ISSN 0012-5008, Doklady Chemistry, 2007, Vol. 416, Part 1, pp. 217–219. © Pleiades Publishing, Ltd., 2007.Original Russian Text © V.I. Ivanenko, E.P. Lokshin, R.N. Osaulenko, V.T. Kalinnikov, 2007, published in Doklady Akademii Nauk, 2007, Vol. 416, No. 2, pp. 200–202.

217

Functional materials containing ferroelectric crys-tals of metatitanates of divalent metals (barium, stron-tium, lead) are promising for manufacturing optical andpiezo ceramics, ceramic memory elements, and artifi-cial photon structures with energy gaps [1–6]. To obtainsuch materials, it is necessary to have single-phase fer-roelectrics with stoichiometric composition in the formof nano- and microcrystalline powders with given nar-row particle size distributions. Methods for producingpowders with a given particle size have not yet beendeveloped.

In this work, by the example of metatitanates ofdivalent metals, we propose a new approach to control-ling the sizes of powders of ferroelectric materials anddescribe the use of this approach for producing nano-sized powders with given narrow particle size distribu-tions.

A suspension containing powders of metatitanatesof divalent metals (barium, strontium, and lead) syn-thesized according to a published procedure [7] wassubjected to hydrodynamic treatment. The stirringintensity was estimated from the modified Reynoldsnumber [8]

where

n

is the stirrer speed, s

–1

;

d

is the stirrer diameter,m;

ρ

is the average heterogeneous density, kg s

2

m

–1

;and

µ

is the dynamic viscosity, kg s m

–2

.An increase in the stirring time led to a uniform

increase in the particle size of

LJíié

3

powders (Fig. 1).The particle size increased only at high stirrer speeds(

n

= 83.3–166.7 s

–1

). At

d

= 0.05 m,

ρ

= 102 kg s

2

m

–1

,

Remnd

2ρµ

------------,=

and

µ

= 10

–4

kg s m

–2

, we have Re

m

= (2.1–4.3)

×

10

5

.Long-term keeping of nanosized particles without stir-ring, as well as stirring of the suspension at low

Re

m

,did not change the particle size.

Comparison of the X-ray diffraction spectra (Fig. 2)of

LJíié

3

single crystals 1–5

µ

m in size grown asdescribed in [9] with those of

LJíié

3

powdersobtained by hydrodynamic treatment showed signifi-cant broadening of peaks in the latter spectra in com-parison with the spectra of single crystals. This broad-ening does not decrease but even somewhat increaseswith an increase in the size of nanosized powders.Short-term ultrasonic treatment of suspensions of pow-ders causes breakup of powder particles into grains nomore than 10–15 nm in size (Fig. 3).

The results obtained cannot be explained on thebasis of concepts of dissolution and recrystallizationsince the increase in the particle size is too rapid andgives rise to morphologically ordered particles withrather narrow particle size distributions (Fig. 1), ratherthan particles with a wide scatter in size.

Production of Nanosized Powders of Ferroelectricswith Narrow Particle Size Distributions

V. I. Ivanenko

a

, E. P. Lokshin

a

, R. N. Osaulenko

b

, and

Academician

V. T. Kalinnikov

a

Received April 16, 2007

DOI:

10.1134/S0012500807090042

a

Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 26A, Apatity, Murmansk oblast, 184200 Russia

b

Petrozavodsk State University, pr. Lenina 33,Petrozavodsk, 185910 Karelia, Russia

CHEMISTRY

100 nm 100 nm(a) (b)

Fig. 1.

Scanning electron microscopy images of

BaTiO

3

powders obtained by hydrodynamic treatment for (a) 40 and(b) 120 min. The images were produced with a LEO-420scanning electron microscope.

218

DOKLADY CHEMISTRY

Vol. 416

Part 1

2007

IVANENKO et al.

Based on the experimental data obtained, theincrease in the particle size of the powders studied canbe represented as the agglomeration of much smallernanosized grains into which particles break up underthe action of ultrasound (Fig. 3).

