Journal of Alloys and Compounds 587 (2014) 420–427

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Journal of Alloys and Compounds

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Facile precipitation of two phase alloys in SnTe0.75Se0.25 with improvedpower factor

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.10.217

⇑ Corresponding author at: School of Materials Science and Engineering, NanyangTechnological University, Singapore 639798, Singapore. Tel.: +65 6790 4140; fax:+65 6790 9081.

E-mail address: ashhhng@ntu.edu.sg (H.H. Hng).

Li Ping Tan a, Ting Sun a, Shufen Fan a, Raju V. Ramanujan a, Huey Hoon Hng a,b,⇑a School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singaporeb Temasek Laboratories @ NTU, Nanyang Technological University, Singapore 638075, Singapore

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 September 2013Received in revised form 28 October 2013Accepted 29 October 2013Available online 9 November 2013

Keywords:Thermoelectric materialsComposite materialsRapid solidification

Multiphase thermoelectric (TE) materials have received great interest in recent years due to the synergis-tic improvements in TE properties. We report for the first time on melt spun samples with nominal com-position of SnTe0.75Se0.25. Interestingly, an orthorhombic second phase was observed to exhibit change inmicrostructure under low applied pressure – the metastable nature leads to an observed increase in massfraction with increased applied pressure – and this second phase can be stabilized by concurrent appli-cation of heat and pressure during hot pressing. The second phase reduces electrical resistivity, whileincreasing Seebeck coefficient values slightly, especially at higher temperatures. A peak power factorof 1.3 mW/m K2 is obtained at 494 K, which is a 1.5 times enhancement over the peak power factor ofpure SnTe. The peak ZT achieved for the alloy is 0.19 at 548 K, which is three times that of pure SnTe.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

There has been immense interest in the development of alterna-tive and renewable energy sources, and energy management sys-tems, in view of issues like the rapid depletion of fossil fuels andglobal warming. Thermoelectric (TE) materials are of particularinterest as they can convert heat into electricity in the presenceof a temperature gradient and vice versa [1–4]. The efficiency ofTE materials is determined by the dimensionless figure of merit,given by ZT = S2rT/k, where S, r, k and T are Seebeck coefficient,electrical conductivity, thermal conductivity and absolute temper-ature respectively. The term S2r is also known as the power factor.In traditional bulk TE materials, these parameters are all inter-related, hence optimization of one property often leads to the dete-rioration of another, making the improvement of TE performancean uphill task. In recent years, there is increasing interest in multi-phase nanocomposites due to the ability to decouple these param-eters, allowing a decrease in thermal conductivity and increase inpower factor to be achieved concurrently, leading to improved ZTvalues [5,6].

The approach of using rapid solidification and annealing is com-monly used to obtain multiphase TE materials [7–10]. The methodallows in situ precipitation of second phase(s) – in the size of a fewnanometers to tens of nanometers – in a bulk material. This leads

to improved TE properties due to effects such as decreased thermalconductivity, ability to tune electrical properties and increasedpower factor. Melt spinning is a popular processing method sinceit leads to metastable, amorphous materials or multiphase materi-als with phases in the nano/micron scale [11,12]. The increasedinterfacial area of the precipitates, and/or reduction in crystal sizeof the materials is beneficial to improve the phonon scatteringeffect in TE materials.

In this work, the SnTe–SnSe material system, in particular meltspun samples with a nominal alloy composition of SnTe0.75Se0.25

were studied and TE properties measurements were performedand compared against the single phase. The presence of a metasta-ble orthorhombic second phase was observed during processing,and counterpart property evaluation was carried out to understandthe effect of the second phase on the TE properties.

The SnTe–SnSe material system is chosen as it has potentialapplications in optical recording, infrared devices and thermoelec-trics, where the constituent phases and microstructures can affectthe properties [13]. SnTe and SnSe possess the cubic and ortho-rhombic crystal structure respectively, and its phase diagram[14] shows two single phase regions (a SnTe-rich and a SnSe-richsolid solution) and a two phase region. The nominal compositionof SnTe0.75Se0.25 is selected as it lies near the phase boundary ofthe single and two phase regions. The fabrication method is viamelt spinning, which allows for rapid solidification. In addition,microstructural optimization through subsequent processing byheat treatment can be carried out [15]. He et al. [16] reported thatin the composite material PbTe–PbSnS2, which has a cubic andorthorhombic crystal structure respectively, a superstructure

L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427 421

formed along the longest axis due to the nature of these two crystalstructures. A variety of effects like phase boundaries, misfit dislo-cations, and displacement layers were also present, which led todecreased thermal conductivity. As our melt spun material systemconsists of both cubic and orthorhombic phases, it is assumed thatperhaps the SnTe–SnSe system can also exhibit unique TEbehavior.

