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–MgOcomposite 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 School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of Chinab 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 2010Accepted 19 October 2010Available online 23 October 2010

Keywords:A. Composites–ceramic matrixC. SinteringE. Electrical

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.10.018

⇑ Corresponding author. Tel.: +86 27 87863277; faxE-mail address: xuqing@whut.edu.cn (Q. Xu).

1 Present address: Department of Chemical and Matof Alberta, Edmonton, AB, Canada T6G 2R3.

a b s t r a c t

Composite ceramics of Ba0.6Sr0.4TiO3 + 60 wt.% MgO were prepared from fine constituent powders by sin-tering at 1200–1280 �C. The composite specimens sintered at the relatively low temperatures showedsatisfactory densification due to fine morphology of the constituent powders. The elevation of sinteringtemperature promoted the incorporation of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phase and graingrowth of the two constituent phases. The dependence of the dielectric properties on sintering temper-ature was explained in relation to the structural evolution. Controlling the sintering temperature of thecomposite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at1230 �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 at10 kHz and 20 kV/cm.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Barium strontium titanate (Ba1�xSrxTiO3, BST) has been consid-ered to be a leading candidate material for tunable microwavedielectric devices by the virtue of strong dielectric nonlinearity un-der bias electric fields and linearly variable Curie temperature withthe content of strontium [1,2]. It has been well recognized that amoderate dielectric constant, a low dielectric loss and a high tun-ability are preferred for the application. BST compositions have rel-atively large dielectric constants, resulting in an impedancematching difficulty for their application in the tunable microwavedevices. Designing composite systems composed of BST and non-ferroelectric constituents has been found to be efficient in over-coming the problem [1]. The basic principle of the compositedesign is to take advantage of each constituent to achieve the pre-ferred nonlinear dielectric properties. Various nonferroelectricconstituents, including MgO [1], Mg2SiO4 [3], Mg2TiO4 [4], Mg2AlO4

[5] and MgTiO3 [6], have been employed to dilute the dielectricconstant of BST. From a viewpoint of materials design, magnesiumis the crucial element of the nonferroelectric constituents. Thus,BST–MgO emerges as the typical system of the ferroelectric/non-ferroelectric composites. In the past decade, BST–MgO compositeshave been the subject of extensive researches in view of the tun-

ll rights reserved.

: +86 27 87864580.

erials Engineering, University

able device application [7–11]. Meanwhile, the system has beenadopted as the basis to develop new BST-based composites by dop-ing various oxides [12–14]. Despite these previous works, therehave been few researches regarding the influence of sintering tem-perature on the nonlinear dielectric properties of the compositesystem with respect to structural evolution. On the other hand,the majority of the prior researches were conducted based onBST–MgO composite ceramics prepared from conventional constit-uent powders, which were sintered at high temperatures(�1350 �C). It has been expected that lowering the sintering tem-perature of BST–MgO composites would leave a larger space fortheir realization in the tunable microwave devices [10]. We believethat utilizing highly-reactive constituent powders is a viable ap-proach to this aim.

In this work, we prepare Ba0.6Sr0.4TiO3–MgO composite ceram-ics at relatively low sintering temperatures of 1200–1280 �C byusing fine Ba0.6Sr0.4TiO3 and MgO powders. Moreover, the depen-dence of the dielectric properties on sintering temperature wasinvestigated from the viewpoint of structural change. The purposeof the research is to specify contributing factors to the nonlineardielectric properties of the composite in the context of low-tem-perature sintering and offer a clue to the design of new BST-basedcomposites for the tunable device application.

2. Experimental

Ba0.6Sr0.4TiO3 powder was synthesized by a citrate method. Re-agent grade Ba(NiO3)2, Sr(NiO3)2, tetrabutyl titanate and citric acid

Fig. 1. FESEM micrographs of (a) Ba0.6Sr0.4TiO3 and (b) MgO powders.

Q. Xu et al. / Materials and Design 32 (2011) 1200–1204 1201

were used as starting materials. Tetrabutyl titanate was first dis-solved into a citric acid solution and various nitrates were thenadded, followed by stirring to yield a transparent aqueous solution.The mole ratio of citric acid to the total metal cation content was1.25. The precursor solution was heated to form a foam-like solidprecursor. The foam precursor was pulverized and calcined at650 �C for 1 h in air. X-ray diffraction (XRD) analysis certified theformation of a pure perovskite phase for the calcined powder.The citrate method synthesis and characterization of the powderhave been described elsewhere [15]. Commercial MgO powder(99.9%, Nanjing High Technology Nano Material Co., Ltd.) wasmixed with the Ba0.6Sr0.4TiO3 powder according to a nominal com-position of Ba0.6Sr0.4TiO3 + 60 wt.% MgO. After through mixing, themixture of the two constituent powders was uniaxially pressedinto discs of 19 mm in diameter and 1 mm in thickness under apressure of 300 MPa. The compacted specimens were sintered at1200–1280 �C for 2 h in air. Ba0.6Sr0.4TiO3 ceramic specimens werealso prepared from the citrate method derived powder by sinteringat 1250 �C for 2 h in air.

