全文获取类型
收费全文 | 901篇 |
免费 | 30篇 |
国内免费 | 5篇 |
专业分类
化学 | 600篇 |
晶体学 | 10篇 |
力学 | 19篇 |
数学 | 107篇 |
物理学 | 200篇 |
出版年
2024年 | 4篇 |
2023年 | 9篇 |
2022年 | 23篇 |
2021年 | 21篇 |
2020年 | 24篇 |
2019年 | 35篇 |
2018年 | 17篇 |
2017年 | 21篇 |
2016年 | 31篇 |
2015年 | 22篇 |
2014年 | 54篇 |
2013年 | 73篇 |
2012年 | 61篇 |
2011年 | 83篇 |
2010年 | 43篇 |
2009年 | 42篇 |
2008年 | 44篇 |
2007年 | 71篇 |
2006年 | 41篇 |
2005年 | 31篇 |
2004年 | 35篇 |
2003年 | 17篇 |
2002年 | 21篇 |
2001年 | 9篇 |
2000年 | 7篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1966年 | 1篇 |
1965年 | 2篇 |
排序方式: 共有936条查询结果,搜索用时 31 毫秒
1.
2.
3.
Songxin Tan Ram M. Narayanan Sunil K. Shetty 《International Journal of Infrared and Millimeter Waves》2005,26(8):1175-1194
There is growing interest in the use of lidar for remote sensing of vegetation owing to the emergence of reliable and rugged lasers and highly sensitive detectors. Lidar remote sensing has a distinct advantage over conventional techniques in vegetation remote sensing due to its capability for three-dimensional characterization of vegetative targets. The Multiwavelength Airborne Polarimetric Lidar (MAPL) system was developed primarily for vegetation remote sensing applications from an airborne platform of up to 1,000 -m altitude. The lidar system has full waveform capture and polarimetric measurement capability at two wavelengths in the near-infrared (1064 nm) and the green (532 nm) spectral regions. This study presents preliminary ground-based lidar reflectance measurements on a variety of deciduous and coniferous trees under fully foliated conditions with a view towards tree species discrimination. Variations in the reflectance characteristics of selected deciduous trees under unfoliated and fully foliated conditions were also investigated. Our study reveals distinct differences in the reflectance characteristics of various trees. 相似文献
4.
The periodic precipitation pattern formation in gelatinous media is interpreted as a moving boundary problem. The time law,
spacing law and width law are revisited on the basis of the new scenario. The explicit dependence of the geometric structure
on the initial concentrations of the reactants is derived. Matalon—Packter law, which relates the spacing coefficient with
the initial concentrations is reformulated removing many ambiguities and impractical parameters. Experimental results are
discussed to establish the significance of moving boundary concept in the diffusion controlled pattern forming systems 相似文献
5.
Anwar Usman Ibrahim Abdul Razak Suchada Chantrapromma Hoong‐Kun Fun Varughese Philip A. Sreekanth M. R. Prathapachandra Kurup 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(11):o652-o654
The title compound, C16H17N5S, is in the thione form and crystallizes with two independent molecules in the asymmetric unit. In both molecules, the pentamethyleneimine five‐membered ring adopts an envelope conformation, and in one of the molecules this ring shows positional disorder. The thione S and hydrazine N atoms are in the Z configuration with respect to the C—N bond. 相似文献
6.
Sunil B. Jadhav 《Tetrahedron letters》2007,48(14):2485-2487
A rapid, mild and high yielding method for the synthesis of methylthiomethyl esters is reported from the corresponding aliphatic, aromatic and unsaturated carboxylic acids under Swern oxidation conditions using dimethylsulfoxide, oxalyl chloride and triethylamine at low temperature. 相似文献
7.
Reichard GA Stengone C Paliwal S Mergelsberg I Majmundar S Wang C Tiberi R McPhail AT Piwinski JJ Shih NY 《Organic letters》2003,5(23):4249-4251
[structure: see text] A highly efficient and practical synthesis of 4,4-Disubstituted-2-Imidazolidinones utilizing a "self-reproduction of the center of chirality" strategy is described. 相似文献
8.
Varughese M. Alexander 《Tetrahedron letters》2005,46(40):6957-6959
Bismuth(III) nitrate pentahydrate is found to be an efficient catalyst for the Pechmann condensation reaction of phenols and β-ketoesters under solvent-free conditions. The reaction protocol is simple and is followed by aqueous work-up leading to the formation of the corresponding coumarin derivatives in good yield and high purity. 相似文献
9.
Junquera E Laynez J Menéndez M Sharma S Penadés S 《The Journal of organic chemistry》1996,61(20):6790-6798
Thermodynamic studies of the binding of a series of p-nitrophenyl glycosides (PNPGly) of varying stereochemistry to alpha-cyclodextrin (alpha-CD) were performed at three different temperatures (25, 35, and 42 degrees C) using a microcalorimetric technique. The system p-nitrophenol (PNP) at pH = 3 and alpha-CD was also studied for the sake of comparison. All these complexes were found to be enthalpy driven with a favorable enthalpic term clearly dominant over an unfavorable entropic term. A clear enthalpy-entropy compensation effect was observed at all the temperatures, with a slope close to unity (alpha = 1.02) and an intercept TDeltaS degrees (o) = 2.91 kcal mol(-)(1). This thermodynamic pattern is in agreement with those usually found for lectin-carbohydrate associations and for the binding processes of several host-guest systems. This pattern is explained in terms of the contribution of primarily two driving forces: the van der Waals interactions between the host and the guest, and the solvation/desolvation processes which accompany the association reaction. The presence of the carbohydrate molecule in the PNP ring causes a slight destabilization of the complex at 25 degrees C with respect to the alpha-CD-PNP (pH = 3) complex, although a different behavior has been observed depending on the axial/equatorial configuration of the glycoside and the temperature. This behavior is modulated by the stereochemistry of the glycoside. Differences were observed between the deoxy-derivatives (LAra and LFuc) and those derivatives with a hydroxymethyl group (Glc, Gal, Man). DeltaC(p) degrees values were obtained from the dependency of DeltaH degrees on temperature (=( partial differentialDeltaH degrees / partial differentialT)(p)). These values are small and negative except for alphaMan complex. For the latter complex, discrepancy between the calorimetric and the calculated van't Hoff enthalpies was observed. Parallels are drawn between the thermodynamics of our model and those proposed for carbohydrate-protein associations. 相似文献
10.
Hiremath Siddalingaiah A. Dodamani Ashok F. Naik Sunil G. 《Transition Metal Chemistry》2002,27(1):47-51
The title complex was prepared and characterized by elemental and spectral analyses. The formation constants of various heterocyclic nitrogen base adducts with the NiII–di(2,4-dibromophenyl)carbazonate have been determined in a monophase by spectrophotometry at 25 ± 0.1 °C. The monodenatate and bidentate bases form penta- and hexa-coordinated adducts respectively with 1:1 stoichiometry for the metal chelate-base, whereas the unsaturated monodentate bases form hexa-coordinated adducts with 1:2 stoichiometry. The behaviour of saturated heterocyclic bases such as pyrrolidine, piperidine, hexamethyleneimine and morpholine towards the metal chelate has been studied, and the results are discussed in terms of steric hindrance, basicity and ring structure. 相似文献