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and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
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aDepartment of Physics, Boston College, Chestnut Hill, MA 02467, USAbDepartment of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 23 February 2012; accepted 25 February 2012Available online 7 March 2012
AbstractThermoelectric figure-of-merit (ZT) of �1.6 at 700 1C is achieved in b-phase copper selenide(Cu2Se) made by ball milling and hot pressing. The b-phase of such material possesses a naturalsuperlattice-like structure that combines ordered selenium (Se) and disordered copper (Cu)layers in its unit cell, resulting in a low lattice thermal conductivity of 0.4–0.5 W m�1 K�1. Al-shaped specific heat peak indicates a phase transition from cubic b-phase to tetragonala-phase at around 140 1C upon cooling and vice versa. An abnormal decrease in specific heat withincreasing temperature was also observed due to the increased random motion of disordered Cuatoms. These features indicate that b-phase Cu2Se could be potentially an interesting thermo-electric material, competing with other conventional thermoelectric materials.& 2012 Elsevier Ltd. All rights reserved.
Introduction
A good thermoelectric material has high dimensionless figure-of-merit ZT: defined as (S2s/k)T, where the S, s, k, and T arethe Seebeck coefficient, electrical conductivity, thermalconductivity, and absolute temperature, respectively. Nor-mally, k is composed of three components, i.e., electroniccontribution (kcar), lattice contribution (klat), and bipolar
contribution (kbipolar). As reducing klat was proven to be theeasiest and most straightforward way to improve ZT, numerousefforts have been devoted in the last two decades in order toincrease the ZT value from the longstanding 1.0 in thermo-electric bulk materials to higher values by minimizing klatthrough the concept of nanocomposite [1–4]. The key idea ofnanocomposite is to create grains or inclusions that scatter thephonons without deteriorating the electron transport. Moregenerally, a good thermoelectric material should behave as a‘‘phonon-glass-electron-crystal’’ with a high charge carriermobility and a low lattice thermal conductivity [5]. Structureswith strong phonon scattering centers, such as skutterudites[6] and clathrates [7] were first demonstrated to have‘‘phonon-glass-electron-cystal’’ type of behavior resulting in
2211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.nanoen.2012.02.010
low klat. Other materials with complex unit cells [8], such asZn4Sb3 and Zintl compounds also have low klat.
Another direction in reducing the lattice thermal conductivityis to use layered structures, both artificial ones such assuperlattices [9,10], and naturally formed superlattices[11,12]. In the artificially grown superlattices, it was knownthat interface roughness reduces the thermal conductivity[13,14]. We noted that lamella crystals—natural superla-ttices—usually have higher lattice thermal conductivity for thein-plane direction than the out-of-plan direction. Increasing thedisturbance at the interfaces of layers and reducing thethickness of the crystalline layers therefore can potentiallylead to further reduction in thermal conductivity. A distortedlattice has recently been reported in the lamella-structuredIn4Se3�d crystals with only 2 atomic layers in the orderedlamella, which has even a lower in-plane lattice thermalconductivity than that of the out-of-plane direction, resulting ina ZT of 1.5 at 705 K [15]. Motivated by these facts, b-phaseCu2Se crystalized in layered cubic structure, as shown inFigure 1(a) and (b), could very likely have a low thermalconductivity and good thermoelectric properties since thesepolycrystals have monoatomic Se layers acting as orderedlamella, and Cu atoms randomly distribute at several latticesites as the disordered lamella (Figure 1(a) and (b)), if one viewsalong the (1 1 1) plane directions [16]. In this paper, we showthat a ZT of about 1.6 at 700 1C could be obtained in b-phaseCu2Se polycrystals with such structure features.
Material and methods
Sample fabrication
In our synthesis, we first prepared Cu2Se nanopowders from Cu(99.5%, Alfa Aesar, USA) and Se (99.99%, 5N PLUS, Canada)elements through high-energy ball milling (Spex 8000M Mixer/Mill). Bulk samples were fabricated by consolidating the as-prepared nanopowders in a graphite die (1/2 or 1 in. indiameter) via a conventional hot pressing method.
