Phase Segregation and Thermoelectric Properties of AgPbmSbTem+2 m=2, 4, 6, and 8

Joseph Sootsmana, Robert Pcioneka, Huijun Kongb, Ctirad Uherb, Mercouri G Kanatzidisa a Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA b Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA ABSTRACT The preparation and characterization of the AgPbmSbTem+2 family of compounds with m=2, 4, 6, and 8 is reported. Phase segregation was observed in all of these materials. The lattice thermal conductivity of these samples is low (<1.1 W/m-K). Powder x-ray diffraction, thermal analysis, and electron microscope investigations of these systems show that ideal solid solutions are not formed. The transport properties of these composite materials are presented and suggest that they could have promising thermoelectric properties when optimized. INTRODUCTION

Recent advancements, largely due to reduced dimensional structuring, have renewed interest in thermoelectric materials for use in thermal to electrical energy conversion [1]. These recent advancements prove that thermoelectrics could someday lead to great improvements in power generation from waste heat. The efficiency of a thermoelectric material is related to its dimensionless figure of merit ZT,

ellat

TSZT

κκσ+

=2

(1)

Where S is the absolute thermopower, σ is the electrical conductivity, T is the temperature, and κlat and κel are the lattice and electronic components of the thermal conductivity respectively [2]. By simultaneously keeping the power factor (S2

σ) high and the thermal conductivity low the efficiency can be increased, however this is difficult due to the interdependence of these properties. The manipulation of thermoelectric materials at the nanoscale may give us the ability to produce efficient thermoelectrics that were not possible previously [3, 4].

One particularly interesting class of nanostructured compounds are in the cubic family AgPbmSbTem+2 [5]. These promising thermoelectrics have high power factors and lower than expected thermal conductivity, possibly due to structuring on the nanometer length scale [6]. The compositional fluctuations, shown to be coherent with the matrix by high resolution transmission electron microscopy, allow electrical carriers to flow through the lattice unhindered while serving as scattering centers for phonons. This decoupling of the thermal and electrical conductivity can yield materials with both a high power factor and low thermal conductivity. The doping of PbTe by silver and antimony has been calculated to have an effect on the band structure which may contribute to the enhanced power factors observed as well [7].

Work in our group suggests that the AgPbmSbTem+2 family of compounds exhibit inhomogeneities on the nanoscale which contradicts the initial work on these systems which claimed these samples to be normal solid solutions [8]. In the original work four decades ago

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verification of “Vegard’s Law” was performed by powder x-ray diffraction. In fact, high ZT p-type behavior was reported [9] while recent work discussed above has proven these materials can also be n-type. The questions raised by these discrepancies and our findings with the high m-value members have prompted us to again investigate these materials over a wide range of compositions with a variety of heating profiles.

Here we present several members of the AgPbmSbTem+2 family with low values of m which were prepared using a heating profile which gives a macroscopically homogeneous material. However, powder x-ray diffraction and thermal analysis studies show that these compounds phase segregate on the micro- and nanoscale. Measurements show that lowering the m-value in these materials further reduces the thermal conductivity. High thermopower values have been observed with these systems. However, because the doping levels have not been optimized the electrical conductivity values are below 50 S/cm in all of the samples. High resolution transmission electron microscopy was used to determine the changes associated with modifying the m-value of the samples and the possible role of nano-structuring in these materials. EXPERIMENTAL

