全文获取类型
收费全文 | 233189篇 |
免费 | 1998篇 |
国内免费 | 801篇 |
专业分类
化学 | 116902篇 |
晶体学 | 3997篇 |
力学 | 10762篇 |
综合类 | 1篇 |
数学 | 25543篇 |
物理学 | 78783篇 |
出版年
2020年 | 1788篇 |
2019年 | 2044篇 |
2018年 | 2721篇 |
2017年 | 2653篇 |
2016年 | 3916篇 |
2015年 | 2392篇 |
2014年 | 3974篇 |
2013年 | 10079篇 |
2012年 | 7540篇 |
2011年 | 9400篇 |
2010年 | 6692篇 |
2009年 | 6630篇 |
2008年 | 8709篇 |
2007年 | 8739篇 |
2006年 | 8282篇 |
2005年 | 7497篇 |
2004年 | 6920篇 |
2003年 | 6118篇 |
2002年 | 6065篇 |
2001年 | 7094篇 |
2000年 | 5380篇 |
1999年 | 4241篇 |
1998年 | 3565篇 |
1997年 | 3474篇 |
1996年 | 3295篇 |
1995年 | 3009篇 |
1994年 | 2952篇 |
1993年 | 3043篇 |
1992年 | 3157篇 |
1991年 | 3335篇 |
1990年 | 3085篇 |
1989年 | 3030篇 |
1988年 | 3067篇 |
1987年 | 2955篇 |
1986年 | 2800篇 |
1985年 | 3815篇 |
1984年 | 4007篇 |
1983年 | 3200篇 |
1982年 | 3412篇 |
1981年 | 3338篇 |
1980年 | 3231篇 |
1979年 | 3338篇 |
1978年 | 3423篇 |
1977年 | 3353篇 |
1976年 | 3448篇 |
1975年 | 3142篇 |
1974年 | 3152篇 |
1973年 | 3196篇 |
1972年 | 2237篇 |
1971年 | 1849篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Structure of anisomycin 总被引:1,自引:0,他引:1
992.
Summary Adaption of theLuís method for the detection of nicotine to microscale, usingEmich's capillary technic proved successful, attaining identification limit of 0,03g.Applying a sub-micro steam distillation effect, the nicotine in a speck of tobacco (1 mm2 and less) could still be detected by using the above reaction.Extension of the above procedure to mixtures of nicotine with substances not volatile with steam, inorganic (halides) as well as organic compounds (other alkaloids, acids, etc.) proved to be successful, even when the nicotine is present in proportions of only 1 part in 1000.
Zusammenfassung Die Anwendung der Kapillartechnik vonEmich zum Nachweis von Nikotin nachLuís war erfolgreich und zeigte eine Erfassungsgrenze von 0,03g.Bei Anwendung einer Ultramikrodampfdestillation kann das Nikotin nach obiger Methode noch in Spuren von Tabak (1 mm2 und weniger) nachgewiesen werden.Die Reaktion ist auch positiv bei Gemischen von Nikotin mit anderen, mit Wasserdampf nicht flüchtigen sowohl anorganischen (Halogenide) als auch organischen Substanzen (andere Alkaloide, Säuren etc.), selbst dann noch, wenn das Nikotin bloß im Verhältnis von 11000 vorhanden ist.
Résumé La technique capillaire d'Emich a été utilisée avec succès pour identifier la nicotine suivant le procédé deLuís; la limite de sensibilité est alors de 0,03g. La mise en oeuvre d'un ultramicroentraînement à la vapeur permet en outre, par application de la précédente méthode, d'identifier la nicotine dans des traces de tabac (moins de 1 mm2). La réaction est également positive en présence d'autres substances non entraînables par la vapeur d'eau, qu'elles soient minérales (halogénures) ou organiques (autres alcaloïdes, acides, etc. ...) même si la proportion de nicotine n'est que de l'ordre de grandeur de 1/1000.相似文献
993.
Direct semi-automatic and automatic derivative potentiometric EDTA procedures for the determination of calcium and/or magnesium with a calcium-selective electrode are described. Calcium is titrated in the presence of magnesium at pH 12.0 and the total calcium and magnesium is determined at pH 9.7. Calcium in the range 1-8 mg and magnesium in the range 0.7-5 mg are determined semi-automatically with average errors of about 0.2 and 0.3% respectively and automatically with average errors of 0.4 and 0.7%. Titration times vary from a few seconds to a few minutes. 相似文献
994.
