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High Thermoelectric Performance and Enhanced Mechanical Stability of p-type Ge1-xSbxTe Suresh Perumal, Subhajit Roychowdhury, Devendra S. Negi, Ranjan Datta, and Kanishka Biswas Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 7, 2015

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Chemistry of Materials

High Thermoelectric Performance and Enhanced Mechanical Stability of p-type Ge1-xSbxTe Suresh Perumal,† Subhajit Roychowdhury,† Devendra S. Negi,±,ǂ Ranjan Datta,±,ǂ and Kanishka Biswas†, * †

New Chemistry Unit, ±Chemistry and Physics of Materials Unit and ǂInternational Centre

for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064 (India) *E-mail: [email protected]

ABSTRACT: High thermoelectric figure of merit, zT, of ~1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge1-xSbxTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300-773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge0.9Sb0.1Te sample shows mechanical stability (Vickers microhardness) of ~206 Hv, which is significantly higher compared to other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se and TAGS.

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INTRODUCTION Thermoelectric materials can convert waste heat to useful electrical energy, thus will play an important role in future energy utilizations and managements.1-4 Thermoelectric conversion efficiency is directly proportional to materials thermoelectric figure of merit, zT = S2σT/κtotal, where, S is Seebeck coefficient, σ is electrical conductivity, κtotal is total thermal conductivity and T is the temperature. Significant efforts were attempted for increasing the numerator (i.e. power factor, S2σ) of the zT equation, through carrier energy filtering,5 generating resonant levels in electronic bands at near to the Fermi level,6,

7

convergence of electronic band

valleys,8-11 and quantum confinement.12 Denominator (i.e. κtotal) of the zT can be decreased through scattering of heat carrying phonons by introducing point defects by alloying, embedding nano-inclusions and second phase precipitates into the matrix,13-15 nano/mesoscale grain boundaries,13-16 and intrinsic bond anharmonicity.17-20 Lead telluride (PbTe) based materials are so far considered to be leading thermoelectric materials for intermediate 600-900 K temperature range power generation.1,3,511,13-15

However, environmental concern about Pb limits its usage in the large scale

thermoelectric applications. Although GeTe based alloys known for their promising thermoelectric properties since 1960s,21 GeTe has not attracted as much attention as others in the IV-VI semiconductor family. Pristine GeTe is a p-type degenerate semiconductor with large carrier density (~1021 cm-3) due to its intrinsic Ge-vacancy,22-24 which is resulting high electrical conductivity of 8500 S/cm, low Seebeck coefficient value of ~34 µV/K, and perversely high total thermal conductivity of ~8 W/mK at room temperature, thus leading to a maximum zT value of 0.8 at 720 K.25 GeTe undergoes the ferroelectric structural phase transition from high temperature cubic (β-phase) to low temperature rhombohedral (α- phase) at ~700 K due to the thermal strain induced shift in Ge atoms (Figure 1(a)), which thrusts the distortion in the unit cell along [111] direction with an angular distortion of α = 1.65˚.26-28 2 ACS Paragon Plus Environment

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Pseudo-binary solid solution of GeTe-AgSbTe2, generally known as TAGS-80 & TAGS-85 depending on composition, illustrated the promising thermoelectric performance. 29-33

Recently, GexPb1-xTe is considered to be high performance thermoelectric materials due

to the donor dopant nature of Pb which increases Seebeck coefficient drastically by reducing carrier density and the presence of low thermal conductivity generated from the thermodynamically driven various nanoscale modulations.34-37 Recently, zT of ~1.9 at 773 K was obtained in 3mol% Bi2Te3 doped Ge0.87Pb0.13Te samples due to the reduction of thermal conductivity by point defect scattering of phonons and enhancement of power factor through valence band convergence.38 Although Pb-free thermoelectric is desirable for mass-market applications, high performance GexPb1-xTe composition contains 13 mol% of Pb. Mention must be made that homologous (GeTe)nSb2T3 (GST) based layered compounds (without any second phase nanostructuring) are known to show zT of 1.3-1.4 at around 723 K.39-42 Here we report an extraordinary thermoelectric performance in p-type Ge1-xSbxTe high quality crystalline ingots with a thermoelectric figure of merit, zT, of ~1.85 at 725 K. We achieve significant enhancement in zT in Ge1-xSbxTe compared to GeTe due to the following collective reasons: (i) aliovalent substitution of Sb3+ at Ge2+ site in GeTe decreases the p-type carrier concentration due to donor dopant nature of Sb3+, which resulted in enhanced Seebeck coefficient; (ii) Sb doping in GeTe significantly increases the cubic nature at room temperature and decreases the rhombohedral to cubic phase transition temperature, thus enhance the valence band degeneracy, which in turn increases Seebeck coefficient, and (iii) significant reduction in the lattice thermal conductivity was obtained due to phonon scattering of various nano/meso-scale structures such as domain variants, twin and inversion boundaries, defect layers, nanoprecipitates and solid solution point defects. A maximum thermoelectric conversion efficiency (ηmax) of ~12% was calculated by considering virtual thermoelectric module consisting of present p-type Ge0.9Sb0.1Te and previously reported n-



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type 8% of Sn doped PbTe-PbS,43 by maintaining the temperature difference of ΔT = 400 K. We show significant high temperature stability with cyclable thermoelectric performance of p-type Ge1-xSbxTe. Mechanical stability is also an important issue for thermoelectric material when it used in devices for power generation. We have measured Vickers micro-hardness (Hv) of ~206 for the present p-type Ge0.9Sb0.1Te, which is higher than that of other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se and TAGS.

