Homogeneous distribution

Abstract: Three typical ceramic processing were respectively used to synthesize (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C high-entropy carbide (HEC) ceramics by spark plasma sintering. Although single-phase composition characterized by X-ray diffraction were obtained by the three processes, the microstructures and elemental distributions are different. The reasons for the formation of these features are preliminarily discussed. The results demonstrate that the particle sizes of the starting metallic powders was a key factor for obtaining a homogeneous distribution of each elements in the HEC. Carbide process with relatively finer starting carbide powders compared to the above metallic starting powders resulted in an HEC with homogeneous distribution of elements, but the obtained ceramics showed the lowest relative density. For oxide process, it is considered that the obviously higher reaction temperature between ZrO2 and graphite resulted in a two-phase structure of an HEC and a zirconium-rich phase, but the obtained HEC showed the highest relative density.

Abstract: Alloying has shown enormous potential for tailoring the atomic and electronic structures, and improving the performance of catalytic materials. Systematic studies of alloy catalysts are, however, often compromised by inhomogeneous distribution of alloying components. Here we introduce a general approach for the synthesis of monodispersed and highly homogeneous Pt-bimetallic alloy nanocatalysts. Pt{sub 3}M (where M = Fe, Ni, or Co) nanoparticles were prepared by an organic solvothermal method and then supported on high surface area carbon. These catalysts attained a homogeneous distribution of elements, as demonstrated by atomic-scale elemental analysis using scanning transmission electron microscopy. They also exhibited high catalytic activities for the oxygen reduction reaction (ORR), with improvement factors of 2-3 versus conventional Pt/carbon catalysts. The measured ORR catalytic activities for Pt{sub 3}M nanocatalysts validated the volcano curve established on extended surfaces, with Pt{sub 3}Co being the most active alloy.