全文获取类型
收费全文 | 1429篇 |
免费 | 50篇 |
国内免费 | 1篇 |
专业分类
化学 | 1025篇 |
晶体学 | 17篇 |
力学 | 80篇 |
数学 | 109篇 |
物理学 | 249篇 |
出版年
2023年 | 6篇 |
2022年 | 33篇 |
2021年 | 47篇 |
2020年 | 34篇 |
2019年 | 26篇 |
2018年 | 35篇 |
2017年 | 31篇 |
2016年 | 56篇 |
2015年 | 55篇 |
2014年 | 45篇 |
2013年 | 70篇 |
2012年 | 103篇 |
2011年 | 129篇 |
2010年 | 72篇 |
2009年 | 70篇 |
2008年 | 85篇 |
2007年 | 82篇 |
2006年 | 74篇 |
2005年 | 73篇 |
2004年 | 76篇 |
2003年 | 47篇 |
2002年 | 50篇 |
2001年 | 13篇 |
2000年 | 13篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 6篇 |
1996年 | 12篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1934年 | 1篇 |
1906年 | 1篇 |
1905年 | 1篇 |
排序方式: 共有1480条查询结果,搜索用时 15 毫秒
101.
M.C.C. Peters I.F. de Oliveira M.G.M. Machado D.C. Ferreira M.H.A. Zanin N. Bou-Chacra 《Materials Today Chemistry》2021
The glucocorticoid derivative of budesonide with a phthalimide group is a drug candidate to treat inflammatory eye diseases; nevertheless, it presents low water solubility. Drug nanocrystals have been proposed to overcome this hurdle. The development of an innovative ophthalmic anti-inflammatory nanosuspension was performed using a design space approach. We obtained the particle size reduction of this glucocorticoid derivative on a nanometer scale (approximately 165.0 nm), applying wet bead milling on a super reduced scale. The design of experiment supported the optimization of the formula evaluating the parameters that influence reducing the particle size and also allowed determining the design space. Considering the two statistical models developed and the size range obtained, we proposed that the optimized formulation for the glucocorticoid derivative nanosuspension may be 1.0 wt% glucocorticoid derivative and 0.092 wt% cetylpyridinium chloride. This formulation was characterized by the morphological, physical–chemical, and mucoadhesive in vitro test and showed potential for ophthalmic use with reduced frequency of product application, improved efficiency, and safety, which may promote better patient compliance. 相似文献
102.
Córdova A Ibrahem I Casas J Sundén H Engqvist M Reyes E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(16):4772-4784
Hexose sugars play a fundamental role in vital biochemical processes and their biosynthesis is achieved through enzyme-catalyzed pathways. Herein we disclose the ability of amino acids to catalyze the asymmetric neogenesis of carbohydrates by sequential cross-aldol reactions. The amino acids mediate the asymmetric de novo synthesis of natural L- and D-hexoses and their analogues with excellent stereoselectivity in organic solvents. In some cases, the four new stereocenters are assembled with almost absolute stereocontrol. The unique feature of these results is that, when an amino acid is employed as the catalyst, a single reaction sequence can convert a protected glycol aldehyde into a hexose in one step. For example, proline and its derivatives catalyze the asymmetric neogenesis of allose with >99 % ee in one chemical manipulation. Furthermore, all amino acids tested catalyzed the asymmetric formation of natural sugars under prebiotic conditions, with alanine being the smallest catalyst. The inherent simplicity of this catalytic process suggests that a catalytic prebiotic "gluconeogenesis" may occur, in which amino acids transfer their stereochemical information to sugars. In addition, the amino acid catalyzed stereoselective sequential cross-aldol reactions were performed as a two-step procedure with different aldehydes as acceptors and nucleophiles. The employment of two different amino acids as catalysts for the iterative direct aldol reactions enabled the asymmetric synthesis of deoxysugars with >99 % ee. In addition, the direct amino acid catalyzed C(2)+C(2)+C(2) methodology is a new entry for the short, highly enantioselective de novo synthesis of carbohydrate derivatives, isotope-labeled sugars, and polyketide natural products. The one-pot asymmetric de novo syntheses of deoxy and polyketide carbohydrates involved a novel dynamic kinetic asymmetric transformation (DYKAT) mediated by an amino acid. 相似文献
103.
