Open access peer-reviewed chapter

Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application

By Yebin Xu and Yanyan He

Submitted: October 22nd 2010Reviewed: March 17th 2011Published: August 24th 2011

DOI: 10.5772/17744

Downloaded: 2001

1. Introduction

Electric field tunable ferroelectric materials have attracted extensive attention in recent years due to their potential applications for tunable microwave device such as tunable filters, phased array antennas, delay lines and phase shifters (Maiti et al. 2007a; Rao et al. 1999; Romanofsky et al. 2000; Varadan et al 1992.; Zhi et al. 2002). Ba1-xSrxTiO3 and BaZrxTi1−xO3 have received the most attention due to their intrinsic high dielectric tunability. However, the high inherent materials loss and high dielectric constant has restricted its application in tunable microwave device. Various methods have been investigated to lower the dielectric constant and loss tangent of pure ferroelectrics.

Forming ferroelectric-dielectric composite is an efficient method to reduce material dielectric constant, loss tangent and maintain tunability at a sufficiently high level. For binary ferroelectric-dielectric composite (such as BST+MgO) (Chang & Sengupta 2002; Sengupta & Sengupta 1999), with the increase of dielectrics content, the dielectric constant and tunability of composites decrease. In order to decrease the dielectric constant of binary composite, it is necessary to increase the content of linear dielectric, and the tunability will decrease inevitably due to ferroelectric dilution. Replacing one dielectric by the combination of dielectrics with different dielectric constants and forming ternary ferroelectric-dielectric composite can decrease the dielectric constant of composite and maintain or even increase the tunability. This is beneficial for tunable application. The Ba0.6Sr0.4TiO3-Mg2SiO4-MgO and BaZr0.2Ti0.8O3-Mg2SiO4-MgO composites exhibited relatively high tunability in combination with reduced dielectric permittivity and reduced loss tangent (He et al. 2010, 2011). With the increase of Mg2SiO4 content and the decrease of MgO content in Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composite, the dielectric constant decrease and the tunability remain almost unchanged. For BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite, an anomalous relation between dielectric constant and tunability was observed: with the increase of Mg2SiO4 content (>30 wt%), the dielectric constant of composite decreases and the tunability increases. The anomalous increased tunability can be attributed to redistribution of the electric field. Ba1-xSrxTiO3-Mg2TiO4-MgO can also form ferroelectric (Ba1-xSrxTiO3)-dielectric (Mg2TiO4-MgO) ternary composite and the dielectric constant can be decreased. With the increase of Mg2TiO4 content and the decrease of MgO content, the tunability of Ba1-xSrxTiO3-Mg2TiO4-MgO composite increase. The multiple-phase composites might complicate method to effectively deposit films, particularly if the dielectrics and ferroelectric are not compatible for simultaneous deposition or simultaneous adhesion with a substrate or with each other. But ferroelectric-dielectric composite bulk ceramics show promising application, especially in accelerator: bulk ferroelectrics composites 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 (Kanareykin et al. 2006, 2009a, 2009b).

Forming ferroelectric-dielectric solid solution is another method to reduce material dielectric constant and loss tangent. Ferroelectric Ba0.6Sr0.4TiO3 can form solid solution with dielectrics Sr(Ga0.5Ta0.5)O3, La(Mg0.5Ti0.5)O3, La(Zn0.5Ti0.5)O3, and Nd(Mg0.5Ti0.5)O3 that have the same perovskite structure as the ferroelectrics (Xu et al. 2008, 2009). With the increase of the dielectrics content, the dielectric constant, loss tangent and tunability of solid solution decrease. Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 shows better dielectric properties than other solid solutions. Compared with ferroelectric-dielectric composite, forming solid solution can decrease the dielectric constant more rapidly when the doping content is nearly the same, and can also improve the loss tangent more effectively. On the other hand, ferroelectric-dielectric solid solution shows lower tunability than composites. The advantage of ferroelectric-dielectric solid solution is that single phase materials is favorable for the thin film deposition. The high dielectric field strength can be obtained easily in thin film to get high tunability.

In this chapter, we summarize the microstructures, dielectric tunable properties of ferroelectric-dielectric solid solution and composites, focusing mainly on our recent works.

