Mechanochemical Activation Assisted Synthesis of Bismuth Layered-Perovskite Bi 4 Ti 4 O 12

A powder mixture of Bi2O3 and TiO2, both monoclinic, was mechanochemically treated in a planetary ball mill in air atmosphere for different time, using zirconium balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 1 h of milling the starting oxides were transformed fully a nanocrystalline Bi4Ti4O12 phase. With increasing the milling time from 3 to 12 h, the particle size of formed Bi4Ti3O12 did not reduced significantly. That was confirmed by IR and TEM analysis. The electron diffraction pattern indicates that Bi4Ti3O12 crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis. After milling for various times the powders were compacted by pressing and isothermal sintering. The dielectric permittivity of the sintered samples significantly depends on the milling time. Sample milled for 12 h and subsequently sintered at 1000°C for 24 h exhibit a hysteresis loop, confirming that the synthesized material possesses ferroelectric properties.


Introduction
Lead zirconate titanate (PZT) has been used in a wide variety of device applications, such as non-volatile memories and piezoelectric products.However, pollution and the destruction of ecological systems, because of illegally-dumped PZT devices, which release toxic lead, have become serious problems.We are trying to develop a novel lead-free ferroelectric that can act as an alternative to PZT.
We have been studying a bismuth layered ferroelectric (Fig. 1), which is a promising candidate for lead-free ferroelectrics.The perovskite layers are composed of m layers of TiO 6 octahedra and bismuth-oxide layers that are alternately stacked in the crystal structure.It is well known that the family of the bismuth layered compounds can be ferroelectric with an m between 1 and 5. Single -crystal Bi 4 Ti 3 O 12 has low dielectric permittivity and a very high Curie temperature (T c = 675 °C), which makes it useful for various applications such as memory elements, optical displays, and piezoelectric converters of pyroelectric divices in a wide temperature range from 20 °C to 600 °C.Above T c , Bi 4 Ti 3 O 12 possesses a paraelectric tetragonal I4/mmm structure proposed by Aurivillius [1].Below the Curie temperature, the structure shows an orthorhombic Fmmm symmetry, which exhibits ferroelectric properties [2].
Fig. 1 Crystal structure of bismuth layered-perovskite.Bi 4 Ti 3 O 12 ceramics are conventionally prepared by a solid-state reaction process, where oxide mixture of Bi 2 O 3 and TiO 2 is ball milled, calcined at an intermediate temperature and finally sintered at high temperature [3,4].The conventional method requires a high calcination temperature, usually leading to inevitable particle coarsening and aggregation of the Bi 4 Ti 3 O 12 powders.The presence of hard particle agglomerates will also result in poor microstructure and properties of the Bi 4 Ti 3 O 12 ceramics.Many efforts have been made to avoid this problem by lowering the calcination temperature [5].The methods reported in literature to prepare Bi 4 Ti 3 O 12 ceramics include chemistry-based preparations [6], such as precipitation [7], sol-gel [8], hydrothermal [9], and molten salt route [10] and recently mechanically assisted synthesis [11].Wet-chemistry involves chemicals that are sensitive to moisture or light, which makes them difficult to deal with, especially for bismuth salts.In addition, those processes are time consuming and most of the chemistry-based processing routes require post-precursor calcinations at elevated temperature to obtain the precursor-toceramic conversion.It is known that the sol-gel process utilizes expensive precursors and that the drying process is a critical operation.The co-precipitation process is limited to cation solutions with similar solubility constants.However, the polymeric precursor method, which employs complexing of cations in an organic media, makes use of low cost precursors resulting in a homogeneous ion distribution at a molecular level [12].Because of the formation of a polyester resin during the synthesis, there is no segregation of cations during the thermal decomposition of organic matter.
Mechanically activated processes have been recently employed by Benjamin and Gilman to prepare nano-sized oxides and compounds [11,13].In many cases, the mechanical technique is superior to both the conventional solid-state reaction and the wet-chemistrybased processing routes for the ceramic powder preparation for several reasons.It uses lowcost and widely available oxides as starting materials and skips the calcinations step at an intermediate temperature, conducting to simplified process [14].Although mechanical activation has been successful with Pb-based electroceramics of perovskite structure, it has not been studied much for layered structured compounds, such as Bi 3 Ti 4 O 12 ceramics.
The objective of this work was to study the feasibility of Bi 3 Ti 4 O 12 formation and ceramics properties obtained from powders prepared by mechanically activating the constituent oxides.

