全文获取类型
收费全文 | 6865篇 |
免费 | 118篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 52篇 |
综合类 | 2篇 |
化学工业 | 850篇 |
金属工艺 | 45篇 |
机械仪表 | 73篇 |
建筑科学 | 104篇 |
矿业工程 | 65篇 |
能源动力 | 86篇 |
轻工业 | 383篇 |
水利工程 | 35篇 |
石油天然气 | 6篇 |
无线电 | 459篇 |
一般工业技术 | 691篇 |
冶金工业 | 3737篇 |
原子能技术 | 45篇 |
自动化技术 | 354篇 |
出版年
2021年 | 27篇 |
2020年 | 31篇 |
2019年 | 46篇 |
2018年 | 51篇 |
2017年 | 58篇 |
2016年 | 55篇 |
2015年 | 48篇 |
2014年 | 72篇 |
2013年 | 182篇 |
2012年 | 97篇 |
2011年 | 144篇 |
2010年 | 102篇 |
2009年 | 100篇 |
2008年 | 145篇 |
2007年 | 127篇 |
2006年 | 103篇 |
2005年 | 115篇 |
2004年 | 99篇 |
2003年 | 88篇 |
2002年 | 94篇 |
2001年 | 72篇 |
2000年 | 98篇 |
1999年 | 159篇 |
1998年 | 1236篇 |
1997年 | 673篇 |
1996年 | 452篇 |
1995年 | 276篇 |
1994年 | 229篇 |
1993年 | 246篇 |
1992年 | 88篇 |
1991年 | 78篇 |
1990年 | 95篇 |
1989年 | 101篇 |
1988年 | 72篇 |
1987年 | 102篇 |
1986年 | 69篇 |
1985年 | 86篇 |
1984年 | 64篇 |
1983年 | 60篇 |
1982年 | 73篇 |
1981年 | 54篇 |
1980年 | 66篇 |
1979年 | 41篇 |
1978年 | 55篇 |
1977年 | 115篇 |
1976年 | 251篇 |
1975年 | 37篇 |
1974年 | 37篇 |
1972年 | 34篇 |
1971年 | 27篇 |
排序方式: 共有6987条查询结果,搜索用时 15 毫秒
21.
Sensitivity to toxicants is a major criterion for selecting organisms for bioassay testing. If a sensitive species is also abundant and occupies a role as prey for many other species within a community, then the species become a valuable tool in environmental monitoring. These features apply to larval midge Chironomus petiolatus in freshwater environments of central Chile. The youngest larval instar is the most sensitive and presents the additional feature of lower survival within control arenas, making it more difficult to discern toxicant-related mortality from background mortality. In this work, we perform acute bioassays with the three larval stages of C. petiolatus and K2Cr2O7 as reference toxicant, with the goal of selecting a particular instar as the best bioassay tool using two criteria: sensitivity and background mortality. Sensitivity is evaluated through Monte Carlo estimation of LC50 and background mortality through bootstrap resampling, and a final Bioassay Performance Index as the product of LC50 and background mortality. For this task we developed a new computationally intensive statistical algorithm. Results show that the best bioassay tool is not the youngest and most sensitive instar but an intermediate one. 相似文献
22.
JF Levine CS Apperson P Howard M Washburn AL Braswell 《Canadian Metallurgical Quarterly》1997,34(6):594-598
Previously archived museum specimens of lizards collected throughout North Carolina were examined for Ixodes scapularis (Say). Lizards (n = 1,349) collected in 80 of North Carolina's 100 counties were examined. Lizards with ticks were collected in 23 (29%) of the 80 counties from which lizards were examined. I. scapularis was detected on 8.7% (n = 117) of the lizards and was the sole species of tick obtained from lizards. Immature ticks were most frequently found on the southeastern five-lined skink, Eumeces inexpectatus, and the eastern glass lizard, Ophisaurus ventralis. Larvae were most frequently found on the six-lined racerunner, Cnemidophorus sexlineatus. One C. sexlineatus harbored 177 larvae and 2 nymphs. Nymphs were most frequently observed on E. inexpectatus. The majority of counties (chi 2, P < 0.01) where ticks were found on lizards were in the Coastal Plain. 相似文献
23.
