Particle Size Control, Sinterability and Piezoelectric Properties of BaTiO<sub>3</sub> Prepared by a Novel Composite-Hydroxide-Mediated Approach

doi:10.4236/msa.2011.27104

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Materials Sciences and Applicatio n, 2011, 2, 758-764

doi:10.4236/msa.2011.27104 Published Online July 2011 (http://www.SciRP.org/journal/msa)

Particle Size Control, Sinterability and

Piezoelectric Properties of BaTiO3 Prepared by a

Novel Composite-Hydroxide-Mediated Approach

Yahong Xie1,3, Takesi Kimura1, Shu Yin1, Takatoshi Hashimoto2, Yuichi Tokano2, Atsushi Sasaki2,

Tsugio Sato2

1Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan; 2NEC Tokin Corporation, Koriyama,

Taihaku-ku, Japan; 3Xin Jiang University, Urumqi, China.

Email: [email protected]

Received Feburary 18th, 2011; revised March 9th, 2011; accepted March 21th, 2011.

ABSTRACT

The size of BaTiO3 particles was controlled by adjusting the molar ratio of the starting materials (BaCl2 + TiO2) to

mineralizer (NaOH + KOH) during a composite-hydroxide-mediated approach using a novel hydrothermal reaction

apparatus with a rolling system. The mean particle diameter decreased from 500 to 50 nm with a decrease in the (BaCl2

+ TiO2)/(NaOH + KOH) molar ratio from 0.44 to 0.04. The powders were sintered by normal one-step sintering at

1200˚C for 5 h and two-step sintering in which temperature was raised to 1200˚C at first and then decreased to 1100˚C

and kept at 1100˚C for 5 h. The BaTiO3 particles prepared with the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio of 0.32

and 0.22 showed excellent sinterability and could be sintered to almost full theoretical density by both method. The sin-

tered bodies obtained by both methods showed similarly excellent dielectric and piezoelectric properties.

Keywords: BaTiO3, Synthesis, Low Temperature Sintering, Piezoelectric Properties

1. Introduction

The excellent dielectric and ferroelectric properties of

barium titanate make it attractive material in the field of

electrceramic and microelectronics. However, the con-

ventional synthetic methods for BaTiO3 involves calcina-

tion of a BaCO3 and TiO2 powder mixture above 1200˚C,

and thus often results in polydispersity and the introduc-

tion of impurities and the powders prepared by this

method consist of non-uniform and coarse particles [1-3].

To overcome these deficiencies, a number of innovative

synthesis methods and chemical prcedures have been

developed [4-7]. Among them, the composite-hydroxide-

mediated (CHM) approach has been considered as an

effective method to synthesize high purity BaTiO3

[8-10].

In the present research a novel ball-milling assisted

hydrothermal reaction was used to promote the diffusion

of chemical species for a higher reaction rate as well as

the uniformity of the product [11-12]. By combining with

the addition of a Li2CO3 and V2O5 mixed sintering addi-

tive [13-16], BaTiO3 could be sintered to almost full

theoretical density at a low sintering temperature of

1200˚C for 5 h.

The tendency of the electronic industry towards mini-

aturization leads to high interest in fabricating some ma-

terials with nanometer-scale structure. The conventional

sintering prcesses are generally accompanied by rapid

grain growth, which has greatly hampered efforts to

produce dense materials with nanometer-scale structure.

Two-step sintering is a promising approach to obtain

dense nanograin ceramics because it suppresses grain

growth in the final stage of sintering. The two-step sin-

tering prcedure consists of the following two steps: (1) at

first step, sintering at a constant rapid heating rate until a

normal sintering temperature; (2) lowering the tempera-

ture by ca.100˚C afterwards followed by sintering at a

lowered temperature until it is fully dense. In this paper,

two-step sintering approach is used to densify the pre-

pared BaTiO3 ceramics at low temperature [17-19].

