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The Influences of Water Content on Synthesis of (Pb,Ba)TiO3 Materials using Acetylacetone as Chelating Agent in Sol-Gel Process

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Dublin Institute of Technology

ARROW@DIT

Articles

Crest: Centre for Research in Engineering Surface

Technology

2004-01-01

Preparation of (Pb,Ba)TiO3 powders and highly

oriented thin films by a sol-gel process

Stacey Boland

Suresh Pillai

Wein-Duo Yang

Sossina Haile

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Recommended Citation

Boland, S. W., Pillai, S. C., Yang, Wein-Doh and Haile, S. H. Preparation of highly-oriented (Pb,Ba)TiO3 thin films by a sol-gel process, Journal of materials research. 19, 2004, 1492-1498

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Preparation of (Pb,Ba)TiO

3

powders and highly oriented thin

films by a sol-gel process

Stacey W. Boland, Suresh C. Pillai, Wein-Duo Yang, and Sossina M. Hailea)

Department of Materials Science, California Institute of Technology, Pasadena, California 91125

(Received 17 September 2003; accepted 9 February 2004)

Solid solution Pb1-xBaxTiO3, with particular emphasis on Pb0.5Ba0.5TiO3, was prepared

using a sol-gel process incorporating lead acetate trihydrate, barium acetate, and titanium isopropoxide as precursors, acetylacetone (2,4 pentanedione) as a chelating agent, and ethylene glycol as a solvent. The synthesis procedure was optimized by systematically varying acetylacetone: Ti and H2O:Ti molar ratios and calcination

temperature. The resulting effects on sol and powder properties were studied using thermogravimetric analysis/differential scanning calorimetry, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller analysis, and x-ray diffraction (XRD).

Crystallization of the perovskite structure occurred at a temperature as low as 450 °C. Thin films were prepared by spin coating on (100) MgO. Pyrolysis temperature and heating rate were varied, and the resultant film properties investigated using

field-emission scanning electron microscopy, atomic force microscopy, and XRD. Under optimized conditions, highly oriented films were obtained at a crystallization temperature of 600 °C.

I. INTRODUCTION

It has recently been shown that the ferroelectric te-tragonal distortion of BaTiO3, in which the a and c lattice

parameters differ by 1.1%, can be used for actuation.1 Strains of approximately 0.9% have been demonstrated. Even greater strain is expected for PbTiO3, which

exhib-its a tetragonality of 6.3%. However, this material re-quires a large coercive field to induce domain wall mo-tion and is prone to brittle fracture. Solid solumo-tions of Pb1-xBaxTiO3, or PBT, with intermediate composition

will presumably present a compromise between large ac-tuation and probability of mechanical failure. In order for PBT to be successfully used in microactuation applica-tions and also be integrated into silicon device technol-ogy, it is necessary to synthesize high-quality epitaxial thin films at low temperatures. Furthermore, if crystalli-zation is carried out below the Curie temperature, struc-tural and microstrucstruc-tural changes associated with the cu-bic to tetragonal phase transition can be eliminated. Though there are several widely used techniques for fab-ricating oxide thin films, including physical vapor depo-sition, ion beam sputtering, electron beam evaporation, and pulsed laser ablation, the sol-gel method has been selected here because of its potential for low-temperature crystallization and the possibility of low-cost fabrication. In the sol-gel process, controlled hydrolysis of dissolved

metalorganic precursors followed by a condensation re-action results in the formation of a three-dimensional network of particles.2Key challenges in the sol-gel syn-thesis of PBT are the identification of a solvent system in which multiple metalorganic precursors are mutually compatible and the preparation of a sol stable against uncontrolled hydrolysis. Moreover, though sol-gel and other solution techniques have widely been used for the fabrication of ferroelectric films of PbTiO3and BaTiO3,

3,4

few studies of solution deposition techniques to prepare PBT thin films have been reported.5 We present here a parametrically optimized sol-gel route for the low-temperature crystallization of powder and thin film PBT and examine the impact of various process parameters on crystallization behavior.