High purity silver, lead, antimony, and tellurium were added in the appropriate molar ratios to fused silica tubes. The tubes were sealed under reduced pressure and heated to 1000˚C over 12 hours. The tubes were kept at 1000˚C for 8 hours to allow the sample to homogenize. The samples were then cooled to 550˚C from the melt over 2 hours. This minimizes the amount of time the compound is kept between the liquidus and solidus, thus reducing the chance for segregation along the length of the ingot. The sample was then cooled from 550˚C to room temperature over 50 hours ensuring the sample will be at equilibrium. The resulting ingots were strong and had few cracks. The samples were then cut for transport, thermal, and microscopic characterization. RESULTS AND DISCUSSION Powder x-ray diffraction patterns for AgPbmSbTem+2 with m=2, 4, 6, 8 are shown in Figure 1. The patterns indicate that the material produced is biphasic. Indexing the reflections reveals that the two phases are not pure PbTe or AgSbTe2, but rather the phases are best described as AgPbm+xSbTem+x+2 and AgPbm-xSbTem-x+2. This notation indicates lead rich and lead poor compositions with respect to the nominal amounts due to the solubility limits of one phase into the other. It is possible that within each phase solid solution behavior exists. In the m=2, and 4 cases there are additional minor phase that could not be indexed. The reflections from this third phase are indicated in Figure 1 by arrows. Powder x-ray diffraction performed along the length of the ingot showed no change in the observed pattern. This reveals the samples are macroscopically uniform from the top to the bottom. Differential thermal analysis (DTA) indicates the samples melt incongruently during heating and multiple crystallizations are observed upon cooling. This is consistent with the multiphase character indicated by powder x-ray diffraction. The multiphase character was retained after the DTA experiments as revealed by powder x-ray diffraction patterns taken after the DTA experiment.

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Figure 1. Powder X-ray Diffraction of AgPbmSbTem+2with m=4, 6, 8 showing phase segregation into AgPbm+xSbTem+x+2 and AgPbm-xSbTem-x+2. Arrows indicate the 2 major phases and the 3rd phase which was not indexed in the m=4 sample. (Cu Kα rad.)

Experimentation with the cooling profile of the m=2 member of the series showed that the phase segregation may be controllable. It is particularly interesting to note that when the m=2 member was quenched from 550˚ C after cooling for 2 hours from the melt a single component sample was produced as judged by powder x-ray diffraction. Further annealing of this sample at 300˚ C resulted in a biphasic system. This clearly indicates that there is a thermodynamic driving force for these materials to segregate. Work with the m=2 system continues and should provide further insight on the mechanism of this transformation. Preliminary electrical conductivity measurements were performed in the temperature range 300-600K using a four-probe configuration. Typical sample dimensions were ~3x3x8 mm. Conductivity values as a function of temperature are shown in Figure 2. The conductivity values are below 50 S/cm and show little variation with temperature. This property may be improved with proper doping to give a higher power factor.

The Seebeck coefficient was measured using a MMR technologies system with a sample (~1x1x4mm) adjacent to the piece used for the electrical conductivity measurement. The collected data are shown in Figure 3. It is generally seen that with increasing the fraction of AgSbTe2 the absolute thermopower increases. In all cases the Seebeck coefficient is negative indicating n-type conduction. This contrasts previous reports on these systems which indicated p-type behavior [9]. In this work we have not encountered any p-type regions or samples. Several samples were examined in order to ensure that the ingots were in fact homogeneous with respect to transport properties. This avoids problems reported recently by Snyder and coworkers that these compounds are inhomogeneous on the macroscale with respect to thermoelectric properties [10].

Thermal conductivity measurements were performed with the flash diffusivity-heat capacity method. Samples in the form of coins ~12 mm in diameter and 1.2 mm thick were used for measurement. From this method thermal conductivity is calculated using the known heat capacity (Cp), density (ρ), and measured diffusivity (D) by ptotal CDρκ = . In these samples the

heat capacity was approximated for each value of m using a weighted average of the heat capacities of lead telluride and silver antimony telluride.

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Figures 2 (left) and 3 (right). (2) Electrical conductivity as a function of temperature for AgPbmSbTem+2 (LAST - m)with m=4, 6, 8. (3)Seebeck coefficient as a function of temperature for AgPbmSbTem+2 with m=4, 6, 8.

The lattice thermal conductivity was calculated using the experimental total thermal conductivity, the measured electrical conductivity, and the Wiedemann-Franz law to subtract the electronic contribution. In these samples the thermal conductivity is largely defined by the lattice component because the electrical conductivity is low and will account only for a small percentage of the total. As expected, increasing the m-value increases the lattice thermal conductivity because the atomic disorder as well as the compositional fluctuations responsible for phonon scattering are reduced. The lattice thermal conductivity value measured for the m=4 sample at room temperature was ~ 0.77 W/mK which agrees well with what was measured by Ono et. al. in 1962 [11] however, values of m>4 were not measured in that study. The m=8 lattice thermal conductivity is also comparable to those measured previously. The m=10 sample reported [12] has a room temperature lattice thermal conductivity of 1.1 W/mK. The behavior of the lattice thermal conductivity does not follow the 1/T dependence predicted by theory [13] and should be investigated further.

