全文获取类型
收费全文 | 241537篇 |
免费 | 2141篇 |
国内免费 | 847篇 |
专业分类
化学 | 122240篇 |
晶体学 | 4076篇 |
力学 | 11039篇 |
综合类 | 1篇 |
数学 | 27125篇 |
物理学 | 80044篇 |
出版年
2020年 | 1878篇 |
2019年 | 2117篇 |
2018年 | 2782篇 |
2017年 | 2714篇 |
2016年 | 4045篇 |
2015年 | 2512篇 |
2014年 | 4115篇 |
2013年 | 10554篇 |
2012年 | 7870篇 |
2011年 | 9829篇 |
2010年 | 6919篇 |
2009年 | 6851篇 |
2008年 | 9127篇 |
2007年 | 9130篇 |
2006年 | 8658篇 |
2005年 | 7874篇 |
2004年 | 7226篇 |
2003年 | 6398篇 |
2002年 | 6382篇 |
2001年 | 7227篇 |
2000年 | 5502篇 |
1999年 | 4328篇 |
1998年 | 3657篇 |
1997年 | 3578篇 |
1996年 | 3420篇 |
1995年 | 3089篇 |
1994年 | 3050篇 |
1993年 | 3145篇 |
1992年 | 3254篇 |
1991年 | 3431篇 |
1990年 | 3180篇 |
1989年 | 3105篇 |
1988年 | 3154篇 |
1987年 | 3044篇 |
1986年 | 2872篇 |
1985年 | 3950篇 |
1984年 | 4144篇 |
1983年 | 3313篇 |
1982年 | 3541篇 |
1981年 | 3482篇 |
1980年 | 3370篇 |
1979年 | 3452篇 |
1978年 | 3550篇 |
1977年 | 3469篇 |
1976年 | 3561篇 |
1975年 | 3243篇 |
1974年 | 3250篇 |
1973年 | 3294篇 |
1972年 | 2297篇 |
1971年 | 1884篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
N. P. Grechkin N. A. Buina I. A. Nuretdinov S. G. Salikhov 《Russian Chemical Bulletin》1968,17(9):2022-2024
Conclusions The reactions of amides of arylphosphorous acids with acyl chlorides take place with replacement of the amide group and formation of amides of carboxylic acids and the corresponding aryl chlorophosphates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2131–2133, September, 1968. 相似文献
982.
V. D. Anand J. P. Srivastava G. S. Deshmukh 《Fresenius' Journal of Analytical Chemistry》1962,191(5):321-329
Summary The polarographic behaviour of nickel and cobalt in the mixed base electrolytes, urea-pyridine and urea-hydrazine has been studied. The effect of pH, concentration of supporting electrolyte and the presence of various other ions was investigated. Well defined polarograms for both the elements were obtained at pH 6.0 and 5.0 respectively. Attempts were made to utilize these observations in the separation of the two cations. While the difference in E
1/2 of the ions in urea-pyridine was greater than –0.3 volts that in urea-hydrazine was about –0.2 volts or slightly less even under the optimum conditions. In a mixed solution the waves for Co and Ni were therefore well developed and separated in the former base electrolyte. The polarograms in urea-hydrazine were, however, of little analytical use because of the washing away of the Ni wave due to the presence of even small amounts of cobalt.
Zusammenfassung Das polarographische Verhalten von Nickel und Kobalt in Harnstoff-Pyridin- und Harnstoff-Hydrazinlösung wurde untersucht und der Einfluß des pH-Wertes, der Leitsalzkonzentration sowie der Gegenwart verschiedener anderer Ionen festgestellt. Bei pH 6,0 bzw. 5,0 wurden für beide Elemente gut ausgebildete Polarogramme erhalten. Es wurde versucht, die erhaltenen Ergebnisse zur Trennung der beiden Ionen zu benutzen. In Harnstoff-Pyridinlösung beträgt der Unterschied der beiden Halbstufenpotentiale mehr als –0,3 V und man erhält gut ausgebildete und voneinander getrennte Stufen. In Harnstoff-Hydrazinlösung dagegen beträgt der Unterschied nur –0,2 V oder etwas weniger, selbst unter optimaler Bedingungen, und die Ni-Stufe wird schon durch kleine Kobaltmengen beeinträchtigt.相似文献
983.
