Research ArticlePHYSICAL SCIENCES

Semiconductor glass with superior flexibility and high room temperature thermoelectric performance

See allHide authors and affiliations

Science Advances  10 Apr 2020:
Vol. 6, no. 15, eaaz8423
DOI: 10.1126/sciadv.aaz8423

Abstract

Most crystalline inorganic materials, except for metals and some layer materials, exhibit bad flexibility because of strong ionic or covalent bonds, while amorphous materials usually display poor electrical properties due to structural disorders. Here, we report the simultaneous realization of extraordinary room temperature flexibility and thermoelectric performance in Ag2Te1–xSx–based materials through amorphization. The coexistence of amorphous main phase and crystallites results in exceptional flexibility and ultralow lattice thermal conductivity. Furthermore, the flexible Ag2Te0.6S0.4 glass exhibits a degenerate semiconductor behavior with a room temperature Hall mobility of ~750 cm2 V−1 s−1 at a carrier concentration of 8.6 × 1018 cm−3, which is at least an order of magnitude higher than other amorphous materials, leading to a thermoelectric power factor also an order of magnitude higher than the best amorphous thermoelectric materials known. The in-plane prototype uni-leg thermoelectric generator made from this material demonstrates its potential for flexible thermoelectric device.

INTRODUCTION

Wearable electronic devices integrating with the natural environment and human experience have been receiving increasing attention (1). Energy conversion devices that can convert ambient energy into electricity are eagerly desired for self-powered wearable devices. In the past decade, several types of flexible energy conversion devices have been developed on the basis of piezoelectricity (2), electrostaticity (3), and triboelectricity (4), but these devices are usually inefficient and costly. Flexible thermoelectric (TE) devices may satisfy the requirements because they can directly convert human body heat into electricity (5). Unfortunately, the development of flexible TE devices suffers from the absence of TE materials with both promising TE properties and flexibilities. Traditional inorganic TE materials normally exhibit poor mechanical properties, which may result in device failure, because they are brittle and prone to rupture under tensile, compression, or bending load (68). Organic TE materials can be flexible, but they display poor electrical properties and low power factors (911). Organic/inorganic TE composites have the combined advantages of flexible polymers and inorganic TE materials. However, the flexibility of the TE device made from organic/inorganic composites is mainly realized by the flexible substrate but not the TE material itself (1214). The inorganic component, which contributes to the TE performance of the device, displays poor flexibility and must be deposited as a thin film (usually only several micrometers thick) on the flexible organic substrate. This results in not only complicated, time-consuming, and costly production process but also uncontrollable device performance because of the problems in homogeneity and thickness variation of the thin film. Research and development of freestanding flexible TE materials are urgently needed to realize self-power supply for wearable electronic devices.

In this contribution, superb flexibility and high TE performance are simultaneously achieved in an amorphized Ag2Te1–xSx–based inorganic material. Compared to recently discovered ductile Ag2S (15), which is crystalline but has very low carrier concentration (~1.4 × 1014 cm−3) and poor electrical conductivity, addition of S into Ag2Te leads to amorphization of Ag2Te1–xSx with exceptional flexibility far superior to crystalline inorganic nonmetal materials. The amorphized Ag2Te0.6S0.4 sample not only displays ultralow lattice thermal conductivity but also still preserves prominent electrical transport properties. An extraordinary room temperature Hall mobility of ~750 cm2 V−1 s−1 at a carrier concentration of 8.6 × 1018 cm−3 is achieved in Ag2Te0.6S0.4, which is an order of magnitude higher than best amorphous materials. A TE figure-of-merit zT value of ~0.2 is obtained at room temperature, and a peak zT around 0.7 at 300°C is reached, the highest among amorphous materials. A flexible in-plane device with a single leg made from the amorphized Ag2Te0.6S0.4 semiconductor glass produces 3.5 μW of power under 50 K temperature difference, demonstrating its great potential as a self-powered cell.