AL Basso E Ricca C Caruso L Ferrara M De Felice 《Canadian Metallurgical Quarterly》1993,144(7):539-545
Acetohydroxy acid synthase (AHAS) and threonine deaminase (TD) activities were found in Streptococcus bovis and shown to be involved in the biosynthesis of the branched chain amino acids isoleucine, leucine and valine. Apparent lack of repression of AHAS synthesis by the end-products and reduced sensitivity of S. bovis growth to analogues of the branched chain amino acids suggested that secretion of isoleucine, leucine and valine in the growth medium may be a consequence of the regulatory features of AHAS. A glycyl-leucine-resistant mutant with reduced TD activity secreted a reduced amount of isoleucine and an increased amount of valine, which might be a result of the reduced rate of synthesis of the isoleucine precursor alpha-ketobutyrate and of a consequent preferential carbon flow through the valine branch of the pathway. 相似文献
24.
25.
26.
27.
The present study was designed to determine if spinal calcium channels, calmodulin, and calcium/calmodulin-dependent protein kinase II were involved in the production of antinociception induced by cold water swimming stress (CWSS). The effects of intrathecal (i.t.) injection of nimodipine, omega-conotoxin GVIA, calmidazolium, or (S)-5-isoquinolinesulfonic acid, 4-[2-[(5-isoquinolinyl-sulfonyl)methylamino]-3-oxo-3-(4-phenyl-1-piperaz inyl)-propyl]phenyl ester (KN-62) on CWSS-induced antinociception were studied in ICR mice. The antinociception was assessed by the tail-flick test. CWSS produced inhibition of the tail-flick response. Various doses of nimodipine (10-40 ng), omega-conotoxin GVIA (5-40 ng), calmidazolium (10-40 ng), or KN-62 (5-40 ng) injected i.t. alone did not show any antinociceptive effect in the tail-flick test. I.t. pretreatment with omega-conotoxin GVIA, calmidazolium, or KN-62 dose dependently attenuated the CWSS-induced inhibition of the tail-flick response. However, i.t. pretreatment with nimodipine did not affect the inhibition of the tail-flick response induced by CWSS. Our results suggest that spinal N-type calcium channel, calmodulin and calcium/calmodulin-dependent protein kinase II may be involved in the production of antinociception induced by CWSS. On the other hand, CWSS-induced antinociception appears not to be mediated via the spinal L-type calcium channel. 相似文献
28.
29.
30.
AL Lindstrom SL Erlandsen JH Kersey CA Pennell 《Canadian Metallurgical Quarterly》1997,90(6):2323-2334
Vascular leak syndrome (VLS) is the dose-limiting toxicity observed in clinical trials of immunotoxins containing ricin toxin A chain (RTA). RTA itself is thought to cause VLS by damaging vascular endothelial cells, but the exact mechanism remains unclear. This is partially due to the paucity of appropriate models. To study VLS, we developed an in vitro model in which human umbilical vein-derived endothelial cells were first grown to confluence on microporous supports and then cultured under low pressure in the presence or absence of RTA. Endothelial cell barrier function was assessed by measuring the volume of fluid that passed through each monolayer per unit time. We found that RTA significantly increased monolayer permeability at times and concentrations consistent with the onset of VLS in patients treated with RTA-based immunotoxins. Scanning electron microscopy showed that intercellular gaps formed in endothelial monolayers exposed to RTA. Intercellular gap formation followed endothelial cell death caused by the enzymatic activity of RTA. We conclude that RTA is directly toxic to endothelial cells in vitro and speculate that this contributes to VLS in vivo. 相似文献