EXPERIMENTAL SECTION Reagents. Germanium (Aldrich 99.999%), Antimony (Alfa Aesar 99.9999%) and Tellurium (Alfa Aesar 99.999+ %) were used for synthesis without further purification. Synthesis. High quality crystalline ingots (∼6 g) of Ge1−xSbxTe (x=0−0.10) were synthesized by mixing appropriate ratios of high purity starting materials of Ge, Sb, and Te in a quartz tube. Typically, to synthesize the Ge0.90Sb0.10Te sample, Ge (1.919 g, 26.418 mmol), Sb (0.3536 g, 2.904 mmol), and Te (3.736 g, 29.279 mmol) were added in a quartz tube of 10 mm diameter. The tube was sealed under vacuum (10−6 Torr) and slowly heated to 1223 K over 10 h then soaked for 6 h, and cooled slowly to room temperature over 10 h. Powder X-ray diffraction. Powder X-ray diffraction for finely ground samples were recorded using a Cu Kα (λ = 1.5406 Å) radiation on a Bruker D8 diffractometer. Temperature dependent XRD patterns were recorded using the attached high temperature chamber. Band gap measurement. To estimate optical band gap of the as synthesized samples, diffuse reflectance measurement has been done with finely ground powder at room temperature using FT-IR Bruker IFS 66V/S spectrometer in the wave number range 4000-400 cm-1 with 2 cm-1 resolution and 50 scans. Absorption (α/Λ) data were calculated from reflectance data using Kubelka-Munk equation: α/Λ=(1−R)2/(2R), where R is the reflectance, α and Λ are the absorption and scattering coefficient, respectively. The energy band gaps were derived from α/Λ vs Eg (eV) plot.


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Electrical transport. Electrical conductivity and Seebeck coefficients were measured simultaneously under He atmosphere from room temperature to 773 K on a ULVAC-RIKO ZEM-3 instrument system. The typical sample for measurement had a parallelepiped shape with the dimensions of ∼2×2×8 mm3. The longer direction coincides with the direction in which the thermal conductivity was measured. Hall measurement. Hall measurement was carried out at room temperature in the home made setup, where fixed magnetic field and dc-current were used to be 0.25 T and 5 mA, respectively. Thermal conductivity. Thermal diffusivity, D, was measured in the range 300−773 K by using the laser flash diffusivity method in a Netzsch LFA-457. Coins with ∼8 mm diameter and ∼2mm thickness were used in all of the measurements. Temperature dependent heat capacity, Cp, was derived using standard sample (pyroceram) in LFA-457, which is in good agreement with Dulong-Petit Cp value. The total thermal conductivity, κtotal, was calculated using the formula κtotal = DCpρ, where ρ is the density. Density measurement. The density, ρ, of all the pellets was measured by Archimedes method and the densities of the measured samples were around ~99% of theoretical density. TEM. TEM imaging was performed using an aberration corrected FEI TITAN cubed 80–300 kV transmission electron microscopes, operating at 300 kV. Mechanical properties. Microhardness of all the samples was measured in the home-made microhardness machine using the diamond indenter on the Vickers hardness scale, where the force was used to be 1 N and the indent was kept for 10s. Vickers hardness (kgf/mm2) values were determined by the equation of Hv = 1. 854xL/(2d)2, where L is the indentation load and 2d is the diagonal length of the indentation. Here, L is 0.1kg and the 2d values of GeTe and Ge0.90Sb0.10Te are 36 µm and 30 µm, respectively (see Figure S9, SI). The error in microhardenss measurement is about 5%.



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Methods for calculation of theoretical device figure of merit and thermoelectric efficiency. (a) Device figure of merit, (ZT)m for single pair (p and n- type legs). (ZT)m value was theoretically calculated from the following equation where previously reported Pb1-xSnxTe-PbS was used as n-type leg43 and present Ge0.90Sb0.10Te was kept to p-type leg, and hot and cold side temperatures are kept to be 700 K and 300 K, respectively. , Where (Sp and Sn), (ρp and ρn) and (κp and κn) represent the Seebeck coefficient, electrical resistivity and total thermal conductivity of p-and n-type materials, respectively. (b) Thermoelectric efficiency. The conversion efficiency of the aforesaid pair of the materials was theoretically calculated from the equation,

Where ΔT/Th refers the Carnot efficiency, Th and Tc are the hot and cold side temperature and (ZT)avg is an average figure-of-merit of a pair of thermoelectric materials.

RESULTS AND DISCUSSION High quality crystalline ingots of pristine GeTe and Ge1-xSbxTe (x = 0.02, 0.05 and 0.1) were synthesized by the melting reaction of elements of Ge, Sb and Te in a vacuum sealed tube at 1223 K and followed by slow cooling to room temperature. Room temperature powder X-ray diffraction (PXRD) patterns of Ge1-xSbxTe (x = 0-0.10) samples could be indexed based on rhombohedral GeTe structure (space group, R3m) with no other impurity phase being observed within the detection limits of PXRD (Figure 1b). Additionally, presence of doublepeaks [(024) and (220)] in between 2θ = 41-45˚ indicates the rhombohedral structure at room 6 ACS Paragon Plus Environment

