Design of a digital compression technique for shuttle television

The determination of the performance and hardware complexity of data compression algorithms applicable to color television signals, were studied to assess the feasibility of digital compression techniques for shuttle communications applications. For return link communications, it is shown that a nonadaptive two dimensional DPCM technique compresses the bandwidth of field-sequential color TV to about 13 MBPS and requires less than 60 watts of secondary power. For forward link communications, a facsimile coding technique is recommended which provides high resolution slow scan television on a 144 KBPS channel. The onboard decoder requires about 19 watts of secondary power

Metastable and transient states of chemical ordering in Fe-V nanocrystalline alloys

Chemical ordering of the disordered alloys Fe0.78V0.22, Fe0.53V0.47, Fe0.39V0.61, and Fe0.37V0.63 was performed by annealing at temperatures from 723 to 973 K. The initial state of chemical disorder was produced by high-energy ball milling, and the evolution of order was measured by neutron diffractometry and by 57Fe Mössbauer spectrometry. The hyperfine magnetic field distributions obtained from the Mössbauer spectra provided quantitative measurements of the number of antisite Fe atoms in the partially ordered alloys. The long-range order parameters in steady state after long annealing times were used as states of metastable equilibrium for a generally successful comparison with the metastable Fe-V phase diagram calculated by Sanchez et al. [Phys. Rev. B 54, 8958 (1996)]. For the metastable equilibrium state of order in Fe0.53V0.47 at low temperatures, the order parameters were smaller than expected. This corresponded to an abundance of antisite atoms, which were not removed effectively by annealing at the lower temperatures

Magnetostriction of single crystal and polycrystalline Tb0.60Dy0.40 at cryogenic temperatures

At cryogenic temperatures, single crystals of TbDy alloys exhibit giant magnetostrictions of nearly 9000 ppm, making these materials promising for engineering service in cryogenic actuators, valves, and positioners. The preparation of single crystals is difficult and costly. Preliminary results on the magnetostriction of textured polycrystalline materials are presented here. For instance, polycrystalline Tb0.60Dy0.40, plane-rolled (one direction of applied stress) to induce crystallographic texture, has shown magnetostrictions at 77 K of 3000 ppm for an applied field of 4.5 kOe and an applied load of 23 MPa, or 48% that of a single crystal under similar conditions. Comparisons are presented between the magnetostrictive response of plane- and form-rolled (two orthogonal directions of applied stress) polycrystalline Tb0.60Dy0.40 at 10 and 77 K. It is reported that at 10 K plane-rolled Tb0.60Dy0.40 exhibits 1600 ppm magnetostriction at an applied field of 4.4 kOe with a minimal applied load of 0.28 MPa. An observed restoration of the initial unstrained state may be a useful feature of polycrystalline materials for engineering service. Finally it is reported that thermal expansion measurements provide a measure of crystallographic texture for comparison with the magnetostriction

Ekman layers and the damping of inertial r-modes in a spherical shell: application to neutron stars

Recently, eigenmodes of rotating fluids, namely inertial modes, have received much attention in relation to their destabilization when coupled to gravitational radiation within neutron stars. However, these modes have been known for a long time by fluid dynamicists. We give a short account of their history and review our present understanding of their properties. Considering the case of a spherical container, we then give the exact solution of the boundary (Ekman) layer flow associated with inertial r-modes and show that previous estimations all underestimated the dissipation by these layers. We also show that the presence of an inner core has little influence on this dissipation. As a conclusion, we compute the window of instability in the Temperature/rotation plane for a crusted neutron star when it is modeled by an incompressible fluid.Comment: 7 pages, 2 figures, revised version to appear in ApJ, March 1

In islands and their conversion to InAs quantum dots on GaAs (100): structural and optical properties

We report growth of crystalline In islands on GaAs (100) by molecular beam epitaxy at low temperatures. The islands have a pyramidlike shape with well defined facets and epitaxial relation with the substrate. They are of nanoscale dimensions with high density. Above a certain substrate temperature, associated with the melting point of In, noncrystalline round shaped islands form with larger size and lower density. Upon conversion of the In islands into InAs islands under As flux, the final shape does not depend on the original crystalline state but on the annealing temperature of the InAs islands. Clear photoluminescence is observed from InAs quantum dots after conversion of the crystalline In islands

Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes

Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes (SWNT) was found to exceed 8 wt.%, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes

Evaluation of the Thermodynamic Properties of H_2 Binding in Solid State Dihydrogen Complexes [M(η^2‑H_2)(CO)dppe_2][BArF^(24)] (M = Mn, Tc, Re): An Experimental and First Principles Study

The solid state complex [Mn(CO)dppe_2][BArF^(24)] was synthesized, and the thermodynamic behavior and properties of the hydrogen absorption reaction to form the dihydrogen complex [Mn(η^2-H_2)dppe_2][BArF^(24)] were measured over the temperature range 313–373 K and pressure range 0–600 Torr using the Sieverts method. The absorption behavior was accurately described by Langmuir isotherms, and enthalpy and entropy values of ΔH° = −52.2 kJ/mol and ΔS° = −99.6 J/(mol K) for the absorption reaction were obtained from the Langmuir equilibrium constant. The observed binding strength was similar to metal hydrides and other organometallic complexes, despite rapid kinetics suggesting a site-binding mechanism similar to physisorption materials. Electronic structure calculations using the LANL2DZ-ECP basis set were performed for hydrogen absorption over the organometallic fragments [M(CO)dppe_2]^+ (M = Mn, Tc, Re). Langmuir isotherms derived from calculation for absorption onto the manganese fragment successfully simulated both the pressure–composition behavior and thermodynamic properties obtained from experiment. Results from calculations for the substitution of the metal center reproduced qualitative binding strength trends of 5d > 3d > 4d previously reported for the group 6 metals

Analysis of surface waves generated on subwavelength-structured silver films

Using transmission electron microscopy (TEM) to analyse the physical-chemical surface properties of subwavlength structured silver films and finite-difference time-domain (FDTD) numerical simulations of the optical response of these structures to plane-wave excitation, we report on the origin and nature of the persistent surface waves generated by a single slit-groove motif and recently measured by far-field optical interferometry. The surface analysis shows that the silver films are free of detectable oxide or sulfide contaminants, and the numerical simulations show very good agreement with the results previously reported.Comment: 9 Figure