全文获取类型
收费全文 | 17993篇 |
免费 | 1241篇 |
国内免费 | 839篇 |
专业分类
化学 | 13718篇 |
晶体学 | 157篇 |
力学 | 438篇 |
综合类 | 66篇 |
数学 | 2829篇 |
物理学 | 2865篇 |
出版年
2023年 | 172篇 |
2022年 | 191篇 |
2021年 | 372篇 |
2020年 | 456篇 |
2019年 | 448篇 |
2018年 | 402篇 |
2017年 | 408篇 |
2016年 | 783篇 |
2015年 | 711篇 |
2014年 | 804篇 |
2013年 | 1276篇 |
2012年 | 1536篇 |
2011年 | 1690篇 |
2010年 | 1076篇 |
2009年 | 888篇 |
2008年 | 1382篇 |
2007年 | 1332篇 |
2006年 | 1124篇 |
2005年 | 1033篇 |
2004年 | 879篇 |
2003年 | 614篇 |
2002年 | 528篇 |
2001年 | 195篇 |
2000年 | 167篇 |
1999年 | 174篇 |
1998年 | 113篇 |
1997年 | 120篇 |
1996年 | 122篇 |
1995年 | 90篇 |
1994年 | 68篇 |
1993年 | 74篇 |
1992年 | 65篇 |
1991年 | 58篇 |
1990年 | 61篇 |
1989年 | 49篇 |
1988年 | 40篇 |
1987年 | 47篇 |
1986年 | 50篇 |
1985年 | 64篇 |
1984年 | 59篇 |
1983年 | 37篇 |
1982年 | 38篇 |
1981年 | 41篇 |
1980年 | 27篇 |
1979年 | 24篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1976年 | 11篇 |
1975年 | 17篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
81.
We focus on continuous Markov chains as a model to describe the evolution of credit ratings. In this work it is checked whether a simple, tridiagonal type of generator provides a good approximation to a general one. Three different tridiagonal approximations are proposed and their performance is checked against two generators, corresponding to a volatile and a stable period, respectively. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
82.
First order necessary conditions and duality results for general inexact nonlinear programming problems formulated in nonreflexive
spaces are obtained. The Dubovitskii–Milyutin approach is the main tool used. Particular cases of linear and convex programs
are also analyzed and some comments about a comparison of the obtained results with those existing in the literature are given. 相似文献
83.
Berry Frank J. Ren Xiaolin Gancedo J. Ramón Marco José F. 《Hyperfine Interactions》2004,156(1-4):335-340
Hyperfine Interactions - Perovskite-related phases of the type LaFe1?x Co x O3 (x=0 and 0.5) have been synthesised by milling techniques. The materials are of smaller particle size and more... 相似文献
84.
Guan Wu WANG* Li Juan JIAO Er Hong HAO Yong Ming LU You Jun YANG Department of Chemistry University of Science Technology of China Hefei 《中国化学快报》2004,15(5)
Water-soluble fullerenes not only have a number of important applications in biological chemistry1, but also form ordered aggregate structures that can be utilized in materials science2. There are reports that some water-soluble fullerene derivatives can form ordered structures in water, such as vesicle, nanorod and nanoparticle2-5. However, relatively few reports on the aggregation process have been reported. Knowledge of factors that affect the cluster formation in solution is of consider… 相似文献
85.
Three novel zinc complexes [Zn(dbsf)(H2O)2] ( 1 ), [Zn(dbsf)(2,2′‐bpy)(H2O)]·(i‐C3H7OH) ( 2 ) and [Zn(dbsf)(DMF)] ( 3 ) (H2dbsf = 4,4′‐dicarboxybiphenyl sulfone, 2,2′‐bpy = 2,2′‐bipyridine, i‐C3H7OH = iso‐propanol, DMF = N,N‐dimethylformamide) were first obtained and characterized by single crystal X‐ray crystallography. Although the results show that all the complexes 1–3 have one‐dimensional chains formed via coordination bonds, unique three‐dimensional supramolecular structures are formed due to different coordination modes and configuration of the dbsf2? ligand, hydrogen bonds and π–π interactions. Iso‐propanol molecules are in open channels of 2 while larger empty channels are formed in 3 . As compared with emission band of the free H2dbsf ligand, emission peaks of the complexes 1–3 are red‐shifted, and they show blue emission, which originates from enlarging conjugation upon coordination. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
86.
