Geometrical Optimization and Thermal-Stability Characterization of Te-Free Thermoelectric Modules Based on MgAgSb/Mg3 (Bi,Sb)2

Interface kinetic manipulation enabling efficient and reliable Mg3Sb2 thermoelectrics

Development of efficient and reliable thermoelectric generators is vital for the sustainable utilization of energy, yet interfacial losses and failures between the thermoelectric materials and the electrodes pose a significant obstacle. Existing approaches typically rely on thermodynamic equilibrium to obtain effective interfacial barrier layers, which underestimates the critical factors of interfacial reaction and diffusion kinetics. Here, we develop a desirable barrier layer by leveraging the distinct chemical reaction activities and diffusion behaviors during sintering and operation. Titanium foil is identified as a suitable barrier layer for Mg3Sb2-based thermoelectric materials due to the creation of a highly reactive ternary MgTiSb metastable phase during sintering, which then transforms to stable binary Ti-Sb alloys during operation. Additionally, titanium foil is advantageous due to its dense structure, affordability, and ease of manufacturing. The interfacial contact resistivity reaches below 5 μΩ·cm2, resulting in a Mg3Sb2-based module efficiency of up to 11% at a temperature difference of 440 K, which exceeds that of most state-of-the-art medium-temperature thermoelectric modules. Furthermore, the robust Ti foil/Mg3(Sb,Bi)2 joints endow Mg3Sb2-based single-legs as well as modules with negligible degradation over long-term thermal cycles, thereby paving the way for efficient and sustainable waste heat recovery applications.

Cellulose binary coatings with spherical envelope structure via structure rearrangement in ball milling for integrated radiative cooling-electricity generation

Passive daytime radiative cooling is a zero-energy consumption cooling technology, which can dissipate heat to outer space via infrared radiation. Recently, coupling radiative cooling technology and thermoelectric devices to generate electricity has attracted much attention. However, existing radiative cooling integrated thermoelectric devices still suffer from low-temperature gradient and output voltage. Here, based on the Mie scattering and internal reflection enhancing principle, an impact-inducing geometry reconstruction approach was proposed to fabricate hierarchical nanostructured cellulosic coatings with good daytime cooling performance to achieve stable electricity generation function, which can be realized by using a scalable and facile wet ball milling technology. Guided by the theoretical simulations of the finite difference time domain method (FDTD), the cellulose and TiO2 nanoparticles can assemble into spherical envelope structured coatings drying by the shear, impact, and friction interaction in the ball milling process, dramatically enhancing the Mie scattering and internal reflection of coatings. The cellulosic coatings exhibit sunlight reflectivity of 0.962 and infrared emissivity of 0.94, resulting in a daytime radiative cooling efficiency of 5.9 °C under direct sunlight. Energy Plus stimulation demonstrated 35 % cooling energy and 468.9 kWh of cooling energy can be saved annually in China. Meanwhile, this cellulosic coating-based thermoelectric device can deliver a high voltage output of 150 mV under 1 Sun due to the strong bonding and high-temperature gradient formation (30 °C), which is higher than previous reports. This study will facilitate the development of sustainable power generation device for the goal of green future.

Revealing the Chemical Instability of Mg3Sb2-xBix-Based Thermoelectric Materials

n-Type Mg3Sb2-xBix alloys have been regarded as promising thermoelectric materials due to their excellent performance and low cost. For practical applications, the thermoelectric performance is not the only factor that should be taken into consideration. In addition, the chemical and thermal stabilities of the thermoelectric material are of equal importance for the module design. Previous studies reported that the Mg3Sb2-xBix alloys were unstable in an ambient environment. In this work, we found that Mg3Sb2-xBix alloys reacted with H2O and O2 at room temperature and formed amorphous Mg(OH)2/MgO and crystalline Bi/Sb. The substantial loss of Mg resulted in a significant deterioration in thermoelectric properties, accompanied by the transition from n-type to p-type. With the increase in Bi content, the chemical stability decreased due to the higher formation energy of Mg3Bi2. A chemically stable Mg3Bi2 sample was achieved by coating it with polydimethylsiloxane to isolate H2O and O2 in the air.

