全文获取类型
收费全文 | 72389篇 |
免费 | 15921篇 |
国内免费 | 240篇 |
专业分类
化学 | 72795篇 |
晶体学 | 267篇 |
力学 | 2741篇 |
综合类 | 1篇 |
数学 | 7248篇 |
物理学 | 5498篇 |
出版年
2024年 | 380篇 |
2023年 | 4135篇 |
2022年 | 1570篇 |
2021年 | 2640篇 |
2020年 | 4810篇 |
2019年 | 2570篇 |
2018年 | 2477篇 |
2017年 | 802篇 |
2016年 | 5925篇 |
2015年 | 5791篇 |
2014年 | 5323篇 |
2013年 | 6412篇 |
2012年 | 4418篇 |
2011年 | 2614篇 |
2010年 | 4181篇 |
2009年 | 3952篇 |
2008年 | 2471篇 |
2007年 | 2215篇 |
2006年 | 1585篇 |
2005年 | 1571篇 |
2004年 | 1353篇 |
2003年 | 1139篇 |
2002年 | 1103篇 |
2001年 | 447篇 |
1997年 | 398篇 |
1996年 | 489篇 |
1995年 | 450篇 |
1994年 | 386篇 |
1993年 | 519篇 |
1992年 | 388篇 |
1988年 | 391篇 |
1987年 | 358篇 |
1985年 | 496篇 |
1984年 | 565篇 |
1983年 | 435篇 |
1982年 | 568篇 |
1981年 | 564篇 |
1980年 | 545篇 |
1979年 | 526篇 |
1978年 | 544篇 |
1977年 | 658篇 |
1976年 | 645篇 |
1975年 | 726篇 |
1974年 | 740篇 |
1973年 | 529篇 |
1972年 | 508篇 |
1971年 | 470篇 |
1970年 | 625篇 |
1969年 | 488篇 |
1968年 | 531篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Oxidation of 4-[3-(dimethylamino)propyl]-3,4-dihydro-2-(1-hydroxyethyl)-3-phenyl-2H-1,4-benzothiazine, hydrochloride (I) with hydrogen peroxide yielded a mixture of two sulfoxides (II). Since this mixture exhibited antiinflammatory activity, the two components (Isomers A and B) were prepared in purified form by oxidation of I with N-chlorosuccinimide and hydrogen peroxide, respectively. Isomer A was more potent than Isomer B in the carrageenin-induced edema test. 相似文献
102.
Dipl.-Ing. Dr. O. Hinterhofer 《Monatshefte für Chemie / Chemical Monthly》1974,105(2):279-290
The selective chlorination of a m/p-xylene mixture, followed by distillation of the unreacted p-xylene, leaves a residue containing up to 90% of monochlorinated m-xylenes. m-Xylene is recovered from the latter by heterogeneous catalytic hydrogenolysis in the gas-phase. It was found that the hydrogenolysis on certain noble metal catalysts proceeds according to an ionic reaction mechanism at temperatures below a definite temperature range. At temperatures above this range hydrogenolysis follows a radical reaction mechanism. 相似文献
103.
Dynamic behaviour of river colloidal and dissolved organic matter through cross-flow ultrafiltration system 总被引:1,自引:0,他引:1
Through cross-flow filtration (CFF) with a 1-kDa regenerated cellulose Pellicon 2 module, the ultrafiltration characteristics of river organic matter from Longford Stream, UK, were investigated. The concentration of organic carbon (OC) in the retentate in the Longford Stream samples increased substantially with the concentration factor (cf), reaching approximately 40 mg/L at cf 15. The results of dissolved organic carbon (DOC) and colloidal organic carbon (COC) analysis, tracking the isolation of colloids from river waters, show that 2 mg/L of COC was present in those samples and good OC mass balance (77-101%) was achieved. Fluorescence measurements were carried out for the investigation of retentate and permeate behaviour of coloured dissolved organic materials (CDOM). The concentrations of CDOM in both the retentate and permeate increased with increasing cf, although CDOM were significantly more concentrated in the retentate. The permeation model expressing the correlation between log[CDOM] in the permeate and logcf was able to describe the permeation behaviour of CDOM in the river water with regression coefficients (r(2)) of 0.94 and 0.98. Dry weight analysis indicated that the levels of organic colloidal particles were from 49 to 71%, and between 29 and 51% of colloidal particles present were inorganic. COC as a percentage of DOC was found to be 10-16% for Longford Stream samples. 相似文献
104.
Soltero-Higgin M Carlson EE Phillips JH Kiessling LL 《Journal of the American Chemical Society》2004,126(34):10532-10533
The flavoenzyme uridine 5'-diphosphate (UDP)-galactopyranose mutase (UGM) plays a key role in the cell wall biosynthesis of many pathogens, including Mycobacterium tuberculosis. Using a synthetic fluorescent ligand, we screened 16 000 compounds in a fluorescence polarization assay. Effective inhibitors of UGM were identified. 相似文献
105.
