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Novel Ge-Sb-Te thermoelectric materials: A demonstration for an efficient diffusion couple technique in expediently exploiting new thermoelectric materials
Ceramics International 45 (2019) 16039–16045
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Novel Ge-Sb-Te thermoelectric materials: A demonstration for an efficient diffusion couple technique in expediently exploiting new thermoelectric materials
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Jian Zhanga, Yonggao Yana,∗, Hongyao Xieb, Ting Zhua, Cheng Zhanga, Junhao Qiua, Lei Yaoa, Xinfeng Tanga,∗∗ a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China Department of Chemistry, Northwestern University, Evanston, IL, 60208, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Combinatorial metrology Diffusion couple Micro-area Seebeck coefficient Ge-Sb-Te thermoelectric materials
An efficient diffusion couple technique is proposed for rapid screening of potential thermoelectric materials and it has been explored in a (GeTe)m (Sb2Te3)n pseudobinary system. In this study, a combinatorial sample library of GeTe-Sb2Te3 diffusion couple with rich compositions and structures was prepared by plasma activation sintering (PAS). The relationship among composition, structure and performance of the materials was revealed by analyzing the chemical composition, crystal structure and thermoelectric properties of the microdomains in the combinatorial sample library, so as to achieve the purpose for rapid identification of Ge-Sb-Te compounds with excellent thermoelectric properties. The scanning results of the Seebeck coefficient of the samples library show that the sample with potential high thermoelectric performance has a chemical composition near Ge33.1Sb13.7Te53.2. The bulk materials with compositions in the vicinity of Ge33.1Sb13.7Te53.2 were prepared by a conventional melting and PAS process. Among these bulk samples, the one with a nominal composition of Ge38Sb10.3Te51.7 achieves a maximum ZT value of 0.55 at 673 K, indicating that the constituent phases Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6 observed in this sample could be potential candidates with excellent thermoelectric properties. These two ternary compounds should be investigated for further optimization. The results of this study provide a reference for the application of combinatorial metrology for screening and optimizing new thermoelectric materials.
1. Introduction Due to the shortage of energy and the serious environmental pollution caused by the burning of fossil fuel, thermoelectric technology has received extensive attention as an environmental-friendly technology for conversion between heat and electricity [1–3]. The performance of thermoelectric materials is generally characterized by the dimensionless figure of merit ZT,calculated by ZT = α2σT/κ,where α is Seebeck coefficient,σ is the electrical conductivity,T is the absolute temperature,κ is the thermal conductivity. The widespread application of thermoelectric technology has not been achieved yet, and it will greatly depend on identifying new thermoelectric materials with excellent performance. Compared with the electrical and thermal conductivity, the Seebeck coefficient α contributes more substantially to the overall thermoelectric properties since α is with a power term in the
∗
formula. Among the above parameters, the Seebeck coefficient is a parameter reflecting the intrinsic band structure of the material, which can only be manipulated within a certain range by controlling the carrier concentration. The thermoelectric material with excellent performance must possess a suitable electronic band structure, and the band structure can be reflected by the Seebeck coefficient of the material. Therefore, it is possible to find some new thermoelectric materials with desirable properties with characterizations of the Seebeck coefficient. A common method for material research is to prepare a sample of one composition at a time, which is a very time-consuming process with a high material cost. While combinatorial materials science has the advantage for exploring a system of multiple constituent elements at a time, if combined with high-throughput characterization techniques, the relation between composition-structure-performance can be
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Yan), [email protected] (X. Tang).
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https://doi.org/10.1016/j.ceramint.2019.05.119 Received 22 April 2019; Received in revised form 8 May 2019; Accepted 12 May 2019 Available online 20 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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attained quickly, which can greatly increase the density of information attained in a single experiment, thus immensely shortening the period of exploring new materials and products [4,5]. Combinatorial metrology has been widely used in the fields of catalysis [6], pharmacy [7] and luminophor [8], nevertheless, it is rarely reported in thermoelectric materials. Although some combinatorial film libraries based on films have been achieved [9–12], the combinatorial research based on bulk materials is still in its infancy for limitations of the preparation technology and the corresponding high-throughput characterization techniques. Zhao et al. [13] studied the solution interval and the thermal properties of Mg2Si–Mg2Sn pseudobinary system. However, the conventional annealing process to obtain a diffusion couple usually takes a few weeks, so it is significant to develop an efficient diffusion couple technique and apply it to exploring the potential thermoelectric materials. In recent years, Ge-Sb-Te compounds have attracted widespread attention as thermoelectric materials [14–16]. Lee Jaeho et al. [17] synthesized the Ge2Sb2Te5 film with a Seebeck coefficient of 371 μV/K. In 2013, Sittner Ernst-Roland et