Thermoanalytical and XRD Investigations on Ba6Ti5O18 Ceramics Synthesized by Auto Combustion

Abstract— The cation deficient perovskite (Ba6Ti5O18) has been successfully synthesized using auto combustion technique. Thermal analysis of obtained powders is carried out to investigate the phase development and to optimize the preparation parameters. The evolution of the desired phase, its structure, lattice parameters and crystallite size variation as a function of calcinations/firing temperature have also been studied using XRD analysis.


I. INTRODUCTION
Barium titanates have emerged as one of the leading option for Dielectric Resonator (DR) materials to be used in Microwave Communication Technology (MCT) because of their suitable values of dielectric constant, highest quality factors & low values of temperature coefficients of resonant frequency [1]. Major phases of these complex titanates include barium titanate BaTiO 3 (perovskite), barium tetra titanate (BaTi 4 O 9 ), barium nona titanate (Ba 2 Ti 9 O 20 ) and cation deficient perovskite etc. These complex cubic perovskites have been extensively investigated in literature but very few reports are there on the relatively less popular hexagonal cation deficient perovskites, though they possess best suited dielectric properties for MCT as compared to simple perovskites [2][3]. A number of synthesis routes have been employed to synthesize these materials in literature but all of them suffers from one or the other problems but sol gel based internal combustion method offers a clean, contamination free and easy method for synthesizing these materials. In the present work, we have synthesized Ba 6 Ti 5 O 18 dielectric ceramics using sol gel combustion method, an energy saving technique. The experimental work is described in section II. Thermal analysis and structural analysis of the synthesized compound have also been carried out in this work. In the section III, the results of these analyses and their discussion have been given. Section IV includes the conclusions of the research work described in the paper.

II. EXPERIMENTAL WORK
In the synthesis of Ba 6  ...i According to above equation, for the synthesis of one mole of Ba 6 Ti 5 O 18 , Ba(NO 3 ) 2 , TiO(NO 3 ) 2 and citric acid should be combined in a molar proportion of 6:5:5.88 or we can say that total metal ion to citric acid molar ratio is 1:0.54.

Gelation and Powder Formation
Dilute ammonium hydroxide NH 4 OH is added i h i j h H 6. I yellow transparent solution. The obtained solution is heated on a hot plate at ~90 o C till the gel formation takes place. Actually on heating this solution undergoes thermal dehydration to form yellow transparent gel. On slowly raising the temperature of the hot plate, the gel swells up and gets ignited at around 240 o C. The ignition takes place for approximately 5 seconds with evolution of large volume of gaseous products. This resulted in the formation of black voluminous powder with little carbon residue. The obtained powder is calcined at 600, 900, 1100, 1200 and 1250 o C.

Characterization
DTA/TGA/DTG analysis of uncalcined powder is carried out under nitrogen atmosphere at heating rate 10 0 C/minute using EXSTAR TG/DTA 6300 instrument. X ray diffraction studies of powders obtained after calcinations at various temperatures a

Thermal Analysis
DTA/TG/DTG curve obtained for uncalcined powder is shown in figure 1. The curve can be divided mainly in four parts. First part is a broad exotherm up to 500 o C with weight loss of 6.52%. This weight loss may be due to dehydration processes, bond breaking of organic compounds and formation of Barium carbonate, Ti complex by oxidation and combustion of organic substance [5][6][7]. The increase in amount of BaCO 3 up to 600 o C is also observed in XRD studies (table 1).
The second part is from 500-600 o C with endothermic peaks at 568 and 586 o C and weight loss of 5%. These peaks correspond to melting of Ba(NO 3 ) 2 and its reaction with TiO 2 to form BaTiO 3 [8]. The following reaction represents the chemical change: XRD results also indicate that, Ba(NO 3 ) 2 which was present in uncalcined powder obtained after auto ignition, completely vanishes at 600 o C and amount of BaTiO 3 has increased (table 1). The third part of curve is from 750-875 o C with endothermic peak at 816 o C and weight loss of 4%. This corresponds to complex reaction between BaCO 3 and TiO 2 to form BaTiO 3 [9]. Similar trends are observed in XRD results, the decrease in amount of BaCO 3 and TiO 2 and increase in BaTiO 3 amount within temperature range of 600-900 o C (table 1) suggests the following reaction:  The various inferences that can be drawn from the XRD pattern are shown graphically. The variation in amount of impurities and required phase with temperature is given in figure 3. The graph has been plotted using normalized intensities. The intensity for impure phases has been normalized by 790.92 (BaCO 3 132), the maximum value of intensity for impure phase whereas the intensity of (110) reflection of required compound Ba 6 Ti 5 O 18 has been normalized by its own intensity i.e. 5926 and intensities of all other peaks of the desired compound have been normalized by 1532, the intensity of (1,0,19) reflection, the second maximum in Ba 6 Ti 5 O 18 XRD pattern.
The percentage amount of impurities i.e. Ba(NO 3 ) 2 , BaCO 3 , TiO 2 and BaTiO 3 and that of required Ba 6 Ti 5 O 18 phase has been calculated at various temperatures and is given in table 1. The graphical variation of percentage amount with temperature has been shown in figure 4.  Temperature (

Lattice Parameters
The indexing of peaks and calculation of lattice parameters were done using analytical method for non-cubic crystals.
i h temperature is shown in figure 5 and 6. As observed from above shown figures, the lattice parameters show decrease with rise in temperature. It can also be observed from XRD patterns that with increase in temperature, the peaks for Ba 6 Ti 5 O 18 phase shift toward higher ϴ value. This is due to decrease in lattice parameters.

Crystallite Size
Th Sh ' q i i lculate the crystallite size [11]. For this full width at half maximum (FWHM) was taken for peak with maximum intensity i.e. (   The variation of crystallite size with temperature is shown in figure 7. It is clear from figure that crystallite size for all the planes increases up to 1200 o C and after that it increases for few planes whereas it showed decrease for some planes. The decrease in crystallite size for few planes may be due to inter-planar shift of atoms. This indicates that crystallization is ongoing up to temperature 1250 o C.

IV. CONCLUSION
It can be concluded from above discussion that the impurities and intermediate phases diminish with increase of temperature and the required phase Ba 6 Ti 5 O 18 is formed by metallurgical reaction of BaCO 3 , TiO 2 and BaTiO 3 but with deficiency of oxygen. A well defined XRD pattern of ceramics obtained after calcinations at 1250 o C, having all the peaks indexed and with negligible amount of impurities indicates the formation of pure, single phase and well crystalline Ba 6 Ti 5 O 18 . The values of l i m ‗ ' ‗ ' as determined from XRD, clearly showed that the synthesized materials have structure similar to the cation deficient pervoskites with n=6. The crystallite size exhibited a critical dependence on calcinations temperature.