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61.
The unit of time of the International System of Units (SI), the “atomic second” was defined through a constant of physics in 1967. It is derived from the frequency of the hyperfine transition of the atom of cesium 133. From the astronomical definition of the second until today, the accuracy of the realization of the second has improved by eight orders of magnitude, with a rate that has increased since the development of the cesium frequency standards, to reach parts in 1016 for the best clocks today. In 2018, when the new SI was adopted, the time metrology community proved that a new generation of frequency standards operating in optical wavelengths has uncertainties at the level of 10–18, and challenge the implementation of high accurate frequency and time comparison techniques to decide on a revision of the definition of the second. Herein, the progress in the definition and realization of the second from astronomy until today is reviewed, an inventory of the present resources is assembled and a brief view for the future given. 相似文献
62.
The detrended fluctuation analysis is used to study the behavior of different time series obtained from the trajectory of the center of pressure, the output of the activity of the human postural control system. The results suggest that these trajectories present two different regimes in their scaling properties: persistent (for high frequencies, short-range time scale) to antipersistent (for low frequencies, long-range time scale) behaviors. The similitude between the results obtained for the measurements, done with both eyes open and eyes closed, indicate either that the visual system may be disregarded by the postural control system while maintaining the quiet standing, or that the control mechanisms associated with each type of information (visual, vestibular and somatosensory) cannot be disentangled with the type of analysis performed here. 相似文献
63.
M. Laura Arias 《Journal of Mathematical Analysis and Applications》2009,355(1):426-1130
In this paper we study some particular solutions of Douglas type equations by means of generalized inverses and angles. We apply this result to characterize positive solutions and some special projections which are symmetrizable for a semidefinited positive bounded linear operator. 相似文献
64.
The compound Ru(bpy)2(dppz-R)(PF6)2, where bpy is 2,2'-bipyridine and dppz-R is 11-(diethoxyphosphorylmethyl)dipyrido[3,2-a:2',3'-c]phenazine, was prepared and anchored to mesoporous nanocrystalline (anatase) TiO2 thin films as a probe of the effects of interfacial water on excited-state charge transfer processes at semiconductor interfaces. In nitrogen-saturated fluid acetonitrile, the Ru(bpy)2(dppz-R)(PF6)2 compound was found to be highly photoluminescent. Water was found to quench the excited state by a mechanism adequately described by the Perrin model, from which the radius of quenching was abstracted, 75 +/- 2 A. The Ru(bpy)2(dppz-R)(PF6)2 compounds were found to bind to the TiO2 thin films in high surface coverages, 5 x 10(-8) mol cm(-2). When these films were immersed in acetonitrile, long-lived excited states (tau = 825 ns) that were quenched by the addition of water were observed. About 30% of the excited states could not be quenched by water. Efficient electron injection, phi(inj) = 0.8, was observed after light excitation of Ru(bpy)2(dppz-R)/TiO2 in a 0.1 M LiClO4/acetonitrile solution. The addition of large concentrations of water, >0.5 M, was found to decrease the injection yield to phi(inj) = 0.3. 相似文献
65.
Gómez-Lopera SA Arias JL Gallardo V Delgado AV 《Langmuir : the ACS journal of surfaces and colloids》2006,22(6):2816-2821
In this work, we describe an experimental investigation on the colloidal stability of suspensions of three kinds of particles, including magnetite, poly(lactic acid) (PLA), and composite core/shell colloids formed by a magnetite core surrounded by a PLA shell. The experiments were performed with dilute suspensions, so that recording the optical absorbance with time gives a suitable indication of the aggregation and sedimentation of the suspensions. The method allowed us to distinguish very accurately between the different surface and magnetic forces responsible for the structures acquired by particle aggregates. Thus, the pure PLA suspensions are very sensitive to ionic strength and almost unaffected by pH changes. On the contrary, the stability of magnetite systems is mainly controlled by pH. The effect of vertical magnetic fields on the stability of magnetite and magnetite/PLA suspensions is also investigated. The PLA shell reduces the magnetic responsiveness of magnetite, but it is demonstrated that the mixed particles can also form structures induced by the field, despite their lower magnetization, and they can be considered in magnetically targeted biomedical applications. 相似文献
66.
Arias M Da Silva-Carballal J García-Río L Mejuto J Núñez A 《Journal of colloid and interface science》2006,295(1):65-70
The capacity of adsorption of P on Fe and Al-oxides-coated quartz sand has been studied. The adsorption process has been described as Freundlich and Langmuir isotherms. In all cases Langmuir equation yields better fits than Freundlich equation. The covering quartz particles with Fe and Al oxides provide a high capacity adsorption and immobilization of P from natural and waste waters. The values obtained for adsorption, desorption and adsorption/desorption cycles show that Al oxides particles provide an excellent material for construction of a P removal system for waste waters. The characteristics of this material would allow the design of a complementary filtration system to reduce P levels in the effluent, reducing contamination risks and combating eutrophication of nearby watercourses. 相似文献
67.
