全文获取类型
收费全文 | 203149篇 |
免费 | 1290篇 |
国内免费 | 428篇 |
专业分类
化学 | 104768篇 |
晶体学 | 3394篇 |
力学 | 10272篇 |
综合类 | 10篇 |
数学 | 21854篇 |
物理学 | 64569篇 |
出版年
2021年 | 1972篇 |
2020年 | 2010篇 |
2019年 | 2567篇 |
2018年 | 3658篇 |
2017年 | 3672篇 |
2016年 | 4815篇 |
2015年 | 2345篇 |
2014年 | 4301篇 |
2013年 | 8592篇 |
2012年 | 6843篇 |
2011年 | 8052篇 |
2010年 | 6458篇 |
2009年 | 6647篇 |
2008年 | 7647篇 |
2007年 | 7607篇 |
2006年 | 6698篇 |
2005年 | 5920篇 |
2004年 | 5637篇 |
2003年 | 5166篇 |
2002年 | 5154篇 |
2001年 | 5606篇 |
2000年 | 4140篇 |
1999年 | 3135篇 |
1998年 | 2700篇 |
1997年 | 2759篇 |
1996年 | 2541篇 |
1995年 | 2211篇 |
1994年 | 2329篇 |
1993年 | 2225篇 |
1992年 | 2522篇 |
1991年 | 2644篇 |
1990年 | 2554篇 |
1989年 | 2623篇 |
1988年 | 2533篇 |
1987年 | 2554篇 |
1986年 | 2414篇 |
1985年 | 3032篇 |
1984年 | 3104篇 |
1983年 | 2598篇 |
1982年 | 2562篇 |
1981年 | 2528篇 |
1980年 | 2335篇 |
1979年 | 2713篇 |
1978年 | 2671篇 |
1977年 | 2871篇 |
1976年 | 2873篇 |
1975年 | 2665篇 |
1974年 | 2602篇 |
1973年 | 2711篇 |
1972年 | 2085篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Photoisomerization of provitamin D 3 (7-dehydrocholesterol) in a nematic liquid crystal (ZLI-1695, Merck) is investigated in detail by UV absorption spectroscopy. It is found that dissolution of chiral molecules of provitamin D 3 induces the cholesteric phase in a nematic. The spectral kinetics of photoisomerization in this phase changes significantly from that in an ethanol solution. A sharp nonmonotonic dependence of the increase in accumulation of trans isomer tachysterol in a liquid crystal matrix with a decrease in the induced cholesteric pitch from 2200 to 25 μm is revealed. 相似文献
72.
Evstafyeva I. T. Bozhenkov G. V. Aizina Ya. A. Rozenzveig I. B. Ermakova T. G. Levkovskaya G. G. Mirskova A. N. 《Russian Journal of Organic Chemistry》2002,38(8):1178-1182
1H-Pyrazoles, triazoles, and imidazoles in reaction with ethoxycarbonylimine and arylsulfonylimines of chloral yield addition products, corresponding 1-(1-amidotrichloroethyl)azoles. Derivatives of 1-alkylpyrazoles and pyrazolones react with chloral 4-chlorophenylsulfonylimine to furnish products of C-amidotrichloroethylation into position 4 of the azole ring. 相似文献
73.
The effect of hydrogen on the photoluminescence and planar conductivity of GaAs/InGaAs quantum-well heterostructures with an island Pd layer at the anodically oxidized surface was studied. Unlike continuous deposited Pd layers, island layers do not cause the formation of defects in the GaAs surface region and yet the Pd layer maintains high catalytic activity with respect to hydrogen. It is found that the thermal treatment of such a structure in a hydrogen atmosphere causes atomic-hydrogen passivation of the defects in quantum wells. Studies of the characteristics of planar photoresistors with an island Pd layer acting as hydrogen sensors show that their hydrogen detectivity is approximately two orders of magnitude higher than that of diode structures with continuous Pd layers. 相似文献
74.
