全文获取类型
收费全文 | 4727篇 |
免费 | 62篇 |
国内免费 | 11篇 |
专业分类
电工技术 | 32篇 |
综合类 | 2篇 |
化学工业 | 593篇 |
金属工艺 | 206篇 |
机械仪表 | 85篇 |
建筑科学 | 94篇 |
矿业工程 | 10篇 |
能源动力 | 53篇 |
轻工业 | 295篇 |
水利工程 | 30篇 |
石油天然气 | 5篇 |
无线电 | 434篇 |
一般工业技术 | 635篇 |
冶金工业 | 1930篇 |
原子能技术 | 51篇 |
自动化技术 | 345篇 |
出版年
2021年 | 35篇 |
2019年 | 40篇 |
2018年 | 37篇 |
2017年 | 26篇 |
2016年 | 50篇 |
2015年 | 31篇 |
2014年 | 46篇 |
2013年 | 152篇 |
2012年 | 92篇 |
2011年 | 131篇 |
2010年 | 88篇 |
2009年 | 117篇 |
2008年 | 135篇 |
2007年 | 119篇 |
2006年 | 118篇 |
2005年 | 111篇 |
2004年 | 105篇 |
2003年 | 101篇 |
2002年 | 91篇 |
2001年 | 81篇 |
2000年 | 94篇 |
1999年 | 105篇 |
1998年 | 442篇 |
1997年 | 307篇 |
1996年 | 241篇 |
1995年 | 168篇 |
1994年 | 121篇 |
1993年 | 177篇 |
1992年 | 86篇 |
1991年 | 75篇 |
1990年 | 70篇 |
1989年 | 67篇 |
1988年 | 74篇 |
1987年 | 93篇 |
1986年 | 56篇 |
1985年 | 70篇 |
1984年 | 39篇 |
1983年 | 45篇 |
1982年 | 45篇 |
1981年 | 49篇 |
1980年 | 54篇 |
1979年 | 30篇 |
1978年 | 50篇 |
1977年 | 63篇 |
1976年 | 107篇 |
1975年 | 40篇 |
1974年 | 33篇 |
1973年 | 32篇 |
1972年 | 31篇 |
1970年 | 33篇 |
排序方式: 共有4800条查询结果,搜索用时 828 毫秒
21.
Synthesis, absolute configuration, and analysis of malathion, malaoxon, and isomalathion enantiomers
Syntheses of the enantiomers of malathion, malaoxon, and isomalathion are reported herein. Malathion enantiomers were prepared from (R)- or (S)-malic acid in three steps. Enantiomers of malathion were converted to the corresponding enantiomers of malaoxon in 52% yield by oxidation with monoperoxyphthalic acid, magnesium salt. The four isomalathion stereoisomers were prepared via two independent pathways using strychnine to resolve the asymmetric phosphorus moiety. The absolute configurations of the four stereoisomers of isomalathion were determined by X-ray crystallographic analysis of an alkaloid salt precursor. A high-performance liquid chromatography technique was developed to resolve the four stereoisomers of isomalathion, and to determine their stereoisomeric ratios. 相似文献
22.
This report contains the experience of our centre, using the transvaginally guided puncture procedure, to reduce the number of fetuses in a multifetal pregnancy to a lower number. The aim of the procedure was to improve perinatal outcome and/or to meet the personal desires of patients and their families. We surveyed 148 multifetal pregnancy reductions. The fetus or fetuses overlying the internal os was most commonly reduced. The total uncorrected loss of the entire pregnancy was 13.4%. The corrected pregnancy loss was 11%. Of the 63 twins left after the reduction, 33 delivered preterm. Of the 36 singletons, two delivered preterm. Our conclusion was that multifetal pregnancy reduction is a safe procedure for the mother and has an acceptable loss rate of the entire pregnancy. The reduction of a fetus overlying the internal os by the transvaginal puncture procedure seems to yield results at least as good as the transabdominally performed puncture procedures for multifetal pregnancy reduction. 相似文献
23.