Structure characterizations
PANalytical multipurpose diffractometer with an X’celeratordetector (PANalytical X’Pert Pro) was used for latticestructure characterizations at both room temperature andhigh temperature. We studied the grain size of bulk sampleson a scanning electron microscope (SEM, JEOL-6340F). Thestructural change versus temperature was monitored byin situ heating experiments inside a high-resolution trans-mission electron microscope (HRTEM, JEOL-2010F). The bulksample was first hand polished and then fixed on a Mo gridwith epoxy (stable up to 1000 1C). Subsequently, thepolished sample was ion milled with a Precision Ion PolishingSystem (model 691, Gatan) till electron transparent andloaded on the heating holder (model 652, Gatan) for in situobservation.
Transport property measurements
We used a commercial four-probe system (ULVAC ZEM-3) tosimultaneously measure electrical resistivity and Seebeck
coefficient. A laser flash system (NETZSCH LFA 457) was usedfor the thermal diffusivity characterization. Specific heat(Cp) data was obtained on a differential scanning calorime-try (NETZSCH DSC 404C) station.
Results
Structural properties
Conventionally, b-phase Cu2Se has been known as a super-ionic conductor [17,18] that crystallizes in an Fm3m type oflattice with Se atoms occupying the (0, 0, 0) site while theknowledge of Cu sites still remains controversial due to itsvariation with temperatures. Figure 1(b) shows a possiblethree-site model for the location of Cu atoms in b-phaseCu2Se at 160 1C where Cu fractionally locates at (0.25, 0.25,0.25), (0.315, 0.315, 0.315), and (0.5, 0.5, 0.5) sites as wasmostly reported [16]. It should also be pointed out that inthose models all the Cu sites fall in the /1 1 1S direction. Ifone views along the (1 1 1) plane direction, a lamellastructure is clearly seen, in which the monoatomic Se layeris separated by two randomly distributed Cu layers. Duringcooling process, b-phase turns into a-phase at temperatureslower than 140 1C and this phase transition was reported tobe reversible [16]. Compared to b-phase that has an FCCstructure, the structure of a-phase Cu2Se is much morecomplicated since it could crystallize in three possible ways:monoclinic, tetragonal, and cubic. Figure 1(c) shows thatthe a-phase Cu2Se nanopowders we obtained from high-energy ball milling are tetragonal phase (PDF# 29-0575) atroom temperature. After being consolidated into bulk formvia hot pressing, those a-phase Cu2Se polycrystals show anincrease of {1 1 1} texturing (see the intensity of planes(1 1 1) and (2 2 2) relatively to that of (4 0 4) in Figure 1(c))with the pressing temperature from 400 to 700 1C. It is notedthat similar XRD patterns were obtained when we measurealong both vertical and parallel directions (to the hotpressing force direction) on those bulk samples. Hightemperature XRD measurements (Figure 1(d)) were alsodone at 200, 400, and 600 1C for the samples pressed at700 1C where we could clearly see the phase transformationbetween 25 and 200 1C and the high-temperature b-Cu2Sephase shows the texturing in {1 1 1} planes as well. We wantto point out that the cubic b-phase has smaller unit cell(e.g., 200 1C, a=b=c=5.8639 A) than the low-temperaturetetragonal a-phase (a=b=11.52 A and c=11.74 A at roomtemperature), which makes the (1 1 1) plane of b-phasecorresponds directly to the (2 2 2) plane of a-phase.Furthermore, we found through the XRD study that thelattice parameter of the cubic b-phase Cu2Se changed from5.8639 to 5.8918 to 5.9172 A when the measurementtemperature increased from 200 to 400 to 600 1C, respec-tively, indicating a possible change in Cu sites with a largethermal expansion of 22� 10�6 K�1.
Figure 2(a) is a room temperature HRTEM image of atypical as-prepared bulk sample, where the ordered tetra-gonal (1 1 1) lattice planes are clearly seen. As the sampletemperature reached 200 1C, we also observed the (1 1 1)lattice planes in b-phase (Figure 2(b)), however, they nowbelong to the FCC crystals instead and correspond to the(2 2 2) planes in tetragonal a-phase lattice as we discussed
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above. This observation suggests the phase change andpossible disordering of Cu sites at elevated temperature,which agrees well with our aforementioned XRD results.Figure 2(c) shows that the grains are typically within a fewmicrometers.