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temperature, but these two peaks approach closer with the increasing of Sb doping concentration in GeTe (see Figure S1, Supporting information (SI)), which suggests that cubic nature of Ge1-xSbxTe increases with increasing Sb doping concentration. High temperature (323-773K) PXRD patterns of Ge0.90Sb0.10Te indicates the rhombohedral to cubic transition temperature to be ~573 K (Figure 1(c)), which is significantly lower than that for pristine GeTe (~700 K, Figure S2, SI). Sb doping in GeTe enhances the cubic nature of the sample, which may have significant impact on the Seebeck coefficient (discussed later). Figure 1(d) represents the optical absorption spectra of Ge1-xSbxTe samples. With increasing the concentration of Sb doping from 0 to 10 mol% in GeTe, the band gap decreases from ~0.2 eV to ~0.08 eV. Electronegitivity of Te (2.10, Pauling electronegative scale) is more than that of Ge (2.01). In GeTe, valence and conduction bands are mostly constituted by Te and Ge orbitals, respectively. Substituting Sb (electronegetivity, 2.05) in place of Ge lowers the conduction band energy, which gives rise to decrease in the band gap Ge1-xSbxTe. Previous first principle electronic structure calculations also indicated decreases in the band gap of GeTe after Sb doping due to lowering of conduction band energy. 44 Aberration corrected transmission electron microscope (TEM) was used to examine the nano/meso-scale architectures of Ge0.90Sb0.10Te sample (see Figure 2). TEM investigation of Ge0.90Sb0.10Te exemplifies the following features: (i) formation of ordered domain variants, surrounded by (001), (0-10) and (11-1) planes, usually referred as herringbone structures with successive bright and dark contrasts; (ii) inversion boundaries and twin boundaries along direction; (iii) nanoscale ordered layer structures of cation vacancies and (iv) the formation of van der Waals gaps due to relaxation of Te atom in the vicinity of the defect layer, which are observed throughout the sample.41,45-48 Figure 2(a) shows bright field TEM image representing the domain variants with bright and dark contrasts in the direction. The sucessive contrast variation occurs due



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to the breakdown of Friedel symmetry of non-centrosymmetric crystals.38 Figure 2(b) represents the high resolution TEM (HRTEM) micrograph of the matrix with the measured interplanar distance of d = 0.34 nm which corresponds to (021) planes of rhombohedral GeTe. Fast Fourier transforms (FFTs) and dark field pattern of electron diffraction analysis confirm the rhombohedral structure. Selected area electron diffraction pattern (SAED) along zone axis obtained from same area indicates the single crystalline nature of the material and rhombohedral structure at room temperature. Figure 2(c) indicates the formation of "reflection twins" along direction with different domain variants in terms of contrast difference. Figure 2(d) shows the zoomed version of twin boundaries where domains were clearly distinguished. In addition, the corresponding SAED pattern (Figure 2 (e)) shows spot splitting, which confirms the twins. Twin free regions (I and II in Figure 2(d), twin planes along and , respectively) showed no spots splitting, whereas overall region (region III) shows clear indication of spot splitting in the electron diffraction patterns, thus confirming the twins (see Figure S3(a,b), SI). Twins are formed due to the cubic to rhombohedral distortion while cooling from melting. Additionally, presence of inversion domain boundaries were clearly seen in Figure 2(f), which is formed because of the existence of two domain region, where trigonal axis are parallel but of opposite polarity.45 Nanoscale planar defect layers were also observed in Ge0.90Sb0.10Te sample (Figure 2(g) and (h)), which are formed due to Ge vacancies. 41,

46-48

These defect layer regions show a diffused SAED pattern, whereas defect free regions depict relatively perspicuous SAED pattern (Figure 2(h)). HRTEM imaging shows the presence of strained endotaxial nanoprecipitates in a few regions of the Ge0.90Sb0.10Te sample (Figure 2(g) and Figure S3(c), SI). Moreover, domain colonies and herringbone-like structures were also seen in Ge0.90Sb0.10Te sample (See Figure S3(d,e), SI).45


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Figure 3 represents the thermoelectric properties of Ge1-xSbxTe (x = 0- 0.10) samples in the measured 300-800 K range. σ values of the Ge1-xSbxTe samples decrease with increasing the temperature (Figure 3(a)), indicating degenerate semiconductor behavior. Substitution of Sb in GeTe drastically decreases the σ from ~8067 S/cm for GeTe to ~1529 S/cm for Ge0.90Sb0.10Te at 300 K. This confirms that the Sb alloying indeed suppresses the Ge vacancies and decrease the carrier concentrations. Typically, the room temperature σ of pristine GeTe is to be ~8067 S/cm, which decreases to ~2152 S/cm at 710 K. Ge0.90Sb0.10Te sample exhibits σ value of ~1529 S/cm at room temperature, which decreases to ~ 778 S/cm at 775 K. Temperature dependent σ curves show a signature of structural phase transition in between 570-670 K Ge1-xSbxTe samples. The Hall coefficients, RH, at room temperature for pristine and Sb doped GeTe samples are positive which indicate p-type conduction in this system. Assuming parabolic bands and a single band conduction process at 300 K, we estimated the carrier concentration, n, from the formula: n=1/eRH, where e is the electronic charge.

GeTe, Ge0.98Sb0.02Te,

Ge0.95Sb0.05Te and Ge0.90Sb0.10Te samples exhibit n values to be ~ 8.72 x 1020 cm-3, ~ 5.8 x 1020 cm-3, ~ 3.68 x 1020 cm-3 and ~2.38 x1020 cm-3, respectively, at room temperature (See Table 1). Decrease in the hole concentrations indicates aliovalent donor doping of Sb3+ in Ge2+ sub-lattice of GeTe. Temperature dependence of Seebeck coefficients (S) of Ge1-xSbxTe (x = 0- 0.10) are presented in Figure 3(b). Positive sign of the S values indicates p-type conduction, which also supports the Hall coefficient data. The S value of pristine GeTe is to be ~32 µV/K at 300 K, which reaches to ~150 μV/K at 710 K. The S value gradually increases with the addition of Sb in GeTe in the 300-773 K temperature range (Figure 3(b)). Typically, Ge0.90Sb0.10Te exhibits a S value of ~107 μV/K at 300 K, which reaches a maximum of ~256 μV/K at 725 K. We have compared our experimental S vs. n data with the previously reported Pisarenko



