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   14949篇
  免费   2148篇
  国内免费   998篇
化学   3668篇
晶体学   81篇
力学   3555篇
综合类   244篇
数学   5336篇
物理学   5211篇
  2024年   40篇
  2023年   234篇
  2022年   467篇
  2021年   460篇
  2020年   449篇
  2019年   374篇
  2018年   373篇
  2017年   640篇
  2016年   720篇
  2015年   643篇
  2014年   798篇
  2013年   1057篇
  2012年   763篇
  2011年   828篇
  2010年   712篇
  2009年   798篇
  2008年   834篇
  2007年   894篇
  2006年   732篇
  2005年   622篇
  2004年   643篇
  2003年   637篇
  2002年   518篇
  2001年   417篇
  2000年   430篇
  1999年   401篇
  1998年   379篇
  1997年   261篇
  1996年   262篇
  1995年   222篇
  1994年   190篇
  1993年   166篇
  1992年   128篇
  1991年   134篇
  1990年   113篇
  1989年   86篇
  1988年   86篇
  1987年   59篇
  1986年   49篇
  1985年   74篇
  1984年   40篇
  1983年   30篇
  1982年   46篇
  1981年   33篇
  1980年   51篇
  1979年   50篇
  1978年   26篇
  1977年   37篇
  1976年   36篇
  1957年   18篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
In recent years there has been considerable interest in the construction of nonlinear models of the dynamics of human behavior. In this exploratory article we argue that attempts at controlling problematic thoughts, emotions, or behaviors can lead to nonlinearity in mental/behavioral dynamics. We illustrate our model by fitting threshold autoregression models to self-recorded time series of the daily highs in intensity of anxiety and obsessive ruminations, kept by an individual in therapy for this problem. In our discussion, we raise the possibility that bifurcations that occur in this nonlinear model may offer insight into mental control paradoxes.  相似文献   
92.
We treat an abstract semilinear control system and study the controllability problem for its trajectories. Assuming a range condition of the control action operator and an inequality condition on the system parameters, we can show that the reachable trajectory set of the semilinear system is equivalent to that of its corresponding linear system.The author wishes to express his deep appreciation to Prof. T. I. Seidman for his many helpful suggestions and to Prof. W. Takahashi for many stimulating conversations.  相似文献   
93.
一种结合了基于任务的访问控制模型TBAC和基于角色的访问控制模型RBAC特点的新型访问控制模型-T-RBAC.该模型采用了以任务为中心的"用户-角色-任务-权限"的四级访问控制结构,较好地解决了当前工作流系统中存在的安全性问题.此外,在新模型的基础上设计了一套描述约束机制的语言ETCL,该语言在RCL2000的基础上扩充了时间特性和任务的概念,使新模型具备了较完整的约束描述能力.  相似文献   
94.
After the laser was invented in 1960, a phase conjugation mirror has been respected to be the most fantastic one for the laser resonator composition because it can compensate any distortions of the laser beams occurred by the many inhomogenuities of the laser media and optical components. Among the many phase conjugation configurations, the stimulated Brillouin scattering phase conjugation mirror is the most simple one and many researchers have tried to utilize it to develop high power/energy laser systems. For realizing a high energy/power laser system the thermal problem is the most difficult to solve, and some researchers suggested a beam combination technique to reduce the thermal load of the big laser media to many small sized ones. To accomplish the beam combination using stimulated Brillouin scattering phase conjugation mirrors (SBS-PCMs), it is necessary to lock/control the phases of the SBS-PCMs. And some researchers have developed several ways for it, but they can lock the phases of a limited number of beams overlapped at the foci less than 5, or lock the phases by back-seeding technique but it loses the phase conjugation characteristics. For realization of the laser fusion driver, it is necessary to combine more than 10 or 100 beams. And the authors have developed recently a new phase controlling/locking technique which is isolated and independent totally from other beams and it can be applied to an unlimited number of beams in principle.  相似文献   
95.
2,2,6,6‐Tetramethyl‐4‐[d‐(+)‐10‐camphorsulfonyl]‐1‐piperidinyloxy was synthesized and used as a chiral nitroxide for the bulk polymerizations of styrene initiated with benzoyl peroxide (BPO), tetraethylthiuram disulfide (TETD), and thermal initiation. The results showed that the polymerizations proceeded in a controlled/living way; that is, the kinetics presented approximately first‐order plots, and the number‐average molecular weights of the polymers with narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight) increased with the monomer conversion linearly. The molecular weight distributions in the case of thermal initiation were narrower than those in the case of BPO and TETD, whereas the polymerization rate with BPO or TETD as an initiator was obviously faster than that with thermal initiation. In addition, successful chain‐extension reactions were carried out, and the structures of the obtained polymers were characterized by gel permeation chromatography and 1H NMR. The specific rotations of the polymers were also measured by polarimetric analysis. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1522–1528, 2006  相似文献   
96.
97.
98.
高分子阳离子絮凝剂用于炼油废水处理研究   总被引:15,自引:0,他引:15  
用聚二甲基二烯丙基氯化铵型高分子阳离子絮凝剂处理炼油废水考察了险油和化学耗氧量的动态变化规律。对产生的浮渣性质及滤饼的成份进行了研究。并且与聚合铝的处理效果进行对照。研究结果表明,HCA除油和化学耗管氧量的性能优于PAC,HCA产生的浮渣是PAC的三分之一,而浮渣中的含油量则是PAC的三倍。  相似文献   
99.
This paper considers an optimal boundary control problem fora hyperbolic system in which constant time lags appear in thestate equation and in the boundary condition simultaneously.Making use of Lion's scheme, necessary and sufficient conditionsof optimality for the Neumann problem are derived.  相似文献   
100.
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