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排序方式: 共有359条查询结果,搜索用时 250 毫秒
61.
Nina‐Luisa Michels Sharon Mitchell Maria Milina Karsten Kunze Frank Krumeich Federica Marone Mandy Erdmann Nadia Marti Javier Pérez‐Ramírez 《Advanced functional materials》2012,22(12):2509-2518
Engineering levels of porosity in hierarchical zeolites is a vibrant area of research with remarkable application potential. To gain practical relevance, the superior properties observed for the as‐synthesized powders have to be preserved when they are shaped into suitable technical geometries. Herein, mechanically stable millimeter‐sized bodies are prepared by granulation of mesoporous ZSM‐5 zeolite powders using an attapulgite clay binder. Alkaline treatment of conventional zeolite granules is demonstrated to be unsuitable for this purpose. Multiple techniques are applied to characterize mesoporous zeolite granules with respect to their conventional zeolite counterparts, thus establishing the impact of binder inclusion and granulation on their respective properties. The intrinsic structure and acidity of the zeolite are retained post‐structuring. Gas adsorption and mercury porosimetry confirm the presence of interconnected micro‐, meso‐, and macropores. A wide range of imaging techniques permits visualization of the particle properties, phase distribution, and consequent origins of the distinct levels of porosity within the zeolite granules. The superior adsorption properties of the hierarchical ZSM‐5 zeolite granules are demonstrated using cyclohexane, toluene, and isopropyl alcohol as probe molecules. 相似文献
62.
Carolin Wang-Hansen Per Ericsson Björn Lundberg Magnus Skoglundh Per-Anders Carlsson Bengt Andersson 《Topics in Catalysis》2013,56(1-8):446-451
The reactivity and reaction kinetics of particulate matter (PM) from direct injected gasoline (GDI) engines has been studied by O2 and NO2 based temperature programmed and isothermal step-response experiments, and the PM nano-structure has been characterized using HRTEM. The reactivity of the PM samples collected in filters during on-road driving was found to increase in the following order: Printex U < diesel < gasoline PI ≈ gasoline DI < ethanol for O2 based combustion. The activation energies for O2 and NO2 based oxidation of PM collected from a GDI engine in an engine bench set-up was estimated to 146 and 71 kJ/mol respectively, which is comparable to corresponding values reported for diesel and model soot. Similar nano-structure features (crystallites plane dimensions, curvature and relative orientation) as observed for diesel soot were observed for gasoline PM. 相似文献
63.
M. Hassan Beyzavi Carolin Nietzold Hans‐Ulrich Reissig Arno Wiehe 《Advanced Synthesis \u0026amp; Catalysis》2013,355(7):1409-1422
meso‐Substituted trans‐A2B2‐porphyrins bearing specific patterns of substituents are crucial building blocks in porphyrin‐based biomimetic systems and molecular materials and can be used for the construction of well‐defined porphyrin‐based architectures. A new stepwise and rational synthesis of functionalized trans‐A2B2‐porphyrins is reported in which for the first time donor–acceptor‐substituted cyclopropane precursors (d–a cyclopropanes) are exploited. The three presented d–a cyclopropanes are readily accessible in a multi‐gram scale and serve as aldehyde equivalents in the reaction with an excess of pyrrole to afford the corresponding dipyrromethanes (DPMs). The three DPMs were synthesized in yields of 60–74%. They are stable in purified form in the absence of light and air and were subsequently condensed with a wide range of aliphatic and aromatic aldehydes bearing electron‐donating or electron‐withdrawing substituents followed by oxidation to form the corresponding trans‐A2B2‐porphyrins. Fourteen functionalized porphyrins were synthesized in yields of 14–31%, indicating the broad scope of the synthetic procedure. The possibility to introduce key functional groups is emphasized, which enables subsequent modification of these porphyrins with moieties inducing biological activity. Modification of the tetrapyrroles may occur by addition to one of the porphyrin peripheral double bonds, the use of substituents of the aryl groups or via the methoxycarbonyl group at two of the meso‐substituents. Three examples of porphyrins were converted into the corresponding 7,8‐dihydroxychlorins by osmium‐mediated dihydroxylation and one of the resulting chlorins was subjected to saponification to give a highly polar chlorin dicarboxylic acid. A 4‐bromophenyl‐substituted d–a cyclopropane was prepared by rhodium‐catalyzed cyclopropanation and then transformed into a DPM which was subsequently condensed to a porphyrin. Its Zn complex allowed a Heck reaction to afford the functionalized bis(alkenyl)‐substituted trans‐A2B2‐Zn‐porphyrin. 相似文献
64.
Gabriel Sas Bj?rn T?ljsten Joaquim Barros Jo?o Lima Anders Carolin 《Canadian Metallurgical Quarterly》2009,13(6):514-534
In this paper the trustworthiness of the existing theory for predicting the fiber-reinforced plastic contribution to the shear resistance of reinforced concrete beams is discussed. The most well-known shear models for external bonded reinforcement are presented, commented on, and compared with an extensive experimental database. The database contains the results from more than 200 tests performed in different research institutions across the world. The results of the comparison are not very promising and the use of the additional principle in the actual shear design equations should be questioned. The large scatter between the predicted values of different models and experimental results is of real concern bearing in mind that some of the models are used in present design codes. 相似文献
65.
