Synthesis and Properties of BaTiO3 Nanopowders

Debasish Sarkarw

Department of Ceramic Engineering, National Institute of Technology, Rourkela 769008, Orissa, India

This work was aimed to produce high tetragonal, pure, andspherical BaTiO3 nanopowder through combined wet-chemicaland rapid calcination process. Lattice parameters, crystal struc-ture, and bonding characteristics of BaTiO3 nanopowder weredetermined from Rietveld refinement and maximum entropymethod. The statistical Weibull distribution analysis describedthe formation of narrow size particle distribution. Selectivesintering additive and optimum temperature profile successfullyconsolidated the nanoparticles without substantial grain growth.The grain exhibited a typical ferroelectric core domain, whereas,the grain shell apparently composed of nonferroelectric phase.The process described here can be implemented to the prepara-tion of other nanoparticles for commercialization that is suitableto be obtained via this approach.

I. Introduction

HIGH-performance multilayer ceramic capacitor with a di-electric thickness below 1 mm has already been commer-

cialized by BaTiO3 nanopowders (r200 nm size), but as thin as0.5 mm or less would be expected for the next-generation com-ponents. The current mainstream Electrical Industry Alliance(EIA) utilizes 0603 (1.6 mm� 0.8 mm) case size for generalelectronic equipment and EIA0402 (1.0 mm� 0.5 mm) for mo-bile equipment. However, the development and large-scale com-mercial use of multilayer ceramic capacitor (MLCC) withEIA0201 (0.6 mm� 0.3 mm) case size is a challenging issuefor capacitors industries, as it can be materialized throughB125nm tetragonal BaTiO3 nanopowders.

1 There are differentchemical methods well established to prepare cubic and/ortetragonal BaTiO3 nanopowders. The room-temperature wet-chemical process dominated to the formation of 30 nm para-electric cubic phase. Hence, subsequent postheat treatment isnecessary to obtain tetragonal phase in BaTiO3 powder; how-ever, it increases the particle size beyond the objective.2–4 In an-other end, the hydrothermal method synthesized o100 nmtetragonal BaTiO3 nanopowders, but process is expensive andintraparticle porosity deteriorates the properties.5,6 In thiscontext, here, we report a new combined synthesis techniquefor the preparation of pure, near spherical, nonporous, highc/a ratio (41.009) of 125 nm grade tetragonal BaTiO3 nano-powders. The sintering of nanopowders with controlled graingrowth and their properties could be considered for electronicsapplications.

II. Experimental Procedure

(1) Preparation and Crystal Structure Evaluation of BaTiO3

Nanopowders

In a typical synthesis, inorganic BaCl2 � 2H2O salt and foodgrade ammonium bicarbonate (NH4HCO3) were dissolved sep-

arately in 1000 and 500 cm3 glass beaker, respectively. The clearammonium bicarbonate solution was slowly added within BaCl2solution at room temperature and kept it for 1 h to complete theformation of BaCO3 nanorods. Subsequently, cosmetic gradecommercially available TiO2 nanopowder (15 nm) was addedwithin as-synthesis BaCO3 nanorods and mixed for another 1 hto obtain homogenous mixed powder precursors. The resultantwhite powder slurry was filtered, washed till removal of chlorideions and dried at 451C for thermal treatment. Before thermaltreatment, a thermogravimetric analysis (TGA, NETZSCH,STA 449C) of the dried precursor powder was carried out inair to study the reaction mechanism of precursor BaCO3 andTiO2 mixed powders. Ten grams of dry precursor (BaCO31TiO2) mixture was placed within 60 cm3 Pt-crucible, heated upto 11001C with a heating rate of 201C/min and isothermaltreated for 2 min. Following that the furnace door was quicklyopened, taken out the Pt-crucible and cooled down to 51Cthrough ice-cooled water. The cooling time from 11001 to 51Cwas B5 s. In a similar fashion, five batches were calcined andcumulative 42 g BaTiO3 nanopowders were used for differentcharacterizations. Normal furnace cooling and another differentcooling media (dry ice and liquid N2) were also used to simulatethe thermal cycle. The phase content, crystallinity, and purityfor different powders were evaluated from synchrotron X-raydiffraction (XRD) pattern and their Rietveld analysis. The ob-served structural factors of 42 reflections derived from theRietveld refinement were used to perform the maximumentropy method (MEM) analysis. The particle sizes and crystalpattern of the BaTiO3 powders were characterized throughtransmission electron microscope (TEM, TECNAI-G2, FEI).The BaTiO3 nanopowder was mixed with ethanol, 1 wt% poly-acrylic acid, and NH4OH and finally mixed through zirconiagrinding media in a high-density polyethylene (Nalgene)jar. Two drops of the mixer solution was taken in experimen-tal vessel and carried out scanning mobility particle size (SMPS,GRIMM, Germany) distribution analysis.

