Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m

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Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m (BTST-m) as Effective Lead-Free Thermoelectric Materials Oliver Falkenbach, Andreas Schmitz, Torben Dankwort, Guenter Koch, Lorenz Kienle, Eckhard Müller, and Sabine Schlecht Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02329 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Chemistry of Materials

Oliver Falkenbach,† Andreas Schmitz,‡ Torben Dankwort,§ Guenter Koch,† Lorenz Kienle,§ Eckhard Mueller,*,†,‡ and Sabine Schlecht† * Corresponding author. E-Mail: [email protected]. † Institute for Inorganic and Analytical Chemistry, Justus-Liebig-University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, ‡ Institute for Materials Research, German Aerospace Center, D-51170 Cologne, Germany and § Institute for Material Science, Christian-Albrechts-University, Kaiserstrasse 2, D-24143 Kiel, Germany

KEYWORDS: Thermoelectric properties, tin telluride, mechanical alloying, compacting methods, temperature treatment ABSTRACT: In the medium temperature range tin telluride has emerged as a compensational thermoelectric material for the well established compound lead telluride. We present tin telluride-based nanocomposites of the composition AgSnmBiTe2+m (Bismuth-Tin-Silver-Tellurium, BTST-m) showing superior thermoelectric performance. Nanopowders with varying concentration ratio between silver bismuth telluride and tin telluride were synthesized by mechanical alloying. Three different compacting routes were applied: cold pressing / annealing, hot pressing and short term sintering. A strong interdependence between compacting method and thermoelectric properties was found with hot pressed and short term sintered samples exhibiting the best results. Alike for the already known antimony analogue, ZT could be increased to around 1. Scanning electron microscopy indicates that the observed tremendous reduction of thermal conductivity originates from precipitation of nanoparticles and a high number of grain boundaries. Powder X-ray diffraction and transmission electron microscopy revealed the formation of the rock salt type structure with linearly varying lattice parameters depending on m according to a solid solution, in agreement to a microscopically homogeneous spatial distribution of the constituting elements. However, imaging on the nanostructure shows the formation of coherently ingrown nanoinclusions and diffuse scattering as an indication of short range ordering.

INTRODUCTION Thermoelectric materials have gained growing attention due to their potential application as electrical power generators for waste heat recovery. The efficiency of such systems is defined by a dimensionless number, the figure of merit ZT = S2∙σ∙κ-1∙T, where S, σ, κ and T represent the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature.1 The total thermal conductivity is the sum of an electronic κe and a lattice contribution κl. A common approach for enhancing ZT is to reduce the lattice thermal conductivity by nanostructuring to cause enhanced scattering of the phonons on lattice strain and grain boundaries.2 Still the most effective thermoelectric materials for the intermediate temperature range are based on lead telluride, for example peaking in a ZT value of 2.2 at 800 K with appropriate content of silver-antimony telluride according to the formal composition AgPbmSbTe2+m, known as LAST-m (Lead-Antimony-Silver-Tellurium).3 The special feature of these quaternary nanocomposites is

the thermodynamically driven formation of minority phase-rich nanodots, which are believed to reduce the phonon mean free path.4 In search for elements that could replace the toxic lead, tin was proposed as a possible candidate. By now, few publications dealing with tin tellurides show convincing thermoelectric performance with ZT values, some of them exceeding unity beyond the intermediate temperature range.5-11 Some early investigations of the thermoelectric transport properties in the quasi-binary system SnTeAgSbTe2 (AgSnmSbTe2+m, Tin-Antimony-Silver-Tellurium, TAST-m) revealed a remarkably low thermal conductivity.12-17 Efforts in optimizing the thermoelectric performance and understanding the complex microstructure of the bulk material were successfully undertaken in the last years.18-22 Nanostructuring via top-down methods was applied and a further reduction of the thermal conductivity could be achieved.23-25 Previously, we emphasized the importance of the thermal history of the sample during

