Search results for " Magnetic materials"
showing 10 items of 4666 documents
Stability analysis of a paramagnetic spheroid in a precessing field
2019
Abstract The stability of a paramagnetic prolate or oblate spheroidal particle in a precessing magnetic field is studied. The bifurcation diagram is calculated analytically as a function of the magnetic field frequency and the precession angle. The orientation of the particle in the synchronous regime is calculated. The rotational dynamics and the mean rotational frequency in the asynchronous regime are also obtained. The theoretical model we describe enables the analytic calculation of the dynamics of the particle in the limiting case when the motion is periodic. The theoretical models were also compared with experimental results of rod like particle dynamics in a precessing magnetic field…
Enhancement of the Multipactor Threshold Inside Nonrectangular Iris
2018
Multipactor breakdown is studied inside the capacitive iris of a rectangular waveguide with a skewed slot along its longitudinal cross section. Both the iris length and height are assumed to be small compared to the electromagnetic wavelength. Therefore, the quasi-static approximation is applied so as to describe the RF field distribution inside the iris gap, whereas a 2-D model is used to analyze the electron motion. The peculiarities of RF field structure are studied using the conformal mapping approach, which shows that the electric field lines can be approximated by circular arcs when the iris length is much larger than its height. The electron motion inside the iris gap is analyzed usi…
Permanent magnet system to guide superparamagnetic particles
2017
A new concept of permanent magnet systems for guiding superparamagnetic particles on arbitrary trajectories is proposed. The basic concept is to use one magnet system with a strong and homogeneous (dipolar) magnetic field to magnetize and orient the particles. A second constantly graded field (quadrupolar) is superimposed to the first to generate a force. In this configuration the motion of the particles is driven solely by the component of the gradient field which is parallel to the direction of the homogeneous field. Then the particles are guided with constant force in a single direction over the entire volume. The direction can be adjusted by varying the angle between quadrupole and dipo…
Analytical induced force solution in conducting cylindrical bodies and rings due to a rotating finite permanent magnet
2020
Abstract Using exact expression of the magnetic field we derive analytical expression for the induced current density and volume force in a solid conducting cylinder and ring due to a coaxial rotating finite permanent magnet with transverse magnetization. The integral torque is calculated from these expressions and validated with numerical and experimental results. Conditions for useful magnetic field approximations are found.
Analytic $JV$ -Characteristics of Ideal Intermediate Band Solar Cells and Solar Cells With Up and Downconverters
2017
The ideal diode equation is regularly used to describe the $\textit {JV}$ -characteristic of single junction solar cells. The connection between the diode equation and fundamental physics is the application of the Boltzmann approximation to describe the fluxes of photons emitted by the cell. In this paper, this approximation is used to derive analytic $\textit {JV}$ -characteristics for three photovoltaic high-efficiency concepts, intermediate band solar cells, and solar cells optically coupled to up and downconverters. These three concepts share the common feature that they allow excitation of electrons between at least three energy levels, which assures a better utilization of the solar s…
Real space observation of two-dimensional Bloch wave interferences in a negative index photonic crystal cavity
2008
We report here the direct observation of two-dimensional (2D) Bloch wave interferences in a negative index photonic crystal by using optical near-field microscopy techniques. The photonic crystal is formed by a defectless honeycomb lattice of air holes etched in III-V semiconductor slab. A scanning near-field optical microscope is used to visualize spatially, as well as spectrally, the light distribution inside the photonic crystal. The recorded near-field spectra and maps presented here unambiguously demonstrate the Bloch wave interferences within the photonic crystal. Then, the spectral and spatial evolution of these interferences allows us to recover experimentally the 2D band diagram of…
Structure and dielectric properties at phase transition of Na1/2Bi1/2TiO3-BaTiO3 solid solutions
2016
ABSTRACTPhase coexistence region is studied by x-ray diffraction for Na1/2Bi1/2TiO3-BaTiO3 solid solutions in the tetragonal phase side from the morphotropic phase boundary. The first order ferroelectric phase transition, determined from a jump in the temperature dependence of dielectric permittivity, is located inside the coexistence region of cubic and tetragonal phases and is below the temperature, where tetragonality disappears. At low BaTiO3 concentrations phase transition into ferroelectric state at cooling is slowly approached in time and is smeared over large temperature range. Rietveld method, applied for more precise evaluation of phase content, reveals large local deformations in…
Defect-induced blue luminescence of hexagonal boron nitride
2016
Abstract Native defect-induced photoluminescence around 400 nm (blue luminescence - BL) was studied in hBN materials with different size and various origins. The following spectral characterizations were used: spectra of luminescence and its excitation, luminescence dependence on temperature, luminescence kinetics, optically stimulated luminescence and infrared absorption. It was found, that the BL is characteristic for all these materials, which were studied. The BL forms a wide, asymmetric and phonon-assisted emission band at 380 nm. This luminescence can be excited either through the exciton processes, or with light from two defect-induced excitation bands at 340 nm and 265 nm. It was fo…
Magnetic field control of gas-liquid mass transfer in ferrofluids
2020
Abstract Gas-liquid mass transfer plays a key role in a broad range of industrial processes. The magnetic field control over the morphology of the gas-liquid interface and solute transport is an attractive feature if it can be realized efficiently. However, the magnetic properties of typical liquids and gases are rather weak. The experimental investigation is carried out to evaluate the effect of the magnetic field, which is mediated by magnetic nanoparticles, on the gas-liquid mass exchange during the sparging run through a hydrocarbon ferrofluid. The results indicate that the gradient field is especially effective at controlling the gas-liquid contact volume: the foaming of the liquid dur…
Spontaneous order in ensembles of rotating magnetic droplets
2019
Ensembles of elongated magnetic droplets in a rotating field are studied experimentally. In a given range of field strength and frequency the droplets form rotating structures with a triangular order - rotating crystals. A model is developed to describe ensembles of several droplets, taking into account the hydrodynamic interactions between the rotating droplets in the presence of a solid wall below the rotating ensemble. A good agreement with the experimentally observed periodic dynamics for an ensemble of four droplets is obtained. During the rotation, the tips of the elongated magnetic droplets approach close to one another. An expression is derived that gives the magnetic interaction be…