Doctor of Philosophy (PhD)
Physics and Astronomy
This dissertation is designed to search for new good thermoelectric materials. Considering that both the low dimensionality and high spin entropy are in favor of thermoelectric performance, we focused on several layered transition-metal compounds including Bi2Sr2Co2O9-δ, Sr3(Ru1-xMnx)2O7, BaMn2Sb2 and Ba2Mn3Sb2O2. By investigating their structural, electrical and thermal transport, and magnetic properties, we discuss the relationship between structure, thermoelectric, and magnetic properties in these materials. For Bi2Sr2Co2O9-δ, both single crystal and thin film forms were studied. Among all films, the film grown 675 ℃ has the best power factor (S2/ρ, where S is Seebeck coefficient and ρ is electrical resistivity), which is also much higher than those of single crystals. By annealing single crystal in Ar-atmosphere, S was enhanced while ρ was increased. Together with the almost unchanged thermal conductivity (κ), figure of merit ZT (Z=S2/ρκ) of the annealed single crystal becomes smaller. For Sr3(Ru1-xMnx)2O7, Mn doping causes metal-insulator transition with much higher ρ. On the other hand, S is found to be slightly enhanced, possibly caused by the improvement of symmetry and increase of degeneracy of Ru by reduction of rotation angle of RuO6-octahedra upon Mn doping. Although Mn doping decreases ZT, the dramatic change of ρ with almost unchanged S provides another route to achieve high ZT. S of BaMn2Sb2 is relatively small with twice sign changes, due to coexistence of both types of carriers. Both ρ and κ decrease with increasing temperature. With no structural transition observed from 6 K to 780 K in the single crystal neutron scattering measurement, BaMn2Sb2 is found to be a G type antiferromagnet (TN = 443 K) with easy axis along c direction. Obtained β is 0.34 (1) which is consistent with 3D Ising model, indicating 3D magnetic structure in the layered crystal structure. Compared to layered BaMn2Sb2, Ba2Mn3Sb2O2 has another Mn-O layer in addition to alternative Ba layer and Mn-Sb layer. S also changes sign twice. In contrast to BaMn2Sb2, ρ and κ has opposite trend with much smaller magnitudes. From single crystal neutron scattering measurement, Ba2Mn3Sb2O2 is found to have two spin sublattice. Mn(2) (Mn-Sb) exhibits G-type antiferromagnetism with TN = 314 K while Mn(1) (Mn-O) has a new magnetic ordering below 40 K.
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Diao, Zhenyu, "Thermoelectric Properties of Novel Layered Transition-Metal Compounds" (2016). LSU Doctoral Dissertations. 4458.
Available for download on Saturday, February 23, 2019