全文获取类型
收费全文 | 73560篇 |
免费 | 5663篇 |
国内免费 | 11803篇 |
专业分类
化学 | 49063篇 |
晶体学 | 1963篇 |
力学 | 782篇 |
综合类 | 474篇 |
数学 | 5369篇 |
物理学 | 14205篇 |
综合类 | 19170篇 |
出版年
2024年 | 191篇 |
2023年 | 1040篇 |
2022年 | 1997篇 |
2021年 | 1989篇 |
2020年 | 2092篇 |
2019年 | 1962篇 |
2018年 | 1666篇 |
2017年 | 2119篇 |
2016年 | 2263篇 |
2015年 | 2122篇 |
2014年 | 2937篇 |
2013年 | 5312篇 |
2012年 | 4262篇 |
2011年 | 4774篇 |
2010年 | 4131篇 |
2009年 | 5007篇 |
2008年 | 4792篇 |
2007年 | 5287篇 |
2006年 | 4907篇 |
2005年 | 4273篇 |
2004年 | 4141篇 |
2003年 | 3500篇 |
2002年 | 2894篇 |
2001年 | 2387篇 |
2000年 | 2210篇 |
1999年 | 1828篇 |
1998年 | 1445篇 |
1997年 | 1300篇 |
1996年 | 1135篇 |
1995年 | 1090篇 |
1994年 | 1019篇 |
1993年 | 852篇 |
1992年 | 802篇 |
1991年 | 612篇 |
1990年 | 436篇 |
1989年 | 384篇 |
1988年 | 317篇 |
1987年 | 246篇 |
1986年 | 172篇 |
1985年 | 154篇 |
1984年 | 130篇 |
1983年 | 46篇 |
1982年 | 94篇 |
1981年 | 126篇 |
1980年 | 104篇 |
1979年 | 121篇 |
1978年 | 89篇 |
1977年 | 94篇 |
1976年 | 61篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
用双-(2-羟乙基)二硫代甲酸铵(HEDC)在反相液相色谱中作检测某些金属离子的衍生化试剂,HEDC的金属螯合物微溶于水,可直接水样注射于C18柱中进行检测,范围为0.006~10mg/L相对偏差1%~2%,检测波长254nm,金属汞的整合物在HPLC分析前进行浓缩富集检测限可低至0.06~25μg/L,相对偏差小于2%。 相似文献
92.
Summary The interfering effect of the -radiation of radioactive isotopes of the rare earths on the non-destructive neutron activation analysis of g amounts of uranium, using the 106 KeV--line of 239Np and NaI (Tl). detector, is estimated. The errors introduced by the interfering isotope in the analysis of uranium, are determined as a function of the ratio of the concentration in the sample of the interfering element and uranium. In addition, the possibilities of determining the quantitative effect of these elements are described. The use of the proposed method is demonstrated by the analysis of some geological samples.
Zerstörungsfreie Neutronenaktivierungsanalyse von Uran über die 106 KeV--Strahlung von Neptunium-239II. Einfluß der Seltenen Erden
Zusammenfassung Es wird die Störung bestimmt, die die -Strahlung der radioaktiven Isotope Seltener Erden bei der zerstörungsfreien Neutronenaktivierungsanalyse von Mikrogramm-Mengen Uran über die 106 KeV--Linie des 239Np mit Hilfe eines NaJ(Tl)-Detektors hervorrufen kann. Die Fehler in der Analyse von Uran, die das Störisotop verursachen kann, werden als Funktion des Verhältnisses der Konzentration des Störelements zu der des Urans in der Probe bestimmt. Es werden Möglichkeiten beschrieben, den Einfluß störender Elemente quantitativ zu erfassen. Die Brauchbarkeit der vorgeschlagenen Methode wurde durch Analyse einiger geologischer Objekte bewiesen.
Part I: Z. Anal. Chem. 257, 23 (1971). 相似文献
93.
