Synergistically Optimized Thermoelectric and Mechanical Properties of Mg3.2Bi1.5Sb0.5-SiC Composites.

Motivated by the surging demand for low-temperature waste heat harvesting, materials with both prominent thermoelectric and good mechanical properties are preferred in practical applications. In this present work, the composite exploration of Te-doped Mg3.2Bi1.5Sb0.5-x vol % nanosized SiC (x = 0, 0.05, 0.1, 0.2, and 0.5) was carried out, where nanosized SiC is physically dispersed in the matrix in the form of a second phase. SiC second phase compositing further optimized the matrix carrier concentration, resulting in a higher power factor in the service temperature range (the highest value from 28.9 to 31.7 µW cm-1 K-2), and the (ZT)ave from 0.91 to 0.96 compared with the matrix sample. In addition, the SiC second phase effectively enhanced the mechanical properties of composite materials, including flexural strength, microhardness, and modulus. Because of the simultaneous optimization of thermoelectric and mechanical properties, the overall performance of Te-doped Mg3.2Bi1.5Sb0.5-0.05 vol % SiC composite is leveraged to meet special requirements of power generation. It is expected that the addition of SiC should be broadly applicable to address the physical performance in other thermoelectric systems.

Performance boost for bismuth telluride thermoelectric generator via barrier layer based on low Young's modulus and particle sliding.

The lack of desirable diffusion barrier layers currently prohibits the long-term stable service of bismuth telluride thermoelectric devices in low-grade waste heat recovery. Here we propose a new design principle of barrier layers beyond the thermal expansion matching criterion. A titanium barrier layer with loose structure is optimized, in which the low Young's modulus and particle sliding synergistically alleviates interfacial stress, while the TiTe2 reactant enables metallurgical bonding and ohmic contact between the barrier layer and the thermoelectric material, leading to a desirable interface characterized by high-thermostability, high-strength, and low-resistivity. Highly competitive conversion efficiency of 6.2% and power density of 0.51 W cm-2 are achieved for a module with leg length of 2 mm at the hot-side temperature of 523 K, and no degradation is observed following operation for 360 h, a record for stable service at this temperature, paving the way for its application in low-grade waste heat recovery.

Breaking the Minimum Limit of Thermal Conductivity of Mg3 Sb2 Thermoelectric Mediated by Chemical Alloying Induced Lattice Instability.

Thermal properties strongly affect the applications of functional materials, such as thermal management, thermal barrier coatings, and thermoelectrics. Thermoelectric (TE) materials must have a low lattice thermal conductivity to maintain a temperature gradient to generate the voltage. Traditional strategies for minimizing the lattice thermal conductivity mainly rely on introduced multiscale defects to suppress the propagation of phonons. Here, the origin of the anomalously low lattice thermal conductivity is uncovered in Cd-alloyed Mg3 Sb2 Zintl compounds through complementary bonding analysis. First, the weakened chemical bonds and the lattice instability induced by the antibonding states of 5p-4d levels between Sb and Cd triggered giant anharmonicity and consequently increased the phonon scattering. Moreover, the bond heterogeneity also augmented Umklapp phonon scatterings. Second, the weakened bonds and heavy element alloying softened the phonon mode and significantly decreased the group velocity. Thus, an ultralow lattice thermal conductivity of ≈0.33 W m-1 K-1 at 773 K is obtained, which is even lower than the predicated minimum value. Eventually, Na0.01 Mg1.7 Cd1.25 Sb2 displays a high ZT of ≈0.76 at 773 K, competitive with most of the reported values. Based on the complementary bonding analysis, the work provides new means to control thermal transport properties through balancing the lattice stability and instability.

Design of N-Type Textured Bi2 Te3 with Robust Mechanical Properties for Thermoelectric Micro-Refrigeration Application.

Thermoelectric refrigeration is one of the mature techniques used for cooling applications, with the great advantage of miniaturization over traditional compression refrigeration. Due to the anisotropic thermoelectric properties of n-type bismuth telluride (Bi2 Te3 ) alloys, these two common methods, including the liquid phase hot deformation (LPHD) and traditional hot forging (HF) methods, are of considerable importance for texture engineering to enhance performance. However, their effects on thermoelectric and mechanical properties are still controversial and not clear yet. Moreover, there has been little documentation of mechanical properties related to micro-refrigeration applications. In this work, the above-mentioned methods are separately employed to control the macroscopic grain orientation for bulk n-type Bi2 Te3 samples. The HF method enabled the stabilization of both composition and carrier concentration, therefore yielding a higher quality factor to compare with that of LPHD samples, supported by DFT calculations. In addition to superior thermoelectric performance, the HF sample also exhibited robust mechanical properties due to the presence of nano-scale distortion and dense dislocation, which is the prerequisite for realizing ultra-precision machining. This work helps to pave the way for the utilization of n-type Bi2 Te3 for commercial micro-refrigeration applications.

Mediating Point Defects Endows n-Type Bi2 Te3 with High Thermoelectric Performance and Superior Mechanical Robustness for Power Generation Application.

Bi2 Te3 -related alloys dominate the commercial thermoelectric market, but the layered crystal structure leads to the dissociation and intrinsic brittle fracture, especially for single crystals that may worsen the practical efficiency. In this work, point defect configuration by S/Te/I defects engineering is engaged to boost thermoelectric and mechanical properties of n-type Bi2 Te3 alloy, which, coupled with p-type BiSbTe, shows a competitive conversion efficiency for the fabricated module. First, as S alloying suppresses the intrinsic B i T e , antisite defects and forms a donor-like effect, electronic transport properties are optimized, associated with the decreased thermal conductivity due to the point defect scattering. The periodide compound TeI4 is afterward adopted to further tune carrier concentration for the realization of an optimal ZT. Finally, an advanced average ZT of 1.05 with ultra-high compressive strength of 230 MPa is achieved for Bi2 Te2.9 S0.1 (TeI4 )0.0012 . Based on this optimum composition, a fabricated 17-pair module demonstrates a maximum conversion efficiency of 5.37% under the temperature difference of 250 K, rivaling the current state-of-the-art Bi2 Te3 modules. This work reveals the novel mechanism of point defect reconfiguration in synergistic enhancement of thermoelectric and mechanical properties for durably commercial application, which may be applicable to other thermoelectric systems.