The enhancement in TE properties over the pure phases ob-tained from multiphase SnTe0.75Se0.25 is reported here for the firsttime. Interestingly, the second phase can be formed at low appliedpressure, and the effect of this second phase on the TE properties inthe temperature range of 300–550 K was studied. This approach issimple and straightforward compared to traditional processingmethods such as solid state synthesis. It was found that the secondphase was useful in improving TE properties of SnTe0.75Se0.25, witha maximum power factor of 1.3 mW/m K2 obtained at 494 K, 1.5times that of SnTe. A peak ZT of 0.19 was achieved at 548 K, 3 timeshigher than the peak ZT of SnTe.

Fig. 2. TEM of as melt spun SnTe0.75Se0.25 at low magnification and the indexeddiffraction pattern corresponding to a cubic phase in the inset.

2. Materials and experimental method

The starting materials were used without further purification: tin telluride(SnTe, 99.999%) and tin selenide (SnSe, 99.999%), both purchased from Alfa Aesar.In a typical experiment, stoichiometric amounts of tin telluride and tin selenidewere cold pressed into pellets and placed into a quartz cylinder of about 10 mmdiameter with a 0.5–1 mm nozzle in the melt spinning system for rapid solidifica-tion in argon atmosphere. The pellets were inductively melted and injected onto theedge of a rotating copper roller with a working distance of 0.5 mm and a frequencyof 60 Hz. The as-obtained thin flakes were then manually ground and ball milledinto powders, before being consolidated into a pellet by hot pressing in vacuum

Fig. 1. (a) Top surface and (b) cross section of SnTe0.75Se0.25 a

at 773 K for 30 min under a pressure of 50 MPa. As SnSe could not be inductivelymelted, as purchased lumps were manually ground into powder and hot pressed.The densities of the hot pressed samples were about 95% of the theoretical values.Thermal conductivity measurement on the hot pressed samples was carried outusing the laser flash method (Netzsch, LFA 447) and TE properties measurementswere done using a commercial instrument ULVAC-Riko, ZEM-3 (M8) over a temper-

nd (c) XRD patterns of melt spun SnTe and SnTe0.75Se0.25.

422 L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427

ature range of 300–550 K. To elucidate the nature of the metastable second phaseand its effect on the TE properties, ball milled powders were cold pressed at variouspressures (1000PSI, 2000PSI and 3000PSI or 6.9 MPa, 20.7 MPa and 34.5 MParespectively) for 10 min. The densities of cold pressed samples were found to beabout 70% of the theoretical values.

Fig. 3. Temperature dependence of thermoelectric properties of SnTe, SnSe and SnTe0.75

conductivity and (e) ZT.

The microstructure was observed using a JEOL-JSM 7600F field emission scan-ning electron microscope (FESEM) and JEOL-JEM 2100F transmission electronmicroscope (TEM). TOPAS was used to calculate the lattice parameters, identifyand calculate the mass percentages of the second phase. This was done via Rietveldrefinement [17] using the following equation:

Se0.25: (a) Electrical resistivity, (b) Seebeck coefficient, (c) power factor, (d) thermal

Fig. 4. XRD patterns of ball milled, cold pressed and hot pressed SnTe0.75Se0.25.

Table 1TOPAS analysis of the mass fraction of phases in the various samples.

Sample Mass fractionof matrix (wt%)

Mass fraction ofsecond phase (wt%)

Sn (wt%)/Se^ (wt%)

S0 – As ball milled 70.0 25.1 4.9S1 – 6.9 MPa 67.5 26.3 6.2S2 – 20.7 MPa 53.1 42.0 4.9S3 – 34.5 MPa 37.2 44.9 17.9S4 – Hot pressed^ 62.4 32.6 2.3/2.7

L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427 423

Wa ¼SaðZMVÞaPiðSiðZMVÞiÞ

where Wa is the weight fraction of phase a, S is the Rietveld scale factor, Z is thenumber of formula units in unit cell, M is the molecular mass of formula unit andV is the unit cell volume.