2. Ferroelectric-dielectric composite

2.1. Ba1-xSrxTiO3 based composites

Various non-ferroelectric oxides, such as MgO, Al2O3, ZrO2, Mg2SiO4 and MgTiO3, were added to Ba1-xSrxTiO3 to reduce the dielectric constant and loss tangent and maintain the tunability at sufficient high level (Chang & Sengupta 2002; Sengupta & Sengupta 1997, 1999). It is better that non-ferroelectric oxide doesn’t react with ferroelectric Ba1-xSrxTiO3. MgO has low dielectric constant and loss tangent, can form ferroelectric (Ba1-xSrxTiO3)-dielectric (MgO) composite. BST-MgO composite shows better dielectric properties. Mg2SiO4 is also a linear dielectrics with low dielectric constant, but it can react with Ba1-xSrxTiO3 to form Ba2(TiO)(Si2O7), as shown in Fig. 1. For 10 mol% Mg2SiO4 mixed Ba0.6Sr0.4TiO3, the major phase is Ba0.6Sr0.4TiO3, and no Mg2SiO4 phase can be found except for two unidentified peaks at 27.6°and 29.7° (relative intensity: ~1%). As the content of Mg2SiO4 increases from 20 to 60 mol%, the impurities phase of Ba2(TiO)(Si2O7) is observed obviously and the relative content is increased with respect to the content of Mg2SiO4. For 60 mol% Mg2SiO4 mixed Ba0.6Sr0.4TiO3 ceramics sintered at 1220°C, the strongest diffraction peak is the (211) face of Ba2(TiO)(Si2O7) (not shown in Fig. 1). Therefore, for Mg2SiO4 added Ba0.6Sr0.4TiO3, it is not as we expected that the ferroelectric (Ba0.6Sr0.4TiO3)-dielectric (Mg2SiO4) composite formed. The dielectric constants and unloaded Q values at microwave frequency were measured in the TE01 dielectric resonator mode using the Hakki and Coleman method by the network analyzer. Table 1 summarizes r and the quality factor (Qf=f0/tan, where f0 is the resonant frequency) at microwave frequencies for some Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics. Increasing the Mg2SiO4 content results in a decrease of dielectric constant but has no obvious effect on the Qf value. The low Qf of Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics restricts their microwave application, and so the tunability has not been measured furthermore. The low Qf is due to Ba2(TiO)(Si2O7) which is a ferroelectrics with promising piezoelectric uses.

Figure 1.

The XRD patterns of Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics. The Mg2SiO4 content is 10-60mol%.

Mg2SiO4 content (mol%)Sintering temperature (°C)f0(GHz)tanQf(GHz)
2012601.79683.70.016112
4012402.98169.20.024124

Table 1.

Microwave dielectric properties of Ba0.6Sr0.4TiO3-Mg2SiO4 ceramics

For Mg2SiO4-MgO added Ba0.6Sr0.4TiO3, ferroelectric (Ba0.6Sr0.4TiO3)-dielectric (Mg2SiO4-MgO) composite is formed, as shown in Fig. 2 (He et al., 2010). With the decrease of MgO content and the increase of Mg2SiO4 content, the diffraction peaks from MgO decrease gradually and the diffraction peaks from Mg2SiO4 increase. Therefore, Mg2SiO4-MgO combination can prohibit the formation of Ba2(TiO)(Si2O7) phase.

Fig. 3 shows the FESEM images of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites sintered at 1350°C for 3h. The FESEM image and element mapping of 40Ba0.6Sr0.4TiO3-12Ba0.6Sr0.4TiO3-48MgO as determined by energy dispersive spectroscopy (EDS) are shown in Fig. 4. Three kind of different grains can be found clearly: light grains with average grain size of about 2m, nearly round larger grains and dark grains with sharp corners. The element mapping of Si K1 and Ti K1 in Fig. 4 can show the distribution of Mg2SiO4 and Ba0.6Sr0.4TiO3 grains clearly. Therefore, we can identify that light grains are Ba0.6Sr0.4TiO3, the dark, larger grains are MgO, and dark grains with sharp corners are Mg2SiO4. With the decrease of MgO content and the increase of Mg2SiO4 content, more and more Mg2SiO4 grains with different size can be found (Fig. 4). It is consistent with the XRD results. We can conclude that Mg2SiO4 and MgO were randomly dispersed relative to ferroelectric Ba0.6Sr0.4TiO3 phase.

Figure 2.

The XRD patterns of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composite ceramics sintered at 1350°C for 3h. From bottom to top, the MgO content is 48 wt%, 36 wt%, 30 wt%, 24 wt% and 12 wt%, respectively.

Figure 3.

FESEM images of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composites ceramics sintered at 1350°C for 3h. From (a) to (e), the MgO content is 48 wt%, 36 wt%, 30 wt%, 24 wt% and 12 wt%, respectively.

Figure 4.

FESEM image and element mapping of 40Ba0.6Sr0.4TiO3-12Mg2SiO4-48MgO as determined by energy dispersive spectroscopy (EDS).

Because of the relatively low dielectric constant and loss tangent of Mg2SiO4 and MgO, it is expected that Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites have lower dielectric constant and loss tangent. Fig. 5 shows the dielectric constant and loss tangent of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composite ceramics at 1MHz. The dielectric constant of composites is much smaller than that of Ba0.6Sr0.4TiO3 (~5160 at 1MHz) (Chang & Sengupta, 2002; Sengptal & Sengupta 1999;). The loss tangent of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites sintered at 1350°C is ~0.0003-0.0006, but the loss tangent of Ba0.6Sr0.4TiO3 is ~0.0096 (Sengptal et al. 1999). Therefore, the composites have much smaller loss tangent than Ba0.6Sr0.4TiO3.

The temperature dependence of dielectric properties for various Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites (sintering temperature: 1350°C) measured at 100kHz is illustrated in Fig. 6. Broadened and suppressed dielectric peaks and shifts of Curie temperature TC are observed. For 40Ba0.6Sr0.4TiO3-12Mg2SiO4-48MgO ceramics, its max is ~ 176.5 at Tc ~224K. As the relative content of Mg2SiO4 increase, Tc is shifted slightly to lower temperatures, thus resulting in a decrease in dielectric constant at a given temperature; at the meantime, max decreases also. For 40Ba0.6Sr0.4TiO3-30Mg2SiO4-30MgO, max is ~140.1 at ~216K and for 40Ba0.6Sr0.4TiO3-48Mg2SiO4-12MgO, max is ~126.8 at ~214K. With the decrease of temperature, the loss tangent increase.

