Trans. Nonferrous Met. Soc. China 31(2021) 753−763 Phase formation and thermoelectric properties of Zn1+xSb binary system Ahmad OSTOVARI MOGHADDAM1, Evgeny TROFIMOV1, Ting ZHANG2, Jordi ARBIOL2, Andreu CABOT3,4 1. Department of Materials Science, Physical and Chemical Properties of Materials, South Ural State University, 76 Lenin Ave, Chelyabinsk, 454080, Russia; 2. Catalan Institute of Nanoscience and Nanotechnology (ICN2), Consejo Superior de Investigaciones Científicas (CSIC) and Barcelona Institute of Science and Technology (BIST), Campus Autonomous University of Barcelona (UAB), Bellaterra, 08193 Barcelona, Spain; 3. Catalonia Institute for Energy Research-IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain; 4. Catalan Institution for Research and Advanced Studies-ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain Received 19 April 2020; accepted 18 December 2020 Abstract: The phase formation and thermoelectric (TE) properties in the central region of the Zn−Sb phase diagram were analyzed through synthesizing a series of Zn1+xSb (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) materials by reacting Zn and Sb powders below the solidus line of the Zn−Sb binary phase diagram followed by furnace cooling. In this process, the nonstoichiometric powder blend crystallized in a combination of ZnSb and β-Zn4Sb3 phases. Then, the materials were ground and hot pressed to form dense ZnSb/β-Zn4Sb3 composites. No traces of Sb and Zn elements or other phases were revealed by X-ray diffraction, high resolution transmission electron microscopy and electron energy loss spectroscopy analyses. The thermoelectric properties of all materials could be rationalized as a combination of the thermoelectric behavior of ZnSb and β-Zn4Sb3 phases, which were dominated by the main phase in each sample. Zn1.3Sb composite exhibited the best thermoelectric performance. It was also found that Ge doping substantially increased the Seebeck coefficient of Zn1.3Sb and led to significantly higher power factor, up to 1.51 mWꞏm−1ꞏK−2 at 540 K. Overall, an exceptional and stable TE figure of merit (ZT) of 1.17 at 650 K was obtained for Zn1.28Ge0.02Sb. Key words: Zn1+xSb; ZnSb/β-Zn4Sb3 composite; thermal stability; thermoelectric properties Seebeck coefficient, σ is the electrical conductivity, 1 Introduction k is the thermal conductivity and T is the thermo- dynamic temperature [3−5]. The design and engineering of thermoelectric Among the known TE materials, the binary (TE) materials are fascinating research areas due to Zn−Sb system provides several environmental the huge economic and social implications of the friendly and relatively low cost phases with direct conversion of heat into electricity and vice excellent TE properties in the intermediate versa. Although, TE modules are relatively simple temperatures range (473−673 K). Zn−Sb materials and reliable, as they contain no fluids or moving could replace current thermoelectric materials parts, their efficiency needs to be improved and operating in this temperature range that generally their cost should be reduced to become cost- contain rare and toxic elements, such as Te and effective for widespread potential application [1,2]. Pb [6]. Among different Zn−Sb phases, the two The efficiency of a TE module can be estimated by with the highest potential are ZnSb and β-Zn4Sb3. a material figure of merit, ZT=S2σT/k, where S is the β-Zn4Sb3 crystallizes in a hexagonal crystal Corresponding author: Ahmad OSTOVARI MOGHADDAM; E-mail: ostovary@aut.ac.ir, ostovarim@susu.ru DOI: 10.1016/S1003-6326(21)65536-X 1003-6326/© 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press 754 Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 structure ( R 3c space group) consisting of 6 Sb 42 ZnSb and β-Zn4Sb3. dimers, 18 isolated Sb3− and 39 Zn2+ cations [7]. In the present work, a series of Zn1+xSb This complex structure provides β-Zn4Sb3 with an (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) materials were ultra-low thermal conductivity and an outstanding synthesized by solid state reaction of precursor TE figure of merit ZT=1.4 at 748 K [8]. However, powders followed by furnace cooling at a low rate thermal instability is a well-known concern in all near to thermodynamic equilibrium. In contrast to β-Zn4Sb3 TE compounds, which has hindered their the conventional synthesis routes of β-Zn4Sb3 that practical applications [9,10]. β-Zn4Sb3 shows two typically employ a high temperature of about polymorphic phase transitions. It undergoes a 904 K, the melting point of Sb (or more routinely of transition to disordered α phase below 263 K and about 1023 °C to speed up the reaction), followed transforms to γ-Zn4Sb3 above 765 K [6]. More by quenching to room temperature [17], we used a importantly, degradation in β-Zn4Sb3 begins at significantly lower temperature of 800 K followed temperatures below the anticipated transition by low rate furnace cooling to hinder the formation temperature (765 K) due to Zn loss and the of unstable phases. The Zn−Sb phases formed in subsequent formation of ZnSb and Zn phases as this process and their TE properties were partial inclusions [11]. Furthermore, there are some thoroughly investigated. reports on the decomposition of β-Zn4Sb3 to ZnSb and Zn even below 500 K [12]. 2 Experimental On the other hand, ZnSb shows suitable thermal stability over the temperature range of Zn1+xSb (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) 300−850 K. In comparison with β-Zn4Sb3, ZnSb materials were synthesized by standard high possesses higher electrical resistivity and larger temperature reaction of the elemental powders Seebeck coefficient [13−15]. Nevertheless, its high mixture. High purity Sb (99.5%, Alfa-Aesar) and thermal conductivity leads to relatively low TE Zn (99.9%, Alfa-Aesar) elemental powders in the performance with a maximum ZT around 0.6 at appropriate ratio were weighed and placed into 550 K [16]. A common route to decrease thermal quartz ampoules. The handling of powders was conductivity is through introducing phonon carried out under argon atmosphere inside a glove scattering centers at different lengths scales such as box. The filled ampoules were then slowly point, line and planar lattice defects, grain evacuated and filled with nitrogen for three times boundaries, embedded nanodomains and secondary and finally sealed under a vacuum of about phases. In our previous study, we demonstrated the 1.33×10−2 Pa. The sealed ampoules were placed potential of using ZnSb/Zn4Sb3 interfaces to vertically inside a furnace, heated to 800 K at a rate promote the phonon scattering and reached a ZT of 100 K/h and soaked for 18 h at that temperature. value of 1.2 in Ge-doped ZnSb/β-Zn4Sb3 nano- The furnace was subsequently turned off and the composites [6]. ampoules were allowed to cool down to ambient In spite of the numerous publications on temperature inside the furnace. The ampoules were Zn−Sb materials, there is still a lack of knowledge broken and the out-coming samples were ground in on the phase formation and TE properties in the an agate mortar inside the glove box. The obtained central region of the Zn−Sb phase diagram between powders were loaded into 8 mm-diameter graphite ZnSb and Zn4Sb3 compositions, i.e. around Zn1.3Sb. dies and uniaxiallly hot pressed at 673 K and After further and thorough study of this system, we 100 MPa for 5 min, under Ar atmosphere. The realized that small off-stoichiometries in the Zn−Sb obtained pellets were then annealed at 653 K for 1 h binary system systematically resulted in the under Ar atmosphere. The relative densities of all simultaneous formation of β-Zn4Sb3 and ZnSb the consolidated samples in this study were ~95% phases. Additionally, the β-Zn4Sb3 decomposition of the theoretical value, as measured by Archimedes’ also occurs during the thermal cycles to which the method. Besides, to