Cyril Opeil

Enhancement of thermoelectric properties by modulation-doping in silicon germanium alloy nanocomposites.

Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si(80)Ge(20))(70)(Si(100)B(5))(30) by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si(70)Ge(30) instead of Si as the nanoparticles and Si(95)Ge(5) as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.

High thermoelectric performance by resonant dopant indium in nanostructured SnTe

From an environmental perspective, lead-free SnTe would be preferable for solid-state waste heat recovery if its thermoelectric figure-of-merit could be brought close to that of the lead-containing chalcogenides. In this work, we studied the thermoelectric properties of nanostructured SnTe with different dopants, and found indium-doped SnTe showed extraordinarily large Seebeck coefficients that cannot be explained properly by the conventional two-valence band model. We attributed this enhancement of Seebeck coefficients to resonant levels created by the indium impurities inside the valence band, supported by the first-principles simulations. This, together with the lower thermal conductivity resulting from the decreased grain size by ball milling and hot pressing, improved both the peak and average nondimensional figure-of-merit (ZT) significantly. A peak ZT of ∼1.1 was obtained in 0.25 atom % In-doped SnTe at about 873 K.

Heavy doping and band engineering by potassium to improve the thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe(1-y)Se(y).

We present detailed studies of potassium doping in PbTe(1-y)Se(y) (y = 0, 0.15, 0.25, 0.75, 0.85, 0.95, and 1). It was found that Se increases the doping concentration of K in PbTe as a result of the balance of electronegativity and also lowers the lattice thermal conductivity because of the increased number of point defects. Tuning the composition and carrier concentration to increase the density of states around the Fermi level results in higher Seebeck coefficients for the two valence bands of PbTe(1-y)Se(y). Peak thermoelectric figure of merit (ZT) values of ~1.6 and ~1.7 were obtained for Te-rich K(0.02)Pb(0.98)Te(0.75)Se(0.25) at 773 K and Se-rich K(0.02)Pb(0.98)Te(0.15)Se(0.85) at 873 K, respectively. However, the average ZT was higher in Te-rich compositions than in Se-rich compositions, with the best found in K(0.02)Pb(0.98)Te(0.75)Se(0.25). Such a result is due to the improved electron transport afforded by heavy K doping with the assistance of Se.

Studies on the Bi2Te3–Bi2Se3–Bi2S3 system for mid-temperature thermoelectric energy conversion

Bismuth telluride (Bi2Te3) and its alloys have been widely investigated as thermoelectric materials for cooling applications at around room temperature. We report a systematic study on many compounds in the Bi2Te3–Bi2Se3–Bi2S3 system. All the samples were fabricated by high energy ball milling followed by hot pressing. Among the investigated compounds, Bi2Te2S1 shows a peak ZT ∼0.8 at 300 °C and Bi2Se1S2 ∼0.8 at 500 °C. The results show that these compounds can be used for mid-temperature power generation applications. The leg efficiency of thermoelectric conversion for segmented elements based on these n-type materials could potentially reach 12.5% with a cold side at 25 °C and a hot side at 500 °C if appropriate p-type legs are paired, which could compete well with the state-of-the-art n-type materials within the same temperature range, including lead tellurides, lead selenides, lead sulfides, filled-skutterudites, and half Heuslers.

n-type thermoelectric material Mg2Sn0.75Ge0.25 for high power generation

Significance Thermoelectric materials have been extensively studied for applications in conversion of waste heat into electricity. The efficiency is related to the figure-of-merit, ZT = (S2σ/κ)T, where S, σ, and κ are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Pursuing higher ZT for higher efficiency has been the focus by mainly reducing the thermal conductivity. In this paper, we point out, for a given ZT, higher power factor (S2σ) should be pursued for achieving more power because power is determined by (Th − Tc)2(S2σ)/L, where Th, Tc, and L are the hot and cold side temperatures, and leg length, respectively. We found a new material, Mg2Sn0.75Ge0.25, having both high ZT and high power factor. Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT, has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor (PF) is equivalently important for high power generation, in addition to high efficiency. A new n-type Mg2Sn-based material, Mg2Sn0.75Ge0.25, is a good example to meet the dual requirements in efficiency and output power. It was found that Mg2Sn0.75Ge0.25 has an average ZT of 0.9 and PF of 52 μW⋅cm−1⋅K−2 over the temperature range of 25–450 °C, a peak ZT of 1.4 at 450 °C, and peak PF of 55 μW⋅cm−1⋅K−2 at 350 °C. By using the energy balance of one-dimensional heat flow equation, leg efficiency and output power were calculated with Th = 400 °C and Tc = 50 °C to be of 10.5% and 6.6 W⋅cm−2 under a temperature gradient of 150 °C⋅mm−1, respectively.

