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The error in VLBI estimates of baseline length caused by unmodelled variations in the propagation path through the atmosphere is greater for longer baselines. We present and discuss series of estimates of baseline lengths obtained using different methods to correct for the propagation delay caused by atmospheric water vapor. The main methods are use of data from a water-vapor radiometer (WVR) and Kalman-filtering of the VLBI data themselves to estimate the propagation delay. Since the longest timespan of WVR data associated with geodetic VLBI experiments was obtained at the Onsala Space Observatory in Sweden, we present results for the following three baselines: (1) Onsala–Wettzell, FRG (920 km), (2) Onsala–Haystack/Westford, MA (5600 km), and (3) Onsala–Owens Valley (7914 km).
We have used very-long-baseline interferometry (VLBI) to make twenty-two independent measurements, between September 1984 and December 1986, of the length of the 3900-km baseline between the Mojave site in California and the Haystack/Westford site in Massachusetts. These experiments differ from the typical geodetic VLBI experiments in that a large fraction of observations are obtained at elevation angles between 4° and 10°. Data from these low elevation angles allows the vertical coordinate of site position, and hence the baseline length, to be estimated with greater precision. For the sixteen experiments processed thus far, the weighted root-mean-square scatter of the estimates of the baseline length is 8 mm. We discuss these experiments, the processing of the data, and the resulting baseline length estimates.
The Commission has been fortunate in being able to continue its work during the last triennium with a minimum of effort, and thanks for this should go largely to the Director of the Bureau, Dr B.G. Marsden, for his untiring dedication, as well as to the Smithsonian Astrophysical Observatory for its general support. If any question has arisen, it was where the line should be drawn between paid and unpaid contributions to the Circulars. This involves a somewhat delicate balance of interests between, on the one hand, the encouragement given to certain types of observations which have traditionally been published in the Circulars, (e.g. accurate positions of comets), and, on the other, the income on whi h the continued successful operation of the Bureau depends. The position will therefore have to be kept under review at all times. At present all evidence points to general satisfaction with current policy.
Work has been in progress for many years at University College London to determine how the intensity spectrum of emitted X-rays changes as a function of time and position on the solar disk. Early spectroheliographs consisted of grazing incidence parabolic mirrors which focussed radiation onto proportional counters from which spectral data were obtained. Such instruments, built in collaboration with the University of Leicester Group, have been flown on two rockets (Negus et al., 1969) and on the OSO V satellite (Herring et al., 1971).
The MSSL/Leicester University package on OSO 5 contained proportional counters having fields of view small compared with the area of the solar disk (Herring et al., 1971). Results discussed here were obtained with a detector sensitive in the band 0.3–0.9 nm. This had an entrance window collimated to examine a strip of angular width 2′ lying across the Sun.
The BBC Voices website will probably already be familiar to those involved in English Language teaching and research. Along with local and national radio programmes about English that were broadcast in the UK in 2005, this site is one of the outputs of Voices, a large collaborative multi-platform project undertaken by the BBC and the University of Leeds in 2004 and 2005. Members of the public were asked to submit to the site the different words they use for a range of concepts, and to air their views on English and language use around the UK. In parallel, regional radio journalists who had been trained by Leeds linguists conducted over 300 sociolinguistic interviews with small groups of speakers, discussing the same set of concepts as the online questionnaire, and similarly eliciting opinions on English and language use.
The effect of hydrogenation on DX centers was evaluated for both Si- and Se-doped AlxGa1-xAs (x=0.26 and 0.23). MBE-grown AIGaAs:Si and MOCVD-grown AIGaAs:Se epilayers were hydrogenated with either monatomic hydrogen or deuterium from a remote plasma at 250°C for 1h. The passivation and subsequent reactivation kinetics were studied with C-V and DLTS techniques. Reactivation was investigated in the space-charge layer of Schottky diodes under different bias conditions. While the Group VI and Group IV deep donors respond similarly to passivation, they display significantly different reactivation kinetics, with thermal dissociation energies of 1.5 eV and 1.2 eV for Se-H and Si-H, respectively. These values are close to the energies previously determined for reactivation of the Si and Se shallow donors in both AIGaAs and GaAs. Therefore, they are not significantly dependent on the Al concentration (x < 0.30) even for donors residing on the As sublattice. Our results are consistent with the Chang-Chadi model of DX centers.
The dynamic aspect of early life growth is not fully captured by typical analyses, which focus on one specific time period. To better understand how infant and young child growth relate to the development of adult body composition, the authors characterized body mass index (BMI) trajectories using latent class growth analysis (LCGA) and evaluated their association with adult body composition. Data are from the Cebu Longitudinal Health and Nutrition Survey, which followed a birth cohort to age 22 years (n = 1749). In both males and females, LCGA identified seven subgroups of respondents with similar BMI trajectories from 0 to 24 months (assessed with bimonthly anthropometrics). Trajectory groups were compared with conventional approaches: (1) accelerated growth between two time points (0–4 months), (2) continuous BMI gain between two points (0–4 months and 0–24 months) and (3) BMI measured at one time point (24 months) as predictors of young adult body composition measures. The seven trajectory groups were distinguished by age-specific differences in tempo and timing of BMI gain in infancy. Infant BMI trajectories were better than accelerated BMI gain between 0 and 4 months at predicting young adult body composition. After controlling for BMI at age 2 years, infant BMI trajectories still explained variation in adult body composition. Using unique longitudinal data and methods, we find that distinct infant BMI trajectories have long-term implications for the development of body composition.
