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Facile precipitation of two phase alloys in SnTe 0.75Se 0.25with improved power

factor

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,Singapore b

Temasek Laboratories @NTU,Nanyang Technological University,Singapore 638075,Singapore

a r t i c l e i n f o Article history:

Received 4September 2013

Received in revised form 28October 2013Accepted 29October 2013

Available online 9November 2013Keywords:

Thermoelectric materials Composite materials Rapid solidi?cation

a b s t r a c t

Multiphase thermoelectric (TE)materials have received great interest in recent years due to the synergis-tic improvements in TE properties.We report for the ?rst time on melt spun samples with nominal com-position of SnTe 0.75Se 0.25.Interestingly,an orthorhombic second phase was observed to exhibit change in microstructure under low applied pressure –the metastable nature leads to an observed increase in mass fraction 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,while increasing Seebeck coef?cient values slightly,especially at higher temperatures.A peak power factor of 1.3mW/m K 2is obtained at 494K,which is a 1.5times enhancement over the peak power factor of pure SnTe.The peak ZT achieved for the alloy is 0.19at 548K,which is three times that of pure SnTe.

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 and global warming.Thermoelectric (TE)materials are of particular interest as they can convert heat into electricity in the presence of a temperature gradient and vice versa [1–4].The ef?ciency of TE materials is determined by the dimensionless ?gure of merit,given by ZT =S 2r T /k ,where S ,r ,k and T are Seebeck coef?cient,electrical conductivity,thermal conductivity and absolute temper-ature respectively.The term S 2r 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 performance an 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 in power factor to be achieved concurrently,leading to improved ZT values [5,6].

The approach of using rapid solidi?cation and annealing is com-monly used to obtain multiphase TE materials [7–10].The method allows in situ precipitation of second phase(s)–in the size of a few nanometers to tens of nanometers –in a bulk material.This leads to improved TE properties due to effects such as decreased thermal conductivity,ability to tune electrical properties and increased power factor.Melt spinning is a popular processing method since it leads to metastable,amorphous materials or multiphase materi-als with phases in the nano/micron scale [11,12].The increased interfacial area of the precipitates,and/or reduction in crystal size of the materials is bene?cial to improve the phonon scattering effect in TE materials.

In this work,the SnTe–SnSe material system,in particular melt spun samples with a nominal alloy composition of SnTe 0.75Se 0.25were studied and TE properties measurements were performed and 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 understand the effect of the second phase on the TE properties.

The SnTe–SnSe material system is chosen as it has potential applications in optical recording,infrared devices and thermoelec-trics,where the constituent phases and microstructures can affect the 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-rich solid solution)and a two phase region.The nominal composition of SnTe 0.75Se 0.25is selected as it lies near the phase boundary of the single and two phase regions.The fabrication method is via melt spinning,which allows for rapid solidi?cation.In addition,microstructural optimization through subsequent processing by heat treatment can be carried out [15].He et al.[16]reported that in the composite material PbTe–PbSnS 2,which has a cubic and orthorhombic crystal structure respectively,a superstructure

?Corresponding author at:School of Materials Science and Engineering,Nanyang Technological University,Singapore 639798,Singapore.Tel.:+6567904140;fax:+6567909081.

E-mail address:ashhhng@https://www.wendangku.net/doc/926109973.html,.sg (H.H.Hng).

TEM of as melt spun SnTe0.75Se0.25at low magni?cation and the

diffraction pattern corresponding to a cubic phase in the inset.

Fig.1.(a)Top surface and(b)cross section of SnTe0.75Se0.25and(c)XRD patterns of melt spun SnTe and SnTe0.75Se0.25.

Temperature dependence of thermoelectric properties of SnTe,SnSe and SnTe0.75Se0.25:(a)Electrical resistivity,(b)Seebeck coef?cient,(c)power factor, conductivity and(e)ZT.

W a?

S aeZMVTa P

eS ieZMVT

where W a is the weight fraction of phase a,S is the Rietveld

number of formula units in unit cell,M is the molecular mass of

V is the unit cell volume.

