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   50686篇
  免费   2605篇
  国内免费   2591篇
化学   28510篇
晶体学   981篇
力学   2531篇
综合类   218篇
数学   5245篇
物理学   18397篇
  2022年   694篇
  2021年   797篇
  2020年   765篇
  2019年   845篇
  2018年   982篇
  2017年   958篇
  2016年   1372篇
  2015年   1127篇
  2014年   1526篇
  2013年   2644篇
  2012年   2731篇
  2011年   3096篇
  2010年   2271篇
  2009年   2331篇
  2008年   2598篇
  2007年   2499篇
  2006年   2346篇
  2005年   1950篇
  2004年   1826篇
  2003年   1568篇
  2002年   1625篇
  2001年   1948篇
  2000年   1373篇
  1999年   1122篇
  1998年   817篇
  1997年   716篇
  1996年   635篇
  1995年   577篇
  1994年   526篇
  1993年   477篇
  1992年   512篇
  1991年   513篇
  1990年   498篇
  1989年   461篇
  1988年   469篇
  1987年   451篇
  1986年   393篇
  1985年   458篇
  1984年   476篇
  1983年   347篇
  1982年   388篇
  1981年   379篇
  1980年   321篇
  1979年   403篇
  1978年   378篇
  1977年   394篇
  1976年   391篇
  1975年   343篇
  1974年   322篇
  1973年   339篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
M.Z.A. Badr  M.M. Aly  S.S. Salem 《Tetrahedron》1977,33(23):3155-3157
Pyrolysis of α-phenylacetanilide resulted in migration of the benzyl group to the o and p positions of the aniline nucleus and formation of CO, NH3, toluene benzaldehyde, dibenzyl, trans stilbene, aniline, 9 phenylacridine and 2,3 diphenyl-indole. With o-toluidine as a solvent the previous products were accompanied by 2,3 diphenyl-7-methylindole and 4 amino 3 methyl diphenylmethane. With isoqumoline 1 benzyl isoquinoline and 1,1' bi-isoquinolyl were also obtained.It is concluded that the pyrolysis of phenylacetanilide depends on the homolytic fission of the amide C-N bond into anilino and phenylacetyl free radicals followed by the interaction of the primary and secondary formed radicals with the rearrangement products and solvent nuclei.  相似文献   
122.
A mixture of isomers corresponding to closing of the ring at the 3 and 4 positions of the thiophene ring is formed during intramolecular acylation of 12-(5-methyl-2-thienyl)-lauryl chloride in a homogeneous medium; the reaction proceeds primarily to favor the formation of the isomer with the larger ring (80%). A method for the synthesis of macrocyclic -alkylcycloalkanones by reductive desulfurization of the corresponding bicyclic ketones, which contain thiophene rings, was developed.See Izv. Akad. Nauk SSSR, Ser. Khim., 2228 (1970) for Communication XI.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1467–1472, November, 1970.  相似文献   
123.
124.
Effect of substitution of Na+ by pyridinium on the surface and pore structures of montmorillonite is studied by CCl4 vapor adsorption.  相似文献   
125.
Thermotropic polyesters based on 4,4-alkane-1-ω-diylbis(4-hydroxybenzoic acid) and 4,4-(pentane-1,5-diyloxy)dibenzoic acid were studied by dynamic mechanical spectroscopy, X-ray scattering, differential scanning calorimetry and polarizing microscopy. The effect of structure changes in the mesogenic group as well as in the flexible spacer, in particular the incorporation of Cl atoms into the mesogen, introduction of ether oxygen into spacer and the number of CH2 groups in spacer was investigated. More complex mechanical and thermal behaviour was found on second heating scan than on first cooling of the isotropic melt; higher values of mechanical functions were observed in the isotropic state than in the nematic state of melts. While an even number of CH2 groups and the presence of ether oxygen in spacer shifts the transition temperatures to higher values, the incorporation of Cl atoms into the mesogen leads to disappearance of ordered structure and the polymers behave as amorphous materials.  相似文献   
126.
The migration of 99Tc in a weak loess aquifer was investigated in-situ with undisturbed aquifer medium columns. The columns were obtained horizontally at a depth of 3236 m in an Underground Research Facility (URF). Quartz containing 3H (HTO) and 99Tc (in the form of 99TcO4 -) was introduced into one end of the columns and the columns were covered tightly. Aquifer water was introduced into the columns directly from an experimental shaft in the UFR. Effluents from the columns were collected and the activity of 3H and 99Tc were determined with a liquid scintillation analyzer. The breakthrough curves of 3H and 99Tc indicate that 99Tc migrates a little faster than that 3H does in the aquifer.  相似文献   
127.
The thermal behaviour of tetrahydroperparine (THP) and its derivatives with various acids has been investigated. The acid is liberated from the formate derivative basically in two steps, while the other aliphatic acids are released quantitatively before the thermal decomposition of the THP molecule. The thermoanalytical curves, electrical conductivity data measured in the molten phase, and infrared spectra prove that while part of the formic acid is hydrogen-bonded the other part is bound ionically in the molecule. It may be assumed that the marked biological activity of the compound can be explained by this difference in the nature of the bonds.  相似文献   
128.
A new method for heat exchange between dispersed flows and a unit for continuous selective heating, comprising a heater and a mass-exchange apparatus, was proposed. A mathematical model of the unit was developed for the cases of direct-flow and counterflow heaters. Analytical solutions to the model equations were obtained. The parameters affecting the result of continuous selective heating of the dispersion medium in the unit were determined.  相似文献   
129.
Evaluation has been carried out of the component of the second virial coefficient of steam related to the formation of dimers. The necessary equilibrium constants of dimerization have been evaluated within the MCY-B water pair interaction potential. The calculated isotopic difference in the second virial coefficient reflects the qualitative features found by observation but, nevertheless, the calculation quite distinctly overestimates the value of this isotopic difference as compared with the observation. The nature of these differences and the relation to the state behaviour of water are discussed.  相似文献   
130.
The acetylation of lagochilin and its derivatives with acetic anhydride in the presence of pyridine has been studied by PMR spectroscopy. It has been shown that the manifestation of the polyfunctionality of lagochilin in acetylation reactions with acetic anhydride is connected with a change in the nature of the process of solvation of the reaction forms with the subsequent replacement of the hydrogen atoms of the hydroxy groups by acetyl groups.V. I. Lenin Tashkent State University. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 46–49, January–February, 1980.  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