Fig. 6 shows the effect of applied field on the tunability of the Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites at 100kHz. The tunability of 40Ba0.6Sr0.4TiO3-12Mg2SiO4-48MgO at 100kHz under at 2kV/mm is 10.5%. With the increase of Mg2SiO4 content, the tunability of 40Ba0.6Sr0.4TiO3-24Mg2SiO4-36MgO decreases slightly to 9.2%. Further increasing Mg2SiO4 content results in a slight increase of tunability: 40Ba0.6Sr0.4TiO3-48Mg2SiO4-12MgO composite has tunability of 10.2%.

Figure 5.

Dielectric constant (solid) and loss tangent (open) of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composite ceramics sintered at different temperature (measure frequency: 1MHz).

Figure 6.

Variation of dielectric constant (solid) and loss tangent (open) with temperature for 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) ceramics measured at 100kHz.

Figure 7.

The tunability of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composites at 100kHz (sintering temperature: 1350°C).

MgO content (wt.%)f0(GHz)tanQf(GHz)
125.7474.590.023250
245.7477.720.019302
305.8077.120.021276
365.9674.390.017351
485.3393.860.014381

Table 2.

Microwave Dielectric Properties of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) ceramics

The room temperature microwave dielectric properties of 40Ba0.6Sr0.4TiO3-60(Mg2SiO4-MgO) composites were summarized in Table 2. With the increase of Mg2SiO4 content, the dielectric constant remain almost the same and the Qf value decrease.

Mg2TiO4 is a low loss tangent linear dielectrics and Mg2TiO4 added Ba1-xSrxTiO3 shows better tuanble dielectric properties (Chou et al. 2007; Nenasheva et al. 2010). The XRD patterns of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 (Fig. 8) show that ferroelectric (Ba0.6Sr0.4TiO3)-dielectric (MgO-Mg2TiO4) composite is formed. On the other hand, impurity phase BaMg6Ti6O19 is found in Mg2TiO4 doped Ba0.6Sr0.4TiO3. The fomation of BaMg6Ti6O19 depends on Ba/Sr ratio. BaMg6Ti6O19 forms in Mg2TiO4 doped Ba0.6Sr0.4TiO3 and Ba0.55Sr0.45TiO3 but not Ba0.5Sr0.5TiO3. Mg2TiO4-MgO combination can prohibit the formation of BaMg6Ti6O19 phase. The FESEM images (Fig. 9) show clearly three kind of grains: Ba0.6Sr0.4TiO3, Mg2TiO4 and MgO.

Table 3 shows the microwave dielectric properties of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 ceramics. With the increase of MgO content, the dielectric constant decrease due to lower dielectric constant of MgO. For x=0-36 wt%, the Qf value remain unchanged. As a whole, the loss tangent is too high to be used for tunable microwave application.

Figure 8.

The XRD patterns of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 ceramics

Figure 9.

FESEM images of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 composites ceramics sintered at 1400°C for 3h.

MgO content (wt.%)f0(GHz)tanQf(GHz)
02.83193.400.03483
122.67226.760.03479
242.96220.250.03585
363.00207.660.03683
483.53199.710.034104
604.80109.630.013369

Table 3.

Microwave dielectric properties of 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4 ceramics

Increasing Sr/Ba ratio can decrease the dielectric constant and loss tangent of Ba1-xSrxTiO3. 40Ba0.5Sr0.5TiO3-xMgO-(60-x)Mg2TiO4 will has lower dielectric constant and loss tangent than 40Ba0.6Sr0.4TiO3-xMgO-(60-x)Mg2TiO4. We prepared 40Ba0.5Sr0.5TiO3-xMgO-(60-x)Mg2TiO4 ceramics and measured the tunability (Fig. 10). With the increase of Mg2TiO4 content, the tunabity of composite increases. The tunability of 40Ba0.5Sr0.5TiO3-12MgO-48Mg2TiO4 is 16.6% at 2kV/mm and 28.5% at 3.9kV/mm, respectively. The corresponding value of 40Ba0.5Sr0.5TiO3-60Mg2TiO4 is 13.6% and 24.0% respectively. The higher tunability of 40Ba0.5Sr0.5TiO3-12MgO-48Mg2TiO4 is due to its higer dielectric constant (=150.2) than 40Ba0.5Sr0.5TiO3-60Mg2TiO4 (=127.8).

Figure 10.

The tunability of 40Ba0.5Sr0.5TiO3-xMgO-(60-x)Mg2TiO4 composites at 10kHz.

2.2. BaZrxTi1-xO3 based composites

BaZrxTi1-xO3 can form ferroelectric-dielectric composite with MgO (Maiti et al. 2007b, 2007c, 2008). High tunability and low loss tangent of the BaZrxTi1-xO3: MgO composites are

Figure 11.

The XRD patterns of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composites ceramics sintered at 1350°C for 3h. (a) x=48wt%, (b) x=36wt%, (c) x=30wt%, (d) x=24wt%, (e) x=12wt%.

Figure 12.

