全文获取类型
收费全文 | 81472篇 |
免费 | 10948篇 |
国内免费 | 2202篇 |
专业分类
化学 | 72270篇 |
晶体学 | 708篇 |
力学 | 1428篇 |
综合类 | 66篇 |
数学 | 8922篇 |
物理学 | 11228篇 |
出版年
2023年 | 418篇 |
2022年 | 426篇 |
2021年 | 867篇 |
2020年 | 2127篇 |
2019年 | 3394篇 |
2018年 | 1951篇 |
2017年 | 1565篇 |
2016年 | 4933篇 |
2015年 | 4765篇 |
2014年 | 4779篇 |
2013年 | 6903篇 |
2012年 | 6307篇 |
2011年 | 5969篇 |
2010年 | 4937篇 |
2009年 | 4512篇 |
2008年 | 5495篇 |
2007年 | 4892篇 |
2006年 | 4240篇 |
2005年 | 4123篇 |
2004年 | 3476篇 |
2003年 | 2884篇 |
2002年 | 3380篇 |
2001年 | 2264篇 |
2000年 | 2036篇 |
1999年 | 950篇 |
1998年 | 467篇 |
1997年 | 370篇 |
1996年 | 499篇 |
1995年 | 341篇 |
1994年 | 312篇 |
1993年 | 378篇 |
1992年 | 336篇 |
1991年 | 226篇 |
1990年 | 227篇 |
1989年 | 182篇 |
1988年 | 183篇 |
1987年 | 179篇 |
1986年 | 184篇 |
1985年 | 280篇 |
1984年 | 228篇 |
1983年 | 166篇 |
1982年 | 182篇 |
1981年 | 171篇 |
1980年 | 156篇 |
1979年 | 151篇 |
1978年 | 140篇 |
1977年 | 118篇 |
1976年 | 127篇 |
1975年 | 103篇 |
1973年 | 110篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The aim of this study was to investigate the influence of cosolvent polarity on the rheological flow properties of aqueous and hydroalcoholic gels obtained from the Carbopol Ultrez 10 base and used for topical applications. Specifically, we have examined the effect of pH (range 4.0--7.0) on the consistency and flow properties of dispersed systems in water and mixtures--15 : 85% v/v of methanol : water, ethanol : water, n-propanol : water and n-butanol : water--at a constant polymer concentration of 0.3% w/w. The gels, which had decreasing polarity values in the jellifying medium, showed qualitatively similar flow behavior, characteristic of pseudoplastic systems, and all of the flow curves were adjusted to the Ostwald model. Sigmoidal dose response functions were calculated to describe the flow and consistency indexes as a function of pH. As a result, the influence of alcohol polarity on the polymer network has been assessed meaningfully using the empirical parameters obtained: maximum consistency index value (k(max)), pH value required for 50% development of polymer network (pH(50)), and asymptotic flow index value (n(min)) for the fully structured gels. 相似文献
992.
The present paper deals with the experimental and theoretical determination of energies of conjugation of organic molecules.A computation process has been applied, first to simple molecules comprising an hydrocarbon (styrene), an oxygen function (benzaldehyde) and a nitrogen function (aniline). The results are compared with those obtained from the literature.The application of this computation process to naphthoic acids permits the determination of energies of conjugation and isomerization of these molecules. 相似文献
993.
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
994.
A noncoded amino acid of cyclomarin A (1) was synthesized in a racemic fashion. The method employs a six-membered ring template to control the relative stereochemistry and introduction of the functional groups. Ultimately, Pd-catalyzed fragmentation of the lactone provided gamma,delta-unsaturated and delta,epsilon-unsaturated alpha-amino acids. A Pd-catalyzed ring opening of a gamma-lactone is also reported. 相似文献
995.
[reaction: see text] Secondary alpha-lithiosulfinyl carbanions react either intermolecularly, after transmetalation into an organocopper derivative in an S(N)2-type process with zinc carbenoid, or intramolecularly via higher-order zincate to give, through a tandem zinc homologation-beta-elimination reaction the corresponding alkenes. alpha,alpha-Disubstituted alkenes are only formed from tertiary alpha-lithiosulfinyl carbanions via the 1,2-metalate rearrangement. 相似文献
996.
