全文获取类型
收费全文 | 4075篇 |
免费 | 77篇 |
国内免费 | 33篇 |
专业分类
化学 | 2709篇 |
晶体学 | 31篇 |
力学 | 204篇 |
数学 | 671篇 |
物理学 | 570篇 |
出版年
2021年 | 31篇 |
2020年 | 33篇 |
2019年 | 26篇 |
2018年 | 42篇 |
2017年 | 24篇 |
2016年 | 74篇 |
2015年 | 57篇 |
2014年 | 70篇 |
2013年 | 216篇 |
2012年 | 135篇 |
2011年 | 194篇 |
2010年 | 108篇 |
2009年 | 108篇 |
2008年 | 157篇 |
2007年 | 166篇 |
2006年 | 170篇 |
2005年 | 186篇 |
2004年 | 198篇 |
2003年 | 188篇 |
2002年 | 180篇 |
2001年 | 80篇 |
2000年 | 89篇 |
1999年 | 49篇 |
1998年 | 44篇 |
1997年 | 47篇 |
1996年 | 71篇 |
1995年 | 58篇 |
1994年 | 58篇 |
1993年 | 55篇 |
1992年 | 47篇 |
1991年 | 47篇 |
1990年 | 48篇 |
1989年 | 51篇 |
1988年 | 47篇 |
1987年 | 40篇 |
1986年 | 38篇 |
1985年 | 65篇 |
1984年 | 71篇 |
1983年 | 59篇 |
1982年 | 68篇 |
1981年 | 62篇 |
1980年 | 73篇 |
1979年 | 68篇 |
1978年 | 56篇 |
1977年 | 54篇 |
1976年 | 44篇 |
1975年 | 49篇 |
1974年 | 61篇 |
1973年 | 39篇 |
1972年 | 35篇 |
排序方式: 共有4185条查询结果,搜索用时 15 毫秒
991.
992.
Terry F. Hamilton Jussi Jernströem Roger E. Martinelli Steven R. Kehl Mats Eriksson Ross W. Williams Marek Bielewski Ariel N. Rivers Thomas A. Brown Scott J. Tumey Maria Betti 《Journal of Radioanalytical and Nuclear Chemistry》2009,282(3):1019-1026
Runit Island on Enewetak Atoll was very heavily impacted by the U.S. nuclear testing campaign in the northern Marshall Islands (1946–58). The primary source of contamination on Runit Island was the 1958 Quince safety test where a large quantity of device plutonium (Pu) was scattered over the area near the GZ. A second low-yield device was detonated on the same site 10 days later, further disturbing the soil and leaving behind a very heterogeneous pattern of contamination including milligram-size particles of plutonium. A limited cleanup of the Fig-Quince zone was carried out in 1979. During this period, the effectiveness of the cleanup operations was primarily evaluated on the basis of bulk soil concentration data with little consideration given to the heterogeneity and long-term material-, biological-, and environmental-specific impacts of residual high activity (hot) particle contamination. The aim of the present study was twofold; (i) to characterize the levels and distribution of residual contamination in the Fig-Quince zone, and (ii) to develop pertinent data on the frequency distribution, elemental and isotopic composition, and physico-chemical properties of hot particles isolated from surface soils from Fig-Quince with a view towards providing recommendations on the future management and possible cleanup of the site. Today, Runit Island remains under an administrative quarantine. 相似文献
993.
Ricardo Godoy Lopez Christophe Boisson Franck D'Agosto Roger Spitz Fernande Boisson Didier Gigmes Denis Bertin 《Journal of polymer science. Part A, Polymer chemistry》2007,45(13):2705-2718
A synthetic method for the production of polyethylene (PE) chains carrying alkoxyamine end‐group has been proposed first by successfully reacting the well‐known 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) and N‐(2‐methyl‐2‐propyl)‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl (commonly called SG1) stable radicals with dipolyethylenylmagnesium compounds to give PE‐TEMPO and PE‐SG1. Since the homolytic cleavage of these two macroalkoxyamines for the production of block copolymers using controlled radical polymerization would require temperatures higher than 160 °C, two original new nitroxides (4‐[(2,2‐dimethyl‐4‐(N‐tert‐Butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxy)‐4‐n‐butoxycarbonyl)butanoyloxyl]‐2,2,6,6‐tetramethylpiperidinyl‐1‐oxy, DD1) and 4‐[(2,2‐dimethyl‐4‐(N‐tert‐Butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxy)‐4‐phenyl) butanoyloxyl]‐2,2,6,6‐tetramethylpiperidinyl‐1‐oxy, DD2) containing a TEMPO moiety and incorporating an SG1‐based alkoxyamine (cleavage temperature: 60 °C) were then synthesized. NMR analyses showed that the resulting PE‐DD1 and PE‐DD2 were obtained using this functionalization strategy though with low to moderate yields (from 17% to 40%). PE‐DD2 (40% functionalization) was used under controlled radical polymerization conditions of n‐butyl acrylate. SEC analyses together with 1H NMR analysis showed that a poly(ethylene‐b‐n‐butyl acrylate) block copolymer was produced and that the polymerization proceeded under control. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2705–2718, 2007 相似文献
994.