(2) Sintering and Properties of BaTiO3 Disks

The as-synthesized BaTiO3 nanopowders were consolidatedthrough uniaxial press (50 ton, Carver) without addition of or-ganic binder; however, different weight percent tetra ethyl or-thosilicate (TEOS) was added to optimize the quantity ofsintering additive. Bulk density and TEM analysis of sintereddisk was estimated to understand the densification behavior andgrain structure. Minimum 2 wt% SiO2 as sintering additive wasrequired to sinter the powder at 12351C for 2 h. The sintereddisks were carefully grinded through diamond wheel and pol-ished down to 100 mm to measure the dielectric and ferroelectricproperties. Remnant polarization and coercive field of sinteredBaTiO3 were measured from a P–E hysteresis curve at 20 Hzdetermined using a ferroelectric test system (FCE-1, Toyo Cor-poration). The dielectric properties of sintered BaTiO3 disks in awide frequency range from 1 kHz to 1 MHz were determined bythe conventional capacitance measurement. Silver (Ag) elec-trodes were formed on the two facing surfaces of the sinteredplates. The capacitance and the loss tangent (tan d) were mea-sured with an impedance analyzer (Agilent 4294A).

A. Bandyopadhyay—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: dsarkar@nitrkl.ac.in

Manuscript No. 27909. Received May 3, 2010; approved July 7, 2010.This work is partially supported by DST (SR/FTP/ETA-088/2009), India.

Journal

J. Am. Ceram. Soc., 94 [1] 106–110 (2011)

DOI: 10.1111/j.1551-2916.2010.04049.x

r 2010 The American Ceramic Society

106

III. Results and Discussion

(1) Synthesis of BaTiO3 Nanopowders

The current synthesis process is very simple and near to 125 nmtetragonal BaTiO3 nanopowders were successfully synthesizedfor commercialization. Key feature of the process is attributedto in situ thermal decomposition of as-synthesis BaCO3 nano-rod, reaction with spherical 15 nm TiO2 particles and incrementof c/a ratio during rapid cooling. Thermogravimetric analysis ofpowder mixture confirmed that the weight loss was as-linearbeyond 11201C. Hence, the BaCO3 was completely decomposedat this temperature; however, a low-temperature isothermaltreatment is beneficial to complete the reaction.7 A combina-tion of processing temperature, time, and cooling environmentwere synchronized to optimize the morphology and crystalstructure of nanoparticles. In the proposed process, the rapidcalcination process has twofold advantages; fast decompositionof BaCO3 and rapid reaction with TiO2. The powder mixturecalcination at 11001C, isothermal heating for 2 min and coolingdown to ice water (51C) temperature was observed as the opti-mum powder synthesis condition. However, in a recent article, alow-temperature (7001–8001C) synthesis process for the 70–85nm BaTiO3 nanopowders starting from BaCO3–TiO2 precursorshas been reported by Buscaglia et al.8 The single-phase BaTiO3

has a specific surface area of 12–15 m2/g, relative density of96.5%–98.3% and tetragonality of 1.005. The obtained particlesize is much lower compared with this proposed method but c/ais lower from our objective. The cooling rate has also significanteffect for the further improvement of c/a ratio. The c/a ratio wasestimated to be B1.008 during normal furnace cooling, whereasit was enhanced through change in cooling media (air to water).Interestingly, the c/a ratio diminishes when cooling down to be-low 51C, for e.g. dry ice and liquid nitrogen. It is presumably dueto the temperature-dependent phase transformation phenome-non of BaTiO3.

