全文获取类型
收费全文 | 1028篇 |
免费 | 34篇 |
国内免费 | 6篇 |
专业分类
化学 | 636篇 |
晶体学 | 9篇 |
力学 | 20篇 |
数学 | 105篇 |
物理学 | 298篇 |
出版年
2023年 | 6篇 |
2022年 | 12篇 |
2021年 | 15篇 |
2020年 | 13篇 |
2019年 | 11篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 34篇 |
2015年 | 22篇 |
2014年 | 16篇 |
2013年 | 49篇 |
2012年 | 55篇 |
2011年 | 53篇 |
2010年 | 28篇 |
2009年 | 30篇 |
2008年 | 77篇 |
2007年 | 81篇 |
2006年 | 45篇 |
2005年 | 51篇 |
2004年 | 52篇 |
2003年 | 30篇 |
2002年 | 40篇 |
2001年 | 26篇 |
2000年 | 26篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 14篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有1068条查询结果,搜索用时 15 毫秒
921.
Organomagnesium‐Catalyzed Isomerization of Terminal Alkynes to Allenes and Internal Alkynes 下载免费PDF全文
Dr. Raphaël Rochat Koji Yamamoto Michael J. Lopez Haruki Nagae Dr. Hayato Tsurugi Prof. Dr. Kazushi Mashima 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8112-8120
Organomagnesium complexes 2 were synthesized from N,N‐dialkylamineimine ligands 1 and dibenzylmagnesium by benzylation of the imine moiety. 3‐Aryl‐1‐propynes reacted with 2 to form the corresponding tetraalkynyl complexes, which acted as catalysts for the transformation of these terminal alkynes into allenes and further to internal alkynes under mild conditions. To the best of our knowledge, this example is the first of an organomagnesium‐catalyzed isomerization of alkynes. Notably, the reactions proceeded through temporally separated autotandem catalysis, thus allowing the isolation of the allene or internal alkyne species in good yields. Mechanistic experiments suggested that the catalytically active tetraalkynyl complexes consist of a tautomeric mixture of alkynyl‐, allenyl‐, and propargylmagnesium species. 相似文献
922.
Christopher E. Rivas Jose C. Alvarado-Monzon Dr. Gerardo Gonzalez-Garcia Dr. J. Oscar C. Jimenez-Halla Jesus Rangel-Garcia Dr. Crispin Cristobal Dr. Jorge A. Lopez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(33):8468-8472
Iridabicycles [Ir{κ3-N,C,O-(pyC(H)=C(C(O)Me)2}(Cl)(L−L)](L−L=cod (cod=1,5-cyclooctadiene), 1 a ; bipy (bipy=2,2’-bipyridine), 1 b ) have been obtained by oxidative coordination of 3-(pyridine-2-yl-methylene)pentane-2,4-dione L1 , to the complexes [{Ir(μ-Cl)(cod)}2] and [{Ir(μ-Cl)(coe)2}2] (coe=cis-cyclooctene), the latter in the presence of bipy. Remarkably, cleavage of the C3−C(O)Me bond of L1 has instead been achieved in the reaction with [Ir(Cl)(dmb)2] (dmb=2,3-dimethylbutadiene), yielding a compound formulated as [Ir{κ2-N,C-(pyC(H)C(C(O)Me))}(CO)(μ-Cl)(Me)]2, 2 . Treatment of dimer 2 with DMSO or PMe3 produced the complexes[Ir{κ2-N,C-(pyC(H)C(C(O)Me)}(CO)Cl(Me)L] (L=DMSO, 3 a ; PMe3, 3 b ). Plausible mechanisms for the reactions leading to complexes 1 and 2 are proposed by means of DFT calculations. 相似文献
923.
Fluorescence and Absorption Contrast Mechanisms for Biomedical Optical Imaging Using Frequency-Domain Techniques 总被引:1,自引:0,他引:1
E. M. Sevick-Muraca G. Lopez J. S. Reynolds T. L. Troy C. L. Hutchinson 《Photochemistry and photobiology》1997,66(1):55-64
Abstract— The ability to optically image or detect diseased tissue volumes located deep within tissues depends upon the degree of contrast provided by differences in local optical properties. In this report, we show that the exogenous contrast offered by fluorescent compounds is superior to that provided by nonfluorescing, light-absorbing compounds when time-dependent measurements are employed. In addition, we show that the induced contrast is not only moderated by the preferential uptake of fluorescent agents into diseased tissue volumes of interest but also by the fluorescent optical properties and the fluorescence dynamics in the specific tissue volume. Using tissue phantom studies, we demonstrated experimentally that near-infrared-absorbing and fluorescent dyes such as in-docyanine green can provide detection of diseased tissue volumes from fluorescence measurements made at the periphery of tissue when there is perfect, 100-fold and 10-fold partitioning in diseased tissues over that in surrounding normal tissues. Experimental results of common laser dyes show the contrast is also mediated by the quantum yield and lifetime parameters that may be dependent upon the local tissue environment. 相似文献
924.
