Bismuth Titanate From Nanocomposite and Sol-gel Processes

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EUEVIER

October 1996

Materials Letters 28 (1996) 469-473

Bismuth titanate from nanocomposite and sol-gel processesA.V. Prasada Rao * , A.I. Robin, S. Komarneni

Intercollege Materials Research Laboratory, The Pennsylvania State Unioersiry, University Park, PA 16802. USAReceived 8 April 1996; accepted 14 April 1996

Abstract

Phase pure bismuth titanate was prepared at 750C from the corresponding hydroxide precursors. Sintering behavior ofthis powder was evaluated in comparison with powder obtained from a sol-gel process. A maximum of about 87% of thetheoretical density was achieved for sintered ceramics processed from powders prepared by the above two methods.Microstructural studies revealed large grains as well as grains less than 1 p,rn in size, the latter forming typical plateletscharacteristic of bismuth titanate.Keywords: Bismuth titanate; Bi4Ti30,2; Synthesis; Low temperature; Microstructure; Nanophase; Ferroelectric; Electrooptic

1. IntroductionBismuth titanate, Bi,Ti,O,,, since its discovery

in 1949 by Aurivillus [11, has been extensively stud-ied for its ferroelectric and related properties espe-cially its electrooptic switching behavior. The initialstudies were carried out on bulk forms synthesizedeither by solid-state reactions [2,3] requiring temper-atures around 1200C or from molten salts [4] withtemperatures around 800C. Most of the subsequentstudies were mainly centered on the fabrication ofthin films by different approaches and especiallysol-gel methods [5-91. At room temperatureBi,Ti,O,, exists in monoclinic symmetry with aCurie temperature of 675C and a spontaneous polar-ization as high as 50 pC/cm2. The crystal structureconsists of three perovskite BiTiO, units betweentwo Bi,O, layers [lo,1 I]. The room temperaturevalues of the dielectric constant, K, were reported to

* Corresponding author.

be K, = 120, K, = 205 and K, = 140. The piezo-electric coefficient of Bi,Ti,O,, is also relativelyhigh making the material suitable for high tempera-ture transducer applications and also as lead freepiezoelectric ceramic [ 12,131. In order to optimizethe piezoelectric properties of plate-like Bi,Ti,O, 2particles different processing techniques such as tapecasting, hot pressing and hot forging were also de-veloped [14,15]. Since Bi,Ti,O,, shows high electri-cal conductivity anisotropy, low temperature synthe-sis and densification of randomly oriented grains aredesirable to control the conductivity paths along thelayers [16]. The present paper describes a low tem-perature synthesis method for the preparation ofBi,Ti,O,, powders by a nanocomposite approachfrom hydroxide precursors of Bi and Ti. Sinteringbehavior of powder obtained in the present method iscompared with powder obtained from the sol-gelmethod, since the sol-gel method is a versatile lowtemperature synthesis method that yields productphases with grain size in the submicrometer range. Inother words, the basic idea of the present investiga-

00167-577X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved.PII SO167-577X(96)00107-3


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470 A. V. Prasada Rao et al. /Maierinls Letters 28 f1996) 469-473tion is to apply the principle of the sol-gel techniqueof mixing precursors at the molecular level to theinexpensive solid-state precursors, but obtain mixingat the nano level to render the process cost effectiveand achieve good densification.

