Developing a Multiband Electronic Band Structure Model and Predictive Maps for Bismuth-Rich Mg3(Sb1-xBix)2 Thermoelectric Materials.

In recent years, bismuth-rich Mg3(Sb1-xBix)2 (x = 0.5-0.8) compositions have generated significant interest due to their excellent thermoelectric (TE) performance near room temperature, making them potential applicants for recovery of low-grade waste heat. The superior performance in these materials is due to its complex electronic band structure (EBS) with presence of multiple near degenerate bands close to the conduction band edge. The position and curvature of these bands strongly depend on the alloy composition, doping amount as well as temperature. Thus, identifying optimal material compositions to get the best TE performance depends on an understanding of the temperature dynamics of EBS and forms the objective of this work. Mg3Sb0.6Bi1.4 (x = 0.7) is chosen for this study due to its reported high near room temperature performance, and compositions with varying doping concentrations (Te used as dopant) have been synthesized. EBS parameters like effective mass and deformation potential of bands, interband separation and band gap values have been estimated using a recently developed refinement approach. Refinement results indicate that the interband separation between conduction bands to be a function of both temperature and doping concentration. Further, thermal conductivity (κ) was estimated for all of the compositions. Utilizing the EBS and κ information, predictive 3D maps indicating the variation in zT (TE figure of merit) with doping concentration and temperature have been generated. The 3D maps reveal an interesting surface topography with a broad peak zT region. This observation explains why these materials have high TE performance and are less sensitive to doping inhomogeneities. Our results provide detailed EBS information and fundamental insights on the TE properties of Mg3Sb0.6Bi1.4. Further, the proposed technique can be utilized to probe other Mg3(Sb1-xBix)2 compositions and TE materials.

Corrosion-Resistant Hydrophobic Thermal Barrier Composite Coating on Metal Strip: A New Dimension to Steel Strips for Roofing Segment.

This study demonstrates a cost-effective, thin, multifunctional composite coating system with outstanding thermal insulation for thermal management and heat shield applications, such as roofs, as well as outstanding resistance to corrosion. The hydrophobic multifunctional epoxy composite coating systems were designed with surface-modified fillers to impart both reduced heat conduction and high infrared reflectance in a thin coating with a 65-100 µm dry film thickness (DFT). With a judicial combination of hollow microspheres (HMS) activated and modified with silica (sHMS) and stearic acid-modified TiO2 (sMO), the developed composite coating attained the highest thermal insulation property with a temperature drop of 21-31 °C at different distances below the coated panel, which is superior to the values of temperature drop reported earlier. The high solar reflectance of the composite coating in the near-infrared (NIR) region exceeds 72% with a low thermal conductivity of 0.178 W m-1 K-1. After 720 h of exposure in a 3.5 wt % NaCl solution, the composite coating revealed a corrosion protection efficiency of 99%. The work demonstrates that high solar reflectivity and low thermal conductivity must be active simultaneously to achieve superior thermal shielding in a thin coating on a metal. A careful selection of fillers and appropriate surface modifications ensures hydrophobicity and proper distribution of the fillers in the coating for a high barrier effect to prevent environmental deterioration. With these superior performance parameters, the developed composite coatings make an essential contribution to energy sustainability and the protection against environmental degradation.

Strained Lamellar Structures Leading to Improved Thermoelectric Performance in Mg3Sb1.5Bi0.5.

