Grain size and grain boundary-related effects on the properties of nanocrystalline barium titanate ceramics
Introduction
Barium titanate BaTiO3 (BT) is one of the ferroelectrics used at the largest scale as base material for multilayer ceramic capacitors, piezoelectric transducers, wireless communication devices, pyroelectric elements and positive temperature coefficient (PTC) sensors. The tendency of the electronic industry towards miniaturization and the need to achieve higher performances in smaller structures lead to high interest in understanding the changes of properties while passing from bulk to the nanosized systems, as well to determine the ultimate structure (particle or grain size (GS), thickness, etc.) whilst still preserving ferroelectric properties.1, 2 The dielectric properties of BT ceramics as a function of GS were extensively investigated, starting in the 1960s, and explained in terms of GS-dependence of the residual stresses, domain wall density, depolarizing fields and surface/interface effects.1, 3, 4, 5, 6, 7 In spite of the large volume of literature dedicated to GS effects in BT ceramics, reliable data have been mostly obtained for ceramics with GS ≥300 nm, due to the difficulty of producing fully dense ceramics below this size. Results previously reported for samples with smaller grains are likely to be strongly affected by the high degree of porosity.1
Dense BT ceramics with grains below 100 nm have been obtained only in recent years, using special densification methods like ultra-high pressure sintering in a multianvil press (GS down to 70 nm, 98% relative density)8 or by a combined sintering method (GS down to 90 nm, 99% relative density).9 A further step towards the production of dense nanocrystalline BT ceramics was made quite recently by using the spark plasma sintering (SPS) technique. Ceramics with GS down to 50 nm and 97% relative density10, 11 were produced by this process. Dielectric measurements8, 9, 10 have shown a significant lowering of the relative dielectric constant in these ceramics in comparison to the coarse ones. This behaviour was interpreted in terms of the presence of a low-permittivity non-ferroelectric GB layer. Since in the studies mentioned above no secondary phases were found, it follows that such a “dead” layer could correspond either to a paraelectric state of BT or to a polar non-switchable state (frozen dipoles) at the surface of the grains.8, 9, 10, 11 However, the real nature of this “modified” region at the GBs and its effect on the properties of the nanosystems is still unclear.
At high temperature BT has a paraelectric cubic (C) perovskite phase; as the temperature decreases, it undergoes three successive transitions to ferroelectric phases with tetragonal (T), orthorhombic (O) and rhombohedral (R) symmetries at around 403, 278 and 183 K, respectively (values characteristics for the single-crystal).12, 13, 14 All these structural modifications correspond to small deformations of the cubic lattice. With decreasing GS, the phase transitions assume a more diffuse character and display a lower transition enthalpy (in particular the R–O and the O–T transitions).1, 4 Although there is not general agreement, there are indications that the relative stability of the different phases is also affected by GS: the C–T transition is shifted towards lower temperatures and the O–T transition to higher temperatures with decreasing GS.1, 9 The possible coexistence of different BT structural modifications in submicron ceramics at room temperature (T and O, C and T) has been also proposed to explain some anomalies observed in the X-ray diffraction (XRD) patterns or discrepancies between XRD and Raman data.4, 15, 16, 17 In coarse BT ceramics, the location of the phase transitions is indicated by anomalies of the relative dielectric constant and of other physical properties.16 However, in nanocrystalline ceramics, this method is less sensitive because the anomalies corresponding to the R–O and R–T transitions are very weak or even absent.1, 12, 13, 14 Raman spectroscopy gives information on the composition and structure of materials, being able to detect modifications of short-range order related to changes in the chemical bond length and angles, thus representing a very useful tool to study order–disorder phenomena and phase transitions.18 Due to its ability to detect local dynamic symmetry in small regions (coherence length lower than 2 nm19), it is particularly useful for probing phase transitions which are barely detectable by other methods, as in case of relaxors,18, 20 to prove non-centrosymmetric local structures in apparent pseudo-cubic nanopowders21 and nanopolar regions in incipient ferroelectrics22 or composition-induced structural transitions between rhombohedral, monoclinic and tetragonal phases in Pb(Zr,Ti)O3 in the range of the famous and still controversial morphotropic phase boundary composition.23 In addition, in case of ultrafine BT structures, mostly the ferro–para phase transition was investigated and no information on the other low-temperature transitions was found in literature.1 In the present work, Raman spectroscopy was used in addition to the XRD, calorimetric and dielectric investigations in order to find evidence of the structural phase transitions in dense nanocrystalline BaTiO3 ceramics.
Section snippets
Experiment
Ultrafine BaTiO3 powders were prepared by precipitation from an aqueous solution of the metallic chlorides in strong alkaline conditions (pH 14) as described elsewhere.24, 25 The specific surface area of the powders was ≈30 m2/g, corresponding to primary particles of ≈35 nm. The Ba/Ti ratio of the powders was controlled to 1.00 ± 0.01, as confirmed by inductively coupled plasma spectroscopy. The main impurities in the powders were Na (≈400 ppm) and Sr (below 50 ppm). The powders were then sintered as
Microstructure and crystalline symmetry
The average GS of the series of samples determined by SEM was 51, 94, 282, 530 and 1200 nm. Representative SEM pictures illustrating microstructures of samples with various GS are shown in Fig. 1. The crystallite size of the nanocrystalline samples was also estimated from the broadening of the (1 1 1) and (2 2 2) XRD peaks, after correction for instrumental broadening, by means of the Scherrer formula. The obtained values are 54 and 96 nm, respectively, in a good agreement with the SEM observations.
Conclusions
Nanocrystalline BaTiO3 dense ceramics down to a GS of 50 nm were prepared from fine nanopowders by SPS. They show all the structural modifications of single-crystal and coarse ceramics (R, O, T and C), with differences resulting from the high density of grain boundaries. With decreasing GS, the phase transitions progressively assume a more diffuse character, taking place in a broader temperature range. A shift of the average Curie temperature with decreasing GS towards lower values (TC ≈ 370 K in
Acknowledgements
This work was performed in the frame of the European COST 525 Action: Advanced Electroceramics–Grain Boundary Engineering. The Italian Ministry of Education, University and Research (PRIN project), the CNCSIS-MEC Romanian grants and the Grant Agency of the Czech Republic also supported this investigation.
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