Filled skutterudites are one of the most promising thermoelectric materials for power generation applications. The choice and concentration of filler atoms are key aspects for achieving high thermoelectric figure of merit values. We report on the high temperature thermoelectric properties in the double-filled skutterudites BaxYbyCo4Sb12. The combination of Ba and Yb fillers inside the voids of the skutterudite structure provides a broad range of resonant phonon scattering and consequently a strong suppression in the lattice thermal conductivity is observed. A dimensionless thermoelectric figure of merit of 1.36 at 800K is achievable for n-type BaxYbyCo4Sb12.

Thermoelectric (TE) power generation has come to be appreciated as an attractive means of low-cost conversion of waste heat to useful electrical energy with a small environmental impact. For a compound to qualify as an efficient thermoelectric material it should exhibit the highest TE figure of merit, ZT, possible at the temperature of operation, T. ZT is defined asand it involves the simultaneous manipulation of the TE power (absolute Seebeck coefficient) S, the electrical conductivity r, and the thermal conductivity j. The search for efficient TE materials mainly focuses on degenerate semiconductors since the underlying physics of these systems allow the coexistence of high thermopower values with high electrical conductivity to achieve high power factors: PF = S 2 r. The Seebeck coefficient is inversely related to the electrical conductivity according to the Boltzmann transport equation, and, as a result, maximization of one cannot be achieved without minimization of the other. An interesting alternative that has been recently suggested to achieve high power factors is the quantum-confinement effect; however, definite experimental verification of this is still lacking.[1]Another route to achieving high-performance TEs is through the minimization of the thermal conductivity. To this end, many suggestions have been made to increase ZT. These include the phonon-glass electron-crystal approach [2] (where loosely bound atoms rattle in cage structures [3] ) as in clathrates, [4] and the thin-film multilayer approach where the introduction of interfaces significantly reduces phonon propagation.[ [10] where compositional fluctuations at the nanoscopic level, resulting in a distinct type of nanostructuring, seem to play a key role in the previously reported very low thermal conductivity. [11] In contrast to the thin-film multilayers, bulk nanocomposite systems offer the advantages of large-scale industrial production and the sustenance of large thermal gradients for extended time. The challenge, therefore, lies in identifying equally efficient p-type materials so that they can be employed in the fabrication of TE modules.Here we report on the Ag(Pb 1 -y Sn y ) m SbTe 2 + m series and show that certain compositions exhibit high performance p-type TE properties (e.g., ZT ∼ 1.45 at 630 K) as a result of their very low thermal conductivity. We show as well that the Ag(Pb 1 -y Sn y ) m SbTe 2 + m systems are in fact bulk nanocomposites. We demonstrate that varying the m and y values, as well as the Ag and Sb concentrations, allows for control over a wide range of properties such as carrier concentration, TE power, and thermal conductivity. These exceptional properties, derived from specific compositions, outperform the standard state-of-the-art p-type systems like TAGS ((AgSbTe 2 ) 0.15 (GeTe) 0.85 , ZT ∼ 1.2 at 720 K [12] ), PbTe (ZT ∼ 0.7 at 740 K [13] ), and Zn 4 Sb 3 (ZT ∼ 1.3 at 670 K [14] ).The electronic-transport properties of the Ag(Pb 1 -y Sn y ) mSbTe 2 + m system can be tuned primarily through carefully control...

Thermoelectric heat-to-electrical-energy converters will play a role in energy management if efficient, stable, and inexpensive materials can be developed. Research to increase the figure of merit ZT is focused on a variety of novel thermoelectric materials and approaches. [1][2][3][4][5][6][7][8][9] Significant advances have come in recent years from the development of nanostructured semiconductors both in thin-film [10] and bulk form. [11][12][13][14] In the thin films, values ZT > 3 have been claimed for PbTe/PbSe superlattice structures, [15] while values of 1.5-1.7 at 700 K have been reported for AgPb 18 SbTe 20, [12] NaPb 20 SbTe 22 , [16] PbTe-PbS, [17] and Tl-PbTe. [18] This progress promises to impact several energy-conversion applications. The advancements have primarily come from sizeable reductions in thermal conductivity, which is largely the result of the scattering of mid-and long-wavelength phonons at the interfaces of the nanoscale inclusions at high temperature. [18,19] Although further increases in ZT can be anticipated by additional reduction of the thermal conductivity, a lower limit is expected to be reached. Thus, dramatic enhancements in ZT can only come from spectacular increases in the power factor (S 2 s). This problem is harder to tackle, since standard charge-transport theory does not offer specific guidance on how to dramatically increase the power factor. Its solution will require breakthroughs in the understanding and control of charge-transport mechanisms in complex materials. An important factor affecting charge transport (e.g. mobility m = et/m, where m is the carrier effective mass) and thus power factor in solids is the relaxation time t, which has an energy dependence [Eq. (1), where E is the energy and r is ascattering parameter]. [20] This relationship implies that the energy dependence of t impacts the mobility, electrical conductivity (s = N em, N = carrier concentration), and thermopower in semiconductors, especially for large values of r owing to their dependence on the relaxation time. In PbTe it is not possible to cause significant changes in the scattering mechanism by making solid solutions or through carrier doping.[21] A more fundamental question is whether the relative contributions of known scattering mechanisms, or even the actual mechanisms themselves in PbTe, can be altered through nanostructuring. For example, can features buried on the nanoscale fundamentally change the electronic structure and therefore charge transport?We report herein a new way to achieve dramatic changes in the carrier transport in PbTe, which leads to large increases in the thermoelectric power factor (S 2 s) at high temperatures. We observe the enhancements only when PbTe is co-nanostructured with two different phases. We show that when lead and antimony are present simultaneously as nanodots throughout the matrix of PbTe, the conductivity behavior becomes novel. We observe unprecedented temperature dependence that is a complicated function of the Pb/Sb ratio. The most important conse...

