全文获取类型
收费全文 | 678篇 |
免费 | 146篇 |
国内免费 | 215篇 |
专业分类
化学 | 63篇 |
晶体学 | 3篇 |
力学 | 568篇 |
综合类 | 29篇 |
数学 | 109篇 |
物理学 | 267篇 |
出版年
2023年 | 11篇 |
2022年 | 27篇 |
2021年 | 35篇 |
2020年 | 17篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 31篇 |
2016年 | 66篇 |
2015年 | 47篇 |
2014年 | 52篇 |
2013年 | 63篇 |
2012年 | 50篇 |
2011年 | 42篇 |
2010年 | 41篇 |
2009年 | 37篇 |
2008年 | 46篇 |
2007年 | 44篇 |
2006年 | 39篇 |
2005年 | 38篇 |
2004年 | 31篇 |
2003年 | 44篇 |
2002年 | 38篇 |
2001年 | 34篇 |
2000年 | 26篇 |
1999年 | 32篇 |
1998年 | 13篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 17篇 |
1992年 | 9篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 2篇 |
1979年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有1039条查询结果,搜索用时 31 毫秒
41.
The zero-asymptotic property of sliding variables in discrete systems is extended to a continuous one and applied to partial
differential equations which describe spatiotemporal chaos. A method of chaos synchronization and parameter identification
is proposed. The synchronization controllers and the parameter recognizers are designed. The uncertain Gray-Scott system is
taken as an example to verify the effectiveness of the method. Simulation results show that the identification variables in
the parameter recognizers may take the place of the unknown parameters in both target and response systems. Global synchronization
of the two spatiotemporal chaotic systems with uncertain parameters may be realized quickly after controllers are added.
Supported by the Natural Science Foundation of Liaoning Province, China (Grant No. 20052151) and the Innovative Team Program
of Liaoning Educational Committee 相似文献
42.
近年来的研究发现固液界面的滑移可以减小表面摩擦,但也会造成流体动力效应下降甚至消失.本文提出了复合表面滑动轴承的概念,轴套表面由具有不同吸附和滑移特性的复合表面组成,发现复合表面轴承比普通轴承有许多优点.通过改变轴套表面的滑移特性可以改变和优化轴承的各种性能,例如摩擦系数、承载力、润滑剂流量、承载角等.数值解表明,在轴承高压区改变轴套表面滑移特性,轴承的整体性能会有大幅度提高.例如,本文给出的初步优化设计方案使得摩擦系数降低50%以上,同时承载力可提高20%,并且承载角可以降低33%.本文提出的设计理念不但可用于设计出具有更优异特性的径向滑动轴承,而且可以设计出具有复合表面的轴向滑动轴承或滑块轴承.复合表面滑动轴承在降低轴承摩擦、提高承载能力方面有很大的空间可以探索. 相似文献
43.
本文详细介绍了滑动频谱方法,并通过模拟仿真和实测资料处理与后传播方法进行比较. 通过对模拟仿真信号反演发现:后传播方法和滑动频谱方法均能削弱大气多路径的影响,后传播方法在一定程度上优于滑动频谱方法;在模拟信号的相位中加入高斯噪声对后传播方法影响不大,但对滑动频谱方法影响较大,尤其在边界层以下. 分别用后传播方法和滑动频谱方法对2007年第71天至73天共约4500个COSMIC掩星数据进行处理. 将折射率反演结果与ECMWF分析场资料进行统计比较,结果显示:滑动频谱方法反演的掩星廓线深度大于后传播方法;后
关键词:
GPS/LEO掩星
多相位屏模型
后传播方法
滑动频谱方法 相似文献
44.
Well‐controlled ATRP of 2‐(2‐(2‐azidoethyoxy)ethoxy)ethyl methacrylate for high‐density click functionalization of polymers and metallic substrates
下载免费PDF全文
![点击此处可从《Journal of polymer science. Part A, Polymer chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The combination of atom transfer radical polymerization (ATRP) and click chemistry has created unprecedented opportunities for controlled syntheses of functional polymers. ATRP of azido‐bearing methacrylate monomers (e.g., 2‐(2‐(2‐azidoethyoxy)ethoxy)ethyl methacrylate, AzTEGMA), however, proceeded with poor control at commonly adopted temperature of 50 °C, resulting in significant side reactions. By lowering reaction temperature and monomer concentrations, well‐defined pAzTEGMA with significantly reduced polydispersity were prepared within a reasonable timeframe. Upon subsequent functionalization of the side chains of pAzTEGMA via Cu(I)‐catalyzed azide‐alkyne cycloaddition (CuAAC) click chemistry, functional polymers with number‐average molecular weights (Mn) up to 22 kDa with narrow polydispersity (PDI < 1.30) were obtained. Applying the optimized polymerization condition, we also grafted pAzTEGMA brushes from Ti6Al4 substrates by surface‐initiated ATRP (SI‐ATRP), and effectively functionalized the azide‐terminated side chains with hydrophobic and hydrophilic alkynes by CuAAC. The well‐controlled ATRP of azido‐bearing methacrylates and subsequent facile high‐density functionalization of the side chains of the polymethacrylates via CuAAC offers a useful tool for engineering functional polymers or surfaces for diverse applications. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1268–1277 相似文献
45.
