Improved Thermoelectric Properties of N-Type Mg3Sb2 through Cation-Site Doping with Gd or Ho

Mg-Defect Compensation to Realize High Performance at Room Temperature in Mg3Bi2-Based Thermoelectric Single Crystals

Mg3Bi2-based materials are a very promising substitute for current commercial Bi2Te3 thermoelectric alloys. The successful growth of Mg3Bi2-based single crystals with high room-temperature performance is especially significant for practical applications. Previous studies indicated that the effective suppression of Mg defects in Mg3Bi2-based materials was crucial for high performance, which was usually realized by applying excessive Mg during syntheses. However, utilization of excessive Mg generates Mg-rich phases between the crystalline boundaries and is unfavorable for the long-term stability of the materials. Here, bulk single crystals with a low-content Mg component such as Mg3.1Bi1.49Sb0.5Te0.01 were successfully grown. For compensating Mg defects, Li was chosen as the additional electron dopant. The results indicate that Li is a very effective electron compensator when low-concentration doping is applied. For high-concentration doping, Mg atoms in the lattice are substituted by Li, leading to decreased electron concentration again. This strategy is very significant for improving the room-temperature performance of Mg3Bi2-based materials. As a result, a record-high figure of merit of 1.05 at 300 K is achieved for Mg3+xLi0.003Bi1.49Sb0.5Te0.01 single crystals.

Porous carboxymethyl cellulose nanocrystalline imprinted composite aerogels for selective adsorption of gadolinium

Gadolinium is widely applied in medical and high-tech materials because of special magnetic properties. Recovery of gadolinium from waste rare earth products has both economic and environmental value. In this experiment, honeycomb porous composite aerogels were constructed using sericin and sodium alginate mixed with functionally modified carboxymethylated cellulose nanocrystals for the adsorption and separation of gadolinium ions. There were large numbers of carboxyl groups as well as hydroxyl groups on the surface of sodium alginate and filamentous protein, which provided more sites for the adsorption of gadolinium ions. Besides, a stable honeycomb structure appeared on the surface of composite aerogels when the mixture of filamentous protein and sodium alginate was 1:1, which increased the specific surface area of materials to 140.65 m2 g-1. Additionally, the imprinted composite aerogels Ic-CNC/SSA were prepared by virtue of the imprinting technology, enhancing the adsorption selectivity of composite aerogels for gadolinium. The adsorption experiments revealed that the maximum adsorption capacity of Ic-CNC/SSA reached 93.41 mg g-1 at pH 7.0, indicating good selective adsorption of gadolinium ions. In summary, such composite aerogels provide great potential and reference value for the selective adsorption of gadolinium ions in industry.

Inhibiting Mg Diffusion and Evaporation by Forming Mg-Rich Reservoir at Grain Boundaries Improves the Thermal Stability of N-Type Mg3 Sb2 Thermoelectrics

N-type Mg3 Sb2 -based thermoelectric materials show great promise in power generation due to their mechanical robustness, low cost of Mg, and high figure of merit (ZT) over a wide range of temperatures. However, their poor thermal stability hinders their practical applications. Here, MgB2 is introduced to improve the thermal stability of n-type Mg3 Sb2 . Enabled by MgB2 decomposition, extra Mg can be released into the matrix for Mg compensation thermodynamically, and secondary phases of Mg─B compounds can kinetically prevent Mg diffusion along grain boundaries. These synergetic effects inhibit the formation of Mg vacancies at elevated temperatures, thereby enhancing the thermal stability of n-type Mg3 Sb2 . Consequently, the Mg3.05 (Sb0.75 Bi0.25 )1.99 Te0.01 (MgB2 )0.03 sample exhibits negligible variation in thermoelectric performance during the 120-hour continuous measurement at 673 K. Moreover, the ZT of n-type Mg3 Sb2 can be maintained by adding MgB2 , reaching a high average ZT of ≈1.1 within 300-723 K. An eight-pair Mg3 Sb2 -GeTe-based thermoelectric device is also fabricated, achieving an energy conversion efficiency of ≈5.7% at a temperature difference of 438 K with good thermal stability. This work paves a new way to enhance the long-term thermal stability of n-type Mg3 Sb2 -based alloys and other thermoelectrics for practical applications.

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.

Mg Compensating Design in the Melting-Sintering Method For High-Performance Mg3 (Bi, Sb)2 Thermoelectric Devices

N-type Mg3 (Bi, Sb)2 -based thermoelectric (TE) alloys show great promise for solid-state power generation and refrigeration, owing to their excellent figure-of-merit (ZT) and using cheap Mg. However, their rigorous preparation conditions and poor thermal stability limit their large-scale applications. Here, this work develops an Mg compensating strategy to realize n-type Mg3 (Bi, Sb)2 by a facile melting-sintering approach. "2D roadmaps" of TE parameters versus sintering temperature and time are plotted to understand the Mg-vacancy-formation and Mg-diffusion mechanisms. Under this guidance, high weight mobility of 347 cm2  V-1  s-1 and power factor of 34 µW cm-1  K-2 can be obtained for Mg3.05 Bi1.99 Te0.01 , and a peak ZT≈1.55 at 723 K and average ZT≈1.25 within 323-723 K can be obtained for Mg3.05 (Sb0.75 Bi0.25 )1.99 Te0.01 . Moreover, this Mg compensating strategy can also improve the interfacial connecting and thermal stability of corresponding Mg3 (Bi, Sb)2 /Fe TE legs. As a consequence, this work fabricates an 8-pair Mg3 Sb2 -GeTe-based power-generation device reaching an energy conversion efficiency of ≈5.0% at a temperature difference of 439 K, and a one-pair Mg3 Sb2 -Bi2 Te3 -based cooling device reaching -10.7 °C at the cold side. This work paves a facile way to obtain Mg3 Sb2 -based TE devices at low cost and also provides a guide to optimize the off-stoichiometric defects in other TE materials.

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.

Large improvement in thermoelectric performance of pressure-tuned Mg3Sb2

The Mg₃Sb₂-based Zintl compound is a promising candidate for a high-performance thermoelectric material with the advantage of the component elements being low cost, non-toxic and earth-abundant. Here, we investigate the influence of pressure on the electronic structure and p-type and n-type thermoelectric transport properties of Mg₃Sb₂ by using density functional theory and Boltzmann transport theory. The energy gaps first increase and then decrease with the increasing of pressure, and a peak value of the valley degeneracy of conduction band occurs at 4 GPa. Based on the calculated band structures, the zT (figure of merit) values of p-type Mg₃Sb₂ under pressure are significantly enhanced, which predominantly originates from the boosted PF (power factor) contributed by the increased carrier's relaxation time. When the carrier concentration reaches 1 × 10²⁰ cm^-3, the PF of p-type Mg₃Sb₂ at 4 GPa is increased by 35% relative to that of the compound at 0 GPa, thus leading to a considerably improved zT of ∼0.62 at 725 K. Under the same conditions, due to the increased density of states effective mass, the n-type Mg₃Sb₂ exhibits a highest PF of ∼19 μW cm^-1 K^-2 and a peak zT of 1.7. Therefore, pressure tuning is an effective method to improve the p-type and n-type thermoelectric transport performance of Mg₃Sb₂-based Zintl compounds. This work on Mg₃Sb₂ under pressure may provide a new mechanism for the experimenters towards the enhancement of the thermoelectric performance of materials.