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   1304篇
  免费   18篇
  国内免费   4篇
化学   915篇
晶体学   11篇
力学   19篇
数学   98篇
物理学   283篇
  2022年   5篇
  2021年   16篇
  2020年   12篇
  2019年   15篇
  2018年   13篇
  2017年   9篇
  2016年   23篇
  2015年   27篇
  2014年   22篇
  2013年   49篇
  2012年   64篇
  2011年   65篇
  2010年   46篇
  2009年   29篇
  2008年   97篇
  2007年   83篇
  2006年   87篇
  2005年   89篇
  2004年   84篇
  2003年   48篇
  2002年   58篇
  2001年   26篇
  2000年   17篇
  1999年   17篇
  1998年   14篇
  1997年   15篇
  1996年   15篇
  1995年   17篇
  1994年   21篇
  1993年   10篇
  1992年   8篇
  1991年   14篇
  1990年   12篇
  1989年   9篇
  1988年   8篇
  1987年   15篇
  1986年   10篇
  1985年   12篇
  1984年   13篇
  1983年   9篇
  1982年   13篇
  1981年   15篇
  1980年   20篇
  1979年   9篇
  1978年   10篇
  1977年   5篇
  1976年   13篇
  1975年   5篇
  1974年   10篇
  1972年   7篇
排序方式: 共有1326条查询结果,搜索用时 15 毫秒
41.
Phosphine oxides bearing two or three 2-pyridyl groups react with organometallic reagents affording 2,2′-bipyridyl, 2-substituted pyridines and pyridine in good yields. Phosphonium salts and phosphine oxides, bearing at least two 2-pyridyl or substituted 2-pyridyl groups also give the corresponding 2,2′-bipyridyls and pyridines upon treatment with acid or neutral solvents such as water and alcohols in substantial yields. The 2,2′-bipyridyls are considered to be formed by ligand coupling within the pentacoordinated phosphorus intermediate formed incipiently during the reaction.  相似文献   
42.
With the progress of LSI technology, the electronic device size is scaled down to the sub 0.1μ m region. In such an ultrasmall device, it is indispensable to take quantum mechanical effects into account in device modeling. In this paper, we present a newly developed quantum Monte Carlo device simulation applicable to ultrasmall semiconductor devices. In this model, the quantum effects are represented in terms of quantum mechanically corrected potential in the classical Boltzmann equation. It is demonstrated that the quantum transport effects such as tunneling and energy quantization in ultrasmall semiconductor devices are obtained for the first time by using the standard Monte Carlo techniques.  相似文献   
43.
Using a complex energy density functional derived from the Reid soft-core potential, the changes in nuclear shapes of two colliding208Pb are investigated. At each separation distanceR, the total binding energy of the Pb + Pb system is minimized w.r.t. the quadrupole deformation and diffuseness parameters of nuclear densities. It is found that the nuclear shapes are strongly affected by the nuclear and Coulomb interactions. A sudden transition occurs from oblate to prolate shapes around R16.8fm and a smooth one from prolate to oblate shapes aroundR?11.76 fm.  相似文献   
44.
Lolitrems are tremorgenic indole diterpenes that exhibit a unique 5/6 bicyclic system of the indole moiety. Although genetic analysis has indicated that the prenyltransferase LtmE and the cytochrome P450 LtmJ are involved in the construction of this unique structure, the detailed mechanism remains to be elucidated. Herein, we report the reconstitution of the biosynthetic pathway for lolitrems employing a recently established genome-editing technique for the expression host Aspergillus oryzae. Heterologous expression and bioconversion of the various intermediates revealed that LtmJ catalyzes multistep oxidation to furnish the lolitrem core. We also isolated the key reaction intermediate with an epoxyalcohol moiety. This observation allowed us to establish the mechanism of radical-induced cyclization, which was firmly supported by density functional theory calculations and a model experiment with a synthetic analogue.  相似文献   
45.
Novel birefringent single-mode fibers are fabricated with a vacant hole on either side of the central elliptical core, referred to as side-hole fiber, and their intrinsic and tension-induced modal birefringences are measured to know their characteristic properties for the purpose of making a strain sensor. Five sampled side-hole fibers are prepared for measurements. The sensitivity of the strain sensor is primarily determined by two parameters, tension-induced and intrinsic modal birefringences. Tension-induced modal birefringence is controllable by optimal arrangement of the size and location of the side-holes in the cross-section of the fiber. A sensitivity diagram for designating and fabricating side-hole fibers is presented.  相似文献   
46.
47.
In a recent development we proposed a quantum chemical approach to compute free energy change for chemical reactions in condensed phases by combining the QM/MM method with the theory of energy representation (QM/MM-ER). We extend in this Letter the novel approach to compute reduction free energy of isoalloxazine ring of FAD (flavin adenine dinucleotide) immersed in water within the framework of the QM/MM-ER method. The characteristic feature of our approach is that the excess electron to be attached on the FAD is identified as a solute. The reduction free energy has been obtained as −80.1 kcal/mol in the aqueous solution.  相似文献   
48.
49.
Divalent metal salts (I) of monohydroxyethyl phthalate have catalytic activity for polymerization of isocyanate groups. The synthesis of novel metal-containing isocyanurate-type crosslinked polyurethanes were investigated by the crosslinking reaction of I in DMF with diisocyanates or urethane prepolymers, that is, diisocyanate adducts of polyethylene glycol. 2,4-Toluene diisocyanate and diphenylmethane diisocyanate were used as diisocyanates, and DMF was the solvent. The crosslinking reactions were followed by determining the gelation time. Gelation time decreased with increasing temperature. The Ca salt was more effective than the Mg salt in the crosslinking reaction. The yields of resulting metal-containing three-dimensional polymers decreased with increasing metal content in the feed. The Ca salt was more easily introduced into the network polymers than the Mg salt was. Decomposition temperatures decreased on introducing metals into the network polymers. Some physical properties of the metal-containing network polymers are also discussed.  相似文献   
50.
A new indole alkaloid with the canthin-6-one skeleton, ishigadine A, has been isolated from an Okinawan marine sponge Hyrtios sp. The structure of ishigadine A was elucidated on the basis of spectroscopic analyses. Ishigadine A is a new canthin-6-one alkaloid possessing a hydroxy group, a 1,3-dimethyl-4-methylthioimidazolium, and a 1-propylguanidine. Ishigadine A is the third canthin-6-one alkaloid from sponges. Ishigadine A might be generated from l-arginine, l-histidine, and l-tryptophan. Ishigadine A exhibited moderate cytotoxicity against L1210 murine leukemia cells.  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