RESULTS AND DISCUSSION

Mechanical properties

Conventional inorganic nonmetal crystals show bad flexibility due to their strong ionic and/or covalent bonds, limiting their potential applications as flexible materials. In contrast to the traditional inorganic TE materials such as Bi2Te3, GeTe, and PbTe (6, 7, 16, 17), our Ag2Te0.6S0.4 sample displays extraordinary flexibility (Fig. 1, A to D). The three-point bending test reveals that the Ag2Te0.6S0.4 sample exhibits a bending strain above 14% under a bending stress as large as 110 MPa (Fig. 1A), which is superior to most metallic glasses (1820). In the compression test (Fig. 1B), the Ag2Te0.6S0.4 sample exhibits an elastic strain limit of 2% before yielding at 70 MPa. However, after yielding, the sample shows stress overshoot, which is commonly observed in the superplastic deformation of metallic glasses (21), and the compression strain of the sample finally reaches 25%. The Ag2Te0.6S0.4 sample displays exceptional plastic property with a tensile strain around 12.5% (Fig. 1C). The measured hardness of Ag2Te0.6S0.4 is 19.5 kgf mm−1, which is much smaller than those of the conventional TE materials (Fig. 1D) (7, 16, 17, 22, 23). In comparison with the previous reported Ag2S showing a record ductility among polycrystalline semiconductors (15), our Ag2Te0.6S0.4 sample exhibits much better plastic deformation property, slightly higher bending property, comparable hardness, and lower but reasonably good compressibility.

Fig. 1 Room temperature mechanical properties of the Ag2Te1–xSx samples (x = 0.1, 0.2, 0.3, 0.4).

(A to C) Strain-stress curves for bending (A), compression (B), and tensile tests (C). (D) Vickers hardness. Typical materials are shown for comparison (8,16,17,22,23,2429).

Because of its extraordinary flexibility (2429), our Ag2Te0.6S0.4 sample displays exceptional processability. The as-prepared Ag2Te0.6S0.4 ingot can be easily pressed into a thin slice by flat hot-pressing at a low temperature (below 200°C, fig. S1). The diameter of the Ag2Te0.6S0.4 ingot can be extended nearly five times (from 11.32 to 49.05 mm), and the thickness can be compressed to 1/20 of its original value (from 5.46 to 0.28 mm, fig. S2). The Ag2Te0.6S0.4 slice obtained by flat hot-pressing can be easily bent with fingers (movie S1) or cut into strips by a stationery scissor (movie S2). The repeated bending test for the Ag2Te0.6S0.4 sample has been carried out under a bending stress of 60 MPa for 200 cycles, and no obvious decrease in the bending stress is detected after the test (fig. S3). The sample shows no cracks on the surface and preserves its original shape after the repeated bending test, indicating the robust flexibility of the Ag2Te0.6S0.4 sample (the electrical properties do not show any obvious changes after the repeating bending test, which will be discussed below).

Structure analysis

In the tensile test, two types of fracture morphologies are observed for the Ag2Te0.6S0.4 sample by scanning electron microscope. The dominant one is the metallic glass type, which displays vein-like structure (Fig. 2A) (29), and the other is the crystalline type, showing a ductile dimple feature (fig. S4) (30). Crossing shear bands, another typical feature of metallic glasses (31), are observed in the fracture surface of the Ag2Te0.6S0.4 sample (Fig. 2B). Thus, the exceptional flexibility of our sample can be mainly ascribed to the formation and evolution of shear bands, which are the dominant processes accounting for the plasticity of bulk metallic glasses (21). It is interesting and important that the Ag2Te0.6S0.4 sample shows the amorphous feature, and its brilliant flexibility superior to conventional inorganic nonmetal crystals must be correlated with its unique disordered structure. Thus, the microscopic structure of Ag2Te0.6S0.4 is further analyzed by transmission electron microscope (TEM). Careful TEM characterization reveals that many crystallites typically with the size of 10 to 30 nm disperse randomly in the glassy matrix (Fig. 2C). Noncrystalline area without lattice fringes is observed in the TEM image (Fig. 2D), and the selected-area electron diffraction (SAED) pattern shows the typical amorphous halo rings (the inset of Fig. 2D), confirming the glassy nature of the Ag2Te0.6S0.4 sample. Similar microstructures are also observed in metallic glasses (32), in which amorphous alloys coexist with crystallites.

Fig. 2 Microstructure of the Ag2Te0.6S0.4 sample.

(A) SEM image of the fracture morphology showing vein-like structure. (B) SEM image of the surface of Ag2Te0.6S0.4 showing crossing shear bands. (C) TEM image showing crystallites embedded in the amorphous matrix. (D) TEM image of pure amorphous phase region and the corresponding SAED pattern. (E) HRTEM image showing a typical crystallite in the amorphous matrix. (F) Corresponding SAED pattern of (E). (G) Room temperature XRD patterns. (H) Heat flow curves. a.u., arbitrary units.