The results obtained when performing a load test to failure of an existing structure are valuable when assessing calculation models, updating finite element models, and investigating the true structural behavior. In this paper a destructive testing and monitoring of a railway bridge in ?rnsk?ldsvik, Sweden is presented. In this particular test the shear capacity of the concrete girders was of primary interest. However, for any reasonable placement of the load (a line load placed transverse to the track direction) a bending failure would occur. This problem was solved by strengthening for flexure using carbon fiber reinforced polymer (CFRP) rectangular rods epoxy bonded in sawed up slots, e.g., near surface mounted reinforcement. The strengthening was very successful and resulted in a desired shear failure when the bridge was loaded to failure. The load-carrying capacity in bending for the unstrengthened and strengthened bridge as well as the shear capacity was predicted with Monte Carlo simulations. The particular calculation presented showed that there was a 25% probability of a bending failure instead of a shear failure. Monitoring showed that the strengthening reduced the strain in the tensile steel reinforcement by approximately 10%, and increased the height of the compressed zone by 100 mm. When the shear failure occurred, the utilization of the compression concrete and CFRP rods were 100 and 87.5%, respectively. This indicates that a bending failure indeed was about to occur, even though the final failure was in shear. 相似文献
66.
67.
Albert Saputra Hossein Talebi Duc Tran Carolin Birk Chongmin Song 《International journal for numerical methods in engineering》2017,109(5):697-738
Digital imaging technologies such as X‐ray scans and ultrasound provide a convenient and non‐invasive way to capture high‐resolution images. The colour intensity of digital images provides information on the geometrical features and material distribution which can be utilised for stress analysis. The proposed approach employs an automatic and robust algorithm to generate quadtree (2D) or octree (3D) meshes from digital images. The use of polygonal elements (2D) or polyhedral elements (3D) constructed by the scaled boundary finite element method avoids the issue of hanging nodes (mesh incompatibility) commonly encountered by finite elements on quadtree or octree meshes. The computational effort is reduced by considering the small number of cell patterns occurring in a quadtree or an octree mesh. Examples with analytical solutions in 2D and 3D are provided to show the validity of the approach. Other examples including the analysis of 2D and 3D microstructures of concrete specimens as well as of a domain containing multiple spherical holes are presented to demonstrate the versatility and the simplicity of the proposed technique. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
68.
69.
B Fülesdi M Limburg D Bereczki RP Michels G Neuwirth D Legemate A Valikovics L Csiba 《Canadian Metallurgical Quarterly》1997,46(11):1840-1845
The early preclinical detection of cerebrovascular complications in individuals with diabetes is one of the goals of care described in the St. Vincent Declaration. In accordance with this goal, the aim of the present work was to investigate whether altered cerebral microvascular function in patients suffering from type 1 diabetes can be detected with a transcranial Doppler probe after the administration of acetazolamide. A total of 72 type 1 diabetic patients and 40 healthy control subjects entered the study. Patients were divided into two groups: those with long-term diabetes (disease duration of >10 years, n = 37) and those with short-term diabetes (disease duration of < or =10 years, n = 35). Mean blood-flow velocity in the middle cerebral artery (MCAV) was measured at rest and at 5, 10, 15, and 20 min after intravenous administration of 1 g acetazolamide with a transcranial Doppler probe and expressed as the percentage change from the pretest measurement. The percentage increase in MCAV (cerebrovascular reactivity) was calculated at each time point and compared between the groups. Cerebrovascular reserve capacity (CRC), expressed as the maximal percentage increase of the MCAV, was compared between the groups. Additionally, a reproducibility study of CRC was performed in 10 patients, using intraclass correlations. Cerebrovascular reactivity in the long-term diabetes group was lower (means +/- SD: 5 min, 23.4 +/- 15.4%; 10 min, 28.8 +/- 17.0%; 15 min, 30.0 +/- 15.6%; 20 min, 24.2 +/- 17.8%) than that of the control subjects (5 min, 43.5 +/- 23.9%; 10 min, 55.3 +/- 24.0%; 15 min, 56.7 +/- 23.8%; 20 min, 54.8 +/- 25.9%) and the short-term diabetic patients (5 min, 43.6 +/- 25.9%; 10 min, 52.2 +/- 27.7%; 15 min, 55.3 +/- 32.2%; 20 min, 45.8 +/- 35.8%). CRC was lower in the long-term diabetes group than in the control group or the short-term diabetes group. Impairment of cerebrovascular reactivity was associated with retino- and nephropathy and increased levels of fibrinogen. In contrast, CRC was independent from actual glucose, insulin, glycosylated hemoglobin, von Willebrand factor antigen, and alpha-2 macroglobulin levels. Transcranial Doppler measurements of the changes in MCAV after stimulation with acetazolamide can detect altered cerebral microvascular function in patients with diabetes. Cerebrovascular reactivity and reserve capacity are reduced in patients with long-term diabetes. Further prospective studies should delineate the clinical significance of our results. 相似文献
70.
Native calf thymus DNA was brominated in high salt to achieve B-->Z conformational transition. Ultraviolet and circular dichroism spectroscopic studies point towards the conformational modification of the native DNA. Specific binding of the monoclonal anti-Z-DNA antibody (Z-22) to the DNA brominated in high salt further confirmed the B-->Z conformational isomerization of native DNA. The role of Z-DNA in the etiopathogenesis of systemic lupus erythematosus has been investigated in the light of the binding of naturally occurring human anti-DNA autoantibodies to the induced Z-DNA. 相似文献