Experimental Procedure
A synthesis procedure for preparation of bismuth titanate from bismuth oxide Bi 2 O 3 and titanium oxide TiO 2 has been already described in paper [15].These oxide powders exhibited a particle size distribution in the range 2-4 μm for TiO 2 and 1-5 μm for Bi 2 O 3 .Mechanical activation was performed in air atmosphere in a planetary ball mill (Fritch Pulverisette 5) for different milling times: 1, 3, 6 and 12 h.Milling conditions were the following: zirconium oxide jars and zirconium oxide balls, ball-to-powder weight ration 20:1, air atmosphere, basic disc rotation speed was 317 min -1 and rotation speed of disc with jars was 396 min -1 .Sintering of BIT was performed in a chamber furnace in closed system at 1000 °C for 24 h with a heating rate of 10 °C min -1 .
Infrared spectra (IR) on BIT samples were measured in the wavenumber range 400 to 4000 cm -1 using a Fourier transforms IR spectrometer (Bomem MB-102).The spectral resolution was 1 cm -1 .The average power density on the sample was about 2 mW mm -2 .The mechanochemically treated powders for various milling times (1-12 h) were cold pressed under a pressure of 210 MPa into pellets 10 mm in diameter and thickness of about 1.5 mm.All characterizations were done at room temperature.Transmission electron microscopy (TEM, Model Philips CM200) was carried out for particle size and powder morphology analysis, operating at 60.0 kV.The powders were compacted into pellets (with several drops of 10 % ethyl alcohol) at 210 MPa.The electron diffraction pattern of TEM was used to study the coexistence of the crystalline/amorphous phase in synthesized BIT powder [16].EDS -PGT digital Spectrometer observations were made to identify the chemical phase composition of sintered BIT sample and estimated ratio of Bi/O, Ti/O and Bi/Ti.The pellets were prepared by pressing at 210 MPa and sintering in a closed system at 1000 °C for 24 h.To calculate the ratio, an average of a least five measurements on the sintered sample was taken.