2. Experimental

2.1. Synthesis Techniques

A mixture of 460 mmol of anhydrous hydroxides (NaOH/

KOH molar ratio = 51.5:48.5) was put into a Teflon®-

Particle Size Control, Sinterability and Piezoelectric Properties of BaTiO Prepared by a Novel 759

Composite-Hydroxide-Mediated Approach

lined stainless steel autclave with 100 cm3 of internal

volume and a 5.5 cm outer diameter, followed by the

addition of 100, 75, 50, 30, 10 mmol of BaCl2 and TiO2

(anatase) with ten Teflon® balls, 1.1 cm in diameter.

Then, the autclave was sealed and placed in an electric

oven and heated at 200˚C for 24 h with a rotation speed

of 100 rpm during the reaction. After the reaction, the

autclave was taken out to allow to cool down to room

temperature. The product was dispersed in deionized

water to remove the hydroxide on the surface of the par-

ticles, then centrifuged and rinsed with ethanol, deion-

ized water and acetone, three times, respectively. Finally,

the obtained powders were dried overnight at 60˚C in a

vacuum oven.

The prepared BaTiO3 powders and commercial Ba-

TiO3 powders (Sakai Chemical Industry Co., BT-05,

which is considered the best commercial pure BaTiO3

having excellent properties) were precalcined at 800˚C

for 1 h followed by mixing well with 0.3 wt% Li2CO 3

and 0.04 wt% V2O5 powder by a wet-ball milling with

acetone and ZrO2 balls of 10 mm in diameter in a Tef-

lon® container at a rolling rate of 90 rpm for 12 h, re-

spectively. Then, the powders were collected, dried and

uniaxially pressed at 20 MPa in a steel die to form pellets,

20 mm in diameter and 3 mm thick, and then isostatically

pressed at 200 MPa. The pellets were then sintered by

normal one-step sintering at 1200˚C for 5 h which was

designated as (1200/5) with a heating rate of 10˚C/min.

The two-step sintering designated as (1200/1100/5) was

also conducted, in which the sample was heated at first

from room temperature to the first sintering temperature

T1 (1200˚C) with a heating rate of 10˚C/min, then the

temperature was held at 1200˚C for 1 min, after that rap-

idly decreased to T2 (1100˚C), and held at T2 for 5 h, as

illustrated in Figure 5 (insert). The densities of the sin-

tered pellets were measured by the Archimedes’ methods.

The rectangular bars of 2 mm × 4 mm × 12 mm were

prepared for piezoelectric measurements. Gold paste was

fired on both sides of the disks at 800˚C as electrode. The

specimens were poled in a stirred silicon oil at 80˚C by

applying DC electrical fields of 2 - 3 kV/mm for 30 min.

About 24 h later of the poling, the dielectric and piezo-

electric properties were measured using an Agilent 4294

A precision impedance analyzer at a frequency of 100

kHz.

2.2. Characterizations

The X-ray diffraction (XRD) analysis of the obtained

powder samples was carried out using CuK radiation

with a pyrolytic graphite monchromator mounted on a

powder diffractometer (Shimadzu XD-D1). Thermogra-

vimetric analysis (TG-DTA, Rigaku, TG8101D) was

performed for the powders from room temperature to

1200˚C with a heating rate of 10˚C/min in air. The parti-

cle morphology and the microstructures of the sintered

bodies were observed by a scanning electron microscope

(SEM; Hitachi S-4100).

3. Results and Discussion

3.1. Crystalline Phase

Figure 1 shows the XRD patterns of the BaTiO3 powders

synthesized by the composite-hydroxide-mediated ap-

proach at 200˚C for 24 h with the molar ratio of starting

materials (BaCl2 + TiO2) to mineralizer (NaOH + KOH)

of (A) 0.44, (B) 0.32, (C) 0.22, (D) 0.12, (E) 0.04. The

peaks of (002) and (200) around 2θ = 45.5˚ were en-

larged in the insert to compare the powder tetragonality.

