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   1053篇
  免费   13篇
化学   774篇
晶体学   10篇
力学   7篇
数学   127篇
物理学   148篇
  2021年   7篇
  2019年   6篇
  2016年   14篇
  2015年   11篇
  2014年   12篇
  2013年   49篇
  2012年   36篇
  2011年   32篇
  2010年   31篇
  2009年   30篇
  2008年   59篇
  2007年   57篇
  2006年   47篇
  2005年   61篇
  2004年   38篇
  2003年   48篇
  2002年   64篇
  2001年   12篇
  2000年   13篇
  1999年   12篇
  1998年   13篇
  1997年   19篇
  1996年   15篇
  1995年   8篇
  1994年   14篇
  1993年   5篇
  1992年   17篇
  1991年   6篇
  1990年   7篇
  1989年   7篇
  1988年   10篇
  1987年   16篇
  1986年   12篇
  1985年   19篇
  1984年   30篇
  1983年   11篇
  1982年   31篇
  1981年   27篇
  1980年   20篇
  1979年   15篇
  1978年   12篇
  1977年   14篇
  1976年   16篇
  1975年   21篇
  1974年   8篇
  1973年   7篇
  1972年   5篇
  1971年   6篇
  1968年   4篇
  1966年   5篇
排序方式: 共有1066条查询结果,搜索用时 531 毫秒
131.
We investigate the effect of incomplete information in a model where a start-up with a unique idea and technology pioneers a new market but will eventually be expelled from the market by a large firm’s subsequent entry. We evaluate the start-up’s loss due to incomplete information about the large firm’s behavior. We clarify conditions under which the start-up needs more information about the large firm. The proposed method of evaluating the loss due to incomplete information could also be applied to other real options models involving incomplete information.  相似文献   
132.
Preface     
  相似文献   
133.
An interband π plasmon and π* edge have been observed in the spectrum of hexagonal boron nitride (h-BN). This result is in good agreement with that  相似文献   
134.
135.
L-Threonine was transformed, stereospecifically, to a versatile β-lactam (5a) in 3 steps. This β-lactam was further converted to a key intermediate (25) for the synthesis of thienamycin and its biologically active analogues. Furthermore, the compound 5a was changed to iodides (18 and 23), cyanides (19 and 24), chloromethylketone (26) and aldehydes (30 and 31) which appear to have a latent potential as precursors for the syntheses of the carbapenems.  相似文献   
136.
New general methods for the synthesis of alkynyl(phenyl)iodonium tetrafluoroborate 2 from alkynyltrimethylsilanes 1 utilizing the combination of iodosylbenzene and triethyloxonium tetrafluoroborate or boron trifluoride etherate were developed. The medium effect observed in the reaction was also discussed.  相似文献   
137.
N-Substituted 2-alkylthio-1,4-dihydro-4,4,6-trimethylpyrimidines easily reacted with methanol to give corresponding 2-methoxy-1,4-dihydropyrimidines, while the other nucleophiles did not react on the C-2 carbon of the pyrimidine rings. However, the substitution of alkylthio group easily proceeded intramolecularly even with weak nucleophilic groups, e.g., phenolic oxygen, thiophenolic sulfur, and anilino nitrogen atom, to yield bi- or tricyclic pyrimidine derivatives.  相似文献   
138.
Adsorption of CO on evaporated Ag and Cu films was studied by ellipsometry and resistivity measurements. Changes in ellipsometric angles δΔ and δψ due to adsorption of CO were analyzed by an improved linear approximation of the stratified layer model of adsorption. Adsorption of CO on Cu induced a 23% increase in the relative resistivity change (δR/R) which was proportional to δψ, while adsorption of CO on Ag induced a 1% increase without the proportionality. The dielectric constant of CO adsorbed on Ag is ?1 = 2.2?0.7i at λ = 1152 nm in comparison to ?1 = 2.1 for gas phase CO; that of CO on Cu is ?1 = ?4.8 ? 8.5i, which is consistent with the prediction by the Bennett and Penn theory. The large difference in \?ge1, of adsorbed CO on Ag and Cu is understood in terms of the energy level of the 2 π molecular orbital of adsorbed CO relative to the Fermi level. A possibility of adlayer plasmon excitation is discussed.  相似文献   
139.
The i.r. absorption intensities have been measured of the fundamental vibrations of water molecules in metal halide aqueous solutions. Pseudo-isosbestic points were observed in the regions of both the stretching and bending vibrations and interpreted in terms of two states of water molecules in the solutions. The intensities of the stretching vibrations of water molecules coordinated to the dissolved ions have been estimated. The intensities become stronger in the order, Li+ > Na+ > K+ for the cations and F- > Cl- > Br- > I- for the anions. These orders are consistent with the strengths of the ion-water interactions, which are related to the dynamical behavior of the water molecules in the primary hydration shell.  相似文献   
140.
Masao Doi 《Chemical physics》1975,11(1):115-121
A simplified version of the closure approximation, presented in the preceding paper, is applied to the reaction of rigid or flexible macromolecular systems. Two effects are found to be important for understanding the characteristic features of the reaction rates of these systems: (i) the orientation effect, which expresses the accessibility of the active sites, and (ii) the relaxation effect, which reflects the nature of the brownian motion of the active sites.  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