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   1026篇
  免费   46篇
  国内免费   8篇
化学   653篇
晶体学   11篇
力学   39篇
数学   157篇
物理学   220篇
  2024年   4篇
  2023年   9篇
  2022年   14篇
  2021年   30篇
  2020年   28篇
  2019年   33篇
  2018年   17篇
  2017年   30篇
  2016年   38篇
  2015年   25篇
  2014年   44篇
  2013年   87篇
  2012年   57篇
  2011年   73篇
  2010年   45篇
  2009年   28篇
  2008年   53篇
  2007年   55篇
  2006年   46篇
  2005年   35篇
  2004年   41篇
  2003年   31篇
  2002年   22篇
  2001年   10篇
  2000年   9篇
  1999年   11篇
  1998年   8篇
  1997年   6篇
  1996年   7篇
  1995年   8篇
  1994年   12篇
  1993年   10篇
  1992年   8篇
  1991年   9篇
  1990年   13篇
  1989年   6篇
  1988年   5篇
  1987年   6篇
  1986年   5篇
  1985年   13篇
  1984年   12篇
  1983年   4篇
  1982年   8篇
  1980年   5篇
  1979年   7篇
  1978年   4篇
  1977年   5篇
  1974年   5篇
  1966年   4篇
  1954年   3篇
排序方式: 共有1080条查询结果,搜索用时 265 毫秒
91.
Andrographis paniculata Nees is an annual erect herb with wide medicinal and pharmacological applications due to the presence of andrographolide and other active chemical constituents. The large-scale cultivation of the kalmegh is not in practice. The aim of this study was to establish sustainable production systems of A. paniculata cv CIM-Megha with the application of different bioinoculants and chemical fertilisers. A. paniculata herb and andrographolide yield in the dried leaves was found to be highest (218% and 61.3%, respectively) in treatment T3 (NPK+Bacillus sp.) compared with T1 (control). The soil organic carbon, soil microbial respiration, soil enzymes activity and available nutrients improved significantly with combined application of bioinoculants and chemical fertilisers.  相似文献   
92.
The reactivity of the 2‐imidazolecarboxaldiimine of triethylenetetraamine (H2imida) towards divalent manganese, cobalt, and nickel ions differs. It forms a cationic nickel(II) complex by acting as a hexa‐dentate ligand, whereas the cobalt(II) complex formation took place by deprotonation of the H2imida forming a neutral complex with imida. Deprotonation of the ligand by H2imida by cobalt(II) ions caused quenching of the fluorescence emission at 312 nm, such a quenching effect was not observed with the nickel ions. On the other hand, a mononuclear manganese(II) complex of a new tetradentate ligand was formed by manganese(II) assisted intramolecular cyclization reactions of H2imida.  相似文献   
93.

In the present study, a series of novel {6-[(1H-1,2,3-triazol-4-yl)methoxy]-3-methylbenzofuran-2-yl}(phenyl)methanones (7a–7o) have been synthesized using click chemistry approach. The structures of all newly synthesized compounds were characterized by FT-IR, 1H and 13C NMR, and MASS spectral data. Most of products demonstrated high antimicrobial activity.

  相似文献   
94.
Entropy solutions have been widely accepted as the suitable solution framework for systems of conservation laws in several space dimensions. However, recent results in De Lellis and Székelyhidi Jr (Ann Math 170(3):1417–1436, 2009) and Chiodaroli et al. (2013) have demonstrated that entropy solutions may not be unique. In this paper, we present numerical evidence that state-of-the-art numerical schemes need not converge to an entropy solution of systems of conservation laws as the mesh is refined. Combining these two facts, we argue that entropy solutions may not be suitable as a solution framework for systems of conservation laws, particularly in several space dimensions. We advocate entropy measure-valued solutions, first proposed by DiPerna, as the appropriate solution paradigm for systems of conservation laws. To this end, we present a detailed numerical procedure which constructs stable approximations to entropy measure-valued solutions, and provide sufficient conditions that guarantee that these approximations converge to an entropy measure-valued solution as the mesh is refined, thus providing a viable numerical framework for systems of conservation laws in several space dimensions. A large number of numerical experiments that illustrate the proposed paradigm are presented and are utilized to examine several interesting properties of the computed entropy measure-valued solutions.  相似文献   
95.
