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ABSTRACT: The solubility of sodium and its effects on phonon scattering in lead chalcogenide PbQ (Q = Te, Se and S) thermoelectric material family was investigated by means of transmission electron microscopy and theoretical calculations. We find that among these three systems, Na has the highest solubility limit, about 2 mol%, in PbS and lowest, ~0.5 mol%, in PbTe. First-principles electronic structure calculations support the conclusions, indicating that Na has the lowest formation energy in PbS and the highest in PbTe. The findings further reveal that in addition to providing charge carriers (holes) for PbQ, Na introduces point defects (solid solution formation) and nanoscale precipitates; both reduce lattice thermal conductivity through scattering heat carrying phonons. These results explain the recent reports of high thermoelectric performance in p-type lead chalcogenide materials, and help lead to further advances in this class of materials.
Journal of the American Chemical Society 02/2013; · 9.91 Impact Factor
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Jiaqing He,
I D Blum,
Hui-Qiong Wang,
S N Girard,
J Doak,
Li-Dong Zhao, Jin-Cheng Zheng,
G Casillas,
C Wolverton,
M Jose-Yacaman,
D N Seidman,
M G Kanatzidis,
V P Dravid
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ABSTRACT: The morphology of crystalline precipitates in a solid-state matrix is governed by complex but tractable energetic considerations driven largely by volume strain energy minimization and anisotropy of interfacial energies. Spherical precipitate morphologies are favored by isotropic systems, while anisotropic interfacial energies give energetic preference to certain crystallographically oriented interfaces, resulting in a faceted precipitate morphology. In conventional solid-solution precipitation, a precipitate's morphological evolution is mediated by surface anchoring of capping molecules, which dramatically alter the surface energy in an anisotropic manner, thereby providing exquisite morphology control during crystal growth. Herein, we present experimental evidence and theoretical validation for the role of a ternary element (Na) in controlling the morphology of nanoscale PbS crystals nucleating in a PbTe matrix, an important bulk thermoelectric system. The PbS nanostructures formed by phase separation from a PbI(2)-doped or undoped PbTe matrix have irregular morphologies. However, replacing the iodine dopant with Na (1-2 mol %) alters dramatically the morphology of the PbS precipitates. Segregation of Na at PbTe/PbS interfaces result in cuboidal and truncated cuboidal morphologies for PbS. Using analytical scanning/transmission electron microscopy and atom-probe tomography, we demonstrate unambiguously that Na partitions to the precipitates and segregates at the matrix/precipitate interfaces, inducing morphological anisotropy of PbS precipitates. First-principles and semiclassical calculations reveal that Na as a solute in PbTe has a higher energy than in PbS and that Na segregation at a (100) PbTe/PbS interface decreases the total energy of matrix/precipitate system, resulting in faceting of PbS precipitates. These results provide an impetus for a new strategy for controlling morphological evolution in matrix/precipitate systems, mediated by solute partitioning of ternary additions.
Nano Letters 10/2012; · 13.20 Impact Factor
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ABSTRACT: The incorporation of PbSnS(2) in PbTe results in a tremendous reduction of the lattice thermal conductivity to 0.8 W/mK at room temperature, a reduction of almost 60% over bulk PbTe. Transmission electron microscopy reveals very high density displacement layers, misfit dislocations, and phase boundaries. Our thermal transport calculations based on modified Debye-Callaway model, well in agreement with the experimental measurements, reveal that the layer structured PbSnS(2) plays an important role in reducing the lattice thermal conductivity.