Piotr Kujawa Annie Audibert‐Hayet Joseph Selb Franoise Candau 《Journal of Polymer Science.Polymer Physics》2004,42(9):1640-1655
Multisticker associative polyelectrolytes of acrylamide (≈86 mol %) and sodium 2‐acrylamido‐2‐methylpropanesulfonate (≈12 mol %), hydrophobically modified with N,N‐dihexylacrylamide groups (≈2 mol %), were prepared with a micellar radical polymerization technique. This process led to multiblock polymers in which the length of the hydrophobic blocks could be controlled through variations in the surfactant‐to‐hydrophobe molar ratio, that is, the number of hydrophobes per micelle (NH). The rheological behavior of aqueous solutions of polymers with the same molecular weight and the same composition but with two different hydrophobic block lengths (NH = 7 or 3 monomer units per block) was investigated as a function of the polymer concentration with steady‐flow, creep, and oscillatory experiments. The critical concentration at the onset of the viscosity enhancement decreased as the length of the hydrophobic segments in the polymers increased. Also, an increase in the NH value significantly enhanced the thickening ability of the polymers and affected the structure of the transient network. In the semidilute unentangled regime, the behavior of the polymer with long hydrophobic segments (NH = 7) was studied in detail. The results were well explained by the sticky Rouse theory of associative polymer dynamics. Finally, the viscosity decreased with an increase in the temperature, mainly because of a lowering of the sample relaxation time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1640–1655, 2004 相似文献
997.
Oliver Moers Violeta Latorre Karl‐Heinz Nagel Armand Blaschette Peter G. Jones 《无机化学与普通化学杂志》2003,629(1):83-90
Polysulfonylamines. CLX. Crystal Structures of Metal Di(methanesulfonyl)amides. 10. The Three‐Dimensional Coordination Polymers M[(CH3SO2)2N], where M is Potassium, Rubidium, Cesium (Isotypic Structures for M = K, Rb) Low‐temperature X‐ray crystal structures are reported for KA (monoclinic, space group P21/c, Z′ = 1), RbA (isotypic and isostructural with KA), and CsA (monoclinic, P21/n, Z′ = 1), where A— denotes the anion obtained by deprotonation of the strong nitrogen acid (MeSO2)2NH. In KA and RbA, the anion is distorted into a rare C1 conformation, whereas the standard C2 conformation is retained in the cesium complex. The structures consist of three‐dimensional coordination networks, in which each cation adopts an irregular (O6N)‐heptacoordination by forming close contacts to one (O, N)‐chelating, one (O, O)‐chelating and three κ1O‐bonding ligands; however, the coordination number for Cs+ is effectively increased to 8 by a very short Cs···Cs contact distance of 422.5 pm. The crystal packings of the isotypic compounds KA and RbA display lamellar layer substructures that involve six independent ligand‐metal bonds and comprise an internal cation lamella and peripheral regions built up from anion monolayers; the 3D framework is completed by one independent M—O bond cross‐linking the layer substructures. In contrast, CsA features anion monolayers that intercalate planar zigzag chains of cations (Cs···Cs alternatingly 422.5 and 487.5 pm, Cs···Cs···Cs 135.7°), whereby each chain is surrounded and coordinated by four anion stacks and each anion stack connects two cation chains. All structures exhibit close C—H···A interanion contacts consistent with weak hydrogen bonding. 相似文献
998.
Jolanta Olkowska‐Oetzel Dieter Fenske Petra Scheer Andreas Eichhfer 《无机化学与普通化学杂志》2003,629(3):415-420
Syntheses and Thermal Properties of Cluster Molecules, formed from Groups 11‐13‐16 Elements In the presence of PPh3, CuX (X = Cl, CH3COO) or AgOC(O)C6H5 and GaCl3 react in THF with S(SiMe3)2 or Se(SiMe3)2 to yield [Cu6Ga8Cl4S13(PPh3)6] ( 1 ), [Cu6Ga8Cl4Se13(PPh3)6] ( 2 ), [Ag6Ga8Cl4S13(PPh3)6] ( 4 ) and [Ag6Ga8Cl4Se13(PPh3)6] ( 5 ). The use of PnPr2Ph instead of PPh3 and subsequent layering with n‐hexane leads to the formation of the cluster [Cu6Ga8Cl4Se13(PnPr2Ph)12] ( 3a , 3b ). Reaction of CuCl, GaCl3 and PnPr3 with Se(SiMe3)2 in THF results in the crystallisation of the ionic cluster (HPnPr3)2[Cu2Ga4Cl4Se6(PnPr3)4] ( 6 ). The structures of 1 — 6 were determined by X‐ray single crystal structure analysis. Thermogravimetric measurements of the cluster molecules and powder diffraction patterns of the remaining powders reveal the potential use of them as single source precursor compounds for the synthesis of the related ternary solid state materials. 相似文献
999.
