Trevor P. Bailey

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.

Partial indium solubility induces chemical stability and colossal thermoelectric figure of merit in Cu2Se

High thermoelectric figure of merit, ZT ∼ 2.1 at 1000 K, have been reported in Cu2−xSe-based materials. However, their deployments in operational devices have been hampered by chemical instability and low average ZT (ZTave) values. Here, we demonstrate improved chemical stability and a record high ZTave ∼ 1.5 over a broad temperature range (T ≤ 850 K) in Cu2Se/CuInSe2 nanocomposites, with ZT values ranging from 0.6 at 450 K to an unprecedentedly large value of 2.6 at 850 K for the sample with 1 mol% In. This remarkable performance is attributed to the localization of Cu+ ions induced by the incorporation of In into the Cu2Se lattice, which simultaneously boost the electrical conductivity and reduce the thermal conductivity of the nanocomposites. These findings pave the way for large-scale utilization of Cu2Se-based materials in thermoelectric generators.

Enhanced ZT and attempts to chemically stabilize Cu2Se via Sn doping

Cu2Se is a p-type semiconducting compound that possesses excellent thermoelectric properties but degrades at elevated temperatures under large currents, precluding it from applications in harvesting waste heat. In this study, we make use of a doping approach to attempt to chemically stabilize Cu2Se while maintaining its superior thermoelectric properties. Specifically, we synthesized Cu2(1−x)SnxSe (x = 0, 0.01, 0.02 and 0.05) via melting, annealing and spark plasma sintering. We found that the ZT was enhanced the most in the x = 0.01 sample, averaging approximately a 15% increase over the pure Cu2Se throughout a broad temperature range of 473–823 K, and achieving a maximum ZT = 1 at T = 823 K. The enhancement is due to an increased power factor and a reduced thermal conductivity, which is a result of point defect scattering from Sn atoms in the Cu2Se matrix and grain boundary scattering from a micron-size secondary phase of SnSe. We further tested the ability of the Sn dopant to prevent material degradation at elevated temperatures under large currents. Increasing the Sn dopant content does indeed decrease the solid Cu precipitation but not enough to resolve the issue of material degradation. As a result, despite its improved ZT, Cu1.98Sn0.01Se is not yet ready for thermoelectric applications, and requires further effort to stabilize the structure.

Grain boundary scattering effects on mobilities in p-type polycrystalline SnSe

The extremely high ZTs of p-type SnSe single crystals have attracted much attention. However, due to the high cost of preparation, SnSe single crystals are difficult to be commercialized. On the other hand, the biggest challenge facing more cost-effective polycrystalline SnSe samples are their inferior electronic properties compared to single crystals. It has been proposed that the crystal orientation is responsible for the difference between the electronic properties of polycrystalline and single crystalline SnSe. To explore the role of the crystal orientation, we synthesized textured pure and Ag-doped polycrystalline SnSe and found that the electronic properties of our most highly oriented polycrystalline SnSe are still not higher than single crystals of SnSe oriented along the a-axis (the least favorable orientation). In this study, we compared the temperature-dependent mobility of Ag-doped polycrystalline samples with Ag-doped single crystals of SnSe. We found that grain boundary scattering is the dominant scattering mechanism in polycrystalline SnSe, and this mechanism is substantially absent in single crystals of SnSe. We conclude that grain boundary scattering, and not an averaging effect of the random grain distribution, is the major reason for the poor performance of polycrystalline SnSe compared to single crystals. Based on our results, improving the thermoelectric performance of polycrystalline SnSe will require identifying a synthesis process that minimizes grain boundary scattering.

Mechanism and application method to analyze the carrier scattering factor by electrical conductivity ratio based on thermoelectric property measurement

Carrier scattering factor is one of the most important parameters for semiconductors. In this paper, we propose the mechanism and the application method to analyze the carrier scattering factor(s) by comparing the ratios of electrical conductivity σ(T)/σ(T0 = 300 K) vs. temperature T in the theoretical calculation and experimental results. It is demonstrated that σ(T)/σ(T0 = 300 K) is only related to the carrier scattering factor when the density of states effective mass, m*, is assumed to be constant in small temperature ranges. Therefore, the carrier scattering factor dependence of the ratios of σ(T)/σ(T0 = 300 K) can be used to pinpoint the carrier scattering mechanism. Taking Bi0.5Sb1.5Te2.7+xSe0.3 as an example, it is found that no matter what theoretical models for the Seebeck coefficient over a range of the reduced Fermi energy are used, the analysis results for the scattering mechanism are unique. The reason behind such an observation is that the ratio of σ(T)/σ(T0) is only dependent on the carrie...

