全文获取类型
收费全文 | 1773篇 |
免费 | 73篇 |
国内免费 | 162篇 |
专业分类
化学 | 1532篇 |
晶体学 | 27篇 |
力学 | 12篇 |
综合类 | 2篇 |
数学 | 21篇 |
物理学 | 414篇 |
出版年
2024年 | 3篇 |
2023年 | 36篇 |
2022年 | 16篇 |
2021年 | 27篇 |
2020年 | 41篇 |
2019年 | 23篇 |
2018年 | 30篇 |
2017年 | 30篇 |
2016年 | 33篇 |
2015年 | 44篇 |
2014年 | 63篇 |
2013年 | 100篇 |
2012年 | 192篇 |
2011年 | 107篇 |
2010年 | 77篇 |
2009年 | 118篇 |
2008年 | 112篇 |
2007年 | 125篇 |
2006年 | 89篇 |
2005年 | 81篇 |
2004年 | 85篇 |
2003年 | 68篇 |
2002年 | 57篇 |
2001年 | 51篇 |
2000年 | 56篇 |
1999年 | 58篇 |
1998年 | 55篇 |
1997年 | 32篇 |
1996年 | 30篇 |
1995年 | 31篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 20篇 |
1991年 | 22篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2008条查询结果,搜索用时 15 毫秒
81.
Manganese is extracted on-line from solid seafood samples by a simple continuous ultrasound-assisted extraction system (CUES). This system is connected to an on-line manifold, which permits the flow-injection flame atomic absorption spectrometric determination of manganese. Optimisation of the continuous leaching procedure is performed by an experimental design. The proposed method allows the determination of manganese with a relative standard deviation of 0.9% for a sample containing 23.4 μg g−1 manganese (dry mass). The detection limit is 0.4 μg g−1 (dry mass) for 30 mg of sample and the sample throughput is ca. 60 samples per hour. Accurate results are obtained by measuring TORT-1 certified reference material. The procedure is finally applied to mussel, tuna, sardine and clams samples. 相似文献
82.
Andriy V. Tkachuk 《Journal of solid state chemistry》2004,177(11):3939-3943
The isostructural ternary transition-metal silicides Zr3Mn4Si6 and Hf3Mn4Si6 can be prepared by direct reaction of the elemental components or by arc-melting. The single-crystal structure of Zr3Mn4Si6 was determined by X-ray diffraction (Pearson symbol tP104, tetragonal, space group P42/mbc, Z=8, , ). Zr3Mn4Si6 is isostructural to Nb3Fe3CrSi6 and contains an essentially ordered arrangement of the transition-metal atoms. Square antiprismatic clusters with Zr and Mn atoms at the corners and Si atoms at the center share opposite faces to form one-dimensional columns extending along the c direction. These columns occupy channels that are outlined by a framework of edge- and face-sharing MnSi6 octahedra. The extensive metal-metal interactions in the structure are complemented by Si-Si bonding in the form of dumbbells, linear chains, and zigzag chains. 相似文献
83.
A new shipboard analytical method for determining picomolar levels of manganese in seawater has been developed. The method is based on a combination of chelating resin column extraction and improved chemiluminescence (CL) detection in a closed flow system. In this method, manganese in sample solution is selectively collected on newly-developed iminodiacetate-immobilized chelating resin, and then eluted with acidic solution containing hydrogen peroxide. The resulting eluent is mixed with luminol solution and aqueous ammonia after removal of iron ions by a chelating resin column, and then the mixture is introduced into the CL cell. The manganese concentration is obtained from the CL intensity. The detection limit (3SD) of manganese is 5 pmol L–1 from 9 mL of seawater sample. The method was applied to seawater samples collected at the Okinawa Trough. 相似文献
84.
Synthesis and characterization of PVP/AAc copolymer hydrogel and its applications in the removal of heavy metals from aqueous solution 总被引:1,自引:0,他引:1
Chelating poly(vinylpyrrolidone/acrylic acid) (PVP/AAc) copolymer hydrogels were prepared by radiation-induced copolymerization. The effects of preparation parameters such as PVP content in the hydrogel and irradiation dose on the swelling behavior of the hydrogel were studied. The pH dependent swelling was investigated. The thermal stability of the prepared hydrogel and the metal chelated ones was characterized by TGA. The removal of Fe(III), Cu(II), and Mn(II) from aqueous solution by the prepared PVP/AAc chelating hydrogel was examined by batch equilibration technique. The influence of treatment time, pH, and the initial feed concentration on the amount of the metal ions removed was studied. The results show that the removal of the metal ion followed the following order: Fe(III) > Cu(II) > Mn(II). The amounts of the removed metal ions increased with treatment time and pH of the medium. To re-use the hydrogel, the metal ions were stripped by using 2 N HCl. 相似文献
85.
