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   7122篇
  免费   419篇
  国内免费   35篇
化学   5023篇
晶体学   17篇
力学   202篇
数学   1153篇
物理学   1181篇
  2023年   76篇
  2022年   58篇
  2021年   103篇
  2020年   189篇
  2019年   190篇
  2018年   115篇
  2017年   104篇
  2016年   294篇
  2015年   271篇
  2014年   246篇
  2013年   435篇
  2012年   523篇
  2011年   622篇
  2010年   356篇
  2009年   296篇
  2008年   482篇
  2007年   440篇
  2006年   419篇
  2005年   383篇
  2004年   326篇
  2003年   296篇
  2002年   213篇
  2001年   81篇
  2000年   56篇
  1999年   50篇
  1998年   46篇
  1997年   92篇
  1996年   89篇
  1995年   75篇
  1994年   58篇
  1993年   58篇
  1992年   48篇
  1991年   31篇
  1990年   33篇
  1989年   27篇
  1988年   32篇
  1987年   23篇
  1986年   29篇
  1985年   24篇
  1984年   21篇
  1983年   14篇
  1982年   22篇
  1981年   16篇
  1980年   18篇
  1979年   10篇
  1978年   21篇
  1977年   13篇
  1975年   11篇
  1974年   13篇
  1970年   8篇
排序方式: 共有7576条查询结果,搜索用时 15 毫秒
1.
Annals of the Institute of Statistical Mathematics - In this paper we study generalized semi-Markov high dimension regression models in continuous time, observed at fixed discrete time moments. The...  相似文献   
2.
(SO4)-rich silicate analogue borosulfates are able to stabilise cationic cluster-like and chain-like aggregates. Single crystals of [Au3Cl4][B(S2O7)2] and [Au2Cl4][B(S2O7)2](SO3) were obtained by solvothermal reaction with SO3, and the electronic properties were investigated by means of density functional theory–based calculations. [Au3Cl4][B(S2O7)2] exhibits a cluster-like cation, and the cationic gold-chloride strands in [Au2Cl4][B(S2O7)2](SO3) are found to resemble one-dimensional metallic wires. This is confirmed by polarisation microscopy.  相似文献   
3.
Recently, chalcogen bonding has been investigated in more detail in organocatalysis and the scope of activated functionalities continues to increase. Herein, the activation of imines in a Povarov [4+2] cycloaddition reaction with bidentate cationic chalcogen bond donors is presented. Tellurium-based Lewis acids show superior properties compared to selenium-based catalysts and inactive sulfur-based analogues. The catalytic activity of the chalcogen bonding donors increases with weaker binding anions. Triflate, however, is not suitable due to its participation in the catalytic pathway. A solvent screening revealed a more efficient activation in less polar solvents and a pronounced effect of solvent (and catalyst) on endo : exo diastereomeric ratio. Finally, new chiral chalcogen bonding catalysts were applied but provided only racemic mixtures of the product.  相似文献   
4.
Fifteen N-butylpyridinium salts – five monometallic [C4Py]2[MBr4] and ten bimetallic [C4Py]2[M0.5aM0.5bBr4] (M=Co, Cu, Mn, Ni, Zn) – were synthesized, and their structures and thermal and electrochemical properties were studied. All the compounds are ionic liquids (ILs) with melting points between 64 and 101 °C. Powder and single-crystal X-ray diffraction show that all ILs are isostructural. The electrochemical stability windows of the ILs are between 2 and 3 V. The conductivities at room temperature are between 10−5 and 10−6 S cm−1. At elevated temperatures, the conductivities reach up to 10−4 S cm−1 at 70 °C. The structures and properties of the current bromide-based ILs were also compared with those of previous examples using chloride ligands, which illustrated differences and similarities between the two groups of ILs.  相似文献   
5.
Herein we report on an analytical study of dry-shredded lithium-ion battery (LIB) materials with unknown composition. Samples from an industrial recycling process were analyzed concerning the elemental composition and (organic) compound speciation. Deep understanding of the base material for LIB recycling was obtained by identification and analysis of transition metal stoichiometry, current collector metals, base electrolyte and electrolyte additive residues, aging marker molecules and polymer binder fingerprints. For reversed engineering purposes, the main electrode and electrolyte chemistries were traced back to pristine materials. Furthermore, possible lifetime application and accompanied aging was evaluated based on target analysis on characteristic molecules described in literature. With this, the reported analytics provided precious information for value estimation of the undefined spent batteries and enabled tailored recycling process deliberations. The comprehensive feedstock characterization shown in this work paves the way for targeted process control in LIB recycling processes.  相似文献   
6.
Bimodal molecular probes combining nuclear magnetic resonance (NMR) and fluorescence have been widely studied in basic science, as well as clinical research. The investigation of spin phenomena holds promise to broaden the scope of available probes allowing deeper insights into physiological processes. Herein, a class of molecules with a bimodal character with respect to fluorescence and nuclear spin singlet states is introduced. Singlet states are NMR silent but can be probed indirectly. Symmetric, perdeuterated molecules, in which the singlet states can be populated by vanishingly small electron-mediated couplings (below 1 Hz) are reported. The lifetimes of these states are an order of magnitude longer than the longitudinal relaxation times and up to four minutes at 7 T. Moreover, these molecules show either aggregation induced emission (AIE) or aggregation caused quenching (ACQ) with respect to their fluorescence. In the latter case, the existence of excited dimers, which are proposed to use in a switchable manner in combination with the quenching of nuclear spin singlet states, is observed  相似文献   
7.
The first total synthesis of resveratrone and iso-resveratrone based on an epoxide olefination approach is described. The pivotal reaction proceeds by insertion of the lithiated epoxide into a boronic ester and subsequent syn-elimination. Resveratrone has been described to have remarkable photophysical properties, including two-photon absorption. Therefore, an azide derivative has been prepared to allow for use as a biological label.  相似文献   
8.
9.

The combinatorial integral approximation decomposition splits the optimization of a discrete-valued control into two steps: solving a continuous relaxation of the discrete control problem, and computing a discrete-valued approximation of the relaxed control. Different algorithms exist for the second step to construct piecewise constant discrete-valued approximants that are defined on given decompositions of the domain. It is known that the resulting discrete controls can be constructed such that they converge to a relaxed control in the \(\hbox {weak}^*\) topology of \(L^\infty \) if the grid constant of this decomposition is driven to zero. We exploit this insight to formulate a general approximation result for optimization problems, which feature discrete and distributed optimization variables, and which are governed by a compact control-to-state operator. We analyze the topology induced by the grid refinements and prove convergence rates of the control vectors for two problem classes. We use a reconstruction problem from signal processing to demonstrate both the applicability of the method outside the scope of differential equations, the predominant case in the literature, and the effectiveness of the approach.

  相似文献   
10.
In the paper mentioned in the title, it is proved the boundedness of the Riesz potential operator of variable order α(x) from variable exponent Morrey space to variable exponent Campanato space, under certain assumptions on the variable exponents p(x) and λ(x) of the Morrey space. Assumptions on the exponents were different depending on whether α ( x ) p ( x ) ? n + λ ( x ) p ( x ) takes or not the critical values 0 or 1. In this note, we improve those results by unifying all the cases and covering the whole range 0 ? α ( x ) p ( x ) ? n + λ ( x ) p ( x ) ? 1. We also provide a correction to some minor technicality in the proof of Theorem 2 in the aforementioned paper.  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