全文获取类型
收费全文 | 2860篇 |
免费 | 84篇 |
国内免费 | 255篇 |
专业分类
化学 | 1106篇 |
晶体学 | 43篇 |
力学 | 78篇 |
综合类 | 43篇 |
数学 | 1716篇 |
物理学 | 213篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 17篇 |
2021年 | 26篇 |
2020年 | 37篇 |
2019年 | 44篇 |
2018年 | 52篇 |
2017年 | 69篇 |
2016年 | 35篇 |
2015年 | 36篇 |
2014年 | 74篇 |
2013年 | 323篇 |
2012年 | 81篇 |
2011年 | 143篇 |
2010年 | 177篇 |
2009年 | 165篇 |
2008年 | 195篇 |
2007年 | 184篇 |
2006年 | 179篇 |
2005年 | 198篇 |
2004年 | 182篇 |
2003年 | 153篇 |
2002年 | 105篇 |
2001年 | 70篇 |
2000年 | 82篇 |
1999年 | 77篇 |
1998年 | 56篇 |
1997年 | 51篇 |
1996年 | 64篇 |
1995年 | 48篇 |
1994年 | 54篇 |
1993年 | 42篇 |
1992年 | 30篇 |
1991年 | 19篇 |
1990年 | 8篇 |
1989年 | 14篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有3199条查询结果,搜索用时 15 毫秒
61.
Young′s moduli of various epoxy coated polyethylene terephthalate (PET) micro-cantilevers were determined from the deflection results obtained using the phase-shift shadow moiré (PSSM) method. The filler materials for epoxy coatings were aluminum and graphite powders that were mixed with epoxy at various percentages. Young′s moduli were calculated from theory based on the deflection results. The PET micro-cantilever coated with aluminum-epoxy coating showed increasing value of Young′s modulus when the ratios of the aluminum-epoxy were increased. The graphite-epoxy coating on the PET micro-cantilever also showed the same trend. The experimental results also show that Young′s modulus of the graphite-epoxy coating is higher than aluminum-epoxy coating in comparison at the same mixing ratio. 相似文献
62.
H. Babacan 《Physics letters. A》2011,375(3):360-362
A relation between the generalized partition function (Tsallis) and density of states is established by using the method of integral transform which enables reducing some integral equations into the algebraic equations. Inverse Mellin transformation of this equation gives the density of states. Similar relation is also hold the for standard partition function (Boltzmann-Gibbs) and the density of states. Using these relations, we recover the density of states for the classical ideal gas within both statistics. 相似文献
63.
Chromones bearing electron-withdrawing substituents at the 2- or 3-position react with nonstabilized azomethine ylides to produce 1-benzopyrano[2,3-c]pyrrolidines in good yields. Reactions of 3-cyanochromones proceed diastereoselectively to give 1-benzopyrano[2,3-c]pyrrolidines and tetrahydro-1H-spiro[chromeno[2,3-c]pyrrol-9,5′-oxazolidine]-9a-carbonitriles, depending on the reactant ratio, as a result of a 1,3-dipolar cycloaddition of the azomethine ylide at the double bond and carbonyl group of the chromone system. The latter undergoes demethylenation and recyclization into a novel hexahydrochromeno[2,3-c:3,4-c′]dipyrrole tetracyclic system on heating with hydrochloric acid. 相似文献
64.
Qun Dang Zhibo Zhang Shuangsheng He Yaohong Liu Tongqian Chen Stephane Bogen Vinay Girijavallabhan David B. Olsen Peter T. Meinke 《Tetrahedron letters》2014
To discover novel nucleosides as potential antiviral agents, 4′-spirocyclic phosphono-nucleosides were designed to mimic the monophosphate of R-1479, a known nucleoside inhibitor of HCV NS5B. Bypassing the first kinase step to nucleoside monophosphate is viewed as advantageous since this phosphorylation is often observed as the rate-limiting transformation to the active NTP for many nucleosides. Efficient synthetic routes were developed with a triphenylphosphine–iodine cyclization reaction as the key step to form the tetrahydrofuran 4′-spirocycle. The desired 4′-spirocyclic phosphono-cytidine analogs 12a, 12b, and 16 were prepared in 11 steps. 相似文献
65.
An efficient synthesis of new pharmaceutically relevant dioxopyrrolidines, spirobenzo thiazine-2,3′-chromans, and benzothiazepines via isocyanide-based multicomponent condensation reactions at room temperature is reported. This synthesis serves as a nice addition to group-assistant-purification (GAP) chemistry in which purification via chromatography and recrystallization can be avoided, and the pure products are obtained simply by washing the crude products with 95% ethanol. 相似文献
66.