In a previous paper we generalized the theory of W?-modules to the setting of modules over nonselfadjoint dual operator algebras, obtaining the class of weak?-rigged modules. At that time we promised a forthcoming paper devoted to other aspects of the theory. We fulfill this promise in the present work and its sequel “Rigged modules II”, giving many new results about weak?-rigged modules and their tensor products. We also discuss the Picard group of weak* closed subalgebras of a commutative algebra. For example, we compute the weak Picard group of H(D), and prove that for a weak* closed function algebra A, the weak Picard group is a semidirect product of the automorphism group of A, and the subgroup consisting of symmetric equivalence bimodules.  相似文献   
96.
Following Elcrat [1] the phenomena associated with the sonic discontinuities in non-equilibrium gasdynamics have been studied here. The sonic wave in non-equilibrium gaseous medium propagates with the frozen speed of sound. The magnitude of discontinuities of the first derivatives of flow quantities in the unsteady flow of relaxing gas are shown to satisfy Riccatti equations along the orthogonal trajectories of surface S(t). In order to integrate them in full generality, they are transformed to an equation along the bicharacteristic curve in the characteristic manifold S(t). These equations have been solved completely. The criteria for decay or blow up of sonic discontinuities are given and the particular cases of plane and spherical waves existing in an ideal dissociative gasdynamics have been discussed. In the case of planewave for uniform propagation, it is shown that the dissociating character of the gas is to decrease the critical time. Other cases of shock formation have been studied in detail.  相似文献   
97.
A general synthetic approach has been developed for heterocyclic prostaglandins (PGF type) starting from the key intermediate ( 6 ). (±) 6 was obtained from (±) ‐Corey lactone. The key intermediate (±) 6 was, in turn, converted in to several new heterocyclic prostaglandins consisting of furan and thiophene moieties in ω‐side chain in two to three steps. Horner–Wadsworth–Emmons reaction and Grignard reaction as the key reactions in these transformations.  相似文献   
98.
Adsorption of pulmonary surfactant to an air-water interface lowers surface tension (γ) at rates that initially decrease progressively, but which then accelerate close to the equilibrium γ. The studies here tested a series of hypotheses concerning mechanisms that might cause the late accelerated drop in γ. Experiments used captive bubbles and a Wilhelmy plate to measure γ during adsorption of vesicles containing constituents from extracted calf surfactant. The faster fall in γ reflects faster adsorption rather than any feature of the equation of state that relates γ to surface concentration (Γ). Adsorption accelerates when γ reaches a critical value rather than after an interval required to reach that γ. The hydrophobic surfactant proteins (SPs) represent key constituents, both for reaching the γ at which the acceleration occurs and for producing the acceleration itself. The γ at which rates of adsorption increase, however, is unaffected by the Γ of protein in the films. In the absence of the proteins, a phosphatidylethanolamine, which, like the SPs, induces fusion of the vesicles with the interfacial film, also causes adsorption to accelerate. Our results suggest that the late acceleration is characteristic of adsorption by fusion of vesicles with the nascent film, which proceeds more favorably when the Γ of the lipids exceeds a critical value.  相似文献   
99.
100.
Based on a multiple scale analysis of a forced reaction-diffusion system leading to amplitude equations, we explain the existence of spiral wave and its photo-induced spatiotemporal behavior in chlorine dioxide-iodine-malonic acid system. When the photo-illumination intensity is modulated, breathing of spiral is observed in which the period of breathing is identical to the period of forcing. We have also derived the condition for breakup and suppression of spiral wave by periodic illumination. The numerical simulations agree well with our analytical treatment.  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