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   79258篇
  免费   7888篇
  国内免费   12406篇
化学   62687篇
晶体学   1883篇
力学   2990篇
综合类   439篇
数学   8938篇
物理学   22615篇
  2023年   904篇
  2022年   1444篇
  2021年   2232篇
  2020年   2941篇
  2019年   2560篇
  2018年   1939篇
  2017年   2267篇
  2016年   2746篇
  2015年   2774篇
  2014年   3560篇
  2013年   6103篇
  2012年   5452篇
  2011年   4759篇
  2010年   3861篇
  2009年   5039篇
  2008年   5180篇
  2007年   5588篇
  2006年   4833篇
  2005年   3918篇
  2004年   3687篇
  2003年   3430篇
  2002年   2883篇
  2001年   2272篇
  2000年   2000篇
  1999年   1670篇
  1998年   1506篇
  1997年   1336篇
  1996年   1385篇
  1995年   1316篇
  1994年   1196篇
  1993年   1048篇
  1992年   991篇
  1991年   624篇
  1990年   475篇
  1989年   360篇
  1988年   380篇
  1987年   337篇
  1986年   318篇
  1985年   433篇
  1984年   309篇
  1983年   189篇
  1982年   346篇
  1981年   517篇
  1980年   472篇
  1979年   499篇
  1978年   388篇
  1977年   292篇
  1976年   254篇
  1974年   86篇
  1973年   174篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
When applying a diagnostic technique to complex systems, whose dynamics, constraints, and environment evolve over time, being able to re-evaluate the residuals that are capable of detecting defaults and proposing the most appropriate ones can quickly prove to make sense. For this purpose, the concept of adaptive diagnosis is introduced. In this work, the contributions of information theory are investigated in order to propose a Fault-Tolerant multi-sensor data fusion framework. This work is part of studies proposing an architecture combining a stochastic filter for state estimation with a diagnostic layer with the aim of proposing a safe and accurate state estimation from potentially inconsistent or erroneous sensors measurements. From the design of the residuals, using α-Rényi Divergence (α-RD), to the optimization of the decision threshold, through the establishment of a function that is dedicated to the choice of α at each moment, we detail each step of the proposed automated decision-support framework. We also dwell on: (1) the consequences of the degree of freedom provided by this α parameter and on (2) the application-dictated policy to design the α tuning function playing on the overall performance of the system (detection rate, false alarms, and missed detection rates). Finally, we present a real application case on which this framework has been tested. The problem of multi-sensor localization, integrating sensors whose operating range is variable according to the environment crossed, is a case study to illustrate the contributions of such an approach and show the performance.  相似文献   
2.
We generalize the Jensen-Shannon divergence and the Jensen-Shannon diversity index by considering a variational definition with respect to a generic mean, thereby extending the notion of Sibson’s information radius. The variational definition applies to any arbitrary distance and yields a new way to define a Jensen-Shannon symmetrization of distances. When the variational optimization is further constrained to belong to prescribed families of probability measures, we get relative Jensen-Shannon divergences and their equivalent Jensen-Shannon symmetrizations of distances that generalize the concept of information projections. Finally, we touch upon applications of these variational Jensen-Shannon divergences and diversity indices to clustering and quantization tasks of probability measures, including statistical mixtures.  相似文献   
3.
Polyacrylonitrile (PAN)/β-cyclodextrin (β-CD) composite nanofibrous membranes immobilized with nano-titanium dioxide (TiO2) and graphene oxide (GO) were prepared by electrospinning and ultrasonic-assisted electrospinning. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) confirmed that TiO2 and GO were more evenly dispersed on the surface and inside of the nanofibers after 45 min of ultrasonic treatment. Adding TiO2 and GO reduced the fiber diameter; the minimum fiber diameter was 84.66 ± 40.58 nm when the mass ratio of TiO2-to-GO was 8:2 (PAN/β-CD nanofibrous membranes was 191.10 ± 45.66 nm). Using the anionic dye methyl orange (MO) and the cationic dye methylene blue (MB) as pollutant models, the photocatalytic activity of the nanofibrous membrane under natural sunlight was evaluated. It was found that PAN/β-CD/TiO2/GO composite nanofibrous membrane with an 8:2 mass ratio of TiO2-to-GO exhibited the best degradation efficiency for the dyes. The degradation efficiency for MB and MO were 93.52 ± 1.83% and 90.92 ± 1.52%, respectively. Meanwhile, the PAN/β-CD/TiO2/GO composite nanofibrous membrane also displayed good antibacterial properties and the degradation efficiency for MB and MO remained above 80% after 3 cycles. In general, the PAN/β-CD/TiO2/GO nanofibrous membrane is eco-friendly, reusable, and has great potential for the removal of dyes from industrial wastewaters.  相似文献   
4.
In this article, a way to employ the diffusion approximation to model interplay between TCP and UDP flows is presented. In order to control traffic congestion, an environment of IP routers applying AQM (Active Queue Management) algorithms has been introduced. Furthermore, the impact of the fractional controller PIγ and its parameters on the transport protocols is investigated. The controller has been elaborated in accordance with the control theory. The TCP and UDP flows are transmitted simultaneously and are mutually independent. Only the TCP is controlled by the AQM algorithm. Our diffusion model allows a single TCP or UDP flow to start or end at any time, which distinguishes it from those previously described in the literature.  相似文献   
5.