FESEM images of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composites ceramics sintered at 1350°C for 3h. From (a) to (e), x=48 wt%, 36 wt%, 30 wt%, 24 wt% and 12 wt%, respectively.

reported, but the sintering temperature is as high as 1500°C. We prepared BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite ceramics at 1350°C (He et al. 2011). The formation of ferroelectric (BaZr0.2Ti0.8O3)-dielectric (Mg2SiO4-MgO) composite was proved by XRD patterns (Fig. 11). Similar to Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composites, three kind of grains: BaZr0.2Ti0.8O3, Mg2SiO4 and MgO, can be identified (Fig. 12 and Fig. 13).

Figure 13.

FESEM image and element mapping of 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO as determined by energy dispersive spectroscopy (EDS).

Figure 14.

Dielectric constant (solid) and loss tangent (open) of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composites ceramics sintered at various temperature (measure frequency: 1MHz).

Fig. 14 shows the dielectric constant and loss tangent of BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite ceramics at 1MHz. With the increase of sintering temperature from 1300°C to 1350°C, the dielectric constant of the composites increase and the loss tangent decrease.

Figure 15.

Variation of dielectric constant (solid) and loss tangent (open) with temperature for 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO ceramics (sintering temperature: 1350°C) measured at 100kHz.

Figure 16.

The tunability of 40BaZr0.2Ti0.8O3-(60-x)Mg2SiO4-xMgO composite ceramics at 100kHz at room temperature (sintering temperature: 1350°C).

Increasing Mg2SiO4 content tends to decrease the dielectric constant of composites. The dielectric constant and loss tangent of composite sintered at 1350°C is ~125-183 and ~0.0010-0.0016, respectively, which is smaller than that of BaZr0.2Ti0.8O3 (Maiti et al. 2007b).

The temperature dependence of dielectric properties for BaZr0.2Ti0.8O3-Mg2SiO4-MgO composites (sintering temperature: 1350°C) measured at 100kHz is illustrated in Fig. 15. Compared with pure BaZr0.2Ti0.8O3 bulk ceramic (Maiti et al. 2007b), broadened and suppressed dielectric peaks and shifts of Curie temperature TC are observed with the addition of Mg2SiO4 and MgO. The results are similar to that of Ba0.6Sr0.4TiO3-Mg2SiO4-MgO. For 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO ceramics, its max decreases to ~ 215.5 and Tc shifts to lower temperature ~246K. For 40BaZr0.2Ti0.8O3-48Mg2SiO4-12MgO, max is ~157.7 at ~240K.

Fig. 16. shows the tunability of the BaZr0.2Ti0.8O3-Mg2SiO4-MgO composites at 100kHz at room temperature. The tunability of 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO under 2kV/mm is 15.6%. With the increase of Mg2SiO4 content, the tunability of 40BaZr0.2Ti0.8O3-30Mg2SiO4-30MgO decreases slightly to 14.2%. Further increasing Mg2SiO4 content results in an anomalous increase of tunability: 40BaZr0.2Ti0.8O3-48Mg2SiO4-12MgO composite has lower dielectric constant than 40BaZr0.2Ti0.8O3-12Mg2SiO4-48MgO but slightly higher tunability (17.9%).

3. Ferroelectric-dielectric solid solution

Forming ferroelectric-dielectric solid solution is another method to reduce material dielectric constant and loss tangent. Some non-ferroelectric complex oxides with perovskite structures have relatively low dielectric constant and low loss tangent. It is expected that they can be combined with barium strontium titanate to reduce material dielectric constant and loss tangent. Furthermore, it is possible for them to form solid solutions with barium strontium titanate because they have the same perovskite structure as barium strontium titanate. Single phase material is favorable for the thin film deposition. On the other hand, some perovskite oxide has positive temperature coefficient of dielectric constant and it can decrease the temperature coefficient of dielectric constant of barium strontium titanate above Curie temperature.

3.1. Ba0.6Sr0.4TiO3-Sr(Ga0.5Ti0.5)O3 solid solution

Sr(Ga0.5Ta0.5)O3 has a comparatively small dielectric constant (27 at 1MHz), a positive temperature coefficient of dielectric constant (120ppmK-1) and a low dielectric loss (Q=8600 at 10.6 GHz) (Takahashi et al. 1997). The lattice constant (a=0.3949nm) of cubic perovskite structure Sr(Ga0.5Ta0.5)O3 is very close to that of Ba0.6Sr0.4TiO3 (a=0.3965nm). Therefore, Sr(Ga0.5Ta0.5)O3 will be possible to form solid solution with Ba0.6Sr0.4TiO3 and reduce the dielectric constant of Ba0.6Sr0.4TiO3. The XRD results (Fig. 17.) prove that solid solution can be formed between Ba0.6Sr0.4TiO3 and Sr(Ga0.5Ta0.5)O3 under the preparative conditions (Xu et al. 2008).

Fig.18 shows the FESEM images of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600°C for 3h. The effect of Sr(Ga0.5Ta0.5)O3 content on the average grain size in not very obvious. We can also see that 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 has higher porosity than other compositions. The morphology of 0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3 shows difference from that of other three compositions.

The temperature dependence of dielectric properties for various Ba0.6Sr0.4TiO3-

Figure 17.

The XRD patterns of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600°C for 3h. From bottom to top, the Sr(Ga0.5Ta0.5)O3 content is 10, 20, 30 and 50mol%, respectively. The intensity is plotted on a log scale.

Figure 18.

FESEM images of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600°C for 3h. From (a) to (d), the Sr(Ga0.5Ta0.5)O3 content is 10, 20, 30 and 50mol%, respectively.