3. Results and discussion

Fig. 1a and b are the FESEM images of the melt spun flakes witha nominal alloy composition SnTe0.75Se0.25, along the top and crosssection respectively. It is seen that melt spinning induced nano tomicron-sized grains in the flakes, with sizes from about 385 nm toabout 1.2 lm. In addition, the features in the cross section are sub-micron size. On the contact surface (left hand side of the flake), theparticles’ sizes are about 200 nm to 350 nm thick; while on the freesurface (right hand side of the flake), there are columnar grainsabout 2 lm long. The XRD results in Fig. 1c indicates that the meltspun samples of SnTe and SnTe0.75Se0.25 is single phase with a cu-bic crystal structure matching that of SnTe (JCPDS No. 65-0322),although SnTe0.75Se0.25 has slight peak shifts due to changes in lat-tice parameters. Minor Sn peaks were also observed in the XRDdata, and their contribution was also included in the subsequentrefinement. Fig. 2 is the TEM micrograph of the melt spun SnTe0.75-

Se0.25 sample with the diffraction pattern in the inset, confirmingthe identity of the cubic phase.

The TE properties of the hot pressed samples of SnTe0.75Se0.25,SnTe and SnSe are shown in Fig. 3a–e. The electrical resistivity ofthe samples in Fig. 3a shows that although SnSe has very highresistivity values, the presence of SnTe in it, in the form of SnTe0.75-

Se0.25, results in resistivity values having the same order of magni-tude as SnTe. This demonstrates the effectiveness of resistivityreduction by the using multiphase concepts. Fig. 3b shows the See-beck coefficient values, which has a trend that is opposite to that ofelectrical resistivity – hence SnSe and SnTe have very high andrelatively low Seebeck coefficient values respectively. The SnTe0.75-

Se0.25 sample exhibits acceptable Seebeck coefficient values in therange of 55–111 lV/K, and its peak value of 111 lV/K at 548 K istwice that of the peak value of SnTe. Coupled with the loweredelectrical resistivity values, the maximum power factor (Fig. 3c)of SnTe0.75Se0.25 is 1.3 mW/m K2 at 494 K, which is 1.5 times thatof pure SnTe, and 56 times that of pure SnSe.

The thermal conductivity of the various samples is shown inFig. 3d. Pure SnTe, being a semi-metal, has a high thermal conduc-tivity of about 10 W/m K at room temperature. Its thermal conduc-tivity decreased with increasing temperature, reaching about7.6 W/m K at 550 K. Pure SnSe, on the other hand, has very lowthermal conductivity values of about 1.1–1.2 W/m K across themeasured temperature range. In SnTe0.75Se0.25, the addition of SnSeto SnTe decreased thermal conductivity by about 50% at room tem-perature. With the benefit of large thermal conductivity reductionand power factor enhancement, the ZT of SnTe0.75Se0.25 shows animprovement over the pure phases of SnTe and SnSe, shown inFig. 3e – the peak ZT is 0.19 at 548 K, 3 times that of SnTe and24 times that of SnSe.

To understand the enhancement in TE properties brought aboutfrom SnTe0.75Se0.25, further characterization was carried out. Whilethe main cubic phase remains stable, there is a metastable ortho-rhombic second phase which appears simply by manual grinding,and its mass fraction can be tuned by application of different pres-sure levels. The evolution of phases at different stages, (a) post ballmilling, (b) cold pressing at different pressures and (c) after hotpressing, are shown in the XRD patterns (Fig. 4). For ease of discus-sion, the samples are labeled S0–S4 (Table 1). With ball milling andcold compaction, an orthorhombic second phase appeared, whichis indexed to SnSe (JCPDS No. 72-1460), and the intensity of thesesecond phase peaks increased with applied pressure for the cold

pressed samples. The hot pressed sample also exhibited peaks forthe SnSe second phase, similar to sample S3 which was coldpressed at 34.5 MPa.

To better understand the evolution of phases in the cold pressedsamples, TOPAS refinement was carried out to determine qualita-tively the mass fraction of the main phase (referred to as matrix)and second phase. These results are summarized in Table 1.

The change from the ball milled powder to the sample coldpressed at 6.9 MPa was quite small, hence the values are quiteclose; for the other samples S1–S3, the mass fraction of the matrixdecreased with increase in applied pressure and the reverse trendwas observed for the second phase. This observation is attributedto the metastable state of the second phase in the ball milledpowder, and small pressure levels caused part of the matrix totransform into the second phase. With increasing pressure, thefraction of the second phase increased to become the majorityphase compared to the matrix. The possible reason for these obser-vations is that the application of pressure facilitates the secondphase precipitation, and has less effect on the matrix hence thereis a decrease in proportion of the matrix phase. It is also possiblethat the application of pressure may induce defects or disorder inthe matrix, such that it forms an amorphous phase, which doesnot recrystallize under pressure in the absence of heat. The differ-ences in the amount of Sn in the samples may be due to batch tobatch differences and are not known to be affected by the process-ing. For the hot pressed sample, the weight percentage of reflectsthe stabilized amount of phases, and a small amount of impurityphase of Se was also observed to be present.

