全文获取类型
收费全文 | 24019篇 |
免费 | 248篇 |
国内免费 | 444篇 |
专业分类
系统科学 | 677篇 |
丛书文集 | 548篇 |
教育与普及 | 89篇 |
理论与方法论 | 93篇 |
现状及发展 | 7160篇 |
研究方法 | 877篇 |
综合类 | 14738篇 |
自然研究 | 529篇 |
出版年
2021年 | 184篇 |
2017年 | 164篇 |
2016年 | 184篇 |
2015年 | 264篇 |
2014年 | 346篇 |
2013年 | 535篇 |
2012年 | 851篇 |
2011年 | 1409篇 |
2010年 | 722篇 |
2009年 | 718篇 |
2008年 | 988篇 |
2007年 | 1056篇 |
2006年 | 996篇 |
2005年 | 889篇 |
2004年 | 654篇 |
2003年 | 540篇 |
2002年 | 534篇 |
2001年 | 679篇 |
2000年 | 723篇 |
1999年 | 489篇 |
1992年 | 316篇 |
1991年 | 282篇 |
1990年 | 268篇 |
1989年 | 257篇 |
1988年 | 232篇 |
1987年 | 239篇 |
1986年 | 216篇 |
1985年 | 271篇 |
1984年 | 254篇 |
1983年 | 179篇 |
1982年 | 192篇 |
1981年 | 171篇 |
1980年 | 213篇 |
1979年 | 481篇 |
1978年 | 366篇 |
1977年 | 405篇 |
1976年 | 288篇 |
1975年 | 345篇 |
1974年 | 487篇 |
1973年 | 411篇 |
1972年 | 405篇 |
1971年 | 448篇 |
1970年 | 519篇 |
1969年 | 462篇 |
1968年 | 452篇 |
1967年 | 473篇 |
1966年 | 410篇 |
1965年 | 315篇 |
1958年 | 236篇 |
1957年 | 187篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
21.
22.
23.
提出了一种基于正六边形线阵CCD探测器的运动目标探测系统.系统不仅可以实现360°范围内环形视场探测,还可以实时显示运动目标的位置及角速度信息,达到搜索跟踪的目的.在硬件设计方面,提出并实现了基于CPLD和AVR的电路方案.在算法设计方面,设计了适于周视场运动目标探测算法. 相似文献
24.
运用临界点理论中的极小极大方法得到一类次二次Hamilton系统的次调和解的存在性定理. 相似文献
25.
Summary Acetone and alcohol extracts of the posterior salivary glands ofOctopus vulgaris, Eledone moschata andEledone Aldrovandi contain a principle which strongly reduces the diuresis in hydrated rats.The antidiuretic principle is absent in all otherOctopus tissues as well as in the salivary glands ofOctopus macropus.It shows a high termostability and is strongly adsorbed by animal charcoal at any reaction.The salivary antidiuretic substance is not identical with the posterior pituitary principle nor with the other active substances found in the salivary glands of Octopoda.The injection of salivary extracts satisfactorily reduces polyuria and polydipsia in human diabetes insipidus.
Le presenti ricerche sono state in parte sovvenzionate da un contributo straordinario del Consiglio Nazionale delle Ricerche. 相似文献
Le presenti ricerche sono state in parte sovvenzionate da un contributo straordinario del Consiglio Nazionale delle Ricerche. 相似文献
26.
27.
Astacin, a digestive zinc-endopeptidase from the crayfish Astacus astacus L., is the prototype for the 'astacin family', which includes mammalian metallo-endopeptidases and developmentally regulated proteins of man, fruitfly, frog and sea urchin. Here we report the X-ray crystal structure of astacin, which reveals a deep active-site cleft, with the zinc at its bottom ligated by three histidines, a water molecule and a more remote tyrosine. The third histidine (His 102) forms part of a consensus sequence, shared not only by the members of the astacin family, but also by otherwise sequentially unrelated proteinases, such as vertebrate collagenases. It may therefore represent the elusive 'third' zinc ligand in these enzymes. The amino terminus of astacin is buried forming an internal salt-bridge with Glu 103, adjacent to His 102. Astacin pro-forms extended at the N terminus, as observed for some 'latent' mammalian astacin homologues, did not exhibit this 'active' conformation, indicating an activation mechanism reminiscent of trypsin-like serine proteinases. 相似文献
28.
Low concentrations of inositol 1,4,5-trisphosphate (InsP3) evoke a very rapid mobilization of intracellular Ca2+ stores in many cell types, which can be followed by a further, much slower efflux. Two explanations have been suggested for this biphasic release. The first proposes that the Ca2+ stores vary in their sensitivity to InsP3, and each store releases either its entire contents or nothing (all-or-none release); the second proposes instead that the stores are uniformly sensitive to the effects of InsP3, but that they can release only a fraction of their Ca2+ before their sensitivity is somehow attenuated (steady-state release). Experiments using purified InsP3 receptor molecules reconstituted into lipid vesicles have shown heterogeneity of the receptors in their response to InsP3 under conditions in which the total Ca2+ level at both sides of the receptor is held constant. We now report that in permeabilized A7r5 smooth-muscle cells incubated in Ca(2+)-free medium, the amount of 45Ca2+ remaining in the stores after the rapid transient phase of release is independent of their initial Ca2+ levels, indicating that partially depleted stores are less sensitive to InsP3. Moreover, if the stores are reloaded with 40Ca2+ after the first stimulus, reapplication of the same low concentration of InsP3 will release further 45Ca2+. This recovery of InsP3 sensitivity is almost complete. Under these conditions, Ca2+ release must thus occur by a steady-state mechanism, in which the decreasing Ca2+ content of the stores slows down further release. 相似文献
29.
30.
研究了Al3+离子的固溶对C-S-H表面吸附Na+离子量的影响.实验结果表明,Al3+离子固溶于C-S-H结构中将使C-S-H表面所带的负电量增大,因而也就增大了C-S-H表面对Na+离子的吸附能力. 相似文献