115 research outputs found
Simplified model of statistically stationary spacecraft rotation and associated induced gravity environments
A stochastic model of spacecraft motion was developed based on the assumption that the net torque vector due to crew activity and rocket thruster firings is a statistically stationary Gaussian vector process. The process had zero ensemble mean value, and the components of the torque vector were mutually stochastically independent. The linearized rigid-body equations of motion were used to derive the autospectral density functions of the components of the spacecraft rotation vector. The cross-spectral density functions of the components of the rotation vector vanish for all frequencies so that the components of rotation were mutually stochastically independent. The autospectral and cross-spectral density functions of the induced gravity environment imparted to scientific apparatus rigidly attached to the spacecraft were calculated from the rotation rate spectral density functions via linearized inertial frame to body-fixed principal axis frame transformation formulae. The induced gravity process was a Gaussian one with zero mean value. Transformation formulae were used to rotate the principal axis body-fixed frame to which the rotation rate and induced gravity vector were referred to a body-fixed frame in which the components of the induced gravity vector were stochastically independent. Rice's theory of exceedances was used to calculate expected exceedance rates of the components of the rotation and induced gravity vector processes
Instrument concept for geophysical fluid flow experiments on the first spacelab mission
A concept is provided for a geophysical fluid flow cell (GFFC) and sufficient detail is given to allow the start of a design effort. A brief background of the scientific studies to be conducted with the GFFC and its theoretical basis for operation are also included
Non-intrinsic origin of the Colossal Dielectric Constants in CaCu3Ti4O12
The dielectric properties of CaCu3Ti4O12, a material showing colossal values
of the dielectric constant, were investigated in a broad temperature and
frequency range extending up to 1.3 GHz. A detailed equivalent circuit analysis
of the results and two crucial experiments, employing different types of
contacts and varying sample thickness, provide clear evidence that the
apparently high values of the dielectric constant in CaCu3Ti4O12 are
non-intrinsic and due to electrode polarization effects. The intrinsic
properties of CaCu3Ti4O12 are characterized by charge transport via hopping of
localized charge carriers and a relatively high dielectric constant of the
order of 100.Comment: 4 pages, 4 figure
Multiferroic behavior in CdCr2X4 (X = S, Se)
The recently discovered multiferroic material CdCr2S4 shows a coexistence of
ferromagnetism and relaxor ferroelectricity together with a colossal
magnetocapacitive effect. The complex dielectric permittivity of this compound
and of the structurally related CdCr2Se4 was studied by means of broadband
dielectric spectroscopy using different electrode materials. The observed
magnetocapacitive coupling at the magnetic transition is driven by enormous
changes of the relaxation dynamics induced by the development of magnetic
order
Colossal magnetocapacitance and colossal magnetoresistance in HgCr2S4
We present a detailed study of the dielectric and charge transport properties
of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in
ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly
enhanced in the region below 60 - 80 K, which can be ascribed to polar
relaxational dynamics triggered by the onset of ferromagnetic correlations. In
addition, the observation of polarization hysteresis curves indicates the
development of ferroelectric order below about 70 K. Moreover, our
investigations in external magnetic fields up to 5 T reveal the simultaneous
occurrence of magnetocapacitance and magnetoresistance of truly colossal
magnitudes in this material.Comment: 4 pages, 4 figure
Multiferroicity and colossal magneto-capacitance in Cr-thiospinels
The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence
of ferromagnetic and ferroelectric properties together with a pronounced
magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is
established, in HgCr2S4 a bond-frustrated magnetic ground state is realized,
which, however, easily can be driven towards a ferromagnetic configuration in
weak magnetic fields. This paper shall review our recent investigation for both
compounds. Besides the characterization of the magnetic properties, the complex
dielectric permittivity was studied by means of broadband dielectric
spectroscopy as well as measurements of polarization hysteresis and
pyro-currents. The observed colossal magneto-capacitive effect at the magnetic
transition seems to be driven by an enormous variation of the relaxation
dynamics.Comment: 10 pages, 11 figure
Glassy freezing of orbital dynamics in FeCr2S4 and FeSc2S4
We report on a thorough dielectric investigation of the glass-like freezing
of the orbital reorientation-dynamics, recently found for the crystalline
sulpho-spinels FeCr2S4 and FeSc2S4. As the orbital reorientations are coupled
to a rearrangement of the surrounding ionic lattice via the Jahn-Teller effect,
the freezing of the orbital moments is revealed by a relaxational behaviour of
the complex dielectric permittivity. Additional conductivity (both dc and ac)
and contact contributions showing up in the spectra are taken into account by
an equivalent circuit description. The orbital relaxation dynamics continuously
slows down over six decades in time, before at the lowest temperatures the
glass transition becomes suppressed by quantum tunnelling.Comment: J. Non-Cryst. Solids, in press. 6 pages, 4 figure
An Arbitrary Curvilinear Coordinate Method for Particle-In-Cell Modeling
A new approach to the kinetic simulation of plasmas in complex geometries,
based on the Particle-in- Cell (PIC) simulation method, is explored. In the two
dimensional (2d) electrostatic version of our method, called the Arbitrary
Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are
carried out in 2d on a uniform grid on the unit square logical domain, and
mapped to a nonuniform boundary-fitted grid on the physical domain. As the
resulting logical grid equations of motion are not separable, we have developed
an extension of the semi-implicit Modified Leapfrog (ML) integration technique
to preserve the symplectic nature of the logical grid particle mover. A
generalized, curvilinear coordinate formulation of Poisson's equations to solve
for the electrostatic fields on the uniform logical grid is also developed. By
our formulation, we compute the plasma charge density on the logical grid based
on the particles' positions on the logical domain. That is, the plasma
particles are weighted to the uniform logical grid and the self-consistent mean
electrostatic fields obtained from the solution of the logical grid Poisson
equation are interpolated to the particle positions on the logical grid. This
process eliminates the complexity associated with the weighting and
interpolation processes on the nonuniform physical grid and allows us to run
the PIC method on arbitrary boundary-fitted meshes.Comment: Submitted to Computational Science & Discovery December 201
Spacelab 3: Research in microgravity
The Spacelab 3 mission, which focused on research in microgravity, took place during the period April 29 through May 6, 1985. Spacelab 3 was the second flight of the National Aeronautics and Space Administration's modular Shuttle-borne research facility. An overview of the mission is presented. Preliminary scientific results from the mission were presented by investigators at a symposium held at Marshall Space Flight Center on December 4, 1985. This special issue is based on reports presented at that symposium
Relaxation dynamics and colossal magnetocapacitive effect in CdCr2S4
A thorough investigation of the relaxational dynamics in the recently
discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has
been performed. Broadband dielectric measurements without and with external
magnetic fields up to 10 T provide clear evidence that the observed
magnetocapacitive effect stems from enormous changes of the relaxation dynamics
induced by the development of magnetic order.Comment: 4 pages, 4 figure
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