Xioo Mo, Jinsong Lio, Guoi Yun, Sh Zhu, Xioo Lei, Lihong Hung,,*,Qinyong Zhng,**, Cho Wng, Zhifeng Ren

aKey Laboratory of Fluid and Power Machinery of Ministry of Education, School of Materials Science & Engineering, Xihua University, Chengdu 610039,China

b Clean Energy Materials and Engineering Center, State Key Laboratory of Electronic Thin Film and Integrated Device, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China

c Department of Physics and TcSUH, University of Houston, Houston, TX 77204, United States

Abstract Bi2Te3 based alloys have been the most widely used thermoelectric material at low temperature for many decades.Here we report Se doped n-type Mg3Bi2 based materials with a thermoelectric figure-of-meri ZT of 0.82 at 300K and a peak ZT of 1.24 at 498K, which is comparable to the n-type Bi2Te3 and Te doped Mg3Bi1.4Sb0.6.The improved thermoelectric performance is benefite from the high carrier concentration and mobility as well as the thermal conductivity reduction.The reduced resistivity increased the power factor at all measured temperatures, leading to a higher engineering ZT (ZTeng) and engineering power factor (PFeng) for n-type Mg3Bi2.The n-type Mg3Bi1.4Sb0.6 materials are promising for thermoelectric power generation and cooling applications near room temperature.

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This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of Chongqing University

Keywords: Mg3Bi2; Zintl compound; Thermoelectric; Se-doping.

1.Introduction

Generally, almost all of the traditional energy sources are converted by thermal energy; however, the energy conversion efficien y is low, and more than half of the energy is discharged in the form of waste heat.Approximately 60% of the unrecovered waste heat is of low-grade (temperature below 500K), and low-temperature waste heat has less thermal and economic values than high-temperature waste heat.Lowgrade waste heat recovery technologies reduce the environmental impacts of fossil fuels and also improve the overall efficien y.Thermoelectric(TE)power generation is one of the most promising technologies for recycling low-quality waste heat and addressing different energy challenges [1].

The conversion efficien y of thermoelectric materials is determined by the dimensionless figure-of-meriZT=S2σT/(κe+κL), where S is the Seebeck coefficientσis the electrical conductivity,Tis the absolute temperature, andκe,κLare the electronic and lattice components of thermal conductivity, respectively [2,3].High TE property needs high Seebeck coefficient high electrical conductivity,and low thermal conductivity.However, it is very hard to independently optimize those three TE performance parameters because they are intercoupled.N-type Bi2Te3-based alloys (ZT≈1 at 400K) are one of the most widely used room-temperature TE materials [4-8].In 2011, Liu et al.[8]reported that Cu0.01Bi2Te2.7Se0.3had a peakZTvalue of 1.06 near 125 °C.

While considering the scarcity and high cost of Te in Bi2Te3alloys, one need to develop new alternative TE materials for large-scale applications at room temperature.

Zintl compound with complex crystal structure is a kind of promising TE materials, meeting the concept of “electron crystal-phonon glass”.Most of the Zintl compounds are ptype intrinsic semiconductor and hard to be doped into ntype.While Mg3Sb2-based materials show prominent n-type TE performance [9-12].In 2016, Tamaki et al.[13]reported that a peak ZT of 1.51 has been obtained for Te doped ntype Mg3.2Sb1.5Bi0.5at 716K, benefite from its multivalley conduction bands with high valley degeneracies.Generally, n-type Mg3Sb2experiences charge scattering by different sources including ionized impurities [11,14,15]and highly-resistive grain boundaries [10,16].Mg3Bi2behaves as a semimetal different from the isomorphic semiconductor Mg3Sb2.Alloying with Mg3Bi2will significantl changes the valence band structure, and also reduce the thermal conductivity[17,18].The effect of alloying on band structure implies that it is possible to adjust the applicable temperature of materials to a lower range by band engineering strategy.In 2019,Liu and co-workers [19]reported that n-type Mg3+δSbxBi2-xexhibited a comparable TE performance of Bi2Te3-xSexmaterials in the temperature range of 50-250 °C, which immediately gravitated many efforts to promote the researches on n-type Mg3+δSbBi based TE materials near room-temperature range [20,21].