All peaks of the synthesized powders were consistent

with the single perovskite phase. The slight peak splitting

around 2θ = 45.5˚ to (002) and (200) suggested the for-

mation of the tetragonal BaTiO3. The tetragonality (c/a)

decreased from 1.0055 to 1.0038, 1.0032, 1.0025 and

1.0013 with a decrease of the (BaCl2 + TiO2)/(NaOH+

KOH) molar ratio from 0.44 to 0.32, 0.22, 0.12 and 0.04,

respectively, which probably due to the difference in the

particle size, since the tetragonality (c/a) of barium titan-

ate is strongly dependent on particle size.

3.2. Thermal Analysis

The TG-DTA curves of the samples are shown in Figure

2. All samples showed three steps of weight losses. The

weight loss up to 200˚C, 200˚C - 800˚C and above 800˚C

may be due to the elimination of water adsorbed on the

surface, dehydration from the OH incorporated in the

lattice and elimination of CO2 from the contaminated

BaCO3, respectively. The weight losses of the present

samples were much smaller than those prepared by con-

ventional hydrothermal reactions [24], especially the

samples with the (BaCl2 + TiO2)/(NaOH + KOH) molar

ratio of 0.32 and 0.22 showed quite little weight losses of

ca.1.05 and 1.30%, respectively, indicating formation of

high purity stoichiometric BaTiO3 powder. According to

the TG-DTA curves, the powders were calcined at 800˚C

for 1 h prior to the sintering.

3.3. Morphology of the Powders

Figure 3 shows the morphologies of the prepared Ba-

TiO3 powders together with those of commercial BaTiO3

powders. All samples showed high quality crystal char-

acteristics of cubes or tetrahedrons. The average particle

size of BaTiO3 prepared in the present study decreased as

400, 200, 100, 60 and 50 nm with a decrease in the

(BaCl2 + TiO2)/(NaOH + KOH) molar ratio as 0.44, 0.32,

Particle Size Control, Sinterability and Piezoelectric Properties of BaTiO Prepared by a Novel

760 3

Composite-Hydroxide-Mediated Approach

Figure 1. XRD patterns of the samples prepared with the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio of (A) 0.44, (B) 0.32, (C)

0.22, (D) 0.12 and (E) 0.04.

Figure 2. TG-DTA curves of the samples prepared with the

(BaCl2 + TiO2)/(NaOH + KOH) molar ratio of (A) 0.44, (B)

0.32, (C) 0.22, (D) 0.12 and (E) 0.04.

0.22, 0.12 and 0.04 (Figure 3 A, B, C, D and E), respec-

tively. The samples A and B showed broad size distribu-

tion from 50 to 500 nm. In contrast, the samples C, D and

E showed relatively narrower size distribution of 30 to

100 nm. Generally, in the solution prcess the formation

of nuclei and growth of crystals cur. When the (BaCl2 +

TiO2)/(NaOH + KOH) molar ratio is low, the

Figure 3. SEM images of the samples prepared with the

(BaCl2 + TiO2)/(NaOH + KOH) molar ratio of (A) 0.44, (B)

0.32, (C) 0.22, (D) 0.12, (E) 0.04 and (F) commercial Ba-

TiO3.

formation rate of BaTiO3 is slow, therefore, the BaTiO3

crystals grow from the initially produced nuclei without

Particle Size Control, Sinterability and Piezoelectric Properties of BaTiO Prepared by a Novel 761

Composite-Hydroxide-Mediated Approach

agglomeration because of the high viscosity of the

NaOH/KOH melt, resulting in final nanostructure with

narrow size distribution (sample C, D and E). On the

other hand, at higher molar ratio (sample A and B) since

the formation rate of BaTiO3 is fast, the nuclei are

formed not only in the initial stage but also in the middle

stage of the reaction, and the agglomeration of particles

and grain growth tends to prceed due to the decrease of

the viscosity because of the formation of water as shown

by Eq. (1).

BaCl2 + TiO2 + 2NaOH = BaTiO3 + 2NaCl + H2O (1)

Therefore, the product consisted of larger crystals with

a large particle distribution. These results indicate that

the lower (BaCl2 + TiO2)/(NaOH + KOH) molar ratio

seems to be beneficial to obtain a narrower size distribu-

tion as well as a smaller particle size. Comparing with

the commercial BaTiO3 particles showing irregular phase

with average particle size of ca. 400 - 500 nm (Figure 3.