2. ExperimentalBismuth nitrate, Bi(NO,), . 5H,O, and titanium

isopropoxide, Ti(OC,H 7)4, from Aldrich ChemicalCompany were the starting materials. Bismuth hy-droxide was prepared by dissolving bismuth nitratein water containing sufficient amount of nitric acidto give a clear solution which was then added slowlyto an equal volume of concentrated ammonia underconstant stirring. The resulting precipitate waswashed with deionized water until it was free fromnitrate and finally with isopropanol and dried at60C. Titanium hydroxide was prepared by dissolv-ing Ti(OC,H,), in isopropanol and adding this solu-tion slowly to a mixture containing 1 : 1 : 2 ammo-nium hydroxide (30%)-isopropanol and deionisedwater while stirring. The precipitate obtained waswashed several times with deionized water and driedat 60C. Known amounts of bismuth and titaniumhydroxides were individually heat treated at 600Cfor 1 h to determine the exact amounts of Bi,O, andTiO, contents in the respective hydroxides, Bismuthhydroxide and titanium hydroxide powders weremixed in the required proportion so as to yield 0.02mol stoichiometric Bi,Ti jO,, and made into a slurrywith 1 : 1 ammonium hydroxide, stirred for about 12h and dried at 60C. The dried powder was used forfurther studies. Bismuth acetate Bi(CH,COO), andTi(OC3H7)4 were the starting materials with 2-methoxy ethanol (2-MOE) and acetic acid (HAC) assolvent for the sol-gel synthesis. 0.04 mol ofBi(CH,COO), was added to 300 cm3 of a mixturecontaining 2-MOE and HAC in the ratio 2 : 1 and theresulting mixture was subjected to distillation at124C to remove 210 cm3 of the mixture. Aftercooling, 95 cm3 of HAC was added and the contentswere heated to 100C to obtain a clear solution. In aseparate flask 26.54 cm3 of Ti(OC,H,), was addedto 1.0 mol of 2-MOE and the solution was refluxedat 124C in argon atmosphere for 6 h. Bismuth andtitanium precursor solutions were then mixed and

refluxed at 124C in argon atmosphere for 6 h, afterwhich the excess solvent along with other unwantedbyproducts were distilled off to obtain a concentratedbismuth and titanium precursor solution which washydrolysed by the addition of water. The wet gel wasdried in an oven at 100C and the gel powder washeat treated at 300C for several hours until all thecarbon was removed.

Phase identification was carried out using an X-raydiffractometer (Scintag model DMC 105) with cop-per Ko radiation. Thermal behavior was investi-gated using a Perkin-Elmer differential thermal ana-lyzer (Model DTA 1700) and a thermogravimetricanalyzer (Delta series TGA7). The densities weremeasured by a liquid displacement method. Mi-crostructural investigations were performed with ascanning electron microscope (Model ISI-DS- 130,Akashi Beam Technology Corporation, Japan).

3. Results and discussionThermal behaviors of bismuth hydroxide, titanium

hydroxide and mixture of bismuth-titanium hydrox-ides are depicted in Fig. 1 and Fig. 2. From thethermograms in Fig. 1 it can be seen that titaniumhydroxide showed a continuous loss from 70 to380C while bismuth hydroxide showed two differ-ent weight losses from ambient to 560C. The exper-imental weight losses for bismuth hydroxide and

75 I 1 I 1 I I I h I I60 I30 200 270 340 410 480 550 620 690 760TEMPERATURE iC)

Fig. 1.Thermogrvimetric analysis curves for (a) bismuth hydrox-ide, (b) mixture of bismuth hydroxide and titanium hydroxide and(c) titanium hydroxide.


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A.V. Prasada Rao et al. /Materials Letters 28 (1996) 469-473 471

II I 1 I ! I I 1 I 1 I60 130 220 270 340 410 480 550 620 690 760TEMPERATURE (C)

Fig. 2. DTA curves for (a) bismuth hydroxide, (b) mixture ofbismuth hydroxide and titanium hydroxide and (c) titanium hy-droxide.

titanium hydroxide suggest the following dehydra-tion reactions:Bi( OH), + BiO( OH) (50-34OC),BiO( OH) -+ Bi,O, (400-550C))TiO( OH) Z + TiOz (50-400C).Corresponding DTA curves for the above samplesare shown in Fig. 2. The DTA curve for Bi-hydrox-ide precursor shows three exothermic peaks at 240,360 and 600C along with three endothermic peaksat 460, 560 and 735C. Since the peaks at 240, 360,460 and 530C all lie within the weight loss regionshown in its thermogram, no emphasis is made toassign these individual peaks even though they maycorrespond to loss of adhering organics (due toisopropanol used in this washing step), crystalliza-tion of BiO(OH) and subsequent dehydration ofBiO(OH) respectively. The remaining peaks at 600and 735C which are outside the weight loss regionare ascribed to crystallization of Bi,O, and phasetransformation of OL + 6Bi,O, (monoclinic to cu-bic). The DTA curve for Ti-hydroxide precursorshows one broad endothermic peak followed by twosharp exothermic peaks. The broad endothermic peakis due to loss of water while the exothermic peakscorrespond to loss of associated organics and crystal-

