Yonggao Yan

Unique nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys

We report a melt spinning technique followed by a quick spark plasma sintering procedure to fabricate high-performance p-type Bi0.52Sb1.48Te3 bulk material with unique microstructures. The microstructures consist of nanocrystalline domains embedded in amorphous matrix and 5–15 nm nanocrystals with coherent grain boundary. The significantly reduced thermal conductivity leads to a state-of-the-art dimensionless figure of merit ZT∼1.56 at 300 K, more than 50% improvement of that of the commercial Bi2Te3 ingot materials.

High thermoelectric performance BiSbTe alloy with unique low-dimensional structure

We report a detailed description of an innovative route of a melt spinning (MS) technique combined with a subsequent spark plasma sintering process in order to obtain high performance p-type Bi0.52Sb1.48Te3 bulk material, which possesses a unique low-dimensional structure. The unique structure consists of an amorphous structure, 5–15 nm fine nanocrystalline regions, and coherent interfaces between the resulting nanocrystalline regions. Measurements of the thermopower, electrical conductivity, and thermal conductivity have been performed over a range of temperature of 300–400 K. We found that MS technique can give us considerable control over the resulting nanostructure with good thermal stability during the temperature range of 300–400 K and this unique structure can effectively adjust the transport of phonons and electrons, in a manner such that it is beneficial to the overall thermoelectric performance of the material, primarily a reduction in the lattice thermal conductivity. Subsequently, this results...

Self-propagating high-temperature synthesis for compound thermoelectrics and new criterion for combustion processing.

The existing methods of synthesis of thermoelectric (TE) materials remain constrained to multi-step processes that are time and energy intensive. Here we demonstrate that essentially all compound thermoelectrics can be synthesized in a single-phase form at a minimal cost and on the timescale of seconds using a combustion process called self-propagating high-temperature synthesis. We illustrate this method on Cu2Se and summarize key reaction parameters for other materials. We propose a new empirically based criterion for sustainability of the combustion reaction, where the adiabatic temperature that represents the maximum temperature to which the reacting compact is raised as the combustion wave passes through, must be high enough to melt the lower melting point component. Our work opens a new avenue for ultra-fast, low-cost, large-scale production of TE materials, and provides new insights into combustion process, which greatly broaden the scope of materials that can be successfully synthesized by this technique.

Multi-Scale Microstructural Thermoelectric Materials: Transport Behavior, Non-Equilibrium Preparation, and Applications

Considering only about one third of the worlds energy consumption is effectively utilized for functional uses, and the remaining is dissipated as waste heat, thermoelectric (TE) materials, which offer a direct and clean thermal-to-electric conversion pathway, have generated a tremendous worldwide interest. The last two decades have witnessed a remarkable development in TE materials. This Review summarizes the efforts devoted to the study of non-equilibrium synthesis of TE materials with multi-scale structures, their transport behavior, and areas of applications. Studies that work towards the ultimate goal of developing highly efficient TE materials possessing multi-scale architectures are highlighted, encompassing the optimization of TE performance via engineering the structures with different dimensional aspects spanning from the atomic and molecular scales, to nanometer sizes, and to the mesoscale. In consideration of the practical applications of high-performance TE materials, the non-equilibrium approaches offer a fast and controllable fabrication of multi-scale microstructures, and their scale up to industrial-size manufacturing is emphasized here. Finally, the design of two integrated power generating TE systems are described-a solar thermoelectric-photovoltaic hybrid system and a vehicle waste heat harvesting system-that represent perhaps the most important applications of thermoelectricity in the energy conversion area.

Ultra-fast synthesis and thermoelectric properties of Te doped skutterudites

The self-propagating-high-temperature-synthesis (SHS) technique is applied here for the first time to synthesize CoSb3 thermoelectric materials. Mixtures of Co and Sb powders were compacted into pellets which were ignited from one end. A single-phase skutterudite material was obtained in a very short period of time using the SHS process which is maintained by the heat released from the chemical reaction of Co with Sb. Thermodynamic parameters and kinetics of the SHS reaction are investigated. The ignition temperature, adiabatic temperature, and the propagation speed of the combustion wave in the synthesis of CoSb3 are 723 K, 861 K, and 1.25 mm s−1, respectively. Using the SHS technique followed by Plasma Activated Sintering (PAS), we synthesized high performance bulk skutterudites of composition CoSb2.85 Te0.15 with a ZT of 0.98 at 820 K, one of the highest ZT values for an unfilled form of skutterudites. Compared with the samples synthesized by the traditional methods, the synthesis time is shortened from the typical several days to less than 20 minutes. Our work opens a new avenue for ultra-fast, low cost, mass production fabrication of skutterudite-based materials, which may also be universally applicable for the synthesis of other thermoelectric materials.

