186,543 research outputs found
Studies on Bis(imido) molybdenum complexes containing unsaturated hydrocarbon ligands
No full textThis thesis describes the synthesis and characterisation of molybdenum bis(imido) complexes containing unsaturated hydrocarbon ligands. A principal objective of the work was to examine the effect of various imido substituents on the coordination number of the complex and the orientations adopted by olefin and acetylene ligands. Chapter One highlights areas of transition metal chemistry relevant to the thesis, with particular emphasis on the psuedo-isolobal analogy between cyclopentadienyl and imido ligands, A convenient one-pot synthesis of molybdenum bis(imido) complexes of the type Mo(NR)(NR')Cl(_2).DME (R=R'=l-adamantyl, 2-t- BUC(_6)H(_4); R=2,6-i-Pr(_2)C(_6)H(_3), R'=t-Bu) is described in Chapter Two. Mo(N-l- adamantyl)(O)Cl(_2).DME has been synthesised, and its structure determined by single crystal X-ray diffraction.- The preparation of olefin complexes Mo(NR)(NR')(C(_2)H(_4))(PMe(_3))n (R=R'=l-adamantyl, n=l; R=R'=2-t-BuC(_6)H(_4), n=2; R=2,6-i-Pr(_2)C(_6)H(_3), R'=t- Bu, n=l) is outlined in Chapter Three. Structural information derived from NMR data has allowed comparison with metallocene-like olefin adducts. Chapter Four describes the synthesis of complexes containing σ-bound phenyl ligands (Mo(NR)(NR')(σ-C(_6)H(_5))(PMe(_3)) (R=R'=l-adamantyl, 2-t-BuC(_6)H(_4); R=2,6-i-Pr(_2)C(_6)H(_3), R'=t-Bu)) as potential precursors to benzyne complexes. Chapter Five describes the preparation of diphenylacetylene complexes Mo(NR)(NR')(PhC=CPh)(PMe(_3)), structural information derived from NMR data allows comparison with previously known metallocene-like acetylene complexes. Full experimental details for Chapters Two to Five are given in Chapter Six
Electrical, morphological and structural properties of RF magnetron sputtered Mo thin films for application in thin film photovoltaic solar cells
No full textMolybdenum (Mo) thin films were deposited using radio frequency magnetron sputtering, for application as a metal back contact material in ‘‘substrate configuration’’ thin film solar cells. The variations of the electrical, morphological, and structural properties of the deposited films with sputtering pressure, sputtering power and post-deposition annealing were determined. The electrical conductivity of the Mo films was found to increase with decreasing sputtering pressure and increasing sputtering power. X-ray diffraction data showed that all the films had a (110) preferred orientation that became less pronounced at higher sputtering power while being relatively insensitive to process pressure. The lattice stress within the films changed from tensile to compressive with increasing sputtering power and the tensile stress increased with increasing sputtering pressure. The surface morphology of the films changed from pyramids to cigar-shaped grains for a sputtering power between 100 and 200 W, remaining largely unchanged at higher power. These grains were also observed to decrease in size with increasing sputtering pressure. Annealing the films was found to affect the resistivity and stress of the films. The resistivity increased due to the presence of residual oxygen and the stress changed from tensile to compressive. The annealing step was not found to affect the crystallisation and grain growth of the Mo films
Nuevos aceros sinterizados de baja aleación Al Cr-Mo : sinterabilidad, comportamiento mecánico y tribológico, tratamientos térmicos y termoquímicos
No full textEn esta Tesis Doctoral se estudia con profundidad el desarrollo de aceros sinterizados de baja aleación aleados con Cr, Mo y Mn, siendo su principal objetivo la optimización del comportamiento mecánico y tribológico de aceros basados en el polvo prealeado Fe 3Cr-0,5Mo, mediante actuaciones en el proceso de conformado y los tratamientos térmicos y termoquímicos.
En metalurgia de polvos existen dos posibles vías que conducen a la mejora del nivel de las propiedades mecánicas y tribológicas de un material: 1) actuar sobre la densidad y, 2) modificar el sistema de aleación, introduciendo nuevos elementos que propicien mejoras por
endurecimiento por solución sólida o bien a través de los tratamientos térmicos.