995.
This paper studies the problem of finding best-possible upper bounds on the Value-at-Risk for a function of two random variables when the marginal distributions are known and additional nonparametric information on the dependence structure, such as the value of a measure of association, is available. The same problem for the Tail-Value-at-Risk is also briefly discussed. 相似文献
996.
Real options analysis (ROA) has been developed to correctly value projects with inherent flexibility, including the possibility to abandon, defer, expand, contract or switch to a different project. ROA allows computing the correct discount rate using the replicating portfolio technique or risk-neutral probability method. We propose an alternative approach for valuing Real Options based on the certainty-equivalent version of the net present value formula, which eliminates the need to identify market-priced twin securities. In addition, our approach can be extended to the case of multinomial trees, a useful tool for modeling uncertainty in projects. We introduce within decision tree analysis (DTA) a method to derive the different discount rates that prevail at different chance nodes. We illustrate the valuation method with an application presented in “A Scenario Approach to Capacity Planning” [Eppen, G.D., Martin, R.K., Schrage, L.E., 1989. A scenario approach to capacity planning. Operations Research, 37 (4)], in which the authors state that for the capacity configuration investment decision studied at General Motors, “… there is no scientific way to determine the appropriate discount rate based on estimated demand.” Our method allows deriving the scientifically correct discount rates. A major result of the analysis is that the discount rates are endogenously derived from the project structure and its behavior in light of prevailing market conditions, instead of being exogenously imposed. 相似文献
997.
Using a relative rate method, rate constants have been measured at 296 ± 2 K for the gas‐phase reactions of OH radicals with 1,2‐butanediol, 2,3‐butanediol, 1,3‐butanediol, and 2‐methyl‐2,4‐pentanediol, with rate constants (in units of 10?12 cm3 molecule?1 s?1) of 27.0 ± 5.6, 23.6 ± 6.3, 33.2 ± 6.8, and 27.7 ± 6.1, respectively, where the error limits include the estimated overall uncertainty of ±20% in the rate constant for the reference compound. Gas chromatographic analyses showed the formation of 1‐hydroxy‐2‐butanone from 1,2‐butanediol, 3‐hydroxy‐2‐butanone from 2,3‐butanediol, 1‐hydroxy‐3‐butanone from 1,3‐butanediol, and 4‐hydroxy‐4‐methyl‐2‐pentanone from 2‐methyl‐2,4‐pentanediol, with formation yields of 0.66 ± 0.11, 0.89 ± 0.09, 0.50 ± 0.09, and 0.47 ± 0.09, respectively, where the indicated errors are the estimated overall uncertainties. Pathways for the formation of these products are presented, together with a comparison of the measured and estimated rate constants and product yields. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 310–316, 2001 相似文献
998.
Danny M. Gelman Penelope A. Mayes Roger Mulder Patrick Perlmutter 《Tetrahedron: Asymmetry》2006,17(24):3341-3350
Reversing the polarity of the cycloaddition partners in Dane’s steroid synthesis provides ready access to enantiomerically pure, functionally-rich steroidal structures. 相似文献
999.
1000.
Christelle Mouveaux Joëlle Levalois-Mitjaville Roger De Jaeger 《Journal of inclusion phenomena and macrocyclic chemistry》2001,39(3-4):253-259
The complexation behaviour between salts of Li+-Rb+ in CD3CN and tris(benzocrown ether)s 2a,bX=P(NMeN=CH-B15C5)3 (X = O, S) and tri[bis(benzocrown ether)][N=P(NMeN=CH-B15C5)2]3
3 was investigated by 13 C NMR spectroscopy. Using the program RMNSTAB, the complexation constants for the different possible complexes (M2L, ML andML2 were L represents one benzo-15-crown-5) were obtained and were compared with those of the corresponding monomer material. A remarkable ``biscrown effect' for compounds 2a,b and 3 was found, especially for potassium and rubidium by the predominant formation of stable ML2 complexes. The strong chelate effect make these ligands highly efficient extracting agents for alkali metal picrate salts of K+, Rb+ and Cs+,as shown by UV-Vis spectroscopy. 相似文献