We investigate homogeneous incompressible turbulence subjected to a range of degrees of stratification. Our basic method is pseudospectral direct numerical simulations at a resolution of . Such resolution is sufficient to reveal inertial power-law ranges for suitably comprised horizontal and vertical spectra, which are designated as the wave and vortex mode (the Craya–Herring representation). We study mainly turbulence that is produced from randomly large-scale forcing via an Ornstein–Uhlenbeck process applied isotropically to the horizontal velocity field. In general, both the wave and vortex spectra are consistent with a Kolmogorov-like range at sufficiently large . At large scales, and for sufficiently strong stratification, the wave spectrum is a steeper , while that for the vortex component is consistent with . Here is the horizontally gathered wavenumber. In contrast to the horizontal wavenumber spectra, the vertical wavenumber spectra show very different features. For those spectra, a clear dependence for small scales is observed while the large scales show rather flat spectra. By modelling the horizontal layering of vorticity, we attempt to explain the flat spectra. These spectra are linked to two-point structure functions of the velocity correlations in the horizontal and vertical directions. We can observe the power-law transition also in certain of the two-point structure functions.
The bacterial and chemical bases for the luminescence of a number of marine animals had been demonstrated by the end of the nineteenth century. Since that time a considerable amount has been learned about the chemical reaction systems of some bioluminescent animals and a generalised reaction sequence can be postulated to encompass the various components extracted from the different species.
The factors which determine, at the various biological levels, the intensity and the spectral and spatial distribution of the emitted light are described and discussed in terms of the chemical nature of the reaction systems and the biology of the animals concerned.
Hybrid proton-carrier polymer composites were fabricated in an effort to develop high-performance high-temperature proton exchange membranes (PEMs) for fuel cell applications in the 100°–200°C range. The solution-cast hybrid membranes comprise a polymer host and a SiO2-based proton-carrier composite that was synthesized via sol gel approach using a functional silane and tetraethoxysilane (TEOS) in acidic conditions. The primary H+-carrying component was either a heteropoly silicotungstic acid (STA) or a sulfonic acid (SFA) that was thermooxidatively converted from a mercapto (-SH) group. The embedding level of STA on the silane-modified SiO2 sol gel composites was strongly affected by the presence and the functional group of the silane. Ion exchange capacity (IEC) of the water-washed, SiO2-based STA and SFA proton-carrier composite powders is in the range of 1.8–3.5 mmol/g, two to three times higher than that for Nafion 117 (0.9 meq/mol). A glycidylmethacrylate-type copolymer, PEMAGMA, which is stable up to ∼225°C, was able to produce mechanically robust and flexible hybrid membranes. Upon curing, the PEMAGMA composite membranes showed a ∼75% gel under the present formulation and retained the "free" STA effectively with slight loss when extracted in an 85°C water. The W12-STA-containing PEMAGMA membranes followed the weight loss trends of water from STA and the SiO2-based sol gel composite, showing a 10 wt% loss at 150°C and a 15 wt% loss at 225°C. Fuel cell performance tests of the preliminary films gave a Voc in the 0.85–0.93 V range, but a low current density of <4 mA/cm2. The resistive characteristics were attributed to inhomogeneous distribution of the sol gel nanoparticles in the PEMAGMA matrix, a result of phase separation and particulate agglomeration during film forming.
Layers of InSb and InAsxSb1-x were grown on GaAs and GaAs on Si substrates and then characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) to determine the epilayer quality. Hall-effect measurements and photoluminescence (PL) were also performed. Single-crystal XRD indicated that the 5-μm InSb layers grown on GaAs had a peak full width at half maximum (FWHM) of 120 arc sec for the (004) reflection. Planar TEM of a 7-μm-thick InSb layer on GaAs(001) indicated a dislocation density of 2 x 106 cm−2 at the top of the layer. Hall effect measurements of an undoped 3.5-μm-thick InSb on semi-insulating GaAs indicated an electron density of 3.7 x 1016 cm−3 at 300K and a mobility of 45,000 cm2 / V-sec. At 77K these values were 2.7 x 1016 cm−3 and 49,200 cm2 / V-sec, respectively. The composition of the InAsxSb1-x was a function of the growth temperature and the As2/In ratio for both Sb2 and Sb4. The XRD (004) peak FWHM increased with the x value, indicating a deterioration in material quality. This may be caused by alloy segregation in InAsxSb1-x. The peak FWHM rapidly increases from x=0.1 to x=0.3 and then its value drops, indicating that the quality of the layers improved. InSb layers displayed a strong PL whereas the PL for the InAs0.5Sb0.5 layers was very weak. We also grew InSb and InAsxSb1-x layers on GaAs on Si. Optical transmission measurements on InSb indicated that the layers were under tensile stress. We believe this tensile stress could be used to lower the bandgap of InAsxSb1-x layers to provide longer cut-off wavelengths for infrared detectors.
