A Study on the Control of Third Generation Spacecraft

An overview of some studies which have recently been carried out on the control of third generation spcecraft, as modelled by the MSAT space vehicle configuration, is made. This spacecraft is highly nonsymmetrical and has appendages which cannot in general be assumed to be rigid. In particular, it is desired to design a controller for MSAT which stabilizes the system and satisfies certain attitude control, shape control, and possibly stationkeeping requirements; in addition, it is desired that the resultant controller should be robust and avoid any undesirable spill over effects. In addition, the controller obtained should have minimum complexity. The method of solution adopted to solve this class of problems is to formulate the problem as a robust servomechanism problem, and thence to obtain existence conditions and a controller characterization to solve the problem. The final controller obtained for MSAT has a distributed control configuration and appears to be quite satisfactory

Si-compatible candidates for high-K dielectrics with the Pbnm perovskite structure

We analyze both experimentally (where possible) and theoretically from first-principles the dielectric tensor components and crystal structure of five classes of Pbnm perovskites. All of these materials are believed to be stable on silicon and are therefore promising candidates for high-K dielectrics. We also analyze the structure of these materials with various simple models, decompose the lattice contribution to the dielectric tensor into force constant matrix eigenmode contributions, explore a peculiar correlation between structural and dielectric anisotropies in these compounds and give phonon frequencies and infrared activities of those modes that are infrared-active. We find that CaZrO_3, SrZrO_3, LaHoO_3, and LaYO_3 are among the most promising candidates for high-K dielectrics among the compounds we considered.Comment: 17 pages, 9 figures, 4 tables. Supplementary information: http://link.aps.org/supplemental/10.1103/PhysRevB.82.064101 or http://www.physics.rutgers.edu/~sinisa/highk/supp.pd

Crystal structure of samarium-strontium-calcium orthoaluminotantalate, (Sm0.40Sr0.50Ca0.10)(Al0.70Ta 0.30)O3

Al2.90Ca0.45O12Sm 1.59Sr2Ta1.10, tetragonal, I4 (no. 82), a = 5.4174(8) Å, c = 7.643(2) Å, V = 224.3 Å3, Z = 1, Rgt(F) = 0.039, wRref(F 2) = 0.1258 , T = 298 K. © 2014 Oldenbourg Wissenschaftsverlag, München

Elastic, phononic, magnetic and electronic properties of quasi-one-dimensional PbFeBO4

The diverse and interesting properties of mullite-type PbFeBO4 have resulted in a growing number of publications, using both experimental and computational methodologies. However, several questions remain to be explored such as the role of the magnetic configuration on the intrinsic potential anharmonicity at a microscopic level, and on the elastic properties and associated pressure-induced response of the nuclear structure. We thus employ the hybrid method PW1PW to study the structural, phononic, magnetic and electronic properties of PbFeBO4 at four different magnetic configurations. The magnetic configuration-driven strong anisotropy of the properties is correlated to two structural features, namely, the one-dimensional chains of FeO6 octahedra and the stereochemical activity of the lone electron pairs of Pb2+ cations. We propose a mechanism to explain the observed axial negative linear compressibility in the b direction. The vibrational features demonstrate insights into the anharmonic behavior of the structure, and a large fraction of modes with negative mode Grüneisen parameters. By optimizing four different magnetic configurations at different pressures the associated spin exchange parameters are calculated; the magnetic configuration considerably affects the magneto-elastic behavior of the structure. Optical absorption spectra calculated by GW0-BSE show a strong anisotropy, associated with the quasi-one-dimensional character of the structure given by the FeO6 chains.Fil: Curti, Mariano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Físicas de Mar del Plata. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Físicas de Mar del Plata; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Biológica; Argentina. Leibniz Universitat Hannover; AlemaniaFil: Murshed, M. Mangir. Universitat Bremen; AlemaniaFil: Bredow, Thomas. Universitat Bonn; AlemaniaFil: Bahnemann, Detlef W.. Saint-Petersburg State Universit; Estados Unidos. Leibniz Universitat Hannover; AlemaniaFil: Gesing, Thorsten M.. Universitat Bremen; AlemaniaFil: Mendive, Cecilia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Físicas de Mar del Plata. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Físicas de Mar del Plata; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Orgánica; Argentin