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   158209篇
  免费   13501篇
  国内免费   18735篇
化学   99310篇
晶体学   2029篇
力学   12653篇
综合类   1455篇
数学   32842篇
物理学   42156篇
  2023年   1604篇
  2022年   2497篇
  2021年   3398篇
  2020年   4520篇
  2019年   4178篇
  2018年   3539篇
  2017年   4723篇
  2016年   4977篇
  2015年   4194篇
  2014年   6710篇
  2013年   12019篇
  2012年   10199篇
  2011年   9662篇
  2010年   8122篇
  2009年   10612篇
  2008年   10779篇
  2007年   11266篇
  2006年   9920篇
  2005年   8162篇
  2004年   7425篇
  2003年   6583篇
  2002年   5669篇
  2001年   4774篇
  2000年   4079篇
  1999年   3682篇
  1998年   3320篇
  1997年   2769篇
  1996年   2427篇
  1995年   2410篇
  1994年   2206篇
  1993年   1854篇
  1992年   1712篇
  1991年   1196篇
  1990年   942篇
  1989年   864篇
  1988年   677篇
  1987年   580篇
  1986年   533篇
  1985年   609篇
  1984年   535篇
  1983年   308篇
  1982年   544篇
  1981年   639篇
  1980年   573篇
  1979年   621篇
  1978年   504篇
  1977年   384篇
  1976年   306篇
  1974年   121篇
  1973年   190篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
利用hirota双线性法,得到(3+1)维孤子方程、(3+1)维KP-Boussinesq方程、(2+1)维修正Caudrey-Dodd-Gibbon-Kotera-S awada方程、Hirota-Satsuma浅水波方程的精确解,并做出一部分解的图形,进一步研究解的结构和性质.  相似文献   
2.
超疏水表面液滴的振动特性与接触线的移动、液滴体积、基底振幅等因素密切相关.本文在基底振幅较小且恒定的条件下,研究了超疏水表面液滴的共振振幅、模式区间、共振频率等振动特性及其与液滴体积(20—500μL)的关系.此外,将基于一般性疏水表面建立的Noblin共振频率计算模型应用于超疏水表面,并提出“虚驻点”的概念,借此对模型进行了误差分析和修正.研究表明:1)共振时,液滴高度变化率即比振幅随体积增大而增大,随阶数增大而减小;2)各模式区间的起止频率首尾相接,其范围随体积增大而减小;3)液滴体积越大,共振频率越小,随着阶数增大,共振频率f与体积V的关系趋于f-V–0.4,不同于一般性疏水表面上的f-V–0.5;4)直接应用Noblin模型计算共振频率会产生较大误差,主要原因在于液滴表面波波段数量统计存在较大偏差,而修正后的模型可以准确计算超疏水表面大体积液滴的共振频率.  相似文献   
3.
为提高K9光学玻璃在一些特殊应用领域(如高压、温度变化剧烈等)的力学性能,并保证其光学性能符合精密光学仪器要求,对K9光学玻璃进行了化学钢化技术研究。以脆性材料断裂过程微裂纹扩展理论为基础,导出化学钢化玻璃强度应力因子计算模型,分析化学钢化表面应力与表面微裂纹深度、韧性之间的关系,指出化学钢化工艺应注意的事项。通过实验研究,分析化学钢化温度和钢化时间对K9光学玻璃抗弯强度、表面应力及应力层厚度的影响,优化得出K9光学玻璃化学钢化温度为400 ℃、钢化时间为40 h。采用优化工艺,获得了表面应力为500 MPa、应力层厚度为50 μm量级及规格为220 mm×110 mm×22 mm的化学钢化K9光学玻璃样件。钢化后,样件抗弯强度提高了3.5倍以上,且表面疵病、光学鉴别率、透过率等光学性能指标未见明显变化。  相似文献   
4.
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   
5.
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   
6.
This paper proposes a method for solving optimisation problems involving piecewise quadratic functions. The method provides a solution in a finite number of iterations, and the computational complexity of the proposed method is locally polynomial of the problem dimension, i.e., if the initial point belongs to the sufficiently small neighbourhood of the solution set. Proposed method could be applied for solving large systems of linear inequalities.  相似文献   
7.
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   
8.
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   
9.
Sol-gel method has been used for the synthesis of biocompatible superparamagnetic nanoferrites of AFeO2 (A = Li, Na, K, Ca). Structural study of the nanoferrites reveals that LiFeO2 exhibits cubic phase on the other hand NaFeO2, KFeO2, CaFeO2 nanoparticles possess orthorhombic phase. Transmission electron microscopy (TEM) suggests that synthesized nanoferrites are nano-sized with spherical morphology. Optical properties confirm that nanoferrites emit and absorb light in a visible range of the electromagnetic spectrum. International Commission on Illumination (CIE) study discloses that the nanoparticles can be used to produce light of various colors. Magnetic study reveals that the nanoferrites exhibit superparamagnetic nature with high values of saturation magnetization 40.26 emu/g, 41.69 emu/g, 57.16 emu/g, and 43.66 emu/g, respectively for LiFeO2, NaFeO2, KFeO2, and CaFeO2. Biocompatibility study of the nanoferrites has been performed using Sulforhodamine B (SRB) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The reason behind the observed properties and behavior has been discussed.  相似文献   
10.
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   
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