Hydrogen passivation and thermal reactivation of Si donors and Be acceptors were investigated in In0.52Al0.48As grown by molecular beam epitaxy. The semiconducting alloy was passivated by exposure to monatomic hydrogen or deuterium from a remote microwave plasma. The passivation was achieved by exposing the samples to monatomic hydrogen at temperatures between 200 and 250 °C for 1h. The electrical activity of the dopants was monitored by spreading resistance and C-V measurements. The samples were homogeneously doped to concentrations of 1.5×1016 or 6×1017 Si / cm3 and 6×1017 Be / cm3. Both dopants were passivated by more than two orders of magnitude through the epitaxial layers. An additional annealing step (440°C, 5 min) resulted in a complete reactivation of the passivated dopants. In addition to the electrical measurements, secondary ion mass spectroscopy showed that for both the Be- and the Si-doped layers the hydrogen profiles were essentially identical to the dopant profiles throughout the epilayers. This behaviour suggests that hydrogen migration is a dopant-trapping-limited process in n-and p-type In0.52Al0.48As.
The mechanical behavior and damage mechanisms of the Ni/TiC microlaminate composites under static and cyclic loading were investigated. The relationship between the ultimate tensile strength and the layer thickness at both room temperature and 600°C was studied. The fatigue life and the evolution of the stiffness reduction under various maximum applied stress levels were determined. The results revealed that the ultimate tensile strength linearly increased as the laminate layer thickness decreased. Also, the microlaminate exhibited a non-progressive fatigue behavior.
The fatigue behavior of the SCS-6 silicon carbide fiber-reinforced Ti-6Al-4V/Ti-25Al- 10Nb hybrid laminated composite was investigated at room temperature. The accumulation of fatigue damage in the form of matrix cracking was measured as a function of loading cycles and applied stress levels. The residual stiffness and residual tensile strength of the post-fatigued specimens were determined. The comparison of the crack growth behavior of the hybrid composite with both the SCS-6/Ti-6-4 and SCS-6/Ti-25-10 composites will also be discussed.
Using a hot-wall rapid thermal system which permits single-wafer processing, thin gate dielectrics consisting of silicon nitride films were fabricated by low pressure chemical vapor deposition (LPCVD). Nitride layers deposited from dichlorosilane (DCS) and ammonia exhibited greatly reduced electrical leakage current compared to silane-based nitride films which are conventionally used in lamp-based single-wafer rapid thermal technology. After a postdeposition anneal, the DCS-based gate dielectric films showed better diffusion barrier properties against boron penetration than silane-based gate dielectrics at a dopant activation temperature of 1000°C.
Surface scratches in a series of controlled epoxy networks (CEN) were measured using a combination of instrumented indentation protocols and laser scanning confocal microscopy. Identical epoxy chemistry with increasing molecular weight between crosslinks provided different viscoelastic relaxation behaviors with the same modulus at ambient conditions. The glass transition temperatures (Tg)ranged between 70°C and 117°C. The high Tg CEN exhibited the lowest penetration depth and the highest elastic recovery. The results are analyzed with respect to the macroscale bulk properties and underlying molecular architecture of the CEN materials.
Significant conductive polymer-based composites consisting of immiscible semi-crystalline polymers PP and PVDF, at different volume ratios loaded with a certain concentration of CB were prepared by blending and sequent hot-pressing technology. The distributing status of CB in the polymers was evaluated through the micrographs of the composite. The percolation threshold of this kind of composite is much lower than those of the individual polymers. Even more, the composite at 1/1 volume ratio of PP and PVDF displays the best conductivity among different ratios at a certain concentration of CB, and it displays an outstanding PTC effect more than five orders of magnitude, and synchronously an enhanced dielectric permittivity about 24.9 at 100 Hz. These inimitable properties may owe to the formation of PP/PVDF co-continuous phases and a double-percolation effect in the composite. The novel polymer-based composite with ultra-low percolation threshold, enhanced PTC effect, as well as the significant dielectric permittivity is promising a potential application.
Interconnected microcellular polymeric monoliths having unexpected high mechanical strength have been prepared using the High Internal Phase Emulsion methodology. Direct concentrated emulsions of aqueous 5-amino-1-vinyl-[1,2,3,4]tetrazole mixed with low molar (5 %) fractions of N,N'-methylenebisacrylamide (MBA) as cross-linking agent were prepared using dodecane as dispersed phase and a mixture of hydrophilic surfactants. “Reverse” polyHIPE materials were obtained after radical copolymerization, solvent extraction and drying. Their morphology, chemical composition and physico-chemical behaviour are discussed.