Zemon S. Pedersen B. Lambert G. Miniscalco W.J. Hall B.T. Folweiler R.C. Thompson B.A. Andrews L.J. 《Photonics Technology Letters, IEEE》1992,4(3):244-247
The performance of Nd3+-doped fibre amplifiers is limited by strong excited-state absorption (ESA) of the signal, and, even for fluorozirconate glasses, ESA prevents the important region below 1320 nm from being used. To quantify this limitation and explore alternative host materials, ESA and stimulated-emission cross sections have been measured for a representative group of glass compositions. These parameters have been used in an accurate fiber-amplifier model to provide the first quantitative comparisons of performance for Nd3+ -doped glasses in the 1300-nm band as a function of host 相似文献
24.
A modification of the classical banyan switching network architecture, called the dilated slipped banyan, is described. This architecture is recursive and switching networks of any size perform permutation switching under a simple switching rule. They also exhibit column-control and dilation, properties that are particularly relevant to guided-wave and free-space photonic technologies. A photonic switching network, with this dilated slipped banyan architecture, is proposed as the hub of an all-optical active-star local-area network. The switching assignment at this hub is time-multiplexed on a fixed schedule that is known to all the terminals. This all-optical local-area network provides the equivalent of full-connectivity with high simultaneous data rates between every pair of terminals. A 16-terminal local-area network with 100 Mb/s of contention-free bandwidth between every pair of terminals is described 相似文献
25.
26.
27.
The coagulation system plays a major role in the formation of the infected endocardial vegetation in bacterial endocarditis. Since monocytes can express tissue factor (TF) on their surfaces, they are thought to be responsible for the extrinsic activation of the coagulation cascade during this disease. The present study used an in vitro model in which fibrin plates, isolated adherent monocytes, and Streptococcus sanguis were used as an analog for endocardial vegetations. Adherence to fibrin by itself was found to stimulate TF expression on the monocytes, but stimulation by S. sanguis significantly increased TF expression, which was found to be maximal at a bacterium-to-monocyte ratio of 9 or more. 相似文献
28.
29.
RA Malik S Tesfaye SD Thompson A Veves AK Sharma AJ Boulton JD Ward 《Canadian Metallurgical Quarterly》1993,36(5):454-459
Twenty diabetic patients with neuropathy underwent clinical and neurophysiological evaluation together with a detailed morphometric assessment of capillary pathology in endoneurial and epineurial microvascular beds of the sural nerve. Morphological data were compared with ten non-diabetic control subjects. There were no significant differences in control subjects between basement membrane area, endothelial cell area, endothelial cell profile number or luminal area of endoneurial when compared with epineurial capillaries. In contrast, when compared with epineurial capillaries, endoneurial capillaries from diabetic patients demonstrated a significant increase in basement membrane (p < 0.001) and endothelial cell (p < 0.001) area and a significant reduction in luminal area (p < 0.001). There was no significant difference in endothelial cell profile number between endoneurial and epineurial capillaries amongst diabetic patients. Previous studies have demonstrated a good correlation between the degree of microangiopathy and measures of neuropathic severity. In the present study increased endoneurial capillary basement membrane area was significantly related to reduced peroneal nerve conduction velocity (p < 0.001), myelinated fibre density (p < 0.001) and elevated vibration (p < 0.05) and thermal (p < 0.001) perception. Increased endothelial cell area and reduced luminal size were related to a reduced peroneal nerve conduction (p < 0.05, p < 0.01, respectively), reduced myelinated fibre density (p < 0.05, p < 0.01) and elevated thermal perception (p < 0.05, p < 0.001). Epineurial capillary basement membrane, endothelial cell and luminal area failed to relate to measures of neuropathic severity.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
30.
We compared the data from four growth hormone (GH) immunoassays for analyzing 24-h GH profiles in four apparently normal subjects and four obese subjects (508 serum samples). The detection limit was 0.02 microgram/L for one immunochemiluminometric assay (ICMA), 0.1 microgram/L for two IRMAs, and 0.4 microgram/L for one RIA. All GH pulses with a peak ICMA value > 1 microgram/L were detected by each of the other methods. Overall, the correlation coefficient between the values obtained with all four assays exceeded 0.90. However, for GH concentrations < or = 0.25 microgram/L, acceptable concordance (r2 > or = 0.80) was reached only between the ICMA and one IRMA; between the ICMA and the RIA, concordance was acceptable only for GH concentrations > or = 10 micrograms/L. In the normal subjects, the percentage of undetectable values was 0% with the ICMA but 29% with one of the IRMAs; in obese subjects, the corresponding values were 12% and 38%. 相似文献