全文获取类型
收费全文 | 488篇 |
免费 | 29篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 12篇 |
化学工业 | 164篇 |
金属工艺 | 2篇 |
机械仪表 | 13篇 |
建筑科学 | 15篇 |
能源动力 | 6篇 |
轻工业 | 58篇 |
水利工程 | 1篇 |
石油天然气 | 1篇 |
无线电 | 20篇 |
一般工业技术 | 83篇 |
冶金工业 | 69篇 |
原子能技术 | 2篇 |
自动化技术 | 72篇 |
出版年
2023年 | 2篇 |
2022年 | 13篇 |
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 14篇 |
2017年 | 18篇 |
2016年 | 12篇 |
2015年 | 23篇 |
2014年 | 19篇 |
2013年 | 39篇 |
2012年 | 40篇 |
2011年 | 46篇 |
2010年 | 21篇 |
2009年 | 23篇 |
2008年 | 30篇 |
2007年 | 25篇 |
2006年 | 19篇 |
2005年 | 14篇 |
2004年 | 13篇 |
2003年 | 17篇 |
2002年 | 11篇 |
2001年 | 8篇 |
2000年 | 9篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1972年 | 3篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有518条查询结果,搜索用时 15 毫秒
61.
In this work, we investigate two four-objective formulations of multicast routing problem, in which a tree must be set to deliver data to a subset of destination nodes in a network, optimizing several conflicting objectives. We propose a routing model based on SPEA2 (Strength Pareto Evolutionary Algorithm 2) to handle it, incorporating a heuristic that performs a reconnection step in crossover and mutation operators in order to produce a new tree. Three different heuristics were designed for such step. Experimental results were conducted to assess convergence and diversity goals over well-known instances of the problem, showing that the heuristic which alternates between shortest path and randomness produced the best results on most cases. It was shown that the proposed model compares well with traditional algorithms, namely, Dijkstra’s algorithm and Takahashi-Matsuyama heuristic. 相似文献
62.
The present study examined the comparability of 4 alternate forms of the Digit Symbol Substitution test and the Symbol Digit Modalities (written) test, including the original versions. Male contact-sport athletes (N=112) were assessed on 1 of the 4 forms of each test. Reasonable alternate form comparability was demonstrated through establishing normality of form distributions and conducting pairwise form comparisons of means, variability, and intraclass correlations. Nonetheless, alternate forms are likely an insufficient means of controlling practice in speeded measures at brief (1-2 weeks) retest intervals. Reliable change indices demonstrated that practice must be accounted for in individual retesting. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
63.
Grimshaw Gina M.; Bryden M. Philip; Finegan Jo-Anne K. 《Canadian Metallurgical Quarterly》1995,9(1):68
Several theorists have proposed that the sex steroid testosterone acts on the fetal brain during a critical period of development to influence cerebral lateralization (N. Geschwind & A. M. Galaburda, 1987; M. Hines & C. Shipley, see PA, Vol 71:8996; S. F. Witelson, see PA, Vol 79:26441. In the present study. relations were examined between prenatal testosterone levels in 2nd trimester amniotic fluid and lateralization of speech, affect, and handedness at age 10. Girls with higher prenatal testosterone levels were more strongly right-handed and had stronger left-hemisphere speech representation. Boys with higher prenatal testosterone levels had stronger right-hemisphere specialization for the recognition of emotion. This pattern of results is most consistent with Witelson's (1991) claim that prenatal testosterone leads to greater lateralization of function. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
64.
Bogdan C. Gagea Andrei N. Pârvulescu Georges Poncelet Vasile I. Pârvulescu 《Catalysis Letters》2005,102(3-4):219-222
Friedel-Crafts alkylations of α-methylnaphthalene with various alkylating agents were first carried out in the presence of methanesulfonic acid (MeSA). The Brønsted acid catalyst MeSA exhibited outstanding catalytic performance, and was found to be excellent catalyst and solvent for alkylation reaction of aromatic hydrocarbon. It’s found that alkenes can be used as excellent alkylating agent for alkylation of α-methylnaphthalene. The effects of various reaction parameters like type of alkylating agent, dosages of catalysts, reaction temperature and reaction time were investigated in detail. Moreover, the performance of reuse for catalysts was also studied. It’s found that, under the optimal reaction conditions, more than 90% of conversion for olefins and 100% of selectivity for the desired products were obtained. Compared with traditional catalysis technology, the reaction, catalyzed by MeSA catalyst, no volatile solvents needed, good selectivity for desired products could be obtained. The catalyst can be isolated easily from the reaction mixtures by decantation, and was successfully reused. The methanesulfonic acid could be considered as environmentally friendly novel catalyst and solvent for long-chain alkylation of α-methylnaphthalene with alkenes. The catalytic reaction mechanism for alkylation in the presence of MeSA was proposed as well. 相似文献
65.
