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Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders
Materials and Design 32 (2011) 1200–1204
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders Qing Xu a,⇑, Xiao-Fei Zhang a, Han-Xing Liu a, Wen Chen a, Min Chen b,1, Bok-Hee Kim b a b
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China Faculty of Advanced Materials Engineering, Chonbuk National University, Jeonju 561756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 June 2010 Accepted 19 October 2010 Available online 23 October 2010 Keywords: A. Composites–ceramic matrix C. Sintering E. Electrical
a b s t r a c t Composite ceramics of Ba0.6Sr0.4TiO3 + 60 wt.% MgO were prepared from fine constituent powders by sintering at 1200–1280 °C. The composite specimens sintered at the relatively low temperatures showed satisfactory densification due to fine morphology of the constituent powders. The elevation of sintering temperature promoted the incorporation of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase and grain growth of the two constituent phases. The dependence of the dielectric properties on sintering temperature was explained in relation to the structural evolution. Controlling the sintering temperature of the composite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was determined to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature attained a tunability of 17.3% and a figure of merit of 127 at 10 kHz and 20 kV/cm. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Barium strontium titanate (Ba1 xSrxTiO3, BST) has been considered to be a leading candidate material for tunable microwave dielectric devices by the virtue of strong dielectric nonlinearity under bias electric fields and linearly variable Curie temperature with the content of strontium [1,2]. It has been well recognized that a moderate dielectric constant, a low dielectric loss and a high tunability are preferred for the application. BST compositions have relatively large dielectric constants, resulting in an impedance matching difficulty for their application in the tunable microwave devices. Designing composite systems composed of BST and nonferroelectric constituents has been found to be efficient in overcoming the problem [1]. The basic principle of the composite design is to take advantage of each constituent to achieve the preferred nonlinear dielectric properties. Various nonferroelectric constituents, including MgO [1], Mg2SiO4 [3], Mg2TiO4 [4], Mg2AlO4 [5] and MgTiO3 [6], have been employed to dilute the dielectric constant of BST. From a viewpoint of materials design, magnesium is the crucial element of the nonferroelectric constituents. Thus, BST–MgO emerges as the typical system of the ferroelectric/nonferroelectric composites. In the past decade, BST–MgO composites have been the subject of extensive researches in view of the tun⇑ Corresponding author. Tel.: +86 27 87863277; fax: +86 27 87864580. E-mail address: [email protected] (Q. Xu). Present address: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2R3. 1
0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.10.018
able device application [7–11]. Meanwhile, the system has been adopted as the basis to develop new BST-based composites by doping various oxides [12–14]. Despite these previous works, there have been few researches regarding the influence of sintering temperature on the nonlinear dielectric properties of the composite system with respect to structural evolution. On the other hand, the majority of the prior researches were conducted based on BST–MgO composite ceramics prepared from conventional constituent powders, which were sintered at high temperatures (1350 °C). It has been expected that lowering the sintering temperature of BST–MgO composites would leave a larger space for their realization in the tunable microwave devices [10]. We believe that utilizing highly-reactive constituent powders is a viable approach to this aim. In this work, we prepare Ba0.6Sr0.4TiO3–MgO composite ceramics at relatively low sintering temperatures of 1200–1280 °C by using fine Ba0.6Sr0.4TiO3 and MgO powders. Moreover, the dependence of the dielectric properties on sintering temperature was investigated from the viewpoint of structural change. The purpose of the research is to specify contributing factors to the nonlinear dielectric properties of the composite in the context of low-temperature sintering and offer a clue to the design of new BST-based composites for the tunable device application. 2. Experimental Ba0.6Sr0.4TiO3 powder was synthesized by a citrate method. Reagent grade Ba(NiO3)2, Sr(NiO3)2, tetrabutyl titanate and citric acid
Q. Xu et al. / Materials and Design 32 (2011) 1200–1204
were used as starting materials. Tetrabutyl titanate was first dissolved into a citric acid solution and various nitrates were then added, followed by stirring to yield a transparent aqueous solution. The mole ratio of citric acid to the total metal cation content was 1.25. The precursor solution was heated to form a foam-like solid precursor. The foam precursor was pulverized and calcined at 650 °C for 1 h in air. X-ray diffraction (XRD) analysis certified the formation of a pure perovskite phase for the calcined powder. The citrate method synthesis and characterization of the powder have been described elsewhere [15]. Commercial MgO powder (99.9%, Nanjing High Technology Nano Material Co., Ltd.) was mixed with the Ba0.6Sr0.4TiO3 powder according to a nominal composition of Ba0.6Sr0.4TiO3 + 60 wt.% MgO. After through mixing, the mixture of the two constituent powders was uniaxially pressed into discs of 19 mm in diameter and 1 mm in thickness under a pressure of 300 MPa. The compacted specimens were sintered at 1200–1280 °C for 2 h in air. Ba0.6Sr0.4TiO3 ceramic specimens were also prepared from the citrate method derived powder by sintering at 1250 °C for 2 h in air. The morphology of the two constituent powders was observed at a Hitachi S-4700 field emission scanning electron microscope (FESEM). The specific surface areas of the two powders were measured by the Brunauer–Emmett–Teller (BET) method at a Micromeritics Gemini 2380 surface area analyzer using liquid nitrogen as the adsorbent. The crystal structure of the ceramic specimens was examined by a Philips X’pert PBO X-ray diffractometer using Cu Ka radiation. The microstructure of the ceramic specimens was observed using polished and thermally-etched surfaces at a Jeol JSM-5610LV scanning electron microscope (SEM) attached with an energy dispersive spectroscopy (EDS) analyzer. The bulk densities of the ceramic specimens were measured by the Archimedes method with ethyl alcohol as the medium. The theoretical densities of the composite specimens were calculated according to the mixing rule using the X-ray theoretical densities and volume fractions of the two constituents. The relative densities of the composite specimens were determined from the measured and calculated data. Silver paste was painted on both surfaces of the ceramic specimens as electrodes. The temperature dependence of dielectric constant (er) was measured by a TH2828 precision LCR meter at 10 kHz and a SCC-M10 environmental chamber between 50 and 120 °C. The polarization vs. electric-field (P–E) relation was measured at room temperature by a Radiant precision workstation based on a Sawyer–Tower circuit at 50 Hz. The nonlinear dielectric properties were measured at room temperature by a TH2818 automatic component analyzer at 10 kHz under external bias electric fields sweeping from zero to 20 kV/cm.