4-Isothiazolin-3-one 1-oxides and 1,1-dioxides react with a variety of cyclic and acyclic dienes to afford cycloaddition products. 相似文献
106.
The stoichiometry and structure of acetophenone benzoylhydrazone and acetophenone salicoylhydrazone chelates with some divalent metal ions are studied by conductometric titration and visible and infra-red absorption spectrophotometry. The results are supported by analysis of the solid complexes. The infra-red study revealed that coordination occurs through C=O and C=N groups. The shift in the C=O and C=N bands is utilised for the determination of bond lengths. 相似文献
107.
H. D. Hollis Showalter John M. Domagala Joseph P. Sanchez 《Journal of heterocyclic chemistry》1981,18(8):1609-1612
Starting from readily available 1,2-dihydro-6-methyl-2-oxo-3-pyridinecarbonitrile, 1, viable synthetic pathways to a number of C-6 functionalized 1,2-dihydro-2-oxo-3-pyridinecarbonitriles and corresponding acids are presented. Through the utilization of dianion chemistry, the C-6 methyl substituent is selectively functionalized to three different oxidation levels. 相似文献
108.
Thermal analysis supported by kinetic calculations was applied exhaustively to these compounds. Under dynamic conditions, tetrafluorometallates(III), which as intermediates, could not be isolated, for the first three compounds. In each case, the final step was the pure metal(III) fluoride. Ammonium hexafluorochromate(III) decomposed directly to the pure chromium(III) fluoride. The decomposition rate of all compounds slowed down towards the end, probably for kinetic reasons. Polymorphic transitions of ammonium hexafluorotitanate(III) were observed at 35 and 100°C. Ammoniumhexafluoroscandate(III) underwent polymorphic transition at 47°C. The decomposition patterns for all these compounds were similar. Conditions for the preparation of pure ammonium tetrafluorometallates(III) of Sc, Ti and V are described. 相似文献
109.
John A. Bishop 《Analytica chimica acta》1973,63(2):305-311
Four different methods for the calculation of the formation constants of fluorescing complexes are discussed. The methods were used to determine constants for the complexes of aluminum(III), gallium(III) and indium(III) with 8-quinolinol-5-sulfonate anion. 相似文献
110.
Zusammenfassung In dieser Arbeit wird die Dreiphononenabsorption in Polymeren, die den Hauptanteil an den dielektrischen Verlusten im Millimeterwellengebiet stellt, theoretisch und experimentell untersucht. Aus der Störungsrechnung bis zur 2. Ordnung wird die Temperaturabhängigkeit der Absorption abgeschätzt. Damit ist eine befriedigende Deutung des gesamten Temperaturverlaufs der dielektrischen Verlust-Kurven möglich. Im Hochtemperaturbereich läßt sich die Mehrphononenabsorption eindeutig gegenüber restlichen Relaxationsprozessen abgrenzen. Im Tieftemperaturgebiet konnte erstmals auch für das Millimeterwellengebiet ein temperaturunabhängiger Bestandteil fehlordnungsinduzierter Einphononenabsorption nachgewiesen werden. Im Übergangsbereich durchlaufen die Absorptionskurven einen Wendepunkt bei einer charakteristischen TemperaturT
0, die dem Schwerpunkt der am Absorptionsprozeß beteiligten Phononenfrequenzen entspricht. Daraus konnten in Übereinstimmung mit dem Experiment die Frequenzen der niedrigsten optisch aktiven Phononen für die verschiedenen Polymeren abgeschätzt werden.
Mit 6 Abbildungen in 8 Einzeldarstellungen
Vorgetragen auf der Arbeitssitzung des Fachausschusses Physik der Hochpolymeren in der Frühjahrstagung 1970 des Regionalverbandes Hessen-Mittelrhein-Saar der Deutschen Physikalischen Gesellschaft in Darmstadt, 10.–13. März 1970. 相似文献
Summary The 3-phonon absorption in polymers, which gives the main contribution to their dielectric loss in the microwave region, is theoretically and experimentally investigated. Perturbation theory up to the second order is used for the evaluation of its temperature dependence. The theoretical results allow an interpretation of the total absorption-temperature-curves: In the high temperature region 3-phonon absorption can be distinguished from residual relaxation losses (above the glass temperature). In the low temperature region the existence of a low frequency tail of temperature independent disorderinduced one-phonon absorption became evident. At intermediate temperatures (50–150°K) the curves show an inflexion point or plateau at a characteristic temperatureT 0, which according toT 0= 0, corresponds to the frequencies of those phonons, that mainly contribute to the multiphonon absorption. The plateau gives, in good agreement with the experiment, the frequencies of the lowest optically active phonons in the various polymers.
Mit 6 Abbildungen in 8 Einzeldarstellungen
Vorgetragen auf der Arbeitssitzung des Fachausschusses Physik der Hochpolymeren in der Frühjahrstagung 1970 des Regionalverbandes Hessen-Mittelrhein-Saar der Deutschen Physikalischen Gesellschaft in Darmstadt, 10.–13. März 1970. 相似文献