全文获取类型
收费全文 | 38206篇 |
免费 | 66篇 |
国内免费 | 100篇 |
专业分类
系统科学 | 175篇 |
丛书文集 | 757篇 |
教育与普及 | 102篇 |
理论与方法论 | 222篇 |
现状及发展 | 16559篇 |
研究方法 | 1581篇 |
综合类 | 18462篇 |
自然研究 | 514篇 |
出版年
2013年 | 200篇 |
2012年 | 502篇 |
2011年 | 1054篇 |
2010年 | 211篇 |
2008年 | 649篇 |
2007年 | 672篇 |
2006年 | 700篇 |
2005年 | 718篇 |
2004年 | 656篇 |
2003年 | 685篇 |
2002年 | 693篇 |
2001年 | 1195篇 |
2000年 | 1114篇 |
1999年 | 757篇 |
1994年 | 340篇 |
1992年 | 720篇 |
1991年 | 574篇 |
1990年 | 625篇 |
1989年 | 601篇 |
1988年 | 586篇 |
1987年 | 622篇 |
1986年 | 631篇 |
1985年 | 777篇 |
1984年 | 621篇 |
1983年 | 512篇 |
1982年 | 445篇 |
1981年 | 451篇 |
1980年 | 555篇 |
1979年 | 1226篇 |
1978年 | 1044篇 |
1977年 | 993篇 |
1976年 | 721篇 |
1975年 | 773篇 |
1974年 | 1072篇 |
1973年 | 916篇 |
1972年 | 969篇 |
1971年 | 1196篇 |
1970年 | 1542篇 |
1969年 | 1171篇 |
1968年 | 1116篇 |
1967年 | 1141篇 |
1966年 | 986篇 |
1965年 | 724篇 |
1959年 | 417篇 |
1958年 | 621篇 |
1957年 | 500篇 |
1956年 | 412篇 |
1955年 | 355篇 |
1954年 | 403篇 |
1948年 | 236篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
971.
Localization of VP4 neutralization sites in rotavirus by three-dimensional cryo-electron microscopy 总被引:35,自引:0,他引:35
Three-dimensional structures of several spherical viruses have been determined by electron microscopy and X-ray crystallography. We report here the first three-dimensional structure of the complex between an intact virus and Fab fragments of a neutralizing monoclonal antibody. The antibody is against VP4, one of the two outer capsid proteins of rotaviruses. These large icosahedral viruses cause gastroenteritis in children and young animals and account for over a million human deaths annually. VP4 in these viruses has been implicated in several important functions such as cell penetration, haemagglutination, neutralization and virulence. Here we demonstrate that the surface spikes on rotavirus particles are made up of VP4. Antigenic sites are located near the distal ends of the spikes and two Fab fragments bind to each of the sixty spikes. The mass of the spike indicates that it is a dimer of VP4. The bilobed structure at the distal end of the spike may be involved in both the attachment to the cell and in viral penetration. A novel feature in the virus-Fab complex is the structural difference between the two chemically equivalent Fab fragments on each spike, which could be indicative of variations in the Fab elbow angles. 相似文献
972.
Cloning of a cDNA encoding a non-isopeptide-selective subtype of the endothelin receptor 总被引:39,自引:0,他引:39
Endothelin-1 was initially identified as a 21-residue potent vasoconstrictor peptide produced by vascular endothelial cells, but was subsequently found to have many effects on both vascular and non-vascular tissues. The discovery of three isopeptides of the endothelin family, ET-1, ET-2 and ET-3, each possessing a diverse set of pharmacological activities of different potency, suggested the existence of several different endothelin receptor subtypes. Endothelins may elicit biological responses by various signal-transduction mechanisms, including the G protein-coupled activation of phospholipase C and the activation of voltage-dependent Ca2+ channels. Thus, different subtypes of the endothelin receptor may use different signal-transduction mechanisms. Here we report the cloning of a complementary DNA encoding one subtype belonging to the superfamily of G protein-coupled receptors. COS-7 cells transfected with the cDNA express specific and high-affinity binding sites for endothelins, responding to binding by the production of inositol phosphates and a transient increase in the concentration of intracellular free Ca2+. The three endothelin isopeptides are roughly equipotent in displacing 125I-labelled ET-1 binding and causing Ca2+ mobilization. A messenger RNA corresponding to the cDNA is detected in many rat tissues including the brain, kidney and lung but not in vascular smooth muscle cells. These results indicate that this cDNA encodes a 'nonselective' subtype of the receptor which is different from the vascular smooth muscle receptor. 相似文献
973.
