全文获取类型
收费全文 | 1156篇 |
免费 | 43篇 |
国内免费 | 11篇 |
专业分类
化学 | 787篇 |
晶体学 | 12篇 |
力学 | 12篇 |
数学 | 145篇 |
物理学 | 254篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 59篇 |
2021年 | 31篇 |
2020年 | 31篇 |
2019年 | 38篇 |
2018年 | 40篇 |
2017年 | 31篇 |
2016年 | 56篇 |
2015年 | 29篇 |
2014年 | 38篇 |
2013年 | 125篇 |
2012年 | 62篇 |
2011年 | 67篇 |
2010年 | 48篇 |
2009年 | 52篇 |
2008年 | 44篇 |
2007年 | 35篇 |
2006年 | 35篇 |
2005年 | 35篇 |
2004年 | 48篇 |
2003年 | 24篇 |
2002年 | 31篇 |
2001年 | 14篇 |
2000年 | 12篇 |
1999年 | 4篇 |
1998年 | 15篇 |
1997年 | 4篇 |
1996年 | 13篇 |
1995年 | 7篇 |
1994年 | 20篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 3篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1977年 | 6篇 |
1975年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1969年 | 2篇 |
排序方式: 共有1210条查询结果,搜索用时 11 毫秒
51.
Oussama Graja Bacem Zghal Kajetan Dziedziech Fakher Chaari Adam Jablonski Tomasz Barszcz Mohamed Haddar 《Comptes Rendus Mecanique》2019,347(1):49-61
Planetary gearboxes are widely used in industrial machines. They usually work in harsh environments giving rise to damages and high maintenance costs. Condition monitoring is a key action allowing one to detect the presence of such damage ensuring healthy running conditions. The knowledge of the dynamic behavior of such a gearbox can be achieved using modeling tools as a primary step before conditioning the monitoring subject. In addition, modeling a gear set can help in the stage of design in order to optimize physical and geometric parameters of the system. Therefore, in this work, a two-dimensional lumped parameter model is adopted to build all vibration sources. The time-varying mesh stiffness is approximated as a square wave form. A novel mathematical formulation is proposed to model the amplitude modulation phenomenon due to the rotational motion of the planets around the center of the gearbox. Finally, the overall vibration signal is concluded as a summation of all vibration components influenced by the modulation function. 相似文献
52.
A novel Schiff base namely (E)-3-((2,6-dihydroxypyrimidin-4-ylimino)methyl)-4H-chromen-4-one and its Co (II), N (II)i, Cu (II) and Cd (II) complexes have been synthesized and proved by elemental analysis, molar conductance, thermal analysis (TGA), Inductive Coupled plasma (ICP), magnetic moment measurements, X-ray powder diffraction, IR, EI-mass,1H NMR, 13C NMR,UV–Vis. and ESR spectral studies. On the basis of these data, it is evident that the Schiff base acts as bidentate via oxygen atom of carbonyl group and azomethine nitrogen atom for Co (II) complex; monobasic bidentate ligand for Ni (II), Cu (II) and Cd (II) complexes via oxygen atom of hydroxyl group and nitrogen atom of pyrimidine ring. The results showed all complexes have octahedral geometry. The average particle size of the ligand and its complexes were found to be 1.010–0.343 nm. The pharmacological action (antioxidant, antimicrobial and anticancer) of the prepared compounds is studied. The antitumor activity of the ligand and its metal complexes is evaluated against human liver carcinoma (HEPG2) cell. The data displayed the Co (II) complexes strong cytotoxicity where IC50 values of Co (II) complex and 5-fluorouracil (stander drug) are 9.33 and 7.86 μg/ml respectively. The Co (II) and Cd (II) complexes have antibacterial activity more than ampicillin (stander drug). The interaction of the synthesized compounds with calf-thymus DNA (CT-DNA) has been performed via absorption spectra and viscosity technique. The DNA- binding constants have been determined. 相似文献
53.
Journal of Thermal Analysis and Calorimetry - Based on the data of the literature reports, the Al–Zr binary system has been re-assessed thermodynamically through the CALPHAD (CALculations of... 相似文献
54.