The morphology of the two constituent powders was observedat a Hitachi S-4700 field emission scanning electron microscope(FESEM). The specific surface areas of the two powders were mea-sured by the Brunauer–Emmett–Teller (BET) method at a Microm-eritics Gemini 2380 surface area analyzer using liquid nitrogen asthe adsorbent. The crystal structure of the ceramic specimenswas examined by a Philips X’pert PBO X-ray diffractometer usingCu Ka radiation. The microstructure of the ceramic specimenswas observed using polished and thermally-etched surfaces at aJeol JSM-5610LV scanning electron microscope (SEM) attachedwith an energy dispersive spectroscopy (EDS) analyzer. The bulkdensities of the ceramic specimens were measured by the Archi-medes method with ethyl alcohol as the medium. The theoreticaldensities of the composite specimens were calculated accordingto the mixing rule using the X-ray theoretical densities and volumefractions of the two constituents. The relative densities of the com-posite specimens were determined from the measured and calcu-lated data. Silver paste was painted on both surfaces of theceramic specimens as electrodes. The temperature dependence ofdielectric constant (er) was measured by a TH2828 precision LCRmeter at 10 kHz and a SCC-M10 environmental chamber between�50 and 120 �C. The polarization vs. electric-field (P–E) relationwas measured at room temperature by a Radiant precision work-station based on a Sawyer–Tower circuit at 50 Hz. The nonlineardielectric properties were measured at room temperature by aTH2818 automatic component analyzer at 10 kHz under externalbias electric fields sweeping from zero to 20 kV/cm.

Fig. 2. XRD patterns in the 2h ranges of (a) 20–80� and (b) 44–48� for the compositespecimens sintered at different temperatures and Ba0.6Sr0.4TiO3 ceramic specimensintered at 1250 �C.

3. Results and discussion

Fig. 1 shows the FESEM micrographs of the two constituentpowders. It was observed that the powders were consisted of fineprimary particles with mild agglomeration. The Ba0.6Sr0.4TiO3 pow-der was composed of superfine particles less than 100 nm, whilethose 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 wereascertained to be 50 and 150 nm, respectively, based on the spe-cific surface area data. The results of the FESEM and BET analysesreveal fine morphology of the two constitute powders.

Fig. 2 shows the XRD patterns of the composite ceramics sin-tered at different temperatures. For comparison purposes, theXRD pattern of the Ba0.6Sr0.4TiO3 ceramic specimen was also shownin Fig. 2. Fig. 2a identified a diphase structure for the compositeceramics, composed of a cubic Ba0.6Sr0.4TiO3 phase and a cubic

1202 Q. Xu et al. / Materials and Design 32 (2011) 1200–1204

MgO phase. The result suggests that chemical reaction between thetwo constituent phases is likely to be insignificant during the sin-tering. Fig. 2b shows the XRD patterns of the specimens in the 2hrange of 44–48�, corresponding to the reflection of the (2 0 0) crys-tallographic plane of the cubic Ba0.6Sr0.4TiO3. Compared with theBa0.6Sr0.4TiO3 specimen, the peaks of the composite specimensmoved to higher diffraction angles. This phenomenon is believedto be caused by the diffusion of Mg2+ into the lattice of theBa0.6Sr0.4TiO3 phase. The effective radii of six-coordinated Mg2+

and Ti4+ are 0.72 and 0.61Å, respectively [16]. Therefore, smallamounts of Mg2+ can substitute for Ti4+ of the Ba0.6Sr0.4TiO3 phasebecause of radius matching, leading to an enlargement of the unitcells of the perovskite phase. The heterovalent substitution waselectrically compensated by forming oxygen vacancies, resultingin 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 theMg2+ doping on the perovskite structure. The present result sug-gests that the oxygen vacancy effect is prevailing, which is respon-sible for the peak shift to high diffraction angles. As for thecomposite specimens, the (2 0 0) peak progressively shifted to-wards higher diffraction angle directions with increasing sinteringtemperature. This behavior infers that elevating sintering temper-ature promoted the diffusion of Mg2+ into the Ba0.6Sr0.4TiO3 phase.