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Fig. 1. FESEM micrographs of (a) Ba0.6Sr0.4TiO3 and (b) MgO powders.
3. Results and discussion Fig. 1 shows the FESEM micrographs of the two constituent powders. It was observed that the powders were consisted of fine primary particles with mild agglomeration. The Ba0.6Sr0.4TiO3 powder was composed of superfine particles less than 100 nm, while those of the MgO powder were comparatively larger (100– 200 nm). The BET measurement indicated that the Ba0.6Sr0.4TiO3 and MgO powders had specific surface areas of 21.6 and 11.2 m2/ g, respectively. The average particle sizes of the two powders were ascertained to be 50 and 150 nm, respectively, based on the specific surface area data. The results of the FESEM and BET analyses reveal fine morphology of the two constitute powders. Fig. 2 shows the XRD patterns of the composite ceramics sintered at different temperatures. For comparison purposes, the XRD pattern of the Ba0.6Sr0.4TiO3 ceramic specimen was also shown in Fig. 2. Fig. 2a identified a diphase structure for the composite ceramics, composed of a cubic Ba0.6Sr0.4TiO3 phase and a cubic
Fig. 2. XRD patterns in the 2h ranges of (a) 20–80° and (b) 44–48° for the composite specimens sintered at different temperatures and Ba0.6Sr0.4TiO3 ceramic specimen sintered at 1250 °C.
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MgO phase. The result suggests that chemical reaction between the two constituent phases is likely to be insignificant during the sintering. Fig. 2b shows the XRD patterns of the specimens in the 2h range of 44–48°, corresponding to the reflection of the (2 0 0) crystallographic plane of the cubic Ba0.6Sr0.4TiO3. Compared with the Ba0.6Sr0.4TiO3 specimen, the peaks of the composite specimens moved to higher diffraction angles. This phenomenon is believed to be caused by the diffusion of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase. The effective radii of six-coordinated Mg2+ and Ti4+ are 0.72 and 0.61Å, respectively [16]. Therefore, small amounts of Mg2+ can substitute for Ti4+ of the Ba0.6Sr0.4TiO3 phase because of radius matching, leading to an enlargement of the unit cells of the perovskite phase. The heterovalent substitution was electrically compensated by forming oxygen vacancies, resulting in a contraction of the unit cells. The peak shift of the Ba0.6Sr0.4TiO3 phase of the composite specimens relative to the Ba0.6Sr0.4TiO3 specimen appears to be dependent on the twofold effect of the Mg2+ doping on the perovskite structure. The present result suggests that the oxygen vacancy effect is prevailing, which is responsible for the peak shift to high diffraction angles. As for the composite specimens, the (2 0 0) peak progressively shifted towards higher diffraction angle directions with increasing sintering temperature. This behavior infers that elevating sintering temperature promoted the diffusion of Mg2+ into the Ba0.6Sr0.4TiO3 phase. The composite specimens sintered 1200–1280 °C attained similar relative densities of around 96%. It has been reported that BST– MgO composite ceramics prepared from conventional constituent powders necessitated sintering temperatures of 1350–1550 °C to gain reasonable densification [3,9,14]. The comparison indicates an improved sinterability for the composite ceramics of the present work, which is believed to be ascribable to high reactivity of the two constituent powders due to their fine morphology. Fig. 3 shows the SEM micrographs of the composite specimens sintered at different temperatures. The composite specimens displayed dense microstructures with two sorts of grains distinct in contrast. The light and dark grains were assigned to the Ba0.6Sr0.4-
TiO3 and MgO phases, respectively, by the EDS analysis. This assignation agrees well with previous results of BST–MgO composite ceramics [7–11]. In general, elevating sintering temperature improved the grain growth of the two constituent phases. The specimen sintered at 1200 °C showed clusters of quite fine Ba0.6Sr0.4TiO3 grains. By comparison, the Ba0.6Sr0.4TiO3 grains of the specimen sintered at 1230 °C became obviously larger and rectangularlyshaped (Fig. 3b). The remarkable grain growth occurred within a small sintering temperature interval is considered to be associated with the nano-sized morphology of the Ba0.6Sr0.4TiO3 starting powder. Afterwards, the grain size of the Ba0.6Sr0.4TiO3 phase gradually enhanced with further increasing sintering temperature. For the specimens sintered at different temperatures, the grain sizes of the two constituent phases were smaller than their counterparts in BST–MgO ceramics prepared from conventional starting powders at sintering temperatures P 1350 °C [12–14]. The reduced grain sizes of the specimens in the present work can be related to the fine morphology of the starting powders and the lower sintering temperatures. Over the whole sintering temperature range, each constituent phase maintained a generally good percolation. The fine morphology of the two constituent powders is considered to be a scenario to interpret the microstructural feature, which favors a uniform dispersion of the starting powders and a good grain connection for each constituent phase in the ceramic specimens. Fig. 4 shows the temperature dependence of the dielectric constant (er) at 10 kHz for the composite specimens sintered at different temperatures. The dielectric data of the Ba0.6Sr0.4TiO3 specimen were also shown in Fig. 4 for comparison purposes. The Ba0.6Sr0.4TiO3 specimen showed a slightly diffuse peak of dielectric constant at around 0 °C (Fig. 4a). This dielectric behavior is well consistent with a prior result of Ba0.6Sr0.4TiO3 ceramic prepared by the conventional solid-state reaction method [17]. As well-known, the dielectric anomaly can be ascribed to a ferroelectric–paraelectric phase transition. The composite specimens presented an analogous dielectric behavior (Fig. 4b). It was noticed that the dielectric constant peaks of the composite specimens became depressed and
Fig. 3. SEM micrographs of the composite specimens sintered at (a) 1200 °C, (b) 1230 °C, (c) 1250 °C and (d) 1280 °C.
