STRUCTURAL STUDIES OF FERROELECTRIC BATIO3 NANO PARTICLES AND VACUUM EVAPORATED BATIO3 NANO SCALE THIN FILMS

Barium titanate (BaTiO3) nanoparticles were prepared by wet chemical method using commercially available materials barium chloride, titanium dioxide and oxalic acid. Nano scale thin films of different thickness were coated on pre cleaned glass substrate by using vacuum evaporation technique under a vacuum of 2x10-5 Torr. The X ray analysis showed that the particles have tetragonal structure. The deposited films of a lower thickness were found to be amorphous in nature, whereas the crystallinity increases with increase of thickness. The estimated value of grain size (D), strain and dislocation density (δ) were also reported in this paper.


INTRODUCTION
The discovery of ferroelectric barium titanate (BaTiO3) opens the present era of ceramic dielectric materials. BaTiO3 having the perovskite structure with tetragonal symmetry at room temperature, possesses a relatively large dielectric constant (ε') and electro optic coefficient. Now-a-days BaTiO3 has become the basic capacitor material in semiconductor technology. BaTiO3 ceramics have a strong piezoelectric effect. These ceramics find wide applications in devices such as microphones, ultrasonic and underwater transducers, sensors and actuators, electro -optic device, multilayer capacitors and spark generators. BaTiO3 is one of the ABO3 type (A = mono or divalent, B = tri-hexavalent ions) ceramic materials which have been examined in search of ferroelectric applications. Due to the desirable properties and applications, over the last few decades, synthesis of BaTiO3 nanopowder and thin film has attracted great attention. Various chemical methods could be employed for the production of these fine particles like sol-gel techniques (Tangwiwat and Milne, 1988), coprecipitation, alkoxide hydrolysis (Kirby et al., 1988), metal-organic processing (Shaikh andVest, 1986), hydrothermal treatment (Boulos et al., 2005) and mechanochemical synthesis (Stojanovic et al., 2005). Wet chemical method is a promising technique that offers relative low cost, uniform size, homogenous powder and high purity of the ceramics. In addition to that different techniques have also been applied to prepare thin film of barium titanate such as r.f.-sputtering (Bhattacharya et al., 1993), pulsed laser ablation (Yoon et al., 1995) and metal-organic chemical vapour deposition (Tahan et al., 1996). Dent et al., have successfully optimized high velocity oxy-fuel (HVOF) spraying for the deposition of barium titanate as dense thick dielectric layer (25-150 μm) and compared the dielectric constant (k) values of these deposits with those (k) values of BaTiO3 layers produced by plasma spraying. The maximum dielectric constant values achieved by HVOF method of deposition are in the range 70-115. Plasma spraying of these materials has produced layers with k values close to 200. However, considerable success has been achieved for both process, some of the problems inherent in each type of deposition are still to be overcome. Despite several techniques have been explored to deposit thin film of BaTiO3, less attention has been devoted to thermal evaporation. In this paper, we have reported about the preparation of nanoscale BaTiO3 thin films by thermal evaporation technique from BaTiO3 nanoparticles synthesised by wet chemical method.
Structure parameters of BaTiO3 nanopartilces and vacuum evaporated nano scale thin films have also been reported.

Synthesis of BaTiO3 nanoparticles
BaTiO3 nanoparticles were synthesized using wet chemical method. The starting materials used were barium chloride (BaCl2.2H2O), titanium dioxide (TiO2) powder and oxalic acid. A solution of barium chloride, titanium dioxide and oxalic acid having mole ratio1: 1: 1 was stirred and evaporated at 80 o C DOI:10.26524/krj55 till a clear, viscous resin was obtained and then dried at 100 o C for 20 hours. The precursor formed was heated at 1000 o C for 2 hours to form BaTiO3 nanoparticles.

BaTiO3 nanoscale thin film preparation
Nano scale thin films of BaTiO3 were prepared by thermal evaporation of BaTiO3 nanoparticles evaporated onto pre-cleaned glass substrate under a vacuum of 2x 10 -5 torr, using a Hind High vacuum coating unit. The growth rate and thickness were measured during growth process by using a quartz crystal oscillator thickness monitor attached inside the vacuum evaporation chamber. The growth rate was adjusted to be as low as 1 Å/sec to avoid the differential evaporation of elements of the alloy.

Structural studies of BaTiO3 nanoparticles and BaTiO3 nanoscale thin films
The XRD patterns of the BaTiO3 nanoparticles and their nanoscale thin films were obtained from Xray powder diffraction with CuKα radiation (λ = 1.5418 Å). The grain size is calculated from the full with half -maximum (FWHM) of the XRD peaks by using Scherrer formula

X -Ray Diffraction Analysis
Where k is the wavelength of the X-rays used, 2θ is the angle between the incident and scattered Xrays, and β is the full width at half maximum. The strain (ɛ) was calculated from the formula ɛ = βcosθ/4 (2) The dislocation density (δ) is defined as the length of dislocation lines per unit volume of the crystal and is given by Table.1 shows the calculated interplanar spacing (d), grain size (D), strain (ɛ) and dislocation density (δ) and the standard'd' values of corresponding predominant peaks. The calculated 'd' values are found to be in agreement with the standard values. The average grain size (D), strain (ɛ) and dislocation density (δ) were found to be 26 nm, 1.40x10 -3 lin -2 m -4 and 1.58x10 15 lin/m 2 .

Fig. 1. XRD Spectrum of BaTiO3 nanoparticles
The X -ray diffraction pattern of the BaTiO3 nano scale thin films of different thickness are shown in figure 2. It reveals that the films of lower thickness (80 nm) appear to be amorphous in nature without well-defined peaks, whereas the films of higher thickness are polycrystalline in nature. It is observed from diffactogram that the crystallites are preferentially oriented along (002) plane of the tetragonal structure. The intensity of the predominant peaks increases with increase in film thickness, indicating the high degree of preferential orientation towards these directions (12). This means that, at the initial state of film formation, i.e.; during the atomistic condensation of the film formation, the deposited atoms are at random orientations. As the film thickness increases, the polycrystalline grains begin to orient along their direction which is evident from the diffractograms of thickness 150 nm and 200 nm. The lattice parameter values determined for the peaks in the diffractograms coincide fairly well with the standard JCPDS data (pattern: 00 -003 -0725). Table. 2 show a comparative look of the grain size, strain and dislocation density of the BaTiO3 films of different thickness. It is observed that the grain size increases with film thickness and attained a value of 23 nm for 200 nm thickness film. Due to the increase in grain size with film thickness, the defect in the lattice are decreased, which is turn reduce the internal microstrain and dislocation density or the columnar grain growth is increased. The strain and dislocation density decreases with increase of thickness, which may be due to increase in crystallinity. Table 1. Structural parameters of BaTiO3 nano particle.

Fig. 2. XRD spectrum of BaTiO3 nano thin film at different thickness
The crystallinity increases with increase of film thickness where as strain and dislocation density decreases with increase of film thickness. The improved crystallinity with temperature and thickness indicated the feasibility of utilising them for sensor applications.

CONCLUSION
Nanoparticle of BaTiO3 were successfully synthesised by low cost wet chemical method using commercially available chemicals such as oxalic acid, TiO2 and BaCl2. Thin films of few hundred nanometer thickness were prepared on well cleaned glass plate for the first time using thermal evaporation method. X-ray analysis showed that the nanoparticles have tetragonal nature and the deposited films at lower thickness have amorphous structure, whereas film of higher thicknesses showed increase in crystallinity.