The composite specimens sintered 1200–1280 �C attained sim-ilar relative densities of around 96%. It has been reported that BST–MgO composite ceramics prepared from conventional constituentpowders necessitated sintering temperatures of 1350–1550 �C togain reasonable densification [3,9,14]. The comparison indicatesan improved sinterability for the composite ceramics of the presentwork, which is believed to be ascribable to high reactivity of thetwo constituent powders due to their fine morphology.

Fig. 3 shows the SEM micrographs of the composite specimenssintered at different temperatures. The composite specimens dis-played dense microstructures with two sorts of grains distinct incontrast. The light and dark grains were assigned to the Ba0.6Sr0.4-

Fig. 3. SEM micrographs of the composite specimens sintered

TiO3 and MgO phases, respectively, by the EDS analysis. This assig-nation agrees well with previous results of BST–MgO compositeceramics [7–11]. In general, elevating sintering temperature im-proved the grain growth of the two constituent phases. The speci-men sintered at 1200 �C showed clusters of quite fine Ba0.6Sr0.4TiO3

grains. By comparison, the Ba0.6Sr0.4TiO3 grains of the specimensintered at 1230 �C became obviously larger and rectangularly-shaped (Fig. 3b). The remarkable grain growth occurred within asmall sintering temperature interval is considered to be associatedwith the nano-sized morphology of the Ba0.6Sr0.4TiO3 starting pow-der. Afterwards, the grain size of the Ba0.6Sr0.4TiO3 phase graduallyenhanced with further increasing sintering temperature. For thespecimens sintered at different temperatures, the grain sizes ofthe two constituent phases were smaller than their counterpartsin BST–MgO ceramics prepared from conventional starting pow-ders at sintering temperatures P 1350 �C [12–14]. The reducedgrain sizes of the specimens in the present work can be relatedto the fine morphology of the starting powders and the lower sin-tering temperatures. Over the whole sintering temperature range,each constituent phase maintained a generally good percolation.The fine morphology of the two constituent powders is consideredto be a scenario to interpret the microstructural feature, which fa-vors a uniform dispersion of the starting powders and a good grainconnection for each constituent phase in the ceramic specimens.

Fig. 4 shows the temperature dependence of the dielectric con-stant (er) at 10 kHz for the composite specimens sintered at differ-ent temperatures. The dielectric data of the Ba0.6Sr0.4TiO3 specimenwere also shown in Fig. 4 for comparison purposes. The Ba0.6Sr0.4-

TiO3 specimen showed a slightly diffuse peak of dielectric constantat around 0 �C (Fig. 4a). This dielectric behavior is well consistentwith a prior result of Ba0.6Sr0.4TiO3 ceramic prepared by the con-ventional solid-state reaction method [17]. As well-known, thedielectric anomaly can be ascribed to a ferroelectric–paraelectricphase transition. The composite specimens presented an analogousdielectric behavior (Fig. 4b). It was noticed that the dielectric con-stant peaks of the composite specimens became depressed and

at (a) 1200 �C, (b) 1230 �C, (c) 1250 �C and (d) 1280 �C.

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 compositespecimens sintered at different temperatures.

Q. Xu et al. / Materials and Design 32 (2011) 1200–1204 1203

broadened compared to the Ba0.6Sr0.4TiO3 specimen. This change isattributed to the diluting effect of the nonferroelectric MgO phaseon the ferroelectricity of the Ba0.6Sr0.4TiO3 phase [9]. Meanwhile,the temperatures of the dielectric constant peaks (Tm) of the com-posite specimens moved to lower temperatures. The Tm decreasemeans that the ferroelectric–paraelectric phase transition of theBa0.6Sr0.4TiO3 phase in the composite specimens occurred at low-ered temperatures. This phenomenon can be explained with thesubstitution of Mg2+ for Ti4+ of the Ba0.6Sr0.4TiO3 phase. The Mg2+

substitution and the resulting oxygen vacancies broke long-rangecoherent interactions of the ferroelectrically active [TiO6] octahe-dra, decreasing the temperature stability of ferroelectric state ofthe Ba0.6Sr0.4TiO3 phase [18]. For the composite specimens, theTm tended to decrease with increasing sintering temperature. Thisbehavior reflects that the elevation of sintering temperature in-creased 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 compositespecimens sintered at different temperatures.

composite specimens exhibited slim but discernable hysteresisloops, which are believed to arise from the Ba0.6Sr0.4TiO3 phaseon account of the nonferroelectric nature of the MgO constituent.Moreover, the hysteresis loops infers an existence of polar micro-regions embedded in the macroscopically paraelectric backgroundof the cubic Ba0.6Sr0.4TiO3 phase at room temperature. Thehysteresis loops turned to be obscure with increasing sinteringtemperature, 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 volumefraction of the polar micro-regions in the Ba0.6Sr0.4TiO3 phase atroom temperature.