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plots38 both at 323 and 623 K and also with the S of previously reported 3mol% Bi2Te3 doped Ge0.87Pb0.13Te,38 Ge1-xPbxTe37 and (GeTe)1-x[(PbTe)(SnTe)(Bi2Te3)]x 34 in Figure 3(c). At 323 K, the experimental S value of Ge0.90Sb0.10Te falls slightly above to Pisarenko line, however, at 623 K, experimental S values lie significantly above the Pisarenko line. This increase in S could be attributed to following collective reasons: (i) reduction of p-type carrier concentration due to the donor dopant nature of Sb in GeTe and (ii) Sb substitution in GeTe enhances the cubic nature of the sample (see PXRD section), which resulted in an increase in valence band valley degeneracy. Valence band valley degeneracy is more in high symmetry cubic phase (Fm-3m) compared to that of the low symmetry rhombohedral (R3m) phase of GeTe (Figure 3(d)). With Sb-doping in GeTe, one T pocket and three L pockets merge to four L pockets; and 6 η pockets and 6 Σ pockets merge into 12 Σ pockets (Figure 3(d)),49 thus valence band degeneracy increases, which resulted in enhanced Seebeck coefficient. We have estimated effective mass (m*) of the carriers in Ge1xSbxTe

using measured S and Hall carrier concentration (n) at room temperature following

the equations (1-3):19, 50, 51

Where, η is the reduced Fermi energy, Fn(η) the nth order Fermi integral, kB the Boltzmann constant, e the electron charge, h the Planck constant and r the scattering factor. By fitting the respective Seebeck data, the reduced Fermi energy was extracted. Although accurate 10 ACS Paragon Plus Environment

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calculation of m* require consideration of non-parabolic band and multiple band model, present calculation of m* considers only single parabolic band with acoustic phonon scattering (r = -1/2) for simplicity. 19, 50, 51 The m* value of Ge0.90Sb0.10Te is to be 2.07m0, which is significantly higher than that of pristine GeTe (1.43m0), which also suggests the effect of valence band degeneracy. Increase in the cubic nature in Ge1-xSbxTe increases the possibility of occupancy of the p-type carriers to the more numbers of degenerate band valleys, which results in increases in the Seebeck coefficient. Temperature dependence of power factors (S2σ) of Ge1-xSbxTe (x = 0- 0.10) are presented in the Figure 3(e). Typically, GeTe show the S2σ value of ~8 μWcm-1K-2 at 300 K, which rises to ~49.4 μWcm-1K-2 at 710 K, whereas Ge0.90Sb0.10Te sample exhibits the S2σ value of ~17.2 μWcm-1K-2 at 300 K, then reaches the maximum value of ~51.4 μWcm-1K-2 at 725 K. Further increasing the Sb concentration to 15 mol%, temperature dependent σ decreases significantly, thus S2σ value decreases with increasing the Sb concentration beyond 10 mol%. The total thermal conductivities, κtotal, of the samples were estimated in the temperature range of 300-773 K using the formula, κtotal = DCpρ, where, D is the thermal diffusivity (Figure S4, SI), Cp is specific heat (Figure S5, SI) and ρ is density (~99 % of the theoretical density) of the sample. Temperature dependent total thermal conductivity (κtotal) of Ge1-xSbxTe (x = 0- 0.10) are illustrated in Figure 4(a). Substitution of Sb in GeTe substantially reduces κtotal value. At 300K, GeTe has the κtotal value of ~8.24 Wm−1K whereas Ge0.90Sb0.10Te shows the κtotal value of ~2.25 Wm−1K

−1

−1

,

, which is about ~77 %

reduction with respect to the κtotal of pristine GeTe. Observed significant reduction in κtotal can be attributed to the reduction in both electronic and lattice thermal conductivity. Mention must be made that slight upturn of κtotal at ~673 K was observed for pristine GeTe, which is attributed to the rhombohedral to cubic structural phase transition. This was earlier observed



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for other GeTe based materials.37, 52 Up-turn temperature decreases to lower value from 673 K to 573 K with increasing the Sb concentration from 0 to 10 mol% (Figure 4(a)). Sb doping in GeTe decreases the rhombohedral to cubic phase transition temperature, which was confirmed by temperature dependent PXRD (see the Figure 1(c)). The electronic thermal conductivities, κe = L∙σ∙T, were presented in Figure S6, SI. Temperature dependent Lorenz number, L, can be obtained based on the fitting of the respective Seebeck values that estimate the reduced chemical potential asuming single parabolic band model as per equation (4) (Figure S7, SI).19, 50, 51 (4) The κe value of all the Ge1-xSbxTe samples are substantially lower than that of pristine GeTe, which is due to the significantly lower electrical conductivity of Sb doped samples compared to that of GeTe. The temperature dependent lattice thermal conductivity (κlat) was estimated by subtracting the κe from κtotal and plotted in Figure 4(b). Sb alloying in GeTe significantly reduces κlat values both at room and high temperatures because of mass-fluctuation phonon scattering where vibrations of atoms with different frequencies damp the phonons transport. In addition, formation of domain structures, twin and inversion boundaries and grain boundaries play a predominant role on reduction in κlat by scattering mid to long wavelength phonons at room temperature. Typically, Ge0.90Sb0.10Te sample exhibits the κlat value of 1.4 Wm-1K-1 at room temperature, which decreases to ~1.1 Wm-1K-1 at 710 K, which is comparable to that of previously reported 3% Bi2Te3 doped Ge0.87Pb0.13Te.38 We have achieved reduction of κlat of ~45 % after Sb (10 mol%) doping in GeTe, which is lower compared to that in reported 3% Bi2Te3 doped Ge0.87Pb0.13Te (~28 % reduction in κlat)38 and Ge-rich Ge1-xPbxTe (~31 % reduction in κlat)35 at room temperatures.


