Magnetic field

Magneto Rheological Fluid (MRF) damper has been opted as a promising semi active element of advanced suspension system. This work focuses on the damping nature of magnetorheological fluid under the influence of a magnetic field strength, size and shape of the of the iron oxide particles. A reduced scale single degree of freedom quarter car model with two stage magnetorheological damper, viscosity measurement setup, synthesized and commercially available Fe2O3 powders are used for the experimental study. An air cooled electro-dynamic shaker, Data acquisition system, accelerometers and the Lab view software acquires and analyzes the response of the quarter car model. X-ray Diffraction and Scanning electron microscopy characterize the morphology of iron oxide particles. The vertical acceleration, the displacement, the transmissibility ratio and the damping characteristics are analyzed based on the response of the top plate at different frequencies of base excitation. The results show t...

The onset of thermal convection of a Boussinesq fluid located in an unbounded layer heated from below and subject simultaneously to rotation and magnetic field, whose vectors act in different directions, is presented. To the knowledge of the authors, the convective thermal instability analysis for this complex problem has not been previously reported. In this paper, we use the Tau Chebyshev spectral method to calculate the value of the critical parameters (wave number and Rayleigh number at the onset of convection) as a function of (i) different kinds of boundaries, (ii) angle between the three vectors, and (iii) different values of the Taylor numberT(rate of rotation) and magnetic parameterQ(strength of the magnetic force). For the classical problems previously reported in the literature, we compare our calculations with Chandrasekhar’s variational method results and show that the present method is applicable.

The effect of a high magnetic field up to 12 T and a high hydrostatic pressure up to 12 kbar on the stability of the metamagnetic isostructural phase transition and the multicaloric effect of Fe49Rh51 alloy has been studied. The phase transition temperature shifts under the magnetic field and the hydrostatic pressure on with the rates of dTm/μ0dH = −9.2 K/T and dTm/dP = 3.4 K/kbar, respectively. The magnetocaloric and multicaloric (under two external fields) effects were studied via indirect method using Maxwell relations. The maximum of the entropy change is increasing toward the high temperature region from ∆S~2.5 J/(kg K) at 305 K to ∆S~2.7 J/(kg K) at 344 K under simultaneously applied magnetic field of 0.97 T and hydrostatic pressure of 12 kbar. The obtained results were explained using the first-principle calculations of Gibbs energies and the phonon spectra of the ferromagnetic and the antiferromagnetic phases. Taking into account the low concentration of antisite defects in ...

We report the results of studies of the magnetic and transport properties of Fe49Rh51 alloy prepared by different sequences of quenching and the annealing process. The temperature dependences of the relative initial magnetic permeability and resistivity are analyzed. An optimal regime consisting of annealing at 1300 K for 440 min and quenching from 1300 K to 275 K is found to observe the desired narrow antiferromagnetic–ferromagnetic transition in Fe49Rh51 alloy under cyclic conditions. This has the potential to increase the efficiency of cooling devices based on the magnetocaloric effect of magnetic materials with a first-order field-induced phase transition.

We discuss low-frequency g modes excited by resonant couplings with weakly unstable oscillatory convective modes in the rotating convective core in early-type main-sequence stars. Our non-adiabatic pulsation analyses including the effect of Coriolis force for $2\, \mathrm{ M}_\odot$ main-sequence models show that if the convective core rotates slightly faster than the surrounding radiative layers, g modes in the radiative envelope are excited by a resonance coupling. The frequency of the excited g mode in the inertial frame is close to |mΩc| with m and Ωc being the azimuthal order of the g mode and the rotation frequency of the convective core, respectively. These g-mode frequencies are consistent with those of photometric rotational modulations and harmonics observed in many early-type main-sequence stars. In other words, these g modes provide a non-magnetic explanation for the rotational light modulations detected in many early-type main-sequence stars.

The Be phenomenon, that is the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT (Convection, Rotation and planetary Transits) satellite brought clear evidence that Be outbursts are directly correlated with pulsations. We found that it may be the transport of angular momentum by waves or pulsation modes that brings the already rapid stellar rotation to its critical value at the surface, and allows the star to eject material. The recent discovery of stochastically excited gravito-inertial modes by CoRoT in a hot Be star strengthens this scenario. We present the CoRoT observations and modeling of several Be stars and describe the new picture of the Be phenomenon which arose from these results.

Photodiodes with a p-MnZnO/n-Si substrate structure were fabricated. Mn-doped p-type ZnO (MnZnO) films were deposited by ultrasonic spray pyrolysis on a (100)-oriented silicon substrate. A dark current and a photocurrent of ∼4.31×10-7 and 6.95×10-4 A, respectively, were measured at a reverse bias of 2 V, and a photocurrent-to-dark current contrast ratio of almost four orders of magnitude was observed. When a p-MnZnO/n-Si structure photodiode was applied a strong magnetic field of 0.5 T, the photocurrent slightly increases to ∼1.24×10-3 A at a reverse bias of 2 V. This may be attributed to the Auger recombination effect due to the applied magnetic field.

The study of stability of lipid-magnetite suspensions (LMS) was carried out using photometry and electronic microscopy. All suspensions are rather stable in time. The best results in stability were demonstrated by suspensions with ratio Fe3O4: SAS=0,02:0,35 g or 0,04 mass % : 0,70 mass % and 0,025: 0,35 g or 0,05 mass % : 0,70 mass %. The sizes of magnetite particles from SAS were determined as– ~76 nm. It was established, that with time (0–48,0 hours) and growth of wave length (210 – 1000 nm) is observed the gradual increase of transmission coefficient from 25 % (210 nm) to 71,9 % (1000 nm) at 0 hours of suspension ageing; from 27,5 % (210 nm) to 81,2 % (1000 nm) at maximal time of suspension ageing (48 hours). There parameters of LMS were determined: concentration of particles – N=1,43 ∙ 1012 cm-3, in 48 hours concentration decreased by 20 % (N=1,19∙1012 cm-3); r=38 nm, n=1,48, κ=0,01. The function of particles distribution by sizes is rather narrow and symmetric that certifies th...

Oscillator strength of excitons in various quantum wells is systematically investigated analyzing Fourier transform reflectance spectra. The temperature dependence of the oscillator strength of the zero-phonon line transition is well described by a modified Debye-Waller expression with an averaged phonon mode. This allows us to investigate the exciton-phonon interaction. The oscillator strength in GaAs/AlGaAs quantum wells increases as the well width is reduced in agreement with the existing theory, while in InGaAs/GaAs and GaAs/GaAsP quantum wells takes a maximum at a certain well width, showing the variation from two- to three-dimensional characters. The oscillator strength in InGaAs/GaAs and GaAs/GaAsP quantum wells is differently dependent on the alloy composition variation.

The effect of local thermal nonequilibrium (LTNE) on the stability of natural convection in a vertical porous slab saturated by an Oldroyd-B fluid is investigated. The vertical walls of the slab are impermeable and maintained at constant but different temperatures. A two-field model that represents the fluid and solid phase temperature fields separately is used for heat transport equation. The resulting stability eigenvalue problem is solved numerically using Chebyshev collocation method as the energy stability analysis becomes ineffective in deciding the stability of the system. Despite the basic state remains the same for Newtonian and viscoelastic fluids, it is observed that the base flow is unstable for viscoelastic fluids and this result is qualitatively different from Newtonian fluids. The results for Maxwell fluid are delineated as a particular case from the present study. It is found that the viscoelasticity has both stabilizing and destabilizing influence on the flow. Incre...