Characterization and microstructure of highly preferred oriented lead barium titanate thin films on MgO (100) by sol-gel process

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Abstract

Highly preferred oriented lead barium titanate (Pb1−x,Bax)TiO3 thin film, with particular emphasis on (Pb0.5,Ba0.5)TiO3, can be obtained by spin-coating on MgO (100) substrate by using the precursor sol, which was synthesized from acetylacetone chelating with titanium isopropoxide and ethylene glycol as a solvent, in the sol-gel process. Film thickness, pyrolysis temperature and heating rate were studied systemically to investigate their influences on the formation of preferred oriented thin films. The highly preferred (001)/(100) oriented thin film could be obtained by the pyrolysis of wet film at 500 °C and annealing at 600 °C at a slow heating rate of 5 °C/min. It is confirmed that the tetragonal perovskite structure of the titanate ceramic decreases with an increase of Ba content in (Pb1−x,Bax)TiO3. The (001)/(100) oriented films were synthesized from all compositions between x = 0.2 and x = 0.8, at a crystallization temperature of 600 °C. In particular, for the Ba content in the range of x = 0.5∼0.6, highly preferred (001)/(100) planes were observed.

Introduction

The solid solution system of lead barium titanate (Pb1−xBaxTiO3, PBT) is a perovskite ferroelectric, which has been recently reported as having extremely large electrostriction [1]. The PbTiO3 based materials are cubic above their Curie temperature and (001) polarized, showing a tetragonal structure, at room temperature. It has been used to manufacture many electronic and optical devices by utilizing their excellent dielectric, piezoelectric and optical properties [2], [3], [4]. For electrostriction applications, the advantage of the anisotropic properties in these ferroelectric materials will be utilized, therefore, it is very important to prepare high-quality epitaxial (001) thin films [5], [6].

There are various methods for manufacturing thin films, for example physical vacuum deposition [7], ion beam sputtering [8], electron beam evaporation [9], and pulsed laser ablation [10]. Although these vacuum deposition techniques are good methods for the epitaxial growth of PBT thin films, they also have a serious problem that occurs due to the processing complexity of multicomponent systems. Recently, sol-gel processing of titanate thin films has been reported, and it has the advantages of precise composition control and homogeneity, low cost, and a short fabrication process [11]. However, the epitaxial growth of sol-gel film is more difficult than the vacuum deposition. Thus, the growth mechanism of the sol-gel process has not been investigated sufficiently.

Recently, many researches have succeeded in growing highly oriented lead-based perovskite thin films [12], [13], [14], [15]. However, these investigations have been carried out using the expensive and oriented single crystal substrates such as sapphire [16], SrTiO3 [17] and MgO [18] to reduce the lattice mismatch between films and substrates, and the growth of thin films has been usually obtained by pulsed laser deposition (PLD) technique [19]. This method can produce a c axis preferred oriented perovskite films that could control the crystal phase of the films. However, the development of PLD method is limited owing to its inadequacy in growing a large area film and as a result it cannot be used practically.

The challenges in preparing multicomponent oxides such as (Pb,Ba)TiO3 by sol-gel methods ensure that all cationic species are sufficiently soluble in the solvent of choice. It has been reported that acetylacetone (2,4-pentanedione) is an excellent chelating agent with metal alkoxide; significantly modified organometallic precursor leads to better homogeneity, enhanced mechanical and physical properties, and reduced in synthesis temperature [20]. However, the (Pb,Ba)TiO3 thin film by sol-gel methods has been studied to an even lesser extent. Giridharan and Tayavel [21] reported the characterizations of Pb0.8Ba0.2TiO3 thin films via a sol-gel route by using acetic acid and 2-methoxyethanol as the chelating agent. More recently, acetylacetone as chelating agent and ethylene glycol as solvent, were first identified for producing (Pb1−xBax)TiO3 precursor sols with good stability. A highly oriented PBT thin film was successfully manufactured on MgO (100) substrate [22].

In this study, mixed (001)/(100) highly preferred oriented PBT thin films on MgO (100) single crystal substrates were prepared by the sol synthesized from the acetylacetone/ethylene glycol route by a sol-gel method. The growths of thin films have been studied as a function of thin film preparation conditions, such as film thickness, pyrolysis temperature and heating rate. In addition, the effects of Ba content on the characteristics of the deposited films were also examined.

Section snippets

Synthesis of Pb-Ba-Ti-sol

Lead barium titanate thin films were prepared with the general chemical formula (Pb1−x,Bax)TiO3, where x = 0.2, 0.4, 0.5, 0.6, and 0.8, but it was prepared with particular emphasis on (Pb0.5,Ba0.5)TiO3 in this study.

For (Pb0.5,Ba0.5)TiO3 sol preparation, barium acetate (99%, 2.103 g) was dissolved in ethylene glycol. Lead acetate (trihydrate 99%, 3.123 g) was dissolved in ethylene glycol on reflux at 120 °C for 1 h, to decrease the residual water. After reflux, the above two acetate solutions

Results and discussion

By using a minimum amount of ethylene glycol as the modified solvent to dissolve lead acetate and barium acetate, the PBT sols can be prepared without any detectable precipitation after stock at room temperature for 12 months at least. During the thin film preparation, it was observed that the annealing temperature was higher, and the heterogeneous nucleation and growth of perovskite grain at the interface between the film and the substrate occurred much easier, because of a higher thermal

Conclusions

The (Pb1−x,Bax)TiO3 thin film with particular emphasis on (Pb0.5,Ba0.5)TiO3, in a highly preferred (001)/(100) orientation can be obtained on (100) MgO by utilizing the precursor sol, which was synthesized by using acetylacetone chelating with titanium isopropoxide and ethylene glycol as the solvent, controlling the hydrolysis and polycondensation in the sol-gel process. The influences of film thickness, pyrolysis temperature and heating rate on the characterization and microstructure of

Acknowledgments

One of the authors thanks the National Kaohsiung University of Applied Sciences, in Taiwan, ROC, for financial support of his work as a visiting scholar at Caltech, USA. In addition, this work has been funded by the United States Department of Defense MURI award administered by the Army Research Office. Additional support has been provided by the National Science Foundation, through Caltech Center for the Science and Engineering of Materials.

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