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Temperature dependence of the dimensionless thermoelectric figure of merit, zT, of Ge1-xSbxTe (x = 0-0.10) samples is illustrated in Figure 5(a). The maximum zT achieved is 1.85 at 725 K for Ge0.90Sb0.10Te, which is ~110 % higher compared to pristine GeTe. Three cycles of heating-cooling (300-773 K) zT vs. T data clearly show high temperature stability and reproducibility of the thermoelectric performance of the Ge0.9Sb0.1Te (Figure 5(b)). Additionally, zT vs T of the Ge0.9Sb0.1Te sample which was annealed at 773 K (above phase transition temperature) for 72 h also confirms long term high temperature stability (Figure 5(b)). Ge0.95Sb0.05Te sample shows the highest zT of ~1.25 at 725 K. Ge0.88Sb0.12Te and Ge0.85Sb0.15Te samples show the highest zT values of ~1.3 and 1.2, respectively at 700 K (Figure S8, SI). Collective theoretical device thermoelectric figure of merit (ZT)m for a pair of legs of p-type (present Ge0.90Sb0.10Te) and n-type (previously reported 8% of Sn doped PbTe-PbS)43 was estimated as a function of temperature (Figure 5(c)) using equation (1), in which the temperature difference was maintained to be 400 K throughout the estimated temperature range. Theoretically calculated average devices’ thermoelectric figure of merit, (ZT)avg, was estimated to be ~0.85 considering ΔT = 400 K. The estimated (ZT)avg was used for theoretical calculation of the thermoelectric conversation efficiency (Figure 5(d)) in equation (2). A maximum efficiency of ~12% was achieved for the temperature difference of 400 K. Besides the high zT and promising theoretical thermoelectric efficiency, one has to look for the good mechanical properties of thermoelectric materials for mass-market applications. We have measured Vicker’s micro-hardness (kgf/mm2)

to estimate the

mechanical strength and hardness of Ge1-xSbxTe (x = 0- 0.10) samples and experimental values are presented in Figure 6. Present micro-hardness (Hv) values were compared with previously reported popular thermoelectric materials, such as Bi2Te3,53 57 xSnxTe,

PbTe,54-56 Pb1-

PbSe,58 Cu2S,59 Cu2Se,59 TAGS-8031 and TAGS-85.31 High performance



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Ge0.90Sb0.10Te exhibits a Hv value of 206, which is higher compared to that of Bi2Te3,53 PbTe,54-56 Pb1-xSnxTe,57 PbSe,58 Cu2S,59 Cu2Se,59 TAGS-80,31 TAGS-8531 and pristine GeTe.

CONCLUSIONS In conclusion, we have synthesized high quality samples of Ge1-xSbxTe (x = 0- 0.10) by simple vacuum sealed tube melting reactions. Substitution of Sb in GeTe significantly decreases the p-type carrier concentration due to the donor dopant nature of Sb, which resulted in enhanced Seebeck coefficients. Sb doping GeTe also increases the cubic nature of the sample and enhances the valence band degeneracy effect, which boosts the Seebeck throughout the measured temperature range. At the same time, significant reduction of thermal conductivity was achieved due to collective phonon scattering of various mesostructured domain variants and twin and inversion boundaries; nano-structured defect layers and precipitates; and solid solution point defects. Highest zT value of ~1.85 at 725 K for ptype Ge0.90Sb0.10Te was achieved, which is ~110 % higher compared to that of pristine GeTe. Additionally, the thermoelectric performance of Ge0.9Sb0.1Te is stable under several heatingcooling cycles. A maximum thermoelectric conversion efficiency (ηmax) of ~12% was estimated by considering virtual thermoelectric module consisting of present p-type Ge0.9Sb0.1Te and previously reported n-type 8% of Sn doped PbTe-PbS,43 by maintaining the temperature difference of ΔT = 400 K. We have achieved Vickers micro-hardness of ~206 Hv for high performance Ge0.9Sb0.1Te, which is significantly higher compared to other high performance thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se and TAGS. Present results on synthesis, fundamental understanding of the structure-property relationship, extraordinary performance, significant temperature and mechanical stability of Sb doped GeTe reflect the promise to do further research in GeTe based materials and replace PbTe for mass-market application. 14 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting information (SI) Zoom PXRD (Figure S1); High temperature PXRD of GeTe (Figure S2); additional TEM micrographs (Figure S3); thermal diffusivity (Figure S4); Cp (Figure S5); electronic thermal conductivity (Figure S6); Lorenz number (Figure S7); thermoelectric figure of merit of Ge1xSbxTe

(x= 0, 0.1, 0.12 and 0.15) (Figure S8) and Vickers microhardness (Figure S9). This

material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was partially supported by DRDO-JNCASR collaborative project and DAE-BRNS project ((37(3)20/01/2015/BRNS). KB acknowledges partial support of DST Ramanujan Fellowship and Sheikh Saqr Laboratory. We acknowlege Ms. Anbukkarasi for help during microhardness measurement.