Thermoelectric properties measurements were carried out onthe cold pressed samples (S1–S3) to elucidate the effect of thesecond phase on the TE properties, and the results are presentedin Fig. 5a–c. S1–S3 are of similar densities to minimize differencesin electrical resistivity due to density differences. In addition,since they are not consolidated via hot pressing, there is anannealing effect observed during the first run, while subsequent

Fig. 6. XRD patterns of SnTe0.75Se0.25 cold pressed at various pressures before andafter TE properties measurements.

424 L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427

measurements are stable. S4 also does not exhibit any metastablebehavior and has consistent TE properties at different runs.

Fig. 5a shows the electrical resistivity as a function of tempera-ture. Based on the trend, samples with lower mass fraction of sec-ond phase exhibit higher resistivity; the values decrease from S1 toS3. The electrical resistivity value of S1 is about 2.2 times higherthan that of S2 and 6.5 times higher values than that of S3. Thistrend of decreased resistivity with temperature is also affectedby the annealing effect of the samples. As S4 has a higher densitythan S3, its electrical resistivity value is an order of magnitudelower.

Fig. 5b shows the Seebeck coefficient which lies between 30 and110 lV/K, and the positive values indicate that it is a p-type semi-conductor. In general, an increase in mass fraction of second phaselowers the Seebeck coefficient, although the values increase withtemperature. S1 and S2 have relatively similar values, althoughthey have different mass fraction of phases. It is possible that thecontribution from the matrix and second phase may have can-celled out each other, especially for S2 where there are nearlyequal mass fractions of both phases. S3 has a lower Seebeck coef-ficient value initially, but it is able to reach values higher than S1and S2 from about 443 K onwards. This indicates that the secondphase is more ‘‘dominant’’ as temperature increases, resulting inhigher Seebeck coefficient values. A maximum value of 107 lV/Kis obtained for sample S3 at about 540 K. S4 has a peak value of

Fig. 5. Temperature dependence of thermoelectric properties of various samp

111 lV/K at 548 K which is about 3.7% higher than that of S3. Sincethe values do not vary much between S3 and S4, it is believed thatcold pressing can achieve equally high Seebeck coefficient values.

les: (a) Electrical resistivity, (b) Seebeck coefficient and (c) power factor.

Table 2TOPAS calculations on the mass fraction of phases after TE properties measurements.

Sample Mass fraction ofmain phase (wt%)

Mass fraction ofsecond phase (wt%)

Sn (wt%)

S1 – 6.9 MPa 93.5 6.1 0.4S2 – 20.7 MPa 89.6 9.3 1.1S3 – 34.5 MPa 84.4 13.0 2.6

L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427 425

The power factor (Fig. 5c) of the three samples exhibit similarpower factor values from room temperature to about 398 K. How-ever, as S3 has the lowest electrical resistivity values initially, andthe highest Seebeck coefficient values beyond 443 K, its power factorincreases quickly to reach a value of 0.328 mW/m K2. This value ofpower factor is about 1.9 times that of S2 and 3.1 times the valuefor S1. S4 has a peak power factor of 1.3 mW/m K2 at 494 K, whichis 4 times higher than the peak value for S3, due to its lower electricalresistivity values and slightly higher Seebeck coefficient value.

No thermal conductivity measurements were made for the coldpressed samples as its density is relatively low, and measurementswould not be accurate. Nevertheless, it is concluded that an increasein applied pressure can increase the mass fraction of second phasepresent and in turn improve the TE properties by reduction ofelectrical resistivity and improved Seebeck coefficient values,compared to the pure SnTe phase, especially at higher temperatures.

Fig. 7. TEM images of ball milled powder at (a) low magnification, (b) HRTEM of cubic mphase separated second phase nanoprecipitates and its diffraction rings in the inset, and

The metastability of the phases was also reflected in the XRDpatterns of the samples after annealing during the TE propertiesmeasurements. From Fig. 6 and Table 2, it can be seen that the sec-ond phase in the samples decreased largely in mass fraction. Thepossible reasons for such an observation may be that the secondphase precipitation is metastable after cold pressing, and annealingallows diffusion to produce the equilibrium values of the massfraction of the two phases.

TEM characterization of the ball milled powders was carried outto confirm the metastability of the cold pressed samples. Uponirradiation of the electron beam, nanoprecipitates of the size oftens of nanometers are observed to be deposited on the carbon filmin the region around the large particle.