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Figure 4. Lattice thermal conductivity data for the series AgPbmSbTem+2 with m=4, 6, 8.

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ELECTRON MICROSCOPY Transmission electron microscopy can give spatial information about the size, distribution and composition of the phases observed by powder x-ray diffraction. Using energy dispersive spectroscopy in the scanning transmission electron microscope (STEM) mode gives the ability to spatially map the composition within the sample. One of these maps is shown in Figure 5. It is clear that regions rich in silver and antimony exist along with regions which are deficient in Pb. The regions observed, on the order of several hundred nanometers, are evenly dispersed through the sample. When these areas are examined under high magnification it appears that they are actually made up of alternating regions of differing composition. These fluctuations are on the order of several nanometers. A typical image of these compositional fluctuations is shown in Figure 6a. In Figure 6b an area within the Ag, Sb-rich region is shown along with the corresponding fast Fourier transform (FFT) of the two regions within the image. This clearly shows the presence of a phase which has approximately double the unit cell of the average NaCl-type lattice (a = 6.141Å)

Figure 5. (A) Electron map and (B) corresponding elemental maps for AgPb4SbTe6 showing the clear difference in the Ag-Sb rich region and Pb rich regions. A JEOL 2200FS was used for STEM mapping.

Figure 6. (A)Transmission electron microscope image showing additional structure within the silver-antimony rich regions and (B) a high resolution image of the boundary between the two phases. The corresponding FFT for the two regions clearly shows a 2-fold supercell which is the second phase.

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CONCLUSIONS AgPbmSbTem+2 materials with m=2, 4, 6, and 8 were prepared and characterized. These systems show phase segregation into components that are best described as AgPbm+xSbTem+x+2 and AgPbm-xSbTem-x+2. Preliminary transport properties in these systems have been measured and show promise, however the low electrical conductivity should be improved. The thermal transport properties of these multiphase compounds appear to be consistent with literature values for the “solid solution” systems. However, these systems do not follow the expected theoretical lattice thermal conductivity for normal “solid solutions”. Transmission electron microscopy is particularly helpful in providing insight into phase segregation in these materials especially on the nanoscale. These compounds, though not optimized, show interesting behavior and we believe they could be doped to give high efficiency thermoelectrics in the future. ACKNOWLEDGEMENTS Financial support from the Office of Naval Research (MURI program) is gratefully acknowledged. REFERENCES

1. Harman, T.C.; Walsh, M.P.; Laforge, B.E.; Turner, G.W. Elect. Mater. 2005. 34, L19. 2. D.M. Rowe, CRC Handbook of Thermoelectrics, (CRC Press, Florida, 1995) 3. Dresselhaus, M.S.; Dresselhaus, G.; Sun, X.; Zhang, Z.; Cronin, S.B.; Koga, T. Phys.

Solid State, 1999, 41 679. 4. Humphrey, T.E.; Linke, H. Phys. Rev. Lett. 2005, 94, 096601. 5. Hsu, K-F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J.S. ; Uher, C.; Hogan, T.; Polychroniadis,

E.K.; Kanatzidis, M.G. Science 2004, 303, 818-821. 6. Quarez, E.; Hsu, K-F.; Pcionek, R.; Frangis, N.; Ploychroniadis, E.K.; Kanatzidis, M.G.

J. Am. Chem. Soc. 2005, 127, 9177-9190. 7. Bilc, D.; Mahanti, S.D.; Quarez, E.; Hsu, K-F.; Pcionek R.; Kanatzidis, M.G. Phys. Rev.

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2005, 87, 171903. 11. Ono, T.; Takahama, T.; Irie, T. J. Phys. Soc. Japan. 1962, 17, 1070. 12. Hsu, K-F.; Loo, S.; Chen, W.; Uher, C.; Hogan, T.; Kanatzidis, M.G.; Mat. Res. Soc.

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