Thermoelectric transport properties
Figure 3 shows the temperature dependent thermoelectricproperties of Cu2Se1.01 samples that were hot pressed atdifferent temperatures. The extra Se was used to compen-sate the potential Se loss during hot pressing due to its
high vapor pressure. Electrical resistivity (Figure 3(a)) ofa typical Cu2Se1.01 sample is around 6–8 mO m at roomtemperature and increases quickly to about 55 mO m at700 1C. With Seebeck coefficient (Figure 3(b)) increasingfrom 75 to 250 mV K�1, these samples have moderate powerfactor (Figure 3(c)) of 750–950 mW m�1 K�1 at room tem-perature and peak around 1125–1250 mW m�1 K�1 at 600 1C.The temperature dependent Cp data is shown in Figure 3(d)where we could see a l-shape peak at around 140 1C as asymbol of the phase transition discussed above. Besides thisfeature, one may also notice that the Cp value is slightlydecreasing with the temperature at above 200 1C. Those b-phase Cu2Se samples also possess low total and lattice
Figure 1 Crystal structure of cubic b-phase Cu2Se. (a) View of (1 1 1) plane. (b) FCC unit cell with proposed possible Cu distributionat (0.25, 0.25, 0.25), (0.315, 0.315, 0.315), and (0.5, 0.5, 0.5) along /1 1 1S direction. (c) Room temperature XRD patterns of Cu2Senanopowders and bulk samples hot pressed at 400 1C (HP400 bulk), 500 1C (HP500 bulk), 600 1C (HP600 bulk), and 700 1C (HP700bulk). (d) Temperature dependent XRD patterns of hot pressed (700 1C) Cu2Se bulk sample measured at 25, 200, 400, and 600 1C.
Figure 2 Microstructure images of as-prepared (pressed at 700 1C) Cu2Se1.01 sample. (a) and (b) HRTEM images at roomtemperature, and 200 1C, respectively. (c) Typical SEM image taken from the same sample to show the grain size.
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thermal conductivities (Figure 3(e)) as we initially expectedfrom their natural superlattice-like structures, where thelattice thermal conductivity is around 0.4 W m�1 K�1 and atotal thermal conductivity of less than 1 W m�1 K�1 at 700 1C.Overall, ZTs of �1.6 at 700 1C were obtained (Figure 3(f)),which are competitive to other traditional medium tempera-ture thermoelectric materials, such as skutterudites [19]and PbTe [20–22], but using abundant and environment-friendly elements. Due to the aforementioned phasetransition between tetragonal a-phase and cubicb-phase, a clear change could be observed in all the curvesof Figure 3 and this sudden change was found to happenat around 140 1C which is slightly higher than the reportedvalue of 130 1C [16]. Furthermore, one may also concludefrom Figure 3 that the thermoelectric properties of as-prepared Cu2Se1.01 are not sensitive to hot pressing tempera-ture (400–700 1C).
We also studied the composition effect on the thermo-electric properties of Cu2Se1+x by changing the amount of Se inthe initial compositions (Figure 4). For all these samples, weused 700 1C for hot pressing to make sure that the samples aremechanically strong as the thermoelectric properties areinsensitive to the hot pressing temperatures as shown inFigure 3. At any given temperature, both electrical resistivityand Seebeck coefficient (Figure 4(a) and (b)) decrease withhigher selenium content indicating an increased hole concen-tration due to more Cu vacancies. Accordingly, the CuSe1.02sample has the highest power factor (Figure 4(c)) due to itslowest electrical resistivity. Figure 4(d) shows the data oftemperature dependent thermal conductivity k while thelattice thermal conductivity klatt (Figure 4(e)) was calculatedby subtracting the carrier contribution kcarr from the total k.The Cu2Se1.02 sample shows not only the highest kcarr due tohigh carrier concentration, but also the highest klatt in this
Figure 3 Temperature dependent thermoelectric properties of Cu2Se1.01 bulk samples prepared with different hot pressingtemperatures. (a) Electrical resistivity. (b) Seebeck coefficient. (c) Power factor. (d) Specific heat (Cp), and thermal diffusivity(HP700 bulk). (e) Total thermal conductivity (filled symbols) and lattice thermal conductivity (open symbols). (f) Figure-of-merit, ZT.