Kan‐Yi Pu Yi Chen Xiao‐Ying Qi Chun‐Yang Qin Qing‐Quan Chen Hong‐Yu Wang Yun Deng Qu‐Li Fan Yan‐Qin Huang Shu‐Juan Liu Wei Wei Bo Peng Wei Huang 《Journal of polymer science. Part A, Polymer chemistry》2007,45(16):3776-3787
In this contribution, we demonstrate a new effective methodology for constructing highly efficient and durable poly(p‐phenyleneethynylene) (PPE) containing emissive material with nonaggregating and hole‐facilitating properties through the introduction of hole‐transporting blocks into the PPE system as the grafting coils as well as building the energy donor–acceptor architecture between the grafting coils and the PPE backbone. Poly(2‐(carbazol‐9‐yl)ethyl methacrylate) (PCzEMA), herein, is chosen as the hole‐transporting blocks, and incorporated into the PPE system as the grafting coils via atom transfer radical polymerization. The chemical structure of the resultant copolymer, PPE‐g‐PCzEMA, was characterized by NMR and gel permeation chromatography, showing that the desirable copolymer was obtained with the narrow polydispersity. The increased thermal stability of PPE‐g‐PCzEMA was confirmed by thermogravimetric analysis and differential scanning calorimetry along with its macroinitiator. The optoelectronic properties of this copolymer were studied in detail by ultraviolet‐visible absorption, photoluminescence emission and excitation spectra, and cyclic voltammogram (CV). The results indicate that PPE‐g‐PCzEMA exhibits the solid‐state luminescent property dominated by individual lumophores, and also the energy transfer process from the PCzEMA blocks to the PPE backbone with a relatively higher energy transfer efficiency in the solid‐state compared to that of the solution state. Additionally, the hole‐injection property is greatly facilitated due to the presence of PCzEMA, as confirmed by CV profiles. All these data indicate that PPE‐g‐PCzEMA is a good candidate for use in optoelectronic devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3776–3787, 2007 相似文献
87.
José Natário 《General Relativity and Gravitation》2006,38(3):475-484
We find a class of warp drive spacetimes possessing Newtonian limits, which we then determine. The same method is used to
compute Newtonian limits of the Schwarzschild solution and spatially flat Friedmann-Robertson-Walker cosmological models.
This work was partially supported by FCT/POCTI/FEDER. 相似文献
88.
89.
This article explores the use of geometric algebra in linear and multilinear algebra, and in affine, projective and conformal geometries. Our principal objective is to show how the rich algebraic tools of geometric algebra are fully compatible with and augment the more traditional tools of matrix algebra. The novel concept of an h-twistor makes possible a simple new proof of the striking relationship between conformal transformations in a pseudo-Euclidean space to isometries in a pseudo-Euclidean space of two higher dimensions. The utility of the h-twistor concept, which is a generalization of the idea of a Penrose twistor to a pseudo-Euclidean space of arbitrary signature, is amply demonstrated in a new treatment of the Schwarzian derivative. 相似文献
90.
There is a general interest in ranking schemes applied to complex entities described by multiple attributes. Published rankings
for universities are in great demand but are also highly controversial. We compare two classification and ranking schemes
involving universities; one from a published report, ‘Top American Research Universities’ by the University of Florida's TheCenter and the other using DEA. Both approaches use the same data and model. We compare the two methods and discover important equivalences.
We conclude that the critical aspect in classification and ranking is the model. This suggests that DEA is a suitable tool
for these types of studies. 相似文献