al. [18] studied a film of Ge8Sb2Te11 with ZTmax = 0.7. In 2015, the Ge2Sb2Te5 compound prepared by Sun Jifeng et al. [19], exhibited the Seebeck coefficient of 200–300 μV/K at room temperature. All studies show that Ge-Sb-Te compounds have the potential to possess an excellent thermoelectric performance. Owing to the number of varieties of Ge-Sb-Te compounds could have [20], the conventional routine to explore the potential high performance materials takes a lot of work. Considering that almost all GeSb-Te compounds can be synthesized from the reaction between GeTe and Sb2Te3, the GeTe-Sb2Te3 diffusion couples was obtained by the PAS technique, then combined with the scanning Seebeck coefficient characterization, the Ge-Sb-Te compounds with potential excellent thermoelectric properties were determined and verified. This study has shown a novel way to screen and optimize thermoelectric materials quickly, efficiently at a low cost. 2. Materials and methods Preparation of the diffusion couples: High purity Sb (bulk, 99.999%) and Te (bulk, 99.999%) were weighed according to the stoichiometry of Sb2Te3. Then the mixtures of Sb and Te were sealed in evacuated quartz tube and heated to 1023 K in 5 h, kept at this temperature for 10 h, and then cooled down in furnace to get a Sb2Te3 ingot. Similarly, a GeTe ingot was obtained with Ge (bulk, 99.99%) and Te (bulk, 99.999%) at the raw materials. In the case of GeTe, the materials melt at 1373 K for 20 h and then quenched in water bath, followed by annealing at 773 K for 5 days. The obtained ingots were grounded into fine powders. As shown in Fig. 1(a), the GeTe powder and Sb2Te3 powder were put inside a mold to form a flat interface. The PAS were processed under a pressure of 40 MPa at 773 K with holding durations of 20 min, 40 min, 60 min, 80 min and 100min, respectively to accelerate the diffusion of atoms between GeTe and Sb2Te3. Then the obtained ingots were cut into several diffusion couples schematically shown in Fig. 1. Preparation of the verifying bulks: Bulk materials with optimized compositions were prepared to verify the results obtained from combinatorial library. The bulk Ge-Sb-Te materials were synthesized by similar processing as the diffusion couples. The mixtures of Ge,Sb and
Te were weighted according to the optimized composition, and sealed in evacuated quartz tubes. The tubes were heated to 1373 K in 10 h, kept at this temperature for 20 h, and then quenched in salt water followed by annealing at 773 K for 5 days. The ingots were grounded into fine powders, and sintered under a pressure of 40 MPa at 773 K for 6 min to obtain dense bulk materials. The bulk materials were cut into slices and bars, and then polished for the following measurements. Measurement of transport properties: the distribution maps of room temperature Seebeck coefficients for the diffusion couples were determined by Potential Seebeck Microprobe (PSM, Panco) with a spatial resolution of 20 μm. The electrical conductivity and the Seebeck coefficient of the bulk samples were measured using a ZEM-3 apparatus (Ulvac Riko Inc.) under helium atmosphere. The thermal conductivity was calculated from κ = DCpρ, where D is the thermal diffusivity measured by the laser flash diffusivity method (LFA 457; Netzsch) under argon atmosphere, and the specific heat capacity (Cp) was calculated by the Dulong-Petit law and the density ρ of the samples was determined by the Archimedes method. All measurements were conducted between 300 K and 673 K. Characterization of the microstructrue and chemical composition: Powder XRD analysis (PANalytical-Empyrean; Cu Kα) was used to identify the phase. The surface morphology and microstructure of the sample were observed by field emission scanning electron microscopy (FESEM, Hitachi S-4800). The composition was determined using electron probe microanalysis (EPMA, JEOL JXA-8230) equipped with an energy dispersive spectrometer (EDS). 3. Results and discussion 3.1. The screening for composition with high performance Fig. 2 shows the distribution maps of the Seebeck coefficient of the GeTe-Sb2Te3 diffusion couples under different holding durations, with the scanning step size of 0.02 mm × 0.02 mm. It can be inferred that the composition at both ends of the sample is uniform, the room temperature Seebeck coefficient of GeTe is about 25 μV/K, and that of Sb2Te3 is about 75 μV/K, both of them are close to the values of pristine bulk materials respectively. There is a peak in the diffusion zone near the GeTe end for all samples, as pictured by the arrow in Fig. 2. With GeTe on the left side and Sb2Te3 on the right side, the correlation of Seebeck coefficient and the chemical composition versus the scanning position for all samples are illustrated in Fig. 3. It is obvious that, with the PAS holding duration increasing, the Seebeck coefficient at the peak increases, i.e., αpeak(20min) = 29.06 μV/K, αpeak(40min) = 29.46 μV/K, αpeak(60min) = 31.90 μV/K, αpeak (80min) = 33.76 μV/K, αpeak(100min) = 37.67 μV/K. More importantly, increasing the holding duration has a significant effect on expanding the width of the diffusion zone from 0.4 mm to 1.0 mm. Compared with the conventional annealing treatment, the application of axial pressure and current during PAS process will facilitate the inter-diffusion between GeTe-Sb2Te3, therefore the diffusion zone obtained by PAS process is apparently broadened. There is a continuous composition variation of Ge, Sb and Te along the diffusion direction. The room temperature Seebeck coefficients of these compounds are below 50 μV/ K, far less than 200 μV/K, which is generally thought as the optimal Seebeck coefficient for a narrow band gap semiconductor. The higher Fig. 1. (a) Schematic diagram showing the preparation process of the diffusion couple; (b) The photo of a Sb2Te3-GeTe diffusion couple obtained via PAS; (c) Schematic diagram showing the cutting position inside the diffusion couple.
