Synergistic Effect of Chemical Substitution and Insertion on the Thermoelectric Performance of Cu26V2Ge6S32 Colusite.

Copper-based sulfides are promising materials for thermoelectric applications, which can convert waste heat into electricity. This study reports the enhanced thermoelectric performance of Cu26V2Ge6S32 colusite via substitution of antimony (Sb) for germanium (Ge) and introduction of copper (Cu) as an interstitial atom. The crystal structure of the solid solutions and Cu-rich compounds were analyzed by powder X-ray diffraction and scanning transmission electron microscopy. Both chemical approaches decrease the hole carrier concentration, which leads to a reduction in the electronic thermal conductivity while keeping the thermoelectric power factor at a high value. Furthermore, the interstitial Cu atoms act as phonon scatterers, thereby decreasing the lattice thermal conductivity. The combined effects increase the dimensionless thermoelectric figure of merit ZT from 0.3 (Cu26V2Ge6S32) to 0.8 (Cu29V2Ge5SbS32) at 673 K.

Thermoelectric Properties and Electronic Structures of CuTi2S4 Thiospinel and Its Derivatives: Structural Design for Spinel-Related Thermoelectric Materials.

We report the preparations, thermoelectric and magnetic properties, and electronic structures of Cu-Ti-S systems, namely, cubic thiospinel c-Cu1- xTi2S4 ( x ≤ 0.375), a derivative cubic and Ti-rich phase c-Cu1- xTi2.25S4 ( x = 0.5, 0.625), and a rhombohedral phase r-CuTi2S4. All samples have the target compositions except for r-CuTi2S4, whose actual composition is Cu1.14Ti1.80S4. All of the phases have n-type metallic character and exhibit Pauli paramagnetism, as proven by experiments and first-principles calculations. The Cu and Ti deficiencies in c-Cu1- xTi2S4 and r-CuTi2S4, respectively, decrease the electron-carrier concentration, whereas the "excess" of Ti ions in c-Cu1- xTi2.25S4 largely increases it. For r-CuTi2S4, the reduced carrier concentration increases the electrical resistivity and Seebeck coefficient, leading to the highest thermoelectric power factor of 0.5 mW K-2 m-1 at 670 K. For all of the Cu-Ti-S phases, the thermal conductivity at 670 K is 3.5-5 W K-1 m-1, where the lattice part of the conductivity is as low as 1 W K-1 m-1 at 670 K. As a result, r-CuTi2S4 shows the highest dimensionless thermoelectric figure of merit ZT of 0.2. The present systematic study on the Cu-Ti-S systems provides insights into the structural design of thermoelectric materials based on Cu-M-S (M = transition-metal elements).

Electronic and thermoelectric properties of Zn and Se double substituted tetrahedrite.

The influence of Zn and Se double substitution on the electronic and thermoelectric properties of tetrahedrite was investigated in this study. The samples Cu11Zn1Sb4S13-xSex (x = 0.25, 0.5, 0.75, 1, and 2) were prepared via solid state synthesis followed by field assisted sintering. The density functional theory (DFT) results showed that Se substitution introduces additional bands near the Fermi level (EF), with lower effective mass compared to Zn (only) substituted sample Cu11Zn1Sb4S13. Consequently, the electrical resistivity decreased with the increase in Se content which is attributed to the enhanced charge carrier mobility caused by the more dispersive Se states as indicated by DFT results. But the Seebeck coefficient was invariant with x, due to the enhancement of the density of states (DOS) at EF. The overall effect was an increase in power factor of the Cu11Zn1Sb4S13-xSex samples compared to Cu11Zn1Sb4S13. The Zn2+ substitution at the Cu1+ tetrahedral site resulted in a decrease of the carrier thermal conductivity due to the decrease in charge carrier concentration. Whereas Se substitution resulted in the decrease of lattice thermal conductivity due to additional phonon scattering caused by the S-Se mass difference. Simultaneous optimization of the power factor and thermal conductivity could thus be achieved via double substitution at Cu and S sites. A maximum thermoelectric figure of merit (zT) of 0.86 at 673 K was exhibited by the Cu11Zn1Sb4S12.75Se0.25 sample due to its relatively high power factor among the samples (0.9 mW m-1 K-2 at 673 K) coupled with very low total thermal conductivity (0.67 W m-1 K-1 at 673 K).

