Teresa Hungría

Sodium niobate ceramics prepared by mechanical activation assisted methods

Abstract Sodium niobate is an interesting material that is nowadays considered to be used as a lead-free piezoelectric ceramic. However, it is very difficult to process fully dense ceramics of this composition starting from crystalline powdered precursors. It seems a priority to develop a new synthesis route for the processing of high quality ceramics. A new synthesis method based on high-energy milling, not yet explored for this type of materials, has been applied to obtain NaNbO 3 . Stoichiometric mixtures of analytical grade Na 2 CO 3 and Nb 2 O 5 were mechanically activated in a vibrating-type mill. A progressive broadening of the different X-ray diffraction peaks was observed when the milling time was increased. The starting products did not become amorphous. The thermal behaviour of the milled samples was studied by high temperature in situ X-ray powder diffraction and thermal analysis techniques. Ceramic materials were processed by natural sintering and hot-pressing or a combination of both. Ferro-piezoelectric properties were measured to assess their practical use as piezoceramics.

Very high remnant polarization and phase-change electromechanical response of BiFeO3-PbTiO3 at the multiferroic morphotropic phase boundary

We have investigated the occurrence of phase-change functional responses in the BiFeO3-PbTiO3 perovskite solid solution, analogous to those anticipated by a recent first-principles study of BiFeO3-BiCoO3. Like the former system, BiFeO3-PbTiO3 shows a morphotropic phase boundary (MPB) between multiferroic polymorphs of rhombohedral and tetragonal symmetries. MPB BiFeO3-PbTiO3 is a high temperature ferroelectric with the phase transition around 900 K, and a room temperature square-shape hysteresis loop with remnant polarization as high as 62 μC cm−2. Strain under the electric field was studied, and a phase-change response was found. Analogous magnetoelectric effects are expected from the multiferroic nature of this MPB.

Mechanosynthesis of the whole xBiFeO3–(1 − x)PbTiO3 multiferroic system: structural characterization and study of phase transitions

Among the recently investigated magnetoelectric materials, those belonging to the solid solution xBiFeO3–(1 − x)PbTiO3 (xBF–(1 − x)PT) seem to be the most promising. However, there is not much information about the characteristics of this system in the whole range of compositions. In this work, the preparation of the system for compositions with 0 ≤ x ≤ 1 is described. The synthesis has been successfully achieved by mechanosynthesis in a high-energy planetary mill. A structural characterization of the mechanosynthesized phases, before and after different thermal treatments, has been carried out by X-ray powder diffraction (XRD), in order to analyze the crystallographic evolution of the phases, correlated to the phase transitions detected by differential thermal analysis (DTA). The structural characterization has allowed the range of compositions of the morphotropic phase boundary (MPB) to be established. First order ferro–paraelectric transitions have been detected by DTA measurements in the whole range of the system and the corresponding Curie temperatures have been determined. Moreover, a comparative microstructural study of the ceramic materials corresponding to the x = 1, 0.7 and 0.6 compositions, prepared by two different processing methods (conventional sintering and spark plasma sintering (SPS)), is reported.

Apparent vanishing of ferroelectricity in nanostructured BiScO3–PbTiO3

Nanostructured ceramics of high-temperature piezoelectric 0.375BiScO3?0.625PbTiO3 were prepared by spark plasma sintering of nanocrystalline powders obtained by mechanosynthesis. The macroscopic electrical properties were characterized on dense ceramics with decreasing average grain size down to 28?nm. Results indicate that the electric field is screened by the electrically insulating grain boundaries at the nanoscale, which needs to be considered when discussing size effects in ferroelectric polycrystalline materials. Moreover, the requirement of increasingly large electric fields to achieve a given polarization with the decrease in the grain size, together with the depletion of the dielectric anomaly associated with the ferroelectric transition until its disappearance, seems to be the result of grain boundary effects.

Study of nanocrystalline BiMnO3-PbTiO3: synthesis, structural elucidation, and magnetic characterization of the whole solid solution.

In the last ten years, the study and the search for new multiferroic materials have been a major challenge due to their potential applications in electronic technology. In this way, bismuth-containing perovskites (BiMO(3)), and particularly those in which the metal M position is occupied by a magnetically active cation, have been extensively investigated as possible multiferroic materials. From the point of view of synthesis, only a few of the possible bismuth-containing perovskites can be prepared by conventional methods but at high pressures. Herein, the preparation of one of these potential multiferroic systems, the solid solution xBiMnO(3)-(1-x)PbTiO(3) by mechanosynthesis is reported. Note that this synthetic method allows the oxides with high x values, and more particularly the BiMnO(3) phase, to be obtained as nanocrystalline phases, in a single step and at room temperature without the application of external pressure. These results confirm that, in the case of Bi perovskites, mechanosynthesis is a good alternative to high-pressure synthesis. These materials have been studied from the point of view of their structural characteristics by precession electron diffraction and magnetic property measurements.

Relaxor behavior in nanostructured Pb(Zn1/3Nb2/3)O3–Pb(Fe1/2Nb1/2)O3–PbTiO3 ceramics

There is controversy about the persistence or not of the relaxor state in the nanoscale. We report here the dielectric properties of nanostructured ceramics of the Pb(Zn1/3Nb2/3)O3–Pb(Fe1/2Nb1/2)O3–PbTiO3 ternary system with an average grain size of 20 nm, which clearly indicate that it persists as long as correlations among polar nanoregions are possible across grain boundaries. Two independent Vogel–Fulcher type relaxations are found in the materials that have a non-negligible width of the size distribution, which are proposed to be associated with intra- and intergranular correlations that show freezing at different temperatures.

Synthesis and Characterization of Sr 2 [Sr n−1 Ti n O 3n+1 ] Series by Mechanochemical Activation

A new synthesis method, based on high-energy milling, has been applied to obtain the n=1 to 3 members of Sr 2 [Sr n-1 Ti n O 3n+1 ] Ruddlesden-Popper series, producing a very important decrease in the temperature and time reactions as compared with the traditional ceramic method. The mechanosynthesis of SrTiO 3 was observed during ball milling processes. Similar results were obtained with vibrating and planetarium milling systems, although the time scales involved are shorter when last one was used. The milled samples were annealed at different temperatures up to the formation of the layered perovskites in one single step. Final products as well as milled powders were studied by thermal analysis, infrared absorption spectroscopy and X-ray diffraction. The microstructure of ceramic samples with Sr 2 TiO 4 and Sr 3 Ti 2 O 7 compositions was investigated.