首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   237篇
  免费   7篇
化学   127篇
晶体学   1篇
力学   22篇
数学   15篇
物理学   79篇
  2022年   2篇
  2020年   6篇
  2018年   5篇
  2017年   2篇
  2016年   6篇
  2015年   4篇
  2014年   7篇
  2013年   15篇
  2012年   14篇
  2011年   14篇
  2010年   11篇
  2009年   7篇
  2008年   5篇
  2007年   10篇
  2006年   5篇
  2005年   6篇
  2004年   5篇
  2003年   7篇
  2002年   2篇
  2001年   3篇
  2000年   2篇
  1999年   2篇
  1996年   1篇
  1995年   3篇
  1994年   3篇
  1993年   11篇
  1992年   6篇
  1991年   5篇
  1990年   1篇
  1989年   3篇
  1988年   3篇
  1987年   1篇
  1986年   1篇
  1985年   5篇
  1984年   2篇
  1983年   8篇
  1982年   5篇
  1981年   5篇
  1980年   7篇
  1979年   8篇
  1978年   1篇
  1977年   6篇
  1976年   6篇
  1974年   2篇
  1973年   1篇
  1972年   1篇
  1971年   2篇
  1970年   1篇
  1966年   1篇
  1965年   1篇
排序方式: 共有244条查询结果,搜索用时 31 毫秒
91.
A new effective interaction potential between an electron and a hole of a Mott-Wannier exciton in a polarizable medium is derived following a procedure similar to that of Haken. The variational polaron wave functions used are those first proposed by Haga. Using this effective interaction potential the values of the binding energy of an exciton are calculated variationally in several polar crystals. The values thus obtained agree very well with those derived from experimental measurements and are always considerably smaller than those calculated using Haken's potential.  相似文献   
92.
The B-A system of CP molecule has been reinvestigated. The rotational structures in the (0, 0) and (1, 1) bands have been photographed in the second order of a 10.6 m grating with 25μ slit width. The molecular constants have been determined using a weighted least squares fit computer program. It is found that the spin-splitting constant in the B-state is negative and not positive as reported by previous workers. An accurate value ofα in the A-state could be determined. The present analysis also suggests that the spin-splitting constant in the ground state of CP should be positive, hence the assignment of theR 1 andR 2 orP 1 andP 2 branches in the B-X system of CP by Barwaldet al should be interchanged.  相似文献   
93.
Engineered hemoproteins have recently emerged as promising systems for promoting asymmetric cyclopropanations, but variants featuring predictable, complementary stereoselectivity in these reactions have remained elusive. In this study, a rationally driven strategy was implemented and applied to engineer myoglobin variants capable of providing access to 1‐carboxy‐2‐aryl‐cyclopropanes with high trans‐(1R,2R) selectivity and catalytic activity. The stereoselectivity of these cyclopropanation biocatalysts complements that of trans‐(1S,2S)‐selective variants developed here and previously. In combination with whole‐cell biotransformations, these stereocomplementary biocatalysts enabled the multigram synthesis of the chiral cyclopropane core of four drugs (Tranylcypromine, Tasimelteon, Ticagrelor, and a TRPV1 inhibitor) in high yield and with excellent diastereo‐ and enantioselectivity (98–99.9% de; 96–99.9% ee). These biocatalytic strategies outperform currently available methods to produce these drugs.  相似文献   
94.
A series of isomeric bis(alkylthiocarbamate) copper complexes have been synthesized, characterized, and evaluated for antiproliferation activity. The complexes were derived from ligand isomers with 3-methylpentyl (H2L2) and cyclohexyl (H2L3) backbone substituents, which each yield a pair of linkage isomers. The thermodynamic products CuL2a/3a have two imino N and two S donors resulting in three five-member chelate rings (555 isomers). The kinetic isomers CuL2b/3b have one imino and one hydrazino N donor and two S donors resulting in four-, six-, and five-member rings (465 isomers). The 555 isomers have more accessible CuII/I potentials (E1/2=−811/−768 mV vs. ferrocenium/ferrocene) and lower energy charge transfer bands than their 465 counterparts (E1/2=−923/-854 mV). Antiproliferation activities were evaluated against the lung adenocarcinoma cell line (A549) and nonmalignant lung fibroblast cell line (IMR-90) using the MTT assay. CuL2a was potent (A549EC50=0.080 μM) and selective (IMR-90EC50/A549EC50=25) for A549. Its linkage isomer CuL2b had equivalent A549 activity, but lower selectivity (IMR-90EC50/A549EC50=12.5). The isomers CuL3a and CuL3b were less potent with A549EC50 values of 1.9 and 0.19 M and less selective with IMR-90EC50/A549EC50 ratios of 2.3 and 2.65, respectively. There was no correlation between reduction potential and A549 antiproliferation activity/selectivity.  相似文献   
95.