Mechanical Compatibility between Mg3(Sb,Bi)2 and MgAgSb in Thermoelectric Modules

Thermoelectric (TE) modules are exposed to temperature gradients and repeated thermal cycles during their operation; therefore, mechanically robust n- and p-type legs are required to ensure their structural integrity. The difference in the coefficients of thermal expansion (CTEs) of the two legs of a TE module can cause stress buildup and the deterioration of performance with frequent thermal cycles. Recently, n-type Mg₃Sb₂ and p-type MgAgSb have become two promising components of low-temperature TE modules because of to their high TE performance, nontoxicity, and abundance. However, the CTEs of n-Mg₃Sb₂ and p-MgAgSb differ by approximately 10%. Furthermore, the oxidation resistances of these materials at increased temperatures are unclear. This work manipulates the thermal expansion of Mg₃Sb₂ by alloying it with Mg₃Bi₂. The addition of Bi to Mg₃Sb₂ reduces the coefficient of linear thermal expansion from 22.6 × 10^-6 to 21.2 × 10^-6 K^-1 for Mg₃Sb_1.5Bi_0.5, which is in excellent agreement with that of MgAgSb (21 × 10^-6 K^-1). Furthermore, thermogravimetric data indicate that both Mg₃Sb_1.5Bi_0.5 and MgAgSb are stable in air and Ar at temperatures below ∼570 K. The results suggest the compatibility and robustness of Mg₃Sb_1.5Bi_0.5 and MgAgSb as a pair of thermoelectric legs for low-temperature TE modules.

Physics-guided co-designing flexible thermoelectrics with techno-economic sustainability for low-grade heat harvesting

Flexible thermoelectric harvesting of omnipresent spatial thermodynamic energy, though promising in low-grade waste heat recovery (<100°C), is still far from industrialization because of its unequivocal cost-ineffectiveness caused by low thermoelectric efficiency and power-cost coupled device topology. Here, we demonstrate unconventional upcycling of low-grade heat via physics-guided rationalized flexible thermoelectrics, without increasing total heat input or tailoring material properties, into electricity with a power-cost ratio (W/US$) enhancement of 25.3% compared to conventional counterparts. The reduced material usage (44%) contributes to device power-cost "decoupling," leading to geometry-dependent optimal electrical matching for output maximization. This offers an energy consumption reduction (19.3%), electricity savings (0.24 kWh W^-1), and CO₂ emission reduction (0.17 kg W^-1) for large-scale industrial production, fundamentally reshaping the R&D route of flexible thermoelectrics for techno-economic sustainable heat harvesting. Our findings highlight a facile yet cost-effective strategy not only for low-grade heat harvesting but also for electronic co-design in heat management/recovery frontiers.

High figure-of-merit and power generation in high-entropy GeTe-based thermoelectrics

The high-entropy concept provides extended, optimized space of a composition, resulting in unusual transport phenomena and excellent thermoelectric performance. By tuning electron and phonon localization, we enhanced the figure-of-merit value to 2.7 at 750 kelvin in germanium telluride–based high-entropy materials and realized a high experimental conversion efficiency of 13.3% at a temperature difference of 506 kelvin with the fabricated segmented module. By increasing the entropy, the increased crystal symmetry delocalized the distribution of electrons in the distorted rhombohedral structure, resulting in band convergence and improved electrical properties. By contrast, the localized phonons from the entropy-induced disorder dampened the propagation of transverse phonons, which was the origin of the increased anharmonicity and largely depressed lattice thermal conductivity. We provide a paradigm for tuning electron and phonon localization by entropy manipulation, but we have also demonstrated a route for improving the performance of high-entropy thermoelectric materials.