Synthesis of submicron PbTiO3 particles by the water-in-oil emulsion process

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ELSEVIER

May 1996

Materials Letters 27 (1996) 13-16

MA

ALS

Law

Rs

Synthesis of submicron PbTiO, particles by the water-in-oil

emulsion process

Chug-Hsin

Lu

* ,

Yann-Pyng Wu

L)epartment of Chemical Engineering, National Taiwan Uniuersiry, Taipei, Taiwan, ROC

Received 29 August 1995; revised 28 September 1995; accepted 3 October 1995

Abstract

Submicron PbTiO, powder with spherical morphology was successfully synthesized by the water-in-oil emulsion process. The aqueous solution containing lead and titanium cations was well emulsified in n-octane solution by adding appropriate surfactant. During the heating of the dried precursors, tetragonal PbTiO, began to form from above 560°C. Heating at 900°C without soaking yielded pure PbTiO,. The particle size of the obtained PbTiO, was within the submicron range, and considerabtf smaller than that prepared by the conventional solid-state reaction route. Agitation time in the emulsification process was found to significantly affect the morphology of PbTiO, powder. Insufficient agitation time resulted in the non-uniform distribution of particle size.

Keywords: Synthesis; PbTiO,; Submicron; Particles; Emulsion; Microstructure

1. Introduction

The properties of (ceramics are greatly affected by the characteristics of the powder, such as particle size, morphology, purity, and chemical composition. Using solution processes, e.g. coprecipitation, sol-gel process, and hydrothermal process, instead of a solid-state mixing process has been confirmed to efficiently control the morphology and chemical composition of synthesized powder. Recently, the emulsion process ha.s been found to be the other promising solution process in the synthesis of fine ceramic powders having a narrow size distribution [l]. Several researchers have utilized this technique

* Corresponding author

to prepare single and multi-component ceramic powder [2-71. The basic concept of the emulsion process is to disperse a solution containing the desirable species in an immiscible liquid. Through adding an appropriate surfactant and using emulsifying treat- ment, the solution having the desirable components can be well dispersed to form tiny liquid droplets in the immiscible liquid. Since each droplet acts as an independent reactor, the morphology of the powder can be easily controlled by the droplet size.

The purpose of this study was to utilize the emulsion process in w/o (water in oil) system to prepare submicron PbTiO, powder. To simplify the synthesis process, the PbTiO, precursors were obtained by directly drying the emulsion solution without adding any precipitant. The crystalline variation and microstructural evolution during the formation process were investigated. Furthermore, the effects 00167-577X/96/$12.0 0 1996 Elsevier Science B.V. All rights reserved

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14 C.-H. Lu, Y.-P. Wu /Materials Letters 27 (19961 13-16

of emulsifying time on the characteristics of PbTiO, powder were also examined.

2. Experimental

The preparation of the aqueous titanium ion solution was similar to Yamamura’s method [8]. TiCl, was dissolved into cold de-ionized water surrounded by an ice bath. Ammonia was added in the solution to precipitate titanium ions. After the precipitates were filtered and washed by de-ionized water repeat- edly to remove chlorine ion, the precipitates were dissolved in HNO, solution to obtain TiO(NO), solution. Then Pb(NO), was added into titanium ion solution at a molar ratio of Pb*+ : Ti4+ = 1 : 1 to obtain the aqueous phase. On the other hand, n-octane was chosen for preparation of the oil-phase. For stabilizing the water-in-oil (w/o) emulsions, 1 wt% of surfactant (Span-80 (Nacalai Tesque, Inc. Japan)) was added into n-octane. After the aqueous phase and the oil phase were prepared, these two phases, mixed at a volume ratio of 3 : 1, were continuously agitated by a homogenizer at 500 rpm for 30 or 70 min for obtaining homogeneous emulsions. Drying was followed to remove the solvents, and then the species present in the emulsions became PbTiO, precursors in solid form.

Differential thermal analysis (DTA) and thermogravimetry analysis (TGA) were conducted at a rate of lO”C/min to detect thermal variations and weight change of the resulting precursors during the heating process. X-ray powder diffraction (XRD) was utilized to identify the compounds present in heat- treated specimens. Scanning electron microscopy (SEMI was used to examine the microstructural evolution and particle size of the specimens.

3. Results and discussion

Fig. 1 shows the DTA and TGA results for PbTiO, precursors prepared by agitating the aqueous and oil phases for 70 min. The endothermic peaks appearing at around 270 and 380°C were attributed to the evaporation of residual organic species. These endothermic reactions were associated with a weight loss of around 4.1%. On further heating, an en-

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Fig. 1. Differential thermal and thermogravimetry analysis of PbTiO, precursors prepared via the emulsion process by agitation for 70 min.

dothennic peak was found at 560°C. No other thermal anomaly appeared at elevated temperatures up to 900°C. For realizing the formation process, PbTiO, precursors were quenched from different temperatures and then analyzed by XRD. Representative XRD patterns for quenched specimens are plotted in Fig. 2. At 500°C PbO and a trivial amount of PbTiO, with undeveloped crystal structure were identified. An unknown phase having a diffraction peak at

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Fig. 2. X-ray diffraction patterns of PbTiO, precursors quenched at (a) 5OO”C, (b) 600°C. (c) 800°C. and (d) 900°C.