F), BaTiO3 powders prepared in the present study exhib-

ited a smaller average particle size (Sample C, D and E)

and a regular cubical shape.

3.4. Sinterablity

The sample powder was sintered at 1200˚C for 5 h with a

heating rate of 10˚C.min–1 with a 0.3 wt% Li2CO3 and

0.04 wt% V2O5 complex sintering additive. The relative

densities and grain sizes of the sintered bodies are shown

in Figure 4 as a function of the (BaCl2 + TiO2)/

(NaOH + KOH) molar ratio. The average grain size

changed as 10 μm, 25 μm, 20 μm, 8 μm and 5μm for the

samples with the (BaCl2 + TiO2)/(NaOH + KOH) molar

ratio of 0.44, 0.32, 0.22, 0.12 and 0.04. The samples with

the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio of 0.32

and 0.22 could be sintered to almost full theoretical den-

sity (> 98%), but the density of other samples were less

than 94%. From Figure 4, it is seen that the powder pre-

pared with the (BaCl2 + TiO2)/(NaOH + KOH) molar

ratio of 0.22 possessed the highest sinterability in the

present samples. These results agreed with the results

that the sample showed the high purity, small particle

size and uniform particle size distribution (Figures 2 and

3).

Morphology of Ceramics Bodies

Figure 5 showed the scanning electron micrographs of the

fracture surfaces of the sintered bodies by normal one-step

sintering (1200/5) and two-step sintering (1200/1100/5)

using the powders with the (BaCl2+ TiO2)

/(NaOH + KOH) molar ratio of 0.22 without and with a

0.3 wt% Li2CO3 and 0.04 wt% V2O5 complex sintering

additive. For comparison the morphology of sintered bod-

ies using the commercial BaTiO3 powder are also shown.

Figure 4. Relative densities and grain sizes of the BaTiO3

ceramics sintered at 1200˚C for 5 h using a 0.3 w t% Li2CO3-

0.04 wt% V2O5 mixed sintering additive as a function of the

(BaCl2 + TiO2)/(NaOH + KOH) molar ratio.

Figure 5. SEM images of the polished surfaces of the sintered

bodies.A, B, C, D: BaTiO3 with the (BaCl2 + TiO2)/(NaOH +

KOH) molar ratio of 0.22., E, F, G, H: commercial BaTiO 3, A,

E: two-step sintering (1200/1100/5) without additive, B, F:

two-step sintering (1200/1100/5) with a 0.3 wt% Li2CO3-0.04

wt% V2O5 mixed sintering additive, F, G: one-step sintering

(1200/5) without additive, D, H: one-step sintering (1200/5)

with a 0.3 wt % Li2CO3-0.04 wt% V2O5 mixed sintering addi -

tive.

Particle Size Control, Sinterability and Piezoelectric Properties of BaTiO Prepared by a Novel

762 3

Composite-Hydroxide-Mediated Approach

Table 1. Density, grain size and dielectric and piezoelectric properties of the BaTiO3 samples prepared with the (BaCl2 +

TiO2)/(NaOH + KOH) molar ratio of 0.22 and commercial BaTiO3 sintered by the normal one-step sintering (1200/5) and

two-step sintering (1200/1100/5).

