全文获取类型
收费全文 | 18803篇 |
免费 | 384篇 |
国内免费 | 496篇 |
专业分类
系统科学 | 405篇 |
丛书文集 | 294篇 |
教育与普及 | 246篇 |
理论与方法论 | 67篇 |
现状及发展 | 4014篇 |
研究方法 | 555篇 |
综合类 | 13930篇 |
自然研究 | 172篇 |
出版年
2022年 | 200篇 |
2021年 | 188篇 |
2016年 | 177篇 |
2015年 | 281篇 |
2014年 | 356篇 |
2013年 | 376篇 |
2012年 | 506篇 |
2011年 | 702篇 |
2010年 | 421篇 |
2009年 | 462篇 |
2008年 | 642篇 |
2007年 | 677篇 |
2006年 | 558篇 |
2005年 | 556篇 |
2004年 | 475篇 |
2003年 | 501篇 |
2002年 | 576篇 |
2001年 | 799篇 |
2000年 | 731篇 |
1999年 | 666篇 |
1998年 | 354篇 |
1997年 | 350篇 |
1996年 | 327篇 |
1995年 | 241篇 |
1994年 | 482篇 |
1993年 | 243篇 |
1992年 | 411篇 |
1991年 | 352篇 |
1990年 | 368篇 |
1989年 | 310篇 |
1988年 | 279篇 |
1987年 | 242篇 |
1986年 | 235篇 |
1985年 | 281篇 |
1984年 | 206篇 |
1983年 | 163篇 |
1979年 | 328篇 |
1978年 | 263篇 |
1977年 | 215篇 |
1976年 | 191篇 |
1975年 | 195篇 |
1974年 | 207篇 |
1973年 | 177篇 |
1972年 | 207篇 |
1971年 | 248篇 |
1970年 | 303篇 |
1969年 | 229篇 |
1968年 | 226篇 |
1967年 | 195篇 |
1966年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 796 毫秒
991.
基于神经网络的多目标综合评价 总被引:11,自引:1,他引:10
本文提出了基于神经网络的多目标综合评价方法。这种方法通过利用神经网络对于以往成功的评价样本的学习, 使得神经网络反映评价指标的偏好, 从而代替评价人进行评价。文章研究了神经网络学习的BP算法, 并分析了某大型水利工程水位方案评价问题。 相似文献
992.
Five hybridomas secreting monoclonal antibody to E. coli L-asparaginase were isolated. These monoclonal antibodies were classified into 3 different subclasses; Ig G1 (1 clone), Ig G2 (2 clones) and Ig G3 (2 clones). One of them possessed anti-L-asparaginase neutralizing activity. Four antibodies examined demonstrated a linear Langmuir binding plot and binding affinities, with equilibrium dissociation constant (Kd) ranging between 2.5 X 10(-9) M and 6.3 X 10(-10) M. The monoclonal antibodies should be useful probes for investigation of the enzyme activity. 相似文献
993.
S. Benvenuti A. Gagliardo T. Guilford P. Luschi 《Cellular and molecular life sciences : CMLS》1996,52(6):608-612
In order to investigate the pigeon's compass mechanism, a series of overcast tests with clock-shifted birds were run at two familiar release sites. While controls were able to assume a correct homeward direction, the experimental birds' initial orientation cannot be explained either on the basis of a time-compensated sun compass or of a time-independent magnetic compass. Speculative explanations of our paradoxical results are attempted. 相似文献
994.
