全文获取类型
收费全文 | 297篇 |
免费 | 8篇 |
专业分类
化学 | 211篇 |
力学 | 8篇 |
数学 | 41篇 |
物理学 | 45篇 |
出版年
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 7篇 |
2020年 | 4篇 |
2019年 | 7篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 7篇 |
2015年 | 9篇 |
2014年 | 2篇 |
2013年 | 32篇 |
2012年 | 6篇 |
2011年 | 9篇 |
2010年 | 11篇 |
2009年 | 6篇 |
2008年 | 19篇 |
2007年 | 23篇 |
2006年 | 9篇 |
2005年 | 10篇 |
2004年 | 17篇 |
2003年 | 7篇 |
2002年 | 14篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1968年 | 1篇 |
1929年 | 1篇 |
排序方式: 共有305条查询结果,搜索用时 437 毫秒
21.
Daniel Zaragoza-Puchol Javier E. Ortiz Alejandro A. Orden Marianela Sanchez Jorge Palermo Alejandro Tapia Jaume Bastida Gabriela E. Feresin 《Molecules (Basel, Switzerland)》2021,26(1)
Plants in the Amaryllidaceae family synthesize a diversity of bioactive alkaloids. Some of these plant species are not abundant and have a low natural multiplication rate. The aims of this work were the alkaloids analysis of a Habranthus cardenasianus bulbs extract, the evaluation of its inhibitory activity against cholinesterases, and to test several propagation strategies for biomass production. Eleven compounds were characterized by GC-MS in the alkaloid extract, which showed a relatively high proportion of tazettine. The known alkaloids tazettine, haemanthamine, and the epimer mixture haemanthidine/6-epi-haemanthidine were isolated and identified by spectroscopic methods. Inhibitory cholinesterases activity was not detected. Three forms of propagation were performed: bulb propagation from seed, cut-induced bulb division, and micropropagated bulbs. Finally, different imbibition and post-collection times were evaluated in seed germination assays. The best propagation method was cut-induced bulb division with longitudinal cuts into quarters (T1) while the best conditions for seed germination were 0-day of post-collection and two days of imbibition. The alkaloids analyses of the H. cardenasianus bulbs showed that they are a source of anti-tumoral alkaloids, especially pretazettine (tazettine) and T1 is a sustainable strategy for its propagation and domestication to produce bioactive alkaloids. 相似文献
22.
This paper extends the canonical small open-economy real-business-cycle model, when considering model uncertainty. Domestic households have multiplier preferences, which leads them to take robust decisions in response to possible model misspecification for the economy’s aggregate productivity. Using perturbation methods, the paper extends the literature on real business cycle models by deriving a closed-form solution for the combined welfare effect of the two sources of uncertainty, namely risk and model uncertainty. While classical risk has an ambiguous effect on welfare, the addition of model uncertainty is unambiguously welfare-deteriorating. Hence, the overall effect of uncertainty on welfare is ambiguous, depending on consumers preferences and model parameters. The paper provides numerical results for the welfare effects of uncertainty measured by units of consumption equivalence. At moderate (high) levels of risk aversion, the effect of risk on household welfare is positive (negative). The addition of model uncertainty—for all levels of concern about model uncertainty and most risk aversion values—turns the overall effect of uncertainty on household welfare negative. It is important to remark that the analytical decomposition and combination of the effects of the two types of uncertainty considered here and the resulting ambiguous effect on overall welfare have not been derived in the previous literature on small open economies. 相似文献
23.
Jocelyn Gonessa 《Archiv der Mathematik》2013,100(5):439-447
In this note we prove a duality theorem for generalized Fock spaces. 相似文献
24.
The atmospheric pressure chemical ionization of the dinitrotoluene isomers in ambient air was studied with a quadrupole mass spectrometer operating in the negative mode. The isomers can be grouped on the basis of the product ions: 2,5-, and 2,6- and 3,5-dinitrotoluene give the molecular anion with little fragmentation; 2,3- and the 3,4-dinitrotoluene behave similarly but with more extensive fragmentation; 2,4-dinitrotoluene gives the quasimolecular [DNT ? H]? ion with little fragmentation. The results are discussed in terms of the molecular structure of the isomers. 相似文献
25.
Pabla?Aguirre Natalia?Mena Victoria?Tapia Miguel?Arredondo Marco?T?Nú?ezEmail author 《BMC neuroscience》2005,6(1):3
Background
Iron is necessary for neuronal function but in excess generates neurodegeneration. Although most of the components of the iron homeostasis machinery have been described in neurons, little is known about the particulars of their iron homeostasis. In this work we characterized the response of SH-SY5Y neuroblastoma cells and hippocampal neurons to a model of progressive iron accumulation. 相似文献26.
In this paper, we revisit the convergence properties of the iterationprocess xk+1=xk–(xk)B(xk)–1f(xk)for minimizing a function f(x). After reviewing some classic results andintroducing the notion of strong attraction, we give necessary andsufficient conditions for a stationary point of f(x) to be a point of strongattraction for the iteration process. Not only this result gives a newalgorithmic interpretation to the classic Ostrowski theorem, but alsoprovides insight into the interesting phenomenon called selectiveminimization. We present also illustrative numerical examples for nonlinearleast squares problems. 相似文献
27.
Jocelyn D Laughton Philippe Bittar Yves Charnay Luc Pellerin Enikö Kovari Pierre J Magistretti Constantin Bouras 《BMC neuroscience》2007,8(1):35
Background
For a long time now, glucose has been thought to be the main, if not the sole substrate for brain energy metabolism. Recent data nevertheless suggest that other molecules, such as monocarboxylates (lactate and pyruvate mainly) could be suitable substrates. Although monocarboxylates poorly cross the blood brain barrier (BBB), such substrates could replace glucose if produced locally. 相似文献28.
The dinitrotoluenes (DNT) are some widely used compounds despite the first general impression one might have. They are the intermediates of several economically important industrial reactions such as the continuous nitration process of trinitrotoluene (TNT).3,4 As we were interested in the spectral characteristics of the various isomers of DNT5, we realized that the 3,5 isomer was not u-sually obtained during the nitration process of toluene6, thus making its availability quite restricted. It appeared relevant to find a useful and practical way to synthesize that particular isomer as it could also lead to the formation of new polyurethanes7, many polymerization processes uses DNT (mainly the 2,4 isomer because of its availability and its low price). 相似文献
29.
Jocelyn S. Y. Chen Tijmen G. Euser Dr. Nicola J. Farrer Dr. Peter J. Sadler Prof. Michael Scharrer Dr. Philip St. J. Russell Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(19):5607-5612
We report the use of a liquid‐filled hollow‐core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow‐core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8 nL cm?1 in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin B12, cyanocobalamin (CNCbl) to hydroxocobalamin ([H2OCbl]+) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real‐time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore, the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly—in our experiments one thousand times faster than in a conventional cuvette. 相似文献
30.