全文获取类型
收费全文 | 6332篇 |
免费 | 290篇 |
国内免费 | 56篇 |
专业分类
化学 | 3744篇 |
晶体学 | 63篇 |
力学 | 251篇 |
综合类 | 2篇 |
数学 | 1453篇 |
物理学 | 1165篇 |
出版年
2023年 | 51篇 |
2022年 | 134篇 |
2021年 | 170篇 |
2020年 | 173篇 |
2019年 | 201篇 |
2018年 | 186篇 |
2017年 | 173篇 |
2016年 | 274篇 |
2015年 | 204篇 |
2014年 | 282篇 |
2013年 | 449篇 |
2012年 | 435篇 |
2011年 | 475篇 |
2010年 | 256篇 |
2009年 | 291篇 |
2008年 | 341篇 |
2007年 | 287篇 |
2006年 | 219篇 |
2005年 | 207篇 |
2004年 | 204篇 |
2003年 | 167篇 |
2002年 | 153篇 |
2001年 | 96篇 |
2000年 | 93篇 |
1999年 | 53篇 |
1998年 | 41篇 |
1997年 | 56篇 |
1996年 | 72篇 |
1995年 | 62篇 |
1994年 | 49篇 |
1993年 | 53篇 |
1992年 | 45篇 |
1991年 | 46篇 |
1990年 | 44篇 |
1989年 | 28篇 |
1988年 | 38篇 |
1987年 | 45篇 |
1986年 | 44篇 |
1985年 | 39篇 |
1984年 | 49篇 |
1983年 | 29篇 |
1982年 | 44篇 |
1981年 | 37篇 |
1980年 | 32篇 |
1979年 | 40篇 |
1978年 | 36篇 |
1977年 | 33篇 |
1976年 | 35篇 |
1975年 | 24篇 |
1974年 | 26篇 |
排序方式: 共有6678条查询结果,搜索用时 15 毫秒
231.
232.
233.
Hou Chien Chang Ching Hsuan Lin Hung Tse Lin Shenghong A. Dai 《Journal of polymer science. Part A, Polymer chemistry》2012,50(5):1008-1017
Generally, protection and deprotection procedures of amino groups are required in preparing propargyl ether‐containing benzoxazines. In this study, we report a facile, deprotection‐free preparation of a propargyl ether‐containing phosphinated benzoxazine (2) from the nucleophilic substitution of a phenolic OH‐containing phosphinated benzoxazine (1) and propargyl bromide in the catalysis of potassium carbonate. The structure of (2) was characterized and confirmed by a high‐resolution mass spectrum, 1H, 13C, 1H‐1H, 1H‐13C nuclear magnetic resonance (NMR) spectra, and X‐ray single crystal diffractogram. infrared (IR) and differential scanning calorimetry were used to monitor the ring‐opening of benzoxazine and crosslinking of propargyl ether. The microstructure and the structure–property relationship of the resulting homopolymers and copolymers are discussed. The Tg of homopolymer of (2) is 208 °C by dynamic mechanical analysis, the coefficient of thermal expansion is 43 ppm/°C, and Td 5% (N2) is 393 °C, respectively, which are higher than those of the homopolymer of (1) . Similar trends were observed in the copolymerization system. The results demonstrate the beneficial effect of crosslinking afforded by the propargyl ether group is higher than that by the phenolic OH group. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
234.
235.
Huy Loc Nguyen James Dedier Jean-Claude Rouillon Gilles Sigaud 《Liquid crystals》2013,40(11):1637-1644
The synthesis of two series of semiperfluoroalkyloxyphenyl and perfluoroalkylphenyl β-D-glucopyranosides is described. Their mesomorphic properties are studied by DSC, polarized optical microscopy and X-ray diffraction. All mesophases are identified as smectic A with a bimolecular in-layer arrangement. 相似文献
236.
Ly Dieu Ha Poul Erik Hansen Fritz Duus Hung Dinh Pham Lien‐Hoa D. Nguyen 《Magnetic resonance in chemistry : MRC》2012,50(3):242-245
Four new prenylated depsidones, oliveridepsidones A–D, were isolated from the bark of Garcinia oliveri collected in Vietnam. Their structures were elucidated using mainly NMR techniques (1H and 13C NMR, HMQC, HMBC and NOE experiments). Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
237.
Julien Iehl Thi Le Anh Nguyen Stéphane Frein Uwe Hahn Joaquín Barberá Jean-François Nierengarten 《Liquid crystals》2017,44(12-13):1852-1860
ABSTRACTLiquid-crystalline [60]fullerodendrimers were constructed via click chemistry based on the reaction between hexa-adducts of [60]fullerene (C60) bearing 12 azide groups and alkyne-terminated cyanobiphenyl dendrons of first- and second-generation. The structure of all the new compounds was confirmed by IR, UV, 1H and 13C NMR spectroscopies and mass spectrometry. The mesomorphic properties were studied by polarised optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. The hexa-adduct of C60 functionalised with the first-generation dendrons gave rise to the formation of a smectic A phase and a rectangular columnar phase (c2mm symmetry) while the hexa-adduct of C60 decorated with the second-generation dendrons displayed only a rectangular columnar phase (c2mm symmetry). Our results show that the hexa-adduct of C60 is a unique synthetic platform for the design of fullerodendrimers and dendronised materials. 相似文献
238.
Stuart Lipsitz Garrett Fitzmaurice Debajyoti Sinha Nathanael Hevelone Jim Hu Louis L. Nguyen 《Journal of computational and graphical statistics》2017,26(3):734-737
Medical studies increasingly involve a large sample of independent clusters, where the cluster sizes are also large. Our motivating example from the 2010 Nationwide Inpatient Sample (NIS) has 8,001,068 patients and 1049 clusters, with average cluster size of 7627. Consistent parameter estimates can be obtained naively assuming independence, which are inefficient when the intra-cluster correlation (ICC) is high. Efficient generalized estimating equations (GEE) incorporate the ICC and sum all pairs of observations within a cluster when estimating the ICC. For the 2010 NIS, there are 92.6 billion pairs of observations, making summation of pairs computationally prohibitive. We propose a one-step GEE estimator that (1) matches the asymptotic efficiency of the fully iterated GEE; (2) uses a simpler formula to estimate the ICC that avoids summing over all pairs; and (3) completely avoids matrix multiplications and inversions. These three features make the proposed estimator much less computationally intensive, especially with large cluster sizes. A unique contribution of this article is that it expresses the GEE estimating equations incorporating the ICC as a simple sum of vectors and scalars. 相似文献
239.
240.