全文获取类型
收费全文 | 254767篇 |
免费 | 3114篇 |
国内免费 | 914篇 |
专业分类
化学 | 140037篇 |
晶体学 | 3748篇 |
力学 | 10092篇 |
综合类 | 2篇 |
数学 | 28693篇 |
物理学 | 76223篇 |
出版年
2021年 | 2028篇 |
2020年 | 2024篇 |
2019年 | 2345篇 |
2018年 | 2707篇 |
2017年 | 2603篇 |
2016年 | 4219篇 |
2015年 | 3007篇 |
2014年 | 4282篇 |
2013年 | 11912篇 |
2012年 | 8865篇 |
2011年 | 10872篇 |
2010年 | 7104篇 |
2009年 | 6988篇 |
2008年 | 9712篇 |
2007年 | 9856篇 |
2006年 | 9008篇 |
2005年 | 8391篇 |
2004年 | 7472篇 |
2003年 | 6613篇 |
2002年 | 6537篇 |
2001年 | 7346篇 |
2000年 | 5483篇 |
1999年 | 4318篇 |
1998年 | 3653篇 |
1997年 | 3665篇 |
1996年 | 3407篇 |
1995年 | 3209篇 |
1994年 | 3025篇 |
1993年 | 3074篇 |
1992年 | 3348篇 |
1991年 | 3398篇 |
1990年 | 3194篇 |
1989年 | 3169篇 |
1988年 | 3216篇 |
1987年 | 3077篇 |
1986年 | 2974篇 |
1985年 | 4047篇 |
1984年 | 4223篇 |
1983年 | 3441篇 |
1982年 | 3813篇 |
1981年 | 3703篇 |
1980年 | 3568篇 |
1979年 | 3597篇 |
1978年 | 3830篇 |
1977年 | 3644篇 |
1976年 | 3845篇 |
1975年 | 3410篇 |
1974年 | 3529篇 |
1973年 | 3834篇 |
1972年 | 2328篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
The role of filamentation instability of quark-gluon plasma, in explaining collective phenomena in relativistic heavy-ion
collisions, has been analyzed. Using equations of SU(2) two fluid color hydrodynamics it is shown that this instability can
significantly enhance nuclear stopping and might contribute to collective sideward flows. 相似文献
992.
Two previously unknown metabolites of halofantrine, a candidate anti-malarial drug, have been isolated by thin-layer chromatography from the plasma of dogs administered a single oral dose of 60 mg/kg. Their identifies were investigated after trimethylsilylation by gas chromatography-mass spectrometry under electron-impact and negative-ion chemical ionization conditions. The structural assignment was further confirmed by using a combination of elemental composition analysis of all the isotope peaks at low mass resolution and isotope pattern matching. These two metabolites were formed by modification of the dibutylaminopropyl side-chain of the parent compound involving deamination and oxidation or reduction. 相似文献
993.
In this paper a single-degree-of-freedom model is developed to predict the dynamic response of an acoustially excited doubly curved sandwich panel. Three variants of the model are investigated, based on differing assumptions regarding the spatial distribution of the applied loading. When the loading is assumed to be uniform then the model reduces to the Miles approach, and when the loading is assumed to conform to the structural mode shape then the method is very similar to the Blevins approach. The third variant involves a more detailed consideration of the travelling wave characteristics of the applied loading, and this is found to give much improved agreement with experimental results obtained in a progressive wave tube facility. In addition, an approach using the finite element method is presented in which the response to grazing incidence excitation is computed, and this is also found to yield good agreement with the experimental results. 相似文献
994.
Herlin-Boime N. Vicens J. Dufour C. Ténégal F. Reynaud C. Rizk R. 《Journal of nanoparticle research》2004,6(1):63-70
Small SiC nanoparticles (10 nm diameter) have been grown in a flow reactor by CO2 laser pyrolysis from a C2H2 and SiH4 mixture. The laser radiation is strongly absorbed by SiH4 vibration. The energy is transferred to the reactive medium and leads to the dissociation of molecules and the subsequent growth of the nanoparticles. The reaction happens with a flame. The purpose of the experiments reported in this paper is to limit the size of the growing particles to the nanometric scale for which specific properties are expected to appear. Therefore the effects of experimental parameters on the structure and chemical composition of nanoparticles have been investigated. For a given reactive mixture and gas velocity, the flame temperature is governed by the laser power. In this study, the temperature was varied from 875°C to 1100°C. The chemical analysis of the products indicate that their composition is a function of the temperature. For the same C/Si atomic ratio in the gaseous phase, the C/Si ratio in the powder increases from 0.7 at 875°C up to 1.02 at 1100°C, indicating a growth mechanism limited by C2H2 dissociation. As expected, X-ray diffraction has shown an improved crystallisation with increasing temperature. Transmission electron microscopy observations have revealed the formation of 10 nm grains for all values of laser power (or flame temperature). These grains appear amorphous at low temperature, whereas they contain an increasing number of nanocrystals (2 nm diameter) when the temperature increases. These results pave the way to a better control of the structure and chemical composition of laser synthesised SiC nanoparticles in the 10 nm range. 相似文献
995.
