N-type Mg3 Research Articles

Thermoelectric Enhancement Achieved by Y and La Co-Doping in N-Type Mg3.2sb1.5bi0.5

Thermoelectric Enhancement Achieved by Y and La Co-Doping in N-Type Mg3.2sb1.5bi0.5

A wearable real-time power supply with a Mg3Bi2-based thermoelectric module

A wearable thermoelectric generator using human body temperature is a promising power supply for wearable electronics. Here we discuss the design and fabrication of one kind of wearable thermoelectric generator constructed by n-type Mg3.2Bi1.498Sb0.5Te0.002 legs, p-type Bi0.4Sb1.6Te3 legs, polyurethane matrices, and flexible Cu/polyimide electrodes. The proposed device has a low thermal bypass and an efficient thermal contact interface, resulting in a peak power density of ∼20.6 μW/cm2 when placed on a human arm at an ambient temperature of 289 K (air velocity, 1.1 m/s) and a high peak power density of 13.8 mW/cm2 at a temperature difference of 50 K. In addition, it can withstand 10,000 bending cycles at a bend radius of 13.4 mm, suggesting that the proposed wearable thermoelectric generator has the potential to be a real-time power supply for certain wearable electronics in daily life.

High thermoelectric energy conversion efficiency of a unicouple of n-type Mg3Bi2 and p-type Bi2Te3

Low-grade heat harvesting by thermoelectric technology can contribute to efficient energy utilization. However, the large-scale application of thermoelectric devices is partially hindered by their unsatisfactory performance and relatively high cost. Here we prepared a cost-effective n-type Mg3Bi2-based compound with a peak zT of 1.24 at 573 K. A unicouple comprised of n-type Mg3.2Bi1.29Sb0.7Te0.01 and p-type Bi0.2Sb1.8Te3 was constructed and its energy conversion efficiency was evaluated. The unicouple exhibited a record-high efficiency of (9.0 ± 0.5)% or (8.3 ± 0.4)% [considering the radiation heat loss] under a temperature difference of 265 K at the hot-side temperature of 573 K. Our results demonstrate the great potential of Mg3Bi2-based materials for low-grade heat recovery.

Towards tellurium-free thermoelectric modules for power generation from low-grade heat

Thermoelectric technology converts heat into electricity directly and is a promising source of clean electricity. Commercial thermoelectric modules have relied on Bi2Te3-based compounds because of their unparalleled thermoelectric properties at temperatures associated with low-grade heat (<550 K). However, the scarcity of elemental Te greatly limits the applicability of such modules. Here we report the performance of thermoelectric modules assembled from Bi2Te3-substitute compounds, including p-type MgAgSb and n-type Mg3(Sb,Bi)2, by using a simple, versatile, and thus scalable processing routine. For a temperature difference of ~250 K, whereas a single-stage module displayed a conversion efficiency of ~6.5%, a module using segmented n-type legs displayed a record efficiency of ~7.0% that is comparable to the state-of-the-art Bi2Te3-based thermoelectric modules. Our work demonstrates the feasibility and scalability of high-performance thermoelectric modules based on sustainable elements for recovering low-grade heat.

Improved thermoelectric performance of n-type Mg3Sb2–Mg3Bi2 alloy with Co element doping

n-type Mg3Sb2–Mg3Bi2 alloy shows as a potential new thermoelectric material (TE) and has been widely researched recently. The pure phase n-type Mg3·20(Sb0·3Bi0.7)1.99Te0.01 were prepared by adjusting Mg content with the Bi impurity phase being effectively suppressed. Then, Co element was doped into the pure phase and the electrical conductivity of samples were improved. With a high power factor of 20.3 μW cm−1K−2 for Mg3·185Co0·015(Sb0·3Bi0.7)1.99Te0.01 at 525 K. Additionally, it was found that the phonon scattering is enhanced due to the larger atomic mass of Co comparing to Mg and the lattice thermal conductivity is reduced. As a result, a high ZT value of ~ 1.03 at 525 K is achieved for the Mg3·185Co0·015(Sb0·3Bi0.7)1.99Te0.01.

Enhanced thermoelectric performance of P-type CaMg2Bi1.98 and optimized CaAl2Si2-type Zintl phase module with equal cross-section area