Synchronization of different chaotic systems via active radial basis functions sliding mode controller
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This paper presents a new method to synchronize different chaotic systems with disturbances via an active radial basis function (RBF) sliding controller. This method incorporates the advantages of active control, neural network and sliding mode control. The main part of the controller is given based on the output of the RBF neural networks and the weights of these single layer networks are tuned on-line based on the sliding mode reaching law. Only several radial basis functions are required for this controller which takes the sliding mode variable as the only input. The proposed controller can make the synchronization error converge to zero quickly and can overcome external disturbances. Analysis of the stability for the controller is carried out based on the Lyapunov stability theorem. Finally, five examples are given to illustrate the robustness and effectiveness of the proposed synchronization control strategy. 相似文献
46.
A two dimensional plane-stress finite-element type of analysis is presented to predict the behaviour of geogrids embedded
in sand under pullout loading conditions. In the analysis the interactions between soil and geogrid are simulated by non-linear
springs. The stiffnesses of the springs can be determined from simple tests in a specially designed pullout box. The proposed
finite element (FE) analysis is applied to interpret test results from a large scale pullout test rig. The predicted behaviour
of the geogrid under pullout load agrees well with the observed data including the load-displacement properties, the displacement
distribution along the longitudinal direction and the mobilisation of the frictional and bearing resistance. 相似文献
47.
48.
The influence of a wall porosity on the pressure distribution in a curvilinear squeeze film bearing lubricated by a lubricant being a viscoplastic fluid of a Vo?adlo type is considered. 相似文献
49.
Bernhard Schweizer 《Nonlinear dynamics》2009,57(4):509-532
High-speed rotors are often supported in floating ring bearings because of their good damping behavior. In contrast to conventional
hydrodynamic bearings with a single oil film, full-floating ring bearings consist of two oil films: An inner and an outer
oil film. As single oil-film bearings, full-floating ring bearings also show the typical fluid-film-induced instabilities
(self-excited vibrations). Both inner and outer oil films can become unstable and exhibit oil whirl/whip instabilities.
The paper at hand considers a Laval (Jeffcott) rotor, which is symmetrically supported in full-floating ring bearings, and
investigates the occurring oil whirl/whip effects by means of run-up simulations. It is shown that the inner oil film, which usually becomes unstable first, gives
rise to a limit-cycle oscillation with an exactly circular rotor orbit, if gravity and imbalance are neglected. Interesting
is the instability generated by the outer oil film. The calculations demonstrate that instability in the outer oil film does
not lead to a simple circular limit-cycle orbit. Whirl/whip-induced limit-cycle oscillations generated by the outer oil film are more complex and entail a coupled circumferential and
radial motion, although the mechanical problem is radially symmetric, if gravity and imbalance are neglected. Thus, whirl/whip instability in the outer fluid film may be interpreted as symmetry breaking.
Finally, a further kind of bifurcation/instability occurring in rotors supported in full-floating ring bearings—called Total Instability in this paper—is analyzed. It is shown that Total Instability is caused by synchronization of two limit cycles, namely synchronization of the inner and outer oil whirl/whip. Total Instability is of practical interest and observed in real technical rotor systems, and frequently leads to complete rotor damage. 相似文献
50.
The study of rotor–fuselage interactional aerodynamics is central to the design and performance analysis of helicopters. However, regardless of its significance, rotor–fuselage aerodynamics has so far been addressed by very few authors. This is mainly due to the difficulties associated with both experimental and computational techniques when such complex configurations, rich in flow physics, are considered. In view of the above, the objective of this study is to develop computational tools suitable for rotor–fuselage engineering analysis based on computational fluid dynamics (CFD). To account for the relative motion between the fuselage and the rotor blades, the concept of sliding meshes is introduced. A sliding surface forms a boundary between a CFD mesh around the fuselage and a rotor‐fixed CFD mesh which rotates to account for the movement of the rotor. The sliding surface allows communication between meshes. Meshes adjacent to the sliding surface do not necessarily have matching nodes or even the same number of cell faces. This poses a problem of interpolation, which should not introduce numerical artefacts in the solution and should have minimal effects on the overall solution quality. As an additional objective, the employed sliding mesh algorithms should have small CPU overhead. The sliding mesh methods developed for this work are demonstrated for both simple and complex cases with emphasis placed on the presentation of the inner workings of the developed algorithms. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献