全文获取类型
收费全文 | 7993篇 |
免费 | 259篇 |
国内免费 | 61篇 |
专业分类
化学 | 5316篇 |
晶体学 | 41篇 |
力学 | 277篇 |
数学 | 1593篇 |
物理学 | 1086篇 |
出版年
2023年 | 47篇 |
2022年 | 113篇 |
2021年 | 189篇 |
2020年 | 155篇 |
2019年 | 162篇 |
2018年 | 139篇 |
2017年 | 114篇 |
2016年 | 276篇 |
2015年 | 234篇 |
2014年 | 240篇 |
2013年 | 445篇 |
2012年 | 531篇 |
2011年 | 594篇 |
2010年 | 361篇 |
2009年 | 353篇 |
2008年 | 518篇 |
2007年 | 486篇 |
2006年 | 488篇 |
2005年 | 412篇 |
2004年 | 353篇 |
2003年 | 283篇 |
2002年 | 292篇 |
2001年 | 102篇 |
2000年 | 73篇 |
1999年 | 67篇 |
1998年 | 79篇 |
1997年 | 87篇 |
1996年 | 98篇 |
1995年 | 59篇 |
1994年 | 82篇 |
1993年 | 56篇 |
1992年 | 57篇 |
1991年 | 43篇 |
1990年 | 54篇 |
1989年 | 48篇 |
1988年 | 42篇 |
1987年 | 47篇 |
1986年 | 33篇 |
1985年 | 46篇 |
1984年 | 65篇 |
1983年 | 41篇 |
1982年 | 45篇 |
1981年 | 31篇 |
1980年 | 40篇 |
1979年 | 29篇 |
1978年 | 30篇 |
1977年 | 27篇 |
1975年 | 14篇 |
1974年 | 15篇 |
1973年 | 14篇 |
排序方式: 共有8313条查询结果,搜索用时 62 毫秒
131.
The technique of bead injection has been adapted for surface enhanced Raman scattering (SERS) to substantially improve precision, long term stability and sensitivity of SERS detection in analytical chemistry. For this purpose a fully automated flow system comprising a dedicated flow-cell has been developed and tested. With the developed flow-cell, which contains two inlet and two outlet channels, it is possible to retain, perfuse and discharge minute amounts of polymer beads while monitoring all steps by Raman spectroscopy. First, beads carrying cation exchanger moieties were retained in the flow-cell and subsequently perfused with a silver nitrate and a hydroxylamine solution using one inlet of the flow cell. By this sequence homogeneous SERS active silver layers were formed on the beads. The uniformity of the achieved silver layer was studied by secondary electron microscopy. For measurement, the analyte was then introduced from the second inlet channel such that the interaction between the activated SERS beads and analyte occurred in close proximity and within the focus of the laser excitation beam. Due to the complete computer control of all experimental steps, including bead entrapment, SERS layer generation, sample introduction and final bead removal, highly reproducible conditions for SERS were achieved. The method was developed using 9-aminoacridine as a test molecule. Quantitative studies were carried out for 9-aminoacridine and acridine showing linear calibrations from 1-100 nmol l(-1) and 50-1,000 nmol l(-1), respectively, using a sample volume of 200 microl each. Typical relative standard deviations were 4.7% for 9-aminoacrine and 5.8% for acridine. 相似文献
132.
Regás D Afonso MM Rodríguez ML Palenzuela JA 《The Journal of organic chemistry》2003,68(20):7845-7852
The reaction of vinyl allenes with imines under Lewis acid catalysis has been explored. Vinyl allenes in which the allenic portion of the molecule is tri- or tetrasubstituted gave octahydroquinoline derivatives as single isomers together with a minor compound formed by an ene reaction of the imine with the allene. Compounds in which the allene is 1,3-disubstituted do not react under the conditions assayed. 相似文献
133.
