Synthesis and Characterization of Ferroelectric

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Motivation and Background
The large scale application of Ferroelectric materials in day to day life in capacitors, sensors, actuators, transducers and memory application demands novel materials of high dielectric constant with low dielectric loss, good piezoelectric as well as pyroelectric properties. Lead-based piezoelectric ceramics with perovskite structure based on lead zirconate titanate (PZT) are widely used for actuators, sensors as well as microelectronic devices because of their excellent piezoelectric properties. However, because of the high toxicity of lead oxide, the use of the lead-based ceramics has caused serious lead pollution and environmental problems devices. The search for novel environmental friendly ferroelectric material is in demand. Among all lead-free piezoelectric materials Bismuth Titanate (BIT), Bi 4 Ti 3 O 12 BIT is one of them about which I will go to discuss in this Thesis.

Bismuth Titanate
Bismuth titanate was discovered by Aurivilius in 1949 [1]. Many of the compositions from the Aurivillius family remain ferroelectric up to significantly higher temperatures than the well known group of ferroelectric perovskite. For example, Bismuth titanate ferroelectric up to 675 0 C as compared to ~ 380 0 C for lead zirconate titanate. Where the ferroelectric phase allows the alignment of dipole (known as poling) and it is essential for piezoelectricity in ceramics. In which bismuth titanate are phase transition temperature (Curie temperature or T c ) at which Ferroelectricity is lost compared to the high temperature piezoelectric ceramics. It is a ferroelectric material with the chemical formula Bi 4 Ti 3 O 12 . It has a high Curie-Weiss temperature, ~ 685 0 C, high dielectric constant, ~200, and highly anisotropic conductivity [2][3][4][5]. Bismuth titanate has recently been receiving attention because of its potential to replace volatile ferroelectrics such as lead containing PZT, as well as for applications in high temperature [2] environments. Some physical properties of bismuth titanate powders are shown in Table.1 [6].

Structure of BIT
BIT has a layered perovskite structure represented by the empirical formula ABO 3 . The BIT is belongs to the family of ferroelectric materials with layered structures and investigated by Aurivillius in the late 1940s and early 1950s [1].
So that the structure are called Aurivillius phases. The layered structure is constructed by alternative stacking of a triple layer of TiO 6 octahedral (perovskite slab) and a monolayer of (Bi 2 O 2 ) 2+ along the c-axis. Single crystal BIT has a low dielectric permittivity and a very high Curie temperature (T c = 675 0 C).
In fig.1 there are differences in the nature of bonding at these sites [8].   [4] polarization [11]. PTCR properties of Bismuth titanate is most found use as a thermistors e.g. in thermal switches. shown to increase because of frequent interaction with the fine powder particles during tape casting. By avoiding pressure densification techniques and using only [5] a small portion of an isometric particle, TGG is a low-cost option for fabricating textured ceramics.

5.
Kong et al., [16]  properties of the films were qualitatively discussed.
6. Su et al., [17] prepared amorphous bismuth titanate nanoparticles (<50nm) dispersed with poly (hydroxyl ethyl methacrylate) via a sol-gel process. With less than 20% by weight of bismuth titanate in the polymer, the material exhibits high refractive indices (>1.6) and high optical transparency (> 90% transmittance from 530 to 800 nm). Furthermore, the material is highly cross linked has an Improved thermal stability and a lower coefficient of Thermal expansion than that of neat polymer. The material displays a high dielectric constant (>10) without Ferroelectricity. The material has potential applications in optical lenses, optical waveguides and capacitors.
7. Gu et al., [18] synthesized Bismuth titanate powders around the nano meter range using sol-gel process. In this work they give heat treatment, crystallization of the gel into mixture phases from 400°C to 800 o C, formed four phases, at 850 o C, finally transformed to single layered perovskite structure. The particle size of powders increased with calcinations temperature.
8. Shen et al., [19] reported thermal conductivity of c-axis textured polycrystalline perovskite Bi 4 Ti 3 O 12 and random polycrystalline material up to 1000°C. Based on measurements of the thermal diffusivity, density and specific heat, the thermal conductivity is lower along the c-axis than in the a-b plane by almost a factor of 2, and the anisotropy persists up to at least 1000°C despite a [6] change in the crystal structure at 675 °C. The exceptionally low (1.0W/mK), temperature-independent conductivity perpendicular to the perovskite layer structure is attributed to the density difference between the pseudo perovskite and fluorite blocks in the unit cell, forming a natural nano structured superlattice. 9. Slavov et al., [20] synthesized Bismuth-titanate ceramics doped with SiO 2 and 11. A very nice study has been carried out by Lazarevic and their group [22].
They prepared ferroelectric bismuth titanate by using different method, depending if the creation will be film coating or ceramics. They observed the structural properties and various other properties of Bismuth titanate which show a significance dependence on the applied synthesis method.

X-Ray Diffraction (XRD)
In X-ray diffraction or scattering (XRD), X-ray photons are utilized to probe the matter. The energy of the emitted radiation is specific for each element. X-rays were discovered by Roentgen, he called them X-rays because their nature at first was unknown so, X-rays were also called Roentgen rays. The X-rays lie in the range of 0.1 Å < λ < 1000 Å. The penetrating power of X-rays depends on energy.

