Monopolar and dipolar relaxation in spin ice Ho 2 Ti 2 O 7
The fractionalization of magnetic dipoles into monopoles is revealed through a crossover in the relaxation dynamics of spin ice.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Engineering topics
Rodriguez, J. A.
Publications and source records attributed to Rodriguez, J. A..
Platinum vs transition metal carbide surfaces as catalysts for olefin and alkyne conversion: binding and hydrogenation of ethylidyne
The development of heterogeneous catalysts with activity for the hydrogenation of unsaturated hydrocarbons is of economic importance. Ethylene (C 2 H 4 ) and acetylene (C 2 H 2 ) are probe molecules useful to understand the hydrogenation mechanisms, where the most studied surfaces are Pt(111) and Pd(111), however, they have a limited activity due to the formation and accumulation of ethylidyne (CCH 3 ) species. Therefore, alternative catalysts should be developed to limit and/or avoid the formation of ethylidyne on the surface. Transition metal carbides has been reported as alternative catalysts, with the additional advantage of lower prices. The thermodynamics of ethylidyne binding and its transformations on δ-MoC(001), TiC(001), and β-Mo 2 C(100) surfaces are studied by means of periodic DFT. The results indicate that ethylidyne could be transformed to ethyl and ethane on δ-MoC(001) and TiC(001) surfaces, which are relevant species to the Horiuti-Polanyi mechanism. Therefore, these surfaces could be an alternative to Pt(111) and Pd(111), since ethylidyne could be transformed to other species, avoiding or limiting their deactivation. Conversely, ethylidyne cannot be transformed to vinyl (CHCH 2 ) or ethylene in a Horiuti-Polanyi-like mechanism; then, it is not thermodynamically feasible to use any of the studied surfaces in the selective hydrogenation of acetylene, since ethylidyne accumulation could poison the surfaces.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Solid-phase reactive inter-diffusion of Mg/B multilayers
A better understanding of the reaction of Mg and B in the solid-phase regime is needed for the low-temperature synthesis of MgB 2 films. Here in this paper, we study the kinetics of reactive inter-diffusion of Mg and B multilayers on glassy carbon substrates in the temperature range of 400-650°C. Results show that, at these temperatures, inter-diffusion is characterized by a single activation energy of ~0.45eV. The formation of the superconducting MgB 2 phase with critical temperatures of 25–31 K occurs at reaction temperatures of 450°C and above, with the rate of inter-diffusion obeying a power law with a kinetic exponent of ~0.3. This suggests that rate-limiting processes are the nucleation and growth of MgB 2 grains rather than diffusion and interfacial reactions. Implications of these results to the low-temperature synthesis of MgB 2 films are discussed.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