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potentially high thermoelectric performance. Due to the fact that the largest diffusion length was observed in the diffusion couple prepared under a holding duration of 100 min, and the corresponding peak value of the Seebeck coefficient is the highest, the composition near peak position of this diffusion couple will be mainly discussed hereafter. Fig. 4 shows the backscattered electron (BSE) image (Fig. 4(a)) and the secondary electron image (Fig. 4(b)) of the abovementioned region corresponding to the peak Seebeck coefficient. A gradient change in composition along the diffusion direction can be observed in Fig. 4(a). From the secondary electron image Fig. 4(b), the pits left by the PSM test can be observed. In order to characterize the composition more accurately, the positions for EPMA avoids the pits and are selected to be right beneath the pits. The pit corresponding to the peak Seebeck value was noted by arrow in Fig. 4(b), and the composition is determined to be Ge33.1Sb13.7Te53.2, as shown in Fig. 5. Since the spatial resolution of the PSM measurement is 20 μm, the composition near the optimum performance point may still possess a high performance, so the optimum composition should be extended to a region of 40 μm in length around the peak point. Seven compositions were taken from this area, to be specifically, 1# Ge42.7Sb5.5Te51.8, 2# Ge38Sb10.3Te51.7, 3# Ge36.5Sb11.2Te52.3, 4# Ge33.1Sb13.7Te53.2, 5# Ge27Sb19.3Te53.7, 6# Ge20Sb25Te55 and 7# Ge15. 5Sb28Te56.5. All corresponding bulk materials have been synthesized to determine the constituent phases and the thermoelectric properties were measured.
Fig. 2. The distribution map of Seebeck coefficient for the diffusion couples prepared by PAS with different holding durations from 20 min to 100 min.
the room temperature Seebeck coefficient is, the larger the possibility for the Seebeck coefficient of the corresponding material reaching 200 μV/K at elevated temperature. The peak of Seebeck coefficients in the composition varying region indicates that the compound with a composition in the vicinity of the corresponding position has
3.2. Verification of the compositions with high performance 3.2.1. Phase analysis Fig. 6 and Figs. S1–S6 show the X-ray diffraction patterns, backscattered electron images and corresponding EDS analysis of the ingots
Fig. 3. Seebeck coefficient and the corresponding chemical composition as a function of detect position for different diffusion couples. 16041
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Fig. 4. SEM images of the polished surface of the diffusion couple processed for 100 min: (a) backscattered electron image; (b) secondary electron image. The arrow shows the detect point with the peak Seebeck coefficient.
Fig. 5. Seebeck coefficient and the corresponding chemical composition as a function of probe position around the position corresponding to the peak Seebeck coefficient for the diffusion couple with 100 min holding duration. The corresponding 7 compositions selected for the verifying bulk materials were also shown.
with different compositions (1#∼7#). In order to improve the measurement accuracy, the EDS results from multiple selected points were averaged. Fig. S1 shows the powder X-ray diffraction pattern (a) and the BSE image with EDS analysis (b) for sample 1#, it is obvious that Sb doped cubic and rhombohedral GeTe coexist, and the excessive Sb in
GeTe leads to precipitation of Ge0.77Sb0.154Te1. For sample 2#, as shown in Fig. 6, Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6 are two main phases identified, and a small amount of bright Sb-rich phase in the BSE image may account for the unidentified peak near 41° in the XRD pattern. It can be seen from Fig. S2 that the main phases in sample 3# are Ge3.37Sb1.63Te6 and Ge0.757Sb0.151Te, besides, another impurity phase with Ge/Sb/Te atomic ratio close to 3:2:5, shown by EDS analysis, exists but below the detection limit of X-ray diffractometer. As shown in Fig. S3, for sample 4#, the bright white region is Ge2Sb2Te5; the gray region corresponds to Ge0.657Sb0.219Te1, and the difference of atomic ratio between those regions might be attributed to the substitution of Ge atom by Sb atom at different percentage for their similar atomic radius. For the sample 5#, it is obvious from Fig. S4 that the bright region in the BSE image corresponds to Ge2Sb2Te5; the gray region and the gray-black region have similar compositions, with a corresponding phase of Ge3.37Sb1.63Te6. For the sample 6#, it can be seen in Fig. S5 that the gray black region corresponds to the phase of Ge3Sb2Te5; the gray region and the gray white region have a corresponding phase of Ge0.95Sb3.01Te4. As shown in Fig. S6, sample 7# comprises of two phases of Ge2Sb2Te5 and Ge0.95Sb2.01Te4. 3.2.2. Transport properties Due to the multi-phase feature of all samples, sample could crake resulting from the differences in thermal expansion coefficients between the phases. Therefore, all thermoelectric properties were measured between 300 K and 673 K to avoid the potential microcracks that could possibly form at elevated temperature. No microcracks were found on the samples after measurements. 3.2.2.1. Electrical transport. Fig. 7 shows the electrical transport
Fig. 6. (a) Powder X-ray diffraction pattern for sample 2#; (b) The backscattered electron image with EDS analysis for the same sample. 16042
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Fig. 7. Temperature dependence of electrical transport for the verifying bulk samples: (a) electrical conductivity; (b) Seebeck coefficient; (c) power factor.