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the compression step for the thermoelectric properties of those tin telluride-based nanocomposites.25 For the present work, the antimony was replaced by bismuth to study the influence of the trivalent pnicogen element in the compound AgSnmBiTe2+m (Bismuth-TinSilver-Tellurium, BTST-m). Quite recently, the thermoelectric properties of this analogue material class have been investigated for the bulk.26 The ternary system silver-bismuth-tellurium that represents the minority component with respect to the tin telluride matrix shows a large range of structural properties. The stoichiometric compound silver bismuth telluride is known to have a disordered cubic crystal structure27-37 and an ordered trigonal one27,28,31-33,36-39. In the latter case, the system can be described with known metrics (a = 4.37 Å and c = 20.76 Å27,28, a = 4.24 Å and c = 20.67 Å33, a = 4.27 Å and c = 29.1 Å38, a = 4.468 Å and c = 20.75 Å39), but it is unclear whether it has a rhombohedral (space group No. 166, R3¯m) or a primitive unit cell (space group No. 164, P3¯m1). Considering the phase diagram the cubic modification represents the high temperature phase, which is thermodynamically stable only beyond 701 K to 716 K.27,31,32,40,41 Below that temperature, it quickly decomposes into silver telluride and bismuth telluride. It has repeatedly been reported that the cubic form of silver bismuth telluride could be synthesized by rapid quenching of the melt under harsh conditions,27,32,34,40,41 though it remains questionable whether it is a single phase.30 EXPERIMENTAL Syntheses. Nanopowders of the composition (SnTe)m(AgBiTe2) with m = 10, 12, 16, 20 and 25 were synthesized via mechanical alloying (MA). Commercially available polycrystalline powders were taken as starting materials: silver (99.999%, -22 mesh, Alfa Aesar), tin (99.85%, -100 mesh, Alfa Aesar), bismuth (99.5%, -200 mesh, Roth) and tellurium (99.999%, -18 to +60 mesh, Alfa Aesar). Mechanical alloying was done by mixing of 4 g from the elemental powders in the desired ratios under an argon atmosphere in a 25 mL stainless steel beaker with eight 4 g stainless steel balls with a diameter of 10 mm, according to a ball-to-powder ratio of 8:1. Milling was performed in a Retsch ‘PM 200’ planetary ball mill for 18 h at 500 rpm with one change of rotational direction per hour. To release the mechanically induced stress and strain in the grains, the as-milled nanopowders were annealed at 773 K for 1 h. This treatment was also important for direct comparison of the influence of the temperature regime and process history of each compacting method. Compacting Conditions. The obtained nanopowders were compacted alternatively via three different procedures: cold pressing / annealing (CPA), hot pressing (HP) and short term sintering (STS). Cold pressing was done by uniaxial compaction of 0.8 g of the nanopowders at 867 MPa at room temperature for 15 min. The resulting discs with a diameter of 12.0 mm

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and a thickness of about 1.0 mm were annealed at 573 K for 24 h under a continuous argon flow for a higher mechanical stability. In the hot pressing procedure, 0.8 g of the nanopowder was uniaxially compacted at 434 MPa for 15 min at 473 K to yield 12.0 mm sized and about 1.0 mm thick pellets. Short term sintering was performed in a direct sinter press ‘DSP 510’ of Dr. Fritsch Sondermaschinen using boron nitride coated graphite dies with diameters of 12.7 mm. During the sintering process, 0.8 g to 0.9 g of the nanopowders were uniaxially compacted at 56 MPa for 11 min at 673 K under application of a continuous direct current. Pellets with a thickness of about 1.1 mm were formed. Characterization Methods and Measurement Parameters. Structural characterization of the nanopowders was accomplished by X-ray diffraction (XRD) on a PANalytical ‘X’Pert Pro’ instrument, working in the reflection mode and using Cu Kα radiation (λ = 1.54 Å) with an operation voltage of 40 kV and a current of 40 mA. The lattice parameter refinement was done using ‘X’Pert HighScore Plus’ version 2.2a (2.2.1). An automatic refinement mode was applied using indexing method ‘Treor’. Microstructural investigations were conducted via transmission electron microscopy (TEM), including energy-dispersive X-ray emission spectroscopy (EDX) and electron diffraction (ED). Two different types of transmission electron microscopes were used, a Philips ‘CM 30’ instrument with a LaB6 cathode, working at an operation voltage of 300 kV, and a Tecnai ‘F30 G2-STwin’ microscope, operated at 300 kV with a field emission gun cathode and a Si/Li detector (EDAX system). The samples were prepared by scratching material of the compacted samples with a blade. The obtained powder contained some larger, more crystalline particles. The largest of them were pounded in a mortar. The obtained powder was mixed with n-butanol. The solution was dispersed on a holey-carbon copper grid. It was waited until the solvent evaporated. To the best of our knowledge, this procedure should not alter the composition and, consequently, the appearance of plate-like precipitates. Data analysis was performed using ‘Digital Micrograph’ (DM) version 1.71.38. Further DM scripts have been used for filtering and processing.42 High resolution scanning electron microscopy (HRSEM) was done on a Zeiss ‘MERLIN’ with Schottky field emitter as electron source. The operating voltage was adjustable between 200 V and 30 kV. The maximum lateral resolution is 0.8 nm. For imaging an EverhartThornley (SE2) detector and an on-axis in-lens secondary electron detector were employed. The thermal conductivity κ of the samples was calculated by κ = α∙ρ∙cp from the thermal diffusivity α, the sample density ρ and the heat capacity cp of the material. The inaccuracy given by this equation adds up to about 10% due to different uncertainties, such as those in the deter-