Fig. 6 shows the dielectric constant as a function of bias electricfield for the composite specimens sintered at different tempera-tures. The composite specimens displayed a typical feature of non-linear dielectrics, with the dielectric constants steadily decliningwith higher bias electric fields. Considering the nature of theMgO constituent as a linear dielectric, the dielectric nonlinearityis attributed to the Ba0.6Sr0.4TiO3 phase of the composite speci-mens. In this sense, the good connection among the grains of theBa0.6Sr0.4TiO3 phase (Fig. 3) is believed to be greatly contributiveto the dielectric nonlinearity. As before-mentioned, the fine mor-phology of the starting powders is regarded to be responsible forthe good connection of the Ba0.6Sr0.4TiO3 grains. Thus, one can sug-gest an effect of the morphology of the starting powders on thedielectric nonlinearity of the composite ceramics.

The tunability was calculated as the percentage of dielectricconstant change under 20 kV/cm. The figure of merit (FOM), de-fined as tunability/dielectric loss (tan d), was determined fromthe tunability and dielectric loss measured at zero bias electricfield. Fig. 7 shows the nonlinear dielectric properties of the com-posite specimens sintered at different temperatures. The dielectricconstants generally increased with sintering temperature. Thistrend can be explained by the grain growth of the Ba0.6Sr0.4TiO3

phase with sintering temperature (Fig. 3), which is known as thegrain size effect [19]. By comparison, the dielectric losses weresomewhat insensitive to sintering temperature, fluctuating within0.13–0.16% over the sintering temperature range. This insensitivityis presumed to be due to the very close densification degrees of thecomposite specimens sintered at the temperatures.

The tunabilities of the composite specimens varied between13.9% and 17.3%, with the specimen sintered at 1230 �C attainingthe maximum value. This result can be understood in light oftwo inverse effects. On one hand, the tunability of the specimensis highly dependent on the size of the Ba0.6Sr0.4TiO3 grains. Thegrowth of the Ba0.6Sr0.4TiO3 grains with increasing sintering tem-perature (Fig. 3) benefits the enhancement of the tunability due

Fig. 6. Dielectric constant (er) as a function of bias electric field for the compositespecimens sintered at different temperatures.

Fig. 7. Nonlinear dielectric properties of the composite specimens as a function ofsintering temperature.

1204 Q. Xu et al. / Materials and Design 32 (2011) 1200–1204

to the grain size effect. On the other hand, the tunability of thespecimens is related with the Tm values. The Tm decrease of thespecimens with elevating sintering temperature (Fig. 4) is unfavor-able to the tunability at room temperature. The variation of thetunability with sintering temperature can be viewed as a resultof the competition of the two effects. The specimen sintered at1230 �C showed remarkably grown Ba0.6Sr0.4TiO3 grains comparedwith the specimen sintered at 1200 �C (Fig. 3a and b), which hasbeen supposed to be related to the nano-sized morphology of thestarting powder. Meanwhile, the specimens sintered at the twotemperatures, respectively, exhibited an identical Tm value of�10 �C (Fig. 4b). In this case, the maximum tunability of the spec-imen sintered at 1230 �C appears to be plausible.

The variation trend of the FOM is basically same to that of thetunability. This sameness is logical on account of the almost unvar-ied dielectric losses with sintering temperature. The subtle changesof the tunability and dielectric loss led to an appreciable variationof the FOM between 87 and 127. This result indicates a significantinfluence of sintering temperature on the parameter. As a compro-mise between the tunability and dielectric loss, the FOM has beenproposed to be a criterion to assess the overall nonlinear dielectricproperties [17]. In terms of the FOM criterion, sintering at 1230 �Cis believed to be preferred for the Ba0.6Sr0.4TiO3–MgO composite,with the specimen sintered at the temperature reaching the largestFOM value of 127 among the investigated specimens. This FOM va-lue rivals literature results measured under the identical condi-tions for composite ceramics composed of BST and MgO ormagnesium-containing complex oxides [8,20]. The relatively lowsintering temperature and comparatively large FOM of the speci-men demonstrate the advantage of preparing Ba0.6Sr0.4TiO3–MgOcomposite from fine constituent powders.