The effective mass of all conduction bands decreases with the increasing Mg3Bi2content in a solid solution because a lighter band mass is favorable for mobility.Imasato et al.[22]reported that the optimum composition is expected to be around 70% of Mg3Bi2, i.e.Mg3Bi1.4Sb0.6, which is the composition with minimum effective mass while maintaining valley degeneracy.Recent studies [10,16]have reported that room-temperature thermoelectric properties can be improved by increasing the grain size to reduce the grain boundary density instead of changing the scattering mechanism, and the increased grain size does not compromise the original low thermal conductivity [22].n-type Te-doped Mg3Sb2and Mg3Bi2are promising thermoelectric materials, while it is necessary to discover new n-type dopants that are richer and cheaper than tellurium for commercial applications.Existing strategies mainly achieve high mobility through effective electron doping and the reasonable control of alloy concentration to simultaneously optimize the band structure and minimizeκL[18,22-24].Previous studies have expounded that excess magnesium helps maximize the electron concentration and also leads to an increased thermal conductivity [25,26].However,due to the high vapor pressure and easy oxidation of magnesium,the loss of magnesium and the generation of magnesium vacancies easily occur during the synthesis process, resulting in poor electronic transport properties and p-type conduction.

In this work, the thermoelectric performances of n-type Mg3.2Bi1.4Sb0.6-based material were researched and optimized, using selenium as an effective electron dopant.In order to ensure high carrier mobility,non-oxidized coarse grains were formed by direct current (DC) hot-pressing technology at 1053K.Finally, Se-doped Mg3Bi2-Mg3Sb2alloy shows a high electron concentration of 3×1019cm-3and an ultrahigh mobility of 150 cm2V-1s-1at 300K.In addition, the as-prepared alloy is an environment-friendly and sustainable high-performance thermoelectric material for low-temperature applications.

2.Experimental details

High-purity magnesium powder(Mg,99.98%,Alfa Aesar),bismuth pieces (Bi, 99.99%, Alfa Aesar), antimony shots (Sb,99.99%, Alfa Aesar), and selenium powder (Se, 99.99%, Alfa Aesar) were weighed according to the nominal composition of Mg3.2Bi1.4Sb0.6-xSex(x=0, 0.005, 0.01, 0.02, 0.04).Small amount of excess Mg was added to compensate the evaporation of Mg.All the elements were weighed in an argonfille glove box, then loaded into a stainless-steel jar, mixed for 0.5h without grinding balls, and then ball-milled continuously for 10h by a SPEX 8000M Mixer/Mill.The obtained powder was loaded into a graphite die with an inner diameter of 12.7mm and consolidated by a direct-current hot pressing under an axial pressure of 45MPa at 1053K for 2min to obtain a disk sample.

The thermal diffusivity (D) was measured on a laser flas system (LFA 457, Netzsch, Germany).The specifi heat capacity(Cp)was measured on a differential scanning calorimetry thermal analyzer (DSC 404 C, Netzsch, Germany).The thermal conductivityκwas calculated viaκ=dDCp, wheredis sample density estimated by the Archimedes method.The electrical conductivity (σ) and Seebeck coefficien (S)from 300 to 623K were measured simultaneously on a ZEM-3 system (ZEM-3, ULVAC Riko, Japan).A four-probe Van der Pauw method was used for Hall coefficien (RH) measurement under a magnetic fiel of 1.5 T.The Hall carrier concentration(nH)was obtained bynH=1/(eRH),and the Hall mobility (μH) was estimated byμH=σRH.

The phase characterization was performed by X-ray diffraction (XRD, D2 PHASER, Bruker) under Cu-Kαradiation.The freshly broken surface of the sample Mg3.2Bi1.4Sb0.59Se0.01was observed by a scanning electron microscope (SEM, Quanta 250G, FEI, USA) to show the particle size.Energy dispersive spectroscopy(EDS)mapping was used to characterize the compositional homogeneity.

Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP)with the projector augmented wave (PAW) method [27,28].The Perdew-Burke-Ernzerhof (PBE) [29]was used as the exchange-correlation, Monkhorst-Packkmesh was used to optimize the structural parameter and calculate the electronic structures.The energy convergence criterion was set at 10-5eV.The spin-orbit coupling (SOC) effect and the modifie Becke-Johnson (mBJ) [30,31]potential were adopted for more accurate electronic structures.In addition,effective band structures of Mg3Bi1.375Sb0.625and Mg3Bi1.375Sb0.5Se0.125were calculated by unfolding the band structures of supercells with 40 atoms (2×2×2 unit cell) into the primitive cells as implemented in BandUP code [32,33].

