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Projects that aim to control invasive species often assume that a reduction of the target species will increase native species abundance. However, reports of the responses of native species following exotic species control are relatively rare. We assessed the recovery of the native community in five tidal wetland locations in which we attempted to eradicate the invasive common reed [Phragmites australis (Cav.) Trin. ex Steud.]. We tested whether 3 yr of treatment were able to eradicate Phragmites and promote recovery of the native plant community. After 3 yr of treatment, Phragmites density declined sharply in all treated stands, though it was not eradicated in any of them. Native plant cover increased significantly in treated areas, and community composition, particularly in smaller stands, converged toward that of uninvaded habitat. Thus, even within the relatively short timescale of the treatments and monitoring, significant progress was made toward achieving the goals of controlling Phragmites infestations and promoting native biodiversity. There was a trend toward greater promise for success in smaller stands than larger stands, as has been observed in other studies. A greater emphasis on monitoring whole-community responses to exotic plant control, across a range of conditions, would enhance our ability to plan and design successful management strategies.
The question of the title of Commission 24, obviously, offers a difficult problem as already mentioned in recent reports. Photographic Astrometry no longer describes the whole scope of the commission. This problem has continued during the last three years especially in view of the preparations for the astrometric tasks of the NASA Space Telescope and of the ESA satellite HIPPARCOS.
The present triennial commission report embraces mainly activities in wide angle, optical astrometry. With the successful development and application of new techniques from Earth (e.g. optical interferometry, CCD’s) and space (Hipparcos mission and new projects) the sub-division between Commissions 8 (Positional Astrometry) and 24 (Photographic Astrometry) has become questionable. During the GA at Kyoto in 1997 all steps for a merger of both commissions have been taken. The final merging will take place at the forthcoming GA in Manchester. For a more complete overview on astrometrical work done in the past triennium the reader should also take notice of the report of Commission 24.
Properties of the microwave emission from HR1099 are examined in an attempt to determine whether the emission arises as gyro-synchrotron radiation from mildly relativistic electrons trapped in magnetic fields above starspots on the active K subgiant component. It is shown that radio curves do not exhibit a systematic variation in phase with the rotation rate, as one might expect for emission from a source situated above a long-lived starspot. However, there is some evidence that the radio flaring occurs at two preferred longitude zones. Whether these zones agree with starspot locations remains to be determined by light curve modelling. What we can say with confidence is that the measured spectral index of the microwave emission does not fit a simple gyro-synchrotron source model, such as that proposed to explain the observed reversal with frequency of the sense of circular polarization.
Inelastic neutron scattering methods were used in conjunction with Raman spectroscopy to probe the vibrational density of states of the hydrofluorocarbons (HFCs) 134 (HF2C-CF2H) and 134a (F3C-CFH2) adsorbed in the cages of dehydrated Na-X zeolite. A comparison of the vibrational spectra of the encaged HFC species with those of their gas-phase analogs indicates that the HFCs adsorb nondissociatively at room temperature and are most likely associated with Na cations in the supercages at the SIII sites. Guest-host interactions are manifested by adsorption-induced perturbations of the gas-phase torsional and C-H stretching vibrations and the presence of additional features presumably due to low-energy whole-molecule vibrations and adsorbate-coupled zeolite framework vibrations. Moreover, although the 134 trans conformer is favored by 5 kJ/mole in the gas phase at 300 K, the gauche conformer seems to be more prevalent in the zeolite at this temperature and below. This suggests that a sizeable fraction of the Na-X adsorption sites provides a stabilizing configuration for the otherwise higher-energy gauche conformation, perhaps due to hydrogen-bonding interactions with the zeolite framework.
We have explored a new technology based on chemically induced phase separation that yields porous epoxies and cyanurates with a closed cell morphology and micrometer sized pores with a narrow pore size distribution. When the precursor monomers are cured in the presence of a low molecular weight liquid, the desired morphology results from a phase separation and a chemical quench. After phase separation, the porosity is achieved by thermal removal of the secondary liquid phase, specifically by diffusion through the crosslinked matrix. In respect to the thermodynamics and kinetics, the origin of the phase separation process can be identified as nucleation and growth. The influence of internal and external reaction parameters, such as chemical nature of the low molecular weight liquid, its concentration and the curing temperature on the final morphology are presented. Thus, the morphology can be controlled ranging from a monomodal to bimodal pore size distribution with pore sizes inbetween 1 to 10 μm. These porous thermosets are characterized by a significantly lower density, without any loss in thermal stability compared to the neat matrix. Such new materials demonstrate great interest for lowering the dielectric constant and for improving the fundamental understanding of the role of voids in stress relaxation and toughening.
