全文获取类型
收费全文 | 1412篇 |
免费 | 51篇 |
国内免费 | 21篇 |
专业分类
化学 | 990篇 |
晶体学 | 9篇 |
力学 | 43篇 |
数学 | 246篇 |
物理学 | 196篇 |
出版年
2022年 | 12篇 |
2021年 | 26篇 |
2020年 | 28篇 |
2019年 | 26篇 |
2018年 | 12篇 |
2017年 | 15篇 |
2016年 | 39篇 |
2015年 | 25篇 |
2014年 | 27篇 |
2013年 | 62篇 |
2012年 | 72篇 |
2011年 | 94篇 |
2010年 | 72篇 |
2009年 | 44篇 |
2008年 | 77篇 |
2007年 | 97篇 |
2006年 | 103篇 |
2005年 | 100篇 |
2004年 | 85篇 |
2003年 | 64篇 |
2002年 | 65篇 |
2001年 | 25篇 |
2000年 | 18篇 |
1999年 | 20篇 |
1998年 | 9篇 |
1997年 | 17篇 |
1996年 | 16篇 |
1995年 | 5篇 |
1994年 | 12篇 |
1993年 | 16篇 |
1992年 | 8篇 |
1991年 | 13篇 |
1990年 | 9篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1979年 | 15篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 9篇 |
1973年 | 11篇 |
1972年 | 5篇 |
排序方式: 共有1484条查询结果,搜索用时 14 毫秒
991.
J. J. Bexon J. F. J. Todd C. N. C. Drey R. J. Ridge 《Journal of mass spectrometry : JMS》1977,12(9):578-581
Mass spectrometry has been used in an attempt to resolve the ambiguity in the structures of the isomeric dihydro-oxazin-6-ones and N-acyl-azetidin-2-ones. 相似文献
992.
A new method of determining the escape depth of electrons in the energy range 0–1000 eV is described. Values obtained were 6.1 Å and 6.8 Å at 234 and 266 eV, respectively, in GeO2. 相似文献
993.
M.J. Todd 《European Journal of Operational Research》1981,8(3):309-310
This work provides a reliability analysis for power units which are deliberately shutdown when undemanded. A gas turbine is a common example of such a unit. Two appropriate interrelated reliability criteria are mathematically formulated and the relation to their ordinary counterparts is investigated. Using semi-Markov and renewal theory methods these criteria are computed under different assumptions with regard to the stochastic behavior of the demand on the unit and the time to failure and repair time distributions. 相似文献
994.
Todd A. Gillespie Laszlo Urogdi Alan R. Katritzky Richard A. Yost 《Journal of mass spectrometry : JMS》1989,24(9):817-822
The positive- and negative-ion chemical ionization (± CI) behavior of phthalic acid and four related compounds (phthalic anhydride, phthalide, biphthalyl, and anthraquinone-2-carboxyIic acid) are characterized by tandem mass spectrometry with low-energy collisionally activated dissociation. Principal fragmentation pathways have been established, and molecular and fragment ions arising from these compounds have been differentiated from adduct and dimer ions, as well as from ions due to impurities. The ubiquitous nature of these compounds in analytical samples makes it important to identify the corresponding ions, and thus to be able to exclude them from consideration in the identification of other sample components. 相似文献
995.
996.
A dynamical analysis of standard procedures for subensemble selection is used to show that the state restriction violation proposal in Part I of the paper cannot be realized by employing familiar correlation schemes. However, it is shown that measurement of an observable not commuting with the superselection operator is possible, a violation of the observable restrictions. This is interpreted as supporting the position that each of these restrictions is sufficient but not necessary for the superselection rule. The results do constitute a proposal for superselection rule violation in theories requiring both restrictions, e.g., the axiomatic treatment by Bogolubov, Logunov, and Todorov. It is also concluded that superselection rules place restrictions on procedures for selective state preparations using correlations. More generally, it is conjectured that a mathematically conceivable decomposition of a given density operator does not necessarily represent a possibility for partitioning of the corresponding ensemble into subensembles by any physically realizable means. 相似文献
997.
998.
Olivetol and 4-nitroso-N,N-dimethylaniline were combined under reflux in acetic acid to produce 1-pentyl-7-dimethylamino-3H-phenoxazine-3-one (1-PDMPO). The fluorescent behavior of the purified compound in a variety of solutions and samples was investigated. Excitation and emission spectra in pure solvents and mixtures of solvents demonstrate solvatochromism, indicating that the fluorescence response of this compound is affected by its environment. Nonpolar, aprotic solvents as well as protic, polar solvents diminish fluorescent emission in the spectral regions examined, and trends in the fluorescence decary lifetimes measured in five pure solvents are consistent with these intensity changes. The anisotropy excitation spectrum taken in glycerol for 1-PDMPO at –15°C appears to be consistent with the presence of a single electronic state upon excitation, with anisotropy values approaching 0.35 over 400–600 nm. Fluorescence emission is also diminished at low acid concentrations in methanol, with smaller decreases observed in more highly concentrated basic solutions. Emission peaks in aqueous sodium dodecyl sulfate solutions, extruded egg phosphatidylcholine vesicles, and fatty acid free bovine serum albumin suspensions all lie above 600 nm, with emission in the albumin suspension displaying a broad shoulder extending to 800 nm. The fluorescent properties of this compound suggest that it or structural homologues may have utility as fluorescent biological probes.Presented in part at the Biophysical Society Meeting, Washington, DC (February 1993). 相似文献
999.
The mass spectra of a series of βγ-unsaturated cyclic and acyclic carboxylic acids are reported. Metastable ion studies and precise mass measurements have been employed in demonstrating that the ten compounds studied conform to one or other of two generalised fragmentation pathways. 相似文献
1000.