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1.
The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution (<0.3 micrometer) and of crushed sifted grains (>0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 10(10) meter (-2)) and natural (1 x 10(10) meter(-2)) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains. 相似文献
2.
Lai Z Bonilla G Diaz I Nery JG Sujaoti K Amat MA Kokkoli E Terasaki O Thompson RW Tsapatsis M Vlachos DG 《Science (New York, N.Y.)》2003,300(5618):456-460
A seeded growth method for the fabrication of high-permeance, high-separation-factor zeolite (siliceous ZSM-5, [Si96O192]-MFI) membranes is reported. The method consists of growing the crystals of an oriented seed layer to a well-intergrown film by avoiding events that lead to a loss of preferred orientation, such as twin overgrowths and random nucleation. Organic polycations are used as zeolite crystal shape modifiers to enhance relative growth rates along the desirable out-of-plane direction. The polycrystalline films are thin (approximately 1 micrometer) with single grains extending along the film thickness and with large in-plane grain size (approximately 1 micrometer). The preferred orientation is such that straight channels with an open diameter of approximately 5.5 angstroms run down the membrane thickness. Comparison with previously reported membranes shows that these microstructurally optimized films have superior performance for the separation of organic mixtures with components that have small differences in size and shape, such as xylene isomers. 相似文献
3.
Lane AL Hord CW West RA Esposito LW Simmons KE Nelson RM Wallis BD Buratti BJ Horn LJ Graps AL Pryor WR 《Science (New York, N.Y.)》1986,233(4759):65-70
The Voyager 2 photopolarimeter successfully completed the Uranus encounter, acquiring new data on the planet's atmosphere, its principal satellites, and its ring system. Spatially resolved photometry of the atmosphere at 0.27 micrometer shows no enhancement in absorption toward the pole, unlike the case for Jupiter and Saturn. Stellar occultation measurements indicate the temperature at the 1-millibar level over the north pole is near 90 kelvins. The geometric albedos of the five large satellites of Uranus were measured at 0.27 and 0.75 micrometer and indicate the presence of low albedo, spetrally flat absorbing material. Titania seems to have a fluffy surface, as indicated by its phase curve. The nine ground-based rings were detected, and their internal structure, optical depths, and positions were determined. The sharp edges of the in ring made it possible to measure its edge thickness (less than 150 meters) and particle sizes (less than 30 meters); little or no dust was detcted. New narrow rings and partial rings (arcs) were measured, and the narrow component of the eta ring was found to be discontinuous. 相似文献
4.
Fast axonal transport in squid giant axon 总被引：35，自引：0，他引：35
R D Allen J Metuzals I Tasaki S T Brady S P Gilbert 《Science (New York, N.Y.)》1982,218(4577):1127-1129
Video-enhanced contrast-differential interference contrast microscopy has revealed new features of axonal transport in the giant axon of the squid, where no movement had been detected previously by conventional microscopy. The newly discovered dominant feature is vast numbers of "submicroscopic" particles, probably 30- to 50-nanometer vesicles and other tubulovesicular elements, moving parallel to linear elements, primarily in the orthograde direction but also in a retrograde direction, at a range of steady velocities up to +/- 5 micrometers per second. Medium (0.2 to 0.6 micrometer) and large (0.8 micrometer) particles move more slowly and more intermittently with a tendency at times to exhibit elastic recoil. The behavior of the smallest particles and the larger particles during actual translocation suggests that the fundamental processes in the mechanisms of organelle movement in axonal transport are not saltatory but continuous. 相似文献
5.
Bubbles of less than 1 micrometer and as large as 13.5 micrometers in diameter, stabilized by an apparent compression of substances sorbed onto their surfaces, were examined to determine their physical and temporal stability. Their ease of formation is related to the qualities of the water in which they are formed. Their presence in the water column must now be considered when interpreting acoustic data gathered to determine marine bubble populations. 相似文献
6.
Mutch TA Arvidson RE Binder AB Huck FO Levinthal EC Liebes S Morris EC Nummedal D Pollack JB Sagan C 《Science (New York, N.Y.)》1976,194(4260):87-91
Drifts of fine-grained sediment are present in the vicinity of the Viking 1 lander. Many drifts occur in the lees of large boulders. Morphologic analysis indicates that the last dynamic event was one of general deflation for at least some drifts. Particle cohesion implies that there is a distinct small-particle upturn in the threshold velocity-particle size curve; the apparent absence of the most easily moved particles (150 micrometers in diameter) may be due to their preferential transport to other regions or their preferential collisional destruction. A twilight rescan with lander cameras indicates a substantial amount of red dust with mean radius on the order of 1 micrometer in the atmosphere. 相似文献
7.
