全文获取类型
收费全文 | 879篇 |
免费 | 23篇 |
国内免费 | 11篇 |
专业分类
化学 | 580篇 |
晶体学 | 3篇 |
力学 | 37篇 |
数学 | 103篇 |
物理学 | 190篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 12篇 |
2020年 | 14篇 |
2019年 | 11篇 |
2018年 | 10篇 |
2017年 | 7篇 |
2016年 | 18篇 |
2015年 | 19篇 |
2014年 | 26篇 |
2013年 | 36篇 |
2012年 | 62篇 |
2011年 | 91篇 |
2010年 | 38篇 |
2009年 | 39篇 |
2008年 | 55篇 |
2007年 | 40篇 |
2006年 | 49篇 |
2005年 | 47篇 |
2004年 | 44篇 |
2003年 | 30篇 |
2002年 | 26篇 |
2001年 | 21篇 |
2000年 | 34篇 |
1999年 | 19篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1935年 | 4篇 |
排序方式: 共有913条查询结果,搜索用时 15 毫秒
791.
Wen-Yi Tseng Yu-Huei Chen Ilias I. Khairullin Soofin Cheng Lian-Pin Hwang 《Solid state nuclear magnetic resonance》1997,8(4):219-229
A solid complex of C60 with γ-cyclodextrin (γ-CyD) was examined with NMR spectroscopic methods in order to understand the dynamics of C60, and the interaction between C60 and γ-CyD. A 13C solid-state cross-polarization magic angle spinning (CP/MAS) NMR spectra shows C60 resonance at 142.6 ppm. This provides the evidence of interaction between 13C spins in C60 and 1H spins in the γ-CyD host. Ambient temperature experiments on the 13C CP/MAS NMR, with varying contact time, shows that the water associated with γ-CyDs plays an important role in the nuclear relaxation processes. The dynamics of C60 in γ-CyD was investigated using temperature and field-dependent 13C spin-lattice relaxation time measurements. The influence of water on the dynamics of C60 was less significant below 250 K. 相似文献
792.
This expository article is motivated by two well-known antinomies. The first is the extended Zeno paradox concerning two persons playing at a ball, passing theball to and fro within 1/2,1/4,1/8,…, minutes successively, and questioning the place of the ball at the end of one minute. The second antinomy is that of Engels concerning the real infinitude of successively generated finite ordinals. In order to explain away or give answer to these two antinomies, we have constructed a kind of non-Cantorian model for the sequence of natural numbers by the aid of Van Osdol-Takahashi's ultrapower (extended real number field) *R. In what follows are a few definitions and some propositions discussed in this article. 相似文献
793.
794.
The reaction of 4,4′‐biphenol and two species of bromoalkanes (e.g., bromoethane and 1‐bromobutane) to synthesize two symmetric products (4,4′‐diethanoxy biphenyl and 4,4′‐dibutanoxy biphenyl) and one asymmetric product (4‐ethanoxy, 4′‐butanoxy biphenyl) was successfully carried out under two‐phase phase‐transfer catalysis conditions. A rational mechanism and kinetic model were built up by considering the reactions both in aqueous phase and in organic phase. The first active catalyst (QO(Ph)2OQ) was also synthesized under two‐phase reaction and was identified by instruments. The experimental data were explained satisfactorily by the pseudo‐steady‐state hypothesis. Two sets of rate constants of organic reactions, i.e. primary (k1 and k2) and secondary (k11, k12, k21, and k22) rate constants participate in the kinetic model. The two primary rate constants were obtained individually via experimental data for synthesizing the symmetric products. The ratios of the other four secondary rate constants were obtained from the reaction of synthesizing asymmetric products and determined from the initial yield rates of symmetric products. The effects of the ratio of bromoethane and 1‐bromobutane, temperature, organic solvents, amount of catalyst, and amount of sodium hydroxide on the reaction rate and the selectivity of products were investigated in detail. The results were explained satisfactorily by the interaction between the reactants and the environmental species. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 139–153, 2003 相似文献
795.