Fig. 7a is the low magnification image of the ball milled powdersample, and Fig. 7b is the HRTEM of the cubic SnTe phase with thediffraction pattern in the inset, with lattice spacing of 2.23 Å and3.15 Å corresponding to the (20�2) and (0�20) planes, respec-tively. Fig. 7c is the high magnification image of nanoprecipitatesas well as its diffraction rings in the inset. Fig. 7d is the HRTEMof the second phase along with the main cubic matrix phase, withlattice spacing of 3.53 Å and 3.39 Å corresponding to (10�2) and(01�2) planes, respectively. The identity of these two phases werefurther confirmed through the diffraction patterns obtained by FastFourier Transformation (FFT), which is compared to the diffractionpattern from the database (refer to Supplementary information).

ain phase with its diffraction pattern in the inset, (c) high magnification images of(d) HRTEM of the second phase with the main cubic matrix phase.

Fig. 8. (a) Projection of SnTe at (101) plane (indicated by the blue plane) and intersection plane (11�1) of the grain boundary (indicated by the yellow plane), (b) projectionof SnSe at (100) plane (indicated by the blue plane) and intersection plane (0�12) of the grain boundary (indicated by the yellow plane), and atom arrangements of theinterface at the boundary for (c) SnTe: the cross plane view (rotated 90�) at the grain boundary of the (11�1) plane and (d) SnSe: the cross plane view (rotated 90�) at thegrain boundary of the (0�12) plane. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

426 L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427

The orientation relationship of the two phases in the ball milledpowders can be described as follows [18,19]: As the orthorhombicunit cell is a slightly distorted version of the cubic unit cell, there isa common plane between these two crystal structures – the (101)plane of cubic phase and (100) plane of the orthorhombic phase.Fig. 8 shows a schematic of the boundary where the two phasescan meet, with Sn as the common atom: Fig. 8a shows the (101)plane of SnTe (indicated by the blue plane) and the intersectionplane (11�1) (taken from the diffraction pattern in the database)of the grain boundary (indicated by the yellow plane), whileFig. 8b is the projection of SnSe at the (100) plane (indicated bythe blue plane) and intersection plane (0�12) of the grain bound-ary (indicated by the yellow plane). The atomic arrangements ofthe grain boundary interfaces after a 90� rotation for SnTe andSnSe is shown in Fig. 8c and d respectively. There is a hexagonalarrangement in both planes, and some degree of distortion is re-quired to match both the structures, hence with different extentof cold pressing, martensitic phase transformation can occur.Subsequently when TE properties were measured, the annealingresults in the diffusion of Te and Se atoms out of the martensite,giving rise to a decrease in mass fraction of second phase. Forthe hot pressed samples, concurrent application of heat and pres-sure stabilized the second phase, hence this behavior was notobserved.

4. Conclusion

The results presented here show the effects of second phases onTE properties in the SnSe–SnTe alloy system: (1) The enhancementof TE properties, in particular the power factor, from multiphaseSnTe0.75Se0.25 over the pure phases is due to the presence of an ortho-rhombic second phase. The second phase is metastable and simplecold pressing can be used to induce the formation of it. (2) Basedon the TE properties measurement on the cold pressed samples, itis noted that the mass fraction of the second phase in SnTe0.75Se0.25

contributes to a reduction in electrical resistivity and an increasein Seebeck coefficient values at higher temperatures. (3) TEM char-acterization shows that the orientation relationship of the twophases provides some insight on how the phase transformations oc-cur, and its reversibility after heat treatment. (4) Although both coldpressing and hot pressing can lead to similar Seebeck coefficient val-ues, hot pressing can achieve higher sample density, and stabilizethe second phase. This preservation of a stable second phase isimportant as it allows the enhanced TE properties over the purephases to be achieved and maintained. (5) Phase stability in TE mate-rials is also important as the material has to survive many runs dur-ing operation. These results give an insight on how second phasescan affect the TE properties in multiphase materials, and may beapplicable to other similar material systems.

L.P. Tan et al. / Journal of Alloys and Compounds 587 (2014) 420–427 427

Acknowledgements

This work is supported and funded by DRTech, Singapore underproject number 9010100257. The electron microscopy and XRDwork were performed at the Facility for Analysis, Characterization,Testing, and Simulation (FACTS) in Nanyang Technological Univer-sity, Singapore. The authors also gratefully acknowledge RepublicPolytechnic, Singapore, for the Laser Flash measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jallcom.2013.10.217.

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