全文获取类型
收费全文 | 4736篇 |
免费 | 117篇 |
国内免费 | 28篇 |
专业分类
化学 | 3086篇 |
晶体学 | 32篇 |
力学 | 87篇 |
数学 | 835篇 |
物理学 | 841篇 |
出版年
2021年 | 40篇 |
2020年 | 42篇 |
2019年 | 61篇 |
2018年 | 32篇 |
2017年 | 42篇 |
2016年 | 74篇 |
2015年 | 66篇 |
2014年 | 75篇 |
2013年 | 200篇 |
2012年 | 182篇 |
2011年 | 255篇 |
2010年 | 147篇 |
2009年 | 130篇 |
2008年 | 243篇 |
2007年 | 200篇 |
2006年 | 251篇 |
2005年 | 227篇 |
2004年 | 237篇 |
2003年 | 178篇 |
2002年 | 183篇 |
2001年 | 65篇 |
2000年 | 76篇 |
1999年 | 53篇 |
1998年 | 42篇 |
1997年 | 45篇 |
1996年 | 86篇 |
1995年 | 46篇 |
1994年 | 66篇 |
1993年 | 49篇 |
1992年 | 60篇 |
1991年 | 42篇 |
1990年 | 47篇 |
1989年 | 61篇 |
1988年 | 46篇 |
1987年 | 35篇 |
1986年 | 42篇 |
1985年 | 64篇 |
1984年 | 61篇 |
1983年 | 54篇 |
1982年 | 66篇 |
1981年 | 66篇 |
1980年 | 63篇 |
1979年 | 57篇 |
1978年 | 64篇 |
1977年 | 73篇 |
1976年 | 53篇 |
1975年 | 54篇 |
1974年 | 65篇 |
1973年 | 66篇 |
1968年 | 29篇 |
排序方式: 共有4881条查询结果,搜索用时 31 毫秒
1.
2.
Vanessa R. Machado Louis P. Sandjo Giovanni L. Pinheiro Milene H. Moraes Mario Steindel Moacir G. Pizzolatti 《Natural product research》2018,32(3):275-281
The natural product lupeol 1 was isolated from aerial parts of Vernonia scorpioides with satisfactory yield, which made it viable to be used as starting material in semisynthetic approach. Ten lupeol derivatives 2–11 were prepared by classical procedures. Including, five new esters derivatives 7–11, which were obtained by structural modifications in the isopropylidene fragment. All semisynthetic compounds and lupeol 1–11 were confirmed by 1H NMR, 13C NMR and HRMS. Their antiprotozoal activity was evaluated in vitro against L. amazonensis and T. cruzi. Derivative 6 showed the best antitrypanosomal activity (IC50 = 12.48 μg/mL) and the lowest cytotoxic derivative (CC50 = 161.50 μg/mL). The mechanism of action of the most active derivatives (4, 6 and 11) is not dependent from the enzyme trypanothione reductase. 相似文献
3.
4.
J. B. Lewis E. L. Baldeschwieler L. D. Tabern E. F. Shelberg D. Hillis S. Cromer I. F. Bingham S. Givaudon M. Aubert M. R. Moutte A. Andant P. Lambert J. Lecomte J. Tausz H. Wolf B. Gordon W. Dachnow I. Perrier M. Lobunetz M. O. Chaikin C. Engler D. Holde 《Analytical and bioanalytical chemistry》1940,119(7-8):307-310
5.
An Application of Branch and Cut to Open Pit Mine Scheduling 总被引:5,自引:0,他引:5
The economic viability of the modern day mine is highly dependent upon careful planning and management. Declining trends in average ore grades, increasing mining costs and environmental considerations will ensure that this situation will remain in the foreseeable future. The operation and management of a large open pit mine having a life of several years is an enormous and complex task. Though a number of optimization techniques have been successfully applied to resolve some important problems, the problem of determining an optimal production schedule over the life of the deposit is still very much unresolved. In this paper we will critically examine the techniques that are being used in the mining industry for production scheduling indicating their limitations. In addition, we present a mixed integer linear programming model for the scheduling problems along with a Branch and Cut solution strategy. Computational results for practical sized problems are discussed. 相似文献
6.
7.
[3,3]-Sigmatropic rearrangements of allyl tetronates and allyl tetramates to give 3-allyltetronic or -tetramic acids, respectively, proceed within 20-60 min under microwave irradiation (300 W, 130-190 °C). Consecutive (homo)sigmatropic [1,5] H-shifts such as oxa-ene reactions are promoted less effectively, which allows the isolation of Claisen intermediates of sigmatropic domino sequences, in contrast to conventional heating. 相似文献
8.
P. Murzyn C. R. Pidgeon P. J. Phillips J. -P. Wells N. T. Gordon T. Ashley J. H. Jefferson T. M. Burke J. Giess M. Merrick B. N. Murdin C. D. Maxey 《Physica E: Low-dimensional Systems and Nanostructures》2004,20(3-4):220
We have made direct pump–probe measurements of spin lifetimes in long wavelength narrow-gap semiconductors at wavelengths between 4 and 10 μm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes, τs300 ps, even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that τs is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of τs in non-degenerate lightly n-type Hg0.78Cd0.22Te of approximately the same band gap as InSb, and find that τs varies from 356 ps at 150 K to 24 ps at 300 K. Our results, both in magnitude and temperature dependence of τs, imply that the Elliott–Yafet model dominates in these materials. 相似文献
9.
Let f :X→X be a continuous map of a compact metric space to itself. We prove that f is topologically conjugate to an adding machine map if and only if X is an infinite minimal set for f and each point of X is regularly recurrent. Moreover, if X is an infinite minimal set for f and one point of X is regularly recurrent, then f is semiconjugate to an adding machine map. 相似文献
10.