全文获取类型
收费全文 | 7269篇 |
免费 | 736篇 |
国内免费 | 362篇 |
专业分类
化学 | 2014篇 |
晶体学 | 43篇 |
力学 | 2587篇 |
综合类 | 31篇 |
数学 | 1129篇 |
物理学 | 2563篇 |
出版年
2024年 | 8篇 |
2023年 | 89篇 |
2022年 | 135篇 |
2021年 | 139篇 |
2020年 | 182篇 |
2019年 | 163篇 |
2018年 | 169篇 |
2017年 | 212篇 |
2016年 | 280篇 |
2015年 | 213篇 |
2014年 | 295篇 |
2013年 | 469篇 |
2012年 | 393篇 |
2011年 | 455篇 |
2010年 | 342篇 |
2009年 | 437篇 |
2008年 | 388篇 |
2007年 | 418篇 |
2006年 | 359篇 |
2005年 | 396篇 |
2004年 | 301篇 |
2003年 | 291篇 |
2002年 | 270篇 |
2001年 | 232篇 |
2000年 | 186篇 |
1999年 | 170篇 |
1998年 | 160篇 |
1997年 | 138篇 |
1996年 | 116篇 |
1995年 | 138篇 |
1994年 | 95篇 |
1993年 | 101篇 |
1992年 | 66篇 |
1991年 | 81篇 |
1990年 | 60篇 |
1989年 | 46篇 |
1988年 | 70篇 |
1987年 | 58篇 |
1986年 | 54篇 |
1985年 | 40篇 |
1984年 | 34篇 |
1983年 | 15篇 |
1982年 | 46篇 |
1981年 | 18篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1971年 | 4篇 |
1957年 | 2篇 |
排序方式: 共有8367条查询结果,搜索用时 0 毫秒
101.
Laloup M Ramirez Fernandez Mdel M Wood M De Boeck G Henquet C Maes V Samyn N 《Journal of chromatography. A》2005,1082(1):15-24
A rapid and sensitive method for the analysis of delta9-tetrahydrocannabinol (THC) in preserved oral fluid was developed and fully validated. Oral fluid was collected with the Intercept, a Food and Drug Administration (FDA) approved sampling device that is used on a large scale in the U.S. for workplace drug testing. The method comprised a simple liquid-liquid extraction with hexane, followed by liquid chromatography-tandem mass spectrometry (LC-MS-MS) analysis. Chromatographic separation was achieved using a XTerra MS C18 column, eluted isocratically with 1 mM ammonium formate-methanol (10:90, v/v). Selectivity of the method was achieved by a combination of retention time, and two precursor-product ion transitions. The use of the liquid-liquid extraction was demonstrated to be highly effective and led to significant decreases in the interferences present in the matrix. Validation of the method was performed using both 100 and 500 MicroL of oral fluid. The method was linear over the range investigated (0.5-100 ng/mL and 0. 1-10 ng/mL when 100 and 500 microL, respectively, of oral fluid were used) with an excellent intra-assay and inter-assay precision (relative standard deviations, RSD <6%) for quality control samples spiked at a concentration of 2.5 and 25 ng/mL and 0.5 and 2.5 ng/mL, respectively. Limits of quantification were 0.5 and 0.1 ng/mL when using 100 and 500 microL, respectively. In contrast to existing GC-MS methods, no extensive sample clean-up and time-consuming derivatisation steps were needed. The method was subsequently applied to Intercept samples collected at the roadside and collected during a controlled study with cannabis. 相似文献
102.
Summary We have investigated to which extent retention data, acquired on single capillary columns, can be used for predicting retention factors in a coupled column system. For this purpose we utilized a model mixture of 18 lipid components with widely different vapor pressures and polarities. The sample was chromatographed on two columns, SB-biphenyl-30 (70% methyl-30% biphenylpolysiloxane) and SB-cyanopropyl-50 (50% methyl-50% cyanopropylsiloxane). Experimental retention factors, acquired in coupled column systems with two columns connected in different order, were thus compared with values calculated from runs on each single column. The agreement between calculated and experimental values generally was better than 5% without any pressure drop correction.To study the possibility of predicting retention behavior in a wide pressure range from a limited number of experiments, we also investigated the relation between solute retention and mobile phase density. We found that all data could be fitted to second order equations, which gives the possibility to optimize the resolution with respect to pressure from a limited number of runs at different pressures. 相似文献
103.
Fatty acid methyl esters from various fats and oils were separated by comprehensive two-dimensional supercritical fluid chromatography with conventional packed columns and FID detection. The first dimension was a silica gel column and the second dimension was an ODS column. This combination was largely orthogonal for the separation of fatty acid methyl esters. The first dimension separations were primarily based on the number of double bonds while the second dimension separations were based on the chain length. The highly-ordered chromatograms and improved resolution allowed the easy detection and identification of minor components. Although the first dimension separations were performed under isobaric conditions where the peak width increased in proportion to the retention, the programming of the sampling duration allowed us to maintain the optimum re-injection frequency (3–4 times) per peak into the second dimension and so to minimize the total analysis time without deteriorating the resolution. 相似文献
104.
Carlos Ariel Gil-Barrera Raymundo Santana Carrillo Guo-Hua Sun Shi-Hai Dong 《Entropy (Basel, Switzerland)》2022,24(5)
In this work, we study the quantum information entropies for two different types of hyperbolic single potential wells. We first study the behaviors of the moving particle subject to two different hyperbolic potential wells through focusing on their wave functions. The shapes of these hyperbolic potentials are similar, but we notice that their momentum entropy densities change along with the width of each potential and the magnitude of position entropy density decreases when the momentum entropy magnitude increases. On the other hand, we illustrate the behaviors of their position and momentum entropy densities. Finally, we show the variation of position and momentum entropies and with the change of the potential well depth u and verify that their sum still satisfies the BBM inequality relation. 相似文献
105.
106.
107.
108.
Snezhana I. Abarzhi Desmon L. Hill Annie Naveh Kurt C. Williams Cameron E. Wright 《Entropy (Basel, Switzerland)》2022,24(6)
Supernovae are explosions of stars and are a central problem in astrophysics. Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities develop during the star’s explosion and lead to intense interfacial RT/RM mixing of the star materials. We handle the mathematical challenges of the RT/RM problem based on the group theory approach. We directly link the conservation laws governing RT/RM dynamics to the symmetry-based momentum model, derive the model parameters, and find the analytical solutions and characteristics of RT/RM dynamics with variable accelerations in the linear, nonlinear and mixing regimes. The theory outcomes explain the astrophysical observations and yield the design of laboratory experiments. They suggest that supernova evolution is a non-equilibrium process directed by the arrow of time. 相似文献
109.
110.