Environmental Friendly Potassium Sodium Niobate Based Thin Films from Solutions

Transcription

1 Environmental Friendly Potassium Sodium Niobate Based Thin Films from Solutions Alja Kupec 1,2,3 Barbara Malič 1,3,4,5 and Marija Kosec 1,2,3 1 Electronic Ceramics Department, Jožef Stefan Institute, Ljubljana, Slovenia 2 Centre of Excellence NAMASTE, Ljubljana, Slovenia 3Jožef Stefan International Postgraduate School, Ljubljana, Slovenia 4Centre of Excellence on Nanoscience and Nanotechnology, Ljubljana, Slovenia. 5Centre of Excellence SPACE-SI, Ljubljana, Slovenia. alja.kupec@ijs.si Abstract. We present the synthesis of ~250 nm thick K0.5Na0.5NbO3 thin films on platinized silicon substrates from alkoxide-based solutions with the stoichiometric composition and with the 5 or 10 mole % potassium acetate excess. The films crystallized into a pure perovskite phase, but depending on the amount of the alkali excess in solutions, they consisted of ~50 nm or of ~200 nm large grains. The fine-grained film from the solution with the 5 mole % alkali excess had the dielectric permittivity and losses of 610 and 1.5 %, respectively, and exhibited a ferroelectric polarisation electric field dependence at room temperature. Keywords: Chemical Solution Deposition, Thin film, Lead-free, Ferroelectric 1 Introduction In the field of piezoelectric materials, lead-based complex perovskite systems are widely used due to their good functional response. The main drawback of these materials is the toxicity of lead compounds and, as a consequence, the research of environmentally friendly ceramic materials has been intensified in the last years. Potassium sodium niobate (K x Na 1-x )NbO 3 has been considered as one of the candidates that could replace lead based perovskites. It is a solid solution of ferroelectric KNbO 3 and antiferroelectric NaNbO 3 with the best dielectric and piezoelectric properties near x = 0.5 (KNN).[1] The major problems related to this material are the humidity, sensitivity and volatilization of alkali compounds, which

2 hinder the control over the composition and may contribute to a major reduction of its functional properties. In the Chemical Solution Deposition of thin films, the alkali losses can be compensated by adding the alkali excess to the starting solution. Based on the reports in the literature, the alkali excess may not be needed or it ranges from up to 10 % to as much as 20 %, depending on the synthesis, deposition and further heating conditions. In order to study the influence of different amounts of the K- excess in solutions on the formation and functional response of the films, we deposited the KNN thin films from alkoxide based solutions with the 0.5/0.5/1, 0.5/0.55/1 and 0.5/0.6/1 Na/K/Nb ratios, respectively. 1 Experimental High purity potassium acetate (KO 2 C 2 H 3, 99+%, Sigma Aldrich), sodium acetate (NaO 2 C 2 H 3, 99.5%, Fluka), and niobium pentaethoxide (Nb(OCH 2 CH 3 ) 5, 99.99%, Starck) were weighted in a stoichiometric ratio and dissolved in 2-methoxyethanol. Upon a 4 h reflux and distillation, the solution concentration was adjusted to 0.4 M and 0, 5 or 10 mole % of the potassium-acetate excess was added to the solutions, further denoted as Stoich, +5K and +10K, respectively. Due to the sensitivity of the starting reagents to the moisture, the solution synthesis was performed in a dry nitrogen atmosphere. The ~240 nm thick films on a platinized silicon substrate (or Pt/Si) were processed by a repeated spin coating and pyrolysis at 300 C, 2 min, followed by final annealing at 750 C for 5 minutes, in synthetic air with the heating rate of 10 K/s. The crystalline structures of the films were investigated by the X-ray powder diffraction (PANalyticalX`Pert PRO MPD) and the microstructure was analysed by the scanning electron microscopy (FE-SEM: Supra 35 VP, Carl Zeiss). For the electric characterization of the thin films, Cr/Au top electrodes with the diameter of 0.4 mm were applied through a shadow mask by sputtering and post annealed at 400 C, for 15 minutes. The room temperature dielectric properties (impedance analyzer HP 4192A) and the polarisation versus electric field dependence (AixACCT TF Analyzer 2000) were measured at 300 K. Further details on the processing and characterization methods can be found elsewhere.[2]

3 2 Results Fig. 1 shows that upon heating to 750 C all films crystallized in a pure perovskite phase, regardless the solution chemistry. The asymmetric shape of the peaks in the Stoich KNN film reveals a decreased symmetry of the unit cell. The ratio of relative intensities between {100} and {110} diffraction peaks is inversed in comparison to the XRD pattern of the randomly oriented powder [3], meaning that the film crystallized with the preferential {100} orientation. A similar XRD pattern was obtained for the +5K film. The +10K KNN film also crystallized with the preferential {100} orientation, but the splitting of the {h00} diffraction peaks at 22 and ~45 2 indicated a pronounced monoclinic distortion of the unit cell (characteristic of KNN) and increased crystallite sizes as compared to the Stoich and +5K KNN films. Figure 1: XRD diffraction patterns of the KNN films prepared from solutions with different amounts of potassium acetate excess.* Substrate. The cross sectional and surface microstructures of the films obtained by FE-SEM are presented in Fig. 2. The microstructure of the ~250 nm thick Stoich film consisted of ~50 nm large equiaxed grains. The +5K film had a similar microstructure, but with a much more uniform grain size distribution. In contrast, the +10K film consisted of large grains of cuboidal shape with only one grain per thickness across.