Mg3Sb2-xBix solid-solutions represent an important class of thermoelectric (TE) materials due to their high efficiency and variable operating temperature range. Of particular significance for midtemperature applications is the Mg3Sb1.5Bi0.5 composition whose superior thermoelectric (TE) performance is attributed to the complex conduction band edge in conjunction with alloy dominated phonon scattering. In this work, we show that microstructure also plays a significant role in lowering the lattice thermal conductivity which in turn affects the overall TE performance (change in peak zT values between 1.1 and 1.4 have been observed). Temperature dependent TE properties of Mg3+xSb1.5Bi0.5 compositions with varying nominal Mg content (x = 0.2, 0.3, 0.4) have been studied. A marked reduction of the lattice thermal conductivity (κL) is observed in compositions with low nominal Mg content (x = 0.2), which is due to the presence of lamellar structures within the grains. These lamellar regions are isostructural to the matrix with a low misfit angle and represent compositional fluctuations in the Bi to Sb ratio. Both the size (200 nm-500 nm) and the interfacial strain contribute to the enhanced phonon scattering. A quantitative estimate of κL reduction due to these structures have been carried out using a mean free path (MFP) spectrum analysis which reveal a good match with experiments at room temperature. Further, the electrical properties are not influenced by these lamellar structures as observed from the similar power-factor (S2σ) and weighted mobilities in all of the compositions. This is due to their similar orientation to the adjacent matrix region. Thus, the zT parameter in the various compositions with similar carrier concentration can be significantly altered (∼25%) by adjusting the nominal Mg content. The results demonstrate that preferential phonon scattering by microstructure modification can be a new route for property improvement in Mg3+xSb2-yBiy solid-solutions.

Highly Stable Metal─Na0.02Pb0.98Te Contacts for Medium Temperature Thermoelectric Devices.

In the medium temperature (600-850 K) range, Na0.02Pb0.98Te is a highly efficient p-type thermoelectric compound. Device fabrication utilizing this compound for power generation demands highly stable low-contact resistance contacts with metal electrodes. This work investigates the microstructural, electrical, mechanical, and thermochemical stability of Na0.02Pb0.98Te-metal (Ni, Fe, and Co) contacts made by a one-step vacuum hot pressing process. Direct contact mostly resulted in either an interface with poor mechanical integrity, as in Co and Fe, or poisoning of the TE compound, as in the case of Ni, which results in high specific contact resistance (rc). In Ni and Co, adding a SnTe interlayer lowers the rc and strengthens the contact. It does not, however, effectively stop Ni from diffusing into Na0.02Pb0.98Te. The bonding is poor in the Fe/SnTe/Na0.02Pb0.98Te contacts due to the absence of any reaction at the Fe/SnTe interface. A composite buffer layer Co + 75 vol % SnTe with SnTe improves the mechanical stability of the Co contact with moderately lesser rc than pure SnTe alone. However, a similar approach with Fe does not yield stable contact. The Co/Co + 75 vol % SnTe/SnTe/Na0.02Pb0.98Te contact exhibits rc less than 50 µΩ cm2 and has good microstructural and mechanical stability after annealing at 723 K for 170 h.

Generic Approach for Contacting Thermoelectric Solid Solutions: Case Study in n- and p-Type Mg2Si0.3Sn0.7.

Metallization (known as contacting) of thermoelectric (TE) legs is vital to the long-term performance of a TE device. It is often observed that the compositional changes in a TE solid solution may render a given contact material unsuitable due to a mismatch in the thermal expansion coefficient values. Finding suitable contact materials for TE solid solutions (which often are the best TE materials) remains a challenge. In this work, we propose a multilayer single-step approach in which the same combination of contact materials can be used for a wide compositional range in a solid solution. The outer layer is a metal foil, which helps in creating an Ohmic contact with the interconnects. The intermediate layer is a mixture of the TE material and a metal powder, which results in the formation of the diffusion barrier. The innermost layer is the TE material, which is the active component of the device. The strategy was applied on n- and p-doped Mg2Si0.3Sn0.7 with elemental Cu and Ni providing the desired interface functionalities. Single-step compaction was carried out using the monoblock sintering technique. Microscopic investigation reveals the formation of a well-bonded crack-free interface. Various intermetallic phases were identified at the interface, and the formation of the MgNi2Sn phase was found to be critical to prevent any interdiffusion of elements. Electrical contact resistance (rc) measurements were conducted, and low values of 3 and 19 µΩ cm2 were measured in n- and p-type legs, respectively. The contacted TE legs were further annealed at 400 °C for 7 days to check their stability. Microstructural and electrical resistance measurements reveal minimal changes in the interface layer and rc values, indicating the workability of the multilayer technique.