optimize TE properties of the material is exposed in working conditions. Thus, we non-stoichiometric Zn1+xSb samples, Ge-doped believe that it is of particular scientific and Zn1.28Ge0.02Sb sample was also produced using technological importance to examine the TE exactly the same procedure detailed above. properties of materials containing both phases, X-ray diffraction (XRD) analysis of the hot- Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 755 pressed samples was carried out on a Bruker AXS and β-Zn4Sb3 phases, without unreacted Zn and Sb D8 ADVANCE X-ray diffractometer using Cu Kα elements. Quantitatively, the amount of β-Zn4Sb3 radiation (λ=0.15406 Å) with a scan rate of 5 s per phase increased from 0 to 11.6%, 38.9%, 49.8%, step and step size of 0.01°. Rietveld refinements 77.1% and 85.6% in samples with x=0, 0.05, 0.1, were performed using the GSAS-II package for 0.15, 0.25 and 0.3, respectively, as determined by quantitative phase analysis [18]. The morphology quantitative Rietveld analyses. and elemental analysis of the samples were examined using a scanning electron microscope (SEM, Zeiss Auriga) equipped with an energy dispersive X-ray spectroscopy (EDS) detector (Oxford Instruments). High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) studies were performed on a field emission gun FEI Tecnai F20 microscope at 200 kV with a point-to-point resolution of 0.19 nm. High angle annular dark-field (HAADF)-STEM was combined with EELS in the Tecnai microscope by using a GATAN QUANTUM filter. Seebeck coefficient and electrical resistivity were measured simultaneously during the sample heating up cycle over the temperature range of 300−650 K in a LSR−3 LINSEIS system under helium atmosphere. Thermal conductivity was calculated as a product of thermal diffusivity (λ), Fig. 1 XRD patterns of Zn1+xSb materials for different heat capacity (cp) and mass density (ρ′) of the values of x samples (k=λꞏρ′ꞏcp). Thermal diffusivity was measured using XFA 600 Xenon Flash apparatus. It has been previously reported that a pure The specific heat (cp) and the density were β-Zn4Sb3 phase could be obtained when the Zn to estimated using Dulong−Petit approximation and Sb ratio falls between 1.3 and 1.33 [19]. If the Zn Archimedes’ method, respectively. content is higher than 1.33%, there will not be enough lattice interstitial sites for all the Zn 3 Results and discussion atoms, resulting in the precipitation of Zn or the formation of ZnSb depending on the amount 3.1 Crystallographic phase of Sb atoms present in the initial composition. The XRD spectra of hot-pressed Zn1+xSb (x=0, BALASUBRAMANIAN et al [20] found that 0.05, 0.1, 15, 0.25, 0.3) samples are shown in Fig. 1. in a Co-doped Co0.05Zn3.95Sb3 sample, various A pure ZnSb phase (JCPDS card 96-900-8883) with combinations of the secondary phases such as no traces of un-reacted Zn and Sb elements nor of ZnSb, Zn and Sb formed in different processing β-Zn4Sb3 (JCPDS card 96-400-1475) was obtained routes. They attributed this observation to the lack for x=0. Using an excess of Zn, the β-Zn4Sb3 phase of a sufficient amount of Zn to form single phase of appeared and its fraction increased with increasing Zn4Sb3. However, we detected a combination of x in Zn1+xSb. At x=0.15, nearly the same amount of ZnSb and β-Zn4Sb3 phases for all Zn1+xSb samples both ZnSb and β-Zn4Sb3 phases was detected. even at Zn content lower than 1.3%, and the Further increasing x up to 0.3 did not allow fraction of β-Zn4Sb3 phase increased with producing pure β-Zn4Sb3 owing to the relatively increasing x in Zn1+xSb. ZHANG et al [21] also low synthesis (reaction) temperature, and even at obtained a combination of ZnSb and β-Zn4Sb3 x=0.3, the synthesized samples contained minor phases when the starting compositions were amount of ZnSb. For all other samples, the XRD Zn56Sb44, Zn57Sb43 and Zn58.5Sb41.5. Recently, LO spectra were consistent with a pure mixture of ZnSb et al [17] reported that pure Zn13−δSb10 (or Zn4Sb3) 756 Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 phase can be synthesized with the Zn to Sb ratio of followed by slow cooling results in materials with a 1.3, and there is no need to use a Zn-richer ratio of superior thermal stability compared to high- 1.33 (Zn4Sb3 composition). They also obtained temperature synthesis followed by quenching. materials containing both ZnSb and β-Zn4Sb3 phases at room temperature in samples with the 3.2 Microstructural characterization starting composition of Zn9Sb7, Zn12.7Sb10 and The microstructural morphologies of Zn1+xSb Zn12.8Sb10. However, our study revealed that not (x=0.3) sample and the EDS spectrum of the only ZnSb forms as the second phase in β-Zn4Sb3 outlined area are shown in Fig. 2(a). Quantitative matrix when the starting composition is EDS results are presented in Table 1. A typical Zn-deficient Zn1.3−δSb, but also the β-Zn4Sb3 morphology of solidified samples, coarse grains secondary phase precipitates in a ZnSb matrix when with unclear interfaces, was observed in the fracture the starting composition is Zn-rich Zn1+δSb. surface of hot-pressed Zn1.3Sb sample. Identical Besides, it should be also noted that we used a morphological characteristics were also observed synthesis temperature of 800 K, where both ZnSb for all the other samples. The results of EDS and β-Zn4Sb3 phases are in solid state. We believe analysis indicated a uniform distribution of Zn and that synthesizing below solidus temperatures Sb elements with no significant segregation on the (b) Fig. 2 SEM image of fracture surface of Zn1.3Sb sample and corresponding EDS spectrum of outlined region (a), and EELS chemical composition maps obtained from red squared area of STEM micrograph of Zn1.1Sb sample (Individual Zn L2,3-edges at 1020 eV (red), and Sb M4,5-edges at 528 eV (green)) (b) Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 757 Table 1 EDS analysis results of regions outlined in Fig. 2 The high-resolution TEM (HRTEM) images of R1 R2 Zn1+xSb (x=0.1) sample are shown in Fig. 3. A Element thorough power spectrum analysis of the HRTEM wt.% at.% wt.% at.% micrograph revealed two different crystallographic Zn 41.93 56.35 39.95 56.45 domains of ZnSb and Zn4Sb3 phases. For both Sb 58.07 43.65 60.05 43.55 rectangular areas in Fig. 3, the crystal structure was Average chemical composition: Zn1.29Sb found to match the orthorhombic ZnSb phase visualized along different zone axes. The power microscopic scale. A slight Zn deficiency was spectrum of Area R1 disclosed the crystal structure observed for all samples, which may be ascribed to to be consistent with the orthorhombic ZnSb phase the p-type conductivity of all Zn1+xSb samples. (space group: Pbca) with a=6.2016 Å, b=7.7416 Å However, the average EDS results were very close and c=8.0995 Å. Besides, the ZnSb lattice fringe to the compositions of starting powders. The distances were measured to be 0.169, 0.193 and elemental distribution within the Zn1+xSb samples 0.280 nm, at 45.26° and 80.65°, which was was further examined by HAADF-STEM interpreted as the orthorhombic ZnSb phase (space characterization and EELS chemical composition group: Pbca), visualized along its [215] zone axis. maps (Fig. 2(b)). EELS elemental maps confirmed For the Area R2, the lattice fringe distances were the homogenous distribution of Zn and Sb all ascertained to be 0.337, 0.192 and 0.279 nm, at through each crystal domain. The results suggested 45.11° and 79.98°, which could be interpreted as that non-stoichiometric Zn1+xSb materials with the orthorhombic ZnSb phase visualized along its uniform chemical distribution could be synthesized [25 1] zone axis. A detailed comparison between at relatively lower temperatures than those reported the theoretical and experimental bulk plane in the literatures [17,22,23]. distances and angles is shown in Table 2. Extensive Fig. 3 HRTEM micrograph of consolidated