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排序方式: 共有425条查询结果,搜索用时 15 毫秒
71.
72.
D. Serantes D. Baldomir M. Pereiro J.E. Arias C. Mateo-Mateo M.C. Buján-Núñez C. Vázquez-Vázquez J. Rivas 《Journal of Non》2008,354(47-51):5224-5226
We have studied the influence of the applied magnetic field on the blocking temperature (TB) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective TB as a function of H. We have focused our study on the limit H → HK (where HK is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of TB. The classical expression predicts TB to disappear for H ? HK, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions TB exists even for fields H > HK. 相似文献
73.
74.
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
75.
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
76.
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
77.
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
78.
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
79.
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
80.
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