Fiame Leo Sadaki Asakuma Tadashi Nakamura Kenji Fukuda Akitsugu Senda Tadasu Urashima 《Journal of chromatography. A》2009,1216(9):1520-1523
An improved analytical scheme for human milk neutral oligosaccharides determination was developed, in which, the oligosaccharides were pooled in two fractions (pools 1 and 2) after gel filtration, and then were quantitatively derivatized with a single fluorescent reagent, 2-anthranilic acid. Separation was by reversed-phase HPLC on an ODS-100Z column with a mobile phase of 50 mM ammonium acetate pH 4.0 and 150 mM citrate buffer pH 4.5 and monitored by a fluorescence detector at 360 nm excitation and 425 nm emission wavelengths. The method improved on the separation of neutral tetra- and hexa-saccharide isomers, namely, lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT) as well as of lacto-N-difucohexaose I (LNDFH I) and lacto-N-difucohexaose II (LNDFH II). The separation of trisacccharide isomers, 3-fucosyllactose (3-FL) and 2′-fucosyllactose (2′-FL) was also successful. Limits of detection and quantification were in the range of 1–10 ng/l and 2–30 ng/l, respectively. The methods’ accuracy was good with its precision at <20% RSD and <1% RSD, respectively, for oligosaccharide concentration and retention time. The recoveries were in the range of 80–100%. This method was successfully applied to the separation and determination of representative neutral oligosaccharide contents in Samoa women milk. 相似文献
67.
Here we employ high-quality samples of (Sr1−xLax)CuO2 and (Nd2−xCex)CuO4 and XANES spectroscopy at O-K, Cu-L2,3 and Ce-M4,5 edges to gain comprehensive understanding of the electronic structure and doping in n-type high-Tc superconductors. Not only common but also slightly different features are revealed for the two systems. From O-K-edge spectra, the UHB is found essentially independent of the electron-doping level for both the systems, in line with our understanding that the doped electrons do not go to the O site in n-type copper-oxide superconductors. Another common observation is that the main CuII peak at the Cu-L3 edge (due to transitions to the CuII-3d orbitals) systematically decreases in intensity upon electron doping, hence verifying the fact that the doped electrons go to the Cu site. The difference then between the two systems is that in (Sr1−xLax)CuO2 the weaker CuII peak due to transitions to the CuII-4s orbital depends on the degree of doping. Moreover, it was found that with increasing x, electron density increases much faster in (Sr1−xLax)CuO2 than in (Nd2−xCex)CuO4. This is a consequence of two phenomena: a tiny increase in oxygen content concomitant to the CeIV-for-NdIII substitution and the somewhat lower Ce-valence value of +3.8 compared to the nominal tetravalent state. 相似文献
68.
A novel tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)32+) cathodic electrochemiluminescence (ECL) was generated at −0.78 V at the Pt electrode in acetonitrile (ACN), which suggested that the cathodic ECL differed from conventional cathodic ECL. It was found that tripropylamine (TPrA) could enhance this cathodic ECL and the linear range (log-log plot) was 0.2 μM-0.2 mM. In addition, hydrogen peroxide (H2O2) could inhibit the cathodic ECL and was indirectly detected with the linear range of 27-540 μM. The RSD (n = 12) of the ECL intensity in the presence of 135 μM H2O2 was 0.87%. This method was also demonstrated for the fast determination of H2O2 in disinfectant sample and satisfactory results were obtained. 相似文献
69.
The effect of a carboxyl group beside nitrogen of aliphatic amines on the tris(2,2′-bipyridine)ruthenium(III), Ru(bpy)33+, chemiluminescent reaction was examined. It has been shown that a carboxylate anion promotes the chemiluminescent reaction at a lower pH and then the aliphatic amines with this substituent can be sensitively detected compared with corresponding aliphatic amines without this substituent. Based on this finding, preliminary studies on simultaneous determination of 4-hydroxyproline, N-methylglycine, N-methylalanine, proline, and pipecolic acid in human serum have been performed using isocratic reversed-phase ion-pair high-performance liquid chromatography (HPLC) with electrogenerated Ru(bpy)33+ chemiluminescent detection. The detection limits (signal-to-noise ratio of 3) with the proposed method were 3.0, 12, 2.7, 4.6, and 10 nM for 4-hydroxyproline, N-methylglycine, N-methylalanine, proline, and pipecolic acid, respectively. 相似文献
70.
A. Paul Krapcho Silvia Sparapani Amber Leenstra Joshua D. Seitz 《Tetrahedron letters》2009,50(26):3195-3747
The synthesis and structural assignments of 9-chloro-1,1-phenanthroline-2(1H)-thione and 1,10-dihydro-1,10-phenanthroline-2,9-dithione have been accomplished. The sulfur-bridged bis-1,10-phenanthroline macrocycle was readily prepared by heating the thione or equimolar amounts of the dithione and 2,9-dichloro-1,10-phenanthroline in diphenyl ether. Displacements of 2-chloro- or 2,9-dichloro-1,10-phenanthroline with N,N-dimethylethylenediamine afforded the corresponding amine and diamino analogues. An amino-substituted-2,2′-bis-1,10-phenanthroline has been prepared. 相似文献