properties as a function of temperature for different compositions. As seen from Fig. 7(a), the electrical conductivity of all samples decreases with temperature, showing a typical behavior of heavily doped semiconductors. As shown in Fig. 7(b), the Seebeck coefficient increases with temperature. The Seebeck coefficient of all samples are in the range of 18–50 μV/K at room temperature, which is consistent with the PSM results, indicating the accuracy and reliability of the characterization of the diffusion couples. The power factor as a function of temperature for all samples are shown in Fig. 7(c). It can be clearly seen that the compounds with a low content of Sb shows a higher power factor than the ones with higher Sb contents. 3.2.2.2. Thermal transport. The thermal conductivity as a function of temperature is shown in Fig. 8. The thermal conductivity of all samples at room temperature is much lower than that of pristine GeTe [21]. For samples 1#, 2# and 3#with low Sb contents, sample 2# possesses the lowest thermal conductivity of 2.3 W m−1K−2 at room temperature. The thermal conductivity decreases slowly with temperature. Compared with samples 1#, 2# and 3#, the samples with a higher Sb content possess a lower thermal conductivity. 3.2.2.3. The dimensionless thermoelectric figure of merit ZT. The dimensionless thermoelectric figures of merit ZT are calculated by ZT = σα2T/κ, as shown in Fig. 9. The ZT values of all samples increases with increasing temperature. It's worth noting that the thermoelectric performance for sample 2# is the highest over the whole temperature range, achieving a maximum value of 0.55 at 673 K and it keeps rising even higher as temperature goes up. 3.2.3. Determination of new Ge-Sb-Te phases with high performance Considering the phase and composition analysis of sample 2# are of
Fig. 9. Temperature dependence of the dimensionless figure-of-merit ZT for the verifying bulk samples.
the most significance for the high thermoelectric performance mentioned above, the following discussion will be focused on that sample. As shown in Fig. 6, sample 2# comprises of two main phases, Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6. The strongest diffraction peaks of the two phases are around 29.4°. The low-temperature crystal structure of Ge0.77Sb0.154Te is an isostructure of the rhombohedral GeTe, which can be described as a cubic NaCl-type structure deformed along one of the 3-fold rotation axis [22], as pictured in Fig. 10(a). A highly symmetrical crystal structure is beneficial to enhance the degeneracy of energy bands, resulting in a desired Seebeck coefficient [23], which account for the largest Seebeck coefficient of sample 2#. Ge3.37Sb1.63Te6 exhibits a layered crystal structure, similar to that of Ge3Sb2Te6, but the cation sites are occupied by Ge/Sb atoms, as shown in Fig. 10(b). The slight disorder for occupancies of Ge/Sb atoms makes the long-range atomic arrangement deviate from the ideal state [24]. This crystal structure is preferred for scattering low-frequency and middle-frequency phonons [23], so as to reduce thermal conductivity, which can explain the low thermal conductivity observed for sample 2#. Therefore, the main phases of sample 2# with promising thermoelectric properties are determined to be Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6. Although the excellent thermoelectric performance is derived from the special crystal structures of the constituent phases, the thermoelectric transport property of single phase Ge0.77Sb0.154Te1 or Ge3.37Sb1.63Te6 have not been explored systematically, which deserves further investigation.
4. Conclusion Fig. 8. Temperature dependence of thermal conductivity for the verifying bulk samples.
To sum up, a high-throughput screening technique has been 16043
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Fig. 10. (a) Crystal structure of Ge0.77Sb0.154Te1 with an atomic arrangement of the distorted NaCl-type; (b) Crystal structure of Ge3.37Sb1.63Te6 with a slight disorder for the occupancies of Ge/Sb atoms.
employed as an effective approach to exploit novel thermoelectric materials, and the Ge-Sb-Te compounds have been chosen as a demonstration. A combinatorial sample library of GeTe-Sb2Te3 diffusion couple which contains continuously varying composition has been synthesized effectively by using PAS process, and the correlations among phase structure, microstructure, and transport properties for the Ge-Te-Sb ternary alloy were investigated. The high-throughput screening result indicated a promising thermoelectric material with the composition of Ge33.1Sb13.7Te53.2. A series of samples with compositions around Ge33.1Sb13.7Te53.2 were prepared by the conventional melting combined with PAS sintering process. Among these bulk samples, the one with a nominal composition of Ge38Sb10.3Te51.7 achieves a maximum ZT value of 0.55 at 673 K. Further investigation indicates the phase segregation in this sample, and two novel thermoelectric materials Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6 have been identified. This study provides new approaches regarding the combinatorial metrology and its utilization in searching for new thermoelectric materials.