Q. Xu et al. / Materials and Design 32 (2011) 1200–1204
Fig. 4. Temperature dependence of dielectric constant (er) at 10 kHz for (a) Ba0.6Sr0.4TiO3 ceramic specimen sintered at 1250 °C and (b) the composite specimens sintered at different temperatures.
broadened compared to the Ba0.6Sr0.4TiO3 specimen. This change is attributed to the diluting effect of the nonferroelectric MgO phase on the ferroelectricity of the Ba0.6Sr0.4TiO3 phase [9]. Meanwhile, the temperatures of the dielectric constant peaks (Tm) of the composite specimens moved to lower temperatures. The Tm decrease means that the ferroelectric–paraelectric phase transition of the Ba0.6Sr0.4TiO3 phase in the composite specimens occurred at lowered temperatures. This phenomenon can be explained with the substitution of Mg2+ for Ti4+ of the Ba0.6Sr0.4TiO3 phase. The Mg2+ substitution and the resulting oxygen vacancies broke long-range coherent interactions of the ferroelectrically active [TiO6] octahedra, decreasing the temperature stability of ferroelectric state of the Ba0.6Sr0.4TiO3 phase [18]. For the composite specimens, the Tm tended to decrease with increasing sintering temperature. This behavior reflects that the elevation of sintering temperature increased the amount of Mg2+ dissolving into the Ba0.6Sr0.4TiO3 phase, coinciding well with the result of the XRD analysis. Fig. 5 shows the P–E relation measured at room temperature for the composite specimens sintered at different temperatures. The
Fig. 5. P–E hysteresis loops measured at room temperature for the composite specimens sintered at different temperatures.
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composite specimens exhibited slim but discernable hysteresis loops, which are believed to arise from the Ba0.6Sr0.4TiO3 phase on account of the nonferroelectric nature of the MgO constituent. Moreover, the hysteresis loops infers an existence of polar microregions embedded in the macroscopically paraelectric background of the cubic Ba0.6Sr0.4TiO3 phase at room temperature. The hysteresis loops turned to be obscure with increasing sintering temperature, showing a decrease of the remanent polarization (Pr). This evolution is regarded to be associated with the Tm decrease with sintering temperature, which reduced the volume fraction of the polar micro-regions in the Ba0.6Sr0.4TiO3 phase at room temperature. Fig. 6 shows the dielectric constant as a function of bias electric field for the composite specimens sintered at different temperatures. The composite specimens displayed a typical feature of nonlinear dielectrics, with the dielectric constants steadily declining with higher bias electric fields. Considering the nature of the MgO constituent as a linear dielectric, the dielectric nonlinearity is attributed to the Ba0.6Sr0.4TiO3 phase of the composite specimens. In this sense, the good connection among the grains of the Ba0.6Sr0.4TiO3 phase (Fig. 3) is believed to be greatly contributive to the dielectric nonlinearity. As before-mentioned, the fine morphology of the starting powders is regarded to be responsible for the good connection of the Ba0.6Sr0.4TiO3 grains. Thus, one can suggest an effect of the morphology of the starting powders on the dielectric nonlinearity of the composite ceramics. The tunability was calculated as the percentage of dielectric constant change under 20 kV/cm. The figure of merit (FOM), defined as tunability/dielectric loss (tan d), was determined from the tunability and dielectric loss measured at zero bias electric field. Fig. 7 shows the nonlinear dielectric properties of the composite specimens sintered at different temperatures. The dielectric constants generally increased with sintering temperature. This trend can be explained by the grain growth of the Ba0.6Sr0.4TiO3 phase with sintering temperature (Fig. 3), which is known as the grain size effect [19]. By comparison, the dielectric losses were somewhat insensitive to sintering temperature, fluctuating within 0.13–0.16% over the sintering temperature range. This insensitivity is presumed to be due to the very close densification degrees of the composite specimens sintered at the temperatures. The tunabilities of the composite specimens varied between 13.9% and 17.3%, with the specimen sintered at 1230 °C attaining the maximum value. This result can be understood in light of two inverse effects. On one hand, the tunability of the specimens is highly dependent on the size of the Ba0.6Sr0.4TiO3 grains. The growth of the Ba0.6Sr0.4TiO3 grains with increasing sintering temperature (Fig. 3) benefits the enhancement of the tunability due
Fig. 6. Dielectric constant (er) as a function of bias electric field for the composite specimens sintered at different temperatures.
