全文获取类型
收费全文 | 330339篇 |
免费 | 3047篇 |
国内免费 | 937篇 |
专业分类
化学 | 164987篇 |
晶体学 | 5374篇 |
力学 | 15536篇 |
综合类 | 7篇 |
数学 | 38569篇 |
物理学 | 109850篇 |
出版年
2021年 | 3136篇 |
2020年 | 3336篇 |
2019年 | 3769篇 |
2018年 | 5205篇 |
2017年 | 5447篇 |
2016年 | 7377篇 |
2015年 | 4149篇 |
2014年 | 7038篇 |
2013年 | 15920篇 |
2012年 | 12397篇 |
2011年 | 14847篇 |
2010年 | 11027篇 |
2009年 | 10925篇 |
2008年 | 13252篇 |
2007年 | 13214篇 |
2006年 | 12152篇 |
2005年 | 10536篇 |
2004年 | 9849篇 |
2003年 | 8698篇 |
2002年 | 8691篇 |
2001年 | 10404篇 |
2000年 | 7670篇 |
1999年 | 5973篇 |
1998年 | 4977篇 |
1997年 | 4814篇 |
1996年 | 4487篇 |
1995年 | 3867篇 |
1994年 | 3826篇 |
1993年 | 3702篇 |
1992年 | 4097篇 |
1991年 | 4307篇 |
1990年 | 4102篇 |
1989年 | 4012篇 |
1988年 | 3782篇 |
1987年 | 3927篇 |
1986年 | 3686篇 |
1985年 | 4631篇 |
1984年 | 4683篇 |
1983年 | 3888篇 |
1982年 | 3976篇 |
1981年 | 3702篇 |
1980年 | 3650篇 |
1979年 | 3916篇 |
1978年 | 3903篇 |
1977年 | 3916篇 |
1976年 | 3885篇 |
1975年 | 3677篇 |
1974年 | 3616篇 |
1973年 | 3656篇 |
1972年 | 2605篇 |
排序方式: 共有10000条查询结果,搜索用时 35 毫秒
991.
Summary Chemiluminescent reactions of ozone with several classes of compounds at different temperatures are described. In a reaction chamber in front of a photomultiplier ozone is mixed with the compounds studied such as alkanes, olefins, alcohols, aromatics, C2H2, NO, vinyl chloride or H2S. The chemiluminescent emission is proportional to the concentration of the compound involved. At room temperature only olefinic hydrocarbons respond but at temperatures above 150°C also a response for alkanes, alcohols, CO etc. is obtained. It is suggested that next to the ozonide-excited formaldehyde mechanism the CO-CO
2
*
mechanism is responsible for the chemiluminescent emission at high temperature. A detection limit of a few ppb is achieved. Possible applications are a selective gas-chromatographic detector, an air pollution monitor for instance for CO, and a photochemical reactivity monitor.
Bestimmung reaktiver Kohlenwasserstoffe durch Chemiluminescenz
Zusammenfassung Chemiluminescenzreaktionen von Ozon mit verschiedenen Verbindungsgruppen (hauptsächlich Kohlenwasserstoffe) bei unterschiedlichen Temperaturen werden beschrieben. In einer Reaktionskammer werden die Verbindungen vor einem Photomultiplier mit Ozon vermischt. Es handelt sich u. a. um folgende Verbindungstypen: Alkane, Olefine, Alkohole, Aromaten, C2H2, NO, Vinylchlorid, H2S. Die Emissionsintensität ist proportional der Konzentration. Bei Raumtemperatur sprechen nur Olefine an, die übrigen Verbindungen erst über 150°C. Die Emission bei höherer Temperatur wird neben der Bildung von Formaldehyd auf die Reaktion CO-CO 2 * zurückgeführt. Die Nachweisgrenze beträgt wenige ppb. Als mögliche Anwendungsgebiete werden ein selektiver GC-Detektor, ein Monitor für Luftverunreinigungen (z.B. für CO) sowie ein Monitor zur Messung photochemischer Reaktivität der Luft genannt.
Presented at the 6th Annual Symposium on Recent Advances in the Analytical Chemistry of Pollutants, April 21–23, 1976; Vienna, Austria. 相似文献
992.
V. P. Zvolinskii P. I. Zakharov V. K. Shevtsov A. V. Varlamov V. G. Pleshakov G. A. Vasil'ev N. S. Prostakov 《Chemistry of Heterocyclic Compounds》1978,14(2):200-203
The dissociative ionization of 4-azafluorene and its methyl and phenyl derivatives was investigated. The relative intensity of the [M — CH3]+ ion peak depends on the position of the CH3 group in the 4-azafluorene ring. It was established that the loss of an RCN particle (R=H, CH3, and C6H5) for unsubstituted 4-azafluorene takes place from the M+ and [M — H]+ ion, exclusively from the [M — H]+ ion for the methyl-substituted compounds, and from the [M — H]+ and [H — 2H]+ fragments for the phenyl-substituted derivatives. Randomization of the deuterium ions in the 9,9-d2-4-azafluorene molecular ion was observed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 246–250, February, 1978. 相似文献
993.
