全文获取类型
收费全文 | 4664篇 |
免费 | 167篇 |
国内免费 | 14篇 |
专业分类
化学 | 2784篇 |
晶体学 | 86篇 |
力学 | 152篇 |
数学 | 333篇 |
物理学 | 1490篇 |
出版年
2023年 | 43篇 |
2022年 | 54篇 |
2021年 | 83篇 |
2020年 | 101篇 |
2019年 | 95篇 |
2018年 | 119篇 |
2017年 | 88篇 |
2016年 | 134篇 |
2015年 | 111篇 |
2014年 | 168篇 |
2013年 | 276篇 |
2012年 | 359篇 |
2011年 | 349篇 |
2010年 | 200篇 |
2009年 | 176篇 |
2008年 | 208篇 |
2007年 | 192篇 |
2006年 | 190篇 |
2005年 | 155篇 |
2004年 | 118篇 |
2003年 | 116篇 |
2002年 | 91篇 |
2001年 | 76篇 |
2000年 | 61篇 |
1999年 | 54篇 |
1998年 | 32篇 |
1997年 | 53篇 |
1996年 | 58篇 |
1995年 | 44篇 |
1994年 | 75篇 |
1993年 | 82篇 |
1992年 | 79篇 |
1991年 | 49篇 |
1990年 | 49篇 |
1989年 | 54篇 |
1988年 | 35篇 |
1987年 | 39篇 |
1986年 | 37篇 |
1985年 | 34篇 |
1984年 | 33篇 |
1983年 | 37篇 |
1982年 | 39篇 |
1981年 | 49篇 |
1980年 | 39篇 |
1979年 | 31篇 |
1978年 | 26篇 |
1977年 | 34篇 |
1975年 | 24篇 |
1974年 | 27篇 |
1973年 | 33篇 |
排序方式: 共有4845条查询结果,搜索用时 375 毫秒
101.
102.
Bankim C. Das I. Dey G. Biswas R. Banerjee Y. Iitaka A. Banerjee 《Journal of chemical crystallography》1993,23(6):509-512
The crystal structure of C6H16N8O2·CuCl2, a biguanide metal complex, has been determined by X-ray diffraction data using MoK radiation. The compound crystallizes in the monoclinic space groupP21/a, witha=11.074(4),b=12.061(4),c=5.312(3)Å and=102.8(1)°.The structure was solved by direct methods and refined by full-matrix least-squares to a finalR value 0.037 with 1245 unique reflections. The complex molecule is centrosymmetric, with the Cu atom (0.5,0.5,0.5) on a crystallographic center of symmetry. The complex molecule is rhombic planar with a trans configuration. 相似文献
103.
Ingo Althöfer Gautam Das David Dobkin Deborah Joseph José Soares 《Discrete and Computational Geometry》1993,9(1):81-100
Given a graphG, a subgraphG' is at-spanner ofG if, for everyu,v V, the distance fromu tov inG' is at mostt times longer than the distance inG. In this paper we give a simple algorithm for constructing sparse spanners for arbitrary weighted graphs. We then apply this algorithm to obtain specific results for planar graphs and Euclidean graphs. We discuss the optimality of our results and present several nearly matching lower bounds.The work of G. Das and D. Joseph was supported by NSF PYI Grant DCR-8402375. The work of D. Dobkin was supported by NSF Grant CCR-8700917. The work of J. Soares was supported by CNPq proc 203039/87.4 (Brazil) and NSF Grant CCR-9014562. This research was accomplished while G. Das was a student at the University of Wisconsin-Madison. A preliminary version was presented at the Second Scandinavian Workshop on Algorithm Theory, Bergen, Norway, 1990, under the title Generating Sparse Spanners for Weighted Graphs, and proceedings appear in the series Lecture Notes in Computer Science, Springer-Verlag. The preliminary version also appears as Princeton University Technical Report CS-TR-261-90, and as University of Wisconsin-Madison Computer Sciences Technical Report 882. 相似文献
104.
105.
The site of proton abstraction by OH? ion in some 4-substituted-3,5,6-triphenyl-Δ′-cyclohexenes has been established from labelling studies. The further fragmentation mode of (M-H)? ion depends on its structure and stereochemistry. The and nitro isomers exhibit stereochemical differences in the elimination of HNO2 while the amino and cyano analogues show stereochemically controlled RDA fragmentation mode. 相似文献
106.
Paritosh K Das 《Tetrahedron letters》1981,22(14):1307-1310
The rate constants for electron transfer reactions from several aromatic carbonyl triplets to paraquat dication leading to the formation of paraquat radical ion have been measured by nanosecond laser flash photolysis and are found to be in the range, 1 – 9 × 109 M?1 s?1. 相似文献
107.
Summary In this study a new approach is presented for on-line radiometric detection in reversed phase LC of medium to low polarity
compounds labelled with14C.
The test compounds,14C-carbaryl and14C-parathion, are extracted post-column into a non-water miscible liquid scintillation cocktail. The segmented two-phase system
formed is introduced into the beta-detector without phase separation and collected in a capillary storage tube.
After completion of the LC separation and detection process, the direction of the flow in the storage system is reversed and
the segmented contents of the loop led at lower flow-rates through the beta-detector again. An enhanced signal, corresponding
to the increase in counting time is obtained without measurable peak broadening.
The lowest possible detection limit of the system is 9 counts per peak corresponding to subnanogram quantities of tested pesticides.
Calibration curves are linear over at least 2 orders of magnitude and have the expected theoretical slopes.
The reproducibility of the system is better than 4 % rel. S.D. An application to a recovery study of parathion shows the practical
potential of this technique.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
108.
Pyridinium methanesulfonate (m.p. 185°C) and pyridinium p-toluenesulfonate (m.p. 121°C) were used as catalysts and media for the esterification of carboxylic acids with primary alcohols
to give the corresponding esters selectively in high yields. The high melting points of these pyridinium salts did not prevent
their application as ionic liquid medium for these reactions which were performed at 90°C. 相似文献
109.
Das A 《Physical review letters》1985,55(20):2126-2128
110.