Two different Li/S cathodes are compared in terms of capacity (mA . h . g sulfur −1 ) and intermediates during discharge and charge. One cathode material is based on fibrous SPAN, a sulfur-containing material obtained via the thermal conversion of poly(acrylonitrile), PAN, in the presence of sulfur. In this material, sulfur is covalently bound to the polymeric backbone. The second cathode material is based on porous activated carbon fibers (ACFs) with elemental sulfur embedded inside the ACFs' micropores. Cyclic voltammetry clearly indicates different discharge and charge chemistry of the two materials. While S-containing ACFs show the expected redoxchemistry of sulfur, SPAN does not form long-chain polysulfides during discharge; instead, sulfide is chopped off the polymer-bound sulfur chains to directly form Li 2 S. The high reversibility of this process accounts for both the high cycle stability and capacity of SPAN-based cathode materials.
Niobium‐alumina composite aggregates with 60 vol% metal content and with particle sizes up to 3150 μm are produced using castable technology followed by sintering, and a crushing and sieving process. X‐Ray diffraction (XRD) analysis reveals phase separation during crushing as the niobium:corundum volume ratios is between 37:57 and 64:31 among the 4 produced aggregate classes 0–45, 45–500, 500–1000, and 1000–3150 μm. The synthesized aggregates are used to produce coarse‐grained refractory composites in a second casting and sintering step. The fine‐ and coarse‐grained material shows porosities between 32% and 36% with a determined cold modulus of rupture of 20 and 12 MPa, and E‐moduli of 37 and 46 GPa, respectively. The synthesized fine‐grained composites reached true strain values between 0.08 at 1100 °C and 0.18 at 1500 °C and the coarse‐grained ones values between 0.02 and 0.09. The electrical conductivity for the fine‐grained and the coarse‐grained material is 448±66 and 111±25 S cm−1, respectively.
Refractory metal (molybdenum, niobium, and tantalum) matrix composite materials in combination with refractory ceramics (alumina, zirconia, and mullite) are of particular interest for high-temperature applications, e.g., in refractory linings in the casting industry. [1] However, besides the high-temperature applicability, other aspects need consideration as well for the choice of material combination, such as: 1) the reaction between the metal and ceramic particles, 2) the chemical interaction with the environment, and 3) the thermal mismatch between the ceramic and the metallic phases. [2] The majority of the past publications dealt with so-called fine-grained refractory composites. [3][4][5][6][7] However, their disadvantages are high shrinkage on sintering, thus resulting in limited thermal shock ability. The refractory metals Nb and Ta are promising candidates for application with Al 2 O 3 due to their similar thermal expansion behavior. Wang et al. [8] and White et al. [9] showed for temperatures between 1000 and 2000 K a similarly linear increase of the linear thermal expansion coefficient for the three elements, with 8.8 to 11.2 Â 10 À6 K À1 for α-Al 2 O 3 , [8] 8.3 to 10.4 Â 10 À6 K À1 for Nb, and 7.1 to 8.4 Â 10 À6 K À1 for Ta. [9] Recently, Zienert et al. [10] demonstrated the fabrication of coarse-grained refractory composites based on alumina with niobium or tantalum, while Weidner et al. [11] reported first mechanical properties at high temperatures (1300-1500 °C) under compressive load. In these coarse-grained refractory composites, alumina with different particle classifications from 0-20 μm up to 2-5 mm (with different volume
Heat treatment of Ta/Nb‐alumina composite parts during fabrication or service can involve the formation of refractory oxide phases if additional oxygen is present. The joint approach of CALPHAD‐based calculations and experimental methods involving electron backscatter diffraction sheds light on why Ta‐containing composites form ternary AlTaO4, but in Nb‐containing samples only the binary compound NbO is found. Further information can be found in article number http://doi.wiley.com/10.1002/adem.202200161 by Michael Eusterholz and co‐workers. (Photo credit: Michael Eusterholz, Institute for Applied Materials – Materials Science and Engineering, and Julian Gebauer, Institute for Applied Materials – Applied Materials Physics, Karlsruhe Institute of Technology)
Parts for metallurgical applications made from refractory metal–ceramic composites offer improved thermal shock resistance due to their capability for resistive heating compared to ones made solely from ceramics such as Al2O3. The combination of Al2O3 and Nb is intriguing as both show similar thermal expansion behavior over a wide temperature range. The high affinity of Nb for O to form nonprotective oxides, however, hampers its use in oxidative environments. Formation of such phases at the ceramic–metal interface can have detrimental effects on the cohesion of the composites. For this work, nanocrystalline Nb films are deposited on sapphire substrates by magnetron sputtering to study diffusion of O and high‐temperature phase formation at a refractory metal–ceramic interface during heat treatment under Ar at 1600 °C. A combined approach of atom probe tomography and transmission electron microscopy for compositional and crystallographic analyses reveals that at triple junctions of the sapphire–Nb interface with Nb grain boundaries, heterogeneous nucleation of nanoscale NbO2 occurs, which further reacts with Al2O3 to form AlNbO4, while the Nb film itself remains metallic. Fast O transport through grain boundaries leads to internal oxidation at the interface, whereas regions further away from Nb grain boundaries remain unchanged.
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