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mination of the pellet thickness and density and the assumptions made for the specific heat. The thermal diffusivity was measured under vacuum with a LINSEIS ‘XFA 500’ instrument, based on the xenon flash technique. The pellet density was determined via the Archimedes method using water. The relative density was calculated according to the ratio of both components using a bulk density of 6.445 g∙cm-3 for SnTe43 and 8.14 g∙cm-3 for AgBiTe228. For the specific heat of the BTST-m samples, values were extracted from the literature, starting from pure bulk SnTe and considering the respective amount of the minority component. This approach, that we used before on the nanostructured TAST-m system,25 was found to be sufficient giving results close to the ones measured on the bulk.19,20 The specific heat of SnTe is expressed by a virial approach based on literature data measured as a function of temperature, which was fitted by us yielding: cp(T) = (58.75 + 1.03∙10-3∙T∙K-1 - 1.1∙106∙T-2∙K-2) J∙mol-1∙K-1.44 This assumption fits quite well to measurements reported elsewhere.45-47 For the contribution of AgBiTe2, the same value of the molar specific heat was used as of the closely related cubic AgSbTe2, where a constant value of cp = 207 J∙kg-1∙K-1 at 300 K – according to the Dulong-Petit law – was assumed.48 Taking into account the ratio of the molar masses, we calculated a value of cp = 175 J∙kg-1∙K-1 for AgBiTe2 as a sufficient approximation. In comparison to other work, the specific heat values of AgSbTe2 as well as those of isostructural ternary chalcogenides differ marginally.31,48-51 The heat capacity of AgBiTe2 itself has never been precisely measured due to a polymorphic phase transition into the low temperature modification around 403 K to 420 K.32,38,40,41 Electrical conductivity and Seebeck coefficient were measured in a LINSEIS ‘LSR-3 Seebeck’ instrument with a setup based on the four-point method. The measurements were made after a first thermal diffusivity measurement cycle on the same samples. The circular pellets were cut into rectangular shape. To achieve a good heat transfer between sample and sample holder during the measurement, the samples were kept under helium atmosphere of 1.1 bar at room temperature. Platinum electrodes (type S) were employed as current feeds for the resistivity measurement, and chromel/alumel (type K) thermocouples were used for measuring the Seebeck voltage. Those measurement results were found to be repeatable. The estimated error of both Seebeck coefficient and electrical conductivity is in the order of about 5% due to irregularities in sample geometry and contacting.

Structural Properties. All BTST-m samples crystallize in the rock salt structure type (space group No. 225, Fm3¯ m), as expected for tin telluride (Fig. 1).52 Silver and bismuth atoms were assumed to be statistically distributed on the tin positions, since the reflections shift continuously towards higher angles with an increased alloying rate. This indicates an incorporation of silver and bismuth on the lattice sites. No secondary phases could be detected. The average crystallite size was determined by the Scherrer equation on the XRD patterns of the annealed nanopowders. A value of about 60 nm was calculated. The main reflection (200) was selected, since it is most meaningful in terms of mathematical accuracy. Nonetheless, this result should be considered being a rough approximation, since it gives only mean values for nanoparticles with a relatively large size and shape distribution which are present after ball milling.

(200) (220)

(420)

Intensity / %

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(222)

(422) (600)

(400) (440)

BTST-10 BTST-12 BTST-16 BTST-20 BTST-25

20

30

40

50

60

70

80

90

100

2 / ° Figure 1. XRD patterns of BTST-m nanopowders after annealing.

Bulk tin telluride is expected to form a solid solution with isostructural silver bismuth telluride.29 Indeed, the lattice parameters of the nanostructured BTST-m samples show a linear dependence on the concentration, decreasing towards a higher amount of minority component, indicating that Vegard’s law is obeyed (Fig. 2). The lattice constant of the solid solution interpolates appropriately between the values of both end components, bulk tin telluride with a = 6.314 Å43 and silver bismuth telluride (a = 6.155 Å27,28, a = 6.16 Å33, a = 6.15 Å34, a = 6.154 Å41).

