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   2518篇
  免费   64篇
  国内免费   8篇
化学   1636篇
晶体学   44篇
力学   87篇
数学   579篇
物理学   244篇
  2023年   12篇
  2022年   35篇
  2021年   67篇
  2020年   40篇
  2019年   44篇
  2018年   33篇
  2017年   23篇
  2016年   68篇
  2015年   69篇
  2014年   56篇
  2013年   127篇
  2012年   113篇
  2011年   149篇
  2010年   91篇
  2009年   83篇
  2008年   126篇
  2007年   110篇
  2006年   135篇
  2005年   130篇
  2004年   107篇
  2003年   92篇
  2002年   82篇
  2001年   30篇
  2000年   21篇
  1999年   29篇
  1998年   22篇
  1997年   21篇
  1996年   42篇
  1995年   26篇
  1994年   37篇
  1993年   28篇
  1992年   20篇
  1991年   21篇
  1990年   22篇
  1989年   13篇
  1988年   18篇
  1987年   16篇
  1986年   9篇
  1985年   25篇
  1984年   16篇
  1983年   20篇
  1982年   23篇
  1981年   20篇
  1980年   31篇
  1979年   28篇
  1978年   17篇
  1976年   16篇
  1975年   12篇
  1915年   9篇
  1906年   7篇
排序方式: 共有2590条查询结果,搜索用时 406 毫秒
91.
92.
Traditional hydrodistillation (HD), supercritical fluid extraction (SFE), organic solvent extraction (SE), and water microwave assisted hydrodistillation (MAHD) techniques were compared and evaluated for their effectiveness in the isolation of rosemary essential oil. The microwave assisted hydrodistillation technique was optimized in terms of both delivered power and time duration. The extracts/distillates were analyzed by GC and GC-MS. Microwave distillation, which exploits the physical action of microwaves on plants, showed a series of advantages over the other approaches: low cost, use of water in sample pre-treatment step, greatly reduced isolation time, and attainment of high quality essential oil distillate. Moreover, the absence of environmental impact of this innovative technique was also emphasized.  相似文献   
93.
The retro 1,3-dipolar cycloaddition induced by electron impact on 3,5-diphenyl-1,2,4-oxadiazole is interpreted as a two-step process on the basis of the energetics and kinetics of the fragments [C7H5NO]+., [C6H5CN]+. and [C6H5CO]+.  相似文献   
94.
The preparation of the nucleophile trans-[RuCl(NO)( 1 )], where 1 is the bidentate ligand Ph2PCH2C18CH2PPh2, and of the five-coordinate species [RuCl(CO)(NO)( 1 )], [RuCl(CO)(NO)(Ph2PCH2Ph)2] and [RuCl(NO)( 2 )( 1 )] are reported. The crystal structure of [RuCl(CO)(NO)( 1 )] shows that the coordination around the metal atom is distorted trigonal bipyramidal with the phosphorus atoms in axial positions. The Ru? N? O bond angle is 142.8°. 1H- and 31P-NMR. and \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \nu $\end{document}NO IR.-data for the above complexes are reported and related to the coordination geometry.  相似文献   
95.
Abstract

The purpose of this study was to conduct a comparative evaluation of some biological properties of methanol and water extracts of leaves of five Juniperus taxa growing in Turkey: J. communis L. var. communis (Jcc), J. communis L. var. saxatilis Pall. (Jcs), J. drupacea Labill. (Jd), J. oxycedrus L. subsp. oxycedrus (Joo), J. oxycedrus L. subsp. macrocarpa (Sibth. & Sm.) Ball. (Jom). The antioxidant properties were examined in vitro; both in the DPPH and in the reducing power tests, Joo methanol extract resulted the most active (IC50?=?0.09?±?0.01mg/mL and ASE/mL?=?2.56?±?0.06). In the TBA assay, Jcs methanol extract exhibited the highest activity (IC50?=?4.39?±?0.47?μg/mL). The extracts displayed bacteriostatic activity against Staphylococcus aureus, and Jd methanol extract resulted the most effective (MIC?=?19.53?μg/mL); no effect on the S. aureus biofilm formation was observed. The extracts resulted non-toxic in the Artemia salina lethality bioassay. Finally, the phenolic profile of the methanol extracts was characterized by HPLC-PDA/ESI-MS.  相似文献   
96.
The relative permittivities for the ternary 1,2-ethanediol (component 1) + 2-methoxyethanol (2) + water (3) solvent system have been measured for 66 mixtures covering the whole mole fraction composition 0X1/X2/X3 1 range at –10, –5 and 0 °C. The experimental data were used to test some empirical relations stating the dependence of = (X1, X2, X3). A comparison between the calculated and experimental data show that these equations can be usefully employed to predict values in correspondence of the experimental data gaps.  相似文献   
97.
Summary In this paper we consider the alternative Cauchy functional equationg(xy) g(x)g(y) impliesf(xy) = f(x)f(y) wheref, g are functions from a topological group (X, ·) into a group (S,·). First we prove that, ifS is a Hausdorff topological group andX satisfies some weak additional hypotheses, then (f, g) is a continuous solution if and only if eitherf org is a homomorphism. Then we describe a more general class of solutions forX =R n .Partially supported by M.U.R.S.T. Research funds (40%)Dedicated to the memory of Alexander M. Ostrowski on the occasion of the 100th anniversary of his birth.  相似文献   
98.
In this paper we propose a recursive quadratic programming algorithm for nonlinear programming problems with inequality constraints that uses as merit function a differentiable exact penalty function. The algorithm incorporates an automatic adjustment rule for the selection of the penalty parameter and makes use of an Armijo-type line search procedure that avoids the need to evaluate second order derivatives of the problem functions. We prove that the algorithm possesses global and superlinear convergence properties. Numerical results are reported.  相似文献   
99.
For classical Hamiltonian systems containing an harmonic oscillator of high frequency, one has the problem of controlling the energy exchange between the oscillator and the remaining slow degrees of freedom; under very general conditions, such an exchange turns out to be exponentially small with the frequency of the oscillator. In the Jeans-Landau-Teller method, one aims to prove the exponential dependence, and to estimate the coefficient of the exponential, by exploiting the analyticity properties of the solution of the differential equations describing the motion of the system. However, in practice, since the exact solution is not known, such properties are inferred from those of an approximate solution, with no control of the difference; this fact might a priori even invalidate the exponential dependence itself. In the present paper a rigorous treatment is given, for a particular model of interest in the domain of atomic collisions, by keeping control of the difference between the exact and the approximate solution.  相似文献   
100.
The preparations of the binuclear hydrido-bridged cations [(terdentate ligand)Pd(μ-H)Pd(terdentate ligand)]+ from [(terdentate ligand)Pd(acetone)]+ and NaO2CH and [(terdentate ligand)Pd(μ-H)Pt(terdentate ligand)]+ from [(terdentate ligand)Pd(acetone)]+ and [(terdentate ligand)PtH] (terdentate ligand = 2,6-(Ph2PCH2)2C6H3) are reported. The preparation of the cation [(terdentate ligand)Pt(μ-H)Pt(terdentate ligand)]+ is also reported.  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