全文获取类型
收费全文 | 515篇 |
免费 | 33篇 |
国内免费 | 5篇 |
专业分类
化学 | 414篇 |
力学 | 11篇 |
数学 | 37篇 |
物理学 | 91篇 |
出版年
2023年 | 6篇 |
2022年 | 18篇 |
2021年 | 16篇 |
2020年 | 37篇 |
2019年 | 35篇 |
2018年 | 43篇 |
2017年 | 40篇 |
2016年 | 44篇 |
2015年 | 30篇 |
2014年 | 44篇 |
2013年 | 55篇 |
2012年 | 33篇 |
2011年 | 24篇 |
2010年 | 24篇 |
2009年 | 9篇 |
2008年 | 26篇 |
2007年 | 20篇 |
2006年 | 11篇 |
2005年 | 3篇 |
2004年 | 5篇 |
2003年 | 6篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 2篇 |
排序方式: 共有553条查询结果,搜索用时 15 毫秒
151.
We present the bound state solution of Schr6dinger equation in D dimensions for quadratic exponential-type potential for arbitrary l-state. We use generalized parametric Nikiforov-Uvarov method to obtain the energy levels and the corresponding eigenfunction in dosed form. We also compute the energy eigenvalues numerically. 相似文献
152.
Spin-half fermions are considered to be limited in a spherical potential well with periodic boundary conditions. The whole system is treated like a relativistic Fermi Gas. Solving the corresponding Dirac equation, the density of states, the Fermi energy, the average energy, the density of states of nucleons and the total energy of the ground-state are obtained. 相似文献
153.
In this study, we simulate numerically quasi pulse propagation in a uniform nonlinear fiber Bragg grating (FBG) made of chalcogenide glasses. Because of bistability and nonlinearity behavior of FBG, the shapes of output pulse vary abruptly and strongly according to the input peak intensity. We take Gaussian pulse as input and produce saw-tooth wave and square wave in the output by suitable input amplitude and FBG length also we could reach pulse chopping and pulse compression. These all optical signal processing achieved for a few length, i.e. 6.6 mm and few intensity, i.e. 35 W/m2 because of high nonlinearity of chalcogenide glasses. 相似文献
154.
This paper considers the problem of hybrid flowshop scheduling. First, we review the shortcoming of the available model in the literature. Then, four different mathematical models are developed in form of mixed integer linear programming models. A complete experiment is conducted to compare the models for performance based on the size and computational complexities. Besides the models, the paper proposes a novel hybrid particle swarm optimization algorithm equipped with an acceptance criterion and a local search heuristic. The features provide a fine balance of diversification and intensification capabilities for the algorithm. Using Taguchi method, the algorithm is fine tuned. Then, two numerical experiments are performed to evaluate the performance of the proposed algorithm with three particle swarm optimization algorithms available in the scheduling literature and one well-known iterated local search algorithm in the hybrid flowshop literature. All the results show the high performance of the proposed algorithm. 相似文献
155.
MohammadReza Elahifard Elham Fazeli Azadeh Joshani MohammadReza Gholami 《Surface and interface analysis : SIA》2013,45(7):1081-1087
Direct CO dissociation is seen the main path of the first step in the Fischer–Tropsch Synthesis (FTS) on the reactive iron surfaces. Cu/Fe alloy film is addressed with various applications over face‐centered‐cubic (fcc)‐Cu and body‐centered‐cubic (bcc)‐Fe in the FTS, i.e. preventing iron carbide formation (through direct CO dissociation) by moderating the surface reactivity and facilitating the reduction of iron surfaces, respectively. In this study by density functional theory, the stable configurations of CO molecule on various Cu/Fe alloys over fcc‐Cu(100) and bcc‐Fe(100) surfaces with different CO coverage (25% and 50%) have been evaluated. Our results showed that the ensemble effect plays a fundamental role to CO adsorption energy on the surface alloys over bcc‐Fe(100); on the other hand, the ligand effect determines the CO stability on the fcc‐Cu(100) surface alloys. CO dissociation barrier was also calculated on the surface alloys that showed although the CO dissociation process is thermodynamically possible on the more reactive surface alloys, but according to their high barrier, CO dissociation does not occur directly on these surfaces. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
156.
