Base-controlled mechanical systems and geometric phases

In this paper, we carry a detailed study of mechanical systems with configuration space Q {long rightwards arrow} Q / G for which the base Q / G variables are being controlled. The overall system's motion is considered to be induced from the base one due to the presence of general non-holonomic constraints. It is shown that the solution can be factorized into dynamical and geometrical parts. Moreover, under favorable kinematical circumstances, the dynamical part admits a further factorization since it can be reconstructed from an intermediate (body) momentum solution, yielding a reconstruction phase formula. Finally, we apply this results to the study of concrete mechanical systems.Facultad de Ciencias Exacta

A generalized Montgomery phase formula for rotating self-deforming bodies

We study the motion of self-deforming bodies with non-zero angular momentum when the changing shape is known as a function of time. The conserved angular momentum with respect to the center of mass, when seen from a rotating frame, describes a curve on a sphere as happens for the rigid body motion, though obeying a more complicated non-autonomous equation. We observe that if, after time Δ T, this curve is simple and closed, the deforming body's orientation in space is fully characterized by an angle or phase θM. We also give a reconstruction formula for this angle which generalizes R, Montgomery's well known formula for the rigid body phase. Finally, we apply these techniques to obtain analytical results on the motion of deforming bodies in some concrete examples.Facultad de Ciencias Exacta

Base-controlled mechanical systems and geometric phases

In this paper, we carry a detailed study of mechanical systems with configuration space Q {long rightwards arrow} Q / G for which the base Q / G variables are being controlled. The overall system's motion is considered to be induced from the base one due to the presence of general non-holonomic constraints. It is shown that the solution can be factorized into dynamical and geometrical parts. Moreover, under favorable kinematical circumstances, the dynamical part admits a further factorization since it can be reconstructed from an intermediate (body) momentum solution, yielding a reconstruction phase formula. Finally, we apply this results to the study of concrete mechanical systems.Facultad de Ciencias Exacta

Aleación mecánica de sistemas Fe-Sn, Fe-Ge y Fe-Si : Evolución temporal y dependencia con la composición

Los métodos de reacción en fase sólida en sistemas finamente divididos concitan considerable interés debido a que permiten acceder a un conjunto de estados metaestables vía la obtención de un primer estado altamente excitado del material. Las transformaciones inducidas pueden conducir a la obtención de amorfos, compuestos estequiométricos, soluciones sólidas de solubilidad extendida, materiales mixtos (compositos) y además pueden producir transiciones orden-desorden, descomposición de compuestos, etc. En este esquema juegan un rol importante las fuerzas termodinámicas que favorecen el estado de equilibrio estable, las condiciones cinéticas experimentales, las energías de activación para el transporte de átomos, y los procesos empleados para crear el estado excitado (transferencia de energía e impulso al sistema por trabajo mecánico con la consiguiente generación de defectos y desorden, producción de movimientos atómicos e interdifusión, etc.). Durante el molido se produce una competencia entre fragmentación y soldadura. Esta competencia da lugar a la creación de una gran área interfacial entre granos cristalinos. En sistemas inicialmente multifásicos el incremento de dicha área y la reducción del tamaño de grano medio conduce a un incremento notable de la interdifusividad que facilita la reacción de estado sólido. El incremento de la concentración de defectos (área intergranular, dislocaciones, vacancias, intersticiales, antisitios, etc.) lleva al sistema inicial a un estado excitado de energía libre relativamente elevada, desde el cual pueden producirse transiciones a estados metaestables. Muchas veces, éstos no pueden alcanzarse por métodos convencionales. Los objetivos de este trabajo fueron investigar experimentalmente los aspectos termodinámicos y cinéticos que controlan la formación de fases por reacción de estado sólido en sistemas metal de transición (MT)- metal sp (Msp).Tesis digitalizada en SEDICI gracias a la Biblioteca de Física de la Facultad de Ciencias Exactas (UNLP).Facultad de Ciencias Exacta

Aleación mecánica de sistemas Fe-Sn, Fe-Ge y Fe-Si : Evolución temporal y dependencia con la composición

Los métodos de reacción en fase sólida en sistemas finamente divididos concitan considerable interés debido a que permiten acceder a un conjunto de estados metaestables vía la obtención de un primer estado altamente excitado del material. Las transformaciones inducidas pueden conducir a la obtención de amorfos, compuestos estequiométricos, soluciones sólidas de solubilidad extendida, materiales mixtos (compositos) y además pueden producir transiciones orden-desorden, descomposición de compuestos, etc. En este esquema juegan un rol importante las fuerzas termodinámicas que favorecen el estado de equilibrio estable, las condiciones cinéticas experimentales, las energías de activación para el transporte de átomos, y los procesos empleados para crear el estado excitado (transferencia de energía e impulso al sistema por trabajo mecánico con la consiguiente generación de defectos y desorden, producción de movimientos atómicos e interdifusión, etc.). Durante el molido se produce una competencia entre fragmentación y soldadura. Esta competencia da lugar a la creación de una gran área interfacial entre granos cristalinos. En sistemas inicialmente multifásicos el incremento de dicha área y la reducción del tamaño de grano medio conduce a un incremento notable de la interdifusividad que facilita la reacción de estado sólido. El incremento de la concentración de defectos (área intergranular, dislocaciones, vacancias, intersticiales, antisitios, etc.) lleva al sistema inicial a un estado excitado de energía libre relativamente elevada, desde el cual pueden producirse transiciones a estados metaestables. Muchas veces, éstos no pueden alcanzarse por métodos convencionales. Los objetivos de este trabajo fueron investigar experimentalmente los aspectos termodinámicos y cinéticos que controlan la formación de fases por reacción de estado sólido en sistemas metal de transición (MT)- metal sp (Msp).Tesis digitalizada en SEDICI gracias a la Biblioteca de Física de la Facultad de Ciencias Exactas (UNLP).Facultad de Ciencias Exacta