Advanced Materials 07/2012; 24(32):4440-4. · 13.88 Impact Factor
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ABSTRACT: We have investigated the possible mechanisms of phonon scattering by nanostructures and defects in PbTe-X (X = 2% Sb, Bi, or Pb) thermoelectric materials systems. We find that among these three compositions, PbTe-2% Sb has the lowest lattice thermal conductivity and exhibits a larger strain and notably more misfit dislocations at the precipitate/PbTe interfaces than the other two compositions. In the PbTe-Bi 2% sample, we infer some weaker phonon scattering BiTe precipitates, in addition to the abundant Bi nanostructures. In the PbTe-Pb 2% sample, we also find that pure Pb nanoparticles exhibit stronger phonon scattering than nanostructures with Te vacancies. Within the accepted error range, the theoretical calculations of the lattice thermal conductivity in the three systems are in close agreement with the experimental measurements, highlighting the important role of misfit dislocations, nanoscale particles, and associated interfacial elastic strain play in phonon scattering. We further propose that such particle-induced local elastic perturbations interfere with the phonon propagation pathway, thereby contributing to further reduction in lattice thermal conductivity, and consequently can enhance the overall thermoelectric figure of merit.
Journal of the American Chemical Society 06/2010; 132(25):8669-75. · 9.91 Impact Factor
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ABSTRACT: The composition and microstructure of five thermoelectric materials, PbTe, SnTe, Pb0.65Sn0.35Te and NaPb18-xSnxBiTe20 (x = 5, 9), were investigated by advanced transmission electron microcopy. We confirm that the pure PbTe, SnTe, and Pb0.65Sn0.35Te have a uniform crystalline structure and homogeneous compositions without any nanoscale inclusions. On the other hand, the nominal NaPb9Sn9BiTe20 phase contains extensive inhomogeneities and nanostructures with size distribution of 3−7 nm. We find that the chemical architecture of the NaPb13Sn5BiTe20 member of the series to be more complex; besides nanoscale precipitates, self-organized lamellar structures are present which were identified as PbTe and SnTe by composition analysis and transmission electron microscopy image simulations. Density functional theory calculations suggest that the arrangement of the lamellar structures conforms to the lowest total energy configuration. Geometric-phase analyses revealed large distributed elastic strain around the nanoscale inclusions and lamellar structures. We propose that interface-induced elastic perturbations in the matrix play a decisive role in affecting the phonon-propagation pathways. The interfaces further enhance phonon scattering which, in turn, reduces the lattice thermal conductivity in these systems that directly results directly in improvement in the thermoelectric figure of merit.
12/2009;
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ABSTRACT: The composition and microstructure of five thermoelectric materials, PbTe, SnTe, Pb(0.65)Sn(0.35)Te and NaPb(18-x)Sn(x)BiTe(20) (x = 5, 9), were investigated by advanced transmission electron microcopy. We confirm that the pure PbTe, SnTe, and Pb(0.65)Sn(0.35)Te have a uniform crystalline structure and homogeneous compositions without any nanoscale inclusions. On the other hand, the nominal NaPb(9)Sn(9)BiTe(20) phase contains extensive inhomogeneities and nanostructures with size distribution of 3-7 nm. We find that the chemical architecture of the NaPb(13)Sn(5)BiTe(20) member of the series to be more complex; besides nanoscale precipitates, self-organized lamellar structures are present which were identified as PbTe and SnTe by composition analysis and transmission electron microscopy image simulations. Density functional theory calculations suggest that the arrangement of the lamellar structures conforms to the lowest total energy configuration. Geometric-phase analyses revealed large distributed elastic strain around the nanoscale inclusions and lamellar structures. We propose that interface-induced elastic perturbations in the matrix play a decisive role in affecting the phonon-propagation pathways. The interfaces further enhance phonon scattering which, in turn, reduces the lattice thermal conductivity in these systems that directly results directly in improvement in the thermoelectric figure of merit.