Fig.2.Calculated electronic band structure of (a) Mg3Bi2 and (b) Mg3Sb2 by PBE functional with SOC and mBJ.Calculated effective band structure of (c)Mg3Bi1.375Sb0.625 and (d) Mg3Bi1.375Sb0.5Se0.125 by PBE functional.

3.Results and discussion

The Zintl compound Mg3Sb2crystallizes in a layered Mn2O3-type structure (space group=P3?m1, Number=164),where each unit cell has three Mg atoms and two Sb atoms,and Mg atoms occupy two distinct crystallographic sites(Fig.1a).XRD patterns of all polycrystalline samples with a nominal composition of Mg3.2Bi1.4Sb0.6-xSex(x=0, 0.005,0.01, 0.02, 0.04) are displayed in Fig.1b.All indexed peaks could be described by the crystal structure of Mg3Sb2phase,and all the samples are single-phase without any impurity within the detection limitation of XRD.

Fig.2a, b show the calculated band structures of Mg3Bi2and Mg3Sb2with a band gap of 0.29 and 0.64eV respectively,using PBE functional with SOC and mBJ.Fig.2c, d show the calculated effective band structure of Mg3Bi1.375Sb0.625and Mg3Bi1.375Sb0.5Se0.125, using only PEB functional without considering SOC and mBJ.The calculation method ofPBE will underestimate the band gap of materials, so that Mg3Bi1.375Sb0.625and Mg3Bi1.375Sb0.5Se0.125seem as metals or semimetals in Fig.2c, d.Actually, Mg3Bi1.375Sb0.625and Mg3Bi1.375Sb0.5Se0.125are semiconductors, which can be confirme by Fig.S2 in the Supplementary Information.The calculated effective band structures of Mg3Bi1.375Sb0.625and Mg3Bi1.375Sb0.5Se0.125is close to our experimental composition of and Mg3.2Bi1.4Sb0.6and Mg3.2Bi1.4Sb0.59Se0.01.Obviously, the band degeneracy is significantl enhanced after the alloying of Mg3Bi2and Mg3Sb2, as shown in Fig.2c.More importantly, the Fermi energy moves near to the bottom of conduction bands after Se doping, indicating a n-type semiconductor behavior for Mg3.2Bi1.4Sb0.59Se0.01, consistent with our experimental results.And the effective band structure of Mg3Bi1.375Sb0.5Se0.125in Fig.2d displays that alloy Mg3.2Bi1.4Sb0.59Se0.01has a high valley degeneracy including the conduction band minimum located at M, K,Γ, and L points, i.e., there will be a significan improvement in electrical performance for Mg3.2Bi1.4Sb0.6alloy via Se doping.

Fig.3.(a-b) SEM image, and (c-f) EDS element mappings of the fracture surface of Mg3.2Bi1.4Sb0.59Se0.01 sample.

Fig.3a, b shows the SEM image of the fracture surface of Mg3.2Bi1.4Sb0.59Se0.01sample, which reveals that the sample was dense and possessed a layered structure.The EDS compositional mappings illustrate that Mg, Sb, Bi, and Se atoms were uniformly distributed in the sample, as shown in Fig.3c-f.