4 Figure 2: The cross-sectional and surface view of the Stoich., +5K and +10K KNN films. Table 1 shows that the room temperature dielectric permittivity ε in Stoich and +5K films are 490 and 610, respectively, at 1 khz, and this value slightly decreases with the increasing frequency. In both films, the losses are lower than 1.6 % in the measured frequency range. The dielectric properties values are in agreement with other reports on KNN thin films.[4] Only poor dielectric properties with high losses were measured in the +10K film, which could be related to the film microstructure. Namely, the grain boundaries, that could provide conduction pathways, are extending across the whole thickness of the film. Table 1: Room temperature dielectric properties of the Stoich and +5K KNN films at 1, 10 and 100 khz. Frequency (khz) Stoich. +5K tan tan The Polarisation-electric field measurements (P-E) of the Stoich, +5K and +10K films at 300 K and 1 khz are collected in Fig. 3. The remnant polarisation (Pr) and coercive field (Ec) of the Stoich KNN film are 5 C/cm 2 and 100 kv/cm, respectively. The ferroelectric properties are slightly improved in the +5K film, reaching the values of the remnant polarisation and coercive field of 8 C/cm 2 and 80 kv/cm, respectively. As expected from the low-field response, the +10K film

5 exhibited a leaky P-E dependence. Wang et al. obtained the values of Pr = 16 C/cm 2 and Ec = 42 kv/cm in about 3500 nm thick KNN films [5], what suggests that the thickness increasing of the +5K film could be advantageous. Figure 3: The polarization versus electric field dependence of the KNN films at 1 khz and at 300 K. 3 Summary Upon a rapid thermal annealing at 750 C, single phase KNN thin films were prepared from the acetate-alkoxide based solutions with the stoichiometric composition and with the 5 or 10 mole % potassium acetate excess. The amount of the potassium excess in solutions contributed to the final properties of the investigated films. The film from the solution with a larger amount of the alkali excess had a columnar microstructure, which consisted of about 200 nm large grains of a cuboidal shape. The grain boundaries extended across the whole thickness of the film and could therefore provide a conduction pathway and contribute to poor dielectric properties. In contrast, the films from the stoichiometric and from the 5 mole % potassium excess solutions, consisted of ~50 nm large equiaxed grains. The addition of a small amount of the potassium excess to the solution, contributed to a more homogeneous microstructure and to a slightly improved functional response. The ~250 nm thick film prepared from the 5 mole % potassium excess solution had the room temperature values of dielectric permittivity, dielectric losses, remnant polarization and coercive field at 1 khz equal to 610, 0.015, 8 C/cm 2 and 80 kv/cm, respectively.

6 References: [1] Y. Saito; H. Takao; T. Tani, et al. Lead-free Piezoceramics. Nature, 432: 84-87, [2] A. Kupec; B. Malič; J. Tellier, et al. Lead-free Ferroelectric Potassium Sodium Niobate Thin Films from Solution: Composition and Structure. Journal of the American Ceramic Society, 95: , [3] J. Tellier; B. Malič; B. Dkhil, et al. Crystal Structure and Phase Transitions of Sodium Potassium Niobate Perovskites. Solid State Sciences, 11: , [4] K. Tanaka; H. Hayashi; K. I. Kakimoto, et al. Effect of (Na,K)-excess Precursor Solutions on Alkoxy-derived (Na,K)NbO3 Powders and Thin Films. Japanese Journal of Applied Physics, 46: , [5] L. Wang; K. Yao; P. C. Goh, et al. Volatilization of Alkali Ions and Effects of Molecular Weight of Polyvinylpyrrolidone Introduced in Solution-derived Ferroelectric K0.5Na0.5NbO3 Films. Journal of Materials Research, 24: , 2009.

7 For wider interest Piezoelectric ceramic materials are used as sensors, actuators and micro-electro mechanical devices (MEMS). The continuous trend in miniaturization of micromechanic and microelectronic components has provided applications for thin films: the nanomaterials with thicknesses of less than 1 m. The properties of thin film-structures often differ from those of bulk ceramics and need to be understood in order to produce new devices. Thin films can be prepared by dry (physical) and wet (chemical) techniques. The former enable the preparation of high quality thin films but with an expensive equipment, while the latter are relatively quick, inexpensive and offer a good variety of possibilities for an easy modification of the composition for improvements in structure properties of functional thin films. The basic steps of Chemical Solution Deposition (CSD) of thin films include the synthesis of the precursor solution, the deposition of the solution on the substrate, and the heat treatment of the deposited film. Among CSD, the alkoxide based solgel route enables the synthesis of different heterometallic solutions and gives the possibility to tailor the reactivity of the starting compounds. The detailed investigations of impacts of precursor solutions, nucleation and growth of the microstructure have led to increase the variety of materials systems that can be prepared and to tremendous improvements in the quality of the films. The lead zirconate titanate based solid solutions (Pb(Zr,Ti)O 3, PZT) are among the most widely studied materials for piezoelectric thin films. However, in the past years the research of lead-free ceramic materials intensified as a consequence of the increased awareness of the society towards the protection of the environment and human health from a hazardous substance, lead.