A cubic room temperature polymorph of thermoelectric TAGS-85+

The alloy (GeTe)(85)(AgSbTe2)(15), commonly known as TAGS-85, is one of the best performing p-type thermoelectric materials in the temperature range 200-500 degrees C. In all reports thus far, TAGS-85 adopts a rhombohedral crystal structure at room temperature and undergoes a reversible transition to a cubic phase in the middle of the operating temperature range. Here, we report on a novel, metrically cubic polymorph of TAGS-85 that can be obtained at room temperature using a particular cooling protocol during initial synthesis. This polymorph transforms irreversibly on initial heating to a 21-layer trigonal structure containing ordered cation vacancy layers, driven by the spontaneous precipitation of argyrodite-type Ag8GeTe6. We show that the precipitation of Ag8GeTe6 is detrimental to the thermoelectric performance of TAGS-85 due to an increase in the vacancy concentration, which makes the samples more metallic in character and significantly reduces the Seebeck coefficient. The precipitation of Ag8GeTe6 can be suppressed by careful control of the synthesis conditions

TEM study of the structural dependence of the epitaxial passive oxide films on crystal facets in polyhedral nanoparticles of chromium

Nanocubes and partially truncated rhombic dodecahedral nanoparticles of Cr have been studied by electron diffraction, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is found that the nanoparticles of Cr are enclosed by the epitaxial passive oxide films. The oxide films on 100\% truncated nanocubes of Cr with only one kind of facets, {100} facets, are face-centered cubic (fcc) structured Cr2O3 with a lattice constant of 0.407 nm. There are two kinds of oxide films in partially truncated nanoparticles of Cr with two kinds of crystallographic facets, {100} and {110}. The same fee Cr2O3 is found on the {100} facets while the rhombohedral alpha-Cr2O3 is found on the {110} facets. This is similar to the two kinds of oxides, fcc and rhombohedral Fe2O3, which have also been observed in polyhedral nanoparticles of Fe. These passive Cr2O3, found in nanoparticles of Cr which have remained unchanged in water for four years, may have important implications for protective oxide films involving Cr. (C) 2003 Elsevier B.V. All rights reserved

Quantitative study on the microstructural evolution and dimensional stability mechanism of 2024 Al alloy during long-term thermal cycling

During the start-stop process of high-precision instruments in service, the critical materials of instruments undergo thermal cycling, resulting in changes in microstructure and dimension. In this paper, the evolution and coupling effects of dislocations and precipitates within 2024 Al alloy during 500 cycles were investigated, and their influence on the dimensional change was quantitatively characterized and decoupling analyzed. Transmission Electron Microscopy (TEM)/aberration-corrected scanning TEM (CS-TEM), X-ray diffraction (XRD), and Three-Dimensional Atom Probe Tomography (3D-APT) were used for microstructural evolution analysis and quantitative statistics. The dislocation density increased from 3.32 × 1014 m−2 to 5.75 × 1014 m−2 after 500 cycles, which accelerated elemental diffusion and provided nucleation sites of precipitates, contributing to an increase in the number density of the S'/S phase from 1.04 × 1024 m−3 to 1.41 × 1024 m−3. Based on the quantitative statistics of precipitate types and corresponding volume fractions, the dimensional change induced by precipitate evolution was −1.25 × 10−4. The dimensional change caused by the free volume introduced by dislocation density was 3.32 × 10−6. Comparing these values with the experimental value of −1.94 × 10−4, it is clear that the precipitate evolution is the main factor that triggers the dimensional change

Rapid liquid phase–assisted ultrahigh-temperature sintering of high-entropy ceramic composites

High-entropy ceramics and their composites display high mechanical strength and attractive high-temperature stabilities. However, properties like strong covalent bond character and low self-diffusion coefficients make them difficult to get sintered, limiting their mass popularity. Here, we present a rapid liquid phase-assisted ultrahigh-temperature sintering strategy and use high-entropy metal diboride/boron carbide composite as a proof of concept. We use a carbon-based heater to fast-heat the composite to around 3000 K, and a small fraction of eutectic liquid was formed at the interface between high-entropy metal diborides and boron carbide. A crystalline dodecaboride intergranular phase was generated upon cooling to ameliorate the adhesion between the components. The as-sintered composite presents a high hardness of 36.4 GPa at a load of 0.49 N and 24.4 GPa at a load of 9.8 N. This liquid phase-assisted rapid ultrahigh-temperature strategy can be widely applicable for other ultrahigh-temperature ceramics as well