Sr(Ga0.5Ta0.5)O3 ceramics (sintering temperature: 1600°C) measured at 100kHz is illustrated in Fig. 19. Broadened and suppressed dielectric peaks and shifts of Curie temperature TC are observed with the addition of Sr(Ga0.5Ta0.5)O3. For 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 ceramics, its max decreases to ~ 686 and Tc shifts to lower temperature ~250K. As more Sr(Ga0.5Ta0.5)O3 is added to Ba0.6Sr0.4TiO3, Tc shifts to lower temperatures, thus resulting in a decrease in dielectric constant at a given temperature and max. For 0.8Ba0.6Sr0.4TiO3-0.2Sr(Ga0.5Ta0.5)O3, max is ~335 at ~200K and for 0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3, max is ~95 at ~100K. On the other hand, loss tangent increases on cooling. For 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 ceramics, there is small peak around ~250K. The loss tangent of 0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3 ceramics (not shown) is almost independent on temperature and fluctuates around 0.004 at the temperature range of 60K-300K.

Figure 19.

Variation of dielectric constant (solid) and loss tangent (open) with temperature for Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 ceramics (sintering temperature: 1600°C) measured at 100kHz: From (a) to (c), the Sr(Ga0.5Ta0.5)O3 content is 10, 20, and 50 mol%, respectively.

Figure 20.

The tunability of 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 and 0.7Ba0.6Sr0.4TiO3-0.3Sr(Ga0.5Ta0.5)O3 at 100 kHz (sintering temperature: 1600°C).

Fig. 20 shows the tunability of the Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 solid solutions at 100kHz, showing that the tunability decreases as the dielectric Sr(Ga0.5Ta0.5)O3 content increases. The decrease in the dielectric constant and tunability of 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 results from the Ga and Ta substitution into B-site Ti and Sr substitution into A-site Ba in barium strontium titanate. 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3 has a dielectric tunability 16% under 2.63kV/mm versus a dielectric constant =534. The tunability of 0.7 Ba0.6Sr0.4TiO3-0.3Sr(Ga0.5Ta0.5)O3 drops to be 5.7% under 2.63 kV/mm.

The microwave dielectric properties of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 solid solutions were listed in Table 4. With the increase of Sr(Ga0.5Ta0.5)O3 content, the dielectric constant decrease and the Qf value increase. The Qf value of the solid solution is not high except 0.5Ba0.6Sr0.4TiO3-0.5Sr(Ga0.5Ta0.5)O3. The low relative density maybe is the main reason: the relative density of 0.9Ba0.6Sr0.4TiO3-0.1Sr(Ga0.5Ta0.5)O3, 0.8Ba0.6Sr0.4TiO3-0.2Sr(Ga0.5Ta0.5)O3 and 0.7Ba0.6Sr0.4TiO3-0.3Sr(Ga0.5Ta0.5)O3 ceramics sintered at 1600°C for 3h is 82%, 89% and 88%, respectively.

Sr(Ga0.5Ta0.5)O3 content(mol%)f0(GHz)tanQf(GHz)
101.73592.40.015115
202.05375.30.013158
302.49236.90.012208
504.6879.60.00391200

Table 4.

Microwave dielectric properties of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 solid solutions

3.2. Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 solid solution

La(Mg0.5Ti0.5)O3 with low dielectric constant and loss tangent can form solid solutions with BaTiO3 or SrTiO3 in the whole compositional range (Avdeev 2002; Lee 2000). As shown in Fig. 21., La(Mg0.5Ti0.5)O3 form solid solution with Ba0.6Sr0.4TiO3 (Xu et al. 2009).

Figure 21.

The XRD patterns of (a) 10, (b) 20, (c) 30, (d) 40, (e) 50 and (f) 60 mol% La(Mg0.5Ti0.5)O3 mixed Ba0.6Sr0.4TiO3 ceramics.

The microwave dielectric properties of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 ceramics were investigated. For different composition, the optimal sintering temperature is different. If the sintering temperature exceeds the corresponding value, the sample’s rim and then interior became dark-blue in color, due to partial reduction of Ti4+ (d0) to Ti3+ (d1) associated with the oxygen loss from the lattice. Fig. 22 show the dielectric constant and Qf of Ba0.6Sr0.4TiO3–La(Mg0.5Ti0.5)O3 ceramics sintered at optimal temperature. decreases with the increase of La(Mg0.5Ti0.5)O3 content, from r=338.2 for 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3 to r=47 for 0.4Ba0.6Sr0.4TiO3-0.6La(Mg0.5Ti0.5)O3. Qf value increases with increasing amounts of La(Mg0.5Ti0.5)O3. High Qf value of 9509 GHz with dielectric constant of 46.7 was obtained for 0.4Ba0.6Sr0.4TiO3-0.6La(Mg0.5Ti0.5)O3 at 5.69 GHz.

Figure 22.

Dielectric constant and quality factor of Ba0.6Sr0.4TiO3–La(Mg0.5Ti0.5)O3 compositions as a function of La(Mg0.5Ti0.5)O3 content.

Figure 23.

The tunability of Ba0.6Sr0.4TiO3–La(Mg0.5Ti0.5)O3 compositions measured at 100kHz and room temperature.