We study the dynamics in the neighborhood of an invariant torus of a nearly integrable system. We provide an upper bound to the diffusion speed, which turns out to be of superexponentially small size exp[-exp(1/)], being the distance from the invariant torus. We also discuss the connection of this result with the existence of many invariant tori close to the considered one. 相似文献
134.
Antonio Cipiciani Stefania Primieri Antonio Arcelli 《Journal of inclusion phenomena and macrocyclic chemistry》1994,15(3):207-214
The hydrolysis reaction of 2-nitro-4-carboxyphenyl acetate was studied in dichloromethane in the presence of an added electrolyte (NaOH) and cationic surfactants systems with varying quantities of added water at 25°C. The kinetic and conductivity data were correlated. 相似文献
135.
We describe a new branch-and-bound algorithm for the exact solution of the maximum cardinality stable set problem. The bounding phase is based on a variation of the standard greedy algorithm for finding a colouring of a graph. Two different node-fixing heuristics are also described. Computational tests on random and structured graphs and very large graphs corresponding to real-life problems show that the algorithm is competitive with the fastest algorithms known so far.This work has been supported by Agenzia Spaziale Italiana. 相似文献
136.
LetK be a field, charK=0 andM
n
(K) the algebra ofn×n matrices overK. If λ=(λ1,…,λ
m
) andμ=(μ
1,…,μ
m
) are partitions ofn
2 let
wherex
1,…,x
n
2,y
1,…,y
n
2 are noncommuting indeterminates andS
n
2 is the symmetric group of degreen
2.
The polynomialsF
λ, μ
, when evaluated inM
n
(K), take central values and we study the problem of classifying those partitions λ,μ for whichF
λ, μ
is a central polynomial (not a polynomial identity) forM
n
(K).
We give a formula that allows us to evaluateF
λ, μ
inM(K) in general and we prove that if λ andμ are not both derived in a suitable way from the partition δ=(1, 3,…, 2n−3, 2n−1), thenF
λ, μ
is a polynomial identity forM
n
(K). As an application, we exhibit a new class of central polynomials forM
n
(K).
In memory of Shimshon Amitsur
Research supported by a grant from MURST of Italy. 相似文献
137.
138.
139.
Fustero S Navarro A Díaz D de La Torre MG Asensio A Sanz F González ML 《The Journal of organic chemistry》1996,61(25):8849-8859
Reaction of azaenolates of 2-alkyl-oxa(thia)zolines 6 with imidoyl chlorides 7 as electrophiles to furnish masked N-substituted beta-enamino acid derivatives 1-2 in 70-90% yield is described. Alternative routes are discussed. Compounds 1-2 generally appear in one tautomeric form, imino or enamino, depending on the nature of the imidoyl chloride. The configuration of the enamino moiety (Z) and the conformation (s-cis) of compounds 1-2 obtained were established by an NMR study and unequivocally set by nuclear Overhauser effect difference experiments. An X-ray structure of compound 1e is also reported, showing a strong intramolecular NH.N hydrogen bond. Ab initio calculations (HF/3-21G and HF/3-21+G) have been carried out on several representative examples (1e, 1p, and 1l) in an attempt to support and provide the correct geometry of these derivatives. Structural considerations among the possible isomers of compounds 1 are discussed. From these studies it was concluded that the theoretical calculations agree with the experimental results. In addition, a very simple one-pot procedure for the preparation of masked N-substituted alpha-alkylated beta-enamino acid derivatives 2 from 6, 7, and different alkyl halides (R(3)Y) is described. 相似文献
140.
The synthesis of tetracyclic polyhydroxyquinones 5a and 5b was achieved through a sequence involving two Diels-Alder reactions with 1-methoxy-1,3-cyclohexadiene: the first with 2-(p-tolylthio)naphthazarin and the second on the resulting tricyclic derivative previously transformed into a (p-tolylsulfinyl)naphthazarin. The success of this strategy stemmed from the efficient remote regiocontrol exerted by the thioether substituent in the first step. 相似文献