The structure and composition of the crystallite have been analyzed by high-resolution TEM (HRTEM) and SAED, and Ag2Te1–xSx–based crystallites with the monoclinic structure are detected in the Ag2Te0.6S0.4 sample. The HRTEM image shows a crystallite, which is larger than typical ones for the convenience of TEM characterization (Fig. 2E). The clear lattice fringes observed in the crystallite indicate the high crystalline nature, and the lattice spacing about 6.79 Å is very close to that of the (100) plane of monoclinic Ag2Te. The SAED pattern of the crystallite reveals the interplanar crystal spacings of 6.79 and 3.23 Å (Fig. 2F), which are in line with the crystallographic plane of (001) and (1¯12) of Ag2Te, respectively. The angle between the two crystallographic planes is approximately 90°, further indicating a projection along [02¯1] of the monoclinic structure of Ag2Te. Both amorphous rings and diffraction spots are found in the SAED pattern (Fig. 2F), further confirming the coexistence of amorphous matrix and crystallites. The SAED pattern of the crystallite matches well with monoclinic Ag2Te, and elemental mapping proves that the composition of the crystallite is Ag2Te1–xSx alloy (fig. S5).

X-ray diffraction (XRD) technique, which measures the “averaged” structure and composition information, displays only the amorphous feature of the Ag2Te0.6S0.4 sample due to the tiny size and random dispersion of the crystallites (Fig. 2G). However, XRD patterns present diffraction peaks from crystalline phase as the sample composition is changed. With decreasing S contents, the diffraction peaks of the crystalline phase become more and more prominent. The Ag2Te0.9S0.1 sample crystallizes in a monoclinic structure with the space group P21/c, and no obvious amorphous phase can be detected in the XRD pattern (Fig. 2G). According to the XRD pattern, the crystalline phase can be indexed as the monoclinic Ag2Te structure, which is consistent with the TEM analysis.

Differential scanning calorimetry (DSC) analysis indicates that the Ag2Te1–xSx sample with lower S content (x = 0, 0.1, and 0.2) shows a phase transition from low-temperature monoclinic phase to high-temperature cubic phase (Fig. 2H and fig. S6), and the phase transition temperature decreases with increasing S content (418 K for Ag2Te, 378 K for Ag2Te0.9S0.1, and 321 K for Ag2Te0.8S0.2). However, for the sample with higher S content (x = 0.3 and 0.4), no obvious phase transition is detected above room temperature by the DSC analysis (Fig. 2H). This indicates that the S alloying decreases the phase transition temperature of Ag2Te1–xSx, which is common in chalcogenides (3335). In addition, the presence and increasing content of the amorphous phase can result in weakened phase transition phenomenon. The heating-cooling cycle during DSC measurement reveals good thermal stability of our Ag2Te1–xSx sample, and the XRD pattern also shows that there is almost no difference before and after flat hot-pressing at the temperature range of 50° to 200°C (fig. S7).

TE transport properties

The dominantly amorphous Ag2Te0.6S0.4 sample, however, shows an unexpectedly high Hall mobility, ~750 cm2 V−1 s−1 (Fig. 3A), at a carrier concentration of 8.6 × 1018 cm−3 (Fig. 3B), which is an order of magnitude higher than other amorphous materials (Fig. 3C) (3640). The Hall mobility and carrier concentration of our flexible Ag2Te0.6S0.4 semiconductor glass are respectively 10 times larger and even four orders of magnitudes higher than those of the ductile Ag2S semiconductor (15). Similar to the pristine Ag2Te (41, 42), our Ag2Te1–xSx sample shows the typical conducting behavior of a degenerate semiconductor (fig. S8). Hall mobilities and carrier concentrations measured during the cycle of heating and cooling show good repeatability (Fig. 3, A and B), indicating the stable electrical properties of the samples. On the basis of Seebeck coefficient measurement (fig. S8B), a room temperature power factor (PF = σα2, σ is the electrical conductivity and α is the Seebeck coefficient) as high as 5.67 μW cm−1 K−2 is achieved for the Ag2Te0.6S0.4 semiconductor glass (Fig. 3D), which is comparable to the polycrystalline Ag2Te (41, 42) and an order of magnitude higher than other amorphous materials at room temperature (4348).

Fig. 3 Electrical transport properties of the Ag2Te1–xSx samples (x = 0.1, 0.2, 0.3, 0.4).

(A) Temperature-dependent Hall mobility and (B) carrier concentration. (C) Room temperature Hall mobility in comparison with typical amorphous materials (3640). (D) Room temperature power factor as a function of electrical conductivity for Ag2Te0.6S0.4 and typical amorphous materials (4348).

