Unraveling the Critical Role of Melt-Spinning Atmosphere in Enhancing the Thermoelectric Performance of p-Type Bi0.52Sb1.48Te3 Alloys.

Melt spinning has proven effective in maintaining chemical homogeneity and introducing multiscale microstructures that can reduce the lattice thermal conductivity and consequently enhance the thermoelectric performance of consolidated bulk materials. In this work, p-type Bi0.52Sb1.48Te3 bulk alloys are fabricated by melt spinning (MS) followed by subsequent plasma activated sintering (PAS). The influence of different MS atmospheres (air, Ar, N2, and He) on the morphologies of MS ribbons and the thermoelectric properties of MS-PAS bulk materials has been investigated systematically. Because of the relatively high thermal conductivity, a He atmosphere expedites the heat dissipation in the MS process and results in severe sublimation of tellurium and thus inferior thermoelectric performance. In contrast, an Ar atmosphere can essentially prevent heat loss of the fusant and suppress the sublimation of tellurium. Consequently, the corresponding Bi0.52Sb1.48Te3 sample (MS in Ar atmosphere) presents the highest peak ZT and average ZT values of 1.09 (at 340 K) and 0.81 (in 300-500 K), respectively. The average ZT of the sample prepared using an Ar atmosphere is almost three times the one prepared using a He atmosphere. This reflects the importance of using the appropriate atmosphere during the melt-spinning process. This result, which indicates that melt spinning in an Ar atmosphere is preferable to avoid heat loss, can also be extended to other materials.

Compressive Fatigue Behavior and Its Influence on the Thermoelectric Properties of p-Type Bi0.5Sb1.5Te3 Alloys.

BiSbTe alloy is one of the most important thermoelectric materials that has been commercialized for large-scale applications in waste heat recovery and spot cooling. However, its practical application often involves complicated service conditions, such as substantial dynamic vibrational stresses as well as long-term exposure to the large thermal gradient that usually generates high thermal stress. Thus, it is of vital importance to investigate the mechanical response and the evolution of microstructure and thermoelectric performance of BiSbTe alloy under the quasi-static force and dynamic compressive stress. Herein, we elucidate the compressive fatigue behavior and its influence on the thermoelectric performance of p-type Bi0.5Sb1.5Te3 materials prepared by melt spinning and subsequent plasma-activated sintering. The fatigue life was tested at various stress ratios ranging from 60 to 90%. The microstructure evaluation indicates that repetitive loading of compressive stress contributes to largely accumulated strains and dislocations near grain boundaries. With the increasing cycling numbers, the magnitude of accumulated strains reaches the critical level and leads to microcrack initiation and propagation of fatigue cracks. The cyclic compressive stress resulted in the marked degradation of thermoelectric performance in Bi0.5Sb1.5Te3 material due to the strong suppression of carrier mobility by the fatigue-induced defects. This work can provide important guidance for the practical applications of p-type Bi0.5Sb1.5Te3 material.

Microstructure and Mechanical Behaviors of Titanium Matrix Composites Containing In Situ Whiskers Synthesized via Plasma Activated Sintering.

In this paper, titanium matrix composites with in situ TiB whiskers were synthesized by the plasma activated sintering technique; crystalline boron and amorphous boron were used as reactants for in situ reactions, respectively. The influence of the sintering process and the crystallography type of boron on the microstructure and mechanical properties of composites were studied and compared. The densities were evaluated using Archimedes' principle. The microstructure and mechanical properties were characterized by SEM, XRD, EBSD, TEM, a universal testing machine, and a Vickers hardness tester. The prepared composite material showed a high density and excellent comprehensive performance under the PAS condition of 20 MPa at 1000 °C for 3 min. Amorphous boron had a higher reaction efficiency than crystalline boron, and it completely reacted with the titanium matrix to generate TiB whiskers, while there was still a certain amount of residual crystalline boron combining well with the titanium matrix at 1100 °C. The composite samples with a relative density of 98.33%, Vickers hardness of 389.75 HV, compression yield strength of up to 1190 MPa, and an ultimate compressive strength of up to 1710 MPa were obtained. Compared with the matrix material, the compressive strength of TC4 titanium alloy containing crystalline boron and amorphous boron was increased by 7.64% and 15.50%, respectively.

Crystalline-Amorphous Silicon Nanocomposites with Reduced Thermal Conductivity for Bulk Thermoelectrics.

Responding to the need for thermoelectric materials with high efficiency in both conversion and cost, we developed a nanostructured bulk silicon thermoelectric materials by sintering silicon crystal quantum dots of several nanometers in diameters synthesized by plasma-enhanced chemical vapor deposition (PECVD). The material consists of hybrid structures of nanograins of crystalline silicon and amorphous silicon oxide. The percolated nanocrystalline region gives rise to high power factor with the high doping concentration realized by PECVD, and the binding amorphous region reduces thermal conductivity. Consequently, the nondimensional figure of merit reaches 0.39 at 600 °C, equivalent to the best reported value for silicon thermoelectrics. The thermal conductivity of the densely packed material is as low as 5 W m(-1) K(-1) in a wide temperature range from room temperature to 1000 °C, which is beneficial not only for the conversion efficiency but also for material cost by requiring less material to establish certain temperature gradient.