Structured Linearization of Discrete Mechanical Systems for Analysis and Optimal Control

Variational integrators are well-suited for simulation of mechanical systems because they preserve mechanical quantities about a system such as momentum, or its change if external forcing is involved, and holonomic constraints. While they are not energy-preserving they do exhibit long-time stable energy behavior. However, variational integrators often simulate mechanical system dynamics by solving an implicit difference equation at each time step, one that is moreover expressed purely in terms of configurations at different time steps. This paper formulates the first- and second-order linearizations of a variational integrator in a manner that is amenable to control analysis and synthesis, creating a bridge between existing analysis and optimal control tools for discrete dynamic systems and variational integrators for mechanical systems in generalized coordinates with forcing and holonomic constraints. The forced pendulum is used to illustrate the technique. A second example solves the discrete LQR problem to find a locally stabilizing controller for a 40 DOF system with 6 constraints.Comment: 13 page

Influence of surface anisotropy on the magnetization reversal of nanoparticles

The influence of surface anisotropy on the magnetization processes of maghemite nanoparticles with ellipsoidal shape is studied by means of Monte Carlo simulations. Radial surface anisotropy is found to favor the formation of hedgehog-like spin structures that become more stable as the surface anisotropy constant at the surface $k_S$ is increased form the value at the core. We have studied the change in the low temperature hysteresis loops with the particle aspect ratio and with $k_S$, finding a change in the magnetization reversal mode as $k_S$ or the particle elongation is increased.Comment: Contribution to SCM2004 (2nd Seeheim Conference on Magnetism), to be published in Physica Status Solidi A. 4 pages, 2 figure

Synthesis of Y1BaCu3O(x) superconducting powders by intermediate phase reactions

A procedure for synthesizing Y1Ba2Cu3O(x) by solid state reactions was developed. The method is based on the use of barium compounds, previously synthesized, as intermediate phases for the process. The reaction kinetics of this procedure were established between 860 C and 920 C. The crystal structure and the presence of second phases were studied by means of XRD. The sintering behavior and ceramic parameters were also determined. The orthorhombic type-I structure was obtained on the synthesized bodies after a cooling cycle in an air atmosphere. Superconducting transition took place at 91 K. Sintering densities higher than 95 percent D sub th were attained at temperatures below 940 C

Surfactant effects in monodisperse magnetite nanoparticles of controlled size

Monodisperse magnetite Fe3O4 nanoparticles of controlled size within 6 and 20 nm in diameter were synthesized by thermal decomposition of an iron organic precursor in an organic medium. Particles were coated with oleic acid. For all samples studied, saturation magnetization Ms reaches the expected value for bulk magnetite, in contrast to results in small particle systems for which Ms is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is also similar to that of bulk magnetite. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder. However, although the saturated state may be similar, the approach to saturation is different and, in particular, the high-field differential susceptibility is one order of magnitude larger than in bulk materials. The relevance of these results in biomedical applications is discussed.Comment: 3 pages, 3 figures. Presented at JEMS 2006 (San Sebastian, Spain). Submitted to JMM

Probing the catalytic activity of sulfate-derived Pristine and post-treated porous TiO2(101) anatase mesocrystals by the oxidative desulfurization of dibenzothiophenes