Pollack JB Ragent B Boese R Tomasko MG Blamont J Knollenberg RG Esposito LW Stewart AI Travis L 《Science (New York, N.Y.)》1979,205(4401):76-79
Several photometric measurements of Venus made from the Pioneer Venus orbiter and probes indicate that solar near-ultraviolet radiation is being absorbed throughout much of the main cloud region, but little above the clouds or within the first one or two optical depths. Radiative transfer calculations were carried out to simulate both Pioneer Venus and ground-based data for a number of proposed cloud compositions. This comparison rules out models invoking nitrogen dioxide, meteoritic material, and volatile metals as the source of the ultraviolet absorption. Models involving either small ( approximately 1 micrometer) or large ( approximately 10 micrometers) sulfur particles have some serious difficulties, while ones invoking sulfur dioxide gas appear to be promising. 相似文献
8.
Recent determinations of high production rates (up to 30 percent of primary production in surface waters) implicate free-living marine bacterioplankton as a link in a "microbial loop" that supplements phytoplankton as food for herbivores. An enclosed water column of 300 cubic meters was used to test the microbial loop hypothesis by following the fate of carbon-14-labeled bacterioplankton for over 50 days. Only 2 percent of the label initially fixed from carbon-14-labeled glucose by bacteria was present in larger organisms after 13 days, at which time about 20 percent of the total label added remained in the particulate fraction. Most of the label appeared to pass directly from particles smaller than 1 micrometer (heterotrophic bacterioplankton and some bacteriovores) to respired labeled carbon dioxide or to regenerated dissolved organic carbon-14. Secondary (and, by implication, primary) production by organisms smaller than 1 micrometer may not be an important food source in marine food chains. Bacterioplankton can be a sink for carbon in planktonic food webs and may serve principally as agents of nutrient regeneration rather than as food. 相似文献
9.
A single kinesin molecule can move "processively" along a microtubule for more than 1 micrometer before detaching from it. The prevailing explanation for this processive movement is the "walking model," which envisions that each of two motor domains (heads) of the kinesin molecule binds coordinately to the microtubule. This implies that each kinesin molecule must have two heads to "walk" and that a single-headed kinesin could not move processively. Here, a motor-domain construct of KIF1A, a single-headed kinesin superfamily protein, was shown to move processively along the microtubule for more than 1 micrometer. The movement along the microtubules was stochastic and fitted a biased Brownian-movement model. 相似文献
10.
A study of absorption bands in simulated lunar glasses resulted in the discovery of a Ti(4+)-O(2-) charge transfer band at 0.26 micrometer and two Fe(2+)-Ti(4+) bands at 0.34 and 0.42 micrometer. These results indicate that the properties of lunar soil are somewhat affected by the glass content but are controlled by other factors. 相似文献
11.
The measured reaction probability, gamma for the reaction of chlorine nitrate (CIONa(2)) with 60 percent (by weight) sulfuric acid aerosol increases monotonically with particle size at 250 kelvin. The reacto-diffusive length (l, the effective liquid depth over which reaction occurs) derived from these experiments is 0.037 +/- 0.007 micrometer (95 percent confidence level for precision). The reaction probability for the reaction of CIONO(2) with 60 percent sulfuric acid aerosol doped with approximately 7 x 10(-4) M hydrochloric acid at 250 kelvin is larger by about a factor of 4 than in the absence of hydrochloric acid and varies less with particle size (l = 0.009 +/- 0.005 micrometer). These results provide a test of the theory for gas-particle reactions and further insight into the reactivity of atmospheric aerosol. 相似文献
12.
The role of surface chemistry in filter feeding by zooplankton 总被引：1，自引：0，他引：1
Surface chemistry of both particles and animals is important in filter feeding at low Reynolds number. Daphnia magna, fed mixtures of three sizes of polystyrene particles, retained particles that were smaller than the mesh size of the animals (1.0 micrometer) at greater efficiencies than predicted by a sieving model. Retention efficiency of the smallest particles (0.5 micrometer) was increased when negative surface charge on the particles was neutralized, and retention was decreased when a nonionic surfactant was, added to reduce wettability. 相似文献
13.
Calcium activated skinned frog muscle fibers develop a large relative force at a sarcomere length of 1.0 micrometer. Since the normal myofilament lattice is perturbed at this length, regularity of the lattice does not appear to be an important factor in the contraction mechanism. 相似文献
14.
Optical and scanning electron microscopy of a chondrule-free clast in the unequilibrated L3 chondrite Khohar revealed a spherical object consisting of an aggregate of small ( approximately 2- micrometer diameter), Ni-poor (0.5 to 2.89 weight percent) metal particles and fine-grained graphite (<1-micrometer diameter). The graphite has large D and 15N excesses (deltaD approximately 1500 per mil and delta15N approximately 1300 per mil) with two isotopically distinct signatures: N rich with a high D/H ratio and N poor with a high 15N/14N ratio. These excesses are the largest D and 15N excesses observed in situ in a well-characterized phase in a meteorite. The isotopic characteristics are suggestive of an interstellar origin, probably by ion-molecule reactions at low temperature in the interstellar molecular cloud from which the solar system formed. The structure and nonchondritic composition of the metal particles suggest they did not form under equilibrium conditions in the solar nebula. 相似文献
15.