The tunability of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 ceramics is shown in Fig. 23. La(Mg0.5Ti0.5)O3 decreases the tunability of Ba0.6Sr0.4TiO3 abruptly. The tunability of 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3 is only 3.7% under 1.67 kV/mm, although its Qf reaches 979GHz. Increasing La(Mg0.5Ti0.5)O3 content decreases the tunability further: the tunability of 0.8Ba0.6Sr0.4TiO3-0.2La(Mg0.5Ti0.5)O3 is 0.5% under 2.08 kV/mm.

The typical FESEM images of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 ceramics sintered at optimal temperature and the energy dispersive spectroscopy of 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3 were shown in Fig. 24. For 0.4Ba0.6Sr0.4TiO3-0.6La(Mg0.5Ti0.5)O3, dense ceramics were obtained, but higher porosity can be observed for the other three compositions. The high Qf value of 0.4Ba0.6Sr0.4TiO3-0.6La(Mg0.5Ti0.5)O3 can be related to its higher relative density. The chemical composition calculated from energy dispersive spectroscopy were listed in Table 5. We can see that the measured At% is consistent with the theoretical value within the error range. The result also proves the formation of solid solution further.

Figure 24.

FESEM images of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 ceramics and the energy dispersive spectroscopy of 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3 (f). From (a) to (d), La(Mg0.5Ti0.5)O3 content is 10, 20, 30 and 60 mol%, respectively.

ElementWt%At%Theoretical At%
OK21.1556.9960.61
MgK00.530.951.01
SrL18.659.187.27
BaL28.769.0310.91
TiK24.1921.7719.19
LaL06.712.082.02

Table 5.

The chemical composition of 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3

3.3. Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 and Ba0.6Sr0.4TiO3-Nd(Mg0.5Ti0.5)O3 solid solution

La(Zn0.5Ti0.5)O3 have a comparatively small dielectric constant (ε=34 at 10GHz), a negative temperature coefficient of the resonance frequency (τf=-52ppmK-1) and a low dielectric loss

Figure 25.

The XRD patterns of (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50 mol% La(Zn0.5Ti0.5)O3 mixed Ba0.6Sr0.4TiO3 ceramics.

Figure 26.

The XRD patterns of (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50 mol% Nd(Mg0.5Ti0.5)O3 mixed Ba0.6Sr0.4TiO3 ceramics.

(Qf=59000GHz) (Cho et al. 1997). For Nd(Mg0.5Ti0.5)O3, the corresponding value is 26, -49 ppmK-1 and 36900GHz, respectively (Cho et al. 1999). XRD analysis showed that they can form solid solution with Ba0.6Sr0.4TiO3 (Fig. 25 and 26), but their microwave dielectric properties is inferior to that of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3.

Figure 27.

Dielectric constant and quality factor of Ba0.6Sr0.4TiO3–La(Zn0.5Ti0.5)O3 compositions as a function of La(Zn0.5Ti0.5)O3 content.

Fig. 27 show the dielectric constant and Qf of Ba0.6Sr0.4TiO3–La(Zn0.5Ti0.5)O3 ceramics. The dielectric constant of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 solid solution decrease as the La(Zn0.5Ti0.5)O3 content increases. The Qf values of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 increase monotonously with increasing La(Zn0.5Ti0.5)O3 content. The highest Qf value of 5674 GHz was achieved in 0.5Ba0.6Sr0.4TiO3-0.5La(Zn0.5Ti0.5)O3 but reduced to 377GHz for 0.9Ba0.6Sr0.4TiO3-0.1La(Zn0.5Ti0.5)O3. The effect of La(Zn0.5Ti0.5)O3 on the microwave dielectric properties of Ba0.6Sr0.4TiO3 solid solution is similar to that of La(Mg0.5Ti0.5)O3. The Qf value of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 is lower obviously than that of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 system although their relative density is higher than that of the corresponding Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3.

Nd(Mg0.5Ti0.5)O3 content (mol%)sintering temperature (°C)f0(GHz)Qf(GHz)
2015002.68198.3535
2015502.83193.0615
3015004.0593.0880
3015504.3094.71137

Table 6.

Microwave dielectric properties of Ba0.6Sr0.4TiO3-Nd(Mg0.5Ti0.5)O3 solid solutions

Table 6 lists the microwave dielectric properties of some Ba0.6Sr0.4TiO3-Nd(Mg0.5Ti0.5)O3 ceramics. Although increasing Nd(Mg0.5Ti0.5)O3 content can increase the Qf value, the Qf value is not ideal: they are even lower than that of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 system.

The tunability of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 ceramics is shown in Fig. 28. The tunability of 0.9Ba0.6Sr0.4TiO3-0.1La(Zn0.5Ti0.5)O3 is only 2.7% under 1.67 kV/mm. It is even smaller than that of 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3 although 0.9Ba0.6Sr0.4TiO3-0.1La(Zn0.5Ti0.5)O3 has higher dielectric constant and loss tangent than that of 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3. Similarly, increasing La(Zn0.5Ti0.5)O3 content decreases the tunability of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3 further. We can see that the dielectric properties of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 is better than that of Ba0.6Sr0.4TiO3-La(Zn0.5Ti0.5)O3.

Figure 28.

The tunability of Ba0.6Sr0.4TiO3–La(Zn0.5Ti0.5)O3 compositions measured at 100kHz and room temperature.

