全文获取类型
收费全文 | 49431篇 |
免费 | 4877篇 |
国内免费 | 8077篇 |
专业分类
化学 | 36851篇 |
晶体学 | 1948篇 |
力学 | 2746篇 |
综合类 | 475篇 |
数学 | 7506篇 |
物理学 | 12859篇 |
出版年
2024年 | 117篇 |
2023年 | 470篇 |
2022年 | 1029篇 |
2021年 | 1048篇 |
2020年 | 1390篇 |
2019年 | 1271篇 |
2018年 | 1160篇 |
2017年 | 1360篇 |
2016年 | 1830篇 |
2015年 | 1720篇 |
2014年 | 2208篇 |
2013年 | 3527篇 |
2012年 | 3764篇 |
2011年 | 2834篇 |
2010年 | 2425篇 |
2009年 | 3007篇 |
2008年 | 3222篇 |
2007年 | 3334篇 |
2006年 | 3051篇 |
2005年 | 2787篇 |
2004年 | 2699篇 |
2003年 | 2397篇 |
2002年 | 2492篇 |
2001年 | 1676篇 |
2000年 | 1665篇 |
1999年 | 1366篇 |
1998年 | 1246篇 |
1997年 | 1021篇 |
1996年 | 955篇 |
1995年 | 836篇 |
1994年 | 780篇 |
1993年 | 603篇 |
1992年 | 598篇 |
1991年 | 359篇 |
1990年 | 299篇 |
1989年 | 233篇 |
1988年 | 218篇 |
1987年 | 152篇 |
1986年 | 136篇 |
1985年 | 163篇 |
1984年 | 131篇 |
1983年 | 101篇 |
1982年 | 108篇 |
1981年 | 94篇 |
1980年 | 88篇 |
1979年 | 78篇 |
1978年 | 62篇 |
1977年 | 58篇 |
1974年 | 48篇 |
1973年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
The structure of the title adduct comprises a phenanthroline derivative 2-phenyl-imidazo[4,5-f]1,10-phenanthroline and a methanol.The composition of the crystalline adduct was characterized as C19H12N4.CH3OH.It belongs to orthorhombic system,space group Pna21 with a=1.3693(4)nm,b=2.2988(7)nm,c=0.51338(15)nm,V=1.6160(8)nm^3.Z=4,and final R1=0.0423.wR2=0.1012 .Crystal structure shows that all the 19 carbon atoms and 4 nitrogen atoms are coplanar.The bond length data indicated that a very extensive conjugation system was formed.This conjugation makes the compound being a potentially excellent energy transformer used for luminescent materials. 相似文献
92.
This paper presents a quantum chemical calculation of native (2–7 fructoside residues) and chemically modified (2–4 fructoside residues) levan molecule models. A levan modification was carried out by oxidation and the following reduction or hyrdazonation of the fructoside rings. The conformational particularity and reaction ability was studied for the native and for the modified levan molecules. 相似文献
93.
ZHOU Ming-Xiua② YANG Chuna DENG Xiao-Yana YU Wei-Feib LI Jin-Shanb a 《结构化学》2006,25(6):647-652
1 INTRODUCTION Silicon and its alloy have been widely applied in such fields as electronic industry, high-temperature structural ceramics, etc. In addition, the researches on silicon and its relevant materials greatly promote the rapid development of modern optics and infor- mation technology. Therefore, more and more at- tention is focused on the structure of silicon, oxide of silicon and the interfaces between silicon and metal or nonmetal. As an ideal passive film on the Si surface, S… 相似文献
94.
Dinesh Topwal U. Manju Sugata Ray S. Raj D. D. Sarma S. R. Krishnakumar M. Bertolo S. La Rosa G. Cautero 《Journal of Chemical Sciences》2006,118(1):87-92
Disordered Sr2FeMoO6 shows a drastic reduction in saturation magnetization compared to highly ordered samples, moreover magnetization as a function
of the temperature for different disordered samples shows qualitatively different behaviours. We investigate the origin of
such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish
that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples, is
responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with
composition fluctuations of the type Sr2Fe1+x
Mo1-x
O6 with Fe-rich (x > 0) and Mo-rich (x < 0) regions.
Dedicated to Prof J Gopalakrishnan on his 62nd birthday. 相似文献
95.
Yong‐Jin Kim Jin‐Woong Kim Jung‐Eun Lee Jee‐Hyun Ryu Junoh Kim Ih‐Seop Chang Kyung‐Do Suh 《Journal of polymer science. Part A, Polymer chemistry》2004,42(22):5627-5635
Mesoporous polymer microspheres with gold (Au) nanoparticles inside their pores were prepared considering their surface functionality and porosity. The Au/polymer composite microspheres prepared were characterized by transmission electron microscope (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) techniques. The results showed that the adsorption of Au nanoparticles could be increased by imparting the pore structure and surface‐functional groups into the supporting polymer microspheres (in this study, poly (ethylene glycol dimethacrylate‐co‐acrylonitrile) and poly (EGDMA‐co‐AN) system). Above all, from this study, it was established that the porosity of the polymer microspheres is the most important factor that determines the distribution and adsorption amount of face‐centered cubic (fcc) Au nanoparticles in the final products. Our study showed that the continuous adsorption of Au nanoparticles with the aid of the large surface area and surface interaction sites formed more favorably the Au/polymer composite microspheres. The BET measurements of Au/poly(EGDMA‐co‐AN) composite microspheres reveals that the adsorption of Au nanoparticles into the pores kept the pore structure intact and made it more porous. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5627–5635, 2004 相似文献
96.
A. Truffert 《Annals of Operations Research》1991,30(1):115-156
The conditional expectation of integrands and random sets is the main tool of stochastic optimization. This work wishes to make up for the lack of real synthesis about this subject. We improve the existing hypothesis and simplify the corresponding proofs. In the convex case we especially study the problem of the exchange of conditional expectation and subdifferential operators. 相似文献
97.
Characteristics and structures of weak efficient surfaces of production possibility sets 总被引:1,自引:0,他引:1
Quanling Wei Hong Yan Gang Hao 《Journal of Mathematical Analysis and Applications》2007,327(2):1055-1074
This paper studies the characteristics and structure of the weak surface of the production possibility set. We apply techniques and methods of transferring a polyhedral cone from its intersection form to its sum form, identify an intersection representation of the production possibility set. We give the structure theorem of weak surface of the production possibility set, which includes three complementary slackness conditions. We define the input weak efficiency and output weak efficiency for different DEA models according to the representation of the intersection form. It investigates the characteristics of the weak surfaces, and proves the structure theorems of input weak DEA efficiency and output weak DEA efficiency. The structure theorems establish weighted combination of inputs and outputs that are weak DEA efficient. Numerical examples are provided for illustration. 相似文献
98.
Zhen‐Feng Chen Hong‐Li Zhou Hong Liang Yan Li Ren‐Gen Xiong Xiao‐Zeng You 《应用有机金属化学》2003,17(11):883-884
The centrosymmetric binuclear structure of [Pb2(H‐Norf)2(ONO2)4]shows the geometry around each lead(II) atom to be distorted trigonal bipyramidal with Pb–O distances ranging from 2.357(3) to 2.769(4) Å. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
99.
本文给出递归集的Hausdorff维数的下界估计,并由此确定了一类递归集的维数,所获结果包含并推广了Bedford,Dekking及文志英、钟红柳等人的有关结果。 相似文献
100.