National Natural Science Foundation of China (No. 51772232). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.05.119. References
Acknowledgment This work was financially supported by National Key Research and Development Program of China (Grant No. 2018YFB0703600), and the 16044
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Novel Ge-Sb-Te thermoelectric materials: A demonstration for an efficient diffusion couple technique in expediently exploiting new thermoelectric materials
T
Jian Zhanga, Yonggao Yana,∗, Hongyao Xieb, Ting Zhua, Cheng Zhanga, Junhao Qiua, Lei Yaoa, Xinfeng Tanga,∗∗ a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China Department of Chemistry, Northwestern University, Evanston, IL, 60208, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Combinatorial metrology Diffusion couple Micro-area Seebeck coefficient Ge-Sb-Te thermoelectric materials
An efficient diffusion couple technique is proposed for rapid screening of potential thermoelectric materials and it has been explored in a (GeTe)m (Sb2Te3)n pseudobinary system. In this study, a combinatorial sample library of GeTe-Sb2Te3 diffusion couple with rich compositions and structures was prepared by plasma activation sintering (PAS). The relationship among composition, structure and performance of the materials was revealed by analyzing the chemical composition, crystal structure and thermoelectric properties of the microdomains in the combinatorial sample library, so as to achieve the purpose for rapid identification of Ge-Sb-Te compounds with excellent thermoelectric properties. The scanning results of the Seebeck coefficient of the samples library show that the sample with potential high thermoelectric performance has a chemical composition near Ge33.1Sb13.7Te53.2. The bulk materials with compositions in the vicinity of Ge33.1Sb13.7Te53.2 were prepared by a conventional melting and PAS process. Among these bulk samples, the one with a nominal composition of Ge38Sb10.3Te51.7 achieves a maximum ZT value of 0.55 at 673 K, indicating that the constituent phases Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6 observed in this sample could be potential candidates with excellent thermoelectric properties. These two ternary compounds should be investigated for further optimization. The results of this study provide a reference for the application of combinatorial metrology for screening and optimizing new thermoelectric materials.
1. Introduction Due to the shortage of energy and the serious environmental pollution caused by the burning of fossil fuel, thermoelectric technology has received extensive attention as an environmental-friendly technology for conversion between heat and electricity [1–3]. The performance of thermoelectric materials is generally characterized by the dimensionless figure of merit ZT,calculated by ZT = α2σT/κ,where α is Seebeck coefficient,σ is the electrical conductivity,T is the absolute temperature,κ is the thermal conductivity. The widespread application of thermoelectric technology has not been achieved yet, and it will greatly depend on identifying new thermoelectric materials with excellent performance. Compared with the electrical and thermal conductivity, the Seebeck coefficient α contributes more substantially to the overall thermoelectric properties since α is with a power term in the
∗
formula. Among the above parameters, the Seebeck coefficient is a parameter reflecting the intrinsic band structure of the material, which can only be manipulated within a certain range by controlling the carrier concentration. The thermoelectric material with excellent performance must possess a suitable electronic band structure, and the band structure can be reflected by the Seebeck coefficient of the material. Therefore, it is possible to find some new thermoelectric materials with desirable properties with characterizations of the Seebeck coefficient. A common method for material research is to prepare a sample of one composition at a time, which is a very time-consuming process with a high material cost. While combinatorial materials science has the advantage for exploring a system of multiple constituent elements at a time, if combined with high-throughput characterization techniques, the relation between composition-structure-performance can be
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Yan), [email protected] (X. Tang).
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https://doi.org/10.1016/j.ceramint.2019.05.119 Received 22 April 2019; Received in revised form 8 May 2019; Accepted 12 May 2019 Available online 20 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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attained quickly, which can greatly increase the density of information attained in a single experiment, thus immensely shortening the period of exploring new materials and products [4,5]. Combinatorial metrology has been widely used in the fields of catalysis [6], pharmacy [7] and luminophor [8], nevertheless, it is rarely reported in thermoelectric materials. Although some combinatorial film libraries based on films have been achieved [9–12], the combinatorial research based on bulk materials is still in its infancy for limitations of the preparation technology and the corresponding high-throughput characterization techniques. Zhao et al. [13] studied the solution interval and the thermal properties of Mg2Si–Mg2Sn pseudobinary system. However, the conventional annealing process to obtain a diffusion couple usually takes a few weeks, so it is significant to develop an efficient diffusion couple technique and apply it to exploring the potential thermoelectric materials. In recent years, Ge-Sb-Te compounds have attracted widespread attention as thermoelectric materials [14–16]. Lee Jaeho et al. [17] synthesized the Ge2Sb2Te5 film with a Seebeck coefficient of 371 μV/K. In 2013, Sittner Ernst-Roland et al. [18] studied