X. Y. Yang H. C. Gao X. L. Tan H. Z. Yuan G. Z. Cheng S. Z. Mao S. Zhao L. Zhang J. Y. An J. Y. Yu Y. R. Du 《Colloid and polymer science》2004,282(3):280-286
1H chemical shift changes of sodium 4-decyl naphthalene sulfonate (SDNS) at 313 K show that its critical micellar concentration lies between 0.82 and 0.92 mmol/dm3, which is in the same range as that of the previous study at 298 K. The spin–lattice relaxation time, spin–spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy experiments give information about the structure of the SDNS micelle and the dynamics of the molecules in the micelle. The size of the SDNS micelle remains almost unchanged in the temperature range from 298 to 313 K as deduced by analyzing the self-diffusion coefficient. Special arrangement of the naphthyl rings of SDNS in the micelles affects the packing of these hydrophobic chains. The methylene groups of the alkyl chain nearest the naphthalene groups penetrate into the aromatic region, which results in a more tightly packed hydrophobic micellar core than that of sodium dodecyl sulfonate. 相似文献
996.
Xia Yin Weiguang Zhang Qijiao Zhang Jun Fan Chian Sing Lai Edward R. T. Tiekink 《应用有机金属化学》2004,18(3):139-140
The centrosymmetric structure of {Cd[S2CN(CH2Ph)2]2}2 features both bridging and chelating dithiocarbamate ligands so that a square pyramidal S5 coordination geometry results. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
997.
R.Thomas Williamson 《Tetrahedron》2004,60(33):7025-7033
Brine shrimp toxicity guided fractionation of the extracts from two mixed Fijian collections of the cyanobacteria Lyngbya majuscula and Schizothrix sp. led to the isolation of eleven novel chlorinated lipids. All of these metabolites show an intriguing constellation of unsaturation (olefinic and acetylenic bonds) and chlorination at the two termini of a 15-carbon chain. The central carbon atom of the chain (C-8) is substituted in each case with an N-acetate function. Taveuniamides A-E have an adjacent carbomethoxy group at C-9 to form a protected β-amino acid while taveuniamides F-K have a methylene group at this position. A standard assortment of 2D NMR techniques in concert with mass spectrometry and other analytical techniques were used to define the structures of these novel metabolites. Taveuniamides F, G and K were the most potent brine shrimp toxins with LD50s between 1.7-1.9 μg/mL. 相似文献
998.
In 1965 Knuth (J. Algebra 2 (1965) 182) noticed that a finite semifield was determined by a 3-cube array (aijk) and that any permutation of the indices would give another semifield. In this article we explain the geometrical significance of these permutations. It is known that a pair of functions (f,g) where f and g are functions from GF(q) to GF(q) with the property that f and g are linear over some subfield and g(x)2+4xf(x) is a non-square for all x∈GF(q)∗, q odd, give rise to certain semifields, one of which is commutative of rank 2 over its middle nucleus, one of which arises from a semifield flock of the quadratic cone, and another that comes from a translation ovoid of Q(4,q). We show that there are in fact six non-isotopic semifields that can be constructed from such a pair of functions, which will give rise to six non-isomorphic semifield planes, unless (f,g) are of linear type or of Dickson-Kantor-Knuth type. These six semifields fall into two sets of three semifields related by Knuth operations. 相似文献
999.
I T Vieira P R Harper A K Shahani V De Senna 《The Journal of the Operational Research Society》2003,54(7):713-722
Mother-to-child transmission (MTCT) of HIV is the most significant source of HIV infection in children below the age of 15 years. In 2000 alone, about 600 000 new infections occurred, the vast majority from mothers living in developing countries who were not aware of their HIV-positive status. To date, at least 4.3 million children have died from AIDS. In this paper, we discuss the development of an operational model that can be used to evaluate intervention options for the prevention of MTCT of HIV. The problem was approached by defining suitable pregnancy risk groups, prevention options and the required model outputs to evaluate different intervention strategies. The method chosen to approach the problem was a discrete-event, three-phase simulation, built in Visual Basic, with a stochastic semi-Markov structure. The developed model takes individual pregnant women through each stage of their pregnancy, labour and birth. Different intervention strategies may be modelled at any time, including short-course antiretroviral drugs and cessation of breastfeeding. The model is demonstrated with data from Botswana, which has one of the highest HIV-infection rates in the world. 相似文献
1000.
We study computable trees with distinguished initial subtree (briefly, I-trees). It is proved that all I-trees of infinite height are computably categorical, and moreover, they all have effectively infinite computable dimension. 相似文献