全文获取类型
收费全文 | 2969篇 |
免费 | 57篇 |
国内免费 | 17篇 |
专业分类
化学 | 1661篇 |
晶体学 | 6篇 |
力学 | 96篇 |
数学 | 518篇 |
物理学 | 762篇 |
出版年
2022年 | 18篇 |
2021年 | 26篇 |
2020年 | 19篇 |
2019年 | 33篇 |
2018年 | 34篇 |
2017年 | 30篇 |
2016年 | 55篇 |
2015年 | 53篇 |
2014年 | 57篇 |
2013年 | 155篇 |
2012年 | 125篇 |
2011年 | 146篇 |
2010年 | 123篇 |
2009年 | 89篇 |
2008年 | 191篇 |
2007年 | 179篇 |
2006年 | 144篇 |
2005年 | 133篇 |
2004年 | 131篇 |
2003年 | 96篇 |
2002年 | 101篇 |
2001年 | 72篇 |
2000年 | 65篇 |
1999年 | 31篇 |
1998年 | 36篇 |
1997年 | 30篇 |
1996年 | 59篇 |
1995年 | 65篇 |
1994年 | 45篇 |
1993年 | 50篇 |
1992年 | 43篇 |
1991年 | 36篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 29篇 |
1987年 | 28篇 |
1986年 | 21篇 |
1985年 | 21篇 |
1984年 | 26篇 |
1983年 | 21篇 |
1982年 | 21篇 |
1981年 | 35篇 |
1980年 | 24篇 |
1979年 | 25篇 |
1978年 | 24篇 |
1977年 | 24篇 |
1976年 | 30篇 |
1975年 | 26篇 |
1974年 | 18篇 |
1973年 | 24篇 |
排序方式: 共有3043条查询结果,搜索用时 56 毫秒
51.
The determination of beryllium and manganese in air particulate matter collected on filter material is discussed. Destruction by digestion with nitric and perchloric acids and by low-temperature ashing with dissolution of the ash in a hydrofluoric-nitric acid mixture were tested. The graphite furnace parameters were investigated for different acid solutions. Interferences of some cations and anions that are abundant in aerosol material are described. Accuracy was checked against standard samples. For manganese, the results are compared with those obtained by energy-dispersive x-ray fluorescence. 相似文献
52.
Gianfranco Denti Mauro Ghedini Jon A. McCleverty Harry Adams Neil A. Bailey 《Transition Metal Chemistry》1982,7(4):222-224
Summary Treatment of [Mo{HB(3,5-Me2C3HN2)3}(NO)I2] with one or two moles of AgPF6 in acetonitrile afforded the paramagnetic (one unpaired electron) complex [Mo{HB(3,5-Me2C3NH2)3}(NO)(NCMe)2][PF6]. The structure of this complex was determined crystallographically, and the six-coordinate geometry of the complex cation confirmed. 相似文献
53.
The reaction of Os3(CO)10(NCMe)[Si(OMe)3](-H),1, with PMe2Ph yielded the new complex Os3(CO)10(PMe2Ph)[Si(OMe)3](-H),2 by substitution of the MeCn ligand with the phosphine ligand. When heated to 125°C compound2 was decarbonylated and transformed into the new unsaturated cluster complex Os3(CO)8[-PMe2(C6H4)][Si(OMe)3](-H)2,3 in 54% yield. Compound3 was characterized by a single crystal X-ray diffraction analysis, osmium bonds. The phenyl ring of the phosphine ligand has undergoneortho-metallation by a neighboring metal atom. A terminally coordinated Si(OMe)3 ligand is coordinated to the third osmium atom. The cluster is unsaturated by the amount of 2 electrons, and there is an open coordination site on the siloxyl substituted osmium atom that is partially filled by a weak interaction with one of the -bonds of theortho-metalled phenyl ring. Complex3 reacts with CO at 1 atm to reform compound2 in 85% yield in 5 h at 40°C. Crystal Data: for3: space group = P21/n,a = 9.911(2) Å,b = 18.451(6) Å,c = 14.872(2) Å, = 95.64(2)°,Z = 4, 1994 reflections,R = 0.028. 相似文献
54.