Jingdong Mao Weiguo Hu Guangwei Ding Klaus Schmidt-Rohr Geoffrey Davies Elham Ghabbour 《International journal of environmental analytical chemistry》2013,93(4):183-196
Qualitative and quantitative analyses of humic acids (HAs) with five different 13C solid-state NMR techniques were assessed using HAs of various origins and locations. The NMR techniques compared are: (1) direct polarization/magic angle spinning (DP/MAS) at 13 kHz, (2) conventional cross polarization (CP)/MAS at 5 kHz, (3) ramp-CP/MAS at 8 kHz, (4) CP/total sideband suppression (TOSS) at 4.5 kHz, and (5) DP/MAS corrected by CP/spin-lattice relaxation with TOSS. The spectra from the five techniques were first compared qualitatively. Then, each spectrum was divided into eight regions for quantitative evaluation. DP/MAS spectra were used as quantitative references. Ramp-CP/MAS and CP/TOSS spectra gave consistently better results than those of the conventional CP/MAS spectra at a 13C frequency of 75 MHz, which were incorrect due to spinning sidebands. CP/MAS at low magnetic fields (22.6 and 50.6 MHz 13C frequency) indicated improved integration results but lower resolution. Correction factors calculated by comparison with DP/MAS will be useful to convert the non-quantitative peak areas in the CP/TOSS and ramp-CP/MAS spectra into more quantitative results. 相似文献
157.
Babak Kaboudin Khavar Moradi Elham Safaei Hossein Dehghan Peyman Salehi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):1521-1527
Abstract A proline-based aminophosphinic acid ligand and it's vanadium complex have been synthesized and characterized by spectroscopic techniques. The inhibitory activity on pancreatic α-amylase and Baker's yeast α-glucosidase has been examined in vitro. The novel complex showed more inhibitory potency against pancreatic α-amylase and Baker's yeast α-glucosidase compared to acarbose as an antidiabetic drug. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
158.
Mohammad Hossein Habibi Elham Askari 《Journal of Thermal Analysis and Calorimetry》2013,111(1):227-233
In the present investigation, a novel synthetic zinc zirconate nanocomposites were prepared by a sol–gel technique using a very stable sol containing zirconium acetylacetonate, zinc acetate, monoethanolamine, and 1,3-propanediol as chelating agent. Thermal analysis results indicated that the decomposition of zinc zirconate precursors occurred at 225 and 234 °C. The influence of thermal annealing (temperatures and duration) on their structural properties has been studied by means of X-ray diffraction and Fourier transform-infrared spectroscopy techniques. The results indicated that a higher percent of zinc zirconate is formed at 800 °C and reached maximum at 1,000 °C in 120 min. The morphology, composition, and bandgap properties of zinc zirconate nanoparticles were characterized by transmission electron microscope, scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy, and ultraviolet diffiuse reflectance. The SEM observation showed that average grain size of zinc zirconate nanopowders was 58 nm. The optical results revealed maximum absorbances at 394, 413, and 438 nm for ZnZrO3 sample annealed at 400, 800, and 900 °C for 30 min, respectively. This is an indication that the nanopowder can absorb lights in the higher wavelength. 相似文献
159.
P. Rezayati Charani M. Dehghani-Firouzabadi E. Afra Å. Blademo A. Naderi T. Lindström 《Cellulose (London, England)》2013,20(5):2559-2567
This work investigated the effect of using Kenaf bast fibre kraft pulps compared to Scotch Pine kraft pulps for producing microfibrillated cellulose (MFC) and its employment for improving mechanical and physical properties of handsheets made from unbleached kraft hardwood pulp. It was shown that MFC based on Kenaf fibres can be produced at higher consistencies [>5 % (w/w)] compared to when Scotch Pine is employed [≈2 % (w/w)] as raw material. The possibility of using a higher consistency when processing Kenaf is beneficial for the processing in microfluidizers. The rheological properties of the products were shown to be consistent with what is known for MFC-based systems. The studies indicate that the mechanical properties of handsheets from unbleached kraft hardwood pulp can be improved by replacing part of the unbleached kraft hardwood pulp fibres with either unbleached kraft Kenaf pulp or unbleached Scotch Pine kraft pulp. However, the same levels of improvements were obtained when using only a small amount [≈6 % (w/w)] of MFC based on Kenaf or Scotch Pine, when introduced into the system either as a dry strength additive or by coating pre-made handsheets. Finally, it was shown that the incorporation of MFC in handsheets decreases the air-permeability; this effect became amplified when the MFC was applied as a coating onto the handsheets. 相似文献
160.
Dirac formalism of Hamiltonian constraint systems is studied for the noncommutative Abelian Proca field. It is shown that
the system of constraints are of second class in agreement with the fact that the Proca field is not gauge invariant. Then,
the system of second class constraints is quantized by introducing Dirac brackets in the reduced phase space. 相似文献