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25. Levin, E. M.; Besser, M. F.; Hanus, R. Electronic and thermal transport in GeTe: A versatile base for thermoelectric materials. J. Appl. Phys. 2013, 114, 083713. 26. Chattopadhyay, T.; Boucherle, J. X.; von Schnering, H. G. eutron diffraction study on the structural phase transition in GeTe. J. Phys. C 1987, 20, 143. 27. Polking, M. J.; Han, M. -G.; Yourdkhani, A.; Petkov, V.; Kisielowski, C. F.; Volkov, V. V.; Zhu, Y.; Caruntu, G.; Alivisatos, A. P.; Ramesh, R. Ferroelectric order in individual nanometre-scale crystals. Nat. Mater. 2012, 11, 700-709. 28. Pereira, P. B.; Sergueev, I.; Gorsse, S.; Dadda, J.; Muller, E.; Hermann, R. P. Lattice dynamics and structure of GeTe, SnTe and PbTe. Phys. Status Solidi B 2013, 250,13001307. 29. Rosi, F. D.; Dismukes, J. P.; Hockings, E. F. Semiconductor Materials for Thermoelectric Power Generation up to700 C. Electrical Eng. 1960, 79, 450-459. 30. Levin, E. M.; Cook, B. A.; Harringa, J. L.; Bud’ko, S. L.; Venkatasubramanian , R.; Schmidt-Rohr, K. Analysis of Ce‐and Yb‐Doped TAGS‐85 Materials with Enhanced Thermoelectric Figure of Merit. Adv. Funct. Mater. 2011, 21, 441-447. 31. Davidow, J.; Gelbstein, Y. A Comparison Between the Mechanical and Thermoelectric Properties of Three Highly Efficient p-Type GeTe-Rich Compositions: TAGS-80, TAGS85, and 3% Bi2Te3-Doped Ge0.87Pb0.13Te. J. Eelectron. Mater. 2013, 42, 1542-1549. 32. Salvador, J. R.; Yang, J.; Shi, X.; Wang, H.; Wereszczak, A. A. Transport and mechanical property evaluation of (AgSbTe)1−x (GeTe)x (x= 0.80, 0.82, 0.85, 0.87, 0.90). J. Solid State Chem. 2009, 182, 2088-2095. 33. Zhang, L.; Wang, W.; Ren, B.; Guo, J. The Effect of Adding Nano-Bi 2 Te 3 on Properties of GeTe-Based Thermoelectric Material. J. Electron. Mater. 2013, 42, 1303-1306. 34. Gelbstein, Y.; Ben-Yehuda, O.; Pinhas, E.; Edrei, T.; Sadia, Y.; Dashevsky, Z.; Dariel, M. P. Thermoelectric properties of (Pb, Sn, Ge) te-based alloys. J. Electron. Mater. 2009, 38, 1478-1482. 35. Gelbstein, Y.; Dado, B.; Ben-Yehuda, O.; Sadia, Y.; Dashevsky, Z.; Dariel, M. P. Highly Efficient Ge-Rich GexPb1−xTe Thermoelectric Alloys. J. Electron. Mater. 2010, 39, 20492052. 36. Gelbstein, Y.; Davidow, J.; Girard, S. N.; Chung, D. Y.; Kanatzidis, M. Controlling metallurgical phase separation reactions of the Ge0.87Pb0.13Te alloy for high thermoelectric performance. Adv. Energy Mater. 2013, 3, 815-820.


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37. Gelbstein, Y.; Davidow, J. Highly efficient functional GexPb1−xTe based thermoelectric alloys. Phys. Chem. Chem. Phys. 2014, 16, 20120-20126. 38. Wu, D.; Zhao, L-D.; Hao, S.; Jiang, Q.; Zheng, F.; Doak, J. W.; Wu, H.; Chi, H.; Gelbstein, Y.; Uher, C.; Wolverton, C.; Kanatzidis, M. G.; He, J. Origin of the high performance in GeTe-based thermoelectric materials upon Bi2Te3 doping. J. Am. Chem. Soc. 2014, 136, 11412-11419. 39. Schneider, M. N.; Rosenthal, T.; Stiewe, C.; Oeckler, O. From phase-change materials to thermoelectrics. Z. Kristallogr. 2010, 225, 463-470. 40. Sankar, R.; Wong, D. P.; Chi, C.-S.; Chien, W.-L.; Hwang, J.-S.; Chou, F.-C.; Chen, L.-C.; Chen, K.-H. Enhanced thermoelectric performance of GeTe-rich germanium antimony tellurides through the control of composition and structure. CrystEngComm, 2015,17, 3440-3445. 41. Rosenthal,T.; Schneider, M. N.; Stiewe, C., Doblinger, M.; Oeckler, O. Real structure and thermoelectric properties of GeTe-rich germanium antimony tellurides. Chem. Mater. 2011, 23, 4349-4356. 42. Schneider, M. N.; Seibald, M.; Oeckler, O. A new series of long-range ordered metastable phases in the system M–Sb–Te (M=Ge, Ag). Dalton Trans. 2009, 2004–2011. 43. Androulakis, J.; Lin, C-H.; Kong, H-J.; Uher, C.; Wu, C-I.; Hogan, T.; Cook, B. A.; Caillat, T.; Paraskevopoulos, K. M.; Kanatzidis, M. G. Spinodal decomposition and nucleation and growth as a means to bulk nanostructured thermoelectrics: Enhanced performance in Pb1-xSnxTe-PbS. J. Am. Chem. Soc. 2007, 129, 9780-9788. 44. Hoang, K.; Mahanti, S. D.; Kanatzidis, M. G. Impurity clustering and impurity-induced bands in PbTe-, SnTe-, and GeTe-based bulk thermoelectrics. Phys. Rev. B 2010, 81, 115106. 45. Lee, H. S.; Kim, B-S.; Cho, C-W.; Oh, M-W.; Min, B-K.; Park, S-D, Lee, H-W. Herringbone structure in GeTe-based thermoelectric materials. Acta Materialia 2015, 91, 83-90. 46. Cook, B. A.; Kramer, M. J.; Wei, X.; Harringa, J. L.; Levin, E. M. Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material. J. Appl. Phys. 2007, 101, 053715. 47. Cook, B. A.; Wei, X. Z.; Harringa, J. L.; Kramer, M. J. In-situ elevated-temperature TEM study of (AgSbTe2)15(GeTe)85. J. Mater. Sci. 2007, 42 7643-7646. 48. Kooi, B. J.; Groot, W. M. G.; De Hosson, J. T. M. In situ transmission electron microscopy study of the crystallization of Ge2Sb2Te5. J. Appl. Phys. 2004, 95, 924-932. 19 ACS Paragon Plus Environment


