全文获取类型
收费全文 | 4294篇 |
免费 | 1318篇 |
国内免费 | 336篇 |
专业分类
化学 | 731篇 |
晶体学 | 49篇 |
力学 | 778篇 |
综合类 | 98篇 |
数学 | 491篇 |
物理学 | 3801篇 |
出版年
2024年 | 19篇 |
2023年 | 36篇 |
2022年 | 102篇 |
2021年 | 120篇 |
2020年 | 130篇 |
2019年 | 117篇 |
2018年 | 134篇 |
2017年 | 173篇 |
2016年 | 205篇 |
2015年 | 186篇 |
2014年 | 369篇 |
2013年 | 300篇 |
2012年 | 294篇 |
2011年 | 318篇 |
2010年 | 255篇 |
2009年 | 265篇 |
2008年 | 296篇 |
2007年 | 285篇 |
2006年 | 295篇 |
2005年 | 268篇 |
2004年 | 208篇 |
2003年 | 192篇 |
2002年 | 164篇 |
2001年 | 182篇 |
2000年 | 143篇 |
1999年 | 129篇 |
1998年 | 119篇 |
1997年 | 102篇 |
1996年 | 96篇 |
1995年 | 78篇 |
1994年 | 62篇 |
1993年 | 37篇 |
1992年 | 41篇 |
1991年 | 50篇 |
1990年 | 26篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 13篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1971年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有5948条查询结果,搜索用时 18 毫秒
91.
92.
烷基极化效应与X=O键伸缩振动频率 总被引:2,自引:0,他引:2
烷基取代物R’X=0的X=0键伸缩振动频率ν与烷基R的极化效应指数PEI(R)的关系可表示为:ν=a bPEI(R)。研究结果表明,烷基的极化效应使X=0键的伸缩振动频率降低。 相似文献
93.
Summary Four most frequently used presentation forms of the ECD response are described. Equations describing the dependence of the
sensitivity, linearity and minimum detectable concentration of the ECD working under constant frequency mode of operation
on the pulse period are derived from Wenthworth’s kinetic model for all four forms mentioned above. The forms presented are
compared on the basis of equations derived. Results of the calculations are given and some predictions are experimentally
tested for sulphur hexafluoride. Good agreement was found between the predicted and observed relationships.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
94.
提出了控制电位电解和恒电流电解两类电解沉积的压电频移分析法测定的理论公式,用实验结果验证了它的正确性. 相似文献
95.
96.
Using geometry optimization and DFT method at the B3LYP/6-31G* level of theory for C30H20, an equilibrium geometry is identified that has the form of polyhedral hydrocarbon with five carbon–carbon single bonds linking a dodecahedrane cage and a pentaprismane cage. Thus, this molecule is a tri-cage molecule with two pentaprismane cages and one dodecahedrane cage. Vibrational frequencies and the infrared spectrum are computed at the same level of theory. The heat of formation for C30H20 has been estimated in this paper. The heat of formation of C30H20 as well as the vibrational analysis indicates that this molecule enjoys sufficient stability to allow for its experimental preparation. 相似文献
97.
A relationship between the X-H (X = N, O, C, and so on) equilibrium bond length in a Morse oscillator and the X-H stretching overtone frequency shifts is obtained theoretically. We use the equation to discuss the empirical linear relationships that have been proposed for heterocyclics, alkanes and fluorinated benzenes. On the other hand, a unified relationship between the X-H bond angles and the experimental quantities (ω(?) and the coupling strength λ) is also presented for XH2, XH, and XH4 molecules or molecular fragments. Calculations of X-H bond angles for a number of molecules show that the results from our equations are in excellent agreement with the experimental values. Also we can extract the information of relative magnitude of bond coupling force field. 相似文献
98.
Uchida K Saito M Murakami A Kobayashi T Nakamura S Irie M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(2):534-542
A photochromic polymer film containing three different diarylethene derivatives, that is, 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyclopentene (1), 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2), and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (3) was prepared. Upon UV irradiation, the three derivatives changed to their closed-ring isomers having different colors, yellow, red, and blue. They showed different spectra not only in UV/Vis region but also in the IR spectral region. Upon irradiation with visible light of appropriate wavelengths, each closed-ring isomer was selectively bleached, and three bits eight states recording was performed. The eight states could be read out nondestructively by using IR light of appropriate wavenumbers. 相似文献
99.
Polymers containing bicarbazolyl moieties in the main chain have been synthesized by the modified Ullmann coupling reaction from 9H,9′H-[3,3′]bicarbazolyl and different dihalo derivatives. The number-average molecular weights of the polymers synthesized were in the range of 2500-6200 with a molecular weight distribution of 1.6-3.1. Well-defined model compounds for the polymers have been synthesized by stepwise reactions. All these compounds have been found to form glasses with glass transition temperatures in the range of 57-119 °C as characterised by differential scanning calorimetry. The electron photoemission spectra of the compounds have been recorded and the ionisation potentials of 5.35-5.4 eV have been established. Room temperature hole drift mobility of the synthesized compounds molecularly dispersed in a polymer host range from 10−6 to 3 × 10−5 cm2/V s at an electric field of 106 V/cm at the room temperature. 相似文献
100.
Digrazia Philip M. Blackburn James W. Bienkowski Paul R. Hilton Barry Reed Gregory D. King J. M. Henry Sayler Gary S. 《Applied biochemistry and biotechnology》1990,(1):237-252
An experimental and mathematical method is developed for the microbial systems analysis of polyaromatic hydrocarbon (PAH)-degrading
mixed cultures in PAH-contaminated “town gas” soil systems. Frequency response is the primary experimental and data analysis
tool used to probe the structure of these complicated systems. The objective is to provide a fundamental protocol for evaluating
the performance of specific mixed microbial cultures on specific soil systems by elucidating the salient system variables
and their interactions. Two well-described reactor systems, a constant volume stirred tank reactor (CSTR) and a plug flow
differential volume reactor, are used in order to remove performance effects that are related to reactor type as opposed to
system structure. These two reactor systems are well-defined systems that can be described mathematically and represent the
two extremes of one potentially important system variable, macroscopic mass transfer. The experimental and mathematical structure
of the protocol is described, experimental data is presented, and data analysis is demonstrated for the stripping, sorption,
and biodegradation of napththalene. 相似文献