Starting from 9-oxo-fluorene-1-carbaldehyde, the title compound, XII, a trioxoderivative of a new biangular diindenofluorene system, has been synthesized in 5 steps (overall yield 37,5%). The corresponding hydrocarbon could not be obtained by reduction of XII. Attempts to synthesize the isomer XX of the trioxoderivative XII are described. 相似文献
796.
Jenn-Yuan Sheu Hsiao-Pei Ku Wei-Chang Tseng Min-Tzu Chen Li-Yu Tsai Yeou-Lih Huang 《Analytical sciences》2003,19(4):621-624
An on-line analytical system for the continuous monitoring of malondialdehyde (MDA) was developed. This method involves the use of microdialysis perfusion, on-line derivatization and on-line HPLC analysis. This method gave a linear response for MDA concentrations and HPLC peak areas in the range from 0.051 microM to 2.43 microM. The intra-day (RSD = 1.6-10.5%) and inter-day (RSD = 1.1-9.3%) precisions were acceptable. The average in vitro probe recovery of MDA standard was 18.4 +/- 1.0%. The detection limit was 0.03 microM, corresponding to 0.6 pmol for an injection volume of 20 microl. This method was used for in vitro peroxidation investigations, which provided evidence for elevated MDA levels following the incubation of metal ions to a linoleic acid solution. 相似文献
797.
Isopropylation of naphthalene over modified-mordenite zeolites 总被引:2,自引:0,他引:2
Isopropylation of naphthalene with propylene was carried out over Ce-, Si-, La-, and Mg-modified mordenite zeolites. The reaction
was performed in a 500 mL batch reactor at 300°C, 8 kg/cm2, and naphthalene to catalyst weight ratio of 10:1, with excess propylene. It was attempted to eliminate the external acid
sites of zeolites by these modifications. The results indicate that the CVD silanation and magnesium or cerium modification
with Mg/Al or Ce/Al atomic ratios less than 0.5 can improve the 2,6-DIPN selectivity of mordenites. The improvement of catalyst
performance is ascribed to the selective deactivation of acid sites on the external surface of HM zeolites. 相似文献
798.
S. J. Yeh C. L. Tseng J. C. Wei 《Journal of Radioanalytical and Nuclear Chemistry》1991,151(1):131-138
It was found that trace amount of various ions could be effectively adsorbed onto hydrous magnesium oxide from aqueous solution. Attempts were made to investigate the feasibility of using hydrous magnesium oxide as preconcentration agent for neutron activation analysis (NAA). The procedure was successfully applied for the measurements of trace amounts of elements in the surface water of a number of large rivers which play very important roles in the lives of local people. Some of them are polluted rather badly. In order to improve the living standards it is rather important to establish reliable analytical methods so that more effective surveillance may be enforced on pollution control problems. NAA is generally accepted as one of the most suitable methods for simultaneous and multielemental determination of trace amounts of elements. In this work polluted surface waters were investigated by NAA using Tsing Hua Open-pool Reactor (THOR). 相似文献
799.
Shin-Shyong Tseng Joseph W. Epstein Herbert J. Brabander Gerardo Francisco 《Journal of heterocyclic chemistry》1987,24(3):837-843
2-Aminobenzimidazoles were reacted with enaminones in acetic acid to give pyrimido[1,2-a]benzimidazoles. With a substituted enaminone only one regioisomer was obtained. Structural assignments based on nmr and uv spectroscopy are presented. Possible pathways leading to the products are discussed. 相似文献
800.
We propose feasible descent methods for constrained minimization that do not make explicit use of the derivative of the objective
function. The methods iteratively sample the objective function value along a finite set of feasible search arcs and decrease
the sampling stepsize if an improved objective function value is not sampled. The search arcs are obtained by projecting search
direction rays onto the feasible set and the search directions are chosen such that a subset approximately generates the cone
of first-order feasible variations at the current iterate. We show that these methods have desirable convergence properties
under certain regularity assumptions on the constraints. In the case of linear constraints, the projections are redundant
and the regularity assumptions hold automatically. Numerical experience with the methods in the linearly constrained case
is reported.
Received: November 12, 1999 / Accepted: April 6, 2001?Published online October 26, 2001 相似文献