Upon heating Ba2+, Sr2+ and Cd2+-exchanged zeolite RHO, abrupt changes have been observed in the cubic unit cell parameters [1–3]. Calculations of the powder x-ray diffraction patterns indicate these changes result from relocation of the extra-framework cations. For Ba2+ and Sr2+-exchanged RHO, a shift from the single 8-ring (S8R) to the double 8-ring (D8R)-site is accompanied by contraction of the unit cell. However, for the Cd2+-exchanged material relocation from the S8R to the single 6-ring (S6R)-site coincides with cell expansion. Further, with relocation of Cd2+ the low temperature acentric (A) form is transformed to a centric (C) structure above 300°C. The shift of Cd2+ ion occurs over a distance of 5.7Å, the largest observed in a zeolite.
The decomposition of zeolites is a novel route for the synthesis of aluminosilicate-based ceramic materials. Zeolites offer a number of advantages as ceramic precursors which allow the formation of dense ceramics at lower temperatures than by conventional methods. Using various cation-exchanged zeolites, ceramics and ceramic composites containing anorthite (CaAl2Si2O8). mullite (Al6Si2O11) cordierite (Mg2Al4Si5O18). celsian (BaAl2Si2O8), and ß-spodumene (LiAlSi2O6) have been formed. Using Extended X-ray Absorption Fine Structure (EXAFS), we have studied the reconstructive transformation of strontium-exchanged zeolite A to Sr-anorthite and have shown that the primary coordination of strontium remains intact during conversion.
Nanochannel glass replica membranes are thin metallic films containing patterned arrays of uniform, nanometer-scale voids whose sizes, positions, geometric patterns, and packing densities may be controlled to a high degree. These membranes have been prepared from refractory and noble metals and are well suited for use as shadow masks in a variety of substrate patterning applications. Here we describe their use in the sub-micron, parallel patterning of Si and GaAs by materials deposition and reactive-ion etching respectively.
The microstructure of carbonaceous materials strongly affect their ability to electrochemically intercalate lithium . The fractional intercalation capacity (x in LixC 6) for various types of amorphous and graphitic carbons can vary over a range between 0 to 1. Capacities exceeding that of graphite (372 mAh/g or LiC6) can be obtained from chemically doped (i.e., with phosphorous ) materials or from carbonized organic precursors pyrolyzed at low temperatures (<900°C) . Additional chemical effects apparently influence the carbon electrochemical behavior in these cases.
Indium arsenide wires have been fabricated in a glass host consisting of a high density array of uniformly shaped channels in a matrix glass. Reactions between organoindium compounds and arsine have produced polycrystalline InAs with crystallite sizes of approximately 10 nanometers when annealed at 400°C. At higher annealing temperatures, the wires exhibit an increase in surface porosity and grain size.
The advent of ambient pressure aerogel technology may offer process advantages compared to supercritical drying. Since low density is required for most applications of interest, precursor gels must be very dilute (e.g. 6–8% SiO2). The remaining 92–94% is solvent which must be carried along for the entire process, then evaporated, condensed, separated, and recycled to be reused. These steps and associated unit operations represent a major cost (capital and operating). The drying step is problematic because of the large mass of solvent and the inherently low thermal conductivity of the gel. A new approach for drying aerogels is described whereby a wet gel containing a solvent is submerged in hot water or similar fluid which results in removal of the solvent without the drying fluid penetrating the gel. This results in a dried gel with aerogel properties but with the advantages of having a high heat transfer rate without using large temperature gradients as well as providing inherent separation of the dried material. We explore the effects of different solvents and drying temperature gradients on resulting gel microstructure.