We report the synthesis of peroxo titanium carbonate complex solution as a novel water-soluble precursor for the direct synthesis of layered protonated titanate at room temperature. The synthesized titanates showed excellent removal capacity for Pb(2+) and methylene blue. Based on experimental observations, a probable mechanism for the formation of protonated layered dititanate sheets is also discussed.  相似文献   
96.
Ultra-narrow wurtzite ZnS quantum wires assembled towards an onion slice shape are synthesized in water using a simple hydrothermal procedure. Wires are formed through an oriented attachment mechanism from wurtzite ZnS spherical particles. Strong confinement effect originated from the narrow width was observed and holds promise for use in broad optoelectronic device application.  相似文献   
97.
Two different chitosan supported palladium based catalysts were prepared, wherein dispersed palladium nanoparticles were obtained via chemical reduction supported on chitosan (Pd/CTS) and amine functionalized modified chitosan (Pd/AFCTS). The catalytic activity of the Pd-based catalysts, Pd/CTS and Pd/AFCTS, were assessed in the hydrogenation of styrene oxide to 2-phenyl ethanol. Both Pd-based catalysts enhanced the formation of the desired 2-phenyl ethanol in contrast to a conventional Pd/C catalyst without the assistance of inorganic or organic base. A considerable influence on the conversion and selectivity was observed in the case of Pd/AFCTS, consisting of palladium nanoparticles stabilized and dispersed on amine-functionalized chitosan matrix, affording complete conversion of styrene oxide with 98% selectivity to 2-phenyl ethanol. The catalyst Pd/AFCTS has also been recycled without significant loss of activity and selectivity.  相似文献   
98.
JPC – Journal of Planar Chromatography – Modern TLC -  相似文献   
99.
外电场下极性量子阱中杂质态结合能   总被引:4,自引:3,他引:1       下载免费PDF全文
我们用变分方法研究了外电场下量子阱中的杂质态结合能,计算中既考虑了电子同体纵光学声子和界面光学声子的相互作用又考虑了杂质中心同体纵光学声子和界面光学声子的相互作用。我们以GaAs/Al0.3Ga0.7As量子阱为例,讨论了结合能随杂质位置、阱宽和电场强度的变化规律。得到了电子-声子相互作用对杂质态结合能和斯塔克效应的修正是相当明显的。  相似文献   
100.
A novel microresonator operating on the principle of nonlinear modal interactions due to autoparametric 1:2 internal resonance is introduced. Specifically, an electrostatically actuated pedal-microresonator design, utilizing internal resonance between an out-of-plane torsional mode and a flexural in-plane vibrating mode is considered. The two modes have their natural frequencies in 1:2 ratio, and the design ensures that the higher frequency flexural mode excites the lower frequency torsional mode in an autoparametric way. A Lagrangian formulation is used to develop the dynamic model of the system. The dynamics of the system is modeled by a two degrees of freedom reduced-order model that retains the essential quadratic inertial nonlinearities coupling the two modes. Retention of higher-order model for electrostatic forces allows for the study of static equilibrium positions and static pull-in phenomenon as a function of the bias voltages. Then for the case when the higher frequency flexural mode is resonantly actuated by a harmonically varying AC voltage, a comprehensive study of the response of the microresonator is presented and the effects of damping, and mass and structural perturbations from nominal design specifications are considered. Results show that for excitation levels above a threshold, the torsional mode is activated and it oscillates at half the frequency of excitation. This unique feature of the microresonator makes it an excellent candidate for a filter as well as a mixer in RF MEMS devices.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号