3.Results and discussion

Fig.1a and b are the FESEM images of the melt

a nominal alloy composition SnTe0.75Se0.25,along the

section respectively.It is seen that melt spinning

micron-sized grains in the?akes,with sizes from

about1.2l m.In addition,the features in the cross

micron size.On the contact surface(left hand side of

particles’sizes are about200nm to350nm thick;

surface(right hand side of the?ake),there are

about2l m long.The XRD results in Fig.1c indicates

spun samples of SnTe and SnTe0.75Se0.25is single

bic crystal structure matching that of SnTe(JCPDS No.65-0322), although SnTe0.75Se0.25has slight peak shifts due to changes in lat-tice parameters.Minor Sn peaks were also observed in the XRD data,and their contribution was also included in the subsequent re?nement.Fig.2is the TEM micrograph of the melt spun SnTe0.75-Se0.25sample with the diffraction pattern in the inset,con?rming the 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 of the samples in Fig.3a shows that although SnSe has very high resistivity 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 resistivity

reduction by the using multiphase concepts.Fig.3b shows the See-beck coef?cient values,which has a trend that is opposite to that of electrical resistivity–hence SnSe and SnTe have very high and relatively low Seebeck coef?cient values respectively.The SnTe0.75-Se0.25sample exhibits acceptable Seebeck coef?cient values in the range of55–111l V/K,and its peak value of111l V/K at548K is twice that of the peak value of SnTe.Coupled with the lowered electrical resistivity values,the maximum power factor(Fig.3c) of SnTe0.75Se0.25is1.3mW/m K2at494K,which is1.5times that of pure SnTe,and56times that of pure SnSe.

The thermal conductivity of the various samples is shown in Fig.3d.Pure SnTe,being a semi-metal,has a high thermal conduc-tivity of about10W/m K at room temperature.Its thermal conduc-tivity decreased with increasing temperature,reaching about 7.6W/m K at550K.Pure SnSe,on the other hand,has very low thermal conductivity values of about1.1–1.2W/m K across the measured temperature range.In SnTe0.75Se0.25,the addition of SnSe to SnTe decreased thermal conductivity by about50%at room tem-perature.With the bene?t of large thermal conductivity reduction and power factor enhancement,the ZT of SnTe0.75Se0.25shows an improvement over the pure phases of SnTe and SnSe,shown in Fig.3e–the peak ZT is0.19at548K,3times that of SnTe and 24times that of SnSe.

To understand the enhancement in TE properties brought about from SnTe0.75Se0.25,further characterization was carried out.While the 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 ball milling,(b)cold pressing at different pressures and(c)after hot pressing,are shown in the XRD patterns(Fig.4).For ease of discus-sion,the samples are labeled S0–S4(Table1).With ball milling and cold compaction,an orthorhombic second phase appeared,which is indexed to SnSe(JCPDS No.72-1460),and the intensity of these second phase peaks increased with applied pressure for the cold pressed samples.The hot pressed sample also exhibited peaks for the SnSe second phase,similar to sample S3which was cold pressed at34.5MPa.

To better understand the evolution of phases in the cold pressed samples,TOPAS re?nement 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 Table1.

The change from the ball milled powder to the sample cold pressed at6.9MPa was quite small,hence the values are quite close;for the other samples S1–S3,the mass fraction of the matrix decreased with increase in applied pressure and the reverse trend was observed for the second phase.This observation is attributed to the metastable state of the second phase in the ball milled powder,and small pressure levels caused part of the matrix to transform into the second phase.With increasing pressure,the fraction of the second phase increased to become the majority phase compared to the matrix.The possible reason for these obser-vations is that the application of pressure facilitates the second phase precipitation,and has less effect on the matrix hence there is a decrease in proportion of the matrix phase.It is also possible that the application of pressure may induce defects or disorder in the matrix,such that it forms an amorphous phase,which does not recrystallize under pressure in the absence of heat.The differ-ences in the amount of Sn in the samples may be due to batch to batch differences and are not known to be affected by the process-ing.For the hot pressed sample,the weight percentage of re?ects the stabilized amount of phases,and a small amount of impurity phase of Se was also observed to be present.

Thermoelectric properties measurements were carried out on the cold pressed samples(S1–S3)to elucidate the effect of the second phase on the TE properties,and the results are presented in Fig.5a–c.S1–S3are of similar densities to minimize differences in electrical resistivity due to density differences.In addition, since they are not consolidated via hot pressing,there is an annealing effect observed during the?rst run,while subsequent XRD patterns of ball milled,cold pressed and hot pressed SnTe0.75

Table1

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

Sample Mass fraction

of matrix(wt%)

Mass fraction of

second phase(wt%)

Sn(wt%)/

Se^(wt%) S0–As ball milled70.025.1 4.9

S1–6.9MPa67.526.3 6.2

S2–20.7MPa53.142.0 4.9

S3–34.5MPa37.244.917.9

S4–Hot pressed^62.432.6 2.3/2.7

L.P.Tan et al./

XRD patterns of SnTe0.75Se0.25cold pressed at various pressures before

TE properties measurements.

