Multi-scale defect engineering and interface modification for enhancement of thermoelectric properties in nanostructured bulk materials

Firstly, the effects of extrinsic point defects, such as rattlers (Ce, In, Ba, Yb), dopants (Co, Ni) and secondary phases on FeSb 3 and CoSb3 based p-type skutterudites on the transport and magnetic properties is studied. ‘Phonon glass and electron crystal’ like behavior was observed in Ni-doped skutterudites. Interestingly, we found that the addition of In facilitated the formation of secondary phases with various morphologies upon surpassing the filling fraction limits. Such in-situ secondary phases were in fact found to be beneficial to the system altering their electrical transport properties, and thereby increasing the ZT of the system as compared to that of the parent compound. The highest ZT value of 0.9 at 650 K was reported for p-type skutterudite sample with nominal composition In0.1Ce0.9Fe3.5 Ni0.5Sb12. In the low temperature regime (T < 150K), the electrical transport and magnetic susceptibility exhibited single-ion Kondo-like behavior. The crystal field effects due to the splitting of ground state of Ce (4f level) in presence of cubic crystalline field were observed to dictate the magnetic properties below 100 K. Further, our magnetic susceptibility data is consistent with a crystal field splitting gap of ∼39 meV (∼450 K).;The intrinsic surface or interfacial defects in elemental Bismuth were introduced by controlling the surface-to-volume ratio using a combination of high energy ballmilling and spark plasma sintering (SPS) processes. The obtained ball-milled powders were SPS processed with different ON-OFF time ratios of the DC current pulses to further modify the nature and extent of these surfaces. The ‘double decoupling’ (simultaneous optimization of the thermopower, electrical conductivity and thermal conductivity) in single element polycrystalline Bi was observed via a combination of an increase in the surface-to-volume ratio achieved by ball milling process and an interface (or grain boundary) modification by the SPS process. As a result, a greater than six-fold improvement in the PF, and hence ZT, was achieved in polycrystalline bulk Bi samples. Our detailed studies of the effect of SPS conditions on the transport properties of polycrystalline Bi strongly suggests that surface states play a prominent role in enhancing the TE performance of Bi.;Lastly, planar or two-dimensional defects were introduced by chemical exfoliation of layered chalcogenide n-type Bi2Te 3. Particularly, chemical exfoliation allows for the introduction of micro-structured scattering centers at multiple length scales while preserving the basal plane properties needed for high ZT values. Mechanical process such as, grinding, sintering and exfoliation are known to generate donor- like defects. In this method, the possible introduction of positively charged defects (TeBi antisites/Te vacancies) on the grain boundaries resulted in: i) the injection of electrons into the bulk increasing carrier concentration, and ii) a potential barrier that selectively filtered low-energy minority carriers (holes in case of n-type Bi 2Te3 samples) and thereby, shifting the bipolar (two carrier contribution) effects to higher temperatures. This effect is clearly reflected in the thermopower and thermal conductivity data. Thus, the shift in the bipolar effects results in the shift of ZT maxima to higher temperature, where peak ZT is broadened over a wide temperature range of ∼ 150 K. In addition to this, the compatibility factor of our samples exhibits smaller changes over the broad operating temperature regime, making it a good candidate for potential device design. (Abstract shortened by UMI.).