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the composite. The increase of sintering temperature promoted the diffusion of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase and grain growth of the two constituent phases. The variation of the dielectric properties with sintering temperature can be explained in relation to the structural evolution. This research demonstrates that controlling the sintering temperature of the composite is important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was ascertained to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature showed a reasonably good tunability of 17.3% and a comparatively large figure of merit of 127 at 10 kHz and 20 kV/cm. The results of the present work may serve to be a clue to the design and preparation of new composite systems composed of BST and magnesium-containing nonferroelectric constituents for the tunable device application. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 51072146, 50932004 and A3 Foresight Program-50821140308) and the Ministry of Education (No. 108092). Fig. 7. Nonlinear dielectric properties of the composite specimens as a function of sintering temperature.
to the grain size effect. On the other hand, the tunability of the specimens is related with the Tm values. The Tm decrease of the specimens with elevating sintering temperature (Fig. 4) is unfavorable to the tunability at room temperature. The variation of the tunability with sintering temperature can be viewed as a result of the competition of the two effects. The specimen sintered at 1230 °C showed remarkably grown Ba0.6Sr0.4TiO3 grains compared with the specimen sintered at 1200 °C (Fig. 3a and b), which has been supposed to be related to the nano-sized morphology of the starting powder. Meanwhile, the specimens sintered at the two temperatures, respectively, exhibited an identical Tm value of 10 °C (Fig. 4b). In this case, the maximum tunability of the specimen sintered at 1230 °C appears to be plausible. The variation trend of the FOM is basically same to that of the tunability. This sameness is logical on account of the almost unvaried dielectric losses with sintering temperature. The subtle changes of the tunability and dielectric loss led to an appreciable variation of the FOM between 87 and 127. This result indicates a significant influence of sintering temperature on the parameter. As a compromise between the tunability and dielectric loss, the FOM has been proposed to be a criterion to assess the overall nonlinear dielectric properties [17]. In terms of the FOM criterion, sintering at 1230 °C is believed to be preferred for the Ba0.6Sr0.4TiO3–MgO composite, with the specimen sintered at the temperature reaching the largest FOM value of 127 among the investigated specimens. This FOM value rivals literature results measured under the identical conditions for composite ceramics composed of BST and MgO or magnesium-containing complex oxides [8,20]. The relatively low sintering temperature and comparatively large FOM of the specimen demonstrate the advantage of preparing Ba0.6Sr0.4TiO3–MgO composite from fine constituent powders. 4. Conclusions Ba0.6Sr0.4TiO3 + 60 wt.% MgO composite ceramics have been prepared from fine constituent powders at sintering temperatures of 1200–1280 °C. Employing fine constituent powders was confirmed to be effective in lowering the sintering temperature of
References [1] Sengupta LC, Sengupta S. Breakthrough advances in low loss, tunable dielectric materials. Mater Res Innovations 1999;2:278–82. [2] Tagantsev AK, Sherman VO, Astafiev KF, Venkatesh V, Setter N. Ferroelectric materials for microwave tunable applications. J Electroceram 2003;11:5–66. [3] Chen Y, Dong XL, Liang RH, Li JT, Wang YL. Dielectric properties of Ba0.6Sr0.4TiO3/Mg2SiO4/MgO composite ceramics. J Appl Phys 2005;98:0641071–75. [4] Chou XJ, Zhai JW, Yao X. Dielectric tunable properties of Ba0.5Sr0.5TiO3– Mg2TiO4 low dielectric constant microwave composite ceramics. Appl Phys Lett 2007;91:1229081–83. [5] Zhang JJ, Zhai JW, Chou XJ, Yao X. Dielectric abnormalities in composite ceramics. J Am Ceram Soc 2008;91:3258–62. [6] Ke SM, Fan HQ, Wang W, Jiao GC, Huang HT, Chan HLW. MgTiO3 doping effect on dielectric properties of Ba0.6Sr0.4TiO3 ceramics via, a molten salt process. Compos Part A-Appl S 2008;39:597–601. [7] Chang W, Sengupta L. MgO-mixed Ba0.6Sr0.4TiO3 bulk ceramics and thin films for tunable microwave applications. J Appl Phys 2002;92:3941–6. [8] Agrawal S, Guo R, Agrawal D, Bhalla AS. Tunable BST:MgO dielectric composite by microwave sintering. Ferroelectrics 2004;306:155–63. [9] Su B, Button TW. Microstructure and dielectric properties of Mg-doped barium strontium titanate ceramics. J Appl Phys 2004;95:1382–5. [10] Hu T, Jantunen H, Deleniv A, Leppavuori S, Gevorgian S. Electric-fieldcontrolled permittivity ferroelectric composition for microwave LTCC modules. J Am Ceram Soc 2004;87:578–83. [11] Chung UC, Elissalde C, Maglione M, Estournes C, Pate M, Ganne JP. Low-losses, highly tunable Ba0.6Sr0.4TiO3/MgO composite. Appl Phys Lett 2008;92:0429021–23. [12] Liang RH, Dong XL, Chen