High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet,
etc.) and mechanical machining (diamond machining, microdrilling, etc.) The lithographic processes require more extensive
processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development
capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The
current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited
in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility
in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small
milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise
and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are
undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which
show that this process can be very effective for the rapid fabrication of molds and mask structures. 相似文献
66.
Micrometer-scale machining: tool fabrication and initial results 总被引:3,自引:0,他引:3
Michael J. Vasile Craig R. Friedrich Bharath Kikkeri Rob McElhannon 《Precision Engineering》1996,19(2-3):180-186
Conventional milling techniques scaled to ultrasmall dimensions have been used to machine polymethyl methacrylate (PMMA) with micrometer-sized milling tools. The object of this work is to achieve machining of a common material over dimensions exceeding 1 mm while holding submicrometer tolerances and micrometer size features. Fabricating the milling tools themselves was also an object of the study. A tool geometry for nominal 25 micrometer diameter cutting tools was found that cuts PMMA with submicrometer tolerances over trench lengths of 2 mm. The tool shape is a simple planar facet cut by focused ion beam milling on ground and polished 25 micrometer diameter steel tool blanks. Pairs of trenches 24 micrometers wide, 26 micrometers deep, 2.3 mm long, with a 14 micrometer separation were milled under various machining conditions. The results indicate that the limits of the machining process in terms of speed, pattern complexity, and tolerances have not been approached. This is the first demonstration of a generic method for microtool making by focused ion beam machining combined with ultraprecision numerically controlled milling. The method is shown to be capable of producing structures and geometries that are considered inaccessible by conventional materials removal techniques, and generally regarded as applications for deep X-ray lithography. 相似文献
67.
The total neutron cross-section of chromium has been measured in the range 1.3 × 10?4 to 30 eV by the transmission method with pulsed neutron time of flight techniques. A theoretical fit was performed giving a value of (3.25 ± 0.15) b for σabs at 0.0253 eV. 相似文献
68.
Cristofor I. Simionescu Constantin Cascaval Vasile Blascu Ioan I. Negulescu 《Polymer》1982,23(13):1862-1863
It is shown that the isomerization of cis polyacetylene at 195°C for long periods is accompanied by structural modifications. Infra-red and pyrolysis gas-chromatography data suggest that the aromatization process occurs even at this temperature. 相似文献
69.
Irina E. Raschip Iryna Yakimets Christopher P. Martin Sabrina S. Paes Cornelia Vasile John R. Mitchell 《Powder Technology》2008,182(3):436-443
In this work, the sorption isotherms of xanthan powder obtained under static and dynamic conditions and their fitting with mathematical models were discussed. The transitions within xanthan-water systems have been investigated using a standard material characterization technique - Differential Scanning Calorimetry (DSC) - and two novel techniques - Dynamic Mechanical Thermal Analysis (DMTA) material pocket and Phase Transition Analysis (PTA). It was found that within water-xanthan systems, the onset temperature for the melting of freezable water obtained from DSC method was in agreement with freezable water melting peaks obtained via DMTA material pocket at levels of hydration when the freezable water was present. Also, low-temperature transitions, related to polysaccharide-water interactions, were observed by both techniques within xanthan systems which contained no freezable water. However, the use of the PTA technique was limited to xanthan systems with low water content. 相似文献
70.
Summary The study of polystyrene (PS)/acrylonitrile-butadiene-styrene (ABS) grafted copolymer/cyclohexanone or methylene chloride system compatibility both in concentrated solutions and solid state has been carried out by phase separation, viscosimetrical measurements and thermal behaviour.Having in view the obtained data it has been ascertained that the systems present different degrees of compatibility depending on the mixtures composition. At higher temperatures, the compatibility of the components is changed. 相似文献