全文获取类型
收费全文 | 1185篇 |
免费 | 62篇 |
国内免费 | 1篇 |
专业分类
化学 | 971篇 |
晶体学 | 15篇 |
力学 | 14篇 |
数学 | 36篇 |
物理学 | 212篇 |
出版年
2023年 | 13篇 |
2022年 | 17篇 |
2021年 | 21篇 |
2020年 | 28篇 |
2019年 | 42篇 |
2018年 | 24篇 |
2017年 | 13篇 |
2016年 | 43篇 |
2015年 | 37篇 |
2014年 | 35篇 |
2013年 | 65篇 |
2012年 | 102篇 |
2011年 | 101篇 |
2010年 | 43篇 |
2009年 | 46篇 |
2008年 | 68篇 |
2007年 | 62篇 |
2006年 | 85篇 |
2005年 | 64篇 |
2004年 | 58篇 |
2003年 | 53篇 |
2002年 | 39篇 |
2001年 | 18篇 |
2000年 | 19篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 9篇 |
1995年 | 3篇 |
1994年 | 11篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1969年 | 4篇 |
1967年 | 2篇 |
排序方式: 共有1248条查询结果,搜索用时 15 毫秒
101.
Masahiro Hotta 《Physics letters. A》2008,372(21):3752-3758
We derive a universal lower bound of actuating energy Es for broadband photon switching by using an uncertainty relation between time and the negative energy density of quantum fields. We find that broadband photon switching between perfect reflection and perfect transmission over a time ts should satisfy . 相似文献
102.
103.
Ruthenium(0)‐Catalyzed Cycloaddition of 1,2‐Diols,Ketols, or Diones via Alcohol‐Mediated Hydrogen Transfer 下载免费PDF全文
Hiroki Sato Dr. Ben W. H. Turnbull Dr. Keisuke Fukaya Prof. Michael J. Krische 《Angewandte Chemie (International ed. in English)》2018,57(12):3012-3021
Merging the characteristics of transfer hydrogenation and carbonyl addition, a broad new class of ruthenium(0)‐catalyzed cycloadditions has been developed. As discussed in this Minireview, fused or bridged bicyclic ring systems are accessible in a redox‐independent manner in C?C bond‐forming hydrogen transfer reactions of diols, α‐ketols, or 1,2‐diones with diverse unsaturated reactants. 相似文献
104.
Stable and Highly Efficient Electrochemical Production of Formic Acid from Carbon Dioxide Using Diamond Electrodes 下载免费PDF全文
Dr. Keisuke Natsui Hitomi Iwakawa Dr. Norihito Ikemiya Prof. Dr. Kazuya Nakata Prof. Dr. Yasuaki Einaga 《Angewandte Chemie (International ed. in English)》2018,57(10):2639-2643
High faradaic efficiencies can be achieved in the production of formic acid (HCOOH) by metal electrodes, such as Sn or Pb, in the electrochemical reduction of carbon dioxide (CO2). However, the stability and environmental load in using them are problematic. The electrochemical reduction of CO2 to HCOOH was investigated in a flow cell using boron‐doped diamond (BDD) electrodes. BDD electrodes have superior electrochemical properties to metal electrodes, and, moreover, are highly durable. The faradaic efficiency for the production of HCOOH was as high as 94.7 %. Furthermore, the selectivity for the production of HCOOH was more than 99 %. The rate of the production was increased to 473 μmol m?2 s?1 at a current density of 15 mA cm?2 with a faradaic efficiency of 61 %. The faradaic efficiency and the production rate are almost the same as or larger than those achieved using Sn and Pb electrodes. Furthermore, the stability of the BDD electrodes was confirmed by 24 h operation. 相似文献
105.
