ICT2014 Abstracts

SrTiO₃ is a promising material for thermoelectric energy conversion as it has a high Seebeck coefficient coupled with chemical and thermal stability at high temperatures~ 1000 K. The limitation however is that it is an electric insulator and hence has extremely small power factor. In order to overcome this limitation, Nb⁵⁺ has been substituted for Ti⁴⁺ so that a n-type compound with increased electrical conductivity can be synthesized. Polycrystalline SrNb_xTi_1-xO₃ with x= 0.05, 0.10 and 0.15 has been synthesized by conventional solid-state reaction method to study systematically the effect of Nb⁵⁺ substitution. The phase purity investigated by x-ray diffraction, shows that all the three compounds are of single phase with a cubic crystal structure. The lattice parameter however is found to increase with Nb substitution from 0.3906 nm to 0.3909 nm implying that Nb⁵⁺ indeed substitutes Ti⁴⁺. Scanning electron microscopy indicates the average grain size to be 300~400 nm in the (sintered) pellet with very little porosity. The thermophysical properties, Seebeck coefficient, electrical and thermal conductivity have been measured from room temperature to 1000 K. The Seebeck coefficient in all the three compounds is negative, confirming substitution of Ti⁴⁺ with Nb⁵⁺ and is found to increase from ~70 µVK^-1 to ~200 µVK^-1  with increasing temperature. The Seebeck coefficient however was found to be nearly independent of extend of Ti⁴⁺ substitution. It is found that a minimum of 15 % Ti-substitution with Nb is required to develop measurable electrical conductivity. The conductivity of SrNb_0.15Ti_0.95O₃ reaches a value of 3000 Ω^-1m^-1 at 1000K. The thermal conductivity on the other hand is found to decrease significantly due to substitution. The room temperature thermal conductivity is found to decrease by ~ 40% while that at 1000 K decreases by 90% to 1.1 Wm^-1K^-1..