Y, Cao F, Wang YL. Effect of ZrO2 doping on the tunable and dielectric properties of Ba0.55Sr0.45TiO3/MgO composites for microwave tunable application. Mater Res Bull 2006;41:1295–302. [13] Liang RH, Dong XL, Chen Y, Cao F, Wang YL. Effect of La2O3 doping on the tunable and dielectric properties of BST/MgO composite for microwave tunable application. Mater Chem Phys 2006;95:222–8. [14] Wang XH, Lu WZ, Liu J, Zhou YL, Zhou DX. Effects of La2O3 additions on properties of Ba0.6Sr0.4TiO3–MgO ceramics for phase shifter applications. J Eur Ceram Soc 2006;26:1981–5. [15] Xu Q, Zhang XF, Huang YH, Chen W, Liu HX, Chen M, et al. Sinterability and nonlinear dielectric properties of derived Ba0.6Sr0.4TiO3 from a citrate method. J Alloy Compd 2009;485:L16–20. [16] Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A 1976;32:751–7. [17] Feteira A, Sinclair DC, Reaney IM, Somiya Y, Lanagan MT. BaTiO3based ceramics for tunable microwave applications. J Am Ceram Soc 2004;87:1082–7. [18] Dai X, DiGiovanni A, Viehland D. Dielectric properties of tetragonal lanthanum modified lead zirconate titanate ceramics. J Appl Phys 1993;74:3399–405. [19] Tang XG, Wang J, Wang XX, Chan HLW. Effects of grain size on the dielectric properties and tunabilities of sol–gel derived Ba(Zr0.2Ti0.8)O3 ceramics. Solid State Commun 2004;131:163–8. [20] Liu P, Ma JJ, Meng J, Li J, Ding LF, Wang JL, et al. Preparation and dielectric properties of BST–Mg2TiO4 composite ceramics. Mater Chem Phys 2009;114:624–8.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders Qing Xu a,⇑, Xiao-Fei Zhang a, Han-Xing Liu a, Wen Chen a, Min Chen b,1, Bok-Hee Kim b a b
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China Faculty of Advanced Materials Engineering, Chonbuk National University, Jeonju 561756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 6 June 2010 Accepted 19 October 2010 Available online 23 October 2010 Keywords: A. Composites–ceramic matrix C. Sintering E. Electrical
a b s t r a c t Composite ceramics of Ba0.6Sr0.4TiO3 + 60 wt.% MgO were prepared from fine constituent powders by sintering at 1200–1280 °C. The composite specimens sintered at the relatively low temperatures showed satisfactory densification due to fine morphology of the constituent powders. The elevation of sintering temperature promoted the incorporation of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase and grain growth of the two constituent phases. The dependence of the dielectric properties on sintering temperature was explained in relation to the structural evolution. Controlling the sintering temperature of the composite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was determined to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature attained a tunability of 17.3% and a figure of merit of 127 at 10 kHz and 20 kV/cm. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Barium strontium titanate (Ba1 xSrxTiO3, BST) has been considered to be a leading candidate material for tunable microwave dielectric devices by the virtue of strong dielectric nonlinearity under bias electric fields and linearly variable Curie temperature with the content of strontium [1,2]. It has been well recognized that a moderate dielectric constant, a low dielectric loss and a high tunability are preferred for the application. BST compositions have relatively large dielectric constants, resulting in an impedance matching difficulty for their application in the tunable microwave devices. Designing composite systems composed of BST and nonferroelectric constituents has been found to be efficient in overcoming the problem [1]. The basic principle of the composite design is to take advantage of each constituent to achieve the preferred nonlinear dielectric properties. Various nonferroelectric constituents, including MgO [1], Mg2SiO4 [3], Mg2TiO4 [4], Mg2AlO4 [5] and MgTiO3 [6], have been employed to dilute the dielectric constant of BST. From a viewpoint of materials design, magnesium is the crucial element of the nonferroelectric constituents. Thus, BST–MgO emerges as the typical system of the ferroelectric/nonferroelectric composites. In the past decade, BST–MgO composites have been the subject of extensive researches in view of the tun⇑ Corresponding author. Tel.: +86 27 87863277; fax: +86 27 87864580. E-mail address: [email protected] (Q. Xu). Present address: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2R3. 1
0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.10.018