4. Discussion

Forming ferroelectric-dielectric solid solution and composite both can reduce the dielectric constant of ferroelectrics efficiently, but has different effect on the dielectric properties of ferroelectrics. (1). Forming solid solution can decrease the dielectric constant of ferroelectrics more rapidly when the doping content is nearly the same. The dielectric constant of 0.5Ba0.6Sr0.4TiO3-0.5La(Mg0.5Ti0.5)O3 is 55, which is far lower than that of 60 wt% MgO-mixed Ba0.6Sr0.4TiO3 (=118) (Chang & Sengupta 2002; Sengupta & Sengupta 1999) although the doping content in 60 wt% MgO-mixed Ba0.6Sr0.4TiO3 is higher and MgO has lower dielectric constant than La(Mg0.5Ti0.5)O3. (2). Forming solid solution can improve the loss tangent of ferroelectrics more effectively. The Qf value of 0.5Ba0.6Sr0.4TiO3-0.5La(Mg0.5Ti0.5)O3 and 60 wt% MgO-mixed Ba0.6Sr0.4TiO3 is 9367GHz and 750GHz (Chang & Sengupta 2002; Sengupta & Sengupta 1999), respectively. Even for loose 0.9Ba0.6Sr0.4TiO3-0.1La(Mg0.5Ti0.5)O3 ceramics, its Qf value (979GHz) is much higher than that of Ba0.6Sr0.4TiO3-Mg2TiO4-MgO. In the preparation process of microwave dielectric ceramics, the formation of secondary phase should be prevented. (3). Forming multiphase composite can maintain sufficiently high tunability. 0.5Ba0.6Sr0.4TiO3-0.5La(Mg0.5Ti0.5)O3 has lost tunability completely, but the tunability of 60 wt% MgO-mixed Ba0.6Sr0.4TiO3 at 2kV/mm and 8kV/mm is 10% and 38%, respectively (Chang & Sengupta 2002; Sengupta & Sengupta 1999).

Some authors addressed the dielectric response of ferroelectric-dielectric composites theoretically and various composite models were used to evaluate the dielectric constant, tunability, and loss tangent (Astafiev 2003; Sherman et al. 2006; Tagantsev et al. 2006). As Tagantsev stated (Tagantsev et al. 2006), mixing a tunable ferroelectric with a linear dielectric may modify the electrical properties of the material due to mainly two effects: (i) “doping effect”,–effect of doping of the ferroelectric lattice via the substitution of the ions of the host material and (ii) “composite effect”–effects of redistribution of the electric field in the material due to the precipitation of the non-ferroelectric phase at the grain boundaries or in the bulk of the material. The first effect results primarily in a shift and smearing of the temperature anomaly of the permittivity. The second effect leads to a redistribution of the electric field in the material. For ferroelectric-dielectric solid solutions, “composite effect“ can be excluded. We can deduce that the addition of low loss perovskite dielectric influenced the chemistry and microstructure of the material, which resulted in the change of dielectric properties of materials. In ferroelectric-dielectric solid solution, a high degree of structural disorder due to random cation arrangement in both A- and B-sites is present, addition of pervoskite dielectrics apparently destroys the ferroelectric state, leading to the sharp decrease of tunability. For ferroelectric-dielectric composites, “doping effect“ can be ignored. The effect of the dilution-driven field redistribution in the material is the main manifestation of addition of the dielectric into ferroelectrics in two-phase or multiphase composite. The reduction of the volume of ferroelectric, which is responsible for tuning, causes suppression of the tunability of the material. In ferroelectric-dielectric composites, ferroelectrics host lattice remains unchanged and the decrease of tunability is mainly due to ferroelectric dilution. “Destruction” in solid solution and “dilution” in composite has different effect on the tunability. Therefore, forming ferroelectric-dielectric solid solution can cause the decrease of tunability more sharply.

In ferroelectric-dielectric composite, the big contrast in the values of dielectric constants of linear dielectrics and the ferroelectric affects the redistribution of the electric field around the dielectrics. The dielectric constant of the ferroelectric under applied electric field becomes in-homogeneously distributed over the volume of the ferroelectric. The overall tunability of the composite, thus changes. Two competitive phenomena affect the tunable properties of the ferroelectric when it is diluted with a dielectric (Sherman et al. 2006). First, the reduction of the volume of ferroelectric, which is responsible for tuning, will cause suppression of the tunability of the material. Second, the redistribution of the electric field surrounding the linear dielectrics will affect the local tuning of the ferroelectric. Depending on the shape of the linear dielectrics and on the dielectric constants of the components, the impact of each of these two effects on the composite tunability is different and the second effect may be stronger (Sherman et al. 2006). In ferroelectric-dielectric composite BaZr0.2Ti0.8O3-Mg2SiO4-MgO, with the increase of Mg2SiO4 content and the decrease of MgO content, the volume of ferroelectric BaZr0.2Ti0.8O3 decrease due to smaller density of Mg2SiO4 than that of MgO, the tunability of composite will be suppressed. It is the fact as MgO content decreases from 48 wt% to 30 wt%. The anomalous increased tunability in BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite with MgO content < 30wt% can be attributed to redistribution of the electric field. Mg2SiO4 and MgO have different dielectric constants, they will have different effects on the redistribution of the electric field. The combination of linear dielectrics with different dielectric constants can result in the change of dielectric constant and loss tangent and even increase the tunabilty by affecting the redistribution of the electric field in the composite. As MgO content decreases from 30 wt% to 12 wt%, the increase of the tunability due to redistribution of the electric field exceeds the decrease of the tunability due to ferroelectric dilution, so the tunability of composite ceramics increases anomalously. The almost unchanged tunability in Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composite can also be explained. No similar result was observed in BST-Mg2TiO4-MgO composite: with the increase of MgO content and the decrease of Mg2TiO4 content, the dielectric constant and tunability decrease monotonously. The tunability of 40Ba0.5Sr0.5TiO3-48Mg2TiO4-12MgO ceramics at 2kV/mm is 16.6%, but the corresponding tunability of 40Ba0.5Sr0.5TiO3-12Mg2TiO4-48MgO is only 5.7%.