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49. Chen, Y.; Jaworski, C. M.; Gao, Y. B.; Wang, H.; Zhu, T. J.; Snyder, G. J.; Heremans, J. P.; Zhao, X. B. Transport properties and valence band feature of high-performance (GeTe)85(AgSbTe2)15 thermoelectric materials. New J. Phys. 2014, 16, 013057. 50. May, A. F.; Toberer, E. S.; Saramat, A.; Snyder, G. J. Characterization and analysis of thermoelectric transport in n-type Ba8Ga16−xGe30+x . Phys. Rev. B 2009, 80, 125205. 51. Zhao, L.-D.; Lo, S.-H.; He, J.; Li, H.; Biswas, K.; Androulakis, J.; Wu, C.-I.; Hogan, T. P.; Chung, D. Y.; Dravid V. P.; Kanatzidis, M. G. High Performance Thermoelectrics from Earth-Abundant Materials: Enhanced Figure of Merit in PbS by Second Phase Nanostructures. J. Am. Chem. Soc. 2011, 133, 20476-20487. 52. Lee, J. K.; Oh, M. W.; Kim, B. S.; Min, B. K.; Lee, H. W.; Park, S. D. Influence of Mn on crystal structure and thermoelectric properties of GeTe compounds. Electron. Mater. Lett. 2014, 10, 813-817. 53. Zhao, L.-D.; Zhang, B.-P.; Li, J.-F.; Zhou, M.; Liu,W.-S.; Liu, J. Thermoelectric and mechanical properties of nano-SiC-dispersed Bi2Te3 fabricated by mechanical alloying and spark plasma sintering. J. Alloys Compd. 2008, 455, 259-264. 54. Ablova, M.S.; Vinogradova, M. N.; Karklina, M. I. Sov. Phys.–Sol. State. 1969, 10, 19291932. 55. Crocker, A. J.; Wilson, M. Microhardness in PbTe and related alloys. J. Mater. Sci. 1978, 13, 833-842. 56. Gelbstein, Y.; Gotesman, G.; Lishzinker, Y.; Dashevsky, Z.; Dariel, M. P. Mechanical properties of PbTe-based thermoelectric semiconductors. Scripta Materialia 2008, 58, 251-254. 57. Cui, J. L.; Qian, X.; Zhao, X. B. Mechanical and transport properties of pseudo-binary alloys (PbTe)–(SnTe) by pressureless sintering. J. Alloy. Compd. 2003, 358, 228-234. 58. Darrow, W. B. W. M. S.; Roy, R. Micro-indentation hardness variation as a function of composition for polycrystalline solutions in the systems PbS/PbTe, PbSe/PbTe, and PbS/PbSe. J. Mater. Sci. 1969, 313-319. 59. Zhao, L.; Wang, X.; Fei, F. Y.; Wang, J.; Cheng, Z.; Dou, S.; Wanga, J.; Snyder, G. J. H igh thermoelectric and mechanical performance in highly dense Cu2−xS bulks prepared by a melt-solidification technique. J. Mater. Chem. A 2015, 3, 9432-9437.


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(a)

(b)

Ge0.90Sb0.10Te

Intensity (a.u.)

Ge0.95Sb0.05Te

> 673K

(024) (220)

(202)

20 25 30 35 40 45 50 55 2 ()

Intensity (a.u.)

(c) Ge0.90Sb0.10Te

)

773 723 673 573 Rhombohedral 473 373 323

(d)

GeTe Ge0.98Sb0.02Te Ge0.95Sb0.05Te Ge0.90Sb0.10Te

mp

era

tur e(

Cubic

41

42

43 2 ()

44

GeTe

/ (a.u.)

at 300K

Ge0.98Sb0.02Te

Te

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45

0.1

0.2

0.3 0.4 Eg (eV)

0.5

Figure 1. (a) Rhombohedral to cubic structural phase transition in GeTe. (b) PXRD pattern of Sb doped GeTe at 300 K. (c) High temperature PXRD pattern of Ge0.9Sb0.1Te showing structural transition at 573K. (d) Optical absorption spectra of Sb doped GeTe.



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(b)

(a)

(021)

0.34 nm

5 nm (c)

-

(d)

I III 200

(g =200)

II 020

--

(f)

(e) 200 020

(g)

Area I

(h)

I

defect layer

II

Overall

Area II

10 nm

Figure 2. (a) Low magnification TEM micrograph of Ge0.90Sb0.10Te showing domain variants called herringbone structure. (b) HRTEM image of Ge0.90Sb0.10Te. SAED pattern along the zone axis as an inset in (b). (c) TEM micrograph shows the twin boundaries along direction. (d) Zoomed version of TEM micrograph in (c) showing the twin boundaries. Inset in (d) shows the schematic twin structure formation. (e) SAED pattern along zone axis of confirms the twin boundaries with clear spots splitting. (f) TEM image shows the presence of inversion boundaries between two domains with different polarity. (g) HRTEM of defect layers. Dotted circle in (g) denotes the nanoprecipitates. (h) Zoomed area of defect layers with SAED pattern in different regions, where defect-free regions (I and II) show clear SAED pattern and overall region shows the diffused SAED pattern,


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8000

GeTe Ge0.98Sb0.02Te

7000

Ge0.95Sb0.05Te

(b)

GeTe Ge0.98Sb0.02Te Ge0.95Sb0.05Te

Ge0.90Sb0.10Te

6000

250 200

S (V/K)

(S/cm)

(a) 9000

5000 4000

Ge0.90Sb0.10Te

150 100

3000 50

2000 1000

0 300 400 500 600 700 800 T (K)

(c)

500

(e)

Ge(Sn,Pb)Te+3 mol% Bi2Te3 (Ref. 34) GexPb1-xBi2Te1+3 (Ref. 37) GexPb1-xBi2Te1+3 (Ref. 37)