Polymeric aluminosilicate gels with Al2O3/SiO2 molar ratios of 1.3/2, 2.5/2, 3/2 and 4.7/2 were prepared by gelling a mixture of tetraethoxysilane and ethyl acetoacetate aluminium diisopropoxide. Mullite, without any other crystalline phase, directly crystallizes from the gel matrix at about 1000 °C for all investigated samples. The Al2O3 or SiO2 crystalline phase can only be detected at relatively high temperatures accompanying the modification of mullite in the lattice structure. 27Al and 29Si MAS-NMR studies indicate that segregation of Al and Si atoms occurs in all samples below 900 °C in the amorphous state, regardless of the gel composition. However, the species of segregated units are very different and strongly dependent on the composition of the starting gels. This segregation may not cause the exothermic effect in differential thermal analysis examination, but it appears to be responsible for the composition change behavior of crystalline mullite with different heat treatment. 相似文献
106.
Kambara O Tamura A Naito A Tominaga K 《Physical chemistry chemical physics : PCCP》2008,10(33):5042-5044
Two structural changes of poly-l-lysine have been studied by various spectroscopic techniques; one is a structural change of a random coil sample in solution to a mixture of alpha-helix and beta-sheet during rapid freezing in the lyophilizing process, and the other is a pressure-induced structural change from an alpha-helix to a beta-sheet structure for a lyophilized sample. 相似文献
107.
3-Ethynyl-chlorin was prepared from methyl pyropheophorbide-d using Bestmann-Ohira reagent. The mono-substituted acetylene was subjected to copper-free, Pd-mediated coupling to form chlorin derivatives possessing a series of substituted ethynyl groups at the C3-position. Its 1,3-dipolar cycloadditions with azido compounds were also demonstrated. 相似文献
108.
Vibrational energy relaxation of the carbonyl CO stretching modes of CH3COOD and CD3COOD in D2O is studied by frequency-resolved infrared pump-probe spectroscopy. The spectral change caused by the vibrational excitation includes two dynamical components with the time constants of 450 and 980 fs for CH3COOD and 390 and 930 fs for CD3COOD. The two dynamical components exhibit different spectral properties. There are two species of acetic acid forming different complexes with solvent water molecules. The time constants are almost the same for CH3COOD and CD3COOD, suggesting that the vibrational energy deposited to the carbonyl group is first distributed among vibrational modes not related to the methyl group. 相似文献
109.
Fürstner A Majima K Martín R Krause H Kattnig E Goddard R Lehmann CW 《Journal of the American Chemical Society》2008,130(6):1992-2004
Reaction of ferrocene with lithium in the presence of either ethylene or COD allows the Fe(0)-ate complexes 1 and 4 to be prepared on a large scale, which turned out to be excellent catalysts for a variety of Alder-ene, [4+2], [5+2], and [2+2+2] cycloadditon and cycloisomerization reactions of polyunsaturated substrates. The structures of ferrates 1 and 4 in the solid-state reveal the capacity of the reduced iron center to share electron density with the ligand sphere. This feature, coupled with the kinetic lability of the bound olefins, is thought to be responsible for the ease with which different enyne or diyne substrates undergo oxidative cyclization as the triggering event of the observed skeletal reorganizations. This mechanistic proposal is corroborated by highly indicative deuterium labeling experiments. Moreover, it was possible to intercept two different products of an oxidative cyclization manifold with the aid of the Fe(+1) complex 6, which, despite its 17-electron count, also turned out to be catalytically competent in certain cases. The unusual cyclobutadiene complex 38 derived from 6 and tolane was characterized by X-ray crystallography. 相似文献
110.
The partition coefficients (Kps) of six anti-inflammatory steroid drugs, dexamethasone (DMS), betamethasone (BMS), triamcinolone acetonide (TCLA), fluocinolone acetonide (FCLA), betamethasone 17,21-dipropionate (BMSDP), and clobetasole propionate (CBSP), for phosphatidylcholine (PC), and PC-cholesterol small unilamellar vesicles (SUVs) were determined by a second-derivative spectrophotometric method. The Kp values were obtained with a relative standard deviation of below 10% and the following order was observed: BMS< or =DMS相似文献