ANTONIO GUERRA MARRERO, PRESIDENTE CÁMARA DE COMERCIO ESPAÑOLA EN INGLATERRA [Material gráfico]

Esta fotografía se incluye en un álbum fotográfico que se encuentra en el Archivo Histórico Municipal de Arucas, publicado por la empresa Guerra & Co. Ldt. en 1952 aproximadamente, con motivo de la Feria de la Alimentación celebrada en Londres en julio de ese año. La nota del reverso se encuentra en la página anterior, dentro de dicho álbum, como entradilla a la presente fotografía.Imagen de don Antonio Guerra Marrero, en su despachoCopia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201

Bond formation at polycarbonate | X interfaces (X = Al2_2O3_3, TiO2_2, TiAlO2_2) studied by theory and experiments

Interfacial bond formation during sputter deposition of metal oxide thin films onto polycarbonate (PC) is investigated by ab initio molecular dynamics simulations and X-ray photoelectron spectroscopy (XPS) analysis of PC | X interfaces (X = Al$_2$O$_3$, TiO$_2$, TiAlO$_2$). Generally, the predicted bond formation is consistent with the experimental data. For all three interfaces, the majority of bonds identified by XPS are (C-O)-metal bonds, whereas C-metal bonds are the minority. Compared to the PC | Al$_2$O$_3$ interface, the PC | TiO$_2$ and PC | TiAlO$_2$ interfaces exhibit a reduction in the measured interfacial bond density by ~ 75 and ~ 65%, respectively. Multiplying the predicted bond strength with the corresponding experimentally determined interfacial bond density shows that Al$_2$O$_3$ exhibits the strongest interface with PC, while TiO$_2$ and TiAlO$_2$ exhibit ~ 70 and ~ 60% weaker interfaces, respectively. This can be understood by considering the complex interplay between the metal oxide composition, the bond strength as well as the population of bonds that are formed across the interface

Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides

Motivated by frequently reported deviations from stoichiometry in cubic transition metal nitride (TMNx) thin films, the effect of N-vacancy concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and MoNx (0.72<x<1.00) is systematically studied by density functional theory (DFT) calculations. The predictions are validated experimentally for VNx (0.77<x<0.97). The DFT results indicate that the elastic behavior of the TMNx depends on both the N-vacancy concentration and the valence electron concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase. These trends can be rationalized by considering vacancy-induced changes in elastic anisotropy and bonding. While introduction of N-vacancies in TiNx results in a significant reduction of elastic modulus along all directions and a lower average bond strength of Ti-N, the vacancy-induced reduction in [001] direction of VNx is overcompensated by the higher stiffness along [011] and [111] directions, resulting in a higher average bond strength of V-N. To validate the predicted vacancy-induced changes in elasticity experimentally, close-to-single-crystal VNx (0.77<x<0.97) are grown on MgO(001) substrates. As the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray diffraction, decreases from 4.128 A to 4.096 A. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus, as determined by nanoindentation. As the experimental data agree with the predictions, the elasticity enhancement in VNx upon N-vacancy formation can be understood based on the concomitant changes in elastic anisotropy and bonding.Comment: 30 pages, 8 figures in the manuscript, 1 figure in supplementary material

Quantenchemische Untersuchungen der Nahordnungseffekte und etwaiger Fehlordnung in κ-Karbiden