Undoped Mg3.2Bi1.4Sb0.6is a n-type semiconductor with ultra-high resistivity and low carrier concentration.The temperature dependent Hall carrier concentration (nH) and Hall mobility(μH)of Mg3.2Bi1.4Sb0.6-xSexare presented in Fig.4a,b, respectively.Hall carrier concentration increases signifi cantly with increasing Se doping content.The room temperaturenHof Se-doped samples is in the range of 1.7×1019cm-3to 3.4×1019cm-3, comparable with Te-doped samples[11,34-36].Here, excess Mg (3+0.2 in the formula) is required to ensure the stable acquirement of n-type properties[37].Fig.4b expresses that temperature-dependentμHof all the Se-doped samples follows a decreasing trend with the relationship ofμH～T-p(1≤p≤1.5), implying a dominant charge scattering by acoustic phonons.The mobility starts to drop above 450K can be attributed to the increasing vacancy defect concentration caused by the loss of Mg at high temperature.In the temperature range of 300-450K,μHdeviated fromT-1toT-0.5due to alloy scattering.The relationships of experimentalnHandμHn-Mg3Sb2TE materials with different dopants are compared in Fig.4c, including our results.Basically, the Hall carrier concentration of Sedoped Mg3.2Bi1.4Sb0.6samples in present work is comparable to that of Te-doped Mg3Sb2[17,34], Y doped Mg3SbBi[23]and Sc doped Mg3SbBi [38], also it is higher than that of Se-doped Mg3Sb2[39,40].Moreover, the Hall mobility is also higher than the results reported in the literatures[17,23,34,38].Undoubtedly, Se is an effective n-type dopant that provides optimum carrier concentration and Hall mobility for Mg3.2Bi1.4Sb0.6, leading to the improved power factor.Pisarenko plots ofSversusnHare calculated based on Eqs.(1)-(6) using a single parabolic band (SPB) model, assuming the acoustic phonon scattering mechanism (scattering factorr=-1/2), the results are presented in Fig.4d [41,42].

Fig.4.Temperature dependent (a) Hall carrier concentration and (b) Hall mobility, (c) Hall carrier concentration as a function of Hall mobility, and (d)Pisarenko plots for Mg3.2Bi1.4Sb0.6-xSex samples at 300, 400 and 500K, data in references are shown for comparison (Se doped Mg3Bi1.5Sb0.5 [39,40], Te doped Mg3Bi1.5Sb0.5 [17,34], Sc doped Mg3BiSb [38]and Y doped Mg3BiSb [23]).The curves are generated by SPB model.

WhereFn(η) is thenth order Fermi integral,ηthe reduced Fermi energy,ethe electron charge,rthe scattering factor,kBBoltzmann's constant,hPlank's constant,rHthe Hall factor, andxthe variable of integration.Based on the experimental Seebeck coefficient and carrier concentrations, a density of state (DOS) effective massm*～1.2mewas derived for Mg3.2Bi1.4Sb0.6-xSexsamples, slightly larger than that of Mg3Bi1.4Sb0.6(m*～1.1me) [22].

Fig.5.Temperature dependent (a) resistivity, (b) Seebeck coefficient (c) power factor (d) total thermal conductivity, (e) electrical thermal conductivity, and(f) lattice thermal conductivity of Mg3.2Bi1.4Sb0.6-xSex samples.

As highly-degenerated multi-valley conduction bands were involved in the charge transportation [13,34,43,44], high mobility and carrier concentration will result in low resistivity and low Seebeck coefficients Fig.5a,b show the temperature dependent resistivity and Seebeck coefficients respectively.Both the resistivity and Seebeck coefficien continuously increase with the increasing temperature, indicating degenerate semiconductor characteristic.With increasing content of Se,the resistivity increases first and reaches the maximum limit whenx=0.01.The significan decrease in resistivity can be attributed to the high carrier concentration and mobility.The room-temperature Seebeck coefficien of Se-doped samples ranged between -175μV K-1and -239μV K-1, which is comparable to the Te-doped samples [13,22].It is evident from Fig.5c that over the entire temperature range, the power factor (PF) of Mg3.2Bi1.4Sb0.6-xSexwas significantl higherthan those of Se-doped samples in the literatures [39,40].The power factor of the sample withx=0.01 was 29μW cm-1K-2at room temperature and decreased to 22μW cm-1K-2at 623K.Such a high power factor was generated from the high Hall mobility, and 70% of Mg3Bi2led to the formation of coarse grains, resulting in ultra-low resistivity [18,22,23].The temperature-dependent total thermal conductivity (κ), electronic thermal conductivity (κe) and lattice thermal conductivity (κL) and bipolar thermal conductivity (κb) of Mg3.2Bi1.4Sb0.6-xSexare presented in Fig.5d-f.κewas estimated according to the Wiedemann-Franz law,κe=LT/ρ, whereLis the Lorenz number.The Lorenz number was determined by the SPB model assuming acoustic phonon scattering [8,45,46].κeshown in Fig.5e increased after Se doping, which mainly due to the reduction of resistivity.As shown in Fig.5f, the sum of the lattice thermal conductivity and bipolar thermal conductivityκL+κbincreases with increasing temperature, this may because the bipolar effect occurs at high temperature (＞450K) in the samples.Looking carefully at our experimental data in Fig.4a and 4b, the carrier concentration increases while the mobility decreases at high temperature, and the change trend of Seebeck coeffi cient changes at 450K, these phenomena should be related to bipolar effects.Mg3.2Bi1.4Sb0.595Se0.01shows a very lowκLof 0.6-0.7W m-1K-1at low temperature range, yielding a high room temperature thermoelectric performance.The reduction ofκLcan be ascribed to alloy scattering caused by the compositional change in Mg3Bi2/Mg3Sb2alloys [22].

