全文获取类型
收费全文 | 1448篇 |
免费 | 38篇 |
国内免费 | 5篇 |
专业分类
化学 | 1001篇 |
晶体学 | 21篇 |
力学 | 32篇 |
数学 | 297篇 |
物理学 | 140篇 |
出版年
2023年 | 9篇 |
2022年 | 11篇 |
2021年 | 41篇 |
2020年 | 34篇 |
2019年 | 30篇 |
2018年 | 20篇 |
2017年 | 11篇 |
2016年 | 59篇 |
2015年 | 39篇 |
2014年 | 38篇 |
2013年 | 67篇 |
2012年 | 87篇 |
2011年 | 82篇 |
2010年 | 63篇 |
2009年 | 51篇 |
2008年 | 92篇 |
2007年 | 79篇 |
2006年 | 79篇 |
2005年 | 82篇 |
2004年 | 68篇 |
2003年 | 43篇 |
2002年 | 57篇 |
2001年 | 16篇 |
2000年 | 19篇 |
1999年 | 20篇 |
1998年 | 17篇 |
1997年 | 9篇 |
1996年 | 20篇 |
1995年 | 11篇 |
1994年 | 9篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 6篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 19篇 |
1986年 | 12篇 |
1985年 | 17篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1959年 | 2篇 |
1894年 | 2篇 |
1891年 | 2篇 |
排序方式: 共有1491条查询结果,搜索用时 766 毫秒
101.
102.
Gabriele Tamborini 《Mikrochimica acta》2004,145(1-4):237-242
In the framework of the projects Environmental Sampling and High Performance Trace Analysis in support to the International Atomic Energy Agency (IAEA) and the Euratom Safeguards Office (ESO), the research has been focused on the identification and characterisation of particles of interest containing mainly U and other actinides.Multiple analyses, elemental and isotopic, of single radioactive particles in the µm-size range are key-tools in environmental research and in nuclear forensics. The main purpose in the analysis of single particles by SIMS is the determination of their isotopic composition that has been demonstrated successfully for uranium and plutonium particles. From these results, the enrichment of the material produced, the process used for the enrichment as well as the starting material employed (natural or reprocessed uranium) can be identified. 相似文献
103.
The effects of 17-ethinylestradiol (EE), an endocrine disruptor, on zoo- and phytoplankton were studied in outdoor 230-L still-water microcosms. Cell density and biomass, diversity, and community composition were analyzed. Five microcosms were treated by controlled release for six weeks, three by direct application of EE. To investigate recovery, sampling was continued for four weeks after treatment. Most characteristics of the zooplankton were not unambiguously affected by EE. Only the relative density of copepods, especially of their larvae, decreased significantly after EE application. For phytoplankton, no unambiguous concentration- or toxodose-correlated effects on any biotic characteristics could be found. However, most properties of the phytoplankton deviated from those of controls, i.e. tended to be smaller (number of species per microcosm, biomass, cell density) or covered a wider range (diversity, evenness). PCA indicated a shift of species structure in the treated microcosms. This was supported by the species scores calculated by the principal response curve method, although the principal response curve itself showed no clear EE-correlated shifts. High variability within the biocenosis between microcosms and over time, probably because of disturbance of the ecosystem before starting of the test, might have superimposed EE-dependent effects.Abbreviations
c
dt
Canonical coefficient
- CRD
Controlled-release device
-
E
Shannon–Wiener index for evenness
- EC10
Effect concentration for 10% of maximum effect
- EC50
Effect concentration for 50% of maximum effect
- EE
17-Ethinylestradiol
-
H
Shannon–Wiener index for diversity
- LPDE
Low density polyethylene
- NOEC
No observed effect concentration
- PCA
Principal component analysis
- PRC
Principal response curve
- RDA
Redundancy analysis
- SD
Standard deviation 相似文献
104.
Filippi A Roselli G Renzi G Grandinetti F Speranza M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):2072-2078
The kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D(1)-3-(p-tolyl)butane have been investigated in the gas phase in the 100-160 degrees C range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-311++G**//HF/6-31+G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions. 相似文献
105.
106.
Fresh (larch and fir, in its white and red varieties) and ancient wood samples (dating respectively to the 13th, 15th and 17th centuries) were subjected to thermogravimetric analysis (TG and DTG). The resulting thermogravimetric data were then used to construct archeometric curves for the wood varieties tested. In a preliminary approach, it was attempted to correlate the onset temperature of the thermogravimetric step corresponding to cellulose decomposition with the age (expressed in centuries) of the samples, although the results obtained were anything but brilliant. More encouraging results were obtained by examining the relationship between wood sample age and the value of the (percent cellulose/percent lignin) ratio computed from the thermogravimetric data. Lastly, a procedure for processing data obtained from the TG curves was applied to a kinetic analysis of the processes that take place when wood samples are subjected to a temperature regime with a constant heating rate, obtaining values for the activation energy of the TG step corresponding to the decomposition of cellulose. Also using these data it was attempted to construct archeometric curves, obtaining results that varied quite significantly according to the wood species tested. 相似文献
107.
Rendiconti del Circolo Matematico di Palermo Series 1 - 相似文献
108.
We study the complete lattice of convergences, that is, of relations between a set and the set of its filters. Various properties of convergences (like isotonicity, antitonicity, being a pseudotopology, a pretopology, an adherence) determine subsets of the lattice of convergences which possess certain particular features called “cyrtological”. The morphisms associated to these sets, said “cyrtomorphisms”, are basic in the study of convergences. A method of generation of GALOIS connections relative to cyrtomorphisms is proposed. Semigroups generated by some cyrtomorphisms are analyzed. 相似文献
109.
Stephan Dahlke Gitta Kutyniok Gabriele Steidl Gerd Teschke 《Applied and Computational Harmonic Analysis》2009,27(2):195-214
In this paper, we study the relationships of the newly developed continuous shearlet transform with the coorbit space theory. It turns out that all the conditions that are needed to apply the coorbit space theory can indeed be satisfied for the shearlet group. Consequently, we establish new families of smoothness spaces, the shearlet coorbit spaces. Moreover, our approach yields Banach frames for these spaces in a quite natural way. We also study the approximation power of best n-term approximation schemes and present some first numerical experiments. 相似文献
110.
Gabriele Bonanno Angela Sciammetta 《Journal of Mathematical Analysis and Applications》2012,390(1):59-67
In this paper, existence results of positive solutions to a Neumann problem involving the p-Laplacian are established. Multiplicity results are also pointed out. The approach is based on variational methods. 相似文献