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   323篇
  免费   18篇
  国内免费   1篇
化学   281篇
晶体学   2篇
力学   4篇
数学   29篇
物理学   26篇
  2023年   3篇
  2022年   7篇
  2021年   9篇
  2020年   9篇
  2019年   4篇
  2018年   6篇
  2017年   5篇
  2016年   21篇
  2015年   16篇
  2014年   8篇
  2013年   29篇
  2012年   17篇
  2011年   20篇
  2010年   11篇
  2009年   12篇
  2008年   19篇
  2007年   14篇
  2006年   32篇
  2005年   10篇
  2004年   15篇
  2003年   7篇
  2002年   11篇
  2001年   11篇
  2000年   3篇
  1999年   5篇
  1998年   3篇
  1997年   3篇
  1996年   2篇
  1995年   4篇
  1994年   1篇
  1993年   2篇
  1992年   3篇
  1991年   1篇
  1990年   1篇
  1987年   1篇
  1985年   2篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
  1976年   1篇
  1974年   1篇
  1972年   1篇
  1971年   1篇
  1959年   1篇
  1958年   2篇
  1956年   2篇
排序方式: 共有342条查询结果,搜索用时 15 毫秒
221.
Aqueous solutions of ascorbic acid in unsupported and supported aqueous solutions and real samples were studied by the kinetics-sensitive double-step voltcoulommetric method with the aim to contribute to a better understanding of its behavior in biological systems. The data obtained from measurements made on analytes prepared in the laboratory, as well as those made on real samples (some commercial orange drinks, flash of the fresh fruits) point to the redox reaction of L-ascorbic acid (L-AH2) being very sensitive to both the presence of dissolved gaseous species (O2, CO2) and the ionic strenght in the analyte. Either the dissolved gaseous species, or the higher ionic strength caused by both the presence of supporting electrolyte and increased total concentration of ascorbic acid, respectively, give birth to the degradation of L-AH2. Naturally, the highest percentage of L-AH2 was spotted in fresh fruit.  相似文献   
222.
The contribution presents a thermionic emission-tunnelling theory of the charge transport through a Schottky contact, valid for low injection. The approach extends the classical thermionic emission theory by incorporating the mechanism of tunnelling across the Schottky barrier. The novelty of this transport model is in considering the transfer of charge carriers in both directions, which is accomplished by introducing the Richardson constant also for electrons in the metal, and in the use of the Global Transfer Matrix technique to compute the relevant transmission coefficients.  相似文献   
223.
224.
We consider the Dirichlet problem for linear nonautonomous second order parabolic equations with bounded measurable coefficients on bounded Lipschitz domains. Using a new Harnack-type inequality for quotients of positive solutions, we show that each positive solution exponentially dominates any solution which changes sign for all times. We then examine continuity and robustness properties of a principal Floquet bundle and the associated exponential separation under perturbations of the coefficients and the spatial domain.  相似文献   
225.
226.
Saucha  Juraj  Rados  Jerko 《Meccanica》2001,36(2):177-190
In Vlasov's approach to the problem of stability of thin-walled elastic beams of open cross section simultaneously subjected to transverse bending and to centric compression or tension, a certain inconsistency in derivation of differential equations of stability has been noticed. A consistently carried through derivation leads to equations that differ from Vlasov's ones. The comparison of Vlasov's equations with the results achieved by the classics in the field and by the more recent authors reveals good correspondence. The equations obtained by a consistent derivation, instead, turn out to be correspondent with the equations obtained by the classics Timoshenko and Bleich and with Ojalvo's equations of a second-order theory which determines the orientation of normal planes with the line of shear centers and assumes the validity of the Wagner hypothesis. Sommario. Nellapproccio di Vlasov al problema di stabilita delle travi elastiche a parete sottile e sezione trasversale aperta, contemporaneamente soggete a flessione trasversale ed a pressione o trazione centrale, e stata notata una certa incoerenza nell derivazione delle equazioni differenziali di stabilita. Un procedimento coerente ci porta delle equazioni che si differenziano da quelle di Vlasov. It confronto tra le equazioni di Vlasovedi risultati ottenuti da autori classici e da quelli piu a recenti mostra che esiste un buon accordo, mentre e evidente che le equazioni, ottenute con il procediments coerente, collimano con le equazioni ottenute dai classici Timoshenko e Bleich e con le equazioni di Ojalvo della teoria del secondo ordine, che definisce lorientazione dei piani normali con la linea dei centri di taglio e che presuppone la validita della ipotesi di Wagner.  相似文献   
227.