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series. As for the ZT (Figure 4(f)), the Cu2Se1.01 sample showsthe highest peak value of �1.6 at 700 1C, while the highestaverage ZT appears in Cu2Se benefiting from its lowest latticethermal conductivity. From practical application point of view,Cu2Se is the preferred composition because of the higheraverage ZT.
Discussion
The good thermoelectric performance of b-phase Cu2Se is adirect result of its unique crystal structure as it possesses lowlattice thermal conductivity and good power factor at thesame time. The disordered Cu atoms at multiple latticepositions in the high temperature b-phase would be a highly
efficient phonon scattering mechanism, which is similar tothe role of Zn in Zn4Sb3 [23]. On the other hand, themonoatomic Se ordered layer may also introduce disturbanceto the phonon propagation. Besides the structure disorder,the abnormal decreasing Cp value at above 200 1C is alsoworth thinking. Normally, the Cp should approach a constantat high temperatures according to Dulong–Petit law or slightincrease with temperature due to the thermal expansion ofthe materials [24]; however, what we observed in ourexperiments is different: a slightly decreasing Cp withtemperature (Figure 3(d)), where similar phenomena werealso reported in Ag2S [25] and AgCrSe2 [26]. Anharmonicphonons usually lead to increasing specific heat withincreasing temperature although theoretically they can alsoreduce the specific heat [27,28]. In the other extreme, many
Figure 4 Temperature dependent thermoelectric properties of Cu2Se1+x with varying selenium content. (a) Electrical resistivity.(b) Seebeck coefficient. (c) Power factor. (d) Thermal conductivity. (e) Lattice thermal conductivity. (f) Figure-of-merit, ZT.
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liquids have shown a reducing specific heat as a function ofincreasing temperature [29–31]. The random distribution ofCu in b-phase Cu2Se among several sites along /1 1 1Sdirection could also be considered as a partial melting of Cuatoms, similar to the reported ‘‘molten sublattice’’ in othersuperionic conductors [32]. Thus it is reasonable to attributethe decreasing specific heat to the increasing anharmonicityin the Cu–Se bonds due to increased random motion of the Cuatoms at high temperature. This bonding change is also thereason for the low total and lattice thermal conductivities(Figure 3(e)) in the b-phase Cu2Se sample.
From Archimedes’ method we found that the volumetricdensities of all the b-phase Cu2Se samples are similar at�6.8 g cm�3, close to the theoretical value of 6.9–7.0 g cm�3,and the SEM study also showed that their typical grain sizes areall in the range of 1–3 mm (Figure 2(c)), which are the reasonsfor their similar thermoelectric properties regardless the hotpressing temperature. Different from other techniques [3,4]utilizing nano-inclusions or nano-grains, the good thermo-electric properties of the b-phase Cu2Se sample mainly rely onits own intrinsic structure features, which was also reported inanother recent publication [33].
Conclusions
In conclusion, low lattice thermal conductivity of 0.4–0.5 Wm�1 K�1 from room temperature to 700 1C was obtained in b-phase Cu2Se polycrystals due to a unique combination ofmonoatomically ordered Se layer and disordered Cu layer intheir crystal structure. The increased random motion of Cuatoms results in slightly decreasing Cp values at above 200 1C.A phase transition from a tetragonal a-phase to the FCC b-phase was indicated at around 140 1C in the plots of theirthermoelectric transport properties, which was also confirmedby our XRD and HRTEM study at different temperatures.Finally, ZT values of�1.6 in Cu2Se and Cu2Se1.01 were achievedin our study, which competes well with other mediumtemperature thermoelectric materials.
Acknowledgments
This work is supported by Solid State Solar–Thermal EnergyConversion Center (S3TEC), an Energy Frontier ResearchCenter funded by the U.S. Department of Energy, Office ofScience, Office of Basic Energy Science under award numberDE-SC0001299 (G. C. and Z.F. R.).
