1,721,009 research outputs found
Entropy-driven expansion of the thermodynamic stability of compositionally complex spinel oxides
Full text linkHigh-entropy ceramics have sparked renewed interest in compositionally complex ceramics since the first introduction in 2015. The kaleidoscopic array of compositions and structures harnessed by this idea has unlocked an unprecedented opportunity to tailor materials for specific applications, including catalysis, thermal barriers, and electrochemical energy storage. Within the family of oxides, a competition exists between rock-salt and spinel structures. The rock-salt structure is highly symmetrical, consisting of a single cation sublattice, while the spinel structure offers more flexibility to accommodate various cations in two distinct sublattices. Herein, we aimed at stabilizing and expanding the thermal stability range of the spinel-structured oxide, successfully synthesizing novel, single-phase, compositionally complex materials by capitalizing on entropy stabilization, all while avoiding the ubiquitous use of nickel and chrome, notorious for their negative environmental impact. The right combination of cations resulted in the synthesis of a seven-metal oxide that is thermally stable up to the remarkable temperature of 1473 K
ZrB2-SiC Sharp Leading Edges in High Enthalpy Supersonic Flows
No full textAero-thermodynamic tests have been carried out in an arc-jet supersonic plasma wind tunnel using a very sharp wedge made of ultra-high temperature ceramic (UHTC) in the ZrB2–SiC system. The comparison with a lower thermal conductivity ceramic material (Si3N4–MoSi2) with the same sharp shape, pointed out at the performance advantages of the UHTC material. When subjected to heat fluxes in the order of 7 MW/m2, the surface temperature of the UHTC wedge increased up to 2450°C near the leading edge. The present study demonstrated that the high thermally conductive UHTC survived such extreme conditions by re-distributing heat over colder regions downstream of the sharp tip. As a consequence, radiative equilibrium temperatures in the range 1400°C–1650°C were established over 85% of the exposed surface. On the other hand, the less thermally conductive Si3N4–MoSi2 material failed to withstand the same heat flux and underwent partial melting with significant mass loss. The post-test microstructural observations of the UHTC wedge proved to be a fundamental source of information which was input into a Computational Fluid Dynamics (CFD) code and by a thermal simulation software to simulate the experimental tests and correlate the in situ observations of the material evolution during testing
Soggetto, Istituzioni, Cultura. Concetti e domande per pensare i sistemi organizzativi di cura
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Compositional pathways and anisotropic thermal expansion of high-entropy transition metal diborides
No full textThe recent discovery of high entropy transition metal diborides (HEBs) has sparked renewed interest in ultra-high temperature ceramics (UHTCs). Presently, transition metal (Me) oxides based boro-carbo/thermal reduction (BCTR) syntheses show great promise as relatively cheap production methods, but also may present limits to attain single phase pure HEBs. Herein, by selectively tuning the concentration of boron and carbon, the reducing agents of Me oxide mixture (Me = Ti, Ta, Nb, Zr and Hf), and exploiting high-resolution synchrotron X-ray powder diffraction, we first identified and quantified the formation of intermediate phases during the BCTR synthesis, with the ultimate intent to achieve a full dense (Ti,Ta,Nb,Zr,Hf)B2 solid solution (SS). Additional insight was obtained by temperature dependent diffraction, which highlighted, for the first time in this class of materials, anisotropic thermal expansion, most likely at the origin of the SS micro-cracking, as was also observed by electron microscopy
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