Tiansong Zhang

High thermoelectric performance in non-toxic earth-abundant copper sulfide

A new type of high performance thermoelectric material Cu_(2-x)S composed of non-toxic and earth-abundant elements Cu and S is reported. Cu_(2-x)S surprisingly has lower thermal conductivity and more strikingly reduced specific heat compared to the heavier Cu_(2)Se, leading to an increased zT to 1.7.

Ultrahigh Thermoelectric Performance in Mosaic Crystals

Successful research strategies to enhance the dimensionless figure of merit zT above 2 rely on either bulk nanomaterials or on single crystals. A new physical mechanism of nanoscale mosaicity is shown that goes beyond the approaches in single crystals or conventional nanomaterials. A zT value of 2.1 at 1000 K in bulk nanomaterials is achieved.

High efficiency Bi2Te3-based materials and devices for thermoelectric power generation between 100 and 300 °C

By suppressing intrinsic excitation in p-type Bi2Te3-based materials, we report maximum and average zT values of up to 1.4 and 1.2 between 100 and 300 °C, respectively. Thermoelectric modules based on these high performance materials show energy conversion efficiencies of up to 6.0% under a temperature gradient of 217 K, and are greatly superior to current Bi2Te3-based modules.

Thermoelectric transport properties of diamond-like Cu1−xFe1+xS2 tetrahedral compounds

Polycrystalline samples with the composition of Cu _(1−x)Fe_(1+x)S_2 (x = 0, 0.01, 0.03, 0.05, 0.1) were synthesized by a melting-annealing-sintering process. X-ray powder diffraction reveals all the samples are phase pure. The backscattered electron image and X-ray map indicate that all elements are distributed homogeneously in the matrix. The measurements of Hall coefficient, electrical conductivity, and Seebeck coefficient show that Fe is an effective n-type dopant in CuFeS_2. The electron carrier concentration of Cu_(1−x)Fe_(1+x)S_2 is tuned within a wide range leading to optimized power factors. The lattice phonons are also strongly scattered by the substitution of Fe for Cu, leading to reduced thermal conductivity. We use Debye approximation to model the low temperature lattice thermal conductivity. It is found that the large strain field fluctuation introduced by the disordered Fe ions generates extra strong phonon scatterings for lowered lattice thermal conductivity.

Thermoelectric transport of Se-rich Ag2Se in normal phases and phase transitions

Small amount of Se atoms are used to tune the carrier concentrations (nH) and electrical transport in Ag2Se. Significant enhancements in power factor and thermoelectric figure of merit (zT) are observed in the compositions of Ag2Se1.06 and Ag2Se1.08. The excessive Se atoms do not change the intrinsically electron-conducting character in Ag2Se. The detailed analysis reveals the experiment optimum carrier concentration in Ag2Se is around 5 × 1018 cm−3. We also investigate the temperature of maximum zT and the thermoelectric transport during the first order phase transitions using the recently developed measurement system.

Ultrahigh thermoelectric performance in Cu2Se-based hybrid materials with highly dispersed molecular CNTs

Here, by utilizing the special interaction between metal Cu and multi-walled carbon nanotubes (CNTs), we have successfully realized the in situ growth of Cu2Se on the surface of CNTs and then fabricated a series of Cu2Se/CNT hybrid materials. Due to the high degree of homogeneously dispersed molecular CNTs inside the Cu2Se matrix, a record-high thermoelectric figure of merit zT of 2.4 at 1000 K has been achieved.

Compound defects and thermoelectric properties in ternary CuAgSe-based materials

CuAgSe is a narrow band gap semiconducting material with superior carrier mobility and low lattice thermal conductivity, which are important and useful for high thermoelectric performance. However, its electrical and thermal transport properties are greatly affected by ionic deficiencies or compositional non-stoichiometry, which lead to a low thermoelectric figure of merit near room temperature. In this work, we systematically studied the compound defects in CuAgSe by tuning its starting chemical composition. We found that its phase purity is very sensitive to nominal chemical compositions. Only a small amount of Ag deficiency is allowed in CuAgSe to maintain phase purity, while the other non-stoichiometric compositions lead to impurity phases. Thermoelectric properties are weakly affected by these compound defects or impurity phases at 300 K, but greatly change at high temperatures. A single type carrier conduction is observed in CuAgSe, but a noticeable two-type carrier conduction is observed in the non-stoichiometric samples. This leads to an evident n to p conduction transition. Consequently, zT values in CuAgSe continuously increase to 0.6 at 450 K while the non-stoichiometric samples display considerably low values due to the contribution from both electrons and holes. The high zT value in n-type CuAgSe suggests that it is a promising thermoelectric material near room temperature.

Evaluating the potential for high thermoelectric efficiency of silver selenide

Measurements and modeling of electronic transport properties of n-type Ag2+xSe suggest that this material could have a thermoelectric figure of merit zT greater than 1 at 300 K and above. The exceptional performance can be traced to the exceptionally high mobility, higher than other optimized thermoelectric materials. Although zT decreases at high temperature because of a transition to a phase with high carrier concentration, our model indicates that reducing the carrier concentration will lead to high thermoelectric performance at room temperature for cooling applications as well as up to 600 K for waste heat recovery.

Optimized thermoelectric properties in pseudocubic diamond-like CuGaTe2 compounds

A pseudocubic structure approach has been proposed recently to screen and design good thermoelectric materials via realizing overlapped band edges for excellent electrical transport properties. A diamond-like compound is a typical example agreeing with the concept of the pseudocubic structure by tuning its lattice distortion parameter to unity. However, besides the band structure, optimized carrier concentration and reduced lattice thermal conductivity are also required for a high thermoelectric figure of merit (zT). In this work, taking CuGaTe2 as an example, we have successfully demonstrated that Cu-deficiency can effectively tune carrier concentrations and In-alloying at Ga sites can effectively lower lattice thermal conductivity. By combining these two strategies, the electrical and thermal transports can be separately optimized in CuGaTe2-based pseudocubic diamond-like compounds, leading to much enhanced zTs, about 24% improvement for Cu0.99In0.6Ga0.4Te2 at 800 K. Furthermore, the average zTs from 300 K to 800 K are improved by 87% compared with that of the CuGaTe2 matrix. This study provides a promising way to optimize the TE performance in pseudocubic diamond-like compounds by simultaneously tuning electrical and thermal transport.

Entropy as a Gene‐Like Performance Indicator Promoting Thermoelectric Materials

High-throughput explorations of novel thermoelectric materials based on the Materials Genome Initiative paradigm only focus on digging into the structure-property space using nonglobal indicators to design materials with tunable electrical and thermal transport properties. As the genomic units, following the biogene tradition, such indicators include localized crystal structural blocks in real space or band degeneracy at certain points in reciprocal space. However, this nonglobal approach does not consider how real materials differentiate from others. Here, this study successfully develops a strategy of using entropy as the global gene-like performance indicator that shows how multicomponent thermoelectric materials with high entropy can be designed via a high-throughput screening method. Optimizing entropy works as an effective guide to greatly improve the thermoelectric performance through either a significantly depressed lattice thermal conductivity down to its theoretical minimum value and/or via enhancing the crystal structure symmetry to yield large Seebeck coefficients. The entropy engineering using multicomponent crystal structures or other possible techniques provides a new avenue for an improvement of the thermoelectric performance beyond the current methods and approaches.