To evaluate the stability of the Ag2Te0.6S0.4 sample, its electrical transport properties have been checked under simulated service conditions. Both the Seebeck coefficient and electrical conductivity, which are very sensitive to the microcracks in samples, do not show any obvious changes after 200 cycles of repeated bending test under a bending stress of 60 MPa (fig. S9). Furthermore, the long-term measurements under different temperature gradients show no change in the electrical properties at 373 K, while a small change is observed at 623 K (fig. S10). Thus, appreciable service performance can be expected for this flexible Ag2Te0.6S0.4 semiconductor glass.

We have measured the total thermal conductivities (κ) of the sample, and they are extremely low mainly due to the disordered structure of amorphous matrix (Fig. 4A). Because of the amorphous lattice thermal conductivity and reasonably good electrical transport properties, a zT (z = σα2/κ) value as high as 0.2 is obtained for the Ag2Te0.6S0.4 semiconductor glass at room temperature and a zT value about 0.7 is achieved at 300°C (Fig. 4B). The TE device figure-of-merit ZT and maximum efficiency η of a single TE leg have been calculated on the basis of the method recently proposed by Snyder and Snyder (49). The calculated efficiency of the Ag2Te0.6S0.4 material can reach 8% under a temperature difference of 450 K (fig. S11).

Fig. 4 Temperature-dependent thermoelectrical properties of the Ag2Te1–xSx samples (x = 0.1, 0.2, 0.3, 0.4).

(A) Total thermal conductivity. (B) zT value.

An in-plane prototype of TE device with a single leg is made from this flexible Ag2Te0.6S0.4 semiconductor glass (fig. S12A), which shows a maximum output power of 3.47 μW under the applied temperature difference of 50 K (fig. S12B). The open-circuit voltages of the device are 0.60, 1.46, 2.38, 3.23, and 4.12 mV under the applied temperature differences of 10, 20, 30, 40, and 50 K, respectively (fig. S12C). As a freestanding flexible TE device, this power generation performance from a crude prototype is superior to most of the state-of-the-art flexible TE devices based on organic materials or organic/inorganic composites, such as Polyaniline (PANI)/Single-walled carbon nanotube (SWNT)/Te films (50), TiS2/hexylamine hybrid film (51), and Polyethyleneimine (PEI)/Single-walled carbon nanotubes (SWNTs) (52).

CONCLUSION

To conclude, a metallic glass–like inorganic semiconductor glass with high mobility has been discovered in the Ag2TexS1–x–based materials. Microstructural analysis shows that the amorphous main phase coexists with tiny crystallites in the Ag2Te0.6S0.4 sample, and the amorphization can be ascribed to the addition of S into Ag2Te. Mechanical tests reveal extraordinarily plastic deformation property of the Ag2Te0.6S0.4 sample, and the repeated bending test confirms that the flexibility is robust. The shear bands, the unambiguous origin of the plasticity of bulk metallic glasses, are observed as the dominant processes in fractured samples, accounting for the exceptional flexibility. Extremely high carrier mobility of ~750 cm2 V−1 s−1 at a carrier concentration of 8.6 × 1018 cm−3 is achieved in the Ag2Te0.6S0.4 sample, which results in a large TE power factor of ~5.7 μW cm−1 K−2 at room temperature. The amorphous Ag2Te0.6S0.4 displays low lattice thermal conductivity due to its structure disorder. As a result, zT ~ 0.22 at room temperature and zT ~ 0.70 at 300°C are achieved in the flexible Ag2Te0.6S0.4 glass, the highest among amorphous materials. The simultaneous realization of superb flexibility and high TE performance establishes the Ag2TexS1–x–based semiconductor glass as a promising flexible TE material. This finding opens up a new way to explore metallic glass–like inorganic semiconductors with exceptional functionalities, such as superior flexibility, high electrical conductivity, and low lattice thermal conductivity.

MATERIALS AND METHODS

Sample synthesis

The Ag2Te1–xSx (x = 0, 0.1, 0.2, 0.3, 0.4) samples were prepared by a conventional melting and annealing route. High-purity elements (Ag 99.9%, Aladdin, China; Te 99.9%, Aladdin, China; S 99.99%, Aladdin, China) were weighed according to the nominal compositions of Ag2Te1–xSx. The mixtures were loaded in a graphite crucible and then sealed into evacuated fused-silica quartz tube (~0.1 Pa). The tubes were slowly heated to 1323 K at the rate of 3 K min−1, held at this temperature for 48 hours, and then annealed at 873 K for 72 hours. Subsequently, the samples were naturally cooled to room temperature in the furnace. The obtained ingots were directly cut into specific shapes such as slices, cuboids, discs, and cylinders by a diamond saw for different characterizations. Thin slices in the thickness of hundreds of micrometers were fabricated by flat hot-pressing.

