全文获取类型
收费全文 | 653918篇 |
免费 | 5308篇 |
国内免费 | 1810篇 |
专业分类
化学 | 337965篇 |
晶体学 | 9291篇 |
力学 | 31443篇 |
综合类 | 21篇 |
数学 | 85699篇 |
物理学 | 196617篇 |
出版年
2021年 | 5743篇 |
2020年 | 6357篇 |
2019年 | 7114篇 |
2018年 | 9572篇 |
2017年 | 9660篇 |
2016年 | 13437篇 |
2015年 | 7419篇 |
2014年 | 12249篇 |
2013年 | 28749篇 |
2012年 | 21990篇 |
2011年 | 26284篇 |
2010年 | 19487篇 |
2009年 | 19263篇 |
2008年 | 25073篇 |
2007年 | 24931篇 |
2006年 | 22900篇 |
2005年 | 20589篇 |
2004年 | 18894篇 |
2003年 | 17084篇 |
2002年 | 16967篇 |
2001年 | 17858篇 |
2000年 | 14005篇 |
1999年 | 10781篇 |
1998年 | 9336篇 |
1997年 | 9253篇 |
1996年 | 8670篇 |
1995年 | 7711篇 |
1994年 | 7718篇 |
1993年 | 7536篇 |
1992年 | 8004篇 |
1991年 | 8483篇 |
1990年 | 8138篇 |
1989年 | 8023篇 |
1988年 | 7850篇 |
1987年 | 7619篇 |
1986年 | 7342篇 |
1985年 | 9450篇 |
1984年 | 9885篇 |
1983年 | 8202篇 |
1982年 | 8684篇 |
1981年 | 8109篇 |
1980年 | 7708篇 |
1979年 | 8263篇 |
1978年 | 8683篇 |
1977年 | 8548篇 |
1976年 | 8496篇 |
1975年 | 8073篇 |
1974年 | 7959篇 |
1973年 | 8308篇 |
1972年 | 5961篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
A generalization of strong regularity around a vertex subset C of a graph Γ, which makes sense even if Γis non-regular, is studied. Such a structure appears, together with a kind of distance-regularity around C , when an spectral bound concerning the so-called predistance polynomial of C is attained. As a main consequence of these results, it is shown that a regular (connected) graph Γwith d + 1 distinct eigenvalues is distance-regular, and its distance- d graph Γ d is strongly regular with parameters a = c , if and only if the number of vertices at distance d from each vertex satisfies an expression which depends only on the order of Γand the different eigenvalues of Γ. 相似文献
92.
D. Citterio Stefan Rásonyi Ursula E. Spichiger 《Fresenius' Journal of Analytical Chemistry》1996,354(7-8):836-840
New chromoionophores have been developed, focused on NIR applications so that optode membranes may be used in monolithically integrated optical sensors. The wavelength of maximum absorbance has been estimated for a new model compound by the Pariser-Parr-Pople (PPP) method. Several cyanine type dyes have been tested as membrane chromoionophores. Membrane composition has been altered to overcome solubility problems. In this way, simple pH-sensitive optode membranes have been produced. 相似文献
93.
H.-J. Sue E. I. Garcia-Meitin N. A. Orchard 《Journal of Polymer Science.Polymer Physics》1993,31(5):595-608
The fracture behavior of a core-shell rubber (CSR) modified epoxy is investigated using both fracture mechanics and microscopy tools. The CSR-modified epoxy is found to be toughened via numerous line-array cavitations of the CSR particles, followed by plastic flow of the epoxy matrix. The toughening effect via the above craze-like damage process is found to be as effective as that of the well-known widespread rubber cavitation/matrix shear yielding mechanisms. The conditions for triggering the craze-like damage appear to be both stress state and rubber concentration dependent. The type of rubber tougheners utilized also plays a critical role in triggering this rather unusual craze-like damage in epoxy systems. © 1993 John Wiley & Sons, Inc. 相似文献
94.
Using the cyclic voltammetry (CV), the electron-transfer kinetics for the reductions of NO+ and NO2+ cations have been studied at the Pt electrode in nitromethane, sulfolane, and propylene carbonate. The heterogeneous rate constants have been determined by two independent procedures from the transfer coefficient α, the diffusion coefficient D, from a detailed examination of the CV-peak separations, and from an inspection of the values of the cathodic peak potentials at different scan rates. The results have been compared to those reported in the literature, and discussed. In the classical model, outer-sphere electron-transfer reactions are considered subject to an activation energy arising from solvent reorganization and bond reorganization processes. The solvent and molecular reorganizational barriers for these electroreductions have been assessed in aprotic media. The Marcus-Hush theory has been applied to the self-exchange reactions of the NO2+/NO2 and NO+/NO couples in an attempt to predict the rate of electron transfer. The findings indicate some improvement between theory and experiment. However, it should be noted that the experimental values of ks found for the NO2+ reduction in the solvents used are still too high in comparison with those determined theoretically. In view of the fairly strong coordination of the solvent molecule(s) as ligand(s) to NO2+ and NO+ cations, we believe that such discrepancies should stem, to some extent, from the involvement of an inner-sphere pathway by generation of an activated complex on the surface of the Pt electrode. © 1993 John Wiley & Sons, Inc. 相似文献
95.
Chemical kinetics of benzonitrile nitration with mixed acid is investigated in the temperature range 283–299 K. Pseudo-first-order rate constants are evaluated by means of rate experiments on homogeneous reacting mixtures having large stoichiometric excesses of nitric acid. The second-order kinetic constants for nitronium ion attack to the aromatic substrate are derived on the basis of the assessed nitration mechanism. An activation energy of 604 ± 37 kJ mol?1 is calculated for this reaction step. © 1993 John Wiley & Sons, Inc. 相似文献
96.
This research extends previous work with dynamic models to manage groundwater quality by using the consumptive nitrate use rate instead of the nitrate application rate. The analysis indicates that misspecification results in overestimation of economic benefits, and supra-optimum nitrogen fertilizer application rates and groundwater nitrate stocks at a steady state. 相似文献
97.
We characterize orbifolds in terms of their sheaves, and show that orbifolds correspond exactly to a specific class of smooth groupoids. As an application, we construct fibered products of orbifolds and prove a change-of-base formula for sheaf cohomology. 相似文献
98.
M. Trunk J. Popp I. Hartmann M. Lankers E. Urlaub W. Kiefer 《Fresenius' Journal of Analytical Chemistry》1996,355(3-4):354-356
In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles. 相似文献
99.
J Z Hu R J Pugmire A M Orendt D M Grant C Ye 《Solid state nuclear magnetic resonance》1992,1(4):185-195
Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described. 相似文献
100.
Comparative measurements were conducted for the backscattered intensities of light from uniform random and fractal aggregated media. Different features are found for the backscattered intensity peak shapes. A crossover between the θ1-D and θ-2 dependences of the backscattered intensity occurs in the case of fractal aggregated medium, where D indicates the fractal dimension. 相似文献