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   223747篇
  免费   2656篇
  国内免费   620篇
化学   125152篇
晶体学   3866篇
力学   8606篇
综合类   11篇
数学   21252篇
物理学   68136篇
  2021年   1994篇
  2020年   2129篇
  2019年   2337篇
  2018年   2598篇
  2017年   2698篇
  2016年   4150篇
  2015年   2741篇
  2014年   4333篇
  2013年   10351篇
  2012年   8094篇
  2011年   9804篇
  2010年   6816篇
  2009年   6628篇
  2008年   8648篇
  2007年   8334篇
  2006年   7984篇
  2005年   7140篇
  2004年   6473篇
  2003年   5895篇
  2002年   5625篇
  2001年   6526篇
  2000年   4995篇
  1999年   3788篇
  1998年   2924篇
  1997年   2874篇
  1996年   2837篇
  1995年   2602篇
  1994年   2511篇
  1993年   2352篇
  1992年   2967篇
  1991年   2840篇
  1990年   2784篇
  1989年   2778篇
  1988年   2771篇
  1987年   2782篇
  1986年   2637篇
  1985年   3367篇
  1984年   3350篇
  1983年   2630篇
  1982年   2755篇
  1981年   2815篇
  1980年   2561篇
  1979年   2909篇
  1978年   2891篇
  1977年   3004篇
  1976年   2850篇
  1975年   2584篇
  1974年   2534篇
  1973年   2493篇
  1972年   1719篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
New environmental applications of radiation technology developed in the laboratory of the author are considered. They include the electron-beam and coagulation purification of molasses slops from a distillery that produces ethanol by fermentation of plant raw materials, the removal of petroleum products (diesel fuel, motor oil, and residual fuel oil) from water by -irradiation, and the electron-beam treatment of multicomponent natural gas mixtures of lower hydrocarbons for the purpose of condensation.  相似文献   
992.
Complexes of CuHg(NCS)4, CuHg(NCS)2 (NCSe)2 and CuHg(NCSe)4 with tetrahydrofuran, dioxane, pyridine, 2-aminopyridine, nicotinamide, bipyridine and phenanthroline have been prepared and comparative studies made. Bipyridine and phenanthroline form cationic—anionic [CuL3]2+ [Hg(SCN)4]2? (L = bipy, phen) complexes with CuHg(NCS)4 and dinuclear bridged complexes with CuHg(NCSe)4 and CuHg(NCS)2 (NCSe)2. For other ligands the nature of the complexes is binuclear or polynuclear. The comparative stability of the -XCN- bridge (X = S, Se) is CuHg(NCSe)4 > CuHg(NCS)2 (NCSe)2 > CuHg(NCS)4.  相似文献   
993.
The present work studies the changes in polymer structure and the mechanism of the decomposition of polytetrafluoroethylene resin (PTFE) exposed to high energy radiation (electron beam). Spectroscopic and kinetic observations are used to interpret the degradation process. For the first time the decomposition of PTFE has been carried out on a preparative scale and new results obtained by analysing the degradation products. The radiation-induced degradation of PTFE is accompanied by thermal degradation under certain irradiation conditions. This is due to an increase in temperature of the polymer caused by retardation of highly accelerated electrons (heat accumulation effect).The kinetics are discussed in terms of the reactions and recombination of radicals produced by high-energy radiation both in the polymer melt and the polymer surface. These are related to the overall rate of decomposition.The primary radicals formed by decomposition of PTFE in an inert atmosphere (N2, Ar) react to produce perfluorinated alkanes and alkenes. In the presence of reactive gases the decomposition fragments originated will react rapidly; e.g. if oxygen is present in the reactive area the radicals form perflourinated peroxyl and oxyl radicals which finally stabilize themselves by CC-scission to perfluorocarbon acid fluorides and carbonyldifluorides.  相似文献   
994.
Summary The number average micellar molecular weights,M n , were determined with the application of membrane osmometry, for various nonionic and a cationic surfactant, in aqueous solutions. These results show that these micelles are monodisperse, as also reported by other investigators. The dependence ofM n on temperature was also studied. It was found that both the nonionic and the cationic micelles showed an abrupt change in logM versus temperature plots. In the case of nonionics, the abrupt change was observed at approximately 30 °C lower than the cloud-point.Presented at the 25th Colloid-Meeting in Munich, October 13–15, 1971.  相似文献   
995.
delta1-Tetrahydrocannabinol, delta6-tetrahydrocannabinol, cannabinol, cannabidiol and several of their monooxygenated derivatives have been separated from each other by a combination of liquid, thin-layer and gas chromatography. Retention volumes (on Sephadex LH-20), Rf values and retention times can be recorded, and may provide guidance in the separation and identification of these known cannabinoids.  相似文献   
996.
The solid + liquid phase diagram, drawn from thermal and dielectric measurements, is presented. It exibits a miscibility gap at 0.08 ≤ χ2 ≤ 0.58, and two eutectics. The occurrence of a (likely 1:1) intermediate compound, which decomposes on melting into two immiscible liquids, is postulated. Mixed crystals are observed in the sulfolane richer region.Deviations of the liquid mixtures from ideality, positive in the case of viscosity and negative in the case of dielectric constants, are evidenced.  相似文献   
997.
998.
The formation (77K) and transformation (300K) of radicals in celluloses with various degrees of crystallinity under -radiation have been studied by the ESR method. The radical products of the low-temperature -radiolysis of cellulose formed in the polymer chain are macroradicals the unshared electrons of which are localized on C(1) C(4), O(O), and C(5). With a rise in the temperature to 300K they undergo substantial changes, which shows the dominating role of secondary reactions of macroradicals in the radiation-chemical transformation of cellulose.A. S. Sadykov Institute of Organic Chemistry, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 62 70 71. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 587–595, July–August, 1996. Original article submitted February 26, 1996.  相似文献   
999.
High-performance liquid chromatography (HPLC) coupled to mass spectrometry (MS) is used to detect 2,4-dinitrophenyl (DNP) derivatives of amines by means of negative chemical ionisation at atmospheric pressure. The high sensitivity and good comparability of UV- and MS-detection of DNP-derivatives of amines is shown by several examples. The high selectivity of the derivatisation and the detection method (UV and MS) is used for the analysis of unknown amines in aqueous phases after hydrolytic degradation of polyamide-amine- or polyamine-epoxide-adducts as well as for the characterisation of commercial products.  相似文献   
1000.
As the application of a dielectric theory proposed previously (J Membrane Sci 64:153–161 (1991)), theoretical formulation and the practical procedure of dielectric analysis are developed to calculate the structural parameters such as the conductivity gradient and the thickness of the concentration polarization layer, the capacitances and the conductances of the two adjoining aqueous phases from the observed dielectric parameters. The procedure of calculation consequent upon the theoretical formulation was applied to double relaxation data observed for cation-exchange membrane systems under application of d.c. bias voltage. As a consequence, the structural parameters of concentration polarization were readily obtained with accuracy.  相似文献   
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