V. F. Burdukovskii D. M. Mognonov S. O. Botoeva Zh. P. Mazurevskaya 《Polymer Science Series B》2006,48(2):92-95
It is shown that polyimidates obtained by polycondensation of bisphenols with imidoyl chlorides may be transformed into N-phenylsubstituted aromatic polyamides. The rearrangement occurs as a result of heating at 260–340°C, and, in a diphenyl ether solution, at 240°C. The resulting polymers are soluble in organic solvents and demonstrate high thermooxidative stability. 相似文献
995.
Close-packed structures with formula AB2O4 were studied in terms of polyhedra arrangement. The junction between octahedral layers (kagome) and mixed (octahedral and tetrahedral) layers were analyzed; this association allows one to determine two types of double layers whose packing leads to three closely related structural types: spinel, double-hexagonal LiFeSnO4, and a hypothetical hexagonal structure. Olivine structure shows a polyhedral arrangement closely related to those of these three structures and can be described alike in terms of mixed layers and double layers. AB2O4 close-packed oxides can exhibit polymorphism; from this analysis a mechanism, involving geometrical operations applied to the double layers, is proposed for the different transitions, really observed as (DH) LiFeSnO4 spinel and olivine-spinel or theoretical as (DH) LiFeSnO4—hypothetical hexagonal and olivine—hypothetical hexagonal. 相似文献
996.
Abstract —Direct photoinactivation by UV light of bovine carbonic anhydrase, as well as its photosensitization by N -formyl-kynurenine, a tryptophan photooxidation product, have been investigated. In the presence of oxygen both methods lead to similar results: the enzyme loses its activity, the tryptophanyl, histidyl and, to a lesser extent, tyrosyl residues being destroyed. In nitrogen-saturated solutions, a dramatic drop is observed in the photoinacitivation yield under the direct action of ultraviolet light, whereas histidyl residues remain intact. Evidence indicates an internal photodynamic action of N -formyl-kynurenine in the protein core produced by the UV photooxidation of tryptophanyl residues. Photoinactivation of oxygenated enzyme solutions by external and internal photodynamic action correlates with histidyl residue destruction via singlet oxygen. The possible importance of the photodynamic ability of N -formyl-kynurenine in the photochemistry of proteins, DNA, and cells is discussed. 相似文献
997.
The enol of acetone, formed by disproportionation reactions of 1-hydroxy-1-methylethyl radicals, is detected by NMR, spectroscopy during photoreactions of 3-hydroxy-3-methyl-2-butanone in acetonitrile and of acetone in 2-propanol and slowly tautomerizes to acetone. The photolysis of 3-hydroxy-3-methyl-2-butanone is shown to proceed via Type I cleavage, predominantly from an excited triplet state. 相似文献
998.
Contributions to the Chemical Transport of Metal Oxides. V. Transport of Ternary and Quaternary Mixed Ferrites by their Chlorides The transport by means of HCl as transporting gas in a closed system in the case of the ternary mixed ferrites (Ni0,8Fe2,2O4, Mn0,75Fe2,25O4, Mn1,286Fe1,714O4) and the quaternary mixed ferrites (Mg0,5Mn0,5Fe2O4, Mg0,75Mn0,536Fe1,714O4, Mg0,281Mn0,469Fe2,25O4, Mn0,5Zn0,5Fe2O4, Mn0,5Zn0,45Fe2,05O4, Ni0,5Zn0,5Fe2O4) between T2 = 1100?1000°C and T1 = 1000?800°C was investigated. Transported crystals were characterized by chemical analysis and the saturation magnetization, the transport rate has been checked. In the case of Mn0,5Zn0,45Fe2,05O4 two phases were transported. By discussing the phase diagram an explanation is given. 相似文献
999.
Extraction and subsequent recovery, in four separate fractions, of naphthenic acids, alkylphenols, pyrrollic compounds and nitrogen bases can be made from petroleum products using macroporous ion-exchange resins. Distinctive features of the technique are the use of dissolved gases in polar solvents as eluants and the application of the carbonate form of anion-exchange resins. The transition metal form of a macroreticular cation exchanger is shown to extract ligands from non-aqueous systems and a specific application to petroleum analysis is given. 相似文献
1000.
The possibility to perform gas chromatographic analysis using both a mass detector and a selective nitrogen-phosphorous detector will enable highly sensitive analytical results and an optimal identification reliability. In this work we describe an easily workable technical solution, which has been accomplished in our laboratory and the application of the above system to analytical procedures for the detection and determination of nitrogenous herbicides in drinking water and phosphorylated pesticides in agricultural products is discussed. 相似文献