Hang Chi

Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe

Heat conversion gets a power boost Thermoelectric materials convert waste heat into electricity, but often achieve high conversion efficiencies only at high temperatures. Zhao et al. tackle this problem by introducing small amounts of sodium to the thermoelectric SnSe (see the Perspective by Behnia). This boosts the power factor, allowing the material to generate more energy while maintaining good conversion efficiency. The effect holds across a wide temperature range, which is attractive for developing new applications. Science, this issue p. 141; see also p. 124 A thermoelectric derived by sodium doping of tin selenide has a high power factor and conversion efficiency over a wide temperature range. [Also see Perspective by Behnia] Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally friendly means of energy savings and power generation. The thermoelectric efficiency is determined by the device dimensionless figure of merit ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Here, we report a record high ZTdev ∼1.34, with ZT ranging from 0.7 to 2.0 at 300 to 773 kelvin, realized in hole-doped tin selenide (SnSe) crystals. The exceptional performance arises from the ultrahigh power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications in the low and moderate temperature range.

Ultrahigh Thermoelectric Performance by Electron and Phonon Critical Scattering in Cu2Se1-xIx

Iodine-doped Cu2 Se shows a significantly improved thermoelectric performance during phase transitions by electron and phonon critical scattering, leading to a dramatic increase in zT by a factor of 3-7 times culminating in zT values of 2.3 at 400 K.

Self-propagating high-temperature synthesis for compound thermoelectrics and new criterion for combustion processing.

The existing methods of synthesis of thermoelectric (TE) materials remain constrained to multi-step processes that are time and energy intensive. Here we demonstrate that essentially all compound thermoelectrics can be synthesized in a single-phase form at a minimal cost and on the timescale of seconds using a combustion process called self-propagating high-temperature synthesis. We illustrate this method on Cu2Se and summarize key reaction parameters for other materials. We propose a new empirically based criterion for sustainability of the combustion reaction, where the adiabatic temperature that represents the maximum temperature to which the reacting compact is raised as the combustion wave passes through, must be high enough to melt the lower melting point component. Our work opens a new avenue for ultra-fast, low-cost, large-scale production of TE materials, and provides new insights into combustion process, which greatly broaden the scope of materials that can be successfully synthesized by this technique.

Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe

We demonstrate a high solubility limit of >9 mol% for MnTe alloying in SnTe. The electrical conductivity of SnTe decreases gradually while the Seebeck coefficient increases remarkably with increasing MnTe content, leading to enhanced power factors. The room-temperature Seebeck coefficients of Mn-doped SnTe are significantly higher than those predicted by theoretical Pisarenko plots for pure SnTe, indicating a modified band structure. The high-temperature Hall data of Sn1-xMnxTe show strong temperature dependence, suggestive of a two-valence-band conduction behavior. Moreover, the peak temperature of the Hall plot of Sn1-xMnxTe shifts toward lower temperature as MnTe content is increased, which is clear evidence of decreased energy separation (band convergence) between the two valence bands. The first-principles electronic structure calculations based on density functional theory also support this point. The higher doping fraction (>9%) of Mn in comparison with ∼3% for Cd and Hg in SnTe gives rise to a much better valence band convergence that is responsible for the observed highest Seebeck coefficient of ∼230 μV/K at 900 K. The high doping fraction of Mn in SnTe also creates stronger point defect scattering, which when combined with ubiquitous endotaxial MnTe nanostructures when the solubility of Mn is exceeded scatters a wide spectrum of phonons for a low lattice thermal conductivity of 0.9 W m(-1) K(-1) at 800 K. The synergistic role that Mn plays in regulating the electron and phonon transport of SnTe yields a high thermoelectric figure of merit of 1.3 at 900 K.

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.

Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi2Te3 Doping

As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (ZT) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ∼1.9 can be achieved at 773 K in Ge0.87Pb0.13Te upon the introduction of 3 mol % Bi2Te3. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge0.87Pb0.13Te + 3 mol % Bi2Te3 was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well.

Non-equilibrium processing leads to record high thermoelectric figure of merit in PbTe-SrTe.

The broad-based implementation of thermoelectric materials in converting heat to electricity hinges on the achievement of high conversion efficiency. Here we demonstrate a thermoelectric figure of merit ZT of 2.5 at 923 K by the cumulative integration of several performance-enhancing concepts in a single material system. Using non-equilibrium processing we show that hole-doped samples of PbTe can be heavily alloyed with SrTe well beyond its thermodynamic solubility limit of <1 mol%. The much higher levels of Sr alloyed into the PbTe matrix widen the bandgap and create convergence of the two valence bands of PbTe, greatly boosting the power factors with maximal values over 30 μW cm−1 K−2. Exceeding the 5 mol% solubility limit leads to endotaxial SrTe nanostructures which produce extremely low lattice thermal conductivity of 0.5 W m−1 K−1 but preserve high hole mobilities because of the matrix/precipitate valence band alignment. The best composition is hole-doped PbTe–8%SrTe.

