F.S. Liu

Enhanced Thermoelectric Performance of Cu2CdSnSe4 by Mn Doping: Experimental and First Principles Studies

Serials of Mn doping by substituting Cd sites on Cu2CdSnSe4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu2Cd1−xMnxSnSe4. Our experimental and theoretical studies show that the moderate Mn doping by substituting Cd sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4. The electrical resistivity is decreased by about a factor of 4 at 723 K after replacing Cd with Mn, but the seebeck coefficient decreases only slightly from 356 to 289 μV/K, resulting in the significant increase of the power factor. Although the thermal conductivity increases with the doping content of Mn, the figure of merit (ZT) is still increased from 0.06 (x = 0) to 0.16 (x = 0.10) at 723 K, by a factor of 2.6. To explore the mechanisms behind the experimental results, we have performed an ab initio study on the Mn doping effect and find that the Fermi level of Cu2CdSnSe4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at the low level doping content. Optimizing the synthesis process and scaling Cu2Cd1−xMnxSnSe4 to nanoparticles may further improve the ZT value significantly by improving the electrical conductivity and enhancing the phonon scattering to decrease the thermal conductivity.

Graphene Quantum Dots Embedded in Bi2Te3 Nanosheets To Enhance Thermoelectric Performance

Novel Bi2Te3/graphene quantum dots (Bi2Te3/GQDs) hybrid nanosheets with a unique structure that GQDs are homogeneously embedded in the Bi2Te3 nanosheet matrix have been synthesized by a simple solution-based synthesis strategy. A significantly reduced thermal conductivity and enhanced powder factor are observed in the Bi2Te3/GQDs hybrid nanosheets, which is ascribed to the optimized thermoelectric transport properties of the Bi2Te3/GQDs interface. Furthermore, by varying the size of the GQDs, the thermoelectric performance of Bi2Te3/GQDs hybrid nanostructures could be further enhanced, which could be attributed to the optimization of the density and dispersion manner of the GQDs in the Bi2Te3 matrix. A maximum ZT of 0.55 is obtained at 425 K for the Bi2Te3/GQDs-20 nm, which is higher than that of Bi2Te3 without hybrid nanostrucure. This work provides insights for the structural design and synthesis of Bi2Te3-based hybrid thermoelectric materials, which will be important for future development of broadly functional material systems.

Heterovalent Substitution to Enrich Electrical Conductivity in Cu 2 CdSn 1-x Ga x Se 4 Series for High Thermoelectric Performances

Serials of Ga doping on Sn sites as heterovalent substitution in Cu2CdSnSe4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu2CdSn1-xGaxSe4 (x = 0, 0.025, 0.05, 0.075, 0.01, and 0.125). Massive atomic vacancies are found at x = 0.10 by the heterovalent substitution, which contributes significantly to the increase of electrical conductivity and the decrease of lattice thermal conductivity. The electrical conductivity is increased by about ten times at 300 K after Ga doping. Moreover, the seebeck coefficient only decreases slightly from 310 to 226 μV/K at 723 K, and a significant increase of the power factor is obtained. As a result, a maxium value of 0.27 for the figure of merit (ZT) is obtained at x = 0.10 and at 723 K. Through an ab initio study of the Ga doping effect, we find that the Fermi level of Cu2CdSnSe4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at low doping levels. Our experimental and theoretical studies show that a moderate Ga doping on Sn sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4.

Controllable Formation of (004)-Orientated Nb:TiO2 for High-Performance Transparent Conductive Oxide Thin Films with Tunable Near-Infrared Transmittance

A niobium-doped titanium dioxide (Nb:TiO2, NTO) film is a promising candidate material for indium-free transparent conductive oxide (TCO) films. It is challenging and interesting to control (004)-oriented growth to decrease resistivity. In this work, NTO films with different fractions of preferential (004) orientation (η(004)) were controllably prepared by direct current sputtering. Notably, the direction of local-ordering of ions-packing could be adjusted by slightly changing the angle between the sputtering source and the glass substrate, which is identified as a key factor to determine the growth direction of a columnar crystal as well as the η(004) of films. Hall effect measurements indicate that NTO films with the highest η(004) present the lowest resistivity (6.4 × 10-4 Ω cm), which originates from super-high carrier concentration (2.9 × 1021 cm-3) and mobility (3.4 cm2 V-1 s-1). The corresponding low sheet resistance (10.3 Ω sq-1) makes it a potential material for commercial TCO films. We also observe that films with higher η(004) show lower transmittance in the near-infrared region.

Dense pure binderless WC bulk material prepared by spark plasma sintering

The phases, microstructures and mechanical properties of binderless WC bulk materials prepared by the spark plasma sintering technique were investigated systematically. The addition of carbon was added to eliminate the impurity phase W2C. The relative density, Vickers hardness and grain size increase obviously with increasing sintering temperature, but increase weakly with increasing pressure or sintering time. The high relative density of 99·1%, HV30 of 27·5 GPa and fracture toughness KIC of 4·5 MPa m1/2 of pure binderless WC bulk with a grain size of 400 nm was obtained by sintering the WC powders with a particle size of 200 nm and the addition of 0·63 wt-%C at 1800°C for 6 min under 70 MPa.

Parameter determination of critical nucleation frequency in solidification of undercooled metallic melts

Abstract A simple method was proposed to calculate the essential parameters correlated with the critical nucleation frequency of undercooled metals and alloy melt. Numerical results show that the calculation accuracy from this method can be improved using the experimental data either with high undercooling or with low undercooling range (the difference of undercoolings between two solidification events). The calculations of the interfacial energy for high undercooling of silver and of the catalytic factor f(θ) for high undercooling of Al, Cu and Al–30 wt-%Cu alloy indicate that the results are consistent with the experimental measurements and with the results of Jian’s model [Metall. Trans. A, 2001, 32A, 391–395]. In addition, by analysing the differential scanning calorimetry data of pure Sn subjected to different cooling rates, similar values of catalytic factor f(θ) are obtained. This further indicates the validity of the current method.

Structural and magnetic properties of DyCo4―xFexGa compounds

The structural and magnetic properties of the DyCo 4− x Fe x Ga compounds with x =0, 0.5, 1, and 1.5 have been investigated by X-ray diffraction and magnetic measurements. Powder X-ray diffraction analysis reveals that each of the DyCo 4− x Fe x Ga compounds has a hexagonal CaCu 5 -type structure (space group P 6/ m m m ). The Fe solubility limit in DyCo 4− x Fe x Ga is x x , the larger the unit-cell parameters a , c , V , and the 3 d -sublattice moment but the smaller the 3 d uniaxial anisotropy. Magnetic measurements show that the Curie temperature of DyCo 4− x Fe x Ga increases from 498 K for x =0 to 530 K for x =1.5, the compensation temperature T comp decreases from 286 K for x =0 to 238 K for x =1.5, and the spin-reorientation transition temperature increases from 403 K for x =0 to 530 K for x =0.5. No spin-reorientation transition was found in the samples with x =1.0 and 1.5. The saturation magnetization of DyCo 4− x Fe x Ga measured at 173 K increases but the magnetization measured at 300 K decreases with increasing Fe content x .