Cesar Garcías-Morales Selene H. Martínez-Salas Armando Ariza-Castolo 《Tetrahedron letters》2012,53(26):3310-3315
The effect of the nitrogen nonbonding electron pair on the 1JC,H values of 1,3-diazaheterocycles was analyzed and compared to 1,5-diazabiciclo[3.2.1]octanes, which have a restricted conformation. The 1JC,H values were measured by observing the 13C satellites in the 1H NMR spectra and then determining the 1H-coupled 13C NMR spectra. The 1JC,H values are 10 Hz larger when the α-hydrogen is synperiplanar rather than antiperiplanar to the nonbonding electron pair on the nitrogen, which serves as experimental evidence of the orbital nN→σ1C,Hap interactions. In addition, the homoanomeric effect from the interactions of the nitrogen lone pair with the antibonding orbital of the equatorial hydrogen, which was in the β position, was discussed (nN→σ1C(β),Heq). 相似文献
104.
Xiao-Jun Yang Feng Gao J. A. Tenreiro Machado Dumitru Baleanu 《The European physical journal. Special topics》2017,226(16-18):3567-3575
In this paper, a new fractional derivative involving the normalized sinc function without singular kernel is proposed. The Laplace transform is used to find the analytical solution of the anomalous heat-diffusion problems. The comparative results between classical and fractional-order operators are presented. The results are significant in the analysis of one-dimensional anomalous heat-transfer problems. 相似文献
105.
Considering the notion of Jacobi type vector fields for a real hypersurface in a complex two-plane Grassmannian, we prove
that if a structure vector field is of Jacobi type it is Killing. As a consequence we classify real hypersurfaces whose structure
vector field is of Jacobi type. 相似文献
106.
107.
Mechanical Behavior of Polymer Gels for RDCs and RCSAs Collection: NMR Imaging Study of Buckling Phenomena
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Erich Hellemann Dr. Rubens R. Teles Prof. Dr. Fernando Hallwass Prof. Dr. W. Barros Jr. Dr. Armando Navarro‐Vázquez Prof. Dr. Roberto R. Gil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(46):16632-16635
Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or ΔνQ), appear in solution‐state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p‐HEMA) is an easy, user‐friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro‐imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross‐linker and the length of the gel. 相似文献
108.
Soraa Bouattour Wannes Kallel Ana Maria Botelho do Rego L.F. Vieira Ferreira I. Ferreira Machado Sami Boufi 《应用有机金属化学》2010,24(10):692-699
This work reports on the synthesis of Li‐doped TiO2 nanoparticles using the sol–gel process and solid‐state sintering, and investigates their potential use as a photocatalyst for degradation under sunlight excitation of different organic model compounds in aqueous solution. The structure of the nanocrystals was examined by X‐ray diffraction, UV‐vis ground state diffuse reflectance absorption spectra and X‐ray photoelectron emission spectroscopy. Results showed that samples prepared by sol–gel process and calcined at 400 °C are composed of a mixture of anatase and rutile phases, in contrast to the one prepared by solid‐state sintering, which exhibits an anatase phase with Li being involved in a spinel phase. The photocatalytic degradation of aqueous solutions of different aromatic compounds was successfully achieved under sunlight excitation in presence of Li‐doped TiO2 prepared via sol–gel process. It was shown that the calcination temperature and the preparation mode greatly affect the photocatalytic efficiency. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
109.
From superhydrophobic‐ to superhydrophilic‐patterned poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) architectures as a novel platform for biotechnological applications
下载免费PDF全文
![点击此处可从《Journal of Polymer Science.Polymer Physics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Vanessa F. Cardoso Ana R. Machado Vânia C. Pinto Paulo J. Sousa Gabriela Botelho Graça Minas Senentxu Lanceros‐Méndez 《Journal of Polymer Science.Polymer Physics》2016,54(18):1802-1810
The manufacture of three‐dimensional patterned electroactive poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) microstructures with tailored architecture, morphology, and wettability is presented. The patterned microstructures are fabricated using a simple, effective, low cost, and reproducible technique based on microfluidic technology. These novel structures can represent innovative platforms for advanced strategies in a wide range of biotechnological applications, including tissue engineering, drug delivery, microfluidic, and sensors and actuators devices. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1802–1810 相似文献
110.
In the current program for the preparation of an urban air map, this paper reports data obtained by the five base stations in the period 1997-2000 regarding traditional pollutants and gives the first results obtained in determining continuously benzene, toulene and PM10. Additional measurements have been carried out by a mobile station at the falcate zone, never tested before, which has very low traffic but with the presence of some small metal industries and, under favourable meteorological conditions, the effects of the harbour. A statistical study on the hourly-trend of particulate suspended matter in two zones of the city which have different traffic volume for the period 1995-1997 is also reported. The ozone characteristic peak at 15.00 is fully confirmed in all stations and over all the period contrarily to most Italian cities, which present the peak about three hours earlier. PM10 annual mean value during 1999 was a little higher than the legal one. Base stations "Archimede" and "Minissale" still present some risk of a stop or limitation of traffic even with respect to traditional or "new" pollutants. Statistical analysis of particulate suspended matter confirm the very different characteristics of the base stations under consideration. 相似文献