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   697篇
  免费   1篇
化学   392篇
晶体学   53篇
力学   12篇
数学   13篇
物理学   228篇
  2022年   7篇
  2021年   13篇
  2020年   13篇
  2019年   17篇
  2018年   18篇
  2017年   20篇
  2016年   18篇
  2015年   11篇
  2014年   17篇
  2013年   29篇
  2012年   19篇
  2011年   36篇
  2010年   29篇
  2009年   42篇
  2008年   23篇
  2007年   20篇
  2006年   21篇
  2005年   19篇
  2004年   27篇
  2003年   16篇
  2002年   15篇
  2001年   12篇
  2000年   10篇
  1999年   7篇
  1998年   12篇
  1997年   13篇
  1996年   20篇
  1995年   10篇
  1994年   18篇
  1993年   7篇
  1991年   5篇
  1990年   8篇
  1989年   9篇
  1988年   6篇
  1987年   7篇
  1986年   6篇
  1983年   4篇
  1979年   8篇
  1978年   6篇
  1977年   5篇
  1976年   8篇
  1975年   5篇
  1974年   9篇
  1973年   5篇
  1972年   8篇
  1971年   8篇
  1970年   10篇
  1969年   7篇
  1968年   8篇
  1967年   6篇
排序方式: 共有698条查询结果,搜索用时 31 毫秒
551.
Reactions of symmetric and unsymmetrical pyrilo-2-cyanines containing polyfluorophenyl substituents with methylamine give rise to the corresponding pyridocyanines, 1,3-cyclohexadiene derivatives, or 1-methyl-2-pyridone. The type of the reaction product depends on the structure of the initial dye and the reaction conditions.  相似文献   
552.
The reaction of ethylene and 1-hexene with butanal was studied using tertiary butyl peroxide initiation at 140°C and a hydrostatic pressure in the range of 250–450 kg/cm2. Based on calculated values of the rate constant ratios for the addition reaction of the n-C3H7O radical to ethylene (k0 e) and 1-hexene (k0 h), we obtained k0 e/k0 h=3.7 as the rate constant of the decarbonylation reaction of this radical (kd). The partial chain transfer constants were calculated for the ethylene-butanal system as Cn: C1=1.0±0.16; C2=1.0±0.14; C3=0.9±0.23. The results obtained indicate the nucleophilic character of the n-C3H7O radical in these reactions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1356–1360, June, 1990.  相似文献   
553.
Babaev  E. V.  Rybakov  V. B.  Orlova  I. A.  Bush  A. A.  Maerle  K. V.  Nasonov  A. F. 《Russian Chemical Bulletin》2004,53(1):176-180
A procedure was developed for the synthesis of representatives of the previously unknown bicyclic mesoionic thiazolo[3,2-a]pyridinium 2-thiolate system by the reaction of 2-X-N-phenacylpyridinium salts (X = Cl, SMe) with CS2 in the presence of Et3N. The three-dimensional structure of 3-(p-nitrobenzoyl)thiazolo[3,2-a]pyridinium 2-thiolate was established by X-ray diffraction analysis.  相似文献   
554.
Heats and constants of 15-crown-5 (R) complexation with silver and lead ions at 298, 308, and 318 K are determined using direct calorimetry. Thermodynamic parameters (G°, H°, S°, and C p°) of formation of AgR+ and PbR2+ complexes at various temperatures are calculated. The effects of the properties of the cations and crown ethers on the thermodynamic parameters of complexation are discussed.  相似文献   
555.
The reactions of carbon monoxide with the palladium nitrosoarene complexes Pd2(-OCOR)2(—CH2C6H4NO)2 (1, R = Me, CF3, But, or Ph) and Pd2(-OCOR)2(PhNC6H4NO)2 (2, R = Me, CF3, But, or Ph) were studied. Complexes 1 contain the o-nitrosotoluene molecule metallated at the methyl group. In complexes 2, the phenyl-o-nitrosophenylamide ligand coordinated via two nitrogen atoms can be considered as a nitrosobenzene derivative bearing the NPh group in the ortho position of the Ph ring. It all cases, carbonylation of the complexes afforded the corresponding aryl isocyanates Ar—N=C=O or the products of their further transformations. The mechanism of reductive carbonylation of nitroarenes catalyzed by palladium compounds and the role of palladium nitrosoarene complexes as possible intermediates in this process are discussed.  相似文献   
556.
A preparative method is proposed for the synthesis of 2,3,4,5-tetrahydro-1,5-benzothiazepine by reductive ring expansion of 4-chromanone and 4-thiochromanone oximes. These compounds were converted to the corresponding N-amino derivatives, which, like arylhydrazines in the Fischer reaction, give new condensed indole systems. The reduction of the oxime (and its tosylate) of 5-oxo-2,3,4,5-tetrahydro-1-benzoxepine was investigated.  相似文献   
557.
The action of methanol on methyl 4, 4-dichlorochromene-2-carboxylate (A) gives a 2-substitution product I, readily hydrolyzed in acid solution to a 2-hydroxy derivative C, which undergoes an allylic rearrangement to an ester of chromane-2-carboxylic acid B. Hydrogenation of compound I gives 2-methoxy-2-carbomethoxychromane (II), which is caused to undergo some chemical reactions.For Part XXIII see [4]  相似文献   
558.
The potential of AAS-ETA, ICP-AES and ICP-MS methods in direct mode or with matrix separation are investigated for the analysis of high purity indium. Optimal conditions for the indium separation as difficult soluble oxychloride are established. Analytical autoclaves which allow to perform both digestion and separation procedures simultaneously are used for this purpose. Using the preconcentration method suggested the detection limit of a number of elements was reduced by a factor of 5-100 comparing to the direct determinations.  相似文献   
559.
This paper reports on a study of the effect of doping with Na2CO3, NaCl, and KClO3 salts on the microstructure and superconducting characteristics of ceramics with nominal compositions Dy1 ? x M x Ba2Cu3 ? y O7 ? δ and DyBa2Cu3 ? y O7 ? δ : M x for M = Na, K; x = 0.2, 0.3, and y = 0, 0.2. The microstructure was characterized by transmission electron microscopy with local energy-dispersive x-ray in situ analysis (probe size ~1 nm). An analysis shows that none of the doping elements (Na, K, or Cl) enters into 123 grains in sizable amounts and that, as a result, the critical temperature of the superconducting transition remains practically constant in the range 90.0–93.5 K. Potassium and chlorine segregate at grain boundaries. It is shown that grain-boundary segregation of chlorine leads to a substantial increase in the superconducting critical current (by a factor 3–5 at 70 K) as compared to the undoped sample. The possible mechanisms accounting for the effect of Cl on intergrain critical current are discussed.  相似文献   
560.
The results are presented that were obtained by measuring the differential cross sections for the reaction 12C(d,d) 12C occurring at E d = 15.3 MeV and leading to the production of a 12C nucleus in the ground and the first excited state. The energy dependences of the differential reaction cross sections were measured for three angles of deuteron emission in the range of projectile-deuteron energies E d between 12 and 15.3 MeV. The double-differential cross sections for the reaction in question were measured for the 2+ state of the 12C nucleus at 4.44 MeV, and the angular dependences of the even spin-tensor components of the density matrix were determined, along with the angular dependences of the populations of magnetic sublevels and the components of the tensors of multipole-moment orientation. These experimental results are compared with their theoretical counterparts obtained under the assumption of various reaction mechanisms, including collective interaction, heavy-particle stripping, a two-step mechanism that takes into account the delay in the interaction, and the mechanism of compound-nucleus formation.  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