Kunpeng Zhao

Multiformity and fluctuation of Cu ordering in Cu2Se thermoelectric materials

Cuprous selenide (Cu2Se) has recently shown a very high dimensionless thermoelectric figure of merit zT as well as a dramatic increase in thermoelectric performance during the critical second-order phase transition. The present study indicates that the ultrahigh thermoelectric performance arises from its specific structural features involving multiformity of Cu ordering and drastic structural fluctuation during phase transition. The Cu2Se sample consists of domains of different ordered lamellar structures of Cu atoms which are coherently immersed in the long-range ordered Se pseudo-fcc framework. The specific self-independent binary-sublattice structures have been found to enhance phonon scattering while still guaranteeing good carrier mobilities. Upon increasing the temperature to near phase transition, the multiple structures undergo intense and stepwise changes including appearance of new ordered structures for copper, disordering and diffusion of Cu atoms across the interlayers, and finally random distribution of Cu in the Se cubic sublattice which alters a little during the phase transition. Such extreme structural fluctuation results in critical electron and phonon scatterings that expedite an exceptional enhancement of thermoelectric performance.

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.

Crystal structure across the β to α phase transition in thermoelectric Cu2−xSe

The average structure of β-Cu2−xSe is reported based on analysis of multi-temperature single-crystal X-ray diffraction data, and structural changes, including a large negative thermal expansion, across the β to α phase transition are discussed. The structural model also describes well high-resolution synchrotron powder X-ray diffraction data.

Extremely low thermal conductivity and high thermoelectric performance in liquid-like Cu2Se1−xSx polymorphic materials

Recently, copper chalcogenides Cu2−xδ (δ = S, Se, Te) have attracted great attention due to their exceptional thermal and electrical transport properties. Besides these binary Cu2−xδ compounds, the ternary Cu2−xδ solid solutions are also expected to possess excellent thermoelectric performance. In this study, we have synthesized a series of Cu2Se1−xSx (x = 0.2, 0.3, 0.5, and 0.7) solid solutions by melting the raw elements followed by spark plasma sintering. The energy dispersive spectroscopy mapping, powder and single-crystal X-ray diffraction and X-ray powder diffraction studies suggest that Cu2Se and Cu2S can form a continuous solid solution in the entire composition range. These Cu2Se1−xSx solid solutions are polymorphic materials composed of varied phases with different proportions at room temperature, but single phase materials at elevated temperature. Increasing the sulfur content in Cu2Se1−xSx solid solutions can greatly reduce the carrier concentration, leading to much enhanced electrical resistivity and Seebeck coefficients in the whole temperature range as compared with those in binary Cu2Se. In particular, introducing sulfur at Se-sites reduces the speed of sound. Combining the strengthened point defect scattering of phonons, extremely low lattice thermal conductivities are obtained in these solid solutions. Finally, a maximum zT value of 1.65 at 950 K is achieved for Cu2Se0.8S0.2, which is greater than those of Cu2Se and Cu2S.

Thermoelectric properties of Cu2Se1−xTex solid solutions

Binary Cu2Se and Cu2Te have gained great attention recently because of their interesting and abnormal physical properties, such as ultralow thermal conductivity, high carrier mobility, large effective mass of carriers and excellent thermoelectric performance. In this study, we find that these two compounds are completely miscible throughout the studied composition range. The trigonal structure of Cu2Se is maintained when the Te content x is 0.2, but a new trigonal structure is formed when the Te content x is between 0.3 and 0.7. The carrier concentration is greatly improved when increasing the Te content in Cu2Se1−xTex solid solutions, resulting in a much reduced electrical resistivity and Seebeck coefficient in the whole temperature range as compared with those of binary Cu2Se. The total thermal conductivity is inversely increased due to the contribution from enhanced carrier thermal conductivity. As a result, the overall thermoelectric performance of Cu2Se1−xTex solid solutions lies between Cu2Se and Cu2Te. We also find that the quality factor of Cu2Se1−xTex is higher than those of most typical thermoelectric materials. Thus the thermoelectric performance can be further improved if the intrinsically high hole carrier concentrations can be reduced in Cu2Se1−xTex.

Self-propagation high-temperature synthesis of half-Heusler thermoelectric materials: reaction mechanism and applicability

Combining excellent thermoelectric and mechanical properties, half-Heusler (HH) materials have been considered as one of the most promising candidates for thermoelectric applications in the high-temperature range. In this work, we apply the self-propagation high-temperature synthesis (SHS), a facile and scalable method, to HH thermoelectric materials. Comparing three families of HH materials, we show that MNiSn and MCoSb (M: Ti, Zr, Hf) can be successfully obtained by the SHS, but RFeSb (R: Nb, V) cannot. Our first-principles calculation suggests that the failure of SHS for RFeSb is because the formation enthalpy of these compounds is too low to overcome the reaction barrier. Our experiment also reveals a two-step reaction mechanism for the SHS of ternary HH compounds, i.e., two elements of lower melting points react first to form binary intermediate products and then the binaries react with the element of the highest melting point, where the second step contributes most of the heat released in the SHS, which is supported by our first-principles calculation. Finally, we optimize the HH materials through doping and show that the thermoelectric properties of SHS-synthesized HH compounds are comparable or even superior to those synthesized by laboratory-scale methods.

Improved Thermoelectric Performance in Nonstoichiometric Cu2+δMn1−δSnSe4 Quaternary Diamondlike Compounds

A novel quaternary Cu2MnSnSe4 diamondlike thermoelectric material was discovered recently based on the pseudocubic structure engineering. In this study, we show that introducing off-stoichiometry in Cu2MnSnSe4 effectively enhances its thermoelectric performance by simultaneously optimizing the carrier concentrations and suppressing the lattice thermal conductivity. A series of nonstoichiometric Cu2+δMn1-δSnSe4 (δ = 0, 0.025, 0.05, 0.075, and 0.1) samples has been prepared by the melting-annealing method. The X-ray analysis and the scanning electron microscopy measurement show that all nonstoichiometric samples are phase pure. The Rietveld refinement demonstrates that substituting part of Mn by Cu well maintains the structure distortion parameter η close to 1, but it induces obvious local distortions inside the anion-centered tetrahedrons. Significantly improved carrier concentrations are observed in these nonstoichiometric Cu2+δMn1-δSnSe4 samples, pushing the power factors to the theoretical maximal value predicted by the single parabolic model. Substituting part of Mn by Cu also reduces the lattice thermal conductivity, which is well interpreted by the Callaway model. Finally, a maximal thermoelectric dimensionless figure-of-merit zT around 0.60 at 800 K has been obtained in Cu2.1Mn0.9SnSe4, which is about 33% higher than that in the Cu2MnSnSe4 matrix compound.