全文获取类型
收费全文 | 20989篇 |
免费 | 155篇 |
国内免费 | 296篇 |
专业分类
系统科学 | 248篇 |
丛书文集 | 274篇 |
教育与普及 | 126篇 |
理论与方法论 | 66篇 |
现状及发展 | 7656篇 |
研究方法 | 670篇 |
综合类 | 12241篇 |
自然研究 | 159篇 |
出版年
2013年 | 223篇 |
2012年 | 323篇 |
2011年 | 512篇 |
2010年 | 223篇 |
2009年 | 283篇 |
2008年 | 437篇 |
2007年 | 471篇 |
2006年 | 428篇 |
2005年 | 439篇 |
2004年 | 424篇 |
2003年 | 412篇 |
2002年 | 419篇 |
2001年 | 617篇 |
2000年 | 629篇 |
1999年 | 554篇 |
1998年 | 213篇 |
1997年 | 209篇 |
1996年 | 241篇 |
1995年 | 216篇 |
1994年 | 468篇 |
1992年 | 392篇 |
1991年 | 345篇 |
1990年 | 364篇 |
1989年 | 343篇 |
1988年 | 317篇 |
1987年 | 323篇 |
1986年 | 310篇 |
1985年 | 345篇 |
1984年 | 237篇 |
1983年 | 257篇 |
1982年 | 212篇 |
1981年 | 204篇 |
1980年 | 245篇 |
1979年 | 508篇 |
1978年 | 434篇 |
1977年 | 390篇 |
1976年 | 314篇 |
1975年 | 386篇 |
1974年 | 474篇 |
1973年 | 423篇 |
1972年 | 431篇 |
1971年 | 562篇 |
1970年 | 608篇 |
1969年 | 531篇 |
1968年 | 532篇 |
1967年 | 459篇 |
1966年 | 408篇 |
1965年 | 294篇 |
1958年 | 322篇 |
1957年 | 253篇 |
排序方式: 共有10000条查询结果,搜索用时 828 毫秒
61.
在形状记忆合金中,存在马氏体穿晶生长现象,它是热弹马氏体相变在小角晶界的行为。 相似文献
62.
A physically based model for ground‐level ozone forecasting is evaluated for Santiago, Chile. The model predicts the daily peak ozone concentration, with the daily rise of air temperature as input variable; weekends and rainy days appear as interventions. This model was used to analyse historical data, using the Linear Transfer Function/Finite Impulse Response (LTF/FIR) formalism; the Simultaneous Transfer Function (STF) method was used to analyse several monitoring stations together. Model evaluation showed a good forecasting performance across stations—for low and high ozone impacts—with power of detection (POD) values between 70 and 100%, Heidke's Skill Scores between 40% and 70% and low false alarm rates (FAR). The model consistently outperforms a pure persistence forecast. Model performance was not sensitive to different implementation options. The model performance degrades for two‐ and three‐days ahead forecast, but is still acceptable for the purpose of developing an environmental warning system at Santiago. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
63.
Itknownthatsmallamountsoffluxonthesurfaceofstainlesssteelcanincreasethedepthofweldpene trationinTIGweldingbyafactorofthree[1] .Thisprocessisreferredtoas”ATIG”orTIGweldingacti vatedbyflux .Therehavebeenthreepublishedphys icalmechanismsthatarepossiblecontri… 相似文献
64.
Rui M Costa Nikolai B Federov Jeff H Kogan Geoffrey G Murphy Joel Stern Masuo Ohno Raju Kucherlapati Tyler Jacks Alcino J Silva 《Nature》2002,415(6871):526-530
Neurofibromatosis type I (NF1) is one of the most common single-gene disorders that causes learning deficits in humans. Mice carrying a heterozygous null mutation of the Nfl gene (Nfl(+/-) show important features of the learning deficits associated with NF1 (ref. 2). Although neurofibromin has several known properties and functions, including Ras GTPase-activating protein activity, adenylyl cyclase modulation and microtubule binding, it is unclear which of these are essential for learning in mice and humans. Here we show that the learning deficits of Nf1(+/-) mice can be rescued by genetic and pharmacological manipulations that decrease Ras function. We also show that the Nf1(+/-) mice have increased GABA (gamma-amino butyric acid)-mediated inhibition and specific deficits in long-term potentiation, both of which can be reversed by decreasing Ras function. Our results indicate that the learning deficits associated with NF1 may be caused by excessive Ras activity, which leads to impairments in long-term potentiation caused by increased GABA-mediated inhibition. Our findings have implications for the development of treatments for learning deficits associated with NF1. 相似文献
65.
从计划角度重点分析了钢铁生产中炼钢、连铸、热轧三者之间联接的较高模式-直接热装轧制计划系统的实现方式,并给出了较为具体的方案。 相似文献
66.
地理信息系统在滑坡预测预报中的应用 总被引:7,自引:0,他引:7
以滑坡监测数据为基础,采用地理信息系统技术,建立实用的滑坡预测预报软件系统。 相似文献
67.
Summary The nature of antigen Dd, an antigen present in the extracts of human dandruff which precipitates human sera selectively, and antibodies reacting with it are reported.This work was supported through U.G.C. Grant No. F.23-230/75/SR II. H.K. participated in this study, first as J.R.F. of U.G.C. and then as S.R.F. of I.C.M.R. We are grateful to Dr Baruch S. Blumberg for his invaluable suggestions and to Professor H. Walter for the gift of IgG, IgM and IgA immune sera. 相似文献
68.
During the evolution of proteins the pressure to optimize biological activity is moderated by a need for efficient folding. For most proteins, this is accomplished through spontaneous folding to a thermodynamically stable and active native state. However, in the extracellular bacterial alpha-lytic protease (alphaLP) these two processes have become decoupled. The native state of alphaLP is thermodynamically unstable, and when denatured, requires millennia (t1/2 approximately 1,800 years) to refold. Folding is made possible by an attached folding catalyst, the pro-region, which is degraded on completion of folding, leaving alphaLP trapped in its native state by a large kinetic unfolding barrier (t1/2 approximately 1.2 years). alphaLP faces two very different folding landscapes: one in the presence of the pro-region controlling folding, and one in its absence restricting unfolding. Here we demonstrate that this separation of folding and unfolding pathways has removed constraints placed on the folding of thermodynamically stable proteins, and allowed the evolution of a native state having markedly reduced dynamic fluctuations. This, in turn, has led to a significant extension of the functional lifetime of alphaLP by the optimal suppression of proteolytic sensitivity. 相似文献
69.
A. E. Yassen H. A. Hassan L. S. Kawashti 《Cellular and molecular life sciences : CMLS》1994,50(11-12):1111-1114
A detailed karyotypic analysis of two Egyptian species of bats,Taphozous perforatus andTaphozous nudiventris, was made on the basis of conventional data and G-band patterns. No detectable karyotypic difference was found between the two species (2n=42, F.N. 64). By comparison of G-band patterns, some differences between the two species were seen in the corresponding autosomal pairs. These results are reported for the first time in Egypt. 相似文献
70.
本文以糠醇为原料,以醇作溶剂,在酸性条件下糖醇开环制得一系列乙酞丙酸酯类化合物。然后再以乙酞丙酸酯类化合物为原料,制得一系列未见报道的此类酯与醇的缩合产物,并用HP5988GC-MS联用仪研究了反应产物的组成。 相似文献