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   5989篇
  免费   290篇
  国内免费   59篇
化学   4218篇
晶体学   20篇
力学   151篇
数学   1055篇
物理学   894篇
  2023年   41篇
  2022年   48篇
  2021年   133篇
  2020年   130篇
  2019年   136篇
  2018年   132篇
  2017年   104篇
  2016年   234篇
  2015年   234篇
  2014年   244篇
  2013年   404篇
  2012年   428篇
  2011年   519篇
  2010年   312篇
  2009年   248篇
  2008年   442篇
  2007年   407篇
  2006年   408篇
  2005年   341篇
  2004年   293篇
  2003年   207篇
  2002年   178篇
  2001年   48篇
  2000年   49篇
  1999年   36篇
  1998年   44篇
  1997年   48篇
  1996年   47篇
  1995年   34篇
  1994年   32篇
  1993年   33篇
  1992年   35篇
  1991年   32篇
  1990年   19篇
  1989年   14篇
  1988年   10篇
  1987年   13篇
  1986年   16篇
  1985年   24篇
  1984年   31篇
  1983年   16篇
  1982年   17篇
  1981年   21篇
  1980年   17篇
  1979年   13篇
  1978年   17篇
  1977年   15篇
  1976年   5篇
  1973年   9篇
  1970年   3篇
排序方式: 共有6338条查询结果,搜索用时 31 毫秒
1.
Journal of Thermal Analysis and Calorimetry - Nanomaterials and nanocomposites have gained relevance in science and technology due to their excellent properties. Therefore, the characterization of...  相似文献   
2.
Meccanica - Fiber reinforced materials are used in assorted engineering application and for this reason, new additive manufacturing technologies have been developed for this type of materials. With...  相似文献   
3.
Entropy makes it possible to measure the uncertainty about an information source from the distribution of its output symbols. It is known that the maximum Shannon’s entropy of a discrete source of information is reached when its symbols follow a Uniform distribution. In cryptography, these sources have great applications since they allow for the highest security standards to be reached. In this work, the most effective estimator is selected to estimate entropy in short samples of bytes and bits with maximum entropy. For this, 18 estimators were compared. Results concerning the comparisons published in the literature between these estimators are discussed. The most suitable estimator is determined experimentally, based on its bias, the mean square error short samples of bytes and bits.  相似文献   
4.
A new family of distorted ribbon-shaped nanographenes was designed, synthesized, and their optical and electrochemical properties were evaluated, pointing out an unprecedented correlation between their structural characteristics and the two-photon absorption (TPA) responses and electrochemical band gaps. Three nanographene ribbons have been prepared: a seven-membered-ring-containing nanographene presenting a tropone moiety at the edge, its full-carbon analogue, and a purely hexagonal one. We have found that the TPA cross-sections and the electrochemical band gaps of the seven-membered-ring-containing compounds are higher and lower, respectively, than those of the fully hexagonal polycyclic aromatic hydrocarbon (PAH). Interestingly, the inclusion of additional curvature has a positive effect in terms of non-linear optical properties of those ribbons.  相似文献   
5.
Journal of Radioanalytical and Nuclear Chemistry - This research aims to contribute to the discussion of ceramic objects found at the Monte Castelo shellmound, an archaeological site located at...  相似文献   
6.
Let G be a finite, connected graph. An arithmetical structure on G is a pair of positive integer vectors d,r such that (diag(d)?A)r=0, where A is the adjacency matrix of G. We investigate the combinatorics of arithmetical structures on path and cycle graphs, as well as the associated critical groups (the torsion part of the cokernels of the matrices (diag(d)?A)). For paths, we prove that arithmetical structures are enumerated by the Catalan numbers, and we obtain refined enumeration results related to ballot sequences. For cycles, we prove that arithmetical structures are enumerated by the binomial coefficients 2n?1n?1, and we obtain refined enumeration results related to multisets. In addition, we determine the critical groups for all arithmetical structures on paths and cycles.  相似文献   
7.
A series of structurally intriguing novel pyrrolo[3,2-c]quinolinone heterocyclic hybrids have been synthesized for the first time via a one-pot multicomponent domino reaction sequence that involves a 1,3-dipolar cycloaddition and two subsequent annulation steps. Baylis-Hillman adducts derived from various substituted benzaldehyde and methyl acrylate in presence of DABCO were used as a dipolarophiles, while the 1,3-dipole components were azomethine ylides, formed in situ from isatin derivatives and l-phenylalanine. The reaction generated five new bonds, three new rings and four contiguous stereocenters, which were created with full diastereomeric control.  相似文献   
8.
In comparison to the extensively studied metal‐catalyzed hydroamination reaction, hydroaminomethylation has received significantly less attention despite its considerable potential to streamline amine synthesis. State‐of‐the‐art protocols for hydroaminomethylation of alkenes rely largely on transition‐metal catalysis, enabling this transformation only under highly designed and controlled conditions. Here we report a broadly applicable, acid‐mediated approach to the hydroaminomethylation of unactivated alkenes and alkynes. This methodology employs cheap, readily available, and bench‐stable reactants and affords the desired amines with excellent functional group tolerance and impeccable regioselectivity. The broad scope of this transformation, as well as mechanistic investigations and in situ domino functionalization reactions are reported.  相似文献   
9.
Full waveform inversion is an advantageous technique for obtaining high-resolution subsurface information. In the petroleum industry, mainly in reservoir characterisation, it is common to use information from wells as previous information to decrease the ambiguity of the obtained results. For this, we propose adding a relative entropy term to the formalism of the full waveform inversion. In this context, entropy will be just a nomenclature for regularisation and will have the role of helping the converge to the global minimum. The application of entropy in inverse problems usually involves formulating the problem, so that it is possible to use statistical concepts. To avoid this step, we propose a deterministic application to the full waveform inversion. We will discuss some aspects of relative entropy and show three different ways of using them to add prior information through entropy in the inverse problem. We use a dynamic weighting scheme to add prior information through entropy. The idea is that the prior information can help to find the path of the global minimum at the beginning of the inversion process. In all cases, the prior information can be incorporated very quickly into the full waveform inversion and lead the inversion to the desired solution. When we include the logarithmic weighting that constitutes entropy to the inverse problem, we will suppress the low-intensity ripples and sharpen the point events. Thus, the addition of entropy relative to full waveform inversion can provide a result with better resolution. In regions where salt is present in the BP 2004 model, we obtained a significant improvement by adding prior information through the relative entropy for synthetic data. We will show that the prior information added through entropy in full-waveform inversion formalism will prove to be a way to avoid local minimums.  相似文献   
10.
Gajbhiye  Sneha  Cabecinhas  David  Silvestre  Carlos  Cunha  Rita 《Nonlinear dynamics》2022,107(3):2291-2308
Nonlinear Dynamics - This paper presents a geometric finite-time inner-outer loop control strategy for slung payload transportation using a quadrotor. The underactuated nature of the quadrotor in...  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