全文获取类型
收费全文 | 19979篇 |
免费 | 133篇 |
国内免费 | 200篇 |
专业分类
系统科学 | 140篇 |
丛书文集 | 307篇 |
教育与普及 | 178篇 |
理论与方法论 | 56篇 |
现状及发展 | 7650篇 |
研究方法 | 669篇 |
综合类 | 11154篇 |
自然研究 | 158篇 |
出版年
2012年 | 313篇 |
2011年 | 444篇 |
2010年 | 215篇 |
2009年 | 245篇 |
2008年 | 403篇 |
2007年 | 423篇 |
2006年 | 386篇 |
2005年 | 388篇 |
2004年 | 411篇 |
2003年 | 395篇 |
2002年 | 383篇 |
2001年 | 585篇 |
2000年 | 574篇 |
1999年 | 490篇 |
1998年 | 186篇 |
1996年 | 188篇 |
1994年 | 406篇 |
1992年 | 346篇 |
1991年 | 285篇 |
1990年 | 323篇 |
1989年 | 292篇 |
1988年 | 294篇 |
1987年 | 304篇 |
1986年 | 306篇 |
1985年 | 349篇 |
1984年 | 235篇 |
1983年 | 257篇 |
1982年 | 212篇 |
1981年 | 204篇 |
1980年 | 245篇 |
1979年 | 508篇 |
1978年 | 434篇 |
1977年 | 390篇 |
1976年 | 314篇 |
1975年 | 386篇 |
1974年 | 474篇 |
1973年 | 422篇 |
1972年 | 431篇 |
1971年 | 562篇 |
1970年 | 608篇 |
1969年 | 531篇 |
1968年 | 532篇 |
1967年 | 459篇 |
1966年 | 408篇 |
1965年 | 294篇 |
1958年 | 322篇 |
1957年 | 253篇 |
1956年 | 202篇 |
1955年 | 200篇 |
1954年 | 193篇 |
排序方式: 共有10000条查询结果,搜索用时 972 毫秒
1.
针对目前高速公路中央分隔带开口处活动护栏防撞等级较低的问题,提出了一种SAm级混凝土活动护栏及一种新型连接装置。基于有限元的方法,建立车辆与护栏的碰撞模型,通过LS-DYNA对车辆与护栏碰撞过程进行求解,根据不同车型的碰撞结果分别在缓冲,阻挡和导向性能上对护栏的安全性能进行评价,其中小型车辆与护栏碰撞后的OIV(Occupant Impact Velocity)值<12 m/s,ORA(Occupant Ridedown Acceleration)值<20 g且车辆没有驶出导向框;大型车辆与护栏碰撞后没有发生穿越、骑跨等现象且车辆没有驶出导向框,均满足护栏安全性能评价标准。 相似文献
2.
3.
4.
过程变量在代表产品或服务过程信息时并非完美,而使用模糊数可能是另一较好途径。文章进一步完善模糊累积和控制图,其中使用中心和扩展具有重抽样分布的模糊随机变量,并给出模拟例证。 相似文献
5.
6.
Mark H. Bickhard 《Foundations of Science》2003,8(3):283-293
If the general arguments concerning theinvolvement of variation and selection inexplanations of ``fit'' are valid, then variationand selection explanations should beappropriate, or at least potentiallyappropriate, outside the paradigm historisticdomains of biology and knowledge. In thisdiscussion, I wish to indicate some potentialroles for variation and selection infoundational physics – specifically inquantum field theory. I will not be attemptingany full coherent ontology for quantum fieldtheory – none currently exists, and none islikely for at least the short term future. Instead, I wish to engage in some partiallyspeculative interpretations of some interestingresults in this area with the aim ofdemonstrating that variation and selectionnotions might play a role even here. Ifvariation and selection can survive in even asinhospitable and non-paradigmatic a terrain asfoundational physics, then it can surviveanywhere. 相似文献
7.
Ten-day treatment with nicotine (5, 25 or 50 micrograms/ml drinking water) dose-dependently intensified gastric ulceration induced by cold-restraint, and emptying rate. Stomach contractions produced by graded doses of bethanechol i.v. were elevated further by nicotine treatment. It is suggested that chronic nicotine administration produces hypersensitivity of the gastric muscarinic receptors; stomach hypermotility contributes to the ulcer-worsening action of the alkaloid. 相似文献
8.
Romeo S Pennacchio LA Fu Y Boerwinkle E Tybjaerg-Hansen A Hobbs HH Cohen JC 《Nature genetics》2007,39(4):513-516
Resequencing genes provides the opportunity to assess the full spectrum of variants that influence complex traits. Here we report the first application of resequencing to a large population (n = 3,551) to examine the role of the adipokine ANGPTL4 in lipid metabolism. Nonsynonymous variants in ANGPTL4 were more prevalent in individuals with triglyceride levels in the lowest quartile than in individuals with levels in the highest quartile (P = 0.016). One variant (E40K), present in approximately 3% of European Americans, was associated with significantly lower plasma levels of triglyceride and higher levels of high-density lipoprotein cholesterol in European Americans from the Atherosclerosis Risk in Communities Study and in Danes from the Copenhagen City Heart Study. The ratio of nonsynonymous to synonymous variants was higher in European Americans than in African Americans (4:1 versus 1.3:1), suggesting population-specific relaxation of purifying selection. Thus, resequencing of ANGPTL4 in a multiethnic population allowed analysis of the phenotypic effects of both rare and common variants while taking advantage of genetic variation arising from ethnic differences in population history. 相似文献
9.
Hellgren M Strömberg P Gallego O Martras S Farrés J Persson B Parés X Höög JO 《Cellular and molecular life sciences : CMLS》2007,64(4):498-505
The metabolism of all-trans- and 9-cis-retinol/ retinaldehyde has been investigated with focus on the activities of human, mouse and rat alcohol dehydrogenase 2
(ADH2), an intriguing enzyme with apparently different functions in human and rodents. Kinetic constants were determined with
an HPLC method and a structural approach was implemented by in silico substrate dockings. For human ADH2, the determined Km values ranged from 0.05 to 0.3 μM and kcat values from 2.3 to 17.6 min−1, while the catalytic efficiency for 9-cis-retinol showed the highest value for any substrate. In contrast, poor activities
were detected for the rodent enzymes. A mouse ADH2 mutant (ADH2Pro47His) was studied that resembles the human ADH2 setup.
This mutation increased the retinoid activity up to 100-fold. The Km values of human ADH2 are the lowest among all known human retinol dehydrogenases, which clearly support a role in hepatic
retinol oxidation at physiological concentrations.
Received 12 October 2006; received after revision 6 December 2006; accepted 8 January 2007 相似文献
10.