全文获取类型
收费全文 | 589篇 |
免费 | 21篇 |
国内免费 | 65篇 |
专业分类
化学 | 403篇 |
力学 | 14篇 |
综合类 | 5篇 |
数学 | 1篇 |
物理学 | 252篇 |
出版年
2023年 | 2篇 |
2021年 | 11篇 |
2020年 | 3篇 |
2019年 | 9篇 |
2018年 | 8篇 |
2017年 | 5篇 |
2016年 | 16篇 |
2015年 | 16篇 |
2014年 | 13篇 |
2013年 | 24篇 |
2012年 | 43篇 |
2011年 | 28篇 |
2010年 | 38篇 |
2009年 | 34篇 |
2008年 | 35篇 |
2007年 | 41篇 |
2006年 | 25篇 |
2005年 | 22篇 |
2004年 | 38篇 |
2003年 | 28篇 |
2002年 | 29篇 |
2001年 | 18篇 |
2000年 | 31篇 |
1999年 | 14篇 |
1998年 | 19篇 |
1997年 | 25篇 |
1996年 | 27篇 |
1995年 | 19篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1986年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有675条查询结果,搜索用时 15 毫秒
41.
化学计量学-同步荧光光谱法同时测定速灭威、残杀威和呋喃丹 总被引:2,自引:0,他引:2
在pH 4.0的B-R缓冲溶液中,速灭威、残杀威和呋喃丹能够产生内源荧光,但光谱严重重叠。当波长差Δλ=30 nm时,加入一定量的β-环糊精,3种农药的荧光强度均有不同程度的加强,且荧光强度与农药的浓度呈良好的线性关系,速灭威、残杀威和呋喃丹单组分的测量线性范围分别为0.2~2.0、0.02~0.38和0.04~0.56μg/mL;检出限分别为0.083、0.015和0.020μg/mL。应用多种化学计量学方法,如经典最小二乘(CLS)、主成分回归(PCR)和偏最小二乘(PLS)对同步荧光光谱严重重叠的3种农药混合体系合成样进行光谱解析并比较其分析能力。结果表明,PLS预报各组分的结果较好,采用该模型分析了一些实际食品样品,获得满意的结果。 相似文献
42.
通过对香豆素343(C343)在不同溶剂中的稳态吸收光谱、稳态荧光光谱和时间分辨荧光光谱的分析,研究了溶剂对C343的光谱性质的影响,并获得了光谱特性与溶剂极性之间的依赖关系. 吸收光谱峰值的红移随着溶剂极性的增加而发生较小的变化. 然而,荧光光谱的峰值对溶剂的极性却很敏感,并随着溶剂极性参数f(ε,n)的增加呈线性增长. 这是由于C343激发态电荷分布的变化导致了它在极性溶剂中第一激发单重态能级的变化. 用溶剂效应测量法和量子化学计算方法确定了C343最低激发态的偶极矩,这两方法所得的结果一致. C343在不同溶剂中的时间分辨荧光光谱研究表明荧光寿命随着溶剂极性的增加而增加,即从甲苯溶液的3.09 ns线性地增加到水溶液中4.45 ns;荧光寿命延长的根源可归因于C343与氢键给体溶剂之间的分子间氢键相互作用. 相似文献
43.
多波长线性回归荧光法同时测定苯酚,间苯二酚和间苯三酚 总被引:4,自引:0,他引:4
酚类物质是环保、卫生、医药、食品等行业分析的重要指标,通常只能测定酚的总量[1~3],最小二乘法[4]、卡尔曼滤波法[5]虽能同时测定各量,但需要用计算机解谱,且只进行了苯酚和间苯二酚两组分的测定.对于结构相似的苯酚、间苯二酚和间苯三酚三组分混合体系... 相似文献
44.
We present a framework aimed to reveal directed interactions of activated brain areas using time-resolved fMRI and vector autoregressive (VAR) modeling in the context of Granger causality. After describing the underlying mathematical concepts, we present simulations helping to characterize the conditions under which VAR modeling and Granger causality can reveal directed interactions from fluctuations in BOLD-like signal time courses. We apply the proposed approach to a dynamic sensorimotor mapping paradigm. In an event-related fMRI experiment, subjects performed a visuomotor mapping task for which the mapping of two stimuli (“faces” vs “houses”) to two responses (“left” or “right”) alternated periodically between the two possible mappings. Besides expected activity in sensory and motor areas, a fronto-parietal network was found to be active during presentation of a cue indicating a change in the stimulus-response (S-R) mapping. The observed network includes the superior parietal lobule and premotor areas. These areas might be involved in setting up and maintaining stimulus-response associations. The Granger causality analysis revealed a directed influence exerted by the left lateral prefrontal cortex and premotor areas on the left posterior parietal cortex. 相似文献
45.