a film of Ge8Sb2Te11 with ZTmax = 0.7. In 2015, the Ge2Sb2Te5 compound prepared by Sun Jifeng et al. [19], exhibited the Seebeck coefficient of 200–300 μV/K at room temperature. All studies show that Ge-Sb-Te compounds have the potential to possess an excellent thermoelectric performance. Owing to the number of varieties of Ge-Sb-Te compounds could have [20], the conventional routine to explore the potential high performance materials takes a lot of work. Considering that almost all GeSb-Te compounds can be synthesized from the reaction between GeTe and Sb2Te3, the GeTe-Sb2Te3 diffusion couples was obtained by the PAS technique, then combined with the scanning Seebeck coefficient characterization, the Ge-Sb-Te compounds with potential excellent thermoelectric properties were determined and verified. This study has shown a novel way to screen and optimize thermoelectric materials quickly, efficiently at a low cost. 2. Materials and methods Preparation of the diffusion couples: High purity Sb (bulk, 99.999%) and Te (bulk, 99.999%) were weighed according to the stoichiometry of Sb2Te3. Then the mixtures of Sb and Te were sealed in evacuated quartz tube and heated to 1023 K in 5 h, kept at this temperature for 10 h, and then cooled down in furnace to get a Sb2Te3 ingot. Similarly, a GeTe ingot was obtained with Ge (bulk, 99.99%) and Te (bulk, 99.999%) at the raw materials. In the case of GeTe, the materials melt at 1373 K for 20 h and then quenched in water bath, followed by annealing at 773 K for 5 days. The obtained ingots were grounded into fine powders. As shown in Fig. 1(a), the GeTe powder and Sb2Te3 powder were put inside a mold to form a flat interface. The PAS were processed under a pressure of 40 MPa at 773 K with holding durations of 20 min, 40 min, 60 min, 80 min and 100min, respectively to accelerate the diffusion of atoms between GeTe and Sb2Te3. Then the obtained ingots were cut into several diffusion couples schematically shown in Fig. 1. Preparation of the verifying bulks: Bulk materials with optimized compositions were prepared to verify the results obtained from combinatorial library. The bulk Ge-Sb-Te materials were synthesized by similar processing as the diffusion couples. The mixtures of Ge,Sb and
Te were weighted according to the optimized composition, and sealed in evacuated quartz tubes. The tubes were heated to 1373 K in 10 h, kept at this temperature for 20 h, and then quenched in salt water followed by annealing at 773 K for 5 days. The ingots were grounded into fine powders, and sintered under a pressure of 40 MPa at 773 K for 6 min to obtain dense bulk materials. The bulk materials were cut into slices and bars, and then polished for the following measurements. Measurement of transport properties: the distribution maps of room temperature Seebeck coefficients for the diffusion couples were determined by Potential Seebeck Microprobe (PSM, Panco) with a spatial resolution of 20 μm. The electrical conductivity and the Seebeck coefficient of the bulk samples were measured using a ZEM-3 apparatus (Ulvac Riko Inc.) under helium atmosphere. The thermal conductivity was calculated from κ = DCpρ, where D is the thermal diffusivity measured by the laser flash diffusivity method (LFA 457; Netzsch) under argon atmosphere, and the specific heat capacity (Cp) was calculated by the Dulong-Petit law and the density ρ of the samples was determined by the Archimedes method. All measurements were conducted between 300 K and 673 K. Characterization of the microstructrue and chemical composition: Powder XRD analysis (PANalytical-Empyrean; Cu Kα) was used to identify the phase. The surface morphology and microstructure of the sample were observed by field emission scanning electron microscopy (FESEM, Hitachi S-4800). The composition was determined using electron probe microanalysis (EPMA, JEOL JXA-8230) equipped with an energy dispersive spectrometer (EDS). 3. Results and discussion 3.1. The screening for composition with high performance Fig. 2 shows the distribution maps of the Seebeck coefficient of the GeTe-Sb2Te3 diffusion couples under different holding durations, with the scanning step size of 0.02 mm × 0.02 mm. It can be inferred that the composition at both ends of the sample is uniform, the room temperature Seebeck coefficient of GeTe is about 25 μV/K, and that of Sb2Te3 is about 75 μV/K, both of them are close to the values of pristine bulk materials respectively. There is a peak in the diffusion zone near the GeTe end for all samples, as pictured by the arrow in Fig. 2. With GeTe on the left side and Sb2Te3 on the right side, the correlation of Seebeck coefficient and the chemical composition versus the scanning position for all samples are illustrated in Fig. 3. It is obvious that, with the PAS holding duration increasing, the Seebeck coefficient at the peak increases, i.e., αpeak(20min) = 29.06 μV/K, αpeak(40min) = 29.46 μV/K, αpeak(60min) = 31.90 μV/K, αpeak (80min) = 33.76 μV/K, αpeak(100min) = 37.67 μV/K. More importantly, increasing the holding duration has a significant effect on expanding the width of the diffusion zone from 0.4 mm to 1.0 mm. Compared with the conventional annealing treatment, the application of axial pressure and current during PAS process will facilitate the inter-diffusion between GeTe-Sb2Te3, therefore the diffusion zone obtained by PAS process is apparently broadened. There is a continuous composition variation of Ge, Sb and Te along the diffusion direction. The room temperature Seebeck coefficients of these compounds are below 50 μV/ K, far less than 200 μV/K, which is generally thought as the optimal Seebeck coefficient for a narrow band gap semiconductor. The higher Fig. 1. (a) Schematic diagram showing the preparation process of the diffusion couple; (b) The photo of a Sb2Te3-GeTe diffusion couple obtained via PAS; (c) Schematic diagram showing the cutting position inside the diffusion couple.
