Nanostructuring of thermoelectric Mg(2) Si via a nonequilibrium intermediate state.

A new route to self-assembled nanocomposite thermoelectric materials is proposed. High-energy mechanical alloying brings materials into a nonequilibrium intermediate state, such as a solid solution with an extended solubility. The large driving force for the transformation to the equilibrium state leads to nanometer-scale microstructure formation, which is ideal for reducing lattice thermal conductivity.

Pore Formation and Shape Control Simulation of Lotus Aluminum by the Phase-Field Method.

We have simulated pore formation and shape control of lotus aluminum by the phase-field method. The simulated material, lotus aluminum, contains anisotropic internal pores, and it is produced by the continuous casting method in a hydrogen atmosphere. Since it is known experimentally that the pore shape of lotus aluminum changes with the pull-out speed, the simulation varied the movement speed of the temperature gradient zone (equivalent to the pull-out speed in the continuous casting method) by proportional differential (PD) control with the pore width as the target value. As a result, a simple PD control ensured the pores closed during the growth process. To keep the pore growth linear, we found that a lower limit of the interface temperature should be set and the temperature gradient zone should be stopped below this lower limit. However, a problem occurred in the pore shape. To mitigate necking of the pore, PD control was done only when the pore width became larger than the target value under the conditions such that the pore expanded easily (i.e., the pull-out movement was stopped for a certain time immediately after nucleation and the initial speed of the temperature gradient zone was decreased). Then, we found the best condition to achieve linear pore growth without necking. Under the same condition, we simulated multiple pore growths by allowing multiple nucleations. As a result, we observed that although the shape control was applied only to a certain single pore, the other pores also grew linearly if the timing of their nucleation was close to that of the target pore.

Fe-Al-Si Thermoelectric (FAST) Materials and Modules: Diffusion Couple and Machine-Learning-Assisted Materials Development.

To lower the introduction and maintenance costs of autonomous power supplies for driving Internet-of-things (IoT) devices, we have developed low-cost Fe-Al-Si-based thermoelectric (FAST) materials and power generation modules. Our development approach combines computational science, experiments, mapping measurements, and machine learning (ML). FAST materials have a good balance of mechanical properties and excellent chemical stability, superior to that of conventional Bi-Te-based materials. However, it remains challenging to enhance the power factor (PF) and lower the thermal conductivity of FAST materials to develop reliable power generation devices. This forum paper describes the current status of materials development based on experiments and ML with limited data, together with power generation module fabrication related to FAST materials with a view to commercialization. Combining bulk combinatorial methods with diffusion couple and mapping measurements could accelerate the search to enhance PF for FAST materials. We report that ML prediction is a powerful tool for finding unexpected off-stoichiometric compositions of the Fe-Al-Si system and dopant concentrations of a fourth element to enhance the PF, i.e., Co substitution for Fe atoms in FAST materials.

Earth-Abundant Fe-Al-Si Thermoelectric (FAST) Materials: from Fundamental Materials Research to Module Development.

To develop an autonomous power generation technology to support an internet-of-things (IoT) society, we proposed low-cost and nontoxic Fe-Al-Si-based thermoelectric (FAST) materials consisting of an Fe3Al2Si3 phase. Because of the cost-effectiveness and easy disposal of FAST materials, they are attractive for autonomous power supplies to drive IoT sensors and devices. While bismuth-tellurium-based thermoelectric power generation modules have been commercialized, the discovery of FAST materials opens an additional route to generate power from waste heat at room temperature under a small temperature difference, which will expand the diversity of applications of thermoelectric power generation modules. This paper reports the thermoelectric properties of FAST materials synthesized by conventional laboratory-scale synthesis and mass production processes, enhancement of power factor less than 600 K through homogenization and removal of metallic precipitations, development of thermoelectric power generation modules, and the results of power generation tests. The operation of temperature/humidity sensors and wireless transmission by Bluetooth low energy communication using FAST materials-based modules under a small temperature difference at room temperature was demonstrated.

Synthesis, structure, and high-temperature thermoelectric properties of boron-doped Ba8Al14Si31 clathrate I phases.

Single crystals of boron-doped Ba8Al14Si31 clathrate I phase were prepared using Al flux growth. The structure and elemental composition of the samples were characterized by single-crystal and powder X-ray diffraction; elemental analysis; and multinuclear (27)Al, (11)B, and (29)Si solid-state NMR. The samples' compositions of Ba8B0.17Al14Si31, Ba8B0.19Al15Si31, and Ba8B0.32Al14Si31 were consistent with the framework-deficient clathrate I structure Ba8Al(x)Si(42-3/4x)cube(4-1/4x) (x = 14, cube = lattice defect). Solid-state NMR provides further evidence for boron doped into the framework structure. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates that transport processes are dominated by electrons. Thermal conductivity is low, but not significantly lower than that observed in the undoped Ba8Al14Si31 prepared in the same manner.

Rapid consolidation of powdered materials by induction hot pressing.

A rapid hot press system in which the heat is supplied by RF induction to rapidly consolidate thermoelectric materials is described. Use of RF induction heating enables rapid heating and consolidation of powdered materials over a wide temperature range. Such rapid consolidation in nanomaterials is typically performed by spark plasma sintering (SPS) which can be much more expensive. Details of the system design, instrumentation, and performance using a thermoelectric material as an example are reported. The Seebeck coefficient, electrical resistivity, and thermal diffusivity of thermoelectric PbTe material pressed at an optimized temperature and time in this system are shown to agree with material consolidated under typical consolidation parameters.