Principle
X-Ray diffraction effects are observed when electromagnetic radiation impinges on periodic structures with geometrical variations comparable to the length scale of the wavelength of the radiation. X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate and directed towards the sample. X-rays are generated when high velocity electrons impinge on a metal target. Approximately 1% of the total energy of the electron beam is converted into X-ray radiation. In order to get a narrow beam of X-rays, the X-rays generated by the target material are allowed to pass through a collimator which consists of two sets of closely packed metal plates separated by a small gap. The collimator absorbs all the X-rays except the narrow beam that passes between the gaps [23,24]. [8]

Bragg's Law
Diffraction is a scattering phenomenon. When X-rays are incident on crystalline solids then they are scattered in all directions. In some of these directions the scattered beams are completely in phase and reinforce one another to form the diffracted beams as shown in fig.2. The Bragg law describes the conditions under which this would occur. It is assumed that a perfectly parallel and monochromatic X-ray beam of wavelength λ is incident on a crystalline sample at an angle θ [25]. The General relationship between the incident ray of wavelength, incident angle and spacing between two rays are diffracted or crystal lattice planes of particle or atoms is known Bragg's law and given relation in Eq. (1) nλ = 2dSinθ………………………… (1) Where d is the spacing between atomic planes in the crystalline phase and λ is the X-ray wave length. The intensity of the diffracted X-rays is measured as a function of the diffraction angle 2θ and the specimen's orientation. [9]

Scanning Electron Microscope (SEM)
The scanning electron microscopy (SEM) is a useful technique to study the topography and morphology of the materials with much higher resolution. The

Hysteresis Measurement
The polarization versus electric field (P-E) hysteresis loop is one of the most important electrical characteristic of ferroelectric ceramics. Hysteresis loop comes to the various sizes and shapes and which can be used to identify the materials. Hysteresis loops can be information for the understanding of ferroelectric materials. For example the materials with a square-like P-E loop as shown in fig.6 have memory ability. A high remanent polarization is related to the high internal [11]  [12] Induced polarization means high electrostriction strain and high electro optic coefficients. A sudden large change in apparent polarization is usually an indication of incipient dielectric breakdown [26]. The ferroelectric hysteresis loop depends on the temperature that mean the temperature increases the loop of width and height will be changing. At the particular temperature behaves of the materials visible and loops are merging to the straight line. This particular temperature called Curie temperature of ferroelectric materials [27].

Synthesis
Ferroelectric materials are found mainly in three form Nano, Bulk and thin film.
There are different methods for the fabrication of these three different forms of ferroelectric materials as shown in Table. 2
Out of these, sol-gel method has many advantages over other methods and was chosen for the fabrication of Bismuth Titanate. Therefore we choose it for our dissertation.

Sol gel synthesis
The sol-gel process is a wet chemical technique widely used in the field of material science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically a metal oxide). The sol-gel process, as the [13] name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The starting material is processed to form a dispersible oxide and forms a sol in contact with water or dilute acid. During the process, the sol yields the gel, and the sol/gel transition controls the particle size  The fourth step is drying of the gel, when water and other volatile liquids are removed from the gel network. This process is complicated due to fundamental changes in the structure of the gel. The drying process has itself been broken into four distinct steps: (i) the constant rate period, (ii) the critical point, (iii) the falling rate period, (iv) the second falling rate period. If isolated by thermal evaporation, the resulting monolith is termed a xerogel. If the solvent (such as water) is [14] extracted under supercritical or near super critical conditions, the product is an aerogel.  The fifth step involves dehydration, during which surface-bound M-OH groups are removed, thereby stabilizing the gel against rehydration.

Sol-Gel Chemistry
The selection of precursors (i.e. starting compound) is very important in sol-gel process. The ideal compound to be used as precursor should satisfy the following criteria:  It should have high metal content: To minimize the volume change during the change from metal-organic solution to inorganic powder.
 High solubility in common solution with other starting compounds. It should be chemically compatible with other compounds. [15]  Cost effective to produce: as the capital equipment requirements are small (e.g. no high vacuum systems are needed), so the cost of sol-gel process is low, and precursor should not change this advantage.
 Thermally decompose without evaporating, melting or leaving carbon content.

Selection of solution
 Solutions have high vaporization rates: they should evaporate as early as possible. Vaporization depends on vapour pressure and interaction between solute and solvent.
 Solvent must be carefully being selected in order to get solution of high concentration of necessary components, proper viscosity and proper surface tension.

