8,721 research outputs found
Transition on the entropic elasticity of DNA induced by intercalating molecules
We use optical tweezers to perform stretching experiments on DNA molecules
when interacting with the drugs daunomycin and ethidium bromide, which
intercalate the DNA molecule. These experiments are performed in the low-force
regime from zero up to 2 pN. Our results show that the persistence length of
the DNA-drug complexes increases strongly as the drug concentration increases
up to some critical value. Above this critical value, the persistence length
decreases abruptly and remains practically constant for larger drug
concentrations. The contour length of the molecules increases monotonically and
saturates as drugs concentration increases. Measured in- tercalants critical
concentrations for the persistence length transition coincide with reported
values for the helix-coil transition of DNA-drug complexes, obtained from
sedimentation experiments.Comment: This experimental article shows and discuss a transition observed in
the persistence length of DNA molecules when studied as a function of some
intercalating drug concentrations, like daunomycin and ethidium bromide. It
has 15 pages and 4 figures. The article presented here is in preprint forma
Non-collinear coupling between magnetic adatoms in carbon nanotubes
The long range character of the exchange coupling between localized magnetic
moments indirectly mediated by the conduction electrons of metallic hosts often
plays a significant role in determining the magnetic order of low-dimensional
structures. In addition to this indirect coupling, here we show that the direct
exchange interaction that arises when the moments are not too far apart may
induce a non-collinear magnetic order that cannot be characterized by a
Heisenberg-like interaction between the magnetic moments. We argue that this
effect can be manipulated to control the magnetization alignment of magnetic
dimers adsorbed to the walls of carbon nanotubes.Comment: 13 pages, 5 figures, submitted to PR
Corrosão de interfaces ti/al2o3 em solucões fisiologicas simuladas
Nos últimos anos o estudo das interfaces metal/cerâmico (M/C) tem despertado grande
interesse devido às diversas utilidades que estas podem oferecer em diferentes campos
de aplicação prática. Um destes campos é o relativo à área das aplicações biomédicas.
De entre os vários processos existentes para a produção de uniões M/C, as técnicas de
união por difusão e de brasagem activa são algumas das mais utilizadas. Em qualquer
uma destas técnicas, a alta temperatura envolvida no processo é um parâmetro que favorece o desenvolvimento de reacções químicas complexas que dão origem a interfaces multi-camadas cujas propriedades físico-químicas são complexas. Esta
complexidade da interface, unida ao facto, já conhecido, de que as propriedades mecânicas, físicas e químicas, incluindo a resistência à corrosão, dependem da composição química e da microestrutura da região interfacial, fazem com que seja necessário um melhor conhecimento da natureza e das características físico-químicas
desta região, de forma a poder controlar mais estritamente as propriedades da interface.
O principal objectivo deste trabalho é o estudo do comportamento à corrosão de interfaces Ti/Al2O3 produzidas pela técnica de união por brasagem activa. Para efectuar este estudo para além da interface, foram produzidas ligas com composições químicas similares a cada camada interfacial, as quais foram testadas através de ensaios electroquímicos de corrente directa, nomeadamente, polarização potenciodinâmica,
resistência à polarização e corrosão galvânica. Neste trabalho, será descrito o comportamento à corrosão da interface, assim como o efeito que a interacção entre as diferentes camadas presentes na interface produz sobre o comportamento à degradação geral da interface Ti/Al2O3.Fundação para a Ciência e a Tecnologia
(FCT), Portugal (Projectos POCTI/CTM/33384/2000 e SFRH/BPD/5518/2001)
Corrosion behaviour of Ti/Al2O3 interfaces produced by an active metal brazing methodology
Metal/ceramic joints are used in a broad range of applications in biomedicine, such as the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or the production of drug delivery systems, biomedical sensors and electrodes. Apart of other characteristics, the corrosion resistance of metal/ceramic interfaces
is of prime importance when biomedical applications are considered. Most of metal/ceramic joints are produced by the active metal brazing technique or by diffusion bonding. Both techniques originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermomechanical discontinuity that characterize these metal/ceramic systems. However, galvanic interactions between those chemically distinct layers are likely to occur, affecting the degradation behaviour of the interface. In this work the corrosion behaviour of Ti-Al2O3 interfaces produced by an active metal brazing methodology was studied. SEM analyses evidenced an interface mainly constituted by four different layers. A first layer rich on titanium and copper, located near to the pure titanium, another layer also rich on Ti and Cu, but with a higher Ag content, an intermediate layer rich in silver which contains some little precipitates of Ti-Cu and finally, a reaction layer with a composition profile containing Ti, Cu, Ag, and Al, located near the alumina part. Potentiodynamic polarisation and electrochemical impedance spectroscopy
(EIS) measurements, carried out in a simulated physiological solution at ambient temperature, revealed a strong influence of the rich silver layer on the passivating behaviour of the interface. On the other hand, the reaction layer appears to be the main responsible for the degradation process of the interface. This degradation is accompanied by a relatively high
release of copper. Through EIS data simulation it was possible to obtain an electrochemical equivalent circuit that describes the corrosion process and allowed an estimation of the polarisation resistance of the constitute layers of the interface. The electrochemical interaction between the different constitutive layers was evaluated, and was correlated with the overall
degradation behaviour of the Ti-Al2O3 interface.Fundação para a Ciência e a
Tecnologia (FCT)
CONSTRUCTAL THEORY APPLIED TO THE GEOMETRIC OPTIMIZATION OF ELLIPTICAL CAVITIES INTO A SOLID CONDUCTING WALL
This work reports, according to Bejan’s Constructal theory, the geometric optimization of an elliptical cavity that intrudes into a solid conducting wall. The objective is to minimize the global thermal resistance between the solid and the cavity. There is uniform heat generation on the solid wall. The cavity is optimized for two sets of thermal conditions: isothermal cavity and cavity bathed by a steady stream of fluid. The solid conducting wall is isolated on the external perimeter. The total volume and the elliptical cavity volume are fixed while the geometry of the cavity is free to vary. The results show that the optimized geometrical shapes are relatively robust, i.e., insensitive to changes in some of the design parameters: the cavity shape is optimal when penetrates the conducting wall almost completely
Matrix Assisted Formation of Ferrihydrite Nanoparticles in a Siloxane/Poly(Oxyethylene) Nanohybrid
Matrix-assisted formation of ferrihydrite, an iron oxide hydroxide analogue
of the protein ferritin-core, in a sol-gel derived organic-inorganic hybrid is
reported. The hybrid network (named di-ureasil) is composed of
poly(oxyethylene) chains of different average polymer molecular weights grafted
to siloxane domains by means of urea cross-linkages and accommodates
ferrihydrite nanoparticles. Magnetic measurements, Fourier transform infrared
and nuclear magnetic resonance spectroscopy reveal that the controlled
modification of the polymer molecular weight allows the fine-tuning of the
ability of the hybrid matrix to assist and promote iron coordination at the
organic-inorganic interface and subsequent nucleation and growth of the
ferrihydrite nanoparticles whose core size (2-4 nm) is tuned by the amount of
iron incorporated. The polymer chain length, its arrangement and crystallinity,
are key factors on the anchoring and formation of the ferrihydrite particles.Comment: 7 pages, 6 figures. To be published in J. Mater. Che
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