Grain size effect on the structural and dielectric properties of Pb0.85La0.15TiO3 ferroelectric ceramic compound
Introduction
Lead titanate, PbTiO3 (PT), is a well-known perovskite-type ferroelectric ceramic characterized by remarkable ferroelectric, piezoelectric and pyroelectric properties [1]. However, it exhibits a structure with a large degree of tetragonality (c/a), i.e., 1.064, resulting in a large stress within the lattice, which hampers the preparation of pure and dense PT ceramics [2]. La3+ has been widely used as a dopant to prepare dense PT solid solutions, and the isomorphic substitution of Pb2+ ions by La3+ ions has improved its mechanical and ferroelectric characteristics [3], [4], [5]. Lead lanthanum titanate, Pb1−xLaxTiO3 (PLT), displays a decrease in both Curie temperature and tetragonality degree as the amount of La increases and the relaxor ferroelectric behavior has been identified in compositions containing more than 24 at.% of lanthanum [1]. These properties have made the PLT system an interesting candidate for technological applications as pyroelectric, piezoelectric or electro-optic devices, optical waveguides, infrared sensors, dynamic random access memories and non-volatile memories [3], [4], [6], [7].
In the last decades, several processing methods to synthesize materials on a nanometer scale have been developed and the study of these nanostructured materials revealed novel properties which cannot be expected for conventional microcrystalline materials [8]. The influence of grain size on the properties of ferroelectric materials has drawn plenty of attention and several studies have been conducted because of their potential technological applications [9]. These studies revealed two critical sizes that exert a major influence on ferroelectricity [10]; one, occurs in the submicron size range and is characterized by a transition from multi-domain to single-domain grains and has been found to be about 300 nm in PbZr0.95Ti0.05O3 crystallites [10]. The second one, involves the disappearance of ferroelectric behavior and studies have reported a critical size as 30 nm for BaTiO3 powder particles and 7–14 nm for PbTiO3 thin films [10].
Nevertheless, most of the studies on the size influence on nanostructured ferroelectric materials have involved the preparation of thin films and relatively simple ceramic compositions, as BaTiO3 [10], [11]. It is well known that the characteristics (purity, size, morphology) of the powder used in the synthesis of dense ceramic samples are crucial to obtain samples with improved properties. Moreover, the use of nanostructured powders has contributed to the sintering process. Plenty of attention has been devoted to the development of new synthesis processes for the densification of nanocrystalline ceramics and, recently, some studies have described the influence of particle size on the ferroelectric properties of ceramic samples [12], [13], [14].
Buscaglia et al. [12] studied dense BaTiO3 nanostructured ceramics varying their average grain size by a spark plasma sintering technique (SPS). Based on dielectric measurements, they showed that the dielectric phase transition progressively assumed a more diffuse character as the particle size decreased [12]. Moreover, a shift in the Curie temperature by decreasing the grain size towards lower values was observed, mainly when the grain size was smaller than 100 nm [12]. Jiang et al. observed a similar behavior when they studied PbTiO3 nanocrystalline ceramics [13].
It has also been shown that the ferroelectric character changes as the average grain decreases [14], [15]. Carreaud et al. [14] observed a decrease in the dielectric maximum resulting in the disappearance of the relaxor state with no change of diffusivity. It is important to note that in this case, they studied ceramic samples with a lower density (around 60%). Algueró et al. [15] observed an evolution from micron-sized lamellar domains towards submicron/nanometer sized crosshatched domains in function of average grain size decrease, resulting in a sample with a certain degree of electrical relaxor behavior.
In the lead-based ferroelectric compounds, the relationship between the local order structure and the ferroelectric properties has been studied in details [16], [17]. However, at the best of our knowledge, the effect of grain size on the short-range order structure and thus on their ferroelectric properties, mainly for complex perovskite ferroelectric ceramic compounds, is not well established.
As the ferroelectric properties have shown to be dependent on the material structure on different scales, the aim of the present study is to provide an analysis of the effect of particle size on the short and long-range order structure and the dielectric behavior of Pb0.85La0.15TiO3 (PLT15) ferroelectric ceramic samples. This composition was chosen because it presents a classic ferroelectric behavior for micrometric grain sizes [1]. Samples of different particle sizes were prepared by the polymeric precursor method modifying their annealing temperature, and their structural, microstructural and dielectric properties were investigated. SPS technique and conventional sintering method were respectively used to prepare dense nano- and micro-crystallized ceramic samples using nanocrystalline PLT15 powder sample.
Section snippets
Experimental procedure
The Pb0.85La0.15TiO3 (PLT15) nanocrystalline powder was prepared by the polymeric precursor method, also referred to as the Pechini method [18]. The details of this preparation, which can be found elsewhere, involve the formation of a polymeric network followed by a heat-treatment at 400 °C for 4 h to eliminate the organic precursors [19]. After this heat treatment, PLT15 nanopowder was annealed at temperatures of 500, 600, 700, 800, 900 and 1000 °C for 2 h. PLT15 samples were labeled as PLT15x,
Results and discussion
Fig. 1(a) shows the FE-SEM image of the PLT15 powder sample calcined at 700 °C for 2 h (PLT15700) which presents an average particle size of 43 ± 5 nm and a significant particle coalescence process. Fig. 1(b) and (c) depict, respectively, the images of the samples annealed at 900 (PLT15900) and 1000 °C (PLT151000). The grains present an average sizes of 157 ± 7 nm for PLT15900 and 191 ± 10 nm for PLT151000. The analyses of the micrographs suggest that larger particles can be formed by a single particle or
Conclusions
The structural and dielectric properties of dense nanostructured and microstructured Pb0.85La0.15TiO3 ferroelectric ceramic samples have been characterized. The XRD results showed a reduction in the degree of tetragonality in the nanostructured ceramic sample whereas XAS and Raman techniques showed that the local order is less affected by the grain size reduction.
The decrease in the average grain size to a nanometer scale induced a diffuse phase transition and a reduction in the permittivity
Acknowledgements
The authors would like to acknowledge R. Camargo (DQ–UFSCar, Brazil) for the SEM measurements. This research was partially developed at LNLS – National Laboratory of Synchrotron Light, Brazil and was supported by FAPESP and CNPq Brazilian agencies.
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