Synthesis of BaTio3 nanocrystals by stearic acid gel method

Journal of Alloys and Compounds, 204 (1994) 33-36 JALCOM 922

33

Synthesis of BaTi03 nanocrystals by stearic acid gel method Xiaohui Wang, Chun Zhao, Zhichen

Wang, Fengqing Wu and Muyu Zhao*

Department of Chemistry, Jilin University, Changchun, 130023 (China

(Received July 1, 1993)

Abstract BaTiO3 nanocrystals were synthesized by the stearic acid gel method using stearic acid, barium stearate and tetrabutyl titanate as raw materials; the synthesis temperature was 650 ~'C and the formation temperature of the perovskite oxide was 950 °C by the conventional solid phase reaction method. Transformations of the precursors to gel and the gel to the crystalline phase were characterized by infrared spectroscopy, differential thermal analysis and thermogravimetric analysis. The nanocrystalline BaTiO3 samples were characterized by transmission electron microscopy and X-ray diffraction, and further analyzed by optical emission spectroscopy and X-ray fluorescence techniques to determine impurity contents and Ba:Ti ratios. Using this synthesis method, smaller, uniform-size and high purity BaTiO3 nanocrystals could be obtained.

1. Introduction BaTiO3 is a very important ferroelectric material. Above 0 °C, it exists in three different solid phases: orthorhombic, tetragonal and cubic. For bulk BaTiO3, the transition temperature of the orthorhombic-tetragonal phase (ferro-ferroelectric) is 5 °C, and that of the tetragonal-cubic (ferro-paraelectric) is 120 °C [1]. The dielectric constant of BaTiO3 ceramics strongly depends on the grain size. Arlt et al. [2] observed that the orthorhombic-tetragonal phase transition temperature increases with decreasing grain size. Kanata et al. [3] studied the grain-size effects on the dielectric phase transition of BaTiO3 ceramics with grain diameters from 0.2 ~m to 20 /zm, and found that the orthorhombic-tetragonal phase transition temperature shifts largely towards higher temperature and the tetragonal--cubic one towards lower temperatures [3]. However, studies on BaTiO3 nanocrystals have not yet been reported. We synthesized nanocrystalline BaTiO3 in order to research its ferroelectric property. Commercial BaTiO3 powders have traditionally been prepared by calcination from oxide or carbonate precursors of Ba and Ti. The relatively high calcination temperatures (>/900 °C) needed for the reaction to occur often result in the formation of coarse aggregates which were difficult to disperse. Recently, alternate methods of powder synthesis have been developed and described by many authors [4-8]. Typically, these methods have involved coprecipitation from inorganic (ni*Author to whom correspondence should be addressed. 0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0925 -8388(93)00922-L

trate, chloride, sulfate) or organic (oxalate, citrate salt solutions, from mixed alkoxide precursors or from hydrothermal solutions. We developed the stearic acid gel method (STM) to synthesize nanocrystalline BaTiO3; this synthetic process is easily controlled, and low-cost in comparison with other methods.

2. Experimental details The raw materials utilized for the preparation of BaTiO3 were stearic acid, barium stearate and tetrabutyl titanate. An appropriate amount of stearic acid was melted, into which stoichiometric barium titanate and tetrabutyl titanate were added. Then the solution was cooled slowly to form a gel. By calcining the gel at a temperature higher than 600 °C in air, white BaTiO3 nanocrystals were obtained. Infrared (IR) spectroscopy was used for monitoring the structural changes occurring during the synthetic process. Differential thermal analysis (DTA) and thermogravimetric analysis (TG) were used to characterize the process of crystallization. The powders were characterized in terms of morphology, grain size, crystalline structure and chemical purity. The morphology of the powders was determined with a Hitachi H-800 transmission electron microscope. Grain sizes, crystalline structure and the distribution of grain diameters were determined using a Higaku X-ray diffractometer. The powders were further analyzed by optical emission spectroscopy and X-ray fluorescence techniques to determine impurity content and Ba:Ti ratios.

X.H. Wang et aL / Synthesis of BaTiO 3 nanocrystals by stearic acid gel method

34

3. Results and d i s c u s s i o n

3.1. Characterization of the synthetic process The synthetic process from precursors to nanocrystalline powders can be divided into three stages: precursors--* sol --*gel --*crystals. IR spectroscopy was utilized for monitoring the structural changes occurring during the process. The IR spectra of the raw materials, the complex gel and nanocrystalline powders are shown in Fig. 1. From Fig. 1 we can see the IR spectra of BaTiO3 nanocrystals obtained without the characteristic bands of organic substances. The two small peaks near 1500 cm-1 are the characteristic peaks of BaTiO3 [9]. Figures 2 and 3 show the results of DTA and TG analysis of the gel. In Fig. 2 there are four peaks: the

1

150

50

I

I

,__1

270 390 510 Temperature (°C)

Fig. 3. T G curve of the gel.

e

950°C, Ih

.O

i

20 ,

4000

I

~

!

,

I

2400 1500 850 l Wavenumber (cm-I)

400

Fig. 1. Infrared spectra of the synthetic process: a, stearic acid; b, barium stearate; c, tetrabutyl titanate; d, gel; e, BaTiO3 nanocrystals obtained after heat treatment at 650 °C for 1 h.