Santoyo E Verma SP Sandoval F Aparicio A García R 《Journal of chromatography. A》2002,949(1-2):281-289
A suppressed ion chromatography (IC) technique has been evaluated as a chemical monitoring tool for detecting major anions (F-, Cl-, NO3- and SO4(2-)) of condensed steam in geothermal power plants. It is shown that the suppressed IC technique provides a suitable means for preventing possible damage to generating equipment in the geothermal industry. An electrical conductivity detector (0.1 microS sensitivity) with an anion-exchange column (IonPac AS4A-SC), a micro-membrane suppressor (AMMS II), and an isocratic high-pressure pump system were successfully used for detecting low concentrations of inorganic anions. Method detection limits for the anions of interest were <0.184 mg/L. Details of the IC methodology as well as some experimental results obtained during its application for the chemical monitoring of geothermal steam pipes are also described. 相似文献
1000.
André Julg 《Theoretical chemistry accounts》1964,2(2):134-157
Résumé L'auteur reprend les principes de la méthode L.C.A.O. améliorée en vue d'une justification plus complète des procédés employés. La théorie des perturbations permet d'une façon simple de tenir compte de la corrélation entre les électrons grâce à l'introduction d'une fonction universelle de la distance entre deux électrons. Le problème est ainsi ramené de l'échelle moléculaire à l'échelle atomique. On montre que la corrélation entre les électrons entraîne la réduction des intégrales atomiques. Le problème de la réduction des intégrales coulombiennes est traité en détail. La possibilité de négliger les autres intégrales biélectroniques est ensuite examiné. Enfin est étudié le problème de la réduction des intégrales de coeur.
Nous tenons à remercier très vivement le Docteur Chr. K. Jørgensen (Cyanamid European Research Institut, Genève) pour les intéressants échanges de vue que nous avons eu sur les divers problèmes évoqués dans cet article ainsi que le Docteur G. Berthier (Institut de Biochimie théorique, Paris) et Monsieur H. v. Hirschhausen. 相似文献
The principles of the improved LCAO theory are investigated to give a better justification for its methods. Perturbation theory allows a simple account for electronic correlation, when a universal function of interelectronic distance is introduced. The problem is hereby reduced from molecular to atomic scale. The reduction of electronic energy integrals by correlation is shown, especially for Coulomb integrals. The possibility of neglecting the other bielectronic integrals is examined. Finally the problem of core integral reduction is studied.
Zusammenfassung Die Grundlagen der verbesserten LCAO-Methode werden untersucht, um dieses Verfahren besser zu rechtfertigen. Mit einer allgemeinen Funktion des Abstandes je zweier Elektronen und der Störungstheorie läßt sich die Elektronenkorrelation auf einfache Weise berücksichtigen. Damit wird das Problem vom molekularen auf den atomaren Maßstab zurückgeführt. Die Elektronenkorrelation bringt eine Reduktion der Elektronenenergieintegrale mit sich; die Coulombintegrale werden im einzelnen behandelt. Anschließend wird die Möglichkeit untersucht, die übrigen Zweielektronenintegrale zu vernachlässigen. Schließlich wird die Reduktion der Rumpfintegrale behandelt.
Nous tenons à remercier très vivement le Docteur Chr. K. Jørgensen (Cyanamid European Research Institut, Genève) pour les intéressants échanges de vue que nous avons eu sur les divers problèmes évoqués dans cet article ainsi que le Docteur G. Berthier (Institut de Biochimie théorique, Paris) et Monsieur H. v. Hirschhausen. 相似文献