全文获取类型
收费全文 | 24767篇 |
免费 | 660篇 |
国内免费 | 142篇 |
专业分类
化学 | 16953篇 |
晶体学 | 123篇 |
力学 | 584篇 |
数学 | 4375篇 |
物理学 | 3534篇 |
出版年
2022年 | 180篇 |
2021年 | 262篇 |
2020年 | 302篇 |
2019年 | 320篇 |
2018年 | 236篇 |
2017年 | 198篇 |
2016年 | 489篇 |
2015年 | 458篇 |
2014年 | 496篇 |
2013年 | 1310篇 |
2012年 | 1147篇 |
2011年 | 1448篇 |
2010年 | 732篇 |
2009年 | 714篇 |
2008年 | 1269篇 |
2007年 | 1297篇 |
2006年 | 1276篇 |
2005年 | 1222篇 |
2004年 | 1061篇 |
2003年 | 923篇 |
2002年 | 826篇 |
2001年 | 353篇 |
2000年 | 285篇 |
1999年 | 259篇 |
1998年 | 279篇 |
1997年 | 314篇 |
1996年 | 365篇 |
1995年 | 243篇 |
1994年 | 289篇 |
1993年 | 272篇 |
1992年 | 244篇 |
1991年 | 247篇 |
1990年 | 198篇 |
1989年 | 233篇 |
1988年 | 240篇 |
1987年 | 202篇 |
1986年 | 195篇 |
1985年 | 297篇 |
1984年 | 340篇 |
1983年 | 219篇 |
1982年 | 370篇 |
1981年 | 337篇 |
1980年 | 319篇 |
1979年 | 323篇 |
1978年 | 327篇 |
1977年 | 298篇 |
1976年 | 283篇 |
1975年 | 254篇 |
1974年 | 250篇 |
1973年 | 243篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
161.
The effects of radiofrequency (RF) (1–4) and magnetic fields (5–9) on the behavior of aqueous solutions and suspensions have been a popular subject in recent years. The mechanism of the magnetic “water memory” effect, though, is still largely unknown (5). In this work, we present evidence that the primary “receptor” of the electromagnetic radiation is a gas/liquid interface. Gas can be either already present in water or produced by the effects of electromagnetic fields. Perturbed gas/liquid interfaces require hours to equilibrate. Certain RF and magnetic signals also produce reactive oxygen and hydrogen species (superoxide, hydrogen peroxide, hydrogen, atomic hydrogen). The perturbed gas/liquid interface modifies the hydrogen bonding networks in water and also the hydration of ions and interfaces. Careful outgassing removes all of the effects of the electromagnetic fields, including the magnetic memory effect. The amplitude of the applied field influences the observed effects. Different amplitudes of RF radiation perturb the interfacial water in different ways and consequently affect the behavior of colloids and ions in specific manners. For instance, the bulk and template precipitation of calcium carbonate, zeta potentials of suspended colloids, rate of dissolution of colloidal silica, and attachment of colloidal silica to metal surfaces are modified in specific ways with the low amplitude or high amplitude RF treatments described in this paper. The solubility/diffusivity of gas species is also modified in a different manner, and it is probably at the core of the specificity of the RF amplitude effects. 相似文献
162.
163.
Trivalent phosphorus compounds are promoters for methyl formate homologation to ethanol and ethyl formate catalyzed by ruthenium compounds in the presence of iodide at 220°C and 27 MPa of synthesis gas. Under these conditions the phosphines are quaternized, but decomposition of phosphonium salts occurs during the reaction. Promotion is also observed for methyltriphenyl-phosphonium bromide and triphenylphosphine sulfide, but benzyltrimethyl-ammonium bromide, triphenylarsine, and triphenylantimony are not effective. The major ruthenium species present is Ru(CO)3I3- but with triphenylantimony a trimethylantimony complex, Ru(CO)2(Sb(CH3)3)2I2, can be isolated in high yield. 相似文献
164.
165.