NMD samples from Brazil have been submitted to magnetic and particle size separations and characterized by X-ray diffraction and fluorescence and thermogravimetric analyses. Results showed that simple physical treatments can lead to more than 60% enriched MnO2 materials which could satistify some electrochemical applications. The electrical properties of the samples conditionated as pressed pellets have been investigated by four-points direct current probe and impedance spectroscopy, varying the conditions of preparation and measurement. It is proposed that the higher frequency impedance is equivalent to the intrinsic electronic resistance of the MnO2 phases while at lower frequencies occurs an interphase charge separation coupled with a possible ionic transport. The corresponding contact resistance depends on the article size distribution of the material, the compactation pressure of pellets and the iron content of the materials. The interphase dielectric relaxation does not behave ideally; the depression of the impedance semicircles as shown in the Nyquist plane is assumed to be related to the roughness of the bulk interfaces.
The electrochemical capacitative behavior of carbon aerogels and selected commercial carbon fiber cloths was studied in 5M potassium hydroxide, 3M sulfuric acid, and 0.5M tetraethylammonium tetrafluoroborate/propylene carbonate electrolytes. The resorcinolformaldehyde based carbon aerogels with a range of density (0.2–0.85 g/cc) have open-cell structures with ultrafine pore sizes (∼5–50 nm), high surface area (400–700 m2/g), and a solid matrix composed of interconnected particles or fibers with characteristic diameters of 10 nm. The commercial fiber cloths in the density range 0.2–0.4g/cc have high surface areas (1000–2500 m2/g). The volumetric capacitances of high-density aerogels are shown to be comparable to or exceeding those obtained from activated carbon fibers. The electrochemical behavior of these types of materials in various electrolytes is compared and related to their physical properties.
Recent NMR studies by Nakayama et al. on sodium zeolite A saturated with potassium metal have implied the presence of the anionic Na− species. This, if confirmed, would represent the first observation in a zeolite and opens up a wide range of possibilities for mixed metal zeolitic systems. We report the results of a number of metal combinations, using both sodium and potassium forms of zeolite A as hosts, studied by ESR, solid state MAS-NMR and powder neutron diffraction.
Elastomeric connectors have been developed for a wide variety of interconnection and test applications which include module-to-board, board-to-board interconnections, high density module, board and LCD testings, as well as chip/wafer testing and burn-in to produce known-good-die. The density, compliance, thickness, pattern and size of these structures can be tailored to meet the requirements of each application. The fabrication processes involve wire bonding and polymer encapsulation. To achieve high compliance with low contact force, alternative means of generating highly compliant elastomers has been developed. This approach involves the incorporation of voids, where the reduction in the compressive force is simply achieved by replacing a portion of the polymer with air. Two approaches of generating porous elastomers are described along with the details of the porosity and mechanical properties.
Yttria-stabilized zirconia was prepared by (i) coprecipitation and (ii) alkoxide hydrolysis. The thermal and X-ray diffraction data were compared. The coprecipitation method yields microporous materials after calcination at 500°C for 4 hours with a narrow pore size distribution at a radius of 2–2.5 nm. Similar results were also seen with the alkoxide hydrolysis.
Mesoporous silicas were modified with titanium alkoxide precursors. Results are consistent with a model that describes the resultant silica surface as a mixture of Si–OH groups, isolated Si–O–Ti type species, and oligomeric (-Ti-O-)n ensembles. Decreasing the rate of hydrolysis of the precursor by using chelated Ti-alkoxides increases the fraction of isolated Ti.
We performed in-situ photoluminescence and Raman measurements on an anodized silicon surface in the HF/ethanol solution used for anodization. The porous silicon thereby produced, while resident in HF/ethanol, does not immediately exhibit intense photoluminescence. Intense photoluminescence develops spontaneously in HF/ethanol after 18–24 hours or with replacement of the HF/ethanol with water. These results support a quantum confinement mechanism in which exciton migration to traps and nonradiative recombination dominates the de-excitation pathways until silicon nanocrystals are physically separated and energetically decoupled by hydrofluoric acid etching or surface oxidation. The porous silicon surface, as produced by anodization, shows large differences in photoluminescence intensity and peak wavelength over millimeter distances. Parallel Raman measurements implicate nanometer-size silicon particles in the photoluminescence mechanism.