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   3605篇
  免费   64篇
  国内免费   10篇
化学   2517篇
晶体学   22篇
力学   102篇
数学   494篇
物理学   544篇
  2021年   20篇
  2020年   24篇
  2019年   28篇
  2018年   29篇
  2017年   21篇
  2016年   47篇
  2015年   49篇
  2014年   62篇
  2013年   183篇
  2012年   127篇
  2011年   153篇
  2010年   128篇
  2009年   105篇
  2008年   166篇
  2007年   163篇
  2006年   151篇
  2005年   172篇
  2004年   155篇
  2003年   171篇
  2002年   161篇
  2001年   61篇
  2000年   60篇
  1999年   45篇
  1998年   29篇
  1997年   37篇
  1996年   58篇
  1995年   43篇
  1994年   44篇
  1993年   40篇
  1992年   34篇
  1991年   51篇
  1990年   28篇
  1989年   40篇
  1988年   27篇
  1987年   27篇
  1986年   28篇
  1985年   85篇
  1984年   55篇
  1983年   39篇
  1982年   68篇
  1981年   71篇
  1980年   84篇
  1979年   66篇
  1978年   61篇
  1977年   78篇
  1976年   63篇
  1975年   48篇
  1974年   42篇
  1973年   40篇
  1972年   24篇
排序方式: 共有3679条查询结果,搜索用时 0 毫秒
11.
The heat capacities per unit volume and the densities of aqueous solutions of 2-propanol, neopentanol, tert-amylalcohol, 2-amino-2-methylpropanol, triethylamine and diethylmethylamine were measured, in many cases as a function of temperature, over the whole mole fraction or solubility range. Apparent and partial molal heat capacities, volumes and expansibilities were derived. The concentration dependence of these functions suggest the existence of transitions in some of these systems, in the water-rich region, qualitatively similar to micellization. The large relaxation contribution observed with some of the thermodynamic functions of hydrophobic alcohols and amines suggests a reinforcement of hydrophobic hydration due to strong hydrogen-bonding interactions of the polar groups with water.  相似文献   
12.
13.
14.
15.
Quasi-Wannier functions for the ammonia crystal have been determined from optimized ab initio calculations of the heptamer (NH3)7, thus accounting for all the first neighbor interactions. The calculated geometry and binding energy are in good agreement with their experimental counterparts.  相似文献   
16.
A general study of the chemical behavior of heterocyclic anions, dianions and dianionic reagents under phase transfer catalysis conditions allowed us to synthesize various heterocyclic compounds such as imidazo[2,1-b]thiazole and derivatives; imidazo[2,1-b]thiazine and imidazo[2,1-b]benzothiazepine. Reaction conditions e.g., catalyst, solvent, temperature, etc., are indicated.  相似文献   
17.
Hydroxylamine-O-sulfonic acid: a convenient reagent for the conversion of aldehydes into nitriles . Hydroxylamine-O-sulfonic acid (HAS) proved to be an excellent reagent for the conversion of aliphatic, aromatic and heteroatomic aldehydes into nitriles. There, aldehydes in the form of suspension in HAS water solution, react rapidely to give the corresponding nitriles in high yield. Only with formylpyridines could the intermediate oxime-O-sulfonic acids be isolated.  相似文献   
18.
Alkylation of ambident-anions derived from 2- and 4-pyridones yield by phase transfer catalysis, 80% of N-substitution and 20% of O-substitution. Change of salts, solvents, temperature and alkyl halide do not interfere very much with the N- and O-alkylation percentage of substitution. Overall yields range from 40 to 80% with primary or secondary alkyl halides.  相似文献   
19.
Abstract— Results concerning Rose Bengal sensitized photoxidation of 2,7-dimethyl-2,6-octadiene (A) are discussed. This diolefin may be considered as a model molecule of polyisoprene of polymerization number two with 1,4-4,1 addition mode.
All the mono- and dihydroperoxides apt to occur are actually obtained. One of the occurring hydroperoxides, 2,7-dimethyl-dihydroperoxy-3,5-octadiene, which has the structure of a conjugated diene, does not lead to an endoperoxide. This is contrary to Kaplan and Kelleher's hypothesis. On the other hand, no trihydroperoxide forms as a result of further addition of singlet oxygen (1O2).
A complete kinetic study has allowed us to determine the rate constants of all the reactions which take place. The reactivity of (A) is twice that of 2-methyl-2-pentene and the addition of 1O2 to one of the double bonds of (A) induces deactivation of the remaining double bond.  相似文献   
20.
The effects of the pressure drop across the column on retention and efficiency in SFC have been studied. Numerical methods are described which enable the prediction of hold-up time and pressure drop in both packed and open tubular columns. Predictions of both hold-up time and pressure drop are in good agreement with experimental data. The density gradient along the column can be calculated using the numerical methods and a procedure is described which enables the calculation of the overall capacity factors of the solutes from the density profile in the column. Significant variations of the capacity factor are observed along the column. The effect of the density gradient along the column on local diffusivity and dispersion is studied. The column efficiency in systems with significant pressure drops is affected by changes in: the linear velocity of the mobile phase; the diffusion coefficients; and the capacity factors of the solutes along the column. The overall efficiency of the chromatographic system can be calculated if, as is the case for open tubular columns, adequate plate height equations are available.  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