En esta Tesis Doctoral se ha actuado en los dos sentidos. Por un lado se ha mejorado la densidad: mediante compactación de polvos precalentados (“warm compaction”), sinterizando a
temperaturas elevadas (1250°C) o modificando el proceso de sinterización introduciendo fases líquidas transitorias. Y también se ha utilizado la segunda vía, utilizándose la adición de Mn como posible camino de mejora de las propiedades.
Todos los materiales desarrollados, han sido optimizados mediante un completo estudio de sinterabilidad, apoyado en técnicas de análisis térmico (dilatometría, y análisis térmico
diferencial con análisis termogravimétrico simultáneo, “STA”), y caracterizados desde el punto de vista mecánico y tribológico, tanto
en estado sinterizado como tratado térmica y termoquímicamente.
Aunque el cromo sea un elemento al uso en los aceros de colada, en los aceros pulvimetallúrgicos, debido a su gran afinidad
por el oxígeno, su implantación se ha visto limitada por el desarrollo tecnológico de hornos y control de atmósferas. Su utilización, al
igual que el manganeso, cayó en desuso a partir de los 80. La década de los 90 y el final de los 80 han estado marcadas por la aceptación de los polvos, bien aleados por difusión o bien prealeados, con contenidos en níquel, molibdeno y cobre fundamentalmente. Solucionar esas limitaciones técnicas, y encontrar la forma para optimizar la sinterización de aceros con contenidos en cromo, es otro de los objetivos de esta investigación.
_________________________________________________The development of this Thesis comprises a deeply study of low prealloyed sintered steels with Cr, Mo and Mn. Its main objective is optimise the mechanical and wear behaviour of sintered steel based
on a prealloyed powder Fe-3Cr-0,5Mo, through improvements on
processing parameters, either on compactation and sintering or on
secondary treatments as heat and thermo chemical treatments.
In powder metallurgy there is two possible ways to lead an
increase of mechanical properties and wear resistance level: 1) it is
possible to act on the density, 2) to modify the alloy system by
adding new alloying elements that promote by solid solution
improvements, or through heat treatments.
Both senses have been taken into account in this Thesis.
Density has been enhanced by warm compaction and high sintering
temperature (1250°C), or by modifying the sintering process
introducing transient liquid phases. Mn additions are the answer
when solid solution effect or heat treatments improvements want to
be achieved.
Ah materials have been optimised through a complete sintered
study, based on thermal analysis (STA and dilatometries). They have
been characterised from the point of view of their mechanical and
wear behaviour, comparing in every case ah the possible finished
states that the material could has.
Although chromium was a common used alloy element on ingot
steels, in powder metallurgy, due to its high oxygen affinity, its
introduction has been imitated for the technical resources as wehl as
for the improper atmosphere control. Its use, with manganese
element, falls down in 80’s. The 90’s decade and also the lasts 80’s
have been marked for the favourable reception of prealloyed steels
with Nickel, Molybdenum and Copper. To solve technical limitations
and to find the appropriate sintering process of chromium-alloyed
steels are also objectives of this research
PHOTO STIMULATED OXYGEN ATOM TRANSFER WITH MO(DIOXO) COMPLEXES ANCHORED ON TIO2
No full textEn este trabajo se presenta la formación de un sistema verde bioinspirado para la Transferencia de Oxígeno (TAO). Se estudió la TAO a Pf3 y etilbenceno empleando oxígeno molecular y luz UV-Vis, el catalizador consiste en complejos del tipo [Mo(=O)2L], L=bipiridil (bipi), bipirazolil (bpz) anclandos sobre TiO2. La TAO se estimula por una corriente electrónica, generada por acción de radiación UV-visible sobre TiO2, que es transferida a la unidad Mo=O para propiciar la TAO. La contribución del anión superóxido O2-, formado por reacción de los e- fotogenerados con el O2, parece jugar un papel importante en la regeneración del centro activo Mo(=O)2 ya que en ausencia de luz la TAO solo es estequiométrica.Se observó que aunque el complejo bipirazolil es un buen agente en la TAO, debido a su propiedad donadora de electrones, el sistema es pobre en la activación de Oxígeno debido a su labilidad a la lixiviación en medio orgánico, para este efecto se obtiene un mejor balance con el ligando Bipi. Con estos sistemas se abre una pers-pectiva para el desarrollo de procesos verdes para la oxidación selectiva de compuestos