Sample characterization

Phase identification and structure analysis were performed on thin slices by XRD using a Rigaku x-ray diffractometer (Smart Lab, Rigaku, Japan) equipped with the Cu Kα radiation. The electrical conductivity σ and Seebeck coefficient α were measured by four-probe method using a TE measurement system (ZEM-3, ULVAC-RIKO, Japan). The total thermal conductivities were calculated by κ = λDCp, where λ is the thermal diffusivity, D is the density of material, and Cp is the specific heat capacity. The thermal diffusivities were measured by the laser flash method (LFA 457, Netzsch, Germany). The densities of samples were measured via the Archimedes drainage method. Cp was measured by a DSC thermal analyzer (DSC 214F1, Netzsch, Germany) at a heating rate of 10 K min−1. The carrier concentrations and mobilities of the samples were measured by van der Pauw technique using a Hall coefficient measurement system under a reversible magnetic field of 0.9 T (8404, Lake Shore, USA). The microstructures of the samples were examined using scanning electron microscope (GeminiSEM 300, ZEISS, Germany) and TEM (Talos F200X, FEI, USA). The cuboid in the size of 1.85 mm × 1.85 mm × 13 mm was used for the bending test (CMT5205, MTS, USA). The cylinder with a diameter of 3 mm and a height of 6 mm was used for the compression test (CMT5205, MTS, USA). The rectangular sample with a size of 20 mm × 1.78 mm × 1.78 mm was used for repeated bending test (INSTRON 8801, USA). The bending and compression tests were carried out at a loading rate of 0.5 mm min−1. A dog bone–shaped specimen was processed by wire cutting and used for tensile test at a loading rate of 1 mm min−1 by a tensile stage (Kammrath & Weiss GmbH, Germany). A polished disc with a thickness of ~0.5 mm was used to measure the Vickers hardness at a 0.098 N (10 gf) load and a 10-s loading time (HXD-1000TM/LCD, Shanghai Taiming Optical Instrument, China).

TE device fabrication and test

The strip Ag2Te0.6S0.4 sample in the thickness of 0.28 mm is directly cut from the flat hot-pressed slice. An in-plane prototype TE device with a single leg is made from the Ag2Te0.6S0.4 strip with an effective length of 10 mm and a width of 4 mm. To measure the output characteristics of the prototype TE device, the strip is stuck to the copper electrode by Ag paste. A temperature control platform consisting of a cooler on the cold side and a heater on the hot side was used to set up the temperature difference, which was also calibrated by the thermocouple meter. The open-circuit voltage and output power of the TE leg were recorded with a source meter (Keithley 2400) using the LabVIEW program. The resistance of the TE leg is ~0.11 ohm, and the contact resistance is 1.2 ohm. If the contact resistance can be further reduced, the output power will be greatly increased.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/15/eaaz8423/DC1

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: We thank G. Wang and K. Hu at Shanghai University for their help on the measurement and analysis of mechanical properties. Funding: This work was supported by the National Key Research and Development Program of China (nos. 2018YFA0702100 and 2018YFB0703600) and the National Natural Science Foundation of China (grant nos. 51632005 and 51772186). G.C. and W. Zhang acknowledge the support of the Centers for Mechanical Engineering Research and Education at MIT and Southern University of Science and technology, China. W. Zhang also acknowledges the support from the Guangdong Innovation Research Team Project (grant no. 2017ZT07C062), Guangdong Provincial Key-lab program (no. 2019B030301001), Shenzhen Municipal Key-lab for Advanced Quantum Materials and Devices, and the Shenzhen Pengcheng-Scholarship Program. Author contributions: J.L. conceived and designed the study. S.H. and Y.L. prepared the samples and carried out the XRD analysis. S.H., Y.L., J.Z., and Z.D. measured and analyzed the electrical and thermal transport properties. S.H. performed the mechanical property test. J.F., W. Zhu, and Y.D. fabricated the TE device and measured the power generation performance. L.L. and Y.J. performed the microstructure analysis on TEM. S.H., J.L., W. Zhang, and G.C. analyzed the experimental results systemically and co-wrote the paper. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from authors.
View Abstract

Stay Connected to Science Advances

Navigate This Article