全文获取类型
收费全文 | 4449篇 |
免费 | 360篇 |
国内免费 | 150篇 |
专业分类
化学 | 3699篇 |
晶体学 | 43篇 |
力学 | 80篇 |
综合类 | 15篇 |
数学 | 87篇 |
物理学 | 1035篇 |
出版年
2024年 | 15篇 |
2023年 | 63篇 |
2022年 | 113篇 |
2021年 | 145篇 |
2020年 | 193篇 |
2019年 | 137篇 |
2018年 | 107篇 |
2017年 | 102篇 |
2016年 | 170篇 |
2015年 | 188篇 |
2014年 | 159篇 |
2013年 | 396篇 |
2012年 | 338篇 |
2011年 | 253篇 |
2010年 | 192篇 |
2009年 | 235篇 |
2008年 | 189篇 |
2007年 | 268篇 |
2006年 | 234篇 |
2005年 | 187篇 |
2004年 | 188篇 |
2003年 | 173篇 |
2002年 | 137篇 |
2001年 | 108篇 |
2000年 | 77篇 |
1999年 | 91篇 |
1998年 | 63篇 |
1997年 | 62篇 |
1996年 | 54篇 |
1995年 | 45篇 |
1994年 | 39篇 |
1993年 | 35篇 |
1992年 | 30篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 12篇 |
1988年 | 21篇 |
1987年 | 15篇 |
1986年 | 12篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1975年 | 8篇 |
1973年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有4959条查询结果,搜索用时 15 毫秒
81.
Spectrafluorimetric method for the rapid screening of toxic heavy metals in water samples 总被引:1,自引:0,他引:1
B. San Vicente de la RivaJ.M. Costa-Fernández R. PereiroA. Sanz-Medel 《Analytica chimica acta》2002,451(2):203-210
A fast and inexpensive sensitive screening test for recognising potential wastewater contamination by the presence of highly toxic heavy metals is described. The test is based on the reaction of the toxic heavy metals Hg(II), Cd(II), Pb(II) and Ag(I) with 6-mercaptopurine (6-MP) to produce highly fluorescent complexes.Optimum experimental conditions include a buffer of pH 7.2 (0.1 M citric acid/0.2 M Na2HPO4), a chelating reagent concentration of 6×10−4 M and the addition of 10−4 M of o-phenanthroline. The fluorescence emitted by the complexes was measured at 380 and 540 nm for excitation and emission wavelengths, respectively.Detection limits of 4, 3, 6 and 3 μg l−1 were achieved for Hg, Cd, Pb and Ag. Relative standard deviation (R.S.D.) were between ±2 and ±6% of the fluorescence signals for five identical samples. Potential interference effects from other heavy metals (Zn, Mn, Co, Fe, Ni and Cu), which could affect the response of the proposed screening test was investigated. Results showed that none of these metals give rise to noticeable fluorescence signals under the above described experimental conditions.Finally, the capability of the proposed heavy metal screening test for the analysis of contaminated water samples is discussed. 相似文献
82.
The microwave-assisted acid-digestion for the determination of metals in coal by ICP-AES was investigated, especially focusing on the necessity of adding HF. By testing five certified reference materials, BCR-180, BCR-040, NIST-1632b, NIST-1632c, and SARM-20, it was found that the two-stage digestion without HF (HNO3 + H2O2 was used) was very effective for the pretreatment of ICP-AES measurement. Both major metals (Al, Ca, Fe, and Mg) and minor or trace metals (Co, Cr, Cu, Mn, Ni, Pb, and Zn) in coal gave good recoveries for their certified or reference values. The possibility of ‘HF-memory effect’ was cancelled by the use of a set of vessels which had been never contacted with HF. Twenty-four Japanese standard coals (SS coals) were analyzed by the present method, and the concentrations of major metals measured by the present method provided very high accordance with those from the authentic JIS (Japanese Industrial Standard) method. 相似文献
83.
Thomas Kauffmann 《Angewandte Chemie (International ed. in English)》1982,21(6):410-429
Aside from elements of the 2nd row, and one element of the 3rd row of the periodic system—Si, P, S, and Se, respectively, whose organoelement groups such as Me3Si and Ph3P⊕ have proven useful in numerous organic syntheses—other elements of the 3rd as well as 4th and 5th row (Ge, As, Sn, Sb, Te, Pb, Bi) can also be used as components of synthetically useful organoelement groups, the elements As, Sn, and Pb, in particular, offering certain advntages over the others. Some of these organoelement groups are suitable equivalents for Li- or halogen-substituents attached to carbon; they stabilize carbanionic centers (minimum of this effect at the 3rd-row elements), and owing to their suitability as leaving groups in β-eliminations, also open up interesting synthetic possibilities. The thermally unduced syn- and silica-gel induced anti-elimination of Ph3Sn, Ph2Sb, Ph3Pb, together with β-OH, are novel. With the newly synthesized compounds PhnEl—Ch2—Li (El = Sn, Pb, As, Sb, Bi) and other α- and β-lithiated RnEl- and Ph2As(O)-reagents such organoelement groups can be introduced into organic compounds and exploited in organic and organoelement synthesis. 相似文献
84.