Fig.5.Temperature dependence of thermoelectric properties of various samples:(a)Electrical resistivity,(b)Seebeck coef?cient and(c)power factor.

The power factor(Fig.5c)of the three samples exhibit similar

The metastability of the phases was also re?ected in the XRD patterns of the samples after annealing during the TE properties measurements.From Fig.6and Table2,it can be seen that the sec-ond phase in the samples decreased largely in mass fraction.The possible reasons for such an observation may be that the second phase precipitation is metastable after cold pressing,and annealing allows diffusion to produce the equilibrium values of the mass fraction of the two phases.

TEM characterization of the ball milled powders was carried out to con?rm the metastability of the cold pressed samples.Upon

Table2

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

Sample Mass fraction of

main phase(wt%)Mass fraction of

second phase(wt%)

Sn(wt%)

S1–6.9MPa93.5 6.10.4

S2–20.7MPa89.69.3 1.1

S3–34.5MPa84.413.0 2.6

milled powder at(a)low magni?cation,(b)HRTEM of cubic main phase with its diffraction pattern in the inset,(c)high

phase nanoprecipitates and its diffraction rings in the inset,and(d)HRTEM of the second phase with the main cubic

L.P.Tan et al./Journal of Alloys and Compounds587(2014)420–427425

The orientation relationship of the two phases in the ball milled powders can be described as follows [18,19]:As the orthorhombic unit cell is a slightly distorted version of the cubic unit cell,there is a common plane between these two crystal structures –the (101)plane of cubic phase and (100)plane of the orthorhombic phase.Fig.8shows a schematic of the boundary where the two phases can meet,with Sn as the common atom:Fig.8a shows the (101)plane of SnTe (indicated by the blue plane)and the intersection plane (11à1)(taken from the diffraction pattern in the database)of the grain boundary (indicated by the yellow plane),while Fig.8b is the projection of SnSe at the (100)plane (indicated by the blue plane)and intersection plane (0à12)of the grain bound-ary (indicated by the yellow plane).The atomic arrangements of the grain boundary interfaces after a 90°rotation for SnTe and SnSe is shown in Fig.8c and d respectively.There is a hexagonal arrangement in both planes,and some degree of distortion is re-quired to match both the structures,hence with different extent of cold pressing,martensitic phase transformation can occur.Subsequently when TE properties were measured,the annealing results in the diffusion of Te and Se atoms out of the martensite,giving rise to a decrease in mass fraction of second phase.For the hot pressed samples,concurrent application of heat and pres-sure stabilized the second phase,hence this behavior was not observed.

4.Conclusion

The results presented here show the effects of second phases on TE properties in the SnSe–SnTe alloy system:(1)The enhancement of TE properties,in particular the power factor,from multiphase SnTe 0.75Se 0.25over the pure phases is due to the presence of an ortho-rhombic second phase.The second phase is metastable and simple cold pressing can be used to induce the formation of it.(2)Based on the TE properties measurement on the cold pressed samples,it is noted that the mass fraction of the second phase in SnTe 0.75Se 0.25contributes to a reduction in electrical resistivity and an increase in Seebeck coef?cient values at higher temperatures.(3)TEM char-acterization shows that the orientation relationship of the two phases provides some insight on how the phase transformations oc-cur,and its reversibility after heat treatment.(4)Although both cold pressing and hot pressing can lead to similar Seebeck coef?cient val-ues,hot pressing can achieve higher sample density,and stabilize the second phase.This preservation of a stable second phase is important as it allows the enhanced TE properties over the pure phases 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 phases can affect the TE properties in multiphase materials,and may be applicable to other similar material

systems.

