Bonding and high-temperature reliability of NiFeMo alloy/n-type PbTe joints for thermoelectric module applications

PbTe is an extremely important thermoelectric (TE) material, due to its high TE conversion efficiency. Consequently, our effort focuses on developing PbTe-based TE modules, which requires developing novel approaches for bonding metallic contacts to PbTe. In this study, Fe, Mo, and NiFeMo alloy foils were directly bonded to n-type PbTe using a rapid hot press at 600, 700, or 800 °C under a pressure of 40 MPa and for various holding times. We find that in the case of Fe and Mo, it is difficult to form a metallurgically bonded high strength joint with PbTe. However, we find that NiFeMo alloy does effectively bond to PbTe at 700 °C, but not at 600 °C. Significant liquid Pb, which might be due to the reaction of PbTe with Ni, is found that penetrates along the NiFeMo grain boundaries near NiFeMo/PbTe joints during bonding at 700 °C where the extent of liquid Pb penetration can be controlled with the time of bonding. Furthermore, the Seebeck coefficient of bulk PbTe with NiFeMo contacts is similar to that without NiFeMo contacts. Finally, the accelerated thermal aging of NiFeMo/PbTe elements at 600 °C for 240 h shows that the failure mechanism of NiFeMo/PbTe joints under operating conditions is the continued formation and penetration of eutectic liquid NiFeMo–PbTe and liquid Pb along the NiFeMo grain boundaries

Temperature dependent band gap in PbX (X=S, Se, Te)

PbTe is an important thermoelectric material for power generation applications due its high conversion efficiency and reliability. Its extraordinary thermoelectric performance is attributed to band convergence of the light L and heavy Σ bands. However, the temperature at which these bands converge is disputed. In this letter, we provide direct experimental evidence combined with ab initio calculations that confirm an increasing optical gap up to 673 K and predict a band convergence temperature of 700 K, much higher than previous measurements showing saturation and band convergence at 450 K

Enhanced thermoelectric performance in the very low thermal conductivity Ag_2Se_(0.5)Te_(0.5)

In this letter, we report the high-temperature thermoelectric properties of Ag_2Se_(0.5)Te_(0.5). We find that this particular composition displays very low thermal conductivity and competitive thermoelectric performance. Specifically, in the temperature region 520 K ≤ T ≤ 620 K, we observe non-hysteretic behavior between the heating and cooling curves and zT values ranging from 1.2 to 0.8. Higher zT values are observed at lower temperatures on cooling. Our results suggest that this alloy is conducive to high thermoelectric performance in the intermediate temperature range, and thus deserves further investigation

Measuring thermoelectric transport properties of materials

In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT. Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed

Optimization of Variable Cross-Sectional Area Thermoelectric Elements Through Multi-method Thermal-Electric Coupled Modeling

A well-posed thermal-electric coupled mathematical-numerical model to optimize the cross-sectional area per length of a thermoelectric (TE) leg is introduced to maximize thermal conversion efficiency (eta) or power output (Po). To employ such optimization, the p- or n-type leg was divided into uniform length segments, wherein the product of the electrical resistance (Rel) and thermal conductance (K) was minimized as to maximize the figure of merit (ZT) of each individual partition. The minimization of RelK was dependent upon the temperature difference established across each segment, which was resolved using a one-dimensional finite difference (FD) scheme of the TE general energy equation (GEQ). The TE GEQ included all pertinent phenomena - conduction, Joule, Peltier and Thomson effects - as well as temperature dependent properties. The boundary conditions of the FD scheme were provided via a one-dimensional thermal resistance network. The current output of the unicouple was determined by the temperature bounds across the junction and the internal resistance of the TE legs, and this was explicitly coupled to the TE GEQ to create a fully-coupled model. The proposed model was validated to a fully-coupled thermal-electric finite volume method model implemented in ANSYS CFX. The proposed optimization process yielded improvements in volumetric efficiency and volumetric power output of 4.60% and 3.75%, respectively, in comparison to conventional constant-area optimization processes

Bonding and interfacial reaction between Ni foil and n-type PbTe thermoelectric materials for thermoelectric module applications

Integration of next generation thermoelectric materials in thermoelectric modules requires a novel or alternative approach for mating the brittle semiconducting thermoelectric materials and the ductile metal interconnects. In this study, pure Ni foil was directly bonded to PbTe-based thermoelectric materials using a rapid hot-press. The materials were sintered at 600 and 650 °C, under a pressure of 40 MPa and for various holding times. The resulting interfacial microstructures of the Ni/PbTe joints were investigated. Additionally, the distributions of elements and the phases formed at the Ni/PbTe interface were analyzed. The β_2 phase (Ni_(3±x) Te_2, 38.8–41 at.% Te) was identified at the Ni/PbTe joints bonded at both 600 and 650 °C. A ternary phase with approximate composition Ni_5Pb_2Te_3 was found at the Ni/PbTe joints bonded at 650 °C. Additionally, the PbTe(Ni) phase was observed along the Ni grain boundaries for both bonding temperatures. Thermodynamics calculation results indicate that only the β_2 phase can be formed at the Ni/PbTe interface at 900 K among the binary nickel tellurides

Interfacial Reaction Between Nb Foil and n-Type PbTe Thermoelectric Materials During Thermoelectric Contact Fabrication

PbTe is a high-conversion-efficiency thermoelectric (TE) material that is commonly used in space exploration applications. Integration of PbTe in TE devices has a significant impact on the conversion efficiency and reliability of TE devices. Hence, our effort focuses on developing novel approaches for bonding metallic contacts to PbTe to improve device performance and reliability. In this study, pure Nb foil was directly bonded to PbTe-based TE materials to fabricate the hot-side contacts of TE elements using a rapid hot-press. The materials were sintered at 700°C under pressure of 40 MPa for various holding times. We found that a reaction layer of needle-like Nb_3Te_4 mixed with Pb forms at the interface of the Nb/PbTe joints and that Pb is distributed in the gaps of the Nb_3Te_4 grains. We analyze the resulting microstructure and finally calculate the time exponent of the growth kinetics of the Nb_3Te_4 layer. Fracture surface analysis showed that the Nb/PbTe joint fractures at the interface between Nb and Nb_3Te_4 and within the PbTe matrix, indicating that the bonding between Nb and Nb_3Te_4 is weak