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排序方式: 共有935条查询结果,搜索用时 15 毫秒
1.
Thermally conductive polymers offer new possibilities for the heat dissipation in electric and electronic components, for example, by a three‐dimensional shaping of the heat sinks. To face safety regulations, improved fire performance of those components is required. In contrast to unfilled polymers, those materials exhibit an entirely different thermal behavior. To investigate the flammability, a phosphorus flame retardant was incorporated into thermally conductive composites of polyamide 6 and hexagonal boron nitride. The flame retardant decreased the thermal conductivity only slightly. However, the burning behavior changed significantly, due to a different heat propagation, which was investigated using a thermographic camera. An optimum content of hexagonal boron nitride for a sufficient thermal conductivity and fire performance was found between 20 and 30 vol%. The improvement of the fire performance was due to a faster heat release out of the pyrolysis zone and an earlier decomposition of the flame retardant. For higher contents of hexagonal boron nitride, the heat was spread faster within the part, promoting an earlier ignition and increasing the decomposition rate of the flame retardant. 相似文献
2.
Vladimir Rozenshtein Alexander Berg Haim Levanon Uwe Krueger Dietmar Stehlik Yuri Kandrashkin Art Van Der Est 《Israel journal of chemistry》2003,43(3-4):373-381
Light-induced spin-polarized transient EPR spectra are reported for several water-soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip-doublet to the trip-quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge-transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed. 相似文献
3.
Dietmar Schulze 《化学,工程师,技术》1995,67(1):60-68
Flowability of Bulk Solids – Definition and Measuring Principles . The flowability of bulk solids has to be known for many practical applications. The present paper describes how the flowability is defined and on which physical properties it depends. Experimental determination of the flowability using a shear test is demonstrated. Conditions influencing measuremed results are described. Finally an overview of various methods used for characterization of the flowability of bulk solids is given. 相似文献
4.
Cell suspensions of Escherichia coli as a model for bacterial populations in wastewaters were treated with ozone as a disinfectant in a continuously operated pilot plant with a plug flow reactor. Suspensions with an initial number of CFU (colony forming units) of 108 mL?1 were ozonized with ozone concentrations up to 12 mg/L. Metabolic activities and membrane functions break down with increasing ozone concentrations. The fluorochromization using 3′,6′-diacetylfluorescein (FDA) proved to be a suitable method for the detection of an alteration in permeability of the cell membranes and an inactivation of metabolic activity. By fluorescence microscopic and photometric investigations it could be clearly demonstrated that, in the case of disinfection with ozone, inactivation of the cells includes first of all a damage of the cell membranes. In contrast to the determination of the number of CFU, fluorochromization allows the detection of alteration in metabolic activities. 相似文献
5.
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7.
In connection with the analysis and the development of failure concepts in fracture mechanics for quasistatic loaded components and elastic-plastic behaviour of the material, tests are also carried out with welded and/or complex shaped specimens or structures. Thereby the difficulty arises of generating reproducible flaws in the form of fatigue precracks in definite positions in the components, respectively in the welded joint. It is reported exemplarily about experiments on different CT25 and CCT specimens and on a pressure vessel which contained a fatigue pre-crack, a 0.2 mm saw cut or notches with notch root radius ≤ ≥ 0.1 mm as flaws. The comparison of the results with regard to J-integral at initiation of stable crack, Ji, and JR curves shows that under certain conditions the 0.2 mm saw cut (notch root radius ≤ ≤ 0.02 mm) is a useful alternative, if reproducible generation of a fatigue pre-crack will not be successful or too expensive. The tests were carried out on StE 460 and on a welded joint of this steel at 25 ± 2°C. 相似文献
8.
JP Antel E McCrea U Ladiwala YF Qin B Becher 《Canadian Metallurgical Quarterly》1998,160(4):1606-1611
Oligodendrocytes and their myelin membranes are the apparent target of the autoimmune response that characterizes the human adult central nervous system-demyelinating disease multiple sclerosis. Human oligodendrocytes do not express MHC class II molecules, a requirement for MHC-restricted injury mediated by myelin-reactive CD4+ T cells, the cell type implicated in initiating the disease process. In this study we observed that human adult central nervous system-derived oligodendrocytes can be susceptible to non-MHC-restricted lysis mediated by myelin basic protein-reactive CD4+ T cell lines. Cytotoxicity was significantly greater (37 +/- 4 vs 7 +/- 3%) with cell lines in which a high proportion of cells (mean, 28 +/- 6%) expressed CD56 compared with cytotoxicity mediated by low CD56 cell lines (8 +/- 2%). High CD56 cell lines, when rested in IL-2, lost cytotoxic activity and had reduced expression of CD56 (mean, 5 +/- 2%). CD4+ T cells isolated from short term (3-day) anti-CD3/IL-2-activated mononuclear cell cultures did not express CD56 and were not cytotoxic to oligodendrocytes unless lectin was added. In contrast, an enriched population of non-T cells extracted from the same activated MNC cultures expressed CD56 as well as other NK cell-associated surface molecules and was cytotoxic. These results indicate the potential susceptibility of human oligodendrocytes to non-MHC-restricted injury mediated by both Ag-reactive and nonspecific cellular immune effector cells, with CD56 expression being a common feature of the effector cells. 相似文献
9.
Paul F. Becher 《Journal of Materials Science》1986,21(1):297-300
The room temperature slow crack growth resistance in air ( 50% relative humidity) and in water for large cracks in MgO-partially stabilized zircoba (PSZ) improves with increase in critical fracture toughness,K
lc. Ageing the as-fired PSZ at 1400° C for 8 h results in decreasing Klc from 8.5 MPa m1/2 to 6 M Pa M1/2. The ageing treatment also promotes the growth of eutectoid decomposition products on grain boundaries that is accompanied by a decrease in the dependence (A the change in Region I crack velocity with a change in the applied stress intensity. Calculated times to failures are markedly decreased in the aged as compared to the as-fired PSZ ceramic. 相似文献
10.
Dietmar Köther 《Journal of Infrared, Millimeter and Terahertz Waves》1987,8(11):1365-1389
In the following paper some different kinds of microwave guides will be inspected under main aspect of higher order modes. The knowledge of these modes is important for field matching problems at discontinuities and for suppressing propagation of higher order modes by geometrical variations. In some cases it is sufficient to know their principal behaviour. They will be calculated in paper by the often used orthogonal expansion. This method allows to determine all kinds of waves: propagating, damping and complex waves. The following guides are inspected:
- rectangular waveguides,
- circular waveguides,
- L-shaped waveguides,
- double ridged rectangular waveguides,
- unilateral finlines with one slot,
- unilateral finlines with two slots,
- shielded microstrip lines,
- circular image guides,
- rectangular waveguides with dielectric slab,
- H-guides.