Zn1.1Sb sample showing both ZnSb and β-Zn4Sb3 phases (Green-circled areas show β-Zn4Sb3 domains; Enlarged images from green and yellow rectangular areas and corresponding power spectrum are also shown, interpreted as orthorhombic ZnSb phase (space group: Pbca), visualized along its [215] and [25 1] zone axes, respectively) Table 2 Comparison between experimental and theoretical bulk plane spacing distances and angles between planes R1 R2 Spot Experimental/nm [215] ZnSb (Pbca)/nm Experimental/nm [25 1] ZnSb (Pbca)/nm 1 0.169 0.164 (240) 0.337 0.339 (204) 2 0.193 (45.26° vs Spot 1) 0.194 (44.83°) (3 1 1) 0.192 (45.11° vs Spot 1) 0.194 (44.49°) (31 1) 3 0.280 (80.65° vs Spot 1) 0.271 (80.45°) (21 1) 0.279 (79.98° vs Spot 1) 0.271 (78.59°) (211) 758 Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 HRTEM analysis also disclosed the presence of Seebeck coefficient of Zn1.3Sb increased from some β-Zn4Sb3 regions, as indicated in Fig. 3. In 124.5 µV/K at room temperature to 189.4 µV/K addition, structural defects such as dislocations at near 650 K. The room temperature Seebeck were also detected in the high magnification coefficient of ZnSb was 286 µV/K and decreased to HRTEM image of ZnSb phase (as indicated in 258 µV/K at 650 K. The other materials exhibited a Fig. 3). These structural defects favor the phonon trend similar to ZnSb or β-Zn4Sb3 depending on the scattering and consequently decrease the thermal percentages of these two phases. conductivity [24]. Both ZnSb and Zn4Sb3 phases exhibited polycrystalline nature with a high degree of crystallinity. 3.3 Thermoelectric properties Figure 4 displays the temperature dependence of the electrical resistivity, Seebeck coefficient and power factor of Zn1+xSb (x=0, 0.05, 0.1, 15, 0.25, 0.3) samples. For pure ZnSb (x=0), the electrical resistivity slightly increased with temperature up to about 500 K, and then decreased with further increase in temperature. The measured electrical resistivity of ZnSb at room temperature was 86.54 µΩꞏm, which was close to the value of ~66 µΩꞏm reported by XIONG et al [25], but it was significantly lower than the value of 250 µΩꞏm reported by VALSET et al [26] for ZnSb. This wide range of measured resistivities may be directly attributed to the different contents of β-Zn4Sb3 phase and its distribution within the ZnSb matrix [6]. The room temperature resistivity of Zn1+xSb samples continuously decreased with increasing Zn content, and reached the value of 21.47 µΩꞏm for x=0.3, which was close to the typical reported resistivity of around 20 µΩꞏm for β-Zn4Sb3 at room temperature [12]. Interestingly, all other samples exhibited an intermediate behavior between ZnSb and β-Zn4Sb3 phases. For 0≤x≤0.15, where ZnSb was the dominant phase in Zn1+xSb samples, the measured resistivities showed a similar trend to that of ZnSb, while for x>0.15, the measured resistivity trend resembled that of β-Zn4Sb3. The Zn1+xSb materials exhibited the p-type conductivity in the entire temperature and compositional range of 0≤x≤0.3. The Seebeck coefficient decreased with increasing x in Zn1+xSb materials, following an identical trend as the Fig. 4 Temperature dependence of electrical resistivity, electrical resistivity. The highest and the lowest  (a), Seebeck coefficient, S (b), and power factor, FP (c), Seebeck coefficients were measured from the nearly for Zn1+xSb samples, indicating decrease in electrical pure ZnSb and β-Zn4Sb3 samples in the entire resistivity and Seebeck coefficient with increasing x in temperature range of 300−650 K, respectively. The Zn1+xSb samples Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 759 The power factor (PF) of Zn1+xSb materials Beside point defects and nanostructure features, also exhibited distinct behaviors in the the intrinsic structural and compositional compositional ranges of 0≤x≤0.15 and x>0.15. distribution is a dominant factor determining When 0≤x≤0.15, an almost identical PF was thermal