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Bo Yu received his Bachelor of Sciencedegree in Materials Science and Engineeringfrom the University of Science and Technol-ogy of China in 2004. After serving as aResearch Scientist at Hefei National Labora-tory for Physical Sciences at Microscales,China, and then Graduate Teaching Assistantin the Chemistry Department, Boston Col-lege, he joined Prof. Zhifeng Ren’s group in2006 as a Ph.D. candidate in Physics. His
research focuses on the design and characterization of novelthermoelectric nanocomposites and the fabrication of solid-statesolar thermal power generation devices.
Thermoelectric properties of Cu2Se with ordered Se and disordered Cu layers 477
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Weishu Liu, Ph.D., is currently a postdoc-toral fellow in the Department of Physics atBoston College. He received his Ph.D.degree in Materials Science from the Uni-versity of Science and Technology Beijing,China, while his doctoral thesis was com-pleted as a member of Prof. Jingfeng Li’sGroup from 2003 to 2008 at TsinghuaUniversity. In 2009, he spent a year as apostdoctoral research associate at the Uni-
versity of Washington, Seattle, and then moved to Boston College.His current research is mainly focused on the design, fabrication,and tailoring of thermoelectric nanocomposites for solid-state solarpower generation and waste heat harvesting.
Dr. Shuo Chen is currently a researchassistant professor in the Department ofPhysics at Boston College. She earned her BSdegree in Physics at Peking University, Chinaand a Ph.D. degree from the Department ofPhysics at Boston College. Her researchinterests cover the material/device fabrica-tion for clean/green energy applicationpurposes and in situ Transmission ElectronMicroscopy. Currently she is also working on
high-temperature thermoelectric materials especially half-Heuslersystem.
Mr. Hui Wang is a Lab Manager in theDepartment of Physics at Boston College. Hereceived his bachelor degree in 2005 fromXiHua Normal University, Nanchong, China,and served as a researcher and lab managerfrom 2006 to 2007 in Prof. Baoqin Zeng’sgroup at the University of Electronic Scienceand Technology of China. In addition toinstrumentation management and mainte-nance, he is also specialized in the fabrica-
tion of carbon nanotubes and thermoelectric nanocomposites.
Dr. Hengzhi Wang is currently a researcher inProf. Zhifeng Ren’s group at Boston College.He received the Ph.D. degree in materialsscience in 2002 from Nanjing University ofScience and Technology and he is a specialiston microscopy and microanalysis (TEM/SEM/EDS/XRD) of nanoscaled materials.
Dr. Gang Chen is currently the Carl RichardSoderberg Professor of Power Engineering atMassachusetts Institute of Technology. Heobtained his Ph.D. degree from UC Berkeleyin 1993 working under then ChancellorChang-Lin Tien, master and bachelor de-grees from Huazhong University of Scienceand Technology, China. He was a facultymember at Duke University (1993–1997),University of California at Los Angeles
(1997–2001), before joining MIT in 2001. He is a member of theNational Academy of Engineering. He is a recipient of the NSF YoungInvestigator Award, the ASME Heat Transfer Memorial Award, andthe R&D 100 Award. He is a Guggenheim Fellow, an AAAS Fellow, andan ASME Fellow. He has published extensively in the area ofnanoscale energy transport and conversion and nanoscale heattransfer. He is the director of Solid-State Solar-Thermal EnergyConversion Center funded by the US DOE’s Energy Frontier ResearchCenters program.
Dr. Zhifeng Ren is currently a professor ofPhysics at Boston College. He obtained hisPh.D. degree from the Institute of Physics,Chinese Academy of Sciences in 1990,master degree from Huazhong University ofScience and Technology in 1987, and bache-lor degree from Sichuan Institute of Tech-nology in 1984. He was a postdoc andresearch faculty at SUNY Buffalo (1990–1999) before joining BC as an Associate
Professor in 1999. He specializes in thermoelectric materials, solarthermoelectric devices and systems, photovoltaic materials andsystems, carbon nanotubes and semiconducting nanostructures,nanocomposites, bio agent delivery and bio sensors, and super-conductors. He is a fellow of APS and AAAS, a recipient of R&D 100award. He has published extensively, and was ranked as the 49th ofthe top 100 Materials Scientists worldwide (by Thomas Reuter).