Recently, Zintl phase Mg3Sb2 has displayed superior n-type thermoelectric properties. However, the corresponding p-type thermoelectric materials with high thermoelectric properties are still in urgent need. CaMg2Bi2-based Zintl compound attracts increasing enthusiasm in recent years due to its high carrier mobility and low lattice thermal conductivity. Here, Yb is used to replace Ca to further tune thermoelectric transport behavior of CaMg1.9Zn0.1Bi1.98, an optimized composition according to previous work. The synergistic effect of the enhanced carrier concentration caused by electronegativity difference in cation site and the suppressed lattice thermal conductivity induced by the stronger point defects scattering leads to the peak ZT of 1.0 in Ca0.65Yb0.35Mg1.9Zn0.1Bi1.98, which exceeds most of reported CaMg2Bi2-based materials. The maximum ZTeng of single p-type leg Ca0.65Yb0.35Mg1.9Zn0.1Bi1.98 is up to 0.54 between Tc = 300 K and Th = 773 K, catching up with the current highest level in 1-2-2 Bi-based materials. Moreover, the interrelationship between conversion efficiency and p-n pair cross-section area ratio (An/Ap) is considered. For n-type Mg3.1Co0.1Sb1.5Bi0.49Te0.01/p-type Ca0.65Yb0.35Mg1.9Zn0.1Bi1.98 pair, a maximum conversion efficiency of ~12% is obtained at the optimum An/Ap approaching to 1. This prominent conversion efficiency and equal cross-section area facilitate the module fabrication process and exhibit bright prospect of CaMg2Bi2-based thermoelectric materials.

Reliable N-type Mg3.2Sb1.5Bi0.49Te0.01/304 stainless steel junction for thermoelectric applications

Mg3Sb2-based materials possess promising thermoelectric performances, however, probably poor interfacial behaviors when connected with the metal electrodes due to the strong chemical activity and volatility of the element Mg. In this paper, a high conversion efficiency of up to 9 % was achieved with a reliable Mg3.2Sb1.5Bi0.49Te0.01/304 stainless steel junction at a temperature difference of 370 K (Th = 673 K, Tc = 303 K). The transformation from a single ferritic structure (Fe) to an austenitic structure (304 stainless steel) insures the lower diffusion coefficient and higher expansion coefficient of 304 stainless steel, which contributes to the suppressed interfacial reaction, ideal electrical properties, and robust mechanical properties from room temperature to higher temperature.

Promoted application potential of p-type Mg3Sb1.5Bi0.5 for the matched thermal expansion with its n-type counterpart

Recently, n-type Mg3Sb1.5Bi0.5-based thermoelectric materials have attracted considerable attention for their extraordinary thermoelectric performance. Ideally, thermoelectric generators should be made of the same material system to avoid thermal mismatch in the practical application. In this work, p-type Mg3Sb1.5Bi0.5 which has almost the same composition as the state-of-the-art n-type Mg3.2Sb1.5Bi0.49Te0.01Mn0.01 was synthesized by ball milling and spark plasma sintering, and then Na was chosen as an acceptor dopant to optimize the carrier concentration and further improve the thermoelectric performance. Na0.0075Mg2.9925Sb1.5Bi0.5 sample gets the highest ZT of ∼0.5 at 773 K. While Na0.005Mg2.995Sb1.5Bi0.5 sample shows the highest average ZT of ∼0.29 in the temperature range of 300–773 K and matched thermal expansion behavior with the state-of-the-art n-type Mg3.2Sb1.5Bi0.49Te0.01Mn0.01, which is of great significance for practical applications. Taking the Joule and Thompson heat into account, a high theoretical conversion efficiency (η) of ∼9.5% was calculated for the thermoelectric module consists of the present p-type Na0.005Mg2.995Sb1.5Bi0.5 and the state-of-the-art n-type Mg3.2Sb1.5Bi0.49Te0.01Mn0.01 with the leg length of 2 mm, and cold and hot side temperature of 300 K and 773 K, respectively, which shows a good potential for the use of this class of materials in the mid-temperature power generation applications.

Metallic n-Type Mg3 Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance.

Mg3 (Sb,Bi)2 alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi2 (Te,Se)3 thermoelectric alloys. Previous theoretical studies predict that single crystals Mg3 (Sb,Bi)2 can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n-type Mg3 (Sb,Bi)2 single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg3 Sb2 single crystals, synthesized by a combination of Sb-flux method and Mg-vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T-1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te-doped Mg3 Sb2 single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge-carrier scattering is crucial for developing high-performance thermoelectric materials and indicates that single-crystalline Mg3 (Sb,Bi)2 solid solutions can exhibit higher zT compared to polycrystalline samples.

Improvement of Low‐Temperature zT in a Mg3Sb2–Mg3Bi2 Solid Solution via Mg‐Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n-Type Mg3 Sb2 -based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain-boundary effect has been limited due to loss of Mg, which hinders a sample's n-type dopability. A Mg-vapor anneal processing step that grows a sample's grain size and preserves its n-type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon-scattering-dominated T-3/2 trend over a large temperature range, further supporting the conclusion that the temperature-activated mobility in Mg3 Sb2 -based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm2 V-1 s-1 in annealed 800 °C sintered Mg3 + δ Sb1.49 Bi0.5 Te0.01 , the highest ever reported for Mg3 Sb2 -based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n-type Bi2 Te3 ) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.