Graphene oxide ‐ Fe3O4 ‐ NH3+H2PW12O40 ‐ magnetic nanocomposite (GO/Fe3O4/HPW) was prepared by linking amino ‐ functionalized Fe3O4 nanoparticles (Fe3O4 ‐ NH2) on the graphene oxide (GO), and then grafting 12 ‐ tungstophosphoric acid (H3PW12O40) on the graphene oxide ‐ magnetite hybrid (GO ‐ Fe3O4 ‐ NH2). The obtained GO/Fe3O4/HPW nanocomposite was well characterized with different techniques such as FT ‐ IR, TEM, SEM, XRD, EDX, TGA ‐ DTA, AGFM, ICP and BET measurements. The used techniques showed that the graphene oxide layers were well prepared and the various stages of preparation of the GO/Fe3O4/HPW nanocomposites successfully completed. This new nanocomposite displayed excellent performance as a heterogeneous catalyst in the oxidation of alcohols with H2O2. The as ‐ prepared GO/Fe3O4/HPW catalyst was more stable and recyclable at least five times without significantly reducing its catalytic activity. 相似文献
55.
56.
Kamal Shanazari Siamak Banei 《Numerical Methods for Partial Differential Equations》2023,39(2):1635-1651
We apply a boundary element dual reciprocity method (DRBEM) to the numerical solution of the forward–backward heat equation in a two-dimensional case. The method is employed for the spatial variable via the fundamental solution of the Laplace equation and the Crank–Nicolson finite difference scheme is utilized to treat the time variable. The physical domain is divided into two non-overlapping subdomains resulting in two standard forward and backward parabolic equations. The subproblems are then treated by the underlying method assuming a virtual boundary in the interface and starting with an initial approximate solution on this boundary followed by updating the solution by an iterative procedure. In addition, we show that the time discrete scheme is unconditionally stable and convergent using the energy method. Furthermore, some computational aspects will be suggested to efficiently deal with the formulation of the proposed method. Finally, two forward–backward problems, for which the exact solution is available, will be numerically solved for two different domains to demonstrate the efficiency of the proposed approach. 相似文献
57.
Kamal K. Bharadwaj Mohan K. Kadalbajoo R. Sankar 《Applied mathematics and computation》1984,15(2):137-149
The symmetric marching technique has been developed to solve the Poisson equation with Dirichlet boundary conditions. The method has been combined with a mesh refinement technique, which is used as an appropriate interpolation scheme to obtain a solution of the problem on finer grids. The effectiveness of the method has been demonstrated by solving some test examples. 相似文献
58.
Unexpected dimerization of 1,3‐dimethyl‐5‐methylenebarbituric acid revealed by a combined experimental and computational study 下载免费PDF全文
Bernd Doser Kamal Sweidan Norbert Kuhn Christian Ochsenfeld 《Journal of Physical Organic Chemistry》2015,28(5):354-357
A comparison of experimental and calculated 13C‐nuclear magnetic resonance chemical shifts reveals the molecular structure of a dimer that was obtained by an unexpected dimerization of 1,3‐dimethyl‐5‐methylenebarbituric acid. Furthermore, the puckering angle of the cyclobutane unit linking the six‐membered rings is discussed in detail. The influence of substituents on 1,3‐position of the cyclobutane ring on the puckering angle is demonstrated based on 1,1,3,3‐tetramethylcyclobutane. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
59.
This article presents the comparison of approximate and exact small-signal theories for analyzing the influence of the higher-order dispersion terms on dispersive optical communication systems operating near zero dispersion wavelength for linear single-mode fiber. For the approximate theory, the generalized conversion matrix has been reported and gives the transfer function of intensity and phase from the fiber input to fiber output for a laser source including the influence of any higher-order dispersion term. In addition, expressions for the small-signal frequency response and the relative intensity noise (RIN) response of an ultrafast laser diode including noises are derived. However, it is observed that the approximation assumed for the second-order dispersion term for the approximate analysis is not valid. From the approximate theory, the exact generalized conversion matrix and exact expressions for small-signal frequency response and relative intensity noise (RIN) are obtained. We show that for the exact theory, the second-order dispersion term has no effect on intensity and frequency response even at large modulating frequencies and large propagation distances contrary to the approximate theory as reported by other authors. But we show that third-order dispersion term certainly has some minute impact on the frequency and RIN response for long distance links at high modulating frequencies. 相似文献
60.
The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP)
were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined
stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace
determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working
conditions for the determination of the compound were established. The peak current was linear over the concentration range
9.90 × 10–8– 5.96 × 10–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos
in commercial formulations and treated waste water was carried out satisfactorily
Received: 10 July 1997 / Revised: 1 April 1998 / Accepted: 6 April 1998 相似文献