able device application [7–11]. Meanwhile, the system has been adopted as the basis to develop new BST-based composites by doping various oxides [12–14]. Despite these previous works, there have been few researches regarding the influence of sintering temperature on the nonlinear dielectric properties of the composite system with respect to structural evolution. On the other hand, the majority of the prior researches were conducted based on BST–MgO composite ceramics prepared from conventional constituent powders, which were sintered at high temperatures (1350 °C). It has been expected that lowering the sintering temperature of BST–MgO composites would leave a larger space for their realization in the tunable microwave devices [10]. We believe that utilizing highly-reactive constituent powders is a viable approach to this aim. In this work, we prepare Ba0.6Sr0.4TiO3–MgO composite ceramics at relatively low sintering temperatures of 1200–1280 °C by using fine Ba0.6Sr0.4TiO3 and MgO powders. Moreover, the dependence of the dielectric properties on sintering temperature was investigated from the viewpoint of structural change. The purpose of the research is to specify contributing factors to the nonlinear dielectric properties of the composite in the context of low-temperature sintering and offer a clue to the design of new BST-based composites for the tunable device application. 2. Experimental Ba0.6Sr0.4TiO3 powder was synthesized by a citrate method. Reagent grade Ba(NiO3)2, Sr(NiO3)2, tetrabutyl titanate and citric acid
Q. Xu et al. / Materials and Design 32 (2011) 1200–1204
were used as starting materials. Tetrabutyl titanate was first dissolved into a citric acid solution and various nitrates were then added, followed by stirring to yield a transparent aqueous solution. The mole ratio of citric acid to the total metal cation content was 1.25. The precursor solution was heated to form a foam-like solid precursor. The foam precursor was pulverized and calcined at 650 °C for 1 h in air. X-ray diffraction (XRD) analysis certified the formation of a pure perovskite phase for the calcined powder. The citrate method synthesis and characterization of the powder have been described elsewhere [15]. Commercial MgO powder (99.9%, Nanjing High Technology Nano Material Co., Ltd.) was mixed with the Ba0.6Sr0.4TiO3 powder according to a nominal composition of Ba0.6Sr0.4TiO3 + 60 wt.% MgO. After through mixing, the mixture of the two constituent powders was uniaxially pressed into discs of 19 mm in diameter and 1 mm in thickness under a pressure of 300 MPa. The compacted specimens were sintered at 1200–1280 °C for 2 h in air. Ba0.6Sr0.4TiO3 ceramic specimens were also prepared from the citrate method derived powder by sintering at 1250 °C for 2 h in air. The morphology of the two constituent powders was observed at a Hitachi S-4700 field emission scanning electron microscope (FESEM). The specific surface areas of the two powders were measured by the Brunauer–Emmett–Teller (BET) method at a Micromeritics Gemini 2380 surface area analyzer using liquid nitrogen as the adsorbent. The crystal structure of the ceramic specimens was examined by a Philips X’pert PBO X-ray diffractometer using Cu Ka radiation. The microstructure of the ceramic specimens was observed using polished and thermally-etched surfaces at a Jeol JSM-5610LV scanning electron microscope (SEM) attached with an energy dispersive spectroscopy (EDS) analyzer. The bulk densities of the ceramic specimens were measured by the Archimedes method with ethyl alcohol as the medium. The theoretical densities of the composite specimens were calculated according to the mixing rule using the X-ray theoretical densities and volume fractions of the two constituents. The relative densities of the composite specimens were determined from the measured and calculated data. Silver paste was painted on both surfaces of the ceramic specimens as electrodes. The temperature dependence of dielectric constant (er) was measured by a TH2828 precision LCR meter at 10 kHz and a SCC-M10 environmental chamber between 50 and 120 °C. The polarization vs. electric-field (P–E) relation was measured at room temperature by a Radiant precision workstation based on a Sawyer–Tower circuit at 50 Hz. The nonlinear dielectric properties were measured at room temperature by a TH2818 automatic component analyzer at 10 kHz under external bias electric fields sweeping from zero to 20 kV/cm.
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Fig. 1. FESEM micrographs of (a) Ba0.6Sr0.4TiO3 and (b) MgO powders.
3. Results and discussion Fig. 1 shows the FESEM micrographs of the two constituent powders. It was observed that the powders were consisted of fine primary particles with mild agglomeration. The Ba0.6Sr0.4TiO3 powder was composed of superfine particles less than 100 nm, while those of the MgO powder were comparatively larger (100– 200 nm). The BET measurement indicated that the Ba0.6Sr0.4TiO3 and MgO powders had specific surface areas of 21.6 and 11.2 m2/ g, respectively. The average particle sizes of the two powders were ascertained to be 50 and 150 nm, respectively, based on the specific surface area data. The results of the FESEM and BET analyses reveal fine morphology of the two constitute powders. Fig. 2 shows the XRD patterns of the composite ceramics sintered at different temperatures. For comparison purposes, the XRD pattern of the Ba0.6Sr0.4TiO3 ceramic specimen was also shown in Fig. 2. Fig. 2a identified a diphase structure for the composite ceramics, composed of a cubic Ba0.6Sr0.4TiO3 phase and a cubic
Fig. 2. XRD patterns in the 2h ranges of (a) 20–80° and (b) 44–48° for the composite specimens sintered at different temperatures and Ba0.6Sr0.4TiO3 ceramic specimen sintered at 1250 °C.