Pedrosa R Andrés C Arias R Mendiguchía P Nieto J 《The Journal of organic chemistry》2006,71(6):2424-2428
Regio- and diastereoselective methoxyselenenylations of double bonds attached to the N,O-ketalic carbon of chiral perhydrobenzoxazines occur in high yields by reaction with benzeneselenenyl chloride in dichloromethane-methanol. The diastereoselection is dependent on the reaction conditions and the structure of the starting compounds and can be rationalized by accepting the coordination of the selenium to the oxygen atom of the heterocycle. 相似文献
68.
M. Rodríguez-Millán A. Vaz-Romero A. Rusinek J. A. Rodríguez-Martínez A. Arias 《Experimental Mechanics》2014,54(5):729-742
This paper presents an experimental investigation on the perforation behaviour of 5754-H111 and 6082-T6 aluminium alloys. The mechanical response of these materials has been characterized in compression with strain rates in the range of $10^{-3}~s^{-1} < \dot {\varepsilon } < 5 \cdot 10^{3}~s^{-1}$ . Moreover, penetration tests have been conducted on 5754-H111 and 6082-T6 plates of $4~mm$ thickness using conical, hemispherical and blunt projectiles. The perforation experiments covered impact velocities in the range of $50~m/s < V_{0} < 200~m/s$ . The initial and residual velocities of the projectile were measured and the ballistic limit velocity obtained for the two aluminium alloys for the different nose shapes. Failure mode and post-mortem deflection of the plates have been examined and the perforation mechanisms associated to each projectile/target configuration investigated. It has been shown that the energy absorption capacity of the impacted plates is the result of the collective role played by target material behaviour, projectile nose shape and impact velocity in the penetration mechanisms. 相似文献
69.
S. Calvo N.P. Arias O. Giraldo A. Rosales-Rivera O. Moscoso 《Physica B: Condensed Matter》2012,407(16):3267-3270
Several Angustifolia Kunth bamboo fibers, which have been previously treated with an alkaline solution, were coated with magnetite particles. The coating of the fibers was achieved by an in-situ co-precipitation method with Fe2+ and Fe3+in NaOH or NH4OH. The fibers were evaluated by chemical analysis using atomic absorption (A.A.) technique, structural characterization by X-ray diffraction (XRD), thermal stability with thermo-gravimetric analysis (TGA) in nitrogen at temperature range between 23 °C and 800 °C and magnetic behavior using vibrating sample magnetometry (VSM) applying a magnetic field between −27 KOe and 27 KOe at room temperature. We found that the thermal stability and magnetization depend of the synthesis method used to cover the Angustifolia Kunth bamboo fibers. In addition, an improved magnetic response was observed when NaOH solution is used to generate the magnetite coating on the fiber surface. 相似文献
70.
Beatriz Pérez-Artacho Visitación Gallardo M. Adolfina Ruiz José L. Arias 《Journal of nanoparticle research》2012,14(4):768
A reproducible methodology is described for the synthesis, by following the double emulsion/solvent evaporation technique,
of magnetic nanocomposites (average diameter ≈ 135 nm) consisting of maghemite nuclei and a biodegradable poly(d,l-lactide-co-glycolide) matrix. The heterogeneous structure of the nanoparticles can confer them the responsiveness to magnetic gradients,
giving both the possibility of their use as a drug delivery system and adequate heating characteristics for a hyperthermia
effect. The physical chemistry of the nanocomposites was extensively characterized, this establishing that their surface properties
were similar to that of pure poly(d,l-lactide-co-glycolide). From an electrokinetic point of view, zeta potential determinations (as a function of the ionic strength, and
pH) pointed out that the nanocomposites were almost indistinguishable from the copolymer. The surface thermodynamic analysis
agreed with the electrophoretic one in suggesting that the coverage of the magnetic nuclei was complete, since the hydrophilic
nature of maghemite was modified and the nanoparticles turned into hydrophobic, just like the copolymer, when they were embedded
into poly(d,l-lactide-co-glycolide). The magnetic behaviours of the composite nanoparticles were also checked. Their heating properties were studied
in vitro in a high-frequency alternating gradient of magnetic field: a stable maximum temperature of 47 °C was satisfactorily
achieved within 45 min. Blood compatibility of the nanocomposites was also defined in vitro. To our knowledge, this is the
first time that such kind of magnetic-sensitive nanoformulation with very promising characteristics (e.g. blood compatibility,
magnetic drug targeting capabilities, and hyperthermia) has been developed for therapeutic purposes. 相似文献