9

(2) Crystal Structure and Morphology of BaTiO3

Nanopowders

The XRD pattern represents the presence of tetragonal BaTiO3

with high K-factor (ratio of the intensity of (200) peak to thatbetween (002) and (200) peaks), which mainly attributed to thehigh tetragonality of BaTiO3 phase. A clear peak splitting within441–451 could be observed with K-factor of 3.35 (Fig. 1). TheRietveld analysis with fit factor; s5 2.13, demonstrates the par-ticles are two phase system; 10 wt% cubic and 90 wt% tetrag-onal BaTiO3 with having a5 3.99682 (3), b5 3.99682 (3), andc5 4.03311 (5). It is interesting to note that the content oftetragonal phase is near to identical with the content of anatasephase present in precursor titania (TiO2) nanopowder. In thepresent work, commercial TiO2 nanopowder has 87% anataseand 13% rutile phase, which near to identical with the presence

of tetragonal and cubic phase in BaTiO3 nanopowders. In recentresearch, a different school proposed that the rutile and anatasephase of TiO2 assists the formation of cubic and tetragonalBaTiO3, respectively.

10,11 The MEM analysis was carried out in128� 128� 128 pixels per lattice parameter of tetragonal phase(Fig. 2(a)). The electron charge-density distributions for (001),(200), and (002) planes are shown in Fig. 2(b)–(d), respectively.No overlapping electron distributions were observed betweenBa–O bonding, as shown in Fig. 2(b), indicating an ionicbonding nature. On the other hand, the (200) and (002) planesrepresenting the electron charge-density distributions betweenTi and O contour, which exhibits the electron overlapping be-tween them or existence of weak covalent bonding betweenTi and O atoms along the a-axis direction. The MEM result re-veals that the tetragonal phase in nano-BaTiO3 is stabilized byionic and weak covalent bonding. The crystal size is reducedfrom higher to lower symmetry by a minute change of latticeparameters. This change consists of a systematic displacementof the equilibrium position of the rattling titanium (Ti) ion fromthe center of the octahedron toward the oxygen ions by anamount of 0.1 A.

The TEM image of BaCO3 and TiO2 represents the formationof elongated and spherical grains, respectively. ElongatedBaCO3 has a wide range of aspect ratio with high crystallinity(Fig. 3(a)). TEM analysis for TiO2 indicates that the particlesare spherical with clear lattice fringes (Fig. 3(b)). The micro-graphs of the as-synthesized BaTiO3 nanoparticles are shown inFig. 3(c). The as-synthesized BaTiO3 nanoparticles with averagediameter of about 120 nm was near spherical and uniform inshape. The specific surface area of this powder was measured7.96 m2/g, indicated the average particles are near to 125 nm insize. Moreover, the crystallinity of BaTiO3 nanoparticles couldalso be confirmed through high-resolution electron microscopy(HRTEM) image as shown in Fig. 3(d). HRTEM image con-firms the clear lattice fringes of BaTiO3 nanoparticles along [101]and [110] directions. The lattice fringes (0.403 nm) observed inthis image is well agreed with the separation between the (100)lattice planes. The SAED pattern obtained from 125 nm nano-particle has a highly symmetrical dotted lattice, which supportsthe nature of crystallinity of BaTiO3 nanoparticles.