We address the problem of telegraphic transport in several dimensions. We review the derivation of two and three dimensional telegrapher’s equations—as well as their fractional generalizations—from microscopic random walk models for transport (normal and anomalous). We also present new results on solutions of the higher dimensional fractional equations.  相似文献   
6.
数学均匀化方法是计算周期复合材料结构的有效方法之一,单胞边界条件施加的合理性直接决定了影响函数控制方程的计算效率和精度,进而影响均匀化弹性参数和摄动位移的计算精度.本文首先将单胞影响函数作为虚拟位移处理,给出了单胞在结构中真实的边界条件,结果表明,四边固支适合作为二维结构单胞边界条件;其次,针对二维结构提出了超单胞周期边界条件,有效提高了影响函数的计算精度,并使用与虚拟位移相对应的虚拟势能泛函验证超单胞周期边界条件的有效性;最后,利用数值分析验证多尺度渐进展开方法的计算精度,强调了二阶摄动的必要性.  相似文献   
7.
三氧化二铋(Bi2O3)是氧离子导电体,为了获得它的原子热振动各向同性温度因子,对该粉末晶体进行X射线衍射实验,建立了晶体结构模型,利用Rietveld 精修方法的RIETAN-2000 程序对所得实验结果进行了晶体结构精修,通过最大熵方法(MEM)解析得到了粉末晶体的等高电子密度分布三维(3D) 和二维(2D)可视化图谱。结果表明,各原子Bi(1)、Bi(2)、O(1)、O(2)和O(3)的原子热振动各向同性温度因子分别为0.004 938 nm2、0.004 174 nm2、0.007 344 nm2、0.007 462 nm2、和0.007 857 nm2,等高电子密度分布的可视化,进一步验证了晶体结构模型和原子位置的准确性,这些参数对研究晶体材料的热性质具有一定参考意义。  相似文献   
8.
运用ANSYS/LS_DYNA软件分析了聚能射流对充液结构的毁伤,初步获得了药型罩壁厚和材料等参数对聚能战斗部水下作用的影响特性。药型罩壁厚在0.04Dk~0.06Dk(Dk为装药直径)之间形成的射流对充液防护结构具有较优的侵彻性能;当δ<0.04Dk时,杆流成型结构较差,在水中的动能抗衰减性能较低;δ>0.06Dk时,射流的初始动能低,靶后效果差。药型罩可采用纯铁、紫铜和钽3种材料,其中纯铁射流的侵彻能力最高,钽射流在水中的动能抗衰减性能最好,紫铜射流具有较好的综合性能。  相似文献   
9.
There is a need to understand the ultrasound-induced changes in the interactions between proteins and phenolic compounds at different pH. This study systematically explored the role of high-intensity ultrasound pre-treatment on the binding mechanisms of β-lactoglobulin (β-LG) to two common phenolic compounds, i.e., (−)-epigallocatechin-3-gallate (EGCG) and chlorogenic acid (CA) at neutral and acidic pH (pH 7.2 and 2.4). Tryptophan fluorescence revealed that compared to proteins sonicated at 20% and 50% amplitudes, 35%-amplitude ultrasound pre-treatment (ULG-35) strengthened the binding affinities of EGCG/CA to β-LG without altering the main interaction force. After phenolic addition, ULG-35 displayed a similar but a greater extent of protein secondary and tertiary structural changes than the native protein, ascribed to the ultrasound-driven hydrophobic stacking among interacted molecules. The dominant form of β-LG (dimer/monomer) played a crucial role in the conformational and interfacial properties of complexes, which can be explained by the distinct binding sites at different pH as unveiled by molecular docking. Combining pre-ultrasound with EGCG interaction notably increased the foaming and emulsifying properties of β-LG, providing a feasible way for the modification of bovine whey proteins. These results shed light on the understanding of protein–phenolic non-covalent binding under ultrasound and help to develop complex systems with desired functionality and delivery.  相似文献   
10.
万世雯  张琦炜  徐鉴 《力学季刊》2021,42(3):429-437
折纸是一门古老艺术,其本质是将平面材料沿着事先设计好的折痕进行折叠,进而形成一个复杂的三维结构.柔性折纸结构是实现三维结构轻量化的重要途径.因此,解析折纸结构几何特性和力学性质十分必要.本文以MSC(Magic Spiral Cube)为研究对象,通过实际折痕和虚拟折痕的方法,建立了该结构的几何模型,确定了实现完全展开和完全折叠对刚性面和可变形面的设计条件,在虚拟折痕上引入了扭转刚度,证明了该扭转刚度与柔性面变形的等效性,从而得到了MSC 折纸结构的弹性势能,得到了使结构变形的力与位移本构.通过力学特性分析,发现了MSC折纸结构具有双稳态特性,这种特性是由面内变形诱发的,与虚拟折痕刚度与弹性折痕刚度的比值有直接的关系.最后,我们对MSC折纸结构进行设计和制备,通过实验,验证了理论 模型的准确性.本文的研究结果不仅进一步加深了我们对于MSC折纸结构力学特性的认识,同时也为其工程应用提供理论基础.  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