Fig.6.(a) Temperature dependent ZT value of Mg3.2Bi1.4Sb0.6-xSex samples, data in references are shown for comparison [8,22], (b) comparison of peak ZT between different material composition [22,24,39,40].

Fig.7.Calculated (a) PFeng, (b) ZTeng, and (c) maximal conversion efficien y of Mg3.2Bi1.4Sb0.6-xSex samples.The hot side temperature was varied up to 623K while the cold side temperature was kept at 300K.

TheZTvalues of Mg3.2Bi1.4Sb0.6-xSexsamples are presented in Fig.6a.A peakZTof 1.24 at 498K was obtained for Mg3.2Bi1.4Sb0.59Se0.01, profi from its high power factor and low thermal conductivity.The Mg3.2Bi1.4Sb0.595Se0.005sample shows the highest room temperatureZTof ～0.82, which is comparable with n-Bi2Te3[8], and is significantl higher than that of Te/Se-doped sample in literatures [22,24,39,40],as shown in Fig.6b.

In addition, as the engineering figur of merit (ZT)engand the engineering power factor (PF)engare widely used to evaluate the conversion efficien y (η) from heat to electricity energy, the (ZT)eng, (PF)eng, andηof Mg3.2Bi1.4Sb0.6-xSexmaterials were calculated by the following equations [47]:

whereS(T),ρ(T), andκ(T) are temperature-dependent thermoelectric properties,ThandTcare the hot side temperature and the cold side temperature, respectively, andαi(i=0, 1,2) is a dimensionless intensity factor of the Thomson effect.As the temperature gradient in power generation applications is generally very large, the cumulative sum of all of the temperature segments is a more accurate quality factor.The calculated temperature-dependent (PF)eng, (ZT)eng, andηat the cold-side temperature of 300K are presented in Fig.7.(ZT)engandηincreased with the increasing Se concentration fromx=0.005 tox=0.01, while started to decrease slightly fromx=0.02 tox=0.04 at high temperatures.Among all samples,Mg3.2Bi1.4Sb0.59Se0.01had the highest (ZT)engandηof about 0.9 and 12%, respectively.

4.Conclusions

A promising n-type Mg3Bi2-based thermoelectric material suitable for the low-temperature TE applications was successfully synthesized by ball milling and hot pressing.Se was found to be an effective and environmentally friendly n-type dopant in comparison with Te.A peakZTvalue of 1.24 at 498K was obtained for Mg3.2Bi1.4Sb0.59Se0.01and a high room temperatureZTof 0.82 was achieved for Mg3.2Bi1.4Sb0.595Se0.005, meaning a good room temperature thermoelectric material attributed to its high power factor and low lattice thermal conductivity.Coarse grains successfully obtained by alloying large amount of Mg3Bi2with Mg3Sb2, which effectively reduces the grain boundary resistance and greatly improves the Hall mobility.Moreover,Mg3.2Bi1.4Sb0.59Se0.01shows high (ZT)engand high (PF)eng,ensuring the possible applications of Se doped Mg3.2Bi1.4Sb0.6materials at low-grade-temperature.

Declaration of Competing Interest

The authors declare no competing interests.

Acknowledgements

The work performed is supported by Young Scientist Fund of National Natural Science Foundation of China (No.51601152), Chunhui Program from Education Ministry of China,Open Research Subject of Key Laboratory of Fluid and Power Machinery of Ministry of Education (No.SZJJ2017-082), and the Sichuan Science and Technology Program (No.2019JDTD0024).