400

S  (Wcm K )

Ge0.87Pb0.13Te +3 mol% Bi2Te3 (Ref.38)

-2

GeTe

Ge0.90Sb0.10Te (This work)

300

623K

200 100

(d)

40

Ge0.98Sb0.02Te Ge0.95Sb0.05Te Ge0.90Sb0.10Te

30 20

0

0.1 1 10 20 -3 Carrier density (x10 cm )

CB

GeTe

10

Pisarenko plot (Ref.38) @ 323K @ 623K

CB

300 400 500 600 700 800 T (K)

2

323K

0

50

-1

S (V/K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CB

CB

300 400 500 600 700 800 T (K)

Effect of Sb doping

EF

EF

CB

CB

VB

VB

VB

VB

VB

VB

1xT

3xL

6xη

6xΣ

4xL

12xΣ

Rhombohedral

Cubic

Figure 3. Temperature dependence (a) electrical conductivity (σ) and (b) Seebeck coefficient (S) of Ge1-xSbxTe (x = 0-0.10) samples. (c) S vs. n data of Ge1-xSbxTe compared with Pisarenko plot of GeTe.38 (d) Schematic shows that the valence band valley degeneracy is more in high symmetry cubic phase (Fm-3m) compared to that of the low symmetry rhombohedral (R3m) phase of GeTe. With Sb-doping in GeTe, one T pocket and three L pockets merge to four L pockets; and 6 η pockets and 6 Σ pockets merge into 12 Σ pockets, thus valence band degeneracy increases,49 which resulted in an enhanced Seebeck coefficient. (e) Temperature dependent power factor (S2σ) of Ge1-xSbxTe (x = 0-0.10) samples. 23 ACS Paragon Plus Environment


(a) 9.0

Ge0.95Sb0.05Te Ge0.90Sb0.10Te

2.0

4.5

1.5

lat (W/mK)

6.0

GeTe Ge0.98Sb0.02Te

2.5

Ge0.95Sb0.05Te Ge0.90Sb0.10Te

3.0

(b) 3.0

GeTe Ge0.98Sb0.02Te

7.5 total (W/mK)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.5

~ 45% reduction

1.0

~ 77% reduction

300 400 500 600 700 800 T (K)

0.5

300 400 500 600 700 800 T (K)

Figure 4. Temperature dependence (a) total thermal conductivity (κtotal) and (b) lattice thermal conductivity (κlat) of Ge1-xSbxTe (x = 0-0.10) samples.


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(b) 2.1

(a) 2.1 GeTe Ge0.98Sb0.02Te Ge0.95Sb0.05Te Ge0.90Sb0.10Te

1.2

1.2

0.9

0.6

0.3

0.3

0.0

Cooling

0.9

0.6

Cycle 1 Cycle 2 Cycle 3

0.0

300 400 500 600 700 800 T (K)

Annealed at 773 K for 72 h

300 400 500 600 700 800 T (K)

(d) 12

T= 400K

1.2

Efficiency, , (%)

10

1.0 0.8 0.6 0.4 0.2

Heating

1.5 zT

zT

1.5

(c) 1.4

Ge0.90Sb0.10Te

1.8

1.8

(ZT)m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


p-type : Ge0.90Sb0.10Te (This work) n-type : 8%Sn doped PbTe-PbS (Ref. 43)

0.0 300

400

500 T (K)

600

8 6 4 2 0

700

Tc= 300K

T= 400K

0

100 200 300 400 Temperature difference (K)

Figure 5. (a) Temperature dependence thermoelectric figure of merit (zT) of Ge1-xSbxTe (x = 0-0.10) samples. (b) Three cycles of heating and cooling temperature dependent zT data of Ge0.9Sb0.1Te. Cycle 2 (red) and Cycle 3 (green) represent the zT vs. T data of the same Ge0.9Sb0.1Te sample which was annealed at 773 K for 72 h. (c) Theoretical device thermoelectric figure of merit (ZT)m for one pair of p-type (present Ge0.90Sb0.10Te) and ntype (previously reported 8% of Sn doped PbTe-PbS)43 legs as a function of temperature. (d) Calculated thermoelectric efficiency as a function of temperature difference.



250 This work Ref. 31

Ge0.90Sb 0.10Te

200

Microhardness, Hv

GeTe

TAGS-85

TAGS-80

Ref. (59)

Cu2Se

Cu2S

Ge0.87Pb0.13Te+3%Bi2Te3

Ref. (59)

Ref. (58)

PbSe

0

Pb1-xSnxTe

50

PbTe Ref. (54, 55, 56)

Ref. (53)

100

Ref. (57)

150

Bi2Te3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Samples

Figure 6. Vickers micro-hardness value (Hv) of Ge1-xSbxTe (x = 0-0.10) samples compared with other thermoelectric materials.


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Table 1. Hall carrier concentration of Ge1-xSbxTe (x = 0-0.10) samples..

Samples GeTe

Carrier concentration, n, (x 1020 cm-3) 8.72

Ge0.98Sb0.02Te Ge0.95Sb0.05Te Ge0.90Sb0.10Te

5.80 3.68 2.38



2.1

This work

0.9

150

300 400 500 600 700 800 T (K)

Table of contents


Samples

TAGS-85

TAGS-80

Cu2Se

Cu2S

0

PbSe

50 PbTe

GeTe Ge0.90Sb0.10Te

Bi2Te3

0.3

Pb1-xSnxTe

100

0.6

0.0

Ge0.90Sb 0.10Te

~ 110 % improved

GeTe

-

Ge0.87Pb0.13Te+3%Bi2Te3

020

Microhardness, Hv

200

200

1.5 1.2

250

1.8

zT

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High Thermoelectric Performance and Enhanced ... - ACS Publications

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