n-type Mg3Sb1.5Bi0.5 intermetallic solid solution is a promising thermoelectric (TE) material for medium temperature range (room temperature to 450⁰ C) applications due to its high figure of Merit ...(peak zT value of 1.5–1.7) and cost-effective constituent elements. Extensive research on materials development has been carried out on this material system but there are limited reports related to its contacting. In this work, a detailed study of the contacting in Mg3.0+ySb1.49Bi0.49Te0.02(y = 0.3) has been carried out using both single layer and multilayer approaches. For single layer contacting studies, the contact metal (Ni/Fe) is directly in contact with TE material while in case of the multilayer contacting process, the TE material powder is sandwiched between two layers (with different functionalities). The top layer is a metal foil (Cu/Ni) and is followed by a composite layer consisting of a mixture of TE material and Ni powder. Compaction in both cases is carried out by simultaneous application of temperature and pressure using the mono-block sintering process. It is observed that the electrical contact resistance (Rc) values for all cases lie in the range of 30‐45μΩcm2. However, a marked difference in the microstructure and nature of bonding is observed between the various contacting approaches. For single layer contacting, Fe is observed to form an abrupt interface with absence of any reactive phases at the interface. However, Ni results in the formation of various intermetallic phases and a diffuse interface. For both Fe and Ni, cracks or delamination are observed at the interface. The multilayer contacting studies result in well bonded and crack-free interface for all the studied compositions. Various intermetallic phases have been identified from an elemental analysis of the interface region. Further, no diffusion of the metal layers is observed into the thermoelectric matrix.
•Single-layer contacting results in crack formation/delamination at the metal-TE material interface.•Nickel metal reacts with the TE material and forms various intermetallic phases (MgNi2, Mg2Ni, Ni3Sb) at the interface.•A mixed layer of Ni and TE material is introduced in multi-layer approach to reduce the thermal stresses at the interface.•The multi-layer contacting technique results in a crack-free and well-bonded interface.
In recent years, bismuth-rich Mg3(Sb1–x Bi x )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–x Bi x )2 compositions and TE materials.
In recent years, bismuth-rich Mg
(Sb
Bi
)
(
= 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. Mg
Sb
Bi
(
= 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
(TE figure of merit) with doping concentration and temperature have been generated. The 3D maps reveal an interesting surface topography with a broad peak
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 Mg
Sb
Bi
. Further, the proposed technique can be utilized to probe other Mg
(Sb
Bi
)
compositions and TE materials.
Parasitic losses at various interfaces play a crucial role in determining the thermoelectric (TE) generator performance. In this work, we adopt an approach wherein the interface contact resistance ...can be minimized by using a combination of the multilayer (ML) contacting technique and transient liquid phase (TLP) bonding. The technique was experimentally verified with doped Mg3Sb1.5Bi0.5. First, a contacted leg consisting of the TE material, the diffusion barrier layer, and an outer metallic (copper) layer was prepared by the ML process. This was then joined to a copper bridge (mimicked by copper holders) by using the TLP process with tin as the brazing element. Our contact resistance measurements indicate an extremely low value corresponding to 2.5% of the total resistance for the TLP-joined layer, which corresponds to a ∼2.6% lowering of the peak efficiency (η max) value compared to that of an uncontacted leg. Thermoelectric generator (TEG) measurements were obtained for the contacted leg. The contacted leg exhibited an η max value of ∼10% and a power output of 80 mW when it was subjected to a temperature gradient (ΔT) of 330 K. These results demonstrate that, by combining ML contacting with TLP bonding, interface contact engineering can be a potential method for enhancing TE performance.
The present study reveals the influences of heating rate, as employed during thermal reduction of graphene oxide (GO) to reduced graphene oxide (rGO), on the degree of exfoliation and defect ...structure of the resulting rGO, and, in turn, the effects of the same on the physical properties, electronic properties, performances as electrode material for electrochemical energy storage and efficacy as mechanical reinforcement in brittle materials. Faster heating (@ 10 °C/min) of GO to 350 °C has been found to result in efficient thermal exfoliation, yielding rGO (R10) with ∼400% increase in specific surface area, but with a highly disordered structure. By contrast, slower heating (@ 1 °C/min; as for R01) causes reduction in surface area by ∼30% (w.r.t. GO) due to failure of exfoliation. However, both the rGOs exhibit high defect density (viz., distance between defect sites; Ld ∼1.12–1.3 nm), corresponding to stage 2 of disorder (as per analysis based on Raman spectra). Higher sp2 C fraction in R01 results in higher electronic conductivity, whereas significantly larger specific surface area, along with relatively lower crystallite size and Ld, bestows R10 with higher capacity and rate-capability in the context of electrochemical Li-/Na-storage. As reinforcement in alumino-borosilicate glass-ceramic, while GO and R10 have been found to improve the resistances towards crack propagation and abrasive wear damage (w.r.t. unreinforced alumino-borosilicate), incorporation of R01 deteriorates both these aspects primarily due to inefficient exfoliation of R01, concomitant poor dispersion, failure of reinforcing mechanisms and presence of thicker flakes acting as sites of excessive material damage.
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Mg3Sb2‑xBi x 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 (S 2σ) 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+x Sb2–y Bi y solid-solutions.