(3) Particle Size Distribution of BaTiO3 Nanopowders

Nanoparticles are more subject to dispersive force due to its highsurface energy, which enhance the agglomeration behavior of

Fig. 1. X-ray diffraction (XRD) pattern and Rietveld analysis ofBaTiO3 nanopowders. A clear splitting could be identified within 21–22and 441–461 (2y).

Fig. 2. Electron-density distributions obtained by maximum entropymethod (MEM) for tetragonal phase in BaTiO3: (a) crystal structureof BaTiO3, (b) (001), (c) (200), and (d) (002) planes, respectively. Theisosurface level is 100 e/A3, represents by color scale bar.

January 2011 Synthesis and Properties of BaTiO3 Nanopowders 107

this class of particles. A detailed investigation of particle sizedistribution can predict the dispersion and consolidation behav-ior of nanopowders. We emphasize that agglomeration duringfabrication, which leads to interparticle and interagglom-erate pores, is a critical problem and leads to inhomogeneousmicrostructure. For nanopowders, interagglomerate poresare typically larger than interparticle pores. Hence, the study of

dispersion behavior is essential to manipulate the sintered mi-crostructure obtained from nanopowders. In a recent article, anew statistical analysis technique has been introduced to analyzenanoparticle size distribution phenomenon, which quantifies thereal meaning of the dispersion behavior of nanoparticles.12 Inbrief, a three-parameter Weibull distribution function has beenintroduced to define the model particle size distribution data

Fig. 3. Transmission electron microscope (TEM) image of as-synthesized BaCO3 nanorods (a), precursor TiO2 (b), as-synthesized BaTiO3

nanopowders (c) and high-resolution electron microscopy (HRTEM) and SAED pattern of BaTiO3 nanoparticles (d).

Fig. 4. Particle size distribution as-obtained from scanning mobility particle size (SMPS) analysis (a) and three-parameter Weibull distributionfunctions, analyzed from SMPS data (b).

108 Journal of the American Ceramic Society—Sarkar Vol. 94, No. 1

obtained from SMPS measurements (Fig. 4(a)):

fðxÞ ¼ 1� exp � x� gb

� �a� �(1)

Here, x, a, b, and g are all positive, and f(x) is the cumulativeundersize percent of particle size x present in the distribution.Three parameters a, b, and g represent shape, scale, and loca-tion, respectively. The values of these parameters can be ob-tained using linear fits between ln[1/(1�f(x))] and ln(x�4)[Fig. 4(b)]. In the present case, a good fit to the SMPS datacould be achieved using the above three-parameter distributionfunction using a, b, and g equal to 4.6, 124, and 8.6, respectively.The shape parameter for the synthesized BaTiO3 nanoparticlerepresents relatively larger value, indicating narrow size distri-bution and most of the particles are of 124 nm in size. The as-realized presence of smaller particles is apparently due to thecontamination from zirconia grinding media during preparationof SMPS sample. The detail properties of as-synthesis powdersand sintered BaTiO3 disks are listed in Table I.

(4) Sintering and Properties of BaTiO3 Disks

A series of sintering profile was considered to optimize theaverage grain size and density of sintered disks. The TEM im-age of selective sintered specimen at 12351C for 2 h exhibits theaverage grain size of about 200 nm with bulk density of 5.94 g/mL (B99% of the theoretical density). Figures 5(a) and (b)

demonstrate the TEM image of sintered BaTiO3. The core–shellgrains are observed with content of strip shape domains. Thecore found in numerous grains shows a typical ferroelectric do-main pattern, whereas the grain shell apparently consists ofnonferroelectric material. The signature of the ferroelectric do-main could be represented as the hysteresis loop.13 This loopdescribes the existence of remnant polarization and coercivefield of the ferroelectric materials. Usually, the polarization isreversible and almost linear at low applied field. However, thepolarization increases considerably due to switching of the ex-isting ferroelectric domains in the present condition. Figure 6(a)reveals that the further increases in the electric field continue toincrease the polarization, but removal of the applied field, spon-taneous polarization does not go to zero rather exhibits a finite1.64 mm/cm2 remnant polarization (Pr). The crystal could not becompletely depolarized until a definite field of magnitude is ap-plied in the negative direction, whereas, an external field (4.91kV/cm) needed to reduce the polarization to zero is expressed asthe coercive field strength Ec. In a recent article, Sakai et al.14

