全文获取类型
收费全文 | 24416篇 |
免费 | 869篇 |
国内免费 | 3201篇 |
专业分类
化学 | 14922篇 |
晶体学 | 219篇 |
力学 | 213篇 |
综合类 | 122篇 |
数学 | 10572篇 |
物理学 | 2438篇 |
出版年
2023年 | 139篇 |
2022年 | 368篇 |
2021年 | 321篇 |
2020年 | 334篇 |
2019年 | 557篇 |
2018年 | 519篇 |
2017年 | 480篇 |
2016年 | 466篇 |
2015年 | 439篇 |
2014年 | 665篇 |
2013年 | 2052篇 |
2012年 | 1029篇 |
2011年 | 1672篇 |
2010年 | 1401篇 |
2009年 | 1607篇 |
2008年 | 1837篇 |
2007年 | 1776篇 |
2006年 | 1515篇 |
2005年 | 1208篇 |
2004年 | 1118篇 |
2003年 | 897篇 |
2002年 | 798篇 |
2001年 | 672篇 |
2000年 | 658篇 |
1999年 | 613篇 |
1998年 | 544篇 |
1997年 | 525篇 |
1996年 | 476篇 |
1995年 | 576篇 |
1994年 | 520篇 |
1993年 | 418篇 |
1992年 | 358篇 |
1991年 | 275篇 |
1990年 | 209篇 |
1989年 | 199篇 |
1988年 | 122篇 |
1987年 | 104篇 |
1986年 | 124篇 |
1985年 | 122篇 |
1984年 | 118篇 |
1983年 | 56篇 |
1982年 | 89篇 |
1981年 | 95篇 |
1980年 | 83篇 |
1979年 | 69篇 |
1978年 | 85篇 |
1977年 | 47篇 |
1976年 | 49篇 |
1975年 | 21篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
71.
72.
We prove a conjecture by F. Ferrari. Let X be the total space of a nonlinear deformation of a rank two holomorphic vector bundle on a smooth rational curve, such that X has trivial canonical bundle and has sections. Then the normal bundle to such sections is computed in terms of the rank of the Hessian of a suitably defined superpotential at its critical points. 相似文献
73.
Jan P. Boroński 《Topology and its Applications》2007,154(18):3156-3166
A set (or a collection of sets) contained in the Euclidean space Rm is symmetric if it is invariant under the antipodal map. Given a symmetric unicoherent polyhedron X (like an n-dimensional cube or a sphere) and an odd real function f defined on vertices of a certain symmetric triangulation of X, we algorithmically construct a connected symmetric separator of X by choosing a subcollection of the triangulation. Each element of the subcollection contains the vertices v and u such that f(v)f(u)?0. 相似文献
74.
New criteria are provided for determining whether an integral representation of a stable process is minimal. These criteria are based on various nonminimal sets and their projections, and have several advantages over and shed light on already available criteria. In particular, they naturally lead from a nonminimal representation to the one which is minimal. Several known examples are considered to illustrate the main results. The general approach is also adapted to show that the so-called mixed moving averages have a minimal integral representation of the mixed moving average type. 相似文献
75.
R. Jayakumar Y.‐S. Lee S. Nanjundan 《Journal of polymer science. Part A, Polymer chemistry》2003,41(18):2865-2878
The calcium salt of mono(hydroxyethoxyethyl)phthalate [Ca(HEEP)2] was synthesized by the reaction of diethylene glycol, phthalic anhydride, and calcium acetate. Calcium‐containing poly(urethane ether)s (PUEs) were synthesized by the reaction of hexamethylene diisocyanate (HMDI) or tolylene 2,4‐diisocyanate (TDI) with a mixture of Ca(HEEP)2 and poly(ethylene glycol) (PEG300 or PEG400) with di‐n‐butyltin dilaurate as a catalyst. A series of calcium‐containing PUEs of different compositions were synthesized with Ca(HEEP)2/PEG300 (or PEG400)/diisocyanate (HMDI or TDI) molar ratios of 2:2:4, 3:1:4, and 1:3:4 so that the coating properties of the PUEs could be studied. Blank PUEs without calcium‐containing ionic diols were also prepared by the reaction of PEG300 or PEG400 with HMDI or TDI. The PUEs were well characterized by Fourier transform infrared, 1H and 13C NMR, solid‐state cross‐polarity/magic‐angle‐spinning 13C NMR, viscosity, solubility, and X‐ray diffraction studies. The thermal properties of the polymers were also studied with thermogravimetric analysis and differential scanning calorimetry. The PUEs were applied as top coats on acrylic‐coated leather, and their physicomechanical properties were also studied. The coating properties of PUEs, such as the tensile strength, elongation at break, tear strength, water vapor permeability, flexing endurance, cold crack resistance, abrasion resistance, color fastness, and adhesive strength, were better than the standard values. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2865–2878, 2003 相似文献
76.
The Li oxides species formed on Li over-deposited V2O5 thin film surfaces have been studied by using X-ray and UV induced photoelectron spectroscopy (XPS and UPS). The photoelectron spectroscopic data show that the Li over-deposited V2O5 system itself is not stable. Further chemical decomposition reactions are taken place even under UHV conditions and lead to form Li2O and Li2O2 compounds on the surface. The formation of Li2O2 causes to arise an emission line at about 11.3 eV in the valence band spectra. 相似文献
77.
To find an effective extraction and removal method for palladium(II), which is one of the main fission products from an acidic nuclear spent fuel solution, the extraction behavior of palladium(II) from a nitric acid medium by an acidic chelating extractant, 1-phenyl-3-methyl-4-trifluoroacetylpyrazolone-5-one (HPMTP) and a tertiary amine of high molecular weight, tri-n-octylamine (TOA), has been studied by spectrophotometry. A noticeable antagonistic extraction effect was observed in the extraction system under the given conditions. To understand this phenomenon, a preliminary investigation was performed to explain the mechanism of this reaction. According to the theory of corresponding solutions (TCS), the association reaction between HPMTP and TOA is proposed in the organic phase. An associated species, HPMTP·TOA, formed through hydrogen bonding in a chloroform medium might be the main reason why an antagonistic extraction effect occurred. The association constant between HPMTP and TOA was calculated to be 2.86±0.05. 相似文献
78.
The thermal rearrangement of trifluoromethyl trifluorovinyl ether (MVE) to pentafluoropropionyl fluoride (PPF) under pressure with and without radical initiators has been studied. The reaction typically gives a mixture of different acyl fluorides. The influence of the reaction parameters (pressure, temperature, concentration, type of initiator and contact time) on the conversion and the selectivity of the process has been carefully examined. In addition, a mechanism which accounts for the formation of all the products has been proposed. 相似文献
79.
Fan Chung 《Annals of Combinatorics》2005,9(1):1-19
We consider Laplacians for directed graphs and examine their eigenvalues. We introduce a notion of a circulation in a directed graph and its connection with the Rayleigh quotient. We then define a Cheeger constant and establish the Cheeger inequality for directed graphs. These relations can be used to deal with various problems that often arise in the study of non-reversible Markov chains including bounding the rate of convergence and deriving comparison theorems.Received September 8, 2004 相似文献
80.
5‐Methylene‐2‐oxazolidinone (1) and 5‐methylene‐1,3‐thiazolidine‐2‐thione (4) react with various isocyanates to give the corresponding urethanes 3 and 5 in high yields in the presence of palladium(0) or palladium(II) catalyst under mild reaction conditions. A mechanism is proposed. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献