Kurenkov V. F. Zaitseva O. A. Kirdyashova G. N. 《Russian Journal of Applied Chemistry》2002,75(4):641-644
The kinetics of radical copolymerization of acrylamide with lithium, sodium, and potassium 2-acrylamido-2-methylpropanesulfonates in concentrated aqueous solutions in the presence of potassium persulfate as initiating agent at pH 9 and 50°C was studied by dilatometry. 相似文献
75.
Bairamov R. K. Ermakov A. I. Vedernikova N. R. 《Russian Journal of Applied Chemistry》2002,75(3):408-410
The influence exerted by the nature and concentration of a number of organic and mineral acids on the composition of products formed in electric-spark dispersion of aluminum was studied. 相似文献
76.
Shunin D. G. Filippova A. G. Okhotina N. A. Liakumovich A. G. Samuilov Ya. D. 《Russian Journal of Applied Chemistry》2002,75(6):1020-1023
Procedures are discussed for production of rubber-bitumen compounds based on spent rubber items. A possibility of using these binders for paving is studied experimentally. 相似文献
77.
This paper deals with mathematical human resource planning; more specifically, it suggests a new model for a manpower‐planning system. In general, we study a k‐classed hierarchical system where the workforce demand at each time period is satisfied through internal mobility and recruitment. The motivation for this work is based on various European Union incentives, which promote regional or local government assistance programs that could be exploited by firms not only for hiring and training newcomers, but also to improve the skills and knowledge of their existing personnel. In this respect, in our augmented mobility model we establish a new ‘training/standby’ class, which serves as a manpower inventory position for potential recruits. This class, which may very well be internal or external to the system, is incorporated into the framework of a non‐homogeneous Markov chain model. Furthermore, cost objectives are employed using the goal‐programming approach, under different operating assumptions, in order to minimize the operational cost in the presence of system's constraints and regulations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
78.
The existence of linear nonstationary optical resonances in a diatomic nanostructural object with a dipole-dipole atomic interaction has been proved. A new solution to the joint system of modified Bloch optical equations and nonlocal field equations is obtained for time intervals much shorter than the times of phase and energy relaxation. Formulas for effective polarizabilities of the object’s atoms, which have a set of dimensional resonances, are derived. The frequencies of these resonances significantly differ from the eigenfrequencies of the object’s atoms, and their properties depend on the interatomic distance, light-pulse duration, initial atomic inversions, and the orientation of the object’s axis relative to the direction of incidence of the external light wave. 相似文献
79.
Afraimovich É. L. Vodyannikov V. V. Voyeikov S. V. Perevalova N. P. Yakovets A. F. 《Radiophysics and Quantum Electronics》2002,45(10):741-757
Using the COPHASE method and the GPS interferometry method for travelling ionospheric disturbances, we analyze in detail the spatio-temporal properties of travelling wave packets (TWP) of total electron content (TEC) disturbances. The analysis is performed on the example of a clearest TWP manifestation observed in California, USA, in October 18, 2001, using the GLOBDET technique, developed at the Institute of Solar-Terrestrial Physics of the Siberian Branch of RAS for global detection and monitoring of natural and technogenic ionospheric disturbances on the basis of TEC variations retrieved from the global network of GPS receivers. In the time domain, TWPs are quasi-periodic TEC oscillations of duration about 1 h, period of 10–20 min, and amplitude exceeding that of the background TEC fluctuations by at least one order of magnitude. The velocity and direction of TWP motion are similar to those of mid-latitude mesoscale travelling ionospheric disturbances, as obtained earlier from the analysis of phase parameters of HF radio signals and the signals of geostationary satellites and discrete space radio sources. 相似文献
80.
A Modified Quasi-Newton Method for Structured Optimization with Partial Information on the Hessian 总被引:2,自引:0,他引:2
This paper develops a modified quasi-Newton method for structured unconstrained optimization with partial information on the
Hessian, based on a better approximation to the Hessian in current search direction. The new approximation is decided by both
function values and gradients at the last two iterations unlike the original one which only uses the gradients at the last
two iterations. The modified method owns local and superlinear convergence. Numerical experiments show that the proposed method
is encouraging comparing with the methods proposed in [4] for structured unconstrained optimization
Presented at the 6th International Conference on Optimization: Techniques and Applications, Ballarat, Australia, December
9–11, 2004 相似文献