Afterwards, the thermal diffusivity of the – now consolidated – samples was measured again since changes occurred during the first cycle. RESULTS AND DISCUSSION



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m 30 25

20

15

10

6.295

Lattice constant a (Å)

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6.290

6.285

6.280

6.275

6.270 3

4

5

6

7

8

9

10

Alloying concentration AgBiTe2 (at.-%) Figure 2. Lattice parameters of BTST-m nanopowders after annealing (line: linear fit).

TEM analyses were exemplarily applied on the BTST-10 sample. As expected from the milling procedure, nanoparticles different in shape and size with diameters about 30 nm to 100 nm but also much finer particles and structures are seen, highly agglomerated up to particles of several hundred nanometers (Fig. SI1). This is roughly in accordance with the approximated crystallite sizes calculated from the XRD patterns. High resolution transmission electron microscopy (HRTEM), ED and EDX elemental maps were further used to investigate microstructure and nanostructure of a hot pressed BTST-10 sample. A representative ED pattern along [110] zone axis proves single crystalline structure with no splitting of reflections or satellite reflections which could be interpreted as incoherently ingrown precipitates (Fig. 3). Diffuse streaks along and are observed, which is an indication for structural disorder.53,54 The insets represent line scans to highlight the intensity of the diffuse scattering. Similar effects are known from lead telluride where lead deficiency leads to randomly dispersed but coherently ingrown plate-like precipitates which are well seen via HRTEM imaging.55 However, HRTEM micrographs for tin telluride of zone axis [110] did not show plate-like precipitates, maybe due to the much lower experimental significance expected for the tin-tellurium vs. lead-tellurium system. A detailed investigation of the origin of the diffuse streaks would need an extensive study of HRTEM micrographs and ED patterns in [100] zone axis which is beyond the scope of this article.

Figure 3. ED on a hot pressed BTST-10 sample. The pattern shows rock salt-type reflections in [110] zone axis. Diffuse streaks are observed in and directions. Inset (I) and (II) highlight the diffuse scattering by means of intensity maxima in the line profile. HRTEM micrographs reveal the presence of precipitates with unknown structure at the grain boundary of a tin telluride grain (Fig. 4). The dashed red line illustrates the boundary between the rock salt-type matrix in [110] zone axis and the unknown precipitate. The precipitate is related to the tin telluride-based phase by showing superstructure reflections at 1/3{11¯1} of the rock salt-type phase resulting in a d spacing of 10.98 Å. Complex tellurates, silver bismuth telluride (P3¯m1), bismuth telluride (R3¯m) and other common phases could be ruled out as possible candidates for the unknown phase due to the huge mismatch of d values. Instead, the superstructure reflections might be understood as an ordering of silver and bismuth along the {111} planes of the tin telluride as known for germanium telluride alloyed with antimony telluride or bismuth telluride.56 Based on this assumption the unknown phase might crystallize in tetradymite structure type (R3¯m) or P3¯m1 considering the [210] zone axis and [100], respectively. For R3¯m, the superstructure reflections with d spacing of 10.98 Å are related to the (003) plane resulting in a lattice parameter of c = 32.94 Å. Further d values of 3.85 Å for (1¯ 20) result in lattice parameters of a = b = 7.70 Å. The authors note equal a and b lattice parameters are uncommon for this type of layered phases. Regarding P3¯m1 in [100] zone axis, measured d values of 10.98 Å and 3.85 Å correspond to (001) and (010) phase, respectively. Consequently, lattice parameters for this cell would be c = 10.98 Å and a = b = 3.85 Å. Based on this result, a cell with space group P3¯m1 and cell parameters as described above was designed. A simulated ED in [100] zone axis of this cell (Fig. 4b) agrees well with the observations (Fig. 4a).


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Chemistry of Materials Figure 5. HRTEM micrograph and FFT of a hot pressed BTST-10 sample showing a region with incommensurable satellite reflections along direction.

Figure 4. HRTEM mircrograph and corresponding FFTs of a hot pressed BTST-10 sample depicted in a) show the tin telluride-based phase in [110] zone axis and an unknown phase in [210] zone axis. The dashed red line indicates the boundary of both phases. b) represents a simulated ED based on a cell of space group (P3¯m1) and cell parameters as described in the text.

A typical SEM image of a fresh fracture face of a cold pressed / annealed BTST-10 sample shows a large number of single heterogeneously shaped particles with a size of several hundred nanometers that are stuck together to form a rough internal structure (Fig. 6). The surface of those large particles itself is all-around decorated with very small nanoparticles with about 10 nm in size, resulting in a finely granulated surface pattern. Some of the particles have been cracked during fracture showing a smooth appearance whereas the material the material has often split along the granularly decorated particle interfaces indicating the lower strength of these.