The influence of the water concentration in the water—n-octane—sodium dodecyl sulfate—n-pentanol microemulsions on the polarity index I
1/I
3, the efficiency of formation of the fluorescent probe (pyrene) excimers I
ex/I
m, and the conductivity of the system in a wide interval of water—oil ratios was studied. Analysis of changes in the polarity index shows that in all types of the microemulsions pyrene is solubilized in the hydrophobic part of the water—oil interface between the surfactant hydrocarbon radicals. Depending on the water—oil ratio, the site of its localization changes, and the effective permittivity of the medium eff varies from 5 to 11. Variations of the conductivity and polarity index allow one to observe three structures in the mucroemulsion, viz., water—oil, bicontinual region, and oil—water. Variation of the I
ex/I
m ratio reveals only the transition from the water—oil microemulsion to the oil—water microemulsion. 相似文献
46.
Ressler T 《Analytical and bioanalytical chemistry》2003,376(5):584-593
Time-resolved X-ray absorption spectroscopy (TR-XAS) possesses excellent capabilities to reveal quantitative phase composition and average valence together with the evolution of the local structure of a system under dynamic reaction conditions. The work discussed here focused on time-resolved in-situ XAS investigations aiming, first, at understanding structural evolution under dynamic conditions and, second, at revealing properties of the system studied not available from investigations under stationary conditions. Hence, not only was the local structure of a material studied under reaction conditions, but characteristic properties of the reaction, such as reaction intermediates or the kinetics of the reaction, were also elucidated. The solid–gas reactions presented here clearly demonstrate the potential of TR-XAS investigations to extend the suitability of XAS for in-situ studies in solid-state chemistry to investigations under dynamic conditions. 相似文献
47.
48.
The photoluminescence (PL) at low temperature of three delta-doped AlGaAs/GaAs heterostructures is investigated under continuous and pulsed excitations. The PL under continuous excitations allows the identification of the trapping centres and probes the carrier's transfer to the GaAs channel. The time-resolved photoluminescence (TRPL) inquires about carrier dynamics and gives radiative lifetimes of different levels. The DX level shows two time constants of the intensity decay relating the splitting of the valence band under impurity strains which reduce the crystal symmetry. The two time constants evolve with temperature and exhibit an increase near T=50 K. 相似文献
49.
The effect of acceptor concentration on the energy transfer from Coumarin 30 (donor) to Rhodamine 6G (acceptor) has been studied.
The nature of energy transfer reaction has been studied through lifetime measurements by recording the time-resolved fluorescence
decay curves. The energy transfer parameters calculated were used to confirm the occurrence of energy transfer on the basis
of the emission-reabsorption effect. 相似文献
50.
In situ simultaneous determination the photolysis of multi-component PAHs adsorbed on the leaf surfaces of living Kandelia candel seedlings 总被引:1,自引:0,他引:1
A fiber-optic fluorimetry for in situ simultaneous determination of fluorine (Flu), phenanthrene (Phe) and fluoranthene (Fla) adsorbed on the leaf surfaces of Kandelia candel (Kc) seedlings was developed. Experimental results showed that the linear ranges for determination of Flu, Phe and Fla adsorbed on Kc leaves were 35-700, 5-900 and 2-450 ng/spot, respectively. The detection limits for Flu, Phe and Fla were 9.11, 1.65 and 0.90 ng/spot and with the relative standard deviations less than 10.32%, 7.56% and 4.29% (n = 9), respectively. The recovery results for Flu, Phe and Fla adsorbed on Kc leaves were 83.0-91.2, 78.5-88.5 and 91.5-107.3%, respectively. Under the laboratory experimental conditions, the photolysis processes of Flu, Phe and Fla individual and in mixtures adsorbed on the leaf surfaces of living Kc seedlings were studied. Results showed that the photolysis of Flu, Phe and Fla individual and in mixtures adsorbed on the leaf surfaces of Kc seedlings followed first-order kinetics with photolysis rates in the order of Flu > Fla > Phe on the Kc leaves. An antagonistic effect was found when the three polycyclic aromatic hydrocarbons (PAHs) were co-adsorbed on living Kc seedlings. The experimental results also indicated that photolysis was the main transformation pathway for Flu, Phe and Fla both individual and in mixtures adsorbed on Kc leaves, whereas disappearance of the adsorbed Flu, Phe and Fla as a result of volatilization and leaf absorption could be negligible during the experimental period. 相似文献