orgánicosIn this paper, the formation of an Oxygen Transfer (OAT) green bioinspired system was developed. The OAT to Pφ3 and ethylbenzene, using molecular oxygen and UV-Vis light is reported. The catalyst consists of com-plexes of the type [Mo(=O)2L], L=bipyriyll(bipi), lbipyrazolyl(bpz) bound to TiO2. The OAT is stimulated by an electron current, generated by the action of UV-visible light on TiO2, which is transferred to the Mo = O unit to promote the OAT. The contribution of the superoxide anion O2-, formed by the action of the e-photogenerated with O2, seems to play an important role in regenerating the Mo(= O)2 active center since in the absence of light OAT is stoichiometric. Although the bipirazolyl is a good OAT agent due to its electron donating property, the system is poor in the activation of oxygen due to its organic media leaching lability, with the ligand Bipi a better balance is obtained. These systems offer a good opportunity to develop green processes for selective oxidation of organic compounds
Commuting difference operators, spinor bundles and the asymptotics of orthogonal polynomials with respect to varying complex weights
No full textAbstractThe paper has three parts. In the first part we apply the theory of commuting pairs of (pseudo) difference operators to the (formal) asymptotics of orthogonal polynomials: using purely geometrical arguments we show heuristically that the asymptotics, for large degrees, of orthogonal polynomial with respect to varying weights is intimately related to certain spinor bundles on a hyperelliptic algebraic curve reproducing formulae appearing in the works of Deift et al. on the subject.In the second part we show that given an arbitrary nodal hyperelliptic curve satisfying certain conditions of admissibility we can reconstruct a sequence of polynomials orthogonal with respect to semiclassical complex varying weights supported on several curves in the complex plane. The strong asymptotics of these polynomials will be shown to be given by the spinors introduced in the first part using a Riemann–Hilbert analysis.In the third part we use Strebel theory of quadratic differentials and the procedure of welding to reconstruct arbitrary admissible hyperelliptic curves. As a result we can obtain orthogonal polynomials whose zeroes may become dense on a collection of Jordan arcs forming an arbitrary forest of trivalent loop-free trees
Mechanical performance and fracture behavior of Fe₄₁Co₇Cr₁₅Mo₁₄Y₂C₁₅B₆ bulk metallic glass
No full textThe mechanical properties of a new Fe₄₁Co₇Cr₁₅Mo₁₄Y₂C₁₅B₆ bulk glassy alloy were studied by impact bending, compression, and hardness tests carried out at room temperature. The compressive fracture strength, elastic strain to fracture, Young’s modulus and Vickers hardness were measured to be 3.5 GPa, 1.5%, 265 GPa, and 1253 kg mm⁻², respectively. The fracture mode of the glassy alloy under uniaxial compression is different from those of other bulk metallic glasses in that this fracture mode causes the samples to be broken, in an exploding manner, into a large number of micrometer-scale pieces. The fracture mechanisms of this bulk glassy alloy under bending and uniaxial compression are discussed based on the observation of the fracture surfaces. Vickers indentation tests indicate that the structure of the glassy ingot may be inhomogeneous
Isomonodromic deformation of resonant rational connections
No full textWe analyze isomonodromic deformations of rational connections on the Riemann sphere with Fuchsian and irregular singularities. The Fuchsian singularities are allowed to be of arbitrary resonance index; the irregular singularities are also allowed to be resonant in the sense that the leading coefficient matrix at each singularity may have arbitrary Jordan canonical form, with a genericity condition on the Lidskii submatrix of the subleading term. We also give the relevant notion of isomonodromic tau function extending the one given for nonresonant deformations by Jimbo, Miwa, and Ueno. The tau function is expressed purely in terms of spectral invariants of the matrix of the connection
Mesoscopic colonization of a spectral band