(101)plane (indicated by the blue plane)and intersection plane (11à1)of the grain boundary (indicated by the blue plane)and intersection plane (0à12)of the grain boundary (indicated by the yellow plane),SnTe:the cross plane view (rotated 90°)at the grain boundary of the (11à1)plane and (d)SnSe:the cross plane.(For interpretation of the references to color in this ?gure legend,the reader is referred to the web

Acknowledgements

This work is supported and funded by DRTech,Singapore under project number9010100257.The electron microscopy and XRD work were performed at the Facility for Analysis,Characterization, Testing,and Simulation(FACTS)in Nanyang Technological Univer-sity,Singapore.The authors also gratefully acknowledge Republic Polytechnic,Singapore,for the Laser Flash measurements. Appendix A.Supplementary material

Supplementary data associated with this article can be found,in the online version,at https://www.wendangku.net/doc/926109973.html,/10.1016/j.jallcom.2013.10. 217.

References

[1]G.J.Snyder,E.S.Toberer,Nat.Mater.7(2008)105–114.

[2]C.J.Vineis,A.Shakouri,A.Majumdar,M.G.Kanatzidis,Adv.Mater.22(2010)

3970–3980.

[3]P.F.P.Poudeu,J.D’Angelo,H.J.Kong,A.Downey,J.L.Short,R.Pcionek,T.P.

Hogan,C.Uher,M.G.Kanatzidis,J.Am.Chem.Soc.128(2006)14347–14355.

[4]L.P.Tan,T.Sun,S.F.Fan,L.Y.Ng,A.Suwardi,Q.Y.Yan,H.H.Hng,Nano Energy2

(2013)4–11.

[5]A.J.Minnich,M.S.Dresselhaus,Z.F.Ren,G.Chen,Energy Environ.Sci.2(2009)

466–479.

[6]W.Liu,X.Yan,G.Chen,Z.Ren,Nano Energy1(2012)42–56.

[7]Y.Z.Pei,J.Lensch-Falk,E.S.Toberer,D.L.Medlin,G.J.Snyder,Adv.Funct.Mater.

21(2011)241–249.

[8]S.N.Girard,J.Q.He,C.P.Li,S.Moses,G.Y.Wang,C.Uher,V.P.Dravid,M.G.

Kanatzidis,Nano Lett.10(2010)2825–2831.

[9]J.R.Sootsman,H.Kong,C.Uher,J.J.D’Angelo,C.I.Wu,T.P.Hogan,T.Caillat,M.G.

Kanatzidis,Angew.Chem.Int.Ed.47(2008)8618–8622.

[10]Y.Z.Pei,https://www.wendangku.net/doc/926109973.html,Londe,N.A.Heinz,G.J.Snyder,Adv.Energy Mater.2(2012)

670–675.

[11]D.G.Ebling,A.Jacquot,M.Jagle,H.Bottner,U.Kuhn,L.Kirste,Phys.Status

Solidi Rap.Res.Lett.1(2007)238–240.

[12]Z.B.Sun,C.Y.Zhang,Y.M.Zhu,Z.M.Yang,B.J.Ding,X.P.Song,https://www.wendangku.net/doc/926109973.html,p.

361(2003)165–168.

[13]C.Y.Chen,H.J.Wu,S.W.Chen,https://www.wendangku.net/doc/926109973.html,p.547(2013)100–106.

[14]A.A.Volykhov,V.I.Shtanov,L.V.Yashina,Inorg.Mater.44(2008)345–

356.

[15]Y.H.Jiang,F.Liu,S.J.Song,B.Sun,J.Non-Cryst.Solids358(2012)1417–

1424.

[16]J.Q.He,S.N.Girard,J.C.Zheng,L.D.Zhao,M.G.Kanatzidis,V.P.Dravid,Adv.

Mater.24(2012)4440–4444.

[17]M.Ahmadi,S.S.Pramana,L.F.Xi,C.Boothroyd,https://www.wendangku.net/doc/926109973.html,m,S.Mhaisalkar,J.Phys.

Chem.C116(2012)8202–8209.

[18]O.Madelung,U.R?ssler,M.Schulz(Eds.),Tin telluride(SnTe)crystal structure,

lattice parameters,Non-Tetrahedrally Bonded Elements and Binary Compounds I,Springer,Berlin Heidelberg,1998,pp.1–8.

[19]W.J.Baumgardner,J.J.Choi,Y.F.Lim,T.Hanrath,J.Am.Chem.Soc.132(2010)

9519–9521.

L.P.Tan et al./Journal of Alloys and Compounds587(2014)420–427427