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potentially high thermoelectric performance. Due to the fact that the largest diffusion length was observed in the diffusion couple prepared under a holding duration of 100 min, and the corresponding peak value of the Seebeck coefficient is the highest, the composition near peak position of this diffusion couple will be mainly discussed hereafter. Fig. 4 shows the backscattered electron (BSE) image (Fig. 4(a)) and the secondary electron image (Fig. 4(b)) of the abovementioned region corresponding to the peak Seebeck coefficient. A gradient change in composition along the diffusion direction can be observed in Fig. 4(a). From the secondary electron image Fig. 4(b), the pits left by the PSM test can be observed. In order to characterize the composition more accurately, the positions for EPMA avoids the pits and are selected to be right beneath the pits. The pit corresponding to the peak Seebeck value was noted by arrow in Fig. 4(b), and the composition is determined to be Ge33.1Sb13.7Te53.2, as shown in Fig. 5. Since the spatial resolution of the PSM measurement is 20 μm, the composition near the optimum performance point may still possess a high performance, so the optimum composition should be extended to a region of 40 μm in length around the peak point. Seven compositions were taken from this area, to be specifically, 1# Ge42.7Sb5.5Te51.8, 2# Ge38Sb10.3Te51.7, 3# Ge36.5Sb11.2Te52.3, 4# Ge33.1Sb13.7Te53.2, 5# Ge27Sb19.3Te53.7, 6# Ge20Sb25Te55 and 7# Ge15. 5Sb28Te56.5. All corresponding bulk materials have been synthesized to determine the constituent phases and the thermoelectric properties were measured.
Fig. 2. The distribution map of Seebeck coefficient for the diffusion couples prepared by PAS with different holding durations from 20 min to 100 min.
the room temperature Seebeck coefficient is, the larger the possibility for the Seebeck coefficient of the corresponding material reaching 200 μV/K at elevated temperature. The peak of Seebeck coefficients in the composition varying region indicates that the compound with a composition in the vicinity of the corresponding position has
3.2. Verification of the compositions with high performance 3.2.1. Phase analysis Fig. 6 and Figs. S1–S6 show the X-ray diffraction patterns, backscattered electron images and corresponding EDS analysis of the ingots
Fig. 3. Seebeck coefficient and the corresponding chemical composition as a function of detect position for different diffusion couples. 16041
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Fig. 4. SEM images of the polished surface of the diffusion couple processed for 100 min: (a) backscattered electron image; (b) secondary electron image. The arrow shows the detect point with the peak Seebeck coefficient.
Fig. 5. Seebeck coefficient and the corresponding chemical composition as a function of probe position around the position corresponding to the peak Seebeck coefficient for the diffusion couple with 100 min holding duration. The corresponding 7 compositions selected for the verifying bulk materials were also shown.
with different compositions (1#∼7#). In order to improve the measurement accuracy, the EDS results from multiple selected points were averaged. Fig. S1 shows the powder X-ray diffraction pattern (a) and the BSE image with EDS analysis (b) for sample 1#, it is obvious that Sb doped cubic and rhombohedral GeTe coexist, and the excessive Sb in
GeTe leads to precipitation of Ge0.77Sb0.154Te1. For sample 2#, as shown in Fig. 6, Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6 are two main phases identified, and a small amount of bright Sb-rich phase in the BSE image may account for the unidentified peak near 41° in the XRD pattern. It can be seen from Fig. S2 that the main phases in sample 3# are Ge3.37Sb1.63Te6 and Ge0.757Sb0.151Te, besides, another impurity phase with Ge/Sb/Te atomic ratio close to 3:2:5, shown by EDS analysis, exists but below the detection limit of X-ray diffractometer. As shown in Fig. S3, for sample 4#, the bright white region is Ge2Sb2Te5; the gray region corresponds to Ge0.657Sb0.219Te1, and the difference of atomic ratio between those regions might be attributed to the substitution of Ge atom by Sb atom at different percentage for their similar atomic radius. For the sample 5#, it is obvious from Fig. S4 that the bright region in the BSE image corresponds to Ge2Sb2Te5; the gray region and the gray-black region have similar compositions, with a corresponding phase of Ge3.37Sb1.63Te6. For the sample 6#, it can be seen in Fig. S5 that the gray black region corresponds to the phase of Ge3Sb2Te5; the gray region and the gray white region have a corresponding phase of Ge0.95Sb3.01Te4. As shown in Fig. S6, sample 7# comprises of two phases of Ge2Sb2Te5 and Ge0.95Sb2.01Te4. 3.2.2. Transport properties Due to the multi-phase feature of all samples, sample could crake resulting from the differences in thermal expansion coefficients between the phases. Therefore, all thermoelectric properties were measured between 300 K and 673 K to avoid the potential microcracks that could possibly form at elevated temperature. No microcracks were found on the samples after measurements. 3.2.2.1. Electrical transport. Fig. 7 shows the electrical transport
Fig. 6. (a) Powder X-ray diffraction pattern for sample 2#; (b) The backscattered electron image with EDS analysis for the same sample. 16042
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Fig. 7. Temperature dependence of electrical transport for the verifying bulk samples: (a) electrical conductivity; (b) Seebeck coefficient; (c) power factor.
