全文获取类型
收费全文 | 31319篇 |
免费 | 519篇 |
国内免费 | 665篇 |
专业分类
系统科学 | 621篇 |
丛书文集 | 595篇 |
教育与普及 | 328篇 |
理论与方法论 | 166篇 |
现状及发展 | 8975篇 |
研究方法 | 783篇 |
综合类 | 20854篇 |
自然研究 | 181篇 |
出版年
2015年 | 378篇 |
2014年 | 449篇 |
2013年 | 508篇 |
2012年 | 733篇 |
2011年 | 993篇 |
2010年 | 491篇 |
2009年 | 568篇 |
2008年 | 774篇 |
2007年 | 790篇 |
2006年 | 735篇 |
2005年 | 873篇 |
2004年 | 1181篇 |
2003年 | 1023篇 |
2002年 | 707篇 |
2001年 | 915篇 |
2000年 | 927篇 |
1999年 | 1007篇 |
1998年 | 571篇 |
1997年 | 561篇 |
1996年 | 618篇 |
1995年 | 531篇 |
1994年 | 539篇 |
1993年 | 365篇 |
1992年 | 622篇 |
1991年 | 541篇 |
1990年 | 488篇 |
1989年 | 489篇 |
1988年 | 430篇 |
1987年 | 373篇 |
1986年 | 362篇 |
1985年 | 377篇 |
1984年 | 264篇 |
1983年 | 242篇 |
1979年 | 614篇 |
1978年 | 503篇 |
1977年 | 464篇 |
1976年 | 364篇 |
1975年 | 435篇 |
1974年 | 610篇 |
1973年 | 492篇 |
1972年 | 516篇 |
1971年 | 607篇 |
1970年 | 749篇 |
1969年 | 603篇 |
1968年 | 586篇 |
1967年 | 568篇 |
1966年 | 481篇 |
1965年 | 406篇 |
1958年 | 332篇 |
1957年 | 251篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
论述了以8031为控制核心的钢丝绳张力单片机检测系统的工作原理、数学模型和系统组成,重点介绍了钢丝绳张力检测的工作原理、主要硬件电路的设计特点及软件设计思想,提供了部分硬件电路图及控制软件的流程图。该系统具有置数、动态显示、数据处理、打印及人机对话等功能,并且使用方便,造价低廉。 相似文献
32.
33.
本文论述了描述微波与生物组织相互作用的数学表达式,讨论工作频率选择、透入深度、反射、极化、泄漏及防护等实际问题。 相似文献
34.
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
35.
M. Lazzari V. Franceschini G. Minelli F. Ciani 《Cellular and molecular life sciences : CMLS》1993,49(4):277-281
The corrosion cast technique provided for the first time an excellent three-dimensional visualization of the vascular pattern of the choroid and iris in the newt eye. The results show the presence of a single arterial afference to the choroidal and iris capillaries: the ophthalmic artery is the origin fo both ciliary arteries and the long posterior ciliary artery. Slightly behind the equatorial circumference of the eyeball the venous drainage consists of a single vessel on the dorsal side and two distinct vessels on the ventral one. It receives blood from both iris and choroid. The surface of the plastic endocasts shows some details of fine luminal structures of the endothelial cells. Shallow depressions may be regarded as imprints of endothelial cell nuclei, and they are distinctly different for arteries and capillaries. The angioarchitecture of the newt eye differs from that of brain in that hairpin-shaped capillary loops are not observed at all. 相似文献
36.
中药材中孳生粉螨的初步调查 总被引:1,自引:0,他引:1
本文采用清水漂浮法和塔氏电热集螨器分离法,共分离中药材样本124种,1240份,从中分离出粉螨45种,隶属7科23属.得出结论:中药材粉螨的污染严重,应加强对中药螨类的防治,以保护中药材及预防人体螨病. 相似文献
37.
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
38.
39.
从嗜热蓝藻鞭枝藻(Mastigocladuslaminosus)中分离出藻胆体,并对其光谱特性进行了研究.完整藻胆体的室温荧光峰位于673um,77K荧光发射光谱中只有一个峰,位于685um,解离的藻胆体的77K荧光峰位于683nm,并在666nm和650nm处形成小的荧光发射峰.严重解离的藻胆体室温荧光峰位于645um,77K荧光峰位于644um,683um荧光显著下降,成为小峰.完整藻胆体与解离的及严重解离的藻胆体的吸收光谱没有显著差别,吸收峰位于622um,在580um处有一吸收肩. 相似文献
40.