W T Clusin 《Nature》1983,301(5897):248-250
Electrical excitation of cardiac muscle may sometimes be due to initiation of inward current by the presence of Ca2+ ions at the inner surface of the cell membrane. During digitalis toxicity and other conditions that abnormally augment cellular Ca2+ stores, premature release of Ca2+ from the sarcoplasmic reticulum leads to a transient inward current, which is large enough to initiate premature beats and is accompanied by a transient contractile response. This inward current may be mediated either by electrogenic sodium-calcium exchange or by specific Ca2+-activated cation channels that have recently been characterized in tissue cultures of cardiac myocytes. An obvious question raised by these observations is whether release of the sequestered Ca2+ stores during each normal beat exerts a similar influence on membrane potential. To explore this, chick embryonic myocardial cell aggregates were voltage-clamped during abrupt exposure to caffeine, which is known to release Ca2+ from the sarcoplasmic reticulum. The speed of the perfusion system and the relative absence of diffusion barriers in the tissue-cultured cells allowed the effects of caffeine-induced Ca2+ release to be studied on a time scale comparable to that of a single normal beat. We report here that abrupt exposure of the cells to caffeine produced a transient inward current having similar features to that of digitalis toxicity, and which was both large enough and rapid enough to potentially contribute to the action potential. 相似文献
995.
Structure of an adenine-cytosine base pair in DNA and its implications for mismatch repair 总被引:30,自引:0,他引:30
Mutational pathways rely on introducing changes in the DNA double helix. This may be achieved by the incorporation of a noncomplementary base on replication or during genetic recombination, leading to substitution mutation. In vivo studies have shown that most combinations of base-pair mismatches can be accommodated in the DNA double helix, albeit with varying efficiencies. Fidelity of replication requires the recognition and excision of mismatched bases by proofreading enzymes and post-replicative mismatch repair systems. Rates of excision vary with the type of mismatch and there is some evidence that these are influenced by the nature of the neighbouring sequences. However, there is little experimental information about the molecular structure of mismatches and their effect on the DNA double helix. We have recently determined the crystal structures of several DNA fragments with guanine X thymine and adenine X guanine mismatches in a full turn of a B-DNA helix and now report the nature of the base pairing between adenine and cytosine in an isomorphous fragment. The base pair found in the present study is novel and we believe has not previously been demonstrated. Our results suggest that the enzymatic recognition of mismatches is likely to occur at the level of the base pairs and that the efficiency of repair can be correlated with structural features. 相似文献
996.
997.
Structure of pre-pro-von Willebrand factor and its expression in heterologous cells 总被引:30,自引:0,他引:30
D T Bonthron R I Handin R J Kaufman L C Wasley E C Orr L M Mitsock B Ewenstein J Loscalzo D Ginsburg S H Orkin 《Nature》1986,324(6094):270-273
Von Willebrand factor (vWF), a multifunctional haemostatic glycoprotein derived from endothelial cells and megakaryocytes, mediates platelet adhesion to injured subendothelium and binds coagulation factor VIII in the circulation. Native vWF is a disulphide-bonded homopolymer; the monomeric subunits, of apparent relative molecular mass (Mr) 220,000 (220K) are derived from an intracellular precursor estimated at 260-275K. Multimer assembly is preceded by the formation of dimers, linked near their C-termini, which then assemble into filamentous polymers. The importance of the removal of the large vWF pro-polypeptide during multimer assembly, and whether this or other stages of the complex post-translational processing require components specific to endothelial cells or megakaryocytes, is unknown. Here we report an analysis of the complete sequence of pre-pro-vWF and expression of the molecule in heterologous cells. The vWF precursor is composed of several repeated subdomains. When expressed in COS and CHO cells, it is cleaved and assembled into biologically active high relative molecular mass disulphide bonded multimers. This suggests that the information for assembly of this complex molecule resides largely within its primary structure. 相似文献
998.
999.
MAPCATI改性环氧丙烯酸双酯的合成及其紫外光固化涂料 总被引:1,自引:0,他引:1
采用自制个甲基丙烯酰氧β-丙氧甲酰氨基甲苯2-异氰酸酯(简称MAPCATI)中的异氰酸基与自制环氧丙烯酸双酯中羟基上的氢反应,制得了改性树脂;并以此为基料配成紫外光固化涂料。所得涂层性能优良,有应用前景。 相似文献
1000.
吴旗苦荞麦粉营养成分分析 总被引:3,自引:0,他引:3
本文分析了吴旗苦荞麦粉中蛋白质、脂肪、淀粉、矿物持、维生素等主要营养成分,为进一步开发利用陕化的苦荞麦资源提供了依据。 相似文献