Powder Metallurgy Progress, Vol.11 (2011), No 3-4 347

PHASE COMPOSITION AND MICROSTRUCTURE OF LEAD-FREE FERROELECTRIC (K, Na)NBO3 THIN FILMS PREPARED BY SOL-GEL METHOD

H. Bruncková, Ľ. Medvecký, P. Hvizdoš

Abstract Environmentally acceptable lead-free ferroelectric KNbO3 (KN) or NaNbO3 (NN) and (K, Na)NbO3 (KNN) thin films were prepared using a modified sol-gel method by mixing K or Na acetates with the polymeric Nb-tartarate complex at 80°C and deposited by spin-coating method on Pt/SiO2/Si substrates. In KN or NN and KNN thin films, the required perovskite KNbO3 or NaNbO3 and K0.65Na0.35NbO3 phases and the additional secondary pyrochlore K4Nb6O17 phase (only in KN film) were revealed after sintering at 650°C. The surface topography and morphology or cross-section of thin films were investigated by the AFM and SEM analysis. In the microstructure of KNN thin films with 100 nm of thickness, the bimodal particle distribution was observed with the small spherical (~50-80 nm) and larger cuboidal particles (~100-150 nm) of the perovskite phase. The compact clusters composed of fine pyrochlore particles ~20-30 nm and perovskite rectangular ~ 100 nm (KN) or needle-like particles ~1 μm (NN) were found. Keywords: sol-gel, ferroelectric, (K,Na)NaNbO3 thin films, spin-coating, pyrochlore, perovskite phase

INTRODUCTION Thin films of alkaline niobates, e.g. the potassium (KNbO3, KN), sodium

(NaNbO3, NN) and (K0.5Na0.5NbO3, KNN), have been intensively studied due to ecological issues [1,2]. The environmental friendly lead-free piezoelectric materials are necessary for applications in the form of thin films in many microelectromechanical systems (MEMS) driven by miniaturization and integration [3]. The CSD method, such as the sol-gel process, is divided into several steps: solution synthesis, coating on a substrate, pyrolysis, crystallization process [4].

The present paper describes the alkaline niobate sols prepared by the modified sol-gel method using polymeric Nb-tartarate complex. The phase composition and the microstructure formation of 2-layered KN, NN and KNN thin films deposited from sols on the Pt/SiO2/Si substrates by spin-coating method and sintered at 650°C were investigated.

EXPERIMENTAL Polymeric Nb-ethylene-glycol-tartarate complex for the KN, NN and KNN film

synthesis was prepared by the modified Pechini PC method [5]. The KNN precursors (sols) were synthesized by the sol-gel method - mixing of the K and Na carbonates dissolved in the acetic acid solution with Nb-complex at 80°C. The mole ratio of K:Na:Nb = 0.5:0.5:1.0. The basic KNN sol was diluted with stabilizer solution (n-propanol and 1,2-propandiol)

Helena Bruncková, Ľubomír Medvecký, Pavol Hvizdoš, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 348 at 1.0 M concentration. Pt/SiO2/Si substrates were spin-coated with the sol precursor at 2000 rpm for 30s and followed by calcining at 400°C for 3 min. The coating-pyrolysis process cycle was repeated twice to obtain 1 and 2-layered thin films. Finally, the films were crystallized via sintering at 650°C for 1 hour in air. Similarly, the KN and NN films were prepared.

The phase composition of KN, NN and KNN films was determined by the X-ray diffraction analysis (XRD) using CuKα radiation. The microstructures of prepared thin films were characterized using a scanning electron microscope (SEM, Jeol-JSM-7000F) equipped with an energy dispersive X-ray (EDX) analyser and atomic force microscopy (AFM, Aicon).

RESULTS AND DISCUSSION The XRD diffractograms of 2-layered KN, NN and KNN thin films deposited on

Pt/SiO2/Si substrates and prepared at 650°C are shown in Figs.1, 2 and 3, respectively. XRD analyses verified formation of the perovskite KNbO3 (PDF4 00-009-0156) and pyrochlore K4Nb6O17 (PDF4 00-014-0287 + PDF4 00-021-1295) phases (Fig.1), pure perovskite NaNbO3 (PDF4 00-014-0603) phase (Fig.2) and pure perovskite K0.65Na0.35NbO3 (PDF4 01-077-0038) phase (Fig.3) with monoclinic symmetry.

Fig.1. XRD diffractogram of the 2-layered KN thin film deposited on Pt/SiO2/Si substrate

after sintering at 650°C (× - perovskite KNbO3 phase, ο - pyrochlore K4Nb6O17.3H2O or ● - K4Nb6O17 phase and s - Pt/SiO2/Si substrate).