EDX elemental maps did not show any significant chemical inhomogeneity (Fig. SI2). These results are in agreement with the XRD patterns which indicate the presence of a solid solution on the microscale. The authors note that EDX measurements are not suitable for quantitative analysis and investigations of inclusions due to multiple superpositions of the characteristic elemental K lines and the small size of possible agglomerates. Besides the formation of the above described unknown ingrown phase also nanoscale domains were observed (Fig. 5). Their structure might be commensurate or incommensurate. The satellite reflections along (111) with varying d spacing do not fit into the reciprocal lattice of the fundamental reflections indicating commensurable or incommensurable structural phenomena. One possible explanation is a partial ordering of silver and bismuth. However, the d spacings are unspecific thus it is hard to distinguish between an ordering effect due to silver or bismuth along the (111) planes or Moiré effects resulting from twisted grains.57 Also domains with satellite reflections in directions different from can be observed. Most of the investigated regions showed either one of the described nano-features. The authors note that density and distribution of them, rather than their nature are contributing to the favorable transport properties.

Figure 6. SEM image of a cold pressed / annealed BTST10 sample after thermal cycles related to the thermoelectric measurements (inset: 4 x magnified). A representative SEM fracture surface profile of a hot pressed BTST-10 sample is much more even with tightly bonded, significantly larger particles, larger smooth areas and a coarser granularity at decorated interfaces where it appears (Fig. 7).



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when prepared under the same conditions. In contrast, it is more evident that the cold pressing / annealing and the hot pressing procedure deliver a higher compaction than short term sintering. That might be due to the relatively low pressure that is used for sintering in comparison to both pressing methods. In a sintering process the compaction proceeds by diffusion induced by the current and the heat, while during a cold pressing step the applied pressure is the most important aspect and hence plastic deformation is the main densification mechanism here. The lower density of the short term sintered samples can be explained with the formation of closed pores in the microstructure (Fig. 8). This is probably a result of limited sintering and diffusion processes or of an earlier closing of porosity. Figure 7. SEM image of a hot pressed BTST-10 sample after thermal cycles related to the thermoelectric measurements (inset: 4 x magnified).

All samples were found mechanically stable. Table 1. Density of nanostructured BTST-m samples. Absolute density (g∙cm-3) /

A short term sintered BTST-10 sample resembles mainly the hot pressed one (Fig. 8). The interface decoration appears as fine as for the cold pressed / annealed material but not as dense. These observations give evidence that local fusing between the nanoparticles could be conceivable during hot pressing and short term sintering, leading to a more homogeneous arrangement of the particles and a better connection between them. The milder cold pressing / annealing approach conserves more of the original nanostructured surface decoration in the pellet. The authors note, technically it cannot be fully excluded that those small particles are a sign of sublimation and re-condensation at free surfaces due to a low local density.

Relative density (%) Composition

CPA

HP

STS

BTST-10

6.365 / 96

6.204 / 94

6.097 / 92

BTST-12

6.306 / 96

6.327 / 96

6.001 / 91

BTST-16

6.348 / 97

6.217 / 95

6.074 / 93

BTST-20

6.282 / 96

6.242 / 96

6.029 / 92

BTST-25

6.370 / 98

6.266 / 96

5.745 / 88

Compared to pure bulk tin telluride, the thermal conductivity of the BTST-m samples has been reduced by a factor of about four due to alloying and nanostructuring.58 Alike for bulk TAST-m most of this effect can be attributed to alloying in general.14-17,20-22 However, the phonon scattering on nanostructures reducing the thermal conductivity has been proven to be additionally effective, with the TAST-m system, before.24,25 Compared to bulk BTST-m, its low thermal conductivity could be further reduced in this work by nanostructuring and density effects.26

Figure 8. SEM image of a short term sintered BTST-10 sample after thermal cycles related to the thermoelectric measurements (inset: 4 x magnified).

