High Performance Magnets - Microstructure and Coercivity

1999 ◽  
Vol 577 ◽  
Author(s):  
J. Fidler ◽  
S. Sasaki ◽  
E. Estevez-Rams
Keyword(s):  
Energy Density ◽  
Oxygen Content ◽  
Coercive Field ◽  
High Performance ◽  
Magnetic Energy ◽  
Low Oxygen ◽  
Critical Temperatures ◽  
High Oxygen Content ◽  
Magnetic Energy Density ◽  
Strip Cast

ABSTRACTThe importance of newly developed permanent magnetic materials in many electro-, magnetomechanical and electronic applications can be attributed to the drastic improvement of the magnetic energy density product and coercive field. A systematic study has been undertaken in order to determine the influence of oxygen content on microstructure and coercivity of high remanence Nd2Fe14B based sintered magnets. The energy density product >400 kJ/m3 (50 MGOe) and the coercive field of 800 kA/m were obtained after a combination of rubber isostatic and transverse die pressing methods. Magnets of the composition Nd151−xFe78+xB6Cu0.03A10.7 [x= 0-2.5] were prepared using strip cast materials. The high oxygen content of the magnets was gradually decreased from values of 4000-6000 ppm to a value < 1000 ppm. Abnormal grain growth (AGG) of the 2:14:1 grains occurred preferentially in magnets with low oxygen content, thus the squareness of the demagnetisation curve drastically decreased. The oxygen content strongly affects the AGG and the magnets with higher oxygen content have the higher critical temperatures at which the AGG occurs. On the other hand, isotropic magnets tend to have the lower critical temperatures than anisotropic magnets by 10-20°C. In the second part of the paper examples of the influence of intergranular phases, grain size and grain shape observed by TEM on coercivity of sintered and melt-spun Nd-Fe-B magnets are compared.

scholarly journals Can the Local Bubble explain the radio background?

2021 ◽  
Vol 502 (2) ◽  
pp. 2807-2814
Author(s):  
Martin G H Krause ◽  
Martin J Hardcastle
Keyword(s):  
Magnetic Field ◽  
Energy Density ◽  
Radio Emission ◽  
Magnetic Energy ◽  
Cosmic Ray ◽  
Observational Constraints ◽  
Local Bubble ◽  
Magnetic Energy Density ◽  
Magnetic Field Model ◽  
Radio Background

ABSTRACT The ARCADE 2 balloon bolometer along with a number of other instruments have detected what appears to be a radio synchrotron background at frequencies below about 3 GHz. Neither extragalactic radio sources nor diffuse Galactic emission can currently account for this finding. We use the locally measured cosmic ray electron population, demodulated for effects of the Solar wind, and other observational constraints combined with a turbulent magnetic field model to predict the radio synchrotron emission for the Local Bubble. We find that the spectral index of the modelled radio emission is roughly consistent with the radio background. Our model can approximately reproduce the observed antenna temperatures for a mean magnetic field strength B between 3 and 5 nT. We argue that this would not violate observational constraints from pulsar measurements. However, the curvature in the predicted spectrum would mean that other, so far unknown sources would have to contribute below 100 MHz. Also, the magnetic energy density would then dominate over thermal and cosmic ray electron energy density, likely causing an inverse magnetic cascade with large variations of the radio emission in different sky directions as well as high polarization. We argue that this disagrees with several observations and thus that the magnetic field is probably much lower, quite possibly limited by equipartition with the energy density in relativistic or thermal particles (B = 0.2−0.6 nT). In the latter case, we predict a contribution of the Local Bubble to the unexplained radio background at most at the per cent level.

scholarly journals Absorption of Curvature Radiation

1979 ◽  
Vol 32 (2) ◽  
pp. 49 ◽  
Author(s):  
VV Zheleznyakov ◽  
VE Shaposhnikov
Keyword(s):  
Magnetic Field ◽  
Energy Density ◽  
Optical Depth ◽  
Magnetic Energy ◽  
Particle Energy ◽  
Relativistic Electrons ◽  
Magnetic Energy Density ◽  
Curvature Radiation ◽  
Maser Effect ◽  
Field Lines

The reabsorption of curvature radiation, i.e. radiation from relativistic electrons moving along curved magnetic field lines, is discussed. The optical depth for the ray path is calculated by use of the Einstein coefficients. It is shown that the optical depth becomes negative (maser effect) if transitions between Landau levels are absent. However, maser action is ineffective if the energy density of the relativistic particles is less than that of the magnetic field. For pulsar radio emission the magnetic energy density is assumed to exceed the particle energy density, so the observed emission cannot be coherent curvature radiation.

Possibility of an inverse cascade of magnetic helicity in magnetohydrodynamic turbulence

1975 ◽  
Vol 68 (4) ◽  
pp. 769-778 ◽  
Author(s):  
U. Frisch ◽  
A. Pouquet ◽  
J. LÉOrat ◽  
A. Mazure
Keyword(s):  
Energy Density ◽  
Large Scale ◽  
Magnetic Energy ◽  
Three Dimensional ◽  
Magnetic Helicity ◽  
Kinetic Energy Density ◽  
Magnetohydrodynamic Turbulence ◽  
Magnetic Energy Density ◽  
Upper Limit ◽  
Cross Correlations

Some of the consequences of the conservation of magnetic helicity$\int \rm{a.b}\it{d}^{\rm{3}}\rm{r\qquad (a\; =\; vector\; potential\; of\; magnetic\; field\; b)}$for incompressible three-dimensional turbulent MHD flows are investigated. Absolute equilibrium spectra for inviscid infinitely conducting flows truncated at lower and upper wavenumberskminandkmaxare obtained. When the total magnetic helicity approaches an upper limit given by the total energy (kinetic plus magnetic) divided bykmin, the spectra of magnetic energy and helicity are strongly peaked nearkmin; in addition, when the cross-correlations between the velocity and magnetic fields are small, the magnetic energy density nearkmingreatly exceeds the kinetic energy density. Several arguments are presented in favour of the existence of inverse cascades of magnetic helicity towards small wavenumbers leading to the generation of large-scale magnetic energy.