Advantage of sol-gel processing
 Versatile: better control of structure, including porosity and particle size possibility of incorporating nanoparticles and organic materials into sol-gel matrix.
 Extended composition ranges: allows the fabrication of any oxide composition but also some non-oxides and hybrid organic-inorganic materials.
 High homogeneity: due to mixing at the molecular level (chemical nanotechnology) high purity.
 Less energy consumption: no need to reach the melting temperature since a dense of network structure can be achieved at lower temperatures near T c .
 Any shape: thin films and coatings, monoliths, composites, porous membranes and powders and fibers.
 Cheap: no need for special or expensive equipment. It provide a simple, economic and effective method to produce high Quality coatings. [16]

Calcination
The well grinded powders were taken in an alumina crucible for calcinations in POT furnace at particular temperatures according to sample. It is a thermal treatment process applied to ores and other materials in ores and other materials in order to bring about a thermal decomposition phase transition or removal of a volatile fraction.
1. It is a heat treatment process which promotes diffusion.
2. Temperature of calcination must be less than the temperature of melting of the materials, which differs from materials to materials.

T calcination < T melting point
At T calcination Gibb's free energy is Zero. It influences density and electromechanical property of final product, inter diffusion of ions of the constituents, and phase transition. Greater T cal suggests greater homogeneity of the final product, as it can be heated to a greater extent.

Sintering
Density of the electronic ceramic is a very sensitive parameter and that directly affects their properties. Therefore, proper sintering of the pellets is essential for electrical measurement. The pellets were taken on an alumina plate and sintered at different temperatures in POT furnace at a heating rate of 5 o C per minute at 750 0 C, for volatile fraction removal if any in the initial heat treatment. [17] It is the process of consolidation of either loose aggregate of powder or a green compact of the desired composition under controlled conditions of temperature and time.
Densification occurs during sintering and solid state sintering is carried out at temperatures where material transport due to diffusion is appreciable. Atomic diffusion is required, since surface diffusion is required. In between the granules, a small granule is present, when pressured the surface area increases and the material gets densified. Densification process is shown in fig 8.  Chapter 3

Results and Discussion
The structural, morphological and ferroelectric properties are carried out by using XRD, SEM and P-E loop respectively. The Results of these experimental investigations are discussed in the subsequent subsection.

XRD Analysis
Calcined powder of Bismuth titanate was subjected to phase analysis by X-ray diffraction. This is done to know the different phases present in the Calcined 3) Fig.10: XRD pattern of Bismuth Titanate ceramics. [20] where k is the shape factor (k = 0.94), λ is the x-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) in radians, and θ is the Bragg's angle, d is the crystallite size. The crystallite size for Bi 4 Ti 3 O 12 nanoparticles was found to be ~19 nm. The XRD pattern shows that the sample contains no impurity peak and match with JCPDS card number 00-035-0795. The pattern in the fig. 9 shows the clear single orthorhombic phase without any secondary phase.

SEM analysis:
SEM micrograph of Calcined powder of Bismuth Titanate with different magnification is shown in the fig.11. Sample shows good surface morphology with varying clusters ranging from 0.5µ to 1µ. Actually these are not representing [21] the grain size because the grain size evaluated by Scherrer's formula is coming out to be ~19 nm, and here the values are much larger than the grain size.
Therefore, SEM micrograph indicates that each and every clusters appearing on the surface is comprised of large no. of crystallites.

P-E loop measurement
The hysteresis measurement is done to confirm the ferroelectric property of the material. A P-E loop tracer shows a plot of the polarization (P) versus applied ac field to (E) at a given frequency. The spontaneous polarization (P s ), remnant polarization (P r ) and the coercive field (E c ) can be measured by studying the hysteresis loop. Fig.12 shows the polarization vs. electric field curves for samples sintered at 750 o C. It can be seen that the value P r and E c is affected by the externally applied field for the drawing the P-E loop. For the higher field, these values are higher as compare to the lower applied electric field (at 12KV/cm 2 , P r = 0.14451µC/cm 2 , E c = 5.597KV/cm and at 10KV/cm 2 , P r = 0.12070µC/cm 2 , E c = 4.6456 KV/cm) When the field is high then the dipole formed between the titanium and oxygen ion is aligning in a better way as compared to lower field. In this way, the applied electric field is serving the purpose of pre-polling. [22] CHAPTER 4

Conclusions and Future Scope
The conclusions of characterization and synthesis are follows:

Conclusions
The discussed results in earlier chapter can be concluded as follows 1. The sample has been successfully synthesized by sol gel method.
2. The XRD studies has confined the BIT crystallizes into single orthorhombic phase with lattice parameter a = c = 5.4489A o , b= 32.8156A o .
3. The crystalline size is calculated by using Debye Scherrer's formula and it is found to be ~19 nm.
4. The SEM studies are showing the good morphology, but not properly sealed grains. But it is representing the cluster; which is the combination of large number of crystallites.
5. The ferroelectric behavior of BIT is confined by studying the P-E loop.
Since, the electric polling of the sample was not done; therefore it is showing the applied electric field dependent E c and P r . Therefore the value of E c and P r at high field is found to be 5.597KV/cm, 0.14451µC/cm 2 resp.

Future Scope
The studies present in this thesis are very preliminary in nature. Therefore, some valuable and applicable investigation of BIT is required. The following are the suggestion for the future scope of studies: 1. The capacitive behavior of BIT should be investigated in detail by making M-I-M capacitor.
2. The composite of BIT with polymer can be used for applying it in modern electronics.
3. The relaxation of dipoles with electric field and temperature is very