A clear trend in the thermal conductivity of compacted tin telluride-based powders regarding different additions of silver bismuth telluride cannot be detected unambiguously. In some cases, even cold pressed / annealed and subsequently sintered bulk samples with a difference of the alloying rate of up to 5 at.-% are barely distinguishable in their thermal conductivity.20 Commonly, in tin telluride variation of the thermal conductivity is observed, depending on sample preparation, sample quality and measurement technique.20-22,24,25,58

Thermoelectric Properties. The powders were compacted leading to a relative density between 88% and 98% (Tab. 1). The density of equivalent samples varies slightly

Modification of the microstructure as a result of the compacting procedure and thermal history has a major effect on the thermal conductivity. The cold pressed / annealed series yielded the highest density in each sample


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and so, for example, the cold pressed / annealed BTST-25 sample shows the highest thermal conductivity over the entire temperature range. On the other hand, the low thermal conductivity of the short term sintered BTST-25 sample can also be explained by the sample density, which is the lowest one in this present study (Tab. 1). Regarding the influence of the sample density for the obtained range, a linear relation between porosity and thermal conductivity can be assumed.59 Thermal conductivity  / Wm-1K-1

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3.0 BTST-10, HP BTST-12, CPA BTST-12, HP BTST-12, STS BTST-16, CPA BTST-16, HP BTST-16, STS BTST-20, CPA BTST-20, HP BTST-20, STS BTST-25, CPA BTST-25, HP BTST-25, STS

2.5

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1.0 300

400

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800

Temperature T / K Figure 9. Thermal conductivity of nanostructured BTSTm samples (stabilized samples, second measurement cycle).

The thermal conductivity of the cold pressed / annealed samples is almost constant over the medium temperature range (Fig. 9). For this series of samples a tendency of reducing the thermal conductivity with increasing alloying rate can be observed. For the hot pressed and short term sintered samples a more pronounced trend of a decreasing thermal conductivity has been found in our measurement, which is resulting from a change of the sample properties during the thermal cycle (Fig. SI3). The first measurement shows an onset of the reduction of the thermal conductivity near 550 K. During the thermal cycle which covers typically about four hours above that temperature, the thermal conductivity decreased noticeably. Hence the scatter of the thermal conductivity values at lower temperatures among both series is related to variation in progress of the ongoing decrease. Usually, relaxation processes of microstructure or lattice defects are related to healing of disorder and are hence linked to a moderate increase of thermal conductivity, which is often observed on a first thermal cycle of powdercompacted samples. Unlike, the effect of a strong reduction observed here in-situ must be of fundamentally different nature. Most probably it is the formation process of nanostructure, which is highly effective in scattering phonons. TEM analysis gave evidence of coherent nanoprecipitations in cycled samples, which we assume to be the origin of the reduction of thermal conductivity. Repeating the measurement over another cycle reveals that the thermal conductivity curves of the hot pressed and the short term sintered sample series flatten towards a monotonic trend slightly decreasing with temperature

(Fig. 9). Finally, after the nanostructure formation has saturated, all samples of a series meet in a relatively narrow band, like for the cold pressed / annealed one, where the order of thermal conductivity values is mainly dominated by alloying concentration and density. Remarkably, our observation implies that the short annealing of the powders at 773 K after mechanical alloying, that all samples were exposed to, did not result in formation of the final nanostructures, whereas the long term annealing step at 573 K after the cold pressing did. This has to be concluded from the fact that the formation occurred in all but the cold pressed / annealed samples. It is known that nanostructuring may modify, among other material properties, solubility of additives, which may be higher than in bulk. We assume that the annealing of the nanopowders has resulted in a diffusive homogenization of the solid solutions, possibly at a higher concentration of the minority component than would be soluble in the bulk. We attribute the origin of that to peculiarities of the lattice dynamics, which are known also for other rock salt-type IV-VI structures.60-62 At elevated temperatures, irreversible phase changes occur breaking the high symmetry in the crystal and thus abruptly restricting the phonon transport. These variations in disorder overlay the influence of phonon scattering caused by an increased alloying rate on the lattice thermal conductivity. As the majority cation (here: tin) finds a longer bonding length in the rock salt telluride structure than optimal, the defect-free lattice is containing a high degree of inherent stress, which results in strong anharmonicity of particular phonon modes and makes the lattice highly perceptive to various kind of structural disorder. In a solid solution with isomorphic additives, this mechanism may favor the formation of coherently incorporated, nanosized domains enriched in or purely consisting of the added compound. Due to a density difference to the matrix, their growth is limited by the strain field spreading and that itself is highly effective in phonon scattering. As a result, a high density of nano-size precipitates will persist stably up to elevated temperature and will fill a large fraction of the overall volume with strain fields reducing thermal conductivity efficiently. We assume that these nanodots will not form in a non-compacted nanopowder since the inherent lattice stress may relax over the size and shape of the individual particles and statistical dissolution of additives. Thus, the relaxation leading to the spontaneous nanostructuring inside the matrix may proceed only in a compacted bulk, where particle shape is fixed due to the fusing with the neighboring grains. We conclude that an annealing step for the hot pressed and short term sintered samples is recommended to reach an equilibrium state, similar to the one that the cold pressed samples were exposed to. Single crystalline tin telluride possesses a high intrinsic carrier concentration far above n = 1020 cm-3.43,58,63-67 Two valence bands and a conduction band form a small band gap of Eg = 0.26 eV at room temperature.67