É. A. Karakhanov S. Kh. Sharipova M. I. Rozhkova E. A. Viktorova 《Chemistry of Heterocyclic Compounds》1978,14(1):15-16
It is shown that the principal pathway in the reaction of benzofuran with a solution of lithium in hexametapol is opening of the furan ring of benzofuran to give a dilithium derivative. Mono- and dialkylation products are formed when the reaction mixture is treated with alkylating agents. It was established by the use of alkylating agents with different degrees of hardness that the oxygen atom is a softer polarizable center than the sp-hybridized carbanion center.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 22–23, January, 1978. 相似文献
994.
G. N. Dorofeenko S. V. Krivun V. G. Korobkova 《Chemistry of Heterocyclic Compounds》1973,9(11):1318-1320
The acylation of esters of 3,4-dimethoxyphenylpyruvic acid with anhydrides of aliphatic acids in the presence of 70% HClO4 leads to the formation of 3-alkoxycarbonyl-1-alkyl-6,7-dimethoxy-2-benzopyrylium salts which, on being boiled with sodium acetate inglacial acetic acid, are converted into 3-alkoxycarbonyl-1-alkyl-6,7-dimethoxyisoquinolines.For Communication XIII, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1458–1460, November, 1973. 相似文献
995.
A. B. Deshpande S. M. Kale S. L. Kapur 《Journal of polymer science. Part A, Polymer chemistry》1973,11(6):1307-1315
Polymerization of methyl methacrylate has been studied with the chromium acetylacetonate–triisobutyl aluminum catalyst system in benzene medium at 40°C. These studies have been carried out at an Al/Cr ratio of 12 to compare the behavior with the previously studied chromium acetyl acetonate–triethyl aluminum catalyst system. The enhanced yield and gelling of polymer suggests a free-radical mechanism of polymerization. Further, the kinetics of polymerization and the heterotactic structure of polymer as determined by NMR examination have led to confirmation of the freeradical mechanism of polymerization of methyl methacrylate by an excess of triisobutylaluminum in the presence of catalyst complex. 相似文献
996.
The thermodynamic quantities associated with ionization of the N1 and N9 protons of adenine have been calorimetrically determined as a function of temperature. The H values for proton dissociation of these groups, with pK values of 4.19 and 9.92, were found to be 5.1 and 9.1 kcal/mole, respectively, at 25°C, =0.025. The C
p
values for proton dissociation of these groups were estimated to be –11 and –17 cal/mole-deg. These results indicate that the large heat capacity changes observed during conformational transitions of polynucleotides are not the result of ionization of the bases. 相似文献
997.
O. V. Krylov 《Kinetics and Catalysis》2002,43(2):284-289
This paper presents an overview of the plenary lectures and some of the oral and poster presentations at the 4th International Congress on Oxidation Catalysis in Potsdam (Germany). The main topics were the partial oxidation of alkanes and the oxidation of olefins by H2O2. Considerable attention is given to the use of modern physical methods in the mechanistic studies of oxidation catalysis and in the studies of catalyst structures. 相似文献
998.
Liposomes with encapsulated carboxyfluorescein were used in an affinity-based assay to provide signal amplification for small-volume fluorescence measurements. Microfluidic channels were fabricated by imprinting in a plastic substrate material, poly(ethylene terephthalate glycol) (PETG), using a silicon template imprinting tool. Streptavidin was linked to the surface through biotinylated-protein for effective immobilization with minimal nonspecific adsorption of the liposome reagent. Lipids derivatized with biotin were incorporated into the liposome membrane to make the liposomes reactive for affinity assays. Specific binding of the liposomes to microchannel walls, dependence of binding on incubation time, and nonspecific adsorption of the liposome reagent were evaluated. The results of a competitive assay employing liposomes in the microchannels are presented. 相似文献
999.
An efficient method was developed for the determination of nanogram levels of lithium in biological samples. Serum samples from human subjects from southeastern Spain, treated or not treated with lithium carbonate, were analyzed by electrothermal atomic absorption spectrometry. The samples were previously treated with a matrix modifier consisting of 0.1% Triton X-100 and injected through a graphite tube with L'vov platform. The Li concentrations measured by the procedure described for the 3 certified reference samples used were not significantly different (p > 0.05) than certified levels. Sample recoveries and variability during several days, with coefficients of variation from 4.00 to 14.8%, demonstrated the reliability and accuracy of this technique. Mean Li concentration determined in the serum of individuals with psychiatric disorders treated with Li (n = 117, 5.077 +/- 1.795 microg Li/mL) was significantly higher (p < 0.001) than that in individuals not treated with Li (n = 24, 1.902 +/- 2.054 ng Li/mL). 相似文献
1000.
S. N. Karyagin V. F. Kiselev G. S. Plotnikov Yu. D. Fomin 《Theoretical and Experimental Chemistry》1990,25(5):504-509
Electrophysical methods and ESR measurements have been used to investigate the changes, photosensitized by molecules of rhodamine B (RB), in the charge states of electron traps created by molecules of p-benzoquinone (pBQ) in the Ge/GeO2 system. The results obtained in studying the quenching of fluorescence of RB molecules indicate that the photodestruction of electron traps is due to transfer of electronic excitation energy traps is due to transfer of electronic oxidation energy from the RB molecules to charged complexes formed upon adsorption of the pBQ. In order to select optimal conditions for recharging of such complexes, a study has been made of the relationship between the relative change in ESR signal and the concentration of adsorbed RB molecules.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 545–550, September–October, 1989. 相似文献