Base-controlled mechanical systems and geometric phases

In this paper, we carry a detailed study of mechanical systems with configuration space Q {long rightwards arrow} Q / G for which the base Q / G variables are being controlled. The overall system's motion is considered to be induced from the base one due to the presence of general non-holonomic constraints. It is shown that the solution can be factorized into dynamical and geometrical parts. Moreover, under favorable kinematical circumstances, the dynamical part admits a further factorization since it can be reconstructed from an intermediate (body) momentum solution, yielding a reconstruction phase formula. Finally, we apply this results to the study of concrete mechanical systems.Facultad de Ciencias Exacta

A generalized Montgomery phase formula for rotating self-deforming bodies

We study the motion of self-deforming bodies with non-zero angular momentum when the changing shape is known as a function of time. The conserved angular momentum with respect to the center of mass, when seen from a rotating frame, describes a curve on a sphere as happens for the rigid body motion, though obeying a more complicated non-autonomous equation. We observe that if, after time Δ T, this curve is simple and closed, the deforming body's orientation in space is fully characterized by an angle or phase θM. We also give a reconstruction formula for this angle which generalizes R, Montgomery's well known formula for the rigid body phase. Finally, we apply these techniques to obtain analytical results on the motion of deforming bodies in some concrete examples.Facultad de Ciencias Exacta

Effects of coexisting spin disorder and antiferromagnetism on the magnetic behavior of nanostructured (Fe79Mn21)1xCux alloys

We report a magnetic study on nanostructured (Fe79Mn21)1xCux (0.00x0.30) alloys using static magnetic measurements. The alloys are mainly composed by an antiferromagnetic fcc phase and a disordered region that displays a spin-glass-like behavior. The interplay between the antiferromagnetic and magnetically disordered phases establishes an exchange anisotropy that gives rise to a loop shift at temperatures below the freezing temperature of moments belonging to the disordered region. The loop shift is more noticeable as the Cu content increases, which also enhances the spin-glass-like features. Further, in the x¼0.30 alloy the alignment imposed by applied magnetic fields higher than 4 kOe prevail over the configuration determined by the frustration mechanism that characterizes the spin glass-like phase.Instituto de Físic

Study of the relation between oxygen vacancies and ferromagnetism in Fe-doped TiO2 nano-powders

In this work, we present an experimental and theoretical study of structural and magnetic properties of Fe doped rutile TiO2 nanopowders. We show that Fe-doping induces the formation of oxygen vacancies in the first-sphere coordination of iron ions, which are in +2 and +3 oxidation states. We found that Fe ions form dimers that share one oxygen vacancy in the case of Fe3+ and two oxygen vacancies in the case of Fe2+. The saturation magnetization is almost independent of iron concentration and slightly increases with the relative fraction of Fe2+. Ab initio calculations show that two Fe ions sharing an oxygen vacancy are coupled ferromagnetically, forming a bound magnetic polaron (BMP), but two neighbor BMPs are aligned antiparallel to each other. Extra electron doping plays a fundamental role mediating the magnetic coupling between the ferromagnetic entities: carriers, possibly concentrated at grain boundaries, mediate between the BMP to produce ferromagnetic alignment.Fil: Mudarra Navarro, Azucena Marisol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: RodrÍguez Torres, Claudia Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Bilovol, Vitaliy. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Laboratorio de Sólidos Amorfos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cabrera, Alejandra Fabiana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Errico, Leonardo Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional del Noroeste de la Provincia de Buenos Aires; ArgentinaFil: Weissmann, Mariana Dorotea. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Effect of nanostructured ferrites MFe₂ O₄ (M = Cu, Co, Mg, Zn) on the thermal decomposition of ammonium nitrate

Ammonium nitrate (NH₄ NO₃) represents a cheap, chlorine-free alternative to ammonium perchlorate for use as an oxidant for solid propellants. But its poor ignitability and low burning rate are all disadvantages to achieve such purposes. For this reason, it is necessary to carry out studies to improve its combustion characteristics, seeking to combine it with catalysts or fuels. The present work explores the possibility of improving its combustion characteristics by adding ferrites as catalysts. Nanostructured ferrites MFe₂ O 4 (M = Mg, Co, Cu, and Zn) synthesized by autocombustion method were tested as catalysts for the thermal decomposition reaction of ammonium nitrate under open, partially open or sealed conditions. The ferrites were characterized by XRD, SEM, UV-vis spectrophotometry and Mössbauer spectroscopy. All the MFe₂ O₄ samples are single phased with a cubic spinel structure and average sizes, L, ranging from about 9 (CoFe₂ O₄) to 25 nm (CuFe₂ O₄). The catalytic effect of MFe₂ O₄ on the thermal decomposition of NH 4 NO 3 was investigated by thermogravimetric analysis and differential scanning calorimetry techniques. The process was also followed in a volumetric Sieverts type apparatus. The results indicate that only under sealed conditions the addition of these ferrites has influence in the decomposition process of AN. The incorporation of any of these ferrites decreases the onset temperature of the process manifested itself through an exothermic reaction, and also increases the amount of heat released in the reaction. The Co-ferrite showed the best efficiency causing the onset temperature to drop around 60 °C. The catalytic performance is correlated with the electronegativity of M 2 + cations, which act as Lewis acid sites that interact with the gas molecules.Instituto de Física La Plat