The fractional frequency stability of a 34-m-diameterbeam-waveguide antenna

Advances in fiber-optic technology have made it possible to develop a new technique to isolate and measure the frequency stability of a large beam-waveguide (BWG) antenna. Through the use of the technique described in the paper, at both 46.5° and 37° elevation angles, under good weather conditions, the degradation of the the fractional frequency stability of signals passing through the antenna path was measured to be between 1.3 and 2.2×10^-15 for sampling intervals of 1024 s. These stability values apply to the portion of the antenna that includes the main reflector, subreflector, tripod legs, and six BWG mirrors. These test results are believed to be the first successful fractional frequency stability measurements made on the microwave optics portion of a large antenna to a level of 1 or 2 parts in 10¹⁵     

A new procedure for nonlinear statistical model extraction of GaAs FET‐integrated circuits

A new statistical nonlinear model of GaAs FET MMICs which allows the representation of distance‐dependent technological parameter variations by means of equivalent circuit parameters, and an automatic extraction procedure, are presented. The capability to reproduce statistical distribution has been successfully checked on S parameters at different distances in the 1–50 GHz frequency range. © 2003 Wiley Periodicals, Inc. Int J RF and Microwave CAE 13, 348–356, 2003.     

Thermal performance analysis for hot-wall condenser and evaporator configurations in refrigeration appliances: Analyse des performances thermiques pour des configurations de condenseurs et d'évaporateurs sous paroi chaude, pour des applications frigorifiques

A sensitivity study is presented about the investigation of the influence exerted by the design parameters on the thermal performance of those refrigeration appliances employing hot-wall condenser and evaporator configurations, compared to the usual refrigerators, in which external heat exchangers are used.The heat transfer rates inside the wall of the hot-wall refrigerator were evaluated by means of a parametric finite element model; in particular, a chest freezer for domestic applications was considered. A thermodynamic approach was employed to assess the relative heat transfer merits of the apparatus (in respect of a usual refrigerator) from the point of view both of the thermodynamic performance and of capacity. The sensitivity analysis of the design parameters affecting the performance was developed (for fixed working temperatures) with reference to the thickness of the metallic plates, the thickness of the insulating foam, the evaporator and condenser tube diameters and pitches, and the thermal contact resistance between the tubes and the plates.The results show the great importance of the 2-D heat conduction in the metallic plates (`fin efficiency effect'), evidencing how the plate thickness and the thermal contact resistance between the refrigerator plates and the tube serpentines affects the global effectiveness of the system.Some practical suggestions are drawn in conclusion on the criteria which should be adopted in the thermal design of a hot-wall refrigerator.     

Incorporation of Mg-Al hydrotalcite into a biodegradable Poly(ε-caprolactone) by high energy ball milling

The technique of high energy ball milling (HEBM) was used to prepare nanocomposites of poly(ε-caprolactone) (PCL) and an organically modified Mg-Al layered double hydroxide. The amount of inorganic material was varied from 0 to 6 wt%, and the samples were melted and quenched in ice-water after milling. The molecular weight of PCL decreased and its distribution increased as a consequence of milling. The structural analysis of the milled samples, conducted by X-ray diffraction and infrared absorption techniques, showed that the 12 hydroxydodecanoates organic modifier was still attached to the inorganic lamellae even if a partial delamination of the layered compounds occurred. The mechanical parameters (modulus, stress at yield point, strain at break point and stress at break values) derived from the stress-strain curves, improved in the composite samples containing up to 2.8 wt% of inorganic filler, with respect to the pure polymer, in spite of the molecular weight decrease. The thermodynamic diffusion coefficient of water vapor in composite samples was lower than in pure PCL, indicating an improvement of the barrier effect.     

Polymerization of styrene with diphenylzinc-cocatalyst systems

Summary The diphenylzinc-H₂O and diphenylzinc-H₂O-Bu^tCl systems were used as catalysts for styrene polymerization in solution at various temperatures and solvents. The systems are greatly influenced by the molar ratio H₂O/Ph₂Zn, and the maximum catalyst activity, in both cases, was found at a molar ratio of 0.75. GPC results strongly suggest the presence of more than one active species. For Ph₂Zn-H₂O-Bu^tCl system, in dichloromethane at-78°C with the molar ratio of H₂O/Ph₂Zn=0.75, the reaction is first order with respect to monomer with k_p=2.45×10^-3 Lmol^-1sec^-1.     

Role of the support and of the preparation method for copper-based catalysts in the 2-propanol decomposition

Pure copper oxide and mixed CuO/ZnO catalysts with different Cu:Zn atomic ratios were tested for the 2-propanol decomposition in order to investigate the nature of the active site and the role of the ZnO support. Fresh catalysts as well as catalysts oxidized in pure oxygen did not exhibit any catalytic activity below 373 K. When reduced either in pure hydrogen or in reaction mixture (helium plus alcohol) both copper oxide and mixed two-phase catalysts showed a dehydrogenating activity in the temperature range 323–423 K. The apparent activation energy for both reduced CuO and reduced CuO/ZnO catalysts was 60 ± 8 kJ mol^–1. The first order rate constants were found to be a linear function of the exposed zero-valent copper area. The comparison of Cu(0) turnover frequency in unsupported Cu(0) and in Cu(0)/ZnO samples did not show any synergic effect of the support. The role of the preparation method on the Cu(0) dispersion is also discussed.     

CEREBRAL BLOOD FLOW AND METABOLISM EVALUATION IN COMATOSE PATIENTS BY DYNAMIC SPECT

Thirty patients in coma state underwent dynamic SPECT with 133Xe, a validated technique for the quantitation of CBF by SPECT, using a new brain dedicated tomograph: CERTO-96. CMRO2 was computed by multiplying the mean CBF by AVDO2 according to the Fick's principle. The mean values of CBF, AVDO2 and CMRO2 in patients with good outcome were significantly different from those with worse outcome. On the basis of the best "discriminant threshold", CBF and AVDO2 demonstrated an intermediate accuracy in separating the two groups, while CMRO2 showed a satisfactory accuracy.