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   221737篇
  免费   2780篇
  国内免费   561篇
化学   121917篇
晶体学   3195篇
力学   8453篇
综合类   5篇
数学   23750篇
物理学   67758篇
  2021年   1660篇
  2020年   2006篇
  2019年   1907篇
  2018年   2517篇
  2017年   2490篇
  2016年   4169篇
  2015年   2766篇
  2014年   3958篇
  2013年   9438篇
  2012年   7702篇
  2011年   9464篇
  2010年   6398篇
  2009年   6274篇
  2008年   8913篇
  2007年   8608篇
  2006年   8174篇
  2005年   7392篇
  2004年   6534篇
  2003年   5904篇
  2002年   5756篇
  2001年   7421篇
  2000年   5509篇
  1999年   4306篇
  1998年   3108篇
  1997年   3180篇
  1996年   3129篇
  1995年   2849篇
  1994年   2879篇
  1993年   2717篇
  1992年   3199篇
  1991年   3087篇
  1990年   2994篇
  1989年   2865篇
  1988年   2901篇
  1987年   2900篇
  1986年   2635篇
  1985年   3297篇
  1984年   3361篇
  1983年   2681篇
  1982年   2768篇
  1981年   2599篇
  1980年   2516篇
  1979年   2820篇
  1978年   2902篇
  1977年   2833篇
  1976年   2701篇
  1975年   2529篇
  1974年   2500篇
  1973年   2553篇
  1972年   1675篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
61.
Images of the surface ordering of 4-n-octyl-4'-cyanobiphenyl, a room temperature smectic liquid crystal deposited upon graphite, have been obtained by scanning tunnelling microscopy. The microscope was operated in air using the constant-current mode. Under certain tunnelling conditions it has been possible to resolve both the aliphatic and aromatic parts of the molecule, and to observe individual benzene rings. Two previously unreported conformations have been observed: an overlapping bilayer structure with spacing 3.7 nm, and a monolayer structure with spacing 2.4 nm. The latter structure may represent the first visual evidence for a surface polar ordered structure.  相似文献   
62.
M. V. Lomonosov Moscow State University. Translated from Funktsional'nyi Analiz i Ego Prilozheniya, Vol. 25, No. 1, pp. 88–91, January–March, 1991.  相似文献   
63.
The mechanism of ozone formation has been studied using 16O and 18O2. High-resolution microwave spectroscopy was used to measure the amounts of the isotopomeric ozone species formed. The study is hampered by the very rapid exchange process between the reactants, that tends to scramble the isotopes and hence give a 2:1 statistical ratio between the two possible isotopomers. We have found a strategy to come around this difficulty and conclude that the mechanism is a simple end-on-addition.  相似文献   
64.
Rate constants have been measured by pulse radiolysis for the reactions of the carbonate radical, CO3·?, with a number of organic and inorganic reactants as a function of temperature, generally over the range 5 to 80°C. The reactants include the substitution-inert cyano complexes of FeII, MoIV, and WIV, the simple inorganic anions SO32?, ClO2?, NO2?, I?, and SCN?, several phenolates, ascorbate, tryptophan, cysteine, cystine, methionine, triethylamine, and allyl alcohol. The measured rate constants ranged from less than 105 to 3 × 109 M?1 s?1, the activation energies ranged from ?11.4 to 18.8 kJ mol?1, and the pre-exponential factors ranged from log A = 6.4 to 10.7. The activation energies for the metal complexes and inorganic anions generally decrease with increasing driving force for the reaction, as expected for an outer sphere electron transfer. For highly exothermic reactions, however, the activation energy appears to increase, probably reflecting the temperature dependence of diffusion. For many of the organic reactants, the activation energies were low and independent of driving force, suggesting that the oxidation is via an inner sphere mechanism.  相似文献   
65.
66.
By applying stepwise phase modulation of recoilless gamma radiation in a coincidence experiment, constructive interference is produced in transmission geometry between the source and the absorber fields. The resulting regenerated decay signal is called a gamma echo. Here it is demonstrated that during the decay of the 14.4 keV state of57Fe multiple echo signals can be generated.  相似文献   
67.
The effect of hormones on progesterone secretion by 6-8 week human trophoblast tissue cultured in serum-free medium has been investigated. GnRH at low concentration (10(-10)-10(-8) mol/L) stimulated progesterone secretion, while high dose (10(-6)-10(-5) mol/L) produced inhibitory effect. The progesterone secretion could be significantly decreased by addition of anti-hCG antiserum or monoclonal anti-hCG IgG in a dose- and time-dependent manner. Various concentrations of TRH, PGE2, PGF2 alpha, testosterone and estradiol were found to be ineffective. These data indicate clearly that progesterone production by human trophoblast tissue at early gestation stage is under the modulation of GnRH and hCG.  相似文献   
68.
Various approaches to the preparation and verification of single-molecule single crystals are discussed for polyethylene and poly (oxyethylene). Analytic tools are electron microscopy, electron diffraction, and differential scanning calorimetry. The main difficulty in producing a single-molecule single crystal is to keep crystals from joining during growth.  相似文献   
69.
We develop a lattice-dynamical model based on the screw symmetry of single-walled carbon nanotubes that allows for reducing the size of the dynamical matrix to six, for all tube chiralities. The model uses force constants derived from fitting to the phonon dispersion of 2D graphite. We present the calculation procedure in a clear and transparent way, making the model easier to follow. We calculate the phonon dispersions of a number of nanotubes of different chiralities. The splitting of two highest Raman active modes and the radial breathing mode frequency are studied by changing the tube diameter and chirality.  相似文献   
70.
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