Extraordinary n‐Type Mg3SbBi Thermoelectrics Enabled by Yttrium Doping

Advancing thermoelectric n-type Mg3 Sb2 alloys requires both high carrier concentration offered by effective doping and high carrier mobility enabled by large grains. Existing research usually involves chalcogen doping on the anion sites, and the resultant carrier concentration reaches ≈3 × 1019 cm-3 or below. This is much lower than the optimum theoretically predicted, which suggets that further improvements will be possible once a highly efficient dopant is found. Yttrium, a trivalent dopant, is shown to enable carrier concentrations up to and above ≈1 × 1020 cm-3 when it is doped on the cation site. Such carrier concentration allows for in-depth understand of the electronic transport properties over a broad range of carrier concentrations, based on a single parabolic band approximation. As well as reasonably high carrier mobility in coarse-grain materials sintered by hot deforming and fusing of large pieces of ingots synthesized by melting, higher thermoelectric performance than earlier experimentally reported for n-type Mg3 Sb2 is found. In particular, the thermoelectric figure of merit, zT, is even higher than that of any known n-type thermoelectric, including Bi2 Te3 alloys, within 300-500 K. This might pave the way for Mg3 Sb2 alloys to become a realistic material for n-type thermoelectrics for sustainable applications.

Achieving band convergence by tuning the bonding ionicity in n-type Mg3 Sb2.

Identifying strategies for beneficial band engineering is crucial for the optimization of thermoelectric (TE) materials. In this study, we demonstrate the beneficial effects of ionic dopants on n-type Mg3 Sb2 . Using the band-resolved projected crystal orbital Hamilton population, the covalent characters of the bonding between Mg atoms at different sites are observed. By partially substituting the Mg at the octahedral sites with more ionic dopants, such as Ca and Yb, the conduction band minimum (CBM) of Mg3 Sb2 is altered to be more anisotropic with an enhanced band degeneracy of 7. The CBM density of states of doped Mg3 Sb2 with these dopants is significantly enlarged by band engineering. The improved Seebeck coefficients and power factors, together with the reduced lattice thermal conductivities, imply that the partial introduction of more ionic dopants in Mg3 Sb2 is a general solution for its n-type TE performance. © 2019 Wiley Periodicals, Inc.

Heat capacity of Mg3Sb2, Mg3Bi2, and their alloys at high temperature

The thermoelectric figure of merit reported for n-type Mg3(Sb,Bi)2 compounds has made these materials of great engineering significance, increasing the need for accurate evaluations of their thermal conductivity. Thermal conductivity is typically derived from measurements of thermal diffusivity and determination of the specific heat capacity. The uncertainty in this method (often 10% or more) is frequently attributed to measurement of heat capacity such that estimated values are often more accurate. Inconsistencies between reported thermal conductivity of Mg3(Sb,Bi)2 compounds may be attributed to the different values of heat capacity measured or used to calculate thermal conductivity. The high anharmonicity of these materials can lead to significant deviations at high temperatures from the Dulong-Petit heat capacity, which is often a reasonable substitute for measurements at high temperatures. Herein, a physics-based model is used to assess the magnitude of the heat capacity over the entire temperature range up to 800 K. The model agrees in magnitude with experimental low-temperature values and reproduces the linear slope observed in high-temperature data. Owing to the large scatter in experimental values of high-temperature heat capacity, the model is likely more accurate (within ±3%) than a measurement of a new sample even for doped or alloyed materials. It is found that heat capacity for the solid solution series can be simply described (for temperatures: 200K≤T≤800K) by the polynomial equation:cp[Jg−1K−1]=3NRMW(1+1.3×10−4T−4×103T−2),where 3NR=124.71Jmol−1K−1, MW is the molecular weight [gmol−1] of the formula unit being considered, and T is temperature in K. This heat capacity is recommended to be a standard value for reporting and comparing the thermal conductivity of Mg3(Sb,Bi)2 including doped or alloyed derivatives. A general form of the equation is given which can be used for other material systems.

Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg3.2Sb1.5Bi0.49Te0.01

Zintl compound n-type Mg3(Sb,Bi)2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material.

Improving the thermoelectric performance in Mg3+xSb1.5Bi0.49Te0.01 by reducing excess Mg

The thermoelectric performance of Mg3+xSb1.5Bi0.49Te0.01 was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal x from 0.2 to 0.01 in Mg3+xSb1.5Bi0.49Te0.01, leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg3+xSb1.5Bi0.49Te0.01; however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material.

Isotropic Conduction Network and Defect Chemistry in Mg3+δSb2‐Based Layered Zintl Compounds with High Thermoelectric Performance

Thermoelectric performance in the layered Zintl phase n-type Mg3+δ (Sb,Bi)2 is reported. Insertion of the excess Mg into the compounds is crucial for realizing n-type carrier transport with multivalley and isotropic character. An excellent ZT of 1.51 ± 0.06 at 716 K is achieved in the sintered polycrystals at the composition of Mg3.2 Sb1.5 Bi0.49 Te0.01 .