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   401篇
  免费   7篇
化学   288篇
晶体学   4篇
力学   1篇
数学   22篇
物理学   93篇
  2020年   4篇
  2018年   3篇
  2016年   3篇
  2015年   3篇
  2014年   3篇
  2013年   22篇
  2012年   13篇
  2011年   23篇
  2010年   3篇
  2009年   6篇
  2008年   17篇
  2007年   27篇
  2006年   23篇
  2005年   28篇
  2004年   24篇
  2003年   27篇
  2002年   30篇
  2001年   13篇
  2000年   8篇
  1998年   4篇
  1997年   5篇
  1996年   5篇
  1995年   3篇
  1994年   2篇
  1993年   4篇
  1992年   6篇
  1991年   5篇
  1990年   6篇
  1989年   5篇
  1988年   3篇
  1987年   4篇
  1986年   4篇
  1985年   2篇
  1984年   5篇
  1983年   3篇
  1982年   2篇
  1981年   6篇
  1980年   3篇
  1979年   3篇
  1978年   5篇
  1977年   3篇
  1976年   7篇
  1975年   4篇
  1974年   5篇
  1973年   3篇
  1972年   4篇
  1938年   2篇
  1912年   2篇
  1911年   2篇
  1909年   2篇
排序方式: 共有408条查询结果,搜索用时 46 毫秒
51.
52.
Starting from readily available (S)‐styrene oxide an asymmetric synthesis is described of the naturally occurring anti‐HIV spirolactone (?)‐hyperolactone C, which possesses adjacent fully substituted stereocenters. The key step involves a stereocontrolled RhII‐catalysed oxonium ylide formation–[2,3] sigmatropic rearrangement of an α‐diazo‐β‐ketoester bearing allylic ether functionality. From the resulting furanone, an acid‐catalysed lactonisation and dehydrogenation gives the natural product.  相似文献   
53.
S K-edge X-ray absorption spectroscopy on the resting oxidized and the S-adenosyl-l-methionine (SAM) bound forms of pyruvate formate-lyase activating enzyme are reported. The data show an increase in pre-edge intensity, which is due to additional contributions from sulfide and thiolate of the Fe(4)S(4) cluster into the C-S σ* orbital. This experimentally demonstrates that there is a backbonding interaction between the Fe(4)S(4) cluster and C-S σ* orbitals of SAM in this inner sphere complex. DFT calculations that reproduce the data indicate that this backbonding is enhanced in the reduced form and that this configurational interaction between the donor and acceptor orbitals facilitates the electron transfer from the cluster to the SAM, which otherwise has a large outer sphere electron transfer barrier. The energy of the reductive cleavage of the C-S bond is sensitive to the dielectric of the protein in the immediate vicinity of the site as a high dielectric stabilizes the more charge separated reactant increasing the reaction barrier. This may provide a mechanism for generation of the 5'-deoxyadenosyl radical upon substrate binding.  相似文献   
54.
Recent work using the concept of alpha-clustering in studies of nuclear structure and nuclear reactions is reviewed. The high symmetry and binding energy of the alpha-particle makes it likely that nucleons inside the nucleus can condense into alpha particles and live long enough to affect many properties of nuclei and also the cross-sections of nuclear reactions, particularly those with alpha-particles as the projectile or ejectile. The alpha-particles inside the nucleus may escape, and the resulting alpha decay is now quite well understood. An incident projectile may collide with a transient alpha-particle in the nucleus and knock it out, thus providing information on the degree of clustering. This model enables the direct part of the cross-sections of (n, α) reactions to be calculated. The alpha-particle mean field unifies many of the characteristics of alpha-particle structure and alpha-particle scattering. Many properties of light nuclei may be simply explained using the concept of alpha-clustering.  相似文献   
55.
The flow capturing and thep-median location—allocation models deal quite differently with demand for service in a network. Thep-median model assumes that demand is expressed at nodes and locates facilities to minimize the total distance between such demand nodes and the nearest facility. The flow-capturing model assumes that demand is expressed on links and locates facilities to maximize the one-time exposure of such traffic to facilities. Demand in a network is often of both types: it is expressed by passing flows and by consumers centred in residential areas, aggregated as nodes. We here present a hybrid model with the dual objective of serving both types of demand. We use this model to examine the tradeoff between serving the two types of demand in a small test network using synthetic demand data. A major result is the counter-intuitive finding that thep-median model is more susceptible to impairment by the flow capturing objective than is the flow capturing model to thep-median objective. The results encourage us to apply the model to a real-world network using actual traffic data.  相似文献   
56.
The multi-service facility (MSF) concept proposes the co-location of a range of human services under a single roof. Thus, for example, services for preschoolers, teens, and seniors might be co-located within a joint facility. MSFs are a response to economies of scale; co-location reduces the monetary costs of providing a variety of services. Yet, the spatial distributions of different societal groups are different within a city – an MSF system will not provide optimum geographical accessibility to individual groups. We introduce two p-median based location–allocation models that trade off the fixed costs of providing services and opening facilities with the travel costs of three societal groups. We observe that some mixes of single- and multi-service facilities can provide efficient service systems without unduly compromising the accessibility needs of individual groups.  相似文献   
57.
Recent applications of the theory of Feshbach, Kerman and Koonin to analyse multistep processes in nuclear reactions are described, and illustrated by detailed comparisons with a range of experimental data for neutron and proton reactions. The techniques used to distinguish between the multistep direct, multistep compound, compound and collective contributions to the cross-sections are described, and their effectiveness assessed. Particular attention is devoted to recent analyses that take into account the transitions from the multistep direct to the multistep compound chain and also to those that evaluate the collective contributions to the continuum region. The variation of the effective nucleonnucleon interaction with incident energy and target nucleus is studied, and the possibility of a parameter-free calculation is discussed. The extensions of the theory to include multiparticle emission from the direct chain and also to studies of alpha-particle emission are described.  相似文献   
58.
59.
The non-thermal, pulsed plasma model of laser annealing was experimentally tested by comparing the effects of 30 ns and 1 μs laser pulses on silicon on sapphire films, impinging from both the front and rear surfaces. The energy density needed to anneal the films and the qualitative behavior of thermal shock damages are in accordance with the simple heating model and inconsistent with the non-thermal model.  相似文献   
60.
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