A sensitive voltammetric method is presented for the determination of tract levels of Mn (II) using carbon film electrodes fabricated from carbon resistors of 2 Ω. Determination of manganese was made by square wave cathodic stripping voltammetry (CSV), with deposition of manganese as manganese dioxide. Chronoamperometric experiments were made to study MnO2 nucleation and growth. As a result, it was found to be necessary to perform electrode conditioning at a more positive potential to initiate MnO2 nucleation. Under optimised conditions the detection limit obtained was 4 nM and the relative standard deviation for eight measurements of 0.22 nM was 5.3%. Interferences from various metal ions on the response CSV of Mn(II) were investigated, namely Cd(II), Ni(II), Cu(II), Cr(VI), Pb(II), Zn(II) and Fe(II). Application to environmental samples was demonstrated. 相似文献
85.
L. A. Chekulaeva I. A. Garbuzova V. V. Gavrilenko L. B. Danilevskaya V. A. Kotel'nikov 《Russian Chemical Bulletin》1996,45(9):2199-2202
Alkali metal salts of acrylamide C3H4NOM (M = Li, Na, and K) were synthesized for the first time by metallation of acrylamide with alkali metals, their alkyl derivatives, or hydrides. The structures of the compounds synthesized were studied by Raman and IR spectroscopy. Based on the results obtained, an ionic structure was proposed for the salts. The salts were tested as initiators of the anionic polymerization of acrylamide. The catalytic activity of C3H4NOM in the polymerization of acrylamide is not lower than that of the well known catalyst, KOBu1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2316–2319, September, 1996. 相似文献
86.
Comprehensive studies have been carried out on the extraction behaviour of thorium matrix vis-a-vis 17 trace metallic elements using a novel extractant viz. Cyanex-923. The near total extraction of thorium and quantitative separation of these metals has been established using inductively coupled argon plasma–atomic emission spectrometry (ICP–AES). The recovery of few representative elements has been confirmed by radio-active tracer studies. The studies carried out here have enabled determination of μg/l amounts of all analyte elements with a precision of better than 1% RSD with prior chemical separation from as low as 1 g thorium sample in just five chemical extractions. 相似文献
87.
The enthalpies of dissolution, transfer, and axial coordination for the Cd(II), Co(II), Mn(III), Fe(III), and Cr(III) complexes of tetraphenylporphine, H2(TPP), in nonpolar (C6H6, CCl4) and electron-donating solvents (DMF, DMSO, Py,c-C5H10NH) have been determined calorimetrically at 298.15 K. On the basis of thermogravimetrical data for the corresponding crystallosolvates the composition, thermal stability, and energy of intermolecular interaction of the metal-porphine complexes with pyridine have been calculated. Complexing in noncoordinating solvents brings about no radical change in the physicochemical characteristics of axial coordination which depend critically on the electron structure of the complex-forming metal.Translated fromfzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 846–850, May, 1993. 相似文献
88.
Single crystals of NaKZnP2O7 were grown, and their crystal structure was determined by X-ray diffraction (space group P21/n, a = 12.585(5) Å, b = 7.277(5) Å, c = 7.428(5) Å, β = 90.00(5)°, Z = 4, 1916 F(hkl), R 1 = 0.0461). The structure contains a 3D tetrahedral zinc phosphate framework with a system of intersecting channels running along the b and c axes. The sodium and potassium atoms are coordinated to six and seven oxygen atoms, respectively, and reside inside these channels; the potassium cations, which are larger than the sodium cations, are located at channel intersections. 相似文献
89.
L. A. Dunyushkina 《Russian Journal of Electrochemistry》2007,43(8):894-900
Results on oxygen-ion, electron, and proton conduction and oxygen penetrability of titanates of alkaline-earth metals doped with acceptor admixtures are briefly reviewed. The applicability of these materials in electrochemical devices, in particular, as oxygen-penetrable membranes, is considered. The focus is on the studies carried out at the Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences. 相似文献
90.
Karsten Schubert Theodor Alpermann Tobias Niksch Helmar Grls Wolfgang Weigand 《无机化学与普通化学杂志》2006,632(6):1033-1042
Synthesis and Analytical Characterization of Functionalized β‐Hydroxydithiocinnamic Acids and their Esters. Complex Chemistry towards Nickel(II), Palladium(II), and Platin(II) Starting from silyl‐protected 4‐hydroxy acetophenone ( 1 ) the 1,1‐ethenedihiolato complexes 3 – 5 were synthesised using carbon disulfide and potassium‐tert‐butylate as a base. After being deprotected, the resulting 4‐hydroxy‐substituted complexes 6 – 8 were esterified with DL‐α‐lipoic acid to obtain the compounds 9 – 11 . The resulting complexes were characterized using NMR spectroscopy, mass spectrometry and IR spectroscopy. 3‐substituted β‐hydroxydithiocinnamic acid methyl ester ( 12 ) was obtained via an analogous path of reaction using silyl‐protected 3‐hydroxy acetophenone ( 2 ), carbon disulfide and methyl iodide. After removing of the silyl group the resulting hydroxy group was esterified with DL‐α‐lipoic acid. Using the dithioacid ester 14 as a ligand the NiII ( 15 ), PdII ( 16 ) and PtII ( 17 ) [O,S] complexes were obtained. 相似文献