We can also see that ternary compositions ferroelectric-dielectric composite shows some advantages over binary compositions. (1). We can decrease the dielectric constant of ternary composites and remain the tunability almost unchanged (in Ba0.6Sr0.4TiO3-Mg2SiO4-MgO), even increase the tunability (in BaZr0.2Ti0.8O3-Mg2SiO4-MgO), without increasing the content of linear dielectrics. In order to decrease the dielectric constant of Ba0.6Sr0.4TiO3-MgO, it is necessary to increase the content of linear dielectrics MgO, and the tunability will decrease inevitably. The tunability of Ba0.6Sr0.4TiO3-MgO decreases with the increase of MgO content (Chang & Sengupta 2002). For Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composite, their dielectric constant decrease to 85-97, the tunability at 2kV/mm can be kept at around 10%. (2). The sintering temperature of ternary compositions BaZr0.2Ti0.8O3-Mg2SiO4-MgO and Ba0.6Sr0.4TiO3-Mg2SiO4-MgO can be reduced to 1350°C, which is 100°C and 150°C lower than the normal sintering temperature of BST-MgO (~1450°C) and BZT-MgO (~1500°C), respectively. The sintering temperature of BST-Mg2TiO4-MgO composite is also lower than that of BST-MgO.

On the other hand, ternary composition is helpful for the formation of ferroelectric-dielectric composite and can prevent from the formation of undesired phase. Forming ferroelectric-dielectric composite is an effective method to reduce the dielectric constant and loss tangent of ferroelectric and maintain higher tunability. The key is that the linear dielectrics with low dielectric constant and loss tangent should not react with ferroelectrics. For binary composition Ba0.6Sr0.4TiO3-Mg2SiO4, it is expected to form ferroelectric (Ba0.6Sr0.4TiO3)-dielectric (Mg2SiO4) composite, but undesired impurity phase Ba2(TiO)(Si2O7) is formed among Ba0.6Sr0.4TiO3-Mg2SiO4 composite. Ba2(TiO)(Si2O7) deteriorate the properties of composites. MgO and Mg2SiO4 combination can prevent from the formation of Ba2(TiO)(Si2O7) and ferroelectric (Ba0.6Sr0.4TiO3) and dielectric (Mg2SiO4 and MgO) composite is obtained. Similarly, Mg2TiO4 can react with Ba1-xSrxTiO3 (x=0.5 and 0.6) to form BaMg6Ti6O19, but no BaMg6Ti6O19 formed in BST-Mg2TiO4-MgO composite. Therefore, maybe ternary compositions can open a new route to decrease the dielectric constant and loss tangent of ferroelectrics and remain higher tunability. In future work, it is necessary to search new combination of linear dielectrics. Even if one linear dielectric may react with ferroelectrics, some linear dielectrics combination is possible to form ferroelectric-dielectric composite with ferroelectrics. This will expand the select range of linear dielectrics.

The multiple-phase ferroelectric-dielectric composites are useful for tunable microwave applications requiring low dielectric constant and make the impedance match more easily. The ferroelectric-dielectric composite bulk ceramics show promising application, especially in accelerator, 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 (Kanareykin et al. 2006, 2009a, 2009b). The ferroelectric bulk ceramics can also be used in ferroelectric lens (Rao et al. 1999).

The ferroelectric-dielectric composites might complicate method to effectively deposit films. Therefore, the advantage of ferroelectric-dielectric solid solution over composite is that single phase materials is favorable for the thin film deposition. At the meantime, the tunability of solid solution can be increase to relatively high level by increasing applied electric field. Generally, linear dielectric with perovskite structure can form solid solution with ferroelectric BST. Different linear dielectrics has different effects on the dielectric properties. Among studied solid solution, Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 shows better properties. It is necessary to increase the density of the solid solution, meanwhile, prevent the reduction of Ti4+.

5. Conclusion

Forming ferroelectric-dielectric composite and solid solution can reduce the dielectric constant of ferroelectrics efficiently, but the mechanisms affecting dielectric properties differ in composites and solid solutions. Forming ferroelectric-dielectric solid solution can improve the loss tangent of ferroelectrics more effectively and is beneficial to film deposition. Forming ferroelectric-dielectric composite is more efficient to decrease the dielectric constant of ferroelectrics to a low value and maintain tunability at a sufficiently high level.

Acknowledgments

This work is supported by the Natural Science Foundation of China under grant number 10975055 and 60771021.

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Yebin Xu and Yanyan He (August 24th 2011). Ferroelectric-Dielectric Solid Solution and Composites for Tunable Microwave Application, Ferroelectrics - Material Aspects, Mickaël Lallart, IntechOpen, DOI: 10.5772/17744. Available from:

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