全文获取类型
收费全文 | 94483篇 |
免费 | 738篇 |
国内免费 | 464篇 |
专业分类
化学 | 34962篇 |
晶体学 | 934篇 |
力学 | 7203篇 |
综合类 | 1篇 |
数学 | 34231篇 |
物理学 | 18354篇 |
出版年
2022年 | 170篇 |
2021年 | 258篇 |
2020年 | 261篇 |
2019年 | 248篇 |
2018年 | 10569篇 |
2017年 | 10363篇 |
2016年 | 6386篇 |
2015年 | 1169篇 |
2014年 | 666篇 |
2013年 | 1228篇 |
2012年 | 4609篇 |
2011年 | 11387篇 |
2010年 | 6210篇 |
2009年 | 6481篇 |
2008年 | 7385篇 |
2007年 | 9509篇 |
2006年 | 1053篇 |
2005年 | 2081篇 |
2004年 | 2249篇 |
2003年 | 2551篇 |
2002年 | 1543篇 |
2001年 | 525篇 |
2000年 | 540篇 |
1999年 | 360篇 |
1998年 | 369篇 |
1997年 | 345篇 |
1996年 | 464篇 |
1995年 | 294篇 |
1994年 | 257篇 |
1993年 | 307篇 |
1992年 | 248篇 |
1991年 | 245篇 |
1990年 | 190篇 |
1989年 | 201篇 |
1988年 | 227篇 |
1987年 | 258篇 |
1986年 | 236篇 |
1985年 | 302篇 |
1984年 | 262篇 |
1983年 | 204篇 |
1982年 | 256篇 |
1981年 | 240篇 |
1980年 | 238篇 |
1979年 | 239篇 |
1978年 | 230篇 |
1977年 | 208篇 |
1976年 | 178篇 |
1975年 | 193篇 |
1974年 | 158篇 |
1973年 | 164篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
A column generation (CG) approach for the solution of timetabling problems is presented. This methodology could be used for various instances of the timetabling problem, although in this paper the solution of the high-school situation in Greece is presented. The results obtained show clearly that the CG approach that has been extremely successful in recent years in the solution of airline crew scheduling problems could also be very efficient and robust for the solution of timetabling problems. Several large timetabling problems corresponding to real problems have been successfully solved, with the solutions obtained feasible and of very high quality in accordance with the problem definition. In addition, none of the solutions contained any idle hour for any of the teachers, which was one of the main goals of this optimization effort. 相似文献
82.
The disregard of causal inference in the methodological literature about qualitative research is highly problematic, since the category of causality is closely linked to the concept of social action. However, it is also clear that causal analysis is burdened with certain difficulties and methodological challenges in the realm of social research. Some of these problems are discussed in this article using Mackie—s concept of 3 “INUS”-conditions. Thereby it will be shown that strategies of causal analysis based on comparative methods proposed for qualitative research, namely “Analytic Induction” and “Qualitative Comparative Analysis” have great difficulties in dealing adequately with these problems. They can only be solved, if case-comparative methods are combined with explorative research strategies which support the researcher in gaining access to the local knowledge of the research field. 相似文献
83.
84.
Qing Shi Dong Yang Yanlei Su Jian Li Zhongyi Jiang Yanjun Jiang Weikang Yuan 《Journal of nanoparticle research》2007,9(6):1205-1210
Lipase from Candida rugosa was covalently anchored onto acid-treated multi-walled carbon nanotubes (MWNTs) through a self-catalytic mechanism. A variety
of characterization techniques including FTIR, Raman spectroscopy, and XPS were employed to demonstrate the formation of the
ester linkage between lipase and MWNTs. The MWNTs-lipase biocomposites showed significantly increased solubility in some common-used
organic solvents, such as THF, DMF and chloroform. This study may offer a novel and facile route for covalent modification
of carbon nanotubes, and expand the potential utilization of both lipases and MWNTs in the fields of biocatalyst and biosensor. 相似文献
85.
Uzi Vishne 《Archiv der Mathematik》2003,80(2):119-122
We prove that assuming enough roots of unity in the base field, a central simple algebra of exponent 2 which is split by a dihedral group, is also split by certain abelian groups. 相似文献
86.
87.
Paul Van Der Meeren Jan Vanderdeelen Leon Baert 《Particle & Particle Systems Characterization》1994,11(4):320-326
Using a monodisperse PMMA dispersion, it was shown that light reflection at the sample cuvette walls may greatly influence the results of both static (SLS) and dynamic (DLS) light scattering experiments. Considering SLS, this reflection phenomenon mostly causes an overestimation of the scattered intensity at high scattering angles, which may give rise to the emergence of an additional, artificial peak in the lower region of the particle size distribution. On the other hand, the influcence of reflection on DLS measurements was shown to be particularly important in the upper region of the particle size distribution. The experimentally observed phenomena were explained from the basic principles of both particle sizing methods. Finally, it was shown that the disturbing effect of reflection could be avoided by modifying either the hardware or the software of the static and dynamic light scattering technique. 相似文献
88.
89.
90.
A well-known problem of prediction in linear regression models is to find a confidence interval for the random value of the dependent variable when the values of the independent variables are given. Such a situation may arise in economic quality control models when the independent variables are costly inputs and the dependent variable is some measure of quality or production. In such a circumstance, an important control objective may be to find values for the inputs that will maximize the lower limit of the prediction confidence interval for a fixed budget, or alternatively, to minimize the cost of the inputs for a fixed lower limit of the confidence interval. In this paper, we shall show that global optima can be found using known algorithms. The special case of simple linear regression is discussed and an illustrative example is provided. 相似文献