conductivity. In β-Zn4Sb3, the anharmonic obtained for all Zn1+xSb samples in the entire motion of the Sb1 atoms coordinated only by Zn temperature range measured, which was atoms results in ultralow thermal conductivities [11]. comparable to the typical behavior of nearly pure On the other hand, the simple crystal structure of ZnSb [25]. Here, the PF of our ZnSb compound ZnSb leads to a relatively high thermal conductivity. was about 0.95 mWꞏm−1ꞏK−2 at room temperature, In Zn1+xSb materials, the phase fraction of β-Zn4Sb3 slightly fluctuated with increasing temperature and significantly increased from 0 for ZnSb to 85.6% reached 0.87 mWꞏm−1ꞏK−2 at 650 K. On the for Zn1.3Sb (x=0.3), as obtained from XRD results. The β-Zn4Sb3 content itself and indirectly the other hand, Zn1.25Sb and Zn1.3Sb samples amount of ZnSb/β-Zn4Sb3 interfaces had a dominate possessed a relatively low PF at room temperature effect on the lattice (κl) and electronic (κe) thermal that significantly increased up to 0.95 and conductivity. The lattice contribution and electronic 1.04 mWꞏm−1ꞏK−2 at temperatures around 650 K, contribution to thermal conductivity of the Zn1+xSb respectively. These results suggested that the TE materials are plotted in Fig. 6. The lattice properties of Zn1+xSb materials were a combination conductivity (κl) was calculated by subtracting the of the TE properties of ZnSb and β-Zn4Sb3 electronic thermal conductivity (κe) from the dominated by the phase with higher content. measured thermal conductivity (κt). The electronic The variation of total thermal conductivity κ thermal conductivity was evaluated using the with temperature for Zn1+xSb samples is presented Wiedemann−Franz equation [24], and the Lorenz in Fig. 5. A relatively high thermal conductivity was number was estimated from the measured Seebeck observed for ZnSb over the entire temperature range measured. The stoichiometric ZnSb sample had a thermal conductivity of 1.81 Wꞏm−1ꞏK−1 at room temperature, while Zn1.3Sb (the nearly pure β-Zn4Sb3) showed a thermal conductivity of only 0.70 Wꞏm−1ꞏK−1 at room temperature. The measured thermal conductivities were in good agreement with the reported values for pure ZnSb [26] and β-Zn4Sb3 [27,28]. Thermal conductivity decreased with increasing temperature for all the samples. Fig. 5 Total thermal conductivities of Zn1+xSb samples as function of temperature, exhibiting characteristic low thermal conductivity of β-Zn4Sb3 phase with increasing x Fig. 6 lattice (a) and electronic (b) thermal conductivities in Zn1+xSb samples of Zn1+xSb compounds as function of temperature 760 Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 coefficient [29]. A lattice thermal conductivity of at 650 K increased from 189.4 µV/K for Zn1.3Sb to 1.76 Wꞏm−1ꞏK−1 was obtained for ZnSb at room 220.7 µV/K for Zn1.28Ge0.02Sb. We tentatively temperature, which was almost four times higher ascribe the measured increase of Seebeck than the value of 0.40 Wꞏm−1ꞏK−1 obtained for coefficient with the substitution of Ge to an increase Zn1.3Sb (x=0.3). The results indicated a substantial in the density of state (DOS) near the Fermi level, role for β-Zn4Sb3 content in the thermal which is indirectly associated with the modification conductivity of Zn1+xSb materials. of the electronic structure through introducing a Figure 7 displays the temperature dependence narrow band of states in the valence band. This may of thermoelectric figure of merit (ZT) for Zn1+xSb result in a large increase in the effective mass and samples. The Zn1.3Sb sample exhibited the best TE improvement of the Seebeck coefficient [32]. performance in the entire temperature range of Consequently, the PF significantly increased in the 300−650 K with a maximum ZT value of 0.99 at entire temperature range measured (insert in 650 K, owing to its low thermal conductivity. The Fig. 8(a)). The thermal conductivity of Zn1.3Sb also highest ZT value obtained here for Zn1.3Sb was