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排序方式: 共有449条查询结果,搜索用时 15 毫秒
391.
Davis TM Drews TO Ramanan H He C Dong J Schnablegger H Katsoulakis MA Kokkoli E McCormick AV Penn RL Tsapatsis M 《Nature materials》2006,5(5):400-408
Precursor nanoparticles that form spontaneously on hydrolysis of tetraethylorthosilicate in aqueous solutions of tetrapropylammonium (TPA) hydroxide evolve to TPA-silicalite-1, a molecular-sieve crystal that serves as a model for the self-assembly of porous inorganic materials in the presence of organic structure-directing agents. The structure and role of these nanoparticles are of practical significance for the fabrication of hierarchically ordered porous materials and molecular-sieve films, but still remain elusive. Here we show experimental findings of nanoparticle and crystal evolution during room-temperature ageing of the aqueous suspensions that suggest growth by aggregation of nanoparticles. A kinetic mechanism suggests that the precursor nanoparticle population is distributed, and that the 5-nm building units contributing most to aggregation only exist as an intermediate small fraction. The proposed oriented-aggregation mechanism should lead to strategies for isolating or enhancing the concentration of crystal-like nanoparticles. 相似文献
392.
An extensive study of UV photon-induced desoprtion, which included mass-spectrometry, surface conductivity and Auger electron spectroscopy measurements, leads us to suggest that carbon reactions have a dominant role in chemisorption and photodesorption from certain semiconductor surfaces. Carbon, which is present as a containment on such surfaces, can be oxidized by gas phase oxygen capturing an electron from the conduction band of the substrate. Photogenerated holes, created by band-gap radiation, can then break this chemisorption bond and CO2 molecules are then desorbed. Experimental data taken on surfaces of ZnO are discussed in detail. Additional results indicate that similar effects occur on other semiconductor surfaces such as TiO2, CdS, the chrome-oxide surface of stainless steel, etc. It is interesting to note that metallic surfaces, carbon contaminated or not, show virtually no photodesorption activity. This behavior is not well understood at this time. 相似文献
393.
394.
A versatile uhv system was designed in order to enable the preparation of thin films of different materials and their analysis to be undertaken. It includes various sources for evaporation: a high-power electron gun, conventional crucibles and Knudsen cells. An airlock is available for fast transfer of prepared films and new substrates. By a special sample holder the thin films can be manipulated to different positions for structural analysis by reflection high energy electron diffraction (RHEED), and for chemical analysis by Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). 相似文献
395.
Joseph Shapira 《International Journal of Wireless Information Networks》1996,3(3):173-179
The emerging generation of personal communications satellites, built to communicate with handheld telephone units, will generate
power flux on earth that is orders of magnitude higher than that of the present satellite service, and its potential interference
with terrestrial-based systems is of concern. It is proposed here that the flux scattered from the ground surface into the
antenna main beam and near sidelobes may cause higher interference than direct coupling into the antenna. This mechanism,
previously observed in terrestrial systems, may not be mitigated by adaptive nulling techniques, and is less angle-dependent.
A simple model is developed, to be used as a computational tool for assessing the interference and its functional dependence
on governing parameters. In the absence of proper terrain scattering models, examples are evaluated by borrowing results from
interference observed in terrestrial microwave links, which indicate the possible severity of the effect. 相似文献
396.
397.
B Gurevich AA Artru AM Lam AL Mueller V Merkind D Talmor L Katchko Y Shapira 《Canadian Metallurgical Quarterly》1998,88(6):1066-1074
OBJECT: The authors sought to determine whether 3,3-bis (3-fluorophenyl) propylamine (NPS 846), a novel noncompetitive N-methyl-D-aspartate receptor antagonist, alters outcome after closed head trauma in rats. METHODS: The experimental variables were: presence or absence of closed head trauma, treatment with NPS 846 or no treatment, and time at which the rats were killed (24 or 48 hours). The NPS 846 (1 mg/kg) was administered intraperitoneally at 1 and 3 hours after closed head trauma or sham operation. Outcome measures were the neurological severity score (NSS), ischemic tissue volume, hemorrhagic necrosis volume, and specific gravity, water content, and concentrations of calcium, sodium, potassium, and magnesium in brain tissue. The following closed head trauma-induced changes in the injured hemisphere (expressed as the mean +/- the standard deviation) were reversed by NPS 846: decreased specific gravity of 1.035 +/- 0.006 at 24 hours was increased to 1.042 +/- 0.004; the decreased potassium level of 0.583 +/- 0.231 mg/L at 48 hours and at 24 hours was increased to 2.442 +/- 0.860 mg/L; the increased water content of 84.7 +/- 2.6% at 24 hours was decreased to 79.8 +/- 2%; the increased calcium level of 0.592 +/- 0.210 mg/L at 24 hours was decreased to 0.048 +/- 0.029 mg/L; and the increased sodium level of 2.035 +/- 0.649 mg/L was decreased to 0.631 +/- 0.102 mg/L. Administration of NPS 846 also lowered the NSS (improved neurological status) at 48 hours (7 +/- 3) and caused no significant changes in ischemic tissue or hemorrhagic necrosis volumes in the injured hemisphere at 24 or 48 hours. CONCLUSIONS: In this model of closed head trauma, NPS 846 improved neurological outcome, delayed the onset of brain edema, and improved brain tissue ion homeostasis. 相似文献
398.
399.
A Twardowski D Heiman MT Liu Y Shapira M Demianiuk 《Canadian Metallurgical Quarterly》1996,53(16):10728-10731
400.
C Chen JL Mobley O Dwir F Shimron V Grabovsky RR Lobb Y Shimizu R Alon 《Canadian Metallurgical Quarterly》1999,162(2):1084-1095
The very late Ag-4 (VLA-4) integrin supports both rolling and firm adhesion of leukocytes on VCAM-1 under shear flow. The molecular basis for the unique ability of a single adhesion molecule to mediate these versatile adhesive processes was investigated. VLA-4 occurs in multiple activation states, with different affinities to ligand. In this study we tested how these states regulate VLA-4 adhesiveness under shear flow in Jurkat T cells and PBL. VLA-4 on nonstimulated Jurkat cells supported rolling and spontaneous arrest on VCAM-1, whereas a Jurkat activation mutant with reduced VLA-4 affinity failed to spontaneously arrest after tethering to or during rolling on VCAM-1. The contribution of VLA-4 affinity for ligand to rolling and spontaneous arrests on immobilized VCAM-1 was dissected using soluble VLA-4 ligands, which selectively block high affinity states. VLA-4 saturation with ligand completely blocked spontaneous adhesion strengthening post-tethering to VCAM-1, but did not impair rolling on the endothelial ligand. High affinity VLA-4 was found to comprise a small subset of VLA-4 on resting Jurkat cells and PBL. This subset is essential for firm adhesion but not for tethering or rolling adhesions on VCAM-1. Interestingly, low and high affinity VLA-4 states were found to mediate similar initial tethering to ligand. High affinity VLA-4, constitutively expressed on circulating T cells, may control their early adhesion strengthening on VCAM-1-expressing endothelium before exposure to vascular chemokines and activation of additional integrins. 相似文献