3000 BTST-10, CPA BTST-10, HP BTST-10, STS BTST-12, CPA BTST-12, HP BTST-12, STS BTST-16, CPA BTST-16, HP BTST-16, STS BTST-20, CPA BTST-20, HP BTST-20, STS BTST-25, CPA BTST-25, HP BTST-25, STS

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700

800

Temperature T / K Figure 10. Electrical conductivity of nanostructured BTSTm samples.

The electrical conductivity of the BTST-m samples decreases likewise towards elevated temperatures, suggesting a semimetal, with values slightly lower than for TASTm with comparable compositions. Some samples with low content of the minority component show the highest electrical conductivity over the entire temperature range (Fig. 10). On the other hand, low values were obtained for the higher alloyed samples. Especially, the hot pressed series of samples shows a distinct trend that an increasing alloying rate decreases the electrical conductivity. The cold pressed / annealed samples exhibit comparably lower electrical conductivity than the similarly doped hot pressed and short term sintered ones. We believe the more inhomogeneous microstructure as imaged by SEM to be the reason for this (Fig. 6). A multitude of interfaces decorated by nano-granular precipitates is expected to lead to considerably reduced effective mobility. The same trend was observed for TAST-m, which underwent an identical preparation treatment.25 Hot pressed and short term sintered samples show a more bulk-like behavior, though – due to the short time of compaction – more extensive grain growth should be avoided since the concentration of the nano-decorated interfaces is reduced and its granularity is coarsened. Due to its defect crystal structure containing cation vacancies, tin telluride possesses a high hole concentration and hence p-type conduction.43,58,63-67 The maximum Seebeck coefficient ranges from about 20 μV∙K-1 to 100 μV∙K-

1 43,58,64,65,67

. This is quite low for common thermoelectric materials. 160

Seebeck coefficient S / VK-1

In this work, we combined two compounds with a high native disorder which probably structurally stabilize each other. Obviously, alloying changes the concentration of impurity states, probably since the inherent stress of the lattice, which requires a high concentration of tin vacancies acting as acceptors, in pure tin telluride is partly relaxed by the formation of nano-precipitations. Hence the equilibrium concentration of the former is less and with it the acceptor density. This leads to a decrease in the electrical conductivity with an increasing concentration of the ternary minority compound, as it has been similarly observed in several studies on the related TASTm system.20-22,25 Electrical conductivity  / Scm-1

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140 120 100 80

BTST-10, CPA BTST-10, HP BTST-10, STS BTST-12, CPA BTST-12, HP BTST-12, STS BTST-16, CPA BTST-16, HP BTST-16, STS BTST-20, CPA BTST-20, HP BTST-20, STS BTST-25, CPA BTST-25, HP BTST-25, STS

60 40 300

400

500

600

700

800

Temperature T / K Figure 11. Seebeck coefficient of nanostructured BTST-m samples.

Higher alloying leads to higher Seebeck coefficient, as clearly seen for the hot pressed series of samples. This trend is in accordance to the observed one of the electrical conductivity due to its reverse dependence on the carrier concentration.1 The Seebeck coefficient of the BTST-m samples is slightly higher than the one of the corresponding TAST-m samples, with no notable difference between the bulk and the nanostructured system.2022,25,26 It is concluded that bismuth can more effectively neutralize holes than antimony.26 By alloying with silver bismuth telluride the Seebeck coefficient could be greatly enhanced up to 150 μV∙K-1 for the hot pressed BTST-10, BTST-12 and short term sintered BTST-12 samples (Fig. 11). The Seebeck coefficient for all hot pressed and short term sintered samples is, independent of the composition, significantly higher than the cold pressed / annealed ones, which only reach maximum values of up to 120 μV∙K-1. A probable reason might be a higher concentration of acceptors whereas the reduced mobility is sufficient to explain the seemingly contradictory trend with the electrical conductivity. The Seebeck coefficient increases almost linearly with increasing temperature until a maximum is reached at around 723 K. The sharp descent at the highest measured temperatures indicates – most likely – a phase transition of the minority component. A plateau in the Seebeck coefficient and, partly, the onset of a moderate decline is also observed in other studies of closely related systems shifting with composition,18,19,21,22,25,26 which is explained by bipolar effects rising with temperature.18 However, the drop of the Seebeck coefficient is sharper here than can be expected from this effect and is not reflected in correlated bowing up of the electrical and thermal conductivity, the latter being the most sensitive parameter to bipolarity. An error in the measurement cannot be excluded as well. As a consequence of the decreased thermal conductivity and a marginally increased Seebeck coefficient the