Mesocrystals (basically nanostructures showing alignment of nanocrystals well beyond crystal size) are attracting considerable attention for modeling and optimization of functionalities. However, for surface-driven applications (heterogeneous catalysis), only those mesocrystals with excellent textural properties are expected to fulfill their potential. This is especially true for oxidative desulfuration of dibenzothiophenes (hard to desulfurize organosulfur compounds found in fossil fuels). Here, we probe the catalytic activity of anatases for the oxidative desulfuration of dibenzothiophenes under atmospheric pressure and mild temperatures. Specifically, for this study, we have taken advantage of the high stability of the (101) anatase surface to obtain a variety of uniform colloidal mesocrystals (approximately 50 nm) with adequate orientational order and good textural properties (pores around 3-4 nm and surface areas around 200 m2/g). Ultimately, this stability has allowed us to compare the catalytic activity of anatases that expose a high number of aligned single crystal-like surfaces while differing in controllable surface characteristics. Thus, we have established that the type of tetrahedral coordination observed in these anatase mesocrystals is not essential for oxidative desulfuration and that both elimination of sulfates and good textural properties significantly improve the catalytic activity. Furthermore, the most active mesocrystals have been used to model the catalytic reaction in three-(oil-solvent-catalyst) and two-phase (solvent-catalyst) systems. Thus, we have been able to observe that the transfer of DBT from the oil to the solvent phase partially limits the oxidative process and to estimate an apparent activation energy for the oxidative desulfuration reaction of approximately 40 kJ/mol in the two-phase system to avoid mass transfer limitations. Our results clearly establish that (101) anatase mesocrystals with excellent textural properties show adequate stability to withstand several post-treatments without losing their initial mesocrystalline character and therefore could serve as models for catalytic processes different from the one studied here.Fil: Rivoira, Lorena Paola. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigación y Tecnología Química. Universidad Tecnológica Nacional. Facultad Regional Córdoba. Centro de Investigación y Tecnología Química; ArgentinaFil: Martinez, Maria Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Tecnológica Nacional. Facultad Regional Córdoba. Centro de Investigación en Nanociencia y Nanotecnología; ArgentinaFil: Falcon, Horacio. Universidad Tecnológica Nacional. Facultad Regional Córdoba. Centro de Investigación en Nanociencia y Nanotecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Beltramone, Andrea Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Tecnológica Nacional. Facultad Regional Córdoba. Centro de Investigación en Nanociencia y Nanotecnología; ArgentinaFil: Campos-Martin, Jose M.. Consejo Superior de Investigaciones Científicas; EspañaFil: García Fierro, José Luis. Instituto de Catalisis y Petroleoquimica-csic; EspañaFil: Tartaj, Pedro. Consejo Superior de Investigaciones Científicas. Instituto de Catálisis y Petroleoquímica; Españ

Crystalline structure of the manganites solid solution RE(Me,Mn)O3, (RE=Gd,Er; Me=Ni,Co)

Las propiedades estructurales de las soluciones sólidas RE(Me,Mn)O3, RE=Gd,Er, Me=Ni,Co, han sido estudiadas por difracción de rayos X, (DRX) y medidas eléctricas. Las fases se sintetizaron por reacción en estado sólido entre los óxidos componentes. La incorporación de los cationes Ni2+ y Co2+,3+ en la red en lugar de Mn lleva a cambios en los parámetros de red y en la simetría de la perovskita, GdMnO3 o del compuesto hexagonal ErMnO3 respectivamente. Las transiciones de fase dependen de la cantidad de Mn3+ sustituido, y por tanto de la debilitación del efecto co-operativo Jahn-Teller. Las soluciones sólidas basadas en GdMnO3 cambian de perovskita tipo O’ a perovskita tipo O. Esta transición ocurre para una menor cantidad de Ni que para Co como sustituyentes. Las soluciones basadas en ErMnO3 muestran un comportamiento algo diferente: la incorporación induce cambios desde la estructura hexagonal a una estructura tipo perovskita en la forma O, para cantidades de ~20 at% Ni y para ~30 at% Co. La influencia del factor estérico en las transiciones observadas parece jugar un papel secundario frente a la desaparición progresiva de los cationes Jahn-Teller Mn3+Peer reviewe

Application of homogeneously precipitated nanosized Fe-doped alumina powders to carbon nanotube growth.

Homogeneous precipitation of hydroxides was investigated as an alternative method to synthesize Fe-doped aluminum oxide (α-Al2−2xFe2xO3) particles over which carbon nanotubes (CNTs) were grown via a catalytic chemical vapor deposition (CCVD) method. Performance of the homogeneously precipitated particles for CNT growth was quantitatively compared with that of the combustion-synthesized particles. The main advantage of the homogeneous precipitation of hydroxides and subsequent calcination process against to the combustion synthesis and other commonly practiced chemical routes is the ability to tailor the Fe-doped Al2O3 precursor powder characteristics such as size and specific surface area (SSA) without requiring any milling step and also to control the phase composition of the oxide powder with high Fe content, and subsequently the quality and quantity of CNTs during CCVD process. The particle size of the precipitated and calcined α-Al2−2xFe2xO3 powders varies between ∼50 and 400 nm for 5–10 cat.% Fe-containing systems. The monodispersed particle size distribution and optimum phase composition of the homogeneously precipitated powders, particularly for a 10 cat.% Fe content in the starting oxide, and their much higher SSA than similar materials prepared by other chemical routes lead to production of high amounts of good quality CNTs

