56,660 research outputs found
Reversal Modes of Simulated Iron Nanopillars in an Obliquely Oriented Field
Stochastic micromagnetic simulations are employed to study switching in
three-dimensional magnetic nanopillars exposed to highly misaligned fields. The
switching appears to proceed through two different decay modes, characterized
by very different average lifetimes and different average values of the
transverse magnetization components.Comment: 3 pages, 4 figure
The effect of hydrogen on the deformation behavior of a single crystal nickel-base superalloy
The effect of hydrogen on the tensile deformation behavior of PWA 1480 is presented. Tensile tests were interrupted at different plastic strain levels to observe the development of the dislocation structure. Transmission electron microscopy (TEM) foils were cut perpendicular to the tensile axis to allow the deformation of both phases to be simultaneously observed as well as parallel to zone axes (III) to show the superdislocations on their slip planes. Similar to other nickel-base superalloys, hydrogen was detrimental to the room temperature tensile properties of PWA 1480. There was little effect on strength, however the material was severely embrittled. Even without hydrogen, the elongation-to-failure was only approximately 3 percent. The tensile fracture surface was made up primarily of ductile voids with regions of cleavage fracture. These cleavage facets are the eutectic (gamma') in the microstructure. It was shown by quantitative fractography that hydrogen embrittles the eutectic (gamma') and causes the crack path to seek out and fracture through the eutectic (gamma'). There was two to three times the amount of cleavage on the fracture surface of the hydrogen-charged samples than on the surface of the uncharged samples. The effect of hydrogen can also be seen in the dislocation structure. There is a marked tendency for dislocation trapping in the gamma matrix with and without hydrogen at all plastic strain levels. Without hydrogen there is a high dislocation density in the gamma matrix leading to strain exhaustion in this region and failure through the matrix. The dislocation structure at failure with hydrogen is slightly different. The TEM foils cut parallel to zone axes (III) showed dislocations wrapping around gamma precipitates. Zone axes (001) foils show that there is a lower dislocation density in the gamma matrix which can be linked to the effects of hydrogen on the fracture behavior. The primary activity in the gamma precipitates is in the form of superlattice intrinsic stacking faults (SISFs). These faults have also been reported in other ordered alloys and superalloys
Two Modes of Magnetization Switching in a Simulated Iron Nanopillar in an Obliquely Oriented Field
Finite-temperature micromagnetics simulations are employed to study the
magnetization-switching dynamics driven by a field applied at an angle to the
long axis of an iron nanopillar. A bi-modal distribution in the switching times
is observed, and evidence for two competing modes of magnetization-switching
dynamics is presented. For the conditions studied here, temperature K
and the reversal field 3160 Oe at an angle of 75 to the long axis,
approximately 70% of the switches involve unstable decay (no free-energy
barrier) and 30% involve metastable decay (a free-energy barrier is crossed).
The latter are indistinguishable from switches which are constrained to start
at a metastable free-energy minimum. Competition between unstable and
metastable decay could greatly complicate applications involving magnetization
switches near the coercive field.Comment: 19 pages, 7 figure
Origin of G Magnetic Fields in the Central Engine of Gamma Ray Bursts
Various authors have suggested that the gamma-ray burst (GRB) central engine
is a rapidly rotating, strongly magnetized, G) compact
object. The strong magnetic field can accelerate and collimate the relativistic
flow and the rotation of the compact object can be the energy source of the
GRB. The major problem in this scenario is the difficulty of finding an
astrophysical mechanism for obtaining such intense fields. Whereas, in
principle, a neutron star could maintain such strong fields, it is difficult to
justify a scenario for their creation. If the compact object is a black hole,
the problem is more difficult since, according to general relativity it has "no
hair" (i.e., no magnetic field). Schuster, Blackett, Pauli, and others have
suggested that a rotating neutral body can create a magnetic field by
non-minimal gravitational-electromagnetic coupling (NMGEC). The
Schuster-Blackett form of NMGEC was obtained from the Mikhail and Wanas's
tetrad theory of gravitation (MW). We call the general theory NMGEC-MW.
We investigate here the possible origin of the intense magnetic fields G in GRBs by NMGEC-MW. Whereas these fields are difficult to
explain astrophysically, we find that they are easily explained by NMGEC-MW. It
not only explains the origin of the G fields when the
compact object is a neutron star, but also when it is a black hole.Comment: 9 pages, accepted for publication in JCA
A Completely Covariant Approach to Transformation Optics
We show that the Plebanski based approach to transformation optics overlooks
some subtleties in the electrodynamics of moving dielectrics that restricts its
applicability to a certain class of transformations. An alternative, completely
covariant, approach is developed that is more generally applicable and provides
a clearer picture of transformation optics.Comment: 10 pages. This version: Additional references added, corrected a
small error in Eq. (28) (Eq. (29) in present version), some revision of the
text, appendix content moved to the main body of the text, figure removed.
Corresponds more closely to published version. Prepared for a special issue
on transformation optics published by Journal of Optic
Direct frequency comb spectroscopy of trapped ions
Direct frequency comb spectroscopy of trapped ions is demonstated for the
first time. It is shown that the 4s^2S_(1/2)-4p^2P_(3/2) transition in calcium
ions can be excited directly with a frequency comb laser that is upconverted to
393 nm. Detection of the transition is performed using a shelving scheme to
suppress background signal from non-resonant comb modes. The measured
transition frequency of f=761 905 012.7(0.5) MHz presents an improvement in
accuracy of more than two orders of magnitude.Comment: 4 pages, 5 figur
Fabrication and test of a space power boiler feed electromagnetic pump. Part 1: Design and manufacture of pump
A three-phase helical induction electromagnetic (EM) pump has been designed and built. This pump was designed for use as the boiler-feed pump of a potassium Rankine-cycle space electric power system. The pump is constructed of high temperature materials including a T-111 duct, Hiperco 27 magnetic material, nickel clad silver conductor wire, and a completely inorganic insulation system. The pump is designed to deliver 3.25 lb/sec potassium at 1000 F with a developed head of 240 psi while being cooled by 800 F NaK. At these conditions, the overall pump efficiency is expected to be 18%
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