II. EXPERIMENTAL

A. Selection of the precursor and solvent system

Two approaches have recently been pursued in the literature as a means of crystallizing the end-member barium titanate from sol-gel methods at temperatures close to ambient. The first, demonstrated by Frey and Payne,6 involves the use of barium and titanium alkox-ides that are relatively stable against rapid hydrolysis, and therefore can be prepared with a high water to metal cation ratio (Rw) for the gelation step. The high water

content apparently ensures that all metal alcohol ligands are completely replaced with metal oxygen bonds during gel formation, and all alcohol by-products are evaporated

a)

Address all correspondence to this author. e-mail: smhaile@caltech.edu

DOI: 10.1557/JMR.2004.0199

J. Mater. Res., Vol. 19, No. 5, May 2004 © 2004 Materials Research Society 1492

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during a drying step carried out under mild heating (50–125 °C). The consequence of eliminating organic components is that barium carbonate, an intermediate product that otherwise requires high-temperature calci-nation for conversion to the perovskite phase, is avoided. Crystallization, using barium and titanium methoxy-ethoxides dissolved in 2-methoxyethanol, has been re-ported to occur at temperatures as low as 120 °C. The second approach, developed by Kuwabara and cowork-ers,7relies on the preparation of highly concentrated al-koxide solutions, without concern for the particular li-gand group used. Such solutions yield very dense gel-structures, which, for reasons that are not entirely obvious, can then readily crystallize at low temperatures. Crystallization at temperatures as low as 50 °C has been obtained when gels (prepared as thin films) were aged under water/alcohol saturated atmospheres.

Low-temperature crystallization of the second end-member in the PBT system, lead titanate, via sol-gel methods has been explored to a much lesser extent. The most important demonstration has been that of Selvaraj et al.,3who obtained oriented thin-film PbTiO3from

so-lutions of lead acetylacetonate and titanium isopropoxide in 2-methoxyethanol at temperatures as low as 425 °C. The use of lead acetylacetonate as opposed to the more typical lead acetate trihydrate apparently ensured the ab-sence of all traces of water from the solution prior to the hydrolysis step and led to more stable gels. It is likely that the acetylacetonate ligand also served as a chelating agent for the titanium in the system, further stabilizing the system. It is not entirely obvious why stable gels should result in lower temperature crystallization.

Synthesis of PBT through sol-gel methods has been studied to an even lesser extent. Indeed, only two reports appear. Meng et al.8 examined particle size effects on Curie temperature for sol-gel prepared materials over the entire range of Pb1-xBaxTiO3. Few synthetic details were

provided, and crystallization was carried out at unspeci-fied temperatures in the range of 500–900 °C. More re-cently, Giridharan and Jayavel5reported the synthesis of

Pb0.8Ba0.2TiO3thin films via a sol-gel route. The starting

materials were barium acetate, lead acetate trihydrate, and titanium butoxide as cation sources, acetic acid as the solvent, and 2-methoxyethanol as the chelating agent. Complete crystallization of films required calcination temperatures of approximately 650 °C (with the onset of crystallization at 400 °C), and films showed a random orientation on the Pt-coated Si and fused quartz sub-strates utilized. Little rationale for the particular chemical system selected was provided.

The studies of the end-member compounds suggest competing approaches to sol-gel systems that yield crys-talline products at low temperatures, which cannot be simultaneously implemented: incorporation of high wa-ter content to avoid carbonate formation; preparation of highly concentrated solutions; and use of an anhydrous lead precursor. Furthermore, the precursors and solvent implied by those earlier investigations, barium methoxy-ethoxide, titanium methoxymethoxy-ethoxide, lead acetylaceto-nate, and methoxyethanol, were found here to be inpatible. That is, a transparent sol based on these com-pounds could not be prepared. In light of this initial result, an extensive survey was carried out to determine a tenable combination of metal sources, chelating agents, and solvents. Ultimately, a system in which lead acetate trihydrate, barium acetate, and titanium isopropoxide serve as metal sources, ethylene glycol as solvent, and acetylacetone as chelating agent, was the first identified for producing PBT precursor sols with good stability, relatively low crystallization temperature, and oriented thin films. A preliminary report has been presented earlier.9