In the present work, quantum-chemical investigations of κ-carbides with the composition [M]3−xAlxCy ([M] = Fe, Mn) have been performed via density-functional theory-based methods. Carbides of said composition are found as precipitate phases in steels with high concentrations of manganese and aluminum as alloying elements. In such steels, the precipitates are located next to ferritic as well as austenitic domains and affect both the macroscopic properties of the material and deformation mechanisms. Detailed knowledge on an atomistic level is necessary to estimate the effects of this influence. Thus, the role of short-range ordering effects and disorder in κ-carbides arising from stoichiometry and spatial configuration was investigated. Calculation of the theoretical enthalpies of formation and bonding analysis using the Crystal Orbital Hamilton Population (COHP) approach were the key tools here. It was found that ferromagnetic, manganese-rich κ-carbides are energetically favored when formed from α-Fe and α-Mn as starting materials, being the allotropes stable in the ground state. Upon formation from γ-Fe and γ-Mn, stable at high temperatures, iron-rich carbides are preferred. In the case of nonmagnetic carbides, iron-rich compositions are preferred regardless of the allotropy of the starting materials. For the ordered quaternary carbides, Fe2MnAlC and FeMn2AlC, five models distinct in spatial configuration were identified, with slightly different enthalpies of formation for a given composition. This difference is stronger for iron-rich, ferromagnetic systems in comparison to manganese-rich ones, while being roughly equally small for nonmagnetic systems, independent of composition. Another observation is the stronger effect of the spatial configuration upon the structure of ferromagnetic κ-carbides in comparison to the nonmagnetic systems. The impact of magnetic effects is especially strong for Mn-rich carbides and is shown to be the dominant contribution to the total energy of the system when compared to structural relaxation effects. These energetic and structural differences between the different spatially ordered models, albeit small, affect macroscopic properties such as elastic moduli: Young’s modulus, bulk modulus, shear modulus and simulated Vickers hardness are highest for the energetically preferred ordered model in FeMn2AlC. Generally, higher elastic moduli and simulated Vickers hardness were observed for Mn-rich carbides. The ternary carbides Fe3AlC and Mn3AlC and all distinct quaternary models were found to be dynamically stable using a lattice dynamics-based approach. In contrast, upon examination of the electronic structure and subsequent bonding analysis, electronic instability was observed in Fe3AlC, stemming from the weakness of the Fe−Fe bonds. Strong antibonding interactions at the Fermi level were found to occur for these bonds, but not for Mn−Mn bonds in ferromagnetic Mn3AlC. This may partially explain why Fe3AlC has yet to be synthesized as a phase-pure compound, while Mn3AlC, which has been successfully synthesized, is electronically stable. Furthermore, in κ-carbides, the Mn−C bond is stronger than the Fe−C one, yielding further energetic gain. The stronger impact of magnetic effects for Mn-rich systems, mentioned above, can also be explained via bonding analysis, since the Mn−Mn interaction turns strongly antibonding in nonmagnetic systems, whereas for Fe−Fe, the difference in character is insignificant. Lastly, disordered carbides with mixed occupancy of the Al positions with Fe and Mn atoms, and vice versa, have been studied. It was found that, for systems with high carbon content, ideal occupancy without disorder is preferred in terms of enthalpy. However, with decreasing carbon content, the formation of antisite defects, with Al situated on Mn or Fe sites, is favored due to the reduced amount of repulsive Al-C interactions. However, high carbon concentrations are generally preferred for phase-pure, Mn-rich κ-carbides at equilibrium conditions. This observation is backed up by experimental investigations of phase-pure Mn3AlC, but has not been observed in precipitates found in steels, where interfacial effects are relevant

Kinetically Limited Phase Formation of Pt-Ir Based Compositionally Complex Thin Films

The phase formation of PtIrCuAuX (X = Ag, Pd) compositionally complex thin films is investigated to critically appraise the criteria employed to predict the formation of high entropy alloys. The formation of a single-phase high entropy alloy is predicted if the following requirements are fulfilled: 12 J∙K&minus;1 mol&minus;1 &le; configurational entropy &le; 17.5 J∙K&minus;1 mol&minus;1, &minus;10 kJ∙mol&minus;1 &le; enthalpy of mixing &le; 5 kJ∙mol&minus;1 and atomic size difference &le; 5%. Equiatomic PtIrCuAuX (X = Ag, Pd) fulfill all of these requirements. Based on X-ray diffraction and energy-dispersive X-ray spectroscopy data, near-equiatomic Pt22Ir23Cu18Au18Pd19 thin films form a single-phase solid solution while near-equiatomic Pt22Ir23Cu20Au17Ag18 thin films exhibit the formation of two phases. The latter observation is clearly in conflict with the design rules for high entropy alloys. However, the observed phase formation can be rationalized by considering bond strengths and differences in activation energy barriers for surface diffusion. Integrated crystal orbital Hamilton population values per bond imply a decrease in bond strength for all the interactions when Pd is substituted by Ag in PtIrCuAuX which lowers the surface diffusion activation energy barrier by 35% on average for each constituent. This enables the surface diffusion-mediated formation of two phases, one rich in Au and Ag and a second phase enriched in Pt and Cu. Hence, phase formation in these systems appears to be governed by the complex interplay between energetics and kinetic limitations rather than by configurational entropy

Lezione 8

κ-carbides of varying composition, seemingly responsible for age hardening in high-Al steel alloys, have been detected to precipitate both at grain boundaries and in the bulk grain of steels. Herein we report the bulk-phase synthesis of “Mn₃AlC” by arc plasma sintering and rapid solidification. Single crystals have been found suitable for X-ray diffraction using Mo radiation and yield a lattice parameter of <i>a</i> = 3.875(2) Å. We find a mixed occupation of the 1<i>a</i> position by Al and Mn, which, together with the C position being fully occupied, leads to the actual composition Mn_3.1Al_0.9C. Additional energy-dispersive X-ray–scanning electron microscopy measurements support the composition and corroborate the homogeneity. SQUID data collected on the polycrystalline ferromagnetic sample exhibit a Curie temperature of about 295 ± 13 K and a soft magnetic behavior. The small but significant nonstoichiometry on 1<i>a</i> leads to a slightly larger lattice parameter, a higher electron count, and, thus, a lowered density of states at the Fermi level, indicative of increased phase stability