Advanced models of penetrant transport and reaction in spatially 3D porous polyolefin particles reconstructed from X‐ray µCT images require proper determination of particle morphology. Moreover, polyolefins exhibit a relatively low absorptivity for X‐rays, therefore their investigation using µCT can be difficult. In this paper, a low‐resolution µCT built into an SEM is used to examine how the µCT resolution and several user‐selected parameters associated with the scanning/reconstruction affect the resulting particle morphology. Using samples with known morphology and independent imaging techniques, the performance of the µCT device is critically assessed. Finally, a method suitable for the reliable reconstruction of polyolefin particles using low‐resolution µCT is proposed.

  相似文献   

228.
The process of CFD model application for atmospheric dispersion modeling is presented. Increasing the CPU power opens new possibilities of the CFD approach application for consequence analysis in real complex urban environments. As successful CFD simulation is directly dependent on the quality and complexity of the computational mesh, a new methodology of transferring the Geographic Information System (GIS) data to the computational mesh can be utilized. A user software for importing and manipulation with the GIS data and their subsequent transfer to an instructional file for the generation of the computational mesh was prepared and tested. The introduced methodology is relatively simple and it requires only a small amount of input data. The process of creating a computational mesh is very straightforward and fast, which enables the application of CFD modeling in urban environments in all fields of engineering applications in safety analysis. Several recommendations concerning proper definition of boundary conditions for atmospheric dispersion modeling were summarized. The presented approach was tested on a realistic case study of liquefied chlorine release in a real town. Results obtained by the CFD approach were compared with those obtained by a simpler but standard integral model.  相似文献   
229.
The novel complex [Cu(men)2][Cu2Cd2Cl2(CN)6] (I) was isolated from the aqueous-ethanol system containing CuCl2, men (men = N-methylethane-1,2-diamine) and K2[Cd(CN)4] in the presence of dilute hydrochloric acid and chemically and spectroscopically characterised. The crystal structure of I consists of [Cu 2 I (CN)6] and [Cd2Cl2(CN)6] building units bridged by cyanide ligands and forms a three-dimensional skeleton with cavities. [Cu(men)2]2+ cations in which two men ligands are chelated (mean Cu-N is 2.033(6) Å) are located in the cavities. The coordination polyhedron around the Cu(II) atoms is formed as a tetragonal bipyramidal by two weaker axial Cu-Cl bonds (2.8642(12) Å) with chlorido ligands from the skeleton. The Cu(I) and Cd(II) atoms in the skeleton exhibit tetra-(CuC4 chromophore) and penta-coordination (CdN3Cl2), respectively. The temperature-dependent susceptibility measurements indicate a Curie-Weiss-like behaviour and the presence of weak anti-ferromagnetic interaction.  相似文献   
230.
In large volumes produced activated sludges from wastewater treatment plants (WWTPs) with low concentrations of heavy metals can be utilized as agricultural fertilizers and soil conditioners. Increased contents of toxic xenobiotics are limiting factors that affect the utilization of these heterogeneous wastes. The main aim of our paper was to show the utilization of dried activated sludge (DAS) from municipal WWTP as potential Co2+ ions sorbent i.e. for non-agricultural purposes. The radio indicator method by radionuclide 60Co and γ-spectrometry for characterization DAS sorption properties was used. DAS soluble and solid fractions were characterized by biochemical, ETAAS and CEC analysis. The sorption of Co2+ ions by DAS was rapid process and equilibrium was reached within 2 h. Sorption capacity of DAS (Q) increased with the initial concentration of CoCl2 in the range from 100 to 4,000 μmol l?1, reaching 20 and 160 μmol g?1. Obtained Q values were depent on pH value from 2.0 to 8.0. The maximum sorption capacity (Q max) of DAS at pH 6 calculated from mathematical model of Langmuir adsorption isotherm was 175 ± 9 μmol g?1. FT-IR analyses showed the crucial role of carboxyl functional groups of DAS surfaces on cobalt uptake. For confirmation ion-exchange mechanism in sorption process of Co2+ ions by DAS scanning electron microscopy and EDX analysis were used.  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