全文获取类型
收费全文 | 2254篇 |
免费 | 83篇 |
国内免费 | 19篇 |
专业分类
化学 | 1495篇 |
晶体学 | 14篇 |
力学 | 50篇 |
数学 | 425篇 |
物理学 | 372篇 |
出版年
2023年 | 12篇 |
2022年 | 16篇 |
2021年 | 26篇 |
2020年 | 41篇 |
2019年 | 41篇 |
2018年 | 30篇 |
2017年 | 31篇 |
2016年 | 75篇 |
2015年 | 64篇 |
2014年 | 85篇 |
2013年 | 96篇 |
2012年 | 121篇 |
2011年 | 157篇 |
2010年 | 97篇 |
2009年 | 90篇 |
2008年 | 138篇 |
2007年 | 149篇 |
2006年 | 131篇 |
2005年 | 112篇 |
2004年 | 125篇 |
2003年 | 55篇 |
2002年 | 62篇 |
2001年 | 30篇 |
2000年 | 31篇 |
1999年 | 26篇 |
1998年 | 23篇 |
1997年 | 27篇 |
1996年 | 30篇 |
1995年 | 23篇 |
1994年 | 27篇 |
1993年 | 18篇 |
1992年 | 15篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 17篇 |
1988年 | 17篇 |
1986年 | 13篇 |
1985年 | 18篇 |
1984年 | 20篇 |
1983年 | 21篇 |
1982年 | 18篇 |
1981年 | 20篇 |
1980年 | 18篇 |
1979年 | 13篇 |
1978年 | 19篇 |
1977年 | 22篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 17篇 |
1973年 | 17篇 |
排序方式: 共有2356条查询结果,搜索用时 0 毫秒
111.
Svante Wold Michael Sjöström Rolf Carlson Torbjörn Lundstedt Sven Hellberg Bert Skagerberg Conny Wikström Jerker Öhman 《Analytica chimica acta》1986
In multivariate data analysis such as principal components analysis (PCA) and projections to latent structures (PLS), it is essential that the training set systems (objects) are selected to provide data with substantial information for model parametrization, and to represent properly any future situations where the multilvariate model is used for predictions. In the framework of multivariate projections (PCA, SIMCA and PLS), elementary concepts of statistical design (fractional factorials and composite designs) can be used with the latent variables (PC or PLS scores) as design variables. The plan of action thus becomes: (1) problem formulation (specify aim and model, make a conceptual division of the investigated system into subsystems); (2) collection of multivariate data for each type of subsystems; (3) estimation of the practical dimensionality of the data for each type of subsystems by PC or PLS analysis; (4) use of the PC or PLS scores (t) as design variables in the combination of subsystems to systems in the training set; (5) measurement of responses (Y); (6) analysis of data by PCA or PLS; (7) interpretation of results with possible feedback to steps 1, 2 or 3. The procedures are illustrated by two problems: a structure/activity relationship for a family of peptides, and optimization of an organic synthesis with respect to system variables (solvent, substrate, co-reactant_) and process variables (temperature, reactant concentrations). 相似文献
112.
Lennart Bergström Sara Stemme Torbjörn Dahlfors Hans Arwin Lars Ödberg 《Cellulose (London, England)》1999,6(1):1-13
Calculations of Hamaker constants using Lifshitz theory require the availability of accurate dielectric data, especially in the visible-ultraviolet region. We present spectroscopic ellipsometry data on well-defined cellulose films of a limited thickness range (100–140 layers) deposited on an oxidised and hydrophobised silicon substrate. The spectral data, representing measurements from a perpendicular orientation to the fibre deposition direction, was used for estimates of the necessary spectral parameters, i.e. the oscillator strengths and characteristic frequencies in the UV-range. Our calculations show that cellulose has a relatively low Hamaker constant in air (58 zJ) and water (8.0 zJ). The implications for the surface energy estimates of cellulose and colloidal interactions between cellulose and various types of fillers and coating colours are indicated. 相似文献
113.
