1,060 research outputs found
Monoclinic and triclinic phases in higher-order Devonshire theory
Devonshire theory provides a successful phenomenological description of many
cubic perovskite ferroelectrics such as BaTiO3 via a sixth-order expansion of
the free energy in the polar order parameter. However, the recent discovery of
a novel monoclinic ferroelectric phase in the PZT system by Noheda et al.
(Appl. Phys. Lett. 74, 2059 (1999)) poses a challenge to this theory. Here, we
confirm that the sixth-order Devonshire theory cannot support a monoclinic
phase, and consider extensions of the theory to higher orders. We show that an
eighth-order theory allows for three kinds of equilibrium phases in which the
polarization is confined not to a symmetry axis but to a symmetry plane. One of
these phases provides a natural description of the newly observed monoclinic
phase. Moreover, the theory makes testable predictions about the nature of the
phase boundaries between monoclinic, tetragonal, and rhombohedral phases. A
ferroelectric phase of the lowest (triclinic) symmetry type, in which the
polarization is not constrained by symmetry, does not emerge until the
Devonshire theory is carried to twelfth order. A topological analysis of the
critical points of the free-energy surface facilitates the discussion of the
phase transition sequences.Comment: 10 pages, with 5 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/dv_pzt/index.htm
Cooperating Agents for 3D Scientific Data Interpretation
Many organizations collect vast quantities of three-dimensional (3-D) scientific data in volumetric form for a range of purposes, including resource exploration, market forecasting, and process modelling. Traditionally, these data have been interpreted by human experts with only minimal software assistance. However, such manual interpretation is a painstakingly slow and tedious process. Moreover, since interpretation involves subjective judgements and each interpreter has different scientific knowledge and experience, formulation of an effective interpretation often requires the cooperation of numerous such experts. Hence, there is a pressing need for a software system in which individual interpretations can be generated automatically and then refined through the use of cooperative reasoning and information sharing. To this end, a prototype system, SurfaceMapper, has been developed in which a community of cooperating software agents automatically locate and display interpretations in a volume of 3-D scientific data. The challenges and experiences in designing and building such a system are discussed. Particular emphasis is given to the agents' interactions and an empirical evaluation of the effectiveness of different cooperation strategies is presented
High-Specific Impulse Hall Thrusters, Part 2: Efficiency Analysis
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77163/1/AIAA-15954-346.pd
The Role of Magnetic Field Topography in Improving the Performance of a High Voltage Hall Thruster
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76735/1/AIAA-2002-4111-487.pd
Neutral density map of Hall thruster plume expansion in a vacuum chamber
A neutral background pressure map of the large vacuum test facility (LVTF) is presented. The LVTF is mapped at cold anode flow rates of 5.25, 10.46, and 14.09âmg/s14.09mgâs. In addition, neutral background pressure maps are created at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46âmg/s10.46mgâs for discharge voltages of 300 and 500âV500V, corresponding to P5 Hall thruster operating conditions ranging from 1.5âtoâ5.0âkW1.5to5.0kW. The chamber pressure is mapped at nominal xenon pumping speeds of 140â000 and 240â000âl/s240000lâs. The pressure map is performed with a rake consisting of five calibrated BayardâAlpert hot-cathode ionization gauges. The plume expansion appears to be independent of anode flow rate and facility background pressure. Analysis of axial pressure profiles on the LVTFâs centerline shows that the plume pressure decreases from a maximum at the thruster exit plane down to the facility background pressure at approximately 2âm2m downstream of the exit plane. Comparison of axial pressure profiles on the LVTFâs centerline shows that the neutral density is nearly the same for cold flow and hot flow. The study shows that a cold flow neutral density background map accurately characterizes the neutral density in an operating Hall thruster plume.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87890/2/053509_1.pd
Nonuniqueness in a minimal model for cell motility
Twoâphase flow models have been used previously to model cell motility, however these have rapidly become very complicated, including many physical processes, and are opaque. Here we demonstrate that even the simplest oneâdimensional, twoâphase, poroviscous, reactive flow model displays a number of behaviours relevant to cell crawling. We present stability analyses that show that an asymmetric perturbation is required to cause a spatially uniform, stationary strip of cytoplasm to move, which is relevant to cell polarization. Our numerical simulations identify qualitatively distinct families of travellingâwave solution that coâexist at certain parameter values. Within each family, the crawling speed of the strip has a bellâshaped dependence on the adhesion strength. The model captures the experimentally observed behaviour that cells crawl quickest at intermediate adhesion strengths, when the substrate is neither too sticky nor too slippy
Hall Thruster Cluster Operation with a Shared Cathode
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76373/1/AIAA-23688-572.pd
Neutral Flow Evolution in a Six-Kilowatt Hall Thruster
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90645/1/AIAA-54141-803.pd
Strongly Non-Equilibrium Bose-Einstein Condensation in a Trapped Gas
We present a qualitative (and quantitative, at the level of estimates)
analysis of the ordering kinetics in a strongly non-equilibrium state of a
weakly interacting Bose gas, trapped with an external potential. At certain
conditions, the ordering process is predicted to be even more rich than in the
homogeneous case. Like in the homogeneous case, the most characteristic feature
of the full-scale non-equilibrium process is the formation of superfluid
turbulence.Comment: 4 pages, revtex, no figures. Submitted to PR
Near-Wall Plasma Properties and EEDF Measurements of a 6-kW Hall Thruster
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76497/1/AIAA-2009-5356-548.pd
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