Page 9 of 12

thermoelectric figure of merit of the nanostructured BTST-m system could be raised compared to the respective TAST-m compositions by about 20% in average for each applied compacting method.25 The highest value was achieved for the hot pressed BTST-20 sample, peaking in a ZT value of 1.2 at 723 K (Fig. 12). Some of the hot pressed and short term sintered samples reach ZT values between 1.0 and 1.1, which is close to the results obtained for the bulk material.26 All cold pressed / annealed samples show distinctly poorer ZT values as a consequence of the deterioration in both the Seebeck coefficient and electrical conductivity. Due to the effects occurring in the hot pressed and short term sintered samples during the first cycle of the thermal conductivity, the resulting ZT values calculated using the results of those non-stabilized samples, is underestimated at lower temperature, but barely affected in the peak ZT values which range between 0.9 and 1.1 at 723 K (Fig. SI4).

ZT

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1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 300

BTST-10, HP BTST-12, CPA BTST-12, HP BTST-12, STS BTST-16, CPA BTST-16, HP BTST-16, STS BTST-20, CPA BTST-20, HP BTST-20, STS BTST-25, CPA BTST-25, HP BTST-25, STS

400

with the structurally complex ternary compound silver bismuth telluride. Though the pellet density of the samples compacted via short term sintering belongs to the lower ones within the series, those samples possess the best thermoelectric performance. Obviously, sintering changes the microstructure of the material significantly. Hot pressing and short term sintering exhibit wide similarities of the thermoelectric properties as a consequence of similar conditions during compaction. The charge carrier transport in BTST-m seems to react sensitively on nanoscopic inhomogeneities in the microstructure. Via scanning electron microscopy we visualized that those exist in particular after cold pressing / annealing when the nanoparticles are more loosely locked, while in hot pressed and short term sintered samples more particles are fused together. In future works, more detailed XRD and SEM studies of the nanopowders prior the compacting step could give more information on the change in crystallite and grain size during the compacting process. It would be interesting to know how compacting leads to more tension in the crystal structure and if the induced heat alters the composition. Going on, it would be worthwhile to use synchrotron X-ray diffraction to study the structural evolution of the observed nanostructures in-situ. This BTST-m system studied here was found to be superior compared to the already known TAST-m material made by similar synthesis routes. This gives rise to the expectation to find other high-performance thermoelectric materials on the basis of tin telluride.

500

600

700

800

Temperature T / K Figure 12. ZT value of nanostructured BTST-m samples (using the thermal conductivity values of stabilized samples, second measurement cycle).

With a further increased fraction of the minority phase or a changed silver-to-bismuth-ratio the relatively high carrier concentration in this type of tin telluride-based nanocomposites could be converged towards the optimum for thermoelectric performance and boost the Seebeck coefficient. CONCLUDING REMARKS In-situ observation of a strong reduction of thermal conductivity during first heating after compaction of BTST-m nanopowders can be explained by the formation of coherent nano-precipitates within tin telluride rock salt lattice driven by complex structural peculiarities of tin telluride alloyed with silver bismuth telluride. Powder Xray diffraction and transmission electron microscopy proved that, microscopically, silver bismuth telluride forms a solid solution with an isostructural matrix of nanostructured tin telluride. The thermoelectric figure of merit of tin telluride could be significantly increased by nanostructuring and alloying

SUPPORTING INFORMATION Figure SI1. TEM image of a BTST-10 nanopowder after annealing. Figure SI2. Representative EDX elemental maps of a hot pressed BTST-10 sample. Figure SI3. Thermal conductivity of nanostructured BTST-m samples (non-stabilized samples, first measurement cycle). The difference in slope from the curves of the hot pressed and short term sintered sample series compared to the second measurement illustrates the reduction of the thermal conductivity during the first temperature treatment after compaction. Figure SI4. ZT value of nanostructured BTST-m samples (using the thermal conductivity values of non-stabilized samples, first measurement cycle). ACKNOWLEDGEMENT This work was financed by the German Research Foundation (DFG) in the priority program SPP 1386 (‘Nanostructured Thermoelectrics’). The authors would like to thank Klaus Peppler (Physical Chemistry Institute, Justus-Liebig-University) for taking the SEM images. REFERENCES



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Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m

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