Fluoroalkylated 1,2,3-triketone 2-arylhydrazones and 2-arylhydrazono-3-oxo esters react variously with methylamine depending on the structure of the fluorinated substituent. 2-Arylhydrazono-1,3-dicarbonyl compounds having “short” fluoroalkyl substituents condense with methylamine at the carbonyl group attached to the non-fluorinated substituent whereas ones containing a lengthy polyfluoroalkyl substituent undergo haloformic cleavage as a result of the amine addition at the carbonyl group bearing such a substituent. The resulting 2-arylazo-3-(N-methyl)amino-1-polyfluoroket-2-en-1-ones and 1-(N-methyl)amino-2-arylhydrazono-3-fluoroalkyl-3-oxopropanamides have complexing properties, and they can bind to nickel(II) and copper(II) ions. Nickel chelates can be obtained by a three-component condensation of 2-arylhydrazono-1,3-dicarbonyl compounds and methylamine in the presence of nickel(II) cations. 相似文献
94.
Ju-Chun Huang Chien Hong Cheng Shyang Roeng Sheen Zhi Ling Lee Chien Ming Lei Maw Kuen Wu 《中国化学会会志》1996,43(2):139-143
The synthesis of bulk Y2Ba4Cu7O15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y2Ba4Cu7O15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y2Ba4Cu7O15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described. 相似文献
95.
The parallel between orbital first and second electric moments and statistical first and second central moments is noted. Three measures of orbital spatial distribution in terms of their moments are proposed, and applied to the LMO's in a series of ten-electron hydrides. Consistent differences between bond and lone pair distributions are found. Using the statistical interpretation, for each LMO an effective solid angle around the central atom is postulated. 相似文献
96.
In distinction to Extended Hückel Theory which predicts as the most stable conformation of free zwitterionic GABA a totally extended form, PCILO and SCF ab initio studies show that the intrinsically preferred conformation of the isolated molecule is a highly folded one, resulting from strong interactions between the two charged ends. Computations are also carried out for hydrated GABA in the supermolecule approach allowing moreover for the flexibility of binding of some of the water molecules of the first hydration shell. They predict the coexistence in solution of a large number of conformations showing different degrees of folding (or extension), a result confirmed by recent NMR studies. This and a number of similar results show that we have to adapt our thinking on the role of conformations in pharmacological activity to this situation, which was frequently obscured by the more abundant results of X-ray crystallography yielding a single conformation. 相似文献
97.
S-Alkylation of 2-acetamido-9-(2-acetoxyethoxymethyl)-6-oxo-8-thioxopurine was used to synthesize its novel S- and N(7)-substituted derivatives. We have established the effect of the structure of the alkylating agent on the reaction conditions and its regioselectivity. We have shown that the synthesized guanine derivatives can be modified further. 相似文献
98.
Richa Pathak 《Tetrahedron》2007,63(2):451-460
Trifluoroacetic acid has been discovered to be a highly effective and efficient reagent for the tandem Claisen rearrangement and cyclization reaction to yield 3-arylmethylene-3,4-dihydro-1H-quinolin-2-ones from compounds obtained from the SN2 reaction between anilines and acetyl derivatives of Baylis-Hillman adducts of acrylates in the presence of DABCO. In contrast, similar compounds obtained from the acetyl derivatives of Baylis-Hillman adduct of acrylonitrile on treatment with trifluoroacetic acid directly furnish 3-arylmethyl-2-amino-quinoline via tandem Claisen rearrangement, cyclization and isomerization. 相似文献
99.
Yuh-Wern Wu 《中国化学会会志》1996,43(6):507-509
The relative rates for the addition reactions of tert-butyl radical to 2-substituted ally chlorides I have een determined. The correlation of log k/k0, vs. σm gives a ρ mvalue of 3.59 with correlation coefficient of 0.930. When the substituent CH2Cl is excluded, correlation coefficient raises to 0.990 and ρ value becomes 3.39. The large p value and the deviation of relative rate of substituent CH2C1 are discussed. 相似文献
100.
Langlet J. Claverie P. Pullman B. Piazzola D. Daudey J. P. 《Theoretical chemistry accounts》1977,46(2):105-116
The effect of water on the conformational preferences of acetylcholine has been studied within the discrete, the continuum and the combined discrete-continuum models described in parts I and II of this series. All the models lead to the conclusion that the trans-gauche form which is, following refined quantum-mechanical computations, the intrinsically preferred one and the one observed in the crystal of acetylcholine and of a number of analogues should remain also the preferred conformation in water. This result agrees with NMR studies. The results of the empirical discrete model used here compare favorably to those obtained by an ab initio super-molecule treatment. The continuum model utilized here represents a net improvement above such models utilized in other works. 相似文献