The relaxation time of the Kerr effect of nitrobenzene and m-nitrotoluene in various mixtures with carbon tetrachloride and various alcohols was determined by measuring the kinetics of the Kerr effect using picosecond laser techniques. These measurements yield information on the rotational motion of molecules in liquids. The relaxation time data are interpreted in terms of an effective local viscosity effect, pair correlation, and coupling of rotational motion with shear modes. 相似文献
984.
The use of nuclide95Zr as radiotracer is often unfavourably influenced by the continuous accumulation of the radioactive daughter nuclide95Nb. The present paper gives a method showing how the daughter radioactivity can influence the value of the distribution coefficient
of zirconium calculated from radioactivity measurements. A simple method of continuous purification of95Zr, which yields a product of the required degree of purity, is also described. 相似文献
985.
The mass spectra of five polycyclic 1,4 benzodioxin derivatives have been studied. The breakdown patterns of the more important ions have been suggested by comparison with three compounds in a deuterated specific position. Some rearrangements are produced by specific hydrogen migration and a few others after scrambling. 相似文献
986.
The pigments Luteoskyrin (Ls) and Rugulosin (Rg) might adopt a priori two extreme conformations, a planar and an angular one. To determine the predominant conformation in solution, the nature of the lowest energy transition of the chromophores and the presence of intramolecular H-bonding have been investigated. The solvent effects on electronic absorption and CD spectra indicated the π-π* nature of the lowest energy transition. Treatments of the electronic absorption results according to McRae and Kosower relations, IR absorption and PMR spectra suggested the presence of strong intra-molecular H-bonds. From these results it was concluded that Ls and Rg adopted in a variety of solvents the most planar conformation. 相似文献
987.
The syntheses of 7′-methoxy- and 5′,7′,6-trimethoxy-coumarino(3′,4′:3,2)coumarones (IIa and IIb) are described. 相似文献
988.
An accurate method for the determination of inorganic sulphate in synthetic detergents has been developed. After addition of a cationic surface-active agent (cetyl pyridinium chloride) to an anionic detergent, sulphate can be determined by means of indirect titration with EDTA, using a mixture of Metalphthalein and Naphthol Green B as indicator. 相似文献
989.
In the interaction of ethioporphyrin and of several of its metal derivatives with negative ions of pyridine and anthracene in tetrahydrofuran solutions, an electron transfer takes place which results in the conversion of the pyridine and anthracene ions into neutral molecules, whereas the molecules of the pigment are first transformed into monovalent anions and subsequently into the di-, tri-, tetra-, and sometimes even into the hexavalent anions. In the first stage of this interaction, ferriethioporphyrin is reduced to ferro-ethioporphyrin, whereas the unmetallized pigment and its copper complex are transformed into the disodium derivative of ethioporphyrin. When the temperature is lowered, the rate of the electron transfer from pyridine ions to zinc-ethioporphyrin is decreased, and illumination leads to a substantial rate increase. The photochemical sensitivity of this system is possibly linked to the formation of stable complexes between pigment molecules and pyridine ions. 相似文献
990.
P. J. Slota C. M. Grieve N. R. Fetter A. J. Bilbo 《Journal of polymer science. Part A, Polymer chemistry》1969,7(8):2051-2057
The number-average molecular weights of beryllium 4-biphenyl(phenyl)phosphinate, di-n-pentylphosphinate, di-n-heptylphosphinate, and trifluoromethyl(phenyl)phosphinate are degraded by the presence of water in toluene. It is proposed that easily hydrolyzable P? O? P bonds contribute, in part, to the bonding of these polymers. 相似文献