Bo Duan

Micro- and Macro- Mechanical Properties of Thermoelectric Lead Chalcogenides

Both n- and p-type lead telluride (PbTe)-based thermoelectric (TE) materials display high TE efficiency, but the low fracture strength may limit their commercial applications. To find ways to improve these macroscopic mechanical properties, we report here the ideal strength and deformation mechanism of PbTe using density functional theory calculations. This provides structure-property relationships at the atomic scale that can be applied to estimate macroscopic mechanical properties such as fracture toughness. Among all the shear and tensile paths that are examined here, we find that the lowest ideal strength of PbTe is 3.46 GPa along the (001)/⟨100⟩ slip system. This leads to an estimated fracture toughness of 0.28 MPa m1/2 based on its ideal stress-strain relation, which is in good agreement with our experimental measurement of 0.59 MPa m1/2. We find that softening and breaking of the ionic Pb-Te bond leads to the structural collapse. To improve the mechanical strength of PbTe, we suggest strengthening the structural stiffness of the ionic Pb-Te framework through an alloying strategy, such as alloying PbTe with isotypic PbSe or PbS. This point defect strategy has a great potential to develop high-performance PbTe-based materials with robust mechanical properties, which may also be applied to other materials and applications.

Ductile deformation mechanism in semiconductor α-Ag 2 S

Inorganic semiconductor α-Ag2S exhibits a metal-like ductile behavior at room temperature, but the origin of this high ductility has not been fully explored yet. Based on density function theory simulations on the intrinsic mechanical properties of α-Ag2S, its underlying ductile mechanism is attributed to the following three factors: (i) the low ideal shear strength and multiple slip pathways under pressure, (ii) easy movement of Ag–S octagon framework without breaking Ag−S bonds, and (iii) a metallic Ag−Ag bond forms which suppresses the Ag–S frameworks from slipping and holds them together. The easy slip pathways (or easy rearrangement of atoms without breaking bonds) in α-Ag2S provide insight into the understanding of the plastic deformation mechanism of ductile semiconductor materials, which is beneficial for devising and developing flexible semiconductor materials and electronic devices.Semiconductors: easy slip explains silver sulfide ductilityWhile semiconductors are usually brittle, the atomic bonds in cubic silver sulfide (α-Ag2S) are flexible, making it ductile at room temperature. Guodong Li from Wuhan University of Technology and colleagues in the USA and Russia used density function theory simulations to examine the bonds between silver and sulphur in α-Ag2S under pressure. They found that shear deformation along specific directions distorted the octagons formed by the Ag-S bonds, while it also created new Ag-Ag bonds to couple the Ag-S octagons, enabling α-Ag2S to retain its structure during deformation. They also found low ideal shear strength along two crystallographic planes, which promoted easy atomic slip while maintaining the integrity of the atomic framework. Research into the atomic origins of ductility in semiconductors may help us better understand and design flexible electronics.

Determining ideal strength and failure mechanism of thermoelectric CuInTe2 through quantum mechanics

CuInTe_2 is recognized as a promising thermoelectric material in the moderate temperature range, but its mechanical properties important for engineering applications remain unexplored so far. Herein, we applied quantum mechanics (QM) to investigate such intrinsic mechanical properties such as ideal strength and failure mechanism along with pure shear, uniaxial tension, and biaxial shear deformations. We found that the ideal shear strength of CuInTe_2 is 2.43 GPa along the (221)[11−1] slip system, which is much lower than its ideal tensile strength of 4.88 GPa along [1−10] in tension, suggesting that slipping along (221)[11−1] is the most likely activated failure mode under pressure. Shear induced failure of CuInTe_2 arises from softening and breakage of the covalent In–Te bond. However, tensile failure arises from breakage of the Cu–Te bond. Under biaxial shear load, compression leads to shrinking of the In–Te bond and consequent buckling of the In–Te hexagonal framework. We also found that the ideal strength of CuInTe_2 is relatively low among important thermoelectric materials, indicating that it is necessary to enhance the mechanical properties for commercial applications of CuInTe_2.

Rapid preparations and thermoelectric properties of bulk skutterudites with in situ nanostructures

In this paper, Ge and Te co-doped skutterudites Co4Sb11Ge1-xTex were synthesized via two rapid preparation methods, melt quenching-spark plasma sintering (MQ-SPS) and high pressure-spark plasma sintering (HP-SPS). Bulk skutterudites can be synthesized in as little as 6 hours by MQ-SPS and under 1 hour by HP-SPS, as shown by both scanning electron microscopy and x-ray diffraction. This is a dramatic improvement over traditional methods requiring a full week of processing. The Seebeck coefficient, electrical conductivity and thermal conductivity across a temperature range of 300 to 800 K where measured. This work shows that the processing by HP-SPS significantly decreases thermal and lattice thermal conductivities, while increasing the temperature-dependent Seebeck maximum. Consequently, the HP-Co4Sb11Ge1-xTex samples show a higher dimensionless figure of merit compared with that of MQ-Co4Sb11Ge1-xTex samples throughout the measured temperature range.In this paper, Ge and Te co-doped skutterudites Co4Sb11Ge1-xTex were synthesized via two rapid preparation methods, melt quenching-spark plasma sintering (MQ-SPS) and high pressure-spark plasma sintering (HP-SPS). Bulk skutterudites can be synthesized in as little as 6 hours by MQ-SPS and under 1 hour by HP-SPS, as shown by both scanning electron microscopy and x-ray diffraction. This is a dramatic improvement over traditional methods requiring a full week of processing. The Seebeck coefficient, electrical conductivity and thermal conductivity across a temperature range of 300 to 800 K where measured. This work shows that the processing by HP-SPS significantly decreases thermal and lattice thermal conductivities, while increasing the temperature-dependent Seebeck maximum. Consequently, the HP-Co4Sb11Ge1-xTex samples show a higher dimensionless figure of merit compared with that of MQ-Co4Sb11Ge1-xTex samples throughout the measured temperature range.

Benificial effect of Se substitution on thermoelectric properties in Co4Sb12-x-yTexSey skutt

Skutterudite-based compounds, Co4Sb12-x-yTexSey (x = 0.4, 0.5, 0.6 and y = 0.0, 0.1), are synthesized by the solid state reaction and spark plasma sintering methods, and their structure and the thermoelectric properties have been investigated systematically. It is found that the doping of Se resulted in a decrease of the lattice parameter and a refinement in the grain size compared with that of Te alone doped samples. The samples doping of Se do not yield a certain increase in the power factor due to the reduced electrical conductivity, but it shows a significant depression in the lattice thermal conductivity because of the enhanced point-defect scattering owing to the larger mass fluctuations and strain field fluctuations. Besides, the effect of the reduced grain size and the nano-precipitates on the lattice thermal conductivity should not be neglected. The highest dimensionless figure of merit ZT = 1.09 is achieved at 800 K in the Co4Sb11.3Te0.6Se0.1 compound, which is improved by 15% as compared with t...