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C.-H. Lu, Y.-P. Wu/Materials Letters 27 (1996) 13-16 15 28 = 34” was also found. After heating at 600°C the

amount of PbTiO, was increased, and the crystal structure became well-developed. From the appear- ance of the diffraction peaks corresponding to (OOl), (loo), (lOl), (1 lo), (0021, and (200) planes, the formed PbTiO, was confirmed to exhibit the tetragonal structure. The tetragonal phase gradually evolved with increasing temlperature. Heating up to 900°C without soaking resulted in pure PbTiO, without any residual compounds. According to the results shown in Fig. 1 and 2, the endothermic reaction at 560°C was confirmed to be the formation reaction of PbTiO,.

To reduce the formation temperature for pure PbTiO,, isothermal calcination for 2 h ranging from 500 to 900°C was Iconducted. After calcination at 500°C PbO and a small amount of PbTiO, were formed in the specimens. At 600°C the amount of PbO decreased. After 700°C calcination, pure PbTiO, formed. At 900°C the crystal structure of PbTiO, was developed further. The crystallization behavior of PbTiO, was similar to that of PbTiO, prepared by the coprecipitation process. The complete crystallization of PbTiO, precipitates required calcination at temperatures higher .than 800°C [9].

The microstructure of quenched and calcined specimens was observed via SEM. Below 600°C no specific morphology could be found in the specimens; however, above 600°C the quenched specimens exhibited particulate morphology, indicating the formation of PbTiO,. The microstructure of the specimen quenched at 700°C showed that the particle size of PbTiO, was around 0.1 to 0.2 Km (see Fig. 3a). After calcination at 9OO”C, PbTiO, particles became coarsened, and the particle size was increased to be 0.2 to 0.4 p,rn (see Fig. 3b). In the quenched and calcined specimens, PbTiO, particles exhibited spherical morphology with a narrow size distribution. The EDS analysis was performed to examine the chemical composition of PbTiO, powder. The results indicated that the molar ratio of lead to titanium was approximately unity. Compared with the precipitation process [lo], the emulsion process was easier in terms of controlling the stoichiometry of PbTiO, powder for preparing the pure compound. The effects of agitation time of aqueous and oil phases on the obtained PbTiO, precursors and powder were further investigated. The agitation time

Fig. 3. Scanning electron micrographs of PbTiO, emulsion precursors prepared by agitation for 70 min. (a) Powder quenched at 7OO”C, and (b) powder calcined at 900°C for 2 h.

during emulsion formation was shortened to 30 min. The other procedures were the same as described in Section 2. The DTA and TGA results for this precur- sor were similar to those shown in Fig. 1. An endothermic reaction was also found at around 560°C at which the PbTiO, phase started to form. After 900°C calcination for 2 h, pure PbTiO, powder was obtained. However its morphology was different from that obtained after 70 min agitation as shown in Fig. 3b. The microstructure of the obtained PbTiO, powder is shown in Fig. 4a. This figure reveals that the PbTiO, powder exhibited a non-homogeneous size distribution, with small particles (0.2 to 0.4 km> and large particles (0.8 to 2.0 km>. These results imply that the variation in agitation time did not influence

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16 C.-H. Lu, Y.-P. Wu/Materials Letters 27 (1996) 13-16

Fig. 4. Scanning electron micrographs of 9OO”Ccalcined PbTiO, precursors prepared via the (a) emulsion process by agitating for 30 min, and (b) solid-state reaction process.

the PbTiO, formation process; however, it did affect the morphology of PbTiO, powder. It is considered that the insufficient agitation time, namely, insufficient mechanical force could not evenly disperse the aqueous solution in the oil phase. In addition, the formed droplets might coalesce to yield larger droplets due to insufficient covering of the surfactant on the droplets. As a result, emulsion droplets with different size were formed during the short agitation period resulting in the non-homogeneous size distribution of PbTiO, powder. Apparently, the emulsification process is critical for tailoring the size distribution of PbTiO, powder. The microstructure of PbTiO, powder prepared by solid-state reaction after calcination at 900°C for 2 h is shown in Fig. 4b. The

shape of PbTiO, grains was faceted, and the particle size ranged from 1.8 to 5.6 pm. Comparing the PbTiO, prepared by the emulsion technique and that prepared via a solid-state reaction, it is obvious that the emulsion process can significantly reduce the particle size and results in a narrow size distribution. It is suggested that the emulsion process may allow effective control of the particle size and particle morphology in the synthesis of other complex-cation oxides.

4.

Conclusion

(i> PbTiO, po w d er was successfully synthesized by the w/o emulsion technique in which the aqueous solution containing lead and titanium cations was emulsified in n-octane oil-phase.

(ii) When heating the obtained precursors, PbTiO, with a tetragonal structure began to form from above 560°C. Heating at 900°C without soaking yielded pure PbTiO,. The particle size obtained PbTiO, was within the submicron range. The particle size was much smaller than that obtained by the conventional solid-state process.

(iii> The agitation time for the emulsification process was found to significantly affect the morphology of the obtained powder. Insufficient agitation time resulted in a non-uniform size distribution of particle size.

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