Developing advanced thermoelectric (TE) materials can impact conversion technologies of waste heat to electrical power. It is well expected that by fabricating TE materials in a nanostructured form their properties can be significantly enhanced. 1 Efficient TE materials must exhibit a large TE figure of merit, ZT, defined as ZT ) σS 2 /κ, where S is the TE power (absolute Seebeck coefficient), σ is the electrical conductivity, and κ is the thermal conductivity. The Seebeck coefficient is generally inversely related to the electrical conductivity and as a result maximization of ZT is difficult.One modality in achieving high performance is through the minimization of the thermal conductivity using nanostructures. Superlattice thin film structures grown by molecular beam epitaxy (MBE) PbSe 0.98 Te 0.02 /PbTe 2-4 have achieved ZT values > 3. 5 These MBE-grown materials contain pyramidal-shaped "nanodots" of PbSe with uniform size (∼20 nm) embedded inside a matrix of PbTe. This assembly possesses record low values of thermal conductivity (∼0.3-0.4 W/(m‚K)) while at the same time retains a high power factor. Therefore, it is of significant interest to devise general, convenient, low cost synthetic methodologies for incorporating similar nanometer scale inclusions into bulk semiconductor materials in an effort to mimic the high ZT superlattice structures. Examples of materials with naturally occurring nanostructuring and enhanced TE properties have been reported. 6,7 Here we describe the intentional preparation of nanometer sized inclusions of Sb, Bi, and InSb in bulk PbTe using a general liquid matrix encapsulation technique. We observe that nanocrystals with large contrast in average mass (e.g., Sb and InSb) with that of the PbTe medium achieve stronger scattering of acoustic phonons than nanocrystals with minimal contrast. We also find that the reduction in lattice thermal conductivity is not monotonic with increasing concentration of nanoparticles, but there is an optimum concentration beyond which the lattice thermal conductivity actually increases. These results are in agreement with theoretical expectations and recent reports that embedded nanocrystals of ErAs promote strong scattering of acoustic phonons in a InGaAs matrix. 8 There is a plethora of published work on the preparation of stable free-standing semiconductor nanocrystals capped with surfactants, 9 embedded in polymers 10 or glasses. 11 However, there is relatively little effort devoted to preparing nanocrystals inside solid matrices or bulk crystals. 12-14 In general, bulk crystals with nanocrystals embedded in them represent a fascinating set of nanostructured materials whose scope extends beyond the field of thermoelectrics especially when the properties of guest/matrix are chosen for specific functions.To achieve nanoscale matrix encapsulation of a minority phase A inside a majority phase B, we choose the former to have very low or no solubility in the solid state but to be completely soluble in the liquid state. We choose the major phase B to ha...

The thermal conductivity of wurtzite and zinc blende indium arsenide nanowires was measured using a microfabricated device, with the crystal structure of each sample controlled during growth and determined by transmission electron microscopy. Nanowires of both phases showed a reduction of the thermal conductivity by a factor of 2 or more compared to values reported for zinc blende indium arsenide bulk crystals within the measured temperature range. Theoretical models were developed to analyze the measurement results and determine the effect of phase on phonon transport. Branch-specific phonon dispersion data within the discretized first Brillouin zone were calculated from first principles and used in numerical models of volumetric heat capacity and thermal conductivity. The combined results of the experimental and theoretical studies suggest that wurtzite indium arsenide possesses similar volumetric heat capacity, weighted average group velocity, weighted average phonon-phonon scattering mean free path, and anharmonic scattering-limited thermal conductivity as the zinc blende phase. Hence, we attribute the differing thermal conductivity values observed in the indium arsenide nanowires of different phases to differences in the surface scattering mean free paths between the nanowire samples.