首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   147082篇
  免费   1294篇
  国内免费   397篇
化学   80252篇
晶体学   2388篇
力学   6063篇
综合类   3篇
数学   13574篇
物理学   46493篇
  2021年   1031篇
  2020年   1184篇
  2019年   1252篇
  2018年   1641篇
  2017年   1689篇
  2016年   2544篇
  2015年   1559篇
  2014年   2349篇
  2013年   6038篇
  2012年   4845篇
  2011年   5939篇
  2010年   4188篇
  2009年   4213篇
  2008年   5732篇
  2007年   5509篇
  2006年   5383篇
  2005年   4946篇
  2004年   4448篇
  2003年   4121篇
  2002年   4011篇
  2001年   5545篇
  2000年   3975篇
  1999年   3074篇
  1998年   2111篇
  1997年   2207篇
  1996年   2071篇
  1995年   1915篇
  1994年   1915篇
  1993年   1726篇
  1992年   2214篇
  1991年   2298篇
  1990年   2134篇
  1989年   2099篇
  1988年   2077篇
  1987年   2088篇
  1986年   1911篇
  1985年   2371篇
  1984年   2392篇
  1983年   1909篇
  1982年   1915篇
  1981年   1753篇
  1980年   1738篇
  1979年   2029篇
  1978年   2101篇
  1977年   2047篇
  1976年   1977篇
  1975年   1860篇
  1974年   1845篇
  1973年   1876篇
  1972年   1236篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
The ability of 3-[-(1,3-dioxo-2-indanyl)benzyl]-4-hydroxy-substituted 2H-benzo [b]pyran-2-ones and benzo[b]thiopyran-2-ones to undergo dehydration under the influence of dehydrating agents to give 7-aryl-6,7-dihydro-8H-indeno[1,2-2,3]pyrano[5,6-c]benzo[c]pyran-6,8-diones and their thio analogs was demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1475–1476, November, 1985.  相似文献   
972.
Summary Two new analytical techniques are described. The so-called injection technique operates on evaluating a dynamic concentration signal as a result of following the concentration change in a certain part of a flow-through channel after injecting a sample or reagent at a small volume into the current of an appropriate solution flowing at a constant rate.The second technique, the programmed coulometric titration technique, also uses the flow-through channel principle; that means this novel technique allows the more precise titrimetric way of analysis in flowing solutions. In the case of both techniques potentiometric and voltammetric detectors were involved. Several examples proved the multifold advantages of these techniques for environmental analysis.
Neue elektroanalytische Verfahren für die Umweltanalyse
Zusammenfassung Zwei neue Methoden werden beschrieben. Die sog. Injektionstechnik wertet ein dynamisches Konzentrationssignal aus, das sich aus der Änderung in einem Durchflußkanal ergibt, wenn Probe oder Reagens in kleinen Volumina in einen konstant fließenden Flüssigstrom injiziert werden. Das zweite Verfahren, die programmierte coulometrische Titration, erlaubt die Anwendung der genaueren titrimetrischen Technik auf fließende Lösungen. Bei beiden Methoden werden potentiometrische und voltammetrische Detektoren verwendet. Mehrere Beispiele zeigen die vielfachen Vorteile dieser Verfahren bei der Anwendung auf die Analyse von Umweltmaterial.
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   
973.
The GPC elution behaviour of a polymer was studied when a solution of another polymer in a liquid was used as an eluent. In ternary systems containing two polymers, GPC results are influenced by the thermodynamics of polymer incompatibility. The incompatibility manifests itself both in the slope of the dependence of elution volume upon the concentration of injected polymer and in the shift of elution volumes extrapolated to zero concentration. Experimental data for systems containing poly(styrene) and poly(methyl methacrylate) have been accounted for qualitatively by theoretical views on coil shrinkage resulting from polymer incompatibility. The concentration effects of the injected polymer and the polymer in the mobile phase on elution volume are compared; further parameters affecting the separation in the systems with polymer solution as eluent are discussed.  相似文献   
974.