Thermoelectric properties of a Mn substituted synthetic tetrahedrite.

Tetrahedrite compounds Cu(12-x)Mn(x)Sb4S13 (0 ≤x≤ 1.8) were prepared by solid state synthesis. A detailed crystal structure analysis of Cu10.6Mn1.4Sb4S13 was performed by single crystal X-ray diffraction (XRD) at 100, 200 and 300 K confirming the noncentrosymmetric structure (space group I4[combining macron]3m) of a tetrahedrite. The large atomic displacement parameter of the Cu2 atoms was described by splitting the 12e site into a partially and randomly occupied 24g site (Cu22) in addition to the regular 12e site (Cu21), suggesting a mix of dynamic and static off-plane Cu2 atom disorder. Rietveld powder XRD pattern and electron probe microanalysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn(2+) at the Cu(1+) site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit ZT decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum ZT = 0.76 at 623 K is obtained for Cu12Sb4S13. The coefficient of thermal expansion 13.5 ± 0.1 × 10(-6) K(-1) is obtained in the temperature range from 460 K to 670 K for Cu10.2Mn1.8Sb4S13. The Debye temperature, Θ(D) = 244 K for Cu10.2Mn1.8Sb4S13, was estimated from an evaluation of the elastic properties. The effective paramagnetic moment 7.45 µB/f.u. for Cu10.2Mn1.8Sb4S13 is fairly consistent with a high spin 3d(5) ground state of Mn.

Charge transfer engineering to achieve extraordinary power generation in GeTe-based thermoelectric materials.

By the fine manipulation of the exceptional long-range germanium-telluride (Ge─Te) bonding through charge transfer engineering, we have achieved exceptional thermoelectric (TE) and mechanical properties in lead-free GeTe. This chemical bonding mechanism along with a semiordered zigzag nanostructure generates a notable increase of the average zT to a record value of ~1.73 in the temperature range of 323 to 773 K with ultrahigh maximum zT ~ 2.7. In addition, we significantly enhanced the Vickers microhardness numbers (Hv) to an extraordinarily high value of 247 Hv and effectively eliminated the thermal expansion fluctuation at the phase transition, which was problematic for application, by the present charge transfer engineering process and concomitant formation of microstructures. We further fabricated a single-leg TE generator and obtained a conversion efficiency of ~13.4% at the temperature difference of 463 K on a commercial instrument, which is located at the pinnacle of TE conversion.

Investigation on the structure and thermoelectric properties of CuxTe binary compounds.

Cu2Te is a superionic conductor that belongs to the Phonon Liquid Electron Crystal class of thermoelectric (TE) materials. Despite the simple chemical formula, the crystal structures and phases in the Cu2Te system have not been understood properly. In this work, we study the structural and TE properties of Cu2Te (CT2), Cu1.6Te (CT1.6) and Cu1.25Te (CT1.25). The samples were synthesized via a solid-state reaction method. Powder X-ray diffraction analysis revealed that the samples have different crystal structures depending upon the Cu : Te stoichiometry. The elemental compositional analysis showed that all the samples are copper deficient. This is due to the precipitation of metallic copper on the surface of the ingot arising from the thermal dissociation of Cu2Te. The transport properties were measured in the temperature range 300 K-600 K. The electrical conductivity (σ) decreases with an increase in temperature indicating a metal-like behaviour for all the samples. The positive Seebeck coefficients (S) for all the samples indicates that majority charge carriers are holes. The sample CT2 has a higher S (29.5 µV K-1 at 573 K) and a lower σ (2513 S cm-1 at 573 K) due to a lower carrier (hole) concentration compared to the other two samples. With the increase in Cu deficiency, the hole concentration increases, and this leads to higher electronic thermal conductivity in the samples CT1.6 and CT1.25. The maximum thermoelectric figure of merit of 0.03 at 524 K is achieved for the sample CT2 owing to its higher power factor (0.24 mW m-1 K-2) and lower thermal conductivity (3.8 W m-1 K-1). The present study bridges the gap between the theoretical predictions and experimental observations involving the various possible structures in this system. Furthermore, we have shown that the Cu vacancies are detrimental to the thermoelectric performance of Cu2Te.

High thermoelectric performance in low-cost SnS0.91Se0.09 crystals.

Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (µ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (µW cm-1 K-2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZT max) of ~1.6 at 873 K and an average ZT (ZT ave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.