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排序方式: 共有1800条查询结果,搜索用时 46 毫秒
151.
Mahadevan Balasubramaniam Yogesh Joshi Daniel Engels Sanjay Sarma Zafar Shaikh 《国际生产研究杂志》2013,51(18):4215-4238
An approach to tool selection and sequencing is presented for three-axis rough machining. The trade-off in the selection of tools is as follows: larger tools have reduced access while smaller tools are capable of reduced cutting speed. Furthermore, every tool change incurs a time penalty. The objective of this paper is to select a tool sequence that minimizes the total rough-machining time. In our approach, the removal volume is stratified into 2.5D machining slabs and, for each tool, the area accessible in each slab is computed incrementally, keeping in mind the cutting portion of the tool and the shape of the tool holder and spindle assembly. This reduces the three-axis problem to a series of two-axis problems with complex precedence constraints. Two models are presented to understand this new form of the problem. First, an integer linear programming formulation is discussed to show the complexity of the task. Second, a network flow formulation is presented, by which we show that it is possible to obtain efficiently an approximate solution of the problem. Examples are discussed to illustrate the algorithms discussed. 相似文献
152.
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154.
Heterogeneous catalysts are now being tried extensively for biodiesel synthesis. These catalysts are poised to play an important role and are perspective catalysts in future for biodiesel production at industrial level. The review deals with a comprehensive list of these heterogeneous catalysts which has been reported recently. The mechanisms of these catalysts in the transesterification reaction have been discussed. The conditions for the reaction and optimized parameters along with preparation of the catalyst, and their leaching aspects are discussed. The heterogeneous basic catalyst discussed in the review includes oxides of magnesium and calcium; hydrotalcite/layered double hydroxide; alumina; and zeolites. Yield and conversion of biodiesel obtained from the triglycerides with various heterogeneous catalysts have been studied. 相似文献
155.
R1 and R2 retrotransposable elements are stable components of the 28S rRNA genes of arthropods. While each retrotransposition event leads to incremental losses of rDNA unit expression, little is known about the selective consequences of these elements on the host genome. Previous reports suggested that in the abnormal abdomen (aa) phenotype of Drosophila mercatorum, high levels of rDNA insertions (R1) in conjunction with the under-replication locus (ur), enable the utilization of different ecological conditions via a population level shift to younger age. We have sequenced the R1 and R2 elements of D. mercatorum and show that the levels of R1- and R2-inserted rDNA units were inaccurately scored in the original studies of aa, leading to several misinterpretations. In particular, contrary to earlier reports, aa flies differentially underreplicate R1- and R2-inserted rDNA units, like other species of Drosophila. However, aa flies do not undergo the lower level of underreplication of their functional rDNA units (general underreplication) that is seen in wild-type strains. The lack of general underreplication is expected to confer a selective advantage and, thus, can be interpreted as an adaptation to overcome high levels of R1 and R2 insertions. These results allow us to reconcile some of the apparently contradictory effects of aa and the bobbed phenotype found in other species of Drosophila. 相似文献
156.
MP Lefranc V Giudicelli C Ginestoux J Bodmer W Müller R Bontrop M Lemaitre A Malik V Barbié D Chaume 《Canadian Metallurgical Quarterly》1999,27(1):209-212
Propofol emulsion supports bacterial growth. Extrinsic contamination of propofol has been implicated as an etiological event in postsurgical infections. When added to propofol, local anesthetics (e.g., lidocaine) alleviate the pain associated with injecting it. Because local anesthetics have antimicrobial activity, we determined whether lidocaine would inhibit microbial growth by comparing the growth of four microorganisms in propofol and in mixtures of propofol and lidocaine. Known quanta of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans were inoculated into solutions of 1% propofol, 0.2% lidocaine in propofol, 0.5% lidocaine in propofol, 0.5% lidocaine in isotonic sodium chloride solution, and 0.9% isotonic sodium chloride solution. All microorganisms were taken from stock cultures and incubated for 24 h. Growth of microorganisms in each solution was compared by counting the number of colony-forming units grown from a subculture of the solution at 0, 3, 6, 12 and 24 h. Propofol supported the growth of E. coli and C. albicans. Propofol maintained static levels of S. aureus and was bactericidal toward P. aeruginosa. The addition of 0.2% and 0.5% lidocaine to propofol failed to prevent the growth of the studied microorganisms. The effect of 0.5% lidocaine in isotonic sodium chloride solution did not differ from the effects of isotonic sodium chloride solution alone. We conclude that lidocaine, when added to propofol in clinically acceptable concentrations, does not exhibit antimicrobial properties. IMPLICATIONS: Local anesthetics such as lidocaine have antimicrobial activity. Propofol supports the growth of bacteria responsible for infection. Bacteria were added to propofol and propofol mixed with lidocaine. The addition of lidocaine to propofol in clinically relevant concentrations did not prevent the growth of bacteria. The addition of lidocaine to propofol cannot prevent infection from contaminated propofol. 相似文献
157.