116例冠心病患者血清中铜铬锰钴镍钒含量的探讨 总被引:1,自引:0,他引:1
测定了116例冠心病患者血清中铜、铬、锰、钴、镍、钒的含量并与正常值比较,显示含量升高才是有铬和镍,降低者有钴和钒、无差异者有铜和锰。 相似文献
86.
Banks CE Kruusma J Moore RR Tomcík P Peters J Davis J Komorsky-Lovrić S Compton RG 《Talanta》2005,65(2):423-429
Three different electroanalytical techniques for the detection of manganese in marine sediments are evaluated. The anodic stripping voltammetry of manganese at an in situ bismuth-film-modified boron-doped diamond electrode and cathodic stripping voltammetry at a carbon paste electrode are shown to lack the required sensitivity and reproducibility whereas cathodic stripping voltammetry at a bare boron-doped diamond electrode is shown to be reliable and selective with a limit of detection, from applying a 60 s accumulation period of 7.4 × 10−7 M and a sensitivity of 0.24 A M−1. The method was used to evaluate the manganese content of marine sediments taken from Šibenik, Croatia. 相似文献
87.
2-Trifluoroacetyl-4,5-dihydrofurans were obtained by manganese(III) acetate mediated radical cyclization of trifluoromethyl-1,3-dicarbonyl compounds (1a-c) with conjugated alkenes (2a-h). The reaction of 1,1,1-trifluoropentane-2,4-dione (1a) with propenylbenzene and 1,1-diphenyl-1-butene surprisingly yielded 3-(dihydrofuran-2(3H)-ylidene)-1,1,1-trifluoroacetones besides 3-trifluoroacetyl-4,5-dihydrofurans. 相似文献
88.
Kentaro Asahi 《Tetrahedron》2005,61(47):11107-11124
The manganese(III)-catalyzed aerobic oxidation of 2,4-piperidinediones was performed in the presence of alkenes at room temperature, producing 1-hydroxy-8-aza-2,3-dioxabicyclo[4.4.0]decan-7-ones in excellent yields. On the other hand, the 6-acetoxy-3-aza-7-oxabicyclo[4.3.0]nonan-2-ones were obtained by the oxidation of the 2,4-piperidinedione-3-carboxylates with manganese(III) acetate in the presence of alkenes at elevated temperature under an argon atmosphere. A similar oxidation using decarboxylated 2,4-piperidinediones produced the 2,3,6,7-tetrahydrofuro[3,2-c]pyridin-4(5H)-ones and/or 2,3,6,7-tetrahydrofuro[2,3-b]pyridin-4(5H)-ones in good yields. The structure determination and the decomposition reaction of the azabicyclic peroxides in acetic acid or acetic anhydride, and the reaction pathway were also described. 相似文献
89.
E. Pollert J. Hejtmnek K. Kní
ek M. Mary
ko J. P. Doumerc J. C. Grenier J. Etourneau 《Journal of solid state chemistry》2003,170(2):368-373
The electric and magnetic properties of the perovskites Nd0.8Na0.2Mn(1−x)CoxO3 (0x0.2) prepared by the usual ceramic procedure were investigated. The insulator-to-metal-like (IM) transition, closely related to a ferromagnetic arrangement, was revealed for the composition of x=0.04 and a similar tendency was detected for x=0. The insulating behavior persists down to low temperatures for higher contents of cobalt ions in spite of the transition to the bulk ferromagnetism. The properties are interpreted in terms of the steric distortion, tilting of the Mn(Co)O6 octahedra and the double-exchange interactions of the type Mn3+–O2−–Mn4+and Mn3.5+δ–O2−–Co2+, respectively. Presence of antiferromagnetic domains in the ferromagnetic matrix for the most of cobalt-substituted samples is supposed. 相似文献
90.
Moonhyun Oh 《Journal of organometallic chemistry》2003,687(1):78-84
The cationic manganese tricarbonyl complexes containing η6-2-methylhydroquinone (2a), η6-2,3-dimethylhydroquinone (3a), η6-2-t-butylhydroquinone (4a), η6-tetramethylhydroquinone (5a) and η6-4,4′-biphenol (6a) are readily deprotonated to the corresponding neutral (η5-semiquinone)Mn(CO)3 (2b-6b) and anionic (η4-quinone)Mn(CO)3− (2c-5c) complexes. The X-ray structures of 2b-6b feature strong intermolecular hydrogen bonding interactions that result in the formation of supramolecular organometallic networks. Significantly, the substitution pattern at the semiquinone ring affects the stereochemistry of the hydrogen bonding interactions. NMR spectra of 2b, 3b and 5b reveal dynamic hydrogen bonding in solution. 相似文献