Differential Scanning Calorimetric Analysis of Ni Doped Sodium Hexa-titanate

Pure and nickel doped alkali titanates Na2Ti6-xNixO13 (where x = 0, 0.04, 0.08, 0.12 mol percentage) were synthesized using conventional solid-state reaction method. Phase of the synthesized samples have been confirmed with the help of X-ray diffraction (XRD) patterns recorded at room temperature (RT). Peak positions of all samples (Na2Ti6-xNixO13) exhibited the monoclinic structure. FE-SEM of these samples has been done at 10kV acceleration voltage, in secondary electron mode, at different magnifications to observe morphology (microstructure) and they have been found as rod shaped. Thermal stability of pure and nickel doped titanates was done with the help of differential scanning calorimetry (DSC) in presence of inert gas (nitrogen) from ambient temperature to 800°C, keeping heating and cooling rates 10°C/minute.


METHOD
Addition of sodium carbonate and titanium dioxide (TiO 2 ) in a proper molar ratio gives sodium hexa-titanate, detailed synthesis process has already been published 4,12 . Na 2 CO 3 + 6TiO 2 → Na 2 Ti 6 O 13 + CO 2 ↑ For the preparation of nickel (Ni) doped sodium hexa-titanate (Na 2 Ti 6-x Ni x O 13 ) for different molar concentration (x=0.0, 0.04, 0.08, 0.12), the equations are given below. Na 2 CO 3 + 5.96TiO 2 +0.04NiO→ Na 2 Ti 5.96 Ni 0.04 O 13 + CO 2 ↑ Na 2 CO 3 + 5.92TiO 2 +0.08NiO→ Na 2 Ti 5.92 Ni 0.08 O 13 + CO 2 ↑ Na 2 CO 3 + 5.88TiO 2 + 0.12 NiO→ Na 2 Ti 5.88 Ni 0.12 O 13 + CO 2 ↑ After adding all components, mixture was ground properly for 6 hours. After grinding, mixture was kept in a programmable muffle furnace for 12 h at 900 o C, the rate of heating was fixed at 4 o C per minute. After that the mixture was cooled in the furnace itself up to room temperature. The cooled material was again ground for 30 min 12, 22-23 . It was then ready for characterization. Figure 1 represents the X-ray diffraction patterns of pure and nickel doped NHT samples recorded at room temperature, which reveals single phase with monoclinic structure. Non-existence of extra peaks in the XRD pattern confirms the single phase of the samples and successful doping of nickel in sodium hexa-titanate. Lattice parameters of the samples can also be calculated using equation (1) given below.

X-ray Diffraction
Where symbols have their usual meanings and already been explained in our previous papers 14, 22-24 .

Fig. 1. XRD Analysis of pure and Ni doped NHT
It has been found that a ≠ b ≠ c, and also α = γ = 90 o ≠ β, as the case should be for monoclinic structure, here the value of β is decreasing on increasing the doping percentage of nickel. The unit volume cell gradually decreases on increasing the Ni in pure as shown in Table 1.

FE-SEM Analysis
Morphology of all synthesized materials have been done by field emission scanning electron microscopy (FE-SEM) at 10kV accelerating voltage. All the samples have been analyzed in secondary mode at various magnifications.
The morphological study showed that all the samples have rod shaped in micron range. It has also been confirmed that these samples have hexagonal structure. Figures 2a, 2b, 2c and 2d show the SEM images (microstructures) of all synthesized samples as rod shaped and particle shape of all samples is hexagonal. Fig. 2a is depicting SEM image of pure NHT and edge to edge distance of the particles lies between 0.1 to 0.25µm and length lies between 0.8 to 1.3µm.  Table 2 and they are also in rod shaped. The small change in the length of the rod has been observed, only because of the doping in pure sodium hexa-titanate. The most appropriate reason behind this is atomic radius of the dopant. Atomic radius of titanium is 215 pico-meter while atomic radius of nickel is 163 pico-meter. When nickel replaces the titanium ions, size of the samples also reduces due to the smaller atomic radius of nickel.

DSC Analysis
The rate of change of heat in energy has been measured using differential scanning calorimetry (DSC), with function of temperature. DSC plot gives the information that at which temperature materials release its heat, known as exothermic peak. Thermal stability of pure and Ni doped sodium hexa-titanate was done with the help of DSC in inert gas (N 2 ) atmosphere from ambient temperature to 800°C, with heating rate of 10°C/minute. Fig. 3a gives the information that a transformation occurred nearly 100°C, because of the presence of some moisture or water in the samples.
After adding 4% and 8% nickel in pure sodium hexa-titanate a small exothermic peak has been obtained nearly at 100°C as shown in Fig. 3b & 3c. In 12% nickel doped sodium hexa-titanate an exothermic peak is obtained at 283 shown in Figure 3d.

CONCLUSION
The alkali titanate, sodium hexa-titanate (pure and Ni doped) were synthesized using conventional solid state reaction method and found that these materials having monoclinic structure and single-phase formation.
Morphological study of all samples shows rod shaped structure and it has been found that on increasing Ni dopant in pure sodium hexa-titanate the particle size decreases.
The most probable reason behind this is replacement of titanium with nickel.
The thermal study of all prepared samples confirms that on increasing the doping percentage at 12% molar concentration an exothermic is obtained 283.

ACKNOwLEDgMENT
One of the author is thankful to TEQIP-III of REC Ambedkar Nagar for providing the financial assistanceship for the project.