scholarly journals Radio-Source Spectra and their Time Variations

1968 ◽  
Vol 1 ◽  
pp. 371-372
Author(s):  
K.I. Kellermann ◽  
I.I.K. Pauliny-Toth
Keyword(s):  
Energy Density ◽  
Flux Density ◽  
Magnetic Energy ◽  
Relativistic Electrons ◽  
Low Frequencies ◽  
High Frequencies ◽  
Magnetic Energy Density ◽  
Straight Line ◽  
Time Variations ◽  
Compact Sources

During the past few years there has been a large increase in the available data on the spectra of radio sources, particularly at short wavelengths, where a number of sources have shown unexpectedly large time variations, with time-scales of 1 year or less.The simple power-law spectrum, which is a straight line on a log-log plot of flux density against frequency, is shown by about 30% of sources. Most sources have a spectrum with negative curvature, which steepens at high frequencies. Many have a sharp cut-off, which is almost certainly due to synchrotron self-absorption, at low frequencies. In several of these sources, such as 3C 48, 3C 147 and 3C 295, the spectrum begins to flatten at a considerably higher frequency than the cut-off frequency. This flattening is too sharp to be caused by a change in the energy distribution of the electrons and is probably due to parts of the source becoming optically thick at higher frequencies. Some sources have components which are optically thick even at centimetre wavelengths. These must have angular sizes of 10−3″ or less. The energy density in relativistic electrons in these compact sources is much larger than the magnetic-energy density, so that the source cannot be stable and variations in the flux density are to be expected.

scholarly journals Relativistic Modifications in Galactic Rotation Curves under a Toroidal Galactic Field

2021 ◽  
Author(s):  
Babur M. Mirza
Keyword(s):  
Energy Density ◽  
Galactic Center ◽  
Magnetic Energy ◽  
Rotational Velocity ◽  
Galactic Rotation ◽  
Axially Symmetric ◽  
Rotation Curves ◽  
Observable Universe ◽  
Magnetic Energy Density ◽  
Dynamics Of Galaxies

Abstract Rotational dynamics of galaxies exhibits an increase beyond the Keplerian velocity which corresponds to a missing mass up to six times the dynamic mass in the observable universe. In this paper we show that the observed increase in galactic rotation velocities is a general relativistic effect resulting from the combined effect of toroidal magnetic energy density in galaxies and spacetime dragging due to the rotating compact mass in galactic center. The effect of magnetic energy density on spacetime vorticity is derived from Maxwell equations in axially symmetric spacetime where the dragging effects are shown to extend farther in the galactic disc via the toroidal field, modifying the rotational speed of the galactic matter. This is shown to lead to the diverse rotation curves of spiral galaxies, along with the Tully-Fisher relation for total galactic mass and maximum rotational velocity.

scholarly journals Tailward Propagation of Magnetic Energy Density Variations With Respect to Substorm Onset Times

2018 ◽  
Vol 123 (6) ◽  
pp. 4741-4754 ◽  
Author(s):  
John C. Coxon ◽  
Mervyn P. Freeman ◽  
Caitriona M. Jackman ◽  
Colin Forsyth ◽  
I. Jonathan Rae ◽  
...  
Keyword(s):  
Energy Density ◽  
Magnetic Energy ◽  
Substorm Onset ◽  
Magnetic Energy Density

The Influence of Diamond Addition to Ni-Al Powder on Oxidation Behavior of Ni-Al During Plasma Spraying for High Performance Oxide-Free Ni-Al Intermetallic Coating

2021 ◽  
Author(s):  
Li Zhang ◽  
Di Wang ◽  
Xian-Jin Liao ◽  
Xiao-Tao Luo ◽  
Chang-Jiu Li
Keyword(s):  
Oxygen Content ◽  
High Performance ◽  
Thermal Spraying ◽  
Low Oxygen ◽  
Mechanically Alloyed ◽  
Intermetallic Coating ◽  
Novel Approach ◽  
Ultrahigh Temperature ◽  
Diamond Powders ◽  
Plasma Sprayed

Abstract Ni-Al intermetallics have excellent corrosion and oxidation resistance, but their use in thermal spraying has been limited due to issues with in-flight oxidation. In this study, a novel approach is proposed to remove oxide from Ni-Al droplets in-flight by adding a deoxidizer (diamond) to the feedstock powder. A mixture of nickel, aluminum, and diamond powders was mechanically alloyed using a combination of cryogenic and planetary ball milling. The resulting Ni/Al/diamond composite powder was then plasma sprayed via the APS process, forming Ni-Al coatings on Inconel 738 substrates. Phase composition, microstructure, porosity, and microhardness of the coatings were characterized by X-ray diffraction, scanning electron microscopy, image analysis, and hardness testing, respectively. Oxygen content measurements showed that the coatings contained significantly less oxygen than coatings made from ordinary Ni/Al powders. In-flight particle temperatures were also measured and found to be higher than 2300 °C. The low oxygen content in the coatings is attributed to the in-situ deoxidizing effect of ultrahigh temperature droplets which are also oxide-free.

Note on the magnetic energy density

1982 ◽  
Vol 50 (8) ◽  
pp. 758-759 ◽  
Author(s):  
N. Gauthier ◽  
H. D. Wiederick
Keyword(s):  
Energy Density ◽  
Magnetic Energy ◽  
Magnetic Energy Density
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