Melanin pigmentation: an in vivo model for studies of melanosome kinetics within keratinocytes 总被引：3，自引：0，他引：3
The phagocytosis of latex beads by epidermal cells is proposed as a model for stludies on melanosome kinetics within the epidermis. Large latex beads (0.8 micrometer) are ingested singly, whereas, small beads (0.1 micrometer) are taken up in groups, results showing that the uptake mechanism depends on the size of thze indlividual particles. This size-dependency may explain the different distribution patterns of melanosomes and thus the differences of skin color in the Caucasoid and Negroid races. 相似文献
16.
The dimensions of the topographical signals for growth orientation and infection structure formation, a cell differentiation event that includes nuclear division, were determined for the stomatal penetrating rust fungus Uromyces appendiculatus. The differentiation signal was found to be a simple ridge on the substrate surface that had a markedly optimum height of 0.5 micrometer. Such ridges were microfabricated on silicon wafers by using electron-beam lithography. A similar ridge, in the form of a stomatal lip, was found associated with the stomatal guard cells of the bean (Phaseolus vulgaris) leaf. Ridge elevations greater than 1.0 micrometer or less than 0.25 micrometer did not serve as effective signals. Germ tubes of the fungus were highly oriented by ridge spacings of 0.5 to 6.7 micrometers. The data indicate that the fungus is able to distinguish uniquely minute differences in leaf surface topography in order to infect the host plant. 相似文献
17.
Three high-temperature, high-density experments were conducted recently with the 10-terawatt, short-wavelength Novette laser system at the Lawrence Livermore National Laboratory. The experiments demonstrated successful solutions to problems that arose during previous laser-plasma interaction experiments with long-wavelength (greater than 1 micrometer) lasers: (i) large-scale plasmas, with dimensions approaching those needed for high-gain inertial fusion targets, were produced in which potentially deleterious laser-plasma instabilities were collisionally damped; (ii) deuterium-tritium fuel was imploded to a density of 20 grams per cubic centimeter and a pressure of 10(10) atmospheres under the improved laser conditions, and compression conditions (preheating and pressure) were consistent with code calculations that predict efficient (high-gain) burn of a large thermonuclear fuel mass when driven with a large, short-wavelength laser; and (iii) soft x-rays were amplified by a factor of 700 by stimulated emission at 206 and 209 angstroms (62 electron volts) from selenium ions in a laser-generated plasma. These small, short-pulse x-ray sources are 10(10) to 10(11) times brighter than the most powerful x-ray generators and synchrotron sources available today. The plasma conditions for these experiments were made possible by advances in Nd:glass laser technology, in techniques to generate efficiently its short-wavelength harmonics at 0.53, 0.35, and 0.26 micrometers, and in diagnostic and computational modeling. 相似文献
18.
Sequential adsorption of a cationic polyelectrolyte and individual sheets of the silicate mineral hectorite has allowed controlled, stepwise formation of multilayered films on silicon wafers. Each component adsorbs rapidly by an ion-exchange mechanism, and x-ray diffractometry indicates structural order even in films with thicknesses greater than 0.2 micrometer. The large lateral extent of the silicate sheets (about 25 to 35 nanometers) allows each layer to cover any packing defects in the underlying layer, thus preserving structural order in the growing film. With careful choice of component materials, this method should allow for the preparation of multilayered films with a variety of technologically important properties. 相似文献
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20.
Oxygen isotope ratios measured by ion microprobe in magnetite from granulite-facies marble of the Adirondack Mountains, New York, range from +2 to +11 per mil (standard mean ocean water) across a single grain that measures 3 millimeters by 5 millimeters. Low values are concentrated in irregular domains near the grain boundary but also occur in the grain's interior. In contrast, grains 1 millimeter in diameter that are from a second nearby sample show no significant heterogeneity, except within 10 micrometers of the grain boundary. These data, including large gradients of up to 9 per mil per 10 micrometers, provide important new constraints on the nature and origins of intragrain isotopic heterogeneity and on oxygen isotope thermometry. The differences between these magnetite grains result from contrasting mechanisms of isotope exchange with fluids after the peak of regional metamorphism. Volume diffusion of oxygen through the crystal structure of magnetite contributed to isotope exchange in the rims of small grains, but larger grains are crosscut by healed cracks that are not readily detected and that short-circuited diffusion. 相似文献