Kudoh M  Kataoka M  Kambara T 《Talanta》1980,27(6):495-498
The construction of a liquid-membrane type periodate ion-selective electrode and its application to the potentiometric titration of alpha-diols and alpha-amino-alcohols are described. The ion-pair of periodate anion with Capriquat (tri-n-octylmethylammonium chloride), is easily extracted into nitrobenzene, and this extract is employed as a liquid ion-exchange membrane. The calibration curve shows Nernstian response towards periodate ion over the concentration range from 10(-1)M to 10(-7)M with a slope of 60 mV/pIO(-)(4). Selectivity coefficients with respect to various ions were evaluated. The electrode potential was independent of pH in the range 2.5-7.5. Some alpha-diols and monoethanolamine were successfully titrated potentiometrically with the aid of the present electrode.  相似文献   
975.
The synthesis and study of some polyenes, polýiminoimides and Schiff polybases with ferrocene obtained by either polymerization or polycondensation are reported.The following monomers were used: ethynylferrocene, 1-chloro-1′-ethynyl-ferrocene, α-chloro-β-formyl-p-ferrocenylstyrene, p-ferrocenylphenylacetylene, p-ferrocenylacetophenone, 1,1′-diacetylferrocene and 1,1′-bis[β-(2-furyl)acryloyl]ferrocene which were characterized by spectral and thermodifferential analyses and Hückel MO calculations. The polymerization was performed in the presence of benzoyl and lauroyl peroxides, triisopropylboron and complex catalysts of [P(C6H5)3]2 NiX2 type. The ferrocene derivatives were polycondensed with biuret, 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl thioether, 4,4′-diamino-2,2′-dinitrodiphenyl disulphide in the presence of metallic salts and p-toluene sulphonic acid as catalysts.Polymers with either linear or tridimensional structure showing good thermal stability and semiconducting properties have been obtained. Some polymers show catalytical activity in the polymerization of chloroformylated vinylic derivatives.  相似文献   
976.
2-(-Oximinoethyl)-2-methylthiirane was obtained by the reaction of 3,4-dibromo-3-methyl-2-butanone oxime with sodium sulfide. At the same time, the O-carbamoyl derivative of the ,-dibromo oxime was converted to the corresponding substituted ,-unsaturated oxime under the same conditions. Thiirane oxime, like its O-carbamoyl derivative, underwent the desulfuration that is characteristic for thirranes under the influence of tributylphosphine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 751–753, June, 1982.  相似文献   
977.
The construction and analytical applications of an improved liquid-membrane electrode for salicylate are described. Tests of various combinations of symmetrical tetraalkylammonium salicylates and solvents showed that the best liquid ion-exchanger was tetraoctylammonium salicylate in p-nitrocumene. Electrode response is Nernstian down to 2 × 10?5 M. Major interferences are perchlorate and periodate; the working pH range is 6–9. The electrode is useful for direct potentiometric determinations of salicylate in pharmaceutical preparations.  相似文献   
978.
The effect of a range of 10 organic nitriles on the radiation-induced polymerization of styrene was studied. A dose rate of 4.4 rad s?1 was used. A rate of polymerization of styrene (1.744 mol L?1 of toluene solution) of 5.0 × 10?7 mol L?1 s?1 was found. With organic nitriles present (styrene:nitrile ratio of 1:0.28) the rate of polymerization increased. Rates in the range of 5.5 × 10?7 ?5.2 × 10?6 mol L?1 s?1, depending on the nitrile present, were obtained. The polymers were partially characterized and evidence of involvement of each of the nitriles in the polymer chains was revealed. The increase in rate of polymerization has been attributed to the part played by nitrile radicals in the initiation of styrene polymerization. Radical yield values [as G(nitrile radical)] were derived from the relevant rate expressions. Values ranged from 2.7 to 49.5, depending on the particular nitrile. Corresponding values of G(nitrile radical) in the range of 5.1–129.4 were obtained by the manipulation of number-average molar mass data. Values of kpkt of approximately 2 × 10?5 L mol?1 s?1 were found. Trommsdorff types of effect are absent from these systems.  相似文献   
979.
Mechanical treatment of hydrargillite Al(OH)3 in a vibrational grinder and in a ball-mill grinder increases the dispersion, the specific surface area, and the unit-cell constants. An x-ray amorphous phase forms. Differences in the nature and in the concentration of paramagnetic centers arise on irradiation (77 K) of the mechanically treated samples. Anion vacancies and oxygen ions with disrupted short-range order appear in the mechanically treated samples. Their concentration increases with increased time of mechanical treatment. Pulsed thermal treatment of hydrargillite forms the Al2O3-Al(OH)3 system and anneals the mechanically induced defects.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1463–1467, July, 1990.  相似文献   
980.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号