Optically active C(2)-symmetric (1S,2S,4S,5S)-bicyclo[2.2.2]octane-2,5-diol ((+)-12; 98% ee) and several selectively protected optically active intermediates useful for synthetic transformations were synthesized via a 1,2-carbonyl transposition route starting from the easily available optically active (1R,4S,6S)-6-hydroxybicyclo[2.2.2]octan-2-one ((-)-2). The synthetic route also allowed the preparation of optically active (1S,4S)-bicyclo[2.2.2]octane-2,5-dione ((+)-14; 98% ee). 相似文献
114.
Novel prostaglandin F(2)(alpha) derivatives, functionalized at C13 and C14, have been prepared. 17-Phenyl-18,19,20-trinorprostaglandin F(2)(alpha) isopropyl ester [(15S)-1] and its epimer [(15R)-1] were stereoselectively epoxidized, using Sharpless conditions, to produce each of the four diastereomeric epoxides (15S)-2, (15S)-3, (15R)-2, and (15R)-3. Treatment of the four epoxides with LiOH stereospecifically-produced the pentahydroxy substituted analogues 12 and 13. Alternatively, epoxides 2 and 3were allowed to react with thiophenolate ion. The attack of the sulfur nucleophile on the epoxide occurred at either C13 or C14 depending on the stereochemistry of the epoxide and of C15. 相似文献
115.
Nicolai Stuhr-Hansen Jakob Kryger Srensen Kasper Moth-Poulsen Jrn Bolstad Christensen Thomas Bjrnholm Mogens Brndsted Nielsen 《Tetrahedron》2005,61(52):12288-12295
Simple and readily accessible aryl bromides are useful building blocks for thiol end-capped molecular wires. Thus, 4-bromophenyl tert-butyl sulfide and 1-bromo-4-(methoxymethyl)benzene serve as precursors for a variety of oligo(phenylenevinylene) and oligo(phenyleneethynylene) wires via efficient synthetic transformations as presented in this paper. 相似文献
116.
Roy A. Meoded Dr. Reiko Ueoka Dr. Eric J. N. Helfrich Dr. Katja Jensen Nancy Magnus Prof. Dr. Birgit Piechulla Prof. Dr. Jörn Piel 《Angewandte Chemie (International ed. in English)》2018,57(36):11644-11648
Enzymatic core components from trans‐acyltransferase polyketide synthases (trans‐AT PKSs) catalyze exceptionally diverse biosynthetic transformations to generate structurally complex bioactive compounds. Here we focus on a group of oxygenases identified in various trans‐AT PKS pathways, including those for pederin, oocydins, and toblerols. Using the oocydin pathway homologue (OocK) from Serratia plymuthica 4Rx13 and N‐acetylcysteamine (SNAC) thioesters as test surrogates for acyl carrier protein (ACP)‐tethered intermediates, we show that the enzyme inserts oxygen into β‐ketoacyl moieties to yield malonyl ester SNAC products. Based on these data and the identification of a non‐hydrolyzed oocydin congener with retained ester moiety, we propose a unified biosynthetic pathway of oocydins, haterumalides, and biselides. By providing access to internal ester, carboxylate pseudostarter, and terminal hydroxyl functions, oxygen insertion into polyketide backbones greatly expands the biosynthetic scope of PKSs. 相似文献
117.
118.
119.
Based on the proposed states of the Metropolis-Hastings (MH) algorithm we construct a MH Importance Sampling estimator for the approximation of expectations. The new approximation scheme is asymptotically correct and numerical experiments indicate that it can outperform the classical MH Markov chain Monte Carlo estimator. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
120.
Torbjörn Kolsrud 《Journal of Theoretical Probability》1989,2(4):399-418
We derive explicit isomorphism formulas between weighted Dirichlet integrals for harmonic functions and boundary Dirichlet forms. Applications yield results on traces of Markov processes and convergence quasieverywhere of harmonic functions. 相似文献