Dongmei Liu

Growth of an oriented Bi40−xInxTe60 (x = 3, 7) thermoelectric material by seeding zone melting for the enhancement of chemical homogeneity

A zone melting technique with seeding has been developed to prepare oriented Bi40−xInxTe60 (at%, x = 3; 7) thermoelectric material with enhanced chemical homogeneity. The respective initial compositions of the sample and the seed were chosen according to the pseudo-binary Bi2Te3–In2Te3 phase diagram that was experimentally redetermined with the aid of a former mushy zone that was resolidified in a temperature gradient. An oriented Bi40−xInxTe60 bulk material with a uniform composition close to the target value over the entire length of the zone-melted region along the growth direction has been successfully manufactured.

Prediction of the solidification path of Al-4.37Cu-27.02Mg ternary eutectic alloy with a unified microsegregation model coupled with Thermo-Calc

Abstract An extended unified microsegregation model with solid back diffusion effects and five different dendrite morphologies was used to investigate the solidification path of Al-4.37Cu-27.02Mg (wt.%) ternary eutectic alloy at different cooling rates and solid back diffusion coefficients, coupled with Thermo-Calc. It was indicated that the cooling rates (Rf) had no obvious effect on the solidification path which was (L + α) → (L + α + T) → (L + α + β + T); but the solid back diffusion coefficient (Φ) had a great effect on the solidification path, which evolved gradually from (L + α) → (L + α + T) → (L + α + β + T) into (L + α) → (L + α + T) when Φ increased from 0 to 1. The volume fractions of primary α phase (Vα), binary eutectic (V2E) and ternary eutectic (V3E) at each solidification path were calculated. It was shown that V2E decreased with the increase of Rf whereas V3E increased and Vα was almost invariant. The dependence of V2E, V3E and Rf were determined by linear regression analysis given as: V2E = −2.5lgRf + 47.5; V3E = 6.4lgRf + 47. Increase in Φ lead to increases in Vα and V2E and decrease in V3E. The predicted soldification paths and volume fractions of Al-4.37Cu-27.02Mg ternary eutectic alloy at different cooling rates were in good agreement with experimental results.

Design of (Nb, Mo) 40 Ti 30 Ni 30 alloy membranes for combined enhancement of hydrogen permeability and embrittlement resistance

The effect of substitution of Nb by Mo in Nb40Ti30Ni30 was investigated with respect to microstructural features and hydrogen dissolution, diffusion and permeation. As-cast Nb40−xMoxTi30Ni30 (xu2009=u20090, 5, 10) alloys consist of primary bcc-Nb phase and binary eutectic (bcc-Nbu2009+u2009B2-TiNi). The substitution of Nb by Mo reduces the hydrogen solubility in alloys, but may increase (xu2009=u20095) or decrease (xu2009=u200910) the apparent hydrogen diffusivity and permeability. As-cast Nb35Mo5Ti30Ni30 exhibits a combined enhancement of hydrogen permeability and embrittlement resistance as compared to Nb40Ti30Ni30. This work confirms that Mo is a desirable alloying element in Nb that can contribute to a reduction in hydrogen absorption and an increase in intrinsic hydrogen diffusion, thus improving embrittlement resistance with minimal permeability penalty.

Anisotropic layered Bi 2 Te 3 -In 2 Te 3 composites: control of interface density for tuning of thermoelectric properties

Layered (Bi1−xInx)2Te3-In2Te3 (xu2009=u20090.075) composites of pronounced anisotropy in structure and thermoelectric properties were produced by zone melting and subsequent coherent precipitation of In2Te3 from a (Bi1−xInx)2Te3 (xu2009>u20090.075) matrix. Employing solid state phase transformation, the Bi2Te3/In2Te3 interface density was tuned by modifying the driving force for In2Te3 precipitation. The structure-property relationship in this strongly anisotropic material is characterized thoroughly and systematically for the first time. Unexpectedly, with increasing Bi2Te3/In2Te3 interface density, an increase in electrical conductivity and a decrease in the absolute Seebeck coefficient were found. This is likely to be due to electron accumulation layers at the Bi2Te3/In2Te3 interfaces and the interplay of bipolar transport in Bi2Te3. Significantly improved thermoelectric properties of Bi2Te3-In2Te3 composites as compared to the single phase (Bi1−xInx)2Te3 solid solution are obtained.

Two-Step Annealing Leading to Refined Bi2Te3-In2Te3 Lamellar Structures for Tuning of Thermoelectric Properties

A two-step annealing process was applied to control the morphology of Bi2Te3-In2Te3 composite materials via precipitation of In2Te3 from supersaturated (Bi,In)2Te3. Finer lamellae were obtained via two-step as compared with single-step isothermal annealing. The microstructure was optimized by exploiting thermodynamic and kinetic effects during nucleation and growth of In2Te3. The relationship between the morphologies and thermoelectric properties was analyzed. With preannealing at a lower temperature, refined morphologies lead to an enhanced power factor and zT in the temperature range from room temperature to ∼100°C. The enhancement is mainly caused by an increased Seebeck coefficient, most probably due to energy-dependent scattering processes. However, the thermal conductivity is dominated by bipolar thermal transport that compensates the low lattice thermal conductivity completely.