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PhD Defense-Nitish Kumar

Defect Mechanisms in Bi-based perovskites

Thursday, September 15, 2016 11:00 AM - 1:00 PM

Abstract: Polycrystalline BaTiO3-Bi(Zn1/2Ti1/2)O3 (BT-BZT) ceramics have been shown to exhibit superior dielectric properties for high temperature and high energy density applications as compared to the existing materials. At first their unique properties and suitability to miniaturization have been demonstrated. The energy density values (~2.8 J/cm3 ) were higher than commercially available devices and the permittivity values were insensitive to temperature over a wide temperature range. Next, using resistivity and activation energy values for conduction, it was concluded that the conduction mechanism transitioned from extrinsic for pure BT to intrinsic-like for BT-BZT suggesting a change in the fundamental defect equilibrium conditions. While pure BT exhibits extrinsic p-type conduction, it is reported that BT-BZT ceramics exhibit intrinsic-like n-type conduction, demonstrated using multiple experimental techniques. These results suggested towards a possible unintentional donor doping upon addition of BZT to the solid solution. Several candidates for donor doping were investigated using various advanced techniques. Using the results obtained and knowledge of thermodynamics, it was concluded that the defects in BT-BZT ceramics have an effect of shifting the conductivity minimum in Kröger-Vink diagram to higher oxygen partial pressure values as compared to unmodified BT, resulting in significantly higher resistivity values and n-type conduction in air atmosphere.

The second Bi-based ceramic system investigated was lead-free Bi(Mg1/2Ti1/2)O3-(Bi1/2K1/2)TiO3- (Bi1/2Na1/2)TiO3 for sensors and actuator applications. There has been a huge drive to replace Pb from widely used Pb-based ceramics due to health and environmental concerns. The system exhibited a classic non-ergodic to ergodic transition with change in temperature and composition. The maximum value of high-field piezoelectric coefficient (d33*) observed was 422 pm/V. Fatigue measurements were conducted on all the compositions. Optical and AC impedance measurements and nonstoichiometry were used to conclude that intrinsic conduction was the dominant conduction mechanism in these ceramics. Low defect concentrations and degree of ergodicity were reported to be responsible for their excellent fatigue behavior.

The results obtained provide an important tool to tailor transport properties and defects in these systems, and bring them one step closer to commercial applications.


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