Low temperature thermoelectric properties of p-type doped single-crystalline SnSe

SnSe single crystals have been widely studied lately as a result of their record high ZT and controversial low thermal conductivity. Much research has focused on the high-temperature properties of single crystals and polycrystalline SnSe, but few studies were carried out on the low-temperature properties of doped single-crystalline SnSe. To study the mechanism of the charge carrier and phonon scattering, and to eliminate the ambiguity of the high temperature thermal conductivity measurement, we performed low temperature transport characterization of Na-doped and Ag-doped single-crystalline SnSe by a longitudinal steady-state technique. The electronic transport property measurements suggest that Na is a more efficient p-type dopant in SnSe than Ag. In the thermal conductivity data, we observe pronounced dielectric peak around 10 K with magnitude dependent on the doping level. In the p-type doped samples, we found that our room temperature lattice thermal conductivities (>1.74 W m−1 K−1) are in general higher than those previously reported. Based on these findings, our study implies that the lattice thermal conductivity values of doped and pure single-crystalline SnSe were underestimated.SnSe single crystals have been widely studied lately as a result of their record high ZT and controversial low thermal conductivity. Much research has focused on the high-temperature properties of single crystals and polycrystalline SnSe, but few studies were carried out on the low-temperature properties of doped single-crystalline SnSe. To study the mechanism of the charge carrier and phonon scattering, and to eliminate the ambiguity of the high temperature thermal conductivity measurement, we performed low temperature transport characterization of Na-doped and Ag-doped single-crystalline SnSe by a longitudinal steady-state technique. The electronic transport property measurements suggest that Na is a more efficient p-type dopant in SnSe than Ag. In the thermal conductivity data, we observe pronounced dielectric peak around 10 K with magnitude dependent on the doping level. In the p-type doped samples, we found that our room temperature lattice thermal conductivities (>1.74 W m−1 K−1) are in general high...

Absence of Nanostructuring in NaPbmSbTem+2: Solid Solutions with High Thermoelectric Performance in the Intermediate Temperature Regime

Thermoelectric devices directly convert heat into electrical energy and are highly desired for emerging applications in waste heat recovery. Currently, PbTe based compounds are the leading thermoelectric materials in the intermediate temperature regime (∼800 K); however, integration into commercial devices has been limited. This is largely because the performance of PbTe, which is maximized ∼900 K, is too low over the temperatures of interest for most potential commercial applications (generally under 600 K). Improving the low temperature performance of PbTe based materials is therefore critical to achieve usage outside of existing niche applications. Here, we provide an in-depth study of the cubic NaPb mSbTe m+2 system of compounds ( m = 1-20) and report that it is an excellent class of low- to medium-temperature thermoelectrics when m = 10-20. We show that the as-cast polycrystalline ingots exhibit degenerate p-type conduction and high maximum ZTs of 1.2-1.4 at 650 K when m = 6-20. Because the ingots are found to be extremely brittle, we utilize spark plasma sintering (SPS) to prepare more mechanically robust samples, and surprisingly, find that SPS results in an undesired change in charge transport toward n-type behavior. We show this unanticipated transition from p-type behavior as ingots to n-type after SPS is due to dissolution of secondary phases that are present in the ingots into the primary matrix during the SPS process, resulting in a transformation from an inhomogeneous state to a solid solution without any observable evidence of nanoscale precipitation. This is in sharp contrast to the seemingly similar AgPbmSbTe m+2 (LAST) system, which is heavily nanostructured. The SPSed NaPb mSbTe m+2 is doped p-type by tuning the cation stoichiometry, i.e., Na1+ xPb m- xSb1- yTe m+2. The optimized compounds have very low lattice thermal conductivities of 1.1-0.55 W·m-1·K-1 over 300-650 K, which enhances the low-intermediate temperature performance and gives rise to maximum ZT values up to 1.6 at 673 K as well as an excellent ZTavg of 1.1 over 323-673 K for m = 10, 20, making Na1+ xPb m- xSb1- yTe m+2 among the highest performing PbTe-based thermoelectrics under 650 K.