A. Lopez Molinero Alberto Morales Anselmo Villareal Juan R. Castillo 《Fresenius' Journal of Analytical Chemistry》1997,358(5):599-603
The generation of volatile species of silicon as a means to introduce silicon into an inductively coupled plasma has been
studied. It is based on the reaction between silicon and fluoride ions in sulfuric acid media and it was carried out using
different flow injection mountings. The first mounting works with an injection of 100 μL concentrated sulfuric acid and 150
μL silicon standard solution in a continuous 0.05 mol L–1 NaF solution flow. The method shows a linear response between the intensity of emission at 251.611 nm line and the silicon
concentration from 0.1 to 200 μg mL–1, with a reproducibility of 2% and a detection limit of 0.004 μg mL–1. The second mounting produces the volatile species by the reaction between two opposed aerosol flows in a home-made nebulization
chamber. This chamber has a Cross-Flow and a Meinhard nebulizer at either end. A linear response ranging from 0.1 to 1000
μg mL–1 of silicon solution is obtained and the reproducibility rises to 8%.The detection limit reached is 0.02 μg mL–1. The silicon content in real water samples was determined by applying both the above-mentioned methods and a third method
for reference.
Received: 6 November 1996 / Revised: 19 December 1996 / Accepted: 3 January 1997 相似文献
925.
The changes in electronic structure of the tetracyanoplatinate complex accompanying the transition [Pt(CN)2?4]n(insulator) + oxidant → [Pt(CN)2 ?q+4]n(conductor) have been investigated with a cluster model approach. Calculations for trimer (n = 3) cluster models of the insulator and conductor have been performed with the self-consistent-field Xα Dirac scattered-wave method. Based on the cluster electronic structure, we infer that the valence band edge in the insulator is not dzz like, but consists of metal (dδ + dπ) and ligand character. Partial oxidation of this band results in a shortening of the inter-stack distance, leading in turn to a partially occupied dzz-like valence band. Thus electron transport occurs along the Pt backbone. 相似文献
926.
We study a two-dimensional semi-totalistic binary cell-state cellular automaton, which imitates a reversible precipitation in an abstract chemical medium. The systems exhibits a non-trivial growth and nucleation. We demonstrate how basic computational operation can be realized in the system when the propagation of the growing patterns is self-restricted by stationary localizations. We show that precipitating patterns of different morphology compete between each other and thus implement basic logical gates. 相似文献
927.
Genaro Lopez Victoria Martin Hong-Kun Xu 《Nonlinear Analysis: Real World Applications》2009,10(4):2369-2383
Perturbation techniques for nonexpansive mappings are studied. An iterative algorithm involving perturbed mappings in a Banach space is proposed and proved to be strongly convergent to a fixed point of the original mapping. These techniques are applied to solve the split feasibility problem and the multiple-sets split feasibility problem, and to find zeros of accretive operators. 相似文献
928.
We analyse the asymptotic behaviour of the solution of a 3Dsingularly perturbed convection–diffusion problem withdiscontinuous Dirichlet boundary data defined in a cuboid. Wewrite the solution in terms of a double series and we obtainan asymptotic approximation of the solution when the singularparameter 0. This approximation is given in terms of a finitecombination of products of error functions and characterizesthe effect of the discontinuities on the small -behaviour ofthe solution in the singular layers. 相似文献
929.
We describe a new method for determining the homogeneous upconversion coefficient by using only three measurements: two fluorescence decays as a function of time and the relationship of normalized fluorescence with excitation power. This technique results in increased accuracy upon employment of a smaller number of measurements. Specifically, the intermediate steps of finding the absorption cross section of the doped material, the emission spectrum of the pumping source, and its spatial power distribution are not necessary. Our technique employs only one experimental setup: changing the pumping conditions. Furthermore, it incorporates dual-function fitting to reduce the uncertainty and error propagation. This method will find ample applications in the study and characterization of erbium-doped materials used in lasers and optical amplifiers, where precise knowledge of efficiency and losses is of uppermost importance. 相似文献
930.