Yuanyue Li

Investigation of deformation instability of Au/Cu multilayers by indentation

Plastic deformation behavior of Au/Cu multilayers with individual layer thicknesses of 25–250 nm was investigated via microindentation experiments. It was found that plastic instability of the Au/Cu multilayer exhibits strong length scale (individual layer thickness and grain size) dependence. The smaller the length scale, the easier shear bands form. In other words, plastic deformation becomes unstable with decreasing length scale. Cross-sectional observation along with plan-view indicates that the occurrence of plastic deformation instability corresponds to transformation of the deformation mechanism associated with geometrical configuration and length scale of the material. At nanometer scale, buckling-assisted interface crossing of dislocations results in local shear band, while, at submicron scale or above, local dislocation pileup-induced interface offset leads to plastic instability. Theoretical analysis is conducted to understand the length scale-dependent plastic deformation behavior of the multilayer.

Two different types of shear-deformation behaviour in Au-Cu multilayers

Localised shear deformation of a material is usually identified as a particular feature of deformation inhomogeneity. Here, we show two different types of shear deformation-behaviour that occurred in Au–Cu multilayers subjected to microindentation load, namely, a cooperative-layer-buckling-induced shear banding in a nanoscale multilayer and a direct localised shearing across a layer interface along a shear plane in a submicron-scale multilayer. Theoretical analysis indicates that the formation of the two different types of shear deformation in the multilayers depends on a competition between the dislocation-pile-up-induced stress concentration at the layer interface and the barrier strength of the layer interface for glissile dislocation transmission.

Transport properties and enhanced thermoelectric performance of aluminum doped Cu3SbSe4

The electrical transport and thermoelectric properties of Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) compounds are investigated in the temperature range of 300–600 K. The results indicate that with increasing Al content from x = 0 to x = 0.03, hole concentration increases monotonically from 8.04 × 1017 to 1.19 × 1019 cm−3 due to the substitution of Al3+ for Sb5+, thus leading to a large decrease in the electrical resistivity of Cu3Sb1−xAlxSe4. Meanwhile, the increase in hole concentration leads to a transition from a non-degenerate (x = 0) to a partial degenerate (x = 0.01, 0.02) and then to a degenerate state (x = 0.03). The power factor (PF) of all the Al-doped Cu3Sb1−xAlxSe4 samples is remarkably improved due to the optimization of hole concentration. Lattice thermal conductivity κL of the heavily doped sample (x = 0.03) is reduced. As a result, a large thermoelectric figure of merit ZT = 0.58 is obtained for Cu3Sb0.97Al0.03Se4 at 600 K, which is around 1.9 times as large as that of the un-doped Cu3SbSe4.

Enhanced thermoelectric performance through carrier scattering at heterojunction potentials in BiSbTe based composites with Cu3SbSe4 nanoinclusions

Thermoelectric materials with the thermoelectric figure of merit, ZT, being much larger than unit at near room temperature are vital for power generation by using low-grade waste heat. Here we show that by incorporating very small proportion (1 vol%) of Cu3SbSe4 nanoparticles into the BiSbTe matrix to form nanocomposites, besides large (∼50%) reduction of lattice thermal conductivity, both enhanced thermopower through energy-dependent scattering and alleviated reduction of carrier mobility via carrier scattering at heterojunction potentials occur at elevated temperatures, which allow the thermoelectric power factor of the composite material to reach ∼37 μW cm−1 K−2 at 467 K. Consequently, a largest value of ZT = 1.6 is achieved at 476 K. Moreover, it has excellent performance in a broad temperature range (say, ZT = 1.0 at 300 K and ZT = 1.5 at 500 K), which makes this material attractive for cooling and power generation.

Enhancement of thermoelectric performance of β-Zn4Sb3 through resonant distortion of electronic density of states doped with Gd

The thermoelectric properties of Gd-doped β-Zn4Sb3 are investigated. The results indicate that Gd-doping not only causes a 41 μV K−1 increase in thermopower owing to resonant distortion of DOS but also results in ∼15% reduction in thermal conductivity at a doping content of 0.2%. Consequently, a largest value of ZT = 1.2 is achieved at 655 K.