Enhanced Thermoelectric Properties in the Counter-Doped SnTe System with Strained Endotaxial SrTe

We report enhanced thermoelectric performance in SnTe, where significantly improved electrical transport properties and reduced thermal conductivity were achieved simultaneously. The former was obtained from a larger hole Seebeck coefficient through Fermi level tuning by optimizing the carrier concentration with Ga, In, Bi, and Sb dopants, resulting in a power factor of 21 μW cm(-1) K(-2) and ZT of 0.9 at 823 K in Sn(0.97)Bi(0.03)Te. To reduce the lattice thermal conductivity without deteriorating the hole carrier mobility in Sn(0.97)Bi(0.03)Te, SrTe was chosen as the second phase to create strained endotaxial nanostructures as phonon scattering centers. As a result, the lattice thermal conductivity decreases strongly from ∼2.0 Wm(-1) K(-1) for Sn(0.97)Bi(0.03)Te to ∼1.2 Wm(-1) K(-1) as the SrTe content is increased from 0 to 5.0% at room temperature and from ∼1.1 to ∼0.70 Wm(-1) K(-1) at 823 K. For the Sn(0.97)Bi(0.03)Te-3% SrTe sample, this leads to a ZT of 1.2 at 823 K and a high average ZT (for SnTe) of 0.7 in the temperature range of 300-823 K, suggesting that SnTe is a robust candidate for medium-temperature thermoelectric applications.

Thermoelectric properties of Ag-doped Cu2Se and Cu2Te

Cu2Se, Cu2Te and Ag-overstoichiometric compounds Cu1.98Ag0.2Se and Cu1.98Ag0.2Te were prepared by melting, annealing, followed by spark plasma sintering compaction. Low and high temperature thermoelectric properties were investigated by measuring the electrical conductivity, Seebeck coefficient, thermal conductivity and Hall coefficient between 2 K and 900 K. Structural analyses were performed by PXRD and SEM-EDX analyses. The Hall and Seebeck coefficients show that holes are the dominant carrier in all compounds. High temperature α–β phase transition in Cu2Se and Cu1.98Ag0.2Se between 350 and 400 K and multiple phase transitions (α–β, β–γ, γ–δ, δ–∈) in Cu2Te and Cu1.98Ag0.2Te between 350 K and 900 K were observed in measurements of heat capacity, temperature dependent PXRD data, and transport coefficients. Low temperature transport measurements (Hall coefficient, electrical conductivity, carrier mobility) strongly suggest the presence of yet another phase transition in Cu2Se, Cu1.98Ag0.2Se, and Cu1.98Ag0.2Te compounds at temperatures between 85 K and 115 K, reported here for the first time. Based on the transport data and structural analysis we conclude that doping Cu2Se and Cu2Te by Ag reduces the density of holes and strongly suppresses the thermal conductivity not only due to a smaller electronic contribution but also due to enhanced point defect scattering of phonons that reduces the lattice portion of the thermal conductivity. Moreover, the phase transition temperature is shifted to lower temperatures upon doping with Ag. The presence of Ag enhances thermoelectric performance of Cu2Te at all temperatures and Cu2Se benefits from Ag doping over a broad range of temperatures up to 700 K. The maximum ZT value of 1.2 at 900 K; 0.52 at 650 K; 0.29 at 900 K; and 1.0 at 900 K were achieved for Cu2Se, Cu1.98Ag0.2Se, Cu2Te and Cu1.98Ag0.2Te, respectively, between 2 K and 900 K.

Enhanced thermoelectric properties of Ba-filled skutterudites by grain size reduction and Ag nanoparticle inclusion

Ba-filled skutterudite compounds, Ba0.3Co4Sb12, with dispersed Ag nanoparticles have been synthesized by ball milling followed by hot pressing. The influence of Ag nanoparticles and their size distribution on electrical and thermal transport properties has been investigated in the temperature range from room temperature to 823 K. It was found that Ag nanoparticles in the Ba0.3Co4Sb12 matrix drastically enhance the electric conductivity and slightly increase the Seebeck coefficient. Surprisingly, Ag nanoparticles do not alter the carrier density of Ag/Ba0.3Co4Sb12 nanocomposites. This large improvement in the electrical conductivity as well as the corresponding power factor is assumed to come primarily from the enhanced mobility in Ag/Ba0.3Co4Sb12 nanocomposites. In addition, a large reduction in thermal conductivity is achieved by grain size reduction resulting from ball milling processing and as a result of the embedded Ag nanoparticles that scatter a wider range of phonon frequencies. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT some 30% higher in comparison to the parent Ba0.3Co4Sb12. Moreover, we found it is advantageous to employ a wider size distribution of Ag nanoparticles to reduce the thermal conductivity to a large degree by enhancing phonon scattering. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of filled skutterudites.