increased with the introduction of Ge. However, Ge higher than that reported by ZOU et al [30] and substitution induced an large increase in ZT because WANG et al [31], but lower than that reported by of the substantial boost in the power factor. A LIN et al [12] and WANG et al [32] for pure notable ZT value of 1.17 was obtained for β-Zn4Sb3. This can be mainly attributed to the Zn1.28Ge0.02Sb at 650 K. Besides, Zn1.28Ge0.02Sb presence of different contents of ZnSb in the exhibited superior thermal stability than β-Zn4Sb3 synthesized materials. On the other hand, the materials synthesized by the conventional melting maximum ZT value of ZnSb was about 0.52 at and quenching routes. Almost identical transport 650 K, close to the reported values of 0.5 by VALSET et al [26] and 0.6 by POTHIN et al [16]. Fig. 7 ZT values for Zn1+xSb materials as function of temperature with Zn1.3Sb showing the best TE performance with maximum ZT value of 0.99 at 650 K To further optimize the TE properties of Zn1+xSb materials, we partially substituted Ge for Zn in the Zn1.3Sb sample. Figure 8 shows the TE properties of Ge-substituted Zn1.28Ge0.02Sb sample as a function of temperature. While the resistivity of Zn1.28Ge0.02Sb sample was nearly identical to the Fig. 8 Temperature dependence of electrical resistivity, resistivity of un-substituted Zn1.3Sb material, the Seebeck coefficient and power factor (insect) (a), and Seebeck coefficient of Ge-substituted Zn1.28Ge0.02Sb total thermal conductivities and ZT values (b) of Zn1.28- significantly increased in the entire temperature Ge0.02Sb as function of temperature with Zn1.28Ge0.02Sb range measured. The maximum Seebeck coefficient exhibiting notable ZT value of 1.17 at 650 K Ahmad OSTOVARI MOGHADDAM, et al/Trans. Nonferrous Met. Soc. China 31(2021) 753−763 761 properties were recorded for Zn1.28Ge0.02Sb during Sb elements or new phases were obtained for all five cycles of repetitive measurements up to 650 K Zn1+xSb samples with x>0. (Fig. 9). This superior stability may be attributed to (2) TE properties of all samples were a the unique characteristics of our synthesis route combination of the properties of ZnSb and β-Zn4Sb3 including reaction in the solidus region of Zn−Sb phases dominated by the phase with higher contents binary phase diagram and cooling in a thermo- in the matrix. dynamically stable situation. (3) Ge substitution in Zn1.3Sb sample led to a substantial increase in the Seebeck coefficient concomitant to a nearly unchanged electrical resistivity, which considerably increased the power factor over the entire temperature range measured. (4) A high ZT value of 1.17 was obtained for Zn1.28Ge0.02Sb at 650 K, which remained almost unchanged after five-cycle measurements. 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Department of Materials Science, Physical and Chemical Properties of Materials, South Ural State University, 76 Lenin Ave, Chelyabinsk, 454080, Russia; 2. Catalan Institute of Nanoscience and Nanotechnology (ICN2), Consejo Superior de Investigaciones Científicas (CSIC) and Barcelona Institute of Science and Technology (BIST), Campus Autonomous University of Barcelona (UAB), Bellaterra, 08193 Barcelona, Spain; 3. Catalonia Institute for Energy Research-IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain; 4. Catalan Institution for Research and Advanced Studies-ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain 摘 要：在 Zn−Sb 二元相图固相线以下使 Zn 和 Sb 粉末反应并随炉冷却，合成一系列 Zn1+xSb (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) 材料，分析 Zn–Sb 相图中心区域的相形成和热电性能。在此过程中，非化学计量比的混合粉末结晶形 成 ZnSb 和 β-Zn4Sb3 相的组合。然后，将材料研磨并热压成致密的 ZnSb/β-Zn4Sb3 复合材料。X 射线衍射、高分 辨率透射电镜和电子能量损失谱分析均未发现 Sb、Zn 元素或其他相。所有材料的热电性能都可以归结为 ZnSb 和 β-Zn4Sb3 相的热电行为的结合, 并由每种材料中的主相所决定，Zn1.3Sb 复合材料具有最好的热电性能。研究发 现，Ge 掺杂可大幅增加 Zn1.3Sb 的 Seebeck 系数且显著提高其功率因数，540 K 时达 1.51 mWꞏm−1ꞏK−2。 总之， Zn1.28Ge0.02Sb 具有优异、稳定的 ZT 值，650 K 时为 1.17。 关键词：Zn1+xSb；ZnSb/β-Zn4Sb3 复合材料；热稳定性；热电性能 (Edited by Bing YANG)