High-Performance and Stable (Ag, Cd)-Containing ZnSb Thermoelectric Compounds

Binary Zn-Sb-based compounds, ZnSb and Zn4Sb3, are promising thermoelectric (TE) materials because they are low-cost and earth-abundant. However, for a long time, their real applications have been limited by the low TE figure-of-merit (zT) of ZnSb and poor thermodynamic stability of Zn4Sb3. In this study, we successfully integrate both high zT and good stability in (Ag, Cd)-containing ZnSb compounds. Alloying Cd in ZnSb greatly suppresses the lattice thermal conductivity to a minimum value of 0.97 W K-1 m-1 at 300 K, while doping Ag significantly enhances the power factor to a peak value of 17.7 μW cm-1 K-2 at 500 K and reduces the bipolar thermal conductivity. As a result of the simultaneously optimized electrical and thermal transport, a peak zT of 1.2 is achieved for Zn0.698Ag0.002Cd0.3Sb at 600 K, which is comparable with the best values reported for Zn4Sb3-based compounds. Moreover, a current stress test confirms that introducing Ag and Cd does not degrade the good stability of ZnSb under an electric field. The phase composition and thermoelectric performance of Zn0.698Ag0.002Cd0.3Sb are not changed even under a high current density of 50 A cm-2, showing much better stability than Zn4Sb3. This study would accelerate the real application of ZnSb-based compounds in the field of waste heat harvesting.

Fast ion transport for synthesis and stabilization of β-Zn4Sb3

Mobile ion-enabled phenomena make β-Zn₄Sb₃ a promising material in terms of the re-entry phase instability behavior, mixed electronic ionic conduction, and thermoelectric performance. Here, we utilize the fast Zn²⁺ migration under a sawtooth waveform electric field and a dynamical growth of 3-dimensional ionic conduction network to achieve ultra-fast synthesis of β-Zn₄Sb₃. Moreover, the interplay between the mobile ions, electric field, and temperature field gives rise to exquisite core-shell crystalline-amorphous microstructures that self-adaptively stabilize β-Zn₄Sb₃. Doping Cd or Ge on the Zn site as steric hindrance further stabilizes β-Zn₄Sb₃ by restricting long-range Zn²⁺ migration and extends the operation temperature range of high thermoelectric performance. These results provide insight into the development of mixed-conduction thermoelectric materials, batteries, and other functional materials.

β-Zn₄Sb₃ has promising thermoelectric performance, but its ionic migration properties make it prone to degradation. Here the authors exploit the ion migration in an electric field-assisted synthesis method, fast producing β-Zn₄Sb₃ with improved phase stability and extended temperature range for the thermoelectric operation.

Thermodynamic Routes to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) research is not only a course of materials by discovery but also a seedbed of novel concepts and methodologies. Herein, the focus is on recent advances in three emerging paradigms: entropy engineering, phase-boundary mapping, and liquid-like TE materials in the context of thermodynamic routes. Specifically, entropy engineering is underpinned by the core effects of high-entropy alloys; the extended solubility limit, the tendency to form a high-symmetry crystal structure, severe lattice distortions, and sluggish diffusion processes afford large phase space for performance optimization, high electronic-band degeneracy, rich multiscale microstructures, and low lattice thermal conductivity toward higher-performance TE materials. Entropy engineering is successfully implemented in half-Huesler and IV-VI compounds. In Zintl phases and skutterudites, the efficacy of phase-boundary mapping is demonstrated through unraveling the profound relations among chemical compositions, mutual solubilities of constituent elements, phase instability, microstructures, and resulting TE properties at the operation temperatures. Attention is also given to liquid-like TE materials that exhibit lattice thermal conductivity at lower than the amorphous limit due to intensive mobile ion disorder and reduced vibrational entropy. To conclude, an outlook on the development of next-generation TE materials in line with these thermodynamic routes is given.

Recent advances in inorganic material thermoelectrics

Time line of representative inorganic bulk thermoelectric materials from 1960s to the present.

High-pressure single crystal X-ray diffraction study of thermoelectric ZnSb and β-Zn4Sb3

The crystal structures of thermoelectric ZnSb and Zn₄Sb₃ have been studied by high pressure single crystal X-ray diffraction and the pressure behavior is different from thermal response.

Graphene boosts thermoelectric performance of a Zintl phase compound

The nanocomposite of Mg₃Sb₂-based Zintl phase compound, fabricated by utilizing, GNS, as a nanocomposite additive, enhances significantly the thermoelectric performance.

Recent advances in thermoelectric materials and solar thermoelectric generators – a critical review

Thermoelectric materials have been extensively used in space satellites, automobiles, and, more recently, in solar thermal application as power generators. Solar thermoelectric generators (STEGs) have enjoyed rapidly improving efficiency in recent years in both concentrated and non-concentrated systems. However, there is still a critical need for further research and development of their materials and systems design before this technology can deployed for large-scale power generation.

Metal nanoparticle decorated n-type Bi₂Te₃-based materials with enhanced thermoelectric performances

In this study, n-type Cu and Zn metal nanoparticle decorated Bi₂(Te₀.₉Se₀.₁)₃ ingots were prepared by a large-scale zone melting technique, with the concept of 'nanoparticle-in-alloy' to separately tune the electrical and thermal transport properties. Cu and Zn additions play multiple but different roles in the materials, whereas both of them form metal nanoinclusions embedded in van der Waals gaps or grain boundaries, exerting influences on thermoelectric properties. Cu addition, accommodated in the tetrahedral vacancies formed by four Te(1) atoms, effectively adjusts the electron concentration by donating its valence electron, and appreciably optimizes the power factor. Coupled with the significant frustration of heat-carrying phonons by Cu nanoinclusions, a highest ZT of 1.15 can be achieved for the 1 at.% Cu sample, which is an ∼20% improvement compared with that of commercial halogen-doped ingots. Zn addition, however, acting as weak donor, noticeably increases the density of state effective mass and Seebeck coefficient, and gives rise to a high ZT of 1.1. In particular, the kilogram-grade production technique coupled with the high ZT makes metal nanoparticle decorated n-type materials very promising for commercial applications.