Date of Award
Doctor of Philosophy (PhD)
Tritt, Terry M
He , Jian
Marinescu , Catalina D
Among various static energy conversion technologies, the thermoelectric (TE) energy conversion has gained the considerable interest due to its reliability and ability to directly convert waste heat into electricity. In TE conversion technology, physical properties such as thermopower (α), electrical (σ) and thermal conductivity (κ) are exploited simultaneously to convert waste heat into electricity. The efficiency of such conversion depends upon various factors such as temperature, figure of merit (ZT= α2σ/ κ) etc. An inherent coupling among α, σ and κ limits ZT and thereby constrains one in achieving high efficiency. Many efforts have been carried out in decoupling TE properties. The inherent coupling of TE properties resulted in a saturation of the field for several years. Recently, it is realized that the inclusion of multi-length scale defects plays an important role in tuning the TE properties of various materials. Defects are often perceived as imperfections in materials that could adversely affect their performance. On the contrary, because of the limited size scale of nanomaterials, the power of defects could be effectively utilized to selectively scatter phonons and filter low-energy carriers. Hence, it is important to control the length scale and nature of these defects to improve the desired TE properties. This dissertation is focused on answering this important question: `Can one achieve control over the nature and length scale of these defects to decouple and and tune the temperature dependence of ZT in nanostructured bulk materials?' To answer this question, three different materials systems were studied in this work demonstrating the role of various length scales and nature of defects.
Firstly, the effects of extrinsic point defects, such as rattlers (Ce, In, Ba, Yb), dopants (Co, Ni) and secondary phases on FeSb3 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 In0.1Ce0.9Fe3.5Ni0.5Sb12. 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 ball-milling 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 Bi2Te3. 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 Bi2Te3 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.
Puneet, Pooja, "Multi-scale defect engineering and interface modifications for enhancement of thermoelectric properties in nanostructured bulk materials" (2013). All Dissertations. 1178.