No full textWe consider the unitary matrix model in the limit where the size of the matrices become infinite and in the critical situation when a new spectral band is about to emerge. In previous works the number of expected eigenvalues in a neighborhood of the band was fixed and finite, a situation that was termed "birth of a cut" or "first colonization". We now consider the transitional regime where this microscopic population in the new band grows without bounds but at a slower rate than the size of the matrix. The local population in the new band organizes in a "mesoscopic" regime, in between the macroscopic behavior of the full system and the previously studied microscopic one. The mesoscopic colony may form a finite number of new bands, with a maximum number dictated by the degree of criticality of the original potential. We describe the delicate scaling limit that realizes/controls the mesoscopic colony. The method we use is the steepest descent analysis of the Riemann-Hilbert problem that is satisfied by the associated orthogonal polynomials
Stacking change in MoS2 bilayers induced by interstitial Mo impurities
No full textWe use a theoretical approach to reveal the electronic and structural properties of molybdenum impurities between MoS2 bilayers. We find that interstitial Mo impurities are able to reverse the well-known stability order of the pristine bilayer, because the most stable form of stacking changes from AA' (undoped) into AB' (doped). The occurrence of Mo impurities in different positions shows their split electronic levels in the energy gap, following octahedral and tetrahedral crystal fields. The energy stability is related to the accommodation of Mo impurities compacted in hollow sites between layers. Other less stable configurations for Mo dopants have larger interlayer distances and band gaps than those for the most stable stacking. Our findings suggest possible applications such as exciton trapping in layers around impurities, and the control of bilayer stacking by Mo impurities in the growth process.Tis work was part fnanced by a Fondecyt grant 1140388 and Anillo Bicentenario de Ciencia y Tecnología, Conicyt grant Act-1204. J.W. González and A. Ayuela acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness MINECO projects FIS2016-76617-P, the Basque Government under the ELKARTEK project(SUPER), and the University of the Basque Country grant No. IT-756-13. N. Cortés acknowledge support from the FSM1204 project, Conicyt grant No 21160844 and the hospitality of CFMMPC and DIPC.Peer reviewe
Metodología de diseño de robots de asistenciales. Aplicación al robot portátil ASIBOT
No full textEl envejecimiento extraordinario de la población provoca la necesidad de desarrollar ayudas técnicas que asistan tanto a personas discapacitadas como a ancianos. Si bien el campo de la robótica asistencial lleva más de 25 años en
desarrollo, todavía no se ha llegado a soluciones útiles y eficientes en cuanto al ratio beneficio-coste para el conjunto de la sociedad, y las actuales soluciones, se limitan a cubrir nichos de mercado.Esta tesis propone una metodología para el diseño de robots asistenciales, que a de partir de un equipo de diseño multidisciplinar e incluir a los usuarios en el proceso de diseño. Permite analizar, con enfoque global y no sólo científico, cómo influyen en la toma de decisiones los factores relacionados con el entorno, las tareas, el usuario y su seguridad. Plantea el diseño con el objetivo de desarrollar productos que llegan a todos y por tanto sean potencialmente económicos.
La aplicación de esta metodología ha dado lugar al desarrollo de un sistema modular asistencial portátil, capaz de integrarse fácilmente en un entorno adaptado y de responder satisfactoriamente a las necesidades de un amplio espectro de personas con movilidad reducida.
___________________________________________Nowadays, the growing number of elderly people in today’s populations has created the need for developing some technical aids that helps disabled as well as elderly people. Although the field of rehabilitation robotics has been developing for over 25 years, still no useful efficient solutions have been achieved
concerning the benefit-cost ratio for the general society, and the solutions that do exist are limited to certain specific market segments.This thesis proposes a methodology for the design of robotic assistant devices, from the creation of a multidisciplinary design team with the participation of the user, to the analysis of how the environment, tasks, users and safety issues, influence the decision making process using a scientific and also a humanistic point of view. The main goal of this methodology is to create robotic products which reach the all the market segments, thereby reducing the costs of these.
The application of this methodology has led to the development of a portable
assistance modular robot that can be easily integrated in to adapted environment and that responds to the special needs of a wide range of people
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