Scan Rate: I0.00 C/rain Atmosphere:

02 20cc/min

30

5'0

40

60

28 ° Fig. 4. X R D patterns of BaTiO3 powders calcined at different temperatures in air.

TABLE 1. The average grain sizes of BaTiO3 powders calcined for 1 h at different temperatures in air Temperature (°C)

650

700

750

800

850

900

950

1000

Size (nm)

18.8

22.0

26.5

33.2

47.7

67.4

114.7

250.0

447

o

525

/I

z V

TABLE 2. The average grain sizes of nanocrystalline BaTiO3 after heat-treatment at different temperatures i

30

,50

-

_i

z 'o

,

I

i

I

390--51o

Temperature (°C) Fig. 2. DTA curve of the gel.

Temperature (°C)

100

200

300

400

450

500

600

Size (nm)

18.8

18.8

18.8

18.8

18.8

18.9

18.9

35

X.H. Wang et al. / Synthesis of BaTi03 nanocrystals by stearic acid gel method

TABLE 3. Constituent analysis of BaTiO3 nanocrystals Constituents

BaO TiO2 SrO

ZrOz SiO2 A1203 Fe203

Content (wt.%) 65.57 34.25 0.06 0.01 0.04 0.05 0.02 Mole r a t i o BaO:TiOz= 0.998 200 180 160 140 E qt~ N

120 I00

¢-

(.9

80 60 40. 20

550 6 0 750 850 950 1050 Temperature (°C)

Fig. 5. Curve of grain size dependence on calcining temperatures. first is due to melting of stearic acid; the second and third were caused by burning of organic substances; the last one (447 °C) is regarded as the temperature of the solid-state reaction. All the organic substances burnt out by 450 °C and there is no loss in weight thereafter.

Fig. 6.

3.2. Characterization for the nanocrystalline powders

By heating the gel at a temperature higher than 600 °C in air, nanocrystalline BaTiO3 powders have been obtained. Figure 4 shows the X-ray diffraction (XRD) patterns of BaTiO3 powders calcined at different temperatures. It can be seen that nanocrystalline BaTiO3 was formed at 650 °C and developed with increasing temperature. The structure of BaTiO3 nanocrystal belongs to the cubic perovskite type. It is very interesting that nanocrystalline BaTiO3 was able to maintain a high-temperature cubic phase at room temperature. This led us to find some new properties of BaTiO3. The grain sizes were determined from the full-width half-maximum (FWHM) of the XRD peak [10]. Table 1 shows the average grain sizes of BaTiO3 powders. Figure 5 shows the dependence of grain sizes on calcining temperature. This indicates that it is important to control calcining temperature for preparing nanocrystalline BaTiO3. Figures 6(a)-6(e) show the morphology of nanocrystalline BaTiO3 powders fired at different temperatures. The nanocrystalline BaTiOa powder fired at 650 °C was fine, of uniform-size and spherical; the grain sizes were in the range 10-20 nm. The average grain sizes increased with calcining temperature and the grains became blocky. Figure 7 is the electron diffraction pattern of nanocrystalline BaTiOa calcined at 650 °C in air. The distribution of the grain sizes of BaTiO3 nanocrystals was calculated using data obtained by X-ray small angle scattering technique. The average size of nanocrystalline BaTiO3 calcined at 650 °C for 1 h was 11 nm smaller than that evaluated using the X-ray line broadening method. Figure 8 shows the grain size distribution of BaTiO3 nanocrystals calcined at 650 °C. The thermal stability of nanocrystalline BaTiO3 synthesized at 650 °C was studied. From Table 2, it can

(continued)

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X.H. Wang et al. / Synthesis of BaTi03 nanocrystals by stearic acid gel method

0

12

18

D(nm)

Fig. 8. The grain size distribution of BaTiO3 nanocrystals calcined at 650 °C in air.

Table 3. From this table, it can be seen that nanocrystalline BaTiO3 prepared using STM was of high purity. Fig. 6. T E M images of nanocrystalline BaTiO3 calcined at different temperatures in air: (a) BaTiO3 nanocrystals calcined at 650 °C for 1 h; (b) BaTiO3 nanocrystals calcined at 650 °C for 1 h (dark field image); (c) BaTiO3 nanocrystals calcined at 800 °C for 1 h; (d) BaTiO3 nanocrystals calcined at 850 °C for 2 h; (e) BaTiO3 nanocrystals catcined at 1000 °C for 1 h.

4. Conclusion

A stearic acid gel method was utilized for the preparation of BaTiO3 nanocrystals. The transformations of precursors to sol, sol to gel and gel to crystalline phase were monitored and characterized. The most important step for preparing nanocrystals was control of the calcining temperature. The nanocrystalline BaTiO3 powders obtained were fine, of uniform-size and of high-purity. Acknowledgments

The project was supported by the National Natural Science,Foundation of China and the Doctorial Foundation of the National Committee of Education of China. References

Fig. 7. The electron diffraction pattern of BaTiO3 nanocrystals calcined at 650 °C in air.

be seen that the grain size of nanocrystalline B a T i O 3 hardly changed when heated at different temperatures below 650 °C. Therefore nanocrystalline BaTiO3 has good thermal stability. Chemical impurities and stoichiometric ratios determined by optical emission spectroscopy and X-ray fluorescence techniques for the powders are listed in

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(1%5) 644. 5 6 7 8 9 10

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