Magnetic Nanoparticles for Power Absorption: optimizing size, shape and magnetic properties

We present a study on the magnetic properties of naked and silica-coated Fe3O4 nanoparticles with sizes between 5 and 110 nm. Their efficiency as heating agents was assessed through specific power absorption (SPA) measurements as a function of particle size and shape. The results show a strong dependence of the SPA with the particle size, with a maximum around 30 nm, as expected for a Neel relaxation mechanism in single-domain particles. The SiO2 shell thickness was found to play an important role in the SPA mechanism by hindering the heat outflow, thus decreasing the heating efficiency. It is concluded that a compromise between good heating efficiency and surface functionality for biomedical purposes can be attained by making the SiO2 functional coating as thin as possible.Comment: 15 pages, 7 figures, 2 table

Aggregation state and magnetic properties of magnetite nanoparticles controlled by an optimized silica coating

The control of magnetic interactions is becoming essential to expand/improve the applicability of magnetic nanoparticles (NPs). Here, we show that an optimized microemulsion method can be used to obtain homogenous silica coatings on even single magnetic nuclei of highly crystalline Fe3-xO4 NPs (7 and 16nm) derived from a high-temperature method. We show that the thickness of this coating is controlled almost at will allowing much higher average separation among particles as compared to the oleic acid coating present on pristine NPs. Magnetic susceptibility studies show that the thickness of the silica coating allows the control of magnetic interactions. Specifically, as this effect is better displayed for the smallest particles, we show that dipole-dipole interparticle interactions can be tuned progressively for the 7 nm NPs, from almost non-interacting to strongly interacting particles at room temperature. The quantitative analysis of the magnetic properties unambiguously suggests that dipolar interactions significantly broaden the effective distribution of energy barriers by spreading the distribution of activation magnetic volumes. Published by AIP Publishing

Iron oxide nanosized clusters embedded in porous nanorods: A new colloidal design to enhance capabilities of MRI contrast agents

“This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Nano, © 2010 American Chemical Society after peer review and technical editing by the publisher. To acces final work see “Iron oxide nanosized clusters embedded in porous nanorods: A new colloidal design to enhance capabilities of MRI contrast agents”, ACS Nano 4.4 (2010): 2095-2103, https://doi.org/10.1021/nn9013388"Development of nanosized materials to enhance the image contrast between the normal and diseased tissue and/or to indicate the status of organ functions or blood flow is essential in nuclear magnetic resonance imaging (MRI). Here we describe a contrast agent based on a new iron oxide design (superparamagnetic iron oxide clusters embedded in antiferromagnetic iron oxide porous nanorods). We show as a proof-of-concept that aqueous colloidal suspensions containing these particles show enhanced-proton relaxivities (i.e., enhanced MRI contrast capabilities). A remarkable feature of this new design is that large scale production is possible since aqueous-based routes are used, and porosity and iron oxide superparamagnetic clusters are directly developed from a single phase. We have also proved with the help of a simple model that the physical basis behind the increase in relaxivities lies on both the increase of dipolar field (interactions within iron oxide clusters) and the decrease of proton-cluster distance (porosity favors the close contact between protons and clusters). Finally, a list of possible steps to follow to enhance capabilities of this contrast agent is also included (partial coating with noble metals to add extra sensing capacity and chemical functionality, to increase the amount of doping while simultaneously carrying out cytotoxicity studies, or to find conditions to further decrease the size of the nanorods and to enhance their stability)We acknowledge financial support from the Spanish Ministerio de Ciencia e innovación trough MAT2008-03224/NAN and from the Comunidad Autónoma de Madrid under Project S-0505/MAT/019