The specific features of the behavior of the powdersstudied can be explained using the fact that metatitan-ates of divalent metals are ferroelectrics. The minimalwidth of a domain of the tetragonal phase of

BaTiO

3

is30–50 nm [10]. Thus,

BaTiO

3

grains constituting largeraggregated particles consist of a single domain eachand, consequently, are spontaneously polarized. Thespontaneous polarization of

BaTiO

3

is rather strong andis 26

µ

C cm

–2

at

23°ë

[11]. Once oppositely chargedfaces have contacted, grains coalesce to form largerparticles, which retain an uncompensated charge on thesurface and are capable of enlarging further. Theformed agglomerate remains spontaneously polarized,and with an increase in its size, its ability to holdattached charged particles increases as in the Madelungmodel for ionic crystals.

The enlargement of agglomerates is accompaniedby their insignificant structural disordering, whichmanifests itself in a small increase in the broadening ofpeaks of the X-ray diffraction spectra with an increasein the particle size (Fig. 2). The coalescence of grains attheir charge-carrying faces determines high preserva-tion of the structural order of the forming polycrystal-line agglomerates. This is the reason why Fig. 2 doesnot show narrowing of peaks for powders 500–600 nmin size in comparison with powders 150–200 nm insize. If the particle enlargement occurred by dissolutionand recrystallization, this narrowing according to the

Selyakov–Scherrer equation should have been 2.5- to4-fold.

The fundamentally possible spontaneous coagula-tion [12] without hydrodynamic treatment of the sus-pension was not experimentally observed, which can beexplained by the presence of oriented hydration layerson the charged surfaces of grains, with the charge ofthese layer being compensated by electrolyte ions.Intense stirring of the suspension decreases the thick-ness of the boundary layer preventing the coagulationof single-domain grains under the action of their sur-face electrostatic charges. At the same time, collisionsof particles may lead to their breakup to form smallerfragments or primary grains. However, it was theoreti-cally proven [12] and experimentally shown [14] thatthe rate of adhesion of small mineral particles to largeones is 400–500 times higher than the rate of coales-cence of small particles. This explains the experimen-tally detected virtually complete absence of smallerparticles in the nanosized powders obtained.

Similar behavior was also observed in production ofnanosized particles of strontium and lead metatitanateswith narrow particle size distributions (the spontaneouspolarization of lead metatitanate at 23

°

C is 50

µ

C cm

–2

[11]). At the same time, in the above experiments withbarium sulfate suspensions, no enlargement of nano-sized particles was observed.

Thus, we have demonstrated the possibility of con-trolling the particle size of nanosized powders ofmetatitanates of divalent metals (strontium, barium,and lead) by hydrodynamic treatment of aqueous sus-pensions to produce powders with narrow particle sizedistributions.

300

250

200

150

100

50

0

I

,

pps

39.6 39.8 40.0 40.2 40.4 40.62

θ

, deg

3

2

1

1

10

2

0

20

30

1

2

3

3

µ

m

µ

m

00

nm

15.0

7.5

0

nm

Fig. 2.

Diffraction peak (101) for the tetragonal phase of

BaTiO

3

:

(

1

) crystalline samples with particles more than1

µ

m in size and powder samples with particles (

2

) 150–200 and (

3

) 500–600 nm in size. The spectra were recordedwith a DRON-6 X-ray diffractometer using

Fe

K

α

radiationmonochromatized by a pyrolytic graphite crystal.

Fig. 3.

Atomic force image of

LJíié

3

powder on a glasssupport after ultrasonic treatment (the particle size of theinitial powder is 100–120 nm). The images were producedwith a Digital Instruments Dimension 3100 atomic forcemicroscope.

DOKLADY CHEMISTRY

Vol. 416

Part 1

2007

PRODUCTION OF NANOSIZED POWDERS OF FERROELECTRICS 219

The detected rapid enlargement of nanosized parti-cles is explained by coagulation under the action ofelectrostatic attraction arising from the presence ofpotentials on the surface of the initial single-domaingrains.

ACKNOWLEDGMENTSThis work was supported in part by the basic

research program of the RAS “Development of Meth-ods for Producing Chemical Substances and Creationof New Materials” and by the Russian Foundation forBasic Research (project no. 06–08–00200-a).

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