Structural characterization of the solvent extracts from four different Pakistani lignitic coals has been carried out by their proximate, elemental analysis and FT-Infrared and 1H n.m.r. spectra. The yield of each extracts was greater with 1:1 benzene-methanol mixture in comparison to the total yield obtained by separate extractions with benzene and methanol. The extracts contained significantly less amount of ash and fixed carbon along with an increase in the percentage of volatile matter. The FTIR and 1H n.m.r. spectra indicated that basically all the extracts contained less condensed aromatic rings in comparison to the coal. The FTIR spectra showed sharp well resolved peaks which have been assigned to various functional groups.
The 1H n.m.r. spectra were used to obtain average structural parameters for all the extracts. A detailed analysis of the FTIR and n.m.r. spectra of the coal and their extracts provided an important in-sight into the differences between various extracts and also between various coals and their corresponding extracts. 相似文献
The 1H n.m.r. spectra were used to obtain average structural parameters for all the extracts. A detailed analysis of the FTIR and n.m.r. spectra of the coal and their extracts provided an important in-sight into the differences between various extracts and also between various coals and their corresponding extracts. 相似文献
158.
Liu Pei Chen Deshu Peng Hua Malik O.P. Hope G.S. 《Power Delivery, IEEE Transactions on》1990,5(1):72-78
An analysis of a scheme of accelerated trips for faults in the second zone of a transmission line distance protection system is given. By the use of the two-port network theory, variations of the sequence currents and voltages are analyzed when the fault occurs on the protected line followed by a trip of the circuit breaker on the other side. The analysis is supported by tests performed on a laboratory model of a power system. The new scheme can improve sensitivity and avoid improper operation. Thus, it may be possible to use this scheme instead of pilot protection and alleviate any problems caused by the nonavailability of a sufficient number of communication channels for remote control and relaying 相似文献
159.
Yunshu Zhang Yogesh Sahai Robert A. Rapp Sanjeev Gupta 《Metallurgical and Materials Transactions B》2002,33(2):315-319
The experimentally well-known alumina solubility in the range of acidic to neutral cryolite-base melts has been modeled thermodynamically
in terms of several oxyfluoride solutes. For an acidic melt, cryolite ratio r=1.5, the dominant solute is monoxygen Na2Al2OF6. In a less acidic regime, dioxygen Na2Al2O2F4 is dominant, whereas for neutral compositions (r=3), Na4Al2O2F6 starts to gain importance. The fit of the model to the experimental solubility data is virtually perfect. The values of the
equilibrium constants for the formation of the individual solutes are reported. The formation and conversion of these oxyfluoride
complexes serve as an effective buffer opposing change in the melt basicity. 相似文献
160.
P Malik TC Fisher LL Barsky L Zeng P Izadi AL Hiti KI Weinberg TD Coates HJ Meiselman DB Kohn 《Canadian Metallurgical Quarterly》1998,91(8):2664-2671
Hemoglobinopathies, such as beta-thalassemias and sickle cell anemia (SCA), are among the most common inherited gene defects. Novel models of human erythropoiesis that result in terminally differentiated red blood cells (RBCs) would be able to address the pathophysiological abnormalities in erythrocytes in congenital RBC disorders and to test the potential of reversing these problems by gene therapy. We have developed an in vitro model of production of human RBCs from normal CD34(+) hematopoietic progenitor cells, using recombinant growth factors to promote terminal RBC differentiation. Enucleated RBCs were then isolated to a pure population by flow cytometry in sufficient numbers for physiological studies. Morphologically, the RBCs derived in vitro ranged from early polylobulated forms, resembling normal reticulocytes to smooth biconcave discocytes. The hemoglobin pattern in the in vitro-derived RBCs mimicked the in vivo adult or postnatal pattern of beta-globin production, with negligible gamma-globin synthesis. To test the gene therapy potential using this model, CD34(+) cells were genetically marked with a retroviral vector carrying a cell-surface reporter. Gene transfer into CD34(+) cells followed by erythroid differentiation resulted in expression of the marker gene on the surface of the enucleated RBC progeny. This model of human erythropoiesis will allow studies on pathophysiology of congenital RBC disorders and test effective therapeutic strategies. 相似文献