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   32194篇
  免费   209篇
  国内免费   282篇
化学   11410篇
晶体学   274篇
力学   1413篇
综合类   12篇
数学   9031篇
物理学   8466篇
无线电   2079篇
  2021年   66篇
  2020年   52篇
  2019年   64篇
  2018年   1252篇
  2017年   1522篇
  2016年   802篇
  2015年   653篇
  2014年   558篇
  2013年   832篇
  2012年   3383篇
  2011年   2548篇
  2010年   1893篇
  2009年   1629篇
  2008年   704篇
  2007年   729篇
  2006年   713篇
  2005年   4891篇
  2004年   4031篇
  2003年   2376篇
  2002年   544篇
  2001年   364篇
  2000年   142篇
  1999年   200篇
  1998年   139篇
  1997年   98篇
  1996年   69篇
  1995年   65篇
  1994年   59篇
  1992年   183篇
  1991年   174篇
  1990年   171篇
  1989年   134篇
  1988年   133篇
  1987年   95篇
  1986年   84篇
  1985年   88篇
  1984年   74篇
  1983年   64篇
  1982年   55篇
  1979年   48篇
  1976年   96篇
  1975年   56篇
  1974年   56篇
  1973年   63篇
  1972年   51篇
  1971年   42篇
  1970年   57篇
  1969年   40篇
  1968年   44篇
  1966年   42篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
1.
We investigate the possibility of phantom crossing in the dark energy sector and the solution for the Hubble tension between early and late universe observations. We use robust combinations of different cosmological observations, namely the Cosmic Microwave Background (CMB), local measurement of Hubble constant (H0), Baryon Acoustic Oscillation (BAO) and SnIa for this purpose. For a combination of CMB+BAO data that is related to early universe physics, phantom crossing in the dark energy sector was confirmed at a 95% confidence level and we obtained the constraint H0=71.03.8+2.9 km/s/Mpc at a 68% confidence level, which is in perfect agreement with the local measurement by Riess et al. We show that constraints from different combinations of data are consistent with each other and all of them are consistent with phantom crossing in the dark energy sector. For the combination of all data considered, we obtained the constraint H0=70.25±0.78 km/s/Mpc at a 68% confidence level and the phantom crossing happening at the scale factor am=0.8510.031+0.048 at a 68% confidence level.  相似文献   
2.
A one-step Rh-catalyzed site-selective ortho-C−H alkynylation of perylene as well as naphthalene mono- and diimides is reported. A single step regioselective access to ortho-C−H alkynylated derivatives of these ryleneimides not only increases the step economy of the ortho-functionalization on these dyes but also provides a quick access route towards highly functionalized dyes that have potential optoelectronic applications. Increased solubility of tetra(triisopropylsilyl)acetylenyl PDIs in organic solvents greatly enhances their utility for further derivatization.  相似文献   
3.
In this paper, we present a novel memory access reduction scheme (MARS) for two-dimension fast cosine transform (2-D FCT). It targets programmable DSPs with high memory-access latency. It reduces the number of memory accesses by: 1) reducing the number of weighting factors and 2) combining butterflies in vector-radix 2-D FCT pruning diagram from two stages to one stage with an efficient structure. Hardware platform based on general purpose processor is used to verify the effectiveness of the proposed method for vector-radix 2-D FCT pruning implementation. Experimental results validate the benefits of the proposed method with reduced memory access, less clock cycle and fewer memory space compared with the conventional implementation.  相似文献   
4.
Mesoscopic modeling at the pore scale offers great promise in exploring the underlying structure transport performance of flow through porous media. The present work studies the fluid flow subjected to capillarity-induced resonance in porous media characterized by different porous structure and wettability. The effects of porosity and wettability on the displacement behavior of the fluid flow through porous media are discussed. The results are presented in the form of temporal evolution of percentage saturation and displacement of the fluid front through porous media. The present study reveals that the vibration in the form of acoustic excitation could be significant in the mobilization of fluid through the porous media. The dependence of displacement of the fluid on physicochemical parameters like wettability of the surface, frequency along with the porosity is analyzed. It was observed that the mean displacement of the fluid is more in the case of invading fluid with wetting phase where the driving force strength is not so dominant.  相似文献   
5.
6.
7.
Thermal effects are becoming an important factor in the design of integrated circuits due to the adverse impact of temperature on performance, reliability, leakage, and chip packaging costs. Making all phases of the design flow aware of this physical phenomenon helps in reaching faster design closure. In this paper, we present an integrated approach to thermal management in architectural synthesis. Our synthesis flow combines temperature-aware scheduling and binding based on feedback from thermal simulation. We show that our flow is effective in preventing hotspot formation and creating an even thermal profile of the resources. Our integrated thermal management technique on average reduces the peak temperature of the resources by 7.34 degC when compared to a thermal unaware flow without increasing the number of resources across our set of benchmarks  相似文献   
8.
This paper presents a stereo vision system for the detection and distance computation of a preceding vehicle. It is divided in two major steps. Initially, a stereo vision-based algorithm is used to extract relevant three-dimensional (3-D) features in the scene, these features are investigated further in order to select the ones that belong to vertical objects only and not to the road or background. These 3-D vertical features are then used as a starting point for preceding vehicle detection; by using a symmetry operator, a match against a simplified model of a rear vehicle's shape is performed using a monocular vision-based approach that allows the identification of a preceding vehicle. In addition, using the 3-D information previously extracted, an accurate distance computation is performed.  相似文献   
9.
Energy-based methods for motion estimation in image sequences process the input data either in the spatiotemporal or in the frequency domain. In both cases, the algorithms already described in the literature often require a huge number of elementary operations. In this paper, we describe a class of velocity selective filters which yield an accurate detection of the edges moving in the sequence. We first present a filtering scheme based on a convolution operation computed on a finite size neighborhood and describe its properties in the spatiotemporal and frequency domains. Then, we show that filters with similar properties can be implemented recursively, i.e., as convolutions computed on infinite-size neighborhoods. As an example, we finally show the filters' responses in the case of two superimposed translational motions.  相似文献   
10.
A fiber optic ring network, such as fiber distributed data interface (FDDI), can be operated over multiple wavelengths on its existing fiber plant consisting of point-to-point fiber links. Using wavelength division multiplexing (WDM) technology, FDDI nodes can be partitioned to operate over multiple subnetworks, with each subnetwork operating independently on a different wavelength, and inter-subnetwork traffic forwarding performed by a bridge. For this multiwavelength version of FDDI, which we refer to as wavelength distributed data interface (WDDI), we examine the necessary upgrades to the architecture of a FDDI node, including its possibility to serve as a bridge. The main motivation behind this study is that, as network traffic scales beyond (the single-wavelength) FDDI's information-carrying capacity, its multiwavelength version, WDDI, can gracefully accommodate such traffic growth. A number of design choices exist in constructing a good WDDI network. Specifically, we investigate algorithms using which, based on prevailing traffic conditions, partitioning of nodes into subnetworks can be performed in an optimized fashion. Our algorithms partition the nodes into subrings, such that the total traffic flow in the network and/or the network-wide average packet delay is minimized  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