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 349

Fig.2. XRD diffractogram of the 2-layered NN thin film deposited on Pt/SiO2/Si substrate

after sintering at 650°C (× - perovskite NaNbO3 phase and s - Pt/Pt/SiO2/Si substrate).

Fig.3. XRD diffractogram of the 2-layered KNN thin film deposited on Pt/SiO2/Si substrate after sintering at 650°C (× - perovskite K0.65Na0.35NbO3 phase and s - Pt/SiO2/Si substrate).

The SEM surface microstructure and cross-section micrographs of 2-layered KN, NN and KNN thin films deposited on Pt/SiO2/Si substrate after sintering at 650°C are shown in Fig.4. KN film (Fig.4a) have heterogenous microstructure, in which two different particle forms the large compact particle clusters (composed of fine particles ~ 30 nm) and coarser rectangular particles ( ~ 100-200 nm). The microporores with size in the range of 100-200 nm are created between rectangular particles as the result of particle sintering and growth. The SEM surface observation of NN thin film showed that the bigger needle-like particles (up to 1 μm), representing agglomerates of smaller spherical particles of ~ 50 nm size (Fig.4b), are surrounded by fine cuboidal, about 100 nm, particles. The small spherical

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 350 particles with diameter of ~50 nm and cuboidal particles size of ~ 80 nm were observed in the dense microstructure of KNN/ film (Fig.4c). Thus, the particle morphology in the KN, NN and KNN film on Pt/SiO2/Si substrates is characterized by the bimodal particle size distribution and films contained spherical and cuboidal particles, whereas the particle size was lower in the case of KNN film. The film thickness increased with the number of layers [6]. The cross-section view of 2-layered KNN film (Fig.4d) shows that the film thickness of 100 nm is quite uniform. As it is shown in the cross-sectional image, the films exhibited a smooth and crack-free surface, composed from densely packed particles.

Tab.1 The values of root mean square roughness (Rg) and average roughness (Ra) over scan areas of 2.5×2.5 μm of the Pt/SiO2/Si substrate, KN, NN and KNN thin films deposited on Pt/SiO2/Si substrate and sintered at 650°C.

sample Rq [nm] Ra [nm] phase composition Pt/SiO2/Si substrate 1 0.8 - KN film 7.4 5.6 KNbO3, K4Nb6O17.3H2O,

K4Nb6O17NN film 15 14 NaNbO3KNN film 11 8.9 K0.65Na0.35NbO3

Fig.4. SEM surface micrographs of (a) KN, (b) NN and (c) KNN thin films deposited on Pt/SiO2/Si substrates after sintering at 650°C and (d) cross-section micrograph of KNN

film.

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Fig.5. 3D AFM topography micrographs of (a) Pt/SiO2/Si substrate and (b) KNN thin film

deposited on Pt/SiO2/Si substrate after sintering at 650°C.

The 3D AFM surface topography micrographs of Pt/SiO2/Si substrate and KNN thin film deposited on Pt/SiO2/Si substrate are shown in Fig.5a, b. The values of root mean square roughness (Rg) and average roughness (Ra) over scan areas of 2.5×2.5 μm of substrate and thin films are present in Tab.1. The results of AFM analyses confirmed that the roughness of Pt/SiO2/Si substrate (~1 nm) was significantly lower than that (~11 nm) of KNN film deposited on Pt/SiO2/Si substrate.

CONCLUSIONS Lead-free ferroelectric 2-layered KNbO3 (KN), NaNbO3 (NN) and (K0.5Na0.5)NbO3

(KNN) thin films were prepared using modified sol-gel method from sols, which were deposited by spin-coating method on Pt/SiO2/Si substrates and annealed at 650°C.

XRD diffractograms showed that the required perovskite KNbO3 phase and the additional secondary pyrochlore K4Nb6O17 phases (in KN film) or pure perovskite phases of NaNbO3 (in NN film) and (K0.65Na0.35)NbO3 (in KNN film) were present.

In the microstructure of KNN thin films with 100 nm of thickness, the bimodal particle distribution was observed with the small spherical (~50-80 nm) and larger cuboidal particles (~100-150 nm) of the perovskite phase. The compact clusters composed of fine pyrochlore ~20-30 nm and rectangular perovskite ~ 100 nm (KN) or needle-like perovskite particles ~1 μm (NN) were found.

Acknowledgement This work was supported by the Grant Agency of the Slovak Academy of Sciences

through project No. 2/0024/11.

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