排序方式: 共有276条查询结果,搜索用时 500 毫秒
271.
272.
R. Arulmurugan G. Vaidyanathan S. Sendhilnathan B. Jeyadevan 《Journal of magnetism and magnetic materials》2006
Mn1−xZnxFe2O4 (with x varying from 0.1 to 0.5) ferrite nanoparticles used for ferrofluid preparation have been prepared by chemical co-precipitation method and characterized. Characterization techniques like elemental analysis by atomic absorption spectroscopy and spectrophotometry, thermal analysis using simultaneous TG-DTA, XRD, TEM, VSM and Mossbauer spectroscopy have been utilized. The final cation contents estimated agree with the initial degree of substitution. The Curie temperature (Tc) and particle size decrease with the increase in zinc substitution. In the case of particles with higher zinc concentration, both ferrimagnetic nanoparticles and particles exhibiting superparamagnetic behavior at room temperature are present. In addition, some of the results obtained by slightly altering the preparation condition are also discussed. The precipitated particles were used for ferrofluid preparation. The fine particles were suitably dispersed in heptane using oleic acid as the surfactant. The volatile nature of the carrier chosen helps in altering the number concentration of the magnetic particles in a ferrofluid. Magnetic properties of the fine particles and ferrofluids are discussed. Ferrofluids having Mn0.5Zn0.5Fe2O4 particles can be used for the energy conversion application utilizing the magnetically induced convection for thermal dissipation. 相似文献
273.
E.C. Passamani C. Larica C. Marques J.R. Proveti A.Y. Takeuchi F.H. Sanchez 《Journal of magnetism and magnetic materials》2006
The present article reports studies on structural and magnetic properties of nanostructured Fe/MnO2 materials prepared by mechanosynthesis method, with Fe to MnO2 ratios of 20/80, 50/50 and 60/40. X-ray diffraction patterns indicate that the milled materials have crystalline grain size in the nanoscale region. Mössbauer spectra of the milled materials suggest the presence of two Fe phases for each sample: a nanocrystalline α-Fe phase with a high degree of disorder/defects and small Fe-oxide particles. The magnetic hysteresis (M(H)) loops, measured at 4.2 K, after the samples were cooled from 300 K in ±10 kOe fields, show unexpected large shifts in both horizontal and vertical directions for the 20/80 sample, while only horizontal shift was detected in the samples with higher Fe concentration. The anomalous vertical shift of the M(H) loop for the 20/80 sample, observed at low cooling field (10 kOe), is being associated with a large contribution from non-collinear magnetic structure of the particles surface. This surface magnetic contribution is strongly influenced by the field cooling magnitude. A simple model is proposed to interpret this result. 相似文献
274.
Babu S Rottereau M Nicolai T Gimel JC Durand D 《The European physical journal. E, Soft matter》2006,19(2):203-211
Off-lattice dynamic Monte-Carlo simulations were done of reversible cluster-cluster aggregation for spheres that form rigid
bonds at contact. The equilibrium properties were found to be determined by the life time of encounters between two particles
(te). te is a function not only of the probability to form or break a bond, but also of the elementary step size of the Brownian motion
of the particles. In the flocculation regime the fractal dimension of the clusters is df=2.0 and the size distribution has a power law decay with exponent τ=1.5. At larger values of te transient gels are formed. Close to the
percolation threshold the clusters have a fractal dimension df=2.7 and the power law exponent of the size distribution is τ=2.1. The transition between flocculation and percolation
occurs at a characteristic weight average aggregation number that decreases with increasing volume fraction. 相似文献
275.
J.C. Lemm J. Uhlig A. Weiguny 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(3):349-366
The paper discusses the reconstruction of potentials for quantum systems at finite temperatures from observational data. A
nonparametric approach is developed, based on the framework of Bayesian statistics, to solve such inverse problems. Besides
the specific model of quantum statistics giving the probability of observational data, a Bayesian approach is essentially
based on a priori information available for the potential. Different possibilities to implement a priori information are discussed in detail, including hyperparameters, hyperfields, and non-Gaussian auxiliary fields. Special emphasis
is put on the reconstruction of potentials with approximate periodicity. Such potentials might for example correspond to periodic
surfaces modified by point defects and observed by atomic force microscopy. The feasibility of the approach is demonstrated
for a numerical model.
Received 29 May 2000 and Received in final form 16 August 2000 相似文献
276.
G. Franzò V. Vinciguerra F. Priolo 《Applied Physics A: Materials Science & Processing》1999,69(1):3-12
A detailed investigation on the excitation mechanisms of rare-earth (RE) ions introduced in Si nanocrystals (nc) is reported.
Silicon nanocrystals were produced by high-dose 80-keV Si implantation in thermally grown SiO2 followed by 1100 °C annealing for 1 h. Subsequently some of the samples were implanted by 300-keV Er, Yb, Nd, or Tm at doses
in the range 2×1012–3×1015 /cm2. The energy was chosen in such a way to locate the RE ions at the same depth where nanocrystals are. Finally an annealing
at 900 °C for 5 min was performed in order to eliminate the implantation damage. These samples show intense room-temperature
luminescence due to internal 4f shell transitions within the RE ions. For instance, luminescence at 1.54 μm and 0.98 μm is
observed in Er-doped nc, at 0.98 μm in Yb-doped nc, at 0.92 μm in nc and two lines at 0.78 μm and 1.65 μm in Tm-doped nc.
Furthermore, these signals are much more intense than those observed when RE ions are introduced in pure SiO2 in the absence of nanocrystals, demonstrating the important role of nanocrystals in efficiently exciting the REs. It is shown
that the intense nc-related luminescence at around 0.85 μm decreases with increasing RE concentration and the energy is preferentially
transferred from excitons in the nc to the RE ions which, subsequently, emit radiatively. The exact mechanism of energy transfer
has been studied in detail by excitation spectroscopy measurements and time-resolved photoluminescence. On the basis of the
obtained results a plausible phenomenological model for the energy transfer mechanism emerges. The pumping laser generates
excitons within the Si nanocrystals. Excitons confined in the nc can either give their energy to an intrinsic luminescent
center emitting at around 0.85 μm nor pass this energy to the RE 4f shell, thus exciting the ion. The shape of the luminescence
spectra suggests that excited rare-earth ions are not incorporated within the nanocrystals and the energy is transferred at
a distance while they are embedded within SiO2. Rare-earth excitation can quantitatively be described by an effective cross section σeff taking into account all the intermediate steps leading to excitation. We have directly measured σeff for Er in Si nc obtaining a value of ≈2×10−17 cm2. This value is much higher than the cross section for excitation through direct photon absorption (8×10−21 cm2) demonstrating that this process is extremely efficient. Furthermore, the non-radiative decay processes typically limiting
rare-earth luminescence in Si (namely back-transfer and Auger) are demonstrated to be absent in Si nc further improving the
overall efficiency of the process. These data are reported and their implications.
Received: 9 April 1999 / Accepted: 10 April 1999 / Published online: 2 June 1999 相似文献