全文获取类型
收费全文 | 12241篇 |
免费 | 201篇 |
国内免费 | 216篇 |
专业分类
化学 | 5614篇 |
晶体学 | 28篇 |
力学 | 717篇 |
综合类 | 5篇 |
数学 | 4030篇 |
物理学 | 2264篇 |
出版年
2024年 | 119篇 |
2023年 | 703篇 |
2022年 | 438篇 |
2021年 | 437篇 |
2020年 | 1491篇 |
2019年 | 1093篇 |
2018年 | 955篇 |
2017年 | 783篇 |
2016年 | 761篇 |
2015年 | 595篇 |
2014年 | 776篇 |
2013年 | 2895篇 |
2012年 | 551篇 |
2011年 | 60篇 |
2010年 | 66篇 |
2009年 | 45篇 |
2008年 | 64篇 |
2007年 | 81篇 |
2006年 | 56篇 |
2005年 | 117篇 |
2004年 | 120篇 |
2003年 | 85篇 |
2002年 | 60篇 |
2001年 | 34篇 |
2000年 | 26篇 |
1999年 | 18篇 |
1998年 | 30篇 |
1997年 | 24篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 22篇 |
1993年 | 20篇 |
1992年 | 13篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1987年 | 3篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
61.
Allyloxy bisbenzo 16-crown-5 trimethoxysilane was first used as precursor to prepare the sol-gel-derived bisbenzo crown ether/hydroxyl-terminated silicone oil (OH-TSO) SPME coating. The coating procedure involving sol solution composition and conditioning process was presented. Compared with commercial SPME stationary phases, the new coatings showed higher extraction efficiency and therefore could provide higher sensitivity for organphosphorous pesticides (OPs). Limits of detection (LODs) were in the range of 0.003-1.0 ng/g for these OPs in food samples (honey, juice, orange and pakchoi). The optimal extraction conditions of the new coatings to OPs in these samples were investigated by adjusting extraction time, salt addition, extraction temperature, and dilution ratios of samples with distilled water by using SPME coupled with gas chromatography (GC)-flame photometric detection (FPD). The method was applied to determine the concentrations of OPs in real samples. 相似文献
62.
The hydrothermal reaction of VOSO4, As2O3, CoC2O4·2H2O and 2,2-bipy yields a novel arsenic-vanadate [CoII(2,2-bpy)2]2[AsIII8VIV 14O42(H2O)]·H2O (1), which is characterized by IR, elemental analysis, TGA, magnetic susceptibility and single crystal X-ray diffraction analysis. X-ray diffraction shows that compound 1 is the first example of tubular arsenic–vanadium cluster containing infinite {[Co(2,2-bpy)2]2[As8V14O42(H2O)]} chain constructed from [As8V14O42(H2O)] clusters interconnected through [Co(2,2-bpy)]2+ units. Crystal data: 1, orthorhombic, P 212121, a=12.1401(2) Å, b=15.8722(1) Å, c=39.9533(5) Å, Z=4.Graphical Abstract: A novel polyoxoarsenicvanadate, [CoII(2,2-bipy)2]2 [AsIII8VIV 14O42(H2O)]·H2O, is depicted along with a tubular hybrid solid with a rhombic tube formed within the chain. 相似文献
63.
《Arabian Journal of Chemistry》2020,13(5):5277-5283
The new title two-dimensional hetero-tetra nuclear Cu3–Na coordination polymer {[NaCu3Cl(cpiap)2(H2O)3]n·6nH2O} (1) consists of crystallographically two-independent copper(II) centers, each bridged by a sodium cation through carboxylate-oxygen of the deprotonated H3cpiap ligand (H3cpiap = 2-(carboxyphenyl)iminoaceticpropanoic acid) to CuII (2) and CuII (2−) cations, and through water molecules to CuII (1) cation. CuII (2) and CuII (1) cations are bridged by carboxylate-oxygen atoms of the ligand in a syn-anti mode which, alternate regularly within the chain being bridged by a tetra coordinated sodium cation. Each CuII (2) and CuII (2−) cation in (1) is in an octahedral environment formed by four carboxylate-oxygens from two cpiap3− ligands, one nitrogen atom and a bridging chloride atom. CuII (1) cation is in a square pyramidal environment formed by three water molecules and two carboxylate-oxygens from two cpiap3− ligands. The ligand acts simultaneously as monodentate and tridentate toward CuII (1) and CuII (2) cations respectively. The lattice water molecules involved in OH···O hydrogen bonding are situated in the void spaces between layers. The zigzag chains, which run along the b-axes further construct three-dimensional metal-organic framework via hydrogen bonding and weak face-to-face π-π interactions. Weak CH···O interactions are also present. 相似文献
64.