High thermoelectric performance of mechanically robust n-type Bi2Te3−xSex prepared by combustion synthesis

The traditional zone melting (ZM) method for fabricating Bi2Te3-based thermoelectric materials has long been considered a time and energy intensive process. Here, a combustion synthesis called the self-propagating high-temperature synthesis (SHS) is employed to synthesize Bi2Te3-based thermoelectric materials. Thermodynamic and kinetic parameters of the SHS process relevant to Bi2Te3 and Bi2Se3 were systematically studied for the first time. SHS combined with plasma activated sintering (PAS) results in a single-phase homogeneous material with precisely controlled composition, no preferential orientation, high thermoelectric performance, and excellent mechanical properties. The technologically relevant average ZT value of SHS–PAS Bi2Te2.4Se0.6 from 298 to 523 K is 0.84, which is an increase of about 25% compared with the ZM sample. The compressive strength and the bending strength of SHS–PAS Bi2Te2.4Se0.6 are increased by nearly 250% and 30%, respectively, compared with those of the ZM samples, measured perpendicular to the c-axis. Moreover, the SHS–PAS process is very fast and shortens the synthesis time from tens of hours to 20 min. On account of the simplicity of the process, short synthesis time, minimal use of energy, and the scalability of the method, SHS–PAS technology provides a new and efficient method for large-scale, economical fabrication of Bi2Te3-based compounds.

Rapid preparation of CeFe4Sb12 skutterudite by melt spinning: rich nanostructures and high thermoelectric performance

In this work, we adopt a non-equilibrium melt spinning technique combined with a subsequent spark plasma sintering technique to successfully synthesize a p-type nanostructured CeFe4Sb12 skutterudite compound with high homogeneity in less than 24 hours. Microstructures of the melt-spun ribbons and the sintered bulk material are systematically investigated. The evolution of multiple-phase melt-spun ribbons into a single-phase skutterudite compound during the heating process is also carefully examined. Greatly refined matrix grains (300–500 nm) and numerous FeSb2 nanodots with sizes below 50 nm are evenly distributed inside the grains, and together contribute to the experimentally observed low lattice thermal conductivity of the sintered bulk material. Both absolute and average ZT values of this melt-spun skutterudite are about 10% higher than in the material of the same composition prepared by traditional melting and long-term annealing. The markedly shortened preparation time coupled with the enhanced thermoelectric performance should make this synthesis process of interest for commercial applications.

Phase Segregation and Superior Thermoelectric Properties of Mg2Si1–xSbx (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.

Low effective mass and carrier concentration optimization for high performance p-type Mg2(1-x)Li2xSi0.3Sn0.7 solid solutions.

Mg2Si1-xSnx solid solutions are promising thermoelectric materials for power generation applications in the 500-800 K range. Outstanding n-type forms of these solid solutions have been developed in the past few years with the thermoelectric figure of merit ZT as high as 1.4. Unfortunately, no comparable performance has been achieved so far with p-type forms of the structure. In this work, we use Li doping on Mg sites in an attempt to enhance and control the concentration of hole carriers. We show that Li as well as Ga is a far more effective p-type dopant in comparison to Na or K. With the increasing content of Li, the electrical conductivity rises rapidly on account of a significantly enhanced density of holes. While the Seebeck coefficient decreases concomitantly, the power factor retains robust values supported by a rather high mobility of holes. Theoretical calculations indicate that Mg2Si0.3Sn0.7 intrinsically possesses the almost convergent double valence band structure (the light and heavy band), and Li doping retains a low density of states (DOS) on the top of the valence band, contrary to the Ga doping at the sites of Si/Sn. Low temperature specific heat capacity studies attest to a low DOS effective mass in Li-doped samples and consequently their larger hole mobility. The overall effect is a large power factor of Li-doped solid solutions. Although the thermal conductivity increases as more Li is incorporated in the structure, the enhanced carrier density effectively shifts the onset of intrinsic excitations (bipolar effect) to higher temperatures, and the beneficial role of phonon Umklapp processes as the primary limiting factor to the lattice thermal conductivity is thus extended. The final outcome is the figure of merit ZT ∼ 0.5 at 750 K for x = 0.07. This represents a 30% improvement in the figure of merit of p-type Mg2Si1-xSnx solid solutions over the literature values. Hence, designing low DOS near Fermi level EF for given carrier pockets can serve as an effective approach to optimize the PF and thus ZT value.

Ultra-fast non-equilibrium synthesis and phase segregation in InxSn1−xTe thermoelectrics by SHS-PAS processing

The self-propagating-high-temperature-synthesis (SHS) in combination with plasma activated sintering (PAS) is applied for the first time to SnTe-based thermoelectric materials and produces single-phase structures. Thermodynamic and kinetic parameters of the SHS process relevant to SnTe compounds were determined. InTe is supersaturated in InxSn1−xTe during the non-equilibrium SHS process. After annealing, doping SnTe with In gives rise to phase separation and the formation of InTe nanoinclusions, which affect the carrier density and, in turn, the transport properties. The presence of the InTe nanophase dramatically reduces the lattice thermal conductivity as low frequency heat carrying phonons are strongly scattered. Moreover, the ensuing deficiency of Te in the SnTe matrix gives rise to Te vacancies which reduce the density of hole carriers and thus enhance the Seebeck coefficient. Compared to samples synthesized by the traditional methods, the SHS-PAS technique shortens the synthesis time from several days to merely 15 min which bodes well for low cost mass production of SnTe-based materials. The phase separation process observed here for the first time effectively adjusts both the microstructure and the carrier density of SnTe-based materials and offers a new approach to optimize their thermoelectric properties.