全文获取类型
收费全文 | 682篇 |
免费 | 18篇 |
国内免费 | 2篇 |
专业分类
化学 | 361篇 |
晶体学 | 23篇 |
力学 | 12篇 |
数学 | 67篇 |
物理学 | 239篇 |
出版年
2022年 | 4篇 |
2021年 | 10篇 |
2020年 | 23篇 |
2019年 | 19篇 |
2018年 | 16篇 |
2017年 | 21篇 |
2016年 | 27篇 |
2015年 | 13篇 |
2014年 | 18篇 |
2013年 | 39篇 |
2012年 | 33篇 |
2011年 | 49篇 |
2010年 | 28篇 |
2009年 | 25篇 |
2008年 | 16篇 |
2007年 | 16篇 |
2006年 | 18篇 |
2005年 | 28篇 |
2004年 | 19篇 |
2003年 | 8篇 |
2002年 | 15篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1997年 | 8篇 |
1996年 | 20篇 |
1995年 | 18篇 |
1994年 | 15篇 |
1993年 | 9篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 12篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1971年 | 4篇 |
1970年 | 5篇 |
1969年 | 4篇 |
1968年 | 4篇 |
1967年 | 7篇 |
排序方式: 共有702条查询结果,搜索用时 296 毫秒
31.
A spectrophotometric method is described for the determination of sulphide or sulphite. The bis(2,9-dimethyl-1,10-phenanthroline)copper(II) ion is reduced at pH 10 by sulphide in the presence of formaldehyde and by sulphide and sulphite in its absence. The resulting copper(I) complex is extracted into chloroform and measured. With any convenient sample volume between 1 and 100 ml, the limits of detection are 0.1 mg for sulphide and 0.25 μg for sulphite. The method is unaffected by iron(II) and nitrite, in concentrations of 100 and 1000 mg l-1, respectively. 相似文献
32.
Oximidobenzotetronic acid is recommended for the separation and gravimetric determination of palladium and cobalt An ethanolic solution of the reagent quantitatively precipitates palladium(II) from solutions which are 0.75 N in acid up to pH 5.1, the complex is weighed as Pd(C9H5NO4)2. Cobalt(II) can be determined in the filtrate after the precipitation of palladium. With 0.5 N acid solutions, no interference was found from Pt(IV), Ir(IV), Rh(III), Ru(III), Os(IV), Au(III), Ag(I), Cu(II), Fe(III), Ni(II), Hg(II). Pb(II), Bi(III), Cd(II), As(V), Se(VI), Te(IV), Mo(VI), Sb(III), Al(III), Cr(III), Zn(II), Ti(IV), Zr(IV). acetate, oxalate, citrate, tartrate, phosphate and fluoride. 相似文献
33.
Propargyloxycarbonyl chloride, 1, has been used to protect the hydroxyl and amino functionalities of amino alcohols and aminophenols in one pot using triethylamine or pyridine as a base. The increased reactivity of benzyltriethylammonium tetrathiomolybdate, 2, toward propargyl carbonates over propargyl carbamates is studied in detail and has been exploited further to develop an orthogonal protection strategy for the hydroxyl and amino functionalities. For example, 2-amino-1-butanol, 6a, was treated with 1 to get the N,O-diPoc compound 7a in 90% yield, which when treated with 1.1 equiv of 2 at room temperature removes the Poc group attached to oxygen while leaving the one attached to nitrogen intact to yield compound 8a in 85% yield. This particular observation offers a new protecting strategy where an amine and an alcohol group can be protected simultaneously in one pot, and in a later synthetic step, if the alcohol group has to be deprotected selectively, it can be achieved with 1 equiv of 2. 相似文献
34.
The spectrophotometric characteristics and the stability constants of the yellow to brown 1:1 and 1:2 complexes of platinum metals with oxine N-oxide (existing as chloro mixed-ligand complexes) have been investigated. Oxine N-oxide can be used as a spectrophotometric reagent for ruthenium(III) and iridium(IV). 相似文献
35.
Syntheses of substituted pyrazolo[3,4-b]quinolines, 3,4-dihydro-4-oxopyriraido[4′,5′:4,5]theino[2,3-b]quinoline and 12-phenylpyrido[1′,2′:1,2[pyrimido[4,5-b]quinoline are described. 相似文献
36.
Sorption of C6 alkanes viz.,n-hexane, 2-methylpentane, 2,2-dimethyl butane, 2,3-dimethyl butane, cyclohexane and methyl cyclopentane in AlPO4-5 is studied and sorption capacity and thermodynamic parameters (H0, H, G, S and Sa) at various sorption coverage for these sorbates have been estimated. The initial heat of sorption (H0) is found to increase with the degree of branching in the alkane chain. The sorbate-sorbate interactions are found to be more prominent during the sorption of doubly branched alkanes and cyclic alkanes and a higher sorption capacity has been observed for cyclic alkanes. Based on the results obtained, a packing model has been proposed for various C6 alkanes inside the AlPO4-5 channel. 相似文献
37.
We present results from a computational study of 4-[4-(dimethylamino)-phenylazo] benzene boronic acid (DABBA) (the 4'-boronic acid isomer of the aminoazobenzene dye N,N-dimethylaminoazobenzene) and its associated anion, as well as, several cyclic esters formed from these azoborates and various conformers of D-glucose. Azo dyes that also contain one or more boronic acid functional groups are of practical importance in the development of chemical sensors for saccharide recognition because of their ability to induce a visible color change upon binding. The lowest-energy DABBA:D-glucose esters found in this investigation consistently involved at least one of the exocyclic hydroxymethyl groups on the D-glucose moiety rather than vicinal cis or trans diol arrangements of hydroxyl groups on the ring. 相似文献
38.
Single‐step oxidative homocoupling of aryl Grignard reagents via Co(II), Ni(II) and Cu(II) Complexes under air 下载免费PDF全文
A simple catalytic system of direct synthesis for the symmetrical biaryls using catalytic amounts of Co(II), Ni(II) and Cu(II) complexes has been developed. The reaction system involves in situ synthesis of Grignard reagents. The complexes, containing bidentate Schiff base and dmit (2‐thioxo‐1,3‐dithiole‐4,5‐dithiolate) ligands, were compatible with diverse functionalities and afford a high yield of biaryls in a single step, proving to be promising catalysts in homocoupling reactions. Atmospheric oxygen is used as an oxidant which renders a green, simple and economical catalytic route. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
39.
40.
Worm-shaped carbon nanofibers (WCNFs) were synthesized in bulk by chemical vapour deposition at 680 °C using iron carboxylate
as catalyst precursors and sodium chloride as catalyst support. The products were characterized by scanning electron microscopy,
transmission electron microscopy, Raman spectroscopy and X-ray diffraction method. The purity of the purified products was
determined by thermal analysis. The WCNF yield was 6700% relative to catalyst. The simplicity, environmental friendliness
and use of easily available low-cost precursors are the advantage of this synthesis technique. 相似文献