Ferroelectric perovskite nanopowders obtained by mechanochemical synthesis

Ferroelectric perovskite nanopowders obtained by mechanochemical synthesis Izabela Szafraniak-Wiza1,2,*, Bozena Hilczer2, Ewa Talik3, Adam Pietraszko4, Barbara Malic5 1Institute of Materials Science and Engineering, Poznań University of Technology, M. Skłodowska-Curie Sq. 5, 60-965 Poznań, Poland 2Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland 3Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland 4Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wrocław, Poland 5Electronic Ceramics Department, Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 9 February 2010; received in revised form 6 June 2010; accepted 29 July 2010


I. Introduction
Electroactive perovskites, the best materials for smart structures, sensors and actuators, are conventionally obtained by solid-state reactions or wet-chemistry.The methods usually need the high temperature treatment (even several hundreds degrees Celsius) in order to obtain the proper crystallographic structure.During last decades a big effort has been done on lowering temperature requires for perovskite preparation and highenergy mechanical milling has been successfully used to synthesize various nano-sized powders of ferroelectrics .Nanostructure materials, with grain size of few nanometers, are produced due to repeated deformation caused by high-energy compressive impact forces.The technique can be used to induce chemical reactions in powder mixtures at room temperature, or at much lower temperatures than normally required to synthe-size, because the large volume fraction of atoms in the grain boundaries enhances the diffusion.The processing of the materials is far from the equilibrium conditions and the constitution of the synthesized powder depends not only on the type of the mill and milling container but also on the milling time, milling speed and the ballto-powder weight ratio.The most important parameter of the mechanosynthesis is the time of milling which has to be optimized for the type of mill used and the particular powders as well as the intensity of milling and the ball-to-powder ratio.From technological point of view mechanochemical synthesis seems to be very attractive also because simple oxides can be used as starting materials for oxide perovskites, the method enables strict control of stoichiometry and eliminates undesirable losses of volatile elements.Moreover, mechanochemical synthesis is a low-temperature solid-state processing technique and therefore limitations imposed by phase diagrams do not apply here.It is especially appropriate for synthesis of lead-based complex perovskites with relaxor properties for which the synthesis by mechanical activation is different from thermally-induced solid state reaction [1][2][3][4][5][6][7][8][9].Though there are numerous papers related to mechanosynthesis of ferroelectric perovskites [8][9][10][11][12][13][14][15][16][17][18] the information on the structure of he nanograins are rather poor.There are however few papers related to the structure of nanograins of nickel ferrite and solid solution of nitrogen in ferrite produced by mechanosynthesis [28,29].Here we would like to present our last achievements related to the characterization of nanosized BaTiO 3 , PbTiO 3 and PZT powder prepared by mechanochemical synthesis.

II. Experimental
Commercially available oxides (BaO, PbO, TiO 2anatase and ZrO 2 purchased from Aldrich, 99% purity or higher) were used as starting materials.Mechanochemical synthesis was performed under normal atmospheric conditions using a SPEX 8000 Mixer Mill.The composition of the starting powder mixture corresponded to the stoichiometry reaction.Each batch contained about 6 g of powders.The round bottom stainless vials were used for the mechanical treatment.The weight ratio of hard steel balls to mixed powder was 2 : 1.The mill was run for various periods t m ranging from 2-120 h.
The obtained powders were characterized by Xray diffraction, electron microscopy and X-ray excited photoelectron spectroscopy (XPS).XRD were performed using an X-ray powder diffractometer with Co Kα or Cu Kα radiation at the various stages of milling.The structure of the powder was studied by high resolution transmission electron microscopy (HRTEM) with Philips CM-20 TEM working at the accelerating voltage of 200 kV.XPS spectra were recorded using a PHI 5700/660 Physical Electronics Photoelectron Spectrometer with monochromatized Al Kα X-ray radiation (1486.6 eV).The energy of the electrons was measured by means of a hemi-spherical mirror analyzer with an energy resolution of about 0.3 eV.In every case the neutraliser was used due to a charging effect that occurs for non-conducting samples.The binding energy was determined by reference to the C1s component set at the energy of 285 eV.The shapes of the bands were fitted after background subtraction, using the Gaussian-Lorenzian function.