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MgO phase. The result suggests that chemical reaction between the two constituent phases is likely to be insignificant during the sintering. Fig. 2b shows the XRD patterns of the specimens in the 2h range of 44–48°, corresponding to the reflection of the (2 0 0) crystallographic plane of the cubic Ba0.6Sr0.4TiO3. Compared with the Ba0.6Sr0.4TiO3 specimen, the peaks of the composite specimens moved to higher diffraction angles. This phenomenon is believed to be caused by the diffusion of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase. The effective radii of six-coordinated Mg2+ and Ti4+ are 0.72 and 0.61Å, respectively [16]. Therefore, small amounts of Mg2+ can substitute for Ti4+ of the Ba0.6Sr0.4TiO3 phase because of radius matching, leading to an enlargement of the unit cells of the perovskite phase. The heterovalent substitution was electrically compensated by forming oxygen vacancies, resulting in a contraction of the unit cells. The peak shift of the Ba0.6Sr0.4TiO3 phase of the composite specimens relative to the Ba0.6Sr0.4TiO3 specimen appears to be dependent on the twofold effect of the Mg2+ doping on the perovskite structure. The present result suggests that the oxygen vacancy effect is prevailing, which is responsible for the peak shift to high diffraction angles. As for the composite specimens, the (2 0 0) peak progressively shifted towards higher diffraction angle directions with increasing sintering temperature. This behavior infers that elevating sintering temperature promoted the diffusion of Mg2+ into the Ba0.6Sr0.4TiO3 phase. The composite specimens sintered 1200–1280 °C attained similar relative densities of around 96%. It has been reported that BST– MgO composite ceramics prepared from conventional constituent powders necessitated sintering temperatures of 1350–1550 °C to gain reasonable densification [3,9,14]. The comparison indicates an improved sinterability for the composite ceramics of the present work, which is believed to be ascribable to high reactivity of the two constituent powders due to their fine morphology. Fig. 3 shows the SEM micrographs of the composite specimens sintered at different temperatures. The composite specimens displayed dense microstructures with two sorts of grains distinct in contrast. The light and dark grains were assigned to the Ba0.6Sr0.4-
TiO3 and MgO phases, respectively, by the EDS analysis. This assignation agrees well with previous results of BST–MgO composite ceramics [7–11]. In general, elevating sintering temperature improved the grain growth of the two constituent phases. The specimen sintered at 1200 °C showed clusters of quite fine Ba0.6Sr0.4TiO3 grains. By comparison, the Ba0.6Sr0.4TiO3 grains of the specimen sintered at 1230 °C became obviously larger and rectangularlyshaped (Fig. 3b). The remarkable grain growth occurred within a small sintering temperature interval is considered to be associated with the nano-sized morphology of the Ba0.6Sr0.4TiO3 starting powder. Afterwards, the grain size of the Ba0.6Sr0.4TiO3 phase gradually enhanced with further increasing sintering temperature. For the specimens sintered at different temperatures, the grain sizes of the two constituent phases were smaller than their counterparts in BST–MgO ceramics prepared from conventional starting powders at sintering temperatures P 1350 °C [12–14]. The reduced grain sizes of the specimens in the present work can be related to the fine morphology of the starting powders and the lower sintering temperatures. Over the whole sintering temperature range, each constituent phase maintained a generally good percolation. The fine morphology of the two constituent powders is considered to be a scenario to interpret the microstructural feature, which favors a uniform dispersion of the starting powders and a good grain connection for each constituent phase in the ceramic specimens. Fig. 4 shows the temperature dependence of the dielectric constant (er) at 10 kHz for the composite specimens sintered at different temperatures. The dielectric data of the Ba0.6Sr0.4TiO3 specimen were also shown in Fig. 4 for comparison purposes. The Ba0.6Sr0.4TiO3 specimen showed a slightly diffuse peak of dielectric constant at around 0 °C (Fig. 4a). This dielectric behavior is well consistent with a prior result of Ba0.6Sr0.4TiO3 ceramic prepared by the conventional solid-state reaction method [17]. As well-known, the dielectric anomaly can be ascribed to a ferroelectric–paraelectric phase transition. The composite specimens presented an analogous dielectric behavior (Fig. 4b). It was noticed that the dielectric constant peaks of the composite specimens became depressed and
Fig. 3. SEM micrographs of the composite specimens sintered at (a) 1200 °C, (b) 1230 °C, (c) 1250 °C and (d) 1280 °C.
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Fig. 4. Temperature dependence of dielectric constant (er) at 10 kHz for (a) Ba0.6Sr0.4TiO3 ceramic specimen sintered at 1250 °C and (b) the composite specimens sintered at different temperatures.
broadened compared to the Ba0.6Sr0.4TiO3 specimen. This change is attributed to the diluting effect of the nonferroelectric MgO phase on the ferroelectricity of the Ba0.6Sr0.4TiO3 phase [9]. Meanwhile, the temperatures of the dielectric constant peaks (Tm) of the composite specimens moved to lower temperatures. The Tm decrease means that the ferroelectric–paraelectric phase transition of the Ba0.6Sr0.4TiO3 phase in the composite specimens occurred at lowered temperatures. This phenomenon can be explained with the substitution of Mg2+ for Ti4+ of the Ba0.6Sr0.4TiO3 phase. The Mg2+ substitution and the resulting oxygen vacancies broke long-range coherent interactions of the ferroelectrically active [TiO6] octahedra, decreasing the temperature stability of ferroelectric state of the Ba0.6Sr0.4TiO3 phase [18]. For the composite specimens, the Tm tended to decrease with increasing sintering temperature. This behavior reflects that the elevation of sintering temperature increased the amount of Mg2+ dissolving into the Ba0.6Sr0.4TiO3 phase, coinciding well with the result of the XRD analysis. Fig. 5 shows the P–E relation measured at room temperature for the composite specimens sintered at different temperatures. The
Fig. 5. P–E hysteresis loops measured at room temperature for the composite specimens sintered at different temperatures.