Chemical manipulation of phase stability and electronic behavior in Cu4−xAgxSe2

Superionic chalcogenides have gained renewed research interest, within the last decade, as emerging thermoelectric materials due to attractive properties, such as glass-like phonon transport coupled with crystal-like carrier transport. Of particular interest has been p-type coinage metal-based materials (Cu2Se, CuAgSe), which have demonstrated figures-of-merit, ZT, exceeding unity through a broad temperature range. However, the lack of n-type counterparts within this class of compounds limits potential module deployment. Here we show that careful stoichiometry control of the Cu4−xAgxSe2 series enables the formation of stable n-type materials throughout the measured temperature range upon substitution of Cu by Ag (1 ≤ x ≤ 3). Thermopower data show that the sample with x = 1 (Cu3AgSe2) undergoes a transition from n- to p-type conducting behavior, whereas samples with x = 2 (CuAgSe) and x = 3 (CuAg3Se2) exhibit n-type character in the whole measured temperature range. The post superionic transition n-type conductivity of CuAgSe is quite surprising and is contrary to the n- to p-type transition previously reported for this composition. Room temperature X-ray diffraction studies indicate the formation of a two-phase mixture for samples with x = 1 (Cu3AgSe2 = α-Cu2Se + α-CuAgSe) and x = 3 (CuAg3Se2 = α-Ag2Se + α-CuAgSe), whereas a single-phase α-CuAgSe is observed for the sample with x = 2. At 523 K, X-ray diffraction patterns show that Cu3AgSe2 (x = 1) and α-CuAgSe (x = 2) transform into single phase structures with the space group Fmm, while the CuAg3Se2 (x = 3) sample remains a two-phase system (CuAg3Se2 = β-Ag2Se + β-CuAgSe) in contrast to previous studies. This structural study is consistent with the observed gradual evolution of the conduction type of Cu4−xAgxSe2 samples between the p-type character of Cu2Se (x = 0) and the n-type semiconducting behavior of Ag2Se (x = 4). This suggests that the conducting behavior in the Cu4−xAgxSe2 is modulated by the Cu : Ag ratio. All Cu4−xAgxSe2 samples exhibit extremely low thermal conductivity after their phase transitions (<0.5 W m−1 K−1), which result in modest ZT values (∼0.45 at 625 K).

Insights on the Synthesis, Crystal and Electronic Structures, and Optical and Thermoelectric Properties of Sr1–xSbxHfSe3 Orthorhombic Perovskite

Single-phase polycrystalline powders of Sr1- xSb xHfSe3 ( x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe3, crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe3 revealed low values ranging from 0.9 to 1.3 W m-1 K-1 from 300 to 700 K, which is further lowered to 0.77 W m-1 K-1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe3 is a p-type semiconductor with thermopower values reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr1- xSb xHfSe3 samples, as determined by density functional theory calculations and the diffuse reflectance method, is ∼1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe3 is essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe3 as absorbing layer in photovoltaic devices.

Soft phonon modes from off-center Ge atoms lead to ultralow thermal conductivity and superior thermoelectric performance in n-type PbSe–GeSe

Historically PbSe has underperformed PbTe in thermoelectric efficiency and has been regarded as an inferior relative to its telluride congener. However, the fifty-fold greater natural abundance of Se relative to Te makes PbSe appealing as a thermoelectric material. We report that the n-type GeSe-alloyed PbSe system achieves a peak figure of merit, ZT, of ∼1.54 at 773 K and maintains ZT values above 1.2 over a broad temperature range from 623 K to 923 K. The highest performing composition is Sb-doped PbSe–12%GeSe, which exhibits an ultralow lattice thermal conductivity of ∼0.36 W m−1 K−1 at 573 K, close to the limit of amorphous PbSe. Theoretical studies reveal that the alloyed Ge2+ atoms prefer to stay at off-center lattice positions, inducing low frequency modes. The Ge atoms also cause the unexpected behavior where the next nearest atom neighbors (6 Pb atoms) oscillate at lower frequencies than in pure PbSe leading to a large reduction of the Debye temperature and acoustic phonon velocity. The Pb0.9955Sb0.0045Se–12%GeSe system also shows Ge-rich precipitates and many aligned dislocations within its microstructure which also contribute to phonon scattering. The resultant average ZT (ZTavg), a broad measure of the materials potential for functional thermoelectric modules, is 1.06 from 400 K to 800 K, the highest among all previously reported n- and p-type PbSe. This value matches or exceeds even those of the best n-type PbTe-based thermoelectric materials.