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   26315篇
  免费   571篇
  国内免费   106篇
化学   14752篇
晶体学   166篇
力学   811篇
综合类   9篇
数学   3621篇
物理学   7633篇
  2023年   119篇
  2022年   347篇
  2021年   431篇
  2020年   388篇
  2019年   347篇
  2018年   425篇
  2017年   393篇
  2016年   695篇
  2015年   561篇
  2014年   657篇
  2013年   1051篇
  2012年   1327篇
  2011年   1520篇
  2010年   864篇
  2009年   828篇
  2008年   1313篇
  2007年   1289篇
  2006年   1141篇
  2005年   1550篇
  2004年   1588篇
  2003年   1103篇
  2002年   754篇
  2001年   617篇
  2000年   578篇
  1999年   358篇
  1998年   298篇
  1997年   302篇
  1996年   364篇
  1995年   326篇
  1994年   320篇
  1993年   333篇
  1992年   328篇
  1991年   281篇
  1990年   232篇
  1989年   213篇
  1988年   190篇
  1987年   186篇
  1986年   179篇
  1985年   230篇
  1984年   216篇
  1983年   184篇
  1982年   192篇
  1981年   186篇
  1980年   172篇
  1979年   154篇
  1978年   176篇
  1977年   143篇
  1976年   136篇
  1974年   133篇
  1973年   123篇
排序方式: 共有10000条查询结果,搜索用时 656 毫秒
151.
A systematic study of the efficiency of protons, Ni, Pd and Th as chemical modifiers for the determination of cadmium by electrothermal atomic absorption spectrometry (ETAAS) using fast temperature programs was made for platform atomization. A comparison was made in terms of the salt type, absorbance-time profiles and elimination of the sodium chloride interference. The results were adapted to develop a method for the ETAAS determination of cadmium in biological and environmental samples. The highest sensitivity to determine cadmium in biological and environmental samples was obtained using nickel (together with protons) as a chemical modifier. The accuracy of the method was tested by the determination of cadmium in different certified reference materials. The best detection limit and the characteristic mass of Cd were found to be 0.03 ng mL(-1) and 0.35 pg, respectively.  相似文献   
152.
153.
154.
Several reported procedures for calibrating glass electrodes in proton concentration are compared. Some recommendations for non-experts are also given. The examined procedures can be classified into two broad categories, namely: those based on direct potential difference measurements of solutions of known proton concentration and those that use one or several pH standards to calibrate the electrode and subsequently measure the pH of solutions containing known proton concentrations. With a single buffer, the two types of procedures lead to equivalent results. However, if two pH buffers are used, the slope of the calibration graph in proton concentration will differ from the real electrode slope to an extent proportional to the difference between the liquid junction potentials of the two buffers. Therefore, any other method is preferable under these circumstances.  相似文献   
155.
156.
157.
A Kind of direct methods is presented for the solution of optimal control problems with state constraints.These methods are sequential quadratic programming methods.At every iteration a quadratic programming which is obtained by quadratic approximation to Lagrangian function and Linear approximations to constraints is solved to get a search direction for a merit function.The merit function is formulated by augmenting the Lagrangian funetion with a penalty term.A line search is carried out along the search direction to determine a step length such that the merit function is decreased.The methods presented in this paper include continuous sequential quadratic programming methods and discreate sequential quadrade programming methods.  相似文献   
158.
It is shown that the non-trivial cocycles on simple Lie algebras may be used to introduce antisymmetric multibrackets which lead to higher-order Lie algebras, the definition of which is given. Their generalised Jacobi identities turn out to be satisfied by the antisymmetric tensors (or higher-order “structure constants”) which characterise the Lie algebra cocycles. This analysis allows us to present a classification of the higher-order simple Lie algebras as well as a constructive procedure for them. Our results are synthesised by the introduction of a single, complete BRST operator associated with each simple algebra. Received: 3 June 1996 / Accepted: 8 November 1996  相似文献   
159.
160.
A class of long-range predictive adaptive fuzzy relational controllers is presented. The plant behavior is described over an extended time horizon by a fuzzy relational model which is identified based on input-output closed-loop observations of the plant variables. In this class of adaptive controllers the control law attempts to minimize a quadratic cost over an extended control horizon. When used with linear models, this approach has revealed a significant potential for overcoming the limitations of one-step ahead schemes, such as the stabilization of non-minimum phase plants. Here, a uniform framework is adopted for implementing both the fuzzy model and the fuzzy controller, namely distributed fuzzy relational structures gaining from their massive parallel processing features and from the learning capabilities typical of the connectivist approaches. Issues such as maintenance during the adaptation process of the meaning of linguistic terms used at both fuzzy systems interfaces are addressed, namely by introducing a new design methodology for on-line fuzzy systems interface adaptation. The examples presented reinforce the claim of the usefulness of this new approach.  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