Enhanced thermoelectric figure of merit in p-type β-Zn4Sb3/Bi0.4Sb1.6Te3 nanocomposites

The thermoelectric properties of Bi0.4Sb1.6Te3-based composites incorporated with β-Zn4Sb3 nanoparticles are investigated in the temperature range from 300 K to 500 K. The results show that ∼5% increase in Seebeck coefficient and ∼32% reduction of lattice thermal conductivity at 443 K are concurrently realized in the nanocomposite system with 1.3 vol% of β-Zn4Sb3, which originates from energy filtering effect as well as enhanced phonon scattering at dispersed nanoparticles and phase boundaries, respectively. As a result, the largest figure of merit ZT = 1.43 is achieved at 443 K for the sample with 1.3 vol% of β-Zn4Sb3 nanoinclusions, which is ∼18% larger than that (=1.21) of the Bi0.4Sb1.6Te3 matrix.

Enhanced thermoelectric performance of CuGaTe2 based composites incorporated with graphite nanosheets

CuGaTe2 based composites incorporated with graphite nanosheets (GNs) CuGaTe2/x G (G = GNs, 0 ≤ x ≤ 3.04 vol. %) were prepared, and the thermoelectric properties of the composites were studied from 300 to 875 K. The results show that the incorporation of GNs into the CuGaTe2 matrix can enhance the Seebeck coefficient and power factor over the whole temperature range investigated due to energy filtering effects, and the reduction of thermal conductivity below 750 K owing to interface scattering. Although the resistivity increases, energy filtering significantly raises the Seebeck component, and the overall effect on power factor is positive. The sample with 2.28 vol. % GNs had the largest ZT value, reaching 0.93 at 873 K, which is a ∼21% improvement on pure CuGaTe2.

Optimized thermoelectric properties of AgSbTe2 through adjustment of fabrication parameters

AgSbTe2 bulk sample is obtained by hot-pressing under different fabrication parameters, and their thermoelectric properties are investigated in the temperature range of 300 - 550 K. The highest ZT = 0.86 is achieved at 475 K for the sample hot-pressed at 423 K and 500MPa due to the lower thermal conductivity and higher power factor. The results indicate that the optimized thermoelectric properties can be obtained for AgSbTe2 compound at the sintering temperature of 423 K under the pressure of 500 MPa.

Direct observation of dislocation plasticity in 100 nm scale Au/Cu multilayers

Dislocation plasticity of Au/Cu multilayer with 100 nm scale was investigated by three-point bending tests. The authors found directly that clear slip lines appeared inside the grains ranging from 60 to 190 nm. The finding implies that the potential dislocation plasticity is still the dominant deformation mechanism in the material. Statistical evaluation of the mean spacing between slip lines reveals the effect of the length scale on homogeneity of plastic deformation. The critical length scale for dislocation-dominated deformation of the nanoscale material is found to be similar to 15 nm.

Thermoelectric transport properties of PbTe-based composites incorporated with Cu2Se nano-inclusions

Thermoelectric transport properties of Lead telluride (PbTe)-based composites incorporated with Cuprous selenide (Cu2Se) nano-inclusions were investigated from 300 K to 800 K. Here, except for the transition from p-type to n-type conduction that occurs in pristine PbTe at ~530 K due to the difference of mobility between thermally electron and hole at high temperature, another transition from p-type to n-type conduction at 300 K with an increasing proportion of Cu2Se could be due to the donor levels introduced by defects and unsaturated bonds at the interfaces. Moreover, by incorporating a small proportion (5 vol.%) of Cu2Se nanoparticles into the PbTe matrix to form nano-composites, both a reduction (~55%) in lattice thermal conductivity and an enhanced electrical conductivity compared with that of pristine PbTe are obtained, which allows the thermoelectric power factor to reach a larger value (~11.2 μW cm−1 K−2). Consequently, a maximum value ZT = 0.91 is obtained at 760 K in the PbTe-5 vol.% Cu2Se sample.