Alkanes (cyclohexane, hexane, heptane isomers) are effectively oxidized in CH3CN at 20–70°C by hydrogen peroxide when catalyzed by a Bu4NVO3-pyrazine-2-carboxylic acid system. Alkyl hydroperoxide is the main product; an alcohol and a ketone or an aldehyde are also formed. Under these conditions benzene is oxidized to give phenol, while alkyl benzenes yield oxygenation products both of the ring and the side chain. It has been assumed that the interaction of H2O2 with VO3
– gives rise to generation of HO radicals and other radical-like vanadium containing species that abstract a hydrogen atom from an alkane, RH. The radical R. formed reacts with O2 to produce ROO. which is then transformed to alkyl hydroperoxide.Presented at the VIII International Symposium on Homogeneous Catalysis (Amsterdam, 1992).Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 64–68, January, 1993. 相似文献
65.
66.
氧化藏红T褪色催化荧光法测定痕量钒 总被引:13,自引:3,他引:13
研究了在酸性介质中柠檬酸存在下,痕量钒(V)催化溴酸钾氧化藏红T的褪色反应及其动力学条件,建立了催化荧光测定痕量钒的新方法。催化反应在沸水浴中进行8min,为假零级反应,反应的表观活化能力为99.21kJ/mol,反应速度常数为0.12/s;该方法的检出限为2.2×10^-8g/L;线性范围为0.2 ̄2.4μg/L研究了共存离子的影响,用于人发和煤灰中痕量钒的测定,结果令人满意。 相似文献
67.
A novel methodology in on-line pyrolysis-gas chromatography (Py-GC) for the fast analysis of fatty acids in vegetable oils with minimal sample treatment and the use of non-toxic reagents is described. Pyrolysis at 500 degrees C for 10 s of sub-microgram quantity of vegetable oil dissolved in dimethyl carbonate (DMC) and in the presence of nanopowder titanium silicon oxide resulted in the production of fatty acid methyl esters (FAMEs) as unique products. Pyrolysis performed by means of a resistively heated filament pyrolyser interfaced to a GC-MS apparatus enabled the direct analysis of evolved FAMEs. The DMC/Py-GC-MS analysis was tested on soybean, coconut, linseed, walnut and olive oil and the results compared to the classical BF(3)-methanol as reference methodology. The DMC method exhibited a lower precision and was biased towards lower levels of polyunsaturated fatty acids (PUFA) in comparison to the BF(3)-methanol method, but was more advantageous in terms of reduced sample treatment, waste generation and risk factors of employed chemicals. 相似文献
68.
Assuming the separation of the intermolecular scattering function into the radial and angular parts and using Egelstaffet al’s orientational model for tetrachlorides, the structure of liquid vanadium tetrachloride has been studied. It has been observed
that such a separation is approximate for this liquid and the introduction of a third correction term is required to account
for the molecular structure function. The chlorine-chlorine partial structure and effective angleaveraged intermolecular chlorine-chlorine
potential in the liquid has been evaluated. Without taking the third correction term, introduced to generate theoretically
the molecular structure function, the centre structure function has been obtained in an approximate way from the experimentally
observed molecular structure function and from it the centre radial distribution function, centre direct correlation function
and the angle-averaged vanadium-vanadium effective potential has been evaluated. 相似文献
69.
A. V. Redkina Yu. V. Belokopytov N. D. Konovalova K. N. Khomenko 《Theoretical and Experimental Chemistry》2007,43(6):405-410
An increase in the propylene output in the oxidative dehydration of propane on V2O5/TiO2-SiO2 was observed after prior reduction of V2O5 in the reaction mixture to V2O4, which reduces the destructive chemisorption of propylene. A low titanium dioxide content in TiO2-SiO2 hinders the deep reduction of V2O5 to V2O3, which reduces the conversion of propane.
__________
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 6, pp. 373–378, November–December, 2007. 相似文献
70.
本文用P204-兰113A-煤油组成的滤膜体系处理钒(Ⅳ)的溶液,并考察了表面活性剂、载体、外相pH值、内相酸度、起始钒(Ⅳ)浓度以及乳水比等因素对分离的影响。 相似文献