Robert K. Tubbs 《Journal of polymer science. Part A, Polymer chemistry》1966,4(3):623-629
Copolymers of vinyl acetate and vinyl alcohol were studied by differential thermal analysis. The melting points of the copolymers are not a simple function of the composition, but depend on the method of preparation of the copolymers. Partial saponification of poly(vinyl acetate) with sodium hydroxide leads to high melting, ordered copolymers, while reacetylation of poly(vinyl alcohol) leads to low melting, random copolymers. Catalytic alcoholysis of PVAc yields copolymers intermediate in melting point and order. The results are treated by assuming that equal melting points indicate similar sequence length distributions of crystallizable units. 相似文献
166.
Norton P. Peet Shyam Sunder Robert J. Barbuch Edward W. Huber Ester M. Bargar 《Journal of heterocyclic chemistry》1987,24(6):1531-1535
Treatment of o-nitrobenzenesulfonyl chloride ( 3 ) with 5-aminotetrazole (5-AT) gave [(2-nitrophenyl)-sulfonyl]carbamimidic azide ( 6 ), a ring-opened isomer of the expected N-(1H-tetrazol-5-yl)-2-nitrobenzenesulfonamide ( 4 ). Sulfonylcarbamimidic azide 6 was converted to 2-amino-N-(aminoiminomethyl)benzene-sulfonamide ( 7 ) with ethanolic stannous chloride, and to 3-amino-1,2,4-thiadiazine 1,1-dioxide ( 8 ) with sodium dithionite. Methanesulfonyl chloride ( 9 ) and 5-AT gave 2-(methylsulfonyl)carbamimidic azide ( 10 ), which isomerized to 5-[(methylsulfonyl)amino]-1H-tetrazole ( 11 ) in warm ethanol. Attempted cycloaddition of 2-(phenylsulfonyl)carbamimidic azide ( 13 ) and ethyl vinyl ether led only to alkylated tetrazole products. In addition, other tetrazole-alkylating reactions are described. Isomers produced from these alkylations were differentiated with 13C nmr spectroscopy. 相似文献
167.
168.
Dr. Gerhard Banik Herbert Stachelberger Alfred Vendl Robert Kellner Manfred Grasserbauer 《Mikrochimica acta》1981,76(1-2):93-109
Summary The fundamental importance of scientific research for the preservation and restoration of works of art is uncontestable. The methods described in this paper, such as light microscopy, microchemical tests, emission spectroscopy, chromatography, scanning electron analysis, electron microprobe analysis and infrared spectroscopy, are of special significance in this filed. Nevertheless a better understanding of the problems that arise out of new special questions of art history can be obtained only by the right interpretation of the data obtained by scientific methods.
Mikrochemische Charakterisierung von Gemälden
Zusammenfassung Die grundlegende Bedeutung wissenschaftlicher Forschung für die Erhaltung und Restaurierung von Kunstwerken ist unbestritten. Die hier beschriebenen Methoden (Lichtmikroskopie, mikrochemische Reaktionen, Emissionsspektroskopie, Chromatographie, Rasterelektronen-Analyse und Infrarotspektroskopie) sind hierfür von zunehmender Bedeutung. Sie ermöglichen ein besseres Verständnis neuerer kunsthistorischer Probleme nur durch die richtige Interpretation der Ergebnisse wissenschaftlicher Methoden.相似文献
169.
Summary The acid-catalysed decarboxylation of thecis-- andcis--[CoL(CO3)]+ complexes (L = 3,6-dimethyl-1,8-diamino-3,6-diazaoctane) have been studied over a range of HClO4 concentrations and the temperatures 25, 35 and 45° at I = 1.0 mol dm–3 (NaClO4). The rate expression takes the form kobs = k0 + k1[H+] where kobs is the observed first order rate constant at constant hydrogen ion concentration. The k0 term makes only a minor contribution to the overall reaction. Both complexes display solvent deuterium isotope effects ofca. 2.6 for the acid-catalysed decarboxylation, consistent with a rapid proton pre-equilibrium mechanism. Activation parameters have been determined and the mechanism of the reaction discussed. The magnitude of the solvent isotope effect is consistent with an A-1 type mechanism involving formation of a 5-coordinate intermediate. 相似文献
170.