4. Conclusions

Ba0.6Sr0.4TiO3 + 60 wt.% MgO composite ceramics have beenprepared from fine constituent powders at sintering temperaturesof 1200–1280 �C. Employing fine constituent powders was con-firmed to be effective in lowering the sintering temperature of

the composite. The increase of sintering temperature promotedthe diffusion of Mg2+ into the lattice of the Ba0.6Sr0.4TiO3 phaseand grain growth of the two constituent phases. The variation ofthe dielectric properties with sintering temperature can be ex-plained in relation to the structural evolution. This research dem-onstrates that controlling the sintering temperature of thecomposite is important to achieve the desired nonlinear dielectricproperties. Sintering at 1230 �C was ascertained to be preferred forthe composite in terms of the nonlinear dielectric properties. Thespecimen sintered at the temperature showed a reasonably goodtunability of 17.3% and a comparatively large figure of merit of127 at 10 kHz and 20 kV/cm. The results of the present workmay serve to be a clue to the design and preparation of new com-posite systems composed of BST and magnesium-containing non-ferroelectric constituents for the tunable device application.

Acknowledgements

This work was supported by National Natural Science Founda-tion of China (Nos. 51072146, 50932004 and A3 Foresight Pro-gram-50821140308) and the Ministry of Education (No. 108092).

References

[1] Sengupta LC, Sengupta S. Breakthrough advances in low loss, tunable dielectricmaterials. Mater Res Innovations 1999;2:278–82.

[2] Tagantsev AK, Sherman VO, Astafiev KF, Venkatesh V, Setter N. Ferroelectricmaterials for microwave tunable applications. J Electroceram 2003;11:5–66.

[3] Chen Y, Dong XL, Liang RH, Li JT, Wang YL. Dielectric propertiesof Ba0.6Sr0.4TiO3/Mg2SiO4/MgO composite ceramics. J Appl Phys2005;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 PhysLett 2007;91:1229081–83.

[5] Zhang JJ, Zhai JW, Chou XJ, Yao X. Dielectric abnormalities in compositeceramics. J Am Ceram Soc 2008;91:3258–62.

[6] Ke SM, Fan HQ, Wang W, Jiao GC, Huang HT, Chan HLW. MgTiO3 doping effecton 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 filmsfor tunable microwave applications. J Appl Phys 2002;92:3941–6.

[8] Agrawal S, Guo R, Agrawal D, Bhalla AS. Tunable BST:MgO dielectric compositeby microwave sintering. Ferroelectrics 2004;306:155–63.

[9] Su B, Button TW. Microstructure and dielectric properties of Mg-doped bariumstrontium titanate ceramics. J Appl Phys 2004;95:1382–5.

[10] Hu T, Jantunen H, Deleniv A, Leppavuori S, Gevorgian S. Electric-field-controlled permittivity ferroelectric composition for microwave LTCCmodules. 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 Lett2008;92:0429021–23.

[12] Liang RH, Dong XL, Chen Y, Cao F, Wang YL. Effect of ZrO2 doping on thetunable and dielectric properties of Ba0.55Sr0.45TiO3/MgO composites formicrowave 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 thetunable and dielectric properties of BST/MgO composite for microwavetunable application. Mater Chem Phys 2006;95:222–8.

[14] Wang XH, Lu WZ, Liu J, Zhou YL, Zhou DX. Effects of La2O3 additions onproperties of Ba0.6Sr0.4TiO3–MgO ceramics for phase shifter applications. J EurCeram Soc 2006;26:1981–5.

[15] Xu Q, Zhang XF, Huang YH, Chen W, Liu HX, Chen M, et al. Sinterability andnonlinear 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 interatomicdistances in halides and chalcogenides. Acta Crystallogr A 1976;32:751–7.

[17] Feteira A, Sinclair DC, Reaney IM, Somiya Y, Lanagan MT. BaTiO3-based ceramics for tunable microwave applications. J Am Ceram Soc2004;87:1082–7.

[18] Dai X, DiGiovanni A, Viehland D. Dielectric properties of tetragonal lanthanummodified 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 dielectricproperties and tunabilities of sol–gel derived Ba(Zr0.2Ti0.8)O3 ceramics. SolidState Commun 2004;131:163–8.

[20] Liu P, Ma JJ, Meng J, Li J, Ding LF, Wang JL, et al. Preparation and dielectricproperties of BST–Mg2TiO4 composite ceramics. Mater Chem Phys2009;114:624–8.