Results and Discussion
The Bi 4 Ti 3 O 12 phase evolution was monitored by XRD.Fig. 2a) refers to the mixture of Bi 2 O 3 and TiO 2 , milled for various times (1, 3, 6 and 12 h).It is evident that before mechanical activation, sharp peaks of crystalline Bi 2 O 3 and TiO 2 did not trigger.XRD analysis of BIT powder, treated for different milling times, showed a rapid increase of Bi 4 Ti 3 O 12 phase formation after 1 h of milling time.All peaks appeared to be very broad as a crystallite size reduction [17].Significant structural changes had already occurred after 1h of milling.Such observations suggest the formation of at least one new phase.On increasing the milling time from 1 to 3 h, the intensity of the starting material decreases while that of the product of the reaction increases.Upon 3 h of milling, the observed peaks at 2θ angles 16.2, 21.3, 22.8, 29.6, 32.2, 39.8, 47.1, 51.4,56.6, 61.9 and 69.5° can be attributed to the formation of Bi 4 Ti 3 O 12 .All peaks are very wide, as a result of downsizing and reduction of the grain size and the presence of micro strain.In the period between 1 and 3 h of initial oxide milling, the expected Bi 4 Ti 3 O 12 phase forms, which is shown on the diffractogram for the sample milled 3 h (Fig. 2a).On the diffractogram of the mixture milled 3 h, one can note all typical peaks of Bi 4 Ti 3 O 12 compound.The crystal structure of the Bi 4 Ti 3 O 12 compound can be orthorhombic or tetragonal.From crystallographic data (JCPDS-card 35-0795, orthorhombic) it is possible to conclude that the structure is orthorhombic.However, it is difficult to differentiate between these two structures based only on the analysis of XRD data due to the intense superposition of the broadened peaks.In the period of 3 to 12 h minor changes occurred.The crystallite size was calculated using Scherrer's equation, where τ is the crystallite size, 0.9 (K) is the shape factor, β τ is the line broadening due to the effect of small crystallites and θ is the diffraction angle [18].Here β τ = (B -b), B being the breadth of the observed diffraction line at its half-intensity maximum, and b the instrumental broadening.The values of the crystallite size calculated by Sherrer`s formula for samples milled for 1, 3, 6 and 12 h are about 15.2, 7.3, 7.2 and 6.9 nm, respectively (Fig. 2b).The effect of mechanical treatment on the crystallite size is quite evident: as the milling time increases (1 to 12 h), the powder becomes more activated and the crystallite size decreases (15.2 to 6.9 nm).Fig. 3 shows the IR spectra of the Bi 4 Ti 3 O 12 powders for different milling times (1 -12 h).These IR data are consistent with that reported by Sych and Titov [19].The similar IR spectra of tetragonal and orthorhombic Bi 4 Ti 3 O 12 may indicate the close similarity between the atomic arrangements in the two types of Bi 4 Ti 3 O 12 crystals [20].As the milling time was increased (1 -12 h), bands were gradually disclosed at ≈ 816 and 500 cm -1 corresponding to Ti-O stretching bands [21].Only two strong peaks near 759 and 494 cm -1 exist.These bands have been assigned to the vibrations of TiO 6 octahedra, which indicate that products are well crystallized.With increasing the milling time from 3 to 12 h, the particle size of formed Bi 4 Ti 3 O 12 did not reduced significantly.That was confirmed by SEM and TEM analysis.The existence of powder agglomerates and changes in their size have been confirmed with SEM analysis [22].The particles size is less than 20 nm and shows a strong tendency to agglomeration.TEM images of the bismuth titanate powder milled for 12 h is presented in Fig. 4. The image present in Fig. 4 reveals that the specimens consists of nanocrystalline particles and an amorphous region.and Bi/Ti = 1.32 ( Fig. 5).The obtained results confirm that single phase of Bi 4 Ti 3 O 12 was obtained.
Fig. 5 EDS analysis of BIT sample prepared from mechanically activated powders for 12 h and sintered at 1000 °C for 24 h.Due to the special structure of Bi 4 Ti 3 O 12, the single crystal is strongly anisotropic in all its ferroelectric properties, including saturated polarization (P s ), remnant polarization (P r ) and coercitive field (E c ).The polarization axis forms an angle of ~ 4.5° off the base plane (in the a-c) of its cell structure, thus giving rise to a much larger in-plane polarization (P s = 50 µC cm -2 ) and E c value for in-plane polarization is 50 kV cm -1 and E c value for c-orientation is less than 5 kV cm -1 [23].For randomly oriented Bi 4 Ti 3 O 12 ceramics or thin films, both remnant polarization and coercitive field have values different than those as above presented.Ferroelectricity in the Bi 4 Ti 3 O 12 ceramics was analyzed with a standardized ferroelectric tester and the results are presented in Fig. 6.A sample milled for 12 h and subsequently sintered at 1000 °C for 24 h exhibited a hysteresis loop, confirming that the synthesized material possesses ferroelectric properties.Representative P-E hysteresis loops of Bi 4 Ti 3 O 12 ceramics prepared from powders obtained by mechanochemical synthesis are almost fully saturated with a remnant polarization of 0.65 µC cm -2 and coercitive field of 1050 kV cm -1 .The grain morphology may be the factor causing a rather large coercive field.The difference in the properties of Bi 4 Ti 3 O 12 ceramics obtained by mechanochemical activating processes can be attributed to the variation in microstructure and grain size.The increase of grain size reduces the coupling between grain boundaries and decrease of the domain wall can appear as a result of a more difficult reorientation and domain wall motion.This then translates to an increase in the domain alignment, corresponding to an increase in the values of remnant polarization and in domain wall mobility [23].

Conclusion
Bismuth titanate has been successfully prepared from nano-sized powders via a highenergy ball milling process directly from oxide through mechanochemically assisted synthesis.A single Bi 4 Ti 3 O 12 phase was obtained after 3 h of milling time.BIT was formed with a small crystallite size of 7-16 nm depending on the milling time.After sintering BIT is well crystallized and the orthorhombic structure becomes the dominant phase.The estimated Bi/O, Ti/O and Bi/Ti ratios confirm the chemical nature of the formed phase indicating the formation of a single Bi 4 Ti 3 O 12 phase.The hysteresis loops and values of the coercive field, spontaneous and remnant polarization, indicates the ferroelectric behavior of the obtained ceramic material.

Fig. 2 a
Fig. 2 a) XRD traces of Bi 4 Ti 3 O 12 prepared mechanochemical activation for 1-12 h and b) crystallite size vs. milling time.

Fig. 4
Fig. 4 TEM image of crystalline Bi 4 Ti 3 O 12 powder obtained after milling of 12 h.

Fig. 6
Fig.6 Hysteresis loop for Bi 4 Ti 3 O 12 sample prepared from mechanically activated powders for 12 h and sintered at 1000 °C for 24 h.