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composite specimens exhibited slim but discernable hysteresis loops, which are believed to arise from the Ba0.6Sr0.4TiO3 phase on account of the nonferroelectric nature of the MgO constituent. Moreover, the hysteresis loops infers an existence of polar microregions embedded in the macroscopically paraelectric background of the cubic Ba0.6Sr0.4TiO3 phase at room temperature. The hysteresis loops turned to be obscure with increasing sintering temperature, showing a decrease of the remanent polarization (Pr). This evolution is regarded to be associated with the Tm decrease with sintering temperature, which reduced the volume fraction of the polar micro-regions in the Ba0.6Sr0.4TiO3 phase at room temperature. Fig. 6 shows the dielectric constant as a function of bias electric field for the composite specimens sintered at different temperatures. The composite specimens displayed a typical feature of nonlinear dielectrics, with the dielectric constants steadily declining with higher bias electric fields. Considering the nature of the MgO constituent as a linear dielectric, the dielectric nonlinearity is attributed to the Ba0.6Sr0.4TiO3 phase of the composite specimens. In this sense, the good connection among the grains of the Ba0.6Sr0.4TiO3 phase (Fig. 3) is believed to be greatly contributive to the dielectric nonlinearity. As before-mentioned, the fine morphology of the starting powders is regarded to be responsible for the good connection of the Ba0.6Sr0.4TiO3 grains. Thus, one can suggest an effect of the morphology of the starting powders on the dielectric nonlinearity of the composite ceramics. The tunability was calculated as the percentage of dielectric constant change under 20 kV/cm. The figure of merit (FOM), defined as tunability/dielectric loss (tan d), was determined from the tunability and dielectric loss measured at zero bias electric field. Fig. 7 shows the nonlinear dielectric properties of the composite specimens sintered at different temperatures. The dielectric constants generally increased with sintering temperature. This trend can be explained by the grain growth of the Ba0.6Sr0.4TiO3 phase with sintering temperature (Fig. 3), which is known as the grain size effect [19]. By comparison, the dielectric losses were somewhat insensitive to sintering temperature, fluctuating within 0.13–0.16% over the sintering temperature range. This insensitivity is presumed to be due to the very close densification degrees of the composite specimens sintered at the temperatures. The tunabilities of the composite specimens varied between 13.9% and 17.3%, with the specimen sintered at 1230 °C attaining the maximum value. This result can be understood in light of two inverse effects. On one hand, the tunability of the specimens is highly dependent on the size of the Ba0.6Sr0.4TiO3 grains. The growth of the Ba0.6Sr0.4TiO3 grains with increasing sintering temperature (Fig. 3) benefits the enhancement of the tunability due
Fig. 6. Dielectric constant (er) as a function of bias electric field for the composite specimens sintered at different temperatures.
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the composite. The increase of sintering temperature promoted the diffusion of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase and grain growth of the two constituent phases. The variation of the dielectric properties with sintering temperature can be explained in relation to the structural evolution. This research demonstrates that controlling the sintering temperature of the composite is important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was ascertained to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature showed a reasonably good tunability of 17.3% and a comparatively large figure of merit of 127 at 10 kHz and 20 kV/cm. The results of the present work may serve to be a clue to the design and preparation of new composite systems composed of BST and magnesium-containing nonferroelectric constituents for the tunable device application. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 51072146, 50932004 and A3 Foresight Program-50821140308) and the Ministry of Education (No. 108092). Fig. 7. Nonlinear dielectric properties of the composite specimens as a function of sintering temperature.
to the grain size effect. On the other hand, the tunability of the specimens is related with the Tm values. The Tm decrease of the specimens with elevating sintering temperature (Fig. 4) is unfavorable to the tunability at room temperature. The variation of the tunability with sintering temperature can be viewed as a result of the competition of the two effects. The specimen sintered at 1230 °C showed remarkably grown Ba0.6Sr0.4TiO3 grains compared with the specimen sintered at 1200 °C (Fig. 3a and b), which has been supposed to be related to the nano-sized morphology of the starting powder. Meanwhile, the specimens sintered at the two temperatures, respectively, exhibited an identical Tm value of 10 °C (Fig. 4b). In this case, the maximum tunability of the specimen sintered at 1230 °C appears to be plausible. The variation trend of the FOM is basically same to that of the tunability. This sameness is logical on account of the almost unvaried dielectric losses with sintering temperature. The subtle changes of the tunability and dielectric loss led to an appreciable variation of the FOM between 87 and 127. This result indicates a significant influence of sintering temperature on the parameter. As a compromise between the tunability and dielectric loss, the FOM has been proposed to be a criterion to assess the overall nonlinear dielectric properties [17]. In terms of the FOM criterion, sintering at 1230 °C is believed to be preferred for the Ba0.6Sr0.4TiO3–MgO composite, with the specimen sintered at the temperature reaching the largest FOM value of 127 among the investigated specimens. This FOM value rivals literature results measured under the identical conditions for composite ceramics composed of BST and MgO or magnesium-containing complex oxides [8,20]. The relatively low sintering temperature and comparatively large FOM of the specimen demonstrate the advantage of preparing Ba0.6Sr0.4TiO3–MgO composite from fine constituent powders. 4. Conclusions Ba0.6Sr0.4TiO3 + 60 wt.% MgO composite ceramics have been prepared from fine constituent powders at sintering temperatures of 1200–1280 °C. Employing fine constituent powders was confirmed to be effective in lowering the sintering temperature of
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