Prepared BaTiO3 Commercial BaTiO3

Two-step sintering

(1200/1100/5)

One-step sintering

(1200/5)

Two-step sintering

(1200/1100/5)

One-step sintering

(1200/5)

Without additive With additive Without

additive With additiveWithout

additive

With addi-

tive

Without

additive

With

additive

Density/% 98.1 96.3 99.1 98.3 89.6 97.4 90.2 96.8

Grain size /μm 1 15 5 20 0.5 30 1 80

33 0



(100KHz) 1785 1256 1668 1274 2478 1452 3081 1074

Qm (-) 476 245 584 749 106 811 123 221

kp (-) 0.27 0.29 0.29 0.38 0.16 0.32 0.18 0.34

S (pm2/N) 8.03 4.8 8.0 7.9 15.0 6.3 17.2 7.3

k31 (-) 0.16 0.26 0.19 0.21 0.05 0.15 0.13 0.20

d31 (pC/N) 56.8 63.5 68.9 67.2 31.4 48.7 46.9 59.0

d33 (pC/N) 173 152 226 181 154 183 168 180

tan δ (%) 0.31 0.96 0.38 0.68 1.13 2.21 0.24 0.40

Although it was difficult to densify the comercial powder

without sintering aid by both sintering prcess (Figure 5

E and G), where the mean grain sizes of the sintered

bodies by two-step sintering and one-step sintering were

0.5 and 1 μm, respectively, the prepared BaTiO3 could be

densified without sintering aid (Figure 5 A and C),

where the mean grain sizes of the sintered bodies by

two-step sintering and one-step sintering were 1 and 8

1μm, respectively. It can be observed that the average

grain sizes of the samples prepared by the two-step sin-

tering were smaller than those by normal one-step sin-

tering. Additionally, the addition of a 0.3 wt% Li2CO3

and 0.04 wt% V2O5 complex sintering additive greatly

increased the average grain size and decreased the poros-

ity, especially in the case of the commercial powder.

3.5. Dielectric Properties of Ceramics Bodies

The relative density, grain size, dielectric constant (ε),

dielectric loss (tan

), piezoelectric constant (d31, d33, k31,

S11), mechanical quality factor (Qm) and electr˚Chemical

coupling factor (kp) of the BaTiO3 samples prepared with

the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio of 0.22

and commercial BaTiO3 sintered by the normal sintering

(1200/5) and two-step sintering (1200/1100/5), respec-

tively, are listed in Table 1. All samples could be sin-

tered to almost full theoretical density using a 0.3 wt%

Li2CO3 and 0.04 wt% V2O5 complex sintering additive,

and the dielectric properties of the sample prepared by

the one-step sintering and two-step sintering did not

change so much, indicating that the two-step sintering is

useful to decrease the sintering temperature without loss

of the dielectric properties. It is notable that the samples

using prepared BaTiO3 powders showed lower dielectric

constant and larger piezoelectric constant than those

prepared using the commercial powder, this result can be

illustrated by the grain size effect and the dielectric con-

stant at room temperature decreased with increasing

mean grain size from grain size of 1μm (GS > 1 μm), but

below 1 μm, dielectric constant at room temperature de-

creased with decreasing the mean grain size for the same

sample. Comparing with the samples sintered without

sintering additive, it can be seen that the addition of sin-

tering additive decreased the dielectric constant at the

room temperature, increased dielectric loss and changed

the piezoelectric properties for the specimens sintered by

both one-step sintering and two-step sintering.

4. Conclusions

The size of BaTiO3 particles was controlled by adjusting

the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio during a

composite-hydroxide- mediated approach using a novel

hydrothermal reaction apparatus with a rolling system.

The mean particle diameter decreased from 500 nm to 50

nm with the decrease of the (BaCl2 + TiO2)/(NaOH+KO

H)molar ratio from 0.44 to 0.04, where the sample with

the molar ratio of 0.22 exhibited the excellent sinterabilty

and dielectric properties due to the small particle size,

uniform morphology and narrow size distribution. The

two-step sintering was useful to decrease the sintering

temperature and to obtain the mean grain size without

loss of the dielectric properties.

5. Acknowledgements

This research was partially supported by the Ministry of

Education, Culture, Sports, Science and Technology,

“Special Education and Research Expenses, Post-Silicon

Materials and Devices Research Alliance” and G-COE

program “International Center of Research & Education

Particle Size Control, Sinterability and Piezoelectric Properties of BaTiO Prepared by a Novel 763

Composite-Hydroxide-Mediated Approach

for Molecular Complex Chemistry (IREMC)”.

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