14,305 research outputs found
A theoretical study of microwave beam absorption by a rectenna
The rectenna's microwave power beam absorption limit was theoretically confirmed by two mathematical models descriptive of the microwave absorption process; first one model was based on the current sheet equivalency of a large planar array above a reflector and the second model, which was based on the properties of a waveguide with special imaging characteristics, quantified the electromagnetic modes (field configurations) in the immediate vicinity of a Rectenna element spacing which permit total power beam absorption by preventing unwanted modes from propagating (scattering) were derived using these models. Several factors causing unwanted scattering are discussed
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Fabrication of Bone Substitute Material by Rapid Prototyping
Bone tissue engineering has gained much attention in recent years. A key requirement in this
field is the development of scaffold structures, on which cells adhere. This can be done by
fabricating scaffolds by direct procedures like 3D-printing or by indirect procedures like casting.
With the 3D-printing process different structures were build up by using hydroxyapatite powder
(HA) and a special binder material. Afterwards the printed 3D structures were sintered.
For the casting process molds have been made of different resins by stereolithography and other
processes using polymers and waxes. These structures were filled by a suspension of HA. By
heating the resulting polymer/ceramic composite to a specific temperature it is possible to
combust the polymer or wax. By further heating the remaining body, the HA is sintered.
Compared to the 3D printing a better resolution can be obtained here. But there are restrictions
regarding the ratio of polymer and the HA ceramic during the heating process which means a
limitation for the level of porosity.Mechanical Engineerin
Microelectromagnets for Trapping and Manipulating Ultracold Atomic Quantum Gases
We describe the production and characterization of microelectromagnets made
for trapping and manipulating atomic ensembles. The devices consist of 7
fabricated parallel copper conductors 3 micrometer thick, 25mm long, with
widths ranging from 3 to 30 micrometer, and are produced by electroplating a
sapphire substrate. Maximum current densities in the wires up to 6.5 * 10^6 A /
cm^2 are achieved in continuous mode operation. The device operates
successfully at a base pressure of 10^-11 mbar. The microstructures permit the
realization of a variety of magnetic field configurations, and hence provide
enormous flexibility for controlling the motion and the shape of Bose-Einstein
condensates.Comment: 4 pages, 3 figure
Nonlinear Dynamics of a Bose-Einstein Condensate in a Magnetic Waveguide
We have studied the internal and external dynamics of a Bose-Einstein
condensate in an anharmonic magnetic waveguide. An oscillating condensate
experiences a strong coupling between the center of mass motion and the
internal collective modes. Due to the anharmonicity of the magnetic potential,
not only the center of mass motion shows harmonic frequency generation, but
also the internal dynamics exhibit nonlinear frequency mixing. We describe the
data with a theoretical model to high accuracy. For strong excitations we test
the experimental data for indications of a chaotic behavior.Comment: 4 pages, 4 figure
The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants
The merger of two neutron stars leaves behind a rapidly spinning hypermassive
object whose survival is believed to depend on the maximum mass supported by
the nuclear equation of state, angular momentum redistribution by
(magneto-)rotational instabilities, and spindown by gravitational waves. The
high temperatures (~5-40 MeV) prevailing in the merger remnant may provide
thermal pressure support that could increase its maximum mass and, thus, its
life on a neutrino-cooling timescale. We investigate the role of thermal
pressure support in hypermassive merger remnants by computing sequences of
spherically-symmetric and axisymmetric uniformly and differentially rotating
equilibrium solutions to the general-relativistic stellar structure equations.
Using a set of finite-temperature nuclear equations of state, we find that hot
maximum-mass critically spinning configurations generally do not support larger
baryonic masses than their cold counterparts. However, subcritically spinning
configurations with mean density of less than a few times nuclear saturation
density yield a significantly thermally enhanced mass. Even without decreasing
the maximum mass, cooling and other forms of energy loss can drive the remnant
to an unstable state. We infer secular instability by identifying approximate
energy turning points in equilibrium sequences of constant baryonic mass
parametrized by maximum density. Energy loss carries the remnant along the
direction of decreasing gravitational mass and higher density until instability
triggers collapse. Since configurations with more thermal pressure support are
less compact and thus begin their evolution at a lower maximum density, they
remain stable for longer periods after merger.Comment: 20 pages, 12 figures. Accepted for publication in Ap
Magnetic nanowires as permanent magnet materials
We present the fabrication of metallic magnetic nanowires using a low
temperature chemical process. We show that pressed powders and magnetically
oriented samples exhibit a very high coercivity (6.5 kOe at 140 K and 4.8 kOe
at 300 K). We discuss the magnetic properties of these metamaterials and show
that they have the suitable properties to realize "high temperature magnets"
competitive with AlNiCo or SmCo permanent magnets. They could also be used as
recording media for high density magnetic recording.Comment: 5 pages, 5 figure
Nodal-to-nodeless superconducting order parameter in LaFeAsPO synthesized under high pressure
Similar to chemical doping, pressure produces and stabilizes new phases of
known materials, whose properties may differ greatly from those of their
standard counterparts. Here, by considering a series of LaFeAsPO
iron-pnictides synthesized under high-pressure high-temperature conditions, we
investigate the simultaneous effects of pressure and isoelectronic doping in
the 1111 family. Results of numerous macro- and microscopic technique
measurements, unambiguously show a radically different phase diagram for the
pressure-grown materials, characterized by the lack of magnetic order and the
persistence of superconductivity across the whole doping
range. This unexpected scenario is accompanied by a branching in the electronic
properties across , involving both the normal and superconducting
phases. Most notably, the superconducting order parameter evolves from nodal
(for ) to nodeless (for ), in clear contrast to other 1111
and 122 iron-based materials grown under ambient-pressure conditions.Comment: 9 pages, 7 figures, Suppl. materia
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