全文获取类型
收费全文 | 805篇 |
免费 | 5篇 |
国内免费 | 17篇 |
专业分类
系统科学 | 20篇 |
丛书文集 | 12篇 |
教育与普及 | 50篇 |
理论与方法论 | 3篇 |
现状及发展 | 190篇 |
研究方法 | 72篇 |
综合类 | 479篇 |
自然研究 | 1篇 |
出版年
2021年 | 3篇 |
2018年 | 25篇 |
2017年 | 20篇 |
2015年 | 9篇 |
2014年 | 6篇 |
2013年 | 3篇 |
2012年 | 16篇 |
2011年 | 30篇 |
2010年 | 16篇 |
2009年 | 56篇 |
2008年 | 38篇 |
2007年 | 34篇 |
2006年 | 32篇 |
2005年 | 29篇 |
2004年 | 88篇 |
2003年 | 29篇 |
2002年 | 16篇 |
2001年 | 78篇 |
2000年 | 70篇 |
1999年 | 52篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 27篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1984年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 6篇 |
1970年 | 8篇 |
1969年 | 8篇 |
1968年 | 3篇 |
1967年 | 4篇 |
1966年 | 2篇 |
1958年 | 2篇 |
1956年 | 2篇 |
排序方式: 共有827条查询结果,搜索用时 218 毫秒
311.
312.
A. Tchernitchin X. Tchernitchin P. Galand 《Cellular and molecular life sciences : CMLS》1975,31(8):993-994
Summary Estradiol-17 β is a stronger estrogen than estriol for the genomic response of estrogens. Estriol is a stronger estrogen than
estradiol-17 β for the estrogeninduced uterine eosinophilia and the 6h increase in the uterine wet weight13. 相似文献
313.
Z Ma S W Morris V Valentine M Li J A Herbrick X Cui D Bouman Y Li P K Mehta D Nizetic Y Kaneko G C Chan L C Chan J Squire S W Scherer J K Hitzler 《Nature genetics》2001,28(3):220-221
t(1;22) is the principal translocation of acute megakaryoblastic leukemias. Here we show this chromosomal rearrangement to result in the fusion of two novel genes, RNA-binding motif protein-15 (RBM15), an RNA recognition motif-encoding gene with homology to Drosophila spen, and Megakaryoblastic Leukemia-1 (MKL1), a gene encoding an SAP (SAF-A/B, Acinus and PIAS) DNA-binding domain. 相似文献
314.
以魏岗乡为例,研究淮河上游沿河平原农村居民点的空间分布模式以及农村居民点空间分布的相关因素.结果表明,该区农村居民点总体呈现一种随机分布的模式,局部呈现聚集、团状、带状分布格局;居民点在各个坡向均有分布,以东南方向最多,正东方向、西北方向、东北方向稍多.农村居民点受道路、海拔、坡度影响比较明显,受坡向影响相对较小.在局部区域内受道路交通、地形制约比较明显;随着海拔及距离的增加,居民点面积呈现先增加后减少的趋势,不同区域范围内居民点面积差别很大;随着坡度的增加,居民点面积呈现出不断减少的趋势. 相似文献
315.
A ductile and super strong nanostructured twinning-induced plasticity(TWIP) steels were fabricated by cold rolling and recovery treatment.This strong and ductile nanostructured alloy can be used for the anti-intrusion part of body-in-white. Cold rolling was used to produce intensive nano-twins so that the microstructure was greatly refined. Recovery is employed to anneal dislocations for improving the ductility. A physical model is proposed to describe the relationship between the yield stress and the twin density. Furthermore,the present work also found that the activation energy for recovery is 160 J/mol,which implies that the recovery mechanism is governed by dislocation core diffusion. 相似文献
316.
Valtonen MJ Lehto HJ Nilsson K Heidt J Takalo LO Sillanpää A Villforth C Kidger M Poyner G Pursimo T Zola S Wu JH Zhou X Sadakane K Drozdz M Koziel D Marchev D Ogloza W Porowski C Siwak M Stachowski G Winiarski M Hentunen VP Nissinen M Liakos A Dogru S 《Nature》2008,452(7189):851-853
Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory. Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later. 相似文献
317.
Since the discovery of superconductivity in the high-transition-temperature (high-T(c)) copper oxides two decades ago, it has been firmly established that the CuO(2) plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO(1-x)F(x) has recently been discovered and the conspicuous lack of the CuO(2) planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO(0.85)F(0.15) with T(c) = 42 K as measured by Andreev spectroscopy. The gap value of 2Delta = 13.34 +/- 0.3 meV gives 2Delta/k(B)T(c) = 3.68 (where k(B) is the Boltzmann constant), close to the Bardeen-Cooper-Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at T(c) in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-T(c) copper oxides. 相似文献
318.
Tracing the stepwise oxygenation of the Proterozoic ocean 总被引:9,自引:0,他引:9
Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column. 相似文献
319.
320.
In this paper, we propose an incremental statistical analysis method with complexity reduction as a pre-process for on-chip power/ground (P/G) networks. The new method exploits locality of P/G network analyses and aims at P/G networks with a large number of strongly connected subcircuits(called strong connects) such as trees and chains. The method consists of three steps. First it compresses P/G circuits by removing strong connects. As a result, current variations (CV) of nodes in strong connects are transferred to some remaining nodes. Then based on the locality of power grid voltage responses to its current inputs, it efficiently calculates the correlative resistor (CR) matrix in a local way to directly compute the voltage variations by using small parts of the remaining circuit. Last it statistically recovers voltage variations of the suppressed nodes inside strong connects. This new method for statistically compressing and expanding strong connects in terms of current or voltage variations in a closed form is very efficient owning to its property of incremental analysis. Experimental results demonstrate that the method can efficiently compute low-bounds of voltage variations for P/G networks and it has two or three orders of magnitudes speedup over the traditional Monte-Carlo-based simulation method, with only 2.0% accuracy loss. 相似文献