We report the evaluation of the thermoelectric performance of polycrystalline p-type SnSe, a material in which unprecedented values of the thermoelectric figure of merit ZT have been recently discovered in single crystals Besides anisotropic transport properties, our results confirm that this compound exhibits intrinsically very low thermal conductivity values The electrical properties show trends typical of lightly doped, intrinsic semiconductors with thermopower values reaching 500 μV K−1 in a broad temperature range An orthorhombic-to-orthorhombic transition sets in at 823 K, a temperature at which the power factor reaches its maximum value A maximum ZT of 05 was obtained at 823 K, suggesting that proper optimization of the transport properties of SnSe might lead to higher ZT values These findings indicate that this system represents an interesting experimental platform for the search of highly efficient thermoelectric materials

Assessment of the thermoelectric performance of polycrystalline p-type SnSe

SnSe has attracted some attention due to the recent report of record high thermoelectric figure-of-merit (ZT) of ∼2.6 at about 923 K in single crystals. Even though the ZT is high in single crystal SnSe, it was very low in its polycrystalline form due to the very low electrical conductivity (<103 S m−1) from 300 K to 700 K. In this work, we report studies on enhancement of electrical conductivity by Na doping to optimize the carrier concentration in polycrystalline SnSe prepared by melting and hot pressing. A room temperature carrier concentration of ∼2.7 × 1019 cm−3 was obtained in 2 atm% Na-doped SnSe samples with the highest power factor obtained in 1.5 atm% Na-doping. A peak ZT of ∼0.8 was achieved at 773 K along the hot pressing direction and the average ZT was improved.

Studies on thermoelectric figure of merit of Na-doped p-type polycrystalline SnSe

The ability of some materials with a perfectly ordered crystal structure to mimic the heat conduction of amorphous solids is a remarkable physical property that finds applications in numerous areas of materials science, for example, in the search for more efficient thermoelectric materials that enable to directly convert heat into electricity. Here, we unveil the mechanism in which glass-like thermal conductivity emerges in tetrahedrites, a family of natural minerals extensively studied in geology and, more recently, in thermoelectricity. By investigating the lattice dynamics of two tetrahedrites of very close compositions (Cu12Sb2Te2S13 and Cu10Te4S13) but with opposite glasslike and crystal thermal transport by means of powder and single-crystal inelastic neutron scattering, we demonstrate that the former originates from the peculiar chemical environment of the copper atoms giving rise to a strongly anharmonic excess of vibrational states.

From crystal to glass-like thermal conductivity in crystalline minerals

Ultralow thermal conductivity is of great interest in a variety of fields, including thermoelectric energy conversion. We report, for the first time, experimental evidence that Ga-doping in SnTe may lower the lattice thermal conduction slightly below the theoretical amorphous minimum at high temperature. Such an effect is justified by the spontaneous formation of nanoprecipitates we characterized as GaTe. Remarkably, the introduction of Ga (2–10%) in SnTe also improves the electronic transport properties by activating several hole pockets in the multivalley valence band. Experimental results are supported by density functional theory calculations. The thermoelectric figure of merit, ZT, reaches ∼1 at 873 K in Sn0.96Ga0.07Te, which corresponds to an ∼80% improvement with respect to pure SnTe.

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Ultralow Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in SnTe:Ga Materials

Polycrystalline samples of the tetrahedrite phase Cu12Sb4−xTexS13 with nominal compositions 0.5 ≤ x ≤ 2.0 were synthesized by two different synthesis routes: from precursors and from direct melting of elements. The crystal structure was verified by single-crystal and powder X-ray diffraction (PXRD), both confirming the successful substitution of Te for Sb in both series. Our chemical analyses evidenced differences between the chemical compositions of the two series of samples likely tied to the synthesis method employed and suggesting off-stoichiometry on the Sb site. High-temperature PXRD and differential scanning calorimetry measurements indicate that these materials are stable up to 623 K. Above this temperature, the decomposition process starts and ends up near 748 K where a Cu2−yS-type phase is solely observed. In agreement with the simple electron counting rule and electronic band structure calculations, the electrical resistivity and thermopower increase with increasing x reflecting the gradual shift from a p-type metallic state (x = 0.0) to a p-type semiconducting behavior (x = 2.0). Combined with extremely low lattice thermal conductivity values (κ ≈ 0.5 W m−1 K−1 at 623 K), this substitution enables us to optimize the power factor leading to a maximum thermoelectric figure of merit ZT of about 0.8 at 623 K. These results parallel those obtained in prior studies dealing with partial substitutions on the Cu site and enlarge the possibilities to tune the electrical properties of tetrahedrites by extrinsic dopants.

Crystal structure, electronic band structure and high-temperature thermoelectric properties of Te-substituted tetrahedrites Cu12Sb4−xTexS13 (0.5 ≤ x ≤ 2.0)

Polycrystalline samples and single crystals of the new compound Ag3In2Mo15Se19 were synthesized by solid-state reaction in a sealed molybdenum crucible at 1300 °C. Its crystal structure (space group R3c, a = 9.9755(1) A, c = 57.2943(9) A, and Z = 6) was determined from single-crystal X-ray diffraction data and constitutes an Ag-filled variant of the In2Mo15Se19 structure-type containing octahedral Mo6 and bioctahedral Mo9 clusters in a 1:1 ratio. The increase of the cationic charge transfer due to the Ag insertion induces a modification of the Mo–Mo distances within the Mo clusters that is discussed with regard to the electronic structure. Transport properties were measured in a broad temperature range (2–1000 K) to assess the thermoelectric potential of this compound. The transport data indicate an electrical conduction dominated by electrons below 25 K and by holes above this temperature. The metallic character of the transport properties in this material is consistent with electronic band structure ca...

Tong Zhou

Papers

Synthesis, Crystal and Electronic Structures, and Thermoelectric Properties of the Novel Cluster Compound Ag3In2Mo15Se19