Molecular cloning of the microtubule-associated mechanochemical enzyme dynamin reveals homology with a new family of GTP-binding proteins 总被引:59,自引:0,他引:59
A complementary DNA encoding the D100 polypeptide of rat brain dynamin--a force-producing, microtubule-activated nucleotide triphosphatase--has been cloned and sequenced. The predicted amino acid sequence includes a guanine nucleotide-binding domain that is homologous with those of a family of antiviral factors, inducible by interferon and known as Mx proteins, and with the product of the essential yeast vacuolar protein sorting gene VPS1. These relationships imply the existence of a new family of GTPases with physiological roles that may include microtubule-based motility and protein sorting. 相似文献
974.
鲁建慧 《黑龙江科技学院学报》1994,(2)
通过实例说明用“异心辅助球面法”求相贯线,不仅适用于回转面相交,对其中一个是非回转面,但只要曲面的截交线为圆时也可使用。既适用于两轴线相交,也适用于两曲面轴线平行的情况。 相似文献
975.
探讨了溅射用铁电陶瓷靶(PZT,PLZT)的烧结工艺,并对其显微结构进行了分析。结果表明,采用新的烧结工艺(含保护措施),可以有效地抑制PbO的挥发,制备出组织结构及成分均匀、PbO含量正常、致密度较高、不变形的符合磁控溅射要求的铁电陶瓷烧结靶。 相似文献
976.
基于对时间序列实质的分析,提出了旨在减少序列的随机误差影响以及提高拟合精度的AR模型参数的积分求解法.重点讨论了AR(1)模型及AR(p)模型参数的积分求解法,并与最小二乘法在计算机上进行了仿真比较.结果表明,采用积分求解法所得的AR模型参数的估计精度比最小二乘法的高. 相似文献
977.
强反射层追踪与相对时差拾取中的“多值”现象严重影响剩余静校正的效果,为此提出的小波包预处理方法,可以实现地震道噪声与信号及不同强反射层的分离,从而能自动识别并提取“纯”的强反射层,提高强反射层追踪的可靠性和相对时差的拾取精度. 相似文献
978.
对几种逆变电源的抗冲击性负荷电路进行分析,给出了一种便于实现且效果很好的电路,实验证明此电路对各种冲击负荷甚至输出短路都能进行很好的限流,可以彻底解决逆变电源在负载冲击下频繁停机的问题。 相似文献
979.
利用Gurson本构理论,导出了损伤材料在平面应力条件下准静态定常扩展I型裂纹近裂纹线应力场和裂纹前方塑性区尺寸的解析表达式。研究结果表明:随着材料损伤量的增加,裂纹尖端处应力降低;裂纹尖端处应力不存在奇异性;裂纹前方塑性区尺寸受材料损伤程度的影响。 相似文献
980.
将煤粉空气两相流在着火前和着火期间基本上仍遵从的射流发展规律作为模型基础,考虑了煤粉颗粒与气相间的速度滑移与温度滑移。气相场按射流规律发展井计及颗粒群对气相温度、速度、浓度变化的影响;颗粒群则按Lagrange处理方法追踪颗粒变化史。模型首次采用了挥发分与环境氧气匹配达着火限方能产生挥发分火焰的概念并取得了与实验现象一致的计算结果。 相似文献