B. Synthesis and characterization

The synthesis procedure used is depicted in Fig. 1. Barium acetate and lead acetate trihydrate were sepa-rately dissolved in ethylene glycol then mixed together to yield a clear solution. Titanium isopropoxide was dissolved in acetylacetone (the chelating agent) and ethylene glycol added to this solution. The acetate and

FIG. 1. FTIR spectra of (a) titanium isopropoxide, acetylacetone, and a mixture of titanium isopropoxide and acetylacetone (RA⳱ 4); and

(b) solutions with RAvalues as indicated.

S.W. Boland et al.: Preparation of (Pb,Ba)TiO3powders and highly oriented thin films by a sol-gel process

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ACKNOWLEDGMENTS

The authors gratefully acknowledge Dr. Chi Ma for his assistance with electron microscopy and Justin S. Boland for his help with atomic force microscopy. This work has been funded by a United States Department of Defense MURI award administered by the Army Research Office. Additional support has been provided by the National Science Foundation, through the Caltech Center for the Science and Engineering of Materials.

REFERENCES

1. E. Burcsu, G. Ravichandran, and K. Bhattacharya, Appl. Phys.

Lett. 77, 1698 (2000).

2. R.J.P. Corriu and D. Leclercq, Angew. Chem. Int. Ed. Engl. 35, 1420 (1996).

3. U. Selvaraj, A.V. Prsadarao, and S. Komerneni, Mater. Lett. 20, 71 (1994).

4. H. Nishizawa and M. Katsube, J. Solid State Chem. 131, 43 (1997).

5. N.V. Giridharan and R. Jayavel, Mater. Lett. 52, 57 (2002).

6. M.H. Frey and D.A. Payne, Chem. Mater. 7, 123 (1995). 7. M. Kuwabara, S. Takahashi, and T. Kuroda, in Better Ceramics

Through Chemistry V, edited by M.J. Hampden-Smith,

W.G. Klemperer, and C.J. Brinker (Mater. Res. Soc. Symp. Proc.

271,Pittsburgh, PA, 1992), p. 365.

8. J.F. Meng, R.S. Katiyar, and G.T. Zou, J. Phys. Chem. Solids 59, 1191 (1998).

9. W-D. Yang, S.C. Pillai, S.W. Boland, and S.M. Haile, in

Ferro-electric Thin Films XI, edited by D.Y. Kaufman, S.

Hoffmann-Eifert, S.R. Gilbert, S. Aggarwal, and M. Shimizu (Mater. Res. Soc. Symp. Proc. 748, Warrendale, PA, 2002) U12.21.1-6. 10. R.T. Brewer, D.A. Boyd, M. El-Naggar, S.W. Boland, S.M. Haile,

D.G. Goodwin, and H.A. Atwater, (204th Mtg. Electrochem. Soc., Orlando, FL, Oct. 2003).

11. C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and

Chemistry of Sol-Gel Processing (Academic Press, New York,

1990), p. 59.

12. R. Caruso, O. de Sanctis, A. Frattini, C. Steren, and R. Gil, Surf.

Coat. Technol. 122, 44 (1999).

13. H.K. Park, D.K. Kim, and C.H. Kim, J. Am. Ceram. Soc. 80, 743 (1997).

14. H.K. Ryu, J.S. Heo, S. Chao, and S.H. Moon, J. Electrochem. Soc.

146,1117 (1999).

15. K. Kitaoka, H. Kozuka, and T. Yoko, J. Am. Ceram. Soc. 81, 1189 (1998).

S.W. Boland et al.: Preparation of (Pb,Ba)TiO3powders and highly oriented thin films by a sol-gel process

J. Mater. Res., Vol. 19, No. 5, May 2004 1498

數據

FIG. 1. FTIR spectra of (a) titanium isopropoxide, acetylacetone, and a mixture of titanium isopropoxide and acetylacetone (R A ⳱ 4); and (b) solutions with R A values as indicated.

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