properties as a function of temperature for different compositions. As seen from Fig. 7(a), the electrical conductivity of all samples decreases with temperature, showing a typical behavior of heavily doped semiconductors. As shown in Fig. 7(b), the Seebeck coefficient increases with temperature. The Seebeck coefficient of all samples are in the range of 18–50 μV/K at room temperature, which is consistent with the PSM results, indicating the accuracy and reliability of the characterization of the diffusion couples. The power factor as a function of temperature for all samples are shown in Fig. 7(c). It can be clearly seen that the compounds with a low content of Sb shows a higher power factor than the ones with higher Sb contents. 3.2.2.2. Thermal transport. The thermal conductivity as a function of temperature is shown in Fig. 8. The thermal conductivity of all samples at room temperature is much lower than that of pristine GeTe [21]. For samples 1#, 2# and 3#with low Sb contents, sample 2# possesses the lowest thermal conductivity of 2.3 W m−1K−2 at room temperature. The thermal conductivity decreases slowly with temperature. Compared with samples 1#, 2# and 3#, the samples with a higher Sb content possess a lower thermal conductivity. 3.2.2.3. The dimensionless thermoelectric figure of merit ZT. The dimensionless thermoelectric figures of merit ZT are calculated by ZT = σα2T/κ, as shown in Fig. 9. The ZT values of all samples increases with increasing temperature. It's worth noting that the thermoelectric performance for sample 2# is the highest over the whole temperature range, achieving a maximum value of 0.55 at 673 K and it keeps rising even higher as temperature goes up. 3.2.3. Determination of new Ge-Sb-Te phases with high performance Considering the phase and composition analysis of sample 2# are of
Fig. 9. Temperature dependence of the dimensionless figure-of-merit ZT for the verifying bulk samples.
the most significance for the high thermoelectric performance mentioned above, the following discussion will be focused on that sample. As shown in Fig. 6, sample 2# comprises of two main phases, Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6. The strongest diffraction peaks of the two phases are around 29.4°. The low-temperature crystal structure of Ge0.77Sb0.154Te is an isostructure of the rhombohedral GeTe, which can be described as a cubic NaCl-type structure deformed along one of the 3-fold rotation axis [22], as pictured in Fig. 10(a). A highly symmetrical crystal structure is beneficial to enhance the degeneracy of energy bands, resulting in a desired Seebeck coefficient [23], which account for the largest Seebeck coefficient of sample 2#. Ge3.37Sb1.63Te6 exhibits a layered crystal structure, similar to that of Ge3Sb2Te6, but the cation sites are occupied by Ge/Sb atoms, as shown in Fig. 10(b). The slight disorder for occupancies of Ge/Sb atoms makes the long-range atomic arrangement deviate from the ideal state [24]. This crystal structure is preferred for scattering low-frequency and middle-frequency phonons [23], so as to reduce thermal conductivity, which can explain the low thermal conductivity observed for sample 2#. Therefore, the main phases of sample 2# with promising thermoelectric properties are determined to be Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6. Although the excellent thermoelectric performance is derived from the special crystal structures of the constituent phases, the thermoelectric transport property of single phase Ge0.77Sb0.154Te1 or Ge3.37Sb1.63Te6 have not been explored systematically, which deserves further investigation.
4. Conclusion Fig. 8. Temperature dependence of thermal conductivity for the verifying bulk samples.
To sum up, a high-throughput screening technique has been 16043
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Fig. 10. (a) Crystal structure of Ge0.77Sb0.154Te1 with an atomic arrangement of the distorted NaCl-type; (b) Crystal structure of Ge3.37Sb1.63Te6 with a slight disorder for the occupancies of Ge/Sb atoms.
employed as an effective approach to exploit novel thermoelectric materials, and the Ge-Sb-Te compounds have been chosen as a demonstration. A combinatorial sample library of GeTe-Sb2Te3 diffusion couple which contains continuously varying composition has been synthesized effectively by using PAS process, and the correlations among phase structure, microstructure, and transport properties for the Ge-Te-Sb ternary alloy were investigated. The high-throughput screening result indicated a promising thermoelectric material with the composition of Ge33.1Sb13.7Te53.2. A series of samples with compositions around Ge33.1Sb13.7Te53.2 were prepared by the conventional melting combined with PAS sintering process. Among these bulk samples, the one with a nominal composition of Ge38Sb10.3Te51.7 achieves a maximum ZT value of 0.55 at 673 K. Further investigation indicates the phase segregation in this sample, and two novel thermoelectric materials Ge0.77Sb0.154Te1 and Ge3.37Sb1.63Te6 have been identified. This study provides new approaches regarding the combinatorial metrology and its utilization in searching for new thermoelectric materials.
National Natural Science Foundation of China (No. 51772232). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.05.119. References
Acknowledgment This work was financially supported by National Key Research and Development Program of China (Grant No. 2018YFB0703600), and the 16044
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