Journal of the American Chemical Society 12/2009; 131(49):17828-35. · 9.91 Impact Factor
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ABSTRACT: Nanoparticles play key roles in reducing thermal conductivity, and hence increasing figure of merit for many thermoelectric materials. We have studied the structure of Ag Pb <sub>18</sub> Sb Te <sub>20</sub> (LAST-18) using high resolution imaging, nanoelectron diffraction, energy dispersive spectrum, and electron energy loss spectrum, and observed a range of nanoparticles with different sizes (from less than 1 nm to more than 10 nm ) and shape (sphere, ellipse, square, etc.). The lattice parameters of the nanoparticles have a wide range from 0.601 to 0.655 nm , while those of the matrix have a range from 0.633 to 0.646 nm . The nanoparticles are formed due to the ordering of Pb and Ag–Sb. There are four ordered structures with primitive cubic, primitive tetragonal (T1, a≈a<sub>0</sub>/√2 , c≈a<sub>0</sub> , here, a<sub>0</sub> is the lattice parameter of the rocksalt-type matrix), primitive tetragonal (T2, a≈a<sub>0</sub>/√2 , c≈2a<sub>0</sub> ), and body-centered tetragonal (T3, a≈a<sub>0</sub>/√2 , c≈3a<sub>0</sub> ) lattices, respectively. Antiphase domains, twins, and phase separations were often observed in the nanoparticles. The strain field in the surrounding matrix due to the presence of nanoparticles was retrieved from the high resolution images. The characteristic that the strain field is anisotropic and extends to large area is considered to enhance the scattering of the phonons. The results provide quantitative structure information about nanoparticles, t-
hat is essential for the understanding of the origin of the high thermoelectric performance in this class of materials.
Journal of Applied Physics 06/2009; · 2.17 Impact Factor
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ABSTRACT: We studied the structure, morphology, and growth mechanism of self-organized La <sub>0.35</sub> Pr <sub>0.275</sub> Ca <sub>0.375</sub> Mn O <sub>3</sub> manganite nanorods grown on Nd Ga O <sub>3</sub> substrates by pulsed-laser deposition. A two-layered structure was revealed: the first layer, about 120 nm thick, was formed via layer-by-layer two-dimensional (2D) growth; the second layer consisted of a three-dimensional assembly of nanorods lying perpendicular to the 2D layer. The nanorods, averaging 50 nm across and 180 nm long, exhibited six crystallographic orientational domains, but only two predominated, both with their b axis lying parallel to that of substrate (parallel to the film normal) and with an in-plane a - and c -axis interchange to minimize local lattice mismatch. We consider that the formation of such self-assembled nanorods is related to the Stranski–Krastanov growth mode and discuss the associated energy terms of such growth based on density functional theory calculations.
Journal of Applied Physics 04/2008; · 2.17 Impact Factor
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ABSTRACT: Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.
Physical Review Letters 12/2005; 95(26):267002. · 7.37 Impact Factor
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ABSTRACT: Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to follow the change of superconductivity with systematic Al doping of MgB$_2$. By using x-ray diffraction and Vegard's law to assess the actual Al content in the samples, changes in behavior were found to be much more in agreement with theoretical predictions than in earlier studies. EELS data show that $\sigma$-band hole states disappear above 33% Al, approximately the composition at which the $\sigma$ band Fermi surface is predicted to lose its cylindrical shape in reciprocal space and break apart into ellipsoidal pockets. At this composition, the $\sigma$ gap obtained from the heat capacity data falls to the level of the $\pi$ gap, implying that band filling results in the loss of strong superconductivity on the $\sigma$ band. However, superconductivity is not quenched completely, but persists with $T_c < 7$ K up to about 55% Al, the Al concentration at which the entire $\sigma$ band is predicted to fall below the Fermi surface. Since, in the region $0.33 \alt x \alt 0.55$, only the $\pi$ band has appreciable density of states, it becomes the stronger of the 2 bands, thus inverting the 2-band hierarchy of MgB$_2$.
04/2005;
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ABSTRACT: We present a detailed first-principles density-functional analysis of the effects of lattice strain on the superconducting transition temperature, Tc, of MgB2, deriving a general rule that governs the enhancement (or suppression) of Tc in strained MgB2 in terms of electronic and phonon contributions. Based on the calculated structural, electronic, vibrational, and superconducting properties of a strained MgB2 superconductor, we show how a higher Tc might be achieved. Several candidate substrates are suggested for growing MgB2 thin films to gain a higher Tc.
Phys. Rev. B. 73(2).