III. Results and discussion
Figs. 1-3 show the XRD patterns of the starting oxides and their stechiometric mixtures after various time of milling.The mechnochemical syntheses of BaTiO 3 , PbTiO 3 and PbZr 0.2 Ti 0.8 O 3 proceed similarly for all investigated compounds.At the first step of high energy milling all XRD peaks of the staring materials have disappeared.This stage has taken between 2 and 10 hours depending on the compositions.After 10-15 h of synthesis several relatively sharp diffraction peaks,    characteristic of the perovskite structure, appear.Longer duration of mechanochemical synthesis (20-30 h milling) produced material with sharper peaks on diffraction patterns.However, for still longer milling periods (more than 30 h) the peaks become broader.This broadening suggests a significant refinement in the crystalline size.The appearance of these diffraction peaks prove that nanocrystalline perovskite can be obtained at room temperature without any additional crystallization steps.
During preparation of nanopowder sometimes intermediated phases have been observed on in the early stage of mechanosynthesis.This was a case for the PbTiO 3 formation when the traces of Ti 10 O 18 and Pb 3 O 4 were apparent (see Fig. 2b).Fig. 5 presents TEM images of the 50 h milled Pb-TiO 3 sample and Fig. 6 shows high resolution structure of the 120 h milled BaTiO 3 .The crystallites form grains that are irregular loose-packed aggregates with lateral size of 70-150 nm.Detailed analysis made at high resolution showed that the grains are built by nanocrystallites.The size of the crystallites has broad distribution between a few nm and 50 nm.Grains larger than 20 nm have well-developed crystallographic structure (see Fig. 6b).All nanocrystallites are surrounded by thin disordered/amorphous 1-2 nm thick layer.In case of nanopowder with the smallest crystallites this amorphous phase can have even ~40% of the total volume.Because the ferroelectric properties are closely related to the proper crystallographic (polar) structure the presence of core/shell structure may have a big impact.
The binding energies of the electrons, characteristic of a given element, are modified due to the valence electron distribution when the element is involved in a compound and moreover the atoms at the surface are in a different potential than the bulk atoms due to the reduced coordination number.The last effect that results in a core-level shift is considerably apparent for the nano-size particles.The X-ray photoelectron spectroscopy was used to characterize the formation of nanopowder from the respective oxides.Fig. 7 shows the main O1s oxygen and Pb4f lead peaks for PbTiO 3 single crystal and nanopowder after different periods of high-energy milling.The binding energy of 1s oxygen electrons in PbTiO 3 single crystal is equal to 530.3 eV and the full width at half maximum (FWHM) of the core O1s line amounts to 1.7 eV.The Pb4f core line exhibits two maxima due to spin-orbital splitting, which  in PbTiO 3 single crystal are located at 143.9 eV (5/2) and 139.0 eV (7/2).In the beginning of the mechanical synthesis the spectra are complex and point to the presence a lead-rich phase, besides the starting compound PbO.As indicated by XRD, Pb 3 O 4 is present in the activated powder in the early stage of the mechanical synthesis.For the powders activated for t m ≥ 20 h the core lines resemble those of PbTiO 3 single crystal but are broader and accompanied on their low-energy side by small components.The additional bands disappear after recrystallization.We ascribe the additional bands at the low energy side to the grain shells since the atoms at the surface are in different potential than that of the bulk atoms due to broken bonds and different mean electron densities at the ions.The spin-orbita splitting of Ti2p peak into at 464.5 eV (1/2) and 458.7 eV (3/2) has been also observed for BaTiO 3 nanopowder (Fig. 8).The nanopowders obtained by mechanochemical synthesis were used for preparation of electroceramics.
The ceramics was fabricated by hot-pressing method appropriate for producing high density ceramics.Ba-TiO 3 nanopowder was pressed at 20 MPa and 1250°C for 2 h.The microstructure was investigated by scanning electron microscopy (Fig. 9a).The hot-pressing conditions for PbTiO 3 nanopowder were found as 20 MPa for 1150°C and 2 h (Fig. 9b).In both cases dense ceramics have been successfully obtained.

IV. Conclusions
Nanostructure materials, with grain size of few nanometers, are produced due to repeated deformation caused by high-energy compressive impact forces.During mechanochemical synthesis high energy milling reduces the particle sizes (increases in the contact area of reactant particles) and the reaction can proceed without thermally-induced diffusion through the product layer, i.e. during mechanochemical synthesis the solid state reaction occurs at lower temperatures.As a result of milling oxides pure perovskite phase nanopowder is formed.The powder consists of loosely packed grains with a broad distribution of sizes between a few nm and 45 nm.All grains have a shell of thin disordered/amorphous layer the crystalline core that may have impact on final properties.
Fig. 2. XRD patterns recorded with Co K of starting oxides and durations of high energy milling and b) traces of additional ph synthesis (diffractogram recorded with Cu K after 2h of high-e

FigureFigure 2 .Fig. 1 .
Figure 1.XRD patterns of starting oxides and BaTiO 3 after different milling periods (squares indicate peaks related to the perovskite phase)

Fig
Fig. 1.XRD patterns of starting oxides and BaTiO 3 after different millin indicate peaks related to the perovskite phase).

Fig. 1 .
Fig. 1.XRD patterns of starting oxides and BaTiO 3 after different milli indicate peaks related to the perovskite phase).

Figure 7 .
Figure 7. XPS spectra of Pb4f and O1s from PbO and TiO 2 after different milling periods (2, 10 and 50 h) and from PbTiO 3 single crystal

Figure 8 .Fig. 1 .
Figure 8. XPS spectra of Ti2p from: a) BaO and TiO 2 high energy milled for 70 h and b) BaTiO 3 single crystal a)