evaluated the remnant polarization and coercive field of BaTiO3

thick film 3.1 mC/cm2 and 1.1 kV/cm at 10 Hz, respectively.Moreover, the ferroelectric properties of this BaTiO3 could bemanipulated with synchronization of additives and sinteringprofile. As defined above, the developed ferroelectric domainis a microscopic region in a crystal in which the polarization ishomogenous. In the presence of electric field, oxygen vacanciesmigrated along the direction of the electric field, and hence thenumber of defects staying in the original state becomes fewer.This makes domain switching easier because there is less resis-

Table I. Physical Properties of BaTiO3 Nanopowders and Electric Properties of Sintered Disks

Weibull parameters

Average grain size of sintered disk (B200 nm)

(diameter +, 15 mm; thickness, 100 mm),

density 5 5.94 g/cm3

Ferroelectric

properties at 20 Hz

Dielectric

properties at 1 kHz

K-factor

c/a

ratio

Bonding

characteristics

Shape

(a)Scale

(b)Location

(g)

Powder

density

(g/cm3)

Specific

surface area

(m2/g)

Particle

size from

SSA (nm)

Remnant

polarization

(mC/cm2)

Coercive

field

(kV/cm)

Dielectric

constant

Dielectric

loss

(tan d)

3.35 1.00908 Ba–O–ionic 4.6 124 8.6 5.98 7.96 126 1.64 4.91 3721 0.0502Ti–O–covalent

The as-synthesized nanopowder was sintered at 12351C for 2 h in air, subsequently grinded and polished down to 100 mm and measured the electric properties. K-factor,

ratio of the intensity of (200) peak to that between (002) and (200) peaks.

Fig. 5. Transmission electron microscope (TEM) image of as-sintered BaTiO3 disks. The average grain size near to 200 nm (a) and ferroelectric domainwith a narrow domain size (b).

January 2011 Synthesis and Properties of BaTiO3 Nanopowders 109

tance from point defects. However, the recoverability becomesweaker. This is because fewer defects stay in the original state;they provide a weak restoring force for reverse domain switch-ing. The dense sintered BaTiO3 exhibits distinct dielectric prop-erties as shown in Fig. 6(b). The highest dielectric constantabout 3700 and the lowest dielectric loss about 5� 10�3 wereachieved when sample sintered at 12351C for 2 h. The highdomain density and vibration enhances the polarization.Moreover, the mismatch crystal lattice symmetry posses spon-taneous lattice strains. This gradient lattice strain region expe-rience high permittivity due to the ionic polarization, whichcould be described as the domain size effect.

IV. Conclusions

A new technique was developed to synthesis of near sphericaland pure 125 nm grade BaTiO3 nanopowders for commercial-ization. The c/a ratio was 1.00908 and rattling Ti ion displacedan amount of 0.1 A along c-axis. Optimum content of sinteringadditive and sintering profile consolidated B99% relative densedisks with 200 nm average grain size. Ferroelectric domain ex-hibited hysteresis loop. The remnant polarization and coercivefield measured from a P–E hysteresis curve were 1.64 mC/cm2

and 4.91 kV/cm at 20 Hz, respectively. The measured dielectricconstant and tan d were 3700 and 0.05 at 1 kHz, respectively.

Acknowledgments

Author would like to thank M. C. Chu and Y. I. Kim, Korea ResearchInstitute of Standards and Science, South Korea for technical assistance and usefuldiscussions.

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Fig. 6. P–E characteristics of sintered disk (+, 15 mm; thickness, 100 mm) at 20 Hz, hysteresis loop observed as a signature of ferroelectric domain (a)and the dielectric properties with respect to frequency (b).

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