Combined Experimental and Theoretical Investigation of the Premartensitic Transition in Ni2MnGa

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity, and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni2MnGa over the temperature range 100<T<250 K. All measurements detect clear signatures of the premartensitic transition (T(PM) approximately 247 K) and the martensitic transition (T(M) approximately 196 K). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at T(PM) located 0.3 eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3 eV in the UPS spectra for T>T(PM) is due to the Ni d minority-spin electrons. Below T(M) this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni d and Mn d electronic contributions to the majority-spin density of states.

Study of the Thermoelectric Properties of Lead Selenide Doped with Boron, Gallium, Indium, or Thallium

Group IIIA elements (B, Ga, In, and Tl) have been doped into PbSe for enhancement of thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity were systematically studied. Room-temperature Hall measurements showed an effective increase in the electron concentration upon both Ga and In doping and the hole concentration upon Tl doping to ~7 × 10(19) cm(-3). No resonant doping phenomenon was observed when PbSe was doped with B, Ga, or In. The highest room-temperature power factor ~2.5 × 10(-3) W m(-1) K(-2) was obtained for PbSe doped with 2 atom % B. However, the power factor in B-doped samples decreased with increasing temperature, opposite to the trend for the other dopants. A figure of merit (ZT) of ~1.2 at ~873 K was achieved in PbSe doped with 0.5 atom % Ga or In. With Tl doping, modification of the band structure around the Fermi level helped to increase the Seebeck coefficient, and the lattice thermal conductivity decreased, probably as a result of effective phonon scattering by both the heavy Tl(3+) ions and the increased grain boundary density after ball milling. The highest p-type ZT value was ~1.0 at ~723 K.

Band Structure of SnTe Studied by Photoemission Spectroscopy

We present an angle-resolved photoemission spectroscopy study of the electronic structure of SnTe and compare the experimental results to ab initio band structure calculations as well as a simplified tight-binding model of the p bands. Our study reveals the conjectured complex Fermi surface structure near the L points showing topological changes in the bands from disconnected pockets, to open tubes, and then to cuboids as the binding energy increases, resolving lingering issues about the electronic structure. The chemical potential at the crystal surface is found to be 0.5 eV below the gap, corresponding to a carrier density of p=1.14 × 10(21)  cm(-3) or 7.2 × 10(-2) holes per unit cell. At a temperature below the cubic-rhombohedral structural transition a small shift in spectral energy of the valance band is found, in agreement with model predictions.

Dramatic thermal conductivity reduction by nanostructures for large increase in thermoelectric figure-of-merit of FeSb2

In this report, thermal conductivity reduction by more than three orders of magnitude over its single crystal counterpart for the strongly correlated system FeSb2 through a nanostructure approach was presented, leading to a significant increase of thermoelectric figure-of-merit (ZT). For the samples processed with the optimal parameters, the thermal conductivity reached 0.34 Wm−1 K−1 at 50 K, leading to a ZT peak of about 0.013, compared to 0.005 for single crystal FeSb2, an increase of about 160%. This work suggests that nanostructure method is effective and can be possibly extended to other strongly correlated low temperature thermoelectric materials, paving the way for future cryogenic temperature cooling applications.

Tin telluride: a weakly co-elastic metal

We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific-heat, and 119Sn Mossbauer spectroscopy measurements on SnTe and Sn0.995Cr0.005Te. Hall measurements at T=77 K indicate that our Bridgman-grown single crystals have a p-type carrier concentration of 3.4×1019 cm−3 and that our Cr-doped crystals have an n-type concentration of 5.8×1022 cm−3. Although our SnTe crystals are diamagnetic over the temperature range 2≤T≤1100 K, the Cr-doped crystals are room-temperature ferromagnets with a Curie temperature of 294 K. For each sample type, three-terminal capacitive dilatometry measurements detect a subtle 0.5 μm distortion at Tc≈85 K. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on Sn0.995Cr0.005Te show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, θD, values of SnTe obtained from 119Sn Mossbauer studies show a hardening of phonons in the range 60–115 K (θD=162 K) as compared with the 100–300 K range (θD=150 K). In addition, a precursor softening extending over approximately 100 K anticipates this collapse at the critical temperature and quantitative analysis over three decades of its reduced modulus finds ΔC44/C44=A|(T−T0)/T0|−κ with κ=0.50±0.02, a value indicating a three-dimensional softening of phonon branches at a temperature T0∼75 K, considerably below Tc. We suggest that the differences in these two types of elastic behaviors lie in the absence of elastic domain-wall motion in the one case and their nucleation in the other.