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   7573篇
  免费   306篇
  国内免费   28篇
化学   4888篇
晶体学   52篇
力学   162篇
数学   1236篇
物理学   1569篇
  2023年   73篇
  2022年   111篇
  2021年   208篇
  2020年   171篇
  2019年   205篇
  2018年   150篇
  2017年   163篇
  2016年   317篇
  2015年   234篇
  2014年   259篇
  2013年   443篇
  2012年   516篇
  2011年   613篇
  2010年   359篇
  2009年   385篇
  2008年   528篇
  2007年   490篇
  2006年   422篇
  2005年   356篇
  2004年   332篇
  2003年   220篇
  2002年   210篇
  2001年   114篇
  2000年   116篇
  1999年   82篇
  1998年   58篇
  1997年   59篇
  1996年   60篇
  1995年   48篇
  1994年   43篇
  1993年   43篇
  1992年   57篇
  1991年   34篇
  1990年   37篇
  1989年   27篇
  1988年   21篇
  1987年   18篇
  1986年   24篇
  1985年   32篇
  1984年   38篇
  1983年   16篇
  1982年   19篇
  1981年   25篇
  1980年   20篇
  1979年   15篇
  1978年   13篇
  1977年   13篇
  1976年   14篇
  1974年   16篇
  1973年   11篇
排序方式: 共有7907条查询结果,搜索用时 421 毫秒
21.
In this paper, we study the consistency of a variant of fractionalstep Runge–Kutta methods. These methods are designed tointegrate efficiently semi-linear multidimensional parabolicproblems by means of linearly implicit time integration processes.Such time discretization procedures are also related to a splittingof the space differential operator (or the spatial discretizationof it) as a sum of ‘simpler’ linear differentialoperators and a nonlinear term.  相似文献   
22.
A six level rate equation system was used to investigate the impact of photodepletion to amplified spontaneous emission (ASE) of intramolecular proton-transfer (IPT) dyes incorporated into polymeric hosts. The model includes the most important transitions for the normal and tautomer form of the molecule as well as intersystem crossing and triplet–triplet transitions. The experimentally observed pulse shape as well as photodepletion phenomena, i.e. a first order exponential decay of the ASE intensity and a shortening in ASE pulse width, have been simulated successfully. Additionally, the model was used to propose an explanation of the unexpected high photodepletion of proton-transfer dyes in solids. The results show that the emission cross section and reabsorption cross section of the tautomer form of the molecule are the most important parameter not only for efficiency but also for photodepletion. The model was tested by comparison with experimental results of 2-(2’-hydroxyphenyl)benzimidazole in polymethylmethacrylate (PMMA), polystyrene (PS) and a 1:1 alternating copolymer matrix PS-co-PMMA. PACS 78.45.th; 42.55.Mv; 42.70.Jk  相似文献   
23.
Facility-location problems have several applications, such as telecommunications, industrial transportation and distribution. One of the most well-known facility-location problems is the p-median problem. This work addresses an application of the capacitated p-median problem to a real-world problem. We propose a genetic algorithm (GA) to solve the capacitated p-median problem. The proposed GA uses not only conventional genetic operators, but also a new heuristic “hypermutation” operator suggested in this work. The proposed GA is compared with a tabu search algorithm.  相似文献   
24.
Sharp upper and lower bounds are obtained for the reliability functions and the expectations of lifetimes of coherent systems based on dependent exchangeable absolutely continuous components with a given marginal distribution function, by use of the concept of Samaniego's signature. We first show that the distribution of any coherent system based on exchangeable components with absolutely continuous joint distribution is a convex combination of distributions of order statistics (equivalent to the k-out-of-n systems) with the weights identical with the values of the Samaniego signature of the system. This extends the Samaniego representation valid for the case of independent and identically distributed components. Combining the representation with optimal bounds on linear combinations of distribution functions of order statistics from dependent identically distributed samples, we derive the corresponding reliability and expectation bounds, dependent on the signature of the system and marginal distribution of dependent components. We also present the sequences of exchangeable absolutely continuous joint distributions of components which attain the bounds in limit. As an application, we obtain the reliability bounds for all the coherent systems with three and four exchangeable components, expressed in terms of the parent marginal reliability function and specify the respective expectation bounds for exchangeable exponential components, comparing them with the lifetime expectations of systems with independent and identically distributed exponential components.  相似文献   
25.
26.
杨光  P. V. Santos 《物理学报》2006,55(8):4327-4331
结合声表面波和光致发光谱在低温(15K)下对非故意掺杂的GaAs(110)量子阱结构的发光特性进行了研究.实验结果表明,由于声表面波的作用GaAs(110)量子阱的发光强度减弱,并且其对应的重空穴能级出现了分裂的现象,当施加的声波强度Prf达到20dBm时,能级分裂ΔE达到了10meV.进一步讨论了声表面波对GaAs(110)量子阱圆偏振光自旋注入的影响. 关键词: 发光 GaAs量子阱 声表面波 自旋极化  相似文献   
27.
The covering graph of a lattice which is contained in an ordered set with a planar covering graph is itself planar.Presented by R. W. Quackenbush.  相似文献   
28.
In this article we prove a local existence and uniqueness theorem for the Kadomtsev-Petviashvili Equation (u t +u xxx +uu x ) x −u yy =0) in the Sobolev spaces of orders≥3, with initial values in the same spaces, and periodic boundary conditions. This theorem improves previous results based upon the application of singular perturbation techniques.  相似文献   
29.
A classical Fermi accelerator model (FAM) is known to show chaotic behavior. The FAM is defined by a free particle bouncing elastically from two rigid walls, one fixed and the other oscillating periodically in time. The central aim of this paper is to connect the quantum and the classical solutions to the FAM in the semiclassical limit. This goal is accomplished using a finite inverted parametric oscillator (FIPO), confined to a box withfixed walls, as an alternative representation of the FAM. In the FIPO representation, an explicit correspondence between classical and quantum limits is accomplished using a Husimi representation of the quasienergy eigenfunctions.  相似文献   
30.
Poly(3-hydroxybutyrate), PHB has been structurally modified through reaction with maleic anhydride, MA. Transesterification reaction was carried out fixing the PHB and MA and besides time and temperature the concentration of the triethylamine (used as catalyst) was changed. Glass transition, melting and crystallization temperature obtained from DSC curves and thermal degradation temperatures obtained from TG traces were used to evaluate the influence of the reaction conditions on the modification of PHB according to factorial design. On the base of the results the optimum conditions are to perform the PHB modification reaction with MA reaction at 110°C for 1 h with 5% v/v triethylamine.  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