Excellent Thermoelectric Materials Research Articles

First-Principles Predictions of Thermoelectric Figure of Merit for Organic Materials: Deformation Potential Approximation.

We propose a combined computational scheme to predict the thermoelectric properties of organic semiconductors, taking α-form phthalocyanine crystals H2Pc, CuPc, NiPc, and TiOPc as examples. This completely parameter-free approach combines first-principles band structure calculations, Boltzmann transport theory, deformation potential theory for electron-phonon coupling, and nonequilibrium molecular dynamics for heat transport. We abandon the constant relaxation time approximation commonly practiced in the literature. Instead, we calculate it from first principles with the deformation potential approximation. The obtained Seebeck coefficients are in good agreement with experimental results, validating our treatment for relaxation time. From the calculated thermoelectric figure of merit (ZT) value, we show that phthalocyanine crystals could be excellent thermoelectric materials when n-doped, with the highest ZT value of 2.5 in NiPc at a doping level of -1.5 × 10(20) cm(-3).

High Performance Thermoelectrics from Earth-Abundant Materials: Enhanced Figure of Merit in PbS by Second Phase Nanostructures

Lead sulfide, a compound consisting of elements with high natural abundance, can be converted into an excellent thermoelectric material. We report extensive doping studies, which show that the power factor maximum for pure n-type PbS can be raised substantially to ~12 μW cm(-1) K(-2) at >723 K using 1.0 mol % PbCl(2) as the electron donor dopant. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding selected metal sulfide phases. The thermal conductivity at 723 K can be reduced by ~50%, 52%, 30%, and 42% through introduction of up to 5.0 mol % Bi(2)S(3), Sb(2)S(3), SrS, and CaS, respectively. These phases form as nanoscale precipitates in the PbS matrix, as confirmed by transmission electron microscopy (TEM), and the experimental results show that they cause huge phonon scattering. As a consequence of this nanostructuring, ZT values as high as 0.8 and 0.78 at 723 K can be obtained for nominal bulk PbS material. When processed with spark plasma sintering, PbS samples with 1.0 mol % Bi(2)S(3) dispersion phase and doped with 1.0 mol % PbCl(2) show even lower levels of lattice thermal conductivity and further enhanced ZT values of 1.1 at 923 K. The promising thermoelectric properties promote PbS as a robust alternative to PbTe and other thermoelectric materials.

Thermoelectric properties and microstructures of AgSbTe2‐added p‐type Pb0.16Ge0.84Te

AbstractThe alloys in the AgSbTe2GeTe system and the AgSbTe2PbTe system are known as excellent thermoelectric (TE) materials, i.e. TAGS and LAST, respectively. Although the TE properties of the AgSbTe2(Pb,Ge)Te system have not been reported. Here, we show the high‐temperature TE properties of the alloys in the AgSbTe2(Pb,Ge)Te system. The samples of (Ag0.5Sb0.5Te)100−x(Pb0.16Ge0.84Te)x (x = 75, 80, 85, and 90) were prepared and the electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were examined in the temperature range from room temperature to 720 K. The samples prepared by hot‐press were composed of two phases: the GeTe‐based matrix phase and the PbTe‐based precipitate. These samples exhibited the moderate ρ and S as well as relatively low κ, leading to high thermoelectric figure of merit ZT (= S2T/ρκ, where T is the absolute temperature). The maximum ZT value was 0.84 at 720 K obtained in the sample of x = 80.

Thermoelectric properties of Ga-added CoSb3 based skutterudites

Filled skutterudite compounds are known as excellent thermoelectric (TE) materials. It is known that the voids in the structure of the skutterudite compounds, such as CoSb3, can be filled or partially filled with a variety of different atoms, and, thus, obtained filled skutterudite compounds exhibit quite low thermal conductivity (κ). In the present study, we tried to fill Ga into the voids of CoSb3. The polycrystalline samples of GaxCo4Sb12 (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) were prepared, and the TE properties were examined from room temperature to 750 K. All the samples were composed of two phases: GaxCo4Sb12 (x = ∼0.02) as the matrix phase and Ga metal as the second phase. All the samples exhibited negative values of the Seebeck coefficient (S). The Hall carrier concentration slightly increased with increasing x, while the carrier mobility decreased. Although the maximum Ga filling ratio was really low, the κ was reduced effectively by Ga adding. The maximum value of the dimensionless figure of merit ZT ( = S2T/ρ/κ, where T is the absolute temperature and ρ is the electrical resistivity) was 0.18 at 500 K obtained for Ga0.25Co4Sb12.

Enhanced thermoelectric performance of PbTe within the orthorhombicPnmaphase

The thermoelectric properties of PbTe within the NaCl (B1) and the orthorhombic $Pnma$ phases are extensively studied by ab initio calculations using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. The calculations of $n$- and $p$-type Seebeck coefficients for the B1 structure suggested that the energy band gap plays an important role in determining the thermoelectric properties, but only at lower carrier concentrations. We found that the $n$-doped $Pnma$ phase at $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ has a larger Seebeck coefficient than that of the $n$-doped B1 phase at zero pressure, but has a comparable electric conductivity. This fact could be well understood by the large density of states in the conduction band and the large anisotropy in the band structure and constant energy surface. Our calculations also predicted that the largest $n$-type ZT values at 300 and $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of the $Pnma$ phase at $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ can reach up to 0.9 and 1.59, respectively, which are two times larger than those in B1 phase at zero pressure. The current theory strongly suggests that the $Pnma$ structure of PbTe is an excellent thermoelectric material. It is desirable to synthesize the $Pnma$ phase of PbTe at ambient pressure by making use of its high performance.

Self-assembly of antimony nanowires on graphite

One-dimensional (1D) semimetals (e.g., antimony and bismuth) are excellent thermoelectric materials. We demonstrate the self-assembly of 1D Sb nanowires on highly oriented pyrolytic graphite, along with structures of other dimensionality. Comparing with an Sb crystal in ambient condition, our scanning tunneling microscopy analysis indicates that these Sb nanowires have a compressed lattice structure, which is likely formed under the Laplace pressure that can be quite large in a nanostructure. The conditions for growing aligned semimetal nanowires exclusively are discussed.