Mike M.K BoysenThisbe K Lindhorst 《Tetrahedron》2003,59(22):3895-3898
Two new pathways to introduce carbohydrate derivatives into dendritic carbosilane structures were explored. Pathway one utilised alcoholysis of a chlorosilane with a protected hydroxyethyl glycoside resulting in Si-O linkages. Pathway two proceeds via a hydrosilylation reaction of a protected allyl glycoside with a carbosilane containing Si-H end groups in the presence of a platinum catalyst, thus leading to Si-C linked structures. 相似文献
55.
Applications of the laser microprobe mass analyzer for characterizing the surface of chemically modified minerals (crown ether/montmorillonite and organosilane/sepiolite) and for detecting micro-crystallites at the surface of an inorganic matrix (brucite surface layer of chrysotile fibers) were investigated using laser desorption conditions. The laser microprobe is shown to be a versatile tool for these applications. 相似文献
56.
The hexanuclear tungsten cluster complex [W6(3-Cl)8Cl6]2–,1 was isolated as a salt of the cation (MeO2CCH2)2N(H)CH2CH2N(CH2CO2Me)2+, EDTEH+, by crystallization from methanol solvent of the product obtained from the reaction of (H3O)2[W6(3-Cl)8Cl3] with the disodium salt of ethylenediaminetetraacetic acid. The compound was charcterized by single-crystal X-ray diffraction analysis. The cluster anion contains an octahedral arrangement of six tungsten atoms with chloride ligands bridging the eight triangular faces of the cluster and one chloride ligand terminally coordinated to each of the six tungsten atoms. The cation (EDTEH)+ achieves its positive charge by protonation of one of the nitrogen atoms. Crystal data: space group = P21/a,a=10.689(2) Å,b=22.931(6) Å,c=12.093(3) Å, =98.41(2)°,Z=2, 2476 reflections,R=0.028. 相似文献
57.
Three categories of tea, black, green and oolong tea, with those varieties in each category were analyzed for their contents of cellulose, hemicellulose, lignin, polyphenols, caffeine and amino acids. The data were subjected to multivarige analysis. Principal component analysis and principal component classification provide discrimination between the different categories and varieties. The quality index, assigned to these tea standards by experts, could be predicted form the principal component scores. 相似文献
58.
Charles M. MarsonJane McGregor Timothy WalsgroveTrevor J. Grinter Harry Adams 《Tetrahedron letters》2003,44(1):141-143
Treatment of an unsaturated 2,3-epoxy alcohol with SnBr4 leads to a stereoselective formation of a cyclopropane ring, and an α-ketol unit as part of a subsequent ring expansion. 相似文献
59.
Stefan TsakovskiKarima Benkhedda Elisaveta IvanovaFreddy C Adams 《Analytica chimica acta》2002,453(1):143-154
8-Hydroxyquinoline (HQ), 2-methyl-8-hydroxyquinoline (CH3-HQ), 5,7-dichloro-2-methyl-8-hydroxyquinoline (Cl2-CH3-HQ), 5,7-dibromo-8-hydroxyquinoline (Br2-HQ), 5-sulfo-7-iodo-8-hydroxyquinoline (ferron) and 5-sulfo-8-hydroxyquinoline (SO3H-HQ) were compared as chelating reagents for on-line sorption preconcentration of cobalt in a knotted reactor (KR) precoated with the reagent. The results obtained with the different HQ derivatives reveal those properties of the chelating reagent responsible for the processes taking place in the KR. The influence of hydrophobicity, acidity, stability of the cobalt chelate and type of substituents in the HQ ring system on the separate steps of the flow injection (FI) preconcentration procedure are discussed. According to the performance characteristics of the different HQ derivatives, the most important parameters for on-line preconcentration in a KR are the hydrophobicity of the reagent and the stability of the chelate complex with the analyte. 相似文献
60.
A non-destructive analytical method for oxygen in silicon by both helium-3 and α-particle activation analysis was applied to different samples, containing 0.05–10 p.p.m. of oxygen. The accuracy and precision of the method were studied. For concentrations above 1 p.p.m., the average coefficient of variation is about 6% for α-activation and 13% for helium-3 activation. For concentrations of about 60 p.p.b., the corresponding figure is about 20%. The results of both methods, which agree within 8%, are compared to infrared analysis. The determination limit of the method is 4 p.p.b. for helium-3 activation and 22 p.p.b. for α-activation. 相似文献