume of the ferroelectric. The overall tunability of the composite changes. Linear dielectrics Mg2SiO4 and MgO have different dielectric constants and shape, and will have different effects on the redistribution of the electric field. On the one hand, with the increase in Mg2SiO4 content and the decrease in MgO content, the volume of ferroelectric (Ba, Sr)TiO3

decreases due to smaller density of Mg2SiO4 than that of MgO, the tunability of composite will be suppressed. On the other hand, the redistribution of the electric field surrounding the inclusion due to the different combination of Mg2SiO4 and MgO results in the increase of tunability of composite ceramics. The increase in the tunability due to redistribution of the electric field exceeds the decrease in the tunability due to ferroelectric dilution, so the tunability of composite ceramics increases anomalously. The increase in tunability should be related to many factors, such as dielectric constant and shape of linear dielectrics and ferroelectrics etc. Certainly, there also exists other possible explanation. The spherical inclusion model does not address the electric field distribution dependence on frequency (tuning frequency versus permittivity measurement frequency). Both dielectric constant and resistive contrast between constituent phases control the electric field distribution in the composite structure. It is expected that the resistive contrast between the BST and Mg2SiO4 becomes more important at low frequency (i.e., tuning frequency). A more likely explanation is that Mg is an acceptor dopant that controls the BST conductivity. A small amount of Mg is seen in the EDAX for the BST grain [Fig. 3(a)]. Perhaps Mg2SiO4 provides the optimum Mg activity for increasing the BST resistivity and hence supports higher electric fields in the BST. Therefore, the tunability increases with the decrease in dielectric constant anomalously. The present work provides a method to decrease the dielectric constant of composite and increase the tunability, which is beneficial for tunable microwave applications. Com-

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Microwave Dielectric Properties of (Ba, Sr)TiO3–MgO–Mg2SiO4 Composites Ceramics

Composition

40Ba0.55Sr0.45TiO3–(60
x)MgO
xMg2SiO4

50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4

Mg2SiO4 content (wt%)

f0 (GHz)

e

tand

Q9f (GHz)

6 30 48 54 5 10 25 40 45

4.25 5.58 5.16 5.15 3.37 3.67 4.26 3.42 3.90

101.61 74.34 63.43 62.69 154.60 138.46 127.43 128.03 113.34

0.0062 0.0075 0.0073 0.0078 0.0048 0.0045 0.0057 0.0072 0.008

686 744 707 660 702 816 747 475 488

pared with binary ferroelectric–dielectric composite, such as BST–MgO, ternary ferroelectric–dielectric composite provide another variable to modify the properties of ferroelectric– dielectric composites. In addition to Ba/Sr and linear dielectrics content, the relative content of different linear dielectrics can also adjust dielectric constant and tunability of composite ceramics. The work on search for new dielectrics combination is in progress. The microwave dielectric properties of some (Ba, Sr)TiO3– MgO–Mg2SiO4 composites ceramics were measured and summarized in Table I. We can see that the dielectric constant of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics is slightly decreased at microwave frequencies compared with that at low frequencies (10 kHz) and the dielectric loss in microwave frequencies is higher obviously than that at 10 kHz. For 40Ba0.55Sr0.45TiO3–(60
x)MgO–xMg2SiO4 composite ceramics, the increase in Mg2SiO4 content results in the decrease of dielectric constant, but has no strong effect on the Q 9 f value. For 50Ba0.5Sr0.5TiO3–(50
x)MgO–xMg2SiO4 composite ceramics, the effect of Mg2SiO4 content on the dielectric constant is similar, but Q 9 f value decreases clearly when Mg2SiO4 content reaches 40 wt%.

IV.

Conclusions

(Ba,Sr)TiO3–MgO–Mg2SiO4 composite ceramics were prepared via the conventional solid-state reaction method. The microstructures, tunability, and microwave dielectric properties of composite ceramics were investigated. With increasing Mg2SiO4 content and decreasing MgO content, the dielectric peaks of (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics were broadened and suppressed. An anomalous dependency of the tunability as a function of dielectric constant was observed in (Ba, Sr)TiO3–MgO–Mg2SiO4 composite ceramics: the tunability increases with the decrease in the dielectric constant. The anomalous increase in tunability is beneficial for tunable microwave applications.

Acknowledgments This work is supported by the Natural Science Foundation of China under grants no. 10975055 and 60771021, Research Fund for the Doctoral Program of Higher Education of China. Dr. Z. X. Tang at the Microwave Center, University of Electronic Science and Technology of China (Chengdu, China) is acknowledged for his kind help in the evaluation of the microwave dielectric properties. The authors wish to acknowledge the Analytical and Testing Center in Huazhong University of Science and Technology for XRD analysis.

References 1

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