lization of the anatase respectively. The crystalliza-tion temperatures of BiO(OH) and anatase alongwith the phase transition temperature of (Y -+6Bi,O, are in agreement with the previously re-ported values in the literature [17]. The DTA curvefor the mixture of Bi and Ti hydroxides shows up to560C peaks characteristic of bismuth hydroxide andtitanium hydroxide. Above 560C the DTA showsone exothermic peak at 715C in contrast to theendothermic peak observed in pure bismuth hydrox-ide. Hence this peak is ascribed to the crystallizationof Bi,Ti30,,. XRD patterns of Bi-hydroxide andTi-hydroxide precursors heat treated at 600C for 1 h(Fig. 3) correspond to Bi,O, and TiO, (anatase) asper the JCPDS (Joint Committee on Powder Diffrac-tion Standards) card numbers 41-1449 and 21-1272respectively. The mixture of Bi and Ti hydroxides,when heated to 750C for 1 h, showed an XRDpattern corresponding to Bi,Ti,O,, (JCPDS card no.35-795) substantiating the assignment of the exother-mic peak at 715C in the DTA curve as due tocrystallization of Bi,Ti,O,,. Stevic et al. [18] re-ported the synthesis of Bi,Ti301? from a coprecipita-tion route using TiCl,/TiCl, and Bi(NO,), andcompared the densification behavior of Bi,Ti 3O, 2powder obtained from the coprecipitation route withthat obtained from mechanically homogenized mix-ture of Bi,O, and TiO,. Bi,TilO,? obtained from

40DEGREES 28 (Cu km)

50 60

Fig. 3. XRD patterns for (a) bismuth hydroxide, (b) titaniumhydroxide both heat treated at 600C for 2 h and (c) mixture ofbismuth hydroxide and titanium hydroxide heat treated at 750Cfor 1 h.


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75OTQL A.- 00Cd\300%

I20I I I I

30 40 50 60DEGREES28 (Cub.)

Fig. 4. XRD patterns of sol-gel powder heat treated for 1 h at300, 500 and 750C.

TiCl, in their study showed a contamination ofBiOCl which subsequently led to the formation ofBi,Ti,O, , that persisted even in ceramics heat treatedat 1100C for 4 h. The TiCl, precursor on the otherhand gave no Bi,Ti,O,, in the final ceramic. Amaximum density of about 8 l-87% was reported inthese ceramics even after double sintering at 1100Cfor 4 h.

Fig. 4. gives the XRD patterns of the sol-gelpowder heat treated at 300, 500C for 1 h and 750Cfor 1 h. The gel powder heat treated at 300C wasyellow in color which subsequently transformed towhite at 750C. Even though Bi,Ti,O,, starts crys-tallizing at 500C fully crystalline monoclinic phaseis observed only after a heat treatment of 750C. Thegel powder heat treated at 300C and mixed hydrox-ide powder dried at 100C were separately pelletizedby cold isostatic pressing and sintered at 950, 1050and 1150C for 2 h. The experimental densities forthe sintered pellets are given in Table 1. Fig. 5shows the microstructure of pellets sintered at1050C. It can be clearly seen from these microstruc-tures that in both cases of hydroxide mixing methodas well as sol-gel method, submicrometer grains are

Table 1Experimental densities for Bi,Ti,O, 7 pelletsSinteringconditions

% theoretical density for pellets derived frompresent method sol-gel method

95OC/2 h 86 85.4105OC/2 h 87 85.61 150C/2 h 87.8 85.3

Fig. 5. SEM micrographs of thermally etched surfaces of bismuthtitanate pellets derived from (a) nano mixing and (b) sol-gelmethod and sintered at 1050C for 2 h.

merging into extended platelets characteristic ofBi,Ti,O,,. This typical platelet formation inBi,Ti30,2 has been previously reported in the litera-ture [15,16].

4. ConclusionsFrom the above results it is apparent that

Bi,Ti,O,, with grain size less than 1 pm can beeasily synthesized from the corresponding hydroxideprecursors at a temperature as low as 750C. Pelletsderived from a mixture of hydroxide precursorsshowed a density close to 88% of the theoreticaldensity which was slightly higher than that of pelletsderived from the sol-gel method. Since the present


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A.V. Prasada Rao et al./ Materials Letters 28 (I9961 469-473 473

method involves no intermediate calcination step andrequires no special handling of precursors as was thecase in the sol-gel method, the present method isexpected to be cost effective.

AcknowledgementsThis research was supported by the Division of

Materials Research, National Science Foundation un-der grant No. DMR-9319809.

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Bismuth Titanate From Nanocomposite and Sol-gel Processes

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